Methane Adsorption Behavior and the Constraint Mechanism of Microbial Degradation in Coals of Varying Ranks

Methane is a renewable and environmentally friendly energy source. Microbial degradation not only increases the methane content of coal seams but also facilitates the migration of coal-bed methane (CBM). However, there is currently limited research on the internal factors that govern the methane adsorption capacity and methane production mechanisms in coals with varying degrees of metamorphism. To investigate the constraints of methane adsorption in coals with different ranks of anaerobic fermentation, Fourier transform infrared spectroscopy, low-temperature nitrogen adsorption, and low-field nuclear magnetic resonance were used. The findings indicate an inverse correlation between the maximum cumulative production of methane and coal rank, while lag time shows a positive correlation with the coal rank. A strong positive correlation between the 2–10 nm pores and methane adsorption capacity suggests that these specific pores play a crucial role in influencing changes in methane adsorption. Considering oxygen-containing functional groups, aliphatic side chain content, methane adsorption sites, and the quantity of adsorbed methane decrease in low-rank coal reservoirs after biodegradation; however, this process enhances both newly generated and inherent methane production from its adsorbed state. These findings demonstrate that low-rank coal reservoirs have the potential for microbial exploitation to promote CBM production.