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.
Tar-rich coal has the potential to substitute the supply of oil-gas resources, which is abundant in China. The effective conversion of tar-rich coal into oil-gas products can promote coal utilization, reduce resource wastage, alleviate environmental pollution, and benefit carbon neutrality. Nevertheless, less work, if any, has been performed on the pyrolysis and mild oxidation behaviors of tar-rich coal in Northwestern China. The influences of limited oxygen addition and an extremely low heating rate on the micromorphology of the residual semi-coke are yet to be fully understood. Here, an experimental study on the pyrolysis and mild oxidation characteristics of tar-rich coal was conducted by the thermogravimetric analysis method, with further elucidation of the physical–chemical properties of the residual semi-coke. Experimental results show that an increase in the ultimate temperature of pyrolysis leads to a decline in the residue mass, while the mass loss from 500 to 550 °C presents the maximum elevation. Volatile matter is inclined to discharge from a certain direction, and the pores formed in various directions hold different possibilities. The organic components undergo both pyrolysis and slow oxidation with limited oxygen in the heating medium. Compared with an inert atmosphere, the mass loss under conditions of a small amount of O 2 is brought forward but prolonged. Compared with a N 2 atmosphere, the oxidation reactions of tar-rich coal are weakened in the presence of CO 2 . A large decrease in the heating rate exerts an unfavorable effect on the production of total volatiles. An extremely low heating rate possibly brings about a change in the mechanism of chemical bond cracking during pyrolysis. More pores can be yielded in tar-rich coal with an increase in the heating rate, and the morphology of the residual semi-coke after pyrolysis is susceptible to the heating rate. The present study offers an improved understanding of the pyrolysis characteristics of tar-rich coal as well as insights into the efficient utilization of tar-rich coal.
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.
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