Objective  Coals with varying metamorphic grades differ significantly in the potential for methane generation through microbial degradation. Medium- to high-rank coals can generate a substantial volume of methane through microbial degradation. However, key factors governing their biogasification potential remain poorly understood. This study aims to identify the dominant factors controlling the potential of medium- to high-rank coals for methane generation through microbial degradation, as well as relevant controlling mechanisms, from the perspective of molecular structures.

Methods The Carboniferous-Permian medium- to high-rank coals along the eastern margin of the Ordos Basin were investigated in this study. Using experiments on the hydrocarbon generation of the coals through the degradation of indigenous microorganisms and the analysis of hydrocarbon generation kinetics, combined with gas chromatography (GC) and Fourier transform infrared spectroscopy (FTIR), this study revealed the biogasification potential of coal samples from different coal seams, along with the laws of changes in the structures of functional groups on the sample surfaces. Furthermore, based on Pearson correlation analysis, the intrinsic relationships between the molecular structural parameters and hydrocarbon generation potential of coals were explored.

Results The microbial degradation-induced methane generation of medium- to high-rank coal samples can be divided into three stages: an initial lag phase (0‒28 d), an exponential growth phase (28‒42 d), and a plateau and attenuation phase (after 42 d). Fitting parameter analysis of the Modified Gompertz, SGompertz, and Logistic models reveals that the Modified Gompertz model exhibited the optimal fitting performance. This model yielded an estimated maximum methane production of 8.94‒14.65 m³/t and a peak methane generation rate reaching up to 2.18 m³/t/d. FTIR analysis indicates that, following microbial degradation, the coal samples showed decreases of 14.3% and 26.8%, respectively, in the absorbance peak intensities of oxygen-containing functional groups and aliphatic structures. Coal samples from different coal seams showed pronounced differences in aliphatic structure, aromatic structure, and the characteristics of oxygen-containing functional groups. Pearson correlation analysis reveals that the maximum methane yield (A₀) exhibited strong positive correlations with both the ratio of aliphatic chain length to aromatic C=C bond abundance (quantified as I_\mathrmH_1 ) and the ratio of aliphatic to aromatic structures (A_al/A_ar). Specifically, coal samples enriched in long-chain aliphatic structures exhibited greater methane generation potential, and a positive correlation was observed between the aliphatic methylene content and the gas production capacity.

Conclusions Compared to low-rank coals, medium- to high-rank coals exhibit a prolonged initial lag phase but an elevated methane production rate in the exponential growth phase. Aliphatic and aromatic structures emerge as key factors influencing the potential of medium- to high-rank coals for methane generation through microbial degradation, with I_\mathrmH_1 and A_al/A_ar serving as effective indicators for evaluating the biogasification potential of the coals. The results of this study provide an important theoretical basis for assessing the biogasification potential of coal seams with medium to high metamorphic grades.