What is Flue Gas?
The increase in all kinds of combustion is contaminating the
environment with ever-greater concentrations of pollutants.
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
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
Units of Measurement
The presence of pollutants in flue gas can be detected from the
concentration of the gas components. The following units are
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) 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).
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
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
Typical value in flue gas oil burners: Smoke spot number 0 or 1
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.