4) What is flue gas?

When fuels are burned there remains, besides ash, a certain number of gas components. If these still contain combustion heat, they are called heating gases. As soon as they have conveyed their energy to the absorbing surfaces of a heat exchanger, they are called flue or stack gases.

The various common flue gas contents were already mentioned in chapter 1.

4.1) Which units are commonly used ?

  The presence of pollutants in a flue gas stream can be expressed in terms of the concentrations of the individual gas components.

The following units are the most common:

ppm (parts per million)

  Like the reading "percent (%) ", ppm expresses a ratio. If there is a concentration of 333 ppm CO in a cylinder and you take one million particles out of that cylinder, 333 particles would be carbon monoxide particles.

For convenience, higher concentrations are generally expressed as a percentage (%). The conversion is as follows:

An oxygen concentration of 21.95 vol.% would equal 219 500 ppm O2, and 10% CO is identical with 100 000ppm CO.

mg/Nm³ or mg/m³ (milligram per cubic meter)

  When using the unit mg/Nm³, the standard volume (standard cubic meter, Nm³) is used as a reference and the mass concentration of the flue gas is given in milligrams (mg). Since this unit is pressure and temperature dependent, the volume is expressed at standard conditions. There are different sets of standard conditions used for different purposes. Flue gas analysis commonly uses the following standard conditions:

onversion to mg/Nm³ from ppm

The conversion factors used in the formula come from the density of the gas components at standard conditions.

For more detailed explanations about the calculations performed in madur instruments read the user's manual of a portable flue gas analyzer, Chapter 3Operating Manual