Influence law of air flow and water immersion duration on the risk of secondary oxidation spontaneous combustion of coal.

To further elucidate the variations of secondary oxidation spontaneous combustion risk of lignite under different air flows and immersion time. Secondary oxidation experiments of short-term water-immersed coal and long-term water-immersed coal were conducted under four air flows. The results show that, the presence of a temperature inflection point during primary oxidation process, when coal temperature exceeds it, both the oxygen consumption rate and heat release intensity of long-term water-immersed coal are lower, furthermore, decrease in air flow leads to reduction in the temperature inflection point. The oxygen consumption rate and heat release intensity during the primary oxidation process exceed those observed during the subsequent secondary oxidation process. In the secondary oxidation process, long-term water-immersed coal exhibits higher rates of oxygen consumption and heat release intensity compared to short-term water-immersed coal. Additionally, the oxygen-consuming activation energy for oxygen consumption of long-term water-immersed coal is lower. The increase in air flow and water immersion time generally leads to the extreme value of the limit parameters, such as the local maximum of minimal thickness of residual coal and the lower limit oxygen fraction, the local minimum of the maximal air leakage intensity develops in the direction of increasing the risk of spontaneous combustion of coal in goaf.