Insight into the chemical reaction process of coal self-heating after N2 drying

There are original active sites on the surfaces of coal pores, and the thermal decomposition of oxygen-containing functional groups also produces secondary active sites, which are capable of producing an oxidative exotherm at room temperature and produce a large amount of CO and CO2 gaseous products. The generation and oxidation of active sites rather than changes in physical structures are the intrinsic reasons why brown coal is more likely to spontaneously combust after N2 drying. [Display omitted] •The room temperature oxidation of N2-dried lignite was studied.•There are original active sites exist on the surface of coal pores.•Thermal decomposition of oxygen-containing functional groups will produce secondary active sites.•The room temperature oxidation of active sites is the reason why N2-dried lignite is easier to spontaneous combustion. It is generally considered that changes in physical rather than chemical structures are responsible for the increase in the spontaneous combustion tendency of coal after inert drying. In this paper, an isothermal flow reactor was combined with FTIR and ESR techniques to study the room temperature oxidation of coal samples under the effects of drying temperatures, numbers of drying-oxidation cycles and anaerobic heating temperatures. Macro-physical parameters such as gaseous generation, oxygen consumption, coal core temperature and micro-chemical structural parameters such as functional groups and free radicals were examined. After coal drying, the thermal decomposition of oxygen-containing functional groups generates free radical active sites, which can be oxidized and are exothermic at room temperature, accompanied by the generation of large amounts of CO and CO2 gaseous products. In addition, the active sites and oxygen-containing functional groups can be transformed into one another under certain conditions. The results indicate that the inherent reason responsible for self-heating and uncontrolled spontaneous combustion of N2-dried coal is the room temperature oxidation of the active sites, especially the room temperature oxidation of the secondary active sites generated by thermal decomposition of the oxygen-containing functional groups. The results presented in this research have applications to reveal the chemical reaction mechanism determining why lignite is more prone to spontaneous combustion after inert drying.