With complete combustion, excess air in percent equals O2 divided by (20.95 minus O2), times 100, using dry flue gas oxygen. At 3 % oxygen that is about 17 % excess air (lambda 1.17); at 5 % it is about 31 %. Each point of oxygen above the target wastes fuel heating air that does no work.
Assumes complete combustion and a dry-basis oxygen reading. Real burners with CO or unburnt fuel need the CO reading too.
How it is calculated
Air is 20.95 % oxygen. If a burner used exactly the air needed to burn the fuel completely (stoichiometric air), the flue gas would hold no oxygen. Any oxygen left over came from air that was not needed. Oxygen in the flue gas therefore measures the air supplied beyond the minimum.
Excess air (%) = O2 / (20.95 - O2) x 100 lambda = 20.95 / (20.95 - O2)
Lambda is the ratio of actual air to stoichiometric air, so lambda 1.00 is exact and 1.17 is 17 % excess.
Worked example
Flue gas oxygen is 3.0 % dry: excess air = 3.0 / 17.95 x 100 = 16.7 %, lambda = 1.167.
The operator trims the burner to 5.0 % oxygen: excess air = 5.0 / 15.95 x 100 = 31.3 %, lambda = 1.31. Nearly twice the excess air must be heated up the stack for the same output.
Why the trade-off matters: too much excess air wastes heat; too little leaves carbon monoxide and soot. The best setting sits just above where CO starts to rise, which is why plants read both oxygen and CO.
Limits
- The formula is for complete combustion and dry-basis oxygen; wet-basis readings are lower. See Dry basis vs wet basis in gas measurement.
- Air leaking into the duct after the furnace adds oxygen and overstates the excess air at the burner.
- In-situ zirconia analysers read on a wet basis; extractive analysers usually read dry.
Frequently asked questions
What excess air should a boiler run at?
It depends on the fuel and burner. Gas burners often run with a few percent oxygen and solid-fuel plants higher. The practical optimum is just above the point where carbon monoxide begins to rise.
How is excess air calculated from oxygen?
Excess air percent equals O2 divided by 20.95 minus O2, times 100, on a dry basis and with complete combustion.
What is lambda in combustion?
The ratio of the actual air supplied to the air needed for exact combustion. Lambda 1.0 is stoichiometric; above 1 is excess air.
Why measure both O2 and CO?
Oxygen shows how much air; CO shows whether combustion is complete. Trimming on oxygen alone can push the burner into incomplete combustion.