|Slag behaviour during entrained flow gasification of Victorian brown coals and other lignites|
|Ms Sunaina Dayal, PhD Student in Energy, Fuels and Reaction Engineering Group, Department of Chemical Engineering, Monash University|
|Oxygen blow, entrained flow gasification of brown coal may be an important future route to low-emissions products. This technology utilises relatively high temperatures, meaning that the ash content of the coal melts to form a flowing slag. Sunaina Dayal reports on her PhD project to investigate the behaviour of brown coal under the conditions that would be encountered in an oxygen-blown gasifier, and her results including predicting and modifying slag viscosity with flux agents.|
|Victorian brown coal is an economical and important source of energy for the state of Victoria, accounting for approximately 85 per cent of all electricity generated. At the current rate of consumption, these brown coal reserves are expected to last for more than 500 years.|
Conventional combustion of brown coal results in high CO2 emissions. However Victorian brown coal contains low levels of ash and is highly reactive once dried - attributes that make Victorian brown coal an ideal candidate for gasification, which can deliver a significantly lower CO2 footprint.
Gasification is a process where solid fuels like coal react with gases like air or oxygen, steam and carbon dioxide to produce fuel gas which can then be used directly for power generation, or as a raw material for manufacture of other chemicals or fuels.
Between 1992 and 2003, considerable research and development work was carried out on brown coal gasification at relatively low temperatures (around 900C) using pressurised fluidized beds and transport gasifiers. Due to the low temperatures, these processes suffer from low carbon conversion and agglomeration.
An alternative process is entrained flow gasification process, where under higher temperatures (resulting in a high carbon conversion) the ash fuses to a molten state to form a flowing slag, avoiding agglomeration. The liquid slag viscosity should be low enough to enable slag to flow down the gasifier walls and drain out from the gasifier in molten form.
The benchmark for continuous slag outflow is set by a maximum slag viscosity value of 25Pa•s (250 Poise).
Overall plant efficiency can be improved by lowering the ash flow temperature (AFT) below the operating temperature of slagging gasifiers (1400-1600oC) – fluxing agents such as limestone and clay can be blended with coals in order to achieve this.
Therefore, information on composition, viscosity and the change of viscosity with temperature for coal ash slags is essential for efficient operation of slagging gasifiers. Other assisting facets such as mineral phase transformations and trace element emissions also require further investigation.
|My project aims to generate this information by focussing on the following aspects:|
Design and construction of a viscosity measurement assembly
This assembly is constructed based on similar experiments previously conducted for high temperature viscometry. The key features of this specially designed assembly include a high temperature furnace, and a viscometer with a spindle and bob arrangement. This arrangement enables measurements of viscosity over a range of temperatures where the slag is expected to be in a molten state.
Examine applicability of existing viscosity models and development of a new model
|Existing viscosity models developed for high-rank coals have been applied to Victorian brown coals to determine their suitability. The predictions varied wildly for the three coals that were tested and trends provided by experimental results also differed largely from those given by these models. Therefore, the next step was to conduct further systematic experimental work to develop new models to yield results specific to these coals. |
|Examine applicability of existing viscosity models and development of a new model|
Existing viscosity models developed for high-rank coals have been applied to Victorian brown coals to determine their suitability. The predictions varied wildly for the three coals that were tested and trends provided by experimental results also differed largely from those given by these models. Therefore, the next step was to conduct further systematic experimental work to develop new models to yield results specific to these coals.
Investigate trends for brown coal using a thermodynamic software package
Various modules of a commercial software are used for estimating different trends for the ashes. The percentage of liquid and solid content at different temperatures, the typical phase changes, and changes in viscosity with temperature are predicted using this software. Effects of flux addition on these properties can also be examined through these modules.
Characterisation of ash and slags
Various analytical techniques such as X-ray fluorescence, In-situ high temperature X-ray diffraction, room temperature X-ray Diffraction, Hot Stage Microscopy, Thermal Analysis, and Scanning Electron Microscopy are used to provide important information such as ash composition, ash fusion temperatures, phase changes in the slag at different temperatures, and trace element composition.
This information along with the experimental and modelling work will allow the establishment of a temperature range for applying entrained flow gasification to Victorian brown coals. The lower the temperature that achieves full carbon conversion at an acceptable fuel gas quality, the better the overall efficiency.
Recently, a synchrotron application for high temperature in-situ x-ray diffraction was approved and experiments conducted for different ash samples including some with flux additives. These experiments will shed more light on the change of different mineral phases with increasing temperature in a CO2 environment and this information will then be related to viscosity changes of the ash as it transforms into slag.
This is the first-ever research project on oxygen-blown entrained flow gasification of Victorian brown coal. The results so far indicate that one coal may not require any fluxing material, while two others will benefit from the addition of calcium based flux. The type of flux and its proportion is being established through this project.
This project has international partnership with Mitsubishi Heavy Industries (MHI), Japan and we have also formed collaboration with Julich in Germany. Both MHI and Julich provide valuable industrial and scientific input into the project, assist in several analyses and measurements, and provide standards for comparisons for our own results.
My project complements two other BCIA-funded projects on entrained flow brown coal gasification carried out by Ms Joanne Tanner and Mr Tao Xu. These research projects are also first-of-their kind on entrained flow gasification of Victorian brown coals and are also supervised by Professor Sankar Bhattacharya.
|Above: Sunaina Dayal preparing for the experiments at the X-Ray Diffraction beamline at the Australian Synchtroton |