Enhancement of biomethane production from coal by supercritical CO2 extraction

The recalcitrant nature of organic matter in coal presents a significant challenge to biomethane production. Supercritical CO2 has gained considerable attention for its unique extraction capabilities. This study uses four coal samples of varying ranks to investigate the promotion mechanism of biomethane production by supercritical CO2 extraction in combination with anaerobic digestion (Sc-AD). Methanogenic potential oxygenated functional groups, intermediate liquid phase products, microbial community structure, and methanogenic metabolic pathways were analyzed between Sc-AD and AD systems. The study found that microorganisms in the Sc-AD system produce more extracellular polymers that enhance the content of aromatic proteins in the anaerobic digestion system. The liquid-phase compounds extracted by supercritical CO2 were consumed by fermentative acidogenic bacteria, significantly increasing the metabolic efficiency of intermediates such as indole and catechol. The reduction in the relative content of carbonyl and carboxyl groups on the coal surface in the Sc-AD system was attributed to the successive attacks by supercritical CO2 and microorganisms on the coal surface. This resulted in the shedding of carbonyl and carboxyl groups from the coal surface, which were then utilized to synthesize small molecule acids. In contrast, hydroxyl groups were retained in the coal. The Sc-AD system exhibited a more stable and efficient microbial community structure in methane production than the AD system. The study demonstrated that supercritical CO2 extraction is a potential method for enhancing CBM recovery and CO2 reduction.