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

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.