Experiments methane production from biodegradation of coal residue caused by water-soaked oxidation in goaf areas

Coalbed gas bioengineering in goaf areas is an effective technical means to realize the resource utilization of residual coal in mines. After oxidation during the coal mining process and water immersion during the later closed period, the structure of the residual coal in the goaf has undergone significant changes, and its biodegradation and utilization efficiency has not yet been fully clarified. In this study, water-soaked oxidized lignite with different degrees of oxidation temperature, water immersion time and pH were simulated and obtained. The evolution laws of methane yield, coal microstructure, degradation metabolites and microbial community structure during the process of microbial degradation of coal to produce methane were analyzed. The results show that the water-soaked oxidized coal (O₁₀₀I₃₀C₆) that has been oxidized at 100 ℃ and soaked in water for 30 days in an environment with a pH of 6 has better bioavailability, and the methane yield is 1.8 times that of raw coal. Even under oligotrophic conditions, the methane yield of coal leachate (O₁₀₀I₃₀W₆) can still reach 327.14 μmol. Oxidation and water immersion disrupt the orderliness of the aromatic layer in coal, making its structure looser and increasing the number of oxygen-containing functional groups in coal, which are more easily utilized by microorganisms. Compared with before degradation, the relative contents of carbonyl groups and ether bonds in O₁₀₀I₃₀C₆ decreased by 69.38% and 28.04% respectively, and the carboxyl groups were completely degraded. During the degradation process of O₁₀₀I₃₀C₆ and O₁₀₀I₃₀W₆, a variety of functional bacterial communities were enriched, including hydrogen and acetogenic bacteria Thermanaerovibrio and Petrimonas, as well as hydrolytic acidifying bacteria Lentimicrobium and Paraclostridium. The oxygen-containing functional groups in coal molecules are gradually transformed into volatile fatty acids under the joint action of the bacterial community, increasing the fatty acid content in the O₁₀₀I₃₀C₆ fermentation broth by 8.96% and that in O₁₀₀I₃₀W₆ by 47.38%. The increase of this type of organic matter stimulated the reproduction of methanogenic archaea. The proportion of methyl-trophic methanogenic archaea, Methanofastidiosales, reached 76.18%, playing a core role in the methane production stage of coal. After the leachate fermented to produce methane under oligotrophic conditions, acetic acid-trophic Methanosaeta dominated, accounting for 70.55%. The above research results indicate that the structure of the coal left in the goaf after oxidation effect of the desiccation zone and oxidation zone and the subsequent water immersion effect is more conducive to biodegradation, with full utilization of oxygen-containing functional groups, active growth of methanogenic bacterial communities, and an increase in methane production. At the same time, the accumulated water in the goaf is also a growth point for biomethane. During the groundwater migration process, the nutrients required by the methanogenic bacterial community are dissolved. The water-accumulated goaf areas combines the advantages of large residual coal volume, large space and abundant water volume, making it a favorable implementation point for coalbed methane bioengineering. The research results provide a theoretical basis for improving the utilization of coal resources in goaf areas and promote the redevelopment and comprehensive utilization of closed/abandoned mines.