Study On The Condition Optimization And Mechanism Of Enhancing CBM In High-rank Coal By Supercritical CO₂ And Microoranisms

Biogenic CBM is an important component of detected CBM,which is formed by coal biodegradation and can be regenerated by anaerobic microorganisms.One of the rate-limiting factors for microbial degradation is the bioavailability of coal molecules,especially for anthracite which is more condense and has higher aromaticity compared with low-rank coal.In order to enhance the contact between microorganisms and degradable components and increase the concentration of effective biological substrates of coal,some technologies can be used to pretreat coal samples before biodegraded by microorganisms.Supercritical CO₂ has amazing solubility.And it has been proved that supercritical CO₂ can extract small molecular organic compounds from coal,and improve the pore and fracture structure of coal.In this paper,Supercritical CO₂ extraction technology was employed to pretreat anthracite from Qinshui Basin to alter the coal structure and facilitate the biodegradation.The residual coal and extracts were degraded by anaerobic microorganisms,and the effects of extraction temperature,pressure and time on the production of biomethane were investigated.The effects of supercritical CO₂extraction on the physicochemical properties of residual coal and the components of extracts were studied.At the same times,the solvent extraction of dichloromethane and non-supercritical CO₂ extraction were designed as controls together to analyze the mechanism of supercritical CO₂ extraction affecting biomethane formation.The main conclusions are as follows:?1?Anthracite was extracted by supercritical CO₂ fluid at different temperatures,pressures and time,and the residual coal samples were degraded by anaerobic microorganisms.The results showed that the extraction rate is very low under all conditions,but supercritical CO₂ extraction has a significant promoting effect on the production of biomethane.Among the temperature and pressure conditions selected in this experiment,the optimum extraction conditions for methane production were 60??10MPa and 8h.The highest methane production is 266.88?mol,increased by 1030.85%compared with raw coal.?2?About the characterization analysis of raw coal and residual coal,the results of industrial analysis showed that supercritical CO₂ extraction reduced the content of water and volatile components,while increased the content of fixed carbon in coal.The results of fourier transform infrared spectroscopy?FTIR?showed that supercritical CO₂ extraction can remove some organic compounds from coal,exposing more functional groups,and reduce the condensation of aromatic structure of coal.The results of BET analysis showed that the specific surface area of anthracite decreased after supercritical CO₂extraction.?3?The results of biodegradation of extracts showed that the extracts can produce biomethane,but the yield of it was very low due to the low volatile content of anthracite and the difficulty of extraction.The results of GC-MS of extracts showed that the extracts contained aromatic compounds,aliphatic hydrocarbons,oxygen-containing compounds and heteroatomic compounds.The components and contents of the extracts were different under different extraction conditions,but the main components were aromatic hydrocarbons and oxygenated compounds.Among them,toluene was the most abundant aromatic hydrocarbon and butyl acetate was the most abundant oxygenated compound.?4?The residual coal was extracted again by dichloromethane solvent and the residual coal degraded by anaerobic microorganisms.The results showed that the residual coal extracted again by dichloromethane solvent produced almost no biomethane.Compared with the pure supercritical CO₂ extraction and pure CH₂Cl₂ extraction,the organic components in the filtrate increased in variety and content.It indicated that supercritical CO₂ extracted some organic molecules in coal,resulting the aromatic structure loosened and more functional groups exposed,which was conducive to the degradation by microorganisms.?5?The anthracite extracted under non-supercritical conditions and the residual coal degraded by anaerobic microorganisms.It is concluded that the mainly reason for increasing production of biological coalbed methane is the extraction effect of supercritical CO₂ fluid.The pressure of supercritical CO₂extraction also contributes to the production of biomethane from anthracite by CO2,but it is not the main factor.