Macroscopic modeling of fluid dynamics in large-scale circultaing fluidized beds
Licentiate thesis, 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.
large-scale circulating fluidized bed