Pore structure characterization of various rank coals and its effect on gas desorption and diffusion

Based on the gas adsorption theory, the low-temperature liquid nitrogen gas adsorption (LT-N₂GA), CO₂ adsorption and scanning electron microscopy (SEM) were used to analyze the pore structures of eight coal samples with different metamorphisms from the aspects of pore volume, specific surface area (SSA), pore size distribution and pore shape.Coal particle gas desorption and diffusion experiments were carried out.Tortuosity of the coal samples and the length of the gas diffusion channels were calculated.The influence and mechanism of pore structures on the gas desorption and diffusion were analyzed.The results show that there are strong differences in gas adsorption capacity between the coals studied.The U-shape relationship between adsorption constant a and volatile component (V_daf) is observed, demonstrating that the adsorption capacity reduces first and then increases as the coalification effect increases, which is consistent with the relationship between the pore specific surface area and coal ranks.To a large extent, the pore shapes observed by SEM are consistent with the analysis of the LT-N₂GA isotherm.The pore shape of coals with different metamorphisms varies a lot, indicating the heterogeneity of coal surface.Adsorption analysis reveals that the mesopore size distributions are multimodal, and the pore volume is mainly contributed by the mesopores of 2-15 nm.The adsorption capacity of the micropores of coal body depends on the 0.6-0.9 nm and 1.5-2.0 nm aperture sections.The influence of coal ranks on gas desorption and diffusion is mainly related to the difference of pore structure.The porosity and tortuosity of various rank coals are different.The length of gas diffusion channels is also different.As the length of gas diffusion channel increases, the initial gas desorption rate decreases exponentially.Gas diffusion in various rank coals is dominated by surface diffusion on the micropores.The larger the pore SSA, the more significant the surface diffusion.The gas desorption quantity and initial diffusion coefficient show an asymmetric U-shaped relationship with coal ranks.For higher rank coals (V_daf< 15%), the gas desorption quantity and initial diffusion coefficient decrease significantly with the increase of V_daf.In the middle and low rank coal stages (V_daf> 15%), both of them increase slowly with the increase of V_daf.