EP0643265B1 - Method and device for controlling excess-air premix gas burners - Google Patents

Description

The invention relates to a method for operating a stoichiometric working Gas burner, a flame characteristic representative of the combustion state measured, a measurand obtained, from which an average signal is formed and this is used to regulate the gas and / or air mass flow.

One measure for reducing NO _x emissions during combustion processes is to lower the flame temperature. This can be done by stoichiometric combustion. In addition to the amount of air required for complete combustion, the burner is supplied with cooling gas, which cooling gas can also include excess air. The combustion state depends on how much excess air or cooling gas is used.

In a known method of the type mentioned (EP0262390B1) is used as flame property representative of the combustion state is the ionization current measured. The mean value signal formed from this has a profile over the air ratio A. on, which decreases with increasing air ratio, whereby the approximation to the Flame stability limit shows that the curve approximates the normal. Such a regulation requires performance-dependent setpoint maps, each only apply to a limited heating and condensing range. Besides, they are Signals due to influences on the electrode and burner components are not stable over the long term. Changes the type of gas therefore requires the controller to be switched externally.

The invention has for its object to remedy this situation and the possibility for automatic detection and consideration of the gas type.

a) der Gas- und/oder Luftmassenstrom solange variiert wird, bis ein vorgegebener Schwellwert der Standardabweichung erreicht ist;

b) der Istivert des Mittelwertsignals bei Erreichen des Schwellwertes der Standardabweichung erfaßt wird;

c) ein Sollwert des Mittelwertsignals in einem vorgegebenen Abstand von dem in Schritt b) erfaßten Istwert des Mittelwertsignals vorgegeben wird;

d) der Gas- und/oder Luftmassenstrom unter Verwendung des in Schritt c) gewonnenen Sollwertes des Mittelwertsignals geregelt wird; und

e) die Schritte a) bis d) bedarfsabhängig und/oder periodisch wiederholt werden.

Dabei ist zu berücksichtigen, daß der vorstehend zitierte Stand der Technik bereits die Erfassung der Standardabweichungen zum Zwecke der Brennerregelung beschreibt, allerdings nur als Alternative zum Mittelwertsignal.To achieve this object, the method according to the invention is characterized in that the standard deviations of the measured value are recorded, assigned to the mean value signal for determining the combustion state and used in addition for setting and control, whereby

a) the gas and / or air mass flow is varied until a predetermined threshold value of the standard deviation is reached;

b) the actual value of the mean value signal is detected when the threshold value of the standard deviation is reached;

c) a target value of the mean value signal is specified at a predetermined distance from the actual value of the mean value signal acquired in step b);

d) the gas and / or air mass flow is regulated using the setpoint value of the mean value signal obtained in step c); and

e) steps a) to d) are repeated as required and / or periodically.

It should be noted that the prior art cited above already describes the detection of standard deviations for the purpose of burner control, but only as an alternative to the mean signal.

The invention, however, is based on the knowledge that a relationship dependent on the type of gas between the mean signal and the standard deviations. Depending on the type of gas, a defined value of the standard deviations is different Assigned mean signal. By capturing these relationships, the control system is able to recognize the type of gas and the setpoint of the mean signal - if necessary taking into account the set performance - to specify accordingly so that the burner in the desired area, e.g. B. in a desired one Distance from the flame stability limit or from a CO emission limit, is operated.

The threshold value can, for example, be set at the flame stability limit. It can also be assigned a critical value for CO emissions. These show the same course as the standard deviations. The regulation ensures that the Burner at a reliable distance from the respective, by the threshold of the standard deviations represented limit remains. The main advantage of this control concept is that no characteristics are required for the mean signal are. The controller only gives a roughly set output for the starting process a "safe" gas / air mass flow ratio. Based on this, he drives then for the first time the threshold of the standard deviations and determines from them automatically the setpoint of the mean signal. For this he only needs one Specification for the distance that this setpoint is from that actual value of the mean value signal which is assigned to the threshold value of the standard deviations is.

In the simplest case, the threshold value of the standard deviations is approached periodically, the length of the periods depending on the stability of the operating conditions can be. However, it is preferable to adapt the Target value, as characterized for example in claims 2 and 3. The The setpoint can be adapted, for example, when changing the gas type Adjustment of performance, a change in air temperature or as a result of any disturbance to the state of the flame may be necessary.

A particularly good control behavior can be achieved in that in addition to the periodic adaptation of the setpoint of the mean value signal as required becomes. The periods can be chosen to be relatively long.

A further improvement of the control concept is achieved in that gas type-dependent Characteristic values for the threshold of the standard deviations, this Threshold value assigned actual value of the mean value signal and the distance of the setpoint of the mean signal from this actual value are specified that when starting the threshold value of the standard deviations from the assigned st value of Average signal the gas type is recognized and that depending on the gas type associated threshold value and the associated distance of the target value of the mean value signal selected from the actual value of the mean value signal assigned to this threshold value will.

In this way, the burner can be very close, for example, to the flame stability limit or the CO emission limit. Because these limits are depending on the gas type, different threshold values of the standard deviations assigned. The distance that the setpoint of the mean value signal from the actual value assigned to the respective threshold value, depending on the gas type to get voted. By specifying these values specifically for the gas type very refine the control concept. However, this advantage is bought by that the controller must be given a larger number of characteristic values.

For example, when approaching the flame stability limit, it is important that Reliable detection of the threshold value of the standard deviations. This is difficult if the measured value represents that of the combustion state Flame property fluctuates due to disturbances as these disturbances go into the standard deviations. In contrast, it has proven to be beneficial proved that the time when approaching the threshold value of the standard deviations Frequency of threshold violations and the threshold of the To allow standard deviations to be considered reached only if the threshold values are exceeded occur with a predetermined frequency. That way ensures that the regulation does not respond to any short-term exceeding of the threshold value responds. The level of the frequency limit (e.g. 1000 / s) is included not critical.

When approaching the flame stability limit, there is a risk that shortly before reaching of the threshold of the standard deviations, the CO emissions rise sharply. This risk increases the higher the performance of the burner and the lower the Calorific value of the gas is. However, the peak CO values occur only very briefly and therefore only slightly deteriorate the total pollutant emissions of the burner. After all, it can be recommended for a high burner output Post-oxidation of carbon monoxide.

The measured value is preferably that which represents the combustion state Flame property used for flame monitoring, d. H. to turn off the Burner if the flame goes out.

An essential development of the invention is as for the combustion state representative flame property to measure the light intensity. It was found that the standard deviations in light intensity particularly at the Flame stability limit a concise course in precisely assignable assignment to the mean signal. Add to that the light intensity is instant The burner is switched off when the flame goes out. You can also use the light intensity Detect in a simple manner and convert it into corresponding measurement signals. Above all, the measurement of the light intensity enables a reliable one Detection of the respective gas type. The measurement is inertia and leaves the detection an integral flame area. Finally it was found that the Light intensity only depends on the power to a comparatively small extent. This is based above all on the map-free, adaptive control concept.

The detection of the radiation of a flame by means of a sensor and use of the Output signal of the sensor for control purposes is known per se, for example from EP 0 408 846 A1.

It is particularly advantageous as a flame property representative of the combustion state measure the light intensity over one or more radiation bands. A suitable selection of the radiation bands can cause interference radiating system components can be avoided. In particular, the different Radiation characteristics of the individual gas types when selecting the Radiation bands are taken into account and thus an optimal measurement signal for the gas type be generated. It is also possible to use the measured value acquisition via or to recognize several radiation bands of gas types.

The infrared radiation is preferably excluded when measuring the light intensity. The measurement is therefore not caused by heated parts in the vicinity of the Flame adulterated.

A photomultiplier or a semiconductor has proven to be an advantageous sensor.

The measurement of the light intensity is preferably carried out within an adjustable range Field of view of the flame area. Optics for determination at the inlet of the light guide the field of view size. The field of vision should be so large that a representative flame area is detected. In the case of premix burners, for example can be worked with a relatively small angle because of the flame has a very uniform structure. This applies all the more, the more intensive the premix is. In the case of muzzle-mixing burners, on the other hand, a relatively large one must be used Flame area can be detected.

The optics can also be designed as a heat shield be so that the light guide is close to the flame can be introduced.

It is also advantageous to have means for cooling the light guide to provide. The light guide can be as close as desired the flame will be approximated. Furthermore, through the use of coolants further optimizes the life of the light guide. It is also possible to use less expensive light guides use lower temperature resistance.

In a further development of the invention, the light guide can with a gas curtain, preferably with an air curtain for Protection against contamination.

Such combinations are also disclosed as being essential to the invention of the features of the present invention that are different from the above discussed links differ.

Fig. 1

eine schematische Darstellung einer erfindungsgemäßen Vorrichtung;

Fig. 2 bis 8

Diagramme der erfindungsgemäß maßgeblichen Kennwerte.

The invention is explained in more detail below on the basis of preferred exemplary embodiments in conjunction with the accompanying drawing. The drawing shows in:

Fig. 1

a schematic representation of a device according to the invention;

2 to 8

Diagrams of the characteristic values relevant according to the invention.

The device according to FIG. 1 comprises a stoichiometric one premixing gas burner 1 with a combustion chamber 2 and one of these upstream mixing chamber 3. To the mixing chamber 3 leads a gas line 4, in which an actuator 5 in the form of a motor-driven gas throttle valve is arranged. Furthermore contains the gas line 4 a safety valve 6. To the mixing chamber 3 also leads an air line 7, in which an actuator 8 in Form a motor-driven air throttle valve is arranged.

The burner 1 is also provided with a light guide 9, which detects the light intensity within the combustion chamber 2. in the In the present case, the light guide 9 observes the combustion chamber 2 through a central opening in a burner plate 10. Das Field of view from a not shown, a heat shield visual optics is narrow, since the burner with intensive premixing works. The main transmission area lies between 200 and 600 nm, so infrared radiation is excluded out.

The light guide 9 is interposed by a transducer 11 in the form of a photomultiplier or semiconductor connected to a controller 12 designed as a computer. This is via a relay stage 13 with the actuators 5 and 8 and with the safety valve 6 in connection. Further the controller receives the feedback from the actuators.

Otherwise, the controller 12 with an operating device 14, a digital-analog stage 15 and an output device 16 provided for the process control variables.

The light intensity represents a flame property that represents the combustion state within the combustion chamber 2 of the gas burner 1. The light intensity is detected by the light guide 9 and fed to the controller 12 as a measurement signal. From the measurement signal, this forms an average signal Um and a signal SN for the averaged standard deviations. The diagram in FIG. 2 shows the course of these two signals, plotted against the air ratio λ, for a given gas type and a given power. As the air ratio increases, the mean signal signal decreases, while the standard deviations increase. When the flame stability limit is reached, the initially gradual increase turns into a steep characteristic curve. The two curves shown in FIG. 2 simultaneously represent the pollutant emissions, namely around the NO _x curve and SN the CO curve, see the diagram according to FIG. 3.

The course of the curves in the diagram according to FIG. 2 is gas type. and performance-dependent. A diagram can be created for each gas type create with groups of curves for the two signals whose Parameter is the performance.

The invention is based on the knowledge that a gas type-dependent Relationship between the mean signal and the Standard deviations is present. So becomes a certain one Value of the standard deviations are defined, so are this value each Different mean signals are assigned according to the gas type. Out In this relationship, the controller recognizes the current gas type and gives a corresponding setpoint for the further control the mean signal before.

After a first development of the invention, the Burner with a roughly specified output and one in safe air range. Then becomes the flame stability limit by increasing the air ratio approached, which is defined as the threshold value of the standard deviations is. As soon as this threshold value is reached, the Controller the associated actual value of the average signal and gives, based on this, a setpoint that is around a certain Amount is higher than the actual value entered. Because the recorded actual value at the flame stability limit, the instantaneous Represents the type of gas, the distance between the setpoint and the actual value can be varied depending on the gas type, if necessary under gas type Adjustment of the threshold.

The controller guides the flame stability limit depending on need, for example when changing the gas type, if the air temperature rises or if other conditions occur Disorders. He recognizes the need for example by that the value of the standard deviations compared to that of Every setpoint adaptation changes the stored value significantly. The ratio of the actual values of the standard deviation can be the same and the mean value signal are monitored. A significant one Deviation of this relationship from the relationship between the a value of the standard deviation stored in the setpoint adaptation and the setpoint itself can be used as an indication be taken for need. Alternatively or in addition a periodic approach to the flame stability limit is provided be.

So that the system does not respond to any brief exceedance of the threshold value defining the flame stability limit of the standard deviations is the frequency the threshold violations are recorded and a corresponding limit value is set, for example 1000 exceedances per second. The level of this limit is not critical.

After a second development of the invention Maps worked as shown in Figures 4 to 8 are. The standard deviations are called long-term standard deviations LSN detected to prevent the Regulator considers short-term malfunctions as a gas type change.

A setpoint characteristic curve of the mean value signal is provided for each gas type To be specified, and plotted against the performance. The corresponding diagram is shown in Fig. 4. The performance is reproduced here and also in the following diagrams as mass air flow.

In addition to the diagram of Fig. 4, for each gas type a map for the long-term standard deviations LSN is specified, and also plotted against the performance, here above the air mass flow. Figures 5, 6 and 7 show corresponding ones Diagrams, namely Fig. 5 for butane, Fig. 6 for Natural gas and Fig. 7 for a known under the name G 110 Test gas, which is 51% hydrogen and 24% nitrogen and consists of 25% methane.

For a more detailed explanation of the control processes, it is assumed that the burner works with natural gas, see Fig. 8, so far corresponds to Fig. 6. The LSN value is there the point A, while the controller does the um-value at this power 4 on the characteristic curve. Now there is a transition on butane, its higher calorific value leads to the average signal To get a rising trend. This tendency the controller counteracts this by reducing the gas supply. At is therefore kept constant. This inevitably increases LSN value and moves towards point B in Fig. 8. Already at point C he leaves the map area. This Transition is detected by the controller and switches to 5 uses the map.

On the other hand, when operating with natural gas, the Test gas G 110 switched, the LSN value drops and leaves 8 at the lower limit. This leads to the controller being on the map according to FIG. 7 toggles.

Both concepts described have their advantages. The des The first concept is that no maps for the Standard deviations are required. There is this when starting the flame stability limit there is a risk that it will there is a sharp rise in CO emissions. The danger is all the more larger, the lower the calorific value of the gas and the higher the performance is. The threshold that defines the flame stability limit the standard deviations must not be chosen too low, because otherwise it does not differ from normal gas Burner operation can be distinguished. You put it on the other hand, close to the flame stability limit, is a corresponding level of CO emissions. The rise However, the CO emission occurs only briefly and can may be tolerated because it does not reduce the total output significantly increased. Otherwise, this disadvantage does not apply if afterburning is provided.

With regard to the specification of the maps, the second is Control concept more complex. For that it can always be in safe Work away from the flame stability limit. The given Setpoints can be used for every gas type and every power level take into account the permissible CO values.

As can be seen from FIG. 1, that of the evaluation device 11 delivered measurement of the light intensity also for Flame monitoring are used, namely to switch off the safety valve 6.

Within the scope of the invention there are possibilities for modification given. So instead of the light intensity, one can other flame property representing the state of combustion be measured, provided the gas type-dependent relationship sufficient between the mean signal and the standard deviations can be clearly recorded. Furthermore, the arrangement not limited to a central alignment of the sensor. Rather, it can be the combustion chamber in any way be assigned. A concept is also theoretically possible in which the controller according to, for example, the flame stability limit representative value of the standard deviations works if the corresponding CO emissions are tolerated can be. There is also the possibility of a distance from this limit. This can, just like the distance the mean value signal in the case of the adaptive control concept, be specified mechanically, for example as a number the adjustment steps of the air and / or gas throttle valve.

Claims (10)

1. Method for controlling an excess-air premix gas burner, wherein a flame property representative of the state of combustion is measured, a measured quantity is obtained, a mean-value signal is formed therefrom and used to control the gas and/or air mass flow,
   characterised in that
   the standard deviation of the measured quantity is recorded, assigned to the mean-value signal for determining the state of combustion and additionally used for adjustment and control, whereby

   a) the gas and/or air mass flow is varied until a specified threshold of the standard deviation is reached;

   b) the actual value of the mean-value signal is recorded on reaching the threshold of the standard deviation;

   c) a setpoint of the mean-value signal is specified at a specified distance from the actual value of the mean-value signal recorded in step b);

   d) the gas and/or air mass flow is controlled using the setpoint of the mean-value signal obtained in step c); and

   e) steps a) to d) are repeated as required and/or periodically.
2. Method according to claim 1, characterised in that

   f) the setpoint of the mean-value signal obtained in step c) is stored:

   g) a value of the standard deviation recorded once on reaching the setpoint of the mean-value signal is stored;

   h) the ratio between the actual values of the mean-value signal and the standard deviation is monitored;

   i) the ratio according to h) is compared with the ratio of the values stored in steps f) and g); and

   k) steps a) to d) are repeated if the ratios compared in step i) diverge significantly.
3. Method according to claims 1 and 2, characterised in that

   a value of the standard deviation recorded once on reaching the setpoint of the mean-value signal is stored;

   the actual value of the standard deviation is monitored;

   the actual value of the standard deviation is compared with the stored value of the standard deviation; and

   steps a) to d) are repeated if the values compared with each other diverge significantly.
4. Method according to any one of claims 1 to 3, characterised in that characteristic values as a function of the type of gas are specified for the threshold of the standard deviations, for the actual value of the mean-value signal assigned to said threshold and for the distance of the setpoint of the mean-value signal from said actual value, that on approaching the threshold of the standard deviations the type of gas is identified from the assigned actual value of the mean-value signal, and that, as a function of the type of gas, the associated threshold and the associated distance of the setpoint of the mean-value signal from the actual value of the mean-value signal assigned to said threshold are selected.
5. Method according to any one of claims 1 to 4, characterised in that on approaching the threshold of the standard deviations the frequency of threshold overruns is recorded, and that the threshold of the standard deviation is considered to have been reached only if the threshold overruns occur with a specified frequency.
6. Method according to any one of claims 1 to 5, characterised in that the measured quantity of the flame property representing the state of combustion is used for flame supervision.
7. Method according to any one of claims 1 to 6, characterised in that the light intensity is measured as the flame property representative of the state of combustion.
8. Method according to any one of claims 1 to 7, characterised in that the light intensity is measured over one or more radiation ranges as the flame property representative of the state of combustion.
9. Method according to claim 7 or 8, characterised in that infra-red radiation is excluded when measuring the light intensity.
10. Method according to any one of claims 7 to 9, characterised in that measurement of light intensity occurs within an adjustable field of view of the flame range.

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