Pore Structure Characterization of Different Rank Coals Using N2 and CO2 Adsorption and Its Effect on CH4 Adsorption Capacity: A Case in Panguan Syncline, Western Guizhou, China

Abstract

To determine the pore structure characteristics of different coal ranks in Panguan syncline, both N2 adsorption–desorption (LP-N2GA) and CO2 adsorption (LP-CO2GA) were carried out with the goal of revealing the differentiation evolution of total pore volume (TPV), specific surface area (SSA), pore size distribution (PSD), and pore shape of 14 coal samples, and the influences of SSA to adsorption capability at different sizes (supermicroporous < 2 nm, micropore: 2–10 nm, transition pore: 10–100 nm) were further discussed. Density functional theory (DFT) is used for the determination of pore structure parameters to ensure its accuracy. Results show that the pore shape in the Panguan area is mostly the semi-open pores with poor connectivity (e.g., wedge-shaped, cylindrical, and slit-shaped pores with one closed side). Only a small amount of the pores is in open shapes (e.g., slit-shaped pores or cylindrical pores with two ends open). For most of the coal samples, the PSDs of pores tested by LP-N2GA appear to be multimodal, and the N2-TPV is mainly from the contribution of the transition pore (73.69–95.21%), followed by the micropore. The PSDs of supermicroporous (tested by LP-CO2GA) appear to be bimodal, with two peak values present at 0.52–0.61 nm and 0.82–0.87 nm, respectively, and a bigger pore volume distribution interval corresponds to a higher specific surface area. The N2-SSA (pore diameter ranges from 2 to 100 nm) mainly comes from the contribution of the pores 2–3 nm in diameter, and a bigger N2-TPV means a higher N2-SSA. The CO2-SSA (pore diameter ranges from 0.489 to 1.083 nm) which has a better positive linear correlation with Langmuir volume is related to CO2-TPV and the distribution frequency of pore volume at different pore sizes, and the contribution rate of CO2-SSA to the total SSA (CO2-SSA + N2-SSA) reaches 99%, indicating that supermicroporous has a significant effect on the coal adsorption capacity. With the increase of the vitrinite reflectance (Ro,ran, 0.83 → 2.27%), the N2-SSA increase gradually, and its increase tendency presents to be a half-reversed “U” shape in the whole, while the CO2-SSA presents an extremely significant linear increase tendency. Accordingly, the Langmuir volume (2.61 → 21.26 m3/t) also displays a linear growth tendency with the increasing vitrinite reflectance. The results of this study will provide guidance to the CBM development in the Panguan area.