Insight into the Chemical Structure Response of Anthracites to Stress by ¹³C NMR and FTIR

Coal and gas outbursts are typically confined to faults and shear zones, where the released gas volume frequently exceeds the maximum gas adsorption capacity of the outburst coal. To study the chemical structure response and gas generation mechanism of coals under stress, a series of high-temperature and high-pressure coaxial compression deformation experiments of anthracites were carried out. ¹³C NMR and FTIR results show a decrease of aliphatic -C-H, f _al and f _al ^O in the deformed anthracites, indicating the aromatization process induced by the stress condensation. Methyl and quaternary carbons are dissociated more easily than methylene and methyne carbons, whereas ether bonds are more susceptible to cleavage than carbonyl carbon under stress. The shedding of these structures increases the aromatic degree and promotes the polycondensation degree of aromatic rings. Besides, the effect of stress on the chemical structure evolution of anthracite is closely related to temperature. At 300 °C, the decrease of aliphatic carbon and the increase of aromatic carbon in the deformed samples are higher than heated samples, while lower at 350 and 400 °C, indicating the suppression of stress at high temperatures. The correlation of the aliphatic carbon content, aromatic carbon content, and polycondensation degree with strain rates is not obvious at different temperatures, which may be related to the inconsistency in the bond-breaking sequence between mechanolysis and pyrolysis, and the uneven distribution of stress. This study provides a theoretical basis for understanding the source of excess gas in coal mines.