Environmental Engineering Reference
In-Depth Information
Under certain conditions, wood pellets can emit appreciable quantities of gase-
ous VOCs, for example aldehydes and ketones, and non-condensable gases (this
latter process is known as off-gassing), primarily CO, CO
2
, and CH
4
. The emis-
sions of condensable gases often give the pellets a pungent smell.
7.2.6
Torrefaction Technology
Torrefaction is a technology for the pretreatment of solid biomass [10]. It is pri-
marily of interest because it generates a product with superior properties to
untreated biomass. In particular, torrefied biomass has a greater energy density
per unit volume and has the added benefit of being hydrophobic.
Broadly speaking, there are three types of thermal treatment that can be used to
torrify woody raw materials:
1.
conventional drying processes
: involves heating at 100-130°C to remove
moisture;
2.
torrefaction
: anoxic thermal treatment at 200-350°C with low particle heating
rates and a long residence time (>1 min); and
3. pyrolysis: anoxic treatment at 400-800°C.
The torrefaction of lignocellulosic biomass is an upgrading process for biomass
drying that removes some VOCs and decomposes the reactive hemicellulose frac-
tion. The moisture content of torrefied biomass is very low. Torrefaction is a mild
pyrolytic process that improves certain properties of the raw material, generating
a product with a higher carbon content and net calorific value than untreated
wood. It involves heating the woody material to 200-350°C for several minutes in
the absence of oxygen. Torrefaction at different temperatures yields products with
different properties and chemical compositions. Notably, the partial removal of
VOCs (around 20% of the VOC content of the raw material is lost) dramatically
changes the material's properties.
Torrefaction is likely to become more important in the future as a method for
the pretreatment of biomass prior to gasification [11]. Torrefied biomass can be
compressed into fuel pellets of high physical and energy density to reduce the
amount of space required for its storage and the cost of transporting it. Torrefied
pellets are less prone to moisture absorption than those made from untreated saw-
dust, and the tendency to absorb moisture diminishes as the severity of the torre-
faction process increases [12].
The pelletization of torrefied Norway spruce was shown to yield pellets with
around 80-90% of the durability of pellets made from untorrefied material [13].
The lack of moisture in the torrefied material increases the glass transition tem-
perature of the remaining carbohydrate polymers and limits the scope for the for-
mation of robust bridges between particles. However, there are several ways of
overcoming this lack of bonding. For example, an additive with a high bonding
capacity could be introduced after the torrefaction process to compensate for the
loss of hydrogen bonding sites due to torrefaction [14].
