Excess air optimization

If combustion air supply is in deficient, proper combustion of fuel may not take place and  hence loss due to unburned fuel will be more. 

If combustion air supply is in excess, loss due to unburned fuel is negligible; whereas  heat loss due to heat energy carried away be flue gas will increase. 

Furnace losses are mainly divided into three parts. 

• Loss due to unburned fuel or incomplete combustion loss 
• Loss due to heat energy carried away by flue gases or flue gas loss 
• Loss due to radiation and wall losses 

Incomplete combustion loss 

When air deficiency is there, the fuel atoms do not find enough oxygen atoms to  get burned. 

Then some combustibles are left unburned. 

This unburned fuel quantity gets reduced when enough or excess combustion air  is supplied. 

The loss due to incomplete combustion will be almost negligible in the excess air  region. 

Flue gas loss 

The heat losses occurring due to the heat energy carried away by flue gases is  called flue gas loss. 

After combustion the hot flue gases transfer heat energy to feed water in the  boiler tubes to saturated steam in the superheater, to cold feedwater in the  economiser and to cold combustion air in the air preheater. 

After transferring heat in the air preheater, flue gases are let into atmosphere  through ID fan chimney. 

They are sometimes called as waste gases because no more heat energy is  extracted from flue gases after air preheater unit. 

Under air deficient conditions the unburned fuel carries heat energy along with  flue gases and in air excess conditions the heat loss is due to unused oxygen  and the accompanying nitrogen carrying away heat energy when discharged into  atmosphere.

There is always a trend of steady increase in flue gas loss as the air combustion goes up. 

Radiation and wall losses 

These losses depend mainly on the combustion chamber temperature and the  refractory wall conditions. 

Radiation and wall losses are relatively constant throughout the operating region  except when the excess air allowed is much more than required. 

In that case the combustion chamber will have a tendency to cool down and  hence the reduction in losses.