The factors which control the densification process are
moisture content, particle size, form, fibrous and non-fibrous
nature as well as lignin contents [19]. Generally, agricultural
residues, due to their bulky forms, are resistant to compression
and consequently resistant to effective densification
[21]. Straws, for example, have structural tubes of highly
crystalline cellulose, protected by a waxy outer surface, and
can be effectively densified only after destroying the structure
through chopping or milling. In contrast, woody residues
like sawdust and wood chips have structures with good
compression properties and contain some inherent binder in
the form of lignin. Where possible, an addition of materials
containing some natural binders such as sawdust, bark, corn
stalk can lead to the production of good briquettes and
pellets. However, the basic problem often encountered in
the use of densified biomass fuels are the costs for
densification [22].
6 J. Werther et al. / Progress in Energy and Combustion Science 26 (2000) 1–27
Table 3
Comparison of pelletization and briquetting methods for straws [7]
Pelletization Briquetting
Moisture content of 10–20% Moisture content of 10–15%
Maximum output of press 4–6
t/h
Maximum output of press 1–
1.5 t/h
Energy consumption (chopping
and pressing) 80–90 kWh/t
Energy consumption (chopping
and pressing) 40–50 kWh/t
Straw must be finely chopped Straw need not be finely chopped
The size of the product is small
Automatized systems can be
used
The same combustion systems as
for peat and wood can be used