Many people my age will remember the television show Mission Impossible from the late 1960s. At the very beginning of each episode, the lead character, Jim Phelps (played by actor Peter Graves) and his team of secret agents would receive a new mission from headquarters, and they would have 60 minutes (of TV time) to execute it in the face of all sorts of obstacles. The instructions and background information for the Impossible Mission Force team were always provided in a large brown envelope and a reel-to-reel audio tape player. At the end of the tape, the voice would always say, “This tape will self-destruct in five seconds. Good luck, Jim.” The tape player would suddenly start smoking and you would see the tape warping and melting as it succumbed to the mysterious fuming and heating. It almost seemed that the device was undergoing spontaneous combustion.
If you think about it, self-destruction is the essence of spontaneous combustion, but spontaneous combustion is not limited to items that have outlived their usefulness. In fact, when spontaneous combustion occurs, it almost never happens because someone intended for the material to be incinerated.
Regardless, the nagging question about spontaneous combustion not how, but why would the material survive for years or decades without undergoing self-ignition, and then one day, its surroundings change and its own tendency to undergo self-heating chemistry launches its voyage toward self-ignition and self-destruction? To paraphrase James Carville, “…It’s the environment, stupid.”
The classic scenario for enabling spontaneous combustion is filling a waste basket with linseed oil-soaked rags and waiting 3 to 12 hours. Linseed oil, like most vegetable oils, contains oxygenated hydrocarbon and these constituents continuously undergo exothermic reactions. Thankfully, the reaction rate is so slow that under normal conditions, the (neat) oils naturally dissipate such heat through conduction and convection and the liquid never warms up or approaches thermal runaway.
Conversely, when a film of the same oil is adhering to the woven interstices of cloth rags accumulated into a pile, the self-heating reactions occurring at the center of the pile deposit their heat into a space that is well insulated from the surroundings, and the heat loss is minimal. This allows the temperature to rise locally, which causes the reaction rate to grow exponentially, and eventually thermal runaway is reached. At that point, the limitation on how much longer it will take for the material to begin smoldering and to ultimately ignite is governed by the ability of fresh air to diffuse through the openings in the rags and to provide oxygen to the reacting organic material at the center of the pile.
NFPA cites the following statistics about spontaneous combustion or chemical reaction caused fires: average of 14,070 fires per year between 2005 and 2009, including 3,200 structure fires, 1,150 vehicle fires, 5,250 outside non-trash fires, and 4,460 outside trash or rubbish fires. The following are known to be capable of spontaneous combustion: Activated carbon, Oily cotton, Paper, Seed cake, Celluloid scrap, Linen stacks, Log piles, Coal piles, Haystacks, Compost piles.
Agricultural raw materials and food or feed products are very likely to undergo spontaneous combustion – especially if they are being heated in an oven or dryer. NFPA 61 states: “Spontaneous ignition is a primary cause of dryer fires and explosions. The requisites of this phenomenon are a heated surface or a hot airstream, a layer of product exposed to this heat, and time.” The four factors that make spontaneous ignition more likely are: (a) size of pile and effectiveness of self-insulation; (b) temperature of surroundings; (c) ability of oxygen to diffuse to the reaction zone; (d) sufficient time for self-heating reactions to accelerate.
Unfortunately, safe handling of linseed oil-soaked rags is not entirely intuitive, and spontaneously ignited fires continue to occur via many different scenarios. Warning labels on cans of linseed oil state: “USE EXTREME CAUTION. Immediately after use and before disposal or storage, you MUST (1) hand-wash rags thoroughly with water and detergent outside in a bucket and rinse. Repeat washing and rinsing until you have removed all oil from all cloths, rags, paper, … and any other materials contacted during use or because of an accidental spill. (2) spread all rinsed materials outside to dry by flattening them out to their full size in an airy spot for at least 24 hours until completely dry.”
This author has investigated fires where spontaneous combustion was the only likely ignition source and others where allegations of spontaneous combustion were proven incorrect upon further investigation.
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