Modeling catalytic oxidation of lean mixtures of methane–air in a packed-bed reactor

A mathematical model developed for simulation of combustion of preheated lean homogeneous methane–air mixtures in an adiabatic catalytic packed bed is described. The unsteady one-dimensional model includes inter-phase heat and mass transfers, longitudinal mass dispersion in the gas phase and longitudinal conduction and radiation heat transfer modes. The solid and gas phases are assumed not to be in local thermal equilibrium. Multi-step reaction mechanisms are employed for both the surface and gas phase chemical reactions. The surface reaction mechanism for the oxidation of methane on Pt includes 36 elementary surface reactions and 11 surface adsorbed species. The gas phase reaction mechanism includes 28 elementary reactions and 21 species. The governing equations are the unsteady equations of conservation of mass, chemical species and separate energy equations for solid and gas phases. These equations were solved using the commercial CFD code ‘Fluent’ and a number of modifying subroutines especially are developed to include the equations of surface reactions in the computations.The model has been used to investigate the effects of operational conditions such as the mixture inlet temperature (600–1250 K), approach velocity (0.5–15 m/s) and equivalence ratio (0.15–0.5) on the oxidation of methane within both the catalytic and non-catalytic packed-bed reactors under adiabatic conditions. The calculated values of methane conversion showed good agreement with the corresponding available experimental data. Moreover the amount of heat release at different inlet velocities was calculated and the optimum value discussed.