Stoichiometric or Theoretical Combustion is the ideal combustion process during which a fuel is burned completely. A complete combustion is a process which burns all the
carbon (C) to (CO2), all hydrogen (H) to (H2O) and all sulphur (S) to (SO2). If there are unburned components in the exhaust gas such as C, H2, CO the combustion process is
uncompleted.

The combustion process can be expressed as:

[C + H (fuel)] + [O2 + N2 (Air)] -> (Combustion Process) -> [CO2 + H2O + N2 (Heat)]

where

C = Carbon

H = Hydrogen

O = Oxygen

N = Nitrogen

To determine the percent excess air or excess fuel at which a combustion system operates, you have to start with the stoichiometric air-fuel ratio. Also known as the perfect, correct or
ideal fuel ratio, the stoichiometric ratio is the chemically correct mixing proportion. When burned, it consumes all the fuel and air without any excess of either left over.

Process heating equipment is rarely run that way, however. Even so-called "on-ratio" combustion, used in boilers and high temperature process furnaces, usually incorporates a modest
amount of excess air -- about 10 to 20% beyond what is needed to burn the fuel completely.

If insufficient amount of air is supplied to the burner, unburned fuel, soot, smoke, and carbon monoxide exhausts from the boiler. This results in heat transfer surface fouling, pollution,
lower combustion efficiency, flame instability and a potential for explosion. To avoid inefficient and unsafe conditions, boilers normally operate at an excess air level. This excess air level
also provides protection from insufficient oxygen conditions caused by variations in fuel composition and "operating slops" in the fuel-air control system. Typical optimum values of excess
air levels are shown here for various fuels.

if the air content is higher than stoichiometric, the mixture is said to be fuel-lean
if the air content is less, the mixture is fuel-rich
Excess air of different fuels
Example - Stoichiometric Combustion of Methane - CH4
The most common oxidizer is air. The chemical equation for stoichiometric combustion of methane - CH4 - with air can be expressed as

CH4 + 2(O2 + 3.76N2) -> CO2 + 2H2O + 7.52N2

If more air is supplied, not all will be involved in the reaction. Additional air is termed excess air, but the term theoretical air may also be used. 200% theoretical air is 100% excess air.

The chemical equation for methane burned with 25% excess air can be expressed as

CH4 + 1.25 x 2(O2 + 3.76 N2) -> CO2 + 2H2O + 0.5O2 + 9.4N2

Excess Air and O2 and CO2 in Flue Gas
Aproximate values for CO2 and O2 in the flue gas as result of excess air are estimated in the table below: