EH-93-4 The Fire Below: Spontaneous Combustion in Coal
ENVIRONMENT SAFETY & HEALTH
BULLETIN
Assistant Secretary for U. S. Department of Energy
Environment, Safety & Health Washington, D.C. 20585
DOE/EH-0320 Issue No. 93-4 May 1993
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THE FIRE BELOW: SPONTANEOUS COMBUSTION IN COAL
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How Coal Self-Ignites
The coal's temperature begins to climb above ambient. At about 150-300
degrees F, it begins to give off minute, but measurable, quantities of
gas--aerosols, hydrogen, and CO(2)--precursors of combustion. As the
temperature increases further--at about 600-700 degrees F--relatively,
large, visible particulates are emitted. Soon, as the heating rate
increases in intensity to about 750-800 degrees F, incipient combustion,
and ultimately self-ignition and flame, will occur.
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At least a dozen coal fires occurred within the Department of Energy (DOE)
over the last decade. Seven sites were involved, with two of the seven
experiencing multiple incidents. Fortunately, none of the fires resulted
in fatalities. However, damages from all but one ran into the thousands
of dollars, and the most severe incident caused a minimum os $800,000
damage.
The risk from fire exists anywhere significant amounts of coal are in use
or storage. Coal is a combustible material, making it susceptible to a
variety of ignition scenarios. One of the most frequent and serious
causes of coal fires in spontaneous combustion, which has been responsible
for a number of incidents within the Department in recent years.
Preventing spontaneous combustion coal fires involves attention to many
different factors. Among the most critical are the type, age, and
composition of coal, how it is stored, and how it is used. Given the
right kind of coal, oxygen, and a certain temperature and moisture
content, coal will burn by itself.
Spontaneous combustion has long been recognized as a fire hazard in stored
coal. Spontaneous combustion fires usually begin as "hot spots" deep
within the reserve of coal. The hot spots appear when coal absorbs oxygen
from the air. Heat generated by the oxidation then initiated the fire.
Such fires can be very stubborn to extinguish because of the amount of
coal involved (often hundreds of tons) and the difficulty of getting to
the seat of the problem. Moreover, coal in either the smoldering of
flaming stage may produce copious amounts of methane and carbon monoxide
gases. In addition to their toxicity, these gases are highly explosive in
certain concentrations, and can further complicate efforts to fight this
type of coal fire.
Even the most universal firefighting substance, water, cannot be used
indiscriminately. Because of the remote possibility of a steam explosion,
it is advisable that water be applied carefully and from a safe distance.
Certain chemicals such as carbon dioxide or nitrogen may mitigate fire
effects, but their use has had mixed success from a DOE perspective. The
above information suggests that coal fires require awareness and prior
planning to extinguish efficiently, completely, and safely.
Recent Coal Fire at DOE Site
In 1992, a DOE site experienced its most recent coal fire initiated by
spontaneous combustion. Due to the nature of the fire and initial
ineffectiveness of the means utilized to fight it, the fire required more
than 28 hours to completely extinguish from the time a hot spot was first
detected in the coal bunker. The initial strategy involved trying to
remove coal from the bunker by feeding it more rapidly to the boiler and
by using a drag chain to move more of it to the field. The drag chain
failed in 30 minutes, however. Subsequent efforts to control the fire
with carbon dioxide applied through inspection ports at the bottom of the
bunker and from the tripper room high above the bunker were ineffective,
and may have worsened the situation. The drag chain emptying coal from
the bunker worked intermittently after being repaired, and finally
stopped. Boiler plant personnel then began to remove burning coal by hand
shovel.
Twenty-one hours after the fire was discovered, it had involved a large
amount of the bunker. At one point, flames appeared at the tripped room
windows, which are approximately 75 feet above the seat of the hot spots.
A strong concern for a steam explosion delayed the application of water,
but the decision was finally made to use water, which was applied without
incident and eventually ended the fire.
Causes of Spontaneous Coal Fires
The incident related above serves as an excellent case study of a
spontaneous combustion coal fire. It illustrated some major causes of
such fires as well as the problems of fighting them effectively and
safety. The following general factors have been mentioned as contributing
causes:
o Coal handling procedures allowed for long-time retention of coal,
which increases the possibility of heating.
o New coal added on top of old coal created segregation of particle
sizes, which is a major cause of heating.
o Too few temperature probes installed in the coal bunker resulted in
an excessive period of time before the fire was detected.
o Failure of equipment needed to fight the fire (drag chain conveyer).
o Ineffective capability and use of carbon dioxide fire suppression
system.
o Delay in the application of water.
o Inadequate policies, procedures, and training of personnel prevented
proper decision making, including the required knowledge to
immediately attack the fire.
o Failure to learn lessons from two previous coal bunker fires at the
same installation.
DOE Coal Fires in the Last Decade
Other coal fires caused by spontaneous combustion have occurred at DOE
facilities in the last decade. In 1985 spontaneous combustion was
responsible for starting two fires in a coal bunker at a DOE site. The
fires were extinguished by injecting gaseous nitrogen into the bunker near
the hot spots. The cause was determined to be the excessive storage time
of the coal. In addition, excessive compaction caused by fines (finely
crushed coal) contributed to the rate of ignition.
In 1991 at another DOE site, a smoldering fire was discovered in an unused
coal bunker which had accumulated 2 cubic feet of coal dust from an
overhead coal conveyer system. The fire was believed to have started
spontaneously, through careless smoking was also listed as a possible
cause. At still another site, coal stored too long in a silo ignited due
to spontaneous combustion.
An unusual fire which occurred in 1991 illustrates the opportunistic
nature of spontaneous combustion on coal. A backhoe had been sitting for
3 hours after the close of work. For the previous 2 days, the backhoe had
been mucking out slag and coal collection ponds to remove coal fines. A
security guard noticed a fire on the backhoe rear tire. Facility
personnel believe the cause of the fire was spontaneous combustion of the
coal fines which had adhered to the tire. Some of the thick coal mud in
the ponds had not been exposed to oxygen until it was churned up by the
tires of the backhoe. Personnel were instructed to wash equipment tires
when leaving the ponds for the day.
Preventing Spontaneous Combustion in Stored Coal
Hugh quantities of coal are stored in bunkers, silos, hoppers and open air
stockpiles. How susceptible such stocks of coal are to fire from
spontaneous combustion depends on a number of factors, from how new the
coal is to how it is piled.
Recommendations:
o Know your coal. Anthracite (sometimes called eastern coal) has a
high carbon content and is much less combustible than low oxygen
content bituminous (or western) coal. Freshly mined coal absorbs
oxygen more quickly than coal mined at an earlier time, and is more
likely to head spontaneously.
o Storing coal with a low sulphur content is helpful. Sulphur
compounds in coal liberate considerable heat as they oxidize.
o Air circulating within a coal pile should be restricted as it
contributes to heating; compacting helps seal air out.
o Moisture in coal contributes to spontaneous heating because it
assists the oxidation process. Moisture content should be limited
to 3 percent; sulphur content should be limited to 1 percent, "as
mined." Coal having a high moisture content should be segregated
and used as quickly as possible. Efforts should be made to keep
stored coal from being exposed to moisture.
o Following the "first in, first out" rule of using stock reduces the
chance for hot spots by helping preclude heat buildup for portions
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of stock which remain undisturbed for a long term. The design of
coal storage bins is important in this regard.
o A high ambient temperature aids the spontaneous heating process.
o Use coal as quickly as practicable. The longer large coal piles are
allowed to sit, the more time the spontaneous process has to work.
o The shape and composition of open stockpiles can help prevent fires.
Dumping coal into a big pile with a trestle or grab bucket can lead
to problems. Rather, coal should be packed in horizontal layers
(opinions range from 1 1/2' to 3' high) which are then leveled by
scraping and compacted by rolling (See Figure 1, not included in
SPMS, see original Bulletin for Figure). This method helps
distribute the coal evenly and thus avoids breakage and segregation
of fine coal. Segregation of coal particles by size should be
strenuously avoided, as it may allow more air to enter the pile and
subsequent heating of finer sizes.
o The height of the coal pile is also important. Limit unlayered,
uncompacted high grade coal to a height of 15' (10' for low grade
coal); maximum height is 26' for layered and packed coal.
o Properly inspect, test and maintain installed fire protection
equipment.
o Maintain an updated pre-fire plan and encourage regular visits to
coal facilities by the site or local emergency response force.
Fig. 1 (Not included, see original Bulletin). Poor distribution of fine
and coarse coal resulting from dumping from a high trestle or grab bucket
without rolling (above), compared with good distribution and exclusion of
air by roll-packing method. (Illustration courtesy of Factory Mutual
Engineering Corp., 1975)
Pertinent Orders, Notices and Rules
DOE Order 5480.7A, Fire Protection, includes requirements for physical
fire protection features, hazards analysis, fire prevention procedures,
and periodic fire safety assessments.
29 CFR, Part 1910, Subpart L, Fire Protection, includes requirements for
fire departments, such as pre-fire planning and training, as well as
criteria for physical fire protection features.
Guidance
The DOE Fire Protection Resource Manual includes model fire prevention
procedures, fire protection system testing and maintenance procedures, and
model fire department operating procedures.
Factor Mutual, Coal and Charcoal Storage, Loss Prevention Date, #8-10,
August 1975, provides guidance on prevention of coal fires.
A Lesson Learned from the Titanic
Deep-seated coal fires are not a new problem. J. Dilley, survivor of the
sinking of the TITANIC, reported to following:
The TITANIC sailed from Southhampton on Wednesday, April 10, 1912,
at noon. I was assigned to the TITANIC from the OCEANIC, where I
served as a fireman. From the day we sailed the TITANIC was on
fire, and my sole duty, together with eleven other men, had been to
fight that fire. We had made no headway against it.
The fire started in bunker No. 6. There were hundreds of tons of
coal stored there. The coal on top of the bunker was wet, as all
the coal should have been, but down at the bottom of the bunker, the
coal had been permitted to get dry.
Two men from each watch of stokers were told off, sir, to fight that
fire. The stokers, you know, sir, work four hours at a time, so
twelve of us was fighting flames from the day we put out of
Southhampton until we hit the iceberg.
No sir, we didn't get that fire out, and among the stokers there was
talk, sir, that we'd have to empty the big coal bunkers after we'd
put our passengers off in New York and then call on the fireboats
there to help us put out the fire. But we didn't need such help.
It was right under bunker No. 6 that the iceberg tore the biggest
hole in the TITANIC, and the floor of water that came through, sir,
put out the fire that our tons and tons of water had not been able
to get rid of.
This Bulletin is one in a series of publications issued by EH to share
occupational safety information throughout the DOE complex. To be added
to the Distribution List or to obtain copies of the publication, call
(615)576-7548. For additional information regarding the publications,
call Barbara Bowers, Safety Performance Indicator Division, Office of
Environment, Safety and Health, U.S. Department of Energy, Washington, DC
20585, (301)903-3016.