The granules in the RFB are uidized layer-by-layer from
the free surface outward at increasing radius as the gas
velocity is increased. This uidizing phenomenon is very
different from that of the conventiona l bed that uidizes
completely at one aeration rate—the minimum uidizing
velocity (Umf ). The uid drag decreases with respect to
radius (r) while the centrifugal force is increasing. The
aeration corresponding to the initial uidization on the inner
free surface is called the surface uidizing velocity (Umfs),
and the aeration corresponding to complete uidization is
called the critical uidizing velocity (Umfc). The difference
between these two velocities increases with the bed
thickness. For a thick bed there is a possibility of partial
uidization where the bed near the surface is uidizing
while the bed near the distributor is still packed. In the
extreme case, particles near the surface might elutriate (if
the uidizing velocity exceeds particle terminal velocity)
while particles near the distributor might not yet be
uidized. Hence, the operation of the RFB incinerator is
limited by the bed thickness. It was found that in order to
achieve complete uidization of bed materials without
excessive particle elutriation, the bed thickness of the
200 mm ID RFB should be kept below 15 mm.
The major contribution to the pressure drop in the . xed
bed region is the drag force per unit volume of uid. In the
uidized bed region, the major contribution is the total mass
of particles. Based on the theoretical analysis developed by
Chen1