Structural Model Construction Of Yangquan 3# Coal Based On HRTEM And Investigation Of Its Adsorption Properties

Coalbed methane(CBM),as a kind of unconventional natural gas,its extensive exploitation and utilization is of great significance to promote the industrial development and change the energy structure in China.The study of the interaction mechanism between coal structure and methane/carbon dioxide molecules is of great significance to understand the distribution characteristics of coalbed methane.However,it is difficult to analyze the adsorption mechanism at the molecular level with macroscopic experimental methods.Therefore,this paper constructs a coal structural model from a microscopic perspective.Based on molecular simulation technology,adsorption mechanism of coal macromolecular structure on CH₄ and CO₂ gas molecules during the adsorption process is explored.In this paper,the research object is the anthracite coal selected from the 3#coal seam of Xinjing mine in Yangquan mining area(Yangquan No.3 anthracite coal,YQ3,volatile matter9.90%,vitrinite reflectance 2.59%).Based on the ultimate analysis and high-resolution transmission electron microscopy(HRTEM),combined with ¹³C nuclear magnetic resonance spectroscopy(¹³C-NMR),X-ray photoelectron spectroscopy(XPS),Fourier transform infrared spectroscopy(FTIR),and other analytical techniques,constructed the YQ3 macromolecular structural model,which is different from the previous small-scale molecular structural models based on average structure characterization,this model captures the diversity of structures and has large-scale characteristics.Materials Studio software(MS)was used to perform molecular mechanics and dynamics simulation of the YQ3 structural model,and the lowest energy configuration was obtained.The single component of CH₄/CO₂ and binary component isothermal adsorption simulations on the structural model were performed.The adsorption configuration was simulated and the dominant adsorption sites and adsorption directions were counted,and the energy of the structure before and after adsorption was analyzed to obtain mechanism of CH₄ and CO₂ competitive adsorption from energy and adsorption configuration perspectives.Some of the layer structures in the model were replaced by functional groups to calculate the adsorption energy and understand the coupling effect between different functional groups and CH₄/CO₂.The conclusions were as follows:(1)Through fringe extraction and length/orientation distribution statistics of HRTEM images,the fringe length has a peak in the range of 0.50-0.70 nm,and the length distribution continues to extend to 4.46 nm,the average is 0.93 nm,and the orientation is also relatively concentrated,with 46.66%of fringe length concentrated in the range of 45°.Analysis of ¹³C-NMR,XPS and FTIR show that the aromatic carbon ratio is 0.91,the nitrogen-containing functional groups are mainly pyrrole and pyridine,hydroxyl,carbonyl groups and ether oxygen are the main forms of oxygen atoms in the model.According to the structural characterization of various functional groups,the structural model of YQ3 was constructed.The molecular formula of the model is C₁₉₃₅H₉₂₈O₄₂N₂₄S₃,and the molecular weight is 25252 Da.The atomic distribution of the structural model is consistent with the results of ultimate analysis.(2)The constructed YQ3 macromolecular structural model was performed to geometrically optimized and annealing dynamic simulation in MS,and the energy composition was calculated.The van der Waals energy accounts for the largest proportion of non-bonding energy in the lowest energy configuration after structural optimization,which plays an important role in maintaining structural stability.During the optimization process,the aliphatic side chain was twisted to different degrees,which made the aromatic structure connected by the aliphatic side chain also deformed to different degrees,and the whole configuration was more three-dimensional and compact.(3)The GCMC method was used to simulate the isothermal adsorption curves of CH₄ and CO₂ in single and binary gas components by YQ₃ molecular structure,and the results show that the three simulated isothermal adsorption curves of CH₄ and CO₂ at the temperature gradient of 10 K in the pressure range of 0-10 MPa are in good agreement with the Langmuir adsorption model.The adsorption amount of CO₂ is greater than that of CH₄ at the same temperature and pressure,the adsorption amount of the two gases at three temperatures is:293 K>303 K>313K,and the temperature increase is not conducive to adsorption.CO₂ molecules show obvious competitive advantage in the adsorption of CH₄/CO₂ binary gas mixture.(4)Based on the GCMC method,the adsorption configuration of CH₄ and CO₂ in the structural model under 8 MPa/293 K conditions was simulated,and the adsorption sites and adsorption directions were counted.The results show that most of the CH₄ are adsorbed on the surface of the structure model in the adsorption directions of Up and 2H,and the adsorption equilibrium distance from Down to Up changed from 4.11(?)to 3.67(?).In the CO₂ adsorption configuration,most of the CO₂ molecules are adsorbed on the surface of the structure in the parallel adsorption direction,and is rarely perpendicular to the coal surface.From parallel to vertical direction,the adsorption equilibrium distance changes from 3.47(?)to 4.07(?).The adsorption sites of CH₄ and CO₂ have a high consistency,CH₄ present an aggregation distribution on the surface of the micropores,while the CO₂ molecules exist in two parallel or intersecting forms.The adsorption equilibrium distance of CO₂is less than that of CH₄.(5)Based on DFT theory,the adsorption energies of CH₄ and CO₂in the pore wall environment with different functional groups in the structural model were calculated and analyzed.The results show that the adsorption energy of CO₂ is greater than that of CH₄ in the same functional group structure model,the presence of oxygen-containing functional groups such as OH,C=O and COOH have a negative effect on the adsorption of CH₄,the order of adsorption strength of the four structures is as follows:C-Layer>C=O-Layer>OH-Layer>COOH-Layer,which is in the order of hydrophobicity and alkalinity.The oxygen-containing functional groups improves the adsorption performance of CO₂,and the adsorption order of the four structures is COOH-Layer>OH-Layer>C=O-Layer>C-Layer,which is the order of polarity.The presence of pyridine in the nitrogenous functional groups also improves the adsorption performance of CH₄ and CO₂,but pyrrole hindered the adsorption of CH₄and CO₂.