Chemical structures of methane-air filtration combustion waves for fuel-lean and fuel-rich conditions

Chemical structures of filtration combustion waves in an inert porous media were analyzed comparatively for lean and rich methane-air mixtures. Temperature, velocity, and chemical products of the combustion waves were studied experimentally in the range of equivalence ratios from 0.2 to 2.5. Downstream (superadiabatic) wave propagation was observed for ultralean (phi less than or equal to 0.45) and ultrarich (phi greater than or equal to 1.7) mixtures. Upstream (underadiabatic) propagation corresponds to the range of equivalence ratios from 0.45 to 1.7. It was found that with the equal heat content, rich mixtures have essentially higher combustion temperatures than corresponding lean mixtures. The products of partial methane oxidation (H-2, CO, and C-2 hydrocarbons) are dominant for ultrarich superadiabatic combustion where up to 60% of methane is converted to CO and H-2. A numerical model, based on a two-temperature approximation and detailed combustion chemistry, is developed to analyze species profiles and the combustion mechanism of the filtration waves. The model predictions for combustion temperatures and chemical products are in good agreement with experimental data. Kinetic simulation revealed the complex chemical structure of the ultrarich superadiabatic waves. It is shown that this wave is composed of an exothermic "partial oxidation" reaction zone followed by an endothermic "steam reforming" zone.