Stoichiometric Combustion

Stoichiometric combustion and excess air.

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Stoichiometric or Theoretical Combustion is the ideal combustion process where fuel is burned completely.

A complete combustion is a process burning all the carbon (C) to (CO₂), all the hydrogen (H) to (H₂O) and all the sulphur (S) to (SO₂).

With unburned components in the exhaust gas such as C, H₂, CO, the combustion process is uncompleted and not stoichiometric .

The combustion process can be expressed:

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

where

C = Carbon

H = Hydrogen

O = Oxygen

N = Nitrogen

To determine the excess air or excess fuel for a combustion system we starts with the stoichiometric air-fuel ratio. The stoichiometric ratio is the perfect ideal fuel ratio where the chemical mixing proportion is correct. When burned all fuel and air is consumed without any excess left over.

Process heating equipment are rarely run that way. "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 to burn the fuel completely.

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.

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 values of excess air are indicated for various fuels in the table below.

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

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 + 3.76 N₂) -> CO₂ + 2 H₂O + 7.52 N₂

If more air is supplied some of the air will not be involved in the reaction. The 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 + 2 H₂O + 0.5 O₂ + 9.4 N₂

Excess Air and O₂ and CO₂ in Flue Gas

Approximated values for CO₂ and O₂ in the flue gas as result of excess air are estimated in the table below:

Carbon Dioxide - CO₂ - in Flue Gas (*% volume*)
Excess Air %	Natural Gas	Propane Butane	Fuel Oil	Bituminous Coal	Anthracite Coal	Oxygen in Flue Gas for all fuels (% volume)
0	12	14	15.5	18	20	0
20	10.5	12	13.5	15.5	16.5	3
40	9	10	12	13.5	14	5
60	8	9	10	12	12.5	7.5
80	7	8	9	11	11.5	9
100	6	6	8	9.5	10	10

Stoichiometric Combustion