Dear Straight Dope:
When reading a safety flier recently, I came across a gem I had not heard before: "Store charcoal in a dry place and prevent it from getting moist or wet (wet charcoal can self-ignite by spontaneous ignition)." What's the deal? True or not?
At first I didn’t want to take this question, since like Hank Hill I grill only with propane so I can “taste the meat, not the heat.” However, since I’m the Straight Dope’s resident Coal Goddess, Cecil ordered me to put aside my prejudices and answer this one for him. So here we are. Short answer: The chances of that bag of charcoal you bought at the Kroger spontaneously catching fire are low – but that’s not to say you should ignore the safety warning. If you heaped up enough of the stuff and got it wet, yes, it could self-ignite.
I’m going to discuss both charcoal and coal in this report, since although they are different substances with different origins, they are somewhat similar in composition and behavior (the key word being “somewhat” – I could ramble on for quite a while about the differences between various types of coal and charcoal). Charcoal, like coal, is made up of many different types of hydrocarbons, sulfur compounds, organic and inorganic chemicals, water, a small amount of oxygen, and even fun trace elements such as mercury and uranium. Most of the combustible matter in charcoal and coal is carbon. When stored in an environment containing oxygen (like, oh, air), the carbon slowly oxidizes to form carbon dioxide and carbon monoxide. This reaction produces heat, and since coal and charcoal are relatively good thermal insulators, much of this heat can be trapped, increasing both the temperature and the rate of oxidation. In fact, the oxidation rate doubles for every rise of 8 to 11 degrees C (15 to 20 degrees F) in coal, and presumably a similar rate applies to charcoal. Depending on how the charcoal or coal is stored, heat production may substantially exceed heat loss to the environment, and the charcoal can self-ignite.
What is the role of water in this? A paper by the Wyoming State Geological Survey on spontaneous combustion of wet coal summarizes it this way:
That role concerns what is known as the heat-of-wetting … Drying coal is an endothermic process [heat is absorbed] and lowers the temperature of the coal … Wetting (or gaining moisture) is an exothermic process and the liberated heat can accelerate the spontaneous heating of the coal.
The most dangerous heating situation is reported to be at the interface between wet and dry fuel. If the stuff is either completely wet or completely dry, the danger is reduced.
The actual risk associated with wet charcoal is hard to quantify. Many home safety guides and public service brochures warn of the danger of storing charcoal while wet, as do some professional guides, such as the National Fire Protection Association handbook. Even the U.S. Code of Federal Regulations warns that:
Special care shall also be taken in the loading of any motor vehicle with Class 4 (flammable solid) or Class 5 (oxidizing) materials, which are likely to become more hazardous to transport by wetting, to keep them from being wetted during the loading process and to keep them dry during transit. Examples of such dangerous materials are charcoal screenings, ground, crushed, or pulverized charcoal, and lump charcoal.
However, according to one technical paper from the 7th International Symposium on Fire Safety Science:
The data show that the largest commercially-available bag of charcoal briquets, 9 kg (20 lb.), cannot self ignite at an ambient temperature below 394 K (121 C or 250 F). All tested variations: size, different formulations, addition of water or dry wood, aging, and different bag configurations, raised this critical temperature even higher. At ambient temperatures (approximately 25 C ) these data show a bag of charcoal briquets would have to exceed the volume of a typical house to self ignite.
I’ve had no personal experience with spontaneous combustion of charcoal nor do I know anyone who has. But spontaneous combustion of coal is a well-known phenomenon, and I have personally witnessed it on numerous occasions with coal from the Powder River basin (PRB) in Wyoming. This is high-moisture, highly volatile sub-bituminous coal that will not only smolder and catch fire while in storage piles at power plants and coal terminals, but will sometimes be delivered to a power plant with the rail car or barge partially on fire! I once sat in the cab of a riverside unloading crane and watched as a scoop of non-burning coal was removed from a barge, revealing a massive glowing luau pit of fire underneath big enough to roast a couple SUVs with a side order of Honda. I wish I’d had a camera. I’ve also walked up to enormous piles of self-heating PRB coal stored at river terminals and, as insulating as coal can be, felt the heat radiating from them while still several feet away.
Spontaneous combustion of coal and charcoal has been more influential in history than you might think. We have evidence that coal seams spontaneously ignited millions of years ago. Early explorers of the Wyoming coal fields noted naturally-occurring coal fires in their reports. Some speculate that the battleship Maine exploded in Havana harbor in 1898 due to spontaneous combustion of coal in its bunkers, and it’s known that the Titanic had a coal bunker on fire from spontaneous combustion from the day she left Southampton to the time she struck the iceberg. Coal mines, not really the safest places to begin with, have been known to have coal seams spontaneously ignite; in some case, these fires can be quite deadly and burn for years.
In all these cases we are dealing with large amounts of combustible material, often in a confined place. My guess is that most of the warnings regarding wet charcoal are focused on bulk quantities that might be stored or transported, such as a large pile at a hardware store or perhaps a loaded truck. However, I’ll concede that, depending on conditions, heat production and the resultant likelihood of spontaneous combustion may vary widely. Some charcoals (such as “rapid light” types) may be more susceptible to spontaneous ignition due to the volatile chemicals added to facilitate lighting. So I advise you to follow the directions given by the safety brochures and not store charcoal where it can get wet – and who wants wet, gummy charcoal for their fire anyhow? You might as well cook over burning newspapers like chestnut vendors do in Paris streets (ugh). Or give up your reliance on dangerous old grilling technology and go with a propane gas grill. After all, what’s safer than a grill with a large tank of highly flammable gas that a 3-year-old can turn on?
“The Fire Below Spontaneous Combustion in Coal,” Environment Safety & Health Bulletin (DOE/EH-0320), No. 93-4 (May 1993), U. S. Department of Energy, Washington, D.C. 20585.
Lyman, Robert M. and Volkmer, John E., "Pyrophoricity (spontaneous combustion) of Powder River Basin coals – considerations for coalbed methane development.” Coal Report CR01-1, Wyoming State Geological Survey, Laramie, Wyoming, March 2001.
Pagni, P.J., Cuzzillo, B.R., Wolters, F.C., and Frost, T.R., “Size Constraints on Self Ignition of Charcoal Briquets,” 7th International Symposium on Fire Safety Science, 16-21 June, 2002, Worcester Polytechnic Institute, Worcester, Massachusetts, USA.
Primer on Spontaneous Heating and Pyrophoricity (DOE-HDBK-1081-94), U.S. Department of Energy, Washington, D.C. 20585
National Fire Protection Association Handbook, 17th edition.
U.S. Code of Federal Regulations, 49 CFR 177.838.
Send questions to Cecil via email@example.com.
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