Environmental Engineering Reference
In-Depth Information
batch to batch. The most important physical parameters are bulk density, pellet
density, and durability (which can be measured by determining the fine particle
content of the raw material). Key physical and chemical parameters include the
pellets' moisture content and their calorific heat value (as an indicator of their
energy content), contents of elements such as K, Mg, Ca, N, S, Cl, ash content,
and ash melting behavior (which determines the degree of ash agglomeration that
will occur in the furnace). The net calorific heat value of typical pellets is about
19 MJ kg
-1
, which is less than half the value for the same quantity of oil. Pellets
contain water, flammable components, and ash. The amount of ash generated dur-
ing pellet combustion is typically less than 0.5% of the pellets' gross weight. Ash
contains several elements such as phosphorus, potassium, calcium, magnesium,
and silicon. The presence of large quantities of fines (notably, sawdust released
from pellets during transportation or otherwise) in pellets increases ash formation
and causes other problems during combustion. In the worst-case scenario, ash
may melt in the burner and damage the furnace. If the pellet combustion process
is not optimized, various harmful emissions will be generated including CO (due
to an inadequate oxygen supply or an insufficiently high combustion tempera-
ture), NO
x
(due to an overly high combustion temperature that allows its forma-
tion from N in the pellets and/or the air), CO
2
, hydrocarbons, and particulate
matter. The chlorine and sulfur contents of the pellets are also important because
they affect the production of HCl, dioxins, and SO
x
during combustion.
7.2.5
Storage of Solid Biomass
Solid biomass is often stored for extended periods of time to facilitate logistics
and to maintain high productivity at pelletization plants on a year-round basis. In
addition, the prolonged storage of sawdust may increase the quality of the result-
ing pellets. The stored biomass often changes during storage. For example, the
storage of pine and spruce sawdust in large quantities causes its fatty and resin
acid content to decline gradually over the first 12 weeks (relative to the dry mass
of the raw material). However, storage for longer periods do not cause any further
reductions, indicating that the sawdust has become mature (i.e. its fatty and resin
acid content stabilized) after 12 weeks of storage [7]. These compounds have
significant effects on pellet production, and pellets made from mature sawdust
often exhibit superior durability [8].
In Sweden, wood pellets are primarily manufactured from spruce and pine saw-
dust and shavings. Because these raw materials contain significant quantities of
readily degraded extractives (notably, fatty and resin acids), it is expected that the
chemical composition of the raw material will change during pelletization and
subsequent storage. This is reflected in the self-heating of pellets and the occur-
rence of odour problems [6, 9]. It is also well known that dry lignocellulosic mate-
rial can generate heat due to moisture uptake, which presents a risk of spontaneous
combustion.
