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SiC-FET Gas Sensors Developed for Control of the Flue Gas Desulfurization System in Power Plants Experimental and Modeling: Experimental and Modeling
Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, The Institute of Technology.
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.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1587
Keyword [en]
SiC-FET sensors, temperature cycle operation, detection mechanism studies, SO2, H2S
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-106224DOI: 10.3384/diss.diva-106224ISBN: 978-91-7519-366-3 (print)OAI: oai:DiVA.org:liu-106224DiVA: diva2:714701
Public defence
2014-06-03, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Funder
Vinnova
Available from: 2014-05-13 Created: 2014-04-29 Last updated: 2015-03-09Bibliographically approved
List of papers
1. SiC-FET based SO2 sensor for power plant emission applications
Open this publication in new window or tab >>SiC-FET based SO2 sensor for power plant emission applications
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2014 (English)In: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, 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
Keyword
SO2 sensors; SiC-FET; Pt; Temperature cycled operation (TCO); Desulphurization; Power plant
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-105569 (URN)10.1016/j.snb.2013.11.089 (DOI)000331575400067 ()
Available from: 2014-03-31 Created: 2014-03-27 Last updated: 2017-12-05Bibliographically approved
2. Hierarchical methods to improve the performance of the SiC - FET as SO2 sensors in flue gas desulphurization system
Open this publication in new window or tab >>Hierarchical methods to improve the performance of the SiC - FET as SO2 sensors in flue gas desulphurization system
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2015 (English)In: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, Vol. 206, 609-616 p.Article in journal (Refereed) Published
Abstract [en]

Experiments were performed both in the laboratory and a desulfurization pilot unit in order to improve the SiC-FET sensor performance using two-step data evaluation. In both cases, a porous Pt-gate enhancement type SiC-FET was utilized in a temperature cycled operation (TCO). Liner Discriminant Analysis (LDA) was chosen as the method for multivariate data analysis. Hierarchical methods with two-step LDA worked quite well in the laboratory tests with SO2 concentrations varied from 25-200 ppm. The same data evaluation was also applied to tests in the desulfurization pilot unit, with higher gas flow and a larger SO2 concentration range (up to 5000 ppm). The results from the SO2 quantification showed a significantly improved fit to corresponding reference instrument (FTIR) values.

Place, publisher, year, edition, pages
Elsevier, 2015
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:liu:diva-106212 (URN)10.1016/j.snb.2014.09.113 (DOI)000345234200079 ()
Available from: 2014-04-29 Created: 2014-04-29 Last updated: 2017-12-05
3. Vibrational analysis of SO2 on Pt / SiO2 system
Open this publication in new window or tab >>Vibrational analysis of SO2 on Pt / SiO2 system
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

In situ diffuse reflectance infrared Fourier transformed spectroscopy was used to study the interactions of SOx species with Pt/SiO2 between 200 and 400°C, and for SO2 concentrations between 10 and 50 ppm, which represents a concentration range where MISFET sensors exhibit good responses. In parallel, first-principles calculations have been carried out to support the experimental interpretations. It was found that sulfate species were formed on the silica surface, accompanied with removal/rearrangement of silanol groups upon exposure to SO2. Both experimental and theoretical calculations also suggest that the surface species were only formed after SO2 oxidation to SO3 on the metal surface. These evidences support the idea of SO2 oxidation to SO3 as the first step in the process of sulfate formation, followed by spillover of SO3 to the oxide, and finally the formation of sulfate species on the hydroxyl positions on the oxide. The results also indicate that the sulfate formation on silica depends both on the temperature and the SO2 concentration. Furthermore, hydrogen exposure was shown to be efficient for sulfur removal from the silica surface.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:liu:diva-106213 (URN)
Available from: 2014-04-29 Created: 2014-04-29 Last updated: 2015-03-09Bibliographically approved
4. Detection mechanism studies of SO2 on Pt / SiO2 system
Open this publication in new window or tab >>Detection mechanism studies of SO2 on Pt / SiO2 system
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Experiment was performed with Pt-gate SiC-FET sensors to study the detection mechanism of the sensors. The sensing measurement showed that oxygen influenced the response quite strongly. The sensor response became larger in the presence of oxygen. Experiment with mass spectroscopy indicated the formation of SO3 during the sensing measurement. Further experiment with DRIFT spectroscopy showed the formation of sulfate species on the oxide surface, accompanied by the disappearance of the silanol groups. An explanatory model was built based on quantum-chemical calculations. The results strengthened the experimental results by showing that it was more energetically favorable for SO2 to oxidize into SO3 before being adsorbed on the oxide surface. It was also observed that the overall adsorption reaction was exothermic, the activation energy for the SO2 oxidation was 48,75 kJ/mol, and the rate limiting step was the desorption of SO3 from the Pt surface.

Keyword
SO2 sensors, SiC-FET, Pt, detection mechanism, quantum chemical calculations
National Category
Physical Chemistry
Identifiers
urn:nbn:se:liu:diva-106214 (URN)
Available from: 2014-04-29 Created: 2014-04-29 Last updated: 2015-03-09Bibliographically approved
5. SiC based field effect transistor for H2S detection
Open this publication in new window or tab >>SiC based field effect transistor for H2S detection
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Pt-gate and Ir-gate SiC-FETs were tested as H2S sensors. Both sensors showed good response towards H2S in dry conditions with oxygen present. Ir-gate sensors showed high sensitivity and low drift, which makes them a suitable candidate for leak detection applications. Further testing was performed with Ir-gate sensors for geothermal applications. This involved humid environments and low oxygen concentrations. The sensitivity of the sensors decreased significantly at these conditions. When propene was added to the gas mixture, crosssensitivity was observed in the sensor signal. Further investigation to reveal the surface chemistry using spectroscopic techniques and modelling is needed to improve the selectivity of Ir-gate sensors in humid conditions and oxygen deficient environments.

Keyword
H2S sensors, SiC-FET, Pt, Ir, geothermal application
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-106215 (URN)
Available from: 2014-04-29 Created: 2014-04-29 Last updated: 2014-05-13Bibliographically approved

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