Macromolecular Model Construction Of Tunlan No.2 Coal And Molecular Simulation Of Interaction Between Coal And CH₄

The study of pores in coal is very important for the exploration of coalbed methane because the pores provide surface area for adsorbing phase methane and provide volume for free gas.The micro-nano pores are the main adsorption pores,and the micropore mechanism is the main adsorption mechanism,especially the ultra-micropores?less than 1 nm?.The heterogeneity of pore structure and pore morphology makes it difficult to explain the adsorption mechanism of methane only by experimental methods.Moreover,these ultramicropores?less than 1 nm?belong to the intermolecular pores.Therefore,the introduction of molecular simulation technology to study the interaction between methane and coal macromolecular structure is particularly critical for the exploration and development of coalbed methane.In this paper,a macromolecular structural model for TunLan No.2 coal was constructed by combining corrected solid-state ¹³C CP/MAS/TOSS nuclear magnetic resonance spectroscopy(¹³C-NMR),Fourier Transform Infrared Spectrometer?FTIR?,X-ray photo electron spectroscopy?XPS?and ultimate analysis.The aforementioned methods yield a molecular model formula of C₁₅₈H₁₁₀O₈N₂S with a weight of 2196.63.The aromatic ring is mainly naphthalene,supplemented by benzene,fluorene and phenanthrene.The fat structure exists in the form of aliphatic side chain,cycloalkane and hydrogenated aromatic ring.The oxygen atom is mainly present in the form of ether oxygen?C-O?in coal,nitrogen atom.It mainly exists in the form of pyridine and pyrrole,and the sulfur atom exists in the form of thiophene sulfur.The aromaticity of the constructed model matches well with that determined by corrected ¹³C-NMR?0.81?and FTIR?0.80?.To further validate the molecular model,we computed its quantum chemical properties.The cleavage sequences of various model bonds agree reasonably well with the study results obtained by TG/MS.By adding periodic boundary conditions,the average density of the simulated model?1.32 g/cm³?is compared with the true relative density?1.34-1.39 g/cm³?probed by experiments;the calculated value is slightly smaller because the model construction does not consider the presence of minerals or small molecules.Based on the Grand Canonical Monte Carlo?GCMC?method,the simulation of isothermal adsorption and saturated adsorption of methane in coal is carried out.It is concluded that the simulated isotherm adsorption curve is similar to the I-type isotherm adsorption curve.It is generally considered to be the characteristic curve of monolayer adsorption theory,and the adsorption model of methane is consistent with the Langmuir adsorption model.The number of molecules of saturated adsorption of methane in the macromolecular model of the No.2 coal is 5.The molecular structure of the coal macromolecular structure was analyzed by molecular probes.The maximum pore size of the molecular model was 1.3 nm and the total porosity was 18.74%and the total specific surface area is 1834.29 m²/g.The porosity measured by helium and nitrogen probe was 13.03%and 9.27%,respectively.By combining the distribution map of the probe molecule in the coal macromolecular model with the adsorption position of methane in the coal,it can be inferred that the minimum accommodating pore of methane adsorbed in the coal is 0.48 nm.There is no clear relationship between the dominant adsorption sites of methane in coal and the pore size of ultramicropores.