Fusion of Coking Blends

The results from a previous ACARP project, "Improved Laboratory Methods to Evaluate Properties of Coking Blends", provided the first direct evidence that when blends of some coals were heated, the coals interacted in a way that affected the extent to which the blends fused. However, the mechanism by which these coals interacted and what impact these interactions had on the properties of cokes made from these blends were unknown.

The objectives of this current work were to elucidate the nature of the interactions that occur between coals in blends during heating and to relate the fusion behaviour of the blends to the properties of the cokes formed from them.

A number of blends of selected coals were examined by proton magnetic resonance thermal analysis (PMRTA) in order to determine relationships between the properties of the components of the blend and the magnitude of the interaction observed between them. The impact of these interactions on coke properties was studied by petrographic examination and reactivity measurements of cokes made from these blends in the laboratory. At the same time, Broken Hill Propriety Co Pty Ltd (BHP) was investigating the effect of varying blend composition on coke properties. Samples of the coals used, and the values of the properties of the cokes formed from their blends were provided to CSIRO so that the relationship between coal and coke properties could be more fully explored.

The major findings were:

• The magnitude of the interaction appeared to be determined solely by differences in the rank and type of coals being blended: the fusion behaviour of coking blends appears to be predictable from the fusion behaviour of the component coals.
• The extent of individual interactions between any given pair of coals in ternary blends was proportional to the product of their proportions in the blend
• Interactions between coals affected the microscopic appearance and reactivity of cokes prepared on a laboratory-scale.

All of the interactions observed can be explained as resulting from physical transfer of volatile material between the coals in the blends during their pyrolysis.

For the blends coked at BHP, rank and type played the major role in determining the coke reactivity index and were important in determining the mean coke size and coke strength after reaction, indicating that rank/type diagrams would be useful in predicting these parameters. Rank and type were not important in determining the surface breakage rate constant.