Impact mechanism of anaerobic digestion of sulfur-rich coal to H2S by graphene

Coalbed Gas Bioengineering (CGB) is a special anaerobic fermentation project, which uses modern engineering techniques to convert coal and CO2 into methane and associated liquid organic matter by using some specific functions of microorganisms. Graphene is regarded as a viable strategy for on-site implementation to enhance the electron transfer rate in anaerobic fermentation systems. Graphene exhibits a positive influence on the biogenic methane production in high organic sulfur coal, while its impact on H2S generation remains uncertain. Using high-sulfur coal extracted from Jincheng as a carbon and sulfur source, an anaerobic fermentation system is constructed with graphene as the conductive material. The anaerobic fermentation process will be analyzed to investigate the evolutionary patterns in gas composition, morphological sulfur in coal, coal molecular structure liquid-phase substances and microbial community structure. The underlying factors contributing to the generation of H2S and the mechanism through which graphene influences its impact have been investigated. The results indicate that graphene reinforcement enhances the production of both biogenic CH4 and H2S in anaerobic fermentation systems. The cumulative CH4 yield in the graphene-enhanced fermentation system reached 4.86 mL/g, exhibiting a significant increase of 77.37% compared to the anaerobic fermentation system without graphene (2.74 mL/g). Additionally, the H2S yield in the presence of graphene was measured at 5.52 mL/g, showing an improvement of 11.74% when compared to the system without graphene (4.94 mL/g). The degradation of organic sulfur was accelerated, and the thiols and thioethers in the residual coal were also completely transformed by microorganisms after the addition of graphene. For the key liquid phase small molecule organic matter, the degradation rate of various substances in the anaerobic fermentation system with graphene was significantly higher than that in the anaerobic fermentation system without graphene. The abundance of Desulfovibrio, Geovibrio, and the archaea Methanosarcina has significantly increased with the introduction of graphene, Mercaptan and thioether in coal are completely transformed by microorganisms after the addition of graphene. The bacterium Geovibrio provides additional electrons to methanogenic archaea. The potential direct interspecific electron transfer (Direct Interspecific Electron Transfer, DIET) between archaea Methanosarcina and bacterium Desulfovibrio is enhanced by the addition of graphene is the reason for influencing the generation of CH4 and H2S. Such electron transfer mode has improved the activity and degradation efficiency of bacterial community. The synthesis of key enzymes in the process of methanogenesis and sulfate dissimilation is accelerated. Meanwhile, there are two distinct mechanisms underlying the formation of H2S. One mechanism involves the direct utilization of methyl groups from organic sulfides by methylotrophic methanogens, resulting in methane production alongside hydrogen sulfide generation. The other mechanism entails a synergistic interplay between Macellibacteroides, a predominant hydrolytic bacterium expressing sulfate esterases, and sulfate-reducing bacteria (SRB) to facilitate hydrogen sulfide formation, which governs the overall anaerobic fermentation process. The comprehension of this concept has necessitated the on-site implementation of coalbed gas bioengineering, when conducting microbial enhanced production in high-sulfur coal reservoirs, it is imperative to incorporate biological inhibitors for suppressing the generation of hydrogen sulfide. Therefore, in the on-site implementation of coalbed methane bioengineering in high-sulfur coal reservoirs, it is necessary to add a biocide to inhibit the generation of biogenic hydrogen sulfide.