Experimental study of effect of liquid nitrogen cold soaking on coal pore structure and fractal characteristics

In this paper, the pore structure and fractal characteristics of coal under liquid nitrogen cold soaking are tested for different coal qualities of anthracite, bituminous coal and lignite, and the variation patterns of BET specific surface area, BJH total hole and adsorption pore specific surface area with liquid nitrogen cold soaking times are studied. Among these coal samples, it is found that the raw bituminous coal has the most internal micropore content and the largest adsorption space for gas, followed by anthracite, and lignite is the last. After one time liquid nitrogen cold soaking, the increase of adsorption and percolation pore contents of each coal sample in the order of: anthracite > lignite > bituminous coal; and with the increase of liquid nitrogen cold soaking times, the internal pore size and number of pores of the three coal samples gradually increases, but the specific surface area of adsorption pores decreases. The reduction amplitude of the specific surface area of coal adsorption pores by one time liquid nitrogen cold soaking gradually increases with the increase of metamorphism. With the increase of liquid nitrogen cold soaking times, the average pore size, total BJH hole and percolation hole of each coal sample increases, while the BET specific surface area and adsorption pore specific surface area decreases gradually. The fractal dimension D1 of each coal sample increases linearly with the increase of liquid nitrogen cold soaking times, and the variation of D2 has an opposite trend. The results of this study can provide theoretical and experimental support for further application of liquid nitrogen to increase the permeability of coal seams.