Fuels - Part VIII

3. Combustion and ignition temperature
Burning of a substance is called combustion. Combustion is sustained by the presence of either pure oxygen or oxygen in air. Oxygen is a supporter of combustion, without it no combustion or burning will take place. Combustion is an exothermic process and the reaction products of this type of chemical reaction are carbon dioxide and water.

The reactions below show the effects of burning methane and charcoal in air.  

If the supply of oxygen is inadequate, then the combustion will be incomplete. Combustion can be of two type : spontaneous and rapid combustion. When substance does not need external heat energy to start the combustion, the process is called spontaneous combustion. For example, when white phosphorus is kept in air for sometime, it will automatically catch fire. The phosphorus reacts with air to form oxide, which is an exothermic reaction. The heat generated by the formation of the oxide ignites the rest of the white phosphorus. Spontaneous combustion is also seen where dry forest or grass is present. The heat in the atmosphere may start the burning. Rapid combustion occurs when a large amount of heat is released in a very short time. Example of rapid combustion can be sighted in case of bombs. Lighting up of LPG in the kitchen, which produces heat and light immediately, is also an example of spontaneous combustion.  

Supporter of combustion :  Oxygen is a supporter of combustion. The sure way to put a fire would be to smother the flame and cut off the oxygen. For chulhas to burn easily you have seen that air has to be blown around the wood or coal; this provides oxygen for the wood or the coal to burn easily.

Ignition temperature : Ignition temperature or ignition point is the temperature at which the fire starts. Below the ignition temperature, even if a combustible substance is present along with oxygen, the fire will not start. Therefore a substance has to be heated to its ignition temperature before it starts burning. For example, the ignition temperature of white phosphorus is 35°C. During summer, ordinary room temperature can rise to about 35°C and thus white phosphorus get ignited spontaneously. Similarly a matchstick can be lighted by striking the match along a roughened surface. The matchstick has red phosphorus along with other chemicals. On striking, friction is created. This generates heat enough to light the match or make the chemical catch fire. The wood used in the match is also of a particular kind whose ignition temperature is as much as the heat produced by the lighted matchstick head. A substance will not catch fire, as long as its ignition temperature is not reached. This is how water can be heated in a paper cup. Take water in a paper cup and heat it over a candle. The paper will not catch fire, but the water will heat up. This is because the water keeps the temperature of the paper lower than its ignition point. This is the same reason that water is used to put out ordinary fire : water reduces the temperature of the substance that has caught fire below its ignition point. The fire thus gets extinguished. If an oil catches fire, pouring water will not help, as the temperature of the oil gets very high. Most of the fire extinguishers work on the principle of either reducing the ignition temperature of the substance that has caught fire or by cutting off oxygen supply.

Flames :  
Flames are associated with fires or combustion. A flame is region where combustion of fuel takes place. The colour of the flame depends on the temperature of the flame, the substance that is burning. In case of hydrocarbons, the flame is either blue or yellow. A blue flame is a non-luminous (not light giving) flame and occurs because of complete burning of hydrocarbons in the fuel. A blue flame is clean and does not leave any residue or any other gases. A blue flame is given out when the oxygen supply is plentiful while the process of burning occurs. A yellow flame comes about when there is insufficient supply of oxygen. It is a luminous (light giving) flame. In this case the hydrocarbons do not burn completely, the temperature of the flame is lower than that of the blue flame. The yellow flame also leaves behind a black soot or residue.

Candle flame : A candle is made of wax made from petrochemicals. The wick is lighted, this melts the wax. The evaporated wax rises and catches fire. As the vapours rise higher, they stay longer in the hot regions of the flame and start burning completely with oxygen.  

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