Monitoring ammonia slip from large-scale selective catalytic reduction (SCR) systems in combined heat and power generation applications with field effect gas sensors

Following tightened regulations, selective catalytic reduction (SCR) of nitrogen oxides (NOinline-formula_x) by ammonia (NHinline-formula₃) has over the last couple of decades found wider adoption as a means of reducing NOinline-formula_x emissions from e.g. power production and district heating plants. As in the SCR process NHinline-formula₃ injected into the flue gas reacts with and reduces NOinline-formula_x to nitrogen (Ninline-formula₂) and water (Hinline-formula₂O) on the surface of a specific catalyst, the NHinline-formula₃ injection has to be dynamically adjusted to match both instant and long-term variations in flue gas nitrogen oxide concentration in order to minimize NOinline-formula_x and NHinline-formula₃ emissions. One possibility of realizing such NHinline-formula₃ dosing control would be the real-time monitoring and feedback of downstream flue gas NOinline-formula_x and NHinline-formula₃ concentrations to the NHinline-formula₃ injection control unit. In this study the sensing characteristics and performance of SiC-based Metal Oxide Semiconductor Field Effect Transistor (MOSFET) sensors with a structurally tailored gas-sensitive gate contact of iridium (Ir) for in situ NHinline-formula₃ monitoring downstream from the SCR catalyst in a combined heat and power (CHP) plant have therefore been investigated and evaluated.

The sensor's NHinline-formula₃ sensitivity and selectivity as well as the cross-sensitivity to common flue gas components – oxygen (Oinline-formula₂), water vapour (Hinline-formula₂O), nitric oxide (NO), nitrogen dioxide (NOinline-formula₂), carbon monoxide (CO), and a model hydrocarbon, ethene (Cinline-formula₂Hinline-formula₄) – were thereby investigated for relevant concentration ranges under controlled conditions in the laboratory. While, at the prescribed sensor operation temperature of 300 inline-formula^∘C, the influence of Hinline-formula₂O, CO, and Cinline-formula₂Hinline-formula₄ on the sensor's NHinline-formula₃ concentration reading could be regarded as practically insignificant, a moderate cross-sensitivity was observed between NHinline-formula₃ and NOinline-formula₂ and, to a lesser extent, between NHinline-formula₃ inline-formula $M30inlinescrollmathml/$ 8pt14ptsvg-formulamathimg5c3774ab0600a2f03e83f0e636ae5ed2 jsss-12-235-2023-ie00001.svg8pt14ptjsss-12-235-2023-ie00001.png  NO and NHinline-formula₃ inline-formula $M32inlinescrollmathml/$ 8pt14ptsvg-formulamathimg5ea72e5fcd9e8768af40707a0416eed7 jsss-12-235-2023-ie00002.svg8pt14ptjsss-12-235-2023-ie00002.png  Oinline-formula₂. As the NOinline-formula_x concentration downstream from the SCR catalyst under normal SCR and power plant operation is expected to be considerably smaller than the NHinline-formula₃ concentration whenever any appreciable ammonia slip occurs, the observed NHinline-formula₃ inline-formula $M37inlinescrollmathml/$ 8pt14ptsvg-formulamathimgf77694ccc584f068782523cf5fd7b6a6 jsss-12-235-2023-ie00003.svg8pt14ptjsss-12-235-2023-ie00003.png   NOinline-formula_x cross-sensitivities may, however, be of less practical significance for ammonia monitoring in real flue gases downstream from the SCR catalyst. Furthermore, if required, the small influence of Oinline-formula₂ concentration variations on the sensor reading may also be compensated for by utilizing the signal from a commercially available oxygen sensor. Judging from in situ measurements performed in a combined heat and power plant, the structurally tailored Ir gate field effect sensors also exhibit good NHinline-formula₃ sensitivity over the relevant 0–40 ppm range when directly exposed to real flue gases, offering an accuracy of inline-formula±3 ppm as well as low sensor signal drift, the latter most likely to further improve with regular zero-point calibration and thereby make the Ir gate MOSFET ammonia sensor a promising alternative for cost-efficient real-time ammonia slip monitoring or SCR system control in heat and/or power production plants.