PRODUCTS OF COMBUSTION When a material (fuel) burns, it undergoes a chemical change. None of the elements making up the material are destroyed in the process, but all of the matter is transformed into another form or state. Although dispersed, the products of combustion equal in weight and volume that of the fuel before it was burned. When a fuel burns there are four products of combustion: • Fire gases • Flame • Heat • Smoke The smoke encountered at most fires consists of a mixture of oxygen, nitrogen, carbon dioxide, carbon monoxide gases, finely divided carbon particles (soot), and a miscellaneous assortment of products that have been released from the material involved. Heat is a form of energy that is measured in degrees of temperature to signify its intensity. In this sense, heat is the product of combustion that is responsible for the spread of fire. In a physiological sense, it is the direct cause of burns and other forms of personal injury. Injuries caused by heat include dehydration, heat exhaustion and injury to the respiratory tract, in addition to burns. Flame is the visible, luminous body of a burning gas. When a burning gas is mixed with the proper amounts of oxygen, the flame becomes hotter and less luminous. This loss of luminosity is because of a more complete combustion of the carbon. For these reasons, flame is considered to be a product of combustion. Heat, smoke and gas, however, can develop in certain types of smoldering fires without evidence of flame. Some materials give off more smoke than others. Liquid fuels generally give off dense black smoke. Oils, tar, paint, varnish, molasses, sugar, rubber, sulfur and many plastics, also generally give off a dense smoke in large quantities. FIRE EXTINGUISHMENT THEORY The extinguishment of fire is based on an interruption of one or more of the essential elements in the combustion process. With flaming combustion the fire may be extinguished by reducing temperature, eliminating fuel or oxygen, or by stopping the uninhibited chemical chain reaction. If a fire is in the smoldering mode of combustion, only three extinguishment options exist: reduction of temperature, elimination of fuel or oxygen. • Extinguishment of Temperature Reduction One of the most common methods of extinguishment is by cooling with water. The process of extinguishment by cooling is dependent on cooling the fuel to a point where it does not produce sufficient vapor to burn. If we look at fuel types and vapor production, we find that solid fuels and liquid fuels with high flash points can be extinguished by cooling. Low flashpoint liquids and flammable gases cannot be extinguished by cooling with water as vapor production cannot be sufficiently reduced. Reduction of temperature is dependent on the application of an adequate flow in proper form to establish a negative heat balance. • Extinguishment by Fuel Removal In some cases, a fire is effectively extinguished by removing the fuel source. This may be accomplished by stopping the flow of liquid or gaseous fuel or by removing solid fuel in the path of the fire. Another method of fuel removal is to allow the fire to burn until all fuel is consumed. • Extinguishment by Oxygen Dilution The method of extinguishment by oxygen dilution is the reduction of the oxygen concentration to the fire area. This can be accomplished by introducing an inert gas into the fire or by separating the oxygen from the fuel. This method of extinguishment will not work on self-oxidizing materials or on certain metals as they are oxidized by carbon dioxide or nitrogen, the two most common extinguishing agents. • Extinguishment by Chemical flame Inhibition Some extinguishing agents, such as dry chemicals and halons, interrupt the flame producing chemical reaction, resulting in rapid extinguishment. This method of extinguishment is effective only on gas and liquid fuels as they cannot burn in the smoldering mode of combustion. If extinguishment of smoldering materials is desired, the addition of cooling capability is required.
Posted on: Wed, 08 Oct 2014 07:12:49 +0000
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