Novel Filtration System and Regime for Removing Particulates...
Transcript of Novel Filtration System and Regime for Removing Particulates...
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Novel Filtration System and Regime for Removing Particulates from Gas at
High Temperatures and Pressures
Sunil D Sharma*, Keith G McLennan, Michael Dolan, Don Chase and Ty
Nguyen
CSIRO Energy Technology,
*10 Murray Dwyer Circuit, Mayfield West, NSW 2304, Australia
Queensland Centre of Advanced Technologies, PO Box 883, Kenmore QLD 4069
Correspondence: [email protected]
Abstract
Continuous increase in demand of energy is a consequence of growing population and
increasing average standard of living in the world. With the existing technological
infrastructure in the current economy, the easiest and fastest way to meet the
increasing energy demand is mainly via carbonaceous fuel within the energy mix
including all other energy sources such as nuclear and renewable. The renewables
and nuclear technologies still have limitations in terms of energy production rate or
availability with respect to time and location. It is certain that in future the renewable
component of per capita energy consumption will be significantly increased for a
greener and cleaner economy, but reliance on fossil fuel for the interim period
between now and the renewable based economy of the future can not be ignored.
Moreover, cleaner fossil fuel based energy source will always be required to meet the
energy demand when and where a renewable source is unavailable and for energy
intensive processes such as metallurgy, heavy transport, chemical production etc.
Therefore it becomes essential to improve the existing fossil fuel based energy
production technologies to minimise their immediate impact on the environment.
The emissions from the fossil fuel based technologies could be reduced by improving
their efficiency, capturing the contaminants produced during the process of energy
production, and reducing the consumption of end products. One of the most effective
ways of clean energy and chemical production from carbonaceous fuels is via clean
syngas production by gasification or partial oxidation to produce syngas at higher
temperature and pressure and achieve a higher overall efficiency. The other
advantages are effective emission control due to separation and removal of solid and
gaseous impurities from the synthesis gas prior to its end use.
The clean syngas, as a feedstock for several chemicals, hydrogen, liquid fuels and a
fuel source for power generation, could be produced from almost every carbonaceous
material including coal, biomass, petroleum crude, shale, petroleum products, natural
gas and municipal waste. The composition of syngas, impurity level and its heating
value depends on the process of production and source material. However, in all
cases the product syngas need to be cleaned to an appropriate level to suit further
processing downstream. A number of gas cleaning processes have been developed to
suit various applications. Conventionally, most of them are wet or semi wet gas
cleaning processes that involve one or several stages of scrubbing with solvent,
usually water. Advanced dry gas cleaning processes currently being developed are
designed to conserve heat, reduce water consumption and reduce waste. However,
their reliability is yet to be proven to make them >95% available for the commercial
scale operations. The main causes for the poor reliability are corrosion, ash fouling
mailto:[email protected]
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and weakening of particulate filters and degeneration of sorbents. This paper
highlights the fundamental reasons for filter failure and proposes a novel concept and
system to prevent or minimise filter failure. Some results are also discussed to prove
the performance of a laboratory scale novel filtration system operated at 400-430 oC
and 2 MPaa.
Keywords: Hot gas cleaning, hot filtration, pulse cleaning.
Introduction
The reliability of coal based advanced power generation systems largely depend on an
effective and reliable gas cleaning process. Ideally, the gas cleaning process should
be able to continuously deliver clean syngas throughout the year; however this is not
achievable due to a number of limitations [Sharma et al 2008] of the existing gas
cleaning process. An availability factor of about 95% will be a reasonable optimum
for the gas cleaning process to make it compatible with conventional power
generation systems, and to allow repair and maintenance of the gas cleaning process
equipment on the same schedule as the other components of the power generation
system. This is the main driver of the research currently being carried out in the gas
cleaning area by a number of organisations [Heindenreich et al 2001, Dahlin et al
2005, Scheibner et al 2002, Suhara 2005, Silmane 2005]. The main constraints which
limit the availability of the gas cleaning process are the failure of candle filters and
degeneration of sorbents. Filter failure is the predominant cause because sorbent
could be replaced without shutting down the gas cleaning plant but filter failure and
subsequent replacement requires shutdown. An expensive alternative is to install two
parallel filter units and switch over to another when one unit fails. However, this
alternative really does not solve the problem as by the time switch over is
accomplished some damage may have already done to the downstream process by the
particles which have escaped or penetrated through the failed filter. Therefore the
filter unit has to be more reliable.
The inclusion of a failsafe is an important modification [Mia et al 2002] to the filter
element design but with additional costs of construction. It also puts additional stress
on the rest of the functional filter elements as soon as the flow through the failed
elements is sealed by their corresponding failsafes.
Another approach is Coupled Pressure Pulse Technique (CPP) which certainly
reduces the chances of filter element failure by regulating the ash built-up on and
pressure drop across the filter elements [Mia et al 2002]. However, the technique
could build some degree of stress on the filter elements as it involves frequent reverse
cleaning regulated by minimum allowed pressure drop build up across the filter
elements. Moreover, it uses larger volumes of syngas for reverse cleaning, a complex
pipework and controls. Frequent pulsing also increases the chances of particulate
penetration, particularly immediately after pulse cleaning when there is no ash cake
present on the filter surface.
Fundamental limitations of hot filtration
It appears that the design of the hot filters is perhaps derived from conventional bag
filters or rather conventional bag filters are adapted to hot conditions by replacing the
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bags by rigid ceramic or metal filter elements that can tolerate a temperature in the
range of 200 to 1000 oC. In order to operate the filter at high pressure the filter
enclosure vessel has been obviously designed appropriately but the mechanism of
filtration and reverse cleaning essentially remains the same as that of the bag filter.
The existing hot filtration system has several design issues rekated to the un-improved
adaptation of the bag filter design. Several fundamental design and operational
limitations could be visualised especially for high temperature and pressure
applications. These limitations are:
1. Rigid filter elements are vulnerable during reverse pulse cleaning as they are weaker for pressure exerted internally (failure under tension) than for exerted
externally (failure under compression). The reverse cleaning puts tensile
stress on the rigid filter (Figure 1). In comparison, bag filters are flexible and
do not experience a significant stress due to reverse pulsing.
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0
5
10
15
20
0 50 100 150 200 250 300
Time (minutes)
Str
ess o
n f
ilte
r (k
g/m
2)
Compression stress on filter during filtration
Tensile stress on filter during jet pulse cleaning
No stress on filter at zero rate of filtration
Figure 1 Variation of mechanical stress resulting from filtration and reverse pulse cleaning
2. The mechanical strength of the rigid filter elements could only be improved to a limited extent because higher mechanical strength of the filter element may
lead to loss of porosity, permeability and thermal conductivity [Sharma et al
2008].
3. The rigid filter elements could not have very high permeability because higher permeability will essentially have more surface area and porosity. The higher
surface area could also enhance the reactivity of ash and impurities with the
filter surface especially at higher temperatures.
Challenges of industrial scale filtration
Industrial scale filter units face another set of challenges besides failure. A number of
filtration units are being tested in leading organisations, however the evaluation and
reporting of the performance does not seem to be based on parameters which can aid
industry to monitor and improve their plant in the future [Sharma 2008]. At present a
number of demonstration scale syngas cleaning systems are operational [Guan et al
2005, Suhara 2005, Salinger et al 2005, Lupion et al 2005] with an objective to test
and improve the performance of various components. The quality of syngas and
levels of impurities varies significantly depending on the type of fuel, gasifier and
oxidant used [Dahlin et al 2005, Guan et al 2005, Salinger et al 2005]. Therefore a
proven component for a gas cleaning process with a quality of raw syngas from a
particular type of gasifier and fuel may not perform equally well with another type of
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gasifier and fuel. Therefore parameters such as the maximum operating period or
availability of any component are only valid for the conditions of its exposure. Any
performance data without detailed operational background variables (OBV) such as
(1) composition of all impurities and fuels, (2) annual maintenance schedule, (3)
component replacement record, (4) period and number of campaigns etc., could be
misleading if used as a basis for performance evaluation, design and scale up of the
gas cleaning process. Although qualitative illustrations of some of these conditions
have been included in some of the publication [Guan et al 2005, Salinger 2005], there
is no systematic method to quantify these variables and the performance of the filters.
The details of a novel approach to calculate availability factors and other performance
parameters using operational background variables are reported elsewhere [Sharma
2008]. However, the availability data reported about demonstration scale hot
filtration units in the literature seems to be insufficient for designing a new unit or
scale up. The published data does not help in selecting a particular type of filter
element or filtration unit in terms of their performance reliability. In other words it
becomes difficult to decide which filtration system and filter elements are the most
suitable for a particular set of conditions.
Despite difficulties of selecting filter elements or scaling up of filtration system the
following challenges can be identified with the industrial scale filtration systems:
1. Failure of filter elements due to fracture, cracks or pinholes which could result from stress due to frequent pulse cleaning or erosion.
2. Permanent residual ash deposition due to interaction between the filter surface and ash and syngas impurities. This situation could result in
reduced filtration capacity at an allowed pressure drop.
3. Rise in the pressure drop across the filter resulting in more and firmer deposition of cake. Higher pressure drop across the filter could produce
compact ash deposits due to excessive pressure exerted on the cake
deposited on the filter surface. In this situation the filter may not be fully
cleaned during reverse cleaning and may require replacement.
4. Corrosion of the filter surface coating and matrix could result in loss of filter material and strength. This may result in formation of pinholes or
cracks and the filter element may need replacement.
5. Reverse pulse pressure is usually twice as high as the filtration pressure and frequent reverse cleaning may the build excessive tensile stress on the
filter element. This stress may increase with permanent residual ash
deposit on the filter element.
6. The compression and thermal energy loss associated with the reverse pulse cleaning could be significantly high depending on the pressure,
temperature and frequency of the pulse cleaning required.
7. There is always some ash penetration through the filter element during the period between pulse cleaning and filtration when there is no cake present
on the filter surface. It is well known that the effective filtration takes
place only on the cake and filter acts like a barrier or support for cake
formation.
8. The reverse pulse cleaning system involves complex pipe work, valves and control. This adds to the thermal mass of the filter system and associated
heat losses. Failure of reverse pulse cleaning would definitely require
shut down.
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A combination or any of these factors could reduce the availability factor. These
design and operational issues are persistent because the existing hot filter design is an
un-improved direct adaption of the conventional bag filter system. Obviously,
significant improvement in the design or development of completely new systems
would be required to improve the reliability and availability of the hot filtration
system to enable successful commercialisation of advanced power generation systems
such as integrated gasification combined cycle (IGCC).
Novel hot filtration regime and system
A number of attempts have been made [Sasatsu et al 2002, Mia et al 2002,
Heidenreich et al 2001] to improve the design of hot filtration systems but they appear
to add more complications and costs to the system. For example, ceramic tube filter
(CTF) significantly reduces the tensile stress on the filter elements during reverse
cleaning as the cake is deposited inside the tube and reverse pulse cleaning requires
flow in the opposite direction which is from the outer surface to the inner surface of
the filter. However, in this design the filter tube will be under more tensile stress and
for longer periods during filtration. This design has not been demonstrated at pilot or
commercial scale.
Coupled pressure pulse technique is a significant improvement but requires
cumbersome pipework and control. The CPP could have significant loss of
compression and thermal energy as it uses frequent, gentle but long duration reverse
pulse [Heindenreich et al 2001, Scheibner et al 2002, Doring et al 2007]. Failsafe are
designed to protect the downstream processes in case a filter element fails. It is a
significant improvement in the filter element design but does not prevent the filter
element failure which may result from tensile stress, corrosion or ash deposition.
An attempt is therefore made in this paper to develop a filtration system and an
operating regime design to minimise the stress, corrosion and ash deposition on filters
and also improve the particulate separation efficiency, simplify the design and
minimise the thermal energy losses.
Pulse-less Filtration Concept
The existing filtration systems do not have any mechanism to stop particulates
breaking through the filter. Frequent ash deposition and reverse pulse cleaning not
only has frequent breakthroughs but could also result in erosion and weakening of the
filter. The coupled pressure pulse (CPP) technique seems to be quite effective in
preventing permanent residual pressure build up but more frequent pulsing
[Heindenreich et al 2001, Scheibner et al 2002, Doring et al 2007] increases the
possibility of particulate breakthrough. The pipe network and controls for reverse
pulsing also appear to be a complex design which could be expensive. In order to
address particulate breakthrough and avoid complex pipe network and control, a novel
concept and design (Figure 2) has been developed and successfully tested. This
design uses an inline jet ejector to create a very high annular (or shear) velocity on the
filter surface to control the cake thickness and allow continuous seepage of gas
through the filter. The shear force on the filter surface keeps ash particles suspended
as shown in Figure 2 and maintains a lower pressure drop across the filter.
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Figure 2 Pulse-less filter concept and regime (A Ashs laden feed gas, B Ash free gas, C
stream with higher ash concentration)
Initial proof of concept at low pressures and temperatures
The concept has been initially tested in a simple laboratory setup (Figure 3)
consisting of a 300 mm long filter element enclosed in a carbon steel jacket designed
to operate below 0.2 MPaa and 200oC. In order to prove the concept the preliminary
tests were conducted with the fly ash entrained in compressed air at various low
temperatures and pressures.
Figure 3 Laboratory setup to prove the pulse-less filtration concept at low pressures and
temperatures
Figure 4 Continuous operation of the pulse-less filter (Feed air ash content = 500 ppmw)
C
B A
FV= dPV* π r2
particle
Vessel or
SleeveFilter
FH= dPH* π r2particle
Agglomerates
Heater
Compressed
air
Fly ash
Screw
feeder
Single
candle filter
Filtered air
Cyclone
Fly ash
Operating limits:
Temperature = 21-100 oC
Pressure = 100- 300 kPa
Air flow = 0- 300 l/m (at 15 oC, 101 kPa)
Heater
Compressed
air
Fly ash
Screw
feeder
Single
candle filter
Filtered air
Cyclone
Fly ash
Operating limits:
Temperature = 21-100 oC
Pressure = 100- 300 kPa
Air flow = 0- 300 l/m (at 15 oC, 101 kPa)
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The concept has been tested in the laboratory at much higher face velocities
than those recommended at higher temperature and pressure for commercial scale
units [Dahlin et al 2005]. The results of a typical operation at 18oC and 130 kPaa
pressure are shown in Figure 4, which clearly indicates a constant pressure drop and a
constant flow rate through the candle filter for five days. This run was conducted
with an ash loading of about 500 ppmw.
Comparison of Performance of Pulsed and Pulse Less Filtration Regimes
In order to evaluate the extent of improvement with the pulse less filter over
the conventional pulsed filter, the filter unit was operated in pulse less and pulsed
modes. The results obtained in the pulse less and pulsed modes are shown in Figure
5.
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 50 100 150 200 250
Time (minute)
dP
ac
ros
s fi
lter
(Pa)
or
air
flo
w
rate
(L
itre
/min
)
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100000
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180000
200000
dP
ac
ros
s r
ig (
Pa
)
dP across filter (Pa)
Air flow (Litre/min)
dP across rig (Pa)
Ash loading 1400 ppmw
0
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0 500 1000 1500 2000 2500 3000
Time (min)
dP
acro
ss f
ilte
r (P
a)
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w
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itre
/min
)
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35000
dP
acro
ss r
ig (
Pa)
dP across filter (Pa)
Air flow rate (Litre/min)
dP across rig (Pa)
Ash loading 1400 ppmw
Figure 5 Change in flow and pressure drop across the filter and whole system with time (left
pulsed mode and right pulse less mode) [Note change of vertical scale for pulse-less mode)
The rise in pressure drop across the filter in the pulsed mode is much faster
compared to the pulse less mode. The characteristics of the pulsed and pulse less
filtration mode are summarised in Table 1 which indicates the possibilities of size
reduction, energy saving and less stress on the filter element operated in the pulse less
regime. In both modes the filter has been operated at about 3 times higher face
velocity than the conventional pulsed filter.
Table 1 Performance characteristics of pulsed and pulse less filter at low pressures
Operating Parameter Conventional
Pulsed Filter
Mode
Pulse less
Filter Mode
Significance
Dust Loading 1400 ppmw 1400 ppmw Industrial loading 100-10,000 ppmw
Face Velocity (cm/s)
(Gas flow/filter area)
7.3
(2.1*)
7.3 3-4 times size reduction likely
Maximum dPfilter (kPa) 4.8 1 Compression energy conserved
Maximum dPoverall (kPa) 180 30 Compression energy conserved
Pulse Frequency 2.5 days Less stress on elements – long life
* Data related to conventional pulsed filter
Laboratory scale High Pressure Pulse-less Filtration System
On the basis of results obtained from the preliminary tests to prove the pulse-less filter
concept, a laboratory scale pulse-less filter unit has been designed to test the pulse-
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less filtration at high temperatures and pressures. The unit also has facility to test
removal of various gaseous impurities present in syngas. The schematic flow diagram
of the laboratory system is shown in Figure 6.
Figure 6 Schematic flow diagram of laboratory scale dry gas cleaning unit including sorbent
injectors (SF1, SF2), sorbent reactors (R1, R2) , cyclone (C1) and particulate filtration system
(PLF1)
The laboratory system has a syngas recirculation system on the left (skid A) and a gas
cleaning system on the right (skid B). Skid B could be separated from skid A and
connected to a gasifier to test and verify the performance of various unit operations of
the dry gas cleaning process being developed at CSIRO.
As shown in Figure 6, the compressed syngas is recycled via a Pump P1
(Manufactured by Haskel). An inline flow meter (GFM2) is used to measure the
volumetric flow. The compressed syngas is then passed through a heat recovery unit
(HE2) which recovers heat from the cleaned hot recycled syngas. The preheated
syngas is then further heated in an electric heater (HE3) up to 650-700 oC). The hot
gas is then doped with water and water soluble impurities via a dosing pump (P2).
The ash particulates could also be injected into hot syngas via an ash feeder (SF3).
Thus a hot simulated syngas could be produced here with some impurities which are
normally present in the real syngas. The simulated syngas is then passed into the dry
hot gas cleaning skid which has a series of sorbent reactors and separators. The
impurities of alkali and chlorides are removed in a sorbent reactor R1 when a sorbent
or a mixture of sorbents is injected through a high pressure sorbent feeder (SF1).
The purpose of the reactor is to provide sufficient residence time to allow effective
sorption of gaseous impurities of alkalis and halides on to the sorbent surface. The
sorbents and ash are then separated in a cyclone (C1). The sulphur impurities from
the syngas is removed by injecting a different sorbent via another sorbent feeder
(SF2). The sorbent is then allowed to be mixed and reacted in the sorbent reactor, R2.
FT1
M1M3
Na/Cl sorbentSulphur sorbent
DealkylDeChlore reactorDesulphur reactor
Ash/NaCl sorbent
removal
Sulphur sorbent
removal
Guard bed
SF1SF3
SV2
VI2
C1
PLF1
GB1
P1 HE1 HE2
DV1
DV2
D1
SV1
SV3SV4
SV6
SV7
VI1VI2
TG1/PG1
R1
R2
TG2/PG2
TG3/PG3
TG4/PG4
GFM1
TG5/PG5
TG6/PG6
GFM2
TG13
TG7
TG8/PG8
TG9
TG10
TG11/PG7TG12
clean
syngas
or N2
inlet
RPV1
PRV2
BD1
CP1
BD3
CP3
BD2
CP2
HE3
Cl, NH3,
Na, K
P2
F1
DP1
SV5
WS1
WC1WC2
WC3
WC4 WC5
WC6
WC7
WC8
CP4
SV8
WC9
WC10
WC11
WC12
WC13
WC14
SV9
F2
PG9
Pilot
air in
Water
cooler
Inlet manifold
Outlet manifold
TG15
TG14
F3
F4
M2
SF2
Gasifier ash
for simulated
run or sorbent
F5
PG10
To various WC
FT1
M1M3
Na/Cl sorbentSulphur sorbent
DealkylDeChlore reactorDesulphur reactor
Electric
heater
Ash/NaCl sorbent
removal
Sulphur sorbent
removal
Guard bed
SF1SF3
SV2
VI2
C1
PLF1
GB1
P1 HE1 HE2
DV1
DV2
D1
SV1
SV3SV4
SV6
SV7
VI1VI2
TG1/PG1
TG2/PG2
TG3/PG3
TG4/PG4
GFM1
TG5/PG5
TG6/PG6
GFM2
TG13
TG7
TG8/PG8
TG9
TG10
TG11/PG7TG12
clean
syngas
or N2
inlet
RPV1
PRV2
BD1
CP1
BD3
CP3
BD2
CP2
HE3
Cl, NH3,
Na, K
P2
F1
DP1
SV5
WS1
WC1WC2
WC3
WC4 WC5
WC6
WC7
WC8
CP4
SV8
WC9
WC10
WC11
WC12
WC13
WC14
SV9
F2
PG9
Pilot
air in
Water
cooler
Inlet manifold
Outlet manifold
TG15
TG14
F3
F4
M2
SF2 F5
PG10
To various WC
From various WCs
Raw
syngas
from gasifier
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The mixture is then passed through a filer (PLF1) to completely remove all the
particles. The filter could either be operated in conventional pulsed regime or novel
pulse-less regime. The particulate free gas from the filter is then passed through a
multi-zoned packed bed of different sorbents to capture trace impurities of S, Se, As,
Hg, NH3, etc. The cleaned hot gas is then recycled after pre-cooling through the heat
recovery unit (HE1) and cooling in water cooler (HE2). The temperature and
pressure in the various parts of the rig was measured via a number of transducers
placed at appropriate positions in the rig. Gas, liquid and solid samples have to be
drawn at various points for analysis purposes. A control system and data logger
(National Instruments) has been used to control the temperature, pressure and flow of
the gas. In the filter testing experiments described in this paper the feeders SF1, and
SF3 were used for injecting ash particles into humidified nitrogen gas which was
recirculated through the unit. The ash feeder (SF2) was not operated.
Results and discussion
The filter testing was carried out by feeding fly ash obtained from Bayswater power
station (Australia) through feeders SF1 and SF3 while Feeder SF2 was isolated. The
carrier gas used was nitrogen from cylinders (commercial grade from BOC gases,
Australia) connected through a gas regulator to the suction line of the gas
recirculation pump P1 as shown in Figure 6. After the unit was pressurised to 2 MPaa
gas pressure the recirculation of the gas through the loop shown by red lines in Figure
6 was started. After ensuring steady flow of gas the water cooler started to ensure
cooling of sample coolers (WC) which were important to allow only cold gas and
solids to pass through the sampling valves into the sampling vessels as they were
designed for operation at a maximum temperature of 200 oC. Moreover, samples
need to be cooled down to a touchable temperatures. After cooling of the sampling
ports was achieved, the heater (HE3) was turned on to heat the recirculating gas and
downstream, reactors (R1, R2), cyclone (C1), filter (PLF1) and guard bed (GB1) up to
400-450 oC. A typical steady state temperature achieved by the various hot
components of the gas cleaning unit is shown in Table 2. The rest of the components
were maintained at room temperature between 20 to 25 oC.
Table 2 Steady state temperatures and pressures of various components of the dry gas cleaning
unit
Components Gas
cooler
(HE1)
Hear
recuperator
(HE2)
Heater
(HE3)
Sorbent
reactor
(R1)
Cyclone
(C1)
Sorbent
reactor
(R2)
Filter
(PLF1)
Guard
bed
(GB1)
Temperature (oC) 24 70 680 587 585 487 486 413
Pressure (MPaa) 19.75 19.75 20.66 20.75 2-75 20.64 19.82 19.49
Note: Rest of the components were maintained at room temperature between 20-25 oC
As soon as the heater (HE3) temperature reached 200 oC, the injection of water into
the gas stream was started by turning on the water pump. The injection of water is
essential to prevent metal dusting [Young 2006] in the presence of carbon monoxide.
During the filtration experiment with the nitrogen gas the injection of water was
mainly to simulate the humid conditions and stickiness of the ash laden moist gas
during filtration. As soon as the filter reached a temperature above 400 oC the
injection of fly ash started from the ash feeders (SF1 and SF2) and hourly sampling of
ash, gas and water also began from various sampling ports. The samples from SV2,
DV1 and SV3 were used to estimate the separation efficiency of the cyclone (C1);
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whereas the samples from SV4, DV2 and SV6 were used for calculating the filtration
efficiency of the filter. In this paper, however, the performance results of the filter
are discussed in detailed.
Due to gas flow in the turbulent region (Reynolds Number, Re > 40,000) throughout
the operation there was no fluid or solid accumulation anywhere in the pipes, the
reactors in the system, and it was assumed that the all ash injected from the SF3 and
which escaped separation in the cyclones was transported on to the filter surface.
Subsequently, all ash was separated at the filter and dropped into the drain vessel
DV2. Therefore, only ash, liquid and gas samples from DV2 were collected on an
hourly basis but and samples from the SV4, SV6 were collected on daily basis. No
ash particles were found in the samples collected from SV6.
Performance of filter during continuous operation in Pulse-less regime
The filter (PLF1 as shown in Figure 6) was continuously operated with hot
compressed nitrogen gas doped with various concentrations of fly ash and moisture
for up to 94 hours. The results are shown in Figure 7.
0
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300
400
500
600
0 20 40 60 80
Time (hours)
dP
fil
ter
(bar)
, P
ressu
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bara
),
Tem
pera
ture
(oC
),
Gas f
low
(l/
min
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co
ncen
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in
feed
gas (
pp
mw
)
dPfilter (Bar)
Filter temperature (C)
Gas flow (l/min)
Filter pressure (Bara)
Ash concentration (ppmw) in feed gas
Figure 7 Continuous record of temperature, pressure, nitrogen flow rate, pressure drop across
the filter and ash concentration in nitrogen gas during filtration in pulse-less regime at 20 MPaa
and 350-450 oC for >90 hours
During this experiment the filter was continuously operated for about a week and data
was continuously recorded in a data logger (National Instrument Field Point System).
Operating variables have been changed, except the operating pressure during the run
to examine the stability of the filtration. As shown in Figure 7, the temperature,
nitrogen flow rate, ash concentration in nitrogen was varied during the run. On the
fourth day the filtration test was carried out in the presence of moisture in the gas.
This was achieved by injecting water into the recirculating nitrogen gas by the water
pump (P2, Figure 6).
The record of pressure drop across the filter during the whole period of the run
(Figure 8) does not show any significant increase and indicates that there was no
significant permanent ash deposition on the filter surface. This is a significant
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reduction in the reverse cleaning frequency normally required in the conventional
filtration regime with reverse pulse cleaning which takes place on a frequency of 15-
30 minutes..
9600
9800
10000
10200
10400
10600
10800
11000
90 90.5 91 91.5 92
Time (hours)
Wate
r co
ncen
trati
on
in
gas (
pp
mw
)
0
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200
300
400
500
600
Gas f
low
rate
(l/
min
) an
d f
ilte
r
tem
pera
ture
(oC
)
Water concentration
(ppmw)
Gas flow (l/min)
Filter temperature
(C)
Figure 8 Record of water injected by water pump (P2) in the recirculating gas stream during last
2.5 hours of the run recoded in Figure 7
Although there was no significant permanent increase in the pressure drop across the
filter, the pressure drop across the filter did vary to some extent as a result of change
in temperature during the heating stage when the flow rate and pressure was
maintained constant. The pressure drop across the filter also varied as a result of
change in moisture content in the gas when the gas flow rate, temperature and
pressure was maintained nearly constant. However, there was insignificant change in
the pressure drop observed with the variation in the concentration of ash while gas
flow rate, temperature and pressure were maintained constant. These effects are
discussed as follows:
Effect of Ash Concentration
Although the average ash concentration in gas was measured around 4000 ppmw, the
ash concentration varied from 500 ppmw to 35,000 ppmw during the run as shown in
Figures 7 and 9.
Average operating conditions
Temperatue = 420 oC
Pressure = 20 bara
Gas flow = 420 l/min
Moisture = 0
0
0.02
0.04
0.06
0.08
0.1
0.12
0 5000 10000 15000
Ash concentration in feed gas (ppmw)
dP
acro
ss f
ilte
r (b
ar)
Operating conditions
Data from whole run,
where temperature,
pressure, flow rate,
and moisture varied
0
0.02
0.04
0.06
0.08
0.1
0.12
0 10000 20000 30000 40000
Ash concentration in feed gas (ppmw)
dP
acro
ss f
ilte
r (b
ar)
Figure 9 Effect of ash concentration in feed gas on the dP across the filter (Left; temperature,
pressure, gas flow was nearly constant, moisture content was zero; Right; data from whole run where
temperature, pressure, gas flow rate and moisture content varied)
9000
10000
11000
90 90.5 91 91.5 92
0
500
1000
-
The left side plot of ash concentration versus pressure drop across the filter in Figure
9, shows a marginal rise in the pressure drop with the ash concentration when the
moisture content in the gas was zero, gas temperature, pressure and flow rate was
maintained constant. The right side plot of ash concentration versus pressure drop
across the filter in Figure 9, shows no rise in the pressure drop when all the operating
conditions were varied. The scattering of the data has been mainly due to sampling
errors and minor influences of fluctuations in gas flow rate and temperature.
Effect of gas flow rate
The effect of variation of gas flow rate on the pressure drop (dP) across the filter is
plotted in Figure 10. In this plot all data recorded during the period between 10 and
70 minutes (Figure 7) when the feed gas flow rate was varied at nearly constant filter
temperature of around 400 oC was plotted. The data when the filter temperature
dropped below 400oC (for the periods 0-10 minutes and 70-80 minutes in Figure 7)
and moisture injected for a period from 80-94 minute in Figure 7), was excluded.
The scattering of data of the left side plot is due to instrumental error and variation of
temperature between 400-420 oC. However a clear trend could be seen with the
average pressure drop across the filter plotted on the right side graph. There marginal
rise in the pressure drop follows Darcy’s law [Sharma and Carras 2009]. The rise in
pressure drop with flow rate could be linear at lower pressure drops or flow rates) but
could be nonlinear at higher flow rates. Physically, when more numbers of molecules
are forced to pass through a porous media of constant permeability, the excess
molecules will tend to bounce off the filter surface. Above a certain flow rate there
will be a steep rise in the pressure drop as beyond this flow rate all excess gas
molecules will bounce off the filter surface.
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0 100 200 300 400 500 600
Gas flow rate (l/min)
dP
acro
ss f
ilte
r (b
ar)
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
0 100 200 300 400 500 600
Feed gas flow rate (l/min)
Avera
ge d
P a
cro
ss f
ilte
r (b
ar)
Figure 10 Effect of variation of gas flow rate on the pressure drop across the filter
Effect of temperature
All data logged during the period between 0 to 30 and 30 to 70 minutes (Figure 7)
when temperature varied at nearly constant gas flow rate and pressure have been
plotted in Figure 11. The trend of the scattered data for the period between 0-30, 30-
70 and 0-70 is shown by plots a, b and c in Figure 11.
The scattering of data was mainly due to the errors of measurement and minor
fluctuations of flow rate during this period. A clear trend could be seen when the
average pressure drop across the filter for different flow rate is plotted as a function of
temperature. The increase in the pressure drop seems to be negligible up to 200oC
-
and linearly increases with the temperature above 200oC and there could be several
reasons for that. The predominant reason could be the rise in viscosity of gas with
temperature and the other less predominant factor could be a change in permeability
of the filter media. It is expected that after certain critical temperature the viscosity
and friction against the flow could steeply rise and lead into a dramatic increase in the
pressure drop. However such a critical temperature limit seems not to have been
reached during this period.
Gas flow rate 520 l/min
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0 100 200 300 400 500
Temperature (oC)
dP
acro
ss f
ilte
r (b
ar)
Gas flow rate 420 l/min
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
360 380 400 420 440 460
Temperature (oC)
dP
acro
ss f
ilte
r (b
ar)
(a) (b)
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0 100 200 300 400 500
Temperature (oC)
dP
acro
ss f
ilte
r (b
ar)
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
0 100 200 300 400 500
Temperature (oC)
Avera
ge d
P a
cro
ss f
ilte
r (b
ar)
Gas flow rate 420-520 l/min
gas flow 420 l/min
Gas flow 520 l/min
(c) (d)
Figure 11 Effect of variation in temperature on the pressure drop (dP) across the filter, ( Gas
flow rate (a) 420 l/min, (b) 520 l/min, (c) 420-520 l/min, and (d) average pressure drop data)
Effect of moisture content of feed gas
The moisture content in the feed gas was varied between 9,600 ppmw and 11,700
ppmw during the period between 80th
to 94th
minute (Figure 7 and Figure 8) where
gas flow rate was maintained around 420 l/min but temperature varied between 400
and 420oC. The plot of moisture has some influence on the pressure drop across the
filter as shown in Figure 12. The left side plot shows a trend of scattered data. The
scattering of the data could be attributed to the minor fluctuation in temperature, gas
flow and measurement errors. The average pressure drop across the filter
exponentially rises as a function of moisture concentration in the gas as shown in right
side plot of Figure 12.
As compared to the rise in pressure drop with temperature (Figure 11) there is
significantly more rise in the pressure drop with the increase in moisture content.
When the moisture content is increased by 1,000 ppmw (only 10% of the full range of
-
moisture varies) from 9,500 to 10,500 ppmw, the rise in pressure drop across the filter
was from 0.02 to 0.05 Bars. When the temperature is increased from 200 to 400 oC
(about 50% increase), the pressure drop is increased from 0.002 to 0.018 Bars.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
9500 10000 10500 11000
Moisture concentration in gas (ppmw)
dP
acro
ss f
ilte
r (B
ar)
0
0.01
0.02
0.03
0.04
0.05
0.06
9500 10000 10500 11000
Moisture concentration in gas (ppmw)
Avera
ge d
P a
cro
ss f
ilte
r (B
ar)
Figure 12 Effect of variation of moisture content in feed gas on the pressure drop across the filter
The rise in pressure drop with moisture is mainly due to increased viscosity as well as
density of the moist gas. This is perhaps due to increase in intermolecular attraction
of the gas phase with the increase in polarity in the medium as the concentration of
water molecules in gas increases. The other reason for the increase in pressure drop
could be due to increase in the stickiness of the ash on the filter surface.
However, the maximum pressure drop across the filter was insignificant under all
conditions of operation tested and therefore there was no need for the reverse pulse
cleaning of the filter element throughout the run.
Comparison of conventional and pulse-less filtration regimes
The comparison of the face velocities used in the conventional filtration and pulse-
less filtration regime reveals that the pulse-less filter operates at very high face
velocity, as shown in Table 3.
Table 3 Comparison of conventional and pulse-less filtration regimes
Filtration Regimes
Pulsed[Heidenreich 2005]
Pulse-less
Average face velocity (m/s) 0.023 0.05
Average pressure drop across filter (bar) 0.025 bar (2.5kPa) 0.0128 bar (1.3kPa)
Average particulate loading (ppmw) 8732 (10g/m3stp N2) 4,161
Maximum particulate loading (ppmw) 8732 (10g/m3stp N2) 35,000
Average moisture loading (ppmw) - 10,000
Gas composition Nitrogen/flue gas Nitrogen
Particles Glass dust Fly ash
Paricle size distribution 6.5 µm median 6.3 µm median
Temperature (oC) 450-525
oC 400-430
oC
Pressure (Bara) 2 (0.2 MPaa) 20 (2 MPaa)
Possible size reduction (%) - 57%
This is perhaps due to the avoidance of thick permanent cake deposition on the filter
in the pulse-less regime. A very thin layer of cake is always present on the surface of
the filter for efficient filtration at significantly lower pressure drops as compared to
what could be observed with the conventional filters with reverse pulse cleaning. A
-
significantly higher face velocity would mean a significant reduction in the size and
therefore cost of the filtration system.
Conclusions
The poor availability of gas cleaning systems is considered as one of the major
hurdles in the commercialisation of IGCC based advanced power generation systems.
One of the main causes for the poor availability appears to be filter failure, which
could be due to corrosion from the syngas impurities, due to permanent residual ash
deposition and frequent reverse cleaning. The frequent pulse cleaning also increases
the chances of particle penetration through the filter especially after completion of
every pulse cleaning. In order to address all these issues a novel pulse-less filtration
concept and system is described. The results obtained from the operation at
atmospheric conditions as well as higher temperatures (400-450 oC) and pressures (20
Bar) have shown that the filter in pulse-less regime could be continuously operated
with up to 35,000 ppmw ash laden nitrogen gas at 400-450 oC and 20 bara. On the
basis of experimental results obtained it has been found that there was a marginal
effect of increase in temperature, flow rate and humidity of gas on the pressure drop
across the filter. No reverse cleaning of the filter was required in the test run
conducted for about 95 hours. These results show a promise towards high reliability
of particulate filtration with significant improvement in the availability and efficiency
of the particulate separation in the hot gas cleaning process.
Moreover the filter was operated at a remarkably high face velocity which was about
two times higher than that used in the conventional filtration regime where very
frequent pulsed cleaning is required. This shows a potential for size and cost
reduction of commercial scale hot gas cleaning systems.
Acknowledgements
Authors wish to acknowledge the support provided by the Centre for Low Emission
Electricity (Australia) and CSIRO National Energy Transformed Flagship (Australia).
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