How does too much excess air impact the boiler combustion and operation?
Combustion efficiency depends on ability to achieve the optimum air-to-fuel ratio.
Air-to-fuel ratio is the amount of air needed to burn a specific fuel. Controlling this parameter at maximum proximity to the optimum is the key to the highest boiler efficiency. Usually fuel burner achieves optimal efficiency balance by operating at 105% to 120% of the optimal theoretical air.
Excess air is the amount of air needed by a burner which is in excess of the amount required for perfect or stoichiometric combustion.
With the load and the firing rate are constant, an increase in the amount of excess air generally reduces the boiler performance:
1) the average temperature in the furnace is decreased by the increased excess air.
2) heat transfer to the water in the boiler tubes is reduced because of the lowered temperature of the combustion gases.
3) the evaporation rate is reduced because of the reduction in heat transfer.
4) the lowered rate of steam generation causes a reduction in the rate of steam flow through the superheater (if installed);
5) the superheater outlet temperature rises as the rate of steam flow through the superheater is decreased.
6) combustion may occur in the tube bank, rather than in the furnace
