Biomass Handling - Biomass Gasification, Pyrolysis and Torrefaction - page 422

12.3.4 Feed Preparation
Biomass received from its source cannot be fed directly into the gasifier for
the following reasons:
Presence of foreign materials (e.g., rocks and metals)
Unacceptable level of moisture
Too large (or uneven in size)
Such undesirable conditions not only affect the flow of solids through the
feeder, but also operation of the gasifier. It is thus necessary to eliminate them
and prepare the collected biomass appropriately for feeding. Foreign materials
pose a major problem in biomass-fired plants. They jam feeders, form arches in
silos, and thus affect the gasifier operation, so it is vitally important to remove
them as much as possible. The three main types of foreign materials are: (1) stones,
(2) ferrous metals (e.g., iron), and (3) nonferrous metals (e.g., aluminum).
Some of the equipment used to remove foreign materials from the col-
lected biomass are as follows:
De-stoner. The basic purpose of a de-stoner is the separation of heavier-
than-biomass materials such as glass, stones, and metals. Typical de-
stoners use vibration in tandem with suitable airflow to stratify heavy
materials according to their specific gravity.
Nonferrous metal separators. Separation of nonferrous metals like alumi-
num has always been a challenge. One solution is an eddy current separa-
tor—essentially a rotor with magnetic blocks that, depending on the applica-
tion, is made of either standard ferrite ceramic or a more powerful rare earth
magnet. The rotors are spun at high revolutions (more than 3000 rpm) to
produce an “eddy current,” which reacts differently with different metals
according to their specific mass and resistivity, to create a repelling force on
the charged particle. If a metal is light yet conductive, such as aluminum, it
is easily levitated and ejected from the normal flow of the product stream,
making separation possible ( Figure 12.11 ). Separation of stainless steel is
also possible by eddy current, but it is more difficult and depend on its grav-
ity. Particles from material flows can be sorted down to a minimum size of
3/32 in. (2 mm) in diameter. Eddy current separators are crucial in the recy-
cling industry because of their ability to separate nonmagnetic materials.
Magnetic metal separation. The use of powerful magnets to separate iron
and other magnetic materials from the feed is a standard procedure in
many plants. Magnets are located at several places along the feed stream.
They are generally suspended above the belt, to attract magnetic materi-
als, which are then discharged away.
12.3.4.1 Size Reducers
Biomass comes from different sources, so the presence of oversized solids or
trash is very common in the fuel delivered. Woody biomass may be sized