Study on the evolution of the pore structure of low rank coal during spontaneous combustion

Studying the evolution of the pore structure of coal during spontaneous combustion is of great value in further understanding the mechanism of coal spontaneous combustion (CSC) and its prevention. In this study, we selected three low-rank coals and used nuclear magnetic resonance (NMR) to visualize the macroscopic evolution of the pore structure of coal after heat treatment and to analyze the effect of temperature (25-500 °C) on the pore structure of coal, including porosity, permeability, and fractal dimensions. The obtained results show that the overall NMR signal in coal increases with increasing temperature, indicating that heat treatment can induce the enlargement, opening, and interconnection of pores and fractures in coal. The equivalent average pore radius (r_m) of coal shows a positive correlation with temperature, with a substantial increase in r_m, especially after temperatures above 200 °C. During heating, the porosity and permeability of all three coals tended to increase with temperature. At temperatures above 300 °C, the permeability of coal dramatically increases, predicting a higher fluid transport capacity. Furthermore, NMR multifractal theory was proposed for quantitative pore space dimensional characterization. The obtained results show that the fractal dimensions of the adsorption space of coal pores increase and then decrease with temperature during heating, while the fractal dimensions of percolation space are negatively correlated with temperature. In addition, the dimensions of adsorption space vary more strongly than those of percolation space, meaning that the adsorption capacity of low-rank coals is more significantly influenced by temperature.