Page 192 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 5 Pyrolysis 169

TABLE 5.5 Effect of Operating Variables on the Pyrolysis Yield
To Maximize Maximum Heating Gas Residence
Yield of Temperature Rate Time
Char Low Slow Long
a
Liquid Low (B500 C ) High Short

Gas High Low Long
a Bridgwater (1999).
Source: Table compiled from Demirbas (2001).

had to be chosen accordingly. These choices also decided what kind of reactor
was to be used. Table 5.5 lists the choice of heating rate, temperature, and gas
residence time for maximization of the yield.
Modern pyrolyzers are more concerned with gas and liquid products,
and require a continuous process. A number of different types of pyrolysis
reactor have been developed. Based on the gas solid contacting mode, they
can be broadly classified as fixed bed, fluidized bed, and entrained bed, and
then further subdivided depending on design configuration. The following
are some of the major pyrolyzer designs in use:
Fixed or moving bed
Bubbling fluidized bed
Circulating fluidized bed (CFB)
Ultrarapid reactor
Rotating cone
Ablative reactor
Vacuum reactor.
Except for the moving bed, other pyrolyzer types are shown in Figure 5.7.

5.6.1 Fixed-Bed Pyrolyzer
Fixed-bed pyrolysis, operating in batch mode, is the oldest pyrolyzer type.
Heat for the thermal decomposition of biomass is supplied either from an
external source or by allowing limited combustion as in a beehive oven
(Figure 5.2). The product may flow out of the pyrolyzer because of volume
expansion while the char remains in the reactor. In some designs, a sweep
gas is used for effective removal of the product gas from the reactor. This
gas is necessarily inert and oxygen free. The main product of this type is
char owing to the relatively slow heating rate and the long residence time of
the product in the pyrolysis zone.