METHOD TO OPERATE A MODULATING BURNER

Abstract

The invention pertains to a method for operating a surface stabilized fully premixed gas premix burner. The burner is adapted to modulate between a minimum load and a full load, the ratio of the full load over the minimum load being at least 4. The method comprises the step of supplying a premix of combustible gas and air to the burner at an air to combustible gas ratio, the combustible gas supplied to the burner comprises at least 20% by volume of hydrogen, In the method, 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.

Claims

1. 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.

2. The method according to claim 1, wherein the air to combustible gas ratio at average load is in relative terms less than 10% higher than the air to combustible gas ratio at full load, and wherein the average load is defined as the average between minimum load and full load.

3. The method according to claim 1, wherein the air to combustible gas ratio of the premix gas supplied to the burner at full load is less than 1.3, preferably less than 1.25.

4. The method according to claim 1, wherein the air to combustible gas ratio of the premix gas which is supplied to the burner is set by the mechanism as a predefined function of the burner load.

5. The method according to claim 4, wherein a pneumatic gas valve is used by the mechanism to set the rate of supply of combustible gas to the burner, in order to set the air to combustible gas ratio of the premix supplied to the burner as a predefined function of the burner load.

6. The method according to claim 5, wherein a pneumatic gas valve is used which comprises a spring, and wherein properties of the spring determine at least in part the predefined function.

7. The method according to claim 1, wherein air or the premix of combustible air and gas is supplied to the burner by a fan, and wherein the amount of air supplied to the burner is measured by a sensor or wherein the fan speed is used as an indication for the amount of air supplied to the burner, and wherein the amount of combustible gas supplied to the burner is set according to a predefined relation to the amount of air supplied to the burner;

8. The method according to claim 1, wherein air or the premix of combustible air and gas is supplied to the burner by a fan, and wherein the amount of combustible gas supplied to the burner is measured by a sensor, and wherein the amount of air supplied to the burner is set according to a predefined relation to the amount of combustible gas supplied to the burner.

9. The method according to claim 1, wherein air or the premix of combustible air and gas is supplied to the burner by a fan, wherein a value providing information of the combustion, of the flue gas and/or of the air to gas mixture supplied to the burner is measured by at least one sensor, and wherein this value is used in combination with a value indicative of the burner load, of the fan speed and/or of the flow rate of air supplied to the burner to set the air to combustible gas ratio.

10. The method according to claim 9, wherein the at least one sensor is or comprises a temperature sensor and wherein the value providing information of the combustion is representative for the flue gas temperature or for the flame temperature of the burner.

11. The method according to claim 9, wherein the burner comprises a burner deck onto which combustion is stabilized when the burner is in operation, and wherein the at least one sensor is or comprises a temperature sensor and the value providing information of the combustion is representative for the temperature of the burner deck of the burner.

12. The method according to claim 9, wherein the at least one sensor is provided to measure a value representative for the oxygen content of the flue gas generated by the burner or representative for the oxygen content of the premix of air and combustible gas supplied to the premix burner.

13. The method according to claim 1, wherein the burner which is used comprises a perforated metal plate onto which the flames are stabilized.

14. A surface stabilized fully premixed gas premix burner system, wherein burner system comprises: a burner comprising a burner deck, which burner deck comprises a plurality of holes, wherein optionally the combined surface area of the holes is up to 5% of the surface area of the burner deck, an air inlet, a combustible gas inlet and a mixer which is in communication with the air inlet and the combustible gas inlet, which mixer is adapted to mix air and combustible gas to a premix of combustible gas and air at an air to combustible gas ratio, wherein the combustible gas inlet is suitable for receiving a combustible gas which contains at least 20% by volume of hydrogen, a burner inlet which is adapted to receive the premix of combustible gas and air and supply it to the burner, a burner load controller, which is adapted to vary the load of the burner between a minimum load and a full load, and wherein the ratio of the full load over the minimum load is at least 4, therewith allowing the burner to modulate between the minimum load and the full load, and a mechanism which is adapted to set the air to combustible gas ratio of the premix of combustible gas and air which is created by the mixer, which setting of the air to combustible gas ratio of the premix of combustible gas and air is at least partially dependent from the load of the burner, 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 the 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.

15. The burner system according to claim 14, wherein the burner system further comprises a controller, which controller is programmed to control the mechanism such that 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 the 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.

16. The burner system according to claim 14, wherein the mechanism is adapted to set the air to combustible gas ratio of the premix gas which is supplied to the burner as a predefined function of the burner load.

17. The burner system according to claim 14, wherein the mechanism comprises a pneumatic gas valve, which pneumatic gas valve is adapted to set the rate of supply of combustible gas to the burner, in order to set the air to combustible gas ratio of the premix supplied to the burner as a predefined function of the burner load.

18. The burner system according to claim 17, wherein the pneumatic gas valve comprises a spring, and wherein properties of the spring determine at least in part the predefined function.

19. The burner system according to claim 14, wherein the burner system further comprises a fan, which fan is arranged to supply air to the mixer or to supply the premix of combustible air and gas to the burner.

20. The burner system according to claim 19, wherein the burner system further comprises: a sensor, which sensor is adapted to measure the amount of air supplied to the burner, a combustible gas supply controller which is adapted to control the amount of combustible gas supplied to the mixer and/or to the burner, which combustible gas supply controller is adapted to set the amount of combustible gas supplied to the burner according to a predefined relation to the amount of air supplied to the burner as measured by the sensor.

21. The burner system according to claim 19, wherein the fan has a variable fan speed, and wherein the burner system further comprises: a combustible gas supply controller which is adapted to control the amount of combustible gas supplied to the mixer and/or to the burner, which combustible gas supply controller is adapted to set the amount of combustible gas supplied to the burner according to a predefined relation to the amount of air supplied to the burner, for which amount of air the fan speed of the fan is a measure.

22. The burner system according to claim 19, wherein the burner system further comprises: a sensor, which sensor is adapted to measure the amount of combustible gas supplied to the burner, an air supply controller which is adapted to control the amount of air supplied to the mixer and/or to the burner, which air supply controller is adapted to set the amount of air supplied to the burner according to a predefined relation to the amount of combustible gas supplied to the burner as measured by the sensor.

23. The burner system according to claim 15, wherein the burner system further comprises: a fan, which fan is arranged to supply air to the mixer or to supply the premix of combustible air and gas to the burner, a sensor, which sensor is adapted to measure a value providing information of the combustion, of the flue gas and/or of the air to gas mixture supplied to the burner, therewith generating measurement data pertaining to said value, wherein the controller is adapted to receive the measurement data pertaining to said value from the sensor and to use said measurement data in combination with a value indicative of the burner load, of the fan speed and/or of the flow rate of air supplied to the burner to control the mechanism to set the air to combustible gas ratio of the premix.

24. The burner system according to claim 23, wherein the at least one sensor is or comprises a temperature sensor and wherein the value providing information of the combustion is representative for the flue gas temperature and/or for the flame temperature of the burner.

25. The burner system according to claim 23, wherein the at least one sensor is or comprises a temperature sensor and the value providing information of the combustion is representative for the temperature of the burner deck of the burner.

26. The burner system according to claim 23, wherein the at least one sensor is adapted to measure a value representative for the oxygen content of the flue gas generated by the burner or representative for the oxygen content of the premix of air and combustible gas supplied to the burner.

Description

[0135] FIG. 1 schematically explains a method of controlling a surface stabilized fully premixed gas premix burner using a constant air to combustible gas ratio over the full range of burner operation from minimum to full burner load.

[0136] FIGS. 2 and 3 schematically explain methods according to the invention.

[0137] FIG. 4 shows an example of implementation of a method according to the invention.

[0138] FIG. 5 shows an example of a predefined function for setting the air to combustible gas ratio of the premix gas supplied to the burner.

[0139] FIG. 6 schematically shows an embodiment of a burner system according to the invention.

[0140] FIG. 7 schematically shows a variant of the embodiment of FIG. 6.

[0141] FIG. 8 schematically shows a further variant of the embodiment of FIG. 6.

[0142] The invention relates to a method to operate a surface stabilized fully premixed gas premix burner wherein the combustible gas supplied to the burner comprises at least 20% by volume of hydrogen and has as benefit that flame flash back is prevented. Although flame flashback is a complex phenomenon, it is related to the ratio of the exit velocity (m/s) of the premix gas through the burner deck to the burning velocity (also in m/s) of the combustible gas. The exit velocity is proportional to the volume flow of the premix gas divided by the surface of the through holes of the burner deck onto which combustion is stabilized. Although the burning velocity depends on a number of parameters, e.g. temperature of the gas premix and/or air to combustion ratio of the gas premix, it can be considered—as a first estimate and other parameters being considered equal—as being constant from minimum to full load of the burner. It has been noticed that when the ratio of the exit velocity (m/s) of the premix gas through the burner deck to the burning velocity (also in m/s) of the combustible gas is becoming low, the risk of flame flash back becomes high.

[0143] FIG. 1 schematically explains a method of controlling a surface stabilized fully premixed gas premix burner using a constant air to combustible gas ratio over the full range of burner operation from minimum to full burner load. FIG. 1(a) shows the air to combustible gas ratio (Y) as a function of the burner load (X, in kVV) from minimum burner load (m) to full burner load (M). In the method of FIG. 1, the air to combustible gas ratio is kept constant. FIG. 1(b) shows—for the method of control as shown in FIG. 1(a)—in vertical axis the ratio (Z) of the exit velocity (m/s) of the premix gas through the burner deck to the burning velocity (also in m/s) of the combustible gas; as a function of burner load (X). This ratio is a straight line, having its minimum—and thus highest risk for flame flash back—at minimum burner load.

[0144] FIG. 2 schematically explains a method for controlling a surface stabilized fully premixed gas premix burner according to the invention. FIG. 2(a) shows the air to combustible gas ratio (Y) used as a function of the burner load (X, in kW), from minimum load (m) to full burner load (M). From full burner load down to burner load A a first value for the air to combustion gas ratio is used; for burner loads below A, a higher value (increasing with decreasing burner load) for the air to combustion gas ratio is set; which is at minimum burner load relatively at least 20% higher than the first valued. The consequence can be noticed in FIG. 2(b) showing the corresponding ratio (Z) of the exit speed over the burning speed for this method of controlling the burner. At burner load level A, the curve changes, wherein the minimum value the ratio (Z) of the exit velocity (m/s) of the premix gas through the burner deck to the burning velocity (also in m/s) of the combustible gas is increased considerably compared to the situation in FIG. 1(a). Therefore, the risk of flame flashback is drastically reduced. Furthermore, the higher amount of premix gas (because more air is supplied) will create an intensified cooling of the burner deck; further reducing the risk of flame flashback. The higher amount of premix reduces the flame speed of the mixture, further reducing the risk of flame flashback.

[0145] FIG. 3 schematically explains a method for controlling a surface stabilized fully premixed gas premix burner according to the invention. FIG. 3(a) shows the air to combustible gas ratio (Y) used as a function of the burner load (X, in kVV), from minimum load (m) to full burner load (M). At minimum load, the air to combustible gas ratio is in relative terms at least 20% higher than at full burner load. The consequence of the air to combustible gas ratio over the burner load can be noticed in FIG. 3(b) showing the corresponding ratio (Z) of the exit speed over the burning speed for this method of controlling the burner. At burner load level B, the curve reaches a minimum. The minimum value of the exit speed over the burner speed is increased considerably compared to the situation in FIG. 1(a). Therefore, the risk of flame flashback is drastically reduced. Furthermore, the higher amount of premix gas (because more air is supplied) will create an intensified cooling of the burner deck; further reducing the risk of flame flashback. The higher amount of premix reduces the flame speed of the mixture, further reducing the risk of flame flashback.

[0146] In the embodiments of FIGS. 1, 2 and 3, normal operating conditions are applied.

[0147] FIG. 4 shows an example of implementation of a method according to the invention. FIG. 4 shows a surface stabilized fully premixed gas premix burner 400. The burner has a cylindrical perforated plate 410 as burner deck. A premix of air and combustible gas is supplied inside the cylindrical perforated plate. The combustible gas supplied to the burner comprises at least 20% by volume of hydrogen. The premix flows through the through holes in the cylindrical perforated plate. The gas is combusted at the outside of the cylindrical perforated plate. The combustion is stabilized at the outside of the cylindrical perforated plate. A fan 420 is provided to supply the premix into the cylindrical perforated plan. The fan aspires air (aspired via supply line 430) and combustible gas (via supply line 440) is fed into the air flow. A pneumatic gas valve 450 is used, the operational characteristics of which are predefined such that the air to combustible gas ratio as a function of the burner load is obtained, e.g. as shown in FIGS. 2(a), 3(a) or FIG. 5.

[0148] Alternatively, the amount of gas can be set as a function of the speed of the fan (which is known via the burner control, or which is measured) via a controlled valve, using a predefined function, thereby implementing the predefined function for setting the air to combustible gas ratio of the premix gas supplied to the burner; e.g. the function as shown in FIGS. 2(a), 3(a) or FIG. 5.

[0149] FIG. 5 shows a practical example of a predefined function for setting the air to combustible gas ratio of the premix gas supplied to the burner. A cylindrical perforated plate burner was operated between minimum load 5 kW and full load 25 kW. The surface stabilized fully premixed cylindrical burner had 63 mm diameter and 158.4 mm length. The cylindrical burner had along its length an unperforated section with length 32 mm at the flange for premix supply; and at the end plate an unperforated length of 14.6 mm. The perforations were circular having 0.8 mm diameter and were uniformly distributed over a length of 111.8 mm of the burner. The porosity of the burner deck was 1.5%. The burner was operated with 100% hydrogen as combustible gas; and using the function as shown in FIG. 5 for the ratio of air to combustible gas (Y-axis) as a function of the burner load (axis X, in kW). The experiments showed total absence of flame flash back.

[0150] FIG. 6 schematically shows an embodiment of a burner system 500 according to the invention.

[0151] The burner system 500 comprises a burner 501 comprising a burner deck 502. The burner deck 502 comprises a plurality of holes 503. In the burner 501 of the burner system 500, the combined surface area of the holes 503 up to 5% of the surface area of the burner deck 502. This allows the burner system to use combustible gas which contains at least 20% by volume of hydrogen. The blind zones 504 do not have holes 503 in them, and therefore are not part of the burner deck 502.

[0152] In the embodiment of FIG. 6, the burner deck 502 is formed by a cylindrical perforated metal plate. The premix of air and combustible gas flows from the inside of the cylindrical perforated metal plate through the perforations (i.e. the holes 503) of the cylindrical perforated metal plate to its outside where it is combusted. Optionally, the cylindrical perforated metal plate is closed at one end by a metal end cap.

[0153] Optionally, the burner 501 comprises a gas distributor which is adapted to distribute the gas over the burner deck in a predetermined way.

[0154] The burner system 500 further comprises an air inlet 510, a combustible gas inlet 520 and a mixer 530 which is in communication with the air inlet 510 and the combustible gas inlet 520. The mixer 530 is adapted to mix air and combustible gas to a premix of combustible gas and air at an air to combustible gas ratio. Optionally, the mixer 530 is or comprises a venturi.

[0155] The combustible gas inlet 520 is suitable for receiving a combustible gas which contains at least 20% by volume of hydrogen. This is for example achieved by that the combustible gas inlet 520 meets any regulatory requirements for retaining such a combustible gas, that the combustible gas inlet is made of a suitable material and/or that the combustible gas inlet is connectable, either directly or indirectly, to a source of combustible gas which contains at least 20% by volume of hydrogen.

[0156] The burner system 500 further comprises a burner inlet 540 which is adapted to receive the premix of combustible gas and air from the mixer 530 and supply it to the burner 501. The burner inlet 540 is arranged upstream of the mixer 530 and downstream of the burner 501, as seen in the direction of the flow of combustible gas and air through the burner system.

[0157] The burner system 500 further comprises a burner load controller 561, which is adapted to vary the load of the burner between a minimum load and a full load. The ratio of the full load over the minimum load is at least 4, optionally more than 4, therewith allowing the burner to modulate between the minimum load and the full load. The burner load controller controls the load of the burner e.g. by a valve or fan 563.

[0158] In the embodiment of FIG. 6, the burner load controller forms part of an overall burner control system 560.

[0159] The burner 501 is adapted to modulate between a minimum load and a full load. The ratio of the full load over the minimum load is at least 4, optionally more than 4; and preferably more than 5, more preferably more than 7, even more preferably more than 10.

[0160] The burner system 500 further comprises a mechanism 570 which is adapted to set the air to combustible gas ratio of the premix of combustible gas and air which is created by the mixer 530. The setting of the air to combustible gas ratio of the premix of combustible gas and air is at least partially dependent from the load of the burner 501. The air to combustible gas ratio of the premix which is supplied to the burner 501 when the burner 501 is operated at minimum load is set by the mechanism 570 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 501 when the burner 501 is operated at full load.

[0161] The mechanism 570 for example comprises a pneumatic gas valve, which is adapted to set the rate of supply of combustible gas to the burner, in order to set the air to combustible gas ratio of the premix supplied to the burner as a predefined function of the burner load. The pneumatic gas valve for example comprises a spring, and wherein properties of the spring determine at least in part the predefined function.

[0162] In the burner system 500 of FIG. 6 further comprises a controller 562. The controller 562 is programmed to control the mechanism 570 such that the air to combustible gas ratio of the premix which is supplied to the burner 501 when the burner is operated at minimum load is set by the mechanism 570 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 501 when the burner 501 is operated at full load.

[0163] In the embodiment of FIG. 6, the controller 562 forms part of the overall burner control system 560.

[0164] In the embodiment of FIG. 6, the fan 563 of the burner load controller 561 is arranged to supply the premix of combustible air and gas to the burner 501.

[0165] FIG. 7 schematically shows a variant of the embodiment of FIG. 6.

[0166] In the embodiment of FIG. 7, the burner system 500 further comprises a sensor 590. The sensor 590 is adapted to measure the amount of air supplied to the burner 501. The sensor 590 is in this embodiment arranged in the burner inlet 540, but alternatively it can be arranged for example in the air inlet 510.

[0167] In the embodiment of FIG. 7, the burner system 500 further comprises a combustible gas supply controller 591 which is adapted to control the amount of combustible gas supplied to the mixer 530, and therewith, also the amount of combustible gas supplied to the burner 501.

[0168] The combustible gas supply controller 591 is adapted to set the amount of combustible gas supplied to the burner 501 according to a predefined relation to the amount of air supplied to the burner 501 as measured by the sensor 590.

[0169] The combustible gas supply controller 591 optionally comprises a fan. This fan optionally forms part of a burner load controller and/or is controlled by a burner load controller or elements thereof. In case such a fan is used in the combustible gas supply controller 591, the fan 563 of the embodiment of FIG. 6 may be dispensed with.

[0170] FIG. 8 schematically shows a variant of the embodiment of FIG. 6.

[0171] In the embodiment of FIG. 8, the burner system 500 further comprises a sensor 592. The sensor 592 is adapted to measure the amount of combustible gas supplied to the burner 501. The sensor 592 is in this embodiment arranged in the burner inlet 540, but alternatively it can be arranged for example in the combustible gas inlet 520.

[0172] In the embodiment of FIG. 8, the burner system 500 further comprises an air supply controller 593 which is adapted to control the amount of air supplied to the mixer and therewith, also the amount of air supplied to the burner 501. The air supply controller 593 is adapted to set the amount of air supplied to the burner 501 according to a predefined relation to the amount of combustible gas supplied to the burner 501 as measured by the sensor 592.

[0173] The air supply controller 593 optionally comprises a fan. This fan optionally forms part of a burner load controller and/or is controlled by a burner load controller or elements thereof. In case such a fan is used in the air supply controller 593, the fan 563 of the embodiment of FIG. 6 may be dispensed with.