Reference
Pyrolysis (or How to Glaze a Judge)
By Mark Hughes, P. Eng.
© 1999 Sintra Engineering Inc.
In cases where no other cause of a fire is apparent, pyrolysis of organic materials is an explanation that some investigators resort to. They reason that pyrolysis can occur in any area where organic materials are present, and requires only a source of heat, not necessarily a source of ignition. While it may seem to some like an easy out, the reality is that pyrolysis is a process that can and does lead to many fires.
As it is a difficult topic to tackle, explaining the concept can cause a glazed look to come across the face of the person you're addressing. As much as you wish this look was on account of your good looks, you suddenly realize that the person you're talking to is completely lost and actually thinks your speaking either Swahili or Urdu.
What is pyrolysis? If you look it up in NFPA 921 or Kirk's Fire Investigation, the explanations focus on the decomposition of wood. They both identify how organic compounds are broken down into simpler compounds that readily burn. In truth, the phenomenon of pyrolysis has been known for a long time but the mechanism itself is only now just starting to be understood. The chemical breakdown of organic materials is as complex and varied as are the materials being broken down.
A simple way of conceptualizing pyrolysis is to consider that hydrocarbons (methane, propane, gasoline, kerosene, etc.) are molecules consisting of hydrogen and carbon. Most hydrocarbons are themselves created by the decomposition of plant matter. (Almost all fuels of importance to fire investigators are derived from organic matter.) Organic matter is composed of the following primary constituents: hydrogen, carbon and oxygen. If you oversimplify the process and think of the decomposition process as just driving the molecular constituents of water, hydrogen and oxygen, out of the organic matter, you can picture the remaining compound being a combination of just carbon and hydrocarbons, both of which readily burn. While the actual mechanism is significantly more complex, this simple model describes an overall effect that is easy to understand.
How does the pyrolysis of organic matter lead to a fire? In essence, the breakdown of the organic matter means that you are creating a newer, more volatile fuel over a very long period of time. This fuel can auto ignite, leading to a fire starting where there is no specific ignition source. The ignition of the pyrolyzed matter is often from the original heat source and as such, there is no change or unusual event that caused the fire to start. Most fire fighters look for things that have recently changed or the presence of new equipment that could have caused the fire. In pyrolysis based fires, there will be no indication of unusual activity prior to the fire. As the process can take years, the clues that indicate how the fire started can be very hard to detect.
An easy to understand analogy involves putting a pop tart in a toaster. If you put a pop tart into a toaster, it turns a little browner than when you started. If you weighed the pop tart after toasting it once, it would weigh less than when you started because some of the moisture has been driven off. If you let the pop tart cool to room temperature and the put the pop tart back in the toaster at the same settings, the pop tart would be darker and weigh still a little less than before. If you repeat the process of cooling and re-toasting the pop tart, you will see the pop tart turn very dark, smoke and possibly burn. However, if you were to put a thermometer into the toaster on the last occasion you would notice that the temperature within the toaster was not higher than the first time it was toasted. Why? The heating and driving off of moisture is a cumulative effect. As the effect accumulates, the pop tart eventually reaches a point where there is no moisture left, only carbon and volatile gases that can be ignited solely by the heat from the toaster.
In practical terms, a stove located near a combustible wall can undergo years of use with no problem until one day when the wall bursts into flames. The cumulative effect on the wall finally results in the catastrophic failure.
return to Reference articles...
