Coal Fluidity, Stockpiling and Coke Quality

One perception in the marketplace is that coking coal quality decreases during transport and stockpiling due to ageing. This ageing is often estimated using plasticity measures with the assumption that plasticity is an indicator of coal oxidation and, therefore, an indicator of potential adverse effect on coke quality. The project addressed the question of "What is the most appropriate measure of coal plasticity to assess change in coke quality?". Specifically a high volatile Australian coal was allowed to naturally age (oxidise). Various coal plasticity measures were evaluated along with the resultant coke quality. In addition, the interaction of the aged coal with fresh coal was studied.

The coal was aged within 200L drums inside an enclosed building. The ageing technique used was designed to be repeatable and should not be considered to be identical to stockpile ageing. The ageing time of samples stored in drums may not be the same as for coal stored in a stockpile. Both traditional and non-standard tests were conducted as the coal was aged. Traditional plasticity measures used included Gieseler plastometry, Ruhr Dilatation and Crucible Swelling Number (CSN, also called Free Swelling Index - FSI). The two non standard techniques used were - Proton Magnetic Resonance Thermal Analysis (PMRTA) and fluorescence. Each coke sample, produced in a 300kg pilot scale coke oven, was characterised by coke mean size, ASTM Stability and Hardness, JIS indices, Micum/Irsid indices and CSR/CRI.

Plasticity of the aged coal, as determined by Gieseler plastometry and Ruhr dilatation showed a significant, rapid decrease in plasticity with ageing. PMRTA showed a steady decrease in Fmax over the period of the study. Plasticity of the blend decreased, to a lesser degree, during the study. Results indicated that adding aged coking coals to fresh coal blends affected the plasticity of the blend only to the extent that the plasticity of the aged coal was affected.

Cold coke quality indices did not change significantly throughout the program for either the coke produced from the aged coal (aged coke) or the blend coal (blend coke), despite a significant change in the plasticity of the aged coal. As such, it was concluded that no relationship existed between coal plasticity and cold strength indices for the coals during the period of the study.

There was, however, a significant change in hot strength of the aged coke, measured by the NSC CSR/CRI test, during the study. The CSR/CRI was constant for the first 51 days of the program, even though the traditional plasticity measures showed a significant change, before decreasing. The best relationship between coal plasticity and hot coke strength was a linear relationship between the PMRTA Fmax and CSR/CRI indices. The change in blend coke CSR/CRI was less than the change in aged coke CSR/CRI. The aged coal was used in the blend at a level of 30%

It was concluded that changes in traditional plasticity measures could not be used to reliably predict changes in resultant coke quality for either the individual coal or blend. There was no significant change in cold coke strength for coke produced from either the aged or blended coals, despite significant changes in plasticity measures. One non-standard plasticity measure (PMRTA) showed a relationship to the change in hot coke strength for the aged coke. Results were less conclusive for the blend. It was unclear why PMRTA Fmax followed the change in hot coke strength better than the traditional plasticity measures. This is an area that could be explored further. The effect of the aged coal in a blend appeared to be a proportional effect.