Technical Market Support » Metallurgical Coal
The coke quality prediction models use thermoplastic terms as key explanatory variables. These terms are derived from standardised tests such as Gieseler fluidity and dilatometer. However, these tests require grinding coal to -400μm and -212μm which differ significantly from industrial operations. The use of pulverised samples in standard tests leads to readings that fail to provide a representative measure of the “real” thermoplastic behaviour of coals. This misrepresentation often leads to a misprediction of coke quality for coals from different measures and geological origins, including competing overseas coals. The issue stems from differences in coal petrography, grain size, maceral size distribution, and the degree of association between macerals, key parameters driving the volatile release, bubble formation and growth in the plastic layer, and ultimately the thermoplasticity of metallurgical coals. Therefore, a deeper fundamental understanding of fluidity development in coal with representative particle size distributions is key to improving coke quality prediction models.
The main objective of this project is to improve the fundamental knowledge of the link between coal grain composition and maceral associations and the thermoplastic behaviour of Australian coals.
The eight coal samples selected were grouped based on the differences as well as similarities in their petrographic and thermoplastic properties:
- Coals with similar vitrinite content and fluidity, but different ranks and dilatation.
- Coals with similar rank and vitrinite content, but different fluidity.
- Higher rank coal with no measured fluidity.
The research involved several complementary parts:
- CGA analysis at CSIRO to determine the maceral composition, size distribution, and association degree of reactive and inert macerals.
- Permeability and dilatation tests at UoN and rheometry tests at UQ to study the thermoplastic behaviour of coals at coke oven feed size. This included analysis of viscoelastic pathways and bubble formation and growth in the plastic phase.
- Larger scale coking tests in the 4kg coke oven at UoN for internal gas pressure (IGP) measurements.
- 3D micro-CT image analysis of quenched plastic layer samples to measure plastic range.
- Linking coal grain composition to the mechanism of thermoplasticity and pressure development to determine promoters/inhibiter of coal fluidity.
Results showed that in most cases standard fluidity and dilatation tests, due to using crushed samples, fail to capture the real thermoplastic behaviour of coals. 3D analysis of plastic layer micro-CT images demonstrated that the standard tests underestimate the plastic range of most coals, especially those containing composite grains.
The influence of coal rank and grain composition as key drivers of fluidity and dilatation at coke oven feed size was studied.
Outcomes suggest that coal grain composition affects the measured fluidity and dilatation by influencing the bubble formation and growth mechanism. In coals containing larger grains with purer reactive macerals, the entrapment of volatiles in the grain leads to the retention of liquid and bubble growth, thus enhancing fluidity and dilatation. In contrast, a higher content of fine composite grains leads to faster escape of volatiles, limited bubble growth, and lower measured fluidity, dilatation, and IGP.
A further key finding is that a low measured fluidity does not necessarily lead to a weak coke. Some tested coals with low measured fluidity and dilatation can form strong cokes. 3D micro-CT image analysis of these coals showed evidence of bubble formation in the plastic phase, which was not detected by the standard tests due to the impacts of composite grains described above.
The findings of this project showed that knowledge of coal grain composition and its influence on fluidity and dilatation behaviour at representative particle sizes provides a new fundamental understanding of drivers of thermoplasticity in coking coals.