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The Volunteer Guide to 21st Century Firefighting: Combustion

For firefighters, it is pretty common for them to interact with, well, fire. While many firefighters go headstrong into a burning building to put the fire out and rescue any victims that may be inside. For you full-time folks the components of fire and the importance of understanding combustion is reviewed everyday but for us small-time, maybe more rural volley fire departments this needs to be reviewed and better appreciated. 

What is Combustion: 

Combustion is a process between a chemical oxidizer and a fuel source to generate light and heat (Gann, Friedman, 2015). In order to support the process of combustion, there are generally four elements that are needed: Fuel, heat, oxygen, and a chemical chain reaction. These components can be depicted in the famous diagram called the fire tetrahedron. 

This tetrahedron is an important aspect to understand when trying to conduct fire operations. In order for combustion to occur all elements of the tetrahedron MUST be present, this will usually support what we as firefighters call "Flames." When trying to contain the fire we must first find a way to take away one of the elements from the tetrahedron by either; reducing the heat (water), removing the oxygen (Starve the fire), or remove the fuel (remove that couch). 

Flamming Combustion vs Nonflamming Combustion

NFPA's Principals of Fire Behavior and Combustion text lists out two different forms of combustion, flaming and nonflaming combustion. In fire fighting, we are more familiar with flaming combustion. In flaming combustion, the tetrahedron elements have come together to generate a free sustaining flame that quickly breaks down the material undergoing combustion. In non-flaming combustion, combustion is still occurring however in a much slower process. Let's take the example for a moment of burning charcoal. The combustion process is much slower and comes in the form of smoldering combustion and promotes free burning. While we may not be called to a scene where the fire was started by charcoal, we do however see instances where a smoldering material was given an additional and forceful amount of oxygen (I.E. wind). 

Pyrolysis 

Let's think for a moment about a piece of wood being burned. As the wood burns, we can see the physical properties begin to change and break down. Pyrolysis is the chemical break down of the material going through combustion (Gann, Friedman, 2015). As this decomposition occurs we see the material chemically changing from its original solid form into the gaseous form that we see in the nature of smoke (Gann, Friedman, 2015). 

Smoke

Let's take a moment before I end here and talk a little about smoke production. Smoke is defined by the NFPA as "the airborne solid and liquid particulates and gases evolved when a material undergoes pyrolysis or combustion together with the quantity of air that is entrained or otherwise mixed into the mass" (Gann, Friedman, 2015). The smoke generated by a fire is dangerous for several reasons that we must take seriously if we are to be safe while working on a fire ground. 

Smoke contains superheated particles that rise until stopped by an outside force. In a structure, we will see this "thermal layer" be at the ceiling level and begin to drop as the heat and the smoke gathers and becomes heavier. 

Smoke can contribute to combustion in that the heat generated can cause charring of other materials which in turn can start flaming combustion (Gann, Friedman, 2015). 

Another dangerous aspect of smoke is the chemicals that are associated with it. Two of the biggest and most dangerous chemicals associated with smoke is Carbon Monoxide and Cyanide. Both of these have a higher affinity to the hemoglobin than oxygen and can cause an abundance of respiratory issues.

The topic of combustion can go so much deeper than this blog goes. Fire behavior and the combustion process is an interesting topic of study and I feel like it can be underappreciated sometimes and needs constant evaluation to help us to predict how things can react and how the course of the fire can change. 

Stay safe and always be learning!

Work Cited: 

Gann, R., & Friedman, R. (2015). Principals of Fire Behavior and Combustion. Quincy, MA: NFPA.

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