To develop the in situ underground pyrolysis process
of tar-rich coal more scientifically, the effect of temperature and
pressure on the distribution of pyrolysis products 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 the main reaction products. 32 typical secondary reactions
were constructed. Based on the thermodynamic analysis strategy, the
variation of the Gibbs free energy and equilibrium constant of secondary
reactions was investigated. The results showed that pressure mainly
affected the reaction characteristics of molecule-increasing reactions.
The Gibbs free energy value of the molecule-increasing reactions increased
with increasing pressure. The trend that the reaction could proceed
spontaneously gradually weakened. The initial temperature of some
reactions that could proceed spontaneously would need to increase
by dozens or even hundreds of degrees. Due to the influence of formation
pressure, the generation of related components of light tar, naphthalene
tar, washing tar, and anthracene tar would be inhibited to varying
degrees in the in situ underground pyrolysis process.
The secondary reactions related to phenol tar were equimolecular reactions,
which were almost unaffected by stratal pressure. Axial pressure and
confining pressure of different coal seam depths should be considered
in the process of in situ underground pyrolysis.