Graphite-enhanced methanogenesis in coal measure shale anaerobic digestion: Implications for increasing gas yield and CO2 utilization

Anaerobic digestion (AD) of coal measure shale (CMS) for methane production is a promising technology for increasing yield of coal measure gas (CMG) wells. However, improving methane production efficiency of CMS remains a challenge. Conductive material has been shown to promote biogas production in different AD systems, but its effectiveness in CMS requires investigation. In this study, graphite was selected as conductive material due to its low cost and suitability for field applications, and its effects on methane production was investigated. The results indicate that as graphite concentration increases from 0 to 1.6 g/L, the methane production of CMS increases. However, when the graphite concentration reaches 2.0 g/L, the methanogenesis is inhibited, resulting in lower methane production compared to the control group. In the group with a graphite concentration of 1.6 g/L, the abundances of Romboutsia and Pseudomonas increase, which improves the compatibility of the microbial community with the CMS AD system and causes the amount of the substrates for methanogenesis to increase. Also, the direct interspecies electron transfer between Methanobacterium, Methanosaeta and Pseudomonas is strengthened, which enhances the methane production. However, excessive addition of graphite (2.0 g/L) intensifies the competitions from dissimilatory iron reduction and sulfate reduction reactions for substrates and electrons with the methanogenesis, causing the methane production to decrease. This study confirms the effectiveness of graphite in enhancing methane production of CMS, and the promoted hydrogenotrophic methanogenesis enhances the CO2 methanation, which offers an efficient pathway for increasing CMG well yield and enabling CO2 underground utilization.