METHOD FOR REGULATING A BURNER AND BURNER ARRANGEMENT WITH ONE BURNER

Description

[0001] The invention relates to a method for controlling a burner. Furthermore, the invention relates to a burner arrangement comprising a burner. The burner is, for example, part of a device for heating ambient air and / or a liquid, e.g., domestic hot water.

[0002] Burners are used in heating systems or water heaters, where the thermal energy generated by burning an air-fuel mixture is transferred via a heat exchanger to room air and / or a liquid, e.g., water. The fuel used can be, for example, propane, butane, gasoline, or diesel.

[0003] To monitor and control the presence of a flame or the combustion quality itself, it is known in the art to use so-called ionization electrodes, which utilize the ionization effect of a flame. The measured ionization signal, in the form of a voltage or current signal, is evaluated and used to control the combustion behavior, for example, by adjusting the air-fuel ratio (also known as lambda or air ratio) as the mass ratio of combustion air to fuel. This is done with the aim of ensuring the cleanest and most efficient combustion possible. For example, a gas valve and a combustion air blower are controlled depending on the ionization signal. A method for monitoring a gas burner using the ionization signal is disclosed, for example, in DE 196 31 821 A1.

[0004] Gas burners, and especially fan-driven gas burners, particularly those used in mobile heating systems, are frequently exposed to changing environmental conditions that can lead to inconsistent combustion behavior (see, for example, DE 102 20 773 A1). Such environmental parameters include atmospheric pressure, combustion air temperature, gas pressure (i.e., the pressure at which the fuel gas is supplied), and the calorific value of the gas. In mobile applications, the composition of the fuel gas can also vary. This is the case, for example, with typical gas mixtures such as LPG (Liquefied Petroleum Gas; autogas). Depending on the gas supply, it is possible for pure propane, pure butane, or an undefined propane / butane mixture to be supplied. In addition to increased emissions, unfavorable combustion conditions can lead to disruptive noises due to thermoacoustic effects, which can also be avoided or at least significantly reduced by adjusting the air-fuel ratio.

[0005] For example, DE 195 02 901 C1 discloses the method of applying an alternating voltage to the ionization electrode and using the resulting fluctuation of the ionization current, which depends on the air-fuel ratio, to adjust the supply of combustion air or gas. According to US 6,356,199 B1, an improved assessment of the combustion behavior is achieved through an extended analysis of the ionization signal by evaluating mean values, signal dispersion, or signal frequencies.

[0006] EP 2 431 663 B1 indicates that the ionization signals are subjected to a Fourier transform and the resulting spectra are evaluated. Reference spectra are generated for different burner types and compared with currently acquired spectra. If combustion instability is detected during operation, the supply of combustion air and / or gas is adjusted until a spectrum indicating stable combustion is measured. The peaks of the spectra are considered individually for evaluation.

[0007] Further methods for controlling the combustion process are disclosed in DE 102 20 772 A1 and DE 195 02 901 C1. EP 0 770 824 A2 discloses a method for controlling a premix burner. Starting from a measured ionization signal, a control variable is determined, and the air-fuel mixture is adjusted depending on the control variable and at least one setpoint. A calibration signal is determined from a frequency range of the ionization signal, and the setpoint is adjusted depending on the calibration signal.

[0008] A disadvantage of the spectral analysis has been found to be that changes to the combustion system, which includes the burner, lead to frequency shifts in the spectral peaks. Such changes include, for example, differing temperature profiles, modifications to the combustion air or exhaust system, or alterations to the burner surface. This reflects the fact that, with the aforementioned method, each burner type, and therefore each system in which the burner is used, requires its own reference spectrum. It should be noted, however, that changes to the system can occur due to use or aging. Furthermore, it is a disadvantage that the spectra exhibit interference originating from outside the system and unrelated to combustion, such as the 50 Hz mains signal. Overall, the analysis is therefore very complex.

[0009] The object underlying the invention is therefore to propose a method for monitoring a combustion process that is as simple as possible and yet reliable.

[0010] The invention solves the problem by a method for controlling a burner, wherein the burner is supplied with an air-fuel mixture, wherein an ionization signal is measured, wherein a control variable is determined from the ionization signal, wherein the air-fuel mixture is adjusted depending on the control variable and at least one setpoint, wherein a spectrum is obtained from the ionization signal, wherein a measure for an area is determined from the spectrum or from at least one frequency range of the spectrum, and wherein the setpoint is adjusted depending on the measure for the area.

[0011] According to the invention, the ionization signal is used to control combustion. For this purpose, a control variable is derived from the ionization signal. Furthermore, a setpoint is used in the control process, which is, for example, initially predefined or determined for the specific application. Based on the ionization signal, the setpoint is adjusted according to the invention. Information is thus extracted from the ionization signal, which is used to correct the setpoint. For example, a disturbance, such as noise generation, can be avoided by using the corrected setpoint, as the control takes place in a different lambda range.

[0012] According to the invention, a frequency spectrum is derived from the time signal. This is done, for example, via a Fourier transform. A value for an area (this measure can also be referred to as the area ratio) is then determined from the frequency spectrum. This is based on the understanding that disturbances, and in particular thermoacoustic effects, manifest themselves as signals in the spectrum. Therefore, the area ratio allows a statement to be made as to whether disturbances are present or whether the combustion process generates noise. In order to leave this noise-laden operating range, the setpoint for the control is adjusted accordingly, e.g., shifted, so that the control takes place in a different air ratio range. In one embodiment, the measure for the area is derived from a spectral range that is free of known disturbances such as mains hum.

[0013] The evaluation of the area ratio has the advantage that frequency shifts due to changed environmental conditions or application conditions do not need to be taken into account or do not change the result of the evaluation.

[0014] Preferably, the setpoint is adjusted if the determined area measurement deviates from a predefined reference value and / or a reference value determined for the burner beyond a tolerance value. In this configuration, the area measurement obtained from the spectrum is compared to a reference value. For example, the difference is calculated. If the difference exceeds a predefined tolerance value, the control device interprets this as indicating that a disturbance, in particular a thermoacoustic resonance, is present or at least incipient.

[0015] One embodiment of the method involves subjecting the ionization signal to a Fast Fourier Transform. The Fast Fourier Transform (FFT) is a very efficient method for transforming discrete-time signals.

[0016] In a further embodiment, the controlled variable is adjusted based on the ionization signal. In this embodiment, the controlled variable is adjusted based on the ionization signal. Information is thus extracted from the ionization signal, which is used to correct the controlled variable and the setpoint. This results, for example, in a corrected controlled variable whose behavior allows for better, or even reliable, control.

[0017] In one design, the air-fuel mixture is adjusted to ensure both the cleanest and quietest possible combustion. Within the air-fuel ratio range where emissions are low, the system is therefore regulated to eliminate or at least reduce resonances caused by thermoacoustic effects.

[0018] A key advantage has been that thermoacoustic effects appear in the ionization signal much earlier than they lead to clearly audible noise.

[0019] According to one embodiment, at least one value representing the magnitude of the ionization voltage is determined from the ionization signal and used as the controlled variable. In this embodiment, the amplitude of the ionization voltage thus serves as the controlled variable. A setpoint is preferably a setpoint of the voltage value.

[0020] An additional or alternative embodiment of the method involves determining several individual values ​​of the ionization voltage magnitude from the ionization signal, calculating the standard deviation from these individual values, and determining the controlled variable as a function of this standard deviation. In this embodiment, individual values ​​for the ionization voltage magnitude are determined from the ionization signal. Based on these individual values, the standard deviation—that is, a measure of the deviation of the individual values ​​from a mean value—is calculated. This standard deviation then serves to correct the controlled variable. This variant of the method is based on the observation that the ionization voltage can change significantly when disturbances, and especially when thermoacoustic effects, are present. The variation in voltage values ​​manifests itself in the standard deviation, thus providing a parameter for further processing and, in particular, for determining a controlled variable.Accordingly, an increasing dispersion of individual values ​​can be interpreted as an indication that, for example, a disturbing noise is occurring. Therefore, it is possible to react and countermeasures earlier.

[0021] In one configuration, the ionization signals are taken from a predefined period during which the air-fuel ratio is essentially constant or changes only within a predefined range. Therefore, no changes are made to the settings during the averaging period.

[0022] In one embodiment, a mean value and the standard deviation are calculated from the individual values, and the controlled variable is determined as the difference between the mean value and the standard deviation. In this embodiment, a mean value of the ionization voltage is calculated over a time period. The standard deviation of the voltage values ​​is then subtracted from this mean value. This difference serves, for example, as the controlled variable. The greater the standard deviation and thus the fluctuation of the ionization voltage, the smaller the controlled variable becomes.

[0023] In one embodiment, the individual values ​​are evaluated using a moving average. In a complementary embodiment, the ionization signals are evaluated at a predefined time interval (e.g., every five minutes) within a time interval of a predetermined width (e.g., measurements within five seconds).

[0024] According to one embodiment, the burner's combustion behavior is essentially continuously controlled, and the variance of individual values ​​is continuously determined using a moving average. In this embodiment, the process is designed, for example, to regulate the burner's operation to a desired, predefined operating point. This could be, for instance, a lambda value of 1.5. If the ambient conditions change—e.g., due to a change in air pressure—in one direction, such as towards a leaner air-fuel mixture, this is reflected in the increasing variance. The variance changes primarily before the mean value shifts outside a predefined tolerance range. If, in particular, the difference between the mean value and the variance is used as the controlled variable, then control towards a richer mixture is triggered.Under normal, undisturbed conditions, the ionization voltage decreases with increasing air-fuel ratio. Experiments have shown that in disturbance-prone areas, the ionization voltage increases or remains constant. The aforementioned configuration, using the difference between the mean and the variance, yields a control variable that can be regulated, for example, using a PID controller.

[0025] One implementation of the method involves adjusting the air-fuel mixture according to the controlled variable, similar to a PID controller. Thus, for example, a PID controller is either present or its behavior is implemented to control the combustion process. A typical PID controller requires a continuous curve for the controlled variable. The aforementioned variations of the method provide this.

[0026] Depending on one implementation of the method, the controlled variable is determined continuously or at predetermined intervals. Thus, there is either constant monitoring, allowing for continuous control, or monitoring and control are only performed at predetermined intervals. The latter is suitable, for example, if changes requiring intervention occur infrequently or do not need to be compensated for immediately.

[0027] According to a further teaching, the invention solves the problem by means of a burner arrangement comprising a burner, a heat exchanger, an ionization electrode, an air-fuel mixture supply, and a control device, wherein the control device receives and evaluates ionization signals measured by the ionization electrode, wherein the control device regulates the air-fuel mixture supply based on the evaluation of the ionization signals, and wherein the control device is designed such that it implements the method according to one of the embodiments described above or below. The explanations and embodiments also apply accordingly to the burner arrangement, so repetition is omitted. The burner arrangement is, for example, part of a device for heating room air and / or a liquid, e.g., water.

[0028] In detail, there are numerous possibilities for designing and further developing the inventive method and burner arrangement. Reference is made, on the one hand, to the claims subordinate to the independent claims, and on the other hand, to the following description of exemplary embodiments in conjunction with the drawings. These show: Fig. 1 a schematic block diagram of a burner arrangement according to the invention, Fig. 2a) and b) spectra without and with thermoacoustic effects, Fig. 3a) and b) curves of the ionization voltage and the area ratio at different burner powers, Fig. 4 curves of the air ratio and the ionization voltage over time and Fig. 5 two curves with a value of the ionization voltage and a control variable determined therefrom as a function of the air ratio.

[0029] The Fig. 1 Figure 1 schematically shows a burner arrangement with a burner 1, which is supplied with an air-fuel mixture via an air-fuel mixture supply 2. The fuel is, for example, a combustible gas such as propane or butane. The flue gas produced during combustion of the air-fuel mixture is fed to a heat exchanger 3, which transfers the thermal energy to water or air. An ionization electrode 4 is provided for monitoring the combustion process. It is positioned relative to the burner 1 such that it protrudes into the flame produced during combustion. Depending on the design, an ionization voltage or an ionization current can be measured as an ionization signal via the ionization electrode 4. The ionization signal is fed to the control device 5 for evaluation.Based on the control variable obtained in this process, the control device 5 acts on the air-fuel mixture supply 2 by, for example, changing the fuel and / or air ratio. This is done with the aim of achieving combustion with the lowest possible emissions and noise levels.

[0030] In the illustrations Fig. 2 bis Fig. 5 The evaluation of the ionization signal is illustrated by way of example, whereby thermoacoustic effects in particular occur as disturbances. These noises are subsequently avoided or at least reduced by changing the mixing ratio.

[0031] The Fig. 2 a) This shows a spectrum of the ionization signal obtained by an FFT without an audible thermoacoustic resonance. The x-axis represents the frequency in Hz. The signal was acquired at an air-fuel ratio of 1.2. A mains voltage signal is visible at 50 Hz.

[0032] At the Fig. 2 b) The spectrum shows a signal around 104 Hz, which is accompanied by an audible thermoacoustic resonance. The spectrum was recorded at an air-fuel ratio of 1.6.

[0033] To measure the resonance within the spectrum Fig. 2 b) To reduce the effect, an area in a frequency range of the spectrum is determined during the evaluation according to a specific design and used for a controlled variable.

[0034] In the Fig. 3 a) und b) The graphs show the average voltage values ​​of the ionization signals (solid line and left y-axis) and the determined area coefficients (dashed line and right y-axis) as a function of the air-fuel ratio. The graphs differ with respect to the power output of the burner: in the Fig. 3 a) The power output is 1 kW and at the Fig. 3 b) 3.5 kW.

[0035] The Fig. 3 a) This illustrates the case where changing the air-fuel ratio does not result in thermoacoustic resonance. The larger the air-fuel ratio becomes, the lower the magnitude of the ionization voltage. Since no noise is produced, no additional signal appears in the spectrum, so the integral of the frequency range, i.e., the surface area ratio, remains constant.

[0036] The behavior changes significantly at higher burner output. In the Fig. 3 b) The magnitude of the ionization voltage decreases again, whereas at an air-fuel ratio of 1.6 (see the Fig. 2 b) A clear increase in the area number can be observed. Note that the ionization voltage exhibits a very flat profile in this range. If the area number changes significantly, the setpoint is adjusted, which is used as the control variable for regulating the combustion process.

[0037] The Fig. 4 This graph shows the fluctuations in the measured ionization voltage values ​​when disturbances occur. The outer y-axis represents the lambda value, the inner y-axis the magnitude of the ionization voltage, and the x-axis represents time. The lambda values ​​were increased in discrete steps, as indicated by the step-like shape of the dashed line.

[0038] The curve generally shows that the ionization voltage decreases with increasing lambda value. It is also evident that there is a direct correlation between the voltage and the set air-fuel ratio. However, it is apparent that the voltage values ​​can fluctuate significantly when disturbances are present. In this experiment, these are clearly audible thermoacoustic resonances that occur at lambda = 1.6 and lambda = 1.7 (from approximately 180 seconds onwards). These fluctuations even have a noticeable effect on the mean value. The scatter alone is therefore also an indicator of the presence of a disturbance.

[0039] In the Fig. 5 Two curves are shown, each displaying a value for the ionization voltage as a function of the lambda value.

[0040] The solid curve represents the unprocessed mean value of the ionization voltage. The dashed curve represents the difference between the mean value and the corresponding standard deviation.

[0041] The solid curve again shows the decrease in voltage. Due to variations in the thermoacoustic effects, the average value is raised in the range between lambda = 1.5 and 1.6 and remains almost constant. This illustrates the impact of this behavior on the control system. If, for example, the setpoint for the ionization voltage were 1.4 V, this would be associated with two lambda values. This means that simply considering the ionization voltage is insufficient for control purposes.

[0042] In the dashed curve, the variation was subtracted from the mean ionization voltage. This shifts the curve downwards. A dramatic effect occurs in the range greater than 1.5 for lambda. The calculated value at lambda = 1.6 differs significantly from the previous value at lambda = 1.5. The increased variation compensates for the increase in the mean value. This results in a steadily decreasing curve, which allows for unambiguous control. Therefore, if the calculated voltage value decreases as the controlled variable in the region of thermoacoustic resonance, the controller would detect that the controlled variable is smaller than a setpoint and would then adjust the operating point to the richer range with a smaller lambda value.

Claims

1. A method of regulating a burner (1), wherein the burner (1) is supplied with an air-fuel mixture, wherein an ionization signal is measured, wherein a regulating variable is determined based on the ionization signal, wherein the air-fuel mixture is set depending on the regulating variable and at least one setpoint value, wherein a spectrum is obtained from the ionization signal - in particular by a Fourier transformation, wherein a measure for a surface area is determined from the spectrum or from at least one frequency range of the spectrum, and wherein the setpoint value is adjusted depending on the measure for the surface area.

2. The method according to claim 1, wherein at least one value of an amount of the ionization voltage is determined from the ionization signal and used as a regulating variable.

3. The method according to claim 1 or 2, wherein a plurality of individual values of an amount of the ionization voltage is determined from the ionization signal, wherein a dispersion is determined from the individual values, and wherein the regulating variable is adjusted depending on the dispersion.

4. The method according to claim 3, wherein an average value and the dispersion are determined from the individual values, and wherein the regulating variable is determined as a difference between the average value and the dispersion.

5. The method according to any of claims 1 to 4, wherein the air-fuel mixture is set depending on the regulating variable in the manner of a PID controller.

6. A burner arrangement comprising a burner (1), a heat exchanger (3), an ionization electrode (4), an air-fuel-mixture supply (2) and a control device (5), wherein the control device (5) receives and evaluates ionization signals measured by the ionization electrode (4), wherein the control device (5) acts in a regulating manner on the air-fuel-mixture supply (2) based on the evaluation of the ionization signals, and wherein the control device (5) is configured such that the control device (5) implements the method according to any of claims 1 to 5.