Study On Macromolecular Modeling And Adsorption Mechanism Of Coal With Different Coal Rank

China is rich in coal bed methane resources,and the development and utilization of coalbed methane has made some progress,but there are still some fundamental problems to be solved to improve the production of coalbed methane.The mechanism of coal’s adsorption of methane and carbon dioxide is a key problem in CBM exploitation.In this paper,chemical structure tests such as 13C-NMR,FTIR and XPS were carried out from the microscopic point of view.In combination with industrial analysis and elemental analysis,three coal macromolecular structural units of Long-flame Coal(Ro≈0.67%),Coking Coal(Ro≈1.50%)and Meager Coal(Ro≈2.21%)were established.The molecular modeling shows that the aromatic carbon in molecule of Long-flame Coal is mainly composed of Two Ring aromatic structure units,Oxygen elements mainly exist in the form of Carbonyl,Hydroxyl,Ethoxyl and Ester bonds,Sulfur elements mainly exist in the form of Thiophene Sulfur,and Nitrogen elements mainly exist in the form of Pyridine and Pyrrole Nitrogen.Among the molecules of Coking Coal,the aromatic carbon is mainly composed of Two or Three Ring aromatic units,the Oxygen element is mainly in the form of Carbonyl Group and Phenolic Hydroxyl Group,the Sulfur element is mainly in the form of Thiophene Sulfur,and the Nitrogen element is mainly in the form of Pyridine and Pyrrole Nitrogen.In Meager Coal molecules,aromatic carbon is mainly composed of Four Ring aromatic structural units,Oxygen is mainly in the form of Carbonyl Group,Ester Group,Ether Oxygen Group and Hydroxyl Group,and Nitrogen is mainly in the form of Pyridine and Pyrrole Nitrogen.The initial structural units of coal macromolecules of different coal grades were optimized by molecular mechanics and molecular dynamics to obtain the lowest energy model,and the model density was simulated.The density of Long-flame Coal,Coking Coal and Meager Coal were 1.3 g/cm3,1.35 g/cm3 and 1.35 g/cm3,respectively.The individual cell sizes are:a=b=c=16.3A,a=b=c=14.9A and a=b=c=14.9A.The Sorption module in MS was used to simulate the saturated adsorption and isothermal adsorption of coal macromolecules.The results showed that the saturated adsorption capacities of methane and carbon dioxide in Long-flame Coal,Coking Coal and Meager Coal models were 7,2 and 3moleculars/u.c respectively,and 9,3 and 4moleculars/u.c respectively.At the same time,the molecular configuration of Coking Coal was analyzed by the dominant adsorption potential of methane and carbon dioxide.Finally,the competitive adsorption simulation of coal macromolecules was carried out,indicating that the adsorption capacity of CH4 and CO2 in coal macromolecular structure models of different coal rank was:CO2>CH4,and in the competitive system of CH4 and CO2,carbon dioxide has the advantage of competitive adsorption.