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Surveillance of De-NOx

For control and surveillance of the De-NOx plant, it is usually recommended to measure NO (possibly NOx), NH3 and H2O before and after the De-NOx reactor.

The purpose of these measurements is to reduce the consumption of raw materials, reduce NOx emission and to minimise and monitor the emission rate of NH3. The reason for measurement of H2O is to convert the results to dry condition (reference state).

If control of the DeNOx-plant is based only on measurement of NOx (feed-forward/feed-back control), high leakages of ammonia may be observed dependant on the following conditions:

  • Stratifications in the gas flow
  • The efficiency of the catalyst
  • Flow in the flue gas cleaning plant

For measurement of NO or NOx, extractive analysis systems are often used. In-situ monitors for measurement of NO or NOx are fairly new and untested on the market.

For measurement of NH3 and H2O, in-situ monitors based on laser light (single line spectroscopy) are used. The LaserGas monitors have very short response times and can be used in applications with high dust loads and high temperatures.

Dust concentration, gas composition and optical path length are important features to evaluate when choosing a laser gas monitor for a power plant. In many coal fired plants, the dust conditions are so critical that it is necessary to use inlet pipes to be able to look across the exhaust gas duct for a reliable measurement. The evaluation of the different monitors is always made in close cooperation with the persons responsible on the power plants in question.

We believe, that the future trend in instrumentation for gas analysis for control and surveillance purposes of the De-NOx plant is laser light; a measurement principle with short response times functioning in high temperatures and high dust loads and which requires a minimum of maintenance.

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