Characterization of elementary reactions inducing coal spontaneous combustion

Twenty-five reactions involving R⋅, ⋅OH, ⋅OOH, R–CH2OH, R–COOH, R–CHO, R–CH3, ROO⋅, R–CH2O⋅, R–C(=O)–O⋅, and R–Ċ=O reactive tissues were studied by computational chemistry to reveal the microscopic mechanism of coal spontaneous combustion (CSC). Results showed that R⋅ chemisorbed oxygen to form ROO⋅. Then, ROO⋅ captured the H atom from alkane or self-structure to generate ⋅OH and R⋅. Accordingly, a cyclic reaction centered on R⋅ was formed. In addition, the reaction of the reactive groups (R–CH3, R–CH2OH, R–COOH and R–CHO) with hydroxyl radicals, which requires low activation energy and is accompanied by exotherm, is the main pathway to produce R⋅, R–CH2O⋅, R–C(=O)–O⋅, and R–Ċ=O. Subsequently, condensation reactions occurred between ⋅OH and R⋅, ⋅OOH and R–Ċ–OH, R–CO⋅ and R⋅, and R–C(=O)–O⋅ and R⋅, which can spontaneously proceed and release large amounts of heat, accelerating the self-heating of the coal. O2 can directly capture the H atom on the alkyl group to generate R⋅ and ⋅OOH as the coal body temperature reaches 30 °C–70 °C.⋅OOH can also capture the H atoms on the alkyl group to generate R⋅. Consequently, another small cycle of radical reactions centered on R⋅ is formed. Meanwhile, the reactions of ⋅OOH and R⋅ with R–CH2OH, R–COOH, and R–CHO provide two additional pathways to generate R–CH2O⋅, R–C(=O)–O⋅, and R–Ċ=O. Finally, the decomposition reactions of R–CH2O⋅, R–C(=O)–O⋅ and R–Ċ=O are the main pathways to produce R⋅, CO2, and CO, respectively. Accordingly, a macrocycle chain reaction of free radicals is formed. Therefore, removal of ⋅OH, R⋅, ⋅OOH, and ROO⋅ is the key to break the chain reaction of the CSC cycle.