P. Ljung

SiC Based Field Effect Gas Sensors for Industrial Applications

The development and field-testing of high-temperature sensors based on silicon carbide devices have shown promising results in several application areas. Silicon carbide based field-effect sensors can be operated over a large temperature range, 100-600 °C, and since silicon carbide is a chemically very inert material these sensors can be used in environments like exhaust gases and flue gases from boilers. The sensors respond to reducing gases like hydrogen, hydrocarbons and carbon monoxide. The use of different temperatures, different catalytic metals and different structures of the gate metal gives selectivity to different gases and arrays of sensors can be used to identify and monitor several components in gas mixtures. MOSFET sensors based on SiC combine the advantage of simple circuitry with a thicker insulator, which increases the long term stability of the devices. In this paper we describe silicon carbide MOSFET sensors and their performance and give examples of industrial applications such as monitoring of car exhausts and flue gases. Chemometric methods have been used for the evaluation of the data.

Investigations on the possibilities of a MISiCFET sensor system for OBD and combustion control utilizing different catalytic gate materials

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.

Evaluation of on-line flue gas measurements by MISiCFET and metal-oxide sensors in boilers

Metal insulator silicon carbide field-effect transistor sensors, metal-oxide sensors, and a linear Lambda sensor in an electronic nose was used to measure on-line in hot flue gases from a boiler. Flue gas from a 100-MW pellets-fuelled boiler has been used to feed the experimental setup. Several reference instruments, which measure the flue gases in parallel to the sensor array, are connected to the electronic nose. Data was collected during six weeks and then evaluated. Using principal component analysis as the data evaluation method, different operating modes for the boiler have been identified in the data set. The different modes could be described in terms of high or low O/sub 2/ and CO concentration. Furthermore, we have shown that it seems possible to use a sensor array to determine the operating mode of the boiler and, by partial least-squares models, measure the CO concentration when the boiler operates in its optimum mode.

Corrosion of TP347H FG stainless steel in a biomass fired PF utility boiler

Detailed microscopic examinations have been conducted on two, temperature-regulated probes which had been exposed to biomass flue gas inside a PF boiler for 3770 h at 600° and 650°C respectively. The two probes were made from commercial grade heat exchanger alloy (TP 347H FG). A significant buildup of potassium chloride and sulphate was found on both probes within the deposits that sinter more strongly on the 650°C probe. Both probes suffer extensive intergranular corrosion as well as general corrosion. The higher temperature enhances scale formation which may provide a beneficial effect with respect to hot corrosion resistance. The intergranular corrosion is linked to the presence of sulphur. The overall material wastage is almost the same for both probes and for different locations covered by deposit.

MISiCFET chemical sensors for applications in exhaust gases and flue gases

A chemical gas sensor based on a silicon carbide field effect transistor with a catalytic gate metal has been under development for a number of years. The choice of silicon carbide as the semiconductor material allows the sensor to operate at high temperatures, for more than 6 months in flue gases at 300degreesC and for at least three days at 700degreesC. The chemical inertness of silicon carbide and a buried gate design makes it a suitable sensor technology for applications in corrosive environments such as exhaust gases and flue gases from boilers. The selectivity of the sensor devices is established through the choice of type and structure of the gate metal as well as the operation temperature. In this way NH3 sensors with low cross sensitivity to NOx have been demonstrated as potential sensors for control of selective catalytic reduction (SCR) of NOx by urea injection into diesel exhausts. Here we show that sensors with a porous platinum or iridium gate show different temperature ranges for NH3 detection. The hardness of the silicon carbide makes it for example more resistant to water splash at cold start of a petrol engine than existing technologies, and a sensor which can control the air to fuel ratio, before the exhaust gases are heated, has been demonstrated. Silicon carbide sensors are also tested in flue gases from boilers. Efficient regulation of the combustion in a boiler will decrease fuel consumption and reduce emissions.

Evaluation of on-line hot flue gas measurements

Using Metal Insulator Silicon Carbide (MISiC) sensors, Semiconducing Metal Oxide sensors (SMO) and a linear lambda sensor in an electronic nose, we measure hot flue gases on-line. Flue gas from a 500 kW pellets fuelled boiler, which is used for heating apartment blocks, has been used to feed the experimental set-up. Several reference instruments, which measure the flue gases in parallel to the sensor array, are connected to the electronic nose. The gases which are interesting to measure are NO, CO, O2 and hydrocarbons, HC. Results on prediction for CO, NO, and O2 in the flue gases based on PLS- (and ANN-) models are here presented.

MISiCFET Chemical Gas Sensors for High Temperature and Corrosive Environment Applications

A chemical gas sensor based on a silicon carbide field effect transistor with a catalytic gate metal has been under development for a number of years. The buried gate design allows the sensor to operate at high temperatures, routinely up to 600degreesC and for at least three days at 700degreesC. The chemical inertness of silicon carbide makes it a suitable sensor technology for applications in corrosive environments such as exhaust gases and flue gases from boilers. The selectivity of the sensor devices is established through the choice of type and structure of the gate metal as well as the operation temperature. In this way NH3 sensors with low cross sensitivity to NOx have been demonstrated as potential sensors for control of selective catalytic reduction (SCR) of NOx by urea injection into diesel exhausts. The hardness of the silicon carbide makes it for example more resistant to water splash at cold start of a petrol engine than existing technologies, and a sensor which can control the air to fuel ratio, before the exhaust gases are heated, has been demonstrated. Silicon carbide sensors are also tested in flue gases from boilers. Efficient regulation of the combustion in a boiler will decrease fuel consumption and reduce emissions.

Corrosion of HR3C heat exchanger alloy in a biomass fired PF utility boiler

Detailed microscopic examinations have been conducted on two, temperature-regulated probes (commercial HR3C heat exchanger alloy) after being exposed to biomass flue gas inside a PF boiler for 3770 h at 600°C and 650°C respectively. Corrosion of the tube proceeds via scale formation and internal element depletion. Three characteristic types of internal corrosion have been identified depending on their position relative to the flue gas passage and deposit/flue gas chemistry. Severe, mainly internal corrosion occurs at down-stream locations where higher potassium chloride content exists within the deposit. Corrosion mechanisms corresponding to each type of internal corrosion have been proposed based on further laboratory tests and thermodynamic analysis. The increased temperature (650°C) causes slightly higher material wastage for the alloy. n n n nKorrosion der Warmetauscherlegierung HR3C in einem mit Biomasse gefeuerten Versorgungskessel n n n nDetaillierte mikroskopische Untersuchungen wurden an zwei, temperatur-regulierten Proben (kommerzielle Warmetauscherlegierung HR3C) nach 3770 h Auslagerung in Biomassenabgas in einem PF-Kessel bei 600 bzw. 650°C durchgefuhrt. Die Korrosion des Rohres erfolgt uber Zunderbildung und innere Elementverarmung. In Abhangigkeit von der relativen Position zum Abgasdurchtritt und von der Ablagerungs-/Abgas-Chemie wurden drei Typen von innerer Korrosion identifiziert. Starke, im wesentlichen innere Korrosion tritt an den stromabwartsgerichteten Stellen auf, wo hohe Kaliumchloridgehalte in den Ablagerungen vorhanden sind. Aufbauend auf weiteren Laborversuchen sowie thermodynamischer Analyse sind entsprechend den einzelnen Typen der inneren Korrosion Korrosionsmechanismen vorgeschlagen worden. Die erhohte Temperatur (650°C) verursacht einen etwas hoheren Materialverlust fur die Legierung.

MISiCFET sensor arrays for on line diagnosis

Catalytic Metal Insulator Silicon Carbide Field Effect Transistor (MISiCFETs) sensors can be processed as a sensor array on a SIC-chip. Different selectivity of the sensors is achieved by the use of different combinations of catalytic metals and insulators. The response patterns are used to evaluate for example different states of a combustion (low/high emissions, efficient/less efficient combustion) or monitoring different components in the exhausts from the combustion.