Molecular Active Groups Reaction of Oxidized Coal under Oxygen-Deficient Environment and Its Pore Structure Development after Water Soaking

The water gushing into the goaf makes the oxidized coal undergo the dual effects of oxidation and water immersion and faces the risk of reburning. The oxidation reaction of molecular active structures on the surface of oxidized coal under varying oxygen concentrations was investigated by using Fourier transform infrared spectroscopy (FTIR). Using scanning electron microscopy (SEM) and liquid nitrogen adsorption technology, the study investigates the evolution of the pore morphology and the development mechanism of its pore structure. The results indicate that higher oxygen concentrations provide greater reaction energy, promoting the consumption of −CH3 groups and facilitating the formation of −CH2– groups. The inadequate supply of reaction energy results in a lower rate of −CH2– generation compared to its consumption. Sufficient oxygen facilitates the release of additional activation energy, promoting the formation of transition-state free radicals and their subsequent combination with oxygen to form −CH2– groups. The −C═O group primarily participates during the intermediate and late stages of coal oxygenation reactions, exhibiting a lower reaction priority. During the low-oxygen phase, the conversion of peroxide radicals to −C-O– is limited, leading to a reduction in the quantity of −C-O– produced. When the oxygen concentration exceeds 9%, it leads to the rapid generation of peroxide radicals. These radicals efficiently extract hydrogen atoms from the fatty chains of coal molecules and other structural components via hydrogen abstraction reactions, generating unstable hydrogen peroxides that are easily decomposed to produce alcohols and ketones containing −C-O–. Oxidation significantly enhances the development of pores compared to raw coal, and the cumulative pore volume and area tend to increase with the degree of oxidation. Although the water soaking resulted in a decrease in the cumulative pore volume compared to oxidized coal, it remained significantly higher than that of raw coal. The distribution of micropores indicates that as oxygen concentration rises, the proportion of micropores in oxidized coal increases progressively. The oxidation process facilitates the transition of coal pores from mesopores (>100 nm) and small pores (10 to 100 nm) to micropores (<10 nm). As the primary site for coal-oxygen adsorption, an increase in the proportion of micropores effectively expands the coal-oxygen reaction interface.