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: sunil.sharma@csiro.au

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

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

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.

-20

-15

-10

-5

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

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.

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.

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 r2particle

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)

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

)

0

20000

40000

60000

80000

100000

120000

140000

160000

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

200

400

600

800

1000

1200

0 500 1000 1500 2000 2500 3000

Time (min)

dP

acro

ss f

ilte

r (P

a)

or

air

flo

w

rate

(L

itre

/min

)

0

5000

10000

15000

20000

25000

30000

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 <3 hours (10 -

15* minutes)

>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-

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

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);

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

100

200

300

400

500

600

0 20 40 60 80

Time (hours)

dP

fil

ter

(bar)

, P

ressu

re (

bara

),

Tem

pera

ture

(oC

),

Gas f

low

(l/

min

)

0

5000

10000

15000

20000

25000

30000

35000

40000

Ash

co

ncen

trati

on

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

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

100

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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