In the pulverized fuel firing system, the coal is reduced to a fine powder with the help of grinding mills and projected into combustion chamber with the help of hot air current. The amount of air required to complete the combustion is supplied separately to the combustion chamber. The resulting turbulence in the combustion chamber helps in the uniform mixing of fuel (coal) and air and through combustion. The amount of air which is used to carry the coal and to dry it before entering into combustion chamber is known as “primary air” and the amount of air which is supplied separately for complete the combustion is known as “secondary air”. The amount of primary air may vary from 10% to almost the entire combustion air requirement as per the type of pulverizer and load on it.