Environmental Engineering Reference
In-Depth Information
Because pellets are a renewable fuel and have a closed CO
2
emission cycle,
their usage represents a step towards a sustainable energy system. Wood pellets
have been used for heat and power generation in both the household and industrial
sectors. They are currently produced in several countries, some of which export
them overseas. For example, Canada ships significant quantities of pellets to
Europe every year [4]. The raw materials used in pellet production are often forest
by-products such as sawdust, mostly from pine and spruce, as well as cutter shav-
ings and, in some cases, the bark of the same species. However, there is consider-
able interest in identifying new raw materials for pellet production because
lignocellulose-rich material from softwoods is also used in the pulp and paper
industry. Moreover, cellulose could also be used in ethanol production. Other bio-
mass sources include straw, various grasses, corn stover, and other agricultural
products [5]. It is possible to use mixtures of different lignocellulose-containing
materials in pellet production and there is ongoing research in Sweden and China
on pelletizing various mixtures including corn stover, rape seed cakes, eucalyptus
leaves, bark, peat, hemp, cassava stems and cotton.
The first-generation pellet presses were developed for feed production. However,
experiments conducted over several years have resulted in the development of mod-
ified presses that have been optimized for use with woody raw materials. In tradi-
tional wood pellet production processes the raw material is dried to a moisture
content of 5-15%, which requires significant amounts of energy. The dried material
is then ground into a fine powder consisting of particles that are less than 3 mm in
diameter. This powder is then pressed through cylindrical holes in a pellet matrix
(die) to form short sticks called pellets. The pressure inside the pellet press is very
high (about 70-100MPa). Production capacity can be increased by treating the
powder with steam before pressing to adjust the moisture content of the raw mate-
rial and soften its lignin/hemicelluloses to improve particle binding. During pelletiz-
ing, the temperature of the material rises to more than 120°C. This further softens
wood components such as lignin and therefore helps to bind the particles together,
eliminating the need for an exogenous binder in many cases. In general, the use of
binders increases production costs and has an adverse effect on the fuel's combus-
tion properties. The newly formed pellets must be cooled to ambient temperature
relatively quickly to prevent the absorption of moisture from the surrounding air.
One drawback of the pelletizing process is its high consumption of electricity.
Another is the relatively large quantity of fine particles and dust that is generated
during pellet production and storage. This increases the cost of pellet production,
because the dust and fines must be removed (to avoid dust explosions, among
other things) before the pellets can be packaged and delivered. It is also important
to remove any potential metal, stone, or sand contaminants that may have become
mixed with the sawdust before it is pelletized. Figure 7.2 outlines the key pro-
cesses that occur in a pilot pellet manufacturing plant.
The interactions between particles during pellet production are governed
by a complex set of forces [5]. Once manufactured, the pellets are much more
