Fluid Dynamics of Fluidized Bed Reactors: Experiments and Simulations with System Interactions

Doktorsavhandling, 2005

This thesis is a study of the interaction between the fluidized-bed dynamics and the pressure and flow pulsations from the air-supply system. The interaction has been identified as a key process to understand in order to correctly formulate inlet boundary conditions in numerical modelling of fluidized beds, operated under industrial conditions. The interaction between pressure waves, resulting from bubble activities in the bed, with pressure and flow pulsations from the air-supply system is recognized as a main mechanism that gives rise to formation of a coupled system, consisting of fluidized bed, air plenum, pipe(s), flow control devices and fan. The interaction between the bed and its air-supply system is studied experimentally, analytically and numerically. The experiments have been performed in a 1/9th scale model of the 12 MW Chalmers research boiler and in a larger cold circulating fluidized bed unit. Simultaneous measurements have been carried out of pressure fluctuations in the bed and in the air plenum and of flow fluctuations in the air-supply ducts. In the scale model, bubble dynamics have been measured by an optical probe. The interaction is shown to depend on the configuration of the air-supply system, such as lengths of pipes, characteristics of fluidizing fan and location of flow control devices. A general model of the response of the fluidized bed to disturbances has been formulated, and the information provided by the model with respect to the dynamics of the bed, the bed plus the air plenum, and the bed plus the entire air-supply system, has been investigated. Different modes of bubble flow, responsible for the creation of pressure waves in the bed, have been studied in the time-frequency plane (wavelet analysis), aiming at localizing the phenomena of interest in time. A robust time series analysis method for evaluating and quantifying the bed dynamics (from measurements of fluctuations of pressure in the bed and of local solids concentration) has been obtained. The method is shown to be applicable in cases where, for various reasons, long data records may not be available. A model of the air-supply system has been coupled with three-dimensional numerical simulations of the flow field in the bed and the air plenum. It is shown that the assumption of a spatially uniform inlet condition of the gas flow over the air distributor, assumed in most previous fluidized-bed studies, is not appropriate.