Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 95

hot substance, dry or wet. In wet torrefaction, the biomass is subjected to
heating in hot compressed water (Yan et al., 2009). Dry torrefaction involves
heating either by a hot inert gas or by indirect heating. The dry process is
the accepted method for commercial torrefaction.
Figure 4.2 describes the sequential process of torrefaction with the help
of photographs of a fresh twig cut from a maple tree and taken through dif-
ferent stages of the process. Mass loss of the wood in each stage is also
shown on the photograph. Figure 4.2A shows a photograph of the twig after
its bark is peeled off exposing its wet surface of the branch. Thereafter, the
twig is left in an air-drying oven at 70 C. After about an hour, its surface
moisture evaporates, and the surface no longer appears wet ( Figure 4.2B ).
The mass loss of the wood at this stage is about 20%.
After that the oven temperature is raised to 110 C for drying when the
inherent moisture within the pores of the wood escapes, and then it is bone
dried at 140 C for an hour. The wood appears a little reddish ( Figure 4.2C ),
and the mass loss nearly doubles because nearly all of the intrinsic moisture
in the biomass is released at this stage. Changes that occurred up to this
stage are primarily physical as very little chemical decomposition took place.
Thereafter, the piece is heated in an inert medium at 200 C for 1 h and
the chemical decomposition starts ( Figure 4.2D ), but the extent of decompo-
sition being small at this temperature, the piece lost mass by only a meager
amount of 4%. The oven temperature is then raised to 250 C when the piece
is baked for another hour. Now one notes evidence of major reaction by the
color change to dark brown and 11% additional loss in mass. At this stage of

Raw sample
20°C

Air dried at
70°C

Bone dried at
140°C

Mildly
torrefied at
200°C

Medium
torrefied at
250°C

Severe
torrefied at
310°C

Retained
mass= 100%

79% 58% 54% 43% 28%

A B C D E F

FIGURE 4.2 Progress of torrefaction of the branch of maple tree.