The Primer on Spontaneous Heating and Pyrophoricity is approved for use by all DOE Components. It was developed to help DOE facility contractors prevent fires caused by spontaneous combustion. Spontaneously combustible materials include those that ignite because of a slow buildup of heat (spontaneous heating) and those that ignite instantly in air (pyrophoricity). The scientific principles of combustion and how they affect materials known to be spontaneously combustible are explained. The fire hazards of specific spontaneously heating and pyrophoric materials are discussed as well as techniques to prevent their ignition. Suitable fire extinguishing agents are included for most materials as well as safety precautions for storage and handling.
The Department of Energy (DOE) Primers are a set of fundamental handbooks on safety-related topics of interest in the DOE Complex. The Primers are written as an educational aid for operations and maintenance personnel. The Primers attempt to supply information in an easily understandable form which will help them perform their duties in a safe and reliable manner. Persons trained in other technical areas may also find the Primers useful as a guide or as a reference source for further investigation.
The DOE Primer series draws heavily upon the subject-specific Primers and training materials previously developed by DOE sites (Savannah River, Rocky Flats, and Mound) and is intended for distribution to all DOE contractors. Information is also drawn from the applicable volumes of the DOE Fundamentals Handbook series developed by the DOE Office of Nuclear Safety Policy and Standards. References to other material sources are indicated in the text where applicable and a bibliography is included.
Beneficial comments in the form of recommendations and any pertinent data that may be of use in improving this document should be addressed to
by using the U.S. Department of Energy Standardization Document Improvement Proposal Form appearing at the end of this document or by letter.
Key words: Combustion, Fire, Spontaneous, Ignition, Heating, Pyrophoricity, Temperature, Hypergolic, Extinguishing Agent, Hydrocarbons, Organic, Heating, Specific Area, Liquids, Gases, Metals, Oxidizer
The Department of Energy Primer on Spontaneous Heating and Pyrophoricity was prepared as an information resource for personnel who are responsible for operation of the Department's nuclear facilities. An understanding of spontaneous heating and pyrophoricity hazards is necessary for DOE facility personnel to operate and maintain facilities and facility support systems in a safe manner.
The Primer on Spontaneous Heating and Pyrophoricity contains an introduction and sections on the following topics:
The information contained in this Primer is by no means all-encompassing. However, enough information is presented to provide the reader with a fundamental knowledge level sufficient to recognize most spontaneous combustion hazards and how to prevent ignition and widespread fires. This Primer is provided as an information resource only, and is not intended to replace any fire protection or hazardous material training.
The Department of Energy wishes to acknowledge the contributions of all those who supplied literature and technical expertise for this Primer. Much of the information contained in this Primer was obtained from the 17th Edition of the National Fire Protection Association (NFPA) Handbook, 1991, and the 2nd Edition of Chemistry of Hazardous Materials, by E. Meyer, 1989.
The purpose of this Primer is to provide operations and maintenance personnel with the information necessary to identify and prevent potential spontaneous combustion hazards. Throughout the history of industry and the DOE Complex, fires caused by spontaneously heating and pyrophoric materials have occurred, sometimes causing personal injury and significant damage to facilities. By its very nature, spontaneous heating and pyrophoricity are among the most insidious types of fire hazards. Many times there is no outward evidence of the potential for fires caused by these phenomena. An understanding of the principles of spontaneous heating and pyrophoricity is necessary for instituting fire prevention measures.
Upon completion of this Primer, the reader should be able to do the following:
Some materials ignite instantly when exposed to air or other oxidizing atmosphere. Other materials such as coal may take weeks to ignite. This difference in time until ignition is the difference between pyrophoricity and spontaneous heating and will be explained in detail later in the text.
Combustion (burning, or fire) falls into a class of chemical reactions called oxidation. Oxidation may be defined as the chemical combination of a substance with oxygen or, more generally, the removal of electrons from an atom or molecule. Oxidation reactions are almost always exothermic, or release heat.
Many materials react with oxygen to some degree. However, the rates of reactions differ between materials. The difference between slow and rapid oxidation reactions is that the latter occurs so rapidly that heat is generated faster than it is dissipated, causing the material being oxidized (fuel) to reach its ignition temperature. Once the ignition temperature of a material is reached, it will continue to burn until the fuel or oxygen is consumed. The heat release during combustion is usually accompanied by a visible flame. However, some materials, such a charcoal, smolder rather than produce a flame.
A familiar slowly occurring oxidation reaction is the rusting of iron. Such a reaction releases heat so slowly that the temperature hardly increases more than a few degrees above the temperature of the surroundings. These slow reactions do not cause fires and are not considered combustion.
Generally, there are three items necessary to support combustion:
These are depicted pictorially in Figure 1, commonly called the Fire Triangle. The Fire Triangle shows that for combustion to occur, fuel, an oxidizing agent, and a heat source must all be present in the same place at the same time. If any one of the legs of the triangle are removed, the fire will be extinguished.
An oxidizing agent (or oxidizer) is a chemical substance that gives up oxygen easily, removes hydrogen from another substance, or attracts electrons. By far the most common oxidizing agent is the oxygen in the earth s atmosphere. However, there are many chemical compounds that also act as oxidizing agents. Some of these agents react with fuels more readily and violently than oxygen. Further information on the relative hazards and classifications of oxidizing agents may be found in Appendix B and in NFPA 43A, Code for the Storage of Liquid and Solid Oxidizing Materials.
A fuel is the substance that reacts with the oxidizing agent during combustion. Fuels can be solids, liquids, gases, and even metals. Familiar fuels are coal, firewood, and gasoline. For the purpose of this document, the discussion of fuels will be limited to those that are known to combust spontaneously.
Normally, a heat source such as a flame of spark is required to ignite a mixture of a fuel and oxidizing agent. That is, heat must be added or the fuel and oxidizing agent will not react. The reactions that are the subject of this document are special instances where no heat source is required to ignite the fuel. These fuels react so readily with oxygen that a heat source is not required for ignition. Ignition for these fuels occurs as a result of spontaneous heating or pyrophoricity.
Spontaneous combustion is the ignition of a combustible material caused by the accumulation of heat from oxidation reactions. Fires started by spontaneous combustion are caused by the following mechanisms:
Spontaneous heating is the slow oxidation of an element or compound which causes the bulk temperature of the element or compound to rise without the addition of an external heat source. Spontaneous heating may be the result of direct oxidation of hydrocarbons (for example, oils, coal, and solvents) or it may occur because of the action of microorganisms in organic materials. A more detailed discussion on spontaneous heating is included in the next section.
Pyrophoric substances ignite instantly upon exposure to air (atmospheric oxygen). A pyrophoric substance may be a solid, liquid, or gas. Most materials are not pyrophoric unless they are in a very finely divided state. Although there are some pyrophoric liquids and gases, most pyrophoric materials are metals. Information concerning specific pyrophoric liquids, gases, nonmetals, and metals are provided in later sections. It should be noted that pyrophoricity is a special case of a hypergolic reaction because the oxidizing agent is restricted to atmospheric oxygen. Hypergolic reactions are described in the following paragraph.
Where pyrophoricity is concerned only with the spontaneous combustion of a material when exposed to air (atmospheric oxygen), a hypergolic reaction describes a material's ability to spontaneously ignite or explode upon contact with any oxidizing agent. The remainder of this document will be concerned only with spontaneous heating and pyrophoricity.
Note: Many of the fuels identified in this handbook react violently or explosively with the oxidizing agents identified in Appendix B and NFPA 43A because of hypergolic reactions.
Some hydrocarbons are capable of spontaneous heating and ignition under proper conditions. Spontaneous heating of hydrocarbons usually involves a combustible liquid hydrocarbon in contact with combustible materials. An example of this would be combustible rags impregnated with oils or solvents. Some solid hydrocarbons, such as coal, can react directly with atmospheric oxygen. Whether spontaneous heating leads to ignition depends on several items:
For spontaneous ignition to occur, the rate of heat being generated through oxidation must exceed the rate of heat removal by conduction, convection, and radiation (thermal). As the temperature of the material begins to rise, the rate of heat generation will often increase. The result is a "runaway" reaction which ultimately causes ignition. If the rate of heat removal exceeds the rate of generation, the material will cool and will not ignite. The rate of heat removal may be increased through physical contact with a thermally conductive surface, by rotating piles of combustibles to cool hot spots, and by circulating inert gases through the piles to cool hot spots and displace oxygen.
The ignition temperature of the materials is obviously of concern and varies widely among materials. Much more stringent controls must be placed on materials which have lower ignition temperatures and those which liberate explosive gases. Although most materials with high ignition temperatures are of lesser concern, some are more explosive than those with lower ignition temperatures. Material Safety Data Sheets (MSDSs) are a source for information such as ignition temperature, vapor pressure, toxicity, and reactivity.
The specific area of a combustible substance is a measure of the surface area of the material exposed to an oxidizing atmosphere per gram of material and is expressed in units of cm{sup 2}/g. Materials which have a high specific area are more prone to heat and ignite spontaneously. For example, it was mentioned earlier that combustible liquids on fibrous material pose a spontaneous fire hazard. This is because the fibers of the material allow the liquid to spread out over a larger surface area, allowing more contact with oxygen. Therefore, porous combustible materials are more likely to ignite than tightly packed solid materials.
These principles are illustrated in Figure 2. Although coal is used as the example in Figure 2, the principles depicted apply to all materials which are known to heat spontaneously (including pyrophoric liquids, gases, metals, and other solids).
It is important to keep potentially spontaneously heating compounds as dry as possible. High ambient temperatures compound moisture problems. As the ambient temperature rises, the rate of spontaneous heat generation will also rise. High ambient temperatures also reduce the rate of heat removal, bringing the hydrocarbon closer to its ignition temperature.
With these facts in mind, the following housekeeping steps will help minimize the threat of spontaneous heating and ignition of hydrocarbon or organic compounds: