Structural Characterization And Molecular Adsorption Property Of CH₄,CO₂ And H₂O For Gas-fat Coal In Jixi Coalfield

The technology of injecting flue gas or CO₂to improve the recovery rate of coalbed methane（CO₂-ECBM）can not only improve the production rate of coalbed methane,reduce coal seam gas content,but also achieve CO₂storage,achieving the goal of reducing greenhouse gas emissions,and has dual economic and environmental benefits.Most of the fractures or pores in coal seams contain formation water,and the flue gas contains about 10%of water vapor.In the research process of improving coalbed methane recovery rate（CO₂-ECBM）by injecting flue gas or CO₂,the influence of water cannot be ignored.This thesis studies the characteristics and micro mechanisms of single component adsorption and diffusion of CH₄,CO₂,and H₂O in coal seams,as well as the competitive adsorption of multiple components.It provides theoretical support for coal mine gas prevention and control,efficient development of coalbed methane,and CO₂geological storage.Taking Jixi gas-fat coal as the research object,the micro structure information of Jixi gas-fat coal was measured by industrial/element analysis,Fourier transform infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）,X-ray diffraction（XRD）and solid nuclear magnetic resonance carbon spectroscopy(¹³C-NMR),and the macromolecular structure model of Jixi gas-fat coal was constructed.A low-temperature CO₂adsorption experiment was conducted on the gas-fat coal,and the microporous specific surface area and pore volume of the gas-fat coal were measured to be 106.87m²/g and 0.055cm³/g,respectively.The distribution characteristics of microporous structure of CH₄,CO₂,and H₂O molecules in gas-rich coal were obtained through probe method testing.The study showed that the smaller the radius of the molecules,the larger the pore space they can enter.Based on the grand canonical monte carlo（GCMC）and molecular dynamics（MD）methods,the adsorption and diffusion characteristics of CH₄,CO₂,and H₂O single component gases in a gas-fat coal macromolecular model were studied using Material Studio software at temperatures ranging from 273.15K to 313.15K and pressures ranging from 0.01MPa to 15MPa.The competitive adsorption behavior of binary（CH₄/CO₂,CO₂/H₂O,CH₄/H₂O）and ternary（CH₄/CO₂/H₂O）mixed components with different molar volume fractions at 293.15K was also studied.The effects of pressure,temperature and molar volume on gas adsorption capacity,equivalent adsorption heat,interaction energy,energy distribution,probability density distribution and diffusion coefficient distribution characteristics are analyzed.Research has shown that the saturated adsorption capacity of Jixi gas-fat coal on H₂O molecules is the highest,followed by CO₂,and the saturated adsorption capacity of CH₄is the lowest.The adsorption of CH₄and CO₂is dependent on physical adsorption,while the adsorption of H₂O molecules exceeds the scope of physical adsorption.With the increase of temperature and pressure,CH₄,CO₂and H₂O all transfer from the preferential adsorption site with higher interaction energy to the secondary preferential adsorption site with lower interaction energy.In a multicomponent system,the larger the molar volume fraction of gas,the greater its adsorption capacity;Multiple competitive adsorption exists,and the competitive adsorption capacity ranges from strong to weak:H₂O>CO₂>CH₄.Low pressure and low molar volume fraction are beneficial for improving competitiveness.The equivalent adsorption heat of weakly adsorbed gas of weakly adsorbed gas fluctuates under the influence of competitive adsorption.The lower the Molar volume fraction of weakly adsorbed gas,the greater the fluctuation.