Experimental study on yield and quality of tar from tar-rich coal under the simulated in-situ conditions

Abstract

Tar-rich coal has a great potential to extract a large body of liquid fuels through thermal treatment. To investigate the effects of in-situ conditions on both the yield and the quality of the tar from tar-rich coal during pyrolysis, an experimental system in which the in-situ conditions can be simulated was built. The simulated conditions included in-situ stress, pyrolysis temperature, heating rate, and moisture content. Furthermore, the underlying mechanisms of in-situ conditions on the tar from tar-rich coal were revealed. The tar yield is increased from 0.83 % to 10.13 % in the temperature range of 200–500 °C, but a considerable drop of the yield occurs around 600 °C due to the cracking of the tar. The heating rate is decreased from 16 °C/min to 2 °C/min, the tar yield is increased from 4.63 % to 10.30 %. At low heating rates, the tar yield can be boosted during the in-situ pyrolysis, in contrast to the high heating rate required in conventional coal pyrolysis. Based on the variations in the average activation energy of pyrolysis kinetic under in-situ conditions, the in-situ pyrolysis can easily form the active free radicals in tar-rich coal but difficultly promote the reactions in the formed active free radicals. The decomposition of the bridge bonds in aromatic, the –CH₂/–CH₃ in aliphatic, the oxygen-containing functional group, and the hydrogen-bonded-OH group for tar-rich coal under in-situ conditions is conducive to producing the high-value tar. The low ratio of chain hydrocarbons to aromatic hydrocarbons and the decreased relative content of oxygenated compounds in the tar under in-situ conditions improve the tar quality. Compared with the tar from tar-rich coal under the conventional pyrolysis, the tar under the in-situ pyrolysis is rich in light oil. This study provides the better understanding of the effects of in-situ conditions on tar from tar-rich coal, and also guides the in-situ pyrolysis of coal to extract high-value tar in applications.