Experimental Study On Supercritical CO₂ Pretreatment To Promote Biodegradation And Methane Production Of Lignite

As an unconventional natural gas,coalbed methane(CBM)is mainly composed of methane,which is a high-quality clean energy.However,the low recovery ratio of CBM has severely restricted the development of the CBM industry.How to increase the yield of CBM has become an important research content for the development of the energy industry.Microbially enhanced CBM technology is a way to generate new CBM by using microorganisms to degrade organics in coal which can increase CBM reserves and promote production.However,the structure of organics in coal is very complex,containing a large amount of organics that is not easily degraded and utilized by microorganisms,which affects the implementation effect of this technology.There is an urgent need to find a new method to improve the structure of coal,promote the anaerobic degradation of microorganisms and increase the production of CBM.In recent years,supercritical CO₂ enhanced CBM have become a research hotspot due to their unique physical and chemical properties in coal modifications,enhanced CBM recovery ratio and environmental friendliness.The intermolecular force of organics in coal is weakened,and the non-covalent bond of organics is broken under the action of supercritical CO₂extraction,which releases low-molecular-weight organic matter from the macromolecular structure of coal.Therefore,supercritical CO₂ extraction can be used as a pretreatment method to improve the bioavailability of coal,which have the potential to increase biogenic CBM production.Taking Baorixile lignite in Inner Mongolia as the research object,the effect of supercritical CO₂ pretreatment on coal biomethane production and structure of coal under different temperature,pressure and time conditions was studied in this article.The bioavailability of the extract was studied.The leading factors for the increase of biomethane through supercritical CO₂ pretreatment were analyzed through experiments of the CH₂Cl₂ extraction and the subcritical CO₂ pretreatment.The effect of supercritical CO₂ pretreatment on biodegradation was studied.The main conclusions are as follows:(1)The methane production of pretreated coal under different conditions also was higher than that of raw coal.The maximum methane production of 245.46?mol/g coal was observed by pretreatment at 40°C and 10MPa for 4h,which was 84.68%higher than that of raw coal,indicating that supercritical CO₂ pretreatment can significantly promote the production of biomethane.(2)After supercritical CO₂ pretreatment,the water content,ash and volatile content of the pretreated coal were reduced,but the fixed carbon content changes in the opposite direction.FTIR results showed that the aliphatic chain of the coal sample become shorter and the degree of branching become higher,which led to the increase of the space occupied by the group accompanied by the weakening of the intermolecular force,which is more conducive to the utilization of organics by microorganisms.The results of low-temperature liquid nitrogen adsorption showed that after supercritical CO₂ pretreatment,the total pore volume of lignite decreased by 30.9%,and the specific surface area decreased by 21%.(3)Abundant organics usable by microorganisms were detected in the extract by GC-MS.The anaerobic degradation to produce methane of the extract showed that the methane production cycle was shorter than that of coal by 5 days,and the biomethane production was about 2 times higher than that of the blank control group.After the gas production is over,the main compounds such as medium-chain aliphatic hydrocarbons,oxygenated compounds and aromatic hydrocarbons with carbon atoms greater than 10 in the extract are degraded into compounds such as fatty acids,aromatic alcohols and aromatic acids with carbon atoms all less than 10.As a result,both the extract and the extracted coal obtained in the experiment also can increase the biomethane production,and finally realize a significant increase in the production of biomethane.(4)The residual coal after CH₂Cl₂ extraction lost the ability of biodegradation to produce methane.Compared with the CH₂Cl₂ extract of raw coal,the CH₂Cl₂ extract of supercritical CO₂ pretreated coal contained more organics that are easy degraded and utilized by microorganisms.The results of subcritical CO₂ treatment experiments showed that the methane yields from residual coals treated by gaseous and liquid CO₂ were137.34-160.45?mol/g coal,which is slightly higher than that of raw coal,but far less than those from supercritical CO₂ pretreated coal.Therefore,the high-temperature or high-pressure conditions during supercritical CO₂ pretreatment have a certain promotion effect on the production of biomethane,but it is not the leading factor in the increase of biomethane.The extraction of organics in coal,a key factor in increasing the production of biomethane,is initiated through the unique physical and chemical properties of supercritical CO₂.(5)After biodegradation,the adhesion of microorganisms and the residue of degradation products in the coal,and the low hydrocarbon generation efficiency result in a decrease in the specific surface area and total pore volume of the raw coal,and an increase in TOC content.After extraction,the expansion of the coal matrix leads to an increase in the biological interaction area between microorganisms and coal,and the change in the organic structure of coal makes it easier for microorganisms to utilize organics in coal.Therefore,after the biodegradation of supercritical CO₂ extracted coal,the specific surface area and total pore volume increase,and the TOC content decreased.In addition,due to the residue of microorganisms and degradation products,the improvement effect of pore structure is more obvious than the above results.