Effect of liquid nitrogen freeze-thawing on coal body pore evolution and adsorption behavior

To investigate the effect of freeze-thawing of liquid nitrogen on the pore structure and adsorption behavior of coal, the pore structure of coal samples before and after freeze-thawing was characterized by low-temperature nitrogen and CO2 adsorption method, and methane isothermal adsorption experiments were carried out under different freeze-thawing times. The results show that the hysteresis index HI of coal samples decreased after freezing and thawing, and the connectivity of the pore system was improved, which is conducive to gas migration in coal. The pores between 2 nm and 100 nm increased to varying degrees after freezing and thawing, and some pores evolved into pores smaller than their own pore size or even less than 2 nm. The microporous pore size distribution has changed, with an increase in microporous pore volume and adsorption volume. With the increase of the number of liquid nitrogen treatments, the effect of liquid nitrogen treatment on the final adsorption amount of methane kept weakening, and after reaching four treatments, the effect of liquid nitrogen treatment on the final adsorption amount of coal body almost reached saturation. The Langmuir parameters VL and pL increased after the coal samples were treated with liquid nitrogen at different times, the larger the pL, the more favorable it was for the extraction of CBM.