METHOD AND DEVICE FOR THE SAFE RESTARTING OF A HIGH-HYDROGEN BURNER

Description

[0001] The invention relates to the safe restart (warm start) shortly after shutdown of a burner that is operated with a fuel gas containing a very high proportion of hydrogen, for example more than 95 vol% [volume percent], in particular 98 to 100%. It is particularly relevant for heating appliances or condensing boilers, for example for heating domestic hot water and / or heating buildings.

[0002] To reduce carbon dioxide emissions in the future, the aim is to operate fuel gas supply networks with decarbonized gases, one possibility being the use of hydrogen as the main component. However, several differences must be considered when burning gases containing high levels of hydrogen compared to the combustion of conventional fuel gases, such as natural gas.

[0003] For the safe operation of a burner, it is essential, for example, to ensure that a flame is always present when fuel gas is supplied to the burner. If the flame goes out unnoticed, fuel gas can escape into the combustion chamber and the surrounding area, potentially even forming an explosive mixture, which must be avoided.

[0004] From DE 101 53 643 A1, a method is known by which the oxidizable components in the exhaust gas of a combustion plant can be detected, and at the same time, the burner flame can be monitored and the combustion optimized. For this purpose, the flame, the oxidizable components of the exhaust gas, and its temperature are monitored simultaneously by a sensor, whereby the monitoring is carried out directly in the exhaust gas stream or in a partial stream representing the exhaust gas stream. The distance between the burner and the sensor is selected such that the effect of the exhaust gas on the sensor's temperature and the associated sensor response time is only a few seconds.

[0005] When using aliphatic fuel gases (e.g., methane, propane, butane), an ionization meter is typically used to monitor the presence of a flame (flame detector) and to control combustion. The presence of an ionization signal indicates that a flame is present. Measurements derived from this ionization signal, in conjunction with the mass flow rate of supplied air, allow for the control of the fuel gas-to-air mixture ratio (lambda value) by adjusting a fuel gas valve.

[0006] All processes that measure an ionization current in the flame are ineffective when hydrogen is used as a fuel gas (even with only small admixtures of aliphatic fuels of a few percent). Furthermore, the flame during hydrogen combustion 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 ensured that even a failure or malfunction of individual components, such as sensors, does not lead to dangerous situations.

[0007] However, methods have already been proposed for operating a burner with air and a fuel gas containing more than 95 vol%, in particular 98 to 100%, hydrogen, which monitor the presence of a flame in a combustion chamber by means of at least one temperature sensor, the measured value of which is continuously or quasi-continuously forwarded to an evaluation and control unit, wherein the evaluation and control unit monitors the measured value and its temporal behavior and compares it with predefinable target ranges, and terminates the fuel gas supply in case of deviations from one of the target ranges.

[0008] For this purpose, calibration data or characteristic curves can be stored or made available in the evaluation and control unit, with which measured values ​​and their time derivatives can be compared.

[0009] If a flame ignites correctly, the temperature sensor measures a temperature increase. If the flame goes out, a temperature drop is measured. If the temperature decrease per unit of time exceeds a previously determined value (e.g., determined experimentally), the evaluation and control unit interprets this as the flame going out and closes the fuel gas valve. The process is reversed when the burner is ignited. If there is no rapid temperature increase at the temperature sensor after ignition, a misfire can be assumed. If the temperature rises at the expected rate, the ignition process is considered successful. Temperature monitoring ensures that hydrogen is combusted. This is particularly important when the temperature in the combustion chamber is > 833 K [Kelvin] (auto-ignition temperature of hydrogen in air) and the air-fuel ratio is > 1 (lambda value > 1).The air-fuel ratio (lambda) relates the actual available air mass to the minimum air mass theoretically required for stoichiometrically complete combustion. Temperature monitoring, functioning as a flame sensor, ensures that combustion occurs and shuts down the system if the flame goes out.

[0010] For redundancy and enhanced safety, two or more temperature sensors can be used. Their readings are compared, and if deviations exceed a predefined threshold, a fault message and / or shutdown is triggered. The measurement signals are transmitted to the evaluation and control unit via separate signal lines. The measured temperatures are compared in the evaluation and control unit. If the temperature difference between the temperature sensors exceeds a defined threshold, this indicates that at least one of the temperature sensors (or its signal lines) is defective or thermally degraded. Preferably, the temperature sensors are identical and can be, for example, thermoelectric temperature sensors (thermocouples), resistance thermometers, or gas or liquid thermometers. Expansion thermometers are also a possible option.

[0011] A corresponding device for operating a burner with combustion air and a fuel gas containing more than 95 vol% 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.

[0012] Even with this equipment, flame monitoring can be performed as described above. Depending on the design and requirements, the temperature sensor can be positioned within the combustion flame or at a distance from it.

[0013] For safety reasons, two or more temperature sensors are often used as flame detectors, connected to an evaluation and control unit via signal lines. This allows faults in one of the temperature sensors or their signal lines to be detected early and trigger a shutdown of the fuel gas supply.

[0014] The system described so far can reliably fulfill its safety function as a flame monitor during burner start-up if it is a so-called cold start, meaning the burner and combustion chamber are at a relatively low temperature (either close to ambient temperature or close to the temperature of a connected heating circuit). The situation can be somewhat different during a restart, a so-called warm start, if the burner has only recently been switched off and is still at an elevated temperature. In this case, undefined temperatures and temperature gradients prevail in the combustion chamber and surrounding components, so that the temperature sensor(s) cannot measure the typical temperature profiles described above when the burner restarts and therefore cannot reliably fulfill its flame monitor function.

[0015] 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 during a hot start of a burner with air and a fuel gas containing more than 95 vol%, in particular more than 98%, hydrogen, as well as an associated computer program product for the control and regulation during the start of the burner.

[0016] To solve this problem, a method, a device, and a computer program product according to the independent claims are provided. Advantageous embodiments and further developments of the invention are specified in the respective dependent claims. The description, particularly in conjunction with the drawing, illustrates the invention and provides preferred embodiments.

[0017] A method for operating a burner with air and a fuel gas containing more than 95 vol% hydrogen contributes to solving the problem, wherein the presence of a flame in a combustion chamber is monitored by means of at least one temperature sensor, the measured value of which is continuously or quasi-continuously transmitted to an evaluation and control unit, wherein the evaluation and control unit monitors the measured value and its temporal behavior and compares it with predefinable target ranges, and closes the fuel gas valve in case of deviations from one of the target ranges, wherein at least one of the following measures is taken when the burner is restarted after a shutdown: a. Restarting is only permitted after a predetermined time interval, the time interval being dimensioned such that the temperature in the combustion chamber is at a level equivalent to that of a cold start; b. Cold air is passed through the combustion chamber before restarting until the temperature in the combustion chamber is at a level equivalent to that of a cold start; c. Restarting is carried out with a higher supply of air and fuel gas than a start with a cold burner.

[0018] This system can utilize a single temperature sensor or multiple temperature sensors and the measured values ​​they generate. The system, consisting of a temperature sensor and an evaluation and control unit, is configured with a continuous or quasi-continuous data connection. The queried or generated measurement data, its temporal progression, and / or rate of change can be compared in real time, or optionally cumulatively / calculated / etc., or with a time delay, against predefined target ranges. These target ranges can be limited by a lower and / or upper threshold. Such a target range can be stored (statically), but it is also possible to adjust the target range to current operating conditions or calculate lower limits. Deviations from one of the target ranges, possibly with a predefined number of exceedances or falls below the target, and / or other relevant factors, can be addressed.If a predetermined tolerance is exceeded, the fuel gas valve can be automatically closed, which is referred to here specifically as "shutdown".

[0019] All three measures a., b., and / or c. for or before restarting (reigniting) the burner can individually ensure a safe restart. For example, simply waiting (the time interval is based on experience) until the temperature reaches a level similar to that of a cold start can eliminate any risk. Such a waiting period can be beneficial for various reasons. For instance, restarting can be accelerated by directing (cold) air, particularly via the burner's fan, through the combustion chamber, thus restoring cold start conditions much more quickly. This also purges the combustion chamber and removes any remaining fuel gases.Furthermore, it may be possible to restart the burner at a higher power output (compared to the initial start or a previous restart) by increasing the supply of air and fuel gas. This allows for characteristic temperature profiles sufficient for flame monitoring, even if the initial temperature does not yet match that of a cold start. Naturally, a combination of these measures is possible and often beneficial.

[0020] It should be noted that measures a., b., and / or c. are preferably only taken if the temperature sensor reading for a desired restart is above a (restart) threshold. Otherwise, a safe restart is possible without further measures. This approach often leads to a faster restart than waiting for a longer lockout period chosen for safety reasons.

[0021] It is particularly advantageous if the type and / or duration of measures a., b., and / or c. are dependent on the temperature sensor reading (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, generally selected to allow a restart as quickly as possible. This can also be accelerated by (continuing to) monitor the temperature in the combustion chamber during the measures, so that a restart can be initiated immediately if a threshold value is undershot.

[0022] In particular, the possibility of restarting with increased power creates considerable scope for accelerating a burner restart, which can be dynamically dependent on the measured temperature and other factors that can be specified to an evaluation and control unit.

[0023] In particular, air (at ambient temperature) can be passed through the combustion chamber until the measured temperature in the combustion chamber falls below a (restart) threshold.

[0024] For safety or redundancy reasons, preferably at least two temperature sensors are present, the measured values ​​of which are compared with each other. If deviations exceed a predefined difference value, at least a fault message or a shutdown is triggered. With such a setup, it is particularly useful to establish defined temperature conditions for a restart, because otherwise deviations could be interpreted as errors, even if they are only due to an elevated initial temperature of one of the sensors.

[0025] To solve the problem, a device for operating a burner with combustion air and a fuel gas containing more than 95 vol% hydrogen is also used, wherein at least one temperature sensor is provided in a combustion chamber as a flame monitor, which is connected via a signal line to an evaluation and control unit, and wherein the evaluation and control unit is configured to perform at least one of the following measures when the burner is restarted after a shutdown, depending on a measured value from the temperature sensor: a. Restarting is only permitted after a predetermined time interval, the time interval being dimensioned such that the temperature in the combustion chamber is at a level equivalent to that of a cold start; b. Air is passed through the combustion chamber before restarting until the temperature in the combustion chamber is at a level equivalent to that of a cold start; c. Restarting is carried out with a higher supply of air and fuel gas than a start with a cold burner.

[0026] Preferably, two or more temperature sensors are used as flame detectors and are connected to the evaluation and control unit via separate signal lines.

[0027] The solution proposed here also involves a computer program product, comprising instructions that cause the described device to execute the described procedure. Generally, an evaluation and control unit will contain at least one microprocessor and data memory to perform the described operations. This requires a suitable program that can be updated as needed, as well as stored calibration data.

[0028] For a characterization of the device, full reference can be made to the explanations of the procedure, and vice versa.

[0029] A schematic embodiment of the invention, to which it is not limited, and the functioning of the method according to the invention are explained in more detail below with reference to the drawing. It shows: Fig. 1 schematically shows the structure of a heating device with sensors and evaluation and control unit.

[0030] Fig. 1Figure 1 shows a heating appliance 1 designed for operation with hydrogen as fuel gas, including associated sensors 10 and 11. The heating appliance 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 mixed with 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 so 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. In the present embodiment, a further temperature sensor 11 is provided, which increases the accuracy, availability, and safety of the system.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, which contains only indicated heat exchanger surfaces. Combustion gases are discharged to the environment via an exhaust system 9. An ignition device 17 is provided for igniting the combustion and is connected to the evaluation and control unit 7 via an ignition line 14. If the burner 3 has been in operation for a period of time and is then switched off, higher temperatures prevail inside the housing 8, particularly in the combustion chamber 15, than during a cold start after a prolonged shutdown of the burner. Due to their heat storage capacity and the surrounding components, the temperatures measured by the temperature sensors 10, 11 are not precisely defined enough to be reliably used directly as flame detectors for a restart (warm start).If the temperature sensors 10, 11 indicate a temperature above a threshold value, measures as described above must be taken to reduce the temperature or change the power output of the heating unit 1 upon restart. Waiting for a lockout period and / or purging the combustion chamber 15 with air are suitable options. It is most advantageous to monitor the effect of these measures on the temperature and trigger a restart if the temperature falls below the threshold value. This can be handled by the evaluation and control unit 7, which is controlled by a suitable algorithm. Combinations of the described measures, particularly with an increase in power output upon restart, are possible.

[0031] The present invention makes it possible to carry out a safe restart after a shortened lockout period in heating appliances that are operated with hydrogen as fuel gas, without impairing the important safety function of at least one temperature sensor as a flame monitor. Reference symbol list

[0032] 1 Heater 2 Blower 3 Burner 4 Air supply 5 Fuel gas valve 6 Fuel gas supply 7 Evaluation and control unit 8 Housing 9 Exhaust system 10 Temperature sensor 11 Additional temperature sensor 12 Signal lines 13 Control lines 14 Ignition line 15 Combustion chamber 16 Flame 17 Ignition device

Claims

1. Method for operating a burner (3) with air and a fuel gas containing more than 95 vol.% hydrogen, wherein the presence of a flame (16) in a combustion chamber (15) is monitored by means of at least one temperature sensor (10, 11), the measured value of which is continuously or quasi-continuously transmitted to an evaluation and control unit (7), wherein the evaluation and control unit (7) monitors the measured value and its temporal behaviour and compares it with predeterminable target ranges, and in the event of deviations from one of the target ranges, closes the fuel gas valve (5) closes, characterised in that, when the burner (3) is restarted after operation and shutdown, at least one of the following measures is taken: a. the restart is only permitted after a preset time interval has elapsed, whereby the time interval is set so that the temperature in the combustion chamber (15) is at a level corresponding to that during a cold start , b. before restarting, cold air is passed through the combustion chamber (15) until the temperature in the combustion chamber (15) is at a level corresponding to that during a cold start , c. the restart is performed with a higher supply of air and fuel gas than a start with a cold burner (3).

2. Method according to claim 1, wherein the measures are only taken if the measured value of the temperature sensor (10, 11) is above a threshold value.

3. Method according to claim 1, wherein at least the type or duration of the measures is made dependent on the measured value of the temperature sensor (10, 11).

4. Method according to one of the preceding claims, wherein air is fed through the combustion chamber (15) until the measured value of the temperature falls below a threshold value.

5. Method according to one of the preceding claims, wherein two temperature sensors (10, 11) are provided, the measured values of which are compared with each other and, in the event of deviations above a predeterminable difference value, at least one fault message or a shutdown is triggered.

6. Device for operating a burner (3) with combustion air and a fuel gas containing more than 95 vol.% hydrogen, wherein at least one temperature sensor (10, 11) is provided in a combustion chamber (15) as a flame monitor, which is connected to an evaluation and control unit (7) via a signal line (12), and characterised in that the evaluation and control unit (7) is designed to perform at least one of the following measures when the burner (3) is restarted after operation and shutdown, depending on a measured value of the temperature sensor (10, 11), to perform at least one of the following measures: a. the restart is only permitted after a preset time interval has elapsed, the time interval being such that the temperature in the combustion chamber (15) is at a level corresponding to that during a cold start; b. before restarting, air is passed through the combustion chamber (15) until the temperature in the combustion chamber (15) is at a level corresponding to that during a cold start; c. the restart is performed with a higher supply of air and fuel gas than a start with a cold burner.

7. Device according to claim 6, wherein two or more temperature sensors (10, 11) are provided as flame monitors and are connected to the evaluation and control unit (7) by means of separate signal lines (12).

8. Computer program product comprising instructions that cause the device according to one of claims 6 or 7 to execute the method according to one of claims 1 to 5.