Introduction
Ask any experienced firefighter, and they will often tell you that a fire that is “fully involved” or “free-burning” is like a living, breathing being. It seems that all it wants to do is eat more fuel, consume more oxygen, and do just about anything it can to stay alive. And to a large degree, this is a fair assessment and is typical of fire behavior. This is because fire is the product of complex chemical reactions.
Chief among the chemical reactions is the fire tetrahedron. One particularly effective analogy which aids in an understanding of the fire tetrahedron is a comparison of a fire to a concert. Although the “headlining event” is the fire tetrahedron, there are opening acts which must appear first before the fire tetrahedron takes the stage.
A free-burning fire is an interesting, but complex, spectacle; it takes quite a few things in the right order for it to appear.
Combustion / Fire Defined
What is a fire?
First of all, it is important to understand the difference between combustion and fire. Unlike the mystery of the chicken or the egg—i.e., which came first?—when discussing fire, the answer is straightforward: combustion is the initiator of fire.
Scientifically speaking, combustion can be defined as a high-temperature exothermic chemical reaction between a fuel and an oxidant (almost always atmospheric oxygen) which produces heat and light in the form of a flame. It is also accompanied by various gaseous byproducts, primarily smoke. Fire, therefore, is the visible effect of the combustion process between a fuel and oxygen in the air.
It should be noted that not all forms of combustion produce visible flames—known as flaming combustion. Some forms of combustion are slower, where the fuel glows but does not flame. This is commonly referred to as smoldering combustion. Smoldering combustion is the process behind cooking on charcoal or can be seen when a flaming combustion fire starts to die down.
What starts the combustion process?
While we have provided the scientific explanation of combustion, it too can be broken down into fairly simple fire chemistry lay terms. Combustion occurs when fuel, heat, and oxygen come together in the proper proportions and under the right conditions. These are the opening acts we referred to at the beginning of the article.
This relationship is often represented by what is known as the fire triangle—or sometimes the combustion triangle—and represented in the graphic shown right.
To better understand the fire dynamics of the combustion process, it helps to have a solid grasp of each element of the fire / combustion triangle.
Fire / Combustion Triangle
Element 1: Fuel
What are the different types of fuels?
For any combustion to occur, there must be a fuel. Fuels can be solids, liquids, or gases; however, not all of these will burn. For the purposes of this discussion, we will be referring to combustible solids, flammable liquids (some agencies also use the term combustible liquid), and flammable gases.
Do different types of fuels produce different types of fires?
There are different types of fire—referred to as classes—each largely based on the type of fuel that is burning, illustrated in the tables below:
| Class of Fire (U.S.) | Material Burning |
|---|---|
| A | Ordinary combustibles (e.g., wood, paper, rubber) |
| B | Flammable liquids (e.g., gasoline, diesel fuel, alcohol, solvents) |
| C | Energized electrical equipment |
| D | Combustible metals (e.g., magnesium, aluminum, titanium, sodium) |
| K | Oils and greases used in commercial kitchens |
| Class of Fire (U.K. / E.U.) | Material Burning |
|---|---|
| A | Ordinary combustibles (e.g., wood, paper, rubber) |
| B | Flammable liquids (e.g., gasoline, diesel fuel, alcohol, solvents) |
| C | Flammable gases (e.g., propane, methane, ethylene) |
| D | Combustible metals (e.g., magnesium, aluminum, titanium, sodium) |
| K | Oils and greases used in commercial kitchens |
NOTE: In the U.K. / E.U., there is no class assigned to electrical fires; rather, the class of fire is based on the material ignited by the electricity.
Understanding the different classes of fire and being able to identify what is burning is a major factor in determining how the fire should be extinguished.
For a thorough discussion of fuels and their interaction with the combustion process, click here.
Element 2: Heat
What is the role of heat in the combustion process?
Of all three segments of the fire / combustion triangle, the heat element is the one that receives the most attention in fire science programs, fire analysis in post-fire investigations, fire prevention, and fire dynamics.
The vast majority of the time, fuels and oxygen manage to peacefully coexist—that is, until heat becomes involved. For virtually every fire, heat is the catalyst. In the study of fire dynamics, the emphasis almost always focuses on heat and its behavior, the reason being that heat provides the activation energy required to break chemical bonds in the fuel. It facilitates the release of flammable vapors that mix with oxygen and ignite.
Without sufficient heat, even the perfect mixture of fuel and oxygen will not ignite. This is why the majority of fire prevention measures are based on eliminating, controlling, or managing heat.
In short, heat is the energy source which initiates and sustains the combustion process. It is responsible for raising the temperature of a material to the point where it can begin to release flammable vapors or gases—a necessary step before ignition can occur.
For a more in-depth discussion of heat in the combustion process, click here.
Element 3: Oxygen
Why is oxygen necessary for a fire to burn?
When a fuel is heated, it begins to release flammable vapors or gases. These vapors must combine with an oxidizing agent—usually oxygen in the air—in order to ignite.
Oxidizing agent:
a substance which provides oxygen—or another element like it—to help a fire burn.
Oxygen reacts chemically with the fuel’s vapor by breaking down molecular bonds, releasing energy in the form of heat and light. Without oxygen, a fire cannot start, grow, or continue burning
For a better understanding of how oxygen and oxidizing agents impact combustion, click here.
The Fire Tetrahedron
What is the fire tetrahedron, and how does it relate to the fire / combustion triangle?
Fire Tetrahedron
Once all three elements of the fire / combustion triangle have come together in the correct proportions and conditions and combustion has begun, a fourth element gets added to the triangle creating what is called the fire tetrahedron.
The fire tetrahedron turns the sometimes-timid combustion process into an all-consuming beast; it will devour everything in its reach as long as there is enough combustible fuel and oxygen available, and that no natural or human-made event stops it. As it grows, it can even produce offspring by starting new fires with the sparks and intense radiant heat it produces. This is all because of the fire tetrahedron.
The Birth of the Fire Tetrahedron
First, the fire / combustion triangle has to occur where fuel is heated, releasing flammable vapors. The vapors mix with oxygen in the air. When the mixture reaches ignition temperature, a flame forms.
In the flame zone:
- Free radicals are produced.
- These free radicals interact rapidly with fuel molecules, causing new reactions.
- Each reaction releases more heat and produces more radicals.
This self-sustaining loop—the fire tetrahedron—keeps the fire burning as long as fuel and oxygen are present and no intervening event stops
Free Radicals in Combustion
Continuing with the fire / concert analogy, with the fire tetrahedron being the headlining event, free radicals (or just radicals for short) are the star performers who make the show worth seeing.
In the context of fire and combustion, free radicals are highly reactive atoms or molecules which have unpaired electrons. These unpaired electrons make radicals unstable, so they aggressively seek out other atoms or molecules to react with.
Free radicals are created when, during the combustion process, heat breaks the chemical bonds in fuel molecules and oxygen. They cause the chemical chain reaction which sustains a fire. The more radicals which are created, the bigger the fire gets, growing exponentially.
The Process Explained
The manner in which radicals enter the event at a fire is all chemistry. This is why the fire tetrahedron is often referred to as the chemical chain reaction. During combustion, several key radicals are continuously formed and consumed. The most important include:
| Radical | Symbol | Role in Fire Chemistry |
|---|---|---|
| Hydrogen | H• | Reacts with oxygen to form other radicals such as OH• |
| Hydroxyl | OH• | Propagates chain reactions by attacking fuel molecules |
| Oxygen atom | O• | Breaks down fuel and helps form more OH• and H• |
| Hydroperoxyl | HO₂• | Forms in cooler flames and contributes to radical recycling |
| Methyl | CH₃• | Forms from hydrocarbon fuels; reacts with O₂ to generate more heat and radicals |
Radicals influence a fire because of how they react with fuel and oxygen as described below:
1. They drive rapid reaction cycles
- Once radicals (e.g., OH•, H•, and O•) are formed, they react with unburned fuel molecules.
- These reactions release energy (heat) and create more free radicals.
- The newly created radicals continue the cycle, attacking more fuel molecules.
2. They enable fire propagation
- Because these reactions happen extremely fast (on the order of microseconds), they create a self-sustaining loop.
- As long as fuel and oxygen remain available, these radicals keep regenerating themselves, creating heat, and allowing the fire to spread and grow.
3. They lower the activation energy
- Free radicals can initiate reactions at lower temperatures than would be needed without them making fuels ignite easier and quicker.
- This makes combustion more efficient and helps maintain the flame, even when external heat sources are removed.
Conclusion
The key to fire protection—fire prevention and fire suppression—is keeping these components apart or, if they do combine to start a fire, disrupting or removing one or more of them. All fire science is centered around these basic concepts.
A thorough understanding of both the fire / combustion triangle and the fire tetrahedron are essential for anyone involved in fire safety, fire prevention, and/or emergency response. The fire / combustion triangle teaches us that fuel, heat, and oxygen are the three essential components required for a fire to start. By identifying and managing these elements, we can reduce the likelihood of ignition—whether by storing flammable materials safely, controlling ignition sources, or managing oxygen and oxidizing agents.
However, the fire tetrahedron takes our understanding one step further by introducing the chemical chain reaction and explaining how a fire sustains and propagates itself once ignited. This fourth component reveals why some fires continue burning even after the original heat source is gone, as well as why some extinguishing agents work more effectively than others. Modern fire suppression systems increasingly rely on disrupting this chain reaction to stop fires quickly and efficiently, especially in high-risk environments such as data centers, chemical facilities, and energy storage systems.
Together, the fire / combustion triangle and fire tetrahedron form the foundation of all fire science. They not only help us understand how fires start and spread, but also guide the design of prevention strategies and suppression technologies that can save lives, protect assets, and reduce environmental impact.
Understanding Fuels in the Combustion Process
As mentioned in the main article, fuels can exist as solids, liquids, or gases. Some examples of each are listed in the tables below:
| Combustible Solids | |
| Class | Examples |
| Biomass | Wood, paper, crop waste |
| Fossil-based | Coal, peat, coke |
| Processed / manufactured | Pellets, charcoal, fire logs |
| Synthetic organic solids | Plastics, rubber, foam |
| Textiles / fibers | Cotton, polyester, upholstery |
| Combustible metals | Magnesium, titanium, aluminum |
| Miscellaneous | Wax, resin, some chemicals |
| Flammable Liquids | |
| Compound Type | Example |
| Hydrocarbons | Gasoline, diesel fuel, naphtha |
| Ketone | Acetone |
| Alcohol | Ethanol, methanol |
| Aldehyde | Formaldehyde |
| Ether | Diethyl ether |
| Amine | Aniline |
| Nitro compound | Nitromethane |
| Glycol | Ethylene glycol |
| Flammable Gases | |
| Chemical Class | Examples |
| Hydrocarbons | Methane, propane, butane, ethylene |
| Hydrogen-based | Hydrogen, ammonia, |
| Carbon-based (non-HC) | Carbon monoxide, acetylene |
| Sulfur | Hydrogen Sulfide |
Solid & Liquid Fuels
While fuels can exist as solids, liquids, or gases, the important thing to realize—often a difficult concept for some to grasp—is that only gases can burn. This means that combustible solids and flammable liquids must be converted into their gaseous or vapor form in order to burn.
So how do combustible solids and flammable liquids become converted into gas? The answer is simple and the element of the fire / combustion triangle we will discuss next: heat.
While the process is similar, the reaction is slightly different when heat is applied to the surface of a combustible solid or flammable liquid fuel. The table below explains how combustible solids and flammable liquids convert into a gaseous form that will burn.
| Stage | Combustible Solid | Flammable Liquid |
| Heat applies energy to the molecules of the material | Heat energy loosens the tight molecular bonds and increases motion, thus moving them apart and starting the creation of a gas.* | Heat energy loosens the already loose molecular bonds and increases motion, thus moving them apart starting the creation of vapor / gas.** |
| Pyrolysis / vaporization occurs | Solids decompose in a process called pyrolysis where complex molecules are broken down into flammable gases, tars, and char (black carbon residue). | The liquid evaporates forming vapors (the gas phase) which are highly flammable. |
| Formation of a flammable gas mixture | The gases given off mix with oxygen from the air and are typically ignitable. | The vapors given off mix with oxygen from the air and If the gas-air mixture reaches the right concentration (within the flammable limits), it becomes ignitable. |
* The greater the surface area exposed to heat, the faster this process evolves; e.g., it is much easier to light sawdust than it is a log of wood.
** This depends on the flashpoint of the flammable liquid. Flashpoint is the minimum temperature at which a liquid will give off vapors sufficient to create a flash across the surface when a spark is applied. Some liquids (e.g., gasoline) have flashpoints well below 0° F, while others (e.g., diesel fuel) have flashpoints well above 100° F. Flashpoint values can vary depending on the reference source used and manufacturer of the flammable liquid.
Gaseous Fuels
Just because a flammable gas is produced or is present does not mean that it will produce flaming combustion. There has to be the correct ratio of flammable gas to oxygen to produce a flame. This range is referred to as the gas’s flammable limits or explosive limits. Every flammable gas has what is known as a lower flammable limit or lower explosive limit (LFL / LEL) and an upper flammable limit or upper explosive limit (UFL / UEL).
The two values—LFL vs. LEL, UEL vs. UFL—are the same. LEL and UEL are most commonly used in industry. LEL means that if that concentration of gas to oxygen is confined, it will explode if exposed to an ignition source. LFL indicates that the mixture will burn.
If a gas / oxygen concentration is below the LEL, it will not burn and is considered “too lean.” If it is above the UEL, it also will not burn because it is considered “too rich.” But anywhere in between the two values, it will burn in an open environment and, if confined, can explode. The danger often arises when a gas leaks into the air, starting below LEL, but as more gas accumulates, it reaches the LEL and burns or explodes.
It is important to understand that different gases can have vastly different flammable limits, with some having very wide ranges and others quite narrow. The limits can also vary depending on the reference source. Some example flammable limits are listed in the below table.
| Product | LFL / LEL | UFL / UEL |
| Gasoline (vapor) | 1.4% | 7.6% |
| Propane | 2.1% | 9.5–10.1% |
| Diesel fuel (vapor) | 0.6% | 7.5% |
| Acetylene | 2.5% | 100% |
| Methyl Ethyl Ketone (MEK) | 1.4% | 11.4% |
| Hydrogen Sulfide | 4.0% | 44.0% |
Pyrophoric Materials
Pyrophoric materials are substances that ignite spontaneously when exposed to air without the need for any ignition source. They are a completely different beast when it comes to substances that will burn. They do not adhere to the same fire dynamics or fire behavior as do common combustible fuels. As such, they require an entirely different approach to fire safety and fire prevention.
Fortunately, pyrophorics are typically only used in very specialized processes and thus are not an everyday product one can expect to encounter. But they are out there, and being ignorant of their reactive and often violent properties could have tragic consequences.
Pyrophoric materials can exist as solids, liquids, or gases. While they share some common combustion characteristics with typical fuels, they often involve unstable chemical bonds or reactive metals, making them hyper-sensitive to oxidation.
Pyrophoric materials bypass the ignition / heat factor—i.e., they combust spontaneously because their chemical structure is so reactive that exposure to air alone is enough. Conversely, typical fuels such as wood or gasoline need that extra nudge of energy (a match, spark, or flame) to get the combustion process going.
The table below sums up the main differences between pyrophoric materials and common combustibles:
| Feature | Pyrophoric Materials | Common Combustibles |
| Ignition source | Spontaneous in air | Requires external ignition (spark, flame, heat) |
| Activation energy | Extremely low | Higher—needs heating to ignition temp |
| Combustion onset | Instant upon air contact | Delayed—requires sustained ignition |
| Hazard profile | Highly unstable – can ignite on contact | Flammable, but more predictable |
| Storage / handling | Requires inert atmosphere (argon/N₂) | Requires sealed containers or routine fire safety measures |
| Examples of each: | Silane, white phosphorus, triethylaluminium | Wood, gasoline, propane |
The Role of Heat in the Combustion Process
Because heat plays such a central role in the combustion process, it is important to understand where the various forms of ignition heat originate and the two types of ignition.
Sources of Ignition Heat
Heat, as well as mechanisms which produce heat, are everywhere around us—all the time, every day. Heat can come from various sources, both natural and human-made. The tables below outline the common sources of fire per the NFPA (National Fire Protection Association) which are heat-related in residential, commercial, and industrial occupancies:
| Causes of Residential Fires | |
| Heat Source | Description |
| Cooking equipment | The leading causes of residential fires, often due to unattended cooking or overheating |
| Heating equipment | Includes furnaces, space heaters, fireplaces, and other heating devices, as well as combustibles placed too close to heating equipment |
| Electrical | Faulty wiring or overloaded circuits |
| Intentional | Arson, children playing with fire, etc. |
| Smoking | Improper use of smoking materials (the #1 cause of fatal residential fire incidents) |
| Natural causes | Lightning strikes and wildfires |
| Causes of Commercial Fires | |
| Heat Source | Description |
| Cooking equipment | Primarily in the food service industry |
| Electrical | Faulty wiring, lighting malfunctions, or overloaded circuits |
| Heating equipment | Includes furnaces and boilers, and poor insulation |
| Intentional | Arson and other fires set intentionally, e.g., by disgruntled employees |
| Smoking | Improper use of smoking materials |
| Natural causes | Lightning strikes and wildfires |
| Causes of Industrial / Manufacturing Fires | |
| Heat Source | Description |
| Equipment or heat source failure | Overheated machinery, mechanical friction, or component failure |
| Electrical distribution & lighting equipment | Power surges, overloaded circuits, or panel faults |
| Hot work processes | Welding, cutting, or grinding that creates sparks near flammable materials |
| Flammable liquids & gases | Leaks, spills, or vapor ignition due to improper handling or equipment failure leading to exposure to a heated ignition source |
| Chemical reactions | Incompatible materials or runaway chemical processes |
| Natural causes | Lightning strikes and wildfires |
Understanding Ignition: Piloted vs. Autoignition
As previously discussed, ignition is the process by which a material begins to burn—but not all ignition happens the same way. There are two primary types of ignition which describe how heat initiates combustion in a fuel: piloted ignition and autoignition.
| Piloted Ignition | |
| Piloted ignition occurs when a material is heated to its ignition temperature and an external ignition source (a pilot)—such as a spark, flame, or ember—is required to start combustion. The key point is that the fuel and oxygen may be ready, but nothing happens until an ignition source is added. | |
| How it works |
|
| Common examples |
|
| Autoignition | |
| Autoignition occurs when a material is heated to its autoignition temperature, and no external ignition source is needed; it catches fire on its own simply due to heat. The key point is that no flame, spark, or other pilot is required—heat alone is enough to ignite the fuel. | |
| How it works |
|
| Common examples |
|
Understanding the difference between the two types of ignition is crucial for fire prevention and fire safety because:
- Piloted ignition helps identify how most everyday fires start—careless flame use, sparks, friction (e.g., striking a match, etc.).
- Autoignition highlights less obvious fire risks, like hot machinery, chemical storage, or oily rags—areas where fires can start without warning.
The Effect of Oxygen & Oxidizing Agents on Combustion / Fire
Aside from just supporting the combustion process, the concentration of oxygen and/or the presence of oxidizing agents can also have a dramatic effect on combustion / fire.
Oxygen Concentration and its Effect on Fire
Normal air contains about 21% oxygen. Most fires require at least 16% oxygen in the surrounding atmosphere to sustain flaming combustion. If oxygen drops below this level (e.g., in a sealed room where an oxygen-displacement fire suppression system has discharged), the flames will be extinguished.
However, in areas where oxygen levels exceed 23% or are considered oxygen enriched, the fire dynamics change completely, and for the worse. In oxygen-enriched environments, combustible materials can:
- Ignite far more easily
- Burn with significantly more intensity and at extremely elevated temperatures
- Present serious fire and explosion risks
Oxygen-enriched environments are not overly typical but can include:
- Healthcare settings where oxygen is used
- Laboratories and industrial facilities
- Vehicles transporting liquid oxygen
Fire science almost always considers a normal oxygen level at 21%. When situations are created where oxygen enrichment occurs, the fire chemistry changes dramatically. Special fire safety precautions are necessary because materials may ignite simply when exposed to compression or friction and cause violent fires.
Here is an example: A truck carrying liquid oxygen experiences a leak onto an asphalt (hydrocarbon) surface. When the driver investigates the leak, the compression of merely stepping onto the asphalt where the liquid oxygen has leaked can cause an explosion and fire.
Oxidizing Agents
The atmospheric air we breathe has more than enough oxygen to support vigorous combustion. However, for fire safety and fire prevention, it is critical to understand that while routine oxygen-enriched environments are not common, oxidizing agents which can create them are, with many found in everyday locations.
The table below describes a variety of oxidizing agents, some of which may currently be at your home, office, or farm and have the potential to dramatically influence the combustion process:
| Product | Use | Hazard |
| Nitrous oxide | Racing, anesthesia | Supports combustion, similar to oxygen |
| Hydrogen peroxide | Rocket fuel, disinfectants | Violent reaction with organics |
| Ammonium nitrate | Fertilizer, explosives | Fire + pressure = detonation risk |
| Potassium chlorate | Pyrotechnics, household matches | Intense oxidizer, shock-sensitive |
| Fluorine / chlorine | Industrial chemicals | Can accelerate combustion |
| Ozone (O₃) | Water treatment, disinfection | Unstable, highly reactive |
| Bleach (sodium hypochlorite) | Disinfecting, whitening clothes, mold removal | Acts as an oxidizer when mixed with flammable substances |
| Pool chemicals | Sanitizing pools and hot tubs | Strong oxidizers |
Using the Fire Triangle & Fire Tetrahedron in Fire Protection
As set out in the main article, the concept of the fire triangle and fire tetrahedron are the basis for most fire-prevention and fire-suppression measures and tactics.
Fire Prevention
In simple terms, fire prevention seeks to stop the combustion process from ever beginning by eliminating or mitigating one (or more) of the elements of the fire / combustion triangle. There are almost countless approaches to this, but following are the basic premises of how each of the three elements can be managed to prevent a fire.
Oxygen
Oxygen is difficult to control or manage because it is in the air everywhere around us at 21%, which is more than enough to support combustion.
With specialty materials such as pyrophorics, storage is often in containers where oxygen is displaced by an inert gas (e.g., argon or nitrogen) or the pyrophoric is submerged in a liquid. Inerting with gas is also commonly used in industry to displace flammable vapors from pipelines, tanks, and processing equipment to eliminate flammable atmospheres.
Another technique is to isolate oxidizing agents from combustible items. While this does not eliminate oxygen, it does mitigate the possibility of an oxygen-enriched situation which can greatly worsen any fire that occurs and make extinguishment infinitely more difficult.
Fuel
Fuel can be another difficult element to manage because, although not as ubiquitous as oxygen, combustible fuels are all around us. One key to managing the fuel risk is to properly store them so they are not exposed to heat sources. Another method, particularly with flammable liquids and gases, is to store them in proper containers specifically designed to contain them without leaking and which have the ability to resist a certain amount of heat exposure. Tactics such as these seek to prevent a fire from occurring in the first place.
Other tactics with fuels attempt to mitigate the fire situation versus preventing it. In this vein, separation of fuels is a commonly used technique. In this way, if one of the fuel sources ignites, it is far enough away from the other fuels that it does not spread to them.
In the petrochemical industry, “tank spacing” is frequently used where, if one tank of flammable liquid ignites, it is far enough away from the others so as not to spread to them. (See image below.) When spacing is not possible, fire-resistant barriers can be placed between the fuel sources. It is also a common practice to limit the amount of combustibles which are present at any one time.
Another fuel-management method is to practice good housekeeping. This includes activities such as monitoring flammable liquid lines and equipment for leaks, preventing the accumulation of combustible dusts, and disposing of combustible wastes regularly and properly.
Heat
As has been mentioned, heat is the one element where we can have the greatest impact on fire prevention. If heat can be controlled and kept away from combustible fuels, fire can easily be prevented. Absent natural occurrences (e.g., a lightning strike), most failures in fire prevention can be attributed to human error in managing heat sources.
The types of heat mismanagement leading to a fire are legion. Poor maintenance, inattention, bad habits, ignorance, defective equipment, and generally poor decision-making are the root causes of most fires. If you keep heat sources away from combustible items, there will be no fire unless nature itself causes it. But in real life, this is not practical. Life requires heat to be in close proximity to combustible items. The key is to be aware of the risk, be smart about your actions (or inactions), and prepare for the worst.
Fire Suppression
Fire suppression is also centered on removing one (or more) of the elements of the fire / combustion triangle. But there is also the option of attacking the fire tetrahedron. However, in the arena of fire prevention, attacking the fire tetrahedron is not an option; that is because the tetrahedron only exists once combustion has begun. As we will explain, it is the “Achille’s heel,” so to speak, of a fire.
First, though, let’s begin with the elements of the fire / combustion triangle.
Oxygen
Just as with fire prevention, oxygen is a difficult element of the fire / combustion triangle to attack because it is all around us. On a small scale, placing a lid over a burning frying pan is effective by depriving the fire of oxygen. On a bit larger scale, using a CO2 fire extinguisher can also extinguish a fire by displacing the oxygen from the fire area. But this is only effective if the fire is small and in a confined area where wind will not blow the CO2 away and bring in more oxygen.
There are large-scale inert gas systems which use CO2 or argon gas to flood an enclosed space, causing the oxygen concentration to decrease to the point where flaming combustion ceases. While this can be effective, it is not without its drawbacks, and some of them are serious.
If the space is large—e.g., the hold of a ship—it can take a tremendous amount of gas to flood it. Plus, the space must be tightly sealed to contain the gas. Should any personnel be in the confined area when the system discharges, they will likely die due to lack of oxygen. Lastly, this method stops flaming combustion, but smoldering combustion will likely continue and can take a significant amount of time to cease and cool. If the space is opened too early, the introduction of oxygen will cause the fire to instantly reignite.
Fuel
Eliminating fuel during a fire is a difficult task. In wildfires, firefighters make fire breaks in the forest to stop the fire from spreading to additional fuels. With flammable liquids, firefighting foam forms a blanket over the top of the liquid to seal the fuel vapors under the foam blanket. (See image below.) Another method in industrial fires is to shut a valve that is supplying flammable liquid or gas to the fire. A last resort is to let the fire consume all of the available fuel and burn itself out.
Heat
Again just as with fire prevention, attacking the heat element is one of the most common types of fire suppression. If the fire can be cooled to the point where ignitable vapors or gases are no longer produced, it will be extinguished. It is seemingly a simple method; the vast majority of people believe that if you put water on a fire, it will go out.
But in reality, it certainly is not that simple. First of all, water is only effective on one class of fire: Class A ordinary combustibles. If used on any of the other classes, not only.
is it ineffective, but it can also be dangerous. But since most fires that the majority of people encounter are Class A—e.g., trash cans, houses, cars, etc.—water is the first extinguishing agent they think of.
Water cools Class A fuels. To some degree, the steam produced also displaces oxygen and fuel vapors. But it mostly works by cooling. Water is easily available in most instances, it is cheap, and it is usually effective.
For water to be effective, it must override the heat production of the fire. This means that a very large fire requires a significant amount of water to defeat the heat production. A substantial building fire can require thousands of gallons of water per minute applied for hours before extinguishment occurs. While effective, water is not very efficient.
One reason for water’s inefficiency is that a significant amount of it, when applied to a fire, does not reach or affect the fire. It gets evaporated as steam and/or just misses the mark and runs off with no effect. Water is also incredibly damaging to anything it touches. It is often the case that in firefighting, water causes more damage than the fire itself.
If water is applied to any class of fuel other than Class A, real problems can occur as outlined in the table below:
| Class of Fire to Which Water is Applied | Result |
| B | Being heavier than flammable liquids, water spreads the fire by making the fuel spill larger. It can also cause dangerous splashing of flaming fuel, endangering the firefighters. |
| C | Water is an excellent conductor of electricity and can lead to electrical shock of those fighting the fire. |
| D | Most Class D fires are also incredibly water reactive. The application of water can cause explosions and intense production of flaming metal sparks which ignite other combustible materials. |
| K | Water applied to a grease fire can cause a massive expansion of steam which launches burning grease over significant distances, burning everything and everyone in its path. |
Fire Tetrahedron
Regardless of the class of fire, the fire tetrahedron can be attacked with specialized chemical agents without the worry of making the situation worse or endangering the firefighter. Because all fires behave in essentially the same way, attacking the chemical chain reaction element makes this mode of fire suppression ideal in most cases.
As a disclaimer, attacking the chemical chain reaction in Class D and Class K fires has not yet been proven to be the most effective tactic. But honestly, these two classes of fires are outliers and not commonly encountered.
When an agent which is designed to interrupt a fire’s chemical chain reaction is applied, here is the process that occurs:
1. The agent breaks the radical loop.
The chemical reacts with the free radicals (e.g., H• and OH•) and neutralizes them by transforming them into stable, non-reactive molecules.
Examples:
- Potassium radicals react with OH• radicals to form potassium hydroxide (KOH), a stable product that removes reactive species from the flame. (See graphic below)
- Clean agents (e.g., halon replacements) react with radicals to form stable halides such as hydrogen chloride (HCl), reducing the number of radicals available to sustain the fire.
2. Radical concentrations drop.
Without enough radicals, the chain reaction cannot continue.
3. Combustion stops.
Since the chain reaction is responsible for generating the heat that keeps the fire alive, disrupting it causes the flame to collapse, even if fuel and oxygen are still present.
Fire suppressants which interrupt the chain reaction work by neutralizing the reactive particles (free radicals) which keep a fire burning. Without those radicals, the fire loses its energy source and quickly goes out.
The important thing to consider is that not all fire suppression agents which attack the fire tetrahedron are created the same—i.e., they have different effects (and side effects) once applied.
Taking a look at the most common agents in this category, all of which are effective, here is a comparison:
| Type | Clean up | Other Considerations |
| Dry chemical
| Can be extensive |
|
| Clean agents (gaseous) | None to minimal |
|
| Condensed aerosol | Minimal |
|
Frequently Asked Questions
What is the difference between the fire triangle and the fire tetrahedron?
The fire triangle consists of fuel, heat, and oxygen, the elements required to start combustion. The fire tetrahedron adds a chemical chain reaction that sustains and propagates a fire after ignition.
What is the fourth element of the fire tetrahedron?
The fourth element is the self-sustaining chemical chain reaction driven by free radicals in the flame zone. Those radicals react with fuel and oxygen, release heat, and create more radicals as long as fuel and oxygen remain available.
Why can a fire keep burning after its original ignition source is removed?
Free-radical reactions can maintain the chemical chain reaction once combustion has begun. They release heat and regenerate radicals, allowing the fire to continue while adequate fuel and oxygen are present.
How can a fire suppression system stop the fire tetrahedron?
A system can stop combustion by removing or disrupting one or more of its essential elements. Modern suppression methods can interrupt the chemical chain reaction, while prevention measures commonly control fuel, heat, or oxygen before ignition occurs.
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