Effects of coal molecular structure and pore morphology on methane adsorption and accumulation mechanism

The adsorption, diffusion, and aggregation of methane from coal are often studied based on slit or carbon nanotube models and isothermal adsorption and thermodynamics theories. However, the pore morphology of the slit model involves a single slit, and the carbon nanotube model does not consider the molecular structure of coal. The difference of the adsorption capacity of coal to methane was determined without considering the external environmental conditions by the molecular structure and pore morphology of coal. The study of methane adsorption by coal under single condition cannot reveal its mechanism. In view of this, elemental analysis, FTIR spectrum, XPS electron energy spectrum, 13C NMR, and isothermal adsorption tests were conducted on the semi-anthracite of Changping mine and the anthracite of Sihe Mine in Shanxi Province, China. The grand canonical Monte Carlo (GCMC) and molecular dynamics simulation method was used to establish the coal molecular structure model. By comparing the results with the experimental test results, the accuracy and practicability of the molecular structure model are confirmed. Based on the adsorption potential energy theory and aggregation model, the adsorption force of methane on aromatic ring structure, pyrrole nitrogen structure, aliphatic structure, and oxygen-containing functional group was calculated. The relationship between pore morphology, methane aggregation morphology, and coal molecular structure was revealed. The results show that the adsorption force of coal molecular structure on methane is as follows: aromatic ring structure (1.96 kcal/mol) > pyridine nitrogen (1.41 kcal/mol) > pyrrorole nitrogen (1.05 kcal/mol) > aliphatic structure (0.29 kcal/mol) > oxygen-containing functional group (0.20 kcal/mol). In the long and narrow regular pores of semi-anthracite and anthracite, methane aggregates in clusters at turns and aperture changes, and the adsorption and aggregation positions are mainly determined by the aromatic ring structure, the positions of pyrrole nitrogen and pyridine nitrogen. The degree of aggregation is controlled by the interaction energy and pore morphology. The results pertaining to coal molecular structure and pore morphology on methane adsorption and aggregation location and degree are conducive to the evaluation of the adsorption mechanism of methane in coal.