Investigating Gas Production Characteristics and Mechanism of Residual Coal via Low-Temperature Aqueous In Situ Oxidation

Abstract Water-saturated residual coal in abandoned seams enables low-carbon in situ gas generation through low-temperature aqueous oxidation. However, the coal-rank-dependent gas generation characteristics and water-promoted reaction mechanisms remain poorly clarified. In this study, three typical coal samples including lignite (CS-1), bituminous coal (CS-2), and anthracite (CS-3) were selected to systematically evaluate the effects of coal rank, temperature, particle size, and salinity on coal oxidation and gas production. Multiscale structural characterization combined with density functional theory (DFT) calculations was performed to reveal the underlying mechanisms. The results indicated that water exerted a strong promoting effect on the gas generation of low-rank coal. Water immersion increased the CO2 and CH4 yields of CS-1 by 123% and 32%, while only slight increments of 5.5% and 18% were observed for CS-3. Coal particle size significantly controls gas production efficiency. Fine particles of 100−200 mesh achieve a peak CO2 generation rate of 19865.42 ppm/d, which is 2.1 times that of 20−60 mesh coarse particles. Salinity presents a dual regulatory effect. Low salinity at 2000 mg/L facilitates methanation, whereas high salinity at 10000 mg/L severely restricts gas production. Microscopically, water infiltration effectively modifies the coal pore network. The total pore volume increases from 0.00097 to 0.01120 cm3/g and the average pore size rises from 5.58 to 18.38 nm. Active oxygen-containing functional groups decrease markedly in water-bearing samples, and the carboxyl content of CS-1 decreased from 24.52% to 14.54%. DFT simulations confirm that water-induced ·OH radicals reduce the oxidation activation energy of coal from 18.71−34.26 kcal/mol to 2.45−8.31 kcal/mol. This work reveals that the coupling of water-induced pore modification and ·OH-driven chemical activation enables sustained low-temperature oxidation and efficient in situ gas generation of residual coal, providing theoretical support for low-carbon exploitation of abandoned coal resources.