Impacts of the caking property of tar-rich coals on their pore structure evolution and seepage during pyrolysis

Objective The in-situ pyrolysis of tar-rich coals stands as a technique whereby coal seams are heated underground to produce tar and gas products. During the in-situ pyrolysis, the injection of heating media and the production of tar and gas are affected by the pore structure and permeability of coal seams. Since pyrolysis of caking tar-rich coals is accompanied by the formation of colloids, the pore structure and permeability of caking tar-rich coals differ from those of non-caking tar-rich coals.

Methods Caking tar-rich coals were pyrolyzed at 300, 400, 500, and 600 ℃. During the pyrolysis, the pore parameters of the semi-coke were tested using the saturated fluid method and nitrogen adsorption method, and the pore structure of the semi-coke was characterized using micro-computed tomography (micro-CT). Meanwhile, using equivalent pore network models established in this study, the laws of changes in parameters such as the pore number, pore radius, and coordination number were analyzed, and the seepage characteristics of high-temperature nitrogen in the pore network were simulated.

Results and Conclusions The results revealed small numbers of fissures in the coal samples after the pyrolysis at 300 ℃, with a total porosity maintained at about 5%. After pyrolysis at temperatures ranging from 400 ℃ to 600 ℃, the total porosity gradually increased to about 50%, while micropores became more abundant only after degassing at 600 ℃. When the pyrolysis temperature rose from 300 ℃ to 400 ℃, the number of pores and throats increased significantly due to the formation and expansion of colloids, whereas the average pore and pore-throat radii were maintained at about 160 μm and 88 μm, respectively. As the pyrolysis temperature increased further from 400 ℃ to 600 ℃, the precipitation of volatile constituents promoted the pore connectivity. Accordingly, pores and throats gradually decreased in number, with equivalent radii trending upward in terms of probability distribution. Specifically, the average pore and pore-throat radii increased to 292.81 μm and 170.60 μm, respectively, and the average coordination number of pores increased from 5.82 to 6.60 (500 ℃) and 6.33 (600 ℃). The increasing porosity and coordination number led to a significant increase in the average simulated permeability of the semi-coke from 246.75 μm² to 1377.49 μm². The findings of this study can serve as a reference for the R&D of the in-situ pyrolysis process of caking tar-rich coals.