CHEMICAL REACTION OF COMBUSTION

Fire is a chemical reaction known as combustion. It is the rapid oxidation of combustible material accompanied by a release of energy in the form of heat and light. There are three components needed for ignition and combustion to occur. A fire requires fuel ­to burn, air to supply oxygen, and a heat source to bring the fuel up to ignition temperature. If any one of these elements is removed, a fire cannot exist. When a fuel burns it undergoes chemical change and there are four products of combustion: fire gases, flame, heat and smoke. The term “fire gases” refers to the vaporized products of combustion. Flame is a visible luminous body of a burning gas, which becomes hotter and less luminous when it is mixed with increased amounts of oxygen. Heat is a form of energy, which is measured in degrees of temperature to signify its intensity. Smoke is a visible product of incomplete combustion.

There are three methods of fire suppression: to remove the fuel, to exclude the oxygen, and to reduce the temperature. The removal of fuel is effective but not always practical or possible. Methods of fuel removal include turning off the fuel supply, pumping flammable liquids from a burning tank, removing unburned portions of large piles of solid combustible materials such as that found in coal piles, diluting liquid material that is burning. Water will dilute materials which are soluble, such as ethyl alcohol. Flammable liquids that are not soluble in water can be diluted with an emulsifying agent which mixes with the top layer of the flammable liquid to stop the vaporization. Flammable gases can also be diluted and can become noncombustible with the addition of an inert gas such as carbon dioxide or nitrogen.

The process of “smothering” or “blanketing” will extinguish fires by separating the oxygen from the other components that make a fire. A common example of this method is extinguishing grease in a frying pan by placing the cover on the pan. But some plastics and metals cannot be extinguished by smothering because they do not depend on an external air supply. Cooling is a widely used method of fire extinguishment. Heat is carried away from a fire by radiation, conduction, convection, and absorption by a cooling agent. Of all the extinguishing agents, water absorbs more heat per volume than any other agent.

There are four classes of fires. Class A fires involve ordinary combustible materials, such as wood, cloth, paper, rubber, and many plastics. Water is used to reduce the temperature of the burning material below the ignition temperature. Class B fires involve flammable liquids, greases and gases. The exclusion of oxygen is most effective. Class C fires involve energized electrical equipment. A non-conducting extinguishing agent sometimes controls this fire. Class D fires involve combustible metals, such as magnesium, titanium, zirconium, sodium and potassium. The extremely high temperature of some burning metals makes water and other common extinguishing agents ineffective. We have no agent available that effectively controls fires in all combustible metals. Special extinguishing agents are available for control of fire in each of the metals and are marked specifically for that metal.