Study on characterization of tri-phase products and microstructure evolution during in-situ pyrolysis of tar-rich coal at different temperatures

The in-situ pyrolysis of tar-rich coal is accompanied by internal migration of volatile matters, which is intimately related to the pore and fracture structure of coal. The pyrolysis products distribution under the in-situ pyrolysis conditions of low heating rate and high pressure are inconclusive. Furthermore, few previous reports have investigated the evolution of microstructure during in-situ pyrolysis sufficiently. It is unable to successfully observe the three-dimensional (3D) anisotropy of various components (mineral, pore-fracture structure and matrix) at the microscopic scale of tar-rich coal. In the present study, the composition distribution of in-situ pyrolysis products at 400-600 degrees C was examined with the tri-phase products being characterized. The evolution of pore-fracture structure was quantitatively described by the X-ray computed tomography (CT) scanning technique. The results indicate that while the pyrolysis gas yield gradually increases, the char yield declines with a rise in temperature. The optimal tar production temperature is 500 degrees C. The formation and development of the interior pore-fracture structure is facilitated by the rise of pyrolysis temperature. At higher temperatures, the samples have greater interior connectivity, which promotes the migration of gaseous products. The largest fracture structure and pore connection group were observed at 600 degrees C. The number of throats with throat diameter ranging from 0.00056 to 0.0011 mm is the largest within the char sample obtained at 600 degrees C, which accounts for 19.2 %. The findings offer theoretical and technological guidance for the microstructure evolution as well as the formation and migration of products during in-situ pyrolysis, which aids in the popularization of tar-rich coal in-situ pyrolysis technology.