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.
In situ underground pyrolysis of tar-rich
coal
is significant for alleviating the scarcity of oil and gas resources
and realizing the green and efficient development and utilization
of coal in China. Tar-rich coal is often subjected to high axial pressure,
surrounding pressure, and pore pressure in the in situ underground pyrolysis environment. Consequently, laboratory simulation
conditions are difficult to meet the actual needs. This paper conducts
a thermodynamic study of the pyrolysis characteristics of tar-rich
coal under an in situ environment. Typical thermodynamic
functions of tar-rich coal, including the standard enthalpy of formation,
standard formation Gibbs free energy, and standard entropy, were determined.
Ten representative primary reactions were constructed with typical
tar-rich coal pyrolysis oil components as a guide. The Gibbs free
energy and equilibrium constant change laws of the above reactions
were analyzed for pyrolysis temperatures from 200 to 800 °C and
pyrolysis pressures from atmospheric pressure to 10 MPa. The results
showed that the standard enthalpy of formation of tar-rich coal was
−72.27 kJ·mol–1, the standard entropy
was −37.79 J·mol–1·K–1, and the standard formation Gibbs free energy was −60.01
kJ·mol–1. When the reaction pressure increased
from atmospheric pressure to 10 MPa, the thermodynamically feasible
initial temperature fractions of the primary reaction of tar-rich
coal pyrolysis all showed different degrees of increase. In the underground
environment, the initial temperature of the primary reaction of in situ underground pyrolysis of tar-rich coal moves to
a higher-temperature gradient to some extent, so the adjustment of
the reaction temperature and pressure could guide the directional
regulation of the in situ underground pyrolysis products
of tar-rich coal.
Rapid pyrolysis of pulverized coal is an essential procedure of the coal upgrading process, but still faced with the problem of high content of heavy components in the tar. The CPD model is a kinetic model that can effectively predict the distribution of coal pyrolysis gasliquid-solid products, but incapable of achieving a detailed description of the specific composition of the tar. Therefore, based on the fundamental assumptions of the molecular structure of coal in original CPD model, a more detailed tar cutting method is carried out. For the tar component only contains one aromatic unit, the aromatic core is divided as 2 types and the yield of tar with less molecular weight can be calculated. By adopting the modifications above, a more accurate prediction of the tar product distribution can be achieved. This study can provide theoretical guidance for the intensification of the pyrolysis process.
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