Chemiluminescence Analyzer (CLA)
When NO and ozone (O 3 ) react a small fraction (about 10% at 26.7° C) of excited NO2* molecules is produced as per the following reactions:
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(4.2) |
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(4.3) |
As the excited molecules of NO2 * decay to ground state, light in the wavelength region 0.6-3.0 µm is emitted. The quantity of excited NO2 produced is fixed at a given reaction temperature and the intensity of light produced during decay of excited NO2 is proportional to the concentration of NO in the sample.
| Figure 4.12 |
Schematic and flow diagram of a c hemiluminescence NOx analyzer. |
A schematic diagram of the chemiluminescence NOxanalyzer is shown in Fig. 4.12.
- The sample containing NO flows to a reactor where it reacts with ozone produced from oxygen in ‘ozonator' .In the reactor NO is converted to NO2 .
- A photomultiplier tube detects the light emitted by the excited NO2 . The signal is then amplified and fed to recorder or indicating equipment.
- For the measurement of nitrogen oxides (NOx ), NO2in the sample is first converted to NO by heating in a NO2- to-NO converter prior to its introduction into the reactor. At 315º C, about 90 percent of NO2 is converted to NO2. The total concentration of NOx in the sample is thus, measured as NO. When the sample is introduced in the reactor bypassing the NO2 - to- NO converter, concentration of NO alone is determined. The difference between the two measurements provides the concentration of NO2in the sample.
- The response of the instrument is linear with NO concentration. The technique is very sensitive and can detect up to 10-3 ppm of NOx.
- The output signal is proportional to the product of sample flow rate and NO concentration. As the method is flow sensitive an accurate flow control is necessary. The calibration and operation are done at the same flow rate and reactor temperature.
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