Methane-involved thermal restructuring of coal enhances low-temperature oxidation reactivity: Experimental and theoretical insights

Igneous intrusion into coal seams creates a high-temperature, high-pressure methane-rich geological environment that substantially modifies coal structure and elevates its spontaneous combustion tendency. Yet the active role of CH 4 during coal thermal restructuring remains poorly understood. In this study, coals were thermally treated under pressurized N 2 and CH 4 atmosphere, respectively. Functional group evolution was characterized by Fourier transform infrared spectroscopy (FTIR) and pyrolysis gases were analyzed by gas chromatography. Quantum chemical calculations were performed to elucidate the reaction mechanisms and low-temperature oxidation behavior. The results demonstrate that the CH 4 atmosphere facilitates coal pyrolysis and significantly increasing aliphatic hydrocarbon content. After treatment at 300 °C, aliphatic content reaches 31.95 % under CH 4 , considerably higher than the 26.45 % observed under N 2 . This arises because highly active coal-derived radicals can abstract hydrogen from CH 4 with a low energy barrier. CH 4 progressively serves as a hydrogen donor as the temperature rises, promoting sustained hydrodeoxygenation, enabling exogenous hydrogen and methyl groups to be incorporated into the coal structure. In addition, comparison of the low-temperature oxidation characteristic temperatures derived from thermogravimetric (TG) analysis shows that coals pretreated in a CH 4 atmosphere possess greater low-temperature reactivity. Consequently, CH 4 -involved thermal restructuring enriches aliphatic hydrocarbons and enhances low-temperature oxidation reactivity. These findings reveal the effect of a CH 4 atmosphere on coal thermal restructuring, providing a theoretical basis for assessing the spontaneous combustion risk in coal seams subjected to magmatic intrusion.