| Summary: | Suspension-to-wall heat transfer in circulating fluidized beds is modeled considering
both the reactor-side and wall-side heat transfer processes. The overall flow structure in
fast fluidized beds is represented by a core-annulus flow pattern with a stagnant particlefree
gas gap between the wall and wall layer. Descending particles are assumed to enter
the heat transfer zone with the same temperature as the core suspension. As particles
descend in the wall layer, they lose heat to the gas by convection and gain heat from fresh
particles arriving from the bulk core region. Gas is dragged downwards in the heat
transfer zone by the rapidly-descending annular particles. The gas receives heat from the
immersed particles by particle-to-gas convection and from the core by conduction. Heat
is then conducted to the wall through the stagnant gas gap, and then through the furnace
wall to the coolant. The model is the first to include the coolant-side heat transfer in the
overall process. Particles also participate in radiation from the core to the wall through
the wall layer. They are assumed to constitute a gray continuous absorbing, emitting and
scattering medium.
The radiation heat transfer process is solved by the two-flux model in a twodimensional
model for CFB units with smooth walls, while the moment method is
employed for the three-dimensional case when membrane walls are present. Under highdensity
CFB operating conditions with smooth walls, the model is extended by allowing
the suspension in the vicinity of the wall to travel intermittently downwards and upwards
as is observed experimentally. The two- and three-dimensional models are validated
using experimental results from the literature and both yield satisfactory predictions of
the suspension-to-wall heat transfer. The influences of key parameters on the heat flux
are analyzed and are found to be consistent with experimental trends where these are
known. The simulation results suggest that the particles participate in a significant way in
determining the radiation flux through the wall layer. Therefore radiation cannot be
uncoupled from particle and gas conduction and convection without introducing
significant error for high temperature systems.
Experiments were conducted in the 76 mm diameter jacketed riser of a dual-loop
high-density CFB facility with FCC particles of 65 pm Sauter mean diameter as bed
material. The superficial gas velocity varied from 4 to 9.5 m/s and the solids circulation
flux was as high as 527 kg/m²s. The suspension temperature and the average and local
suspension-to-wall heat transfer coefficients were measured. The suspension temperature
distributions indicate that the particles in the vicinity of the wall do not move in one
direction only, but oscillate downward and upward, leading to higher local heat transfer
coefficients at the ends of the heated section. Experimental results also show that
suspension-to-wall heat transfer coefficients are strongly influenced by suspension
density. However, they are not significantly influenced by superficial gas velocity at a
constant suspension density. By superimposing the heat transfer results when the
suspension in the vicinity of wall is allowed to move downwards and upwards separately,
the model predicts the experimental results well.
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