Quantum Chemical Calculation of Original Aldehyde Groups Reaction Mechanism in Coal Spontaneous Combustion

摘要

Oxygen functional groups play a key role in the process of coal spontaneous combustion, and aldehyde groups are one of the main oxygen functional groups, but their reaction pathways are still unclear. Based on the quantum chemical calculation method, this study used the density functional theory (DFT) of Gaussian software to explore the oxidation and self-reaction pathways of aldehyde groups in the process of coal spontaneous combustion. The Ph–CH_(2)–CHO was selected as the characterization of a coal molecule containing the aldehyde group, and the results showed that the C–H bonds of the aldehyde groups formed by s–sp~(2) hybridization are the active sites. During the oxidation reaction process, the hydrogen atoms in aldehyde groups can be captured by oxygen to generate the −·C═O free radicals. The enthalpy change and activation energy of the reaction are 136.87 and 149.53 kJ/mol, respectively, indicating that the reaction can occur in the middle and later stage of coal spontaneous combustion (70–200 °C), which can greatly enhance the self-heating of the reaction system. During the self-reaction process, aldehyde groups can react with the −·CH_(2) free radicals and the ·OH free radicals, and both reactions can generate the −·C═O free radicals, but the thermal effects are not obvious. The activation energies of the two reactions are 63.76 and 22.23 kJ/mol, respectively, which indicates that the former can occur in the middle stage of coal spontaneous combustion (30–70 °C) and the latter can occur in the initial stage of coal spontaneous combustion (room temperature). One part of the generated −·C═O free radicals will directly undergo decarbonylation to generate CO, and the enthalpy change and activation energy are 9.62 and 37.69 kJ/mol, respectively. This reaction can be regarded as the main source of CO in the initial stage of coal spontaneous combustion (room temperature). Another part of the generated −·C═O free radicals can adsorb free O atoms to generate the −COO· free radicals and undergo a decarboxylation reaction to generate CO_(2). The total enthalpy change and activation energy of these reactions are 6.12 and 73.11 kJ/mol, respectively, which can occur in the middle stage of coal spontaneous combustion (30–70 °C). The results can be helpful to the study of coal spontaneous combustion mechanism.