Experimental Study Of Liquid CO₂ Acidification Antireflection Coal And Pore Structure Reconstruction Mechanism

Liquid CO₂ fracturing coal seam anti-reflection technology can alleviate greenhouse effect while controlling mine gas disaster and improving coal bed methane production,and has gradually become a research hotspot.In this paper,based on the characteristics of liquid CO₂ itself,such as low temperature,high pressure,non-polar and water acidification after phase change,by building an experimental platform for liquid CO₂ leaching and acidification of coal,liquid CO₂ leaching and acidification of coal have been carried out.A series of test methods,such as nitrogen adsorption,XRD,XRF,ICP-OES,FTIR and SEM-EDS,were used to study the interaction between liquid CO₂ and coal at the liquid phase and acidification stage,as well as the process mechanism of pore structure reconstruction of coal body.The following conclusions were drawn:By measuring the mineral content in coal,the element content of coal body and reaction water sample,pH and other parameters,the evolution mechanism of minerals was analyzed.The results show that liquid CO₂ leaching has little effect on the minerals in coal body,which is mainly due to the partial collapse and trace migration of coal body minerals caused by low temperature damage and mechanical migration of liquid CO₂.The content of most minerals decreased significantly after liquid CO₂ acidification and was sensitive to pressure.The acidification of liquid CO₂ mainly leads to the dissolution reaction,transformation and precipitation of coal and the hydration and expansion of clay,among which the dissolution reaction is stronger.Carbonate content in anthracite and long-flame coal decreased the most during dissolution,followed by illite,quartz,pyrophyllite and rutile content changed slightly.The higher the pressure,the faster the H+consumption of the reaction water sample is,and the stronger the dissolution reaction of the mineral componen ts in the coal body is,resulting in the greater the element dissolution and mobility of each component.Among them,the Ca element dissolution and mobility are the largest in the anthracite and long-flame coal,which is mainly caused by the dissolution of carbonate.By measuring the functional group structure content of organic matter in coal and analyzing its evolution mechanism,the results show that the functional group structure content of organic matter in liquid CO₂ leaching and acidification process is controlled by swelling,dissolution,extraction,bond dissociation reaction,addition reaction,substitution reaction,etc.After liquid CO₂ leaching,the relative contents of functional groups in the aromatic structure of anthracite and long-flame coal at 2H,3H and 5H increased overall except for 1H.The contents of non-polar or weakly polar functional groups such as aliphatic ether,aromatic ether and carbonyl C=O decreased after extraction.The bond dissociation reaction destroyed part of the C=C structure of the aromatic structure and caused a large number of aliphatic hydrocarbon structures to fall off,and the content of OH-O and OH-OH of self-associating hydroxy hydrogen bonds decreased.After liquid CO₂ acidification,the content of aromatic structure of each component of anthracite increased further.The content of 1H,3H and 5H of long flame coal increases,while the content of 2H structure decreases.The contents of non-polar or slightly polar functional groups such as aliphatic ethers,aromatic ethers and carbonyl C=O of the two coal samples are still reduced by extraction,and the-COOH structure content is significantly increased,the overall aliphatic hydrocarbon structure content is further reduced,and the overall hydroxyl functional group content is significantly increased.In addition,after liquid CO₂ leaching and acidification,both aromaticity and condensation degree of the two coal samples were improved,and after liquid CO₂ acidification,both aromaticity and condensation degree showed an overall trend of increasing with the increase of pressure.By measuring the pore structure parameters of coal and combining with the evolution mechanism of mineral components and organic matter,the pore structure reorganization mechanism was analyzed.The results showed that:After liquid CO₂ leaching,the specific surface areas of both anthracite and long-flame coal decreased,and the number of micropores decreased,and the reduction rate of long-flame coal was greater than that of anthracite,and the peak pore size also shifted to the direction of large pore size.The macropore,mesopore and micropore volume of both coals increased significantly.Through analysis,low temperature caused some minerals containing combined state water is advantageous to the crack formation collapse and increase pore connectivity,but the mineral and coal particles mechanical migration on the one hand,make coal surface is not opened,the pore is shaken after stripping effect is opened and form a big hole,but there are also some big hole was occurred jams migration of particles;After liquid CO₂ leaching,the number of micropores in anthracite and long-flame coal decreased mainly because of the swelling of coal matrix and the increase of aromatic ring condensation degree,which resulted in the decrease of molecular gap and the increase of order degree,while the dissolution of non-polar small molecules and functional groups resulted in the increase of micropore volume.After liquid CO₂ acidification,the specific surface area of both anthracite and long-flame coal increases,and the peak range of pore size shifts to micropore direction.The micropore and macropore volume of anthracite increase obviously,while the mesopore volume of anthracite decreases.The mesopore and large pore capacities of long flame coal increase and show a trend of increasing with the increase of pressure.In addition,some of the macropores were blocked by leaching,but were reopened by acidification.The potassium feldspar on the surface of anthracite formed a small amount of quartz precipitation,and the semi-closed pores were opened due to the dissolution reaction between kaolinite and calcite.Due to the uniform mineral distribution of long flame coal,the flake coal falls off after acidification,and the coal tends to be loose and granulated,forming a large number of dissolution holes and dissolution debris,and with the increase of pressure,the coal surface gradually tends to granulated.After liquid CO₂ acidification,the mineral evolution shows the effect of increasing pore size and expanding pore size on mesopores and macropores.However,the micropore development of anthracite and long-flame coal is mainly due to the acidification process promoting the swelling and dissolution process of macromolecular phase.After the swelling and dissolution process is accelerated,the disorder between macromolecular phase and macromolecular phase is increased,which leads to the expansion of intermolecular gap,the increase of the number of micropores and the pore volume.In addition,acidification makes a large number of edge groups such as methyl,methylene,aliphatic ethers,aromatic ethers and carbon based C=O structures to be extracted,detached and dissolved,further leading to the increase of micropore volume.The research results of this paper can provide theoretical basis and data support for liquid CO₂ fracturing coal seam to increase CBM production,which has academic research and engineering application value.