|Steam fluidized bed drying of Victorian brown coal|
|By David Stokie, PhD Student in Energy, Fuels and Reaction Engineering Group, Department of Chemical Engineering, Monash University|
|Coal drying and upgrading is of growing commercial importance in Australia. The physical and chemical changes that that coal undergoes through these processes can have dramatic effects on the end product, in terms of coal handling and moisture re-absorption. David Stokie provides an update on his PhD investigations into steam fluidised bed drying of coal.|
|Brown coal represents a large resource for Victoria, with reserves sufficient for over 500 years at the current rate of consumption. However, the high moisture content of Victorian brown coal impedes its efficient utilisation. Current estimates show that the energy needed to dry the coal represents 20 – 25 per cent of the fuel’s heat of combustion. |
Coal drying is essential for improving the efficiency of brown coal power plants and processes. Many different drying technologies have been successfully commercialised or are in the late stages of development, however none have succeeded in completely addressing the environmental, economic and efficiency challenges inherent in power generation from moist, low rank coals.
The wide range of methods is due to the heterogeneous nature of low rank coals, which vary in moisture content and in physical and chemical composition. Due to the specific nature of the drying technologies, many of which have been developed with a particular fuel in mind, an ideal drying technology for one low rank coal may be inefficient and ineffective for another.
Steam fluidised bed drying (SFBD) was originally developed at Monash University by Professor Owen Potter in the 1970s. While the original process involved a single stage fluidised bed, many variations have been developed subsequently to increase the drying efficiency.
RWE in Germany has developed the internal waste heat utilisation system (WTA), which uses slightly superheated steam to dry the coal. Victorian brown coal power stations do not produce superheated steam, but it would be possible to integrate SFBD into a power station by using low grade steam, resulting in a cycle which has comparable or lower energy requirements for moisture removal.
Information on drying kinetics, chemical composition, physical changes, and the resultant combustion properties are not readily available. Investigating these factors will give a greater understanding of the practicality of steam fluidised bed drying for Victorian brown coal.
|Figure 1: Continuous fluidised bed|
|Such information is also required for scale up and eventual commercialisation of steam fluidised bed drying in Victoria. Investigation of steam fluidised bed drying at Monash University has involved the design and building of several small-scale fluidised beds (see Figure 1) and has been split into several objectives:|
|Analysis of the physical characteristics of fluidised bed dried Victorian brown coals|
|Changes in the internal structure of the coal particles during drying are not well understood. The surface area and porosity of the coal may affect the rate of moisture re-adsorption after drying.|
Particle attrition of Victorian brown coal is a significant issue for the drying of brown coals. The vigorous nature of fluidisation, coupled with the comparatively weak structural strength of Victorian brown coals, has the capacity to cause a large amount of particle damage.
The decreasing particle size increases elutriation loss and changes the fluidisation characteristics in the bed. Understanding how the particle size changes as a function of moisture content, particle size and residence time is imperative to better control of fluidised bed conditions.
|Analysis of the chemical characteristics of fluidised bed dried Victorian brown coals|
|During the drying process, it is expected that Victorian brown coal will undergo a change in chemical composition, which may affect downstream processes. |
There is limited information available on the changes in chemical composition during drying, particularly to oxygen functional groups, and how this is affected by specific drying variables. We are working to develop a better understanding of these changes.
Another form of chemical analysis of the dried coal generated is the effect of drying conditions on the impact of combustion and gasification reactivity.
Investigation of combustion and gasification at laboratory scale can be used as a measure of the effectiveness of the coal for power generation and production of value-added chemicals. Understanding and confirming the applicability of specific drying techniques is essential for use in an industrial implementation.
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