Molecular Simulation Of Gas Adsorption Based On Coal Pore Structure And Surface Functional Group Characteristics

China is rich in coal seam gas reserves.The basic premise of coal seam gas drainage and utilization is the accurate prediction of gas content and its enrichment area.The main place of coal gas adsorption is on the surface of micropores,fissures and groups of coal body.The change of coal surface microstructure affects the gas adsorption of coal body,and then causes the unevenness of coal seam gas occurrence.Therefore,using the combination of experimental research,theoretical analysis and numerical simulation,this paper takes Dongsheng lignite,Daliuta bituminous coal and Chengzhuang anthracite as the research object,starts with their pore structure and surface functional group characteristics,and deeply studies the changes of different coal gas adsorption characteristics,so as to provide theoretical guidance for understanding the influence mechanism of coal pore structure and surface functional group characteristics on gas adsorption.In this paper,the pore structure and surface functional group characteristics of these three coal samples were studied through liquid nitrogen adsorption experiments,¹³C-NMR experiments,and Fourier transform infrared spectroscopy experiments.Both pore volume and pore specific surface area show a U-shaped change trend in value,and there are certain differences in the pore shape characteristics of the three coal samples;From lignite to anthracite,the aromaticity of coal samples and the degree of condensation of the macromolecular structure of aromatic compounds gradually increase,and the oxygen-containing functional groups and side chains gradually decrease.The types of oxygen-containing functional groups are mainly carboxyl,hydroxyl,carbonyl,ether,etc.Among them,carboxyl group It mainly exists in lignite and bituminous coal with low and medium metamorphism.The macromolecular models of Dongsheng lignite and Chengzhuang anthracite were constructed by using Chemdraw software and Materials Studio software combined with the experimental results,and their molecular formulas were finally determined as C₁₅₃H₁₃₉N₃O₂₉and C₁₈₈H₁₃₈N₂O₇respectively.After optimizing the model,the pore structure of lignite was measured by molecular probe technology based on Connery principle.The specific surface area of lignite was 40.5707 m²/g and the pore volume was 0.022873 cm³/g;The specific surface area of anthracite is 45.5005 m²/g and the pore volume is 0.034971 cm³/g.The adsorption of lignite and anthracite macromolecular models were simulated by using the numerical simulation method and Materials Studio software.According to the isothermal adsorption simulation results of CH₄,CO₂and H₂O,it was found that the adsorption isotherm accorded with the Langmuir equation.The higher the temperature,the smaller the adsorption capacity.The adsorption amount of the three adsorbents is CH4,CO2 and H2O from small to large.For the anthracite macromolecular model with the addition of acidic oxygen-containing functional groups,the adsorption capacity of CH₄was simulated by Monte Carlo method.It was found that the adsorption capacity of CH₄decreased to a certain extent,indicating that the addition of acidic oxygen-containing functional groups has a certain inhibitory effect on the adsorption of CH₄by anthracite.In terms of inhibition effect,carboxyl group is the best,phenolic hydroxyl group is in the middle,and lactone group is the smallest.