FEBRUARY 25, 2015
SAN ANTONIO, TEXAS
(AP)
Authorities say
three workers were injured in an explosion at a San Antonio cement plant. All workers had burns from the explosion.
Spokeswoman Deborah
Foster of the San Antonio Fire Department says the Tuesday afternoon blast
occurred in a coal mill that led to a fire at Capitol Aggregates on
Nacogdoches.
The company's carbon dioxide suppressant system had already
put out the fire by the time firefighters arrived at the scene. Authorities haven't announced what caused the
explosion or the identities of the workers.
However, the use of coal in mills in the concrete industry has been known
to be susceptible to explosion and fire risks.
At least one of the employees was taken to a hospital, where
his or her condition wasn't immediately known.
Phenomena of a Coal Dust
Explosion
To understand fully the hazards of using pulverized coal as a
fuel in a cement plant, one must become familiar with the factors that typically
enter into the development of an explosion.
A typical pulverized coal fuel system must process bulk coal
into a form that can be efficiently utilized as a fuel to heat the kiln for calcining
the raw materials (clay, limestone, and so forth) into clinker.
This is usually accomplished by grinding and drying
the bulk feed in a pulverizer so that the coal emerging from the pulverizer will
consist of 70 to 80% particulate that will pass through a 200-mesh U.S. screen sieve
(that is, particles with a diameter of 74 km or less).
High temperature air from the clinker cooler is
often used to dry the coal and convey it from the pulverizer to the burning pipe
in the kiln. The coal pulverizer is one of
the most hazardous pieces of equipment from a fire and explosion viewpoint because
fuel and oxygen for combustion are always present.
A dust explosion is often described as a rapid burning of combustible
particulate within a confined area, which results in the generation of intense heat
and corresponding pressure rise. If not vented
adequately, the rise in pressure caused by the rapid buildup of heat can cause damage
to the confining vessel and the surroundings. The following factors must be present
to cause a dust explosion:
1.
the presence of dust in suspension in a concentration
above its flammable limit,
2.
sufficient oxygen to enable combustion of the fuel,
3.
a source of energy to ignite the fuel, and
4.
a certain degree of confinement of the suspended
dust mixed with oxygen.