U. S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
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EMISSIONS FROM COAL-FIRED POWER PLANTS:
A COMPREHENSIVE SUMMARY
'Stanley T. 'Cuffe
and
Richard W. Gerstle
National Center for Air Pollution Control
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Bureau of Disease Prevention and Environmental Control
Cincinnati, Ohio
1967
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The ENVIRONMENTAL HEALTH SERIES of reports was estab-
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Public Health Service Publication No. 999-AP-35
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CONTENTS
Page
Abstract v
Introduction 1
Sampling and Analytical Techniques 1
Description of Units 2
Vertically Fired Unit 2
Corner-Fired Unit 6
Front-Wall Horizontally Fired Unit 6
Horizontally Opposed Fired Unit 6
Cyclone-Fired Unit 10
Spreader-Stoker Fired Unit 11
Discussion of Results 12
Oxides of Sulfur 12
Oxides of Nitrogen 14
Solid Particulate 16
Polynuclear Hydrocarbons 19
Emissions of Trace Contaminants 21
Summary 23
References 25
iii
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ABSTRACT
The Public Health Service and the Bureau of Mines conducted a
study of air pollutant emissions from the six main types of coal-burning
power plants. The components tested include sulfur oxides, nitrogen
oxides, polynuclear hydrocarbons, total gaseous hydrocarbons, solid
particulates, formaldehyde, organic acids, arsenic, trace metals, and
carbon monoxide. This report relates the effects of variables such as
method of operation, type of boiler furnace and auxiliaries, reinjection
of fly ash, and type of coal burned to the concentrations of gaseous and
particulate pollutants in the products of combustion.
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EMISSIONS FROM COAL-FIRED POWER PLANTS:
A COMPREHENSIVE SUMMARY
INTRODUCTION
Total requirements for power plant energy from the combustion
of coal are expected to increase from 211,000,000 tons per year in
1963 to over 600,000,000 tons per year in 2000. This 300 percent
expansion is expected in spite of the increased use of nuclear and
petroleum fuels.
Most of this coal will be burned in large power plants with steam
capacities in excess of 1 million pounds per hour. In 1962, over 60
percent of the steam generators sold in the United States had capacities
of 2 million pounds of steam per hour; this trend toward large-capac-
ity generators is expected to continue as the need for more economical
power production increases.
To assess the contribution of coal-fired power plants to the over-
all air pollution burden, the Public Health Service and the Bureau
of Mines conducted a joint study to determine atmospheric emissions
from the main types of coal-fired power plants. The objective of the
study was the assessment of a number of flue-gas-stream components
of interest in air pollution. The components determined were sulfur
oxides, nitrogen oxides, polynuclear.hydrocarbons, total gaseous
hydrocarbons, solid particulates, formaldehyde, trace metals, carbon
monoxide, carbon dioxide, and oxygen. When possible, the effects of
variables such as method of operation, type of boiler furnace, type of
coal burned, and reinjection of fly ash were related to the type and
amount of pollutant emitted.
The six typical designs of coal-fired steam generators tested
were vertically fired; corner-fired; front-wall horizontally fired;
horizontally opposed fired, wet-bottom; cyclone-fired wet-bottom;
and traveling-grate spreader-stoker fired. Results of tests on the
first four designs have been published. 2,3 This report presents
comparative data for all six types of boilers.
SAMPLING AND ANALYTICAL TECHNIQUES
Summarized descriptions of flue-gas sampling and analytical
techniques used for the power plant study have been published. 2,3,4
Standard methods were used for sulfur oxides, 5,6 nitrogen oxides, 1
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polynuclear hydrocarbons, 8,9,10 total
particulate, formaldehyde, H arsenic,
oxide, carbon dioxide, and oxygen.
;aseous hydrocarbons, solid
2 trace metals, carbon mort-
Solid particulate was collected by simultaneously traversing the
inlet and outlet ducts of the fly-ash collector with the sample train
illustrated in Figure 1. Isokinetic sampling rates were maintained by
continually measuring the velocity with a pitot tube attached to the
probe. Particulate obtained from the cyclone and filters, in addition
to that obtained by brushing the train and filtering the bubbler and
wash water, was used for determining total particulate weight after
drying at 105°C.
Samples of coal entering the furnaces were collected hourly
during the test periods. Proximate and sulfur analyses were made
on each sample, and ultimate analyses were made on composite
samples (Table 1).
DESCRIPTION OF UNITS
Boiler operating conditions and flue-gas data for all six types
of boilers at both full and partial load are given in Table 2.
Vertically Fired Unit The vertically fired dry-bottom unit (Figure 2)
is rated at 1,100,000 pounds of steam per hour at 1,900 psig and 1,000°F.
Coal, pulverized in two ball mills, is conveyed to 16 sets of burner
ports by four exhausters. Combustion air is supplied by two 210,000-cfm
forced-draft fans. The air enters the furnace through the burner ports
QTHESE COMPONENTS ARE INSERTED
BEHIND BUBBLERS FOR COLLECTION
OF POLYNUCLEAR HYDROCARBONS
HOT
FILTER
-THERMOMETER
U-TUBES IN DRY-ICE ALCOHOL BATH
THERMOMETERS
BUBBLERS IN ICE-WATER BATH
AIR TIGHT ORIFICE
PUMP RATE
METER
Figure 1. Particulate sampling train.
EMISSIONS FROM COAL-FIRED
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i
M
Table 1. ANALYSES OF COALS BURNED
Component
Proximate Analysis
(as -fired), %
Moisture
Volatile matter
Fixed carbon
Ash
Ultimate analysis
(as -fired), %
Hydrogen
Carbon
Nitrogen
Oxygen
Sulfur
Ash
Heating value , Btu/lb
Vertical
Pa.
1.1
30.8
48.3
19.8
4.6
65,8
1.4
6.1
2.3
19.8
11,820
Corner
Ohio
2.8
37.2
44.0
16.0
5.0
64.2
1.3
11.8
1.8
16.0
11,480
W. Va.
District 8
1.8
32.9
53.6
11.7
4,8
72.1
1.4
9.0
1.0
11.7
12,645
Front-wall
Ky.
Strip
2.3
38.3
49.6
9.8
5.1
70.9
1.5
10.4
2.3
9.8
12,640
W. Va.
Deep Mine
1.2
36.2
54.5
8.1
5.1
75.9
1.5
8.2
1.2
8.1
13,540
Spreader
stoker
111.
4.1
42.9
44.8
8.2
5.5
70.0
1.4
12,4
2.5
8.2
12,650
Cyclone
Pa.
1.1
37.0
54.5
7.4
5.2
77.4
1.4
6,1
2.4
7.5
13,910
Horizontally
opposed
m.
2.0
36.5
53.6
7.9
5. 1
73.7
1,6
9;4
2,3
7.9
13,195
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Table 2. SUMMARY OF BOILER AND FLUE GAS DATA
Type of boiler firing
Full-load tests0
Vertical
Corner
Front-wall
Spreader-stoker
Cyclone
Horizontally opposed
Partial-load testsd
Vertical
Corner
Front-wall
Spreader-stoker
Cyclone
Horizontally opposed
Boiler conditions
Steam
rate,
IbAr
1,100,000
960,000
920,000
150,000
1,332,000
149,000
815,000
696,500
628,000
119,500
1,022,000
108,000
Coal
rate ,
ton/hr
65.6
56.1
52.2
9.2
64.4
9.6
41.1
40.8
38.8
6.9
41.3
6.6
Flue -gas
volume ,
mscfm
Be
397.4
362.9
329.0
53.9
557.6
62.2
297.2
283.8
254.0
44.0
443.5
43.8
Ae
409.9
351.0
328.0
59.6
500.8
62.2
303.8
264.2
256.0
46.7
404.1
44.6
Average
flue-gas
temp, °F
Be
258
283
257
426
279
315
245
253
244
396
265
314
A*
268
275
255
328
256
310
235
252
244
327
240
308
Moisture ,
%
Be
6.4
7.0
5.9
7.8
6.3
6.8
6.4
8.0
5.9
7.4
6.6
6.6
Af
5.7
8.1
6.3
7.5
5.9
6.5
6.6
7.4
6.0
7.0
6.4
6.6
C02,
%
Be
12.6
14. 4a
13.6
12.1
12.8
13.2
12.5
14. 9a
13.3
12.1
12.0
12.8
Af
12.2
14. 3a
13.0
11.8
12.7
13.0
12.2
14. 7a
13.0
12.0
12.2
12.4
°J>
%
Be
6.2
4.7
5.3
6. 6
6.4
5.9
6.6
4.2
5.6
6.9
6.8
6.0
Af
6.4
4.8
5.6
7.0
6.3
6.1
7.0
4.4
5.8
7.2
6.8
6.5
Excess air,
%
Be
41.0
28.6
32.9
44.5
42.6
38.6
45.0
24.2
35.0
47.8
46.0
38.4
A*
45.3
29.1
35.5
44.8
42.0
40.7
48.8
25.6
37.7
51.1
46.2
43.7
aCalculated values, based on oxygen values and fuel analysis.
^Measured at fly-ash collectors.
cAverage values for three or four tests at each unit under normal steam load conditions.
^Average values for two tests at each unit.
eB: Before fly-ash collector.
^A: After fly-ash collector.
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PULVERIZER
REHEAT
DAMPER CONTROL
mCONOMIZER
LET HEADER
REHEAT
INLET HEADER
REHEAT
OUTLET HEADER
SUPERHEAT
OUTLET HEADER
Figure 2. Boiler outline for vertically-fired unit showing sampling positions.
POWER PLANTS: A SUMMARY
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with the coal and through additional ports interspersed between the
burner ports. The steam generator consists of a water-wall furnace
section, a combined radiant and convection superheater, a reheater,
an economizer, and an air heater. Flue gas, after passing through the
regenerative air heaters, is divided into two parallel ducts and drawn
through the fly-ash collector by two 300,000-cfm induced-draft fans.
The clean gas is then discharged through a brick-lined steel stack.
The fly-ash control system includes a cyclone-type separator followed
by an electrostatic precipitator.
A high-volatile bituminous coal from Pennsylvania was burned
in the vertically fired unit. Three tests were run at full load and two
at three-quarter load.
Corner-fired Unit The corner-fired dry-bottom boiler is of the mono-
tube type, i.e., without a steam drum (Figure 3). It is completely water-
cooled and rated at 960,000 pounds per hour steam at 2,565 psig and
1,050°F. The 16 tangential corner-mounted burners receive pulverized
coal from 4 ball mills. Two 185,000-cfm forced-draft fans supply com-
bustion air. The flue gas leaving the boiler is carried through two
regenerative-type air preheaters, fly-ash collectors, a low-level heat
economizer, and two 275,000-cfm induced-draft fans before it is dis-
charged through a stack to the atmosphere. The fly-ash collector
consists of a cyclone-type separator in series with an electrostatic
precipitator.
The high-volatile bituminous coals burned in the corner-fired
unit were obtained from West Virginia and Ohio (Table 1). Normal
amounts of excess air were used during three tests at full boiler load
and two tests at three-quarter load.
Front-Wall Horizontally Fired Unit The front-wall-fired dry-bottom
unit is illustrated in Figure 4. At full load, the unit produces 920,000
pounds of steam per hour at 1,000°F and 1,900 psig. Coal is pulverized
in 4 rotary mills and conveyed to 24 front-wall burners. Combustion
air from the regenerative preheaters enters the furnace as primary air
with the coal and as secondary air through the annular ports of each
burner. Hot gas recirculates from the economizer outlet to the bottom
of the furnace. The convection heat-transfer section of the water-
cooled furnace includes superheater, reheater, and economizer units.
Flue gas from the two air heaters enters two parallel electrostatic
precipitators for collection of fly ash.
The coals burned in this unit were supplied from a Kentucky strip
mine and a West Virginia deep mine. Three full-load tests and two
partial-load tests were conducted at normal excess air conditions.
Horizontally Opposed Fired Unit The turbo-fired wet-bottom unit
(Figure 5) burns either pulverized coal or gas and is designed to re-
inject all fly-ash. Steam production is rated at 150,000 pounds per
hour at 1,000 psig and 835°F. The water-cooled furnace is designed
for continuous drip removal of slag. The fly-ash collector consists of
6 EMISSIONS FROM COAL-FIRED
-------
MECHANICAL
DUST COLLECTOR
,-LOW LEVEL
/ ECONOMIZER
TO STACK
BURNERS
EXHAUSTER
Figure 3. Boiler outline for corner-fired unit showing sampling positions.
POWER PLANTS: A SUMMARY
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PULVERIZER
Figure 4. Boiler outline for front-wall fired unit showing sampling positions.
EMISSIONS FROM COAL-FIRED
-------
SUPERHEAT,!
OUTLET HEADER
SUPERHEATER
Figure 5. Boiler outline for horizontally-opposed firing unit showing sampling positions.
POWER PLANTS: A SUMMARY
-------
a cyclone-type separator with facilities for storage or reinjection of
fly ash. Six tests were run on the turbo-fired unit to evaluate emis-
sions at full and three-quarter load both with and without fly-ash
reinjection. A high-volatile bituminous coal from Illinois was burned
during all tests.
Cyclone-Fired Unit In the cyclone-type furnace, a mixture of crushed
coal and air is injected tangentially into a horizontal, cylindrical com-
bustion chamber. Essentially all of the combustion takes place in this
SECONDARY SUPERHEATER
AND REHEAT HEADERS
COAL SAMPLING
POINT
COAL FEEDERS
COAL CRUSHERS
FLUE GAS TEMPERING DUCT
Figure 6. Boiler outline for cyclone type unit showing sampling positions.
10 EMISSIONS FROM COAL-FIRED
-------
water-cooled chamber. A substantial amount of ash is melted in the
cyclone and removed from the furnace as slag. This boiler is rated
at 1,360,000 pounds of steam per hour at 2,400 psig and 1,050°F.
Two forced-draft fans with a capacity of 370,000 scfm supply
combustion air to the furnace and maintain positive pressure through-
out the furnace-boiler system. Flue gas leaving the boiler passes
through secondary and primary superheater sections, an economizer,
an air preheater, and finally a fly-ash collector, as shown in Figure 6.
The fly-ash collectors include two parallel electrostatic precipitators.
Collected fly ash is normally reinjected into the furnace.
A single type of high-volatile bituminous coal from Pennsylvania
was burned during all tests. Three tests were run at approximately
full load, two of which included fly-ash reinjection. Two additional
tests were run at three-quarter load, both with fly-ash reinjection.
All tests were conducted with normal amounts of excess air.
Spreader-Stoker Fired Unit In this traveling-grate type of spreader
stoker, as shown in Figure 7, crushed coal is gravity-fed to rotating
blades which distribute the coal over a slowly revolving continuous
grate. Collected fly ash is also reinjected at the rear of the grate.
^SUPERHEAT
I Q^OUTLET
HEADER
COAL SAMPLING
POINT
Figure 7. Boiler outline for spreader stoker unit showing sampling positions.
POWER PLANTS: A SUMMARY
11
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The coal passes slowly through the combustion zone where it is burned,
and the remaining ash is discharged from the opposite end of the re-
volving grate into an ash hopper. The grate has an effective area of
362 square feet and serves a boiler with a nominal capacity of 220,000
pounds of steam per hour at 875 psig and 760°F.
Combustion air is supplied by both forced-draft fans and overfire-
air fans. Flue gases leaving the grate pass up through a water-cooled
boiler-furnace section, superheater, economizer, cyclone-type fly-ash
collector, air preheater, and induced-draft fan before entering the stack.
A high-volatile bituminous mixed coal from Illinois was burned
during all tests.
DISCUSSION OF RESULTS
1. Oxides of Sulfur
The average sulfur content of coals used for electric-power
production in the United States is about 2.5 percent. The sulfur content
of the coals burned in the six units during testing varied from 1.6 to
2.9 percent (Table 3), with an average of 2.4 percent.
The sulfur in the coal appears mainly as sulfur dioxide in the
flue gas. The balance of the residual sulfur in the fuel appears as
sulfur trioxide, sulfuric acid mist, or as other compounds in the fly ash
or bottom slag. The sulfur content of either the fly ash or slag from
five power plants tested was normally 0.1 to 0.2 percent. Since the
average ash content of the coal from these units was about 10 to 12
percent, less than 1 percent of the sulfur in the coal appeared in the
ash. The spreader stoker had approximately 1.0 percent sulfur in
the reinjected particulate and 0.25 percent in the grate ash; this ac-
counted for about 2 percent of the sulfur in the coal. The remainder
of the sulfur should therefore appear as sulfur oxides in the flue gas.
Other investigators have estimated that between 90 and 100 percent
of sulfur entering the boiler in the coal would be expected to appear
as sulfur oxides in the flue gas. 13
The concentrations of sulfur dioxide in the flue gas leaving the
fly-ash collector varied from 1,000 to 1,730 ppm for all six units
(Table 3). No significant changes in concentrations of sulfur dioxide
were noted in full- and partial-load operation. Material balances for
conversion of sulfur in the coal to sulfur dioxide in the flue gas showed
conversions ranging from 91 to 105 percent. Since sulfur conversion
must be below 100 percent, the errors involved in calculating theoret-
ical emissions and in sampling and analysis for sulfur oxides become
apparent. The change in sulfur content of coals during the sampling
runs, non-uniform mixing of flue gases, and slight inaccuracies in
sampling and analysis could account for errors of 5 to 10 percent. The
range of conversions does not include concentrations of sulfur dioxide
from the front-wall-fired unit, which averaged 120 percent of theoretical.
These high values for sulfur dioxide were due probably to interference
12 EMISSIONS FROM COAL-FIRED
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i
Table 3. TEST CONDITIONS AND MAJOR POLLUTANT EMISSIONS
Type of boiler firing
Full-load testsa
Vertical
Corner
Front-wall
Spreader-stoker
Cyclone
Horizontally opposed
Partial-load testsb
Vertical
Corner
Front-wall
Spreader-stoker
Cyclone
Horizontally opposed
Coal
Sulfur,0
%
2.9
1.7
2.3
2.8
2.4
2.4
2.8
1.6
1.8
2.5
2.4
2.9
Ash,d
%
20.2
14.9
10.3
8.4
7.7
8.2
19.0
13.5
9.2
8.7
7.4
7.8
Emissions
gr/scf6
Fly-ash
BS
4.8
3.7
2.5
2.3
1.5
4.9
4.7
2.9
2.4
1.5
1.8
2.9
Ah
0.18
0.23
0.44
0.38
0.39
0.68
0.11
0.13
0.22
0.19
0.22
0.61
ppm by volume , dry basis
Nitrogen oxides*
BS
221
526
416
431
1204
393
161
393
500
430
742
395
Ah
310
413
606
437
1160
350
171
325
453
390
784
328
Sulfur dioxide
BS
1450
1150
2120
1380
1350
1560
1700
1120
1080
1280
1380
1780
Ah
1730
1130
1680
1570
1360
1380
1640
1000
1460
1240
1370
1680
Sulfur trioxide
BS
66
8
11
58
21
10
46
10
3
52
13
6
Ah
9
12
7
76
31
9
10
12
20
69
22
8
>
CD
aAverage values from three or four tests.
b Aver age values from two tests.
cMoisture- and ash-free basis.
^Moisture-free basis.
eCorrected to 12% CC-2, dry basis.
fReported as NO2.
SBefore fly-ash collector.
hAfter fly-ash collector.
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from organic acids and/or other trace mineral acids before the an-
alytical technique was modified to preclude this interference.
It was theorized that the percentage conversion of sulfur in the
coal to sulfur dioxide in the flue gas would be considerably lower in
wet-bottom units. This theory was based on the assumptions that (1)
some of the sulfur in the coal would contact the furnace bottom slag
and form molten iron sulfide, particularly with low excess air, or (2)
the iron content of recirculated fly ash would catalytically oxidize the
sulfur dioxide in the flue gas to sulfur trioxide. Results did not sub-
stantiate this theory. Between 95 and 100 percent of the sulfur that
entered in the coal appeared as sulfur dioxide in the stack gases in
the wet-bottom units.
The concentrations of sulfur trioxide in the flue gas leaving the
fly-ash collector varied between 7 and 76 ppm for all six units. The
average outlet concentration of sulfur trioxide at both full and three-
quarter load was 24 ppm. The total gaseous sulfur oxides found to be
sulfur trioxide varied from 0.3 to 4.4 percent for measurements at
the fly-ash collector inlet; the arithmetic mean was 1.7 percent. Under
conditions of thermodynamic equilibrium, the percentage of sulfur
trioxide should have been negligible at flame temperatures, and over
99 percent of the sulfur oxides after the air heater. 14 The measured
concentrations, however, correspond to equilibrium values for con-
ditions of gas temperature and oxygen content near the furnace outlet.
The rapid cooling of gases in the convective-heat-transfer system
near the furnace outlet appears to quench further oxidation of sulfur
dioxide to sulfur trioxide.
In the vertically fired unit, which produced the highest concen-
trations of sulfur oxides, the sulfur trioxide concentration was reduced
appreciably in passage through the fly-ash collector (Table 3). It was
initially theorized that cooling of flue gases in the fly-ash collector
at points of influent air leakage could result in condensation of sulfur
trioxide and formation of sulfuric acid mist. The acid mist could then
be adsorbed on the fly-ash particles and their removal effected by the
collection of fly ash. This theory does not appear valid in all cases,
because none of the other units realized significant reductions of sulfur
trioxide in passage through the fly-ash collector.
2. Oxides of Nitrogen
Oxides of nitrogen are formed largely by high-temperature oxida-
tion of atmospheric nitrogen during combustion. Nitric oxide (NO) is
the primary combustion product resulting from nitrogen fixation in
the furnace. Theoretical equilibrium concentrations of nitric oxide
may be as high as 1,000 ppm at 2,500°F and as low as 1 ppm or less
at the lower temperatures (250-300°F) of the fly-ash collector. 15
Concentrations of nitrogen oxides measured at the inlet of the
fly-ash collectors in all units except the cyclone-fired furnace ranged
from 221 to 526 ppm at full load and from 161 to 500 ppm at three-
14 EMISSIONS FROM COAL-FIRED
-------
quarter load (Table 4). These concentrations were generally equilib-
rium levels for temperatures near the furnace outlet. The nitrogen
oxides tended to decompose as the gases cooled. Apparently, the
concentrations of nitrogen oxides are determined by flame temperature,
incomplete decomposition as the gases flow from the flame to the
furnace outlet, and rapid quenching of the decomposition reaction as
the gases cool in the convective heat-transfer system.
The higher temperatures encountered in the cyclone-fired
furnace resulted in expected higher levels of nitrogen oxides at the
fly-ash collector inlet -- 1,204 ppm at full load and 742 ppm at three-
quarter load (Table 4). The wet-bottom turbo-fired furnace, which
also operated with higher temperatures in the slagging zone, was also
expected to produce higher concentrations of nitrogen oxides than the
395 ppm measured at the inlet of the fly-ash collector (Table 4). By
concentration of combustion in the slagging zone at the bottom of the
Table 4. NITROGEN OXIDE CONCENTRATIONS
Full-load testsa
Type of boiler firing
Vertical
Corner
Front-wall
Spreader-stoker
Cyclone
Horizontally opposed
ppm*)
Bd
221
526
416
431
1204
393
Ae
310
413
606
437
1160
350
Lb/106Btu
Bd
0.38
0.95
0.68
0.65
2.5
0.65
Ae
0.55
0.71
0.95
0.76
2.2
0.59
Partial-load testsc
Type of boiler firing
Vertical
Corner
Front-wall
Spreader-stoker
Cyclone
Horizontally opposed
ppnv3
Bd
161
393
500
430
742
395
Ae
171
325
453
390
784
328
Lb/106 Btu
Bd
0.28
0.73
0.82
0.73
1.9
0.66
Ae
0.31
0.57
0.74
0.68
1.8
0.56
aAverage values for three or four tests at each unit.
^Reported as NO2, at stack conditions.
cAverage values for two tests at each unit.
dB: Before fly-ash collector.
eA: After fly-ash collector.
POWER PLANTS: A SUMMARY
15
-------
furnace, however, a relatively long and gradual cooling of combustion
products was provided in the upper part of the furnace and probably
accounted for the decomposition of the nitrogen oxides. The result
was a lower concentration of nitrogen oxides than would have been
expected on the basis of combustion temperature alone.
In two of the four units equipped with electrostatic-type fly-ash
collectors (the vertically-fired and front-wall-fired units), the con-
centrations of nitrogen oxides measured at the outlet of the fly-ash
collectors were 40 to 45 percent higher than those measured at the
inlet during full-load tests. This increase may be attributed to
formation of ozone and atomic oxygen in the corona discharge of the
electrostatic precipitator and their subsequent reaction with nitrogen
to form additional nitrogen oxides. No increase in nitrogen oxide
levels was found, however, at reduced loads for the vertically-fired
and front-wall-fired units or at either load condition for the corner-
fired or cyclone-fired boilers in passage through the electrostatic
precipitators. Since the literature gives little information on this
subject, additional studies are required to explain this phenomenon.
3. Solid Participate
Efficient fly-ash-control equipment has enabled operators of the
modern coal-burning power plant to reduce particulate emissions
considerably. Ply-ash collection efficiencies of over 95 percent are
not uncommon today.
The three pulverized-coal-burning units, i.e., the vertically
fired, the corner-fired, and the front-wall-fired boilers are operated
with bituminous coals having ash contents ranging from approximately
10 to 20 percent. It has been estimated that over 75 percent of the
ash in dry-bottom pulverized-coal-burning power plants leaves the
furnace with the flue gas. The average grain loadings at the fly-ash
collector inlet for these three units during full-load operation were
4.8, 3.7, and 2.5 grains per standard cubic foot respectively (Table 5).
Ash leaving the furnace and entering the fly-ash collector amounted to
about 60 percent for the vertically-fired unit, 80 percent for the corner-
fired unit, and 75 percent for the front-wall-fired unit. A vertically-
fired unit would be expected to retain more fly ash than a corner- or
horizontally-fired unit because of the downward direction of flow
resulting in fly-ash impaction on the bottom of the furnace. This
condition would allow more fly ash to settle in the furnace. The lower
ash retention for the corner-fired and front-wall-fired units would
also be expected because their horizontal firing arrangement hinders
ash dropout to the furnace floor.
The combination mechanical-electrostatic fly-ash collectors for
both the vertically- and corner-fired units effected average collection
efficiencies of 96.4 and 93.9 percent, respectively, at full load (Table 5).
The average collection efficiencies at three-quarter load increased
to 97.5 percent for the vertically-fired unit and 95.7 percent for the
corner-fired unit. The electrostatic fly-ash collector for the front-wall-
16 EMISSIONS FROM COAL-FIRED
-------
Table 5. FLY-ASH CONCENTRATIONS AND COLLECTION EFFICIENCIES
Type of boiler firing
Full-load tests6
Vertical
Corner
Front-wall
Spreader-stoker
Cyclone
Horizontally opposed
Partial-load testsf
Vertical
Corner
Front-wall
Spreader-stoker
Cyclone
Horizontally opposed
Ash in
coal,a
%
20.2
14.9
10.3
8.4
7.7
8.2
19.0
13.5
9.2
8.7
7.4
7.8
Concentrations
gr/scfb
BS
4.8
3.7
2.5
2.3
1.5
4.9
4.7
2.9
2.4
1.5
1.8
2.9
Ah
0.18
0.23
0.44
0.38
0.39
0.68
0.11
0.13
0.22
0.19
0.22
0.61
lbs/1000 lbc
dry flue gas
BS
8.8
6.9
4.6
4.2
2.8
8.9
8.7
5.5
4.4
2.8
3.1
5.1
Ah
0.27
0.42
0.82
0.66
0.62
1.27
0.21
0.21
0.41
0.35
0.36
1.1
Type of
fly-ash
collector^
C, E
C, E
E
C
E
C
C, E
C, E
E
C
E
C
Collector
efficiency,
%
96.4
93.9
83.1
83.9
74.5
83.9
97.5
95.7
91.3
87.3
86.3
77.7
aMoisture-free basis.
^Corrected to 12 percent CO2, dry volume basis.
C1000 pounds of dry flue gas corrected to 50 percent excess air.
dc designates cyclone; E designates electrostatic precipitator.
eAverage values for either three or four tests of each unit.
^ Average values for two tests at each unit.
SBefore fly-ash collector.
h After fly-ash collector.
-------
fired unit operated with an average collection efficiency of 83.1 percent
at full load and 91.3 percent at three-quarter load. Although good
reproducibility for fly-ash collection efficiency was obtained for both
the vertically- and the corner-fired units, the collection efficiency
varied appreciably at full-load operation of the front-wall and cyclone-
fired units. No reason was apparent for this variation.
The spreader-stoker unit was fired with crushed coal having an
average ash content of 8.2 percent. The average grain, loading at the
inlet to the mechanical fly-ash collector during normal-load operation
was 2.3 .grains per standard cubic foot, which represents about 47 per-
cent of the particulate entering the boiler as ash in the coal and as
reinjected material. Average collection efficiency of the mechanical
fly-ash collector was 83.9 percent .at normal load and 87.3 percent
at partial load.
The cyclone-fired unit and the horizontally opposed, downward-
inclined fired unit are both wet-bottom boilers that normally operate
with fly-ash reinjection. Both-were fired with bituminous coals having
ash contents ranging from about 7 to 8 percent. The average grain
loadings at the inlet to the fly-ash collector during full-load operation
were 1.5 grains per standard cubic foot for the cyclone unit and 4.9
grains per standard cubic foot for the horizontally opposed fired unit
(Table 5). The amounts of ash leaving the boiler and entering the
fly-ash collector were about 50 percent for the cyclone-fired unit and
70 percent for the horizontally opposed fired unit.
The electrostatic fly-ash collector for the cyclone-fired unit
operated with average collection efficiencies of 74.5 percent at full
load and 87.8 percent at three-quarter load. The mechanical fly-ash
collector for the horizontally opposed fired unit effected average col-
lection efficiencies of 83.9 percent at full load and 77.7 percent at
three-quarter load. Particulate emissions from the electrostatic
precipitators of the three pulverized-coal-burning units and of the
cyclone unit would .meet the old American Society of Mechanical
Engineers standard of 0.£5 pound particulate per 1,000 pounds of
dry flue gas corrected to 50 percent excess air. 1° Particulate
emissions from the mechanical collectors of the horizontally opposed
fired unit would not meet this standard.
Average fly-ash collector efficiencies of plants in operation a
number of years were lower than those originally guaranteed by the
manufacturer.
Table 6 lists the guaranteed fly-ash collector efficiencies and
the actual efficiencies measured during the tests.
In general, operation at partial load showed higher fly-ash
collector efficiencies. This indicated an over-loaded condition during
normal operation. Decreases in fly-ash collector efficiencies were
largely due to blockage of air passage with dust, which increased
velocities; breakage and corrosion of electrostatic precipitator
18 EMISSIONS FROM COAL-FIRED
-------
Table 6. COMPARISON OF FLY-ASH COLLECTOR EFFICIENCIES
Type of firing
Vertical
Corner
Front-wall
Spreader-stoker
Cyclone
Horizontally opposed
Type
of
collectora
C & E
C & E
E
C
E
C
Fly-ash collector efficiency, %
Guaranteed by
manufacturer
98.2
97.5
95.0
93.1
95.0
89.7
Obtained in tests
Normal
load
96.4
93.9
83.1
83.9
74. 6b
88. 5b
Partial
load
97.5
95.7
91.3
87.3
86. 3b
78. Ob
aC denotes a cyclone type collector;
E denotes an electrostatic precipitator.
blncludes only tests with fly-ash reinjection, which was normal operating
procedure at these plants.
electrodes; and changes in fly-ash characteristics due to variations
of coal or boiler operation,, or both.
The amount and composition of mineral matter in the coal largely
determines the concentrations of trace metals in the fly ash. Seventeen
common trace metals were determined in fly-ash samples from each
of the six units (Table 7) during full-load operation. The estimated
accuracy of these values is + 50 percent of the measured value. The
spectrographic analyses of fly-ash samples were intended to determine
any significant difference in collector efficiency for individual minerals
as indicated by the content of the various metals. The average collector
efficiencies of trace metals for any of the six power plants were nearly
the same as the fly-ash collection efficiencies for each unit; for the
spreader-stoker unit, however, metal collection efficiency was only
47 percent.
4. Polynuclear Hydrocarbons
Although polynuclear hydrocarbons normally occur in minute
concentrations, these compounds are of interest from an air pollution
standpoint because several of them have exhibited carcinogenic
properties in animal studies. 17> 18 Previous work has shown that
polynuclear hydrocarbons result from the incomplete combustion of
organic fuels; 19 thus, when a combustion process is poorly controlled,
emissions of polynuclears may be high. Since combustion control in
power plants was generally good and fuel-burning methods efficient,
emissions of polynuclear hydrocarbons were low. The results are
POWER PLANTS: A SUMMARY
19
-------
Table 7. METALS ANALYSIS FOR FULL-LOAD TESTS (grains/scf x 10-4)a
Type of
boiler
firing
Vertical
Corner
Front-wall
Spreader -
stoker
Cyclone
Horizontally
opposed
Avg.
coll.
eff. ,
%
89
94
86
4.7
69
QA
Sam-
pling
point
B*
Ac
Bb
Ac
Bb
Ac
Bb
Ac
Bb
Ac
Bb
AC
Cd
Td
Td
<0.42
<0.024
0.73
0.26
Td
Td
0.30
0.12
<0.97
0.58
Ba
9.5
0.34
20.8
1.4
20.4
3.4
3.65
0.94
27.2
7.5
6.8
1.1
Be
0.24
0.02
0.42
0.024
0.60
0.11
0.20
0.06
0.28
0.08
0.94
0.14
Fe
480
17
1900
102
480
. 58
1100
380
1360
380
6800
730
Pb
3.6
1.1
4.2
0.24
12.5
1.2
4.8
7.4
11.4
3.8
68
14
Cr
0.95
0.08
8.3
0.58
4.8
0.68
1.95
1.52
8.2
2.2
9.7
1.8
Cu
9.5
0.87
25
1.1
3.6
0.88
1.9
1.1
3.2
0.8
20
4.4
Sn
Td
0.04
<0.42
<0.22
<0.26
0.26
Td
0.17
0.65
0.26
<0.68
0.32
Sb
Td
Td
<4.2
<0.24
<0.8
<0.8
<2.4
<0.4
<1.4
<0.4
<6.7
<0.8
Mn
7.2
0.26
4.2
0.44
17.0
1.6
6.1
1.3
5.7
1.2
10.6
0.73
Ni
4.8
1.1
8.8
0.58
12.5
0.76
3.6
1.5
10.3
2.2
20
3.0
Mo
0.95
0.17
<1.22
0.10
3.8
0.58
0.73
0.37
1.14
0.38
10.6
2.2
V
9.5
0.88
24.8
1.4
24
2.4
6.1
1.5
13.6
4.7
42
6.6
Ti
95
3.4
420
22
460
48
48
17
136
38
540
73
Zn
Td
0.34
12.2
<0.72
<24
< 2.8
<7.3
3.0
<4.2
< 1.2
42
14
Co
0.48
0.06
1.22
0.082
2.0
0.37
<0.73
0.21
2.2
0.8
5.1
0.66
As
1.4
0.11
2.6
1.6
0.66
0.30
4.0
0.54
aBased on particulate grain loading. Each value is the average of at least two tests.
^Before fly-ash collector.
cAfter fly-ash collector.
dlrace; blank indicates no data.
-------
shown in Table 8. Figure 8 and Table 8 compare concentrations of
seven polynuclears for which the analytical technique was most accurate.
Concentrations of polynuclears are shown for the fly-ash-collector
outlet only because early tests showed significant recovery of poly-
nuclears by the fly-ash collectors. The levels of polynuclear hydro-
carbon emissions for all units were well below the levels that result
from the firing of coal in smaller furnaces with less precise control
of the combustion process. Benzo(a)pyrene emissions from small
furnaces varied from 3,800 to 400,000 micrograms per million Btu
heat input; *$ average benzo(a)pyrene emission from six plants ranged
from 19 to 223 micrograms per million Btu.
5. Emissions of Trace Contaminants
Concentrations of carbon monoxide and gaseous hydrocarbons
were low for all six units (Table 9). These low values indicated a
high degree of combustion efficiency. Concentrations of these gases
did not change significantly in any unit during operation at full and
partial load, nor did passage of the flue gas through the fly-ash
collector affect the concentrations.
Formaldehyde concentrations from all units were very low,
ranging between 0.03 and 0.25 ppm. The fly-ash collector reduced
formaldehyde concentrations in five of the six units, with an average
reduction of 45 percent. Removal of formaldehyde in the fly-ash
collectors indicated adsorption of this compound by the fly-ash
particles that were removed. The slight increase indicated for
formaldehyde in the spreader-stoker test was probably due to an
excessive amount of participate in the outlet sampling train. No
Table 8. POLYNUCLEAR HYDROCARBON CONCENTRATIONS3"
(micrograms /lO^ Btu heat input)
Type of boiler firing
Compound
Fluoranthene
Pyrene
Benzo (a)pyr ene
Benzo(e)pyrene
Benzo(ghi)perylene
Coronene
Perylene
Vertical
200
155
19
Corner
390
140
140
86
150
7
71
Front-wall
80
180
19
23
7
Spreader
stoker
50
105
< 20
30
5
Cyclone
79
1025
223
395
198
6
17
Horizontally
opposed
188
91
81
265
645
56
aAfter fly-ash collector during full-load operation. Average values for two
tests at each unit. A blank indicates that the compound was not detected.
POWER PLANTS: A SUMMARY
21
-------
I
*l
8
O
O
M
0
1200
1000
= 900
1
800
600
500
400
TYPE OF BOILER FIRING - FULL LOAD TESTS
VERTICAL
II || || SPREADER- || I | HOI
—1 U CORNER -J U- FRONT WALL -—4 U STOKER J U CYCLONE J U
i;:g;:;| FLUORANTHENE
^3 PYRENE
^^ BENZO (a) PYRENE
[^j BENZO (e) PYRENE
ffi\ BENZO (ghi) PERYLENE
^| CORONENE
tS^) PERYLENE
HORIZONTALLY-
OPPOSED -
Figure 8. Polynuclear hydrocarbon concentrations at fly-ash collector outlet.
-------
Table 9. SUMMARY OF TRACE GASEOUS EMISSIONS
(ppm by volume, dry basis)
Full-load testsa
Type of boiler firing
Vertical
Corner
Front-wall
Spreader-stoker
Cyclone
Horizontally opposed
Carbon
monoxide
Bd
17
11
5
29
f
44
AS
11
16
6
32
f
51
Hydrocarbons13
Bd
17
7
18
15
0
2
Ae
14
9
6
8
0
0
Formaldehyde
Bd '
0.25
0.17
0.14
0.06
0.17
0.10
Ae
0.12
0.12
0.08
0.10
0.07
Partial-load testsc
Type of boiler
Vertical
Corner
Front-wall
Spreader -stoker
Cyclone
Horizontally opposed
Carbon
monoxide
Bd
13
39
12
13
15
69
Ae
19
33
5
17
10
64
Hydrocarbons
Bd
17
6
10
3
0
6
Ae
14
6
7
2
0
6
Formaldehyde
Bd
0.26
0.11
0.14
0.03
0.15
0.11
Ae
0.07
0.04
0.06
0.06
0.11
0.09
aAverage values for three or four tests at each unit.
^Gaseous hydrocarbons at room temperature expressed as a single carbon
atom hydrocarbon.
°Average values for two tests at each unit.
"Before fly-ash collector.
eAfter fly-ash collector.
%o data.
significant changes in concentrations of any of these trace contaminants
resulted from operation at either full or partial load.
SUMMARY
A series of tests of six coal-burning power plants was conducted
to determine certain stack-gas components of interest in atmospheric
pollution. The six units tested included three dry-bottom pulverized-
coal-burning units, two wet-bottom units, and a large spreader-stoker
traveling-grate unit.
POWER PLANTS: A SUMMARY
23
-------
Measurements of sulfur oxides indicated that essentially 90 to
100 percent of the sulfur in the coal appeared as sulfur oxides in the
stack gas. Of this amount 1 to 2 percent was in the form of sulfur
trioxide and the balance was sulfur dioxide. Neither the type of
furnace, the conditions of firing, nor the reinjection of fly ash affected
sulfur oxide emissions significantly. Thus, concentrations of sulfur
oxides are essentially determined by the amount of sulfur in the coal
entering the furnace.
Concentrations of nitrogen oxides varied widely, ranging from
221 ppm for the vertically fired unit to 1,204 ppm for the cyclone-type
furnace. Concentrations of nitrogen oxides apparently are determined
by initial flame temperatures in the firebox, decomposition in the high-
temperature region of the furnace, and quenching of the decomposition
reaction as the gases are cooled in the boiler section of the furnace.
Control of particulate emissions varied considerably in coal-
fired power plants. Combination cyclone and electrostatic-precipitator-
type fly-ash collectors gave collection efficiencies of about 96 percent
and an outlet grain loading of 0.20 grain per standard cubic foot at full
load. Electrostatic precipitators and mechanical cyclone collectors,
when used separately, gave average collection efficiencies ranging
from 75 to 85 percent, with loadings at the fly-ash collector outlet
varying from 0.19 to 0.68 grain per standard cubic foot.
Other emissions were determined including polynuclear hydro-
carbons, carbon monoxide, gaseous hydrocarbons, formaldehyde, and
trace metals. None of these components were found in appreciable
quantities during normal furnace-operating conditions.
24 EMISSIONS FROM COAL-FIRED
-------
REFERENCES
1. Rohrman, F. A., B. J. Steigerwald. Some Potential Air Pollution
Problems of the Future. Unpublished report. Division of Air
Pollution, U.S. Public Health Service, R. A. Taft Sanitary
Engineering Center, Cincinnati, Ohio (April 1965).
2. Cuffe, S. T., R. W. Gerstle, A. A. Orning, C. H. Schwartz. Air
Pollutant Emissions from Coal-Fired Power Plants; Report No. 1.
J. Air Pollution Control Assoc. 10:9 353-363 (Sept. 1964).
3. Gerstle, R. W., S. T. Cuffe, A. A. Orning, C. H. Schwartz. Air
Pollutant Emissions from Coal-Fired Power Plants; Report No. 2.
J. Air Pollution Control Assoc. 15:2 59-64 (Feb. 1965).
4. Cuffe, S. T. Techniques for Evaluating Air Pollutants from Power
Plants. Arch. Environ. Health. 6:422-427 (March 1963).
5. Determination of Sulfur Dioxide and Sulfur Trioxide in Stack Gases.
Emeryville Method Series 4S16/59a, Shell Development Company
Analytical Department, Emeryville, California (1959).
6. Berk, A. A., L. R. Burdick. A Method of Test for SO2 and 803 in
Flue Gases. Bureau of Mines Report of Investigations 4618 (Jan.
1950).
7. Beatty, R. L., L. B. Berger, and H. H. Schrenk. Determination of
the Oxides of Nitrogen by the Phenoldisulfonic Acid Method. Bureau
of Mines Report of Investigations 3687 (Feb. 1943).
8. Stenburg, R. L., D. J. Von Lehmden, and R. P. Hangebrauck". Sample
Collection Techniques for Combustion Sources -- Benzopyrene
Determination. Amer. Ind. Hyg. Assoc. J. 22: 271-275 (Aug. 1961).
9. Commins, B. T., P. J. Lawther. Volatility of 3, 4 Benzpyrene in
Relation to the Collection of Smoke Samples. Brit. J. Cancer
12: 351-354 (Sept. 1958).
10. Sawicki, E., T. W. Stanley, T. R. Hauser, and F. T. Fox. The
Detection and Determination of Polynuclear Hydrocarbons in
Urban Airborne Particulates -- 1, The Benzopyrene Fraction.
Intern. J. Air Pollution 2: 273-282 (1960).
11. Altshuller, A. P., D. L. Miller, and S. F. Sleva. Determination of
Formaldehyde in Gas Mixtures by the Chromotropic Acid Method.
Anal. Chem. 33: 621 (Apr. 1961).
12. The Silver Diethyldithiocarbamate Method of Arsenic Determination.
Technical Data 142, Fisher Scientific Co. (Feb. 1960).
POWER PLANTS: A SUMMARY 25
-------
13. Committee on Air Pollution, Interim Report. Cmd. 9011, Her
Majesty's Stationery Office, London (1953).
14. Orning, A. A., C. H. Schwartz, J. F. Smith. Minor Products of
Combustion in Large Coal-Fired Steam Generators. Presented at
the Winter Annual Meeting of the Am. Soc. Mech. Engrs., New
York, New York (Nov. 29 Dec. 4, 1964) Paper No. 64-WA/FU-2.
15. Orning, A. A. Air Pollutants from Coal-Fired Electric Power
Plants. National Conference on Air Pollution, Washington, D. C.
PHS Publication No. 654, 159-160 (Nov. 1958).
16. Stern, A. C. Air Pollution. Academic Press, New York and London.
Vol II, Chapter 37 (1962).
17. Sawicki, E., K. Cassel, (ed.). Analysis of Carcinogenic Air Pollu-
tants. Natl. Cancer Inst. Monograph No. 9 (Aug. 1962).
18. Hartwell, I. L. Survey of Compounds Which Have Been Tested for
Carcinogenic Activity. Second Edition, Public Health Service
Publication No. 149 (1951).
19. Hangebrauck, R. P., D. J. Von Lehmden, J. E. Meeker. Emissions
of Polynuclear Hydrocarbons and Other Pollutants from Heat-
Generation and Incineration Processes. J. Air Pollution Control
Assoc. 14: 7 267-278 (July 1964).
26
-------
BIBLIOGRAPHIC: Cuffe, S. T. and Gerstle, R. W.
Emissions from coal-fired power plants: a compre-
hensive summary. PHS Publ. No. 999-AP-35. 1967. 26 pp.
ABSTRACT: The Public Health Service and the Bureau
of Mines conducted a study of air pollutant emissions
from the six main types of coal-burning power plants.
The components tested include sulfur oxides, nitrogen
oxides, polynuclear hydrocarbons, total gaseous
hydrocarbons, solid particulates, formaldehyde,
organic acids, arsenic, trace metals, and carbon
monoxide. This report relates the effects of
variables such as method of operation, type of
boiler furnace and auxiliaries, reinjection of fly
ash, and type of coal burned to the concentrations
of gaseous and particulate pollutants in the products
of combustion.
ACCESSION NO.
KEY WORDS:
BIBLIOGRAPHIC: Cuffe, S. T. and Gerstle, R. W.
Emissions from coal-fired power plants: a compre-
hensive summary. PHS Publ. No. 999-AP-35. 1967. 26 pp.
ABSTRACT: The Public Health Service and the Bureau
of Mines conducted a study of air pollutant emissions
from the six main types of coal-burning power plants.
The components tested include sulfur oxides, nitrogen
oxides, polynuclear hydrocarbons, total gaseous
hydrocarbons, solid particulates, formaldehyde,
organic acids, arsenic, trace metals, and carbon
monoxide. This report relates the effects of
variables such as method of operation, type of
boiler furnace and auxiliaries, reinjection of fly
ash, and type of coal burned to the concentrations
of gaseous and particulate pollutants in the products
of combustion.
ACCESSION NO.
KEY WORDS:
BIBLIOGRAPHIC: Cuffe, S. T. and Gerstle, R. W.
Emissions from coal-fired power plants: a compre-
hensive summary. PHS Publ. No. 999-AP-35. 1967. 26 pp.
ABSTRACT: The Public Health Service and the Bureau
of Mines conducted a study of air pollutant emissions
from the six main types of coal-burning power plants.
The components tested include sulfur oxides, nitrogen
oxides, polynuclear hydrocarbons, total gaseous
hydrocarbons, solid particulates, formaldehyde,
organic acids, arsenic, trace metals, and carbon
monoxide. This report relates the effects of
variables such as method of operation, type of
boiler furnace and auxiliaries, reinjection of fly
ash, and type of coal burned to the concentrations
of gaseous and particulate pollutants in the products
of combustion.
ACCESSION NO.
KEY WORDS:
------- |