Macroscopic modeling of fluid dynamics in large-scale circultaing fluidized beds

Licentiatavhandling, 2005

Satisfactory knowledge of the fluid dynamics governing the gas-solid flow in large-scale Circulating Fluidized Bed (CFB) units is still lacking, although the CFB technology is widely used for power (and heat) generation. Due to the complex two-phase flow in large CFB units fluid dynamical modeling of practical use is so far limited to a macroscopic approach and available models generally focus in describing one of the following key-phenomena: main gas-solid flow pattern, mixing of fuel particles and gas mixing. The two first phenomena are addressed in this work. Firstly, a model for the macroscopic gas-solid flow pattern of the entire circulating loop of a large-scale CFB unit is presented. The model aims at a solid base for future development of a comprehensive CFB model including combustion and heat balance. The fluid dynamical model is established by linking a selected set of submodels of particular zones or phenomena in the CFB unit. The submodels were taken both from literature as well as developed for the present purpose. Modeled results show good agreement when compared with experimental data from large-scale industrial CFB units (ranging from 30 to 226 MWth). The second phenomenon addressed is fuel mixing, which has important implications for further development of the proposed model. The fuel mixing has been studied through digital image analysis by means of a novel particle tracking technique applied to a 2-dimensional cold CFB unit with the tracer particle simulating a fuel particle (with respect to size and density). Characteristic flow patterns of the tracer particle under different fluidization regimes are identified and the dependence of the dispersion coefficient on main operational variables has been studied.