Catalytic combustion properties of Fe−Co/Al₂O₃ porous media catalysts for low-concentration coal mine gas

Addressing the low utilization and massive venting of low-concentration methane (LCM) in coal mines (< 8%), which results in energy waste and greenhouse effect. Fe−Co/Al₂O₃ bimetallic porous media catalyst loaded with Fe₂O₃ and Co₃O₄ as dual active components was prepared using ultrasonic-assisted impregnation method to investigate the combustion performance of LCM. The effects of excess air ratio φ_a and flow rate f on combustion characteristics were examined, and long-term combustion stability was tested. The results indicated that the Fe and Co in Fe−Co/Al₂O₃ synergistically engaged in CH₄ catalytic reaction. Fe³⁺ and Co³⁺ were partially reduced to Fe²⁺ and Co²⁺ species, and the formation of CoFe₂O₄ spinel structure improved thermal stability and resistance to sintering of the catalyst, enabling it to maintain good catalytic activity during high-temperature lean combustion. CH₄ oxidation reaction followed the MvK mechanism, in which lattice oxygen participated in the cleavage and activation of the C—H bond in CH₄, with its consumption forming oxygen vacancies. Adsorbed oxygen dynamically replenished lattice oxygen to maintain the migration and cycling of oxygen species, while further promoting the oxidation of intermediates. Excess air ratio and flow rate significantly influenced temperature distribution and flame migration, low excess air ratio or low flow rate tended to cause combustion flashback, whereas high excess ratio or high flow rate could lead to the risk of blow off. The thermal buffering effect generated by the gradient pore structure of the porous media, combined with the sustained heat release of the catalytic reaction, alleviated temperature fluctuation and enhanced flame stability. Within the range of φ_a = 2.44—2.56 and f = 50—65 L/min, the flame was anchored in the middle of the burner, the combustion temperature was maintained above 800 ℃, and NO_x emissions were extremely low. At φ_a = 2.56, the preheating zone temperature remained around 220 ℃ even after 72 h of combustion, and the CH₄ conversion approached 100% throughout the process, confirming that the coupling of gradient porous media and catalytic reaction significantly improved lean-burn performance and stability. This study provides a novel insights and technical support for the clean and efficient utilization of coal mine LCM.