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Abstract
The heterogeneous oxidation of methane-air mixture in a honeycomb catalytic reactor is investigated numerically in the present study. An improved multi-step surface reaction mechanism for methane oxidation on platinum is proposed so that surface ignition of lean methane-air mixtures is better modeled. First, this surface mechanism is used to determine the apparent activation energy of methane-air catalytic combustion. The predicted activation energies are found to agree well with the experimental data by Trimm and Lam (1980) and by Griffin and Pfefferle (1990). The chemical model indicates that, depending on the surface temperature, the surface reaction rate is dominated by either the oxygen desorption rate or by the methane adsorption rate.
Second, the surface chemistry model is used to model a methane-air catalytic reactor with a two-dimensional flow code. The substrate surface temperatures are solved directly with a thermal boundary condition derived by balancing the energy fluxes at the gas-catalyst surface. Predictions of gas phase CO profiles and methane conversion at low surface temperatures are improved over those calculated in a previous study (Bond et al., 1996). The numerical model indicates that surface reaction becomes diffusion controlled soon after the surface is ignited. Since the surface ignition point is located near the entrance region, the catalytic combustion process is largely diffusion limited. A parametric study of pressure effects on the methane catalytic combustion is performed with the present numerical model. The predicted methane conversion rate does not decrease monotonically with pressure as expected for diffusion limited reactions. The model predicts that the methane catalytic combustion rate is limited to an even greater extent by gas phase diffusion when the pressure exceeds 2 atm.