POWER August 2012 - 47

PLANT DESIGN
Flow Control Chutes Reduce
Fugitive Coal Dust
Moving thousands of tons of coal per hour at high speeds through a complex
handling system is a main cause of airborne coal dust in a coal-fired plant.
Depending upon the coal's characteristics, that dust can become explosive
when its concentration reaches 80 g/m3 and, hence, a threat to life
and property. The best option is to stop the dust from becoming airborne
in the first place.
By Daniel Mahr, PE, Energy Associates PC and Michael A. Schimmelpfennig, PE, Ameren Missouri
T
he dangers and causes of explosions
in power plants caused by fugitive
coal dust have been discussed
at length in POWER and have been addressed
extensively by the Occupational
Safety and Health Administration's regulatory
response. (See " Coping with Coal
Dust, " March 2012; " Proactive Strategies
for Dealing with Combustible Dust, " May
2011; and " A Burning Concern: Combustible
Dust, " May 2010, all available in the
archives at www.powermag.com.) Owners
of new or soon-to-be upgraded coal-handling
systems should be proactive in reducing
fugitive dust emissions by using flow
control chute technology. In this article we
examine the features, benefits, and limitations
of flow control chutes and report on a
good application of this technology.
Identifying Dust Sources
High-capacity conveying systems handle
thousands of tons of coal per hour, and there
are a number of conditions, situations, and locations
on a coal conveyor where dust can be
produced. Even when a small fraction of the
coal throughput becomes airborne as dust, it
can become an unacceptable safety hazard,
particularly when dust levels reach explosive
concentrations.
Any coal or other solid fuel that contains
small, dry particles is a prime dust
source. Moisture content and particle size
are important properties to consider when
controlling dust. The obvious control option
is to add moisture. Coal stockpiles are
often sprayed by water cannons to control
windblown dust from the surface. Spray
headers at coal-handling system transfer
points also are used to wet coal as it is
discharged from one conveyor to another
or to the stockpile. Inevitably, some moist
coal particles will adhere to the surface
of the conveyor belt. These particles start
48
to dry as the belt travels along the return
strand. Flexing of the belt around pulleys
and over return idlers releases the bond, so
larger particles spill onto the floor below,
and the smallest particles become an airborne
cloud.
Coal spillage from an operating conveyor
increases the dust dilemma. Spillage can be
caused by momentary overloads or surges,
off-center loading of the belt, poorly fitted
seals, holes in worn plates, and inspection
ports that are left open or are poorly fitted.
Worn or inadequate belt cleaners can also
cause spillage. This is most clearly seen
behind the conveyor's discharge, at vertical/horizontal
take-up areas, and at the tailloading
section of the conveyor. However,
the primary airborne dust sources in most
coal-fired plants are the conveyor transfer
points. Therefore, the most economic design
approach is to first focus on the transfer
points to get the best return on the plant's
investment dollar.
Classic Chute Design
Belt conveyors were once designed with
relatively low capacity and slow belt speeds.
The discharge of coal from the head pulley
of a conveyor fell (usually) vertically onto
shallow sloped chute surfaces, which slowed
the speed of the falling coal stream. The geometry
allowed for a relatively low transfer
height, which by default helped to minimize
dust. Historically, plants handled hard coals
rather than a product with a high percentage
of small particles, such as today's Powder
River Basin (PRB) coals, which are more friable
and prone to spontaneous combustion.
Classically designed chutes on older plants
burning hard, washed coals had fewer dust
problems because of higher surface moisture
and fewer small particles.
1. Classic discharge trajectory. For this 72-inch-wide belt operating at 760 feet per
minute, the stream of coal impacts a curved directional plate, which can slide and rotate to turn
the falling coal and better center it on the receiving belt. Courtesy: Energy Associates PC
Discharge head pulley
Coal stream trajectory
Top belt
Baffle plate
Return belt
Liner plate
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POWER August 2012

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