Both Primary air and Secondary air have an important role to play in fuel combustion and must be provided in different ratios according to the fuel quantity.
For combustion of any fuel to take place, 3 basic ingredients are needed – Fuel, Air (Oxygen to specific) and heat. Air provided for combustion reacts with the fuel according to its stoichiometric ratio. But In real some percentage of air which has been provided for combustion goes unreacted. This generally happens because of the quantity of fuel that is fed at once and variation in the sizes of fuel particles. So, some amount of an extra air (usually some percentage of an air provided initially) must be provided to compensate for an air that went unreacted.
Steam boilers having an external furnace are generally provided with both Primary and Secondary air inlets and both play an important role in the combustion of fuel. Primary air is generally the basic amount of air required for complete combustion of fuel and it depends upon the composition and quantity of fuel required by the boiler. Different fuels will require a different amount of primary air. Primary air is generally provided from the bottom through the Airbox and the fuel is fed either manually or through the screw feeder into the furnace. Primary air also helps in segregating the fuel particles so that each fuel particle could come in direct contact with the air resulting in efficient combustion of fuel particles. Secondary air, on the other hand, is generally provided to compensate for the primary air that went unreacted. It is generally said to be an extra air that must be provided for complete combustion of fuel. Secondary air is generally provided through the nozzles installed on the walls of the furnace.
A forced draft fan designed for a particular boiler must be able to provide sufficient quantity of air which is the sum of both Primary and Secondary air. The air before entering the boiler is made to pass through an Air Preheater (APH) where the inlet air to the boiler gets heated by the outgoing flue gases and then this preheated air helps in reducing the moisture content of the fuel and further increases the boiler efficiency. Providing air at different stages generally reduces the combustion temperature and will provide more convective heat while single stage combustion will increase the combustion temperature and will provide more radiative heat transfer as compared to convective heat transfer. Multi-staging combustion also reduces the NOx formation which in turn will reduce the pollution caused by exhaust gases.