METHOD FOR OPERATING A PREMIX GAS BURNER, A PREMIX GAS BURNER AND A BOILER

Abstract

A method for operating a premix gas burner wherein an air flow rate and/or a fuel gas flow rate are controlled so as to generate heat with the premix burner in accordance with a heat demand related value. The fuel gas comprises hydrogen and the method further provides a desired air excess factor relation of the air/fuel gas mixture which defines the relation between a desired air excess factor and an input variable like the heat demand related value, an air flow rate related value, or a fuel gas flow rate related value. The desired air excess factor is not a constant factor but varies for different input variable values. The fuel gas flow rate and/or the air flow rate are controlled such that an actual air excess factor converges towards the desired air excess factor while meeting the heat demand.

Claims

1.-25. (canceled)

26. A premix gas burner comprising: a burner housing with a burner deck; a supply channel configured to supply a combustible mixture to the premix gas burner; a fan configured to supply air or the combustible mixture to the supply channel; a fuel gas supply including a fuel gas control valve configured to supply a fuel gas to the supply channel; a mixing area configured to mix the air and the fuel gas so as to form the combustible mixture; and an electronic controller configured to control the rotational speed of the fan and the fuel gas control valve; wherein the electronic controller comprises a memory in which a desired air excess factor relation (λ.sub.d(I)) of the mixture is stored which defines the relation between a desired air excess factor and a chosen input variable (I), wherein the input variable (I) is one of the following parameters: a heat demand related value (Q); an air flow rate related value (F) indicative of a flow rate of the flow of air; or a fuel gas flow rate related value (FG) indicative of an actual fuel gas flow rate, wherein the desired air excess factor (λ.sub.d(I)) is not a constant factor but varies for different values of the input variable (I), wherein the desired air excess factor (λ.sub.d(I)) is defined as a ratio of the desired air to fuel gas ratio of a mixture relative to a stoichiometric air to fuel gas ratio; wherein the electronic controller is configured to: determine an actual value (I.sub.a) of the input variable (I); determine an actual air excess factor (λ.sub.a), wherein the actual air excess factor (λ.sub.a) is defined as the ratio of the actual air to fuel gas ratio of the mixture relative to the stoichiometric air to fuel gas ratio; control the fuel gas flow rate (FG) and/or the air flow rate (F) such that: the actual air excess factor (λ.sub.a) converges towards the desired air excess factor (λ.sub.d(I)) belonging to the actual value (I.sub.a) of the input variable (I); and that the heat generated with the premix burner remains in accordance with an acquired heat demand related value (Q).

27. The premix gas burner according to claim 26, wherein the electronic controller is configured to control the air flow rate (F) in dependence of the heat demand related value (Q), and wherein the fuel gas flow rate (FG) is controlled in dependence of the actual air flow rate related value (F.sub.a), wherein the chosen input value (I) of desired air excess factor relation (λ.sub.d(I)) of the mixture is the air flow rate related value (F) indicative of a flow rate of the flow of air.

28. The premix gas burner according to claim 26, wherein the electronic controller is configured for controlling to control the fuel gas flow rate (FG) in dependence of the heat demand related value (Q), and wherein the air flow rate (F) is controlled in dependence of the actual fuel gas flow rate related value (FG.sub.a), wherein the chosen input variable (I) of desired air excess factor relation (λ.sub.d(I)) of the mixture is the actual fuel gas flow rate related value (FG.sub.a) indicative of a flow rate of the fuel gas.

29. The premix gas burner according to claim 26, wherein the electronic controller is configured to directly controlling both the fuel gas flow rate (FG) as well as the air flow rate (F) in dependence of the heat demand related value (Q), wherein the chosen input variable (I) of desired air excess factor relation (λ.sub.d(I)) of the mixture is the heat demand related value (Q).

30. The premix gas burner according to claim 26, wherein the stored desired air excess factor relation (λ.sub.d(I)) of the mixture as a function of at least the input variable value (I) defines a working curve in an input variable/air excess factor-diagram in which the input variable value (I) is defined on a horizontal axis and the desired air excess factor (λ.sub.d(I)) is defined on a vertical axis, wherein the working curve extends in a work area which, along a working range of the input variable value (I), is bounded at an underside by an emission limit line, at an upper side by a blow off limit line, and at a left-hand side by a flashback limit line, which flashback limit line strictly increases when the input variable value (I) decreases.

31. The premix gas burner according to claim 30, further comprising: a temperature sensor configured to detect a signal which is indicative of a burner deck temperature (T); wherein the electronic controller is configured to adapt an existing working curve in response to a detection of a too low or too high burner deck temperature (T) at a given input variable value (I) so as to obtain an adapted working curve.

32. The premix gas burner according to claim 30, further comprising: a sensor configured to detect a signal which is indicative of a flashback; wherein the electronic controller is configured to adapt the existing working curve in response to a detection of a flashback so as to obtain an adapted working curve.

33. The premix gas burner of claim 32, wherein the sensor comprises at least one of: a temperature sensor configured to sense a temperature (T) of the burner deck; a pressure sensor configured to sense a pressure (p) in the supply channel; and a sound sensor configured to measure a sound intensity level (S) of the premix gas burner.

34. The premix gas burner according to claim 26, further comprising a mass flow determining unit configured to determine the mass flow of the air which is supplied to the supply channel by the fan, wherein a determined actual mass flow value of the air is the air flow rate related value (F), wherein the controller is configured to supply of an amount of fuel gas in dependence of the mass flow value of the supplied air such that the air excess factor (λ) is varied in accordance with a working curve.

35. A boiler for heating water, comprising: the premix gas burner according to claim 26; and a heat exchanger having a combustion chamber, wherein the burner deck of the premix gas burner is positioned in the combustion chamber.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0071] FIG. 1 shows schematically an example of a boiler according to the invention;

[0072] FIG. 2 shows an air flow/air excess factor-diagram in which an example of a working curve according to the invention is shown;

[0073] FIG. 3 shows a schematic representation of the control of the gas burner according to an example of the invention;

[0074] FIG. 4 shows a schematic representation of the control of the gas burner according to another example of the invention.

DETAILED DESCRIPTION OF THE FIGURES

[0075] In this application similar or corresponding features are denoted by similar or corresponding reference signs. The description of the various embodiments is not limited to the example shown in the figures and the reference numbers used in the detailed description and the claims are not intended to limit the description of the embodiments, but are included to elucidate the embodiments by referring to the example shown in the figures.

[0076] In general, the invention relates to a method for operating a premix gas burner 12. The method comprises: [0077] providing the premix gas burner 12 having a burner housing with a burner deck 24; [0078] acquiring a heat demand related value ; [0079] supplying a flow of fuel gas and supplying a flow of air to form a mixture; [0080] supplying the mixture to the premix gas burner 12 to burn the mixture; [0081] controlling in dependence of the acquired heat demand related value at least one of: [0082] an air flow rate, and [0083] a fuel gas flow rate,
so as to generate heat with the premix burner in accordance with the acquired heat demand related value .

[0084] The method is characterized in that the fuel gas comprises hydrogen (H.sub.2) and in that the method further comprises: [0085] providing a desired air excess factor relation λ.sub.d(I) of the mixture which defines the relation between a desired air excess factor and a chosen input variable I, wherein the input variable I is one of the following parameters: [0086] the heat demand related value ; [0087] the air flow rate related value F indicative for a flow rate of the flow of air; or [0088] the fuel gas flow rate related value FG indicative of the actual fuel gas flow rate,

[0089] wherein the desired air excess factor λ.sub.d(I) is not a constant factor but varies for different input variable values I,

[0090] wherein the desired air excess factor λ.sub.d(I) is defined as the ratio of the desired air to fuel gas ratio of the mixture relative to the stoichiometric air to fuel gas ratio; [0091] determining an actual value I, of the input variable I; [0092] determining an actual air excess factor λ, wherein the actual air excess factor λ.sub.a is defined as the ratio of the actual air to fuel gas ratio of the mixture relative to the stoichiometric air to fuel gas ratio; [0093] controlling the fuel gas flow rate FG and/or the air flow rate F such that: [0094] the actual air excess factor λ.sub.a converges towards the desired air excess factor λ.sub.d(I) belonging to the actual value I, of the input variable; and that [0095] the heat generated with the premix burner remains in accordance with the acquired heat demand related value .

[0096] The invention also provides a premix gas burner comprising: [0097] a burner housing with a burner deck 24; [0098] a supply channel 30 for supplying a combustible mixture to the premix gas burner 12; [0099] a fan 34 for supplying air or the combustible mixture to the supply channel 30; [0100] a fuel gas supply 42 including a fuel gas control valve 36 for supplying a fuel gas to the supply channel 30; [0101] a mixing area 38 for mixing the air and the fuel gas so as to form the combustible mixture; and [0102] an electronic controller 26 for controlling the rotational speed of the fan 34 and the fuel gas control valve 36.

[0103] The premix gas burner is characterized in that the electronic controller 26 comprises a memory in which a desired air excess factor relation λ.sub.d(I) of the mixture is stored which defines the relation between a desired air excess factor and a chosen input variable I, wherein the input variable I is one of the following parameters: [0104] the heat demand related value ; [0105] the air flow rate related value F indicative for a flow rate of the flow of air; or [0106] the fuel gas flow rate related value FG indicative of the actual fuel gas flow rate,

[0107] wherein the desired air excess factor λ.sub.d(I) is not a constant factor but varies for different values of the input variable I,

[0108] wherein the desired air excess factor λ.sub.d(I) is defined as the ratio of the desired air to fuel gas ratio of the mixture relative to the stoichiometric air to fuel gas ratio;

[0109] wherein the electronic controller 26 is configured to: [0110] determine an actual value I.sub.a of the input variable I; [0111] determine an actual air excess factor λ.sub.a, wherein the actual air excess factor λ.sub.a is defined as the ratio of the actual air to fuel gas ratio of the mixture relative to the stoichiometric air to fuel gas ratio; [0112] control the fuel gas flow rate FG and/or the air flow rate F such that: [0113] the actual air excess factor λ.sub.a converges towards the desired air excess factor λ.sub.d(I) belonging to the actual value I.sub.a of the input variable I; and that [0114] the heat generated with the premix burner 12 remains in accordance with the acquired heat demand related value .

[0115] The mixing area 38 may comprise a mixing device 40. The mixing device 40 may be embodied as a venturi with a throat as schematically indicated in FIG. 1. The mixing device 40 may also be embodied in other ways. The mixing area 38 may be formed by (a part of) the supply channel 30. For example, a part of the supply channel 30 which is downstream of the point where the fuel gas supply 42 is connected to the supply channel 30. The mixing area may also include the fan 34 when the fuel gas supply 42 is connected upstream from the fan 34 to the supply channel 30.

[0116] The invention further provides a boiler 10 for heating water, e.g. for central heating and/or for tap water heating. As is visible in the schematic example shown in FIG. 1 the boiler 10 comprises the premix gas burner 12 according to the invention, and a heat exchanger 11 having a combustion chamber 13. The burner deck 24 of the premix gas burner 12 is positioned in the combustion chamber 13.

[0117] The effects and advantages of the method, the premix gas burner and the boiler 10 have been described in the summary section and these effects and advantages are inserted here by reference.

[0118] In a first embodiment of both the method and the premix gas burner 12, the air flow rate F is controlled in dependence of the heat demand related value . The fuel gas flow rate FG is controlled in dependence of the actual air flow rate related value F.sub.a. The chosen input variable I of desired air excess factor relation λ.sub.d(I) of the mixture is the air flow rate related value F indicative for a flow rate of the flow of air. In this embodiment, in which the fuel gas flow rate FG follows the air flow rate related value F.sub.a which is advantageous because the fuel gas flow rate FG can be controlled very rapidly by means of the controlling the fuel gas control valve 36. Consequently, the desired air excess factor relation λ.sub.d(I) of the mixture can be achieved very quickly when the air flow rate related value F.sub.a varies, e.g. as a consequence of a increased heat demand related value .

[0119] In a second embodiment of both the method and the premix gas burner 12, which second embodiment is an alternative to the first embodiment, the fuel gas flow rate FG is controlled in dependence of the heat demand related value . The air flow rate F is controlled in dependence of the actual fuel gas flow rate related value FG.sub.a. The chosen input variable I of desired air excess factor relation λ.sub.d(I) of the mixture is the actual fuel gas flow rate related value FG.sub.a indicative for a flow rate of the fuel gas. In this second embodiment, for example the rotational speed of the fan is varied in dependence of the actual fuel gas flow rate related value. Also this type of control is a feasible solution.

[0120] In a third embodiment of both the method and the premix gas burner 12, which third embodiment is an alternative to the first and the second embodiment, both the fuel gas flow rate FG as well as the air flow rate F are directly controlled in dependence of the heat demand related value , wherein the chosen input variable I of desired air excess factor relation λ.sub.d(I) of the mixture is the heat demand related value . It will be clear that in this embodiment, the control is also very quick because both the fuel gas flow rate FG as well as the air flow rate F are controlled simultaneously.

[0121] In an embodiment of the method, the fuel gas may in addition to hydrogen (H.sub.2) also comprise methane. In the shift from natural gas to a hydrogen dominated society, there will probable an intermediate period in which a mixture of natural gas and hydrogen is supplied in the gas supply network. The method according to the invention also relates to burning a mixture of natural gas, which mainly comprises methane, and hydrogen in a premix burner 12. The premix burner 12 according to the invention is also well suited for burning mixtures of methane and hydrogen.

[0122] In a preferred embodiment, the desired air excess factor relation λ.sub.d(I) of the mixture defines a working curve 14 in an input variable/air excess factor-diagram (an I/λ.sub.d(I)-diagram) in which the input variable I is defined on the horizontal axis and the desired air excess factor λ.sub.d(I) is defined on the vertical axis. An example of such a diagram is shown in FIG. 2. The working curve 14 extends in a work area which, along a working range of the input variable I, is bounded at an underside by an emission limit line 16, at an upper side by a blow off limit line 18, and at a left-hand side by a flashback limit line 20, which flashback limit line 20 strictly increases when the value of the input variable I decreases.

[0123] Below the emission limit line 16 there is not enough air for the hydrogen comprising combustible mixture to fully combust, and/or a flame temperature may become too high, resulting in too high nitrogen oxides NO.sub.x emissions. The working curve 14 should therefore be kept above the emission limit line 16, preferably at a safety margin from the emission limit line 16. Above the blow off limit line 18 the mixture velocity is so high that the flame is not stable anymore and a blow-off occurs. Left of the flashback limit line 20 the mixture velocity is that low that it is smaller than the flame speed or the length of the flames is so short that the temperature of the upstream side of the burner deck will exceed the auto-ignition temperature of the mixture, both meaning a flashback will occur. In accordance with this embodiment, the working curve 14 extends in the area bounded by these limit lines 16, 18, 20. Preferably, the air excess factor λ is kept as low as possible so as to achieve the highest efficiency with acceptable nitrogen oxides NO.sub.x emissions.

[0124] In a further elaboration of this embodiment, the emission limit line 16 comprises a constant emission limit line air excess factor λ.sub.ELL. The emission limit line air excess factor λ.sub.ELL may be between 1 and 1.5, preferably around 1.2.

[0125] As explained, an air excess factor of 1 means that the mixture comprising hydrogen and air is a stoichiometric mixture, which means that the mixture comprises exactly enough oxygen molecules to bond with every fuel molecule. Because these molecules can freely move, it is hard for the last fuel molecule to find the last oxygen molecule. It is therefore advantageous to have a small air excess factor λ, so that each fuel molecule, even the last one, is able to readily bond with an available oxygen molecule. It turns out that it is advantageous for the efficiency of the combustion to have an air excess factor between 1 and 1.5, preferably around 1.2. It is therefore best to choose a constant emission limit line air excess factor λ.sub.ELL with this value. The working curve 14 should be kept above this value.

[0126] In a further elaboration of the first embodiment, the blow off limit line 18 is determined by tests in which for a number of air flow rate related values F the flow rate FG of the fuel gas (the test gas can be chosen in accordance with the desired gas quality) is varied until a fuel gas flow rate value is reached at which the premix gas burner 12 starts to blow off and by calculating a blow off limit line air excess factor λ.sub.BOLL belonging to that air flow rate related value F and fuel flow rate value FG. The number of blow off limit line air excess factors λ.sub.BOLL thus determined define points of the blow off limit line 18.

[0127] In a further elaboration of the second embodiment, the blow off limit line 18 is determined by tests in which for a number of gas flow rate related values FG the flow rate of the air is varied until a air flow rate value F is reached at which the premix gas burner 12 starts to blow off and by calculating a blow off limit line air excess factor λ.sub.BOLL belonging to that air flow rate related value F and fuel flow rate value FG. The number of blow off limit line air excess factors λ.sub.BOLL thus determined define points of the blow off limit line 18.

[0128] A fuel gas comprising hydrogen is more reactive for combustion than e.g. natural gas. This means that the fuel molecules are more readily to react with oxygen molecules. As a result, the combustion of these fuel molecules can be performed with a lower concentration compared with the fuel molecules of natural gas. In other words, with a higher air excess factor λ for a fuel gas comprising hydrogen the combustion can still take place without the occurrence of a blow-off. The values of the air excess factor λ.sub.BOLL above which the premix gas burner 12 starts to blow off are dependent on the characteristics of the premix gas burner 12, such as its size, shape, maximum load etc. and can be determined by tests. The working curve 14 should be kept below these values.

[0129] In an embodiment, of which an example is shown in FIG. 2 the desired air excess factor λ.sub.d(I) of the working curve 14 is substantially constant (meaning with a tolerance of up to ±0.05λ) for values of the input variable I which are higher than a cut-off input variable value I.sub.e. For input variable values I which are lower than the cut-off input variable value I.sub.e the desired air excess factor λ.sub.d(I) of the working curve 14 strictly increases when the input variable value I decreases. The desired air excess factor λ.sub.d(I) of the working curve 14 is between 1.0 and 1.6, preferably between 1.2 and 1.4, more preferably around 1.3 for input variable values I which are higher than the cut-off input variable value I.sub.e.

[0130] A small air excess factor λ is usually preferable, because that is the most efficient. According to this embodiment, a small desired air excess factor λ.sub.d(I) is chosen for input variable values I which are higher than the cut-off input variable value I.sub.e. At input variable values I higher than the cut-off input variable value I.sub.e, the lower limit of the area in which the working curve 14 must extend is not formed by the flashback limit line 20 but by the emission limit line 16. However, for input variable values I lower than the cut-off input variable value I.sub.e, the lower limit of the area may well be formed by the flashback limit line 20 which has a downwardly directed slope when viewed from low input variable values I to higher input variable values I. According to this embodiment, for input variable values I which are lower than the cut-off input variable value I.sub.e the desired air excess factor λ.sub.d(I) of the working curve 14 strictly increases when the input variable value I decreases. This means that for every input variable value I.sub.2 which is smaller than input variable value I.sub.1, the corresponding desired air excess factor λ.sub.d(I) on the working curve 14 is higher. Or in a formula, for every I<I.sub.e, if I.sub.1>I.sub.2, then λ(I.sub.1)<λ(I.sub.2). The actual cut-off input variable value I.sub.e will, amongst others, depend on the configuration of the premix gas burner 12 and the maximum air flow rate F, i.e. the maximum heat, the premix burner 12 can deliver.

[0131] In an embodiment, of which a schematic representation is shown in FIG. 3, the method includes acquiring an actual air flow rate related value F.sub.a indicative of the actual flow rate of the air, wherein the acquiring includes at least one of: [0132] measuring the actual air mass flow rate in the supply of air; [0133] measuring the actual air volume flow rate in the supply of air; and [0134] determining the rotational speed or rotations per time unit of the fan;

[0135] Additionally, this embodiment of the method includes acquiring an actual fuel gas flow rate related value FG.sub.a indicative of the actual fuel gas flow rate, wherein this acquiring includes at least one: [0136] measuring the actual fuel gas mass flow rate in the supply of fuel gas; [0137] measuring the actual fuel gas volume flow rate in the supply of fuel gas.

[0138] Further, in this embodiment of the method the determining of the actual air excess factor λ.sub.a is effected by: [0139] calculating the actual air excess factor λ.sub.a from the actual flow rate FG.sub.a of the fuel gas, the actual air flow rate related value F.sub.a and the stoichiometric air to fuel ratio.

[0140] In an embodiment of the premix gas burner 12, the calculating may be performed by a controller 26. The controller 26 may have the stoichiometric air to fuel ratio stored in memory.

[0141] In an alternative embodiment, of which a schematic representation is shown in FIG. 4, the determining of the actual air excess factor λ.sub.a may be effected by measuring a temperature T of the burner deck 24 and by acquiring a matching air excess factor λ.sub.a from an air excess factor map, curve or formula stored in a memory. The air excess factor map, curve or formula defines the air excess factor λ.sub.a in dependence of both the burner deck temperature T and the actual value of the input variable I.sub.a. Again, the acquiring of the matching air excess factor λ.sub.a may be performed by a controller 26. The controller 26 may comprise the memory in which the air excess factor map, curve or formula is stored.

[0142] In an embodiment of the method and of the premix gas burner, the premix gas burner is provided with a temperature sensor 22 capable of detecting a signal which is indicative of a burner deck 24 temperature T. In this embodiment, the existing working curve 14 is adapted in response to a detection of a too low or too high burner deck temperature T at a given input variable value I so as to obtain an adapted working curve 14′. The electronic controller 26 of the embodiment of the premix gas burner 12 is configured to perform this adaptation.

[0143] A too high temperature T of the burner deck 24 at given input variable value I is an indication of a too high flame temperature which may result in the emission of unwanted nitrogen oxides NO.sub.x or risk of flashback. A too low burner deck temperature T at given input variable value I is an indication for an excess of air and thus an inefficient combustion. When for some reason, the circumstances in the premix gas burner 12 are such that the burner deck temperature T becomes too high or too low at given input variable value I, the method of this embodiment will detect the exceeding temperature T and adapt the working curve 14 so that the chance of a too high or too low temperature in future is reduced.

[0144] In an embodiment of the method and the premix gas burner 12 in which the working curve 14 is adapted, the adapting the existing working curve 14 in response to a detection of a too low or too high burner deck temperature T at given input variable value I comprises at least one of: [0145] shifting the working curve 14 in the input variable/air excess factor-diagram to the left with respect to the input variable value I when the burner deck temperature is too low in relation to the input variable value F, such that the new working curve 14′ as a function of the input variable value I relates to the former working curve 14 in that: λ.sub.new(I)=λ(I+ΔI), wherein ΔI is the shift in input variable value I; [0146] shifting the working curve 14 in the input variable/air excess factor-diagram downwardly with respect to the air excess factor λ when the burner deck temperature is too low in relation to the input variable value I, such that the new working curve 14′ as a function of the input variable value I relates to the former working curve 14 in that: λ.sub.new(I)=λ(I)−Δλ, wherein Δλ is the shift in air excess factor λ; [0147] making the slope of the working curve 14 less steep for input variable values I which are lower than the cut-off input variable value I.sub.e when the burner deck temperature is too low in relation to the input variable value I; and [0148] shifting the cut-off input variable value I.sub.e in the input variable/air excess factor-diagram to the left when the burner deck temperature is too low in relation to the input variable value I, such that the new cut-off input variable value I.sub.e′ relates to the former cut-off input variable value I.sub.e in that: I.sub.e′<I.sub.e; [0149] shifting the working curve 14 in the input variable/air excess factor-diagram to the right with respect to the input variable value I when the burner deck temperature is too high in relation to the input variable value I, such that the new working curve 14′ as a function of the input variable value I relates to the former working curve 14 in that: λ.sub.new(I)=λ(I−ΔI), wherein ΔI is the shift in input variable value I; [0150] shifting the working curve 14 in the input variable/air excess factor-diagram upwardly with respect to the air excess factor λ when the burner deck temperature is too high in relation to the input variable value I, such that the new working curve 14′ as a function of the input variable value I relates to the former working curve 14 in that: λ.sub.new(I)=λ(I)+Δλ, wherein Δλ is the shift in air excess factor λ; [0151] making the slope of the working curve 14 steeper for input variable values I which are lower than the cut-off input variable value I.sub.e when the burner deck temperature is too high in relation to the input variable value I; and [0152] shifting the cut-off input variable value I.sub.e in the input variable/air excess factor-diagram to the right when the burner deck temperature is too high in relation to the input variable value I, such that the new cut-off input variable value I.sub.e′ relates to the former cut-off input variable value I.sub.e in that: I.sub.e′>I.sub.e.

[0153] The shifting of the working curve 14 with respect to the input variable value I may be executed such that the new working curve 14 as a function of the input variable value I relates to the former working curve 14 in that: λ.sub.new(I)=λ(I−ΔI) for a shift to the right, and in that: λ.sub.new(I)=λ(I+ΔI) for a shift to the left, wherein ΔI is the shift in the input variable value I. The shifting of the working curve 14 with respect to the air excess factor λ may be executed such that the new working curve 14 as a function of the input variable value I relates to the former working curve 14 in that: λ.sub.new(I)=λ(I)+Δλ for an upward shift, and in that: λ.sub.new(F)=λ(F)−Δλ for a downward shift, wherein Δλ is the shift in air excess factor λ. The shifting of the cut-off input variable value I.sub.e may be executed such that the new cut-off input variable value I.sub.e′ relates to the former cut-off input variable value I.sub.e in that: I.sub.e′>I.sub.e for a shift to the right, and in that: I.sub.e′<I.sub.e for a shift to the left.

[0154] As explained above, one of the drawbacks of using a fuel gas comprising hydrogen in a premix gas burner 12 is the possibility of the flame temperature become too high. The use of the working curve 14 is aimed at avoiding this drawback in an optimal way. An existing working curve 14 may, e.g. due to a change in external circumstances, become ill-fitted for the new circumstances, meaning that the burner deck temperature T may become too high at a given input variable value I, which is indicative of a too high flame temperature, or too low, which is indicative of an inefficient combustion. Detecting these too high and/or too low temperatures T and adapting the existing working curve 14 will additionally optimize the working curve 14. According to this embodiment, the adaptation of the working curve in response to the detection of a too high or too low burner deck temperature T at given air flow rate related value F involves moving the working curve 14 away from or towards the flashback limit line 20 and/or the emission limit line 16, making the occurrence of a too high or too low burner deck temperature T even less likely. Of course, combinations of these adaptations may be used as well.

[0155] In an embodiment of the method, the premix gas burner 12 comprises a sensor 22, 28, 32 capable of detecting a signal which is indicative of a flashback. The method further comprises adapting the existing working curve 14 in response to a detection of a flashback so as to obtain an adapted working curve 14′.

[0156] In an embodiment of the premix gas burner, the premix gas burner comprises a sensor 22, 28, 32 capable of detecting a signal which is indicative of a flashback. This embodiment of the premix gas burner has an electronic controller 26 which is configured to adapt the existing working curve 14 in response to a detection of a flashback so as to obtain an adapted working curve 14′.

[0157] When for some reason, the circumstances in the premix gas burner 12 are such that a flashback occurs, the method of this embodiment will detect the flashback and adapt the working curve 14 so that the chance of a flashback in the future is reduced.

[0158] In a further elaboration of the embodiment with the flashback detection, the adapting the existing working curve 14 in response to a detection of a flashback comprises at least one of: [0159] shifting the working curve 14 in the input variable/air excess factor-diagram to the right with respect to the input variable value I, such that the new working curve 14′ as a function of the input variable I relates to the former working curve 14 in that: λ.sub.new(I)=λ(IF−ΔI), wherein ΔI is the shift in input variable value I; [0160] shifting the working curve 14 in the input variable/air excess factor-diagram upwardly with respect to the air excess factor λ, such that the new working curve 14′ as a function of the input variable value I relates to the former working curve 14 in that: λ.sub.new(I)=λ(I)+Δλ, wherein Δλ is the shift in air excess factor λ; [0161] making the slope of the working curve 14 steeper for input variable values I which are lower than the cut-off input variable value I.sub.e; and [0162] shifting the cut-off input variable value I.sub.e in the input variable/air excess factor-diagram to the right, such that the new cut-off input variable value I.sub.e′ relates to the former cut-off input variable value I.sub.e in that: I.sub.e′>I.sub.e.

[0163] As explained above, another risk of using a fuel gas comprising hydrogen in a premix gas burner 12 is the higher probability of the occurrence of flashbacks. The use of the working curve 14 is aimed at avoiding these flashbacks. An existing working curve 14, e.g. due to a change in external circumstances may become ill-fitted for the new circumstances, meaning that a flashback may occur. Detecting these flashbacks and adapting the existing working curve 14 will additionally optimize the working curve 14, meaning that in future the chance of an occurrence of a flashback is reduced. According to this embodiment, the adaptation of the working curve in response to a flashback occurrence involves moving the working curve 14 away from the flashback limit line 20, making the occurrence of a flashback even less likely. Of course, a combination of these adaptations may be used as well.

[0164] In an embodiment, the sensor 22, 28, 32 capable of detecting a signal which is indicative of a flashback comprises a temperature sensor 22 which measures the temperature T of the burner deck 24. A said flashback is determined by an increase ΔT in the temperature T of the burner deck 24 within a certain time period Δt.sub.0 which increase is bigger than a preset temperature increase threshold ΔT.sub.0.

[0165] In a further elaboration of this embodiment, the time period Δt.sub.0 may equal 2 seconds and the temperature increase threshold ΔT.sub.0 may equal 100° C. The temperature sensor 22 may be connected to the burner deck 24. For example, the sensor 22 may be a thermocouple which is welded to the burner deck 24.

[0166] Thermocouples are reliable and relatively cheap. Consequently, their application as a sensor 22 for detecting flashbacks is preferred. Alternatively to the thermocouple which is welded to the burner deck 24, the sensor 22 may also be embodied as an infrared sensor which is placed at a distance from the burner deck 24 and detects infrared radiation from the burner deck 24.

[0167] Additionally or alternatively, the sensor 22, 28, 32 capable of detecting a signal which is indicative of the flashback may be a pressure sensor 28 which measures a pressure p in a supply channel 30 through which supply channel 30 the mixture is supplied to the premix gas burner 12. A flashback is determined by an increase Δp in the pressure p in the supply channel 30 within a certain time period Δt.sub.0 which increase is bigger than a preset pressure increase threshold Δp.sub.0.

[0168] The pressure sensor 28 may be e.g. a pressure transducer.

[0169] In yet another alternative or additional embodiment, the sensor 22, 28, 32 capable of detecting a signal which is indicative of a flashback may be a sound sensor 32 which measures a sound intensity level S of the premix gas burner 12. A flashback is determined when the sound intensity level S is higher than a preset sound intensity threshold S.sub.0.

[0170] When a flashback occurs, the wave front of the flashback flame travels in the upstream direction. Effects of a flashback are e.g. an increase in temperature of the burner deck 24, an increase in pressure in the supply channel 30, and an audible sound. With the sensors 22, 28, 32 described above, each of or any combination of these effects may be monitored. In each of the three alternative embodiments discussed above, one of these effects is monitored. It has been determined that the temperature on the burner deck 24 may rise more than 100° C. within 2 seconds in case of a flashback. The burner deck 24 is therefore well suited to place the temperature sensor 22. The flashback propagating within the supply channel 30 will result in a considerable increase in pressure. The supply channel 30 is therefore well suited to place the pressure sensor 28.

[0171] In an embodiment of the method, the air flow rate related values F may be mass flow rate values of the air which is supplied to the premix burner 12. Instead of mass flow rate values, volume flow rate values of the supplied air may be used as the air flow rate related value. Further, the rotational speed of the fan 34 may be used as the air flow rate related value. For example, the rotations per minute of the fan 34 may be used as the air flow rate related value. Optionally, the inlet temperature of the air and/or a pressure difference between the environment and a pressure downstream of the fan 34 in the supply channel 30 may be taken into account. For example, the mass flow rate of the air flow may be calculated based upon air inlet temperature and rotations per minute of the fan 34, an outside environmental pressure and a pressure measured in the supply channel 30.

[0172] In an embodiment, the premix gas burner may comprise a mass flow rate determining unit for determining the mass flow rate of the flow of air which is supplied to the supply channel 30 by the fan 34. In this embodiment, the determined actual mass flow rate value of the flow of air may be used as the air flow rate related value F for controlling the actual air excess factor λ.sub.a. In this embodiment, the electronic controller 26 is configured for controlling supply of the amount of fuel gas in dependence of the mass flow rate value of the supplied air such that the actual air excess factor λ.sub.a is varied in accordance with the working curve 14.

[0173] The controlling of the flow rate FG of the fuel gas may be effected with a fuel gas control valve 36 which is connected to the electronic controller 26 and which is accommodated in the fuel gas supply 42.

[0174] Of course, the electronic controller 26 may also be configured to control the fan 34 so that the air flow rate of the premix burner 12 can be varied, for example in response to a heat demand from a central heating system or a tap water demand.

[0175] The various embodiments which are described above may be used implemented independently from one another and may be combined with one another in various ways. The reference numbers used in the detailed description and the claims do not limit the description of the embodiments nor do they limit the claims. The reference numbers are solely used to clarify.

LEGEND

[0176] 10—central heating boiler [0177] 11—heat exchanger [0178] 12—gas burner [0179] 13—combustion chamber [0180] 14—working curve [0181] 16—emission limit line [0182] 18—blow off limit line [0183] 20—flashback limit line [0184] 22—temperature sensor [0185] 24—burner deck [0186] 26—electronic controller [0187] 28—pressure sensor [0188] 30—supply channel [0189] 32—sound sensor [0190] 34—fan [0191] 36—fuel gas control valve [0192] 38—mixing area [0193] 40—mixing device [0194] 42—fuel gas supply [0195] F—air flow rate related value [0196] F.sub.a—actual air flow rate related value [0197] F.sub.e—cut-off air flow rate [0198] FG—flow rate of the fuel gas [0199] I—input variable [0200] I.sub.a—actual value of the input variable I [0201] S—sound intensity level [0202] S.sub.0—sound intensity threshold [0203] p—pressure [0204] T—temperature [0205] ΔF—shift in air flow rate related value [0206] Δp—increase in pressure [0207] Δp.sub.0—pressure increase threshold [0208] Δt.sub.0—time period [0209] ΔT—increase in temperature [0210] ΔT.sub.0—temperature increase threshold [0211] Δλ—shift in air excess factor [0212] λ.sub.a—actual air excess factor [0213] λ.sub.d(I)—desired air excess factor at a given value of the input variable [0214] λ.sub.ELL—emission limit line air excess factor [0215] λ.sub.BOLL—blow off limit line air excess factor