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What is Flue Gas?

What is Flue Gas?

The increase in all kinds of combustion is contaminating the environment with ever-greater concentrations of pollutants.

Smog formation, acid rain and the growing numbers of allergies are direct consequences of this development. The solution to environmentally sound energy production must therefore involve reducing pollutant emissions.

Pollutants in flue gas can only effectively be reduced if existing plants operate as efficiently as possible or noxious boilers are shut down. Flue gas analysis offers a means of determining pollutant concentrations and adjusting heating installations for maximum efficiency.

Units of Measurement

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

The components of flue gas are listed below in the order of concentration in the gas.

Nitrogen (N2)

Nitrogen (N2) is the main component (79 vol.%) of the air we breathe. This colourless, odourless, tasteless gas plays no part in combustion. It is drawn into the boiler as ballast, heated and sent to the stack.

Typical values in flue gas: Oil/gas burners: 78 % - 80 %

Carbon dioxide (CO2)

Carbon dioxide is a colourless, odourless gas with a slightly sour taste. Under the influence of sunlight and the green leaf colour, chlorophyll, plants convert carbon dioxide (CO2) into oxygen (O2).

Human and animal respiration converts the oxygen (O2) back into carbon dioxide (CO2).

This creates an equilibrium which gaseous products of combustion distort. This distortion accelerates the greenhouse effect. The threshold limit value is 5000 ppm. At concentrations of over 15% volume (150000 ppm) in breath, loss of consciousness occurs immediately.

Typical values in flue gas: Oil burners: 12.5 % - 14 % and gas burners: 8 % - 11 %

Water vapour (Humidity)

The hydrogen contained in the fuel combines with oxygen to form water (H2O). This escapes with the water from the fuel and the combusted air, depending on the flue gas temperature (FT), in the form of flue gas moisture (at a high flue gas temperature FT) or as condensate (at a low flue gas temperature).

Oxygen (O2)

The remaining oxygen not used in combustion in the case of excess air appears as gaseous flue gas and is used to measure combustion efficiency. It is used to determine flue gas loss and carbon dioxide content.

Typical values in flue gas: Oil burners: 2% - 5% and Gas burners: 2% - 6%

Kane 455 Infra-Red Analyser

Testo 327-1 Flue Gas Analyser

Crowcon Tempest 100 Flue Gas Analyser

Carbon monoxide (CO)

Carbon monoxide is a colourless, odourless breathing poison and is the product of incomplete combustion. In too high a concentration, it prevents the blood from absorbing oxygen. If for example the air for breathing in a room contained 700 ppm CO, a human being breathing in that air would be dead in 3 hours. The threshold limit value is 50 ppm.

Typical values in flue gas: Oil burners: 80 ppm - 150 ppm and Gas burners: 80 ppm - 100 ppm

Nitrogen oxides (NOX)

At high temperatures (combustion), the nitrogen (N2) present in the fuel and in the ambient air combines with the oxygen of the air (O2) to form nitrogen monoxide (NO). After some time, this colourless gas oxidises in combination with oxygen (O2) to form nitrogen dioxide (NO2).

NO2 is a water-soluble respiratory poison which causes severe lung damage if breathed in and contributes to ozone formation in combination with ultraviolet radiation (sunlight). The NO and NO2 components together are called nitrogen oxides (NOX).

Typical values in flue gas: Oil/gas burners: 50 ppm - 100 ppm

Sulphur dioxide (SO2)

Sulphur dioxide (SO2) is a colourless, toxic gas with a pungent smell. It is formed by the sulphur present in the fuel. The threshold limit value is 5 ppm. Sulphurous acid (H2SO3) is formed in combination with water (H2O) or condensate.

Typical value in flue gas of oil burners: 180 ppm - 220 ppm

Unburned Hydrocarbons (HC)

Unburned hydrocarbons (HC) are formed when combustion is incomplete and contribute to the greenhouse effect. This group includes methane (CH4), butane (C4H10) and benzene (C6H6).

Typical value in flue gas of oil burners: under 50 ppm


Soot is almost pure carbon (C) resulting from incomplete combustion.

Typical value in flue gas oil burners: Smoke spot number 0 or 1

Particulate Matter

Particulate matter (dust) is the name given to the smallest solids distributed through the air. These may occur in any shape and density. Particulate matter is formed by the ash and mineral components of solid fuels.

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