Stoichiometric air for combustion

Incorrect amount of air in fuel combustion accounts for the largest losses in combustion systems.

All fuels consist mostly of atomic Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Sulfur (S), minerals (ash) and water (H2O).

In practice, mixing is never perfect, a certain amount of excess air is needed to complete combustion and ensure that release of the entire heat contained in fuel oil.
One kg of fuel requires a certain minimum of ambient air to be fully combusted. We call this minimum amount of air the stoichiometric air.When fuel and oxygen in the air are in perfectly balance - the combustion is said to be stoichiometric.

If an insufficient amount of air is supplied to the burner, unburned fuel, soot, smoke, and carbon monoxide exhausts from the boiler - resulting in heat transfer surface fouling, pollution, lower combustion efficiency, flame instability and a potential for explosion.

 If too much air than what is required for completing combustion were allowed to enter, additional heat would be lost in heating the surplus air to the chimney temperature. This would result in increased stack losses. Less air would lead to the incomplete combustion and smoke. Hence, there is an optimum excess air level for each type of fuel.

if air content is higher than the stoichiometric ratio - the mixture is said to be fuel-lean.

if air content is less than the stoichiometric ratio - the mixture is fuel-rich.

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Stoichiometric air for combustion