Microscopic fracturing and unfrozen water hysteresis effects analysis of lignite and anthracite

China possesses widely distributed low-permeability coal seams of various ranks, where differences in coal rank determine fracture mode selection and optimization of fracturing effects. This study investigates the response of low-rank lignite and high-rank anthracite to liquid nitrogen freeze–thaw. Using nuclear magnetic resonance technology, we examined T2 relaxation curves, porosity, and pore throat changes during freeze–thaw processes, focusing on unfrozen water from a microscopic perspective. Experimental results indicate that lignite exhibits significantly higher T2 relaxation amplitudes compared to anthracite, with a predominance of adsorption pores. Lignite shows a more pronounced response to freeze–thaw cycles, resulting in increases of 0.9% in cumulative porosity and 0.9% in pore throat count for lignite, while anthracite shows increases of 0.1% and 0.13%, respectively. The average aperture of flow pores increased by 45.2% and 49.4%. Upon returning to room temperature, lignite shows a two-stage increase in porosity loss rate after initial fluctuations, while anthracite exhibits a slightly fluctuating trend. Both lignite and anthracite demonstrate a lag effect in unfrozen water during freeze–thaw processes, with maximum lag percentages at −5 °C and 5 °C recorded as 51.07% and 67.75%, respectively. The primary factors contributing to the unfrozen water lag effect are the supercooling effect during freezing and changes in pore ice melting points due to water-ice phase transitions. The rapid temperature differential from low-temperature liquid nitrogen triggers uneven thermal stress within the coal body and freeze expansion, optimizing coal pore structure and enhancing connectivity and permeability.