Experimental and theoretical study of the oxidation of ventilation air methane over Fe2O3 and CuO

Abstract

Coal mine ventilation air methane (VAM) is an important contributor to methane emissions from the energy sector. Although various technologies are under development, treatment of the VAM with an efficient and cost-effective approach has been an ongoing challenge due to massive flow rates of the ventilation air and low and variable methane concentrations. Recently a new concept based on the principle of chemical looping combustion (CLC) has been proposed for VAM abatement (Appl. Energy, 2014, 113, 1916), in which oxidation of low-concentration CH₄ balanced by N₂ with Fe₂O₃ or CuO as the oxygen carrier was studied. Here, we thoroughly examined the feasibility of CLC of VAM based on experimental study and theoretical calculations. Reduction of Fe₂O₃ and CuO and evolution of gas products during CH₄ oxidation were investigated using TGA-MS under two reaction atmospheres: 1 vol% CH₄ balanced by N₂ and the simulated VAM containing 1 vol% CH₄, 20 vol% O₂, 0.4 vol% CO₂ and balance N₂. It was found that the CLC of VAM is fundamentally infeasible because the reduced phase of Fe₂O₃ and CuO cannot be formed for chemical looping when reacting with the simulated VAM containing abundant oxygen. Theoretical calculations revealed that Fe₂O₃ and CuO remain stable without the transition to the reduced phase as the generated oxygen vacancy on the surface of metal oxides during CH₄ oxidation can recover quickly with O₂ adsorption and dissociation. Calculations confirmed that both Fe₂O₃ and CuO play a role of surface catalyst in VAM oxidation. More importantly, it was found that the low-coordinated metal atoms and oxygen vacancies can stabilize CH_x radicals to promote the dissociation of CH₄, which is generally the rate-determining step for CH₄ oxidation. Such findings are useful for new development and understanding of high-performance and low-cost metal oxide catalysts for CH₄ oxidation.