Chemical structure transformation during the later stage of plastic layers during coking using Synchrotron infrared microspectroscopy technique

During the coking process, the carbon structure of coal evolves after the re-solidification of the plastic layers to form a solid residue, i.e., semi-coke, which is known to determine the structure and quality of the final coke product. This paper reports the chemical structural transformations of Australian coking coals during the later stage of the plastic layers that may be related to the evolution of the carbon structures. The plastic layers and semi-coke/coke samples were prepared from four Australian coking coals of different ranks and vitrinite contents in a lab-scale coke oven. The Synchrotron attenuated total reflection Fourier transform infrared (ATR-FTIR) microspectroscopy (Synchrotron IR) was used in combination with the X-ray photoelectron spectroscopy (XPS) to investigate the changes in the chemical structure during coke formation. The Synchrotron IR, as a nondestructive analytical technique, proved to be a useful tool to examine the change in functional groups in the semi-coke samples. The XPS analysis was employed to study the conversion of sp²- and sp³-bonded carbons. The results suggested that a dramatic chemical structure change took place during the stages following the plastic layers through to the coke/semi-coke regions. The chemical structure changes were strongly impacted by the properties of the parent coals. The carbon structure changes observed in XPS spectra were in good agreement with the aromatic ring condensation degree, which was indicated by the ratio of out-of-plane aromatic C-H to C=C bonds in aromatic rings. The results showed that the carbon structure evolution took place during the later stages of the thermoplastic ranges, forming the semi-coke.