Transportation - Engineering and Science of Biomass Feedstock Production and Provision - page 148

Fig. 6.5 Energy requirement in PIHV for compression of the sample shown in Fig. 6.4 , versus the
“in-mold” density

Finally, there is a common misconception that compressing biomass may dimin-
ish its inherent energy-producing potential. No evidence has been shown in the lit-
erature that this is the case. Further research is needed to determine the effect of
compression on the biomass conversion efficiency.

6.3.4 Pelletization

Pellets, briquettes, and cubes made from biomass have the advantage of yielding a
flowable material form, which is suitable for long-distance rail transportation [ 4 , 11 ].
To produce pellets, biomass feedstock needs to be ground to a particle size of
approximately 2-8 mm and compressed while potentially applying increased tem-
peratures and binding agents. The energy requirement for pelletization is a function
of the particle size, the required pellet material density, and the required pellet qual-
ity which includes durability, a function of the pellet hardness [ 4 , 36 ]. Typical bio-
mass pellets have a diameter ranging from 12 to 15 mm [ 32 ].
One of the drawbacks of pelletized material in bulk form is that it exhibits poros-
ity due to air pockets among the pellets. Various mathematical models predict the
porosity of randomly packed cylindrical particles in a bulk material as a function of
the pellets' aspect ratio, which, for typical biomass pellets, ranges from 1.5 to 2.5.
In this range, the so-called Z-Y model predicts a porosity of approximately 0.32
[ 37 ]. To reduce the porosity, secondary compression of bulk pellets may be feasible;
however, this method may compromise the integrity of the pellets.
A second limiting factor is long-term post-compression rebound, which is
defined as the increase in volume of the pellet after the pressure applied during