Ind. Eng. C h e m . Res. 1994,33,2846-2852
2846
Influence of the Temperature and Oxygen Concentration on NO, Reduction in the Natural Gas Reburning Process Rafael Bilbao,' Angela Millera, and Maria U. Alzueta Department of Chemical and Environmental Engineering, Faculty of Science, University of Zaragoza, 50009 - Zaragoza, Spain
The reburning process with natural gas is an effective technique which can be applied in existing coal combustors in order to reduce the NO, emissions. The influence of the reburning temperature (1200-1500 "C) and the oxygen concentration coming from the primary combustion zone (0-5%) have been studied. Experimental results of NO, HCN, "3, CO, COZ,and CH4 concentrations in the exit gas have been determined. The influence of the natural gas decomposition on these results has been analyzed. The experimental trends obtained have been compared with those calculated with the model of Kilpinen et al. (1992).
Introduction
CH,
The combustion of pulverized coal in conventional power stations can produce appreciable quantities of NO,, with outlet concentrations higher than those permitted in the legislation. Therefore, the study of different methods of NO, reduction in power station emissions is very important. There are several techniques of NO, abatement that can be applied. Reburning with coal, natural gas, or other hydrocarbons has been demonstrated to be an effective and simple technique appropriate for existing pulverized coal boilers with low investment costs. Reburning with natural gas has advantages over coal because of its characteristics, in particular its high heating value and its low or complete lack of nitrogen and sulfur content (Chen et al., 1986; Mereb and Wendt, 1990; Burch et al., 1991). This method allows us to reach NO, reductions greater than 50% in full scale plants (Bartok et al., 1990; Folsom and BrowningSletten, 1990; Glarborg, 1990) and can be applied simultaneously with other reduction methods of NO, andor SOz. The reburning process can be divided into three zones. In the primary zone, most of the fuel is burned with a slight air excess. The typical combustion products, including NO,, are produced in this zone. Downstream, the reburning fuel is injected into the so-called reburning zone, creating a fuel rich zone where the NO, coming from the primary zone is reduced by the action of hydrocarbon radicals (Myerson, 1974)giving essentially Na, although other unwanted intermediate species (mainly HCN and "3) can be produced. Finally, additional air is added into the burnout zone to obtain the complete combustion oxidizing any remaining fuel fragment. The NO, reduction takes place in the reburning zone in a reducing environment through a complex reaction pathway which includes a great number of reactions. Several authors have proposed different mechanisms for this reduction (Burch et al., 1991; Glarborg, 1990; Kilpinen et al., 1990, 1992). An example of the main reaction steps involved in the NO reduction are shown as follows. (i) Formation of hydrocarbon radicals, such as CHi. (ii) Reactions of NO with hydrocarbon radicals, to give mainly HCN which is a very important intermediate nitrogenous species in the NO reduction.
* To whom correspondence
should be addressed.
0888-5885/94/2633-2846$04.50/0
+ NO - HCN + ...
(1)
(iii) Conversion of HCN to other nitrogenous species.
+ (H, 0, OH) - NH, + ... HCN + (0,OH) - HNCO + ...
HCN
(2) (3)
Afterwards, the species NHi and HNCO can evolve into other nitrogenous species and radicals, finally producing Nz or NO. It is important to know the influence of different operating conditions in the reburning zone. These variables are, principally, mixing conditions, temperature, stoichiometry, gas residence time, and the inlet NO, concentration or primary NO,. The reburning stoichiometry, usually called SRz, is defined as the ratio between the oxygen amount available for reaction and the oxygen amount necessary t o obtain the complete combustion of the species entering into the reburning zone. Therefore, S& depends on the oxygen concentration coming from the primary zone and on the reburn fuel fraction, ~ R B . This term, ~ R B refers , to the fraction in energetic content of the reburning fuel with respect to the total fuel used in the boiler. Several studies have been performed on the influence of the values of these operating conditions on NO, reduction. Consequently, in the bibliography, different results have been shown for reburning temperatures ranging between 1000 and 1400 "C (Lanier et al., 1986; Miyamae et al., 1986; Fujima et al., 1990; Kremer et al., 19901, oxygen concentrations coming from the primary zone between 1 and 2% (Chen et al., 1986; Glarborg, 1990; Mereb, 1991), and reburn fuel fractions between 0.15 and 0.20 (Chen et al., 1986; Wiersma et al., 1986). Taking into account these studies, the aim of the present work is to obtain a wider and more detailed knowledge of the influence of the oxygen concentration and its interconnectionwith the temperature on the NO, reduction. Experimental results have been obtained in a range of values which has been extended with respect to those that appeared in the literature. Thus, the oxygen concentration a t the inlet of the reburning zone ranged between 0 and 5% and the temperatures studied ranged between 1200 and 1500 "C. Taking into account the influence of the carbonaceous species on the NO, reduction, it has been considered useful t o study the trends obtained for these concentrations. These trends
0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2847
FLOW CONTROLLERS
-7-7-
Natural gas
NO
Electric furnace
Continuous analyzers NON02, COlC02,Oz
1 Gas chromatograph I Figure 1. Experimental installation. 2000
(CGTKiq
T system 1200QC O
Pe
1300'C
x
6
400t
U
0
300
600 900 1200 Reactor length (mm)
1500
Figure 2. Temperature profiles inside the reburning reactor.
have been analyzed both experimentally and theoretically using a model proposed in the literature.
Experimental Method The experimental installation, Figure 1, consists basically of a reaction system, a gas feeding system, and a gas analysis system. The reaction system includes a ceramic reactor heated by an electric hrnace that allows us to reach up to 1500 "C. The reactor is an alumina tube of 20 mm i.d. and 2500 mm in length. Some previous experiments were performed in order to know the behavior of the reaction system. The objective of these experiments was to determine the longitudinal temperature profiles inside the reburning reactor for different system temperatures and to determine the possible formation of thermal NO at different temperatures, flow rates, and oxygen concentrations. The results obtained showed that the shape of the temperature profiles is very similar for all the system temperatures and flow rates and that there is a central zone (=800 mm in length) where the temperature can be considered to be constant. An example of the longitudinal temperature profiles obtained for a nitrogen flow rate of 900 NVh (referred to 1 atm and 0 "C) and different system temperatures is shown in Figure 2. Moreover, it was also observed that the thermal NO formation is appreciable at temperatures higher than 1350 "C, being significant only a t temperatures near t o 1500 "C.
In the reburning experiments, a gas consisting of 0 2 , N2, and C02 was prepared and bubbled into a water container until saturation, in order to reach a desired moisture a t a given temperature. The gas was mixed with NO and natural gas, and the mixture was fed into the reburning reactor. In all the experiments, fixed concentrations of C02 (20%) and natural gas (1.7%) in dry basis were used. For a coal with a heating value of 3850 kcalkg, this natural gas concentration was in the ~ R range B between 0.15 and 0.20, the range normally used. The natural gas used had an average volume composition of 90.5% CHI, 8.5% C2H6,0.5%C3H8, 0.4% N2, and 0.1% C, ( n = 4-7). Because methane is the main component of natural gas, in this work the methane concentration will be considered as representative of the natural gas concentration. Taking into account the primary NO originated in pulverized coal combustors, usually called (NO),, in this work a value of (NO), of 900 ppm was chosen. In the different experiments, the 0 2 concentration in the inlet gas of the reburning zone was varied between 0 and 5%. The N2 necessary to fulfill the gas balance in dry basis was introduced. Steam was added to the total gas flow rate, representing an extra 6%. The concentration values of steam (6% extra) and C02 (20%) have been chosen because they were representative of those obtained in several Spanish coal power plants. The temperature used in the different experiments ranged between 1200 and 1500 "C. With respect to the gas residence time in the reburning zone, this variable can be a limiting factor when this process is retrofitted to existing installations. Therefore, the determination of the residence time necessary to reach the desired NO, reduction is essential. The real residence time in a boiler is determined by the mixing time of the flue gas coming from the primary zone with the reburning fuel and the reaction time. In this work, a flow rate of 900 NVh (referred to 1 atm and 0 "C, in dry basis) was used, corresponding to gas residence times between 98 and 280 ms for temperatures between 1200 and 1500 "C, respectively, including only the reaction times because the flow gas enters the reaction zone already well mixed. In some previous studies carried out (Bilbao et al., 19931, these residence times were found to achieve significant NO reduction, consistent with the results of other authors (Myerson, 1974; Chen et al., 1986; Lanier et al., 1986; Miyamae et al., 1986; Mereb, 1991). The contents of NO, (NO and Nod, 0 2 , CO, and CO2 in the exit of the reburning zone were measured by continuous analyzers. 02, CO, CO2, CHI, and other hydrocarbons were determined by gas chromatography. The NH3 concentration was determined by passing a known volume of gas through an aqueous acid solution, which was subsequently analyzed using the Nessler colorimetric method (Clesceri et al., 1989). The HCN concentration was also determined by collecting a gas sample in an aqueous basic solution and analyzing it using the barbituric pyridine colorimetric method (Clesceri et al., 1989).
Results and Discussion The oxygen that enters the reburning zone is a very important parameter that influences the NO reduction and is related t o the air excess used in the primary combustion zone. The NO reduction for different oxygen concentrations and temperatures, using a natural gas concentration of 1.7%, is shown in Figures 3 and 4.
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994
2848
100 I o .. 0
.
.
1.0 I
1
1.7 % Natural gas (NO) I)= 900 ppm
0
. .
0.8
I
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1.7 % Natural gas (NO) = 900 ppm
. e
. U
0
2
1
3
6
5
4
0
1
2
0 2 (%)
Figure 3. NO reduction versus peratures. 1
concentration for low tem-
X
I
A
'
*
6
Figure 6. HCN selectivity versus 02 concentration for different temperatures.
1.7 % Natural gas (NO) = 900 ppm
.
800
i
t
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i A
A A
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0
2
I
3 02 (%)
Figure 4. NO reduction versus peratures.
w 300 /
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1450 "C 1500°C
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0
o
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e
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z
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.
Figure 7. TFN concentration versus 0 2 concentration for different temperatures.
reburning zone. For a given fRB value, this reducing environment is determined by the oxygen concentration. At temperatures of 1200 and 1300 "C, the HCN concentration presents a maximum with respect to the oxygen concentration. The low concentrations of HCN originating at very low oxygen concentrations would be due to the small amount of NO reacted in these conditions, Figure 3. This fact can be corroborated if the selectivity between the HCN formed and the NO reacted is analyzed. These values for different temperatures and oxygen concentrations are shown in Figure 6. It can be observed that the selectivity values diminish when the oxygen concentration and the temperature increase. The values of TFN (total fixed nitrogen) concentration, for different temperatures and oxygen concentrations, are shown in Figure 7. The TFN value represents the sum of the NO, HCN, and NH3 concentrations, although the NH3 concentrations can be considered as negligible in all the experiments carried out because the values obtained were always below 5 ppm. Logically, the TFN values can be obtained from those data shown in Figures 3-5. It can be observed that for a given temperature the values of TFN concentration present a minimum with respect to the oxygen concentration, while they diminish as the temperature increases. It has been suggested that in the reburning process the NO reduction takes place through reactions between NO and hydrocarbon radicals. Therefore, and in order to explain the results obtained, it is interesting to analyze the influence of the operating conditions on the formation of free radicals from natural gas. It is important to indicate that, given the existing NO, concentrations, the consumption of natural gas to
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2849 71
14000
I
[Q-smauhl
Q = 900 N l h 1.7 % Natural gas
12000
1.7 % NaNral gas
10000
-[
8000 i
1450 "C 1500°C
6000 4000 2000 0
..
.
0
1
a 0
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4
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4
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5
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5
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6
Figure 8. C& concentration in the outlet gas versus 0 2 concentration for different temperatures.
Figure 9. CO concentration in the outlet gas versus tration for different temperatures.
produce hydrocarbon radicals involved in the NO, reduction is low. However, a reducing environment in the reburning zone seems to be necessary for the generation of these radicals and for their reaction with NO. It is known that the thermal decomposition of hydrocarbons is a process that involves intermediate free radicals (Rokstad et al., 1991; Weil et al., 1992; Billaud et al., 1993). Therefore, the formation of free radicals can take place through pyrolysis reactions which are accelerated when the temperature increases (Albright and Marek, 1988; McCaffrey and Harkleroad, 1988; Velenyi et al., 1991). The yield to the different products depends on the temperature and the residence time, and a significant coke formation is observed at temperatures of 1400-1500 "C (Holmen et al., 1976; Rokstad et al., 1992; Billaud et al., 1993). For low oxygen concentrations, the pyrolysis reactions of natural gas can be the main factor responsible for the formation of hydrocarbon radicals, especially at high temperatures (Myerson, 1974; Billaud et al., 1993). At lower temperatures (1200-1300 "C), the pyrolysis of natural gas is less significant. The presence of a certain quantity of oxygen favors the natural gas decomposition and the formation of hydrocarbon radicals (Myerson, 1974; Glarborg, 1990)because of the increase of radicals which are responsible for the initiation of the decomposition reactions (Lanier et al., 1986; Kilpinen et al., 1992). As the oxygen concentration increases, the methane (the main component of natural gas) can be oxidized through a mechanism giving CO and C02 (Kilpinen et al., 1990), which causes a diminution of hydrocarbon radicals available to react with NO. In order to analyze these facts, the concentrations of CH4, CO, and C 0 2 in the different experiments were determined and their trends were studied where oxygen concentration and temperature are varied. The experimental results of the methane concentration in the outlet gas of the reburning zone for different oxygen concentrations and temperatures are shown in Figure 8. At high temperatures, the decomposition of natural gas is almost complete €or every oxygen concentration. When the oxygen concentration is low, the pyrolysis of natural gas is the predominant process, with the corresponding formation of hydrocarbon radicals available to react with NO and the high NO reduction obtained, Figure 4. A fact which could be indicative of the predominance of the pyrolysis reactions in these conditions is that, for the highest temperatures and the lowest oxygen concentrations, the presence of a pulver-
ized carbonaceous material that deposited on the reactor walls and the installation pipes was detected. For high oxygen concentrations, the reactions of hydrocarbon oxidation would be dominant with the resulting fall of NO reduction, Figure 4. At the lowest temperatures and for low oxygen concentrations, an appreciable amount of methane is observed in the outlet gas of the reburning zone. This fact causes a lower formation of hydrocarbon radicals and a relatively low NO reduction, Figure 3. As the oxygen concentration increases, the CH4 concentration in the outlet gas diminishes because the amount of oxygen existing in the reburning zone begins to be enough to promote the natural gas decomposition. The hydrocarbon radicals necessary to carry out the NO destruction are produced, and therefore an increase in NO reduction is observed, Figure 3. A greater increase of the oxygen concentration produces a total decomposition of C&, but the oxidation reactions predominate and the NO reduction diminishes, Figure 3. With respect to the concentrations of CO and C02 in the outlet gas from the reburning zone, the experimental results are shown in Figures 9 and 10. For low oxygen concentrations, an increase in the temperature causes a significant increase in the amounts of CO produced accompanied by a diminution of COz. This effect could be due t o the predomination of the natural gas pyrolysis when the temperature increases, producing hydrogen and carbon which would react with CO2 giving CO. At these high temperatures, an increase in the oxygen concentration produces a lower prevalence of the pyrolysis reactions and a lower amount of CO is generated. For the lowest temperatures, the process of pyrolysis is not so significant and the concentration of CO versus 0 2 presents the typical maximum. With respect to the COz obtained, an increase in the oxygen concentration implies a greater importance of the natural gas oxidation reaction and a logical increase of C02 concentration. It can be observed that for a high oxygen concentration the values of CO2 obtained are quite similar. Although for the reasons mentioned above, for low oxygen concentrations the values of C 0 2 diminish when the temperature increases. An analysis of the trend of the variation of CH4, CO, and COz concentrations when the operating conditions are varied has also been performed using a kinetic model developed by Kilpinen et al. (1992). This model was proposed t o explain the reburning chemistry and consists of a reaction set including 225 reversible elementary gas-phase reactions and 48 chemical species. This model includes a mechanism for the decomposition
0 2
concen-
2850 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 24 I
1
I .7 % Natural gas
Fl
2-
10000
I\
1500 "C
I.*
0
2
1
3
5
4
0
6
2
I
3
5
4
6
0 ,(90)
0,(%)
Figure 10. COZ concentration in the outlet gas versus concentration for different temperatures.
0 2
Figure 12. Theoretical concentration of CHI versus 0 2 concentration for a temperature of 1200 "C, according to the Kilpinen et al. model. 5
..
1.7 % Natural gas
.,.
15000
4
I
.
1200 "C
- 3
s 8
2 1
0 1000
1200
1400
1600
1 am
Temperature ("C)
Figure 11. Theoretical concentration of CH4 versus temperature for 0% 0 2 , according to the Kilpinen et al. model.
of natural gas (considered as consisting basically of methane), and the main reactions coincide with those proposed by other authors (Glarborg et al., 1986; Miller and Bowman, 19891, although in some cases with slight modifications. The mathematical resolution of the model has been performed using the chemical kinetic software called CHEMKIN (Kee et al., 19891, and some examples of the results obtained are shown. The influence of the temperature on the disappearance of CH4 in the absence of oxygen has been analyzed. It can be observed, Figure 11,that when the temperature increases the CH4 decomposition does increase by means of the pyrolysis reactions, being almost total at temperatures higher than 1400 "C. The influence of the oxygen concentration in the inlet gas has been analyzed for those temperatures at which the amount of methane in the outlet gas was still appreciable in the absence of oxygen. The results obtained at a temperature of 1200 "C are shown in Figure 12. It can be observed that the existence of 02 promotes the decomposition of methane, being practically total from 2% 0 2 . The trends obtained for CO and COn have been likewise analyzed when the temperature and the oxygen concentration are varied. The examples which correspond to the lowest and highest temperatures studied (1200 and 1500 "C) are shown in Figures 13 and 14. The tendencies observed are similar to those obtained experimentally, although the values are rather different and a maximum is also observed for CO concentration at 1500 "C when the oxygen concentration is changed. Finally, the values of NO and HCN obtained experimentally have been compared with those predicted by the model of Kilpinen et al. (19921, Figures 15 and 16. It can be observed that the data show some dispersion
0
2
I
3
02
4
5
(%o)
Figure 13. Theoretical concentration of CO versus 0 2 concentration for different temperatures, according to the Kilpinen et al. model.
19
-
ia
-
11
. - - - -- -
0
1200 "C 1500 OC
,' 1
2
3
4
5
0, (%)
Figure 14. Theoretical concentration of COZversus 0 2 concentration for different temperatures, according to the Kilpinen et al. model.
for NO and a significant dispersion for HCN. The experimental NO and HCN concentrations obtained are lower than those theoretically predicted.
Conclusions For low oxygen concentrations, the NO reduction obtained in the reburning process with natural gas increases significatively with the temperature. An increase of 0 2 concentration (higher than approximately 3%)causes a significant reduction in the NO reduction. The ratio between the HCN produced and the NO reacted diminishes when the temperature and the oxygen concentration increase. An analysis of the decomposed CHI allows us t o explain the trends of the
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2851
U
0
200
400 600 800 NO calculated (ppm)
loo0
1200
Figure 16. Comparison between the experimental and calculated (Kilpinen et al. model) NO values.
.
.*
0
7
0
200
.
.
*
.
400
b
600
800
HCN calculated (ppm)
Figure 16. Comparison between the experimental and calculated (Kilpinen et al. model) HCN values.
results obtained. At high temperatures, the decomposition of natural gas is almost total for every oxygen concentration, the pyrolysis of natural gas being dominant for low oxygen concentrations. At lower temperatures, the pyrolysis of natural gas is less important and the presence of a certain amount of oxygen favors the natural gas decomposition. The results obtained with the model of Kilpinen et al. (1992) follow the same tendencies as those obtained experimentally, although there are significant differences in the values.
Acknowledgment The authors express their gratitude to the ENAGAS, ENDESA, and SEVILLANA companies, to OCIGAS, OCIDE, and CYCIT (Project AMB92-0888)for providing financial support for this work, and also to Ministerio de Educacih y Ciencia (Spain) for a research grant awarded to M. U. Alzueta.
Literature Cited Albright, L. F.; Marek, J . C. Coke Formation During Pyrolysis: Roles of Residence Time, Reactor Geometry, and Time of Operation. Znd. Eng. Chem. Res. 1988,27, 743-751. Bartok, W.; Folsom, B. A.; Elbl, M.; Kurzynske, F. R.; Ritz, H. J. Gas Reburning - Sorbent Injection for Controlling SO, and NO, in Utility Boilers. Environ. h o g . 1990, 9 (l), 18-23. Bilbao, R.; Alzueta, M. U.;Millera, A.; Lezaun, J.; Adanez, J. Gas Reburning. Influence of Different Variables on the NO, Reduction. Proceedings of the Second International Conference on Combustion Technologies for a Clean Environment, Lisbon, Portugal, July 1993; Vol. 11, pp 39.1/1-39.1/8. To be published in “Combustion Technologies for a Clean Environment” within the book series “Combustion, Energy and the Environment”; Gordon and Breach Science Publishers: New York.
Billaud, F.; Broutin, P.; Busson, C.; GuBret, C. Coke Formation during Hydrocarbons Pyrolysis. Part two. Methane Thermal Cracking. Rev. Znst. Fr. P&. 1993, 48 (21, 115-125. Burch, T. E.; Tillman, F. R.; Chen, W. Y.; Lester, T. W.; Conway, R. B.; Sterling, A. M. Partitioning of Nitrogenous Species in the Fuel-Rich Stage of Reburning. Energy Fuels 1991,5 (21, 231237. Clesceri, L. S., Greenberg, A. E., Trussell, R. R., Eds. Standard Methods for the Examination of Water and Wastewater; APHA-AWWA-WPCE: Washington, DC, 1989; Chapter 4, pp 20-42, 111-126. Chen, S. L.; McCarthy, J. M.; Clark, W. D.; Heap, M. P.; Seeker, W. R.; Pershing, D. W. Bench and Pilot Scale Process Evaluation of Reburning for In-Furnace NO, Reduction. Proceedings of the 21st Symp. (Znt.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; pp 1159-1169. Folsom, B. A.; Browning-Sletten, M. Evaluation of Gas Reburning and Low NO, Burners on a,Wall-Fired Boiler. Proceedings of the Reburning Workshop, Orenas Slott, Sweden, Nov. 1990; Nordic Gas Technology Centre: Horsholm, Denmark, 1990; pp 87-102. Fujima, Y.; Takahashi, Y.; Kunimoto, T.; Kaneko, S. Field Application of MACT. Proceedings of the Reburning Workshop, drenas Slott, Sweden, Nov. 1990; Nordic Gas Technology Centre: Horsholm, Denmark, 1990; pp 7-25. Glarborg, P. Unresolved Questions in Natural Gas Reburning. Proceedings of the Reburning Workshop, Orenas Slott, Sweden, Nov. 1990; Nordic Gas Technology Centre: Horsholm, Denmark, 1990; pp 313-325. Glarborg, P.; Miller, J . A,; Kee, R. J . Kinetic Modeling and Sensitivity Analysis of Nitrogen Oxide Formation in WellStirred Reactors. Combust. Flame 1986, 65, 177-202. Holmen, A.; Rokstad, 0. A.; Solbakken, A. High-Temperature Pyrolysis of Hydrocarbons. 1. Methane to Acetylene. Znd. Eng. Chem. Process Des. Dev. 1976, 15 (3),439-444. Kee, R. J.; Miller, J. A.; Jefferson, T. H. CHEMKIN: a GeneralPurpose, Problem Independent, Transportable, FORTRAN Chemical Kinetics Code Package. Report No. SAND/80-8003; Sandia National Laboratories: Livermore, CA, 1989. Kilpinen, P.; Hupa, M.; Glarborg, P.; Hadvig, S. Parametric Study of Natural Gas Reburn Chemistry using Kinetic Modelling. Report No. 90/7; Nordic Gas Technology Centre: Turku, Finland, 1990. Kilpinen, P.; Glarborg, P.; Hupa, M. Reburning Chemistry: A Kinetic Modeling Study. Znd. Eng. Chem. Res. 1992,31,14771490. Kremer, H.; Klocke, B.; Mechenbier, R. NO, Reduction Potential with Pulverized Coal Combustion by Reburning using Methane and Optimum Conditions for Mixing Reburning Fuel with Flue Gases. Proceedings of the Reburning Workshop, Orenas Slott, Sweden, Nov. 1990; Nordic Gas Technology Centre: Horsholm, Denmark, 1990; pp 205-223. Lanier, W. S.; Mulholland, J. A.; Beard, J. T. Reburning Thermal and Chemical Processes in a Two Dimensional Pilot-Scale System. Proceedings of the 21st Symp. (Znt.) on Combustion;The Combustion Institute: Pittsburgh, PA, 1986; pp 1171-1179. McCafEey, B. J.; Harkleroad, M. Combustion Efficiency, Radiation, CO and Soot Yield from a Variety of Gaseous, Liquid, and Solid Fueled Buoyant Diffusion Flames. Proceedings of the 22nd Symp. (Znt.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 1251-1261. Mereb, J . B. Nitrogen Oxide Abatement by Distributed Fuel Addition. Ph. D. Dissertation, University of Arizona, Tucson, Az,1991. Mereb, J. B.; Wendt, J. 0. L. Reburning Mechanism in a Pulverized Coal Combustor. Proceedings of the 23rd Symp. (Znt.) on Combustion, Pittsburgh, PA, 1990; pp 1273-1279. Miller, J. A.; Bowman, C. T. Mechanism and Modeling of Nitrogen Chemistry in Combustion. Prog. Energy Combust. Sei. 1989, 15, 287-338. Miyamae, S.; Ikebe, H.; Makino, K.; Suzuki,K.; Mogi, S. Evaluation of In-Furnace NO, Reduction. Proceedings of the Joint Symp. on Stat. Combust. NO, Control, EPRLZPA San Francisco, CA, 1986; Vol. I, pp 24/1-24/20. Myerson, A. L. The Reduction of Nitric Oxide in Simulated Combustion Effluents by Hydrocarbon-Oxygen Mixtures. Proceedings of the 15th Symp. (Znt.) on Combustion; The Combustion Institute: Pittsburgh, PA, 1974; pp 1085-1092. Rokstad, 0. A.; Olsvik, 0.; Holmen, A. Thermal Coupling of Methane. In Natural Gas Conversion; Holmen, A., Jens K.-J.,
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Wiersma, S. J.; Pratapas, J. M.; Kurzynske, F. R.; Berkau, E. E. The Use of Natural Gas for Pollution Control. Gas Waerme Znt. 1986,35,479-484.
Received for review March 14, 1994 Revised manuscript received July 12, 1994 Accepted August 1, 1994@
@
Abstract published in Advance ACS Abstracts, October 1,
1994.
