Biomass Handling - Biomass Gasification, Pyrolysis and Torrefaction - page 414

interlocking, with the particles mechanically locking to form the obstruction,
or it can be held simply by particle cohesion. Coarse particles can also form
an arch while competing for an exit, as a traffic jam results from a large
number of automobiles trying to pass through a narrow road in an unregu-
lated manner. By making the outlet size at least 8

12 times the size of the
largest particle, this type of arching can be avoided (Jacob, 2000).
Flushing results in uncontrolled flow of fine solids like Geldart's group A
or group C particles (Basu, 2006, p. 443) through the exit hole. It is uncom-
mon for relatively coarse biomass, but it can happen if the hopper is improp-
erly aerated in an attempt to collapse a rat hole.
Another problem influenced by hopper design is inadequate emptying.
This can happen if the base of the hopper is improperly sloped, causing
some solids to remain on the floor that cannot flow by themselves.
Erratic flow from an inappropriately designed hopper often results from
alternating between an arch and a rat hole. Interestingly, a rat hole could col-
lapse because of an external force of a flow-aid device such as an air cannon
or vibrator, or even vibrations created by a passing train or a plant pulverizer
(mill). Some biomass discharges as the rat hole collapses, but the falling
material can compact over the outlet and form an arch. The arch may break
because of a similar external force, and the material flow will resume until
the flow channel is emptied and a rat hole is once again formed (Hossfeld
and Barnum, 2007).
Material discharge problems can also occur if the biomass stays in the
bunker for a very long time, forming cakes because of humidity, pressure,
and temperature. This easily results in arching or rat holes. To avoid this,
renewal of solids in the hopper is necessary.
Besides solids flow there are some special problems in fuel-handling sys-
tems. For example, spontaneous ignition of biomass can occur if fine bio-
mass particles stay stagnant in a bunker for too long. Even in an operating
silo, a stagnant region can be a problem for fuels like biomass or coal, which
are prone to spontaneous combustion. Dust explosion that could occur in
fine dust in the silo is another problem.
If the fuel flows through a channel in the silo, the fuel outside the chan-
nel remains stagnant for a long time. The residence time of such fuels in the
silo should be reduced by emptying the silo frequently or by using a first in
first out mass-flow pattern ( Figure 12.6B ), where all of the material is in
motion whenever the fuel is discharged.

12.3.2.4 Achieving Mass Flow
To achieve mass flow, the following conditions are to be met:

The hopper wall must be sufficiently smooth for mass flow.