Fire Phenomena – Understanding the Process of Combustion
July 2021 - Fire Phenomena
The bedrock foundation for understanding the phenomena of fire is understanding what makes a fire. As a kid, I was mildly fascinated with fire – not that I was a pyromaniac, but I enjoyed watching the flames lick up and dance around the campfire logs or in the wood burning stoves in our home. I am convinced the beautiful flames are one reason why people enjoy sitting around the campfire for hours on end. It can be quite mesmerizing watching the logs burn.
If one were to look closely, though, you would find that the logs themselves are not actually burning; not really anyway. Look very closely next time you have the opportunity, and you’ll see the flames are dancing just above the surface of the wood. So, what is going on here? There are a couple of things to know.
The Process of Combustion
First, the process of combustion requires four things: fuel, heat, an oxidizer, and an uninhibited chain reaction. These four items make up what is known as the fire tetrahedron (see NFPA 921, 2021 edition, section 5.1.5 for further information). Take any one of those four items away, and you don’t have a fire – combustion stops.
Some quick and simplified definitions of these terms would be helpful here. These topics are defined and discussed in much greater detail in the document called Guide for Fire and Explosion Investigations (often referred to as NFPA 921) Chapters 3 and 5. Here are some simplified definitions for our discussion:
- Fire – A rapid oxidation process, which is an exothermic chemical reaction, resulting in the evolution of light and heat in varying intensities.
- Fuel – any substance that can undergo combustion.
- Oxidizing Agent – in the vast majority of situations this is oxygen in the atmosphere.
- Heat – A form of energy…capable of initiating and supporting chemical changes of state.
- Uninhibited Chain Reaction – this one is a little more complicated. Combustion is a complex set of chemical reactions that result in the rapid oxidation of a fuel. This rapid oxidation produces heat, light, and a variety of chemical by-products. The uninhibited chain reaction occurs when the combustion produces enough excess heat to feed back into the fuel to continue the process of vapor generation and ignition. In contrast, slow oxidation generally does not result in a fire. Examples of slow oxidation would be the yellowing of newspaper over time, or the generation of rust, again, over time. These slow oxidation reactions produce heat much too slowly to initiate flaming combustion. See NFPA 921 chapter 5 for further reading on this topic.
In our campfire example, the fuel is the wood logs; the oxidizer is the available oxygen in the air; the heat, initially, would come from a small starter fire (a match, for example), and once established the heat would then come from the combustion of the logs themselves. Finally, the uninhibited chain reaction is partly the result of a combination of the three previous legs of the fire tetrahedron. Take away, or inhibit, if you will, any of those legs, and you disrupt the chain reaction. For example, if you inhibit the fuel source by separating the logs from one another, combustion will eventually stop. If you inhibit the introduction of oxygen by covering the fire with a fire-retardant blanket, or say, a “blanket of water,” combustion stops. By the way, the introduction of water will inhibit oxygen introduction and remove heat, further inhibiting the chemical chain reaction and potentially ending the combustion process.
Now, back to those wood logs in our campfire example. The logs are just a repository for fuel that needs to be “extracted,” so to speak, in order for the logs to “burn.” When heated sufficiently, wood will undergo a process called pyrolysis and begin to char. Continue adding heat, and flaming combustion will eventually ensue. Pyrolysis is essentially the process that releases the fuel (the volatiles) trapped inside the wood. Burning of wood logs goes something like this: (1) pyrolysis initiates [the releasing of flammable vapors] which in turn (2) mix with available oxygen, (3) form an ignitable mix above the surface of the wood, (4) ignite, and then (5) leave behind the black carbon char we sometimes refer to as charcoal. Left to burn to completion, the char will eventually turn to ash.
The vapors (or volatiles) generated from the pyrolysis process are what is actually combusting and providing those wonderful colors of orange, red, yellow and sometimes blues, just above the surface of the wood. By the way, this is true of liquid fuels as well. For instance, liquid gasoline does not burn, but its vapors do. Don’t try this at home, NO REALLY, DON’T, but igniting the vapors from a small pan of gasoline will reveal that the flames will quickly settle and hover just above the liquid fuel. It will also produce an enormous smoky billowing mess, but that is a topic for another newsletter.
So, next time you find yourself sitting around the campfire, take a look at the flames with an added measure of scientific interest. You’ll see the flames dance, leap, whirl, and boogie across the upper wood surface, while not actually seating themselves on the wooden dancefloor itself. This is because the volatiles are releasing, mixing with air, and only igniting once an ignitable mix is achieved, typically just above the surface. Now, go put that in your Gee Whiz! collection for campfire discussion.
Cord is a licensed Private Investigator in the state of Arkansas, and New Mexico. He is a member of the National Association of Fire Investigators (NAFI), a member of the National Fire Protection Association (NFPA), and the International Association of Arson Investigators (IAAI). Give Cord a call to discuss more on fire phenomena or anything fire related!
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