EP3712501B1 - Method and device for regenerating an electrode for ionization measurement in a flame area of a burner - Google Patents

Description

The invention relates to a method and a device for regenerating an electrode for measuring the ionization in a flame area of a burner, in particular a burner operated with a fuel gas and air. Such measurements can be used to control and regulate many devices, in particular for hot water preparation or heating, and must then deliver values that are as reliable as possible over long periods of time.

The basic structure of burners with measuring systems for ionization measurement and their use to control a burner are for example from the EP 0 770 824 B1 and the EP 2 466 204 B1 known. This is particularly about regulating the ratio of air to fuel gas, the so-called lambda value.

From the WO2020 / 020494 A1 or the US2006 / 0257801A1 it is also already known to alternately use two different frequencies and / or voltages for generating an ionization signal. However, this is done to improve the regulation or monitoring of a heater and does not serve to regenerate an ionization electrode.

It is also known that ionization electrodes used for measurement are subject to high thermal and / or corrosive loads. In the case of the ionization electrodes, special metallic alloys are usually used, which also contain a proportion of aluminum. This forms when used during the operation of the Burner over time an aluminum oxide layer on the surface of the electrode, which protects against corrosion, but is electrically and thermally insulating. This in turn has the consequence that a measurement signal supplied by the electrode is weakened and possibly even completely suppressed in the cold state. The thermal insulation provided by the aluminum oxide layer also prevents the electrode from heating up quickly. The measurement signal is particularly severely impaired in a cold start phase on the one hand because of the only slowly rising temperature and on the other hand because of the electrical insulation. Often, as a result of alternating thermal loads, cracks form in the aluminum oxide layer afterwards, which can even lead to partial areas of the aluminum oxide flaking off, making the measurement signal stronger again. However, this does not help in all cases, so that a reliable measurement cannot always be ensured, especially in a cold start phase of the burner.

One mechanical approach would be to make at least a portion of the ionization electrode made of a material, e.g. B. a nickel-tungsten alloy, which does not form an oxide layer under the conditions in the flame area.

The object of the present invention is to at least partially solve the problems explained with reference to the prior art and, in particular, to create a method and a device which, in the case of an ionization electrode of any design, in particular also in the case of a conventional ionization electrode with aluminum content, regeneration and thus enable reliable measurement over long periods of time and especially during cold start phases of a burner.

A method, a device and a computer program product according to the independent claims serve to solve this problem. Advantageous further developments and refinements to which the invention is not limited, however, are specified in the respective dependent claims. The description, in particular in connection with the figure, specifies the subject matter of the invention and cites further exemplary embodiments.

The method proposed here for regenerating an ionization electrode for a measurement of the ionization in a flame area of a burner with a first alternating voltage of a first frequency applied to the ionization electrode with a second alternating voltage of a second frequency, which is higher, after the burner has been started for a predeterminable time interval Δt than the first frequency used for continuous operation.

The method could be carried out after each cold start, but it can be useful to carry out a regeneration only when certain predefinable criteria are reached. Since the application of the invention requires the presence of plasma in the area of the ionization electrode, the second alternating voltage with the second frequency should only be applied when the burner is running stably with its usual regulation of the lambda value after a cold start. Such cold starts are not very easy to carry out in terms of control technology, because a good signal is not always available from the ionization electrode, but they have already been controlled and / or regulated in a stable manner through the use of empirical values or similar measures. Suitable criteria for regeneration can e.g. B. derived from conventional control electronics, for example, if a drift of the ionization electrode has exceeded a threshold value. Of course, it is also possible to simply carry out a regeneration after a certain number of operating cycles or after certain time intervals. The burner is preferably restarted after a regeneration in order to enable the control electronics to be updated.

The second frequency of the second alternating voltage is preferably in the range from 10 to 100 MHz [MegaHertz], in particular in the range from 13.5 to 50 MHz.

The second alternating voltage is preferably in a range from 100 to 300 V [volts], particularly preferably between 100 and 200 V.

The first frequency of the first alternating voltage corresponds to the values suitable for such ionization measurements and is preferably in the range from 50 to 1000 Hz [Hertz], the voltage being between 100 and 300 V [volts]. In particular, an alternating voltage of 170 V and 107 Hz has proven to be suitable.

In a special embodiment of the invention there is a switching device which determines from sensor data or other data whether the burner is in a (predefined) cold or (predefined) warm state, and which, when started in a determined warm state, only uses the ionization electrode applied to the first alternating current of the first frequency. In other words, the application of the second alternating voltage and the second frequency to the ionization electrode during the ionization measurement can be suppressed despite the presence of the predeterminable criteria. This avoids unnecessary effort during a warm start and regeneration is made up at a suitable point in time.

An electronic module preferably evaluates the electrical current flowing through the ionization electrode and uses this measurement signal in a known manner to regulate the burner, specifically to regulate the air-to-fuel ratio (lambda value), in the case of a cold start a regulation and / or control is initially carried out until stable combustion is reached, is then replaced by a control for the predefinable time interval Δt and after the predefinable time interval Δt the first alternating current is regulated again at the first frequency. A “control” is understood here in particular to mean that the lambda value is specified or set without the actual lambda value being taken into account. A “regulation” is understood here in particular to mean that the lambda value is set, with this setting measuring the current ACTUAL lambda value using the ionization current and adjusting it to the specified target lambda value.

The predeterminable time interval Δt is preferably in the range from 10 to 100 s [seconds], preferably from 20 to 30 s.

Since no good measurement signal for the ionization current in the flame area can be derived from the second alternating current, it is advantageous not to select the time interval Δt too long, because optimal control of the combustion process may not be possible during this time.

It is particularly advantageous if the second alternating voltage and second frequency are selected to be so high during the predeterminable time interval Δt that the plasma generated by the combustion is additionally heated in the vicinity of the ionization electrode. On the one hand, this leads to a reduction in the thickness of an oxide layer on the ionization electrode due to the impact of fast ions and, on the other hand, promotes the oxide layer cracking open or flaking off due to thermal effects, so that aging of the ionization electrode is at least partially reversed.

The object of the invention is also achieved by a device, in particular for carrying out the method described above. For this purpose, there is an ionization electrode which can be arranged in a burner in such a way that it can measure an ionization current in a flame area when the burner is in operation. There is also a first Alternating current source for a first alternating current with a first frequency used for continuous operation and a second alternating current source for a second alternating current with a second, higher frequency. During operation, a switching device switches on the second alternating current source for a specifiable time interval (Δt) according to specifiable criteria. An electronics module is used to regulate the burner and is set up for regulation by means of an ionization current determined during operation of the second alternating current source, with the second alternating current source (6) being switched on for the predeterminable time interval during operation of the second alternating current source if the predeterminable criteria are present this regulation is switched off, and is replaced by a control according to specifiable criteria. Thus, after a cold start, the burner can be controlled for a short time according to empirical values, in which the ionization electrode is heated up and regenerated, while the usual regulation with the first alternating current for measuring the ionization is then resumed.

The second alternating current source is preferably set up for a frequency between 10 and 100 MHz, in particular for 13.5 to 50 MHz. Such a frequency range has proven to be suitable for rapid heating of the ionization electrode.

The first alternating current source is set up for a frequency between 50 and 1000 Hz and a voltage between 100 and 300 V. The first alternating current source does not have to differ from previously known alternating voltage sources for ionization measurements, but can also be designed differently through the additional use of the second current source.

As already mentioned, the second alternating current source should preferably be set up for a second frequency and a second alternating voltage which are so high that during their operation the plasma in the vicinity of the ionization electrode is heated to an excess temperature. It is precisely because of this that the use of the second alternating current source can develop its best effect.

The switching device is preferably connected to sensors, e.g. temperature sensors and / or data sources of the electronics module, which enable a distinction between the cold and warm state of the burner, so that the second alternating current source cannot be switched on or blocked when it is warm. In the simplest case, it is sufficient if the electronic module saves the time since the burner was last switched off. This information alone can be used to determine whether a cold start is present or not. Measured values of the temperature of the burner or the ionization electrode are of course more precise.

The switching device and / or the second alternating current source are preferably designed in such a way that the second alternating current source can only be switched on for a predeterminable time interval Δt of 10 to 100 s, preferably 20 to 30 s.

Despite the large frequency difference, the first alternating current source and the second alternating current source can be formed by a single alternating voltage source which can be changed or switched in frequency and voltage, which does not change anything in the other functions described.

Furthermore, a computer program product is also proposed which comprises commands which cause the device explained here to carry out the proposed method.

Fig. 1:

schematisch eine Vorrichtung zur Durchführung eines Verfahrens zur Messung der Ionisation in einem Flammenbereich eines Brenners mit einer lonisationselektrode.

A schematic embodiment of the invention, to which it is not limited, and the mode of operation of the method according to the invention will now be explained in detail with reference to the drawing. It shows:

Fig. 1:

schematically a device for performing a method for measuring the ionization in a flame area of a burner with an ionization electrode.

Figure 1 illustrates that a flame area 2 is formed in a burner 1 during operation, in which an ionization current is to be measured. For this purpose, an ionization electrode 3 protrudes into the flame area 2. A metallic component in the area of the entry of fuel gas and air into the burner 1 typically serves as the counter electrode 4. The counter electrode 4 is typically electronically connected to ground. After a cold start, ionization electrode 3 and counter electrode 4 are connected to a second alternating current source 5, which supplies an alternating current of high frequency, which leads to rapid heating of plasma in the vicinity of ionization electrode 3 and thus also ionization electrode 3 itself. After a short time interval .DELTA.t, a switching device 7 switches from the second alternating current source 5 back to a first alternating current source 6, the properties of which can correspond to known alternating current sources for ionization measurements. Their measurement signal can be fed via a measurement signal line 13 to an electronics module 10, which carries out a conventional control of the burner 1 with the measurement signal, which is now reliable. Such a regulation typically takes place in that commands are given to actuators in an air inlet 11 and / or fuel gas inlet 12 via an actuating signal line, so that an optimal mixture of air and fuel gas is always supplied. The switching device 7 is connected to at least one sensor 8 for determining the burner temperature and / or via a data line 9 to other data sources of the electronic module 10 in order to be able to decide whether a cold start is present or not. This data line 9 can also be used in the event of a cold start in order to provide the electronics module 10 with the information that a cold start has been initiated and that the combustion process should therefore not be regulated by means of ionization current, but rather briefly. Even while the Regeneration by means of the second alternating current is controlled according to empirical values.

Even in long-term operation, the present invention avoids malfunctions during cold starts of a burner due to measurement errors in the ionization current and enables regeneration of the ionization electrode through accelerated heating during a cold start at predefinable time intervals and / or according to predefinable criteria to ensure further interference-free control.

List of reference symbols

1

burner

2

Flame area

3

ionization electrode

4th

Counter electrode (ground)

5

first AC power source

6th

second AC power source

7th

Switching device

8th

Sensor (temperature)

9

Control line

10

Electronics module

11

Air inlet

12th

Fuel gas inlet

13th

Measuring signal line

14th

Control signal line

Claims (13)

1. Method for regenerating an ionisation electrode (3) for measuring the ionisation in a flame area (2) of a burner (1) having a first alternating voltage with a first frequency, wherein after a start of the burner (1), when predeterminable criteria are present for a predeterminable time interval (Δt), a second alternating voltage with the second frequency, which is higher than the frequency used for a continuous operation, is applied to the ionisation electrode (3).
2. Method according to claim 1, wherein the second frequency is in the range from 10 to 100 MHz.
3. Method according to claim 1 or 2, wherein the first frequency is in the range from 50 to 1000 Hz.
4. Method according to any of the preceding claims, wherein a switching device (7) is present which determines from sensor data or other data whether the burner (1) is in a cold or warm state, and at a start in a determined warm state, despite the presence of the predeterminable criteria, does not apply the second alternating current of the second frequency to the ionisation electrode (3).
5. Method according to any of the preceding claims, wherein an electronics module (10) evaluates the electrical current flowing through the ionisation electrode (3) and uses it for the closed-loop controlling of the burner (1), and specifically for the closed-loop controlling of the ratio of air to fuel (lambda value), and wherein in the case of a start in cold state and in the presence of the predeterminable criteria, the closed-loop controlling is replaced by a controller for the predeterminable time interval (Δt) and after the predeterminable time interval (Δt) a closed-loop controlling takes place again on the basis of the first alternating current with the first frequency.
6. Method according to any of the preceding claims, wherein the predeterminable time interval (Δt) is in the range from 10 to 100 s.
7. Method according to any of the preceding claims, wherein during the predeterminable time interval (Δt) the second alternating voltage and second frequency are selected to be so high that plasma in the vicinity of the ionisation electrode (3) is heated to an overtemperature.
8. Device having an ionisation electrode (3) which can be arranged in a burner (1) such that during operation of the burner (1) it is able to measure an ionisation current in a flame area (2), having a first alternating current source (5) for a first alternating current with a first frequency used for a continuous operation, having a second alternating current source (6) for a second alternating current with a second, higher frequency and a switching device (7) which during operation switches on the second alternating current source (6) for a predeterminable time interval (Δt) according to predeterminable criterion, and wherein an electronics module (10) is present for the closed-loop controlling of the burner (1), which electronics module is configured for a close-loop controlling by means of an ionisation current determined during operation of the first alternating current source (6), and in order that, in the presence of the predeterminable criterion, the second alternating current source (6) is switched on for the predeterminable time interval (Δt), during the operation of the second alternating current source (5) this closed-loop controlling is switched off, and is replaced by a controller according to predeterminable criteria.
9. Device according to claim 8, wherein the second alternating current source (5) is configured for a frequency between 10 and 100 MHz.
10. Device according to claim 8 or 9, wherein the first alternating current source (6) is configured for a frequency between 50 and 1000 Hz and a voltage between 100 and 300 V.
11. Device according to any of claims 8 to 10, wherein the switching device (7) is connected with sensors (8) and/or data sources of the electronic module (10) which facilitate a differentiation between cold and warm state of the burner (1), such that the second alternating current source (5) cannot be switched on in the warm state.
12. Device according to any of claims 8 to 11, wherein the first (5) and second (6) alternating current source are formed by one single alternating voltage source (5, 6) which can be changed or switched in frequency and voltage.
13. Computer programme product comprising commands which cause the device according to any of claims 8 to 12 to carry out the method according to any of claims 1 to 7.

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