Environmental Engineering Reference
In-Depth Information
Chipped biomass
Shredder (coarse
mill)
Fine mill
Mixer
Steam
generator
Saw dust
Cooler
Pellet mill
Storage and resting silos
Figure 7.2
Key processes in pilot pellet manufacturing plant Biofuel Technology Center at SLU
in Umeå. Reproduced with permission from Håkan Örberg of Biofuel Technology Center at SLU.
amenable to extended storage and transportation without loss of quality than saw-
dust, which has a moisture content of 45-55% and requires large volumes of stor-
age space. In addition, unprocessed sawdust is much more sensitive to microbial
activity and can undergo various chemical changes during storage.
When used as fuels, pellets have several advantages over the raw materials used
in their production (primarily sawdust):
• a lower moisture content (about 40-45% less water content);
• more homogeneity and so less variation in moisture content and particle size
(pellets therefore have superior combustion properties);
• more even combustion and lower levels of undesirable emissions;
• higher density and lower transportation costs;
• higher energy density and much easier feeding of burners;
• lower storage costs; and
• the ability to be stored for extended periods of time without risk of mold or
other microbiological degradation.
Pellets are usually stored in closed warehouses or silos to ensure that they do not
absorb water. During prolonged storage or transportation, wood pellets may emit
some volatile organic compounds (VOCs) such as aldehydes and low-molecular-
weight carboxylic acids into the storage room or container. These compounds
may have unpleasant smells and can potentially represent health hazards. They are
formed by the oxidation of fatty and resin acids present in the wood pellets [6].
The pellets are delivered to customers in bulk by lorry, train, or boat, in large or
small sacks. They are burned in pellet furnaces at small-, medium-, and large-
scale facilities to generate heat. The quality of the pellets can be evaluated using a
few key parameters, which is important because their properties may vary from
