xvEPA
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 2771 1
EPA-600,'7-79-019
January 1979
Alternatives for
High-temperature/
High-pressure
Participate Control
Interagency
Energy/Environment
R&D Program Report
-------
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protect the public
health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
energy supplies in an environmentally-compatible manner by providing the nec-
essary environmental data and control technology. Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
effects; assessments of, and development of, control technologies for energy
systems; and integrated assessments of a wide range of energy-related environ-
mental issues.
EPA REVIEW NOTICE
This report has been reviewed by the participating Federal Agencies, and approved
for publication Approval does not signify that the contents necessarily reflect
the views and policies of the Government, nor does mention of trade names or
commercial products constitute endorsement or recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/7-79-019
January 1979
Alternatives for
High-temperature/
High-pressure
Participate Control
by
Richard Parker and Seymour Calvert
Air Pollution Technology, Inc.
4901 Morena Boulevard, Suite 402
San Diego, California 92117
Contract No. 68-02-2190
Program Element No. 1NE624
EPA Project Officer: Dennis C. Drehmel
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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ABSTRACT
High temperature and pressure (HTP) particulate control
offers efficiency and potential economic advantages over cold
gas cleanup in pressurized fluidized bed combustion (PFBC) and
low-BTU coal gasification (LBCG) combined-cycle power generation
systems. However, considerably more development will be neces-
sary in order to demonstrate the technical and economic feasi-
bility of HTP gas cleanup on a commercial scale.
This report presents the status of the most promising
HTP particulate control devices currently being developed. Avail-
able data are presented and anticipated performance and develop-
ment problems are discussed.
The alternative of recuperative heat exchange coupled with
low temperature, high pressure particulate control is reviewed
with regard to power system efficiencies for PFBC and LBCG com-
bined-cycle processes. Successful hot gas cleanup has clear thermal
efficiency advantages (1% to 7%) over cold gas cleanup. The
economics of hot gas cleanup, however, are very speculative at
the current state of development.
The relative cost of HTP, pre-turbine particulate control
using cyclones, multiclones, and granular bed filters is compared
with low temperature and pressure (LTP) post-turbine control
using conventional electrostatic precipitators. HTP control
equipment costs are estimated to be significantly higher than
LTP equipment costs. However, LTP costs are significant and
should not be neglected when considering the feasibility of hot
gas cleanup for turbine protection followed by post-turbine
fine particulate control to meet emissions regulations.
111
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TABLE OF CONTENTS
Page
ABSTRACT iii
FIGURES v
TABLES Viii
ACKNOWLEDGEMENT x
SUMMARY AND CONCLUSIONS 1
INTRODUCTION 8
Particulate Removal Requirements 9
Background 11
Pressurized Fluidized Bed Coal Combustion 11
Low-BTU Coal Gasification 16
PARTICULATE CONTROL DEVICES 24
Primary and Secondary Collection 24
Tertiary Collection 29
Cyclones 32
Granular Bed Filters 41
Scrubbers 62
Electrostatic Precipitation 73
Fiber Filtration 78
Membrane Filtration 88
HOT VERSUS COLD GAS CLEANUP 94
Introduction 94
Pressurized Fluidized Bed Coal Combustion 95
PFB Boiler Process 95
Air-Cooled PFBC Process 99
Adiabatic PFBC Process 104
Post-Turbine Particulate Control 107
Low-BTU Coal Gasification Processes 108
REFERENCES • . 120
IV
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FIGURES
Number Page
1 Water-cooled combustor 12
2 Air-cooled combustor 12
3 Adiabatic combustor 12
4 LBCG combined-cycle system 18
5 Three stage hot gas cleanup -25
6 Fractional efficiency for secondary cyclone (from
Hoke, et al. 1978) 27
7 HTP particulate control system for Westinghouse
PFBC design (from Beecher, et al. 1976) 28
8a Efficiency vs. temperature for high temperature
cyclone (from Parent, 1946) 33
8b Efficiency vs. pressure drop for high temperature
cyclone (from Parent, 1946) 33
9 Rotary flow cyclone 36
10 Performance of rotary flow cyclone (from Ciliberti
and Lancaster, 1976) 36
11 Cyclocentrifuge (from McCabe, 1977) 38
12 Comparison between estimated performance of cyclo-
centrifuge and conventional cyclone (from McCabe,
1977) 40
13 Cold flow granular bed filter parameters 42
14a Fractional collection efficiency, nominal configura-
tion,
45
14b Influence of operational parameter combinations on frac-
tional efficiency (16" filter, 2 mm collector gran-
ules 45
v
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FIGURES, continued
Number Page
14c Fractional efficiency performance, small media
configuration. . 46
15a Influence of pressure drop function on overall
collection efficiency, nominal configuration .... 46
15b Influence of pressure drop function on overall
collection efficiency, thick bed configuration ... 47
15c Influence of pressure drop function on overall
collection efficiency, small media 47
16 Rexnord gravel bed filter 50
17 Experimental grade efficiency curve of a Rexnord
gravel bed filter (McCain, 1976) 52
18 Ducon granular bed filter 53
19 Fractional penetration curve for Ducon granular
bed (from Kalen and Zenz, 1973) 54
20a Schematic of a single Exxon filter bed 55
20b Modified filter bed 55
21 Fractional efficiency data for Ducon GBF (from
Bertrand, et al. , 1977) 59
22 Predicted GBF performance 61
23 Schematic diagram of A.P.T. dry scrubber system. . . 63
24 Predicted performance for A.P.T. dry scrubber. ... 65
25 Comparison of experimental with theoretical particle
collection characteristics of the A.P.T. dry
scrubber 67
26 Comparison of particle collection characteristics of
the A.P.T. dry scrubber with the A.P.T. cut/power
relationship 68
27 Viscosity/temperature relationship determined by
NBS on synthetic slags formulated to represent the
average compositions of fly ash from Montana Rosebud
and Illinois no. 6 coal types 72
VI
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FIGURES, continued
Number Page
28 Predicted performance for ceramic fiber filter. . . 82
29 3M crossflow ceramic monolith 89
30 Pore size distribution for Thermacomb ceramic
fiber material (from Poe, et al., 1977) 90
31a Pressurized fluidized bed/combined cycle system
water cooled combustor with hot gas cleanup .... 97
31b Pressurized fluidized bed/combined cycle system
water cooled combustor with cold gas cleanup. ... 98
32a Curtiss-Wright pressurized fluidized bed/combined
cycle system hot gas cleanup with aerodyne cyclones 102
32b Curtiss-Wright pressurized fluidized bed/combined
cycle system cold gas cleanup system with heat
recovery 103
33a Pressurized fluidized bed combustion system with
dual admission steam turbine 105
33b Pressurized fluidized bed combustion system with
dual admission steam turbine, cold gas cleanup -
low pressure evaporator added for heat recovery . . 106
34a Lurgi air blow gasifier - cold purification case. . 112
34b Lurgi air blown gasifier study - hot purification
case
113
35a Air blown entrained bed gasifier study - cold
purification case 114
35b Air blown entrained bed gasifier study - hot
purification case 115
36a Slagging gasification study - cold purification
case 116
36b Slagging gasification study - hot purification
case 117
VII
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TABLES
Number Page
1 Summary of FBC Particulate Emissions Characteristics 15
2 Gas Analysis from Exxon Miniplant PFBC Unit 17
3 Summary of Particulate Emissions Data for Coal
Gasification Processes 20
4 Estimated Gas Cleaning Equipment Performance
(Beecher, et al. , 1976) 30
5 Estimated Temperature and Pressure Losses for Gas
Cleanup System (Beecher, et al. 1976) 30
6 Estimated Gas Cleanup Equipment Costs Per Gas
Turbine (from Beecher, et al., 1976) 31
7 Fractional Efficiencies for Multiclone at 538°C
(from Yellott and Broadley, 1955) 34
8 Estimated Performance of Aerodyne Cyclone (from
Klett, et al., 1977) 37
9 Test Parameters for CPC Moving Bed Filter (from
Wade, et al. , 1978) 43
10 Granular Bed Filter Performance (from Bertrand,
et al., 1977) 58
11 Salt Composition Used in PDU Demonstration Runs. . . 70
12 Particle Behavior: Molten Salt Pilot Plant Run
Number 3 70
13 Particle Behavior: Molten Salt Pilot Plant Run
Number 6 71
14 Test Data Summmary for ESP at 900°C, 4.4 atm (from
Brown and Walker, 1971) 74
15 Recommended Design Parameters for HTP ESP (from
Feldman, et al. , 1977) 76
Vlll
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TABLES, continued
Number Page
16 Effect of Temperature on Filtration Efficiency
(from First, et al., 1955) 80
17 Fractional Efficiency for Composite Filter Tests. . 80
18 Ceramic Fiber Test Data 84
19 Summary of Hot Tests with 3M Thermacomb 92
20 System Parameters for PFBC Analysis (from Klett,
et al., 1977) 96
21 Effectiveness Ranges for Recuperative Heat
Exchangers 100
22 Performance of PFBC Processes with Hot and Cold
Gas Cleanup (from Klett, et al., 1977) 101
23 Comparison Between Pre-turbine and Post-turbine
Equipment Costs 109
24 Summary of Estimated Thermal Efficiencies for Hot
Versus Cold Gas Cleanup in Coal Gasification Pro-
cesses (from Jones and Donohue, 1977) HI
IX
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ACKNOWLEDGEMENT
A.P.T., Inc. wishes to express its appreciation to Dr. Dennis
Drehmel, EPA Project Officer, for excellent technical coordination
and assistance in support of all our efforts under this contract.
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SUMMARY AND CONCLUSIONS
INTRODUCTION
High temperature and pressure (HTP) particulate control
offers system efficiency and potential economic advantages over
cold gas cleanup in pressurized fluidized bed combustion (PFBC)
and low-BTU coal gasification (LBCG) combined-cycle power genera-
tion systems. However, considerably more development will be
necessary in order to demonstrate the technical and economic
feasibility of HTP gas cleanup on a commercial scale.
Although HTP particulate control has been a recognized
technical problem for over thirty years, no satisfactory solu-
tions have been demonstrated. Renewed efforts have been directed
at this problem during the past few years with the hope of develop-
ing new concepts and equipment for HTP particulate control.
This report presents the status of the most promising devices
currently being developed. Available data are reported and anti-
cipated performance and problems are discussed.
Alternative approaches using recuperative heat exchangers
coupled with cold gas cleanup are reviewed with regard to their
effect on power system efficiencies for PFBC and LBCG combined-
cycle processes. The relative costs of HTP gas cleanup for tur-
bine protection are compared with the cost of post-turbine clean-
up for emissions control.
PRIMARY AND SECONDARY COLLECTION
Most proposed HTP particulate control systems use one or
two stages of cyclones for primary and secondary particulate
removal. The primary cyclone recycles unreacted carbon to the
combustor or gasifier. The secondary cyclone reduces the mass
loading and size of particulates which pass through to the final
collection stage-
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Secondary cyclones may be multiclones, rotary flow cyclones,
or other high efficiency cyclone designs. Cyclones may be subject
to plugging if condensed tars are present. Primary and secondary
cyclones are designed to collect particles with diameters larger
than 10 to 20 ym. They generally are not effective for particles
smaller than about 5 ym.
Primary and secondary particle collection equipment are
commercially available although HTP applications are scarce and
there is room for improved materials, designs, and engineering
models.
TERTIARY COLLECTION
The third stage (tertiary) collection device must be capa-
ble of reducing the particle size and mass loading to a level
which is compatible with gas turbine specifications and environ-
mental emissions regulations. This most likely will require
90 to 99% collection efficiency on a particle size distribution
with a mass median diameter of about 4 ym and a geometric stan-
dard deviation of 3 (based on data from the Exxon PFBC miniplant).
Cyclones
Conventional cyclones have been tested at high temperature
and pressure and generally have been found to be inadequate for
tertiary cleanup requirements.
High efficiency rotary flow cyclones have been proposed for
the tertiary collection stage. To date, experimental data have
not demonstrated that adequate collection efficiencies can be
maintained with rotary flow cyclones. More high temperature
and pressure tests are being planned.
The "cyclocentrifuge" is a device under preliminary develop-
ment which uses a high reaction turbine to drive a centrifuge
which serves as the exit tube in a cyclone. High collection effi-
ciencies for fine particles have been predicted but experimental
data are not available yet to validate the predictions. Opera-
tion of the centrifuge bearings at high temperature and pressure
in a dirty environment is likely to present the most difficult
development problem with this device.
2
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Granular Bed Filters
Granular bed filters (GBFs) are often proposed as tertiary
cleanup devices. A granular bed filter is defined as any filtra-
tion system comprised of a stationary or slowly moving bed of
discrete, relatively closely packed granules as the filtration
medium.
Some laboratory data are available which show that very high
collection efficiencies (99+%) can be achieved if a filter cake
is allowed to form on the surface of the granular bed. Large
scale pilot plant and industrial GBF' s have not been able to
establish or maintain a filter cake and much lower efficiencies
have resulted.
Extensive HTP tests on a fixed bed, pilot plant GBF were
conducted at the Exxon PFBC miniplant. Efficiencies as high as
97% were achieved but could not be maintained for more than a
couple of hours operation. In all runs in which more than one
outlet concentration was measured, the efficiency was found to
decrease with time. Further HTP tests are planned on a smaller
scale GBF. If this GBF can be modified to meet emissions requirements
over a prolonged run additional full scale tests will be conducted.
High temperature tests on a moving bed GBF (Combustion Power
Company) are being conducted. Cold tests demonstrated that fine,
submicron particles could be collected with greater than 90%
efficiency using the proper velocities, dust loadings, and granule
flow rates. No high temperature data are available.
The intermittently moving panel bed filter being developed
at CCNY (Squires design) has obtained high collection efficiency
(99+%) by establishing a filter cake. The major problem with
this design is the requirement for low gas capacity and thus
relatively large capital costs for a HTP installation. There are
not many HTP data available for this device. Establishment of
a good cake and high collection efficiency may be more difficult
at high temperatures.
At this time, GBF's must be considered to be in a
highly developmental stage. Design improvements and more data
-------
at HTP operating conditions are required before their feasibility
as tertiary collectors can be adequately evaluated.
Scrubbers
Conventional wet scrubbers are not generally suitable for
high temperature gas cleaning because they necessarily cool
the gas. However, dry scrubbers and molten salt scrubbers are
being developed as alternatives for HTP particulate control.
The A.P.T. dry scrubber (PxP) system uses large collector parti-
cles as collection centers for the fine dust particles. In the
configuration presently being developed, the solid collectors
are fed into a high velocity throat and contacted with the dust par-
ticles. The collectors are removed from the gas in an inertial
separation device.
Low temperature and pressure data have been obtained which
show that the primary collection efficiency (>90% at 1.0 ymA) is
as would be expected from theoretical predictions for SL venturi
scrubber. A high temperature pilot demonstration of the PxP system
is currently underway. The feasibility of electrostatic augmen-
tation for improving the collection efficiency, lowering the pres-
sure drop, and increasing particle-collector adhesion is being
investigated.
Molten salt scrubbing is being investigated for simultaneous
particulate and H2 S control. Pilot plant data indicate that the
mass loading of particles leaving the molten salt venturi scrub-
ber may be close to the anticipated environmental emissions
standards. Tests will be conducted on a full-scale demonstration
plant. No data are available yet.
Electrostatic Precipitation
The operation of an electrostatic precipitator at high
pressures and temperatures has been demonstrated. Stable corona
can be maintained at higher electric field strengths than in con-
ventional precipitators. There are few studies providing data
on fine particle collection at high temperature and pressure.
Future development problems will involve electrode rapping and
alignment, reentrainment, and materials problems associated with
HTP designs.
4
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Practical HTP electrostatic precipitation will be limited
by thermal ionization at temperatures above 1,100 to 1,300°C.
Also a positive barrier-type backup device may be required
to prevent catastrophic turbine damage during possible electri
cal outages.
Fiber Filtration
High temperature filtration using metal or ceramic fiber
filters is being investigated. In practice it is expected that
the effect of temperature and pressure on filtration mechanisms
will not be a limiting factor in the overall collection efficiency,
The main problems will be the physical and chemical effects of
a high temperature environment on the filter materials. These
effects may appear as reduced mechanical strength and resilience
or loss of adhesion, leading to mechanical leakage and decrease
in efficiency.
Recent studies with ceramic fabric bags have shown that some
have good properties for high temperature and pressure applica-
tions. More data are required to adequately establish the useful
bag life and other important design parameters. Blanket or
felted ceramic materials look most promising because they combine
good filtration properties with relatively high strength. How-
ever, superficial gas velocities (air-to-cloth ratios) are low in
comparison with granular bed filters and the large volume required
for fabric filtration may present some economic limitations.
Membrane Filtration
Laboratory tests on ceramic membrane and honeycomb filtra-
tion materials have been conducted. The honeycomb materials
were able to obtain high collection efficiencies (averaging
96+%) on fine limestone particles at temperatures up to 815°C.
More work is needed to determine optimum configurations for
filtration and for cleaning. Also the durability and erosivity
limitations need to be determined. The major advantage of these
filtration media is that they have very high surface to volume
ratios. Superfical velocities are similar to those for fabric
filters.
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HOT VERSUS COLD GAS CLEANUP
Pressurized Fluidized Bed Combustion
The relative advantages of hot versus cold gas cleanup are
different for the three PFBC processes currently under development.
These are: the water-cooled PFBC boiler, the air-cooled PFBC
process, and the adiabatic PFBC process.
The efficiency loss between hot and cold gas cleanup is
greatest for the adiabatic combustor configuration because all
the working fluid passes through the bed and must be cleaned.
The air-cooled and water-cooled combustors appear to be
capable of using cold gas cleanup techniques with fairly small
(about 1 to 2% system efficiency) performance penalties. The
Phase II EGAS studies showed the PFBC boiler process with hot
gas cleanup to have a 1% advantage over conventional coal-fired
boilers using flue gas scrubbing for SO control.
3C
In order to fully assess the cold cleanup alternatives, re-
cuperative heat exchangers must be studied more closely, especi-
ally regarding their effectiveness, availability, and cost for
high temperature and pressure applications.
Post-Turbine Particulate Control
If gas turbines which have relatively high tolerance for
fine particles can be developed, then it may be feasible to use
currently available hot gas cleanup devices (cyclones, multiclones)
to protect the gas turbine. In such cases the emissions regula-
tions would have to be met by applying conventional particulate
control equipment downstream from the gas turbine.
The cost of post-turbine particulate control equipment is
noticeably less (about 10 to 30?0 than the cost of hot gas
cleanup as estimated in the EGAS Phase II design studies. How-
ever the post-turbine equipment costs are significant and must
be considered in the overall capital cost for gas cleaning
equipment when considering the feasibility of pre-turbine/post-
turbine control systems.
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Low-BTU Coal Gasification
Gas cleanup in LBCG processes must consider acid gas removal
(principally H2S and COS), tar removal, removal of alkali metal
vapors and compounds, and fine particle removal. There is little
incentive for hot particulate removal if the gas must be cooled
and scrubbed to remove H2S.
The presence of tars in LBCG processes presents serious
problems.' They will contaminate or plug subsequent H2S and
particulate removal systems unless they can be kept in the vapor
phase. They may be removed in a quench scrubber before H2S and
particulate removal, however, this wastes approximately 20% of
the available energy through sensible heat loss, and as much as
20% of the available chemical energy in the fuel gas. Hot H2S
and particulate control would save these energy losses and enable
the tars to be burnt in the gas turbine combustor.
For all LBCG gasifiers where tar removal is not a problem,
thermal efficiency advantages associated with hot gas cleanup
(H2S and fine particulate matter) appear to be marginal (1 to
2%). The Phase II EGAS studies showed LBCG combined-cycle
processes with cold gas cleanup to have a 7% advantage over a
conventional coal-fired boiler using flue gas scrubbing for SOX
control.
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INTRODUCTION
The economic attraction of many advanced energy processes
depends on the development of technology for cleaning gases at
high temperatures and pressures. Two processes requiring high
temperature and pressure gas cleanup are: 1) pressurized fluidized
bed coal combustion, and 2) the combined open-cycle gas turbine-
steam turbine system integrated with a low-BTU coal gasifier.
These processes are among those recommended for further develop-
ment by the Energy Conversion Alternatives Study (Lewis Research
Center, 1976 and 1977).
Both of these processes require expension of the gas through
a gas turbine at high temperature and pressure. The gas must be
cleaned to remove corrosive gases, condensible vapors and particu-
late matter which could potentially damage the gas turbine. Any
temperature and pressure losses during gas cleanup will reduce the
overall thermodynamic efficiency of the process. Therefore it is
desirable to clean the gas at system temperature and pressure.
Equipment for removing particulate matter from high tempera-
ture (HTP) gas streams has been under development to various de-
grees for over thirty years. Recently there has been renewed
interest in this problem and a number of new concepts and improved
materials are now being investigated.
In this report we review the present state-of-the art for
HTP particulate collection and discuss the process requirements
and possible alternative approaches to hot gas cleanup. In many
situations, the requirement for hot particulate removal is coupled
with a need for removing acid gas, and/or alkali metal vapor
at high temperature and pressure. Therefore, systems which poten-
tially can remove particulates, gases, tars and condensible vapors
will be especially attractive.
It has been assumed that readers are basically familiar with
conventional particulate control technology. Details on conventional
control equipment are presented by Calvert, et al. (1972), Billings
and Wilder (1970) and Stern (1977).
8
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PARTICULATE REMOVAL REQUIREMENTS
The degree to which particles must be removed from low-BTU
coal gasification (LBCG) and pressurized fluidized bed combustion
(PFBC) processes depends on the gas turbine tolerances for particu-
late matter, the size distribution and mass loading of particulate
matter, the gas composition, turbine blade material, gas tempera-
ture, other process parameters and environmental regulations.
There are no current New Source Performance Standards (NSPS)
for advanced fossil-fuel conversion processes. In order to allow
for industry growth while maintaining regional air quality stan-
dards, it would be necessary to control particulate emissions from
new sources more stringently than current standards. The NSPS
for particulate emissions from advanced fossil fuel conver-
sion processes will most likely be as stringent as the anti-
cipated new standards for boilers. That is, <0.05 lb/106 BTU
(21.5 mg/MJ), or at least be based on BACT (best available con-
trol technology).
The gas turbine specifications also impose a gas cleanup
requirement. The useful life of a gas turbine depends on the
extent of erosion and corrosion damage to the internal components
of the turbine. The extent of damage depends upon the concentra-
tion and size of particulate matter entrained in the gas, upon
the chemical composition of the gas and particles, and upon'
the gas temperature, turbine design and other operating para-
meters .
Erosion damage results from the inertial bombardment of
particles onto the stator and rotor blades of the gas turbine.
The erosion damage is proportional to the kinetic energy of the
particulate matter striking the turbine blades. Therefore the
damage is more severe when larger more massive particles are pre-
sent in the gas stream. Large concentrations of very small
particles may be even less harmful than their total mass would
imply because their trajectories would tend to follow the gas
streamlines and thus would be less likely to strike the turbine
blades.
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Corrosion damage depends on the amount of particulate matter
that adheres to the turbine surfaces as well as the chemical com-
position of the gas and particles. In general, the most corrosive
compounds are those containing sodium and potassium. Liquid
deposits of such compounds can form inside the turbine at tem-
peratures between about 500°C and 1,000°C. These molten films
attack the protective oxide scale on the blade material, and thus
initiate accelerated oxidation of the turbine components.
The buildup of particulate deposits on the turbine blades
also can significantly impair the aerodynamic performance of the
blades. Furthermore, large agglomerates can break off from such
deposits and cause additional erosion damage.
There is much debate regarding the definition of acceptable
particle concentrations for gas turbines. There is general
agreement that large particles must be removed from the gas.
Robson (1976) recommends a maximum allowable concentration of 2.7
mg/Nm3 (0.0012 gr/SCF) for particles larger than 2 ym in diameter.
There is less agreement on the allowable turbine tolerance
for fine particles. A Westinghouse study (Westinghouse, 1974)
proposed that a concentration of 340 mg/Nm3 (0.15 gr/SCF) of
particles smaller than 2 ym could be tolerated. Sverdrup and
Archer (1977) recommended that the concentration be maintained
below 4.6 mg/Nm3 (0.002 gr/SCF) for all particle sizes with no
particles larger than 6 ym.
The variation in turbine tolerance estimates is not sur-
prising considering the scarcity of direct experience with large
coal-fired industrial gas turbines, the difficulty of extrapola-
ting data from small turbines, the variations in existing and
proposed turbine materials and blade temperatures, and variations
and uncertainties in the chemical and physical properties of the
particles and gases.
These uncertainties pose a major problem to engineers
throughout this report we have used the convention of "ym" for physical
diameter and "ymA" for aerodynamic diameter. Particle densities are given
whenever they are known.
10
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involved in the development and evaluation of advanced energy
processes. Developers of hot gas cleanup devices do not know
how efficient their equipment must be at removing fine particles,
At the same time, estimates of the cost of electricity and capi-
tal investment for advanced energy processes are strongly depen-
dent on assumptions concerning the cost (and indirectly the effi-
ciency) of gas cleanup equipment.
This predicament results in part from the necessity for
parallel development of advanced industrial scale gas turbines
and high temperature and pressure gas cleanup technology. The
cost of HTP gas cleanup will vary inversely with the allowable
particle concentration entering the gas turbine. However, the
process emissions must satisfy all environmental regulations.
Gas turbines which can tolerate relatively large concen-
trations of fine particles may necessitate particulate control
equipment downstream from the gas turbine. The cost of this
control equipment must be considered in the overall economic
evaluation of the process. If expensive control equipment is
required to protect the turbine, economics may require that this
equipment satisfy the emissions regulations as well.
BACKGROUND
Pressurized Fluidized Bed Coal Combustion
The combustion of coal in a fluidized bed of limestone or
dolomite offers many advantages over conventional coal-fired
boilers. The primary advantage is the ability to burn high sul-
fur coals without requiring flue gas desulfurization equipment.
Also PFBC processes have the potential for lower introgen oxide
emissions, higher thermodynamic efficiencies, more efficient
fuel utilization, lower cost of electricity and lower capital
costs compared with conventional coal-fired power plants using
stack gas scrubbers for SO control.
J\.
There are three approaches currently being considered for
PFBC processes. The first is the PFBC boiler illustrated in
Figure 1. Combustion takes place at a pressure of 10 atm and
a temperature around 900°C. The combustion temperature is
11
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AIR
COMPRESSOR
STACK
ADDITIVE
COAL
ASH DISPOSAL
Figure 1. Water-cooled combustor.
STACK
STEAM TURBINE |
CONDENSER
FEED WATER
AIR
STACK
STEAM TURBINE t
BOILER FEED WATER
AIR
WASTE HEAT
BOILER
GAS TURBINE
PARTICULATE
REMOVAL
ADDITIVE
COAL
ASH DISPOSAL
Figure 2. Air-cooled combustor.
ADDITIVE
COAL
U^
COMPRESSOR
~i
I
w
Si
0
u
t=J
\
1
GAS TURBINE
PARTICULATE
REMOVAL
ASH DISPOSAL
Figure 3. Adiabatic combustor.
12
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maintained using water-cooled heat exchanger tubes. The steam
generated is used to drive a steam turbine. The products of
combustion leave the combustor at high temperature and pressure,
and pass through the HTP gas cleanup system before being expan-
ded through the gas turbine.
Beecher, et al. (1976) reported the conceptual design of
an advanced steam plant using a PFBC boiler. The plant gener-
ates a net electric power of 679 MW. Four PFBC boiler modules
are used to fire two gas turbine generators. The turbine inlet
temperature is 959°C. Each turbine handles a gas flow of 345 kg/s
(760 Ib/s). Each boiler has two stages of cyclones for dust re-
moval. The first stage removes large particles and conveys them
to the carbon burnup cell. The second stage is a multiclone de-
vice which removes coarse and fine particles and thereby reduces
the loading to the tertiary (third stage) cleanup devices. Four
granular bed filters (43 kg/s or 95 Ib/s each) are proposed for
tertiary cleanup for each boiler.
The overall cost of electricity will be affected by the
capital cost and operating cost of the particulate control equip-
ment. The capital cost is a function of the size and number of
units required, which depend primarily on the total volumetric
gas flow to be treated at high temperature and pressure. The
operating cost depends on the power requirement of the control
equipment and the temperature and pressure losses which occur.
In the PFBC boiler system, approximately 80% of the total elec-
tricity is generated in the steam turbine cycle. Only 20% is
generated downstream of the hot gas cleanup system in the gas
turbine cycle.
An air-cooled PFBC process is being developed by Curtiss-
Wright Corporation under D.O.E. sponsorship. A schematic dia-
gram of this process is shown in Figure 2. The FBC unit is
cooled by passing approximately two-thirds of the compressed
air through heat exchanger tubing in the combustor. The other
third of the compressed air is used for combustion.
13
-------
The effluent gas from the combustor is cleaned before it
is mixed with the hot gas from the heat exchanger tubes. The
mixed gas is then expanded through the gas turbine and passed
through a waste heat boiler.
The principal advantage of this process is that it dilutes
the gases leaving the cleanup equipment thereby reducing the mass
loading entering the gas turbine. The total flow to the gas
turbine is increased threefold. For equivalent size plants, the
combustion gas flow will be approximately the same as that re-
quired with the PFBC boiler. Depending on the heat loss during
cleanup and the efficiency of heat transfer, the mixed gas
may attain a higher or lower temperature than the effluent gas
temperature from the PFBC boiler. All electricity is generated
downstream from the control equipment.
The third type of PFBC process currently being developed
uses the adiabatic fluidized bed combustor. This process
has been developed by the Combustion Power Company for refuse
combustion and also is being investigated for its potential in
burning coal.
The adiabatic system is illustrated in Figure 3. The bed
temperature is maintained suitably low by using large volumes
of excess air (300%J. Thus the gas flow rate is approximately
the same as the total flow used in the air-cooled PFBC process.
In the adiabatic combustor system, all the gas passes
directly through the combustor and must be cleaned. Therefore
the size (and capital cost) of the cleanup equipment will be
significantly more than in the other designs. Furthermore, all
the power generation occurs downstream from the cleanup equip-
ment and therefore the cost of electricity will be especially
sensitive to temperature and pressure losses during cleanup.
The particulate emissions and high temperature and pressure
cleanup requirements for fluidized bed combustion processes has
been reviewed by Parker and Calvert (1977). Particulate emis-
sions data from their survey are presented in Table 1.
14
-------
TABLE 1. SUMMARY OF FBC PARTICULATE EMISSION CHARACTERISTICS
Temperature, °C
Pressure, atm
Mass loading, g/Nm3
Mass loading, gr/SCF
Mass median diameter, ym
Geometric standard deviation
Exxon
Miniplant*
820-950
5-10
1.8-2.8
0.8-1.8
4-8
2.7
Argonne
Natl. Labs
790-900
to 8
0.5-4.8
0.2-2.1
Combustion
Power Cot*
760-980
3-7
0.09
0.04
1.2
1.9
National
Coal Board
(G.B.)
750-950
1-5
3.0
1.4
Pope, Evans
§ Robbins**
300-400
1
0.2-1.7
0.5-4
4.6
2
* Particle density estimated to be 1.5 g/cm3
*** Waste-fired combustor
-------
The particulate removal requirements for pressurized
fluidized bed combustion processes will depend on the turbine
tolerances for fine particles and the emissions regulations
for coal-fired boilers.
An emission of 0.05 lb/106 BTU corresponds to a particu-
late loading of approximately 57 mg/DNm3 (0.025 gr/SCF) based
on the heat input of the coal and emissions data from the Exxon
PFBC miniplant (Hoke, et al., 1978). The emissions regulations
will be approximately the same for all three PFBC designs.
Particulate control to protect the gas turbine is coupled
with the problem of alkali metal vapor removal. HTP control
equipment for PFBC processes may need to remove alkali metal
vapors as well as fine particles.
Recent data taken at the Exxon miniplant (Bertrand, et al.
1978) indicated that 2 ppm sodium vapor, 0.05 ppm potassium
vapor were present in the effluent gas. Gas analyses for the
same run showed very low H2S and hydrocarbon concentrations and
285 ppm S02. These data are listed in Table 2.
Low-BTU Coal Gasification
Low-BTU coal gasification processes are under development
as a possible means of obtaining electrical energy from coal
while satisfying all environmental regulations. Preliminary
economic analyses (Lewis Research Center, 1976 and 1977) in-
dicate that a combined cycle gas turbine-steam power generation
cycle fired by gas from an integrated low-BTU coal gasifier
offers significant economic advantages over conventional coal-
fired boilers in terms of more efficient coal usage and lower
cost of electricity. A typical process flow diagram is shown
in Figure 4.
The basic gasification process converts solid coal into
a combustible gaseous fuel by reacting it with air and steam.
The product gas contains hydrogen, carbon monoxide, carbon
dioxide, nitrogen, methane and hydrogen sulfide. The product
gas leaves the gasifier at high temperature (500 to 1,500°C)
16
-------
TABLE 2. GAS ANALYSIS FROM EXXON MINIPLANT PFBC UNIT
COMPONENT
CO 2
02
S02
(std.dev.)
N0x
CO
H2S
COS
CS2
CHi,
C2H6
C3 to C6
Na
K
CONCENTRATION
13.5 I
4.5 %
285 ppm
(110 ppm)
50 ppm
600-750 ppm
1 ppm
not detectable
not detectable
18 ppm
14 ppm
not detectable
2 ppm
0 . 5 ppm
17
-------
00
RECYCLE GAS TO GASIFIER
CLEAN LOW
GAS
PRETREATED
LIMESTONE
r
COMPRESSED
AIR
CHAR
STRIP
*_STEAM
COMBUSTOR
AIR
GAS TURBINE
-*- ELECTRICITY
SULFUR (^BOILER )—>• TO STACK
WATER
-^AMMONIA
DUST
STEAM
GASIFIER
ELECTRICITY
,. STEAM TURBINE
SLAG
Figure 4. LBCG combined-cycle system.
-------
and often at high pressure (10 to 20 atm). The gas usually
will contain large concentrations of entrained particulate
matter (ash, carbon, and possibly tars). Alkali metal vapors
and other contaminants also may be present.
There are many different processes for gasifying coal,
and the gaseous and particulate emissions can vary widely from
one process to another. The most popular classification of
gasification processes is classification according to the flow
of the gas relative to the coal. The four basic gasifier types
classified in this manner are:
1. Fixed or slowly moving beds of solids
2. Entrained solids
3. Fluidized beds
4. Molten baths
Gasifiers also may be classified with regard to the ash
removal method. At temperatures below approximately 1,000°C
the mineral matter in the ash remains dry. At temperatures
somewhat higher the ash becomes tacky and tends to agglomerate.
At even higher temperatures the ash melts. Molten ash usually
becomes free-flowing at temperatures of about 1,500°C to 1,600°C.
Therefore coal gasifiers can be classified as dry bottom, ash
agglomerating, or slagging gasifiers with regard to ash removal.
Gasifiers also can be classified as to pressure level, number
of reaction stages, and the source of oxygen (either air blown
or oxygen blown).
In general, particulate emissions are expected to be greater
for entrained bed and fluidized bed dry bottom gasifiers. Par-
ticulate emissions should be less for ash agglomerating and
molten bath gasifiers. Parker and Calvert (1977) reviewed the
available data on particulate emissions from coal gasification
processes and their results are summarized in Table 3.
Particulate control for coal gasification processes is
closely coupled with acid gas removal (principally H2S, COS,
and Ca), tar removal, and alkali vapor removal. In some processes
there may be a need for NH3 removal also. There is little
19
-------
TABLE 3. SUMMARY OF PARTICULATE EMISSION DATA FOR COAL GASIFICATION PROCESSES
Process
I Fixed Beds
Lurgi
USBM Stirred Bed
Gasifier Exit
Temperature
400-600
500-650
II Dry Fluidized Beds
Winkler
USMB Synthane
COa Acceptor -
Gasifier
C02 Acceptor -
Regenerator
800-1,000
400-750
800-850
1,000
BCR Fluidized Bed 1,000-1,150
III Ash Agglomerating
Fluidized Bed
Battelle -
Union Carbide
IGT U-Gas
Westinghouse
•
1,100
1,000
750-900
Gas Cleanup
Temperature
°C
200
150-200
250
200
1,000
650
--
400
Pressure
atm
20-30
7
1
10
10
10
20
7
20
10-15
Mass Loading
g/Nm3
24
24
--
--
20
18
--
--
--
gr/SCF
10
10
--
--
8.8
7.8
--
--
--
Control Devices
Used or
Anticipated
scrubbers
scrubbers
cyclones,
scrubbers,
electrostatic
precipitators
scrubber
venturi
scrubber
cyclones, sand
bed filters
--
cyclones
proprietary
process >
rotary flow
cyclones, granu
lar bed filters
Remarks
tars present
tars present
tars § heavy
hydrocarbons
present
IX)
O
-------
TABLE 3. Continued
Process
IV S lagging-Entrained
Flow
Koppers-Totzek
Combustion
Engineering
Foster Wheeler
BYU
Texaco
V Molten Bath
ATC Molten Iron
Atomics Internal ' 1
VI In-Situ
LERC
Gasifier Exit
Temperature
°C
1,200-1,300
900
1,000
650-1,300
1,400
1,100
950
—
Gas Cleanup
Temperature
°C
200
150
100
--
300
--
—
250-350
Pressure
atm
1
10
30
1
15
1
5
2-7
Mass Loading
g/Nm3
40
_ _
—
--
--
—
—
--
gr/SCF
17.5
--
—
—
—
—
—
Control Devices
Used or
Anticipated
two disintegra-
tor or venturi
scrubber in
series
scrubber
scrubber
--
--
--
—
scrubbers
Remarks
MMD l,000ym
a ~ 15
g
bench scale
development
alkali meta:
fumes presei
tars presem
-------
advantage to hot particulate control unless acid gases and
metal vapors also can be controlled at high temperature.
Gas cleanup at low temperature (system pressure) is being
proposed for most coal gasification processes. There are three
basic reasons for this:
1. The energy content of the gas is predominantly chemical
energy rather than sensible heat. Sensible heat accounts
for approximately 10 to 20% of the total energy content
of the gas.
2. High pressure/low temperature gas cleanup equipment is
available commercially although it has not necessarily
been optimized for coal gasification applications.
No hot gas cleanup systems have been demonstrated to
be satisfactory and they must be considered as not
commercially available.
3. Coal gases which contain tars must be cooled in the
tar removal quencher to prevent tars from depositing
in downstream system components.
If the gas can be kept hot enough it is conceivable that
tar condensation can be prevented and tars can be built in the
gas turbine combustor. In this case hot gas cleanup would save
the sensible heat in the gas and the chemical energy in the tars.
Tars can contain as high as 201 of the chemical energy in the
gas leaving the gasifier (MERC, 1978). However, most LBCG pro-
cesses which emit tars propose a quench to remove the tars. Al-
though relying on existing technology, this severely limits the
thermal efficiency of the process. Tar removal is a problem
with fixed bed and slowly moving bed gasifier designs where
the gasifier temperature is too low to crack tars.
If the process temperature is high enough to prevent the
formation of tars (as in many entrained bed and fluidized bed
gasifier designs), it usually is proposed that the gas be cooled
in a heat exchanger to provide process steam for the gasifier or
superheat steam for the steam turbine cycle of the process.
22
-------
Hot gas cleanup has some advantages over cold gas cleanup
for coal gasification processes in terms of the overall thermo-
dynamic efficiency. However, the availability and cost of hot
gas cleanup systems (particulate removal and acid gas removal)
may hinder realization of the higher thermodynamic efficiency
in practice.
In the next section of this report we review the current
developmental status and anticipated problems or limitations
of high temperature and pressure particulate control devices.
In the final section we review the advantages and disadvantages
of hot versus cold gas cleanup for pressurized fluidized bed
combustion and low-BTU coal gasification combined-cycle processes
23
-------
PARTICULATE CONTROL DEVICES
PRIMARY AND SECONDARY COLLECTION
Most proposed high temperature and pressure particulate
control systems use three particle collection stages as illus-
trated in Figure 5. The first stage usually is a large cyclone
which removes coarse particles and recycles them to the combustor
or gasifier. The particles removed in the primary cyclone are
predominantly unreacted carbon.
In the PFBC system these large particles can ignite and
cause fires in the primary cyclone. Fires may cause excessive
temperatures and corrosion in the cyclone. Very sticky particles
may also be present and can eventually cause the cyclone to become
plugged. This problem has been encountered in the fluidized bed
combustion of refuse although it has not been a serious problem
with coal combustion. The PFBC systems use limestone or dolo-
mite as bed sorbent material to react with sulfur from the coal.
The primary cyclone also will collect and recycle large sorbent
particles.
In gasification processes where tar removal is required,
plugging of the primary cyclone may be a problem. In such situ-
ations the primary collection stage is usually a quench tower
which cools the gas, and condenses and removes tars and other
coarse particles.
In processes where tars are not a problem or where hot gas
cleanup is viable, primary cyclones may be used. Because of the
reducing atmosphere and the presence of hydrogen sulfide, materials
requirements will be different than for combustion processes.
The purpose of the secondary collection stage is to remove
particles larger than approximately 20 ym in diameter. This
24
-------
PRIMARY
COLLECTION!
COAL
FEED.
TERTIARY
COLLECTION
TO GAS TURBINE
i
Y
ASH
T
ASH
SECONDARY
COLLECTION
GAS
Figure 5. Three stage hot gas cleanup
25
-------
reduces the particle size and mass loading to the third stage
or tertiary collector. Proposed designs for secondary collectors
use various high efficiency cyclones or multiclones. In some
cases it may be desirable to eliminate the secondary collector
in order to increase the size and loading of particles to the
tertiary collector. An example of this might be a ceramic fiber
baghouse in which large particles and high dust loadings are required in
the formation of a dust cake which is necessary to maintain high
collection efficiency.
Experimental measurements of the fractional collection effi
ciency for the conventional secondary cyclone at the Exxon PFBC
miniplant have been reported by Hoke, et al., (1978). Their
data are plotted in Figure 6.
Beecher, et al. (1976) described the design of a 680 MWe
power plant. Four pressurized fluidized bed boilers supplied
gas to two gas turbines. Each turbine handled 345 kg/s (760
Ib/s) of gas. Each boiler had four primary cyclones, one secon-
dary collector, and four tertiary collectors associated with it.
Each cylone handled about 43 kg/s of gas or approximately 850
m3/min (30,000 ft /min) at 966°C (1,770°F) and 10 atm. A flow
diagram of the cleanup system is shown in Figure 7.
The first stage of particulate removal is a group of four
cyclone collectors housed in a pressure vessel. The solids
collected by these cyclones consist of dolomite fines, coal
ash, and unburnt char. To maximize the combustion efficiency
of the system, these solids are fed to a carbon burn-up cell
operating at approximately 1,000°C. The combustion products
from the burn-up cell mixed with the gases leaving the primary
cyclone and pass on to the secondary collector.
The primary cyclones are made from Haynes Alloy 188 which
is a cobalt-based high temperature, corrosion-resistant super-
alloy. The inner surfaces are lined with a hard refractory. Main-
tenance of these surfaces is anticipated at approximately 2 year
intervals.
26
-------
fO
U
2
100
90
80
70
60
tu
IX,
u 50
u
8
40
30
20
10
0
\
20
30 40 50
Figure 6.
2 3 4 5678 10
PARTICLE SIZE, ym
Fractional efficiency for secondary cyclone (from Hoke, et al., 1978).
-------
MULTICLONE
CARBON
BURNUP
CELL
RECUPERATOR
WATER
BOOSTER
COMPRESSOR
TO
WASTE
TREATMENT
Figure 7. HTP particulate control system for Westinghouse PFBC
design.
28
-------
The second stage collector is a multiclone (multiple cy-
clone) . Approximately 1% of the gas flow is bled off and used
to transport the collected solids. The solids are removed from
the bleed stream by a separate cyclone, then are depressurized,
and deposited in an ash lock-hopper. The bleed gas is cooled in
a recuperator, scrubbed, compressed, reheated and used as the re-
verse cleaning gas for the tertiary collectors (granular bed filters)
The particulate mass loadings and overall efficiencies
for each cleanup stage are listed in Table 4. The estimated
temperature and pressure losses are shown in Table 5. The
estimated gas cleanup costs per gas turbine are listed in Table
6.
Primary and secondary collection equipment are commercially
available although high temperature and pressure applications
are scarce and there is room for substantial improvements in
materials, designs, and engineering models. Also, high tempera-
ture and pressure conditions necessitate the use of expensive
materials and fabrication techniques.
In the design reported by Beecher, et al. (1976), eight
primary cyclones served each gas turbine at an average major
component cost of $117,250 per cyclone (1975 dollars). Each
cyclone handled 850 m'/min (30,000 ACFM) of gas.
For comparison we have used the curves presented by
Neveril, et al. (1978) to estimate the cost of a conventional
cyclone of similar capacity. A 10 gage stainless steel cyclone
handling 850 m3/min would cost approximately $23,000 (1975
dollars). If the materials were 10 gage carbon steel the cost
would be approximately $11,000. It should be noted that the
HTP cyclone handles approximately 2.2 times the mass flow of gas
handled by the conventional cyclone in this comparison.
TERTIARY COLLECTION
The final collection stage must reduce the mass loading
of particles to a level compatible with gas-turbine operating
specifications and environmental standards. This most likely
will require 90 to 99% collection efficiency on a particle
29
-------
TABLE 4. ESTIMATED GAS CLEANING EQUIPMENT PERFORMANCE
(BEECHER ET AL., 1976)
Stage
Primary
Cyclone
Inlet Loading, Outlet Loading, Efficiency
g/g gas g/g gas %
0.0245
0.0005
97.9
Multiclone
Granular
Bed Filter
0.01521
0.00075
0.00075
0.000014*
95.1
98.1
* Equivalent to 16.9 mg/Nm3 (0.0074 gr/SCF) entering the
gas turbine
TABLE 5. ESTIMATED TEMPERATURE AND PRESSURE
LOSSES FOR GAS CLEANUP SYSTEM
(BEECHER, ET AL. 1976)
Description
Primary Cyclone
Multiclone
Granular Bed Filters
Temperature
Drop, °C
2.2
3.1
11.8
Pressure
Drop*, %
0.6
2.0
0.6
* % of gas turbine discharge pressure
30
-------
TABLE 6. ESTIMATED GAS CLEANUP EQUIPMENT COSTS PER GAS TURBINE
BEECHER ET AL., 1976)*
Major Comp.
Material Installation Total
' CycLfleparators $ 938>°°°
1,594,064
_. ccn
34'550
27,900
24,900
21'600
2. Multiclones
3. Multiclone Bleed
Cyclone
4. Multiclone Bleed
Cooler
5. Multiclone Bleed
Scrubbers
6. Multiclone Bleed
Cooler^
462>000 $1,400,000
785,136 2,379,200
0
8'650 43>200
4,500
7,500
32,400
32,400
22>600
7. Multiclone Bleed
Recuperator
8. Multiclone Bleed
Compressor
'
'
'
205,500
10. Granular Bed
r 1 -L L- Ci 5
11. Balance of
Plant Materials
and Installation
SUBTOTAL
36,800
2,452,200
3,680
1,207,800
1,066,800
40,480
3,660,000
1,066,800
$ 5,334,214 $ 3,571,566 $ 8,905,780
*Costs are based on mid-1975 dollars.
31
-------
size distribution which is approximately log-normal with a mass
median diameter of about 4 ym and a geometric standard deviation of 3.
In recent years cyclones and granular bed filters have
received the most attention as potential tertiary collection
devices. To date, neither device has been demonstrated to be
efficient enough to satisfy the emissions regulations or the
turbine specifications. It is apparent that the current state
of the technology is not sufficient and more reliable and effi-
cient cleanup devices are required.
Economic studies of the pressurized fluidized bed combus-
tion process have generally assumed that granular bed filters
in their current state of development will be sufficient, or at
least representative of the tertiary collector costs. Recent
experience at the Exxon miniplant has indicated that granular
bed filters need further development before they can be con-
sidered commercially viable for PFBC applications. The economic
feasibility of pressurized fluidized bed combustion processes
must remain highly speculative until a satisfactory solution to
the hot gas cleanup problem has been demonstrated.
Cyclones
Background
The application of cyclone separators for particulate re-
moval at high temperature and pressure has been considered for
over thirty years. Parent (1946) tested small sampling cyclones
(2 and 3 inch diameter) at temperatures up to 1,400°F and
pressures to 6.8 atm (100 lb/in2). Dust loadings from 0.34
to 6.86 g/Nm3 (0.15 to 3.0 gr/SCF) were also considered. His
results are shown in Figure 8.
Figure 8a shows the decrease in overall efficiency as tem-
perature increases for a constant pressure drop. Figure 8b in-
dicates that an increase in pressure drop from about 10 cm W.C0
to 25 cm W.C. is required to maintain the overall collection
efficiency at 951 when the temperature increases from 24°C to
540°C. Parent 's data also indicate that there was no significant
effect of mass loading on the efficiency for the parameters studied
32
-------
100
0\°
tu
p-
o
w
o
u
OH
0-
PJ
z
o
I— I
H
u
8
90 •
80
i i i
AP = 7.6 cm W.C.
100 200 300 400
TEMPERATURE, °C
500
600
Figure 8a. Efficiency vs. temperature for high temperature cyclone
(from Parent, 1946).
100
90
80
i l l l l
i i I i i
l I I
540°C
l I I l I l i i
24
8 10 12 14 16 18 20 22 24 26
PRESSURE DROP, cm W.C.
Figure 8b. Efficiency vs. pressure drop for high temperature cyclone
(from Parent, 1946) .
33
-------
Yellott and Broadley (1955) studied the efficiency and
pressure drop of cyclones operating at high temperatures. They
also found that efficiency decreased with increasing temperature,
Their study included a 10-inch multiclone for which the frac-
tional efficiency is presented in Table 7. These data are for
fly ash particles at 1 atm and 538°C (1,000°F).
TABLE 7. FRACTIONAL EFFICIENCIES FOR MULTICLONE AT 538°C
(FROM YELLOTT AND BROADLEY, 1955)
Pressure
Drop ,
kPa
0.75
1.00
1.34
Air Flow,
m3/min
21.4
24.1
28.0
Size analysis of dust
Particle Size, ym
0-10
50.3%
55.2
60.6
46.3
10-20
91.51
92.0
92.6
17.8
20-44
97.5%
97.5
97.5
15.8
+ 44
99.0%
99.0
99.0
20.1
Advanced Cyclone Designs
Experience with cyclones and multiclones generally confirms
that these devices are not sufficient either for protecting the
turbine or for satisfying environmental regulations.
Recent data obtained at the Exxon miniplant (Bertrand, et
al., 1978) show unusually high collection efficiency for a ter-
tiary cyclone of conventional design. Efficiencies in excess
of 80% for 1 ym diameter particles were reported. The size dis-
tribution of particles entering the cyclone was not measured
directly and it may be that particles were agglomerating some-
where upstream. These results need to be looked at more care-
fully in order to identify the cause of the high efficiencies
measured.
34
-------
Rotary flow cyclone - A rotary flow cyclone design by
Aerodyne Corporation has the potential for higher collection
efficiency than conventional cyclones or multiclones and has
been proposed as a possible tertiary cleanup device. This
cyclone is illustrated in Figure 9.
In the rotary flow cyclone, the primary flow (to be
cleaned) enters through a set of vanes located at the base
of the unit. A secondary flow is introduced around the circum-
ference of the unit at the top through tangential inlet nozzles.
The secondary flow is approximately 601 of the primary flow.
As particles in the primary flow are forced towards the
wall they are swept downward to the collection hopper by the
secondary flow. The secondary flow may be either clean or dirty
gas.
A commercially available Aerodyne rotary flow cyclone, rated
at 2.3 m3/min (80 CFM))was tested at Westinghouse Research
Laboratories (Ciliberti and Lancaster, 1976). They used a
portion (30 CFM) of the dirty gas stream as the secondary
flow. Their results are presented in Figure 10. Also shown
are their theoretical predictions. Both their theoretical and
experimental performance curves showed lower efficiency than the
manufacturers performance curves which claim 50% efficiency for
approximately 0.5 ym particles.
From the Westinghouse data, it seems unlikely that the
rotary flow cyclone will be able to collect any particles
smaller than about 1 to 2 ym. From particle size data obtained
at the Exxon miniplant, approximately 30% of the mass leaving
the secondary cyclone is smaller than 2 ym. This corresponds
to a mass loading of roughly 0.69 g/Nm3 (0.3 gr/SCF).
Westinghouse plans further tests on the Aerodyne cyclone
at high temperature and pressure. Although it may not be
sufficient as a tertiary collector, it may be a useful alter-
native to multiclones as the secondary collection device.
35
-------
CLEAN GAS
TANGENTIAL
NOZZLES
TURNING VANES
tu
»-H
u
tL.
U.
o
I-H
U
U]
o
u
PRIMARY I
GAS FLOW I —•"
PRIMARY GAS FLOW
UPWARD SPIRAL
(ROTATIONAL FLOW)
SECONDARY GAS FLOW
SECONDARY GAS FLOW
DOWNWARD SPIRAL
(POTENTIAL FLOW)
DUST
COLLECTED DUST
Figure 9. Rotary flow cyclone.
100
90
80
70
60
50
40
30
20
10
0
I i I
PRESENT MODEL
PREVIOUS MODEL
_°~° EXPERIMENTAL
Ib/ft-s-
ft
lb/ft;
CFM
50 CFM
i
1
PARTICLE DIAMETER, ym
Figure 10. Performance of rotary flow cyclone (from Ciliberti
and Lancaster, 1976).
36
-------
Klett, et al. (1977) reported on a high temperature and
pressure design for the Aerodyne cyclone in which the inlet
gas is split to provide both primary and secondary flows. They
predicted outlet loadings from this cyclone based on the manu-
facturers performance curves and available data on emissions
from PFBC processes. Their predictions are presented in
Table 8. Even if this level of performance is attained the
emissions will exceed the anticipated emissions regulations
(approximately 57.2 mg/Nm3 or 0.025 gr/SCF for a PFBC process).
TABLE 8. ESTIMATED PERFORMANCE OF AERODYNE
CYCLONE (from Klett, et al. 1977)
Particle Diameter
ym
0-2
2-3
3-4
4-5
5-6
6-10
10-20
+ 20
TOTAL
Inlet
mg/Nm
460.0
153.3
153.3
153.3
153.3
460.0
766.6
13,032
15,332
Loading,
3 (gr/SCF)
(0.201)
(0.067)
(0.067)
(0.067)
(0.067)
(0.201)
(0.335)
(5.695).
(6.700)
Outlet
mg/Nm3
96.1
22.9
13.7
9.2
6.9
4.6
0.0
0.0
153.4
Loading,
(gr/SCF)
(0.042)
(0.010)
(0.006)
(0.004)
(0.003)
(0.002)
(0.000)
(0.000)
(0.067)
Efficiency,
%
79
85
91
94
96
99
99.9
100
99.0
Cyclocentrifuge - Mechanical Technology, Inc. is developing
a gas cleanup device which they call a "cyclocentrifuge"
(McCabe, 1977). The basic design of this device is illustrated
in Figure 11. It is a hybrid device using a rotating assembly
37
-------
SHAFT
CLEAN GAS OUTLET
BEARING ASSEMBLY
DIRTY GAS INLET
CENTRIFUGE SHELL
SWIRL AUGMENTATION
BLADES
CYCLONE SHELL
DRIVE TURBINE
ASSEMBLY
JOURNAL BEARING
TO HOPPER
Figure 11. Cyclocentrifuge (from McCabe, 1977)
38
-------
called the "centrifuge" and a stationary assembly called the
"cyclone". The centrifuge is driven by a high reaction axial
turbine driven by energy extracted from the process gas.
The predicted fractional efficiency for this device was
presented by McCabe and is shown as Figure 12. The design
parameters are listed below.
• Volume flow 3,457 Nm3/min (125,260 SCFM)
• Inlet pressure 17 atm (250 lb/in2)
• Inlet temperature 538°C (1,000°F)
• Pressure drop 1 atm (14.6 lb/in2)
The explanation given for the improved efficiency as com-
pared to a conventional cyclone is as follows:
• The length from the inlet duct to the centrifuge is
longer than in a conventional cyclone.
• The centrifuge collects particles that are reentrained
from the cyclone section.
• The centrifugal force in the centrifuge is larger than
in a cyclone.
• Agglomeration will be increased by the increased cen-
trifugal force field.
If the cyclocentrifuge works as predictedsthe outlet load-
ing still may exceed the environmental regulations, depending
on the mass loading of particles smaller than 2 ym.
One of the most attractive aspects of cyclones for dust
removal is that they have no moving parts. This is not true
for the cyclocentrifuge. Operation of the centrifuge bearings
at high temperature and pressure in a dirty environment is
likely to present the most difficult development problem with
this device.
Assuming the predicted performance can be achieved and
mechanical problems are not severe, the economic analysis pre-
sented by McCabe indicates that the cyclocentrifuge is economi-
cally viable for low-BTU coal gasification processes.
39
-------
CONVENTIONAL CYCLONE
4 56789
PARTICLE DIAMETER, ym
10 11
12
Figure 12. Comparison between estimated performance of
cyclocentrifuge and conventional cyclone
(from McCabe, 1977).
40
-------
Granular Bed Filters
Granular bed filter technology has recently been reviewed
by Yung, et al. (1977a, 1977b, 1978) to assess the state-of-the-
art and to evaluate the feasibility of granular bed filters for
high temperature and pressure applications.
Granular bed filters may be defined as any filtration
system comprised of a stationary or slowly moving bed of discrete,
relatively closely packed granules as the filtration medium.
With respect to motion of the granules, granular bed filters
may be classified as continuously moving, intermittently moving,
and fixed bed filters.
Moving Bed Filters
The continuously moving bed filter is usually arranged in
a cross-flow configuration. The bed is a vertical layer of
granular material held in place by louvered walls. The gas
passes horizontally through the granular layer while the granules
and collected dust move continously downward and are removed
from the bottom. The dust is separated from the granules by
mechanical vibration. The cleaned granules are then returned
to the overhead hopper and the panel by a granule recirculation
system.
The Combustion Power Company's dry scrubber is an example
of a continuously moving bed filter. The system is shown in
Figure 13. The granular bed material flows downward between
two concentric cylinders. The gas passes through the bed and
is filtered by the granules. The granules are recycled pneu-
matically and the collected dust particles are disengaged from
the granules and sent to a conventional baghouse.
The performance of this device has been reported by Wade,
et al. (1978). They conducted extensive cold flow tests to in-
vestigate the effects of bed depth,granule diameter, amd other
parameters on the collection efficiency. Test parameters for
the nominal, thick bed, and small collector granule configurations are listed
in Table 9. Particulate loadings ranged from 0.46 to 4.6 g/Nm3
41
-------
MEDIA CLEANING AIR
FILTERED AIR
GRANULAR
BED
MEDIA INJECTION AIR
TO BAGHOUSE
INLET
OUTLET SCREEN
TRANSPORT PIPE
MEDIA TRANSPORT AIR
Figure 13. Cold flow granular bed filter parameters.
42
-------
TABLE 9. TEST PARAMETERS FOR CPC MOVING BED FILTER
(FROM WADE ET AL., 1978)
Mass Median
Particle
Configuration
Nominal
Thick Bed
Bed Depth,
mm
203.2
406.4
Medium Diameter,
mm
2.0
2.0
Diameter,
Vim
3.2
2.6
Small Granules
203.2
0.8
7.0*
* Correlation was not materially influenced by deletion of
four data points resulting in an average median diameter
of 2.5 ym
43
-------
(0.2 to 2.0 gr/SCF). The superficial gas velocity was varied
from 20 to 80 cm/s (40 to 160 ft/min). The medium flow rate was
varied from 0.4 to 1.6 kg granules/kg air. Pressure drop ranged
from 1.2 to 5.7 kPa (5 to 23 in.W.C.).
Fractional efficiency curves are shown in Figures 14a, b,
c. The overall penetration (and efficiency) are correlated with
the pressure drop function,0, in Figures 15a, b, c. The pressure
drop function was defined in English units as:
AP L.
e - —^ (i)
V M
where AP = pressure drop, in.W.C.
L. = mass loading of dust, gr/SCF
V = superficial velocity, ft/min
M = media rate, Ib granules/lb air
In general the CPC moving bed filter was found to be capa-
ble of particulate removal efficiencies in excess of 98% for
particles in the 1 to lOymA diameter range. Submicron particles
were collected at an efficiency in excess of 90% in cases with
high velocities, high loadings, and low granule rates. Beds with
larger thickness to granule diameter ratios were most effective
in the capture and retention of particles in the 2 to 5 ymA dia-
meter range. Also, intermittent granule movement was shown to
improve efficiency by a few percent. However, the economics of
this operational technique have not been analyzed.
High temperature tests of the moving bed filter are planned.
No high temperature data are available at this time.
The major advantage of the moving bed filter design is that
the bed granules are removed and cleaned out of the primary gas
stream. This enables efficient cleaning and a relatively steady
collection efficiency. Also it is not necessary to isolate filter
units during cleaning so that the total filter area open to gas
flow is available for filtration at any time.
44
-------
0\°
U
2
u.
UH
o
u
100
a 90
80
u
m
70
12 4 6 8 10 12 14
AERODYNAMIC PARTICLE DIAMETER, ym
16
Figure 14a. Fractional collection efficiency, nominal configuration.
100
0\°
u
I—I
0-
U-,
tu
§
I—I
H
u
tu
»J
o
u
90
80 U
12 4 6 8 10 12 14 16
AERODYNAMIC PARTICLE DIAMETER, ym
Figure 14b. Influence of operational parameter combinations
on fraction efficiency (16" filter, 2mm media).
45
-------
u
u
h- 1
UH
B-
U4
100
90
80
70
4 6 8 10 12
AERODYNAMIC DIAMETER, umA
14
16
Figure 14c. Fractional efficiency performance, small media
configuration.
10
-1
g 10
o
cfl
£ io-2
10
_3
10
-2
cor
10
-1
0.5
0.9
0.95
§
•H
4->
U
rt
u
0.99
0.995
10
Figure 15a. Influence of pressure drop function on overall
collection efficiency, nominal configuration.
46
-------
10
o
o
oj
10
-1
£ io-2
10"
_ 3
o;s
0.9 u
0.95 g
hH
U.
W
0.99
0.995
10
-2
10
- 1
10
0
Figure 15b. Influence of pressure drop function on
overall collection efficiency, thick bed
configuration.
g 0.10
O
cU
(H
0.01
g
I-H
U
tu
W
0.01
0.1
1.0
e
Figure 15c. Influence of pressure drop function on
overall collection efficiency, small media.
47
-------
The moving bed design also has some limiting operating
characteristics. The granule recirculation system adds signifi-
cantly to the operating cost. Particle reentrainment caused
by the relative motion of the granules limits the granule flow
rate and affects the overall collection efficiency. Erosion of
the retaining grids, louvers, and transport system components
may be a problem, especially in high temperature and pressure
systems. The collected dust particles cannot form a filter cake
so that the operating efficiency will be essentially that of a
clean bed. Temperature losses may be large and will be propor-
tional to the energy required to keep the granules hot during
recirculation.
It may be possible to resolve most of these problems
through further development and testing. Performance data at ,
high temperatures and pressures will be important in identifying
the most serious operational problems.
Intermittently Moving Bed
In the late 1950s, Squires modified the continuously moving
bed design to obtain a fixed bed device with an intermittent
movement of granular solids. The bed is stationary during fil-
tration. The accumulated filter cake and the surface layer of
granules are ejected from the panel by a sharp backwash pulse and
fall to the bottom of the filter vessel. The expelled granules
are immediately replaced by downward movement of fresh granules
from the overhead hoppers.
Intermittent movement is normally limited to vertical panel
filters. The granules are intermittently removed in a cross.-flow
arrangement panel bed. The advantage of this type of bed struc-
ture is the capability for external granule/dust separation with
minimum disturbance to the rooting cake. A rooting cake is the
foundation for the formation of a surface cake. After cleaning,
the surface cake is formed readily without disturbing the rooting
cake and filtration efficiencies can be much higher.
The intermittently moving bed also has the advantage that
granule cleaning is off-line and potentially more effective.
48
-------
The major disadvantage is that the gas capacity is lower
than for other granular bed filter designs and this results in
high capital costs for a given installation. During cleaning,
about two to three layers of granules are removed from the bed.
To prevent the dust from being carried deep into the bed by the
gas, CCNY recommends the velocity be kept as low as possible
to reduce the aerodynamic drag force- They usually operate the
panel bed filter at about 15 cm/s (30 ft/min). This velocity
is about one third the velocity used in the fixed bed and con-
tinuously moving bed GBFs. Thus, more filtration area is required.
Bed plugging also can be a problem if the surface cake is
not formed properly. Erosion of the retaining grids, louvers,
and other components may be a problem. Granule recirculation
temperature losses, and the requirement for blow-back air pulses
during cleaning add to the overall operating costs.
Recent work on the CCNY panel bed was reported by Lee,
et al. (1977).
Fixed Bed Filters
Fixed bed filters operate in two modes; the filtration
mode and the cleaning mode. During filtration the bed is sta-
tionary. The gas passes through the bed and collected
particles are deposited within the bed and on the bed surface.
During cleaning the bed is isolated from the main flow and
agitated mechanically or pneumatically by a reverse flow of gas.
There are two fixed bed devices currently being developed;
the Rexnord gravel bed filter and the Ducon granular bed filter.
The Rexnord filter (Figure 16) uses a rake-shaped stirring
device to agitate the bed during cleaning. This loosens the
filter cake which is then removed by a reverse flow of clean air.
Rexnord granular bed filter - No Rexnord filters have been
tested at high temperature and pressure, however, McCain (1976)
reported the results of a performance test on a Rexnord filter
used to control the emissions from a clinker cooler in a Portland
cement plant.
49
-------
OPERATING PHASE
BACKFLUSH PHASE
12
14
BACKFLUSH
AIR
1. INLET CHAMBER
2. PRIMARY COLLECTOR (CYCLONE)
3. DOUBLE TIPPING GATE (DUST DISCHARGE)
4. VORTEX TUBE
5. FILTER CHAMBER
6. GRAVEL BED
7. SCREEN SUPPORT FOR BED
8. CLEAN GAS COLLECTION CHAMBER
9. EXHAUST PORT
10. BACKWASH CONTROL VALVE
11. BACKWASH AIR INLET
12. VALVE CYLINDER
13. STIRRING RAKE
14. STIRRING RAKE MOTOR/REDUCERS
Figure 16. Rexnord gravel bed filter.
50
-------
Samples were taken simultaneously at the filter inlet and
outlet with cascade impactors. The operating conditions of
the gravel bed were:
Gravel diameter = 4 mm
Face velocity = 73 cm/s
Gas temperature = 174°C
Pressure drop = 25.4 cm W.C.
The grade penetration curves for these tests are shown
in Figure 17. The predicted curve was obtained using the model
presented by Yung, et al. (1977b) which assumes the bed is clean
and particulate collection results from the inertial impaction
mechanism only.
Ducon granular bed filters - The Ducon granular bed filter
cleans the bed by a reverse flow of gas which fluidizes the bed
and elutriates the fine collected particles. The filtration
and cleaning modes of the Ducon filter are illustrated in Figure
18.
The Ducon filter was tested on the effluent from a fluid
bed catalytic cracking unit regenerator at an oil refinery (Kalen
and Zenz, 1973). The gas was at 370°C to 480°C and 1 to 1.5 atm.
The dust loading ranged from 0.34 to 1.94 g/m3 (0.15 to 0.85 gr/
ACF) . A collection efficiency of 85-98% was obtained on dust
with a mass median diameter of 35 ym and a geometric standard
deviation of about 4. Yung, et al. (1978) estimated fractional
penetrations from their data and the results are shown in Figure
19.
A high temperature and pressure design of the Ducon filter
was tested at the Exxon miniplant (Hoke, et al., 1978). Ini-
tially severe plugging of the bed retaining grids was encountered,
This problem was resolved by eliminating the grids and redesign-
ing the bed housing to provide sufficient freeboard above the
bed to allow cleaning of the bed without loss of bed material.
The modified filter module is illustrated in Figure 20.
A number of operating problems were encountered during the
51
-------
1.0
0.5
o
I
o
DJ
o.i
tu
_J
y 0.05
(X
0.01
0.1
DATA
J I I I
THEORY
0.5 1.0
AERODYNAMIC PARTICLE DIAMETER, ymA
10
Figure 17. Experimental grade efficiency curve of a Rexnord gravel bed
filter (McCain, 1976).
52
-------
FILTRATION CYCLE
BLOWBACK CYCLE
FILTER
MEDIA
CLEAN
GAS EXIT
DIRTY GAS
RETAINING SCREEN
^
7
*,v>
^^m
m '"*.
-"'••:•
*2
***/"'
^
1
1
^
f
':•'."
*^ •
•^1:
.'-V
^»*.-.
^
'•%'•
&
m
\
FLY ASH
f-
*~ DIRTY GAS
FLUIDIZED
FILTER MEDIA
LOCK
HOPPER
LOCK
HOPPER
Figure 18. Ducon granular bed filter.
53
-------
1.0
0.05
u
P-.
oi
w
z
tu
0.1
0.05
0.01
I I I i i
0.5
5 10
PARTICLE DIAMETER, ym
I I
J I i
15
Figure 19. Fractional penetration for Ducon granular bed (from
Kalen and Zenz, 1973).
54
-------
DIRTY GAS
TOP RETAINING
SCREEN
BOTTOM RETAINING
SCREEN
FILTER
MEDIUM
CLEAN
GAS
Figure 20a. Schematic o£ a single Exxon filter bed.
DIRTY GAS
INLET
-*- CLEAN GAS
OUTLET
FLUIDIZING GRID
I i
Figure 20b. Modified filter bed.
55
-------
Exxon tests of the modified Ducon filter. The lowest demon-
strated particulate outlet concentration was 68.6 mg/Nm3 (0.03
gr/SCF) which was considered to be too large to protect a gas
turbine and borderline for meeting current emissions regulations,
The use of smaller filter media could be expected to improve
efficiency. However, at times the filtration efficiency was
very poor and the outlet particulate concentrations were as high
as 700 to 1,200 mg/Nm3 (0.3 to 0.5 gr/SCF). It was also ob-
served that the efficiency decreased with time in the longer
runs, dropping from 90% initially to about 50% later in the run.
Loss of filter medium during blow back was another recurring
problem. Further attempts were made to use 50 mesh retaining
screens but they failed because of plugging. Additional tests made
with 10 mesh screens also resulted in significant screen plug-
ging.
A large buildup of particles in the filter beds
also was observed amounting to about 30% of the weight of the
filter medium. A possible steady long term increase in filter
pressure drop may result because of this. However, no signifi-
cant increase in filter pressure drop was noted during any of
the shakedown runs.
It was observed that the particles were not only
building up in the beds, but were uniformly mixed with the fil-
ter medium. It is possible that the buildup and mixing of parti-
cles in the bed could be responsible for the increase in the
particle concentration in the outlet gas with time.
Another potential problem with the current design was its
vulnerability to upsets. When upsets occurred, such as bed plugging
or loss of filter medium, the operating problems caused by such
upsets required shutdown of the system. Another problem which
may be unique to the miniplant was the interaction of the granu-
lar bed filter with the rest of the FBC system during the blow
back cycle. An increase in system pressure was noted during
blow back resulting in problems with the coal feed system
56
-------
which is controlled by the differential pressure between the
coal feed vessel and combustor. This required modifications
to the coal feed control system to minimize these effects.
Granular bed filter performance data for all runs through
November, 1977 are listed in Table 10. The efficiencies are
based on an inlet concentration of 2.3 g/Nm3 (1.0 gr/SCF) which
is the average for the emissions from the secondary cyclone.
Fractional efficiency data are presented in Figure 21.
The granular bed filter test program was suspended in
November, 1977. In all runs in which more than one outlet con-
centration was measured, it was observed that the outlet con-
centration increased with time. They were not able to demon-
strate that the current EPA emission standard (0.1 lb/106 BTU
or 0.05 gr/SCF) could be met for more than a few hours of opera-
tion. In no run was the anticipated new standard (0.05 lb/106
BTU or approximately 0.025 gr/SCF) satisfied.
Further tests are planned on a 0.85 m3/min (30 ACFM) slip
stream from the miniplant combustor. The slipstream will be at
870°C and 9 atm pressure. If the filter can be modified to
satisfy the EPA regulations for a prolonged run, it will
be tested on the full miniplant flow stream.
Summary
At this time granular bed filters have not been demonstra-
ted to be efficient enough to perform as tertiary collectors
in high temperature and pressure gas cleanup systems.
High collection efficiencies may be obtained if a filter
cake is formed on the surface of the bed. This has been accom-
plished with the CCNY (Squires) design at low temperature and
pressure operating conditions. Efficiencies in excess of 99.9%
have been measured. However, when this filter was tested at the
Morgantown Energy Research Center at 1,000°F, no filter cake
was formed, although 99% efficiency was obtained.
Operation of the Ducon granular bed filter at the Exxon
miniplant also indicated that no filter cake was formed.
57
-------
TABLE 10. GRANULAR BED FILTER PERFORMANCE
(FROM BERTRAND ET AL., 1977)
Outlet Concentration
Run Number
54
57
59
59
59
61
62.
62.
63
63
63
64
64
64
65
65
66
gr/SCF
0.
69
0.04-0.08
(Sample
(Sample
(Sample
1
3
(Sample
(Sample
(Sample
(Sample
(Sample
(Sample
(Sample
(Sample
1)
2)
3)
1)
2)
3)
1)
2)
3)
1)
2)
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
08
28
54
46
03
21
05
07
12
28
29
27
05
06
06
g/m3
1.
Collection Efficii
57
0.09-0.18
0.
0.
1.
1.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
18
64
23
05
07
48
11
16
27
64
66
61
11
14
14
31.
0
92.0-96.0
92.
72.
46.
54.
97.
79.
95.
93.
88.
72.
71.
73.
95.
94.
94.
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
* Based on a 2.3 g/Nm3(1.0 gr/SCF) inlet concentration
58
-------
en
100
96
w 92
o
3 88
u
Si 84
80
I
I
I
6 8 10 12
PARTICLE DIAMETER, ym
14
16
18
Figure 21. Fractional efficiency data for Ducon GBF (from Bertrand, et al.)
1977).
-------
Therefore, the current state of development of granular bed
filters for high temperature applications seems to be pre-
dominantly limited to clean bed filtration.
The model developed by Yung, et al. (1977b, 1978) can be
used to predict performance for clean bed granular bed filters.
This has been done using typical operating parameters from the
Exxon tests, as listed below.
Granule diameter = 400 ym
Bed depth = 3.8 cm
Superficial velocity = 45 cm/s
Particle mass median diameter = 3.5 ym
Particle density = 1.5 g/cm3
Bed porosity = 0.25
The results are presented as Figure 22. The collection
efficiency decreases (penetration increases) radically as the
gas conditions go from ambient to high temperature and pressure
This effect should be remembered when interpreting low tempera-
ture and pressure performance data for granular bed filters.
There are many operational problems and uncertainties
which need to be resolved before HTP granular bed filters can
be considered sufficiently reliable for commercial application.
These problems include the needs to:
1. Prevent particle seepage through the bed (during
cleaning or filtration.
2. Reduce temperature losses (especially during
cleaning).
3. Improve the efficiency and reduce the cost of
granule regeneration and recirculation.
4. Prevent attrition of granules causing particle
reentrainment.
5. Prevent sintering of granules.
6. Prevent plugging of retaining grids.
7. Reduce pressure drop across the bed.
60
-------
1.0
0.5
o
O
0.1
jg
g 0.05
0.01
10 atm
20°C
I I I i
GRANULE DIAMETER: 400 ym
BED DEPTH: 3.8 cm
SUPERFICIAL VELOCITY = 45 cm/S~
PARTICLE DENSITY =1.5 g/cm3 _
POROSITY =0.25
10 atm
870°C
1 apn
870 C
i i
i i i i i i
0.1 0.5 1.0
PARTICLE DIAMETER, ym
Figure 22. Predicted GBF performance.
10
61
-------
Resolving these problems may provide a solution to the HTP
particle collection problem, and will improve granular bed
filter technology for many other applications, especially where
hot, corrosive gases are encountered.
Scrubbers
Wet scrubbers are not generally suitable for high tempera-
ture and pressure gas cleaning because they necessarily cool
down the gas. It is possible to cool the gas in a regenerative
heat exchanger and then use a wet scrubber to clean the gas at
high pressure before the gas is reheated. Wet scrubbers also
are used for tar removal, H2S removal, and particulate removal
in many coal gasification process designs.
Dry scrubbers and molten glass or metal salt scrubbers are
being developed for hot gas cleanup applications. These systems
are described and available data are reported below.
A.P.T. Dry Scrubber
A.P.T., Inc. is developing a dry scrubber system, called
the PxP (particle collection by particles) system which can
be used for high temperature and pressure gas cleaning. This
system has been reported by Calvert et al.(1977) and Patterson, et al.(1978).
The PxP system is somewhat similar to a venturi scrubber sys-
tem in that it uses relatively large particles as collection
centers for the fine particles in the gas stream. The principal
advantage with this system is that it maximizes the collection
efficiency of individual collector particles and thereby reduces
the number of collectors that need to be cleaned and recycled.
The collector particles introduced to the gas stream
collect fine particles by mechanisms such as diffusion, inertial
impaction, interception and electrophoresis. The larger size
of the collector particles allows easy separation from the gas
stream by methods such as cyclones and gravitational settling.
Figure 23 is a functional diagram of the process steps
for a representative PxP system. The functions represented on
this diagram could occur concurrently or separately in several
62
-------
CONTACTOR
SEPARATOR
DUSTY GAS
COLLECTOR
PARTICLES
CLEAN GAS
DISCARD
+- DISCARD
COLLECTOR
CLEANER
Figure 23. Schematic diagram of A.P.T. dry scrubber system.
63
-------
types of equipment.
The first step involves introducing the collectors to
the gas stream. This process can involve pneumatic or mechanical
injection. The second stage involves contacting the collectors
with the gas in such a way as to encourage the movement of the
fine particles towards the collectors. A venturi device can be
used for the contactor in which case the system would be analogous
to a venturi scrubber with solid collectors used instead of
liquid drops.
The next process step is to remove the collector particles
after they have captured the fine particles initially present in the
gas. This is accomplished by using the large size and mass of
the collector particles to separate them from the gas. A cyclone
separator, gravity settler, or virtual impactor could be used
for this step. Two streams are shown leaving the separator:
the cleaned gas leaves the process at this point and the second
stream represents the flow of collector particles to the next
step. The final process involves either discarding the collector
particles or cleaning them for recycle and disposing of the
particulate matter collected from the gas stream.
The particle collection efficiency and pressure drop for
an A.P.T. dry scrubber with cocurrent flow can be predicted
with the same relationships that define cocurrent wet scrubber
performance. The theoretical performance of the PxP scrubber
has been determined based on the venturi scrubber performance
model of Yung, et al. (1977). Figure 24 shows the predicted
efficiency at 20°C and 820°C. Efficiencies will be somewhat
lower at high pressure for the same pressure drop.
Experimental work has been done by A.P.T. to determine
fine particle collection efficiency in a PxP scrubber in order
to confirm the predictions obtained from available mathematical
models. A dibutylphthalate (DBP) aerosol was used in collection
efficiency experiments with 100 ym mean diameter sand as collec-
tor particles. The DBP aerosol had a mass median aerodynamic
diameter of 1.3ymA and a geometric standard deviation of 2.0.
64
-------
1.0
0.05
Z
O
O
cu
w
Cu
0.1
0.005
0.001
0.0005
0.001
820°C
0.1 0.2 0.5 1.0 2 5
PARTICLE AERODYNAMIC DIAMETER, ymA
10
Figure 24. Predicted performance for A.P.T. dry scrubber.
65
-------
The resulting penetration data are shown in Figure 25.
The prediction is also shown in Figure 25 and compares well with
the experimental curve. Higher collection efficiencies can be
achieved using denser collector particles. For this reason
experiments were also conducted with 125 ym nickel beads.
Particle penetration data for all runs with nickel and sand
collectors are represented in Figure 26 in terms of the 50%
cut diameter as a function of gas pressure drop. The line
represents the best available relationship for industrial
scale wet venturi scrubbers. Therefore the A.P.T. dry scrubber
follows the same primary collection efficiency/power relation-
ship as venturi-type wet scrubbers.
The overall efficiency of the PxP system will depend on
the reentrainment characteristics of the specific system confi-
guration in addition to the primary collection efficiency.
Particle and collector properties, system geometry, flow rates,
and other parameters will influence reentrainment.
A.P.T. has built an atmospheric fluidized bed coal combus-
tor which will be used for testing a pilot plant PxP system at high tem-
perature. Electrostatic augmentation is also being investigated
as a means of increasing the collection efficiency independently
of pressure drop, and possibly improving the adhesion of fine
particles to collector particles. The economics of the PxP
system for high temperature and pressure particulate control
will be analyzed in connection with the pilot plant test program.
Molten Salt Scrubber
Battelle-Northwest has constructed a process demonstration
unit for the molten salt scrubber. The scrubber is designed to
remove H2S as well as particulates from the high temperature
and pressure gases produced in low-BTU coal gasification pro-
cesses. Minor impurities such as halogens, volatile metals
and non-metals, and ammonia also potentially could be removed
from the gas with this system.
Details of the system design and pilot plant data were
presented by Moore, et al. (1977). The system consists of a
66
-------
1.0
o
t-H
E~
U
I-H
H
W
O-
0.1
0.01
\
\
I
THEORETICAL
EXPERIMENTAL
0.1 0.5 1.0 2.0
AERODYNAMIC DIAMETER, ymA
5.0
Figure 25. Comparison of experimental with theoretical
particle collection characteritics of the A.P.T
dry scrubber.
67
-------
oi
tu
H
Q
U
Q
§
UJ
3.0
2.0
1.0
0.7
0.5
0.3
0.2
0.1
I r
I I
HORIZONTAL Q
VERTICAL FLOW Q
THEORETICAL
I I I I I
0.1 0.2 0.3 0.5 0.7 1 23
GAS PHASE PRESSURE DROP, kPa
I I
7 10
Figure 26. Comparison of particle characteristics of the A.P.T. dry
scrubber with the A.P.T. cut/power relationship.
68
-------
venturi scrubber followed by a packed tower entrainment separator.
The venturi is operated vertically to avoid the need for a mechan-
ical pump to feed the molten salt.
The salt composition used is shown in Table 11. The parti-
culate removal efficiency was measured using an Andersen impactor.
The particle size distribution and mass loadings leaving the
scrubber are shown in Tables 12 and 13.
Poe, et al. (1977) evaluated the molten scrubber process
for particulate control. They pointed out numerous potential
problems including material corrosion, alkali metal vapor emis-
sions, line clogging due to precipitation of metal oxides, par-
ticulate buildup in the molten salt and particulate solubility
in the molten salt.
Their analysis indicated that, from an economic standpoint,
molten scrubbing appears to be a promising approach for high
temperature fine particle collection. They did not consider the
effect of gas density on the scrubber performance at high pres-
sure. Theoretical considerations based on the venturi scrubber
model of Yung, et al. (1977) indicate that high temperature and
high pressure operation may be less favorable in terms of the
pressure drop required to achieve a desired efficiency.
Hot Gas Cleanup by Molten Glass
The General Electric Company is investigating the use of
coal slag based glasses for hot gas cleanup. Preliminary work
has been presented by McCreight, et al. (1977) and Fedarko, et al. (1978).
Figure 27 shows viscosity curves for typical coal slags
as a function of temperature. Particulate collection studies
are being carried out by inertial impingement onto plates coated
with glycerine. In the temperature range -20 to 30°C glycerine
has a similar viscosity to that of coal slag in the range 1,200
to 1,600°C.
The work is in a very early stage and there are no data
available to indicate the potential particle collection efficiencies
and operational problems that can be expected for large scale
molten glass scrubbing equipment.
69
-------
TABLE 11. SALT COMPOSITION USED IN PDU DEMONSTRATION RUNS
(FROM MOORE, ET AL., 1977)
Component
Li2C03
Na2C03
K2C03
CaC03
Mole %
18.0
37.3
29.6
15.1
Weight %
13.0
36.0
37.3
13.8
TABLE 12. PARTICLE SIZE DATA; MOLTEN SALT PILOT PLANT RUN NUMBER 3
(FROM MOORE, ET AL., 1977)
Inlet Gas; Total Particle Burden = 0.109 gr/SCF, % H20 =9.1
Outlet Gas; Total Particle Burden = 0.077 gr/SCF, % H20 = 4.5
Anderson Head Particle Size Distribution
Plate
No.
1
2
3
4
5
6
7
8
Filter
Total
Wt.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
Calculated
Gain Effective
g Cut-off Dia, ym %
0048
0042
0327
0498
0405
0381
0289
0314
0948
3253
17.
11.
7.
5.
3.
1.
1.
0.
—
6
2
5
3
5
7
0
7
1.
1.
10.
15.
12.
11.
8.
9.
29.
5
3
1
3
5
7
9
7
2
Concentration in Gas
gr/SCF
0.
0.
0.
0.
0.
0.
0.
0.
0.
00114
00099
00774
0118
0096
0091
0069
0075
0224
0.
0.
00
0.
0.
0.
0.
0.
0.
g/Nnr'
0026
0023
0176
0270
0220
0208
0158
0172
0513
70
-------
TABLE 13. PARTICLE SIZE DATA; MOLTEN SALE PILOT PLANT RUN NUMBER 6
(FROM MOORE, ET AL., 1977)
Inlet Gas; Total Particle Burden = 0.074gr/SCF, % H20 = 11.2
Outlet Gas; Total Particle Burden = 0.024 gr/SCF, % H20 =3.3
Anderson Head Particle Size Distribution
Plate
No.
1
2
3
4
5
6
7
8
Filter
Total
Wt. Gain
g
0.0015
0.0013
0.0016
0.0019
0.0026
0.0029
0.0030
0.0047
0.0106
0.0301
Calculated
Effective
Cut-off Dia, ym
20.1
10.5
7.0
5.0
3.1
1.6
1.0
0.6
--
%
5.0
4.3
5.3
6.3
8.6
9.6
10.0
15.6
35.2
Concentration
gr/SCF
0.0012
0.0011
0.0013
0.0015
0.0021
0.0021
0.0024
0.0038
0.0086
in Gas
g/Nm3
0.0027
0,0025
0.0030
0.0034
0.0048
0.0048
0.0055
0.0087
0.0197
71
-------
pq
en
i—i
O
o.
8
AVERAGE MONTANA ROSEBUD
SLAG (K-875)
TYPICAL AVG.
SLAG (K-884)
AVERAGE ILLINOIS
NO. 6 SLAG (K-878)
1200
1300
1400
1500
1600
1700
TEMPERATURE, °C
Figure 27. Viscosity/temperature relationship determined by NBS on synthetic
slags formulated to represent the average compositions of fly ash
from Montana Rosebud and Illinois No. 6 coal types.
-------
Electrostatic Precipitation
The normal operating temperature range for industrial
electrostatic precipitators is 20 to 300°C. In a few special
applications they are used at temperatures up to about 550°C.
The application of electrostatic precipitation at higher tem-
peratures and pressures has been considered in many studies
over the past 30 years.
Roller and Fremont (1950) studied the corona characteristics
of precipitators at temperatures to 500°C and pressures to 5
atm. Thomas and Wong (1958) conducted a similar study to 800°C
and 8 atm. Both studies showed that the current-voltage charac-
teristics were predominantly a function of gas density rather than
temperature and pressure independently. Thomas and Wong exper-
ienced regions of instability at temperatures above approximately
800°C at atmospheric pressure. At 815°C and pressures above 4.6
atm (relative density >_ 1.25) adequate electric field can be
maintained for effective electrostatic precipitation.
Shale and Faaching (1969) reported on the operation of a
high temperature (800°C) and pressure (6.4 atm) electrostatic
precipitator. Dust removal efficiency ranged from 91 to 96%
for negative corona with a voltage of 36.5 kV and power input
averaging 6.4 kW. For positive corona the efficiency was only
75 to 77% even though a higher operating voltage was possible
(54 kV) at a lower power input (2.0kW). The particulate matter
had a mass median diameter of approximately 30 to 40 ym. There
was no significant mass below 5 ym.
The input power-removal efficiency relationships at high
temperature (800°C) compared favorably with those found in con-
ventional industrial precipitation. No apparent insurmountable
electronic problems were encountered, however, thermal misalignj
ment of the tube and mechanical difficulties with the tube
rapper presented problems.
Brown and Walker (1971) operated an electrostatic precipi-
tator at about 900°C and 4.4 atm. Particle removal efficiencies
ranged from 25% for positive polarity to 87% for negative polarity,
Test data are summarized in Table 14.
73
-------
TABLE 14. TEST DATA SUMMARY FOR ESP AT 900°C,
4.4 ATM (FROM BROWN S WALKER, 1971)
Run
No.
1
2
3
4
5
6
Polarity
Negative
Positive
Negative
Positive
Negative
Negative
Applied
Voltage,
kV
29
47.5
34
25.5
42
44.5
Current ,
mA
1.9
4.6
3.7
2.4
3.5
11.0
Field
Strength,
kV/cm
2.9
4.6
3.3
2.5
4.1
4.4
Power,
W
55
230
126
61
147
490
Efficiency,
%
37.7
25.0
59.5
16.0
73.5
78.1
Effective
Migration
Velocity,
cm/s
1.8
0.9
3.4
0.6
5.2
5.8
74
-------
The effective migration velocity for negative polarity
averaged about 4.0 cm/s. This is about half that ex-
pected for a conventional precipitator operating at a similar
field strength and low temperature. The lower migration velocity
can be accounted for adequately by considering the effect of high
temperature on the gas viscosity.
More recent work on high temperature and pressure electro-
static precipitation has been reported by Feldman, et al. (1977,
1978) and by Robinson (1978). They reported work conducted at
Research-Cottrell which demonstrated that stable operating condi-
tions can be maintained at temperatures and pressures up to
1,100°C and 35 atm.
Their major conclusions are summarized below:
1. There are no temperature or pressure limitations to
electrostatic precipitation over the range studied. Practical
high temperature precipitation may be limited by thermal ioniza-
tion at temperatures exceeding 1,100 to 1,300°C.
2. High temperature and pressure enables operation at higher
voltages than is possible at standard conditions. This is in
part due to the supression of back-corona by operation at high
pressure. The higher operating voltage should more than compen-
sate for the adverse effect of high temperature gas viscosity on
the migration velocity and collection efficiency.
3. The critical pressure (above which sparkover results)
increases with temperature. Negative polarity gives higher
critical pressure than positive polarity.
4. In most cases negative currents are larger than positive
currents.
They prepared a preliminary design for a high temperature
and pressure electrostatic precipitator to be used in a pressurized
fluidized bed combustion unit. The operating temperature and
pressure are 815°C, 10 atm. Their recommended design parameters
are presented in Table 15.
75
-------
TABLE 15. RECOMMENDED DESIGN PARAMETERS FOR HTP ESP
(FROM FELDMAN ET AL., 1977)
Number of Precipitators per
Boiler Module
Pipes per Precipitator
Pipe Size
Vessel Diameter
Vessel Height
Steel Thickness
Vessel Weight
Capacity
MW of Plant capacity
Collecting Surface Area
Discharge Electrode
Length
Specific Collection
Area
Expected Efficiency
Power Supply
Voltage
Current
Discharge Electrode
320
0.229 x 3.05 m
7.0 m
10.7 m
0.038 m
52.8 x 103 kg
19.82 m3/s
42,000 ACFM
106,000 SCFM
75 MW
700.5 m2
975.4 m
35.3 m2/m3/s
180 ft2/103 ACFM
98-99%
150 kV
3,0:00 mA
3.08 mA/m
76
-------
TABLE 15. Continued
Current Density/Collecting 4.31 mA/m2
Surface
Specific Power 1.462 x 10"W/m3/s
6.9 W/ACFM
Capital Costs
Per Precipitator - Less Engineering
Stainless steel internals $1,945,000
Inconel internals $2,185,000
Per 300 MW Plant - Including Engineering
Stainless steel internals $7,980,000
Inconel internals $8,940,000
Per kW of Plant Capacity
Stainless steel internals $26.60/kW
Inconel internals $29.80/kW
77
-------
The recent Research-Cottrell work did not include the effects
of dust particles on the electrical performance. In general, the
presence of dust particles suppresses current flow and creates
a space charge between the electrodes. This enables higher
operating voltages than are possible in a clean gas. However,
as dust accumulates on the collection surface the sparkover
voltage decreases and may to some extent nullify the effect of
space charge.
Many practical problems are likely to be encountered in
the further development and demonstration of HTP electrostatic
precipitation. The very low electrical resistivity of ash par-
ticles at temperatures above 400-500°C may result in excessive
reentrainment during electrode rapping operations. Also mechani-
cal problems such as electrode alignment and strength after many
rapping cycles will need to be resolved.
Also it will probably be necessary to follow the precipita-
tor with a barrier collection device such as a granular bed
filter because even brief outages of the precipitator can cause
catastrophic damage to the gas turbine.
Plans are being considered for testing the Research-Cottrell
high temperature and pressure electrostatic precipitator on a slip'
stream from the Exxon PFBC miniplant. It would operate at 870°C
9 atm, and would handle 0.85 m3/min (30 ACFM) of gas containing
2.3 g/Nm3 (1.0 gr/SCF) of fly ash particles with a mass median
diameter of about 4 urn and a geometric standard deviation of
approximately 3.
Fiber Filtration
Conventional fabric filters are limited to operating tempera-
ture below 250°C. The maximum temperature varies with the spe-
cific fabric and is determined as the temperature at which accel-
erated fabric deterioration or abrasion occurs.
Glass fiber bags are the most common type used for higher
temperature applications and are limited to about 300°C. The
glass fibers are coated with a silicone, silicone-graphite, or
equivalent finish in order to provide lubrication between the
78
-------
fibers. Unfinished glass fibers experience extreme abrasion and
unsatisfactory bag life. The temperature limit of glass fiber
fabrics is directly related to the temperature limit of the
finish.
Lundgren and Gunderson (1976) reviewed the filtration
characteristics of glass fiber filters at elevated temperatures
(to approximately 500°C). Their review indicated that in practice
the effect of temperature and pressure on filtration mechanisms
was not a determining factor in the application of high effi-
ciency filters. The main problems are the physical and chemical
effects of a high temperature environment on the filter materials.
These effects may appear as reduced mechanical strength and re-
silience or loss of adhesion, leading to mechanical leakage,
decrease in efficiency and eventually mechanical failure.
High Temperature Filtration Studies
Filtration media for extreme temperatures and pressures has
been investigated by many authors. Silverman and Davidson (1956)
suggested the use of ceramic fibers sandwiched between layers
of woven metallic or ceramic fabrics for filtration at temperatures
to 1,100°C and higher. Billings, et al. (1955) and Silverman
(1962) discussed the use of metallic fiber "slag/wool" filters
for high temperature filtration of open-hearth furnace fume.
They tested the slag-wool filter at temperatures from 300 to
650°C and dust loadings from 0.1 to 1.1 g/Nm3 (0.1 to 0.5 gr/SCF).
Efficiencies ranged from 75 to 98%. A continuous slag-wood fil-
ter was designed and tested at 300-400°C. The efficiency ranged
from 10 to 80% for Fe20a particles with a mass median diameter
of 0.65 ym.
First, et al. (1956) and Kane, et al. (1960) reported on
the use of ceramic fiber filters capable of withstanding tem-
peratures up to 1,100°C. First, et al. (1955) measured collec-
tion efficiencies at 21°C and 760°C. The mass median particle
diameter was 8.5 ym with a particle density of 6.4 g/cm3. The
effect of temperature on efficiency for individual fiber dia-
meters is shown in Table 16.
79
-------
TABLE 16. EFFECT OF TEMPERATURE ON FILTRATION
EFFICIENCY (FROM FIRST, ET AL. 1955)
Filter Diameter,
ym
20
8
4
Filter Depth,
cm
10.16
3.81
2.54
Superficial
Velocity
cm/s
203.2
203.2
203.2
Efficiency,
Wt. %
21°C
85
99
98
760°C
82
94
91
TABLE 17. FRACTIONAL EFFICIENCY FOR
COMPOSITE FILTER TESTS
(FROM FIRST, ET AL., 1955)
Particle Diameter Range Collection Efficiency
0-1 urn 28.4%
1-2 ym 95.4%
2-5 ym 99.4%
>5 ym 99.4%
80
-------
A composite filter comprised of fibers from 4 ym to 20 ym
in diameter gave over 99% collection for all temperatures. The
fractional efficiency for the composite tests is shown in Table
17.
Theoretical Predictions
Filtration theory has been reviewed by many authors including
Davies (1973) and Pich (1966). We have used the theory presented
by Davies to predict the collection efficiency of a clean fiber
filter at high temperature and high pressure. The following
parameters were assumed:
Filter weight = 0.026 g/cm2
Fiber density = 2.53 g/cm3
Fiber diameter = 5.0 ym
Superficial velocity = 12 cm/s
Particle density = 2.5 g/cm3
Air properties at:
Temperature = 20°C and 1,100°C
Pressure = 1 atm and 15 atm
These parameters simulate a typical aluminum-silicate
ceramic paper. The results are presented in Figure 28.
At high temperature and low pressure the Brownian diffusion
regime becomes very significant and the collection efficiency
of particles smaller than approximately 0.5 ym increases drama-
tically. At high temperature and high pressure this effect is
less apparent.
In the inertial impaction regime, high temperature and high
pressure reduce the collection efficiency from that obtained
at standard conditions. Even at high temperature and pressure,
however, the predicted collection efficiency is effectively 100%
for particles larger than 2 ym. This is consistent with the
data reported earlier from the work by First, et al. (1955).
Current Development Work
Recent development work on high temperature and pressure
ceramic fiber filters is being carried out by Acurex Corporation
81
-------
oo
100
90
80
70
w 60
U,
UH
UJ
g
50
w 40
30
20
10
7 / INERTIAL IMPACTION
/ / REGIME
1 atm
15 atm
I
I
I I I
0.1
0.2
1.0
0.5
PARTICLE DIAMETER, ym
Figure 28. Predicted performance for ceramic fiber filter.
2.0
-------
and has been reported by Shackelton (1977, 1978).
They have conducted a detailed survey and evaluation of
ceramic fiber media with potential for high temperature filtration,
Available ceramic fiber configurations can be classified into
the following three groups of materials:
1. Woven structures - Cloth woven from long-filament yarns
of ceramic fibers.
2. Papers - Ceramic structures produced from short lengths
of fibers, generally held together with binders.
3. Felts - Structures produced to form mats of relatively
long fibers. These materials are known as blankets in
the insulation industry. They tend to be less tightly
packed than conventional felt materials.
A summary of room temperature data is presented in Table 18.
The last three materials are conventional fabric filter media
which are only included for purposes of comparison. A number
of the media show high collection efficiencies for 0.3 ym dia-
meter DOP aerosol particles. It should be noted that this is
the efficiency of a clean filter media and higher efficiencies
can be anticipated for a dirty filter operating with some residual
dust deposited between the fibers.
Permeability is measured as the flow per unit area at a
constant pressure drop. Thus, a material with low permeability
offers a high restriction to gas flow and one with high perme-
ability allows more gas to penetrate for a given pressure drop.
Table 18 shows that some ceramic materials are available which
have low permeability, while others have high permeability.
Most of the paper and felt materials have permeability similar
to that of commonly used filter materials.
Ceramic fiber filters have two major drawbacks regarding
application at high temperature and pressure. First of all they
must be very durable. Conventional filter bags are expected to
last at least one year (see Billings and Wilder, 1970). Bag
life at the Nucla Power Plant was estimated to be 5+ 1.3 years
83
-------
TABLE 18. CERAMIC FIBER TEST DATA (FROM SHACKLETON
AND DREHMEL, 1978)
(W) Woven
(P) Paper
(F) Felt
1. Carborundum Fiberfrax cloth
(W) with nichrome wire insert
2. Zircar Zirconia felt ZFY-100
(F)
3. ICI Saffil alumina paper
(P) with binder
4. ICI Saffil mat
(F)
5. Babcock S Wilcox Kaowoll
(F)
6. Carborundum Fiberfrax
(F) durablanket
7. John-Mansville Fiberchrome
(F)
8. Stevens Astroquartz
(W) style 581
9. Hitco Refrasil C-100-96
(W) heat cleaned
10. Hitco Refrasil C-100-48
(W) not heat cleaned
11 . Stevens Astroquartz cloth
Basis
Weight
g/m2
1366
615
165
355
746
1363
1297
283
1284
667
667
Permeability Percent Efficiency on
cm3/s/cm 2 ft3/min/ft2 0.3 ym OOP at cm/s
1.27 cm H20 at 0.5 in. W.C. 2.68 5.35 14.22
8.7
10.9
9.3
12.4
8.1
5.6
11.9
37.2
1.2
3.1
22.8
17.1 45
21.4 75
18.3 82
24.4 79
15.9 96.5
11.0 97.1
23.4 78
73.3 0
2.4 0
6.1 0
44.8 0
47
78
65
80
93.5
94.6
73
9
19
11
13
50
72
62
73
86
90.5
74
12
34
10
32
(W) style 570
continued
-------
TABLE 18. Continued
00
in
(W) Woven
(P) Paper
(F) Felt
12. 3M AB-312 basket weave
(W) cloth
13. 3M AB-312 twill weave
(W) cloth
14. HITCO Refrasil cloth
(W) UC-100-48
15. Zircar Zironia cloth
(W) ZFY-30A
16. FMI -Stevens Astroquartz
(W) cloth crowfoot satin
17. 3M AB-312 twill weave
(W) cloth coated with 3M coating
18. 3M AB-312 basket weave
(W) cloth coated with 3M coating
19. 3M AB-312 twill weave
(W) cloth Menarde coating
20. HITCO Refrasil cloth
(W) UC-100-96 not heat cleaned
21 . Carborundum Fiberfrax
(W) no insert wire L-126TT
22. HITCO Refrasil batt B100-1
Basis
Weight
g/m2
311
231
643
608
352
227
281
254
1249
1544
807
Permeability Percent Efficiency on
cm3/s/cm 2 ft3/rain/ft2 0.3 urn OOP at cm/s
1.27 cm H20 at 0.5 in. W.C. 2.68 5.35 14.22
13.6
28.4
8.7
5.8
16.6
65.2
47.6
51.2
3.4
7.4
8.9
26.7 058
56.0 032
17.1 043
11.4 29 37 34
32.6 056
128.3 0 3 10
93.7 070
100.8 0 6 10
6.7 0 11 16
14.7 55 55 57
17.5 84 79 72
(F)
-------
TABLE 18. Continued
oo
(W) Woven
(P) Paper
(F) Felt
23. HITCO Refrasil standard
(W) not heat cleaned very
thin UC-100-28
24. HITCO Irish Refrasil
(W) chromized C-1554-48
25 . Carborundum Fiberf rax
(P) paper (with binder) 970J
26. ICI Saffil Zirconia paper
(P) (with binder)
27. Carborundum Fiberf rax
(P) paper (no binder) 970-AH
28. 3M AB-312 double thick
(W) plain weave
29. FMI crowfoot satin cloth
(W) astroquartz
30. 3M AB-312 12 harness satin
(W) weave
31. 630 Tuflex fiberglass*
(W)
32. 15-011-020 woven filment*
(W) polyester
33. 25-200-070 polyester felt*
(F)
Basis Permeability Percent Efficiency on
Weight cm3/s/cm2 ft3/min/ft2 0.3 ym OOP at cm/s
g/m 1.27 cm H20 at 0.5 in. H.C. 2.68 5.35 14.22
335 11.9 23.4 010
683 5.2 10.1 2 8 10
604 26.9 53.0 99.5 99.0 97.6
212 8.8 17.1 83 78 74
152 12.4 24.4 88 -- 73
1035 84.8 167.0 0 10 41
905 62.1 122.2 0 10 32
675 75.5 148.7 0 8 24
564 16.0 31.6 10 9 19
175 6.8 13.4 604
524 11.9 23.4 34 24 29
*These materials are conventional (not ceramic) media.
-------
(Ensor, et al. 1976). For typical cleaning pulse frequencies,
a bag may have to withstand a few million pulses in its lifetime.
For this reason, blanket or felted ceramic fiber materials are
expected to be the most promising. They combine good filtration
properties with relatively high strength.
The second drawback is the size of typical fabric filter
installations. Most conventional fabric filters operate at super-
ficial velocities in the range of 1 to 3 cm/s. Somewhat higher
velocities up to 10 or 20 cm/s are possible with some felted
fabrics although bag life will be shortened.
In comparison, granular bed filters operate at superficial
velocities from 40 to 80 cm/s. For a given gas flow rate, fabric
filters will require from 4 to 20 times the surface area required
by granular bed filters. This is especially important at high
pressure where the cost of the pressure vessel can be a signifi-
cant fraction of the capital cost. Both baghouses and GBFs will
have to be designed so as to maximize the surface to volume ratio,
These problems are being considered in the EPA-sponsored
development program. Preliminary experience at high tempera-
ture and pressure indicate that at least three configurations
show promise, having survived 50,000 cleaning pulses at 815°C
and 9 atm. Test conditions were as follows:
Temperature - 815°C
Pressure - 930 kPa (9 atm)
Air-to-cloth-ratio - 5 to 1 (2.54 cm/s)
Cleaning pulse pressure - 1,100 kPa
Cleaning pulse interval - 10 s
Cleaning pulse duration - 100 m/s
Dust - redispersed fly ash
The three filter media configurations tested were:
1. Saffil alumina mat contained between an inside and an
outside layer of 304 stainless steel knit wire screen.
2. Woven Fiberfrax cloth with nichrome wire scrim insert.
3. Fiberfrax blanket contained between an inside and an
87
-------
outside cylinder of 304 SS square mesh screen similar
to common window screen. The ceramic fiber blanket
was held in position between the screens with 302 SS
wire sewn between the screens.
Pressure drop during the tests was controlled by rapid
cleaning pulses and in general remained less than about 5 kPa
(20 in.W.C.). Formation and removal of the filter cake for these
configurations and test conditions presented no problems.
The average outlet loading during the Fiberfrax blanket
test was 0.0055 g/Nm3 (0.0024 gr/SCF). The fly ash dust dis-
persion apparatus used was suitable for filter loading tests
but may not be representative of the dust characteristics and
size distribution to be encountered in a real application.
In order to obtain test data in a real PFBC application,
the Acurex ceramic baghouse is to be tested on a slipstream at
the Exxon miniplant. Installation has begun and the test pro-
gram is scheduled to start in late November, 1978.
Membrane Filtration
Several available ceramic materials in many configurations
have been evaluated as possible high-temperature filters by
Ciliberti (1977) and Poe, et al. (1977). One of the most promising
materials tested was a ceramic cross flow monolith produced by
3M Company under the trade name of Thermacomb. This material
is composed of alternate layers of corrugations separated by
thin filtering barriers. This type of configuration affords a
large amount of filter surface in a very small volume. Figure 29
shows a piece of this material and indicates the cross flow con-
figuration. The material has an average pore size of 10 ym with
a range as shown in Figure 30.
The Thermacomb cross flow structure is made up of several
layers in the following pattern: a thin (0.25 - 1.5 mm) porous
cordierite sheet, a layer of cordierite corrugations similar in
88
-------
CLEAN
GAS
GAS
FLOW
Figure 29. 3M crossflow ceramic monolith.
89
-------
100
80
UJ
IS!
in
o
o
ex
UH
o
60
40
20
0
0.001
I I I
0.01 0.1 1.0
PORE DIAMETER (MICRONS)
10
100
Figure 30. Pore size distribution for Thermacomb ceramic fiber
material (from Poe, et al., 1977).
90
-------
appearance to those used in cardboard, another flat sheet of
cordierite followed by another layer of corrugations oriented
90° from the corrugations below. The presently available forms
of the material have 1.97, 3.15 or 4.72 corrugations per cm. A
similar material is manufactured by W.R. Grace § Company. This
material has perpendicular dividers which given rectangular
holes rather than the triangular holes seen in the Thermacomb.
The Grace material tested had approximately 8.5 holes per cm
and an equal number of layers per cm.
These materials have many properties that make them attrac-
tive as filters. Among these are (1) working temperature to
1,200°C, (2) very good mechanical strength despite thin separators,
(3) excellent resistance to thermal shock, (4) excellent resis-
tance to corrosive atmospheres and, (5) very high surface area
to volume ratios. Ciliberti (1977) estimated that the Therma-
comb material tested had 3.27 cm2 filter area per cm3 of element
while the W.R. Grace § Company material had approximately 6.52 cm2
of usable area per cm3 of element.
Cascade impactors were used to measure the size of the
limestone test dust. The mass median diameter was typically
1.4 ym and the geometric standard deviation was 3.0.
Results of the high temperature Thermacomb tests are pre-
sented in Table 19. The overall collection efficiency averaged
96.4% with some tests showing 100% collection. No problems were
encountered in cleaning the filter media by reverse pulses of
compressed air. It was possible to clean the filter and return
to a stable pressure drop even the relatively heavy dust loadings.
Similar results were obtained in a limited number of tests
on the W.R. Grace material.
Although ceramic honeycomb filters operated successfully
on limestone particulate in bench scale tests, there are a number
of uncertainties regarding their application as tertiary cleanup
devices. Further development work is needed to resolve the fol-
lowing major questions.
91
-------
TABLE 19. SUMMARY OF HOT TESTS WITH
3M THERMACOMB (FROM CILIBERTI,
1977)
Temperature,
°C
815
750
720
360
520
710
700
680
690
680
695
680
660
650
700
630
620
615
Superficial
Velocity
cm/s
11.6
7.3
7.0
4.5
5.7
7.3
6.9
6.8
6.8
6.8
6.9
2.9
2.8
2.7
2.2
1.6
1.5
1.4
Inlet
Concentration
g/m3
1.2-2.2
1.8-2.1
8.4-12.0
3.6
5.2
4.5
2.7-7.9
2.6
3.8
3.2
5.4
0.05
0.11
0.11
1.1
0.06
0.12
0.08
Overall
Efficiency,
%
95.5
85.0
95.0
100.0
100.0
99.6
99.9
99.6
100.0
100.0
100.0
100.0
100.0
93.0
99.0
88.0
92.0
81.0
92
-------
1. Are ceramic honeycombs susceptible to clogging with
actual process particulate matter?
2. Can these filter media be redesigned or optimized with
regard to particle collection, pressure drop, permeability?
3. What are the temperature and pressure losses associated
with alternative cleaning methods?
4. How durable are these media over a prolonged high tem-
perature and pressure run, and how serious are the pro-
blems of erosion and degradation of the media ?
93
-------
HOT VERSUS COLD GAS CLEANUP
INTRODUCTION
Because of the technical and economic uncertainties associa-
ted with high temperature and pressure particulate control, there
is a possibility that the only practical approach to particulate
removal which will allow for reasonable turbine life is low
temperature, high pressure cleanup.
In most cases, low temperature-high pressure gas cleaning
equipment can be based on conventional technology. However, the
specific design configurations for high pressure applications
will generally be different than for low pressure applications.
For example, tubular electrostatic precipitators are more suit-
able than parallel plate designs for operation at high pressure.
This is essentially a matter of packaging the precipitator in
a pressure vessel. However, increased pressure increases pre-
cipitation efficiency by reducing ion mobility and thereby
enabling operation at higher electrical potential. Therefore
particle collection should not present problems.
Fabric filtration baghouses should operate adequately at
high pressure. However the expense of high pressure housings
may require special design to minimize the baghouse volume
and maximize the superficial velocity (air-to-cloth ratio).
Wet scrubbers can also be designed to work well at high
pressure. However, large gas densities may require higher
liquid-to-gas ratios, and in some configurations high pressure
frictional losses may become more significant than in conven-
tional applications.
In general, low temperature-high pressure particulate
control technology is feasible and to some extent commercially
available. This is the major advantage to cold gas cleanup
94
-------
as 'Compared to hot gas cleanup. On the other hand, hot gas
cleanup can achieve higher overall thermal efficiencies
and therefore more efficient fuel usage.
The relative process efficiencies for hot and cold gas
cleanup depend on the specific process configuration. Hot ver-
sus cold gas cleanup for pressurized fluidized bed combustion
and low-BTU coal gasification processes are reviewed in this
section.
PRESSURIZED FLUIDIZED BED COAL COMBUSTION
Klett, et al. (1977) carried out an analytical study to
determine the performance penalties which accompany cold gas
cleanup for representative pressurized fluidized bed combustion
combined-cycle systems. They considered a cold gas cleanup
cycle consisting of a recuperator followed by a baghouse or
electrostatic precipitator. The PFBC boiler, air-cooled combus-
tor, and adiabatic combustor designs were considered. The
system parameters are listed in Table 20.
PFB Boiler Process
The G.E. 1,000 MW commercial scale combined-cycle design
was used as a basis for the water cooled PFBC process. The
system is illustrated in Figure 31. Water and steam pass
through tubes in the bed to control bed temperature. Combustion
is carried out with approximately 20% excess air and the combus-
tion gas is cleanetd prior to entering the gas turbine.
The proposed G.E. system cleans the combustion gas at
high temperature ( 955°C ) using two stages of cyclone separation
and a final gas cleanup stage. Klett, et al. (1977) also con-
sidered cold gas cleanup with heat recovery. For all cases,
the temperatures out of the bed were held constant at 955°C
which are consistent with those chosen by G.E. for their pre-
liminary base case.
The tertiary cleanup device was a moving granular bed
filter (GBF) . A pressure drop of 0.34 atm and a temperature drop
of 14°C across the bed were assumed. A 1% mass flow loss due
to medium recirculation and leakage was also assumed for the moving
bed filter.
95
-------
TABLE 20. SYSTEM PARAMETERS FOR PFBC ANALYSIS
(FROM KLETT, ET AL., 1977)
vo
PFBC Temperature (°C)
PFBC Pressure (atm)
Excess Air for Combustion (%)
Gas Cleanup Temperature (°C)
RHX Effectiveness
Gas Turbine Inlet Temperature
Steam Turbine Pressure/
Temperature (atm/°C)
Steam Turbine Pressure/
Temperature (psia/°F)
Stack Temperature (°C)
Gas Turbine Electrical Output
Steam Turbine Electrical Outpi
Adiabatic
(CPC-400
Hot Gas
Cleanup
983
6.3
300
969
N/A
(°C) 933
31.6/426-
7.14/204
465/800-
105/400
163
(MW) 12.5
it 7.4
Combustor
System)
Cold Gas
Cleanup
983
6.3
300
316
0.80
802
31.6/426-
70 14/204
465/800-
105/450
133
9.7
7.8
Air Cooled
Combustor
(Curtiss -Wright System)
Hot Gas
Cleanup
899
6.9
33
899
N/A
916
54.4/440
800/825
149
304
190
Cold Gas
Cleanup
899
6.9
33
316
0.80
864
54.4/440
800/825
149
280
194
Water Cooled
(GE CFCC
Hot Gas
Cleanup
955
10
20
955
N/A
926
238/538/538
Combustor
System)
Cold Gas
Cleanup
955
10
20
316
0.80
814
238/538/538
3500/1000/1000 3500/1000/1000
149
222
731
149
174
768
(MW)
-------
T - TEMPERATURE, F
P - PRESSURE, PSIA
AIR IN
59T
14.7P
COMP.
147P
667T
4358P
FEED
WATER
FURB.
1700T
PFBC
J2 STAGE
— CYCLONE
17OTT
1000T
574P
1003T
3575P
926T
TO STACK
0.98P
Figure 31a. Pressurized fluidized bed/combined cycle system, water cooled combustor with hot gas cleanup.
-------
00
T - TEMPERATURE, F
P - PRESSURE, PSIA
810T
AIR IN
59T
14.7P
COMPR
147P
FEED
WATER
TURB.
TO STACK
1500T
PFBC
RECUP.
HEAT
EXCHANGER
1750T 1725T
837T
1003T
3575P
Figure 31b. Pressurized fluidized bed/combined cycle steam, water cooled combustor with cold gas cleanup.
-------
The cold gas cleanup system used a pressurized baghouse, in
conjunction with a gas/gas recuperative heat exchanger. An effec-
tiveness of 80% and a pressure drop of 76 cm W.C. per side were
assumed for the heat exchanger. These values are consistent with
those found on gas/air heat exchangers used on regenerative gas
turbines in electric power generation service. Effectiveness
ranges for various commerically available recuperative heat ex-
changer designs are given in Table 21.
The heat removed from the cleanup stream prior to reentering
the recuperator was recovered in a bottoming cycle auxiliary econo-
mizer. The baghouse filter inlet temperature was set at 316°C
based on design limitations for available material.
The results are presented in Table 22.
Air-Cooled PFBC Process
The air-cooled PFBC analysis was based on the Curtiss-Wright
500 MW combined cycle system. This system is illustrated in Figure
32. Approximately two-thirds of the total gas flow is passed as
cooling air through tubes in the combustor bed to control the
bed temperature. Combustion is carried out with approximately
33% excess air. The combustion gas is cleaned and mixed with the
heated air prior to entering the combustion turbine.
The proposed Curtiss-Wright system cleans the combustion gas
at high temperature (899°C) using two stages of Aerodyne rotary flow
cyclones. Klett, et al. considered alternative hot gas cleanup
systems as well as cold gas cleanup with heat recovery. For all
cases, the temperature out of the bypass air heat exchanger was
held constant at 856°C.
The alternative hot gas cleanup method studied was a gravel
bed filter (GBF). Two types of GBF were considered: fixed bed
and moving bed. For both filters, a pressure drop of 34.5 kPa
(5 psij and a temperature drop of 14°C across the bed were assumed.
A 1% flow loss due to media recirculation and leakage was assumed
for the moving bed filter. A 0.25% flow leakage was assumed for
the fixed bed filter.
99
-------
TABLE 21. EFFECTIVENESS RANGES FOR
RECUPERATIVE HEAT EXCHANGERS
Heat Exchanger Type Effectiveness*
Shell $ Tube 48-57.5%
Single Pass
Shell § Tube 60-73%
Double Pass
Shell § Tube 81-83%
Three Pass
Rotary (Heat Wheel) 75-80%
(3 or 4 units)
*Effectiveness is the percentage of available
temperature differential recovered.
100
-------
TABLE 22. PERFORMANCE OF PFBC PROCESSES
WITH HOT AND COLD GAS CLEANUP
(FROM KLETT, ET AL., 1977)
Process
Configuration
I. PFB Boiler
A. Hot Gas Cleaning
1. Moving GBF
B. Cold Gas Cleanup
1 . Pressurized baghouse
II. Air-Cooled PFBC
A. Hot Gas Cleanup
1. Two stages
Aerodyne cyclones
2. Moving GBF
3. Fixed GBF
B. Cold Gas Cleanup
1. Baghouse/low
pressure heat
2 . Baghouse/high
pressure heat recove
II. Adiabatic PFBC
A. Hot Gas Cleanup
1. Moving GBF
B. Cold Gas Cleanup
1. Baghouse high
pressure steam with
heat recovery
Gas Turbine
Inlet Temp.°C
926
814
871
866
867
828
ry 828
932
802
Thermodynamic
Efficiency, %
40.6
39.6
38.8
37.1
37.3
35.5
36.0
37.1
32.6
Heat Rate,
BTU/kW-hr
8,399
8,624
8,805
9,200
9,154
9,621
9,476
9,203
10,450
Net Power
MW
953
942
458.1
435.4
440.7
419.3
425.7
19.9
17.5
101
-------
AIR IN
o
txj
T - TEMPERATURE, F
P - PRESSURE, PSIA
Figure 32a. Curtiss-Wright pressurized fluidized bed/combined cycle system, hot gas cleanup
with aerodyne cyclones.
-------
AIR IN
T - TEMPERATURE, F
P - PRESSURE, PSIA
Figure 32b. Curtiss-Wright pressurized fluidized bed/combined cycle
system cold gas cleanup system with heat recovery.
-------
A baghouse, in conjunction with a gas/gas recuperative heat
exchanger, was considered for cold gas cleanup. As with the
preceding study, an effectiveness of 80% and pressure drops of
76 cm W.C. per side were assumed for the heat exchanger. The heat
removed from the cleanup stream prior to reentering the recupera-
tor was used to generate additional steam. in one system, a second
low pressure evaporator was added to recover the heat. In a second
system, the heat was recovered by adding a high pressure econo-
mizer .
The results are presented in Table 22.
Adiabatic PFBC Process
The Combustion Power Company's adiabatic PFBC system was
used as the basis for the adiabatic process. The system incor-
porates a moving bed granular bed filter (GBF) for high tem-
perature cleanup of the combustion gas prior to entering the
gas turbine. The system is illustrated in Figure 33.
The cold gas cleanup method was a baghouse with fiberglass
filters. A pressure drop of 13 cmW»C. through the baghouse was
assumed. The maximum operating temperature of the baghouse is
about 315°C (600°F). The combustion gas was cooled by means of
a gas/gas heat exchanger which was added to the system ahead of
the baghouse. The heat exchanger effectiveness and pressure
drop per side were assumed to be 80% and 76 cm W0C. The results
are presented in Table 22.
The efficiency loss between hot and cold gas cleanup is
greatest for the adiabatic combustor configuration, since all the
working fluid passes through the bed and therefore must be cooled
and cleaned. Future equipment, capable of handling higher product
gas temperature, could reduce the performance penalties associated
with cold gas cleanup and probably justify the additional hard-
ware complexity.
The systems using air and water cooled combustors appear
to be capable of using current cold gas cleanup techniques with
fairly small system performance penalties (1 to 2%). The Phase
II EGAS studies (Lewis Research Center, 1977 and General Electric
104
-------
AIR IN
o
tn
DUAL ADMISSION
STEAM TURBINE
FUEL IN
Figure 33a. Pressurized fluidized bed combustion system with dual admission steam turbine.
-------
269T
AIR IN
872T
FUEL IN
ASH
1
-
802'
GEN.
r
f
f
'HP ' HIGI
SUPHTF If
"^
V T r»i*r
PRES.
rEVAP.
1
3.
HIGH
PRES.
* — FTHM * —
550P 0
A >
P - PRESSURE, PSIA
T - TEMPERATURE, F
E
Figure 33b. Pressurized fluidized bed combustion system with dual
admission steam turbine cold gas cleanup - low pressure
evaporator added for heat recovery.
DUAL ADMISSION
STEAM TURBINE
-------
Company 1976) showed the PFBC boiler process with hot gas clean-
up to have a 7% thermal efficiency advantage over conventional
coal-fired boilers with wet scrubbers for SO control.
J\.
In order to fully assess the cold cleanup alternatives,
recuperative heat exchangers must be studied more closely, es-
pecially regarding effectiveness, availability and cost for high
temperature and pressure applications.
Further economic evaluation of hot versus cold gas cleanup
must consider the cost and effectiveness of high temperature-high
pressure and low temperature-high pressure particulate control
devices, recuperator performance and cost, and the specific par-
ticulate control requirements. Much more development work is
required to provide this information.
Post-Turbine Particulate Control
If gas turbines can be developed which have relatively
high tolerance for fine (<5 ym) particles, then it may be fea-
sible to use conventional cyclones and multiclones to protect
the turbine. In such cases the emissions regulations would have
to be met by applying conventional particulate control equipment
downstream from the gas turbine.
Using the Westinghouse EGAS Phase II PFBC design (Beecher,
et al. 1976), each gas turbine would handle 345 kg/s of gas
flow. At a stack temperature of 150°C, this would correspond
to 24,900 m3/min (879,000 ACFM) of gas to be controlled.
Data from the Exxon miniplant indicate that about 0.23 to
2.3 g/Nm3 (0.1 to 1.0 gr/SCF) of particulate matter penetrates
the second stage cyclone. The proposed New Source Performance
Standard for coal-fired boilers of 0.05 lb/106BTU corresponds
to about 0.046 g/Nm3 (0.025gr/SCF) for the Exxon system. There-
fore collection efficiencies of 80 to 98% may be required down-
stream from the turbine.
The estimating curves presented by Neveril, et al. (1978)
were used to estimate the cost of electrostatic precipitators
with design efficiencies of 80, 90, 95, and 98%. Typical effec-
tive migration velocities (precipitation rate parameters) of 9
and!2 cm/s (0.3 and 0.4 ft/s) were assumed for these predictions.
107
-------
The results are shown in Table 23, along with the costs for high
temperature and pressure particulate control equipment reported
by Beecher, et al. All prices have been adjusted to mid-1978
dollars using the Marshall and Stevens equipment cost index.
When interpreting Table 23 it should be remembered that the
granular bed filter costs are highly speculative in that they are
based on an unproven technology at high temperature and pressure.
Also the electrostatic precipitator costs assume that there are
no problems with using standard cold-side designs for this appli-
cation. High resistivity problems necessitating hot-side electro-
static precipitation will cause the estimated costs to increase
by a factor of two or more.
The cost of post-turbine cleanup equipment is noticeably less
expensive than hot gas cleanup. However, the post-turbine equip-
ment costs are significant and must be considered in the overall
capital cost for gas cleaning equipment. Any economic advantages
associated with using post-turbine cleanup very likely will depend
on the cost and availability of gas turbines which can tolerate
relatively heavy loadings of fine particulate matter.
LOW-BTU COAL GASIFICATION PROCESSES
The incentives for hot gas cleanup in coal gasification pro-
cesses are different than for PFBC processes. The major differences
are:
1. The available sensible heat in the coal gas is only
about 20% or less of the total energy available from the gas.
Therefore heat transfer inefficiencies associated with the cold
gas cleanup approach are less severe.
2. Maximum turbine inlet temperature is assured because
of the heat released by combustion of the gas.
3. High temperature particulate removal is coupled to hot
H2S removal. There is little incentive for hot particulate
removal if the gas must be cooled for HZS removal.
108
-------
TABLE 23. COMPARISON BETWEEN PRE-TURBINE AND
POST-TURBINE EQUIPMENT COST
CONTROL DEVICE
(Number per Turbine)
A. HOT GAS CLEANUP:*
1. Primary Cyclones (8)
2. Multiclones (2),
including bleed lines
3. Granular Bed Filters (8)
TOTAL ESTIMATED MAJOR
COMPONENT COST PER
TURBINE, $
1,140,000
2,360,000
2,980,000
B. POST-TURBINE CLEANUP:
Electrostatic Precipitators :
(Cold Side)
1. 80% collection
2. 901 collection
3. 95% collection
4. 98% collection
Migration Velocity
9 cm/s 12 cm/s
400,000 500,000
530,000 630,000
640,000 750,000
750,000 960,000
*From Beecher, et al. (1976) converted to mid-1978 dollars.
109
-------
4. In some processes quenching is required to remove tars.
The presence of tars can plug hot gas cleanup devices unless the
tars can be prevented from condensing. Quenching loses the
gas sensible heat and the heating value of the tars which can be
as much as 20% of the total heating value of the fuel (MERC,
1978). Also tar disposal or recovery from the scrubber liquor
can present problems. Hot gas cleanup which enables the tars
to be burnt in the gas turbine combustor is highly desirable.
Jones and Donohue (1977) reported on a comparative evalua-
tion of high and low temperature gas cleaning for coal gasifica-
tion combinedrcycle power systems. They were concerned primarily
with hot H2S removal and for the purposes of their evaluation
they assumed that suitable high temperature and pressure parti-
culate control equipment would be developed. However, they noted
that it is impossible to develop better than rough cost estimates
for hot gas cleanup equipment at the current state of development.
Process evaluations were performed for five different coal
gasification schemes.
1. Air blown, dry ash, moving bed gasifiers (Lurgi)
2. Oxygen blown, dry ash, moving bed gasifiers (Lurgi)
3. Oxygen blown, slagging, moving bed gasifier (British
Gas Corporation)
4. Oxygen blown, two-stage entrained bed gasifier
(Foster-Wheeler)
5. Air blown, two-stage entrained bed gasifier (Foster-Wheeler)
The air blown systems and the oxygen blown slagging gasifier
system are illustrated in Figures 34, 35 and 36. The Morgantown
iron oxide system was used for hot HaS removal. The CCNY (Squires)
granular bed filter system was used for hot particulate control.
Cold HaS removal was achieved using the proprietary Benfield pro-
cess. Wet scrubbers were used for low temperature particulate
removal.
The results of their study are summarized in Table 24. It
appears that the greatest thermal benefits are to be derived
from applying hot gas purification to Lurgi gasifiers. It is
110
-------
TABLE 24. SUMMARY OF ESTIMATED THERMAL EFFICIENCIES FOR HOT VERSUS
COLD GAS CLEANUP IN COAL GASIFICATION PROCESSES (FROM
JONES AND DONOHUE, 1977)
PURIFICATION
COLD GAS
CLEANUP
HOT GAS
CLEANUP
COLD GAS
CLEANUP
HOT GAS
CLEANUP
Gas Turbine Inlet
Temperature, °C
Lurgi (02)
1,063
1,063
1,315
Thermal Efficiency, %*
Heat Rate, BTU/kW-hr
1,315
Thermal Eff.
Heat Rate
Lurgi (Air)
Thermal Eff.
Heat Rate
Slagging (02)
Thermal Eff.
Heat Rate
Entrained Bed (Air)**
Thermal Eff.
Heat Rate
Entrained Bed (02)**
Thermal Eff.
Heat Rate
29.4
11,628
31.0
10,994
36.5
9,352
38.0
8,982
35.4
9,641
35.4
9,630
37.0
9,223
37.5
9,095
38.4
8,879
36.6
9,334
32.4
10,544
34.5
9,907
39.6
8,624
40.8
8,359
37.8
9,028
39.9
8,558
41.2
8,285
40.6
8,409
41.6
8,215
39.3
8,688
'Thermal Efficiency (%) » (Delivered kW) (3412.75) (100)
(Coal Ib/hr) (Coal HHV BTU/lb)
**Foster Wheeler Gasifier. The notation Air or Og indicates the oxidant
employed in each type of coal gasifier.
Ill
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BENFIELD H2S REMOVAL PLANT
ts)
COAL •
H2S
ABSORBER
H2S
STRIP
.LP STN
BENFIELD
SOL'N
SULFUR
RESATURATOR
QUENCH
SCRUBBER
/ V STACK
WATER
TREATMENT
GAS
TURBINE
HEAT
RECOVERY
NAPHTHA
OILS,PHENOLS ^
COMPRESSOR NHs STEAM
SOL'N TURBINE
AIR
Figure 34a. Lurgi air blow gasifier - cold purification case.
-------
COAL
AIR
MP STM
COMPRESSOR
REMOVAL REMOVAL
REGENERATION SULFUR TURBINES
STACK
Figure 34b. Lurgi air blown gasifier study - hot purification case.
-------
BENFIELD H2S REMOVAL PLANT
COAL
TRANSPORT
GAS
MP
STEAM
GLAUS PLANT
OFFGAS
TO
BOILER
SULFUR
WATER PLANT
1 20%
AMMONIA
SOLUTION
AIR
&
STEAM TURBINES
j
COMPRESSOR ^>
TRANSPORT
GAS TO
GASIFIER
Figure 35a. Air blown entrained bed gasifier study - cold purification case.
-------
ENTRAINED
PARTICLE REMOVAL
AIR
COAL
BED
GASIFIER
COMBUSTOR
COMPRESSOR
BFW
HOT IRON
OXIDE H2S
REMOVAL
TRANSPORT
GAS,
CHAR
HOPPER
TURBINE
GUARD
FILTER
REDUCING GAS
HEAT RECOVERS
SLAG
COMPRESSOR
STEAM
TURBINES
IRON
OXIDE
REGEN.
CYCLE
RECYCLE
SYSTEM
OXYGEN PLANT
MP STM
Figure 35b. Air blown entrained bed gasifier study - hot purification case.
-------
BENFIELD H2S REMOVAL PLANT
COAL
MP STEAM
STEAM
TURBINES
Figure 36a. Slagging gasification study - cold purification case.
-------
AIR
MP STM
J COMPRESSOR
HOT IRON
PARTICLE OXIDE
BTIIB PAC REMOVAL H,S REMOVAL
FLUE GAS «~ 2
STACK
STE/
TURBINES
Figure 36b. Slagging gasification study - hot purification case.
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important to remember that the underlying assumption used for
these cases is that tars will pass uncondensed through the hot
iron oxide beds directly into the gas turbine combustor.
For all cases other than the Lurgi dry ash gasifiers,
thermal efficiency advantages associated with hot gas cleanup
appear to be marginal (1 to 2%). The Phase II EGAS studies
(Lewis Research Center, 1977 and General Electric Co., 1976)
showed the LBCG combined-cycle system to have a 7% advantage
over conventional coal-fired boilers with stack gas scrubbing
for SO control.
JC
There are two major reasons why high temperature purifi
cation proved to be of such benefit to the Lurgi systems and
of little advantage to the other gasification systems studied:
1. The Lurgi gasifiers were the only systems considered
to have a net production of tars. Tars produced by the BGC slag-
ger were separated from the gas stream prior to desulfurization and
were recycled to extinction to the gasifier. Tars were considered
to be absent in the crude gas from the entrained gasifier. The
presence of tars in a crude fuel gas has major impact on the com-
parison between high and low temperature cleaning schemes. For
the high temperature case, tars are assumed to pass through the
iron oxide system and are converted to electricity at combined-
cycle efficiency, (40-50%). With low temperature cleaning systems,
tars are scrubbed from the crude gas by direct quench and are
eventually converted to electricity at only the steam cycle effi-
ciency, (30-40%) .
2. The Lurgi gasifiers consume large quantities of steam to
prevent ash matter from clinkering in the bottom. Most of this
steam passes through the gasifier unconverted and is condensed
in the gas quench operation necessary for low temperature gas
cleaning. Therefore, if a quench is necessary, most of the sen-
sible heat in the steam is unavailable for power generation. If
high temperature desulfurization is employed, the steam passes
through the iron oxide beds, and its sensible heat is converted
into electricity in the combined-cycle plant.
118
-------
The BGC slagger required only 13% of the steam required
by the dry ash oxygen blown Lurgi gasifier due to the higher
bottom temperature required for ash slagging. It also converts
approximately 901 of the gasifier steam to hydrogen and carbon
monoxide. Therefore, steam losses due to cooling of the slagging
gasifier effluent in the quench operation are negligibly small.
Steam consumed by both the air and oxygen blown entrained
gasifier is approximately the same as that consumed by the slag-
ger resulting in the same negligibly small steam losses on gas
cooling.
Thermal efficiency advantages for hot gas cleanup may be
even lower than predicted if satisfactory high temperature
particulate removal cannot be achieved without significant
temperature and pressure losses.
Also both HTP H2S and particulate removal systems are
required. Therefore technology development needs may be greater
than for combustion processes.
Existing technology is capable of satisfying cold gas
cleanup needs. This should be sufficient for first generation
LBCG processes. The development of HTP particulate and HaS
removal systems may be helpful in improving the performance of
second generation systems.
HTP control equipment also will make the fixed or moving
bed processes more economically competitive by enabling the
conversion of sensible heat and tar heating value into electri-
cal energy at combined-cycle efficiency.
119
-------
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-------
REFERENCES,,continued
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Fedarko, W., A. Gatti, and L.R. McCreight. High Temperature
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Feldman, P., J. Bush and M. Robinson. High Temperature, High
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and Low Temperature Gas Cleaning for Coal Gasification-
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REFERENCES, continued
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73515, NSF/RA-770133. NTIS PB 270-017. April, 1977.
Lewis Research Center (NASA). Comparative Evaluation of Phase
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Lundgren, D.A. and T.C. Gunderson. Filtration Characteristics
of Glass Fiber Filter Media at Elevated Temperatures.
EPA 600/2-76-192. July, 1976.
McCabe, J.T. Cyclocentrifuge Development for Particulate Control;
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McCain, J.D. Evaluation of Rexnord Gravel Bed Filter. EPA
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122
-------
REFERENCES, continued
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Scrubber for Particle Collection at High Temperature and
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Yung, S.C., et al. Granular Bed Filter for Particle Collection
at High Temperature and Pressure. Proceedings of EPA/DOE
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124
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-79-019
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Alternatives for High-temperature/High-pressure
Particulate Control
5. REPORT DATE
January 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Richard Parker and Seymour Calvert
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Air Pollution Technology, Inc.
4901 Morena Boulevard, Suite 402
San Diego, California 92117
10. PROGRAM ELEMENT NO.
1NE624
11. CONTRACT/GRANT NO.
68-02-2190
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PER
Final; 10/77 - 10/78
ERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES EERL-RTP project officer is Dennis C. Drehmel, MD-61, 919/541-
2925.
16. ABSTRACT The report gjves tne status of the most promising high-temperature/high-
pressure (HTP) particulate control devices being developed. Data are presented and
anticipated performance and development problems are discussed. HTP particulate
control offers efficiency and potential economic advantages over cold gas cleanup in
pressurized fluidized-bed combustion (PFBC) and low-Btu coal gasification (LBCG)
combined-cycle power generation systems. However, considerably more develop-
ment will be necessary in order to demonstrate the technical and economic feasi-
bility of HTP gas cleanup commercially. The alternative of recuperative heat ex-
change coupled with low-temperature/high-pressure particulate control is reviewed
with regard to power system efficiencies for PFBC and LBCG combined-cycle pro-
cesses . Successful hot gas cleanup has clear efficiency advantages (1-7%) over cold
gas cleanup. The economics of hot gas cleanup, however, are very speculative at
the current state of development.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
Pollution Coal Gasification
Dust Gas Scrubbing
High Temperature Tests
High Pressure Tests
Combustion
Fluidized Bed Pro-
cess ins;
Pollution Control
Stationary Sources
Particulate
High Temperature/
Pressure Control
Gas Cleanup
Combined-cycle systems
13B
11G
14B
21B
13H.07A
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
155
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
125
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