Page 195 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

172 Biomass Gasification, Pyrolysis and Torrefaction

gas containing bio-oil vapor leaves through another tube, while the solid
char and sand spill over the upper rim of the rotating cone into a fluidized
bed surrounding it, as shown in Figure 5.7E. The char burns in the fluid-
ized bed, and this combustion helps heat the cone as well as the solids
that are recycled to it to supply heat for pyrolysis. Special features of this
reactor include very short solids residence time (0.5 s) and a small gas-
phase residence time (0.3 s). These typically provide a liquid yield
of 60 70% on dry feed (Hulet et al., 2005). The absence of a carrier gas
is another advantage of this process. The complex geometry of the system
may raise some scale-up issues.

5.6.7 Vacuum Pyrolyzer
A vacuum pyrolyzer, as shown in Figure 5.7F, comprises a number of

stacked heated circular plates. The top plate is at about 200 C while the

bottom one is at about 400 C. Biomass fed to the top plate drops into suc-
cessive lower plates by means of scrapers. The biomass undergoes drying
and pyrolysis while moving over the plates. No carrier gas is required in
this pyrolyzer. Only char is left when the biomass reaches the lowest plate.
Though the heating rate of the biomass is relatively slow, the residence
time of the vapor in the pyrolysis zone is short. As a result, the liquid yield
in this process is relatively modest, about 35 50% on dry feed, with a high
char yield. This pyrolyzer design is complex, especially given the fouling
potential of the vacuum pump.

5.7 PYROLYZER DESIGN CONSIDERATIONS

This section discusses design considerations in the production of liquid fuel
and charcoal through pyrolysis.

5.7.1 Production of Liquid Through Pyrolysis
Pyrolysis is one of several means of production of liquid fuel from biomass.
The maximum yield of organic liquid (pyrolytic oil or bio-oil) from thermal
decomposition may be increased to as high as 70% (dry weight) if the bio-
mass is rapidly heated to an intermediate temperature and if a short residence
time in the pyrolysis zone is allowed to reduce secondary reactions.
Table 5.2 gives the effect of heating rate, pyrolysis temperature, and resi-
dence time on the pyrolysis product. These findings may be summarized as
follows:
A slower heating rate, a lower temperature, and a longer residence time
maximize the yield of solid char.