Ultra-lean methane as a kinetic promoter of early oxidation in Fe-based oxygen carriers

Abstract

Methane is a potent greenhouse gas, and large volumes of ultra-lean methane are continuously emitted through coal mine ventilation systems. Although the methane concentration is typically below 1 vol%, the enormous gas flow rates result in significant cumulative emissions. Chemical looping oxidation offers a promising pathway for the safe and efficient mitigation of such dilute methane streams, yet the influence of methane on the oxidation behavior of oxygen carriers remains insufficiently understood. In this work, the effect of ultra-lean methane on the early oxidation behavior of Fe-based oxygen carriers was investigated by combining thermogravimetric analysis with reactive molecular dynamics simulations. The experiments reveal a non-monotonic kinetic response: a trace amount of methane (0.5 vol%) accelerates the initial oxidation rate, whereas higher methane concentrations suppress oxygen uptake. Atomic-scale simulations demonstrate that ultra-lean methane promotes oxygen activation and leads to more uniform lattice oxygen incorporation, accompanied by smoother strain evolution and faster Fe redox conversion. However, increasing methane concentration enhances oxygen diversion toward methane-involved pathways, forming intermediates such as OH and products including H₂O, CO, and CO₂. This competitive oxygen consumption reduces the fraction of oxygen incorporated into the lattice. These results reveal a dual regulatory mechanism governed by the balance between synergistic oxygen activation and competitive oxygen diversion. The findings provide new insight into gas–solid coupled oxidation mechanisms under ultra-lean fuel conditions and offer guidance for the design of chemical looping systems for methane mitigation.

Novelty and significance statement: The novelty of this study lies in the detailed investigation of the regulatory effect of ultra-lean methane on the oxidation behavior of Fe-based oxygen carriers through a combined thermogravimetric and reactive molecular dynamics approach. By integrating macroscopic kinetic measurements with atomic-scale simulations, this work elucidates how trace methane influences oxygen activation, lattice oxygen incorporation, and the evolution of reaction intermediates. Furthermore, the underlying mechanism governing the transition from oxidation promotion to inhibition with increasing methane concentration is systematically clarified.

This research is significant because ultra-lean methane emitted from coal mine ventilation systems represents a major source of fugitive greenhouse gas emissions, yet its efficient mitigation remains a technological challenge due to the extremely low fuel concentration. Chemical looping oxidation offers a promising strategy for the safe and efficient treatment of such dilute methane streams. However, the interaction between methane and oxygen carriers under ultra-lean conditions remains insufficiently understood, particularly regarding the coupling between gas-phase reactions and lattice oxygen incorporation. The mechanistic insights obtained in this study provide useful guidance for understanding gas–solid oxidation processes and for optimizing chemical looping technologies for ultra-lean methane mitigation.