Page 194 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 5 Pyrolysis 171

maximum of 5 s, which gives a liquid yield as high as 83% for wood
(Hulet et al., 2005).

5.6.4 Ultrarapid Pyrolyzer
High heating rate and short residence time in the pyrolysis zone are two
key requirements of high liquid yield. The ultrarapid pyrolyzer, shown in
Figure 5.7C, developed by the University of Western Ontario provides
extremely short mixing (10 20 ms), reactor residence (70 200 ms), and
quench (B20 ms) times. Because the reactor temperature is also low

( B650 C), one can achieve a liquid yield as high as 90% (Hulet et al., 2005).

The inert gas nitrogen is heated at 100 C above the reactor temperature
and injected at very high velocity into the reactor to bombard a stream of
biomass injected in the reactor. The reactor can also use a heat-carrier
solid like sand that is heated externally and bombarded on a biomass stream
through multiple jets. Such a high-velocity impact in the reactor results in an
exceptionally high heating rate. The biomass is thus heated to the pyrolysis
temperature in a few milliseconds. The pyrolysis product leaves the reactor
from the bottom and is immediately cooled to suppress a secondary reaction
or cracking of the oil vapor. This process is therefore able to maximize the
liquid yield during pyrolysis.

5.6.5 Ablative Pyrolyzer

This process, shown in Figure 5.7D, involves creation of high pressure
between a biomass particle and a hot reactor wall. This allows uninhibited
heat transfer from the wall to the biomass, causing the liquid product to melt
out of the biomass the way frozen butter melts when pressed against a hot
pan. The biomass sliding against the wall leaves behind a liquid film that
evaporates and leaves the pyrolysis zone, which is the interface between bio-
mass and wall. As a result of high heat transfer and short gas residence time,
a liquid yield as high as 80% is reported (Diebold and Power, 1988). The
pressure between biomass and wall is created either by mechanical means or
by centrifugal force. In a mechanical system, a large piece of biomass is
pressed against a rotating hot plate.

5.6.6 Rotating-Cone Pyrolyzer

In this process, biomass particles are fed into the bottom of a rotating
cone (360 960 rev/min) together with an excess of heat-carrier solid
particles (Figure 5.7E). Centrifugal force pushes the particles against the
hot wall; the particles are transported spirally upward along the wall.
Owing to its excellent mixing, the biomass undergoes rapid heating
(5000 K/s) and is pyrolyzed within the small annular volume. The product