Phenomenon of Spontaneous Ignition Is Still Misunderstood by Some
A $1.8 million fire destroyed a portion of a large multistore occupancy in Wisconsin (Fire Engineering, September 1979). The cause: spontaneous heating of rags soaked with thioglycolic acid and sodium bromate, chemicals used by the hairdresser in the shopping center.
Ten months earlier, a fire occurred in a silo in New York containing spent grain from brewing operations. Again, the cause was spontaneous ignition.
These incidents are not singled out because they are unusual, however, but rather because they are quite typical.
But just what is spontaneous ignition? We can read that it is “the ignition of a substance or body from the rapid oxidation of its own constituents, without heat from an external source.” The requirements for fuel, an oxidizing agent and temperature exist just as they do for any other fire. It’s just that the temperature rise is generated within the fuel rather than by an external source. The term spontaneous combustion is synonymous with spontaneous ignition, although the latter is currently preferred.
Note however that nothing is mentioned about a time factor in this definition. This is as it should be, for there is nothing about the spontaneous ignition process that makes time a critical factor. A common misconception is found in the fire service, though, and indeed has even found its way into some textbooks, to the effect that ignition is virtually instantaneous, that flames suddenly appear on the surface of a fuel in spontaneous ignition. It can indeed be instantaneous, although usually it is not so. More commonly, spontaneous heating and ignition require a period of time that may range from several minutes to several years!
The word spontaneous simply means that the process arises from internal forces or causes, that it is unplanned and that it is a natural process.
Conditions for heating
Despite the myriad circumstances in which spontaneous heating occurs, there are three general conditions that govern whether or not heat will build up to the point of ignition including:
- The rate at which heat is generated within the fuel,
- The oxygen supply at the point of heat buildup in the fuel mass,
- The rate of heat loss to the surrounding environment.
Virtually all substances of biological origin are chemically capable of reacting with atmospheric oxygen with an accompanying release of heat. These oxidation reactions are typically very slow, however, and the heat is dissipated to the surrounding atmosphere as quickly as it is released. Thus the temperature within the fuel does not increase very much, and certainly not to a dangerous level.
But this is not the case with materials that are subject to what we know as spontaneous heating and spontaneous ignition. When materials of this latter group are oxidized, heat accumulates much faster than it is dissipated. As a result, the temperature of the fuel rises and ignition can ultimately occur.
The volume of air (oxygen) present is critical in determining whether heating will occur. There must be adequate oxygen to support the oxidation reaction, yet the air flow in and around the fuel must not be great enough to dissipate the heat by convection as it is released.
Method of storage
Consider a cotton rag that has been soaked with linseed oil. As fire fighters, we know that this is a potentially dangerous situation. Linseed oil is often highly subject to spontaneous heating if the rags are piled in a poorly ventilated corner somewhere. If these same rags are hung outside on a clothesline, however, spontaneous ignition will not occur because the heat generated by chemical oxidation is dissipated into the air just as rapidly as it is released by the reactions.
So, spontaneous heating and ignition depend not only upon the chemical composition of the potential fuel but also upon the conditions surrounding its storage at the time the heating occurs.
Despite the many types of materials that are subject to spontaneous heating under varying conditions, the basic causes of the heating are just two: biological processes (metabolism) and chemical reactivity.
Biologically induced heating occurs primarily in agricultural products such as hay, feeds, grains and manures that have been incompletely dried before being stored. Fire fighters in rural areas may recall a rash of barn fires each autumn as farmers hasten to get their hay crop inside before winter.
Bacteria are the agents responsible for initial heating in these materials. These microscopic organisms are inherent to all agricultural products. Like every living organism, bacteria respire. (There is an important difference between breathing and respiring. One of the products of respiratory metabolism in any organism is heat.) Bacteria buried deep within a pile of hay, for example, respire, raising the temperature in the hay to about 160 degrees, if the pile is poorly ventilated so that heat cannot be dissipated. Bacteria, just like humans, cannot survive very long at high temperatures, however. So at about 160 degrees the bacterial cells die. Respiratory metabolism and biological heat production cease.
At 160 degrees or so, a second process is initiated, that of chemical oxidation. And, as you know, chemical oxidation is accompanied by heat release also. Thus, the temperature within the hay pile continues to rise, eventually reaching the ignition point for the hay. When this happens, the hay deep within the pile begins to smolder. The temperature rises even more rapidly, the smoldering increases, and very soon the hay may start burning freely.
The potential for biologically induced spontaneous heating can be eliminated by storing agricultural products under low moisture conditions and in wellventilated areas. The danger period for most agricultural products is between two and six weeks after being stored.
The frequency of barn fires has declined sharply in the last few years, not because of successful fire prevention but rather due to a major change in farming practices. Most farmers now cut and bale hay whereas previously it was mowed and stored in bulk. The bales are wrapped tightly with wire so there is little internal air space, and without adequate oxygen there can be no heating and ignition.
One of the most widely recognized types of spontaneous heating is that which occurs in oily rags. Let us understand, however, that it is only oils of vegetable and animal origin that spontaneously heat and ignite, not petroleum oils. Most people outside of the fire service do not realize this, and attempts to mislead fire investigators with petroleum (mineral) oil-soaked rags have been made.
Fires attributable to paint-soaked cloths and brushes also belong in the oily-rag category because it is the vegetable oils in the paint that are the heating agents.
When oil or paint-soaked rags are piled in a poorly ventilated corner of a room or other confined space, spontaneous heating is likely to occur. This begins with a slow combination of atmospheric oxygen with the oils. The heat released by this oxidation increases the temperature within the mass of rags, causing oxidation to proceed increasingly faster because the heat cannot be dissipated by convection. It is not possible to estimate just how long it may take for ignition to occur; there are too many variables involved. It may be accurately said, though, that improper disposal of vegetable and animal oils has historically been a major cause of spontaneously ignited fires.
The problem of spontaneous heating in painting supplies has been lessened considerably in recent years with the introduction of water-soluble latex paints. These paints lack the oils in their composition that are subject to such heating.
Different vegetable and fish oils vary widely in their susceptibility to spontaneous heating. It isn’t possible to establish an absolute measure of the spontaneous heating hazard for any material, but there are chemical tests that can be conducted to determine a relative tendency to do so. Based on these tests, the following oils have a high tendency to heat: fish oils and meals (cod liver and herring), linseed oil and most oil-based pigments.
Corn, olive and cottonseed oils have a moderate tendency to heat spontaneously under certain ill-defined conditions, as do sunflower and tobacco seed oils.
Those products with a slight propensity to heating including cotton, castor, coconut and palm oils, and turpentine.
These abbreviated lists are offered only as examples of materials in each category. Detailed information for virtually every material with any tendency to heat spontaneously may be found in the current edition of the NFPA Handbook of Fire Protection.
Many metals are pyrophoric, meaning that they ignite spontaneously under certain conditions. The list includes relatively common metals like cobalt, copper, magnesium, calcium, sodium and potassium, as well as more exotic metals such as thorium, hafnium, zirconium, uranium and plutonium. The conditions under which each of these metals may ignite differ greatly from one metal to the other, so it isn’t possible to generalize very much about them.
Metals that a fire fighter is likely to encounter in large quantities are not usually subject to spontaneous heating unless they are in the form of powder or fine lathe turnings. If the metal powder is moist, the chances of its heating are greatly enhanced. The subject of pyrophoricity of metals is complex, however. Therefore, it you should come upon such a questionable metal during a fire prevention inspection or a fire investigation, consult a chemist knowledgeable about these materials.
Numerous chemicals react in the presence of water, some violently and others slowly. Most water-reactive chemicals fall within the broad categories of the alkalies, carbides, anhydrides, hydrides and oxides.
As with any material subject to spontaneous ignition, the key to avoiding fires from water-reactive chemicals is prevention through proper storage and handling. Obviously they shouldn’t be placed in an area where an accidental sprinkler discharge would shower water over them, for example. Also, these materials must be stored in a noncombustible room or building where all potential sources of water have been eliminated.
Air-reactive chemicals are usually stored under water or oil in unbreakable containers. Fires have occurred in classrooms and laboratories when an air-reactive material was dropped or spilled. The protective oil or water is gone and ignition occurs quickly. Fires in these materials are best fought with large volumes of water and, in some cases, with dry chemicals. Remember that the material will reignite if the water runs off or the dry extinguishing agent dissipates and it once again becomes exposed to air.
Coal and charcoal
Charcoals in general can spontaneously heat, especially those freshly made from hardwoods such as hickory, oak and ash. The heating is most likely to occur if the charcoal has not been cooled sufficiently or is moist when it is bagged or stored. Some economyminded people routinely extinguish charcoal with water after cooking with it. It is then dried and saved for future reuse. This is a very dangerous practice because it is not likely that the charcoal will be adequately dried prior to storage. If this must be done, recommend storage in a tightly closed metal container.
Some types of coal also spontaneously heat. The bituminous, or soft, coals pose the greatest danger in this regard. Anthracite, or hard coal, is not usually dangerous.’ Lignite, or brown coal, is a variety of coal between peat and bituminous. It, too, spontaneously heats if not properly stored.
Spontaneous heating in piles of lignite and soft coal is most apt to occur if the ambient temperature exceeds 80 degrees. It is virtually certain that selfheating to the ignition point will continue once the temperature within the coal pile reaches 140 degrees. Combustible impurities in the pile increase the risk of self-ignition.-
The first detectable signs of heating deep within a bulk pile of material may be an unusual odor or perhaps a trace of smoke in the air. Closer inspection may reveal slightly elevated surface temperatures.
If these conditions are found, do not open the pile to search for the source of the odor or smoke. Sudden exposure of smoldering material to air may cause almost instantaneous ignition of the entire mass of material. Instead of opening the pile, probe into it with a thermometer or thermocouple. If the temperature within the pile is much higher (more than 10 to 15 degrees) than the ambient’ temperature, potentially dangerous heating is occurring.
If heating is discovered, lay hose lines before attempting to open the pile. Although the size of the pile is the governing factor, at least two 2 ⅛ -inch lines should be positioned and charged. Slowly open the pile, thoroughly wetting down any hot spots as they are dug out. Don’t rush the job—you may cause the entire mass of material to ignite.