United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-84-075 Sept. 1984
Project Summary
Combustion Modification Tests
on a Subscale Cement Kiln for
NOX Reduction
W.A. Carter and R.C. Benson
Field tests were conducted on a
subscale rotary cement kiln to evaluate
the effect of various combustion
modifications on gaseous emissions.
The test program was conducted with a
research kiln donated by a cement
company. The test unit was 8.2 m (27
ft) long and 0.38 m (15 in.) inside
diameter and will produce 68 kg (150
Ib) of clinker per hour. A coal burner was
designed and built for the test program,
and the kiln was reworked to incorporate
heated secondary air and flue gas
recirculation (FGR). The effect on NO
emissions of FGR, diluted primary air,
primary/secondary air ratio, burner tip
velocity, and secondary air temperature
were evaluated. The most effective
combustion modification for coal firing
was determined to be dilution of
primary air with inert gas which reduced
NO by nearly 40 percent when the
primary-air oxygen concentration was
reduced to 12 percent.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory. Research Triangle
Park. NC, to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
KVB has reported on previous EPA-
sponsored sub- and full-scale cement kiln
combustion modification tests that
evaluated the effects on NO* emission of
several types of modifications including air
preheat, fuel injection velocity, and
oxygen content of the primary combustion
air stream. Tests in a combustion tunnel
simulating kiln conditions showed dilution
of the primary air stream with an inert
gas, altered fuel injection velocity, and
combustion air preheat to be most
effective in lowering the NO emission
level.
This report summarizes results of tests
of a subscale rotary cement kiln firing
natural gas and pulverized coal. The kiln
was modified to incorporate flue gas
recirculation (FGR) and combustion air
preheat. A coal burner wasfabricated and
installed in the kiln. The FGR system was
designed to inject flue gas into the
secondary combustion air stream or into
a tertiary air stream in the burner.
The overall objective of the contract,
under which the work reported here was
conducted, was to investigate advanced
combustion modification concepts re-
quiring relatively minor hardware modifi-
cations that could be used by operators
and/or manufacturers of industrial
process equipment to reduce NOX emis-
sions. Another objective was to investigate
the feasibility of these modifications to be
readily adopted by equipment manufac-
turers and operators.
The specific object of the test program
reported here was to evaluate FGR,
vitiated primary air, primary/secondary
air ratio, temperature of combustion air,
and coal-carrier air velocity as they affect
emission of NOX in a subscale rotary
cement kiln provided by a major cement
manufacturer. The test results were to be
compared with previous laboratory-scale
tests and recommendations formulated
for tests at larger scale.
The initial phase of testing was
conducted with natural gas fuel to define
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kiln operation over a range of operating
variables; e.g., excess air, primary-to-
secondary-air ratio, and combustion air
preheat. When the performance of the
kiln was documented with natural gas
fuel, firing was changed to coal. Kiln NO
emissions were evaluated as a function
of combustion air temperature, primary
air velocity, primary-to-secondary-air
ratio, FGR rate, and primary (carrier) air
oxygen content.
Previous Work
The present program is a follow-on
study, intended to build on the results of
an earlier program in a laboratory
combustion tunnel. The objective of the
earlier effort was to determine the effect
of burner parameters on near-flame NO
levels for both natural gas and coal fuels.
This laboratory effort determined that
combustion air preheat, fuel injection
velocity, and oxygen content of the
primary combustion air stream have first-
order effects on NO levels. These
parameters were then selected for
implementation on the subscale kiln.
Program Approach
During the earlier program, several
subscale test facilities were visited and
evaluated. A test program was presented
to the operators of these facilities, and
estimates of test costs were received
from each. The selected subscale kiln
offered the most operational flexibility
and opportunity for the maximum data at
minimum cost.
Prior to the subscale tests, KVB
conducted laboratory tests in a kiln-
simulation furnace to define key param-
eters influencing NO formation in the
near-burner region. The data gained from
this program were used to design the
subscale test plan.
A coal burner and feed system were
built and installed at the test site, and the
kiln was modified to incorporate combus-
tion air preheat and flue gas recirculation.
The initial portion of the test program was
conducted firing natural gas to define kiln
operation over a range of operating
variables; e.g., excess air, primary-to-
secondary-air ratio, and combustion air
preheat.
When the performance of the kiln was
documented with natural gas fuel, firing
was changed to coal. Kiln NO emissions
were evaluated as a function of combus-
tion air temperature, primary air velocity,
primary-to-secondary-air ratio, tertiary
air injection FGR rate, and primary-air
oxygen content.
Tables 1 and 2 summarize the signifi-
cant results obtained from the test
program for natural gas and coal firing,
respectively.
Conclusions
The following conclusions were made
from the field tests and the analyses of
the data:
• The subscale tests corroborated
laboratory combustion tunnel test
results. Combustion tunnel tests
indicated that reducing the coal
carrier air oxygen content was a
promising method of NO reduction,
and the subscale tests verified this.
Figure 1 compares the data taken
during the laboratory program with
those from the subscale kiln tests.
The data trends are similar: the kiln
was more sensitive to carrier-gas
oxygen content probably due to
higher secondary air preheat tem-
perature.
• The subscale kiln tests showed that
NO emissions are quite sensitive to
kiln operating 02 level. Figure 2
shows the effect of kiln oxygen level
on NO emissions for 1100°F (593°C)
secondary air temperature. This
figure shows that a 1-percent
reduction in Oa level results in a NO
reduction of about 63 ppm.
• Flue gas recirculation (FGR) was
found to be more effective with gas
firing than with coal firing. Gas
firing reduced NO by about 20
percent from the baseline condition
at a FGR rate of 34 percent. Coal
firing reduced NO by only 7 percent
from the baseline with 21 percent
FGR.
• Diluting coal carrier air with an inert
gas reduced NO by about 30 percent
from baseline conditions. Diluting
the primary air (coal carrier gas) was
found to be the most effective
combustion modification.
• Further evaluation of the dilution of
coal carrier air in larger scale kilns
should provide valuable information.
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Table 1. Summary of Test Results: Gas Firing - Subscale Cement Kiln*
SA Temp
Test Series"
i Primary
Air
%FGR
Fuel
MW
Firing Rate
(W6 Btu/hr)
Kiln Exit
Oz
<%. dry)
NO fppmv, @
3% Oz, dry)
Maximum %
COz NO Reduction
(%, dry) from Baseline
PA/SA Variation Baseline
(20/80J
PA/SA Variation Baseline
(33/67)
SA Temp.
Variation
FOR
Baseline
Baseline
1100 867
900 756
600 589
300 422
Amb.
1200 922
600 589
300 422
Amb.
1000 811
900 756
800 700
700 644
Amb.
1168 904
19.1
33.0
14.6
28.0
0
21.6
28.6
32.6
33.8
0.21
0.21
0.19
0.19
(0.70)
(0.70)
(0.64)
(0.64)
2.0
2.0
2.0
2.0
640
600
400
243
228
300
194
180
170
404
419
364
350
212
340
163
135
110
100
11.2
11.5
14.2
15.8
64.4
43.3
47.5
70.6
*AII CO data for the nominal Oz's listed are less than 200 ppmv.
*PA = Primary Air; SA = Secondary Air; FOR = Flue Gas Recirculation.
Table 2. Summary of Test Results: Coal Firing - Subscale Cement Kiln
Date
(1982)
7/27
7/27
8/04
7/29
8/12
8/12
8/12
8/12
8/12
8/11
7/29
8/04
8/04
8/04
8/04
8/10
8/10
8/10
8/12
8/12
8/12
SA Temp."
°F
1099
1003
1145
1167
1162
1158
1166
1169
1161
1187
1167
1153
1130
1144
1160
1140
1160
1140
1160
1160
1160
(K)
(866)
(816)
(892)
(904)
(901)
(899)
(903)
(905)
(900)
(915)
(904)
(896)
(883)
(891)
(900)
(889)
(900)
(889)
(900)
(894)
(900)
PA Velocity"
fpm
3338
3338
3338
3232
2950
2504
2285
2128
2504
2915
3232
3268
3440
3390
3440
3120
2890
3120
2500
2500
2500
(m/s)
(17)
(17)
(17)
(16.4)
(15)
(12.7)
(11.6)
(10.8)
(12.7)
(14.8)
(16.4)
(16.6)
(17.5)
(17.2)
(17.5)
(15.8)
(21.7)
(15.8)
(12.7)
112,7)
(12.7)
%PA
28.4
27.9
29.4
25.8
21.0
19.3
17.1
16.2
9.3
22.2
25.4
29.8
30.1
29.7
30.1
22.5
21.7
22.5
19.3
18.4
19.7
Avg. Oz
(%)
3.5
3.7
3.8
3.6
3.7
3.6
4.1
5.3
4.3
3.6
3.6
4.1
5.4
5.0
5.6
2.9
3.7
2.3
5.0
4.0
3.8
Avg. NO
Avg. CO fppm corrected COz
(ppm) to 3% Oz. dry) (%)
25 753
35 794
559 826
125 848
148 806
396 643
303
963
293
608
136
268
452
79
167
481
104
550
188
87
708
773
711
775
822
848
847
836
788
727
700
613
771
635
440 544
16.4
15.4
14.6
15.4
15.5
17.8
15.3
15.5
17.6
15.1
15.8
14.6
13.8
16.4
13.7
15.3
13.5
16.2
17.5
17.9
17.6
Comments
Evaluate the effect of SA temperature on NO
emissions; PA = constant Oz = constant.
Velocity variation; varied primary air flow.
SA &P was relatively constant.
PA/SA variation tests. Varied PA flow and SA
flow at constant Oz.
Baseline for series to evaluate FGR.
19% FGR*
21% FGR
Baseline for PA dilution tests\
18% O2 in PA >
12% Oz in PA 1
Baseline for PA dilution tests)
15%OzinPA }
12% Oz in PA )
O%FGR
High PA
Velocity
Low PA.
Velocity
*SA = Secondary Air; PA = Primary Air; FGR = Flue Gas Recirculation.
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1000
900
800
700
600
<5 50°
£ 400
§
300
200
too
Subscale Kiln
Results
Laboratory Tunnel
Results
Western Bituminous Coal
Coal Nitrogen Content = 1.1%
Kiln Exit O2 = 4.0%
Combustion Air Temperature = 894 K (1150°F)
\
A
I
2.5
7.5 10 12.5 15 17.5
Primary Air Oxygen Content, %, dry
20
22.5 25
Figure 1. Effect of primary-air oxygen content on NO emissions.
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1200
1100
1000
900
800
700
600
500
400
300
200
700
I I I I I I
Western Bituminous Coal
Secondary Air Temperature = 567 K (1100°F)
I
I
456
Kiln Exit Oa %, dry
9 10
Figure 2. Effect of kiln oxygen level on NO emissions.
W. A. Carter andR. C. Benson are with KVB, Inc., Irvine, CA 92714.
Robert E. Hall is the EPA Project Officer (see below).
The complete report, entitled "Combustion Modification Tests on a Subscale
Cement Kiln for /VO« Reduction," (Order No. PB 84-223 502; Cost: $10.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC27711
•frl). S. GOVERNMENT PRINTING OFFICE: 1984/759-102/10659
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Environmental Protection
Agency
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