Effect of water immersion on active functional groups and characteristic temperatures of bituminous coal

In the mining of multiple coal seams, geological fracture may cause water from aquifers to accumulate in the overlying gob. Residual coal in the overlying gob after water immersion and air exposure is regarded as soaked coal, which is at a high risk of coal spontaneous combustion. In the present study, Fourier transform infrared spectrometry and synchronous thermal analysis were performed to investigate the active functional groups, characteristic temperatures, and exothermic properties of bituminous coal (selected due to its abundance in northwestern China) immersed in water for different durations (150, 180, 210, and 240 days). On the basis of reaction mechanism equations, two thermokinetic models were used to calculate the apparent activation energy (AAE) in different stages of coal pyrolysis. Results revealed that soaked coals were more prone to spontaneous combustion than raw coal (WW0) because of changes in the numbers of functional groups and characteristic temperatures. The critical soaking duration of 180 days increased the exothermic strength of coal to its maximum. The AAE of WW0 was higher than that of soaked coals in Stages 1 and 4 of coal pyrolysis, indicating that soaked coals were more likely to react.