Promoted microbial degradation of lignite by supercritical CO2 extraction to enhance coalbed methane production

Supercritical CO2 (ScCO2) extraction can weaken the intermolecular forces and break the non covalent bonds of organics in coal, resulting in the separation of low molecular weight organics from its macromolecular structure. Therefore, the ScCO2 extraction can be used as a pretreatment method to improve the bioavailability of coal, which have the potential to increase biogenic coalbed methane (CBM) production. The experiments of the ScCO2 extraction at different temperatures and pressures, and the biodegradation of lignite were carried out to determine the biomethane production from extracted coal and extractions, the ScCO2 extraction yield of lignite, and the composition of extractions.The methane production from non supercritical CO2 treated coal, the secondary extraction of ScCO2 extracted coal with dichloromethane (DCM), and low temperature liquid nitrogen adsorptiontesting were performed to further analyze the mechanism of ScCO2 extraction toincrease biogenic CBM production.The results showed that the biomethaneproductions of extracted coal at different temperatures and pressureswereall higher than that of raw coal. The maximum methane production of 245.46 μmol/g coal was observed after ScCO2 extractionat 40 ℃-10 MPa,which was 84.68%higher than that from raw coal.The extractions contained various bioavailable organics which could be utilized to produce methane by anaerobic microflora as revealed by anaerobic degradation experiments.Therefore, the ScCO2 extraction could promote the microbial degradation of lignite to increase biomethane production.In addition, the methane productions of residual coal treatedby subcritical CO2 were significantly lower than that of ScCO2 extracted coal, indicating that the ScCO2 extraction of organics was crucial tostimulatebiomethane production, not temperature and pressure. However, the extraction yields of ScCO2 were low and abundant bioavailable organics were detected after the secondary extraction with DCM, indicating that a large number of organics still remained in the coal after ScCO2 action.Theresults of low temperature liquid nitrogen adsorption test showed that the total pore volume and specific surface area of the ScCO2 extracted coal decreased, and the pore structure distribution was changed after ScCO2 extraction, indicating that the extract was transported and adsorbed by ScCO2, resulting in most of the organic matter remaining in the coal.These results revealed that part of the organics in coal was separated from coal matrix by ScCO2 extraction,while a large number of bioavailable organics was migrated andremoldedin coal during ScCO2 extraction, which improved the bioavailability of residual coal, resulting in the increment of biomethane production.