BCIA’s annual program of postgraduate research scholarships is part of our commitment to strategic investment in skills development to secure the scientific, engineering and trades expertise required for the development of new low-emissions brown coal technologies.

To date, BCIA has awarded six research scholarships to PhD candidates at top-ranking Australian universities.

In this edition of
Perspectives on Brown Coal, BCIA scholarship recipient, Hui-En Teo from Melbourne University, provides an update on her PhD research project below.

A Novel Dewatering Approach to Victorian
Brown Coal

By Hui-En Teo, Melbourne University PhD Candidate and BCIA Scholarship Awardee

The brown coal mined in the Latrobe Valley, Victoria, is abundantly available and contains remarkably low levels of inorganic impurities (ash < 5wt%). The biggest challenge is that it contains up to two-thirds water, which reduces the heating value of the coal.

It is therefore highly desirable to develop a continuous, economical technology that could be retrofitted with minimal impact to existing power stations.

Such a technology would not only upgrade the quality of the brown coal boiler feed, but could also potentially create an opportunity for large-scale exportation of Victorian brown coal.

A wide variety of processes have been developed for dewatering or drying brown coal, but none have stood out as being particularly economically optimal. My project involves an investigation of dewatering using a simple modification of existing high pressure grinding rolls (HPGRs).

In standard operating conditions for HPGRs, the feed enters from above and is forced through the gap between two rollers, which usually rotate at equal speed.

While this would expose the coal to a compressive force, there is also a lot of slippage, so the pressures experienced are relatively low.

My work focusses on the case where one of the rollers turns more slowly than the other. This creates an environment of combined shear and compression which is more effective for dewatering the coal.

My goal is to develop a fundamental understanding of the forces acting on the brown coal particles under these conditions and to develop a modelling approach that can be used for process design and scale-up.

Brown coal represents quite a complex system for study. It is more liquid than solid, and can be regarded as a saturated gel network of humic acid molecules. The water is held within the structure in three forms:

1. chemisorbed, which is bound to brown coal through hydrogen bonds,
2. adsorbed, which loosely attaches to surface sites of the brown coal, and
3. bulk water, that exists outside of brown coal pores.

Currently, the science of suspension rheology can be used to account for the movement of water when brown coal is exposed to either shear or compression forces in isolation. However, the behaviour under a combination of shear and compressive forces is not well understood.

The greatest challenge I face is to develop robust experimental methods that allow precise control over loading conditions, enabling the realisation of fundamental material properties. I have tried several different approaches, and I am now working to develop the theoretical basis for future work using a model coagulated inorganic system.

To date, I have discovered that the results of one-dimensional shear stress relaxation tests are dependent on the strain rate used. Thus, when dealing with shear related mechanisms including combined shear and compressive forces, it is also necessary to control the shear strain rate applied. This knowledge will be used in future experiments concerning brown coal.

A number of conference presentations have been given this year with regards to the findings on superposed shear and compression. Two poster presentations have also been exhibited at various conferences; at the Australian Colloid and Surface Science Student Conference in Newcastle, NSW in February, and more recently in September at the Suspension Processing and Suspension Engineering Rheology meeting in Cambridge, UK.

In the following months, the novel characterisation techniques will be fine-tuned and employed on brown coal and a model coagulated inorganic suspension to test their yielding behaviour under varying combinations of mechanical forces. It is expected that experimental outcomes will contribute significantly to improve the fundamental understanding of multi-dimensional suspension rheology.

This fundamental information will then be useful for process optimisation of brown coal under mixed loadings, as well as for many other pertinent industrial materials, such as mineral tailings, sludges and pharmaceutical products.

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LEFT: Brown coal boilers on site at EnergyAustralia Yallourn power station in Latrobe Valley, Victoria
RIGHT: Post-crushing brown coal feed as it enters the boiler at EnergyAustralia Yallourn power station in Latrobe Valley, Victoria