When magnesium ribbon is heated in air, it catches fire and produce heat and light. Similarly when a piece of paper is burnt, it produces heat and light. The material which on heating in the presence of air or oxygen, catch fire easily and produce heat and light energy are called combustible materials. The chemical process in which a substance burns in air or oxygen with the release of heat and light energy is called combustion. The substance that undergoes combustion called a combustible substance or fuel.
Conditions required for combustion
Oxygen present in air helped in the combustion of a fuel and so is called supporter of combustion.Also in pure oxygen, a substance burns five times faster than air.A piece of paper burns quickly when a burning matchstick is brought near it. On the other hand, a piece of wood cannot be burnt with the help of a matchstick. These experiments tell us that different substances catch fire at different temperatures.The lowest temperature at which a substance (combustible) catches fire is called ignition temperature or kindling temperature.
Let us take some examples to prove that the ignition temperature is necessary for combustion.
1. We can boil water in a paper cub, on a spirit lamp. It is because the heat energy supplied to the paper cup rapidly passes on to water. Thus, ignition temperature of paper is not achieved, and hence, it does not catch fire.
2. Ignition temperature of white phosphorus is 35 C. During summer, room temperature can rise to about 35 C and thud, white phosphorus can catch fire spontaneously.
3. A matchstick can be lighted by striking the match along a rough 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 matchstick is also of a particular kind whose ignition temperature is as much as the heat produced by the lighted matchstick head.
4. Flame of burning candle goes off, when we blow over it strongly. It is because the air current lowers the temperature of burning wax vapours below their ignition point and carbon dioxide in our breadth acts as a fire extinguisher. Thus, the flame goes off.
We conclude that the following conditions are necessary for producing or sustaining combustion:
(i) There must be a combustible substance.
(ii) There must be a continuous supply of oxygen or air.
(iii) The temperature of combustible material should be above its ignition temperature.
Types of Combustion
1. Slow combustion
When a combustible material burns at slow or moderate rate, it is called slow combustion. Slow combustion occurs when there is an insufficient supply of air. Therefore, combustion is never complete. Burning of cow dung cakes, wood etc. are examples of slow combustion.
2. Rapid Combustion
When a combustible substance burns fast to produce heat, it is called rapid combustion. Generally, a gaseous fuel undergoes rapid combustion.
Rapid combustion occurs when there is sufficient supply of air and hence, the combustion is complete.
3. Spontaneous Combustion
When a combustible substance catches fire even at room temperature, spontaneous combustion takes place. For example, when a small piece of white phosphorus or sodium is kept in air for sometime, it suddenly catches fire. It is because these material form oxides in the presence of air with the production of heat energy. The temperature roses above the ignition temperature and so the substance, catches fire.
4. Explosive Combustion
When a mixture of combustible materials and air completely burns in a very short span of time, is some closed space, explosive combustion takes place. when a combustible material burns uncontrollably in free supply of air, it releases a very large amount of heat. For example, when a cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound. Such a reaction is called explosion
In any kind of fire, it is the combustible material which catches fire. We must take precautions, so that the combustible substance does not get heated up to its ignition temperature. Places like petrol stations have highly combustible substance like petrol, diesel, etc. No one should be allowed to take any burning material within the premises of such places. Even a burning matchstick can ignite petrol vapours. The same precautions should be taken at LPG godowns, fire cracker factories and ammunition depots. Care should be taken while placing a burning candle in the house at the time of power failure. Once a fire has started, it can be stopped by either cutting off the supply of air or lowering the temperature of combustible substance below its ignition temperature. For example, if the clothes of person catch fire, the person should be immediately wrapped in a thick blanket. This will cut off the supply of air and the fire will stop. Sometimes sand also be used to cut off the supply of air in the case of fire produced by kerosene and the fire will stop. Water can also be poured over fire to lower the temperature of the burning material.
Q- What happens when there is incomplete combustion?
A- During incomplete combustion-
1. A part of the unburnt carbon passes into atmosphere in the form of soot. This not only wastes the fuel, but also pollutes the atmosphere.
2. Carbon monoxide is formed. this gas is highly poisonous and causes respiratory problems and may prove to be fatal also.
When you burn a piece of paper or a wax candle, a flame is produced. flame is defined as the region of burning gases.
Observe a flame carefully and you will notice:-
It has three zones-
1. Innermost zone: This zone consist of unburnt wax vapour given off by the molten wax. It is the coldest part of the flame.
2. Middle zone or Luminous zone: In this zone, a partial combustion of wax vapours takes place with the liberation of lot of energy. This energy partly decomposes the wax vapours into carbon particles. The luminous zone of the flame is hotter than dark inner zone and is yellow in color.
3. Outermost zone or non-luminous zone: It is a zone of complete combustion of wax vapours and carbon particles. The air from the sides of the flame mixes with unburnt wax vapours and carbon particles from the luminous zone and completely burns them to form carbon dioxide gas and water vapour. It is the hottest part of the flame.
Fuel and Calorific value
The amount of heat energy produced on completely burning one kilogram of a fuel is pure oxygen is called calorific value of a fuel. A fuel which produces more heat energy per kilogram is said to have high calorific value. The more the calorific value, the better is the efficiency of a fuel. The calorific value is expressed in kilojoules per kilogram(kj/kg0
Characteristics of a good fuel
1. It should be cheap and readily available.
2. It should be easy to store.
3. It should have a high calorific value.
4. It should not produce harmful gases as pollutants.
5. It should have low ignition temperature, but not below room temperature.
6. It should undergo complete combustion and should be controllable.
Amongst the physical state of fuels, the fuel in gaseous state are the best because of the following reasons:
(i) Gaseous fuels are supplied in cylinders or through pipelines, so, the transportation cost is lowered.
(ii) Gaseous fuels release large amount of energy and do not leave behind any ash or solid residue on combustion.
(iii) Gaseous fuels produce least amount of harmful gases as compared to solid and liquid fuels.
(iv) Gaseous fuels can be easily ignites with a small spark and the rate of combustion can be easily controlled with the help of a control valve.
Harmful effects of fuels
Increases fuel consumption has led to harmful effects on the environment in the following ways:
1. Carbon fuels like wood, coal, petroleum release unburnt carbon particles of smoke or ash get suspended int he air. Excessive amount of these particles in the air causes breathing problems. In winter, these particles produce 'smog'. It is very harmful to plans as well as animals.
2. Incomplete combustion of carbon fuels give carbon monoxide gas. It is a very poisonous gas. Even small amounts of carbon monoxide in air can cause breathing problems. Large amount of it can cause death.
3. Combustion of most fuels release carbon dioxide in the environment. no doubt it acts as food for the plants but excess of carbon dioxide in air traps heat energy which raises the temperature of the earth. This results in the melting of polar glaciers, which leads to a rise in the sea level, causing floods in the coastal areas. This is termed as global warming.
4. Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating gas. It also dissolve in water vapour to produce suphuric acid. When it rains, the acid in it damages buildings, plants and trees. Such rain is called acid rain.
The use of diesel and petrol as fuels in automobiles is being replaced by CNG ( Compressed Natural Gas) because CNG produces less harmful products and is a cleaner fuel.