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

3.2 Measurement Techniques and Data Acquisition
Calibration of Chemiluminescent NO x Analyzer
The chemiluminescent NO x analyzer was calibrated with NO at concentrations
of 2.00, 8.66, and 9.80 ppm in an N 2 balance [111]. However, when sampling
from combustion systems, additional species apart from N 2 are introduced,
including H 2 O, CO 2 , CO, H 2 , UHCs, and soot. Under certain circumstances,
the composition of the exhaust can affect the measurement value indicated
for NO due to differences in third body quenching reactions between
the span gas used and the final sample gas (Eq. (3.4)). Fitz and Welch [136],
for instance, report on an NO concentration that is 30% lower than the actual
concentration, using synthetic exhaust simulated for a gas-fired power
plant with water vapor at 17% by volume. For such an exhaust analysis, the
water must either be removed prior to NO x detection, the detector be calibrated
with saturated span gas, or the NO x reading be corrected on the basis
of third body quenching. The third body quenching effect is most crucial in
samples that have large concentrations of non-diatomic constituents. Gärtner
[144], Matthews et al. [273], and Tidona et al. [439] investigated interferences
in the chemiluminescent measurement of NO and NO 2 emissions from combustion
systems. Generally, the NO x measurement error increases with an increasing
H/C ratio of the fuel and an increasing equivalence ratio φ. Measurements
taken in very fuel-rich systems or in the fuel-rich regions of diffusion
flames may be subject to error well above 20% due to the potential presence
of polyatomic fuel molecules with high quenching characteristics.
As third body quenching may be such a significant source of error in NO
measurement with the effect of excessively low NO x indication, the relative
quenching efficiencies of various third bodies were included in the postprocessing.
The calculation follows the procedure of Matthews et al. [273],
which was also employed in the work of Gärtner [144], Baessler [31], and
Brückner-Kalb [57]. If the concentrations of the important third bodies are
known, the actual NO concentration can be calculated from the concentration
indicated using the following relations:
where
[NO] actual
= X NO,actual
= 1+
[NO] indicated X NO,indicated
J∑
(R M − 1), (3.7)
M=1
R M ≡ I N 2
I M
. (3.8)
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