Biomedical Engineering Reference
In-Depth Information
A kinetic model (nth-order, shrinking particle, and shrinking core) may
also be used to determine the residence time, the net solid holdup, and there-
fore the height of the dense bed.
8.8.2.5 Freeboard Height
Entrainment of unconverted fine char particles from the bubbling bed is a
major source of carbon loss. The empty space above the bed, the freeboard,
allows entrained particles to drop back into it. A bubbling, turbulent, or
spouted fluidized bed must have such a freeboard section to help avoid
excessive loss of bed materials through entrainment and to provide room for
conversion of finer entrained char particles. The freeboard height must be
sufficient to provide the required residence time for char conversion. It can
be determined from experience or through kinetic modeling.
A larger cross-sectional area and a taller freeboard increase the residence
time of gas/char and reduce entrainment. From an entrainment standpoint,
the freeboard height need not exceed the transport disengaging height of a
bed because no further reduction in entrainment is achieved beyond this.
8.9 ENTRAINED-FLOW GASIFIER DESIGN
Because the gas residence time in an entrained-flow reactor is very short—
on the order of a few seconds—to complete the reactions, the biomass parti-
cles must be ground to extremely fine sizes (less than 1 mm). The residence
time requirement for the char is thus on the order of seconds. Section 8.9.1
describes some important considerations for entrained-flow gasifier design.
Although an entrained-flow gasifier is ideally a plug-flow reactor, in
practice this is not necessarily so. The side-fed entrained-flow gasifier, for
example, behaves more like a CSTR. As we saw in Figure 8.15 , at a certain
distance from the entry point, fuel particles may have different residence
times depending on the path they took to arrive at that section. Some may
have traveled a longer path and so have a longer residence time. For this rea-
son, a plug-flow assumption may not give a good estimate of the residence
time of char.
8.9.1 Gasifier Chamber
Most commercial entrained-flow gasifiers operate under pressure and there-
fore are compact in size. Table 8.9 gives data on some of these operating in
the United States and China.
A typical downflow entrained-flow gasifier is a cylindrical pressure ves-
sel with an opening at the top for feed and another at the bottom for dis-
charge of ash and product gas. The walls are generally lined with refractory
and insulating materials, which serve three purposes: (i) they reduce heat
Search WWH ::




Custom Search