Adsorption Heat Response Law And Mechanism Of Tectonic Coal After SC-CO₂ Extraction

The prominent problem of energy shortage and environmental problems especially the problem of excessive greenhouse gas emissions need to be solved urgently with the rapid development of China’s economy in recent years.Many researchers prefer to store carbon dioxide（CO₂）in coal seams because of its advantages that can improve the recovery of coalbed methane and help to control the greenhouse effect so as to realize the dual significance of carbon dioxide resource reuse and environmental protection.Studies have shown that CO₂ is in supercritical state in coal seams deeper than 800 m,because the temperature and pressure of coal seams at this position can reach the CO₂ critical point（Tc=31.06,Pc=7.39 MPa）.Consequently,it is helpful to explore the differences of sorption of coal samples with different degrees of damage before and after supercritical CO₂（SC-CO₂）extraction from a microscopic perspective to deepen and reveal more accurately the mechanism of gas sorption/desorption and diffusion in coal bodies.Coal samples with different damage degrees are taken as the research object,this paper reveals the influence mechanism of SC-CO₂ extraction on the adsorption thermal response of structural coal based on the effective information of coal samples from modern testing techniques such as XRD,low-temperature liquid nitrogen sorption and heat of sorption.Based on the results of sorption heat test,the effects of inorganic mineral composition and nanoscale pores in coal on the thermal response pattern of gas sorption were investigated,and the mechanism of coal composition and structural evolution on gas sorption was elaborated from the microscopic perspective.The results show that.（1）Under the action of SC-CO₂,the minerals dissolved in the coal were calcite,dickite,dolomite and kaolinite in order from strong to weak.The extraction effect of SC-CO₂ fluid becomes more and more significant with the increase of coal body destruction.Among them,this phenomenon that the types and quantity of minerals are changed in fragmented coal compared with the relative content of minerals changes significantly in primary structure coal and cataclastic coal after extraction may be related to the size of coal particles or the development degree of pores and fissures,and it may be that the pores and fissures in fragmented coal are more developed,which is conducive to the infiltration of SC-CO₂.Further,it was found that the dissolution of carbonate minerals in coal was the largest,followed by clay minerals,while oxide minerals were more stable and did not change much before and after extraction.（2）SC-CO₂ has the effect of pore expansion is more significant,that is,the leaching of small molecule compounds and minerals converts some micropores into mesopores or macropores,which is reflected in the obvious increase of mesopore volume above 3nm and the decrease of micropore content below 3nm.However,due to the extraction yield and the retention of the extract,the total pore volume and stage pore volume are slightly reduced after the lean coal is extracted,and the"hysteresis loop"of the adsorption-desorption curve is reduced,the pore connectivity becomes worse,while the anthracite shows that the pore volume increases and the pore connectivity improves.（3）Before and after SC-CO₂ fluid extraction.The integral adsorption heat of lean coal showed an increasing and decreasing trend with the increase of the damage degree,and the anthracite showed an increasing trend.The adsorption time showed an increasing and decreasing trend with the increase of the damage degree at the adsorption pressure of1 MPa and 2 MPa,and when the adsorption pressure is 3 MPa and 4 MPa,the adsorption equilibrium time increases continuously.However,after SC-CO₂ fluid extraction,On the whole,the integral adsorption heat of two types of structural coals with poor coal increased.When the adsorption pressure of anthracite is 1 MPa and 2 MPa,the integral adsorption heat has increased,the integral adsorption heat has decreased obviously at the adsorption pressure is 3 MPa and 4 MPa.（4）Combined with the changes of pore content at different scales before and after extraction,it was concluded that the increase and weakening of coal sorption capacity by the extraction effect of SC-CO₂ fluid was mainly related to the change of porosity:On the one hand,some minerals dissolved out of the coal body so that the porosity is increased,which improved the sorption capacity of the coal sample to the gas;On the other hand,the pore expansion of fluid leads to the reduction of micropores and the decrease of pore specific surface area,which leads to the decrease of adsorption capacity.This paper contains 39 figures,10 tables,and 124 references.