Thermophysical Response Patterns of Pore Structures in Coals of Different Ranks

Understanding the differential evolution characteristics of pore structures in coals of various ranks under heat treatment conditions is fundamental for developing coal thermal mining technologies. This study systematically collected coal samples from the Zhangjiamao Mine in the Ordos Basin, the Dahebian Mine in western Guizhou, and the Sihe Mine in the Qinshui Basin, representing a series of samples with different coal ranks. Through drying, saturation, and heating processes, combined with low-field nuclear magnetic resonance (NMR) testing and pore fractal geometry theory, the effects of heat treatment at 200 °C, 300 °C, and 400 °C on NMR T ₂ spectra, pore size distribution, two-dimensional relaxation distribution, and pore fractal dimensions of coal samples were investigated. The results indicate that after heat treatment, the porosity of coal samples across all coal ranks increased, with low-rank coal exhibiting the greatest increase, suggesting that low-rank coal pore structures possess the strongest temperature sensitivity. When the temperature reached 300 °C, the porosity of high-rank coal began to increase significantly, suggesting that its pore structure requires a higher activation temperature for thermal evolution. The DHB coal sample exhibited caking properties, and significant thermal expansion of the coal matrix after heating led to a smaller porosity at 300 °C compared to ZJM and SH samples. Heat treatment promoted the transformation of micropores into mesopores and macropores. Low-rank coal was characterized by an increase in mesopore volume, the pore connectivity of medium-rank coal was optimized, and the macropore volume of high-rank coal increased markedly with rising temperature. After heat treatment, the fractal dimensions of mesopores and macropores generally decreased, indicating reduced pore surface irregularity and enhanced connectivity, which facilitates fluid seepage. This study reveals the thermophysical differential response patterns of pores in coals of different ranks. Heat treatment affects coal pores though expansion, increase in number, and improved connectivity, creating favorable conditions for the fluid product during thermal extraction. The research findings provide theoretical and parametric support for geological site selection and process control of thermal mining, thereby promoting the development of thermal extraction technologies for coal resources.