EPA-650/2-74-038
May 1974
Environmental Protection Technology Series
-------
EPA-650/2-74-038
COMBUSTION CONTROL OF POLLUTANTS
FROM MULTI-BURNER
COAL-FIRED SYSTEMS
by
C. McCann, J. Demeter,
R. Snedden. and D. Bienstock
U.S. Bureau of Mines
4800 Forbes Avenue
Pittsburgh, Pennsylvania 15213
Interagency Agreement EPA-IAG-020(D)
ROAP No. 21ADG-81
Program Element No. 1AB014
EPA Project Officer: David G. Lachapelle
Control Systems Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
May 1974
-------
This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
-------
ABSTRACT
An experimental 500 lb/hr multi-burner pulverized-coal fired furnace was
utilized to determine the effects of several combustion modifications on
nitrogen oxides emissions. Techniques investigated were reduced excess air,
staged combustion, flue gas recirculation to both primary and secondary
combustion air streams, and combinations of all techniques.
Reduction of about 70% in NO emissions were achieved by reduction of
excess air from 207. to 27., however, the NO reduction was accompanied by a
significant decrease in carbon conversion at the lower excess air levels.
Staged combustion resulted in about a 507. reduction in NO emissions,
with little increase in carbon loss over conventional combustion.
Thirty percent flue gas recirculation also resulted in an NOx reduction
of about 50%, but was accompanied by a significant reduction in carbon
combustion efficiency similar to that experienced in the low excess air tests.
Various combinations of the individual NOx reduction techniques were tested
but they did not offer any advantage as the effects were not cumulative.
This report was submitted in fulfillment of Interagency Agreement Number
EPA-IA -020(D) by the U.S. Bureau of Mines under the sponsorship of the
Environmental Protection Agency. Work was completed as of February 1, 1974.
].L1.
-------
ACKNOWLEDCMENTS
The authors wish to express their appreciation to Messrs. John Dzubay,
Herbert L. Vandale, John T. Hoffman and Forrest E. Walker for their contri-
butions to completion of the program through furnace operation, instrumentation,
sampling, and analyses.
iv
-------
TABLE OF CONTENTS
Page
I. INTRODUCTION 1
II. DESCRIPTION OF FACILITY 2
III. RESULTS AND DISCUSSION 11
A. Baseline Tests 11
B. Low Excess Air Tests 11
C. Two-Stage Combustion Tests 13
D. Bias Firing Test 20
E. Flue Gas Recirculation Tests 22
1. Recirculation to Secondary Air 22
2. Recirculation to Primary Air 25
F. Combinations of Combustion Modifications 27
IV. CONCLUSIONS 32
V
-------
List of Tables
Table Page
1 Typical Analysis of Coal Fired 8
2 Summary of Tests at Various Excess Air Levels 13
3 Results of Two-Stage Combustion Tests 17
4 Summary of Data: Two-Stage Combustion 19
5 Results of Bias Firing Tests 20
6 Flue Gas Recirculation to Secondary Air 23
7 Flue Gas Recirculation to Primary Air 25
8 Combinations of Techniques with Two-Stage Combustion 29
9 Combinations of Techniques with Bias Firing 31
vi
-------
List of Figures
FiRure Page
1 View of 500 lb/hr Coal-Fired Furnace 3
2 Flowsheet of Pulverized-Coal-Fired Facility 4
3 Half-section View of Principal Components of the Combustion
System 5
4 Multi-Fuel Burner Assembly 7
5 N0 Formation as a Function.of Excess Air 12
6 Carbon Combustion Efficiency as a Function of Excess Air 12
7 Water-Cooled Second-Stage Air Injector 14
8 Points of Second-Stage Air Introduction 15
9 NOx Formation with Two-Stage Combustion 16
10 NO Formation with Bias Firing 21
11 N0 Formation with Flue Gas Recirculation to Secondary Air - -- 24
12 NO Formation with Flue Gas Recirculation to Primary Air 26
vii
-------
I. INTRODUCTION
It is generally known that coal-fired utility boilers are the largest
st .Itionary source contributor of nitrogen oxides (NO ) in the U.S , Compounding
the NO problem from coal are a variety of industrial applications where coal
is the fuel. Recent estimates indicate that about 427 of the total NO emissions
from stationary sources result from coal combustion.
Combustion modification techniques offer the most promising cost-
effective short-term approach to control of NO from stationary combustion
sources regardless of fuel. This study, however, deals with techniques
applicable to multi-burner coal-fired combustion systems. The techniques
studLed WLII h.ive the greatest beneficial impact on utility and industrial
size boilers.
The purpose of the study was to investigate in further detail the
effects of specific combustion modification techniques and assess the impact
that they have on NO and combustion-related pollutants, C, CO, and hydrocarbons.
AdditLonally the study provided an insight into the impact that combustion
modifications have on combustion efficiency as measured by carbon losses.
The e cper.Lmental studies should provide more definitive guidance as
to the merits and limitations of these techniques for subsequent application
to field units..
1
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II. DESCRIPTION OF FACILITY
Combustion tests were conducted in an experimental, pulverized-coal-
fired furnace designed to simulate the performance of an industrial steam
generating unit used in power generating plants. The wall-fired, dry-bottom
furnace is capable of burning 500 pounds of coal per hour, with an exit gas
temperature of 2000°F. Heat release rate is about 16,000 Btu/hr.-cu.ft. A
photograph of the combustor is shown in Figure 1. The furnace has water-
cooled walls with refractory applied in the burner zone to provide flame
stability and to prevent excessive heat transfer to the walls in the vicinity
of the burners. Coal is burned in a direct-fired system through four burners
in the front wall of the 7 ft. wide, 12 ft. high and 5 ft. deep rectangular
furnace. A simplified flowsheet of the combustion system is shown in Figure 2.
Provision has been made to preheat secondary air and to vary distribution of
combustion air between the primary and secondary streams. Variations in coal
feed rate can result in pockets deficient in either fuel or oxygen, producing
fluctuations in fuel-air ratio. Consequently a recycle loop was provided in
the primary air-coal transport line to obtain a more uniform coal feed rate and
thus minimize these fluctuations. Figure 3 shows the principal components of
the combustion system in half section. Shown are the combustor, the convective
heat-transfer section, a duct designed for emission measurements, and the
recuperative air preheater. Combustion products flow through the convective
heat transfer section decreasing the gas temperature to 1000°F; through the
air heater, used to preheat secondary air; then through a mechanical dust
2
-------
Plqure I. View of
500
coal
- fired furnace.
• Mi
.j&j
U
3
-------
Figure Flowsheet of pulverized coal - fired facility.
-------
Convective heat transfer s.ction...\
t
f
Figure 3. Half section view of principal components
of tñe combustion system.
Recuperative
ojr Preheoter .\
U
Second stags air
Dust loading duct\
4-
5
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collector. The cross section of a multi-fuel burner is shown in Figure 4.
The burners were designed to impart swirl in both primary and secondary air
streams. The flame profile can be continuously varied from a short, bushy
pattern to a relatively long, narrow pattern by adjustment of swirl induced
in the secondary air stream.
The four front-wall burners were designed to fire natural gas and/or
pulverized solid fuel. Prior to each test period, the experimental furnace
was fired with natural gas to preheat the refractory and to provide a source
of preheat for secondary air. During this period combustion air flows were
established and necessary secondary air swirl adjustments were made to provide
flames that were attached to the burners, but not drawn into the burner tubes.
Preheating was then continued until secondary air temperature reached 550°F.
Natural gas flow to each burner was then reduced by 507., and pulverized coal
feed was started at a rate of about 250 lb/hr. From this point oxygen content
of the flue gas was used as a guide in the fuel changeover. As coal feed rate
was increased, natural gas input was decreased to maintain a constant oxygen
level. When the desired coal feed rate was reached, all natural gas input was
eliminated, and the unit was allowed to reach thermal equilibrium.
6
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cconaory
air
Figure 4. Multi-fuel burner assemb(y.
Primory
air-cool
---
Pilot
gas
Auxiliory
gas
Pilot gas
spark igniter
Primary air
swirl inducer
Secondary air
swirl vanes
7
-------
During the course of the experimental program, the coal-firing rate
was maintained at 500 lb/br, coal fineness at 757. through 200 mesh, and
secondary air temperature at 600°F. Table 1 shows proximate and ultimate
analyses of the coal fired. Sufficient coal was obtained to minimize variations
in analysis. As a result, nitrogen content of 1.4% was maintained throughout
the prpgram.
TABLE 1.- TYPICAL ANALYSIS OF COAL FIRED
As received,
Proximate wt-7 .
Moisture 2.2
Volatile matter 36.5
Fixed carbon 51.5
Ash 9.8
Ultimate
Hydrogen 5.3
Carbon 73.0
Nitrogen 1.4
Oxygen 8.4
Sulfur 2.1
Ash 9.8
Heating value 13,060 Btu/lb
8
-------
Emission measurements were made during each test period, using
instrumented methods where possible. Samples were taken from the dust
loading duct for analysis of gaseous components and carbon content of the
fly ash. Gaseous species and the instrumentation used for their measurement
were:
Oxygen Catalytic combustion
Polarographic electro-chemical
Carbon Dioxide Non-dispersive infrared
Carbon Monoxide Non-dispersive infrared
Nitric Oxide (NO) Non-dispersive infrared
NO /NO Chemi. luminescent
Sulfur Dioxide Electro-chemical
Hydrocarbons Flame ionization
Throughout this report, NO emissions are expressed either as ppm corrected
to 07. 02 or as g N02/10 6 cal heat input. This latter method is consistent
with requirements of the Federal Register, Vol. 36, No. 247-Thursday, December
23, 1971. To convert from g N0 2 /lO 6 cal to lb NO 2 /10 6 Btu, divide by 1.8.
9
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To minimize sample time lag, a massive sample was taken from the duct,
passed through glass wool filters to remove particulate, than cooled in ice
baths. From this stream proportional samples were fed to each instrument
using the shortest possible conductor length. Excess gas in the major stream
was vented to the atmosphere. The ice baths were designed to provide limited
gas-liquid contact. In addition, spot samples were obtained for wet chemical
analysis. For NOB, the EPA-recommended phenoldisulphonic acid technique was
used, while the Shell technique was used for sulfur oxides.
Samples for particulate determination were obtained using apparatus
manufactured to Environmental Protection Agency specifications.
10
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III. RESULTS AND DISCUSSION
A. Baseline Tests
Prior to initiating the experimental study of combustion modifications,
a baseline NO emission level was established for the combustion unit. This
study was conducted firing at rated capacity, 500 lb/hr, with 20°h excess air and
secondary air preheated to 600°F, representing a situation somewhat typical of
those encountered in conmercial practice. These conditions served as a basis
to define the process conditions and modifications used in later phases of the
program. Tests at baseline conditions were repeated several times throughout
the program with excellent reproducibility. Nitrogen oxides emissions were
1.46 g N0 2 /10 6 cal. Oxides of sulfur were on the order of 1600 ppm. Concen-
tration of hydrocarbons in the flue gas were typically about I ppm.
B. Low Excess Air Tests
The initial phase of the study of combustion modifications dealt with
variations in excess air level. Results of the tests are shown in Table 2. As
indicated, excess air was reduced from a 207. baseline condition, to as low as
27., with resulting nitrogen oxide reduction from 1.46 g N0 2 /10 6 cal to about
0.5 g N0 2 /10 6 cal. These results are shown in Figure 5. Sulfur oxides emissions
appeared to be a function only of the sulfur content of the coal. About 2.57. of
the total sulfur in the coal was found in the ash, the remainder being in the
effluent gas. Variations in sulfur oxide concentration, shown iii Table 2, are
mainly the result of dilution by excess air.
Also shown in Table 2 is the effect of reduced excess air on carbon conversion.
At 207. excess air, 99.57. of the carbon in the coal was burned, while at 27. excess air
carbon combustion efficiency was reduced to 96.2°!.. The effect of reduced excess air
on carbon combustion efficiency is shown in Figure 6.
11
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1.6 —
IJ0 S 10
EXCESS AIR , percent
IS
FIGURE 5.
NO, Formation as a function of excess air.
U
100
98
96
94
92
90
0 5 SO IS 20 25 30
EXCESS A FR, percent
FIGURE 6. Carbon combustion efficiency as a function of excess air.
D l
‘I
20
12
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TABLE 2.- SUMMARY OF TESTS AT VARIOUS EXCESS AIR LEVELS
N0 ,
Excess as ppm Carbon Furnace
air, N0 , gs corrected Ca, SO 2 , combustion outlet
g N0 2 1 10 cal to 0% 0 ppm ppm efficiency, % temperature, °F
20 1.46 694 30 1620 99.5 2025
15 1.17 560 60 1770 98.8 2040
10 0.97 466 100 2030 98.2 2060
5 0.74 327 1000 2150 96.5 2075
2 0.52 253 5000 - 96.2 2100
C. Two-Stage Combustion Tests
A water-cooled second-stage air injector that could be installed at
various positions in the furnace was constructed for the two-stage combustion
studies. As shown in Figure 7 it was equipped with 14 equally spaced nozzles
to provide uniform distribution of second stage air over the entire width of the
furnace. Initially the injector was located at the furnace outlet, point A in
Figure 8. The nozzles were aimed toward the burner wall at a 45° angle from the
horizontal. Results of tests with second stage air introduced in this manner
are shown in Table 3 and Figure 9. Air to the first combustion stage (burners)
was varied from 105 to 80% of the stoichiometric requirement. Overall excess
air as measured in the sampling duct was maintained at 20% in all tests. Nitrogen
oxides emissions were 1.1 g N0 2 /10 6 cal with 1057. to the first stage and 0.77 g
N0 2 /10 6 cal with 80% of stoichiometric supplied to the first stage. These values
13
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Air
inlet
Cooling
water
outlet
.1:-
14- “2 Couplings spaced 6on
center to accept variable
size jets
Coo/ing
water
Figure 7 Water-coo/ed second stage air injector.
-------
Bias firm
ports
Burners
Figure 8.
Points of second stage air 1 t oduct1O
(A,8 and c) and bias firing ports.
15
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1.6 —
1.4 —
0
b
I2
0
U
0 —
U
0
U
o 0.8 -
c0.6
0.4 —
70 80 90 100 110 120
AIR To FIRST STAGE, percent of stoicflfometrlc
FIgure 9. N0 formation wltñ two-stage combustion.
_EPA regulation, coal-fIred plants
0
Probe location - A
Angle • 45’
Excess air 20 %
I I I I I
16
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111(1st h c IIllI)J red wi th I . 46 No 2 / w 6 c i , m s s i !:r rn Convent ton combu St ion
Thu ,, Lwo-stdge combustion wLth 8070 of stoichLometric air supplied to the first
s t ige r ’su I ted in bou t 477. reduction in NO emissions. Furthermore, this
reduc Lion was accomplished with carbon combustion efficiency of nearly 99°! ., j; st
slightLy [ ower than that obtained under conventional combustion conditions in the
tx1)erIIu(nL .I I I iirii.I e
TABLE 3.- RESULTS OF TWO-STAGE COMBUSTION TESTS
(Second stage air introduced at furnace outlet)
Air to
first
sL Ige,
7.
stoich Lu-
metric
g
NON,
N0 9 /lO cal
NOR,
.is ppm
corrected
to 07. 02
CO,
ppm
Hy ro-
carbon,
ppm
S02,
ppm
Carbon
corn-
bustion
effi-
ciency,
7.
Furnace
ouilet
temper-
ature, °F
105
1.20
581
140
1.3
1490
98.9
2050
[ 00
I. [ 3
548
180
0.4
1470
99.0
2050
90
0.88
426
160
0.6
1480
99.0
1970
80
0.77
374
180
0.8
1480
98.8
1950
17
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To investigate the effect of position and angle of air introduction,
the probe was rotated on its axis at each point of introduction. These points
(A, B and C) and angles of air introduction are shown in Figure 8. Since the
provision of 807. of stoichiometric air to the first stage resulted in the most
effective NO reduction in the initial tests, this level was used to study the
effect of position and angle of second stage air introduction. The probe was
installed at points A, B and C, Figure 8, and rotated on its axis through
angles corresponding to positions 1 through 12. Overall excess air was 207..
Results of the combustion tests are given in Table 4. Stable combustion could
not be maintained at several angles. Table 4 therefore contains no data for
these points. Although several positions in Table 4 indicate NO reductions
greater than those obtained in the initial staged combustion tests, these
reductions were achieved only with a sacrifice in carbon combustion efficiency.
The data suggest that the original point of air introduction (A) was probably
the most favorable for optimizing both NO reduction and combustion efficiency.
Observations of the flames were made during the course of these tests.
When second stage air was introduced at an angle at which the air penetrated the
primary combustion zone, combustion intensity increased, resulting in high NO
emissions. Additional1y the flames were forced down along the front sloping
wall, resulting in overheating of the lower furnace section. When the probe
was rotated such that the second stage air was directed toward the rear wall,
some combustion air short circuited to the furnace outlet, resulting in high
arbon loss.
18
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TABLE 4.- SUMMARY OF DATA: Tb )-STAGE COMBUSTION
Carbon
Point of
NOx
corn-
introduc-
as ppm
Hydro-
bustion
tion of 2nd
stage air
Posi-
tion
N0 ,
g N09/10
g 8
cal
corrected
to O7 O
CO,
ppm
carbons,
ppm
effi-
ciency, %
A 1 0.72 350 100 0.8 96
2 0.67 332 100
2¾ 0.76 359 120 0.8 98.8
3 0.74 353 140 0.8
4 0.81 400 200
5 0.74 353 220
11 0.61 296 360 0.8
12 0.63 302 360 0.8 94.6
B 1 0.64 302 220 0.7
2 0.94 453 200 0.4
3 0.94 362 210 0.6 98.4
4 0.79 386 180 0.6 97
5 0.86 416 180 0.5
6 0.74 356 220 0.7
7 0.70 326 240 1.0 96.7
8 0.52 253 200 1.2
9 0.40 193 400 1.3 95
10 0.45 217 320 0.9
11 0.60 278 260 0.8
12 0.60 278 260 0.75
C 1 0.77 374 340 0.7
2 0.74 362 340 0.7
3 0.67 326 280 0.65
4 0.77 374 260 0.6
5 0.76 368 340 0.6
6 0.79 386 240 0.9
7 0.70 338 360 0.65
8 0.54 266 400 0.85
9 0.54 260 420 0.9 94
10 0.59 290 360 0.8
11 0.59 290 380 0.8
12 0.77 374 440 0.8
19
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D. Bias Firing Tests
It would probably be difficult to adapt the method of second stage air
distribution used in the 500 lblhr furnace to large commercial units. As a
result the combination of combustion modifications study was duplicated,
substituting bias firing for staged combustion with second stage air intro-
duced at the furnace outlet. For these tests an air supply system was installed
above the burners to simulate an additional row of burners. The location of
this air supply is shown in Figure 8.
Thus, for bias firing tests, the top row of “burners” was operated
with air only, and the two lower rows of burners were operated fuel rich. Table
5 and Figure 10 present the results of this combustion modification for two
levels of overall excess air. The NO levels were nearly identical to those
obtained in the two-stage combustion tests.
TABLE 5. - RESUI2S OF BIAS FIRINC TESTS
Excess
air, Bias NOx, as
firing’ g N0 2 /10 6 cal
20 80 0.68
5 80 0.63
20 100 1.19
5 100 0.98
1
Percent of stoichiometric air supplied in first
combustion stage.
20
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1.6 —
1.4—
1.2
‘.0-
0.8-
0.6-
0.4—
70
14
b
4,
4.
b
U
0
U
S
0
U
0
0
a
0
— EPA regulation, coal-fired plants
• 20 Z Excess all
g 5 ° Z Excess alr
I I
80 90 100
AIR TO FIRST STAGE, percent of stoicñiometric
Figure /0. NO formation wltñ bias firing.
21
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E. Flue Gas Recirculation Tests
1. Recirculation to Secondary Air
The furnace was provided with a piping system which allowed equal
distribution of recirculated flue gas to the four secondary air supplies just
before entry into the burners.
Results of combustion tests with various percentages of flue gas
recirculation (FGR) supplied to the secondary air are given in Table 6. Percent
FGR is defined as:
7 FGR = 100 x
Wa + Wf
where Wfg = wet mass of flue gas taken from the flue and recirculated
Wa = n ss of inlet air (corrected to 80°F and 607 relative humidity)
Wf = mass of fuel burned.
22
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TABLE 6. - FLUE GAS RECIRCULATION TO SECONDARY AIR
The temperature of the recirculated flue gas was about 300°F. Furnace
outlet temperature decreased with increased FGR. Figure 11, a plot of N0 con-
centration vs flue gas recirculation to secondary air, indicates that N0 reduction
is directly proportional to the 7, FGR, and that about 45% NO reduction was obtained
with 30% FGR. However, as can be seen in Table 6, carbon combustion efficiency was
significantly reduced. With reduced excess air, reductions of N0 on the order of
707, were also achieved with similar carbon loss. It should also be pointed out
that the reduction obtained from FGR to secondary air must be weighed against
increased blower requirement and wear on recirculation components.
Percent
FGR
g
NOx, as
NO /l0 6
cal
NOR,
as ppm
corrected
to 0% 02
CO,
ppm
Hydro-
carbon
ppm
Carbon
combustion
efficiency,
7.
Furnace
outlet
temper-
ature,
°F
0
1.46
694
30
-
99.5
2025
10
1.26
585
140
-
98.8
2020
15
1.19
543
160
0.6
98.2
2005
20
1.00
482
180
0.75
98.2
1980
23
0.91
446
200
0.9
96.2
1950
24.3
0.86
422
200
0.9
96.2
1930
28
0.87
422
210
1.1
96.0
1870
31
0.81
398
240
-
95.9
-
31
0.79
386
260
1.2
-
-
23
-------
I ,
0
a.
0
S
0
U
a
0
U
0
a8
5 10 IS 20 25 30 35 40 ‘15
PERCENT FLUE GAS RECIRCULATION
Figure ii. NO formation with flue gas r.circuiatioy, to secondary air.
24
-------
2. Recirculation to Primary Air
Various amounts of flue gas, together with primary air were used to
sweep the pulverizer. Percent FGR was as previously defined. With conventional
combustion in the furnace, about 157. of total air was supplied as primary air.
For the modification studies, the levels of FGR used were about 6, 9, and 127..
Air displaced by flue gas in the primary stream was added to the
secondary air stream, so that the total standard cubic feet of the primary
stream remained constant over the range of FGR studied. Overall excess air
was maintained at 207, in all cases. Experimental results of the tests are
shown in Table 7 and Figure 12. The data indicate that FGR to primary air
TABLE 7. - FLUE GAS RECIRCULAT ION TO PRIMARY AIR
Hydro- Carbon Furnace
Percent NOx, as co, carbons, combustion outlet
FGR g N0 9 /10 6 cal ppm ppm efficiency, 7. temperature, °F
0 1.46 30 - 99.5 2050
6.6 1.35 90 0.4 99.1 1970
9.3 1.33 90 0.4 99.1 1940
12 1.17 120 0.9 98.4 1950
25
-------
I,
b 1.6-
I
U
00.8-
Exc.ss air = 20 %
cr0.4’
I I I I I I
0
0 2 6 8 10 12 I’i 16
PERCEWT FLUE GAS RECIRCULATION
Figure /2. NO formation witñ flue gas r.circulation to primary air.
26
-------
provided about the same degree of NO reduction as FGR to secondary air. However,
127. was the highest primary FGR obtainable in the 500 lb/hr furnace. Attempts to
exceed 12% resulted in flame instability. There are two probable causes: (1)
lack of sufficient oxygen, since that level of FGR produced a primary stream
containing nearly 857. flue gas by volume, and (2) the huge proportion of hot
flue gas increased the temperature of the primary stream and the expanded volume
caused excessive burner port velocities.
F. Combinations of Combustion Modifications
An extensive investigation was conducted on the effects of combinations
of combustion modifications on emissions. The study included all possible com-
binations of the modifications investigated earlier in the program. Experience
gained in the earlier phases of the program was used to determine ranges of
excess air level, and distribution of air for two stage combustion. The parameters
and their ranges are:
Excess air 5 and 207.
Two-stage combustion 80 and 1007. of stoichiometric to
the first stage
Flue gas recirculation to
primary air 0 and 6%
Flue gas recirculation to
secondary air 0 and 207.
27
-------
The values used for FGR to primary and secondary were selected since
their total was approximately equal to the maximum volume capability of the
FGR blowers. Combustion appeared stable under all combustion conditions.
Experimental results of combination tests, ‘with second stage air supplied at
the furnace outlet, are given in Table 8. Reduced excess air provided further
NO reduction ‘when used in combination ‘with two-stage combustion. Again,
however, the reduction was accompanied by a decrease in carbon combustion
efficiency. Flue gas recirculation provided little further NO reduction
when used in combination with staged combustion and excess air variation.
28
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TABLE 8. - COMBINATIONS OF TECHNIQUES WITH TWO-STAGE COMBUSTION
(Second stage air introduced at furnace outlet)’
Excess
air,
%
Two-
stage
firing 2
Flue-gas
recycle,
prima y
air
Flue gas
recycle
second ry
air
g
NOx,
NO lO
cal
NOR,
as ppm
corrected
to 07. 02
Hydro-
carbons,
ppm
CO.
ppm
Carbon
combustion
efficiency,
7.
Furnace
outlet
temperature
°F
20
5
80
80
0
0
0
0
0.77
0.63
359
261
0.95
0.70
200
160
98.8
93.0
1950
1940
20
5
100
100
0
0
0
0
1.13
1.10
532
463
0.95
0.45
190
130
99.0
97.8
2050
2070
20
5
80
80
6
6
0
0
0.85
0.63
410
263
0.50
-
70
-
98.9
-
-
-
20
5
100
100
6
6
0
0
1.10
1.07
527
448
0.70
0.40
450
200
97.8
95.6
1980
-
20
5
80
80
0
0
20
20
0.75
0.70
353
290
0.98
0.63
340
240
93 54
91.6
1830
1970
20
5
100
100
0
0
20
20
0.96
0.76
468
318
1.65
0.60
400
480
93•94
92.0
1860
1880
20
5
80
80
6
6
20
20
0.74
0.59
358
245
1.00
1.70
190
525
97.1
90.4
1830
-
20
5
100
100
6
6
20
20
1.01
0.83
478
347
-
2.40
125
510
98.9
97.2
1800
1900
‘Second stage air introduced at an angle of 45° from horizontal.
2 Percent of stoichiometric air supplied in first combustion stage.
3 Weight percent of recirculated flue gas to total input products.
4 C.C.E. values appear low, probably due to instability while ash samples were obtained.
-------
The investigation of combinations of combustion modifications was
repeated using bias firing in place of the two-stage combustion mode previously
used since the bias firing techniques was somewhat more representative of what
could be employed on a commercial unit. Experimental results of these combination
tests are given in Table 9.
Examination of the data in Tables 8 and 9 indicate that bias firing
o two-stage combustion used in combination with other techniques are equally
effective. However, there appears to be no advantage to combining bias firing
or two-stage combustion with FGR.
Comparing Tables 8 and 9, it may be noted that some discrepancies
appear in the carbon combustion efficiencies. For example, the efficiency
obtained with bias-firing at 57. excess air and 207. flue gas recycle to secondary
air is significantly lower than that obtained with two stage combustion and the
same conditions. It must be assumed that the ash samples taken for these
determinations were In error, and that the efficiency is actually nearer the
higher value, since other values obtained are nearly identical. The tests were
not repeated because the carbon-combustion efficiency data were not required to
evaluate the tests.
30
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TABLE 9.- COMBINATIONS OF TECHNIQUES WITH BIAS FIRING
Excess
air,
%
Bias
firing 1
Flue-gas
recycle,
primary
air 2
Flue gas
recycle
secondary
air 2
N0 ,
as ppm
NOx,as corrected
g N0 2 /l0 cal to 0% 09
Hydro-
carbons,
ppm
CO,
ppm
Carbon
combustion
efficiency,
7.
Furnace
outlet
temperature, °F
20 80 0 0 0.68 324 0.30 80 98.6 1930
5 80 0 0 0.63 260 0.30 - 93.4 1870
20 100 0 0 1.19 567 0.30 55 98.7 1930
5 100 0 0 0.98 405 0.30 130 97.3 1930
20 80 6 0 0.83 396 0.70 120 97.9
5 80 6 0 0.65 273 0.35 170 96.8
20 100 6 0 1.29 618 0.60 80 98.8
5 100 6 0 1.02 421 0.30 170 98.1
20 80 0 20 0.71 340 0.90 - 95.8 1830
5 80 0 20 0.66 275 - 540 92.1 1910
20 100 0 20 1.05 507 1.25 690 97.3 1860
5 100 0 20 0.80 328 0.55 250 84.9 1840
20 80 6 20 0.72 341 1.00 230 96.9 1830
5 80 6 20 0.53 222 1.75 345 91.7 1880
20 100 6 20 1.04 494 - 350 98.2 1850
5 100 6 20 0.80 333 1.95 250 94•73 1940
‘Percent of stoichiometric air supplied in first combustion stage.
eight percent of recirculated flue gas to total input products.
3 C.C.E. values appear low, probably due to instability while ash samples were obtained.
-------
IV. CONCLUSIONS
Combustion modifications of the type studied in this program should be
adaptable to many existing combustion systems, and could be incorporated into
the design of new units.
Environmental Protection Agency New Source Performance Standards for new
coal-fired steam generating units of over 250 million Btu/hr limit NO
emissions to 1.26 g N0 2 /l0 6 cal. Conventional firing En the experimental
furnace produced NO emissions of 1.46 g N0 2 /10 6 cal.
Operation at low excess air would be satisfactory where a relatively
small reduction is required to meet regulations, i.e., operation at l5 excess
air rather than 2O , result in a 207. reduction in NOx with a 0.77. loss in carbon
combustion efficiency from normal operation. If higher reductions of NOx are
required, they can be achieved via staged combustion or bias firing.
Two-stage combustion or bias firing reduced NOx emissions to 0.77 g 1402/10
cal and 0.68 g N0 2 /l0 6 cal respectively with an overall excess air of 207.. This
corresponded to reductions of 477. and 537. with little effect on carbon combustion
efficiency. When used in combination with reduced excess air (570) two-stage
combustion and bias firing both resulted in N0 reductions of 577.. however, at
this reduced excess air level, carbon combustion efficiency was adversely affected.
32
-------
The indication that bias firing produced NO emission reductions comparable
to that of staged firing with second stage air supplied at the furnace outlet
is significant. Bias firing would probably be more easily achieved in many
existing combustion units (although some load reduction could occur because
of limiting pulverizer capacity).
Flue gas recirculation to primary and secondary air streams was effective
in reducing NO emissions, but increased carbon loss also resulted. It did
not offer any advantage over low excess air firing. In addition, the effective-
ness of flue gas recirculation as an NO control modification must be weighed
against increased blower requirements and wear on the recirculation system.
Combination of combustion modifications do not appear to offer any
significant advantage over staged combustion or bias firing. Apparently the
advantages of individual techniques are not cumulative.
Throughout the program, NO 2 was about 5 to 7% of the total nitrogen oxides
emission.
Sulfur oxides emissions were not influenced by combustion modifications,
except by dilution with excess air. Approximately 2.5% of the total sulfur in
the coal remained in the fly ash, the balance was emitted in the flue gas.
33
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Carbon monoxide levels were generally on the order of 30 to 60 ppm for
conventional combustion tests, and did not exceed 250 ppm for staged ccimbustion
tests. However, with 5% excess air, CO increased to 1000 ppm and with 2% excess
air increased further to 5000 ppm.
Total hydrocarbons emissions were generally low over the entire test
program, on the order of 0.5 to 0.8 ppm in tests conducted at 20% excess air.
Highest values, 2 to 5 ppm, were detected at reduced excess air levels.
ffects of combustion modifications on slagging and conditions arising due
to local reducing atmospheres are difficult to ascertain because of the
relatively short period of operation at a given test condition. It is
recommended that, if possible, these evaluations be made on a large combustion
unit where the unit could be operated for an extended time period.
34
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TECHNICAL REPORT DATA
P!casc read /najucftons on the rci’crse bcjore coin pletingJ
1 REPORT NO. 12.
EPA-350/2-74-038 I
3. RECIPIENTS ACCESSIOFNO.
4 TITLE AND SUBTITLE
Combustion Control of Pollutants from
Multi-burner Coal-fired Systems
5. REPORT OATE
May 1974
6. PERFORMING ORGANIZATION CODE
7 AUTHOR(S)
C. R. McCann, J. J. Demeter, R. B. Snedden, D. Bienstoc]
8. PERFORMING ORGANIZATION REPORT NO.
PERFORMING ORGANIZATION NAME AND ADDRESS
U. S. Bureau of Mines
4800 Forbes Avenue
Pittsburgh, Pennsylvania 15213
10. PROGRAM ELEMENT NO.
LABO14; ROAP 2IADG-8l
11. CONTRACT/GRANT NO.
EPA-1AG020(D)
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
ERC-RTP, Control Systems Laboratory
esearch Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final- - Through 2/1/74
14• SPONSORING AGENCY CODE
15 SUPPLEMENTARY NOTES
16 Ru Ir1MCT
P he report gives results of an investigation, utilizing an experimental 500-lb/hr
nulti-burner pulverized-coal-fired furnace, to determine the effects of several
combustion modifications on nitrogen oxide (NOx) emissions. Techniques investigated
were: reduced excess air, staged combustion, flue gas recirculation to both primary
md secondary combustion air streams, and combinations of all techniques. Reducing
xcess air from 20 to 2 percent reduced NOx emissions by about 70 percent; however,
he NOx reduction was accompanied by a significant decrease in carbon conversion
it the lower excess air levels. Staged combustion reduced NOx emissions by about
5C percent, with little increase in carbon loss over conventional combustion. Using
30 percent flue gas recirculation also reduced NOx emissions by about 50 percent,
)Ut was accompanied by a significant reduction in carbon combustion efficiency,
;imilar to that experienced in the low excess air tests. Various combinations of the
ndividual NOx reduction techniques were tested but, since the effects were not
mmulative, they did not offer any advantage.
Ii. KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IOENTIFIERSIOPEN ENDED TERMS
C. COSATI FICId/Group
Ur Pollution
itrogen Oxides
ombustion
oal
!urnaces
ftoichiometry
Flue Gases
Air Pollution Control
Stationary Sources
Combustion Modification
Reduced Excess Air
Staged Combustion
Flue Gas Recirculation
3B
)7B
1B
1D
3A
07D
13 DISrAIRUTION STATEMENT
.
Unlimited
19 SECURITY CLASS (This Report)
Unclassified
21. .OF PAGES
‘
20 SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220.1 (9.73)
35
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