Since fire is an oxidation reaction, the fire tetrahedron is an important way to think about what is happening. The four components required for a fire are:

• Fuel;
• Oxidizing Agent;
• Heat; and
• Uninhibited Chemical Chain Reaction.

For the fire to continue burning, all four sides of the tetrahedron must be present in the right combinations. In other words, removing any one of the four sides of the tetrahedron will extinguish the fire. Knowing this enables us to begin to explain many different aspects of fires.

Flaming Combustion

Flames are the visible, luminous indication of where oxidation (combustion) reactions are taking place. Both the oxidizer and fuel are gases. The reaction and heat release occur in the gas phase.

Flame: A visible, luminous body where the oxidation reaction is occurring.

Flaming combustion is the process occurring when the flames and plume of a fire are visible. In flaming combustion, the fuel and air are both gases. The reactions and heat release occur in the gas adjacent to the liquid or solid surface. There are two ways in which flaming combustion can take place depending upon how the fuel and oxidizer mix before entering into the combustion reaction. The flame can burn either as a premixed or as a diffusion flame.

Premixed flames occur when the fuel and oxidizer are well mixed before they ignite and begin to combust. Flaming combustion in fires over liquid and solid fuels usually involves diffusion flames, which the most commonly encountered flames by fire safety professionals. In diffusion flames, the oxidation reactions occur in regions where the gaseous fuel (fuel vapours) and oxidizer locally mix by diffusion processes (the boundary between fuel and oxidizer). Combustion occurs where oxygen is drawn from the surrounding area and diffused into the fuel vapour. The flame must generate enough heat so that new fuel vapour is continually generated to feed the reaction. The rate of burning of a diffusion flame is therefore tightly linked to how much vapour can be produced from the fuel.

Fire Tetrahedron - Fuel

Fuel: Any substance that can undergo combustion

Fuels can be classified in various ways. One method is to classify them by their physical properties. Based on this, fuels can either be:

• Solids: Materials that have a definite shape and volume.
• Liquids: Materials that have a definite volume but will take the shape of the container that holds them.
• Gases: Materials that have no natural shape or volume and will thus spread out evenly to take both the shape and volume of the container or space that holds them.

Chemical Nature of Fuels

Another way to classify fuels is by their chemical nature. The majority of fuels that we see are organic, meaning they are made up of carbon-based compounds. They are often hydrocarbons, which are materials containing molecules made largely of carbon and hydrogen atoms.

Under the right conditions, these fuels will produce CO, CO2 and water when they burn. Depending on the availability of fuel vapour and air, other organic compounds (unburned fuel) and soot can also be found in the gases emitted from the fire plume. Organic fuels include woods and other natural fibres, plastics and hydrocarbon liquids such as gasoline. Inorganic fuels include metals such as magnesium or sodium.

Pyrolysis, Vaporization and Sublimation

As solid fuels are exposed to heat, they absorb energy and the temperature of the molecules increases. A certain amount of energy is necessary to produce a flammable vapour from a fuel that is at ambient temperature. This energy is called the heat of gasification . Of two fuels that release approximately the same amount of energy when they burn, the one with the lower heat of gasification will burn faster (as long as there is enough available oxygen). This is because it takes less energy from the oxidation reactions to vaporize fuel to feed the fire.

As different types of solids (different materials) heat up, they will behave in different ways. There are also many other variables that may determine how a solid will react to heat. Some solids first change into a liquid (melt) before they form fuel vapour and burn. Other solids form vapour directly upon heating. This vapour is often composed of decomposed parts of the molecules that were in the original solid because the molecules break down (decompose) as their temperature increases.

Pyrolysis: A process by which a material is decomposed, or broken down, into simpler molecular compounds by the effects of heat alone.

Pyrolysis typically starts between 100°C and 250°C. Fuel pyrolysis vapours are the fuel vapours that burn in a fire. If pyrolysis happens in the presence of oxygen, which is commonly the case in fires, it is called oxidative pyrolysis. The vapours will be composed of different materials than those occurring when only thermal pyrolysis takes place.

In addition to the formation of flammable vapours, some materials will also char.

Char is defined as a carbonaceous material that has been burned or pyrolyzed and has blackened appearance .

It is this char that reacts directly with oxygen on the surface during non-flaming combustion. Both vaporization and sublimation are endothermic processes. Vaporization is the phase change from liquid to vapour state, which is what occurs in a boiling pot of water. Sublimation is a phase transition of a substance directly from the solid to the gas phase without passing through the intermediate liquid phase. An example of this would be dry ice.

Solid Evaporation Pathways

Since fire is a chemical process, it is important to learn about the chemistry of fire. This relates to changes of state of different materials, decomposition of materials by pyrolysis when they are heated, the heat released and products of the combustion reaction. How the fire will develop depends on the details of the combustion reactions that are taking place. Given the complexity of the chemical reaction, we instead look for key indicators that will tell us what state the fire is in, what hazards the fire might pose and what the fire might do next.

As liquid and solid fuels are heated, they can take many different routes to become vapours. Solids can sublimate directly into gases, or they can decompose (pyrolyze) and then evaporate. Solids can melt into liquids, retaining their original chemical composition, or they can melt and decompose at the same time. Similarly, liquids can evaporate and retain their chemical composition, or they can decompose and evaporate.

Prior to ignition, a fuel can be a solid, liquid or vapour. Fuels can undergo various chemical changes or changes of state before they become involved in a fire. For example, liquid fuels must vaporize by boiling or evaporating before they can burn, such as water turning to steam. Evaporation of water into steam is also critical in the understanding of the role water plays in suppression activities. Solid fuel may melt and then evaporate (similar to the foam shown in the centre image of Figure 13), or decompose chemically into other substances through pyrolysis.

a. Boiling

Melting and decomposition

Sublimation

Figure 13: Fuel and its response to heat

Fuel Classifications

Fuels can also be classified by the recommended methods to extinguish a given class of fire. In North America, there are five main fuel classifications. It is important to note that there are differences in the classifications of fuels and suppression agents in different countries. Since this system is not universal, it is an important consideration to make when using this method to teach fire dynamics. The different classifications are shown in Table 1.

.

Table 1: Extinguisher classifications

Description	Ordinary Solid Combustibles	Flammable Liquids	Flammable Gasses	Energized Electrical Equipment	Combustible Metals	Cooking Oils & Fats
NFPA Class
European Class	A	B	C	Not Classified	D	F
Australian Class	A	B	C	E	D	F
Suitable Suppression	Water Foam Dry Chem	Foam CO2 Dry Chem		CO2 Dry Chem	Class D Powder	Foam CO2

Heat of Combustion

The amount of energy that can be generated from a certain fuel under ideal conditions is termed the heat of combustion.

Heat of combustion: The total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions.

Figure 14: Heat of combustion of common materials

Heat Release Rate (HRR)

The heat energy in a fire is produced through exothermic oxidation reactions between fuel and air. To understand fire development, it is important to take this concept one step further and recall that heat (energy) can be absorbed or released over time. The rate at which energy changes over time is power; therefore, the rate of heat release, or heat release rate (HRR) of a fire, driven by the ongoing chemical reactions, is the power of the fire. Power is measured in Watts, which are equal to J•s. One Watt (W) is equal to 1 J of energy being released per second.

Fire HRRs are usually quite large, so they are given in units of kilowatts, which is 1000 W, or in megawatts, which is 1 million Watts. As this energy is released and transferred through the environment, it increases the temperature of the smoke plume and vaporizes more fuel, keeping the fire burning.

Once the fire is established and begins to grow, it is important to understand the HRR from the burning fuel. This determines how much energy the fire will be able to exchange with the surroundings and also relates to the rate of growth of a fire. It is also closely related to how the energy released from a burning item would change the thermal conditions in a fire room or how it might ignite other nearby fuels via heat transfer. Therefore, having an appreciation of the HRR expected from different types of fuels helps to determine the possible impact of a fire on people, structures and the environment.

The HRR of the fire is controlled by many parameters. Some of the most important are the chemical and physical properties of the fuel, the geometry of fuel, the level of fuel containment, the ventilation patterns and whether there are surrounding fuels.

Table 2: Energy released by different common fuels

Fuel	Energy Released/kg of O2 (MJ/kg)
Organic liquids and gas fuels	12.72
Synthetic polymers	13.03
Natural fibers (cotton, cardboard, newspaper, etc.)	13.21
Cellulose (in well-ventilated and under-ventilated conditions)	13.37 - 13.59
Polypropylene, polyacrylonitrile, polytetrafluoroethylene	10.76 - 13.91

Standard tests (called cone calorimeter tests) are conducted to determine the HRR of different materials and objects under well-ventilated burning conditions and thereby to rank the HRR that can be expected from different fuels and objects when they are involved in a fire.

When looking at published values of HRR for a given material or object, it should always be remembered that these values come from laboratory tests. The HRR from that same fuel in real fire situations can vary significantly from the published value.

Knowledge Check What are the sides of the fire tetrahedron? In what state is fuel during flaming combustion? Define fuel. What are pyrolysis, vaporization and sublimation? What is the heat of combustion and what are the units of measure? What is the heat release rate and what are the units of measure?

Fire Tetrahedron - Oxygen

Oxidizing Agent

In most fire situations, the oxidizing agent is usually the oxygen in the air. The air contains approximately 21% oxygen at sea level. Under certain conditions there is also oxygen mixed in with fire gases. This can provide the oxidizing agent in some cases and can result in rapid and unexpected fire spread situations.

Figure 15: Fertilizer fire in West Texas

Some fires also involve other chemical oxidizers. Some common ones are ammonium nitrate fertilizer (NH4NO3), potassium nitrate (KNO3), as well as various perchlorate materials or hydrogen peroxide (H2O2). The molecules in these types of compounds can rapidly break down and recombine with other molecules, resulting in rapid burning rates and flame propagation as well as explosions, as shown in Figure 15.

Effect of Oxygen on Flame

The amount of oxygen in the ambient environment will affect the intensity, growth and size of the flame:

Oxygen-Enriched

• Combustion intensity may increase.
• Fire spread may be accelerated, particularly when new oxygen is added to an oxygen-depleted fire environment.

Oxygen-Depleted

• Often the case in real fires.
• Fire spread is decelerated.
• Can undergo transition to smouldering combustion

Oxygen is a key element in determining the dynamics of fires. Increasing the amount of available oxygen will accelerate combustion (for example, an oxygen respirator), while limiting the oxidizer will result in smoldering, or non-flaming combustion. The lower limit of oxygen concentration that will support combustion is called the Limiting Oxygen Index (LOI).

Limiting Oxygen Index (LOI): The lowest oxygen concentration in nitrogen that will support flaming combustion

Typical values of LOI at room temperature are 10%-14% by volume for many materials. If the fire uses up the available oxygen in a compartment and the overall oxygen concentration falls below these levels, then the fire will become poorly ventilated and may even extinguish.

Thornton's Rule

In 1917, W.M. Thornton discovered that there is a relatively constant 13.1 MJ of energy released for every 1 kg of oxygen used in the combustion of most organic materials, regardless of the material. This conclusion, known as Thornton's Rule, has since been verified using oxygen consumption calorimetry. It indicates that the heat released by organic combustibles is dependent upon the amount of oxygen consumed in the process and can therefore be calculated if we know how much oxygen is consumed. If we assume that air contains approximately 21% oxygen, we can further calculate that for every 1 kg of air consumed, approximately 2.75 MJ of energy are released. If 1 m3 of air has a mass of 1.2 kg, for every 1 m3 of air consumed, approximately 3.3 MJ of energy are released.

This is critical to understanding how a structure fire might develop. Thornton's rule states that high HRR fuels (which we find in modern furnishings and building materials) will consume large quantities of oxygen rapidly when they burn. This can result in oxygen being consumed faster than it can be replaced in a compartment or room, contributing to the onset of ventilation-controlled fires. When a fire becomes ventilation controlled, the HRR will decrease again, but the material certainly may be hot enough to continue emitting fuel vapours. Then, if oxygen is re-introduced to the fire compartment or is introduced at a greater rate (i.e., when the compartment is ventilated), extremely high rates of heat release can rapidly develop. This can lead to very dangerous situations such as flashover or backdraft. These concepts are discussed in detail later in the curriculum.

To reiterate, what this means is that in an enclosure fire, the HRR of the fire cannot exceed what the available oxygen can support, and so controlling that oxygen availability becomes paramount. It is this understanding that supports how ventilation without extinguishment can impact any enclosure fire, and how the introduction of air directly influences the potentially rapid increase in HRR of that fire when any ventilation, either planned or unplanned, occurs.

Fire Tetrahedron - Heat

Heat plays several key roles in the fire tetrahedron, both by vaporizing and pyrolyzing liquid and solid fuels as well as providing the ignition energy to initiate and maintain the chemical reaction.

The heat component of the tetrahedron indicates that for a fire, we need heat energy above the minimum level necessary to release fuel vapours.

Fuel vapours from a solid or liquid fuel are produced through changing the state of the fuel, with or without pyrolysis. The initial heat energy can be provided through outside sources such as a pilot flame, flame radiation, vibration, friction, light, electricity or compression. These may initiate vaporization and pyrolysis of a material and often provide enough energy to ignite the vapours produced.

Chemical Chain Reaction

The heat side of the triangle is also closely linked to the uninhibited chain reaction, the last side of the tetrahedron. Heat provides energy to initiate and sustain the chemical reaction: once the chain reactions have started, they must continue uninhibited. For this, the exothermic chemical chain reaction between the fuel and air must release enough energy to sustain combustion. A self-sustained reaction occurs when sufficient excess heat from an exothermic reaction radiates back to fuel to produce vapours, which then mix with air and ignite in the absence of the original ignition source.

There are many ways the chemical chain reaction can be interrupted. One important way is by the introduction of suppression agents such as halons where the chlorine, fluorine and/or bromine atoms interrupt the exothermic reactions taking place. Some fire retardants also work to slow down the chain reactions so that they are no longer vigorous enough to supply the energy needed to maintain combustion.

Ignition

Ignition is the process of initiating self-sustained combustion. The form, mechanism and energy required for ignition vary with the form of the fuel (gas, liquid, solid), the chemical properties, geometry of the fuel, as well as the form and intensity of heat input into the fuel to initiate the process.

Independent of how ignition takes place, there is a minimum amount of energy that must be transferred to a substance for it to ignite under specific test conditions. Energy is needed to heat up (specific heat) and vaporize the fuel (latent heat of vaporization). Additional energy is also required to account for any energy lost during the process, as well as to keep the oxidation reactions going after they have started. Ignition, then, is very dependent on the original state of the fuel in a given situation as well as how the energy is applied.

Fuels are tested to determine their minimum ignition energy, listed in units of energy intensity (Watts/m2) or total amount of energy (Watt or Joule). However, the ease of ignition and the energy required for ignition are also dependent on many other factors.

It is important to realize that ignition temperatures are not the same as ignition energy. There are values for critical temperature for ignition in the literature, but the value for a certain fuel is NOT a property of a fuel and can vary from a listed value for various reasons. A measured (reported) value of ignition temperature is determined by a specific test method and may not reflect the actual temperature that a material has to heat up to in order to ignite.

If fuels are already producing sufficient vapours for the flammable limit to be reached without applying an external heat source, they will require only small amounts of energy input to ignite and burn. For other liquid and solid fuels, there must be enough energy in the ignition source to locally heat the fuel so that the fuel begins to pyrolyze, and for enough pyrolysis vapours to form and mix with air to form a flammable volume. Once this has occurred, an additional amount of energy is still needed to initiate the chain reactions that will sustain flaming combustion over the fuel, as long as enough fuel vapour and air are still available to react.

There are three main modes of ignition that can initiate the burning of a fuel, thus potentially leading to a fire. These are termed piloted ignition, auto-ignition and spontaneous ignition.

Piloted Ignition

In piloted ignition, a flammable mixture is formed above the bulk fuel. An external source, direct input, of energy (i.e., flame or spark) then ignites the mixture. The energy needed to ignite the material may come in the form of radiant energy, convective energy or a combination of both.

In Figure 17, we see that as the flame from the barbeque lighter is brought near the pool of liquid fuel, it ignites a vapour-air mixture near the edge of the pool. The flame then propagates around and across the surface of the liquid fuel (anywhere there is a flammable mixture) and finally grows large enough to establish a burning fire plume above the entire container of fuel. This is an example of how fires ignite locally and the energy produced by the initial reactions is enough for the flame to grow and spread across the available fuel surface.

Auto-Ignition

In contrast to piloted ignition, auto-ignition is the process in which fuel vapour is formed and mixes with oxygen and then is raised to a temperature high enough that it will ignite without the presence of a flame, spark or other external (piloted) source of ignition.

In Figure 18, a block of wood is exposed to heat radiated from an electrical heating element, which increases the surface temperature of the sample. Initially, pyrolysis vapours are formed and rise from the sample as the wood begins to decompose. Once the surface of the wood reaches its auto-ignition temperature, the lowest temperature at which a gas-air mixture can ignite and continue to burn, the block ignites without the presence of an additional source of energy.

Figure

18: Auto-ignition of composite wood

Spontaneous Ignition

Spontaneous ignition is a complex process that begins as an exothermic chemical reaction in which a material self-heats. It most often occurs in organic materials including animal and vegetable fats and oils, which react with oxygen (non-flaming combustion) to emit heat. Sometimes referred to as biological activity, such as that found in haystacks, decomposition oxidation can also result in self-heating. Spontaneous ignition requires the self-heating process to proceed to what is known as thermal runaway, the point at which a material is generating more heat than it is losing. There is a significant increase in the rate of temperature rise, and temperatures increase to a point where ignition can occur. There are four main processes that can lead to spontaneous ignition: microbiological heating; oxidative heating; moisture induced heating; and, in certain cases, other chemical processes. Examples of materials that may spontaneously ignite due to these processes are shown in Table 3.

Table 3: Spontaneous combustion materials

Microbiological Heating	Oxidative Heating	Moisture Induced Heating	Other Chemical Processes
Bagasse (Sugar Cane Residue)	Activated Carbon	Chlorinated Oxidizers	Monomers
Compost	Coal	Calcium Oxide	Nitrocellulose
Grains	Cotton	Cotton Bales	Peroxides
Hay (moist)	Foam Rubber	Dry Paper Rolls
Mulch (moist)	Metal Filings & Powder	Insulating Boards
Pecans	Particleboard	Potassium Phosphide
Sewage Sludge	Peat	Wool Bales
Soy Beans	Sawdust
Walnut	Wood Chips

Factors in Ignition

Many variables influence spontaneous ignition, including the ambient temperature, humidity, and the availability of oxygen to sustain the reactions. In addition, whether or not a material can spontaneously ignite is influenced by the other more general factors that influence ignition and early flame spread. These include fuel geometry, fuel density, thermal inertia, critical mass, surface area, and irradiance, as shown in Figure 19.

Spontaneous ignition is very rare because it can be prevented by any one of the following:

1. Insufficient surface area
2. Insufficient oxygen
3. Ambient temperature is too low
4. Insufficient insulation - heat radiated away
5. Insufficient critical mass of fuel (e.g., material)

Many of these same factors, such as the material properties and configuration of the fuel, are critical in piloted ignition and auto-ignition as well. Some rules of thumb are as follows: thin materials ignite more easily than thicker ones; low-density materials ignite more easily than high-density materials; and it is easier to ignite something on its edge than near its centre.

As a final note on ignition and initiation of a fire, there can often be a time delay between the application of an ignition source to a fuel and the time at which open flaming combustion starts. This can be due to the type of material or also because there is a non-flaming (smouldering) fire for a period of time after ignition.

Figure 19: Conditions required for spontaneous combustion

In real fire situations, it is important to remember that the time between the start of smouldering and flaming combustion is unpredictable. If a material smoulders for a long period of time, pyrolysis gases build up and when conditions are right (fuel, air and energy), a fire can grow quickly. This will be discussed in more detail when the different HRR profiles from fires are reviewed later in the curriculum.

Knowledge Check How does oxygen concentration affect combustion? What are the implications of Thornton's Rule? What role does heat play in the fire tetrahedron? Name three types of ignition.