Excess air is necessary for complete combustion and for temperature control.
If excess air is not provided, there is a large risk for incomplete combustion and combustible gases such as CO will leave with the flue gas. CO is poisonous, unstable and represents an energy loss.
On the other hand too much excess air will increase the risk of thermal NOx production (see later this chapter) and increase the amount of energy lost with the flue gas.
Thus, the excess air should be as little as possible, but enough for complete combustion.
The air excess factor for combustion is often denoted m. For stoichiometric conditions m=1, i.e. the stoichiometric air amount was supplied for the combustion process.
If excess air is supplied, m > 1.