Reaction pathways and cyclic chain model of free radicals during coal spontaneous combustion

•The cyclic model of free radicals R1 ~ R9 during coal self-heating was proposed.•Most reactions in free radicals are exothermic with a maximum heat release of 167.113 kJ/mol.•The initial reactions (R1 and R2) can accumulate heat and generate R-O-O and OH.•Both OH and R play vital roles in consuming active groups in coal continuously.•R R-O-O and OH should be eliminated for terminating the cyclic chain reaction. Reactions of free radicals play essential roles in the evolution of the spontaneous combustion of coal. However, their elementary reaction pathways and details during this process have not been revealed by far. This paper established reaction pathways of free radicals during coal spontaneous combustion based on typical active free radicals by quantum chemistry method. The specific structural parameters and thermodynamic data for reactions (R1 ~ R9) are calculated to reveal the reaction process. The cyclic model of free radicals during coal self-heating has been proposed for the first time. The results indicate that most of the reactions involved in free radicals are exothermic reactions with heat release ranging from 7.568 ~ 167.113 kJ/mol. The initial reactions (R1 and R2) can accumulate heat and generate active peroxide free radical (R-O-O) as well as hydroxyl radical (OH). Both of them can occur easily at the normal temperature. The heat release increases the ambient temperature, which causes non-spontaneous reactions to proceed successively. The decomposition of peroxide free radicals triggers the reaction pathways of the cyclic model. The hydroxyl radical can lead to the reaction with active sites in coal, which can accumulate plenty of heat and supply the R for initiating reaction R(1) in the cycle. Both hydroxyl radicals and hydrocarbon radicals play vital roles in consuming active groups in coal and oxygen continuously. The results demonstrate that to fundamentally prevent coal spontaneous combustion, it is necessary to eliminate the key active free radicals (R, R-O-O, and OH) to terminate the cyclic chain reaction.