Evolution regularities of solid, liquid and gas phases during coal anaerobic fermentation

Coal anaerobic fermentation technology utilizes indigenous or exogenous microorganisms to degrade coal organic matter, converting it into clean energy sources such as methane and hydrogen. The core scientific challenge lies in elucidating the multi-scale synergistic mechanisms involving coal structure evolution, metabolite transfer, and microbial community response. This study systematically reviews the evolution law of solid, liquid, and gas during coal anaerobic fermentation and establishes a joint characterization and analysis technology system for the components of coal anaerobic fermentation. The analysis of solid phase components encompasses coal pretreatment, pore structure, molecular structure, and model construction techniques, and different pretreatment techniques have been shown to significantly increase biomethane production under laboratory conditions, ranging from 24.98% to 2430%, but the in-situ effect under complex geological conditions needs to be further verified. As the primary medium for interfacial reactions between microorganisms and the coal matrix, the structural characteristics of the coal reservoir pore system undergo significant changes under anaerobic microbial activity. Specifically, the proportion of micropores decreases by 12.38%, whereas the proportions of mesopores and macropores increase by 15.84% and 352.89%, respectively. Porosity and permeability also exhibit notable increases, ranging from 4.73% to 64.9% and 33.3% to 40.0%, respectively. The research on the characterization of coal molecular structure and model construction techniques has provided direct evidence for the functional groups on the surface of coal, the carbon skeleton, and the positions of chemical bond breakage during the process of biogasification. Microbial degradation was found to reduce Van der Waals energy by 8.74% to 9.27%, followed by decreases in non-covalent bond energy and hydrogen bond energy. Liquid phase component analysis primarily encompasses molecular biology and intermediate metabolite analysis. Molecular biology overcomes the cognitive bottleneck of the life activities of native microorganisms by analysing the diversity characteristics and metabolic potential of coal seam microorganisms, thereby providing a new approach for the study of biological methane generation pathways. Meanwhile, the characterization of intermediate metabolites, such as Dissolved organic matter, low-molecular-weight organic acids, and sugars, offers significant data support for the research on the transmission of metabolites. Furthermore, the quantitative characterization of gas phase components such as methane, hydrogen, and carbon dioxide facilitates the understanding of biogasification potential and methane generation pathway. Future research should prioritize breakthroughs in key technologies, including multi-field coupling mechanism analysis, metabolic network engineering, and dynamic environmental regulation, in order to provide robust theoretical support for the intelligent bioconversion and industrial application of CBM resources.