POWER July 2010 - 58

PLANT SAFETY
The CSB Investigation Report also
found that 18% of all incidents and
fatalities involved dust collectors such
as baghouses.
the early 1990s, PRB mines were very busy
building new facilities and were required
to install baghouse collection systems as
BDT. Since this rush to build baghouses,
there have been many baghouse fires and
some violent explosions caused by the
inherent properties of PRB coal. Power
plants burning PRB coal were also having
the same experiences. "
The CSB Investigation Report also found
that 18% of all incidents and fatalities involved
dust collectors such as baghouses.
The risk of fires, explosions, and noncompliance
has prompted nearly every
PRB coal mine and more than 100 coalfired
power plants to phase out mechanical
dust collection in favor of a fogging
system. With dry fog, there is no explosive
exhaust stream.
One of the first PRB coal mines to make
the switch to dry foggers was Triton Coal
(now Black Thunder). The transition was
difficult because regulators required the
mine to keep the mechanical dust collection
systems in service until they could
demonstrate that the foggers worked at
least as well as the mechanical systems.
The foggers proved their value, and, as
they say, the rest is history. Wyoming, the
home of PRB coal, eventually permitted
fogging systems as best available control
technology (BACT) for PRB coal conveyor
transfer points, giving fog systems a Zero
Visible Emission Rating. Today, nearly every
PRB mine has a fog system as part of
its dust control strategy.
Word of the success of this " new " technology
at the PRB coal mines trickled into
the power generation industry, especially at
plants that were converting to burn PRB coal.
PRB coal mines are concerned about handling
the coal from the mine to the freight
cars. Coal-fired plants are concerned about
unloading the coal, but they're even more
concerned about storing, handling, and pulverizing
a coal with extremely friable outer
layers that dry quickly, which makes more
fines that increase the potential for baghouse
fires and dust explosions. They soon became
interested in the potential of fogging.
Fog System Application Guidelines
Like any technology, fog systems must be
properly applied to achieve the desired re58
sults.
With fog systems, a fuel-handling
transfer point is a " scrubbing chamber. "
Gilroy says that each chamber must be designed
so that airflow is properly managed,
such as by having adequately sized conveyor
skirt covers at the conveyor load point.
This is an important consideration for foggers
because air movement must be slow so
that the fog droplets injected into the transfer
point will have sufficient retention time
to attach to the airborne dust particles and
settle them out on the belt. The design must
have enough cover height for the air to expand
and enough length for agglomeration
to occur (Figures 1-4).
In the past few years, controlled-flow
chutes have taken these principles one step
further by shaping the material flow and reducing
material impact within the chute. Gilroy
notes that although some manufacturers
claim to have a " dustless " transfer chute design,
his customers say that controlled flow
transfer points have helped reduce dust generation;
however, in many cases, that is not
enough, and a fog system must be added to
achieve the desired results.
Gilroy pointed out an interesting recent
project where dry foggers were added to
an existing PRB coal-handling system. Dry
Fork Mine (part of Basin Electric Power Cooperative)
is a PRB coal mine that installed
a passive control chute under a primary
crusher to achieve the specified amount of
dust control. The staff at Dry Fork Mine has
long experience with fog systems, having
previously replaced their mechanical dust
collectors at the silo fill and conveyor transfer
points. When the conveying system was
designed for supplying coal from the mine
to the Dry Fork Power Station now under
construction, Basin Electric specified dry
fog combined with passive control chutes
for optimum control.
Basin Electric is also installing fogging
systems at its new 400-MW Dry Fork Station
in Gillette, Wyo. Each transfer point
will use both a controlled flow chute and a
fog system to achieve the highest degree of
dust control efficiency. ■
-Dr. Robert Peltier, PE is editor-in-chief
of POWER
MAXIMIZE EFFICIENCY
Revolutionize your plant to
ViSit uS At
CoAl-gen
bootH 1438
Air HAndling SyStemS | CuStom FAn deSign | FAn & VibrAtion AnAlySiS
FAbriCAtion & inStAllAtion | Field SerViCe And 24/7 emergenCy repAir
total systems solutions
visit: www.processbarron.com/power | 866-307-4318
CIRCLE 24 ON READER SERVICE CARD
www.powermag.com
POWER | July 2010
http://www.processbarron.com/power http://www.powermag.com
POWER July 2010

Table of Contents for the Digital Edition of POWER July 2010
