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 ------------------------------------------------------------------------- THE FIRE BELOW: SPONTANEOUS COMBUSTION IN COAL ------------------------------------------------------------------------- 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. ------------------------------------------------------------------------- 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 :: 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.