CONTROL METHOD FOR A GAS BOILER

Description

[0001] The invention relates to a method for controlling the operation of 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, a data processing apparatus comprising a processor for executing said computer program product, and a computer readable data carrier having stored thereon the computer program product. In addition, the invention relates to a combustion appliance comprising means for carrying out the method.

[0002] 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, etc. 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.

[0003] 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 can also be indicated as lambda). Around lambda 1 the flame speed is the highest and if lambda increases the flame speed decreases.

[0004] 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).

[0005] Methods are known in prior art for controlling the value of lambda during the operation 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, the ratio between the lambda value at the minimum load and the lambda value at the maximum load is higher than 1.2, wherein the value at the maximum load is less than 1.3. However, this could lead to a higher risk of flashback, higher emissions in the flue and higher presence of nitrogen oxides (NOx).

[0006] US 2022 / 0163203 A1 describes a method for operating a surface stabilized fully premixed gas premix burner, wherein the burner is adapted to modulate between a minimum load and a full load, and wherein the ratio of the full load over the minimum load is at least 4, wherein the method comprises the step of supplying a premix of combustible gas and air to the burner at an air to combustible gas ratio, wherein the combustible gas supplied to the burner comprises at least 20% by volume of hydrogen, and wherein the air to combustible gas ratio of the premix which is supplied to the burner when the burner is operated at minimum load is set by a mechanism to be in relative terms at least 20% higher than the air to combustible gas ratio of the premix which is supplied to the burner when the burner is operated at full load.

[0007] It is therefore desirable to provide a method for controlling the operation of a gas boiler that allows a bigger margin of operation of the combustion appliance as well as a mitigation of the risk of flashback, emissions in the flue and NOx without strongly affecting the efficiency of the combustion appliance.

[0008] The object is solved by a method according to claim 1.

[0009] Advantageously, the value of the air to the fuel gas ratio in the gas mixture is controlled to avoid the risk of flashback and at the same time to reduce the emissions in the flue and the presence of NOx.

[0010] The lambda value is a function of the load (Q), wherein in a load range between the minimum load (Q min ) and the maximum load (Q max ), the lambda value first decreases and then stops to decrease. Defining a lambda value comprises defining and setting a lambda value as a function of the load, based on which the one or more actuators of the combustion appliance control the air flow and / or the fuel gas flow.

[0011] In one example, in a load range between the minimum load and the maximum load, the lambda value first decreases and then increases. Thus, stopping the decrease of lambda value can occur by increasing the lambda value. In particular, the lambda value is not continuously lowered when passing from the minimum load to the maximum load but there is a range wherein said value increases. It is noted that this trend goes beyond the normal fluctuations of the measured value of lambda. As a matter of fact, the load range between the minimum load and the maximum load comprises a first load range (comprising the minimum load), wherein the lambda value continuously decreases, and a second load range (consecutive the first load range and comprising the maximum load), wherein the lambda value does not decrease any more, i.e., continuously increases.

[0012] In another example, the lambda value stops to decrease when the load has reached at least 80% of the maximum load, in particular 90% of the maximum load. In particular, the lambda value stops to decrease at a reference load value, wherein in a load range between the reference load value and the maximum load, the lambda value increases, in particular continuously increases. In particular, the reference load value is a load value close to the maximum load. In other words, the behaviour of stopping decreasing the lambda value occurs when the combustion appliance is almost operating at the maximum load.

[0013] 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.

[0014] In one example, the lambda value is higher than 1.3, in particular higher than1.5, when the combustion appliance is operated at the maximum load. For example, the lambda value can be 1.6 when the combustion appliance is operated at the maximum load.

[0015] Also, the lambda value is higher than 1.7, in particular 1.8, when the combustion appliance is operated at the minimum load. For example, the lambda value can be 1.92 when the combustion appliance is operated at the minimum load.

[0016] According to an example, in the load range between an average load and the maximum load, the lambda value is, in particular continuously, lowered, in particular by less than 2%, and then is increased or is constant, the average load being the average value of the load in the load range between the minimum load and the maximum load. Specifically, the average load is defined as the difference between the maximum load value and the minimum load value divided by 2 and added to the minimum load value. In particular, the lambda value at the average load is higher than 1.5, in particular is comprised between 1.51 and 1.56. In other words, once the combustion appliance is operating at the average load, the lambda value has almost reached the minimum value.

[0017] In a further example, the maximum load is comprised between 27kW and 29 kW, in particular 28kW and the minimum load is comprised between 6kW and 7kW, in particular 6.5kW.

[0018] 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.

[0019] In another aspect, 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. Also, a computer readable data carrier is provided, the carrier having stored thereon the inventive computer program product.

[0020] According to one aspect of the invention, a combustion appliance, in particular a gas boiler, is provided, the combustion appliance comprising a data processing apparatus for carrying out the inventive method. Examples of combustion appliances can include furnaces, water heaters, boilers, direct / in-direct make-up air heaters, power / jet burners and any other residential, commercial or industrial combustion appliance.

[0021] 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).

[0022] In one example, the combustion appliance comprises a burner, wherein the ratio between the perforated surface area of the burner and the total surface area of the burner is comprised between 15% and 20%, in particular 18.7%. Such a perforated surface area enables combustion of fuel gas having more than 20 mol% hydrogen in a safe manner.

[0023] It is noted that the particular configuration of the burner in terms of perforated surface area as well as of the manifold mixer in terms of cross section area can reduce the pressure loss of the combustion appliance. The reduction of pressure loss can affect the setting parameters to achieve a bigger margin of operation.

[0024] 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 1shows a schematic representation of a combustion appliance according to an example. Figure 2shows a flow chart of a method for controlling the operation of a combustion appliance according to an example. Figure 3show the variation of the lambda value as a function of the load according to an example.

[0025] 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.

[0026] 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 2 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.

[0027] 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.

[0028] By acting on the fan element 2 and / or the gas valve 3, 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.

[0029] Figure 2 shows a flow chart of the method 100 for controlling the operation of the combustion appliance 1 and in particular for operating a gas boiler, as described above, when operating between the minimum load and the maximum load.

[0030] 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 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. In the load range between the minimum load and the maximum load, the lambda value first decreases and then stops to decrease. A decrease interruption is intended here that the lambda value can increase or can remain constant.

[0031] 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 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 is the same or lower than a fuel flow rate 18 of the operation phase (OP). The ignition value of the gas flow rate is a value above a predetermined ignition threshold. The ignition value of the fuel flow rate is the value at which the mixture ignites. The ignition value of the air flow rate is the value at which the mixture ignites.

[0032] The mixture of air and fuel gas having the defined lambda value is supplied to the burner 5.

[0033] The trend of the lambda value in the load range is shown in figure 3. 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. 3 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.

[0034] 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 (Q 1 ) representing the lowest load value (for example 5kW) and a second load value (Q 2 ) representing the highest load value (for example 25kW). Usually, at the first load value (Q 1 ) lambda has the highest value (for example more than 1.8) and at the second load value (Q 2 ) 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 (Q 1 , Q 2 ), the lambda value curve 12 decreases in a steep way. The ratio between the lambda value at the first load (Q 1 ) and the lambda value at the second load (Q 2 ) is more than 1.2, in particular 1.5.

[0035] 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 (Q min ) and the lambda value at the maximum load (Q max ) is less than 1.2, in particular 1.13. Furthermore, in the load range between the minimum load (Q min ) and the maximum load (Q max ) the lambda value decreases, in particular continuously decreases, in a range between the minimum load (Q min ) and the reference load 13 (first load range), and then stops to decrease in a final load range 14 between the reference load 13 and the maximum load (Q max ) (second load range). In particular, in the final load range 14, the lambda value can either increase (dashed line) or remain constant (straight line). It is furthermore noted that to have more margin (to consider also the tolerances of the system), the maximum load (Q max ) is higher than the second load (Q 2 ) of the second lambda value curve 12. For example, the maximum load (Q max ) is at 28kW.

[0036] 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 (Q min ) is 1.7 and the lambda value at the maximum load (Q max ) is 1.5 or 1.6, so that the ratio between the lambda value at the minimum load (Q min ) and the lambda value at the maximum load (Q max ) is 1.13 or 1.06. Of course, other specific lambda values can be considered.

[0037] What is important is that the lambda value curve does not have a steep behavior in the load range between the minimum load (Q min ) and the maximum load (Q max ). In particular, the ratio between the lambda value at the minimum load (Q min ) and the lambda value at the maximum load (Q max ) needs to be less than, or equal to, 1.2. For example, the lambda value at the minimum load (Q min ) can be 1.8 and the lambda value at the maximum load (Q max ) can be 1.5. In another example, the lambda value at the minimum load (Q min ) can be 1.92 and the lambda value at the maximum load (Q max ) can be 1.6. Additionally or alternatively the non-steep behaviour is realized in that a slope value of the first lambda curve 11 is between 0 and 5%, in particular between 0 and 4%, in particular between 0 and 3%, in particular between 0 and 2,5%.

[0038] In the load range between the minimum load (Q min ) and the maximum load (Q max ), an average load (Q ave ) can be defined. The average load (Q ave ) is the average value of the load in the load range between the minimum load (Q min ) and the maximum load (Q max ). For example, in case the maximum load (Q max ) is 28 kW and the minimum load is 6.5 kW, the average load (Q ave ) is [(Q max -Q min ) / 2) + Q min ], i.e., [(28 kW-6.5 kW) / 2 + 6.5 kW] = 17.25 kW. It is noted that in the load range between the average load (Q ave ) and the maximum load (Q max ), the lambda value is lowered less than 2% and then is increased or is constant. In particular, the lambda value at the average load (Q ave ) is higher than 1.5, in particular is comprised between 1.51 and 1.53, in particular 1.52. Accordingly, the lambda value at the average load (Q ave ) is lower than the lambda value at the maximum load (Q max ) or is slightly higher than said lambda value at the maximum load (Q max ). On the other hand, taking the second lambda value curve 12 into account, the lambda value at the corresponding average load (i.e., 15 kW) is about 1.25 and is lowered more than 2% before reaching the lambda value at the second load (Q 2 ).

[0039] It is noted that these results can be achieved also due to a combination of parameters affecting the pressure loss of the system.Reference Signs

[0040] 1Combustion appliance 2Fan element 3Gas valve 4Control unit 5Burner 6Manifold mixer 7Heat exchanger 8Gas mixture channel 9Air supply line 10Gas supply line 11First lambda value curve 12Second lambda value curve (prior art) 13Reference load value 14Final load range 15Heating system Q min Minimum load Q max Maximum load Q 1 First load Q 2 Second load 100Method

Claims

1. Method (100) for controlling the operation of a combustion appliance (1), in particular a gas boiler, wherein the combustion appliance (1) is operable between a minimum load (Qmin) and a maximum load (Qmax) and uses a fuel gas having 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, the method comprising: defining (S101) a lambda value during an ignition phase (IP) and / or an operation phase (OP) of the combustion appliance (1), the lambda value being an air to fuel gas ratio of the mixture, controlling (S102) one or more actuators (2, 3) of the combustion appliance (1) to control the air flow and / or the fuel gas flow to achieve the defined lambda value, wherein in a load range between the minimum load (Qmin) and the maximum load (Qmax), the lambda value first decreases and then increases.

2. Method (100) according to any one of claims 1, wherein: a. the lambda value stops to decrease when the load has reached at least 80% of the maximum load (Qmax), in particular 90% of the maximum load (Qmax); and / or b. the lambda value stops to decrease at a reference load value (13), wherein in a load range between the reference load value (13) and the maximum load (Qmax), the lambda value increases, in particular continuously increases.

3. Method (100) according to any one of claims 1 to 3, wherein a. the ratio between the lambda value at the minimum load (Qmin) and the lambda value at the maximum load (Qmax) is lower than 1.20, in particular 1.13, and / or b. wherein a slope 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%.

4. Method (100) according to any one of claims 1 to 3, wherein: a. the lambda value is higher than 1.3, in particular higher than 1.5, when the combustion appliance (1) is operated at the maximum load (Qmax); and / or b. the lambda value is 1.6 when the combustion appliance (1) is operated at the maximum load (Qmax); and / or c. the lambda value is higher than 1.7, in particular 1.8, when the combustion appliance (1) is operated at the minimum load (Qmin); and / or d. the lambda value is 1.92 when the combustion appliance (1) is operated at the minimum load (Qmin).

5. Method (100) according to any one of claims 1 to 4, wherein a. in the load range between an average load (Qave) and the maximum load (Qmax), the lambda value is, in particular continuously, lowered, in particular by less than 2%, and then is increased or is constant, the average load (Qave) being the average value of the load in the load range between the minimum load (Qmin) and the maximum load (Qmax); and / or b. the lambda value at the average load (Qave) is higher than 1.5, in particular is comprised between 1.51 and 1.56.

6. Method (100) according to any one of claims 1 to 5, wherein the maximum load (Qmax) is comprised between 27kW and 29 kW, in particular 28kW and the minimum load (Qmin) is comprised between 6kW and 7kW, in particular 6.5kW.

7. Computer program product comprising instructions which, when the program is executed by a computer or control unit (4), cause the computer or the control unit (4) to carry out the method according to one of the claims 1 to 6.

8. Data processing apparatus comprising a processor for executing the computer program product according to claim 7.

9. Computer readable data carrier having stored thereon the computer program product according to claim 7.

10. Combustion appliance (1), in particular a gas boiler, comprising a data processing apparatus for carrying out the method (100) according to any one of claims 1 to 6.

11. Combustion appliance (1) according to claim 10, wherein the combustion appliance (1) comprises a burner (5), wherein the ratio between the perforated surface area of the burner (5) and the total surface area of the burner (5) is comprised between 15% and 20%, in particular 18.7%.