Combustion Excess Air from O₂ Calculator
Find the excess combustion air and the air ratio (lambda) from the dry flue gas oxygen reading, or the oxygen setpoint for a target excess air, for burner tuning and O₂ trim control.
- EA = O₂ ÷ (20.9 − O₂) × 100 = 3 ÷ 17.9 × 100 = 16.76 %
- λ = 1 + EA ÷ 100
Excess Air Formula
λ = 1 + excess air ÷ 100
O₂ for target = 20.9 × EA ÷ (100 + EA)
O₂ is the oxygen in dry flue gas in %. This widely used approximation assumes complete combustion and is accurate for most hydrocarbon fuels; exact values depend slightly on fuel composition (the CO₂ and water formed).
Worked example
A dry flue gas O₂ of 3 % means about 16.8 % excess air (λ ≈ 1.17). To run at 15 % excess air, set the O₂ trim to about 2.7 %.
Frequently Asked Questions
What is excess air?
Air supplied above the theoretical (stoichiometric) amount needed to burn the fuel, given as a percentage.
Why not run at zero excess air?
Mixing is never perfect, so some excess air is needed to avoid CO, soot and unburned fuel.
Wet or dry O₂?
The formula uses dry O₂, as measured by extractive analyzers. In situ zirconia probes read wet O₂, which is a little lower.
Learn more on the blog
Instrumentation and control standards
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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