Environmental Engineering Reference
In-Depth Information
Cellulose bundle
Lignin cover
Hemicellulose
Figure 3.2
Structural arrangement of lignocellulose.
In addressing those challenges, the EU is increasingly shifting towards policies
favoring use of renewable energy sources. Currently biomass delivers around 4%
of the EU's primary energy (Eurostat), and in order to reach the future targets set
by the EU significant amounts of biomass will be required in the majority of
European countries [7]. Since the resources of biomass are situated large distances
from energy users, in such countries there will be additional logistic costs and
material losses. However, it has been shown that densification prior to interna-
tional transportation of biomass is crucial, as converting biomass into a densified
intermediate can save transport and handling costs. Consequently, pretreatment is
a key step in the total supply chain [8].
The different types of biomass densification are bailing, briquetting, extrusion,
and pelletization. With respect to the production of high-density solid energy car-
riers from biomass, briquetting and pelletization are the two most widely used
methods. Both processes take place at high pressure and are very closely related
techniques. The basic processing difference is the use of a piston or screw press
for briquetting, while biomass pellets are produced in a pellet mill. The size of the
final products is different; pellets have a cylindrical shape and are only about
6-25 mm in diameter and 3-50 mm in length [9].
The European and North America pellet markets have been studied in great
detail by Sikkema
et al.
[10] and Spelter and Toth [11]. Wood pellet production in
the EU was estimated to be about 10 million tons and 6.2 million tons for North
America in 2009. Recent studies suggest a strong growth for both the European
and North American pellet market, and the globally installed pellet production
capacity for 2011 was estimated to be about 30 million tons [9].
At present, wood pellet production is limited to sawdust and cutter shavings (dry
and wet), predominantly from spruce and pine tree species, as a raw material. As
forest-based by-products, feedstocks for conventional wood pellets are finite
resources and, due to the existing supply chain, pellet production has a strong
dependence on the productivity of the wood-processing sector. To satisfy demand
for pellet fuels, agricultural residues and industrial food by-products are being
