Emission Concentration Oxygen Correction Calculator
Emission limits are stated at a reference oxygen content. This calculator corrects a measured concentration to the reference O₂ level and converts between ppm and mg/Nm³, as needed for CEMS reporting.
- Factor = (20.9 − O₂ref) ÷ (20.9 − O₂meas) = (20.9 − 3) ÷ (20.9 − 8) = 1.3876
- Value already in mg/Nm³
- Corrected = measured × factor = 208.1 mg/Nm³
Oxygen Correction Formula
mg/Nm³ = ppm × MW ÷ 22.414 (0 °C, 101.325 kPa)
Both concentrations and O₂ are on a dry basis. 20.9 is the oxygen content of air. Normal conditions (Nm³) are 0 °C and 101.325 kPa, where one mole occupies 22.414 L. NOx is conventionally reported as NO₂. Check the reference O₂ and conditions in your permit.
Worked example
NOx of 150 mg/Nm³ measured at 8 % O₂, corrected to the 3 % reference for a gas boiler: 150 × 17.9 ÷ 12.9 = 208.1 mg/Nm³.
Frequently Asked Questions
Why correct emissions to reference oxygen?
Adding air dilutes the flue gas and lowers measured concentration without reducing the pollutant. Correction removes the effect of dilution.
Wet or dry basis?
Correction uses dry values. Convert wet readings to dry first using the flue gas moisture content.
What reference O₂ applies to my plant?
It is set by the permit or regulation, commonly 3 % for gas and oil boilers, 6 % for solid fuel and 15 % for gas turbines.
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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