The closed pores of tectonically deformed coal studied by small-angle X-ray scattering and liquid nitrogen adsorption

Abstract

To study the variation in the characteristics of closed pores in coal under tectonic deformation, three coal samples are selected that differ only slightly in metamorphic degree but differ sharply in deformation degree, with vitrinite reflectance range from 0.88% to 0.96% and deformation degrees corresponding to weak brittle, strong brittle and strong ductile deformations. The experimental methods used are small-angle X-ray scattering and liquid nitrogen adsorption, and the pore diameters obtained by combining the two methods are within the mesopore (2–50 nm) size range. The deformation of coal narrows the distribution of pore diameters. Changes in pore shape are described according to fractal theory: the pore fractal dimension is larger in coal with a greater pore size, and tectonic movement promotes irregularity and fracturing of the original pores. The specific surface area of pores is calculated in this paper. We conclude that the proportion of closed pore specific surface area reaches the peak with the increase of the deformation degree, and then falls gradually. Pore diameters greater than 3.2 nm and less than 3.2 nm correspond to the two parts in the liquid nitrogen adsorption data, with the fractal dimension of the former being larger than that of the latter. The results also show that the closed pore volume increases but the proportion of the total pore decreases with increasing deformation degrees.