Structural Differences Between Coking Coals of the Sydney Basin and Other Sources

Cokes made from Australian coals of relatively low fluidity can have better strength and reactivity values than their equivalents prepared from European or American coals. Although this systematic difference is well known, it has long been an issue in contract negotiations in sales of Australian coking coals. There have been many attempts to identify the cause of this difference. Early work found that some inertinites in Australian coals are fusible to some extent, but this didn't resolve the problem since some inertinites in Carboniferous American and European coals were found to be fusible as well. There was some evidence that in some cases a greater fraction of the Australian inertinites was fusible than those sourced from other countries but the results were not unequivocal, and there was no mechanistic connection between this result and coke properties. The connection between inertinite fusibility, low fluidity of Australian coking coals and coke properties remains unexplained. These coals are the Permian coals from the Sydney and Bowen basins from Eastern Australia, which dominate Australian coking coal exports. In this report 'Sydney basin coals' is used as short hand to describe these coals.

Recently a fundamental, physical difference was found between the inertinites in Sydney Basin coals and Carboniferous coals from America and Europe. Using Small Angle Neutron Scattering (SANS) it was found that gases can flow into (and out of) nanopores, 10-20 nm in size, in inertinites in the Sydney basin coals; however these gases do not readily penetrate the same-sized nanopores in inertinites in American and European coals. These pores are much too small to be detectable by optical microscopy, which is one reason why this difference hasn't been observed before. The initial study was extended to include some West Canadian coals, which also have low fluidity and good coking propensity, and it was found that their inertinites were almost as permeable to gases as the inertinites in Sydney Basin coals. Thus the nanostructure of coals affects gas transport in the coal. Gas transport in the coal and coke during the coking process plays an important role in determining swelling, coke oven wall pressure and the properties of the final coke.

If the reason for the differences in coking behaviour of Sydney Basin and Carboniferous coals from Europe and America is due to the nanostructure of the inertinites affecting the flow of gas inside coal particles, then there are a number of questions arising from this interpretation. For instance:

1. What is the nanostructure of inertinites in coals from other regions? Is it like Sydney Basin coals, European coals or different again?

2. Can these regional differences in inertinites be picked up by other techniques? Gas sorption rates, for example.

3. Do these differences in structure carry over to the cokes?

This project was designed to answer these three questions.