THERMOCHEMICAL CONVERSION: TORREFACTION - Biomass as a Sustainable Energy Source for the Future - page 391

Hemicellulose Lignin Cellulose

Extensive
devolatilization
and

E E

300 300

carbonization
(E)

D

250 250

D

Limited
devolatilization
and
carbonization (D)

C

200 200

C

Depolymerization
and
recondensation
(C)

150 150

A

Glass transition/
softening (B)

Drying (A) A

100 100

Hemicellulose Lignin Cellulose

FIGURE 12.2 Stages of torrefaction (Bergman et al., 2005b).

acid, methanol, and formic acid. With increasing torrefaction severity, carbon
monoxide is also present along with traces of phenols, furfural, and ammonia
(Bridgeman et al., 2008). On a molecular level, nuclear magnetic resonance
(NMR) studies suggest that torrefaction leads to removal of the acetyl group in
hemicelluloses along with the cleavage of the aryl ether bonds in lignin and
changes in the cellulose structure like depolymerization (Ben and Ragauskas,
2012; Melkior et al., 2012).
The macroscopic structure and bulk volume of the biomass are not affected greatly
by torrefaction, and consequently, the volumetric energy density of the fuel is lower
than that of the input. To overcome the reduction in energy density, the torrefaction
process can be followed by densification of the biomass to form pellets or briquettes.
Densification can be particularly important for economic viability in situations where
the torrefied biomass is to be transported over long distances or where the inventory
costs are high.