Enhancement of Biogenic Coalbed Methane by ScCO₂–H₂O Treatment

Supercritical CO2 (ScCO2) extraction could enhance biogenic methane production from coal by optimizing the coal structure, and water acted as an entrainer to improve the extraction efficiency of ScCO2. In this study, the methane production experiments were conducted on anthracite with different moisture contents treated with ScCO2. The changes in coal structure, organic matter, and elemental content caused by ScCO2–H2O treatment were analyzed. The methane production increased by 46.1% as moisture content rose from 0% to 20%, but did not continue to increase at 30% due to nutrient leaching. Under water-immersed conditions, although the methane production from ScCO2 treated coal decreased, the total methane production from ScCO2 treated coal and leachate reached 314.95 μmol/g coal. This value was increased by 61.59% compared with that of raw coal. These findings highlighted the dual role of moisture in facilitating ScCO2 extraction and shifting methane generation from coal to leachate. Structural analysis confirmed that moisture enhanced the effect of ScCO2 on functional groups in coal. Specifically, the total amount of aromatics with two substitutions and aromatics with three substitutions decreased progressively from 67.98% to 54.27% with increasing moisture content. The same phenomenon was also observed for carboxylic acids and C=O functional groups with maximum reductions of 100% and 71.51%. Instead, the CH3/CH2 ratio was positively correlated with moisture content. The concentrations of TOC, straight-chain alkanes, iron, nickel, and cobalt in leachate treated with ScCO2–H2O increased by 5.52, 1.35, 11 592.79, 229.17, and 27.76 times, respectively, compared to those in leachate treated with water alone, supporting the high methane production in leachate. It highlighted that moisture-driven structural modifications and nutrient mobilization were key mechanisms for optimizing the enhancement of ScCO2 on microbially enhanced coalbed methane (MECBM). These findings suggested that water-bearing anthracite coal seam would be a suitable stratum for ScCO2 enhanced MECBM.