Catalytic Combustion of Methane/Air Mixtures over Platinum

The homogeneous ignition of fuel-lean methane/air mixtures is investigated numerically in laminar plane channel configurations with platinum-coated isothermal walls and uniform incoming properties. Parametric studies are carried out to determine the dependence of the homogeneous ignition distance (xig) on the fuel-to-air equivalence ratio (), the wall temperature (TW), the inlet temperature (TIN), the inlet velocity (UIN), and the channel wall separation (2b). Computations are performed with elliptic and parabolic two-dimensional numerical codes, both with elementary heterogeneous and homogeneous chemical reaction schemes. The applicability of the parabolic approach (boundary layer approximation) in assessing homo-geneous ignition is investigated. The elliptic approach yields shorter xig compared to those of the parabolic approach, but as UIN increases, their difference diminishes, and for UIN greater than a minimum value Um,IN, both computations give the same xig. Um,IN depends strongly on and ranges from 8 m/s ( 0.35) to 15.5 m/s ( 0.55) at atmospheric pressure. An analytical homogeneous ignition criterion based on activation energy asymptotics, a one-step gaseous reaction and a mass-transport-limited surface reaction, is presented for catalytic channel configurations and adapted to lean methane/air combustion. The mass-transport-limited assumption is shown to be valid only at atmospheric pressure. A one-step gaseous reaction with a methane order of 0.33 and an activation energy of 243.4 kJ/mol yields, in conjunction with the analytical ignition criterion, homogeneous ignition distances at atmospheric pressure within 9.2 % of those numerically predicted over a wide range of operating conditions (0.35 0.55, 1380 K TW 1600 K, 623 K TIN 743 K and 1.5 mm b 15 mm). The negative methane reaction order (methane self-inhibition) results in shorter xig for the leaner mixtures. The apparent activation energy is higher than that of purely homogeneous combustion (200 kJ/mol) due to catalytic inhibition via radical adsorption