Origin and Radical Chain Formation Mechanism of Abnormal CO Emissions from Jurassic Low-Rank Coal at Ambient Temperature

Abnormally high CO concentrations in the return air of Jurassic low-rank coal mines in the Shenfu-Dongsheng Coalfield hinder spontaneous-combustion forecasting and threaten occupational health. To determine the source of this anomaly, four representative mines were examined using in situ borehole measurements, ambient-temperature cyclic oxidation tests, in situ EPR analysis, and DFT calculations. Results show that abnormal CO emissions originate from ambient-temperature coal oxidation rather than preexisting CO in the seam. All samples continuously consumed O2 and generated CO and CO2 at ambient temperature. Gas production was not simply related to particle size, and the 0.180–0.425 mm fraction was most suitable for oxidation studies. Reducing O2 concentration markedly suppressed, but did not fully stop, oxidation. EPR peak deconvolution revealed decreasing alkyl radicals and increasing oxygen-containing radicals, indicating that side-chain alkyl groups are the main reactive sites. DFT analysis based on bond dissociation energies and condensed Fukui functions identified side-chain methylene radicals (-CH2·) as key active structures initiating ambient-temperature oxidation. Reaction-path calculations further demonstrated that CO formation is exothermic and has a low energy barrier, proceeding via a radical-chain mechanism. These findings clarify the origin of abnormal CO emissions and support inhibitor development targeting primary active structures.