February 2000 ECN-C--00-028

THERMOCHEMICAL CONVERSION OF WILLOWFROM SHORT ROTATION FORESTRY

Ludger Dinkelbach

Revisions

A Draft 16 February 2000

B Final 14 June 2000

Made by:

L. Dinkelbach

Approved by:

J. Prij

ECN Biomass

Checked by:

J. Beesteheerde

Authorised by:

H.J. Veringa

2 ECN-C--00-028

AbstractThis report is one of three technical reports that are to form the basis of a written guide toenergy from willow – the REGROW Guide. This report describes the thermochemicalconversion technologies that are most relevant for energy production from willow biomass. Theother two technical reports cover the production of willow and its pre-treatment prior to energyconversion and are written by the Swedish University of Agricultural Science (SLU) and theScottish Agricultural College (SAC), respectively.A broad variety of conversion technologies are available. Combustion technology is alreadyapplied commercially, whereas gasification and pyrolysis are still in a demonstration stage. Ifonly heat is generated, combustion appears most suitable. If electricity is to be produced at highefficiency, gasification and pyrolysis are promising options. The latter (with flash pyrolysis asan exception) is applied for contaminated streams rather than clean biomass such as willowfrom short rotation forestry.The respective suitability of the different available technologies depends on the type of fuel tobe used (e.g. moisture content, ash content, particle size), the capacity of the installation (small-scale or large-scale application), and the envisaged product (heat, electricity, CHP).

KeywordsWillow, fuel characteristics, combustion, gasification, pyrolysis

ECN-C--00-028 3

COLOPHON

This report is written as part of the project “A Dynamic Information Structure to Improve Co-ordination on Willow Biomass for Energy” (CONTRACT No XVII/4.1030/99-335). Thisproject is carried out by the Scottish Agricultural College (SAC), the Swedish University ofAgricultural Science (SLU), and the Netherlands Energy Research Foundation (ECN) in theframework of the ALTENER-programme of the Commission of the European Communities.

The report is written byNetherlands Energy Research Foundation (ECN)P.O. Box 1NL-1755 ZG PETTENThe NetherlandsPhone: +31 224 56 4663Fax: +31 224 56 3487E-mail: biomass@ecn.nlContact person: Ludger Dinkelbach

Management and co-ordination of the ALTENER project XVII/4.1030/99-335 is carried out byScottish Agricultural College (SAC)West Mains RoadUK-Edinburgh EH9 3JGUnited KingdomPhone: +44 131 535 3030Fax: +44 131 535 3031Email: a.hunter@ed.sac.ac.ukContact person: Alastair Hunter

ECN’s contribution to the ALTENER-project is also supported within the framework of theDutch EWAB programme (Energy production from waste and biomass). The EWAB projectnumber is 355198/5060. Co-ordination of the EWAB programme is the responsibility of

The Netherlands Agency for Energy and Environment (Novem)Catharijnesingel 59P.O. Box 8242NL-3503 RE UTRECHTThe NetherlandsPhone: +31 30 239 34 58Fax: +31 30 231 64 91Email: a.de.zeeuw@novem.nlContact person: A.A. de Zeeuw

No rights can be derived from this report. Use and publication from this report is admitted,provided proper reference is made.

4 ECN-C--00-028

ECN-C--00-028 5

CONTENTS

CONTENTS 5

1. INTRODUCTION 7

2. WILLOW AS AN ENERGY SOURCE 92.1 Relevant characteristics of solid biofuels 92.2 Data on characteristics of willow - the Phyllis database 102.3 Measurement of biomass characteristics 10

3. THERMOCHEMICAL CONVERSION OF (WILLOW) BIOMASS 113.1 Combustion 113.1.1 Classification of combustion technologies 123.2 Gasification 153.2.1 Classification of gasification technologies 153.2.2 Most relevant types of gasifiers 153.2.3 Definition of most relevant gasification systems for CHP production 163.3 Pyrolysis 18

4. LITERATURE 21

6 ECN-C--00-028

ECN-C--00-028 7

1. INTRODUCTION

This report is ECN’s contribution to the project “A Dynamic Information Structure toImprove Co-ordination on Willow Biomass for Energy” (CONTRACT No XVII/4.1030/99-335). This project is carried out by the Scottish Agricultural College (SAC) together with theSwedish University of Agricultural Science (SLU), and the Netherlands Energy ResearchFoundation (ECN) in the framework of the ALTENER-programme of the Commission of theEuropean Communities.

The objective of this ALTENER-project is to explain the entire process of producing energyfrom willow biomass in a way that aids general understanding. The most important deliverableof the project is a guide to sustainable energy from willow – the REGROW Guide - written in aclear, readable and informative style.

The REGROW Guide will be worked out by the Scottish Agricultural College (SAC) based onthree separate technical reviews written by the three project partners, see Figure 1-1:1. Willow biomass production by SLU [1],2. Biomass supply logistics by SAC [2], and3. Thermochemical conversion of willow by ECN (this report).

Figure 1-1 Contributions to the REGROW Guide, position of this report

This report consists of• A description of the characteristics of willow as fuel (chapter 2),• An overview of relevant conversion technologies for energy from willow (chapter 3), and• A list of references (chapter 4).

1. Willow Production

(SLU)

2. Logistics/Pretreatment

(SAC)

3. Energy Conversion

(ECN)

REGROW Guide

this report

8 ECN-C--00-028

ECN-C--00-028 9

2. WILLOW AS AN ENERGY SOURCE

2.1 Relevant characteristics of solid biofuelsWhenever biomass is used as an energy source, information on the characteristics of thematerial is required. In Table 2-1 those characteristics of solid biofuels are summarised thatappear most relevant in the context of energy conversion. The list is taken from [3]. Note thatthe order of the properties does not necessarily correspond to their importance.

Table 2-1 List of properties and characteristics of biomass [3]Property Symbol Unit Commentmoisture content mc %dimension(s) mm sawdust, shavings, chips, lumps, logs, trunksshape cubes, pellets, sticks, fibrescalorific value LHV kJ/kgtype (species)ash content a %ash composition % oxides i.e. Fe2O3, CaO, SiO2, Al2O3 etc.

raw bulk density kg/m3

true specific gravity kg/m3

angle of repose °“flowability” important for handling and storage (bridging)

volatile matter vm % in particular light weight components that tend to escapewhile drying reported separately

strength, durabilityfor coal standards for the “grindability” have beendeveloped (i.e. Hardgrove) also “size stability”,“friability” and “test for dustiness” may be useful

fermentation degradation of large stacks of (moist) wood reduceheating values and increase danger of fire

dust explosions ignition energy for dust generated from wood in handlingand drying equipment

sulphur S %nitrogen N %chlorine Cl %fluorine F %composition:- carbon- hydrogen- oxygen

CHO

%%%

major dilutions mg/kg Al, Si, K, Na, Ca, Mg, Fe, P, Ti...minor dilutions mg/kg As, Ba, Cd, Be...

heavy metals mg/kgSb, Pb, Cr, Cu, Mn, V, Sn, As, Ni, Se, Te the sum ofthese is specifically limited in Dutch emissionregulations

volatile heavy metals mg/kg Cd and Hg reported separatelyenergy density kJ/m3

ash fusion behaviour t °C initial deformation, softening, hemispherical and fluidtemperature for oxidising and reducing conditions

ash fouling potentialfixed carbon cf %C/N ratio c/n -

10 ECN-C--00-028

2.2 Data on characteristics of willow - the Phyllis databaseIn 1998, ECN has developed the database “Phyllis” on data of the characteristics of biofuels [5].With financial support from Novem, Phyllis was made available for third parties. Since May1999, the database is accessible through Internet: http://www.ecn.nl/Phyllis. Table 2-2 shows anexample of the composition of willow as it can be found in the Phyllis database.

Table 2-2 Example of a set of data on the composition of willow [source: Phyllis database]proximate analysis [wt-% as received] ultimate analysis [wt-% on dry basis]

moisture 17 C 47,3volatiles 67,1 H 5,66fixed carbon 13,6 N 1,4ash 2,3 O 42,8

heating value [kJ/kg as received] S 0,05LHV a.r. (as received)1 15.400 Cl 0,018

elementary analysis [mg/kg on dry basis]Al 70 Mg 600As 1,3 Mn 11B 12 Na 190Ca 7.700 Ni 25Co 0,6 P 860Cr 8,2 Si 220Cu 7,6 Ti 2,8Fe 81 V 0,2K 2.700 Zn 130

2.3 Measurement of biomass characteristicsLittle (international) standard procedures are available to determine the characteristics of solidbiofuels as given in Table 2-1. Application of different methods e.g. to determine the moisturecontent lead to different results. It is acknowledged that European standards are needed on thatissue [3].

In [4] an overview is given on relevant literature related to the standardisation and classificationsolid biofuels. In March 2000, the Commission of the European Communities has given themandate to the European standardisation body CEN to set up a Technical Committee (TC) withthe objective to work out European standards on sampling, testing and classification of cleansolid biomass.

ECN-C--00-028 11

3. THERMOCHEMICAL CONVERSION OF (WILLOW) BIOMASS

When biomass is used for energetic purposes, chemically “stored” energy is converted into heatby means of combustion. The best known application of biofuels is direct combustion of woodin open fireplaces. Of course, modern biomass conversion technology is not that simple.

A broad variety of biomass-to-energy processes are available. Commonly, a distinction is madebetween thermochemical, biochemical, and physical processes. For woody biofuels such aswillow from short rotation forestry, thermochemical processes are most suitable. Depending onthe amount of oxygen that is supplied to the process (indicated by the stoichiometric coefficientvalue lambda), the following distinction is made:• (Direct) combustion. Supply of a sufficient amount of oxygen to achieve complete

combustion (lambda ≥ 1). The main product is a hot flue gas mainly consisting of thecombustion products carbon dioxide (CO2) and water (H2O) as well as nitrogen (N2).Combustion technology is described in chapter 3.1.

• Gasification. A certain amount of oxygen is supplied that is not sufficient to achievecomplete combustion (0 < lambda < 1). The main product is a gas containing thecombustible components carbon monoxide (CO), hydrogen (H2), and methane (CH4). Thisso-called producer gas can be burnt in boilers, engines or gas turbines. Gasificationtechnology is described in chapter 3.2.

• Pyrolysis. No oxygen is supplied (lambda = 0). Heating of the biomass results in solid(pyrolysis char), liquid (pyrolysis oil) and gaseous (pyrolysis gas) products. Pyrolysistechnology is described in chapter 3.3.

All these processes are based on1. thermal decomposition of the primary fuel and then2. combustion of the products of decomposition.Though, in the case of (direct) combustion, steps 1. and 2. take place in the same reactorwhereas they are spatially separated in the cases of gasification and pyrolysis. The mostimportant advantage of this spatial separation is the possibility to apply “prime movers”(internal combustion engines, gas turbines) that require liquid or gaseous fuels leading to higherelectrical efficiencies.

In practice, the different processes are not always distinguished that sharply: It might appearthat the term pyrolysis is used despite the supply of a small amount of oxygen; in many cases,combinations of the processes are applied; sometimes, a name for a certain process might evenbe chosen for political rather than technical reasons. In the following, the different processes aredescribed in more detail.

3.1 CombustionCombustion is the thermal conversion of solid, liquid or gaseous fuels to CO2 and H2O with(excess) air. The generated heat is used for heating, steam production or electricity productionvia steam engines or steam turbines. Combustion is more common than gasification orpyrolysis, and there is much more experience on combustion so that this technology can beconsidered proven. The number of manufacturers offering commercial combustion technologyis much higher compared to gasification and pyrolysis technology.

The flue gases from biomass combustion contain contaminants. Nitrogen, sulphur and chlorinecomponents in the biomass can lead to the production of NOx, SO2 and HCl, respectively, in theflue gas. In addition to “fuel NOx”, “thermal NOx” is produced from N2 and O2 at high

12 ECN-C--00-028

combustion temperatures (especially > 1400 °C). Incomplete combustion leads to (enhancedconcentrations of) CO and hydrocarbons. Besides, flue gases contain dust and, under specificconditions, dioxins and or heavy metals. There are different principles to reduce emissions:• Primary measures (choice of fuels leading to low emissions),• Secondary measures (measures during combustion, e.g. staged supply of combustion air),• Tertiary measures (“end of pipe” cleaning measures like filtration of flue gases).

3.1.1 Classification of combustion technologiesThere are a broad variety of combustion technologies, the majority of which is commerciallyavailable. The technology that is chosen for a specific application depends on the size of theinstallation, the characteristics of the fuel (moisture content, particle size) and the envisagedapplication (heat, steam and/or electricity). Capacities of combustion plants vary from verysmall (15 kW thermal capacity) to very large (electricity production based on coal combustion:several hundreds of MW). In the case of biomass combustion, the upper scale limit isdetermined by local energy demand and availability of biomass rather than restrictions related tocombustion technology. Overview on biomass combustion technologies are given in [6] - [12].

Wood stovesWood stoves are used for domestic heating. Both manually and automatically fed wood stovesare available. Manually fed wood stoves mostly use wood briquettes or blocks, whereasautomatically fed stoves mostly use chips or pellets. The smallest commercially available woodstoves have a thermal capacity of ca. 15 kW. In this case, the maximum consumption of wood isca. 3,5 kg per hour.In many small-scale combustion units, combustion air is supplied at different locations(primary, secondary, sometimes tertiary air supply) to ensure complete combustion and toreduce flue gas emissions.Investment costs mainly depend on the capacity and on the degree of automation. In the case ofthe Austrian manufacturer Fröling, prices vary between 120 EURO/kW and 280 EURO/kW forcapacities of 50 kW and 15 kW, respectively. These figures do not include the connection of thewood stoves to the central heating system. A new development is the so-called micro-CHPmaking use of (part of) the heat to produce electricity via a Stirling engine.When applying small-scale wood stoves, flue gas emission can cause problems. There are noEuropean laws on emissions, but the current tendency is to sharpen emission limits in severalEuropean countries.Automatically operated wood stoves are mostly designed as screw stokers. Biomass is(automatically) fed to the combustion chamber by means of a transport screw. A broad varietyof screw stokers is commercially available up to several MW thermal capacity. An example of ascrew stoker is given in Figure 3-1.

Entrained combustionThe fuel is pneumatically fed to the combustion chamber. Part of the transport air is used asoxidising agent. This principle requires small fuel particles (max. 5 x 5 x 5 mm). Because of thehigh amount of excess air, a high volume of flue gas is produced causing a relatively lowefficiency.

Grate burnersGrate burners are commercially available in different designs based on fixed grates, movinggrates or vibrating grates. Grate burners are suitable for a broad range of fuels and are also usedfor the combustion of domestic waste.In fixed grate burners, fuel is fed from the top of a sloping grate and moves downwards due togravimetrical forces. At the bottom, the fuel is completely combusted, and the ash is discharged.The last part of the grate is horizontal. Primary air is supplied below the grate, secondary andtertiary air is supplied from above the grate. Disadvantages of fixed grates are the danger of fuel

ECN-C--00-028 13

avalanches, an uneven distribution of fuel over the cross-section of the grate, and difficulty insteering the process conditions.A modification of a fixed grate is a moving grate. In this case, the grate is a conveyor belt or aslowly moving platform leading to the advantage of an even distribution of fuel.In vibrating grate burners, movement of the (biomass) fuel is caused by periodical vibration ofthe grate leading to the advantage of an even distribution of the fuel. In some installations,water-cooled grates are used.

1. combustion chamber 3. heat exchanger 4. fire hearth5. primary air inlet 6. secondary air inlet 8. blower9. automatic ignition 10.ash removal 11.insulation13.reburn valve 14.sprinkler connection 15. transport screw16.bunker 17.switch box 19.security plug

Figure 3-1 Example of an underfeed screw stoker (type Fröling Lambdamatic)

Pile burnersPile burners consist of a refractory lined combustion chamber with a grate on the bottom. On thegrate, biomass is piled up. Mostly, pile burners consist of several identical cells. Primary air issupplied from the bottom through the grate. A disadvantage of this technology is that there islittle possibility to control the process conditions. Locally, high temperatures can occur leadingto high emissions of NOx. In order to discharge the ash, a cell must be shut down and cleanedmanually after cooling. Pile burners are simple in design and suitable for a broad range ofbiomass fuels. Though, they do not represent state-of-the-art technology because of a poorprocess control. A new technology is being developed combining the concepts of both pileburners and grate burners to the so-called “whole tree burning” concept.

Fluidised bed combustionIn fluidised bed combustors, the biomass particles (e.g. chips) are fluidised together with inertbed material (sand, limestone) by supplying air at a certain velocity. In bubbling fluidised beds,the gas velocity is relatively low (1-3 m/s) so that the bed material is fluidised but nottransported by the gas stream. The bed shows similarities with a boiling fluid. Heat isdischarged from the bed by means of a heat exchanger. In a circulating fluidised bed, the gasvelocity is higher (3-10 m/s) so that part of the bed material leaves the reactor vessel. Via acyclone and a return pipe, these solids are transported back to the reactor vessel. In both types of

14 ECN-C--00-028

fluidised bed, the combustion temperature is kept relatively low (ca. 850 °C) and can easily becontrolled so that NOx emissions can be kept low. By means of additives (e.g. limestone),sulphur and/or chlorine can be bound to reduce emissions of SO2 and HCl, respectively.

In Table 3-1 the advantages and disadvantages of grate combustion and fluidised bedcombustion are summarised [9]. In Table 3-2 characteristics of most relevant combustionsystems are summarised.

Table 3-1 Advantages and disadvantages of grate combustion and fluidised bed combustion [9]advantages disadvantages

grate burnerslow investment costs for installations < 10 MWlow operational costslow dust emissionsgood carbon conversion efficiencypartload operation possibleless slagging/fouling compared to fluidised beds

NOx reduction measures requiredhigh excess air leading to a low efficiencyinhomogeneous combustion conditions

bubbling fluidised bed combustionlow investment costs for installations > 10 MWno moving parts in combustion chamberNOx reduction by staged air supplyhigh flexibility towards fuelslow excess air leading to a high efficiency

higher operational costsmore dust in flue gaspartload operation more difficultslagging/fouling can lead to troubledanger of wearing on heat exchangers

circulating fluidised bed combustionno moving parts in combustion chamberNOx reduction by staged air supplyhigh flexibility towards fuelshomogeneous combustion conditionshigh specific heat transferin-bed removal of sulphur possible

high investment costs (for installations > 30 MW)high operational costshigh amount of dust in flue gaspartload operation is difficultloss of bed materialslagging/fouling can cause troubledanger of wearing on heat exchangers

Table 3-2 Types, capacities and fuel specifications of most relevant combustion technologiescapacity (input) type of fuel particle size

wood stoveblocks/briquettes

chips/pellets15 kW – 150 kW15 kW - 1,2 MW

wood blocks/briquetteswood chips/pellets

> 20 cm; < 50 cm1 – 5 cm

entrained combustion 80 kW – 1,2 MW5 MW – 10 MW

(saw)dust, 0,5 x 0,5 x 0,5 cm< 0,5 cm

grate combustionfixed grate

moving gratevibrating grate

400 kW - 30 MW biomass, waste< L: 30 x D:20 cm< 30 x 10 x 5 cm

BFBatmosphericpressurised

4 MW- 200 MW5 MW – 15 MW

< 5 cm< 1 cm

CFBatmosphericpressurised

4 MW – 250 MW15 MW – 100 MW

< 5 cm< 1 cm

pile burner 4,4 MW - 110 MW wood, waste 3 - 8 cm

ECN-C--00-028 15

3.2 GasificationGasification is the thermal conversion of solid (in exceptional cases liquid) organic material to acombustible gas. However, the term gasification is often used in a more general sense for theentire process not only consisting of the production of the fuel gas but also its application togenerate heat and/or electricity as well as additional steps like cooling or cleaning of the gas.

The most important reasons why to convert solid fuels to a combustible gas is the possibility touse gases in prime movers (gas engines, gas turbines, fuel cells) to generate electricity at a highefficiency and the fact that only a small volume of (producer) gas must be cleaned instead of ahigh amount of (flue) gas. If only heat is to be generated or electricity is to be generated bymeans of a conventional steam process, combustion (paragraph 3.1) appears more suitable inmost cases. Overviews on biomass gasification technology are given in [13] - [16].

3.2.1 Classification of gasification technologiesThere are different gasification technologies of which the suitability for the respectiveapplication mainly depends on the type of biomass, the envisaged capacity of the gasifier, andthe application of the produced fuel gas. The most important characteristics of the differentgasification processes are the type of (fuel bed in the) reactor, the way the necessary heat issupplied, and the pressure in gasifier.

The type of the fuel bed in the reactor: fixed bed vs. fluidised bed gasificationIn fixed bed reactors, biomass is normally fed from the top and forms a fixed bed of lossmaterial that slowly moves downwards as a consequence of decomposition of the lower layersof biomass. In fluidised beds, biomass particles are fluidised by a gas stream. In entrained flowreactors, pulverised biomass is pneumatically transported. Because of the high heat and masstransfer, there is - in opposite to fixed bed reactors - no technical scale-up limit.

The way of heat supply: direct vs. indirect gasificationThe heat that is required for the endothermic gasification reactions is mostly supplied bycombustion of part of the biomass that is to be gasified (autothermal or direct gasification). Inthese cases, air is mostly used as gasifying agent, sometimes pure or enriched air. If heat issupplied from an external heat source (allothermal or indirect gasification), steam is used asgasifying agent. In principle, other oxygen containing gases (e.g. carbon dioxide) could also beused as gasifying agent.

The pressure level in the reactor: atmospheric vs. pressurised gasificationMost gasifiers work at (almost) atmospheric pressure, a certain pressure difference is alwaysnecessary to force movement of the gases. In large-scale installations with a gas turbine,gasification at higher pressures (10-20 bar) is sometimes preferred to avoid costly gascompression prior to the turbine.

3.2.2 Most relevant types of gasifiersTheoretically, any combination of the principles described above is possible. Practically, there isa limited number of combinations leading to the following “basic technologies”.

Downdraft gasifiers. Most fixed bed gasifiers are downdraft gasifiers operating at atmosphericpressure with air as gasifying agent. Both gas and biomass move from the top of the reactor tothe bottom. Characteristics of downdraft gasifiers are:• Requirements on biomass specifications (moisture content, particle size/shape, mechanical

stability of particles etc.) are strict compared to updraft gasifiers or fluidised bed gasifiers. • Downdraft gasification is only suitable for installation up to a few MW capacity (the upper

limit depends on the design of the gasifier and is not exactly known).

16 ECN-C--00-028

• The quality of the fuel gas is good (low amount of tar) compared to updraft gasifiers andfluidised bed gasifiers.

Updraft gasifiers. In updraft gasifiers, the gas flow is directed upwards. This principle is usedin a number of commercial gasifiers for heat production. Though, updraft gasification is hardlysuitable for electricity generation in prime movers because of the bad quality of the producedfuel gas. Characteristics of updraft gasifiers are:• Requirements on biomass specifications (moisture content, particle size/shape, mechanical

stability of particles etc.) are flexible compared to downdraft or fluidised bed gasifiers. • Updraft gasification is suitable for installation of several 10 MW capacity.• The quality of the fuel gas is bad (very high amount of tar) compared to downdraft and

fluidised bed gasifiers.

Other fixed bed gasifiers. There is a large number of variations of the two basic principles offixed bed gasifiers described above, e.g.• Supply of oxygen as gasifying agent instead of air• Open reactor instead of closed reactor• Combination of both downdraft and updraft principle by supplying the gasifying agent at

several heights of the reactor• Feeding from the bottom by means of a transport screw instead of feeding from the top.There is only a small number of gasifiers making use of one or several of these variations.

Fluidised bed gasifiers. In bubbling fluidised bed gasifiers, the biomass particles (e.g. chips)are fluidised together with inert bed material (mostly sand) by means of the gasifying agent thatis fed at relatively low velocity. In a circulating fluidised bed gasifier, the gas is supplied athigher velocity so that sand and biomass circulate in a system consisting of reactor vessel,cyclone, and a return pipe. Important characteristics of fluidised bed gasifiers are:• The size of biomass particles has to be reduced, mostly to several cm length.• There is no technical scale-up limit. The scale of an installation is determined by economic

or logistic aspects (lower limit by the “economy of scale”, upper limit by availability ofbiomass and the local energy demand).

• Most fluidised bed gasifiers operate at atmospheric pressure using air as gasifying agent.Pressurised gasification is an option if gas turbines are applied above about 100 MW.

• Because of the relatively low temperature in fluidised bed gasification of about 850 °C, thedanger of ash agglomeration is relatively low compared to gasifiers operating at highertemperature.

Entrained flow gasifiers. In entrained flow gasifiers, the gasifying agent is supplied at such ahigh velocity that the (pulverised) biomass is pneumatically transported through the reactor.Important characteristics of entrained flow reactors are:• Biomass must be pulverised.• In order to achieve a high conversion efficiency despite a low residence time of the biomass

in the reactor, oxygen is used as gasifying agent instead of air.• Gasification with oxygen is only economically feasible above a certain size of min.

200 MW thermal capacity. For biomass gasification, entrained flow technology is onlyinteresting in exceptional cases because of logistics (availability of biomass).

3.2.3 Definition of most relevant gasification systems for CHP productionAmong the various options of biomass gasification, a limited number of technologies appearmost suitable. The choice of “most promising options” made below is based on the followingstarting points:• Only technologies making use of prime movers (gas engines, gas turbines, fuel cells) for

electricity production at high efficiencies (with or without application of heat) are

ECN-C--00-028 17

considered “promising”. Stirling engines, steam engines and indirectly fired gas turbines arenot taken into account. Steam turbines are only considered interesting as part of a combinedcycle consisting of both gas turbine and steam turbine. Updraft gasifiers are hardly suitablefor electricity production in prime movers and are therefore left out.

• It is assumed that the gasification plants are to be operated 8.000 hours per year using100 % biomass as fuel. 200 MW thermal input is regarded as maximum capacity because ofthe limited availability of biomass. Larger biomass plants will only be applied in exceptionalcases. Because of this upper limit, only air-blown gasifiers appear relevant. Entrained flowgasifiers and other oxygen-blown gasifiers are not taken into account.

• The following ranges of capacities are considered feasible for the different technologies:lower limit 2) upper limit

Internal combustion engines 50 kWe 10 MWe

gas turbines 1 MWe ∞ 1)

steam turbines 10 MWe ∞ 1)

fuel cells no limit (stacks of small units)1) There are upper limits above the regarded capacities of 200 MWth2) Smaller capacities are technically possible but economically feasible in exceptional cases only

The following three combinations appear most suitable for future application of biomassgasification. Note that the three systems described below are considered examples. Many othertechnologies and capacities are possible.A. Downdraft gasification, atmospheric pressure, air-blown, connected to an internal

combustion engine. Thermal input 3 MWin, electrical output 0,75 MWe.B. Circulating fluidised bed gasification, atmospheric pressure, air-blown, connected to a gas

turbine and a steam turbine (combined cycle). Thermal input ca. 100 MWin, electrical output44 MWe. (example see Figure 3-2)

C. Circulating fluidised bed gasification, pressurised, air-blown, connected to a gas turbine anda steam turbine (combined cycle). Thermal input ca. 150 MWin. Electrical output 72 MWe.

Because taking both small-scale (A.) and large-scale systems (B. and C.) are taken into account,a large part of the potential for biomass gasification can be covered by these three systems. Thecombination of biomass gasification and fuel cells will probably not be available on a short termand is not taken into account. In Table 3-3, an overview is given of key characteristics of mostrelevant gasification technologies. Note that figures are taken from several references and arerelated to the 10th commercial installation of the respective type.

Table 3-3 Characteristics of most suitable gasification systems (10th installation of its kind)unit type A type B type C

capacity (input) MWin 3 100 150electrical efficiency % 25 44 48output electrical MWe 0.75 44 72output thermal MWth 1.5 18 24total efficiency % 75 62 64investment per kWe EURO 2800 1800 1800total investment kEURO 2100 79200 129600personnel (50 kEURO/man-year) kEURO/a 20 1000 1250O/M (5 % of the total investment) kEURO/a 105 3960 6480biomass 1) kEURO/a 240 8000 12000

1) based on a biomass price of 10 EURO/MWh (fuel input) and 8000 hours of operation per year

18 ECN-C--00-028

GA

C1 C2 BH

AB

AIR

APGC

SC

SF

WT

GR

CC

CO GT

GE1

BS

SF

ST

GE2

CTCD

FWP

IMPURITIES

FGB

DRSB

OIL

BIOMASS

SD

HRB

EC

EV

SH

ST

:

GAS CYCLE - Nuovo PignoneCOCCGTGE1BS

::

::

CompressorCombustion ChamberGas TurbineGenerator 1Bypass Stack

CW

GASIFICATION PROCESS - LURGIGAC1, C2ABAP

GC

BHSC

WT

SFGRFGBSBDRST

::::

:

::

:

::::::

GasifierCyclone 1, 2Air BlowerAir Preheater

Gas Cooler

Bag House FilterScrubber

Water Treatment

Start-up FlareSyngas RecompressorFlue Gas BlowerStart-up BurnerDryerStack

CW : Cooling Water

Gasification Process

Gas Cycle

Steam Cycle

AIR

Water/SteamDry BiomassImpuritiesAirFlue GasSyngas

SB

::

FWPSF

Feed Water PumpSupplementary Firing

:::

ECSHEV

EconomizerSuperheaterEvaporator

:CP Circulation Pump

STGE2

::::::

CDCTHRBSD

Generator 2

STEAM CYCLE - ENELSteam Turbine

CondenserCooling TowerHeat Recovery BoilerSteam Drum

:SB Syngas Blower

Oil

CP

˜

˜

Figure 3-2 Biomass Gasification Integrated in a Combined Cycle: The Italian THERMIEEnergy Farm project [13]

3.3 PyrolysisPyrolysis is the thermal degradation of organic material in the absence of oxygen. Typicalproducts are gas, a mixture of oil/tar/water, and char. The amount and composition of pyrolysisproducts depends on pyrolysis temperature, heating rate and residence time (and of course onthe composition of the fuel). In general three regimes can be distinguished:

Heating rate [K/s] Residence time T-max in °C Main product

slow pyrolysis << 1 hours/days5-30 min

400600

Chargas,oil,char

fast pyrolysis 500-100.000 0,5-5 s 650 Oil1)

flash pyrolysis >105 < 1 s< 1 s<0,5 s

< 650> 6501000

Oil1)

GasGas

1) Pyrolysis oil mostly needs further treatment before being used as fuel

Slow pyrolysis is typically used for charcoal production from biomass. Under these conditions,low temperature and long residence time, maximal charcoal yield is reached. High temperature

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(>700°C) and short residence time (flash pyrolysis) results in maximal gas production (up to80%). Fast and flash pyrolysis at intermediate temperature (500-650°C) and short residence time(<2 s.) results in maximal oil production (up to 80%).

Charcoal production from wood (including willow) is state of the art technology and is being usedon large scale world-wide. Technologies being used are mainly batch type processes oralternatively rotary kilns.

Oil production from wood is under development. Intensive research is being done world-wide,either on reactor development for improved process economy, oil yield and quality and on oilupgrading. Several pilot-plants have been realised with capacities up to 650 kg/h. A wide range ofreactors have been operated:– Bubbling and circulated fluidised beds– Entrained flow– Ablative pyrolysis– Rotating cone reactor– Vacuum pyrolysis

Comprehensive overviews are given in [17]-[19].

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4. LITERATURE

[1] Christersson, L.: Willow Power. Contribution to the REGROW Guide. SwedishUniversity of Agricultural science, Uppsala, 2000 (in preparation).

[2] Boyd, J.: Willow harvesting, drying, storage and transport. Contribution to theREGROW Guide. Scottish Agricultural College, Edinburgh, 2000 (in preparation).

[3] Dinkelbach, L. (ed.): Standardisation of Solid Biofuels in the Netherlands. Contributionto the project FAIR4-CT97-3952 “Standardisation of Solid Biofuels in Europe”. ReportEWAB-9923, Petten, 1999.

[4] Pike, D.C.: A review and critical assessment of ten reports relevant for to standardssolid biofuels. Report to a meeting of CEN/BT/WG 108 on Solid Biofuels held inStuttgart, Germany, March 1999.

[5] van der Drift, A.; van Doorn, J.: Phyllis. Operationeel maken – fase 2 (in Dutch). ECN-CX—98-138, Netherlands Energy Research Foundation, Petten, November 1998.

[6] Madsen, M.: Combution of Biomass – An Overview. Proceedings 10th EuropeanConference and Technology Exhibition “Biomass for Energy and Industrie”, Würzburg(Germany), June 1998, edited by H. Kopetz et al., pp. 213-215.

[7] Ortinger, W.; Fink, M.; Wanner, L.; Weber, Th.: Survey Concerning Biomass-to-Energy in CHP Systems and Power Systems and Heating Systems in Bavaria.Engineering-Economy-Ecology. Proceedings 10th European Conference andTechnology Exhibition “Biomass for Energy and Industrie”, Würzburg (Germany), June1998, edited by H. Kopetz et al., pp. 1283-1286.

[8] van den Broek, R.; Faaij, A.; van Wijk, A.: Biomass combustion power generationtechnologies; part of: Energy from biomass: an assessment of two promising systemsfor energy production, RUU report: 95029, May 1995.

[9] Obernberger, I.: Decentralized biomass combustion: state of the art and futuredevelopment; Biomass and Bioenergy, vol. 14, no. 1, p. 33, 1998.

[10] Padinger, R.; Stanzel, W.: Wood chip furnaces for district heating in Austria; technicaldevelopments and operational performance; 8th European Conference on Biomass forEnergy, Environment, Agriculture and Industry, 3-5 October, Vienna, Austra, 1994.

[11] Bain, R.L.; Overend, R.P. and Craig, K.R.: Biomass fired power generation, Fuel Proc.Techn. vol. 54, 1, 1998.

[12] Nussbaumer, T. Automatische Holzfeuerungen; Wärmetechnik-Versorgungstechnik12/1999.

[13] Kaltschmit, M.; Rösch, C.; Dinkelbach, L. (eds): Biomass Gasification in Europe. Finalreport, 1998.

[14] Solantausta, Y.; Bridgwater, A.; Beckman, D.: Electricity production by advancedbiomass power systems. VTT research notes 1927, 1996.

[15] Faaij, A.; van Ree, R.; Meuleman, B.: Long term Perspectives of Biomass IntegratedGasification with Combined Cycle technology, rapport EWAB 9840, 1998.

[16] Koljonen, T.; Solantausta, Y.; Salo, K.; Horvath, A.: IGCC Power Plant integrated to aFinnish pulp and paper mill. VTT research notes 1954, 1999.

[17] Bridgwater, A.V. et al.: Fast pyrolysis of biomass: A Handbook, ISBN 1 872691 07 2,May 1999.

[18] Rosillo-Calle, F. et al.: The charcoal dilemma, ISBN 1 85339 322 3, 1996.[19] Kitani, O. ; Hall, D.: The biomass handbook, ISBN 2 88124 269 3, 1989.