Molecular structural controls on biomethane generation during coal biodegradation

Abstract

Biogenic coalbed methane is a unconventional natural gas resource. To clarify the influence of coal rank on the kinetics and molecular mechanisms of microbial methane generation, this study conducted laboratory primary microorganism degradation simulations combined with multiple analytical techniques, systematically investigating the methane production kinetics, chemical structural evolution, and key molecular parameters of coals across different ranks. Results show that low-rank coal exhibits early gas production initiation, high methane yield, and fast rates, reaching a maximum cumulative production of 263 µmol/g within 14 days. Medium-rank coal shows a delayed peak, mainly between days 21–35, reaching a maximum cumulative production of 275 µmol/g within 14 days while high-rank coal starts late, proceeds slowly, and yields significantly less methane. The total methane production has reached 191 µmol/g. Microbial activity preferentially degrades aliphatic structures in low- and medium-rank coals, enriching aromatic structures. In high-rank coal, it promotes condensation and reorganization of aromatic units, increasing stacking layers. Correlation analysis confirms that aliphatic abundance (A_al/A_ar), chain length/branching (CH₂/CH₃), and Raman D₃ band intensity are positively correlated with maximum methane production, indicating aliphatic components as key molecular determinants. This study provides new insights into the structural evolution and methane generation mechanisms during microbial degradation of coals at different metamorphic stages.