On the Formation of Nitrogen Oxides During the Combustion of ...

3.2 Measurement Techniques and Data Acquisition
use a metal converter at 400 ◦ C and a molybdenum converter at 350 to 370 ◦ C,
respectively [16, 111, 113–115, 150].
NO+O 3 → NO 2 + O 2 (80 %) (3.1)
NO+O 3 → NO
∗
2 + O2 (20 %) (3.2)
NO
∗
2 → NO2 + hν (3.3)
NO
∗
2 + M → NO2 + M (3.4)
NO 2 + C→NO+CO (3.5)
When quantifying NO x concentrations, measuring NO 2 is generally the primary
problem. As this species is highly soluble in water, water condensation
in the sample line as well as in the supply line of the analyzer has to be
prevented. Furthermore, the significance of the emission data rests on reliable
converter components, in particular for low concentrations, because the
NO 2 /NO x ratio tends to increase with overall decreasing NO x levels [371].
FT-IR Spectroscopy
Since the spectroscopic FT-IR method is ideally suited for capturing the concentrations
of a larger number of species, it was used to determine the major
combustion products including CO 2 , CO, and H 2 O. Oxides of nitrogen (NO,
NO 2 , and N 2 O) were measured by FT-IR as a back-up solution to the chemiluminescence
method. The FT-IR method also solely utilizes physical exhaust
properties, ignoring chemical properties. It is based on Beer’s law, which relates
the absorption of light to the concentration and path length of the material
through which the light is traveling [97, 158, 420, 434–438]. Equation (3.6)
shows the correlation for a given wavelength λ:
( ) I
A λ =−log = c ǫ λ l. (3.6)
I 0
The absorbance A λ is the ratio of the intensity of light I that has passed
through the sample and the intensity of light I 0 before it enters the sample,
expressed as a logarithm with base 10. A linear correlation is valid for the concentration
c of the absorbing species, molar absorptivity (molar extinction co-
87