Page 138 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 115

The information on the effect of torrefaction on density is important for
the design of a torrefaction plant as well as for a detail analysis of the pro-
cess. For a good understanding of the effect of torrefaction on biomass den-
sity, it is worth recalling three types of densities explained in Section 3.5.1.

1. True density of particle (based on solid or cell wall volume alone)
2. Apparent density of particle (based on biomass solid and internal pore
volume)
3. Bulk density of particles packed in an enclosure or piled on a surface
(based on biomass solid, pore volume, and the void between particles in
the packing).
The density of the cell walls that is the “true density” for most lignocellu-
3
lose biomass is typically of the order of 1400 kg/m (Jenkins, 1989). After
torrefaction, there is only a marginal (,5%) reduction in the true density
(Phanphanich and Mani, 2011), but the reduction in the apparent density is
noticeable (Table 4.6). The bulk density of biomass in packing also reduces
with torrefaction temperature.
Exploratory work carried out on coarse pieces of poplar wood showed
(Basu et al., 2013b) that it is apparent density under torrefied condition
decreases with severity and temperature of torrefaction. The rate of this
decline, however, reduces at higher temperatures. The external volume of the
torrefied biomass also reduces with increasing torrefaction temperature but
to a lesser degree than done by its mass. This causes the apparent density to
reduce with torrefaction temperature.

4.6.2 Grindability
Raw biomass is highly fibrous in nature, and its surface fibers often lock in
with each other like in Velcro. This greatly increases interparticle friction.

TABLE 4.6 Change of Densities with Torrefaction Temperature for Several
Types and Sizes of Wood
Temperature ( C) 25 200 220 225 240 250 260 275 280 300

a
3
Bulk density (kg/m ) 381 342 332 376 400 b
Apparent density c 500 489 445 444 395 340 297
3
(kg/m )
a
3
True density (kg/m ) 1400 1410 1400 1370 1340
a
Pine wood chips 20.94 70.59 mm long, 1.88 4.94 mm thick, and 15.08 39.70 mm wide
(Phanphanich and Mani, 2011).
b
This reduced density could be a result of an experimental error.
c
Poplar wood 25.4 mm diameter 3 32 mm (Basu et al., 2012).