Study on evolution law of pore characteristics of freeze-thaw coal based on fractal theory

Abstract

To investigate the characteristics of pore structure change of coal under freeze-thaw cycles, the surface and internal pore structure distribution of coal samples with long flame and different freeze-thaw times were measured by scanning electron microscopy (SEM) and low-temperature nitrogen adsorption method, and the law of pore structure change of coal after freeze-thaw was analyzed by fractal theory. The results of scanning electron microscopy experiments show that after freeze-thaw, the phenomenon of crack pore enlargement on the surface of coal samples occurs to different extents, and the phenomenon of crack pore enlargement on the surface of coal samples increases with the duration of freeze-thaw; at the same time, the porosity and fractal dimension DS of microcracks on the surface increased by 214.2% and 8.8%, respectively, corresponding to the change of macrocracks. The low-temperature nitrogen adsorption results show that the freezing and thawing changes the internal pore structure from simple to complex, and the maximum nitrogen adsorption capacity, specific surface area and pore volume in the pore parameters increase by 67.57%, 49.12% and 35.55%, respectively, compared with those before freezing and thawing. After freezing and thawing, the fractal dimension of the pores decreased to different degrees in the high-pressure and low-pressure stages. It decreased by 6.6% in the high pressure stage and by 8.9% in the low pressure stage. The change in fractal dimension DL in the low-pressure stage is greater than that of fractal dimension DH in the high-pressure stage, and the difference between the two decreases with increasing number of freeze-thaw cycles.