Filtration combustion of a methane wave in air for oxygen-enriched and oxygen-depleted environments

In this work, filtration combustion waves formed in a packed bed of randomly arranged alumina pellets are studied experimentally. Methane is used as a fuel; the oxygen content of the oxidizer is varied from 10% to 30%. Five oxidizer compositions (10, 15, 21, 25, and 30%) are comparatively analyzed focusing on combustion temperatures, combustion wave velocities, and pollutant characteristics. The studied range of equivalence ratios spans from 0.1 to 2 covering the superadiabatic and underadiabatic combustion waves formed in ultralean, lean, rich, and ultrarich regions. It is found that combustion temperature drops with oxygen enrichment while the absolute propagation rate increases. The opposed variation of adiabatic and combustion temperatures strongly affects regimes of superadiabatic (downstream) and underadiabatic (upstream) wave propagation. For certain oxidizer compositions, the combustion temperatures more than twice exceed the adiabatic temperatures of respective homogeneous flames. To help understand the thermal and chemical structure of filtration combustion wave, two important minor products, oxides of nitrogen (NOx) and carbon monoxide (CO) emissions, were sampled from the product stream. The analysis of emission levels suggests strong influence of equivalence ratio and oxygen enrichment on CO emissions due to the variation of the combustion chemistry and contraction/extension of the reaction zone relative to the characteristic pore size. The prompt mechanism is identified as a major pathway of NOx formation. Variation of NO with equivalence ratio in filtration combustion reproduces a characteristic bell shape pertinent to homogeneous flames, extending it to the ultralean and ultrarich regions.