Method and device for protecting a heater during the ignition of a mixture of air and hydrogen-containing fuel gas

Abstract

The invention relates to a method and a corresponding device for igniting a combustion process in a combustion chamber (13) of a heating appliance (1) which is operated with hydrogen or a hydrogen-containing fuel gas and has a flame detector (11) for detecting a flame with a response time (x), wherein, after the start of the supply of an ignitable mixture of air and fuel gas to the combustion chamber (13), an ignition process is triggered for a maximum duration of a predefinable ignition time interval (z), which is shorter than a safety time interval (y), after the expiry of which the supply of ignitable mixture is stopped if no flame has been detected by then. The present invention increases the safety when starting a heating appliance operated with hydrogen or a hydrogen-containing fuel gas and enables particularly gentle and safe ignition processes.

Description

[0001] The invention relates to a method and a device for protecting a heating appliance when igniting a mixture of air and hydrogen-containing fuel gas, in particular for preventing damage caused by pressure pulses.

[0002] Modern heating systems are often operated with premix burners, in which air and fuel gas are mixed in a suitable ratio. The ignitable mixture is then fed by a blower through openings into a burner body and into a combustion chamber, where it is ignited and burned. Great care has always been taken with the ignition process, as a timing malfunction can lead to the accumulation of an explosive mixture in the combustion chamber or other system components, which, upon subsequent ignition, can cause uncontrolled deflagrations (so-called misfires or hard ignitions).

[0003] When using hydrogen as a fuel gas or a fuel gas with a high hydrogen content of, for example, more than 50%, especially more than 97%, it must also be taken into account that such deflagrations can be significantly more violent than with conventional fuel gases (e.g., natural gas), since high flame velocities and large pressure gradients occur when hydrogen and air are burned, and hydrogen flames can also shoot back into the burner body through very small openings.

[0004] EP 3 336 427 A1 describes a gas burner with gas distribution means configured to direct combustible gas to a mixing zone and air distribution means configured to direct air to the mixing zone. The combustible gas can be hydrogen, as well as natural gas, liquefied petroleum gas (LPG) such as propane or butane, and hydrogenated mixtures of natural gas and / or biogas containing up to 25% or even up to 50% hydrogen. The gas burner enables the necessary air to be efficiently directed to the combustible gas, ensuring rapid and effective combustion when the ignition source, e.g., a spark generator, is activated.

[0005] US Patent 4,299,556 A relates to a timer circuit with so-called flip-flop devices in a digital combustion control system for a burner assembly. If ignition fails within a time set by a safety timer, i.e., if no flame signal is received, the safety timer generates an ignition failure signal at the end of the preset time. A safety device is then activated to shut off the fuel valve, ignition device, and blower, thereby interrupting the operation of the burner assembly.

[0006] The Kromschröder company brochure "Automatic Burner Controls" ed. 5.05 describes an automatic burner control unit in which an ignition device is configured to ignite a pilot flame as the ignition point for a main flame. For this purpose, the ignition time of the pilot flame is set to be less than a safety time. This brochure cannot solve the aforementioned problems with hydrogen combustion.

[0007] According to current technology, when the heating appliance is started, the blower is switched on first, then a suitable quantity of fuel gas is added to create an ignitable mixture, and after a suitable time interval (based on experience, e.g., depending on the length of the paths in the premixing system), the actual ignition process is initiated. Here and in the following, it is assumed that the ignition process is achieved by electronically generated sparks via an ignition electrode, i.e., an arc or a rapid succession of sparks is initiated and then remains lit. For safety reasons, current technology always includes a flame detector in or on the combustion chamber, which detects whether a flame is burning or not. Its signal is also used to terminate the ignition process (but not the supply of the mixture) as soon as a flame is detected.If no flame is detected, the ignition process and at least the fuel gas supply are terminated after a safety time interval, according to current technology. With conventional fuel gases, standard flame detectors (e.g., ionization meters or optical sensors) respond very quickly, so ignition of the mixture can be detected almost without delay. If no flame is detected within the safety time, the ignition process is aborted because otherwise too much ignitable mixture could accumulate in the combustion chamber. This can then be followed by purging with air and repeating the entire start-up process, and / or a fault message or shutdown of the system.

[0008] However, the situation is different when using hydrogen or hydrogen-containing fuel gases as a fuel gas for two reasons. Firstly, as explained above, there is a higher potential for damage in the event of a deflagration, and secondly, flame detectors for such fuel gases are based on different measurement principles and take significantly longer to detect the presence of a flame. Therefore, due to their design, they have a typical response time, meaning that an ignited flame can only be detected after a delay. This response time can range, for example, from 0.1 to 5 seconds. The response time can depend on the intensity of the flame to be detected, although safety considerations always assume the maximum possible response time for a weak flame.

[0009] The object of the present invention is to take into account the problems described with reference to the prior art in an ignition process in a heating appliance and in particular to achieve the safest possible operation of ignition processes in heating appliances operated with hydrogen or hydrogen-containing fuel gas.

[0010] To solve this problem, a method, a heating 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 further exemplary embodiments.

[0011] In the inventive method for igniting a combustion process in a combustion chamber of a heating appliance which is operated with hydrogen or a hydrogen-containing fuel gas and has a flame detector for detecting a flame with a response time, an ignition process is triggered after the start of the supply of an ignitable mixture of air and fuel gas to the combustion chamber for a maximum duration of a predefinable ignition time interval, which is shorter than a safety time interval, after the expiry of which the supply of ignitable mixture is stopped if no flame could be detected by then, wherein the ignition time interval is at most as long as the safety time interval minus the response time of the flame detector.

[0012] As explained, an ignitable mixture containing hydrogen has a higher potential for damage than other ignitable mixtures. Therefore, for safety reasons, ignition of such a mixture in the combustion chamber should only occur as long as a limited maximum quantity is involved. After this maximum quantity is reached, ignition should no longer be possible, even if more mixture continues to flow. This means that the ignition process (spark) may only last until the maximum quantity is reached. This can be a relatively short ignition time interval of, for example, 1 to 5 seconds, particularly 2 to 3 seconds. The safety time interval, on the other hand, must be longer, especially because the flame sensor needs at least this response time to detect whether the ignition process was successful. According to the invention, the ignition time interval and the safety time interval are therefore defined independently of each other according to the characteristics of a heating appliance and the safety requirements to be met.Under favorable conditions, it is possible that the flame detector may recognize a flame during the ignition time interval and (prematurely) end the ignition process, but the ignition time interval may also be shorter than the response time of the flame detector, which means that the ignition process is ended before the flame is detected.

[0013] In a particular embodiment, measures to intensify combustion can be taken during and / or after the ignition interval within the safety time interval. Since a flame cannot initially be detected due to the response time, and ignition is no longer to be expected after the end of the ignition interval, the mixture supply can be increased or its composition (lambda value – the ratio of air to fuel gas compared to a stoichiometric mixture) can be changed. This is done under the assumption that successful ignition has occurred but has not yet been detected. If this assumption is correct, combustion becomes more intense and flame detection is easier. The probability that a successfully ignited flame will not be detected by the end of the safety time interval is thereby reduced (and the desired heating output is reached more quickly).If ignition is unsuccessful or no flame is detected by the end of the safety time interval, the starting process is terminated without the change in mixture supply causing any damage.

[0014] Particularly suitable and preferred measures include increasing the speed of a blower and / or changing the position of a fuel gas valve. Increasing the speed improves combustion efficiency and, in the case of a pneumatic fuel gas-air mixture, lowers the lambda value. Changing the position of a fuel gas valve also affects efficiency and / or lambda value.

[0015] In a typical embodiment of the invention, at least the fuel gas supply is terminated at the end of the safety time interval. This prevents any further ignitable mixture from entering the combustion chamber. If the blower continues to run, the mixture already present in the combustion chamber is even diluted with air and purged, so that a subsequent start-up process again has the original initial conditions.

[0016] The described procedure is preferably repeated automatically, with the combustion chamber being purged with air before each repetition. A failed start attempt of a heating appliance is not uncommon and should not necessarily be immediately classified as a fault. If ignition is successful on a second or subsequent attempt, no further action is generally required.

[0017] To increase the chances of successful ignition, different parameters for the formation of the ignitable mixture are automatically adjusted with each repetition of the process, specifically parameters modified to make ignition more likely. Although an initial attempt is made to start the engine at the lowest possible power to protect the heating element, the probability of success increases at higher power, possibly also with a lower lambda value, which is exploited here.

[0018] According to the claimed invention, the ignition time interval is at most as long as the safety time interval minus the response time of the flame detector. This is based on the consideration that ignition shortly before the end of the safety time interval would be pointless if the flame detector, due to its (long) response time, would not have enough time to detect a developing flame, thus aborting the start-up process despite the flame having ignited. This can be taken into account when specifying a maximum length for the ignition time interval.

[0019] In a device according to the invention for igniting combustion in a combustion chamber of a heating appliance, the heating appliance is operable with a mixture of air and hydrogen or a hydrogen-containing fuel gas, and a flame detector for detecting the presence of a flame in the combustion chamber is arranged on or in the combustion chamber with a response time, wherein the heating appliance has an automatic ignition system which is configured to trigger an ignition process after the start of a supply of ignitable mixture to the combustion chamber for the duration of a predefinable ignition time interval, which is shorter than a safety time interval after which the supply of ignitable mixture is stopped if no flame has been detected, wherein the ignition time interval is at most as long as the safety time interval minus the response time of the flame detector.In general, the automatic ignition system is part of a control and regulation unit for the entire operation of a heating appliance.

[0020] Preferably, the automatic ignition system is configured to initiate measures to intensify combustion during and / or after the ignition time interval, up to a maximum of the end of the safety interval. This increases the probability that a successfully ignited flame can be detected by the end of the safety time interval. More intensive combustion shortens the response time of the flame detector and / or increases its measurement signal.

[0021] In a typical embodiment of the invention, the automatic ignition system is configured to trigger a purging process of the combustion chamber after an ignition process has been aborted and then to repeat the ignition process of the heating device.

[0022] The automatic ignition system is particularly preferably designed such that different parameters for the supply of ignitable mixture are set with each repetition of the ignition process, namely parameters changed in the direction that makes ignition more likely.

[0023] To ensure the gentle operation of a heating device, the automatic ignition system is specifically designed to carry out the ignition process with the smallest possible volume flow of mixture during the first ignition attempt and to increase the speed of a blower and / or the supply of fuel gas with each repetition.

[0024] Another aspect of the invention relates to a computer program product comprising commands that cause the described device to execute the described method. An electronic unit with automatic ignition requires a program and data for controlling the heating device and evaluating the signals from the flame sensor, both of which must or may need to be updated occasionally, for which a computer program product can be used.

[0025] The explanations of the method can be used to further characterize the device, and vice versa. The device can also be configured to carry out the method.

[0026] A schematic embodiment of the invention, to which it is not limited, and the functioning of the method will now be explained in more detail with reference to the drawing. The drawing shows: Fig. 1: schematically a heating device according to the invention with automatic ignition and flame sensor and Fig. 2: a diagram to illustrate the temporal sequences according to the invention when starting a heating device.

[0027] Fig. 1 Figure 1 schematically shows a heating device 1 according to the invention, in which air is drawn in from an air inlet 4 by a blower 2 and conveyed to a burner 3. Hydrogen or a hydrogen-containing fuel gas from a fuel gas supply 8 is mixed with the air via a fuel gas valve 5. The resulting fuel gas-air mixture is ignited and combusted in a combustion chamber 13 by means of an ignition electrode 12. The resulting exhaust gases transfer most of their heat to a heat exchanger 14 and are then discharged into the environment via an exhaust system 10. An automatic ignition system 6, usually integrated into a control unit 7, controls each start of the heating device and the actual ignition process (the generation of ignition sparks). A flame detector 11, which is specifically designed for the detection of hydrogen flames, is arranged on or in the combustion chamber 13.Such a flame detector has a response time before it can reliably detect a flame after it has ignited. The control unit 7 is equipped with a display 9 that can show the system status and / or warning signals.

[0028] Fig. 2The invention is illustrated by an example of the time sequences during repeated ignition processes with changing parameters. The x-axis of the diagram represents time (t) in seconds. Curve A shows the time course of the fan speed 2. It can be seen that the speed increases incrementally with each start attempt, and that there are brief periods with additional increases in between. Curve B shows the opening time of the fuel gas valve 5. The respective opening time during a start attempt corresponds to the safety time interval y. Five start attempts are shown, the last of which is successful. Curve C shows the times during which the actual ignition process (ignition spark active) takes place. Five start attempts are shown, each with an ignition time interval z, where z is smaller than y according to the invention. Curve D shows the times during which a flame is present.During three start attempts, a flame did indeed occur, but it could not be detected twice, which is why the respective start attempts were aborted despite the flame. During the last start attempt, as curve E shows, a flame was detected, so the combustion process continued. The time delay between the appearance of the flame in curve D and the time of detection corresponds to the response time x of the flame detector 11. Had no flame been detected again, the start attempts would have been terminated completely and the combustion chamber 13 would have been purged with air. Curve A also shows that more air was supplied with each start attempt and that after the end of the respective ignition time interval z, the speed of the blower 2 was increased to intensify combustion (if ignition had occurred). Furthermore, the blower 2 continues to run after the fuel gas valve is switched off in order to purge the combustion chamber 13.Finally, the rotational speed is increased to the desired power output of the heating device.

[0029] The present invention increases safety when starting a heating appliance operated with hydrogen or a hydrogen-containing fuel gas and enables particularly gentle and safe ignition processes. Reference symbol list

[0030] 1 Heater 2 Blower 3 Burner 4 Air inlet 5 Fuel gas valve 6 Automatic ignition 7 Control and regulation unit 8 Fuel gas supply 9 Display 10 Exhaust system 11 Flame sensor 12 Ignition electrode 13 Combustion chamber 14 Heat exchanger A-curve: blower speed; B-curve: fuel gas valve position; C-curve: ignition spark; D-curve: flame; E-curve: flame detection; x-response time; y-safety time interval; z-ignition time interval

Claims

1. Method for igniting a combustion process in a combustion chamber (13) of a heating appliance (1) which is operated with hydrogen or a hydrogen-containing combustible gas and has a flame monitor (11) for detecting a flame with a response time (x), wherein, after the start of the supply of an ignitable mixture of air and combustible gas to the combustion chamber (13), an ignition process is triggered for a maximum duration of a predeterminable ignition time interval (z) that is shorter than a safety time interval (y), after which the supply of the ignitable mixture is terminated if no flame has been detected by that time, wherein the ignition time interval (z) is at most as long as the safety time interval (y) minus the response time (x) of the flame detector (11).

2. Method according to claim 1, wherein during and / or after the ignition time interval (z) has elapsed, measures are taken within the safety time interval (y) to intensify combustion.

3. Method according to claim 2, wherein the measures include increasing the speed of a blower (2) and / or changing the position of a fuel gas valve (5).

4. Method according to one of claims 1 to 3, wherein at least the fuel gas supply is terminated at the end of the safety time interval (y).

5. Method according to one of the preceding claims, wherein the method is repeated automatically, wherein the combustion chamber (13) is flushed with air before a repetition.

6. Method according to claim 5, wherein each time the method is repeated, other parameters for forming the ignitable mixture are automatically set, namely parameters changed in such a way that ignition becomes more likely.

7. Heating appliance (1) with a device for igniting combustion in a combustion chamber (13) of the heating appliance (1), wherein the heating appliance (1) can be operated with a mixture of air and hydrogen or a hydrogen-containing fuel gas, and a flame detector (11) for detecting the presence of a flame in the combustion chamber (13) is arranged at or in the combustion chamber (13) with a response time (x) and wherein the heating appliance (1) has an automatic ignition system (6) which is designed to trigger an ignition process for the duration of a predeterminable ignition time interval (z) after the start of a supply of ignitable mixture to the combustion chamber, which is shorter than a safety time interval (y), after the expiry of which the supply of ignitable mixture is terminated if no flame has been detected, wherein the ignition time interval (z) is at most as long as the safety time interval (y) minus the response time (x) of the flame detector (11).

8. Heating appliance (1) according to claim 7, wherein the automatic ignition device (6) is designed to initiate measures to intensify combustion during and / or after the ignition time interval (z) up to a maximum of the end of the safety interval (y).

9. Heating appliance (1) according to claim 7 or 8, wherein the automatic ignition device (6) is designed to trigger a flushing process of the combustion chamber (13) after termination of an ignition process and then to repeat the ignition process of the heating appliance (1).

10. Heating appliance (1) according to claim 9, wherein the automatic ignition system (6) is designed to set different parameters for the supply of ignitable mixture ( ) each time the ignition process is repeated, namely parameters that are changed in such a way that ignition becomes more likely.

11. Heating appliance (1) according to claim 10, wherein the automatic ignition system (6) is designed to carry out the ignition process during a first ignition attempt with the smallest possible volume flow of mixture and to increase the speed of a fan (2) and / or the supply of fuel gas with each repetition.

12. Computer program product comprising instructions that cause a heating appliance (1) according to one of claims 7 to 11 to execute a method according to one of claims 1 to 6.

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