Pore Microstructure and Dynamic Evolution Characteristics of Unfrozen Water during the Freezing Process of Coal with Different Moisture Contents

China possesses diverse coal-bearing strata types, and the moisture content in these coal seams exhibits significant variations due to differences in the coal quality and geological conditions. To investigate the impact and mechanism of saturation on the fracturing effects of coal bodies during liquid nitrogen freezing, two groups of coal samples, air-dried and saturated anthracite from the Hengyi coal mine, underwent liquid nitrogen immersion treatment. Low-field nuclear magnetic resonance (NMR) technology was employed to measure the T2 curves, porosity, and changes in pore throat distribution of coal samples at different freezing temperatures. Quantitative analysis was conducted on the pore structure characteristics and the occurrence and evolution patterns of unfrozen water within the pores. Experimental results indicate that during the liquid nitrogen freezing process, the saturated coal samples exhibit significantly higher rates of decline in cumulative porosity, maximum effective diameter, and cumulative pore throat distribution compared to the air-dried coal samples. Both coal samples experience three stages of variation in unfrozen water content: a rapid decrease in stage I (15 to −50 °C), a slower rate in stage II (−50 to −100 °C), and a gradual stabilization in stage III (−100 to −196 °C). However, the average freezing rate of saturated coal samples in stage III is much higher than that of air-dried coal samples. This suggests that the initially higher moisture content and greater free water content in saturated coal samples allow for a faster transformation from liquid to solid during the freezing process compared to air-dried coal samples. Conversely, the bound water in air-dried coal samples may require more time to undergo the phase transition, resulting in a lower freezing rate.