&EPA
United States Industrial Environmental Research EPA-600/9-82-005a
Environmental Protection Laboratory July 1982
Agency Research Triangle Park NC 2771 1
Research and Development
Third Symposium on the
Transfer and
Utilization of Particulate
Control Technology:
Volume I. Control of
Emissions from Coal
Fired Boilers
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EPA-600/9-82-005a
April 1982
THIRD SYMPOSIUM ON THE
TRANSFER AND UTILIZATION OF
PARTICULATE CONTROL TECHNOLOGY
VOLUME I. CONTROL OF EMISSIONS FROM COAL FIRED BOILERS
Compiled by:
F.P. Venditti, J.A. Armstrong, and M. Durham
Denver Research Institute
P.O. Box 10127
Denver, Colorado 80208
Grant Number: R805725
Project Officer
Dale L. Harmon
Office of Environmental Engineering and Technology
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
Prepared for:
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NC 27711
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DISCLAIMER
This report has been reviewed by the Industrial Environmental
Research Laboratory-Research Triangle Park, North Carolina, Office of
Research and Development, U.S. Environmental Protection Agency, and
approved for publication. Approval does not signify that the contents
necessarily reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
11
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ABSTRACT
The papers in these four volumes of Proceedings were presented at the
Third Symposium on the Transfer and Utilization of Particulate Control
Technology held in Orlando, Florida during 9 March through 13 March 1981,
sponsored by the Particulate Technology Branch of the Industrial Environ-
mental Research Laboratory of the Environmental Protection Agency and
coordinated by the Denver Research Institute of the University of Denver.
The purpose of the symposium was to bring together researchers,
manufacturers, users, government agencies, educators and students to
discuss new technology and to provide an effective means for the transfer
of this technology out of the laboratories and into the hands of the users.
The three major categories of control technologies —electrostatic
precipitators, scrubbers, and fabric filters — were the major concern of the
symposium. These technologies were discussed from the perspectives of
economics; new technical advancements in science and engineering; and
applications. Several papers dealt with combinations of devices and
technologies, leading to a concept of using a systems approach to partic-
ulate control rather than device control. Additional topic areas included
novel control devices, high temperature/high pressure applications. fugitive
emissions, and measurement techniques.
These proceedings are divided into four volumes, each volume contain-
ing a set of related session topics to provide easy access to a unified
technology area.
iii
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VOLUME I
Page
VOLUME II. ELECTROSTATIC PRECIPITATORS--CONTENTS ... ix
VOLUME III. PARTICULATE CONTROL DEVICES--CONTENTS . . . xiii
VOLUME IV. ATYPICAL APPLICATIONS—CONTENTS xviii
Section A - Fabric Filters
COAL PROPERTIES AND FLY ASH FILTERABILITY 1
R. Dennis, J.A. Dirgo and L.S. Hovis
PULSE-JET FILTRATION WITH ELECTRICALLY
CHARGED FLYASH 11
R.P. Donovan, L.S. Hovis, G.H. Ramsey and J.H. Abbott
ELECTRICALLY CHARGED FLYASH EXPERIMENTS IN A
LABORATORY SHAKER BAGHOUSE 23
L.S. Hovis, J.H. Abbott, R.P. Donovan and C.A. Pareja
ELECTROSTATIC AUGMENTATION OF FABRIC FILTRATION .... 35
D.W. VanOsdell, G.P. Greiner, G.E.R. Lamb and L.S. Hovis
FABRIC WEAR STUDIES AT HARRINGTON STATION 45
R. Chambers, K. Ladd, S. Kunka and D. Harmon
SPS PILOT BAGHOUSE OPERATION 55
K. Ladd, W. Hooks, S. Kunka and D. Harmon
REVIEW OF SPS INVESTIGATION OF HARRINGTON STATION
UNIT 2 FABRIC FILTER SYSTEM 65
K. Ladd, S. Kunka
A SUMMARY OF PERFORMANCE TESTING OF THE APITRON
ELECTROSTATICALLY AUGMENTED FABRIC FILTER 75
D. Helfritch and L. Kirsten
FABRIC FILTER OPERATING EXPERIENCE FROM SEVERAL
MAJOR UTILITY UNITS 82
O.F. Fortune, R.L. Miller and E.A. Samuel
EVALUATION OF THE 25 MW KRAMER STATION BAGHOUSE:
TRACE ELEMENT EMISSION CONTROL 94
M.W. McElroy and R.C. Carr
CHARACTERIZATION OF A 10 MW FABRIC FILTER
PILOT PLANT 96
W.B. Smith, K.M. Gushing and R.C. Carr
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VOLUME I CONTENTS (cont.)
SPECIFYING A FABRIC FILTER SYSTEM 107
R.L. Ostop and D.A. Single
EVALUATION OF THE 25 MW KRAMER STATION BAGHOUSE:
OPERATIONAL FACTORS IN PARTICULATE MATTER
EMISSION CONTROL 118
R.C. Carr and M.W. McElroy
PULSE-JET TYPE FABRIC FILTER EXPERIENCE AT AIR TO
CLOTH RATIOS OF 5 TO 1 ON A BOILER FIRING PULVERIZED
COAL 120
G.L. Pearson
SELECTION AND OPERATION OF BAGHOUSES AT R.D. NIXON
STATION, UNIT #1 129
R.C. Hyde, J. Arello and D.J. Huber
POTENTIAL FOR IMPROVEMENT IN BAGHOUSE DESIGN 138
R.M. Jensen
REVIEW OF OPERATING AND MAINTENANCE EXPERIENCES WITH
HIGH TEMPERATURE FILTER MEDIA ON COAL-FIRED BOILERS . . .148
L.K. Crippen
Section B - Electrostatic Precipitators
PILOT DEMONSTRATION OF THE PRECHARGER-COLLECTOR
SYSTEM 157
P. Vann Bush, Duane H. Pontius
REMEDIAL TREATMENTS FOR DETERIORATED HOT SIDE
PRECIPITATOR PERFORMANCE 165
R.E. Bickelhaupt
EVALUATION OF THE UNITED McGILL ELECTROSTATIC
PRECIPITATOR 176
D.S. Ensor, P.A. Lawless, A.S. Damle
PREDICTING THE EFFECT OF PROPRIETARY CONDITIONING
AGENTS ON FLY ASH RESISTIVITY 185
R.J. Jaworowski and J.J. Lavin
SO, CONDITIONING TO ENABLE ELECTROSTATIC
PRECIPITATORS TO MEET DESIGN EFFICIENCIES 197
J.J. Ferrigan, III
vi
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VOLUME I CONTENTS (cont.)
ENHANCED PRECIPITATOR COLLECTION EFFICIENCIES
THROUGH RESISTIVITY MODIFICATION 206
D.F. Mahoney
DEVELOPMENT OF A NEW SULFUR TYPE ASH CONDITIONING . . . .216
R.H. Gaunt
OPERATING EXPERIENCE WITH FLUE GAS CONDITIONING
SYSTEMS AT COMMONWEALTH EDISON COMPANY 226
L.L. Weyers and R.E. Cook
THE APPLICATION OF A TUBULAR WET ELECTROSTATIC
PRECIPITATOR FOR FINE PARTICULATE CONTROL AND
DEMISTING IN AN INTEGRATED FLY ASH AND SO2 REMOVAL
SYSTEM ON COAL-FIRED BOILERS 236
E. Bakke and H.P. Willett
FIELD EVALUATIONS OF AMMONIUM SULFATE CONDITIONING
FOR IMPROVEMENT OF COLD SIDE ELECTROSTATIC PRECIPITATOR
PERFORMANCE 237
E.G. Landham, Jr., G.H. Marchant, Jr., J.P. Gooch and
R.F. Altman
EVALUATION OF PERFORMANCE ENHANCEMENT OBTAINED
WITH PULSE ENERGIZATION SYSTEMS ON A HOT-SIDE
ELECTROSTATIC PRECIPITATOR 253
W. Piulle, L.E. Sparks, G.H. Marchant, Jr. and J.P. Gooch
A NEW MICROCOMPUTER AND STRATEGY FOR THE CONTROL
OF ELECTROSTATIC PRECIPITATORS 265
K.J. McLean, T.S. Ng, Z. Herceg and Z. Rana
ASSESSMENT OF THE COMMERCIAL POTENTIAL FOR THE HIGH
INTENSITY IONIZER IN THE ELECTRIC UTILITY INDUSTRY . . . .272
J.S. Lagarias, J.R. McDonald and D.V. Giovanni
APPLICATION OF ENERGY CONSERVING PULSE ENERGIZATION
FOR PRECIPITATORS--PRACTICAL AND ECONOMIC ASPECTS . . . .291
H. H. Petersen and P. Lausen
Section C - Dry SO9 Scrubbers
S02 REMOVAL BY DRY INJECTION AND SPRAY ABSORPTION
TECHNIQUES 303
E.L. Parsons, Jr., V. Boscak, T.G. Brna and R.L. Ostop
DRY SCRUBBING SO2 AND PARTICULATE CONTROL 313
N.J. Stevens, G.B. Manavizadeh, G.W. Taylor and M.J. Widico
vii
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VOLUME I CONTENTS (cont.) pg(
-i -»
FIBER AND FABRIC ASPECTS FOR SO2 DRY SCRUBBING
BAGHOUSE SYSTEMS
L. Bergmann
TWO-STAGE DRY FLUE GAS CLEANING USING CALCIUM
ALKALIS 333
D.C. Gehri, D.F. Dustin and S.J. Stachura
CONTROL OF SULFUR DIOXIDE, CHLORINE, AND TRACE
ELEMENT EMISSIONS FROM COAL-FIRED BOILERS BY FABRIC
FILTRATION 341
R.J. Demski, J.T. Yeh and J.I. Joubert
Section D - Scrubbers
FLYASH COLLECTION USING A VENTURI SCRUBBER—MINNESOTA
POWER'S COMMERCIAL OPERATING EXPERIENCE 352
C.A. Johnson
AUTHOR INDEX 361
viii
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VOLUME II
ELECTROSTATIC PRECIPITATORS
Section A - Fundamentals
Page
MATHEMATICAL MODELING OF IONIC
CONDUCTION IN FLY ASH LAYERS 1
R.B. Mosley, J.R. McDonald and L.E. Sparks
MEASUREMENTS OF ELECTRICAL PROPERTIES
OF FLY ASH LAYERS 13
R.B. Mosley, P.R. Cavanaugh, J.R. McDonald and L.E. Sparks
LASER DOPPLER ANEMOMETER MEASUREMENTS OF PARTICLE
VELOCITY IN A LABORATORY PRECIPITATOR 25
P.A. Lawless, A.S. Damle, A.S. Viner, E.J. Shaughnessy and
L.E. Sparks
PROGRESS IN MODELING BACK CORONA 35
P.A. Lawless
A COMPUTER MODEL FOR ESP PERFORMANCE 44
P.A. Lawless, J.W. Dunn and L.E. Sparks
MEASUREMENT AND INTERPRETATION OF CURRENT
DENSITY DISTRIBUTION AND CHARGE/MASS DATA 54
M. Durham, G. Rinard, D. Rugg and L.E. Sparks
THE RELATIONSHIP BETWEEN GAS STREAM TURBULENCE
AND COLLECTION EFFICIENCY IN A LAB-SCALED
ELECTROSTATIC PRECIPITATOR 66
B.E. Pyle, J.R. McDonald, W.B. Smith
PARTICLE DEPOSITION PROFILES AND REENTRAINMENT
IN A WIRE-PLATE ELECTROSTATIC PRECIPITATOR 76
E. Arce-Medina and R.M. Felder
PARTICLE TRANSPORT IN THE EHD FIELD 87
T. Yamamoto
SURFACE REENTRAINMENT OF COLLECTED FLY ASH IN
ELECTROSTATIC PRECIPITATORS 97
M. Mitchner, M.J. Fisher, D.S. Gere, R.N. Leach and S.A. Self
ELECTROMECHANICS OF PRECIPITATED ASH LAYERS 109
G.B. Moslehi and S.A. Self
ix
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VOLUME II CONTENTS (cont.)
EXPERIMENTAL MEASUREMENTS OF THE EFFECT OF
TURBULENT DIFFUSION ON PRECIPITATOR EFFICIENCY
G.L. Leonard, M. Mitchner and S.A. Self
CAN REENTRAINMENT BE EXPLAINED USING A NEW
PRECIPITATOR FORMULA? 1JU
S. Maartmann
A LABORATORY FURNACE FOR THE PRODUCTION OF
SYNTHETIC FLY ASH FROM SMALL COAL SAMPLES 141
K.M. Sullivan
COMPUTER SIMULATION OF THE WIDE PLATE
SPACING EFFECT 149
E. A. Samuel
SIMULTANEOUS MEASUREMENTS OF AERODYNAMIC SIZE
AND ELECTRIC CHARGE OF AEROSOL PARTICLES IN REAL
TIME ON A SINGLE PARTICLE BASIS 160
M.K. Mazumder, R.G. Renninger, T.H. Chang,
R.W. Raible, W.G. Hood, R.E. Ware and R.A. Sims
APPLICATION OF LASER DOPPLER INSTRUMENTATION TO
PARTICLE TRANSPORT MEASUREMENTS IN AN ELECTROSTATIC
PRECIPITATOR 169
M.K. Mazumder, W.T. Clark III, R.E. Ware, P.C. McLeod,
W.G. Hood, J.E. Straub and S. Wanchoo
THE APPLICATION OF MEASUREMENTS OF AEROSOL
CHARGE ACQUISITION BY BIPOLAR IONS TO THE PROBLEM
OF BACK CORONA 179
R.A. Fjeld, R.O. Gauntt, G.J. Laughlin and A.R. McFarland
IDENTIFICATION OF BACK DISCHARGE SEVERITY 189
S. Masuda and Y. Nonogaki
Section B - Operations and Maintenance
MODELING OF ELECTROSTATIC PRECIPITATORS WITH RESPECT
TO RAPPING REENTRAINMENT AND OUTLET OPACITY 199
M.G. Faulkner, W.E. Farthing, J.R. McDonald and L.E. Sparks
NEW PRECIPITATOR TECHNOLOGY FOR PARTICULATE
CONTROL 208
J.R. Zarfoss
X
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VOLUME II CONTENTS (cont.)
Page
AN APPLICATION SUMMARY OF HIGH ENERGY SONIC
CLEANING APPLIED TO ELECTROSTATIC PRECIPITATORS 218
M.J. Berlant
THE IMPACT OF INTELLIGENT PRECIPITATOR CONTROLS 230
N.Z. Shilling, R.O. Reese and J.A. Fackler
AN ENERGY MANAGEMENT SYSTEM FOR
ELECTROSTATIC PRECIPITATORS 242
R.R. Crynack and M.P. Downey
RELATIONSHIP BETWEEN ELECTROSTATIC PRECIPITATOR
PERFORMANCE AND RECORDKEEPING PRACTICES 252
S.P. Schliesser
AN OPERATION AND MAINTENANCE PROGRAM FOR
A PHOSPHATE ROCK ELECTROSTATIC PRECIPITATOR 262
D.B. Rimberg
Section C - Advanced Design
ELECTROSTATIC PRECIPITATOR PERFORMANCE
WITH PULSE EXCITATION 273
D. Rugg, M. Durham, G. Rinard and L.E. Sparks
DEVELOPMENT OF A CHARGING DEVICE FOR HIGH-RESISTIVITY
DUST USING HEATED AND COOLED ELECTRODES 283
G. Rinard, M. Durham, D. Rugg and L.E. Sparks
THE EVALUATION OF NOVEL ELECTROSTATIC PRECIPITATOR
SYSTEMS USING A TRANSPORTABLE PROTOTYPE 295
G. Rinard, M. Durham, D. Rugg, J. Armstrong,
L.E. Sparks and J.H. Abbott
ANALYSIS OF THE ELECTRICAL AND CHARGING
CHARACTERISTICS OF A THREE ELECTRODE PRECHARGER . . . .304
K.J. McLean
PARTICLE CHARGING IN AN ELECTROSTATIC
PRECIPITATOR BY PULSE AND DC VOLTAGES 314
L.E. Sparks, G.H. Ramsey, R.E. Valentine and J.H. Abbott
PARTICLE COLLECTION IN A TWO STAGE ELECTROSTATIC
PRECIPITATOR WITH VARIOUS COLLECTOR STAGES 326
L.E. Sparks, G.H. Ramsey, R.E. Valentine and J.H. Abbott
HIGH INTENSITY IONIZER DEVELOPMENT 334
M.H. Anderson, J.R. McDonald, J.P. Gooch and D.V. Giovanni
xi
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VOLUME II CONTENTS (cont.)
DEMONSTRATION OF AIR POLLUTION SYSTEMS HIGH
INTENSITY IONIZER/ELECTROSTATIC PRECIPITATOR ON
AN OIL-FIRED BOILER
G.A. Raemhild, A. Prem and F. Weisz
PRIMARY AND SECONDARY IONIZATION IN AN
ELECTRON BEAM PRECIPITATOR SYSTEM
W.C. Finney, L.C. Thanh, J.S. Clements and R.H. Davis
INFLUENCE ON PARTICLE CHARGING OF ELECTRICAL
PARAMETERS AT DC AND PULSE VOLTAGES ......... 370
H.J. Joergensen, J.T. Kristiansen and P. Lausen
BOXER-CHARGER MARK III AND ITS
APPLICATION IN ESP'S ................ 380
S. Masuda, H. Nakatani and A. Mizuno
THE PERFORMANCE OF AN EXPERIMENTAL
PRECIPITATOR WITH AN ALL-PLATE ZONE .......... 390
J. Dalmon
THE PHYSICS OF PULSE ENERGIZATION OF
ELECTROSTATIC PRECIPITATORS ............. 404
L. Menegozzi and P.L. Feldman
ADVANCED ELECTRODE DESIGN FOR
ELECTROSTATIC PRECIPITATORS ............. 405
S. Bernstein, K. Ushimaru and E.W. Geller
Section D - Industrial Applications
PROBLEMS IN APPLYING AN ELECTROSTATIC
PRECIPITATOR TO A SALVAGE FUEL-FIRED BOILER ....... 415
C.R. Thompson
THE APPLICATION OF ELECTROSTATIC PRECIPITATORS
TO BOILERS FIRING MULTIPLE FUELS ........... 425
R.L. Bump
AUTHOR INDEX ................... 435
xii
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VOLUME III
PARTICULATE CONTROL DEVICES
Section A - Scrubbers
Page
THE CALVERT SCRUBBER 1
S. Calvert, R.G. Patterson and S. Yung
FLUX FORCE/CONDENSATION SCRUBBER SYSTEM
FOR COLLECTION OF FINE PARTICULATE EMISSIONS
FROM AN IRON MELTING CUPOLA 10
S. Calvert and D.L. Harmon
DEMONSTRATION OF HIGH-INTENSlTY-IONIZER-ENHANCED
VENTURI SCRUBBER ON A MAGNESIUM RECOVERY
FURNACE FUME EMISSIONS 21
A. Prem, M.T. Kearns and D.L. Harmon
A NEW ENTRY IN THE HIGH EFFICIENCY SCRUBBER FIELD .... 33
L.C. Hardison and F. Ekman
PERFORMANCE OF PARTICULATE SCRUBBERS AS
INFLUENCED BY GAS-LIQUID CONTACTOR DESIGN
AND BY DUST FLOCCULATION 43
K.T. Semrau and R.J. Lunn
INVESTIGATION OF VENTURI SCRUBBER EFFICIENCY
AND PRESSURE DROP 51
R. Parker, T. Le and S. Calvert
SCRUBBER TECHNOLOGY AND THE INTERACTION OF
A UNIQUE STRUCTURE AS MIST ELIMINATOR 60
G.C. Pedersen
NOVEL ANNULAR VENTURI SCRUBBER DESIGN REDUCES
WASTE DISCHARGE PROBLEMS 71
H.P. Beutner
CONSIDERATION OF THE PERTINENT DESIGN AND
OPERATING CHARACTERISTICS ESSENTIAL FOR
OPTIMIZATION OF VENTURI SCRUBBER PERFORMANCE 80
H.S. Oglesby
APPLICATION OF SCRUBBERS FOR PARTICULATE
CONTROL OF INDUSTRIAL BOILERS 90
M. Borenstein
xiii
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VOLUME III CONTENTS (cont.)
Page
APPLICATION OF HIGH ENERGY VENTURI SCRUBBERS
TO SEWAGE INCINERATION ...............
F.X. Reardon
AN INCINERATOR SCRUBBER THAT WORKS:
A CASE STUDY ................... m
C. Menoher
EVALUATION OF ENTRAINED LIQUOR CONTRIBUTION TO
TOTAL MASS EMISSIONS DOWNSTREAM OF A WET SCRUBBER . . .119
W. David Balfour, L.O. Edwards and H.J. Williamson
Section B - Fabric Filters
A DUAL-BEAM BACKSCATTER BETA-PARTICLE GAUGE
FOR MEASURING THE DUST CAKE THICKNESS ON OPERATING
BAG FILTERS INDEPENDENT OF POSITION .......... 128
R.P. Gardner, R.P. Donovan and L.S. Hovis
DIAGNOSING FILTER FABRIC CAPABILITIES WITH LIGHT
SCATTERING AND NUCLEI DETECTING INSTRUMENTATION . . . .140
R. Dennis, D.V. Bubenick and L.S. Hovis
ACID DEWPOINT CORROSION IN PARTICULATE
CONTROL EQUIPMENT ................. 150
T.E. Mappes, R.D. Terns and K.E. Foster
SECOND GENERATION OF EMISSIONS CONTROL
SYSTEM FOR COKE OVENS ............... 160
J.D. Patton
EFFECTS OF FLYASH SIZE DISTRIBUTION ON THE
PERFORMANCE OF A FIBERGLASS FILTER .......... 171
W.F. Frazier and W.T. Davis
FUNDAMENTAL STUDY OF A FABRIC FILTER
WITH A CORONA PRECHARGER .............. 181
K. linoya and Y. Mori
ECONOMIC EVALUATION FACTORS IN BID
EVALUATIONS— A SENSITIVITY ANALYSIS .......... 193
J.G. Musgrove and J.E. Shellabarger
FLY ASH RE-ENTRAINMENT IN A BAGHOUSE—
WHAT DOES IT COST? ................. 201
J.G. Musgrove
xiv
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VOLUME III CONTENTS (cont.)
Page
WHY PERFORM MODEL STUDY OF FABRIC FILTER
COLLECTOR? 211
W.T. Langan, N.Z. Shilling, W.A. Van Kleunen and O.F. Fortune
EXPERIENCES OF A SMALL INSULATION MANUFACTURER
IN MAINTAINING COMPLIANCE WITH AIR POLLUTION
CONTROL REGULATIONS 221
R.L. Hawks
ADVANCED FABRIC FILTER TECHNOLOGY FOR
DIFFICULT PARTICULATE EMISSIONS 228
H.P. Beutner
DEVELOPMENT OF GUIDELINES FOR OPTIMUM BAGHOUSE
FLUID DYNAMIC SYSTEM DESIGN 238
D. Eskinazi, G.B. Gilbert and R.C. Carr
THEORETICAL ASPECTS OF PRESSURE DROP REDUCTION
IN A FABRIC FILTER WITH CHARGED PARTICLES 250
T. Chiang, E.A. Samuel and K.E. Wolpert
EXPERIMENTAL CORRELATION OF DUST CAKE POROSITY,
AIR-TO-CLOTH RATIO AND PARTICLE-SIZE DISTRIBUTIONS . . . .261
T. Chiang and R.L. Ostop
MODEL FOR DUST PENETRATION THROUGH A
PULSE-JET FABRIC FILTER 270
D. Leith and M.J. Ellenbecker
PERFORMANCES OF DUST LOADED AIR FILTERS 280
C. Kanaoka, H. Emi and M. Ohta
ELECTROSTATICALLY ENHANCED FABRIC
FILTRATION OF PARTICULATES 290
T. Ariman and S.T. McComas
A STAGGERED ARRAY MODEL OF A FIBROUS FILTER
WITH ELECTRICAL ENHANCEMENT 301
F. Henry and T. Ariman
Section C - Granular Beds
AEROSOL FILTRATION BY A COCURRENT MOVING
GRANULAR BED: PENETRATION THEORY 311
T.W. Kalinowski and D. Leith
FUNDAMENTAL EXPERIMENTS ON A GRANULAR BED FILTER . . . .321
K. linoya and Y. Mori
XV
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VOLUME III CONTENTS (cont.)
DRY DUST COLLECTION OF BLAST FURNACE
EXHAUST GAS BY MOVING GRANULAR BED FILTER
A. Wakabayashi, T. Sugawara and S. Watanabe
Section D - Novel Devices
IRON AND STEEL AIR POLLUTION CONTROL
USING MAGNETIC SEPARATION 341
D.C. Drehmel, C.E. Ball and C.H. Gooding
TECHNICAL AND ECONOMIC EVALUATION OF TWO
NOVEL PARTICULATE CONTROL DEVICES 353
R.R. Boericke, J.T. Kuo and K.R. Murphy
TM
THE ELECTROSCRUBBER111 FILTER—APPLICATIONS
AND PARTICULATE COLLECTION PERFORMANCE 363
D. Parquet
HIGH EFFICIENCY PARTICULATE REMOVAL WITH
SINTERED METAL FILTERS 373
B.E. Kirstein, W.J. Paplawsky, D.T. Pence and T.G. Hedahl
APPLICATION OF ELECTROSTATIC TECHNIQUES TO
THE REMOVAL OF DUST AND FUME FROM THE
INDUSTRIAL ENVIRONMENT 382
S.A. Hoenig
THE DRY VENTURI 393
AJ. Teller and D.RJ. Roy
FIBER BED FILTER SYSTEM CONTROL OF
WELDING PARTICULATES 398
J.A. Bamberger and W.K. Winegardner
THE USE OF GLASS CAPILLARY FILTERS TO
CLASSIFY ACTINOLITE FIBERS 406
J.W. Gentry, T.C. Chen, S.W. Lin and P.Y. Yu
ULTRA-HIGH EFFICIENCY FILTRATION SYSTEMS
(AIR RECIRCULATION) 417
R.W. Potokar
THE WET WALL ELECTROSTATIC PRECIPITATOR 428
J. Starke, J. Kautz and K-R. Hegemann
xvi
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VOLUME III CONTENTS (cont.)
Section E - Mechanical Collectors
TROUBLESHOOTING MULTIPLE CYCLONES ON
FUEL-OIL-FIRED BOILERS 438
F. Crowson and R.L. Gibbs
COLLECTION EFFICIENCIES OF CYCLONE SEPARATORS 449
P.W. Dietz
ELECTROSTATICALLY AUGMENTED COLLECTION
IN VORTICAL FLOWS 459
P.W. Dietz
HIGH PERFORMANCE CYCLONE DEVELOPMENT 468
W.G. Giles
AUTHOR INDEX 481
xvii
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VOLUME IV
ATYPICAL APPLICATIONS
Section A - Advanced Energy Applications
Page
HIGH TEMPERATURE PARTICLE COLLECTION WITH
A.P.T. EPxP DRY SCRUBBER .......... • • • • L
S. Yung, T. Lee, R.C. Patterson, S. Calvert and D.C. Drenmel
PARTICLE COLLECTION IN CYCLONES AT HIGH TEMPERATURE
AND HIGH PRESSURE
R. Parker, R. Jain, S. Calvert, D.C. Drehmel and J. Abbott
OPERATING RESULTS OF ELECTROSTATIC PRECIPITATORS
AT HIGH TEMPERATURE AND HIGH PRESSURES ........ 3
P.L. Feldman and K.S. Kumar
CONTROL OF PARTICULATES IN PROCESS AREA 12, SOLVENT
REFINED COAL PROCESS ................ 15
W.H. Wilks, P.D- Wilkinson and J.A. Schlosberg
NON- PLUGGING RETAINING STRUCTURE FOR GRANULAR
BED FILTER FOR HTHP APPLICATION ........... 26
A.M. Presser and J.C. Alexander
PARTICULATE EMISSIONS CONTROL FROM A COAL-FIRED
OPEN-CYCLE MAGNETOHYDRODYNAMICS/STEAM POWER PLANT ... 36
H.H. Wang and T.E. Dowdy
REAL TIME COARSE PARTICLE MASS MEASUREMENTS IN
A HIGH TEMPERATURE AND PRESSURE COAL GASIFIER
PROCESS TREATMENT ................. 46
J. Wegrzyn, J. Saunders and W. Marlow
THE DESIGN, ENGINEERING, AND STARTUP OF A VENTURI
SCRUBBER SYSTEM ON AN OIL SHALE OFF-GAS INCINERATOR ... 55
P. A. Czuchra and J.S. Sterrett
FLUIDIZED-BED COMBUSTION HOT FLUE GAS CLEANUP
PERSPECTIVE ON CYCLONES AND OTHER DEVICES ....... 63
R.F. Henry and W.F. Podolski
PRESSURIZED AND NON-PRESSURIZED ACOUSTIC
AGGLOMERATORS FOR HOT-GAS CLEANUP APPLICATIONS ..... 73
K.H. Chou and D.T. Shaw
xviii
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VOLUME IV CONTENTS (cont.)
ALKALIS AND THEIR CONTRIBUTIONS TO CORONA CURRENT
AT HIGH TEMPERATURE AND HIGH PRESSURE 74
R.W.L. Snaddon
HOT GAS CLEANUP IN PRESSURIZED FLUIDIZED
BED COMBUSTION 83
L.N. Rubow and M.G. Klett
VENTURI SCRUBBING FOR CONTROL OF PARTICULATE
EMISSIONS FROM OIL SHALE RETORTING 95
G.M. Rinaldi and R.C. Thurnau
OVERVIEW OF THE DEPARTMENT OF ENERGY'S PRESSURIZED
FLUIDIZED-BED COMBUSTOR CLEANUP TECHNOLOGY PROGRAM . . .105
W.E. Moore
THE CYCLOCENTRIFUGE™—AN ADVANCED GAS/SOLIDS
SEPARATOR FOR COAL CONVERSION PROCESSES 116
P.R. Albrecht, J.T. McCabe and W. Fedarko
Section B - Fugitive Emissions
DEMONSTRATION OF THE USE OF CHARGED FOG IN
CONTROLLING FUGITIVE DUST FROM LARGE-SCALE
INDUSTRIAL SOURCES 125
E.T. Brookman, R.C. McCrillis and D.C. Drehmel
THE CONTROL OF FUGITIVE EMISSIONS USING WINDSCREENS . . .135
D. Games and D.C. Drehmel
THE INFLUENCE OF AGGREGATE PILE SHAPE AND
ORIENTATION ON PARTICULATE FUGITIVE EMISSIONS 145
D. Martin
SPRAY CHARGING "AND TRAPPING SCRUBBER FOR
FUGITIVE PARTICLE EMISSION CONTROL 155
S. Yung, S. Calvert and D.C. Drehmel
IMPROVED STREET SWEEPER FOR CONTROLLING URBAN
INHALABLE PARTICULATE MATTER 156
S. Calvert, H. Brattin, S. Bhutra, R. Parker and D.C. Drehmel
A WIND TUNNEL FOR DUST ENTRAINMENT STUDIES 168
A.S. Viner, M.B. Ranade, E.J. Shaughnessy, D.C. Drehmel
and B.E. Daniels
xix
-------
VOLUME IV CONTENTS (cont.) Page
TECHNIQUES AND EQUIPMENT FOR MEASURING INHALABLE
PARTICULATE FUGITIVE EMISSIONS ............
H.J. Kolnsberg
BALLOON SAMPLING TO CHARACTERIZE PARTICLE
MISSIONS FROM FUGITIVE SOURCE;
J.A. Armstrong and D.C. Drehmel
EMISSIONS FROM FUGITIVE SOURCES 188
AN ELECTROSTATICALLY CHARGED FOG GENERATOR FOR
THE CONTROL OF INHALABLE PARTICLES ..........
C.V. Mathai, L.A. Rathbun and D.C. Drehmel
RELATIVE EFFECTIVENESS OF CHEMICAL ADDITIVES
AND WIND SCREENS FOR FUGITIVE DUST CONTROL ....... 210
D.C. Drehmel and B.E. Daniel
PARTICULATE IMPACT COMPARISON BETWEEN CONTROLLED
STACK EMISSIONS FOR A 2000 MW ELECTRICAL GENERATING
STATION ..................... 222
H.E. Hesketh and F.L. Cross
OPERATING EXPERIENCE AND THE TECHNIQUES IN THE
CONTROL OF COAL DUST EMISSIONS FROM LARGE
STORAGE PILE AT NANTICOKE TGS ............ 232
N. Krishnamurthy, W. Whitman and Y.V. Nguyen
Section C - Opacity
MODELING SMOKE PLUME OPACITY FROM PARTICULATE
CONTROL EQUIPMENT ................. 242
D.S. Ensor, P. A. Lawless, S.J. Cowen
TETHERED BALLOON PLUME SAMPLING OF A PORTLAND
CEMENT PLANT ................... 252
J.A. Armstrong, P. A. Russell, M.N. Plooster
THE RELATIONSHIP OF FLY ASH LIGHT ABSORPTION TO
SMOKE PLUME OPACITY ................ 264
S.J. Cowen, D.S. Ensor
Section D - Measurements
A SPECIAL METHOD FOR THE ANALYSIS OF
SULFURIC ACID MISTS ................ 275
P. Urone, R.B. Mitchell, J.E. Rusnak, R.A. Lucas and
J.F. Griffiths
XX
-------
VOLUME IV CONTENTS (cont.)
A MICROCOMPUTER-BASED CASCADE-IMPACTOR
DATA-REDUCTION SYSTEM 285
M. Durham, S. Tegtmeyer, K. Wasmundt and L.E. Sparks
DEVELOPMENT OF A SAMPLING TRAIN FOR STACK
MEASUREMENT OF INHALABLE PARTICULATE 297
A.D. Williamson, W.B. Smith
INHALABLE PARTICULATE MATTER SAMPLING
PROGRAM FOR IRON AND STEEL: AN OVERVIEW
PROGRESS REPORT 306
R.C. McCrillis
DEVELOPMENT OF IP EMISSION FACTORS 317
D.L. Harmon
INHALABLE PARTICULATE EMISSION FACTOR PROGRAM
PURPOSE AND DEVELOPMENT 326
P.M. Noonan and J.H. Southerland
INHALABLE PARTICULATE EMISSION FACTORS FOR BLAST
FURNACE CASTHOUSES IN THE IRON AND STEEL INDUSTRY . . . .335
P.O. Spawn, S. Piper and S. Gronberg
INHALABLE PARTICULATE EMISSIONS FROM VEHICLES
TRAVELING ON PAVED ROADS 344
R. Bonn
QUALITY ASSURANCE FOR PARTICLE-SIZING MEASUREMENTS . . .353
C.E. Tatsch
PARTICULATE EMISSIONS CHARACTERIZATION FOR
OIL-FIRED BOILERS 363
D. Mormile, S. Hersh, B.F. Piper and M. McElroy
A CONTINUOUS REAL-TIME PARTICULATE MASS MONITOR
FOR STACK EMISSION APPLICATIONS 373
J.C.F. Wang, H. Patashnick and G. Rupprecht
Section E - Mobile Sources
STUDIES OF PARTICULATE REMOVAL FROM DIESEL EXHAUSTS
WITH ELECTROSTATIC AND ELECTROSTATICALLY-
AUGMENTED TECHNIQUES 383
J.L. DuBard, M.G. Faulkner, J.R. McDonald, D.C. Drehmel
and J.H. Abbott
xxi
-------
VOLUME IV CONTENTS (cont.)
v Page
STUDIES OF PARTICULATE REMOVAL FROM DIESEL EXHAUSTS
WITH MECHANICAL TECHNIQUES 395
M.G. Faulkner, J.L. DuBard, J.R. McDonald, D.C. Drehmel
and J.H. Abbott
UPDATE ON STATUS OF CONNECTICUT'S CONTROL PROGRAM
FOR TRANSPORTATION-RELATED PARTICULATE EMISSIONS . . . .406
H.L. Chamberlain and J.H. Gastler
AUTHOR INDEX 413
xxii
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COAL PROPERTIES AND FLY ASH FILTERABILITY
Richard Dennis
John A. Dirgo
GCA/Technology Division
213 Burlington Road
Bedford, MA 01730
Louis S. Hovis
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
ABSTRACT
The results of a survey of U.S. coal sources, types, compositions,
production, and use are described with the emphasis on coals whose combustion
effluents are controllable with fabric filters. Physical and chemical
properties of these coals and their mineral constituents were evaluated for
potential impact on resultant fly ashes. Several characterizing criteria
commonly used to delineate coal fouling and slagging properties were examined
for probable impact upon fly ash size and surface properties. Consideration
was also given to the method of coal combustion, operating temperatures,
fusion and hardness properties of mineral constituents, and slag viscosity as
possible factors in determining fly ash filtration characteristics. High
alkalinity contents were examined for their potential to increase fly ash gas
and moisture sorption. A major purpose of the survey was to provide a
rationale for selecting representative fly ash samples for laboratory
determination of the specific resistance coefficient (K2) and the cleaning
parameter (ac).
BACKGROUND
Reliable predictions of fabric filter performance depend upon accurate
determination of two important variables: 10), the specific resistance
coefficient for the dust; and ac, the cleaning parameter.1»2 Although
many factors influence the filtration process, the variable indicating the air
permeability properties of the dust layer (K2) and the parameter defining
the fraction of the surface dust layer removed by the cleaning action (ac)
play commanding roles in those filter systems cleaned by reverse flow air
and/or by mechanical shaking. Because of limitations in the existing theory,
direct measurements of K£ and ac represent the only safe way to evaluate
these parameters. For this reason, GCA has performed under EPA sponsorship
several filtration research programs involving both the prediction and direct
measurement of K£ and ac: Contracts 68-02-1438, Task 5; 68-02-2607, Tasks
7 and 8; and 68-02-2607, Technical Directive 35. The objective of the
-------
on-going program (Contract No. 68-02-3151) is to augment the available
K2-ac data base by searching for definitive relationships between parent
coal properties and the filterability of the resultant fly ashes.
This paper reviews the background phase of this study involving the actual
sample selection and the identification of relevant coal and fly ash
properties. First, a rationale for the selection of specific coal fly ashes
for laboratory evaluation was established based upon a detailed survey of U.S.
coal production statistics and the projected use of these coals in energy
generation processes where fabric filtration might be used for control of
particulate emissions. Second, after determining what appeared to be
representative statistics for fly ash sources and coal consumption, coal
properties and other factors affecting the filterability of the resultant fly
ashes were considered. These included the mineralogical composition of coal
seam overburdens, floors, and partings; probable changes associated with the
preparation of coal for combustion; and the impact of the combustion process
itself upon fly ash characteristics.
PRELIMINARY SURVEY OF U.S. COAL PRODUCTION AND UTILIZATION
Coal production and use statistics given in Table 1 show the estimated
tonnages of lignite, subbituminous, and bituminous coals that are mined in the
six major U.S. coal regions and combusted in utility and industrial
energy-generating operations. These data show that nearly 80 percent of the
coal is produced in the Eastern and Midwestern U.S. whereas current Western
output is only about 20 percent.3 However, the increase in coal production
to supply new, large (2,000 MW) power stations in the West is expected to
change this balance. It should also be noted that the latter region is the
source and the primary user of lignite coals.
Figure 1 shows the geographical distribution of major U.S. coal fields,
the types of coal mined, and the locations of coal-burning industries and
utilities that now use fabric filtration for particulate collection.^ Coal
delivery statistics demonstrate that most coal is burned in the region in
which it is mined, and the average transportation distance of U.S. coal is
only 592 km.5 Detailed information concerning coal production and
properties for various regions, districts, and seams may be found in the
Keystone Coal Manual and related publications.69?
The production statistics in Table 1 were used to establish a preliminary
weighting for the selection of fly ash samples based upon emission potential.
This weighting, however, could be modified to take into account any unusual
coal and fly ash properties that might conceivably bias the original selection.
-------
ANALYSES OF COAL AND FLY ASH PROPERTIES
Proximate Analyses and Sulfur Content
Proximate analyses, which describe the properties used to assign coal
rankings, do not include those factors suspected to exert first order effects
on fly ash filterability via their impacts on particle size. However, the
moisture and ash content can affect significantly the magnitude of the
uncontrolled emissions and, in turn, the design capacity and frequency of
baghouse cleaning. It is also possible that the fuel composition with respect
to moisture, ash, volatiles, and fixed carbon may have some bearing on the
ultimate size of the non-coal mineral constituents (overburdens, partings, and
floors) discharging from the pulverizers. Additionally, as ash content
increases, less heat may transfer to the particles for a fixed energy input,
thus retarding or preventing particle transformation to the fluid state.
Approximate average values for volatiles, sulfur, ash, and moisture
contents for the six coal-producing regions are shown in Figure 2. Despite
the broad range in coal analyses (and with allowances for variability in
mining, cleaning, and assaying techniques), three general coal groups are
discernable. Eastern coals represented by Regions I through III are
characterized by medium to high volatile content, ~2 percent sulfur, low (6 to
8 percent) ash, and low (4 percent) moisture. Because of the high ash and
moisture levels for Midwestern coals, the heating values are usually lower
while the high sulfur content adds to the overall emissions control problems.
In the case of Western Region VI coals, sulfur contents are consistently low,
the volatile contents high, and, with the exception of lignites, the ash and
moisture contents are similar to those for Midwestern coals. The fact that
electrostatic precipitators may perform less effeciently with low sulfur coals
suggests that the filtration option may be the better approach with many
Western coals.
Mineralogy of Coal Seam Overburdens, Partings, and Floors
Certain minerals are associated with the common rock structures
surrounding or separating coal seams as distinguished from the inorganic
materials encapsulated in the parent vegetable matter from which the coals
were formed. The relative hardness, chemical constituents, and fusion
properties of such minerals are expected to affect particle size properties,
chemical activity, and hygroscopicity of the fly ash. Generally, the
similarities among the mineral fractions of most coals tend to outweigh the
differences. Claystones, shales, sandstones, and slate represent typical
constituents that appear in many combinations in U.S. coal formations."''
In the case of sandstones with high quartz contents, one might expect to find
larger ash particles and fewer fusion products (cenospheres) in the fly ash
because of the hardness and high melting point for quartz. Conversely, if
dolomitic species are the predominant sandstone component, pulverizing should
produce finer particles because of increased friability. Reaction of calcined
components with sulfur oxides may also produce a family of hygroscopic
reaction products in the form of sulfites, sulfates, and bisulfites that may
contribute to the formation of a "sticky" dust layer.
-------
Chemical assays alone may not be sufficient to determine what effect the
mineral content of the coal has upon the actual fly ash filtration
characteristics. For example, it is expected that any inorganic species
originating within the coal will appear in a finely subdivided form such that
its capacity to react rapidly with flue gas components and to form finer
particles far exceeds that of the coarser rock phase. The specific
combinations of inorganic constituents may also play important roles in
controlling the fusion and slag forming properties of the coal ash which, in
turn, may impact strongly upon the quantity, shape, and size of the
uncontrolled fly ash emissions.
Physical Properties and Characterizing Chemical Ratios for Typical U.S. Coal
Ashes
The manner in which certain coal properties and various characterizing
chemical ratios are postulated to affect fly ash collectability is indicated
in Table 2. Although this listing was developed mainly from selected
information sources on boiler slagging and fouling problems,^>° these data
also provide useful guidelines pertaining to the structure of the coal fly ash
particles and their ultimate behavior in a filter system. Exclusive of those
ash properties that relate to the combustion and heat transfer aspects of the
boiler operation, four factors may be identified that affect the fly ash
filterability. These are ash melting or fusion temperatures, ash viscosity,
the capacity to react with gases and/or to absorb moisture, and the ability to
retain or lose electrical charge as a function of fly ash resistance
properties.
Acid/Base Ratios—
Listings of the principal elements found in various U.S. coal ashes
expressed as their percent oxide equivalent are presented in Table 3 along
with their characterizing acid/base (A/B) ratios. As indicated in Table 2,
very high or very low A/B ratios are usually associated with high melting
point ashes. Conversely, minimal melting points with a rapid transition from
the solid to the liquid phase are observed with a 50:50 mix, Figure 3.
Assuming that ash fusion temperatures are roughly proportional to the
softening temperatures, data presented in Table 3 appear to support the
suggested correlation. Further details on mineral constituents and
characterizing ratios, Table 4 and Figure 4, while indicating the variability
in components from one area to another, also show that the A/B ratios for
coals mined in the same regions or from seams that cross regional boundaries
are generally similar. The numerical indices refer only to contiguous or
geologically related seams and not to the previously defined coal Regions I
through VI.
Silica/Alumina Ratio and Dolomite Percentage—
Examination of the softening temperatures given in Table 3 indicates no
apparent correlation with either the silica/alumina ratio or the dolomite
percentage. According to the theoretical effects proposed in Table 2,
softening and melting point temperatures and slag viscosity should increase as
either ratio increases. Figure 4 also reveals that as far as the
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silica/alumina ratio is concerned, there are no characterizing values
associated with the arbitrary geographical (1 through 3) locations.
Iron Oxide (Percent of Total Oxides) —
With few exceptions, iron oxide (Fe203> concentrations expressed as
percent of total oxides also showed very consistent relationships with respect
to the three geographical areas cited in Table 4 and Figure 4. Additionally,
F6203 exhibited a good inverse correlation with the A/B ratio indicating
that the Si02 and Al£03 contents increased at the expense of a dimin-
ished Fe203 content. It was concluded that high Fe2<>3 concentrations
are indicative of A/B ratios close to unity. Therefore, one can infer that a
lower melting point and less viscous slag will result with the potential to
generate a larger population of smaller cenospheres in the fly ash. This
concept is also partially supported by the softening temperature versus
data shown in Table 3.
Sulfur-Iron Balance —
The sulfur-iron balance indicates the extent to which the total sulfur
content can be removed by pyrite separation in coal upgrading. A low ratio
suggests that the coal sulfur is mainly organically bound and thus more likely
to appear in the flue gas where it can adsorb on, or react with, the alkaline
constituents of the ash. Conversion to soluble sulfur compounds such as
alkali metal sulfites, bisulfites, and sulfates that are subsequently
collected on a filter may contribute to potential plugging unless baghouse
temperatures are maintained well above the dewpoint. At high total sulfur
contents, a smaller fraction of iron should appear in the pyrite form.
Furthermore, it has been suggested that a reduction in the pyrite content
might indicate more iron in the lower oxidation state, H a condition
observed to improve ash fluxing properties and to decrease fusion
temperatures. Total sulfur contents of several Alabama coals** were examined
as potential melting property indicators. When ash softening temperatures
were plotted against total sulfur content, the random point scatter failed to
support any positive correlation between the two variables or between fluxing
effects and the implied iron valence state.
Total Ash Content —
The effect of ash content upon ash fusion properties was examined to
determine if an increase in coal ash content might conceivably result in less
heat transfer to individual mineral particles because of the dilution factor
(less heat per particle). Such a process might be expected to produce
generally coarser and more irregularly shaped fly ash particles. Although
analyses of field data indicated a possible correlation between ash content
and softening points, the relationship was too weak statistically to be of
predictive value. Regardless of any possible impact upon particle size
parameters or softening temperatures, filtration demands (cloth and fan
capacity) will automatically relate to the volume of fly ash produced which,
in turn, should relate directly to the amount of mineral present in the parent
coal.
-------
Coal Firing Method Versus Fly Ash Properties
The method of coal firing is expected to influence fly ash size properties
in two ways. The degree of size reduction prior to firing determines the
parent size of the fly ash constituents of the coal; i.e., that fraction of
the inorganic solids contributed by the coal seam overburden, parting, or
floor minerals. In the case of pulverized coal-fired boilers, the typical
aerosolized material is described as 70 percent by weight less than 200 mesh
(~74 pm). Cyclone-fired boilers ordinarily use a coarser grind, 4-14 mesh,
whereas stoker coals are only crushed sufficiently to permit uniform grate
loadings by the feeding mechanism.
If the size reduction process alone were the controlling factor, the ash
from a pulverized coal firing system should be the finest followed by the
cyclone boiler and stoker-fired products, respectively. However, the added
inertial separation feature of a cyclone boiler produces a fly ash effluent
even finer than that generated by pulverized coal firing. It should be noted
that the potential for producing high NOX concentrations makes doubtful the
large-scale growth of this combustion system. In the case of stoker-fired
boilers, the fly ash in the uncontrolled effluent is not only the coarsest but
also likely to contain more unburned coal than the other effluents. As
expected, the use of fly ash re-injection with spreader stokers results in a
somewhat finer ash. At this time, the relationships between fly ash size and
firing method are restricted to qualitative applications because of measuring
problems, variations in system geometries and particulate residence times in
the systems, and settlement factors (bottom versus fly ash). Nevertheless, it
is important that the firing method be considered along with other factors
suspected to affect the ultimate filterability of coal fly ash.
SUMMARY AND CONCLUSIONS
An investigation of coal production and utilization statistics in
conjuncton with the identification of coal properties, treatment, and
combustion procedures that might affect fly ash filterability was performed to
establish a basis for selecting representative fly ash samples for detailed
study. A determination of key filtration parameters, K£ and ac,
describing filter resistance and fabric cleanability, respectively, is now
underway to permit broader applications of the EPA filtration model.2
Analyses of the data base developed for this study provided the rationale for
the conclusions on fly ash selection listed below:
1. The number of fly ash samples to be investigated for any coal type
should reflect the best projections for current and future use of
that coal in applications where fabric filters can provide the
particulate emission control. For example, Region VI coals have been
given an increased weighting because of projections for much
increased future usage and the unusually high ash contents of some
lignite coals.
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2. Basic coal properties, as delineated by proximate analyses, are
similar for Regions I, II, and III. Thus, fly ash samples obtained
from cooperating utilities and industries that burn coal from these
regions can be treated as a single class. The approximate number of
samples will be based on the combined regional output used for power,
steam, and heat generating purposes.
3. The distribution of fly ash samples should be representative of the
principal coal firing methods. Although pulverized coal firing is
far more common on the basis of fuel consumption, there are fewer and
sometimes more conflicting data available on the properties of
uncontrolled fly ash emissions from stoker-fired boilers. Therefore,
increased sampling above that suggested by the fuel use criterion is
required for stoker-fired systems.
4. The physical properties of the mineral constituents of a coal
considered most likely to affect fly ash properties are hardness and
melting (and softening) point. Both may affect fly ash size
properties, the latter through its impact on slag viscosity. Of the
several characterizing ratios for mineral constituents claimed to
affect slag viscosity, the acid/base ratio alone appears to have a
predictive value. Thus, when more than one sample is selected from
any region, at least two A/B levels should be tested.
5. High calcinable mineral and/or sulfur contents contribute to
increased fly ash hygroscopicity, which may present difficulties when
baghouse temperatures are allowed to fall below the dewpoint. Excess
moisture condensation causes reduced dust cake porosity that leads to
prohibitively high filter pressure loss. On the other hand, although
low sulfur coals present few condensation problems, an excess
electrical charge accumulation may interfere with normal filtration
processes. Therefore, the fly ash sampling should include both low
and high sulfur coals.
ACKNOWLEDGMENTS
This project has been funded at least in part with Federal funds from the
U.S. Environmental Protection Agency under Contract 68-02-3151. This paper
does not necessarily reflect the views or policies of the U.S. Environmental
Protection Agency, nor does mention of trade names, commercial products, or
organizations imply endorsement by the U.S. Government.
REFERENCES
1. Dennis, R. and J.A. Dirgo. Comparison of Laboratory and Field Derived
K£ Values for Dust Collected on Fabric Filters. Proceedings,
U.S.-Japan Scientific Seminar, Measurement and Control of Particulates
Generated from Human Activities. Kyoto, Japan. November 10-14, 1980.
2. Dennis, R. and H.A. Klemm. A Model for Coal Fly Ash Filtration. J. Air
Pollut. Control Assoc. 29:230-234 (1979).
-------
3. Energy Data Report. Coal-Bituminous and Lignite in 1976. DOE/EIA-0118/1
(1976). Prepared in the Office of Energy Data and Interpretation, U.S.
Department of Energy. December 18, 1978.
4. Gibbs and Hill, Inc. Coal Preparation for Combustion and Conversion.
Prepared for Electric Power Research Institute. EPRI AF-791, Project
466-1, Final Report. May 1978.
5. The Direct Use of Coal: Prospects and Problems of Production and
Combustion. Prepared by the Office of Technology Assessment, Congress of
the United States. OTA-E-86. April 1979.
6. Nielson, G.F. (Editor-in-Chief). 1979 Keystone Coal Industry Manual.
McGraw-Hill, Inc. New York, New York. 1979.
7. Nielson, G.F. (Editor-in-Chief). U.S. Coal Mine Production by Seam -
1976. McGraw-Hill, Inc. New York, New York. 1977.
8. Winegartner, E.G. (Editor). Coal Fouling and Slagging Parameters, Report
by ASME Research Committee on Corrosion and Deposits from Combustion
Gases, ASME (1974).
9. White, H.J. Electrostatic Precipitation Research, Precipitator Design.
J. Air Pollut. Control Assoc. 27:206-217 (1977).
10. Perry, R.H., C.H. Chilton, and S.D. Kirkpatrick (Editors). Chemical
Engineer's Handbook, 4th Ed. Section 9, Table 9-2. McGraw-Hill, Inc.
New York, New York. 1969.
11. Plumley, A.L. Fossil Fuel and the Environment, Present Systems and Their
Emissions. Combustion Engineering, Inc. October 1971.
WESTERN REGION
IE EASTERN
MIDWEST REGION
I NORTHERN APPALACHIAN
REGION
SOUTHERN
APPALACHIAN
REGION
HI ALABAMA
REGION
Figure 1. Coal Fields of The United States and Sites
Selected to Obtain Fly
8
-------
20 •
j.i (0) UP
t UP TO 28% WITH LIGNITE
COAL PRODUCING REGION
Figure 2. Averaged coal
analyses for
major coal
Regions.
1600
1500
UJ
I30°
1200
1100
O DATA POINTS
FROM REF. 9
x
o
0
i
20 50 80
PERCENT ACID
0.25 1.0 4.0
100
!
ACID/BASE RATIO
I SILICA/ALUMINA RATIO)
I F«2°3
ACID/BASE RATIO
Figure 3, Effect of acid/base
ratio on ash fusion
temperature.^
CC I0|- < 4
ro
O
CJ
II) (2) 13)
REGION OR SEAM LOCATION
Figure 4. Characterizing ratios
versus location of
coal seam. Refer to
Table 4.
TABLE 1.
ESTIMATED COAL PRODUCTION USED BY UTILITIES AND
INDUSTRY FOR ELECTRICAL POWER, STEAM, OR
HEATING PURPOSES
Region
I .Northern Appalachian
II Southern Appalachian
III Alabama
IV Eastern Midwest
V Western Midwest
VI Western
Production
States 106 metric tons/year
PA,WV(n)*,OH,MD,MI
WV(s),VA,KY(e),TN(n)
AL,GA,TN(s)
KY(w),IN,IL
AR,IA,OK,KS,MO,TX
CO,WY,MT,SD,ND,DT,
NM,AZ,ID,WA,AK
128
110
13
120
23
95
(26. 2) f
(22.5)
(2.6)
(24.6)
(4.6)
(19.5)
*Letters in parentheses refer to north, south, east, and west.
Numbers in parentheses refer to percent of total.
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TABLE 2. CHARACTERIZING COAL PROPERTIES AND PARAMETERS THAT
MAY AFFECT FLY ASH FILTERABILITY
Property or parameter
Probable effect on fly ash
and Its fllterabtllty
1. Ash Fusion Temperature
2. Hardness (Grlndabillty)
3. Silica/Alumina Ratio
4. Acid/Base Ratio (A/B)
SiO,, AljOi. TIP,
FejOj, CaD, MgO, K20, Ha20
5. Dolomite Percentage (D.P.)
100x
Fe203. CaO, MgO, K20, Ha20
6. Sulfur-Iron Balance
TotajMtejQj ini__ash
Fe20j equivalent of 50 percent
of sulfur as FeSj
7. Total Alkali Content (T.A.)
(Na20 -f K20) x (Ash)
8. Water Soluble Content
Na and K content by boiling
and reflux in water.
More cenospheres, finer particles, and higher K2
at low fusion temperature. More and larger irregular
particles at high fusion temperature and easier cleaning.
Larger particles, fewer cenospheres. and lower K2 with
harder minerals.
Lower ratio decreases viscosity and particle size with
higher Kz. more difficult cleaning.
Lower melting point and viscosity as A/B ratio approaches
unity; finer particles with higher K2 and more difficult
cleaning.
Slag viscosity and size Increase with dolomite percentage;
lower K2 and easier cleaning.
High sulfur to iron balance increases fly ash acidity and
hygroscoplclty; fabric plugging and chemical attack, minimal
electrical charge effects.
High alkali content increases fly ash hygroscopicity and
stickiness (increased electrical conduction may Improve
ESP performance)
Increased solubles (usually Na and K) increase fly ash hygro-
scopicity and stickiness, reduced electrical resistivity.
: 1,3-5 ; Item 2'; Ite
TABLE 3. SOFTENING TEMPERATURES, ASH CONSTITUENTS AND CHARACTERIZING
RATIOS FOR SEVERAL U.S. COALS10
Coal Source
Montana
Subbltunlnous
Illinois
Bituminous
Pennsylvania
Bituminous
West Virginia
Bituminous
Kentucky
Bituminous
Softening
temper atur
•c
1130
1270
1370
1500
>1595
S10Z
30.7
46.2
49.7
51.0
58.5
Coal ash
A120,
19.6
22.9
26.8
30.9
30.6
Fe20,
18.9
7.7
11.4
10.7
4.2
analysis* percent
T102
1.1
1.0
1.2
1.9
1.8
CaO
11.3
10.1
4.2
2.1
2.0
MgO
3.7
1.6
o.a
0.9
0.4
Na20
KiO
2.4
1.5
2.9
1.4
1.6
Acid/
SOa ratio
12.2 1.42
8.9 3.35
2.5 4.03
0.6 5.55
0.9 11.1
Silica/
Alumina
ratio
1.57
2.02
1.85
1.65
1.91
Dolomite
percentage
41
56
26
20
29
TABLE 4. CHEMICAL CONSTITUENTS OF TYPICAL COAL ASHES
EXPRESSED AS PERCENT OF TOTAL OXIDES^
Silica/
Acid/base* alumina
S102 A1203 Fez03 CaO MgO Ha,0 KjO SOj ratio ratio
Alabama,
Tuscaloosa County
Illinois,
Stark County
Schuyler County
Kentucky,
Pike County
Bell County
Ohio County
Ohio,
Belmont County
Jefferson County
Noble County
Pennsylvania,
Bedford County
Allegheny County
Butler County
West Virginia,
Barbour County
Fayette County
Kanawha County
48.2
42.1
48.0
52.0
46.9
49.3
42.4
50.4
36.7
57.7
49.2
26.9
52.5
48.5
37.9
28.4
20.1
14.7
34.5
28.4
19.4
19.6
23.2
19.9
32.1
24.8
18.2
35.6
30.1
23.8
15.1
22.8
18.7
6.4
12.0
27.4
27.5
21.9
37.8
5.1
20.3
52.5
6.1
13.6
31.4
1.8
6.6
10.4
1.9
2.7
1.8
4.8
1.9
1.8
1.7
1.8
1.2
1.8
1.8
2.0
0.8
1.1
0.6
0.6
2.0
0.7
1.2
0.5
0.6
0.9
0.5
0.4
0.6
0.8
0.5
0.3
0.5
0.2
0.5
0.2
0.2
0.2
0.1
0.2
0.2
0.2
0.2
0.2
0.7
0.5
1.2 2.4
1.9 2.0
1.8 2.1
3.3 0.8
2.0 4.5
1.9 0.2
1.6 2.0
1.0 0.4
1.4 0.7
2.6 0.2
3.6 1.3
1.0 0.8
1.9 0.9
2.6 1.3
0.5 2.9
4,
1.
2,
6.
4,
2,
1,
2,
2,
a,
2,
134
.95
.04
.85
.54
.22
.81
.90
.08
.57
.84
0.83
8.
4,
1.
.39
.02
.85
(1)
(2)
(2)
(1)
(1)
(2)
(2)
(2)
(2)
(3)
(2)
(2)
(3)
CD
(2)
1.7
2.09
3.26
1.51
1.66
2.54
2.16
2.17
1.84
1.80
1.98
1.48
1.47
1.61
1.60
*Numbers in parentheses designate arbitarary geographical locations for coal se
expected to display similar mineral assays.
10
-------
PULSE-JET FILTRATION WITH ELECTRICALLY CHARGED FLYASH
By: R.P. Donovan
Research Triangle Institute
P.O. Box 12194
Research Triangle Park, N.C. 27709
L.S. Hovis, G.H. Ramsey, J.H. Abbott
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, N.C. 27711
ABSTRACT
Pulse-jet performance equations differ from those of reverse-air-cleaned
or shaker-cleaned baghouses, especially when operated at high air-to-cloth
ratios as has been described by Leith, Ellenbecker, Dennis, and others. These
differing performance equations imply that dust electrical charge should
influence pulse-jet performance differently than, say, shaker-baghouse per-
formance. Measurement protocol distinguishing between conventional pulse-jet
operation and a "dust cake" mode of pulse-jet operation is described and the
initial results of an experimental program derived from this distinction are
presented. Comparisons with the shaker baghouse are emphasized.
TEXT
Electrical charge as a means of enhancing fabric filtration performance
is receiving a "big play" in the contemporary fabric filtration literature
(Cooper, 1979; Ariman and Helfritch, 1980). Previously reported EPA in-house
research with actively charged flyash systems has been carried out with the
shaker baghouse (Hovis, et al., 1981; Donovan, et al., 1981). This paper
describes complementary work initiated in the EPA pulse-jet baghouse in which
flyashes similar to those tested in the shaker baghoue are used in various
operating modes of the pulse jet. The capability of experimenting with simi-
lar flyashes, either uncharged or charged, provides insights into the mecha-
nisms of the two types of filtration equipment and also helps to isolate
charge effects and charge-affected mechanisms of filtration.
This paper begins by reviewing important differences between pulse-jet
fabric filtration and shaker-cleaned fabric filtration. Models of charge-
enhanced fabric filtration often attribute the enhancement to a charge-
induced increase in dust-cake porosity (Cooper, 1979). Because of significant
differences between the dust cake on a shaker-cleaned woven fabric and the
dust layer on a pulse-jet-cleaned felt fabric, such dust-cake porosity models
immediately alert one to anticipate that a different charge interaction could
dominate each of these two common techniques of fabric filtration.
Much of the work to be presented here was undertaken in an attempt to
distinguish between these two operating modes and the effect of dust electri-
cal charge on each; particular emphasis is placed on operating the pulse-jet
equipment in the "dust cake" mode. This experimental, non-practical mode of
11
-------
pulse-jet operation simulates the shaker cycle in that it features long filtra-
tion periods (9-120 minutes), followed by a multi-pulsed (40 pulses), off-line
cleaning prior to the next filtration cycle. The reason for such non-standard
operation is to be able to form a dust cake in the pulse jet and to produce a
dust-cake-layer-dominated AP characteristic by which performance can be
measured, and from which charge dependence of the performance can be detected.
In the description of the pulse-jet apparatus, emphasis is placed on
those equipment properties that are expected to influence charged-dust filtra-
tion differently than in the shaker baghouse. The Results section also includes
shaker-baghouse results with which various performance comparisons are made.
Background
Conventional pulse-jet operation refers to fabric filtration which features,
in addition to bag cleaning by a short (typically 50-150 ms) burst of compres-
sed air fired down the bag through a venturi nozzle: outside collection, on-
line cleaning, higher superficial gas flows than shaker or reverse-air cleaned
units, and shorter intervals between cleaning cycles than the typical shaker or
reverse-air unit. Departures from one or more of these properties are common-
place; however, not all pulse jets use Venturis (some don't even use an aimed
blow pipe above the bags but resort to a more remote pulse-control mechanism
as in the plenum-pulse design), not all pulse jets are outside collectors (some
are cageless with strictly induced pulse cleaning initiated at the bag bottom) ,
and more and more pulse jet operators are resorting to off-line cleaning. In
short, the term pulse jet no longer adequately defines a specific standardized
fabric filtration operation—if it ever did. The key properties of what is
defined in this paper as "conventional" pulse-jet operation are:
1. a residual dust load in/on the fabric that is large with respect to
the fresh dust fed between successive cleaning pulses;
2. a low cleaning efficiency in the sense that only a small percentage
of the dust load on the fabric is transferred to the hopper in any
single cleaning cycle; and
3. significant dust redeposition after a cleaning pulse because of the
on-line cleaning.
These properties of the "conventional" mode of operating a pulse jet fol-
low from the process descriptions of Dennis and Klemm (1980), Ellenbecker and
Leith (1979, 1980), Leith and First (1977), Leith and Ellenbecker (1980), and
Leith, et al., (1980). These publications emphasize the differences between
pulse-jet fabric-filter performance equations and the ideal Darcy equations
which have been used to predict shaker or reverse-air baghouse performance by
modeling filtration as a dust-cake dominated process.
Particular attention is drawn to the presence of the larger quantity of
dust that remains on the bag during conventional pulse-jet filtration when
compared to dust-cake filtration processes. This retained dust is not neces-
sarily all residual dust that is never removed from the bag, but may consist of
cleaned dust that has redeposited on the bags rather than fall to the hopper.
Regardless, its presence adversely affects conventional pulse-jet performance
12
-------
and limits the range of practical operating gas-to-cloth ratios.
That the performance of a conventional pulse jet depends so strongly on
retained dust implies that understanding the effect of dust electrical charge
in a conventional pulse jet will require knowledge of not only the incoming
dust charge but also that of the retained dust layer. In addition the influ-
ence of dust charge upon the various interactions (such as initial capture,
cleaning efficiency, and redeposition) must be known. From these considera-
tions the conventional pulse-jet problem is expected to be more complicated
than dust-cake filtration in, say, the shaker baghouse.
For these reasons a preliminary experimental program has been defined
prior to addressing the conventional pulse-jet problem. In this initial
phase of the EPA/IERL-RTP in-house program, the pulse-jet equipment is
operated in a "dust-cake" mode which implies the following modifications:
1. filtration periods sufficiently long to build a dust cake on the
fabric, analogous to that in a shaker operation; and
2. off-line cleaning in which multiple pulses are fired through all
bags simultaneously and in sufficient number to reduce the residual
dust areal density to 250-300 g/m2.
The fabrics and superficial velocities of filtration used, however, are
those typical of conventional pulse-jet filtration. The goal is to under-
stand the dust collection process and the influence of dust charge upon it
before considering the effects of dust charge upon the dominating effects of
conventional pulse-jet cleaning. In addition the pulse-jet equipment lends
itself to crude bag weighing and thus enables direct measurements of total
dust gain by the bags during a given filtration cycle. This information is
particularly valuable in separating charge-related dust-cake porosity effects
from precharger dust collection effects, a difficult separation to make with
the shaker baghouse (Hovis, et al., 1981).
Apparatus
The in-house laboratory pulse-jet baghouse is a commercially available,
nine-bag Mikro-Pul baghouse operated as previously described by Turner (1977).
It consists of three rows of three bags each. Each bag is mounted on the
shoulder of a venturi nozzle which itself is sealed to a tube sheet. A blow
pipe is positioned above each row with an orifice opposite the bag mouth.
Cleaning pulses were drawn from a 90-psi (620 kPa) reservoir of compressed air
for all data to be reported.
Dirty air enters the hopper beneath the bags and flows through the bags
outside-in. The bags are mounted on steel cages to prevent collapse during
filtration.
To this standard apparatus various additions have been made as shown in
Figure 1. A prime addition is that of the corona precharger shown in the in-
let duct. This precharger consists of a single wire mounted along the center-
line of the inlet duct. The corona discharge by which the incoming dust is
13
-------
charged occurs along the 1/16-inch (1.6 mm) welding rod shown protruding from
the larger diameter mount. The larger diameter mount is held in position by
two Teflon standoffs which extend across the diameter of the duct and through
which the high voltage is routed to the wire.
This location of a precharger suffers from two shortcomings:
1. the remoteness of its location from the bags; and
2. the high speed of the gas flow through the precharger and hence the
short charging time available for charging the dust.
Simple calculations suggest that, in spite of the short residence time in
the corona region, significant dust charging can occur with this precharger
design (Appendix 1). Charge loss between the precharger and the bags proved
difficult to sample and measure accurately so a second precharger design
(Figure 2) was built to mount immediately beneath the bags in a region that is
both nearer the bags and in a region of lower linear gas flow than the duct
precharger. Most of the dust-cake mode data were taken with this second pre-
charger design, called the plenum precharger.
The plenum precharger design, however, does not match up well with the
study of conventional pulse-jet service because it is located between the dust
on the bags and the hopper. Dust removed from the bag by the pulse-jet clean-
ing action then must pass through the precharger to reach the hopper and hence
is subject to capture by the precharger. Even some dust that normally would
be redeposited could be captured by the precharger electric field because of
its location. Hence, this precharger location complicates data interpretation.
The duct charger, on the other hand, is out of the dust path between the
bags and the hopper and, in addition, operates with little dust capture in the
duct itself, although wall capture by the hopper and lower baghoue chamber will
still be enhanced when the precharger is ON. Stable precharger operation is
easier to achieve with this arrangement, however.
Other ports have also been added to the baghouse as illustrated in Figure
1. Two ports for inserting an impactor into the baghouse chamber now exist.
This in-the-housing capability minimizes probe losses during impactor sampling.
The test dusts used in the pulse-jet experiments are the same Southwestern
Public Service (SPS) flyash and Detroit Edison flyash (DBF) used in the shaker-
baghouse experiments (Hovis, et al.. 1981). While the inlet dust concentra-
tion in the pulse jet is generally higher than in the shaker baghouse (4 gr/
ft3 [9 g/m3] vs 3 gr/ft3 [7 g/m3]), the hopper fallout characteristic of this
particular pulse-jet operation (~ 50%) reduces the dust concentration reaching
the bags to about 2 gr/ft3 (4.5 g/m3). Hopper fallout in the top fed shaker
unit is essentially zero.
Most of the data to be reported in the pulse jet were taken at an oper-
ating air-to-cloth ratio of 9 ft/min (4.5 cm/sec). This value is more than
double the 4 ft/min (2 cm/sec) superficial velocity at which the shaker oper-
ates. Relative humidity in both baghouses is controlled by the same Inreco®
14
-------
controller— the same feedhouse that encloses the pulse- jet hopper also en-
closes the shaker-baghouse hopper.
The only fabric used in the pulse- jet tests to be reported is a needled
polyester felt weighing 16 oz/yd2. These bags were purchased from Mikro Pul.
This fabric style is the second major difference (next to cleaning tech-
nique) between the pulse-jet experiments and the shaker experiments.
Results
Most of the evaluations of charged dust effects in pulse- jet fabric
filtration carried out in the EPA pulse jet have been made in the dust-cake
mode prevously described. This mode allows a separation of charge effects on
fresh incoming dust from those on the retained or redeposited dust. In the
dust-cake mode, only off-line cleaning is carried out; the only evaluating
measurements, in addition to the usual pressure drop and flow-rate records,
are total bag weight before and after the cleaning. The measurement procedure,
beginning with cleaned bags, thus is:
1. Remove bags from baghouse and weigh, one at a time. Reinstall bags
in identical slots from which removed.
2. Run under desired test conditions.
3. At end of run, slip a plastic cover over each bag; remove and weigh
each bag.
4. Reinstall bags and clean with 40 pulses at 90 psi (620 kPa) .
5. Repeat Step 1.
For these dust-cake mode measurements, only three bags were used; the
other six slots of the nine-bag baghouse were capped off. The three bags were
mounted on one row (Row 2) so that they were cleaned simultaneously; the
cleaning control was rewired so that only this row was actuated for cleaning.
Figure 3 is a sketch of a typical pressure trace operating in the dust-
cake mode. In contrast to the shaker baghouse pressure trace, all the pulse-
jet pressure traces initially exhibited what Dennis and Klemm (Nov., 1980)
describe as concave-up behavior; but, if allowed to continue for long periods
of time and to build up large pressure drops, the traces eventually displayed
linear behavior from which a specific dust-cake resistance (Kj) could be esti-
mated. Because the length of this initial non-linear region is a significant
fraction of the typical filtration period, the K2 calculation was modified as
follows:
(AP - AP )A
where APF, ^PDCQ) tj, and t2 are defined in Figure 3;
A = total fabric area;
15
-------
m = total weight gain of all bags during the filtration period tz; and
V = the gas-to-cloth ratio.
The (t2-ti)/t2 correction assumes that only that fraction of the total
dust gain of the bags that occurred during the linear buildup region is appro-
priate for the K.2 calculation—the earlier dust deposit is at some lower
effective K£ rate and represents a dust/felt interface (or other non-dust-cake)
property. The corresponding pressure drop (AP - APDCQ) of the linear region
is used in the numerator of Equation (1).
Tables 1 and 2 summarize measurements for DBF and SPS flyash, respectively.
K.2 in these tables has been calculated by Equation (1), using the (t2-ti)/t2
correction factor when a clearly defined linear region exists in the experi-
mental pressure trace. Otherwise (t2-tj)/t2 was set equal to 1. A major
effect of turning the plenum precharger ON is to reduce the quantity of flyash
reaching the bags, presumably because of precharger and wall capture. This
effect repeats the observations of the shaker baghouse (Hovis, et al., 1981).
The improved precharger performance with relative humidity (in the sense of en-
hanced flyash capture) also repeats in this pulse-jet arrangement. At 70%
relative humidity the mass of flyash reaching the bags when the precharger is
ON is only 20-30% of that when the precharger is OFF. At 30% relative humidity
this ratio is greater.
Values of K£ calculated by the mass corrected expression given by Equation
(1) do not show a significant dependence on dust charge for either of these
flyashes. As was true for the shaker results, the dramatic change in the Ap
trace upon turning ON the precharger (Figure 4) is primarily explained as fly-
ash capture by the precharger, thus reducing the effective flyash feed rate to
the bags. When this flyash loss is accounted for by direct bag weighing, the
K2 values calculated from Equation (1) are similar for the charged and the
uncharged flyashes.
As with the shaker data, this conclusion does not rule out the charge-
induced porosity increase reported by others (Chudleigh and Bainbridge, 1980;
Ariman and Helfritch, 1980; linoya and Mori, 1980) so much as demonstrate
that such an effect is smaller in magnitude than the precharger collection
effect. The bag weighing technique employed here is still a coarse measure-
ment that is time consuming and technique dependent. Nonetheless the pre-
charger mass collection effect is clear, while the dependence of flyash
porosity on electrical charge is not.
Conventional Pulse-Jet Mode Results
Some preliminary conventional pulse-jet operation was carried out prior
to the dust-cake mode experiments just described. These experiments were per-
formed with the full nine-bag complement of polyester felt bags filtering DBF.
The air-to-cloth ratio was 6 fpm (3 cm/sec) with an inlet dust concentration
of 4 gr/ft3 (9 g/m3). Cleaning pulse pressure was 90 psi (620 kPa) and the
time interval between successive cleaning pulses was 60 sees so that each row
of three bags was cleaned on-line every 3 min. Total airflow through the bag-
house was 250 acfm (7.08 m3/min) implying an average upward linear gas flow
16
-------
between the hopper and the bags of 46 ft/min (14 m/min).
Only the duct precharger (Appendix 1) was used in these experiments and
the conclusion reached in a series of 6-hour runs comparing precharger ON with
precharger OFF performance was that the precharger had no significant influence
on either collection efficiency or pressure drop.
This observation is consistent with the model of conventional pulse-jet
filtration described in the Background section of this paper in which the
filtration properties are depicted as more dependent on the retained dust load
than on the dust freshly fed between cleaning pulses. The implication of that
model is that the bag cleaning and the redeposition processes dominate the
performance rather than a dust parameter such as inlet concentration. Varia-
tions in inlet concentration are less important than the areal mass density of
the retained dust (Leith and Ellenbecker, 1980); the retained dust density
depends more on cleaning parameters than inlet dust concentration (Dennis and
Klemm, 1980).
Should the dominating effect of precharger operation prove to be a reduc-
tion in the rate of dust arriving at the bags, as the dust-cake mode measure-
ments suggest, then the influence of precharger operation is predicted by
these models to be small.
A new series of runs in the conventional pulse-jet mode is now underway.
A major difference between this series and that previously carried out is that
continuous operation around-the-clock over an extended period is planned, as
was done in the shaker baghouse experiments reported by Hovis, et al., (1981).
To accommodate this goal other modifications have been required:
1. the complement of bags has been reduced from nine to three—one bag
per row, and
2. the total gas flow has been reduced from 250 acfm (7.08 m3/min) to
125 acfm (3.54 m3/min), raising the air-to-cloth ratio from 6 ft/min
(3 cm/sec) to 9 ft/min (4.5 cm/sec)—the same as in the dust-cake
pulse-jet work just described.
Air pressure in the pulse reservoir remains at 90 psi (620 kPa); the interval
between successive pulses is 47 sec so that each bag is cleaned every 141 sec.
The flyash concentration in the inlet remains at 4 gr/ft3 (9 g/m3).
One observation already apparent in this new series is the higher dust
loads of DEF building up on the fabric compared to the SPS flyash loads. This
result is surprising because of the higher AP'S and K^'s of the SPS flyash in
the shaker experiments. But after 24-hr operation in the conventional pulse-
jet mode, a virgin polyester felt fabric weighed in with more than 2-1/2 times
as much DEF as did a similar virgin fabric operating with the same filtration
parameters but using SPS flyash (432 g/m2 DEF vs 154 g/m2 SPS flyash) .
Establishment of long-term operating equlibriums, with the precharger
both ON and OFF, is now underway.
17
-------
Conclusions
1. The recognized differences between conventional pulse-jet operation and
dust-cake filtration imply a different role of dust electrical charge in
the two processes.
2. Operating the pulse-jet equipment in a "dust cake" mode shows the domi-
nant effect of charging the flyash with a corona precharger is to
reduce the dust loads on the bags.
3. Initial precharger-ON measurements in the conventional pulse-jet mode of
operation fail to identify any performance effect of dust charging.
References
1. Ariman, T. and D.J. Helfritch. "Pressure Drop in Electrostatic Fabric
Filtration." Second Symposium on the Transfer and Utilization of Parti-
culate Control Technology. Vol. Ill, EPA-600/9-80-039c (NTIS PB81-144800),
September 1980, pp.222-236.
2. Chudleigh, P.W. and N.W. Bainbridge. "Electrostatic Effects in Fabric
Filters During Build-up of the Dust Cake." Filtration and Separation,
July/August 1980, pp.309-311.
3. Cooper, D.W. "Mechanisms for Electrostatic Enhancement of Filter Perfor-
mance." Invited paper presented to The Fiber Society and the Filtration
Society Joint Symposium on Fibers, Electrostatics and Filtration,
Princeton, NJ, November 14-15, 1979.
4. Donovan, R.P., et al. "Electrostatic Augmentation for Particulate
Removal with Fabric Filters." 5th International Fabric Alternatives
Forum Proceedings, January 1981, Scottsdale, AZ, American Air Filter,
Louisville, KY.
5. Dennis, R. and H.A. Klemm. "Modeling Concepts for Pulse Jet Filtration."
J. Air Poll. Cont. Assn., 30, January 1980, pp.38-43.
6. Dennis, R. and H.A. Klemm. "Recent Concepts Describing Filter System
Behavior." U.S./Japan Scientific Seminar on Measurement and Control of
Particulates Generated from Human Activities. Kyoto, Nov. 10-14, 1980.
7. Ellenbecker, M.J. and D. Leith. "Theory for Dust Deposit Retention in a
Pulse-Jet Fabric Filter." Filtration and Separation. November/December
1979, pp.624-629.
8. Ellenbecker, M.J. and D. Leith. "The Effect of Dust Retention on Pres-
sure Drop in a High Velocity Pulse-Jet Fabric Filter." Powder Technology,
25, 1980, pp.147-154.
9. Hovis, L.S., et al. "Electrically Charged Flyash Experiments in a
Laboratory Shaker Baghouse." Presentation to the Third Symposium on the
Transfer and Utilization of Particulate Control Technology. March 1981,
Orlando, FL (this proceedings).
18
-------
10. linoya, K. and Y. Mori. "Effects of a Corona Precharger and Relative
Humidity on Filter Performance." Second Symposium on the Transfer and
Utilization of Particulate Control Technology. Vol. Ill, EPA-600/9-80-
039c (NTIS PB81-144800), September 1980, pp.237-250.
11. Leith, D. and M.W. First. "Pressure Drop in a Pulse-Jet Fabric Filter."
Filtration and Separation, September/October 1977, pp.473-480.
12. Leith, D. and M.J. Ellenbecker. "Theory for Penetration in a Pulse-Jet
Cleaned Fabric Filter." J. Air Poll. Contr. Assn., 30. August 1980, pp.
877-881.
13. Leith, D., et al. "Performance of a High-Velocity Pulse-Jet Filter II."
EPA-600/7-80-042 (NTIS PB80-183866), March 1980.
14. Oglesby, S. and G.B. Nichols. Electrostatic Precipitation, Marcel Dekker
Inc., New York, NY (1978).
15. Turner, J.H. "EPA Research in Fabric Filtration: Annual Report on IERL-
RTP In-House Program." EPA-600/7-77-042 (NTIS PB-267441), May 1977.
TABLE 1. DETROIT EDISON FLYASH FILTRATION IN THE PULSE JET (DUST CAKE MODE)
Charger
Status
OFF
ON
OFF
ON
OFF
ON
R.H.
(%)
70
70
30
30
70
70
Filtration
Time (min)
60
120
60
120
40
80
Total Dust
on Bags (g)
844
483
775
972
387
295
Feed Rate to
Bags (g/min)
14.1 )
4.0 )
12.9 )
8.1 j
9.7 )
3.7 )
Feed-Rate
Ratio
ON/OFF
0.28
0.63
0.38
Kj Ratio
ON/OFF
—
0.85
0.90
TABLE 2. SOUTHWESTERN PUBLIC SERVICE FLYASH FILTRATION IN THE PULSE JET (DUST CAKE MODE)
Relative Filtration Total Flyash Feed Rate Feed Rate R- K- Ratlo
Charger Humidity Time Added to Bags Ratio *
Status (%) (min) (g) (g/min) (ON/OFF) (in. of H20-min-ft/lb[m]) (ON/OFF)
OFF
ON
OFF
OFF
ON
OFF
30
30
30
70
70
70
18.5
40
13.2
18.6
40
9.0
443
312
304
400
169
189
23.9
7.8
23.0
21.5
4.2
21.0
\ 0.33
\ 0.34
1 0.20
I 0.20
13.8
12.9
13.2
15.1
13.9
12.4
>
}
/
>
}
0.93
0.98
0.92
1.12
* 4-1
1 (in. of H-O-min-ft/lbfm]) = 10 sec
19
-------
L 0.71 m (28 in.)
Clean
Air
Out
0.86 m
(34 in.)
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To
High Voltage #2
I
To
High Voltage #1
Figure 1. Pulse jet schematic.
(APF - APD c Q )A
Figure 2. Plenum precharger.
Figure 3. Pulse jet pres-
sure trace (DC
mode).
Precharger
OFF
Figure 4.
Precharger ON
Pulse jet filtra-
tion of Detroit
Edison flyash.
10 20 30 40
50 60 70
Time Imin)
80 90 100 110 120
20
-------
APPENDIX 1: PREDICTED PRECHARGER PERFORMANCE
A sketch of the duct precharger appears in Figure Al along with typical
current-voltage levels. All corona current is assumed to flow through the
protruding short length, £, of welding rod. For a typical experiment this
length was 5-in. (12.7 cm) and was mounted coaxially within a 3-in. (7.6 cm)
inside diameter duct. Current flow through other sections of the structure,
such as the support mount to which the welding rod is attached, is ignored.
To calculate the charging-time constant, T, of this precharger at typi-
cal power settings, an expression given by Oglesby and Nichols (1978, p.63)
is used:
4e0 4e0 ,.,.
T = ——£• = 2. (Al)
N0eb a '
where e0 = permitivity of free space = 8.85 x 10 12 (C2s2/kg-m3);
No = charge carrier concentration;
e = carrier charge;
b = carrier mobility; and
a = electrical conductivity.
From the power supply current and voltage, the medium conductivity can be
estimated as:
a = Y- (A2)
where J = I/duct wall area adjacent to corona wire = J/2-rr£r;
Eo = V/r; and
I,V = power supply current and voltage.
Substituting the values from Figure Al in Equations (A2) and (Al) yields:
a) (A3)
= 4(8.85 x
2.51 x 10
The typical gas flow rate through the inlet duct is 125 acfm (3.54 m3/
min). This volume flow corresponds to a linear flow rate of approximately 44
ft/sec (13 m/s) which implies that the time required to pass by the 5-in. (12.7
cm) length of corona wire is about 9.8 ms. Using the charging expression
from Oglesby and Nichols (1978),
(A5)
21
-------
where q(t) = particle charge;
q = particle saturation charge;
s
t = charging time; and
T = charging time constant;
and the estimated values of t (9.8 ms) and T (1.41 ms) just calculated, Equa-
tion (A3) predicts a q(t)/q ratio of 0.87, implying that the flyash particles
should achieve 87% of their saturation charge when traversing the precharger
sketched in Figure Al. Doubling the volume flow through the inlet duct cuts
the charging time in half. Thus, a total air-flow rate of 250 acfm (7.08 m3/
min) reduces charging time to 4.9 ms which, by Equation (A5), implies that
particles will be charged to 78% of their saturation values.
The expression for saturation charge q , under field charging conditions,
is given by Oglesby and Nichols (1978, p.63f as:
qa = 12 —£— TT£oa2E0 (A6)
S K I™ £.
where K = particle dielectric constant;
a = particle diameter; and
all other symbols are as before.
By dividing a flyash sample into increments according to size as in an impactor
sampling, q can be computed from Equation (A6) for the mean diameter of each
increment. Assigning appropriate weightings to each increment, the total
saturation charge of a given mass of sample can be estimated.
Following this procedure yielded a saturation charge/mass estimate for
the Detroit Edison flyash of 57 /nC/g, assuming that K/K+2 in Equation (A6) is
equal to 1. At 87% charging efficiency, the estimate reduces to about 50 yC/
g. As measured by the Faraday cage, values of Q/M have typically been 0.7-1.1
with the precharger ON. When the precharger is OFF, Q/M ~0.1
e
(~5in. [12.7cm])
T
Estimated Charging Efficiency 3- ~ 78%
at 250 act. ft3/min (7.1 m3/min)
Figure Al. Duct precharger.
22
-------
ELECTRICALLY CHARGED FLYASH EXPERIMENTS IN A LABORATORY SHAKER BAGHOUSE
By: L.S. Hovis, J.H. Abbott
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, N.C. 27711
R.P. Donovan, C.A. Pareja
Research Triangle Institute
P.O. Box 12194
Research Triangle Park, N.C. 27709
ABSTRACT
As has been demonstrated for numerous dust/fabric systems, increasing the
electrical charge of the dust particles dramatically reduces the KZ values of
the dust cake formed by such particles. This demonstration has been repeated
in a room-temperature-operated shaker baghouse, using redispersed flyash from
both Southwestern Public Service and Detroit Edison with silicone-graphite
finished fiberglass bags. The observed dependencies of this effect upon
operating time and relative humidity are presented. Interpreting the results
in traditional K.2 terms requires corrections for flyash collection by the pre-
charger which is shown to vary with precharger cleaning techniques.
Prolonged operation with a charged flyash gas stream at 50 percent rela-
tive humidity causes the low K£ values, characteristic of highly charged fly-
ash, to gradually increase to the range characteristic of uncharged flyash.
Increasing the operating relative humidity to 70 percent rapidly restores the
low K2 operation. These observations are interpreted in terms of flyash elec-
trical resistivity and its role in the operation of the precharger of an
electrically enhanced dust filtration system.
TEXT
Increasing the magnitude of the electrical charge on a dust sample
improves its filtration properties by increasing its collectability (increas-
ing the efficiency of the filter collecting it) and lowering its specific cake
resistance, as has now been shown by many researchers (Chudleigh and Bainbridge,
1980; Ariman and Helfritch, 1980; and linoya and Mori, 1980). Commercial
apparatus seeking to capitalize on this improved performance now exists
(Helfritch, 1977 and 1979) even though the basic mechanisms producing this
type of electrically enhanced performance are not well understood. The goal
of the experiments to be reported here is to develop fabric filter performance
models that include electrical charge as a parameter and permit the prediction
of electrically enhanced performance. This goal is ambitious since only under
severely restricted conditions can fabric filtration performance be predicted
for the non-electrically enhanced case. Nonetheless this paper describes the
initial EPA experiments toward that goal.
23
-------
Apparatus
All experiments were performed in the one-bag shaker baghouse described
by Durham and Harrington (1971). This system operates at room temperature
and uses redispersed dusts of various types as a test material for bag perfor-
mance measurements. Several modifications are worth noting, however. The
variable speed dust feeder is now enclosed in a separate feed house, the
humidity of which is controlled by an Inreco® humidifier. Thus, the entire
hopper and its dust load are stored at the desired humidity. The flyash-laden
stream passed to the baghouse draws its makeup air from this same humidity-
controlled chamber,
A corona charging section has also been added to the top entry of the
baghouse as illustrated in Figure 1. The high-voltage electrode consists of a
steel tube positioned along the centerline of a section of the entry pipe.
The steel tube is electrically isolated by Teflon® standoffs through which the
high voltage power line is routed. On the downstream end of the tube, an
adjustable length of 1/16 in. (1.6 mm) welding rod protrudes. Its dimensions
control the corona discharge. The distance between the end of the corona wire
and the top of the bag varies between 14 and 18 in. (36 and 46 cm) .
A 1/4-in. (6 mm) line for drawing a sample through a Faraday cage is
located immediately downstream of the corona wire. In the absence of flyash
feed, no significant current is detected by the Faraday cage even when opera-
ting above the maximum corona power used in any of the experiments.
With typical flyash concentration in the inlet (~3 gr/ft3 [6.9 g/m3]),
the charge/mass (Q/M) of the flyash increases by approximately one order of
magnitude (to 2 to 5 jnC/g) when the corona power is 15-20 kV, 0.1 to 0.2 mA.
This operation constitutes the "charged" flyash condition used as a variable
in the performance experiments. Charger-OFF (no high voltage to the corona
wire) typically means the flyash Q/M is 0.2 to 0.5 yC/g, although excursions
from this range occur periodically. The sign of the flyash charge with the
corona ON is always negative, corresponding to the power supply polarity.
Charger-OFF operation sometimes results in positive flyash charge but is not
predictable.
Two types of flyash are used in these experiments: from the hopper of an
electrostatic precipitator (ESP) at Detroit Edison and from the hopper of a
baghouse at Harrington Station No. 2 Boiler (Southwestern Public Service).
The Detroit Edison flyash (DBF) was further classifed by passing it through a
laboratory ESP. Once loaded into the baghouse-feed hopper, it is dispersed
into the dirty air stream, collected by the bag, dumped into the baghouse hop-
per, and eventually returned manually to the feed hopper from which the cycle
is repeated. The size distribution of this DEF, after several months of such
cycling, is shown in Figure 2. (These plots were actually made in the labora-
tory pulse-jet baghouse, which used the same flyash, and in the same recycling
mode. The pulse jet permits an impactor to be inserted into the baghouse so
that sampling losses are confined to the impactor inlet alone.)
No impactor analyses have been made on the Southwestern Public Service
(SPS) flyash since it has been used only in the shaker baghouse. This flyash
24
-------
is the same as that described by Ladd, et al. (1980) and Lipscomb, et al.
(1980).
The filter bags used in all tests reported here were silicone-graphite
finished fiberglass, W.W. Criswell Style 445-04. These bags are similar to
those installed initially at Harrington No. 2. Their properties and perfor-
mance in the Harrington baghouse were discussed by Ladd, et al. (1980);
Lipscomb, et al. (1980) evaluated this fabric, along with other fabrics in the
EPA mobile baghouse operating on a slipstream from Harrington No. 1. Unused
bag samples prepared for these latter tests were furnished to the EPA in-house
facility by Lipscomb. These bags were cut and shaped to fit the shaker bag-
house, resulting in a bag area of approximately 7.4 ft2 (0.69 m2) . The bags
were mounted with their texturized surface facing the flyash stream.
The shaker baghouse has a top entry through which the dirty-air stream
from the feedhouse is pulled. A long 1-in. (2.5 cm) diameter hose connects
the feedhouse to the top of the baghouse. At the top of the baghouse the gas
stream passes through a flow straightener element shaped like a witch's hat,
traverses the corona precharger section, and then enters the top of the verti-
cally hung bag. A sealed hopper beneath the bottom of the bag catches all
dust during filtration and cleaning. In-situ measurements show essentially
no dust collection by the hopper during filtration—the hopper fallout in this
configuration is essentially zero. For shake-cleaning, the bottom of the bag
is attached to a tapered outlet section which is coupled by a flexible joint
to the hopper entry pipe. The tapered section is oscillated by an eccentri-
cally driven rod at 4 cycles/sec. The amplitude of the rod displacement from
its center position is about 7/8-in. (2.2 cm).
The typical cleaning cycle used in most of the experiments reported here
consists of a 1-min pause after the end of the filtration cycle, a 2-min shake-
clean as described above, and finally a 1-min pause before the beginning of
the next filtration cycle. For some data the shake-clean portion of the cycle
was shortened to either 60 or 30 sees, primarily to minimize bag wear under
special test conditions. This modification is assumed not to alter the clean-
ing significantly, based on beta backscattering measurements of SPS flyash
removal during cleaning (Gardner, et al., 1981). These removal measurements
suggest that virtually all flyash that is removed during the overall cleaning
cycle is removed during the first 20 sees of the shake-clean, a conclusion
similar to that reached earlier by Walsh and Spaite (1962) and by Dennis and
Wilder (1975) although these researchers used different experimental tech-
niques.
In the typical routine operation, pressure drop across both the bag and
a venturi neck (to measure flow) is monitored continuously. In addition,
special sensors have been incorporated for certain experiments. Among these
sensors have been the previously mentioned beta gauge (Gardner, et al., 1981),
an in-situ load cell for weighing the hopper dust, and various electrodes for
measuring charge accumulation in or on the bag (Donovan, et al., 1980). These
outputs will not be discussed further in this paper.
Periodic sampling of the inlet and outlet dust streams is also routine in
the operation of the laboratory baghouse. The quantity of dust fed into the
25
-------
baghouse by the Acrison screw feeder is sampled daily. This feed rate is
usually very stable so long as the hopper is above 10 percent capacity. The
outlet dust concentration is also monitored during several filtration cycles
per day by isokinetically drawing a sample from the outlet line and collecting
it on a filter for weighing. Typically this outlet sampling takes place only
during the forward filtration flow and extends over only one such cycle, but
in any event is of at least 20-min duration.
Inlet dust Q/M is also sampled periodically, typically daily, through a
port just downstream from the corona wire (Figure 1). This measurement con-
sists of collecting a sample on a filter that is electrically isolated from
ground. During sampling the charge accumulation on the filter is measured by
a Keithley electrometer (in the charge measurement mode). The dust mass is
determined by a post-sampling weighing and both a Q/M and a space-charge
density (Q/volume of gas passed through the filter) are calculated. The total
mass collected by the filter is reduced when the precharger is ON. The Q/M
reported is that of the mass that is collected and the charge it carried with
no correction attempted for any sampling bias that might be introduced by the
charge.
Precharger operation was at predetermined values of either current or
voltage. The corresponding power levels were recorded at the beginning of the
experiments and monitored periodically through the runs but no continuous
record of these levels was made.
Results
A test series was conducted with each of the two test flyashes. The SPS
flyash series was conducted first and consisted of three separate sequential
runs, each starting with a new bag which was operated around the clock over a
time period of 2-3 weeks. One run with DBF then followed, again beginning
with a fresh, unused fiberglass bag.
The test plan consisted of comparing performance, chiefly pressure drop,
between precharger-ON and precharger-OFF operation. Implementation consisted
of operating the baghouse at fixed conditions until an equilibrium was
achieved, then switching the precharger to the opposite state and re-establish-
ing equilibrium. Performance comparisons were then made between the two
equilibrium states.
One question addressed immediately was the significance of the charge
state of the initial flyash collected by the virgin fabric. The break-in
period of a fabric is critical and under some conditions interactions occurring
then can influence fabric performance far beyond this period of initial use
(Davis, 1980). Specifically the desire to confirm that performance differences
measured by the switching procedure outlined in the previous paragraph did not
depend on break-in led to a comparison of two bag break-in histories—one
broken in with the precharger OFF (Bag 1) and the second broken in with the
precharger ON (Bag 2).
Each of these bags operated with a fixed 80-min filtration cycle followed
by the cleaning cycle of 1-min pause, 2-min shake, and 1-min pause. Typical
26
-------
AP traces during these cycles appear in Figure 3, one showing precharger ON;
the other, precharger OFF.
The persistent observation of all the experiments, regardless of dust type,
is contained in Figure 3— turning ON the precharger significantly reduces the
rate at which Ap increases with time. This conclusion holds for bags broken
in with the precharger ON as well as for those broken in with the precharger
OFF. Indeed the status of the precharger during break- in appears to be unim-
portant to the laboratory observations, as shown in Figure 4. The same general
shaded bands of K'2 and Ap^ (Figure 4) apply to both bags; hence the conclusion
that ordering sequence is relatively unimportant for determining AP and K^ .
In Figures 5 and 6 similar data are shown for the DBF. Here, only one
bag was evaluated and the filtration period was extended to 180 minutes because
of the lower values of K£ and Ap As in Figures 3 and 4 the influence of
turning ON the precharger is evident in this flyash system as well, although
the KZ values of the DBF are lower than those of the SPS flyash (indeed the
precharger OFF values for DEF are about the same as those measured for the SPS
flyash with the precharger ON) .
Other researchers have attempted to interpret the reduced K2 values of
charged dust in terms of an increased dust-cake porosity (Cooper, 1979). While
this effect may contribute to the flyash observations reported here, other
factors prove to be more important. The Kjj data presented in Figures 4 and 6
have been calculated from the following expression:
(AP -AP )
where AP = the pressure drop across the bag at the end of the filtration
cycle;
AP = the effective pressure drop at the beginning of the filtration
cycle (the intersection of the linear portion of the AP trace with
the t = 0 axis) ;
C. = dust concentration in the inlet;
V = the superficial filtration velocity (the gas-to-cloth ratio); and
t = the time of a filtration cycle.
In Equation 1, the factor C.VtF represents the mass/area of the dust cake
producing the change in pressure drop across the bag. Using Equation 1 is
therefore equivalent to assuming that all dust fed into the baghouse ends up
evenly distributed on the fabric. In particular the use of Equation 1 in
conjunction with precharger-ON data is suspect, since flyash collected by the
precharger section is not adequately accounted for. Rewriting Equation 1 as:
F
(2)
27
-------
where A = the total fabric area; and
m = the total dust mass added to the fabric during the filtration cycle;
makes clear the fact that a change in the flyash ending up on the bag from that
assumed by setting HL, = C Vt A changes the computed value of K£ . Failure to
include such corrections leads to Ka errors. No such corrections have been
made in the data presented in Figures 4 and 6.
To use Equation 2, which requires a measurement of mT> the experimental
procedure was changed to include a hopper weighing after selected shake-clean-
ings. In addition, in certain experiments, the precharger section above the
bag was cleaned by manual rapping after the bag itself had been cleaned by the
conventional mechanism. A crude mass balance could be carried out by dividing
the incoming flyash into four possible destinations:
1. the hopper,
2. the outlet duct,
3. the bag, and
4. the precharger.
As mentioned previously, the hopper fallout proved negligible in this top-
entry system. The flyash penetrating the bag to reach the outlet duct was
assumed to be less than 1 percent and hence also negligible. The conclusion
is that the two flyash weighings—that of the flyash shaken from the bag during
the shake-clean portion of the cycle and that dislodged from the precharger
section during manual rapping—should account for most of the flyash entering
the baghouse during a given cycle.
Table 1 summarizes the results of such measurements carried out with the
DBF. All measurements were made after a fixed 3-hour filtration cycle in
which approximately 1170 g of flyash entered the baghouse. The amount of fly-
ash ending up on the bag decreased with precharger voltage as shown. Using
the actual mass of flyash collected on the bag as m in Equation 2 results in
the corrected Kj values shown while using Equation I yields the uncorrected Ka
values. These data show that when the precharger is cleaned each cycle, the
reduced K^ values portrayed in Figures 4 and 6 are attributable primarily to
flyash collection by the precharger.
When, however, the precharger section is not cleaned and the collected
flyash is allowed to accumulate, a steady state is reached in which the flyash
leaving the precharger is nearly the same as the quantity entering. As shown
in Table lb, under operation tfith no precharger rapping, 90 percent-of the fly-
ash mass fed into the baghouse ends up on the bag. Corrected K^ values calcu-
lated from Equation 2 now show a decrease when the precharger is ON. This mode
of operation is that in which the data of Figures 4 and 6 were collected.
The conclusions emerging from these corrected K.2 measurements therefore
are that with a regularly cleaned precharger, the dominant mechanism of reduced
bag Ap is flyash capture by the precharger. Without regular precharger clean-
ing, however, the reduced AP is still observed but cannot be attributed to
precharger collection. A true dust-cake effect is one possible explanation,
but precharger efficiency and performance must be monitored more carefully
28
-------
before conclusive statements can be made.
Certain instabilities were noted under the no-clean mode of operation with
Bag 3, the third bag run with the DEF. Bag 3 operated in a constant AP mode—
the shake-cleaning portion of the cycle was initiated when the AP acrossFthe
bag reached 6.6 in. of H20 (1.6 kPa) . Filtration time becomes a variable in
this operating mode rather than a fixed value as it was for the cycles sketched
in Figures 3 and 5. Long-term operating stability in this mode was monitored
by running the bag around the clock over a period of several weeks, the only
interruptions being for insertion and removal of efficiency measuring probes.
Precharger power was ON throughout and the precharger section was left unrapped.
As evident in Figure 7, the charge-enhanced operation had essentially
disappeared by Day 8; that is, the early filtration time period was on the
order of 170 minutes (Day 2) while the Day 8 filtration time decreased to 70-
80 minutes. The corresponding Kj values are equivalent to those of the corona-
ON band (Figure 4) for the Day 2 operation and the corona-OFF band for the Day
8 operation, even though the corona power has actually been ON throughout.
That this deterioration in performance is caused by precharger inefficiency
is suggested by the subsequent Day 12 observation (not shown) in which the
longer filtration time (lower Kj) operation is restored by operating at 70 per-
cent relative humidity. This dependence on relative humidity is interpreted as
a flyash-resistivity effect, the added mositure serving as a "conditioning"
agent which lowers the electrical resistivity of the flyash and restores the
charging efficiency of the precharger.
Conclusions
1. When the precharger section is cleaned by manual rapping during each
cycle, the dominant mechanism of the charge-enhanced performance is dust
removal by the precharger so that the system should be modeled as a
serial combination of a precharger ESP and a conventional fabric filter.
This conclusion does not reject previous reports of K^ reduction with
increasing electrical charge as will be discussed in the following para-
graph; it does imply that the contribution of any such true K2 reduction
is less than that attributable to the mass removal effect—at least for
the experiments carried out in the EPA shaker baghouse with regular pre-
charger cleaning.
2. If the precharger section is not cleaned, a significant enhancement con-
tinues to occur. Under these conditions, a steady state is reached between
the dust entering the precharger and the dust leaving the precharger. The
dust reaching the bag during any given filtration cycle then is approxi-
mately equal to that fed into the system so that the precharger collection
efficiency is effectively zero. Under these conditions, enhanced filtra-
tion still occurs as is apparent by the mass-corrected values of K^. This
mode of operation appears to be unstable as was illustrated by the loss
of enhancement under long-term operation (~8 days) at 50 percent relative
humidity. That the enhancement could be re-established by raising the
relative humidity to 70 percent suggests a flyash-electrical resistivity
effect whereby the precharger operation has been restored by the use of a
more conductive flyash.
29
-------
In short, stable precharger operation is essential for stable enhanced
filtration via charged flyash. Optimum precharger design is still under
development. Whatever design is chosen, it will have to be operated and main-
tained along with the bags in order to realize and maintain enhanced filtra-
tion. The incorporation of a precharger upstream of a fabric filter therefore
adds its own maintenance requirements to those of the baghouse. Although what
constitutes either optimum or adequate precharger operation is not yet defined,
the precharger probably will have modest performance requirements compared to
those of a conventional electrostatic precipitator, and appears to possess the
potential for improving overall baghouse performance whether as a pre-collector
or a method of modifying dust-cake porosity or a combination of both.
Metric Conversions
1 in. of H20 = 0.25 kPa; 1 gr/ft3 = 2.29 g/m3; 1 (in. of H20/f t/min)/lb/f t2 =
"
References
Ariman, T. and D.J. Helfritch, "Pressure Drop in Electrostatic Fabric Filtra-
tion," Second Symposium on the Transfer and Utilization of Particulate
Control Technology, Vol. Ill, EPA-600/9-80-039c (NTIS PB 81-144800),
September 1980, pp. 222-236.
Chudleigh, P.W. and N.W. Bainbridge, "Electrostatic Effects in Fabric Filters
During Build-up of the Dust Cake," Filtration and Separation, July/ August
1980, pp. 309-311.
Cooper, D.W., "Mechanisms for Electrostatic Enhancement of Filter Performance,"
Invited paper presented to The Fiber Society and the Filtration Society
Joint Symposium on Fibers, Electrostatics and Filtration, Princeton, N.J.,
November 14-15, 1979.
Davis, W.T. and W.F. Frazier, "Conditioning of Fiberglass Fabric Filters for
Improved Performance," TVA Report, October 1980, Department of Civil
Engineering, University of Tennessee, Knoxville, Tennessee 37916.
Dennis, R. and J. Wilder, "Fabric Filter Cleaning Studies," EPA-650/ 2-75-009
(NTIS PB 240372), January 1975.
Donovan, R.P., J.H. Turner, and J.H. Abbott, "Passive Electrostatic Effects in
Fabric Filtration," in Second Symposium on the Transfer and Utilization
of Particulate Control Technology, Vol. I, EPA-600/9-80-039a (NTIS PB 81-
122202), September 1980, pp. 476-493.
Durham, J.F. and R.E. Harrington, "Influence of Relative Humidity on Filtra-
tion Resistance and Efficiency of Fabric Dust Filters," Filtration and
Separation, July /August 1971, pp. 389-398.
Gardner, R.P., R.P. Donovan, and L.S. Hovis, "A Dual-Beam Backscatter Beta-
Particle Gauge for Measuring the Filter Cake Thickness on Operating Bag
Filters Independent of Position," presentation to the Third Symposium on
30
-------
the Transfer and Utilization of Particulate Control Technology, March
1981, Orlando, FL (this symposium).
Helfritch, D.J., "Performance of an Electrostatically Aided Fabric Filter,"
Chem. Eng. Prog., _73, August 1977, pp.54-57.
Helfritch, D.J., Apitron, Product Brochure, 1979, Apitron Division, American
Precision Industries, 12037 Goodrich Dr., Charlotte, NC 28217.
linoya, K. and Y. Mori, "Effects of a Corona Precharger and Relative Humidity
on Filter Performance," Second Symposium on theTransfer and Utilization
of Particulate Control Technology, Vol. Ill, EPA-600/9-80-039c (NTIS PB
81-144800), September 1980, pp.237-250.
Ladd, K.L., R. Chambers, S. Kunka, and D. Harmon, "Objectives and Status of
Fabric Filter Performance Study," Second Symposium on the Transfer and
Utilization of Particulate Control Technology, Vol. I, EPA-600/9-80-039a
(NTIS PB 81-122202), September 1980, pp.317-341.
Lipscomb, W.O., S.P. Schliesser, and S. Malane, "EPA Mobile Fabric Filter—
Pilot Investigation of Harrington Station Pressure Drop Difficulties,"
Second Symposium on the Transfer and Utilization of Particulate Control
Technology, Vol. I, EPA-600/9-80-039a (NTIS PB 81-122202), September
1980, pp.453-475.
Walsh, G.W. and P.W. Spaite, "An Analysis of Mechanical Shaking in Air Filtra-
tion," J. Air Pollut. Control Assn., 12, No. 2, February 1962, pp.57-61.
Table 1. Kj Values of Detroit Edison Flyash Corrected for Mass Loss to the
Precharger* (3-hr filtration at 3 gr/ft3 => 1170 g total feed).
a) Precharger Cleaned Each Cycle bv Manual Rapping
Precharger
Voltage Dust Q/M
(kV) (nC/g)
Dust on Bag Uncorrected Kj Corrected Kj
(g) (from Equation 1) (from Equation 2)
0
10
12.5
15
17.5
20
Precharger
(kV
17.
17.
17.
17.
0
Kj in uni
_
1.5
1.9
3.1
3.5
-
Voltage
)
5
5
5
5
Fin.
tsof [_
1,152
999
615
615
413
443
5.4
4.8
3.7
3.0
2.5
1.7
Dust on Bag Uncorrected Kj
(g) (from Equation 1)
1,060
1,071
1,012
900
908
of H^O/ft/min]
lb/ff J
2.5
1.9
2.1
2.3
5.4
5.5
5.6
7.0
5.7
6.9
4.4
Corrected Kj
(from Equation 2)
2.7
2.1
2.5
3.0
6.1
31
-------
HIGH VOLTAGE
TO FARADAY
CAGE
Figure 1. Shaker baghouse inlet modified to accommodate a corona wire.
MASS CONC. (g/m3)
RUN *
PJ61A1
PJ61A2
PJ61B1
PJ61C1
PJ61D1
PJ61D2
PJ61 El
PJ61 F2
RELATIVE
HUMIDITY
50% 0
50% 0
30% n
70% A
70% A
70% A
39%
50% n
MMD Ifiml
4.90
5.09
4.88
4.93
4.98
4.75
4.53
4.85
W/PROBE
2.63
2.31
2.73
2.61
2.89
2.79
2.70
2.24
W/O PROBE
2.31
2.02
2.27
2.20
2.29
2.24
2.20
1.88
SAMPLE
TIME (mini
7
7
6
4
3
3
3
3
j—.—i 1—i I i i i
99.8
99
98
95 Z
I
to
w
UJ
70
SO
50 O
20 P
<
_l
10 S
o
5
2
1
0.5
0.2
0.1
0.2 0.3 0.4 0.6 0.8 1.0 2.0 3.0 4.0 6.0 8.0 10.0
PARTICLE SIZE (micrometers)
Figure 2. In-situ impactor analyses of Detroit Edison flyash.
32
-------
Laboratory Shaker Baghouse/Redispersed
SPSFIyAsh
Laboratory Shaker Baghouse/Corona Charged
Redispersed SPS Fly Ash
c 4
8084
TIME (min)
164
Figure 3.
Reduced pressure drop of charged
Southwestern Public Service flyash.
1 J
_l
' 8
7
6
B
4
3
~1
8
<
'
7
6
5
4
3
x-
r
•
i ,
NEW
•
CORONA
*- OFF -».
-*
-
«
* X \
X.
BAG OF
1
,^
0
3tt
0 *
OFF
• 1
* •
X X X X
*
^
ON
*
XXX
OFF
x
BAG OPERATING TIME (DAYS)
I
30
20
10
L.
.
-
• • • •
5
BAG 2
•
1
10
•
•
.
yrtsT-r
\
ON
i
I
15
^
Figure 4. Filtration performance of Southwestern Public Service
flyash/Criswell silicone-graphite fiberglass bag.
33
-------
Laboratory Shaker/Detroit Edison Fly Ash -
Laboratory Shaker/Corona Charged
Detroit Edison Fly Ash
1 2
TIME (hr)
Figure 5. Reduced pressure drop of
charged Detroit Edison
flyash.
-OFF-
///;//////
CORONA
ON
i — i
i
- "^T"—^
//SSSMSSWWSSSSSS,/,,,,,,^
Figure 6. Specific cake resistance of
Detroit Edison flyash.
,
s 2 3 4 5 6 7 8 9 10 11
OPERATING TIME (days)
1 in.of H2O
A DAY 2. CORONA ON
1 in.of H2O
B DAY 8, CORONA ON
Figure 7. Degradation of electrically enhanced filtration of
Southwestern Public Service flyash.
34
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ELECTROSTATIC AUGMENTATION OF FABRIC FILTRATION
By: D. W. VanOsdell
Research Triangle Institute
P. 0. Box 12194
Research Triangle Park, North Carolina 27709
G. P. Greiner
ETS, Inc.
3140 Chaparral Drive, S. W. (Suite C-103)
Roanoke, Virginia 24018
G. E. R. Lamb
Textile Research Institute
P. 0. Box 625
Princeton, New Jersey 08540
L. S. Hovis
Industrial Environmental Research Laboratory
MD-61
U. S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
ABSTRACT
The concept of electrostatic augmentation of fabric filtration (ESFF)
has been investigated at pilot scale. The pilot unit consisted of a con-
ventional baghouse in parallel with an ESFF baghouse, allowing direct
comparison. All results reported in this paper are for pulse-cleaned bags
in which the electric field was maintained parallel to the fabric surface.
The performance of the ESFF baghouse has been superior to the parallel con-
ventional baghouse by several measures. The ESFF baghouse demonstrated:
1) a reduced rate of pressure drop increase during a filtration cycle,
2) lower residual pressure drop, 3) stable operation at higher face veloc-
ities, and 4) improved particle removal efficiency. These benefits can be
obtained with only minor modifications to conventional pulse-jet hardware
and at low electrical power consumption. The indicated ability to operate
at increased face velocities with only modest expenditure for electrical
hardware leads to very favorable economic projections.
INTRODUCTION
Electrostatic Augmentation of Fabric Filtration (ESFF)
A number of researchers have proposed and investigated various concepts
for the use of electrostatic forces to improve fabric filtration. The work
described in this paper used as a starting point work done by the Textile
Research Institute (TRI).(l) Briefly, the TRI concept of ESFF consisted of
the use of an electric field parallel to the fabric surface. The field was
produced by raising alternate parallel wires to a high potential. Corona
particle charging was not necessary. The electrode wires were on the
35
-------
dirty-gas side of the filter, in contact with, but not incorporated into, the
fabric. .It was found that fabric surface characteristics had a major effect
on the amount of enhancement possible. A low fabric surface density appeared
to improve performance compared to a smooth, "cake release" fabric.
Figure 1 is a schematic of the TRI "harness," which was developed over
the course of the TRI work and used at bench scale to evaluate the ESFF ef-
fect. As shown, the harness was tied to the outside of the bag and elec-
trically connected so that alternate electrode wires were at high electrical
potential. An important practical aspect of this design is that the cage
within the filter bag must be insulated to prevent electrical shorting
through the fabric. In general, the TRI investigation was carried out at
ambient temperature with the bag operating at low pressure drops (about 0.25
kPa) . The dust was a resuspended fly ash, with the bench pulse-jet unit
cleaned on a 15-minute cycle.
The major advantages found for ESFF during the TRI investigation were a
reduced rate of pressure drop increase and reduced particle penetration. A
pressure drop ratio (PDR) was defined as:
CAP - AP )
j r
_
f r control
where AP is the pressure drop across the bags: f refers to the final state
(just prior to cleaning), r refers to the residual state (just after clean-
ing), and control refers to a baghouse without electrical enhancement.
For an idealized filter cycle in which the bag pressure drop increases
linearly with time at constant dust loading from AP to APf, it can be shown
that: r
PBK - (2,
(K2) control
where K.2 is the specific cake resistance. PDR, then, is the ratio of the
flow resistance of a dust cake collected by an ESFF system to that of a con-
ventional fabric filter.
The Program
A program to evaluate ESFF at pilot scale is being carried out by a
team consisting of the Research Triangle Institute (RTI) , ETS, Inc., and
TRI under contract to the U.S. Environmental Protection Agency. RTI, prime
contractor, has directed the technical effort and performed electrical hard-
ware development and construction. ETS, Inc., designed, built, and is
operating the pilot unit.
The pilot unit will be operated in both pulse-jet cleaning (outside
collection) and reverse-air cleaning (inside collection) modes. The program
has been underway for about one and a half years. Most of that time has
36
-------
been spent in the pulse-jet operating mode; thus, all of the data reported
in this paper are from pulse-jet operation. The principal parameters of the
study have been pressure drop, mass removal efficiency, and removal effici-
ency by particle size. In addition, an economic analysis of the ESFF con-
cept has been conducted. The program is presently continuing with the
baghouses in the reverse-air operating mode.
EXPERIMENTAL SYSTEM
Pilot Baghouses
The pilot plant which was designed and built to investigate ESFF con-
sists of two identical baghouses operating in parallel from a flue gas slip-
stream. One baghouse is electrostatically augmented, while the other is
not. Figure 2 is an isometric drawing of the pilot unit. Each baghouse can
accept up to 13 pulse-cleaned, 11.4 cm (4.5 in.) bags, each 244 cm (8 ft)
long. Each pilot baghouse is about 85 cm (33 in.) square and 4.3 m (14 ft)
high. Construction is carbon steel throughout. Three inches of insulation
was used on both the baghouses and on all ductwork. The pilot units are
fairly typical of the smallest industrial unit with two exceptions: 1)
access doors were added on three sides of the baghouses, and 2) center-to-
center bag spacing was increased slightly to allow clearance for the elec-
trical hardware.
We feel that the use of parallel control and experimental units has
made the difference between success and failure for the ESFF program. As is
described below, the dust source has been highly variable, and it would not
have been possible to speak with confidence about the experiment if we had
operated serially, even with much more sophisticated instrumentation. Bag-
house performance has been good. Additional baghouse insulation, wall heat-
ers, and improved door seals were added during the checkout period to
correct a low temperature problem. Otherwise, the baghouses performed as
designed.
The pilot unit was installed on a slipstream from an industrial boiler
house. Four pulverized coal boilers are in use with a normal load of about
160,000 kg/hr (350,000 Ib/hr) of steam. The coal fed to the boilers changes
frequently, with sulfur content varying from less than one percent to about
two percent and ash content ranging from five to 15 percent. The boilers
are sometimes co-fired with oil. Inlet dust loadings at the pilot plant
have varied from 0.6 to 5 g/scm (0.25 to 2 gr/scf); 0.7 g/scm (0.3 gr/scf)
is typical.
High Voltage Electrical Hardware
The electrical requirements of the ESFF system were relatively low.
The power consumption measured by TRI (and since confirmed in the pilot
unit) was about 1 W/m2 (~ 0.1 W/f t2) . For comparison, energy requirements
for gas moving at 3 cm/sec (6 ft/min) and 0.25 kPa (1 in. H20) are about
10 W/m2 (~ 1 W/ft2). At a potential difference between electrodes of 10 kV,
one pilot unit bag required about 100 yA of current. The entire 13-bag
complement, under normal conditions, requires less than 1.3 mA at 10 kV.
37
-------
The electrical system used at the pilot unit consisted of a variable DC
power supply (0 to 20 kV at 5 mA) for each of five power supply net-
works. Voltage was the controlled variable. Current and primary voltage
were measured separately in each network. Current limiting and meter pro-
tection circuits were also used. The pilot unit was operated with no more
than five bags and, consequently, only one bag on each network.
As described earlier, the TRI electrode "harness" consists of an array
of small diameter vertical wires glued to horizontal yarns. The entire as-
sembly was tied to the outside of the bag. Several problems were inherent
in this design. The "harness" had the potential of restricting bag clean-
ing, was relatively fragile and difficult to install, presented materials
problems in the high temperature flue gas environment, and required an in-
sulated cage. In spite of these anticipated difficulties, harnesses were
constructed for the pilot unit and were used to collect a significant por-
tion of the data which is presented in this report. From the standpoint of
operating a research pilot plant, the failures of the insulating coating on
the cages and of the harness adhesive were the most significant problems.
The materials used deteriorated within one to two months and required fre-
quent attention.
The problems discussed above were largely overcome with the design and
construction of a new electrode assembly. The electrodes were moved to the
clean side of the bag and performed the dual function of cage and electrodes
(Figure 3). Early laboratory data taken by TRI indicated that performance
with this electrode arrangement would be reduced from that possible with the
outside harness. Pilot plant results for the two designs have been essen-
tially identical, however. The inside electrodes have now been used
successfully for several months.
PILOT UNIT OPERATION
Pilot Unit Checkout/Bag Conditioning
The initial portion of the pilot unit test program was intended to
demonstrate the crucial point that the two baghouses really were parallel
units with identical inlets. To accomplish this, both houses were operated
as standard fabric filters. After collecting enough data to be confident
that the two baghouses were comparable, the test bags were installed. Four
bags were installed in each baghouse; the 'electrical harnesses were installed
on the bags in the ESFF baghouse. The bags were broken in at a face velocity
of 1 cm/sec. Following the conditioning phase, the face velocity was raised
to 3 cm/sec and the test program was initiated.
Test Program
Normal operating procedure was to conduct the test program during the
day and to leave the baghouses on automatic operation overnight and on week-
ends. Several cleaning cycles were used during the test program; the face
velocity was varied between 1 and 5 cm/sec. Operation was evaluated at
field strengths between 0 and 5 kV/cm. Both the TRI harness and the clean-
side cage/electrode described earlier were used to produce the electric
30
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field. Three fabrics were tested: 1) 23 oz Teflon® felt, mechanically
napped to reduce the surface density; 2) 17 02 Teflon® felt, an experimental
fabric; and 3) 17 oz woven glass fabric, ten percent Teflon® B finish, bulked
in the fill yarn.
RESULTS AND OBSERVATIONS
Ob servations
Visually, the dust cake was somewhat thinner near the electrodes; this
was true for both outside and inside electrodes. The outside harness con-
tinued to function even with a dust cake several times the electrode diameter
on the bag. Cleaning of the bottom half of the bag was restricted by the
harness, however. We were not able to resolve whether a very thick dust cake
would reduce the ESFF effect, although some data indicated reduced perfor-
mance after several hours without cleaning.
Normal, "good" operation of the ESFF baghouse required a current of
only 1 to 10 yA at 8 to 10 kV on the electrodes. Variation in dust loading
and normal boiler variations did not adversely affect operation. High dust
loadings, for instance, caused high pressure drops or more frequent cleaning
for the baghouses, but the ESFF baghouse retained its advantage. Dust char-
acteristics do affect ESFF operation, however. Operation below the acid dew
point was not possible because of high dust conductivity. In addition,
short periods of high bag current (and reduced ESFF effect) due to dust
properties have occurred two or three times. The ESFF baghouse recovered
after cleaning. The cause of these upsets has not been determined, but soot
and/or excessive carbon in the ash is suspected.
Rate of Pressure Drop Increase
The most obvious of the effects of ESFF is that of a reduced rate of
pressure drop increase for a given dust loading rate. This is the effect
which has been reported extensively by TRI. Figure 4 presents data from
one cleaning cycle for the pilot baghouses. There is an evident difference
in the rate of pressure drop increase. Expressed as PDR, the difference is
even greater. The pressure drop in the control baghouse is not linear ini-
tially but becomes so after the cake has formed. The ESFF trace does not
exhibit a cake repair period. Other data do show a cake repair period for
the ESFF baghouses, although it is generally not as pronounced as that for
the conventional baghouse.
Figure 5 is a presentation of PDR data as a function of average elec-
tric field. Teflon® felt and fiberglass bag data are included. A curve
representing the performance of the TRI bench-scale baghouse is also pre-
sented. For the Teflon® felt bags, the pilot unit data are very similar to
the TRI data in spite of markedly different operating conditions. The fiber-
glass bags, which do not have the low density surface characteristic of the
Teflon® bags, do not exhibit as great an ESFF effect. It is worth noting
that inlet mass loadings and hopper dust weights have remained about the
same for both baghouses.
39
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Residual Pressure Drop
The data presented in Figure 4 exhibit another of the ESFF effects, a
lower residual pressure drop. The residual pressure drop of the ESFF unit
was 20 to 80 percent of that of the conventional baghouse operated under
identical conditions. It was possible through multiple off-line cleaning to
return both baghouses to essentially the same residual pressure drop, but
the residual pressure drop difference soon returned once normal operation
was resumed.
Stability at Increased Face Velocity
The pilot unit has shown that the ESFF baghouse can be operated without
blinding at a higher face velocity than can a conventional baghouse. Fig-
ure 6 presents some data which clearly show the improved performance possible
with the ESFF baghouse. At 4 cm/sec, the conventional baghouse pressure drop
became too high for the pilot unit blower, and the gas rate was actually
dropping off at the high pressure drop. Mass collection efficiency decreased
at high face velocities, as expected.
Particle. Collection Efficiency
The pilot unit data indicate an increase in size dependent particle
collection efficiency with the ESFF baghouse. Figure 7 is a plot of frac-
tional penetration as determined by multiple cascade impactor tests. The
ESFF baghouse shows consistently better collection efficiencies over the
range 0.4 to 4.0 ym during the test period. However, penetrations are also
influenced by more frequent cleaning of the conventional baghouse, so the
improvement may not be because of improved collection as much as reduced
losses through the cloth. Mass efficiency data have not shown a clear ad-
vantage for the ESFF baghouse.
Economics
The capability to operate at an increased face velocity for a particu-
lar dust gives the ESFF concept a large cost advantage over conventional
fabric filtration. Compared to a conventional baghouse operating at
2 cm/sec (4 ft/min), an ESFF baghouse operating at 3 cm/sec (6 ft/min) might
be expected to control the same emission problem for a 30 percent lower
total annual cost. It may be possible to operate at 4 cm/sec (8 ft/min) as
shown in Figure 6 at even lower cost.
CONCLUSIONS
ESFF as developed in the laboratory has been transferred successfully
to a pilot unit on coal-fired boiler flue gas. The ESFF baghouse can be op-
erated at increased face velocities compared to a conventional baghouse on
the same flue gas. Other manifestations of the effect of the electric field
on the dust collection are a reduced rate of pressure drop and a reduced
residual pressure drop for an ESFF baghouse operated at the same face veloc-
ity as a conventional baghouse. In addition, at constant face velocity, par-
ticulate control is improved by the use of ESFF.
40
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RECOMMENDATIONS
Pulse-Jet Baghouse
Although the initial pilot plant experience has been very positive and
may be commercially viable, additional development work is required. Scale-
up has not yet been investigated, nor truly long-term operation, and
additional engineering effort is sure to be required in these areas. In
addition, work is needed in the areas of cage insulators and the entire
electrical system to reduce operator involvement and improve reliability.
The use of different fabrics and application to other dust control problems
are also subjects that should be investigated.
Reverse-Air Baghouse
The use of ESFF in reverse-air baghouses has not been tested at pilot
scale. Bench work at TRI has been somewhat successful. The woven glass
fabrics normally used do not have favorable surface characteristics, al-
though they do exhibit an ESFF effect. In addition, the clean-side
electrode/cage which was very desirable for the pulse-jet unit may not be
best for the reverse-air system. The indicated need, then, is for develop-
ment of fabric/electrode combinations that are optimal for ESFF. We will
be working on this matter during the continuation of the pilot unit program.
ACKNOWLEDGEMENTS
This work was supported by Contract No. 68-02-3186 from the EPA
Industrial Environmental Research Laboratory, Research Triangle Park, North
Carolina. The assistance provided by E. I. DuPont de Nemours and Company,
Inc., is gratefully acknowledged. The pilot unit was located at their
Waynesboro, Virginia, plant and Teflon® fabric was donated to the test
program by DuPont.
ENDNOTES
1. Lamb, G. E. R., and P. A. Costanza. A Low-Energy Electrified Filter
System. Filtration and Separation. 17:319, 1980.
41
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TO POWER SUPPLY
FIBERGLASS YARN-
-ELECTRODES
STAINLESS STEEL
WIRE-0.58 mm
O)
Hh
1.5 cm
HIGH VOLTAGE HARNESS HARNESS INSTALLED ON BAG
Figure 1. TRI high voltage harness.
Figure 2. ESFF pilot unit.
42
-------
FILTER
BAG
SUPPORT
ROD/ELECTRODE
INSULATING
RING
ALTERNATE ELECTRODES
CONNECTED ELECTRICALLY
, FILTER
BAG
Figure 3. Inside electrodes-cage assembly.
O 0.75
£ 0.50
0.25
ESFF Baghouse
Face velocity: 3 cm/sec
Field: 4 kV/cm
30
TIME, minutes
Figure 4. Bag pressure drop performance-single cycle,
43
-------
5 0.6
0.0
TEFLON® FABRIC @
GLASS FABRIC
23
ELECTRIC FIELD (kV/cm)
Figure 5. Pilot plant PDR data.
£
2-0
a
LLJ
CC
1.0
[off-scale]
CONVENTIONAL
2.5 cm/sec (5 ft/min)
ESFF
4 cm/sec (8 ft/min)
_l 1_
J_
34 5 67 89
TIME FROM START OF TEST (hours)
Note: Each test period preceded by
multiple-pulse bag cleaning
Figure 6. Pilot unit performance at two face velocities.
J I
10 11 12 13 14
02 0.3 0.4 0.5 0.6 0.8 1.0
2.0 3.0 4.0 5.0 6.0 8.0 10."
Aerodynamic Particle Diameter, taa
Figure 7. Size dependent particle penetration.
44
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FABRIC WEAR STUDIES AT HARRINGTON STATION
By: Richard Chambers, Kenneth Ladd, and Sherry Kunka
Southwestern Puhlic Service Company
P. 0. Box 1261
Amarillo, Texas 79170
Dale Harmon
Industrial Environmental Research Laboratory
Environmental Protection Agency
Research Triangle Park, North Carolina 27711
ABSTRACT
The performance and service life of filtration fabrics in the utility
industry is a subject of interest to many fabric filter users. South-
western Public Service Company (SPS), among several other utilities, has
developed a fabric evaluation program in conjunction with an Environmental
Protection Agency (EPA) study being done at SPS's Harrington Station. This
paper will discuss SPS's experience with fabrics and some of the conclu-
sions that have been drawn.
Among the topics addressed will be:
1. The use of systematic physical testing on fabrics to monitor
fabric wear and eventually predict fabric life.
2. An analysis of fabric wear mechanisms at Harrington Station.
3. Fabric failures by compartment position.
4. Controlling fabric wear by reducing shaker amplitude.
INTRODUCTION
Fabric filtration used in association with coal-fired utility boilers
remains a relatively new air quality control technique. One of the most
critical aspects of a successful fabric filter application is proper fabric
selection. To date, however, conclusive fabric performance data has been
unavailable to those involved in fabric selection for baghouse installa-
tions . Performance trends for a variety of fabrics are now beginning to
emerge from a study being conducted by Southwestern Public Service Company
(SPS) for the United States Environmental Protection Agency (EPA).
SPS is an electric utility based in Amarillo, Texas. The Company
provides electricity to a 117,000-square kilometer (45,000-square mile)
service area. Traditionally a gas-fired utility, SPS began its switch to
coal-fired generation in 1976 when Harrington Station Unit 1 was put into
service. Harrington Station is located approximately 8.1 km (5 miles)
north of Amarillo. The facility consists of three 360 MW units. Unit 2,
in 1978, was the first new electric utility boiler to be equipped with a
fabric filter system for emission control. The boiler utilizes pulverized,
low sulfur Western coal to produce 1,220,000 kg (2,688,000 Ib) steam/hour.
Flue gas from the boiler flows through the preheater directly through the
fabric filter system and then out the stack.
The fabric filter system (or baghouse) is a Wheelabrator-Frye, Inc.
(WFI) structural baghouse with shake/deflate cleaning. It consists of 28
compartments with 204 bags per compartment for a total of 5,712 bags; each
45
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bag is 0.3 m (11.5 in.) in diameter and 9.3 m (30 ft, 6 in.) long. The
baghouse was specified at 2,800,000 m /h (1,650,000 acfm) of flue gas at
156°C (313°F), thus yielding a design air-to-cloth ratio of 3.17 to 1
(gross) or 3.4 to 1 (net). Harrington's Unit 3 is equipped with a WI
fabric filter which also employs shake/deflate cleaning. The Unit 3 bag-
house, however, has four more compartments which yield air-to-cloth ratios
of 2.78 to 1 (gross) and 2.98 to 1 (net).
Fabric Testing Results
Immediately following the rebagging of the Harrington Unit 2 fabric
filter system, a fabric evaluation program was initiated for the new fab-
rics. The purpose of this program was to evaluate the wear characteristics
and through-put for each of the fabrics under test in the unit. Table I
shows the fabrics being evaluated.
TABLE I. Test Fabrics
Criswell 442-57DC2
Criswell 449-57DC2
Criswell 449-1580
Criswell 449 COF
Menardi Southern 601-T
Globe-Albany
Allspun Nomex
0.34 kg/nu
0.48 kg/nu
0.48 kg/m
0.34 kg/ml
0.34 kg/rn
(10 oz/ydb
(14 oz/yd,)
(14 oz/yd )
(10 oz/yd*)
(10 oz/yd )
0.19 kg/m
10% Teflon coating
10% Teflon coating
Silicone/graphite/
Teflon coating
Proprietary coating
10% Teflon coating
2
(5.5 oz/yd ) Permaguard coating
In order to examine the various fabrics for wear, samples were removed
from each of the test compartments on a periodic basis and sent to Environ-
mental Consultants, Inc., for evaluation. Each sample was subjected to the
following tests:
1. Breaking strength.
2. Mullen Burst.
3. MIT Flex -0.08 cm (0.03-in.) jaw @ 1.8 kg (4 Ib).
4. Loss on ignition @ 650°C (1200°F).
5. Permeability.
According to the testing program conducted by Environmental Consult-
ants for SPS, the weakened area along the most severe fold lines were the
ones to be tested for mechanical strength rather than the stronger areas
away from the fold lines. SPS believes that testing results obtained in
this manner are more representative of the rate of decay in mechanical
properties for a material since it will be the weakened areas that will
determine the fabric life.
The results of Mullen Burst
the fabrics are shown in
materials show much higher initial values-for both Mullen Burst and break-
ing strength than the 0.34 kg/m (10 oz/yd ) fabrics, yet the rate of decay
of these values is seen to be much higher. The higher initial values are
due to the stronger fill yarn construction of the heavier fabrics while the
difference in the rate of decline of the Mullen Burst and breaking strength
is due to the different reaction of the fabrics to the baghouse environment
and wear during the cleaning process.
46
Burst and breaking strength testing for each of
Figures 1 and 2. The 0.48 kg/m (14 oz/yd )
-------
On both plots, the highest initial values are for the proprietary
Cris-0-Flex (COF) coated material. This is due to the fact that the fabric
was not heat-cleaned prior to application of the coating. Since heat
cleaning weakens the fabric to some extent, the COF-finished material has
the initial advantage.- This advantage is seen to decline, however, as all
three of the 0.48 kg/m (14 oz/yd ) materials appear to have similar Mullen
Burst values after a year in service.
Nomex, the only synthetic fabric in service, is shown on each of the
plots at the bottom. This synthetic material, having inferior tensile
properties to glass, nevertheless excells in the critical area of flex
strength.
Aside from showing some different characteristics of fabrics of dif-
ferent weight, the Mullen Burst and breaking strength plots are disappoint-
ing in that there is nothing in the appearance of these curves to indicate
that two of the test fabrics have reached termination of their useful life.
The only indication of incipient failure obtained from the fabric testing
results is shown on the MIT Flex plot in Figure 3. Here the reduced MIT
value (MIT Flex/Initial MIT Flex, M/M ) is plotted vs. service life. An
indication of incipient failure is obtained by looking at the steep slope
and low values for M/M for the 449 COF and 449-1580 fabrics. Both these
fabrics are scheduled €o be replaced due to the high failure rate at Har-
rington. That the MIT Flex tests should correlate best with the progress
of fabric wear is certainly understandable since, of all the testing meth-
ods, the MIT Flex most accurately duplicates what the fabric undergoes
during cleaning.
The existence of a critical value for M/M can be inferred from the
simultaneous failure of the two fabrics as they approach the same reduced
MIT Flex value (r*0.07). If such a critical value exists at which termina-
tion of fabric life can' be determined, it should become evident as the
remainder of the test fabrics fail.
The data for Nomex is shown on the reduced MIT Flex plot but is not
directly comparable to the glass fabrics since the test conditions were not
the same. The MIT values for Nomex were obtained with a 2.3 kg (5-lb)
weight instead of the usual 1.81 kg (4-lb) weight used for glass in order
to generate numbers in a reasonable amount of time. The Nomex data is
interpreted such that the fabric appears to yield acceptable remaining life
and is in no way inferior to the glass materials. A recent paper titled
"SPS Experience With Fabric Filtration" presents a more detailed discussion
of the correlation between fabric testing results and actual operating
conditions (1).
Failures by Compartmental Position
Any given section of bags within a compartment would be expected to
have roughly the same failure rate as any other section of bags, all things
being equal. In the case where bag failures are not equally distributed
throughout a compartment, the cause of higher failure rates should be
identified and eliminated, if possible, to save costly bag replacements and
to allow faulty design concepts to be identified and eliminated.
To analyze the failure rate by compartment position at Harrington
Station, the failures that have occurred to date have been superimposed
onto a single plot by thimble position as shown in Figure 4. Two types of
failures have been noted. Any small hole in a fabric is referred to as a
47
-------
"pinhole;" whereas, a large section ripped out of the fabric is shown as a
"blowout."
The pinhole type failure is by far less prevalent now than during the
first year of operation where it was the main failure type with the orig-
inal fabric. These initial failures were thought to be due to fabric
stretching resulting in a lack of tension and blousing over the thimble.
The data indicate that there is no discernible pattern to the pinhole type
of failure. Therefore, in Figure 4, only the blowout location is shown.
The blowout failures definitely appear to occur more often in certain
locations. The outside two rows account for 73 percent of all the blowout
failures. Calculating the failures per thimble position for the outside
two rows yields 0.91 failures per thimble vs. an overall average of 0.39-
Narrowing the failure area to the four corner areas two bags deep and
five bags wide yields the failure rate shown in Figure 5. The two areas
adjacent to the interior wall are seen to have the greatest failure rates
with 1.8 and 1.2 failures per thimble compared to 0.21 for thimbles not in
the corner areas. The corner areas adjacent to the outside walls also show
significantly higher failure rates with 0.8 and 0.7 failures per thimble
position. Factors contributing to these higher failure rates are thought
to be hopper shed plate design and turbulent flow distribution in the ash
hopper.
Failure Mechanisms in Filtration Fabrics
The two fabric styles undergoing premature failure at Harrington,
Criswell's 449 COF and 449-1580, appear to be experiencing a wear pattern
that is typical of the other fabrics being tested except that these two
fabrics are damaged to a much greater degree. The typical wear pattern
consists of a severely abraded area along a fold line in the fabric origi-
nating at the top and proceeding down the fabric approximately one-third
the length of the bag. Along these abraded fold lines (called "wooley
worms") pinhole failures occur at irregular spacings. A single bag may
have as few as two such lines or as many as six. Eventually, failure will
occur in the 0.48 kg/m (14 oz/yd ) fabrics of these "wooley worms" as a
vertical split up to several meters (feet) long. The lighter, Teflon-
coated fabrics, however, most often exhibit a rectangular-shaped blowout
where the fabric appears to have had a section of cloth cut out of it.
This behavior_is believed to be due to the weaker warp yarn construction of
the 0.34 kg/m (10 oz/yd ) material.
To have a closer look at a "wooley worm" failure, one of the affected
fabrics was removed from the baghouse and dissected. The fabric chosen was
one that had a vertical split along a fold line. In Photo 1, the inside
portion of the fabric is shown just above the tear. From this photo the
full extent of the massive fiber damage along these fold lines can be
observed. Although the example shows only a single abraded area, it is not
uncommon for two such areas to be very close to one another, suggesting
that the fabric was actually folding back on itself and undergoing abra-
sion.
The deflation process is expected to play only a minor role in the
development of the "wooley worm" type failure observed at Harrington since
the deflation levels employed are very low (approximately 0.5 cm (0.2 in.)
w.g.) and observation shows no discernible flexing of the material. In
order to examine the effect of shaking on the fabric, one compartment of an
EPA-sponsored pilot baghouse currently in operation at Harrington was re-
48
-------
bagged with one new Criswell 449-1580 0.48 kg/m (14 oz/yd ) graphite/
Teflon/silicone (G/T/S) coating) and one Criswell 442-57DC2 0.34 kg/m (10
oz/yd ) Teflon-coated). The other thimbles were left unbagged to allow for
better observation. The effect of the shaking forces on these fabrics was
then viewed by entering the compartment at both the upper and lower levels.
The motions of the fabric were complex; however, it was clearly apparent
that the shaking forces were responsible for causing folding and flexing
along certain lines in the fabric.
The two fabrics appeared to have somewhat different reactions to the
shaking forces. The 0.34 kg/m (10-oz/yd ), Teflon-coated material seemed
to transmit the intensity of shake farther down the fabric than the 0.48-
kg/ m (14-oz/yd ) material. By comparison, the heavier fabric reacted to
the same shaking forces in a more sluggish and lethargic manner than the
lighter material. The 0.34-kg/m (10-oz/yd ) Teflon-coated fabric appeared
to have a very "live" reaction to shaking. The degree to which flexing
occurred in the cloth was also different for the two fabrics. The rela-
tively limp 14-oz. material appeared to be undergoing a more severe folding
action while the rigid Teflon-coated fabric tended to resist the folding
forces.
It is felt that this difference in reaction to the shaking forces may
play the principal role in the premature failure of the 449 fabrics instal-
led at Harrington Station. The more rigid Teflon coating may be protecting
the fabric by mechanically restricting its motion during the cleaning
cycle. The reaction of fabric to the forces employed during cleaning is
undoubtedly one of the most important factors in determining fabric life.
Further studies of this type are planned for the pilot unit in order to
better define the reaction of various fabrics to shaking forces and its
relation to fabric wear.
Reducing Fabric Wear
After an appreciation has been developed for the mechanisms involved
in fabric wear, the next logical step is to attempt to minimize these
actions by some means . One method of reducing the severity of the fold-
ing and flexing wear occurring during the shaking process that appears to
hold promise is that of employing smaller shake amplitude by replacing the
existing eccentric with one having less of a throw. The intention of such
a change is that the fabric would undergo less severe flexing; however, if
this is the only change made, cleaning will suffer as more dust cake will
be left on the fabric. Dennis (2) has shown that dust removal is a linear
function of the acceleration applied to a fabric. The Acceleration, a,
imparted to a fabric during shaking is proportional to Af where A is the
amplitude and f is the frequency. Dennis gives the equation
(1)
where k - 0.7.
The ratio of acceleration compared to some standard values for A and f on a
given system can be calculated from
2 2
(acceleration ratio) a_ _ Af „ _ Yd (2)
a ~ A f Y A 2
s s s s s
49
-------
where
d = sheave diameter
Y = eccentric throw
If, for example, the fabric was being shaken by a mechanism employing a
13.5-cm (5.3-in.) drive pulley and a 1.3-cm (1/2-in.) throw eccentric and a
change of eccentric is accomplished to one with a 1 cm (3/8-in.) throw,
then the acceleration would be only 75 percent of its former value.
A study was undertaken at Harrington to determine the impact of ampli-
tude and frequency changes on cleaning by placing different sizes of eccen-
trics and shaker motor pulleys (sheaves) on several compartments and mea-
suring the change in flow through the compartment with manometers installed
across the outlet dampers.
The relative flow after cleaning vs. the acceleration ratio plot is
shown in Figure 6. The plot confirms the results reported by Dennis in
that a directly proportional measure of dust removal, through-put after
cleaning, is linearly related to the acceleration imparted to the fabric
during shaking.
From Figure 6 it is apparent, then, that reducing the eccentric throw
from 1.3 cm (1/2-in.) to 1 cm (3/8-in.) would cause an unacceptable lower-
ing of flow through a compartment if no other changes were made. However,
as can also be seen from the plot, the installation of a 15.2-cm (6-in.)
pulley brings the acceleration ratio back close to 1.0 and the flow comes
back to the same level as under the original conditions. Therefore, in-
creasing sheave size can be used in conjunction with smaller shake ampli-
tudes without incurring unacceptable losses in cleaning. In light of this,
studies are now underway to determine the effect on fabric wear of employ-
ing 1-cm (3/8-in.) eccentrics and 15.2-cm (6-in.) shaker motor pulleys
instead of the 13.5-cm (5.3-in.) pulleys and 1.3-cm (1/2-in.) eccentrics
currently being used.
CONCLUSION
The results of fabric filtration analyses are now beginning to reveal
performance trends for various types of fabrics. In the coming months, SPS
plans to look at dust and flow distribution by analyzing individual com-
partment pressure drop behavior. SPS hopes to make more compartment obser-
vations and conduct individual compartment through-put measurements. In
addition, SPS will continue to-evaluate different fabrics, including two
entire compartments of 0.34 kg/m (10 oz/yd ) Acid Flex coated bags and one
compartment of 0.19 kg/m (5.5 oz/yd ) Nomex bags. As results of these
studies become available, SPS looks forward to sharing them with other
utilities considering fabric selection in association with coal-fired
boilers.
ENDNOTES
1. Chambers, R. C., Ladd, K. L., and Kunka, S. L., "SPS Experience With
Fabric Filtration," presented at 5th International Fabric Filter
Forum, Phoenix, Arizona, January 1981.
2. Dennis, R.C., Cass, R.W., Cooper, D.W., Hall, R.R., Hampl, V., Klenan,
H.A., Langley, J.E., and Stern, R.W., GCA Corporation. "Filtration
Model for Coal Fly Ash With Glass Fabrics," EPA-600/7-77-084 (NTIS No.
PB 276 489), August 1977. 50
-------
Ui «
IOO
200 30O
Days In Service
350!
30O-
442 Tef Ion
100
—r-
200 300
Days in Service
400 500
Figure I. Mullen Burst
Figure 2. Breaking Strength
-------
20O 300
Days in Service
Figure 3. Reduced MIT Flex vs. Time
Figure 4. Failures by Thimble Position
-------
Interior Wall
i.e
1.2
0.21
Failed Bags Per
Thimble
0.8
0.7
Exterior Wall
Overall Average - 0.39 Failures by Thimble
o> 1.0
* £
O c
» 0.
0.7
0.6
6.0 in. Pulley
3/8 in.Eccentric
5.3in.PuH«y
l/4 in. Eccentric
i. Pulley
'/4 in. Eccentric
1cm - 0.39 in.
o.s
Acceleration —
Ratio BS
o Afz
5.3in. Pulley
''1 in. Eccentric
t.o
Figure 5. Failure by Compartment Position
Figure 6. Effects of Various Sheave
And Eccentric Sizes on Cleaning
-------
Photo 1. Dissection of "Wooley Worm" Failure
54
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SPS PILOT BAGHOUSE OPERATION
By: Kenneth Ladd, Wanda Hooks, and Sherry Kunka
Southwestern Public Service Company
P. 0. Box 1261
Amarillo, Texas 79170
Dale Harmon
Industrial Environmental Research Laboratory
Environmental Protection Agency
Research Triangle Park, North Carolina 27711
ABSTRACT
In 1977, the Environmental Protection Agency (EPA) executed a contract
with Southwestern Public Service Company (SPS) to assess the performance of
a large fabric filter system used on a new utility boiler burning low
sulfur Western coal. One option of this contract provided for the instal-
lation of a pilot filter system on-stream with Harrington Station Unit 2.
This paper outlines the overall objectives of the pilot unit program. In
addition, proper system design, problems in start-up and operation, and a
fabric evaluation program are discussed.
INTRODUCTION
Southwestern Public Service Company's fabric filter study contract
(EPA No. 68-02-2659) with the EPA included a provision to exercise an
option for a pilot baghouse. In 1980, the EPA elected to exercise this
option. Southwestern agreed to operate and maintain a pilot unit at Har-
rington Station in conjunction with Unit 2 for a 3-year period.
The objectives of this option are (1) to operate the slipstream unit
under the same operating parameters as the full-scale unit, and (2) to
develop scale-up parameters from the slipstream unit to the large opera-
tion. Additionally, optimization of operating techniques will be deter-
mined on the pilot baghouse and applied to the full-scale unit. Another
phase of the pilot project is to conduct detailed studies on fabric per-
formance. Individual fabrics will be examined to assess the performance of
bags with different coatings. The cleaning effect of various weaves and of
different sizing removal procedures (chemical cleaning, heat cleaning,
coronizing) will also be studied.
Description
The pilot unit is a Wheelabrator-Frye, Inc., Model 366, Series 11.5 RS
Dustube Dust Collector. It has two 6-bag compartments and initially was
fitted with Criswell Style 442 10 percent Teflon-coated fabric. The bags
were 29.2 cm (11.5 in.) in diameter and 9302cm (366 in.) long, untensioned.
Cloth area per compartment is 51 m (549 ft )
The unit is operated by a 480-V control panel and an instrumentation
panel. The 480-V control panel houses the damper controls, hopper heater
controls, cleaning cycle timers, fan start/stop controls, and bypass auto-
matic controls. The instrumentation panel housesAP, flow, and temperature
monitoring devices as noted in Table 1.
55
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TABLE 1. INSTRUMENTATION PANEL
AP west recorder
Flow west recorder
Outlet temp recorder
West compartment flow
Inlet temp—temp control
Dampers
AP east recorder
Flow east recorder
Inlet temp recorder
East compartment flow
Off preheater, hot and cold
Installation
The installation of the pilot baghouse began when the modules arrived
on the job site around June 1, 1979. The pilot is installed at the south-
east corner of the east baghouse on Harrington's Unit 2. The inlet flue
gas to the pilot is pulled from the east inlet duct of the main baghouse
and mixed with the hot preheater flue gas with a mixing valve to maintain
temperature. (See Figure 1, Pilot Baghouse Schematic.)
The actual installation consists of the following:
41 cm (16-in.) I.D.
30 cm (12-in.) I.D.
20 cm (8-in.) O.D.
36 cm (14-in.) I.D.
20 cm (8-in.) O.D.
41 cm (16-in.) I.D.
15 cm (6-in.) O.D.
36 cm (14-in.) I.D.
36 cm (14-in.) I.D.
41 cm (16-in.) I.D.
Elbow from preheater prior to reduction
From preheater
Bypass
To east and west inlet before branch
To east and west inlet after branch
From main baghouse inlet duct
Recirculation
From pilot outlet plenum to main fan
Crossover from main fan
From main fan crossover to main baghouse duct
Deflation
Main fan
Shaker
Variable-speed
shaker
Motors
7.5 kW (10 H.P.) 3600 rpm
37.3 kW (50 H.P.) 3600 rpm
0.75 kW (1 H.P.) 900 rpm
3.75 kW (5 H.P.) 1710 rpm
Siemens - Allis, Inc.
Siemens - Allis, Inc.
3/60/230/460
Siemens - Allis, Inc.
Reliance
30 cm (12-in.)
15 cm (6-in.)
20 cm (8-in.)
10 cm (4-in.)
Dampers
Outlet - Poppet
Deflation - Butterfly
Bypass - Butterfly
Recirculaton-Manual Operation - Recirc. Deflation
Dust Air - Butterfly
56
-------
Fans
Deflation 222-4-15 PBF Thermal-aire Design
Garden City Fan Co.
Main 15RF2 Thermal-aire Design
Garden City Fan Co.
SCOPE OF WORK
The pilot unit is designed to be used over a 3-year period for a
series of research activities. Certain "technical directives," or tasks,
have been set out in the contract f6r the pilot study. When these tasks
are accomplished, they will be written up as task reports and published as
individual publications under EPA cover.
SPS and the EPA discussed several areas of potential testing and
developed a list of tasks which reflected the interest of both parties.
These study areas include:
Filtration parameters - Average K_C and AP daily so
that these parameters may be studied versus air-to-cloth ratio.
Particulate Tests - Inlet/outlet grain loading and particulate size
distribution to determine fabric efficiency, size distribution, and
comparison of grain loadings to main unit.
Modeling Parameter - Find K_, AP_ as a function of V by varying
Z Ci
air-to-cloth ratio, to be used in developing a filtration empirical
model.
Extended Filtration Time - To determine the effect of extending fil-
tration time on system pressure drop (clean better but less often).
Scale-up - Develop scale-up equations from a pilot unit to a multi-
compartment fabric filter system.
Reverse Air Cleaning - Information will also be used to develop model-
ing parameters for reverse air cleaning.
Fabric Testing - Evaluate fabrics not already under test and exotic
fabrics too expensive for full-compartment assessment.
Future Studies - Investigate potential of extending studies to higher
grain loadings and utilizing other fly ashes because of variability of
coal supplies to other utilities.
Start-up Plan
Prior to the actual start-up, a preliminary equipment checkout was
conducted. The 480-V control panel was energized with all valves from flue
57
-------
gas sources closed. Hopper heaters were set at 132°C (270°F) and allowed
to heat for 16 hours prior to start-up. With all controllers in the closed
position, the outside dampers were manually opened and the following steps
taken.
1. Open damper from preheater.
2. Open flue gas inlet damper.
3. Open return damper.
4. Open inlet damper to main fan.
5. Open bypass butterfly valve.
6. Close bypass valve.
7. Open outlet dampers.
8. Start main fan.
9. Activate east and west flow controllers.
10. Activate temperature controller.
11. Start deflation fan.
12. Set cleaning cycle to automatic.
The inlet temperature was controlled automatically at 232°C (450°F)
for the first 24 hours, at which time it was decreased to 204°C (400°F).
The inlet flow was 935 m /h (550 acfm) for 2 hours at an air-to-cloth ratio
of 1. At the end of the 2-hour period, the air-to-cloth ratio was in-
creased, but not to exceed 2 for the first 24 hours.
The cleaning cycle was not to be activated until theAP reached 13 cm
(5 in.) w.g. At 13 cm (5 in.) w.g., the cleaning cycle was set up on a
20-sec shake. Each compartment was to clean on a 33-min cycle, 25-sec
settle, and 35-sec final settle. Ash was to be manually removed once a
day, one hopper at a time, until automatic mode modifications could be
made.
OPERATION
The pilot baghouse was first placed in service on October 23, 1979;
however, painting, insulating, guard rail installation, and construction
cleanup continued for about 8 weeks. On initial start-up, excessive con-
densation was present. Therefore, to prevent moisture condensation on
future start-ups, the manual inlet from the air preheater was opened to
allow compartments to heat.
Prior to the annual Harrington Unit 2 outage, numerous problems were
encountered. Immediate problems after start-up were (a) high temperature
loss; (b) constant tripping of the variable speed shaker motor; (c) improp-
er wiring; (d) inadequate bracing of the inner compartment wall; and (e)
in-leakage. These problems were minor and were resolved shortly after
start-up.
The biggest problem, in-leakage, was believed to be due to warped
doors, all four of which it was decided to replace. By the end of November
1979 the new doors had arrived; installation was completed in 4 days.
Ultrasonic leak testing was performed by SPS and WFI engineers and small
leaks in the back wall were repaired.
Modifications and Problems
During the 1979 fall outage (October 26-December 10) the following
pilot modifications were made for ease of operation:
58
-------
1. Made ash system automatic with main unit.
2. Installed compartment isolation switches.
3. Relocated cleaning cycle timers to enclosure on back of 480-V
breaker for ease of access.
After the annual outage, the pilot was again started up on January 3,
1980. Start-up followed the standard procedure established for the initial
start-up with 1700 m /m (1000 acfm), 232°C (450°F) on inlet, and automatic
cleaning at 33-min intervals. After 4 days, the air-to-cloth ratio was
decreased to 2 to see if lower flow would increase the temperature loss.
The temperature loss, however, held stable at 43°-46°C (110°-115°F).
During the following few days the instrument shop confirmed that both inlet
and outlet thermocouples were performing accurately.
By mid-January, it was obvious that the temperature loss due to possi-
ble air leakage was going to have to be dealt with. Faulty outlet damper
packings were found to be pulling air in directly on top of the outlet
thermocouple. Both sides were packed and have worked well ever since.
The temperature loss problem still had not been completely solved,
however. A new temperature tap was put in the outlet plenum to see if the
outlet thermocouple and the one in the plenum read the same; they did. At
this time temperature probes were placed in manometer taps at the cell
plate level to see if temperature loss was uniform throughout each com-
partment: some evidence pointed to ash hopper or below the cell plate
in-leakage. A check of the in-leakage ash through the hopper dump gates
failed to reveal significant in-leakage.
In February, SPS began a series of four 0_ tests, performed by the SPS
environmental test group. These tests still pointed to high in-leakage
with the temperature loss at a 3.4 air-to-cloth ratio at 17°C (63°F).
Eventually, a faulty fabrication weld in the west ash hopper was discover-
ed. The joint had been tack-welded and never seal-welded. This was prompt-
ly repaired.
In April 1980, leaks were repaired in the roof at the wall joint in
both the east and west compartments. In addition, leakage was repaired
below the cell plate on the east side. By mid-April, CL testing had shown
that in-leakage was at an acceptable level. The pilot unit was scheduled
for official start-up, and the doors were sealed with silicone to prevent
future in-leakage.
By May 5, 1980, the pilot unit was on line and was operating satis-
factorily. The most recent in-leakage test proved acceptable: it showed a
temperature loss of only 8°C (47°F) at an air-to-cloth ratio of 3.4.
Special Testing
For 4 days in mid-July 1980, the SPS environmental test group tested
the pilot baghouse to measure particulate and gaseous emissions. Test
procedures were those specified for EPA Methods 1, 2, 3, 4, and 5 (40 CFR
60, Appendix A). Results are shown in Table 2.
This limited amount of testing indicates that flue gas constituents
for the pilot baghouse are equivalent to those of the main baghouse. Fog
instance, an average inlet particulate concentration of 4.6 (±0.7) g/m3
(2.0 (±0.3) gr/scf) for the pilot baghouse is comparable to 3.9 (±0.9) g/m
(1.7 (±0.4) gr/scf) for the main baghouse. (Note that averages for the
pilot baghouse are based on three samples, and averages for the main bag-
house are based on 21 samples.) On the pilot baghouse outlet side, an
59
-------
TABLE 2. PARTICULATE TEST SUMMARY
Orsat Gas
Analysis, %
CO,
CO"
°2
N2
Particulate
Results
% Moisture
Wet Stack Gas
Mol. Wt.
Particulate Con-,
centration g/m~
Run 1
13.7
0.0
5.7
80.6
11.3
29.0
5.5
K2.4)
Inlet
Run 2
13.9
0.0
5.5
80.7
11.3
29.0
4.4
(1.9)
Run 3
14.8
0.0
5.3
80.5
11.1
29.1
3.9
(1.7)
Run 1
13.0
0.0
6.5
80.5
11.1
29.0
0.0
Outlet
Run 2
13.4
0.0
6.1
80.5
11.0
29.0
0.0
Run 3
13.4
0.0
6.0
80.6
10.5
29.1
0.0
Isokinetic
Variation, %
97.9
96.7
96.9
101.8
101.7
99.5
-------
average particulate emission rate of 1.3(±0.4)ng/J(0.003 (±0.001) lb/106
Btu) compares gto an average particulate emission rate of 13(±4)ng/J (0.03
(±0.01) lb/10 Btu) for the main baghouse (pilot baghouse averages are
based on 3 samples and main baghouse averages based on 10 samples).
Similar tests in the future will determine the value of using pilot
flue gas testing results as scale-up factors to the main baghouse to com-
pare fabric efficiency, size distribution, and grain loadings. Factors
such as the length of time bags have been in service and pressure drop,
however, may also be needed before drawing conclusions about baghouse
performance based on data from the pilot unit.
Data Collection
Official data on the Criswell 442-57DC2 bags was collected from May
through September 1980. This data was collected at air-to-cloth ratios of
1.94, 2.20, 2.50, 2.80, and 3.40. (See Table 3, Data Collection Averages.)
Each average in this table represents 48 cleaning cycles with 96 data
points each.
To determine the specific resistance coefficient (K_), the pressure
drop across the bags at both the beginning (&Pp) and the end (j^PT) of the
cleaning cycle must be known. The pressure drop is recorded on a paper
chart recorder. To obtain numbers for calculating AP_ and K«, the strip
chart must be unrolled, the cleaning cycles measured^ and a line drawn
through the linear part of the graph. AP_, is measured by an extrapolation
of this line. Values for APE andAP are then used to calculate an average
value for K-C with the aid of a programmable calculator.
The cleaning cycle is about 30 minutes long, and there are two com-
partments to be calculated. This means that for every 15 minutes, this
process must be performed. It takes approximately 60 man-hours per month
to obtain values for K~ and APp by this method (2,880 cleaning cycles per
30-day period, 5,760 points to read AP_ andAP for each 30-day cleaning
cycle, 28,800 points per data chart).
Reverse Air Experiments
During the Unit 2 1980 fall outage (October 3-November 8), plans were
made for a reverse air experiment on the west side of the pilot baghouse.
By October 23, 1980, the Criswell 442-57DC2 bags had been removed. The
east compartment was fitted with six used Nomex bags' and the west with six
new ringed Kennecott 0.28 g (10 oz) bags. In order to avoid ash carryover
from one compartment to the other during cleaning, the Nomex compartment
employed shake cleaning only, while the test compartment used reverse air
cleaning.
For the reverse air experiment, the cleaning cycle timers were set as
follows:
First settle - 30 sec.
Reverse air - 45 sec.
Second settle - 30 sec.
45-min intervals for 3 days.
60-min intervals for duration of test.
The reverse air start-up followed standard procedure (204 C (400 F)
inlet temperature, air-to-cloth ratio 2.0). Performance of the pilot unit
during the reverse air mode was satisfactory with only minor mechanical
61
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TABLE 3. DATA COLLECTION AVERAGES
N5
Air-to-Cloth
Ratio May_ June
APE APT APE APT
1.94
2.20
2.50
2.80 4.62 5.39
3.40 5.93 7.34 5.12 6.45
Air-to-Cloth
Ratio May June
AP£ APT APE APT
1.94
2.20
2.50
2.80 4.44 5.12
3.40 4.93 6.41 4.74 5.89
Criswell 442-57DC2 Fabric
Inlet Temperature 204°C (400°F)
Cleaning Interval - 33 minutes, 20-second shake
Each average represents 48 cleaning cycles, 96
EAST
July
APE
2.65
3.36
4.67
6.17
WEST
July
APE
3.14
3.59
4.65
5.74
data points.
~A JTr«
2.97
3.68
5.39
7.64
APT
3.43
4.03
5.39
7.12
K,
August September
APE APT AP£ AP.
3.09 3.44 3.60 3
4 . 66 5
6.93 8.25 6.38 7
August September
APE APT AP£ AP,
3.32 3.64 2.98 3
4.03 4
6.20 7.36 6.40 7
= 15-28 in. w.fc. min. ft.
r
.89
.13
.62
T
.29
.46
.61
AP_ ia pressure drop across bags at beginning of cleaning cycle.
AP_ is pressure drop across bags at end of cleaning cycle.
Averages reported in in. w.g. (1 cm = 0.39 in.).
Ib
m
-------
problems, if the Nomex fabric was cleaned every hour without deflation.
The Kennecott fabric averaged a residualAP of 16.0 cm (6.31 in. w.g.); a
terminal AP of 19.8 cm (7.78 in. w.g.) for a 60-min cleaning cycle at an
air-to-cloth ratio of 2.3; and an inlet temperature of 204°C (400°F),
during the 14-day test.
On December 31, 1980, the pilot was taken off line for installation of
the second test fabric (Fabric Filters 502-1 Acid Flex). To obtain faster
break-in, cleaning was set at automatic 30-min. intervals regardless of
pressure drop. Following the break-in period, the cleaning cycle was set
to 60-min and the test extended to 3 weeks.
The SPS reverse air testing program will run through July 1981. SPS
plans to run 11 tests during this time, involving various fabric finishes
and numbers of rings in the bag. Table 4 shows the fabrics to be evalu-
ated.
TABLE 4. PILOT BAGHOUSE TEST FABRIC SELECTION
Bag Type Bag Weight g (oz) Supplier
1. Kennecott 0.28 (10) Kennecott
2. 502-1 Acid Flex 0.40 (H) Fabric Filters
(Burlington 1625)
3. Q78-877 0.40 (14) Globe-Albany
4. 449-COF(Heat
cleaned) 0.40 (14) W. W. Criswell
5. Fabric Filters 504-1
Tex Flex 0.28 (10) W. W. Criswell
6. Fabric Filters 504-1
Tuff Coat 0.28 (10) W. W. Criswell
7. 502-1 Tuff Coat
(Burlington) 0.40 (14) Fabric Filters
8. 601 Tuflex 0.28 (10) Menardi Southern
9- Best candidate from previous with seven rings.
10. Same as Item 9, except five rings.
11. Same as Item 9, except nine rings, if time allows.
ENDNOTES
1. Chambers, R. C., Ladd, K. L. , and Kunka, S. L. , "SPS Experience With
Fabric Filtration" presented at 5th International Fabric Filter Forum,
Phoenix, Arizona, January 1981.
2. Dennis, R. C., Cass, R. W. , Cooper, D. W. , Hall, and Stern, R. W. GCA
Corporation. "Filtration Model for Coal Fly Ash With Glass Fabrics,
"EPA-600/7-77-084 (NTIS No. PB 276 489), August 1977.
63
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ON
-C-
AIR
GAS DUCT
TO AIR
PREHEATEF
370° G
(70O
-------
REVIEW OF SPS INVESTIGATION OF
HARRINGTON STATION UNIT 2 FABRIC FILTER SYSTEM
By: Kenneth Ladd and Sherry Kunka
Southwestern Public Service Company
P. 0. Box 1261
Amarillo, Texas 79170
ABSTRACT
For the last three years, Southwestern Public Service Company (SPS)
has tested and monitored the Harrington Station Unit 2 fabric filter system
in an effort to characterize its operation. This fabric filter system was
one of the first large air quality control devices to be used in associa-
tion with a low-sulfur, coal-fired boiler. Start-up, monitoring, testing,
and operation of the fabric filter system since an EPA-sponsored program
began in October, 1977 are discussed in this paper.
In addition, the operation and maintenance problems encountered during
the first year of the program are reviewed. A summary of the extensive
flue gas and particulate sampling performed by SPS and its subcontractor,
GCA Corporation, is presented. A brief discussion of fabric problems
encountered during the last three years is also included.
INTRODUCTION
Background
Southwestern Public Service Company (SPS), an electric utility with
headquarters in Amarillo, Texas, undertook a project with the Environmental
Protection Agency (EPA) in 1977 to assess a new application for fabric
filter technology. The site selected for the study was Harrington Station,
SPS's first coal-fired generating plant. In 1978 when it was put on-line,
Harrington's Unit No. 2 was the first new utility boiler to be equipped
with a fabric filter system (baghouse) for particulate control. Its effec-
tiveness in controlling emissions from a coal-fired boiler was of special
interest to the EPA in its effort to set standards for air quality control
equipment.
The objectives established for the study are described below. These
objectives were formulated to help utilities and other coal-burning facil-
ities obtain information for evaluating air control alternatives. SPS
especially hopes to aid those burning low-sulfur Western coal.
1. Determine operating and maintenance costs of an operating fabric
filter system over an extended period of time in order to give
other utilities data on the economic feasibility of baghouses.
2. Characterize performance of the unit by looking at specific
parameters which include (a) an assessment of the effect of
pressure drop versus time; (b) removal efficiency to be deter-
mined by special manual testing; (c) continuous opacity monitor-
65
-------
ing; (d) an investigation of the long-term reliability of the
systems; and (e) determination of optimum operating conditions.
3. Bag performance and bag life will be studied by (a) periodic
removal of test bags for analysis by an independent consultant;
(b) analysis of pressure drop and thru-put on an individual
compartment basis; and (c) other special fabric studies.
The study is now in its fourth year. The remaining project time will
be spent analyzing continuous monitoring data and assessing the cost of
operation and maintenance activities. Project activities during the first
two years of study are discussed in the first and second annual reports (1,
2).
Facility Description
Harrington Station, SPS's first coal-fired plant, is located approxi-
mately five miles northeast of Amarillo, Texas. The first 360 MW unit went
into operation July, 1976; a second 360 MW unit went on-line in 1978 and
Unit 3 was brought into service in 1980.
The particulate emission control device selected for Unit 2 is a
Wheelabrator-Frye, Inc. (WFI) fabric filter baghouse system. It is de-
signed to operate at a flue gas flow of 1,650,000 acfm at 313°F. Minimum
design efficiency is 98.6 percent, which would permit 0.1 pounds of partic-
ulate per million BTU out the stack. The exterior of the baghouse has 3.5
inches of fiberglass insulation; there is no insulation between plenums and
compartments. Other design parameters are summarized in Table I.
Table I. Fabric Filter System Design Parameters
Compartments 28
Bags per compartment 204
Bag diameter 11.5 inches
Bag length 30.5 feet
Bag spacing, center to center 14.0 inches
Air-to-cloth ratio, gross 3.16:1
Air-to-cloth ratio, net 3.40:1
Bag reach 2
Start-up
SPS formulated a start-up procedure for the Harrington Station fabric
filter system after reviewing start-up experience of other utilities and
after consulting individuals known to have experience in start-up of these
systems. A set of guidelines was written for the actual start-up and these
were described in an earlier paper devoted to start-up of the fabric filter
system (3).
No start-up difficulties were experienced. Problems associated with
the acid dewpoint had been of concern but neither bag blinding or corrosion
due to excessive moisture was noted. As the final bypass damper closed,
the opacity dropped to one percent. This drop was signalled dramatically
by the perfectly clear stack.
Approximately three weeks after the baghouse was initially started,
SPS was able to operate Unit 2 at full load with only coal in service.
Since that time, the unit has operated at loads consistently above 200 MW
and during peak periods, loads over 350 MW.
66
-------
OPERATING AND MAINTENANCE EXPERIENCE
The fabric filter system has functioned very well in terms of overall
performance. Consistent stack opacity monitor readings in the two to four
percent range indicate that the baghouse is doing an excellent clean-up
job. Specific areas of operating and maintenance experience are discussed
below. Despite the favorable removal efficiency of flyash from the low
sulfur coal, some major concerns arose. The first major difficulites to be
encountered were high pressure drop and problems with cleaning the bags.
Later, fabric wear and performance resulted in a need for evaluation and
change.
Deflation
Unit 2 started up and operated until mid-June, 1979 without good
deflation control. WFI recommended that deflation levels be at 0.5 inches
w=g., which was very difficult to maintain manually.
During this period of time, SPS ran tests to see if bags were pancak-
ing at 0.5 inches w.g. deflation pressure; results indicated that severe
pancaking was taking place. By watching bags through observation ports in
one compartment and lowering the deflation pressure drop until pancaking no
longer occurred, 0-1 to 0.2 of an inch w.g. deflation pressure was deter-
mined to be an appropriate level to prevent pancaking. A control unit was
installed by WFI, which gave operators the capability to change deflation
from the control room, enhancing the ability to keep deflation at indicated
levels.
Shake
Correction of deflation improved baghouse operation, but it was ap-
parent that the bags still were not being adequately cleaned. More force
was needed to release the fly ash from the cloth. To determine the effect
of increased acceleration on cleaning, SPS decided to run some experimental
tests with 6-inch shaker pulleys in place of the 4-inch diameter pulleys
originally provided. Modifying the amplitude of the shaker to increase
fabric acceleration during cleaning would have required fabrication of a
special eccentric. Shaker frequency, however, is modified simply by chang-
ing the main drive pulley on the shaker motor shaft.
Table II shows a summary of compartment pressure drop comparisons for
the 4-inch and 6-inch diameter shaker pulleys.
Table II. Summary of Compartment AP Comparisons*
AP Difference Compartment AP Difference
A
17
21
23
9
8
8
.75
.30
.65
0.
1.
1.
45
45
10
(4-inch)
(4-inch)
(4-inch)
17
21
23
10
7
7
.37
.13
.33
0
3
3
.00
.24
.04
(4-inch)
(6-inch)
(6-inch)
* @ 350 raw.
It appeared that the increased shake doubled or tripled the decrease
in pressure drop. As a result of the experimental work with 6-inch dia-
meter shaker pulleys, SPS decided to replace all of the 4-inch pulleys with
5.3-inch pulleys in all of the compartments.
67
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Adjustment to Shaker Mechanisms
Increased shaker frequency (or rapidity) resulted in problems with the
existing pillow blocks. The original structure simply had pillow blocks
bolted to a plate without any lateral support and at least a quarter inch
of shims was used to properly locate the pillow block. After installation
of 6-inch pulleys on shaker mechanisms, the higher forces caused movement
of pillow blocks and eventual damage to shaker mechanisms.
To remedy the situation, SPS designed a modified pillow block support
structure. Since this modification was made in August, 1979, no failures
have been observed due to loose pillow blocks.
Ventilation System
The design of the Harrington Station Unit 2 baghouse leaves the inside
compartment and duct walls uninsulated in order to allow the compartments
to be heated while the flue gas bypasses the baghouse. This design pre-
vents condensation and corrosion problems during start-up. Single compart-
ment inspections can be performed with the unit on-line as long as it is
not necessary for personnel to enter the upper level of the compartment.
Since replacement of blown-out bags requires upper level entry where tem-
peratures may be greater than 130°, a ventilation system became necessary
for each compartment.
The ventilation system consists of a single fan on the ground level in
duct work to and dampers for each of the 28 compartments. The system has
been sized to deliver approximately 60,000 cfm of ambient air for compart-
ment cooling.
Replacement of Hopper Heaters
Original hopper heaters in the Unit 2 fabric filter system were made
of silicone rubber and applied with an adhesive to the hopper shell.
During the first year of operation, 99 percent of these heaters failed.
Beginning in September of 1979, they were replaced with a modular box-style
heater.
Safety Features
During a Unit 2 outage in April of 1979, safety devices to prevent
doors from blowing shut were placed on all compartment doors. Seals on all
compartment doors were also replaced on an as-needed basis during the
second year of study.
Opacity
The Lear-Siegler RM41 Opacity Monitor appears to be a very stable and
dependable instrument. To date, the device has needed very few adjustments
and has had few maintenance problems. The sensitivity of the unit has
allowed it to be used in precise pinpointing of failed bags by compartment.
In order to get good measurements, however, it is necessary to do a
clear stack alignment while the unit is off-line, i.e., during an overhaul.
All attempts to do bench alignments have proven unsatisfactory.
68
-------
SPECIAL TESTING
One of the major aims of the SPS/EPA study is to determine mass emis-
sions of particulate, sulfur dioxides, and oxides of nitrogen. The con-
tract specifies a special testing program designed to measure these para-
meters and relate results to the collection efficiency of the fabric filter
system.
The original work plan called for three special tests by SPS. How-
ever, the test dates were altered in order to allow for changes in oper-
ating conditions and budget limitations. Tests were conducted by SPS at
the following times :
Phase I December 11-15, 1978
Phase II October 8-12, 1979
Phase III March 24-25, 1980
Phase IV August 26-29, 1980
The contract also called for three more specialized tests to be con-
ducted by an outside consultant under subcontract to SPS. GCA Corporation
was selected to sample at five locations for the following parameters:
particulate, C.,-C17 organic compounds, C.-C.., organic compounds, C09, 0~,
CO, SO,, S03, N0x, and particulate particle size distribution.
GCA conducted their first test in February, 1979. Results from this
test led SPS and the EPA to a decision to reduce this phase of the test
program to only one more test to be performed as a joint test by SPS and
GCA. Furthermore, the test parameters were reduced to the following:
particle size distribution by three methods and SO,, and SO., sampling at
three locations. This series of tests was performed in June, i980.
Procedures followed during periods of special testing were implemented
by the Environmental Group of the SPS System Laboratory for collection,
recovery, and analysis of samples for the determination of particulate,
S02, NO , and combustion gases (0^ and C02) in flue gases during special
test periods. In addition, the System Lao fabricated certain pieces of
test equipment, followed standard calibrating procedures, and employed
quality control/quality assurance policies. The procedures for the special
test program were basically the same as EPA Methods 3, 5, 6, and 7 as
outlined in the Federal Register (40 CFR 60, Appendix A).
A crew of test personnel from SPS power plants was assembled at Har-
rington Station for the first week-long series of tests. Approximately 26
stack sampling team members participated in the test program. By taking
samples at three locations rather than five, experienced personnel were
able to work with the less experienced ones. As a result, personnel at all
the sampling locations had some degree of experience during the remaining
tests.
Analyses of the special testing program are shown in Tables III, IV,
V, and VI. The fundamental conclusion to be drawn from these results
confirms that Unit 2 can operate in compliance with the New Source Perform-
ance Standard of 0.1 lb/10 BTU. The special testing phase of the SPS/EPA
study has been completed. A report which deals exclusively with this
portion of the program is currently being prepared.
FABRIC EXPERIENCE
The Unit 2 fabric filter system was originally equipped with bags of
fiberglass with a silicone/graphite coating (W.W. Criswell Style 445-04).
69
-------
Table III. Results of Particulate Testing
Phase I, December 1978
Run
No.
I17
\H
&
East
Inlet
gr/scf
2.28
2.04
1.67
West
Inlet
gr/scf
2.74
2.56
1.63
V
Theoretical-' ,,
Inlet Stack-' .
gr/scf gr/scf lb/10°Btu
2.09
1.96
2.27
.053
.050
.033
.106
.097
.061
Phase II, October 1979
Run
No.
1
2
3
East
Inlet
gr/scf
1.40
1.61
1.61
West
Inlet
gr/scf
1.41
1>415/
.74^
Theo- ,,
retical-
gr/scf
1.79
1.78
1.81
East
Outlet
gr/scf
.014
.014
.027
West
Outlet
gr/scf
.012
.024
.028
Stack ,
gr/scf lb/10°Btu
.010
.011
.011
.020
.021
.022
Phase III, March 1980
1
2
3
4
11
1.43
.97-,,
y
y
1.35
1.14
1.50
1.82
1.95
2.03
2.04
.008
.013
.011
.007
.012
.038
.022
.020
.009
.013
.018
.024
.017^
.024
•032
.044^
Phase IV, August 1980
Run
No.
1
2
3
East
Inlet
gr/scf
1.64
1.88
1.35
West
Inlet
gr/scf
1.92
1.67
1.44
Theoretical—' Stack ,
gr/scf gr/scf lb/10bBtu
1.99
1.97
1.96
.016
.009
.015
.031
.018
.030
NOTES
I/ Sootblowing continuously.
2/ Not sootblowing.
3/ Assumes 70% fly ash, no consideration for sootblowing.
4/ The concentrations of particulate obtained from the stack are biased
high because of a reaction that took place in the unheated Inconel
probe liner.
5/ One sample of probe wash was lost during transport from sample site
to the lab, causing this sample to be biased low.
6/ Based upon stoichiometric flue gas flow, coal flow, ash analysis;
70% fly ash generation; and 30% bottom ash and economizer ash gen-
eration.
y No data.
8/ Not isokinetic.
9/ Failed leak check test.
1_0/ Based upon stoichiometric flue gas flow, coal flow, coal analysis
and 70% fly ash generation (dry basis).
70
-------
Phase I, December 1978-'
Table IV. Results of NO Testing
I/
Run
No.
1
2
3
East
Inlet
Method 7
.68
.71
.67
East
Outlet
Method 7
.64
.68
.71
West
Inlet
Method 7
.61
.59
.62
West
Outlet
Method 7
.62
.62
.62
Stack
Method 7
.63
.66
.64
NOTES
I/ NO sampling was only performed one time.
X
71
-------
Table V. Results of SO,, Testing
Phase I, December 1978
Run
No.
1
2
3
I/
East
Inlet
Method 6-lb/106Btu
.53
.59
.62
I/
West
Inlet
Method 6-lb/106Btu
.36
.32
.20
21
Stoichiometric-
lb/106Btu Method
.76
.84
.88
Stack
6-lb/10
.73
.78
.84
6Btu
Phase II, October 1979
East
Run Inlet
No. lb/106Btu
1 .71
2 .74
3 .66
West
Inlet
lb/106Btu
•743/
•79I/
.71-'
Theoretical
lb/106Btu
1.03
.95
.99
East
Outlet
lb/106Btu
.70
•793/
.78^
West
Outlet
lb/106 Btu
.76
.82
.78
Stack
lb/106Btu
.79
1.01
.96
Phase III, March 1980
Run
No.
1
2
3
East
Inlet
lb/106Btu
•764/
H
West
Inlet
lb/106Btu
.69
.67
.67
Theoretical
lb/106Btu
.82
.81
.81
East
Outlet
lb/106Btu
.70
.69
.65
West
Outlet
lb/106Btu
.684/
\l
Stack
lb/106Btu
.81
.75
.78
Phase IV, August 1980
Run
No.
1
2
3
East
Inlet
lb/106Btu
.84
.89
.88
West
Inlet
lb/106Btu
.76
.79
.78
Theoretical
lb/106Btu
.96
.94
.96
Stack
lb/106Btu
.84
.95
.83
NOTES
\j These concentrations are suspected of being low because of the high
negative pressure pulling the absorbing solutions forward, thereby
resulting in the absorbed SO,, not being analyzed.
2/ Assumes all sulfur is converted to S0?.
3/ Based upon only one of two samples for this run.
4/ No data.
72
-------
Tabla VI. CCA Tsst Results
Test
I, February
1979
Results Of
Run
Ho.
1
2
3
4
5
6
Run
Ho.
2
4
6
Run
Ho.
2
4
6
Run
Ko.
2
4
6
East
Inlet
gr/scf
1.03
0.99
1.34
2.21
1.53
1.36
East
Inlet
lb/106Btu
I/
0.98
1.10
East
Inlet
lb/106Btu
11
0.35
0.63
East
Inlet
ppm
0.27
2.07
2.56
West
Inlet
f?r/scf
1.57
1 68
1.20
1.36
1.02
2.36
Results
Wast
Inlet
Particulate Testing SPA Method 5
East West
Outlet Outlet Stack
gr/scf gr/scf gr/'sicf
C.011 0.019 -f
0.007 0.005 0.009
0.004 0.008 0.003
0.004 0.007 0.012
0.001 0.002 O.OC4
0.005 0.042 0.017
of S02 Testing EPA Method 6
East West
Outlet Outlet
lb/106Btu lb/106Btu lb/106Btu
0.91
0.80
0.61
Results
West
Inlet
0.82 0-95,,
0.94 -'
0.68 0.64
of NO^ Testing EPA Method 7
East West
Outlet Outlet
lb/106Btu lb/10°Btu lb/1068tu
11
0.50
0.53
Results
West
Inlet
ppm
0.79
0.67
1.96
0.60 0.69
0.52 0.53
0.48 0.55
of SO, Testing EPA Method 8
East West
Outlet Outlet Stack
ppm ppm ppm
0.9S 0.72 1-10,/
0.60 0.82 -'
1.81 1.67 1.86
Stack,
lb/10°Btu
i/
0.018
0.016
0.024
0.007
0.034
Stack
lb/106Btsi
1.10.,
'LI
0.74
Stack
lb/106Btu
\l
0.51
0.47
Test II, June 1980
SPS/GCA Joint Test
Results of Particulate Testing
East
Run Inlet
So. gr/scf
i y
2 1.92
3 1.38
West
Inlet
gr/scf
2.30
1.86
1.31
Taeo- -'
retical
gr/scf
2.13
2.22
1.95
r
Stack £
gr/scf
.015
.019
.008
Ib/lOTBtu
.032
.037
.016
NOTES
I/ No data this run.
2/ Invalid test.
Til Based upon stoichiometric flue gas fiow, coal flow, coal analysis
and 70% fly ash eenerstion (dry basis).
73
-------
After the first several months of operation it became apparent that the
original choice of fabric was not going to give the expected pressure drop.
Several more months of operation also made it clear that the fabric was
experiencing a high rate of failure.
Failure of the original fabric made it necessary for SPS to engage in
a fabric filter evaluation program in order to find a fabric that would
give satisfactory performance. A review of SPS's experience with fabric
testing and analysis of those tests is presented in a recent paper titled
"SPS Experience With Fabric Filtration" (4).
In May, 1979, a decision was made to rebag the entire fabric filter
system before the summer peak load. Not only was the pressure drop exces-
sive, but the original bags had a high failure rate, and SPS could not risk
curtailing generating capacity because of the fabric filter system.
The Unit 2 baghouse was rebagged in June, 1979. Most of the com-
partments were equipped with fiberglass bags finished with 10 percent
Teflon (Criswell Style 442-57DC2) or another fiberglass bag coated with a
proprietary finish (Criswell Style 449-1580). Selection of these cloths
was based on an evaluation of test compartments 21 and 23 which were rebag-
ged in January, 1979. SPS will continue to document its experience with
fabric wear and share its findings with other utilities interested in
filtration technology.
CONCLUSION
Despite first-time difficulties with operating and maintenance param-
eters, SPS is pleased with the overall performance of the Harrington Unit 2
baghouse. The EPA study allowed SPS to focus on problems unique to a
baghouse as large and with as many variables as the one at Harrington.
Results of the special testing program are significant in that they
show Unit 2 can operate consistently with the old New Source Performance
Standard (0.1 Ib /10 BTU). However, the applicability of using SPS's
experience to set industry-wide standards may be questioned because of
SPS's unique fabric experience, cleaning problems, and less than ideal
sampling locations. Improvements in particulate removal will need, to be
made before Harrington's Unit 2 can consistently meet .03 lb/10 BTU,
although the data SPS collected during its special testing program is
consistent with Unit 2 operating conditions.
SPS will continue to monitor the performance of the baghouse for the
next four years. In addition, an economic analysis will be performed and
fabric studies will be conducted in an EPA-sponsored pilot baghouse on
stream with Harrington Unit 2.
ENDNOTES
1. Ladd, K.L., Faulkner, G.R., and Kunka, S.L., "Fabric Filter System
Study: First Annual Report," EPA-600/7-79-183, August 1979-
2. Ladd, K.L., Chambers, R.L., Plunk, O.C., and Kunka, S.L., "Fabric
Filter System Study: Second Annual Report," Contract No. 68-02-2659,
August 1980-
3. Faulkner, G.R., Ladd, K.L., "Start-up, Operation and Performance
Testing of Fabric Filter System," 3rd International Fabric Filter
Forum, Phoenix, Arizona, September 1978.
4. Chambers, R.L., Ladd, K.L., and Kunka, S.L., "SPS Experience With
Fabric Filtration," 5th International Fabric Filter Forum, Phoenix,
Arizona, January 1981. -
-------
A SUMMARY OF PERFORMANCE TESTING
OF THE APITRON ELECTROSTATICALLY
AUGMENTED FABRIC FILTER
by
Dennis Helfritch
and
Lotar Kirsten
AIR QUALITY DIVISION
AMERICAN PRECISION INDUSTRIES, INC.
Charlotte, N.C.
ABSTRACT
The Apitron filter is currently the only commercially operating
electrostatically augmented fabric filter. As such it has been
intensively investigated by industry and governmental agencies
over the past several years. A large amount of performance
data has been gathered. Following a brief description of
Apitron operating principles, this data is reviewed and
compared for consistency. General conclusions are drawn from
the data regarding performance improvements which result from
electrostatic augmentation.
INTRODUCTION
In recent years a considerable amount of interest has been
directed toward augmenting the filtration process by means of
electrostatics. Many researchers have been actively engaged in
theoretical and experimental aspects of electrostatic
filtration, and funding for this type of research has
increased dramatically. A recent symposium sponsored by the
Fiber Society and the Filtration Society was devoted entirely
to this subject.1 The reasons for the interest in
electrostatic filtration center about the potential of these
techniques to reduce energy usage and increase submicron
collection efficiency.
There are several techniques by which electrostatics can be
applied to filtration. Particles or fibers or both may be
charged, and electrostatic fields may be applied in different
ways. These techniques and several combinations have all been
attempted, and the interesting result is that they all improve
filtration performance.
75
-------
Almost all of the work, both theoretical and experimental, that
has been done in this field so far has been in the area of
basic studies. Modeling work, involving single and multiple
fibers and particle trajectories under the influence of various
electrostatic forces has been done. Experimental work,
primarily with bench scale equipment and patch filters, has been
done. While this work is essential and provides a base for
future work, the results so far achieved cannot be used to
predict the performance of commercial units, which are operated
continuously for periods of years, during which the fabric is
frequently pulsed, shaken or flexed. Because of this, the
field testing of commercial and pilot scale Apitron units can
yield valuable information.
Figure 1
The Apitron electrostatically aug-
mented fabric charges particles
prior to their filtration by
conventional bags. As shown in
Figure 1, incoming airflow enters
the charging section in the area
below the walkway. The flow then
passes upward through the tubes,
where the particulate is charged
and where some is deposited. Flow
continues upward past the tubes,
into and through the bags, where
the final filtration of the charged
particulate takes place. Clean
air exits the unit at the
exhaust, located at the top of the
baghousing section.
Periodically, the deposit of particulate is cleaned from the
tube and fabric. Six bags and tubes are cleaned of deposited
dust at one time. This cleaning is initiated when an
electrical pulse from the control box opens one of the twelve
diaphragm valves for 1/10 second. Compressed air jets
downward from six nozzles, each directly above a tube and
concentric to a discharge electrode. The jet of air flowing
downward through the tube entrains and mixes with a secondary
air flow, and the collector tube is swept clean of deposited
dust by the mixture of high velocity air. The secondary
airflow, passing from the outside to the inside of the bag,
snaps the bag inward and dislodges dust deposit from it, see
figure 2.
76
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Figure 2
THREE TEST PROGRAMS
Most test programs involving novel devices seek to document the
efficiency and energy consumption of the device and then
compare these results with those previously obtained for
conventional devices. These comparisons are usually hindered by
the fact that the conditions under which the two devices had
been tested are invariably somewhat different. Completely
valid comparisons can only be made when the two devices are
simultaneously tested under identical conditions. The testing
of electrostatically augmented filtration devices, however,
offers the opportunity to switch off the electrostatics and
hence tests the device as a conventional filter. This type of
test minimizes the problem of comparisons.
We will therefore be primarily concerned with Apitron testing
in which data was taken with and without electrostatic
augmentation, and we will not make a strong attempt to compare
Apitron performance with that of conventional fabric filters.
The Apitron has been tested by several industrial and
governmental agencies, as well as its manufacturer and users.
All testing has included data concerning collection efficiency
and pressure drop, and data has only been recorded after
several hundred hours of continuous operation. We will review
three of these test programs individually.
AMERICAN FOUNDRYMEN'S SOCIETY
The object of this study 2 was to identify control devices
which would be suitable for use in foundry process air
recirculation. The foundry cleaning room was used as a
potential recirculation application, and five conventional
baghouses were efficiency tested as controls. Subsequently,
several pilot scale model devices, including the Apitron, were
then efficiency tested on the cleaning room exhaust of the
General Motors Central Foundry. The inlet particulate was
900mg/m3 of mostly iron oxide and silica, with a mass mean
diameter of 3 microns. Only total particulate efficiency
testing was done, and only for the electrostatically augmented
case. The air cloth ratio was 5 CFM/sq. ft. and bag cleaning
was off line, once every two hours. The results are show in
Table 1.
77
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Table 1
AMERICAN FOUNDRYI«EN'S SOCIETY
Recirculation -Test Results
EPA
Best AFS Baghouse
Worst AFS Baghouse
Average AFS Baghouse (5 tested)
Required for Recirculation
Apitron (OHS #1)
Apitron (OHS #2)
Inlet,
(MG/l-r)
1049
1714
1755
1755
1054
743
Outlet
>
Effici-
ency
66.1
39.7
1.5
.19
.11
96.14
97.75
29.91
99.982
99.985
In 1978 a full scale Apitron module was tested under the EPA
novel devices evaluation program 3. 13 x 10* mg/m3 of
redispersed silica dust with a mass mean diameter of 25 microns
was filtered at a 6 CFM/sq. ft. air cloth ratio. Bag cleaning
was on line, each bag cleaned every eight minutes. Fractional
efficiencies and pressure drops were measured for operation
with and without electrostatic augmentation. A chronological
view of the testing is shown in Figure 3. In this Figure the
application of particle charging is given in terms of corona
power (voltage x current), and obviously no particle charging
occurs at zero corona power. The effect of particle charging
on penetration and pressure drop can be easily seen.
Outlet
Participate
MG/M3
Corona
Power
Watt/CFM
Figure 3
1.5
1.0
.5
1.0
.5 .
Pres sure
Drop
Inches Water
10
6
2 .
I I
'1600
1700
1800
1900
2000
2100
2200
2300
Hours Continuous Operation
Figure 4 shows typical fractional efficiency curves for this
application for filtration with and without electrostatic
augmentation. It can be seen from these curves that, for this
particular test, electrostatics exerts a stronger influence on
larger particles than on smaller particles, there being an
order of magnitude difference in penetration at 4 microns, but
only a factor of two at 0.4 microns.
78
-------
Figure 4
NIOSH
rticle Size, Mic
As with the American Foundrymen's Society, the object of this
study was to identify control devices which would be suitable
for plant air recirculation applications. Several pilot scale
devices, including the Apitron, were efficiency tested on a
weld shop ventilation system. The inlet particulate was 12
mg/m3 of welding fume, with a mass mean diameter of .65
microns. Bag cleaning was off-line, once every eight hours,
and air cloth ratio was 6.5 CFM/sq. ft. The results of
fractional efficiency testing is shown in Figure 5. In this
case, in contrast to the EPA tests, electrostatics has the
largest influence on small diameter particles.
Figure 5
Without
Particle
Charging
With
Particle
Charging
SUMMARY OF RESULTS
At this point we wish to formulate some general statements
concerning electrostatically augmented fabric filtration, based
upon the results of the work reviewed above. There are
several similarities and several differences among the
operational parameters of the three test programs. Gas
temperature for all tests was ambient (about 70 F.). The
filtration fabric for all tests was 11 ounce/sq. yd.
polyester felt, and the corona power level for all tests was
between .23 and .29 watts per CFM. Basic differences from test
to test was of course the amount and the size distribution of
the inlet particulate, and the type of bag cleaning (on vs. off
line).
79
-------
Table 2 summarizes the test parameters and results. The
independent variables of the Table are essentially the inlet
concentration and mass mean particle diameter, and the dependent
variables are penetration and filter drag. Corona power and
air cloth ratio do not vary in a significant way and can be
considered fixed variables. In order to assess the influence
of the independent variables on performance, graphs were
constructed of each dependent variable versus each independent
variable. The only paring which produced a physically
meaningful result was that of % penetration versus mass mean
diameter. This is shown in Figure 6, and the strong effect of
particle size on penetration is obvious. Figure 6
Table 2
SUMMARY OF RESULTS
10"
Test Corona Pwr. Inlet Particle MMD ACR Penetration Drag °
(watt/CFM) (Mg/m3) (micron) (CFM/ft2) (*) ("H20/FPM) •£ -1
.017
.16
.52
.29
0
12.7 x 10*
5.8
5.0
1.3 X 10"'
3.8 x 10
"3
.48
1.72
NIOSH
.23
12.4
0
6.5
.65
6.5
.30
.90
1.54
2.54
10
CONCLUSIONS
.1 1 10 100
Mass Mean Diameter. Microns
Table 3 summarizes the effect on performance caused by the
utilization of electrostatics, where the penetration and filter
drag results are given in terms of the ratio of their values
obtained with and without electrostatics. Considering that
three substantially different applications were used as a
source of inlet particulate, the degree of performance
enhancement brought about by particle precharging is remarkably
consistent. From Table 3 we can say that electrostatic
augmentation by means of particle charging prior to filtration
yields approximately a threefold decrease in penetration and a
twofold decrease in filter drag.
Table 3 EFFECT OF PARTICLE CHARGING
Test
% Penetration
Elect/non-elect
Filter Drag
Elect/non-elect
AFS
EPA
NIOSH
.34
.33
.31
.28
.61
AVE.
.34
.40
80
-------
It should be remembered that these conclusions are derived from
& varied, but extremely small data base. They are meant only
to be qualitative guidelines as to what can be expected from
electrostatic augmentation. In addition, these conclusions can
only apply to the case of charged particles, and the
application of electrostatic fields to filtration cannot be
expected to produce similar results.
On the other hand, the results given are currently the only
available results for long term operation of a commercial scale
electrostatically augmented fabric filter. As such, the
results can provide valuable insights to this emerging
technology.
ENDNOTES
1) E.R. Frederick, "Fibers, Electrostatics, and Filtration: A
Review of New Technology, "JAPCA, Vol. 30, No. 4,pp.426-
431.
2) R. W. Potokar, "Foundry Process Air Recirculation, " JAPCA,
Vol. 29, No. 1, pp 18-21.
3) L. G. Felix and J. D. McCain, "Apitron Electrostatically
Augmented Fabric Filter Evaluation, "EPA-600/7-79-070.
4) M. L. Holcomb and R. C. Scholz, "Recirculation of
Industrial Exhaust Air Pilot Study, "To be published by
NIOSH.
81
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FABRIC FILTER OPERATING EXPERIENCE
FROM SEVERAL MAJOR UTILITY UNITS
By: Owen F. Fortune
Richard L. Miller
Eric A. Samuel
Buell Emission Control Division
Envirotech Corporation
Lebanon, Pennsylvania 17042
ABSTRACT
This paper presents the field operating experience from several major
utility units. Reverse air fabric filter units operating experience from
pulverized coal and cyclone-type boilers are presented. Startup experience
with different firing techniques (coal, oil assist, gas) are presented.
Field pressure drop data are correlated with a model. This field correlation
demonstrates the dependence of pressure drop with the flyash particle size
distribution, grain loading density, cleaning, and air-to-cloth ratio. This
model explains the high pressure drop experienced by some units on lignite
coal. Flyash particle size distributions from various compartments and
within compartment are compared for a few units to investigate the industry's
conjecture of variable particle size to different compartments.
INTRODUCTION
The successful use of baghouses in minimizing particulate emissions from
coal-burning power plants is now in its seventh year. Yet, there is still
debate over what factors are important in successfully designing and operat-
ing a utility baghouse. The purpose of this paper is to recount the opera-
tional factors that Buell has found to be important, and some of the conclu-
sions that we have drawn from our experience and mathematical modeling work.
BAGHOUSE DUST AND GAS DISTRIBUTION
Much has been written (1, 2) about the problems of obtaining an even
distribution amongst both the compartments and the bags of a large baghouse.
This debate has gone on in spite of the fact that, unlike an electrostatic
precipitator, the pressure drop across the cleaning elements is much greater
(2 to 4" W.G.) than the velocity head of the dusty gas (typically 1/2" W.G.
in the inlet manifold). Thus, if initially one area of the compartment has a
thinner filter cake on the bags than in the rest of the compartment, the flow
to that area will increase, and soon cause the filter cake in this area to
increase more rapidly than in the rest of the compartment. Soon, cake thick-
ness and bag gas flows will equalize throughout the compartment.
Our experience has been that obtaining equitable dust/gas distribution
is a relatively easy thing to do, provided that four simple rules are
followed:
82
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A. Tap off from only the bottom - or at lea«t. n« ^ u
manifold, r at ieast near the bottom, of the inlet
B. Taper the inlet manifold so that rou»hlv M,
tained along its length, ^ the Same §as velocity is maln_
C. Be sure that the hydraulic diameter nf ^
small relative to the local h^aulic dia .^f ^ lnlet Valves is
D. Decelerate the gas and dust Sterin* the b T the talet -^"oW, and
leaving it, in easy, gradual stages baShouse> and accelerate the gas
(A) is important because hori^rmi-ai A *
often long enough (several hundred eet to^ermit th/l*
30 microns) dust particles to settle towards til 1 ^ (§reater than
Tapering the floor - rather than th* ^°Wfds *he lower P^rt of the duct.
off the manifold's up£r potion practical^ '
heavier dust particles will be s^pt to tS L
P the most
es w e spt to t '
compartments. P the most d°wnstream baghouse's
500 fn« wM? ^ 8 "f thr°Ugh the ±nlet ValV6S and then to the order of
500 fpm while passing under the bags. Fine-tuning the details of the ho^er
Sudf (3)!S the m°St imP°rtant reaS°n beh±nd doln^ a ba^— geometric model
The importance of Items (C) and (D) is illustrated in Table 1 In an
* • t7bagh handlln§ 38°'°00 ACM' inlet
for a velo • fH ' ' Vaves -re a
for a velocity of about 1000 fpm, while later designs have valve entrance
design velocities between 2000 and 2500 fpm. In the former case, a coarser
dust was found in the filter cake of bags in the upstream compartment indi-
cating a mildly uneven dust distribution along the baghouse length. For the
high-speed inlet valve design, there is no significant difference in filter-
cafce mean particle size between compartments.
TABLE 1 - BAG FILTERCAKE, MEAN PARTICLE SIZE (HICRONS)
380,000 ACFM BAGHOUSE
"LOW VELOCITY INLET VALVES"
UPSTREAM COMPARTMENTS: 10.3
MIDDLE COMPARTMENTS: 7.4
DOWNSTREAM COMPARTMENTS: 7.0
83
600,000 ACFM BAGHOUSE
"HIGH VELOCITY INLET VALVES"
6.0
6.1
5.9
-------
Two other dust distribution questions are whether coarser dust winds up
going up to the bags on one side of a compartment, and finer dust to the bags
at the other end, and whether the filtercake at the bottom of a bag has
coarser dust than the filtercake at the top of the bag. Buell's experience
- as illustrated in Figure 1 - is that with properly designed inlet valves
and hopper flow control devices, there is NO noticeable particle size
gradient in a baghouse compartment (4).
30'
FIGURE I
Compartment Filtercake Mean Particle Size
Dust Distribution (Microns)
Using Stokes Law, the maximum terminal velocity for a falling particle
in a gas flow can easily be computed:
Maximum Drop Velocity (fpm) = 7.2 X 10
6 (dust
(gas density) I size (micronsj
For a 35' long, 12" diameter bag, the bulk velocity through the entrance
thimble is of the order of 250 fpm. Thus, for 30 micron particles, only the
top 6% of the bag would have a noticeably coarser dust. These theoretical
considerations are in agreement with our field experience:
TABLE II - VERTICAL BAG __FI LTERCAKE MEAN PARTICLE SIZE
01.5" 0 x 34 '-8" L. BAG)
SI a (MICRONS!
TOP - 5.2
MIDDLE - 5.2
BOTTOM - 5.0
BAG CLEANING DESIGN PHILOSOPHY
Once the dust and gas have been evenly brought to the thousands of bags
in a baghouse, the next design consideration is how to set up the bag clean-
ing system. Two important factors to consider are:
84
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A. Optimum Number of Compartments
Typically, this is in the range of 8 to 16 compartments per collector,
with the average being 10 to 12. This results because:
1. Having too few compartments causes large pressure drop spikes as a
result of taking too large a percentage of cloth off line during
cleaning, while
2. using too many compartments can cause the time-averaged baghouse
pressure drop to be high due to the fact that it will take hours to
properly clean down all the compartments. This forces you into a
continuous cleaning mode just to maintain reasonable pressure drops
(i.e., if you initiate cleaning at 4.5" W.G., by the time you cleaned
all the compartments, it would be time to start all over again).
Continuous cleaning does not allow any margin of safety in case of
upset conditions, such as a wet coal pile, high ash coal, boiler tube
leaks, etc.
B. Optimum Number of Bags per Compartment
As shown in Figure 2, the current trend in reverse air cleaned bag-
houses is towards more and more bags per compartment. Since it is rela-
tively easy to get good dust and gas distribution in large compartments,
the limiting factors on compartment size become (1) how long it will take
to install a new set of bags, and (2) at what point does the width and
depth of the compartment result in a hopper whose height and weight is
excessive.
5MV-
500-
450-
400-
300-
?50-
FIGURE 2
—I—
WR
1
1Q71
1QBO 19S11 1W
OPriWIONM VFAR
19R3
85
-------
A three-man crew working in a reasonably cool (80 to 100PF) compart-
ment can be expected to handle (remove old bags, install new bags, care-
fully check bag tensioning) about 50 bags in an eight-hour shift. Thus,
a well ventilated compartment with three sets (tubesheet and bag top) of
walkways could contain up to 450 bags and still be rebagged by nine men
in three shifts. Similarly, it is logical to have as many as 600 bags in
a compartment with four sets of doors.
REVERSE AIR CLEANING SYSTEM
How Not to Clean Bags
a. Using excessively high velocity reverse air flows, which can damage the
bags by overflexing them.
b. Using cold reverse air (gas) can cause sub-dewpoint conditions in the
compartment, resulting in an acid rain on the bags. This will result in
high pressure drops and reduced Bag life.
c. Too frequent cleaning of the filter cake exposes the fabric interstices.
This causes "puffing" and opacity excursions due to particle migration
through the fabric.
Proper Design Practice
a. Reverse air capacity at between 1-1/2 to 2 ACFM/FT^ of active cloth per
compartment. Buell typically designs for a ratio of 1,75:1 with at least
a 14" W.G. capacity fan to provide the margin of safety needed in the
cleaning system to handle conditions of high moisture, high dust loadings,
sticky cake, etc., which can occasionally occur.
b. Slowly repressurizing a compartment after cleaning ensures that ,the bags
will not suddenly reinflate ("pop") when the compartment is brought back
into service after cleaning.
c. Maintaining reverse air relief flow at low levels (20% of design) at all
times to keep reverse air ducts hot. This avoids sub-dewpoint reverse air
temperatures and, hence, the spraying of acid rain on the bags during
cleaning.
Cleaning Cycle Philosophy
The two common strategies used in cleaning reverse air baghouses are
"Batch" cleaning and "continuous" cleaning. Batch Cleaning is initiated when
the baghouse flange-to-flange pressure loss reaches a given level. All the
compartments are cleaned in rapid sequence, and then left on line luntil the
baghouse pressure loss again builds up to the trigger point. Continuous
cleaning has the compartments cleaned on a rigid time schedule, regardless of
the baghouse pressure drop. The advantages to batch cleaning are that the
time-averaged baghouse pressure loss (and ID fan power consumption) is mini-
mized. Continuous cleaning, however, avoids the brief (a few minutes) pressure
spike that occurs when the first dirty compartment of the batch is cleaned.
Maximum Baghouse Pressure Drops
A considerable number of baghouse mathematical simulation models (5, 6,
86
-------
Lrlt-! £r SVrP°8ed recently to try to explain why one baghouse will
operate with a 5" W.G. pressure loss while another struggles with an 11" W.G,
loss.^ Buell s experience is that a relatively simple extension of the
classic Carman-Kozeny model, coupled with an elementary knowledge of boiler
type (cyclone, tangential fired, etc.) and the expected coal and ash proper-
ft
LJ
CO
§'
CD
CO
CO
O
i_
OPERATING CONDITIONS /-EXPECTED MAXIMUM
285°r 4 8AGHOUSEPRESSURE
5.9GRAINS/ACF LOSS IS 7V !N.W.G,
-5'/8 IN.W.G,
TIME-AVERAGED
PRESSURE LOSS
PRESSURE LOSS
— W CM
PREDICTED BAGHOUSE OPERATING
HISTORY WITH ONE COMPARTMENT
— OFF-LINE FOR MAINTENANCE AND
ONE COMPARTMENT AT A TIME
BEING CLEANED.
1
0 1/2 1 11/2
1
2
TIME (HOURS)
o '—
FIGURE 3
ties are sufficient to yield a realistic forecast of what air-to-cloth ratios
are needed to meet a given maximum draft loss specification. The model Buell
uses was adapted by T. Bechtel and 0. Fortune from the Carman-Kozeny model.
The C-K model was rewritten in terms of the important physical phenomena:
where
AP
Ap
A
t
D
A +
A2 t D
(eq. 2)
draft loss across filter cake ("W.G.)
air-to-cloth ratio (fpm)
time required for a complete cleaning cycle (hours)
flyash flow rate into the baghouse (gr/acf)
Since the field data used to establish k^ and k2 would be gathered from
plants at different altitudes (and atmospheric pressures) and with different
87
-------
baghouse gas densities and temperatures, reference terms were added to the
model to account for this:
Ap = ktA + k2A2 t D f = kjA + k3A2 t (eq. 3)
where H = atmospheric pressure at plant site ("Hg)
T = gas temperature (°Rankine = °F + 460°)
r = reference quantity
Since baghouse pressure drops have always been high for metallurgical
applications involving submicron particles, and low for cement plant appli-
cations with coarse duct (greater than 30 microns), plotting data for a
variety of industrial and utility applications leads to the conclusion that:
AP o< d ~°'8 (eq- 4)
where: d = dust mean particle size (microns)
Buell's flyash pilot baghouse testing ( 8 ) later roughly confirmed this
deduction by finding
Thus, our model became n R
u. o
AT, 1 A , *2 DH fdr\ . . , . Az t DH (eq. 5)
AP = k, A + k- A t — (T/ = k, A + k/, M
Buell has-found that this relatively simple model is a good pressure drop
prediction forecaster and also helps explain the high pressure drop problems
at installations such as TUSI Monticello and SWEPCO Harrington. The results
of a parameter study using the Bechtel-Fortune model are shown in Figure 4.
The results agree with the conclusion of the flyash baghouse survey of Noll
and Patel (7); that provided the air-to-cloth ratio is under 2:1, baghouse
pressure loss is not a problem for utility applications.
BAGS, COATINGS AND WEAR
A. The vast majority of current utility boiler filter bags are made of
fiberglass cloth. The only other present candidate fabrics are Nomex
and acrylics. Both of these materials are seldom used because:
1. The maximum continuous temperature allowed with acrylics is 265°F.
While some boilers do operate in this temperature range, it does not
allow any room for excursions, and could lead to having to reduce
load on hot summer days to protect the bags.
2. Nomex has a temperature limitation of 4009F and has poor resistance
to acids. With some special finishes currently available, a maximum
of 1-1/2% sulfur exposure is allowable but field data is minimal.
Also, Nomex is approximately 50% more expensive than fiberglass.
88
-------
FIGURE 4
SET GAS-TO-CLOTH 5ATIO
(AC7M/FT2)
Thus, the final decision regarding the bag material will usually boil
down to what weight of fiberglass cloth to use (9-1/2 oz./yd.^ versus
13-1/2 oz./yd^)and what type of coating (Teflon B or acid resistant) to
apply to the untreated fiberglass. Buell's standard practice is to
prolong bag life by using 13-1/2 oz./yd^ cloth where the design coals
have high ash contents (i.e., over 15%), high sulfur contents (i.e., over
1-1/2%), or operating temperatures near the dewpoint. The more flexible
9-1/2 oz./yd2 cloth is used for less vigorous, base loaded systems where
we can take advantage of its greater flexibility and lower capital cost.
B. Fabric Protective Coatings
Fiberglass cloth ("Greige Goods") must receive proper chemical treatments
(1) to protect the glass fibers from both acid and alkali attack, (2) to
lubricate the fibers and thus improve abrasion resistance and flex endur-
ance, and (3) to enhance its particulate release properties (allowing
easier clean down and hence lower pressure drops). The_two most_commonly
.. 3 *._•„ _ _ m^jTi Tt ~.«.j 4-v.A rt«_rtrtii/-\^ cicicl"—ITGS is 13nt f in is IIGS
used coatings are Teflon B, and the so-called
89
-------
(i.e., Q78, Acid Flex, Chemflex, etc.). The latter consist of silicon,
graphite, teflon, and organic resins. While Teflon B does offer excel-
lent lubrication and release properties, it does not offer the high
degree of acid resistance needed with (1) high sulfur coals, (2) lower
operating temperature, and (3) cyclic boiler operation. This is due to
the fact that the various "acid-resistant" coatings encapsulate over two-
thirds of the glass filaments, while a pure Teflon B coating encapsulates
over one-third.
C. Bag Tensioning and Ring Spacing
1. Filter bags must be kept taut enough to ensure that the bags will not
"pancake" together during cleaning; thus, both trapping dust and
abrading the bag material, and slack enough so that their flexing
during cleaning will remove at least half of the dust cake. This is
achieved by sewing horizontal steel rings into the bags. Using cable
theory, it can be calculated that the optimal ring spacing is a vari-
able pattern (2 diameters at the bottom to a maximum of 4-1/2 dia-
meters at the top) which provides a more uniform fabric flex angle
along the bag. This reduces the chance of having localized flexure
failure, particularly at the bottom of the bag.
2. Premature bag failure can be caused by both too little and too much
tension (.10) . Recommended bag tension is approximately 50# for 8"
diameter bags, and 75# for 12" diameter bags.
Tensioning of the bags in a compartment has some similarity to tuning
a piano. Start from the outside edges of the compartments and work
toward the middle. Then wait a day or two and re-tune. After a few
months on line, the bags should be re-checked for slackness due to
creep-relaxat ion.
D. Quality Assurance and Quality Control for Bag Manufacture
To get high reliability out of bags calls for a firm QA/QC program. A
good program (3) provides the baghouse supplier with the ability to trace
all raw materials used during manufacture back to the original supplier,
and due to various inspections and certifications, also ensures that
these raw materials meet the specifications as required by the customer.
Buell's experience is that good QA/QC procedures mean the difference
between having on-site bag rejection rates of less than 1%, as opposed to
as high as 20%, when a QA program was not carried out.
E. Bag Maintenance and Inspection
Periodic inspection and maintenance of bags is essential to prolong bag
life. To provide a safe and workable environment, the following are
desirable:
1. Compartment ventilation system is needed to cool internal compartment
temperatures to approximately 20°F above ambient. Often the most
90
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economical system is a combination of partial internal insulation and
partial ventilation. Without this type of workable environment, the
most one can expect of a man working in a HOT compartment is about 15
minutes inside, 20 minutes outside. Poor maintenance conditions will
in the long run cause a reduction in bag care (i.e., bags near the
hot wall not properly tensioned).
2. Doorway ventilation velocities under 1000 fpm. with maximum door open-
ing being no wider than the lower walkways to avoid exposing the bags.
3. Opacity monitoring is a good method of checking for damaged bags. By
cross-checking any opacity spikes with the cleaning cycle, one can
identify the compartment with damaged bags.
PREPARATION FOR OUTAGES
For short outages (under two days) , the baghouse can be bottled up if it has
(1) good insulation (3" of mineral wool at least), and (2) the hopper heaters
are left on. For long outages, the bags should be cleaned down at least 1/2
dozen times before the unit is brought off-line.
BAGHOUSE OPERATION DURING BOILER START-UP AND UPSET CONDITIONS
1. Start-Ups - Using Oil and Natural Gas
It is necessary to prevent the aromatic compounds formed during incomplete
combustion of oil from entering the filter cake. So the baghouse is
bypassed when burning either 100% oil or gas, until (a) baghouse internals
are over 200°F, and (b) several coal mills are in operation.
Using a total liquids trap in the natural gas supply line to remove impur-
ities helps minimize the problem in plants using gas fire-up.
2. Bypass Switchover Temperature
This is essentially a function of coal sulfur content.
o.i o.n i.t
i.t M) *•«
CtWl SM-flin CWITHIT («)
91
-------
The minimum operating temperatures shown are typically lower than pure
gas-liquid 803 dewpoint curves due to the fact that they take into
consideration the flyash alkali content.
3. Boiler Tube Leaks
Should a boiler tube leak occur, the baghouse chould be bypassed immedi-
ately to avoid possible damage to the bags. With superheater tube leaks,
as long as the baghouse temperature remains above the new dewpoint temper-
ature, this small increase in moisture will not affect the performance of
the unit.
If a large tube rupture should occur in the economizer section of the
boiler, a large amount of water will enter the baghouse, leaving a wet,
muddy cake on the bags. Immediately, the baghouse should be put through
5-6 cleaning cycles to clean off as much of the cake as possible. Then,
it should be bypassed and bottled up. After the damage to the boiler has
been repaired, bring the baghouse on line on bypass until the bags can be
dried out.
4. Air Heater Stoppages
This is not a problem for a baghouse, since you have about 5 minutes to
bypass the baghouse before the temperature spike hits the bags, and the
baghouse steel takes hours to heat up to over 500°F.
To avoid boiler upsets, the design bypass /!pshould be equal to average
baghouse Ap. This means velocities in the range .of 5,000 to 7,000 fpm
through the bypass valves.
CONCLUSIONS
We have attempted to show in this paper that by paying attention to a
relatively small number of design parameters (dust loading, mean particle size,
gas density, coal sulfur content) it is easy to rationally size a baghouse to
operate with minimal, maximum and average draft losses. Then, given proper
attention to bag manufacture and tensioning, the baghouse will perform success-
fully.
REFERENCES
1. P. R. Belkus. Dust and Flow Distribution on a 10 MW Unit. Fifth Inter-
national Fabric Alternatives Forum, Phoenix, Arizona.
2. P. Bowen. Fabric Filter Model Studies Update. Fifth International Fabric
Alternatives Forum, Phoenix, Arizona.
3. J. A. Hudson, et. al. Design and Construction of Baghouses for Shawnee
Steam Plant. Second Symposium on the Transfer and Utilization of Particu-
late Control Technology, July, 1979.
92
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4. W. Van Kleunen. Particulate Distribution Within TVA Shawnee Baghouses,
ECD Ref.: 27475/S.O. 8898, January, 1981
5. R. Dennis, H. A. Klemm. A model for Coal Flyash Filtration, Journal Air
Pollution Control Association 29:230 (1979).
6. J. R. Koscianowski, et. al. Filtration Parameters for Dust Cleaning
Fabrics. EPA-600/7-79-031. January, 1979-
7. K. E. Noll, M. Patel. Evaluation of Performance Data From Fabric Filter
Collectors on Coal Fired Boilers. Filtration and Separation, May, 1979.
8. E. A. Samuel. Research and Development of Test Baghouse at Martin Drake
No. 6, City of Colorado Springs Mathematical Modeling Studies, December,
1979.
9. F. A. Horney. Coal Ash Characteristics: Do They Allow Predictability of
Fabric Filter Performance. Fourth International Fabric Alternatives
Forum, Phoenix, Arizona.
10. P. R. Campbell. Make fiberglass Bags Last Longer by Maintaining Proper
Tension. Power. March, 1980.
11. E. W. Stenby, F. A. Horney, R. W. Scheck, D. M. Shattuck. Minimizing
Boiler/Baghouse Impact. Power. December, 1979.
93
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EVALUATION OF THE 25 MW KRAMER STATION BAGHOUSE:
TRACE ELEMENT EMISSION CONTROL
By
M. W. McElroy
R. C. Carr
Electric Power Research Institute
Palo Alto, California
The Electric Power Research Institute (EPRI) is currently supporting a
major research program characterizing the emission control performance of
fabric filter baghouses in coal-fired utility boiler applications. An
integral part of this program is the determination of particulate matter
chemical composition and collection. One such effort reported here is an
evaluation of the Kramer Station baghouses of the Nebraska Public Power
District.
Chemical analysis of particulate matter samples shows that the
collection efficiency of over 35 individual major and trace elements was
essentially equivalent to that of total mass. Size dependent chemical
analysis results for nearly 30 elements over a 0.05-10 micrometer diameter
size range further reveals that the vast majority of elements exhibit
penetration profiles remarkably similar to total mass (Figure 1). A notable
exception was selenium, which exhibited much higher penetration. Inlet size
distributions indicated fine particle enrichment for several elements
relative to the matrix elements. This enrichment behavior was still evident
at the outlet of the baghouse (Figure 2), despite very low outlet emissions.
Typical outlet size distributions are illustrated in Figure 3. A complete
discussion and presentation of results are provided in EPRI Report CS-1669,
"Kramer Station Fabric Filter Evaluation" (January 1981), prepared by
Meteorology Research, Inc.
Penetration (percent)
100 i
10
0.1
0.01
0.01
Collection Efficiency (percent)
Selenium
90
99
99.9
I I I I I 11
99.99
0.1
10
Diameter (^m)
Figure 1. PARTICLE SIZE DEPENDENT
ELEMENTAL PENETRATIONS.
Baghouse penetration profiles
of most elements are contained in
a relatively narrow band and
closely resemble total mass. Thus,
the total mass penetration may be
a useful indicator of elemental
penetrations. Overall baghouse
collection efficiency was 99.7% at
the time of these tests. Elements
listed are all those for which
complete inlet/outlet size dis-
tributions were obtained.
94
-------
Enrichment ~
(M/[Fe))10)U71
Figure 2. ENRICHMENT OF SELECTED
ELEMENTS AT BAGHOUSE OUTLET
Enrichment is defined here as the
concentration ratio of an element to
iron at a specific particle diameter
divided by the concentration ratio at
a particle diameter of 10 micrometers.
Iron was selected as the reference
element because of the similarity of
its size distribution to that of total
mass..
1 -
0.01
0.1 1
Particle Diameter,
dM/d log D (ptg/m3)
10*
103
102
10
= Low-pressure cascade impactor
- neutron activation analysis
Total
mass
10-1
10-2
10-3
10-4 =-
10-5 I I I II
Potassium
Barium
Strontium
Selenium
Arsenic
Cesium
I i
I I II
INI
0.01 0.1 1 10
Particle Diameter, D (/urn)
100
Figure 3. SIZE DISTRIBUTION OF SELECTED
ELEMENTS AT BAGHOUSE OUTLET
Only 1 of the over 30 elements
analyzed are shown for clarity. Elements
are present over the entire particle size
range and exhibit size distributions
qualitatively similar to total mass.
Results here and in the previous figures
are based on neutron activation analysis
of low pressure cascade impactor samples.
Electrical aerosol size analysis (dashed
line) independently confirms submicrometer
size distributions.
95
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CHARACTERIZATION OF A 10 MW FABRIC FILTER PILOT PLANT
By: W.B. Smith and K.M. Gushing
Southern Research Institute
2000 Ninth Avenue South
Birmingham, AL 35255
R.C. Carr
Electric Power Research Institute
P.O. Box 10402
Palo Alto, CA 94304
ABSTRACT
Data are reported describing the initial operating phase of EPRI's 10 MW
Fabric Filter Pilot Plant. During these tests the main variable studied was
the method of cleaning the bags. One of the four compartments was operated in
a shake/deflate mode and the other three in reverse air cleaning mode. Signif-
icant differences were found in the operating pressure losses for different
cleaning modes. The lowest pressure loss was experienced with Shaker/deflate
cleaning while the highest was with reverse air and near continuous cleaning.
Extended dwell times resulted in lower operating pressure for the compartments
with reverse air cleaning. Detailed measurements were made to relate the
operating pressure losses to the properties of the dust cake.
INTRODUCTION
Although the use of fabric filters in controlling particulate emissions
from industrial processes is an established technology; their application to
large electrical generating systems is relatively new. As more large units
come on line and experience is gained, it is clear that the fundamental mecha-
nisms that govern the operating pressure loss and reliability are not predic-
table or well understood. In most instances1'2'3'\ however, the emissions
are very low.
EPRI is conducting detailed tests of a 10 MW Fabric Filter Pilot Plant
(FFPP) at the Arapahoe Emissions Control and Test Facility to build a data
base relating the design, startup, and operating parameters to the efficiency,
pressure loss, cost, and reliability. These data will support the electric
utilities in designing more efficient full-scale systems.
The FFPP is a versatile system allowing flexibility in selecting most of
the operating parameters (air/cloth ratio, cleaning mode, cleaning intensity,
flow distribution, ash concentration, dwell time, bag materials, bag size, pre-
coating, and preheating). During the test reported here, the variables tested
were the bag cleaning method and dwell (or filtering) time.
The following sections contain a description of the performance of the
FFPP during the first few months of operation and of detailed experiments that
were performed to relate the measured performance to the properties of the
96
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aerosol/filter system. In the discussion section, the data are summarized and
the dependence of the operating pressure loss on the properties of the dust
cake is discussed.
OPERATION OF THE FABRIC FILTER PILOT PLANT
The FFPP contains four compartments, each equipped for independent opera-
tion. The system is designed for a nominal flowrate of 30,000 ft3/min, but
individual compartments can be operated at flowrates up to 15,000 ft3/min. As
shown in Figure 1, each compartment contains 36 bags in a 6x6 array. Shaker,
reverse air, or combinations of these are available cleaning methods. The bag
support assemblies and tube sheets are adjustable, allowing the evaluation of
22, 26, 30, and 34 foot bags of various diameters. Each compartment is equip-
ped with a dirty gas inlet, compartment bypass, reverse air, clean air preheat,
and clean air outlet duct. This arrangement allows complete flexibility in
selecting the operating conditions for each compartment. Provision is made
for the injection of flyash into the inlet to allow testing under a wide range
of dust concentration. The entire system can be decoupled from the boiler for
air load testing. The FFPP is controlled and all of the operating flowrates,
pressures, and temperatures monitored by a dedicated micro-processor.
The FFPP is operated on a sidestream from a 110 MW, pulverized coal boiler
burning "Energy" (0.3% S) fuel. In order to study the dependence of the FFPP
operation upon the boiler condition, logs are kept of the primary operating
parameters of the latter. Coal and ash samples are taken frequently for
analysis, and NO and Q% monitors are installed in the gas stream.
A variety of test methods are being used to measure the performance of
the FFPP. For investigations of gas flow, s-type pitots, hot-wire anemometers,
and smoke are used. For particulate measurements at the inlets, cascade
impactors, cascade cyclones, mass trains, and a transmissometer are used. For
particulate measurements at the outlets, cascade impactors, mass trains,
special photometers, and a Fine Particle Sizing Stack Spectrometer (FSSS)* are
used. An Electrical Aerosol Size Analyzer (EASA) ** is used at the inlet and
outlet to measure the size and concentration of submicron particles. Windows
and lights are distributed throughout the system to allow visual observations
during operation. Samples are cut from the bags for detailed analysis of the
dust cake.
In preparation for startup the compartments were all equipped with woven
glass fiber bags coated with silicon/teflon/graphite (Albany International
Q53-S3016 "Tri-Coat") 34 feet long and one foot in diameter. Compartment A
was programmed for shaker/deflate cleaning. Compartments B, C, and D were
programmed for reverse air cleaning. The bags in compartment A contain no
rings. The others have eight support rings spaced about four feet apart. The
ringed bags were tensioned to 75 Ibs and the nonringed bags to 60 Ibs. It was
decided not to preheat or precoat any of the compartments because those
procedures do not represent typical industry practice.
*Particle Measurements Systems, Boulder, Colorado
**Thermosystems Inc., Minneapolis, Minnesota
97
-------
When shutting down, it was planned to purge the compartments with clean,
heated air to avoid condensation of flue gas upon cooling, although two
unplanned shutdowns occurred where purging was not possible with no apparent
permanent detrimental effects. Arrangements were made to check all flows,
pressures, temperatures, and particulate monitors around the clock.
After the initial startup period, all of the compartments with reverse
air cleaning were put on time initiation instead of AP initiation because the
increasing pressure loss led to continuous cleaning when operated in the AP
initiation mode. Each compartment was set to a different dwell time in order
to further investigate the effect of dwell time upon the pressure drop observed
previously at the Kramer station.3 Ultimately the system settled into a stable
operating mode as shown in Figure 2. The operating parameters of the FFPP for
these tests are summarized in Table 1.
Referring to Figure 2 the phenomenon first noticed at the Kramer station
is confirmed. If a compartment is left on line longer without cleaning, the
residual AP after cleaning and the slope in the AP vs. time curve are lower.
This appears to be a clear indication that heavier dust cakes are easier to
dislodge, which might be deduced intuitively. On the other hand, the depen-
dence of the slope on the time between cleaning cycles is not as easily
explained. Perhaps this is an indication that certain areas on fabric surface
are more nearly cleaned to a "like new" condition.
TABLE 1. OPERATING PARAMETERS DURING THE FFPP TESTS
Compartment Air/Cloth Ratio Dwell Period Cleaning Method
A 2.0 ft3/acfm ^ 2 hrs Shake/deflate, AP initi-
ate, +_ 2 inches, 2Hz
B " 1 hr Reverse air, timed, 1.5
ft3/ac£m
C " 3 or 3.5 hrs
D " 5 hrs
The curves shown in Figure 2 also offer a possible explanation for the
reason that the tests where the reverse air cleaning was AP initiated (4.5 iwc)
were unsuccessful. As the time between cleaning (dust load in bags) is
reduced, the residual AP increases and the slope increases. Thus, any pertur-
bation resulting in an increased AP will create an unstable situation driving
the system toward a condition of continuous cleaning.
Two positive aspects of the bag cleaning experience can be derived from
Figure 2. The first is that shaker/deflate cleaning appears to be a viable
method, although not optimized during these tests. The second is that it can
be seen to be advantageous to employ timed cleaning, rather than AP initiation,
and extend the time—perhaps beyond five hours. From the second observation
98
-------
it was concluded that additional data and understanding of the cleaning
phenomenon might be gained through artificially increasing the inlet mass
concentration (dust accumulation rate) by injecting fly ash at the inlet.
The results of the ash injection tests are shown in Figures 3, 4, and 5
for compartments B, C, and D, respectively. No data are shown for compartment
A because it was operated at a lower flow during these tests to reduce the
load on the booster fan. Sufficient ash was injected to increase the mass
concentration from 2.5 to 11.4 gr/scf. The pressure loss before cleaning and
after cleaning and the rate of increase in pressure loss (slope of curves in
Figure 2 are plotted vs. time. All three of these parameters were affected
immediately by the injected ash. Filter cycles that were in progress when the
injection began were characterized by increases in the slope of AP vs time and
higher pressure drops. In subsequent cycles, however, the residual pressure
drop (after cleaning) decreased for all three compartments. The AP before
cleaning and slope were significantly lower in compartment B, but essentially
unchanged in C and D. It should be noted that the total test period, 17 hours,
was too short for any of the compartments to reach a stable condition. Com-
partment D, for example, only cleaned 3 times during the test. It is clear
from these data that the additional dust load resulted in better cleaning
within all three compartments. The improvement was greatest in compartment B
where the dust loading on the bags was least under ordinary conditions. The
improvement in compartment D was much smaller, perhaps indicating that the 5
hour cleaning period was near optimal for the normal (2.5 gr/scf) mass concen-
tration and our cleaning sequence.
The data are shown plotted another way in Figure 6. Here the pressure
loss before (AP ) and after (AP ) cleaning are shown plotted as functions of
filtering time,Bwith and without the ash injection. It appears from the curves
that no advantage can be gained by extending the dwell time beyond about five
hours. Although it is possible to reduce AP by extending the filtering time,
the problem of a high average AP remains. Tnus further studies are planned to
investigate means of cleaning the bags further.
Throughout the tests the efficiency of the compartments was very high—on
the order of 99.99+% for the reverse air compartments and 99.95+% for the
shaker compartment. Typical recordings of the outlet opacity (undiluted and
normalized to a 10M stack) are shown in Figure 7. The emissions increase by
about two orders of magnitude immediately after cleaning, but quickly subside
to yield opacities less than 0.05% for the reverse air and 0.2% for the shaker
compartments.
In an effort to interpret and understand the operating conditions of the
FFPP observed during the startup and early operation periods, detailed analyses
of fabric samples were undertaken. The thrust of these analyses was to charac-
terize the residual dust layer as thoroughly as possible and to relate the
observed properties to the operating AP and clean down effectiveness. The
measurements included permeability tests, higher reverse air/cloth, photo-
graphs, thickness profiles, mass density, and size analysis. All of the
measurements were made using swatches carefully cut from the top, middle, and
bottom sections of the bags after the cpmpartments were shut down.
99
-------
Recall that during operation the bags in compartment A were shaken with
reverse air assist at a AP of 4.5 iwc. Those in compartments B, C, and D were
cleaned using reverse air (A/C of 1.5:1) at intervals of 1, 3, and 5 hours,
respectively. Figure 9 contains photographs illustrating the remarkable
differences in appearance from compartment to compartment and top to bottom.
Notice the common formations of "nodular deposits" in compartments B, C, and D.
Upon close examination, the nodules can be seen to be attached to fibers
protruding from the main body of the bafric. Both the nodules and smooth cake
are rather frangible and can be disturbed easily with a pencil. There is no
evidence of any "cement" like properties. The swatches taken from the tops of
the bags appear visually to contain a heavier dust cake then those from the
bottom. It is surmised that an avalanche effect occurs during the cleaning
cycle and that the lower parts of the bags tend to be swept clean. Vertical
lines of smoother material can be seen in the swatches from compartments B, C,
and D at positions when the bags folded during reverse-air cleaning.
The pressure loss vs. A/C of the filter material and residual dust layers
was measured by clamping the samples into a standard 8 x 10 inch high-volume
filter holder and measuring the pressure drop as flow was increased in the
forward and reverse direction. Figure 8 shows AP vs air/cloth measured in the
forward direction before and after running the swatches at reverse A/C of
4.1:1. It was surprising to find the more permeable cakes at the top in
compartments C and D because the dust cakes appeared visually to be much
heavier, and that although the bags had already been cleaned in the compartment
at an A/C of 1.5:1, additional reductions in pressure loss were achieved by
increasing the reverse air/cloth ratio to 4:1.
From these data it can be concluded that the flow through the bags during
the filter cycle is strongly stratified immediately after cleaning from top to
bottom of the bags. Greater flow occurs at the top in the reverse air compart-
ments despite heavier dust loads and greater flow at the bottom in the shaker
compartment. The pressure loss does not correlate well with mass of dust cake
per unit area indicating that the packing density or size distribution may be
different.
DISCUSSION
From the data described above it appears that the details of the gas flow
and pressure loss are intimately related to properties of the dust cake. This
dependence results in large variations in flow along the length of the bags
after cleaning. It is not known how long this unequal flow persists into the
dwell period.
Heavier dust cakes are easier to remove than light ones, indicating that
extended dwell times might allow operation at lower average pressure than very
short dwell times or continuous cleaning. Some increase in maximum Ap may be
required to accumulate the heavier cakes, however, and additional tests will
be required to optimize the operating cycle.
Published literature on the removal of collected dust from bags and the
factors determining the pressure loss are scarce. Dennis5 has observed that
100
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"tinder field conditions, dust removal from woven glass fabrics cleaned by
collapse and reverse flow may range from 5 to 35 percent, whereas mechanical
shaking (2.5 cm amplitude at 8 Hz), may provide ~50 percent removal." Recently
a paper written by Lembach and Penney6 contained a discussion of "nodular
deposits" formed in fabrics after many cleaning cycles. It is thought
(although not confirmed) by the authors that these deposits, which cling to
protruding fibers, are composed of fine particles and act as check valves
offering little resistance to reverse flow but moving into voids to offer a
high resistance to forward flow. These deposits have been observed by others
(e.g. at Four Corners) and by us. There is some speculation that the forma-
tion of hydrated salts in the cake make the particles more adhesive.
0
Ariman and Helfritch have found that dust is removed more easily under
conditions of high relative humidity. Sproull9 reports that thick layers are
removed (from ESP plates) more easily than thin ones and that the dust is more
easily dislodged at high temperatures.
Although the physical forces governing particle cohesion and adhesion are
such that individual small particles cling more tightly to one another and
other surfaces than large particles. It is generally thought that layers of
fine particles are less porous, but this rule of thumb is not supported by
experimental data. linoya has measured the porosity for polydisperse aggre-
gates of particles of different size and composition. His data show that dis-
tributions of small mass median diameter have higher porosity (See Table 3.).
The relationship between the porosity measured in bulk powders and the
operating fabric filter systems is yet to be established. The fabric structure
may play an important role and the cake may collapse under the gas pressure to
present a more impermeable barrier to flow.
Further studies will be required to relate the properties of the aerosol
and dust cake to the operation of fabric filter systems. Future plans include:
Tests with variable compartment flow to simulate large installation, long-term
fly ash injection tests, a range of magnitude in the reverse air/cloth, ex-*
tended dwell time for the shaker/deflate compartment, and a range of magnitude
in the filtering air/cloth ratio.
TABLE 3. POROSITY* OF ACCUMULATED POWDERS (AFTER IINOYA AND YAMAMURA )
Powder
Talc
Flour
Flour
Clay
Wood Dust
Flue Dust
Flue Dust
Surface Mean
1.
33
12
4.
6.
5.
0.
Diameter, ym
6
6
0
6
24
Porosity {£)
0.74 -
0.51 -
0.62 -
0.68 -
0.90 -
0.74 -
0.
of Deposit
0.81
0.61
0.70
0.71
0.91
0.77
94
*Porosity, e, is defined in terms of the pressure drop by D'Arcy's
101
-------
Equation:
AP = c(l-e)2S2 yvr or AP - CyVT
v
e"3"
where
P K
AP = the pressure loss observed,
e = the porosity,
S = the surface area of a unit volume of the cake,
y = the viscosity of the gas,
V = the face velocity or A/C,
K = the permeability, and
T = the thickness of the cake.
ACKNOWLEDGEMENTS
Several individuals and organizations have contributed to the design and
evaluation of the FFPP. The mechanical design, flow modeling, and construction
was done by Lodge Cottrell Division of Dresser Industries. Lou Rettenmair,
Richard Hooper, and Walter Puille of EPRI have made significant contributions
during the construction, planning, and startup programs. Kaiser Engineers
provide continuing support for the operation and modification of the unit.
We have benefited significantly from the advice of Charles Gallaer, Engineering
Consultant.
The data reported here were taken by Ray Wilson, Annette Duncan, and
Myron McCallum of Southern Research Institute.
The continuing support of Public Service of Colorado is supporting the
emissions control test facility and cooperating with us in this pilot program
is greatly appreciated.
This work was supported by the Electric Power Research Institute under
contract RP1129-8.
102
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ENDNOTES
1. Ensor, D.S., R.G. Hooper, and R.W. Sheck. Determination of the Fractional
Efficiency, Opacity Characteristics, Engineering and Economic Aspects of
a Fabric Filter Operating on a Utility Boiler, EPRI Report No. FP-297,
November 1976.
2. Huber, D.J. Start-up and Initial Operation of the City of Colorado
Springs Unit #1, Baghouse Filter. (Presented at the 5th Fabric Filter
Forum, Phoenix, Arizona, January, 1981).
3. Ensor, D.S., S. Cowen, A. Shendrikar, G. Markowski, G. Woffinden,
R. Pearson, and R. Sheck. Kramer Station Fabric Filter Evaluation, EPRI
Report No. CS-1669, January 1981.
4. Chambers, R. Operating Data - Harrington Station. (Presented at 5th
Fabric Filter Forum, Phoenix, Arizona, January, 1981) .
5. Dennis, R. and H.A. Klemm. Recent Concepts Describing Filter System
Performance. (Presented at the Joint U.S.-Japan Seminar on Measurement
and Control of Particulates Generated from Human Activities, Kyoto, Japan,
November, 1980).
6. Lembach, R.F. and G.W. Penney. Nodular Deposits in Fabric Filters. J.
APCA 29(8), August 1979.
7. Singh, U. Four Corners Units Four and Five Particulate Removal Project
Filter-House Pilot Program. (Presented at 5th Fabric Filter Forum,
Phoenix, Arizona, January 1981).
8. Ariman, T. and D.J. Helfritch. How Relative Humidity Cuts Pressure Drop
in Fabric Filters. Filtration and Separation. March/April 1977.
9. Sproull, W.T. Fundamentals of Electrode Rapping in Industrial Electrical
Precipitators. J. APCA 15(2), February 1965.
10. linoya, K. and M. Yamamara. Fundamental Experiments with Fabric Dust
Collectors. Chem. Eng. (Japan) 20, 1956.
103
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AHTM6NT |
SHELL MM- VIEW)
L_|
rvi
««jr\ /
HWVEH \ /
"rj—i ^-T-T;
w— COMPARTMENT B /
f 7 /
,1-
1 2 3 4 B
WLTtaiNQ TIMS, rcw* '*»'-*>
Figure 1. Sketch of bag compartment with sections Figure 2. Typical AP versus time data for reverse
broken away to show arrangement of air and shaker cleaning.
internal details.
I COW»AaT\tErjT B t ;
-•A fl
- vMVfT
i S 1/V^vT
MH IMJ^CTItlN
RATE OF uP IMCREASE
INLET -.OAntMQ
WITHOUT INJECTION
1C II 12 13
RATE OF APINCREASE
H^rvjhEN CLEAWNG
•' (iNcHEsmouni
JMLET LOADING
WITHOUT 1KJEC7I;
WTH 1-JJHCriQN
1 ' **'
Figure 3. Tube sheet AP before and after cleaning Figure 4. Tube sheet AP before and after cleaning
and rate of AP increase for Compartment and rate of AP increase for Compartment
B during the period December 9-13,1980.
Ash injection was on for a period of seven-
teen (17) hours.
C during the period December 9-13,1980.
Ash injection was on for a period of seven-
teen (17) hours.
104
-------
1 MT! Of itf INCBtAH
•ETWEEN CLEANING
tNLET tOACTM
*ITHOUT INJECTION
Figure 8. Pressure loss before (AP BC) and after
(AP AC) cleaning the bags by reverse air
for different filtering times.
Figure 5.
Tube sheet AP before and after cleaning
and rate of AP increase for Compartment
D during the period December 9-1 3, 1 980.
Ash injection was on for a period of seven-
teen (17) hours.
I*
j
AWCLOTH RATIO.
Figure 8. AP versus air/cloth in samples from
Compartment C before and after reverse
air tests at air/cloth of 4.1 acfm/ft2.
Figure 7. Typical response of the opacity monitors
on Compartments A and B.
105
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st#%MM* I :' • ••
BOTTOM
Figure 9. Photographs of swatches taken from bags in each compartment of
the FFPP.
106
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SPECIFYING A FABRIC FILTER SYSTEM
By: Ronald L. Ostop
Department of Public Utilities
Colorado Springs, Colorado 80947
David A. Single
Buell Emission Control Division
Lebanon, Pennsylvania 17041
ABSTRACT
In the last decade environmental regulations have required demanding
technological advances on particulate control techniques. One particular
challenge has been collecting fly ash from the combustion of low sulfur coal.
Ten years ago, two of the choices available for collecting high "resistivity"
coal ash were hot side precipitators and cold side precipitators with flue
gas conditioning. Five years ago, fabric filtration began emerging as
another alternative.
With the new application of this old technology, fabric filter baghouse
applications have resulted in many successes, and in some instances, systems
that were not so successful. So, the question arises, "How does one design
and specify a fabric filter baghouse system that will be successful?"
Based upon actual design and operating experience, this paper presents
the basic information needed to specify a fabric filter system that will pro-
vide cost effective operation. The discussion will include the conceptual
requirements for specifying such parameters as air-to-cloth ratio, pressure
drop, bypass capability, inlet and outlet valve requirements, pneumatic and
electronic control systems, emergency control instrumentation, etc.
The operating principles of a baghouse are simple. By following funda-
mental steps in specifying a baghouse, the subsequent purchase and installa-
tion can be equally uncomplicated.
INTRODUCTION
Fabric filter baghouses are highly efficient and reliable particulate
collection systems. These systems also demonstrate high availability with
relatively low operating and maintenance costs. The key to such success is
properly specifying a system which will meet the established need.
The first step is to define the parameters of the process system to be
controlled. Such parameters include:
- Whether the boiler unit is continuous or cyclic.
- Whether the boiler is stoker-fired or burns pulverized fuel.
- What is the fuel used for ignition?
- What are the fuel characteristics?
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- What is the maximum air flow rate?
- What are the maximum and minimum operating temperatures?
- Is there any flue gas desulfurization prior to the fabric filter
system?
- What is the maximum ash and moisture content of the fuel?
- What operating pressure drop will be compatible with the complete
process system?
The careful examination of the application and the emphasis of the phys-
ical phenomena of mass, momentum and heat transfer will avoid such problems
at excessively high differential pressures and corrosion of the baghouse
structure and auxiliary equipment. It will also help avoid erosion of the
filter media and rotating equipment, while still resulting in no visible
emissions being emitted from the stack.
Operating Principles
The application of fabric filtration is found in that branch of fluid
mechanics describing the flow of fluids through a porous material. In
theory, the collection mechanisms are described by processes of impaction,
interception and diffusions. In practice, hot dust laden flue gases pass
through a fabric filter leaving approximately 99-9% of the dust behind.
The fabric itself does not collect all of these particles. The fibers
act as a substrate to capture and hold the larger particles which in turn
act as the filter media for capturing the smaller particles. Therefore, the
development of the filter cake during the initial conditioning period will
determine the success of the operation of the baghouse.
Operational observations have shown that the filter efficiency increases
with the increase of pressure drop across the filter during the development
of the filter cake. Once the filter cake has been established, the increase
of pressure differential will stabilize and the ultimate collection effic-
iency will be established.
Conceptual Design Considerations
Air-to-Cloth Ratio
The air-to-cloth ratio is the major parameter in determining the cost
of the fabric filter system. It will determine the physical size of the
baghouse and its auxiliary equipment, thus being a major factor in the cost
of material and labor. The air-to-cloth ratio is also directly proportional
to the pressure drop across the collection equipment. Pressure drop is the
main parameter in determining the operating cost of the fabric filter system.
The cost of bag replacement is second to the pressure drop costs, but they
are related.
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Low air-to-cloth ratios will lead to lower pressure drops across the
filter and longer bag life, thus resulting in higher capital costs with lower
operation and maintenance costs. On the other hand, higher air-to-cloth
ratios will increase pressure drop, decrease bag life and possibly affect the
reliability and availability of the collection system.
An optimum solution can be found. The choice must be based upon what
the site specific need is and what the overall utility philosophy is. The
following are considerations which should be incorporated in determining the
air-to-cloth ratio:
- Determine the maximum amount of land area available for installing
the fabric filter system. This is of major importance if the system
is being retrofitted on an existing boiler unit. Installations at
new power plant sites usually allow for more flexibility.
- Determine the maximum possible flue gas flow rate that can be emitted
to the baghouse. This should include burning the fuel with the worst
combustion characteristics that the boiler can handle; i.e. lowest
BTU per pound, highest excess air, highest moisture content, etc. The
calculation should also include leakage into the flue gas which would
increase the flow rate such as air heater leakage, tempering air,
soot blowing, etc. Air flows used for design purposes which are ob-
tained from measuring the flow rates of existing units may be mis-
leading. These flow rates are usually lower than the maximum and will
more than likely result in higher air-to-cloth ratios and higher
pressure drops than desired.
- Determine the maximum pressure drop that can be handled by the fans
while maintaining maximum load while burning the fuel with character-
istics likely to produce the most particulate.
Theory shows a relationship between pressure drop across the filter
with respect to air-to-cloth ratio as follows:
P = Sv
Where: AP is the average pressure drop across the baghouse (inches
of water),
v is the air-to-cloth ratio, (ft./min.),
S is the filter drag (inches of water/ft./min.)
Field measurements on a Colorado Springs system have shown a filter
drag of approximately 2.5 inches of water/ft./min. 0) This is from fly
ash from northwest Colorado coal in a pulverized coal boiler. The mass mean
diameter of the fly ash particles ranged from 2 to 50 ym. (2) It should be
noted that the AP calculated for all compartments in the filtering mode will
not be the maximum pressure drop that will be used to design the fan. When
compartments are in the cleaning mode or out of service for maintenance, the
air-to-cloth ratio will increase, thus increasing the pressure drop.
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If there is a strict size limitation due to available land area, the
maximum size of the baghouse may already be determined. The determination
to be made then is how many compartments should the baghouse have and what
should the size of the fan be to move the air through the baghouse. Be
careful of promises to provide low pressure differentials with high air-to-
cloth ratios. This is contrary to the laws of physics. Such promises may
lead to undersizing the fan, thus load limiting the boiler unit.
If land area is not a limiting factor, an optimum size can be determined
by performing a cost analysis. Remember, a lower air-to-cloth ratio will in-
crease capital costs by increasing materials and labor and a higher air-to-
cloth ratio will increase operation and maintenance costs by increasing
pressure drop across the system and reducing bag life. However, it is
possible to run into operating problems if the air-to-cloth ratio is too low.
If the velocity in a compartment is reduced and the temperature is low, which
is typical of low load operation, moisture in the flue gas stream may con-
dense in areas of no flow and begin corroding. It should be noted that
maintaining a sufficient velocity in a compartment or in the ductwork will
minimize if not eliminate condensation, even if the temperatures are below
the dewpoint.
To give an example, the City of Colorado Springs has installed two
fabric filter systems within the last two and a half years. One system was
retrofitted on a 400,000 ACFM 85MW unit and the other was installed on a new
1,071,600 ACFM 200MW unit. Both systems were specified with the same con-
servative conceptual design parameters. Both fabric filter systems operate
at an efficiency exceeding 99.9% at average pressure drops of approximately
4.5 inches of water, even though they are built by different vendors. The
City did not issue a design specification, but instead it specified a per-
formance contract, so each manufacturer was to use his best engineering
practices considering the following conceptual parameters:
- An independent cleaning cycle operation was provided for nominally
every 500,000 ACFM.
- The air-to-cloth ratio was 2:1 for each independent cleaning cycle
with one compartment out of service and one compartment in the clean-
ing mode (two compartments out of service).
- The bag cloth area was defined to be the active filter area and shall
not include surfaces occupied by anti-collapse rings, caps, cuffs,
seams, or the length of the inlet thimble penetration.
- The maximum composite pressure drop across each baghouse module gas
inlet and outlet manifold shall not exceed 5.0 inches of water gauge.
Both baghouse systems operate with similar results. Both exceed state
and federal collection requirements by about 75%. Both systems have negli-
gible maintenance cost and the system availability exceeds 99%. The 85MW
unit has one clean cycle operation with twelve compartments and 198 bags
thirty feet long and twelve inches in diameter in each compartment. The
200MW unit has two independent cleaning cycle operations. Its thirty-six
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compartments are housed in two modules of eighteen compartments each with
156 bags thirty-one feet long and 12 inches in diameter in each compartment.
Filter Cleaning Systems
During the filtering process, fly ash builds up on the inside of the
fabric. The velocity through the fabric will remain constant, but the filter
drag will increase, thus increasing pressure drop. In order to continue to
operate the baghouse, without exceeding the capabilities of the fan in the
system, the bags must be periodically cleaned. Systems utilized in cleaning
the bags include reverse air, high pressure pulse, and shaking the bags.
It should be noted that the primary mechanism for cleaning the bags is
mechanical action. That is, during the filtering process, the ash forms a
"cake" on the inside wall of the bag. Changing the shape of the bag during
the cleaning period will break this cake away from the bag wall and allow it
to fall into the collection hopper.
Reverse air cleaning is one of the most widely used cleaning mechanisms.
The filter bags are nominally 30 to 35 feet long and 12 inches in diameter
with steel anti-collapse rings spaced throughout the length of the bag. When
a compartment of bags is put in the cleaning mode, it is isolated from the
flue gas stream, and clean hot flue gas enters the compartment in the reverse
direction. This will collapse the bags, thus breaking off the filter cake.
The anti-collapse rings will prevent this long bag from "pancaking" (collaps-
ing to the point where the bag becomes flat thus not allowing the filter cake
to fall into the storage hopper). The amount of reverse air used should be
only as much as is needed to collapse the bags. Under normal conditions
too much reverse air will tend to overclean the bags, thus reducing overall
collection efficiency, and reducing bag life.
Shaking the bags is the second most widely used method for cleaning.
The compartment is isolated from the flue gas stream, the bags are gently
shaken and then subjected to a gentle reverse air flow. This will change
the shape of the bags, which will then break the fly ash cake away from the
wall of the bag. Bags in a shaker system do not have anti-collapse rings.
With the shaking action, these rings will cause wear and premature failure.
As a result, if the bag is too long, pancaking may occur. Experience in
Australian baghouse applications indicates that a baghouse with a shaker
type cleaning method should not have bags exceeding 20 feet in length and
six inches in diameter (3). This will result in an effective cloth area
per bag about one third of that for a bag used in a reverse air cleaning
system. For the same air-to-cloth ratio, this will result in a baghouse
structure taking up at least three times the ground area. Utilizing larger
bags in the shaker cleaning type baghouse will result in less efficient
cleaning which will increase the filter drag, thus increasing pressure
differential across the system.
High pressure pulse cleaning is limited by the size of the filter bags.
The compressed air used to emit the pulse to the bag is only effective to
bags up to approximately 15 feet long. Pulse jet baghouses are generally
designed at an air-to-cloth ratio approximately twice that of reverse air or
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shaker baghouses. Even so, a pulse jet would require twice the land area of
a reverse air type baghouse. The pulse jet tends to overclean the bags which
will increase emissions. On the average, reverse air type baghouses have
offered the most cost effective operation on large utility boilers to date.
Bag Specifications
The typical fabric used in constructing the fabric filter bags for use
on coal-fired boilers is fiber glass. With the recent introduction of spray
absorbtion techniques for flue gas desulfurization, inlet baghouse tempera-
tures will be lower. This will lead to use of other types of fabrics being
used in fabric filter collection systems. It should be noted that if low
temperature fabrics are used in installations following spray absorbtion
systems, the availability of the boiler unit may be lowered. If there is a
malfunction with the spray absorber unit, the entire boiler and baghouse
unit will have to be shut down until the malfunction is corrected. The low
temperature bags will not be able to withstand the higher temperatures re-
sulting from a spray absorber which has been bypassed.
Varying finishes are available with fiber glass bags to prevent self-
abrasion of the fiber glass. Some typical finishes include teflon, silicon
graphite, and a tri-coating which uses a combination of teflon, silicon
graphite and an acid resistant coating. The finish specified is to the
customer's preference. An investigation should be conducted on the desirable
bag performance and baglife characteristics before specifying and accepting
a particular bag finish from a vendor.
The bag specification should include the number and type of anti-
collapse rings if the baghouse incorporates a reverse air cleaning system.
A typical specification for anti-collapse rings may be as follows:
Bags shall be provided with a minimum of seven sewn-in anti-
collapse rings (bags nominally 30 feet long and 12 inches in diameter).
The rings shall be 3/16 inches minimum thickness steel located 30
inches from the bottom of the bag and on four centers.
It should also be specified that the top end of the bag be provided
with a steel cap with an eye bolt (4). It is advisable to specify stainless
steel rings and caps to minimize deterioration due to the corrosive flue gas
environment.
The bags provide the collection mechanism of the system. Special atten-
tion should be given to their selection. The proper bag given the proper
care will lead to a highly efficient cost-effective collection system.
Compartment, Hopper, and Inlet and Outlet Arrangements
The arrangement of the compartment and hoppers will more than likely be
dictated by the space available for the installation. However, the arrange-
ment of the inlet manifold and the inlets and outlets to the compartments are
crucial to a cost effective operation.
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The inlet manifold should be designed to approach an equal distribution
of particles to each compartment. The inlet to the hopper area and outlet of
the compartment should be located to provide an equal distribution of flow to
all the bags. Also, the velocities entering and leaving a compartment should
be low enough not to cause turbulent flow in the hopper area. Turbulence
will cause increased pressure drop and decrease the effective cloth area of a
compartment. Typical velocities entering a compartment which will reduce
turbulence are 20 to 25 feet per second. The typical outlet velocity range
is from 30 to 40 feet per second.
It is advisable to require a fluid flow model study in the specification
to arrive at the proper arrangement to maximize the effectiveness of the
fabric filter system.
Operation Control Systems
The type of logic control system is again a matter of customer prefer-
ence. New microprocessor control systems provide more flexibility, are
easier to reprogram and utilize approximately one-one hundredth of the space
of the relay-time type logic control systems. Ease of trouble shooting and
longer component life are also advantageous to the microprocessor controllers.
If a pneumatic actuator system is used, the following should be provided
to insure a reliable operation:
- A sufficiently sized air dryer to eliminate moisture in the system
which could freeze in cold weather or cause corrosion in the system.
- A sufficiently sized air receiver to provide quick recovery of the
air supply system following an emergency bypass and isolation
situation.
- Air regulators and lubricators at each pneumatic operator to provide
consistent reliable service.
Some instrumentation which will be useful during start-up and operation
of a fabric filter system includes:
- Inlet and outlet temperature recorders.
- Individual compartment temperature recorders.
- Flange to flange pressure drop recorder.
- Inlet flow rate recorder.
- Reverse air flow rate recorders.
- Visual emissions monitor recorder.
- Compartment operational mode indicators.
Bypass and Emergency Control Provisions
Many new installations provide for a baghouse bypass which is used
during start-up of a boiler unit and for emergency isolation during a serious
boiler system upset condition. The bypass must be designed so that there is
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no leakage of dirty flue gas to the stack during normal operation. A typical
way to avoid leakage is to provide a double louvre damper system in the by-
pass duct work. The plenum between the louvre dampers is then pressurized
with a seal air fan or a globe valve so that leakage is from the plenum to
the bypass duct work in both directions, thus eliminating fly ash infiltra-
tion around the baghouse.
The baghouse bypass is an advantage during the start-up period of the
boiler unit. It allows the boiler to be started up and stabilized on clean
fuels, such as natural gas and No. 2 fuel oil. The baghouse can then be put
on-line with hot flue gas from these fuels just before going to coal firing.
This will minimize the potential for condensation forming during the initial
start-up stages which could lead to blinding the bags.
Emergency bypass and isolation operations should be provided to prevent
irreparable damage during certain upset conditions. Such conditions would
probably lead to boiler unit shutdown so, in these cases, system protection
is essential. Emergency conditions of great concern are excessive differ-
ential pressure across the baghouse and high temperatures entering the bag-
house. High differential pressure can be caused by a massive boiler tube
leak. High inlet temperatures can be caused by a malfunctioning air pre-
heater. Both situations will result in damage to the bags if not isolated.
Also, these emergency situations will cause a boiler shutdown; therefore,
continued exceedences of emission standards are unlikely.
A low temperature bypass trip, which has been used in some baghouse in-
stallations is unnecessary. In fact, it may cause more harm than good. The
only time a low temperature situation arises is during a controlled shutdown.
If warm moisture laden flue gas is isolated in a bypassed baghouse during the
shutdown process, these stagnated gases will slowly cool down in the com-
partments and the moisture will condense on the compartment walls and the
bags. This will result in corrosion of the compartment walls and operating
valves. The condensed moisture will also form small nodules with the ash in-
creasing filter drag, thus resulting in higher operating pressure drops.
If low load operation results in dangerously low baghouse inlet tempera-
tures, an air preheated bypass can be provided to increase the temperature
to the baghouse.
If the baghouse is bypassed and isolated as a result of an upset con-
dition, provisions should be made in the operating instructions to purge
the baghouse of the moisture laden flue gases. Ambient air contains less
than one-tenth of the moisture than flue gas.
Other Options
With the introduction of dry FGD systems preceding a fabric filter
system, inlet temperatures to baghouses are lower. A spray absorbtion
system reduces sulfur content of the flue gas. Acids resulting from carbon
dioxide and oxides of nitrogen can be formed at temperatures approaching
adiabatic saturation of the flue gas. During full load situations when
compartment velocities are their highest, the probability of low pH conden-
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sation is minimal. However, at reduced load situations when air-to-cloth
ratios are lowered, acidic condensation may develop. Therefore, it is ad-
visable to investigate reheat systems which will meet the need of the
specific boiler unit.
Some fabric filter installations have incorporated a preheat system to
prevent moisture condensation during start-up and shutdown. This is an
expensive luxury which can be avoided by developing and implementing good
start-up, shutdown and operating procedures. This is possible even for a
cycling unit.
Operation and Maintenance Instructions
The specifications should require detailed operation and maintenance
instructions not only for the entire system as a whole, but also for indi-
vidual components. These instructions should include trouble shooting pro-
cedures, periodic component checks, calibration procedures and procedures
for start-up, shutdown, and emergency contingencies.
The start-up and shutdown procedures are of particular importance.
These procedures are unique to each installation and must be customized.
They must be incorporated in the boiler and turbine start-up and shutdown
procedures. Because of environmental restrictions, the fabric filter system
has become an integral part of coal-firing to produce electricity. These
procedures are as important as the components of the baghouse for properly
operating the system.
Guarantees and Warranties
The best specification that can be written will state the desired
results and allow the vendor to utilize his expertise in designing and in-
stalling his product. Unfortunately, the process of competitive bidding
may short circuit this ideal. As a result, a specification must be written
to obtain the desired results without inhibiting the vendor's talent and
expertise. This is commonly referred to as a performance specification.
A performance specification should be based upon rendering final com-
pensation to the vendor in return for specific performance criteria. The
criteria should be based on results. These results should reflect economics,
operationability, and compliance with applicable environmental standards.
Therefore, criteria for achieving the desired performance must be specific.
Examples of performance criteria are:
- Maximum pressure drop across the fabric filter system.
- Maximum temperature drop across the fabric filter system.
- Minimum acceptable collection efficiency under specified operating
conditions of the baghouse.
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All tests performed to prove compliance with the performance criteria
must be specific and relative to the purpose of the operation of the entire
system within the constraints of environmental standards.
Design Input to Vendors
In order to obtain what is needed for particulate control, the proper
input must be provided to the vendor. This again will be unique to each
specific application. The following is a suggested list of parameters which
will facilitate the design and installation of the equipment to meet your
needs:
- Size and type of combustion unit.
- Primary fuel and ignition fuel.
- Static pressure at baghouse inlet.
- Expected flue gas temperature at full load and maximum load.
- Maximum flue gas temperature.
- Maximul fuel firing rate.
- Flue gas flow rate at maximum load and minimum load.
- Coal analysis to include moisture content, sulfur content, ash
content, and heat rate in BTU/lb.
- Maximum fly ash rate to baghouse.
- Elevation at plant site.
- Flue gas temperature for equipment selection and structural design.
- Snow loads, wind loads, and seismic loads.
- Internal pressure design.
- Fly ash density.
- Minimum fly ash storage capacity.
- Dust load build-up conditions in ductwork.
- Maximum acceptable pressure and temperature drop across system.
- Bag specifications including bag reach.
- Maximum velocity through each compartment inlet and outlet.
- Maximum air-to-cloth ratio.
- Installation space limitations.
- Model study results.
- Calculations deriving air-to-cloth ratio with respect to pressure
drop, damper leakage, thermal movement, temperature differential
throughout the system, hopper sizing.
Conclusion
A fabric filter has proven to be a very cost-effective means to obtain
high collection efficiency of particulates produced by utility boilers.
These systems, if properly designed and operated, can provide a means of
controlling particulate emissions with high availability with reasonable
operating and maintenance costs. The operation of a fabric filtration
system is not complicated. Specifying such a system can be equally as
uncomplicated by determining the specific need and results required.
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ENDNOTES
1. Test Report for Research and Development on Test Baghouse at Martin
Drake Unit No. 6, City of Colorado Springs, Volume II, Mathematical
Modeling, Eric A. Samuel, 1980.
2. Test Report for Research and Development on Test Baghouse at Martin
Drake Unit No. 6, City of Colorado Springs, Volume II, Main Baghouse
and Pilot R & D Baghouse Particle Loading, T. K. Chiang, 1980.
3. Discussion with Electricity Commission of New South Wales, Sidney,
Australia, February 9, 1981.
4. Specifications for Modular Baghouse Fabric Filter System for the
Ray D. Nixon Power Plant, 1976.
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EVALUATION OF THE 25 MW KRAMER STATION BAGHOUSE:
OPERATIONAL FACTORS IN PARTICULATE MATTER EMISSION CONTROL
By
R. C. Carr
M. W. McElroy
Electric Power Research Institute
Palo Alto, California
The Electric Power Research Institute (EPRI) is currently supporting a
major research program characterizing the emission control performance of
fabric filter baghouses in coal-fired utility boiler applications. One such
effort reported here is an evaluation of the Kramer Station baghouses of the
Nebraska Public Power District. These units represent the first application
of fabric filter control technology to pulverized-coal fired utility boilers
burning a western, low-sulfur sub-bituminous coal.
As summarized below and in Figure 1, the baghouse cleaning cycle had
the greatest effect on particulate matter removal efficiency compared to any
other operational parameters. Implementation of a "preferred" 100-minute
cleaning cycle, obtained by extending the dwell time between compartment^
cleaning from zero to eight minutes, reduced particulate matter penetration
by 50 percent without an increase in pressure drop. Total particulate
matter collection efficiency measured for the baghouse with the preferred
cycle in effect averaged 99.92 percent, with associated outlet emissions and
stack opacity of 0.001 lb/106 Btu (0.43 ng/J) and 0.07 percent, respectively.
The 0.07 percent opacity is equivalent to a 20 kilometer stack "visibility."
As shown in Figure 2, the baghouse size dependent mass collection efficiency
exceeded 99 percent over the measured 0.02 to 10 micrometers diameter size
range. The clear stack and low outlet emissions were maintained throughout
the nine-month test program despite intentional variations in baghouse
operating parameters and uncontrolled fluctuations in coal ash content and
boiler combustion conditions. The continuing good performance during these
episodes suggests that the baghouse is a very effective and forgiving
particulate emission control device for utility application. The particulate
mass removal efficiencies and outlet emissions measured for Kramer are
remarkably similar to results obtained in a previous EPRI study of the Nucla
Station baghouse. This comparison is very significant since there are major
design differences between the two baghouse installations and boilers (Nucla
is spreader-stoker whereas the Kramer units are pulverized-coal).
KRAMER UNIT 3 BAGHOUSE EMISSION SUMMARY
Average Concentration
(gr/sct) Emissions (lb/106 Btu)
Cleaning Air/Cloth Ratio Efficiency Opacity
Cycle (acfm/ft2) Inlet Outlet (%) (%) Total <2fim
Continuous,
10-minute cycle 1.80 1.00 0.0014 99.86 0.08 0.002 0.0005
Preferred,
100-minute cycle 1.86 0.69 0.0006 99.92 0.07 0.0009 0.0002
No cleaning,
350 minutes 1.57 1.38 0.00032 99.98 0.02 0.0005 0.00008
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Figure 1
OUTLET CONCENTRATION VS AIR-TO-CLOTH RATIO
Outlet Concentration
(gr/scf X 10-2)
1.0
0.1
0.01
Approximate Emissions
(lb/106Btu)
0.01
Kramer 10-
minute cycle
Kramer 100-
minute cycle
Nucla
continuous cleaning "7
•
/Nucla
hourly cleaning
Nucla no
cleaning
Kramer no cleaning
1.0 1.4 1.8 2.2 2.6
Air-to-Cloth Ratio (acfm/ft2)
0.001
0.0001
3.0
Figure 2
SIZE DEPENDENT PENETRATION
Penetration (%) Collection Efficiency (%)
"3 90.0
Air-to-cloth ratio 1.73 acfm/ft2
100-minute cleaning cycle
Kramer
electric-aerosol
size analyzer
0.01
0.01
0.1 1.0
Particle Diameter
— 99.0
'— 99.9
99.99
10.0
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PULSE-JET TYPE FABRIC FILTER EXPERIENCE AT AIR TO CLOTH
RATIOS OF 5 TO 1 ON A BOILER FIRING PULVERIZED COAL
By: G. L. Pearson
Adolph Coors Company
Golden, Colorado 80401
ABSTRACT
Since November, 1979, a 12 module baghouse of Carter-Day design has been
in operation successfully controlling emissions from the Boiler No. 5 pulver-
ized coal fired unit rated at 450,000 LB/HR of steam. Bags are made of 22-
ounce felted "Ryton" and cleaning is accomplished only as required via a low
pressure-moderate volume, pulse-jet technique.
This paper describes the system and covers the operating experiences with
this installation during the first 15 months of operation. Data on outlet
particulate emissions, pressure drop, and cleaning cycles is presented.
INTRODUCTION
Energy availability considerations and cost projections motivated Coors
to proceed with their second coal fired boiler (Unit No. 5) in late 1976.
This unit was scheduled for start-up and went into service in November, 1979.
In the installation of this coal fired boiler, it was important to the Company
that not only permit requirements were met, but that emissions were minimized
to the lowest levels possible with current technology. However, both space
and capital dollars were at a premium to accomplish these objectives. After
study and careful consideration, a pulse-jet type modular system, offered by
Carter-Day of Minneapolis, was selected to control particulate emissions.
This modular system was ordered in April, 1978, and went into service with
the boiler in November, 1979. It has operated successfully for the last 15
months.
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BOILER DESCRIPTION
The boiler is a Combustion Engineering pulverized coal VU-40 type unit
rated at 450,000 LB/HR of steam at 825 PSIG and 850°F. Coal is pulverized
with three Raymond RB-613 bowl mills. A large fin tube boiler feedwater
economizer is utilized to cool the flue gas to within the temperature range
of 300°F to 360°F depending upon boiler load. The boiler was designed for,
and operates with, a 3 to 1 turn-down capability.
BAGHOUSE MODULE DESCRIPTION
The basic configuration of the Carter-Day Model 376 RF10 high temperature
unit is illustrated in Figure 1. Each circular unit has 376 oval pattern
bags, 10 feet long for a filter area of 4800 sq. ft. per module. (The 12
module system has 4512 bags and a filter area of 57,600 sq. ft.) Bags are
installed over and supported by carbon steel 9-gauge wire frame cages. Fly-
ash cake is accumulated on the outside of the bags and the filtered flue gas
leaves at the top of the units and goes to the induced draft fan and stack.
Bags are installed or removed through the top clam shell doors at the top of
the units.
Bags are made of 22-ounce per square yard "Daytex" felted fabric. This
fabric is essentially felted and needled "Ryton" fibers on a "Gortex" scrim.
"Ryton" is the trademark name which Phillips Petroleum has given to Polypheny-
lene Sulfide. This material has both excellent acid resistance and tempera-
ture resistant characteristics up to 370°F.
Cleaning of the filter cake from the outside of the bags is accomplished
by opening the diaphragm valve at the top of the unit and discharging the 7.5
PSIG reservoir tank into the inside of the bags directly under the nozzles on
the distribution manifold. Each reservoir tank is charged to 7.5 PSIG by a
Miehle-Dexter Model Number 46-4006 positive displacement blower. Since each
module has its own blower, tank, valve, etc., the reliability of the overall
system is quite good. Tripping of the diaphragm valve is signaled by a pres-
sure switch mounted on the reservoir tank. The distribution manifold is
turned at one rotation per minute during cleaning via a small h HP motor and
gear box to the side of the diaphragm valve. When in the cleaning mode, the
diaphragm valve pulses approximately every 3.7 seconds.
BAGHOUSE INSTALLATION AND AUXILIARIES
To insure a good, economical and easy to operate and maintain system,
considerable thought and engineering went into the installation. Figures 2
and 3 show the basic module installation configuration. The inlet and out-
let plenums were designed to balance the distribution to each module and keep
the ductwork free of ash drifts. In order to minimize troublesome and expen-
sive expansion joints, all modules and some flue ducts are supported by hang-
ing rods which allow adequate movement due to thermal expansion. The entire
system has only 10 expansion joints. Each module has a minimum leakage type
outlet damper, which can be operated remotely from the control room to either
take a module off-line or to accomplish off-line cleaning. When this damper
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CEA-Carter-Day RF376 FILTER
FOR HIGH-TEMPERATURE FLY ASH
COLLECTION IN INDUSTRIAL-UTILITY
COAL-FIRED BOILERS
DRIVE MOTOR
FOR REVERSE PULSE
AIR CLEANING SYSTEM
DIAPHRAGM
VALVE
AIR RESERVOIR TANK
FOR REVERSE PULSE
AIR CLEANING SUPPLY
FILTER TUBE SHEET
FOR HOLDING FILTER
TUBE CAGE ASSEMBLY
CEA-Carter-Day
DAYTEX FILTER TUBES
AIR INLET
COLLECTED
PARTICULATE
OUTLET
FACTORY INSTALLED
INSULATED TOP ACCESS
DOORS FOR FILTER
TUBE INSPECTION/SERVICE
HIGH TEMPERATURE
ACID-RESISTANT
GASKETING
CLEAN
AIR OUTLET
CLEANING AIR
MANIFOLD
NOTE:
FILTER BODY AND HOPPER
INSULATED IN FIELD
HOPPER ACCESS
HOLE NOT SHOWN
60° CONICAL HOPPER
FIGURE 1 MODULE GENERAL CONFIGURATION
~"~""~~~ 122
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U>
FIGURE 2 PLAN VIEV! OF MODULE ARRANGEMENT
ILLUSTRATING INLET PLENUM
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FIGURE 3
SECTIONAL ELEVATION OF MODULE ARRANGET£MT ILLUSTRATING
BY-PASS VALVES, INLET PLENUM, OUTLET PLENUM, MODULE SUPPORT
RODS, AID CONFIGURATION OF BUILDING ENCLOSURE.
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1S closed, a small valve at the top of the unit opens, automatically letting
air in to purge that particular module and prevent acid corrosion Each
module is equipped with a manually operated inlet damper for use when that
module is in a maintenance mode. There are two 60-inch diameter bypass poppet
type valves in the system which are used during start-up and oil firing of
the boiler. These bypass valves are operated from the control room and can
open against 40 inches W.G. pressure.
The conical ash hoppers are insulated with 10 inches of MT-8 mineral wool
insulation and electric blanket heaters are used on the bottom 3'-6" portion
of the cone to insure that the flyash will stay dry and convey easily. The
main portion of the module and all flue ducts are insulated with 4 inches of
MT-8 mineral wool insulation. The modules and flue ducts were metal lagged
with "Reynolds Rainlock Rib" diamond embossed aluminum 0.0165 inches thick.
The conical hoppers, and other difficult areas, were lagged with flat diamond
embossed aluminum 0.025 inches thick. High ash level alarms are installed in
each hopper.
Because of the location of the facility (adjacent to the main entrance
and visitors area of the plant), an enclosure was built for the facility to
satisfy aesthetic considerations. This enclosure is shown in Figures 2 and 3.
An opacity meter (Lear Siegler RM-41) is installed in the 13 foot diam-
eter stack downstream of the baghouse and induced draft fan. Readout of
opacity in the control room is located so as to be easily visible from the
baghouse operator panel.
The operator panel for the baghouse, located in the control room, has the
following features:
1. Pressure drop across the baghouse (inlet plenum to outlet plenum) is
displayed on a meter. The operator can select the high pressure
that he wants cleaning to start at and the low pressure where he
wants cleaning to cease on this meter.
2. Temperature of the flue gas going to the baghouse is displayed on a
meter on the control panel.
3. Conditions of either high baghouse pressure drop or high temperature
are alarmed via red lights and buzzers.
4. The module outlet dampers can be either closed or opened from this
panel to isolate a module or accomplish off-line cleaning. Damper
position is indicated by status lights.
5. Bypass valves can be opened or closed from this panel. Status
lights indicate their location at all times.
6. The status of which modules are in the clean mode is indicated on
this panel to the operator via blower motor running status lights.
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The total installed turn-key cost of this 12 module system, including the
enclosure building, was approximately $2,500,000, or roughly $8 per design
ACFM.
BAG CLEANING CYCLE LOGIC
During the design phase, Coors engineers and Carter-Day engineers mutu-
ally agreed to set up the system so that normally, cleaning would be accom-
plished only as needed to enhance bag life, blower life, and to save energy.
The blower and manifold turning mechanism motors on all modules would come on
when the pressure across the baghouse reached the high set point (for example,
5 in. W.G.). The modules would be in the clean mode until the pressure drop
across the baghouse reached the lower set point (for example, 2 in. W.G.) and
cleaning would cease until the pressure built up to the high set point again.
Shortly after start-up, this was found to not be very practical because the
cleaning was so effective that the pressure drop across the baghouse would
drop from 5 in. W.G. to 2 in. W.G. in only 20 to 30 seconds and the stack
opacity became higher than desirable for a short period of time.
To reduce the effect of this problem and to insure that nearly all bags
get pulsed when a module is cleaned, the PLC (programmable logic computer)
software was changed to clean only 4 modules at a time and run the clean
mechanism for 5 minutes at a time. Using this scheme, 8 modules are always
in the passive no-clean mode. The four modules cleaned are sequentially
stepped by the software so that each module gets the same number of 5 minute
clean cycles over a period of time. This cleaning method on a demand basis
has worked well over the current operating range of the boiler (150,000 LB/HR
to 400,000 LB/HR of steam). Generally, the baghouse is in the no clean mode
with none of the 12 clean blowers running 50% to 70% of the time.
EMISSION TESTS
Emission tests were performed at the stack of this unit on April 17 and
April 18, 1980, as part of the Compliance Tests required on the permit for
this coal fired boiler. The results of the EPA Method No. 5 particulate tests
are shown in Figure 4. The average of the three tests via F Factor method is
.023 LB per million BTU particulate (roughly \ of the permit requirements.)
Since this ratio is energy rate dependent, data is also presented with respect
to the energy bases of steam rate out and coal consumed.
The coal burned during the compliance tests had a heating value of 10,700
BTU per LB., a moisture content of 13.64%, an ash content of 5.89%, and a sul-
fur content of 0.53%, all on an as-received basis. This coal comes from Routt
County, Colorado.
During operation of this unit, the plume has been essentially non-visible
day-in and day-out to a human observer.
126
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COMPLIANCE TESTS ON BOILER NO. 5
ADOLPH COORS COMPANY
(DATA PER EPA METHOD NO. 5)
PARTICULATE EMISSION RATE
(BASED ON F FACTOR)
PARTICULATE EMISSION RATE
(BASED ON STEAM RATE)
PARTICULATE EMISSION RATE
(BASED ON COAL RATE)
BOILER STEAM OUTPUT
ACTUAL VOLUME OF
FLUE GAS THRU FILTER
AIR TO CLOTH RATIO
TEST #1
4-17-80
1200-1355
0.01 9 LB
106BTU
0.01 8 LB
1Q6BTU
0.01 9 LB
106BTU
405,000 LB
HR
276,000
ACFM
4.79
TEST #2
4-18-80
0902-1047
0.031 LB
1Q6BTU
0.029 LB
106BTU
0.021 LB
1Q6BTU
420,000 LB
HR
293,000
ACFM
5.08
TEST #3
4-18-80
1240-1424
0.01 8 LB
1Q6BTU
0.01 6 LB
1Q6BTU
0.01 6 LB
106BTU
420,000 LB
HR
288,000
ACFM
5.00
AVG. OF
3 TESTS
.023 LB
1Q6BTU
.021 LB
106BTU
.019LB
106BTU
FIGURE 4 EMISSION TEST RESULTS
127
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SUMMARY OF EXPERIENCE WITH SYSTEM
During the first 15 months of operation, this system has operated very
well and up to expectations. All felted "Ryton" bags have maintained their
integrity and there have been no bag failures. All mechanical components
have functioned without failure or repair except for a few cases with the
diaphragm valves. On a few occasions, they would stick and require some
minor repair, which generally consisted of replacing the coil spring in the
valve and adjusting it. This was accomplished by the boiler operator on duty
in about 10 minutes.
The clean only-as-required and clean only 4 modules at a time (1/3 of
the 12 modules) has generally worked well throughout the current normal opera-
ting range of the boiler (150,000 LB/HR to 400,000 LB/HR), maintaining the
pressure drop across the baghouse within the range of 2.5 to 5 inches W.G..
Through experience, it has been found that operation at a pressure drop
greater than 6 inches W.G. causes fine flyash to be driven through the felted
"Ryton" causing a higher than desired opacity. It has also been learned
that at air-to-cloth ratios, slightly above approximately 5 to 1, on-line
cleaning is not effective over a long period of time in cleaning and discharg-
ing the flyash from the bags because of reintrainment of fine flyash. This
reintrainment causes the pressure drop across the baghouse to gradually
increase. Operation at higher than roughly 5 to 1 air to cloth ratios or
recovery from a high pressure drop requires occasional off-line cleaning.
Currently, this is done by the boiler operator in the control room by closing
the module outlet damper during cleaning. Eventually, we plan to incorporate
this feature automatically via software logic in the PLC computer. These
techniques have worked quite well in bringing the baghouse back into an
acceptable pressure drop range without taking any other corrective action
such as reducing boiler load.
02/13/81
re/2U
128
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SELECTION AND OPERATION OF BAGHOUSES AT
R. D. NIXON STATION, UNIT #1
By: R. C. Hyde
JOY Manufacturing Company
4565 Colorado Boulevard
Los Angeles, California 90039
J. Arello
Lutz, Daily & Brain, Consulting Engineers
6400 Glenwood
Overland Park, Kansas 66204
D. J. Huber
City of Colorado Springs
Ray D. Nixon Power Plant
Fountain, Colorado 80817
ABSTRACT
This paper discusses the selection criteria and the subsequent success-
ful operation of two (2) baghouses placed in service at the R. D. Nixon
Station, Unit #1. These baghouses serve one (1) 220 MW boiler and have been
in continuous service since April, 1980.
This paper identifies the specification criteria deemed most important
for a successful baghouse installation and reviews the architect's decision-
making process in selection of this type of air pollution control equipment.
Results from the first eight (8) months of operation are presented including
pressure drop, opacity, outlet emission, etc.
To date, the unit has operated with low outlet emissions and low pres-
sure drop (3-4 inches, W.C.). Additionally, start-up, shut-down, and main-
tenance procedures are discussed.
INTRODUCTION
re-
Forecasted load growth and replacement of ret ired
quired that the City of Colorado Springs, Colorado install additional elect
rical generating capacity. On April 4, 1980 the Ray D. Nixon ^Jlant,
Unit #1 located in Fountain, Colorado, some 17 miles south of Colorado
Brings; "began commercial production of 220 ^ of electrical energy The
power'piant'design engineering was undertaken by Lutz Daily J Brain.
Equipment located at the plant includes a Babcock & Wilcox ^^ £
per hour steam generator, General Electric 220 MW turbine generator and
JOY Manufacturing Company "THERM-0-FLEX" baghouse system.
AIR POLLUTION CONTROL CONSIDERATIONS
The steam generator purchased by the City of Colorado Springs was a ba-
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lanced draft, pulverized coal fired unit. The unit was designed to burn a
low sulfur Western coal with a typical coal and ash composition as follows:
Coal Analysis, % Ash Analysis, %
Moisture 28.11 Phosphorus 0.85
Carbon 50.19 Silica 29.83
Hydrogen 3.54 Ferric oxide 4.88
Nitrogen 0.67 Alumina 15.75
Chlorine 0.01 Titania 1.18
Sulfur 0.32 Lime 23.52
Ash 4.87 Magnesia 5.00
Oxygen 12.29 Sulfur trioxide 15.31
Potassium oxide 0.35
Sodium oxide 1.09
Undetermined 2.24
When the Utility used a low sulfur Western fuel and acquired an
Environmental Construction Permit prior to the enactment of a
more stringent state sulfur dioxide requirement, the installation of a
wet scrubber was averted. However, it was recognized that a reliable and ef-
ficient particulate collection device would be required in order to operate
within compliance of the newly adopted New Source Performance Standards.
This meant a reduction in outlet emissions to a value no greater
than 0.1 Ib per million Btu heat input. It was also felt that although the
requirements for sulfur dioxide removal were forestalled, the ever-changing
regulations may dictate installation at a later date. A design philosophy
was therefore established. An efficient particulate removal system would be
specified and provisions for incorporation of sulfur dioxide removal equip-
ment would be made.
The state of the art in particulate removal for low sulfur Western coal
during the period in which the R.D. Nixon Station was in design was either a
hot side precipitator or a gas conditioned cold side unit. However, the
Utility's experience with both of these particulate control devices at the
Martin Drake Station spawned interest in an investigation of a fabric filter
collector, more commonly referred to as a baghouse. Little was known of this
newcomer to the utility industry and a careful review of the few utility in-
stallations was made. After an exhaustive study, most of the skepticism con-
cerning the use of the fabric filter subsided and a decision was made to is-
sue two separate sets of design specifications. One set would incorporate
the installation of a hot side electrostatic precipitator on which the utility
had experience, and the other set of specifications would be for a fabric
filter collector. Both sets of specifications announced the intent that bids
were being taken for both a hot side precipitator and a baghouse, only one of
which would be purchased by the Utility after a through evaluation.
BAGHOUSE SPECIFICATIONS
As a review was made into the possible operating parameters the baghouse
would encounter, a specification started to evolve. The filter materials
must be capable of withstanding the flue gas temperatures encountered in-
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eluding excursions. This entailed the use of fiberglass filtering material
In order to reduce wear during cleaning a reverse gas cleaning system in lieu
of a shaker or pulse jet was specified. Coupled with the reverse gas clean-
ing system, came the decision that the ratio of gas volume to cloth area
should be kept low in an effort to extend the time between cleaning cycles
thereby increasing bag life. This would have an additional benefit of an
overall reduced pressure drop.
After reviewing the various air-to-cloth ratios and their relationships
to the baghouse size and cost, a 1.9:1 gross air-to-cloth ratio was chosen.
In recognizing the size and possible problems with one unit, a minimum of two
baghouse units would be required. To keep the unit operational at a conserv-
ative air-to-cloth ratio during periods of compartment cleaning or maintenance,
the air-to-cloth ratio was specified to not exceed 2.1:1 during a period of
two compartments off-line. This required more compartmentalization but in-
sured that enough bag material would be available for safe operation.
In an attempt to provide a safe and dependable system, other features in-
corporated into the specification included a compartment door interlock sys-
tem which prevented unauthorized entrance and weather protection by hopper
and penthouse enclosures. Another important concern during the operation of
the units was the consequences of an upset condition, i.e. high temperature
or pressure. A bypass was incorporated into the design in response to the up-
set requirements.
The bypass required the tightness of a guillotine but the response of a
louver. After a review of various dampers available, a double louver system
was chosen. In order to maintain zero leakage a vent opening was placed in
the center thereby allowing ambient air to leak into the bypass instead of
particulate laden gases through the dampers.
In order to deter proposals from baghouse manfacturers who were inexper-
ienced in the operation of utility boilers, proposals were accepted from
qualified manufacturers only. A qualified supplier was one having at least
one unit of a size capable of handling gases of a quantity equal to or great-
er than 100,000 ACFM on a utility boiler. As a result, bids were received
from three manufacturers. The experience requirement and conservative spec-
ification notwithstanding, some skepticism still remained.
SELECTION OF TYPE OF EQUIPMENT AND VENDOR
As bids were received, a careful review was made of the qualified bid-
ders and a separate evaluation was performed for the precipitators and bag-
houses. Items evaluated included pressure drop, power consumption, operat-
ing and maintenance cost, escalation, etc. As a result of the evaluation,
Western Precipitation was evaluated low bidder on both the precipitator and
baghouse. These two (2) particulate collection devices were again evaluated a-
gainst each other with a resulting cost difference, based on a ten-year life,
of $305,754 in favor of the fabric filter. The evaluation was based on a two-
year life of the filter bags and maximum pressure loss through the baghouse.
It is now evident with the number of baghouses in operation that these as- ^
sumptions were quite conservative resulting in a greater cost difference be
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tween the two devices.
It was also recognized that the psychological advantages of very low
opacity with a baghouse, irrespective of load, coal or operating procedures
is difficult to assess monetarily and because of the experience with both
hot and cold side precipitators at the Martin Drake Station, there was some
doubt of the precipitator's ability to perform continuously in a satisfactory
manner at the high collection efficiency required.
Therefore a difficult decision was made; purchase a pollution control
device which did not have a significant amount of operating time but which
promised to be the state of the art in particulate collection.
CHRONOLOGY OF BAGHOUSE INSTALLATION
An award was made to JOY Manufacturing Company, Western Precipitation
Division on August 10, 1976 for the purchase of two (2) baghouses. The con-
tract encompassed design, fabrication, erection and start-up of the bag-
houses. Milestones achieved during the performance of this work are shown
below:
Purchase Order: August, 1976
Completion of Engineering: January, 1978
Commence Material Shipments: March, 1978
Mobilization at Job Site: February, 1978
Completion of Material Shipments: June, 1978
Completion of Erection: August, 1979
Initial Start-up: March, 1980
Commercial Operation: April, 1980
BAGHOUSE SELECTION CRITERIA
The baghouse system selected to filter flue gases emanating from the
boiler is described as follows:
Total Flue Gas Flow Rate: 1,071,600 ACFM
Temperature: 295*F
Inlet Particulate Concentration: 5.25 gr/ACF
Maximum Sulfur Content: 1.5%
Guaranteed Outlet Particulate Concentration: .0074 gr/ACF
Efficiency: 99.86%
Number of Baghouses: 2
Compartments Per Baghouse: 18
Filter Bags
Quantity/Compartment: 156
Quantity/Baghouse: 2808
Material: Fiberglass, Teflon coating
Size (Nominal): 12 in. X 31 ft. - 9 in.
Total Filter Area* (2 baghouses): 542,650 ft2
Gross Air-to-Cloth Ratio: 1.97:1
Net Air-to-Cloth Ratio: 2.09:1
Net/Net Air-to-Cloth Ratio** : 2.22:1
* excluding cuffs and seams
** with four (4) compartments out for cleaning and maintenance
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BAGHOUSE SEQUENCE OF OPERATION
The baghouse system supplied to the Utility was similar to that applied
at the Sunbury Steam Electric Station of Pennsylvania Power and Light Company
i.e. fiberglass filter media with straight reverse air cleaning. Sunbury was'
the first electric utility to control steam generator emissions with a bag-
house and had been in successful operation since February 1973. The reverse
air cleaning concept can be broken down into four (4) distinct operations.
1. Filtering - Dirty flue gas enters compartment hoppers through the ser-
vice damper. The gas flowing from the inside to the outside of the
filter media is relieved of particulate. Clean gas then passes through
the outlet dampers and exits the baghouse through the outlet plenum.
2. Settling - The first phase of the cleaning cycle begins as the gas out-
let dampers in one compartment close. This establishes a "no-flow" or
settle period in the compartment, allowing some of the captured partic-
ulate to descend into the hoppers for removal.
3. Reverse Air Cleaning - Gas flow is reversed through the bags when the
reverse air damper is opened exposing the section to positive pressure
from the clean side of the bags. Reverse air is provided by a fan
which draws hot filtered gas from the baghouse outlet plenum. If nec-
essary this reverse air period can be followed by another dust settling
period and then another reverse air period before the section is return-
ed to the filtering mode.
To return the compartment to the normal filtering mode, the reverse air
damper is closed and the gas outlet dampers opened. These two (2) out-
let dampers are opened on a staggered basis and with very slow cylinder
travel to reinflate the filter bags gently and avoid "popping" which
can occur if the filtering elements are returned to full flow service
instantaneously.
4. Isolation - By closing the reverse air damper, gas outlet dampers and
service damper, the compartment can be completely isolated from the gas
stream. When the upper and lower access doors are opened, fresh ambient
air flows freely through the compartment. Personnel can then enter for
inspection or maintenance.
BAGHOUSE DESIGN FEATURES
The design features deemed most important to the eventual successful
operation of the baghouses are enumerated below. These items were either
specified by the Architect/Engineer or offered as the manufacturer's stand-
ard design.
1. Low Inlet Velocity to Compartments - Flue gas is introduced into the
compartments at velocities less than 15 fps. This allows fallout of
larger dust particles prior to contact with the filtering elements,
minimizes abrasion and reduces pressure losses.
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2. Stepped Thimble Floor - In conjunction with the low velocity inlet the
stepped thimble floor promotes even dust distribution across the com-
partment, which extends filter life.
3. 12" Long Thimbles - Thimbles of one bag diameter were supplied to min-
imize abrasion as the flue gas enters each bag. Thimbles are welded in
place to ensure a 100% seal in the clean/dirty interface. Bags are at-
tached to thimbles with no tools, reducing potential for damage during
installation.
4. Ample Bag Spacing - 14" center-to-center spacing between bags ensures
that adjacent bags will not touch, eliminating a common mode of bag
failure.
5. Ratchet Tensioner - A "no-tool" stainless steel tensioner provided to
allow quick, precise tensioning of filtering elements.
6. Enclosed Access - All devices requiring routine maintenance (pneumatic
operators, high ash level detectors, etc.) are enclosed.
7. Fiberglass Filter Bags - Fiberglass filtering bags, 12" in diameter,
31'-9" long were provided. Bags supplied weighed 10 oz./yd.^ and in-
cluded a 10% Teflon coating to resist the corrosive and abrasive poten-
tial in the flue gas. Seven (7) anti-collapse rings were provided a-
long the length of the bag to ensure an open path for dust to flow dur-
ing cleaning and to minimize fold lines in the fabric which can become
points of failure.
START-UP OF THE BAGHOUSE SYSTEM
The boiler was initially fired on oil during December 1979 and January
1980 to complete the boiler boilout and steam blows. During this initial
firing of the boiler, the baghouse was bypassed. While on bypass, the vent
hatches for the compartments were opened slightly to allow outside air to
enter the compartment and exit-through the partially open outlet valve.
This eliminated the possibility of any damaging flue gas entering the com-
partments and also protected against condensation.
It was found through experience that if the compartments were closed
up tight, considerable condensation occurred.
On Monday, March 3, following completion of the baghouse checkout, bag-
house "B" was first put in service. To approximate design air-to-cloth
ratios, only a portion of the baghouse was put in service. The boiler was
operated at approximately 35% load, 70-80 MW's, hence 10 of 36 total com-
partments were initially cut in. Since the fixed point in the baghouse
structure is located in the center, the baghouse compartments were brought
into service beginning with the center compartments and working toward the
outboard compartments. This was done in an attempt to have the baghouse
expand outward from the fixed points. By maintaining design air-to-cloth
ratios, the individual compartments were heated more quickly and conden-
sation/ dewpoint concerns were minimized. As the temperature of these com-
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partments stablized, the remaining compartments were added. Within 45 min-
utes, the entire B baghouse was in service. After the initial bleed
through of particles, the filter cake formed on the bags and the opacity
stabilized at 2.5% . On Tuesday, March 4, at 9:00-a.m. the "B" baghouse was
cleaned for the first time. After one complete cleaning cycle, the differ-
ential pressure dropped to 0.9" water column.
"A"
On Tuesday, March 4, load was increased to approximately 65% and the
'A" baghouse was put in service using the same procedure. Both baghouses
were then cleaned on Thursday, March 6, when the differential pressure for
"A" baghouse was 4.1" water column and the differential pressure for "B"
baghouse was 5.2" water column. After cleaning unit "A", the differential
pressure dropped to 0.7" water column and unit "B" dropped to 1.1" water
column. The unit was at 120MW or 60% load at this point. The operating
temperature of the unit ranged from 300 F to 330 F.
Once full load operation was attained, a continuous cleaning cycle was
initiated.
The unit was declared commercial on April 1, 1980. Since that time the
unit has been base loaded at 220 MW generally between 8:00 a.m. and midnight.
From midnight to 8:00 a.m. the load is normally dropped to 50-80%.
PERFORMANCE OF THE BAGHOUSE SYSTEM
Particulate Emissions
Tests were conducted at both the inlet and outlet of the baghouse sys-
tem since system efficiency, rather than an outlet emission, was guaranteed.
All particulate tests were performed according to E.P.A. Method 17 pro-
cedures as described in the February 23, 1978, Federal Register. This pro-
cedure uses a pitot tube alongside the filter holder or thimble holder to
measure the gas velocity at the sampling points for the calculation of
isokinetic sampling. The tests consisted of 240 minutes of sampling time
with 48 test points per duct on the inlet and 10 minutes sampling time for
24 test points at the stack.
In order to conduct the tests at the design net air-to-cloth ratio of
2.09:1 a measurement of the gas volume was performed before each test
to determine the number of compartments to remove from service. Two (2) com-
partments were removed from service per unit, before the first and third
tests, and three (3) compartments per unit before the second test.
A summary of the particulate test results is contained in Table #1.
All inlet test values contained in the table are composites of the "A" & "B"
baghouse inlet test runs. Values expressed in dry standard cubic feet, DSCF,
are based on 70°F and 29.92" Hg. Efficiencies were based on the measured
inlet and outlet concentrations expressed in gr/DSCF. These measurements
for the three (3) tests were respectively: 99-94%, 99.90%, and 99.93%.
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Each test consisted of nearly simultaneous runs on the north and south
inlet ducts and the stack. All tests were conducted during normal operation
at full boiler production. No changes were made to the fabric filter clean-
ing schedule. Boiler soot blowing was conducted during the tests.
TABLE 1
Test Number 11 22 33
Location Inlet Outlet Inlet Outlet Inlet Outlet
Temperature (°F) 295 302 299 302 302 303
Particulate
Emission (gr/ACF) 1.80 0.0011 1.73 0.0017 2.02 0.0015
Particulate
Emission (gr/DSCF) 3.39 0.002 3.28 0.0032 3.84 0.0028
Efficiency 99.94 99.90 99.93
Outlet emissions on similar installations have ranged from the above
level to 0.01 gr/ACF.
Pressure Drop
The average pressure drop across the fabric filter during the tests was
3.5" water column. It ranged from 3.8" to 4.1" on the north or "A" unit and
3.1" to 3.2" on the south or "B" unit. The measured concentration of the
north inlet was greater on all tests than the south inlet by 12. to 26%. The
higher grain loading noted on the north unit influences the higher pressure
drop noted.
Opacity
The opacity at the stack during the test program was approximately four
(4) percent.
Stack opacity consistently ranges from 2-4% with no visible emissions.
Temperature Drop
With an average inlet temperature of 300-325°F temperature drop across
the baghouse system is 10-12°F.
Filter Bag Failures
Since the initial start-up, it has been necessary to replace a total of
seven (7) bags. These were all due to cuts, etc. and can be attributed to
the installation.
OPEBATIONAL AND MAINTENANCE PROBLEMS
1. One of the most time consuming problems with the baghouse is the main-
tenance and operation of the key interlock system. The Nixon baghouse has 36
compartments: each compartment has two (2) upper doors and two (2) lower doors
plus one door on the hopper totaling 180 doors. Each compartment has four (4)
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dampers (1-inlet, 1-reverse air, 2-outlet) totaling 144. The doors and door
locks on the baghouse are of the highest quality obtainable. Because the locks
on these systems are not used regularly and because of the extreme heating
and cooling and exposure to moisture and fly ash, the locks and linkages
tend to bind up. During a unit outage, when it is necessary to open all doors
for inspection, opening and closing of the baghouse doors becomes a major
project. We have found that it takes two (2) men 16 hours to open or close all
doors. To date, we have broken seven (7) keys on the interlock system due to
jamming. The interlock system on the baghouse is a carryover from the elect-
rostatic precipitator and it is felt that there is only limited need for such
a system on a baghouse. Rather, it would be sufficient to simply provide a
means of padlocking dampers in the safe position for maintenance while on
line. It should be noted that maintenance of the baghouse while on line is
less dangerous than maintenance on other power plant equipment, such as pul-
verizers, breakers, large motors, fans, etc., and that the real protection is
adherence to a good tagging and clearance procedure. The Owner's recommend-
ation on future units would be to delete the key interlock system.
2. An additional maintenance problem that has been experienced on the unit
is the expansion joints in the reverse air system. The design of the ex-
pansion joints was inadequate resulting in the failure of said joints after
approximately four weeks of operation. This did not present an operational
problem in that cleaning the bags was possible even with the expansion joints
completely missing on three sides. These expansion joints were replaced by
Western Precipitation and again failed after approximately four weeks of
operation. A third generation of expansion joints, specially designed and
manufactured by Western Precipitation, has replaced the purchased joints pre-
viously supplied. These joints include revised mounting flanges which have
resolved the problem.
CONCLUSIONS
The success of the R.D. Nixon installation, i.e. low outlet emissions,
low pressure drop and zero visible emissions stems from a number of factors:
1) Conservative air-to-cloth ratio - 2.09:1 net.
2) Large number of compartments with comparatively few bags
per compartment (156). This minimizes large-scale swings
in a A P during cleaning and maintenance operations.
3) Sound engineering procedures regarding start-up, i.e. no
precoat and bypass during oil light-off.
4) Vigilant maintenance procedures.
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POTENTIAL FOR IMPROVEMENT IN BAGHOUSE DESIGN
By: Robert M. Jensen, Bechtel Power Corporation, San Francisco, CA
ABSTRACT
This paper describes the potential for improvement in design of large,
structural, reverse air baghouses of the type now being applied to remove
fly ash from large coal-fired power plants. It is intended to demonstrate
that some currently acceptable and recommended design details cause unneces-
sary pressure loss, increase the energy required for cleaning, and decrease
bag life. In addition, the discussion indicates that these improvements are
more significant for the utility industry than for industrial users because
of differences in evaluation methods.
INTRODUCTION
The baghouse designs under consideration are limited to "large struc-
tural" or "large custom design" baghouses, especially those now being used
to remove fly ash from the flue gas of large coal-fired power plants. Shaker-
cleaned and pulse-jet baghouses are not considered. The design elements in
need of improvement are as follows.
Hopper Fallout
In technical literature, commercial brochures, and advertisements, hop-
per fallout is described as a desirable feature. It is claimed that hopper
fallout reduces the amount of material to be collected by the bags, thus
reducing cleaning frequency and resulting in less cleaning energy and longer
bag life. These claims may not be justified; in fact, the opposite may be
true.
The generally accepted expression for fabric filtration pressure loss
across the cloth and filter cake is:
AP = k x C/7000 x t x Vn
Where: AP = pressure loss in inches of water column
k£ = a constant, in. wg -r (Ib/ft )(ft/min)
Ci ~ inlet grain loading, grains/actual cu ft (ACF)
t = time in minutes between cleanings
V = cloth ratio in ft/rain
n = a number between 1 and 2.
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In using this expression for pressure loss, it is customary to use th«
value of k2 for the median particle size in the filter cake even though
it varies with particle density, particle size, gas viscosity, and gas
temperature. For a given application we can assume that particle density
gas viscosity, and gas temperature are constants. '
Particle size distribution in filter cake and hopper fallout has been
reported by Robinson et al (1) and is reproduced in this paper as Figure 1.
The variation of k^ with particle size has been reported by Noll, et al (2)
and is reproduced in this paper as Figure 2. The median size of particles
in the cake, taken from Figure 1, is shown on Figure 2. The median size of
particles in the inlet gas stream, assuming 15% wt fallout, is calculated
from Figure 1 and shown on Figure 2.
With no hopper fallout kX would be about 9.5, as opposed to 13 with
15% wt fallout. Applying this reduction as a percentage to a typical cloth
and cake differential of 4 in. wg would reduce the differential by approxi-
mately 1 in. wg.
We can hypothesize that the representative value of kj, without hopper
fallout is less; than with fallout due to the change in composition of the
filter cake. With hopper fallout the particles in the cake are small and
are all in a narrow size range. Small particles pack into a low-porosity,
dense cake. Without hopper fallout the particles in the cake have a larger
median size and a wide range of particle sizes. Filter cake with a wide
range of particle sizes is more porous and has a lower pressure loss than a
cake entirely of small particles. It also requires less cleaning energy.
If we can design an improvement to eliminate hopper fallout we can
reduce pressure loss, increase bag life, and reduce both the frequency of
and the energy required for cleaning.
Redeposition
As dirty gas rises in a bottom inlet bag its vertical velocity dimin-
ishes. In a 12-in. diameter, 30-ft long bag at a cloth ratio of 2 fpm, the
entering velocity will be about 240 fpm. Near the top of the bag the
velocity will approach zero. This velocity reduction limits the ability of
the gas to carry particles upward in the bag. As a consequence, the bottom
inlet bag serves as a sort of classifier, with the fine fraction of par-
ticles accumulating in the top of the bag.
During reverse air cleaning some of the fines at the top of the bag
will have agglomerated and will fall out; some will adhere to the cloth; and
some will float in the top of the bag and again return to it as soon as the
bag comes back on line. The latter is called redeposition.
If we take a solid sphere of fly ash, 1/2 micron in diameter, 80 lb/
cu ft specific density, and use Stokes1 Law to calculate its terminal
velocity in still air, the particle will reach its terminal velocity in less
than a second. At that velocity the time to travel 30 ft from the top of
139
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the bag to the hopper is approximately 1 year. In reality a single small
particle will never make the trip alone because its fall will be opposed by
the stack effect, or natural draft, in the bag.
We can assume that after a bag has been in service for a period of time,
a certain portion of the fines in the top of the bag will be dislodged and
returned to the bag every cleaning cycle. Obviously, this is not a cumula-
tive process because the bags do not plug solid. Therefore, it is likely
that some combination of conditions at the top of the bag releases some
agglomerated fines during cleaning.
A typical pressure loss versus time plot for a single bag or compart-
ment is shown as curve A on Figure 3. In our formula for AP we can assume
that ki = 50, C. =3, n = 2, and using values for AP and t for points on
curve fl of Figure 3, we can solve for V, the cloth ratio. A plot of V
obtained in this way, versus time, is shown on Figure 4. Note that for
curve A cloth ratio decreases with time. We can assume that the cloth ratio
of a bottom inlet bag is variable and approaches zero near the top where
redeposited fines form an impermeable cake. This cake blinds the top of the
bag and, in effect, reduces its cloth area. Thus redeposition results in
increased pressure loss and filtering capacity of a conventional baghouse
compartment during each cleaning cycle. The reduction must be compensated
by an actual cloth ratio for the baghouse that is higher than its design
cloth ratio.
A well-documented characteristic of conventional baghouses is the
gradual increase of pressure loss in new bags with time in service. It
is the author's opinion that this is caused by a decrease in filtering
capability at the top of the bag with steady usage over an extended time
period. This gradual increase in pressure loss probably occurs at a faster
rate as bag length increases.
If we assume that 10% of the bag goes blind at the top there will be an
increase of 10% in cloth ratio which will increase pressure loss by approxi-
mately 0.6 in. wg.
Spring Tension
The notion that there is a single: correct tension for each application
of bottom inlet bags is not consistent with the actual necessity to
re-tension after the bags have been in service for a period of time. It is
also inconsistent with a type of bag damage caused by inadequate tension.
A typical spring might have a spring constant of 40 Ib/in. A 12-in.
diameter, 30-ft long bag might be installed with 40 Ib of tension by com-
pressing the spring 1 in. If we assume that a new bag weighs 10 Ib (not
including the chain nor the cap, but including the anti-collapse rings), we
find that tension in the warp fibers at the top of the bag is 40 Ib while
tension at the bottom of the bag is 30 Ib. If, during a cleaning cycle, the
bag accumulates 30 Ib of particulate, the tension at the top of the bag is
70 Ib while there is zero tension at the bottom.
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The so-called correct tension is one that does not make the bag so taut
that it will not clean, nor so slack that the bottom of the bag will droop
over the top of the thimble. The initial tension may satisfy these two
requirements at certain times, but it cannot always be adequate because the
bag's weight changes during each cleaning cycle. Moreover, the tension is
always greatest at the top of the bag and least at the bottom. During
reverse air cleaning the air flow (clean gas flow) is from outside to inside
Cleaning is accomplished by: a change in cloth shape, which releases the
cake; reverse flow through the cloth, which also helps to release the cake;
and downflow in the bag, which assists gravity in moving the released par-
ticulate to the hopper.
Anti-collapse rings, spaced along the length of the bag, are used to
limit shape change. The warp fibers change from straight to a catenary
shape between the rings. In cross section the fill fibers change from a
circular shape to a multi-lobed ring with the least diameter midway between
the rings. If tension is too great these shape changes may not be adequate
to break the cake loose. If tension is too little the rounded, fluted lobes
may collapse into vertical creases and the diameter between rings may be so
small that the fall of the particulate will be impeded resulting in bag abra-
sion. Because of this variation in bag tension, there is more need for
anti-collapse rings at the bottom of the bag, where tension is reduced, than
at the top where tension is greatest. Variable tension and the irregular
spacing of anti-collapse rings in the bag cause unequal shape change during
cleaning. As a result, cleaning effectiveness is not uniform over the
length of the bag.
We can assume that cleaning efficiency is greatest at the bottom of the
bag where there is a high concentration of friable filter cake and a maximum
change in bag shape. The initial reverse flow cleans the bottom of the bag
which then becomes the path of least resistance for the balance of the
reverse flow. The top of the bag has the highest tension, the greatest
amount of cake, and a cake composition that is more difficult to remove.
Thus we have the most difficult cleaning to do at the top of the bag where
we have the poorest cleaning efficiency.
Another undesirable aspect of spring tension is bag stretch which makes
it necessary to re-tension the bags from time to time. With inadequate ten-
sion the bottom of the bag hangs down on the outside of the thimble and forms
a pocket which collects particulate. Motion of the bag causes the cloth to
rub against the particulate resulting in abrasion in this part of the bag.
When the bag goes into the cleaning mode, the excess cloth at the bottom is
blown to the inside of the thimble where it hangs down in folds. Material
falling down the bag hits one side of these folds, thus causing a second
kind of abrasion on the same part of the bag. When the bag is put back on
line it is blown to its position hanging down outside the thimble. Ihe
incoming flow of dirty gas strikes the other side of the folds, causing a
third kind of abrasion on the same cloth. This scenario, illustrated by
Figure 5, explains why the bottom of this type of bag is prone to pmhoiing,
a very common type of damage leading to bag failure.
141
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A popular explanation maintains that pinholing is caused by high inlet
gas velocities as the gas enters the bottom of the bag. One remedy, assuming
that the cause is high velocity, is to make the thimbles at least one bag
diameter in length. The vena contracta caused by the entrance will then
expire within the thimble and the flow leaving will be in streamlines paral-
lel to the cloth. However, although long thimbles may be beneficial they
cannot correct the damage caused by loose, extra cloth at the bottom of the
bag. In addition, there is some evidence that gas velocity, in the range of
200 to 300 fpm, is not the cause of pinholing.
Pinholing at the bottom of the bag may be aggravated by null periods.
Null periods are short intervals during a cleaning cycle when there is no
pressure differential across the bags. They are favored by some suppliers
who insist that they provide an opportunity for the particulate to settle
out. This theory should be questioned for several reasons. During a null
period the only force available to cause particles to settle is gravity,
which is not adequate to overcome the buoyancy of gas in the compartment
when there is a thermal gradient from the bottom to the top. If there is
any slack at the bottom of the bag, the slack will hang down inside the bag,
as shown by Figure 5, but it will move down at the beginning of the null
period and up e.t the end of the null period. This movement aggravates wear
on this portion of the bag. Null periods also lengthen the cleaning cycle,
increasing the length of time that cloth area is out of service, and pro-
longing the pressure loss increase when a compartment is off line for
cleaning. Finally, null periods increase the number of times a bag will be
flexed, which diminishes bag life. In sum, null periods do not help the
cleaning process, but rather add to pressure loss, increase cleaning energy,
and diminish bag life.
In conclusion, spring tension is detrimental to bag life and may be the
cause for particle penetration. It also contributes to uneven cleaning and
increased cleaning energy.
Recirculation
The clean reverse flow entering the compartment during reverse air
cleaning becomes a dirty flow leaving the hopper and entering the inlet duct.
This dirty flow has to be cleaned by the on-line compartments. It is a flow
that has already been cleaned and whose particulate has been previously col-
lected in other compartments. This process of returning the reverse flow
and some particulate to the inlet duct is called recirculation.
The amount of the reverse flow is; determined by design and included in
the so-called net cloth ratio. The amount of particulate in the reverse
flow is indeterminate. Recirculation undoubtedly entails pressure loss;
costs an increment of bag life; and uses an excessive amount of cleaning
energy. If recirculation increases the grain loading at the inlet to the
bags by 10%, the increase in the pressure loss will be approximately
0.4 in. wg.
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Ventilation
Before workmen may enter a compartment for inspection or maintenance
work the compartment must be purged of gas, cooled to a tolerable tempera-
ture, and ventilated by fresh air. This is usually done by closing the
outlet, reverse air, and inlet dampers so that access doors can be opened
After the access doors have been opened the inlet damper can be partially
opened so that outside air will be sucked in to purge, cool, and ventilate
the compartment. In addition, this arrangement prevents the escape of
fugitive dust from the compartment.
Without a separate ventilation system, the outside air drawn through
the compartment enters the inlet duct, Air temperature is lower than gas
temperature and air may have a significant moisture content. The ventila-
tion flow thus cools the gas and may increase its moisture content. This
can be detrimental to the bags and the house if the system is operating
near the acid dew point. Ventilation flow occurs at a time when the cloth
area of the baghouse has been reduced by taking a compartment off line and
is added to the normal flow, thus causing an increase in cloth ratio and
pressure loss. These disadvantages are diminished if the baghouse is
equipped with a separate ventilation system.
A compartment may be off line for several hours before it is safe and
cool enough for workmen to enter. Work in the compartment may be a matter
of minutes or hours. In any case, baghouse pressure loss is increased for
the duration of the outage.
Some filter cakes are hygroscopic and will pick up water from the air
during a compartment outage. The resultant wet cake, which may be somewhat
pozzolithic, may set into a hard cake, or glaze, when the bags come back on
line. During subsequent cleaning cycles some of this hard cake will fall
off and some will not. The hard cake that remains will be in patches with
sharp, jagged edges. Bag shape changes will be resisted by the remaining
hard patches of cake, and fibers will be damaged by the jagged edges.
This ventilation procedure also makes it necessary to subject the bags
and the interior of the compartment to two excursions through the acid dew
point: once on the way down and once on the way up in temperature. During
such excursions through the dew point, the cloth, the thread used in making
the bag, and the finish on the cloth suffer some damage. Damage varies in
relation to the amount of sulfur trioxide present in the cake on the bags.
These excursions will also result in corrosion of bag hardware, access door
gaskets, and the house.
Ventilation is, thus, a cause of pressure loss and a detriment to bag
life. These shortcomings can be alleviated by reducing the time required to
prepare for entry after the access doors are opened; and by reducing the
time required to clean the tubesheet, remove defective bags, and install new
bags. With or without a separate ventilation system, a better procedure for
gaining access to a compartment is one of the most needed design improvements,
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Re-entrainment
Some dust collector users store the collected material in the collector
hoppers for many hours, typically one or two shifts of 8 hours. With a
baghouse, this length of storage will span a number of cleaning cycles. As
an extreme example, a baghouse cleaning cycle might cover every compartment
twice an hour and the storage might be for two shifts. In this case material
collected during 32 compartment cleanings would be stored in the hoppers.
Depending on the inlet grain loading and the hopper size and shape,
the level of material in the hopper may be near the bottom of the inlet
duct. This would be likely in the aforementioned extreme case. If the
level of stored material is high, or if the inlet is baffled in a way that
turns the incoming gas flow downward, material may be swept off the surface
of the collected material and redeposited on the bags. This process is
called re-entrainment. One safeguard against re-entrainment requires
emptying the hoppers before the level of stored material gets too high. If
re-entrainment occurs, it leads to recollecting and recleaning which once
again wastes time and energy; increases pressure loss; and decreases bag life.
If re-entrainment were to increase the grain loading at the inlet to the
bags by 5%, the pressure loss would increase about 0.2 in. wg.
Rehandling
In this description of potential design improvements, it has been noted
that some of the particulate is collected, removed from the cloth, rede-
posited, recirculated, and re-entrained. We might call these processes
rehandling.
In Figure 6 the numbered circles are as follows:
1 - The grain loading in the gas leaving the process; i.e., leaving the air
preheaters of a coal-fired power plant.
2 - Hopper fallout.
3 - Material collected in the bags and transferred to the hopper during a
cleaning cycle.
4 - The fine particle sizes at the top of the bag which adhere to the bag
or move off of the bag at the start of cleaning and back on the bags at
the end of cleaning, i.e. redeposition.
5 - Particulate in the reverse flow returned to the inlet duct during
reverse air cleaning, i.e., recirculation.
6 - The actual inlet grain loading to each compartment. The sum of No. 1
and No. 5.
7 - Material picked up from the surface of material stored in the hopper
and redeposited on the bags, i.e., re-entrainment.
This schematic diagram illustrates the number of ways and suggests the
number of times that particulate is handled and rehandled.
It takes energy to move particulate around and to move gas through a
filter cake. If particulate in the cake is collected repeatedly, there is
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unnecessary energy loss that could be avoided through elimination of rede-
position, recirculation, and re-entrainment. Rehandling also entails
greater cleaning frequency, and greater bag abrasion, both of which reduce
bag life. The ideal baghouse design would provide for the handling of a
particle only once: collect it once and transfer it from the bag to the
hopper once.
Redeposition at about 0.6 in. wg, plus recirculation at 0 4 in we
added to re-entrainment at 0.2 in. wg., together with a hopper fallout pres-
sure loss of about 1 in. wg give a total of 2.2 in. wg, which a utility
would evaluate at approximately $2,000,000 for a 500-MW plant (3).
Conclusion
Design improvements are needed to reduce operating costs, which are of
greater concern for utility applications than for most industrial applica-
tions. Industrial users want a fast payback; they do not evaluate baghouse
use for a 35 to 40 year equipment life. Operating cost can be recovered in
the selling price of their product which is not price-regulated by a public
commission. As long as all baghouses remain so similar in design and
operating cost, the industrial user has little incentive to reduce operating
cost because his competition is buying the same kind of baghouse with the
same capital and operating costs. He is especially not interested in saving
operating cost if he must pay a higher capital cost.
Utilities, on the other hand, can add capital cost to their rate base
and may qualify for tax credit for purchasing pollution control equipment.
Utilities are very concerned with operating costs because they affect the
selling price of electricity which is regulated by a public commission and
because they have to be evaluated for a 35 to 40 year plant life.
When the market for coal-fired power plant baghouses started to develop
in the early 1970s, the expedient method of supply was with the conventional
baghouse designs that had been developed for the industrial market. Now, in
the early 1980s, at a time when there is great emphasis on conserving energy
and reducing production costs, we are still using the industrial baghouse
design for both industrial and utility applications.
In this paper the author has attempted to identify some potential
design improvements. Although individual improvements may contribute only a
small saving of pressure loss, it is possible that the total pressure loss
that could be saved would be significant. Similarly, although individual
design details may contribute only a small amount of bag damage, it is
possible that improving a number of them would result in significantly
longer bag life. In addition, the individual corrections may be syner-
gistic; correcting one deficiency may make it possible to correct another.
It is the author's hope that this paper will stimulate the development of
design improvements.
145
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3 uj go
-SIZE DISTRIBUTION IN HOPPER FALLOUT WITH
ROXIMATELY 15% FALLOUT
MEDIAN SIZE IN HOPPER FALLOUT—*
I
I
I II
10 20 100 200 300
PARTICLE SIZE, MICRONS
FIGURE 1 PARTICLE SIZE DISTRIBUTION OF
FLY ASH IN FILTER CAKE AND
HOPPER FALLOUT
300
200
SPECIFIC DENSITY OF PARTICLE «0 LB/FT3
GAS TEMPERATURE 318°F
CAKE WITH 15* HOPPER FALLOUT ^
£*i.L2.uX
20 3D 40
TIME ON STREAM, MINUTES
FIGURE 3 PRESSURE LOSS VERSUS TIME
DURING ONE CLEANING CYCLE
10 20 30 40
TIME ON STREAM. MINUTES
50 60
FIGURE 4 CLOTH RATIO VERSUS TIME
DURING ONE CLEANING CYCLE
1.0 10.0 20
PARTICLE DIAMETER. MICRONS
FIGURE 2 VARIATION OF k'2 WITH PARTICLE
SIZE
CLEANING
FIGURE 5 BAG BEHAVIOR AT THIMBLE
WITH INADEQUATE TENSION
146
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ON LINE
CLEANING
FIGURE 6 PARTICULATE RE-HANDLING
ENDNOTES
1.
2.
3.
Robinson, J.W., R.E. Harrington, and P.W. Spaite, A New Method of
Analysis for Multicompartmented Fabric Filtration. Atraos. Environment
1, 495 (1967).
Noll, K.E., W.T. Davis, and S.P. Shelton, New Criteria for the Selection
of Fabric Filters for Industrial Application. (Paper 73-301 presented
at A.P.C.A. Annual Meeting 1973).
Jensen, R.M., Baghouse Bid Evaluation.
Fabric Alternatives Forum, 1977).
(Presented at 2nd International
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REVIEW OF OPERATING AND MAINTENANCE EXPERIENCES WITH
HIGH TEMPERATURE FILTER MEDIA ON COAL-FIRED BOILERS
By
L. K. Crippen, Ph.D.
Marketing Representative
E. I. du Pont de Nemours and Company, Inc.
Textile Fibers Department
Centre Road Building
Wilmington, Delaware 19898
ABSTRACT
Experiences on full-scale coal-fired industrial boiler applications
using high velocity outside collectors fitted with Teflon® TFE-fluorocarbon
fiber felts is reviewed. Key operating and maintenance concerns which can
influence bag life and performance at high operating temperatures is
summarized.
INTRODUCTION
Today, I will narrow my presentation to cover some of our experiences
with particulate control on coal-fired boilers which use high velocity
outside collectors with high temperature filter media. I will use a
number of case studies to give a brief description of a variety of our
actual operating and maintenance experiences. In addition, key operating
and maintenance concerns as well as factors which can influence bag life
and performance are summarized.
Fisher Body's Side Stream Separator
Fisher Body, a division of General Motors Corporation, employs a
Western Precipitation pulse-jet baghouse to collect the fly ash exhausted
from the hopper of a multicyclone which acts as the primary collector
(1). Western Precipitation originally used a multicyclone instead of a
baghouse behind the initial multicyclone. Optimization of the primary
multicyclone led to collection efficiencies in the range of 9U$ - 96%, and
outlet particulate concentrations in the range of 0.30 - 0.35 Ib/MM Btu
with no hopper exhaust gas flow. When 15% - 20% of the flue gas was
exhausted through the hopper to the baghouse, the stack emission was
reduced 50% resulting in emissions of 0.12 - 0.16 Ib/MM Btu.
This improvement far exceeded what one would expect from simply
filtering 15$ - 20% of the flue gas. That would reduce the emission to
0.255 - 0.298 Ib/MM Btu, so the hopper exhaust apparently does improve the
flow pattern in the primary cyclone and increase its efficiency.
Fisher Body has installed eleven of these systems in their retrofit
program on spreader-stoker boilers. Ten of the operating units have bags
of felted Teflon® fiber and one had bags of glass felt. The Fisher Body
148
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a
design project engineers reported typical A P for Teflon®
fiber felt from U-S.. and glass felt at 5-7". The first
was installed at the Hamilton, Ohio plant of Fisher Body in September
1978, on a 52 000 Ib/hr boiler. None of these installations have had
single Teflon® bag failure. However, the glass felt has just recently
failed presumably due to the frequent dew point excursions associated with
their week-end energy saving shut downs.
Du Pont Plant Update
The Du Pont Company operates a plant in West Virginia where the
spreader-stoker boilers are equipped with a combination of multicyclone
collectors and baghouses. Boilers 2, 4, 5, and 6 have single stage
mechanicals preceding the baghouse. This installation was the first
coal-fired powerhouse equipped with pulse-jet baghouses with bags of
Teflon® felt, and several papers (2, 3, 4, 5) have been written about the
design and early performance of the equipment. Today, I will present an
update on this operation.
Operating Temperature
The baghouses run in a temperature range of 330°F (166°C) to
400°F (20iJ°C). Below 300°F (1U9°C) there is danger of sulfuric
acid corrosion to the baghouse. The coal contains about 2.6% sulfur which
does not damage the bags, but it does corrode the baghouse casing.
Bag Replacement
The baghouses have been in operation nearly six years now, and many
of the original bags are still in service. Table I shows the annual bag
replacement since start-up. So, at the end of 1980, after five years, 63$
of the bags have been replaced, approximately 59% due to wear. However,
after the first three years, no bag failures had occurred due to wear; in
the fourth year, only 1% were replaced due to wear.
The failure rate is plotted in Figure 1 (see back). The dashed line
shows the annual failures from Table I and the solid line shows cumulative
failures, with linear extrapolation past 1980. This line intersects the
"complete set" ordinate at about March of 1982, so we predict complete
failure of the original set of bags at that time. The annual failure rate
follows a bell-shaped curve, in whioh the average bag has a life of 5.3
years, and the oldest bag is expected to fail after 6.5 years.
The bags are replaced when the stack opacity increases.
Transmissometers located in the baghouse exit breeching tell the operator
that fly ash is escaping. The operator then switches compartments off
line until he finds the compartment which when valved out, makes the stack
clear up. That is the compartment with the leaking bags. The boiler is
shut down and a fluorescent powder is fed into the baghouse inlet
breeching (6). This powder, Visolite, is sensitive to black light which
makes it fluoresce. The compartment is again valved out of service and
tagged, locked, cleared, and tried. Then the operator enters the
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penthouse, opens the access doors, turns off the lights in the penthouse,
and scans the Venturis with black ultraviolet light. The Venturis that
fluoresce are marked for bag replacement.
The maintenance crew removes the marked bags, vacuums and inspects
them. A bag which has only one hole, perhaps because of a broken cage
wire, or possibly a stitching failure in the bottom disk, is sent off to a
repair shop. Bags which show general wear are discarded. New bags are
installed as needed, but so far the plant has not found it necessary to
replace all of the bags in one compartment at the same time.
TABLE I
DU PONT WEST VIRGINIA PLANT BAG FAILURES
Year Failures Remarks
1975 99 54 burned in hopper fire, 45 bottoms ripped out.
1976 39 17 burned by weld spatter, 22 bottoms ripped out.
1977 25 2 came undamped, 23 bottoms ripped out.
1978 40 Wear
1979 1069 Wear
1980 1252 Wear
End 1980 total replaced = 2,524
Complete Set of Bags = 4,018
Tennessee Plant
Du Pont operates a plant in Tennessee, which has four 145,000 Ib/hr
boilers burning 3% sulfur coal. Boilers 1 and 2 are spreader-stokers, and
3 and 4 are chain-grate stokers. Boilers 1 and 4 are equipped with
2-stage multicyclones, and boilers 2 and 3 have pulse-jet baghouses
similar to boiler 5 at the West Virginia plant. Bag life has been similar
to that at the West Virginia plant.
Staclean™ Diffusers
A. S. Johnson, Jr., Partner of Carolina Stalite Company of Salisbury,
North Carolina visited the Tennessee Plant in December, 1978 and learned
that the Teflon® TFE-fluorocarbon fiber bags operated successfully.
However, he noticed some wear had occured on the inside of some of the
bags after three years. The wear was fairly uniformly distributed around
the inner surface of the bag and limited to the top three or four feet of
the bag and was attributed to fly ash erosion.
A. S. Johnson thought about the wear and invented a solution to it.
He sketched a perforated tube which could be placed inside the cage, to
shield the bag from the erosion flow of fly ash (7). His idea developed
into a product known as a Staclean™ diffuser which has been patented in
the U.S. and has foreign patents pending. The manufacturer claims that in
addition to reducing wear, use of the diffusers results in improved bag
150
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cleaning and reduced pressure drops.
In the summer of 1980, the Tennessee plant installed a complete set
of Staclean1^ diffusers in the two baghouses serving Boiler 3 The
baghouse pressure drop was reduced from 10" water to the range of 5" to 6"
water, and that level has been maintained for several months.
Calculated Savings
It is a bit early to say how much bag life has been extended, but
after discussion with plant personnel, the following calculated savings
justify the installation of Staclean™ diffusers on Boiler 5 at the
Du Pont plant in West Virginia (Table II).
Cost for the 1,176 Staclean™ diffusers is $35 each, plus $25 each
for transportation and installation; $60 per diffuser for a total of
$71,000. Investor's method return for the installation is 59$. The
entire calculations are too lengthy to present here.
The West Virginia Plant has purchased diffusers for the #5 boiler
baghouses, and performance data are expected shortly. The Staclean™
diffusers will be installed in dirty bags to give a direct measure of
pressure drop savings attributable to the diffusers.
TABLE II
CALCULATED COST SAVINGS WITH DIFFUSERS*
Horsepower $15,000/yr
Compressed Air 8,000/yr
Bag Replacement 9,000/yr
Bag Washing 3.000Vyr
TOTAL SAVINGS $35,000/yr
* Tennessee Plant
ETS Monitored Baghouse
An Enviro-Systems baghouse on a stoker-boiler has been monitored by
ETS, Inc. through a contract from the Environmental Protection Agency.
Six hundred and forty-eight bags of Teflon® TFE-fluorocarbon fiber have
been in service for a total of three and one half years. After 27 months
at the first site, a fire occured in the main plant; however, the boiler,
baghouse, and bags were not damaged. The boiler and baghouse were moved
to a new site. The Teflon® bags were boxed and shipped to the new site
where they were reinstalled and have been in service now for an additional
15 months.
The Enviro-System's baghouse at the original installation used
reverse air flow for cleaning. However, pulse assist using approximately
90 psi was added. The baghouse currently operates between 400°F -
450°F (20U°C - 232°C) (Table III). The operating and maintenance
procedures used at this site have been well developed over the years to
151
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react to a variety of potential problems (8).
The baghouse operates 5 to 5-1/2 days a week and has gone through
many shut-downs and total cold re-starts. Eventually, off-line cleaning
is used to remove the cake which built up during cycling through dew
point. Nodules also developed and became attached to the bags at the
original site. Eventually, the bags would have to be cleaned in place
using a combination of vacuum suctioning and scraping. This procedure has
not been necessary since the addition of pulse assisted cleaning.
Despite the severe chemical and physical environment, the bag
failure rate was approximately 5%/yr for the first 27 months and
approximately 5.7%/yr for the last 15 months.
TABLE III
FLUE GAS PARAMETERS*
Type
1st Site
30,000-34,000
300°F
2nd Site
35,000-38,000
5 - 5-1/2 days/wk
- 450°F
- 232°C)
0.2 - 0.5 typical
5.7*/yr
5 - 5-1/2 days/wk
Gas Flow (ACFM):
Gas Temperature:
Inlet Loading: (gr/DSCF) 0.5 typical
Bag Failures:
Operating Cycle:
* ETS monitored baghouse.
Routine Operating and Maintenance Considerations
We have learned a lot from our field experiences in relation to
routine operating and maintenance procedures and bag use. Much has been
written about general operating and maintenance practices and procedures
(6, 9, 10). However, we are all still learning about the use of filter
media in this application. I would like to summarize some general points
which I have found to be key from ray field experiences.
Get to know your entire system.
System variables you need to become very familiar with include the
design and operation of the boiler including start-up and shut-down
procedures; baghouse design including the cleaning mechanism, fly ash
storage and removal; monitoring instrumentation; filter media; the type of
coal, fly ash characteristics, and gas stream chemistry.
Experiment with the operation of the system.
152
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to *»*»>* a good »omng relationship
Keep excellent records.
Record keeping can offer valuable assistance in solving the simple to the
complex problems. Keep accurate records of bag replacement and a diagram
showing replacement patterns for your entire baghouse. This record should
include the date of the bag change, the type of bag installed, the
location, detailed description of the failure, and an evaluation of what
contributed to the bag failure.
Develop a step-by-step problem solving approach.
Develop a step-by-step problem solving plan based on operation of
your system. The plan should be practiced and modified when necessary.
Quick action is often necessary when a problem is encountered; hence, the
need for a plan.
Obtain assistance when needed.
Unusual operating conditions should be noted immediately and
determination made as to possible causes. Be sure to examine the entire
system. Get people involved who understand all segments of the operation
including the various types of filter media. However, you should start
with your original equipment manufacturer.
Filter Media Considerations
From our field experience, I have compiled a checklist (Table IV).
TABLE IV
FILTER MEDIA CHECKLIST
Concern Check
Bag Installation Recheck bag seating/reduce bag change outs
Bag Construction Reinspect bags before installation
Chemical Degradation Choose chemical resistant media and thread
Mechanical Failure Periodic bag evaluation to predict bag life
Bag Cleaning Experiment with cleaning mode and timing;
check pulse action; evaluate air flow
Bag Sizing/Fit Follow recommendation for specific filter
media; Teflon® TFE-fluorocarbon fiber
oversized to recommendation
Cake Release Try off-line cleaning; refurbish bags, e.g.
vacuum/wash
Embers in Baghouses Cyclones before baghouse/pull ash
Bag Sizing
Our experiences in the field and laboratory indicate the need for
153
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correct bag sizing with Teflon® TFE-fluorocarbon fiber bags. Your felt
supplier, either Du Pont Fabric and Finishes Department or Albany
International, should recommend the percent of oversizing for your
specific operating conditions based on their expected shrinkage at your
operating temperature. This oversizing allows the bag to inflate properly
when pulsed thus allowing the cake to blow off. Teflon® fiber bags which
are sized too small may eventually shrink tight on the cages which can
lead to poor bag cleaning.
Due to the pliable nature of this felt, problems with flex failures
have not been observed. Stiff fabrics when oversized can wrinkle and fail
due to flexure; hence, their sizing requirements differ. Due to the range
of performance for fibers and filter media, you should use recommendations
for your specific filter media.
Washing a correctly sized bag should present no problems when done
properly. This can be a big advantage to your operating and maintenance
cost since a process upset or problem does not mean that you have to throw
all the bags away. The bags should be sent to a cleaner who has had
previous experience in refurbishing bags. It is also possible to repair
bags where small holes have occurred. In many cases, the organization
refurbishing your bags will determine whether the bag is worth repairing.
Bag Life
Bag life varies to a large extent depending on: dust characteristics,
operating conditions, operating history (temperature surges, weld holes,
embers), maintenance, and filter media characteristics.
SUMMARY
There is considerable interest in the use of filter media on high
velocity outside collectors for coal-fired boilers. From our involvement
in particulate control using fabric filters, we have learned the
importance of good operating and maintenance procedures as well as the
need for systematic problem solving techniques. Bags of Teflon® fiber
have demonstrated good performance and long life which can often reduce
operating and maintenance expenditures. Proper selection and use of bags
can contribute to good operation.
154
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REFERENCES
1. Anon., "The Side Stream Separator", General Motors Corp. (1980).
2. Lucas, R. L., "Gas-Solids Separations: State of the Art", Chemical
Engineering Progress, 70, 52 (December 1974).
3. Barrosse, B. A., "Baghouses on Stoker-Fired Boilers", American Power
Conference Proceedings, 37, 720 (1974).
4. McKee, D. E., et al, "A Review of Pulse Jet Baghouses and Dual
Mechanical Collector Performance on Spreader Stoker-Fired Boilers",
Paper No. 77-26.4, 1977 Air Pollution Control Association Annual
Meeting, Toronto.
5. Smith, J. E., "Case History of Bag Filter Operation on Spreader Stoker
Boilers", ASME Industrial Power Conference, Cleveland, OH, Oct. 23-26,
1977.
6. Bundy, R. P., "Operation & Maintenance of Fabric Filters", Journal of
The Air Pollution Control Association, 30, 754, (July 1980).
7. Lucas, et al., "The Staclean™ Diffuser Increases Capacity and
Reduces Bag Wear in Pulse-Jet Baghouses", Paper No. 80-30.4, 1980 Air
Pollution Control Association Annual Meeting, Montreal.
8. ETS, Inc., "Fabric Filter Seminar Handbook".
9. Romanski, J. E., "Reducing Opacity by Optimizing Maintenance and
Operating Procedures", Journal of the Air Pollution Control
Association, 30, 748, (July 1980).
10. Rullman, D. H., "Baghouse Technology: A Perspective", Journal of the
Air Pollution Control Association, 26, 16, (January 1976).
ACKNOWLEDGEMENTS
1. G. A. Faber and J. P. Pagan, E. I. du Pont de Nemours and Company,
Inc., Fabrics & Finishes Department.
2. T. T. Gniewek and R. H. Me Coy, E. I. du Pont de Nemours and Company,
Inc., Chemical 4 Pigments Department.
3. G. P. Greiner, ETS, Inc.
4. A. S. Johnson, Jr., Staclean™ Diffuser Company.
5. R. L. Lucas, E. I. du Pont de Nemours and Company, Inc., Engineering
Department.
6. J. N. Shah, E. I. du Pont de Nemours and Company, Inc., Textile Fibers
Department.
7. R. J. Tessier, General Motors Corporation, Fisher Body Division.
155
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CD
O
a
c.
CD
cn
O)
03
CD
"o
"c
a
6
«^uu
3600
3000
2400
1800
1200
600
f— *+\J I O *** ' ' V >»X Vr/ 1 1 1 J-/I W * V. W \— t i«l 1 k^ <-l "y*1
- 1
— 1
_ /
: /
— . ^
/
/
Cumulative /
: /
' *-+
L'" %\ Annual Failure
I // \ of Original
/' \ Set of Bags
// i
'- — — •>/ \
0
1 ! ^ 1 1*1 VI!
i i
74 76 78 '80 '82 '84
Year Average Bag Has
— Life 5.3 Years — -
After 6.5 Years Expect
Figure 1
Bag Repiacement at
Du Pont West Virginia Plant
156
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PILOT DEMONSTRATION OF THE PRECHARGER-
COLLECTOR SYSTEM
By: P. Vann Bush and Duane H. Pontius
Southern Research Institute
Birmingham, AL 35255
ABSTRACT
Results from the evaluation of a 1000 ACFM pilot scale precharger-collec-
tor system in September, 1979 justified the development of a larger scale two-
stage precipitator system. A pilot demonstration system with a 30,000 ACFM gas
volume capacity was designed and fabricated, and it was installed at TVA's Bull
Run Steam Plant. There is sufficient flexibility in the system to enable a tho-
rough demonstration of the technology.
Continuous monitors interface to a computer data acquisition system to
provide real-time mass efficiency, outlet particle size distribution, SOa con-
centration, and precipitator voltages and currents. These measurements will be
supplemented with standard stack sampling techniques for selected precipitator
conditions. Startup information and preliminary data as available at the time
of the Symposium will be presented.
INTRODUCTION
Southern Research Institute (SoRI) has a contract with the EPA Industrial
Environmental Research Laboratory (EPA/IERL) to demonstrate on a large pilot
scale the precharger-collector system for high resistivity ash collection. The
technical and economic feasibility of this two-stage design was confirmed by a
small pilot scale system evaluation. SoRI contracted Lodge-Cottrell Division
of Dresser Industries to design a two-stage electrostatic precipitator (ESP)
system with a 30,000 ACFM gas volume capacity based upon the design of the
small pilot scale system. Lodge-Cottrell was also contracted to fabricate and
erect the pilot demonstration ESP.
The evaluation of the system performance will include a computerized con-
tinuous monitoring network. Component instruments in this automated data acqui-
sition system are mass concentration monitors, particle size analyzer, S02 con-
centration monitor, total power consumption monitor, thermocouples, and voltage
and current transducers. These instruments will be used to determine the opti-
mum operating condition for the ESP. Once stable operation in this mode is
achieved, supplemental measurements and analyses of the ESP system performance
will be made using conventional stack sampling techniques.
157
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DESIGN
The design of the pilot demonstration ESP was based upon the electrode
design used in small pilot scale tests and standard ESP design practices. The
precharger stage of the ESP is a three-electrode configuration patented by
Pontius and Smith of SoRI. The arrangement of the three electrodes is shown
schematically in Figure 1. Due to the proximity of the screen electrode to the
grounded plate electrode and the necessity for separate high voltage sources
for the screen and corona discharge electrodes, the design developed for the
large pilot scale precharger had to incorporate techniques for fabricating and
erecting the component parts within tolerances more restrictive than typical in
ESP design.
The precharger stage has an active plate length in the direction of gas
flow of 0.22m. The electrodes have an active height of 3.12m. There are eight-
een gas passages in the precharger, with a plate-to-plate spacing of 0.18m, giv-
ing a total plate area of 24.7m . A 50 kV, 50 mA power supply is used on the
corona discharge electrodes which allows a maximum current density of 200 nA/cm2.
The screen electrodes are powered with a 20kV, 500 mA supply.
The collector stage of the pilot demonstration ESP was designed to allow
flexibility in the type of discharge electrode used. Otherwise, the collector
stage consists of four standard Lodge-Cottrell design electrical fields of
2.74m, 1.83m, 1.83m, and 2.74m lengths. There are three rapping fields, with
fields two and three sharing 3.66m long collection plates. The plate height in
all fields is 3.12m. There are thirteen gas passages in the collector stage,
with a plate-to-plate spacing of 0.25m, giving a total collection plate area of
742.8m2. Each electrical field is energized with a transformer-rectifier set
having a capacity of 55 kV and 200 mA.
The discharge electrode designs selected for study in the collector stage
are 2.5 cm mesh x 0.32 cm diameter wire mounted on standard Lodge-Cottrell masts
and 0.95 cm diameter rods spaced 9.2 cm apart on standard Lodge-Cottrell masts.
These special configurations were selected for their abilities to produce a low,
stable current at a high voltage, which is a desirable feature for the collector
stage of a two-stage ESP.
A flow model study was conducted by Lodge-Cottrell to assist in the design
of flow distribution splitters for the inlet nozzle to the ESP, baffles between
sections, and the flow straighteners in the inlet and outlet. The flow model
was built at one-half scale which allowed greater precision in developing design
parameters than typical of full-scale units which are modeled at one-sixteenth
or one-eighth scale.
The internal arrangement of the precharger-collector system is shown in the
schematic sectional side view in Figure 2.
INSTALLATION
The pilot demonstration ESP system was installed at the TVA Bull Run Steam
Plant. This plant was chosen because the coal source used provides a consistent
grade low-sulfur coal which produces a high resistivity ash (SxlO1 ^hm-cm at
158
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130 C) Combustion exhaust gases are extracted immediately downstream of the
air preheater, pulled through the pilot scale system, and reentered into the
plant duct. A schematic illustration of the pilot demonstration site arrange-
ment is given in Figure 3. ncmgt;
The collector-stage electrodes were installed in the ESP casing and then
aligned. The 0.95 cm diameter rod discharge electrode masts were installed
in the four collector fields. Standard Lodge-Cottrell design cam-type drop
rods are used for electrode rapping in the collector sections.
The precharger electrodes were assembled on temporary scaffolding outside
the ESP casing. This made it easier to properly align the three-electrode ar-
rangement. The assembled precharger was set into the ESP casing and final
alignment of the electrodes was performed. The electrode spacing tolerances
were not difficult to attain due to the careful fabrication and treatment of
the electrodes. Electric vibrators are used to provide ash removal from the
precharger electrodes.
The Laboratory/Control Building for the pilot demonstration ESP is repre-
sented in Figure 3. The control panels for the transformer-rectifier sets,
the power supply controls for the precharger stage, precharger vibrator control
panel, rapper controls, hopper controls, and power distribution panels are
located in the building. Also, the minicomputer used for the precharger-col-
lector system evaluation is in the laboratory.
EVALUATION
A computerized data acquisition system is used to monitor long-term oper-
ating stability and the effects of changes in the system operating values.
The secondary voltage and current values from each of the transformer-rectifier
sets and from the precharger power supplies are monitored. A wattmeter is
used to log total power consumption. A P-5A Particulate Monitor made by
Environmental Systems Corporation is installed in the ESP inlet ductwork to
measure the inlet mass loading. Another P-5A Particulate Monitor is located
in the outlet ductwork to measure the outlet mass loading. Thermocouples are
used to monitor inlet and outlet flue gas temperatures. A Lear Siegler SOa
monitor measures the S02 concentration in the flue gas; which, together with
the temperature, can be used to detect a possible change in the ash resisti-
vity. A Fine Particle Stack Spectrometer System (FPSSS) by Particle Measuring
Systems is installed in the outlet ductwork to monitor the particle size dis-
tribution.
These continuous monitors are interfaced to a POP 11/23 minicomputer. A
block diagram of the data acquisition system is shown in Figure 4. A data
handling program has been developed which acquires data from the monitors,
checks the integrity of the data, stores the acquired data on diskettes, com-
putes mass efficiency from the two P-5A monitors, displays selected data, and
plots the data if desired. The frequency and number of data samples are sel-
ectable. The computer monitor continuously displays an updated summary of the
data record.
159
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STARTUP INFORMATION
The startup of the precharger-collector system consisted of primarily six
phases: collector stage rapping check, precharger vibrator sequencer program-
ming, transformer-rectifier checkout, precharger power supply checkout, hopper
heater/ash disposal system check, and monitor data acquisition check. There
were no difficulties in energizing and operating the collector stage rapping
and transformer-rectifier sets. The precharger vibrators were programmed and
activated. The hopper heaters were turned on and the thermostatic controls
set. The ash disposal system which was integrated into the plant system was
modified to improve the operation.
As expected in a prototype system, the precharger startup was not as
straightforward as the collector stage startup. One of the precharger power
supplies was incorrectly wired. This has since been corrected. The collector
stages and the data acquisition system have been operated. We have not been
operating long enough to present any data at this time.
We plan to operate the system continuously for several weeks before a
complete characterization of performance is made, since long-term operating
experience and an assessment of reliability are essential components of the
ultimate evaluation of the system.
ACKNOWLEDGEMENTS
The authors wish to express their gratitude to Morey Nunn of Lodge-
Cottrell for his technical management of the design, fabrication, and erection
of the pilot demonstration ESP.
We appreciate the work of William Steele and Bobby Pyle of SoRI in the
development of the data acquisition programs.
160
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PRECHARGER ELECTRODE CONFIGURATION
TUBULAR
SUPPORT
GAS
FLOW
CORONA
DISCHARGE
ELECTRODE
SCREEN
ELECTRODES
GROUNDED
PLATE ELECTRODES
TO DOWNSTREAM
COLLECTOR
4100-11
Figure 1. Precharger electrode configuration.
161
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TWO-STAGE ESP INTERNAL ARRANGEMENT
FIELD NO. 1
FIELDS 2 &3
FIELD NO. 4
PRECHARGER
FIELD
GAS FLOW
DISCHARGE RAPPING
ARRANGEMENT
COLLECTOR GUIDES
& BAFFLES
BOTTOM
DISCHARGE
FRAME
— COLLECTOR
ASSEMBLY
620-249
Figure 2. Two-stage ESP internal arrangement.
-------
PILOT DEMONSTRATION ESP FACILITY
AIR PREHEATER
EXHAUST DUCT
PILOT DEMONSTRATION
ESP
ESP LABORATORY-CONTROL ROOM
620-250
Figure 3. Pilot demonstration ESP facility.
163
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DATA ACQUISITION SYSTEM
COMPUTER
P-5A MONITORS,
VOLTAGE & CURRENT
TRANSDUCERS (12),
POWER MONITOR,
SO2 MONITOR
DIFF. VOLT
AMP & I/V CONV.
SCANNING
THERMOMETER
COMMAND/STATUS ^
DATA
FPSSS
A
16 CHANNEL
A/D MUX
16 BIT
PARALLEL
I/O
INTERFACE
J
SERIAL
INTERFACE
(RS 232)
4100-101
Figure 4. Block diagram of the data acquisition system.
-------
REMEDIAL TREATMENTS FOR DETERIORATED HOT SIDE
PRECIPITATOR PERFORMANCE
By: Roy E. Bickelhaupt
Southern Research Institute
Birmingham, Alabama 35255
ABSTRACT
Laboratory experiments with parallel plate resistivity cells were
conducted to provide chemical evidence in support of the sodium depletion
phenomenon as the cause of time dependent deterioration of hot side precipi-
tator performance. Using a wire/plate corona discharge device with hand-
placed untreated and sodium conditioned ash, the effect of commercial sodium
conditioning was successfully recreated in the laboratory. Chemical analyses
and current density/time relationships suggest that the conditioning effect
occurs because sodium from the conditioned ash diffuses chemically toward the
collection plate into the zone depleted of sodium. Apparently an equilibrium
is established between sodium chemically diffusing toward the plate and
sodium migrating away from the plate under the influence of the electric
field Collection plate doping was found to be a potential remedial treat-
ment. This technique involves placing a substance having a high concentration
of charge-carrying ions on the steel plate to relieve the blocking effect of
the collection Electrode. A thin layer of high sodium, borosilicate glass
sJmulatinTa porcelain enameled plate and a film of sodium carbonate dried
from an aqueous solution worked well.
INTRODUCTION
One of the techniques used to combat «
ash is to install the electrostatic precipitator on the hot side ot
de ".r.turo 50 C a »os a l-
preheater. At the typical hot side ".r.tur^o
10 ohm cm
ashes have a resistivity lower than 1 x 10 ohm cm
C
about fly ash compositional variations.
A large number of hot side P-ipit-tors have
several years. One of the most ^^cult^roble^ OCCUIied even though
oration of performance as a function of ™*e' are properiy executed.
the precipitator design, Construction and Deration a ^ ^^
As the performance deteriorates, the electric p corona> Back
attenuates, and the current/voltage relationship sugges
165
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corona is a manifestation of dielectric breakdown of the collected ash layer
due to intense electric fields caused by excessively high resistivity.
Inconsistently, in-situ and laboratory resistivity measurements indicate that
back corona should not occur.
In a previous paper(l) an interpretation of the observed phenomenon was
given. It was suggested that a thin layer of fly ash tenaciously adheres to
the surface of the collection plate even though a good rapping schedule is
followed. After a long time under electric stress, the thin layer is
depleted of mobile charge carriers, sodium ions. Current flow through the
fly ash layer is greatly attenuated, because conduction is then dependent on a
different, less favorable mechanism. The thin, more-or-less permanent ash
layer develops a high resistivity, and consequently, the collection plate
surface becomes coated with a very good electric insulator.
To overcome this problem, the formation of the tenaciously adherent
layer must be prevented, or a source of charge-carrying ions must be made
available to the sodium depleted zone. This paper describes the results of
laboratory experiments conducted to better understand the phenomenon and to
evaluate several remedial techniques.
CHEMICAL EVIDENCE OF THE SODIUM DEPLETED LAYER
Evidence(2) obtained with chemical transference experiments to demon-
strate ionic conduction showed an increase in sodium at the cathode and a
decrease in sodium at the anode. However, these experiments could not
illustrate the very steep sodium concentration gradient suggested in
reference 1 for the ash contiguous with the electrodes, because
the thickness of the ash layers chemically analyzed were too great. Attempts
have been made to show the sodium gradient using fly ash removed from com-
mercial precipitator collection plates. While it is found that the sodium
concentration in the ash layer decreases as the collection plate surface is
approached, the very steep gradient cannot be shown. Several reasons are
suggested to explain this.
One argument is that the very thin layer depleted of sodium becomes
intermixed with the oxidation/corrosion product on the collection plate
surface. It is very difficult to remove successive 50 to 100 micron thick
layers of ash from a collection plate and maintain the cut parallel to the
metal plate. It is also conceivable that sodium can migrate back toward the
depleted zone because: a) the plates remain hot without power to the precip-
itator at the start of a shutdown, or b) the high ambient relative humidity
during shutdown facilitates reverse migration. It is believed that operators
have noted improved performance resulting from a short shutdown because of
the above factors.
To demonstrate the substantial depletion of sodium in an ash layer con-
tiguous with the anode (collection plate), several experimental prerequisites
must be met to overcome the specific points mentioned immediately above. Two
stainless steel electrodes were fabricated with recesses 0.01 cm and 0.02 cm
deep in the anode and cathode surfaces, respectively. Large fly ash particles
166
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and strong agglomerates of particles would present a problem when only the
ash contained in the recess was to be removed. Therefore the fly ash was
passed through a 200 mesh screen before use. The minus 200 mesh ash con-
tained 0.34 percent sodium expressed as oxide in weight percent. A 0.4 cm
layer of this ash was placed between the described electrodes.
To make sure that a significant chemical change occurred within the ash
layer in a reasonable time, the experiment was conducted at 470°C (880°F) in
an environment of air containing 1 percent mositure. The ash layer was
maintained under a voltage gradient of 5 kV/cm for 7 days. Conducting the
experiment near the upper end of the hot side precipitator temperature range
produced a high current density that caused the depleted zone to develop
quickly. To transport the same quantity of electric energy at about 350°C
would require in excess of 70 days, an undesirable laboratory situation.
Current density as a function of time of applied voltage gradient is
shown in Figure 1. This curve shape is typical for the current density/time
relationship during which sodium depletion takes place. The initial part of
the curve probably represents the time during which a dimensionally stable
layer of sodium depleted ash is developed. Later the charge transport
through the depleted zone is dependent on a mechanism other than sodium
migration, and the current density asymptotically approaches an equilibrium
value for this mechanism. If the anode (collection plate) were capable of
injecting carrier ions into the contiguous fly ash and the cathode were a
nonblocking electrode, the observed current density attenuation would not
occur.
The resistivity values shown in Figure 1 are from calculations made at
the start and termination of the test. The 2.5 x 10 ohm cm value shown for
the 0.4 cm ash layer at the end of the test is the effective resistivity for
several resistances in series. The most influential resistance in this
series is the sodium depleted zone.
After completing the test, the ash layer was cooled under electric
stress in dry air. The ash contained in the recessed faces of the electrodes
was quickly removed for chemical analysis. The 0.01 cm layer of ash con-
tiguous with the anode contained 0.06% sodium as oxide in weight percent,
and the 0.02 cm layer adjacent to the cathode contained 0.77%. These data
are plotted as part of a hypothetical sodium concentration profile in
Figure 2. For lack of data, the exact profile is not known. However, based
on other investigations, it seems correct that the curve should pass through
the coordinates 0.25% sodium oxide and 0.25 millimeters and that the
composition at the centerline should be equal to the starting composition.
This is the expected profile for an ionic conduction process operating with
a fixed number of mobile charge carriers between blocking electrodes. In a
precipitator showing performance deterioration, the rapping process dislodges
all of the ash layer except some fraction of a millimeter nearest the
collection plate or anode.
167
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Two somewhat crude but interesting calculations can be made using these
data. Assuming that sodium is the only conducting species with respect to
all the electric energy passed in Figure 1, about 5 x 10~ grams of sodium
per cm2 of electrode area would have migrated out of the ash layer contiguous
with the anode. Considering the subtle factors such as the ash layer poros-
ity, it was computed that the 0.01 cm thick ash layer contained about
4 x 10~5 grams of sodium per cm2. This suggests that all of the mobile
sodium in the recessed ash layer migrated.
Assume that the terminal resistivity of 2.5 x 10 ohm cm was
the resultant for two uniform layers. One layer has a thickness of 0.01 cm
and an unknown resistivity, while the other layer has a resistivity of
1.3 x 109 ohm cm and thickness of 0.39 cm. From these data and assumptions,
the resistivity of the thin, sodium depleted zone is calculated as about
1 x 1012 ohm cm. If this experiment had been conducted for a much longer
time at a conventional hot side temperature of 350°C, the computed resis-
tivity value for the depleted zone would be about 1 x 1013 ohm cm. In a
precipitator, the situation is additionally aggravated by lower temperatures
prevailing when the power generating system is not at full load and
especially during the time that the unit is being brought to full load.
RESULTS USING REMEDIAL TECHNIQUES
Reversable Polarity
In reference 1, several remedial techniques were suggested to overcome
the degraded precipitator performance caused by the development of a sodium
depleted layer of ash. One of the principal experimental techniques used to
define the character of the sodium depletion phenomenon was polarity
reversal. It was shown that 'a degraded current density/voltage relationship
created with a given polarity could be caused to recover by reversing the
corona polarity. Subsequently this technique was attempted in the field
using commercial equipment. The results were negative, and the tests termi-
nated. In principal the procedure has merit; however, the conventional
precipitator designed for negative corona is incapable of operating under
positive polarity(3).
Sodium Conditioning
Another remedial technique that has received commercial application is
sodium conditioning. Apparently some success has been obtained by injecting
dry sodium compounds into the ductwork preceding a hot side precipitator(4).
On the other hand, a totally negative result was obtained when this technique
was tried with a cold side precipitator(S).
During the past year, sodium conditioning by the addition of a sodium
compound to the coal feed has been investigated, and an interim report has
been published(6). It was found that the sodium compound decomposed and/or
volatilized in the furnace and the vapor condensed on the fly ash surface
becoming an integral part of the ash. When this conditioned ash was
168
-------
precipitated on top of the thin, sodium depleted ash layer adhering to the
collection plates, an interesting result occurred. The effect is shown in
Figure 3 where total power to the precipitator and the particulate emission
rate are illustrated as a function of time.
Precipitator washing is a common technique to obtain relief from the
sodium depletion problem for a short time, perhaps up to 60 days. Although
the washing operating does not necessarily remove the fly ash that is
tenaciously adherent to the oxidized surface of the mild steel collection
plate, the procedure apparently promotes the movement of sodium ions back
toward the collection plate. The water, penetrating the adherent layer,
carries sodium in solution or increases the surface diffusion of the sodium
toward the plate. It is believed that this is what occurs when a precipita-
tor is shut down and exposed to ambient conditions. The shutdown usually
results in improved performance for a very short time.
In Figure 3 it is shown that 50 days after washing, the precipitator
power had decreased an order of magnitude and the emission rate approached
0.3 lbs/106 BTU. At that point, sodium conditioning was started, and within
a few days, the power to the precipitator had increased about 5 times and the
emission rate was less than 0.05 lbs/106 BTU. This significant recovery
occurred even though the collection plate surfaces were covered with a thin
layer of sodium depleted ash. It would seem improbable that sodium-rich,
conditioned ash could penetrate the interstices of the sodium depleted layer
to provide a source of sodium at the plate. Therefore, it was assumed that
sodium from the conditioned ash chemically diffused toward the collection
plate.
After some period of time with a given level of sodium conditioning, the
performance characteristics are apparently controlled by an equilibrium estab-
lished between two competing processes. On the one hand, sodium ions serving
as charge carriers are motivated away from the collection plate under the
influence of the electric field created in the ash layer. On the other hand,
sodium ions from the surface of the sodium-rich, conditioned ash migrate to-
ward the collection plate under the influence of the negative gradient of the
chemical potential, i.e., the difference in sodium concentration between the
conditioned ash and the sodium depleted ash. The chemical diffusion idea is
especially plausible since the increased sodium concentration for the condi-
tioned ash is associated with the ash surface thereby offering potentially
high diffusivity coefficients.
A laboratory experiment was conducted to: a) determine whether the data
shown in Figure 3 could be reproduced in the laboratory and b) determine
whether the chemical diffusion of sodium into the depleted zone could be
detected. A previously described wire/plate corona discharge device (7) that
utilizes a 0.5 cm layer of hand-placed ash was used. An ash containing 0.32%
sodium by weight as oxide was placed on the plate and a negative 11 kV
voltage was applied. After 264 hours, the current density/time relationship
shown in Figure 4 as Phase 1 was established. At this point, the ash layer
was cooled in the dry air environment, removed from the test chamber, and
while still hot to the touch, the plate was placed on edge and gently tapped.
169
-------
About 0.5 mm of ash adhered to the "collection plate". This was quickly
overlayed with conditioned ash containing 1.8% sodium by weight as oxide and
returned to the test apparatus. The assembly was quickly heated to 345°C in
dry air, and the negative 11 kV voltage was reapplied. That portion of
Figure 4 labeled Phase 2 was then generated.
It is obvious that the combination of Phase 1 and Phase 2 in Figure 4
produces the same curve shape as that for precipitator power versus time in
Figure 3. In Figure 4, the solid symbols represent the actual data taken.
It is believed that the offset in the data was due to experimental problems
related to starting and stopping the experiment and removing and replacing
ash by hand. The dashed curve simply shows the data as if the offset had not
occurred.
In a companion experiment, the 0.5 mm layer of ash adhering to the
"collection plate" at the end of Phase 1 was removed for chemical analysis
rather than overlayed with conditioned ash. This adherent layer had a sodium
content of 0.29% by weight as oxide in contrast to the 0.32% determined for
this ash prior to the test. Chemical analysis of the 0.5 mm layer of ash
contiguous to the "collection plate" at the end of the experiment involving
the overlay of conditioned ash (Phase 2) revealed a sodium concentration of
0.39%. A very significant difference in percent of combustibles between the
original ash and the overlayed, sodium conditioned ash gave evidence that it
was unlikely that mechanical mixing of the two ash layers provided the
chemical change determined for sodium concentration.
The described experiment illustrated three important points. First,
overlaying a sodium depleted ash layer with a second layer of ash that had
received a surface enrichment of sodium by conditioning caused current density
to increase even though a voltage was continuously applied. Second, the cur-
rent density increased at a decreasing rate for three days as it approached a
limit value. Third, chemical analyses suggest the sodium concentration in the
sodium depleted zone increased slightly due to the sodium-rich, overlayed ash.
Since the positive electrode, "collection plate", was at all times
covered with a continuous layer of sodium depleted ash and there was no con-
ceivable way for the conditioned ash to penetrate this layer, it is concluded
that the sodium from the conditioned ash chemically diffused into the sodium-
depleted zone. Apparently, it required about three days for the competing
processes, inward sodium migration due to chemical diffusion and outward
migration due to electric field polarity, to equilibrate under the
particular set of circumstances.
Collection Plate Doping
Since a steel collection plate cannot furnish or give up sodium ions to
an adherent ash layer, it is called a blocking electrode and electrode
polarization occurs under negative corona. If the collection plate possessed
an unlimited source of sodium or other mobile charge-carrying ions, the
electrode blocking effect and the time dependent attenuation of current
density would not occur. It is believed that the time dependent deterioration
170
-------
of cold side precipitator performance is not common because there usually is
an unlimited source of alternate charge carriers, adsorbed sulfuric acid
vapor.
It follows that a source of charge carriers provided to the collection
plate should overcome the effect of the blocking electrode. Therefore
several laboratory experiments were conducted to evaluate the effect of col-
lection plate doping; i.e., providing the collection plate surface with a
large quantity of charge carriers. This was done in two ways: a) an aqueous
solution containing 10 percent sodium carbonate was allowed to evaporate on
an electrode surface leaving a white film, and b) a 200-micron-thick layer of
minus 325 mesh borosilicate glass containing 16.8% sodium as oxide in weight
percent was placed in a position contiguous with an electrode surface.
Three experiments were conducted using a parallel plate resistivity cell.
In each case a voltage of 2 kV was continuously applied to a 0.4 cm layer of
fly ash that contained 0.34% sodium by weight as oxide. The test cell was
maintained at 470°C in an air environment containing 1% water vapor. Anode
conditions evaluated included clean stainless steel surfaces and the two
doped surfaces described above.
Current was monitored as a function of time of applied voltage, and these
data are given in Figure 5. In each case for a very short time, the current
increased. This could be due to the establishment of good particle contact
and/or the migration of the carrier ions located in most favorable positions.
The rapid and pronounced attenuation of current with time for the electrode
set without doping is typical for the voltage applied and the test tempera-
ture. In 7 days, the current decreased by a factor of 20. Doping the anode
surface with either a thin layer of high sodium glass or a film of sodium
carbonate caused a great reduction in the rate at which the current/time
relationship decreased.
The thin layer of pulverized glass represented a simulation of a high
sodium porcelain enamel coating on a steel collection plate. After 132 hours,
the current flowing through this composite of 0.02 cm of glass and 0.38 cm of
ash was still over an order of magnitude greater than the current flowing in
the undoped specimen. The effect would be even more pronounced if the test
were run using constant current density instead of constant applied voltage.
In the case of doping with sodium carbonate, an attempt was made to
simulate the effect of ending a precipitator washing operation by saturating
the adherent ash layer with a 10% aqueous solution of sodium carbonate. This
procedure caused a great change in the current/time relationship. After 180
hours, the current flowing through the layer doped with sodium carbonate was
almost 50 times as great as the current passing through the ash layer with no
doping. Assuming that the current/time relationship for the sodium carbonate
doping experiment would eventually develop the same curve shape as the other
two curves shown in Figure 5, one would anticipate that this remedial
technique would be required on a very infrequent basis.
171
-------
To demonstrate that the desirable effect due to the deposition of a thin
film of sodium carbonate on the anode was truly related to sodium migration,
a 0.02 cm layer of ash contiguous with the cathode was chemically analyzed
after the 180 hour test was completed. This sample of ash, which at the
start of the test contained 0.34% sodium as oxide in weight percent, now
contained 4.5% sodium.
The effect of collection plate doping was also evaluated using a wire/
plate corona discharge device. A layer of minus 325 mesh borosilicate glass
0.01 cm thick was placed on the "collection plate" and was covered with 0.49
cm of low sodium ash. Current density was determined as a function of time
for a voltage of -10 kV in dry air at 372°C.
After 132 hours, the current density had decreased from an initial value
of 81 nA/cm2 to 50 nA/cm2 for an experiment without doping. In the same time
period, the current density had increased about 20% for a similar test with
doping.
CONCLUSIONS
• To accompany previously described electrical data, chemical evidence
has been obtained to demonstrate the depletion of sodium in a thin layer of
ash contiguous with a positive electrode. From the data acquired, one can
calculate that the resistivity of this thin layer of ash can be in excess of
1 x 1013 ohm cm under certain conditions.
• A laboratory experiment to simulate the effect of sodium conditioning
in a hot side precipitator has been successfully executed. These data
suggest that sodium enriched fly ash collected on top of a sodium depleted
ash layer conditions this high resistivity material by the chemical diffusion
of sodium ions toward the collection plate.
• Laboratory experiments indicate that electrode doping, supplying the
collection electrode surface with a source of charge-carrying ions, is a
potential remedial procedure to overcome the problem of time dependent
precipitator performance loss.
ACKNOWLEDGMENT
Funds for this research were principally supplied by Southern Company
Services, Inc., the Electric Power Research Institute, and the U. S. Environ-
mental Protection Agency.
ENDNOTES
1. R. E. Bickelhaupt, "An Interpretation of the Deteriorative Performance of
Hot-Side Precipitators," JAPCA j30_:882 (1980).
2a. R. E. Bickelhaupt, "Electrical Volume Conduction of Fly Ash," JAPCA
24:251 (1974).
172
-------
2b. R. E. Bickelhaupt, "Volume Resistivity-Fly Composition Relationship "
ENVIRON. SC. TECH £:336 (1975).
3. Private communication with G. B. Nichols, Southern Research Institute.
4. P. B. Lederman, P. B. Bibbo, and J. Bush, "Chemical Conditioning of Fly
Ash for Hot-Side Precipitation," EPA-600/7-79-044a, pp 79-98, Environ-
mental Protection Agency, Research Triangle Park, NC, February 1979.
5. J. P. Gooch, R. E. Bickelhaupt, and L. E. Sparks, "Fly Ash Conditioning
by Co-Precipitation-with Sodium Carbonate," EPA-600/9-80-039a, pp 132-153,
Environmental Protection Agency, Research Triangle Park, NC,
September 1980.
6. J. P. Gooch, et al, "Improvement of Hot-Side Precipitator Performance
with Sodium Conditioning - An Interim Report," JAPCA 3l_: (1981).
7. R. E. Bickelhaupt, "High Resistivity Behavior of Hot-Side Electrostatic
Precipitators," EPA-600/7-80-076, Environmental Protection Agency,
Research Triangle Park, NC, April 1980.
173
-------
CM
o
1
. RHO = 1.3 x 10** ohm-cm
ENVIRONMENT: AIR/1% WATER
TEMPERATURE: 471°C (880°F)
ASH: -200 MESH, 0.34% Na20
1-
tn
2
iu 2
a
Y-
z
111
x
§ 1
u
n
L E: 5 kV/cm
T TEST CELL: PARALLEL PLATE
•x
** — »^L^^ RHO = 2.5 x
I I I 1 1 I
""""
-
1010 ohm-cm
\
1 <
20 40 60 80 100
TIME, hours
120
140
160
Figure 1. Current density vs. time of applied voltage.
0.0
1 2 3
DISTANCE FROM ANODE, mm
4
4260-59
Figure 2. Suggested sodium concentration profile.
174
-------
©COMMENCE TESTING
© ADD SODIUM
50 60 70
DAYS AFTER WASHING
® 0.96% Na20
© 1.91% NajO
Figure 3. Chronological display of Lansing Smith Data. Data taken from
ESP which is followed by a cold-side ESP (from Reference 6).
hot-side
100
90
80
70
60
50
40
30
20
10
ENVIRONMENT: DRY AIR AT345°C
CONDITIONED ASH PLACED OVER ADHERENT
LAYER OF "DEGRADED" ASH
•f*— PHASE 2—»|
,cP"~
o-—•o _
Figure 4. Laboratory simulation of the effect of sodium conditioning.
1
a 10'4
1 1
%
cc
1 10-5
10'6
c
1 1 1 1 1
,,-—•—•• • •
•^*^ — •
— • NO DOPING ' ' ._ ~
• ANODE DOPED WITH SODIUM CARBONATE FILM
* ANODE DOPED WITH BOROSILICATE GLASS
11 1 '
30 60 90 120 150 18
TIME, hours ,„„_,
Figure 5. Effect of anode doping on attenuation of current/time relationship.
175
-------
EVALUATION OF THE UNITED McGILL ELECTROSTATIC PRECIPITATOR
By: David S. Ensor, Phil A. Lawless, and Ashok S. Damle
Research Triangle Institute
P. 0. Box 12194
Research Triangle Park, North Carolina 27709
ABSTRACT
A United McGill electrostatic precipitator installed on an industrial
coal-fired boiler was evaluated during a field test. Included in the testing
were measurements of particle size distribution from 0.05 to 10 ym diameter,
opacity, mass concentration, resistivity, and plant parameters. The particle
size dependent efficiency, rapping losses, and power requirements are re-
ported for the unit.
INTRODUCTION
Objective
The objective was the evaluation of the technology associated with the
United McGill electrostatic precipitator. The data were used to develop a
computer model describing the specific conditions in the precipitator. A
secondary objective not reported in this paper because of its exploratory na-
ture was the development of new test methods to characterize the flow distri-
bution in precipitators. This paper reports the overall performance of the
precipitator and the particle size distribution results.
Background
The United McGill is a precipitator of innovative design. Energized
plates rather than wires are used for discharge electrodes. The unusual de-
sign motivated a two-week field test program.
DESCRIPTION OF THE ELECTROSTATIC PRECIPITATOR
Design
The precipitator consists of rows of plates parallel to the direction of
gas flow. Alternate plates have needles installed on the leading and trailing
edges. These points act as corona sites when the plate is energized. The
other plates are grounded and act as collectors. Both the energized and
grounded plates are rapped from the side. Specific design features are
listed in Table 1. The plate spacing of 0.08 m is much narrower than con-
ventional precipitators. The precipitator is modular in design. It is fab-
ricated in sections at a central manufacturing facility and assembled at the
site. 176
-------
Site
The precipitators were installed on an industrial boiler fired with
eastern subbituminous coal. The coal typically was 12 percent ash, two per-
cent sulfur, and had a heat value of 12500 BTU/lb. Two separate precipita-
tors were installed in parallel with installation gate valves at each inlet.
During the tests, the boiler load was low enough to allow diversion of all
the flue gas through one precipitator.
TEST METHODS
Overview
A series of comprehensive test methods were used during a two-week field
test. However, no compliance test methods were used. The particle size
distribution and the charge-to-mass ratio of ash entering and leaving the
precipitator were determined. Supporting measurements included: ash re-
sistivity, flue gas sulfur oxides, boiler operation, and precipitator
controls.
Size Distribution
The size distribution was measured with cascade impactors for particle
diameters greater than 0.5 ym and with electrical aerosol analyzers and
optical particle counters for particles less than 5 ym diameter. The
Meteorology Research, Inc., Model 1502 Cascade Impactor was used at both the
inlet and outlet of the precipitator. Generally, the procedures described
by Wilson(1) were followed. Short sampling times of about one minute were
used at the inlet. One half of the jet holes on the last stage of the impac-
tors used in the inlet tests were blocked to reduce the particle size cutoff
by 40 percent. The outlet impactor tests were conducted for about two hours
to obtain weighable samples. The substrates, Apiezon M coated stainless
foil, were weighted on a Perkin-Elmer microbalance to 0.01 mg. Each day
four tests were conducted at the inlet and two parallel tests were conducted
at the outlet of the precipitator. The rapping emissions were also conducted
by sampling alternately with two separate pairs of impactors. One pair was
used during rapping, and the other was used between rapping.
Supporting measurements included velocity traverse and Orsat measure-
ments. Two ultrafine sampling systems were used, one at the inlet and the
other at the outlet. Both systems contained an electrical aerosol analyzer
and a Climet optical particle counter. The inlet system had three porous
wall dilution stages. One was located in the stack; the other two stages,
outside. The combined maximum dilution range was up to 2000 to 1. The out-
let dilution system was a single porous wall diluter in the stack.
Charge-to-Mass Probe
The charge-to-mass of the particulate was measured at the inlet and
outlet of the precipitator with a filter probe similar to that developed by
Denver Research Institute. A standard 47 mm filter holder was insulated
from the probe and placed in a Faraday cage. A curved inlet sampling nozzle
177
-------
was used to obtain isokinetic sampling. The charge collected by the filter
holder was measured with an electrometer with a strip chart output.
Ash Resistivity
The ash resistivity was measured with an in-situ probe developed by
Southern Research Institute using the procedures described by Smith et al.(2)
In addition, ash samples were analyzed for chemical composition and the re-
sults used in a resistivity prediction program described by Bickelhaupt.(3)
Sulfur oxides were measured by the controlled condensation method de-
scribed by Cheney and Homolya.(4) The condensate and impinger catches were
analyzed with ion chromatography.
TEST RESULTS
Overview
A summary of the test results will be presented with emphasis on the
size distribution results. The details of the study and the results of the
special test work is summarized in the final report.(5) The overall results
will be covered, first, with a discussion of the size distribution data and,
finally, aspects of the electrostatic precipitator performance evaluation.
Overall Results
The overall test results are sumarized in Table 2. The tests were con-
ducted with gas volumetric flow rates slightly higher than design. The ef-
ficiencies were consistent with previous tests at the site, and the emis-
sions were within any applicable limitations. Of particular interest was
the large emission of particles re-entrained during rapping. Often during
the test, rapping puffs were visible from the stack.
Particle Size Distribution
The results of the size distribution measurements are shown in Fig-
ures la to 2b. The data are presented in the form of dM/dlogD as a function
of particle diameter on semilogarithmic coordinates. The area under the
curve is proportional to total mass concentration. Because of the wide
range in concentrations, the cascade impactor data are plotted separately
from the fine particle results. The distribution was bimodal, similar to
that reported by Markowski et al.(6)
The rapping loss size dependence is shown in Figure 3. Over 80 percent
of the emission was due to rapping re-entrainment. The cumulative distribu-
tion of the rapping puff is shown in Figure 4. The mean diameter and
standard deviation are similar to that reported by Gooch and Marchant.(7)
Size Dependent Penetration
The particle size dependent penetration was computed by taking the
ratio of outlet to inlet particle size distributions. The particle size
178
-------
dependent penetrations are shown in Figure 5. The measurements from cascade
impactors and the fine particle measuring system are shown. It was noticed
that a discrepancy existed between the impactors taken under rapping condi-
tions and the ultrafine particle results. While good agreement existed for
the nonrapping case. The rapping emissions were of sufficient concentration
to cause unstable conditions even though an averaging volume was used.
Thus, the stable in between rapping conditions were used in the fine parti-
cle results.
Electrical Conditions
Voltage-Current Curves
The voltage-current (V-I) curves were taken at the end of each day with
the boiler and precipitator operating identically to the test condition.
One section at a time was evaluated with the other sections energized. The
V-I curves changed during the test with the operating currents increasing
by 25 to 30 percent during the test. The change in the V-I characteristic
is believed to be due to ash layer changes caused by the combustion of oil
during a pulverizer malfunction midway through the test. The average opera-
tion power is presented in Table 3. The difference between the character-
istics of Section 3 and the other sections. Hypothesis of plate alignment
or heavy plate ash deposits are inadequate to explain the discrepancy.
Power Input
The power input was obtained from the secondary voltage and current
and is reported in Table 3. The power was slightly lower on the low load
cases than reported for conventional precipitators.
Resistivity
The resistivity was measured with an in-situ probe, reported by Smith
et al.(2) and a computer prediction reported by Bickelhaupt(3). The results
are summarized in Table 3 and Figure 5. The precipitator was operated at a
temperature corresponding to the maximum on the curve.
The difference between the in-situ data and the prediction is believed
to be due to the high carbon content of the ash. The SO3 content was found
not to be important in the determination.
Modeling
The model of the precipitator is described by Lawless et al.(8) and
for that reason will not be described here. The geometry was approximated
by a single wire instead of needles.
CONCLUSIONS
The precipitator was performing as designed, in compliance with appli-
cable regulations. However, about 80 percent of the emissions measured re-
sulted from plate rapping rather than direct penetration through the
179
-------
precipitator. In addition, the emissions were found to be very dependent on
gas velocity.
ACKNOWLEDGMENT
This work was conducted as part of a Cooperative Agreement No. R-805897-03
with the U. S. Environmental Protection Agency. The cooperation of
Mr. V. F. Wilkerson of the North Carolina Finishing Division of Fieldcrest
Mills, Inc., and Dr. C. G. Noll of the United McGill Corporation is greatly
appreciated.
ENDNOTES
1. Wilson, R., Jr., P. Cavanaugh, K. Gushing, W. Farthing, and W- Smith.
Guildelines for Particulate Sampling in Gaseous Effluents from Industrial
Process. U. S. Environmental Protection Agency. EPA-600/ -79-028, 1979.
2. Smith, W. B., K. M. Gushing, J. D. McCain. Procedures Manual for Electro-
static Precipitation Evaluation. U. S. Environmental Protection Agency.
EPA-600/7-77-059.
3. Bickelhaupt, R. E. A Technique for Predicting Fly Ash Resistivity.
U. S. Environmental Protection Agency. EPA-600/7-79-204, 1979-
4. Cheney, J. L., and J. B. Homolya. Sampling Parameters for Sulfate Meas-
urement and Characterization. EST. 13:584-588, 1979.
5. Ensor, D. S., P. A. Lawless, A. S. Damle, A. D. Shendrikar, A. S. Viner,
and E. R. Kashdan. Evaluation of the United McGill Electrostatic Pre-
cipitator. Report in preparation to EPA as part of Cooperative Agreement.
6. Markowski, G. R., D. S. Ensor, R. G. Hooper, and R. C. Carr. A Submicron
Aerosol Mode in Flue Gas from a Pulverized Coal Utility Boiler. EST 14:
1400, 1980.
7. Gooch, J. P., and G. H. Marchant. Electrostatic Precipitator Rapping Re-
Entrainment and Computer Model Studies. Electric Power Research Institute
Report No. EPRI FP-792, 1978.
8. Lawless, P. A., J. W. Dunn, and L. E. Sparks. A Computer Model for ESP
Performance. Presented at the EPA Third Symposium on the Transfer and
Utilization of Particulate Control Technology, Orlando, Florida,
March 9-12, 1981.
180
-------
TABLE 1. DESIGN SPECIFICATIONS
Manufacturer
Model
Gas volume treated
(both precipitators)
Plate area
Plate height
Design SCA
Fields
Plate spacing
Plate thickness
Collector
Discharge
United McGill
4-400 x 2 EP
70.8 n>3/sec at 193°C (150,000 acfm at 380°F)
8.070 m2 (86,880 ft2)
3.05 m (10 ft)
108 m2/m3/sec (549 ft2/k acfm)
4
0.08 m (3.15 in.)
2.1 mm (0.082 in.)
1.9 mm (0.075 in.)
TABLE 2. SUMMARY OF TEST RESULTS
Condition
High load
Low load
During rapping
(high load)
Between rapping
(high load)
Boiler
/k Ib steam \
V hr /
131
126
132
132
SCA
[m2/(m3/sec) ]
92
100
96
96
Inlet
(mg/DNm3)
3970
3860
2980
2980
Outlet
(mg/DNm3)
383
99.7
828
18
Penetration
(percent)
9.65
2.58
27.8
0.6
Efficiency
(percent)
90.4
97.4
72.2
99.4
Design SCA - 108 m2/(m3/sec)
TABLE 3. OPERATING CONDITIONS
Resistivity
Measured
Predicted from ash composition
Power
93 - 122 w/(acfm/1000)
Secondary Voltage and Current:*
1
2
3
4
1 x 1013 fi-cm
2 x 1012 n-cm
Voltage (kV)
30
23.2
20
24.5
Current (ma)
27
70
110
Sulfuric Acid
Sulfur Dioxide
Excess Oxygen
Average Opacity
12 ppm
1430 ppm
9%
2-7%
* Low load conditions.
181
-------
1NLET DISTRIBUTION
300-1
INLET ULTRHFINE DISTRIBUTION
7-
6-
I
o
g
3-
2-
1-
1 10 102
PHTSICRL DIRMETER (MICROMETERS)
PHTSICfiL DIBMETER (MICROMETERS)
Figure la. Inlet particle size dis-
tribution determined by
cascade impactor.
Figure Ib. Inlet particle size dis-
tribution determined by
ultrafine sampling system.
.5-
.4-
.1-
OLJTLET DISTRIBUTION
8-
6-
4-
2-
.1
1 10
PHTSICRL DIRMETER (MICROMETERS)
OUTLET ULTRRFINE DISTRIBUTION
.01
-I—i—I I I i I I
PHTSICRL DIRMETER (MICROMETERS)
Figure 2a. Outlet particle size dis- Figure 2b. Outlet particle size dis-
tribution determined by
tribution determined by
cascade impactor.
ultrafine sampling system.
182
-------
2.S-,
RflP - NO RHP DISTRIBUTION
1.5H
1 10
PHTSJCRL DIRMETER (MICROMETERS)
IBB
99.9-1
99.64
99. &J
70-
30-
2B-
10-
5-
2-
1- '
.5-
.2-
.1--
.1
RRPPINB PUFF DISTRIBUTION
Figure 3.
Comparison of rapping
emission between rapping.
Figure 4,
PMTSIOHL DIRMETER (MICROMETERS)
Cumulative particle size
distribution of the
rapping puff.
§
i
-Bid
PENETRRTION
OPTICHL
PBRTICLE COUNTER
ELECTRICflt
REROSOL HNHLTZEfl
1MPRCTOR - NO RRP
1E-B31 1 1 iiin
.1 1 IB
PHTSICBL OIHMETER (MICROMETERS)
Figure 5. Particle size dependent
penetration through the
baghouse.
183
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X In Situ Measurements
• Computed from Ash Composition
Average
Precipitstor
Operating
Temperature
2.2 2.0
1000/T, IT1
359 441
Temperature (°F)
Figure 6. Particle resistivity from in-situ
measurement and prediction from
composition following Bickelhaupt(S)
184
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PREDICTING THE EFFECT
OF PROPRIETARY CONDITIONING AGENTS ON FLY ASH RESISTIVITY
Raymond J. Jaworowski, Technical Director, International Operations
3ohn 3. Lavin, Associate Director, Applications Engineering
Apollo Technologies Inc.
One Apollo Drive
Whippany, New Jersey 07981
ABSTRACT
Fly ash resistivity plays an important role in the performance of electrostatic
precipitators. As ash resistivities approach approximately 5 x 10 ohm-cm, opera-
tional problems occur.
Recognition of flue gas conditioning agents as a means to lower ash resistivity has
placed increased emphasis on the development of predictive methods to determine how
the resistivity will be influenced by a specific chemical.
Based on previous work by Bickelhaupt and Sparks, a correlation has been
developed to predict the effects of proprietary chemicals on fly ash resistivity. The
correlation is relatively insensitive to temperature, field strength, and coal type.
Data from field trials are used to demonstrate the effectiveness of this approach.
Excellent agreement between predicted and measured results has been obtained.
INTRODUCTION
Fly ash resistivity plays a significant role in the design of electrostatic precipi-
tators. As fly ash resistivities approach and exceed approximately 5 x 10 ohm-cm,
operational problems occur. Many of these problems can be controlled in the
specification stage, by changing various parameters in the precipitator design such as
collecting area, residence time, precipitator power, and the use of "hot side" precipi-
tators. However, when a precipitator already exists it is virtually impossible to change
most of these parameters, with the possible exception of additional power. In the case
where resistivity problems impair performance or when a projected shift in coal supply
might result in a resistivity problem it becomes important to be able to measure or
predict the resistivity of the fly ash as it exits the boiler.
With problem resistivities it is equally important to be able to predict if a
significant reduction in resistivity can be accomplished through the use of chemical
conditioning agents. This information can then be used to determine the degree of
improvement in precipitator performance to be expected through the use of fly ash
conditioning .(1)
A significant volume of work exists describing relationships between fly ash
resistivity and various other parameters, including chemical composition, flue gas
temperature, dew point, etc. (2,3,4,5) Recently Bickelhaupt and Sparks (6) published a
185
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comprehensive technique for predicting fly ash resistivity, which is based on Bickel-
haupt's previous work on calculating bulk resistivity from fly ash elemental analysis.
This report includes a computer program to calculate bulk resistivity and then allows
adjustments for flue gas moisture concentration, applied electrical stress, and sulfur
trioxide (SO-) content in the flue gas. Development of the SOjresistivity relationship
involved modifying the ASTM bulk resistivity probe (7) to allow measurement of a 1-mm
ash layer after an equilibration time of 16-18 hours. This process was then extrapolated
to estimate the effect on resistivity of additional SO, conditioning.
In view of the importance of resistivity modification to the utility industry, a
study was undertaken to correlate data obtained from actual field trials with the
existing Bickelhaupt and Sparks program, to determine if a procedure to predict the
effect of proprietary flue gas conditioning agents on fly ash resistivity could be
developed. This report describes the results of this investigation.
BICKELHAUPT/SPARKS CALCULATIONS
Bickelhaupt and Sparks have developed an expression to calculate the basic (bulk)
resistivity as a function of ash composition. This equation was then modified to include
expressions for temperature, moisture, sulfur trioxide concentration, and the ash layer
field strength. The results have been incorporated into a computer program which is
part of the published paper. The input data required and output from the program are
shown in Table I. While complete ultimate and ash analyses are desirable, the program
actually requires knowledge of only moisture and sulfur dioxide (SO_) concentrations in
the flue gas and atomic concentrations of lithium, sodium, magnesium, calcium, and
iron. The program makes a few assumptions, such as flue gas composition is based on
30% excess air, the SO, concentration is 0.4% of the SO- level in the flue gas, and that
the ash resistivity is measured at a field strength of TO kilovolts per cm. Based on
these assumptions, the output data can be plotted as shown in Figure 1 to yield a typical
temperature resistivity curve. Field tests at six power stations comparing the
predicted resistivity with actual in situ measurements showed good correlation.
CORRELATION OF APOLLO FIELD DATA
Five flue gas conditioning trials were selected where sufficient information had
been obtained to use in a correlation analysis. Table II lists the pertinent boiler,
precipitator, flue gas, and fly ash information for each station. Table III lists the
chemical additive type and feed rate, the resulting measured baseline (untreated) and
treated resistivities, and the calculated baseline resistivities. Actual moisture and SO-
concentrations were used to calculate baseline predicted resistivity values, as ultimate
coal analyses were not performed during these early trials. Field resistivities were
obtained in situ using a probe of the SRI design.
The effect of resistivity modifiers can be calculated using the expression for the
sum of parallel resistances (Equation 1) stated in terms of resistivity if the correlation
factor is known.
/>t= Pbxpc (1)
186
-------
where:
pt = treated (final) resistivity
pb = baseline (untreated) resistivity
pc = surface resistivity resulting from adsorption of
additive (correlation factor)
Rearranging Equation 1 to allow direct calculation of the additive surface
resistivity component yields the expression shown in Equation 2:
fc= Pbxpt
The additive surface resistivity values then were calculated using the measured
baseline and treated resistivity values, and are also listed in Table III. A plot of these
results vs. additive concentration is shown in Figure 2. A correlation coefficient of 0.901
was obtained from the 11 data points.
These results were very encouraging and further examination of the field data
indicated the following conclusions:
A. Field resistivity measurements were insensitive to the field strength used in
the in situ resistivity probe.
B. Temperature effects were minimal. This is due to the fact that the
additives have extremely low vapor pressures as compared to SO., over the
temperature range evaluated.
C. Additive performance response did not appear to be directly related to coal
type. The resulting equation fit Bickelhaupt's classification for eastern and
western coals, as well as Texas Lignite.
FIELD TRIAL RESULTS
Three different sites were chosen to evaluate the ability to predict resistivity
modification through the use of the correlation factor just developed. Table IV lists the
pertinent boiler, precipitator, and fly ash information for these trials. Flue gas analysis
(percent moisture, ppm SO2) was used in the program rather than the ultimate coal
analysis to calculate th^ flue gas chemistry, and the field strength was assumed to be
10 KV/cm. In all cases the baseline resistivity was calculated using the Bickelhaupt/
Sparks equation, and that result was used to calculate treated resistivities using the
correlation factor shown in Figure 2. The additive shown had been selected for use at
the stations based on previous evaluations. These results are shown in Table V.
Field measurements of resistivity were then made for both the untreated
(baseline) and treated cases. In addition, a second set of resistivity calculations was
made for the treated case using the measured baseline resistivity. All of these results
are shown in Table V. At two of the three stations (Ma and BB) excellent agreement
was obtained between the calculated and measured resistivities for both the treated and
untreated cases. In addition, the calculated resistivities from the measured baseline
were also in excellent agreement with the measured treated readings. These results
show that in most cases chemical additive performance can be predicted from either a
calculated or measured baseline resistivity level. The agreement obtained between
calculated and measured results is well within the experimental reproducibility normally
187
-------
associated with in situ field resistivity measurements.
At station "FC" (Table V) as well as at station "A" (Table III) the predicted
resistivities were significantly lower than those measured during the field trials. The
reasons for this are not fully understood at this time, but it is possible that errors in
coal analysis due to nonhomogeneity of the coal supply may have occurred. A second
possibility is that the higher ratio of sodium, potassium, and iron to calcium and
magnesium in these two cases may bias the computer calculation. Additional data is
currently being examined to determine if this inconsistency is real.
PRECIPITATOR PERFORMANCE
A significant improvement in precipitator performance was found with the use of
additives at all the stations in this study, both those for the correlation development
and for the final field trials. In many cases, improvements were greater than those
expected through theoretical considerations. (1) The field data is currently being
reviewed to determine the contribution to performance improvement by both resistivity
changes and other factors such as reentrainment, space charge modification, etc. This
information will be reported as soon as the investigation is complete, but some
preliminary results are listed in Table VI.
CONCLUSION
The Bickelhaupt/Sparks equation is a useful tool for predicting resistivity from fly
ash chemistry. By extending this equation, it is now possible to predict the effect of
proprietary chemical additives on fly ash resistivity. The developed correlation has
been tested in three field trials with excellent results. In addition to demonstrating
resistivity predictability, significant relationships between electrostatic precipitator
performance and resistivity have been observed. These relationships are being fully
evaluated and will be reported in a subsequent work.
188
-------
ENDNOTES
1. Sparks, L. E., SR-52 Programmable Calculator Programs for Venturi Scrubbers
and Electrostatic Precipitators, EPA-600/7-78-026, 1978.
2. Wagoner, C. L., et al, Fuel and Ash Evaluation to Predict Electrostatic Precipi-
tator Performance. Presented at the ASME/IEEE 3oint Power Generation Confer-
ence, Long Beach, Calif., Sept. 18-21, 1977.
3. Bickelhaupt, R. E., Influence of Fly Ash Compositional Factors on Electrical
Volume Resistivity, EPA-650/2-74-074, 197*.
*. Bickelhaupt, R. E., Effect of Chemical Composition on Surface Resistivity of Fly
Ash, EPA-600/2-75-017, 1975.
5. Maartmann, Sten, The Effect of Gas Temperature and Dew Point on Dust
Resistivity and thus the Collecting Efficiency of Electrostatic Precipitators,
Second International Clean Air Congress of the International Union of Air
Pollution Prevention Association, Washington, D.C., 1970.
6. Bickelhaupt, R. E. and Sparks, L. E., A Technique for Predicting Fly Ash
Resistivity, EPA 600/7-79-20*, 1979.
7. American Society of Mechanical Engineers, Power Test Code 28, Determining the
Properties of Fine Particulate Matter; Section 4.05, Method for Determination of
Bulk Electrical Resistivity, 1965.
189
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TABLE I: PROGRAM DATA INPUT AND OUTPUT"
Data Input
1. Ultimate Coal Analysis (%)
C, H, O, N, S, HO, ash
Coal Ash Analysis (%)
Li2O,
CaO,
Ti02,
, MgO,
S0
Field Strength (KV/cm)
Actual or 10
Used by Program
1. Flue Gas Composition
% H2O, ppmy SO2
2. Atomic
Li + Na, Mg + Ca, Fe
3. Same
Typical Program Output
TEMP 1QOO/T(K) DEG K
1.*
1.6
1.8
2.0
2.2
2.4
2.6
2.8
DEGC
DEGF RHO(VS)
RHO(VSA)
714
625
556
500
455
417
385
357
441
352
283
227
182
144
112
84
826
666
541
441
359
291
233
183
2.2E+09
1.6E+10
1.1E+11
6.3E+11
1.2E+12
7.3E+11
2.6E+11
2.1E+10
2.2E+09
1.6E+10
1.1E+11
3.9E+11
7.1E+10
5.5E+09
-
_
where:
RHO(VS) = Resistivity excluding SO- effect
RHO(VSA) = Resistivity including SO, effect
190
-------
TABLE II; BOILER AND FLY ASH DATA FOR CORRELATION
Station:
Load (Mw):
Boiler Manufacturer:
ESP SCA (ft2/ 1000 acfm):
ESP Temperature (°F):
Firing Method:
C
350
C.E.
135
250
P.C.
Coal Type: Western
Proximate Coal Analysis
Ash (%):
Sulfur (%):
Moisture (%):
Flue Gas Analysis
Moisture (%):
SO2 (ppmy):
Elemental Coal Analysis*
Si02 (%):
A1203(%):
Fe203 (%):
MgO (%):
CaO (%):
Na-O (%):
/
K20 (%):
Li2O (%):
6
0.5
12
6
370
64.68
22.23
3.37
0.40
7.34
0.69
1.29
#•»
G
100
C.E.
209
360
P.C.
Western
5-8
0.6
6.5
7
500
54.7
18.5
8.9
0.8
16.7
0.5
0.8
*#
Mo
575
C.E.
186
380
P.C.
Lignite
13.5
0.5
31
15.5
490
55.92
26.02
4.30
2.58
10.32
0.11
0.75
**
A
146
C.E.
352
350
P.C.
Eastern
13.5
1.7
3.7
7
1250
52.4
29.8
11.6
0.1
2.3
4.9
2.7
0.1
B
700
C.E.
145
275
P.C.
Eastern
13.8
1.5
6.5
8
800
50.82
24.07
19.21
0.52
1.45
0.62
3.31
**
*Corrected to 100% total.
**Not analyzed; assumed less than 0.1%.
191
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TABLE III: CHEMICAL ADDITIVES AND RESISTIVITY DATA FOR CORRELATION
Station
Mo
A
B
Measured
Surface
Fly Ash Resistivity
Additive
LPA-40
LPA-410M
LPA-40
LPA-410
LPA-40
LPA-40
(GPT)
0
0.10
0.15
0.15
0.30
0.40
0.50
0
0.20
0
0.15
0.20
0
0.15
0
0.15
Treatment Rate
(ppm )
™»_*_™™y_
0
3.2
4.7
2.1
4.8
6.3
7.9
0
6.9
0
9
12
0
4.4
0
5.1
Resistivity
Factor
(10 ohm-cm) (10 oTim-cm)
6.9 (3.0)*
3.0
1.7
2.4
1.5
0.9
0.7
5.1 (3.5)*
0.8
3.5 (5.8)*
0.5
0.2
10.0 (0.03)*
3.5
1.0 (2.4)*
0.40
5.3
2.3
3.7
1,9
1.1
0.8
_
0.95
—
0.6
0.2
_
5.4
_
0.7
*Calculated Baseline Resistivities from Bickelhaupt/Sparks Program
GPT = Gallons of Additive per Ton of Coal Fired
192
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TABLE IV; BOILER AND ASH DATA FROM FIELD TRIALS
Station:
Load (Mw):
Boiler Manufacturer:
ESP S.C.A. (ft /1000 acfrn):
ESP Temperature (°F)
Firing Method:
Coal Type:
Proximate Coal Analysis
Ash (%):
Sulfur (%):
Moisture (%):
Flue Gas Analysis
Moisture (%):
S02 (ppmv):
Elemental Coal Analysis*
Si02 (%):
A1203 (%):
Fe203 (%):
MgO (%):
CaO (%):
Na20 (%):
K20 (%):
Li2O (%):
Ma
400
C.E.
175
280
P.C.
Eastern
16.4
1.5
1.1
7
8000
64.2
26-4
5.7
0.1
1.0
0.7
3.5
**
BB
575
C.E.
156
370
P.C.
Lignite
14
0.75
24
13
1000
62.0
17.4
6.6
1.5
13.3
1.1
1.2
*#
FC
800
B&W
165
225
P.C.
Western
23
0.8
4.3
9
750
58.91
28.11
4.72
0.64
3.97
2.04
1.61
**
*Corrected to 100% total.
**Not analyzed; assumed less than 0.1%.
193
-------
TABLE V; RESISTIVITY RESULTS
Treatment Rate Fly Ash Resistivity (10 ohm-cm)
Station Additive (GPMl(ppm ) Calculated* Measured Calculated**
Ma LPA-410M 0 0 3.0 1.3
0.10 4.1 1.* 0.81 0.89
0.20 8.2 0.5* 0.34 0.44
BB LPA-410 0 0 2.0 3.0
0.20 9 0.4 0.4 0.42
FC LPA-40 0 0 0.33 3.9
0.10 3.7 0.21 2.3 1.7
*From calculated baseline
**From measured baseline
TABLE VI; MEASURED PRECIPITATOR PERFORMANCE
Treatment Rate Predicted Measured
Station Additive (GPT) Efficiency (%) Efficiency (%)
A LPA-40 0 94.9 92.2
0.15 94.9 99.7
Ma LPA-410M 0 92.0 91.5
0.20 97.4 98.6
BB LPA-410 0 85.3 95-9
0.20 95.1 99.3
194
-------
1013
1012--
o
I
'.c
o
101!.
cc
re
00
109
LEGEND:
O RHO (VS)
A RHO (VSA)
1000/T (OK) 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2
(°C) 64 84 112 144 182 227 283 352 441 560
(°F) 141 183 233 291 359 441 541 666 826 1041
TEMPERATURE
FIGURE 1 - PREDICTED RESISTIVITY FOR THE COAL AND ASH USED
TO ILLUSTRATE THE COMPUTER PROGRAM(6)
195
-------
o
I
o
o
O
<
TO
111
114-
00
LU
CtL
LU
C_3
10
10
log f>c = 12.0607 - (0.1515 x ppmv additive)
O
(D
LEGEND:
O LPA-40
0 LPA-410
0 LPA-410 M
Coefficient of Correlation
R = -0.901
O
4 56 7 8 9 10
ADDITIVE CONCENTRATION (ppmv)
11
12
FIGURE 2 - EFFECT OF ADDITIVE ON SURFACE RESISTIVITY FACTOR
196
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S03 CONDITIONING
TO ENABLE
ELECTROSTATIC PRECIPITATORS TO MEET
DESIGN EFFICIENCIES
BY:
J. J. FERRIGAN III
WAHLCO, INC.
SANTA ANA, CALIFORNIA 29704
This is a case study involving a particular utility's and precipitator
manufacturer's problem of selecting an effective and reliable method of
increasing the collection efficiencies of two new electrostatic precipitators
to their design specifications. This paper traces the study from the time
of erection of the two cold-side precipitators in the early 1970's to the
final solution which was 803 flue gas conditioning in 1979.
We detail the mechanical modifications and consultant's recommendations
which were undertaken to no avail. We point out that after all possible
mechanical fix-ups had been completed, proprietary chemical conditioning
of the flue gas was tried, which proved to be more detrimental than good.
Finally, S03 flue gas conditioning was tried, and it brought both cold-side
precipitators into design compliance. These facts are supported by
precipitator outlet grain loading tests, which show that 803 flue gas
conditioning enabled the precipitators to operate under the .02 gr/scf limit.
INTRODUCTION
This is a case study concerning two cold-side electrostatic
precipitators designed to collect fly ash generated from the burning of
low sulfur Eastern Bituminous Coal. (See Table 1). The two electrostatic
precipitators referred to as "A" and "B", were erected in 1973 and designed
to accommodate a flue gas volume of 110,000 ACM each. The total specific
collecting area (SCA) for each precipitator is 600-K The first three fields
have transformer-rectifiers rated at 700 MA and the last four fields are
rated at 1000 MA. The two "NEW" electrostatic precipitators are preceded
by two "OLD" electrostatic precipitators which had been in operation for
twenty years (See Figure 1). The two "NEW" electrostatic precipitators were
designed to operate at 99.8% efficiency with the "OLD" precipitators off line.
197
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In the first quarter of 1975 the precipitator manufacturer conducted
the first acceptance tests on "A" precipitator. (See Table 2). These
results indicated that there was trouble ahead. The migration velocities
were extremely low with an average of 2.54 cm/sec. The outlet grain loading
was a factor of ten away from the design outlet grain loading option of
0.02 gr/scf. The power levels of the precipitators were also extremely low.
The highest reading obtainable on all seven T/R sets was 100 MA on the last
field. These results pointed to a variety of possible problems.
SOLUTIONS
The utility and precipitator manufacturer spent the next two years
investigating the problems with "A" and "B" precipitators. They tried
several methods of increasing the efficiency of the precipitators. Listed
below are the actions that were taken to increase the efficiency and the
results of ehese actions.
1. Power off rapping was initiated by the precipitator manufacturer
which resulted in no improvement of precipitator efficiency.
2. Saddle weights were added to the existing wire weights to reduce
slack in the discharge electrodes. This step was initiated by the precipitator
manufacturer with no improvement of precipitator efficiency,
3. Flail hammer rappers were installed to measure the effectiveness
of this form of rapping versus the low frequency magnetic impulse rappers.
This program was initiated by the precipitator manufacturer with no
improvement of precipitator efficiency.
4. Lowering of the air heater outlet temperature to reduce the resistivity
of the fly ash was attempted. This step was initiated by the utility and
resulted in no improvement of precipitator efficiency.
5. An independent consultant was retained to determine why the
precipitator would not perform when low sulfur coal was being fired in the
boilers. The consultant's final conclusion was that further rapping improvement
efforts should be initiated. He also stated that the gas distribution was
acceptable.
6. A second consultant was retained. His conclusions were that
the gas distribution was poor and the subject precipitators should be fully
optimized by including a new rapping system, correcting gas distribution
and adding transformer-rectifiers. He also stated that gas conditioning,
utilizing some form of sulfur oxide input into the gas stream, would be
quite effective in lowering resistivity and increasing the efficiency of
the precipitators.
7. Finally the precipitator manufacturer considered the use of flue
gas conditioning as a possible solution and decided to pursue proprietary
chemical additives as the method.
198
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PROPRIETARY CHEMICAL GAS CONDITIONING
The proprietary chemical gas conditioning system was selected over S03
flue gas conditioning solely due to capital cost. The total capital cost
for the proprietary chemical gas conditioning system vas approximately
$60,000.00 versus $180,000.00 for the S03 flue gas conditioning system.
The proprietary chemical gas conditioning system chosen is a process
which involves injecting a liquid additive conditioning agent upstream of
the air preheater to reduce the resistivity of the fly ash. The chemical is
injected upstream of the air preheater to allow for vaporization of the
conditioning agent. The chemical constituents of the additive would not be
divulged by the manufacturer. The initial injection rate of the conditioning
agent was 0.1 gal/ton of coal which was eventually raised to 0.3 gal/ton.
This conditioning agent had no effect on either the stack opacity or the
power in the subject precipitator. Eventually, their chemical caused pluggage
of the air preheater (5" pressure drop across the air preheaters) . The utility
had the chemical analyzed and found it to be ammonium sulfate, (Nlfy^SO/!, and
60% H20.
After failing with the ammonium sulfate, the utility was approached by
the precipitator manufacturer and asked their permission to. inject a new
chemical. The utility had this new chemical analyzed and found it to be
Sodium Bisulfate (NaHSOz^) and 60% 1^0. This chemical was very acidic (PH<1)
and was very corrosive and difficult to handle. The utility allowed this
new conditioning agent to be tried for a thirty-day period. (See Table 3).
The initial injection rate of this conditioning agent was 0.1 gal/ton and
eventually was raised to 0.5 gal/ton with no effect on the outlet grain
loading. The emission rate was not within a factor of ten of meeting the
outlet option for contract design guarantee which was 0.02 gr/scf. In addition,
after only one week of injection of the chemical, the 304-stainless steel
atomizer tip on the injection nozzle was completely corroded away.
Another major disadvantage of the proprietary chemical conditioning
system was the operating cost. The operating cost for injecting 0.3 gal/ton
of this chemical into each boiler "A" and "B" was approximately $18,000.00/month.
Needless to say the utility's view of flue gas conditioning was not
very optimistic after this episode, but nevertheless 803 flue gas conditioning
had to be considered the only other viable solution to the existing problems
with the subject precipitators.
S03 FLUE GAS CONDITIONING
From correspondence with other major utilities it was discovered that
they were using, quite successfully, S03 flue gas conditioning to help
maintain particulate collection efficiency and opacity requirements. After
consulting these electric utilities, field trips were conducted to observe
the systems in operation. The major point that was made clear by the other
operating utilities was that S03 injection would allow the precipitators to
perform as well as they would with high sulfur coal.
199
-------
The utility had very good results when burning high sulfur coal in
boiler "A". One specific group of tests conducted in 1974, by the utility,
with coal that had an average sulfur content of 3.94% and an average ash
content of 20.75%, yielded an outlet grain loading of 0-011 gr/scf for
precipitator "A". Based on this and other data, it was decided that the
precipitator manufacturer should pursue 863 flue gas conditioning as a final
solution.
The precipitator manufacturer purchased 863 flue gas conditioning
equipment to condition the fly ash going to both "A" and "B" precipitators.
This was an S02 source system consisting of a liquid 802 storage tank, liquid
S02 vaporizer, air filters, fans, SCR controlled air heaters, 802/803 catalytic
converters and injection probes. Process instrumentation provided failsafe
operation and automatic adjustment of 803 production in response to boiler load.
The system was designed to inject 40 PPM of 803 into both flue gas streams of
110,000 ACFM at 30QOF.
TEST RESULTS WITH S03 FLUE GAS CONDITIONING
In October 1979, the 803 flue gas conditioning system was installed and
operational. The same quality low sulfur coal was being fired as when the
proprietary chemical gas conditioning system was being tested. The utility
had agreed that the outlet option of 0.02 gr/scf would be sufficient in lieu
of a precipitator efficiency test. Initially there was a seven-day optimization
period while injecting 803 prior to testing. Improved electrical readings
were observed during this time. (See Table 5). After, this waiting period,
outlet grain loading tests were conducted on "A" precipitator. The first
three tests surpassed the guarantee, averaging 0.0157 gr/scf. (See Table 4).
After these tests were completed "B" precipitator was tested and also surpassed
the 0.02 gr/scf limit.
Not only did the 803 flue gas conditioning system enable the precipitator
to meet design specifications, it also allowed the utility to support and
maintain a system with a low operating cost. Based on an injection rate of
20 PPM, the total operating cost for both Units "A" and "B" was only $2,400/month.
CONCLUSION
The final decision to install an 803 flue gas conditioning system was a
mutual agreement reached by both the precipitator manufacturer and the utility.
Thousands of dollars could have been saved if high resistivity had been
recognized initially as the problem with "A" and "B" precipitators. Tests conducted
in 1974 proved that the burning of high sulfur coal enabled the precipitators to
meet design specifications.
Hopefully this case study will enable precipitator manufacturers and
the electric utilities to apply this information to the design of cold-side
electrostatic precipitators in the future. The feeling for many years was
that if you make them big enough they will work. This is not always true as
shown in this case. After evaluating all possible solutions, one must agree
that 803 flue gas conditioning is a reliable and cost efficient method in
200
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enabling cold-side electrostatic precipitators to meet federal particulate
emission standards while collecting lox* sulfur coal fly ash.
Table 1 DESIGN SPECIFICATIONS FOR ELECTROSTATIC PRECIPITATORS "A" & "B"
Process
Suspended Material
Fuel
Gas Source
Gas Volume
Gas Temperature
Gas Moisture
Pressure
Fuel Specifications:
Ash
Sulfur
Guaranteed Efficiency
Outlet Option
Surface Collecting Area
Steam Generation
Fly Ash and Fume
Coal (Eastern Bituminous)
Boilers (Steam Generators A & B)
110,000 ACFM (Each)
325 to 350°F
4 to 5% By Weight as Fired
Negative
25%
1%
99.8%
0.02 Gr/SCF
600-f SCA (Each)
201
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Table 3 PRECIPITATOR TEST RESULTS USING PROPRIETARY CHEMICAL
CONDITIONING ON PRECIPITATOR "A" (1978)
(TESTS CONDUCTED BY INDEPENDENT CONSULTANT)
PROPRIETARY
TEST NO.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
DATE
8/10
8/10
8/10
8/11
8/11
8/14
8/14
8/15
8/15
8/15
8/16
8/16
8/16
8/17
8/17
8/17
8/18
8/18
8/18
8/21
8/21
8/21
8/22
8/22
8/23
8/23
8/23
8/24
8/24
8/24
STEAM FLOW
(103 LBS/HR)
180
182
185
181
185
162
171
171
170
174
176
182
180
178
179
180
183
185
187
175
177
178
163
172
168
166
173
161
166
166
Oo
£.
3.5
3.5
3.7
3.6
3.8
3.2
3.7
3.4
3.5
3.4
3.4
3.5
3.3
3.6
3.6
3.8
3.8
3.8
3.7
3.7
3.7
4.0
3.1
3.8
3.7
3.8
3.1
3.7
3.6
3.6
CHEMICAL
(GPT)
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0
0
0
0.1
0.1
0.1
0
0
0
0.2
0.3
0.3
0.4
0.4
0.4
0.5
0.4
0.5
0.5
0.5
0.5
0.5
0.5
EMISSION RATE
(GR/SCF)
0.33
0.31
0.39
0.33
0.34
0.31
0.28
0.32
0.27
0.31
0.33
0.25
0.31
0.38
0.37
0.36
0.40
0.28
0.27
0.40
0.27
0.35
0.18
0.26
0.24
0.24
0.27
0.29
0.28
0.25
Note: Outlet Option for Contract Design Guarantee was 0.02 Gr/SCF
202
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Figure 1. Typical Schematic of Gas Flow For Units "A" & "B"
TABLE 2
Precipitator "A" Test Results, Acceptance Tests M975) Without SO3 Conditioning
(Tests Conducted by Precipitatoi Manufacturer)
Test
No.
j
2
3
4
5
6
Inlet
Loading
(Gr/SCF)
2.85
5.05
5.20
4.52
4.67
4.39
Outlet
Loading
(Gr/SCF)
0.212
0.277
0.269
0.250
0.186
--
to
CM/SEC
2.199
2.450
2.507
2.531
2.728
Efficiency
<%)
92.56
94.51
94.83
94.47
96.02
Gas
Volume
(ACFM @ Outlet)
125,100
97,000
97,700
107,600
98,800
n^ f\r\f\
Temperature
(°F Outlet)
288
276
275
279
287
077
7
8
9
10
11
3.84
4.88
4.82
4.97
0.216
0.173
0.231
0.225
0.171
2.549
2.622
2.583
2.593
2.636
95.08*
95.49
95.27
95.33
95.56
97,900
90,600
91,800
99,100
96,400
267
270
268
276
•Based on Test 6 Inlet, Test 7 Outlet, Standard Conditions 70°F, 29.92" Hg.
Note- Guaranteed Design Efficiency was 99.8%.
Note: Tests 1 And 8 Were Recorded With The Old "A" Precipitator On Line.
Typical Coal Analysis
Proximate Analysis as Received
Moisture, % 3.45
Heating Value, BTU/Lb 10,698
Ash, % 23.13
Sulfur, % 1.04
203
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Table 4 PRECIPITATOR TEST RESULTS USING S03 CONDITIONING ON
PRECIPITATOR "A" (1979)
(TESTS CONDUCTED BY PRECIPITATOR MANUFACTURER)
TEST NO.
Location
% Moisture
% Oxygen
% C02
Gas Temperature (°F)
Gas Volume (ACFM)
Emission Rate (Lbs/MMBTU)
Gr/SCF
Outlet
5.7
5.0
1A.5
300°F
85,710
0.0147
0.0186
Outlet
5.2
5.0
14.5
300°F
85,621
0.0112
0.0140
Outlet
7.2
5.0
14.5
300°F
85,981
0.0117
0.0145
Note: Outlet Option for Contract Design Guarantee was 0.02 Gr/SCF
Note: 1. Average Outlet Grain Loading from New "A" Precipitator
with S03 Conditioning 0.0157 Gr/SCF
2. Average Inlet Grain Loading into New "A"
Precipitator with Old "A" Off 4.81 Gr/SCF
3. Estimated Efficiency Based on Inlet
Assumptions
A. Migration Velocity Based on Above Data
99.9967%
8.74 Cm/Sec.
TYPICAL COAL ANALYSIS
Proximate Analysis as Received
Moisture, % ...
Heating Value, BTU/Lb . . .
5.51
10,613
204
Ash, %
Sulfur, %
23.28
1.20
-------
Table 5 ELECTRICAL READINGS OF PRECIPITATOR "A" WITH AND WITHOUT
S03 INJECTION WHILE BURNING LOW SULFUR COAL
1. WITH S03 INJECTION, 10/19/79
DC-KVOLTS AC-VOLTS
SPKS
AC-AMPS DC-MA
Fields A
B
C
D
E
•p
45
55
55
50
55
290
290
300
380
400
v-; oi A i
30
40-50
0-20
0-20
0-20
25
20
25
60
85
100
75
100
380
560
S03 Injection Rate - 23 PPM
Opacity - 5-7%
2. WITHOUT S03 INJECTION, 11/2/79
DC-KVOLTS AC-VOLTS SPKS AC-AMPS DC-MA,
Fields A
B
C
D
E
t?
43
48
47
45
47
230
200
220
250
260
Field
20
40
20
30
20
Trioped Off
10
10
10
15
26
50
10
10
50
120
503 Injection Rate - 0 PPM
Stack Opacity - 25-30%
205
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ENHANCED PRECIPITATOR COLLECTION EFFICIENCIES
THROUGH RESISTIVITY MODIFICATION
By: Dennis F. Mahoney
Apollo Technologies Inc.
Whippany, New Jersey 07981
ABSTRACT
High resistivity process dusts and fly ashes from low sulfur coal may be
modified by chemical treatment to improve cold-side precipitator performance.
A new series of additives which are effective in lowering resistivity is
described. The additives are non-corrosive and of neutral pH. Laboratory
resistivity test results are presented to show that treatment is insensitive
to ash composition or coal variations. Reductions from lO^-lO12 ohm-cm to
<108 ohm-cm have been achieved. Field results on a pilot precipitator and a
utility boiler show power and opacity improvements.
INTRODUCTION
It is well known that ash from low sulfur coals presents collection
problems for electrostatic precipitators (ESP's). This is especially true
for those that were designed for high sulfur coal or those that are in poor
mechanical condition. The reason for these collection difficulties is also
well known: ESP's are most effective in collecting fly ash or other particles
with resistivity values of about 10" ohm-cm. Fly ash resistivity values
are typically higher than this at cold-side temperatures; however, if suffi-
cient sulfur is present in the coal, some of it will be oxidized during
combustion to give 803 which may then condense on the fly ash surface to
give a conductive film of sulfuric acid. Without this conductive film the
resistivity will be a function of the particle composition and temperature.
The resistivity will be determined by the pathway giving the lowest value.
Above 400°F or in dry air the water layer is desorbed and the bulk resistiv-
ity values are measured. Bulk resistivity is inversely proportional to
temperature while the surface resistivity is determined by adsorption mech-
anisms which are directly proportional to temperature. In general, fly ash
bulk resistivities are too high for effective collection at cold-side ESP
temperatures of 275-375°F, and with low sulfur coals the S03 and moisture
levels are too low to reliably condition the ash at these temperatures.
In this paper the degradation mechanisms and some of the cures for
ESP's collecting high resistivity fly ash will be briefly reviewed. The
main portion of the paper will then report on new flue gas conditioners
(FGC's) which are designed to overcome some of these difficulties.
EFFECT OF RESISTIVITY ON ESP OPERATION
Ash particle charging and the subsequent collection are determined by a
number of factors including field strength, particle size and dielectric
constant. Particle resistivity affects the collection process indirectly; as
ash is collected a layer builds up on the plates, increasing the resistance
206
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across the precipitator. This lowers the voltage and in turn the collection
efficiency.
The poor charge transfer of high resistivity ash also makes it more
difficult to rap from the plates. Consequently, the clean plate powers seen
when a unit comes back from an outage or after an air sweep will deteriorate
with time. Table 1 shows the timewise degradation of an electric utility ESP
collecting ash from low sulfur coal.
Table 1. EFFECT OF TIME ON ESP POWERS*
Days After Airsweep Precipitator Powers (KVA)
0 179
1 174
2 148
3 160
4 134
5 79
* Operating temperature is approx. 275°F. Coal supply
is 1.0-1.2% sulfur.
A number of different approaches have been used to counteract the
effects of high resistivity ash. These include larger ESP's, hot-side
ESP's, S03 injection and flue gas conditioning.
Increasing ESP size suffers from the obvious disadvantage of increasing
construction costs. For example, a recent article (1) in the Journal of the
Air Pollution Control Association estimated that ESP construction costs were
$35/kW for high sulfur coal and $85/kW for low sulfur coal. This difference
is due to the greater collection area and higher field strengths specified
by ESP manufacturers for high resistivity ash.
Another mechanical alternative is the hot-side precipitator. By
positioning the ESP before the air heater, these are designed to take ad-
vantage of the lower bulk resistivity in the 500-700° region. Unfortun-
ately, a number of the low sulfur coals burned also have high bulk
resistivities resulting in a similar problem.
One alternate approach to mechanical modifications is to lower the ash
resistivity to match it to the particular ESP. As mentioned above this can
be done with S03, NH3 or other flue gas conditioners. Sulfuric acid has a
higher dew point than water, but its performance is also limited. It is re-
ported (2) to be most effective on basic ashes and to lose its effectiveness
as the temperature increases. Other FGC additives are usually solutions
which spray dry or vaporize in the gas stream. A well known example of
these is Apollo's LPA-40®. FGC's are commonly molten salts which prevent
rapper reentrainment or lower resistivity. As with sulfuric acid they may be
ash specific or react with the ash to be deactivated.
207
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RESULTS
In trying to develop an FGC additive to avoid these problems, a number
of factors were considered including
a liquid material for easy handling
neutral pH to prevent corrosion or injury
• low feed rates for economy and ease of injection
ash independence to minimize the effects of coal changes
chemically inert to prevent pluggage or deactivation.
The results obtained can be understood in terms of physical properties, fri-
ability and resistivity lowering.
Physical Properties
The additives discussed here are similar to previous FGC's in that they
are water based. They are also near neutral in pH. Ash reactive chemicals
are not present or are in relatively low concentrations resulting in less
interaction with the ash.
Deposit Friability
Under ideal conditions FGC's are sprayed into the gas stream evenly at
low levels of 0.1-1% of the ESP ash loading. In practice, however, air
heaters, cold spots and internal ductwork often provide sites where the
additive and ash can collect together to give deposits which may disrupt or
block the gas flow. If the two then react together or sinter, the deposit
may become hard enough to resist easy removal. This behavior is most ex-
pected with high salt additives and reactive ashes.
With the new materials hard buildups do not occur. Laboratory
simulations were made by mixing up to 5-10% of additives with ash and heating
at 400°F for 4-16 hours. The result was a dry friable ash. In field trials
a worst case situation was simulated by placing a cooled deposit probe di-
rectly in front of an FGC injection nozzle. This resulted in a moist
deposit buildup which rapidly eroded after the FGC was shut off. By con-
trast a standard high salt FGC had a hard, sintered deposit which could only
be removed with great difficulty.
Resistivity Modification
Even fly ashes with different compositions typically have very similar
bulk resistivities. Figure 1 shows the dry air resistivity curves for two
bituminous and one lignitic ash. Table 2 gives their elemental analyses.
208
-------
13
E 12
o
11
110
O
8
200 250 300 350 400 500
Temperature, (° F)
600
Figure 1. Resistivity Of Various Fly Ashes As A Function Of Temperature
In Dry Air. A And C Are Bituminous Ashes And B Is Lignitic.
Analyses Are In Table 2.
209
-------
TABLE 2. FLY ASH COMPOSITION
Oxide. % A B
Si02
A1203
Fe20i!
CaO
MgO
Na20
45.9
21,2
11.2
2.2
0.5
0.3
32.3
12.4
5.6
12.4
6.3
4.2
50.6
24,4
6,4
2,1
0,8
1.3
Ashes A and C are bituminous while B is lignitic. The most noticeable
difference in this composition is the higher calcium oxide leyel in B, but
the iron levels also vary.
The surface resistivities of these ashes vary considerably as can be
seen by Figure 2. Here the water is much more strongly adsorbed onto ash B,
resulting in greater resistivity reductions. This is more representative of
actual field conditions where flue gas moisture is 5^10%,
Conventional flue gas conditioners such as ammonium sulfate lower these
resistivity values. Typically, 0,5% by weight on the ash lowers the resis-
tivity by about one half order of magnitude,
Our first efforts to develop liquid modifiers yielded thermally unstable
materials which were very effective at low temperatures but actually in-
creased the resistivity at ESP temperatures. Figure 3 illustrates the effect
of one of these formulations on ash A. Comparison of the curves at different
moisture levels shows that the additive is much more effective than water at
low temperatures, but that as the temperature increases the additive effec-
tiveness drops off, and resistivity actually rises above the water
conditioned values. The shaded areas in Figure 3 indicate regions in a nor-
mal ESP temperature range where resistivity reductions (T<300°F) and
increases (T>300°F) would be seen.
This problem was overcome and treatment rates greatly reduced by
reformulation. The curves in Figure 4 show the results of treating ash B.
The shaded region between the untreated curves is bounded by the operating
temperatures and moisture levels found in a typical cold^side ESP. Treated
resistivity over this temperature range is low and essentially constant,
with performance only beginning to drop off above 350-400°F. Lower treat-
ment levels of less than 1% give similar curves but displaced slightly up-
ward. Higher treatment values give off-scale resistivities.
In actual practice it may not be desirable to lower resistivity by such
a large amount. Figure 5 illustrates the effect of a lower activity addi-
tive on two different fly ashes. Note that ash C is modified considerably
over all ESP operating temperatures but that ash B, which has a lower resis-
tivity, is only improved slightly in a narrower range. With sufficient
experience these effects can be predicted by the ash elemental analysis and
210
-------
13
E 12
o
11
110
O
8
200 250 300 350 400 500
Temperature, (°F)
Figure 2. Effect Of Surface Conditioning By
20% H20 In Air On Resistivity
600
211
-------
13 L
8
200
Treated (5%)
250 300 350 400 500 600
Temperature, (°F)
Figure 3. Effect Of Additive X On Ash Resistivity
See Discussion In Text.
212
-------
13 -
3.1% Additive Y
(10% H2O)
200 250 300 350 400 500 600
Temperature, (° F)
Figure 4. Effect Of Additive Y At 1% On Ash
Resistivity As A Function Of Temperature
1. Untreated, 5% H20; 2, Untreated,
10% H20; 3. 1% Additive Y; 10% H20
213
-------
13
E 12
o
11
110
O
8
3. Ash B
Treated
200 250 300 350 400 500
Temperature, (°F)
600
Figure 5. Effect Of Additive Z On Differing
Fly Ashes Using 10% H20 Atmosphere
Curve 1 - Ash C, Untreated;
Curve 2 - Ash B, Untreated;
Curve 3 - Ash C, treated
214
-------
untreated resistivity curves.
Actual Precipitator Studies
Effects on an actual ESP bear out the resistivity reductions seen in
the laboratory. Table 3 shows the effects on powers and opacities on a pilot
ESP derated with high resistivity ash. Table 4 shows similar results on an
actual field unit.
TABLE 3. PILOT PLANT RESULTS*
Untreated
ESP Voltage (kV)
• inlet
• outlet
Opacity
21
24
18
Treated
30
36
8
Percent Change
+42
+50
-55
* 270-310°F operating temperature, ash C
TABLE 4. FIELD ESP RESULTS
Untreated Treated Percent Change
ESP Powers (kVA)
• inlet 45 57 +27
• outlet 15 19 +27
In the field case testing was carried out at a time when ESP powers were not
the limiting factor in collection efficiency; hence, the increase in powers
did not materially lower opacity.
CONCLUSION
Ash resistivity can be an important factor in lowering ESP collection
efficiency. The additives described here are independent of ash composition
and can yield varying treated resistivity values depending on the additive
chosen and amount used. The laboratory results have been substantiated in
pilot plant and field precipitators.
A patent on this new process is being applied for and further results
and discussion will be forthcoming.
ENDNOTES
1. Komanoff, C. Pollution Control Improvements in Coal Fired Electric
Generating Plants: What They Accomplish, What They Cost. JAPCA. 30 (9)
p. 1051 (1980).
2. Oglesby S. et al. A Manual of Electrostatic Precipitator Technology,
Part I: Fundamentals. 1970. PP 19, 173. Report No. APTD-0610 (Avail.
NTIS).
215
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DEVELOPMENT OF A NEW SULFUR TYPE ASH CONDITIONING
By: Robert H. Gaunt, Senior Research Engineer
Air Correction Division/UOP, Inc.
Norwalk, Connecticut 06856
ABSTRACT
Various types of chemical ash conditioning agents are used to alter fly-
ash resistivity, and thereby improve electrostatic precipitator performance.
Air Correction Division, UOP, Inc. has had experience with many chemicals and
processes on a pilot and full scale basis. Sulfuric acid conditioning has
advantages over other sulfur conditioning systems in availability, cost of
chemicals, ease of handling, process control, and relative lack of corrosion
problems. In the past, operating costs (power consumption) of sulfuric acid
systems has not been particularly attractive in large power plants. ACD/UOP
has developed a new system that utilizes the advantages of sulfuric acid while
reducing the capital and operating costs to competitive levels with other
systems. The new process utilizes existing energy in the compressed and
heated plant combustion air. A prototype unit at a 46 MW station improved
precipitator collection efficiency to 99.8% from 85.0% at an operating cost
of $40.00 per day.
INTRODUCTION
As emission standards have become more stringent over the past decade,
design and operation of pollution control equipment has become more and more
critical. This particularly applies to the sizing and design of electrostatic
precipitators (ESP's) for fossil fueled power plants.
One of the basic parameters that effects ESP performance is the resisti-
vity of the flyash. ESP's operating on highly resistive flyash (>10^^ ohm-cm)
(see figure 1) are faced with problems of ash build up on electrodes, and poor
electrical operating characteristics such as low voltage-current, back corona,
and early sparking. (1) (see figure 2) These problems can, to some extent,
be dealt with by specific ESP designs for highly resistive ash. However, this
increases the size and complexity of an ESP and adds considerably to the cost.
The resistivity of the ash is dependent on a number of factors which can
be altered or used advantageously. One is temperature. Typical resistivity
temperature curves are shown in figure 1. The conventional ESP is placed
after the air preheater which frequently is near the point of highest ash
resistivity. Moving the ESP to the hot side of the preheater overcomes this
problem, but adds to the ESP size since greater gas volume must be handled,
and structures are more complex to deal with higher temperatures. Hot side
ESP's are not a cure all since they too can have resistivity problems,
particularly with low sodium coals.
A second approach to processing with highly resistive ash is to chemical-
ly alter the flyash and thereby change the resistivity to a more favorable
range. Chemical conditioning agents can be introduced in the power plant's
216
-------
process stream in several places. The agent can be added to the coal iniect-
ed in the combustion zone, or injected in the flue gas stream. The first two
locations may create slagging problems in the boiler. Flue gas injection
generally involves more equipment, but has fewer operational difficulties.'
The chemicals typically added to alter resistivity for cold side ESP's
are sulfur compounds, such as, sulfur-trioxide (S03), as well as ammonia (NH4),
water, and other proprietary chemicals. Sodium compounds are generally used
to condition hot side ESP's. Air Correction's experience with cold side con-
ditioning is the subject of this report.
ASH CONDITIONING
Two mechanisms are involved in passing an electric charge thru flyash.
Volume conductivity is dominant at temperatures above 450°F. (2) As the
temperature increases, the resistivity of the ash particle drops allowing
passage of an electric current thru the body of the particle. As the tempera-
ture falls below 350°F, surface conductivity becomes dominant. In this process
the charge is carried by the adsorbed chemical impurities on the surface of
the particle rather than thru the particle. Since this temperature break is
also at the upper range of cold side ESP application, most cold side condition-
ing agents are adsorbed by the flyash and act by improving surface conductivi-
ty-
In the 1960's it was found that the addition of sulfates (S04) greatly
increased the conductivity of flyash. This happens naturally when most coal is
burned. Sulfur is usually present in all coal to some degree. When burning
it forms sulfur dioxide (S02) and a few percent of sulfur trioxide (803). (2)
The 863 in combination with the water in the flue gas acts as a natural condi-
tioning agent with most flyash. As power plants began to burn low sulfur coal
(under 1.5% to 1.0%) to meet S02 emission requirements, the amount of 803 was
not enough to keep the ash resistivity in a reasonable range for good opera-
tion of ESP's. This resulted in increased particulate emissions as the sulfur
emissions fell.
ACD/UOP EXPERIENCE
Interest in ash conditioning began to develop with the need to burn lower
sulfur coals. ACD first tried the obvious step of injecting 803 in a flue gas
stream on an experimental basis. This produced improvement in ESP operation
and also pointed out the difficulties in handling S03. The test system used
stabilized 803 liquid, heated to a vapor, and injected it in the flue duct.
However, when cooled below 90°F, S03 starts to solidify in storage tanks,
pumps, heat exchangers, pipes, etc. Heating it as a solid frequently causes
it to sublime in which case, the gas may be trapped by the solid and under
high pressure. As a gas it reacts extremely exothermically with water. With
water it forms fuming sulfuric acid which is extremely corrosive until suffi-
cient dilution occurs. The S03 studies were not carried past the experimental
stage.
Ammonia was tried and found to produce improvement in ESP operation
few special instances. Water was also tried but was found to be ineffective
in a
217
-------
except in very large amounts or at very low flue gas temperatures. Sulfur
trioxide was almost always effective, but a more practical way to introduce
it in the flue gas was required.
One novel method tried was to pass a slip stream of flue gas through a
catalyst bed to increase the SO^ content, and then pass it back into the flue
gas. Ash plugging problems were encountered and operation on extremely low
sulfur coal was not feasible. Another method was to take liquid S02, heat it
to a vapor, pass it over a catalyst bed to convert a large quantity to 803,
and inject it into the flue gas. While this looked promising, a third method
looked even better. This was to vaporize liquid sulfuric acid (H2S04) and
thermally dissociate a portion of the vapor to 803 and water (H2S04*H20 + 863)
The temperature required to vaporize the acid was around 450°F to 500°F.
Then the vapor was passed over a superheater to raise the temperature to
around 800°F to 900 F. This has the advantage of using sulfuric acid as a
conditioning agent. It is readily available, comparitively cheap, and fairly
safe to handle. As a common industrial reagent, there is a large amount of
technical experience on its handling, storage, transport, and use. With the
93% (66°Baume) concentration used, serious corrosivity exists only when
changing state from a liquid to a gas or the converse. Further work proved
that acid dissociation is not required, and the superheater could be eliminat-
ed.
The use of sulfuric acid vapor as a conditioning agent produces the same
effect as conditioning with 803 and has some handling and process advantages.
When 803 i-s injected into a flue gas stream, it immediately reacts with the
moisture in the gas stream to form H2S04, which is adsorbed on the flyash.
Injecting H2S04 to start with has the identical effect on the ash. It has
been found that the lances and nozzles used to inject 803 must be kept at
800°F or higher to keep the 803 from immediately combining with the moisture
at the nozzle and thereby corroding it quickly away. In contrast, the nozzles
and lances used to inject sulfuric acid vapor need only be kept above the
vapor dew point, generally under 400°F. This can be accomplished with the
heat of the vapor with short runs that are well insulated. Heat tracing is
not required. A flow diagram of this conditioning system is shown in figure
3. This system was installed at Cameo Station, Public Service Company of
Colorado in 1968, and resulted in the first utility order for a commercial
ash conditioning unit in the United States.
At this point it would be good to reexamine sulfur emissions and the
operation of sulfur conditioning systems. At first glance, it appears that
with sulfur conditioning, sulfur is being added to replace that reduced by
burning low sulfur coal. This is not the case since most sulfur from coal
combustion is emitted in the form of 802> which is released to atmosphere.
Large percentages of 803 or H2S04 are absorbed on the flyash and collected by
the ESP. This is confirmed in EPA Report, EPA-600/7-79-104a. The acidity of
the ash increases but not greater than that from burning high sulfur coal.
For determining the conditioning feed rate two methods are generally
used. Sampling the stack gas and operating at a rate just below that where
sulfur emissions begin to increase is possible. The second is considerably
less difficult. Assuming the plant has been operating without conditioning,
218
-------
injection is initiated at a low rate,and slowly increased over a period of at
least 24 hours. The ESP voltage-current readings will increase as the condi-
tioning rate increases up to the point of acid carry over. The ESP readings
then stabilize or fall off slightly and a blueish tint to the plume may be
noticiable. The optimum conditioning rate is slightly below this point. It
should be noted that it may take 18 to 24 hours for the full effect of condi-
tioning to take place.
It has been ACD's experience that 12 to 20 ppm of acid is all that is
required to condition the ash. Most of our units are sized for 30 ppm
maximum and some for 45 ppm for unusual conditions or customers request. This
extra capacity has never been required and is a margin of safety. To date no
corrosion problems have occurred to the flue ducts or ESP internals due to
the ash conditioning.
ACD DEVELOPMENT PROGRAM
The early work by ACD in sulfuric acid conditioning resulted in ACD's
patented sulfuric acid vaporizing system. As is seen in figure 3, clean air
is compressed to around 2 psig by a blower; the air is heated to 485°F by an
electric air heater and mixed with a metered amount of acid in a glass lined
packed column. The acid is pumped from a day tank and injected into the
vaporizer by a metering pump. All equipment is mounted on a single skid near
the flue duct. From the vaporizer the acid vapor is carried in a short
length of glassed steel pipe to the stainless steel injection lances where it
is dispersed in the flue gas.
This system has been developed to be quite reliable. Changes from early
units have simplified controls and improved materials to withstand system
upset conditions without damage. These units are most attractive to smaller
plants (see table 1). Their performance is quite good. The main draw back
is their relatively high operating cost due to electric power consumption
heating the process air.
ACD recognized the operating cost problem and set out to develop a
system that would utilize sulfuric acid with its advantages, but do so in a
more economical manner. The first approach was that of direct spray into the
flue duct. Acoustic atomizing nozzles were used to produce a very fine "fog"
of atomized liquid sulfuric acid. This was by far the simplist and cheapest
approach. In still air these nozzles worked quite well, however, after tests
in three locations, it was determined that the acid was not dispersing
properly in the flue ducts and was in fact wetting streams of flyash that
passed near the nozzles. This caused the locally damp ash to adhere and
build up on the first surface it came in contact with. Usually, this would
be turning vanes or the perforated plate across the face of the ESP. No
practical way to prevent this was found and work on the direct spray approach
was suspended.
The next step was to try to maintain the lance dispersion method of the
vaporizer system with the efficiency of the direct spray approach. It was
decided to try spraying the acid in a slip stream of hot air or gas in a
219
-------
controlled manner so that the acid would vaporize and could be Injected into
the flue duct as a vapor. The initial step was to develop a spray chamber to
vaporize the acid. This was done with a laboratory model using air and water.
One requirement was to vaporize the acid completely before it came in contact
with any surface so as to eliminate corrosion problems. A cylinderical
chamber was devised with the air flow in and out of tangential openings and
r.the acid sprayed down the center axis from one end. (see figure 4). After
a working model was developed in the lab, a chamber was made up to try with
acid. This project was done at a power plant with one of our vaporizer
conditioning units so the acid vapor could be fed into the flue duct and
collected by their ESP. The vapor system rate was turned down accordingly
when the chamber was in operation. The hot air for the chamber was taken
from the hot air side of the Eundstrom type air preheater. The amount of air
is about 14-15 scfm per megawatt of power produced. This is below the normal
leakage rate associated with a Eundstrom air heater. Using this test setup,
exact chamber, nozzle, and internal flow device dimensions were developed for
the chamber; and air flow, temperature, and acid flow rate limits were
developed.
When the basic spray chamber was perfected the next step was to incorpo-
rate it in an operating system. A prototype system was built and installed
at the same plant so that the new system could be tested with the vaporizer
system as back up and as a comparison. This new system consists of a control
and metering pump skid, two chambers, two lances, and the associated air and
acid piping. It is shown in figure 5. This system was operated by both ACD/
UOP and power plant personnel to discover any operational bugs and correct
them prior to offering the system as part of our product line. Also, life
expectancy of the components could be determined and improved if necessary.
The only probleHis which appeared were in the area of automating the shut down
procedure to prevent acid from sitting stagnent in the hot lines near the
spray nozzle. This was accomplished with an automatic air purge.
The new process is called the SAAC System (Sulfuric Acid Ash Condition-
ing) . It avoids several limitations of the vaporizer column. It can handle
dirty process air. The flow thru the chamber is clear so most particulate
is carried along with the air. This is not the case with a packed vaporizer
column which must have filtered air and quite clean acid. The chamber can
operate over a wide temperature range (500°F to 750°F),where the vaporizer
glass and teflon must be closely held between 475°F to 500°F for proper
operation. No expensive materials are required for the chamber. The
tangential air flow keeps the acid out of contact with the chamber until it
is vaporized. The only exotic material used is the acoustic spray nozzle
which is made out of tantalum. This is required since it is at a liquid/
gaseous interface where extreme corrosion occurs.
In operation the SAAC system avoids the operational expense of our
previous system. Hot air is supplied by the existing plant air heater. In
some cases a start-up trim heater may be recommended depending on operating
:conditions. The pressure differential between the air heater outlet and in-
let to the ESP produced by the plant combustion air blowers is sufficient for
the process so no blower is required. The compressed air for the nozzles is
one cfm per nozzle.
220
-------
The sulfuric acid is kept as an easily handled ambient liquid up until
mixing with hot air. This is done adjacent to the duct work to keep the
vapor piping as short as possible. The control and metering pump skid can be
mounted remotely at a convenient location.
In summary, ash conditioning with sulfur compounds is the most success-
ful method of lowering the resistivity of high resistivity ash to a range
needed for good electrostatic precipitator operation. Air Correction Division
of UOP (ACD/UOP) has been a leader in researching this field and led the
field in early pilot work. ACD/UOP has offered a reliable, effective ash
conditioning system over the past decade. Now ACD/UOP has developed a new
sulfur conditioning process that retains the advantages of the old system
and has greatly lowered operating costs associated with sulfur conditioning.
REFERENCES
1. White, H.J. Resistivity Problems in Electrostatic Precipitators, APCA,
Vol 24, No. 4, April 1974.
2. Oglesley, S. Jr. and Nichols, G.B. Electrostatic Precipitation. New
York, N.Y., Marcel Dekker, Inc. 1978.
TABLE 1
UTILITY
UNIT
MW
ACFM
TEMP
EFFICIENCIES (%)
WITHOUT
CONDITIONING
WITHOUT
CONDITIONING
Public Service Co. of
Colorado - Cameo Station
44 221,300 280
50.0
94.0
General Motors Corp. 1&2
Chevrolet Detroit-Forge 7&8
150,000 350
150,000 350
73.0
48.4
93.3
93.3
Public Service Co. of 2 46 261,000 315
Colorado-Arapahoe Station 3 44 250,000 350
51.5
42.2
97.5
94.8
Luzern Electric Div., UGI
Corp. Hunlock Creek Station 6
46 225,000 350
85.0
99.8
Pennsylvania Electric Co. 9
Front Street Station 10
270,000 400
270,000 400
90.0
91.0
99.1
99.2
Detroit Edison 23
Pennsalt Plant 24
Wyandotte, Michigan
90.1
87.4
94.4
93.0
221
-------
10'
10V
0.5-1 %
1.5-2%
-2.5 - 3%
200 250 300 350 400 450
TEMPERATURE, °F
FIGURE 1
RESISTIVITY OF FLYASH FOR VARIOUS SULFUR CONTENTS
0.6
5
u
s
t-^"
z
z
o
8
0.5
0.4
0.3
0.2
10
SPARK
^=2.5 « 10
= 10"
30 40 50 60
70
80 90
CORONA VOLTAGE, KILOVOLTS
FIGURE 2
CORONA CURRENT-VOLTAGE DISTORTIONS CAUSED
BY DUST LAYERS ON GROUNDED PLATES DC VOLTAGE
(1)
222
-------
VENT
LEVEL
SENSOR
HI & LO
LEVEL
GAGE -
PULSATION
DAMPER
ACID FILL LINE FROM SUPPLY TANK
FILTER
METERING
PUMP
DAY
TANK
(ACID)
^
BPJLE_R_SJGNAL
120~PsiG~ _
INS~T7~AiR~
CONTROL SYSTEM
PNEUMATIC
ELECTRICAL
CABINET
LOW
FLOW SENSOR
HEATER
CONTROLS
&
Hit LO
TEMPS.
ALARMS -
MISC.
CONTROLS
SIGNALS,
ETC.
S
c
R
480V
30
FILTER
AIR
HEATER
480V
30
z
CS PIPE
(HOT AIR)
VAPORIZER
FRESH AIR
TURBO
COMPRESSOR
DRAIN
LANCES
'GLASSED
TRANSFER PIPE
(ACID VAPOR)
EXPANSION JOINTS
FIGURE 3
TYPICAL VAPORIZER SYSTEM FLOW DIAGRAM
223
-------
SAAC CHAMBER
HOT AIR
INLET
ACOUSTIC
ATOMIZING
NOZZLE
AClb
fR
FLUE
k
FIGURE 4
SAAC CHAMBER ARRANGEMENT
224
-------
600 SCFM PER CHAMBER
HOT
AIR
PIPE '
ATOMIZING AIR LINE 80 TO 100 PSIQ
—* T] * PLANT AIR
FILTER 60 SCFH PER NOZZLE
LIQUID ACID LINE
LIQUID ACID
LINE
0-3 QAL/HR
PER NOZZLE
LEVEL
GAGE j
INLET VENT
J t_
ACID DAY
TANK
FILTER 80 To 100
PLANT AIR i—i
AIR PURGE LINE
VENT
CONSOLE
MOUNTED
EQUIPMENT
I—(-O) DUPLEX
FILTER SET
PUMP
_EQUIPMENT -l
FIGURE 5
TYPICAL SAAC SYSTEM FLOW DIAGRAM-2 CHAMBER
225
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OPERATING EXPERIENCE WITH FLUE GAS CONDITIONING SYSTEMS
AT~ COMMONWEALTH EDISON COMPANY
By: L. L. Weyers, R. E. Cook
Commonwealth Edison Company
Chicago, Illinois 60690
ABSTRACT
The use of western low sulfur coal, to reduce sulfur oxide emissions,
has resulted in decreased electrostatic precipitator collection
efficiencies. In an effort to restore precipitator performance, a flue gas
conditioning program was established by the company in the early seventies.
This paper is a history of Commonwealth Edison Company's experience
with flue gas conditioning agents over the last eight years. Extensive
testing of these systems has supplied valuable information which is
presently being used as a basis in design of future plant additions to our
systems.
TEXT
The change to low sulfur western coals in the early seventies resulted
in unacceptable particulate emissions due to the high resistivity of the
fly ash. This condition required that Edison either install additional
precipitator sections, new precipitators, or reduce the resistivity of the
fly ash by the addition of chemicals. Each unit was examined individually
and two hot side precipitators and one cold side were installed for low
sulfur coal. The remaining twelve units, which were scheduled to burn low
sulfur coal, had large enough precipitators to meet particulate limits if
the resistivity of the fly ash could be improved and the boiler operated at
design gas flow.
Edison, after testing several proprietary chemicals and various SOg
injection systems, including a liquid sulfur trioxide system, decided that
the most cost effective and potentially most reliable system was the sulfur
burner type SO^ system.
The experience gained from a pan-type sulfur burner installed on a 230
megawatt pulverized coal fired boiler at State Line Station in 1973
indicated a considerable reduction in particulate emissions could be
expected. The precipitator efficiency increased from 84% to 97% when
feeding 50 ppm of 863 and the corona power input exhibited a great
improvement with 363 injection. By reducing the sparking in the
precipitator, the power input level was increased from 25 watts per 1,000
ACFM to 250 watts per 1,000 ACFM; a ten-fold increase.
Operating experience with the pan-type burner, however, was not good.
The sulfur feed method was unreliable and required extensive operator
attention. In addition, there was little turn-down capability of the
226
-------
burner. It was possible to increase the turn-down capability to five to
one. However, because of the uncertainty of future coals for the stations,
it was felt that any flue gas conditioning system must be automated, simple
to operate, and have a turn-down ratio of at least ten to one.
Wahlco, Inc., had submitted a proposal for a sulfur burning type flue
gas conditioning system. Their system utilized a drip-type burner with a
guaranteed ten to one turn-down ratio. The flue gas conditioning system,
shown schematically in Figure One, was quite simple and had the advantage
of being compactly mounted on a skid. This was an important consideration
because previous experience had shown that the 803 source should be
located as close to the injection probes as possible, to minimize heat loss
in the piping system. In addition, their system, because of its
compactness, could be installed in a rather small area.
A flue gas conditioning system was ordered for State Line Unit Three in
1974, but due to commitments to the Illinois EPA and the City of Hammond
EPA, Edison was required to order flue gas conditioning equipment for
eleven other units, totaling approximately 3,000 megawatts before the first
system at State Line could be tested. All of these units had to be
operational before June, 1976.
As with any new system, a certain number of problems were expected.
The first to occur was the sulfur pumping system. Liquid sulfur requires a
temperature range of 245°F to 310°F. Below 245°F it solidifies, and
above 310°F it becomes extremely viscous. Many pump failures occurred
which were usually attributed to the pump diaphragms being ruptured when
bringing the system on line. These failures also occurred when the sulfur
in the stand-by pump was allowed to solidify. By changing to steam
jacketed pumps instead of electrically heated, and adding additional steam
tracing and insulation on the pump piping and valves, the problem was
corrected.
The next problem that occurred was the failure rate of the calrod air
heaters. Even though Edison had specified an in-line spare heater, the
failure rate often resulted in a burner being out-of-service for
replacement of the-'heater elements. This problem was corrected by
improving the quality control at the heater manufacturer's plant.
The third problem did not become apparent until after several months ^of
operation. Some of the injection probe insulation, which was covered by
fiberglass and epoxy, was being eroded by the fly ash particles. These
failures occurred in installations where the probes were experiencing high
'as velocities because of their location in the duct-work. Only a few
units had this problem, and it was corrected by installing a steel liner
over the insulation.
The original specification for the sulfur burner units called for a
capability of producing at least 60 ppm of S03 with a turn-down ratio of
ten to one. The 60 ppm was based on precipitator test results with the
original burner and Arch Mineral Coal. An analysis of Arch Mineral and
227
-------
NJ
N)
00
Liquid
Sulfur
Storage
Liquid Sulfur
250-300°F
Ambient
Air In
Metering
Pump
Injection
Probes
l
Controlled to
800-825°F
Sulfur
Burner
Boiler Flue
FLOW DIAGRAM
FLUE GAS CONDITIONING UNIT
Figure 1
Converter
Air/SOs—
800-IIOO°F
\. 7 Conditioned
—*» Flue Gas To
s Precipitator
-------
other coals utilized by Edison is shown in Table One. When the Wahlco
System became operational, precipitator tests indicated that less than 20
ppm of S03 was the optimum feed, even on Arch Mineral Coal with its
extremely high resistivity (greater than 1013 ohm-cm). The only
explanation for this change was the much more extensive injection probe
system installed by Wahlco which resulted in better S03 distribution.
As more sulfur burners became operational at the various stations, it
became apparent that some low sulfur coals required very little S03 to
improve or optimize precipitator performance. The oversized burners,
therefore, aggravated the problem since operation at minimum sulfur feed
rate often created a condition whereby the system would trip-out due to low
temperatures which indicated flame failure of the burner.
The coals that need little S03, such as Decker and Big Horn,
fortunately also have a very low ash content. Therefore, even if the
precipitator was not at optimum performance levels because of the sulfur
burner being out-of-service, the low inlet dust loadings resulted in
acceptable particulate emissions. Consequently, many of the sulfur burners
were left in a "stand-by" mode of operation when the stations were burning
Decker or Big Horn coal.
This problem was examined in detail in 1977 when an experimental
program was undertaken to determine quantitive relationships between
precipitator performance parameters and other system variables at State
Line Station, Unit Three. This is a Combustion Engineering twin furnace
steam generator. The Research Cottrell precipitator was designed for 98%
collection efficiency with a gas volume of 657,700 ACFM at 305°F and has
a SCA of 137 square feet/1,000 ACFM. Investigated were the effects of
variation in boiler steaming rate, the S03 injection rate, and the
relationship derived from the individual automatic voltage control
operating parameters of voltage, current and spark rate. The independent
variables of the experiment were the S03 injection rate and the boiler
steaming rate. The AVC parameters are, in effect, dependent variables for
two reasons. First, they control themselves in secondary currents.
Second, the currents and spark rates are affected by the S03 injection.
Contrary to normal operation of the boiler, the boiler steaming rate
was held constant. The S03 injection rate was then held constant for
long periods of time to observe the effects. However, several times during
the test period the steaming rate was reduced significantly. The
transmissometer readings, however, remained constant at a relatively low
level.
A data acquisition system was installed and operated for a seven day
period. During this time, two different periods of "constant coal" were
arranged utilizing Decker and Arch Mineral. These coals were chosen^
because of their chemical differences as shown in Table One. Arch Mineral,
with its low sodium content, results in very high resistivity fly ash;
while Decker, with its relatively high sodium content, results in a fly ash
resistivity below LO^- ohm-cm.
229
-------
TABLE 1. TYPICAL COAL AND ASH ANALYSIS
"As
Moisture
Sulfur
Ash
BTU/lb.
Silica
Alumina
Iron Oxide
Calcium Oxide
Magnesium Oxide
Potassium Oxide
Sodium Oxide
Received" Coal Analysis (% By Weight)
Decker
24.2
0.4
4.3
9,511
Ash Analysis
23.6
20.5
7.5
17.7
3.6
0.6
6.4
Arch Mineral
13.8
0.6
12.0
9,760
(% By Weight)
35.5
18.1
14.6
17.0
3.3
1.2
0.7
Big Horn
23.5
0.5
5.4
9,329
29.8
17.3
9.0
15.6
4.8
1.2
2.5
230
-------
The data acquisition system was designed to collect data on a sample
basis every five minutes from six different AVC's and three other analog
signals representing the outputs of the transmissometer, a boiler steam
rate measurement and an S03 injection rate measurement. Every half hour
(or six five-minute samples) a block of data was transferred from the
computers memory and recorded on the cassette tape contained in the unit.
The data indicated that when the system performance is measured by the
optical transmissometer, there is an optimum value for the SOo injection
rate. In particular for the coals tested, including a blend of the two
coals, the optimum injection rate was in the range of five ppm to 20 ppm.
Values above 20 ppm were definitely shown to be less effective in reducing
opacity. The effect of 803 conditioning on opacity with Decker and Arch
Mineral coals is shown in Figure Two. The higher ash content of the Arch
Mineral coal results in an overall higher opacity as would be expected.
The optimum 803 injection rate for Decker and Arch Mineral based on this
test period was five ppm and 20 ppm respectively.
The data also clearly indicated that the optimal value of 803 did not
occur when the AVC currents were highest or when the spark rate was a
minimum. Figure Three shows the current, opacity and 863 feed. As can be
seen, the opacity decreases with increasing 803 feed rate and increasing
power until approximately 20 ppm of 863 is injected. Further increases
in the 863 feed rate results in additional power input; however, the
opacity also increases.
In October of 1979, extensive testing of Crawford Station Unit Seven
precipitator was begun with varying 803 feed rates. The coals used for
these tests were Decker and Big Horn, and their analysis is shown in Table
One.
A total of 24 tests were conducted on Crawford Unit Seven. This is a
220 megawatt unit with a Combustion Engineering twin furnace steam
generator. The Research-Cottrell precipitator was designed for 98%
collection efficiency with a gas volume of 690,000 ACFM at 300°F and has
a SCA of 147 square feet/1,000 ACFM. The precipitator was tested at 200
megawatts to obtain near design gas volume and the 803 feed rate was
adjusted in increments from 0 to 35 ppm. The S03 feed was held constant
for a minimum of twelve hours prior to each test.
The tests conducted with Big Horn and Decker Coals, which are very
similar chemically, indicated the lowest outlet dust loadings were obtained
when the 863 feed rate was set at six ppm.
The results of these tests verified our earlier findings at State Line
Station that maximum precipitator power with flue gas conditioning is not
necessarily optimum removal efficiency.
Since the earlier flue gas conditioning units were installed,
additional units have been purchased and installed because of fuel changes
at the stations. The additional units, as well as those already installed,
are shown in Table Two. After the installation of the original flue gas
231
-------
to
u>
Opacity
%
40-
0
0
10 15 20
S03 - RPM.
OPACITY VS SOs FEED RATE
Figure 2
35
-------
30C
o
^ 200H
IOO
I 25
I
>\
'o
o
o.
O
0
40 n
0
863 Feed Increasing-
Power Increasing
Opacity Increasing-
$03 Increasing
Opacity Decreasing
503 Increasing
10 20
SOj Concentration- P.P. M.
POWER INPUT AND OPACITY VS S03 FEED
Figure 3
233
-------
TABLE 2. FLUE GAS CONDITIONING SYSTEMS INSTALLED
STATION
Fisk
Crawford
Joliet
Powerton
State Line
Waukegan
UNIT
19
7
8
5*
6
7
8
5
1*
2*
3
4
5*
6
8
MW (NET)
341
222
326
117
344
537
537
850
206
150
190
318
117
88
358
DATE
OPERATIONAL
April, 1979
March, 1979
March, 1979
December, 1975
December, 1975
June, 1976
June, 1976
July, 1979
September, 1975
September, 1975
December, 1974
August, 1975
October, 1975
October, 1975
May, 1976
Will County
520
June, 1979
*Units No Longer in Operation
234
-------
conditioning systems, some of the generating units were retired, however,
at the present time, sulfur burner type S03 systems are installed on
twelve units totaling over 4,600 megawatts.
The new flue gas conditioning systems have been greatly improved over
the original units and require very little operator attention or
maintenance. The diaphragm type pumps used on the original units have been
replaced with positive displacement piston type pumps. They are now
properly sized and will produce as little as four ppm 803, which will
allow conditioning of coals, such as Decker to obtain optimum particulate
removal.
Our experience with these systems indicates that they are reliable and
greatly improve precipitator performance at a comparatively low investment
and operating cost. The sulfur trioxide flue gas conditioning systems have
saved Edison several hundred million dollars in capital costs which
otherwise would have been required to replace or increase the size of
existing precipitators.
The increasing size and cost of precipitators to meet the new source
EPA standards has made a combination of flue gas conditioning and
subsequently smaller precipitators a viable option. When purchasing new
precipitators in the future, the specification will require proposals for a
precipitator with and without flue gas conditioning. An economic analysis
will then be performed to determine the most cost effective way to meet
particulate emission requirements.
235
-------
THE APPLICATION OF A TUBULAR WET ELECTROSTATIC PRECIPATOR FOR FINE
PARTICULATE CONTROL AND DEMISTING IN AN INTEGRATED FLY ASH AND SC>2
REMOVAL SYSTEM ON COAL-FIRED BOILERS
By: Dr. Even Bakke
Howard P. Willett
Peabody Process System
835 Hope Street
Stamford, CT 06907
ABSTRACT
The development of a combined, tubular, wet electrostatic preclpitator
(WESP) and vapor condensing heat exchanger mounted in the top section of an
S02 absorber will be discussed.
Pilot plant data on an integrated fly ash and S02 removal system with
a variable pressure drop venturi, a high velocity spray tower and the WESP
will be presented. Data will show that it is possible to operate with a
specific collection area in the WESP of only 20 to 30 sq. ft/1000 ACFM,
tube velocities of 16 to 20 ft/sec, overall system pressure drop of 4 to 8
in. W.G. and still have fly ash removal efficiencies in the range from
99.25 up to 99.65% or outlet emissions from 0.015 gr/scfd to 0.006 gr/scfd.
Heat recovery and flue gas reheat options made possible by the WESP/
heat exchanger combination will be discussed. Commercial installations of
this system on industrial boilers will be reviewed. Initial and annualized
costs and space-saving benefits will be presented.
NOTE:
Please contact the authors for information regarding this paper.
236
-------
FIELD EVALUATIONS OF AMMONIUM SULFATE CONDITIONING FOR IMPROVEMENT
OF COLD SIDE ELECTROSTATIC PRECIPITATOR PERFORMANCE
By E. C. Landham, Jr.
G. H. Marchant, Jr.
J. P. Gooch
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35255
and
Ralph F. Altman
Electric Power Research Institute
516 Franklin Building
Chattanooga, Tennessee 37411
ABSTRACT
Measurement and analysis of the improvement in cold side electrostatic
precipitator performance through the use of ammonium sulfate conditioning
agents were conducted at two electric utility generating stations. One plant
was burning a low sulfur, high alkalinity Western coal and the other a moder-
ate sulfur, low alkalinity Eastern coal. Comprehensive field tests were
performed with and without the agent in use to evaluate the change in perfor-
mance as well as to determine the mechanisms involved. The measurements
conducted included total mass and fractional efficiencies, particle size
distributions, rapping emissions, in situ resistivity, ash and flue gas
analyses, and voltage-current characteristics of the power supplies. _Measure-
ments were made with a proprietary formulation of ammonium sulfate injected
on the hot and cold sides of the air heater and with a generic formulation
injected on the cold side. The performance of the precipitators was compared
with predictions of a theoretical model, and an engineering analysis of the
installations was performed.
INTRODUCTION
Electrostatic precipitators (ESPs) are the most widely
control devices in the utility industry due to inherently 1 be
maintenance requirements. However, the performance of these devices can be
severely limited by high ash resistivity, ^lc\cau8!%^12eSrode rap-
corona, and can also be limited by dust reentrainment du™ *^°f ™J
Ping. Operating problems have developed at a number of -tallations wher
utilities have changed to low sulfur coals in an effort to comp y x
237
-------
emission limitations. At some of these installations conditioning of the
flue gas or the coal supply with certain chemical compounds has significantly
improved ESP performance. Even so, successful application of conditioning
agents has not been consistent primarily due to a lack of knowledge of the
mechanisms involved. This paper reports the results of studies conducted
under the sponsorship of the Electric Power Research Institute (EPRI) at two
power plants using ammonium sulfate as a conditioning agent. Ammonium sulfate
was chosen for study because: 1) it has the potential for enhancing ESP
performance through several different mechanisms, 2) it is a relatively
innocous chemical, and 3) it is economical and readily available. Further-
more, it has received fairly widespread usage in full scale plants as an
ingredient of one or more proprietary formulations. The studies were
designed to: 1) quantify the improvement in performance of the control
device due to the use of the agent and, 2) determine the mechanisms by which
the agent effected the improvement.
The two power plants included in the research program were the Corette
Station of Montana Power Company and Unit 6 of the Gannon Station of the
Tampa Electric Company. Both stations use ammonium sulfate for conditioning
in a cold side precipitator. At Corette, injection of ammonium sulfate
solution is made ahead of the air preheater, where the temperature is high
enough to decompose ammonium sulfate into ammonia, sulfur trioxide, and
water vapor. At Gannon 6, injection of ammonium sulfate may be made either
before or after the air preheater. In the latter event, decomposition of
the agent is not expected to be complete.
Conditioning Mechanisms
The mechanisms by which a conditioning agent may improve ESP performance
are:
• Reduction of resistivity and/or alteration of the electrical break-
down strength of fly ash allowing more favorable ESP electrical
operating conditions.
' Reduction of reentrainment by changing the cohesive properties of fly
ash.1'2
• Enhancement of the collecting electric field through the formation
of a space charge in the region between the discharge and collecting
electrodes. This might be accomplished by the formation of a sub-
micron particulate fume of ammonium sulfate.3
• Agglomeration or growth of small particles to form larger particles."*
Measurements
In order to determine which of these mechanisms were involved in any ESP
performance improvement, a test plan was developed which included various
measurements conducted both with and without the conditioning agent in use.
The measurements were conducted by techniques described by Smith et al.5
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They included the following:
• Impactor sampling - Determine inlet size distribution for modeling
purposes, fine particle collection with and without conditioning, and
changes in inlet and outlet size distribution as a result of condition-
ing agent usage.
• Mass train sampling - Determine particle mass concentrations and over-
all performance with and without conditioning, quantify rapping
reentrainment losses, and collect samples for chemical analysis.
• Resistivity measurements - Determine in situ resistivity of fly ash
with and without conditioning.
• Ion mobility measurements - Determine whether the conditioning agent
significantly alters the charge carrying characteristics of the flue
gas.
• Ultrafine particle measurements - Determine whether the conditioning
agent produces a space charge fume of ultrafine particles (at Corette
and at Gannon), and (at Corette only) determine the ultrafine particle
collection efficiency of the precipitator.
• Chemical analysis - Determine the concentrations of flue gas and fly
ash components related to the conditioning agent and monitor coal
supply for variations.
• ESP electrical characteristics - Determine the changes in the electri-
cal performance of the ESP caused by the conditioning agent.
CORETTE TEST
Plant Description
The J. E. Corette Station of Montana Power Company has a single 180 MW
pulverized coal-fired boiler designed by Combustion Engineering, Inc., to
burn a low sulfur subbituminous coal from the Rosebud seam of the Colstrip
Mine. The electrostatic precipitator is a small Research Cottrell unit with
two electrical fields in the direction of gas flow and a specific collection
area of 27.5 m2/(m3/sec) (140 ft2/1000 ACFM). A hot air bypass is used to
keep the temperature of the ESP below 130°C (270°F) to lower the ash resis-
tivity and improve ESP performance. The chemical injection system was
provided by Apollo Technologies, Inc., and introduces the proprietary con-
ditioning agent ahead of the horizontal superheater.
Test Program
Testing at the Corette Station was conducted in three phases: 1) a "con-
ditioning pretest" to obtain some preliminary information on the fate of the
injected ammonium sulfate, conducted in August 1979, 2) a conditioning test
with comprehensive measurements as outlined previously with the proprietary
conditioning agent injected ahead of the air preheater, conducted in
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October 1979, and 3) a baseline test with comprehensive measurements without
the conditioning agent in use, conducted in November 1979.
Results
The following results have been derived from the laboratory and field
investigation concerning the Corette Station.
Collection Efficiency
The collection efficiency of the precipitator, on an overall mass basis,
was increased from ^77% to ^98% as a result of the use of the conditioning
agent. Table 1 presents overall performance data obtained during the test
program. The size-dependent data indicate that the cumulative mass collection
efficiency for particles smaller than 2.0 ym diameter increased from 57% to
92% as a result of conditioning agent usage.
Chemical Analyses
The results of chemical analysis of the proprietary agent used at
Corette indicate that the primary active constituent of the agent was ammonium
sulfate, which represented about 38% of the agent by weight (most of the
balance was water). As indicated in subsequent discussion, the test results
can be explained by considering the effects of the decomposition products of
ammonium sulfate (ammonia and sulfur trioxide) on fly ash resistivity. No
effort was made to determine the effect that a low concentration of urea
(1.8%), which was also detected in the agent, might have had on precipitator
performance, but there was no evidence of a significant effect.
Proximate and ultimate analyses of coal samples collected during the
separate tests during August, October, and November 1980, are given in
Table 2. These indicate that the coal was reasonably uniform throughout the
test series.
Analytical data for the fly ash from the separate tests are given in
Table 3. The concentration of the elemental oxides (the first 11 components
listed) indicate that the primary differences related to electrical resis-
tivity were in the sodium oxide; the values were approximately 1.0% in
August, 0.4% in October, and 0.6% in November. These variations are noted
because of the' importance of sodium ions as a charge carrier in the precipi-
tated ash.6'7 The variations are significant in that the conditioning
process had to overcome a higher inherent resistivity level during the
October conditioning test than that in the August conditioning test or that
indicated by the baseline data acquired in November.
The results of determinations of ammonia and sulfur trioxide in the flue
gas and on the fly ash during the conditioning tests in August and September
are presented in Table 4. These measurements show clear evidence that the
ammonium sulfate did decompose into ammonia and sulfur trioxide as expected.
Apparently, the use of only two injection nozzles was not sufficient to
produce a uniform distribution of the conditioning agent. This is illustrated
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by the high concentrations of ammonia and sulfur trioxide measured at the
air heater inlet port and by the variation in ammonia concentration from port
to port at the ESP inlet. A substantial loss of ammonia (and perhaps an
equivalent amount of sulfur trioxide) occurred across the air heater, which
may be attributed to deposition of ammonium sulfate as a solid deposit on the
surfaces of the air heater or to surface-catalyzed oxidation of ammonia to
elemental nitrogen and water vapor. Most of the sulfur trioxide was taken
up by the alkaline ash whereas most of the ammonia that remained in the gas
phase after the air heater was discharged through the stack.
Electrical Resistivity Studies
Measurements of the electrical resistivity of the fly ash in situ were
made during each of the field tests with a point-plane resistivity probe by
procedures described in another document.5 In addition, laboratory resis-
tivity measurements were performed to substantiate observations made in the
field and to provide information that could be used to interpret the action
of the conditioning agent.
Field- and laboratory-measured resistivity values and predicted resis-
tivity values are compared in Figure 1. The field results without condition-
ing are represented by the upper shaded area and those with conditioning by
the lower shaded area. Laboratory results with sulfur trioxide concentrations
of 0, 1.8, and 2.5 ppm at a selected temperature (about 140°C) are shown by
the solid triangles. Finally, predicted resistivity values through a range
in temperature for sulfur trioxide concentrations of 0, 1, and 3 ppm are
portrayed by the three curves. The field measurements without conditioning
were obtained at sulfur trioxide concentrations below 0.5 ppm; the measure-
ments with conditioning were obtained at an average sulfur trioxide concen-
tration around 1.0 ppm from the ammonium sulfate conditioning agent. It is
thus appropriate to compare the baseline field data with the single laboratory
data point obtained with no added sulfur trioxide (the uppermost triangle)
and the predicted curve for no added sulfur trioxide. The agreement is
excellent. It is further appropriate to compare the conditioning test data
from the field at about 1 ppm of sulfur trioxide with an interpolated
location between the two laboratory'data points at 0 and 1.8 ppm (the upper
two triangles) and the middle of the three predicted curves (for 1 ppm of
sulfur trioxide). Again the agreement is very good.
General observations to be made from Figure 1 are as follows: 1) the
resistivity of the ash is highly sensitive to temperature in the operating
range of the Corette plant, below 150°C (300°F), and 2) the resistivity is
also highly responsive to the presence of even minor concentrations of sulfur
trioxide.
Three other laboratory resistivity measurements were conducted on the
ash to investigate possible mechanisms by which the ash resistivity might be
lowered by a factor of ten as observed with the field measurements. These
consisted of:
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1. Mixing ammonium sulfate participate with the ash at a concentration
of 1% by weight to simulate co-precipitation of ammonium sulfate in
the precipitator.
2. Exposure of the ash to sulfur trioxide followed by exposure to
ammonia to attempt to reform ammonium sulfate as an essentially
continuous surface deposit.
3. Exposure of the ash to ammonia vapor alone.
In each of these experiments the ash was subjected to concentrations far in
excess of those encountered in the field, but none of these techniques pro-
duced more than a factor of two change in the resistivity.
In summary, the laboratory studies showed that only sulfur trioxide pro-
duced an effect on resistivity of the same magnitude as that observed in the
field. In addition, the effect produced by sulfur trioxide was quantitatively
comparable with that observed in the field.
Precipitator Electrical Characteristics
The electrical operation of the precipitator power supplies during the
test series was consistent with the in situ and laboratory measured values of
resistivity for the with- and without-agent conditions. Comparison of the
baseline and conditioning test data indicate that the overall useful average
precipitator current density increased from 2 to 24 nA/cm when the condi-
tioning agent was used.
The average operating points and voltage-current relationships for the
precipitator indicate that the baseline data exhibit severe limitations due
to high resistivity, whereas the conditioning agent data sets show that
increased values of voltage and current are possible before back corona or
sparking is encountered. However, under certain high load, high temperature
conditions, the resistivity reduction produced by the conditioning agent was
not sufficient to allow the desired level of collection efficiency to be
achieved.
Reduction of Reentrainment (Increased Cohesiveness of the Precipitated Ash)
Measurements of ESP collection efficiency with the electrode rappers
energized and de-energized indicate that the fraction of emissions attribut-
able to rapping did decrease with conditioning agent usage. However, the
amount of emissions due to rapping with the agent in use were not signifi-
cantly less than those predicted from a correlation developed for ESPs with
no conditioning agent employed.1 These results suggest that the most impor-
tant mechanism in the reduction of rapping emissions was the improved
electrical characteristics of the precipitator, which allowed a greater mass
fraction of the dust to be collected in the inlet fields during the with-'
agent test series. This, in turn, allowed lower mass collection rates in the
outlet fields, and thus less dust was collected between raps for possible
reentrainment in the outlet fields. Since a precipitator follows an
exponential mass collection efficiency relationship with collection area,
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higher overall collection efficiencies generally result in reduced mass
collection rates in outlet fields.
Space Charge Enhancement
Particle concentration measurements with an ultrafine particle sizing
system with agent injection on and off provided no evidence of a submicron
fume as a result of the ammonium sulfate injection. Similarly, the ESP
voltage-current relationships gave no evidence of space charge enhancement
resulting from introduction of a submicron particulate.
Ion mobility determinations were made with a wire-pipe corona device in-
serted in the flue for the with- and without-agent test series. The data from
this instrument indicated no significant differences in the mobility of the
charge-carrying components of the flue gas as a result of conditioning agent
usage. If a less-mobile component of the flue gas were to assume a signifi-
cantly increased fraction of the charge flux, the result would be a space
charge enhancement similar to that described with the introduction of a sub-
micron fume.
Particle Agglomeration
Inlet size distribution data obtained from impactor traverses for the
baseline and the conditioning test series show that, within the accuracy of
the instrumentation, the conditioning agent does not influence the inlet size
distribution over the particle size range resolved by the impactors.
Similarly, the ultrafine sizing system detected no significant particle con-
centration or size distribution changes at the ESP inlet over the particle
size range from approximately 0.05 to 0.5 ym diameter.
Performance Analysis
A computer program developed to simulate ESP operation8'9 was used with
the data obtained at Corette as input for the baseline and for the condition-
ing agent test series to project overall mass efficiency as a function of
specific collection area. The results from these simulations are plotted in
Figure 2, along with the experimental data obtained with mass trains. The
results indicate the mathematical model was reasonably successful in predict-
ing the overall collection efficiencies for both test series, although the
baseline efficiency was significantly lower than the value the model predicted.
This results from the unusually large reentrainment emissions obtained with a
two-stage ESP with poor electrical operating conditions, and from the
difficulty of estimating useful electrical conditions when the ESP power
supplies are operating with severe sparking or back corona. The correlation
used in the model for estimating reentrainment was obtained using data from
relatively high efficiency ESPs with several fields in the direction of gas
flow.
The significance of these results is illustrated by projecting the
measured efficiency point from the with-agent test series to the required value
of specific collection area on the curve generated by the model for the base-
line electrical conditions. This exercise indicates that, without condition-
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ing, a specific collection area of approximately 375 ft2/1000 acfm, i.e. an
ESP over two and one-half times as big, would be required to give the 97.5%
efficiency level obtained with conditioning agent in use. The baseline model
projections represent the worst case of high temperature, low sodium, high
resistivity conditions observed during the test program.
GANNON 6 TEST
Plant Description
Gannon Unit 6 is a 350 MW pulverized coal-fired Riley Stoker boiler built
in 1967. It utilizes a Research-Cottrell cold side precipitator that treats
a flue gas volume of 1,350,000 ACFM with a specific collection area of 64.0
m2/(m3/sec) (326 ft2/1000 ACFM). The ESP has four identical chambers each
with eight electrical sections in the direction of gas flow.
The gas conditioning system was designed and installed by Apollo Tech-
nologies, Inc. It is similar to that used at the Corette Station. It can be
used for injection of an ammonium sulfate solution prior to either the air
preheater or the ESP (hot side or cold side injection). It was operated in
both modes during the investigation discussed in this report. Its normal mode
of operation is now cold side injection; originally the normal mode was hot
side injection as at Corette.
Test Program
Testing at the Gannon Station consisted of four phases: 1) a baseline
test with no conditioning agent in use, conducted in December 1979, 2) a "cold
side proprietary agent test", in which measurements were performed in January
1980 with the normal plant procedure of injecting the proprietary conditioning
agent downstream of the air preheater, 3) a "cold side generic agent test",
in which a solution of 36% ammonium sulfate in water was injected downstream
of the air preheater, also in January 1980, and 4) a "hot side proprietary
agent test" in which the proprietary agent was injected ahead of the air
preheater. This last phase was performed during the period January 30 -
February 1, 1980. As a result of T-R set outages on one side of the precipi-
tator, most of the measurements were obtained across only one of the four
chambers (Chamber 4) during all four phases.
Results
The results of experiments performed at Gannon Unit 6 and supporting
experiments performed in the laboratory are described below.
Collection Efficiency
Normal precipitator mass collection efficiency data were obtained during
three of the four test phases using EPA Method 17 mass concentration measure-
ments across Chamber 4 of the precipitator. In the fourth series of tests,
which were conducted with proprietary agent injected ahead of the air pre-
heater, only the "rap/no-rap" procedure employing mass train measurements was
performed. The purpose of this procedure was to determine whether hot side
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injection of ammonium sulfate resulted in significant differences in reentrain-
ment emissions when compared with cold side injection. Mass efficiency data
are shown in Table 5, with data obtained by the rap/no-rap method shown in
parentheses.
It is apparent from these data that the precipitator was performing at a
high level of collection efficiency even when the conditioning agent was not
in use. However, there is considerable evidence from resistivity measurements
and T-R set data that the baseline measurements were unfortunately not repre-
sentative of normal baseline conditions. This problem will be considered
further in the subsequent discussion of conditioning mechanisms. Even though
the baseline collection efficiency is considered to be higher than normal,
the efficiency data consistently show that reduction in penetration, or
increases in collection efficiency, occurred as a result of agent usage.
The mass penetration of particles smaller than 2.0 ym diameter was
reduced by 40 - 60% as a result of the injection of ammonium sulfate.
Chemical Analyses
Two conditioning agents were used in the studies at Gannon Unit 6. One
was a proprietary agent supplied to Tampa Electric Company by Apollo Technol-
ogies, Inc. The second agent was a "generic" agent—an aqueous solution of
ammonium sulfate purchased by Tampa Electric from another chemical supplier.
The analyses of the two agents confirmed that ammonium sulfate was the predom-
inant component of each. The data also show that urea was present at a low
concentration in the proprietary agent but absent from the generic formulation.
The results of coal analyses (Table 6) indicated that no unacceptable
variation occurred during the different phases of testing at Gannon Unit 6.
Comparison of the coal properties with those of the coal burned at Corette
reveal that the coal at Gannon was of higher rank and contained a higher level
of sulfur.
Analytical data for samples of hopper ash obtained during each of the
field tests at Gannon are given in Table 7. The major difference between this
ash and that encountered at Corette is the much lower alkalinity of the
Gannon ash (as indicated by the calcium oxide content). The uptake of ammonia
by the ash was increased significantly due to this reduced alkalinity.
The data in Table 8 show that significant concentrations of ammonia and
sulfur trioxide were found in the gas phase at the inlet of the precipitator,
indicating a substantial degree of thermal decomposition of ammonium sulfate
even with cold side injection and a significant persistence of the gaseous
decomposition products at the reduced temperature following hot side injection.
Comparison of the concentrations of ammonia and sulfur trioxide in the
gas phase with those in the fly ash shows that a substantial fraction of each
substance occurred in each phase. Thus, the markedly preferential uptake of
sulfur trioxide over that of ammonia that was observed at Corette was not
observed at Gannon. The difference in behavior of the two ashes in this
regard is attributed to the lower alkalinity of the Gannon ash, which makes
this ash less hostile to the uptake of ammonia.
245
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Electrical Resistivity Studies
Measurements of the electrical resistivity of fly ash were made for
Gannon Unit 6, as at Corette, in both the field and the laboratory. Also,
predicted resistivities were compared with the field- and laboratory-measured
values. The general purposes of this work were -to quantitate the effect of
the conditioning agent on resistivity, to elucidate the mechanism of resis-
tivity attenuation (through the action of undecomposed ammonium sulfate or the
individual decomposition products), and to test the agreement between measured
and predicted values.
Figure 3 includes a presentation of field-measured resistivity values.
The two shaded areas and the two data points therein show the regions occupied
by the data and the averages calculated from the data for tests with and
without conditioning. The upper area is for the baseline test; the lower area
is for all three conditioning tests and reveals a reduction of resistivity of
almost one order of magnitude as the result of conditioning.
Figure 3 also presents laboratory data obtained at three concentrations
of sulfur trioxide (0, 2, and 6 ppm). Comparison of the baseline field data
with the laboratory data shows reasonable agreement at a laboratory concen-
tration of zero sulfur trioxide. Comparison indicates, further, that the
field data obtained with conditioning would agree with laboratory data at a
concentration between the two experimental values of 2 and 6 ppm. Field
concentrations for sulfur trioxide were (1) less than 1 ppm in the baseline
test (determined at the inlet of Chamber 1) and (2) 0.8 to 2.4 ppm in the
three conditioning tests (determined at the inlet of Chamber 4). Thus, the
overall agreement between the field and laboratory data is reasonably satis-
factory and indicates that the sulfur trioxide produced by thermal decomposi-
tion of the ammonium sulfate is the probable explanation of the reduction in
resistivity observed during the field conditioning tests. However, since some
differences existed between the Corette and Gannon ashes, the experiments to
evaluate the effects of co-precipitation of ammonium sulfate and treatment by
gaseous ammonia were repeated. The results showed, as at Corette, less than
a factor of two change in resistivity due to these mechanisms.
In conclusion, therefore, the apparent mechanism of resistivity attenua-
tion is the uptake by ash of sulfur trioxide resulting from the thermal
decomposition of ammonium sulfate.
Precipitator Electrical Characteristics
Electrical characteristics of the precipitator power supplies showed
evidence of the attenuation of electrical resistivity that was revealed by
both the field- and laboratory-measured values discussed above. Secondary
voltages and currents of the power supplies were recorded daily during each
of the test phases, and total power inputs were calculated from these data.
Average daily power inputs to the east side of the Gannon Unit 6 precipitator
(serving Chambers 3 and 4) during each phase of the testing show an average of
158 KW during the baseline test versus 254 KW for the conditioning tests.
246
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There was no evidence of back corona without conditioning, but the
current density attainable without sparking was enhanced by conditioning. The
absence of back corona in the baseline test was confirmed by high voltage
wave-form photographs.
There was evidence of a slow degradation of the ESP performance during the
baseline test. Although the average total power input to the eight electrical
sections was about 158 KW during nine days of measurements, there was a
gradual decrease from about 200 KW on the first day to about 100 KW on the
ninth day. The favorable subsequent effect of conditioning was thus greater
than that indicated by the average value from the baseline test. The gradual
degradation of the ESP during the baseline test may have been caused by a
delay in elimination of residual effects of ammonium sulfate that had accumu-
lated during prior operation with conditioning, perhaps a gradual evolution of
sulfur trioxide from a deposit of ammonium sulfate in the duct leading to the
ESP.
Reduction of Reentrainment (Increased Cohesiveness of the Precipitated Ash)
Tests to compare emissions under rap and no-rap conditions were conducted
at Gannon Unit 6 during three of the test series: baseline, generic agent
with cold side injection, and proprietary agent with hot side injection
(similar results for cold side injection of the proprietary and generic agents
was expected). Rapping emissions were quantified by two measurement systems—
Method 17 mass trains and a Large Particle Sizing System (LPSS) designed by
this Institute to determine real-time particle concentrations in five size
ranges.
The results obtained with the Method 17 mass trains reveal that both rap
and no-rap emissions were reduced on an absolute basis with conditioning in
progress. The data were in quantitative agreement with predictions made on
the basis of no assumed change in dust cohesiveness. The inference, therefore,
is that no large change in cohesiveness occurred.
The LPSS data were in agreement with the Method 17 data. The primary
additional information provided by the LPSS was an indication that, as
expected, most of the rapping emissions were of the larger particles, specif-
ically those larger than 3 ym.
Space Charge Enhancement
Three types of data provided the basis for deciding whether or not the
use of ammonium sulfate conditioning produced a fine fume that could have
enhanced space charge and thus improved electrical conditions. First was
direct measurement of concentrations of ultrafine particles (from about 0.01
to 2.0 ym). Data obtained in the baseline test fell between results of two
conditioning tests, and cumulative concentrations were all within a factor of
two of each other; thus, no evidence of a fine fume during conditioning was
obtained. Second were the voltage-current density curves of the precipitator
transformer-rectifier sets; current density was shifted to higher values by
conditioning, giving evidence that resistivity attenuation overwhelmed any
contrary shift associated with space charge enhancement. Third were
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determinations of ion mobilities without conditioning at Gannon Unit 6 and
with conditioning at Gannon Unit 5 (equipment problems prevented measurements
at Unit 6 with conditioning); mobility values for both circumstances were
within the normal range and gave no evidence of the suppression associated
with fine particles.
Particle Agglomeration
Particle size distributions at the precipitator inlet with and without
conditioning, as obtained with impactors and the ultrafine particle sizing
system give no indication of particle growth as the result of conditioning.
Performance Analysis
Figure 4 gives the result of computer model simulation of the Chamber 4
precipitator with three electrical data sets: 1) those observed during base-
line conditions, 2) those observed with cold side injection of the generic
conditioning agent, and 3) a current density of 5 nA/cm2, which would repre-
sent the expected allowable current density if the in situ resistivity
increased by a factor of four over the measured values observed at the
entrance to Chamber 4 during the baseline test series. This factor of four
increases in resistivity would represent the expected average value of resis-
tivity across the inlet to the precipitator in the absence of any residual
effects from the conditioning agent.
These computer simulations illustrate how the performance of the unit
can become marginal with electrical conditions consistent with the expected
average values of baseline resistivity. The loss of one equivalent electrical
section would lower the efficiency to a range in which consistent compliance
with the local emission standard of 0.1 lb/106 Btu would become doubtful,
ECONOMIC ANALYSIS
At both the Corette and Gannon installation, the major cost element
associated with use of the conditioning agent is the purchase cost of the
chemical compounds. At Corette, the cost of the conditioning agent is
0.143 mills/kWh, and at Gannon, the cost is 0.08 mills/kWh.
At both installations, the use of the conditioning agent is cheaper, on
the basis of levelized annual costs, than the alternative option of adding
precipitator plate area to achieve the levels of particulate control which
the conditioning agent allowed to be obtained with the existing plate area.
The plate area requirements without the use of conditioning agent were esti-
mated using the EPA-SoRI mathematical model of electrostatic precipitation.
CONCLUSIONS
The following conclusions about the use of ammonium sulfate to condition
cold side precipitators can be drawn from the results discussed in this report.
• The active ingredient of both the proprietary and generic conditioning
agents was ammonium sulfate. A trace of urea occurred in the
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proprietary agent, but apparently did not affect precipitator perfor-
mance.
t Decomposition of the ammonium sulfate to ammonia and sulfur trioxide
occurred with both hot side and cold side injection. However, decom-
position was less complete at the lower injection temperature and
approximated the degree predicted from thermodynamic data.
• Addition of ammonium sulfate improved the collection efficiency of the
two precipitators tested on both a total mass and fine particle basis.
The performance improvements ranged from 20% to 90% reduction in mass
penetration of the precipitator.
• The primary mechanism by which ESP performance was improved was a
decrease in fly ash resistivity which allowed the precipitator to
operate at higher power levels. The major part of the reductions in
resistivity can be attributed to the effect of sulfur trioxide alone.
• The conditioning agent does not significantly change particle size
for particle sizes larger than 0.05 ym diameter. Thus, it gives no
evidence of conditioning through the mechanisms of fly ash agglomera-
tion or production of an ultrafine fume.
• The reduction in rapping emissions was primarily due to improved
collection in the inlet fields and not to a large change in the
cohesive properties of the dust.
• The use of ammonium sulfate as a conditioning agent can be economically
favorable method of improving performance in comparison to additions
of precipitator plate area. However, operational problems associated
with its use can include air heater pluggage with hot side injection
and duct buildups, if distribution is inadequate, with cold side
injection. Moreover, under certain operating conditions, the resis-
tivity reduction achieved by ammonium sulfate conditioning may be
insufficient.
ACKNOWLEDGMENTS
Officials of Montana Power Company and Tampa Electric Company were most
cooperative in providing field test facilities and adjusting normal plant
operating procedures to accomodate the needs of this research program.
Assistance by Carlton Grimm and Fred Walter of Montana Power and James L.
Hudson, Jr., of Tampa Electric was especially noteworthy.
ENDNOTES
1. J. P. Gooch and G. H. Marchant, "Electrostatic Precipitator Rapping Re-
entrainment and Computer Model Studies", Report EPRI FP-792, Volume 3,
Electric Power Research Institute, Palo Alto, California 1978.
2. J. Dalmon and D. Tidy, Atmos. Environ. 6^, 81 (1972).
249
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3. E. B. Dismukes, "Conditioning of Fly Ash with Sulfur Trioxide and Ammonia,"
Report EPA-600/2-75-015 (TVA-F75 PRS-5), U.S. Environmental Protection
Agency, Research Triangle Park, North Carolina (Tennessee Valley Authority,
Chattanooga, Tennessee), 1975.
4. E. C. Potter and C. A. J. Paulson, Chem. Ind. 1974, 532.
5. W. B. Smith, K. M. Gushing, and J. D. McCain, "Procedures Manual for ESP
Evaluation," Report EPA-600/7-77/059, U.S. Environmental Protection
Agency, Research Triangle Park, North Carolina, 1977.
6. R. E. Bickelhaupt, "Influence of Fly Ash Compositional Factors on Elec-
trical Volume Resistivity," Report EPA-600/2-75/074, U.S. Environmental
Protection Agency, Research Triangle Park, North Carolina, 1974.
7. R. E. Bickelhaupt, "Effect of Chemical Composition on Surface Resistivity
of Fly Ash," Report EPA-600/2-75/017, U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina, 1975.
8. J. R. McDonald, "A Mathematical Model of Electrostatic Precipitation
(Revision 1): Volume I, Modeling and Programming," Report EPA-600/7-78-
llla, U.S. Environmental Protection Agency, Research Triangle Park,
North Carolina, 1978.
9. J. R. McDonald, "A Mathematical Model of Electrostatic Precipitation
(Revision 1): Volume II, User Manual," Report EPA-600/7-78-lllb, U.S.
Environmental Protection Agency, Research Triangle Park, North
Carolina, 1978.
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SUMMARY OF METHOD 17 AND CONDITIONING AGEN'
Table 3.
ANALYSIS OF FLY ASH FROM CORETTE
ftugust October November
August October November conditioning conditioning baseline
T „„ Q . conditioning conditioning baseline Component" pretest testc testc
Parameters Baseline Conditioning Proximate analyst
a
Efficiency, % 77.30 97.50 * V° atlle
(69.46) (97.13) » Fixed carbon
SCA,ftJ/1000 acfm 147 143 BtU/U>
% Sulfur
Total 1.78 o 13 ultimate analysis
"2 Um °'12 °'025 Number o£ samples
Conditioning agent % Moisture
Addition rate, gal/ton coal 0 0.096 % carbon
°F - 1,000-1,150 * Hydrogen
% Nitrogen
% Chlorine
a. Estimated useful voltage and current from voltage-current % Sulfur
curves.
% Ash
b. Data in parenthesis are from rap/no-rap measurements. % Oxygen
7 17 9
11.66 + 1.74 12.85 + 1.56 11.68 £ 1.29
11.04 + 0.52 10.61 + 0.43 10.68 + 0.39
34.61 + 0.63 33.84 + 0.79 33.65 £ 0.56
42.69 + 1.00 42.68 + 0.87 43.99 + 0.91
10,172 + 236 10,079 + 200 10,206 + 154
1.07 -I- 0.11 0.71 + 0.10 0.74 + 0.03
b
a . The data given are avei
the standard deviations
b. Not performed for the ?
Table 4
Ul COMPARISON OF OBSERVED AMMONIA AND SULFUR TRIOXIDE
H- ' CONCENTRATIONS IN FLUE GAS AND FLY ASH WITH
INJECTED CONCENTRATIONS AT CORETTE
Date of Injected Concn, pprn Sampling Port Gas Phase Fly Ash
test NH 3 SO 3 Location No. NH 3 SO 3 NH 3 S03
Aug. 6.8 3.4 AH Inlet - 11.3 4.5 0.035 26
I012
ESP Inlet 2 2.5 <0.5
3 1,2 <0.5 -
5 1.6 - 0. 73 12 1
o
8 3.4 <0.5 ^
10 4.2 - p
ESP Outlet - 0.21 - jjj
Oct. 7.4 3.7 AH Inlet - 13.1 1.3 0.085 18
1010
ESP Inlet 1 1.3
2 1.2 0.5
5 1.7 -
0.33 12
7 - 1.0
IB*
9 4.0 - 1000'T(
10 3.9
LSP OiitU't - -0.7 0.018 0.95
Stack 2.7 --0.5 - F
i 1 .
r
i ' (
1 r SHA
; 290
! SYM
AVE
(BAS
: i
N
:••• |
K) — 3.2
°C — 40
op — 103
3 3
13.34 + 0.74 11.31 + 0.77
58. 56 + 0.72 59.61 + 0.48
3.94 + 0.06 3.95 + 0.05
1.03 + 0.10 1.11 ^ 0.05
0.10 + 0.01 0.03 + 0.01
0.71 + 0.08 0.72 t 0.03
10.37 + 0.36 10.64 + 0.32
11.96 + 0.18 12.63 + 0.68
Li2O
Na20
KjO
MgO
CaO
A1203
Fe20j
Si02
Ti02
S03
0.04 0.03 0.04
0.99 0.38 0.60
0.67 0.55 0.37
3.4 5.0 5.6
16.2 20.7 19.0
23.3 21.7 21.4
5.9 4.6 6.2
44.3 41.8 42.2
1.8 2.1 1.2
0.4 0.3 0.4
1.1 l.l 1.4
Total 98.10 98.26 98.41
NH3 0.010 0.004 0.0001
SO,,"2 0.98 0.70 0.60
ignited. The
ignition by ex
ages for the numbers of samples indicated plus
from the avetages. b- For * samPle c
c. For a proporti
ugust pretest. ESP inlet and
99.9
99.8
$03 (TOP, MIDDLE, AND BOTTOM CURVES, !
RESPECTIVELY!. 1 JJ-tWJ. '
j Jlij || | 1 1 f '' ** -\ 1- -
DED AREA T "7^5 ~ " "V ^ - ' N ' \ ['
IAGE VALUE!" fl^; , j J ' I
ELINE) J }7hf / \
/ m
i y/?- r -SHADED AREA | I • i ' ;
f~ ~//f~ "j " " 36 DATA POINTS .1 ' ' \ \ '•
}~ ~y]\; SYMBOL IS \\ it
I/ COND TION NG) \\
I T • i • ji i j ~r i i
| - 1 M 1 1 1 1 i • I
OF S03 TOP, MIDDLE, AND BOTTOM
|| '| | ; ; . , , ! . | j
99.7
99.6
99.5
99.3
- 99.2
99.1
i 99
K 98
u,
97
j
95
94
93
92
: 91
3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1 4 1.2
60 84 112 144 182 227 283 352 441 560
141 183 233 291 359 441 541 666 826 1041 BQ
TEMPERATURE oto-tB
7D
tracting soluble NHj and SO.,'2 with water.
sllected in the mass train at the ESP inlet.
jnate blend of individual samples collected from the
jutlet hoppers.
1 I 1 1
WITH AND WITHOUT AGENT
— MODELED WITH AGENT 9 _
S • I
-D /
_ » A MEASURED WITH AGENT
_ / Q MEASURED PRETEST
— / O MEASURED WITHOUT AGENT
/
/ AGENT V = 47 kV
/ j = 24 nA/cm2
' MODELED WITHOUT
AGENT V » 30 kV i
j • 1 7 nA/cm2(8ACK CORONA
O
1 1 , 1
gure 1. Field, laboratory, and predicted resistivity values fur Corette. 100 200 300 400 500 eoO
SCA, ft2/1000 ACFM j,,, ,
-------
SUMMARY OF METHOD 17 AND CONDITIONING AGENT
DATA FOR CANNON UNIT 6
r A • Y • •
Lonaitioning
Average electrical conditions
Voltage, kV
Precipitator
Efficiency, Z
SCA, ftVlOOO acfm
Outlet emissions, lb/106 Btu
Total
<2 um
Conditioning agent
Addition rate, gal/ton coal
Temperature of addition, °C
(99.73)d
0.006
0.001
99.92
(99.91)d
0.002 0.006
0.0006 0.0004
0.182 0.189
179 175
355 347
(99.84)a
335
0.159
ca 540
ca 1,1000
a. Pr/CS = proprietary agent with cold-side injection.
Ge/CS = generic agent with cold-side injection.
Pr/HS = proprietary agent with hot-side injection.
b. Estimated useful voltage and current from voltage-current cur
c. No result is available.
d. Data in parenthesis are from rap/no-rap measurements.
Ln
to
COMPARISON OF AMMONIA AND SULFUR TRIOXIDE
CONCENTRATIONS IN FLUE CAS AND FLY ASH
WITH INJECTED CONCENTRATIONS AT GANNON
Type o
jrlstf
Pr/CS
Ge/CS
Pr/HS
HH3
SOj
12.2 6.1
12.8 6.4
10.6 5.3
Sampling
ESP Inlet
ESP Outlet
ESP Inlet
ESP Outlet
ESP Inlet
ESP Outlet
•age Observed Cone
Fly Ash
NH3 SO,
2.6
0.3
2.4
0.9
1.2
1.9
0.8
0.6
4.5
0.1
7.7
0.1
7.3
0.1
2.8C"
<1'°x
~c c°
Table 6.
ANALYSIS OF COAL FROM CANNON UNIT 6
a. See Table 5 for definition of abbreviations.
b. Calculated as the increment over the S03 equivalent in baseline ash.
Based on samples collected from the flue gas ducts in mass trains,
but in reasonable agreement with data for hopper samples (Table 4-9).
Maxim
SO-,-2
Proximate analysis
Number of samples
% Moisture
% Ash
% Volatile
% Fixed carbon
Btu/lb
2 Sulfur
Ultimate analysis
Number of samples
% Moisture
% Carbon
% Hydrogen
% Nitrogen
% Chlorine
% Sulfur
2 Ash
% Oxygen
test
8
8.
10.
31.
49.
12,
1.
3
8.
67.
4.
1.
0.
1.
10.
6.
13
24
84
79
033
14
73
90
31
42
16
15
19
14
Pi
3
8
9
31
49
12
1
2
8
68
4
1
0
1
10
6
r/CS
.51
.90
.87
.72
,036
.08
,14
.60
.16
.52
.18
.10
.05
.25
G,
5
8
9
31
49
12
1
2
8
67
It
1
0
1
9
6
s/CS
.48
.71
.82
.99
,039
.10
.76
.40
.28
,44
.14
.14
.94
.91
Pr/HS
3
7.
9.
31
51.
12 :
1.
2
7
69.
4.
1
0.
1
9
6.
96
.01
.93
10
.277
.19
.67
.29
.69
.50
.13
.21
.04
,48
FIELD RESULTS WITHOUT
CONDITIONING
1 : I M I !
FIELD RESULTS WITH
CONDITIONING
144 182 227
291 359 441
TEMPERATURE
ANALYSIS OF FLY ASH FROM CANNON
t
0
0
3
2
2
23
13
51
est
.01
.68
.5
.0
.0
.6
.6
.6
1.1
0
0
.29
.30
Pr/CS
0.01
0.69
4.3
1.9
2.0
25.0
13.4
48.7
1.3
0.45
0.50
Ge
0
0
4
1
1
23
13
/cs
.01
.63
.2
.8
.8
.3
.4
48.0
1
0
0
.3
.34
.90
Hr
0
0
3
1
2
23
13
53
1
0
0
/HS
.01
.63
.9
.7
.0
.5
.8
.7
.1
.20
.40
Total 98.68
8.25 95.68 100.94
0.050 0.088 0.046
NH3 0.003
2
SO, 0.51 1.00 0.99 1.00
Computed for proportionate blends of inlet and outlet hopper sample
The first 11 components were determined after the sample had been
ignited. The last 2 components were determined prior to sample
ignition by extracting soluble NHa and S0i,~2 with water.
Table 10.
50
100 150 200 250 300
SPECIFIC COLLECTION AREA, ft2/1000 ACFM)
I 1 I I I I
NO. C
fields 01
from 332
4. Collection efficiency as a function of SCA or number of
ervice at Gannon Unit 6. (Actual SCA values ranged
342 ft2/1000
-------
EVALUATION OF PERFORMANCE ENHANCEMENT OBTAINED WITH PULSE
ENERGIZATION SYSTEMS ON A HOT-SIDE ELECTROSTATIC PRECIPITATOR
By Walter Piulle
Electric Power Research Institute
3412 Hillview Avenue
Palo Alto, California 94303
L. E. Sparks
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, N.C. 27711
and
G. H. Marchant, Jr.
J. P. Gooch
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35255
ABSTRACT
Two pulse energization systems of differing design were temporarily in-
stalled on separate chambers of a hot-side electrostatic precipitator. Meas-
urements of overall and size dependent collection efficiency were performed
using conventional and pulse energization for transformer-rectifier sets on
each of the two separate chambers. The enhancement in performance resulting
from the use of pulse energization was evaluated on an overall mass and on a
particle-size-dependent basis. The results obtained from the full-scale
installation are compared with results obtained in a pilot precipitator at
EPA's Industrial Environmental Research Laboratory, Research Triangle Park
N.C. The precipitator was performing below design values without the use of
sodium conditioning and had experienced a pattern of perf°^^d^°™*1O
with accumulated time of operation following plant outages during which the
precipitator was cleaned by water washing. Sodium conditioning is normally
used to maintain desired levels of performance.
INTRODUCTION
Developments within the electronic industry during the past de^e have
resulted in the promotion of the concept of pulse energization ^ ^r°v
of ESP performance. Pulse energization was investigated by H J. ™^
late 1940's and early 1950's but, due to the "^atxons of the then available
equipment, the concept was not pursued. Pulse energization basically in
253
-------
superimposing high voltage pulses on top of the normal precipitation voltage
waveform. Several ESP manufacturers in the U.S. and abroad are now offering
pulse energization systems on a commercial scale.
Advantages claimed for pulse energization in comparison with conventional
DC energization include: 1) higher peak voltage without sparking; 2) more
controllable corona current; 3) more uniformly distributed corona discharges;
and 4) higher input power.
Two pulse energization systems of differing design were temporarily in-
stalled on separate chambers of the hot-side ESP at the Lansing Smith gener-
ating station of Gulf Power Company. Side A of the ESP was equipped with a
pulsing system supplied by Buell Envirotech, and side B was equipped with a
pulsing system supplied by Research-Cottrell, Inc. A research program was
sponsored by the Electric Power Research Institute, the U.S. Environmental
Protection Agency, and Southern Company Services, Inc. for the purpose of
evaluating the effectiveness of pulse energization as a means of improving
hot-side ESP performance. This paper gives preliminary results obtained from
the evaluation program.
BACKGROUND
In 1977, hot-side ESP's were retrofitted on both units at Plant Lansing
Smith to allow the plant to meet environmental regulations when they began
firing low-sulfur, low-sodium South African coal. The units initially passed
normal compliance tests, but performance deficiency problems appeared in 1978.
A number of potential remedial approaches, such as power-off rapping, the use
of improved transformer-rectifier (T-R) set controls, reversible-polarity T-R
sets, ammonia conditioning, ESP washdowns, and sodium conditioning, were
attempted. With the exception of ESP washdowns and sodium conditioning, these
procedures did not produce the desired level of ESP performance. Each ESP
washdown did improve performance, but ESP performance degraded over a period
of from 1 to 2 months until it reached unacceptable levels. An outage would
then be required to perform another washdown. In early 1980, a research pro-
gram conducted by Southern Research Institute, under the sponsorship of the
Electric Power Research Institute, the Environmental Protection Agency, and
the Southern Company Services, Inc., was begun to evaluate sodium conditioning
as a potential solution to the problem. Unit 2 at the Lansing Smith Station
(rated at 196 MW) had been operating since May 1980 (7 months) with sodium
conditioning, except for the 8 day period at the beginning of the evaluation
period for the pulsers, without an ESP washdown, with acceptable performance
levels.
TEST PLAN
The initial evaluation program called for data to be obtained from each
chamber of the Unit 2 ESP with each vendor's pulsing system operational and
with no sodium conditioning of the coal supply. These data were to be compared
with baseline data taken previously on Lansing Smith Unit 2 when the ESP was
in a degraded condition without sodium conditioning. A test series was also
planned which consisted of ESP efficiency measurements for each chamber to
determine the effectiveness of pulse energization for sustaining desired
254
-------
levels of performance for an extended operating period (2 to 3 months)
Although_it was desirable to arrange the pulsing systems for each chamber in
an identical configuration for evaluation purposes, equipment limitations
prevented this goal from being accomplished. Figure 1 (top) gives the original
electrical sectionalization of the ESP, and the electrical sectionalization
and areas pulsed during the pulse energization test program (bottom). As the
diagram indicates, the outlet half of the plate area was pulsed on the A side,
whereas the inlet 80% of the plate area was pulsed on the B side.
The test program began at Plant Lansing Smith the first week of November,
1980, with no sodium conditioning of the coal supply. After the unit had
operated for 7 days without sodium conditioning, ESP performance had degraded
to the point that, even with both pulsers operational, compliance with local
emission codes could not be maintained unless unit load was reduced, or unless
the sodium conditioning system was placed back in service. Plant management
then ordered the activation of the sodium conditioning system to increase the
sodium oxide content in the fly ash. From previous conditioning tests, the
level required for acceptable performance was determined to be 'VL%. On
November 17, the sodium oxide content in the fly ash was '^1%, and testing was
performed with impactors on the B side ESP with and without pulser operation.
At this level of sodium addition, ESP performance was relatively high (^13%
plume opacity) , and the effect of the B side pulser on outlet emissions was
difficult to determine. A tube leak necessitated the unit to be off-line on
November 18, 1980. After start-up, plant management agreed to lower the
sodium addition rate in order to degrade the electrical operating conditions
so that the effects of the pulsers could be discerned at a lower ESP perfor-
mance level. The sodium oxide content of the fly ash was reduced to approxi-
mately 0.7%. At this level of sodium conditioning, stack emissions were at a
level which could be tolerated, and effects of the pulse energization systems
were discernible. As a result of the inability of the ESP to maintain compli-
ance without sodium conditioning, the evaluation of the capability of pulsing
to maintain a given level of performance for an extended period was not
conducted.
RESULTS
Averaged results from the Method 17 mass sampling system are presented in
Table 1. Table 2 summarizes results from a previously conducted sodium condi-
tioning test series for comparison. Coal and ash analyses are given in
Tables 3 and 4. For reasons previously discussed, only a limited amount of
data were obtained with no sodium addition to the coal supply. The results
obtained indicate that penetration decreased from 3.33% to 1.57% on the B side,
and from 3.76% to 1.68% on the A side as a result of pulse energization with
no sodium addition. However, the increase in performance on the A side
during the November 13, 1980, test appears to have been associated at least in
part with a coal composition change due to sodium sulfate contamination of the
coal supply. Chemical analysis of fly ash obtained on November 13, 1980,
gave an Na20 content of 0.41% in the fly ash, whereas the normal baseline Na20
content of ash from the South African coal is 0.32%.
For the test condition of ^0.7% Na20 in the fly ash, the averaged mass
train results indicate that penetration decreased from an average of 1.671 to
255
-------
0.93% on the A side, and from 1.76% to 0.58% on the B side as a result of
pulse energization. Individual efficiencies from which these average values
were obtained are given in Table 1.
Total mass loadings obtained from traverses of the inlet and outlet
sampling ports with impactors are given in Table 5. Since it is necessary to
maintain constant flow through an impactor to obtain size resolution, isokinet-
ic traverses are not possible. However, traverses were made at a flow rate
isokinetic to the average gas velocity in the duct at the sampling location.
The results from these measurements indicate, as stated previously, no
significant change in performance due to pulse energization on the B side
(0.80% vs. 0.71% penetration) with sodium conditioning at a level equivalent
to ^1% Na20 in the fly ash. Side A was not tested due to pulser equipment
failure. For the M3.7% NazO addition level, the impactor data indicate pene-
tration decreased from 2.01% to 1.25% on the A side, and from 1.82% to 1.17%
on the B side as a result of pulse energization.
Figure 2 gives efficiency as a function of particle diameter for the A
side of the ESP with and without pulsing at approximately 0.7% Na20 level.
Also shown for comparison are analogous data obtained during the sodium
condition test series with sodium at approximately the 1% Na20 level. The
data indicate that significant improvements in collection efficiency as a
result of pulse energization were detected only for particle diameters larger
than about 2.0 ym. Size dependent efficiency data for the B side are pre-
sented in Figure 3. These data indicate that, up to about 1.5 ym particle
diameter, efficiency changes due to pulsing are either marginal or not
detectable by the impactor sampling systems. For particle diameters larger
than 1.5 ym, significant collection efficiency improvement due to pulsing is
indicated. Size-dependent cumulative emission data for the two pulsing
systems are given in Figures 4 and 5; sodium conditioning test data are shown
for comparison. These cumulative emission data also illustrate that the
performance changes due to pulsing are observed primarily for particles larger
than 1.5 to 2.0 ym diameter.
In addition to the impactor sampling system, an extractive sampling
system (which employs a diluter, an electrical aerosol analyzer, and an optical
particle counter) was used at the outlet of each chamber. This system is
described in detail elsewhere.1 The system samples at a single point, and
uses electrical mobility and the optical properties of the particles as a
basis for obtaining particle size distribution data. It also enabled pulser-
on/pulser-off comparison on a real time basis. Outlet particle concentrations
were obtained with pulsing systems on and off, and the observed changes due to
pulse energization were reported as a percent reduction at a specified parti-
cle diameter. These results are displayed in Table 6. The percent reductions
shown approach the "noise level" of this type of instrumentation. These
results are interpreted as supporting the conclusion, derived from the impactor
data, that large changes in collection efficiency were not achieved by the
pulsing systems in the fine particle size bands.
Figure 6 presents data obtained at EPA's IERL-RTP laboratories in a pilot
unit with and without pulse energization. These results are discussed in
detail elsewhere,2 and were obtained under conditions which were significantly
256
-------
different (cold-side conditions with resuspended ash) from those encountered
at Lansing Smith. However, it is interesting to note that these data also
indicate significant performance enhancements due to pulsing for particle
diameters larger than about 1.5 ym.
A modified Millikan cell was used at the outlet of each chamber to obtain
charge as^a function of particle diameter using a technique described by
McDonald. The purpose of these measurements was to obtain insight into the
mechanisms through which pulsing influences ESP performance. Examination of
the data obtained in the particle size range 1.0 to 1.6 ym diameter with and
without pulsing has led to the following observations:
• On the A side, there was no evidence of increased particle charge
due to pulsing.
• On the B side, there is an indication of a 25% to 30% increase in
particle charge due to pulsing on November 20. On November 21,
the charge enhancement due to pulsing showed up only toward the
upper end of the size range and only to the extent of about 15%.
This indicates that the charge gain due to pulsing was varying
with time during the test program.
The significance of these observations is that, although similar overall
mass reductions were achieved by both pulsing systems, the mechanisms through
which the enhancements were accomplished were quite different. On side A, the
performance enhancement is thought to have been accomplished primarily by
reduction of large particle emissions which seem to be associated with reen-
trainment. On side B, the increase in particle charge associated with pulsing
is qualitatively consistent with the collection efficiency improvements (shown
in Figure 3) measured for particles in the diameter range 1.0 to 1.6 ym. That
is, the ratios of individual particle size-dependent migration velocities
ranged from 1.05 to 1.15 when pulser-on conditions were ratioed to pulser-off
conditions. These ratios for both the A and B sides are shown in Figure 7.
Note that the pulser-on/pulser-off ratios for side A show essentially no en-
hancement for particle diameter less than 2.0 ym.
Electrical operating conditions for the T-R sets not pulsed were governed
by the existing automatic controllers. Each pulsing system manufacturer
established the operating conditions of its system. The waveforms delivered
by the pulsing system are not presented in this paper due to a confidentiality
agreement with the manufacturers. Generalized waveform, pulse width, and pulse
repetition rate will be given in the final report. Also included will be
averaged and detailed electrical readings of voltage and current, and power
consumption for each system.
DISCUSSION
Although the pulsing systems installed at Lansing Smith were unable to
allow the ESP to operate in compliance, it is of interest to consider the
implications of the performance enhancements which were achieved. A practical
method of determining these implications is to estimate the plate area
equivalent of the performance gains effected by the pulsing systems. Unfortu-
257
-------
nately, since the mechanisms of performance enhancement are not described by
existing physical models, a precise method of calculating the plate area
equivalent is not available.
An estimation of the plate area equivalent of the performance gains,
however, may be obtained by using the fractional efficiency data from the
impactors, and the Deutsch equation, for individual particle sizes
V , / 100 \
wi ' A ln
to. = apparent migration velocity for the i particle diameter,
V
where -r = ratio of volume flow to plate area, and
H . = efficiency for the i particle diameter.
The impactor data sets from both chambers were used to compute 01^ values,
which were used, in turn, to calculate new values of collection efficiency
for each particle size band in a histogram distribution at larger values of
specific collection area. Overall mass collection efficiencies were obtained
by summing over the particle size distribution. (That is, n = I, P. r\. where
P. is the fraction of the inlet mass in the ith particle size band in the inlet
particle size distribution.) Results from these calculations are displayed by
the dotted lines in Figures 8 and 9. The solid lines labeled with 0)^ were
obtained with another method which will be discussed later. This calculation
assumes that the size dependent migration velocities will remain constant if
each chamber is increased to a larger specific collection area in the existing
respective configurations. Although this assumption is subject to some ques-
tion, it is believed that this method is the more conservative for extrapolat-
ing the results to larger specific collection areas.
Also shown in Figures 8 and 9 are mass train data obtained under the
indicated conditions. Since the data from the mass trains show significantly
different results in overall mass collection efficiency from those with im-
pactors, these data were extrapolated to larger specific collecting areas
graphically by drawing curves parallel to the curves obtained from the pre-
viously described numerical integration procedure with the impactor data.
Results from this exercise are interpreted as follows for the ^0.7% Na20
level:
t On the A side, the extrapolation indicates that pulsing allows
a performance level to be achieved at an SCA of 66.93 m2/(m3/
sec) (340 ft2/1000 acfm) which would require an SCA of 77.76
m2/(m3/sec) (395 ft2/1000 acfm) without pulsing. This is
equivalent to a 16% increment in plate area. The equivalent
plate area enhancement is approximately the same for both mass
train and impactor data sets.
• On the B side, extrapolation of the impactor data indicates
that pulsing allows a performance level to be achieved at an
258
-------
SCA of 64.96 in /(m3/sec) (330 ft2/1000 acfm) which would
require an SCA of 73.62 m2/(m3/sec) (375 ft2/1000 acfm)
without pulsing. This is equivalent to a 14% increment
in plate area. Similar extrapolation of the mass train
data indicates a 32% gain in plate area (from 320 to 420
ft /lOOO acfm).
The authors believe that these differing results were each representative
of the ESP performance at the time the measurements were made. The wide
variation in enhancement (14 vs. 32% is thought to be associated with the
sensitivity of indicated enhancement to the level of sodium oxide actually
present in the ash undergoing collection. Recall that with 1% Na20 in the ash
(Table 1), no enhancements were discernible.
Another approach for estimating the improvement in ESP performance due to
pulsing was recently suggested by Feldman.1* This approach is based on a modi-
fied Deutsch equation in the form,
n = 100 (l-exp-[(ajkA/V)m])
in which n = overall mass efficiency,
to, = modified overall precipitation race parameter,
A = collection area
V = volumetric flow rate, and
m = exponent depending on the inlet particle size distribution.
An "enhancement factor" based on this approach is defined as the ratio of the
to, values for pulsed and unpulsed performance. Table 7 provides enhancement
factors calculated by this definition for both sides of the ESP for the
averaged mass train and impactor data sets at the ^0.7% NaaO level condition,
with values of m of 0.4, 0.5, and 0.6. Efficiency versus SCA lines in
Figures 8 and 9 are shown derived from the (% approach with m = 0.6. It should
be noted, however, that this calculation procedure does not account for the
particle size-dependent nature of the response to pulsing which was observed
during this test program. It can be seen from the graphs that this method
results in larger apparent values of plate area gain than does the more con-
servative method used by the authors.
CONCLUSIONS
Although several undesirable circumstances were encountered during the
test program, it was possible to evaluate the capability of the pulsing sys-
tems for improving the performance of the hot-side ESP at Lansing Smith. The
following conclusions have been derived from the evaluation program:
• With no sodium addition to the coal supply, the pulsing systems
decreased penetration from 3.33% to 1.57% on the B side, and
259
-------
from 3.67% to 1.68% on the A side. These penetration reduc-
tions were the largest observed on an absolute value basis.
However, neither system allowed the hot-side ESP to operate
in compliance (0.1 lb/106/Btu, or 43 ng/J) or to approach
level of performance (0.36% penetration)5 achieved with sodium
conditioning.
• With ^0.7% NaaO in the fly ash, the penetration (emissions) on
the A side with pulsing decreased from 1.67 to 0-93% (mass
train data), and from 2.01 to 1.25% (impactor data) penetra-
tion without pulsing. It must be noted that the pulser system
on A side operated only during the testing period and not on a
24 hour basis.
t With ^.7% Na20 in the fly ash, the penetration on the B side
with pulsing decreased from 1.76% to 0.58% (mass train data),
and from 1.82 to 1.17% (impactor data).
• For particle diameters smaller than about 1.5 ym, the changes
in collection efficiency due to pulsing were relatively small
and were near the resolution limits of the instrumentation.
For particle diameters larger than 2.0 ym, significant
improvements in collection efficiency as a result of pulsing
were observed.
• The long-term capability of pulse energization to sustain a
given level of performance improvement could not be deter-
mined due to the limited duration of the test program.
• These results were obtained with a hot-side ESP which had
experienced performance degradation due to a sodium deple-
tion process adjacent to the collection electrode. The
results, therefore, are not necessarily applicable to cold-
side applications where different charge transport mechanisms
through the dust layer may be involved.
A more detailed presentation of results, along with discussions of possi-
ble mechanisms involved in the observed performance enhancements, will be
presented in the project final report.
260
-------
ENDNOTES
1. Wilson, R. R. , et al. Guidelines for Participate Sampling in Gaseous
Effluents from Industrial Processes. EPA-600/7-79-028 (NTIS PB 290899)
January 1979.
2. Rugg, D. , et al. "Electrostatic Precipitator Performance with Pulse
Excitation." To be Published.
3. McDonald, J. R., M. H. Anderson, R. B. Mosley, and L. E. Sparks.
J. Appl. Phys. 51, pp. 3632, 1980.
4. Feldman, Paul L. "Pulse Energization: Present Status." Presented at
the "Second Conference on Air Quality Management in the Electric Power
Industry." January 22-25, 1980, Austin, Texas.
5. Gooch, J. P., et al. "Improvement of Hotside Precipitator Performance
with Sodium Conditioning - An Interim Report." To be Published.
LANSING SMITH PULSER EVALUATION
METHOD 17 MASS RESULTS
Date
Particulate
Mass Loading, Precipitator
mg/DNm3 Efficiency SCA
Inlet Outlet % m2 /(ms/sec)
Loc
Sal
:ation
ipling
Condition
NO SODIUM ADDITION
11/11/80
11/12/80
11/12/80
11/13/80
11/20/80
11/20/80
11/20/80
11/20/80
11/23/80
11/23/80
11/24/80
11/24/80
11/25/80
11/25/80
15
16
13
14.
,786
,227
,881
.327
526.4
255.2
509.2
240.3
96.67
98.43
96.33
98.32
65.26
63.19
69.29
67.22
B
B
A
A
Side
Side
Side
Side
Pulser
Pulser
Pulser
Pulser
Off
On
Off
On
Table 2
RESULTS FROM SODIUM CONDITIONING TEST SERIES
Efficiency SCA Sampling
Dace Z • mz/(»J/sec) Location Condition
3/14-19/80 99.88 63.78 A i B Baseline 1
Sides After Washdow
4/22-5/2/80 98.21 61.61 A 5 B Baseline 2 -
Sides After Degradatioi
13
16
14
14
16
15
14
14
14
13
SOD
,778
,479
,602
,213
,341
,472
,991
,178
,098
,984
IUM ADDITION;
66.4
128.2
82.4
124.7
272.4
337.6
156.8
154.5
263.2
MX
99.60
99.12
99.42
99.24
98.24
97.75
98.89
98.90
98.12
,7% Na;0 IN FLY
61.61
61.42
64.37
67.91
62.11
60.63
63.09
66.93
67.81
68.90
ASH
B
B
A
A
B
B
A
A
A
A
Side
Side
Side
Side
Side
Side
Side
Side
Side
Side
Pulser
Pulser
Pulser
Pulser
Pulser
Pulser
Pulser
Pulser
Pulser
Pulser
Off
On
Off
On
On
Off
Off
On
On
Off
5/14-24/80 99.64 63.39 A i B Conditioning
1
Sides Sodium Sulfate
Conditioning to
M.OZ Na20
a. All outlet loadings were determined at the outlet of the hot-side precipi-
tator which is followed by a cold-side preclpitator.
b. Probable coal change.
TYPICAL ASH ANALYSES
(EXPRESSED AS % OP IGNITED SAMPLE)
Table 3
TYPICAL COAL ANALYSIS
Date
7. Moisture
% Carbon
% Hydrogen
% Nitrogen
% Chlorine
% Sulfur
% Ash
7. Oxygen
% Volatile
% Fixed Carbon
Cal/g
Btu/lb
11-11-80
As Received
4.82
69.82
3.89
1.71
0.02
P. 85
12.86
6.69
25.98
56.34
6630
11935
Dry
-
72.66
4.09
1.80
0.02
0.89
13.51
7.03
27.30
59.19
6966
12539
261
Sample
JOJUH J.H
Li^O
NajO
K20
MgO
CaO
FejOj
AljOs
Si02
TiOz
PaOs
SO,
LOI
No Sodium Addition
0.08
0.39
0.52
2.5
9.6
5.8
33.2
38.8
1.7
2.7
1.1
17.0
MJ.7Z Na20 Level
0.09
0.88
0.52
2.6
10.3
5.6
33.1
39.0
1.8
2.6
0.88
17.5
-------
AVERAGE MASS LOADING CALCULATED
FROM CASCADE IMPACTOR DATA
Participate
Mass Loading,
mg/DNm3
Inlet Outlet
Precipitator
Efficiency SCA
% mVdn'
Sampling
c) Location Condition
SODIUM ADDITION M.(K Na20 IB FLY ASH
11/17/80
11/17/80
15,699 110.7
15,699 125.2
99.29
99.20
59.79
59.79
B Side Pulser On
B Side Pulser Off
SODIUM ADDITION M).7% NaaO IN FLY ASH
11/21-22/80 15,657 183.6 98.83 64.83
11/21-22/80 14,784 269.0 98.18 64.83
11/24-25/80 13,959 174.3 98.75 66.68
11/24-25/80 16,263 326.1 97.99 66.68
B Side Pulser On
B Side Pulser Off
A Side Pulser On
A Side Pulser Off
a. All outlet loadings were determined at the outlet of the hot-side precipi-
tator which is followed by a cold-side precipitator.
Size, V
Percent
Size, u
Percent
Table 6
ULTRAFINE SYSTEM DATA
B Side
m 0.075 0.24 0.59
Reduction 18 17 29
A Side
m 0.075 0.24 0.59
Reduction 7 19 8
1.2
41
1.2
22
Table 7
CALCULATIONS USING VARIOUS VALUES FOR m
Pulsing Ho Pulse
m
0.4
0.5
0.6
0.4
0.5
0.6
0.4
0.5
0.6
0.4
0.5
0.6
CD, , cm/sec OL
A SIDE
Mass Trains"
71.2
32.9 ..
19.7
Impactors
62.0
25.6
18.1
B SIDE
Mass Trainsc
98.0
43.2
25.0
Irapactors
62.6
29.7
18.0
,, cm/sec
50.9
25.2
15.7
46.5
23.5
14.9
53.4
26.6
16.7
48.2
24.1
15.2
Ratio
1.40
1.31
1.25
1.33
1.26
1.21
1.84
1.62
1.50
1.30
1.23
1.18
a. SCA 66.5 mVdn'/sec), r\ 99.07 and 98.33
b. SCA 64.83 m2/dn'/«ec), n - 98.75 and 97.99
c. SCA 61.44 mVdn'/sec), n 99.42 and 98.24
d. SCA . 66.68 mVdn'/sec), n 98.83 and 98.18
PLATE AHEA PLATE AREA
2,890
31,104 2.890
31,104 2.890
23,328
23,328
15,552
2,167
2.167
1,445
31,104
31.104
31,104
23,328
23,328
15,552
2,890
2.890
2.890
2,167
NORMAL ELECTRICAL SECTIONALIZATION
.AREA PULSED BY BUELL
x"'
FLOW
\
•^
\
/rBs
f- — —-
\ "
C^)
\
frS-
Vi>
ft
^
i
j
I
I
S)
J
(T»
@
" "1
!
^\^ FT2OF FULL OR «2 OF
^ TR PLATE AHEA PULSED HALF WAVE PLATE AREA
S|_ A 31.104 NO FU Z890
E 15.552 NO FU 1,445
jS J 38,880 YES FU 3,612
^/^ G 38,880 YES FU 3,612
^^ B 31,104 YES HA 2.890
D 31,104 YES HA 2.890
H 15.552 YES FU 1,445
K 15.5S2 YES FU 1,445
.S^ M 31.104 NO FU 2,890
ELECTRICAL SECTIONALIZATION DURING PULSER TESTING
$ 95.0 _
1.0
5.0
100
10.0
PARTICLE DIAMETER, ,um (eoo^
Figure 2. Fractional efficiency of A side ESP • pulsar on and off.
Figure 1. Schematic of Lansing Smith Unit 2 hot-side ESP, transformer-rectifier arrangement.
262
-------
•—CONDITIONING 1 TEST SERIES
MAY 1980 (A&B SERIES)
— B SIDE PULSE
B SIDE PULSE
0.7% NazO
A SIDE PULSER OFF
1.0 5.0 10.0
PARTICLE DIAMETER, urn
PARTICLE DIAMETER, *
Figure 4. Cumulative outlet emissions versus particle diameter - A side impactor data,
11/24-25/80*
Figure 3. Fractional efficiency of B side ESP - pulsar on arid off.
a. Emission from hot-side ESP upstream of cold-side ESP.
1.0 10.0 100.0
PARTICLE DIAMETER, jj.m mo T
Figure 5. Cumulative outlet emissions versus particle diameter - B side impactor data,
11/21-22/80.3
1.0
PARTICLE DIAMETER, (I
Figure 6. Fractional efficiency data from U.S. EPA, IERL, RTP, pulse energization tests.
a. Emission from hot-side ESP upstream of cold-side ESP.
263
-------
s
3-
BSIDE
1—I I I I 11
I i i .
0.5 1-0 2.0 5.0 10
PARTICLE DIAMETER, micron*
350 400 450
SPECIFIC COLLECTION AREA, f
70 80 90
SPECIFIC COLLECTION AREA,
Figure 7. Effect of pulsing on the migration velocities of various particle sizes.
O MASS TRAIN DATA WITH 0.7% Na^O, 11/20, 24-25/80
A IMPACTOR DATA WITH 0.7% NayO. 11/24-25/80
D MASS TRAIN DATA WITH CONDITIONING, 5/8 (1-0% Nl^O)
OPEN SYMBOLS - PULSER OFF CLOSED SYMBOLS PULSER ON
Figure 8. Effect of increasing collection area on collection efficiency of A side.
350 400 450 WO
SPECIFIC COLLECTION AREA. ft2/kacfm
60 70 SO 90 100 11
SPECIFIC COLLECTION AREA, m2(m3/i>e)
O MASS TRAIN DATA WITH 0.7% NajO, 11/208.23/80
A IMPACTOR DATA WITH 0.7% N>20. 11/21-22/80
O MASS TRAIN DATA WITH CONDITIONING, 5/80 (1.0)1 N»2°>
OPEN SYMBOLS . PULSER OFF CLOSED SYMBOLS - PULSER ON
Figure 9. Effect of increasing collection area on collection efficiency of 8 side.
264
-------
A NEW MICROCOMPUTER SYSTEM AND STRATEGY FOR THE
CONTROL OF ELECTROSTATIC PRECIPITATORS
By: K.J. McLean
T.S. Ng
Z . Herceg
Z . Rana
University of Wollongong
Wollongong. N.S.W. 2500
Australia
ABSTRACT
The paper outlines a new microcomputer control system for electrostatic
precipitators. The system comprises three stages of development. In the
first stage, a microcomputer, together with appropriate interfacing hardware,
replace the existing analogue controllers and implement the present control
strategy. In the second stage, the computer will carry out diagnostic
analysis of voltage and current relationships for each zone and then initiate
remedial action if necessary. In the third stage, the dust monitor output
will be used to adaptively fine tune the control settings.
INTRODUCTION
Electrostatic precipitators are one of the most efficient and economical
means of removing particulates from effluent gases at large coal-fired
installation^n! are extensively used on boilers at electrical generating
sSionf ?ne basic principle of operation design and applications of
electrostatic precipitators have been well documented (1, 2, 3).
The most efficient operating conditions '""^ J^Sf ^"ow In
maintained at or near their maximum value. Since Australian coals are low
sulphur content, the resistivity of the resul tant fl y ash is very hig
this adversely affects the precipitator P"*^^4^ ^elcing the
'back corona' formed on the collecting P1^6* j^^f "barging of the
sparkover voltage, increasing *^™*^,^£^ t^lL stream. The
suspended particles by the positi ed into tneg
in existing electrostatic ^^07. These
l conditi
in existng eec 0. Tese
regulated from some Predetermined electrical conditi coliected
may vary with the manufacturer and ^jope^ minimum sparkover
but are usually based on spark rate, cu«f^/^ settings for the control
voltage or some combination of these T^ ^^ J[ch may be up-dated
systems is based on a series of special tuning t conditions
from time to time. This approach assumes that the opti ^
remain unchanged for all operating modes ^a
precipitator condtions and the inevitable coal
-------
With the recent development of reliablle optical monitors and the
availability of relatively cheap microcomputers, we now have the opportunity
to develop a new control philosophy for electrostatic precipitators.
The overall aim of the programme being undertaken by the Electrostatic
Precipitator Research Group at the University of Wollongong is to develop a
comprehensive computer control system comprising three development stages.
In the first stage, a microcomputer, together with appropriate interfacing
hardware, replace the existing analogue controllers and the microcomputer is
programmed to adjust the corona voltage at five minute intervals to a level
just below the sparkover voltage. Spark rate, current and voltage levels are
monitored continuously and provision is made for a suitable protection and
alarm system. In the second stage, the computer will carry out a diagnostic
investigation of each zone using the voltage-current relationships and then
initiate special remedial action if required. For the final stage, signals
from the optical monitor, placed at the output of the precipitator and
adaptive control techniques, will be used to fine tune the spark rate, voltage
and current levels. The system described in this paper is designed to be
installed to one pass of an electrostatic precipitator at Munmorah Power
Station, N.S.W., Australia. Each of the development stages will now be
discussed in more detail.
Stage 1 - Computer Control System
Figure 1 shows a block diagram of the power supply and existing control
scheme for one zone of the electrostatic precipitator. The saturable reactor
is controlled by a dc current supplied from the saturable reactor control
unit. This control current is obtained from a full wave rectified voltage
switched through an S.C.R. Its switching angle is controlled by feedback
signals provided by the automatic controller and maintains a predetermined
current and voltage level, and spark rate.
The area within the dotted line is the block diagram of the computer
control system for five zones of one pass. The existing automatic analogue
and the prototype computer control system can be separately switched into the
same line providing the main control signal to the dc control for the
saturable reactor.
A Rockwell AIM 65 single board microcomputer is used in this unit. It
comprises a 6502 C.P.U. with eight K bytes of memory and six, eight bit
bidirectional I/O ports.
The overall computer control system is shown in Figure 2. One analogue
multiplexer is used to read voltages-and currents for the five zones, using
time multiplexing techniques. The spark status is counted independently for
all the zones and is read continuously. The output control signals for the
five zones are converted back to analogue voltages and transferred to the
saturable reactor control units through voltage to current converters.
266
-------
HIGH VOLTAGE
TRANSFORMER
PSECIPITATOR
ZONE 1
Figure 1: Block diagram showing the interconnection of the
existing analogue control system with the microcomputer.
ZONE 2
1 ZONE 3
A ZONE
SPARK INPUT FROM
ALL ZONES
A
ZONE 5
A
iES
1 1
D/A
LATCHES
D/A
|
LATCHES
1
1
D/A
LATCHES
f)
D/A
LATCHES
'
TO MUL
LATCHE
JJ
MM Nil
DECODERS
V AND 1 INPUTS
Figure 2: Overall organisation of the computer control system.
267
-------
At this stage, the software is written to adjust the corona voltage to a
value slightly below the sparkover level at five minute intervals. The normal
control loop then goes through each zone at one second intervals to measure
the voltage, current and spark rate. If the spark rate exceeds the pre-
determined value, the voltage is immediately reduced to just below the spark-
over level. Similarly, if the current exceeds a predetermined value (possibly
due to excessive back corona), the control signal is adjusted to reduce the
current to some predetermined value.
At the time this paper was written, the microcomputer control unit had
been constructed and tested at Munmorah Power Station. During the tests the
dust emissions were reduced using the computer control system when compared
with that obtained using the existing analogue system (see Figure 3). However,
much more extensive testing is necessary in order to obtain more data to cover
all possible operating conditions.
Figure 3:
Variation of the
optical monitor
output with type
of control
A. Computer control on.
B. Normal analogue control
on
TIME (HOURS)
Stage 2 - Diagnostic Tests and Remedial Action
Preliminary tests have been carried out on two of the eight passes of an
electrostatic precipitator at Munmorah Power Station and some typical V-I
characteristics are shown in Figure 4.
These characteristics change with time and have a tendency to operate
in a high voltage low current mode as dust layer builds up on the collecting
plates. These characteristics can be changed as shown in Figure 4 if special
rapping cycles are introduced.
268
-------
0.10 -
S 0.05 *
o
cr.
o
Figure 4: Variation of votlage current characteristics with
collecting plate contamination.
1. Very dirty plates. 2. After period of intensive rapping.
3. Relatively clean plates.
In the second stage of the system development, the computer will be
programmed to evaluate the characteristics of each zone and take the
appropriate remedial action. For example, when a particular zone is operating
in the strong back corona mode, indicated by curve 3, the computer could be
programmed to detect this and adjust the control signal so that its operating
point is close to or just on the onset of back corona.
A further extension of this mode of control would be to regulate the
rapping of each zone. Once the effects of dust thickness have been
characterised, an optimum rapping sequence could be programmed into the
computer based on V-I characteristics. Special intensive rapping cycles can
also be initiated for individual zones, aided by the switching off of the
power supply, when the dust build up becomes excessive.
Stage 3 - Adaptive Control
In order to fully utilise the potential of a computer control system,
certain optimal control theory could be made to apply to the precipitator.
With the development of reliable dust monitors, the computer could be
programmed to implement a model reference adaptive control system. Figure
5 shows the block diagram for such a system.
In Figure 5, GI to 65 represent the invariant transfer function for the
control system for each zone (see also Figure 1) and G6 represents the time
varying transfer function for the electrostatic precipitator. The adaptive
269
-------
1 v
ADAPTIVE
CONTROLLER
* —
•*
-i
.r
\
1
.r
S
•\ ,
?
r v
•\ 1
p
\
Gl
I, SR
I
i
1
G5
, I. SR
DISTURB
1
_Ai
^G6
-*
DUST
MONITOR
OUTPUT
Figure 5: Block diagram of an adaptive controller.
controller continuously up-dates the parameters of Ge based on the dust
monitor output and the voltage, current and spark rate feedback from the
control unit of each zone. Using certain optimal control techniques such as
self-tuning regulations and minimum variance control techniques (5, 6),
the settings for spark rate, current and voltage levels can be continuously
adjusted to obtain the maximum possible performance of the electrostatic
precipitator.
ACKNOWLEDGEMENT
The authors are grateful for the financial and technical support given
by the Electricity Commission of New South Wales and the National Energy
Research, Development and Demonstration Programme administered by the
Australian Commonwealth Department of National Development and Energy.
ENDNOTES
1. White, H.J. Industrial Electrostatic Precipitation. Reading, Mass.,
Addison-Wesley, 1963.
2. Hall, H.J. Design and Application of High Voltage Power Supplies in
Electrostatic Precipitation, J. Air Poll. Cont. Ass., 25: 132-38, 1975.
3. Oglesby, S. and Nichols, G.B. Electrostatic Precipitation. New York,
M. Dekker, 1978.
270
-------
4. McLean, K.J. and Kahane, R.B. Electrical Performance Diagram for Pilot-
Scale Electrostatic Precipitators. (Presented to the Symposium on
Transfer and Utilisation of Particulate Control Technology, Denver,
U.S.A., 1978).
5. Astrom, K.J. Introduction to Stochastic Control Theory. New York,
Academic Press, 1970.
6. Astrom, K.J., Borisson, V., Ljung, L. and Wittenmark, B. Theory and
Application of Self-Tuning Regulators. Automatica, 13: 457, 1977.
7. Lamb, A.N. and Watson, K.S. Electrostatic Precipitation of Fly Ash from
Low Sulphur Coal in Power Stations. (Presented to the Symposium on the
Changing Technology of Electrostatic Precipitation, Adelaide, Australia,
1974).
271
-------
ASSESSMENT OF THE COMMERCIAL POTENTIAL FOR THE HIGH INTENSITY IONIZER
IK THE ELECTRIC UTILITY INDUSTRY
By: John S. Lagarias
Kaiser Engineers, Inc.
Oakland, California
Jack R. McDonald
Southern Research Institute
Birmingham, Alabama
Dan V. Giovanni
Electric Power Research Institute
Palo Alto, California
ABSTRACT
The High Intensity Ionizer (HII) has reached a level of development where
projections can be made of its potential in the electric utility industry.
Computer simulations have been conducted to predict the maximum electrostatic
precipitator (ESP) performance enhancement of HII applications for high- and
low-resistivity fly ash applications. Future electrostatic precipitator per-
formance requirements have been forecast based on an assessment of possible
changes to regulations for mass emissions, fine particulate, and opacity. A
forecast is made of the equipment which may be used over the next 15 years for
upgrading existing precipitator installations. Alternative means of upgrading
ESP performance were also considered. We conclude that the HII may be one ap-
plicable method for upgrading electrostatic precipitators to meet possible
more stringent regulations and may be used by as much as 20 to 25 percent of
the upgrade market.
1. INTRODUCTION
This report examines the commercial potential for the High Intensity Ion-
izer (HII) to upgrade existing electric utility precipitators over the next
15 years. Situations where the HII may apply and the segment of the market
which the HII may capture are estimated. In making the analysis, competing
alternative processes for improving precipitator performance have also been
considered, such as adding more specific collection surface area, gas condi-
tioning, and pulse energization.
EPRI has been involved with the development of the HII technology since
1974 including benchscale, pilot, prototype, and full-scale testing. Much of
the experimental evaluation of the HII has been cenducted at the EPRI Arapahoe
Test Facility in Denver, Colorado. The HII development has now reached a
stage where projections of its commercial performance may be made. To assess
the commercial potential of the HII, however, a brief review of the current
ESP situation is appropriate.
Fly ash ESP's have been used by utilities since 1923, but it was not un-
til the 1950's, when pulverized coal-fired boilers became common and started
272
-------
to Increase significantly in size, that ESP performance became essential to
utility operations. Since their initial usage, about 1,900 utility fly ash
ESP s have been installed in the United States, most (1,400) since 1950. Of
this number, 550 were installed in the last decade (1970-1980) including ap-
proximately 100 "hot" ESP's for high resistivity fly ash applications.
Over the past 30 years, the collection efficiency of installed ESP's has
Improved dramatically through advances in equipment performance and design.
For example, the average design efficiency for fly ash ESP's selected in 1950
was 93.5%. By 1970, the average design efficiency had increased to 99.0%
and, by 1980, performance specifications quite normally called for efficien-
cies in excess of 99.5%.
Today, however, ESP's face an uncertain future. Questions of applicabil-
ity, cost, and most of all, reliable performance, are being raised. Changes
in environmental regulations, particularly those involving opacity and fine
particulate, stipulate levels of emission control which may be difficult to
sustain. Competitive particulate reduction methods such as the use of fabric
filter collectors or process modifications (solvent refined coal, combined
gas/particulate controls, coal gasification, etc.) are starting to be used.
These actions suggest that the potential of the electrostatic precipitator to
the electric utility industry be reviewed critically. The issues are parti-
cularly significant as this country moves to a greater usage of coal.
The approach used in this analysis was to combine:
(1) An engineering assessment of the commercial performance and cost of the
HII based on extrapolated results of the HII at the Arapahoe Test Facil-
ity;
(2) A scenario of possible future particulate regulatory changes;
(3) A survey of experienced designers, users, and scientists of ESP's as to
the performance of existing utility fly ash ESP's;
(4) An identification of other available particulate control methods;
(5) An assessment of the type of upgrading that ESP's would require to meet
the particulate scenario; and
(6) A projection of the commercial usage for the HII to the electric utility
industry over the next 15 years.
In making an engineering assessment of the commercial performance and
cost of the HII, we assumed that the increased charging capability of the HII
demonstrated at Arapahoe could be obtained in full-scale units and incorpo-
rated into existing ESP's. This analysis would thus indicate the maximum
performance we could expect from the HII.
To understand j-ust what performance levels the utility ESP's would be re-
quired to meet in the future, it was necessary to create a scenario of pos-
sible future regulations which precipitator performance should address. We
identified mass emissions, fine particulate, and opacity as being of greatest
concern. For the purposes of our analysis, we assumed a number of regulatory
changes to occur over the next 15 years which would require existing utility
fly ash precipitators to be upgraded.
273
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To assess the condition of existing precipitators, a survey was made of
knowledgeable precipitator users, suppliers, and scientists. We asked for
their assessment as to the ability of existing precipitators to comply with a
scenario of assumed regulatory changes. The survey was entirely subjective
and based on the best judgment and experience of those interviewed. The con-
sensus was translated into a numerical assessment.
We identified the performance improvements that existing precipitators
wou^d have to meet and further identified competing technologies, in addition
to the HII, that might be used to upgrade ESP's. Cost and performance compa-
risons were made of the available alternative upgrading methods and the HII
which could be used to meet performance improvement previously identified.
Comparisons were made by simulation studies of two specific installations for
which performance data were available.
Combining all of the above information, we then forecast the number of
existing ESP's which could require upgrading during each of the next three
five-year periods, 1980-1984, 1985-1989, and 1990-1995. The number of instal-
lations where the HII might be used, and the potential magnitude of the HII
market, were identified.
2. ASSESSMENT OF PERFORMANCE AND COST
Tests made by Southern Research Institute (SoRI) of the performance of an
HII array on a pilot ESP at the EPRI Arapahoe Test Facility have demonstrated
that the HII can charge individual fly ash particles to much higher levels
than a conventional ESP. However, some loss of charge has occurred downstream
of the HII, and methods are still to be developed to reduce this loss. For
the purpose of this analysis, however, it is assumed that in a commercial in-
stallation the HII will be able to fully charge entering fly ash particles in
agreement with theory and experimental results, and that an existing ESP will
be able to collect the highly charged particles.
To evaluate commercial potential, an analysis was made of the capability
of the HII, along with three alternative methods, to upgrade precipitators to
meet certain target performance levels. Two existing precipitators were
selected, one handling high resistivity fly ash and the other handling low
resistivity fly ash. Precipitator and boiler data for the two installations
are given in Table 3-1. The efficiency required for each precipitator to
meet a target emission level of 0.1 lb/10 Btu was calculated and shown on
Table 3-2. It should be noted that the two precipitators had significantly
different ratings (180 and 92 SCA ft /1000 acfm).
Options for Upgrading ESP's
HII Addition
According to theory, the High Intensity Ionizer (HII), installed as a
precharger to an existing precipitator, creates an extremely high initial
charging field and is especially effective for fine particle charging. The
HII would be used where, for a number of reasons, inadequate charging is ob-
tained in a conventional electrostatic precipitator and would be powered
independently of the precipitator power system.
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The performance of the High Intensity Ionizer was evaluated at Southern
Research Institute (SoRI) using an EPA/SoRI computer simulation model (EPA-
600-7-78-111A) of an electrostatic precipitator preceded by an HII array.
The expected HH-precipitator efficiency was calculated as a function of ioni-
zer voltage (Figure 3-1). The ionizer voltage required to meet the desired
outlet emission level was determined from this information. Computer analy-
ses were also made of the anticipated performance of the High Intensity Ioni-
zer added to an ESP for fine particulate under 2.5 microns (Figure 3-2). The
results indicated that a substantial improvement could be obtained for the
removal of fine particulate using the HII, especially when high resistivity
fly ash is involved and thus could prove effective in meeting opacity require-
ments.
Additional Plate Area
Computer model studies were made to predict the anticipated efficiency
of adding additional collecting surface to each precipitator. In each case,
the computer model was corrected for nonuniform gas flow and for reentrain-
ment loss. Precipitator efficiency was identified as a function of specific
collection area (Figure 3-3). The specific collection area required to
achieve a performance level of 0.1 lb/10 Btu was obtained using Figure 3-3.
Pulse Energization
At least two companies offer pulse energization systems to enhance par-
ticle charging using high voltage pulses superimposed on conventional DC
voltage power supplies. Pulse energization would be proposed for high re-
sistivity fly ash applications and is considered to be at an engineering de-
monstration stage. Model studies of the anticipated precipitator performance
by the addition of pulse energization were calculated as projected increases
in migration velocity based on published enhancement values.
Gas Conditioning
Conditioning of fly ash through flue gas additives is used where high
resistivity fly ash conditions exist, especially where a utility may have
switched from high sulfur to low sulfur coal. Sodium, sulfur, and proprietary
chemical compounds have been used as conditioning agents. Over 150 flue gas
conditioning systems are estimated to be operating or are in the process of
being installed. Flue gas conditioning is in the commercial stage. Model
studies of anticipated precipitator performance by gas conditioning were simu-
lated through an increase in precipitator current and voltage as a result of
the conditioning.
Precipitator Performance
The model simulations showed that permissible emissions could be achieved
by the addition of the HII or by more plate area for both the low or the high
resistivity fly ash precipitators. SO, gas conditioning and pulse energiza-
tion would be effective for the high resistivity precipitator but could not
improve the performance of the low resistivity fly ash precipitator to meet
the permissible emission requirements (Table 3-3).
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Capital Costs
Estimates of 1980 capital costs to upgrade the two precipitators were pre-
pared following standard procedures. Cost data for cold side precipitators
were used to estimate equipment costs for upgrading precipitators with dif-
ferent SCA (specific collection area) values.
Vendor-supplied equipment costs for the ionizer were used on a unit cost
basis. Cost data for ionizer electrical components were updated to 1980 cost
levels from an earlier EPRI study and adjusted for the number of TR sets used
in the system.
Information on cost estimates for the equipment required in the SO, gas
conditioning system was vendor-supplied.
Cost estimates for field material and installation labor were based on
in-house experience with comparable equipment and on published cost factors.
Total capital Investment required to achieve the desired performance for
each upgrading alternative and 30-year levelized power costs are summarized in
Table 3-4.
Total capital investment for each upgrading alternative was obtained by
combining costs for equipment, field material, installation, engineering, and
contingency.
For the high resistivity fly ash application examined, the capital cost
of the High Intensity Ionizer would be substantially lower than the conven-
tional plate retrofit. The operating cost for the HII would be higher prin-
cipally due to the cost of steam addition to the purge gas (42 percent of the
total levelized operating cost). However, 803 conditioning and possibly pulse
energization would be more economical to achieve the desired performance level
(0.1 lb/106 Btu).
The capital and operating cost for upgrading the low resistivity fly ash
precipitator with the High Intensity Ionizer would be higher than a conven-
tional plate retrofit. The other alternatives, SOj conditioning and pulse
energization, could not give sufficient improvement to the performance of low
resistivity fly ash precipitator to meet the desired performance level.
Operating Costs
The operating costs for upgrading by each of the alternatives were de-
veloped using EPRI economic premises and methodology (EPRI Technical Assess-
ment Guide PS-1201-SR). The fixed operating cost included operating, mainten-
ance, administrative labor, and material. Variable operating costs included
precipitator and miscellaneous power, ionizer power, steam, chemicals, etc.
Over the life of the plant, the operating costs tended to increase due to in-
flation. A single "levelized" value was computed using the "present worth"
concept of money to represent the varying requirements for fixed and variable
costs. A 30-year levelizatlon factor for operating and maintenance costs and
for the total capital requirement was computed on the basis of an EPRI-assumed
inflation rate and discount rate. The yearly operating and capital costs were
276
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converted to 30-year levelized costs and represented in terms of power cost
(mills/kwh). Operating cost data for each of the upgrading alternatives are
also presented in Table 3-4.
3. A SCENARIO OF FUTURE PERFORMANCE REQUIREMENTS
Forecasting possible future regulatory changes is always difficult. It
is particularly so at this time because of philosophical changes occurring in
government administration and policy. However, it is conceivable that exist-
ing precipitators could require upgrading if particulate regulations became
more stringent. For the purpose of this analysis, we have taken a scenario
where changes in regulations would impact on upgrade requirements.
Mass Emission Regulations
The New Source Performance Standards (NSPS) for utility precipitators
promulgated in 1980 lower permissible particulate emissions from 0.1 lb/106
Btu heat input to 0.03 lb/106 Btu. While existing precipitator installations
are exempted from NSPS, stricter requirements for existing precipitators could
occur as state and local regulatory agencies comply with the Clean Air Act and
Amendments in updating State Implementation Plans (SIP's).
We have taken a scenario wherein changes in present regulations would re-
quire existing installations to meet the former NSPS standard (0.1 lb/106 Btu).
In the immediate future, however, say up to 1985, we would anticipate that
only a small number of the existing utility precipitators would have to meet
a 0.1 lb/106 Btu requirement because most state and local regions are in reas-
onable compliance with total suspended particulate (TSP) standards. However,
we could foresee that, by 1990, most existing ESP's could have to meet a 0.1
lb/106 Btu emission level as a result of changes to regulations.
Opacity Regulations
For the purposes of this study, we assume that changes to State Implemen-
tation Plans, local regulations, and federal requirements would tighten opa-
city regulations. Current requirements of the Clean Air Act Amendments call
for State Implementation Plans to be updated by July 1, 1982. There already
is a trend among regulatory agencies to require existing precipitators to meet
20% opacity as a minimum with others requiring a 10% opacity or even a clear
stack. Many of the existing installations cannot meet a 10% opacity require-
ment, although it appears that precipitators installed since the late 70's
are meeting 10% opacity in many instances.
Fine Particulate Regulations
Only one state, New Mexico, has a fine particulate emission regulation
(particulate emissions, 2 microns or smaller, are not to exceed 0.04 lb/10
Btu). However, we can postulate a situation where specific fine particulate
emission regulations are adopted by other states.
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As a scenario, we would forecast that existing installations would not be
subject to a federal fine particulate regulation but that some state or local
agencies would adopt fine particulate emission regulations. For the purpose
of our analysis, we consider that a small number of the existing precipitators
would be subject to a New Mexico-type fine particulate emission regulation by
1985. By 1990, however, a larger number of the existing installations could be
required to upgrade to meet fine particulate regulations.
4. ASSESSMENT OF EXISTING PRECIPITATOR PERFORMANCE
To estimate the immediate and future requirements for upgrading existing
utility precipitators, a Delphi-type survey was conducted among 10 knowledgeable
precipitator and utility representatives. A Delphi survey seeks to obtain a
consensus on certain identified issues by reviewing the extreme positions taken
in the survey and narrowing differences of opinion. This survey assessed the
ability of existing precipitators to meet possible future regulatory require-
ments. Respondents were selected who, in the judgment of the authors, were
knowledgeable of the operation of fly ash precipitators and the regulatory pro-
cesses.
Questions asked were: Based on the interviewees' experience, what per-
centage of the existing utility fly ash precipitators
a) encounter high resistivity fly ash?
b) meet their existing particulate regulations?
c) could meet a 0.1 lb/106 Btu regulation?
d) could meet a 0.04 lb/106 Btu (minus 2 micron) regulation?
e) could meet a 10% opacity regulation?
The response (Table 4-1) was that, of the existing utility fly ash pre-
cipitator installations,
a) 30 percent experience high resistivity fly ash
b) 75 percent meet their existing regulations
c) 45 percent could meet a 0.1 lb/10° Btu regulation
d) 30 percent could meet a 0.04 lb/106 Btu (fine particulate) regulation
e) 20 percent could meet a 10% opacity regulation
Impact on Precipitator Performance Requirements
The Delphi-type survey estimated the present level of performance of ex-
isting fly ash precipitators. The assessment of regulations which may apply
to these precipitators ever the next 15 years established a scenario. A com-
parison can be drawn of the current level of precipitator performance to the
potential regulation changes and a forecast made of precipitator upgrade re-
quirements .
The following assessment is made of the number of existing utility fly
ash precipitators that would have to be upgraded as a result of projected
changes in regulations (Table 4-2).
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1980-1984
Minor changes to mass, fine participate, and opacity regulations are pro-
jected to occur. ESP's not always in compliance, or operating under variances,
would be required to meet the existing particulate regulations on a regular
basis. The Delphi survey estimated that 25 percent (350) of the existing pre-
cipitators would have been subject to upgrading. Of this number, 230 would
be retired during this period for being over 30 years old. The remaining 120
ESP's would be subject to upgrade requirements.
1985-1990
Existing precipitators would be subject to stricter controls. The 1981
National Council on Air Quality report to Congress expresses strong concern
over long-range transport and visibility and will precede congressional review
of the Clean Air Act and Amendments. Changes to the Clean Air Act and Amend-
ments cannot be forecast. However, as a scenario, we could project that 20
percent of the then existing utility fly ash precipitators would have to be up-
graded to meet a 0.1 lb/106 Btu regulation (100 units). The current NSPS of
0.03 lb/106 Btu would continue to apply to new installations only.
An additional 15 percent of the existing precipitators might be subject
to upgrading to meet new fine particulate regulations (30 units). If a 10%
opacity regulation were adopted, an additional 20 percent of the existing pre-
cipitators would require upgrading (20 units). Thus, 45 percent of the exist-
ing precipitators were projected as subject to upgraded requirements in the
1985-1990 period (150 units).
Post-1990
A further tightening of particulate regulations may be projected to occur.
In the post-1990 time period, 710 ESP's would still remain of the 1,400 units
existing in 1980. One could project 300 ESP's requiring upgrading. Of this
number, 230 units would be subject to a 0.1 lb/106 Btu regulation, 30 units
subject to fine particulate regulations, and 40 units subject to opacity regu-
lations. This number (300) would not include units upgraded in the 1985-1990
time period, or units which were upgraded to meet fine particulate regulations
and thereby met opacity regulations. Two hundred thirty precipitators are as-
sumed to be capable of meeting any of the hypothesized regulation modifications.
5. PRECIPITATOR UPGRADE ALTERNATIVES
Prechargers are only one of several alternatives being developed or al-
ready available which may be used to upgrade precipitators, and the High In-
tensity Ionizer is only one of several prechargers currently under development
in the United States and Japan. Over the next 15-year period, only those al-
ternatives which are currently either at a commercial or an engineering devel-
opment stage would be considered as available for utility integration.
Selection of the appropriate alternative to improve precipitator perfor-
mance is site specific. The precipitator should first be made to operate as
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effectively as originally designed. The internals must be in good shape or re-
paired; warped or misaligned plates should be straightened or replaced; rappers
made to operate properly; flow adjusted; and power supplies made fully and ef-
fectively operable. An assessment can then be made of the performance limita-
tions of the precipitator and a recommended course of action identified.
Mass Emission Reduction
Mass emission reduction requirements may vary substantially, from a condi-
tion where marginal precipitator performance is obtained (a regulation is met
part of the time) to a condition where the current precipitator performance is
completely inadequate. A mass reduction requirement is considered slight when
penetration must be reduced by a factor of two. It is considered significant
when penetration must be reduced by a factor greater than two.
Significant reductions in penetration will most often be achieved by ad-
ding new equipment such as pulse energization, the High Intensity Ionizer or
a fabric filter, or by equipment modifications such as additional plate sur-
face capacity.
Fine Particulate
The alternatives for upgrading existing precipitators to meet regulations
involving fine particulate fc2 microns) will probably require the use of new
technology such as pulse energization or the High Intensity Ionizer. Fine
particulate removal will require a system which can apply sufficient forces
to the individual small particles so as to separate them from the gas stream.
Both pulse energization and the High Intensity Ionizer are intended to bring
this about.
Opacity
All upgrading alternatives will reduce emissions to a degree, but sig-
nificant reductions in opacity will depend on the ability of the alternative
to reduce fine particulate emissions.
6. FORECAST OF MARKET REQUIREMENTS
The methods which could be used to upgrade existing precipitators to meet
possible regulatory changes include: additional collecting plate area, the
High Intensity Ionizer, pulse energization, or gas conditioning. No one method
would be expected to be used for all situations. However, based on the projec-
ted performance of the HII through tests made at Arapahoe, and reports of the
performance of other upgrade methods (1,2,3), one can examine future market re-
quirements.
To forecast the market for the High Intensity Ionizer (HII) through 1995,
the following assumptions were made:
o Those precipitator installations which must upgrade to meet current regu-
lations will use either additional plate capacity or gas conditioning.
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o The HII and pulse energization will prove to be effective, reliable, and
commercially available by late 1983. Prototype and demonstration testing
will have been completed by 1982.
o The HII will compete with plate addition and fabric filtration to meet
fine particulate emission regulations.
o The HII will compete with the addition of more plate capacity to meet 10%
opacity regulations.
o The HII will not compete with the addition of more plate capacity for most
applications involving low resistivity fly ash.
Equipment performance is only one criterion used in equipment selection.
Other criteria include cost, vendor marketing effectiveness, acceptance by in-
dustry, regulatory acceptance, and the performance and cost of competing alter-
natives. The number of utilities which would use the High Intensity Ionizer
will depend, to a certain extent, on the number of major vendors offering it.
Some utilities have a definite vendor preference based on previous experience
and geographical location.
If several of the major vendors offer the HII, the number of precipitator
installations which would consider the HII would increase proportionately.
Conversely, if only one of the smaller vendors were to offer the HII, the num-
ber of HII installations would not be as great because that vendor reaches a
lesser number of the utilities. Thus, to be used extensively, the HII—or any
other alternative, for that matter—should be available through a significant
number of the major precipitator vendors.
A forecast of the future of the .approximately 1,400 precipitators in ex-
istence in 1980 is illustrated in Figure 6-1. We anticipate that older pre-
cipitators will gradually be retired, middle-aged precipitators will be up-
graded to meet regulatory changes, and newer precipitators will not require
any modification. The cumulative impact of these effects on the existing pre-
cipitators is shown starting for the five-year period to 1985, the 10-year
period to 1990, and the 15-year period to 1995. An assessment of these
changes is discussed below.
Assessment of the 1980-1995 Utility Precipitator Market
The assessment of the upgrade precipitator market for the 1980-1995 period
is based on the following premises:
o Precipitators are projected to have a 30-year life and will be retired at
a uniform rate (3.3%/year). There are approximately 1,400 utility fly
ash precipitator units currently Installed in the United States.
o The Energy Use Act would preclude the use of gas or oil for new utility
installations after 1990, placing a greater emphasis on the use of coal
as a utility fuel.
o Possible changes to mass, fine particulate, and opacity regulations may
place increased performance requirements on existing precipitators.
o By 1990, at least one other new alternative, not presently identified,
will have been developed and used to upgrade existing precipitators.
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o New coal-fired installations will use either precipitators or fabric fil-
ter dust collectors, with electrostatic precipitators being used in slight-
ly more than half of the new applications. New precipitator installations
will also be able to incorporate the High Intensity Ionizer, other pre-
chargers, or pulse energization in their design.
1980-1984
Existing precipitators upgraded during this period will use proven up-
grading alternatives such as adding more plate capacity or gas conditioning.
It is estimated that a two-year evaluation period and an additional one-year
full-scale successful testing period would be required before the HII would
have demonstrated acceptable performance to the utility industry. One or two
demonstration-type HII units may be installed during this period but a substan-
tial number of ionizer applications is not anticipated to occur. A shorter
acceptance period is forecast for pulse energization although there may be
some question about the charging of fine particulate by pulse energization.
1985-1989
During this period, 150 existing precipitators would require upgrading to
meet possible new emission regulations. Approximately one-fourth of the pre-
cipitator installations (37) are estimated to encounter high resistivity fly
ash. Upgraded precipitators will handle high resistivity fly ash by one of
three methods: gas (803) conditioning, pulse energization, or the use of a pre-
charger such as the High Intensity Ionizer.
The SOg gas conditioning system has already been proven commercially while
pulse energization and the HII have still to demonstrate reliable performance.
However, concern over sulfate and SOg emissions may result in SOg conditioning
systems not being universally acceptable for all high resistivity applications.
Nevertheless, it is projected that 50 percent of all high resistivity fly ash
precipitator upgrading applications would use 863 conditioning. The other 50
percent would be divided equally between pulse energization and the High In-
tensity Ionizer. Thus, an estimated 6.3 percent (nine) of the total high
resistivity fly ash precipitators to be upgraded during this period would use
the HII.
The precipitator applications encountering medium or low resistivity fly
ash (113) have other upgrading alternatives available. 803 conditioning and
pulse energization are not as effective for a low resistivity ash and would
not be considered. Installations having medium or low resistivity fly ash
could either add more plate capacity or use the HII.
The adding of pulse energization to existing power supplies for high re-
sistivity fly ash applications could be quite attractive. It is still to be
demonstrated, however, that sufficient useful power is developed by pulse
energization to obtain the desired upgrading performance and that the opera-
ting long-term costs are acceptable.
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It is forecast that the 113 existing low and medium resistivity fly ash
installations would probably add additional plate capacity in 50 percent of
the upgrade applications. Pulse energization would be used by 30 percent and
the HII by 20 percent. The 20% share for the HII of the medium/low resistiv-
ity fly ash units being upgraded is equivalent to 15 percent of the total num-
ber of precipitators. Together with the 6.3 percent of the high resistivity
fly ash applications that would use the HII, approximately 20 percent of the
total number of precipitators being upgraded could use the HII. Quantitative-
ly, this would be about 30 precipitator installations (Figure 6-1). Assuming
the cost of an average HII installation for a 200-MW station to be $2.2 x 10°,
total investment for HII installations would be $66 million x 10^ ($15.2 mil-
lion per year).
1990-1995
During this time period, it is postulated that a large number (300) of
the existing fly ash precipitator installations would be subject to opacity
and fine participate regulations. Precipitators erected prior to 1960 would
have been retired.
Gas conditioning systems to upgrade precipitators could decrease if regu-
latory concern focuses on sulfate and 803 emissions.
With its ability to charge fine particulate, the HII could become an at-
tractive method for precipitator upgrading as regulations for fine particulate
centrol become prevalent.
The existing high resistivity fly ash installations to be upgraded (25
percent of total installations) would use alternatives such as pulse energiza-
tion, gas conditioning, or the HII. If each alternative captured an equal
share of the high resistivity upgrade applications, 11 percent of the total
high resistivity upgrade applications would use the HII.
For the medium and low resistivity fly ash precipitators subject to up-
grade (75 percent of the total) , we postulate that the upgrading of the medium
and low resistivity fly ash precipitators would be divided among four alterna-
tives—the High Intensity Ionizer, additional plate capacity, pulse energiza-
tion, or a fourth undefined, still to be developed system. Assuming 20 percent
of these medium/low resistivity fly ash precipitators use the HII, 30 percent
use pulse energization, and 50 percent use all other systems, then 15 percent
of the total nuufcer would be upgraded with the HII. Adding the 11 percent of
the high resistivity fly ash applications (and rounding off), we project that
25 percent (75 of the 300 upgraded precipitators) could use the HII (Figure
6-1). At an average cost of $2.2 million, a market of $165 million for the
High Intensity Ionizer could result. Over a five-year period, this would re-
present an annual market of $33 million.
7. SUMMARY
Computer simulation studies have been made of the anticipated performance
of the HII in full-scale installations assuming charge losses noted at Arapahoe
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could be overcome. Sizing data from two existing installations were used:
one involving high resistivity fly ash and the second, low resistivity fly
ash. Comparative capital and operating coets were also developed. Compari-
son analyses were made with alternative methods of upgrading precipitator
performance including additional plate area, SO, gas conditioning, and pulse
energization.
Each of the four methods was capable of upgrading the high resistivity
precipitator to a target emission level of 0.1 Ib/lO^ Btu. For the low re-
sistivity precipitator, conventional plate addition and the HII were the vi-
able alternatives.
Comparison of the capital and operating costs for upgrading a high re-
sistivity precipitator showed 803 gas conditioning to have the lowest costs,
followed by a conventional upgrade and the HII upgrade. Data were not avail-
able to project the cost for pulse energization. This example of one high
resistivity precipitator is illustrative only and site-specific analyses at
other installations would still be required. For the low resistivity pre-
cipitator, the lowest capital and operating cost could be obtained by a con-
ventional upgrade over the HII for the example selected. Gas conditioning
or pulse energization could not meet performance requirements.
A scenario of possible future regulatory requirements would indicate that
tighter mass emission regulations, more stringent opacity regulations and
fine particulate regulations might occur. On this basis the performance of
existing precipitators to meet such regulations suggests that a substantial
number of existing units would require upgrading. An assessment of the 1980-
1995 utility precipitator market for the HII indicates the following:
a. One or two demonstration-type HII units could be Installed during the
1980-1985 period, but a substantial number of ionizer applications is
not anticipated to occur.
b. During the 1985-1989 period, It is estimated that about 6 percent of the
total number of fly ash precipitators to be upgraded would use the HII
for high resistivity application, and 15 percent of the total number of
precipitators would use the HII for medium and low resistivity fly ash
applications. About 30 precipitator installations could be upgraded with
the HII during this period.
c. During the 1990-1995 time period, 11 percent of the total number of pre-
cipitators would use the HII to upgrade high resistivity precipitators
and 15 percent of the total number would use the HII for low and medium
resistivity fly ash applications. It is estimated that roughly 25 per-
cent of the precipitators could be upgraded during this period with the
HII.
The HII may be in an advantageous commercial position in the following situa-
tions :
a. High and low resistivity fly ash upgrade applications where fine partic-
ulate control and/or opacity are major concerns;
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c. High resistivity fly ash upgrade applications requiring significant eais-
sion reductions.
The HII may be in a disadvantageous commercial position in the following
situations: 6
a. Either high or low resistivity fly ash applications needing slight emis-
sion reductions;
b. Upgrade applications where cost alone is the major concern;
c. Low resistivity fly ash installations where the existing collecting sur-
face area is too small to benefit from HII upgrading.
REFERENCES
1. Feldman, P. L., and H. I. Milde, Pulsed Energization for Enhanced Electro-
static Precipitation in High Resistivity Applications, Symposium on the
Transfer and Utilization of Particulate Control Technology, EPA-60017-79-
044a, February 1979.
2. Lederman, et al, Sodium Conditioning Aids Precipitation, Symposium on
the Transfer and Utilization of Particulate Control Technology, EPA/DRI,
Denver, Colorado, July 1978.
3. Cook, R. E., Sulfur Trioxide Conditioning, Symposium on Electrostatic
Precipitators for the Control of Fine Particles, EPA-650/2-75-016, Janu-
ary 1975.
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Table 3-1
PRECIPITATOR AND BOILER DATA
(High Resistivity)
(Low Resistivity)
Gas and Particulate Conditions
Average gas flow, acfm
Gas temperature, F
Dust loading, gr/acf
Fly ash resistivity, ohm-cm
Particulate density, Ibs/ft
Geometry and ESP Performance
SCA, ft2/!,000 cfm
Number of fields
Total plate area, ft
Plate to plate spacing, Inches
Wire to wire spacing, inches
Operating average voltage, kv
Operating average current, nA/cm
Precipitator efficiency, %
Boiler Conditions (Estimated
Boiler capacity, Mw
Turbine heat rate, Btu/kwh
Boiler efficiency, %
Gross heat rate, Btu/kwh
1,200,000
225
2.91
6.0 x 1012
141.7
180
3
214,400
9
9
35.3
3.5
96.7
300
9,016
NA
NA
240,000
- 300
0.5
2.0 x 109
162.3
92
2
22,080
9
9
46.7
42.5
91.75
75
7,871
87.3
9,016
Table 3-2
PRECIPITATOR EFFICIENCY REQUIRED
TO MEET UPGRADING REQUIREMENT
Installation
A
B
Fly Ash
Resistivity
(ohm cm)
High (6 x 101?)
Low (2 x 109)
Current
Performance
Efficiency
(Z)
96.7
91.75
Desired
Performance Level
(0.1 lb/10 Btu)
98.9
93.6
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Table 3-3 COMPARISON OF UPGRADE PRECIPITATOR PERFORMANCE REQUIREMENTS
Precipitator
A
B
Existing
Precipitator
High Resistivity
Average voltage KV 35.3
Current nA/cm2 3.5
SCA ft2/!, 000 cfm 180
Incremental plate
area, ft2/103 cfm
Efficiency, % 96.7
Low Resistivity
Average voltage KV 46.7
Current nA/cm2 42.5
SCA ft2/!, 000 cfm 92
Incremental plate —
area, ft2/103 cfm
Efficiency, % 91.75
Performance Level
(0.1 lb/106 Btu)
Conventional Ionizer SO 3 Gas
Upgrade Upgrade Conditioning
57<» 45.6
1.5(2> 50.4
260 180 180
80
98.9 98.9 98.9
70(1)
2.5(2)
120 92
28
93.6 93.6 (3)
Pulse
Energizer
180
—
98.9
—
—
—
(3)
(1) Ionizer only.
(2) Ionizer current ma/ ionizer.
(3) Performance level efficiency not reached.
Table 3-4 PRECIPITATOR UPGRADE COST COMPARISONS
Installation
A
B
High Resistivity ESP
Capital Cost $ x 106
Levelized Operating Cost
Capital mills/kwh
Operating mills/kwh
Total Levelized Operating Cost mills/kwh
Low Resistivity ESP
Capital Cost $ x 106
Levelized Operating Cost
Capital mills/kwh
Ooeratine mills/kwh
Performance Level
(0.1 lb/106 Btu)
Conventional Ionizer SO 3 Gas
Upgrade Upgrade Conditioning
3.32 2.6 1.96
0.325 0.254 0.191
0.103 0.346 0.113
0.428 0.600 0.304
0.38 0.70 (2)
0.149 0.274
0.143 0.179
Pulse
Energizer
1.29<»
NA(1>
-
(2)
-
Total Levelized Operating Cost
mills/kwh
0.292
0.453
(1) Estimated.
(2) Efficiency level is not achievable.
(3) 30-year levelized power cost.
287
-------
Table 4-1 QUALITATIVE ASSESSMENT OF EXISTING UTILITY FLY ASH PRECIPITATORS
N
REVIEWER
A
B
C
D
E
F
G
H
I
J
Range
Concensus*
MEET
HANDLE HIGH EXISTING
RESISTIVITY REGULATIONS
FLY ASH (%) (%)
95
20 70
40 40-50
33 95
35 75
20 90
35 75
33 60
30 90
20 30
20-40 30-95
(%) 30 75
(Delphi Approach)
COULD MEET A
0.1 lb/106 BTU
REGULATION (%)
75
40
20
80
—
70-80
90
10
20
10
10-90
45
COULD MEET A
0.4 lb/106 BTU
(MINUS 2 MICRON)
REGULATION (%)
30
20
20
80
20
30-40
50
10
25
—
10-80
30
COULD MEET A
10% OPACITY
REGULATION (%)
20
10
10
25
35
10
50
20
10
30
10-50
20
*Highest and lowest estimates discarded.
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
L.
M.
Table 4-2 FORECAST* OF
Existing Precipitators
Units Retired (3.3%/yr)
Precipitators Subject to Upgrade
Do not Meet Existing Regulations
Would Require Upgrading
Could Not Meet 0.1 lb/106 Btu
Regulation
Would Require Upgrading
Meets F But Not 0.04 lb/106 Btu
(minus 2 micron)
Would Require Upgrading
Meet I But Not 10% Opacity
Would Require Upgrading
Total Requiring Upgrade
Meet All Requirements
TOTAL
UTILITY PRECIPITATORS REQUIRING UPGRADING
1980-
1984
1,400
230
1,170
(% of A) 25
(No.) 350
(% of D) 100,
(No.) 120^
(% of C)
(No.)
(% of F)
(No.)
(% of C)
(No.)
(% of H)
(No.)
(% of C)
(No.)
(% of J)
(No.)
120
Time Period
1985-
1990
1,170
230
940
—
__
1)
55
520
20
100
15
140
20
30
10
90
20
20
150
1990+ Total
940
230 690
710
__
—
55
390
60
230
15
110
130
30
10
70
60
40
300 570
140
1,400
*Based on survey of knowledgeable precipitator specialists
(l)Excludes precipitators scheduled for retirement.
288
-------
ttJB
*
•9.9
e».o
OO
90.0
HIGH RESISTIVITY
INSTALLATION "A"
160 SCA
COLLECTION
EFFICIENCY
X
LOW RESISTIVITY-
INSTALLATION "B" ^
92 SCA ^,.-*^wg«r. »-x
OVERALL
BRECIPITATOR EFFICIENCY
VS. H1I VOLTAGE
4O SO 60 TO 00
HIGH INTENSITY IONIZER VOLTAGE - KV
90
»o
to
to.
«0
80
40
90
Vlgur* 1-2
ESP « HIGH INTENSITY IONIZER
PARTICLE SIZE-EFFICIENCY
-WITH HI!
COLLECTION
EFFICIENCY
CURRENT PERFORMANCE
(ESP ONLY)
HIGH RES!ST?VITY
INSTALLATION A
ISO SCA
i.O
I.S
2.Q
to
to
70
so
50
4O
WITH H1I
COLLECTION
EFFICIENCY
CURRENT PERFORMANCE
(ESP OMLY)
LOW RESISTIVITY
INSTALLATION B
02 SCA
I.O I.S E.O
PARTICLE DIAMETER IN MICRONS
iO
-------
•*.»
•S.S
»t.o
•9.0 -
• 4.0
LOW RESJSTiyirr
INSTALLATION
—HIGH RESISTIVITY
INSTALLATION V
COLLECriON
EFFICIENCY
PRECIPITATOR EFFICIENCY
VS.
SCA
100 200 jco 400
SPECIFIC COLLECTING AREA (SCA)
aoo coo
ft'/IOOOcfm
USiT*
Aoo
Zoo
-------
APPLICATION OF ENERGY CONSERVING PULSE ENERGIZATION
FOR PRECIPITATORS
PRACTICAL AND ECONOMIC ASPECTS
By
Helge Hoegh Petersen
Preben Lausen
F.L. Smidth & Co. A/S
77, Vigerslev Alle, DK-2500 Valby
Copenhagen, Denmark
ABSTRACT
Performance of precipitators collecting high resistivity dust can be
improved considerably by pulse energization. The energy consumption, however,
is a major problem for its application on large precipitators, where consider-
able amount of energy is required to charge the precipitator capacitance to
a high pulse voltage level. The energy conserving pulse energization system
dealt with here solves this problem, as the energy stored in the precipitator
during the pulse and not used in the precipitation process is recovered to be
used for the next pulse application. Operating results and experience from
full-scale field tests over a period of more than two years are presented.
The tests have demonstrated that the system, utilizing high-power electronic
components, has the required degree of reliability for practical application.
Further, the obtained improvement of precipitator performance in relation to
installation and operating costs makes it an attractive option to new precip-
itators for high resistivity dust as well as to existing precipitators with
resistivity problems. Based on these findings, practical and economic aspects
of installation and operation of the system on new as well as on existing pre-
cipitator installations are discussed.
INTRODUCTION
It has been known for many years that the performance of a precipitator
collecting high resistivity dust can be improved by pulse energization.
By superimposing short duration high voltage pulses on the DC voltage by
means of a suitable pulse generator, higher peak voltage is obtainable without
sparkover. The use of pulse voltage gives a number of advantages including
improved particle charging, higher collection field strength and better
current distribution on the collection plates (1), (2). ,
Further, the discharge current can be regulated independently of the pre-
cipitator voltage by variation of pulse repetition frequency and pulse height
(3). This makes it possible to reduce the discharge current to the threshold
limit for back corona with a high resistivity dust, without reducing the pre-
cipitator voltage. This means more favourable electrical energization for
such applications than is obtainable with conventional DC operation, where the
291
-------
current cannot be regulated independently of the precipitator voltage.
Earlier pulse energization work was hampered by the lack of reliable
high-power switch elements for the pulse generator. Recent years technical
progress has changed this situation. Various pulse energization systems are
now under development in the U.S.A., Japan and Europe, with some of them at
or very close to a commercial stage.
An inherent problem in pulse energization is the considerable amount of
energy required for repetitive charging of the precipitator capacitance to a
high pulse voltage level. Only a minor part of this energy is necessary for
the discharge current in the precipitator. For reasons of economy, recovery
of the surplus energy is important, particularly for pulse energization of
large precipitators.
Some of the pulse energization systems under development are capable of
conserving the capacitive pulse energy by recovering it from the precipitator
after the pulse and using it for the next pulse. Such an energy conserving
pulse energization system developed by F.L. Smidth (3) is described in the
following, and the results obtained from industrial scale tests are presented.
F.L. SMIDTH ENERGY CONSERVING PULSE ENERGIZATION SYSTEM
General Principle
-120
-100
Si -80
$
s
3
'a.
'<3
S?
QL
-so
-40
-20 -
Pulse repetition frequency 200 pps
0.5
5.5
Time, ms
Figure 1. Precipitator voltage wave-form.
292
-------
The system is used with conventional precipitators and does not require
any special electrode arrangements. Short duration high voltage pulses are
repeatedly superimposed on an operating DC voltage as shown in Figure 1. The
pulse duration is within the range 50-200 Vis, and pulse repetition frequen-
cies from 25 to 400 pulses per second are used.
The level of the DC voltage depends on dust and gas characteristics.
For high resistivity dust the DC voltage is maintained at or slightly below
corona onset in order to extinguish the corona discharge after each pulse.
This allows control of the discharge current by means of pulse height and
repetition frequency.
Idealized wave-forms of the precipitator pulse current and voltage are
shown in Figure 2. During the first half of the pulse time a negative cur-
rent flows from the pulse generator to the precipitator, charging the pre-
cipitator capacitance from the operating DC voltage level to the pulse peak
voltage.
-400
-120
200
100
Figure 2. Ideal wave-forms of precipitator
current and voltage.
For high pulse voltages only a minor part of this charge is emitted as
discharge current during the pulse in order to maintain a suitable collec-
tion plate current density. The remaining and major part of the charge is
returned to the pulse generator as a reversed current flow during the second
half of the pulse time, thus bringing the precipitator voltage back to the
operating DC voltage level, refer Figure 2.
293
-------
The Energy Conserving Pulse Energization System
DC supply
-^
Charger
1—
H
1=
—
,
Tr
'i
1 I I
LOG
(
Switch
C^>PI
Diode s '••
_ .--..L
•\ „
•'C"
IJ
: Precipitator
>ulse transformer
Figure 3. The F.L. Smidth energy conserving
pulse energization system.
Figure 3 is a basic diagram of the energy conserving pulse energization
system. The DC operating voltage is maintained by a DC supply with a block-
ing inductor L preventing the pulse voltage from entering the supply. A
coupling capacitor C blocks the DC operating voltage from the pulse trans-
former. The pulse circuit includes a charger supplying a storage capacitor
C , a thyristor switch, a feed-back diode, and a series inductance L . The
storage capacitor C , the series inductance L together with the pulse trans-
former leakage inductance, the coupling capacitor C_, and the precipitator
capacitance C,, form a series oscillatory circuit.
r
The storage capacitor C is charged to a controlled DC level by the
charger. The thyristor switch is turned on, and the precipitator represented
by the capacitance C is charged to the maximum pulse voltage level by the
first half period of the oscillatory current. Because of the series oscilla-
tion, the energy supplied to C and not used for the corona discharge is re-
turned to CR through the feed-back diode by the current in the second half
period. During this interval, the thyristor switch is turned off, and the
current in the pulse circuit is blocked until the next ignition of the
thyristor switch. The returned energy is stored in C during the interval
between the pulses and is used for the next pulse.
2
An industrial precipitator may have a capacitance of 30-40 pF per m
(3-4 pF per square foot) collection area. Charging this capacitance to a
high pulse voltage level a large number of times per second requires a con-
siderable quantity of energy. Depending on pulse voltage level and repeti-
tion frequency, the charging energy might amount to several times the energy
absorbed by the precipitator as useful corona discharge energy. For reasons
294
-------
of economy, recovery of the charging energy is consequently extremely advan-
tageous for pulse energization of large precipitators (4).
GENERAL ARRANGEMENT
Industrial versions of the energy conserving pulse energization system
are now being manufactured in the United States as well as in Europe by manu-
facturers, who also provide field service and stock spare parts. It is pre-
sently available in two standard unit sizes capable of energizing a precip-
itator fie.ld or cell with a collection area of 2500 m (25000 square feet) up
to 3000 m (30000 square feet), depending on unit size, gas temperature and
precipitator duct width. Larger units, capable of energizing up to 5000 m
(50000 square feet) of collection area each, are being developed and will
become available early 1982.
CONTROL PANEL
FLANGE
DC SUPPLY PULSE SUPPLY
CHARGING
TRANSFORMER/RECTIFIER
Figure 4. Arrangement of the pulse
energization system.
A system for energizing one field or cell comprises the following
equipment, refer Figure 4:
A. A control panel containing
- Thyristor controllers for the DC high voltage supply and pulse charger
- storage capacitor
- thyristor switch for firing the pulses
- feed-back diode for returning the charge from the precipitator to the
storage capacitor
- series inductance for matching circuit impedance to precipitator
capacitance
295
-------
- instruments for measuring DC voltage level, pulse peak voltage,
precipitator current, and pulse repetition frequency
- manual controls
- automatic controls continuously adjusting DC voltage level, pulse
peak voltage and pulse repetition frequency for optimum precipitator
efficiency on basis of criteria related to control of corona discharges
between pulses, total precipitator current, sparkover rate and spark-
over intensity. The automatic controls further ensure fast precipi-
tator voltage recovery after sparkover.
B. A transformer/rectifier for charging of the storage capacitor to a con-
trolled voltage level corresponding to the desired pulse peak voltage.
C. A pulse power supply comprising the pulse transformer and the coupling
capacitor in a common oil tank. The tank further contains a voltage
divider for direct high voltage measurements. The precipitator can be
connected either to the pulse supply or to ground for safety, through a
manually operated high voltage change-over switch in the oil.
D. A DC high voltage supply comprising an oil emerged three phase trans-
former/rectifier with a blocking inductor preventing the pulse voltage
from entering the supply. By means of a manually operated high voltage
change-over switch in the oil, the precipitator can be connected either
to the DC supply or to ground for safety during repairs.
Where pulse energization systems are installed at an existing precipi-
tator installation the existing conventional single phase transformer/
rectifiers may be used instead of the 3 phase DC supply. The single
phase transformer/rectifier would have to be tied in with the controls
for the pulse energization system. Also a smoothing filter for reduction
of its ripple voltage would have to be added together with the necessary
blocking inductor to prevent the pulse voltage from entering the DC supply.
Installation of the pulse system at existing or new precipitator instal-
lations is simple. The pulse power supply and the DC high voltage supply may
be placed on the precipitator roof for direct connection to the field, or they
may be placed at any other suitable outdoor or indoor location in the vicinity
of the precipitator, and connected to this through high voltage busbars pro-
tected by trunking.
Where the size of the fields or cells is relatively small two fields or
cells may be energized from the same pulse energization unit.
The control panel requires a dust-free environment and ambient tempera-
tures maintained between 0°C (32°F) and 40°C (104°F).
For 6 mobile test and demonstration pulse energization units, 2 of which
were manufactured in Europe and 4 in the United States, a suitable environ-
ment for the control panels was obtained by placing them in a 20' container
with filtered ventilation, and heating and cooling facilities.
296
-------
Similar control house arrangements may be advantageous also for permanent
installations, particularly at existing precipitator installations where
building layout makes it difficult to find suitable space and environment for
the control panels. Further, the container solution has the advantage of
minimizing field wiring work and installation time. Up to four control panels
serving four high voltage units may be placed in one container.
Remote control and remote instrumentation is easily incorporated, either
in a separate small remote control panel or built into a central control panel
placed at any desired distance from the pulse energization system.
FULL SCALE TEST RESULTS
Precipitator for a Lime Burning Rotary Kiln
From early 1979 full scale tests have been carried out on a precipitator
dedusting the 350°C hot exit gases from a 290t/24h lime burning rotary kiln in
Denmark. The precipitator consists of two fields in series with collection
areas of abt. 1100 m* and 1400 m , respectively. In the first field the duct
width is 300 mm, and the discharge electrodes are of a rigid pin-type design.
In the second the duct width is 250 mm, and the discharge electrodes are of
the conventional 2.7 mm diameter helical type. The gas velocity is about
0.6 m/s, giving a total treatment time of 12 seconds.
O
The gas volume treated in the precipitator is about 115,000 m /h, temp-
erature about 350°C, the water content about 15% by volume, and the dust load
about 20 g/m-'. Particle size median is about 17V-m and dust resistivity
varies from about 10^-0 to 10 ohm-cm, depending on temperature, raw materials
and kiln operation.
Each of the fields can be energized from either a pulse energization
system or a conventional single phase, full-wave raw rectified DC power supply.
The precipitator efficiency and the w, migration velocity were determined
from measurements of the inlet and outlet dust concentrations and the gas
volume flow rate. An improvement factor was defined as the ratio between the
w values for pulse and DC energization for optimum operation conditions. The
results confirmed earlier findings with a double pipe test precipitator (3) ,
which showed that the improvement factor was strongly dependent on the degree
of back ionization as judged from resistivity measurements and the corres-
ponding current-voltage curve for DC energization, refer the table below.
OPERATION CONDITION
No back ionization,
No back ionization,
Moderate back " ,
Severe back " ,
IQlO
1011
io12
1013
ohm-
ohm-
ohm-
ohm-
cm
cm
cm
cm
w, IMPROVEMENT
k
1.
1.
1.
2.
1
2
6
2
FACTOR
297
-------
During 1980 the pulse system has been running continuously on the first
section of the precipitator with a total of about 6500 hours of operation
without operation failure or need for service, and thus proved that the system
has the required degree of reliability for practical application.
Precipitator for a 4-Stage Preheater Kiln
Two mobile pulse units have been operated for about 3 months on a preci-
pitator for a 2800 t/24 h, 4-stage preheater cement kiln with raw mill in
Spain. The installation has a conditioning tower in parallel with the raw
mill in order to maintain a gas temperature of about 150°C, which is necessary
to prevent back ionization (5). The precipitator has two chambers each with
two fields in series with a collection area of about 2500 m^ per field.
The gas volume treated is about 390000 m3/h, and the dust load about
30 g/m3. Particle size median is about 5jmm.
The degree of back ionization can be controlled simply by reducing the
water injection in the conditioning tower, which results in higher tempera-
ture, reduced moisture content, and higher dust resistivity.
One pulse unit supplies one field, and each of the four fields can be
connected to a pulse unit. The dust emission can be measured at each of the
chambers separately, and the installation thus makes it possible to test
performance of pulsing on the two inlet fields, the two outlet fields, and
the two fields in one chamber.
During the tests the temperature has been varied between 140 and 200°C,
the moisture content in the range 15-8% by volume, and the dust resistivity
from 10^-1 to 5-10^- ohm-cm. This resistivity is measured on samples of dust
scraped off the collection electrodes. Samples from the dust extraction
equipment show resistivities about one decade lower.
Through these ranges of variation the w, improvement factor remained
constant at about 2 calculated for the pulse energized part.
ECONOMICS
Capital Expense for New Installations
Pulse energization can be used either to improve the performance of an
existing precipitator, or to reduce the required collection area for a new
precipitator installation for a high resistivity application.
The w, improvement factor used above is useful, since the necessary col-
lection area is inversely proportional to w, . The earlier showed relation-
ship between w, improvement factor and resistivity level found for the lime
kiln precipitator can be transformed into a set of curves as shown in
298
-------
Figure 5, which illustrates the saving in collection area for pulse energi-
zation compared with DC energization.
3001
10
1011 10
Resistivity, ohm • cm
12
10
13
Figure 5. Relative collection area for
DC and pulse energization.
The equipment cost on installed basis of the described type of pulse
energization system is presently in the order of $6 per square foot of collec-
tion area energized. Further development of the technology, larger units and
the general downward trend in prices of high power electronic switch elements
can be expected to lower this figure in the future. However, already at a
cost of $6 per square foot of plate area energized pulse energization becomes
economically interesting for new installations even at medium level resisti-
vities producing moderate w, improvement factors.
This can be shown by the following simple way of reasoning with an im-
provement factor of 1.5 as example. The cost of pulse energizing two square
feet plate area would be 2 x 6 = $12. With an improvement factor 1.5 the two
square feet of pulse energized plate area would perform as well as 2 x 1.5 = 3
square feet of plate area energized with conventional DC. There is thus saved
one square foot of plate area by using $12 for the pulse energization system.
With costs of new large precipitator installations going as high as $22 per
square foot of collection area on installed basis for a 500 MW unit it is
seen that pulse energization has a considerable potential for savings with
high resistivity fly ash.
Upgrading of Existing Installations
For upgrading of older precipitators pulse energization, where applicable,
may be preferred to 803 conditioning for its simplicity, low power requirements,
high reliability and low maintenance costs characteristic of solid-state TR sets.
299
-------
For a backfit situation, assume a cold side precipitator on a 500 MW
(net) plant with a SCA=300 square feet per 1000 ACFM achieving 97% efficiency
on a high resistivity ash. In such cases an 803 conditioning system having an
enhancement factor of 2 on migration velocity w^ might be a logical choice to
achieve 99.4% efficiency. Pulse energization with a similar enhancement
factor would also be a very viable alternative. Either of these would be much
less expensive than adding an additional precipitator with SCA = 300 to achieve
about 99.4% efficiency. Some cost comparisons for this case may be noted as
follows; (1.9 million ACFM at 300°F).
Capital Expense
New Backfit ESP at $22/sq.ft. installed $12.5 million
Add Pulse Energization at $6/sq.ft. $3.42 million
SO,, System installed - S burner type $2.8 million
Annual power costs, however, would be much less for the pulse system than
for 803 conditioning. This is in particular the case for the energy conser-
ving system dealt with here. During operation on medium resistivity dust at
150°C with a collection plate current density of 10/XA per square foot the
power supplied from the mains to the DC as well as the pulse unit was measured
to about 0.5 watts per square foot of collection plate area. At a price of
$0.04/kWh for 7000h operation this is about $80,000.
This shall be compared with annual operating cost of $417,000 for the S0~
conditioning composed of $210,000 for power to the precipitator and sulfur at
$50,000 ($0.032/Tn. coal burned) and incremental maintenance, labor, utilities
and overhead of about $157,000.
Other proprietory chemical conditioning systems, where applicable, have
very low capital costs, but cost of chemicals may be high - typically
$0.3-0.5/Tn. coal burned.
CONCLUSIONS
Full scale field tests with the energy conserving pulse energization
system described in the paper have shown that the performance of precipitators
collecting high resistivity dust can be improved considerably by pulse energi-
zation. The improvement results in capital savings from reduced size require-
ments for new installations, and in upgraded performance of existing install-
ations.
The tests have demonstrated that the automatic controls of the system
are able to maintain a stable operation and to adjust continuously the DC
voltage level, pulse peak voltage, and pulse repetition frequency for optimum
precipitator performance under the various operation conditions experienced.
Further, a test with continuous operation of a unit for about one year
has shown that the equipment has the required degree of reliability for
300
-------
practical application. The pulse energization system is now being manufac-
tured in the United States as well as in Europe.
The practical arrangement of the system gives an easy installation with
a high degree of flexibility on new as well as on existing precipitators. The
high voltage DC and pulse supplies may be placed on the precipitator roof or
at any other suitable location in the vicinity of the precipitator. For old
installations the existing DC high voltage supply may be used. The control
panels require a dust-free environment and suitable ambient temperatures. If
desired, the panels can be placed in a container delivered with the system.
Comparison of installation costs for new precipitators shows that even at
medium resistivity levels, where only moderate w, improvement factors are
attained, pulse energization with this system is an interesting alternative to
a large precipitator with conventional DC energization.
The advantage is even more pronounced for upgrading of existing precipi-
tators, where the installation costs for additional collection area in an
example far exceed the costs of the alternative pulse solution.
In the latter case an SO,, conditioning system giving a similar enhance-
ment factor as the pulse solution may be installed at somewhat lower costs
than for the pulse system.
A comparison of the operating costs for the two solutions, however, shows
that pulse energization with the energy conserving pulse energization system
can be performed at much lower annual costs than SO,, conditioning.
301
-------
REFERENCES
1. Lausen, P.: Improved Precipitation by Pulse Energization. Proc.
of US-Japan Seminar on Measurement and Control of
Particulates Generated from Human Activities, Nov.
1980, Kyoto, Japan.
2. Joergensen, H.J., Influence on Particle Charging of Electrical Para-
J.T. Kristiansen meters at DC and Pulse Voltages. 3rd Symposium on
and P. Lausen: The Transfer and Utilization of Particulate Control
Technology. March 1981, Orlando, Florida, U.S.A.
3. Petersen, H.Hoegh
and P. Lausen:
4. Lausen, P.:
5.
Precipitator Energization Utilizing an Energy
Conserving Pulse Generator. Proc. of 2nd Symposium
on The Transfer and Utilization of Particulate
Control Technology. July 1979, Denver, Colorado, U.S.A.
Energy Considerations on Pulse Energization of
Electrostatic Precipitators. Proc. of US-Japan
Seminar on Measurements and Control of Particulates
Generated from Human Activities, Nov. 1980, Kyoto,
Japan.
Petersen, H.Hoegh: Electrostatic Precipitators for Cement Mills, Kilns,
and Coolers - Control of Operating Conditions During
Transition Periods. Presented at IEEE Cement Industry
Technical Conference. May 1978, Roanoke, Virginia,
U.S.A.
302
-------
SO? REMOVAL BY DRY INJECTION AND
SPRAY ABSORPTION TECHNIQUES'
By: Edward L. Parsons, Jr.
Buell Emission Control Division
Envirotech Corporation
Lebanon, Pennsylvania 17042
Vladimir Boscak
Chemico Air Pollution Control Corporation
Envirotech Corporation
New York, New York 10001
Theodore G. Brna
Industrial Environmental Research Laboratory
United States Environmental Protection Agency
Research Triangle Park, North Carolina 27711
Ronald L. Ostop
Department of Public Utilities
City of Colorado Springs
Colorado Springs, Colorado 80903
ABSTRACT
The initial phases of EPA-sponsored test programs on dry injection and
spray absorption were executed from December 1979 through July 1980 at
Buell's Colorado Springs test facilities and established a solid base of
nrocess design data. This paper presents the results of EPA-funded follow-
u^udies Dieted in October 1980, which addressed iss-* °f J^*^^
disposal for both dry FGD methods, and further *xP^es parametric °Ptlmlza
tion of the more commercially important spray absorption process.
characteristics.
hot gas bypass reheat on spray ""JJJeltSne were evaluated and
testing.
INTRODUCTION
The technical feasibility °~f^r £™ fquel hL'bfen
fired boilers by dry injection and spray a»sorpc ± x 4llttle has been
widely demonstrated la P^^-f^^^fp^ducrSracterLation and/or
published, however, pertaining to ^te pro auc process. For dry
environmentally acceptable disposal Method 8 «*e^ ben£ successful demon-
injection or spray absorption V^ff^tant "o rhe future development
regains to be
303
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in scientifically controlled tests. In EPA-sponsored tests conducted at
Envirotech's Lebanon R&D laboratory and Battelle's Columbus Laboratories,
representative samples of spray absorption and dry injection waste product
obtained from Buell's Colorado Springs pilot plant were subjected to various
stabilization processes and were evaluated for environmental acceptability
in simulated landfill disposal.
With 13 utility systems totaling over 4000-MW capacity on order, spray
absorption FGD stand-s on the threshold of its first generation of large-
scale commercial application. However, even as the startup dates for major
installations in the 400- to 600-MW class approach, much remains to be
investigated in regard to flue gas pretreatment, the effects of ash chemistry,
and alternate sorbent types and preparation methods. These and other issues
were pursued in an EPA-funded follow-up study completed in October 1980,
which was a continuation of the original spray absorption test program
conducted by Buell and Anhydro A/S at their Colorado Springs test facility (1),
The 2-MW equivalent pilot plant used in both programs was installed in
slipstream configuration at the inlet of the 189 m3/s (400,000 acfm) Buell
reverse air filter operating on Boiler No. 6 of the City of Colorado Springs'
Martin Drake Station, a 85-MW pulverized coal-fired steam generator burning
low-sulfur Northwest Colorado coal.
SPRAY ABSORPTION WASTE PRODUCT STUDIES
Relative ease of handling and disposal of spray absorption waste
products is widely claimed as an advantage over wet FGD processes, but is
supported by little in the way of published geo-technical data. It does
seem reasonable that the mixture of fly ash and calcium salts would undergo
pozzolanic reactions after mixing with water and landfilling, rendering the
material acceptable from the standpoint of unconfined compressive strength,
permeability, and leachability. To confirm this hypothesis, representative
samples of lime-based spray absorption waste product from the Colorado
Springs pilot plant were tested at Envirotech's R&D laboratory in Lebanon, PA.
Waste product samples were obtained from the absorber and baghouse
hoppers for test points representing straight-through (no waste product
recycle) operation at a low stoichiometric ratio as well as a high stoichio-
metric ratio run with 50 percent recycle of the total waste product. The
results of chemical analysis of the samples are presented in Table 1 and
indicate 20 to 25 percent calcium species for the low stoichiometry case
(Sample 1) and 40 to 50 percent for the higher stoichiometry (Sample 2)-
TABLE 1. CHEMICAL ANALYSIS OF DRY FGD WASTE PRODUCT
FROM SPRAY ABSORPTION PILOT PLANT
Sample 1 Sample 2
Low Stoichiometry High Stoichiometry
Straight Through 50% Recycle
Constituent Percent Weight Percent Weight
Absorber Baghouse Absorber Baghouse
Solids 99.9 99.7 96.8 98.5
CaSOo-1/2 H20 3.9 2.3 19.2 20.8
CaS04'2 H20 1.2 2.2 9.3 2.3
CaC03 10.7 10.7 9.9 11.9
Mg 0.9 1.0 0.2 3.0
Acid Insoluble 80.4 77.5 59.6 62.1
304
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The samples were mixed with varying amounts of water, and simulated
landfill specimens were prepared using a Harvard miniature compactor and
cured in constant humidity chambers. For the high stoichiometry case,
baghouse and absorber waste product samples were prepared separately; for
the low stoichiometry case, equal parts of baghouse and absorber waste
product were mixed. Specimens were tested for unconfined compressive strength
and permeability after two different curing periods. The results of these
tests are summarized in Table 2 and show that the highest strength and lowest
permeability coincided for all samples: 80 percent solids gave the best
results for Sample 1 and 75 percent solids was best for Sample 2. The results
indicate ample compressive strength for the support of men and equipment
(19.5 tonne/m2), and the permeability is acceptable
TABLE 2. UNCONFINED COMPRESSIVE STRENGTH AND PERMEABILITY FOR
SIMULATED LANDFILL DISPOSAL OF DRY FGD WASTE PRODUCT
Percent Solids
SAMPLES FROM SPRAY ABSORPTION PILOT PLANT
1SL Zl -§°-
Compressive Strength, tonne/m2(ton/ft2)
Sample 1 - Mixture of spray absorber
and baghouse waste product
After 7 days 32.8
(3.36)
After 26 days 31.5
(3.23)
Sample 2 - Spray absorber waste product
After 10 days
After 21 days
Sample 2 - Baghouse waste product
After 10 days
After 21 days
20.9
(2.14)
54.0
(5.53)
27.5
(2.82)
29.7
(3.04)
219
(22.4)
284
(29.1)
Permeability, 106cm/s(10~6 in./sec)
Sample 1 - Mixture of spray
absorber and baghouse waste product
After 7 days
After 26 days
60
(24)
45
(18)
Sample 2 - Spray absorber waste product
After 10 days
After 21 days
Sample 2 - Baghouse waste product
After 10 days
25
(9.8)
9.3
(3.7)
2.3
(0.91)
2.0
(0.79)
0.25
(0.10)
(0.26)
39.6
(4.06)
61.7
(6.32)
19.7
(2.02)
17.7
(1.81)
84.0
(8.60)
101
(10.4)
10
(3.9)
7.1
(2.8)
5.1
(2.0)
3.4
(1.3)
2.0
(0.79)
(0.67)
5
24.0
(2.46)
29.4
(3.01)
18.1
(1.85)
11.1
(1.14)
118
(12.1)
106
(10.9)
44
(17)
18
(7.1)
9-7
(3.8)
10
(3.9)
3.5
(1.4)
(1.4)
305
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Table 3 summarizes the results of leachate tests performed on equal
mixtures of absorber and baghouse waste product using EPA's extraction
procedure (Proposed Rules Part 250 - Hazardous Waste Guidelines and Regula-
tions, Federal Register, Volume 43, No. 243, December 18, 1978). The
results show that the levels of mercury approximate the proposed level for
hazardous waste, but that the seven other heavy metals are far below the
proposed levels. It should be pointed out, however, that the extraction
tests were performed on the "as received" dry powder samples which had not
undergone pozzolanic (hydraulic cement forming) reactions. Thus, the expected
contaminant levels from an actual landfill would be lower.
TABLE 3. LEACHATE ANALYSIS FROM EPA'S EXTRACTION
PROCEDURE PERFORMED ON "AS RECEIVED" DRY
FGD WASTE FROM SPRAY ABSORPTION PILOT
SPRAY ABSORBER AND
BAGHOUSE WASTE PRODUCT
Extract Sample Proposed Level
Contaminant _ mg/1 _ _ mg/1 _
Sample 1 Sample 2
Arsenic 0.012 0.015 0.5
Barium 0.029 0.117 10.0
Cadmium 0.006 0.012 0.1
Chromium 0.009 0.021 0.5
Lead 0.015 0.027 0.5
Mercury 0.017 0.021 0.02
Selenium 0.006 0.016 0.1
Silver 0.001 0.011 0.5
DRY INJECTION WASTE PRODUCT STUDIES
Spent material consisting of fly ash and sodium salts from the dry
injection FGD pilot program at Colorado Springs' Martin Drake Station (1,2)
was analyzed to establish a cost-effective process for the environmentally
acceptable disposal of the waste materials. In this study, Battelle's
Columbus Laboratories evaluated the insolubility of sodium salt wastes after
treatment. The treatment involved pelletizing the wastes after addition of
about 5 percent lime and sintering the pellets at a temperature up to 1000°C
(1843°F) while noting S02 evolution. Testing of the stabilized pellets as
aggregates for leaching and strength characteristics was also performed.
Based on these analyses, an optimum process, balancing S02 evolution during
curing and leaching of salts from the stabilized pellets, was defined and
costed. The economic analysis for the waste stabilization process used the
premises given in the most recent TVA/EPA report for a typical Upper Great
Plains 500-MW power plant (3). This analysis indicates that with pelletiza-
tion and sintering of the sodium salt wastes the dry injection FGD process
has a higher cost per kW than lime-based spray absorption when waste
disposal costs are included.
SPRAY ABSORPTION PARAMETRIC TESTS
Pilot Plant Description
The slipstream pilot plant used for the follow-on spray absorption
tests was essentially the same as described previously (1), except that a
spray system and a powder injection system were added upstream of the
absorber for pretreatment of the gas with water sprays and/or injection of
additional fly ash. The 3.8-m (12.5-ft) diameter Anhydro spray absorber had
306
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«n,"P A ?-£f 4™ ?,/? (85°° acfm) at optimum (10-s) residence time and
r ??o PEh WK x 3°^ £4°~hp) belt-driven Anhydro centrifugal atomizer.
Gas from the absorber discharge was directed through ductwork designed to
rer!rLre?eaf uy.e"h?r absorber or preheater bypass gas prior to entering a
reverse air fabric filter equipped with full-size (0.305-m diameter x 9.30 m
long (1-ft diameter x 30.5 ft long)) Teflon-coated fiberglass filter bags.
Lime slurries were prepared using a full-scale commercial package-type lime
storage handling, and slaking system, and recycle slurries were prepared by
hand and agitated in a 1325-1 (350-gal.) tank. Lime and recycle slurries were
circulated to their points of application by independent flooded loops, and
the process flow of each slurry was automatically controlled by a variable
speed progressing cavity slurry pump. An S02 vaporizing and injection system
enabled spiking the slipstream gas flow up to a level of 2500 ppm SO?, and a
??o!r7onoit heat exchan§er allowed slipstream gas preceding by as much as
44 C (.80 F). The pilot plant was fully instrumented to measure SOo con-
centrations, gas and slurry flow rates, temperatures, and pressures. All
instrument channels were scanned, data were converted to engineering units,
and all data were printed on both an instantaneous and time-averaged basis by
a Monitor data logger.
Alkaline Fly Ash
Since the Martin Drake Station burns a northern Colorado coal with low
available alkalinity when slurried (0.4 to 0.6 percent expressed as Ca(OH)2),
the effect of alkaline ash could be investigated only by introducing such ash
into the slipstream gas flow. A supply of moderately alkaline Texas lignite
ash was obtained from the Monticello Station of Texas Utilities Services to
study'the effect of ash alkalinity. The available alkalinity of this ash
when slurried is about 2.5 percent alkalinity (expressed as Ca(OH)2). The
total alkaline content of this ash is given in Table 4.
The alkaline fly ash was pneumatically injected into the slipstream
flow upstream of the absorber at a rate of 0.080 kg/m^ (3.5 gr/ft*) of flue
gas. Performance tests were run at various approaches to saturation tempera-
ture at the absorber outlet, with and without waste product recycle, to
assess the impact of ash alkalinity. In Figure 1, the results of these tests
are compared with the lime data obtained with nonalkaline ash in the original
spray absorption test program (1). Since the FGD reactions in lime-based
spray absorption take place only in an aqueous medium, the alkaline ash would
be expected to have little or no effect on straight-through performance.
This is borne out by Figure 1. For the same reason, recycle performance is
noticeably better than without recycle because utilization of ash alkalinity
made available in the recycle slurry provides a benefit over a nonalkaline
ash.
Component
SiOo
A1203
Fe203
Ti02
TABLE 4. ANALYSIS OF TEXAS LIGNITE ASH FROM
TEXAS UTILITIES SERVICES' MONTICELLO STATION
Na20
Percent Weight
60.05
21.91
3.09
1.61
0.10
0.31
0.46
Component
K20
Li20
CaO
MgO
LOI*
Undetermined
Percent Weight
0.89
0.02
8.87
2.04
0.10
0.55
*Loss on ignition
307
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Absorber Inlet Prequench
Diffusion of S02 into the sorbent droplets was shown to be rate-
controlling for SC>2 removal across the absorber (1). Since SC>2 must diffuse
counter to water vapor evolving from the droplet surface, greater 809 mass
transfer would be expected if the initial water evaporation rate could be
retarded. Prequenching the absorber inlet gas flow with water sprays would
reduce the initial driving forces for the evaporation rate as decreased
temperature of the flue gas gives a lower gas-liquid temperature differential,
while higher gas moisture reduces the vapor pressure-partial pressure differ-
ential. Both of these forces tend to reduce the evaporation rate. However,
prequench reduces the percent of total water evaporated which passes through
the absorber atomizer in the process, and since there is an upper limit on
the solids concentration which can be used for lime and recycle slurries, the
ability of the system to utilize recycle is effectively reduced. Thus for a
net benefit, prequench must more than balance the loss of recycle capacity it
entails.
Performance tests were run using a spray system to prequench the absorber
inlet gas from its normal temperature of about 163 to 149°C (325 to 300°F)
and 135°C (275°F) by means of water sprays. The results of these tests are
compared to the lime data in Figure 2, which indicate only a very slight
benefit due to prequench at 135°C (275°F), well within the "noise" level of
the data. Since about 25 percent of the total water must be diverted to the
sprays to get the 135°C (275°F) temperature, the capacity to recycle is
seriously reduced. Thus, it appears that prequench offers no advantage to
the spray absorption process.
Lime With Adipic Acid Additive
Adipic acid was tested as a means of increasing the rate of dissolution
of Ca(OH)2 in the lime slurry and thus improving SO? removal. The results
of tests conducted with 1500 and 3000 ppm adipic acid in the lime slurry are
compared to the lime data in Figure 2. A significant benefit was realized
only with recycle and 3000-ppm adipic acid. A greater effect would be
expected with recycle since recovery of residual adipic acid in the waste
product will increase the total adipic acid concentration in the slurry
system. It is also reasonable to suppose that a sufficiently high level of
adipic acid will reduce the recycle slurry pH to the level where alkalinity
in the waste product will have increased availability. The level of benefit
shown merits further investigation of the effect of adipic acid on the lime-
based spray absorption process, including cost impacts.
Dolomitic Lime
In some spray absorption applications, particularly in the Northeast,
a dolomitic lime may be more readily available than high calcium lime, and
thus would be desirable as a sorbent, provided that reasonable performance
was obtainable. This would be a problem with most sources of dolomitic
lime, since magnesium oxide is usually "hard-burned" in the calcining
process, and therefore will not explosively slake to form the highly reactive
lime slurry needed for best results in spray absorption FGD. A commercial
soft-burned dolomitic lime was used in the tests described here.
Dolomitic lime requires special slaking methods because of the unusual
slaking characteristics of MgO. Slaking water must be preheated to 77°C
(170°F) to obtain the optimum slaking temperature range of 88 to 93°C (190
to 200°F) at a water-to-lime ratio of 4:1, and slaker residence time must be
extended to at least 30 minutes to allow a complete slaking reaction to
308
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occur. Under these optimum conditions, an extremely thixotropic lime slurry
is produced which requires about double the agitator power of a normal lime
slurry of the same solids concentration. The results of a straight-through
performance test are compared with the lime data base in Figure 3 and
indicate performance essentially equivalent to high calcium lime for a 11 to
17°C (20 to 30°F) approach to saturation at the absorber outlet. Due to the
hydroscopic nature of the magnesium sulfites and sulfates in the waste
product, a minimum approach to saturation of 14°C (25°F) is recommended to
obtain dry powder. It is emphasized that these results were obtained with a
soft-burned dolomitic lime and that the normal hard-burned variety would
likely produce less satisfactory results.
Limestone with Prequench and Adipic Acid
Further attempts were made in this test program to improve the poor SC>2
removal results obtained in the original test program with limestone slurries.
Differing concentrations of adipic acid, recycle of waste product, and
prequench were all tried with disappointing results. The best performance
results for 200 to 500 ppm S02 at the absorber inlet and a 14°C (25°F)
approach are plotted in Figure 4. These data show that S(>2 removal is
severely rate-limited by the slow reaction with CaC03 and thus increasing
stoichiometry has little or no effect on S02 removal. Further tests were
run at 1000 ppm S02 with worse results. The highest S02 removal was observed
at the lowest SC>2 concentration (214 ppm), which further confirms that
reactions with CaC03 are rate-limiting.
Trona
Attempts were made to improve the trona results presented earlier (1)
by means of partial recycle of the waste product. Due to the very high
reactant utilization, little or no performance improvement was achieved.
Hot Gas Bypass Reheat
Reported test results (1) establish the importance of operating at an
exit gas temperature from the spray absorber as close as possible (about
11°C (20°F)) to the adiabatic saturation temperature to attain the optimum
S02 removal efficiency. Yet, many spray absorption system specifications
require baghouse inlet temperatures as much as 44°C (80°F) higher than
saturation temperature. Hot gas bypass reheat is often the most cost-
effective means of accomplishing the reheat but entails the loss of S02
removal efficiency due to bypassing part of the flue gas around the absorber.
For reheat up to about 28°C (50°F) above the saturation temperature, this is
the only penalty paid for reheat as the percentage S02 removal across the
baghouse is essentially unchanged. Higher levels of reheat, however, reduce
S09 removal in the baghouse as well and, thus, impose a double penalty. With
reheat of 42 to 47°C (75 to 85°F) above saturation, the baghouse flange-to-
flange SO? removal percentage is reduced to 20 percent of the value which
would be experienced with no reheat. Apparently, the extreme degrees of
reheat dry out the residual moisture in the sorbent particles leaving the
absorber to the point that the reaction rate with S02 in the filter cake is
substantially reduced.
CONCLUSIONS
The series of follow-up tests described in this paper was undertaken
to resolve issues of waste product disposal and process optimization. A
technically feasible, although economically unattractive, disposal method
was demonstrated for dry injection waste product, and the applicability of
309
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conventional fly ash disposal methods to spray absorption waste product was
confirmed. Alkaline fly ash was found to have a beneficial effect on spray
absorption process efficiency when recycle was used. Soft-burned dolomitic
pebble lime was found to be an acceptable reagent if the special requirements
for its use can be met. Adipic acid additive to lime slurry was found to
improve performance when used at the 3000-ppm level and with waste product
recycle. Prequench of the absorber inlet gas with water spray was found to
be relatively ineffective in improving SO? removal with either lime or
limestone sorbent. The low levels of performance for limestone sorbent
reported earlier were not improved by recycle, higher stoichiometry, or
higher levels of adipic acid additive. Recycle tests with trona sorbent
failed to improve upon results previously observed for straight-through
operation.
ACKNOWLEDGEMENT
The authors acknowledge with appreciation the contributions of Edward
Tomeo, James Utt, and Thomas Griffen of Buell/Envirotech, and Fritz Paulsen
of Anhydro A/S for their assistance in the collection and reduction of the
data presented here. This work was supported under EPA Contract No. 68-02-
3119, Mod. 3 and with the cooperation of the City of Colorado Springs.
REFERENCES
1. E. L. Parsons, Jr., L. F. Hemenway, 0. T. Kragh, T. G. Brna, and R. L.
Ostop, "S02 Removal by Dry FGD," presented at the U.S. Environmental
Protection Agency's Sixth FGD Symposium, Houston, TX, October 1980.
2. D. A. Furlong, T. G. Brna, and R. L. Ostop, "SO? Removal Using Dry
Sodium Compounds," presented at the AIChE 89th National Meeting,
Portland, OR, August 1980.
3. T. A. Burnett, K. D. Anderson, and R. L. Torstrick, "Spray Dryer FGD:
Technical Review and Economic Assessment," presented at the U.S.
Environmental Protection Agency's Sixth FGD Symposium, Houston, TX,
October 1980.
310
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3.0-
LEAST SQUARES FIT FOR LIME DATA BASE,
INLET PPM S02 = 1000
2.0-
E = S02 REMOVAL FRACTIOM
1.0-
50% RECYCLE
STRAIGHT
THROUGH
SYMBOL tp F RECYCLE?
o
a
A
20 NO
20 YES
30 NO
30 YES
40 NO
40 YES
1.0 2.0
STOICHIOMETRIC RATIO
3.0
Figure 1. Comparison of SC>2 removal data for
lime with injection of 3.5 gr/ACF
alkaline ash (see Table 4) with
lime data base, at different values
of approach temperature, tp. All
data for 1000 ppm SO2 at inlet and
10 second residence time.
3.0-
2.fi-
1.0-
LEAST SQUARES FIT FOR LIME DATA BASE,
INLET PPM S02 = 1000, tp = 2QOF
RECYCLE
E = S02 REMOVAL FRACTIOM
INLET F RECYCLE7
300 MO
NO
YES
YES
RECYCLE?
NO
YES
NO
YES
1500
3000
3000
1.0 2.0
STOICHIOMETRIC RATIO
3 0
Figure 2. Comparison of SO2 removal for lime
with prequench, recycle and adipic
acid additive to lime data base.
All data for 1000 ppm SO2 at inlet,
approach temperature, tp = 20°F,
and 10 second residence time.
-------
U>
I—l
K5
LEAST SO.UARES FIT FOR
LIME DATA BASE (HIGH CALCIUM) STRAIGHT
1.0 2.0
STOICHIOMETRIC RATIO
1.0-
0.5-
SYMBOL
O
€
2.0
50*
NO
NO
NO
YES
YES
MO
YES
INLET
QUENCH
RECYCLE TO 275°F
NO
NO
NO
NO
NO
YES
YES
PPM
ADIPIC
ACID
1500
3000
1500
3000
1500
1500
O
S02 REMOVAL FRACTION
3.0 4.0
STOICHIOMETPIC RATIO
5.0
Figure 3. Comparison of SC>2 removal data for
soft-burned dolomitic lime with
high calcium lime data base, with
approach temperature, tp = 20°F and
30°F. All data for straight through
operation and 10 second residence
time.
Figure 4. SC>2 removal data for Q4 grind lime-
stone with prequench, recycle, and
adipic acid additive. All data
for 200 to 500 ppm S02 at inlet
and 10 second residence time.
Approach temperature, tp = 25°F.
-------
DRY SCRUBBING SO AND PARTICULATE CONTROL
By: Nicholas J. Stevens
Ghassem B. Manavizadeh
George W. Taylor
Michael J. Widico
Research-Cottrell, Inc.
Somerville, New Jersey 08876
ABSTRACT
In dry SO scrubbing, the spray dryer and fabric filter are employed to a
significant extent for both S02 removal and particulate solids collection.
Pilot test results were obtained on low sulfur fuels to elucidate the roles of
each control device in the dry scrubbing system.
SO removal in the spray dryer and fabric filter is compared and the im-
portant process variables identified. Particulate collection of fly ash/FGD
solids mixtures by the spray dryer and fabric filter is examined. The relative
amounts of solids collected at different operating conditions are presented.
Fabric filter pressure drop variation with time and the effect of dry scrubbing
solids on the specific resistance coefficient are described. Spray dryer and
fabric filter solids are examined for particle size distribution, chemical
composition and moisture content.
For Presentation at the U.S. EPA Third Symposium
on the Transfer and Utilization of
Particulate Control Technology
Orlando, Florida
March 9-12, 1981
TEXT
The spray dryer is the primary SO control element while the fabric filter
is the primary particulate control unit. This paper presents the extent and
limits of each device for S02 and particulate removal based on the results of
field pilot test programs.
313
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Pilot Plant Description
The pilot test system (See Figure 1), designed to treat up to 10,000 ACFM
of flue gas, consists of a spray dryer followed by a fabric filter. Dirty flue
gas containing S02 and flyash continuously enters the top of the spray dryer
where it is intimately contacted with finely atomized lime slurry.
Flue gas from the spray dryer enters the bottom of the fabric filter unit
and leaves from the top. From the fabric filter, the flue gas flows to an I.D.
fan and then to the stack. The induced draft fan moves the treated flue gas
through the system and a second "reverse air" fan periodically cleans solids
from the fabric filter cloth.
Spray Dryer
The pilot spray dryer is an 8'0 x 35' high unit equipped with a variable
speed rotary disc atomizer. Intimate contact between finely atomized lime
slurry and SO in the flue gas and large interfacial area produced by ato-
mization in the spray dryer result in very rapid SO, absorption. In the pilot
unit, coarse solids tend to settle in the conical Bottom of the dryer and are
discharged through a rotary valve to receiving drums. The scrubbed flue gas
containing finer particles leaves the dryer through a side exit port and flows
to the fabric filter.
Temperature drop across the spray dryer is controlled by total water flow
to the unit. Water for temperature control is metered as a trim water flow to a
mixing tee immediately upstream of the spray dryer atomizer. Increasing lime
reagent flow increases the SO« removal efficiency at a given water flow, gas
flow and sulfur dioxide content. System SO responses to changes in stoi-
chiometry are measured by a duPont UV analyzer which samples both inlet and
outlet streams from the spray dryer and fabric filter.
Fabric Filter
The fabric filter unit size is 10' x 15' x 55' and it contains two-sixteen
bag compartments each designed to process about 5,000 cfm of flue gas. Each of
the fabric filter bags is of commercial utility dimensions, 12" 0 x 30" high,
and contains about 94 ft of bag surface.
During an operating cycle, a thin layer of solid particles containing un-
reacted lime continually builds up on the surface of the filter bags. Flue gas
from the spray dryer with a reduced SO concentration flows through the bed of
finely divided solids. S0_ removal occurs although the gas contact time in the
bed is short.
1 EPA policy is to express all measurements in Agency documents in metric
units. This paper uses English units which are customary in the U.S. to im-
prove clarity of presentation. Conversion factors are provided in the
Appendix.
314
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To limit pressure drop across the filter bags resulting from the accumu-
lation of collected solids, a flow of air periodically is passed through the
bags in the reverse direction for a short period of time (one-two minutes per
hour). The "reverse air" flow causes most of the solids deposited to dislodge
from the bag surface and drop into the collection hopper where they are dis-
charged through rotary valves. (During the brief bag cleaning period, flue gas
is bypassed around the fabric filter.)
When high SC>2 removal is desired, reagent use becomes excessive unless
solids recycle is practiced. These recycle solids are obtained from the
baghouse and/or spray dryer and contain unreacted lime. The solids are fed
into the recycle tank where they are reslurried with process water. Slurry
from the recycle tank is combined with the slaked lime slurry and the mixture
is metered into the spray dryer atomizer.
Relative SO Removal
S02 removal in the dry scrubbing system is strongly influenced by a
limited number of process variables. Stoichiometric ratio (S.R.) and tem-
perature are two of the more significant parameters that dictate SO removal
both in the spray dryer and in the fabric filter. The spray dryer, however, is
the key SO removal device in the dry scrubbing system. At the conditions
tested in the pilot plant, at least 75% of the total S09 removal in the dry
scrubbing system is accomplished in the dryer, according to Figure 2.
As Figure 2 indicates, a decreasing percentage of the total S0? removal
takes place in the spray dryer as stoichiometry is increased. Of course, com-
bined SO removal in the spray dryer/fabric filter system is greater at higher
stoichiometry. At lower values of S.R. (£1.0 moles slaked lime/mole SO in),
lime utilization tends to be higher in the spray dryer because lime becomes the
limiting reagent. A greater fraction of SO removal takes place in the spray
dryer at lower S.R. values since relatively little lime reagent is present in
the FGD solids to the fabric filter from the spray dryer. At higher S.R., more
unreacted lime is conveyed to the fabric filter and is available for reaction
because utilization is lower in the spray dryer.
Figure 2 shows that relative SO- removal is affected significantly by the
spray dryer and fabric filter operating temperatures. Lower operating tem-
perature results in greater SO2 removal in both the spray dryer and fabric
filter. As the fabric filter operating temperature is decreased between 200 F
and 145°F, S0« removal in the fabric filter increases significantly and rela-
tive SO removal shifts toward the fabric filter.
S0_ removal in the spray dryer is favored by keeping the atomized lime
slurry as wet as possible for as long as possible. Lower operating temperature
produces improved S09 removal but results in a moist solids product that is
difficult to discharge from the spray dryer bottom. The optimum spray dryer
operating temperature is arrived at by striking a balance between high SO2
removal and smooth handling of moisture-laden product solids. But, the fabric
filter operating temperature often must be controlled at a higher level than
that of the spray dryer. Avoidance of dew point problems (condensation), bag
blinding and low buoyancy conditions in the stack are common reasons for
315
-------
increasing the fabric filter operating temperature. As a consequence, rela-
tively less SO- removal takes place in the fabric filter under these con-
ditions.
SO,, Removal vs. Pressure Drop
Fabric filter SO removal varies not only with the key process variables,
stoichiometric ratio and temperature, but also with time between cleaning
cycles. After reverse air cleaning of the filter bags, fabric filter pressure
drop (Ap) and SO removal increase with time. Figure 3 presents SO removal in
the fabric filter as a function of the Ap increase during different operating
cycles. Percent SO- removal initially tends to increase linearly with pressure
drop and then begins to level off at higher AP values near the end of the
cleaning cycle. In these runs, pressure drop increased from about 2.0 in. HO
initially to between 4 and 5 in. H.O at the end of operating cycles of 60-90
minutes duration.
Figure 3 also shows the strong effects of stoichiometric ratio and fabric
filter inlet temperature on %SO- removal in the fabric filter. Percentage
removals of SO- entering the fabric filter vary from as low as 20-30% at S.R. =
1.0 and an inlet temperature of 175°F to as high as 60-70% at S.R. = 2.25 and a
temperature of 155 F. Figure 3 indicates that reducing the fabric filter inlet
temperature from 175 F to 155 F has at least as great an effect on %SO_ removal
as increasing S.R. from 1.0 to the range of 2.1 to 2.25.
Fly Ash vs. FGD Solids
The relative amounts of fly ash and FGD solids in the mixture collected in
the spray dryer and fabric filter depend primarily on three factors:
1. Fly ash grain loading in the flue gas entering the dry scrubbing
system
2. SO concentration of the flue gas
3. Lime stoichiometric ratio
Fly ash grain loadings of 1.0-2.5 grains/actual cubic foot (dry) were
measured in the flue gas at both Big Brown (TUSI) in Texas and at Comanche
Station (P.S. of Colorado) in Colorado. The amount of FGD solids collected is
directly related to the arithmetic product of the incoming flue gas SO con-
centration and the lime stoichiometric ratio used in a particular run to remove
SO . SO levels experienced varied from 200 to 2000 ppm while stoichiometric
ratios tested ranged from about 0.5 to 6.0 moles slaked lime/mole SO- fed to
the spray dryer.
For a given stoichiometry, the amount of FGD solids on a weight basis
depends on the extent of conversion of lime to sulfur products, i.e., reagent
utilization, during dry SO- scrubbing. The weight of FGD solids collected is
also slightly affected by the lime converted to calcium carbonate by reaction
with a^ and by the degree of oxidation that occurs. The extent of the
oxidation reaction determines the ratio of calcium sulfite to calcium sulfate
316
-------
found in the product solids. The sulfite/sulfate ratio of the product solids
in dry SO scrubbing is significantly greater than is generally found in wet
FGD scrubbing. The larger ratio indicates that considerably less oxidation
occurs in the dry scrubbing systems where conditions are not as favorable as in
wet FGD systems. Table 1 shows typical chemical compositions of product solids
collected from the spray dryer and fabric filter under recycle and lime-only
dry scrubbing operations.
TABLE 1. TYPICAL PRODUCT SOLIDS CHEMICAL COMPOSITIONS
(Weight %)
Component
CaS03 • 1/2
Ca(Oll)
CaC03
Flyash
Product Solids
R-425
(Recycle)
R-501R
(Lime Only)
SD
18.6
6.7
0.5
8.7
65.5
100.0
SD
23.9
4.8
<0.1
17.5
53.7
100.0
FF
23.0
4.9
1.3
13.2
57.6
100.0
The product solids collected in the spray dryer are coarser than those
collected in the fabric filter. Table 2 shows the mean particle diameter and
the geometric standard deviation of spray dryer and fabric filter solids ob-
tained from pilot plant runs at a number of different conditions. From the
runs, the average mean particle diameter (50 wt.% solids less than the mean
value) of the spray dryer solids is about 30-35 microns and the average for
fabric filter solids is about 10 microns. Figure 4 presents typical particle
size distributions of solids collected in the spray dryer and fabric filter.
As Figure 4 also shows, the fabric filter fly ash/FGD solids particle size in
this case is nearly identical to that of fly ash itself. The relative dif-
ference in particle size of the spray dryer and fabric filter solids is illu-
strated by the selected scanning electron micro (SEM) photographs of Figures 5
and 6. For comparison, fly ash particulate is depicted in Figure 7.
TABLE 2. TYPICAL PRODUCT SOLIDS PARTICLE SIZES
Spray Dryer
Fabric Filter
Run No.
105
106
115
130
603
928
Average
Mean
(Microns)
35.0
26.
42.
19.0
35.0
.2
,5
39.0
32.8
Std. Dev.
3.2
3.2
3.5
3.6
4.2
5.0
3.8
Mean
(Microns)
8.1
9.8
6.8
11.0
6.2
14.0
9.3
Std. Dev.
2.8
2.9
2.5
3.3
3.1
4.0
3.1
317
-------
Spray dryer solids, according to Figure 5, tend to be composed of a
mixture of large discrete particles, as well as agglomerates of flyash and FGD
solids. From Figure 6, fabric filter solids are made up of finer individual
particles and smaller agglomerates. Particulate size and the tendency to
agglomerate appear to be related to the moisture content of the solids col-
lected. Figure 8 shows that the percentage of the total solids collected in
the system that drops out in the spray dryer increases directly as the moisture
content of the spray dryer solids increases. In normal operation, spray dryer
solids contain more moisture (10-25 wt.%) than fabric filter solids (<5 wt.%).
As Figure 1 shows, the flue gas and particulate leaving the spray dryer
must turn essentially 180 in direction in order to enter the outlet duct
located near the bottom of the spray dryer. In turning, the larger, more
agglomerated solids tend to collect in the spray dryer hopper while the finer
particles are carried by the flue gas into the fabric filter.
Additional solids drying occurs between the time the flue gas leaves the
spray dryer and its particulate is collected in the fabric filter hopper. It
is likely that the additional drying causes some agglomerates to disintegrate
and results in the collection of finer particulate in the fabric filter.
Fabric Filter Pressure Drop
Particulate collection in the fabric filter during dry SCL scrubbing pro-
duces an accumulation of the flyash/FGD solids mixture on the filter cloth with
time during an operating cycle. Figure 9 shows typical linear pressure drop
increases as a function of time during the cycle for Big Brown fly ash alone
and for mixtures of fly ash and FGD solids. Dry scrubbing system operation
with lime slurry feed, but with no recycle, produces about the same rate of
pressure drop increase vs. time as operation with fly ash alone. The pressure
drop buildup rate is comparable in the two cases even though the grain loading
in the runs containing FGD solids is nearly double that present in the fly
ash-only run (4.6 vs. 2.5 gr/acf) . When recycle solids are introduced into the
system, in addition to the lime slurry feed and fly ash already present in the
flue gas, a significantly greater increase in pressure drop with time is
experienced. In this case under recycle conditions, the total particulate
grain loading in the system approaches twice that encountered in the lime-only,
no-recycle run (7.9 vs. 4.6 gr/acf), and more than three times greater than
that for the fly ash-only run.
Specific Resistance Coefficient
Specific resistance coefficients computed from pressure drop vs. time
data show that the fabric filter cake resistance decreases as the FGD solids
content in the fly ash/FGD solids mixture increases. Figure 10 presents a
correlation of the specific resistance coefficient, k , versus the slaked lime
content fed to the spray dryer/fabric filter system. Values of k range from
about 30 in. H20-min.-ft./lb. for fly ash alone to as low as 5 in. H 0-min.-
ft./lb. as the lime grain loading is increased up about 3.5 gr/acf or
5 x 10 Ibs. slaked lime/acf. of flue gas fed to the fabric filter. In these
runs, the lime grain loading is directly proportional to the stoichiometric
318
-------
ratio used since the S02 concentration in the inlet flue gas to the system was
held constant at 800 ppm.
The effect of lime grain loading appears to be more important in de-
termining the specific resistance coefficient than the total grain loading or
the recycle solids grain loading. For example, k decreased from about 15 in.
H2O-min.-ft./lb. in the recycle runs to near 5 in. H O-min.-ft./lb. in the
lime-only test run at high lime grain loadings, although the total particulate
grain loadings were approximately the same. The fly ash grain loading entering
the dry scrubbing system in the flue gas was constant at 2.5 gr/acf. in all
runs but the recycle solids and lime grain loadings varied considerably from
run to run.
From these results, it appears that a specific resistance coefficient of
about 15 in. H O-min.-ft./lb. is experienced in the fabric filter operation
when Big Brown fly ash is utilized over a fairly wide range of practicable
operating conditions for dry S0« scrubbing.
ACKNOWLEDGEMENTS
We thank the host utilities, Public Service of Colorado (Comanche Sta-
tion) and Texas Utilities (Big Brown Station), and their employees for the help
and cooperation in enabling Research-Cottrell to conduct the pilot test pro-
grams. We also wish to thank Dr. Theodore G. Brna, EPA Project Officer, for
his aid and direction in conducting the EPA-funded portion of the work reported
in this paper.
APPENDIX
CONVERSION FACTORS
To Convert From To Multiply
English Metric By
scfm (60°F) nm3/hr (O°C) 1.61
cfm m /hr 1.70
°F °C (°F-32)/1.8
ft m 3 0.305
gr/scf gm/m 2.29
in. cm 2.54
in. H00 mm Hg 1-87
lb 2 gm 454
319
-------
iL iL
IDS RECYCLE 1
I.D. FAN STACK
REAGENT
PREPARATION
FIGURE 1
RESEARCH-COTTRELL
DRY SCRUBBING PILOT FLOW DIAGRAM
TSD=145'F, TFF = 2
0.5 1.0 1.5 2.0 2.5 3.0 3.5
STOICHIOMETRIC RATIO, MOLES LIME/MOLE SO, INLET
FIGURE 2 RELATIVE SOj REMOVAL
U>
tsJ
o
8 30
TFF«155'F
0123456
PRESSURE DROP, IN. H,0
FIGURE 3 FABRIC FILTER SO2 REMOVAL
5
S "
so
Z 40
• SPRAY DRYER SOLIDS
• BAGHOUSE SOLIDS
X FLYASH
FIGURE 4
> 10 20 50 100 200 500 1000
PARTICLE DIAMETER, MICRONS
PARTICLE SIZE DISTRIBUTIONS
-------
;: ;; y :|*^2%
-------
CO
M
N3
20 30
SPRAY DRYER SOLIDS MOISTURE CONTENT, WT. %
FIGURE 8 RELATIVE PRODUCT SOLIDS COLLECTION
FIGURE 9
20 40 60 60 100 120 140
OPERATING CYCLE TIME, MINUTES
FABRIC FILTER PRESSURE DROP
• LIME-ONLY
• RECYCLE OPERATION
X FLY ASH
FIGURE 10
1.0 1.5 2.0 2.5
LIME QRAIN LOADING, OR./ACF.
EFFECT of LIME CONTENT on K,
-------
FIBER AND FABRIC ASPECTS FOR SO? DRY SCRUBBING BAGHOUSE SYSTEMS
By: Lutz Bergmann, V.D.I.
President, Filter Media Consulting, Inc.
P. 0. Box 2189
LaGrange, Georgia 30241
SUMMARY/ABSTRACT
For more than 5,000 MW generating capacity (Table 1) - foremost in the
utility industry - orders are placed for a relatively new technology in the
particulate/S02 pollution control field. This technology offers many aspects
to the heart of these systems, the filter fabric handling the collection of
the combined product. This paper addresses some basic and more specific
aspects of the fibers and the fabrics already used and to be selected in
future installations.
PREFACE
Economically, S02 dry scrubbing systems offer distinct advantages - at
least under certain conditions - not only in initial capital investment, but
more importantly, in operating and maintenance.(1) Some major differences or
a comparison between wet and dry systems may be summarized:
1. A dry S02 scrubber has less equipment than a wet scrubber. No equipment
such as thickness, centrifuges, vacuum filters, and mixers is required
to handle the waste product.
2. Waste products from the dry scrubber are collected along with fly ash and
can be handled with conventional dry material handling systems.
3. Scaling and plugging problems due to wet/dry interface are not expected
with a dry system since the interface is a point in space. Only dry
powder strikes the vessel walls (theoretically).
4. Pilot plant work has indicated that corrosion problems are not experien-
ced in dry systems provided the spray dryer outlet temperature is main-
tained at a proper level.
5. The dry system has flexibility of operation. Feed rates can be immedi-
ately adjusted with little concern for pH control. Reagent consumption
closely follows inlet S02 concentration.
6. The first cost of the dry system is considerably less than a comparable
specified wet scrubbing system.
7. Considerably fewer operators and maintenance personnel will be required
for a dry system.
323
-------
8. The dry system is expected to use only 25% to 50% of the energy required
for a wet system.
9. The gas volume of the spray dryers are reduced below those leaving the
air heater with the resulting savings and costs for particulate collec-
tion equipment.
10. With a dry system, the ID fan can be safely located just ahead of the
stack without fear of fan corrosion and imbalance, and without need for
reheating.
11. The water requirements for a dry system are much less than for a wet
system.
12. Although the process of dry scrubbing is new, the components of the sys-
tem have substantial operating experience with high availability of
plants throughout the world.
13. Sludge disposal is nonexistent and consequently impact on environmental
aspects is considerably reduced in utilizing dry systems.
(From Vendor Literature)
324
-------
TABLE 1. COMMERCIAL SPRAY-DRYER FGD SYSTEMS
Installation
UTILITY BOILER
Coyote
Unit 1
Antelope Valley
Unit 1
La ramie River
Unit 3
Stanton
Unit 2
Springerville
Unit 1 & 2
Rawhide
Unit 1
Riverside
Unit 6 & 7
Sunflowers
Hoi comb Unit 1
Niagara Power
Tonawanda, NY
Colorado Ute,
Craig St. #3
Tuscon Elec. Po.
Springville
Unit 1 & 2
Platte River
Rawhide #1
Antelope Valley
Unit 2
Light & Power
Marquette
Size
MW
410
440
575
63
350
250
100
300
100
450
350
250
440
44
%SO? Alkali Startup
Fuel Type, % S
Lignite, 0.78
Lignite, 0.68
Subbituminous,
0.54
Lignite, 0.77
Subbituminous,
0.69
Subbituminous,
0.28
Blend, subbit-
uminous, bitu-
minous, petro-
leum coke
Subbituminous
8950 Btu/lb.,
0.70
Subbituminous,
0.53 - 0.69
Subbituminous,
1.3
Lignite
INDUSTRIAL BOILER
Strathmore Paper
Company
Celanese Corp.
University of
Minnesota
Calgon Corp.
Pittsburgh
Argone National
Lab.
RI/WF - Rockwell
pitation Div. of
14
22
83
22
Bituminous,
2 - 2.5
Bituminous,
1 - 2
Subbituminous,
0.6 - 0.7
1.0 - 2.0 Sul-
phur +HC1
Removal Material
70
62/78
85
73
61
70
70
80
87
61
80
80
75
70/80
70
75
I nternati onal /Wheel abrator
Joy
Manufacturing CO/Niro
Soda Ash
Lime
Lime
Lime
Lime
Lime
Lime
Lime
Soda Ash
Lime
Lime
Lime
Lime
Lime
Lime
Lime
Lime
Soda Ash
Lime
--Frye; Joy/
Date
4/81
4/82
4/81
9/82
2/85
9/86
12/83
9/80
4/83
82
4/83
84/86
83
10/82
6/79
1/80
9/81
6/81
Niro -
Atomizer Inc. ; B&W
Vendor
RI/WF
Joy/Niro
B&W (ESP)
RC
Joy/Niro
Joy/Niro
Joy/Niro
Joy/Niro
(ESP)
B&W (ESP)
Joy/Niro
Joy/Niro
Joy/Niro
Buell/Anhydro
Mikropul
RI/WF
Carborundum
Joy/Niro
Joy/Niro
Western Preci-
- Babcock and
Wilcox; RC - Research-Cottrell
325
-------
Spray Drying Principle
Spray drying is the transformation of a pumpable fluid feed (solution
slurry) into a particulate dried product, in a single drying process. It has
to be assumed that the basic principle of spray drying is well documented,
although, the method of operation of the spray dryer absorber is relatively
simple. The sorbent solution of the slurry is atomized into the incoming
flue gas stream to increase the liquid/gas interface and to promote mass
transfer of the S02 from the gas to the slurry droplets, where it is absorbed.
Simultaneously, the energy of the gas evaporates the water in the droplets to
produce a dry powder mixture of sulfite/sulfate and some unreacted alkali.
Gas then passes through the second stage fabric filter where the dry product
is collected and a percentage of the unreacted alkali reacts with the S02 for
further removal.(2)
Chemical Reaction/Temperature
A great similarity to pure coal-fired boiler/fly ash applications can't
be denied; however, the most significant difference between fly ash collec-
tion and conditions in twin technology applications is continuous operation
temperature.
The chemical interactions - when lime particles and S02 in the flue gas
form sulfite, respective sulfate - takes place most economically closest to
the moisture dewpoint - in the wet phase - thus representing an operation
temperature range between 130°F to 160°F in the spray dryer absorber. It is
the upset conditions - the fact that the lime/water injection is "off-line" -
when suddenly the baghouse inlet temperature may raise to heat exchanger out-
let conditions between 340°F to 380°F and the temperature requirement for the
filter bag fabric has to accommodate these conditions. Practically only
glass - woven, or needled fabrics - can withstand these requirements, provid-
ed that more expensive fabrics like Teflon^ or PBI^ or others would not be
considered viable candidates.
Bypass or dilution would basically solve any sudden temperature surge in
the baghouse. It is then obvious that the more attractive homopolymer
acrylic fabrics - woven or needled - not only would cost less to install, but
promises to be used at higher air-to-cloth ratios than glass. The decision,
however, as to which system should get preference is site specific and can
probably not generally be addressed. It also depends sometimes on regulatory
aspects, which may differ from state to state.
Combustion Efficiency
Combustion efficiency has not been necessarily a major factor in the de-
sign stage of a number of existing units. As it has turned out, at least in
one installation, the coal quality and the amount of unburned carbon had
different effects on baghouse performance. A larger amount of unburned car-
bon would allow very fine particles to reach the filter bag surface. Natur-
ally, this can contribute to blinding. If, in addition, moisture - in the
liquid phase - is present, this problem multiplies itself and should either
326
-------
be operationally resolved or considered in a more conservative design ratio.
The inlet loading expected, for instance, at 1 - 2 gr/cu.ft. may increase to
as much as 15 gr/cu.ft. hence, creating a much thicker dust layer on the
fabric. This requires either a more frequent cleaning or cleaning at higher
pressure, in long term, however, affecting filter bag life especially with
the more fragile glass fabrics.
Boiler Load
Boiler load may vary particularly in an industrial application, so it is
recommended that air-to-cloth ratios are based on maximum load rather than on
average load, since it is very difficult to predict how frequent maximum load
conditions may prevail. If sometimes in the winter season a boiler has to
meet additional heating requirements, it is advisable to consider these fac-
tors at an early stage of design, since maximum load conditions may last sev-
eral months of operation.
Temperature Control
Temperature control in.S02 dry scrubbing systems is based on the amount
of water being needed to cool the incoming gas to the desired "interface"
condition. One is trying to operate these systems as close as practically
possible to the dewpoint. One has to prevent condensation conditions, on the
other hand, since excessive water will eventually blind the filter bags and
also create problems with corrosion in the baghouse and ash handling system,
notably in the hopper section. If, for instance, the spray dryer in the
startup mode is fed with water only to balance the temperature prior to intro-
ducing lime slurry, a very small amount of $03 content in the flue gas raises
the acid dewpoint significantly. This may have disasterous effects on the
filter material as well as eventually on the metal surfaces like cages, walls,
doors, etc. Such conditions should be avoided or at least reduced to a
minimum.(Table 2)
In summary, the temperature range at which systems are operated obviously
has the most profound effect on S02 removal efficiency and, therefore, main-
taining this temperature range is vital to a fully economic and successful
installation.
Fibers and Fabrics
Basically speaking, as of today only two fibers are generally considered
for these systems. These are glass and homopolymer acrylic. Both fibers are
available in fabrics made from woven yarns, respective from needled
material.(3) Each of the four types are currently under full-scale tests:
1. Needled homopolymer acrylic at Strathmore with excellent results for more
than 18 months.
2. Needled felt glass at Celanese with interesting, although conflicting,
results with the first set of bags for about 10 months, but the second
set working very well since November, 1980.
327
-------
TABLE 2
CO
LU
ec.
a.
2!
UJ
I—
C3
CL.
O
s
K
Generic Name
Fiber
Trade Name
Recommended continuous
operation temperature
(dry heat)
Water vapor saturated
condition (moist heat)
Maximum (short time)
operation temperature
(dry heat)
Specific density
Relative moisture
regain in % (at 68 F
& 65% relative moisture)
Supports combustion
Biological resistance
(bacteria, mildew)
Resistance to alkalies
Resistance to mineral
acids
Resistance to organic
acids
Resistance to oxidizing
agents
Resistance to organic
solvents
Comments
Cotton
180°F
82°C
180°F
82°C
200°F
94°C
1.50
8.5
Yes
No,
if not
treated
Good
Poor
Poor
Fair
Very
Good
Wool
200°F
94°C
190°F
88° C
230°F
no°c
1.31
15
No
No,
if not
treated
Poor
Good
Good
Fair
Very
Good
Polyamid
Nylon 66
200° F
94°C
200°F
94° C
250°F
121°C
1.14
4. - 4.5
Yes
No
Effect
Good
Poor
Poor
Fair
Very
Good
Polypropylene
P
Herculon*
200°F*
94°C
200°F
94°C
225^F
107°C
0.9
0.1
Yes
Excellent
Excellent
Excellent
Excellent
Good
Excellent
*250°F for
Type 154
(under devel-
opment 8/80
328
-------
TABLE 2
GO
LU
0£
CD
•z.
*—I
-------
TABLE 2
-------
3. Woven glass at Northern States Power in operation since early 1981.
4. Woven homopolymer acrylic fabric at Coyote in operation since early 1981.
Depending on the baghouse system (inside or outside collection) the
fabric selection depends on the cleaning method.(4)
The cleaning method, on the other hand, depends on the type of baghouse,
whereas Reverse Air and Shaker baghouses (the names originate from the clean-
ing action) require woven fabrics, whereas the pulse jet/cage type (outside
collection system) calls for the more efficient needled fabrics. Needled
felts are of a textile structure in which single fibers are mechanically
interlocked. Normally, a scrim is sandwiched in the center of the felt be-
tween two fiber layers. The unique feature of needled felts is that the
single fiber is the active element in removing submicron size particles.
Sometimes special woven glass fabrics are being used on cage type collectors,
but this represents, basically speaking, an exception.
Woven glass which is successfully used in large-scale coal-fired utility
applications, allows temperatures up to 500°F and short surges up to 550°F.
Chemically treated glass fabrics have a very good chemical resistancy,
however, being used in Reverse Air systems, with air-to-cloth ratios mainly
at 2:1, represents for many users a compromise. It is only when woven glass
fabrics (special constructions) are being used on outside/cage type collect-
ors with special design cages, off-line cleaning, and the other important
precautions, that woven glass fabrics can be used at 4:1 and 4^:1 air-to-
cloth ratios.
Needled Glass
Needled glass, today, domestically available from two sources, is so far
successfully applied to outside cage type collectors. The use of these
fabrics in S02 dry scrubbing systems is currently being tested. The temper-
ature capability of these fabrics is approximately the same as for woven
glass fabrics. The air-to-cloth ratio should be approached rather conserva-
tively since experience has shown that high amounts of unburned carbon as
well as moisture can lead to blinding and so the air-to-cloth ratio of 3Jg to
4:1 seems appropriate for twin technology applications. Addressing air-to-
cloth ratios like in fly ash applications, the kind of boiler (stoker vs.
P.C.) has great influence on the design ratio.
Homopolymer Acrylic
Homopolymer acrylic is considered already in one large S02 dry scrubbing
system, but is a viable fabric in many metal applications for many years.
Woven homopolymer acrylic fabrics can be used in Reverse Air as well as
Shaker baghouses and can provide approximately 50% higher air-to-cloth ratios
as compared to woven glass on inside collecting systems (woolen system yarn
fabrics). Homopolymer acrylic Dralon T is more economical than glass; the
temperature limitation, however, is 284°F.
331
-------
Homopolymer acrylic needled felts can be used at air-to-cloth ratios be-
tween 4 and 6:1 if the fabric is special manufactured and, most importantly,
exhibits a special surface treatment. The difference between standard
Dralon T needled felts and special manufactured, special surface treated
Dralon T felts is significant, particularly looking at the application re-
quirement in S02 dry scrubbing systems. The system very well may go through
upset conditions, thus, could create a very moist environment in the baghouse.
On the other hand, different load factors may provide significant differences
in inlet loading conditions, at the same time, unburned carbon may accumulate
much finer dust on the surface. It is for this reason that special surface
treated felts should be chosen over somewhat less expensive regular felts,
which simply do not hold up as well under such delicate working conditions.
These special treatments (HCE II, Permaguard™, Hi-Rel™, Swiftclean™,
and Mirror Finish™, etc.) are available from different manufacturers.
Specific fabric specifications are available upon, request, but not part
of this paper.
Endnotes
1. "Spray Dryer FGD Capital and Operating Cost", M. Drabkin and E. Robinson,
Houston, October 1980.
2. "Modeling The Spray Absorption Process For S0£ Removal", Journal APCA,
December 1979.
3. "Needled Felts for Fabric Filters", ENVIRONMENTAL SCIENCE AND TECHNOLOGY,
December 1979, Lutz Bergmann, Filter Media Consulting, Inc.
4. "Fiber/Fabric Selection for SOo Dry Removal Baghouse Systems", POWER
ENGINEERING, October 1980, Lutz Bergmann, Filter Media Consulting, Inc.
332
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TWO- STAGE DRY FLUE GAS CLEANING
USING CALCIUM ALKALIS
By: D. C. Gehri, D. F. Dustin, and S. J. Stachura
Energy Systems Group
Rockwell International
8900 De Soto Avenue
Canoga Park, California 91304
ABSTRACT
Most of the dry flue gas cleaning (FGC) systems that have been sold to
date will utilize high-calcium lime as the alkali reactant for S02 removal.
In a two-stage, dry FGC system, optimum performance and utilization of lime
is achieved by the special technique of gas bypass around the first-stage
spray dryer and/or by recycle of particulates removed in the second-stage
collector.
This paper presents three examples of dry FGC system performance with
lime using the special techniques mentioned above. Three types of coal are
considered: (1) a low-sulfur western subbituminous, (2) a medium-sulfur lig
nite and (3) a high-sulfur eastern bituminous. The required FGC systems are
discussed in terms of the lime preparation equipment, the flue gas cleaning
equipment, and waste disposal techniques.
An economic evaluation of the three dry FGC systems is given along with
a comparison to equivalent electrostatic precipitator-wet scrubber combxna-
tlons. As shown by the comparisons, a two-stage, dry FGC system can be
economically employed for all the given fuel types.
INTRODUCTION
A two-stage, dry FGC system consists of a first-stage spray dryer which
functionTas the primary S09 absorber, followed by a second-stage fabric fil-
5££ coitions to produce a reactive slurry containing CaWH), parties
of minimum size and maximum specific surface area.
0ptimiZed performance i
let temperatures as close as practical
f 4-v.o fahr-fr filter typically requires inlet temper-
Reliable operation of ^^f^ay^yer outlet temperature. This can
atures somewhat ^ove the opt^im spray dry<* ? ^ t
-------
SYSTEM APPLICATIONS
In order to illustrate the performance of two-stage, dry FGC systems,
we have selected three examples of 500-MW utility boilers, each fired with a
different type of fuel. The essential boiler, fuel, and flue gas character-
istics for these applications are given in Table 1. These characteristics
were derived from TVA study premises with two exceptions and one addition.
The exceptions are:
1) It was assumed that all of the fuel sulfur converts to S02 in
the flue gas and that the FGC system must meet NSPS require-
ments based on that assumption. The TVA premises account for
some sulfur retention in the ash and reduce SO^ removal
requirements to reflect that retention. Our assumption clearly
imposes a more stringent performance requirement on the FGC
system and is consistent with the typical commercial specifica-
tions for performance guarantees.
2) The lignite example is for a fuel containing 1.5% sulfur (dry
basis), whereas the TVA base case uses 0.9% sulfur. This
higher sulfur level was chosen to reflect a typical maximum
sulfur for North Dakota lignite and to provide an example of
intermediate (~80%) S0~ removal requirements.
The one addition to the TVA premises is in the category of active ash
alkalinity. The numbers given in Table 1 reflect our experience in titrating
the given types of fly ashes to ascertain their potential value for recycle.
It should be noted that this parameter is indicative, but not necessarily
definitive as a measure of fly ash SO- absorption capacity in a dry FGC system.
The Western Subbituminous Application
For the western subbituminous application, the FGC system has been opti-
mized using warm gas bypass. This technique involves bypassing flue gas from
downstream of the air heater to provide reheat before the fabric filter. As
shown in Table 2, a 10% bypass of the 300°F flue gas provides 15°F of reheat.
With a 17 F approach to the adiabatic saturation temperature (!.„) at the
dryer outlet and the 15°F of reheat, the 165°F fabric filter inlet temperature
is 35 F above the water dew point (T,). Note that T, is typically 2 or 3°F
less than T . With the specified 50% recycle (defined as the fraction of
total collected product that is recycled), the "apparent utilization" in the
system is 100%. This measure of system performance is the amount of S0»
removal compared to that which could be removed by reaction of the total
available CaO in the fresh lime feed.
As shown in Table 2, the contribution of the fly ash alkalinity is sig-
nificant. The difference between the actual lime utilization of 86.7% and
the "apparent utilization" of 100% reflects that contribution. In general,
the relative contribution of an active fly ash is greatest for the low-sulfur,
low-removal applications in which recycle can be maximized without slurry
pumping problems. In fact, "apparent utilizations well above 100% can often
334
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be achieved." In this particular example, a potential of 120% exists. How-
ever, the only reliable technique to establish the optimum recycle fraction
is to test. Frequently, the incremental utilization improvements beyond a
certain recycle fraction do not justify the costs involved.
TABLE 1. EXAMPLE APPLICATIONS - 500-MW BOILERS
Fuel Type
Boiler Heat Rate
(Btu/kWh)
Fuel Sulfur
(%, dry basis)
Flue Gas Flow
(acfm, @ 300°F)
S09 in Flue Gas*
Tlb/h)
Fly Ash in Flue Gas
Western
Subbituminous
10,500
0.7
2,030,000
6,150
32,000
North Dakota
Lignite
11,000
1.5
2,160,000
15,800
48,000
Eastern
Bituminous
9,500
3.5
1,690,000
27,300
49,000
Active Alkalinity§
in Fly Ash
(lb/h of CaO)
1,000
2,000
500
Adiabatic Saturation
Temperature ( F)
NSPS SO- Removal (%)
133
70
137
79.1
122
89.5
Assumes all of sulfur in fuel converts to SO
Assumes 80% of ash in fuel becomes fly ash
§
Typical quantities for given fuel type based on mineral composition of
fly ash.
The Lignite Application
Warm gas bypass has also been selected for the lignite application. In
this case, the lowest recommended approach to T. of 15 F is used, and the
12% bypass fraction provides 18°F of reheat. The resulting 170 F fabric fil-
ter inlet temperature is again 35 F above T,. With the specified 60% recycle
fraction, the "apparent utilization" is 96%. This recycle fraction is about
the maximum possible without reaching the slurry pumping limits. Fly ash
alkalinity utilization of 59% has, therefore, been optimized.
Even though the SO removal is greater, the actual lime utilization of
85.7% is close to that for the previous example. This is partially due to
the closer approach to T and partially due to the greater recycle fraction.
335
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TABLE 2. SYSTEM PERFORMANCE SUMMARIES
Western North Dakota Eastern
Fuel Type Subbituminous Lignite Bituminous
FGC System Temperature
Profile (°F)
Dryer Inlet
. Dryer Outlet
. Fabric Filter Inlet
Bypass Gas
Bypass Fraction (%)
Recycle Fraction (%)
Pebble Lime* Used
(Ib/h)
SO Removal (%)
Apparent Utilization (%)
Actual Lime Utilization (%)
Actual Active Fly Ash
Utilization (%)
Product for Disposal
300
150
165
300
10
50
4,200
70.2
100
86.7
50
41,200
300
152
170
300
12
60
12,700
79.4
96
85.7
59
75,400
300
137
155
750
3
50
28,200
89.6
84
83.3
49
105,800
ft
Assumes 90% available CaO in the lime
Assumes average molecular weight of CaSO product is 128.
X
The only possibility for further utilization improvement is in the selection
and/or slaking of the pebble lime. All of the examples of this paper are
based on the implicit assumptions that the lime used is at least of "medium
reactivity" as defined by the AWWA slaking rate test (1), and that the slak-
ing temperature is controlled at 185° +15 F using ball mills or paste slakers.
The use of a "high reactivity" lime and/or closer control of slaking tempera-
ture (i.e., 190 +5 F) might provide some small improvement. As a practical
matter, such improvement would have a limited impact on system economics.
The Eastern Bituminous Application
For this application, the FGC system has been optimized using hot gas
bypass. This technique involves bypassing flue gas from upstream of the air
heater and results in a minor penalty to the boiler heat rate. However, it
is usually the choice when 90% or greater SO removal is required. A 15°F
approach to T is again employed in the spray dryer, and the 3% hot gas by-
pass results in a fabric filter inlet temperature 35 F above Td- The
recycle fraction of 50% results in a slurry of maximum practical density, so
the system has been optimized.
336
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The important thing to note about this example is the relatively small
contribution of the fly ash alkalinity. Recycle is primarily useful in pro-
viding a chance for the excess unreacted Ca(OH) to aid in SO removal. The
importance of high-quality pebble lime and proper slaking techniques are
obvious. The key concern, however, is not a few percentage points of utili-
zation. Rather, it is the economics of a two-stage, dry FGC system using
lime, versus a wet scrubber using limestone, for this application.
Waste Disposal
The product of a two-stage, dry FGC system is a dry mixture of fly ash,
CaSO^, CaSO^, and unreacted lime. For all of the examples in this paper, the
fraction of unreacted lime is less than 5 wt %. The fly ash/CaSO ratio
varies from 4:1 for the western subbituminous application to 1:1 ?or the
eastern bituminous application. In its dry state, the waste material can be
handled in the same manner and with the same equipment as pure fly ash. A
simple landfill operation is a suitable disposal technique (2), although
alternate disposal methods and/or end product uses are being explored. For
purposes of economic comparisons, landfill will be the basic disposal method
evaluated.
ECONOMICS
This paper will base its economic comparisons on a report prepared by
TVA for EPA comparing a "generic" lime spray dryer FGD system with a wet
limestone-electrostatic precipitator system for a low-sulfur, western subbitu-
minous application (3). That report provides the "base case" data from which
all other economics in the paper have been extrapolated. These extrapolation
techniques were devised by the authors of this paper who take full responsi-
bility for their validity. No endorsement by TVA or EPA is implied nor
intended.
Capital Investment
The capital investment comparison of Table 3 are based on mid-1982 costs.
For the dry FGC systems, the base system costs were taken directly from the
EPA-TVA report (3) . Extrapolation techniques used for the other cases are
listed below:
1) Material Handling and Feed Preparation
1.3 x base for lignite system
1.5 x base for eastern system
2) Gas Handling - Directly proportional to total gas flow
3) SO Absorption - Directly proportional to gas flow to spray
dryers
4) Particulate Removal - Directly proportional to total system
gas flow
337
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5) Recycle
2 x base for lignite and eastern systems
6) Disposal - Directly proportional to quantity of waste.
TABLE 3. CAPITAL INVESTMENT* COMPARISONS - 500-MW SYSTEMS
System
Material
Handling
and Feed
Prep.
Gas
Handling
S0_
Base
TVA-Dry
3.04
7.19
Western
Dry
3.0
7.2
Lignite
Dry
3.0
7.7
Eastern
Dry
4.5
6.2
Base
TVA-Wet
1.99
9.92
Western
Wet
2.0
9.9
Lignite
Wet
2.6
10.4
Eastern
Wet
3.0
8.4
Absorption 7.17 7.7 8.2 7.2 13.73 13.7 17.2 15.7
Particulate
Removal 11.13 11.1 11.6 10.0 12.40 12.4 12.9 10.5
Recycle 1.43 1.4 2.8 2.8 0000
Disposal 1.22 1.2 2.2 3.2 6.16 6.1 9-0 12.2
Other 34.99 35.4 40.6 38.1 48.91 48.9 57.9 55.2
(including
indirects)
Totals 66.17 67 77 72 93.11 93 110 105
*
All costs shown in millions of dollars.
For the wet FGD systems (including an ESP), the base system costs were
again taken from the referenced report. Extrapolation techniques for all
cost categories were the same as used for dry FGC systems, except that dis-
posal system costs are not proportional to waste quantity, but rather 1.5 x
base for lignite and 2 x base for eastern. This is significantly lower than
an extrapolation based on waste quantity.
The two-stage, dry FGC systems are much lower in capital cost than wet
scrubber-ESP combinations for all of the example applications. It is inter-
esting to note that the 500-MW lignite-fired boiler requires a more expensive
wet or dry system than its high-sulfur eastern counterpart.
Revenue Requirements
The revenue requirement comparisons of Table 4 are based on 1984 costs
and include a $75/ton cost for lime and an $8.50/ton cost for limestone. The
levelizing factor used in all costs, except capital charge, was 1.886. The
capital charge is a levelized cost of 14.7% of the total investment.
338
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TABLE 4. REVENUE REQUIREMENT* COMPARISONS - 500-MW SYSTEMS
System
Raw Materials
Labor and
Supervision
Electricity
Maintenance
Capital
Charge
Other
TVA-Dry
1.03
0.95
1.48
1.94
9.73
1.91
Dry
0.9
1.0
1.5
2.0
9.8
1.9
Dry
2.6
1.2
1.6
2.2
11.3
1.9
Dry
5.8
1.4
1.3
2.0
10.6
1.9
TVA-Wet
0.13
1.32
1.76
3.47
13.70
3.12
Wet
0.2
1.3
1.8
3.5
13.7
3.1
Wet
0.6
1.5
1.9
4.2
16.2
3.1
Wet
1.2
1.7
1.6
4.0
15.4
3.1
First Year
Annual 17.04 17.1 20.8 23.0 23.50 23.6 27.5 27.0
Levelized
Annual 23.52 23.6 29.2 34.0 32.19 32.4 37.5 37.3
*
All costs shown in millions of dollars.
Extrapolation techniques used are as follows:
1) Raw Materials - See Table 2 for lime quantites and assume 4,
12, and 25 tons/h or limestone for the three wet scrubber
cases.
2) Labor and Supervision - Add $200,000 to base for lignite and
$400,000 to base for eastern.
3) Electricity - Proportional to total gas flow.
4) Maintenance - Proportional to direct capital investment.
5) Other - No change from base.
As with the capital investments, the revenue requirements for the two-
stage, dry FGC systems are all lower than those for the corresponding wet
scrubber-ESP combinations. However, within the probable comparative accuracy
of the original base case estimates (+10%) and the additional uncertainties
caused by extrapolation, the first year annual and the levelized annual
revenue requirements for the two eastern, high-sulfur systems could be about
equal.
The obvious reason is the cost penglty of $4.6 x 10 for the use of lime,
which on a levelized basis is $8.7 x 10 per year.
339
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Conclusions
Two-stage, dry FGC systems have lower capital investments than comparable
wet scrubber-ESP systems for all cases evaluated. They also have lower annual
revenue requirements, except for the eastern, high-sulfur applications where
the two alternatives could be about equal. On the basis of these comparisons,
it is concluded that dry FGC systems can be economically employed on boilers
burning coal with up to 3.5% sulfur.
ENDNOTES
References
1. A.W-W.A, "Standard for Quicklime and Hydrated Lime," Std B202-65.
2. Buschmann, J. C., et al., "Disposal of Wastes from Dry SO. Removal Pro-
cesses," presented at Joint Power Generating Conference (September 1980).
3. EPA-TVA, "Preliminary Economic Analysis of a Lime Spray Dryer FGD System,"
EPA-600/7-80-050 (March 1980).
340
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CONTROL OF SULFUR DIOXIDE, CHLORINE, AND TRACE ELEMENT EMISSIONS
FROM COAL-FIRED BOILERS BY FABRIC FILTRATION
By: R. J. Demski, J. T. Yeh, and J. I. Joubert
Pittsburgh Energy Technology Center
U. S. Department of Energy
Pittsburgh, Pennsylvania 15236
ABSTRACT
Experimental programs carried out at the Pittsburgh Energy Technology
Center (PETC) have demonstrated that fabric filtration systems are effective
in controlling emissions of a number of pollutants resulting from the combus-
tion of bituminous coal in boilers. In studies conducted in a 500 Ib/hr
coal-fired furnace equipped with a baghouse, it was found that the baghouse
filter cake removed significant portions of the toxic trace elements mercury,
selenium, arsenic, beryllium, lead, and cadmium. When operating the baghouse
in combination with injection of dry sorbents such as nahcolite, trona, and
sodium bicarbonate, approximately 95% removal of sulfur dioxide and chlorine
was obtained.
In this paper, the previously reported study relating to control of trace
elements is summarized. Emphasis is placed on the recently completed investi-
gation of S02 control by dry sorbent injection. In the latter study, tests
were conducted with coals ranging in sulfur content from 1 to 3 percent
Operating variables considered included baghouse temperature, baghouse cleaning
cjcle time, sorbent particle size, and the ratio of sorbent to sulfur. With
the exception of baghouse cleaning cycle rate, each parameter had a significant
effect on S02 removal.
INTRODUCTION
Until the latter half of the 1970's, the preferred method for participate
control in the utility industry was electrostatic precipitation However
sulfur (>3 percent) coal is burned (2).
341
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Dry Flue Gas Desulfurization
In parallel with the increasing interest in baghouse filtration for
particulate control, a considerable amount of development work has been con-
ducted utilizing baghouses in conjunction with dry flue gas desulfurization
systems (4-7). A dry FGD system is one in which an alkaline sorbent is
injected into the boiler flue gas as a dry powder or aqueous slurry; the
sorbent reacts with SC>2 to form a dry product containing sulfates and sulfites.
The mixture of spent material and fly ash is separated from the gas stream by
the baghouse, where additional reaction between the sorbent and SOg occurs.
While ESP's can also be used in dry FGD systems, the use of baghouses has been
favored.
Data reported in the literature indicate that sodium compounds are con-
siderably more reactive than calcium compounds when injected into flue gas as
dry powders, although reactivities of the calcium compounds (particularly
lime) are increased when they are injected via a spray dryer system. The
study described in this paper was confined to dry-powder injection tests with
sodium bicarbonate, nahcolite (a sodium bicarbonate mineral), and trona (a
sodium carbonate/bicarbonate mineral).
The objective of the tests conducted at PETC was to evaluate the relative
effectiveness of the three NaHCOs sorbents mentioned in removing S02 from flue
gas streams. The tests were carried out in an experimental furnace designed
to burn 500 Ib/hr of pulverized coal. Parametric studies were conducted to
determine the effect of operating parameters on SC>2 removal efficiency and
sorbent utilization (gram atoms Na converted to Na2S04/gram atoms Na injected).
Parameters varied were sorbent/sulfur ratio, sorbent particle size, and bag-
house temperature and cleaning cycle time. Three types of bituminous coal
were burned, ranging in sulfur content from 1 to 3.1 percent; the heating
value of each coal was about 13,500 Btu/lb. All tests were conducted at an
excess air level of 20 percent.
Control of Chlorine Emissions
The chlorine content of most American coals ranges from 0.01 to 0.5
percent. The chlorine content of Western coals is generally low, but higher
levels are present in Central and Appalachian coals. Nearly all of the chlo-
rine contained in coal is emitted to the atmosphere as hydrogen chloride when
the coal is burned (8,9).
Chlorine emissions from large fossil-fuel burning installations are not
currently regulated. However, because of the large quantities of coal burned
in the United States, the quantity of hydrogen chloride emitted to the atmos-
phere is significant (=1.5 million tons/year). These emissions may contribute
to acid rain formation as well as create an air pollution hazard in the vicini-
ty of large power plants burning high-chlorine coal. Hence, while chlorine
emissions are not currently regulated, the desirability of minimizing the
emissions is obvious. In this paper, data are presented that indicate high
levels of removal of chlorine from flue gases can be achieved in conjunction
with dry sorbent flue gas desulfurization.
342
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Control of Trace Element Emissions
Although toxic elements are present in very small quantities in coal
fissions of these elements to the atmosphere are not insign f cant ™s ng
niinn T ™ ^1 ^^ element l6VelS ^ 101 C°alS a"3lyZed by thC
Illinois State Geological Survey (10), the potential total tonnages of trace
elements released to the atmosphere are
Element
Lead
Arsenic
Cadmium
Selenium
Beryllium
Mercury
Amount in Coal, PPM
34.8
14.02
2.52
2.08
1.3
0.2
Amount Emitted, Tons/Year
24,673
9,940
1,787
1,475
922
142
A study was conducted in the PETC 500 Ib/hr combustion test facility to
determine to what extent each of these trace elements is retained in the fly
ash captured in a baghouse filter (11). The effects of baghouse temperature,
fly ash loading on the filter bags, and carbon content in the fly ash were
investigated. Fly ash loading was controlled by baghouse pulse cycle, and the
carbon content was controlled by varying the amount of excess air fed to the
furnace.
EXPERIMENTAL
Combustion Test Facility
The 500 Ib/hr combustion test facility is shown schematically in Figure 1.
The furnace was designed to simulate the performance of an industrial steam
generator. The unit is 7 feet wide, 5 feet deep, and 12 feet high, and has a
volumetric heat liberation rate of about 16,000 Btu/hr-ft3 at a thermal input
of 6.5 million Btu/hr. The furnace walls are refractory-lined and water-cooled.
Coal is charged to the hopper, pulverized to a size consist of 70% minus
200 mesh, and then is conveyed by the primary air into a recycle coal loop
where intimate mixing of coal and air occurs. Four adjustable exit tubes are
connected to the recycle loop; these convey the primary air-coal mixture to
each of the four burners. Secondary air at 600°F is fed through adjustable
swirl vanes surrounding each burner. The flue gas exits the furnace at about
2000°F, passes through a convective heat transfer section, and is then used to
preheat the secondary air to the desired inlet temperature. By controlling
the air flow through the recuperative air preheater, the flue gas exit tempera-
ture can be maintained in the range of 300°-475°F.
343
-------
The baghouse is a Mikro-Pulsaire* model manufactured by the Pulverizing
Machinery Division of the Slick Corporation, and is 6 feet, 6 inches in
diameter and 9 feet, 10 inches high. It contains 57 Nomex bags, 8 feet long x
4.5 inches OD. The unit is normally operated at air/cloth ratios of 4-4.5
feet/minute.
Dry Flue Gas Desulfurization Tests
The dry-sorbent injection system is shown schematically in Figure 2. This
rather simple system provides reliable and accurate feeding of solid sorbent.
In most of the tests, dry sorbent was injected into the 12-inch diameter
flue-gas duct at the exit of the preheater, 26 feet upstream of the baghouse.
The nominal flue gas velocity in the duct is about 50 feet/second, which
results in a gas/solid contact time of about 0.5 second prior to entering the
baghouse. A few tests were conducted while injecting sorbent at the inlet of
the baghouse.
Gas analyses, as well as temperatures, pressures, and flows, are recorded
with a computerized data collection system. The flue gas is analyzed at four
locations (see Figure 1): at the furnace outlet; at the air preheater outlet
(prior to sorbent injection); at the baghouse inlet; and at the baghouse outlet.
Typical analyses of the nahcolite and trona used in the dry sorbent tests
are given in Table 1. The nahcolite was supplied by Superior Oil Company from
a mine near Rifle, Colorado. The trona was obtained from a Stauffer Chemical
Company mine in Rock Springs, Wyoming. The sodium bicarbonate used was USP
grade and was =» 100 percent NaHCOs- Much of the parametric testing was con-
ducted with this material because it is well characterized chemically and is
available in carefully graded size consists with the following industrial
designations: No. 3 (32 micron mean particle diameter); No. 1 (69 micron);
No. 2 (110 micron); No. 4 (115 micron); and No. 5 (180 micron).
TABLE 1. TYPICAL ANALYSES OF NAHCOLITE AND TRONA
Nahcolite , . ,. . Trona
(Weight Percent)
Not Detected 24.3
62.7 25.3
Na 22.1 24.9
K 0.1 0.2
Ca 0.65 1.3
Mg 0.5 0.6
86.05 76.6
-"Reference herein to any specific commercial product, process, or service is
to facilitate understanding and does not necessarily imply its endorsement
or favoring by the United States Department of Energy.
344
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Most of the tests were conducted with a 1 .6-percent-sulfur Pittsburgh
seam coal. Twelve tests were carried out with a 3.1-percent-sulfur West
Virginia coal, and two tests with a 1-0-percent-sulfur Kentucky coal.
Chlorine Removal Tests
The chlorine removal study was conducted during two dry-sorbent injection
tests with nahcolite and one test with sodium bicarbonate. Pittsburgh seam
coal, containing 0.11 percent chlorine, was burned in each test, and the
baghouse temperature was maintained at 400°F.
Gas samples were extracted from the stack (after the baghouse) through a
glass probe and tubing. Approximately 100 ft3 of gas was drawn through a
fritted-disc bubbler immersed in an ice bath; this was followed by a dry
ice/acetone bath to condense residual moisture. The Volhard technique was
used to determine the amount of chlorine in solution.
Trace Element Removal Tests
The trace element removal tests were conducted while burning Pittsburgh
seam coal containing the following levels of trace elements:
Element Amount, PPM
Arsenic 8.3
Lead 7.9
Beryllium 1.38
Selenium 1-25
Mercury 0.15
Cadmium 0.14
The baghouse was operated at temperatures ranging from 275° to 365°F.
The fly ash loading on the filter bags was varied over the range of 0.0003 to
0.05 lb/ft2 by adjusting the baghouse cleaning cycle rate, and the carbon
content of the fly ash was controlled at values ranging from 5 to 20 percent
by varying the excess air level. Details of the analytical procedures used in
this study were reported previously (11).
RESULTS AND DISCUSSION
Dry Flue Gas Desulfurization Tests
Results of the dry flue gas desulfurization tests are summarized here.
A more detailed report of this investigation will be published in the near
future (12).
345
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Sodium Bicarbonate Tests
Most of the parametric and exploratory testing was carried out with USP
sodium bicarbonate. In general, tests with the other sorbents were confined
to varying the Na2/S ratio at a fixed set of operating conditions.
The effect of baghouse temperature was examined over the range of 350°-
450°F. Number 1 (69 micron) sodium bicarbonate was used at a Na2/S ratio of
1.3. The S02 removal efficiency was found to level off at temperatures in
excess of 400°F. With the other sorbents, a temperature of 400° also appeared
near optimal in terms of 862 removal, and this temperature level does not
represent a severe operating condition for the baghouse. Thus, most of the
testing was carried out at approximately 400°F.
To determine the effect of sorbent particle size on S02 removal, a series
of tests was conducted with sodium bicarbonate at a Na2/S ratio of 2 and a
baghouse temperature of 400°F. The S02 removal efficiency varied approxi-
mately linearly with the mean particle diameter of the sorbent. The data
indicated that S02 removals of 90 percent are achievable at mean sorbent
diameters of 65 microns or less. For a stoichiometric ratio of 2, this
corresponds to a sorbent utilization of 45 percent. However, in a series of
tests with No. 3 sodium bicarbonate (32 micron diameter), it was found that 90
percent S02 removal could be attained at a Na2/S ratio of 1.3, corresponding
to about 70 percent sorbent utilization.
Tests with Nahcolite
A series of tests was conducted with Pittsburgh seam coal while injecting
nahcolite with a mean particle diameter of 37 microns. Figure 3 shows the
effect of Na2/S ratio on 862 removal at 400° and 420°F. As expected from the
earlier sodium bicarbonate tests, the slight difference in temperature appears
to have no major effect on the removal achieved.
It should be noted that varying the baghouse cycle rate over the range 6
to 30 minutes also had no apparent effect on S02 removal. This was found to
be true in all other tests performed, regardless of the sorbent employed. It
is believed that this is an indication that reactions are confined to only a
thin outer layer of sorbent deposited on the filter bags.
For the conditions stated in Figure 3, 90 percent S02 removal is achieved
at a Na2/S ratio of 1.1. This corresponds to 82 percent sorbent utilization.
Two tests were conducted with the Kentucky coal containing 1.0 percent
sulfur, and six tests were conducted with the West Virginia coal containing
3.1 percent sulfur. Baghouse temperature was maintained at 400°-420°F, and
the cycle times were either 15 or 30 minutes.
The results for the Kentucky coal, at Na2/S ratios of 1.1 and 1.2, were
consistent with the results obtained with the Pittsburgh seam coal indicated
in Figure 3. However, the S02 removal efficiencies achieved when burning the
3.1%-S West Virginia coal were somewhat lower than those obtained with the
Pittsburgh seam coal; a Na2/S ratio of 1.5 was required to achieve 90 percent
802 removal. The reason for this is not apparent. Intuitively, one would
346
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anticipate greater reactivity with the West Virginia coal, as initial S02
concentrations should be higher than when burning Pittsburgh seam coal. The
same phenomenon has been noted by others (13).
Tests with Trona
A series of ten tests was conducted with trona with a mean particle
diameter of 59 microns while burning 1.6%-S Pittsburgh seam coal. Another
series of seven tests was carried out with a second batch of trona (32 microns)
while burning 3.1%-S West Virginia coal. The results are plotted in Figure 4.
As in the case with nahcolite, higher S02 removal efficiencies were achieved
while burning Pittsburgh seam coal than with West Virginia coal, although here
the difference is more pronounced. In fact, if one were to adjust the West
Virginia coal curve to account for particle size effects, the difference would
be even greater (a decrease in S02 removal efficiency by approximately 3
percentage points at each Na2/S ratio).
Chlorine Removal Tests
Results of the chlorine removal tests, carried out in conjunction with
three dry FGD tests, are given in Table 2. All tests were conducted at a bag-
house temperature of 400°F. In one of the tests, the sorbent Na2/S ratio was
varied over the range of 0.84-1.12, while in the other two tests, the Na2/S
ratios were maintained at 0.99 and 1.04, respectively.
Chlorine removal efficiencies ranged from 95.6 to 98.8 percent. These
results indicate that even without attempting to optimize operating parameters,
dry, Na-containing sorbents are extremely effective in reducing emissions of
chlorine from coal-fired boilers.
TABLE 2. REMOVAL OF CHLORINE IN BAGHOUSE WHILE BURNING PITTSBURGH SEAM COAL
CONTAINING 0.11 PERCENT
Sorbent
Mean Particle Diameter, (Jm
Na2/S Ratio
Chlorine Fed with Coal,
Ib/hr
Chlorine in Flue Gas
(after baghouse), Ib/hr
fVil ^v--i no Romirwa 1 Pprrpnt
CHLORINE; BAGHOUSE
Nahcolite
37
0.84-1.12
0.55
0.024
95.6
TEMPERATURE :
Nahcolite
37
0.99
0.55
0.0066
98.8
400°F
Sodium
Bicarbonate
69
1.04
0.54
0.011
98.0
Removal of Trace Elements
The amounts of the trace elements mercury, selenium, arsenic, beryllium,
lead, and cadmium retained by baghouse fly ash and furnace bottom ash are
ziven in Table 3 (These tests were conducted without sorbent injection.)
347
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The contribution of the bottom ash is small, since only 10 percent or less of
the coal ash remains in the furnace during a given test.
The retention of mercury and selenium was sensitive to baghouse loading.
Loadings of 0.03 to 0.04 lb/ft2 were required to achieve high levels of reten-
tion; these loadings correspond to baghouse cycle times of 35 and 60 minutes.
Baghouse loading had no effect on retention of the other trace elements.
Baghouse temperature also affected the retention of mercury and selenium.
Lower temperatures (270°-300°F) produced higher levels of retention.
The retention of mercury also appeared to increase with increasing carbon
content of the fly ash. It is believed that this may be due to adsorption of
mercury by the carbon.
Comparing these results to results reported by others (14,15), it is
apparent that significantly higher retention of mercury, selenium, arsenic,
and beryllium is possible with a baghouse than with an electrostatic precipi-
tator. However, the levels of retention of lead and cadmium reported here are
lower than values reported in other studies involving power plants equipped
with ESP's. Material balances on these two elements indicated a significant
fraction (25-60 percent) of each was unaccounted for in each test. Hence, the
elements or their combustion products may have condensed out of the gas phase
in cooler sections of the boiler upstream of the baghouse.
TABLE 3. TRACE ELEMENT RETENTION BY FLY ASH AND BOTTOM ASH
Element Average Retention, Percent Range, Percent
Mercury 100118*
Selenium 100±13*
Arsenic 91 74-120
Beryllium 77 62-94
Lead 63 53-77
Cadmium 55 52-65
"'"Optimum Conditions
CONCLUSIONS
The FGD study discussed in this paper has confirmed that high levels of
S02 removal can be achieved via dry powder injection of NaHC03-bearing sor-
bents into the flue gas of coal-fired boilers. The mineral nahcolite is
particularly effective. Commercially produced sodium bicarbonate is slightly
less reactive than nahcolite. The mineral trona was found to be less effec-
tive, probably due to the lower bicarbonate content of this material. With
all three sorbents, S02 removal efficiencies in excess of 90 percent could be
348
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achieved, although in some cases at fairly low sorberit utilization effi-
ciencies. Future investigations at PETC will focus on improving sorbent
utilization while burning high-sulfur coals. A spray dryer system has been
added to the test facility, and a sorbent recycle system will be installed.
The planned work will include an evaluation of the effectiveness of lime and
limestone slurry injection.
In tests conducted concurrently with the dry FGD tests, it was found that
greater than 95 percent removal of chlorine was obtained with nahcolite and
sodium bicarbonate. While emissions of chlorine from power plants are not
currently regulated, this nevertheless represents a valuable additional bene-
fit of dry FGD technology.
The emissions of the trace elements mercury, selenium, and arsenic can be
effectively controlled via fabric filtration. Significant amounts of the
elements beryllium, lead, and cadmium are also retained in the baghouse fly
ash. The best results were obtained at baghouse temperatures lower than that
required («s400°F) for S02 emission control by dry sorbent injection. How-
ever, when injecting sorbents as slurries using a spray dryer system, high S02
removal efficiencies can be achieved at flue gas temperatures significantly
lower than 400°F. It appears probable, therefore, that boilers equipped with
spray dryer/baghouse systems can be operated with minimal emissions of not
only S02 and Cl2, but also the major toxic trace elements.
REFERENCES
1. Reigel, S. A. and R. P. Bundy. Why the Swing to Baghouses? Power.
January 1977. p. 68.
2. Anon. Chemical Engineering. December 31, 1979. p. 19.
3. Pruce, L. M. Interest in Baghouses on Upswing. Power. February 1980.
p. 86.
4. Midkiff, L. A. Spray-Dryer System Scrubs S02. Power, 123, No. 1, 29,
1979.
5. Lutz, S. J., R. C. Christman, B. C. McCoy, S. W. Mulligan, and K. M.
Slimak. Evaluation of Dry Sorbents and Fabric Filtration for FGD.
Prepared by TRW, Inc. for U. S. EPA, EPA-600/7-79-005, January 1979.
6. Blythe, G. M. , J. C. Dickerman, and M. E. Kelly. Survey of Dry S02
Control Systems. Prepared by Radian Corp. for U. S. EPA, EPA-600/7-
80-030, February 1980.
7. Burnett, T. A., K. D. Anderson, and R. L. Torstrick. Spray Dryer FGD:
Technical and Economic Assessment. Symposium on Flue Gas Desulfuriza-
tion, Houston, Texas, October 28-31, 1980.
8 Kear R W and H M. Menzies. Chlorine in Coal: Its Occurrence and
Behaviour during Combustion and Carbonisation. BCURA Monthly Bulletin,
Vol. XX, No. 2, p. 53, February 1956.
349
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9. lapalucci, T. L., R. J. Demski, and D. Bienstock. Chlorine in Coal
Combustion. U. S. Bureau of Mines, RI 7260, 1969.
10. Ruch, R. R., H. J. Fluskoter, and N. F. Shirap. Occurrence and Distribu-
tion of Potentially Volatile Trace Elements in Coal. Report, Illinois
State Geological Survey, August 1974.
11. Yeh, J. T., C. R. McCann, J. J. Demeter, and D. Bienstock. Removal of
Toxic Trace Elements from Coal Combustion Effluent Gas. U. S. Energy
Research and Development Administration, PERC/RI-76/5, September 1976.
12. Yeh, J. T., R. J. Demski, and J. I. Joubert. Control of 863 Emissions by
Dry Sorbent Injection. To be presented at the ACS Symposium on Advances
in Flue Gas Desulfurization, Atlanta, Georgia, March 29-April 3, 1981.
13. Parsons, E. L., L. F. Hemenway, 0. T. Kragh, T. G. Brna, and R. L. Ostop.
S02 Removal by Dry FGD. Presented at the Symposium on Flue Gas Desul-
furization, Houston, Texas, October 28-31, 1980.
14. Bolton, N. E., J. A. Carter, J. F. Emergy, C. Feldman, W. Fulkerson, L.
0. Hulett, and W. E. Lyon. Trace Element Mass Balance Around a Coal-
Fired Steam Plant. Fuel Chem. Preprints, 18(4):114-123, 1973. (ACS
166th National Meeting, Chicago, Illinois.)
15. Klein, D. H., et al. Pathways of Thirty-Seven Trace Elements Through
Coal-Fired Power Plant. Environmental Science and Technology. October
1975. p. 973.
350
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Coal hopper
©@©@
Flue gas sample positions
Boqhouse
Secondary
Air flow
control valve
Sorbent
hopper
Sight glass
Rotary feeder
*• To injection
point
Figure !. Simplified flowsheet of 500lb/hr pulvenred-coal-fired furnace
Figure 2. Dry sorbent injection system
0.7 0.8 0 9 1 0 1 1 1 2 1.3 1 4 1 5 1.6 1 /
Na2/S RATIO
FIGURE 3. SO2 REMOVAL WITH NAHCOLITE WHILE
BURNING PITTSBURGH SEAM COAL
80
SORBENT: TRONA
BAGHOUSE TEMPERATURE. 400'F
O • PITTSBURGH SEAM COAL (1.6% S)
(S9-MICRON SORBENTI
a - WEST VIRGINIA COAL (31% S)
(32-MICRON SORBENT)
1.5 2.0
Na2/S RATIO
2.5
3.0
FIGURE 4. SO2 REMOVAL WITH TRONA WHILE BURNING
PITTSBURGH SEAM AND WEST VIRGINIA COAL
351
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FLYASH COLLECTION USING A VENTURI SCRUBBER
MINNESOTA POWER'S COMMERCIAL OPERATING EXPERIENCE
By: Carlton A. Johnson
Peabody Process Systems, Inc.
835 Hope Street
Stamford, Connecticut 06907
ABSTRACT
Minnesota Power elected to use a venturi scrubber as a particulate re-
moval device for Clay Boswell Station, Unit No. 4 (500 MW). The selection was
based upon significant cost savings when compared to conventional precipitator
technology.
Prior to start up of the full scale system, extensive pilot plant test
work was conducted to determine performance characteristics related to both
particulate removal and opacity- Unit No. 4, now in commercial operation,
provides comparison of predicted performance based on pilot plant data with
full scale system performance. This paper discusses the comparison of actual
performance versus predicted performance.
INTRODUCTION
In December 1976 Minnesota Power awarded a contract to Peabody Process
Systems for an Air Quality Control System for the 500 MW Clay Boswell Unit
No. 4, Cohasset, Minnesota. The system was designed for integral particulate
and S02 removal using a venturi and spray tower absorber. Selection of a
venturi for particulate removal gave a 25 million dollar capital cost savings
compared to using conventional electrostatic precipitator.
The design of the Air Quality Control System was based upon burning a
sub-bituminous coal from the "Big Sky" mine at Coalstrip, Montana, where the
Rosebud and McKay seams vary in sulfur content from 0.4% to 2.8% with approxi-
mately 10% ash content.
Prior to start up of the full scale system, extensive pilot plant test
work was conducted to determine performance characteristics relating to SOo
removal, particulate removal and opacity. Clay Boswell Unit No. 4, 500 MW
started commercial operation in March of 1980. The performance of Unit No. 4
the full scale system, has since been compared with the earlier pilot plant
results.
System Description
The system design criteria for Clay Boswell Unit No. 4 are shown in
Table 1. Raw flue gas containing S02 and particulate enters the air quality
control system (AQCS) via a ductwork plenum. The ductwork plenum distributes
the raw flue gas to any three or four gas cleaning trains. Under full load
conditions three gas cleaning trains are operational. The fourth gas
cleaning train is a spare. Each train includes a venturi and absorber and is
designed to function independently.
352
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Table 1
DESIGN BASIS
Unit Generating Capacity 500 m
Coal Sulfur Content 2 87
S02 Emission Standards lYlbs. SOo/mm BTU
Particulate Emission Standards Q 1 Ibs /mm BTU
Flue Gas Volume 2,207,000 ACFM
Percent of Gas Scrubbed 95 to 100%
No. of Gas Cleaning Trains 3 Operating/one spare
No. of Venturi Recycle Pumps/Trains 1 Operating/one spare
No. of S02 Absorber Recycle Pumps/Trains 2 Operating/one spare
Flue Gas Inlet Grain Loading 11 gr./SCF
Particulate Removal Efficiency 99.7%
S02 Removal Efficiency 90%
Alkali Lime/flyash
Waste Solids Disposal Methods Ponding
Reheat Method None or 800°F
Bypass gas
The raw flue gas distributed by the ductwork plenum enters the gas
cleaning trains at the top of the venturi in each operating train where first
stage cleaning - removal of particulate is accomplished.
The venturi selected for particulate removal is based on the radial-flow
design concept. The venturi design must contend with the problems of abrasion
and solids build-up due to hot gas contacting the slurry used for particu-late
removal. To avoid solids build-up, the venturi is designed using a "dentist
bowl" concept. The upper section of the venturi consists of a conical section
in which slurry is introduced tangentially. The quality of slurry used is
several times greater than the potential evaporative capacity of the hot flue
gas.
The excess quantity of slurry insures that the conical section is com-
pletely wetted thus eliminating solids build-up. Raw flue gas enters the
venturi through an insulated thimble section which introduces the gas below
the slurry injection point, keeping the gas/slurry contact point below the wet-
dry line and eliminating solid build-up.
Inherent in the design of a venturi are three distinct areas where ab-
rasion can occur. The first is where the gas makes a 90 degree turn to flow
through the cylindrical orifice. Abrasion in this area is avoided by having
the gas impinge on a pan filled with slurry, which absorbs the impact of the
gas. This pan is maintained full by the flow of slurry from the "dentist
bowl" and supplemental slurry added via a bull nozzle. The slurry required
for the "dentist bowl" and the bull nozzle is pumped from the recycle tank.
The slurry overflow from the pan and the raw gas are mixed intimately in
the cylindrical orifice around the pan. It is at this point that particulate
removal is achieved.
353
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The second area where abrasion can occur is at the orifice which cannot
be eliminated. However, the wear surface in the orifice area is fabricated
from disposable sections easily replaced which simplifies maintenance.
The third area is the wall section in the main shell of the venturi. The
gas and slurry mixture leaving the orifice has a high velocity which if allowed
to impact on a surface, also could cause severe abrasion problems. This situ-
ation is avoided by maintaining a sufficient distance between the orifice area
and the wall of the venturi. This allows the gas to expand and slow down
sufficiently to be nonabrasive. As an added precaution, the wall is rubber
lined to withstand any residual abrasive impact.
The flue gas and slurry mixture then flow into and through the base of
the absorber into the recycle tank. The slurry mixture is then pumped to the
spray headers in the absorber for removal of SC>2 from the gas.
The absorber is a high-velocity spray tower design. Each absorber con-
sists of six spray banks through which a slurry containing flyash is sprayed
countercurrent to the gas flow. The gas, as a result of being contacted with
the slurry is cleaned of SC>2. Under design conditions only four spray banks,
two per recycle pump, are used. The other two spray banks and one recycle
pump are spares.
After leaving the absorber zone, entrained slurry in the flue gas is
removed by means of a two stage mist elimination section. The first stage is
a weeping sieve tray deluged with a mixture of reclaimed water and river water.
Final de-entrainment, particularly of gas entrained water, is accomplished
in a second stage which is a chevron type mist eliminator.
The wet gas leaving the absorber flows to a ductwork plenum from which it
is distributed to four I.D. fans and then discharged to the stack. The gas
system downstream of the absorber is designed to run with or without reheat.
When reheat is desired, a slip stream of 800 F flue gas taken ahead of the air
preheater is injected into the outlet duct of each absorber via spargers. The
reheat gas which amounts to approximately 5% of the total flue gas stream is
cleaned of particulate by means of a precipitator prior to injection into the
scrubbed gas.
Each module has a recycle tank which collects the slurry draining from
both the venturi and the absorber. One pump circulates slurry to the venturi
and three pumps are installed for circulation of slurry to the absorber. Two
absorber spray headers are dedicated to each recycle pump only two of which
operate at design conditions.
Waste slurry overflows from the recycle tank to a waste slurry sump which
also collects all drainage and water used for system flushing. The waste slurry
is then transported from the sump to a pond in which the solids are allowed to
settle. The water reclaimed from the slurry is recycled back to the AQCS system
for reuse. The system operates on a totally closed loop water balance basis.
354
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Fresh water is added to the system to make up for losses resulting from evapo-
ration and water bound with the waste solids.
In the event a purchased alkali (lime) should be required, a lime slaking
system has been incorporated in the system. The lime is prepared as a 20 wt.
percent slurry and fed to a storage tank. A lime slurry recirculation system
transports the lime slurry to the modules.
As a site convenience, the owner elected to dispose of flyash collected in
baghouses from Units No. 1 and 2 with the waste solids from Unit No. 4. To
accomplish this the dry flyash from Units No. 1 and 2 is prepared as a slurry
and fed to the Unit No. 4 AQCS system waste solids disposal.
Pilot Plant Testing and Results
The contract between Minnesota Power and Peabody guarantees both the
particulate and S02 removal efficiencies. To further Minnesota Power
personnels' indoctrination of the operating characteristics of the system and
confirm the guarantees, a pilot plant test program was originated. The test
program extended over a eighteen month period, objectives of the program
included the following:
1. Confirm the pressure drop in the venturi required to meet
particulate and opacity emission standards.
2. Confirm the system SC>2 removal efficiency of 90%, when
burning coals containing up to 2.8% sulfur.
3. Demonstrate that the system can operate on a closed-loop
water balance.
4. Determine the alkali utilization of the flyash.
5. Evaluate alternative alkalis.
6. Define waste solids characteristics such as settling rates,
percent moisture, and settled solids.
The pilot plant (Fig. 1) to achieve the above objectives was installed
on Unit #3 at the Clay Boswell station. Unit #3 has the same boiler design
and is burning the same coals which are to be burned in Unit #4. Thus, the
results obtained from the pilot plant could be considered representative of
the results to be expected for Unit #4.
The pilot plant system was identical in concept to a proposed full-scale
unit with some modifications based on practical economical considerations.
The lime slaker system was not installed. An in-line electric reheater was
used in lieu of the hot gas bypass system. A thickener and vacuum filter
were used in lieu of a waste solids pond system.
355
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The pilot plant data demonstrated that the venturi performance is signi-
ficantly affected by particle size distribution, as evidenced by particulate
removal curve. (Fig. 2)
For particle sizes of 2.5 microns or larger, the percent removed is
essentially 100%, regardless of venturi pressure drop. The percent removed
drops off slightly as particle size decreases from 2.5 microns to 0.8 microns.,
It is in the less than 0.8 micron size range that the venturi pressure drop
has the greatest significance.
Data was also plotted (Fig. 3 & Fig. 3A) showing the total outlet grain
loading as a function of venturi pressure drop. The curves for both coals are
shown. The venturi system had been based upon achieving a 0.03 gr./SCFD (0.06
Ibs./MM BTU) using a 12 inch w.c. pressure drop. The curves show that with
Rosebud coal the 0.03 gr./SCFD could be achieved with a 6 inch w.c. drop
pressure, whereas the McKay coal required an 8 inch w.c. pressure drop. Both
curves appear to indicate that regardless of pressure drop, outlet grain load-
ings significantly less than 0.01 gr./SCFD (0.02 Ibs./MM BTU) are not achieva-
ble. Particulate removal performance was guaranteed by Peabody at a venturi
pressure drop of 12 inch w.c.. The test data confirmed that this value could
easily meet the required emission standards.
Opacity is a different problem. Test results showed that even with
particulate loading significantly less than that required to meet emission
standards a 20% opacity was not achievable. This is partially attributed to
the fact that stack geometry is an important factor in estimating the measured
opacity. The larger the stack diameter, the more difficult it becomes to
achieve a low opacity value.
A significant cost benefit was also established for integral particulate
and S02 removal. During the S02 removal study phase of the test program it
was established that S02 emission standards could be met without the need for
purchased alkali. The alkali in the ash is sufficient to satisfy S02 removal
requirements. This cost benefit had not been included in the economic evalu-
ation of the system.
Commercial Operating Results
Clay Boswell Unit No. 4 was started up in March 1980. During the start-up
period the site was experiencing a strike of its operating and maintenance
personnel. Consequently this was started up with a skeleton crew consisting
of Minnesota Power supervisory personnel. Despite the lack of operation and
maintenance resources the Air Quality Control System started up with only
minor problems. The system entered formal commercial operation in May 1980.
At Design Sulfur Coal Conditions (2.8%) to comply with the 1.2 Ibs.,
S02 MM BTU emission standard, a 90% S02 removal is necessary. However, at
less than design sulfur coals environmental permit requirements also dictate
a minimum 862 removal of 60%. Based on the actual coals burned, (Fig. 4) the
60% S02 minimum removal is the controlling emission criteria. Operation of
the AQCS is therefore geared to meeting the 60% S02 removal standard.
356
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The AQCS has the flexibility to save pumping power by allowing the recycle
pumps for the absorber to be turned on and off as required. Generally, when
less than design sulfur coals are burned, the quantity of slurry recirculated
in the AQCS can be reduced. Two absorber recycle pumps are required for the
maximum design sulfur coals. Under actual sulfur coal conditions, only one
of the pumps is used, resulting in an operating power saving.
Pilot plant operation indicated that the recycle slurry should be con-
stituted with a low pH to maximize the extraction of the alkali from the fly-
ash. To this end a pH value of 4 or less has been maintained. SINCE COM -
MERCIAL OPERATION OF THE SYSTEM THE S02 EMISSION STANDARD HAS BEEN MET WITHOUT
THE NEED TO USE ANY PURCHASED ALKALI.
Performance tests have shown that the S02 and particulate emission stand-
ards have been met. However, as predicted, stack opacity is significantly
greater than 20%. These results confirm the pilot plant test data.
The full load operating requirements for Unit No. 4 are set forth in
Table 2 below:
Table 2
DESIGN NORMAL
% S COAL % S COAL
No. of Gas Cleaning Trains 3 3
No. of Operating Venturi
Recycle pumps/trains 1 1
No. of Operating Absorber
Recycle pumps /trains 2 1
System Pressure Drop
inches w.c. 20 18
Power Requirement Excluding
Draft Loss KW 4416 2771
% Rated Capacity 0.88 0-55
Lime Requirement-lbs. /hr . 32241 0
At present two operators per shift (4 shift basis) are required. Minnesota
Power had originally budgeted a greater number of operators, however, during
start-up it became evident that two men/shift were adequate.
Since start-up, the system has evidenced almost a 100% availability.
Except for approximately nine (9) hours during which a problem with a
programmable controller was encountered, the AQCS has not restricted the
generating capability of Clay Boswell Unit #4. (Fig. 5)
Conclusion
ri«v Roswell Unit #4 has demonstrated that a venturi integrated with an
Clay Boswell unit ** reliable Air Quality Control System. How-
~J " f •
-------
As emission standards are tightened the pressure drop required will
increase. As determined in the pilot plant tests, the key to venturi
performance is the amount of sub-micron particles in the raw flue gas. For
some coals the quantity of sub-micron particles is relatively low so that a
reasonable pressure drop is needed to meet the current standard of 0.03 Ibs./
MM BTU. For other coals, particularly western coals, the flyash contains a
large portion of sub-micron particles. This requires a very large pressure
drop (20 w.c. or greater) which imposes a very severe energy penalty on the
system. However, a new technology has emerged which promises to offset that
penalty.
The new technology has been developed by Peabody and is labeled "Heatron".
It is a wet tubular electrostatic precipitator which replaces the mist elimi-
nator in the upper section of the 862 absorber. This device can achieve
extremely high particulate removal efficiency with a minimal energy con~
sumption.
Thus, today, an Air Quality Control System equipped with a "Heatron"
and venturi or a modified version thereof can meet emission control require-
ments and achieve significant cost savings when compared to conventional
systems using a dry electrostatic precipitator or baghouse.
358
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Figure 1
Figure 2
Figure 3
Minnesota Power and Light
Pilot Plant Installation
Clay Boswell Station-Unit no. 3
Boiler
Krebs
Participate
Scrubber
Venturi
I.D.
Spray Fan
Tower
Absorber
% Particle Removal vs. Panicle Diameter
as a Function ol Venturi Pressure Drop
Particle Diameter. Microns
n Loading, Qrain/SCFD
s s s s s sss;
Outlet Grai
n 01 -g 03 «>2
Ifl
III
•j
^
'rft
s
iSI
X
fft
v
Dr
x
W
Si
?s
n
iiif
^^
t (
~-<
>ra
^
n
Cc
<
• —
nil
al:
—
rlii
R<
g
se
)Ul
g 8 10 12 14 16 18 20 22
Venturi Pressure Drop, Inches W.C.
359
-------
Figure 3A
Figure 4
£ 06
O .07
8 .oe
2 .05
a
? "
2 4 6 B 10 12 14 16 1B 20 22 24 26 28
Venturi Pressure Drop, Inches W.C.
Figure 4
Minnesota Power and Light
Clay Boswell Station
Daily Coal Sulfur Content Variations
2.0
CO
8
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31
July 1980
Figure 5
Figure 5
Minnesota Power & Light Clay Boswell Station
Unit No 4 Availability
100 "~
90
80
| 70
Is 60
50
40
30
20
10
0
J FMAMJJASOND
1980
360
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AUTHOR INDEX
AUTHOR NAME
Albrecht, P.R.
Anderson, M.H.
Arce-Medina, E.
Ariman, T.
Armstrong, J.A.
Bakke, E.
Half our, W.D.
Bamberger, J.A.
Bergmann, L.
Berlant, M.J.
Bernstein, S.
Beutner, H.P.
Bickelhaupt, R.E.
Boericke, R.R.
Bohn, R.
Borenstein, M.
Brookman, E.T.
Bump, R.L.
Bush, P.V.
Calvert, S.
Carries, D.
Carr, R.C.
Chamberlain, H.L.
PAGE
IV-116
11-334
11-76
III-290
IV-188, IV-252
1-236
111-119
III-398
1-323
11-218
H-405
111-71, III-228
1-165
III-353
IV-344
111-90
IV-125
11-425
1-157
III-l, 111-10, IV-156
IV-135
1-118
IV-406
361
-------
AUTHOR INDEX (cont.)
AUTHOR NAME PAGE
Chambers, R. I-45
Chiang, T. m-250, III-261
Chou, K.H. IV'73
Cowen, S.J. IV-264
Crippen, L.K. I-148
Crowson, F. III-438
Crynack, R.R. H-242
Czuchra, P.A. IV-55
Dalmon, J. H-390
Demski, RJ. 1-341
Dennis, R. 1-1, HI-140
Dietz, P.W. III-449, III-459
Donovan, R.P. 1-11
Drehmel, D.C. III-341, IV-210
DuBard, J.L. IV-383
Durham, M. 11-54, IV-285
Ensor, D.S. 1-176, IV-242
Eskinazi, D. III-238
Faulkner, M.G. 11-199, IV-395
Feldman, P.L. IV-3
Ferrigan III, J.J. 1-197
Finney, W.C. 11-358
Fjeld, R.A. 11-179
Fortune, O.F. 1-82
362
-------
AUTHOR INDEX (cont.)
AUTHOR NAME
PAGE
Frazier, W.F. III-171
Gardner, R.P. III-128
Gaunt, R.H. 1-216
Gehri, B.C. I_333
Gentry, J.W. III-406
Giles, W.B. III-468
Hardison, L.C. 111-33
Harmon, D.L. IV-317
Hawks, R.L. III-221
Helfritch, D. 1-75
Henry, F. III-301
Henry, R.F. IV-63
Hesketh, H.E. IV-222
Hoenig, S.A. HI-382
Hovis, L.S. 1-23
Hyde, R.C. J'129
lionya, K. m-181, HI-321
Jaworowski, R.J. 1-185
Jensen, R.M. l~138
Joergensen, H.J. n-370
Johnson, C.A. I"352
Kalinowski, T.W. In-311
Kanaoka, C. m-280
Kirstein, B.E. II][-373
JDJ
-------
AUTHOR INDEX (cont.)
AUTHOR NAME PAGE
Kolnsberg, HJ. IV-179
Krishnamurthy, N. IV-232
Ladd, K. 1-55, 1-65
Lagarias, J.S. l~212
Landham, Jr., E.G. J'237
Langan, W.T. III-211
Lawless, P.A. H-25, 11-35, 11-44
Leith, D. IH-270
Leonard, G.L. H-120
Maartmann, S. 11-130
Mahoney, D.F. 1-206
Mappes, T.E. III-150
Martin, D. IV-145
Masuda, S. 11-189, 11-380
Mathai, C.V. IV-200
Mazumder, M.K. 11-160, 11-169
McCrillis, R.C. IV-306
McElroy, M.W. 1-94
McLean, KJ. 1-265, 11-304
Menegozzi, L. 11-404
Menoher, C. Ill-Ill
Mitchner, M. 11-97
Moore, W.E. IV-105
Mormile, D. IV-363
364
-------
AUTHOR INDEX (cont.)
AUTHOR NAME pAQE
Moslehi, G.B. IM09
Mosley, R.B. ^ n_13
Musgrove, J.G. m_193/ In.201
Noonan, P.M. IV-326
Oglesby, H.S. m_80
Ostop, R.L. j_107
Parker, R. m_51/ IV_2
Parquet, D. III-363
Parsons, Jr., E.L. 1-303
Patton, J.D. III-160
Pearson, G.L. 1-120
Pedersen, G.C. 111-60
Petersen, H.H. 1-291
Piulle, W. 1-253
Potokar, R.W. III-417
Prem, A. HI-21
Presser, A.M. IV-26
Pyle, B.E. I][-66
Raemhild, G.A. H-349
Reardon, F.X. III-102
Rimberg, D.B. 11-262
Rinaldi.. G.M. IV"95
Rinard, G. "'283, H-295
Rubow, L.N. IV"83
365
-------
AUTHOR INDEX (cont.)
AUTHOR NAME PAGE
Rugg, D. 11-273
Samuel, E.A. 11-149
Schliesser, S.P. 11-252
Semrau, K.T. 111-43
Shilling, N.Z. 11-230
Smith, W.B. 1-96
Snaddon, R.W.L. IV-74
Sparks, L.E. 11-314, 11-326
Spawn, P.O. IV-335
Starke, J. III-428
Stevens, N.J. 1-313
Sullivan, K.M. 11-141
Tatsch, C.E. IV-353
Teller, AJ. III-393
Thompson, C.R. 11-415
Urone, P. IV-275
VanOsdell, D.W. 1-35
Viner, A.S. IV-168
VVakabayashi, A. III-332
Wang, H.H. IV-36
Wang, J.C.F. IV-373
Wegrzyn, J. IV-46
Weyers, L.L. 1-226
Wilks, W.H. IV-15
366
-------
AUTHOR INDEX (cont.)
AUTHOR NAME
PAGE
Williamson, A.D. IV-297
Yamamoto, T. 11-87
Yung, S. IV-l, IV-155
Zarfoss, J.R. 11-208
367
6USGPO: 1982 — 559-092/0429
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