Comparative analysis of pore structure parameters of coal adsorbed by different gas probe molecules

The accurate characterization of the microscopic pore structure of coal is of great significance to the in-depth understanding of the internal structure of coal and the characteristics of gas adsorption, desorption and diffusion. Under the condition of liquid nitrogen (77 K), the low pressure argon adsorption experiment (LPGA-Ar), low pressure nitrogen adsorption experiment (LPGA-N₂) and low pressure oxygen adsorption experiment (LPGA-O₂) are carried out on the coal with three kinds of metamorphic degrees, and the changes of coal adsorption are analyzed. Using experimental data and non-local density functional theory (NLDFT), the pore structure parameters of coal are characterized. The results show that the adsorption processes of Ar, N₂ and O₂ by coal include micropore filling, single-layer adsorption, multi-layer adsorption and capillary condensation. The adsorption capacity of N₂ by coal is slightly higher than that of Ar, and the adsorption amount of O₂ is 2−8 times that of N₂. The hysteresis types of coal adsorption/desorption isotherms are of the same type in different probe molecular tests, that is, H3 type, which reveals that coal contains a large number of slit pores. At the test temperature of 77 K, the pore size range of different probe molecules is different, that is, the Ar probe molecules are 0.87−12 nm; the N₂ probe molecules are 1.09−250 nm; the O₂ probe molecules are 0.81−50.82 nm. However, the pore size distribution of coal samples shows a “multi-peak” distribution under different probe molecular tests, and it shows a “single-peak” characteristic in the range of micropores. Compared with N₂ and O₂ probe molecules, Ar probe molecules can accurately analyze 2−10 nm pore parameters, and Ar and O₂ probe molecules can enter narrow micropores (0.7−2 nm). It is difficult to accurately characterize the pore volume of the three probe molecules, especially N₂ probe molecule. In addition, the contribution rates of micropores and mesoporous specific surface areas of coal samples under different probe molecular tests are different, indicating that micropores are the main factor determining the adsorption of N₂ by coal, while the adsorption of Ar and O₂ is dominated by mesoporous pores.