Influence of methane-oxygen coupled gas field on the transformation mechanism of microporosity and active structure of cryogenic pre-oxidized residual coal

Due to the impact of coal seam mining processes and the release of adsorbed gases in residual coal, a mixed gas field of methane and oxygen is formed in the goaf, which affects the oxidation process of the residual coal over a long period. In this study, to explore the low-temperature oxidation mechanism of residual coal in a hypoxic methane-containing environment, Low temperature nitrogen adsorption (LTNA) experiments and Fourier transform infrared spectroscopy (FTIR) were utilized to examine the pore development properties of residual coal and the transformation mechanisms of reactive micro-groups. The results indicate that increases in temperature and methane proportion promote the transition of various pores to larger scales, with 3 % methane inducing the formation of fine micropores, thereby increasing the specific surface area of the coal. High temperatures (80℃,100℃) and increased proportions of oxygen and methane synergistically elevate the fractal dimensions. Methane inhibits the consumption of hydroxyl groups, and a low oxygen concentration (5 %) impedes the gas production from coal oxidation. The presence of both oxygen and methane exhibits a compound effect; the strongest tendency towards spontaneous combustion occur at 1 % methane and 9 % oxygen. The findings provide valuable insights for controlling spontaneous combustion disasters in goafs.