Flue gas monitoring sensors

  • September 02, 2014
  • News

By Tom Gurd, City Technology

The main products of incomplete combustion are usually CO2, CO, H2 & NO; to ensure safe combustion we measure CO and the remaining O2 after combustion to calculate a CO:CO2 ratio – the accepted benchmark of performance. Of course there is much more to understand regarding efficiency or environmental emissions. CO sensors are also sensitive to H2 so in our more demanding markets, we measure the H2 to compensate the CO reading. The combustion process is complex, requiring the right mix ratio, turbulence, temperature and time for reactants to combine. Poor fuel mixing or too little air produces carbon monoxide (CO) and soot. If the flame temperature is too high, nitric oxide (NO) and nitrogen dioxide (NO2) are produced. A fuel containing sulphur will produce sulphur dioxide (SO2) - all these gases are toxic and indicate inefficient combustion, which is both dangerous and costly in terms of fuel consumption. Oxygen (O2) is essential for combustion so the gases detected by a typical flue gas analyser are O2, CO, H2, CO2, NO, NO2, SO2 & H2S. There is a lot of focus on the analyser specification itself, but it’s also important to remember that an analyser is only as effective as the sensors it uses. Because of the risks associated with combustion emissions, safety and reliability are critical. Due to the nature of the typical toxic gas sensor used in a flue gas analyser, emissions gases and products of combustion can produce cross-sensitivity issues. Field failures can be dangerous and costly and lives depend on a good sensing technology. What is less widely recognised are the other economic benefits a gas sensor can bring, by reducing an analyser’s servicing needs and operational costs. Long life oxygen sensors for example are designed to work as long and hard as a typical analyser, compared with a standard O2 sensor with a two year life expectancy that must be changed 2-3 times in the life of an instrument. In a real-world context, a combustion check can take up to 15 minutes and is typically carried out up to 8 times in each working day. With improved response times, new sensors can allow the same combustion check to be carried out in half the time or less. This means much less time waiting for the reading to stabilize – and ultimately a valuable opportunity to carry out more services in the field. Faster response Is what engineers really value as they have to wait less time for the reading to stabilize which can lead to more accurate readings as they are less likely to take the reading too quickly. Flue gas analysis in domestic and light commercial boilers can present a challenge for sensors; they are subjected to high humidity, temperature and pressure changes, corrosive and acidic gases, and cross-sensitivity from emissions gases. This can cause field failures, compromise sensor integrity and create considerable servicing needs. Every component in a sensor should be built to a high specification using ultra resistant materials, so they can work effectively in these adverse conditions without affecting accuracy and reliability and deliver the fast response times. Sensor design is essential in achieving this. Long life oxygen sensors are built using “electrochemical pump” technology which is proven in the field for 4 years and provides a viable lead fee replacement. A high range CO monitoring application, which traditionally requires a second sensor under EN50379-2, benefits from the use of a robust, high sensitivity solution like our A5F+ CO sensor, which can monitor up to 20,000ppm. The combination of high capacity filtration to remove SOx and NOx and auxiliary electrodes to compensate for hydrogen, provides the highest reliability.

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