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EPSRC THERMAL MANAGEMENT OF
INDUSTRIAL PROCESSES
Case Study Thermal Design of a Biomass Drying Process Using Low Grade Heat from Steel Industry
(July 2011)
Report Prepared by SUWIC Sheffield University
Researcher Dr J Zhou
Investigators Professor Jim Swithenbank Professor Vida N Sharifi
Sheffield University Waste Incineration Centre (SUWIC) Department of Chemical and Biological Engineering Sheffield University
- 1 -
Executive Summary
A considerable amount of waste heat is available from process industries in the
form of cooling water and flue gases Depending on their temperatures these
sources of low grade heat can be utilised in a number of ways such as district heating
systems heat pumps condensing boilers and drying of biomass
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased
In accordance with our EPSRC grant proposal Sheffield University has conducted
an extensive literature review of biomass drying looking into various technologies
and the associated costs In addition calculations were carried out as part of a case
study in order to investigate the thermal design of a biomass drying system using the
waste heat from steel industry The possible use of each flue gas stream in drying
biomass was analysed Additional calculations were conducted in order to estimate
the capital and running costs of the process In addition the effects of different
initial and final moisture contents of the biomass material on the performance of dryer
and the associated drying costs were evaluated This report presents the results
obtained from the above studies
Acknowledgements
The authors would like to thank the Engineering and Physical Science Research
Council (EPSRC Thermal Management of Industrial Processes Consortium) for their
financial and technical support for this research work
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List of Contents
1 Introduction3
2 Literature Review Biomass Drying 5
21 Drying Process and Mechanism5
22 Dryer Principle6
221 Heating Source6
222 Heating Method 8
223 Types of Dryer 9
23 Selection of Dryers 15
24 Capital and Running Costs16
25 Safety and Environmental Issues 18
3 Case Study Biomass Drying Process Design Using Low Grade Heat 20
31 Low Grade Heat from Steel Production Process 20
32 Drying System Design 25
321 Thermal Design Methodology 25
322 Dryer Capacity 28
323 Drying Curve 32
324 Dryer Parameters 36
33 Cost Estimation41
331 Capital Costs 41
332 Running Costs48
333 Profitability 49
4 Conclusions54
References55
- 3 -
1 Introduction
Biomass has some environmental advantages over fossil fuels as it generates lower
level of pollutants such as SO2 and CO2 Therefore biomass as the only significant
source of carbon-based renewable fuel can replace fossil fuels for heating power
generation and transport
The combustion of biomass can be divided into several processes ie drying
pyrolysis gasification and combustion The moisture content of biomass typically
varies between 50-63 wt (wet basis) depending on the season weather and the type
of material (Holmberg and Ahtila 2004) The high moisture content in biomass
requires more energy for evaporation of water in the combustion chamber which
cannot be utilized in the power generation Hence the energy input into the process
is decreased which consequently results in a reduced heat andor electricity
production Table 11 presents data for the combustion of wood fuel with different
moisture contents (Wimmerstedt 2006) Here it is assumed that the flue gas
temperature is constant at 150 ordmC and the feeding air temperature is 40 ordmC The
calculation is based on 1 kg of dry material
Table 11 Combustion of wood fuel with different moisture contents
Moisture contents () 65 50 15
Water amount (kgkgdm) 19 10 02
Anticipated excess air level 16 14 12
Low calorific value (MJkg) 144 165 186
Flue gas volume at 1 bar 0 ordmC (m3kg) 103 88 62
Flue gas sensible heat loss (MJkg) 21 18 13
Efficiency 085 089 093
Adiabatic combustion temperature (ordmC) 900 1200 1800
As shown in table 11 a high level of excess air ratio is required when burning a
wood fuel with high moisture content This results in lower temperatures in the
boiler which in turn is highlighted by the adiabatic combustion temperatures In
addition there is a significant increase in the amount of flue gases due to the
evaporated water and the higher level of excess air ratio Therefore the flue gas heat
loss increases at higher fuel moisture content and the boiler efficiency is decreased
Some of the main reasons for drying biomass are highlighted in the lsquoHandbook of
Biomass Combustion and Co-firingrsquo (Loo and Koppejan 2008) These are as
follows
1 The heating value of the fuel (based on NCV) is affected by its moisture content
Therefore the efficiency of the combustion system increases as the moisture content
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decreases
2 A more complex combustion technology and process control system is required if
the moisture content of the fuel varies which will add extra investment costs
3 The moisture content of the fuel should be below 30 wt (wet basis) for
domestic applications This is because the long-term storage of wet biomass fuels
causes problems with dry-matter loss and hygiene
4 The moisture content of material should be about 10-30 wt (wet basis) for use
in a small scale furnace
5 The moisture content of the raw material must be about 10 wt (wet basis) for
production of pellets
Although it is known that drying biomass fuel provides significant benefits these
benefits must be balanced against increased capital and operating costs occurred by
the drying process
In UK a large amount of low grade heat is generated by the process industry In
July 2008 the market potential for surplus heat from industrial processes was
estimated at 65 PJ by the Governmentrsquos Office of Climate Change (BERR 2008) and
36-71 PJ in a report by McKenna (McKenna 2009) Low grade heat (ie flue gas
hot water and steam) from the process industries can be used as a source of energy for
drying biomass This is beneficial to both process industries and the industries
utilizing biomass
This report presents the results obtained from the literature review work (ie drying
mechanism and biomass drying technologies) It also presents the results obtained
from a series of calculations which were carried out in order to investigate the design
of a biomass drying process using the low grade heat from process industries as the
heating source
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2 Literature Review Biomass Drying
21 Drying Process and Mechanism
Drying is a process that removes moisture thermally to yield a solid product It is
a complex operation involving transient transfer of heat and mass During the
thermal drying two processes occur simultaneously (1) the energy (mostly as heat) is
transferred from the surrounding environment to evaporate the surface moisture by
the means of convection conduction or radiation (2) the internal moisture is
transferred to the surface of solid and then evaporated due to the process (1) In
process (1) the removal of water as vapour from the material surface is determined by
the external conditions such as temperature air humidity and flow area of exposed
surface and pressure In process (2) the transport of moisture within the solid
depends on the physical nature of the solid the temperature and its moisture content
(Mujumdar 2006)
The drying behaviour of solids can be characterized by measuring the moisture
content loss as a function of time Figure 21 shows a typical drying rate curve of a
hygroscopic product Three stages can be distinguished during the drying process
In the first constant drying rate stage the external free water attached to the product is
removed The rate-controlling step in this drying stage is the diffusion of the water
vapour across the air-moisture interface and the rate at which the surface for diffusion
is removed Towards the end of the constant drying rate stage moisture has been
transported from the inside of the solid to the surface by capillary forces and the
drying rate may still be constant The drying rate starts to fall when the average
moisture content reaches the critical moisture content This leads to the second
falling rate stage of unsaturated surface drying The internal diffusion of water to
the surface of the product takes place in this period It proceeds until the surface
film of liquid is entirely evaporated In the following third drying stage the
controlling step is the rate at which moisture may move through the solid due to the
concentration gradients between the deeper parts and the surface The heat transfer
to the surface and the heat conduction in the product are both active and the latter
influences the drying rate increasingly The rate of drying reduces even more rapidly
than before and drying stops once the moisture content falls down to the equilibrium
value for the prevailing air humidity The second and third stages can also be
combined together since the both experience the falling drying rate
The understanding of drying process and drying mechanism is extremely important
when drying biomass It is on the one hand expected to operate the drying at high
temperature in order to accelerate the heat transfer and minimize the equipment size
but on the other hand there are concerns with regard to the ignition of the biomass
The risk of biomass being ignited usually occurs at two points during the drying
process The first one is just at the end of the constant drying rate period when the
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surface moisture has evaporated but an appreciate amount of water has not moved
from the inside to the surface In this short period the temperature at the surface
increases quickly since there is no water vapour near the surface to keep the biomass
particles cool The second point occurs when the biomass is over dried The
biomass could be ignited when it reaches its combustion temperature or the emitted
gases reach their flash point Over drying only happens during the upset conditions
or when using unsuitable dryers
Figure 21 Typical rate-of-drying curve (Mujumdar 2006)
22 Dryer Principle
The drying system needs to meet three requirements heat source drying method
and the form of agitation to expose new material for drying (Liptaacutek 1998a) The
different methods to achieve these requirements result in different dryers These
three requirements are consistent with the principle factors suggested by Keey (Keey
1972 1978) which could be used to classify dryer manner of heat supply to the
material temperature and pressure of operation and manner to handle the material
within the dryer
221 Heating Source
Drying mediums are mostly flue gas air and steam For drying within a biomass
fired combustion plant possible sources of heating are from (1) exhaust gases from
hot furnace engine or gas turbine (2) high-pressure steam from a steam or combined
cycle plant (3) warm air from an air-cooled condenser in a steam or combined cycle
plant and (4) steam from dedicated combustion of surplus biomass or diverted
product gas char or bio-fuel (Fagernaumls et al 2010) Figures 22 and 23 show the
principles of a flue gas dryer and a superheated steam dryer in combination with a
- 7 -
boiler respectively For the flue gas dryer the flue gases after the boiler are taken
through a fuel dryer in which the fuel used in the boiler is dried For superheated
steam dryer the steam is extracted from the boiler and the evaporated water from the
dryer is recovered as low-pressure steam The superheated steam dryers have some
advantages compared to the flue gas dryers The total energy efficiency is increased
due to the possibility of reuse the latent heat of evaporation No oxidation or
combustion reaction is possible which eliminates the risks of explosions and hazards
And steam dryers have higher drying rates than flue gas dryers However steam
dryers also have some disadvantages Due to the high temperature level they have
problems with temperature-sensitive materials (Beeby and Potter 1985) For drying
biomass using steam dryer volatile organic materials contained in the biomass may be
emitted increasingly together with the water vapour at higher temperature steam
drying This would reduce the heating value of the biomass and increase the costs of
treating the exhaust steam In addition superheated steam dryers are difficult to
achieve low moisture content and the initial condensation may increase the total
drying time The systems also become more complex compared to those using flue
gas dryers
Figure 22 Principle of a flue gas dryer in combination with a boiler (Wimmerstedt
2006)
Figure 23 Principle of a superheated steam dryer in combination with a boiler
(Wimmerstedt 2006)
- 8 -
222 Heating Method
Convection conduction and radiation are three commonly used methods in
industrial drying In most cases heat is transferred to the surface of the product and
then to the interior However using dielectric radio frequency (RF) or microwave
freezing drying methods heat is generated internally within the product and then
transfers to the exterior surface
Convection
Convection is possibly the most common mode of drying Heat for evaporation is
supplied by convection to the exposed surface of the material and the evaporated
moisture is carried away by the drying medium Air inert gas (eg N2) direct
combustion gas or superheated steam can be as the drying source Convective
dryers can also be called direct dryers During the constant drying rate period the
solid surface takes on the wet bulb temperature which is determined by the ambient
air temperature and humidity at the same location While during the falling rate
period the solidsrsquo temperature approaches the dry bulb temperature of the drying
medium It should be noted that when using superheated steam as the drying
medium the solidsrsquo temperature corresponds to the saturation temperature at the
operating pressure
Conduction
Conduction or indirect drying is more suitable for thin products or for very wet
solids Heat is supplied through heated surfaces (stationary or moving) placed
within the dryer to support convey or confine the solids The evaporated moisture
is carried away by vacuum operation or by a stream of gas as a carrier of moisture
The thermal efficiency of conductive dryers is higher than convective dryers as the
latter loses a considerable amount of enthalpy with the drying medium
Radiation
Infrared radiation is often used in drying coatings thin sheets and films Although
most moisture materials are poor conductors for 50-60 Hz current the impedance falls
dramatically at radio frequency Hence such radiation could be used to heat the
solid volumetrically Energy is absorbed selectively by the water molecules Thus
less energy is required as the material becomes drier Since the capital and operating
costs are high for radiation drying it is usually to dry high unit value products or to
finally correct the moisture profile wherein only small quantities of hard-to-get
moisture are removed
It is noteworthy that sometimes the different drying methods can be combined
together For example a fluid bed dryer with immersed heating tubes or coils
- 9 -
combines advantages of both direct and indirect heating It can be only one third the
size of a purely convective fluid bed dryer for the same duty The combination of
radiation and convection is also feasible such as infrared plus air jets or microwave
with impingement
223 Types of Dryer
The dryers can be classified according to the method of heating transfer or the
drying source Using the former classification the dryers can be divided into
convective dryers conductive dryers radiative dryers and dielectric dryers Using
the later classification the dryers can be broadly divided into airflue gas dryers and
superheated steam dryers In biomass drying the most common types of flue gas
dryers are rotary drum dryers and flash dryers And the commercial scale steam
dryers for biomass reported so far are tubular dryers fluidized bed dryers and
indirectly flash dryers (Wimmerstedt 2006 Fagernaumls et al 2010) In this section
some widely used biomass dryers will be review briefly
Rotary Dryer
Rotary dryer has been used for a long time in drying biomass and is by far the most
common dryer type in the existing large scale bioenergy plants It consists of a
slightly inclined rotating cylindrical shell fitted with a number of longitudinal flights
The flights lift the material and cascade it in a uniform curtain through the passing
gases Wet biomass is fed into the upper end of the dryer moves through it by virtue
of rotation head effect and slope of the shell and withdraws at the lower end finally
A schematic diagram of a direct co-current rotary dryer is shown in Figure 24 The
shell diameter can range from lt 1 m to gt 6 m depending on the throughput And the
drum rotating speed is varied from 2 to 8 rpm The direction of drying medium can
be either co-current or counter-current relative to the solids The biomass and hot
airflue gas normally flow co-currently through the dryer The hottest flue gas
contacts with the wettest biomass and the cooled flue gas contacts with the dried
biomass which could reduce the fire risk The exhaust gases leaving the dryer pass
through a cyclone multicyclone baghouse filter scrubber or electrostatic precipitator
(ESP) to remove any fine material entrained in the air According to the dryer
configuration an ID fan may be required which can be placed before the emission
control equipment to reduce erosion of the fan or after the first cyclone to provide the
pressure drop
Indirectly heated rotary dryers are widely used for the materials that would be
contaminated by the drying medium The heat source passes through the outer wall
of the dryer or through an inner central shaft to heat the dryer by conduction A
combined directindirect rotary dryer also exists where very hot flue gases enter the
dryer through a central shaft and initially provide heat indirectly by conduction then
the same gases pass through the dryer coming into direct contact with the wet
- 10 -
material During the second pass the indirect heating warms the flue gas and
material In this way a high flue gas temperature can be used for heating while the
fire risk is reduced by limiting the temperature of the gas in direct contact with the
biomass
Figure 24 Schematic diagram of direct rotary dryer (Krokida et al 2006)
The inlet gas temperature to rotary biomass dryer can vary from 230 ordmC to 1100 ordmC
and the outlet gas temperature varies from 70 ordmC to 110 ordmC To avoid the
condensation of acids and resins the outlet gas temperature is normally higher than
104 ordmC The retention time in rotary dryers can be less than a minute for small
particles and 10 to 30 minutes for larger material (Haapanen et al 1983
Intercontinental Engineering Ltd 1980 MacCallum et al 1981 Wardrop
Engineering Inc 1990)
The advantages of rotary dryers include (1) they are less sensitive to particle size
and can accept the hottest flue gas of any type of dryer (2) they have low
maintenance costs and the greatest capacity of any type of dryer (Intercontinental
Engineering Ltd 1980) But the material moisture is hard to control in rotary dryers
due to the long lag time for material in the dryer (Fredrikson 1984) Rotary dryers
also present the greatest fire hazard and require the most space (Intercontinental
Engineering Ltd 1980)
Conveyor Dryer
The conception of conveyor dryer (belt dryer) is simple The material is spread on
to a horizontally moving permeable belt in a continuous process and the heating
medium is forced through the bed of product by fans The drying medium is usually
either air or flue gas and its flow can be upward or downward Conveyor dryers are
very versatile and can handle a wide range of materials making them attractive for
biomass feedstocks Figure 25 is the conveyor dryer developed by Swiss Combi
(2009)
According to conveyor and airflow arrangement conveyor dryers generally have
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 1 -
Executive Summary
A considerable amount of waste heat is available from process industries in the
form of cooling water and flue gases Depending on their temperatures these
sources of low grade heat can be utilised in a number of ways such as district heating
systems heat pumps condensing boilers and drying of biomass
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased
In accordance with our EPSRC grant proposal Sheffield University has conducted
an extensive literature review of biomass drying looking into various technologies
and the associated costs In addition calculations were carried out as part of a case
study in order to investigate the thermal design of a biomass drying system using the
waste heat from steel industry The possible use of each flue gas stream in drying
biomass was analysed Additional calculations were conducted in order to estimate
the capital and running costs of the process In addition the effects of different
initial and final moisture contents of the biomass material on the performance of dryer
and the associated drying costs were evaluated This report presents the results
obtained from the above studies
Acknowledgements
The authors would like to thank the Engineering and Physical Science Research
Council (EPSRC Thermal Management of Industrial Processes Consortium) for their
financial and technical support for this research work
- 2 -
List of Contents
1 Introduction3
2 Literature Review Biomass Drying 5
21 Drying Process and Mechanism5
22 Dryer Principle6
221 Heating Source6
222 Heating Method 8
223 Types of Dryer 9
23 Selection of Dryers 15
24 Capital and Running Costs16
25 Safety and Environmental Issues 18
3 Case Study Biomass Drying Process Design Using Low Grade Heat 20
31 Low Grade Heat from Steel Production Process 20
32 Drying System Design 25
321 Thermal Design Methodology 25
322 Dryer Capacity 28
323 Drying Curve 32
324 Dryer Parameters 36
33 Cost Estimation41
331 Capital Costs 41
332 Running Costs48
333 Profitability 49
4 Conclusions54
References55
- 3 -
1 Introduction
Biomass has some environmental advantages over fossil fuels as it generates lower
level of pollutants such as SO2 and CO2 Therefore biomass as the only significant
source of carbon-based renewable fuel can replace fossil fuels for heating power
generation and transport
The combustion of biomass can be divided into several processes ie drying
pyrolysis gasification and combustion The moisture content of biomass typically
varies between 50-63 wt (wet basis) depending on the season weather and the type
of material (Holmberg and Ahtila 2004) The high moisture content in biomass
requires more energy for evaporation of water in the combustion chamber which
cannot be utilized in the power generation Hence the energy input into the process
is decreased which consequently results in a reduced heat andor electricity
production Table 11 presents data for the combustion of wood fuel with different
moisture contents (Wimmerstedt 2006) Here it is assumed that the flue gas
temperature is constant at 150 ordmC and the feeding air temperature is 40 ordmC The
calculation is based on 1 kg of dry material
Table 11 Combustion of wood fuel with different moisture contents
Moisture contents () 65 50 15
Water amount (kgkgdm) 19 10 02
Anticipated excess air level 16 14 12
Low calorific value (MJkg) 144 165 186
Flue gas volume at 1 bar 0 ordmC (m3kg) 103 88 62
Flue gas sensible heat loss (MJkg) 21 18 13
Efficiency 085 089 093
Adiabatic combustion temperature (ordmC) 900 1200 1800
As shown in table 11 a high level of excess air ratio is required when burning a
wood fuel with high moisture content This results in lower temperatures in the
boiler which in turn is highlighted by the adiabatic combustion temperatures In
addition there is a significant increase in the amount of flue gases due to the
evaporated water and the higher level of excess air ratio Therefore the flue gas heat
loss increases at higher fuel moisture content and the boiler efficiency is decreased
Some of the main reasons for drying biomass are highlighted in the lsquoHandbook of
Biomass Combustion and Co-firingrsquo (Loo and Koppejan 2008) These are as
follows
1 The heating value of the fuel (based on NCV) is affected by its moisture content
Therefore the efficiency of the combustion system increases as the moisture content
- 4 -
decreases
2 A more complex combustion technology and process control system is required if
the moisture content of the fuel varies which will add extra investment costs
3 The moisture content of the fuel should be below 30 wt (wet basis) for
domestic applications This is because the long-term storage of wet biomass fuels
causes problems with dry-matter loss and hygiene
4 The moisture content of material should be about 10-30 wt (wet basis) for use
in a small scale furnace
5 The moisture content of the raw material must be about 10 wt (wet basis) for
production of pellets
Although it is known that drying biomass fuel provides significant benefits these
benefits must be balanced against increased capital and operating costs occurred by
the drying process
In UK a large amount of low grade heat is generated by the process industry In
July 2008 the market potential for surplus heat from industrial processes was
estimated at 65 PJ by the Governmentrsquos Office of Climate Change (BERR 2008) and
36-71 PJ in a report by McKenna (McKenna 2009) Low grade heat (ie flue gas
hot water and steam) from the process industries can be used as a source of energy for
drying biomass This is beneficial to both process industries and the industries
utilizing biomass
This report presents the results obtained from the literature review work (ie drying
mechanism and biomass drying technologies) It also presents the results obtained
from a series of calculations which were carried out in order to investigate the design
of a biomass drying process using the low grade heat from process industries as the
heating source
- 5 -
2 Literature Review Biomass Drying
21 Drying Process and Mechanism
Drying is a process that removes moisture thermally to yield a solid product It is
a complex operation involving transient transfer of heat and mass During the
thermal drying two processes occur simultaneously (1) the energy (mostly as heat) is
transferred from the surrounding environment to evaporate the surface moisture by
the means of convection conduction or radiation (2) the internal moisture is
transferred to the surface of solid and then evaporated due to the process (1) In
process (1) the removal of water as vapour from the material surface is determined by
the external conditions such as temperature air humidity and flow area of exposed
surface and pressure In process (2) the transport of moisture within the solid
depends on the physical nature of the solid the temperature and its moisture content
(Mujumdar 2006)
The drying behaviour of solids can be characterized by measuring the moisture
content loss as a function of time Figure 21 shows a typical drying rate curve of a
hygroscopic product Three stages can be distinguished during the drying process
In the first constant drying rate stage the external free water attached to the product is
removed The rate-controlling step in this drying stage is the diffusion of the water
vapour across the air-moisture interface and the rate at which the surface for diffusion
is removed Towards the end of the constant drying rate stage moisture has been
transported from the inside of the solid to the surface by capillary forces and the
drying rate may still be constant The drying rate starts to fall when the average
moisture content reaches the critical moisture content This leads to the second
falling rate stage of unsaturated surface drying The internal diffusion of water to
the surface of the product takes place in this period It proceeds until the surface
film of liquid is entirely evaporated In the following third drying stage the
controlling step is the rate at which moisture may move through the solid due to the
concentration gradients between the deeper parts and the surface The heat transfer
to the surface and the heat conduction in the product are both active and the latter
influences the drying rate increasingly The rate of drying reduces even more rapidly
than before and drying stops once the moisture content falls down to the equilibrium
value for the prevailing air humidity The second and third stages can also be
combined together since the both experience the falling drying rate
The understanding of drying process and drying mechanism is extremely important
when drying biomass It is on the one hand expected to operate the drying at high
temperature in order to accelerate the heat transfer and minimize the equipment size
but on the other hand there are concerns with regard to the ignition of the biomass
The risk of biomass being ignited usually occurs at two points during the drying
process The first one is just at the end of the constant drying rate period when the
- 6 -
surface moisture has evaporated but an appreciate amount of water has not moved
from the inside to the surface In this short period the temperature at the surface
increases quickly since there is no water vapour near the surface to keep the biomass
particles cool The second point occurs when the biomass is over dried The
biomass could be ignited when it reaches its combustion temperature or the emitted
gases reach their flash point Over drying only happens during the upset conditions
or when using unsuitable dryers
Figure 21 Typical rate-of-drying curve (Mujumdar 2006)
22 Dryer Principle
The drying system needs to meet three requirements heat source drying method
and the form of agitation to expose new material for drying (Liptaacutek 1998a) The
different methods to achieve these requirements result in different dryers These
three requirements are consistent with the principle factors suggested by Keey (Keey
1972 1978) which could be used to classify dryer manner of heat supply to the
material temperature and pressure of operation and manner to handle the material
within the dryer
221 Heating Source
Drying mediums are mostly flue gas air and steam For drying within a biomass
fired combustion plant possible sources of heating are from (1) exhaust gases from
hot furnace engine or gas turbine (2) high-pressure steam from a steam or combined
cycle plant (3) warm air from an air-cooled condenser in a steam or combined cycle
plant and (4) steam from dedicated combustion of surplus biomass or diverted
product gas char or bio-fuel (Fagernaumls et al 2010) Figures 22 and 23 show the
principles of a flue gas dryer and a superheated steam dryer in combination with a
- 7 -
boiler respectively For the flue gas dryer the flue gases after the boiler are taken
through a fuel dryer in which the fuel used in the boiler is dried For superheated
steam dryer the steam is extracted from the boiler and the evaporated water from the
dryer is recovered as low-pressure steam The superheated steam dryers have some
advantages compared to the flue gas dryers The total energy efficiency is increased
due to the possibility of reuse the latent heat of evaporation No oxidation or
combustion reaction is possible which eliminates the risks of explosions and hazards
And steam dryers have higher drying rates than flue gas dryers However steam
dryers also have some disadvantages Due to the high temperature level they have
problems with temperature-sensitive materials (Beeby and Potter 1985) For drying
biomass using steam dryer volatile organic materials contained in the biomass may be
emitted increasingly together with the water vapour at higher temperature steam
drying This would reduce the heating value of the biomass and increase the costs of
treating the exhaust steam In addition superheated steam dryers are difficult to
achieve low moisture content and the initial condensation may increase the total
drying time The systems also become more complex compared to those using flue
gas dryers
Figure 22 Principle of a flue gas dryer in combination with a boiler (Wimmerstedt
2006)
Figure 23 Principle of a superheated steam dryer in combination with a boiler
(Wimmerstedt 2006)
- 8 -
222 Heating Method
Convection conduction and radiation are three commonly used methods in
industrial drying In most cases heat is transferred to the surface of the product and
then to the interior However using dielectric radio frequency (RF) or microwave
freezing drying methods heat is generated internally within the product and then
transfers to the exterior surface
Convection
Convection is possibly the most common mode of drying Heat for evaporation is
supplied by convection to the exposed surface of the material and the evaporated
moisture is carried away by the drying medium Air inert gas (eg N2) direct
combustion gas or superheated steam can be as the drying source Convective
dryers can also be called direct dryers During the constant drying rate period the
solid surface takes on the wet bulb temperature which is determined by the ambient
air temperature and humidity at the same location While during the falling rate
period the solidsrsquo temperature approaches the dry bulb temperature of the drying
medium It should be noted that when using superheated steam as the drying
medium the solidsrsquo temperature corresponds to the saturation temperature at the
operating pressure
Conduction
Conduction or indirect drying is more suitable for thin products or for very wet
solids Heat is supplied through heated surfaces (stationary or moving) placed
within the dryer to support convey or confine the solids The evaporated moisture
is carried away by vacuum operation or by a stream of gas as a carrier of moisture
The thermal efficiency of conductive dryers is higher than convective dryers as the
latter loses a considerable amount of enthalpy with the drying medium
Radiation
Infrared radiation is often used in drying coatings thin sheets and films Although
most moisture materials are poor conductors for 50-60 Hz current the impedance falls
dramatically at radio frequency Hence such radiation could be used to heat the
solid volumetrically Energy is absorbed selectively by the water molecules Thus
less energy is required as the material becomes drier Since the capital and operating
costs are high for radiation drying it is usually to dry high unit value products or to
finally correct the moisture profile wherein only small quantities of hard-to-get
moisture are removed
It is noteworthy that sometimes the different drying methods can be combined
together For example a fluid bed dryer with immersed heating tubes or coils
- 9 -
combines advantages of both direct and indirect heating It can be only one third the
size of a purely convective fluid bed dryer for the same duty The combination of
radiation and convection is also feasible such as infrared plus air jets or microwave
with impingement
223 Types of Dryer
The dryers can be classified according to the method of heating transfer or the
drying source Using the former classification the dryers can be divided into
convective dryers conductive dryers radiative dryers and dielectric dryers Using
the later classification the dryers can be broadly divided into airflue gas dryers and
superheated steam dryers In biomass drying the most common types of flue gas
dryers are rotary drum dryers and flash dryers And the commercial scale steam
dryers for biomass reported so far are tubular dryers fluidized bed dryers and
indirectly flash dryers (Wimmerstedt 2006 Fagernaumls et al 2010) In this section
some widely used biomass dryers will be review briefly
Rotary Dryer
Rotary dryer has been used for a long time in drying biomass and is by far the most
common dryer type in the existing large scale bioenergy plants It consists of a
slightly inclined rotating cylindrical shell fitted with a number of longitudinal flights
The flights lift the material and cascade it in a uniform curtain through the passing
gases Wet biomass is fed into the upper end of the dryer moves through it by virtue
of rotation head effect and slope of the shell and withdraws at the lower end finally
A schematic diagram of a direct co-current rotary dryer is shown in Figure 24 The
shell diameter can range from lt 1 m to gt 6 m depending on the throughput And the
drum rotating speed is varied from 2 to 8 rpm The direction of drying medium can
be either co-current or counter-current relative to the solids The biomass and hot
airflue gas normally flow co-currently through the dryer The hottest flue gas
contacts with the wettest biomass and the cooled flue gas contacts with the dried
biomass which could reduce the fire risk The exhaust gases leaving the dryer pass
through a cyclone multicyclone baghouse filter scrubber or electrostatic precipitator
(ESP) to remove any fine material entrained in the air According to the dryer
configuration an ID fan may be required which can be placed before the emission
control equipment to reduce erosion of the fan or after the first cyclone to provide the
pressure drop
Indirectly heated rotary dryers are widely used for the materials that would be
contaminated by the drying medium The heat source passes through the outer wall
of the dryer or through an inner central shaft to heat the dryer by conduction A
combined directindirect rotary dryer also exists where very hot flue gases enter the
dryer through a central shaft and initially provide heat indirectly by conduction then
the same gases pass through the dryer coming into direct contact with the wet
- 10 -
material During the second pass the indirect heating warms the flue gas and
material In this way a high flue gas temperature can be used for heating while the
fire risk is reduced by limiting the temperature of the gas in direct contact with the
biomass
Figure 24 Schematic diagram of direct rotary dryer (Krokida et al 2006)
The inlet gas temperature to rotary biomass dryer can vary from 230 ordmC to 1100 ordmC
and the outlet gas temperature varies from 70 ordmC to 110 ordmC To avoid the
condensation of acids and resins the outlet gas temperature is normally higher than
104 ordmC The retention time in rotary dryers can be less than a minute for small
particles and 10 to 30 minutes for larger material (Haapanen et al 1983
Intercontinental Engineering Ltd 1980 MacCallum et al 1981 Wardrop
Engineering Inc 1990)
The advantages of rotary dryers include (1) they are less sensitive to particle size
and can accept the hottest flue gas of any type of dryer (2) they have low
maintenance costs and the greatest capacity of any type of dryer (Intercontinental
Engineering Ltd 1980) But the material moisture is hard to control in rotary dryers
due to the long lag time for material in the dryer (Fredrikson 1984) Rotary dryers
also present the greatest fire hazard and require the most space (Intercontinental
Engineering Ltd 1980)
Conveyor Dryer
The conception of conveyor dryer (belt dryer) is simple The material is spread on
to a horizontally moving permeable belt in a continuous process and the heating
medium is forced through the bed of product by fans The drying medium is usually
either air or flue gas and its flow can be upward or downward Conveyor dryers are
very versatile and can handle a wide range of materials making them attractive for
biomass feedstocks Figure 25 is the conveyor dryer developed by Swiss Combi
(2009)
According to conveyor and airflow arrangement conveyor dryers generally have
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 2 -
List of Contents
1 Introduction3
2 Literature Review Biomass Drying 5
21 Drying Process and Mechanism5
22 Dryer Principle6
221 Heating Source6
222 Heating Method 8
223 Types of Dryer 9
23 Selection of Dryers 15
24 Capital and Running Costs16
25 Safety and Environmental Issues 18
3 Case Study Biomass Drying Process Design Using Low Grade Heat 20
31 Low Grade Heat from Steel Production Process 20
32 Drying System Design 25
321 Thermal Design Methodology 25
322 Dryer Capacity 28
323 Drying Curve 32
324 Dryer Parameters 36
33 Cost Estimation41
331 Capital Costs 41
332 Running Costs48
333 Profitability 49
4 Conclusions54
References55
- 3 -
1 Introduction
Biomass has some environmental advantages over fossil fuels as it generates lower
level of pollutants such as SO2 and CO2 Therefore biomass as the only significant
source of carbon-based renewable fuel can replace fossil fuels for heating power
generation and transport
The combustion of biomass can be divided into several processes ie drying
pyrolysis gasification and combustion The moisture content of biomass typically
varies between 50-63 wt (wet basis) depending on the season weather and the type
of material (Holmberg and Ahtila 2004) The high moisture content in biomass
requires more energy for evaporation of water in the combustion chamber which
cannot be utilized in the power generation Hence the energy input into the process
is decreased which consequently results in a reduced heat andor electricity
production Table 11 presents data for the combustion of wood fuel with different
moisture contents (Wimmerstedt 2006) Here it is assumed that the flue gas
temperature is constant at 150 ordmC and the feeding air temperature is 40 ordmC The
calculation is based on 1 kg of dry material
Table 11 Combustion of wood fuel with different moisture contents
Moisture contents () 65 50 15
Water amount (kgkgdm) 19 10 02
Anticipated excess air level 16 14 12
Low calorific value (MJkg) 144 165 186
Flue gas volume at 1 bar 0 ordmC (m3kg) 103 88 62
Flue gas sensible heat loss (MJkg) 21 18 13
Efficiency 085 089 093
Adiabatic combustion temperature (ordmC) 900 1200 1800
As shown in table 11 a high level of excess air ratio is required when burning a
wood fuel with high moisture content This results in lower temperatures in the
boiler which in turn is highlighted by the adiabatic combustion temperatures In
addition there is a significant increase in the amount of flue gases due to the
evaporated water and the higher level of excess air ratio Therefore the flue gas heat
loss increases at higher fuel moisture content and the boiler efficiency is decreased
Some of the main reasons for drying biomass are highlighted in the lsquoHandbook of
Biomass Combustion and Co-firingrsquo (Loo and Koppejan 2008) These are as
follows
1 The heating value of the fuel (based on NCV) is affected by its moisture content
Therefore the efficiency of the combustion system increases as the moisture content
- 4 -
decreases
2 A more complex combustion technology and process control system is required if
the moisture content of the fuel varies which will add extra investment costs
3 The moisture content of the fuel should be below 30 wt (wet basis) for
domestic applications This is because the long-term storage of wet biomass fuels
causes problems with dry-matter loss and hygiene
4 The moisture content of material should be about 10-30 wt (wet basis) for use
in a small scale furnace
5 The moisture content of the raw material must be about 10 wt (wet basis) for
production of pellets
Although it is known that drying biomass fuel provides significant benefits these
benefits must be balanced against increased capital and operating costs occurred by
the drying process
In UK a large amount of low grade heat is generated by the process industry In
July 2008 the market potential for surplus heat from industrial processes was
estimated at 65 PJ by the Governmentrsquos Office of Climate Change (BERR 2008) and
36-71 PJ in a report by McKenna (McKenna 2009) Low grade heat (ie flue gas
hot water and steam) from the process industries can be used as a source of energy for
drying biomass This is beneficial to both process industries and the industries
utilizing biomass
This report presents the results obtained from the literature review work (ie drying
mechanism and biomass drying technologies) It also presents the results obtained
from a series of calculations which were carried out in order to investigate the design
of a biomass drying process using the low grade heat from process industries as the
heating source
- 5 -
2 Literature Review Biomass Drying
21 Drying Process and Mechanism
Drying is a process that removes moisture thermally to yield a solid product It is
a complex operation involving transient transfer of heat and mass During the
thermal drying two processes occur simultaneously (1) the energy (mostly as heat) is
transferred from the surrounding environment to evaporate the surface moisture by
the means of convection conduction or radiation (2) the internal moisture is
transferred to the surface of solid and then evaporated due to the process (1) In
process (1) the removal of water as vapour from the material surface is determined by
the external conditions such as temperature air humidity and flow area of exposed
surface and pressure In process (2) the transport of moisture within the solid
depends on the physical nature of the solid the temperature and its moisture content
(Mujumdar 2006)
The drying behaviour of solids can be characterized by measuring the moisture
content loss as a function of time Figure 21 shows a typical drying rate curve of a
hygroscopic product Three stages can be distinguished during the drying process
In the first constant drying rate stage the external free water attached to the product is
removed The rate-controlling step in this drying stage is the diffusion of the water
vapour across the air-moisture interface and the rate at which the surface for diffusion
is removed Towards the end of the constant drying rate stage moisture has been
transported from the inside of the solid to the surface by capillary forces and the
drying rate may still be constant The drying rate starts to fall when the average
moisture content reaches the critical moisture content This leads to the second
falling rate stage of unsaturated surface drying The internal diffusion of water to
the surface of the product takes place in this period It proceeds until the surface
film of liquid is entirely evaporated In the following third drying stage the
controlling step is the rate at which moisture may move through the solid due to the
concentration gradients between the deeper parts and the surface The heat transfer
to the surface and the heat conduction in the product are both active and the latter
influences the drying rate increasingly The rate of drying reduces even more rapidly
than before and drying stops once the moisture content falls down to the equilibrium
value for the prevailing air humidity The second and third stages can also be
combined together since the both experience the falling drying rate
The understanding of drying process and drying mechanism is extremely important
when drying biomass It is on the one hand expected to operate the drying at high
temperature in order to accelerate the heat transfer and minimize the equipment size
but on the other hand there are concerns with regard to the ignition of the biomass
The risk of biomass being ignited usually occurs at two points during the drying
process The first one is just at the end of the constant drying rate period when the
- 6 -
surface moisture has evaporated but an appreciate amount of water has not moved
from the inside to the surface In this short period the temperature at the surface
increases quickly since there is no water vapour near the surface to keep the biomass
particles cool The second point occurs when the biomass is over dried The
biomass could be ignited when it reaches its combustion temperature or the emitted
gases reach their flash point Over drying only happens during the upset conditions
or when using unsuitable dryers
Figure 21 Typical rate-of-drying curve (Mujumdar 2006)
22 Dryer Principle
The drying system needs to meet three requirements heat source drying method
and the form of agitation to expose new material for drying (Liptaacutek 1998a) The
different methods to achieve these requirements result in different dryers These
three requirements are consistent with the principle factors suggested by Keey (Keey
1972 1978) which could be used to classify dryer manner of heat supply to the
material temperature and pressure of operation and manner to handle the material
within the dryer
221 Heating Source
Drying mediums are mostly flue gas air and steam For drying within a biomass
fired combustion plant possible sources of heating are from (1) exhaust gases from
hot furnace engine or gas turbine (2) high-pressure steam from a steam or combined
cycle plant (3) warm air from an air-cooled condenser in a steam or combined cycle
plant and (4) steam from dedicated combustion of surplus biomass or diverted
product gas char or bio-fuel (Fagernaumls et al 2010) Figures 22 and 23 show the
principles of a flue gas dryer and a superheated steam dryer in combination with a
- 7 -
boiler respectively For the flue gas dryer the flue gases after the boiler are taken
through a fuel dryer in which the fuel used in the boiler is dried For superheated
steam dryer the steam is extracted from the boiler and the evaporated water from the
dryer is recovered as low-pressure steam The superheated steam dryers have some
advantages compared to the flue gas dryers The total energy efficiency is increased
due to the possibility of reuse the latent heat of evaporation No oxidation or
combustion reaction is possible which eliminates the risks of explosions and hazards
And steam dryers have higher drying rates than flue gas dryers However steam
dryers also have some disadvantages Due to the high temperature level they have
problems with temperature-sensitive materials (Beeby and Potter 1985) For drying
biomass using steam dryer volatile organic materials contained in the biomass may be
emitted increasingly together with the water vapour at higher temperature steam
drying This would reduce the heating value of the biomass and increase the costs of
treating the exhaust steam In addition superheated steam dryers are difficult to
achieve low moisture content and the initial condensation may increase the total
drying time The systems also become more complex compared to those using flue
gas dryers
Figure 22 Principle of a flue gas dryer in combination with a boiler (Wimmerstedt
2006)
Figure 23 Principle of a superheated steam dryer in combination with a boiler
(Wimmerstedt 2006)
- 8 -
222 Heating Method
Convection conduction and radiation are three commonly used methods in
industrial drying In most cases heat is transferred to the surface of the product and
then to the interior However using dielectric radio frequency (RF) or microwave
freezing drying methods heat is generated internally within the product and then
transfers to the exterior surface
Convection
Convection is possibly the most common mode of drying Heat for evaporation is
supplied by convection to the exposed surface of the material and the evaporated
moisture is carried away by the drying medium Air inert gas (eg N2) direct
combustion gas or superheated steam can be as the drying source Convective
dryers can also be called direct dryers During the constant drying rate period the
solid surface takes on the wet bulb temperature which is determined by the ambient
air temperature and humidity at the same location While during the falling rate
period the solidsrsquo temperature approaches the dry bulb temperature of the drying
medium It should be noted that when using superheated steam as the drying
medium the solidsrsquo temperature corresponds to the saturation temperature at the
operating pressure
Conduction
Conduction or indirect drying is more suitable for thin products or for very wet
solids Heat is supplied through heated surfaces (stationary or moving) placed
within the dryer to support convey or confine the solids The evaporated moisture
is carried away by vacuum operation or by a stream of gas as a carrier of moisture
The thermal efficiency of conductive dryers is higher than convective dryers as the
latter loses a considerable amount of enthalpy with the drying medium
Radiation
Infrared radiation is often used in drying coatings thin sheets and films Although
most moisture materials are poor conductors for 50-60 Hz current the impedance falls
dramatically at radio frequency Hence such radiation could be used to heat the
solid volumetrically Energy is absorbed selectively by the water molecules Thus
less energy is required as the material becomes drier Since the capital and operating
costs are high for radiation drying it is usually to dry high unit value products or to
finally correct the moisture profile wherein only small quantities of hard-to-get
moisture are removed
It is noteworthy that sometimes the different drying methods can be combined
together For example a fluid bed dryer with immersed heating tubes or coils
- 9 -
combines advantages of both direct and indirect heating It can be only one third the
size of a purely convective fluid bed dryer for the same duty The combination of
radiation and convection is also feasible such as infrared plus air jets or microwave
with impingement
223 Types of Dryer
The dryers can be classified according to the method of heating transfer or the
drying source Using the former classification the dryers can be divided into
convective dryers conductive dryers radiative dryers and dielectric dryers Using
the later classification the dryers can be broadly divided into airflue gas dryers and
superheated steam dryers In biomass drying the most common types of flue gas
dryers are rotary drum dryers and flash dryers And the commercial scale steam
dryers for biomass reported so far are tubular dryers fluidized bed dryers and
indirectly flash dryers (Wimmerstedt 2006 Fagernaumls et al 2010) In this section
some widely used biomass dryers will be review briefly
Rotary Dryer
Rotary dryer has been used for a long time in drying biomass and is by far the most
common dryer type in the existing large scale bioenergy plants It consists of a
slightly inclined rotating cylindrical shell fitted with a number of longitudinal flights
The flights lift the material and cascade it in a uniform curtain through the passing
gases Wet biomass is fed into the upper end of the dryer moves through it by virtue
of rotation head effect and slope of the shell and withdraws at the lower end finally
A schematic diagram of a direct co-current rotary dryer is shown in Figure 24 The
shell diameter can range from lt 1 m to gt 6 m depending on the throughput And the
drum rotating speed is varied from 2 to 8 rpm The direction of drying medium can
be either co-current or counter-current relative to the solids The biomass and hot
airflue gas normally flow co-currently through the dryer The hottest flue gas
contacts with the wettest biomass and the cooled flue gas contacts with the dried
biomass which could reduce the fire risk The exhaust gases leaving the dryer pass
through a cyclone multicyclone baghouse filter scrubber or electrostatic precipitator
(ESP) to remove any fine material entrained in the air According to the dryer
configuration an ID fan may be required which can be placed before the emission
control equipment to reduce erosion of the fan or after the first cyclone to provide the
pressure drop
Indirectly heated rotary dryers are widely used for the materials that would be
contaminated by the drying medium The heat source passes through the outer wall
of the dryer or through an inner central shaft to heat the dryer by conduction A
combined directindirect rotary dryer also exists where very hot flue gases enter the
dryer through a central shaft and initially provide heat indirectly by conduction then
the same gases pass through the dryer coming into direct contact with the wet
- 10 -
material During the second pass the indirect heating warms the flue gas and
material In this way a high flue gas temperature can be used for heating while the
fire risk is reduced by limiting the temperature of the gas in direct contact with the
biomass
Figure 24 Schematic diagram of direct rotary dryer (Krokida et al 2006)
The inlet gas temperature to rotary biomass dryer can vary from 230 ordmC to 1100 ordmC
and the outlet gas temperature varies from 70 ordmC to 110 ordmC To avoid the
condensation of acids and resins the outlet gas temperature is normally higher than
104 ordmC The retention time in rotary dryers can be less than a minute for small
particles and 10 to 30 minutes for larger material (Haapanen et al 1983
Intercontinental Engineering Ltd 1980 MacCallum et al 1981 Wardrop
Engineering Inc 1990)
The advantages of rotary dryers include (1) they are less sensitive to particle size
and can accept the hottest flue gas of any type of dryer (2) they have low
maintenance costs and the greatest capacity of any type of dryer (Intercontinental
Engineering Ltd 1980) But the material moisture is hard to control in rotary dryers
due to the long lag time for material in the dryer (Fredrikson 1984) Rotary dryers
also present the greatest fire hazard and require the most space (Intercontinental
Engineering Ltd 1980)
Conveyor Dryer
The conception of conveyor dryer (belt dryer) is simple The material is spread on
to a horizontally moving permeable belt in a continuous process and the heating
medium is forced through the bed of product by fans The drying medium is usually
either air or flue gas and its flow can be upward or downward Conveyor dryers are
very versatile and can handle a wide range of materials making them attractive for
biomass feedstocks Figure 25 is the conveyor dryer developed by Swiss Combi
(2009)
According to conveyor and airflow arrangement conveyor dryers generally have
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 3 -
1 Introduction
Biomass has some environmental advantages over fossil fuels as it generates lower
level of pollutants such as SO2 and CO2 Therefore biomass as the only significant
source of carbon-based renewable fuel can replace fossil fuels for heating power
generation and transport
The combustion of biomass can be divided into several processes ie drying
pyrolysis gasification and combustion The moisture content of biomass typically
varies between 50-63 wt (wet basis) depending on the season weather and the type
of material (Holmberg and Ahtila 2004) The high moisture content in biomass
requires more energy for evaporation of water in the combustion chamber which
cannot be utilized in the power generation Hence the energy input into the process
is decreased which consequently results in a reduced heat andor electricity
production Table 11 presents data for the combustion of wood fuel with different
moisture contents (Wimmerstedt 2006) Here it is assumed that the flue gas
temperature is constant at 150 ordmC and the feeding air temperature is 40 ordmC The
calculation is based on 1 kg of dry material
Table 11 Combustion of wood fuel with different moisture contents
Moisture contents () 65 50 15
Water amount (kgkgdm) 19 10 02
Anticipated excess air level 16 14 12
Low calorific value (MJkg) 144 165 186
Flue gas volume at 1 bar 0 ordmC (m3kg) 103 88 62
Flue gas sensible heat loss (MJkg) 21 18 13
Efficiency 085 089 093
Adiabatic combustion temperature (ordmC) 900 1200 1800
As shown in table 11 a high level of excess air ratio is required when burning a
wood fuel with high moisture content This results in lower temperatures in the
boiler which in turn is highlighted by the adiabatic combustion temperatures In
addition there is a significant increase in the amount of flue gases due to the
evaporated water and the higher level of excess air ratio Therefore the flue gas heat
loss increases at higher fuel moisture content and the boiler efficiency is decreased
Some of the main reasons for drying biomass are highlighted in the lsquoHandbook of
Biomass Combustion and Co-firingrsquo (Loo and Koppejan 2008) These are as
follows
1 The heating value of the fuel (based on NCV) is affected by its moisture content
Therefore the efficiency of the combustion system increases as the moisture content
- 4 -
decreases
2 A more complex combustion technology and process control system is required if
the moisture content of the fuel varies which will add extra investment costs
3 The moisture content of the fuel should be below 30 wt (wet basis) for
domestic applications This is because the long-term storage of wet biomass fuels
causes problems with dry-matter loss and hygiene
4 The moisture content of material should be about 10-30 wt (wet basis) for use
in a small scale furnace
5 The moisture content of the raw material must be about 10 wt (wet basis) for
production of pellets
Although it is known that drying biomass fuel provides significant benefits these
benefits must be balanced against increased capital and operating costs occurred by
the drying process
In UK a large amount of low grade heat is generated by the process industry In
July 2008 the market potential for surplus heat from industrial processes was
estimated at 65 PJ by the Governmentrsquos Office of Climate Change (BERR 2008) and
36-71 PJ in a report by McKenna (McKenna 2009) Low grade heat (ie flue gas
hot water and steam) from the process industries can be used as a source of energy for
drying biomass This is beneficial to both process industries and the industries
utilizing biomass
This report presents the results obtained from the literature review work (ie drying
mechanism and biomass drying technologies) It also presents the results obtained
from a series of calculations which were carried out in order to investigate the design
of a biomass drying process using the low grade heat from process industries as the
heating source
- 5 -
2 Literature Review Biomass Drying
21 Drying Process and Mechanism
Drying is a process that removes moisture thermally to yield a solid product It is
a complex operation involving transient transfer of heat and mass During the
thermal drying two processes occur simultaneously (1) the energy (mostly as heat) is
transferred from the surrounding environment to evaporate the surface moisture by
the means of convection conduction or radiation (2) the internal moisture is
transferred to the surface of solid and then evaporated due to the process (1) In
process (1) the removal of water as vapour from the material surface is determined by
the external conditions such as temperature air humidity and flow area of exposed
surface and pressure In process (2) the transport of moisture within the solid
depends on the physical nature of the solid the temperature and its moisture content
(Mujumdar 2006)
The drying behaviour of solids can be characterized by measuring the moisture
content loss as a function of time Figure 21 shows a typical drying rate curve of a
hygroscopic product Three stages can be distinguished during the drying process
In the first constant drying rate stage the external free water attached to the product is
removed The rate-controlling step in this drying stage is the diffusion of the water
vapour across the air-moisture interface and the rate at which the surface for diffusion
is removed Towards the end of the constant drying rate stage moisture has been
transported from the inside of the solid to the surface by capillary forces and the
drying rate may still be constant The drying rate starts to fall when the average
moisture content reaches the critical moisture content This leads to the second
falling rate stage of unsaturated surface drying The internal diffusion of water to
the surface of the product takes place in this period It proceeds until the surface
film of liquid is entirely evaporated In the following third drying stage the
controlling step is the rate at which moisture may move through the solid due to the
concentration gradients between the deeper parts and the surface The heat transfer
to the surface and the heat conduction in the product are both active and the latter
influences the drying rate increasingly The rate of drying reduces even more rapidly
than before and drying stops once the moisture content falls down to the equilibrium
value for the prevailing air humidity The second and third stages can also be
combined together since the both experience the falling drying rate
The understanding of drying process and drying mechanism is extremely important
when drying biomass It is on the one hand expected to operate the drying at high
temperature in order to accelerate the heat transfer and minimize the equipment size
but on the other hand there are concerns with regard to the ignition of the biomass
The risk of biomass being ignited usually occurs at two points during the drying
process The first one is just at the end of the constant drying rate period when the
- 6 -
surface moisture has evaporated but an appreciate amount of water has not moved
from the inside to the surface In this short period the temperature at the surface
increases quickly since there is no water vapour near the surface to keep the biomass
particles cool The second point occurs when the biomass is over dried The
biomass could be ignited when it reaches its combustion temperature or the emitted
gases reach their flash point Over drying only happens during the upset conditions
or when using unsuitable dryers
Figure 21 Typical rate-of-drying curve (Mujumdar 2006)
22 Dryer Principle
The drying system needs to meet three requirements heat source drying method
and the form of agitation to expose new material for drying (Liptaacutek 1998a) The
different methods to achieve these requirements result in different dryers These
three requirements are consistent with the principle factors suggested by Keey (Keey
1972 1978) which could be used to classify dryer manner of heat supply to the
material temperature and pressure of operation and manner to handle the material
within the dryer
221 Heating Source
Drying mediums are mostly flue gas air and steam For drying within a biomass
fired combustion plant possible sources of heating are from (1) exhaust gases from
hot furnace engine or gas turbine (2) high-pressure steam from a steam or combined
cycle plant (3) warm air from an air-cooled condenser in a steam or combined cycle
plant and (4) steam from dedicated combustion of surplus biomass or diverted
product gas char or bio-fuel (Fagernaumls et al 2010) Figures 22 and 23 show the
principles of a flue gas dryer and a superheated steam dryer in combination with a
- 7 -
boiler respectively For the flue gas dryer the flue gases after the boiler are taken
through a fuel dryer in which the fuel used in the boiler is dried For superheated
steam dryer the steam is extracted from the boiler and the evaporated water from the
dryer is recovered as low-pressure steam The superheated steam dryers have some
advantages compared to the flue gas dryers The total energy efficiency is increased
due to the possibility of reuse the latent heat of evaporation No oxidation or
combustion reaction is possible which eliminates the risks of explosions and hazards
And steam dryers have higher drying rates than flue gas dryers However steam
dryers also have some disadvantages Due to the high temperature level they have
problems with temperature-sensitive materials (Beeby and Potter 1985) For drying
biomass using steam dryer volatile organic materials contained in the biomass may be
emitted increasingly together with the water vapour at higher temperature steam
drying This would reduce the heating value of the biomass and increase the costs of
treating the exhaust steam In addition superheated steam dryers are difficult to
achieve low moisture content and the initial condensation may increase the total
drying time The systems also become more complex compared to those using flue
gas dryers
Figure 22 Principle of a flue gas dryer in combination with a boiler (Wimmerstedt
2006)
Figure 23 Principle of a superheated steam dryer in combination with a boiler
(Wimmerstedt 2006)
- 8 -
222 Heating Method
Convection conduction and radiation are three commonly used methods in
industrial drying In most cases heat is transferred to the surface of the product and
then to the interior However using dielectric radio frequency (RF) or microwave
freezing drying methods heat is generated internally within the product and then
transfers to the exterior surface
Convection
Convection is possibly the most common mode of drying Heat for evaporation is
supplied by convection to the exposed surface of the material and the evaporated
moisture is carried away by the drying medium Air inert gas (eg N2) direct
combustion gas or superheated steam can be as the drying source Convective
dryers can also be called direct dryers During the constant drying rate period the
solid surface takes on the wet bulb temperature which is determined by the ambient
air temperature and humidity at the same location While during the falling rate
period the solidsrsquo temperature approaches the dry bulb temperature of the drying
medium It should be noted that when using superheated steam as the drying
medium the solidsrsquo temperature corresponds to the saturation temperature at the
operating pressure
Conduction
Conduction or indirect drying is more suitable for thin products or for very wet
solids Heat is supplied through heated surfaces (stationary or moving) placed
within the dryer to support convey or confine the solids The evaporated moisture
is carried away by vacuum operation or by a stream of gas as a carrier of moisture
The thermal efficiency of conductive dryers is higher than convective dryers as the
latter loses a considerable amount of enthalpy with the drying medium
Radiation
Infrared radiation is often used in drying coatings thin sheets and films Although
most moisture materials are poor conductors for 50-60 Hz current the impedance falls
dramatically at radio frequency Hence such radiation could be used to heat the
solid volumetrically Energy is absorbed selectively by the water molecules Thus
less energy is required as the material becomes drier Since the capital and operating
costs are high for radiation drying it is usually to dry high unit value products or to
finally correct the moisture profile wherein only small quantities of hard-to-get
moisture are removed
It is noteworthy that sometimes the different drying methods can be combined
together For example a fluid bed dryer with immersed heating tubes or coils
- 9 -
combines advantages of both direct and indirect heating It can be only one third the
size of a purely convective fluid bed dryer for the same duty The combination of
radiation and convection is also feasible such as infrared plus air jets or microwave
with impingement
223 Types of Dryer
The dryers can be classified according to the method of heating transfer or the
drying source Using the former classification the dryers can be divided into
convective dryers conductive dryers radiative dryers and dielectric dryers Using
the later classification the dryers can be broadly divided into airflue gas dryers and
superheated steam dryers In biomass drying the most common types of flue gas
dryers are rotary drum dryers and flash dryers And the commercial scale steam
dryers for biomass reported so far are tubular dryers fluidized bed dryers and
indirectly flash dryers (Wimmerstedt 2006 Fagernaumls et al 2010) In this section
some widely used biomass dryers will be review briefly
Rotary Dryer
Rotary dryer has been used for a long time in drying biomass and is by far the most
common dryer type in the existing large scale bioenergy plants It consists of a
slightly inclined rotating cylindrical shell fitted with a number of longitudinal flights
The flights lift the material and cascade it in a uniform curtain through the passing
gases Wet biomass is fed into the upper end of the dryer moves through it by virtue
of rotation head effect and slope of the shell and withdraws at the lower end finally
A schematic diagram of a direct co-current rotary dryer is shown in Figure 24 The
shell diameter can range from lt 1 m to gt 6 m depending on the throughput And the
drum rotating speed is varied from 2 to 8 rpm The direction of drying medium can
be either co-current or counter-current relative to the solids The biomass and hot
airflue gas normally flow co-currently through the dryer The hottest flue gas
contacts with the wettest biomass and the cooled flue gas contacts with the dried
biomass which could reduce the fire risk The exhaust gases leaving the dryer pass
through a cyclone multicyclone baghouse filter scrubber or electrostatic precipitator
(ESP) to remove any fine material entrained in the air According to the dryer
configuration an ID fan may be required which can be placed before the emission
control equipment to reduce erosion of the fan or after the first cyclone to provide the
pressure drop
Indirectly heated rotary dryers are widely used for the materials that would be
contaminated by the drying medium The heat source passes through the outer wall
of the dryer or through an inner central shaft to heat the dryer by conduction A
combined directindirect rotary dryer also exists where very hot flue gases enter the
dryer through a central shaft and initially provide heat indirectly by conduction then
the same gases pass through the dryer coming into direct contact with the wet
- 10 -
material During the second pass the indirect heating warms the flue gas and
material In this way a high flue gas temperature can be used for heating while the
fire risk is reduced by limiting the temperature of the gas in direct contact with the
biomass
Figure 24 Schematic diagram of direct rotary dryer (Krokida et al 2006)
The inlet gas temperature to rotary biomass dryer can vary from 230 ordmC to 1100 ordmC
and the outlet gas temperature varies from 70 ordmC to 110 ordmC To avoid the
condensation of acids and resins the outlet gas temperature is normally higher than
104 ordmC The retention time in rotary dryers can be less than a minute for small
particles and 10 to 30 minutes for larger material (Haapanen et al 1983
Intercontinental Engineering Ltd 1980 MacCallum et al 1981 Wardrop
Engineering Inc 1990)
The advantages of rotary dryers include (1) they are less sensitive to particle size
and can accept the hottest flue gas of any type of dryer (2) they have low
maintenance costs and the greatest capacity of any type of dryer (Intercontinental
Engineering Ltd 1980) But the material moisture is hard to control in rotary dryers
due to the long lag time for material in the dryer (Fredrikson 1984) Rotary dryers
also present the greatest fire hazard and require the most space (Intercontinental
Engineering Ltd 1980)
Conveyor Dryer
The conception of conveyor dryer (belt dryer) is simple The material is spread on
to a horizontally moving permeable belt in a continuous process and the heating
medium is forced through the bed of product by fans The drying medium is usually
either air or flue gas and its flow can be upward or downward Conveyor dryers are
very versatile and can handle a wide range of materials making them attractive for
biomass feedstocks Figure 25 is the conveyor dryer developed by Swiss Combi
(2009)
According to conveyor and airflow arrangement conveyor dryers generally have
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 4 -
decreases
2 A more complex combustion technology and process control system is required if
the moisture content of the fuel varies which will add extra investment costs
3 The moisture content of the fuel should be below 30 wt (wet basis) for
domestic applications This is because the long-term storage of wet biomass fuels
causes problems with dry-matter loss and hygiene
4 The moisture content of material should be about 10-30 wt (wet basis) for use
in a small scale furnace
5 The moisture content of the raw material must be about 10 wt (wet basis) for
production of pellets
Although it is known that drying biomass fuel provides significant benefits these
benefits must be balanced against increased capital and operating costs occurred by
the drying process
In UK a large amount of low grade heat is generated by the process industry In
July 2008 the market potential for surplus heat from industrial processes was
estimated at 65 PJ by the Governmentrsquos Office of Climate Change (BERR 2008) and
36-71 PJ in a report by McKenna (McKenna 2009) Low grade heat (ie flue gas
hot water and steam) from the process industries can be used as a source of energy for
drying biomass This is beneficial to both process industries and the industries
utilizing biomass
This report presents the results obtained from the literature review work (ie drying
mechanism and biomass drying technologies) It also presents the results obtained
from a series of calculations which were carried out in order to investigate the design
of a biomass drying process using the low grade heat from process industries as the
heating source
- 5 -
2 Literature Review Biomass Drying
21 Drying Process and Mechanism
Drying is a process that removes moisture thermally to yield a solid product It is
a complex operation involving transient transfer of heat and mass During the
thermal drying two processes occur simultaneously (1) the energy (mostly as heat) is
transferred from the surrounding environment to evaporate the surface moisture by
the means of convection conduction or radiation (2) the internal moisture is
transferred to the surface of solid and then evaporated due to the process (1) In
process (1) the removal of water as vapour from the material surface is determined by
the external conditions such as temperature air humidity and flow area of exposed
surface and pressure In process (2) the transport of moisture within the solid
depends on the physical nature of the solid the temperature and its moisture content
(Mujumdar 2006)
The drying behaviour of solids can be characterized by measuring the moisture
content loss as a function of time Figure 21 shows a typical drying rate curve of a
hygroscopic product Three stages can be distinguished during the drying process
In the first constant drying rate stage the external free water attached to the product is
removed The rate-controlling step in this drying stage is the diffusion of the water
vapour across the air-moisture interface and the rate at which the surface for diffusion
is removed Towards the end of the constant drying rate stage moisture has been
transported from the inside of the solid to the surface by capillary forces and the
drying rate may still be constant The drying rate starts to fall when the average
moisture content reaches the critical moisture content This leads to the second
falling rate stage of unsaturated surface drying The internal diffusion of water to
the surface of the product takes place in this period It proceeds until the surface
film of liquid is entirely evaporated In the following third drying stage the
controlling step is the rate at which moisture may move through the solid due to the
concentration gradients between the deeper parts and the surface The heat transfer
to the surface and the heat conduction in the product are both active and the latter
influences the drying rate increasingly The rate of drying reduces even more rapidly
than before and drying stops once the moisture content falls down to the equilibrium
value for the prevailing air humidity The second and third stages can also be
combined together since the both experience the falling drying rate
The understanding of drying process and drying mechanism is extremely important
when drying biomass It is on the one hand expected to operate the drying at high
temperature in order to accelerate the heat transfer and minimize the equipment size
but on the other hand there are concerns with regard to the ignition of the biomass
The risk of biomass being ignited usually occurs at two points during the drying
process The first one is just at the end of the constant drying rate period when the
- 6 -
surface moisture has evaporated but an appreciate amount of water has not moved
from the inside to the surface In this short period the temperature at the surface
increases quickly since there is no water vapour near the surface to keep the biomass
particles cool The second point occurs when the biomass is over dried The
biomass could be ignited when it reaches its combustion temperature or the emitted
gases reach their flash point Over drying only happens during the upset conditions
or when using unsuitable dryers
Figure 21 Typical rate-of-drying curve (Mujumdar 2006)
22 Dryer Principle
The drying system needs to meet three requirements heat source drying method
and the form of agitation to expose new material for drying (Liptaacutek 1998a) The
different methods to achieve these requirements result in different dryers These
three requirements are consistent with the principle factors suggested by Keey (Keey
1972 1978) which could be used to classify dryer manner of heat supply to the
material temperature and pressure of operation and manner to handle the material
within the dryer
221 Heating Source
Drying mediums are mostly flue gas air and steam For drying within a biomass
fired combustion plant possible sources of heating are from (1) exhaust gases from
hot furnace engine or gas turbine (2) high-pressure steam from a steam or combined
cycle plant (3) warm air from an air-cooled condenser in a steam or combined cycle
plant and (4) steam from dedicated combustion of surplus biomass or diverted
product gas char or bio-fuel (Fagernaumls et al 2010) Figures 22 and 23 show the
principles of a flue gas dryer and a superheated steam dryer in combination with a
- 7 -
boiler respectively For the flue gas dryer the flue gases after the boiler are taken
through a fuel dryer in which the fuel used in the boiler is dried For superheated
steam dryer the steam is extracted from the boiler and the evaporated water from the
dryer is recovered as low-pressure steam The superheated steam dryers have some
advantages compared to the flue gas dryers The total energy efficiency is increased
due to the possibility of reuse the latent heat of evaporation No oxidation or
combustion reaction is possible which eliminates the risks of explosions and hazards
And steam dryers have higher drying rates than flue gas dryers However steam
dryers also have some disadvantages Due to the high temperature level they have
problems with temperature-sensitive materials (Beeby and Potter 1985) For drying
biomass using steam dryer volatile organic materials contained in the biomass may be
emitted increasingly together with the water vapour at higher temperature steam
drying This would reduce the heating value of the biomass and increase the costs of
treating the exhaust steam In addition superheated steam dryers are difficult to
achieve low moisture content and the initial condensation may increase the total
drying time The systems also become more complex compared to those using flue
gas dryers
Figure 22 Principle of a flue gas dryer in combination with a boiler (Wimmerstedt
2006)
Figure 23 Principle of a superheated steam dryer in combination with a boiler
(Wimmerstedt 2006)
- 8 -
222 Heating Method
Convection conduction and radiation are three commonly used methods in
industrial drying In most cases heat is transferred to the surface of the product and
then to the interior However using dielectric radio frequency (RF) or microwave
freezing drying methods heat is generated internally within the product and then
transfers to the exterior surface
Convection
Convection is possibly the most common mode of drying Heat for evaporation is
supplied by convection to the exposed surface of the material and the evaporated
moisture is carried away by the drying medium Air inert gas (eg N2) direct
combustion gas or superheated steam can be as the drying source Convective
dryers can also be called direct dryers During the constant drying rate period the
solid surface takes on the wet bulb temperature which is determined by the ambient
air temperature and humidity at the same location While during the falling rate
period the solidsrsquo temperature approaches the dry bulb temperature of the drying
medium It should be noted that when using superheated steam as the drying
medium the solidsrsquo temperature corresponds to the saturation temperature at the
operating pressure
Conduction
Conduction or indirect drying is more suitable for thin products or for very wet
solids Heat is supplied through heated surfaces (stationary or moving) placed
within the dryer to support convey or confine the solids The evaporated moisture
is carried away by vacuum operation or by a stream of gas as a carrier of moisture
The thermal efficiency of conductive dryers is higher than convective dryers as the
latter loses a considerable amount of enthalpy with the drying medium
Radiation
Infrared radiation is often used in drying coatings thin sheets and films Although
most moisture materials are poor conductors for 50-60 Hz current the impedance falls
dramatically at radio frequency Hence such radiation could be used to heat the
solid volumetrically Energy is absorbed selectively by the water molecules Thus
less energy is required as the material becomes drier Since the capital and operating
costs are high for radiation drying it is usually to dry high unit value products or to
finally correct the moisture profile wherein only small quantities of hard-to-get
moisture are removed
It is noteworthy that sometimes the different drying methods can be combined
together For example a fluid bed dryer with immersed heating tubes or coils
- 9 -
combines advantages of both direct and indirect heating It can be only one third the
size of a purely convective fluid bed dryer for the same duty The combination of
radiation and convection is also feasible such as infrared plus air jets or microwave
with impingement
223 Types of Dryer
The dryers can be classified according to the method of heating transfer or the
drying source Using the former classification the dryers can be divided into
convective dryers conductive dryers radiative dryers and dielectric dryers Using
the later classification the dryers can be broadly divided into airflue gas dryers and
superheated steam dryers In biomass drying the most common types of flue gas
dryers are rotary drum dryers and flash dryers And the commercial scale steam
dryers for biomass reported so far are tubular dryers fluidized bed dryers and
indirectly flash dryers (Wimmerstedt 2006 Fagernaumls et al 2010) In this section
some widely used biomass dryers will be review briefly
Rotary Dryer
Rotary dryer has been used for a long time in drying biomass and is by far the most
common dryer type in the existing large scale bioenergy plants It consists of a
slightly inclined rotating cylindrical shell fitted with a number of longitudinal flights
The flights lift the material and cascade it in a uniform curtain through the passing
gases Wet biomass is fed into the upper end of the dryer moves through it by virtue
of rotation head effect and slope of the shell and withdraws at the lower end finally
A schematic diagram of a direct co-current rotary dryer is shown in Figure 24 The
shell diameter can range from lt 1 m to gt 6 m depending on the throughput And the
drum rotating speed is varied from 2 to 8 rpm The direction of drying medium can
be either co-current or counter-current relative to the solids The biomass and hot
airflue gas normally flow co-currently through the dryer The hottest flue gas
contacts with the wettest biomass and the cooled flue gas contacts with the dried
biomass which could reduce the fire risk The exhaust gases leaving the dryer pass
through a cyclone multicyclone baghouse filter scrubber or electrostatic precipitator
(ESP) to remove any fine material entrained in the air According to the dryer
configuration an ID fan may be required which can be placed before the emission
control equipment to reduce erosion of the fan or after the first cyclone to provide the
pressure drop
Indirectly heated rotary dryers are widely used for the materials that would be
contaminated by the drying medium The heat source passes through the outer wall
of the dryer or through an inner central shaft to heat the dryer by conduction A
combined directindirect rotary dryer also exists where very hot flue gases enter the
dryer through a central shaft and initially provide heat indirectly by conduction then
the same gases pass through the dryer coming into direct contact with the wet
- 10 -
material During the second pass the indirect heating warms the flue gas and
material In this way a high flue gas temperature can be used for heating while the
fire risk is reduced by limiting the temperature of the gas in direct contact with the
biomass
Figure 24 Schematic diagram of direct rotary dryer (Krokida et al 2006)
The inlet gas temperature to rotary biomass dryer can vary from 230 ordmC to 1100 ordmC
and the outlet gas temperature varies from 70 ordmC to 110 ordmC To avoid the
condensation of acids and resins the outlet gas temperature is normally higher than
104 ordmC The retention time in rotary dryers can be less than a minute for small
particles and 10 to 30 minutes for larger material (Haapanen et al 1983
Intercontinental Engineering Ltd 1980 MacCallum et al 1981 Wardrop
Engineering Inc 1990)
The advantages of rotary dryers include (1) they are less sensitive to particle size
and can accept the hottest flue gas of any type of dryer (2) they have low
maintenance costs and the greatest capacity of any type of dryer (Intercontinental
Engineering Ltd 1980) But the material moisture is hard to control in rotary dryers
due to the long lag time for material in the dryer (Fredrikson 1984) Rotary dryers
also present the greatest fire hazard and require the most space (Intercontinental
Engineering Ltd 1980)
Conveyor Dryer
The conception of conveyor dryer (belt dryer) is simple The material is spread on
to a horizontally moving permeable belt in a continuous process and the heating
medium is forced through the bed of product by fans The drying medium is usually
either air or flue gas and its flow can be upward or downward Conveyor dryers are
very versatile and can handle a wide range of materials making them attractive for
biomass feedstocks Figure 25 is the conveyor dryer developed by Swiss Combi
(2009)
According to conveyor and airflow arrangement conveyor dryers generally have
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 5 -
2 Literature Review Biomass Drying
21 Drying Process and Mechanism
Drying is a process that removes moisture thermally to yield a solid product It is
a complex operation involving transient transfer of heat and mass During the
thermal drying two processes occur simultaneously (1) the energy (mostly as heat) is
transferred from the surrounding environment to evaporate the surface moisture by
the means of convection conduction or radiation (2) the internal moisture is
transferred to the surface of solid and then evaporated due to the process (1) In
process (1) the removal of water as vapour from the material surface is determined by
the external conditions such as temperature air humidity and flow area of exposed
surface and pressure In process (2) the transport of moisture within the solid
depends on the physical nature of the solid the temperature and its moisture content
(Mujumdar 2006)
The drying behaviour of solids can be characterized by measuring the moisture
content loss as a function of time Figure 21 shows a typical drying rate curve of a
hygroscopic product Three stages can be distinguished during the drying process
In the first constant drying rate stage the external free water attached to the product is
removed The rate-controlling step in this drying stage is the diffusion of the water
vapour across the air-moisture interface and the rate at which the surface for diffusion
is removed Towards the end of the constant drying rate stage moisture has been
transported from the inside of the solid to the surface by capillary forces and the
drying rate may still be constant The drying rate starts to fall when the average
moisture content reaches the critical moisture content This leads to the second
falling rate stage of unsaturated surface drying The internal diffusion of water to
the surface of the product takes place in this period It proceeds until the surface
film of liquid is entirely evaporated In the following third drying stage the
controlling step is the rate at which moisture may move through the solid due to the
concentration gradients between the deeper parts and the surface The heat transfer
to the surface and the heat conduction in the product are both active and the latter
influences the drying rate increasingly The rate of drying reduces even more rapidly
than before and drying stops once the moisture content falls down to the equilibrium
value for the prevailing air humidity The second and third stages can also be
combined together since the both experience the falling drying rate
The understanding of drying process and drying mechanism is extremely important
when drying biomass It is on the one hand expected to operate the drying at high
temperature in order to accelerate the heat transfer and minimize the equipment size
but on the other hand there are concerns with regard to the ignition of the biomass
The risk of biomass being ignited usually occurs at two points during the drying
process The first one is just at the end of the constant drying rate period when the
- 6 -
surface moisture has evaporated but an appreciate amount of water has not moved
from the inside to the surface In this short period the temperature at the surface
increases quickly since there is no water vapour near the surface to keep the biomass
particles cool The second point occurs when the biomass is over dried The
biomass could be ignited when it reaches its combustion temperature or the emitted
gases reach their flash point Over drying only happens during the upset conditions
or when using unsuitable dryers
Figure 21 Typical rate-of-drying curve (Mujumdar 2006)
22 Dryer Principle
The drying system needs to meet three requirements heat source drying method
and the form of agitation to expose new material for drying (Liptaacutek 1998a) The
different methods to achieve these requirements result in different dryers These
three requirements are consistent with the principle factors suggested by Keey (Keey
1972 1978) which could be used to classify dryer manner of heat supply to the
material temperature and pressure of operation and manner to handle the material
within the dryer
221 Heating Source
Drying mediums are mostly flue gas air and steam For drying within a biomass
fired combustion plant possible sources of heating are from (1) exhaust gases from
hot furnace engine or gas turbine (2) high-pressure steam from a steam or combined
cycle plant (3) warm air from an air-cooled condenser in a steam or combined cycle
plant and (4) steam from dedicated combustion of surplus biomass or diverted
product gas char or bio-fuel (Fagernaumls et al 2010) Figures 22 and 23 show the
principles of a flue gas dryer and a superheated steam dryer in combination with a
- 7 -
boiler respectively For the flue gas dryer the flue gases after the boiler are taken
through a fuel dryer in which the fuel used in the boiler is dried For superheated
steam dryer the steam is extracted from the boiler and the evaporated water from the
dryer is recovered as low-pressure steam The superheated steam dryers have some
advantages compared to the flue gas dryers The total energy efficiency is increased
due to the possibility of reuse the latent heat of evaporation No oxidation or
combustion reaction is possible which eliminates the risks of explosions and hazards
And steam dryers have higher drying rates than flue gas dryers However steam
dryers also have some disadvantages Due to the high temperature level they have
problems with temperature-sensitive materials (Beeby and Potter 1985) For drying
biomass using steam dryer volatile organic materials contained in the biomass may be
emitted increasingly together with the water vapour at higher temperature steam
drying This would reduce the heating value of the biomass and increase the costs of
treating the exhaust steam In addition superheated steam dryers are difficult to
achieve low moisture content and the initial condensation may increase the total
drying time The systems also become more complex compared to those using flue
gas dryers
Figure 22 Principle of a flue gas dryer in combination with a boiler (Wimmerstedt
2006)
Figure 23 Principle of a superheated steam dryer in combination with a boiler
(Wimmerstedt 2006)
- 8 -
222 Heating Method
Convection conduction and radiation are three commonly used methods in
industrial drying In most cases heat is transferred to the surface of the product and
then to the interior However using dielectric radio frequency (RF) or microwave
freezing drying methods heat is generated internally within the product and then
transfers to the exterior surface
Convection
Convection is possibly the most common mode of drying Heat for evaporation is
supplied by convection to the exposed surface of the material and the evaporated
moisture is carried away by the drying medium Air inert gas (eg N2) direct
combustion gas or superheated steam can be as the drying source Convective
dryers can also be called direct dryers During the constant drying rate period the
solid surface takes on the wet bulb temperature which is determined by the ambient
air temperature and humidity at the same location While during the falling rate
period the solidsrsquo temperature approaches the dry bulb temperature of the drying
medium It should be noted that when using superheated steam as the drying
medium the solidsrsquo temperature corresponds to the saturation temperature at the
operating pressure
Conduction
Conduction or indirect drying is more suitable for thin products or for very wet
solids Heat is supplied through heated surfaces (stationary or moving) placed
within the dryer to support convey or confine the solids The evaporated moisture
is carried away by vacuum operation or by a stream of gas as a carrier of moisture
The thermal efficiency of conductive dryers is higher than convective dryers as the
latter loses a considerable amount of enthalpy with the drying medium
Radiation
Infrared radiation is often used in drying coatings thin sheets and films Although
most moisture materials are poor conductors for 50-60 Hz current the impedance falls
dramatically at radio frequency Hence such radiation could be used to heat the
solid volumetrically Energy is absorbed selectively by the water molecules Thus
less energy is required as the material becomes drier Since the capital and operating
costs are high for radiation drying it is usually to dry high unit value products or to
finally correct the moisture profile wherein only small quantities of hard-to-get
moisture are removed
It is noteworthy that sometimes the different drying methods can be combined
together For example a fluid bed dryer with immersed heating tubes or coils
- 9 -
combines advantages of both direct and indirect heating It can be only one third the
size of a purely convective fluid bed dryer for the same duty The combination of
radiation and convection is also feasible such as infrared plus air jets or microwave
with impingement
223 Types of Dryer
The dryers can be classified according to the method of heating transfer or the
drying source Using the former classification the dryers can be divided into
convective dryers conductive dryers radiative dryers and dielectric dryers Using
the later classification the dryers can be broadly divided into airflue gas dryers and
superheated steam dryers In biomass drying the most common types of flue gas
dryers are rotary drum dryers and flash dryers And the commercial scale steam
dryers for biomass reported so far are tubular dryers fluidized bed dryers and
indirectly flash dryers (Wimmerstedt 2006 Fagernaumls et al 2010) In this section
some widely used biomass dryers will be review briefly
Rotary Dryer
Rotary dryer has been used for a long time in drying biomass and is by far the most
common dryer type in the existing large scale bioenergy plants It consists of a
slightly inclined rotating cylindrical shell fitted with a number of longitudinal flights
The flights lift the material and cascade it in a uniform curtain through the passing
gases Wet biomass is fed into the upper end of the dryer moves through it by virtue
of rotation head effect and slope of the shell and withdraws at the lower end finally
A schematic diagram of a direct co-current rotary dryer is shown in Figure 24 The
shell diameter can range from lt 1 m to gt 6 m depending on the throughput And the
drum rotating speed is varied from 2 to 8 rpm The direction of drying medium can
be either co-current or counter-current relative to the solids The biomass and hot
airflue gas normally flow co-currently through the dryer The hottest flue gas
contacts with the wettest biomass and the cooled flue gas contacts with the dried
biomass which could reduce the fire risk The exhaust gases leaving the dryer pass
through a cyclone multicyclone baghouse filter scrubber or electrostatic precipitator
(ESP) to remove any fine material entrained in the air According to the dryer
configuration an ID fan may be required which can be placed before the emission
control equipment to reduce erosion of the fan or after the first cyclone to provide the
pressure drop
Indirectly heated rotary dryers are widely used for the materials that would be
contaminated by the drying medium The heat source passes through the outer wall
of the dryer or through an inner central shaft to heat the dryer by conduction A
combined directindirect rotary dryer also exists where very hot flue gases enter the
dryer through a central shaft and initially provide heat indirectly by conduction then
the same gases pass through the dryer coming into direct contact with the wet
- 10 -
material During the second pass the indirect heating warms the flue gas and
material In this way a high flue gas temperature can be used for heating while the
fire risk is reduced by limiting the temperature of the gas in direct contact with the
biomass
Figure 24 Schematic diagram of direct rotary dryer (Krokida et al 2006)
The inlet gas temperature to rotary biomass dryer can vary from 230 ordmC to 1100 ordmC
and the outlet gas temperature varies from 70 ordmC to 110 ordmC To avoid the
condensation of acids and resins the outlet gas temperature is normally higher than
104 ordmC The retention time in rotary dryers can be less than a minute for small
particles and 10 to 30 minutes for larger material (Haapanen et al 1983
Intercontinental Engineering Ltd 1980 MacCallum et al 1981 Wardrop
Engineering Inc 1990)
The advantages of rotary dryers include (1) they are less sensitive to particle size
and can accept the hottest flue gas of any type of dryer (2) they have low
maintenance costs and the greatest capacity of any type of dryer (Intercontinental
Engineering Ltd 1980) But the material moisture is hard to control in rotary dryers
due to the long lag time for material in the dryer (Fredrikson 1984) Rotary dryers
also present the greatest fire hazard and require the most space (Intercontinental
Engineering Ltd 1980)
Conveyor Dryer
The conception of conveyor dryer (belt dryer) is simple The material is spread on
to a horizontally moving permeable belt in a continuous process and the heating
medium is forced through the bed of product by fans The drying medium is usually
either air or flue gas and its flow can be upward or downward Conveyor dryers are
very versatile and can handle a wide range of materials making them attractive for
biomass feedstocks Figure 25 is the conveyor dryer developed by Swiss Combi
(2009)
According to conveyor and airflow arrangement conveyor dryers generally have
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 6 -
surface moisture has evaporated but an appreciate amount of water has not moved
from the inside to the surface In this short period the temperature at the surface
increases quickly since there is no water vapour near the surface to keep the biomass
particles cool The second point occurs when the biomass is over dried The
biomass could be ignited when it reaches its combustion temperature or the emitted
gases reach their flash point Over drying only happens during the upset conditions
or when using unsuitable dryers
Figure 21 Typical rate-of-drying curve (Mujumdar 2006)
22 Dryer Principle
The drying system needs to meet three requirements heat source drying method
and the form of agitation to expose new material for drying (Liptaacutek 1998a) The
different methods to achieve these requirements result in different dryers These
three requirements are consistent with the principle factors suggested by Keey (Keey
1972 1978) which could be used to classify dryer manner of heat supply to the
material temperature and pressure of operation and manner to handle the material
within the dryer
221 Heating Source
Drying mediums are mostly flue gas air and steam For drying within a biomass
fired combustion plant possible sources of heating are from (1) exhaust gases from
hot furnace engine or gas turbine (2) high-pressure steam from a steam or combined
cycle plant (3) warm air from an air-cooled condenser in a steam or combined cycle
plant and (4) steam from dedicated combustion of surplus biomass or diverted
product gas char or bio-fuel (Fagernaumls et al 2010) Figures 22 and 23 show the
principles of a flue gas dryer and a superheated steam dryer in combination with a
- 7 -
boiler respectively For the flue gas dryer the flue gases after the boiler are taken
through a fuel dryer in which the fuel used in the boiler is dried For superheated
steam dryer the steam is extracted from the boiler and the evaporated water from the
dryer is recovered as low-pressure steam The superheated steam dryers have some
advantages compared to the flue gas dryers The total energy efficiency is increased
due to the possibility of reuse the latent heat of evaporation No oxidation or
combustion reaction is possible which eliminates the risks of explosions and hazards
And steam dryers have higher drying rates than flue gas dryers However steam
dryers also have some disadvantages Due to the high temperature level they have
problems with temperature-sensitive materials (Beeby and Potter 1985) For drying
biomass using steam dryer volatile organic materials contained in the biomass may be
emitted increasingly together with the water vapour at higher temperature steam
drying This would reduce the heating value of the biomass and increase the costs of
treating the exhaust steam In addition superheated steam dryers are difficult to
achieve low moisture content and the initial condensation may increase the total
drying time The systems also become more complex compared to those using flue
gas dryers
Figure 22 Principle of a flue gas dryer in combination with a boiler (Wimmerstedt
2006)
Figure 23 Principle of a superheated steam dryer in combination with a boiler
(Wimmerstedt 2006)
- 8 -
222 Heating Method
Convection conduction and radiation are three commonly used methods in
industrial drying In most cases heat is transferred to the surface of the product and
then to the interior However using dielectric radio frequency (RF) or microwave
freezing drying methods heat is generated internally within the product and then
transfers to the exterior surface
Convection
Convection is possibly the most common mode of drying Heat for evaporation is
supplied by convection to the exposed surface of the material and the evaporated
moisture is carried away by the drying medium Air inert gas (eg N2) direct
combustion gas or superheated steam can be as the drying source Convective
dryers can also be called direct dryers During the constant drying rate period the
solid surface takes on the wet bulb temperature which is determined by the ambient
air temperature and humidity at the same location While during the falling rate
period the solidsrsquo temperature approaches the dry bulb temperature of the drying
medium It should be noted that when using superheated steam as the drying
medium the solidsrsquo temperature corresponds to the saturation temperature at the
operating pressure
Conduction
Conduction or indirect drying is more suitable for thin products or for very wet
solids Heat is supplied through heated surfaces (stationary or moving) placed
within the dryer to support convey or confine the solids The evaporated moisture
is carried away by vacuum operation or by a stream of gas as a carrier of moisture
The thermal efficiency of conductive dryers is higher than convective dryers as the
latter loses a considerable amount of enthalpy with the drying medium
Radiation
Infrared radiation is often used in drying coatings thin sheets and films Although
most moisture materials are poor conductors for 50-60 Hz current the impedance falls
dramatically at radio frequency Hence such radiation could be used to heat the
solid volumetrically Energy is absorbed selectively by the water molecules Thus
less energy is required as the material becomes drier Since the capital and operating
costs are high for radiation drying it is usually to dry high unit value products or to
finally correct the moisture profile wherein only small quantities of hard-to-get
moisture are removed
It is noteworthy that sometimes the different drying methods can be combined
together For example a fluid bed dryer with immersed heating tubes or coils
- 9 -
combines advantages of both direct and indirect heating It can be only one third the
size of a purely convective fluid bed dryer for the same duty The combination of
radiation and convection is also feasible such as infrared plus air jets or microwave
with impingement
223 Types of Dryer
The dryers can be classified according to the method of heating transfer or the
drying source Using the former classification the dryers can be divided into
convective dryers conductive dryers radiative dryers and dielectric dryers Using
the later classification the dryers can be broadly divided into airflue gas dryers and
superheated steam dryers In biomass drying the most common types of flue gas
dryers are rotary drum dryers and flash dryers And the commercial scale steam
dryers for biomass reported so far are tubular dryers fluidized bed dryers and
indirectly flash dryers (Wimmerstedt 2006 Fagernaumls et al 2010) In this section
some widely used biomass dryers will be review briefly
Rotary Dryer
Rotary dryer has been used for a long time in drying biomass and is by far the most
common dryer type in the existing large scale bioenergy plants It consists of a
slightly inclined rotating cylindrical shell fitted with a number of longitudinal flights
The flights lift the material and cascade it in a uniform curtain through the passing
gases Wet biomass is fed into the upper end of the dryer moves through it by virtue
of rotation head effect and slope of the shell and withdraws at the lower end finally
A schematic diagram of a direct co-current rotary dryer is shown in Figure 24 The
shell diameter can range from lt 1 m to gt 6 m depending on the throughput And the
drum rotating speed is varied from 2 to 8 rpm The direction of drying medium can
be either co-current or counter-current relative to the solids The biomass and hot
airflue gas normally flow co-currently through the dryer The hottest flue gas
contacts with the wettest biomass and the cooled flue gas contacts with the dried
biomass which could reduce the fire risk The exhaust gases leaving the dryer pass
through a cyclone multicyclone baghouse filter scrubber or electrostatic precipitator
(ESP) to remove any fine material entrained in the air According to the dryer
configuration an ID fan may be required which can be placed before the emission
control equipment to reduce erosion of the fan or after the first cyclone to provide the
pressure drop
Indirectly heated rotary dryers are widely used for the materials that would be
contaminated by the drying medium The heat source passes through the outer wall
of the dryer or through an inner central shaft to heat the dryer by conduction A
combined directindirect rotary dryer also exists where very hot flue gases enter the
dryer through a central shaft and initially provide heat indirectly by conduction then
the same gases pass through the dryer coming into direct contact with the wet
- 10 -
material During the second pass the indirect heating warms the flue gas and
material In this way a high flue gas temperature can be used for heating while the
fire risk is reduced by limiting the temperature of the gas in direct contact with the
biomass
Figure 24 Schematic diagram of direct rotary dryer (Krokida et al 2006)
The inlet gas temperature to rotary biomass dryer can vary from 230 ordmC to 1100 ordmC
and the outlet gas temperature varies from 70 ordmC to 110 ordmC To avoid the
condensation of acids and resins the outlet gas temperature is normally higher than
104 ordmC The retention time in rotary dryers can be less than a minute for small
particles and 10 to 30 minutes for larger material (Haapanen et al 1983
Intercontinental Engineering Ltd 1980 MacCallum et al 1981 Wardrop
Engineering Inc 1990)
The advantages of rotary dryers include (1) they are less sensitive to particle size
and can accept the hottest flue gas of any type of dryer (2) they have low
maintenance costs and the greatest capacity of any type of dryer (Intercontinental
Engineering Ltd 1980) But the material moisture is hard to control in rotary dryers
due to the long lag time for material in the dryer (Fredrikson 1984) Rotary dryers
also present the greatest fire hazard and require the most space (Intercontinental
Engineering Ltd 1980)
Conveyor Dryer
The conception of conveyor dryer (belt dryer) is simple The material is spread on
to a horizontally moving permeable belt in a continuous process and the heating
medium is forced through the bed of product by fans The drying medium is usually
either air or flue gas and its flow can be upward or downward Conveyor dryers are
very versatile and can handle a wide range of materials making them attractive for
biomass feedstocks Figure 25 is the conveyor dryer developed by Swiss Combi
(2009)
According to conveyor and airflow arrangement conveyor dryers generally have
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 7 -
boiler respectively For the flue gas dryer the flue gases after the boiler are taken
through a fuel dryer in which the fuel used in the boiler is dried For superheated
steam dryer the steam is extracted from the boiler and the evaporated water from the
dryer is recovered as low-pressure steam The superheated steam dryers have some
advantages compared to the flue gas dryers The total energy efficiency is increased
due to the possibility of reuse the latent heat of evaporation No oxidation or
combustion reaction is possible which eliminates the risks of explosions and hazards
And steam dryers have higher drying rates than flue gas dryers However steam
dryers also have some disadvantages Due to the high temperature level they have
problems with temperature-sensitive materials (Beeby and Potter 1985) For drying
biomass using steam dryer volatile organic materials contained in the biomass may be
emitted increasingly together with the water vapour at higher temperature steam
drying This would reduce the heating value of the biomass and increase the costs of
treating the exhaust steam In addition superheated steam dryers are difficult to
achieve low moisture content and the initial condensation may increase the total
drying time The systems also become more complex compared to those using flue
gas dryers
Figure 22 Principle of a flue gas dryer in combination with a boiler (Wimmerstedt
2006)
Figure 23 Principle of a superheated steam dryer in combination with a boiler
(Wimmerstedt 2006)
- 8 -
222 Heating Method
Convection conduction and radiation are three commonly used methods in
industrial drying In most cases heat is transferred to the surface of the product and
then to the interior However using dielectric radio frequency (RF) or microwave
freezing drying methods heat is generated internally within the product and then
transfers to the exterior surface
Convection
Convection is possibly the most common mode of drying Heat for evaporation is
supplied by convection to the exposed surface of the material and the evaporated
moisture is carried away by the drying medium Air inert gas (eg N2) direct
combustion gas or superheated steam can be as the drying source Convective
dryers can also be called direct dryers During the constant drying rate period the
solid surface takes on the wet bulb temperature which is determined by the ambient
air temperature and humidity at the same location While during the falling rate
period the solidsrsquo temperature approaches the dry bulb temperature of the drying
medium It should be noted that when using superheated steam as the drying
medium the solidsrsquo temperature corresponds to the saturation temperature at the
operating pressure
Conduction
Conduction or indirect drying is more suitable for thin products or for very wet
solids Heat is supplied through heated surfaces (stationary or moving) placed
within the dryer to support convey or confine the solids The evaporated moisture
is carried away by vacuum operation or by a stream of gas as a carrier of moisture
The thermal efficiency of conductive dryers is higher than convective dryers as the
latter loses a considerable amount of enthalpy with the drying medium
Radiation
Infrared radiation is often used in drying coatings thin sheets and films Although
most moisture materials are poor conductors for 50-60 Hz current the impedance falls
dramatically at radio frequency Hence such radiation could be used to heat the
solid volumetrically Energy is absorbed selectively by the water molecules Thus
less energy is required as the material becomes drier Since the capital and operating
costs are high for radiation drying it is usually to dry high unit value products or to
finally correct the moisture profile wherein only small quantities of hard-to-get
moisture are removed
It is noteworthy that sometimes the different drying methods can be combined
together For example a fluid bed dryer with immersed heating tubes or coils
- 9 -
combines advantages of both direct and indirect heating It can be only one third the
size of a purely convective fluid bed dryer for the same duty The combination of
radiation and convection is also feasible such as infrared plus air jets or microwave
with impingement
223 Types of Dryer
The dryers can be classified according to the method of heating transfer or the
drying source Using the former classification the dryers can be divided into
convective dryers conductive dryers radiative dryers and dielectric dryers Using
the later classification the dryers can be broadly divided into airflue gas dryers and
superheated steam dryers In biomass drying the most common types of flue gas
dryers are rotary drum dryers and flash dryers And the commercial scale steam
dryers for biomass reported so far are tubular dryers fluidized bed dryers and
indirectly flash dryers (Wimmerstedt 2006 Fagernaumls et al 2010) In this section
some widely used biomass dryers will be review briefly
Rotary Dryer
Rotary dryer has been used for a long time in drying biomass and is by far the most
common dryer type in the existing large scale bioenergy plants It consists of a
slightly inclined rotating cylindrical shell fitted with a number of longitudinal flights
The flights lift the material and cascade it in a uniform curtain through the passing
gases Wet biomass is fed into the upper end of the dryer moves through it by virtue
of rotation head effect and slope of the shell and withdraws at the lower end finally
A schematic diagram of a direct co-current rotary dryer is shown in Figure 24 The
shell diameter can range from lt 1 m to gt 6 m depending on the throughput And the
drum rotating speed is varied from 2 to 8 rpm The direction of drying medium can
be either co-current or counter-current relative to the solids The biomass and hot
airflue gas normally flow co-currently through the dryer The hottest flue gas
contacts with the wettest biomass and the cooled flue gas contacts with the dried
biomass which could reduce the fire risk The exhaust gases leaving the dryer pass
through a cyclone multicyclone baghouse filter scrubber or electrostatic precipitator
(ESP) to remove any fine material entrained in the air According to the dryer
configuration an ID fan may be required which can be placed before the emission
control equipment to reduce erosion of the fan or after the first cyclone to provide the
pressure drop
Indirectly heated rotary dryers are widely used for the materials that would be
contaminated by the drying medium The heat source passes through the outer wall
of the dryer or through an inner central shaft to heat the dryer by conduction A
combined directindirect rotary dryer also exists where very hot flue gases enter the
dryer through a central shaft and initially provide heat indirectly by conduction then
the same gases pass through the dryer coming into direct contact with the wet
- 10 -
material During the second pass the indirect heating warms the flue gas and
material In this way a high flue gas temperature can be used for heating while the
fire risk is reduced by limiting the temperature of the gas in direct contact with the
biomass
Figure 24 Schematic diagram of direct rotary dryer (Krokida et al 2006)
The inlet gas temperature to rotary biomass dryer can vary from 230 ordmC to 1100 ordmC
and the outlet gas temperature varies from 70 ordmC to 110 ordmC To avoid the
condensation of acids and resins the outlet gas temperature is normally higher than
104 ordmC The retention time in rotary dryers can be less than a minute for small
particles and 10 to 30 minutes for larger material (Haapanen et al 1983
Intercontinental Engineering Ltd 1980 MacCallum et al 1981 Wardrop
Engineering Inc 1990)
The advantages of rotary dryers include (1) they are less sensitive to particle size
and can accept the hottest flue gas of any type of dryer (2) they have low
maintenance costs and the greatest capacity of any type of dryer (Intercontinental
Engineering Ltd 1980) But the material moisture is hard to control in rotary dryers
due to the long lag time for material in the dryer (Fredrikson 1984) Rotary dryers
also present the greatest fire hazard and require the most space (Intercontinental
Engineering Ltd 1980)
Conveyor Dryer
The conception of conveyor dryer (belt dryer) is simple The material is spread on
to a horizontally moving permeable belt in a continuous process and the heating
medium is forced through the bed of product by fans The drying medium is usually
either air or flue gas and its flow can be upward or downward Conveyor dryers are
very versatile and can handle a wide range of materials making them attractive for
biomass feedstocks Figure 25 is the conveyor dryer developed by Swiss Combi
(2009)
According to conveyor and airflow arrangement conveyor dryers generally have
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 8 -
222 Heating Method
Convection conduction and radiation are three commonly used methods in
industrial drying In most cases heat is transferred to the surface of the product and
then to the interior However using dielectric radio frequency (RF) or microwave
freezing drying methods heat is generated internally within the product and then
transfers to the exterior surface
Convection
Convection is possibly the most common mode of drying Heat for evaporation is
supplied by convection to the exposed surface of the material and the evaporated
moisture is carried away by the drying medium Air inert gas (eg N2) direct
combustion gas or superheated steam can be as the drying source Convective
dryers can also be called direct dryers During the constant drying rate period the
solid surface takes on the wet bulb temperature which is determined by the ambient
air temperature and humidity at the same location While during the falling rate
period the solidsrsquo temperature approaches the dry bulb temperature of the drying
medium It should be noted that when using superheated steam as the drying
medium the solidsrsquo temperature corresponds to the saturation temperature at the
operating pressure
Conduction
Conduction or indirect drying is more suitable for thin products or for very wet
solids Heat is supplied through heated surfaces (stationary or moving) placed
within the dryer to support convey or confine the solids The evaporated moisture
is carried away by vacuum operation or by a stream of gas as a carrier of moisture
The thermal efficiency of conductive dryers is higher than convective dryers as the
latter loses a considerable amount of enthalpy with the drying medium
Radiation
Infrared radiation is often used in drying coatings thin sheets and films Although
most moisture materials are poor conductors for 50-60 Hz current the impedance falls
dramatically at radio frequency Hence such radiation could be used to heat the
solid volumetrically Energy is absorbed selectively by the water molecules Thus
less energy is required as the material becomes drier Since the capital and operating
costs are high for radiation drying it is usually to dry high unit value products or to
finally correct the moisture profile wherein only small quantities of hard-to-get
moisture are removed
It is noteworthy that sometimes the different drying methods can be combined
together For example a fluid bed dryer with immersed heating tubes or coils
- 9 -
combines advantages of both direct and indirect heating It can be only one third the
size of a purely convective fluid bed dryer for the same duty The combination of
radiation and convection is also feasible such as infrared plus air jets or microwave
with impingement
223 Types of Dryer
The dryers can be classified according to the method of heating transfer or the
drying source Using the former classification the dryers can be divided into
convective dryers conductive dryers radiative dryers and dielectric dryers Using
the later classification the dryers can be broadly divided into airflue gas dryers and
superheated steam dryers In biomass drying the most common types of flue gas
dryers are rotary drum dryers and flash dryers And the commercial scale steam
dryers for biomass reported so far are tubular dryers fluidized bed dryers and
indirectly flash dryers (Wimmerstedt 2006 Fagernaumls et al 2010) In this section
some widely used biomass dryers will be review briefly
Rotary Dryer
Rotary dryer has been used for a long time in drying biomass and is by far the most
common dryer type in the existing large scale bioenergy plants It consists of a
slightly inclined rotating cylindrical shell fitted with a number of longitudinal flights
The flights lift the material and cascade it in a uniform curtain through the passing
gases Wet biomass is fed into the upper end of the dryer moves through it by virtue
of rotation head effect and slope of the shell and withdraws at the lower end finally
A schematic diagram of a direct co-current rotary dryer is shown in Figure 24 The
shell diameter can range from lt 1 m to gt 6 m depending on the throughput And the
drum rotating speed is varied from 2 to 8 rpm The direction of drying medium can
be either co-current or counter-current relative to the solids The biomass and hot
airflue gas normally flow co-currently through the dryer The hottest flue gas
contacts with the wettest biomass and the cooled flue gas contacts with the dried
biomass which could reduce the fire risk The exhaust gases leaving the dryer pass
through a cyclone multicyclone baghouse filter scrubber or electrostatic precipitator
(ESP) to remove any fine material entrained in the air According to the dryer
configuration an ID fan may be required which can be placed before the emission
control equipment to reduce erosion of the fan or after the first cyclone to provide the
pressure drop
Indirectly heated rotary dryers are widely used for the materials that would be
contaminated by the drying medium The heat source passes through the outer wall
of the dryer or through an inner central shaft to heat the dryer by conduction A
combined directindirect rotary dryer also exists where very hot flue gases enter the
dryer through a central shaft and initially provide heat indirectly by conduction then
the same gases pass through the dryer coming into direct contact with the wet
- 10 -
material During the second pass the indirect heating warms the flue gas and
material In this way a high flue gas temperature can be used for heating while the
fire risk is reduced by limiting the temperature of the gas in direct contact with the
biomass
Figure 24 Schematic diagram of direct rotary dryer (Krokida et al 2006)
The inlet gas temperature to rotary biomass dryer can vary from 230 ordmC to 1100 ordmC
and the outlet gas temperature varies from 70 ordmC to 110 ordmC To avoid the
condensation of acids and resins the outlet gas temperature is normally higher than
104 ordmC The retention time in rotary dryers can be less than a minute for small
particles and 10 to 30 minutes for larger material (Haapanen et al 1983
Intercontinental Engineering Ltd 1980 MacCallum et al 1981 Wardrop
Engineering Inc 1990)
The advantages of rotary dryers include (1) they are less sensitive to particle size
and can accept the hottest flue gas of any type of dryer (2) they have low
maintenance costs and the greatest capacity of any type of dryer (Intercontinental
Engineering Ltd 1980) But the material moisture is hard to control in rotary dryers
due to the long lag time for material in the dryer (Fredrikson 1984) Rotary dryers
also present the greatest fire hazard and require the most space (Intercontinental
Engineering Ltd 1980)
Conveyor Dryer
The conception of conveyor dryer (belt dryer) is simple The material is spread on
to a horizontally moving permeable belt in a continuous process and the heating
medium is forced through the bed of product by fans The drying medium is usually
either air or flue gas and its flow can be upward or downward Conveyor dryers are
very versatile and can handle a wide range of materials making them attractive for
biomass feedstocks Figure 25 is the conveyor dryer developed by Swiss Combi
(2009)
According to conveyor and airflow arrangement conveyor dryers generally have
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 9 -
combines advantages of both direct and indirect heating It can be only one third the
size of a purely convective fluid bed dryer for the same duty The combination of
radiation and convection is also feasible such as infrared plus air jets or microwave
with impingement
223 Types of Dryer
The dryers can be classified according to the method of heating transfer or the
drying source Using the former classification the dryers can be divided into
convective dryers conductive dryers radiative dryers and dielectric dryers Using
the later classification the dryers can be broadly divided into airflue gas dryers and
superheated steam dryers In biomass drying the most common types of flue gas
dryers are rotary drum dryers and flash dryers And the commercial scale steam
dryers for biomass reported so far are tubular dryers fluidized bed dryers and
indirectly flash dryers (Wimmerstedt 2006 Fagernaumls et al 2010) In this section
some widely used biomass dryers will be review briefly
Rotary Dryer
Rotary dryer has been used for a long time in drying biomass and is by far the most
common dryer type in the existing large scale bioenergy plants It consists of a
slightly inclined rotating cylindrical shell fitted with a number of longitudinal flights
The flights lift the material and cascade it in a uniform curtain through the passing
gases Wet biomass is fed into the upper end of the dryer moves through it by virtue
of rotation head effect and slope of the shell and withdraws at the lower end finally
A schematic diagram of a direct co-current rotary dryer is shown in Figure 24 The
shell diameter can range from lt 1 m to gt 6 m depending on the throughput And the
drum rotating speed is varied from 2 to 8 rpm The direction of drying medium can
be either co-current or counter-current relative to the solids The biomass and hot
airflue gas normally flow co-currently through the dryer The hottest flue gas
contacts with the wettest biomass and the cooled flue gas contacts with the dried
biomass which could reduce the fire risk The exhaust gases leaving the dryer pass
through a cyclone multicyclone baghouse filter scrubber or electrostatic precipitator
(ESP) to remove any fine material entrained in the air According to the dryer
configuration an ID fan may be required which can be placed before the emission
control equipment to reduce erosion of the fan or after the first cyclone to provide the
pressure drop
Indirectly heated rotary dryers are widely used for the materials that would be
contaminated by the drying medium The heat source passes through the outer wall
of the dryer or through an inner central shaft to heat the dryer by conduction A
combined directindirect rotary dryer also exists where very hot flue gases enter the
dryer through a central shaft and initially provide heat indirectly by conduction then
the same gases pass through the dryer coming into direct contact with the wet
- 10 -
material During the second pass the indirect heating warms the flue gas and
material In this way a high flue gas temperature can be used for heating while the
fire risk is reduced by limiting the temperature of the gas in direct contact with the
biomass
Figure 24 Schematic diagram of direct rotary dryer (Krokida et al 2006)
The inlet gas temperature to rotary biomass dryer can vary from 230 ordmC to 1100 ordmC
and the outlet gas temperature varies from 70 ordmC to 110 ordmC To avoid the
condensation of acids and resins the outlet gas temperature is normally higher than
104 ordmC The retention time in rotary dryers can be less than a minute for small
particles and 10 to 30 minutes for larger material (Haapanen et al 1983
Intercontinental Engineering Ltd 1980 MacCallum et al 1981 Wardrop
Engineering Inc 1990)
The advantages of rotary dryers include (1) they are less sensitive to particle size
and can accept the hottest flue gas of any type of dryer (2) they have low
maintenance costs and the greatest capacity of any type of dryer (Intercontinental
Engineering Ltd 1980) But the material moisture is hard to control in rotary dryers
due to the long lag time for material in the dryer (Fredrikson 1984) Rotary dryers
also present the greatest fire hazard and require the most space (Intercontinental
Engineering Ltd 1980)
Conveyor Dryer
The conception of conveyor dryer (belt dryer) is simple The material is spread on
to a horizontally moving permeable belt in a continuous process and the heating
medium is forced through the bed of product by fans The drying medium is usually
either air or flue gas and its flow can be upward or downward Conveyor dryers are
very versatile and can handle a wide range of materials making them attractive for
biomass feedstocks Figure 25 is the conveyor dryer developed by Swiss Combi
(2009)
According to conveyor and airflow arrangement conveyor dryers generally have
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 10 -
material During the second pass the indirect heating warms the flue gas and
material In this way a high flue gas temperature can be used for heating while the
fire risk is reduced by limiting the temperature of the gas in direct contact with the
biomass
Figure 24 Schematic diagram of direct rotary dryer (Krokida et al 2006)
The inlet gas temperature to rotary biomass dryer can vary from 230 ordmC to 1100 ordmC
and the outlet gas temperature varies from 70 ordmC to 110 ordmC To avoid the
condensation of acids and resins the outlet gas temperature is normally higher than
104 ordmC The retention time in rotary dryers can be less than a minute for small
particles and 10 to 30 minutes for larger material (Haapanen et al 1983
Intercontinental Engineering Ltd 1980 MacCallum et al 1981 Wardrop
Engineering Inc 1990)
The advantages of rotary dryers include (1) they are less sensitive to particle size
and can accept the hottest flue gas of any type of dryer (2) they have low
maintenance costs and the greatest capacity of any type of dryer (Intercontinental
Engineering Ltd 1980) But the material moisture is hard to control in rotary dryers
due to the long lag time for material in the dryer (Fredrikson 1984) Rotary dryers
also present the greatest fire hazard and require the most space (Intercontinental
Engineering Ltd 1980)
Conveyor Dryer
The conception of conveyor dryer (belt dryer) is simple The material is spread on
to a horizontally moving permeable belt in a continuous process and the heating
medium is forced through the bed of product by fans The drying medium is usually
either air or flue gas and its flow can be upward or downward Conveyor dryers are
very versatile and can handle a wide range of materials making them attractive for
biomass feedstocks Figure 25 is the conveyor dryer developed by Swiss Combi
(2009)
According to conveyor and airflow arrangement conveyor dryers generally have
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 11 -
three configurations that are single passsingle-stage dryers single passmulti-stage
dryer and multiple pass dryers
Figure 25 A conveyor dryer developed by Swiss Combi (1 Wet product 2 Feeding
screw 3 Pre-dried product discharge screw 4 Feeding screw 2nd layer 5 Dry product
discharge 6 Heat exchanger 7 Extraction fan 8 Cleaning brush 9 Belt cleaning
system) (Swiss Combi 2009)
Single passsingle-stage conveyor dryer is the simplest conveyor arrangement A
continuous belt runs the whole length of the dryer The advantages of this
configuration are that the heating medium temperature and velocity can be controlled
easily as the material progresses through the dryer and the bed cleaning accessories
are easy to access the bed as the bed can be returned under the dryer But its main
disadvantage is the same bed depth must be used throughout the complete drying
process
Single passmulti-stage conveyor dryer is the most versatile dryer configuration
available It overcomes the disadvantage of single passsingle stage dryer since the
bed depth can be varied during drying The single passmulti-stage dryer can adjust
the speed of beds in each stage Therefore the bed depth can be increased when the
following stage is slower than the preceding stage As a result the retention time for
a given product can be achieved in a smaller dryer Similar to the single
passsingle-stage configuration the single passmulti-stage one can also control the
heating medium temperature and velocity flexibly The only shortcoming of this
configuration is the higher cost and relatively large floor space requirement
Multiple pass conveyor dryer has same benefits as the single passmulti-stage one
but needs a much smaller footprint This is because the conveyor beds are arranged
one above the other running in opposite directions The material enters the dryer on
the top bed and cascades down to the lower beds The multiple pass dryer is the
most popular conveyor configuration in many industries due to its relatively low cost
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 12 -
small footprint and the ability to control the bed depths
The uniformity of drying in conveyor dry is very good due to the shallow depth of
material on the belt Conveyor dryers are better suited to take advantage of waste
heat recovery opportunities since they operate at lower temperatures than rotary
dryers The typical maximum drying air temperature is from 90 ordmC to 120 ordmC
Hence they can be used in conjunction with a boiler stack economizer to take
maximum advantage of heat recovery from boiler flue gas The lower temperature
also implies a lower fire hazard and lower emission of volatile organic compounds
(VOCs) from the dryer
Flash Dryer
Flash or pneumatic dryer achieves rapid drying with short residence time by fully
entraining the material in a high velocity gas flow (usually 15-35 ms) A simple
flash drying system (without scrubber) is presented in Figure 26 It includes the gas
heater the wet material feeder the drying duct the separator the exhaust fan and a
dried product collector The wet material is suspended in the drying medium
usually hot flue gas which flows up the drying tube
Figure 26 Schematic process of a flash dryer (Borde and Levy 2006)
The flash dryer is normally used for small particles and its gas stream velocity must
be higher than the free fall velocity Since the thermal contact between the
conveying gas and the solids is very short it is most suitable to remove the external
moisture The solid and gas are separated using a cyclone and the gases continue
though a scrubber to remove any entrained fine particles For wet or sticky
materials some of the dry material can be recycled back and mixed with the incoming
wet material to improve material handling Meanwhile the recirculation of the
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 13 -
material can also shorten the drying time Gas temperature of flash dryers is slightly
lower than rotary dryers but is still above the combustion point The solid residence
time in a flash dryer is typically less than 30 seconds to minimize the fire risk
The main advantages of flash dryer include (1) it can dry thermolabile materials
due to the short contact time and parallel flow (2) the dryer needs a very small area
and can be installed outside a building (3) it is easy to be controlled (4) the capital
and maintenance costs are low (5) simultaneous drying and transportation is useful
for material handling process While its main disadvantages are (1) high efficiency
of gas cleaning system is required (2) toxic materials cannot to be dried due to
powder emission but it can be avoided when using superheated steam as the heating
medium (3) lumped materials cannot be dried as they are difficult to disperse (4) it
needs to be operated carefully to avoid flammability limits in the dryer (Borde and
Levy 2006)
Cascade Dryer
Cascade or sprouted dryers were extensively in Nordic Countries especially in
Sweden for drying grain but they can be used for other types of biomass It
consists of five main components fan cyclone superheater drying chamber with a
conical bottom and material inletoutlet as shown in Figure 27 Wet material is
introduced to the dryer with a high velocity flue gas stream at atmospheric pressure
and whirls around a cascading bed where the material is dried The coarse material
is removed from the drying chamber by an overflow The fine particles leave the
dryer with the exiting gas and are separated in a cyclone The typical residence time
for a cascade dryer is a couple of minutes Applications have been mainly as
pre-dryer in combination with wood-fuel boilers in saw and pulp mills
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 14 -
Figure 27 Schematic diagram of a cascade dryer (Berghel et al 2008)
Cascade dryers operate at intermediate temperatures between those of rotary and
conveyor dryers They have a small footprint than rotary and conveyor dryers A
disadvantage is that they are more prone to corrosion and erosion of dryer surfaces
and thus require high maintenance costs
Fluidized Bed Dryer
Fluidized bed dryers are used extensively for the drying of wet particulate and
granular materials that can be fluidized For drying powders in the particle size
range of 50 microm to 2000 microm fluidized bed dryers compete successfully with other
drying techniques such as rotary dryers and conveyor dryers Conventional
fluidized bed dryer using hot air or flue gas could be suitable for biomass feedstocks
Figure 28 shows a typical fluidized bed drying system zones in a fluidized bed with
its corresponding solids hold-up and types of perforated distributor plates The
drying system consists of gas blower heater fluidized bed column and gas cleaning
systems (eg cyclone bag filters precipitator and scrubber) To save energy
sometimes the exit gas is partially recycled A gas stream is supplied to the bed
through a special perforated distributor plate and is uniformly distributed across the
bed As shown in Figure 28 there are four common types of distributors that are (i)
ordinary (ii) sandwiched (iii) bubble cap tuyere and (iv) sparger The gas stream
velocity is high enough to support the weight of whole bed in a fluidized status and
makes the solids suspend in the upward flowing gas The bubbling fluidized bed is
divided vertically into two zones namely a dense phase at the bottom and a freeboard
phase above the denser phase (Figure 28 upper right side) In the freeboard phase
the solids are held up and their density decreases with height Since the gas phase
and solid phase are mixed well the fluidized bed dryer can achieve a uniform and fast
evaporation
Figure 28 Typical fluidized bed drying set up (Law and Mujumdar 2006)
Steam fluidized bed dryer can combine the advantages of superheated steam drying
and the excellent heat and mass transfer characteristics of a fluidized bed A
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 15 -
pressurized steam fluid bed dryer developed by Niro Inc has been installed in a
biofuel plant in Sweden for drying wood chips and barks (Jensen 1995) The
efficiency of this dryer is high because of the utilization of the recovered steam
And it has excellent environmental performance since the system is fully closed with
no gaseous emissions to atmosphere But the cost of this dryer is high and it is
likely only suited to relatively large-scale plant
A recently developed method for biomass drying is sub-fluid bed drying In this
dryer solid particles are brought in a fluid state by an upward-moving flow of gas in
combined with vertical mechanical shaking of the fluid bed distributor plate Thus
the gas and solids are intensively mixed resulting in high heat transfer rates and
proper drying conditions It is possible to have the residence time up to 2 hours and
the drying temperature up to 600 ordmC
The main advantages of fluidized bed dryer include high rate of moisture removal
high thermal efficiency easy material transport inside dryer easy control and low
maintenance cost And its limitations include high pressure drop high electricity
consumption poor fluidization quality of some particulate products non-uniform
product quality for certain types of fluidized bed dryers erosion of pipes and vessels
(Law and Mujumdar 2006)
23 Selection of Dryers
In the view of the enormous choices of dryer types one could possibly deploy for
most products selection of the best type is a challenging task
Drying kinetics plays a significant role in the selection of dryers Location of
moisture (whether near surface or distributed in the material) nature of moisture (free
or strongly bound to solid) mechanisms of moisture transfer (rate-limiting step)
physical size of material conditions of drying medium (eg temperature humidity
and flow rate) pressure in dryer etc should be considered for selecting the suitable
dryer as well as the optimal operating conditions (Mujumdar 2006)
In terms of the size of the material to be dried triple-pass rotary dryers can accept
larger material but may experience plugging with very large material For very
large or variable size material a single pass rotary dryer might be best choice
Cascade dryers require a very uniform particle size For flash dryers and fluidized
bed dryers a small particle size is needed so that the material can be suspended in a
moving gas or steam stream
In terms of the heat source and temperature flue gas is an efficient source of heat
but the temperature may be too low to provide enough heat for complete drying
Using a process stream for heating may be energy efficient but requires the capital
investment in a heat exchanger Superheated steam dryers require a high
temperature heat source It is necessary to determine what excess heat is available in
the system and then to design the drying system to take advantaged of it If no extra
heat can be utilized it has to install a burner with an auxiliary fuel source to provide
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 16 -
the heat for drying (Amos 1998)
In addition the product quality requirement has an overriding influence on the
selection process For high-value products the selection of dryers depends mainly
on the value of the dried products since the cost of drying becomes a small fraction of
the sale price of the product On the other hand for very low value products the
choice of drying system depends on the cost of drying so the lowest cost drying
system is selected Since the energy cost is a large part in the life cost of a dryer and
the cost of energy continues to rise in the future it is important to select an
energy-efficient dryer where possible even at a higher initial cost In return the
higher efficiency translates better environmental implications
Although the focus is to select the dryer it is noted that in practice pre-drying stages
(eg mechanical dewatering evaporation preconditioning of feed and feeding) as
well as post-drying stages (eg exhaust gas cleaning product collection partial
recirculation of exhausts cooling of product etc) are included in the drying system
The optimal cost-effective choice of dryer will depend in some cases significantly on
these stages
Based on the above discussion the selection of dryer is rather complex Several
different choices may exist but the final choice rests on numerous criteria Finally
even if the dryer is selected correctly the dryer needs to be operated properly to
achieve the desired product quality and production rate at minimum total cost
24 Capital and Running Costs
Costs of drying are varied depending on the types of dryers the material to be dried
and the plant in which the dryers are installed Table 21 lists the costs of rotary
dryers in $kgh of water removed from various sources in 1998 USD The heat
requirements were 3000 ndash 8100 kJkg of water removed with most estimates in the
range of 3500 ndash 4700 kJkg (Intercontinental Engineering Ltd 1980 Mercer 1994)
It should be noted that the first five cases only calculated the dryer unit costs but the
last four cases were the installation costs The installation costs were found to be
very site-specific
Some capital costs of flash dryers are listed in Table 22 in 1998 USD The
estimated energy requirement for flash drying is around 3700 kJkg of water removed
(Intercontinental Engineering Ltd 1980) The first two cases show the dryer unit
costs while the last two cases give the installation costs
Capital costs of three cascade dryer installations were identified from technical
articles by Bruce and Sinclair (1996) as shown in Table 23 They also compared
the capital costs of rotary flash and cascade dryers as listed in Table 24 The
material handling equipments such as conveyors feeders and bins were not included
in the cost information The heating medium is flue gas entering the dryer at 300 ordmC
and leaving at 105 ordmC As shown in these tables the flash dryer requires higher
equipment and installation costs than the rotary and cascade dryer while the costs of
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 17 -
the rotary dryer is similar to the cascade dryer
Table 21 Rotary dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Single-Pass 26 Fredrikson 1984
Triple-Pass 22 Fredrikson 1984
Stearns amp Roger 40 - 71 Intercontinental Engineering Ltd 1980
Aeroglide 24 - 46 Intercontinental Engineering Ltd 1980
Heli 37 - 106 Intercontinental Engineering Ltd 1980
Rotary 224 Frea 1984
Rotary 176 Technology Application Laboratory 1984
Flue Gas 761 Wardrop Engineering Inc 1990
Rotary 300 - 796 MacCallum et al 1981
Table 22 Flash dryer capital costs in 1998 USD (Amos 1998)
Dryer type Capital costs ($kgh) Source
Flash 18 - 35 Fredrikson 1984
Williams Hot Hog 53 - 160 Intercontinental Engineering Ltd 1980
Bark 335 Haapanen et al 1983
Flash 550 - 1600 MacCallum et al 1981
Table 23 Cascade dryer capital costs (Bruce and Sinclair 1996)
Throughput Capital cost Source
9 th 5 million USD in 1995 Cadcades Inc East Angus Quebec
36 th 85 million CAD in 1986 Fletcher Challenge Crofton BC
32 th 63 million USD in 1992 Alabama River Pulp Claiborne AL
Table 24 Comparison of costs of flue gas dryers (Bruce and Sinclair 1996)
Dryer Moisture content Equipment costs Installed costs
type In () Out () (k$th) (k$th)
Rotary 55 40 45 ndash 80 370 ndash 160
Cascade 55 40 45 ndash 70 360 ndash 200
Flash 55 15 180 ndash 70 860 ndash 330
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 18 -
Note the first value is about 4 th and the second about 35 th
Costs of conveyor dryer in biomass drying have been calculated by Holmberg and
Ahtila (2004) Two alternative drying processes were considered multi-stage drying
and single-stage drying with multi-stage heating Air was used as the drying
medium and heat transfer from the heat source (secondary heat back pressure steam
and extraction steam) into the drying system occurred in indirect heat exchanges It
was found that for the multi-stage drying process the annual investment costs varied
from 359 keuro to 932 keuro based on the price level in 2002 For the single-stage drying
the annual investment costs varied from 323 keuro to 949 keuro They suggested that
single-stage drying could be a more economic way to carry out the drying if the
amortisation time were short otherwise multi-stage drying is generally more
economic because of the increasing running costs
According Bruce and Sinclair report (1996) installation costs of a high pressure
steam dryer developed by Imatran Voima Oy (IVO) were expected to be of the order
of 300 ndash 400 k$tevaph in 1996 USD which included a pulveriser for some size
reduction Wade (1998) provided costs of superheated steam dryers in a case study
for three dryer configurations The dryers were sized for a 55 th boiler drying
material from 60 to 40 moisture content The capital costs were 45 million
CAD for a MoDo type steam dryer 70 million CAD for a steam dryer with a heat
exchanger and 54 million CAD for a diskporcupine dryer All costs were based on
the price level in 1990 (Wardrop Engineering Inc 1990)
In addition to the equipment and installation costs the running costs are important
factors Power and maintenance costs are the main parts of running costs Power
consumptions based on the oven dry throughput are 8 ndash 14 kWht for rotary dryers
15-20 kWht for cascade dryers and 16-38 kWht for flash dryers (Bruce and Sinclair
1996) Holmberg and Ahtila (2004) estimated that the heat and electricity costs were
17-211 keuroyear for a multi-stage conveyor dryer with a throughput of 1 kgdms and
17-177 keuroyear for a single-stage conveyor dryer having the same throughput The
maintenance costs of equipment are not reported in the literature Generally an
allowance of 2 of total installation costs of the drying system is suggested as the
basis for preliminary evaluation purpose
25 Safety and Environmental Issues
A dryer fire or explosion can arise from ignition of a dust cloud if substantial
amounts of fines are present or from ignition of combustible gases released from the
drying material The combustion temperature of biomass released during drying is
in the range of 204 ndash 260 ordmC with an auto-ignition temperature of 260 ndash 288 ordmC
(MacCallum et al 1981) However most dryers can operate at much higher
temperatures The low drying temperature is beneficial to reduce the fire risk in the
dryer but decreases the drying rate
In addition the oxygen concentration in the dryer is also a serious concern In
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 19 -
most dryers the risk of fire or explosion becomes significant if the drying medium
has an oxygen concentration over about 10 vol In order to maintain a low
oxygen concentration the amount of excess air needs to be controlled or exhaust
gases can be recirculated to the dryer inlet Recirculation also increases the thermal
efficiency of the dryer Flue gas dryers typically operate at higher temperatures than
indirectly heated air dryers partly because of the lower oxygen content of the gas
(Intercontinental Engineering Ltd 1980) It should be noted that a high drying
temperature could create a risk of spark development and the release of carbon
monoxide through slow pyrolysis and smouldering Carbon monoxide together with
dust creates a risk of hybrid explosion which is very dangerous With carbon
monoxide present in atmosphere the safe oxygen level needs to be decreased
substantially ie below 8 vol
In terms of the drying time the longer residence time of material exposed to high
temperature medium and the lower the moisture content the greater the fire risk
Rotary dryers have the highest fire risk because of their longest retention times
Equipments to control fires include fire detection equipment fuel and air shut-offs
deluge showers steam or water sprays and fire dumps to prevent smouldering
material from reaching fuel stockpiles (Intercontinental Engineering Ltd 1980)
Another cause of fires in biomass dryers is the condensation of resins that are
released from the wood during drying If the dryer exhaust gases cool or come in
contact with cold surfaces the resin vapours may condense and then attract dust
This dust and resin mixture is very flammable and may build up and ignite at some
later time (Mercer 1994 Lamb 1994)
For superheated steam dryers the fire risk is minimal The guaranteed absence of
air and oxygen eliminates fire and explosion risk (Deventer 2004) The only risk is
when the dried material leaving the dryer is still hot and comes in contact with air
(Haapanen 1983 Wardrop Engineering Inc 1990 Ceckler 1994)
During the biomass drying process organic compounds are released as a result of
volatilization steam distillation and thermal destruction In the directly heated flue
gas dryers since the drying temperature is relatively high the organic compounds
released are diluted in the flue gas and emitted through the chimney The installation
of flue gas clean-up equipments depends on the local regulation Solid particulates
are usually removed by cyclones or bag filters Direct-heated rotary dryers have
greater emission of VOCs and particulates than indirect-heated dryers Conveyor
dryers have lower emissions of VOCs and particulates than rotary dryers
Superheated steam dryers by nature do not have air emissions but may have
contaminated condensate that must be treated by precipitation and biological
oxidation before being led to a recipient (Vidlund 2004 Berghel and Renstroumlm
2003) The non-condensable stream can be burned in an existing boiler
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 20 -
3 Case Study Biomass Drying Process Design Using Low
Grade Heat
31 Low Grade Heat from Steel Production Process
As part of EPSRC Thermal Management programme our academic partner
Newcastle University carried out a study to identify and classify the sources of low
grade heat available from steel industry in the UK (University of Newcastle 2011)
The report prepared by Newcastle University included the data gathered from a
thermal energy audit of one of Corusrsquos steelworks This plant produces nearly 5
million tonnes of steel slabs every year
Sheffield University has used the data provided by Newcastle University in order to
carry out a series of calculations The following sections present the results obtained
form Sheffield University case study calculations
Case Study
The flue gas waste heat and cooling water streams are released from the following
individual processes in this steel plant coke oven sinter blaster furnace (BF) basic
oxygen furnace (BOF) continuous casting hot mill cold mill annealing processing
line and power plant The gas and cooling water streams identified in the above
processes were classified in terms of their exergy as shown in Tables 31 and 32
respectively
Based on the temperature and gas composition the low grade gas streams listed in
Table 31 can be further divided into three groups The first group is
non-recoverable waste heat because of their low temperature such as fume and
extraction gas from cold mill fumes from BOS primary and secondary fumes from
casthouse and sinter gas from sinter dedust The composition of these gas streams is
identical to air and their highest temperature is only 50 ordmC which is too low to be
used in drying process Hence these gas streams were excluded from our
calculations The second group is the recoverable and combustible flue gas streams
ie BOF primary gas and flare BF gas These gases must be burnt completely before
they can be recovered In order to calculate the flue gas products after combustion
following assumptions were made These were
1 - During the combustion the excess air ratio is 12
2 - 10 of the released heat is used to heat up the post-combustion products
Based on the above two assumptions the flue gas composition and temperature
after combustion were calculated as shown in Table 33 The last group is the
recoverable gas streams that can be used directly such as sinter gas NH3 combustion
gas and mixture of BF and coke oven gas Their temperatures are between 130 ordmC to
220 ordmC Table 33 lists the recoverable low grade gas streams used in our case study
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 21 -
The flare BF gas from lsquoBF arsquo and lsquoBF brsquo are combined together as their compositions
and temperatures are exactly the same Although the sinter gas 1 from the main
stack the sinter gas 2 from the end of sinter strand and the NH3 combustion gas from
the ammonia incinerator all have high oxygen content (above 17 by volume) the
gas temperatures are in the range of 403 K to 483 K hence there will be a very remote
chance of fire incident during the drying process (Roos 2008) The moisture
content in these gas streams is low (the highest one is only 85 by volume) so it
will be difficult to recover the latent heat of water vapour from them
The low grade cooling water streams temperatures varies between 33 ordmC to 50 ordmC as
shown in Table 32 Therefore it is not beneficial to use these water streams in
biomass drying process However they can be used as the boiler feed water if the
drying process is integrated with the biomass power and CHP plant
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 22 -
Table 31 Classification of low grade flue gas streams in the steel industry (University of Newcastle 2011)
Location Type Composition (by volume) Temperature Quantity Exergy
H2 O2 N2 CO2 CO SO2 NO2 H2O (ordmC) (kgs) (MW)
Cold mill Fume 0 021 079 0 0 0 0 0 30 12 0002
Cold mill Extraction gas 0 021 079 0 0 0 0 0 40 22 0014
BOS primary Pouring fume 0 021 079 0 0 0 0 0 50 60 0088
BOS secondary Fume 0 021 079 0 0 0 0 0 50 86 0125
BOS primary BOS gas 002 0 013 015 07 0 0 0 70 32 0125
BOS secondary Pouring fume 0 021 079 0 0 0 0 0 40 191 0125
BF a Flare BF gas 003 0 058 013 026 0 0 0 200 3 0126
BOS primary BOS gas 002 0 013 015 07 0 0 0 150 10 0148
Casthouse (north) Fume 0 021 079 0 0 0 0 0 50 185 0229
Casthouse (south) Fume 0 021 079 0 0 0 0 0 50 185 027
Sinter dedust Sinter gas 0 021 079 0 0 0 0 0 50 245 027
BF b Flare BF gas 003 0 058 013 026 0 0 0 200 10 036
End of sinter strand Sinter gas 0 021 079 0 0 0 0 0 180 36 0443
Ammonia incinerator NH3 combustion gas 0 018 068 001 001 007 0 005 210 193 0734
Coke oven gas BF and coke oven gas 0 007 072 013 0 0 0 009 220 100 0827
Main stack Sinter gas 0 017 076 004 0 0 0 003 130 388 5128
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 23 -
Table 32 Classification of low grade cooling water streams in the steel industry (University of Newcastle 2011)
Type Location Temperature (ordmC) Quantity (kgs) Exergy (MW)
Cooling water Sinter 50 9 0016
Cooling water BF b 41 257 0311
Cooling water Hot mill 38 233 0337
Cooling water Hot mill 38 218 0353
Cooling water BF a 35 307 0466
Cooling water Caster 3 40 200 0535
Coolingquench water Hot mill 35 444 0599
Cooling water Caster 3 33 542 062
Cooling water BF a 35 665 0651
Cooling water BF b 40 1405 0701
Cooling water BF b 37 417 081
Cooling water BOS primary 35 565 0824
Cooling water BF b 36 511 0882
Cooling water Caster 1 42 316 1019
Cooling water Caster 2 40 486 1296
Cooling water Caster 1 40 495 132
Cooling water Caster 2 40 497 132
Cooling water Coke oven 40 556 1518
Dirty water return Hot mill 35 1827 2457
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 24 -
Table 33 Classification of recoverable low grade gas streams in the steel industry
Location Type Composition (by mass) Temperature Quantity Enthalpy
O2 N2 CO2 SO2 H2O (K) (kgs) (MW)
Main stack Sinter gas 1 0184 0731 0063 0 0021 403 388 4158
End of sinter strand Sinter gas 2 0233 0767 0 0 0 453 36 565
Ammonia incinerator NH3 combustion gas 0187 063 0014 0138 0031 483 193 334
Coke oven gas BF and coke oven gas 0079 0679 0190 0 0052 493 100 2058
BF a and b Flare BF gas 0017 0652 0320 0 0010 546 235 594
BOS primary BOS gas 1 0026 0551 0419 0 0004 5408 955 2329
BOS primary BOS gas 2 0026 0551 0419 0 0004 6277 299 1016
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 25 -
32 Drying System Design
In this case study the low grade gas streams (Table 33) emitted from the steel
industry are used as the heating source White pine wood chip is the chosen biomass
material for this study with the net heating value of 1666 MJkg (dry basis) The
performance of each gas stream in drying biomass is calculated and analysed
Figure 31 illustrates the mass and energy balances of the adiabatic drying process
utilizing these gas streams Properties of waste flue gas are known so the next stage
of work concentrates on finding the mass flow rate of biomass during the drying
process These calculations are repeated for a range of biomass materials with
different moisture contents It is intended to dry the biomass material as much as
possible using the existing waste flue gases
Figure 31 Mass and energy balances of adiabatic drying
321 Thermal Design Methodology
As discussed in section 223 industrial conveyor dryers are the most popular
family of dryers for drying agricultural products A single passsingle stage
cross-flow conveyor dryer where the waste flue gas passes through the perforated tray
is used in this study A typical flow sheet of a conveyor dryer is presented in Figure
32 The following initial assumptions are made
1) dry mass flow of the biomass fuel through the dryer is constant
2) air velocity is constant
3) bed height does not change during drying
The wet feed at flow rate fmamp (kgdms) temperature tinf (K) and humidity uin
(kgkgdm) is distributed on the belt as it enters the dryer The dried biomass leaves
the dryer at the same flow rate fmamp (kgdms) temperature toutf (K) and moisture
content uout (kgkgdm) The belt is moving at a velocity of υc (ms) and requires an
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 26 -
electrical power Eb (kW) The air which is used for drying enters the dryer at a flow
rate of gmamp (kgdms) temperature ting (K) and humidity xin (kgkgdm) and leaves the
dryer at a temperature toutg (K) and humidity xout (kgkgdm) An electrical power Ef
(kW) is used to operate the fan
Figure 32 Side view of a continuous crossflow dryer
The mathematical model of the dryer involves heat and mass balances of flue gas
and product streams as well as heat and mass transfer phenomena that take place
during drying The total humidity balance in the dryer is given by the following
equations
( ) ( )inoutgoutinf xxmuum minus=minus ampamp (31)
The total energy balance assuming negligible heat losses is given as follows
( ) ( )goutgingfinfoutf hhmhhm minus=minus ampamp (32)
where hing is the specific enthalpy of gas stream at the dryer inlet (kJkg) houtg the
specific enthalpy of gas stream at the dryer outlet (kJkg) houtf the specific enthalpy
of fuel stream at the dryer out (kJkg) and hinf the specific enthalpy of fuel stream at
the dryer inlet (kJkg) Here the specific enthalpy of gas stream can be written as
( )[ ])()( gwrefgwprefggpg TiTTcxTTch +minussdot+minus= (33)
where cpg is the specific heat capacity of non-condensable gases cpw the specific heat
of the water vapour iw the latent heat of water and x the molar fraction of water
vapour in the flue gases And the specific enthalpy of biomass stream can be
expressed as
( )15273)( minussdot+minus= fwrefffpf TcuTTch (34)
where cpf is the specific heat capacity of dry biomass which is assumed to be 25
kJkgk cw the heat capacity of liquid water
It is assumed that the heat transfer coefficient takes a value high enough to allow the
product stream (ie biomass material after drying) leaving the dryer to be in thermal
equilibrium with the air stream leaving the product Therefore heat transfer within
the dryer is expressed by means of the following equation
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 27 -
goutfout tt = (35)
Furthermore thermodynamics indicates that the moisture content of the product
stream leaving the dryer should be greater than the corresponding moisture content at
an equilibrium imposed by the air operating conditions in the dryer as proposed by
the following relation
SEout uu ge (36)
where uSE is the equilibrium moisture content (EMC) of fuel stream (kgkgdm) The
Hailwood-Horrobin equation is often used to approximate the relationship between
EMC temperature (T) and relative humidty (h) for wood (Figure 33) (Hailwood and
Horriobin 1946 Eleoteacuterio et al 1998)
++
++
minus=
22
211
22
211
1
2
1
1800
hkkkkhk
hkkkkhk
kh
kh
WM eq (37)
20041504520330 TTW ++= (38)
274 10448106347910 TTkminusminus timesminustimes+= (39)
254
1 1035910757346 TTk minusminus timesminustimes+= (310)
252
2 1004910842091 TTk minusminus timesminustimes+= (311)
where Meq is the EMC (percent) T the temperature (ordmF) h the relative humidity
(fractional)
This equation does not account for slight variation with wood species state of
mechanical stress andor hysteresis In this case study equation 37 is used to
calculate the EMC of white pine wood chips when the temperature T and the relative
humidity h are known
In order to obtain the maximum drying throughput the flue gas at the dryer outlet is
expected to be saturated However since the saturated state is difficult to achieve
the maximum relative humidity can be estimated as 90
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 28 -
Figure 33 Equilibrium moisture content of wood versus humidity and temperature
according to the Hailwood-Horrobin equation (Eleoteacuterio et al 1998)
322 Dryer Capacity
Based on the assumptions made and the mass and energy balance equations in
section 321 the dryer capacity was calculated using Microsoft Excel Two group
cases were investigated In group 1 the initial moisture content of biomass is 15
kgkgdm and the final moisture content changes between 04 03 02 to 01 kgkgdm
In group 2 the initial moisture content is 10 kgkgdm and the final moisture content
changes from 04 03 02 to 01 kgkgdm The biomass throughput capacity and the
evaporation rate of water from biomass for each gas stream heating source are shown
in Figures 34 and 35 respectively
It can be seen that lsquosinter gas 1rsquo from main stack stream has the maximum dry
biomass throughput and the highest water evaporation rate among the gas streams
from steel industry due to its large quantity The gas streams in order of the drying
capacity from high to low are lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and coke oven gasrsquo
lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo The drying
capacities for these streams are consistent with their enthalpy levels This implies
that the energy content of the gas stream determines its performance in the drying
process Even though the temperature level of some gas streams is relatively low
they can still possess a large amount of energy because of their considerable quantity
In addition it is clear that for a fixed initial moisture content of biomass the increase
in final moisture content will increase the throughput of biomass However this will
slightly reduces the evaporation rate of water When comparing two group cases with
different initial moisture contents it is noted that group 1 cases with higher initial
moisture content (15 kgkgdm) process less biomass whereas there is not much
change in terms of the evaporation rates of water for these cases This is because all
the gas streams are supposed to be nearly saturated when leaving the dryer
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 29 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 34 Dry biomass throughput varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 30 -
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 35 Evaporation rate of water varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
The biomass power plant sizes using different gas streams in the drying process are
also calculated and presented in Figure 36 It can be concluded that using the waste
heat of steel industry to dry the biomass is promising in practical application The
input heating value of power plant can be varied from 13 MW to 187 MW and 20
MW to 334 MW at initial moisture content of 15 kgkgdm and 10 kgkgdm
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 31 -
respectively Assuming that the efficiency of the power plant is 30 we can
generate up to 100 MW electricity
(a) Initial fuel moisture 15 kgkgdm
(b) Initial fuel moisture 10 kgkgdm
Figure 36 Biomass power plant size varies with the final moisture content using
different heating sources at (a) initial moisture content of 15 kgkgdm and (b) initial
moisture content of 10 kgkgdm
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 32 -
323 Drying Curve
Drying curve is an important characteristic curve for designing a conveyor dryer as
discussed in section 21 Each material at a specific condition has its distinct drying
curve The drying rate and the variation of moisture with time are normally obtained
from the experiments The drying rate curve is also important for determining the
residence time of solid materials in the dryer which is an essential parameter in dryer
design However in the current study due to the time limitation it is not possible to
carry out the experiments in order to obtain the drying curve of white pine wood chips
For this reason the drying curve used in this study was taken from literature
Gigler et al (2000) carried out thin layer forced convective drying experiments on
willows chips and simulated the drying process for the same chips Two bed heights
were tested one with 1 cm height and the other with 8 cm height Their
corresponding superficial velocities of the air were 012 ms and 017 ms
respectively The drying curves shown in Figure 37 indicated that a good fit was
achieved between the simulated and experimental drying curves Two drying stages
(constant drying rate stage and falling drying rate stage) can be observed from Figure
37 At the constant drying rate stage (from 0 s to 05times105 s approximately)
convective heat transfer dominates the drying process leading to a fast drop of
moisture content with time As the drying process continues there is less water
contact at the surface and the internal diffusion of water inside the solid becomes
more significant hence slowing down the drying rate The drying process enters
into the falling drying rate stage after around 05times105 s Figure 37 also suggests that
with an increase in the bed height the drying process was retarded during the constant
drying rate stage But at the falling drying rate stage the drying curves at two bed
heights are nearly identical
Figure 37 Comparison of simulated (continuous line) and experimental (discrete
symbols) drying curves for willow chips at the bed height of 1 cm () and 8 cm ()
(Gigler et al 2000)
They also carried out deep bed drying experiments and simulations The total
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 33 -
quantity of willow chips was 1440 kg and the bed height was 1 m The air velocity
used for the simulation was 055 ms The drying air left the chip bed fully saturated
until the EMC was nearly reached The measured and simulated average moisture
content of the willow chips bed as a function of time is shown in Figure 38 It is
found that the drying model described the drying curve well except for the time
interval 90 ndash 120 h
Figure 38 Measured () and simulated (continuous line) chip bed moisture content
for a chip bed of 1 m (Gigler et al 2000)
Holmberg et al (2004 2005) investigated the drying of regularly shaped wood
particles in a fixed-bed reactor The flow sheet of the reactor is shown in Figure 39
The initial moisture of the particles was 15 kgkgdm and the dry mass flow rate of the
material through the dryer was 1 kgdms The temperature difference between the dry
bulb temperature (tdb) and the wet bulb temperature (twb) in the test were 32 ordmC 46 ordmC
61 ordmC 78 ordmC 95 ordmC and 100 ordmC respectively The dry bulb temperature can be
expressed as a function of wet bulb temperature as follows (Lampinen 1997)
)()(
wbv
pa
wbwbdb tl
c
xtxtt
minusprime+= (312)
( )( )
( )( )230
64997811
5
230
64997811
5
10
106220)(
+
minus
+
minus
minus
=prime
wb
wb
wb
wb
t
t
o
t
t
wb
ep
etx (313)
wbwbv ttl 23402501000)( minus= (314)
where x is the air moisture at dryer inlet and )( wbtxprime is the saturated air moisture
According to the equations 312 ndash 314 the corresponding dry bulb temperatures
were 54 ordmC 74 ordmC 93 ordmC 115 ordmC 134 ordmC and 152 ordmC respectively The bed height
was kept at 200 mm and the air velocity through the bed was constant at 065 ms
The drying time was determined by measuring the values of the inlet and outlet air
moistures as a function of time as shown in Figure 310 In addition the regression
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 34 -
curves (Figure 311) provided the correlation between drying time and the
temperature difference tdb-twb which were determined based on Figure 310
Figure 39 Test rig used for the determination of drying curves (x air moisture
transmitter t thermocouple m mass flow control) (Holmberg et al 2005)
Figure 310 Drying curves determined from experiments for various temperature
differences tdb-twb (Holmberg et al 2005)
For a certain fuel moisture decrease uin-uout the correlation can be expressed as
follows
BttA wbdbdry +minus= )ln(τ (315)
where A and B are two coefficients Figure 311 presents four examples of
regression curves and the corresponding correlation equations
In this case study since the initial moisture contents of the biomass are assumed to
be 15 kgkgdm and 10 kgkgdm it is feasible to use the drying curves provided by
Holmberg et al (2004 2005) to calculate the drying time However as shown in
Figure 311 the lowest final fuel moisture content available in the correlation equation
is only 04 kgkgdm and the temperature difference (tdb-twb) is at the range of 32 ordmC to
110 ordmC Considering the final moisture contents and the temperatures of gas streams
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 35 -
used in this case study we had to extrapolate the regression curves in Figure 311
The regression curves for the final moisture contents of 03 02 and 01 kgkgdm are
added as shown in Figure 312 And their corresponding correlation equations are
as follows
12768)ln(82469 +minusminus= wbdbdry ttτ for 01 kgkgdm final moisture (316)
11417)ln(92209 +minusminus= wbdbdry ttτ for 02 kgkgdm final moisture (317)
10065)ln(11950 +minusminus= wbdbdry ttτ for 03 kgkgdm final moisture (318)
Figure 311 Regression curves and correlation equations (Holmberg et al 2005)
Figure 312 Calculated regression curves used to calculate the drying time at the initial
moisture content of 15 kgkgdm
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 36 -
As shown in Figure 312 the biomass material with higher final moisture content
requires less drying time whereas the material with lower final moisture content
needs more drying time Furthermore for a given moisture reduction the required
drying time decreases with an increase of drying temperature difference It also
implies that the effect of drying temperature difference on the drying time becomes
limited as the drying temperature difference increases further In other words it is
believed that the drying time for a certain fuel moisture reduction will not be
decreased further when the drying temperature difference is high enough Under this
circumstance the correlation equation 315 is not valid In this case study since the
gas stream temperature can rise as high as 345 ordmC (which corresponds to a drying
temperature difference of 297 ordmC) some assumptions have to be made in order to
calculate the drying time Here it is assumed that when the drying temperature
difference is higher than 120 ordmC the drying time will not be affected So the drying
time equations can be expressed as follows
BttA wbdbdry +minus= )ln(τ for tdb-twb le 120 ordmC (319a)
BAdry += )120ln(τ for tdb-twb gt 120 ordmC (319b)
But this equation is only valid when the initial moisture content of biomass is 15
kgkgdm and the bed height is 200 mm It is not applicable to the cases with the
initial moisture content of 10 kgkgdm Hence in the following sections only the
group with the initial moisture content of 15 kgkgdm will be calculated and
discussed
324 Dryer Parameters
In sections 322 and 323 the properties of the biomass and flue gas at the dryer
inlet and outlet and the drying time results were presented In this section other dryer
parameters such dryer size mass hold up will be analysed
The mass holdup Mf and volume holdup Vf of the conveyor dryer can be written as
follows
)1( infdryf umM += ampτ (320)
dm
f
f
MV
ρε )1( minus= (321)
where ε is the volume fraction of air in the bed and ρdm the dry biomass density
The geometrical distribution of the volume holdup on the conveyor can be expressed
as
BLZV f = (322)
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 37 -
where B is the width of the conveyor L the length of the conveyor Z the bed height
(loading depth) In this case study the volume fraction of air ε is set as 05 the dry
biomass density ρdm as 700 kgm3 and the loading depth Z as 02 m The bed width
is changeable to make sure that the ratio of bed length to bed width is realistic The
bed height is the same as the one reported in the fixed-bed experiment study so that
the drying curves obtained from the experiments are applicable to the conveyor dryer
In addition the study by Holmberg and Ahtila (2004) indicated that the bed height
should not be too high or too small Too high bed height will cause a high pressure
drop as the pressure drop is proportional to the bed height whereas too small bed
height cannot ensure the flue gas reaches its saturation point before the end of the bed
The selection of 02 m bed height has been verified in Holmberg and Ahtila
experiments (2004) Once the volume holdup bed width and bed height are known
the conveyor length L can be obtained from equation 322
The cross-sectional area of conveyor dryer Ab is easy to calculate using the bed
width and length
)51()51( +sdot+= LBAb (323)
Here a 5 extra width and length is taken into consideration in the design The
moving velocity of the conveyor υc is also available
dryc L τυ = (324)
The cross-sectional area of the covering is 10 larger than that of the conveyor
The height and the wall thickness of the covering are assumed to be 6 m and 35 mm
respectively
In order to size the fan the flue gas velocity and the pressure drop of flue gas
through the loaded bed should be known The equations calculating these two
parameters are listed here
dg
g
gBL
m
ρυ
amp= (325)
2
1
k
gZkp υ=∆ (326)
where υg is the gas velocity ρdg the dry flue gas density ∆p the pressure drop of flue
gas and k1 and k2 the fitted parameters which depend on the size and shape of the
drying material The both parameters can be determined from experimental data
Gustafsson (1988) Kofman and Spinelli (1997) and Nellist (1997) derived values of
the fitted parameters for wood chips In the size range 10 ndash 25 mm average values
of the fitted parameters are k1 = 1755 and k2 = 167 In the ldquoHandbook of Industrial
Dryingrdquo (Mujumdar 2006) k1 = 20000 and k2 = 2 for an unknown material
Holmberg and Ahtila (2004) estimated the pressure drop for regularly shaped spruce
particles during each drying stage is 500 Pa In this case study the pressure drop is
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 38 -
estimated to be 400 Pa
Table 34 lists the summary of conveyor dryer design parameters at the initial
moisture content of 15 kgkgdm It is found that the throughput of biomass and the
drying rate (ie the water evaporation rate) determine the size of the dryer The
dryer using lsquosinter gas 1rsquo as the heating source has the highest drying rate in
comparison to other dryers As a result its mass and volume holdup are also larger
than others as well as its dryer size which may lead to a higher capital and running
costs The dryer using lsquoNH3 combustion gasrsquo as the heating source provides the
lowest drying rate Therefore dryer size and the biomass massvolume holdup on the
conveyor are much smaller than other dryers
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 39 -
Table 34 Conveyor dryers design parameters
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1316 193 112 633 197 773 334
Dryer length (m) 4989 975 1133 2128 994 2599 1688
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3491966 272989 158597 893886 278443 1091749 472558
Volume holdup (m3) 9977 78 453 2554 796 3119 1350
Belt area (m2) 49885 3900 2266 12770 3978 15596 6751
Gas velocity (ms) 060 072 062 060 042 041 030
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1325 194 113 639 199 779 338
Dryer length (m) 5354 1054 1228 2310 1078 2817 1832
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3747868 295045 171889 970135 301827 1183257 512869
Volume holdup (m3) 10708 843 491 2772 862 3381 1465
Belt area (m2) 53541 4215 2456 13859 4312 16904 7327
Gas velocity (ms) 056 066 057 055 039 038 028
Loading depth (m) 02 02 02 02 02 02 02
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 40 -
Table 43 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1332 195 114 644 200 785 341
Dryer length (m) 5671 1123 1311 2470 1152 3009 1959
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 3969580 314348 183591 1037225 322422 1263673 548394
Volume holdup (m3) 11342 898 525 2964 921 3610 1567
Belt area (m2) 56708 4491 2623 14818 4606 18052 7834
Gas velocity (ms) 053 062 054 051 036 036 026
Loading depth (m) 02 02 02 02 02 02 02
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Drying rate (kgs) 1338 196 115 649 202 790 343
Dryer length (m) 5940 1181 1382 2605 1214 3170 2066
Dryer width (m) 10 4 2 6 4 6 4
Mass holdup (kg) 4158215 330540 193465 1093968 339809 1331379 57846
Volume holdup (m3) 11881 944 553 3126 971 3804 1653
Belt area (m2) 59403 4722 2764 15628 4854 19020 8264
Gas velocity (ms) 050 059 051 049 034 034 024
Loading depth (m) 02 02 02 02 02 02 02
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 41 -
33 Cost Estimation
The cost of conveyor dryer was evaluated as part of our case study The overall
total cost (also known as lifetime costs) consists of the capital running and
maintenance costs The capital costs cover the costs associated with design
materials manufacturing (machinery labour and overhead) testing shipping
installation and depreciation The running costs consist of the costs associated with
the energy costs including heat and electricity warranty insurance maintenance
repair cleaning lost productiondowntime due to failure and decommissioning costs
It should be noted that it is very difficult to find accurate cost data for drying system
and the costs are variable depending on where the drying system is installed Some
researchers (Holmberg et al 2004 and 2005 Mujumdar 2006) have calculated the
drying costs using their own data sources
The following sections present the results obtained from cost calculations using the
methods provided by Holmberg et al (2004 2005) and from the lsquoHandbook of
Industrial Dryingrsquo (Mujumdar 2006)
331 Capital Costs
Capital costs are usually divided into direct and indirect costs which can be
expressed as
IDCDCC CostCostCost += (327)
where CCost is the total capital costs DCCost the direct capital costs IDCCost the
indirect capital costs Direct capital costs are calculated by multiplying purchased
equipment costs by a given factor (termed as ldquoLang factorrdquo (Brennan 1998)) Then
it is can be written as
eqDC CostGCost sdot= (328)
where G represents the Lang factor and eqCost the equipment costs The Lang
factor is a sum of several factors which are applied for the estimation of costs such as
instrumentation electrical erection structures and lagging It can be expressed as
sum=
+=m
j
jgG1
1 (329)
where g is the individual factor and m the number of the factors The value of
individual factor depends on the purchased equipment costs Some approximate
values for these factors are listed in (Brennan 1998) The Lang factor in this case
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 42 -
study is assumed to be 16 which is determined based on the following factors
electrical 01 instrumentation 01 lagging 005 civil work 015 and installation 02
(Brennan 1998) However it should be noted that the actual values of these factors
are heavily site dependent and can deviate considerably from those used in the current
case study In order to estimate the factors more precisely detailed formation about
the investment costs concerning the dryer projects should be known However such
information is not available easily
Purchased equipment costs are usually presented in chart form and are correlated
with a capacity factor using the relationship as follows
b
eq kYCost = (330)
where k is the proportionality factor Y the capacity parameter and b the exponent
Exponent b is typically within a range of 04 ndash 08 (Brennan 1998) In drying
systems the main pieces of equipment are conveyors heat exchanges (if required) air
ducts covering and fans Without considering the original capacity factor of each
piece of equipment (eg cross-sectional area in the case of conveyors) they are all
dependent on the dry mass flow of drying medium (ie hot air or flue gas) Hence
the flue gas mass flow is selected as the capacity factor Y for each piece of
equipment in equitation 330 in the current case study The cross-sectional area of
the air ducts and the covering of the dryer are also proportional to the air mass flow
If the length of the air ducts and the height of the dryer are known their costs can also
be calculated as a function of air mass flow Cost data for dryer equipment are
obtained from published data sources equipment seller quotations and constructors
(Holmberg and Ahtila 2004) Proportionality factor k and exponent b are
determined on the basis of the cost data
In this case study the purchased costs (in Euros) of the main equipment are
calculated using the relationships shown in Table 35 which are taken from Holmberg
and Ahtila (2004) The relationships for conveyor and fan are based on the cost
information from Finnish equipment seller quotations The relationships for air
ducts and covering are defined by assuming that they are black iron with the density
of 9500 kgm3 and the price of 35 eurokg The length and the wall thickness of the air
ducts are assumed to be 30 m and 35 mm respectively Since these relationships
were obtained in 2000 and 2002 a 5 annual increase in price has been added to the
original prices
Substituting equations 329 and 330 into equation 328 and using gas mass flow as
the capacity parameter the direct capital costs of the dryer can be expressed as
follows
)()1(11
sumsum==
sdot+=n
i
b
dgi
m
j
iDCimkgCost amp (331)
where n is the number of the pieces of equipment purchased
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 43 -
Table 35 Cost functions for main components of the dryer (Holmberg and Ahtila
2004)
Equipment Relationship Capacity parameter Y Year
Conveyor 2700Y Cross-sectional area 2000
Air duct 3770Y05 Air mass flow 2002
Fan 09∆pY07 Air mass flow 2002
Covering 1200Y05 Cross-sectional area 2002
Note ∆p is the pressure drop of drying stage
Indirect costs include engineering and project management as well as a
contingency allowance which can be considerable in pilot plans Indirect capital
costs are not dependent on the dimension of the dryer In order to simplify the
calculations they are usually added as a percentage of direct capital costs Here the
indirect costs are defined as 5 of direct costs
Assuming the life time of the dryer lf is 10 years and the interest rate ir is 5
The capital recovery factor can be calculated from the following equation
1)1(
)1(
minus+
+=
f
f
l
r
l
rr
i
iie = 013 (332)
The capital costs of conveyor dryer at different operating conditions are listed in
Table 36 It can be seen that due to the large size of dryer and high biomassgas
flow rate the dryer using lsquosinter gas 1rsquo as the drying source has a relatively higher
capital cost The annual capital costs for ldquosinter gas 1 dryerrdquo are about 18 times
higher than the costs for the smallest dryer using lsquoNH3 combustion gasrsquo Analysing
the data presented in Table 36 indicates that the conveyor costs are the dominate part
of the total capital costs The final moisture content of biomass does not affect the
capital costs significantly As shown in Table 36 the lower the final moisture
content of the biomass the higher the capital costs
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 44 -
Table 36 Costs analysis of conveyor dryer at different final moisture contents
Final moisture content 04 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 253978 19855 11535 65014 20252 79405 34370
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4579 1280 976 2317 1293 2560 1684
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 437922 40529 24835 119332 39823 142465 63763
Indirect costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Total capital costs (keuro) 459818 42555 26076 125298 41814 149589 66952
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 59546 5511 3377 16226 5415 19372 8670
Running costs
Electrical load (kW) 315618 10159 6582 65537 9860 94980 26809
Electricity costs (keuro) 291631 9387 6081 60556 9110 87762 24771
Maintenance costs (keuro) 21896 2026 1242 5967 1991 7123 3188
Other costs (keuro) 4379 405 248 1193 398 1425 638
Annual running costs (keuro) 317906 11818 7571 67716 11500 96310 28597
Total annual costs (keuro) 377455 17330 10948 83942 16915 115682 37268
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 45 -
Table 36 continued
Final moisture content 03 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 272591 21459 12502 70560 21953 86061 37302
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4744 1331 1016 2413 1346 2665 1755
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 467966 43177 26446 128360 42629 153283 68567
Indirect costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Total capital costs (keuro) 491364 45336 27768 134778 44761 160947 71995
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 63632 5871 3596 17454 5797 20843 9323
Running costs
Electrical load (kW) 312616 10128 6593 65828 9880 95164 26931
Electricity costs (keuro) 288857 9359 6092 60825 9129 87932 24884
Maintenance costs (keuro) 23398 2159 1322 6418 2131 7664 3428
Other costs (keuro) 4680 432 264 1284 426 1533 686
Annual running costs (keuro) 316935 11949 7679 68527 11687 97129 28998
Total annual costs (keuro) 380569 17820 11275 85981 17484 117972 38322
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 46 -
Table 36 continued
Final moisture content 02 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 288723 22863 13352 75440 23449 91906 39888
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4882 1374 1050 2496 1391 2754 1815
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 493999 45491 27861 136298 45096 162777 72801
Indirect costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Total capital costs (keuro) 518699 47765 29254 143113 47351 170916 76441
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 67171 6186 3788 18533 6132 22134 9899
Running costs
Electrical load (kW) 307560 10029 6554 65541 9823 94527 26819
Electricity costs (keuro) 284185 9267 6056 60560 9076 87343 24781
Maintenance costs (keuro) 24700 2275 1393 6815 2255 8139 3640
Other costs (keuro) 4940 455 279 1363 451 1628 728
Annual running costs (keuro) 313825 11996 7728 68738 11782 97110 29149
Total annual costs (keuro) 380999 18182 11516 87272 17914 119244 39049
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 47 -
Table 36 continued
Final moisture content 01 kgkgdm
Heating source Sinter gas 1 Sinter gas 2 NH3 combustion gas BF and coke oven gas Flare BF gas BOS gas 1 BOS gas 2
Capital costs
Conveyor costs (keuro) 302442 24040 14070 79567 24713 96838 42075
Air duct costs (keuro) 11520 3509 2568 1380 2835 5717 3196
Fan costs (keuro) 3624 686 443 1403 509 1359 602
Covering costs (keuro) 4997 1409 1078 2563 1428 2827 1864
Lang factor 160 160 160 160 160 160 160
Direct costs (keuro) 516133 47430 29053 143009 47177 170785 76378
Indirect costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Total capital costs (keuro) 541940 49802 30506 150160 49536 179324 80197
Annual recovery factor 013 013 013 013 013 013 013
Annual capital costs (keuro) 70181 6449 3951 19446 6415 23222 10386
Running costs
Electrical load (kW) 300924 9865 6467 64722 9690 93134 26481
Electricity costs (keuro) 278054 9116 5975 59803 8954 86055 24469
Maintenance costs (keuro) 25807 2372 1453 7150 2359 8539 3819
Other costs (keuro) 5161 474 291 1430 472 1708 764
Annual running costs (keuro) 309022 11962 7718 68383 11784 96302 29051
Total annual costs (keuro) 379206 18411 11669 87830 18199 119526 39437
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 48 -
332 Running Costs
During the drying process the running costs cover all those costs associated with
the operation of the dryer The most important running costs include the use of heat
and electricity and maintenance costs The former is dependent on the annual
running time of the dryer and the price of energy while the latter is usually estimated
as a percentage of direct capital costs Typically its value is in the range of 2 to
11 averaging around 5 to 6 (Brennan 1998) Personnel costs and insurance
costs are also considered in the running costs However since they are heavily site
dependent it is difficult to define them accurately If the running time of the dryer is
τ hyear the annual running costs become
xmehRUN CostCostbEbCost +++Φ= ττ (333)
where Φ is the heat consumption (W) E the electricity consumption (W) bh the price
of heat be the price of electricity Costm the maintenance costs and Costx all other
running costs Here Costm and Costx are assumed to be 5 and 1 of the direct
capital costs respectively And the annual running time of the dryer is assumed to
be 8400 hyear Since the heating source is the waste heat from steel industry the
price of heat is defined as zero The main consumers of electricity are fan and belt
driver and their electricity consumptions can be calculated as follows (Mujumdar
2006)
dg
dg
f mp
E ampηρ
∆= (334)
finb muLeE amp)1(1 += (335)
bf EEE += (336)
where Ef is the required electrical power to operate the fan Eb the required electrical
power to move the belt ∆p the pressure drop of the dryer η the mechanical efficient
of the fan which is 70 in this case study and e1 the constant defined as 2
Once the capital costs the capital recovery factor and the annual running costs are
known the total annual costs (TAC) can be calculated as follows
RUNC CosteCost +=TAC (337)
The running costs and the total annual costs of conveyor dryers at different final
moisture contents are also listed in Table 36 The dominant contributor to the
running costs is the energy costs 80 ndash 90 of running costs are spent for electricity
consumption And the annual running costs are found to be about 2 ndash 5 times of the
annual capital costs when the lifetime of dryer is 10 years and interest rate is 5
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 49 -
The total annual costs for each dryer at different final moisture contents are presented
in Figure 313 The total annual costs of the lsquosinter gas 1rsquo dryer can go up to 3700
keuro while it is only about 110 keuro for the lsquoNH3 combustion gasrsquo dryer Even though
the lsquosinter gas 1rsquo dryer can provide the highest drying capacity among the dryers
investigated here its total annual costs are significantly higher than others hence
making its profitability questionable The profitability of each dryer will be
discussed in the next section Figure 313 also shows that the total annual costs at
different final moisture contents of biomass are similar indicating that the final
moisture content has very limited influence on the capital and running costs
Figure 313 Total annual costs of dryers at different final moisture contents
333 Profitability
Drying biomass increases the net heating value of the biomass material and
improves the performance of the boiler As a result the biomass consumption for a
given energy output is reduced and the boiler efficiency is increased In addition
recovering the waste heat is also beneficial to the steel industry
Based on the cumulative cash flow the profitability can be evaluated in terms of
time cash and percentage return on the investment Payback period is generally the
main concern for investors Sometimes it is taken as the time from commencement
of the project to recovery of the initial capital investment Normally it is taken as
the time from the start of production to recover the fixed capital expenditure only
However the payback period analysis method has serious limitations as it does not
consider the time value of money risk financing and other important issues Thus
it should not be used in isolation for investment decisions
Alternatively in this case study the earnings of the drying are calculated using net
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 50 -
present value (NPV) which takes into account both incoming and outgoing cash
flows during the economic lifetime of the dryer It is an indicator of how much
value an investment can add The capital and running costs are considered as
negative cash flows the NPV for the dryer is
C
k
tt
r
RUN Costi
Costminus
+
minussdot=sum
=0 )1(
)NI(NPV
τ (338)
where NI is the net income of drying in a time unit ir the interest rate t the individual
year and k the total number of years If NPV gt 0 it means that the investment would
add values to the investors and the project is profitable Otherwise the investment
would subtract the values from the investors and the project is not deserved to be
invested economically
The NI in this case study can be defined as hourly price in saved fuel When the
water content in biomass is reduced the net heating value of the biomass will be
increased and the energy required to evaporate the water in the fuel will be reduced
As a result marginal fuel can be replaced with biomass The saved marginal fuel
consumption can be considered as a positive cash flow which can be written as
( ) fuelwoutinf biuum minus= ampNI (339)
where iw is the latent heat of water (MJkg) bfuel the marginal fuel price (euroMWh)
The marginal fuel price is dependent on the type of fuel time and other factors
Here peats are assumed as the marginal fuel and its price is set as 14 euroMWh which
includes cost of emission trade
Figure 314 shows the net present values as a function of dryer operation duration at
different final moisture contents It can be seen that the NPVs for most of dryers
turn to be positive within two years except the lsquosinter gas 1rsquo dryer which needs at
least ten years to return the costs This suggests that the lsquosinter gas 1rsquo dry is not
economically applicable on account of its large investment costs and slow return of
money However for the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer they
become profitable after 12 yearsrsquo operation and the increase rates of earnings are
much higher than other dryers In addition both of them have relatively large drying
capacities which could process 6 ndash 7 kgs dry biomass Therefore they are more
attractive to be used The change of final moisture content from 04 kgkgdm to 01
kgkgdm has little effect on the NPV as shown in Figure 314a and d
The NPVs of each dryer at 10 years lifetime are shown in Figure 315 As shown
the lsquoBOS gas 1rsquo dryer and lsquoBF and coke oven gasrsquo dryer have much more profits than
other dryers The former earns more than 8000 keuro and the latter earns more than
7500 keuro after 10 yearsrsquo operation However the lsquosinter gas 1rsquo dryer in spite of its
large drying capacity produces very limited profits in its lifetime Therefore it is
believed that among these waste gas streams released from steel industry the gas
streams with relatively high temperature and large quantity (eg BOS gas 1 and BF
and coke oven gas) can not only dry a large amount of biomass but also bring
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 51 -
considerable profits to the investors Using the flue gas streams such as BOS gas 2
flare BF gas and sinter gas 2 in biomass drying can also generate the profits over
2900 keuro in the dryerrsquos 10 years lifetime
(a) Final moisture content 04 kgkgdm
(b) Final moisture content 03 kgkgdm
For caption see overleaf
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 52 -
(c) Final moisture content 02 kgkgdm
(d) Final moisture content 01 kgkgdm
Figure 314 Net present value as a function of dryer operation duration
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 53 -
Figure 315 Net present value as a function of final moisture content at economic
lifetime of 10 years
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 54 -
4 Conclusions
The main conclusions from this case study are as follows
1 It is feasible to use the low grade heat from steel industry to dry biomass
material
2 The energy (enthalpy) that the gas stream contains determines its performance in
the drying process Since the lsquosinter gas 1rsquo from main stack has the largest quantity
the dryer using this flue gas stream as the heating source produces the highest biomass
throughput and water evaporation rate The dried biomass can be used as the fuel in
a power plant with a capacity of up to 100 MW The gas streams in order of the
drying capacity from high to low are as follows lsquosinter gas 1rsquo lsquoBOS gas 1rsquo lsquoBF and
coke oven gasrsquo lsquoBOS gas 2rsquo lsquoflare BF gasrsquo lsquosinter gas 2rsquo and lsquoNH3 combustion gasrsquo
3 The thermal design of a single passsingle stage conveyor dryer shows that the
throughput of biomass and the drying rate determine the size of the dryer The
higher the throughput of the biomass the larger the size of the dryer will be
4 The estimated capital costs for dryers range from 3377 keuro to 70181 keuro and the
annual running costs from 7571 keuro to 317906 keuro The NPVs of dryers at 10 years
lifetime indicate that most of dryers turn to be profitable within two years except the
lsquosinter gas 1rsquo dryer which needs at least ten years to return the costs Therefore the
lsquosinter gas 1rsquo dry is not economically applicable on account of its large investment
and slow return of money The main benefits of using the lsquoBOS gas 1rsquo dryer and
lsquoBF and coke oven gasrsquo dryer are i) their relatively large drying capacities ii) their
short payback period and iii) high profits resulted from their use
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 55 -
References
Amos WA Report on biomass drying technology National Renewable Energy
Laboratory Golden Colorado Report NRELTP-570-25885 1998
Beeby C Potter OE Steam drying In Drying rsquo85 Selection of papers from 4th
International Drying Symposium Kyoto Japan Toei R Mujumdar AS editors
Hemisphere Washington 1985 p 41-58
Berghel J Nilsson L Renstroumlm R Particle mixing and residence time when drying
sawdust in a continuous spouted bed Chemical Engineering and Processing 2008 47
p 1246-1251
BERR Heat call for evidence Department for Business Enterprise amp Regulatory
Reform London 2008
Brennan D Process industry economics United Kingdom Institution of Chemical
Engineers 1998
Bruce DM Sinclair MS Thermal drying of wet fuels opportunities and technology A
report prepared by H A SIMONS Ltd 1996 Available from
httpmydocsepricomdocspublicTR-107109pdf
Deventer van HC Industrial superheated steam drying TNO-report R2004239 2004
Eleoteacuterio JR Cloacutevis RH Nestor PG A program to estimate the equilibrium moisture
content of wood Ciecircnicia Florestal 1998 8 1 p 13-22
Fagernas L Brammer J Wilen C Lauer M Verhoeff F Drying of biomass for second
generation synfuel production Biomass and Bioenergy 2010 34 p 1267-1277
Fredirkson RW Utilisation of wood waste as fuel for rotary and flash tube wood dryer
operation In Biomass Fuel Drying Conference Proceedings 1984 University of
Minnesota p 1-16
Gustafsson G Forced air drying of chips and chunk wood In Production storage and
utilization of wood fuels Proceedings of IEABE Conference Task III 6-7 December
Uppsala Sweden vol II Swedish University of Agricultural Sciences Garpenberg
Sweden 1988 p 150-162
Haapanen AP Heikkila L Ijas M Valkamo P Enso uses flash-dried pulverized bark
to replace coal as boiler fuel Pulp and Paper 1983 p 70-77
Hailwood AJ and Horriobin S Absorption of water by polymers analysis in terms of
a simple model Transactions of the Faraday Society 1946 42B p 84-102
Holmberg H Ahtila P Comparison of drying costs in biofuel drying between
multi-stage and single-stage drying Biomass and Bioenergy 2004 26 p 515-530
Intercontinental Engineering Ltd Study of hog fuel drying systems Canadian
Electrical Association Prepared by Intercontinental Engineering Ltd 1980
Jensen A Industrial experience in pressurised steam drying of beet pulp sewage
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 56 -
sludge and wood chips Drying technology 1995 13 p 1377-1393
Keey RB Drying Principles and Practice Pergamon Press Oxford 1972
Keey RB Introduction of industrial drying operations Pergamon Press New York
Chapter 2 1978
Kofman PD Spinelli R Storage and handling of willow from short rotation coppice
Elsamprojekt Fredericia Denmark 1997
Krokida M Marinos-Kouris D Mujumdar AS Rotary drying In AS Mujumdar
editor Handbook of Industrial Drying Chapter 7 CRC press 3rd edition 2006
Lampinen MJ Chemical Thermodynamics in Energy Engineering Publications of
laboratory of applied thermodynamics Finland Helsinki University of Technology
1997 [in Finish]
Law CL Mujumdar AS Fluidized bed dryers In AS Mujumdar editor Handbook of
Industrial Drying Chapter 8 CRC press 3rd edition 2006
Liptaacutek B Optimizing dryer performance through better control Chemical
Engineering 1998 105 2 p 110-114
Loo SV Koppejan J Handbook of biomass combustion amp co-firing Earthscan UK
2008
MacCallum C Blackwell BR Torsein L Cost benefit analysis of systems using flue
gas or steam for drying of wood waste feedstocks Final Report DSS Contact
42SSKL229-0-4002 Work performed by Sandwell amp Company Ltd Vancouver
British Columbia Canada 1981
Maroulis ZB Saravacos GD Mujumdar AS Spreadsheet-aided dryer design In AS
Mujumdar editor Handbook of Industrial Drying Chapter 5 CRC press 3rd edition
2006
McKenna RC Industrial energy efficiency Interdisciplinary perspectives on the
thermodynamic technical and economic constraints PhD thesis Department of
Mechanical Engineering University of Bath 2009
Mujumdar AS Principles classification and selection of dryers In AS Mujumdar
editor Handbook of Industrial Drying Chapter 1 CRC press 3rd edition 2006
Nellist ME Storage and drying of short rotation coppice ETSU BW200391REP
ETSU Harwell UK 1997
Poirier D Conveyor dryers In AS Mujumdar editor Handbook of Industrial Drying
Chapter 17 CRC press 3rd edition 2006
Roos CJ Biomass drying and dewatering for clean heat amp power WSU Extension
Energy Program WSUEEP08-015 Northwest CHP Application Centre 2008
Swiss Combi Swiss Combi belt dryer Available from
httpwwwswisscombichfilesdownloadsenSWISS_COMBI_Belt_Dryerpdf
University of Newcastle National sources of low grade heat available from the
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006
- 57 -
process industry EPSRC Thermal Management of Industrial Processes UK 2011
Vidlund A Sustainable production of bioenergy product in the sawmill industry
Licentiate thesis Stockholm Sweden Dept of Chemical Engineering and
TechnologyEnergy Processes KTH Royal Institute of Technology 2004 57
Wardrop Engineering Inc Development of direct contact superheated steam drying
process for biomass Report of Contact File No 34SZ23283-7-6040 Bioenergy
Development Program Energy Mines and Resources Canada Work performed by
Wardrop Engineering Inc Winnipeg Manitoba Canada 1990
Wimmerstedt R Drying of peat and biofuels In AS Mujumdar editor Handbook of
Industrial Drying Chapter 32 CRC press 3rd edition 2006