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3.4.3 MEXA 1170Nx Working Principles The MEXA-1170NX is the NH
3
measuring unit combining NO NOx detector based on the Chemiluminescence CLD method and the oxidation catalyst. The sample gas is divided into two
lines. One line SUM line would go through the catalyst inside the oxidation furnace at around 850
O
C. The other line NOx line would skip the oxidation furnace. At the catalyst, NH
3
is oxidized into NO by the reaction as follows:
4NH
3
+ 5O
2
== 4NO + 6H
2
O
Since the oxidation efficiency in the oxidized catalyst is not 100, the measured value is compensated using the confirmed oxidation efficiency value. The unit is equipped with an
adjustment function to minimize the response gap between detectors in each line that may cause error at drastic concentration change. The analyser is capable of switching between two modes. By
default in the NO
2
mode the oxidation catalyst would be turned off but optionally could be turned on for fast switching option. The carbon converter is gradually consumed by reduction process and
requires periodic replacement.
3.4.3a Working Principle of Chemiluminescence CLD
The details of CLD working principles are described in the MEXA 1170Nx user manual [Horiba MEXA 1170Nx operating manual 2004]. CLD is widely used as the measurement method of NO and NOx in
exhaust gases from engines because it is highly sensitive to NO and is not easily interfered by other components. When sample gas containing NO is mixed with ozone O
3
gas in a reactor, NO is oxidized and is transformed to NO
2
as shown in the reaction: NO + O
3
== NO
2
+ O
2
Some of the formed NO
2
molecules here is in excited state, which means its energy is higher than normal. Excited NO
2
molecules release excitation energy as light when returning to the ground state following these reactions:
NO + O
3
== NO
2
+ 0
2
NO
2
:NO
2
molecules in excited state NO
2
== NO
2
+ hv This phenomenon is called Chemiluminescence, and the light intensity is directly proportional to the
quality of NO molecules before the reaction. Thus, NO concentration in the sample can be estimated by measuring the amount of radiated light.
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3.4.3b Interference of CO
2
and H
2
O
Also noted from the MEXA 1170Nx user manual is the effect from interference of CO
2
and H
2
O to the measurements. Some of exited NO
2
molecules lose excitation energy by collision with another molecule before returning to the ground state by emitting light. In this case, NO
2
returns to ground state, but chemiluminescence does not occur as shown in reaction;
NO
2
+ M == NO
2
+ M where M: Other molecules
The probability of energy loss depends on the kind of the collision opponent, and the species and concentrations of co-existing gas components may affect NO sensitivity of the CLD method. The
probability of energy loss by CO
2
and H
2
O is larger than that by N
2
and O
2
in the components of typical engine exhaust gas. Therefore the change of CO
2
and H
2
O concentration in the sample tends to cause the change of NO sensitivity. In general, to lessen the interference of CO
2
and H
2
O inside of a reactor is maintained to a low pressure condition.
3.4.3c Measurement of NOx
Based on the working principles of CLD described in MEXA 1170Nx user manual, it is obvious that the NO
2
originally included in a sample cannot be measured by CLD, because it does not cause chemiluminescence. To measure the NO
2
, it is converted to NO using NOx converter before measurement. This is shown in the following reactions:
NO
2
+ C == NO + CO 2NO
2
+ C == 2NO + CO
2
From the above reaction, it is clearly seen that carbon C, which is the main substance of the NOx converted is being consumed by the reduction process. Therefore, as mention earlier, the periodic
check and replacement of the NOx converter is required.
3.4.4 NOx measurement in NH
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