Critical thresholds of pre-oxidation in coal spontaneous combustion: microstructural drivers and kinetic implications

Abstract

During mining, the residual coal in the goaf and roadway undergoes continuous oxidation in a complex environment characterized by air leakage. The oxidation temperature and oxygen concentration fluctuate with the leakage of air, which leads to changes in the spontaneous combustion characteristics and microstructure of the residual coal. To investigate the impact of pre-oxidation on the spontaneous combustion characteristics and microstructure of loose coal, coking coal was pre-oxidized in the laboratory under varying temperature and oxygen concentration conditions. The pore structure, functional group content, gas oxidation products, and exothermic characteristics of both the pre-oxidized coal and the raw coal were compared using programmed heating, simultaneous thermal analysis, Fourier-transform infrared spectroscopy, and low-temperature nitrogen adsorption techniques. The results indicated that the temperature range in which the pre-oxidized coal is more prone to spontaneous combustion than the raw coal is 70–100 °C, with an oxygen concentration range of 5 %–10 %. Under these pre-oxidation conditions, the oxygen consumption during secondary oxidation increases, leading to higher concentrations of indicator gases such as CO and C₂H₄. Additionally, the cross-point temperature decreases, while the ignition point temperature and temperature of the maximum oxidation rate are lowered. The active group content increases, the pore structure develops, and the specific surface area expands, thereby increasing the risk of spontaneous combustion. When the pre-oxidation temperature and oxygen concentration are excessively high, the spontaneous combustion tendency of the coal is reduced, and the initial generation temperature of C₂H₄ during secondary oxidation increases. This study provides valuable insights into the development of coal spontaneous combustion early warning systems in coal mines.