Thermal effects and active group differentiation of low-rank coal during low-temperature oxidation under vacuum drying after water immersion

Reasons for the more spontaneous combustion of coal in the goaf after being immersed were investigated. Using simultaneous thermal analysis, heat flow and thermal mass curves with temperature were measured for two low-rank raw and water-immersed coals, obtained from the Xinjiang and Pingshuo regions of China, with different water contents after air drying. The influence of water content on the low-temperature oxidation and exothermic characteristics of coal during the process of immersion and air drying was analyzed. Three-dimensional in situ Fourier Transform infrared spectra, measured during the low-temperature oxidation of water-immersed coal that easily spontaneously combusted and raw coal, enabled comparison and analysis of differences in the initial functional group structures of the two coal samples and their evolution during the oxidation process. The results showed that there is a range of moisture contents experienced during the process of water immersion and air drying that renders the coal more prone to spontaneous combustion than raw coal. Within this range, one specific moisture content was most susceptible to spontaneous combustion. When water-immersed coal was air-dried to the critical moisture content that was most likely to self-ignite, the contents of associated hydroxyl and aliphatic hydrocarbon functional groups were larger than those of the raw coal. The oxidation reaction rates of these two groups were accelerated. These changes promote the rapid formation of aliphatic chain cyclic reactions and increase the initial heat release, resulting in such coal being more spontaneously combustible.