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

3 Experiments on Droplet Array Combustion
vapor and acidic gases such as NO 2 . A constant flow rate of 3.0 l min −1 was ensured.
Development of the analytical methods was done in the software package
QuantPad (version: 6.1), followed by implementation into the software
package OMNIC ® (version: 6.1a), both supplied by Thermo Nicolet [435, 436].
One spectral region was regularly selected for the determination of a particular
component (species). This region had to include portions without significant
absorption, as they built the baseline to determine the absorbance
A λ (cf. Eq. (3.6)). Furthermore, each spectral region needed to be subdivided
into different subregions to allocate the component-specific bands employed
for spectral analysis. Interference occurring on account of cross-sensitivity
additionally increased the fragmentation within a spectral region. Crosssensitivity
due to H 2 O and CO 2 renders measurement extremely difficult in
the infrared spectrum. It is high for NO, in particular under ambient pressure.
Even though tilted, shifted, and curved baselines are feasible and baseline correction
may be applied by the user, it is a disadvantage to have wide-spread
spectral regions and high absorbance A λ of, for instance, 0.8 to 1.0. The latter
may occur due to strongly varying levels of concentration. If absorption and
hence signal-to-noise ratio (SNR) get too large with an increase of concentration,
switching to a different spectral region might deliver better results.
Consequently, more than one spectral region might be required for a single
component under certain circumstances [435–438].
Since light gases, such as NO and CO, absorb rotational energy in addition to
vibrational energy, a polynomial instead of a linear curve fit became necessary
for the calibration of these components. This involved a larger number of
span gases because the number of available data points must outnumber the
polynomial order.
To cover all these effects, both calibration standards (Tabs. 3.7 and 3.8) were
employed in the present study on a “competitive” basis, as they represent differing
approaches. Nevertheless, they showed consistent results that were in
good agreement with theory (see Chap. 5). In the end, the reliability of the results
depends on the maintenance and calibration of the FT-IR spectrometer,
which is time-consuming in comparison with other analytical methods, but
the analysis time itself is short.
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