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Investigations on the possibilities of a MISiCFET sensor system for OBD and combustion control utilizing different catalytic gate materials
Linköping University, Department of Physics, Chemistry and Biology, Applied Physics. Linköping University, The Institute of Technology.
Vattenfall Utveckling AB, Älvkarleby, Sweden.
Vattenfall Utveckling AB, Älvkarleby, Sweden.
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
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2004 (English)In: Topics in catalysis, ISSN 1022-5528, E-ISSN 1572-9028, Vol. 30-31, no 1, 365-368 p.Article in journal (Refereed) Published
Abstract [en]

Different catalytic materials, like Pt and Ir, applied as gate contacts on metal insulator silicon carbide field effect transistors — MISiCFET—facilitate the manufacture of gas sensor devices with differences in selectivity, devices which due to the chemical stability and wide band gap of SiC are suitable for high temperature applications. The combination of such devices in a sensor system, utilizing multivariate analysis/modeling, have been tested and some promising results in respect of monitoring a few typical exhaust and flue gas constituents, in the future aiming at on board diagnostics (OBD) and combustion control, have been obtained.

Place, publisher, year, edition, pages
2004. Vol. 30-31, no 1, 365-368 p.
Keyword [en]
platinum, iridium, sensor system, field effect sensors, SiC, OBD, combustion control, car exhaust, flue gases
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-13092DOI: 10.1023/B:TOCA.0000029776.18603.74OAI: oai:DiVA.org:liu-13092DiVA: diva2:17822
Available from: 2008-04-01 Created: 2008-04-01 Last updated: 2017-12-13Bibliographically approved
In thesis
1. SiC based field effect sensors and sensor systems for combustion control applications
Open this publication in new window or tab >>SiC based field effect sensors and sensor systems for combustion control applications
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Increasing oil prices and concerns about global warming have reinforced the interest in biofuels for domestic and district heating, most commonly through combustion of solid biomass like wood logs, hog fuel and pellets. Combustion at non-optimal conditions can, however, lead to substantial emissions of noxious compounds like unburned hydrocarbons, carbon monoxide, and nitrogen oxides as well as the generation of soot.

Depending on the rate of combustion more or less air is needed per unit time to completely oxidize the fuel; deficiency of air leading to emissions of unburned matter and too much of excess air to slow combustion kinetics and emissions of mainly carbon monoxide. The rate of combustion is influenced by parameters like fuel quality – moisture and ash content etc. – and in what phase the combustion takes place (in the gas phase through combustion of evaporated substances or on the surface of char coal particles), none of which is constant over time.

The key to boiler operation, both from an environmental as well as a power to fuel economy point of view, is thus the careful adjustment of the air supply throughout the combustion process. So far, no control schemes have been applied to small-scale combustors, though, mainly due to the lack of cheap and simple means to measure basic flue gas parameters like oxygen, total hydrocarbon, and carbon monoxide concentrations.

This thesis reports about investigations on and characterization of silicon carbide (SiC) based Metal Insulator Semiconductor (MIS) field effect gas sensors regarding their utility in emissions monitoring and combustion control applications as well as the final development of a sensor based control system for wood fired domestic heating systems.

From the main sensitivity profiles of such sensor devices, with platinum (Pt) and iridium (Ir) as the catalytic metal contacts (providing the gas sensing ability), towards some typical flue gas constituents as well as ammonia (NH3), a system comprising four individual sensors operated at different temperatures was developed, which through the application of Partial Least Squares (PLS) regression, showed good performance regarding simultaneous monitoring of propene (a model hydrocarbon) and ammonia concentrations in synthetic flue gases of varying content. The sensitivity to CO was, however, negligible. The sensor system also performed well regarding ammonia slip monitoring when tested in real flue gases in a 5.6 MW boiler running SNCR (Selective Non-Catalytic reduction of nitrogen oxides with ammonia).

When applied to a 200 kW wood pellet fuelled boiler a similar sensor system was, however, not able to follow the flue gas hydrocarbon concentration in all encountered situations. A PCA (Principal Components Analysis) based scheme for the manipulation of sensor and flue gas temperature data, enabling monitoring of the state of combustion (deficiency or too much of excess air), was however possible to develop. The discrepancy between laboratory and field test results was suspected and later on shown to depend on the larger variation in CO and oxygen concentrations in the flue gases as compared to the laboratory tests.

Detailed studies of the CO response characteristics for Pt gate MISiC sensors revealed a highly non-linear sensitivity towards CO, a large response only encountered at high CO/O2 ratios or low temperatures. The response exhibits a sharp switch between a small and a large value when crossing a certain CO/O2 ratio at constant operating temperature, correlated to the transition from an oxygen dominated to an almost fully CO covered Pt surface, originating from the difference in adsorption kinetics between CO and O2. Indications were also given pointing towards an increased sensitivity to background hydrogen as being the mediator of at least part of the CO response. Some general characteristics regarding the response mechanism of field effect sensors with differently structured metal contacts were also indicated.

The CO response mechanism of Pt metal MISiC sensors could also be utilized in developing a combustion control system based on two sensors and a thermocouple, which when tested in a 40 kW wood fired boiler exhibited a good performance for fuels with extremely low to normal moisture content, substantially decreasing emissions of unburned matter.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2007
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1077
Keyword
SiC, Field Effect Sensors, Combustion Control
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-11415 (URN)978-91-85715-56-5 (ISBN)
Public defence
2007-03-23, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
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Supervisors
Available from: 2008-04-01 Created: 2008-04-01 Last updated: 2014-01-09
2. Development and characterization of SiC based field effect gas sensors and sensor systems for emissions monitoring and control of biomass combustion
Open this publication in new window or tab >>Development and characterization of SiC based field effect gas sensors and sensor systems for emissions monitoring and control of biomass combustion
2005 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Increasing oil prices and environmental levies have reinforced the interest in biofuels for domestic and district heating, most commonly through combustion of solid biomass like wood logs, hog fuel and pellets in water based heating systems. The combustion process itself proceeds through three elementarysteps; drying, where fuel moisture is driven off, followed by pyrolysis and finally combustion of the remaining charcoal. Given the sufficient amount of air, good mixing and long enough residence time at elevated temperatures, the short-chained hydrocarbons and carbon monoxide formed during pyrolysis andchar combustion will burn to completion leaving only CO2 and H2O in the flue gases.

In case of air deficiency, combustion will be incomplete, leaving noxious compounds, like certain hydrocarbons and CO, behind. Too much of excess air, on the other hand, will lower the temperature of the combustion chamber, giving rise to both emissions of unburned material and, due to the forcedconvection of heat out the chimney, an impaired boiler efficiency. The key to boiler operation, both from an environmental as well as a power to fuel economy point of view, is thus the careful adjustment of the airflows during combustion. The amount of air needed for complete oxidation of the fuel varies with the phase of combustion, fuel, fuel quality and load, however, why an active control of the airflows is considered a prerequisite. So far, nocontrol schemes have been applied to small- and medium-sized combustors, though, mainly dependent on the lack of cheap and simple means to measure basic flue gas parameters, like oxygen, hydrocarbons and CO.

Here is reported about the possible use of a system comprising SiC based field effect sensors to monitor the state of combustion, applicable to domestic heating systems, where only a rough picture of the air to fuel relationship is needed. Furthermore, it has been shown possible to obtain a multivariate linear regression model for propene (a model hydrocarbon) by the application of an array of SiC field effect sensors in a varying background of typical flue gas constituents, as long as thevariation is not too large. This model could possibly be applied to a control scheme for medium sized boilers, where smaller variations of flue gas constituents are encountered, and the possibility of simultaneous ammonia estimations has also opened up the field of flue gas after-treatment controlapplications, monitoring ammonia slip from selective noncatalytic reduction of nitrogen oxides by ammonia. The quantitative estimation of hydrocarbons over a wider range of concentrations and backgrounds, as well as of minor flue gasspecies, NO and CO, is however not possible with the SiC sensors currently comprising the sensor system.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2005. 61 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1155
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-28434 (URN)13574 (Local ID)91-85297-88-7 (ISBN)13574 (Archive number)13574 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-21

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Andersson, MikeLöfdahl, MikaelLloyd Spetz, Anita

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