Reactive Membrane Separations for Power-Generation Applications

The results of ongoing studies of methane reforming for power-generation applications are discussed. Of interest are processes involving chemical recu...
0 downloads 0 Views 115KB Size
Download PDF
Ind. Eng. Chem. Res. 2001, 40, 5155-5161

5155

Reactive Membrane Separations for Power-Generation Applications: Pollutant Emission Aspects J.-Y. Ren,† F. N. Egolfopoulos,† and T. T. Tsotsis*,‡ Department of Aerospace and Mechanical Engineering and Department of Chemical Engineering, University of Southern California, Los Angeles, California 90089

The results of ongoing studies of methane reforming for power-generation applications are discussed. Of interest are processes involving chemical recuperation for power generation utilizing the endothermic methane reforming reaction and combining both reactive separation and combustion. This study focuses on the dynamics and structure of flames of lean mixtures of air with CH4/H2/CO/CO2/H2O, a five-gas fuel mix resulting when the product exiting a reformer is combined with fresh CH4 prior to entering the combustor. Experiments are carried out in the single jet-wall, stagnation-flow configuration using laser Doppler velocimetry for determination of the axial velocity profiles. The concentrations of stable species are measured by mass spectrometry, nitrogen oxides are measured by chemiluminescence analysis, and the temperature distribution along the centerline is measured using fine-wire thermocouples. Numerical simulations are conducted to solve for the steady-state mass, momentum, energy, and species conservation equations for the stagnation streamline in a finite domain with the addition of radiation transfer. The experimental results compare favorably with the predictions. Introduction In recent years, the development of efficient, cleanburning natural gas combustion systems has attracted the attention of many scientists and engineers, as natural gas is the least polluting of all hydrocarbon fuels. Although improving efficiency remains an important goal of such efforts, the key impetus is the attempt to meet increasingly more stringent pollutant emission control requirements. Lean premixed combustion is the emphasis of most research today, because of its promise for reducing NOx emissions. Lower flame temperatures, however, characterize lean premixed combustion. This, in turn, implies reduced energy/power-generation output for the combustor. Burning too lean, in addition, results in flame stability problems such as flame blowout. The optimal operation of fuel-lean combustors requires a fine balance between burning lean enough to obtain low NOx levels and not so lean that high power output together with reduced CO and unburned hydrocarbon (UHC) emissions cannot be attained. It is essential for the proper design of such combustors that one has an accurate knowledge of flame speeds and extinction conditions for a given combustible mixture. Maintaining combustion stability under fuel-lean conditions is a technically challenging task with significant environmental and economic implications. One idea that has been proposed is the addition to natural gas of another gaseous fuel such as hydrogen (H2). The latter has a higher burning intensity, thus creating a fuel mixture with improved combustion stability characteristics under lean-burning conditions.1-6 Generally, most of the prior studies report that the addition of H2 to natural gas results in a combustion mixture with enhanced combustion stability and high power/energy output. * Author to whom correspondence should be addressed. † Department of Aerospace and Mechanical Engineering. ‡ Department of Chemical Engineering.

However, the economic production of H2 for use in such power/energy-generation applications still remains a significant technological challenge. Among the advanced cycle concepts for power/energy generation involving the use of H2 that have been studied in recent years,6,7 the most promising involve waste heat recovery from the combustion system exhaust. In this advanced concept, H2 is produced through the use of an endothermic reaction (reforming, cracking, etc.) of the fuel, and at least part of the energy required is “recouped” in the form of flue gas exhaust “waste heat.” For powergeneration applications involving natural gas, the most promising avenue for chemical recuperation of the “waste heat” is the endothermic catalytic reforming process of methane (the key component of natural gas) described by the following two reactions

CH4 + H2O T 3H2 + CO

(1)

CO + H2O T H2 + CO2

(2)

In gas turbine power-generation applications, such advanced concepts involving chemical recuperation are termed the CRGT cycle. The design characteristics and performance of the CRGT cycle have been discussed in a prior publication by our group.6 One of the simplest such cycles, for example, is shown in Figure 1 and involves recouping the flue gas energy through conventional heat exchangers. From the perspective of improving overall energy conversion efficiency, it has been suggested8 that part of the exhaust gases be recycled and directly mixed with the natural gas prior to injection into the reformer. CO2 is also thought to be capable of directly reacting with CH4 according to the catalytic reaction

CH4 + CO2 T 2H2 + 2CO

(3)

Reforming in the presence of substantial amounts of CO2 (in addition to steam), i.e., “dry reforming”, in

10.1021/ie001054k CCC: $20.00 © 2001 American Chemical Society Published on Web 10/04/2001

5156

Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001

Figure 1. Schematic of the CRGT cycle for the case of reforming through external H2O addition.

addition to improving the energetics of the CRGT cycle, also provides a plausible means for recycling the CO2 found in the flue gas to produce valuable H2 and synthesis gas. It also makes the CRGT concept potentially feasible in areas where water is not abundant. Because reaction 3 is even more endothermic than reaction 1 (31 vs 26% in LHV), the use of dry reforming also allows for the recovery of larger amounts of waste heat. Of all available options, the most promising one involves the direct separation of CO2 from the flue gas. Because minimizing waste heat losses is of paramount importance in this application, the most plausible means for separating the CO2 is through the use of hightemperature membranes. The same membranes could find application in the reforming step, where a catalytic membrane reactor could be the preferred configuration from a heat recuperation standpoint over a more conventional reformer. In the CRGT cycle and related applications, the gaseous products exiting the reformer, either intact or having undergone membrane separation to remove undesirable components (see subsequent discussion), are allowed to mix with fresh natural gas and are then fed into the combustor. The open literature technical knowledge on the combustion characteristics of the resulting five-gas fuel mixture (CH4/H2/CO/CO2/H2O) is virtually nonexistent.9 The knowledge regarding the flame structure of binary fuel mixtures containing CH4 and either H2, CO, H2O, or CO2 is also somewhat limited.10 Flame studies on the effect of hydrogen addition to methane have been restricted to flame speed and ignition delay measurements.1,11 No information has been provided in terms of flame extinction and pollutant emissions. More studies have been devoted to the effect that the addition of such compounds to natural gas has on NOx production. Maughan et al.12 report that hydrogen addition results in a somewhat increased NOx production. Studies by Tuzson13 and Maughan et al.12 indicate that steam injection can reduce NOx emissions by reducing flame temperature. As with the flame studies, however, these studies, although they contain practical information, do not provide detailed kinetic mechanistic data that would help generate substantial physicochemical insight. Most recently Gerhard et al.14 have studied the effect of H2 and CO addition on NOx emissions in diesel engines or gas turbines. Ladommatos et al.15 and Groppi et al.16 have investigated the effect

of CO2 and H2O addition on NOx production in diesel engines and gas turbines. Judging from the above considerations, our group has undertaken a research project whose goal is to investigate technical and design aspects of the CRGT and other relevant power-generation cycles involving CH4 reforming. As part of this project, various power-generation configurations are being analyzed, and their relative merits are addressed in terms of technical feasibility and overall thermal efficiency improvement. The results of this part of our studies have been reported recently.6 Overall thermal efficiency improvement depends strongly on the degree and means (direct mixing vs use of conventional heat exchangers) of waste heat recouping and the type (conventional vs membrane reactor) and operating conditions of the reformer. Detailed experimental and numerical studies are conducted on the dynamic behavior and structure of lean CH4/air flames in the presence of species resulting from the reforming processes. We have, so far, reported on the characteristics of simple binary mixtures of methane with these species. In a recent paper, for example, we examined the effect of CO2 addition on flame dynamics and structure.17 Laminar flame speeds, extinction strain rates, and detailed flame structures were determined for conditions of relevance to practical combustors, and the effect of CO2 addition on the NOx emissions was assessed. Results indicated that the presence of CO2 in the fuel feed results in a substantial reduction of the laminar flame speeds and extinction strain rates. Even though CO2 is an active participant in combustion reactions, detailed path analysis reveals that flame dynamics and structure have a minor effect on such reactions, and the effect of CO2 on the combustion characteristics was found to be mainly of a thermal nature. Fundamental flammability limits were also calculated, and it was found that, as the CO2 concentration increases, the flammable range noticeably decreases. Finally, our studies revealed that the addition of CO2 increases the NOx emissions per gram of CH4 consumed. The results of our studies on the effect of individual additions of H2, CO, or H2O species to CH4/air flames are reported in two recent publications.6,18 It was found that the presence of H2 and CO enhances flame stability by increasing the flame speeds and the combustion mixture’s resistance to extinction. The presence of H2O, as with CO2, might have a negative impact on the overall flame stability improvement. It was shown that H2 enhances flame stability by generating more H radicals, thus favoring the main branching reaction (H + O2 f OH + O), which, in turn, enhances the burning rate. The addition of CO results in stronger flames by its participation in the chain carrier reaction (CO + OH f CO2 + H). In this paper, the emphasis is on the combustion characteristics of the full mixture of CH4 with the four other gases found in the product stream exiting the reformer. First, results on the laminar flame speed characteristics of this mixture will be presented. These results will be compared with preliminary results on the extinction strain characteristics of such mixtures.18 From the experimental findings with binary mixtures on the negative impacts of H2O and CO2,6,18 results will be presented discussing the beneficial effects, if any, that the separation of these gases (e.g., through the use of high-temperature membranes) has on the flame

Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5157

stability characteristics and intensity during combustion. Finally, the pollutant formation characteristics of such mixtures will be discussed. The goal of these studies is to improve the understanding of the physicochemical mechanisms involved during the combustion of such mixtures. As a result, one might be able to increase their burning stability and flame intensity while simultaneously reducing the NOx emissions. This work is carried out in tandem with a parallel experimental investigation by our group that focuses on the development of membrane and reactive membrane separation technologies of relevance to CRGT and other similar concepts. The emphasis in this effort is on the development of CO2 and H2O permselective membranes for use with reformate or flue gas mixtures,19-21 the study of membrane reactors for steam22-24 and dry reforming25,26 of CH4, and the development of steamresistant, high-temperature SiC membranes.27 Because of space limitations, we provide no additional details about this aspect of our work in this paper, but we will discuss key aspects at the meeting. Experimental Approach The combustion experiments reported in this paper were carried out using the stagnation-flow experimental configuration. Planar flames are established between a aerodynamically shaped nozzle from which the reactive gases are exiting and a variable-temperature plate that acts as the stagnation plane.28 Measurements were conducted along the stagnation streamline. They included the determination of axial profiles along the stagnation streamline of the flow velocities, concentrations of stable species including NOx, and temperatures. The flow velocities were determined through laser Doppler velocimetry (LDV). Typically, the measured velocity profile in this experimental configuration has a near zero 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 is defined as the imposed strain rate, K, and the minimum velocity as the reference upstream flame speed, Su,ref.29-31 To determine the laminar flame speed, Sou, Su,ref is plotted with respect to K, and Sou is found by linearly extrapolating to K ) 0. In the stagnation-flow experimental configuration, as the nozzle exit velocity increases, K increases, and simultaneously the flame is pushed toward the stagnation plane. A critical value exists beyond which the flame cannot be sustained, which is defined as the extinction strain rate, Kext.29 The concentrations of all stable species were determined by molecular beam mass spectrometry. A quenching quartz probe with an orifice diameter of about 20 µm was used to extract samples from the center of the flame. The sampling line was maintained at 270 Pa pressure and was connected to the mass spectrometer. A small part of the sample was allowed into the mass spectrometer chamber through a pinhole. A mechanical pump withdrew the rest of the sample. The sample passed into the second chamber through a skimmer to form a pseudo-molecular beam. The beam was chopped to provide background discrimination and focused into the ionizer region of the quadrupole mass spectrometer. The ion signals were detected with a lock-in amplifier. Calibration of the mass spectrometer was achieved by experimentally determining relative sensitivities. The

experimental data and models (see discussion below) showed good agreement, providing validation for both the models and the experiments. Chemiluminescence was used for the measurement of NOx again using a quenching quartz probe with an orifice diameter of about 20 µm. The instrument (model 42C, Thermo Environmental Instruments Inc., Franklin, MA) was calibrated using standard gases. Temperatures were measured using fine-wire thermocouples aligned with the flame surface (and thus isothermal contours) to minimize conductive losses. Corrections for radiation effects were performed during data processing. In all experiments reported here, the plate temperature was maintained at 900 K, the diameter of the nozzle was 22 mm, and the separation distance between the nozzle and the plate was 16 mm. The sampling position for the NOx measurements was 4 mm away from the plate. Further details about the experimental system, the experimental techniques used and their limitations, and the errors in the experimental measurements can be found in our recent publications.6,17,18 Numerical Approach The basic idea behind the concept being studied here is to use the enthalpy contained in the combustor exhaust gases to promote natural gas catalytic reforming, thus chemically recouping the waste heat. There are a number of different concept configurations. One of the simplest involving a gas turbine and flue gas energy recouping through heat exchangers is shown schematically in Figure 1. Other more elaborate schemes might involve direct mixing of the gas to be reformed with the flue gas or the CO2, which would, in this case, be separated out of the flue gas through the use of hightemperature, CO2 permselective membranes. One can opt to use a conventional reformer or, preferably from a heat recuperation standpoint, a catalytic membrane reactor. A simulation program has been developed to systematically investigate many of these concepts. The characteristics of two CRGT cycles including the one shown in Figure 1 have been discussed, for example, in a prior publication,6 and a more comprehensive discussion can be found elsewhere.32 No further analysis will be provided here. All further calculations will be with the CRGT configuration of Figure 1. One important parameter is the ratio R1, which, as indicated in the Figure 1, is defined as the ratio of the amount of CH4 directly fed into the reformer to the total CH4 (that fed into the reformer plus the amount fed directly into the combustor). Other important parameters are the reactor temperature (unless otherwise noted taken to be 823 K), and the steam-to-methane ratio (unless otherwise noted taken to be 2:1). A conventional reactor was assumed, and the exit composition was calculated using the STANJAN code33 that is used to calculate equilibrium properties. The laminar flame speeds were calculated using the 1-D Premix code of Kee et al.34 The numerical simulations of the thermal and concentration flame structures were conducted using a one-dimensional stagnation flow code35 that solves for the coupled 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 using the optically thin assumption was included.36 The codes were linked to the Chemkin II37 and Trans-

5158

Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001

Figure 2. Numerically and experimentally determined laminar flame speeds and species mole fractions of (five-gas fuel)/air mixtures as functions of R1 (φ ) 0.65 for the R1 ) 0 case).

port38 subroutine packages. The GRI 3.0 mechanism39 was used for the description of the C1, C2 , and C3 hydrocarbon oxidation reactions and the NOx formation kinetics. Results and Discussion Laminar Flame Speeds. For the CRGT configuration of interest here (Figure 1), the reactor uses an external steam supply. The products exiting the reactor are then fed into the combustor, together with the main CH4/air flows, to produce the high-energy combustion product mixtures, which drive the turbine. As detailed further in a prior publication,6 the overall system thermal efficiency increases with reformer reactor operating temperature and R1. The proper choice for R1 will depend on process economics, technical feasibility, which includes considerations of burning stability and pollutant emissions, and appropriate reactor sizing. Only aspects of burning stability and pollutant emissions will be further discussed here. We first report on the burning characteristics of the CRGT mixtures. Figure 2 depicts experimentally determined laminar flame speeds of such combusting mixtures as functions of R1. Also shown in Figure 2 are the numerically calculated results. Good agreement between theory and experiments is found, with the difference between theory and experiments typically being less than 5%. Shown, in addition, in Figure 2 are the species mole fractions of the mixtures entering the combustor. These results indicate that increasing R1 results in moderate decreases of Sou. Loosely interpreted, these results indicate that combustion of such mixtures results in somewhat lower intensity flames. Prior experimentation with binary mixtures of CH4 with the individual components indicated that the additions of CO and H2 have a favorable impact on laminar flame speeds, whereas the additions of CO2 and H2O have a negative impact. The fact that the laminar flame speed decreases with increasing R1 indicates that the negative effects of H2O/CO2 addition outweigh the positive effects of H2/CO addition. Interestingly enough, as reported previously,6 for the same flames, one observes that the extinction strain rate Kext shows a slight increase with R1, indicating that the presence of hydrogen tends to improve the stability characteristics of what otherwise appear to be weaker flames. Of course, these conclusions depend on the reactor conversion, as can be seen in Figure 3, which indicates the effect of R1 on laminar

Figure 3. Numerically determined laminar flame speeds of (fivegas fuel)/air mixtures as functions of R1 for various reactor temperatures (φ ) 0.65 for the R1 ) 0 case).

flame speeds for four different reactor operating temperatures (500, 550, 600, and 650 °C). As the reactor temperature increases, the content of H2 in the mixture increases, whereas that of H2O decreases, thus favorably impacting the laminar flame speed characteristics. For the higher reactor temperatures, the laminar flame speed increases with R1 similarly to the extinction strain rate behavior previously reported. However, the higher the reactor temperature, the more limited the opportunities for direct waste heat utilization. From the discussion presented above, one expects that removing H2O and CO2 from the products exiting the reformer (e.g., through the use of high-temperature membranes) before they enter the combustor will have beneficial effects in terms of the laminar flame speed effects. This is shown schematically in Figure 4a, which depicts the laminar flame speeds of various combustion mixtures corresponding to different R1 values as functions of the fraction of the water in the reformer exit stream that is being removed. Figure 4b is a threedimensional version of the results shown in Figure 4a. As expected, increasing the fraction of H2O that is being removed strongly affects the laminar flame speed. When no H2O is removed, the laminar flame speed decreases as a function of R1, as also expected on the basis of the behavior exhibited in Figure 2. Separating the water from the reactor exit stream before it enters the combustor results in higher laminar flame speeds, with this effect being stronger for the larger R1 values. Comparing, for example, the corresponding behaviors for the R1 ) 0.1 and 0.5 mixtures, the laminar flame speed for the R1 ) 0.1 mixture increases only mildly with the fraction of H2O removed, whereas the change is much more significant for the R1 ) 0.5 mixture. As a result, there is a definite value in the fraction of water that is removed beyond which the mixture corresponding to the higher R1 has a higher laminar flame speed. One must be cautious, however, in deciding what fraction of water to remove, as the presence of H2O is known to have beneficial effects in reducing NOx emissions; relevant discussion follows. CO2 removal has similar effects on the laminar flame speeds. The effects are more visible, however, for higher reactor temperatures or after the H2O itself has been removed. For lower reactor temperatures, H2O removal benefits tend to mask those corresponding to CO2 removal. NOx Formation. We have also investigated the NOx formation characteristics of various combustion mixtures corresponding to different R1 values. It is impor-

Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5159

(b)

Figure 4. (a) Numerically determined laminar flame speeds of (five-gas fuel)/air mixtures as functions of R1 and the amount of H2O removed (φ ) 0.65 for R1 ) 0 case). (b) Three-dimensional representation of numerically determined laminar flame speeds of (five-gas fuel)/air mixtures as functions of R1 and the amount of H2O removed (φ ) 0.65 for the R1 ) 0 case).

tant during investigations of the NOx emission problem to take burning intensity into account, as quantified, for example, by the laminar flame speed (Sou) experimental measurements of such flames. In comparisons of two combustion mixtures in terms of their NOx emission characteristics, it is important that the two mixtures have similar burning intensities (i.e., laminar flame speeds). Flames with lower burning intensities might result in partial or complete quenching during lean premixed combustion and, therefore, in significant emissions of CO and unburned hydrocarbons (UHCs). A comparison of the propensities of two combustion mixtures with the same laminar flame speed to form NOx is a sensitive indicator of the temperature characteristics of the corresponding flames. For example, if one were to maintain the same Sou for a given mixture while decreasing its flame temperature, common wisdom dictates that this would result in lower NOx emissions, as the thermal (Zeldovich) mechanism of NOx formation is of significance in fuel-lean combustion. Figure 5 summarizes the results of our studies of NOx formation for combustion mixtures corresponding to different R1 values for flames with the same laminar flame speeds. Note that, to attain the same laminar flame speeds with all of these different fuel mixtures, one must adjust the fuel-to-air ratio in the combustor appropriately. Figure 5 depicts the experimentally measured NOx concentrations at a distance of 4 mm from the stagnation plate for fuel mixtures corresponding to different R1 values scaled with respect to the NOx measurement of the R1 ) 0 fuel mix. On the same figure, we also plot for comparison the integral (i.e., total) N2 destruction (i.e., reaction) rates for the same fuel

Figure 5. Experimentally determined NOx emission ratio and numerically determined integrated N2 destruction ratio as functions of R1 for a fixed value of the laminar flame speed (φ ) 0.65 for R1 ) 0 case).

Figure 6. Experimentally determined NOx emission ratio and numerically determined integrated N2 destruction ratio as functions of R1 for a fixed value of Tmax (φ ) 0.65 for the R1 ) 0 case).

mixtures numerically calculated using the 1-D Premix code of Kee et al.34 and scaled again with respect the integral N2 destruction rates of the R1 ) 0 fuel mix. Because the fuel mixtures studied here contain no organically bound nitrogen, the rationale for comparing these two variables should be obvious. As Figure 5 indicates, combining the reformate mixture with CH4 has a positive influence on NOx reduction for flames with the same Sou value, with this reduction being more profound as R1 increases. From a power/energy-generation perspective, especially in applications involving gas turbines, it is important that, when the NOx-forming propensities of various fuel mixtures are compared, flames that have the same maximum temperature, Tmax, are considered. Figure 6 summarizes the results of our studies of NOx formation for combustion mixtures corresponding to different R1 values for flames with the same Tmax. Similarly to the conditions shown in Figure 5, to maintain the same Tmax with all of these different fuel mixtures, one must adjust the fuel-to-air ratio in the combustor appropriately. Figure 6 depicts the scaled, experimentally measured NOx concentrations and the calculated integral N2 destruction rates. The results in Figure 6 indicate that combining the reformate mixture with CH4 has a similar positive influence on NOx reduction for flames with the same Tmax value, with this reduction again being more profound as R1 increases. Comparing NOx production among flames with the same Tmax value, in addition to allowing one to establish an “equivalent” power-generation basis, also helps to provide additional insight into other factors beyond the thermal effects that might be responsible during NOx formation. Because the thermal effects in NOx formation are expected to be similar for such flames, observed

5160

Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001

Figure 7. Normalized sensitivity coefficients on the NO concentration for a φ ) 0.65 CH4/air flame at a distance of 4 mm from the stagnation plane.

differences in the NOx generation rates must to due to other-than-thermal effects. Figure 7 depicts the normalized sensitivity coefficients of the most important reactions on NO production for a stagnation flame with φ ) 0.65 at a location corresponding to the point at which measurements were conducted, i.e., 4 mm from the stagnation plane. The observed reduction of NO emissions as R1 increases can be explained by noticing the substantial negative sensitivity of the H + O2 + H2O f HO2 + H2O reaction. This large negative sensitivity stems from the fact that H is needed for the promotion of the forward rate of the CH-producing CH2 + H f H2 + CH reaction. The produced CH subsequently is chiefly responsible for the initiation of the prompt mechanism via the reaction CH + N2 f HCN + N. While other reactions such as the main branching reaction H + O2 f O + OH also have significant negative sensitivity as they consume H radicals, the promotion of the forward rate of the H + O2 + H2O f HO2 + H2O reaction appears to be responsible for the NO reduction as R1 increases. More specifically, as R1 increases, the concentration of H2O increases for two reasons. First, H2O is added as a result of the reforming process. Second, as more H2 is added as a result of the reforming process, more H2O is produced. As a result, the forward rate of the H + O2 + H2O f HO2 + H2O reaction is enhanced, and the CH-producing reaction CH2 + H f H2 + CH slows because of the competition for the H radicals. Thus, the effectiveness of the prompt mechanism is reduced, and the overall NO emissions are lower. Summary This study focuses on the dynamics and structure of flames of lean mixtures of air with CH4/H2/CO/CO2/H2O, a five-gas fuel mix resulting when the products exiting a reformer are combined with fresh CH4 before entering the combustor. Experiments were carried out in the single-jet-wall, stagnation-flow configuration using laser Doppler velocimetry for the determination of the axial velocity profiles. The concentrations of stable species were measured by mass spectrometry, nitrogen oxides were measured by chemiluminescence, and the temperature distribution along the centerline was measured using fine-wire thermocouples. Numerical simulations were conducted solving the steady-state mass, momentum, energy and species conservation equations for the stagnation streamline in a finite domain with the inclusion of radiation transfer. The effect of adding varying amounts of the reformate mixture (H2/CO/H2O/CO2/CH4) to lean premixed meth-

ane/air flames was studied. A key parameter here is the mixing ratio R1, defined as the ratio of the amount of CH4 that is directly fed to the reformer to the total CH4 feed. It has been observed that increasing the R1 value decreases the laminar flame speed. Removing H2O or CO2 from the reformate mixture before mixing it with the fresh CH4 helps to alleviate the negative impacts on the laminar flame speeds, but it might prove to be counterproductive in terms of NOx formation. Both experiments and simulations of NOx formation were carried out for flames with either the same laminar flame speed or the same maximum temperature. Numerical predictions and experiments of NOx emissions indicate that, in both cases, i.e., for the same laminar flame speed or for the same maximum temperature, lower NOx emissions are achieved as R1 increases. Through a detailed sensitivity analysis, the key step responsible for the reduction of NOx emissions has been identified. The reduction in NO that occurs as R1 increases appears to be due to the promotion of the forward rate of the H + O2 + H2OfHO2 + H2O reaction. Acknowledgment This study was performed under the support of the Southern California Gas Company. Literature Cited (1) Yu, G.; Law, C. K.; Wu, C. K. Laminar Flame Speeds of Hydrogen + Air Mixtures with Hydrogen Addition. Combust. Flame 1986, 63, 339. (2) Sher, E.; Ozdor, N. Laminar Burning Velocities of n-Butane/ Air Mixtures Enriched with Hydrogen. Combust. Flame 1992, 89, 214. (3) Karim, G. A.; Wierzba, I.; Alalousi, Y. Methane/Hydrogen Mixtures as Fuels. Int. J. Hydrogen Energy 1996, 21, 625. (4) Karbasi, M.; Wierzba, I. The Effects of Hydrogen Addition on the Stability Limits of Methane Jet Diffusion Flames. Int. J. Hydrogen Energy 1998, 23, 123. (5) Bell, S. R.; Gupta, M. Extension of the Lean Operating Limit for Natural Gas Fueling on a Spark Ignited Engine Using Hydrogen Blending. Combust. Sci. Technol. 1997, 123, 23. (6) 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. (7) Harvey, S.; Kane, N. Analysis of a reheat gas-turbine cycle with chemical recuperation using ASPEN. Energy Convers. Manage. 1997, 38, 1671. (8) Harvey, S. P.; Knoche, K. F.; Richter, H. J. Reduction of Combustion Irreversibility in a Gas-Turbine Power Plant Through Off-Gas Recycling. J. Eng. Gas Turbines Power 1995, 117, 24. (9) Lowder, R. Gas Turbine Operates on Catalytic Reformer Gas. Oil Gas J. 1989, Oct 9, 83. (10) Masri, A. R.; Dibble, R. W.; Barlow, R. S. Chemical Kinetic Effects in Nonpremixed Flames of H2/CO2 Fuel. Combust. Flame 1992, 91, 285. (11) Fotache, C. G.; Kreutz, T. G.; Law, C. K. Ignition of Hydrogen-Enriched Methane by Heated Air. Combust. Flame 1997, 110, 429. (12) Maughan, J. R.; Bowen, J. H.; Cooke, D. H.; Tuzson, J. J. Reducing Gas-Turbine Emissions Through Hydrogen-Enhanced Steam-Injected Combustion. J. Eng. Gas Turbines Power 1997, 116, 78. (13) Tuzson, J. Status of Steam-Injected Gas Turbines. J. Eng. Gas Turbines Power 1992, 114, 682. (14) Gerhard, L.; Verina, J. W.; Franz, W.; Hermann, H. Decomposition of Nitrous Oxide at Medium Temperatures. Combust. Flame 2000, 120, 427. (15) Ladommatos, N.; Abdelhalim, S.; Zhao, H. Control of Oxides of Nitrogen from Diesel Engines Using Diluents While Minimizing the Impact on Particulate Pollutants. Appl. Therm. Eng. 1998, 18, 963.

Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5161 (16) Groppi, G.; Lietti, L.; Tronconi, E.; Forzatti, P. Catalytic Combustion of Gasified Biomasses over Mn-Substituted Hexaaluminates for Gas Turbine Applications. Catal. Today 1998, 45, 159. (17) Qin, W.; Egolfpoulos, F. N.; Tsotsis, T. T. Fundamental and Environmental Aspects of Landfill Gas Utilization for Power Generation. Chem. Eng. J. 2001, 82, 157. (18) Ren, J. Y.; Qin, W.; Egolfopoulos, F. N.; Tsotsis, T. T. Strain-Rate Effects on Hydrogen-Enhanced Lean Premixed Combustion. Combust. Flame 2001, 124, 717. (19) Zhu, Y.; Minet, R. G.; Tsotsis, T. T. Continuous Pervaporation Reactor for the Study of Esterification Reactions Using a Composite Polymeric/Ceramic Membrane. Chem. Eng. Sci. 1996, 51, 4103. (20) Sedigh, M. G.; Onstot, W. J.; Xu, L., Peng, W. L.; Tsotsis, T. T.; Sahimi, M. Experiments and Simulation of Transport and Separation with Carbon Molecular Sieve Membranes. J. Phys. Chem. A 1998, 102, 8580. (21) Sedigh, M. G.; Xu, L.; Tsotsis, T. T.; Sahimi, M. Preparation and Transport Characteristics of Polyetherimide-based Carbon Molecular Sieve Membranes. Ind. Eng. Chem. Res. 1999, 38, 3367. (22) Minet, R. G.; Tsotsis, T. T. Catalytic Membrane Steam/ Hydrocarbon Reformer. U.S. Patent 4,981,676, Jan 1, 1991. (23) Minet, R. G.; Vasileiadis, S. P.; Tsotsis, T. T. Experimental Studies of a Ceramic Membrane Reactor for the Steam/Methane Reaction at Moderate Temperatures, 400-700 °C. In Proceedings of Symposium on Natural Gas Upgrading; Huff, G. A., Scarpiello, D. A., Eds.; 1992; Vol. 37, p 245. (24) Tsotsis, T. T.; Champagnie, A. M.; Vasileiadis, S. P.; Ziaka, Z. D.; Minet, R. G. The Enhancement of Reaction Yield through the Use of High Temperature Membrane Reactors. Sep. Sci. Technol. 1993, 28, 397. (25) Onstot, W. J. Membranes for Use in CO2 Reforming Environments. Ph.D. Dissertation, University of Southern California, Los Angeles, CA, 2001. (26) Onstot, W. J.; Minet, R. G.; Tsotsis, T. T. Design Aspects of Membrane Reactors for Dry Reforming of Methane for the Production of Hydrogen. Ind. Eng. Chem. Res. 2001, 40, 242. (27) Suwanmethanond, V.; Goo, E.; Johnston, G.; Liu, P. K. T.; Sahimi, M.; Tsotsis, T. T. Porous SiC Sintered Substrates for HighTemperature Membranes for Gas Separations. Ind. Eng. Chem. Res. 2000, 39, 3264. (28) Egolfopoulos, F. N.; Zhang, H.; Zhang, Z. Wall Effects on the Propagation and Extinction of Strained, Laminar, Premixed Flames. Combust. Flame 1997, 109, 237. (29) Law, C. K. Dynamics of Stretched Flames. Proc. Combust. Inst. 1988, 22, 1381.

(30) Egolfopoulos, F. N.; Cho, P.; Law, C. K. Laminar Flame Speeds of Methane-Air Mixtures Under Reduced and Elevated Pressures. Combust. Flame 1989, 76, 375. (31) Egolfopoulos, F. N.; Zhu, D. L.; Law, C. K. Experimental and Numerical Determination of Laminar Flame Speeds: Mixtures of C2-Hydrocarbons with Oxygen and Nitrogen. Proc. Combust. Inst. 1990, 23, 471. (32) Ren, J.-Y. Methane Reforming and its Potential Effect on the Efficiency and Pollutant Emissions of Methane-Air Combustion. Ph.D. Dissertation, University of Southern California, Los Angeles, CA, 2001 (33) Reynolds, W. J. The Element Potential Method for Chemical Equilibrium Analysis: Implementation in the Interactive Program STANJAN; Department of Mechanical Engineering, Stanford University: Stanford, CA, 1986. (34) 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. (35) Egolfopoulos, F. N.; Campbell, C. S. Unsteady, Counterflowing, Strained Diffusion Flames: Frequency Response and Scaling. J. Fluid Mech. 1996, 318, 1. (36) Egolfopoulos, F. N. Geometric and Radiation Effects on Steady and Unsteady Strained Laminar Flames. Proc. Combust. Inst. 1994, 25, 1375. (37) 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. (38) 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. (39) Bowman, C. T.; Frenklach, M.; Gardiner, W. R.; Smith, G. The “GRI 3.0” Chemical Kinetic Mechanism; University of California: Berkeley, CA, 1999 (http://www.me.berkeley.edu/gri_mech/).

Received for review December 4, 2000 Revised manuscript received July 27, 2001 Accepted July 30, 2001 IE001054K