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SiC-FET based SO2 sensor for power plant emission applications
Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
Alstom Power AB, Sweden .
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2014 (English)In: Sensors and actuators. B, Chemical, ISSN 0925-4005, Vol. 194, 511-520 p.Article in journal (Refereed) Published
Abstract [en]

Thermal power plants produce SO2 during combustion of fuel containing sulfur. One way to decrease the SO2 emission from power plants is to introduce a sensor as part of the control system of the desulphurization unit. In this study, SiC-FET sensors were studied as one alternative sensor to replace the expensive FTIR (Fourier Transform Infrared) instrument or the inconvenient wet chemical methods. The gas response for the SiC-FET sensors comes from the interaction between the test gas and the catalytic gate metal, which changes the electrical characteristics of the devices. The performance of the sensors depends on the ability of the test gas to be adsorbed, decomposed, and desorbed at the sensor surface. The feature of SO2, that it is difficult to desorb from the catalyst surface, makes it known as catalyst poison. It is difficult to quantify the SO2 with static operation, even at the optimum operation temperature of the sensor due to low response levels and saturation already at low concentration of SO2. The challenge of SO2 desorption can be reduced by introducing dynamic operation in a designed temperature cycle operation (TCO). The intermittent exposure to high temperature can help to desorb SO2. Simultaneously, additional features extracted from the sensor data can be used to reduce the influence of sensor drift. The TCO operation, together with pattern recognition, may also reduce the baseline and response variation due to changing concentration of background gases (4-10% O-2 and 0-70% RH), and thus it may improve the overall sensor performance. In addition to the laboratory experiment, testing in the desulphurization pilot unit was performed. Desulphurization pilot unit has less controlled environment compared to the laboratory conditions. Therefore, the risk of influence from the changing concentration of background gas is higher. In this study, linear discriminant analysis (LDA) and partial least square (PLS) were employed as pattern recognition methods. It was demonstrated that using LDA quantification of SO2 into several groups of concentrations up to 2000 ppm was possible. Additionally, PLS analysis indicated a good agreement between the predicted value from the model and the SO2 concentration from the reference instrument of the pilot plant.

Place, publisher, year, edition, pages
Elsevier , 2014. Vol. 194, 511-520 p.
Keyword [en]
SO2 sensors; SiC-FET; Pt; Temperature cycled operation (TCO); Desulphurization; Power plant
National Category
Engineering and Technology
URN: urn:nbn:se:liu:diva-105569DOI: 10.1016/j.snb.2013.11.089ISI: 000331575400067OAI: diva2:708869
Available from: 2014-03-31 Created: 2014-03-27 Last updated: 2015-03-09Bibliographically approved
In thesis
1. SiC-FET Gas Sensors Developed for Control of the Flue Gas Desulfurization System in Power Plants Experimental and Modeling: Experimental and Modeling
Open this publication in new window or tab >>SiC-FET Gas Sensors Developed for Control of the Flue Gas Desulfurization System in Power Plants Experimental and Modeling: Experimental and Modeling
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Electricity and power generation is an essential part of our life. However, powergeneration activities also create by-products (such as sulphur oxides, nitrogen oxides,carbon monoxide, etc), which can be dangerous when released to the atmosphere.Sensors, as part of the control system, play very vital role for the fluegas cleaning processes in power plants. This thesis concerns the development ofSilicon Carbide Field Effect Transistor (SiC-FET) gas sensors as sensors for sulfurcontaining gases (SO2 and H2S) used as part of the environmental control systemin power plants. The works includes sensor deposition and assembly, sensinglayer characterization, operation mode development, performance testing of thesensors in a gas mixing rig in the laboratory and field test in a desulfurization pilotunit, and both experimental and theoretical studies on the detection mechanismof the sensors.

The sensor response to SO2 was very small and saturated quickly. SO2 is a verystable gas and therefore reaction with other species requires a large energy input.SO2 mostly reacts with the catalyst through physisorption, which results in lowresponse level. Another problem was that once it finally reacted with oxygen andadsorbed on the surface of the catalyst in form of a sulfate compound, it is desorbedwith difficulty. Therefore, the sensor signal saturated after a certain timeof exposure to SO2. Different gate materials were tested in static operation (Pt,Ir, Au), but the saturation phenomena occurred in all three cases. Dynamic sensoroperation using temperature cycling and multivariate data analysis could mitigatethis problem. Pt-gate sensors were operated at several different temperatures in acyclic fashion. One of the applied temperatures was chosen to be very high for ashort time to serve as cleaning step. This method was also termed the virtual multisensor method because the data generated could represent the data from multiplesensors in static operation at different temperatures. Then, several features of thesignal, such as mean value and slope, were extracted and processed with multivariatedata analysis. Linear Discrimination Analysis (LDA) was chosen since itiiiallows controlled data analysis. It was shown that it was possible to quantify SO2with a 2-step LDA. The background was identified in the first step and SO2 wasquantified in the second step. Pt sensors in dynamic operation and 2-step LDAevaluation has also demonstrated promising results for SO2 measurement in thelaboratory as well as in a desulfurization pilot unit. For a commercial sensor, algorithmhave to be developed to enable on-line measurement in real time.

It was observed that Ir-gate sensors at 350oC were very sensitive to H2S. The responseobtained by Ir sensors to H2S was almost five times larger than that of Ptsensors, which might be due to the higher oxygen coverage of Ir. Moreover, Irsensors were also more stable with less drift during the operation as a result ofhigher thermal stability. However, the recovery time for Ir sensors was very long,due to the high desorption energy. Overall, the Ir sensors performed well whentested for a leak detection application (presence of oxygen and dry environment).The geothermal application, where heat is extracted from the earth, requires thesensor to be operated in humid condition in the absence (or very low concentration)of oxygen, and this poses a problem. Temperature cycle operation and smartdata evaluation might also be an option for future development.

Along with the sensor performance testing, a study on the detection mechanismwas also performed for SO2 sensor, both experimentally and theoretically. The experimentincluded the study of the species formed on the surface of the catalystwith DRIFT (diffuse reflectance infrared frourier transform) spectroscopy and theanalysis of the residual gas with mass spectroscopy. Explanatory investigation ofthe surface reactions was performed using quantum-chemical calculations. Theoreticalcalculations of the infrared (IR) vibration spectra was employed to supportthe identification of peaks in the DRIFT measurement. Based on the study on theresidual gas analysis and quantum-chemical calculations, a reaction mechanismfor the SO2 molecule adsorption on the sensor surface was suggested.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 44 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1587
SiC-FET sensors, temperature cycle operation, detection mechanism studies, SO2, H2S
National Category
Engineering and Technology
urn:nbn:se:liu:diva-106224 (URN)10.3384/diss.diva-106224 (DOI)978-91-7519-366-3 (print) (ISBN)
Public defence
2014-06-03, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Available from: 2014-05-13 Created: 2014-04-29 Last updated: 2015-03-09Bibliographically approved

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