Page 121 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 99

final temperature (T p ) of the product for further processing or storage.
By extracting the energy, Q cool , in the form of either hot air or vaporized
liquid, the product is cooled.
Q cool 5 M f ð1 MÞ MY db C pt ðT t 2 T p Þ (4.5)
where C pt is the specific heat of torrefied biomass and MY db is the mass
yield.
The extracted energy, Q cool , may be partially recovered in the form of
hot air or vaporized liquid like steam, which could be gainfully utilized in
providing a part of the energy required for drying or preheating the biomass.

4.4.2 Mechanism of Torrefaction

The thermochemical changes in biomass during torrefaction may be divided
into five regimes following Bergman et al. (2005a):

1. Regime A (50 120 C): This is a nonreactive drying regime where there
is a loss in physical moisture in biomass but no change in its chemical
composition. The biomass shrinks but may regain its structure if rewetted
(Tumuluru et al., 2011). Upper temperature is higher for cellulose.
2. Regime B (120 150 C): This regime is separated out only in case of lig-

nin that undergoes softening, which make it serve as a binder.

3. Regime C (150 200 C): This is called “reactive drying” regime that
results in structural deformity of the biomass that cannot be regained
upon wetting. This stage initiates breakage of hydrogen and carbon bonds
and depolymerization of hemicellulose. This produces shortened polymers
that condense within solid structures (Bergman et al., 2005a).
4. Regime D (200 250 C): This regime along with regime (E) constitutes

torrefaction zone for hemicellulose. This regime is characterized by lim-
ited devolatilization and carbonization of solids structure formed in
regime (C). It results in the breakdown of most inter- and intramolecular
hydrogen, CaC and CaO bonds forming condensable liquids and non-
condensable gases (Tulumuru et al., 2011).
5. Regime E (250 300 C): This is the higher part of torrefaction process.

Extensive decomposition of hemicellulose into volatiles and solid pro-
ducts takes place. Lignin and cellulose, however, undergo only a limited
amount of devolatilization and carbonization. Biomass cell structure is
completely destroyed in this regime making it brittle and nonfibrous.
Major devolatilization and carbonization of the biomass polymers take
place in a different temperature range. Some qualitative values taken from
Prins (2005, p. 89) are given below.
Hemicellulose: 225 300 C

Cellulose: 305 375 C

Lignin: 250 500 C