Porosity Changes in Thermally-Treated Bituminous Coal During Step-by-Step Crushing: Implications for Closed Pore Variations with Temperature

Coal has many closed pores that limit fluid escape. However, the evolution of these closed pores during pyrolysis has rarely been studied. Based on the method of opening closed pores through step-by-step crushing, this study investigated the pore structure of thermally-treated bituminous coal during crushing using low-field nuclear magnetic resonance, low-temperature nitrogen adsorption, and thermogravimetry. The differences in pore structures were combined with pyrolysis to clarify the evolution of closed pores during pyrolysis. The results showed that closed pores with sizes less than 0.09 μm increased with temperature. However, for a narrow range of closed pores (2–50 nm), a different characteristic exhibited that closed pores first decreased and then increased with increase in temperature. Thermal swelling of the coal matrix occurred at 300 °C. This made the partial mesopores disappear and closed the narrow pore throats and cracks, resulting in the generation of closed pores ranging from 50–90 nm. Many volatiles were generated, forming large bubbles inside the coal at 500 °C, which reduced the number of closed mesopores and increased the number of large closed pores. In addition, condensation polymerization played a crucial role at 700 °C, increasing the number of closed pores of each pore size.