POWER August 2012 - 55
PLANT DESIGN
10. Discharge chute dust curtain and
belt seals. Source: Energy Associates PC
Dust curtain
Entrance to head
discharge chute
Skirtboard exit
Dust curtain
11. Examples of dust/flow control skirtboard designs. On the left is a dust
curtain installed on the skirtboard exit. On the right is skirtboard with an adjustable flow gate.
Source: Energy Associates PC
Adjustable flow gate
Skirtboard
Top belt
Top belt
Top belt
Rubber belt
Rubber seals
start of the workday, when bunker levels
are low. At one retrofit location, the flow
control chutes plugged at such a time during
their first commissioning test. The
plant's fueling operation degenerated with
the silos' coal tonnage precipitously falling
to near-empty levels before an emergency
silo-filling procedure could be
applied. This 1,500-MW coal-fired power
plant reportedly came within 30 minutes
of an emergency plant shutdown.
Inducing Airflow
Computational fluid dynamic computer
tools are used to predict the aerodynamic
drag, turbulence, and vortices that form as
coal flows along the surface of a flow control
chute and the movement of air around
the stream of coal. These tools can be used
to understand aerodynamic forces and the
flow paths that dust-size particles will
take. DEM does not consider these forces,
nor dust-size particles, which would be too
numerous for analytical modeling, as seen
in Figure 7.
A falling stream's bulk density (lb/ft3
)
decreases as air becomes entrained around
the moving matrix of particles. This is an
indicator that a dust cloud is probably being
formed. It is a reason why one of the
basic design techniques for flow control
chutes is to keep the moving stream of coal
in contact with chute surfaces.
Air is induced into the expanding stream
as gravity accelerates particle speeds. This
moving stream of coal particles and air
creates suction that pulls air into the head
chute. At the bottom of the chute, the induced
air is exhausted from the slowing
stream that is condensing as it loads on the
receiving conveyor. Depending upon the degree
of control at the loading location, large
56
amounts of air can be exhausted from the
coal stream as it funnels onto the receiving
conveyor. Typically, this dust-laden air will
blow out through the skirtboard. This is a
reason that one of the primary objectives for
flow control chutes is to attempt to match
the speed of the coal at the loading point
with the speed of the belt. Another reason is
to minimize frictional belt wear, as particles
slide/accelerate onto the moving belt.
To reduce the amount of air that is
pulled into the head chute, dust curtains
and belt seals at the discharge are often
used and are Ameren's standard. Figure
10 illustrates a typical arrangement. The
dust curtain is slit and cut to conform to
the surface contour of coal on the belt.
The belt seals are arranged to lightly contact
the bottom of the carrying strand and
both the top and bottom of the return belt
strand. The entrance to the discharge chute
is behind the conveyor's head pulley, in
the zone where the belt transitions from a
35-degree trough to a flat, horizontal contour
at the head pulley.
To reduce the amount of air that is exhausted
at the loading point, skirtboards
are normally fitted with dust curtains at
the skirtboard exit. An additional method
that has gained a following is to add dust
curtains and flow control plates within the
length of the skirtboard.
Figure 11 illustrates an example of this.
The dust curtains provide additional barriers
to airflow, to reduce air speed and turbulence.
The flow control plate at the right
in Figure 11 also helps to control surges
and overflows, particularly following an
emergency stop, when the lower portions
of the transfer may fill due to differential
conveyor stopping times. If you see a large
pile of coal at the exit of a skirtboard,
chances are that it was caused by such a
spill event.
Reducing Chute Wear
Materials of construction are important
for flow control chutes. Because the coal
www.powermag.com
stream maintains contact with chute surfaces,
chute and liner wear will increase
dramatically. In one case, holes were worn
through flow control chutes in a matter of
weeks, instead of years, due to errors in
selecting and inspecting fabrications and
the materials of construction.
Typically, chutes are constructed with
replaceable liners. Liners are a wear item.
For many chutes, replacement liners can
be sourced from local fabricators who can
follow the existing shape and attachment
patterns.
Because of the high wear rates associated
with flow control chutes, some manufacturers
and proponents like Weba Chute
Systems have adopted a design feature of
uniformly spaced micro-ledges that capture
the bulk material being handled, like
a rock box that is used in some industries.
The spacing between ledges is a function
of particle size and the product's rill angle.
The flow and impact forces can pack
the bulk material tightly. The bulk material
wears on itself, rather than a liner plate,
so liner maintenance and replacement is
often reduced. That's fine for minerals
like iron ore, but it can be problematic for
highly volatile bituminous and PRB coal.
Some suppliers have adopted complex
curved and circular shapes for chute sections.
Their intricate patterns make the
design and eventual retrofit of worn liners
difficult. Instead, the complete chute
section is fabricated from steel with a
hardened, finished surface. The chutes
themselves become a spare parts replacement
business. Special care is needed in
these cases, because welding steel with a
hardness gradient requires an atypical procedure
during fabrication; uneven welded
joints become the wear locations. ■
-Daniel Mahr, PE (danmahr@energy-pc
.com) is a project manager with Energy Associates
PC. Michael A. Schimmelpfennig,
PE (mschimmelpfennig@ameren.com) is a
consulting engineer for Power Operations
Services, Ameren Missouri.
POWER | August 2012
http://www.powermag.com
POWER August 2012
Table of Contents for the Digital Edition of POWER August 2012
Contents
POWER August 2012 - Cover1
POWER August 2012 - Cover2
POWER August 2012 - Contents
POWER August 2012 - 2
POWER August 2012 - 3
POWER August 2012 - 4
POWER August 2012 - 5
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