Study on the Law and Risk of Spontaneous Combustion of Residual Coal during Water Drainage in Goaf

Water–gas displacement occurring during the drainage of water-soaked goafs facilitates the oxidation of water-soaked coal. The characteristics of oxygen migration and the oxidation and spontaneous combustion (SC) of soaked residual coal during goaf drainage were explored through laboratory research, water drainage simulation and on-site measurement. The results reveal that compared with raw coal samples, the amount and rate of gas products of water-soaked coal samples are higher in the heating oxidation process, demonstrating a strengthened spontaneous combustion (SC) propensity. Its cross-point temperature falls and the apparent activation energy decreases by 1.43–8.75%, that is, the soaked coal sample is easier to spontaneously combust during the drainage of water-soaked goafs. Through simulation, it is found that after water is drained, air leakage in the goaf is significantly intensified, and the pressure difference inside and outside the goaf reaches 498 Pa. By taking the air inlet roadway as the air leakage point for fitting, it is found that the oxygen concentration in the air leakage range increases to 18% during water drainage. The simulation results are basically consistent with the on-site measurement. The on-site monitoring result shows that during water drainage of 7 2 25 goaf in Qinan Coal Mine, water-immersed coal is more prone to spontaneous combustion, and air leakage leads to low-temperature oxidation of water-immersed coal, which increases the on-site temperature rapidly and increases the risk of spontaneous combustion in the goaf. With respect to water drainage in the goaf, an optimization measure of fixed-point and quantitative nitrogen injection during water drainage was put forward on site.