The presence of pollutants in flue gas can be detected from the concentration of the gas components. The following units are generally used:

ppm (parts per million)

Like "per cent (%)", ppm describes a proportion. Per cent means "x number of parts in every hundred parts", while ppm means "x number of parts in a million parts". For example, if a gas cylinder contains 250 ppm carbon monoxide (CO), then if one million gas particles are taken from that cylinder, 250 of them will be carbon monoxide particles. The other 999,750 particles are nitrogen dioxide (N2) and oxygen particles (O2).

The unit ppm is independent of pressure and temperature, and is used for low concentrations. If larger concentrations are present, these are expressed as percentages (%). The conversion is as follows:

10 000 ppm = 1 %
1 000 ppm = 0.1 %
100 ppm = 0.01 %
10 ppm = 0.001 %
1 ppm = 0.0001 %

An oxygen concentration of 21% volume would be equivalent to a concentration of 210,000 ppm O2.

mg/Nm3 (milligrams per cubic metre)

With the unit mg/Nm3, the standard volume (standard cubic metres, Nm3) is taken as a reference variable and the mass of the pollutant gas given in milligrams (mg). Because this unit varies with pressure and temperature, the volume in normal conditions is taken as reference. Normal conditions are as follows:

Temperature: 0 °C
Pressure: 1013 mbar (hPa)

However, this information alone is not sufficient, because the respective volumes in the flue gas change according to the proportion of oxygen (dilution of flue gas with ambient air). The measured values therefore have to be converted to a particular volume of oxygen, the reference oxygen content (reference O2).

Only data with the same reference oxygen content can be directly compared. The measured oxygen content (O2) of the flue gas is also needed when converting ppm into mg/Nm3.

mg/kWh (milligrams per kilowatt-hour of energy)

Calculations are made with fuel-specific data in order to determine pollutant gas concentrations in the energy-related unit mg/kWh.There are thus different conversion factors for each fuel.

The conversion factors for ppm and mg/m3 into the energy-related unit mg/kWh are shown below. Before converting to mg/kWh, the measured emission value concentrations first have to be converted to undiluted flue gas (0 % reference oxygen content).

The conversion factors for solid fuels also depend on the form in which the fuel is available (in one piece, as chippings, powder, shred etc.) For this reason the factors should be checked carefully.

Components of Flue Gas