Fire Science: Put the Wet Stuff on the Red Stuff

By Michael J. Klemenz, Upstate Fire Protection Engineering

We all know the old fire service adage “Put the wet stuff on the red stuff.” But, just what is the “red stuff?”

Understanding basic fire chemistry allows us to anticipate a fire’s next move, which comes in quite handy when making tactical decisions on the fireground. And this knowledge is not reserved for Chiefs and company officers. It can be applied by all personnel operating on the fire incident scene. Awareness of fire chemistry is essential to gain the upper hand in the battle with the “red stuff.”

As I often explain to my fire chemistry students on the first day of class, the general public has a gross misunderstanding of fire and how it operates. This comes from the misrepresentation of fire and firefighters in movies and TV shows as far back as the 1970s.

Firefighting is not quite as glamorous as it is depicted on the big or little screen. If you are a 70-year-old fire service veteran, you already know this. If you are a 17-year-old “newbie,” you will find out soon. And unlike Hollywood, there are no “reshoots” when something goes wrong.

NFPA 921, Guide for Fire and Explosion Investigations (1), defines fire as “a rapid oxidation process, which is a chemical reaction resulting in the evolution of heat and light in varying intensities.” Simply put, fire results when fuel and oxygen molecules are chemically combined under conditions of high heat to produce even more heat, light and combustion products. When properly controlled and contained, “good” fire is the result (think summer barbecue). But, when we lose control and containment, “bad” fire is an unfortunate result.

Scientists and engineers conduct experiments and full-scale burn tests to more fully understand fire, predict its behavior and determine how fire reacts to various fire suppression activities. Compartment fire research continues to this day.

More recently, scientific research has shifted to include wildland fires. Since 2010, massive wildfires have burned an average of 6.9 million acres of land in the U.S. each year (2). Many lives and thousands of structures have been lost in these wildland urban interface (WUI) fires.

What do a birthday candle, a house fire and a 100,000-acre wildland megafire have in common? The chemical reaction is exactly the same … just on a different scale!

In order for fire to occur, at least three things must be present: 1) A fuel to burn, 2) sufficient oxygen for combustion and 3) a source of ignition heat. These components are often represented by the popular “fire triangle.”

A fourth component known as an “uninhibited chemical chain reaction” is sometimes included in discussions on fire chemistry but it is not addressed in this article.

A fuel is any substance that is combustible. Fuels occur in all three phases of matter: solid, liquid and gas. Numerous examples of fuels are available. They include common construction materials like wood and polymers such as polyurethane, which is a component of modern furniture. Liquid fuels include the motor fuels gasoline and ethanol. Gaseous fuels include propane and methane, two common ignitable gases used in heating systems. Just about everything we see can become a fuel under the right conditions.

The oxygen (O2) needed for combustion normally comes from the earth’s own air supply. Ambient air is comprised of about 21 percent oxygen and 78 percent nitrogen (N2) by volume. In more exotic fire situations involving DOT Class 5 hazardous materials, the O2 can be chemically contained within the fuel itself or may be from a separate oxidizing agent.

Just about everything we see can become a fuel under the right conditions.

Heat is a form of energy that causes irreversible changes to the chemical composition of fuels. Heat causes ignition of fuel vapors and is produced in great quantities during combustion.

The quantity of heat that can be released from a particular fuel is referred to as the heat of combustion, or the Hc, for that fuel. In English units, heat of combustion is expressed in terms of BTU/lb. The measurement of 1 BTU is the quantity of heat energy required to increase the temperature of one pound of liquid water by 1 degree Fahrenheit.

Heats of combustion for various fuels are measured in a laboratory. They range from about 6,500 BTU/pound for dry wood, 12,800 BTU/pound for ethanol and 61,000 BTU/pound for hydrogen (think Hindenburg disaster).

Under conditions of high heat, fuel molecules combine with O2 in the air to form new chemical products. Relatively innocuous combustion products include solid carbon particulates, carbon dioxide (CO2) gas and water vapor (H2O).

Other combustion products such as carbon monoxide (CO) gas and hydrogen cyanide (HCN) gas are extremely toxic. This is why we wear self-contained breathing apparatus, even during overhaul.

What do a birthday candle, a house fire and a 100,000-acre wildland megafire have in common? The chemical reaction is exactly the same … just on a different scale!

Recent studies attribute increased fire service cancer risks to frequent exposure to combustion products.

Technically speaking, only gases burn. This sounds like a play on words, but it is true that solid fuels and liquid fuels must first be converted to gas before combustion can occur. When a solid or liquid fuel is exposed to a sufficient heat source, the molecules near the surface of the fuel are driven off in the form of a gas. This heating process is called “pyrolysis” for solid fuels and “vaporization” for liquid fuels.

Fuels that are already in the gas phase (like methane) and have the correct air/ fuel mixture do not require pre-heating for combustion to occur. A competent ignition source is all that is needed to ignite the fuel/air mixture.

Back to the fire triangle. When all three components of the fire triangle (fuel, oxygen and heat) converge, fire can occur. If, however, we remove any side of the fire triangle, fire cannot exist.

If you think about it, our attempts to extinguish fires depend on controlling at least one side of the fire triangle.

Application of water via hose streams removes the “heat” side of the fire triangle by cooling the fuel and preventing further pyrolysis. Closing the door to the fire compartment removes the “oxygen” side of the fire triangle by stopping, or at least limiting, airflow into the compartment. Cutting down vegetation ahead of a wind-driven wildfire removes the “fuel” side of the fire triangle by eliminating the fuel available for combustion.

Of course, these are very simple examples of how to manipulate the fire triangle. The trick is being able to anticipate the fire’s next move and perform the right action at the right time.

Fire chemistry is not just for firefighters. It is also extremely useful to fire investigators, fire protection system designers, code enforcement officials and others involved with public safety. There is much more to fire chemistry, including science and math.

1. National Fire Protection Association, 1 Batterymarch Park, Quincy, MA 02169-7471.

2. National Interagency Fire Center, Boise, Idaho

Michael J. Klemenz is Principal and Fire Protection Engineer at Upstate Fire Protection Engineering, PLLC. An adjunct member of Onondaga Community College’s Department of Fire Protection faculty, he holds a bachelor of science in Fire Protection Engineering from the University of Maryland at College Park. Klemenz is a licensed professional engineer in New York, North Carolina and Pennsylvania. He is retired from the Liverpool Fire Department and past member of Prince Georges County (Maryland) and Minetto Fire Departments.