Stoichiometric, or theoretical, combustion is the ideal combustion process where fuel is burned completely. To calculate the excess air or excess fuel for a combustion system, the stoichiometric air-fuel ratio is determined. The stoichiometric ratio is the ideal fuel ratio where the chemical mixing proportion is correct. When burned, all fuel and air is consumed without any left over.
Furnaces, boilers, and process heating equipment cannot run at the stoichiometric ratio. "On-ratio" combustion used in boilers and high temperature process furnaces usually incorporates a modest amount of excess air (about 10 to 20% more) than what is needed for the fuel to completely burn. If an insufficient amount of air is supplied to the burner, unburned fuel, soot, smoke, and carbon monoxide exhausts from the boiler will result in heat transfer surface fouling, pollution, lower combustion efficiency, flame instability, and a potential for explosion.
To avoid unsafe conditions, which are far worse than the lost efficiency associated with some excess air, boilers normally set to operate on the fuel-lean side of the combustion curve. This approach will also accommodate variations in the fuel-air control system.
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