Simulation Study On Adsorption And Diffusion Properties Of CO₂ And CH₄ Gas By Coal Molecular In Chicheng Coal Mine

The study on the adsorption and diffusion mechanism of CH₄and CO₂in coal is very important for the development of coalbed methane and the key to improve the recovery of coalbed methane.In this paper,non-sticky coal from Chicheng Coal Mine is taken as the research object.Based on the microstructure information of coal samples measured by experiments,a macromolecular structure model of non-sticky coal is constructed.The adsorption and diffusion characteristics and microscopic mechanism of CH₄ and CO₂in coal under different conditions are studied by molecular simulation method.In order to lay a theoretical foundation for revealing the adsorption-diffusion characteristics of coalbed methane in coal and provide technical support for further improving coalbed methane extraction,the main research contents are as follows:The contents of C,H,O,N and S in coal samples were obtained by industrial/elemental analysis.Fourier infrared spectroscopy（FTIR）was used to obtain the structural characteristics of aromatic,fatty and oxygen-containing functional groups of experimental coal samples.The occurrence states of N and S elements in coal samples were obtained by X-ray photoelectron spectroscopy（XPS）experiment,and the structural information of carbon skeleton in coal samples was obtained by nuclear magnetic resonance(¹³C NMR)experiment.On this basis,the macromolecular structure model of non-sticky coal from Chicheng mine was constructed,and the ¹³C NMR spectra of the constructed model were in good agreement with the experimental¹³C NMR spectra.The micropore distribution characteristics of He,CO₂ and CH₄ gases in the coal macromolecule model were obtained by using the Materials Studio（MS）molecular simulation software based on the coal macromolecule model.The smaller the molecular dynamics radius of the gas,the larger the pore space in the coal.Based on molecular dynamics（MD）method and Monte Carlo（GCMC）method,molecular simulation analysis was performed under different pressure（50k Pa-10MPa）,temperature（293.15K-313.15K）and water content（1%-5%）.The variation rule and microscopic mechanism of CO₂and CH₄gas molecular adsorption capacity,equal adsorption heat,interaction energy,diffusion coefficient and diffusion activation energy in the coal macromolecular structure model of Chicheng mine show that:The higher the temperature,coal-gas system in the adsorption amount and the smaller the interaction energy,the changing rule of the two is consistent,and the same conditions,the adsorption quantity,amount of adsorption heat and CO₂ and interaction energy are greater than CH₄,the influence of temperature on the adsorption performance of CO₄ than CH₄,two kinds of gas in coal adsorption belongs to physical adsorption,The diffusion activation energy of CO₂ is 4.57k J.mol^-1,and that of CH₄ is 3.56k J.mol^-1.The diffusivity of gases in the system increases with the increase of temperature,and the diffusivity of CO₂ is greater than that of CH4 under the same conditions.With the increase of pressure,the interaction of the two kinds of CH4 and CO₂ gas can in dry coal samples and the adsorption quantity and water cut were increased in the coal sample,amount of CO₂ adsorption heat in dry（wet）and decreased with the increase of stress in coal,the amount of CH₄ adsorption heat with the increase of pressure in the dry coal samples first increase in reducing,in aqueous coal samples have been reduced,The diffusivity of both gases decreases with the increase of pressure.With the increase of water content,the adsorption capacity and interaction energy of CH₄ and CO₂in coal samples gradually decrease,the adsorption heat of the same amount gradually increases,and the diffusion coefficient gradually decreases.