The Chemistry of Fire

Before we can discuss fire safety awareness in the workplace, you first need to understand how fire works. One of the simplest techniques for understanding fire is to use the fire triangle, a simple and fundamental concept used to understand the three essential components required for a fire to occur and sustain itself: fuel, heat, and oxygen. The process referred to as a fire is more accurately defined as combustion. Combustion is the process of combining a fuel and oxygen and then adding an ignition source. Once you have those three elements in the proper proportion, the end result is fire. Fire, like a three-legged stool, will fail if you remove one of the supporting elements. Fuel provides the base substance which, when heated enough, breaks down into molecules, releasing gaseous energy which then ignites, creating heat and light. Heat is the energy that must be maintained to sustain the chemical breakdown of the fuel, providing the base for combustion. The final side of the fire triangle is oxygen which reacts with the fuel molecules that break apart and combine with other atoms and molecules.

To extinguish a fire, you will need to remove one of the three key elements: fuel, heat, or oxygen. This is performed by different processes depending upon the fuel or ignition present. Fuel and ignition define what classification of fire it is. Classes of fire will be discussed later.

Classifications of Fire

Fires are classified into different categories based on the type of fuel involved. The classification system helps explain the nature of the fire and determines the appropriate firefighting methods and extinguishing agents to be used. The most commonly used classification system is the one developed by the National Fire Protection Association (NFPA) in the US which categorizes fires into five classes (O’Connor, 2023).

Class A: These fires involve ordinary combustible materials, such as wood, paper, cloth, rubber, and some plastics. These fires typically leave behind ash after combustion. Class A fires are extinguished using water, which cools the fuel and removes heat from the fire triangle.

Class B: Class B fires involve flammable liquids or gases, such as gasoline, oil, grease, propane, butane, and certain paints and solvents. These fires are characterized by their ability to spread rapidly and may produce large flames or explosive conditions. Class B fires are extinguished using foam, carbon dioxide, or dry chemical agents that smother the fire and eliminate the oxygen supply.

A flammable liquid is any liquid having a flashpoint at or below 199.4 °F (93 °C).

As per its standard on flammable liquids (OSHA, 1970a), the OSHA Office of Training and Education (n.d.) states:

It should be mentioned that flash point was selected as the basis for classification of flammable liquids because it is directly related to a liquid’s ability to generate vapor, i.e., its volatility. Since it is the vapor of the liquid, not the liquid itself, that burns, vapor generation becomes the primary factor in determining the fire hazard. (p. 3)

Class C: Fires in this class involve energized electrical equipment, such as electrical appliances, wiring, and circuit breakers. The key hazard in these fires is the risk of electrical shock rather than the actual burning of materials. It is important to note that water should never be used to extinguish class C fires because it conducts electricity and can lead to electrocution. Class C fires are extinguished using dry chemical agents or carbon dioxide which do not conduct electricity.

Class D: Class D fires involve combustible metals such as magnesium, titanium, potassium, sodium, and certain types of powdered metals. These fires require specialized extinguishing agents including dry powder extinguishing agents specifically designed for use on metal fires. Water, foam, or carbon dioxide should not be used to extinguish class D fires since they can react with the metal and potentially exacerbate the fire.

Class K: These fires are found primarily in commercial kitchens. The use of high-temperature cooking oils creates a unique environment, which requires this classification. This classification of fire also has a unique fire extinguisher. They use a fine wet mist consisting of an alkaline mixture, such as potassium acetate, potassium carbonate, or potassium citrate that forms a soapy foam as it is applied to the cooking oil or other substance. Class K fires should only be extinguished with class K extinguishers and should never be sprayed with water as this can increase the spread of the cooking liquids.

Attributions

1. Figure 2.2: The fire triangle by Gustavb is released under CC BY-SA 3.0
2. Figure 2.3: Classifications of fire by Nicolas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0