Evolution of pore and fracture of coal under heating-freezing effects: An experimental study

To explore the evolution of pores and fractures of coal under heating-freezing effects, a tube furnace was used to heat samples. The coal sample was heated to 100 degrees C and 300 degrees C in air, and liquid nitrogen was used to freeze the heated coal sample. Low-temperature nitrogen adsorption experiments were used to analyse the damage degree of the coal pore structure quantitatively, and the surface characteristics of the coal sample were qualitatively analysed using a scanning electron microscope. Experimental studies have shown that liquid nitrogen injection increases the pore volume of coal. By injecting liquid nitrogen into the heat-treated coal sample, the pore volume of the macropores of the water-saturated coal sample increased by 19.61%. When the heat treatment temperature reached 300 degrees C, the macroporosity of the dry coal sample increased by 38.75%. In addition, the fractal dimension of the pores increased after liquid nitrogen treatment, indicating that the freezing of liquid nitrogen caused the surface of the coal sample to become rough. The development of pores and fractures caused by liquid nitrogen injection results from the low-temperature fracturing characteristics of liquid nitrogen and the volume expansion caused by the freezing of pore water.