POWER January 2013 - 16

throat. The temperature below the yoke is close to many subbituminous
coals' auto-ignition temperatures, Storm said. As a result, coal
spillage into the wind belt under the yoke is a common cause of mill
fires. Rejected coal quickly dries and ignites in the high-temperature,
oxygen-rich environment.
Coal feed interruptions also are a potential source of fires,
Storm said. In this case, raw coal supply is interrupted due to imprecise
feeder control and stoppages above and below the feeder.
With no supply of moist coal, the higher temperatures and airto-fuel
ratios present under the yoke migrate upwards into the
grinding zone. This is also a risk in the case of mill trips or shutdown.
Accumulations of debris or coal anywhere in the pulverizer
also will increase the chance of a mill fire because accumulation
and settling in the pulverizer components allow coal to dry and
may lead to spontaneous ignition.
Storm said excessive airflow to the pulverizer provides an
abundant source of air to combust ignition sources, including
smoldering coal in the classifier, pulverizer, or raw coal under the
yoke. Smoldering coal from the bunker reaches a point of deflagration
as it travels through the feeder and moves into the mill.
Smoldering coal, which has no access to oxygen in the tightly
packed bunker, will suddenly be exposed to oxygen as it breaks
apart in transit. That, coupled with a decrease in particle size,
can compound the danger of a fire.
Storm said fundamental precautionary methods to reduce the
chance of a pulverizer " puff " include the following:
■ Ensuring that pulverizer airflow is adequate to facilitate stable
transport of coal without settling in the burner but not excessively
high to provide an abundant source of air for combustion
in the presence of an ignition source.
■ Taking all measures to prevent coal from accumulating or settling
in any of the pulverizer components.
■ Ensuring that raw coal to the pulverizer remains uninterrupted
and controllable. This can be done through precise feeder control
and minimizing stoppages above and below the feeder.
■ Ensuring that no hot smoldering or burning raw fuel is anywhere
in the pulverizer system. It is imperative that raw coal
spillage into the under bowl area be prevented.
For more information on the Asian Sub-bituminous Coal Users'
Group, visit www.asiansbcusers.org. More information on the Powder
River Basin Coal Users' Group may be found at www.prbcoals.com.
-David Wagman is executive editor of POWER.
Controlling Fugitive Combustible Coal Dust
Regardless of how much prevention is employed to mitigate combustible
dust in coal-fired power plants, fugitive coal dust is pervasive
and can be dangerous. In coal-fired power plants, mechanical transfer
points are leading sources for airborne fugitive dust. However, because
coal dust travels quickly over large areas with minimal airflow,
fugitive combustible dust settles in many areas.
Primary dust explosions occur when combustible dust is present,
forms a dust cloud (in sufficient amounts) in an enclosed
environment with an ignition source and oxygen (Figure 3). If
one were to put a flame to a layer of combustible dust on a desk,
the dust would burn, but not explode. Fanning the dust with
a piece of paper to make the dust particles airborne, however,
would create a dust cloud that could blow up.
Catastrophic secondary explosions occur when the force from
the primary explosion dislodges fugitive dust that has been allowed
to accumulate on walls, floors, and other horizontal surfaces
such as equipment ledges, above suspended ceilings, and
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POWER | January 2013
Dispersion of dust particles
on other concealed surfaces, producing more dust clouds and
creating a domino effect that causes further explosions.
Preventing Explosions by Using Appropriate Vacuum
Cleaners
Bill Bobbitt of Bobbitt Associates Environmental Systems, who's
been working in the safety field for more than 25 years, said, " I
always tell my clients, it not a matter of if, but when. Conditions
have to be perfect and that 'when' can be 30 years from now, or
it could be next week. But if you eliminate the fugitive dust, it
cannot create a secondary dust explosion. "
The National Electrical Code (NEC) defines hazardous locations
as those areas " where fire or explosion hazards may exist due to
flammable gases or vapors, flammable liquids, combustible dust,
or ignitable fibers or flyings. " Hazardous locations are classified
in three ways by the NEC: type, condition, and nature. Class
II locations are those areas made hazardous by the presence of
combustible dust. Finely pulverized material, suspended in the
atmosphere, can cause powerful explosions.
The NEC also specifies that hazardous material may exist in several
different kinds of conditions, which, for simplicity, can be described
as normal conditions (Division 1) and abnormal conditions (Division
2). In the normal condition, the hazard would be expected to be
present in everyday production operations or during frequent repair
and maintenance activity. When the hazardous material is expected
to be confined within closed containers or closed systems and will be
present only through accidental rupture, breakage, or unusual faulty
operation, the situation would be called abnormal.
As the first line of defense in housekeeping routines to prevent
catastrophic explosions caused by combustible dust and comply with
regulatory agencies, plant personnel need to employ industrial vacuum
cleaners that are built from the bottom up to be used in a variety
of Class II, Division 2 areas (Figure 4). Redundantly grounded indus3.
Explosive situation. Primary dust explosions occur when
combustible dust such as coal dust is present, forms a dust cloud (in
sufficient amounts) in an enclosed environment with an ignition source
and oxygen. If any one of these elements is missing, there can be no
explosion. Source: National Fire Protection Association
Ignition
Confinement of dust cloud
Combustible dust
Oxygen in air
4. Housekeeping helper. Cleaning up the abundant dust around
the boot seals in this cement plant is more effective with the VACU-MAX
air-operated industrial vacuum cleaner than with shovels and
wheel barrows. Courtesy: VAC-U-MAX
http://www.asiansbcusers.org http://www.prbcoals.com http://www.powermag.com
POWER January 2013

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