Kinetic effects of hydrogen addition on the catalytic self-ignition of methane over platinum in micro-channels

Abstract

The hetero-/homogeneous chemistry coupling of hydrogen and methane over platinum in catalytic micro-channels was investigated numerically for hydrogen–methane–air mixtures with lean methane–air equivalence ratios φ=0.7, 0.8 and 0.9, containing 0–5% hydrogen (on a molar basis). Simulations were carried out with a 2-D computation that included detailed hetero-/homogeneous chemistry and transport, with emphasis on the kinetic effects of hydrogen addition on the catalytic self-ignition of methane. Simulations indicated that hydrogen can successfully cause catalytic self-ignition of methane–air mixtures in micro-channels, eliminating the need for startup devices. While, methane kinetically inhibits catalytic combustion of hydrogen at low hydrogen compositions, and a two-stage catalytic ignition can be observed under certain conditions. A minimum hydrogen composition for catalytic self-ignition of methane–air mixtures is found to be relatively constant, which is at the level of approximately 3.2% molar fraction, irrespective of the methane–air composition. Furthermore, kinetic analysis suggested that this constant is exactly the critical point from thermal effect (preheating region) to kinetic effect (self-ignition region) with the increase of hydrogen composition, resulting in a sharp reduction in ignition temperature, and providing a good startup strategy and an opportunity to self-ignite hydrocarbons. However, once the hydrogen concentration in the feed is sufficiently low, i.e., below the above critical point, external heating is necessary to ignite methane–air mixtures. Finally, surface coverage simulations in catalytic micro-channels revealed that hydrogen promotes methane catalytic combustion because of the increase in free Pt(s) sites, and richer hydrogen composition or lower temperature enhances this kinetic effect.