Combustion

 3.7 COMBUSTION 

The oxidation of fuel or its chemical combination with oxygen to produces light and heat is called combustion. In fuel, mainly hydrogen and carbon are present. During combustion, this carbon and hydrogen react with oxygen to form carbon dioxide and water vapours. 

For combustion, following three things are required:

• Fuel

• Oxygen

• Three Ts

Fuel

We have discussed about fuel in previous sections. Mostly, it contains carbon, hydrogen and some amount of oxygen and sulphur.

Oxygen 

For combustion process, oxygen is obtained from air. Air contains 79% nitrogen and 21%oxygen by volume or 77% nitrogen and 23% oxygen by weight. As the Nitrogen percentageis more in air, so large amount of air is required to get the required amount of oxygen.This nitrogen does not take part in combustion but is required to be heated during combustion. 

To obtain 1 kg of oxygen, we require 4.35 kg of air and for 1 m3 of oxygen, we require 100/21 = 4.76 m3 of air.

Three Ts

Time, temperature and turbulence are called as three Ts. These three Ts are important for burning process.

Time: For complete combustion of fuel, sufficient time is required.

Temperature: The fuel should be at ignition temperature for burning. Different fuels are having different ignition temperatures. Temperature at which fuel starts to burn is called as ignition temperature.

Turbulence: For complete combustion, the fuel and air should mix properly. By creatingturbulence, the fuel can be burnt with less excess air. Turbulence ensures the supply of oxygen to each molecule of the fuel.

In a boiler, total air for combustion is divided into two parts. One part is primary air which supports the burning of fuel initially. Second part of the air is called as secondary air. This air is supplied into the boiler furnace to create turbulence and ensure complete combustion.

The combination of fuel, heat and oxygen is called as Fire triangle. Absence of anyone from these three can extinguish the fire.

3.7.1 Chemistry of Combustion 

As discussed earlier, fuel contains some basic element like hydrogen, carbon and sulphur. These elements react with oxygen during burning and heat is released. Some of the important