Gas adsorption analysis of pore structure differences and influencing factors in coal with varying metamorphic grades

The heterogeneity structural characteristics of the pores in the coal matrix largely determine the storage and transport capacity of gas. In order to quantitatively characterize the pore structure in coal matrix and analyze the influencing factors, the filled pores (0.38-1.5 nm) and diffusion pores (1.5-100 nm) of seven coal samples with different metamorphic degrees were measured by low-temperature liquid nitrogen adsorption and carbon dioxide adsorption experiments, and combined with the theory of multifractality, the filled pores and adsorbed pores of coal samples with different degrees of metamorphism were characterized and discussed. The multiple fractal characteristics and influencing factors of the filled and adsorbed pores of coal samples with different metamorphic degrees were characterized and discussed. The results show that both diffusion pores and filled pores have multiple fractal characteristics; the more metamorphism, the more developed the filled pores and the stronger the heterogeneity. High-order coals had a strong heterogeneity of diffusion pores, while low-order coals exhibited a strong heterogeneity of filled pores. The heterogeneity of filled pores was negatively correlated with specific surface area and pore volume, while the opposite was true for diffusion pores. The relationship between pore heterogeneity and fixed carbon content showed a "U" distribution.