Kinetics and structural evolution mechanism of bio-gasification in low-rank coals with different macerals

Bio-gasification of low-rank coal is a promising strategy for increasing coalbed methane production. Its metabolic processes and biomethane yield are critically governed by coal macerals, yet the underlying regulatory mechanisms remain unclear. Therefore, this study selected low-rank coal from the Haerwusu Mine in the Ordos Basin for bio-gasification experiments. Using gas composition analysis, coal petrography, and molecular characterization, we thoroughly investigated how macerals regulate the kinetics and metabolic pathways. Our findings demonstrate that the gas production potential and kinetic characteristics are significantly governed by macerals. The semi-bright coal produced the highest CH4 yield of 6.13 cm3/g, attributable to the fact that its vitrinite-rich structure provided abundant degradable components (e.g., aliphatic chains and oxygen-bearing groups), which dominated the early-stage gas production. The liptinite and mineral groups sustained the later gas generation by supplying continuous carbon sources and regulating the micro-environment. In contrast, the dull coal, which had a high inertinite content, produced the lowest gas yield, as microbes could only utilize oxygen-containing groups on aromatic edges and adjacent aliphatic chains. The differences in the molecular structures of the macerals led to distinct microbial degradation pathways, essentially reflecting the coupling between the macerals, molecular structure, and microbial metabolic capacity. Therefore, the bio-gasification potential of coal depends on the composition and spatial distribution of the active components (e.g., collinite, sporinite, resinite, and calcite) and inhibitory components (e.g., fusinite, and vitrodetrinite). These findings provide critical parameters for identifying microbial sweet spots and improving the bio-conversion efficiency of low-rank coal.