Investigation of the Links Between Microstructure Development in Softening Coal and the Characteristics Controlling Coke Quality

In an earlier project (C23048) computed tomography determination of three dimensional structure of coke was developed as a tool for understanding the impact of coke microstructure on coke strength. This project provides insights for the development of improved prediction of the coking behaviour of coals and how to optimise blends. In particular, it further developed understanding of the relationships between key microstructural features of coke and coke failure mechanisms and strength indices, microstructure features of carbonisation and how different inertinite types can influence structure development by modifying processes in the plastic layer.

A series of coals were studied in a high temperature rheometer at different temperatures and partially coked in a sole heated oven (SHO) under different confining loads. Micro-CT analysis was performed on both sets of samples to determine structural characteristics.

In the SHO, little direct relationship between pore size (connected pore) in the plastic layer or the softening coal and available coal properties was observed. Neither volatile matter nor vitrinite showed a reliable correlation with change in pore diameter between the plastic layer and the semicoke. The increase in confining pressure had small effects on the pore properties of the samples. In general similar trends were seen in results from the rheology experiments. Comparison of the results between the rheometer and the SHO showed a number of similarities supporting the combined use of techniques to better understand the mechanism of coking. Bubble nucleation and growth is broadly understood but there are still questions about the mechanisms of transition to semicoke and the densification that occurs.

It is recommended in the future that, unless mass of samples is limited, the 4kg coke oven at Newcastle should be used for quenched plastic layer samples.

A novel technique was developed for controlled studies of the interface formed during coking between vitrinites and inerts from different coals. The technique allowed the porosity of the interface as a function of inert source and vitrinite source to be estimated from CT scans. Some clear differences were seen between samples indicating that binding of inerts into the coke structure can vary with the choice of both the inert and vitrinite, i.e., the results suggest that the nature of the interfaces between vitrnite and inert depend on both the inert and vitrinite chosen; that is vitrinites are not the same in their ability to bind inerts, and inerts are not the same in their ability to interact with vitrinite. Combined with other novel techniques for characterising inerts it may be possible to determine the source of differences in the ability of coals to bond inerts into cokes.