POWER May 2011 - 77

HEALTH & SAFETY
Proactive Strategies for
Dealing with Combustible Dust
The challenges of using Powder River Basin (PRB) coal are as significant as the
rewards. The subbituminous coal contains lower amounts of sulfur dioxide
than bituminous coal but can be prone to combustible dust explosions
if it is not properly managed. To eliminate such hazards, plant personnel
need to establish best practices for the safe operation and maintenance
of PRB coal-handling and -storage systems based on best available technologies.
By
Mark R. Collett, Plant Professionals
T
here are strong justifications for the use
of Powder River Basin (PRB) coal as
a fuel for electricity generation. Specifically,
PRB coal has many valuable advantages,
including its competitive price, low
caloric value, and lower emissions of sulfur
oxides and nitrogen oxides compared with
other types of coal. In addition, other fuels
used for power generation have significant
drawbacks. For example, there are troubling
questions about the long-term price stability
of natural gas (currently a relatively low-cost
fuel) and the safety of nuclear energy.
On the flip side, PRB coal's main challenge
is that its use can lead to combustible
dust explosions if plant personnel fail to
handle it correctly. In order to continue utilizing
this environmentally friendly PRB in
our power generation facilities, we need to
make protecting workers and managing the
risks top priorities.
Despite PRB coal's volatile properties,
there is a race to find methods for redistributing
PRB coal to Europe and Asia. For example,
I found it interesting that many of the
power generation facilities I recently visited
in mainland China were more interested in
PRB coal than Indo-coal.
Handling Challenges Related to
Moisture and Friability
PRB coal has high moisture, generally 28%
to 30% when transported by rail cars. The
reduction of PRB coal's high moisture content
through transport or placement into new
environs is normally followed by the coal's
re-adsorption of moisture. While the coal
establishes a new moisture equilibrium, exothermic
reactions can result in self-heating.
Incidents of spontaneous combustion can
then lead to combustible dust explosions.
Another safety concern related to PRB
coal is its friability. When it crumbles, it exposes
many times the surface area per pound
compared with bituminous coal. Friability
can result in dangerous quantities of fine coal
fragments, dangerous combustible dust, and
higher dust emissions (Figure 1).
Problems with Dust Dispersion
Most material-handling systems do not have
the containment features and new technolo1.
Falling to pieces. This 5-day-old undisturbed coal fractured simply as a result of exposure
to oxygen and the rapid decrease in moisture content. Courtesy: Benetech Inc.
gies required to keep the material and dust
from exiting the process flow at the point of
source (POS). As a result, dust can be dispersed
into the air and, when exposed to oxygen
and a source of ignition, may become a
significant danger by causing an explosion
and/or flash fire before the dust settles for the
first time (Figure 2).
In a typical power plant setting, dispersed
combustible dust eventually finds its way to
horizontal and near-horizontal surfaces (up to
an angle of approximately 65 degrees). Any
surface covered with this combustible dust is
as dangerous as a pool of gasoline. Even a
minor primary or secondary explosion elsewhere
in the plant can disperse dust sitting
on rafters, pipe racks, or structural members
back into the air, forming a lethal mixture.
Combustible dust piles on floors and equipment
at power plants can likewise be lofted
from their resting places and cause an insidious
dispersion of dust that lacks only an
ignition source to become a life-threatening
hazard.
Many areas of the fuel yard and fuelhandling
system also experience flow,
2. Deep dust. Dangerous levels of combustible
dust accumulations. Courtesy: Plant
Professionals (P2), a division of Benetech Inc.
76
www.powermag.com
POWER | May 2011
http://www.powermag.com

POWER May 2011

Table of Contents for the Digital Edition of POWER May 2011

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