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© KWB – Kraft und Wärme aus Biomasse GmbH IM Easyfire 10/2008 Page 71
7 Flue gas measurement 7 Flue gas measurement
7 MEASURING FLUE GAS – GENERAL INFORMATION
7.1 Brief information on flue gas values
Designation Abbreviation Good value Comment
Carbon dioxide CO
2
10 - 11.5 % Jointly responsible for the greenhouse effect. The CO
2
that
occurs with combustion is bonded as a plant nutrient by the
growing wood. Consequently wood is “CO
2
neutral”.
Oxygen O
2
8.5 - 10 % Residual oxygen from the combustion air
Carbon monoxide CO <500 -
1000 mg/Nm³
13 % O
2
Strong blood toxin; prevents transport of oxygen in the blood,
is quickly decomposed in air.
Nitrogen oxide NO
x
<200 -
500 mg/Nm³
13 % O
2
Respiratory toxin; highly dependent on the nitrogen content
of the fuel; the limit values must only be complied with for
nitrogen content < 0.15 % by weight.
Hydrocarbons HC A wide variety of harmful compounds, “wood fire odour”, they
behave like CO, complex measurement
Dust <50 mg/Nm³
13 % O
2
Fine fly ash and particles of soot, depending on the dust content
of the fuel
Firing efficiency
Eta [
η
]
>88 - 95 % depends on CO
2
content and flue gas temperature, (approximate
boiler effi 2 %)
7.1.1 Emission value units
Unfortunately a standard unit is not used for emission values in the different directives and standards. To be
precise, the conversion is quite complex because many influence factors must be taken into consideration. The
accompanying table suffices for an approximate estimate calculation.
Abbreviation Designation Comment
% CO
2
and O
2
Percentage by volume (m³/m³) Measured in dry flue gas (without water vapour)
ppm parts per million (cm³/m³) = 0.0001 percentage by volume
mg/Nm³ 10 % O
2
Milligrams per standard cubic
meterResidual oxygen in the flue
gas 10 % Reference value is 1 cubic meter flue gas at 0 °C and 1.013 bar
and a residual oxygen content of the flue gas of 10 %
mg/MJ Milligrams per mega joule Reference value is the lower heating value of the fuel in MJ
mg/kWh Milligrams per kilowatt hour Reference value is the lower heating value of the fuel in kWh
MA-Easyfire 10.2008_EN.indd 71 MA-Easyfire 10.2008_EN.indd 71 30.03.2009 13:30:11 Uhr 30.03.2009 13:30:11 Uhr