METHOD AND ARRANGEMENT FOR IGNITING A FUEL-AIR MIXTURE IN A HEATING APPLIANCE THAT CAN BE OPERATED WITH DIFFERENT FUELS

Description

[0001] The invention relates to a method and a system for igniting a fuel-air mixture in a heating device that can be operated with different fuels. In particular, so-called adaptive combustion systems, e.g., systems controlled by an ionization current, are capable of combusting a broad spectrum of fuel gases. Natural gases from various sources or even with admixtures of hydrogen can be used. This applies not only to large systems, but also to wall-mounted water heating devices and, in general, to heating devices for heating buildings and / or providing hot water.

[0002] Combustion in such systems is generally carried out in a closed-loop control system, ensuring a desired fuel-to-air mixture (often characterized by a lambda value, which indicates the ratio of air to fuel relative to a stoichiometric ratio). Such a control system typically has a feedback loop that takes into account the characteristics of a fuel. This allows the control system to detect different fuels and react accordingly, ensuring equally clean combustion.

[0003] DE 20 2018101271 U1 relates to a fuel gas-operated heater for implementing a method for fuel gas type detection. The heater has a gas mass sensor through which the fuel gas flows, which measures both the fuel gas mass supplied to the burner and another physical property of the fuel gas, which allows conclusions to be drawn about the composition of the fuel gas. A control unit then determines the fuel gas type from this measured property of the fuel gas. The control unit then regulates a gas actuator, depending on the specific fuel gas type, to a first starting gas mass flow that lies below an ignition limit of the fuel gas of the specific fuel gas type. The supplied gas mass flow is then increased, starting from the starting gas mass flow, with constant air volume flow and continuous ignition attempts by the ignition unit until an ignition range of the previously determined fuel gas type is exceeded and ignition occurs.

[0004] DE 102 00 128 A1 discloses a method for detecting gas types in a combustion device for air-gas mixtures. In a first method step, the air volume flow contributing to the air-gas mixture and / or the gas volume flow contributing to the air-gas mixture is varied until the air-gas mixture ignites in the combustion chamber or until the flame of an ongoing combustion process is extinguished. In a second method step, the gas type is determined from the air volume flow and gas volume flow contributing to the air-gas mixture at the time of ignition or at the time of extinguishment. It has been shown that the mixing ratio of air and gas at the time of ignition or at the time of extinguishment is characteristic and therefore different for each gas type, so that the gas type can be determined with a high degree of certainty.

[0005] However, during the ignition of the fuel-air mixture, which is largely controlled and occurs without feedback, information about the supplied fuel gas is not (yet) available. However, since different fuel gases only ignite reliably at different lambda values, the lambda value is varied during ignition within a time interval over a predeterminable range (mixture range), within which ignition can be expected for all fuel gases present. According to the state of the art, this is achieved by varying the fuel gas supply with a constant air supply. The fuel gas supply can be changed very quickly, which is advantageous given the typically very short time intervals for ignition (e.g. 0.5 to 5 s [seconds]) and thus for varying the mixture.

[0006] One disadvantage, however, is that a change in the fuel gas supply also changes the fuel energy (ignition load), which indicates how much energy is available and released (suddenly) during ignition. If more fuel is supplied, the energy released during ignition changes accordingly. With a high ignition load and delayed ignition, this can lead to violent explosions or even damage to components. Estimates show that the ignition load can change upwards (or downwards) by more than 30% with typical variations in the fuel gas supply.

[0007] The object of the present invention is to at least partially solve the problems described with reference to the prior art. In particular, a method and an arrangement are to be created in which the ignition load changes little or not at all despite variations in the lambda value during a time interval prior to ignition.

[0008] To achieve this object, a method and an arrangement, as well as a computer program product, are provided according to the independent claims. Advantageous embodiments and refinements of the invention are specified in the respective dependent claims. The description, particularly in conjunction with the drawings, illustrates the invention and provides further exemplary embodiments.

[0009] A method for igniting a fuel-air mixture in a heater operable with different fuels, in particular with different fuel gases, contributes to this. This heater has a premix burner to which air can be supplied via an air supply and fuel via a fuel supply in a premixed mixture ratio. Within a preset time interval for ignition, the fuel-to-air mixture ratio is varied to cover a preset mixture range, specifically by varying the air supply.

[0010] The process can be used to ignite various fuel-air mixtures. The fuel can be provided in liquid (oil, etc.) and / or solid (pellets, etc.) and / or gaseous form. The fuel is particularly preferably a combustible gas, so this application is particularly emphasized here—without intending to limit the use of the process.

[0011] The air supply is generally provided by a fan, which generates a volume flow dependent on its speed. By varying the speed, the volume flow and thus the air supply can be changed. This is a preferred method of varying the air supply, but it does not preclude other methods, such as changing the position of a throttle valve, orifice plate, or the like.

[0012] The time interval and mixture range are selected so that a maximum ignition load is not exceeded by the end of the time interval, regardless of the type of fuel or fuel gas supplied. This ensures that even under the most unfavorable conditions, the fuel or fuel gas generating the highest ignition load cannot cause excessively violent (hard) ignitions, allowing for a safe ignition to be successfully performed or for a repeat of the ignition process to be initiated.

[0013] Preferably, the fuel supply is kept constant during the ignition interval. This is always the simplest and preferred approach when the options for varying the air flow rate are sufficient to cover the entire mixture range within the time interval.

[0014] In a special variant, the air supply is varied as much as possible within the time interval, and the fuel or combustion gas supply is only varied to the extent necessary to cover the entire mixture range. This can be necessary in particular if a fan, whose rotating parts cannot be accelerated or decelerated at will (i.e. whose so-called speed dynamics are not sufficient for a desired rapid change), is not sufficient on its own to achieve the necessary mixture change within the time interval. In this case, the supply of fuel or combustion gas is also varied, but only to the smallest possible extent due to the disadvantage of this procedure described above. For this purpose, the position of a combustion gas valve can preferably be changed, but here too, other ways of changing the combustion gas supply are in principle possible.

[0015] To achieve the object, an arrangement for igniting a fuel-air mixture in a heating device which can be operated with different fuels and has a premix burner to which air can be supplied by an air supply and fuel by a fuel supply in a premixed mixture ratio, wherein an ignition control is provided which is set up to vary the air supply during a time interval for ignition in order to cover a predeterminable mixture range.

[0016] The ignition control is also set up in such a way that the time interval and the mixture range are selected so that a maximum ignition load is not exceeded until the end of the time interval, regardless of the type of fuel or fuel gas supplied.

[0017] In a particular embodiment, the ignition control is configured to vary a fan for supplying air with a maximum speed dynamic and additionally to vary the fuel supply if the speed dynamic is not sufficient to cover a mixture range within the time interval.

[0018] A further aspect also relates to a computer program product comprising instructions that cause the described arrangement to execute the described method. Implementing the described ignition sequence requires a program and data, both of which must be updated occasionally.

[0019] The explanations of the procedure can be used to further characterize the arrangement, and vice versa. The arrangement can also be designed to carry out the procedure.

[0020] 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. They show: Fig. 1: a heater with an automatic ignition system and Fig. 2 diagrams illustrating the ignition load of the method proposed here in comparison to the prior art.

[0021] Fig. 1 shows a schematic of a heater 1 with an air supply 2, which is equipped with a fan 4 and transports air to a burner 6. Via a fuel supply, here in the form of a fuel gas supply 3 with a fuel gas valve 5, fuel gas G is mixed with the air L. The resulting fuel gas-air mixture is ignited and burned in a combustion chamber 7 by means of an ignition device 10. The resulting exhaust gases give off most of their heat to a heat exchanger 9 and are then discharged into the environment through an exhaust system 8. An ignition control 12, usually integrated into a control and regulation unit 11 of the heater 1, controls each start-up of the heater 1 and the actual ignition process that takes place during this process. The ignition control 12 controls the fan 4 and the fuel gas valve 5 via data or control lines 13. An ignition line 14 connects the ignition control 12 to the ignition device 10, in most cases an ignition electrode for generating ignition sparks.

[0022] Fig. 2 illustrates the invention using examples of the temporal sequences and the resulting mixture ratios and ignition loads. The upper diagram shows the prior art and the resulting ignition load Z (curve A) when the mixture ratio of air L and fuel gas G is changed by varying the fuel gas supply 3. The ignition load Z increases in direct proportion to an increasing fuel gas supply 3, while the mixture ratio lambda (curve B) decreases. The time t is plotted on the x-axis (e.g. in seconds), with the time interval Δt for ignition here being 10 s. The ignition load Z is plotted on the left y-axis, and the mixture ratio lambda on the right y-axis.

[0023] In the middle diagram, time t is plotted on the X-axis, a mass flow of air L on the right Y-axis and a mass flow of fuel gas G on the left Y-axis. According to the invention, the fuel gas supply 3 (curve C) is kept constant, but the air supply 2 (curve D) is reduced in the time interval Δt.

[0024] The result can be seen in the lower diagram, in which the ignition load Z (curve E) remains constant while the mixture ratio lambda (curve F) changes (in the same way as in the upper diagram according to the state of the art). The representation in the lower diagram corresponds to the upper one, and it can be seen that when passing through the same mixture range, the ignition load Z is constant and thus significantly lower than in the state of the art. Here, too, the ignition load Z is practically proportional to the fuel gas supply 3, and this remains constant throughout the entire time interval Δt.

[0025] The present invention makes it possible to safely ignite a heating device 1 which can be operated with different fuel gases G, without taking the risk of an undesirably high ignition load Z with possible consequential damage. List of reference symbols

[0026] 1Heater 2Air supply 3Fuel gas supply 4Blower 5Fuel gas valve 6(Premix) burner 7Combustion chamber 8Exhaust system 9Heat exchanger 10Ignition device 11Control and regulation unit 12Ignition control 13Control lines 14Ignition line AKurve Ignition load according to the state of the art BCurve Mixture ratio (lambda) when changing the fuel gas supply CCurve Fuel gas DCurve Air EKurve Ignition load when changing the air supply FCurve Mixture ratio (lambda) when changing the air supply GBuction gas LAir ZZignition load tTime Δ t(Delta t) Time interval

Claims

1. Method for igniting a fuel / air mixture in a heating appliance (1) which can be operated with different fuels with a premix burner (6), to which air (L) is supplied through an air supply (2) and fuel through a fuel supply (3) in a premixed state in a mixing ratio, wherein within a predeterminable time interval (Δt), within which a ignition of the various fuels is expected, the mixing ratio of fuel to air (L) for ignition is varied to cover a predeterminable mixture range, namely by varying the air supply (2), and the time interval (Δt) and the mixture range are further selected such that a maximum ignition load (Z) is not exceeded by the end of the time interval (Δt), independently of the type of fuel supplied.

2. Method according to claim 1, wherein the fuel supply (3) is kept constant during the time interval (Δt) for ignition.

3. Method according to claim 1, wherein in the time interval (Δt) the air supply (2) is varied in the maximum possible manner and the fuel supply (3) is only varied to the extent necessary to cover the entire mixture range.

4. Method according to any one of the preceding claims, wherein the ignition is controlled and takes place without feedback.

5. Arrangement for igniting a fuel / air mixture in a heating appliance (1) that can be operated with different fuels, with a premix burner (6), to which air (L) can be supplied through an air supply (2) and fuel through a fuel supply (3) in a premixed state in a mixing ratio, wherein an ignition control (12) is provided which is configured, during a time interval (Δt) within which the ignition of the various fuels is expected, to vary the air supply (2) for ignition in order to cover a predeterminable mixture range, and the time interval (Δt) and the mixture range are further selected such that a maximum ignition load (Z) is not exceeded by the end of the time interval (Δt), independently of the type of fuel supplied.

6. Arrangement according to claim 5, wherein the ignition control (12) is configured to vary a fan (4) of the air supply (2) with a maximum speed dynamic and to additionally vary the fuel supply (3), if the speed dynamic is not sufficient to cover a mixture range within the time interval (Δt).

7. Computer program product, comprising commands, which cause the arrangement according to any one of claims 5 or 6 to execute the method according to any one of claims 1 to 4.