Effects of exogenous CO₂ injection on biomethane production in in-situ anaerobic digestion system of coal measure shale

The integration of increasing coal measure gas (CMG) production and CO₂ resource utilization aligns with China’s “dual carbon” goals and can also promote the large-scale development and circular growth of the CMG industry. Studies have shown that coal measure shales (CMS), in addition to serving as a reservoir for CMG and a substrate for methanogenesis, can also drive the bioconversion of CO₂ to methane, indicating their potential as a “CO₂ methanation factory”. However, research on the CMS anaerobic digestion (AD) system containing exogenous CO₂ is still lacking, hindering the development and application of this technology. Middle Jurassic CMS from Yima Coalfield was used as the object, and AD experiments under in-situ temperature and pressure conditions were conducted to investigate the impact of exogenous CO₂ injection on biomethane production in the CMS AD system. The results indicate that both the structures and metabolic functions of bacterial and archaeal communities in the in-situ AD system underwent substantial changes, and hydrogenotrophic methanogenesis remained the dominant pathway throughout the process. The acetate produced from the degradation of organic matter during the late stage of AD was utilized, effectively prolonging the methanogenesis period. Under the effect of exogenous CO₂, the biomethane production increased significantly, with the cumulative methane production reaching 2.12 times that of atmospheric pressure AD system. In the in-situ AD system, under the mediation of hydrogen-producing bacteria such as Clostridium_sensu_stricto_1, the H⁺ generated after the injection of exogenous CO₂ combined with electrons through the “catalytic electrode” function of hydrogenases such as ferredoxin hydrogenase, producing substantial H₂. This achieved endogenous H₂ supply and provided abundant substrates for hydrogenotrophic methanogenesis. However, the accumulation of volatile fatty acids in the system exerted a pronounced acid inhibition effect on both bacterial and archaeal communities, leading to reduced microbial diversity and abundance, as well as weakened metabolic activity. As a result, the methanogenesis process terminated at 30 days despite sufficient CO₂ and H₂ being available. Exploring effective strategies to alleviate acid inhibition is crucial for enhancing the application potential of this system. Based on these findings, CMS is proposed to possess the attributes of a “CO₂ methanation factory”. In this conceptual factory, exogenous CO₂ serves as the raw material, the pore-fracture network provides the reaction space, the organic matter in the CMS acts as the activator, and methanogenic microbial consortia serve as low-cost labor. Methane is produced as a clean product, providing an efficient and synergistic pathway for both the enhancement of CMG production and the resource utilization of CO₂.