INVESTIGATION OF THE PORE STRUCTURE OF LIGNITE AT DIFFERENT TEMPERATURES AND PRESSURES

Underground coal gasification and in situ pyrolysis exploitation techniques have attracted renewed interest to meet the demand for clean and efficiently mined lignite. The investigation of the pore structure evolution during coal pyrolysis is beneficial for understanding the pyrolysis mechanisms and guiding the practical application of in situ pyrolysis exploitation. Furthermore, it was found to be essential to study the structure of coal under conditions that mimic the in situ environment. In this work we first designed a new high-temperature gas heating reaction system, which is heated by fluidic N2 gas to simulate the in situ pyrolysis condition. With this new reaction system, the pore structure and structural parameters of lignite from room temperature to 500°C at pressures of 0.1, 0.5, and 1 MPa were systematically investigated by X-ray computed tomography (CT). The results show a systematic evolution of the pore structure of lignite under high temperature and pressure. By analyzing the structural parameters, including porosity, percolation probability, specific surface area, and cluster size, it was found that pressure can promote the development of new pore clusters and the interconnection of existing pore clusters. At 500°C, the porosities of lignite at 0.1, 0.5, and 1 MPa were 23.01%, 27.11%, and 28.63%, respectively, indicating good interconnectivity and permeability of lignite. The results also show that the pyrolysis of lignite can be affected by dynamic competition between externally applied pressure and internal pressure from pyrolysis products, which in turn influences the development of pore structure.