WO2016012336A1

WO2016012336A1 - Method for operating a gas sensor to improve the long-term stability of the gas sensor

Info

Publication number: WO2016012336A1
Application number: PCT/EP2015/066256
Authority: WO
Inventors: Sabine Fischer; Maximilian Fleischer; Erhard Magori; Ralf Moos; Roland Pohle
Original assignee: Siemens Aktiengesellschaft
Priority date: 2014-07-23
Filing date: 2015-07-16
Publication date: 2016-01-28
Also published as: EP3152561A1; DE102014214413A1

Abstract

The invention relates to a method for operating a gas sensor for detecting nitrogen oxides in a gas mixture, comprising the following steps: provision of a gas sensor with at least two electrodes made from the same material and arranged on an oxygen ion conductor, wherein both electrodes contact the gas mixture during operation. A polarization of the electrodes is subsequently carried out in such a way that a voltage is changed from a first voltage to a second voltage. The depolarization of the electrodes in the presence of the gas mixture is recorded. The electrodes are subsequently short-circuited in such a way that the first voltage is applied at the electrodes after the short-circuiting.

Description

description

Method for operating a gas sensor to improve the long-term stability of the gas sensor

The invention relates to a method for operating a gas ^sensor to improve the long ^- term stability of the gas ^¬ sors. Rising requirements with regard to the emission of exhaust gases and the efficiency in the operation of power plants, combustion plants, waste incineration plants, gas turbines and engines of all kinds can be countered by, among other things, determining the composition of gases in the respective plants during operation and evaluating them for improved operation becomes. This results in a need for sensors for Be ^{¬ determination} of components of a gas mixture. An example of this is the ever-increasing number of motor vehicles, for which stricter exhaust gas regulations have to be complied with in order to limit the damage to the environment and health caused by combustion exhaust gases. Besides schädli ^¬ chen exhaust gas components such as sulfur oxides and carbon dioxide it is increasingly becoming the group of nitrogen oxides, NOx briefly called to the fore. To wrestlers to comparable nitrogen oxide emissions, both technically and financially enormous effort will be ^¬ exaggerated. Examples include exhaust gas recirculation and selective catalytic reduction. To monitor the functi ^¬ on these procedures and to reduce operating costs, a continuous monitoring of the NOx concentration in the exhaust gas of the vehicle is necessary.

Especially in automotive applications is required in certain countries, that the functioning of the exhaust aftertreatment ^¬ is diagnosed in the vehicle itself. The car manufacturer must ensure that a randomly selected vehicle complies with emission regulations even after a long period of use. Especially for diesel vehicles is the monitoring of NOx storage catalytic converters and SCR (selective catalytic reduction) catalysts for reducing NOx emission is a task that is being intensively worked on. Known sensors for the measurement of NOx are optical or chemoluminescence-based systems. In addition to the high price be ^¬ these systems have the disadvantage that an extractive Mes ^¬ solution is necessary, ie a gas extraction is necessary. For ^many applications this is associated with great expense.

Known sensors that overcome these disadvantages are based on yttrium-stabilized zirconia (YSZ) and are similar in construction to the conventional lambda probe. There are thereby Elect ^¬ roden same material used tin for example, from plan-. In the classical operating principle of this sensor, a simultaneous measurement of oxygen and NOx is carried out in a two-chamber system. The disadvantage of this typi ^¬ rule Sensorprinizip but always a complex structure of the sensor and thus a high price. A second, newer option for a functional principle of the gas sensor is disclosed in the German patent application 102013222195.9. This gas sensor comprises an oxygen ion-conducting material and at least two electrodes arranged on the ion-conducting material. The electrodes are made of the same material in this gas sensor. The gas sensor is furthermore configured in such a way that, when the gas sensor is in operation, both ^{electrodes come} into contact with the gas mixture. It is therefore not necessary in this case that the gas sensor has a two-chamber system. This greatly simplifies the construction of the gas sensor. The measurement of these gas sensors is based on a polarization method in which the NOx concentration is measured by means of voltage pulses and a subsequent depolarization. Disadvantageously, these sensors show a change in both the polarization current and the discharge curves during prolonged use. This change results in a noticeable degradation of the sensor signal. It is therefore an object of the present invention to provide a method which overcomes the disadvantages mentioned and improves the long-term stability of the gas sensor. The object is achieved by a method according to claim 1.

The inventive method for operating a gas sensor for the detection of nitrogen oxides in a gas mixture comprises several steps. First of all, a gas sensor with at least two electrodes arranged on an oxygen ion conductor is provided from the same material, with both electrodes coming into contact with the gas mixture during operation of the gas sensor. The electrodes are polarized such that a voltage is changed from a first voltage to a second voltage. The depolarization of the electrodes in the presence of the gas mixture will be indicative ^¬ net. Subsequently, the electrodes are short-circuited in such a way that, after short-circuiting, the first voltage is applied to the electrodes.

Advantageously, prevents the short-circuiting the electric ^¬ of the inventive process that due to the accelerated ^¬ be depolarization of the electrodes in NO-containing atmosphere the voltage after depolarization is different in dependence of the NO concentration. These different voltage differences between the first and second voltage disadvantageously add up without the short-circuiting of the electrodes, so that the sensor signal degrades in the case of long-term ^measurements . This degradation of the sensor signal is advantageously reduced by the method according to the invention. In particular, the first voltage after short-circuiting is close to 0V.

In one embodiment of the invention, the short-circuiting lasts less than 20 ms. The polarization typically lasts from 100 ms to 1 s and the depolarization from 1 s -3 s. Advantage- Thus, short-circuiting hardly prolongs the analysis time of the gas sensor.

In a further embodiment of the invention, the method repeats cyclically with mutual polarity.

In a further embodiment of the invention, the method repeats cyclically with the same polarity. A first of the two electrodes then is advantageously a measuring electrode and a second of the two electrodes, a Polarisationselekt ^¬ rode. Advantageously can in this case material einspa ^¬ reindeer, since the second electrode only serves for the polarization and thus can be carried out significantly smaller. The first electrode can be optimized as a measuring electrode advantageous in terms of functionality, design and appropriate placement.

In a further advantageous embodiment of the invention, the polarization of the electrodes is carried out with a time constant voltage. Alternatively, the polarization can also be done with a time variable voltage.

The invention will now be explained in more detail with reference to drawings.

The figures show by way of example:

FIG. 1 shows a gas sensor for operation with the short-circuit method;

FIG. 2 shows a method for operating the gas sensor with short-circuit and mutual polarization;

3 shows a method for operating the gas sensor with short-circuit and one-sided polarization. FIG. 1 shows a gas sensor 10 in a highly schematic manner. This comprises a block 11 of yttrium-stabilized

Zirconia (YSZ) as an oxygen ion conductor. On a ^¬ ers th page of this block 11, a first platinum electrode 12 is arranged, while 13 is placed ^¬ introduced on a second side opposite the first side opposite a second platinum electrode. The platinum electrodes 12, 13 are electrically connected to a device 14 for generating and measuring voltage US. Not shown in Figure 1 are means by which the first gas sensor 10 can be placed in a space filled with the gas mixture to be measured space, for example, a flange for screwing into a correspondingly shaped opening. These means and the gas sensor 10 are designed so that after attaching the gas sensor 10, both the first and the second platinum electrode 12, 13 are in direct contact with the gas mixture. A contact of the block 11 with, for example, the ambient air, however, is thereby expediently avoided. In the operation of the gas sensor 10 is alternately by means of

Device 14, a voltage US between the platinum electrodes 12, 13 applied and measured the voltage waveform. The gas sensor in this example operates at a temperature of 350 ° C. An exemplary profile of the voltage US with the use of a short circuit is shown in FIG. FIG. 2 shows the voltage curve of a typical NO measurement of the gas sensor plotted over the time of the measurement. In the first measuring cycle 1, the electrodes of the gas sensor are brought from a first voltage U _M to a second voltage + Uo for a time trough to. + Uo in this example is 1 V, U _M ^¬ typi cally almost 0V. The time to is typically Zvi ^¬ rule 100 ms and 1 s. In this example, t0 = 0.4 s. Subsequently, the depolarization of the electrodes takes place in the NO-containing gas mixture. Typically, the measured value is the

Voltage U * recorded after a time t *. T * is ty ^¬ pisch legally 1s. After time ti, the short circuit occurs for a period of time t ₂ . The time ti is typically between 1 and 3 s. The short circuit itself, ie t ₂ , takes only 10 ms in this example. After this short circuit, the voltage is again U _M. Thereafter, the second measurement cycle 2 begins, wherein now the voltage of U _{M is brought} to -Uo. The polarization takes again to = 0.4 s. This is followed by the depolarization for a time ti and the short circuit for the time t ₂ .

FIG. 2 shows an alternative arrangement of the measurement cycles with only one-sided polarization. The first measurement cycle 1 is analogous to Figure 1 shown in the first measuring cycle 1. As a second measurement cycle 2 but then is followed by a measurement ^¬ cycle, in which in turn is carried by the polarization of a voltage U _M to + Uo. This is advantageously possible because the voltage U _{M has} been reached again by the short circuit and there is no residual voltage after time ti.

Claims

claims

1. A method for operating a gas sensor for the detection of nitrogen oxides in a gas mixture with the following steps:

Providing a gas sensor with at least two electrodes arranged on an oxygen ion conductor of the same material, wherein during operation of the gas ^sensor both electrodes come into contact with the gas mixture,

Polarizing the electrodes such that a voltage is changed from a first voltage to a second voltage,

Recording the depolarization of the electrodes in the presence of the gas mixture,

- Shorting the electrodes such that after short-circuiting the first voltage is applied to the electrodes.

2. The method according to any one of the preceding claims, wherein the short-circuiting takes less than 20 ms.

3. The method of claim 1 or 2, wherein the process repeats cyclically with mutual polarity.

4. The method of claim 1 or 2, wherein the method is repeated cyclically with the same polarity.

5. The method of claim 4, wherein a first of the two

Electrodes, a measuring electrode and a second two Elect ^¬ roden a polarizing electrode.

6. The method according to any one of the preceding claims, wherein the polarizing of the electrodes takes place with a time constant voltage.