Page 180 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 5 Pyrolysis 157

(F)
Scrapper driver

200°C Vapor

400°C
Liquid

Condenser
FIGURE 5.7 (Continued)

5.2.2.4 Ultrarapid Pyrolysis
Ultrarapid pyrolysis involves extremely fast heating of biomass. In one
method, heat-carrier solids impact on biomass steam, resulting in a very high
(Figure 5.7C) heating rate. A rapid quenching of the primary product follows
the pyrolysis, occurring in its reactor. A gas solid separator separates the hot
heat-carrier solids from the noncondensable gases and primary product vapors
and returns them to the mixer. They are then heated in a separate combustor.
Then a nonoxidizing gas transports the hot solids to the mixer as shown in
Figure 5.7C. A precisely controlled short uniform residence time is an impor-
tant feature of ultrarapid pyrolysis. To maximize the product yield of gas, the

pyrolysis temperature is around 1000 C for gas and around 650 C for liquid.

5.2.2.5 Pyrolysis in the Presence of a Medium
Normal pyrolysis is carried out in the absence of a medium such as air, but a
special type is conducted in a medium such as water or hydrogen.
Hydropyrolysis is one such type where this thermal decomposition of bio-
mass takes place in an atmosphere of high-pressure hydrogen. Hydropyrolysis
can increase the volatile yield and the proportion of lower-molar-mass
hydrocarbons (Rocha et al., 1997). This process is different from the hydro-
gasification of char. Its higher volatile yield is attributed to hydrogenation of
free-radical fragments sufficient to stabilize them before they repolymerize
and form char (Probstein and Hicks, 2006, p. 99).
Hydrous pyrolysis is the thermal cracking of the biomass in high-
temperature water. It could convert, for example, turkey offal into light
hydrocarbon for production of fuel, fertilizer, or chemicals. In a two-stage