Transformation of cross-linking structures in the plastic layers during coking of Australian coals and its role in coke formation

This project aimed to:

• Characterise the transformation of cross-linking structures from the plastic layer to coke by using the 4 kg lab-scale coke oven facility and various advanced analytical techniques.
• Explore the underlying mechanisms of how the cross-linking structures in coal are transformed into stronger cross-linking structures in the semi-coke after the solidification, in combination with radical reaction mechanisms.
• Understand how the coal properties (coal rank, vitrinite content, and coal fluidity) would impact on the cross-linking structure changes, and any quantitative correlations between the properties of coal and the cross-linking structure parameters.

It was conducted in a series of steps:

• A suite of Australian coking coals varying in rank, vitrinite content, and fluidity was selected to prepare the plastic layer samples using a 4kg laboratory-scale coke oven facility, which include characteristic layers such as the plastic layer and coke/semi-coke.
• The transformation of the cross-linking structures across the plastic layer sample was characterised by Carbon-13 Nuclear Magnetic Resonance (13C NMR), electron spin resonance (ESR) spectroscopy and synchrotron attenuated total reflection Fourier-transform infrared (ATR-FTIR) microspectroscopy (Synchrotron IR), and X-ray photoelectron spectroscopy (XPS). Correlations between the transformation and the carbon layer structure development from the plastic layer to coke were investigated by a high-resolution transmission electron microscope (HRTEM), and a HRTEM image analysis technique was developed to quantify microstructural parameters from the obtained HRTEM images.
• These results were correlated to coal properties and coke strength indices such as CSR, CRI and M10 to explore the underlying mechanisms of how the cross-linking structures in coal are transformed into stronger cross-linking structures and investigate the impacts of coal properties not just on the cross-liking transformation but also on the coke strength.

Some key results from this work are that:

• Coal rank, fluidity and oxygen content were found to strongly affect the transformation of cross-linking structure and microstructure development during coking.
• High-rank coal with lower fluidity formed a higher number of c-bearing cross-links during the resolidification, thereby forming larger aromatic layer structures with lower tortuosity and narrower interlayer spacing. Subsequently, the precursor structures evolved into large aromatic layer structures with less tortuosity and narrow interlayer spacing in the coke.
• Low-rank coal with high fluidity and high oxygen content showed an increase in oxygenbearing cross-links (ether bonds) during the plastic layer formation, which prevented the growth and stacking of aromatic layer structures. These precursor structures evolved into the coke microstructures with larger frindge sizes, higher tortuosity and wider interlayer spacing.
• Coke microstructural parameters such as mean fringe size, interlayer spacing and tortuosity were correlated to coal properties and coke strength indices. It was found that such microstructural properties of higher-rank coal were correlated to lower M10, indicating stronger cold strength.

This project provided an insight into how the carbon structures of coke evolve in relation to the crosslinking structure transformation from the plastic layer to coke, and thus better understood underlying mechanisms of the coke microstructure formation. We envisage a range of future works for some of the techniques developed in this study. In particular, we are proposing to advance the HRTEM and IR analyses used in this study through which more representative and comprehensive data could be obtained possibly to distinguish the chemical structure transitions of individual macerals and also accurately examine their chemical interactions during coking. These future works could be used to investigate chemical interactions between coal blending components, thereby providing a useful tool to advance coke strength prediction models.