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

3 Experiments on Droplet Array Combustion
analyzer was the system of choice for determining the exhaust gas concentrations.
In the former case a NEXUS ® 470 unit of Thermo Electron was used, in
the latter a CLD 700 EL ht as well as a CLD 700 LEV ht unit of ECO PHYSICS
[111–115, 434–438]. 3 As the constituents of the exhaust were not separated for
identification and measurement, and no moisture removal was utilized, the
approach chosen for this research can be classified as “hot/wet method without
separation”.
NO/NO x Chemiluminescence Analysis
In an early work, Allen [14] reviews numerous methods of analysis for oxides
of nitrogen, namely nitric oxide (NO) and nitrogen dioxide (NO 2 ). The suitability
of the chemiluminescence method, which is widely seen as state of the
art in NO x measurement, is evaluated in the work of Allen et al. [16], Black
and Sigsby Jr. [46], and Sigsby Jr. et al. [397]. A chemiluminescent analyzer
measures the light emitted from an excited NO 2 molecule (NO 2
∗ , see Eq. (3.3)).
This technique provides exceptional sensitivity with a feasible lower detection
limit of less than 0.1ppb. However, only NO can be measured directly, as indicated
by the initial reactions of Equations (3.1) and (3.2). For this purpose,
ozone (O 3 ) is generated in the analyzer and added to the gas under investigation.
The NO 2 formed is promoted to an excited state in 20 % of the reactions
(Eq. (3.2)), and it returns to its ground state according to Equation (3.3) by
emitting a photon. The resulting electromagnetic emission spectrum is in the
range of 600 to 3000 nm with the intensity maximum at 1200 nm. In order to
detect this chemiluminescence, the photoelectric effect is employed, and as
long as O 3 is in excess, proportionality to the actual NO concentration is given.
As a linear correlation prevails between absolute pressure and the probability
of Equation (3.4), the analyzer is generally operated at a low pressure of
40±10 mbar to achieve a high degree of light efficiency. Furthermore, a reducing
converter is used in the analyzer forcing the reaction of Equation (3.5), in
which NO 2 is reduced to NO prior to its detection. Consequently, the total NO x
is determined as one quantity from the original NO plus the NO resulting from
the NO 2 to NO conversion; and the NO 2 concentration is calculated by the difference
of the NO and NO x values measured. The reaction of Equation (3.5) is
outlined for carbon here, but the CLD 700 EL ht and the CLD 700 LEV ht unit
3 The CLD 700 LEV ht is a modified CLD 700 AL unit, that features a heated gas inlet.
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