On the applicability of MISiCFET gas sensors regarding ammonia slip monitoring in small-scale boilers running SNCR

• Gas sensitive Metal Insulator Silicon Carbide Field Effect Transistor – MISiCFET – devices have shown good possibilities of realizing sensors for high temperature applications. One such application could be the monitoring of ammonia slip from boilers running SNCR – Selective Non-Catalytic Reduction of nitrogen oxides (NOx) with ammonia (NH3). In this study a number of MISiCFET gas sensors operated at different temperatures and with both platinum (Pt) and iridium (Ir) as the gate contact have been tested for their ability to detect and quantify ammonia slip in flue gases from a 5.6 MW wood fired boiler during a test of a new SNCR-system. The individual sensors have been evaluated and compared to each other for the sensitivity towards NH3 and possible cross-sensitivities to other flue gas species through the comparison of the sensor signals with the signals from analytical instruments like FTIR – Fourier Transform Infrared spectroscopy – for nitrogen oxides (NO + NO2), NH3, and carbon monoxide (CO) and an FID – Flame Ionization Detector – for the Total Hydrocarbon Concentration (THC). The ability of a combination of sensors to provide extra or more accurate information about the NH3 concentration was also evaluated through the construction and validation of a Partial Least Squares – PLS – regression model based on all the sensor signals. Under the assumption that the sensors’ responses follow a logarithmic dependence on NH3 concentration the results regarding  ammonia slip quantification were promising both for a single Ir sensor and for the system based on all sensors. There is still a question mark for the long-term stability of the devices in real flue gases, however.

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