The Evolution of Thermal Conductivity and Pore Structure for Coal under Liquid Nitrogen Soaking

An experimental system for liquid nitrogen soaking and real-time temperature measurement was designed and implemented to investigate the characteristics of temperature field changes in coal under liquid nitrogen soaking. Then, the heat conduction law of the coal in the process of liquid nitrogen soaking and room temperature recovery for dry and water-saturated coal were examined. The microstructure characteristics of the coal before and after liquid nitrogen soaking were analyzed with nuclear magnetic resonance (NMR) technology. The results showed that, during the liquid nitrogen cold soaking process, the heat transfer law of the dry and water-saturated coal samples exhibited a notable three-stage distribution. For the room temperature recovery process, the dry and water-saturated coal samples exhibited rapid heating characteristics, and the cooling rate gradually decreased to zero. NMR test results indicated that the liquid nitrogen soaking increased the number of micro and small pores in the coal. Thermal stress analysis revealed that the thermal stress generated by the dry coal was larger than that produced by the saturated coal, and the damage was primarily caused by thermal stress. However, the permeability of the saturated coal was better than that of the dry coal. The damage on the saturated coal was caused by the volume expansion of pores and fissures caused by water-ice phase transition.

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