The invention relates to a method for controlling the ignition of a burner in a combustion appliance, in particular a gas boiler. Also, the invention relates to a computer program product executed by a computer or control unit carrying out the above method, and to a control unit for performing the method. In addition, the invention relates to a combustion appliance, in particular a gas boiler, comprising said control unit.

Gas boilers combust gas fuel to heat water for domestic use and/or central heating system facilities in buildings. The boilers can be used to operate in different modes, such as continuous-flow heaters, for preparing hot water. In gas boilers, the power output is substantially determined by the setting of the supply of fuel gas and air and by the mixture ratio between gas and air that is set. The temperature produced by the flame is also, among other things, a function of the mix ratio between fuel gas and air. An important factor influencing the safety of the boiler is the flame or burner stability, which is defined in terms of a stable combustion and thus no or next to no occurrence of flashbacks.

The flame speed is an important factor on the flame stability and a high flame speed can cause a flashback. In case the flame speed becomes greater than the mixture velocity, the flame can traverse in the upstream direction, which is toward the burner deck and even across the burner deck into the burner causing a so-called flashback. Flashback can be triggered e.g. by a change in a ratio of the air to the fuel gas in the mixture, by a change in composition of the fuel gas. When the mixture velocity becomes too high and rises above a so-called blow-off speed, blow-off may occur which means that the flame is blown-off the burner deck, with the consequence that the flame extinguishes or suffers incomplete combustion. For these reasons, the mixture velocity and/or the flame speed needs to be controlled. The flame speed is a function of the ratio of air to fuel gas in the mixture (in the following this ratio is indicated as lambda). Around lambda 1 the flame speed is the highest and if lambda increases the flame speed decreases.

The control of mixture velocity is much more important when the boiler uses hydrogen as fuel gas rather than other fuel gasses such as methane. In fact, flashback can occur more easily in hydrogen boilers since the laminar flame speed of hydrogen air mixture is around eight times higher than the flame speed for methane air mixture (with reference to the stoichiometric condition). Methods are known in prior art for controlling the value of lambda during different operations of a combustion appliance. In operation between a minimum load and a maximum load for example, the value of lambda can be controlled to continuously lower its value. Usually, during the start-up phase the premixed gas is ignited at a first lambda value that is much higher than the lambda value during the operation phase.

It is therefore desirable to provide a method for controlling the ignition of a burner of a combustion appliance in which the risk of flashback is reduced.

The object is solved by a method for controlling the ignition of a burner in a combustion appliance, in particular a gas boiler, wherein the combustion appliance is operable in an ignition phase and in an operation phase after the burner has ignited, wherein the burner is supplied with a mixture of air and fuel gas, the method comprising: defining a lambda value during the ignition phase and/or the operation phase of the combustion appliance, the lambda value being an air to fuel gas ratio of the mixture, controlling one or more actuators of the combustion appliance to control the air flow and/or the fuel gas flow to achieve the defined lambda value, wherein the lambda value at the ignition phase is below than 1.8, in particular between 1.5 and 1.7.

Advantageously, the value of the air to the fuel gas ratio in the gas mixture and thus lambda is controlled to avoid the risk of flashback. The lambda value depends on the load of the combustion appliance. By selecting a lambda value that is below 1.8 in the ignition phase, steep lambda changes as a result of load change are avoided. Such a control enables to secure that the lambda values are sufficiently apart from critical lambda values at which flashback can occur. Thus, the inventive method improves the safety of the operation of the combustion appliance.

According to one example, the ignition phase comprises a spark ignition phase. The spark ignition phase can be executed at a middle power range of the combustion appliance. In particular, the spark ignition phase can be executed when a value of an air flow rate reaches an ignition value, in particular a local maximum. The ignition value is arranged above a predetermined threshold and/or is the value at which the mixture of ar and fuel ignites. Additionally or alternatively the spark ignition phase can be executed when a value of the fuel gas flow rate reaches a ignition value, in particular a local maximum. The ignition value is above a predetermined threshold. According to an example, in the load range between the minimum load and the maximum load, the ratio between the lambda value at the minimum load and the lambda value at the maximum load is lower than 1.20, in particular 1.13. Additionally or alternatively a slope value of a lambda curve is between 0 and 5%, in particular between 0 and 4%, in particular between 0 and 3%, in particular between 0 and 2,5%. The aforementioned slope values are absolute values and thus can be positive or negative. In these ways, there is a limited variation of the lambda value when the combustion appliance is operating at the minimum load and at a maximum load resulting in a combustion appliance that can be safely operated.

The middle power range is defined as a range having a middle power value being 50% of the maximum load of the combustion appliance and having a low border value being lower than the middle power value and a high border value being higher than the middle power value. A distance between the low border value and the middle power value corresponds to 20% of the maximum load of the combustion appliance. A distance between the high border value and the middle power value corresponds to 20% of the maximum load.

In the spark ignition phase the lambda value is lower than 1.7, in particular between 1 ,65 and 1 ,55. Additionally or alternatively in the spark ignition phase the ignition value of the air flow rate and/ or the fuel flow rate, in particular the respective local maximum, is the same or lower than a maximum value of an air flow rate and/ or a fuel flow rate in the operation phase. Such lambda values are farer away from critical lambda values for which a flashback can occur than lambdas resulting from known control methods.

In one example, the combustion appliance operates between a minimum load and a maximum load, the maximum load being between 27kW and 29 kW, in particular 28kW, and the minimum load being between 6kW and 7kW, in particular 6.5kW.

In examples, in the ignition phase the power of the combustion appliance is between 12 kW and 15 kW. In particular, the ignition phase occurs at about half of the maximum power of the combustion appliance.

According to an example, the ignition phase further comprises a pre-ventilation phase (PV) prior to the spark ignition phase (SI) and a post spark ignition phase (PSI) after the spark ignition phase. In the pre-ventilation phase (PV) the value of the air flow rate increases and the value of the fuel gas flow rate is zero. In the spark ignition phase (SI) the value of the air flow rate has reached the ignition value, in particular a local maximum value, and/or the value of the fuel gas flow rate increases in the spark ignition phase. In particular, the fuel gas flow rate continuously increases until it reaches the ignition value.

In examples, in the post spark ignition phase (PSI) the value of both the air flow rate and the fuel gas flow rate decreases compared to the ignition value of the spark ignition phase. The value of the air flow rate and/or the fuel gas flow rate has a local minimum at the transition between the ignition phase and the operation phase.

In one example, in the operation phase the value of both the air flow rate and the fuel gas flow rate increases to reach an air operation value and a gas operation value, respectively. The operation value is selected such that an excess of air remains.

According to one aspect of the invention, a computer program product is provided. This product comprises instructions which, when the program is executed by a computer or control unit, cause the computer or the control unit to carry out the inventive method. Also, a computer readable data carrier is provided, the carrier having stored thereon the inventive computer program product.

In another aspect of the invention, a control unit is provided, the control unit is configured to perform the inventive method. The control unit can comprise at least one processor or be a processor.

According to one aspect of the invention, a combustion appliance, in particular a gas boiler, is provided, the combustion appliance comprising: a burner for receiving a mixture of air and fuel gas from a gas mixture channel and for combusting said mixture; one or more actuators located upstream the gas mixture channel to control the air flow and/or the fuel gas flow; and the inventive control unit for controlling a lambda value during the operation of the combustion appliance, the lambda value being an air to fuel gas ratio of the mixture wherein the control unit controls the one or more actuators. In one example, the combustion appliance comprises at least two actuators. One of the actuators comprises a fan element located in an air supply line. The other of the actuators comprises gas valve located in a gas supply line.

In particular, the appliance including the present system can be a gas boiler for the combustion of fuel gas.

Fuel gas can comprise more than 20 mol% hydrogen, in particular more than 30 mol%. In particular, fuel gas can comprise more than 50 mol%, in particular more than 90 mol% hydrogen or be pure hydrogen. Pure hydrogen is defined as comprising at least 98 mol% hydrogen (hydrogen-fire gas appliance guide PAS4444:2020).

Also, a computer readable data carrier is provided, the carrier having stored thereon the inventive computer program product.

In the figures, the subject-matter of the invention is schematically shown, wherein identical or similarly acting elements are usually provided with the same reference signs.

Figure 1 shows a schematic representation of a combustion appliance according to an example.

Figure 2 shows a flow chart of a method for controlling the operation of a combustion appliance according to an example.

Figures 3A-B show the variation of the lambda value as a function of the load and of the air and gas flow rate as a function of the time according to an example.

Figure 1 illustrates a combustion appliance 1 such as gas boiler used for the combustion of fuel gas, for example containing hydrocarbons and/or hydrogen. The fuel gas is mixed with air and is provided to the burner 5 through a gas mixture channel 8, the burner 5 being coupled to a heat exchanger 7 for heating water for domestic use and/or central heating system facilities in buildings. The gas mixture channel 8 receives air from an air supply line 9 and fuel gas from a gas supply line 10.

The flow of air - and correspondingly the flow of the air/fuel gas mixture - can be controlled by a fan element 2a located in the air supply line 9. Advantageously, the fan element 2a is located upstream the region where the fuel gas is inserted into the gas mixture channel 8. The gas supply line 10 is provided with a gas valve 3a for controlling the fuel gas flow entering the gas mixture channel 8.

The combustion appliance 1 comprises furthermore a control unit 4 for controlling a lambda value and two actuators 2, 3. A first actuator 2 comprises the fan element 2a and a non-shown actuating element by means of which the fan speed can be controlled. A second actuator 3 comprises the gas valve 3 and a non-shown actuating means by means of which an opening section through which the gas flows can be controlled. The control unit 4 controls the first actuator 2 and the second actuator 3 to control and eventually adapt the air to fuel gas ratio. The control unit 4 controls the two actuators 2, 3 by sending at least one control signal to the respective actuator. A manifold mixer 6 is provided in gas mixture channel 8 at the joint region where the gas supply line 10 is connected to the gas mixture channel 8.

By acting on the fan element 2a and/or the gas valve 3a, the control unit 4 can control the air to fuel gas ratio in the gas mixture channel 8 that is supplied to the burner 5. Accordingly, the value of lambda can be varied during the operation of the combustion appliance. For example, if the combustion appliance 1 is operating between a minimum load to a maximum load, the value of lambda can be changed in the load range between the minimum load and the maximum load.

Figure 2 shows a flow chart of the method 100 for controlling the ignition of the burner 5 of the combustion appliance 1 and in particular for operating a gas boiler, as described above, when operating in an ignition phase (IP) and in an operation phase (OP).

At step S101 , the air to fuel gas ratio of the mixture to be supplied to the burner 5 is defined. This ratio is the lambda value. At step S102, the method comprises controlling one or more actuators 2, 3 to control the air flow and/or the fuel gas flow to achieve the defined lambda. As mentioned above the first actuator 2 can comprise the fan element 2 to control the air flow and the second actuator can comprise the gas valve 3 to control the fuel gas flow. During the ignition phase, the lambda value is below 1.8. In particular, this value is between 1.5 and 1.7.

If the combustion appliance 1 is operated in the operation phase, the control unit 4 ensures at step 103 that the ignition value of the air flow rate 13 is the same or lower than a maximum value of the air flow rate 17 in the operation phase OP. Additionally or alternatively the control unit 4 ensures at step 103 that the ignition value of the fuel flow rate 14 is the same or lower than a fuel flow rate (18) of the operation phase (OP). The ignition value of the gas flow rate 14 is a value above a predetermined ignition threshold.

The mixture of air and fuel gas having the defined lambda value is supplied to the burner 5.

The trend of the lambda value in the load range is shown in figure 3A. In particular, the figure illustrates a comparison between a first lambda value curve 11 according to the present disclosure (thicker line) and a second lambda value curve 12 according to prior art (thinner line) describing the variation of the lambda value as a function of the operating load of the combustion appliance 1. Fig. 3A does not show a curve separating an operation range of the combustion appliance, in which flashbacks occurs, from an operation range of the combustion appliance in which no flashback occurs. Said curve is usually straight and passes from a lambda value to load value. The first lambda curve 11 and the second lambda curve 12 are provided in the operation range in which no flashback results during operation. The second lambda curve 12 is arranged closer to the non-shown curve as the first lambda curve 11 .

According to the second lambda value curve 12, or prior art lambda value, the value of lambda continuously decreases within a load range defined by a first load value (Qi) representing the lowest load value (for example 5kW) and a second load value (Ch) representing the highest load value (for example 25kW). Usually, at the first load value (Qi) lambda has the highest value (for example more than 1.8) and at the second load value (Q2) lambda assumes the smallest value (for example less than 1.3, in particular less than 1.2). It is noted that in the load range between the first and the second load values (Qi, Q2), the lambda value curve 12 decreases in a steep way. The ratio between the lambda value at the first load (Qi) and the lambda value at the second load (Q2) is more than 1.2, in particular 1.5.

The first lambda value curve 11 according to the present disclosure behaves in a completely different way. First of all, it is noted that the first lambda value curve 11 is less steep compared to the second lambda value curve 12. As a matter of fact, the ratio between the lambda value at the minimum load (Qmin) and the lambda value at the maximum load (Q _ma x) is less than 1.2, in particular 1.13. Furthermore, in the load range between the minimum load (Qmin) and the maximum load (Q _ma x) the lambda value decreases, in particular continuously decreases, and then stops to decrease in a final load range at the maximum load (Q _ma x) (second load range). In particular at the maximum load (Qmax), the lambda value can either increase (dashed line) or remain, in particular almost, constant (straight line). It is furthermore noted that to have more margin (to consider also the tolerances of the system), the maximum load (Q _ma x) is higher than the second load (Ch) of the second lambda value curve 12. For example, the maximum load (Qmax) is at 28kW.

The first lambda value curve 11 is an example of a possible behavior of the lambda value as a function of the load according to the present disclosure. In this case, the lambda value at the minimum load (Qmin) is 1.7 and the lambda value at the maximum load (Qmax) is 1 .5 or 1.6, so that the ratio between the lambda value at the minimum load (Qmin) and the lambda value at the maximum load (Qmax) is 1.13 or 1.06. Of course, other specific lambda values can be considered. What is important is that the lambda value curve does not have a steep behavior in the load range between the minimum load (Qmin) and the maximum load (Qmax). This results in that first lambda value curve 11 is farer away from the non shown curve than the second lambda value curve 12.

It is noted that the ignition happens at a middle power range of the combustion appliance. In the shown embodiment, the middle power range is about 12-15kW (Qmid). In this case, the lambda value is between 1 ,5 and 1 ,7 that is far from 1 .85 of prior art.

Once the burner 5 is ignited, the system modulates following the heating request in the operation phase OP.

With reference to figure 3B, the air flow rate 13a, the gas flow rate 14a and the fan speed 16a are shown as a function of time, in particular during the ignition phase (IP) and the operation phase (OP) of the combustion appliance 1. The ignition phase (IP) comprises a pre-ventilation phase (PV), a spark ignition phase (SI) and a post spark ignition phase (PSI). In the pre-ventilation phase (PV), the value of the air flow rate 13 increases and the value of the fuel gas flow rate 14 is zero. As a matter of fact, after a heating request, the fan element 2 is activated and an air sensor is able to detect the air flow. However, the gas valve 3 is still close.

Afterwards, in the spark ignition phase (SI) the value of the air flow rate 13 reaches the ignition value, in particular its local maximum, and the value of the fuel gas flow rate 14 firstly increases and then slightly decreases. When the air flow reaches the correct value (at not fixed rpm), the gas valve 3 opens. A gas sensor is able to measure the gas flow and the ignitor sparks to ignite the air/gas mixture. Afterwards, in the post spark ignition phase (PSI) the value of both the air flow rate 13a and the fuel gas flow rate 14a decreases until it reaches a local minimum 19. The local minimum 19 is arranged at the transition between the ignition phase and the operation phase. In the operation phase (OP), the value of both the air flow rate 13a and the fuel gas flow rate 14a increases again to reach an air operation value 17 and a gas operation value 18, respectively.

It is noted that the control unit 4 ensures that the ignition value, in particular local maximum value, of the air flow rate 13 reached in the ignition phase (IP) corresponds to the air operation value 17 in the operation phase or is lower than the air operation value 17 in the operation phase.

In a similar way, the ignition value, in particular the local maximum value, of the fuel gas flow rate 14 reached in the ignition phase (IP) corresponds to the gas operation value 18 in the operation phase or is lower than the a gas operation value 18 in the operation phase..

A dedicated software can be used to build an air/gas curve step by step from 0% to 100% and to modify the lambda value in each point (with some software limits to have a safe behaviour).

Reference Signs

1 Combustion appliance

2 First actuator

2a Fan element

3 Second actuator

3a Gas valve

4 Control unit

5 Burner

6 Manifold mixer

7 Heat exchanger

8 Gas mixture channel

9 Air supply line

10 Gas supply line

11 First lambda value curve

12 Second lambda value curve (prior art)

13 Value of air flow rate

14 Value of gas flow rate

13a Air flow rate

14a Gas flow rate

16a Fan speed

17a Air operation value

18a Gas operation value

19 local minimum in post spark ignition phase

Qmin Minimum load

Qmax Maximum load

Qi First load

Q ₂ Second load

Qmid Middle power

IP Ignition phase

OP Operation phase

PV Pre ventilation phase

SI Spark ignition phase

PSI Post spark ignition phase

100 Method