Hong Life CalculatorsUnderstanding the emission O₂ reference correction calculator — concept and principle
Stack emission limits for pollutants such as NOx, CO, SO₂ and dust are usually expressed at a fixed reference oxygen content. This calculator converts a measured concentration to that reference O₂ level, so that a boiler, engine or incinerator cannot appear cleaner simply by diluting its flue gas with extra air.
How it is calculated
- C_ref = C_meas × (20.9 − O₂ref) ÷ (20.9 − O₂meas)
- C_meas is the measured concentration in any unit (mg/Nm³, ppm); the result is in the same unit.
- O₂meas is the measured oxygen in the flue gas (% by volume, normally dry basis).
- O₂ref is the reference oxygen level set by the applicable rule.
- 20.9 is the oxygen content of dry ambient air in percent. Some regulations write 21 instead.
The principle
Excess combustion air does not change how much pollutant is produced, but it adds volume and so lowers the concentration. The term 20.9 − O₂ measures how much oxygen has been consumed by combustion: in undiluted gas with 0% O₂ it is 20.9, and in pure air it is 0. The ratio (20.9 − O₂ref) ÷ (20.9 − O₂meas) is therefore the ratio of flue-gas volumes at the two dilution levels. Multiplying by it rescales the concentration to what it would be if the gas contained exactly the reference amount of oxygen, putting all plants on the same footing.
Worked example
- Inputs: measured concentration 150, measured O₂ = 9%, reference O₂ = 6%.
- Numerator: 20.9 − 6 = 14.9.
- Denominator: 20.9 − 9 = 11.9.
- C_ref = 150 × 14.9 ÷ 11.9 = 150 × 1.2521 = 187.8, in the same unit as the measurement.
| Measured O₂ (%) | Corrected concentration |
|---|---|
| 2.25 | 119.8391421 |
| 4.5 | 136.2804878 |
| 6.75 | 157.95053 |
| 9 | 187.8151261 |
| 11.2 | 231.6062176 |
| 13.5 | 302.027027 |
| 18 | 770.6896552 |
How to read the result
When the measured O₂ is higher than the reference, the corrected value is higher than the measurement, as in the example; when it is lower, the correction reduces the value. Compare only the corrected figure with the limit. Reference levels differ by plant type; in EU rules, for example, 6% is used for solid-fuel combustion plants, 3% for liquid and gaseous fuels, 15% for gas turbines and 11% for waste incineration. Check your own permit for the exact value.
Common mistakes
- Using wet-basis O₂ with a dry-basis concentration, or the reverse.
- Applying the correction when the measured O₂ is close to 20.9%: the denominator approaches zero and the result becomes unreliable.
- Correcting a mass emission rate (kg/h), which does not depend on dilution and must not be corrected.
Frequently asked questions
Why does my plant look worse after correction?
Because it runs with more excess air than the reference level. The correction removes the dilution benefit, which is exactly its purpose.
Should I use 20.9 or 21?
Use the value written in your regulation. The difference is small, but legal reports must follow the official formula.
Sources: Directive 2010/75/EU on industrial emissions, Annexes V and VI (reference oxygen conditions) · EN 15259:2007, Air quality — Measurement of stationary source emissions