Experimental Study On Synergistically Enhancing CBM In Middle-rank Coal By Supercritical CO₂ Extraction And Microbial Degradation

As a kind of clean energy,the development and utilization of coalbed methane?CBM?can greatly reduce the exploitation cost,improve the structure of energy supply,alleviate the energy crisis of insufficient supply of fossil energy effectively and ensure the sustainable development.There are many ways to improve the production of coalbed methane.With the characteristics of green and pollution-free,Microbially Enhanced Coal Bed Methane?MECBM?has become a hot topic in the research.Methanogenic bacteria produce biomethane through the reduction of CO₂,in which CO₂ plays a key role.CO₂ displacement happens to be a popular technology for increasing production of coalbed methane,which can convert CO₂ injected into coal seam into methane.When the CO₂ injected into the coal seam reaches the supercritical state,the organic matter from the complex compound can be extracted effectively.Therefore,combined with supercritical CO₂ extraction and MECBM,it is meaningful to have a research on the optimal extractionconditions and stimulation mechanism,which is of great significance for the effective utilization of resources and geological storage of CO₂.In this paper,we use the self-developed CO₂-microorganism device for increasing the production of CBM.The bituminous coal was taken as the research object.In the research,coal sample was extracted by supercritical CO₂ under different temperature,pressure,extraction time and sample particle size,and the optimal condition for the methane increase was determined by microbial degradation of residual and extracts.On this basis,the physicochemical properties of residual coal and the composition of the extract were analyzed in order to explore the effect of supercritical CO₂ extraction on the yield of biomethane.In addition,in order to further clarify the mechanism of synergistic effect of supercritical CO₂ extraction and microbial degradation,the secondary extraction experiment of organic solvent and the experiment of high temperature and pressure of non-supercritical CO₂ were carried out as control.The main conclusions are as follows:?1?The results of supercritical CO₂ extraction under different conditions showed that the best supercritical temperature,pressure and extraction time were 80 °C,20 MPa and 12 h respectively,the optimum sample size was 80-100 mesh,and the highest methane yield was145.68%.?2?The results of GC-MS analysis of supercritical CO₂ extractsshowed that there were no significant differences in the types of extracts under different conditions,among which the content of aromatic hydrocarbon was the highest,accounting for about 90%;The four samples which have the highest amount of organic matter in the extract were consistent with the four samples with the highest gas production.?3?The results of physicochemical properties showed that after supercritical CO₂ treatment,the water content of the sample decreased,the ash content increased,and the volatile content decreased compared with the raw coal.With the increase of the fixed carbon content,the reduced volatile content was transferred to the extract,which resulted in the degradation of the extract to produce a huge amount of methane.The fat chain shortened and the branch chain increased,which was more conducive to the degradation of microorganisms.The increase of specific surface area proved that the extraction of supercritical CO₂ made the contact area between microorganisms and coal bodies larger,which was in favor of the adhesion and degradation of microorganisms.?4?The results of the experiment in which the coal was treated with CO₂ at high temperature and high pressure under non-supercritical condition showed that the methane production increased by about 11% to29%.There was a great difference compared with raw coal,indicating that both high temperature and high pressure treatment had an impact on the increase of bio-methane,but it is not the most important factor toachieve increased production.?5?The experimental results of organic solvent extraction showed that both the residuals extracted by secondary extraction and the raw coal extracted directly by organic solvent had little value of microbial utilization,and the production of methane was very low.On the contrary,the yield of methane in liquid phase extracts was higher than control,and the methane production of the secondary extracted sample was35.72%³32.59% higher than that of the organic solvent extracted raw coal.The results showed that after supercritical CO₂ treatment,the extractable organic matter increased and the number of small molecular substances which could be directly used by microorganisms to produce methane increased as well.?6?Finally,it was concluded that supercritical CO₂ extraction can destroy the weak complexing force in coal molecular structure,so that a part of soluble small molecular matter in coal was released and taken away by solvent,and some of the small molecules that could not be transferred to the solvent also became loose and unstable because of the extraction of CO₂,and the amount of organic compounds that could be used by microorganism to produce methane increased significantly,thus achieving the methane increase of coal sample.