Experimental and numerical investigation of the ignition of methane combustion in a platinum-coated honeycomb monolith

Abstract

This paper represents an experimental and numerical study of the ignition of catalytic combustion of methane in a cylindrically shaped honeycomb monolith coated with platinum. The objective is the achievement of a better understanding of transient processes in catalytic combustion monoliths. In the experiment, cold methane/oxygen/argon mixtures are fed into the monolith, which is placed in a furnace used to heat up the monolith until ignition occurs. The ignition process is monitored by thermocouples and mass spectroscopy. In the numerical study, the time-dependent temperature distribution of the entire catalytic solid structure and the two-dimensional laminar flow fields of the single monolith channels are simulated. The latter predict the gaseous velocity, species concentrations, and temperature based on a boundary-layer approximation. A multistep heterogeneous reaction mechanism is used, and the surface coverage with adsorbed species is calculated as function of the position in the monolith. The heat balance for the solid structure is coupled with the single channel simulations by axial wall temperature profiles, representing the temperature boundary condition in the single channel simulation, and by heat source terms, derived from the gaseous heat convection and chemical heat release in the single channels. The procedure employs the difference in timescales of the temperature variation of the solid, which is on the order of seconds, and of the flow, which is on the order of miliseconds. Experimentally determined and numerically predicted ignition temperatures, as well as time-varying monolith exit temperatures, and fuel conversion during ignition are compared for several CH4/O2 ratios. At the conditions applied, ignition starts at the rear end in the outmost channels.