How the units fit together
Emission limit values in ordinances, BAT conclusions and permits almost always apply to dry gas at standard conditions (0 °C and 101.325 kPa), converted to a reference oxygen content. The unit Nm3, a cubic metre of dry gas at those conditions, is what makes measured values from different plants and different days comparable. Which reference oxygen content applies is stated in the regulation or in the permit: 6 % for solid fuels, 3 % for oil and gas in boilers, 11 % for waste incineration, 15 % for gas turbines and engines.
The oxygen correction rests on the fact that excess air dilutes the flue gas without changing the amount of the component you are measuring. The more air, the higher the O2 and the lower the measured concentration; the correction converts back to the oxygen content that the emission limit value refers to. At a very high measured O2, close to the 21 % of air, the factor becomes large and small errors in the oxygen measurement carry through to the result.
Instruments often show ppm, while emission limit values are given in mg/Nm3. The conversion goes through the molar mass: one ppm of a gas corresponds to M/22.414 mg per Nm3. For NOx, the molar mass of NO2 is used regardless of how much of the nitrogen oxides is NO, because the limit values are given as NO2.
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