Improving Understanding of the Factors that Contribute to Thermoplasticity during Cokemaking and Laboratory Carbonising Tests and Impact on Coke Strength

This project encompassed two key components:

• Development of inertinite analogues to investigate the link between inertinite attributes and the development of coke strength attributes, and
• Investigation into whether laboratory carbonising test results reflect the impact of maceral associations on coke strength using coal grain analysis.

Three coals were selected:

• Coal A, a high rank coal from the Moranbah coal measures;
• Coal B, a medium rank coal from the Moranbah coal measures;
• Coal D, a medium rank coal from the Rangal coal measures.

The key findings of the first project component, whereby inertinite analogues (graphite and charcoal) were used to investigate the link between inertinite attributes and coke strength, were:

• Graphite showed higher interfacial boundary quality with the RMDC than charcoal, which is likely owing to its ability to provide pathways for the volatiles generated during coking to escape but they then become trapped within the melt.
• Unlike graphite, charcoal was found to have bottle-neck shaped pores at its surface, which may slow the release of gas out of the pores. The combination of charcoal's high surface area and bottle-neck pore shape is expected to trap gas, to the extent that the charcoal is less available to bond to the metaplast during coking.
• The thermoplastic behaviour of blends of each of the three parent coals with 5 wt. % charcoal, graphite or a 50:50 mix of the two.

The key findings of the second project component, whereby the impact of coal grain composition on laboratory carbonising and coke strength test results was investigated, were:

• Coals A and D showed an increase in vitrite (i.e. liberated vitrinite) in the Gieseler and dilatation samples compared with the coke oven feed. However, the reverse trend was observed for Coal B. Five additional samples were prepared for Coal B, which were anticipated to provide insights for the unexpected result.
• These additional 'grind' samples were prepared so that the overall composition and grind stayed the same but the proportions of vitrinite-rich and inertiniterich material at each size fraction differed.

The key recommendations are:

• Continued efforts to understand the fundamental factors contributing to the complex phenomenon of coal thermoplasticity, including its dependence on coal geographical origin, which is critical to ensure accurate valuation of Australian coals in a competitive market.
• In this project, Gieseler and dilatation tests were unable to detect differences in coal thermoplastic behaviour and coke strength as a function of coal grind characteristics. Herein, it is important to understand what these standard laboratory tests actually measure, and how these test results relate to actual thermoplastic behaviour of coals and coal blends during commercial cokemaking.

Other recommendations to improve the experimental procedure and analysis of the results are detailed within the report.