Enhanced combustion stability of low-concentration methane though a flame buffer zone in a variable pore-density porous media burner

Porous media combustion (PMC) is considered as a feasible solution for addressing low-concentration methane (LCM) in coal mines. Conventional straight PM burners are suboptimal in ameliorating LCM combustion stability. Herein, a four-layer PM burner assembled with variable pore-density Al2O3 foam ceramics was designed to improve the combustion stability. The effects of PM arrangement, equivalence ratio (phi), and inlet velocity (v) on combustion performance was systematically evaluated. Finally, the heat transfer processes and oxidation pathways were also elucidated in details. The results indicated that the burner with a gradual pore-density PM configuration (40-30-20-10 PPI) achieved more uniform temperature distribution and superior combustion stability. The presence of flame buffer zone in the structure attenuated flame propagation speed and harmonized equivalence ratios and velocities to achieve an efficient dynamic heat balance, which is conducive to improving the stability of the combustion system under lean combustion conditions. At phi = 0.37, v = 0.104 m/s, the flame position was anchored near 60 mm with a moderate peak temperature of 834 degrees C and the maximum temperature of 811 degrees C were obtained with a flat rate of change of 0.02 similar to 0.07 degrees C/s. The flame still sustained the desirable submerged combustion with a final temperature of 144 degrees C under this lean combustion scenario. The pollutant emissions in the PMC process were overall at lower levels and the NOx emissions were lower than 3.60 ppm for all operating conditions. This study provides a worthy reference in designing and optimizing burner structures for stable and efficient combustion of LCM.