Dynamic physical simulation for microbial mining of residual coal in abandoned coal mine goafs

Coal mine goaf is an important breakthrough for my country to achieve the "dual carbon" goal. There are a lot of residual coal, thin coal seams and dispersed organic matter in the goaf, and a green and low-carbon technology is urgently needed for its secondary resource utilization. The birth of coalbed gas bioengineering provides theoretical and technical support for the microbial mining residue in goaf. To this end, this paper uses the independently developed in-situ dynamic and static anaerobic fermentation device of coal reservoir. The Inner Mongolia lignite as the fermentation substrate and the in-situ microorganisms of coal reservoir as the source of microbial community, and conducts dynamic and static anaerobic fermentation experiments of coal by physically simulating the temperature (35℃), pressure (0.5 MPa) and dynamic supply of mine water (0.020 mL/min) in the coal mine goaf. The evolution of gas production characteristics, key liquid phase substances, microbial community structure, and key metabolic pathways under in-situ dynamic and static conditions is compared and analyzed, revealing the microbial methanogenesis mechanism of residual coal in coal mine goaf under static conditions and dynamic groundwater recharge conditions. The results show that the methanogenesis effect of the in-situ dynamic physical simulation experiment is better than that of the static experiment. The cumulative methane production was 5.72 mL/g, which was 37.9% higher than that of the conventional static anaerobic fermentation system. In addition, through the mine water recharge, the content of small molecular alcohols and volatile fatty acids such as acetic acid in the system increased by about 20%, and the abundance of key hydrolytic bacteria Romboutsia and Aminobacterium also increased by 10%, indicating that the mine water recharge provided more nutrients for the fermentation system; at the same time, it promoted the activity of hydrolytic bacteria and the degradation of organic matter, providing more available substrates for methanogens. After the mine water was recharged, the abundance of Methanosaeta increased significantly, and acetyl-CoA synthetase, as a key enzyme for acetic acid-type methanogenesis, increased by 17%, and the aceticlastic methanogenesis in the system was improved. Therefore, when the groundwater recharge rate in the weak runoff zone matches the metabolic cycle of microbial anaerobic fermentation, sustainable gas production can be achieved, and the potential of microbial mining of residual coal is strong; the nutrients available to microorganisms in the groundwater retention area are limited, and the gas production potential is weaker than that in the runoff zone. The results of the experiment shows that by injecting high-efficiency methanogens and nutrient solution into the goaf, microbial methanogenesis of residual coal can be achieved, providing an experimental basis for microbial mining of residual coal in goafs of coal mines.