Examination of Contraction Pre and Post Resolidification Using a High Temperature Dilatation Rig

This project examined the contraction of coking coal samples, both pre-resolidification and post-resolidification. The means for performing the study consisted of:

• An in-situ dilatation/contraction rig;

• Graphite crucibles in a muffle furnace;

• TGA mass loss measurements;

• Gieseler and dilatometer tests; and

• A detailed mathematical model.

In previous observations some high fluidity coals appeared to exhibit significant contraction before resolidification had occurred in an earlier version of the in-situ rig. Should such contraction occur, it may be a factor to take advantage of in blends, to aid in control of oven wall pressure, pushing force etc.

After significant modification of the in-situ rig it was determined that pre-resolidification contraction is present for most of the coals studied here, but the magnitude of this contraction varied considerably. The highest fluidity coal exhibited the highest such contraction. The small crucible results, combined with TGA measurements, showed there was a clear relationship between volatile mass loss and contraction. They also supported the results of the in-situ rig contraction measurements, in terms of pre-resolidification contraction, although there was no clear correlation between the in-situ rig results and the graphite crucible results.

The measurements were supplemented by the model, which also was able to “predict” pre-resolidification contraction, by fitting the model predictions to the measured data. The model incorporates a large amount of detail regarding the fundamental phenomena involved in carbonisation, including vitrinite particles swelling due to bubble growth within the particles, bubble coalescence within particles, particles bursting to cause fusion of the RMDC, gas pressure in the connected porosity between particles and gas flow through that porosity. Finally, contraction of the solid phase was related to the mass loss measured in the TGA. A key finding from the model analysis is that the pre-resolidification contraction in the in-situ rig occurs due to the fusion of the softened vitrinite particles when the particles burst, and the temperature at which this phenomenon occurs, in relation to the resolidification temperature. The coal that had the largest amount of pre-resolidification contraction exhibited the largest temperature difference between these two events, and a general relationship between the predicted temperature difference and the amount of pre-resolidification contraction appears to exist. This fundamental behaviour seems to be of some importance and deserves to be investigated further.

Measurement in graphite crucibles was found to be a better method for measuring overall semicoke contraction than measurement in the in-situ rig. The former method is much more straightforward and the results more meaningful than the latter, but lack the detailed insight provided by the in-situ rig, which is challenging to optimise.