Experimental Study On Microstructure Characteristics Improvement Of Low-Permeability Coal Treated By Chemical Solvents

China is rich in coal resources.However,there are more coal seams with high gas contents and low permeability.As a complex porous medium,in addition to the macromolecular skeleton,the pores and macromolecular networks of coal matrix also contain many minerals and soluble small molecular organic matter.Their existence is closely related to the adsorption and fluidity of gas.Chemical solvent treatment is one of the most effective means to improve pore structure of low-permeability coal and increase production of coalbed methane.In this study,tetrahydrofuran(THF),carbon disulfide(CS2)and hydrochloric acid(HCl)were used to extract and dissolve coal samples.With the help of low temperature liquid nitrogen adsorption method,carbon dioxide adsorption method,X-ray diffraction mineral analysis and Fourier transform infrared spectroscopy,combined with density functional theory,fractal dimension model and peak fitting method,the changes of pore structure parameters and surface functional groups of coal before and after chemical solvent treatment were studied.The methane adsorption characteristics of coal were tested by methane isothermal adsorption experiment.Based on the Langmuir adsorption theory model,the influence mechanism of pore structure and chemical composition change on methane adsorption characteristics was further explored.The main conclusions are as follows :(1)The pore size of coal samples after solvent treatment is mainly distributed in the range of 0-10 nm.HCl treatment can open a large number of closed pores,and the proportion of micropores increases.At the same time,the pore specific surface areas of the two samples(sample I and II)increase by 55.21% and 31.36%,respectively.THF extraction increased the pore volume and specific surface area,and increased the openness of some pores.After CS2 extraction,the dissolution of organic matter increases the pore size and the volume of transition pores and mesopores.The ultra-micropores less than 2 nm in the coal sample show a bimodal distribution.HCl treatment causes decrease in D1 value of pores.However,in organic-solvent-treatment cases,solvent residues retain on coal pores,resulting in growth in D1 value and pore surface becoming rougher.Proportion of total pore volume of transition pores and mesopores of acidized coal decreases and D2 value rises.Pore distribution is more uneven.(2)The content of hydrogen bonds(OH-OH)formed by self-associated hydroxyl groups is the largest in the three raw coal samples.After HCl and THF treatment,the hydroxyl content of the three coal samples decreased.The bridge bond between the metal cation and the-OH group corresponding to the HCl treatment was destroyed.After the organic solvent treatment,the self-associated hydroxyl hydrogen bond in the coal sample was more easily dissolved than other hydrogen bonds.Raw coal samples mainly contain asymmetric-CH2 groups,and organic solvents are more likely to react with methylene in coal,shortening the length of aliphatic chains and increasing the number of branched chains.THF is easier to dissolve the C-O-C group and carbonyl C = O group of phenol,alcohol and ether in coal samples.CS2 as a nonpolar solvent has a great influence on aromatic hydrocarbons.The change of aromatic structure of coal samples after HCl treatment is closely related to the substitution reaction of aromatic hydrocarbons,and organic solvents can destroy the π...π bond between aromatic nuclei,thus dissolving more aromatic substances.(3)Under the same temperature and pressure,the adsorption capacity of three kinds of raw coal samples for methane is: I > III > II.Except sample I-HCl and sample III-THF,the methane adsorption capacity of other coal samples is lower than that of raw coal.Methane is mainly adsorbed on the inner surface of micropores,but micropore structure is not the only factor leading to differences in methane adsorption.The ability of coal to adsorb methane is controlled by micropore structure and surface functional groups.The relevant results are helpful to reveal the mechanism of action of chemical solvents on the microstructure and composition of coal,provide a theoretical reference for the optimization of chemical solvent permeability technology parameters,and help gas efficient extraction and safe coal mining.Figure 37 Table 10 Reference 100...