Quantum chemical calculation of coal spontaneous combustion mechanism promoted by transition metal ions

In order to study the effect of typical transition metal ions on coal spontaneous combustion under low temperature oxidation conditions, the transition metal ions of lignite (HM), gas coal (QM) and fertilizer coal (FM) were determined, and Fe (Ⅲ), Mn (Ⅲ) and Cu (Ⅱ), which are abundant in transition metal ions and have strong oxidation, were selected as the research objects. The charge distribution, frontier orbitals and Laplacian bond levels were analyzed by the quantum chemistry calculation method, and the existence of active sites was proved. Two reaction processes of Ar—CH2—CH3 to Ar—CO—CH3 with or without transition metal ions were investigated, and the thermodynamic parameters were obtained by model optimization, transition state calculation and IRC verification. The calculation results show that the activation energy barrier is 164.18 kJ/mol without transition metal ion and 158.72 kJ/mol with transition metal ion. The effect of transition metal ion on the total reaction rate is not significant. With the participation of Fe (Ⅲ), Mn (Ⅲ) and Cu (Ⅱ), the total heat releases of the reaction are 1535.52, 1834.97 and 365.93 kJ/mol, respectively, which are higher than the heat release from the oxidation of aliphatic hydrocarbons by oxygen molecules (319.93 kJ/mol). The free energy barriers of C—H in oxidized aliphatic hydrocarbons are 42.79, 4.30 and 117.29 kJ/mol, respectively, which are all lower than the activation energies of C—H oxidation by oxygen molecules (146.38 kJ/mol). The order of C—H oxidation capacity of transition metal ions from high to low is Mn (Ⅲ), Fe (Ⅲ) and Cu (Ⅱ). During the reaction process, the formation of ·OH and H+ is accompanied by further reaction with the coal structure to accelerate the spontaneous combustion process of coal. In order to verify the accuracy of the simulation results, the time and concentration of CO gas products in four different coal samples were measured by temperature programmed gas chromatography. It was found that the temperature points of CO gas products in four coal samples were similar. The coal samples with manganese ion added produced CO gas products at 90 ℃, while the other three coal samples produced CO gas products at 100 ℃. The concentration of CO gas products from large to small is Mn (Ⅲ), Fe (Ⅲ), Cu (Ⅱ) and raw coal sample, which is consistent with the calculated results. With the increase of temperature, the effect of Mn (Ⅲ) on increasing the rate of CO production gradually weakens, and the effect of iron and copper ions on catalytic coal spontaneous combustion is gradually significant. The experimental results have a certain correlation with the simulation results. Compared with oxygen molecules, transition metal ions in coal are more likely to oxidize C—H, which not only causes the free radical chain reaction to occur earlier, but also increases the heat release of the reaction and the production of ·OH and H+, thus promoting spontaneous combustion of coal. This has reference significance for the mechanism of spontaneous combustion of transition metal catalyzed coal.