Spontaneous combustion oxidation characteristics and microscopic mechanism of coal with different dehydration degree

Coal spontaneous combustion has seriously hindered the sustainable development of coal mines, resulting in a large amount of resource loss and serious environmental pollution. To study the oxidation reaction characteristics of active functional groups in coal with different degrees of dehydration, this paper employed density functional theory (DFT) within quantum chemistry methods to construct models of six typical active group structures in coal. Using the B3LYP method to optimize the model structures and calculate their frequencies, we obtained the electrostatic potential, frontier molecular orbitals, surface electron cloud distribution, enthalpy change, and Gibbs free energy change values for the six models, revealing the impact of moisture on the reactivity of active groups in coal. In situ FTIR and DSC were used to investigate the effects of different dehydration levels on the microstructure and thermal effects of coal. The results showed that in the presence of water, water acted as a hydrogen transfer medium, promoting the reactions of Ar-CH2OH, Ar-CH(CH3)OO•, C4H5N, and C6H6S with O2, while inhibiting the reaction of Ar-COOH with O2. Compared to raw coal, the characteristic temperature points of dried coal samples shifted to lower temperatures, and the exothermic amount increased. During the oxidation and dehydration of coal, a certain amount of moisture (12.12%-16.82%) acts as a hydrogen transfer medium to form a hydrogen bond network, which promotes the chemical adsorption of oxygen and the formation and decomposition of intermediate complexes. In the dried coal samples, numerous active sites were exposed on the surface, forming more peroxide complexes with oxygen and accelerating the chain reaction of coal radicals. The findings of the research will provide optimum moisture conditions for coal pile storage, and have important theoretical guiding significance for the management and prevention of coal spontaneous combustion.