Molecular Simulation on Competitive Adsorption Differences of Gas with Different Pore Sizes in Coal

Abstract

Micropores are the primary sites for methane occurrence in coal. Studying the regularity of methane occurrence in micropores is significant for targeted displacement and other yield-increasing measures in the future. This study used simplified graphene sheets as pore walls to construct coal-structural models with pore sizes of 1 nm, 2 nm, and 4 nm. Based on the Grand Canonical Monte Carlo (GCMC) and molecular dynamics theory, we simulated the adsorption characteristics of methane in pores of different sizes. The results showed that the adsorption capacity was positively correlated with the pore size for pure gas adsorption. The adsorption capacity increased with pressure and pore size for competitive adsorption of binary mixtures in pores. As the average isosteric heat decreased, the interaction between the gas and the pore wall weakened, and the desorption amount of CH4 decreased. In ultramicropores, the high concentration of CO2 (50–70%) is more conducive to CH4 desorption; however, when the CO2 concentration is greater than 70%, the corresponding CH4 adsorption amount is meager, and the selected adsorption coefficient SCO2/CH4 is small. Therefore, to achieve effective desorption of methane in coal micropores, relatively low pressure (4–6 MPa) and a relatively low CO2 concentration (50–70%) should be selected in the process of increasing methane production by CO2 injection in later stages. These research results provide theoretical support for gas injection to promote CH4 desorption in coal pores and to increase yield.

The role of water bridge on gas adsorption and transportation mechanisms in organic shale

“…4 . The CO 2 absolute adsorption under 10 MPa is 8.6 mmol/g, 7.4 mmol/g, and 6.7 mmol/g at respectively 308 K, 338 K, and 368 K. This computed adsorption amount is in line with the observations by Raza, Ottiger, and other studies 35 – 37 , 39 , 40 , presenting 10.5 mmol/g in simulation, and 9.0 mmol/g by simulation under the same conditions. Figure 4 a–c demonstrate a decreasing trend in CO 2 adsorption amount with increasing temperature in all directions.…”

Construction of Macromolecular Structural Models of Coal and Simulation of Adsorption under Acidification

“…Meng et al explored the adsorption mechanism of coal molecules for CH 4 using the GCMC method and density functional theory and found that compared with oxygen-containing functional groups, molecules with aromatic structures within coal molecules showed a greater adsorption capacity for CH 4 . To investigate the adsorption characteristics of coal under different pore size conditions, Han and Mosher et al , used the vacuum layer thickness of the slit model to describe the pore size conditions of coal and applied GCMC and molecular dynamics theories to simulate the adsorption of coal with different pore sizes for CH 4. . Compared with the experimental test, computer simulation of the macromolecular structure can not only provide information that is difficult to measure through experiments but also reduce the experimental costs and provide a better understanding of the mechanism of interaction of substances in the chemical system.…”