Bioconversion of coal to Biogas: Insights into the microbial degradation mechanisms and molecular structure transformations

Abstract

This study explores the microbial degradation mechanisms and molecular structural transitions of coal during the bioconversion process. We built macromolecular structure models of coal samples at various stages of anaerobic fermentation by analyzing its elemental makeup, carbon framework, surface groups, and pore changes, and using molecular simulation. Experimental results indicate that: the pore size of the coal sample increased from 3.04 nm to 5.0 nm, accompanied by a slight increase in the interlayer spacing d₀₀₂ of the aromatic layers, with a decrease in both the microcrystalline extension L_a and the stacking height L_c, suggesting the disruption of the coal sample’s microcrystalline structure. During the biogas production process, the molecular structural transformations are primarily focused on the side chains, specifically characterized by a reduction in − CH₂ − groups within the aliphatic chains, consumption of − OH groups, and the formation of − COOH groups. Notably, the cleavage of benzene rings occurs at the initial stage of biogas production, while the degradation of naphthalene rings takes place during the biogas production peak, indicating that the aromatic structures significantly influence the biogas production process in lignite. GC–MS analysis of the fermentation liquid revealed that benzene compounds are the main constituents, suggesting that the degradation of naphthalene rings occurs through an open-ring mechanism rather than a direct degradation pathway. During microbial fermentation, the model’s total potential energy drops significantly, making it more stable. This change increases non-six-membered rings and lattice defects, affecting the coal’s pore structure and reducing its surface fractal dimension.