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Coal spontaneous combustion (CSC) is a disaster that seriously threatens safe production in coal mines. Revealing the mechanism of CSC can provide a theoretical basis for its prevention and control. Compared with experimental research is limited by the complexity of coal molecular structure, the quantum chemical calculation method can simplify the complex molecular structure and realize the exploration of the mechanism of CSC from the micro level. In this study, toluene and phenylacetaldehyde were used as model compounds, and the quantum chemical calculation method was adopted. The reaction processes of the methyl and aldehyde groups with oxygen were investigated with the aid of the Gaussian 09 software, using the B3LYP functional and the 6-311 + G(d,p) basis set and including the D3 dispersion correction. On this basis, the generation mechanisms of CO and CO2, two important indicator gases in the process of CSC, were explored. The calculation results show that the Gibbs free energy changes and enthalpy changes in the two reaction systems are both of negative values. Accordingly, it is judged that the reactions belong to spontaneous exothermic reactions. In the reaction processes, the activation energy of CO is less than that of CO2, indicating that CO is formed more easily in the above-two reaction processes. In addition, the variations in concentrations of important oxidation products (CO and CO2) and main active functional groups (such as methyl, carboxyl and carbonyl) with temperature were revealed through a low-temperature oxidation experiment. The experimental results verify the accuracy of the above quantum chemical reaction path. Moreover, it is also found that the generation mechanisms of CO and CO2 in coal samples with different metamorphic degrees are different. To be specific, for low-rank coal (HYH), CO and CO2 mainly come from the oxidation of alkyl side chains; for high-rank coal (CQ), CO is produced by the oxidation of alkyl side chains, and CO2 is attributed to the inherent oxygen-containing structure. [ABSTRACT FROM AUTHOR]

• The relative content of main active groups was obtained by Infrared spectral peak fitting. • The initial and critical temperatures of CO and CO 2 released from coal samples with different degrees of metamorphism were obtained by low temperature oxidation program temperature experiment. • Correlation of various active groups was obtained by Pearson linear analysis. • The relationship between CO, CO 2 and main active groups was obtained by principal component analysis. • Quantum chemical calculation method is used to study the mechanism of free radical reaction between C = O· and OH· At present, the mechanism of CO and CO 2 production in coal spontaneous combustion is controversial. With the hope of exploring the generation mechanism of CO and CO 2 in the low-temperature oxidation of coal, in this study, the correlation between the relative contents of three active functional groups, –CH 3 /–CH 2 , -C = O, and –COOH and the generation law of CO and CO 2 in the low-temperature oxidation of coal samples with different metamorphic degrees was explored by means of principal component analysis, Pearson linear analysis, and Fourier transform infrared spectroscopy. The results indicate that –CH 3 /–CH 2 , -C = O, and –COOH in low-rank lignite (HC) are significantly correlated with the concentrations of CO and CO 2. The -C = O and –COOH in medium-rank bituminous coal (BC) are closely correlated with the concentrations of CO and CO 2. The concentrations of CO and CO 2 in high-rank anthracite (CQ) show the highest correlation with the relative content of –CH 3 /–CH 2 , followed by their correlation with the relative content of -C = O, and their correlation with the relative content of –COOH is relatively low. Thus, it can be concluded that the generation mechanism of CO and CO 2 varies with the metamorphic degrees of coal in coal spontaneous combustion. CO and CO 2 generated from low-rank HC mainly come from continuous generation and oxidation of inherent and secondary alkyl chains and those generated from medium-rank BC mainly come from the oxidation of oxygen-containing functional groups. Besides, for high-rank CQ, CO is mainly generated from the process in which alkyl chains in coal oxidize to form carbonyl groups which then break to form CO, and a small part of CO comes from the coal-oxygen free radical reaction. As for CO 2 , its sources are more complex. A big portion of CO 2 comes from the free radical reaction and CO 2 may also come from the direct conversion of CO at high temperature and in the presence of water. Finally, the reaction mechanism of C = O· and ·OH free radicals was studied by quantum chemical calculation. The calculation results reveal that the free radical reaction can occur spontaneously and release a certain amount of heat under certain conditions, thus playing a positive role in promoting coal spontaneous combustion. [ABSTRACT FROM AUTHOR]