Reaction mechanisms involving the hydroxyl radical in the low-temperature oxidation of coal

To explore reaction mechanisms in coal involving the hydroxyl radical (·OH), its abstraction of hydrogen atoms from toluene and peroxyl radicals and hydroxyl, aldehyde, and carboxyl groups has been analyzed by quantum chemical calculations. Further quantum chemical calculations and electron paramagnetic resonance (EPR) experiments have been performed on the inhibitory effect of hydroxytyrosol (HTY) on coal spontaneous combustion (CSC). Our results indicate that ·OH is mainly formed through the decomposition of ·OOH or ROO·. The bond dissociation energy of ·OOH is 317.69 kJ/mol, whereas hydrogen transfer within ROO· needs to overcome an energy barrier of 86.64 kJ/mol. Therefore, ·OH is primarily produced from ROO· in the initial stage, but the decomposition of ·OOH gradually becomes the main pathway with increasing temperature. H in hydrocarbons and oxygen-containing groups is abstracted by ·OH to form C·, -C(OH)·, -C(O)·, –CO(O)· radicals and H2O. The C· radicals then adsorb O2 to generate ROO·, which can then decompose to produce ·OH. Therefore, the whole process of coal oxidation manifests as a cyclic oxidation reaction. The reactions of hydrocarbons and hydroxyl groups with ·OH need to surmount energy barriers of 39.38 and 23.63 kJ/mol, respectively, with corresponding heat releases of 18.38 and 154.90 kJ/mol. H abstractions from aldehyde and carboxyl groups need to overcome energy barriers of 68.26 and 73.51 kJ/mol, respectively, and absorb heats of 97.14 and 49.88 kJ/mol. The O18-H19 bond in HTY, after overcoming a barrier of 36.75 kJ/mol, can eliminate ·OH with a heat release of 23.63 kJ/mol, the rate constant for which is 1.60 × 109 s−1 m−1. When HTY inhibits CSC, the EPR g factor is lowered and the linewidth becomes greater. The results indicate that HTY effectively removes oxygen-containing and other heteroatom-containing free radicals from coal.