Ill I II Hi la

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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y. 117 The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed In its publications. Discussion is printed only If the paper is published in an ASME Journal. Papers are available from ASME for 15 months after the meeting. Printed in U.S.A. Copyright 1994 by ASME 94-GT-432 SIMPLIFIED MODELS FOR NO PRODUCTION RATES IN LEAN-PREMIXED COMBUSTION David G. Nicol, Philip C. Matte, and Robert C. Steele Department of Mechanical Engineering Combustion Laboratories University of Washington Seattle, Washington Ill I II Hi la Abstract Simplified models for predicting the rate of production of NOx in lean-premixed combustion are presented. These models are based on chemical reactor modeling, and are influenced strongly by the nitrous oxide mechanism, which is an important source of NOx in leanpremixed combustion. They include 1) the minimum set of reactions required for predicting the NOx production, and 2) empirical correlations of the NOx production rate as a function of the CO concentration. The later have been developed for use in an NOx post-processor for CFD codes. Also presented are recent laboratory dam, which support the chemical rates used in this study. Under lean conditions, the NH formed by reaction (3) is converted to NO. This occurs mainly through reaction of the NH to N and HNO, followed by reaction of these intermediates to NO. Under the conditions of this study, the steady-state assumption is generally valid for the NH, N, and HNO. Thus, the rate of NO production by the nitrous oxide mechanism is: dfl91/dt = 2k2[192][] + 2k3(1921[H] Eq(1) Under the present conditions, the NO production by the Zeldovich mechanism can be expressed in terms of the initial Zeldovich reaction: NO Reactions in Lean-Premixed Combustion N2 + --> NO + N Rx(4) The production of NO in lean-premixed (4.4 to.6) combustors operated at the pressures (P=1 to 4atm) of stationary gas turbine engines and fired on natural gas occurs principally by the nitrous oxide and Zeldovich mechanisms. Under the leanest conditions, effecting the smallest NOx emission (i.e., about loppmv or less), the nitrous oxide mechanism is predominant and can account for essentially all of the NO formed (Nicol et al ). There are three reactions which must be considered for the nitrous oxide mechanism for high pressure, lean-premixed combustion. These are as follows: The nitrous oxide is formed by reaction (1): N2 + + M N2 + M Rx(1) For high pressure combustion, the N2 is destroyed mainly by the reverse of reaction (I). This is different from the situation for atmospheric pressure combustion, in which case destruction by free radical attack (i.e., especially reaction N2 + H --> N2 + OH) is quite important. Nitric oxide is formed from the NIO by reactions (2) and (3): N2 + --> NO + NO N2 + H --> NO + NH Rx(2) Rx(3) leading to the following well known equation for the rate of production of NO by the Zeldovich mechanism: d[no]/dt = 21:4[N2][] Eq(2) As P increases, and 4) and inlet mixture temperature decrease, the ratio of d[no]/dt by the nitrous oxide mechanism to that by the Zeldovich mechanism increases. The four reactions listed above form the minimum set of nitrogen reactions required to predict the NO for high pressure, lean-premixed combustion. In addition, reactions are required in order to predict the free radical concentrations. For fast chemical reactor computations, an abbreviated mechanism of 25 reactions and 15 species has been developed for lean-premixed. methane-air combustion with NO production by the nitrous oxide and Zeldovich mechanisms, for both atmospheric pressure and high pressure combustion. The abbreviated mechanism and its comparison to the full mechanism are given by Nicol (1994). The third NO mechanism of interest is the prompt mechanism, i.e., fixation of N2 by hydrocarbon attack leading to NO. This mechanism based on available rate data, generally exerts a weak effect on high pressure, lean-premixed combustion. However, for the flame zone of latm, lean-premixed combustors, the mechanism can be significant and should be considered. For the present work, the initiating reaction is taken to be: Presented at the International Gas Turbine and Aeroengine Congress and Exposition The Hague, Netherlands June 13-16, 1994 This paper has been accepted for publication in the Transactions of the ASME Discussion of it will be accepted at ASME Headquarters until September 3, 1994 Downloaded From: on 5/12/218 Terms of Use:

2 N2 + CH --> HCN + N leading to the following reaction rate: R5 =k5[n21[ch1 Rx(5) Eq(3) 2) For Fig. 1, the rate of NO production remains close to the maximum rate of 1.6ppmv/Ins through the WSR into the PFR. Fall-off in the rate does not occur until the CO concentration decreases to about 4ppmv, which corresponds approximately to a total residence time of 1.ms. The HCN forms in the flame zone, followed rapidly by complete conversion to NO for lean conditions. A second effect is the reduction of NO by hydrocarbon attack, leading to additional HCN and other cyano species (e.g., HCNO). These molecules form in the flame zone and then are reconverted to NO. With respect to NO production, the net effect of the attack of hydrocarbons on the nitrogen system is the formation of NO in the flame zone at a rate less than 2R5, because some of the nitrogen is tied up as cyano molecules, followed by a yield of NO as the cyano molecules are convened to NO. Under some circumstances, the reduction of NO by hydrocarbon attack exceeds the yield of NO by reaction (5), and thus, the net effect of the attack of hydrocarbon on the nitrogen system is a loss of NO in the flame zone. In this paper, the terms "NO production" and "NOx production" are interchangeable. This is because the NO2 forms only through oxidation of NO. On the other hand, the exhaust emissions and the measurements are expressed in terms of NOx. Throughout this paper, concentrations are expressed as ppmv. on a wet, actual-2 basis, with the expection of some NOx data denoted as NOx (15% 2 dry). Comparison of NO Production Rates Rates of NO production are compared based on chemical reactor modeling. In Fig. 1, the rates are shown for atmospheric pressure combustion, and in Fig. 2, the rates are shown for combustion at 14.5bar. For each figure, the fuel is methane, premixed with air at 4)=.5. The mixture inlet temperature (Ti n) is 385 C in the case of Fig. 1, and 4 C in the case of Fig. 2. The concentrations are expressed as parts per million by volume (ppmv), on a wet, actual- 2 basis. The rates are expressed as ppmv per millisecond (ms). The rate of NO production by reaction (2) is written "N2+," that by reaction (3) is written "N2O+H," that by the sum of these reactions, i.e., equation (1), is written "TOTAL BY N2," and that by reaction (4), i.e., equation (2), is written "ZELD." The total rate of NO production predicted by the chemical reactor modeling is written "GRAND TOTAL." The difference between the total rate and the sum of the rates due to reactions (2), (3), and (4) is the net rate of NO production by the attack of hydrocarbon on the nitrogen system; this is written "PROMPT." The kinetic mechanism used throughout this paper is that of Miller and Bowman (1989). The few minor modifications made to this mechanism for the present work are listed by Nicol et al. (1993). The chemical reactor model consists of a micromixed, well stirred reactor (WSR) followed by a plug flow reactor (PFR). The WSR is assigned a mean residence time of.35ms in Fig. 1, and.14ms in Fig 2, which is the value for incipient blowout. The results are plotted versus the CO concentration. Right-to-left in CO-space represents increasing time. The point of maximum CO concentration is the WSR condition. All other points lie within the PFR, whose residence time is varied from zero to the equilibrium condition. These points are indicated on Fig. I. The WSR and the early PFR are taken to represent the flame zone, and the remainder of the ['FR is taken to represent the post-flame zone. The main features of Figs. 1 and 2 are as follows: 1) For the WSR of Fig. 1, reactions (2), (3), and (4), and the net prompt effect, exhibit approximately equal rates; i.e., all of the reaction routes to NO contribute approximately equally in the latm, lean-premixed flame zone. This is consistent with the results of Con et al. (1992) and Nicol et al. (1993). NO Production Rate (ppmv/ms) 3) For the high pressure combustion shown in Fig. 2, reaction (2) is the predominant source of the NO. Reaction (3) is relatively unimportant, and thus, the assumption of steady-state behavior of the NH and HNO no longer has much bearing on the predicted rate of NO production; the Zeldovich mechanism is also relatively unimportant; and the net prompt effect is negligible, and thus, is not plotted in Fig. 2. 4) For the combustion at 14.5bar, the maximum rate of NO production is about 16pprn/ms, which is ten-fold greater than that computed for the latm case. 5) For the high pressure combustion, the rate of NO production by the nitrous oxide mechanism exceeds that by the Zeldovich mechanism for essentially all of the CO-space. This is noted in Fig. 2b, which is an enlargement of Pig. 2a near the equilibrium condition; the Zeldovich rate does not overtake the nitrous oxide NO rate until the CO falls to a low level, i.e., 7ppmv. For the latm combustion, as noted in Fig. I, the nitrous oxide mechanism also persists into the PFR. Its rate exceeds that of the Zeldovich mechanism until the CO concentration decreases to about 2ppmv, which corresponds to a total residence time of about 5.ms. The only prompt mechanism (i.e., mechanism restricted mainly to the flame zone) is the fixation of N2 by hydrocarbon attack GRAND TOTAL O TOTAL BY N2 O N2. A N2.11 ZELD PROMPT _.1 4 PER increasing time W R CO (ppmv) Figure 1. Rates ol NO production (ppmv/ms) by the nitrous oxide, Zeldovich, and net prompt mechanisms versus CO concentration (ppmv). Results computed for a.35ms-wsr + PFR. Combustion of premixed CI-14-air at =.5, P = 1atm, and inlet mixture temperature =- 385C. 2 Downloaded From: on 5/12/218 Terms of Use:

3 production rate by the nitrous oxide mechanism might prove to be beneficial for lean-premixed combustion. Of the combustion variables predicted directly by the CFD code, the CO concentration is the most indicative of the free radical chemistry, and thus, perhaps the most satisfactory for linking the NO prediction. Empirical results generated by chemical reactor modeling could be the basis of the NO Production Ra te (ppmv/ms) O N2+ A N2O+H ZELD a TOTAL BY N2 GRANO TOTAL N2. A R2..H ZELD Tarps BY N2 GRAND TOTAL. I jar NOx post-processor. For example, for given, P. and Ti n, the chemical reactor modeling could be used to generate the rate of NO production by the nitrous oxide mechanism as a function of the CO concentration. In addition, information on the -atom could be used to upgrade the prediction of the Ztldovich NO. Inclusion of this information into the CFD code, followed by integration of the chemical rate and species diffusion would yield the NO field. By such an approach, the NO is predicted based on the time mean temperature and composition fields. In order to assess the influence of temperature and concentration fluctuations on the rate of NO production, chemical reactor modeling with coalescence-dispersion theory included'(e.g., see Pratt, 1979) could be performed. For this paper. in order to examine NO production rate as a function of CO, several chemical reactor zone arrangements are used, as shown in Fig. 3. Arrangement B, involving the WSR+PFR, has been used in the preceding section. Other arrangements for simulation of the combustor involve the following: A. a single WSR; C, a WSR followed by a recycle zone treated as a PER; and D, a series of WSRs or PFRs followed by a PFR recycle zone. Arrangement D could represent the flame of a premixed combustor -- for example, the flame stabilized behind a flow blockage. The flame front is divided into several WSRs or PFRs, each of which is fed a perfect mixture of fuel and air and reacting gas from the previous reactor. At the last WSR or PFR part of the gas exits the flame and enters the post-flame zone; the remainder of the gas is recirculated in order to provide for the flame stabilization CO (ppmv) [A] Figure 2. Rates of NO production (ppmv/ms) by the nitrous oxide and Zeldovich mechanisms versus CO concentration (ppmv). Results computed for a.1 4MS- WSR + PFR. Combustion of premixed CFI4-air at o.5, P = 14.5bar, and inlet mixture temperature = acmc. Upper figure (a) covers the full CO-space. Lower figure (b) gives an enlargement of the CO-scale for low values of the CO concentration. IC] wsr WSR PER PER NO Production Rates as a Function of CO In this section, additional results are presented on the NO production rates expressed as a function of the CO concentration. This is done because curves of NO production rate versus CO obtained using different chemical reactor arrangements are found to be nearly identical, indicating the passibility of universal behavior. Furthermore, there is the potential of using the predictions of NO production rate versus CO as the basis of an NOx post-processor for CFD codes. [D] _L W5R W5R WSR 5R o or PPR PFR PFR PFR PER Computational fluid dynamics (CFD) codes can directly handle short, simplified chemical mechanisms. An example is a 2-to-3 step, global, chemical energy release mechanism, leading to the prediction of temperature and the concentrations of the unburned fuel, CO, CO2,1-12, and 2. Thermal NOx and prompt NOx are treated with post-processors. In this regard, an NOx post-processor which could treat the super-equilibrium -atom concentration and the NO Figure 3. Chemical reactor zone arrangements used in this study. 3 Downloaded From: on 5/12/218 Terms of Use:

4 In Fig. 4, results are shown for the nitrous oxide and -atom concentrations for six chemical reactor zone arrangements. In Fig. 5 the results are shown for the rate of production of the NO by the nitrous oxide and Zeldovich mechanisms. The conditions are methane-air combustion with =.5, P=14.5bar, and Ti n=4 C. These six arrangements are as follows: I) WSR of mean residence time varied from the value for incipient blowout (.14ms) to infinitely long (i.e., the equilibrium condition). 2) WSR of.14ms mean residence time followed by a PFR extended to infinitely long residence time. This is the blowout-wsr + PER model. 3) WSR of.43ms residence time followed by a PFR. 4) Arrangement C (Fig. 3). for a total residence time of 1.ms, with the fraction of material recycled varied from R=.1 to E 6 a. a. 4 2, r -ig FA... 1/1 Ily ' :.., WSR.14ms-WSR + PFR.43ms-WSR + PPR WSR + Recycle A 4 WSRs + Recycle 4 PFRs + Recycle 5) Arrangement D (Fig. 3), with the flame front treated as a series of WSRs. The overall residence time is held at 1.ms; and the fraction of material sent through the recirculation zone is varied from.1 to.99. 6) Arrangement D (Fig. 3) with the flame front treated as a series of PFRs. Other conditions are the same as in case 5. Although many other arrangements could be treated, the six cases chosen for Figs. 4 and 5 illustrate the important features of this type of modeling. It is seen (Fig. 4a) that the nitrous oxide concentration is relatively insensitive to the chemical reactor arrangement. This is also seen for the -atom concentration, provided that the residence time has reached about.4ms. This corresponds approximately to 4ppmv CO on Fig. 4b. Early in the CO-space, however, the blowout-wsr + PER model gives a greater -atom concentration (Fig. 4b) and greater NO production rate (Fig. 5) than the WSR model. Peak values for these variables occur in the very early part of the PER, which corresponds to the flame zone. This behavior is characteristic of highly loaded (i.e., short residence time) flame zones. The contributions of the reactions responsible for the NO production for the blowout-wsr + PFR case have been plotted above in Fig. 2. The information contained in these figures can be transformed into algebraic equations or tables. Although not presented in this paper, it has been found possible to express results for -atom and NO production rate for a range of O's, P's, and Ti n's in terms of a few algebraic equations. Equations for the WSR case and the blowout- WSR + PER case permit one to cover most situations, and provide for a relatively simple-to-use NOx post-processor. However, if preferred, the chemical reactor code can be called directly and used to predict the -atom and NO production rate versus CO for exactly the case being treated with the CFD model. This method (regardless of how the NO rates versus CO are called) is appropriate provided that the combustion process is adiabatic (or nearly adiabatic) and the CO oxidation process is not quenched. -atom (ppmv) [b] Adilli 12 Fi 2... a 1. is... 8 a ' :....4 LI a WSR IL..14ms-WSR + PFR ms-WSR + PFR.43ms-WSR + PFR 4 ri:.43ms-wsr + PFR WSR + Recyde PP WSR + Recycle A 4 WSRs + Recycle A 4 WSRs + Reryde..1 4 PFRs + Recycle d 4 PFRs + Recycle n , CO (ppmv) Figure 4. Nitrous oxide (upper figure a) and -atom (lower figure b) concentrations versus CO concentration for six chemical reactor zone arrangements. Combustion of premixed CH4-air at Q..5, P = 14.5bar, and inlet mixture temperature = 4C. NO Production Rate (p pmv/ms) CO (ppmv) Figure 5. Rate of NO production versus CO concentration for six chemical reactor zone arrangements. Combustion of premixed CI-14-air at O =.5, P = 14.5bar, and inlet mixture temperature = 4C. 4 Downloaded From: on 5/12/218 Terms of Use:

5 Effects of Pressure and Inlet Mixture Temperature The maximum rate of NO production is dependent on pressure. This has been shown above in the comparison of the results for I atm (Fig. I) and 14.5bar (Fig. 2) combustion. for the same fuel-air equivalence ratio (i.e., =.5) and essentially the same inlet mixture temperature. For I atm, the maximum rate is 1.6ppmv/ms. whereas for 14.5bar, the maximum rate is 16ppmv/ms. Additional results are shown in Fig. 6. Rates of NO production by the sum of the nitrous oxide and Zeldovich mechanisms are plotted versus the adiabatic equilibrium temperature for four combinations of pressure (1 and 3atm) and inlet air temperature (6 and 8K). The fuel temperature is 395K: the values of lie between about.4 and.6; and the WSR + PER model is used. The mean residence time of the blowout-wsr varies from about.5ms in the case of 3atm combustion at the highest temperature shown to about.5ms in the case of loatrn combustion at the lowest temperature shown. The maximum rates occur in the PFRs immediately downstream of the WSRs operated at incipient blowout. This is the situation for the cases plotted in Fig. 6a. The NO production rates show sensitivity to P and Ti n at the highest temperatures examined. At the lowest temperatures, the sensitivity is weak. When the model is modified so that the mean residence time of the WSR is held constant, little sensitivity to P and Ti n is found for the rate of NO production plotted versus the adiabatic equilibrium temperature. This is the situation for Fig. 6b, in which case the WSR is held at.5ms mean residence time. As indicated above, this is the blowout value for the leanest loatm case considered. Also for the conditions of Fig. 6b, the maximum rate of NO production is essentially the rate found in the WSR, i.e., there is essentially no increase in the rate in the early-pfr. By the WSR + PFR model, the exit plane NOx concentration (which is not plotted) also has little sensitivity to the P and Ti n, regardless of the mean residence time assumed for the WSR. This is consistent with the results of Leonard and Stegmaier (1993), for which it was claimed that the NOx measured for very lean-premixed flames stabilized in a laboratory burner had little dependence on P and Ti n. The extent to which this is a general result, valid for fuels other than methane, and for complex flows and flame zones, remains to be verified, however. Experimental Comparison In order to test the chemical rate data used in this study, experiments have been performed for lean-premixed combustion of methane-air and CO-H2-air in an atmospheric pressure jet-stirred combustion reactor. Future experiments will take place in a high pressure jet- stirred reactor. The results for the methane-air experiments of t:3.5 to.6, conducted for the shortest mean residence time run in the reactor (I.7ms, nominal), are presented herein. The balance of the experimental results are discussed by Steele et al. (1994). The reactor is similar to the reactor used in the experiments of Corr et al. (1992). However, the single, relatively large feed-jet used in that reactor has been replaced by a nozzle block made of Inconel, in which eight very small jet holes have been drilled. These jets, comprised of premixed reactants preheated electrically to 325 C, and operated sonically, are arranged in a diverging pattern. That is, the jets are caused to sweep along the inside side-walls of the reactor. reflect at the downstream end-wall, and are backmixed along the centerline axis of the reactor. Upon recirculation, the hot, burning gases mix with the fresh reactants in the jets, thereby stabilizing the combustion. Row out of the reactor occurs through four drain holes located near the upstream end of the reactor (see Corr et al ). NO Product io n Rate (ppmv/ms) NO Product ion Rate (p pmv/ms) t loatm/6k 1atnV8K 3atm/6K 3atrn/8K El _---, O loatnv6k O loatm/8k 3atm/6K 3atm/8K : Adiabatic Equil Temp (K) Figure 6. Rate of NO production versus adiabatic equilibrium temperature for four cases of pressure and mixture inlet temperature. Lean-premixed, methaneair combustion. WSR + PFR model. Upper figure (a) assumes the WSR to be at incipient blowout. The NO rate is the maximum rate, which occurs just into the PFR. Lower figure (b) assumes a WSR of.5ms mean residence time. The NO rate is for the WSR. Through comparison to chemical reactor modeling, the reactor is found to nearly exhibit well-stirred behavior for lean-premixed. methane-air combustion. A small plug flow component is indicated, because the measured temperature is slightly above the value computed by modeling the reactor as an adiabatic WSR, and the CO concentration is below the computed value. Some of the CO difference, but not all, can be attributed to oxidation in the sample probe. Therefore, in order to account for the small PFR component, the reactor is modeled as case C shown in Fig. 3. Temperature measurement and species probe sampling take place in the middle part of the reactor, in a region nearly uniform in temperature and composition. For purposes of the model, this is assumed to be the middle of the recycle PFR. Probe reaction is accounted for by adding a short (.5ms). low pressure (.25atm) PER to the sample stream. Thus, there are three reactor zones: the WSR for flame stabilization, the PFR for recycle, and the PER for probe reaction. The later includes wall-loss reactions for the -atom. H-atom, and OH-radical. 5 Downloaded From: on 5/12/218 Terms of Use:

6 Temperatures are measured with a platinum/rhodium thermocouple coated with a material to prevent catalytic reaction at the thermocouple bead surface. These measurements are corrected for radiation loss by calculation. The sample probe is a quartz micro-probe of the design used by Corr et al. (1992) and Kramlich and Malte (1978). Gas analysis is performed for the following species: 1) NO and NO/ are measured with a chemiluminescent NO- NOx analyzer. Condensation in the sample line and watertrap impingers is maintained low in order to prevent loss of NO2. 2) N2 is measured by injecting gas samples into a gas chromatograph equipped with an electron capture detector. The lower detection limit of this method is about.ippmv. Temperatu re (K) Measured O Single WSR A WSR+recyc+probe Equilibrium 3) Cl to C3 hydrocarbons are measured by the flame ionization detector of the gas chromatograph. Typically, this measurement yields mainly methane, in the 1 to loppmv range. 4) CO and CO2 are measured by non-dispersive infrared analyzers. 5)2 is measured by a process gas analyzer. The experimental and computed results are shown in Figs. 7a, 7b, 8a, and 8b, respectively, for the temperature, CO, NOx, and N2. The model is operated by matching the predicted CO to the measured CO. The parameter adjusted to accomplish this is the recycle fraction, R. This is found to lie between.8 and.9, which is expected for a jet-stirred reactor operating under the present conditions. The trend is increasing R with decreasing O. That is, the reactor approaches the WSR condition (R=1.) as the Damkohler number decreases Measured O Single WSR/meas. T O Single WSR/adiabatic Also shown in these figures are results computed assuming the reactor to be a single WSR operating adiabatically, and a single WSR operating at the measured temperature. There is no probe reactor in these cases. The results shown for temperature in Fig. 7a indicate close agreement between the measured temperature and that predicted by the. WSR+recycle+probe model. The single WSR under-predicts the temperature. In Fig. 7b, the measured CO is seen to be less than the single WSR prediction by a factor 1.5 to 3.. When "recycle+probe" is added is to the single WSR model, the predicted CO is decreased to match the measured values. In Fig. 8a, the predicted NOx is seen to closely match the measured NOx, except when the single WSR is assigned the measured temperature. In that case, the CO and -atom concentrations are over-predicted. In the case of the adiabatic WSR, the reduced value of predicted temperature off-sets the effect of the over-predicted - atom, leading to the apparent agreement with the measured NOx. In Fig. 8b, the predicted N2 is seen to be essentially insensitive to the model arrangement selected. Good agreement between the measurements and model is shown. The relative contributions of the nitrous oxide. Zeldovich, and prompt mechanisms to the NOx measured at =.5 are essentially equivalent to the results noted in Fig. 1 at the condition of 3ppmv CO. At this condition the nitrous oxide and Zeldovich rates are nearly equal, and the prompt rate is negligible. Thus, the experiment provides a valid test of the nitrous oxide mechanism, though not as much so as for high pressure combustion, since in that case the NO would form essentially exclusively by the nitrous oxide mechanism. Based on the good agreement shown between the modeled and Fuel-Air Equivalence Ratio Figure 7. Measured and modeled temperature (upper figure a) and CO concentration (lower figure b) versus. Atmospheric pressure, premixed methane-air combustion in jet-stirred combustion reactor. Inlet mixture temperature = 325C. Concentrations expressed as ppmv, wet actual-2 basis..6 measured NOx and N2, the chemical mechanism used herein appears appropriate for lean-premixed, methane-air combustion, at least at latm. In Fig. 9, the measured rate of NOx production is plotted. This rate is based on the assumption of reactor uniformity. That is, the rate of NO production is taken as the measured NOx divided by the mean residence time of the reactor. The log of the measured rate is seen to be linear in the reciprocal of the measured temperature. The activation energy inferred from the plot is 47.3 kcal/gmol. This value should be valid for the flame zones of atmospheric pressure, leanpremixed. methane-air combustors. Finally, in Fig. 1, the NOx measurements are compared to the results of Leonard and Stegmaier (1993). The present measurements agree well with the lower temperature range of the Leonard and Stegmaier (1992) results, though the temperatures may not be exactly equivalent. The present measurements are plotted versus the 6 Downloaded From: on 5/12/218 Terms of Use:

7 Measured '... _.. Single WSR/meas. T Single WSR/adiabatie A WSR.reeye+probe NOx Produ ction Rate (pp mv/ms) d(nx)rdt (2.6.7)exp(-23911) o..6 CA.2 - _. :--,_...- Single WSR/meas. T _ o Single WSR/adiabatie A WSR+reeye+probe Measured,._ Fuel-Air Equivalence Ratio Figure 8. Measured and modeled NOx (upper figure a) and N2 (lower figure b) concentrations versus e. Atmospheric pressure, premixed methane-air combustion in jet-stirred combustion reactor. Inlet mixture temperature = 325C. Concentrations expressed as ppmv, wet actual-2 basis. experimental temperature.. The temperature in the Leonard and Stegmaier paper is not defined, though it may be the adiabatic equilibrium temperature. Replotting of the present measurements versus the adiabatic equilibrium temperature would move the curve about 3 C to the right. NOx (pp mv, 15% 2 dry) 1,/T(K) Figure 9. Rate of NOx production based on the measurements, assuming a uniform reactor, versus reciprocal measured temperature. Experimental rate (ppmvfms) shown. Band tor lab burner data ot Leonard & Slegmeler ( 993) Measurements-- this sudy Temperature(K) Figure 1. Comparison of NOx (ppmv adjusted to 15% 2 dry basis) measured for the jet-stirred reactor to that measured by Leonard and Stegmaier (1993) for a lean-premixed laboratory burner. Conclusions Chemical reactor modeling has shown the reactions mainly responsible for NO production in lean-premixed, methane-air combustion. Rates of NO production have been determined for both atmospheric and high pressure combustion, though without the effect of turbulent fluctuations on the chemical rates. Methods of computing the rates of NO production have been discussed. Measurements of NOx and N2 for a jet-stirred combustion reactor operated at a mean residence time of nominally I.7ms have provided experimental confirmation of the chemical rates used in this study, at least for atmospheric pressure combustion. The measured rate of NOx production is applicable to the flame zone of lean-premixed, atmospheric pressure, methane-air combustors. Areas for future work include the following: I) measurements of NOx and N2 for a high pressure, high intensity, laboratory combustion reactor; 2) extension of the chemical reactor modeling to fuels of practical interest other than methane; and 3) assessment of the influence of turbulent temperature and concentration fluctuations on the modeled rate of production of NO. Acknowledgements The authors wish to acknowledge the support of the USEPA, for the nitrous oxide measurements and modeling. Additional support for 7 Downloaded From: on 5/12/218 Terms of Use:

8 the modeling has been provided by a member company of the gas turbine industry. The chemical reactor code used in this study was developed by Professor David T.Pratt of this university. The authors acknowledge his helpful suggestions and cooperation. References Corr, R.A., Malte, P.C., and Marinov, N.M. (1992). "Evaluation of NOx Mechanisms for Lean. Premixed Combustion," Transactions of the ASME, bourn. of Engr. for Gas Turbines and Power 114, pp Leonard, G. and Stegmaier, J. (1993). "Development of an Aeroderivative Gas Turbine Dry Low Emissions Combustion System," ASME paper 93-GT-288, presented at the International Gas Turbine and Aeroengine Congress, Cincinnati, Ohio, May, Kramlich, J.C. and Malie, P.C. (1978). "Modeling and Measurement of Sample Probe Effects on Pollutant Gases Drawn from Flame Zones," Combust. Science and Technol. 18, pp Miller, ID. and Bowman, C.T. (1989). "Mechanism and Modeling of Nitrogen Chemistry in Combustion," Prog. in Energy and Combust. Science 15, pp Nicol, D.G. (1994). "Modeling NOx for Lean-Premixed Combustion," in preparation: PhD Thesis, Department of Mechanical Engineering, University of Washington. Seattle, Washington. Nicol, D.G., Steele, R.C., Marinov, N.M., and Mahe, P.C. (1993). "The Importance of the Nitrous Oxide Pathway to NOx in Lean- Premixed Combustion." ASME Paper No. 93 -GT-342, to appear in Transactions of the ASME, Journal of Engineering for Gas Turbines and Power, Pratt, D.T. (1979). "Coalescence -Dispersion Modeling of High- Intensity Combustion," AIAA Journal of Energy 3, pp Steele, R.C., Nicol, D.C., and Mahe, P.C. (1994). "NOx Formation in a Lean-Premixed, High-Intensity Combustion Reactor," paper submitted for the International Symposium on Combustion, Irvine, California, August, 1994.' 8 Downloaded From: on 5/12/218 Terms of Use: