Abstract: There exist coal seams of various ranks in China. Due to differences in coal quality and geological conditions, the occurrence of water in different coal seams is different. Coal rank and water saturation are two important factors affecting the cracking effect of liquid nitrogen at low temperature, which are worthy of further study. Therefore, three ranks of coals, including lignite, bituminous and anthracite coal samples, were chose to be treated as coal samples with 0%, 33% and 99% water saturation respectively for liquid nitrogen immersion. The camera was used to shoot and observe the variation of macroscopic cracks on the surface of coal samples before and after treatment. The coal sample was tested with nitrogen seepage. The experimental results were as follows. 1) The lignite after liquid nitrogen immersion was destroyed as a whole due to one and several penetrating cracks. New cracks were created on the surface of bituminous coal, and the original macroscopic cracks were expanded and extended compared with those before treatment. Macroscopic cracks on the surface of anthracite have no visible changes. 2) The higher the saturation of coal sample was, the better the fracturing and enhancing permeability effect of liquid nitrogen was. 3) The ranking of fracturing and enhancing permeability effect of liquid nitrogen immersion on three coal rank coal samples is as follows: lignite> bituminous coal> anthracite coal, but in the completely dry state, enhancing permeability effects of the bituminous coal and anthracite coal were approximately the same because the thermal stress was not enough to destroy the inter particle linkage. 4) Liquid nitrogen immersion could effectively crack the completely dry lignite, and the permeability of treated lignite increased by 559.35% on average. Liquid nitrogen immersion could significantly crack the bituminous coal with 33% and 99% water saturation, and the average increase of permeability of treated bituminous coal was 330.60% and 448.77%, respectively. For the anthracite with 33% and 99% water saturation, average permeability increase was 185.53%.

Key words: coal rank, water saturation, liquid nitrogen immersion, dimensionless permeability, permeability increase