Page 142 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 119

Table 4.8 compares energy density and several related characteristics of
wood, torrefied wood, pelletized greenwood, andpelletizedtorrefied
wood. Data for a typical subbituminous coal is also given here for
reference.
Pelletization increases the bulk density of green or untreated biomass
because of its higher apparent density and regular shapes. The densification
of biomass through pelletization can be made more effective if the wood is
torrefied and then pelletized. In a typical process, the biomass is torrefied,
cooled, ground to required size and then subjected to densification under
pressure and slight heating.
Densification through pelletization increases the mass energy density of
wood from 10.5 to 20.7 MJ/kg and volume energy density from 5.8 to
3
16.6 GJ/m . Thus, further densification of wood through pelletization and
torrefaction could make transportation and handling of wood competitive
with that of coal.
Torrefaction could also remove a major limitation of raw wood pellets.
When stored for long periods of time, the pellet absorbs much moisture
reducing the strength of the pellet, even to the extent of crumbling
when wet.
Densification of raw biomass through pelletization requires it to be
ground typically to 3.2 6.4 mm size (Mani et al., 2006). After that the
ground biomass is compressed into pellets to increase its density. Some form
of external binding agent needs to be used to give the pellet a good binding
strength. Alternatively, the natural binder of biomass, lignin, may also be

TABLE 4.8 Comparison of Mass and Energy Densities

Pellet of
Fresh Pellet of Torrefied Torrefied Subbituminous
Wood Wood a Wood Wood a Coal a
Moisture 35 8.5 3 3 31
content (%)
Mass energy density 10.5 15.9 19.9 20.7 20.4 HHV
(LHV, as-received)
(MJ/kg)
Volume energy 5.8 9.2 4.6 16.6 26.6
3
density (GJ/m )
3
Bulk density (kg/m ) 550 575 230 800 850
a
Average values taken form Bergman et al. (2005b). Volume energy density calculated based on
3
apparent density of 1300 kg/m .
#Rank III, Group 2, Steam, p. 9 7.