Reaction Mechanism and Thermodynamic Properties of Aliphatic Hydrocarbon Groups during Coal Self-Heating

To further understand the development of coal self-heating, the reaction sequences and thermal properties of aliphatic hydrocarbon groups during coal self-heating were analyzed. The structural parameters, frontier orbital characteristics, molecular orbital, and perturbation energy of aliphatic hydrocarbons and oxygen were analyzed by the quantum chemistry method. Then, the reaction pathways of aliphatic hydrocarbon groups and the corresponding reaction model were proposed. The results indicate that the reactions of aliphatic hydrocarbon groups include three kinds, i.e., the hydrogen capture by oxygen, reaction between aliphatic hydrocarbon radicals and the hydroxyl radical, and reaction between aliphatic hydrocarbon radicals and oxygen. The main reactions include the reaction between carbon free radicals and oxygen (E1), the reaction between aliphatic hydrocarbon and the hydroxyl radical (E2), the reaction between methyne and oxygen (E3), and other spontaneous reactions caused by E1 and E3 (E2). There is only one reaction procedure for the carbon free radical (R•) to turn into peroxide (R–O–O•) and cause more heat release. Reaction E3 generates less heat compared to reaction E1, but it can form hydroxyl radicals, which then result in other spontaneous reaction sequences (E2) to generate R•, which will easily start reaction E1. Therefore, reaction sequence E3 includes two aspects of influence during coal reaction with oxygen, i.e., heat supply and the trigger of other further reactions. It shows that the reaction sequence of aliphatic hydrocarbon groups releases more heat than other reaction sequences and plays more important roles in the temperature rise process of coal self-heating. The results are useful for further exploring the reaction mechanism of spontaneous combustion of coal.