BURNER CONTROL SYSTEM

Abstract

A burner control system for controlling the operation of a fuel burner arranged to burn a combination of a supply of fuel and a supply of air is provided. The burner control system is arranged to receive from an exhaust gas analyzer one or more signals, each signal being indicative of the level of an exhaust gas emitted by the fuel burner; receive from a photodetector a signal indicative of a level of electromagnetic radiation output by the flame of the fuel burner; and control at least one of the supply of fuel and the supply of air to the burner based on the one or more signals received from the exhaust gas analyzer and the signal received from the photodetector.

Claims

1. A burner control system for controlling the operation of a fuel burner arranged to burn a combination of a supply of fuel and a supply of air, wherein the burner control system is arranged to: receive from an exhaust gas analyzer one or more signals, each signal being indicative of the level of an exhaust gas emitted by the fuel burner; receive from a photodetector a signal indicative of a level of electromagnetic radiation output by the flame of the fuel burner; and control at least one of the supply of fuel and the supply of air to the burner based on the one or more signals received from the exhaust gas analyzer and the signal received from the photodetector.

2. The burner control system as claimed in claim 1, wherein the burner control system is further arranged to determine a level of oxygen emitted by the fuel burner from the signal received from the photodetector.

3. The burner control system as claimed in claim 1, wherein a signal of the one or more signals received from the exhaust gas analyzer is indicative of the level of oxygen emitted by the fuel burner.

4. The burner control system as claimed in claim 3, wherein the burner control system is further arranged to determine a level of oxygen emitted by the fuel burner from a combination of the signal received from the photodetector and the signal indicative of the level of oxygen received from the exhaust gas analyzer.

5. The burner control system as claimed in claim 1, wherein the signal received from the photodetector is indicative of a level of ultraviolet radiation output by the flame.

6. The burner control system as claimed in claim 1, wherein the signal received from the photodetector is a single value indicative of a total level of electromagnetic radiation output by the flame.

7. The burner control system as claimed in of claim 1, wherein the burner control system is further arranged: when in a first operative state, to control at least one of the supply of fuel and the supply of air to the burner based on the signal received from the photodetector; and when in a second operative state, to control at least one of the supply of fuel and the supply of air to the burner based on the one or more signals received from the exhaust gas analyzer.

8. The burner control system as claimed in claim 7, wherein the burner control system is further arranged to move from the first operative state to the second operative state when the signal received from the photodetector is within a predetermined threshold for a predetermined period of time.

9. The burner control system as claimed in claim 8, wherein the burner control system is further arranged to move from the second operative state to the first operative state when the signal received from the photodetector moves outside of the predetermined threshold.

10. The burner control system as claimed in claim 7, wherein the burner control system is further arranged to move from the second operative state to the first operative state in response to a change in the level of supply of fuel to the fuel burner.

11. A burner control system as claimed in claim 7, wherein the burner control system is further arranged to move from the second operative state to the first operative state when a signal of the one or more signals received from the exhaust gas analyzer indicates that the level of a first exhaust gas being above a predetermined threshold.

12. The burner control system as claimed in claims 11, wherein the first exhaust gas is oxygen.

13. The burner control system as claimed in claim 7, wherein the burner control system is further arranged to move from the first operative state to the second operative state when a signal of the one or more signals received from the exhaust gas analyzer indicates that the level of a second exhaust gas has risen above a predetermined level.

14. The burner control system as claimed in claim 13, wherein the second exhaust gas is carbon monoxide.

15. A fuel burner arranged to burn a combination of a supply of fuel and a supply of air, comprising: an exhaust gas analyzer arranged to generate a one or more signals, each signal being indicative of the level of an exhaust gas emitted by the fuel burner; a photodetector arranged to generate a signal indicative of a level of electromagnetic radiation output by the flame of the fuel burner; and a burner control system arranged to: receive from an exhaust gas analyzer one or more signals, each signal being indicative of the level of an exhaust gas emitted by the fuel burner; receive from a photodetector a signal indicative of a level of electromagnetic radiation output by the flame of the fuel burner; and control at least one of the supply of fuel and the supply of air to the burner based on the one or more signals received from the exhaust gas analyzer and the signal received from the photodetector.

16. The fuel burner as claimed in claim 15, wherein the photodetector is located in the combustion chamber of the fuel burner.

17. The fuel burner as claimed in claim 15, wherein the photodetector is an ultraviolet photodetector.

18. The fuel burner as claimed in claim 15, wherein the photodetector is a photodiode.

19. A method of commissioning the fuel burner as claimed in claim 15, comprising: operating the fuel burner at a plurality of combinations of operational parameters, the operational parameters including at least the level of supply of fuel and level supply of air to the fuel burner; determining for each combination of operational parameters a level of oxygen emitted by the fuel burner; recording for each combination of operational parameters the one or more signals generated by the exhaust gas analyzer and the signal output by the photodetector; and determining a mapping from the operational parameters, one or more signals generated by the exhaust gas analyzer and signal output by the photodetector to the level of oxygen emitted by the fuel burner.

20. The method of commissioning a fuel burner as claimed in claim 19, further comprising: for each combination of operational parameters, determining air rich and air lean combinations of operational parameters on either side of the combination of operational parameters; and recording for the air rich and air lean combination of operational parameters the one or more signals generated by the exhaust gas analyzer and the signal output by the photodetector.

Description

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0050] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

[0051] FIG. 1 shows a schematic view of a fuel burner system according to a first embodiment of the invention;

[0052] FIG. 2 is a graph showing ultraviolet level against O.sub.2 level for different firing rates of a fuel burner;

[0053] FIG. 3 is a graph showing the signal from an ultraviolet photodiode of the fuel burner system of the first embodiment averaged over different time periods;

[0054] FIG. 4 is a flow chart showing the operation of the fuel burner system of the first embodiment; and

[0055] FIG. 5 is a graph showing the response of the ultraviolet photodiode and an exhaust gas analyzer of the fuel burner system of the first embodiment in response to a change in the firing rate of the fuel burner system.

DETAILED DESCRIPTION

[0056] FIG. 1 shows a schematic view of a fuel burner system according to a first embodiment of the invention. The fuel burner system 200 is of a type suitable for use as part of a commercial boiler installation which may for example be employed in the process or heating system of large premises, for example a factory, offices, a hotel or hospital. However, it will be appreciated that in other embodiments other types of fuel burner systems could be used.

[0057] The fuel burner system 200 comprises a fuel burner 1 to which fuel is fed via a duct 2, in which is located a fuel damper 4 to control the supply of fuel to the fuel burner 1. In addition, air is drawn from outside the fuel burner system 200 by a fan 6, and fed to the fuel burner 1 by a duct 5, in which is located an air damper 7 to control the supply of air to the fuel burner 1. In the present embodiment the fuel burner 1 is a gas burner supplied with natural gas, hydrogen gas or the like, but in other embodiments other types of fuel may be used, for example oil fuel or biomass.

[0058] In the fuel burner 1, the gas and air are mixed and combustion takes place, creating a flame 17. The heat generated by the combustion heats a supply of water 11. The exhaust gases and other combustion products that follow combustion by the fuel burner 1 are emitted via an exhaust 8.

[0059] An exhaust gas analyzer 9 is positioned in the exhaust 8. The exhaust gas analyzer 9 extracts and analyses the levels of exhaust gases exiting through the exhaust 8. In the present embodiment, the levels of O.sub.2, CO and CO.sub.2 are measured, though in other embodiments levels of further and/or other exhaust gases may be measured.

[0060] In addition, an ultraviolet photodiode 16 (i.e. a photodiode that converts ultraviolet radiation to an electrical current) is located in the combustion chamber of the fuel burner 1, where it receives electromagnetic radiation (e.g. visible light, ultraviolet and infrared), and particularly ultraviolet radiation, generated by the flame 17.

[0061] The ultraviolet photodiode 16 is a simple photodiode, without any filtering or electronic processing to select wavelengths or the like (though as discussed below it comprises a variable gain circuit, but only to amplify the signal it generates). In other embodiments, other photodiodes may be used, for example photodiodes that convert visible light, infrared, or any combination thereof (including in combination with ultraviolet) to an electrical current. The photodiode may also be located other than in the combustion chamber, as long as it is still able to receive electromagnetic radiation generated by the flame. In particular, the photodiode should be positioned so that it has a good view of the heart of the flame, as this maximizes the size of the ultraviolet response, and ensures that any changes in the ultraviolet level are directly proportional to the amount of ultraviolet radiation emitted by the flame. Having an obscured view of the flame, or pointing the photodiode towards the edge of the flame, may result in an erroneous assessment of the actual total ultraviolet being emitted by the flame. A control unit 10 controls the operation of the fuel burner 1, by controlling the fuel damper 4 and air damper 7 to adjust the gas and air flow rates to the fuel burner 1. The operation of the fan 6 is also controlled by the control unit 10. In this way, the control unit 10 controls the amounts of gas and air burnt by the gas burner, so controlling the operation of the gas burner.

[0062] Further, the control unit 10 receives signals from the exhaust gas analyzer 9 indicating the levels of O.sub.2, CO and CO.sub.2 emitted through the exhaust 8. As in known systems, the control unit 10 is able to use these levels to optimize the operation of the fuel burner 1, in particular the level of O.sub.2 emitted as an exhaust gas, by adjusting the fuel damper 4, air damper 7 and fan 6 as to give the fuel to air ratio required to give the desired level of O.sub.2.

[0063] However, the control unit 10 also receives a signal from the ultraviolet photodiode 16. This gives a single level indicative of the ultraviolet radiation the ultraviolet photodiode 16 has received from the flame 17. As mentioned above, no filtering or processing of the ultraviolet photodiode 16 or its signal is performed, the signal it outputs is purely an electrical current generated in response to the ultraviolet radiation it receives. It has been found that the amount of ultraviolet radiation emitted by a flame varies linearly with that of the percentage of O.sub.2 present in the exhaust gases of the boiler, for a given firing rate. The measured UV level of the flame is observed to decrease linearly as the excess oxygen level increases, for a given firing rate. This can be seen in FIG. 2, which shows the ultraviolet level against O.sub.2 level for different firing rates of a fuel burner.

[0064] In contrast to the signals from the exhaust gas analyzer 9, it has been found that the signal from the ultraviolet photodiode 16 is subject to considerable fluctuation, due to flickering of the flame 17. To mitigate this, the signal from the ultraviolet photodiode 16 is averaged over a time period prior to being used to determine the O.sub.2 level, for example for 10 seconds or 20 seconds. FIG. 3 shows an example of the signal ultraviolet photodiode 16 over a time period, averaged over one second (which it can be seen still gives considerable fluctuation), 10 seconds, and 20 seconds. The averaging is done by the control unit 10, but in other embodiments may be done at the ultraviolet photodiode 16 prior to transmitting to the control unit 10.

[0065] As mentioned above, the ultraviolet photodiode 16 comprises a variable gain circuit to amplify the signal it generates. This allows an optimum gain to be set during commissioning of the fuel burner system 200, so that the signal is as large as possible without going into saturation at the peak firing rate, so provides good signal strength throughout the firing range.

[0066] As well as being used to determine the level of O.sub.2, the ultraviolet photodiode 16 may be used to as a safety feature to determine the presence or absence of a flame, similarly to a standard flame scanner. The control unit 10 can then use this to ensure safe combustion.

[0067] The control unit 10 can then use the signal from the ultraviolet photodiode 16 as measure of O.sub.2 level, in addition to the levels of O.sub.2, CO and CO.sub.2 from the exhaust gas analyzer 9, to control the fuel burner 1. FIG. 4 is a flowchart showing the operation of the fuel burner system 200.

[0068] In a first step, the fuel burner 1 is started (step 101). In standard operation (step 102), the control unit 10 uses the signals received from the exhaust gas analyzer 9 to perform trimming (known as the “EGA trim”), i.e. to adjust the fuel damper 4 and air damper 7 as required to maintain optimum combustion. Any changes in the firing rate are detected (step 103). If no changes are detected, the standard EGA trim cycle is continued.

[0069] However, if a change in the firing rate is detected, instead of using EGA trim the control unit 10 uses the signal from the ultraviolet photodiode 16 to determine how to trim the system (“UV trim”), i.e. to move the air damper 7 to the desired position (step 104). The calculation uses the commissioned, air rich and air lean ultraviolet measurements recorded during commissioning of the fuel burner system 200 to calculate the change in ultraviolet that is needed for the desired O.sub.2 level to be occurring, and thus the change in the angle of the air damper 7 that is required. Linear interpolation is used to determine the required offset in ultraviolet reading for any firing rate in between the points used during commissioning. The air damper 7 is moved to the new position in small steps to prevent it overshooting the desired angle and causing an oscillating behavior. Alternatively changing the fan 6 speed may be used as an alternative to moving the air damper 7 to control the O.sub.2 level.

[0070] While UV trim is being used, the exhaust gas analyzer 9 continues to be used to measure the CO level in the exhaust 8 (step 105). If this rises to an undesirable level, the angles of the air damper 7 and fuel damper 4 are moved to those determined during commissioning (step 107), and the control unit 10 returns to using EGA trim again (step 102 again), so that safe operation is resumed, even if it may not immediately be optimal in terms of the O.sub.2 level. In fact, the exhaust gas analyzer 9 also continues to be used to measure the other exhaust gases, to identify if safe combustion is not occurring so that steps can be taken if required. In addition, it may be identified that the O.sub.2 level indicated by the ultraviolet photodiode 16 is inaccurate, which can occur at high temperatures for example, in which case the O.sub.2 level measurements from the exhaust gas analyzer 9 can be used to correct the O.sub.2 level determined from the signal from the ultraviolet photodiode 16.

[0071] Again while UV trim is being used, the control unit 10 periodically checks if the ultraviolet level as determined by the ultraviolet photodiode 16 has stabilized (step 106). If not, a period of time is waited before checking again (step 109, then returning to step 104). This period of time may be 5 or 10 seconds, for example.

[0072] However, if it is found that the ultraviolet level as determined by the ultraviolet photodiode 16 has settled, then the control unit 10 returns to using EGA trim (step 102).

[0073] In this way, the well-established EGA trim method of controlling the fuel burner system 200 can be used when the fuel burner system 200 is in the steady state. However, when the firing rate is changed, which creates a significant change in the operating conditions of the fuel burner 1 in a short amount of time, UV trim can be used to make changes to the fuel burner 1, with rapid feedback as to their effects being available due to the quick response of the ultraviolet photodiode 16, to enable optimum conditions to be quickly returned to. FIG. 5 is a graph showing example UV trim reading and EGA trim readings following a change in the firing rate over time. As can be seen, the UV trim is able to react to and adjust to take account of the firing rate change very quickly, compared to the EGA trim, which reacts much more slowly.

[0074] In the above embodiment, the signal from the ultraviolet photodiode 16 is used by the control unit 10 to control the operation of the fuel burner 1 when the firing rate is changed, with the signals from the exhaust gas analyzer 9 being used in standard operation. However, in other embodiments the control unit 10 could use the signals in other ways. For example, in an embodiment the ultraviolet photodiode 16 could be used for standard operation of the fuel burner system 200, effectively as a replacement for the exhaust gas analyzer 9, with the exhaust gas analyzer 9 only being used to detect unsafe behavior due to excess CO or the like, or to detect and correct the O.sub.2 level indicated by ultraviolet photodiode 16 when it becomes inaccurate, for example. In another embodiment, the signal from the ultraviolet photodiode 16 could be used as simply another parameter taken into account during commissioning, so that rather than the fuel burner 1 being controlled based on a mapping of exhaust gas analyzer 9 measurements and operating parameters (e.g. firing rate, damper angles) determined during commissioning of the fuel burner system 200, the ultraviolet photodiode 16 measurement is also incorporated as a parameter of the mapping. In another embodiment, the O.sub.2 level from the ultraviolet photodiode 16 could be used to supplement the O.sub.2 level from the exhaust gas analyzer 9, for example by averaging the two levels. It will be appreciated that various other control methods could be used in other embodiments of the invention.

[0075] While the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.

[0076] Where in the foregoing description, integers, or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.