Improved Laboratory Methods to Evaluate Properties of Coking Blends

A range of Australian coking coals were blended and their behaviour during carbonisation examined to see if their fusibility was affected by interactions between them. The effects of additives, such as petroleum coke and charcoal, on the fusibility of blends were also investigated. Fusibility was measured by both Gieseler plastometry and proton magnetic resonance thermal analysis (PMRTA). PMRTA was found to give better repeatability of the magnitude of interactions than did Geiseler plastometry.

Interactions were found and quantified and mechanisms for the interactions were proposed and tested. Both positive interactions (degree of fusion of the blend greater than expected if the coals had acted independently) and negative interactions (degree of fusion of the blend less than expected if the coals had acted independently) were found. If interactions occurred between two coals, their magnitude was greatest when they were in approximately equal proportion in the blend.

The degree of interaction generally increased with increasing differences in rank between the coals being blended. Coals of similar rank did not show any evidence that they interacted. Both semi-anthracite and petroleum coke were found to reduce the fusibility of bituminous coals by more than if they behaved simply as diluents.

The interactions appear to be mediated by gas-phase transfer of volatile material between the coals.

It is concluded that because interactions were so common, attempts to accurately predict the behaviour of blends under carbonisation conditions from the behaviour of the component coals under the same conditions require both theoretical models to predict the properties of the blend in the absence of interactions and experimental work to identify specific interactions. However, it appears that once the magnitude of an interaction between two coals has been established for one blend of the two coals, the magnitude of the interaction can be predicted over the full range of blend compositions of these coals.

Cokes prepared on a laboratory scale were characterised by their reactivity to carbon dioxide and microstrength. The variation of these coke properties with blend composition was studied to see if their properties were affected by the interactions observed in the their fusibility behaviour. Intrinsic reactivity was found to be largely linear with blend composition and microstrength index appeared to be most strongly related to coke density. Thus it was not established whether the interactions that occur between coals during carbonisation impact on these coke properties.