Flue Gas CO2 Calculator

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Tracking Combustion Tuning From the Other Direction

While O2% rises with excess air, CO2% falls — both track the same underlying dilution effect from opposite directions, which is why some combustion analyzers and older instruments use CO2 as the primary tuning indicator instead of O2.

The Formula

CO2% = CO2max% × (100 / (100 + Excess Air%))

CO2max is the fuel's theoretical CO2 at zero excess air (roughly 11.7-12% for natural gas, ~15-16% for coal).

Worked Example

Natural gas (CO2max = 11.7%) burned with 15% excess air:

CO2% = 11.7 × (100 / 115) = 10.17%

How to Use This Calculator

  1. Enter the fuel's theoretical CO2max at zero excess air.
  2. Enter the Excess Air percentage.
  3. Select Calculate to get expected flue gas CO2%.

Related Calculations

Need the oxygen side of the same tuning check? See the Flue Gas Oxygen Calculator, or find nitrogen by difference in the Flue Gas Nitrogen Calculator.