Numerical studies of methane catalytic combustion inside a monolith honeycomb reactor using multi-step surface reactions

The heterogeneous oxidation of methane-air mixture in a honeycomb catalytic reactor is investigated numerically in the present study. An improved mult i-step surface reaction mechanism for methane oxidation on platinum is prop osed so that surface ignition of lean methane-air mixtures is better modele d. First, this surface mechanism is used to determine the apparent activati on energy of methane-air catalytic combustion. The predicted activation ene rgies are found to agree well with the experimental data by Trimm and Lam ( 1980) and by Griffin and Pfefferle (1990). The chemical model indicates tha t, depending on the surface temperature, the surface reaction rate is domin ated by either the oxygen desorption rate or by the methane adsorption rate . Second, the surface chemistry model is used to model a methane-air catalyti c reactor with a two-dimensional flow code. The substrate surface temperatu res are solved directly with a thermal boundary condition derived by balanc ing the energy fluxes at the gas-catalyst surface. Predictions of gas phase CO profiles and methane conversion at low surface temperatures are improve d over those calculated in a previous study (Bond et al., 1996). The numeri cal model indicates that surface reaction becomes diffusion controlled soon after the surface is ignited. Since the surface ignition point is located near the enhance region, the catalytic combustion process is largely diffus ion limited. A parametric study of pressure effects on the methane catalyti c combustion is performed with the present numerical model. The predicted m ethane conversion rate does not decrease monotonically with pressure as exp ected 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.