The Myth About Storing Wet Charcoal

What Is The Myth?
You will see warnings on various fire department "Summer Safety Tips" webpages that say if you have a bag of wet charcoal you should discard it, keep it in a well-ventilated area or dry it out in a well-ventilated area. Most of these web pages don't explain why they warn you about storing wet charcoal. So, after seeing several of these warnings we wanted to know what the problem is with wet charcoal. We decided to investigate and eventually found warnings indicating that the problem was that the charcoal could spontaneously ignite and burn. Here are some examples of warnings:

The Felton, California Volunteer Fire Department says:

"Keep damp or wet coals in a well ventilated area. During the drying process, spontaneous combustion can occur in confined areas."
The Andrews Air Force Base newsletter advises:
"Store charcoal in a dry place and prevent it from getting moist or wet. Wet charcoal can self-ignite by spontaneous ignition."
The police department in the town of Ossining, New York recommends:
"...dispose of bags of damp or wet charcoal, as it can spontaneously combust."

We Investigate
This really made no sense to us, so we then decided to try to find out the theory behind this spontaneous ignition. Filtering out all the kook sites that claim people are bursting into flames, we found a web page from the U.S. Department of Energy that states:

"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."
However, note that they are talking about coal, not charcoal. We also found the following regulations from the U.S. Department of Transportation:
[Code of Federal Regulations]
[Title 49, Volume 2]
[Revised as of October 1, 2002]
From the U.S. Government Printing Office via GPO Access [CITE: 49CFR177.838]

Table of Contents Subpart B--Loading and Unloading Sec. 177.838 Class 4 (flammable solid) materials, Class 5 (oxidizing) materials, and Division 4.2 (pyroforic liquid) materials.

"(b) Articles to be kept dry. Special care shall be taken in the loading of any motor vehicle with Class 4 (flammable solid) or Class 5 (oxidizing) materials which are likely to become hazardous to transport when wet, to keep them from being wetted during the loading process and to keep them dry during transit. 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."
Ok, we are getting closer. They mention charcoal. Then we found some specific information on the topic in the 19th Edition of the National Fire Protection Association Handbook on page 6-328:
Under certain conditions, charcoal reacts with air at a sufficient rate to cause the charcoal to heat spontaneously and ignite. Charcoal made from hard wood by the retort method apears to be particularly susceptible. Spontaneous heating occurs more readily in fresh charcoal than in old material; the more finely divided it is, the greater the hazard. Among the conditions that can lead to spontaneous heating of charcoal are 1) lack of sufficient airing and cooling before shipment; 2) charcoal becoming wet; 3) friction in grinding of finer sized, particularly of material insufficiently aired before grinding; and 4) carbonizing of wood at too low a temperature, leaving the charcoal in a chemically unstable condition.
Alright! Now they even mention wet charcoal. But they seem to be talking about enormous volumes of coal or charcoal, not the small quantities you would find in your typical home. More on this later!

Typical Examples Of Fires Blamed On Storing Wet Charcoal
We have received emails telling us of specific home fires started by wet charcoal. These stories go something like this:

"I know someone who's house burned down. They had left a bag of charcoal outside. When the fire trucks came and put out the fire, the firemen said it was probably wet charcoal spontaneously igniting."

We also found reports in the news media about fires started by spontaneous ignition of charcoal. For example, the following press release comes from the Hawaii County Police Department on the Big Island:

"A Hilo home was extensively damaged by fire Sunday (July 2, 2000) after a bucket of wet charcoal next to the house ignited itself. Responding to a 10:39 a.m. call, police and fire units arrived to find about half of the house, located at 33 Noeau Street, engulfed in flames.

Fire investigators said the blaze was caused when a bucket of wet charcoal at the southeast corner of the house dried out and ignited itself."

And this comes from the Benton County, Oregon Sheriff's Department:
"Wednesday, August 1, 2001 -- Benton County, Oregon

At 2:40 am, July 16, 2000 an alarm came in to the Albany Fire Department for an apartment complex fire. A fire on the balcony of one of the apartments led to the early morning evacuation of several tenants and in the end, an estimated $25,000 damage to the building structure....

The source of the fire was determined to be a sack of charcoal briquettes that “spontaneously combusted.” No matches required. According to Albany Fire Department’s Public Information Officer, Wanda Omdahl, the charcoal briquettes had gotten wet and as they dried, created the necessary heat and unstable conditions that led to their spontaneous ignition. “The sun participated in the drying process, but really had nothing to do with starting the fire,” according to Omdahl. It’s a situation similar to one that can occur in a stack of drying hay or a wad of fabric that has been contaminated with linseed oil. It’s a hazard that can be avoided by drying combustibles flat and away from ignition sources rather than gathered together or crumpled up.

This summer (last week), another local house fire is suspected to have been ignited in the same manner."

We note that these stories always seem to have one or both of two things in common. The report usually comes second hand, and the conclusion that the cause of the fire was spontaneous ignition of wet charcoal comes after the fire has consumed the home and been put out. We have yet to see a report of someone finding a bag of wet charcoal that is hot to the touch or smoking, about to ignite. Nope. What they always find is a house that has burned down and the assumption is that the charcoal spontaneously ignited.

We find it somewhat interesting in the Hilo case that the bucket of charcoal was wet. Why was it wet? Perhaps because they had been burning it? Is it possible they didn't wet it enough to douse the entire bucket of charcoal? And notice that both reports blame it on drying charcoal. Check out this information from the paper "Pyrophoricity (spontaneous combustion) of Powder River Basin coals– considerations for coalbed methane development" by Robert M. Lyman and John E. Volkmer at the Wyoming State Geological Survey on spontaneous combustion of wet coal:

"Drying coal is an endothermic process 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."

"Significantly higher temperatures were recorded at the interface between the dry and wet coal, and this interface was the location along which spontaneous combustion started."

The fire department/fire investigator blames the fire on drying charcoal, when drying will actually cause the temperature of the charcoal to fall. And that bucket of wet charcoal, if it was truly completely wet as the owners probably would claim, was actually less likely to spontaneously ignite than a bucket that was only partially wet.

We Experiment
We were so intrigued by these reports that we actually attempted to cause spontaneous combustion of wet charcoal. We placed dry charcoal in a bucket. We placed wet charcoal in a bucket. We placed burning charcoal in a bucket, doused it with water, drained it and left it still steaming. We placed a whole bag of charcoal out on the driveway. We wet a whole bag of charcoal and placed it out on the driveway. The charcoal sat out there for weeks in sun and rain. At no time in any of these tests, did the temperature of the center of the bucket or bag of charcoal ever exceed the ambient air temperature, let alone get hot enough to ignite.

Our Challenge
If charcoal has been spontaneously igniting and setting homes ablaze since Kingsford has been selling charcoal, why can't we even get it to warm up, let alone ignite? So we challenge our readers to provide us with a procedure that we can use to reliably cause charcoal to spontaneously ignite. If it is so common, we should be able to make it happen ourselves. This challenge is open to all, amateurs and professionals alike. Send us the procedure and if we can get a bag of briquettes to spontaneously ignite, we'll publish it right here!

The Definitive Answer
Of course, we are saving the best for last. A little searching on self heating and spontaneous combustion of charcoal will eventually lead you to a paper presented at the 7th International Symposium on Fire Safety Science, 16-21 June, 2002 Worcester Polytechnic Institute, Worcester, Massachusetts titled "Size Constraints on Self Ignition of Charcoal Briquets" by P.J. Pagni, B.R. Cuzzillo, F.C. Wolters and T.R. Frost. You can view the abstract online, but to read the actual paper, you must join the International Association for Fire Saftey Science. So we did. You see before you member number 1604.

The first interesting bit of information in this paper is essentially the origin of this myth. It's a bit lengthy but we thought it was pretty interesting:

"In the 1950s, the National Fire Protection Association developed an excellent classification scheme for ranking the fire hazard of flammable and combustible liquids as IA through IIIB, in order of increasing flashpoint, which is still in use today. Buoyed by this success with liquids, they attempted a similar classification for solids, ranking the tendency of various materials to self heat from “very slight” to “high,” but neglecting the critical roles of system size and exposure temperature on self heating. The table, “Materials Subject to Spontaneous Heating,” first appeared in the twelfth (1962) edition of the NFPA Fire Protection Handbook. The NFPA “Committee on Spontaneous Heating and Ignition,” which generated that table with little underlying scientific analysis, has been disbanded. However, the original incorrect 1962 table remains unaltered in the current edition of the NFPA Fire Protection Handbook. Table A-10 should also be abandoned, since it misleads fire investigators by failing to identify the inherent connection between system size and self ignition. They consequently may erroneously claim spontaneous combustion as a fire cause."
The paper's abstract contains this information:
"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 25°C (77°F ) these data show a bag of charcoal briquets would have to exceed the size of a typical house (>103 m3) to self ignite. Self ignition at ambient temperatures of bagged charcoal briquets in commercially available sizes is impossible."
And finally, some of their conclusions from the paper:
"1. A temperature of at least 394 K (121ºC, 250ºF) is required for self ignition of charcoal briquets to occur in the largest commercially available bag size, 9 kg (20 lbs.).

2. The minimum ambient temperature required for ignition increases for smaller sample sizes, for samples containing lignite char, for wet samples, for samples not enclosed in bags, and for partially filled bags.

4. Spontaneous combustion of bagged charcoal briquets in commercially available sizes is impossible under any normal ambient conditions."

And there are other references that confirm this conclusion.

  • The 19th edition of the NFPA Fire Protection Handbook states on page 6-331 that:
    "Small quantities of charcoal are normally stored in heavy paper bags. The spontaneous heating hazard of individual small bags, as might be found in a dwelling, is not serious."
  • NFPA 921 (2008 Edition) “Guide for Fire and Explosion Investigations” states:
    "Spontaneous combustion of bagged charcoal briquettes in commercially available sizes is not possible under any normal ambient conditions."
  • Kirk's Fire Investigation, Fifth Edition, John D. DeHaan, page 151 also states:
    "Activated charcoal can self-heat in masses of a few pounds and requires several hours to a few days (but charcoal barbecue briquets are unlikely to self-heat to ignition unless presented in large masses [more than 50 pounds (20 kg)] or at high ambient temperatures (over 100°C)."

So in conclusion, yes, you should exercise care in the storage and use of your charcoal, but there is no evidence to suggest that wet charcoal, in the amounts stored and used by homeowners, can spontaneously ignite and thus, no reason to believe all these reports and warnings which litter the internet. We feel badly for anyone who loses their home to fire, but there is no reason to spread misinformation that flies in the face of all scientific knowledge. We also feel that some fire investigators would do well to heed the words of John DeHaan, again from Kirk's Fire Investigation, Fifth Edition, John D. DeHaan, on page 152:

"There is a temptation to label many accidental fires as 'spontaneous' because there is no identifiable ignition source or obvious human intervention, but this is not correct. If the materials and processes cannot be specifically characterized as susceptible to self-heating under the prevailing conditions, then the cause must be considered to be unknown. Spontaneous ignition (with very rare exceptions) does not occur instantaneously and the time frame for development is linked to the chemistry and mass of the reactant. Flaming ignition is always preceded by smoke and odors that should be detectable by anyone in the vicinity for some time prior to flaming ignition."
Just use some common sense and enjoy your cooking!

Update (07/27/10): As an exclusive feature for our readers, we offer the following document for you to use if you should find yourself involved in a house fire that a fire marshall has blamed on wet charcoal:
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