Page 84 - Wastewater Solids Incineration Systems

Combustion Technology 53

two size extremes will be in a state of fluidization. To avoid carry-over of solids from
a bed, the gas velocity for fluidized bed operations should be kept somewhere
between U and U . In calculating U , the mean particle diameter for the size distri-
mf t mf
bution actually present in the bed must be used. However, in calculating U , the
t
smallest size of sand solid in the bed must be considered.
For WWTP applications, the fluid bed incinerator is typically designed based on
a fluidizing gas velocity U equal to 2.5 to 3 U .
0 mf
2.5 Transport Disengaging Height

In a bubbling fluid bed, bubbles once formed in the bed start rising, grow in size, coa-
lesce, reach the bed surface, and finally erupt. When these bubbles erupt at the sur-
face they scatter solids into the region above. Erupting gas bubbles do splash solids
far into the freeboard or the region above the surface of the bed. If the gas exit were
situated immediately above the top of the bed, then the gas would entrain a consid-
erable amount of solids. With a higher gas exit, the amount of entrainment is smaller,
and finally a level is reached above which entrainment becomes approximately con-
stant. The entrainment increases with the gas velocity or when the size of solids
decreases. This height of exit above the top of the bed where entrainment becomes
approximately constant is known as the transport disengagement height (TDH).
Findings from Zenz and Othmer (1960) on TDH at different gas velocity are illus-
trated in Figure 5.4. The solids used in this research are a combination of sizes
between 20 and 150 μm. In this application, depending on the gas velocity, TDH
ranges from 0.2 to 0.3 m above the surface of the bed. The TDH increases by approxi-
mately 70% for a doubling in gas velocity.
The height of the freeboard of a WWTP fluid bed incinerator is typically 4.6 m
(15 ft) minimum (between the fluidized bed surface area and the exhaust gas duct). This
height is determined by the freeboard gas residence time of (minimum) 6.5 seconds.

2.6 Freeboard Gas Velocity

Zenz and Othmer (1960) (Figure 5.4) show that for a given solids and vessel, the
entrainment is not only sensitive to height of the freeboard, but also to gas spatial
2
4
velocity. The entrainment increases strongly with gas velocity. It varies as U to U .
f f
Freeboard gas velocity U is, therefore, an important parameter of design. In WWTP
f
applications, the freeboard gas velocity ranges from 0.64 to 0.76 m/s (2.1 to
2.5 ft/sec), calculated based on combustion gases. The velocity should be small