EP4089329A1 - Method and device for safe restart of a burner operated with a high proportion of hydrogen - Google Patents

Abstract

The invention relates to a method and a device for operating a burner (3) with air and a fuel gas containing more than 95% by volume of hydrogen, the presence of a flame (16) in a combustion chamber (15) being detected by means of at least one temperature sensor ( 10, 11) is monitored, the measured value of which is forwarded continuously or quasi-continuously to an evaluation and control unit (7), the evaluation and control unit (7) monitoring the measured value and its behavior over time and comparing it with predefinable target ranges, and in the event of deviations from one of the target ranges, the fuel gas valve (5) closes, with at least one of the following measures also being taken when the burner (3) restarts after it has been switched off: a) restarting is only permitted after a specifiable time interval has elapsed, b) it is stopped before the Restart air passed through the combustion chamber (15), c) the restart takes place with a higher supply of air and fuel gas as a cold burner start (3). The present invention allows heaters (1) that are operated with hydrogen as the fuel gas to be restarted safely after a shortened blocking time without impairing the important safety function of at least one temperature sensor (10, 11) as a flame monitor.

Description

The invention relates to the safe restart (warm start) shortly after switching off a burner that is operated with a fuel gas that contains a very high proportion of hydrogen, for example more than 95% by volume [percent by volume], in particular 98 to 100%. In particular, it is about heaters or condensing boilers, for example for heating domestic water and/or heating buildings.

In order to reduce carbon dioxide emissions in the future, efforts are being made to operate fuel gas supply networks with decarbonized gases, one way of which is the use of hydrogen as the main component. In the combustion of gases with a high hydrogen content, however, there are various differences compared to the combustion of conventional fuel gases, such as e.g. g. natural gas.

For safe operation of a burner it is z. B. necessary to ensure that there is always a flame when fuel gas is supplied to the burner. If the flame goes out unnoticed, fuel gas can escape into a combustion chamber and the surrounding area and possibly even form an explosive mixture, which must be avoided.

When using aliphatic fuel gases (e.g. methane, propane, butane), a so-called ionization measuring device is usually used to monitor the presence of a flame (flame detector) and to regulate the combustion. The presence of an ionization signal indicates that a flame is present. Measured values derived from the ionization signal in conjunction with the measurement of the mass flow of air supplied allow the mixing ratio of fuel gas to air (lambda value) to be controlled by adjusting a fuel gas valve.

However, all processes in which an ionization current is measured in the flame do not work with the combustion of hydrogen as fuel gas (not even with only small admixtures of aliphatic fuels of a few percent). In addition, when hydrogen is burned, the flame is almost invisible to the human eye and cannot be detected with simple optical sensors. Detection in the ultraviolet range is possible, but very complex. Finally, for safe operation, it must also be taken into account that even a failure or malfunction of individual parts, e.g. B. of sensors, must not lead to dangerous situations.

However, methods for operating a burner with air and a fuel gas containing more than 95% by volume, in particular 98 to 100%, hydrogen have already been proposed, which monitor the presence of a flame in a combustion chamber by means of at least one temperature sensor whose measured value is forwarded continuously or quasi-continuously to an evaluation and control unit, wherein the evaluation and control unit monitors the measured value and its behavior over time and compares it with definable target ranges, and stops the fuel gas supply if there are deviations from one of the target ranges.

For this purpose, calibration data or characteristic diagrams can be stored or made available in the evaluation and control unit, with which measured values and their time derivatives can be compared.

If a flame ignites properly, the temperature sensor measures an increase in temperature. If the flame goes out, a drop in temperature is measured. If the temperature drop per unit of time exceeds a previously z. B. experimentally determined value, this is interpreted as the extinguishing of the flame by the evaluation and control unit and the fuel gas valve is closed. The procedure is reversed when the burner is ignited. If there is no rapid rise in temperature at the temperature sensor after ignition, misfiring can be concluded. If the temperature rises at the expected speed, the ignition process is rated as successful. Temperature monitoring can ensure that hydrogen is burned. This always applies in particular when the temperature in the combustion chamber is > 833 K [Kelvin] (self-ignition temperature of hydrogen in air) and the air ratio is > 1 (lambda value > 1). The combustion air ratio (lambda) puts the air mass actually available in relation to the minimum necessary air mass that is theoretically required for stoichiometrically complete combustion. Temperature monitoring with the function of a flame monitor can therefore ensure that combustion takes place or that the device is switched off if the flame goes out.

For reasons of redundancy and for a higher safety standard, two or more temperature sensors can be present, with the measured values being compared with one another and a fault message and/or shutdown being triggered if there are deviations above a definable threshold value. The measurement signals are separated by Signal lines transmitted to the evaluation and control unit. The measured temperatures are compared in the evaluation and control unit. If the temperature difference between the temperature sensors exceeds a defined threshold value, this is an indication that at least one of the temperature sensors (or its signal lines) is defective or thermally degraded. The temperature sensors are preferably identical in construction and can be designed, for example, as thermoelectric temperature sensors (thermocouples) or resistance thermometers or gas or liquid thermometers. Expansion thermometers can also be considered.

A corresponding device for operating a burner with combustion air and a fuel gas containing more than 95% by volume hydrogen was also proposed, which has at least one temperature sensor as a flame monitor in a combustion chamber, which is connected to an evaluation and control unit via a signal line .

As described above, flame monitoring can already be carried out with this equipment. Depending on the design and requirements, the temperature sensor can be arranged in the area of a combustion flame or at a distance from it.

For safety reasons, two or more temperature sensors are often used as flame monitors, which are connected to an evaluation and control unit via signal lines. In this way, errors in one of the temperature sensors or their signal lines can be detected at an early stage and trigger the fuel gas supply to be switched off.

The system described so far can reliably fulfill its safety function as a flame monitor when a burner is started if it is a so-called cold start, i.e. the burner and the combustion chamber are at a relatively low temperature (either close to the ambient temperature or close to a temperature of a connected heating circuit ) is located. The situation can be somewhat different with a restart, a so-called warm start, i.e. if the burner was only switched off a short time ago and is still at an elevated temperature. Temperatures and temperature gradients that are difficult to define then prevail in the combustion chamber and the surrounding components, so that the temperature sensor(s) does not measure the typical temperature curves described when the burner is restarted and therefore cannot reliably fulfill the function of a flame monitor.

The object of the present invention is therefore to at least partially solve the problems described with reference to the prior art and in particular to create a method and a device for safe operation even with a warm start of a burner with air and a fuel gas that contains more than 95 vol -%, in particular more than 98%, contains hydrogen, as well as an associated computer program product for controlling and regulating when the burner is started.

A method and a device as well as a computer program product according to the independent claims serve to solve this task. Advantageous refinements and developments of the invention are specified in the respective dependent claims. The description, particularly in connection with the drawing, illustrates the invention and specifies preferred exemplary embodiments.

1. a. der Wiederstart wird erst nach Ablauf eines vorgebbaren Zeitintervalls erlaubt,
2. b. es wird vor dem Wiederstart Luft durch den Verbrennungsraum geleitet,
3. c. der Wiederstart erfolgt mit einer höheren Zufuhr an Luft und Brenngas als ein Start bei kaltem Brenner.

A method for operating a burner with air and a fuel gas that contains more than 95% by volume of hydrogen helps to solve the task, with the presence of a flame in a combustion chamber being monitored by means of at least one temperature sensor whose measured value is continuously or quasi-continuously connected to a evaluation and control unit, the evaluation and control unit monitoring the measured value and its behavior over time and comparing it with predefinable target ranges, and if there are deviations from one of the target ranges, the fuel gas valve closes, with at least one of the the following measures are taken:

1. a. the restart is only permitted after a specified time interval has elapsed,
2. b. air is passed through the combustion chamber before restarting,
3. c. the restart takes place with a higher supply of air and fuel gas than a start with a cold burner.

A single temperature sensor or a plurality of temperature sensors or the measured values obtained with them can be used here. The system of temperature sensor and evaluation and control unit is set up in such a way that they are equipped with a continuous or quasi-continuous data connection. The measured value data queried or generated in this way, their progression over time and/or their degree of change can be cumulated/calculated/etc. in real time or if necessary. or possibly compared with a time delay with predefinable target ranges. These target ranges can be limited by a lower and/or upper threshold value. Such a target range can be (rigidly) stored, but it is also possible to adapt the target range to current operating conditions, if necessary, or to calculate it, taking this into account. In the event of deviations from one of the target ranges, possibly also with a specified number of or falls below and/or possibly a predetermined tolerance, the fuel gas valve can be closed automatically, which is referred to here in particular as "shutdown".

All three measures a., b. and/or c. for or before the restart (re-ignition) of the burner can already individually bring about a safe restart. If you wait e.g. B. just as long (time interval corresponds to an empirical value) until the temperature is at a level that corresponds approximately to that of a cold start, so any risk can be avoided. Such a blocking period can be useful for various reasons. A restart can also be accelerated, for example, by directing (cold) air through the combustion chamber, in particular by means of the burner fan, as a result of which the conditions for a cold start are achieved again much more quickly. This also allows the combustion chamber to be flushed and any fuel gas residues to be removed. In addition, it may be possible to restart the burner with a higher output (compared to the initial start or a previous restart) by means of a higher supply of air and fuel gas, which means that characteristic temperature profiles that are sufficient for flame monitoring can be achieved, even if the initial temperature does not yet correspond to that of a cold start. Of course, a combination of the individual measures is possible and often useful.

It should be mentioned that measures a., b- and/or c. preferably only be taken if the measured value of the temperature sensor for a desired restart is above a (restart) threshold value. Otherwise, a safe start is possible without further measures. This procedure often leads to a quicker restart than waiting for a blocking time chosen for security reasons.

It is particularly advantageous if the type and/or duration of measures a., b. and/or c. be made dependent on the measured value of the temperature sensor (at the time of a desired restart). Depending on the temperature in the combustion chamber, different measures or combinations of measures can then be taken, these generally being selected in such a way that a restart is permitted as quickly as possible. This can also be accelerated by (further) observing the temperature in the combustion chamber during the measures, so that a restart can take place immediately if a threshold value is undershot.

In particular, due to the possibility of restarting with increased power, there is considerable scope for accelerating a restart of the burner, which can be made dynamically dependent on the measured temperature and other factors that can be predetermined by an evaluation and control unit.

In particular, air (at ambient temperature) can be conducted through the combustion chamber until the measured value of the temperature in the combustion chamber falls below a (restart) threshold value.

For reasons of safety or redundancy, there are preferably at least two temperature sensors whose measured values are compared with one another, with deviations above a definable differential value triggering at least one error message or a shutdown. Especially with such equipment, it makes sense to bring about defined temperature conditions for a restart, because otherwise deviations could be interpreted as errors, e.g. B. based only on an increased output temperature of one of the sensors.

1. a. der Wiederstart wird erst nach Ablauf eines vorgebbaren Zeitintervalls erlaubt,
2. b. es wird vor dem Wiederstart Luft durch den Verbrennungsraum geleitet,
3. c. der Wiederstart erfolgt mit einer höheren Zufuhr an Luft und Brenngas als ein Start bei kaltem Brenner.

A device for operating a burner with combustion air and a fuel gas that contains more than 95% by volume of hydrogen also serves to solve the task, with at least one temperature sensor being present in a combustion chamber as a flame monitor, which is connected to an evaluation and control unit via a signal line is connected and the evaluation and control unit is set up to carry out at least one of the following measures when the burner restarts after it has been switched off, depending on a measured value of the temperature sensor:

1. a. the restart is only permitted after a specified time interval has elapsed,
2. b. air is passed through the combustion chamber before restarting,
3. c. the restart takes place with a higher supply of air and fuel gas than a start with a cold burner.

Two or more temperature sensors are preferably present as flame monitors and are connected to the evaluation and control unit by means of separate signal lines.

The solution proposed here also relates to a computer program product comprising instructions which cause the device described to carry out the method described. In general, an evaluation and control unit will contain at least one microprocessor and data memory in order to carry out the processes described. A suitable program is required for this, which can also be updated if necessary, as well as stored calibration data.

To characterize the device, full reference can be made to the explanations of the method, and vice versa.

Fig. 1

schematisch den Aufbau eines Heizgerätes mit Sensorik und Auswerte- und Regeleinheit

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

1

schematic of the structure of a heater with sensors and evaluation and control unit

1 1 shows a heater 1 designed for operation with hydrogen as the fuel gas, with associated sensors 10, 11. The heater 1 has a blower 2, which supplies a burner 3 with air from an air supply 4. Fuel gas (here hydrogen or a fuel gas mixture consisting predominantly of hydrogen) from a fuel gas supply 6 is added to the air via a fuel gas valve 5 . An evaluation and control unit 7 controls the blower 2 and the fuel gas valve 5 via control lines 13 in such a way that a mixture suitable for ignition and/or continuous operation is produced. When this mixture is burned in a combustion chamber 15, flames 16 are produced, the presence of which is monitored by at least one temperature sensor 10. A further temperature sensor 11 is present in the present exemplary embodiment, as a result of which the accuracy, availability and safety of the system is increased. Each temperature sensor 10, 11 is connected to the evaluation and control unit 7 via a signal line 12. The combustion chamber 15 is surrounded by a housing 8 in which heat exchanger surfaces are only indicated here. Combustion gases produced are discharged to the environment via an exhaust system 9 . To ignite the combustion, there is an ignition device 17 which is connected to the evaluation and control unit 7 by means of an ignition line 14 . If the burner 3 was in operation for a while and was then switched off, higher temperatures prevail within the housing 8, in particular in the combustion chamber 15, than in the case of a cold start after the burner has been switched off for a long time. the The temperatures measured by the temperature sensors 10, 11 are not defined so precisely because of their heat storage capacity and the surrounding components that they could be used directly and safely as a flame monitor for a restart (warm start). If the temperature sensors 10, 11 indicate a temperature above a threshold value, measures as described above must therefore be taken to reduce the temperature or change the output of the heater 1 when restarting. Waiting for a blocking time and/or flushing the combustion chamber 15 with air are suitable, it being most favorable to observe the effect of these measures on the temperature and to trigger a restart if the temperature falls below the threshold value. This can be taken over by the evaluation and control unit 7, which is controlled by a suitable algorithm for this purpose. Combinations of the measures described, in particular with an increase in power when restarting, are possible.

The present invention allows heaters that are operated with hydrogen as the fuel gas to be restarted safely after a shortened blocking time, without impairing the important safety function of at least one temperature sensor as a flame monitor.

Reference List

1

heater

2

fan

3

burner

4

air supply

5

fuel gas valve

6

fuel gas supply

7

Evaluation and control unit

8th

Housing

9

exhaust system

10

temperature sensor

11

Another temperature sensor

12

signal lines

13

control lines

14

ignition wire

15

combustion chamber

16

flame

17

ignition device

Claims (8)

Method for operating a burner (3) with air and a fuel gas containing more than 95% by volume of hydrogen, the presence of a flame (16) in a combustion chamber (15) being monitored by means of at least one temperature sensor (10, 11), whose measured value is forwarded continuously or quasi-continuously to an evaluation and control unit (7), with the evaluation and control unit (7) monitoring the measured value and its behavior over time and comparing it with predeterminable target ranges, and in the event of deviations from one of the target ranges, the fuel gas valve (5 ) closes, with at least one of the following measures being taken when the burner (3) restarts after it has been switched off: a. the restart is only permitted after a specified time interval has elapsed, b. air is passed through the combustion chamber (15) before restarting, c. the restart takes place with a higher supply of air and fuel gas than a start with a cold burner (3). Method according to Claim 1, in which the measures are only taken if the measured value of the temperature sensor (10, 11) is above a threshold value. Method according to Claim 1, in which at least the type or duration of the measures are made dependent on the measured value of the temperature sensor (10, 11). Method according to one of the preceding claims, in which air is passed through the combustion chamber (15) until the measured temperature falls below a threshold value. Method according to one of the preceding claims, wherein two temperature sensors (10, 11) are present, the measured values of which are compared with one another and at least one error message or shutdown is triggered in the event of deviations above a predeterminable differential value. Device for operating a burner (3) with combustion air and a fuel gas that contains more than 95% by volume of hydrogen, with at least one temperature sensor (10, 11) as a flame monitor being present in a combustion chamber (15), which is connected via a signal line (12 ) is connected to an evaluation and control unit (7) and wherein the evaluation and control unit (7) is set up, when the burner (3) restarts after it has been switched off, depending on a measured value of the temperature sensor (10, 11), at least one carry out the following measures: a. the restart is only permitted after a specified time interval has elapsed, b. air is passed through the combustion chamber (15) before restarting, c. the restart takes place with a higher supply of air and fuel gas than a start with a cold burner. Device according to Claim 6, in which two or more temperature sensors (10, 11) are present as flame monitors and are connected to the evaluation and control unit (7) by means of separate signal lines (12). A computer program product comprising instructions which cause the device according to any one of claims 6 or 7 to carry out the method according to any one of claims 1 to 5.

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