POWER March 2012 - 38

PLANT DESIGN
7. Old, second-generation transfer
chute at Ameren's Meramec Plant.
Courtesy: Energy Associates PC
Violent collisions such as these produce
a lot of dust. Some lumps of coal fracture,
which creates smaller, lighter dust particles
with newly exposed dry surfaces. These
are entrained with air and then carried
along in the newly formed aerated mixture.
The falling aerated stream of coal creates a
draft at the entrance to the discharge hood,
pulling more air into the chute to continue
the process. The slightly higher pressure
within the chute allows floating dust to escape
through poorly sealed openings and
flanges.
In the mid-1980s, new transfer designs
8. New, third-generation flow control
chute. Courtesy: Ameren Missouri/
Michael Schimmelpfennig
using fluid flow principles were introduced
that reduced coal dust generation
produced by high-capacity conveyor systems.
The new approach focuses on transfer
chute design to control the flow of coal
within the chute. This technique avoids the
direct impact of the coal stream on chute
surfaces. Instead, the stream is guided. Its
velocity and direction are controlled with
intersecting, contoured, and form-fitting
plate surfaces. The objective is to channel
the coal stream so its speed, as it is being
loaded onto the downstream belt, closely
matches the speed of that belt. This also
reduces turbulence and sliding friction at
the loading point, which is a primary cause
of belt cover wear.
One of the all-too-common problems
9. Illustration of the new flow
control chutes at Ameren's Meramec
Plant. Courtesy: Ameren/ Michael
Schimmelpfennig
10. Dust collection test methodology.
Courtesy: Ameren Missouri/Michael
Schimmelpfennig
experienced with coal chutes is plugging.
Ameren has installed flow control chutes
at several of its coal-fired plants, including
the Meramec Plant; Figures 7 and 8 are
before and after photos of that plant's tripper
transfer. This was a particularly challenging
transfer system design because of
an extremely short skirt board length. This
design, common in the 1950s, featured a
slow-moving belt fitted with a bend pulley
for transitioning from the inclined loading
section to the horizontal length of the belt.
As units were added, belt speeds increased
and the bend pulley was replaced by a convex
belt curve. The original transfer chute/
skirtboard was replaced with a secondgeneration
design.
Meramec next switched to Powder River
Basin coal, which increased the amount of
spillage and dust to unacceptable levels. To
alleviate the problem, a third-generation
design using flow control technology was
installed. A " hood " funnels the flow of coal
from the discharging conveyor. The coal is
then guided into a concentrated stream by
intersecting plates and is directed toward
the tail pulley of the tripper conveyor.
This is different from how most chutes
are designed, where the primary objective
is to direct the flow in the direction of the
outbound belt. As seen in Figure 9, there is
38
www.powermag.com
a lower spoon chute section that does this,
but the coal takes an atypical zigzag route.
The objective is to gently lower the flowing
stream by maintaining plate contact to
avoid an uncontrolled freefall where gravity's
acceleration expands the falling coal
stream and separates it into individual particles
or lumps that allows the finer particles
to float into a cloud of dust. The lower
loading chute has an internal " spoon " that
directs the coal stream onto the tripper belt
at a reasonable speed and angle to minimize
turbulence, belt wear, and particle
size degradation. The spoon is enclosed in
a " stilling box, " which also helps to reduce
airborne dust.
As a proof of concept, Ameren conducted
an acceptance test over a three
day-period. As seen in Figure 10, sample
collecting pans were employed. The dust
samples collected in these pans confirmed
that coal spillage and dust accumulations
just outside of the transfer chute were reduced
by 98.8%, from an average of 1.810
to 0.022 grams/ton for those samples that
were collected
Meramec's new tripper conveyor loading
chutes illustrate an issue that is common
for many existing power plants. Existing
transfers were not originally designed for
flow control chutes. The available transfer
height and conveyor arrangement was
fixed long ago and are definitely not ideally
suited for the features employed in flow
control technology. Besides the zigzag
flow path, the loading point for Meramec's
tripper conveyor is moved back toward the
tail pulley. This is a compromise for the
belt troughing transition distance, which is
normally established to avoid overstressing
the outer edges of the belt as it transitions
from flat to troughed contours as it
moves from the tail pulley to the troughing
impact idlers. Overstressing a conveyor
belt can lead to other problems including
poor tracking due to belt cupping and
splice failures.
Materials of construction are also important
when specifying flow control
chutes. Because the coal stream maintains
contact with chute surfaces, chute or liner
wear will increase dramatically. In one
case, chutes have worn through in a matter
of weeks instead of years due to errors in
selecting and inspecting materials of construction
for the particular application.
Good Housekeeping Approval
Vacuum and washdown systems provide the
tools necessary to clean up spills and dust
accumulations that will inevitably occur.
Designs run the gamut from rudimentary to
sophisticated. Choices are often determined
POWER | March 2012
http://www.powermag.com
POWER March 2012

Table of Contents for the Digital Edition of POWER March 2012
