Thermodynamic Analysis on In-Situ Underground Pyrolysis of Tar-Rich Coal: Secondary Reactions

Abstract

To design the in-situ underground pyrolysis process of tar-rich coal more scientifically, the effect of temperature and pressure on the pyrolysis product distribution of tar-rich coal should be clarified. This paper selected the typical components in five distillates of light tar, phenol tar, naphthalene tar, washing tar and anthracene tar as products, and 32 typical secondary reactions were constructed. Based on the thermodynamic analysis strategy, the variation laws of Gibbs free energy and equilibrium constant of the secondary reactions were investigated. The results showed that the pressure mainly affected the molecule-increasing reactions. The Gibbs free energy value of the molecular-increasing reactions increased with the pressure rise. The trend of the reactions that could be spontaneous was weakened. The initial temperature of some reactions that could be spontaneous would need to increase to dozens or even hundreds of degrees. In the in-situ underground pyrolysis, the generation of related components of light tar, naphthalene tar, washing tar, and anthracene tar would be inhibited to varying degrees. The secondary reactions related to phenol tar were equimolecular reactions, which were almost unaffected by stratal pressure. Therefore, the axial compression and confining pressure of coal seam at different depths should be considered in the in-situ underground pyrolysis of tar-rich coal. A reasonable pyrolysis process should be scientifically set.

Keywords: Tar-rich coal, In-situ underground pyrolysis, Secondary reaction, Thermodynamics analysis