Study on the Mechanism of Antioxidants Affecting the Spontaneous Combustion Oxidation of Coal

In this paper, in order to further reveal the mechanism of oxidation inhibitors inhibiting coal spontaneous combustion, the mechanism of typical antioxidants butylated hydroxytoluene (BHT) and vitamin C (VC) inhibiting coal low-temperature oxidation was studied by thermogravimetry (TG), Fourier transform infrared spectroscopy, and quantum chemical simulation. This paper studies the thermal reaction process of coal by TG. The experimental results showed that the characteristic temperature of coal samples increased significantly after adding an antioxidant, and the maximum exothermic peak temperature of coal samples with BHT was 36 °C higher than that of raw coal, and the dehydration activation energy was 1.22 times higher than that of raw coal. Then, through the Fourier transform infrared spectroscopy experiment, it is concluded that antioxidants have a great influence on the content of active groups in the process of oxidative heating of coal, and the hydroxyl structure changes in coal are the most obvious. After BHT and VC were added to the coal, the hydroxyl content in the coal was reduced by 5.90 and 4.20%, respectively. Finally, the molecular models of antioxidants and oxygen-containing functional groups (RCOOH, R–CHO, and R–CH2–CH2–OH) in coal are constructed using quantum chemical theory, and their frontier orbitals and thermodynamic data are calculated according to density functional theory. The reaction mechanisms of antioxidants reacting with hydroxyl groups and with hydroxyl groups and oxygen-containing functional groups were comparatively analyzed. The results of orbital gap analysis indicated that antioxidants react more easily with hydroxyl groups than with oxygen-containing functional groups, and the order of their reaction activity is BHT > VC > R–CH2–CH2–OH > R–CHO > RCOOH. Compared to reactions containing oxygen-containing functional groups and hydroxyl groups, the thermodynamic reaction of hydroxyl groups and antioxidants is more active. Because antioxidants preferentially react with hydroxyl radicals and inhibit part of the chain cycle reaction during spontaneous combustion of coal, antioxidants play a good inhibitory role in spontaneous combustion of coal.