Effects of calcite on biogenic methane production and microbial community structure in coal

Coal can be used to produce methane under anaerobic conditions through the action of microorganisms. A variety of environmental factors, including minerals, pH, and temperature, influence this process. The effect of a common carbonate mineral in coal, i.e., calcite, on the biogenic gas production was examined in this work. Biomimetic gas production was conducted by calcite amendments at various concentrations to an anaerobic fermentation system containing a high-volatile bituminous coal from Daliuta Coal Mine, Shaanxi, China. Changes in CH₄ content, total volatile fatty acid (VFA) concentrations, coenzyme F₄₂₀ activity, coal surface functional groups, and microbial community structure were analyzed. The results demonstrated that calcite addition promoted gas production, with the highest yield of 78.44 μmol CH₄/g coal observed at a 4 % amendment. Calcite concentrations below 8 % effectively enhance the utilization of substrates such as acetic acid by methanogens, while 2 % and 4 % calcite addition significantly increased the activity of coenzyme F₄₂₀. Functional groups on the coal surface, such as –OH, –NH- and –NH₂, were utilized by microorganisms, which all contributed to methane generation. In terms of microbial communities, calcite addition increased the abundance of Firmicutes in the bacterial phylum and Halobacter in the archaeal phylum. At the genus level, Paraclostridium and Proteiniphilum were most abundant, suggesting their roles in the acid-producing stage and the subsequent conversion of fatty acids to methane. Based on these findings, we developed a regulatory model for methanogenic metabolism and microbial community dynamics, incorporating four regulatable points. In summary, the findings demonstrate that calcite addition at specific concentrations enhances the production of biogenic CBM through mechanisms involving microbial metabolism, community shifts, and substrate utilization.