Mechanism of carbon structure transformation in plastic layer and semi-coke during coking of Australian metallurgical coals

The transformation of the carbon structure of coal during the coking process influences the coke microstructure and microtexture, and ultimately, the coke quality. This study investigates the impacts of parent coal properties on the evolution of carbon structures of selected Australian coals from the plastic layer to semi-coke stage during coking using electron spin resonance (ESR), Synchrotron attenuated total reflection Fourier transform infrared microspectroscopy (Synchrotron ATR-FTIR) and solid-state carbon-13 nuclear magnetic resonance (13C NMR) analyses. The stable radical concentration, the g-value, and the linewidth measured by ESR were combined with IR and Solid-state 13C NMR results to improve the understanding of carbon structural transformation at the plastic and post-resolidification stages of coke formation. In addition, micro gas chromatography (micro-GC) was used to study the evolution of gaseous species. The results suggested that the coal undergoes crosslinking reaction, condensation, and re-polymerization within the thermoplastic range, resulting in loss of oxygen to form condensed carbon-bearing crosslinking structures. Due to differences in their chemical structure, macerals significantly influenced crosslinking structures during plastic layer formation. Higher rank coals generated more stable radicals in the plastic phase due to their lower H/C and O/C ratios than low-rank coals with higher vitrinite contents. Lower fluidity and lower rank coals formed oxygen-bearing cross-links at the early plastic stages, hindering fluidity development and carbon ordering at high temperatures. Above the resolidification point, the continuous transformation of C-O and C–H bonds to C–C bonds was accompanied by the release of H2 and CO2, leading to increased ordering and anisotropy of coke carbon structures.