Mechanism of microstructural evolution in coke during the co-pyrolysis of coking coal with organic additives

To deepen understanding of the evolution of coal char microstructural properties of coal char during the co-pyrolysis of coking coal with additives, this study incorporated two typical additives, coal tar pitch (CTP) and waste plastic (HDPE), into a blended coal sample and carried out pyrolysis experiments. The pyrolysis process and the microstructure of char were systematically characterized using various analytical techniques, including thermogravimetric analysis (TGA), X-ray diffraction (XRD) and Raman spectroscopy. Data correlation analysis was performed to reveal the mechanism of carbon structural ordering evolution within the critical temperature range (350−600 °C) from colloidal layer formation to semi-coke conversion in coking coal, and to elucidate the regulatory effects of different additives on coal pyrolysis pathways. The results indicate that HDPE releases free radicals during high-temperature pyrolysis, accelerating the pyrolysis reaction and increase the yield of volatile components. Conversely, CTP facilitates pyrolysis at low temperatures through its light components, thereby delaying high-temperature reactions due to the colloidal layer’s effect. XRD results indicate that during the process of pyrolysis, there is a progressive decrease in the interlayer spacing of aromatic layers (d002), while the aromatic ring stacking height (Lc) and lateral size (La) undergo significant of carbon skeleton ordering. Further comparative reveals that CTP partially suppresses structural ordering at low temperatures, whereas HDPE promotes the condensation and alignment of aromatic clusters via a free radical mechanism. Raman spectroscopy reveals a two-stage reorganization mechanism in the microstructure of the coal char: the decrease in the ID/IG ratio between 350 and 550 °C is primarily attributed to the cleavage of aliphatic side chains and cross-linking bonds, leading to a reduction in defective structures; whereas the increase in ID/IG between 550 and 600 °C is closely associated with enhanced condensation reactions of aromatic structures. Correlation analysis further demonstrates progressive graphitization during pyrolysis, with a significant positive correlation (R2 > 0.85) observed between d002 and the full width at half maximum of the G-band (FWHM-G).