High-Temperature Catalytic Combustion and Its Inhibition of Gas

This inhibition of gas-phase combustion by the catalytic reactions is ... The inhibition seen in the monolith burners was observed in more detail in t...
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High-Temperature Catalytic Combustion and Its Inhibition of Gas-Phase Ignition V. Dupont,* S.-H. Zhang, and A. Williams Department of Fuel and Energy, The University of Leeds, Leeds, LS2 9JT, United Kingdom Received June 5, 2002

The emissions characteristics of the combustion of fuel-lean mixtures of methane and air were studied in steady-state conditions using two catalytic reactors: a honeycomb monolith burner and a stagnation point flow reactor (SPFR). The investigations showed that the gas-phase fuel/ air mixture does not ignite until the catalytic surface is significantly above the ordinary autoignition temperature. This inhibition of gas-phase combustion by the catalytic reactions is desirable, since it extends the range over which clean and stable oxidation conditions persist. The inhibition seen in the monolith burners was observed in more detail in the SPFR experiments. The analysis of the experimental results was assisted by numerical simulations of both reactors using kinetic models consisting of heterogeneous and homogeneous elementary reactions as well as global chemical kinetic mechanisms found in the literature. The measured peak selectivity of the C-containing products was in agreement with the detailed chemical simulations. However, the latter underestimated the temperature range of the inhibition, indicating that the current description of the free radicals formation needs further investigation.

Catalytic monolith burners can currently be found in combustors of stationary gas turbines for power generation1,2 and for heating purposes.3-6 Catalytic burners yield lower emissions than those using conventional combustion, as well as increased stability. The benefits of using catalytic combustion technology stem from the fact that ultra-lean fuel/air mixtures can ignite heterogeneously below the conventional lean flammability limit. Despite the drop in adiabatic temperature resulting from using ultra-lean mixtures, the thermal output of catalytic burners can be used for their radiant power and for their hot exhaust gases. These can originate either from complete combustion, or from partial oxidation used to stabilize homogeneous combustion downstream of the catalyst, the latter can be achieved using fuel/air staging. As ultra-lean fuel mixtures can burn catalytically, the operational conditions are designed so that bulk gas temperatures do not reach the threshold of NOx formation. In addition, the formation of CO as a final gas product in fuel-lean conditions with a hydrocarbon oxidation catalyst only arises from the homogeneous gas-phase reactions when present, although it is * Corresponding author. Ph: +44 1132332507. Fax: +44 113 3440572. E-mail: [email protected]. (1) Dalla Betta, R. A.; Rostrup-Nielsen, T. Catal. Today 1999, 47, 369-375. (2) Beebe, K. W.; Cairns, K. D.; Pareek, V. K.; Nickolas, S. G.; Schlatter, J. C.; Tsuchiya, T. Catal. Today 2000, 59, 95-115. (3) Seonhi, R.; Scholten, A. 20th World Gas Conference Proceedings; Copenhagen, 1997. (4) Scholten, A.; Van Yperen, R.; Emmerzaal, I. J. Fourth International Workshop on Catalytic Combustion; Gaz de France, Catalytica, ORNL and Gastec, 1999. (5) Kang, S. K.; Jeong, N. J.; Ryu, I. S.; Baek, Y. S. Fourth International Workshop on Catalytic Combustion; Gaz de France, Catalytica, ORNL and Gastec, 1999. (6) Cho, W.; Baek, Y. S.; Oh, Y.; Lee, Y.; Park, D.; Pang H. Fourth International Workshop on Catalytic Combustion; Gaz de France, Catalytica, ORNL and Gastec, 1999.

an intermediate of the combustion on the catalyst surface. Fuel conversion efficiency can therefore attain 100% via catalytic reactions. As heterogeneous solidgas reactions are extremely fast due to their low activation energy, and because catalysts are designed with an abundance of active sites in the form of dispersed catalyst in the washcoat, the conditions for zero unburnt fuel emissions are easily reached, albeit requiring the use of initial preheating of the catalyst in order to reach light-off. Therefore, not counting CO2 and H2O, overall zero emissions are achievable while remaining in the purely heterogeneous combustion mode. Of particular interest to the applications mentioned earlier is the high-temperature region of the catalytic mode, because it allows maximum thermal output in near-zero pollutant emissions (except CO2) and high stability conditions. Unfortunately, this region is usually associated with the deactivation of the catalyst and the degradation of the support, particularly by sintering. The latter is a common problem that occurs with loss of active surface area. To extend the life of the catalyst, this region is usually avoided by careful design of the reactive flow conditions. This is the case of gas turbines catalytic combustors, where the catalysts are divided into various sections with different active component and surface area according to each section’s operating temperature. However, some applications such as water heaters and cookers burn natural gas in catalytic monoliths in this high-temperature region. Because these catalysts have to withstand the highest temperatures, noble metals are usually chosen as the active component. Of these, platinum is the most popular as it has been shown to be a more effective catalyst than Pd under conditions of catalytic combustion,7 in addition to the more stable nature of the Pt active sites compared

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to those of Pd. In the present study, we show that the current assumptions as to the chemistry that takes place in the catalytic high-temperature region underestimate the upper limit for which pure catalytic oxidation of methane on Pt persists. This means that the high-temperature catalytic mode could be better exploited in the relevant applications, while also explaining the reason they perform better than expected. Experimental Methods Honeycomb Monolith Catalytic Burners Experiments. The experimental setup and measurements methods are explained in detail in ref 8. To summarize, ∼5% CH4/air mixtures were fired with flow rates between 60 and 100 L/min (STP) into a vertically positioned honeycomb monolith (Johnson Matthey, Royston, U.K.), with a cordierite support and a washcoat rich in γ-alumina that could contain different amounts of Pt or Pd. The monoliths were 10.16 cm diameter and contained 62 channels per square centimeter, corresponding to approximate square cells of 1.27 mm sides. The ignition process involved preheating the air until the gas temperature below the catalyst reached ca. 180 °C. At this point, a nearstoichiometric flow was ignited in the gas phase above the catalyst, yielding a flat blue flame. The combustion then penetrated down the catalyst, as the latter heated to light-off temperature. This was accompanied by the disappearance of the blue flame in favor of an intense red glow coming from deep into the channels. The preheating was then discontinued and the combustion was allowed to stabilize in ultra-lean conditions to reach steady state. Measurements of O2, CO, and CO2 of the exhaust using on-line paramagnetic and IR absorption analyses, respectively, of the axial temperatures inside the channels (bulk gas and wall using type K thermocouples of 0.5 mm and 1 mm sheath diameter, respectively) and of the average downstream temperature of the catalyst using an IR pyrometer were performed. Details of the radiant heat rate output as well as the minimum CH4 concentration necessary to achieve stable combustion of the various conditions tested are also given in ref 8. Here we present the bulk-gas and catalyst temperature profiles measured inside a Pd catalyst and a Pt catalyst. These are subsequently used in a plug flow model of a channel using homogeneous chemistry only. These temperatures are only indicative of the bulk-gas and of the catalyst wall temperature, as their measurement was subject to many sources of errors that were unavoidable and could not be quantified easily.8 The measurements were performed with the thermocouple inserted from the top of the channels (downstream end) and started from the bottom of the channel, progressing the measurements by pulling it upward and downstream, to introduce the unavoidable flow obstructions downstream of the measurement. However, the measured bulk gas temperatures are known to be significantly larger than the true gas temperatures due to the increasing radiation fluxes from the channel walls as the thermocouple travels in the downstream direction. The simulations based on these temperature profiles therefore have a tendency to underestimate the CO yield because of the falsely favored oxidation conditions defined by the overestimated gas temperatures. The Stagnation Point Flow Reactor (SPFR) Experiments. In a stagnation point flow catalytic reactor, the catalyst is flat and the reactant flow is fired at right angles with the catalyst surface and with a uniform velocity, while the back surface is inert and not exposed to the flow. The onedimensionality permits a detailed treatment of both the heterogeneous and homogeneous chemistries without being too (7) Burch, R.; Loader, P. K. Appl. Catal. B: Environ. 1994, 5, 149164. (8) Dupont, V.; Moallemi, F.; Williams, A.; Zhang, S. H. Int. J. Energy Res. 2000, 24, 1181-1201.

Figure 1. Schematic of the stagnation point flow reactor (SPFR). computationally expensive. The combination of experimental data and modeling SPFR is an ideal method for validation and development of chemical kinetics mechanisms.9-16 Our previous studies using an experimental SPFR had provided validation data for CH4/O2/N2 mixtures oxidation on Pt, but had also highlighted the limitations of the reactor.9,10 These limitations arose from discrepancies between the finite geometry of the experimental reactor and the infinite one of its theoretical model. An important consequence of these discrepancies was that a correction factor accounting for the radial expansion of the gases at the edges of the flow had to be applied to the predicted fuel conversions and to the selectivity of the products. But the small size of the previous SPFR and the large scaling ratio of the separation distance injector-catalyst (L) to their diameter (D) were also invoked to explain the differences between the experimental and the predicted selectivity of CO by affecting the free radicals diffusion in the radial direction, a factor that the model does not include. Both these shortcomings have now been addressed by using a new reactor that is as close as possible to the ideal geometry. A diagram of this reactor is shown in Figure 1. In this new design, the following features have been implemented: (i) The geometry is now almost entirely axi-symmetric, and the heated surface of the Pt catalyst foil is a disk, like the injector area; (ii) The radial flow is extended beyond the foil area by the thickness of the power connectors (10 mm), thus edge-flow effects are not felt directly under the catalyst, but away from it in a colder and nonreactive environment; (9) Dupont, V.; Zhang, S. H.; Williams, A. Int. J. Energy Res. 2000, 24, 1291-1309. (10) Dupont, V.; Zhang, S. H.; Williams, A. Chem. Eng. Sci. 2001, 56, 2659-2670. (11) Song, X.; Williams, W. R.; Schmidt, L. D.; Aris, R. Combust. Flame 1991, 84, 292-311. (12) Ikeda, H.; Sato, J.; Williams, F. A. Surf. Sci. 1995, 326, 1126. (13) Deutschmann, O. Ph.D. Thesis, Heidelberg University, 1996. (14) Bui, P.-A.; Vlachos, D. G.; Westmoreland, P. R. Twenty-Sixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1996; pp 1763-1770. (15) Veser, G.; Schmidt, L. D. AIChE J. 1996, 42 (4), 1077. (16) Harle, H.; Lehnert, A.; Metka, U.; Volpp, H.-R.; Willms, L.; Wolfrum, J. Chem. Phys. Lett. 1998, 293, 26.

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(iii) The power input is now ac, allowing a surface temperature measurement of the foil by using a 50 µm wires thermocouple type R, welded at the back; (iv) The reactants’ injector is stacked with stainless steel gauzes to achieve the necessary flat velocity profile at the outlet; (v) The scaling ratio L/D defined earlier is reduced to 0.34 following the recommendations in ref 12; (vi) A fine sheet of mica on the back of the foil prevents catalytic reactions on the back of the foil; the power connectors also act as a barrier to the flow at the back of the foil; (vii) Water cooling is used to protect the electrical cables and the thermocouple wires from the radiation from the foil, and maintains the reactor at a low temperature away from the stagnation point flow region; (viii) Finally, the tension in the foil can be adjusted to maintain its flatness at increasing temperatures. The measurement techniques used previously9,10 consisted of mass flow controllers for the relevant reactant flows, online analyses of O2 and CO in the exhausts, gas-temperature measurements along the symmetry axis of the reactor using a silica-coated (exposed junction) type R thermocouple (50 µm wires) inserted in a 0.8 mm external diameter ceramic sheath. The catalyst surface is measured by a type R thermocouple of 50 µm diameter wires welded on the back of the catalyst. In addition, the exhausts were collected in 5 L Tedlar bags and analyzed off-line by gas chromatography (FID and TCD detectors) for analysis of higher hydrocarbons (C2H4, C2H6, C3H8, C4H10) as well as back-up measurements of O2, respectively. The latter were usually within 1% error with the online paramagnetic analyzer’s readings and occasionally rose to 5%.

Modeling The code SENKIN17 was used in the plug-flow mode to simulate the flow in a central channel, by imposing the measured bulk gas temperatures and assuming homogeneous chemistry only. The latter was represented using different mechanisms of varying complexity. These were (i) a one global reaction of combustion (S),18 (ii) a two-step reaction mechanism (D),19 and (iii) a detailed elementary reaction mechanism of methane combustion GRI-Mech 2.11 or (G2).20 The latter has been subjected to an extensive program of experimental validation20 and in the fuel-lean conditions of the present study is expected to reproduce the CO emissions from the gas-phase combustion with accuracy. The numerical code SPIN21 was used to simulate the SPFR experiments. This code solves the conservation equations for the flow situated between the injector and the catalyst (the stagnation plane), assuming an infinitely large radius for both of them and a separation distance (here called L). The vertical flow at the injector converts gradually to a radial flow as it approaches the stagnation plane (the catalyst surface) and all flow (17) Lutz, A. E.; Kee, R.; Miller, J. A. SENKIN: A Fortran program for predicting homogeneous gas-phase chemical kinetics with sensitivity analysis. Sandia Report SAND87-8248-UC-301, 1988. (18) Coffee, T. P. Combust. Sci. Tech. 1985, 43, 333-339. (19) Dryer, F. L.; Glassman, I. Fourteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1973; pp 987-1003. (20) Bowman, C. T.; Hanson, R. K.; Davidson, D. F.; Gardiner, W. C.; Lissianski, V.; Smith, G. P.; Golden, D. M.; Frenklach, M.; Goldenberg, M. GRI-Mech 2.11; 1996. http://www.me.berkeley.edu/ gri•mech. (21) Coltrin, M. E.; Kee, R. J.; Evans, G. H.; Meeks, E.; Rupley, F. M.; Grcar, J. F. SPIN (version 3.83): A Fortran programme for modeling one-dimensional rotating disk/stagnation flow chemical vapour deposition reactors. SANDIA report SAND91-8003, 1991.

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exiting the stagnation point flow domain does so in the radial direction. The axial velocity (U) and the scaled radial velocity (V/r) are known everywhere in the gap between the injector and the catalyst from the solution of the conservation equations, but there are no axial velocity, concentration, or temperature gradients in the radial direction. This is why in order for the experimental flow to approach the model, edge effects from the finite geometry have to be minimized. This was achieved by choosing a scaling ratio L/D as small as possible, which implies as large as possible a common diameter of both the catalyst and the injector. According to ref 12, the minimum value of 0.28 for L/D is recommended for an experimental SPFR to fulfill the theoretical requirements. A complication arises, as the injector gets closer to the heated catalyst and heat conducting back to the fresh reactant mixture alters the inlet boundary conditions of each experiment, thereby increasing both the velocity and the gas temperature. While this preheating was quantified in the present study from the gas-temperature measurement along the reactor’s axis of symmetry, the profile of the gas temperature at the injector outlet needed to be flat or near flat, also as required by the inlet boundary conditions of the model. This on the other hand implied a minimum value of L. The necessary compromise to achieve a small L/D ratio resulted in the chosen separation distance of injectorcatalyst L of 8 mm for a catalyst/injector diameter D of 23.5 mm, yielding a scaling ratio L/D of 0.34, i.e., slightly higher that the recommended minimum. We shall see later that the flow was nevertheless correct. The code SPIN was run using two options: one that solved the energy conservation equation and thus calculated a gas-temperature profile along the axis, and one that used the imposed measured gas-temperature profile and no longer solved the energy equation. Ideally, the first option should yield the same gas temperatures as the measured ones, if the model were a good representation of the experiment. In addition to the temperature profiles, the comparisons of the model outputs with their experiments were carried out based on the overall reactor’s fuel conversions and the selectivity of the principal carbon-containing products. This approach was chosen over measurements in the boundary layer for the following reasons. First, measurements of concentrations in the boundary layer are usually intrusive, particularly in the case where L and D are small, and the probe of nonnegligible diameter. This is to the exception of optical techniques such as laser diagnostics (Raman, Rayleigh scattering, LIFS) in which case, and second, the small distance L and the material presence of the reflecting surfaces provided by the catalyst and the injector are a hindrance to multiple optical paths. Finally, provided the scaling ratio L/D allows the validity of the SPFR theory, as is the case here, a number of common on-line diagnostics techniques can simply be employed in the reactor’s exhaust, i.e., away from the reaction zone, and reactants conversion and products selectivity calculated from them and compared to their theoretical values. The relationships used to calculate these are explained in refs 9,10 and are based on species balances carried out on a cylinder volume that encloses the region bounded by the injector and the catalyst. The balance takes into

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Figure 2. Gas and wall temperature profiles inside monoliths. (a) 50 mm long monolith (Pt catalyst with 5.5% CH4 in air, total flow 60 L/min STP, and Pd catalyst with 5% CH4 in air, total flow 94 L/min STP, (b) Pd catalyst shortened to 15 mm, same flow as (a).

account the particularity of the known two components of the flow (axial and radial) of the stagnation point geometry, and therefore it incorporates the fact that some of the reactants in the flow will bypass the catalyst and leave the cylinder volume radially. All radial reactant “leakage” is accounted for in the balance and therefore the latter can be used to compare conversions and products selectivity from experimental measurements in the reactor’s exhausts to the predicted ones from the relevant balance. However, and now in complete agreement with the theory as opposed to the results of refs 9,10, the correction factor for radial expansion of the gases (edge effects), no longer necessary, was set to zero due to the improvements in the new reactor. This is later justified as the fuel conversions found experimentally in the plateau of CH4 conversion due to the diffusion rate-controlled heterogeneous oxidation matched those found with the raw simulations, deprived of correction factor, as shall be seen in the Results section. Results and Discussion Honeycomb Monolith Burner Studies. Figure 2a,b represents the profiles of the bulk gas and catalyst wall temperatures inside centrally located channels of 50 mm long Pd and Pt monoliths (Figure 2a), and the Pd monolith shortened to 15 mm (Figure 2b). The total flow rates used were 94 L/min (STP) with 5% CH4 for the Pd monolith, and 60 L/min with 5.5% CH4 for the Pt monolith. Higher temperatures were observed in the less fuel-lean concentration used with the Pt catalyst than with the Pd catalyst, as the adiabatic temperature was increased. However, the reaction zones in Figure a, evidenced by the sharp temperature increase reaching a peak, with higher wall temperatures than for the bulk gas, indicated that the reaction took place at the bottom of the catalyst and must have occurred at the walls. The final dip in the temperature was due to the heat losses by radiation from the downstream surface of the catalyst to its laboratory surroundings. The profiles shown in Figure 2b were lower because radiant losses to the laboratory surroundings affected the entire short length

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Figure 3. Predicted CH4 and CO concentrations using the plug flow model and homogeneous gas-phase chemical mechanism (S, D, G2), for reactant mixture of 5% CH4 in air with Pd catalytic monolith and total flow rate of 94 L/min STP. (a) Monolith 50 mm long: Simulations with global 1-reaction mechanism (S), global 2-reactions mechanism (D), and the detailed mechanism (G2). (b) Monolith 15 mm long, G2 mechanism only.

of this catalyst. In all the runs, the complete conversion of the methane to CO2 and H2O was observed, with no detection of CO by the on-line analyzers. Figure 3a,b show the predicted CH4 and CO profiles using the plug flow option of the SENKIN code17 with the measured gas-temperature profile, and the various chemical mechanisms mentioned earlier, for the runs with the Pd catalyst in the same conditions as in Figure 2a,b. In the case of the 50 mm long monolith, all three models predicted a complete combustion using the measured gas-temperature profiles, including sub-ppm CO emissions for the detailed gas-phase chemical mechanism G2. As mentioned earlier, the bulk-gas-temperature profiles used in these simulations were larger than the true profiles due to the radiation fluxes from the channel inner walls onto the thermocouple’s hot junction. These effects and other sources of errors on the thermocouple measurements into the channels are described in detail in ref 8. To summarize, the CO emissions predicted by the gas-phase mechanism using the measured gas temperatures are underestimated, as the true lower temperatures would hinder the CO oxidation to CO2. Thus, the conditions in the 50 mm monolith could possibly have been due to homogeneous combustion, since the lack of predicted emissions is consistent with the experiments. However, the simulations for the shorter monolith shown in Figure 3b leave no doubt that this was not the case, as the prediction of the emission of a minimum of several hundred ppm of CO were not found experimentally. Therefore, despite conditions conducive to gas-phase combustion inside the monoliths, the heterogeneous catalytic combustion prevailed. This was a result of the inhibition of gas-phase ignition by catalytic combustion processes. Ideally, one should model the reactive flows inside the monoliths with detailed coupled heterogeneous-homogeneous chemistry in order to explore this phenomenon in the same conditions as those of practical catalytic burners. However, while monolith combustion models are available, because they are computationally expensive due to the complexity of the heat and mass transfer involved, they make use of simplified chemistry. This

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consists most often of global pseudo-homogeneous reaction mechanisms, with rate constants that were originally derived at much lower temperatures, in isothermal conditions, and often lower pressures. More recently, studies of detailed heterogeneous chemical kinetics in the literature predicted heterogeneous ignition points with accuracy.13,22 This was despite the very large uncertainties surrounding the kinetic rates of the elementary heterogeneous reactions compared to those usually claimed for their gas-phase counterparts. The same mechanisms were then used to model conditions at higher temperatures and in coupled heterogeneoushomogeneous conditions, and turbulent flow.23 The main advantage of detailed mechanisms should be their confident use in a wider range of conditions than those over which the simplified global mechanisms are valid, as well as providing additional information that can assist in complex burner design. The question is therefore whether this is actually the case. Before this can be answered, it is necessary to assess whether such detailed mechanisms can predict simple model flows with well-known catalysts. The SPFR studies here endeavor to shed light on this subject for the catalyst platinum and the model of a stagnation point flow. The advantages of the latter are that gas-phase chemistry coupled with solid-gas chemistry, and axial species as well as thermal diffusions can take place without invalidating the assumption of the model, as opposed to a well-stirred or a plug flow reactor which are also employed in similar studies. It is therefore ideally suited to the investigation of homogeneous ignition in the presence of a catalyst. SPFR Studies. In the experimental studies, CH4/O2/ N2 mixtures represented by a parameter R between 0.1 and 0.2, where R ) V˙ CH4/(V˙ CH4 + V˙ O2), were fired with a velocity U0 between 4.5 and 4.7 cm s-1 (corrected to 298 K at atmospheric pressure), on to a polycrystalline Pt foil of 99.95% purity, 100 µm thick, and 2.35 cm in diameter, from the distance of 8 mm. The amount of N2 used in the mixture, represented by the mole fraction XN2, was between 0.84 and 0.87. Studies with an earlier reactor design9,10 had showed that in the purely heterogeneous oxidation regime, a single-step global reaction mechanism was sufficient to reproduce the CH4 conversion dependence on the catalyst temperature. In ref 10, the rate constant of the single-step reaction was optimized via the fitting of two experimental points to its predicted values. We show in the conversion curves of Figure 4, obtained this time without correction factor, that with the new reactor design, the optimization of the heterogeneous reaction rate constant could be carried out via the fitting of just one conversion at a medium-low temperature (1100 K). To model the CO conversion, a more detailed chemical mechanism was necessary. The GRI-Mech 324 coupled with the detailed heterogeneous mechanism of CH4 oxidation on Pt from13,23,25 were used, assuming a site density of Pt active sites of 2.7 × 10-9 mol cm-2. This (22) Deutschmann, O.; Behrendt, F.; Warnatz, J. Catal. Today 1994, 21, 461-470. (23) Mantzaras, J.; Appel, C.; Benz, P.; Dogwiler, U. Catal. Today 2000, 59, 3-17. (24) 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. The GRI-Mech 3; 1999. http://www.me.berkeley.edu/gri•mech.

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Figure 4. Comparison of experimental CH4 conversion in the SPFR with predicted conversions using optimized heterogeneous single reaction global mechanisms. Reaction rate given by ωHET ) A × exp(-E/RT) × [CH4]1 × [O2]0.5. In GL* A ) 2.228 × 1010 and E ) 135.0 kJ (from ref 10). In GL**, the best fit uses A ) 2.228 × 1010 and E ) 144.7 kJ. Experimental conditions are for R ) 0.2, XN2 ) 0.877, and U0 ) 4.73 cm s-1.

value was estimated in ref 22, which contained the earlier version of the mechanism proposed in ref 13, and later tested in refs 23 and 25. As shown below, with the new reactor design we obtain much higher CO yields, supporting the assumption made in ref 9, that radial gradients were too large in the old reactor to be consistent with the SPFR model. We believe that most of the CO measured in the present SPFR study, which, we remind the reader, concentrated on mixtures with excess oxygen, had originated in the large majority from the gas phase, a fact that was confirmed by the marked change in the gas-temperature profile that took place simultaneously and evidenced a mild gas-phase ignition. The gas-temperature profiles in the reaction zone are not shown here but similar profiles are presented in ref 9. The origin of the CO was also corroborated by the modeling study in ref 9 for R ) 0.3, which determined that the CO selectivity of heterogeneous oxidation origin was smaller compared to that of the gas phase in this temperature region. In addition, the CO could not have desorbed from the surface and subsequently oxidized completely in the gas phase, because the flow geometry stipulates that a certain proportion of any species present in the gas phase should leave the reactor carried by the flow in the radial direction, and is then measured in the exhaust. However, adsorbed CO is accepted in the exhaustive literature available on this subject to be a short-lived intermediate in the complete lean heterogeneous oxidation of CH4 to CO2 on noble metals, which is sustained by the availability of excess oxygen on the surface even at the high temperatures. Figure 5 shows the experimental CH4 conversion and products selectivity dependence on the Pt catalyst temperature for R ) 0.15, XN2 ) 0.874, and the initial velocity (corrected for 298 K) U0 ) 4.6 cm s-1. It (25) Raja, L. L.; Kee, R. J.; Deutschmann, O.; Warnatz, J.; Schmidt, L. D. Catal. Today 2000, 59, 47-60.

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Figure 5. Dependence of experimental CH4 conversion, CO, C2H4, and C2H6 selectivities on Pt catalyst temperature in the SPFR for R ) 0.15, XN2 ) 0.874, and U0 ) 4.6 cm s-1. DETHET CH4 conv. is the CH4 conversion predicted with the detailed heterogeneous chemical mechanism on its own.

indicates that heterogeneous combustion prevailed up to 1400 K, a temperature for which small amounts of C2H6 first appeared in the products. When CO suddenly increased between 1450 and 1500 K with the characteristic change of gas-temperature profile, the CH4 conversion also increased abruptly, exhibiting the onset of gas-phase combustion (i.e., mild homogeneous ignition). The selectivity of CO, C2H6, and C2H4 increased simultaneously during gas-phase ignition but C2H6 peaked at lower catalyst temperatures, followed by C2H4 and CO with increasing temperature. It is worth reminding that these measurements are all carried out at steady-state, as the catalyst temperature is increased by 20 K steps from heterogeneous ignition. Once in the ignited gas-phase region, we found that for those mixtures with R > 0.2, decreasing the temperature would cause hysteresis.10 At temperatures where the mixture was previously catalytically ignited, it would now remain ignited in the gas phase, with the characteristic higher fuel conversion and concurrent CO and C-2 species emissions. This is evidence of an inhibition of the heterogeneous combustion by the gas-phase combustion, an effect that was previously reported in ref 12 for the catalytic combustion of H2 on Pt. The effect was reproduced in our model when using the imposed gas temperatures, effectively yielding two solution branches that can be seen in ref 10. The hysteresis branch was not further investigated in the present study and the experiments reported here were therefore obtained by increasing the catalyst temperature, while the simulations yielded solutions at steady state. Also included in Figure 5 is the CH4 conversion curve predicted using the heterogeneous detailed mechanism alone (DETHET).13,25 This indicates that the heterogeneous detailed mechanism predicted a slightly higher conversion curve at lower temperatures than the experimental one, and thus would benefit from further optimization as done previously in ref 10. However, it principally shows that, in the mass transport (diffusion) rate control regime corresponding to the plateau of CH4 conversion between 1200 and 1500 K, the match with the experimental CH4 conversion was excellent without

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Figure 6. Comparison of the experimental dependence of the CH4 conversion curves with predicted ones using (i) the detailed heterogeneous mechanism only (DET, HET), (ii) the detailed homogeneous mechanism only (DET, HOM), (iii) the detailed coupled heterogeneous/homogeneous mechanism imposing the measured gas temperatures (DET, HET+HOM), and (iv) the detailed coupled heterogeneous/homogeneous mechanism letting the code solve the energy equation (DET, HET+HOM, ENRG). Experimental conditions are as in Figure 5.

any correction factor applied to it. This diffusion rate control is due to the lack of CH4 near the surface caused by fast solid-gas oxidation reactions and the inability of the flow to replenish the pool of the deficient reactant (CH4) via diffusion sufficiently quickly. Therefore in this region, the kinetic rates chosen for these reactions are irrelevant as long as they are fast, and the fuel conversion is controlled by the in-flow of reactants and its fate in the reactor, itself dependent on the flow geometry. The good match in these conditions and in the absence of correction factor indicates that the flow in the experimental SPFR was a good representation of the ideal flow. The simulations were stopped at a temperature just under the homogeneous ignition point, as the heterogeneous mechanism alone could not model it. Figure 6 shows a comparison of the CH4 conversions of Figure 5 with predicted ones using the detailed coupled heterogeneous-homogeneous mechanism HET+HOM.13,24,25 One predicted curve was found by imposing the measured gas-temperature profiles, the other by solving the energy equation (ENRG). Also shown is the predicted CH4 conversion curve using only the homogeneous gas-phase mechanism (HOM), representing the effect of an inert surface. Below 1150 K, all the mechanisms tested that incorporated the detailed heterogeneous mechanism predicted the same conversion, indicating that for all these simulations the gas-phase conversion was inactive. Above 1150 K, both simulations using the coupled heterogeneous-homogeneous mechanisms predicted the onset of homogeneous combustion, visible in the figure by an increase in conversion. To further demonstrate that this was not happening in the experiments, Figures 7-9 plot the predicted and experimental selectivity of CO, C2H4, and C2H6 corresponding to the CH4 conversion curves of Figure 6. All the results shown in Figures 6-9 contribute to demonstrate that the detailed coupled heterogeneoushomogeneous mechanism, whether used solving the

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Figure 7. Comparison of experimental CO selectivity dependence on catalyst temperature with predicted ones with the same mechanisms as Figure 6 and for the same conditions.

Figure 8. Comparison of experimental C2H4 selectivity dependence on catalyst temperature with predicted ones with the same mechanisms as Figure 6 and for the same conditions.

Figure 9. Comparison of experimental C2H6 selectivity dependence on catalyst temperature with predicted ones with same mechanisms as Figure 6 and for the same conditions.

energy equation or imposing the experimental gas temperatures, predicted a gas-phase ignition 350-400 K too low. They also indicate that for all the catalyst temperatures below the gas-phase ignition point, the heterogeneous mechanism alone (global or detailed) can rightly predict the nearly complete conversion of CH4 to CO2. A further finding was that the predicted CH4 conversion, using the detailed homogeneous mechanism on its own and shown in Figure 6, indicated that an inert

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surface must have witnessed gas-phase ignition between 1250 and 1350 K, that is, 100-200 K higher than the prediction for the catalytic surface (1150 K), and 150200 K lower than the experiments. The simulations obtained here for R ) 0.15 therefore attribute a promoting effect on the gas-phase ignition by the catalytic combustion, whereas the experiments prove an inhibiting effect. Incidentally, this contradicted the results found in our previous studies9,10 for R ) 0.3, where an inhibiting effect was found both experimentally and in the models. Further optimization of these mechanisms seems therefore to be required to extend their range of validity. The adjustments should not, however, be significant, as the experimental curves of selectivity of CO, C2H4, and C2H6 shown in Figures 7-9 differ only from those of the models by their temperature range and not their values or curve profiles. The inhibition mechanism experimentally observed here was, however, clearly stronger than expected, revealing a large region at high temperatures where the catalytic combustion still dominates. Thus, it became necessary to explore what the causes of this inhibition could be in order to promote its accurate reproduction in the future. Two opposing theories can be found presently in the literature. The first one attributes the inhibition to the depletion of fuel in the bulk gas by the heterogeneous oxidation reactions, and seems to be supported by the results of experiments on Pt gauzes and accompanying modeling.26 These studies found that the OH radical profiles measured in the wake of CH4/O2/N2 mixtures flowing through heated Pt gauzes were consistent with a heterogeneous oxidation that was physically and chemically separate from the homogeneous oxidation (no coupling). However, gauze experiments where the flow is perpendicular to the gauze would inherently disturb the interactions of the two chemistries via the strong convective flow through the gauze. The gauze experiments are therefore not the best model representatives of what could occur in a monolith channel or a plate reactor. On first inspection of Figure 6, our results would not seem to be in disagreement with this theory, as the heterogeneous mechanism alone predicted well the experimental CH4 conversion curve all the way to gas-phase ignition. Unfortunately, this theory does not explain why the inhibition becomes weaker for leaner fuel mixtures (lower R). The latter is evidenced in Figure 10, which shows the CH4 conversions and selectivity of CO for the three lean values of R (0.1, 0.15, and 0.2). Figure 10 indicates that, in agreement with our previous studies,9-10 the homogeneous formation of CO occurred at lower catalyst temperatures for decreasing fuel strength, i.e., smaller R values. As the absolute measurements of CO selectivity from the present study are much higher than those of refs 9,10 because of the reactor improvements, the gas-phase ignition point is also much more obvious and more reliable. The second version of the causes of inhibition is more complex27 and uses thermo-kinetic interaction theory.28 Many phenomena are involved in addition to the depletion of the fuel by the heterogeneous oxidation reactions, (26) Davis, M. B.; Pawson, M. D.; Schmidt, L. D. Combust. Flame 2000, 123, 159-174. (27) Vlachos, D. G. Chem. Eng. Sci. 1996, 51, 2429-2439. (28) Griffiths, J. F.; Scott, S. K. Prog. Energy Combust. Sci. 1987, 13, 161-197.

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Figure 10. Experimental CH4 conversion and CO selectivity dependence on Pt catalyst temperature for various lean mixtures (R ) 0.1, 0.15, 0.20) with XN2 ) 0.84, U0 ) 4.5-4.7 cm s-1.

which was the sole factor in the theory discussed earlier. Diffusion of CH4 toward the catalyst, further depriving the bulk gas of fuel in the regions conducive to gas ignition, is invoked. According to refs 27,29 the desorption of large amounts of water vapor by the active catalyst would be responsible for the results of Figure 10. Water vapor participates as a third body (M) in two reactions of significance in preventing the conditions for gas-phase ignition. They do so by consuming the free radicals H and CH3, and by forming the more stable species HO2 and C2H6, respectively, via the following reactions:

H + O2 + M w HO2 + M

(R1)

2CH3 + M w C2H6 + M

(R2)

As the fuel strength is lowered, the concentration of H2O decreases and the inhibition weakens accordingly. This effect was well illustrated in refs 14, 27 for R1, and in refs 29-30 for R2. There is, however, a slight difference in the way the R1 was described in refs 14, 27 with the mechanisms used here. The earlier versions of the present mechanism attributed a collision efficiency of 18.6 (the highest) to the third body H2O31 and the same was used in refs 14, 27. The more recent versions (GRIMech 2.11 and 3) used here, assigned a zero value to this collision efficiency, but instead included separately the reaction H + O2 + H2O w HO2 + H2O with a different kinetic rate. Further simulations were carried out for R ) 0.15 and XN2 ) 0.87 in order to determine whether arbitrarily increasing the collision efficiency to 18.6 for H2O in R1 within the mechanism GRI-Mech 3 would increase the temperature of gas-phase ignition. The results, not shown here, indicated that indeed it occurs 100 K higher than previously. This was still short by 250-300 K of predicting homogeneous ignition at the expected 1450-1500 K. Other factors have to intervene in order to reflect the observed strength of the inhibition and correct the mechanism. An important aspect not entirely accounted for in the current detailed heterogeneous mechanism is the ad(29) Park, Y. K.; Vlachos, D. G. AIChE J. 1997, 43, 2083-2095. (30) Park, Y. K.; Vlachos, D. G. Combust. Flame 1998, 114, 214230. (31) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287-338.

sorption and subsequent fate of the free radicals originating from the bulk gas. It is generally assumed that strong fluxes of adsorption of H, OH, or O to the surface and recombination to H2O would hamper gas-phase ignition, and indeed the question of radical fluxes to and from Pt surfaces has been investigated before.32-34 The detailed heterogeneous mechanism used in the present study only accounts for the adsorption of the free radical OH. The latter is recognized to play a crucial role in gas-phase ignition, but so is H, the species responsible for attacking CH4, yielding the CH3 radicals involved in R2 but also the precursors of CO via CH2, CH, and CH2O formation, part of the gas-phase ignition process. The H, CHi, and CH2O radicals could also interact with the Pt surface but this is not represented in the mechanism tested here. Optimization methods of the heterogeneous mechanisms of oxidation of H2, CO, and CH4 on Pt are currently being developed35-37 using thermodynamics and transition state theory. The combination of such methods with an extensive experimental database would give a strong basis for the development of detailed mechanisms of heterogeneous oxidation. The present work, which includes the selectivity of CO and of some C-2 species, as well as the overall fuel conversion, goes toward the building of such a database. On their own, our experimental results present evidence that the heterogeneous oxidation of lean CH4/O2/N2 mixtures on Pt is maintained over a much larger catalyst temperature range than expected by current detailed kinetic models, and that their optimizations should be continued in order to provide better burner design tools. It is to be expected that similar results would be obtained for other catalysts such as Pd, which was not investigated in the SPFR, but also showed the inhibition effect in the monolith burners. Conclusions The combustion of ultra-lean methane/air mixtures in precious metal-based catalytic honeycomb monolith burners remained catalytic at temperatures high enough to have supported gas-phase combustion. This was concurrent with near-zero emissions, apart from CO2 and H2O, and high stability. This phenomenon was examined in more controlled conditions in a stagnation point flow reactor with Pt, paired with its theoretical model, and making use of global and detailed coupled heterogeneous-homogeneous chemistry. While the reactor design showed that it accurately reproduced the reactive flow of the ideal setup, without the need for correction factors, the state-of-the-art mechanisms used here underestimated the strength of the inhibition of gas-phase ignition by predicting it would occur 300400 K lower, effectively wrongly indicating a promoting (32) Pfefferle, L. D.; Griffin, T. A.; Winter, M.; Crosley, D. R.; Dyer, M. J. Combust. Flame 1989, 76, 325-338. (33) Pfefferle, L. D.; Griffin, T. A.; Winter, M.; Crosley, D. R.; Dyer, M. J. Combust. Flame 1989, 76, 339-349. (34) Forsth, M.; Gudmundson, F.; Persson, J. L.; Rosen, A. Combust. Flame 1999, 119, 144-153. (35) Park, Y. K.; Aghalayam, P.; Vlachos, D. G. J. Phys. Chem. A 1999, 103, 8101-8107. (36) Aghalayam, P.; Park, Y. K.; Vlachos, D. G. Proc. Combust. Inst. 2000, 2017-2029. (37) Aghalayam, P.; Park, Y. K.; Vlachos, D. G. AIChE J. 2000, 46, 2017-2029.

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effect. The shortcomings of these mechanisms are likely to be simple because the discrepancies between model and experiments are seen only in the temperature range and not in the magnitude or curve profile of the products selectivity. They are most probably due to the weakness of the representation of the free radicals population in the chemical mechanisms. The established methodology with the SPFR can be used for the optimization of such mechanisms where gas-phase and heterogeneous chem-

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istry are coupled, and here provides a valuable tool that can ultimately assist in the design of catalytic burners. Acknowledgment. We are thankful to the CVCP for an ORS award to S.-H. Zhang, to Dr. M. V. Twigg, Johnson Matthey, Royston, U.K., for providing the catalytic monoliths, and to Mr. R. Holt for the design of the SPFR reactor. EF020125K