P5.7 - NOx gas sensor designed for integration on hot components

SENSOR+TEST Conferences 2009
2009-05-26 - 2009-05-28
Congress Center Nürnberg
Proceedings SENSOR 2009, Volume II
P5 - Gas-Sensors
B. Saruhan, M. Stranzenbach - German Aerospace Center (DLR), Cologne, Germany
411 - 416


In lean-burn engines operating under excess oxygen conditions (air/fuel-ratio >> 14.5), emission control becomes a challenging task to deal solely with conventional after treatment systems. Modern after treatment systems use catalysts and methods that are capable of reducing and controlling NOx-emission at higher oxygen containing atmospheres. In vehicles, emission control sensors are connected to the OBD-system to record the history of the emission reduction and to track the function of the catalytic converter. This is done mainly by employing single-unit upstream and downstream gas sensors. High efficiency mobile or stationary turbine engines which are predestined to produce larger NOx-emission do not contain emission after treatment and control system in operation. Instead, emission is defined externally, that means either by measuring emission at periodical testing phases or by calculating the emission characteristics typical to turbine. These do not reflect the emission state ruling under service conditions. Calculation of emission characteristics considers the ideal conditions. Moreover, the emission concentration behind the turbine outlet is influenced by environmental factors. It is needless to say that it is almost impossible to replace a single-unit gas sensor within a gas turbine due to the present harsh environments and turbine’s tightly built structure. Turbine geometries can not be easily changed and the aerodynamic and flow conditions should not be disturbed, since these determine the turbine capability and flight conditions.
In the past decade, the efforts into developing powerful single-unit NOx-sensors were directed to those basing on amperometric and potentiometric measurement principles. Amperometric sensors measure diffusion-limited current by employing metal oxide electrodes to optimize the response. In order to overcome the cross-selectivity towards oxygen, a separate pumping cell is used typically to maintain a constant O2-concentration at the electrode which requires a complicated device design. The classical potentiometric sensors in turn measure open circuit potential by means of two separate electrodes one of which being in the reference gas. Moreover, the potentiometric sensors suffer under the opposite sign for the EMF-potential of NO and NO2. Monitoring total NOx with these sensors becomes difficult. The primary requirement for new generation NOx-sensors is quantitative detection of total NOx with high selectivity at elevated operating temperatures. Other requirements are low cross-sensitivity towards other emission gases without the need for a reference gas. A planar design impedance-metric sensor with NiOSE and EB-PVD manufactured YSZ-electrode may overcome these challenges.
Electrochemical solid-state gas sensors employ yttria-stabilized zirconia (YSZ) as electrolyte which is manufactured typically by tape-casting method if planar design is used. This method is cost-effective and easy to handle, but not suitable for integration of the sensors on to the components. Thermal barrier coatings (TBCs) are currently used in the hot gas path of aero-engines and land-based gas turbines on highly loaded turbine blades and vanes to increase turbine efficiency. They typically comprise of a ceramic top coating – partially yttria stabilized zirconia (PYSZ) - of low thermal conductivity that reduces the metal temperature and smoothen the temperature peaks during the transient stage of turbine operation. Utilising the EB-PVD manufactured thermal barrier coatings as electrolytes of gas sensors, the vanes and blades of turbines can be re-functionalized and serve as active systems. This study suggests a novel planar-type NOx-sensor design which allows the integrated built of the sensors onto the hot parts of engines and turbines.