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

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