Pyrolysis characteristics and kinetic analysis of coating pitch derived from ethylene tar using model-free and model-fitting methods

This work conducted the pyrolysis of coating pitch derived from the ethylene tar through thermogravimetry analysis. The thermogravimetry-mass spectrometry and solid-state ¹³C nuclear magnetic resonance spectroscopy were performed to correlate the structural characteristics with the pyrolysis behavior. Meanwhile, the pyrolysis kinetics were estimated with model-free and model-fitting methods. The TG-DTG results showed that the pyrolysis process was generally divided into drying, fast pyrolysis, and polycondensation stages, accompanied by the decomposition of active functional groups like the aliphatic carbons bonded to oxygen ($f_{\mathrm{al}}^O$), the pyrolysis of relatively stable groups like the non-protonated aromatic carbon ($f_a^H$), and the condensation of high bonding-energy aromatic structure like aromatic bridgehead carbon ($f_a^B$). The activation energy obtained from model-free analysis ranged from 78.12 to 150.92 kJ/mol with the increasing conversion, indicating the pyrolysis process as a multi-step reaction mechanism. Furthermore, the reaction model A1/3 ($g\left(\alpha \right)={[-\ln (1-\alpha \left)\right]}^3$) was identified as the most suitable model for the pyrolysis of coating pitch in the conversion range of 0.25–0.8. The modeling values matched well with the experimental data, indicating the accuracy and feasibility of the results.