Environmental Engineering Reference
In-Depth Information
Rotating die
Internal rolls
Axis
Pellet
Pellet
Knife
Knife
FIGURE 8.5
Working principle of a pelletizing machine.
drying), the pelletizing itself, and posttreatment (cooling, packaging). During the
major processing phase of pelletizing, biomass is pressed through a press matrix
and shaped into extrudates; also a ring matrix can be used. Figure 8.5 shows the
working principle of a pelletizing machine.
Common dimensions of the pellets produced are diameters of 6
8 mm, though
also larger diameter pellets are manufactured, and lengths varying from 3 to 50 mm
(tinyurl.com/pq2f6us). The process demands a feedstock consisting of small
particles with diameters preferably lower than ~6
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8 mm (Samson et al., 2005) and
moisture levels lower than 10
15 wt%, though a piston press can handle feedstock
of a moisture content of up to 20 wt% (Uslu, 2005). Pelletizing is carried out with peak
temperatures of about 150
C, because lignin starts softening at 100
C and then acts as a
cellulose binder allowing the feedstock to fuse to form pellets. The moisture content of
the feedstock is a crucial parameter, because biomass that is either too wet or too dry
can cause a significant increase of the pressure required (Reed and Bryant, 1978).
Pelletizing can be combined with torrefaction as is done in the development of
the torrefaction and pelletizing process (TOP) technology, which is described
in Chapter 12.
Intake fresh biomass generally has a moisture content of 50 wt%, but before the
pelletizing step, this is reduced to ~15 wt%. The moisture content of the final product
is 10 wt%. Since no significant volatilization occurs during the process, the total mass
loss is due to water evaporation. Pelletizing typically has a thermal efficiency of 94%
(94% of the initial energy remains in the pellet product), and the net efficiency, taking
into account the energy consumption of the process, was calculated by Uslu to be
around 87% (Uslu, 2005).
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