What is Chemistry of Fire

Fire has always fascinated man. Since it was first discovered it has been the basis of mystery and marvel. Magicians and sorcerers have used it to amaze and frighten people. Properly harnessed and controlled, fire warms our shelter, cooks our food, and forges and fires our industrial products. Unfortunately, most people know very little about fire and its behavior.

Fire is a chemical process. Whenever oxygen in any form combines with the fuel, a chemical reaction known as oxidation occurs. The process generates heat. Oxidation can take place at various rates. Probably the most familiar type of oxidation is the slow combination of oxygen with steel to form rust. Because this reaction takes place very slowly the heat evolved dissipates and there is no detectable rise in temperature. However, if we take another material, such as paper, and expose it to heat and oxygen, the combination of the paper (fuel) and the air (oxygen) occurs at such a rapid rate that a great deal of heat and light are liberated. This rapid chemical reaction is what we call fire.

Fire is like a double-edged sword – it can either help us or hurt us. The type of fire which we use in a gas range to cook our food is a fire under our control and which is serving us. This is known as friendly fire.

If the box of a paper napkin is sitting too close to the burner, it may ignite. The burning napkins represent a fire that is not serving us, and which may endanger us. This type of fire is known as hostile fire.

In the work environment, we usually have numerous types of friendly fires or devices that make use of friendly fire. Our concern in this program is the hostile fire which most frequently stems from a lack of understanding of what fire is and how it behaves.

 The Fire Triangle

For many years the basic combustion process has been explained by the use of the fire triangle. Each of the three legs of the fire triangle represents one of the basic elements required for the combustion process. These are fuel, heat, and oxygen.

Fuel

The base of the fire triangle is formed by the fuel. Fuels may be in the form of solids, liquids, or gases. However, with the exception of combustible metals, only gases will burn. Solids and liquids must be converted to a gas before they will burn. As we examine the combustion process further, we will see how this occurs.

Before we look at the role of fuel in a fire it will be helpful to understand some of the terms that are used to define and explain the burning characteristics of fuels.

Classes of Fire

To help identify fire potential within the work environment, fire is classified into one of four classes. Class A fires involve fuels commonly referred to as ordinary combustibles, such as wood and paper products. Class B fires involve Class B fuels which are usually liquids, such as gasoline and cooking oil, or solids capable of liquefying. Some plastics, although solid in nature, can decompose and burn with characteristics of a flammable or combustible liquid. Class C fires involve energized electrical equipment, such as switchgear, or a computer. Once de-energized, the Class C fire generally becomes a Class A or Class B fire. Class D fires involve Class D fuels which are combustible metals, such as magnesium or titanium.

Temperature

Temperature is a measurement of hotness or coldness. Temperature values are expressed in terms of degrees. On the Fahrenheit scale, 32℉ represents the freezing point of water and 212oF represents the boiling point of water. The Celsius scale measures degrees in terms of degrees centigrade (℃). This scale is based on 0℃ as the freezing point of water and 100℃as the boiling point of water. 

British Thermal Unit (Btu)

A unit of heat measurement; the amount of heat energy needed to raise the

the temperature of one pound of water 1℉ is called 1BTU

Calorie (cal)

A unit of heat measurement; 1 cal is the amount of heat energy needed to raise the

the temperature of one gram of water 1℃

Ignition Temperature. This temperature is the lowest temperature at which a fuel

generates sufficient combustible vapors to be ignited and sustain a fire. This is also the

the temperature that a heat source must have in order to ignite the vapors given off by the

fuel.

Auto-ignition Temperature

This is the temperature to which material must be heated for it to ignite without the need for an outside ignition source. For example, the auto-ignition temperature of gasoline ranges from 500 to 850℉. If a sample of gasoline was heated to a temperature within that range, it could self-ignite. 

Solid Fuels

When solid fuel burns there are several factors that determine the rate of the combustion process. Among these are the properties of the material itself and the configuration of the material. Using wood as an example let's look at the combustion process. If we take a small piece of wood and apply heat to it from a flame the wood begins to increase in temperature and undergo a chemical change. As the heating continues the wood begins to breakdown and release a number of different gases and vapors. This process is known as pyrolysis. Many of these gases and vapors are combustible and when they are released in large enough quantities they begin to burn and sustain a flame. Soon the gases and vapors are being released at such a rate that the external ignition source can be removed and the fire will sustain itself. As the fire continues to burn, it heats adjacent portions of the wood which in turn evolve more gases that add to the size of the fire.

The ease of ignition and the burning characteristics of the solid fuel are greatly influenced by its configuration. For example, if we take a large chunk of wood, such as 2" x 4", and try to ignite it with a match, it is impossible. However, if we take the same piece of wood and cut it into very small pieces the size of toothpicks, we can easily ignite the pile with a match. This is because the mass of the wood now has much more surface area. And, when the mass of the material stays the same while the surface area increases, we increase the ease with which the fuel can ignite. This same concept holds true for nearly all solid materials. The smaller the size of the fuel or the greater the surface is to mass ration, the more easily it ignites and the faster it burns.

Liquid Fuels

Liquid fuels burn much like solid fuels with the exception that they generally take less heat to evolve enough vapors to support flaming combustion. In fact, liquids classified as flammable liquids can evolve enough vapors at room temperature (72℉) to form ignitable mixtures near the surface. All they need is a heat source with enough energy to ignite the vapors.

Flammable and combustible liquids have contributed to many serious industrial fires. Nearly every facility, regardless of the product manufactured, uses one or more flammable or combustible liquids in its operations. They are most commonly used for finishing processes, degreasing, lubrication, and cleaning. Within a few seconds of ignition, a flammable/combustible liquids fire can easily reach a temperature of 2000℉.

A flammable liquid is any liquid with a flash point less than 100℉. A combustible liquid is any liquid with a flashpoint of 100℉ or greater. NFPA standards further subdivide both flammable and combustible liquids into several sub-categories based on the flashpoint and the boiling point of the liquid.

Some combustible liquids, such as heavy oils, must be heated the same as solid fuels before they will evolve enough vapors to support combustion. However, once they are ignited they burn as fiercely as any of the more common flammable liquids.

The ease with which a flammable liquid is ignited is measured by its flashpoint. The flashpoint is the temperature at which the liquid gives off sufficient vapors to form an ignitable mixture near the surface of the liquid.

For example, gasoline has a flashpoint of about -45℉. This means that at that temperature gasoline will produce enough vapors to form an ignitable mixture near the surface of the liquid. At the flashpoint, the vapors will flash, but the fire will not sustain itself. The lower the flashpoint of a liquid the more severe the fire potential is at normal temperatures.

Gas Fuels

Gas fuels burn in the same manner as solids and liquids, with the exception that they are more easily ignited because they do not require the addition of any heat in order to vaporize the fuel. All they require is an ignition source. Fuel gases such as propane, methane, and acetylene burn readily and have been the cause of numerous industrial disasters.

Heat

An ignition source with sufficient heat energy is needed before a fire can occur. Heat may be supplied from any number of sources which are classified as chemicals, electrical, mechanical or nuclear heat sources. Our concern with fire in the work environment does not involve nuclear (fusion or fission) heat sources.

Chemical Heat Sources

Heat can be generated by chemical reactions. Spontaneous heating of oily rags is a good example. If rags soaked with vegetable oil are piled together, they begin to heat as the oil oxidizes. The oxidation process generates beat which may be trapped in the pile. As the heat continues to evolve, the temperature in the pile may be elevated to the point where the rags ignite. This same phenomenon can occur in other materials as well.

Chemical reactions that generate heat are known as exothermic reactions and are used in many industries. Some processes produce so much heat that they may require large volumes of cooling water to maintain stable temperatures. Loss of cooling can lead to an uncontrolled or run-away chemical reaction.

In addition to these chemical heat sources, some chemicals such as ammonium nitrate can undergo chemical decomposition when heated and cause violent explosions.

Electrical Heat Sources

Short circuits and other electrical problems have been blamed for many fires. Resistance heating of a conductor can occur whenever there is an electric current flowing through it. The principle of generating heat due to resistance in a conductor is used in toasters and electric furnaces that heat many homes. When a breakdown of an electrical circuit occurs, wiring and components can be subjected to high currents that they are not designed to carry. Resistance heating can occur which can easily result in a fire. Electric arcs can also be a source of heat and ignite a fire in many cases if it is handled unsafely.

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