Mechanisms underlying variations in pore structures and permeability in response to thermal treatment and their implications for underground coal utilization

Coal remains a predominant energy source in today's society. However, the coal mining process is associated with severe environmental hazards, and combustion for power generation leads to pollution and carbon dioxide emissions. As a result, many countries have imposed restrictions on underground coal mining. Underground coal gasification, direct combustion for heat recovery and thermal treatment provides serval promising way for cleanly and safely utilizing coal resources. However, the successful operation of these technologies needs to comprehensively understand the physical and chemical properties of coal responding to high temperature. In particular, the pore structures and permeability of coal which are closely related to the transport of gas and liquid in coal seams. Therefore, serval advanced instruments were employed to investigate the variation of the functional groups, pore structures, permeability from room temperature to 500 °C. Their connection was discussed in detail to reveal the mechanism of thermal treatment on the pore structures and permeability of coal. The variations of all physical properties with temperature demonstrated a trend of gradual change followed by a sudden change. This indicated that coal samples possess relatively good stability up to 300 °C. Experimental results revealed significant alterations in the pore structure and permeability of the coal at 400 °C and 500 °C. Correspondingly, the mass of coal samples and the CC functional groups, which constitute the main molecular structure of coal, underwent significant changes at these temperatures. This suggested that the pyrolysis of coal beyond 400 °C led to an increase in both pore volume and permeability. Moreover, observations through scanning electron microscopy indicated that the increase in pore volume at 400 °C and 500 °C also resulted from the thermal stress induced cracks between minerals and organic matters. These mechanisms significantly enhance the pore volume and permeability of coal seams, facilitating the transport of coalbed gases and liquids during the production process of underground coal utilization. [Display omitted] •The changes in coal porosity and permeability are closely related to its pyrolysis and thermal stress.•The pores and fractures induced by coal pyrolysis and thermal stress mainly occurs after 400 degrees.•Coal properties exhibit a slight change followed by a significant change with increasing temperature.