Mutual conversion of active sites and oxygen-containing functional groups during low-temperature oxidation of coal

The room temperature oxidation of active sites is believed as the initial heat source responsible for coal self-heating, however its specific microscopic reactions in coal oxidation are still unclear. The interconversions between oxygen-functional groups and active sites are therefore investigated. Low-temperature oxidation combined with the low-temperature pyrolysis process was employed, the macroscopic gas concentrations of the two processes were first measured, the microscopic content of free radicals and oxygen-containing functional groups was then determined using in-situ FTIR and in-situ EPR. It was found that the low-temperature pyrolysis of coals led to the decomposition of oxygen-functional groups and the generation of alkyl radicals. During oxidation, ambient oxidation of alkyl radicals was observed, followed by an increase in alkoxy radicals, and then a decrease in alkyl structures. The results revealed that the active sites are alkyl radicals that can exist stably in an inert environment and can be transformed into oxygen-functional groups during oxidation. In addition, the oxidation of active sites resulted in chain reactions, which further led to the dehydrogenation of alkyl chains to free radicals. The increase in the free radical active sites and their interconversion with oxygen-functional groups result in heat accumulation and uncontrolled spontaneous combustion.