Page 246 - Biosystems Engineering

224 Cha pte r Se v e n

Reactor
Type Advantages Disadvantages
Ablative Can process feedstocks Mechanical char abrasion is very
with large particles sizes important
Has compact design Heat supply is difficult
Circulating Has high heat-transfer Char abrasion is very important
fluid bed rates Needs very high volumes of
The solid is recycled carrier gases
Fluid bed Has high heat-transfer Char abrasion is not very important
rates Uses large volumes of carrier gases
Particle size limit <2 mm
Vacuum Can process feedstocks System is operated under vacuum
moving bed with large particles Has a relatively low external heat
pyrolysis Does not have carrier gas transfer coefficient

TABLE 7.2 Characteristics of Some Fast Pyrolysis Reactors

(Meier and Faix 1999; Bridgwater and Peacocke 2000; Czernik and
Bridgwater 2004; Kersten et al. 2005; Mohan et al. 2006).
The most important fast pyrolysis reactors developed so far are
(1) fluidized beds, (2) circulating fluid beds, (3) ablative pyrolysis,
and (4) vacuum pyrolysis (Scott et al. 1999). Table 7.2 shows some of
the most important features, advantages, and disadvantages of these
reactors (Bridgwater et al. 1999, 2001).
Bubbling fluidized beds are the most popular fast pyrolysis reactors
in operation today. These reactors are easy to operate and can be read-
ily scaled up (see Fig. 7.8) (Vanasse et al. 1988; Bridgwater et al. 1999;
Huber and Dumesic 2006). Almost 90 percent of the heat is trans-
ferred by direct contact between the biomass and the fluidized-bed
material (usually sand). The convective heat transfer from the gas only
represents around 10 percent of the total heat transferred (Bridgwater
et al. 1999). Basically, these systems are formed by a pyrolysis reactor;
one or two cyclones to separate the char particles and condensers
where the oils are collected (see Fig. 7.8). The separation efficiencies
of cyclones are not high enough to retain all the charcoal Thus, unless
hot filtration is used, char particles will find their way into the bio-oil.
The alkalis contained in the charcoal will be leached out, additionally
decreasing bio-oil thermal stability. The charcoal or the gases can be
used to supply the energy needed in the process. Better temperature
control, efficient heat transfer, and short residence times for vapors
are the main advantages of fluidized beds. The main drawback of this
technology is the use of large volumes of carrier gas and very small
particles (diameter less than 2 to 3 mm). This type of reactor is not well
adapted to operate as part of mobile units. The high gas-to-biomass