SURFACE STABILIZED FULLY PREMIXED GAS PREMIX BURNER FOR BURNING HYDROGEN GAS, AND METHOD FOR STARTING SUCH BURNER

Abstract

Method for starting a burner wherein a premixed gas comprising a combustible gas and air is supplied, wherein the combustible gas comprises at least 50% by volume of hydrogen. The method comprises the following steps: during a start-up phase: supplying premixed gas having a first lambda-value to the burner surface, wherein the first lambda-value is at least 1.85, and igniting the supplied premixed gas having the first lambda-value using an ignition source. During an operation phase after the premixed gas has been ignited: supplying premixed gas having a second lambda-value to the burner surface, wherein the first lambda-value is larger than the second lambda-value. Independent claims for a burner and a heating appliance are included.

Claims

1. A method for starting a burner wherein a premixed gas comprising a combustible gas and air is supplied to a burner surface of the burner, wherein the combustible gas comprises at least 50% by volume of hydrogen, a lambda-value is defined as a ratio between an actually supplied quantity of air and the quantity of air required for stoichiometric combustion of the premixed gas, the burner is a surface stabilized fully premixed gas premix burner, the burner is configured to be modulated between a minimum load and a full load, wherein the method comprises the following steps: during a start-up phase: supplying premixed gas having a first lambda-value to the burner surface, wherein the first lambda-value is at least 1.85, and igniting the supplied premixed gas having the first lambda-value using an ignition source, during an operation phase after the premixed gas has been ignited: supplying premixed gas having a second lambda-value to the burner surface, wherein the first lambda-value is larger than the second lambda-value.

2. The method according to claim 1, wherein the lambda-value is controlled during the start-up phase by controlling the quantity of air supplied by an air channel and/or the quantity combustible gas supplied by a combustible gas channel.

3. The method according to claim 1, wherein the burner comprises a premixed gas supply circuit, comprising an air channel for supplying air, a combustible gas channel for supplying combustible gas, a mixing channel for mixing air supplied by the air channel and combustible gas supplied by the combustible gas channel into the premixed gas to be supplied to the burner surface, and at least one channel obstruction element for partially obstructing the combustible gas channel and/or the air channel, wherein the method further comprises the following steps: during the start-up phase: partially obstructing the combustible gas channel with the at least one channel obstruction element, such that less combustible gas is provided to the mixing channel during the start-up phase relative to the operation phase, and/or during the operation phase: partially obstructing the air channel with the at least one channel obstruction element, such that more air is provided to the mixing channel during the start-up phase relative to the operation phase.

4. The method according to claim 3, wherein the at least one channel obstruction element is arranged in a rest position in the start-up phase, wherein the method further comprises the step of actuating the channel obstruction element to arrange the channel obstruction element in an actuated position during the operation phase.

5. The method according to claim 1, wherein the first lambda-value is larger than 2, e.g. between 2-6, preferably larger than 3, e.g. between 3-5, more preferably larger than 4, e.g. between 4-5.

6. The method according to claim 1, wherein the second lambda-value is between 1-2, preferably between 1.05-1.5, more preferably between 1.05-1.3.

7. The method according to claim 1, the first lambda-value is at least 1.5 times as large as the second lambda-value, preferably at least 2 times as large, e.g. at least 3 times as large.

8. The method according to claim 1, wherein the combustible gas comprises at least 75% by volume of hydrogen, preferably at least 80% by volume of hydrogen, more preferably at least 95% or at least 98% by volume of hydrogen.

9. The method according to claim 1, wherein the start-up phase lasts at least 1 second, preferably at least 2 seconds, and even more preferably at least 3 seconds, e.g. between 3-6 seconds.

10. The method according to claim 1, wherein the burner is started at a start-up load in the start-up phase which is different from a desired load in the operation phase, wherein the method further comprises a transition phase to transition from the start-up phase to the operation phase after the premixed gas has been ignited, wherein transition phase comprises a step of changing the load to the desired load.

11. The method according to claim 1, wherein the method further comprises a step of maintaining the ignition source in an ignition state for an ignition period after it has been detected that the supplied premixed gas having the first lambda-value has been ignited.

12. A burner configured to perform the method according to claim 1, the burner being a surface stabilized fully premixed gas premix burner.

13. A burner for burning a combustible gas comprising at least 50% by volume of hydrogen, wherein said burner is a surface stabilized fully premixed gas premix burner, and wherein said burner is configured to be modulated between a minimum load and a full load, said burner comprising: a burner surface, a premixed gas supply circuit, comprising i. an air channel for supplying air, ii. a combustible gas channel for supplying combustible gas, iii. a mixing channel for mixing air supplied by the air channel and combustible gas supplied by the combustible gas channel into a premixed gas to be supplied to the burner surface, wherein a lambda-value is defined as a ratio between an actually supplied quantity of air and the quantity of air required for stoichiometric combustion of the premixed gas, an ignition source for igniting the premixed gas supplied to the burner surface, a controller configured to control the lambda-value of the supplied premixed gas by controlling the quantity of air supplied by the air channel and/or the quantity combustible gas supplied by the combustible gas channel, wherein the controller is configured to i. supply premixed gas having first lambda-value during a start-up phase of the burner wherein the ignition source is configured to ignite the supplied premixed gas having the first lambda-value, wherein the first lambda-value is at least 1.85, and ii. supply premixed gas having a second lambda-value during an operation phase of the burner after the ignition source is configured to ignite the supplied premixed gas having the first lambda-value, wherein the first lambda-value is larger than the second lambda-value.

14. The burner according to claim 13, further comprising at least one channel obstruction element for partially obstructing the combustible gas channel and/or the air channel, wherein the controller further is configured to control the at least one channel obstruction element to partially obstruct the combustible gas channel during the start-up phase and/or partially obstruct the air channel during the operation phase.

15. The burner according to claim 14, wherein the at least one channel obstruction element has an actuated position and a rest position, wherein the at least one channel obstruction element is configured to be in the actuated position during the operation phase and in the rest position during the start-up phase.

16. The burner according to claim 14, wherein the burner comprises a gas valve in addition to the at least one channel obstruction element, wherein the gas valve arranged in the combustible gas channel, wherein said gas valve has a closed position wherein the combustible gas is prevented from flowing through the combustible gas channel, and an open position wherein the combustible gas is able to flow through the combustible gas channel.

17. The burner according to claim 14, wherein the channel obstruction element is a valve, e.g. an electronically actuated control valve.

18. The burner according to claim 13, further comprising at least one oxygen sensor configured to measure a value representative of an oxygen content of a flue gas generated by the burner or representative of an oxygen content of the premixed gas supplied to the burner surface.

19. The burner according to claim 13, further comprises at least one flame detector configured to detect when the supplied premixed gas is ignited and/or burning, and generate a corresponding flame signal, wherein preferably the controller is further configured to control the premixed gas to have the second lambda-value after having received the flame signal from the detector.

20. The burner according to claim 13, wherein the burner comprises a perforated metal plate for stabilizing flames when the supplied premixed gas is burning.

21. A hydrogen gas fired heating appliance comprising a burner according to claim 13.

Description

[0100] In the figures:

[0101] FIG. 1 illustrates a burner according to first embodiment of the invention;

[0102] FIG. 2 illustrates an example of the lambda-value in function of time;

[0103] FIG. 3 illustrates a few factors that in optional embodiments can be taken into account when deciding the first and/or second lambda-value;

[0104] FIG. 4 shows a second embodiment of the burner according to the invention;

[0105] FIG. 5 shows a third embodiment of the burner according to the invention;

[0106] FIG. 6 schematically illustrates the steps of a method for starting a burner in accordance with a possible embodiment of the invention.

[0107] FIG. 1 schematically illustrates a burner 100 first embodiment of the invention. The burner 100 preferably is a surface stabilized fully premixed gas premix burner, which can be modulated between a minimum load and a full load. The burner 100 comprises a burner surface 123, to which premixed gas is supplied by a premixed gas supply circuit. In the shown example, the burner surface 123 comprises perforations through which the premixed gas flows into a combustion chamber 130. The combustion chamber 130 may e.g. be part of heating appliance, wherein in particular water is heated. An ignition source 124 is further provided for igniting the supplied premixed gas. In the shown embodiment the burner surface 123 is schematically depicted as being round. However, in practice the burner surface 123 can have any suitable shape, e.g. round, curved or flat. The shape of the burner surface 123 may be dependent on the shape of the combustion chamber 130, and/or vice versa.

[0108] The premixed gas comprises combustible gas and air. Therefore, the premixed gas supply circuit comprises a combustible gas channel 111, which is connected to a combustible gas supply 114. The combustible gas supply 114 in the shown example is a tank, but other options include a distribution network similar as to what is known for the distribution of traditional hydrocarbon gasses such as methane in municipal or industrial areas. In the context of the present invention, the combustible gas comprises at least 50% by volume of hydrogen, in some embodiments at least 80%, at least 95% or at least 98%.

[0109] In the combustible gas channel 111 a gas valve 112 is provided, with which the quantity of combustible gas that flows through the combustible gas channel 111 can be regulated. In the shown example, the gas valve 112 is an electronically actuated control valve, controlled by an electronic actuator 113. However, it is also known to design the gas valve 112 such that it opens based on pneumatic forces. For example, the gas valve 112 may be biased to a closed position by a spring force, but automatically open to let the desired quantity of combustible gas through when an under pressure downstream of the gas valve 112 is created by a flow of air.

[0110] In order to be able to ignite the combustible gas, oxygen is required. In the present invention, air is used for supplying said oxygen. The premixed gas supply circuit therefore comprises an air channel 101 for providing air. Preferably, a fan 102 is provided for providing the air to flow. Although in the shown example the fan 102 is provided upstream of the location where the air channel 101 and the combustible gas channel 111 meet, in some embodiments the fan 102 may be arranged downstream of said location. It is also possible to arrange multiple fans, optionally on multiple locations. The air channel 101 is further upstream connected to an air supply (not shown). Usually, the air supply simply is the environmental air. For example, the air channel 101 may connected to the outside air, e.g. through a hole in a wall, and the fan 102 provides a suction force for sucking the air into the air channel 101.

[0111] FIG. 1 further shows that optionally the air channel 101 comprises a narrower portion 121, i.e. being narrower than the more upstream part of the air channel 101. The flow speed of the air increases in the narrower portion, and therefore the pressure reduces, as follows from Bernoulli's principle. The combustible gas channel 111 is connected to this narrower portion 121. Because of the reduced pressure of the air, a venture effect is created, since a suction force on the combustible gas is provided, resulting in improved mixing of the combustible gas and the air.

[0112] The premixed gas supply circuit further comprises a mixing channel 122. In the mixing channel 122 the air supplied by the air channel 101 and combustible gas supplied by the combustible gas channel 111 into a premixed gas to be supplied to the burner surface 123. Based on the composition of the combustible gas, a certain quantity of oxygen is required for complete combustion of the combustible gas. Based on the composition of the air, the quantity of required air can be derived from the quantity of required oxygen. Since in practice, the quantity of air will differ from this, a lambda-value is defined as a ratio between an actually supplied quantity of air and the quantity of air required for stoichiometric combustion of the premixed gas.

[0113] Usually the burner 100 is started following the following procedure. First, the fan 102 is started such that air flows through the air channel 101. Then, the ignition source 124 is started, but since there is no combustible gas yet, there will not be any combustion. Thereafter, the gas valve 112 is opened such that combustible gas can flow in the combustible gas channel 111.

[0114] The combustible gas and air are mixed in the mixing channel 122 and through perforations of the burner surface 123 the premixed gas enters the combustion chamber 130. The ignition source 124, which is still activated, ignites the supplied premixed gas, and a combustion and flame are present in the combustion chamber 130.

[0115] If there is a malfunction or failure, however, for example of the ignition source 124, there may not be an immediate combustion of the supplied premixed gas. Consequently, premixed gas comprising the combustible gas will accumulate in the combustion chamber 130. The same will occur during a delayed ignition test. Test have shown that in case the combustible gas comprises a substantial quantity of hydrogen, several problems can occur if the accumulated premixed gas then after a certain amount of time is ignited. These problems can lead to undesired damage and/or danger. For example, a flame flashback can occur, i.e. the flame can propagate back through the burner surface 123. It can also happen that an explosion occurs in the combustion chamber 130.

[0116] The present invention provides a solution by providing an additional excess of air during a start-up phase in comparison to an operation phase. An example of the lambda-value in function of time is shown in FIG. 2. As can be seen, the lambda-value is at 4 between the first and sixth second. Note that initially only the fan is started to provide air, and after 1 second combustible gas is added. After the eighth second, the lambda-value in the shown example is at approximately 1.3, although the exact lambda-value may be dependent on the load. Test have shown that the increased lambda-value during the start-up phase decreases the problems above. Note further that the load during the start-up phase may be different than the load in operation phase. During the transition from the start-up phase to the operation phase, which in FIG. 2 corresponds with the time period 6-8 seconds, the fan may also be adapted to provide a different flow.

[0117] With reference to FIG. 1, an embodiment of the implementation of the invention will further be elaborated. The burner 100 comprises a controller 150. The controller 150 is configured to control the lambda-value of the supplied premixed gas. In the shown example, the controller 150 does this by controlling the gas valve 112. In particular, the controller 150 has an output terminal 150.1 for sending a control signal 151 to an input terminal 113.1 of the actuator 113 of gas valve 112. By controlling the position of the gas valve 112, the quantity of combustible gas that enters the mixing channel 122 is controlled, and as such the ratio air to combustible gas and the lambda-value. It should be noted, however, that several other possibilities can be applied as alternative or in combination of the electronically actuated control gas valve 112, some of which are elaborated herein.

[0118] In accordance with the invention, the controller 150 is configured to supply premixed gas having first lambda-value during a start-up phase of the burner 100. The period before the ignition source 124 ignites the supplied premixed gas having the first lambda-value, is part of the start-up phase. Also the ignition itself is during the start-up phase. The controller 150 is further configured to supply premixed gas having a second lambda-value during an operation phase of the burner. The operation phase commences after the ignition source 124 has ignited the supplied premixed gas having the first lambda-value. According to the invention, the first lambda-value is larger than the second lambda-value.

[0119] In case of a failure or during a delayed ignition test, the premixed gas having the first lambda-value may accumulate in the combustion chamber 130, until it is ignited. Because the premixed gas that is initially ignited has the lower, first lambda-value, the flame speed is reduced. The risks of flame flashback and explosion are as such reduced.

[0120] Preferably, the first lambda-value is at least 1.85. It has been found that this is a practical lower limit with which satisfactory results can be achieved.

[0121] The burner 100 preferably comprises at least one channel obstruction element 112, which in the example shown in FIG. 1 is embodied as the gas valve 112. The channel obstruction element 112 in this embodiment is arranged such that it can partially obstruct the combustible gas channel 111. The controller 150 can control the channel obstruction element 112 by outputting a control signal 151 via output terminal 150.1 to input terminal 113.1 of actuator 113. During the start-up phase, the controller 150 controls the channel obstruction element 112 such that the combustible gas channel 111 is partially obstructed. As such, less combustible gas is supplied to the premixed gas, resulting in the first lambda value being larger.

[0122] Preferably, the channel obstruction element 112 is in a rest position during the start-up phase. So, the gas valve 112 may be biased to be partially closed, for example by means of one or more springs. By providing a force with the actuator 113, the gas valve 112 can be further opened to an actuated position during the operation phase, such that more combustible gas is supplied to the premixed gas. However, in case there is a failure, e.g. in the controller 150 or actuator 113, the gas valve 112 will remain in the rest position even during the operation phase, and the premixed gas in the operation phase will have the first lambda-value. Although this may lead to an inefficient combustion, safety is guaranteed, because it is avoided that during such failures the lambda-value during the start-up phase is too low.

[0123] There are several possible ways to implement when the transition from the start-up phase to the operation phase can be done. Preferably, the start-up phase lasts at least 1 second, preferably at least 2 seconds, and even more preferably at least 3 seconds, e.g. between 3-6 seconds. In some embodiments, the controller 150 can be configured to automatically switch to the operation phase after a predetermined amount of time.

[0124] FIG. 1 shows that an optional flame detector 131 is provided in the combustion chamber 130. The flame detector 131 is configured to generate a flame signal 153 when it detects a flame in the combustion chamber 130, which indicates that that the supplied premixed gas is ignited and/or burning. The flame detector 131 may be embodied according to any of the known suitable principles for flame detection. The flame signal 153 is outputted to the controller 150 via output terminal 131.1 and input terminal 150.3. The controller 150 can use the information provided by the flame signal 153 in several ways. For example, the controller 150 can be configured to only actuate the gas valve 112 to the actuated position after a flame is detected, thereby avoiding that premixed gas with the second lambda-value reaches the combustion chamber 130 before the already present premixed gas is ignited. This can be done as an alternative or in addition to waiting the predetermined amount of time as explained above. It is also possible that the controller 150 is able to control the ignition source 124, as e.g. indicated in FIG. 1 wherein a control signal 152 can be send via output terminal 150.2 and input terminal 124.1. In that case, the controller 150 can be configured to stop the ignition source 124 from igniting the premixed gas if no flame has been detected by the flame detector 131 after a certain amount of time. This would avoid dangerous situations when a substantial quantity of premixed gas has accumulated in the combustion chamber 130 without being ignited. It is noted that some standards prescribe this as a mandatory measure. On the other hand, by controlling the ignition source 124, it is also possible to ensure that the supplied premixed gas in the combustion chamber 130 only is ignited, when said premixed gas has a satisfactory lambda-value. It is also possible that the controller 150 controls the ignition source 124 to stay in an ignition state for an ignition period after detection of the initial ignition of the premixed gas. As such, accumulated premixed gas may be burned even when the flame has moved away from said accumulated premixed gas.

[0125] FIG. 3 illustrates a few factors that in optional embodiments can be taken into account when deciding the first and/or second lambda-value. These factors can be taken into account separately or in combination of each other. On the horizontal axis of FIG. 3, the load of the burner is expressed, and on the vertical axis the lambda-value is expressed. Each of the lines in the graph represent a different factor, which will be explained below. For each line, an arrow is provided indicating on which side of the respective line the lambda-value preferably should be.

[0126] The burner is configured to be modulate between a minimum and a full load. For example, for household heating appliances the full load may be 24 kW. Whereas traditionally the modulation ratio, i.e. the ratio of the full load over the minimum load, was around 4:1-5:1, recently modulation ratios of up to 10:1 have been proposed. In FIG. 3, line 3.6 illustrates the lower limit of 20% when the modulation ratio is 5:1, and line 3.7 shows the lower limit of 10% when the modulation ratio is 10:1.

[0127] The second lambda-value is normally in the range of 1.05-1.3, in particular at high or full load. A small excess of air is provided to avoid incomplete combustion in cases where the air and combustible gas are not sufficiently mixed, or the composition of the air and/or combustible gas deviate. Line 3.8 in FIG. 3 illustrates an example of the second lambda-value in function of the load. It has been found that when the combustible gas comprises a significant quantity of hydrogen, it may be optimal to adapt the lambda-value based on the load during the operation phase, thus e.g. the second lambda-value. As explained in European patent application with application Ser. No. 19/162,278, the lambda-value at the minimum load may be at least 20% higher than at full load, and optionally at average load the lambda-value may be less than 10% higher than at full load. In general, the burner will be started on a load within the modulation range. In accordance with the invention, the first lambda-value at any given load is higher than the second lambda-value at said load, when the burner is started at said load. This is indicated by line 3.10 in FIG. 3, which corresponds to line 3.8 multiplied by 1.5. In some embodiments, however, the burner may be started at a load that is below the minimum load, although this is limited by a reduced minimum load, since below that minimum load it may not be able to determine the flame or whether the burner is on or off within an acceptable time.

[0128] Preferably the first lambda-value is below a blow-off value. The blow-off value is the lambda-value at which there is so little combustible gas relative to air in the premixed gas, that any flame at the burner surface is blown out by the premixed gas, because there is not sufficient combustible gas to keep the flame burning.

[0129] Preferably, the first lambda-value is such that the concentration of combustible gas in the premixed gas is below an upper flammability limit, also referred to as UFL, indicated in FIG. 3 by line 3.2. Preferably, the first lambda-value is such that the concentration of combustible gas in the premixed gas is above a lower flammability limit, also referred to as LFL, indicated in FIG. 3 by line 3.1. Otherwise, it may not be possible to ignite the premixed gas, since the premixed gas is too rich or too lean, respectively. It should be noted that the concentration of combustible gas in the premixed gas being above a certain threshold, corresponds with the lambda-value being below a lambda-value corresponding with said threshold. It should further be noted that the upper and lower flammability limit is determined by the composition of the combustible gas, but also dependent on factors such as temperature and pressure.

[0130] In practice, the first lambda-value may also be limited by the fan, in particular in embodiments where the lambda-value is adjusted by partially obstructing the air channel. The maximal capacity or power of the fan determines the maximal quantity of air that can flow through the air channel, which with a given quantity of supplied combustible gas determines the lambda-value of the premixed gas. Of course, in theory it is possible to provide a larger fan, but in practice this may not be desirable because of cost considerations. Therefore, the first lambda-value preferably corresponds with a quantity of air that is lower than a quantity of air provided by the fan, when said fan is on full load. This is indicated in FIG. 3 by line 3.3. It is noted that the quantity of combustible gas may also be determined by the fan, in particular when the fan is arranged downstream of the location where the combustible gas channel and the air channel meet.

[0131] Preferably, the first lambda-value is such that the concentration of combustible gas in the premixed gas is below a lower explosion limit, also referred to as LEL, meaning that the first lambda-value should be above a lambda-value corresponding with the LEL, indicated in FIG. 3 by line 3.4. It may be preferable to control the first lambda-value to differ more than a predetermined safety margin from the lower explosion limit, e.g. the safety margin being a factor 1.2 or 1.5, indicated in FIG. 3 by line 3.5. This ensures safety start-up, even when the actual composition of the air or combustible gas differs from expectation.

[0132] Preferably, the first lambda-value is below lower temperature value, indicated in FIG. 3 by line 3.9. In this context the lower temperature value is defined as the value which causes the flame of ignited premixed gas to be at such a low temperature that it extinguishes. In case the combustible gas comprises exclusively hydrogen, said temperature is approximately 571 degrees Celsius.

[0133] As can be seen in FIG. 3, when following all of the above optional limitations, an ideal range for the first lambda-value becomes apparent, which is indicated by reference numeral 3.50 in FIG. 3. This can be used to determine optimal first lambda-value, based on the composition of the combustible gas and the air, as well as environmental conditions such as temperature and pressure. Depending on how many of the above factors are taken into account, this range can be determined with more precision. However, in some cases estimates or standard values may be used for one or more of the factors.

[0134] In practice, however, it may be cumbersome to determine the first and second lambda-value by determining all of the lines shown in FIG. 3. Out of tests and simulations, the applicant has found that in general the following rules of thumb give satisfactory results. The first lambda-value is at least 1.85, preferably at least 1.9, preferably larger than 2, e.g. between 2 and 5, preferably larger than 3, e.g. between 3 and 5, more preferably larger than 4, e.g. between 4 and 5. The second lambda-value can be taken between 1 and 2, preferably between 1.05 and 1.5, more preferably between 1.05 and 1.3. In general, the first lambda-value is preferably at least 1.5 times as large as the second lambda-value, preferably at least 2 times as large, e.g. at least 3 times as large.

[0135] FIG. 4 shows a second embodiment of the burner 300 according to the invention. The burner 300 shown in FIG. 4 differs from the burner 100 shown in FIG. 1 in the channel obstruction element and the gas valve. In FIG. 4, the channel obstruction element 312 is not the same element as the gas valve 212, on the contrary, the channel obstruction element 312 is present in addition to the gas valve 212. In addition, in the shown embodiment the gas valve 212 is not an electronically actuated control valve, but it is a mechanism that opens based on the pneumatic force balance upstream and downstream of the valve 212; however, this is not a requirement for the embodiment of the channel obstruction element 312 as shown in FIG. 4.

[0136] The channel obstruction element 312 is configured to be arranged in the combustible gas channel 111 in a rest position, as is shown in FIG. 4. In a non-shown actuated position, the channel obstruction element 312 is not in the combustible gas channel 111, or at least the channel obstruction element 312 is obstructing the combustible gas channel 111 less in comparison to the rest position. An actuator 313 is provided to move the channel obstruction element 312 from the rest position into the actuated position. The channel obstruction element 312 is preferably biased into the rest positon, such that reverse movement back into the rest position can be done making use thereof, e.g. including spring forces or gravity forces. The actuator 312 can be configured to move the channel obstruction element 312 based on pneumatic, hydraulic, mechanical, and/or magnetic forces. The controller 150 is configured to control the actuator 313 with control signal 351 via output terminal 150.1 and input terminal 313.1. The controller 150 is configured to arrange the channel obstruction element 412 in the rest position during the start-up phase, and in the actuated position during the operation phase. The channel obstruction element 312 itself can take any suitable shape and form.

[0137] FIG. 5 shows a third embodiment of the burner 400 according to the invention. The burner 400 shown in FIG. 5 differs from the burner 100 shown in FIG. 1 in the channel obstruction element and the valve. In FIG. 5, the channel obstruction element 412 is not the same element as the valve 212. In addition, in the shown embodiment the valve 212 is not an electronically actuated control valve, but it is a mechanism that opens based on the pneumatic force balance upstream and downstream of the valve 212; however, this is nota requirement for the embodiment of the channel obstruction element 412 as shown in FIG. 5.

[0138] The channel obstruction element 412 is configured to be arranged in the air channel 101 in an actuated position, as is shown in FIG. 5. In a non-shown rest position, the channel obstruction element 412 is not in the air channel 101, or at least the channel obstruction element 412 is obstructing the air channel 101 less in comparison to the actuated position. An actuator 413 is provided to move the channel obstruction element 412 from the rest position into the actuated position. The channel obstruction element 412 is preferably biased into the rest positon, such that reverse movement back into the rest position can be done making use thereof, e.g. including spring forces or gravity forces. The actuator 412 can be configured to move the channel obstruction element 412 based on pneumatic, hydraulic, mechanical, and/or magnetic forces. The controller 150 is configured to control the actuator 413 with control signal 451 via output terminal 150.1 and input terminal 413.1. The controller 150 is configured to arrange the channel obstruction element 412 in the rest position during the start-up phase, and in the actuated position during the operation phase. The channel obstruction element 412 itself can take any suitable shape and form.

[0139] In this embodiment, differently from the embodiments shown in FIG. 1 and FIG. 3, the quantity of air is reduced during the operation phase rather than reducing the quantity of combustible gas during the start-up phase.

[0140] In a non-shown embodiment, the channel obstruction element 412 is configured to be arranged in narrower portion 121. Since narrower portion 212 is even narrower in that case, the speed increased further and the pressure reduces further, and more combustible gas will be sucked in by the reduced pressure.

[0141] FIG. 6 schematically illustrates the steps of a method for starting a burner in accordance with a possible embodiment of the invention. In step 1001, there is a heat demand. The heat demand may e.g. arise from the heating in a building being turned on, or warm water being requested by a tap or shower. The heat demand may optionally initiate a pre-purge in step 1002. A pre-purge entails that air is blown through the burner to ensure there are no combustible gasses present. After the pre-purging, premixed gas having the first lambda-value is supplied in step 1003 and an ignition source is controlled to be in an ignition state in step 1004. In the ignition state the ignition source is adapted to ignite the premixed gas having the first lambda-value. It is possible that step 1004 is performed before step 1003 or simultaneously with step 1003. Preferably the first lambda-value is at least 1.85. Optionally in step 1005 the ignition source is controlled to be no longer in the ignition state before in step 1006 a flame detection is performed with a flame detector. Step 1005 may in particular be beneficial if the ignition source, e.g. being a spark igniter, would otherwise adversely detect the flame detection, which also depends on the type of sensor being used as flame detector. The steps 1001-1006 are part of a start-up phase 1100.

[0142] In case no flame is detected in step 1006 after a safety time, step 1007 provides a restart, in which the pre-purge in step 1002 ensured that the non-combusted premixed gas is not present in the burner anymore. Optionally step 1007 can only be performed a predetermined number of times, such that the burner is completely shut down if it cannot be started after e.g. five tries. The safety time may be in accordance with EN15502.

[0143] If a flame is detected in step 1006, the method may optionally comprise a transition phase 1200 after the start-up phase 1100. The transition phase 1200 may in particular be beneficial if the burner is started at a start-up load in the start-up phase 1100 which is different from a desired load. In the shown embodiment, the transition phase 1200 comprises a step 1009 changing the lambda-value of the premixed gas that is being supplied to a second-lambda value associated with the start-up load if the load in the operation phase would be equal to said start-up load. The transition phase 1200 then comprises a step 1010 of changing the load to the desired load and changing the lambda-value to the second lambda-value associated with said desired load. Other embodiments of the transition phase 1200 are possible, as explained herein.

[0144] After the transition phase, the method may comprise an operation phase 1400. The operation phase 1400 comprises a step 1012 of supplying premixed gas having a second lambda-value to the burner surface. The first lambda-value is larger than the second lambda-value.

[0145] FIG. 6 further shows an optional ignition period 1300. The ignition period 1300 starts at a step 1008 in which the ignition source is controlled to be in the ignition state, and stops at a step 1011 in which the ignition source is controlled to not be in the ignition state. In the shown example, the ignition period 1300 corresponds with the end of the start-up phase 1100, the transition phase 1200, and the beginning of the operation phase 1400.

[0146] Although shown herein as separate embodiments, it is noted that is possible to combine one or more of the embodiment of FIG. 1, i.e. the gas valve 112 functioning as channel obstruction element 112; the embodiment of FIG. 4, i.e. the channel obstruction element 312 arranged in the combustible gas channel 111; and the embodiment of FIG. 5, i.e. the channel obstruction element 111 arranged in the air channel 101.

[0147] As required, this document describes detailed embodiments of the present invention. However it must be understood that the disclosed embodiments serve exclusively as examples, and that the invention may also be implemented in other forms. Therefore specific constructional aspects which are disclosed herein should not be regarded as restrictive for the invention, but merely as a basis for the claims and as a basis for rendering the invention implementable by the average skilled person.

[0148] Furthermore, the various terms used in the description should not be interpreted as restrictive but rather as a comprehensive explanation of the invention.

[0149] The word “a” used herein means one or more than one, unless specified otherwise. The phrase “a plurality of” means two or more than two. The words “comprising” and “having” are constitute open language and do not exclude the presence of more elements.

[0150] Reference figures in the claims should not be interpreted as restrictive of the invention. Particular embodiments need not achieve all objects described.

[0151] The mere fact that certain technical measures are specified in different dependent claims still allows the possibility that a combination of these technical measures may advantageously be applied.