Biogas production characteristics of lignite and related metabolic functions with indigenous microbes from different coal seams

Abstract

To investigate the effects of different exogenous microorganisms on coal biomethanation and its related microbial metabolic functions. Bacterial sources were obtained from the mine water of the Xinsheng, Qinan, Fangzhuang, and Tianyuan coal mines, and lignite was utilized for conducting experiments to produce biomethane. The biomethane yield and statistical analysis based on KEGG metabolic pathways were used to assess the biomethane production potential and the difference of metabolic functions during anaerobic fermentation. The results showed that the accumulative production of biomethane in the anaerobic fermentation system with microorganisms from Qinan mine was 1.80 times, 1.18 times and 1.13 times that of the fermentation systems with other microbes from the Xinsheng, Fangzhuang and Tianyuan mine, respectively. Based on the geological environment of groundwater, it was observed that the moderate water-rich fractures in the coal seam fractures of Qinan Mine are relatively developed, which facilitates the transport of organic matter during microbial metabolism. On the other hand, the Fangzhuang and Tianyuan coal mines are situated in the groundwater runoff zone, their fracture development characteristics are comparatively lower than those of the Qinan mine. In contrast, the coal seam roof and floor of Xinsheng mine are characterized by a stable aquifer comprising thick mudstone and claystone, which presents difficulties for hydrogen-producing bacteria and methanogenic bacteria in obtaining liquid nutrients. This leads to a lower potential for methane production in the Xinsheng mine as compared to the other coal mines. The prediction based on 16S rRNA gene function showed that the microorganisms in Qinan mine had the largest abundance of annotated homologous genes involved in glycolysis, fatty acid synthesis, and methane metabolism, which had a significant positive correlation with methane production. An important observation was the absence of acetate-CoA ligase in the glycolytic pathway, which rules out the acetate-trophic pathway as a significant contributor to coal biodegradation and methanogenesis. Moreover, the abundance of genes related to the hydrogenotrophic methanogenesis pathway was found to be the highest during methane metabolism, indicating that the CO2 reduction pathway plays a significant role during the anaerobic fermentation of coal to produce biomethane.