Effect of organic solvent extraction treatment on biogenic gas production of lignite

The bioavailability of organic components in coal is a core research direction in the field of coalbed methane bioengineering. Although organic solvent extraction has been proven to enhance coal’s biogas production efficiency, its mechanism remains unclear. Shengli No.5 lignite from Inner Mongolia was used as the research object, and a solvent polarity gradient treatment system was constructed using methanol, ethyl acetate, petroleum ether, and tetrahydrofuran. Through ultrasonic extraction treatment and biogas production experiments, combined with GC-MS, GC, SEM, XRD, FT-IR, and high-throughput sequencing technologies, the mechanism by which changes in coal quality characteristics induced by organic solvent extraction affect the biogas production characteristics and microbial community structure of residual coal was systematically investigated. The results showed that the extracts of the four organic solvents only produced H₂ under the action of methanogenic bacteria, with no CH₄ detected. The order of methane production capacity of the residual coal samples was: methanol > ethyl acetate > tetrahydrofuran > petroleum ether. Among them, the methane (with the highest polarity index) production of methanol-extracted coal reached the highest at 31.12 μmol/g, significantly higher than that of raw coal (11.86 μmol/g), while the methane production of the coal residue extracted with petroleum ether, which has the lowest polarity, is close to that of the raw coal. Solvent extraction altered the physical and chemical properties of coal by selectively removing low-molecular organic components. Proximate analysis showed that the volatile matter of residual coal decreased while fixed carbon increased; ultimate analysis indicated increased oxygen content and decreased carbon-hydrogen content. XRD, FT-IR, and SEM results revealed that after solvent treatment, the aromatic layer spacing of coal samples increased, the intensity of absorption peaks of oxygen-containing functional groups enhanced, and porosity increased, indicating loosened coal structure and improved bioavailability. Microbial community analysis showed that the diversity and richness of bacteria in the residual coal system were higher than those in the raw coal system. The abundance of Firmicutes increased, with the most significant change observed in the abundance of norank_f__Spirochaetaceae within Spirochaetes. In the archaeal community, the abundances of Euryarchaeota, Methanosarcina, and Methanobacterium changed significantly. Correlation analysis indicated that Proteobacteria (bacteria) and Euryarchaeota (archaea) were positively correlated with CH₄ accumulation, serving as key functional phyla for gas production. Halobacterota (archaea) was negatively correlated with dissolved organic carbon, suggesting its ability to decompose organic carbon in coal and convert it into CH₄. In conclusion, the effect of organic solvents on coal biogas production follows a “solvent polarity-coal phy-chemical structure variation-microbial metabolism” cascade effect. Polar solvents (especially methanol) improve coal loosening and functional group exposure through selective extraction, reshape microbial niches, and enhance methanogenic activity. This study provides a theoretical reference for deepening the understanding of coal structure changes and microbial gas production mechanisms.