Catalytic combustion of methane/air mixtures over platinum: Homogeneous ignition distances in channel flow configurations

Abstract

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 (χ ig ) on the fuel-to-air equivalence ratio (), the wall temperature ( T W ), the inlet temperature ( T IN ), the inlet velocity ( U IN ), and the channel wall separation (2 b ). 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 homogeneous ignition is investigated. The elliptic approach yields shorter x ig compared to those of the parabolic approach, but as U IN increases, their difference diminishes, and for U IN greater than a minimum value U m, IN , both computations give the same x ig . U m, IN depends strongly onand 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≤ T W ≤1600 K. 623 K≤ T IN ≤743 K and 1.5 mm≤ b ≤15 mm). The negative methane reaction order (methane self-inhibition) results in shorter x ig 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.