Basic Aspects of Combustion Stability and Pollutant Emissions of a

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Ind. Eng. Chem. Res. 2002, 41, 4543-4549

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Basic Aspects of Combustion Stability and Pollutant Emissions of a CO2 Decomposition-Based Power-Generation Cycle Jyh-Yih Ren,† Theodore T. Tsotsis,*,‡ and Fokion N. Egolfopoulos† Department of Aerospace and Mechanical Engineering and Department of Chemical Engineering, University of Southern California, Los Angeles, California 90089

Results are reported on an investigation of the technical feasibility of separating and decomposing CO2 within the context of power/energy-generation. The power/energy-generation environment is well-suited for such an application because it provides the high-temperature conditions and/ or electric energy needed. The emphasis in this paper is on the combustion aspects of this integrated CO2-capture/power-generation advanced cycle. As part of this effort, experimental and numerical studies were carried out on the dynamics and structure of lean atmospheric CH4/ air flames in the presence of binary CO/CO2 mixtures that would result from the CO2 decomposition process. For the lean flames that were considered in this investigation, application of the integrated CO2-capture/power-generation advanced cycle was found to provide important benefits in terms of both flame stability and flame intensity for a range of conditions, in addition to allowing for waste heat utilization. For another range of conditions, significant improvements in pollutant emissions were identified. Generally, improvements in combustion intensity and stability can be achieved but at a cost of increased NOx emissions, and vice versa. A range of conditions also exists for which the cycle allows for effective waste heat utilization without an undue impact on combustion stability and pollutant emissions. Introduction The main contributor to the greenhouse effect is CO2. The 1995 Intergovernmental Panel on Climate Change has predicted that, without additional measures being taken, atmospheric CO2 emissions will increase from 7.4 billion tons of carbon in 1997 to 26 billion tons by 2100 and that the CO2 atmospheric content will double by the middle of next century.1 Although the effect of CO2 emissions on climate can be debated, the general agreement is that doubling the atmospheric CO2 concentration will have serious environmental effects.2,3 The development of a strategy to address the climate change problem is urgently needed, and improving the energy efficiency of power plants must be a key component. CO2 sequestration using current techniques can raise the cost of electricity production by 20-30 mills/kWh, at this time an economically unacceptable prospect. Future technologies, therefore, must be more efficient, compact, and able to be retrofitted within existing power plants, and they must follow approaches commonly practiced in the industry involving thermal integration and waste heat recycling. Furthermore, they must be applicable to small-scale devices, including internal combustion engines and microturbines, which will play an increasingly important future role as the emphasis in industry shifts progressively toward distributed generation. Even though natural gas combustion produces the smallest amount of CO2 per unit of thermal output, it still results in significant CO2 emissions, almost a 3:1 ratio of grams of CO2 per gram of CH4. In terms of addressing the CO2 pollution issue, the natural gas power/energy industry is, by some estimates, at the * To whom correspondence should be addressed. † Department of Aerospace and Mechanical Engineering. ‡ Department of Chemical Engineering

same point as it was in its NOx efforts in the early 1980s.4 One novel concept currently being investigated by our group to address the CO2 issue is a powergeneration cycle incorporating recuperation of waste heat through CO2 decomposition (reaction below) in a membrane reactor with the aid of high-temperature solid oxide membranes.

2CO2 f 2CO + O2

(R1)

A first step in this concept is the separation of CO2 from the flue gas. CO2 decomposition into CO and O2 is a highly endothermic reaction (552 kJ/mol) that lends itself easily to CO2 emissions reduction as part of a power/energy-generation cycle involving waste heat recuperation. The concept is an equally or even more effective means for waste heat recouping and utilization as catalytic reforming-based technologies. Waste heat recuperation significantly improves power/energy production efficiency.5 This, in turn, minimizes the use of fossil fuel resources and the release into the atmosphere of additional amounts of CO2. Moreover, CO2 decomposition technology has the additional benefit that it produces pure O2. This might significantly impact the economics of power/energy-generation in select industries (e.g., glass manufacturing, waste incineration, etc.). It might also result in additional substantial reductions in CO2 emissions, because the production of oxygen is currently an energy-intensive operation. Although innovative for energy applications, CO2 decomposition carried out in a membrane reactor is not an untried concept.6 For example, the technology has been studied for a number of years in the space program for manned missions to Mars7 and has been proven technically feasible. The technical challenge that our research is currently addressing is how to adopt the existing reactor technology to meet the demands of the power-generation marketplace. The emphasis is on the

10.1021/ie020106y CCC: $22.00 © 2002 American Chemical Society Published on Web 07/17/2002

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use of materials that have both good electronic and ionic conductivities, thus eliminating the need to maintain an external electronic circuit. The long-term performance of these materials in the power/energy-generation environment remains a key focus. Another aspect of the ongoing effort relates to the combustion stability and performance of the proposed integrated process, because little knowledge exists on the flame characteristics of the combustible mixtures that will result under the pertinent process conditions. Even though the combustion characteristics of CH4 have been extensively studied over the years, knowledge regarding the combustion of fuel mixtures containing CH4, CO, and CO2 is quite limited.8-10 In the implementation of the cycle, all three components will be present in significant amounts in the fuel mixture, and each component is expected to affect the overall combustion process. The maintenance of consistently good combustion stability for the wide variety of fuel compositions that can be employed in the cycle is an issue with significant economic and environmental implications, which might turn out to be the principal technological challenge. NOx emissions remain a concern, as it is undesirable to have the CO2 remediation efforts result in an increase in the emissions of other key pollutants. The emphasis in this paper is on the combustion issues related to the integrated CO2-capture/powergeneration cycle. Laminar flame speeds, extinction strain rates, and NOx emissions for such flames were measured and are presented. Numerical simulations of the same flames are also described. Good agreement was found between experiment and simulation. This provides confidence in the use of detailed numerical simulations to probe further the complex aspects of flame stability and pollutant emissions. Experimental Approach The experiments were carried out using a stagnationflow experimental configuration, in which planar flames are established between a nozzle and a variable-temperature plate that acts as the stagnation plane.11 Measurements were conducted along the stagnation streamline and included the determination of flow velocities, stable species concentrations, and NOx. The flow velocities were determined through laser Doppler velocimetry (LDV). A typical velocity profile has a nearzero gradient at the nozzle exit and gradually develops an increasing slope, which reaches its maximum just before the minimum velocity point, where heating starts. This maximum velocity gradient in the hydrodynamic zone is defined as the imposed aerodynamic stretch, K, and the minimum velocity as a reference upstream flame speed, Su,ref, as proposed by Law and co-workers.12 To determine the laminar flame speed, Sou, for a given mixture, Su,ref is plotted with respect to K, and Sou is determined by linear extrapolation to K ) 0. Stable combustion requires a balance between fluid mechanics, heat and mass transfer, and chemical kinetics. When this balance is perturbed, flame extinction occurs, characterized by a sharp reduction in the chemical activity and heat release; local and global extinction events in a combustor can lead to lower combustion efficiency and increased pollutant emissions. By increasing the nozzle exit velocity, K increases, and the flame is pushed toward the stagnation plane; at a

Figure 1. Schematic of the CO2 decomposition-based power generation system.

critical value, Kext, extinction is eventually obtained.12 In the present study, the extinction strain rates, Kext, were also determined. The NOx concentrations were determined by quartz microprobe sampling coupled with a chemiluminescence NO-NO2-NOx analysis. To avoid surface catalytic effects, the probe was water-cooled when sampling the high-temperature combustion products. The accuracy of the measured NOx concentrations is estimated to be within 5%. All measurements were conducted at atmospheric pressure, with a burner nozzle 22 mm in diameter and a separation distance between the nozzle exit and the stagnation plate of 16 mm. Numerical Approach. The laminar flame speeds were calculated using the one-dimensional Premix code of Kee et al.13 The numerical simulation of the counterflow configuration for the determination of the extinction strain rates, concentration flame structures, and NOx contents was conducted by using an onedimensional stagnation flow code.14 The code integrates the steady-state mass, momentum, energy, and species conservation equations along the stagnation streamline in a finite domain. In both codes, radiative transfer from CH4, H2O, CO2, and CO was included.15 The codes were linked to the Chemkin-II16 and Transport17 subroutine libraries. The GRI 3.0 mechanism18 was used for the description of the C1- and C2-hydrocarbon oxidation and NOx kinetics. Results and Discussion Figure 1 presents a schematic of the proposed integrated CO2 decomposition/power-generation process. Part of the flue gas exiting the power/energy-generating unit (e.g., turbine, furnace, boiler) flows through a separation unit, where CO2 is separated and fed directly into a membrane reactor. The reactor utilizes a solid oxide membrane that is permeable only to O2. If the O2 flux though the membrane is sufficiently high because of electrochemical and/or chemical pumping (i.e., through the maintenance of a very low oxygen fugacity on the permeate side by reacting the O2 away), the reactor conversion rate can be very high. The power-generation environment provides advantages for either mode of operation. In the configuration shown in Figure 1, the product exiting the reactor mixes directly with CH4 in the combustion chamber, while the O2 that is produced is diverted elsewhere. This is likely to be the preferred configuration when an electrochemical membrane reac-

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Figure 2. Experimental and simulated laminar flame speeds as functions of CO addition and φinit in CH4/air flames.

Figure 3. Experimental and simulated laminar flame speeds as functions of CO2 addition and φinit in CH4/air flames.

tor is utilized, and waste heat recouping can then be implemented through the heating of the CO2 feed stream entering the reactor. For furnace/boiler-type applications, a conventional membrane reactor system might be most appropriate, in which case use will be made of the produced O2 in the combustion chamber. This configuration lends itself more appropriately to staged combustion. In either case, the feed entering the combustor contains substantial quantities of CO and CO2. It is essential for the proper design of such combustors that laminar flame speeds and the extinction condition for a given combustible mixture are known. Figure 2 depicts the experimentally measured Sou values for CH4/air mixtures as a function of CO addition for three different initial equivalence ratios, φinit, equal to 0.63, 0.68, and 0.73; φinit is the equivalence ratio of the CH4/air mixture before CO is added, defined as φinit ) (XCH4/XO2)actual/ (XCH4/XO2)stoich. The CO mole fraction in the fuel is defined as XCO/(XCH4 + XCO), where Xi represents the mole fraction for species i in the total reacting mixture. Note that Sou gradually increases as XCO increases. This is not a surprising result because adding CO, which itself is a fuel, to a fuel-lean mixture increases the overall equivalence ratio φtotal {defined in this case as [(XCO+XCH4)/XO2]actual/[(XCO+XCH4)/XO2]stoich} and enhances the overall reaction intensity. The lines on the same figure represent the results of the simulations; they are in good agreement with the experimental data. Figure 3 depicts the effect of CO2 addition on Sou for three different values of φinit. In contrast to the CO case,

Figure 4. Experimental and simulated extinction strain rates as functions of the mole fraction of CO or CO2 added to CH4/air flames (φinit ) 0.65).

the addition of CO2 decreases the laminar flame speed. A detailed analysis reported previously10 indicates that CO2 addition has a significant thermal effect on the intensity of burning through dilution and radiation. CO2, being inert, does not affect the overall equivalence ratio of the mixture, so, upon addition, φtotal remains constant and equal to φinit. The effect of CO and CO2 addition on the flame stability is shown in Figure 4, which depicts the experimental and predicted Kext values for a given value of φinit and different levels of CO and CO2 additions. Increasing the fraction of CO noticeably increases Kext, thus improving, the flame stability characteristics. Increasing the CO2 molar fraction, on the other hand, decreases Kext, thus resulting, under realistic conditions, in flames with inferior stability characteristics. The results of Figure 4 also indicate close agreement between experiments and simulations. The effect of CO and CO2 addition to CH4/air flames on the maximum NOx concentration has also been investigated experimentally in the stagnation-flow configuration by our group, and detailed discussions have been presented elsewhere.19 To summarize the findings for the addition of CO, two types of experiments were carried out. In the first series of experiments, the CH4/ air ratio was kept constant while CO was added to the reactive mixture (equivalent to keeping φinit constant). In the second series of experiments, upon addition of CO, the air flow was adjusted so that the total equivalence ratio, φtotal remained constant. For both types of experiments, the NOx concentration profiles were measured, and the maximum mole fraction was identified. The key finding for both experiments was that, with increasing CO mole fraction in the fuel at constant φinit or φtotal, the attendant maximum NOx mole fraction increased, but the increase was smaller in the constantφtotal case. Given the close agreement that has been observed between experiments and numerical predictions for all cases investigated, additional simulations were performed to probe the complex aspects relating to pollutant formation processes. Figure 5, for example, depicts predicted normalized (by the 0% CO addition case) and actual (inset) (XNOx)max values for (CH4 + CO)/air flames. For the flames in this figure, when the CO mole fraction in the fuel is increased, φtotal is appropriately adjusted to maintain a constant maximum flame temperature, Tmax; Tmax is used as an indicator of the energetic output

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Figure 5. Simulated maximum NOx concentrations for CH4/CO/ air flames as functions of CO addition and Tmax normalized by the 0% CO case. The inset depicts the absolute values (φinit ≈ 0.7).

Figure 6. Simulated maximum NOx concentrations for CH4/CO2/ air flames as functions of CO2 addition and Tmax normalized by the 0% CO2 case. The inset depicts the absolute values (φinit ≈ 0.7).

of a reacting mixture. Judging from the results in Figure 5, CO addition appears to increase (albeit slightly) the rate of NOx formation. The effect is more pronounced for small Tmax. The strong effect that Tmax itself exerts on (XNOx)max is also obvious from the inset of Figure 5. Analysis of the flame structures revealed that the addition of CO favors the conversion of N2O to NO via the reaction

N2O + CO f NCO + NO

(R2)

The N2O is chiefly produced by the three-body reaction

N2 + O + M f N2O + M

(R3)

As Tmax increases, the relative contribution of the thermal NO-production mechanism increases over that of the three-body mechanism, and the promoting effect of CO on NO formation is diminished. Adding CO2 to the reactive mixture while maintaining a constant φinit decreases NOx emissions.10 The agreement between theory and experiment is again generally satisfactory. The decreasing trend of emissions with CO2 addition is of a thermal nature. With increasing CO2 mole fraction in the fuel (at constant CH4/air ratio), the flames are further diluted, and Tmax is reduced, thus affecting the effectiveness of the thermal NOx mechanism. Figure 6 depicts the results of simulations for a (CH4 + CO2)/air flame, in which, as the CO2 mole fraction in the fuel increases, the molar flow of air decreases to keep Tmax constant. Note that, as the CO2 mole fraction increases, the NOx formation rate increases (albeit slightly). This interesting behavior occurs because,to maintain the same Tmax with CO2 added to the reactive mixture, the equivalence ratio, φ, must be increased. Increasing φ enhances the CH radical concentration, thus increasing the reaction rate of the ratelimiting CH + N2 f HCN + N reaction, which, in turn, increases NOx production through the prompt mechanism. Figure 7 depicts experimental (points) and predicted (lines) values of Sou for CO2/CO/CH4/air flames. The composition of these mixtures is determined on the basis of the configuration shown in Figure 1. On the horizontal axis is indicated the cumulative fraction of CO2 and CO in the fuel fed into the combustor, which is defined as (XCO2 + XCO)/(XCO2 + XCO + XCH4). The parameter in this figure is the membrane reactor conversion, which

Figure 7. Experimental (points) and calculated (lines) laminar flame speeds as functions of the (CO + CO2) mole fraction in the fuel and reactor conversion in CH4/air flames (φinit ) 0.7).

is equal to the ratio XCO/(XCO2 + XCO) in the feed to the combustor. For both the experiments and the simulations, the CH4/air ratio is kept constant, while CO and CO2 are being added to the reactive mixture (φinit ) 0.7). Both the cumulative fraction of CO2 and CO and the membrane reactor conversion significantly affect Sou. For lower levels of reactor conversion, Sou decreases as the cumulative fraction of CO2 and CO increases. As the level of reactor conversion increases, the decrease in Sou is no longer as pronounced. For high levels of conversion, Sou increases as the cumulative fraction of CO2 and CO increases. The behavior shown in Figure 7 can be explained in terms of the effects that CO2 and CO individually have on Sou (Figures 2 and 3). For low reactor conversions, mostly CO2 enters the combustor, which was shown previously (Figure 3) to have a negative impact on Sou. For high reactor conversions, more CO is produced and enters the combustor, which, as previously noted, has a positive effect on the burning intensity. Figure 8 presents again simulated values of Sou for CO2/CO/CH4/air flames. In this figure, however, upon addition of CO/CO2 the air flow is adjusted so that the total equivalence ratio, φtotal, remains constant and equal to 0.7. Although the behavior in Figure 8 is qualitatively similar to that in Figure 7 the level of reactor conversion above which Sou increases with in-

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Figure 8. Calculated laminar flame speeds as a function of the (CO + CO2) mole fraction in the fuel and reactor conversion in CH4/air flames (φtotal ) 0.7).

Figure 9. Experimental (points) and calculated (lines) extinction strain rates as functions of the (CO + CO2) mole fraction in the fuel and reactor conversion in CH4/air flames (φinit ) 0.7).

creasing cumulative (CO2 + CO) fraction is significantly greater. This is because, to maintain φtotal constant as CO/CO2 is added, the CH4 concentration must be reduced for a fixed flow of air, which reduces the reactivity of the mixture. Thus, the positive effect of the addition of CO on Sou is reduced and becomes more apparent at higher cumulative (CO2 + CO) fraction. Figure 9 depicts experimental and predicted values of Kext for CO2/CO/CH4/air flames pertinent to the process configuration shown in Figure 1 for constant φinit ) 0.7. The behavior in Figure 9 is similar to that exhibited by Sou. For lower levels of conversion and for a given reactor conversion, Kext decreases as the cumulative (CO2 + CO) fraction in the feed to the combustor increases. As the level of conversion increases, however, the decrease becomes more gradual, and eventually, Kext increases with the cumulative (CO2 + CO) fraction. The behavior shown in Figure 9 can again be explained in terms of the effects that CO2 and CO individually have on Kext (Figure 4). For low reactor conversions, mostly CO2 enters the combustor, which was shown previously (Figure 4) to have a negative impact on Kext. For higher reactor conversions, more CO is produced and enters the combustor, which, as previously noted, has a positive effect on the flame stability characteristics. Additional simulations of Kext values for CO2/CO/CH4/air flames were carried out, in which, upon addition of CO/CO2, the air flow is adjusted so that the total equivalence

Figure 10. Experimental (points) and calculated (lines) maximum NOx concentrations as functions of the (CO + CO2) mole fraction in the fuel and reactor conversion in CH4/air flames (φinit ) 0.7).

ratio, φtotal, remains constant and equal to 0.7. The behavior found is qualitatively similar to that in Figure 9, but the level of reactor conversion above which Kext increases with increasing cumulative (CO2 + CO) fraction is significantly greater. The observed response of Kext with reactor conversion and cumulative (CO2 + CO) fraction is similar to that observed for the laminar flames speeds in Figures 7 and 8, which is anticipated given that both propagation and extinction are hightemperature phenomena and are controlled by similar kinetics. The NOx formation characteristics of various CO2/CO/ CH4/air combustion mixtures resulting from the process configuration shown in Figure 1 were also investigated. Figure 10 depicts experimental and predicted values of the maximum NOx concentration as a function of cumulative (CO2 + CO) fraction in the feed, for various levels of membrane reactor conversion and φinit ) 0.7. Again, for lower levels of conversion and for a given reactor conversion, NOx emissions decrease with the cumulative (CO2 + CO) fraction. As the level of conversion increases, the decrease is no longer as great. For higher levels of conversion, NOx emissions increase with the cumulative (CO2 + CO) fraction. This behavior can be explained by considering the individual effects that CO and CO2 have on NOx emissions. Increasing the CO mole fraction results in higher NOx production, given that the CO addition increases the flame temperature. On the other hand, adding CO2 tends to decrease the flame temperature and, consequently, the NOx production, given that CO2 acts mostly as a diluent. Maximum NOx concentrations for CO2/CO/CH4/air flames for constant φtotal ) 0.7 were also calculated. As with the Kext and Sou behavior, the behavior observed is qualitatively similar to that shown in Figure 10, but the level of reactor conversion above which the maximum NOx concentration increases with increasing cumulative (CO2 + CO) fraction is significantly greater. Figure 11 displays the results of extensive numerical simulations that are indicative of flame intensity and combustor stability (e.g., Sou, Kext) and NOx emissions of CO2/CO/CH4/air combustion mixtures resulting from the process configuration shown in Figure 1. The results shown in Figure 11 demonstrate that there is a broad range of operating conditions (high membrane reactor conversions) for which the process of Figure 1, in addition to utilizing waste heat and improving overall combustor efficiency, also positively impacts flame

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Figure 11. Calculated values of (a) Sou, (b) NOx, and (c) Kext as functions of the (CO + CO2) mole fraction in the fuel and reactor conversion in CH4/air flames (φinit ) 0.7).

intensity and combustor stability. For the most part, this improvement comes at the cost of increased NOx emissions. For lower membrane reactor conversions, the resulting flames are characterized by lower NOx emissions, but they are also less intense and less stable. From an operating standpoint intermediate levels of reactor conversion might be more appropriate. Under such conditions, flame intensity, stability, and NOx emissions are only marginally affected (the effects of CO counterbalance those of CO2), while the process still allows for substantial waste heat recovery and oxygen production. Concluding Remarks A combined experimental and detailed numerical investigation was conducted on the effects of the addition of CO/CO2 mixtures on the burning and pollutant emission characteristics of lean atmospheric CH4/air flames. The study is of relevance to a proposed advanced power-generation cycle involving waste heat utilization through CO2 decomposition using a high-temperature membrane reactor that also produces pure O2 that can be selectively removed. The resulting mixtures from the decomposition of CO/CO2 can be fed into the combustor along with CH4 and air. It was found that laminar flame speeds, extinction strain rates, and NOx emissions gradually increase as XCO increases in lean CH4/air flames. This is a result of

the extra fuel (i.e., CO) that is added to lean flames resulting in increased flame temperatures and more intense burning. The increase in NOx emissions appears to be mostly of a thermal nature, although there is still a slight increase in NOx emissions even for flames with constant maximum flame temperature. Laminar flame speeds and extinction strain rates, on the other hand, were found to decrease as CO2 was added to CH4/air flames. CO2 addition has a significant thermal effect on the burning intensity through dilution and radiation effects. It was found that, for flames with the same maximum flame temperature, the prompt mechanism (CH + N2 f HCN + N) plays a prominent role for the positive (albeit small) effect on NOx formation. Considering the flame stability and pollution emissions of various CO2/CO/CH4/air mixtures, it was found that, for low reactor conversions, NOx reductions can be achieved, but with reduced laminar flame speeds and extinction strain rates. For high reactor conversions, increased laminar flame speeds and extinction strain rates are observed, accompanied, however, by an increase in NOx emissions. Accounting for both process economics and technical feasibility in overall system design, the optimal choice might be intermediate reactor conversions, which allow for substantial waste heat recovery without unduly impacting combustion efficiency and pollutant emissions.

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Acknowledgment The support of the California Energy Commission through the Energy Innovations Small Grant (EISG) program is gratefully acknowledged. Literature Cited (1) Reische, D. Working Paper on Carbon Sequestration, Science and Technology; Office of Science/Office of Fossil Energy, U.S. Department of Energy (DOE), U.S. Government Printing Office: Washington, DC, 1999. (2) Crane, R. G.; Hewitson, B. C. Double CO2 Precipitation Changes for the Susquehanna Basin: Down-Scaling from the Genesis General Circulation Model. Int. J. Climatol. 1998, 18, 65. (3) Betts, R. A.; Cox, P. M.; Woodward, F. I. Simulated Responses of Potential Vegetation to Double-CO2 Climate Change and Feedbacks on Near-Surface Temperature. Global Ecol. Biogeogr. 2000, 9, 171. (4) Herzog, H. J., Ed. Proceedings of the Third International Conference on Carbon Dioxide Removal, Cambridge, MA, USA. Energy Convers. Manage. 1997, 38, S1-S2 Suppl. S. (5) Ren, J. Y.; Qin, W.; Egolfopoulos, F. N.; Mak, H.; Tsotsis, T. T. Methane Reforming and Its Potential Effect on the Efficiency and Pollutant Emissions of Lean Methane-Air Combustion. Chem. Eng. Sci. 2001, 56, 1541. (6) Itoh, N.; Sanchez, M. A.; Xu, W. C.; Haraya, K.; Hongo, M. Application of a Membrane Reactor System to Thermal Decomposition of CO2. J. Membr. Sci. 1993, 77, 245. (7) Frankie, B. M.; Zubrin, R. Chemical Engineering in Extraterrestrial Environments. Chem. Eng. Prog. 1999, 95, 45. (8) Lowder, R. Gas Turbine Operates on Catalytic Reformer Gas. Oil Gas J. 1989, 9, 83. (9) Masri, A. R.; Dibble, R. W.; Barlow, R. S. Chemical Kinetic Effects in Non-Premixed Flame of H2/CO2 Fuel. Combust. Flame 1992, 91, 285. (10) Qin, W.; Egolfopoulos, F. N.; Tsotsis, T. T. A Detailed Study of the Combustion Characteristics of Landfill Gas. Chem. Eng. J. 2001, 3773, 1.

(11) Egolfopoulos, F. N.; Zhang, H.; Zhang, Z. Wall Effects on the Propagation and Extinction of Strained, Laminar, Premixed Flames. Combust. Flame 1997, 109, 237. (12) Law, C. K. Dynamics of Stretched Flames. Proc. Combust. Inst. 1988, 22, 1381. (13) Kee, R. J.; Grcar, J. F.; Smooke, M. D.; Miller J. A. A Fortran Program for Modeling Steady Laminar One-Dimensional Premixed Flames; Report SAND85-8240; Sandia National Laboratories: Albuquerque, NM, 1985. (14) Egolfopoulos, F. N.; Campbell, C. S. Unsteady, Counterflowing, Strained Diffusion Flames: Frequency Response and Scaling. J. Fluid Mech. 1996, 318, 1. (15) Law, C. K.; Egolfopoulos, F. N. A Unified Chain-Thermal Theory of Fundamental Flammability Limits. Proc. Combust. Inst. 1992, 24, 137. (16) Kee, R. J.; Rupley, F. M.; Miller J. A. Chemkin-II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics; Report SAND89-8009; Sandia National Laboratories: Albuquerque, NM, 1989. (17) Kee, R. J.; Warnatz, J.; Miller J. A. A Fortran Computer Code Package for the Evaluation of Gas-Phase Viscosities, Conductivities, and Diffusion Coefficients; Report SAND83-8209; Sandia National Laboratories: Albuquerque, NM, 1983. (18) Smith, G. P.; Golden, D. M.; Frenklach, M.; Moriarty, N. W.; Eiteneer, B.; Goldenberg, M.; Bowman, C. T.; Hanson, R. K.; Song, S.; Gardiner, W. C., Jr.; Lissianski, V. V.; Qin, Z. GRI-Mech 3.0; Gas Research Institute (GRI): Chicago, IL, 1999. Available at http://www.me.berkeley.edu/gri_mech/. (19) Ren, J. Y.; Tsotsis, T. T.; Egolfopoulos, F. N. Methane Reforming and NOx Emission Control of Lean Methane/Air Combustion with Addition of Methane Reforming Products. Combust. Sci. Technol. 2002, 174 (4), 181.

Received for review February 4, 2002 Revised manuscript received May 23, 2002 Accepted May 25, 2002 IE020106Y