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Environmental aspects of low Btu gas combustion
ENVIRONMENTAL
ASPECTS
OF LOW
BTU GAS COMBUSTION
M. P. HEAP, T. J. TYSON, J. E. CICHANOWICZ, R. GERSHMAN, AND C. J. KAU Uhrasystems, Inc., Irvine, California AND
G. B. MARTIN AND W. S. LANIER United States Environmental Protection Agency, Research Triangle Park, North Carolina Nitrogen oxide emissions from power plants burning low Btu gas derived from coal were investigated using a kinetic model which included over 100 reactions. The model was validated by comparison with the best available experimental data and then applied to idealized combustor configurations for two representative combined cycle systems: (1) an advancedtechnology high-temperature gas turbine with a waste heat boiler, and (2) a supercharged boiler with a current-technology gas turbine. In both cases staged combustion schemes involving rich primary zones and controlled mixing secondary zones were found to provide for minimum NO~ emissions. The calculated minimum NO x levels (corrected to three percent 02, dry) were 105 ppm for the high-temperature turbine case and 100 ppm for the supercharged boiler case. These results indicate that combustor design has a strong potential for controlling NOx from advanced systems fired with ammonia-containing low Btu gas. 1. Introduction The United States is expected to place heavy reliance on the use of coal in order to reduce the amount of i m p o r t e d oil. Although there are a b u n d a n t supplies of coal in the United States, it is perhaps the least desirable fuel from an environmental aspect. Consequently, several options are b e i n g explored w h i c h will allow the increased use of coal without degradation of the environment. While each of these options can potentially decrease emissions of sulfur oxides to acceptable levels, it is also important to consider the associated emissions of nitrogen oxides (NOx). This p a p e r is concerned with the NO~ aspects of one option, the use of coal gasifier off-gas in c o m b i n e d cycle systems for the p r o d u c t i o n of electricity. Technical and e c o n o m i c obstacles limit direct fossil fuel fired steam generating plants to overall efficiencies in the range of 33 to 39 percent. To be competitive, coal-gas fired generating plants will require d e v e l o p m e n t of i m p r o v e d industrial gas turbines. 1 These turbines can then be used in advanced high efficiency power cycles w h i c h will more than compensate for losses suffered in the processes 535
involved in the conversion of coal to a clean fuel. The energy losses associated w i t h coal gasification must be r e d u c e d to a m i n i m u m if the overall energy conversion is to prove economically attractive. T y p i c a l product gases leave the gasifier with a sensible heat w h i c h is equivalent to 10-15 percent of the heat of c o m b u s t i o n of the coal. T h e raw product gas m a y then pass through a high temperature clean-up process a n d into a c o m b i n e d cycle system with u n d i m i n i s h e d temperature in order to utilize all of the sensible heat effectively. Thus the characteristics of hot clean-up systems are most important with regard to energy conversion efficiency. Although identified b y the single term low Btu gas, the nature of the fuel is as varied as the processes used to p r o d u c e it. Quantities of a m m o n i a in the p r o d u c t gas a n d the fate of this a m m o n i a in sulfur clean-up processes are unclear at this time. Robson et al 2 estimate up to 600 p p m of a m m o n i a in a low Btu gas with low temperature h y d r o g e n sulfide cleanup and 3,800 p p m from high temperature clean-up processes w h i c h are u n d e r development.
536
ENERGY PRODUCTION FROM COAL
A large body of experimental information is available concerning the conversion of ammonia to nitric oxide in combustion systems. a,4,5 Under premixed conditions the amount converted is dependent upon mixture stoiehiometry and can be as high as 80 percent. In diffusion flames up to 50 percent of the ammonia can be converted to NO x depending on the method of ammonia addition and the rate of f u e l / a i r mixing. Consequently, the use of a high temperature H 2S clean-up system may well provide a fuel gas with the potential to produce more NO x than the parent coal. This paper addresses the question of whether combustor design can provide for significant reduction of NO~ emissions when burning low Btu gas with high ammonia content. The approach used here was to develop and validate a model capable of predicting NO~ formation rates for fuels consisting of carbon monoxide, hydrogen and methane with trace quantities of ammonia and then to apply this model to a series of "limit cases" based on operating constraints imposed by practical systems. Although macro- and micro-scale mixing phenomena will dictate the NO~ emission levels in any practical system, modeling the chemistry of the combustion process can establish upper and lower limits on the emission levels and thus give an indication of the feasibility of direct use of high ammonia fuel gases. 2. A Kinetic Model for NO Formation in Ammonia Containing Fuel Gases An essential component of the analytical tool used to assess the NO emission potential of low Btu gas fired combustors is a kinetic model capable of describing both NO formation and destruction in flames. This kinetic model should be able: - - t o describe the heat release mechanism for the fuel gases commonly found in low Btu gases (in this work the fuel components were restricted to Ha, CO and CH 4 ); - - t o predict both thermal and fuel NO formation when the fuel nitrogen compound is ammonia; - - t o predict the destruction of nitric oxide under fuel rich conditions. The experimental data used to assess the validity of the model was that reported by Bartok et al. a for NO formation in a Longwell well-stirred reactor. This data was chosen because the combustion rates were limited by chemical kinetics as opposed to transport el-
fects and experimental data was available for methane air mixtures with and without the addition of NO or NH a. The data has two limitations as far as the present investigation is concerned: all the experiments were conducted at approximately atmospheric pressure and no nitrogen species other than NO~ were determined. Engleman et al. 6 have reported calculations of NO~ emissions from the same experimental series. These workers were successful in matching calculated and experimental values for carbon monoxide and hydrogen; however, for p r o p a n e / a i r combustion the calculations underpredicted exhaust NO concentrations by a factor of four for fuel lean conditions and an order of magnitude for fuel rich conditions. The present investigation followed upon the work of Waldman, Wilson and Maloney 7 who also attempted to predict the data of Bartok et al for m e t h a n e / a i r combustion. Waldman was able to obtain reasonable predictions using a 33 reaction set which included a reaction sequence of the type R + N2---~ ... ---~NO where R is a hydrocarbon fragment. For the present study, the original master set listed by Waldman et al was used as the basis for the generation of a new larger set of reactions which included those reactions relevant to ammonia oxidation as well as those concerned with the formation and destruction of nitric oxide in combusting m e t h a n e / a i r mixtures. Thus the reaction set includes reactions CHz + Nz---~ CHN + NH
(1)
CH + N 2 ---, CHN + N
(2)
because they are of the type suggested by Fenimore s to account for rapidly formed NO in rich hydrocarbon flames. Myerson 9 h a d reported results showing that nitric oxide contained in simulated combustion products could be reduced by the addition of hydrocarbon fuels and reactions of the type suggested by him to account for this phenomena were included: CH + N O ~
CHN + O
(3)
CH + N O ~
CHO + N
(4)
A complete list of the reactions and the reaction rate constants used in the subsequent calculations is presented in the Appendix. Most of the rate data was based on either the recent
ASPECTS OF GAS COMBUSTION
537
RESIDENCE TIMES
1300 PPM NH 3
EXPERIMENTAL DATA
o EXPERIMEi~TAL DATA
i00
o
1.3 MSEC.
0
1.5 MSEC.
i00
9 CALCULATIONS
0 1.8 MSEC. V
9
8
2.3 MSEC.
0
/', 3.0 MSEC
~0 5O
CALCULATIONS
~7
:E
~o
0
0
9 2.0 MSEC. 0
[]
/x 0 ,
50
9
8e 9
-~-O
,
,
50
,
,
,
,
/
i00
c> o o~o, 150
0 50
200
100
150
200
PERCENT STOICHIOMETRIC AIR
PERCENT STOICHIOMETRIC AIR
Fro. 1. Validation of the model by comparison with data of Bartok 3 for methane/air in jet-stirred reactor.
Flc. 3. Validation of the model by comparison with data of Ba~ok a for methane/air doped with ammonia in jet-stirred reactor.
survey b y E n g l e m a n 1~ or the estimates of Benson et a l . " Where rate data was not available in the literature, it was estimated from b o n d energy data using the m e t h o d of T u n d e r et al. a2 Figures 1 through 4 present comparisons b e t w e e n the Bartok data a n d calculations for a 2 msec perfectly stirred reactor with a specified heat loss of 0.028 c a l / s e c ~ The calculations were carried out using a fully implicit numerical technique d e v e l o p e d b y Tyson 13 w h i c h allows the integration of the stiff differential equations w h i c h govern the kinetic system. Initially it was necessary that some of the rate constants be adjusted slightly to obtain the results p r e s e n t e d in Fig. 1. Reaction rates were varied w i t h i n the limits of accuracy
given b y E n g l e m a n and Benson. F a i r l y good agreement was also o b t a i n e d b e t w e e n measured and calculated stirred reactor temperature (Fig. 2). W i t h o u t m o d i f y i n g the reaction set in any way, calculations were then made for the cases where N H 3 or NO was a d d e d to the premixed m e t h a n e / a i r mixture feeding the reactor. The results presented in Figs. 3 and 4 show that the kinetic model was capable of simulating both the conversion of a m m o n i a to nitric oxide and the reduction of nitric oxide b y rich c o m b u s t i b l e mixtures. Although the kinetic m e c h a n i s m model a p p a r e n t l y predicts NO X emissions from the well-stirred reactor adequately there is one gap in the experimental data w h i c h prevents complete acceptance of the model. Reactions 1,
3500 RESIDENCE TIMES EXPERIMENTAL DATA B 1.3 NSEC. 0 1.8 MSEC. A 5.0 MSEC. CALCULATIONS
3000
"_ 0~
9
-
Z~&
9
'-
~9
[
~
~-
A 0
z o r--
I 9 2,0 MSEC.
O0 O00O
i00
50
u.l
o
EXPERIMENTALDATA
o 9
o 200 PPMNO
o 2500
/', 1300 PPMNO
0
CALCULATIONS 9 1300 PPMNO
,A.
50
100
150
200
PERCENT STOICHiOMETRIC AIR
Fro. 2. Validation of the model by comparison with data of Bartok 3 for methane/air in jet-stirred reactor.
0
50
i00
150
200
PERCENTSTOICHIMETRICAIR
FIC. 4. Validation of the model by comparison with data of Bartok 3 for methane/air doped with nitric oxide in jet-stirred reactor.
ENERGY PRODUCTION FROM COAL
538
TABLE I Fuel composition--potential air blown Lurgi gas Specie
Mole %
H20 Hz CO CO~ CH 4 N2
10.1 19.6 13.3 13.3 5.5 37.6
2 and 3 all produce H C N and the calculations indicate considerable quantities of H C N in the exhaust from the well-stirred reactor. No quantitative data is available for species other than NO x and it is not possible to check the accuracy of the prediction. However, since HCN has been observed in the primary reaction zone of rich premixed flames a n d in quantities greater than the NOx concentration,5 it appears that the kinetic model is qualitatively correct in this regard.
3. NO, Emissions from Low Btu Gas Combustors The o p t i m u m use of low Btu gas for power generation in c o m b i n e d cycles depends u p o n allowable gas t u r b i n e inlet temperature. Robson et al. z identified the high temperature gas t u r b i n e / w a s t e heat boiler combination as the most promising concept for t u r b i n e inlet ten> peratures in excess of 2600~ (Although it is EPA policy to use metric units, temperatures have been expressed in English units here for the convenience of the reader.) C o n v e n t i o n a l industrial gas t u r b i n e technology is limited to turbine inlet temperatures which are considerably lower than this in which case a supercharged boiler plus gas turbine appears to offer the most advantageous use of low Btu gasJ 4 Thus two types of combustor have b e e n conceptually modeled: 1) an adiabatic gas
FUEL
1
generator with outlet temperature of 2800~ suitable for use with high temperature fuel gas, and 2) a supercharged boiler fired b y low temperature fuel gas. The objective of the model calculations was not to simulate a specific piece of hardware, but to examine simple idealized flow situations to indicate potential emission levels. The calculations were carried out with a fuel gas which is considered representative of a potential second-generation air-blown Lurgi system whose major constituents are listed in Table I. This particular fuel gas was used because of its projected near-term availability and its high methane content relative to presently available gasifiers. It was assumed that all H 2 S and COS was removed from the raw product gas by an unspecified process w h i c h gave a m m o n i a concentrations of 4000 p p m in a 1500~ fuel gas a n d 500 p p m in a low temperature fuel gas. 3.1 Adiabatic Gas Generator An exit temperature of 2800~ from the combustor was selected because it was considered that this was representative of the most severe case for N O X control. I n an adiabatic combustor this corresponds to an overall equivalence ratio of 0.45 if a fuel temperature of 1500~ and an air temperature of 1000 ~ are assumed. Preliminary calculations indicated that the autoignition temperature was ap'proximately 1400~ for a broad range of f u e l / a i r ratios, thus an ignition source was required. Figure 5 presents a generalized schematic of a simple idealized reactor sequence used in the numerical experiments. A 1 msec stirred reactor provides the ignition source for a primary plug flow reactor which feeds a secondary plug flow reactor. Calculations for a case with no staging, complete mixing a n d an equivalence ratio of 0.45 indicated that 86 percent of the a m m o n i a contained in the fuel gas would be converted to NO giving a con-
SECONDARY AIR
llltl
?i TTi
TERTIARY AIR
r
FIG. 5. Reactor sequence used to model adiabatic gas generator.
ASPECTS OF GAS COMBUSTION centration of greater than 900 p p m of NO in the combustion products after 500 msec. Any residual nitrogen c o m p o u n d s remaining at the exit of the p r i m a r y combustor are likely to be converted to nitric oxide d u r i n g secondary b u r n o u t and d i l u t i o n as the products are brought to the desired exit temperature.
Primanj Zone Optimization As indicated earlier, the kinetic model allows the synthesis of H C N t h r o u g h reactions 1, 2 and 3; thus the major nitrogen species (excluding N 2) present at the e n d of the p r i m a r y zone reactor are residual NH3, H C N a n d NO. T h e concentrations of N O 2, N z O and nitrogen containing radicals are negligible. Numerical experiments were c o n d u c t e d to determine the o p t i m u m equivalence ratio and residence time in the primary reactor for m i n i m u m nitrogen species. F i g u r e 6 shows the sum' of the mole fractions of the nitrogen containing species n o r m a l i z e d b y the initial a m m o n i a mole fraction (NH3) o plotted against equivalence ratio at various residence times. F o r lean mixtures thermal NO p r o d u c t i o n dominates with peak value occurring at a p p r o x i m a t e l y q~ = 0.9 ( T = 3600~ A m i n i m u m residual N O fraction is shown for rich mixtures with equivalence ratios of 1.4 and its m a g n i t u d e decreases w i t h 2.5
i
i
i
~
i
i
i
539
time. Further calculations indicated that for 0.8 -< q~ -< 1.7 a n d t -< 200 msec the absolute value of the sum of the nitrogen fractions is i n d e p e n d e n t of the initial a m m o n i a content. For ~ --- 1.1 this is due to the reactor achieving near e q u i l i b r i u m conditions. For rich mixtures the kinetic model indicates the interchange of NO, H C N and N H through the formation of nitrogen atoms. H C N concentrations are i n d e p e n d e n t of a m m o n i a d o p i n g a n d a m m o n i a can be synthesized in the absence of initial d o p i n g due to a chain w h i c h is initiated b y the p r o d u c t i o n of N H via reaction 1. Based u p o n these results it was d e c i d e d that a primary reactor w i t h 1.33 -< s -< 1.45 a n d residence time in excess of 200 m s e c w o u l d be the o p t i m u m configuration.
Secondary Zone Optimization F o u r methods of a d d i n g the secondary dilution air to the exit of the o p t i m u m primary reactor were considered (see Fig. 5): 1) instantaneous m i x i n g of all the secondary air with the p r i m a r y products at the beginning of the secondary region; 2) mixing the dilution air u n i f o r m l y into the primary products over a 50 msec period; 3) mixing the p r i m a r y products into the dilution air u n i f o r m l y over a 50 msec period; and
i
500 MSEC
1000
AIR ADDEDOVER50 MSEC
4000
2.0
vZ~l. 5
:~
T
3000
z + m
~- I!
+ 1.0
o
1 MSEC
0.5
50 MSEC 200 MSEC 500 MSEC o
L
o.N
t
o'.8
'
1'.2
'
1'.6
'
2.o
'J
I i
,1 ,'/t 1 ,
i /"
./
I 1 ~, "UEATEDO~US~ONFLAME /
F ~NTO _ _ '_~US " "XI N I G_..
I
U ....
t
ZOO
N ISTANTANEOUS
MIXING
<~ ~-
I i000
EQUIVALENCE RATIO
FIG. 6. Optimization of primary zone residence time and equivalence ratio for adiabatic gas generator firing Lurgi low Btu gas. Air temp. = 1000~ fuel temp. = 1500~ pressure = 10 atm, fuel ammonia concentration = 4000 ppm.
TIME AFTER INITIATION OF SECONDARYAIR INJECTION (MSEC)
F]c. 7. Effect of secondary zone mixing method on NO formation in adiabatic gas generator firing Lurgi low Btu gas.
540
ENERGY PRODUCTION FROM COAL HEAT TRANSFER FUEL , f IGNITION S O U R C E [ ~ . ~
AIR
PRIMARY ZONE
$iiii.
LLLLL
HEAT TRANSFER
SECONDARY AIR
FIG. 8. Reactor sequence used to model supercharged boiler.
4) a d d i n g the fuel to the air stream b y a series of 3 m s e c stirred reactors f e e d i n g into the secondary air stream at three stations in order to model diffusion zone effects. (The three previous cases assumed instantaneous mixing.) The results of calculations with these four methods of air introduction following a q~ = 2.0 primary zone are presented in Fig. 7. M i n i m u m final N O levels are obtained w i t h methods 1 and 3 while m a x i m u m emissions occur with m e t h o d 2. T h e difference in endssion levels can be explained b y c o m p a r i s o n of the temperature curves. W h e n the air is a d d e d to the fuel the mixture progresses from rich to lean giving peak temperatures in excess of 3600~ with the subsequent formation of thermal NO. This c o u l d be prevented b y extracting heat from the primary zone w i t h the dilution air, thus r e d u c i n g peak temperatures during burnout. T h e 3 msec stirred reactor "flame" simulators gave a final N O level approximately one-half that p r o d u c e d b y progressively mixing the air with the fuel. Calculations for a ~p = 1.33 primary zone followed b y a m e t h o d 1 secondary zone y i e l d e d an NO x emission of 105 p p m (corrected to three percent 0 2 , dry). While method 1 m a y be difficult to a p p r o a c h in a practical system, Fig. 7 indicates that similar results c o u l d be obtained using m e t h o d 3 w h i c h could be followed in practice. 3.2 Supercharged Boiler Supercharged boilers have been used in Europe for some time; they offer the advantage that their compact size reduces capital costs. The idealized m o d e l used to simulate a supercharged boiler c o m b u s t o r is shown in Fig. 8. The heat extraction rate was uniform for the primary zone a n d was arbitrarily assigned a distribution in the secondary zone with the peak rate occurring at one quarter of the residence time w i t h a m a g n i t u d e equal to three times the m i n i m u m rate. Without staged corn-
bustion and a s s u m i n g complete premixing, NO emissions were f o u n d to be 350 ppm. T h e objective of the numerical experiments was to assess h o w to a p p l y staged c o m b u s t i o n techniques to o b t a i n m i n i m u m emissions.
Prima~ Zone Optimization The kinetic m o d e l was used to determine the o p t i m u m c o m b i n a t i o n of primary equivalence ratio and residence time for the fuel identified in T a b l e I with an a m m o n i a content of 500 ppm. F u e l and air inlet temperatures were 150 oF and 600 ~F respectively. M i n i m u m residual nitrogen content at the end of the primary zone was obtained with an equivalence ratio of 1.15. A 500 msec residence time was selected to p e r m i t decay of NO concentration (see Fig. 9). Heat extraction lowering the primary zone exit temperature b y 2 0 0 ~ was found to have little effect on the residual nitrogen concentration for these conditions. The conditions in the primary zone are most d e p e n d e n t u p o n the h y d r o c a r b o n content of
[ ' 1 3 0 1 -
'
.~'PRIMARYZONE' ' ' ~ SECONDARY AIR ADDITION
3100 2700 2300
~ L~
rn
~1
7O
1900
50
1500 ~CN
30
ii00
10
7OO 100
300
500
700 900 TIME (MSEC)
1100
13'00
1500
F1G. 9. NO formation for optimized staging in supercharged boiler firing Lurgi low Btu gas. Air temp. = 600~ fuel temp. = 150~ pressure = 10 atm, fuel ammonia content = 500 ppm.
ASPECTS OF GAS COMBUSTION 150
I
counts for the more r a p i d b r e a k d o w n of ammonia. Further consideration is required regarding the influence of higher h y d r o c a r b o n and sulfur-containing species, b u t this was b e y o n d the scope of the work d e s c r i b e d here.
I
5.5;I CH4 IN FUEL ---
\
0.0% CH4 IN FUEL
!
\
541
Secondary Zone Optimization
i00
A variety of secondary m i x i n g schemes were considered based on an overall equivalence ratio of 0.95. A m i n i m u m NO concentration was obtained when the air was a d d e d gradually over a 125 msec period (see Fig. 9). Correcting to three percent O~, dry, gives an emission level of 100 p p m for this case.
\ c~
".
NO
50
4. Conclusions NH3_ .- --.z.:.S.LL/"
-
- ........
HCN
0 0
i00
200 300 TIME (MSEC)
4DO
500
Flc. 10. Effect of hydrocarbon content on NO formation in supercharged boiler firing low Btu gas. Air temp. = 600~ fuel temp. = 150~ fuel ammonia content = 500 ppm, equivalence ratio = 1.33.
the fuel gas. This is illustrated in Fig. 10 w h i c h shows the concentration of NO in a rich p r i m a r y p l u g flow reactor as a function of time. T h e solid lines refer to calculations made with the fuel listed in T a b l e I. The calculated concentrations shown dotted in Fig. 10 were m a d e assuming an instantaneous conversion of methane to carbon monoxide, thus preventing the formation of b o t h h y d r o c a r b o n radicals a n d HCN. Table II compares O, O H and H concentrations at the outlet of the 1 msec ignition reactor for the two cases. The increased concentration of these radicals ac-
TABLE II Concentrations of O, OH and H at the outlet of the 1 msec ignition reactor
O OH H
Fuel as shown in Table I
Instantaneous conversion of CH 4 to CO and H 2
9.5 ppm 1.09 ppm 1.61 ppm
33 ppm 608 ppm 1200 ppm
F o r both concepts s t u d i e d it was f o u n d that careful design of the combustor can contribute significantly to reduction of NO~ emissions from low Btu gas systems. Staging of the c o m b u s t i o n process was f o u n d to b e desirable in both cases. The o p t i m u m design for an adiabatic gas generator b u r n i n g high a m m o n i a content fuel gas was f o u n d to be a rich (1.33 < q~ < 1.45) primary reactor section with a residence time of at least 200 msec f o l l o w e d b y a gradual mixing of the p r i m a r y products into the dilution air stream. NO emissions for such a system could potentially be lower than those for an unstaged system b y an order of magnitude. F o r the supercharged boiler case the o p t i m u m configuration consisted of a rich p r i m a r y zone (~0 = 1.15) a n d a secondary zone with gradual air introduction. T h e potential NO x reduction c o m p a r e d with the u n s t a g e d configuration is a factor of three in this case. W h i l e it must be recognized that this study considered idealized combustor configurations and utilized a kinetic model that has not been c o m p l e t e l y validated, the results can be taken to indicate that there is an excellent likelihood that NO~ emissions from low Btu gas p o w e r generation systems can be controlled by combustor design. Although the two cases are not directly comparable, the results of this study indicate that a d v a n c e d systems fired with low Btu gas can potentially satisfy current New Source Performance Standards for natural gas fired units. The combustor concepts d e s c r i b e d here will be further o p t i m i z e d computationally, including consideration of other fuel compositions, a n d then will be verified in experimental combustors as part of a n e w E P A project (contract n u m b e r 68-02-2196).
542
ENERGY PRODUCTION FROM COAL Acknowledgment
The work described in this paper was carried out under U.S. Environmental Protection Agency contract number 68-02-1361.
6.
REFERENCES
7.
1. SPA1TE, P. W.: "Liquefaction and Gasification of Solid Fuel, Prospects for Production of Synthetic Boiler Fuels from Coal." Introductory report presented at Economic Commission of Europe, Second Seminar on Desulfurization of Fuels and Combustion Gases, Washington, D.C., November 11-20, 1975. 2. ROBSON,F. L., BLECHER, W. A. AND GIRAMONT1, A. J.: Symposium Proceedings: Environmental Aspects of Fuel Conversion Technology, II, p. 359, EPA-600/2-76-149, 1976. 3. BARTOK,W., ENGLEMAN,V. S. AND DEL VALLE, E. G.: "Laboratory Studies and Mathematical Modeling of NO x Formation in Combustion Processes." Exxon Research and Engineering Company Report No. GRU-3GNOS-71, EPA No. APTDl168, NTIS No. PB 211-480, 1972. 4. SAROF1M,A. F., WILLIAMS, G. C., MODELL, M. ANDSLATER,S. M.: "Conversion of Fuel Nitrogen to Nitric Oxide in Premixed and Diffusion Flames." Paper presented at the AIChE 66th Annual Meeting, Philadelphia, 1973. 5. DE SOETE, G. G.: " L a Formation Des Oxydes D'Azote dans la Zone D'Oxydation des Flammes d'Hydrocarbures," Compte rendu final des tra-
8.
9.
10.
11.
12.
13.
14.
vaux Contrat No. 73-56 avec le Ministere de la Protection de la Nature et de l'Environnement. Institut Francais de Petrole, June 1975. ENGLEMAN,V. S., BARTOK,W., LONGWELL,J. P. AND EDELMAN, R. B.: Fourteenth Symposium (International) on Combustion, p. 755, The Combustion Institute, 1973. WALDMAN,C. H., WILSON,JR., R. P. ANDMALONEY, K. L.: "Kinetic Mechanism of Methane/Air Combustion with Pollutant Formation," EPA650/2-74-045, 1974. FENIMORE, C. P.: Thirteenth Symposium (International) on Combustion, p. 373, The Combustion Institute, 1971. MYERSON,A. L.: Fifteenth Symposium (International) on Combustion, p. 1085, The Combustion Institute, 1974. ENCLEMAN,V. S.: "Survey and Evaluation of Kinetic Data on Reactions in Methane/Air Combustion," EPA-600/2-76-003, NTIS No. PB 248-139/AS, January 1976. BENSON, S. W., GOLDEN, D. M., LAWRENCE, R. W., SHAW,R., ANDWOOLFOLK,R. W.: Final Report EPA Grant No. R-800798, 1975. TUNDER, R., MAYER, S., COOK, E. AND SnmLER, L.: Aerospace Corp. Report No. TR-001 (921002)-1, 1967. TYsoN, T. J.: "'An Implicit Integration Method for Chemical Kinetics," TRW Report No. 98406002 RU00 September, 1964. HEAP, M. P., TYSON, T. J., AND BROWN, N. D.: Proceedings of the Stationary Source Combustion Symposium, p. II1-119, EPA-600/2-76-152, 1976.
APPENDIX Elementary Reactions and Kinetic Parameters k = A . Z -N 9 exp(-E/RT)
A (cc, mole, sec) Reactions Describing CH 4 CH 4 + M = CH 3 + H + M CHzO + M = CH20 + H + M CH20+ M=CHO+ H + M CHO + M= CO + H + M CO z + M = CO + O + M H z+ M= H + H + M HzO + M= HO + H + M H+O+M = OH + M
N
E (kcal)
References
2.00 E + 17
0.0
88.0
10
4.00 E + 40
7.5
22.6
11
8.00 E + 33
4.5
87.0
11
2.50 E + 20
1.5
16.8
11
1.00 E + 15 2 . 0 0 E + 14
0.0 0.0
100.0 96.0
10 10
3.00 E + 15
0.0
105.0
7
8.00 E + 15
0.0
0.0
10
Oxidation Mechanism
ASPECTS OF GAS COMBUSTION
543
APPENDIX (continued) Elementary Reactions and Kinetic Parameters k = A 9 T -N 9 exp(-E/RT) A (co, m o l e , see) H+O2+M= HO 2 + M CH + CH 4 = CH 2 + CH 3 CH+CH 3=CH 2+CH 2 CH + HO=CHO+ H CH 3 + OH = CH z + H20 CH 2 + H 2 =CH 3 + H CH 3 + CH20 = CH 4 + CHO CH z + CHeO = CH 3 + CHO CH z+ H = CH+H e CH4+OH=CH3+HeO CH 4 + H = CH 3 + H e CH 4 + O = CH 3 + HO CH 3 + O = CHzO + H CH 3+O 2=CH30+50 CH 3+HO e=CH 4+O e CH3+O2=CHeO+HO CH e +O 2=CHeO+O CHO + O = CO + HO CHeO + HO = CHO + HeO CHO+HO=CO+HeO CHeO+O=CHO+HO CHO + H = CO + H e CH + O e = CHO + O CH e+HO=CH+HeO CO+HO=CO e + H CO + O e = CO e + O CO+ HO e= CHO+ 0 2 HO e + H = O e + H e HO z+ HO=H20+ Oe HO + HO = HzO + O HO +H e = H + HeO H + HO 2 = HO + HO HO+O=H +O z H+HO=He+O Reactions Involving N, NO, NeO + M = N 2 + O +M N e + M= N + N + M NO e + M = NO + O + M O e + N + M = NO 2 + M HO + N = H +NO N + NO = N e + O tN + O z = N O + O NO + NO e = NeO + O e H + N zO= HO+N e
N
E (kcal)
References
15 12 12 11 12 12
0.0 0.0 0.0 -0.5 -0.7 0.0
1.0 17.1 8.0 10.0 2.0 7.0
10 11 11 10 10 11
4.00 E + 10
0.0
0.0
10
1.50 E + 1.00 E + 3.20E+ 5.00E+ 1.00 E + 3.00E+
2.00 E + 3.00E+ 3.00E+ 5.00 E + 1.00 E + 2.00 E + 2.50E+ 1.00E+ 3.00E+ 5.00E+ 3.00 E +
11 11 13 10 10 12 9 11 13 11 11
0.0 -0.7 0.0 -1.0 -1.0 -0.5 -1.0 -0.5 0.0 -0.5 -1.0
6.5 26.0 5.0 10.0 8.0 -0.3 28.5 6.0 30.O 7.O 0.5
11 7 10 7 7 7 12 10 10 10 11
1.00 E + 3.00E+ 2.00E+ 3.00 E + 5.00 E + 5.00E+ 5.60E+ 1.00E+ 3.00E+ 2.50E+ 5.00E+ 6.00 E + 2.50E+ 2.50E+ 2.50E+ 8.00 E +
14 10 11 10 11 11 11 13 12 13 13 12 13 14 13 9
0.0 -1.0 -1.0 -1.0 -0.5 -0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -1.0
0.0 0.0 4.4 0.0 6.0 6.0 1.08 60.0 37.1 O.7 0.0 1.0 5.2 1.9 0.0 7.0
11 11 11 11 10 10 11 10 11 10 10 10 o 10 10 10 10
1.00 E + 14 4.00E+21
0.0 1.6
50.0 225.0
10 10
1.00 E + 16
0.0
65.0
10
1.0 -0.5 0.0 -1.0 0.0 0.0
0.0 8.0 0.334 6.3 60.0 15.0
12 11 10 10 10 10
N e O, or NO z
1.00 E + 6.00E+ 3.10E+ 6.00E+ 1.00E+ 8.00E+
19 11 13 9 12 13
544
ENERGY PRODUCTION FROM COAL A P P E N D I X (continued) Elementary Reactions and Kinetic Parameters
k = A " T -N 9 e x p ( - E / R T ) A (cc, m o l e , sec) N20 + O = NO + NO N20 + O = N z + 0 2 N + NO 2 = NO + NO N + NO 2 = N 2 + 0 2 NO+ N20-- NO z + N 2 NO 2 + O = NO + 0 2 NO+HO 2=NO 2+HO NO + HO = HO 2 + N H +NO+ M= HNO + M H + HNO = H 2 + NO HNO+NO=HO+N 20 HNO + HNO = N20 + HzO HNO + O = HO + NO HNO+HO=H20+NO
N
E (kcal)
References
14 14 12 12 12 13 11 10
0.0 0.0 0.0 0.0 0.0 0.0 -0.5 -0.5
28.0 28.0 0.0 0.0 40.0 1.0 3.52 96.5
10 10 10 10 12 10 12 12
2.00 E + 16 1.00 E + 13 2.00E+ 12
0.0 0.0 0.0
0.0 2.5 26.0
12 10 10
1.00 E + 10 5.00 E + 11 1 . 0 0 E + 12
-0.5 -0.5 0.0
41.55 0.0 1.0
12 10 11
5.00E+ 11 8.20 E + 11 1.90 E + 11
-0.5 -0.5 -0.67
56.0 0.0 3.4
12 12 12
4.00 E + 1.00E+ 9.20 E + 1.40 E + 3.00E+ 1.20E+ 1.70E+ 1.70 E + 2.40 E + 1.00 E + 3.60 E + 5.00 E + 5.00 E + 6.00 E + 1.00 E + 5.00 E + 2.00 E +
-0.68 0.0 -0.5 -0.67 -0.68 0.0 -0.63 -0.7 0.0 -0.68 -0.55 -0.5 -0.5 -0.5 -0.5 -0.5 -0.5
1.1 50.5 0.0 4.3 1.3 0.0 3.6 0.1 0.0 1.9 1.9 2.0 5.0 0.0 30.0 2.0 23.0
12 12 12 12 12 12 12 12 12 12 12 12 10 10 10 10 10
1.00 E + 1.00 E + 4.00 E + 1.00 E + 2.00E+ 1.00 E + 5.00E+ 5.00 E +
Reactions of A m m o n i a and Intermediates NH 3+O 2=NH 2+ HO 2 NH 3 + O = NH 2 + HO NH 3 + H = NH 2 + H 2 NH 3 + HO = NH 2 + H20 NH 2+O 2 = NH + HO 2 NH 2 + O = NH + HO NH 2 + H = NH + H 2 NH 2+HO=NH+H20 NH 2+ NO= N z+ H20 NH 2+NH 2=NH 3+NH NH + O = N + HO NH + NO = N 2 + HO NH + H = N + H z NH + NH = N 2 + H 2 NH + HO = NO + H 2 NH + O = NO + H NH + N = H + N 2 H + N20 = H + NO NH + HO = H20 + N H + HNO = NH + OH
10 13 11 11 10 10 11 10 12 12 11 11 11 11 11 11 11
Reactions Between Carbon, Hydrogen and Nitrogen CHN + N = CH + N 2 CH 2+N 2=CHN +NH CHO + N = CH + NO CHN + O = CH + NO CN + NH = CH + N 2 CN + NO = CO + N 2 CN + O 2 = CO + NO CN + O = CO + N CHN+OH=CN+H20 CO 2 + N = CO + NO CH 20+ NO= HNO + CHO
5.00 E + 1.00E+ 1.00 E + 1.00 E + 1.00 E + 3.00 E + 3.00 E + 5.00 E + 2.00E+ 2.00 E +
11 14 14 13 14 11 11 11 11 11
0.0 0.0 0.0 0.0 0.0 0.0 0.0 -0.5 -0.6 -0.5
16.0 55.0 48.6 72.0 40.0 0.0 0.0 9.63 5.0 25.0
11 11 11 12 11 10 10 12 10 10
5.00 E + 11
-0.5
27.0
12
ASPECTS O F GAS C O M B U S T I O N
545
APPENDIX (continued) Elementary Reactions and Kinetic Parameters k = A " T -N " exp(-E/RT)
A (cc, m o l e , sec) CH30 + NO = HNO + CH20 CO+HNO=CO 2+NH CH 3 + HNO = CH 4 + NO CHO + NO = CO + HNO
N
E (kcal)
5.00 E + 11 5.00 E + 8
-0.5 -0.5
4.23 7.0
12 12
5.00 E + 11 2.00 E + 11
-0.5 -0.5
0.0 2.0
10 10
References
COMMENTS John B. H e y w o o d , M . L T., USA. In practical gas turbine combustors, nonuniformities in fuel and air distribution throughout the system have a very important impact on nitric oxide formation rates. The effect is to flatten out the curve of nitric oxide emissions versus equivalence ratio. It has been shown that this flattening out effect occurs with both thermal and fuel nitrogen NO. It has been found necessary to introduce small-scale fuel nonuniformities into any modelling program to get reasonable agreement with experimental data. The mixing models used in your paper do not include these small-scale nonuniformities. Thus, I question whether the strong trends in nitric oxide emissions with equivalence ratio w h i c h your theoretical model
predicts, would in fact be achieved in a practical turbulent combustion system. Authors" Reply. Nonuniformities in turbulent diffusion flames will certainly affect the formation of nitric oxides; however, since we are dealing with a new generation of combustion systems it should not be too difficult to design these systems to minimize any adverse effects of nonuniformities. The results of this study indicate that combustor design holds great potential for reduction of nitric oxide emissions in the low Btu gas combustion. The idealized combustor configurations modeled here can serve as targets for designers of low Btu gas systems.
ASPECTS
OF LOW
BTU GAS COMBUSTION
M. P. HEAP, T. J. TYSON, J. E. CICHANOWICZ, R. GERSHMAN, AND C. J. KAU Uhrasystems, Inc., Irvine, California AND
G. B. MARTIN AND W. S. LANIER United States Environmental Protection Agency, Research Triangle Park, North Carolina Nitrogen oxide emissions from power plants burning low Btu gas derived from coal were investigated using a kinetic model which included over 100 reactions. The model was validated by comparison with the best available experimental data and then applied to idealized combustor configurations for two representative combined cycle systems: (1) an advancedtechnology high-temperature gas turbine with a waste heat boiler, and (2) a supercharged boiler with a current-technology gas turbine. In both cases staged combustion schemes involving rich primary zones and controlled mixing secondary zones were found to provide for minimum NO~ emissions. The calculated minimum NO x levels (corrected to three percent 02, dry) were 105 ppm for the high-temperature turbine case and 100 ppm for the supercharged boiler case. These results indicate that combustor design has a strong potential for controlling NOx from advanced systems fired with ammonia-containing low Btu gas. 1. Introduction The United States is expected to place heavy reliance on the use of coal in order to reduce the amount of i m p o r t e d oil. Although there are a b u n d a n t supplies of coal in the United States, it is perhaps the least desirable fuel from an environmental aspect. Consequently, several options are b e i n g explored w h i c h will allow the increased use of coal without degradation of the environment. While each of these options can potentially decrease emissions of sulfur oxides to acceptable levels, it is also important to consider the associated emissions of nitrogen oxides (NOx). This p a p e r is concerned with the NO~ aspects of one option, the use of coal gasifier off-gas in c o m b i n e d cycle systems for the p r o d u c t i o n of electricity. Technical and e c o n o m i c obstacles limit direct fossil fuel fired steam generating plants to overall efficiencies in the range of 33 to 39 percent. To be competitive, coal-gas fired generating plants will require d e v e l o p m e n t of i m p r o v e d industrial gas turbines. 1 These turbines can then be used in advanced high efficiency power cycles w h i c h will more than compensate for losses suffered in the processes 535
involved in the conversion of coal to a clean fuel. The energy losses associated w i t h coal gasification must be r e d u c e d to a m i n i m u m if the overall energy conversion is to prove economically attractive. T y p i c a l product gases leave the gasifier with a sensible heat w h i c h is equivalent to 10-15 percent of the heat of c o m b u s t i o n of the coal. T h e raw product gas m a y then pass through a high temperature clean-up process a n d into a c o m b i n e d cycle system with u n d i m i n i s h e d temperature in order to utilize all of the sensible heat effectively. Thus the characteristics of hot clean-up systems are most important with regard to energy conversion efficiency. Although identified b y the single term low Btu gas, the nature of the fuel is as varied as the processes used to p r o d u c e it. Quantities of a m m o n i a in the p r o d u c t gas a n d the fate of this a m m o n i a in sulfur clean-up processes are unclear at this time. Robson et al 2 estimate up to 600 p p m of a m m o n i a in a low Btu gas with low temperature h y d r o g e n sulfide cleanup and 3,800 p p m from high temperature clean-up processes w h i c h are u n d e r development.
536
ENERGY PRODUCTION FROM COAL
A large body of experimental information is available concerning the conversion of ammonia to nitric oxide in combustion systems. a,4,5 Under premixed conditions the amount converted is dependent upon mixture stoiehiometry and can be as high as 80 percent. In diffusion flames up to 50 percent of the ammonia can be converted to NO x depending on the method of ammonia addition and the rate of f u e l / a i r mixing. Consequently, the use of a high temperature H 2S clean-up system may well provide a fuel gas with the potential to produce more NO x than the parent coal. This paper addresses the question of whether combustor design can provide for significant reduction of NO~ emissions when burning low Btu gas with high ammonia content. The approach used here was to develop and validate a model capable of predicting NO~ formation rates for fuels consisting of carbon monoxide, hydrogen and methane with trace quantities of ammonia and then to apply this model to a series of "limit cases" based on operating constraints imposed by practical systems. Although macro- and micro-scale mixing phenomena will dictate the NO~ emission levels in any practical system, modeling the chemistry of the combustion process can establish upper and lower limits on the emission levels and thus give an indication of the feasibility of direct use of high ammonia fuel gases. 2. A Kinetic Model for NO Formation in Ammonia Containing Fuel Gases An essential component of the analytical tool used to assess the NO emission potential of low Btu gas fired combustors is a kinetic model capable of describing both NO formation and destruction in flames. This kinetic model should be able: - - t o describe the heat release mechanism for the fuel gases commonly found in low Btu gases (in this work the fuel components were restricted to Ha, CO and CH 4 ); - - t o predict both thermal and fuel NO formation when the fuel nitrogen compound is ammonia; - - t o predict the destruction of nitric oxide under fuel rich conditions. The experimental data used to assess the validity of the model was that reported by Bartok et al. a for NO formation in a Longwell well-stirred reactor. This data was chosen because the combustion rates were limited by chemical kinetics as opposed to transport el-
fects and experimental data was available for methane air mixtures with and without the addition of NO or NH a. The data has two limitations as far as the present investigation is concerned: all the experiments were conducted at approximately atmospheric pressure and no nitrogen species other than NO~ were determined. Engleman et al. 6 have reported calculations of NO~ emissions from the same experimental series. These workers were successful in matching calculated and experimental values for carbon monoxide and hydrogen; however, for p r o p a n e / a i r combustion the calculations underpredicted exhaust NO concentrations by a factor of four for fuel lean conditions and an order of magnitude for fuel rich conditions. The present investigation followed upon the work of Waldman, Wilson and Maloney 7 who also attempted to predict the data of Bartok et al for m e t h a n e / a i r combustion. Waldman was able to obtain reasonable predictions using a 33 reaction set which included a reaction sequence of the type R + N2---~ ... ---~NO where R is a hydrocarbon fragment. For the present study, the original master set listed by Waldman et al was used as the basis for the generation of a new larger set of reactions which included those reactions relevant to ammonia oxidation as well as those concerned with the formation and destruction of nitric oxide in combusting m e t h a n e / a i r mixtures. Thus the reaction set includes reactions CHz + Nz---~ CHN + NH
(1)
CH + N 2 ---, CHN + N
(2)
because they are of the type suggested by Fenimore s to account for rapidly formed NO in rich hydrocarbon flames. Myerson 9 h a d reported results showing that nitric oxide contained in simulated combustion products could be reduced by the addition of hydrocarbon fuels and reactions of the type suggested by him to account for this phenomena were included: CH + N O ~
CHN + O
(3)
CH + N O ~
CHO + N
(4)
A complete list of the reactions and the reaction rate constants used in the subsequent calculations is presented in the Appendix. Most of the rate data was based on either the recent
ASPECTS OF GAS COMBUSTION
537
RESIDENCE TIMES
1300 PPM NH 3
EXPERIMENTAL DATA
o EXPERIMEi~TAL DATA
i00
o
1.3 MSEC.
0
1.5 MSEC.
i00
9 CALCULATIONS
0 1.8 MSEC. V
9
8
2.3 MSEC.
0
/', 3.0 MSEC
~0 5O
CALCULATIONS
~7
:E
~o
0
0
9 2.0 MSEC. 0
[]
/x 0 ,
50
9
8e 9
-~-O
,
,
50
,
,
,
,
/
i00
c> o o~o, 150
0 50
200
100
150
200
PERCENT STOICHIOMETRIC AIR
PERCENT STOICHIOMETRIC AIR
Fro. 1. Validation of the model by comparison with data of Bartok 3 for methane/air in jet-stirred reactor.
Flc. 3. Validation of the model by comparison with data of Ba~ok a for methane/air doped with ammonia in jet-stirred reactor.
survey b y E n g l e m a n 1~ or the estimates of Benson et a l . " Where rate data was not available in the literature, it was estimated from b o n d energy data using the m e t h o d of T u n d e r et al. a2 Figures 1 through 4 present comparisons b e t w e e n the Bartok data a n d calculations for a 2 msec perfectly stirred reactor with a specified heat loss of 0.028 c a l / s e c ~ The calculations were carried out using a fully implicit numerical technique d e v e l o p e d b y Tyson 13 w h i c h allows the integration of the stiff differential equations w h i c h govern the kinetic system. Initially it was necessary that some of the rate constants be adjusted slightly to obtain the results p r e s e n t e d in Fig. 1. Reaction rates were varied w i t h i n the limits of accuracy
given b y E n g l e m a n and Benson. F a i r l y good agreement was also o b t a i n e d b e t w e e n measured and calculated stirred reactor temperature (Fig. 2). W i t h o u t m o d i f y i n g the reaction set in any way, calculations were then made for the cases where N H 3 or NO was a d d e d to the premixed m e t h a n e / a i r mixture feeding the reactor. The results presented in Figs. 3 and 4 show that the kinetic model was capable of simulating both the conversion of a m m o n i a to nitric oxide and the reduction of nitric oxide b y rich c o m b u s t i b l e mixtures. Although the kinetic m e c h a n i s m model a p p a r e n t l y predicts NO X emissions from the well-stirred reactor adequately there is one gap in the experimental data w h i c h prevents complete acceptance of the model. Reactions 1,
3500 RESIDENCE TIMES EXPERIMENTAL DATA B 1.3 NSEC. 0 1.8 MSEC. A 5.0 MSEC. CALCULATIONS
3000
"_ 0~
9
-
Z~&
9
'-
~9
[
~
~-
A 0
z o r--
I 9 2,0 MSEC.
O0 O00O
i00
50
u.l
o
EXPERIMENTALDATA
o 9
o 200 PPMNO
o 2500
/', 1300 PPMNO
0
CALCULATIONS 9 1300 PPMNO
,A.
50
100
150
200
PERCENT STOICHiOMETRIC AIR
Fro. 2. Validation of the model by comparison with data of Bartok 3 for methane/air in jet-stirred reactor.
0
50
i00
150
200
PERCENTSTOICHIMETRICAIR
FIC. 4. Validation of the model by comparison with data of Bartok 3 for methane/air doped with nitric oxide in jet-stirred reactor.
ENERGY PRODUCTION FROM COAL
538
TABLE I Fuel composition--potential air blown Lurgi gas Specie
Mole %
H20 Hz CO CO~ CH 4 N2
10.1 19.6 13.3 13.3 5.5 37.6
2 and 3 all produce H C N and the calculations indicate considerable quantities of H C N in the exhaust from the well-stirred reactor. No quantitative data is available for species other than NO x and it is not possible to check the accuracy of the prediction. However, since HCN has been observed in the primary reaction zone of rich premixed flames a n d in quantities greater than the NOx concentration,5 it appears that the kinetic model is qualitatively correct in this regard.
3. NO, Emissions from Low Btu Gas Combustors The o p t i m u m use of low Btu gas for power generation in c o m b i n e d cycles depends u p o n allowable gas t u r b i n e inlet temperature. Robson et al. z identified the high temperature gas t u r b i n e / w a s t e heat boiler combination as the most promising concept for t u r b i n e inlet ten> peratures in excess of 2600~ (Although it is EPA policy to use metric units, temperatures have been expressed in English units here for the convenience of the reader.) C o n v e n t i o n a l industrial gas t u r b i n e technology is limited to turbine inlet temperatures which are considerably lower than this in which case a supercharged boiler plus gas turbine appears to offer the most advantageous use of low Btu gasJ 4 Thus two types of combustor have b e e n conceptually modeled: 1) an adiabatic gas
FUEL
1
generator with outlet temperature of 2800~ suitable for use with high temperature fuel gas, and 2) a supercharged boiler fired b y low temperature fuel gas. The objective of the model calculations was not to simulate a specific piece of hardware, but to examine simple idealized flow situations to indicate potential emission levels. The calculations were carried out with a fuel gas which is considered representative of a potential second-generation air-blown Lurgi system whose major constituents are listed in Table I. This particular fuel gas was used because of its projected near-term availability and its high methane content relative to presently available gasifiers. It was assumed that all H 2 S and COS was removed from the raw product gas by an unspecified process w h i c h gave a m m o n i a concentrations of 4000 p p m in a 1500~ fuel gas a n d 500 p p m in a low temperature fuel gas. 3.1 Adiabatic Gas Generator An exit temperature of 2800~ from the combustor was selected because it was considered that this was representative of the most severe case for N O X control. I n an adiabatic combustor this corresponds to an overall equivalence ratio of 0.45 if a fuel temperature of 1500~ and an air temperature of 1000 ~ are assumed. Preliminary calculations indicated that the autoignition temperature was ap'proximately 1400~ for a broad range of f u e l / a i r ratios, thus an ignition source was required. Figure 5 presents a generalized schematic of a simple idealized reactor sequence used in the numerical experiments. A 1 msec stirred reactor provides the ignition source for a primary plug flow reactor which feeds a secondary plug flow reactor. Calculations for a case with no staging, complete mixing a n d an equivalence ratio of 0.45 indicated that 86 percent of the a m m o n i a contained in the fuel gas would be converted to NO giving a con-
SECONDARY AIR
llltl
?i TTi
TERTIARY AIR
r
FIG. 5. Reactor sequence used to model adiabatic gas generator.
ASPECTS OF GAS COMBUSTION centration of greater than 900 p p m of NO in the combustion products after 500 msec. Any residual nitrogen c o m p o u n d s remaining at the exit of the p r i m a r y combustor are likely to be converted to nitric oxide d u r i n g secondary b u r n o u t and d i l u t i o n as the products are brought to the desired exit temperature.
Primanj Zone Optimization As indicated earlier, the kinetic model allows the synthesis of H C N t h r o u g h reactions 1, 2 and 3; thus the major nitrogen species (excluding N 2) present at the e n d of the p r i m a r y zone reactor are residual NH3, H C N a n d NO. T h e concentrations of N O 2, N z O and nitrogen containing radicals are negligible. Numerical experiments were c o n d u c t e d to determine the o p t i m u m equivalence ratio and residence time in the primary reactor for m i n i m u m nitrogen species. F i g u r e 6 shows the sum' of the mole fractions of the nitrogen containing species n o r m a l i z e d b y the initial a m m o n i a mole fraction (NH3) o plotted against equivalence ratio at various residence times. F o r lean mixtures thermal NO p r o d u c t i o n dominates with peak value occurring at a p p r o x i m a t e l y q~ = 0.9 ( T = 3600~ A m i n i m u m residual N O fraction is shown for rich mixtures with equivalence ratios of 1.4 and its m a g n i t u d e decreases w i t h 2.5
i
i
i
~
i
i
i
539
time. Further calculations indicated that for 0.8 -< q~ -< 1.7 a n d t -< 200 msec the absolute value of the sum of the nitrogen fractions is i n d e p e n d e n t of the initial a m m o n i a content. For ~ --- 1.1 this is due to the reactor achieving near e q u i l i b r i u m conditions. For rich mixtures the kinetic model indicates the interchange of NO, H C N and N H through the formation of nitrogen atoms. H C N concentrations are i n d e p e n d e n t of a m m o n i a d o p i n g a n d a m m o n i a can be synthesized in the absence of initial d o p i n g due to a chain w h i c h is initiated b y the p r o d u c t i o n of N H via reaction 1. Based u p o n these results it was d e c i d e d that a primary reactor w i t h 1.33 -< s -< 1.45 a n d residence time in excess of 200 m s e c w o u l d be the o p t i m u m configuration.
Secondary Zone Optimization F o u r methods of a d d i n g the secondary dilution air to the exit of the o p t i m u m primary reactor were considered (see Fig. 5): 1) instantaneous m i x i n g of all the secondary air with the p r i m a r y products at the beginning of the secondary region; 2) mixing the dilution air u n i f o r m l y into the primary products over a 50 msec period; 3) mixing the p r i m a r y products into the dilution air u n i f o r m l y over a 50 msec period; and
i
500 MSEC
1000
AIR ADDEDOVER50 MSEC
4000
2.0
vZ~l. 5
:~
T
3000
z + m
~- I!
+ 1.0
o
1 MSEC
0.5
50 MSEC 200 MSEC 500 MSEC o
L
o.N
t
o'.8
'
1'.2
'
1'.6
'
2.o
'J
I i
,1 ,'/t 1 ,
i /"
./
I 1 ~, "UEATEDO~US~ONFLAME /
F ~NTO _ _ '_~US " "XI N I G_..
I
U ....
t
ZOO
N ISTANTANEOUS
MIXING
<~ ~-
I i000
EQUIVALENCE RATIO
FIG. 6. Optimization of primary zone residence time and equivalence ratio for adiabatic gas generator firing Lurgi low Btu gas. Air temp. = 1000~ fuel temp. = 1500~ pressure = 10 atm, fuel ammonia concentration = 4000 ppm.
TIME AFTER INITIATION OF SECONDARYAIR INJECTION (MSEC)
F]c. 7. Effect of secondary zone mixing method on NO formation in adiabatic gas generator firing Lurgi low Btu gas.
540
ENERGY PRODUCTION FROM COAL HEAT TRANSFER FUEL , f IGNITION S O U R C E [ ~ . ~
AIR
PRIMARY ZONE
$iiii.
LLLLL
HEAT TRANSFER
SECONDARY AIR
FIG. 8. Reactor sequence used to model supercharged boiler.
4) a d d i n g the fuel to the air stream b y a series of 3 m s e c stirred reactors f e e d i n g into the secondary air stream at three stations in order to model diffusion zone effects. (The three previous cases assumed instantaneous mixing.) The results of calculations with these four methods of air introduction following a q~ = 2.0 primary zone are presented in Fig. 7. M i n i m u m final N O levels are obtained w i t h methods 1 and 3 while m a x i m u m emissions occur with m e t h o d 2. T h e difference in endssion levels can be explained b y c o m p a r i s o n of the temperature curves. W h e n the air is a d d e d to the fuel the mixture progresses from rich to lean giving peak temperatures in excess of 3600~ with the subsequent formation of thermal NO. This c o u l d be prevented b y extracting heat from the primary zone w i t h the dilution air, thus r e d u c i n g peak temperatures during burnout. T h e 3 msec stirred reactor "flame" simulators gave a final N O level approximately one-half that p r o d u c e d b y progressively mixing the air with the fuel. Calculations for a ~p = 1.33 primary zone followed b y a m e t h o d 1 secondary zone y i e l d e d an NO x emission of 105 p p m (corrected to three percent 0 2 , dry). While method 1 m a y be difficult to a p p r o a c h in a practical system, Fig. 7 indicates that similar results c o u l d be obtained using m e t h o d 3 w h i c h could be followed in practice. 3.2 Supercharged Boiler Supercharged boilers have been used in Europe for some time; they offer the advantage that their compact size reduces capital costs. The idealized m o d e l used to simulate a supercharged boiler c o m b u s t o r is shown in Fig. 8. The heat extraction rate was uniform for the primary zone a n d was arbitrarily assigned a distribution in the secondary zone with the peak rate occurring at one quarter of the residence time w i t h a m a g n i t u d e equal to three times the m i n i m u m rate. Without staged corn-
bustion and a s s u m i n g complete premixing, NO emissions were f o u n d to be 350 ppm. T h e objective of the numerical experiments was to assess h o w to a p p l y staged c o m b u s t i o n techniques to o b t a i n m i n i m u m emissions.
Prima~ Zone Optimization The kinetic m o d e l was used to determine the o p t i m u m c o m b i n a t i o n of primary equivalence ratio and residence time for the fuel identified in T a b l e I with an a m m o n i a content of 500 ppm. F u e l and air inlet temperatures were 150 oF and 600 ~F respectively. M i n i m u m residual nitrogen content at the end of the primary zone was obtained with an equivalence ratio of 1.15. A 500 msec residence time was selected to p e r m i t decay of NO concentration (see Fig. 9). Heat extraction lowering the primary zone exit temperature b y 2 0 0 ~ was found to have little effect on the residual nitrogen concentration for these conditions. The conditions in the primary zone are most d e p e n d e n t u p o n the h y d r o c a r b o n content of
[ ' 1 3 0 1 -
'
.~'PRIMARYZONE' ' ' ~ SECONDARY AIR ADDITION
3100 2700 2300
~ L~
rn
~1
7O
1900
50
1500 ~CN
30
ii00
10
7OO 100
300
500
700 900 TIME (MSEC)
1100
13'00
1500
F1G. 9. NO formation for optimized staging in supercharged boiler firing Lurgi low Btu gas. Air temp. = 600~ fuel temp. = 150~ pressure = 10 atm, fuel ammonia content = 500 ppm.
ASPECTS OF GAS COMBUSTION 150
I
counts for the more r a p i d b r e a k d o w n of ammonia. Further consideration is required regarding the influence of higher h y d r o c a r b o n and sulfur-containing species, b u t this was b e y o n d the scope of the work d e s c r i b e d here.
I
5.5;I CH4 IN FUEL ---
\
0.0% CH4 IN FUEL
!
\
541
Secondary Zone Optimization
i00
A variety of secondary m i x i n g schemes were considered based on an overall equivalence ratio of 0.95. A m i n i m u m NO concentration was obtained when the air was a d d e d gradually over a 125 msec period (see Fig. 9). Correcting to three percent O~, dry, gives an emission level of 100 p p m for this case.
\ c~
".
NO
50
4. Conclusions NH3_ .- --.z.:.S.LL/"
-
- ........
HCN
0 0
i00
200 300 TIME (MSEC)
4DO
500
Flc. 10. Effect of hydrocarbon content on NO formation in supercharged boiler firing low Btu gas. Air temp. = 600~ fuel temp. = 150~ fuel ammonia content = 500 ppm, equivalence ratio = 1.33.
the fuel gas. This is illustrated in Fig. 10 w h i c h shows the concentration of NO in a rich p r i m a r y p l u g flow reactor as a function of time. T h e solid lines refer to calculations made with the fuel listed in T a b l e I. The calculated concentrations shown dotted in Fig. 10 were m a d e assuming an instantaneous conversion of methane to carbon monoxide, thus preventing the formation of b o t h h y d r o c a r b o n radicals a n d HCN. Table II compares O, O H and H concentrations at the outlet of the 1 msec ignition reactor for the two cases. The increased concentration of these radicals ac-
TABLE II Concentrations of O, OH and H at the outlet of the 1 msec ignition reactor
O OH H
Fuel as shown in Table I
Instantaneous conversion of CH 4 to CO and H 2
9.5 ppm 1.09 ppm 1.61 ppm
33 ppm 608 ppm 1200 ppm
F o r both concepts s t u d i e d it was f o u n d that careful design of the combustor can contribute significantly to reduction of NO~ emissions from low Btu gas systems. Staging of the c o m b u s t i o n process was f o u n d to b e desirable in both cases. The o p t i m u m design for an adiabatic gas generator b u r n i n g high a m m o n i a content fuel gas was f o u n d to be a rich (1.33 < q~ < 1.45) primary reactor section with a residence time of at least 200 msec f o l l o w e d b y a gradual mixing of the p r i m a r y products into the dilution air stream. NO emissions for such a system could potentially be lower than those for an unstaged system b y an order of magnitude. F o r the supercharged boiler case the o p t i m u m configuration consisted of a rich p r i m a r y zone (~0 = 1.15) a n d a secondary zone with gradual air introduction. T h e potential NO x reduction c o m p a r e d with the u n s t a g e d configuration is a factor of three in this case. W h i l e it must be recognized that this study considered idealized combustor configurations and utilized a kinetic model that has not been c o m p l e t e l y validated, the results can be taken to indicate that there is an excellent likelihood that NO~ emissions from low Btu gas p o w e r generation systems can be controlled by combustor design. Although the two cases are not directly comparable, the results of this study indicate that a d v a n c e d systems fired with low Btu gas can potentially satisfy current New Source Performance Standards for natural gas fired units. The combustor concepts d e s c r i b e d here will be further o p t i m i z e d computationally, including consideration of other fuel compositions, a n d then will be verified in experimental combustors as part of a n e w E P A project (contract n u m b e r 68-02-2196).
542
ENERGY PRODUCTION FROM COAL Acknowledgment
The work described in this paper was carried out under U.S. Environmental Protection Agency contract number 68-02-1361.
6.
REFERENCES
7.
1. SPA1TE, P. W.: "Liquefaction and Gasification of Solid Fuel, Prospects for Production of Synthetic Boiler Fuels from Coal." Introductory report presented at Economic Commission of Europe, Second Seminar on Desulfurization of Fuels and Combustion Gases, Washington, D.C., November 11-20, 1975. 2. ROBSON,F. L., BLECHER, W. A. AND GIRAMONT1, A. J.: Symposium Proceedings: Environmental Aspects of Fuel Conversion Technology, II, p. 359, EPA-600/2-76-149, 1976. 3. BARTOK,W., ENGLEMAN,V. S. AND DEL VALLE, E. G.: "Laboratory Studies and Mathematical Modeling of NO x Formation in Combustion Processes." Exxon Research and Engineering Company Report No. GRU-3GNOS-71, EPA No. APTDl168, NTIS No. PB 211-480, 1972. 4. SAROF1M,A. F., WILLIAMS, G. C., MODELL, M. ANDSLATER,S. M.: "Conversion of Fuel Nitrogen to Nitric Oxide in Premixed and Diffusion Flames." Paper presented at the AIChE 66th Annual Meeting, Philadelphia, 1973. 5. DE SOETE, G. G.: " L a Formation Des Oxydes D'Azote dans la Zone D'Oxydation des Flammes d'Hydrocarbures," Compte rendu final des tra-
8.
9.
10.
11.
12.
13.
14.
vaux Contrat No. 73-56 avec le Ministere de la Protection de la Nature et de l'Environnement. Institut Francais de Petrole, June 1975. ENGLEMAN,V. S., BARTOK,W., LONGWELL,J. P. AND EDELMAN, R. B.: Fourteenth Symposium (International) on Combustion, p. 755, The Combustion Institute, 1973. WALDMAN,C. H., WILSON,JR., R. P. ANDMALONEY, K. L.: "Kinetic Mechanism of Methane/Air Combustion with Pollutant Formation," EPA650/2-74-045, 1974. FENIMORE, C. P.: Thirteenth Symposium (International) on Combustion, p. 373, The Combustion Institute, 1971. MYERSON,A. L.: Fifteenth Symposium (International) on Combustion, p. 1085, The Combustion Institute, 1974. ENCLEMAN,V. S.: "Survey and Evaluation of Kinetic Data on Reactions in Methane/Air Combustion," EPA-600/2-76-003, NTIS No. PB 248-139/AS, January 1976. BENSON, S. W., GOLDEN, D. M., LAWRENCE, R. W., SHAW,R., ANDWOOLFOLK,R. W.: Final Report EPA Grant No. R-800798, 1975. TUNDER, R., MAYER, S., COOK, E. AND SnmLER, L.: Aerospace Corp. Report No. TR-001 (921002)-1, 1967. TYsoN, T. J.: "'An Implicit Integration Method for Chemical Kinetics," TRW Report No. 98406002 RU00 September, 1964. HEAP, M. P., TYSON, T. J., AND BROWN, N. D.: Proceedings of the Stationary Source Combustion Symposium, p. II1-119, EPA-600/2-76-152, 1976.
APPENDIX Elementary Reactions and Kinetic Parameters k = A . Z -N 9 exp(-E/RT)
A (cc, mole, sec) Reactions Describing CH 4 CH 4 + M = CH 3 + H + M CHzO + M = CH20 + H + M CH20+ M=CHO+ H + M CHO + M= CO + H + M CO z + M = CO + O + M H z+ M= H + H + M HzO + M= HO + H + M H+O+M = OH + M
N
E (kcal)
References
2.00 E + 17
0.0
88.0
10
4.00 E + 40
7.5
22.6
11
8.00 E + 33
4.5
87.0
11
2.50 E + 20
1.5
16.8
11
1.00 E + 15 2 . 0 0 E + 14
0.0 0.0
100.0 96.0
10 10
3.00 E + 15
0.0
105.0
7
8.00 E + 15
0.0
0.0
10
Oxidation Mechanism
ASPECTS OF GAS COMBUSTION
543
APPENDIX (continued) Elementary Reactions and Kinetic Parameters k = A 9 T -N 9 exp(-E/RT) A (co, m o l e , see) H+O2+M= HO 2 + M CH + CH 4 = CH 2 + CH 3 CH+CH 3=CH 2+CH 2 CH + HO=CHO+ H CH 3 + OH = CH z + H20 CH 2 + H 2 =CH 3 + H CH 3 + CH20 = CH 4 + CHO CH z + CHeO = CH 3 + CHO CH z+ H = CH+H e CH4+OH=CH3+HeO CH 4 + H = CH 3 + H e CH 4 + O = CH 3 + HO CH 3 + O = CHzO + H CH 3+O 2=CH30+50 CH 3+HO e=CH 4+O e CH3+O2=CHeO+HO CH e +O 2=CHeO+O CHO + O = CO + HO CHeO + HO = CHO + HeO CHO+HO=CO+HeO CHeO+O=CHO+HO CHO + H = CO + H e CH + O e = CHO + O CH e+HO=CH+HeO CO+HO=CO e + H CO + O e = CO e + O CO+ HO e= CHO+ 0 2 HO e + H = O e + H e HO z+ HO=H20+ Oe HO + HO = HzO + O HO +H e = H + HeO H + HO 2 = HO + HO HO+O=H +O z H+HO=He+O Reactions Involving N, NO, NeO + M = N 2 + O +M N e + M= N + N + M NO e + M = NO + O + M O e + N + M = NO 2 + M HO + N = H +NO N + NO = N e + O tN + O z = N O + O NO + NO e = NeO + O e H + N zO= HO+N e
N
E (kcal)
References
15 12 12 11 12 12
0.0 0.0 0.0 -0.5 -0.7 0.0
1.0 17.1 8.0 10.0 2.0 7.0
10 11 11 10 10 11
4.00 E + 10
0.0
0.0
10
1.50 E + 1.00 E + 3.20E+ 5.00E+ 1.00 E + 3.00E+
2.00 E + 3.00E+ 3.00E+ 5.00 E + 1.00 E + 2.00 E + 2.50E+ 1.00E+ 3.00E+ 5.00E+ 3.00 E +
11 11 13 10 10 12 9 11 13 11 11
0.0 -0.7 0.0 -1.0 -1.0 -0.5 -1.0 -0.5 0.0 -0.5 -1.0
6.5 26.0 5.0 10.0 8.0 -0.3 28.5 6.0 30.O 7.O 0.5
11 7 10 7 7 7 12 10 10 10 11
1.00 E + 3.00E+ 2.00E+ 3.00 E + 5.00 E + 5.00E+ 5.60E+ 1.00E+ 3.00E+ 2.50E+ 5.00E+ 6.00 E + 2.50E+ 2.50E+ 2.50E+ 8.00 E +
14 10 11 10 11 11 11 13 12 13 13 12 13 14 13 9
0.0 -1.0 -1.0 -1.0 -0.5 -0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 -1.0
0.0 0.0 4.4 0.0 6.0 6.0 1.08 60.0 37.1 O.7 0.0 1.0 5.2 1.9 0.0 7.0
11 11 11 11 10 10 11 10 11 10 10 10 o 10 10 10 10
1.00 E + 14 4.00E+21
0.0 1.6
50.0 225.0
10 10
1.00 E + 16
0.0
65.0
10
1.0 -0.5 0.0 -1.0 0.0 0.0
0.0 8.0 0.334 6.3 60.0 15.0
12 11 10 10 10 10
N e O, or NO z
1.00 E + 6.00E+ 3.10E+ 6.00E+ 1.00E+ 8.00E+
19 11 13 9 12 13
544
ENERGY PRODUCTION FROM COAL A P P E N D I X (continued) Elementary Reactions and Kinetic Parameters
k = A " T -N 9 e x p ( - E / R T ) A (cc, m o l e , sec) N20 + O = NO + NO N20 + O = N z + 0 2 N + NO 2 = NO + NO N + NO 2 = N 2 + 0 2 NO+ N20-- NO z + N 2 NO 2 + O = NO + 0 2 NO+HO 2=NO 2+HO NO + HO = HO 2 + N H +NO+ M= HNO + M H + HNO = H 2 + NO HNO+NO=HO+N 20 HNO + HNO = N20 + HzO HNO + O = HO + NO HNO+HO=H20+NO
N
E (kcal)
References
14 14 12 12 12 13 11 10
0.0 0.0 0.0 0.0 0.0 0.0 -0.5 -0.5
28.0 28.0 0.0 0.0 40.0 1.0 3.52 96.5
10 10 10 10 12 10 12 12
2.00 E + 16 1.00 E + 13 2.00E+ 12
0.0 0.0 0.0
0.0 2.5 26.0
12 10 10
1.00 E + 10 5.00 E + 11 1 . 0 0 E + 12
-0.5 -0.5 0.0
41.55 0.0 1.0
12 10 11
5.00E+ 11 8.20 E + 11 1.90 E + 11
-0.5 -0.5 -0.67
56.0 0.0 3.4
12 12 12
4.00 E + 1.00E+ 9.20 E + 1.40 E + 3.00E+ 1.20E+ 1.70E+ 1.70 E + 2.40 E + 1.00 E + 3.60 E + 5.00 E + 5.00 E + 6.00 E + 1.00 E + 5.00 E + 2.00 E +
-0.68 0.0 -0.5 -0.67 -0.68 0.0 -0.63 -0.7 0.0 -0.68 -0.55 -0.5 -0.5 -0.5 -0.5 -0.5 -0.5
1.1 50.5 0.0 4.3 1.3 0.0 3.6 0.1 0.0 1.9 1.9 2.0 5.0 0.0 30.0 2.0 23.0
12 12 12 12 12 12 12 12 12 12 12 12 10 10 10 10 10
1.00 E + 1.00 E + 4.00 E + 1.00 E + 2.00E+ 1.00 E + 5.00E+ 5.00 E +
Reactions of A m m o n i a and Intermediates NH 3+O 2=NH 2+ HO 2 NH 3 + O = NH 2 + HO NH 3 + H = NH 2 + H 2 NH 3 + HO = NH 2 + H20 NH 2+O 2 = NH + HO 2 NH 2 + O = NH + HO NH 2 + H = NH + H 2 NH 2+HO=NH+H20 NH 2+ NO= N z+ H20 NH 2+NH 2=NH 3+NH NH + O = N + HO NH + NO = N 2 + HO NH + H = N + H z NH + NH = N 2 + H 2 NH + HO = NO + H 2 NH + O = NO + H NH + N = H + N 2 H + N20 = H + NO NH + HO = H20 + N H + HNO = NH + OH
10 13 11 11 10 10 11 10 12 12 11 11 11 11 11 11 11
Reactions Between Carbon, Hydrogen and Nitrogen CHN + N = CH + N 2 CH 2+N 2=CHN +NH CHO + N = CH + NO CHN + O = CH + NO CN + NH = CH + N 2 CN + NO = CO + N 2 CN + O 2 = CO + NO CN + O = CO + N CHN+OH=CN+H20 CO 2 + N = CO + NO CH 20+ NO= HNO + CHO
5.00 E + 1.00E+ 1.00 E + 1.00 E + 1.00 E + 3.00 E + 3.00 E + 5.00 E + 2.00E+ 2.00 E +
11 14 14 13 14 11 11 11 11 11
0.0 0.0 0.0 0.0 0.0 0.0 0.0 -0.5 -0.6 -0.5
16.0 55.0 48.6 72.0 40.0 0.0 0.0 9.63 5.0 25.0
11 11 11 12 11 10 10 12 10 10
5.00 E + 11
-0.5
27.0
12
ASPECTS O F GAS C O M B U S T I O N
545
APPENDIX (continued) Elementary Reactions and Kinetic Parameters k = A " T -N " exp(-E/RT)
A (cc, m o l e , sec) CH30 + NO = HNO + CH20 CO+HNO=CO 2+NH CH 3 + HNO = CH 4 + NO CHO + NO = CO + HNO
N
E (kcal)
5.00 E + 11 5.00 E + 8
-0.5 -0.5
4.23 7.0
12 12
5.00 E + 11 2.00 E + 11
-0.5 -0.5
0.0 2.0
10 10
References
COMMENTS John B. H e y w o o d , M . L T., USA. In practical gas turbine combustors, nonuniformities in fuel and air distribution throughout the system have a very important impact on nitric oxide formation rates. The effect is to flatten out the curve of nitric oxide emissions versus equivalence ratio. It has been shown that this flattening out effect occurs with both thermal and fuel nitrogen NO. It has been found necessary to introduce small-scale fuel nonuniformities into any modelling program to get reasonable agreement with experimental data. The mixing models used in your paper do not include these small-scale nonuniformities. Thus, I question whether the strong trends in nitric oxide emissions with equivalence ratio w h i c h your theoretical model
predicts, would in fact be achieved in a practical turbulent combustion system. Authors" Reply. Nonuniformities in turbulent diffusion flames will certainly affect the formation of nitric oxides; however, since we are dealing with a new generation of combustion systems it should not be too difficult to design these systems to minimize any adverse effects of nonuniformities. The results of this study indicate that combustor design holds great potential for reduction of nitric oxide emissions in the low Btu gas combustion. The idealized combustor configurations modeled here can serve as targets for designers of low Btu gas systems.