Study on spontaneous combustion characteristics of air-dried water-soaked lean coal in a gas-containing environment

To clarify the spontaneous combustion risk of residual coal under long-term water immersion and gas-containing airflow in underground mines and investigate the effects of dual water immersion and gas action on its spontaneous combustion characteristics, water-soaked coal samples were prepared by immersing partial coal samples in distilled water at a constant temperature of 30 ℃ for 30 days. Programmed temperature rise experiments for coal spontaneous combustion, simultaneous thermal analysis, and infrared spectroscopy were conducted to analyze the temperature-dependent variations in oxidation products, apparent activation energy, and heat release intensity during the low-temperature oxidation process of air-dried raw coal and water-soaked coal under three gas volume fractions (0, 1.5%, and 3%). Additionally, the evolution characteristics of functional groups after low-temperature oxidation. The results show that with the increase of gas volume fraction during low-temperature oxidation, the oxygen consumption rate, oxidation product generation rate, total heat release, and relative content of active functional groups of two coal samples gradually decrease, with more significant reductions observed in water-soaked coal compared to raw coal; the temperatures corresponding to the occurrence of C₂H₄, the achievement of thermal equilibrium, and the peak of heat flux all increase with the rise in gas volume fraction, Showing an obvious temperature hysteresis, and the increase in water-soaked coal is greater than that of raw coal; the activation energy required for low-temperature oxidation increases progressively, with a larger increment in water-soaked coal. Under the same gas volume fraction, water-soaked coal demonstrates higher oxygen consumption rate, oxidation product generation rate, total heat release, and relative active functional group content than raw coal; the appearance of C₂H₄, thermal equilibrium, low-temperature oxidation activation energy, and heat flux peak temperature are lower in water-soaked coal than in raw coal. Comprehensive analysis reveals that water-soaked coal exhibits stronger spontaneous combustion tendency than raw coal, indicating that gas exerts a more stronger inhibitory effect on the oxidation of water-soaked lean coal at the same gas volume fraction. However, the promoting effect of water immersion on the spontaneous combustion of lean coal outweighs the inhibitory effect of gas on water-soaked coal. In environments with low gas volume fractions, water-soaked lean coal is more prone to oxidation and spontaneous combustion than raw coal.