CFD Modeling and Simulation on Hydrodynamics of CFB Biomass Gasifier using FLUENT
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Date
2011-07
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Addis Ababa University
Abstract
Biomass gasification in Circulating fluidized beds (CFB) is one of the most promising
conversion processes in meeting future ecologically compatible and sustainable energy demand,
based on a combination of flexibility, efficiency, and environmental acceptability. Widespread
industrial use of CFB technology in coal and biomass combustion and gasification depends on
improved control of the fluidization process that demands a better understanding of fluidized bed
hydrodynamics. However, its application to CFB systems is limited due to the high
computational requirements for understanding complex fluid flow behavior. The effect of
different operating parameters on the hydrodynamic behavior of a two-phase gas-solid CFB
biomass gasifier were studied in this thesis work systematically using computational fluid
dynamics (CFD) software FLUENT.
Both 2-D and 3-D computational fluid dynamics (CFD) model based on Eulerian-Eulerian
approach coupled with granular kinetic theory is developed to simulate the hydrodynamics and
flow structures of the CFB under different operating conditions. A parametric analysis is
performed to comprehensively investigate the influences of particle properties (Bagasse, sand
and its sizes), operating parameters (gas velocity and solid circulation rate) and gasifier
geometries (inlet structure/position).
The dynamic characteristics obtained from CFD simulation have been compare
ed with the some similar conditions experimental data obtained from open literatures and in
general a good agreement has been observed. Eulerian-Eulerian granular multiphase flow model
approach is capable of predicting the core-annulus structure in CFB. The lower region of the
CFB riser is denser than the upper-dilute region. Back-mixing behavior or accumulation of
particles near the wall has been perfectly exist in the CFB. Increasing solids flux slows down the
flow development, but increasing superficial gas velocity makes the flow development faster.
Superficial gas velocity has a strong influence on the axial solids velocity. The volume fraction
of big particles is lower in the upper. This hydrodynamics model provides the powerful
theoretical basis for next steps on CFB designs. This study proposes that the operating
parameters (solid circulation rate and gas velocity) and solid inlet geometry may be the critical
consideration in order to reduce non-uniform distribution of gas and solid in CFB gasifier.
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Process Engineering