PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 258

biomass is most often compacted. Compaction (or densification) has already
been applied for a long time in food, feed, and pharmacy industrial sectors.
Before giving an overview of the technologies, we first discuss some definitions of
density.
The apparent density, ρ app (also called bulk density), of a heap of biomass is the

mass of all particles forming it divided by the total volume of the heap. This contains a
substantial void fraction,

ε , not occupied by particles. Its relation to the particle

density, ρ p , is as follows:

ρ app = ρ p 1 ð − ε Þ ð Eq : 8 : 22 Þ

The particle density in its turn is related to the true material density but lower in value
as the particle has a certain porosity, χ :

ρ p = ρ true 1 ð − χ Þ ð Eq : 8 : 23 Þ

Wetted particles, containing a fluid (e.g., moisture) of density ρ f , have a density of

ρ w = ρ p + χρ f ð Eq : 8 : 24 Þ

In order to increase the apparent density of biomass, two techniques exist (we exclude
baling as this is described as part of the harvesting phase in which it takes place),
namely, briquetting and pelletizing.
An excellent overview of all densification techniques is provided by Tumuluru
et al. (2011), but for the production of solid bioenergy carriers, the aforementioned
technologies are most prominent.
Other advantages associated with the increase of the apparent density of bio-
mass are:

￿ The energy density is increased.

￿ Handling of biomass is simplified (storage and logistics).

￿ Easier dosing is possibly realized depending on which technique is used.

￿ Lower water reabsorption can be realized.

￿ Pressing biomass can be combined with adding additives for improving

downstream processes.

￿ Less dust is released.

￿ The stability is increased, preventing biological decay.

￿ A customer-tailored commodity product may be created.

8.9.1 Briquetting

Briquettes are commonly fabricated in an elongated press with extrusion of the
biomass realized by a plunger or a screw (see Figure 8.4 for a schematic). The strain