Page 193 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

170 Biomass Gasification, Pyrolysis and Torrefaction

5.6.2 Bubbling-Bed Pyrolyzer
Figure 5.9A shows a bubbling fluidized-bed pyrolyzer. Crushed biomass
(2 6 mm) is fed into a bubbling bed of hot sand or other solids. The bed is
fluidized by an inert gas such as recycled flue gas. Intense mixing of inert
bed solids (sand is commonly used) offers good and uniform temperature
control. It also provides high heat transfer to biomass solids. The residence
time of the solids is considerably higher than that of the gas in the pyrolyzer.
The required heat for pyrolysis may be provided either by burning a
part of the product gas in the bed, as shown in Figure 5.5, or by burning
the solid char in a separate chamber and transferring that heat to the
bed solids (Figure 5.7B). The pyrolysis product would typically contain
about 70 75% liquid on dry wood feed. As shown in the figure, the
char in the bed solids acts as a vapor-cracking catalyst, so its separation
through elutriation or otherwise is important if the secondary cracking is
to be avoided to maximize the liquid product. The entrained char particles
are separated from the product gas using single- or multistage cyclones.
A positive feature of a bubbling fluidized-bed pyrolyzer is that it is relatively
easy to scale up.

5.6.3 CFB Pyrolyzer

A CFB pyrolyzer, shown in Figure 5.7B, works on the same principle as the
bubbling fluidized bed except that the bed is highly expanded and solids
continuously recycle around an external loop comprising a cyclone and loop
seal (Basu, 2006, p. 35). The riser of the CFB operates in a special hydrody-
namic regime known as fast bed. It provides good temperature control and
uniform mixing around the entire height of the unit. The superficial gas
velocity in a CFB is considerably higher than that in a bubbling bed. High
velocity combined with excellent mixing allows a CFB to have large through-
puts of biomass. Here, gas and solids move up the reactor with some degree of
internal refluxing. As a result, the residence time of average biomass particles is
longer than that of the gas, but the difference is not as high as it is in a bubbling
bed. A major advantage of this system is that char entrained from the reactor
is easily separated and burnt in an external fluidized bed. The combustion heat
is transferred to the inert bed solids that are recycled to the reactor by means
of a loop seal.
Rapid thermal pyrolysis (RTP), a commercial process developed by
Ensyn of Canada probably originated from the ultrarapid fluidized-bed
pyrolyzer developed at the University of Western Ontario in Canada. RTP
uses a riser reactor. Here, biomass is introduced into a vessel and rapidly

heated to 500 C by a tornado of upflowing hot sand; it is then cooled

within seconds. The heating rate is on the order of 1000 C/s, and the
reactor residence time is from a few hundredths of a millisecond to a