SAXS-based nano-scale pore structure characteristics of coals with different metamorphic degrees

In order to reveal the characteristics of nano-scale pores in coals with different metamorphic degrees, the small-angle X-ray scattering method (SAXS) is used to obtain the porosity, pore size distribution, specific surface area and fractal dimension of 15 samples whose vitrinite reflectance R_max is 0.31%-6.24%, and the low temperature CO₂ and N₂ adsorption DFT model results are used to verify the pore size distribution. The results show: during coalification process, when R_max is < 0.5%, the porosity and the specific surface area of coal increase with the increase of metamorphic degree, the micropores(< 2 nm) content increases seldomly, the mesoporous(2-50 nm) and microporous(50-100 nm) content increase greatly, and the surface of coal is gradually smooth. When R_max=0.5%-1.4%, the porosity and the specific surface area decrease, and each type of pore content is reduced, and the surface of coal is gradually smooth. When R_max=1.4%-4.0%, the porosity and specific surface area of coal rock increase, the microporous content increases, and the mesoporous and microporous content are nearly stable, and the surface of the coal is gradually rough. When R_max > 4.0%, the porosity and specific surface area of coal increase, the microporous growth is slow, and the surface of coal is gradually smooth.