An Introduction to the Chemistry of Fire: Going Beyond the Basics

By Joshua Scalf

Now is an interesting and exciting time to be involved in the American fire service; it seems like the list of new gadgets and tools continues to expand almost endlessly as each year passes. Several new items can now be added to this list.

Fire departments across the country and, presumably, the globe, are looking for innovative ways to incorporate the latest technology into the daily operation of career and volunteer houses. In addition to the progress being made to enhance the modern fire service, you should take steps to enhance your knowledge of fire behavior.

As a firefighter, fire is your enemy. The more that you know about it, the better your chances are of completing your mission successfully. I believe that no one area of study—outside of building construction—can improve your knowledge of fire behavior than that in the field of chemistry. This article and future articles will enhance your knowledge of chemistry by building on your existing training.

According to the International Fire Service Training Association (IFSTA), fire is defined as a rapid, self-sustaining oxidation process accompanied by the evolution of heat and light of varying intensities.”¹ This definition can vary depending on the source referenced, but most support a similar version. For this article, the two most important parts of this definition are the words “self-sustaining” and “oxidation.” The key that ties these two parts together is the underlying chemical chain reaction.

For chemists, there are numerous, almost dizzying, chemical chain reactions to be studied, practiced, and perfected. Chemists can make reactions almost as broad or specific as they need, depending on the expected outcome. Fortunately, however, this article will attempt to limit itself to just one type of chemical reaction: the combustion reaction. This reaction should become as familiar to the black-hat firefighter as the coffee pot in the kitchen at the station!

The combustion reaction is arguably the most important chemical reaction for a firefighter to understand because fire simply would not exist without it. With that said, following are the five basic steps of the combustion reaction:

1. Hydrocarbon must be present to burn.
2. Oxygen is added to the hydrocarbon.
3. Ignition source introduced.
4. Carbon dioxide (CO₂) and Water (H₂O) are given off.
5. Energy is released from the reaction

Those five simple steps are the fundamentals of the combustion reaction. Understanding these steps can greatly improve a firefighter’s knowledge of what causes fire. For example, consider a call that comes into the station for a propane tank fire. The first-due engine arrives on scene to find a residential propane tank with gas and heavy fire coming out of a burst line. This is a great example of a combustion reaction in progress. Here is what the reaction would look like from a chemical standpoint:

            C₃H₈            +     5O₂    + Ignition       →   3CO₂       +     4H₂O                +       ∆H

                           (Propane)          (Oxygen)                             (Carbon Dioxide)   (Water)         (Heat released)           

In this example, propane serves as the hydrocarbon (step 1), and oxygen is the oxidizing agent (step 2). Now that both a fuel source and an oxidizing agent are present, an ignition source is needed (step 3). This ignition source may be electrical, heat, or pressure, but whatever its form, it initiates the chain reaction. As a result, CO₂ and H₂O are given off as the products of combustion (step 4). In this case, the breaking of the bonds in this reaction means that energy is released (step 5) in the form of heat and light. Also, energy is given off in a larger volume than what is needed to start the combustion process. This is how the chain reaction becomes self-sustaining; it simply carries on until there is no more fuel left to burn. The combustion reaction can be summarized by saying that anytime you add oxygen to a hydrocarbon and throw in an ignition source, you will always get CO₂, H₂O, and energy as the products. It is important to recognize, however, that these rules only apply in the case of complete (clean) combustion, which is shown in the above example.

In relating chemical reactions to structure fires, you have to consider what happens when the combustion is incomplete. However, for now, complete combustion will be the extent of this article. I have completed an important step in expanding your knowledge of the chemistry of fire by introducing or refreshing you on the combustion reaction. The next article of this series will attempt to show the complexity of different hydrocarbons and how they affect the quantity of CO₂, H₂O, and heat that are produced. I will also take a look at another reaction that is common to Firefighter/emergency medical technicians: the breakdown of glucose in the body.

References

1. IFSTA-Essentials of Fire Fighting Fourth Edition. Pg. 40.

Josh Scalf is a firefighter/EMT and 10-year member for the Lafayette Township Fire Protection District in Floyds Knobs, Indiana. He is also an instructor II/III and a member of the rope and water rescue teams. Scalf is a student at Indiana University Southeast and has an associate of arts degree in General Studies. He is actively pursuing a bachelor of science in Chemistry-American Chemical Society certified degree.