Page 266 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

242 8 Pre-combustion Air Emission Control

8.4.1.2 Wood Pellets
Biomass can be processed into wood pellets for effective transportation and efficient
combustion too. The feedstock can be any lignocellulosic biomass, and wood is the
most widely used one. Wood pellets can be burned in stove for indoor air heater or
in specialized furnace for energy production. They burn like high quality coal, with
much lower air emissions.
There are several steps that we can follow to make wood pellets. While the exact
procedure may be different from plant to plant and the product quality may vary
accordingly [30], the common steps are summarized in Fig. 8.3 for guidance only.
• The first step is biomass drying. The moisture content in the raw biomass must
be reduced to a level of about 10 % before the pelletizing process begins.
Otherwise, it is difficult to pelletize and consequently affects the quality of the
final product. This step is especially important when recycled biomass is used as
a feedstock.
• The next step is to remove the impurities from the dried wood. Metal, for
example, can be removed by magnets and a screen. This step is especially
important for recycled biomass too.
• After the removal of the impurities, the raw material is ground in a hammer mill
and it becomes wood powder. The powderous particles can be separated using a
cyclone or surface filter. This size reduction is necessary for pelletization.
Typical powder sizes are below 5 mm in diameter.
• Right before pelletization, the powder is heated to 70 °C or so. Heating and
softening ensure that the lignin in the wood is released and the particles can be
bonded effectively together in the final product.

Biomass drying

Impurity removal
Biomass Rolling
Milling to power powder

Powder heating

Pelletization

Wood pellet
Pellet cooling

Fig. 8.3 Pellet processing