Development of a numerical model of the flow processes in a fluidised bed

In Australia and throughout the World coal provides a major source of thermal energy for the generation of electricity and will continue to do so for the foreseeable future. Victoria has large reserves of Low-Rank coal but its use in current pulverised coal fired furnaces results in large capital intensive plant with low thermal efficiencies. Fluidised bed technology in the form of coal gasifiers and combustors has the potential to improve efficiency, whilst reducing emissions and capital cost. Before fluidised bed technologies can be widely adopted reliable techniques for predicting system behaviour and scaling plant designs need to be developed. Traditionally scaleup techniques such as dimensional analysis suffer from the strong non-linearity inherent in fluid dynamics. Computational Fluid Dynamics (CFD) is one technique which has a demonstrated ability to predict the behaviour of complex flows for applications with a single fluid and is being applied to an increasing number of multiphase flow applications. CFD has previously, in a limited way, been applied to fluidised bed systems but most work is of a research nature and uses specially written codes with simple rectangular grid systems. The present work develops a time dependant Eulerian-Eulerian finite volume CFD model for fluidised bed systems based on the commercial code CFX (formally CFDSFLOW3D). An advantage of adopting a modern commercial code is that complex geometry can be handled relatively easily and they contain efficient solver algorithms. From a research viewpoint this allows more time to be spent on improvements to the modelling technique and understanding the system's physics. However the major advantage is that it provides a much simpler means of adapting the model for use by plant designers and engineers. To model the hydrodynamics of fluidised beds in CFX requires the addition of constitutive relationships for physical properties which include the transfer of momentum between phases and solid pressure. To allow prediction of wall to bed heat transfer coefficients, a heat transfer model which includes constitutive models for thermal conductivity and interphase energy transfers are added. Extension of the hydrodynamic and heat transfer models by including chemical reactions provides the ability to simulate coal gasification in a fluidised bed and predict product gas composition. The constitutive terms are based on published models and experimental work. A number of improvements to the numerical technique are developed to improve accuracy of the model. Validation of the model is achieved by comparing model predictions with published experimental and numerical modelling results to obtain a degree of confidence in the model. Initially a one dimensional hydrodynamic model is used to demonstrate and validate both the solution procedure and the improved numerical techniques developed as part of this research project. An extensive study of bubble formation in a two dimensional bubbling bed demonstrates the ability of the model to predict bubble formation and the general behaviour of bubbling bed systems. The model is also used to assess the influence of numerical techniques including higher order differencing, asymmetry and the form of the momentum equations on the model's predictions. A complete isothermal Circulating Fluidised Bed system including riser, cyclone and return leg is modelled in two dimensions. The CFB simulation demonstrates that the model is capable of predicting on a qualitative basis the formation of clusters, coreannular flow structure and solids recirculation typical of CFB systems. The model's ability to handle three dimensions is demonstrated by modelling a cylindrical bubbling bed riser. Results for the model are in reasonable agreement with tomographic imaging data. Predicted values of the heat transfer coefficient between a heated wall and a two dimensional bubbling fluidised bed are in excellent agreement with other published models. Large values of the heat transfer coefficient typically associated with fluidised beds are predicted and found to vary rapidly as bubbles move through the system. The ability to couple chemical reactions to the hydrodynamic model is demonstrated by modelling a coal gasifier operating in a slugging mode in two dimensions. Outlet gas properties predicted by the model are in reasonable agreement with published experimental values and consistent with result for simple one dimensional gasification models. Changes in temperature and gas composition can effect the hydrodynamics of the gasifier and are accounted for by the model. From the extensive validation exercises the ability of the model to predict the qualitative behaviour of typical fluidised bed systems is demonstrated. Where possible quantitative validation is reasonable however is limited due to the lack of reliable high quality experimental data and the large computing requirements of the model. Animation of model results provides an insight into the complex behaviour of the fluidised bed plant which can not currently be obtained by other techniques.