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US2013260321
COOLED ELECTRODE AND BURNER SYSTEM INCLUDING A COOLED ELECTRODE



    
Also published as: WO2013126143 / EP2817566 /   CN104136849 / CA2862808
     
According to embodiments, an electrode configured to provide an electric field to a flame or combustion gas produced by a flame may receive heat from the flame or the combustion gas. The electrode may be cooled to remove the heat received from the flame or combustion gas.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority benefit from U.S. Provisional Patent Application No. 61/601920, entitled “COOLED ELECTRODE AND BURNER SYSTEM INCLUDING A COOLED ELECTRODE”, filed Feb. 22, 2012; which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

SUMMARY

[0002] According to an embodiment, an electrode system for a burner may include a thermally coupled electrode configured to apply an electric field to a region corresponding to a flame or combustion gas produced by the flame and to receive heat from the flame or the combustion gas. A cooling apparatus may be operatively coupled to the thermally coupled electrode and configured to remove the heat received by the electrode from the flame or the combustion gas.

[0003] According to another embodiment, a method of cooling an electrode subject to heating by a flame or a combustion gas produced by the flame may include applying an electric field to a flame or combustion gas produced by the flame with an electrode, causing a detectable response in the flame or the combustion gas responsive to the electric field, receiving heat from the flame or the combustion gas with the electrode, and cooling the electrode to remove the heat received from the flame or the combustion gas.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] FIG. 1 is a diagram showing a burner and a cooled electrode system for the burner, according to an embodiment.

[0005] FIG. 2 is a diagram showing the electrode of FIG. 1 with a thermo-electric cooler configured to remove the heat from the electrode, according to an embodiment.

[0006] FIG. 3 is a diagram showing the electrode of FIG. 1 with a cooling apparatus including a heat pipe, according to an embodiment.

[0007] FIG. 4 is a diagram showing the electrode of FIG. 1, wherein the electrode includes a flow channel for a cooling fluid and an aperture for outputting the heated cooling fluid to the flame or combustion gas, according to an embodiment.

[0008] FIG. 5 is a diagram showing the electrode of FIG. 1, wherein the electrode includes first and second fluid flow channels for carrying a cooling fluid through the electrode, according to an embodiment.

[0009] FIG. 6 is a diagram showing an electrically isolated cooling fluid source configured to provide a cooling fluid to a thermally coupled electrode, according to an embodiment.

[0010] FIG. 7 is a flowchart illustrating a method for cooling an electrode subject to heating by a flame or a combustion gas produced by the flame, according to an embodiment.



DETAILED DESCRIPTION

[0011] In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.

[0012] FIG. 1 is a diagram showing a burner 102 configured to support a flame 106 and a cooled electrode system 101 for the burner 102, according to an embodiment. The electrode system 101 for the burner 102 may include a thermally coupled electrode 104 configured to apply an electric field or eject electrically-charged ions to a region corresponding to the flame 106 or combustion gas 108 produced by the flame 106. The thermally coupled electrode 104 may receive heat from the flame 106 and/or the combustion gas 108. A cooling apparatus 110 may be operatively coupled to the thermally coupled electrode 104 and configured to remove the heat received by the electrode 104 from the flame 106 or the combustion gas 108.

[0013] A volume in which the flame 106 and combustion gases 108 are at least transiently held may be referred to as a combustion volume 111. The electrode 104 may be disposed at least partially within the combustion volume 111 to receive the heat from the flame 106 and/or the combustion gas 108. Alternatively, the electrode 104 may be outside the combustion volume 111, but thermally coupled to the flame 106 or the combustion gas 108 to receive heat therefrom.

[0014] The electrode 104 and cooling apparatus 110 are shown in block diagram form. Their physical form and location may vary from what may be indicated in FIG. 1. As will be appreciated from the description below, the cooling apparatus 110 may be disposed substantially within the electrode 104, may be adjacent to the electrode 104, and/or may include relatively extensive apparatus separated from the electrode 104.

[0015] According to an embodiment, at least a majority of heat removed by the cooling apparatus 110 from the thermally coupled electrode 104 may correspond to heat received from the flame 106 and/or combustion gas 108 produced by the flame 106. Additionally or alternatively, heat removed by the cooling apparatus 110 from the thermally coupled electrode 104 may include heat caused by dissipation from electrical modulation of the thermally coupled electrode 104 and heat received from the flame 106. Optionally, the burner and electrode system 101 may include a plurality of thermally coupled electrodes 104 and/or additional non-thermally coupled electrode(s) (not shown).

[0016] The burner 102 may include a fuel source 112 configured to provide fuel for the flame 106 and an oxidizer source 116 configured to provide oxidizer for the flame 106. Electrical isolation 114 may be configured to electrically isolate the fuel source 112 from ground or voltages other than voltages corresponding to the thermally coupled electrode 104.

[0017] An electrode controller 118 may be configured to apply a voltage corresponding to the electric field to the thermally coupled electrode 104 through one or more electrical leads 120. The electrode controller 118 may include a waveform generator 122 and an amplifier 124. The waveform generator 122 may be configured to provide a time-varying voltage. The time varying voltage may be at least partially periodic and may have a frequency between about 50 Hz and 10 kHz, for example. The amplifier 124 may amplify the time varying voltage received from the waveform generator 122 to a working voltage. The working voltage may be conveyed to the thermally coupled electrode 104 by the one or more electrical leads 120. The working voltage may range between about ±1000 V (e.g., as a time varying voltage formed as a sinusoid or a square wave that cycles between +1 kV and -1 kV) to about ±500,000 V (±500 kV). Experiments were run by the inventors using voltages between ±4 kV and ±40 kV. Typically, current dissipated by the electrode(s) 104 is low -- for example, 50 milliamperes or less. Accordingly, the electrode(s) 104 do not typically undergo any significant Joule heating responsive to the applied amplified electrical waveform. Rather, a majority to substantially all of the heat removed by cooling may be attributed to radiant and/or convective heat transfer from the flame 106 or the combustion gas 108 to the electrode 104.

[0018] The cooling apparatus 110 may be operatively coupled to and controlled by the electrode controller 118. Alternatively, the cooling apparatus 110 may be not controlled by the electrode controller 118. The block diagram connection 126 shown in FIG. 1 between the electrode controller 118 and the cooling apparatus 110 may be omitted when there is no operative coupling between the cooling apparatus 110 and the electrode controller 118.

[0019] A heat sink (not shown) may be operatively coupled to the cooling apparatus 110 and configured to receive the heat received by the thermally coupled electrode 104 from the flame 106 or the combustion gas 108, and removed from the thermally coupled electrode 104 by the cooling apparatus 110. For example, the cooling apparatus 110 may be configured to output heat from the thermally coupled electrode 104 to a heat sink (not shown) including a heat exchange surface (not shown) configured to pre-heat an oxidizer 116 or gas fed to the flame 106. For example, the oxidizer 116 may include oxygen carried in air. The heat sink (not shown) may include fins configured to pre-heat the air before the air flows into and past the flame 106.

[0020] Alternatively or additionally, the cooling apparatus 110 may be configured to output heat from the thermally coupled electrode 104 to a heat sink (not shown) including a heat exchange surface (not shown) configured to pre-heat fuel fed to the flame 106. For example, the heat sink (not shown) may include fins (not shown) disposed along a non-conductive portion 114 of a fuel supply tube. Such a heat sink and fins (not shown) may be formed, for example, by embedding the heat sink and/or fins in a cast fuel supply tube 114 or by co-molding the heat sink and/or fins in a thermoplastic fuel supply tube 114. Additionally or alternatively, the heat sink (not shown) may be formed as a conductive portion 112 of the fuel supply. This arrangement may be especially suitable for gaseous fuels.

[0021] According to another example, the heat sink (not shown) may be configured to be at least partially immersed in a liquid fuel, such as heating oil or bunker fuel. According to an embodiment, the heat sink (not shown) may be configured as at least a portion of a fuel intake mechanism (not shown) that pre-heats the liquid fuel for easier pumping and passage through a nozzle 112. This arrangement may be especially suitable for a viscous fuel such as bunker fuel.

[0022] The cooling apparatus 110 may be configured to output heat from the thermally coupled electrode 104 to a combustion volume 111 corresponding to the flame 106 or the combustion gas 108. For example, the cooling apparatus 110 may include a forced or natural convection system (not shown) configured to pass overfire air through a hollow thermally coupled electrode 104. The overfire air may pass through the electrode and out an orifice (such as an open end of the electrode, for example) at an overfire air injection location. In this case, the overfire air (or other fluid passing through the thermally coupled electrode 104) may itself act as the heat sink. This approach is described more fully in conjunction with FIG. 4 below.

[0023] The cooling apparatus 110 may be configured to output heat from the thermally coupled electrode 104 to a liquid, gas, or solid heat sink (not shown) that is not thermally coupled to the flame 106 or the combustion gas 108. For example, the liquid, gas, or solid heat sink may be electrically isolated from a secondary coolant (not shown) configured to remove the heat from the heat sink.

[0024] According to an embodiment, an electrical isolation system (not shown) may be configured to reduce or substantially prevent current leakage from the thermally coupled electrode 104 to a heat sink (not shown) configured to receive heat removed from the thermally coupled electrode by the cooling apparatus 110.

[0025] Various types of cooling apparatuses 110 are contemplated.

[0026] FIG. 2 is a diagram 201 showing the thermally coupled electrode of FIG. 1 with a thermo-electric cooler 202 configured and operatively coupled to remove the heat from the thermally coupled electrode 104, according to an embodiment. Thermo-electric coolers may typically operate according to the Peltier effect.

[0027] According to embodiments, the thermally coupled electrode 104 may be configured to be fluid cooled. Fluid cooling may take several forms including gas cooling, liquid cooling, phase change cooling; open and closed tips (respectively allowing and not allowing the fluid to be launched toward the flame 106); and/or with various heat sinking approaches. Generally speaking, a common theme may include electrical isolation of the electrode with respect to external fluid systems and/or with respect to grounding to the burner 102 and associated apparatuses. Electrode isolation may be intrinsic in the case of a non-conducting heat sink such as combustion air, or may include relatively sophisticated isolation approaches.

[0028] FIG. 3 is a diagram showing the thermally coupled electrode 104 of FIG. 1 with a cooling apparatus 110 including a heat pipe 302, according to an embodiment. The heat pipe 302 may be configured to receive heat from the flame 106 via evaporation at an evaporator end 304 and output the heat from the flame via condensation at a condenser end 306. Heat pipes are self-contained coolers that do not receive any power input or fluid flow from an outside source. At the evaporator 304, a liquid form of the working fluid in contact with a thermally conductive solid surface 312 turns into a vapor by absorbing heat from the surface. The vapor form of the working fluid traverses the length of the heat pipe (or depth of the heat pipe, depending on the particular physical form of the electrode 104) through a vapor space 308 to the condenser 306. At the condenser 306, the vapor condenses back into the liquid, releasing the latent heat that was absorbed at the evaporator 304. The liquid then returns to the evaporator 304 through either capillary action or gravity action where it evaporates once more and repeats the cycle. Typically, the liquid returns from the condenser to the evaporator via a wicking layer 310 adjacent to the vapor space 308.

[0029] According to an embodiment, a wall 312 of the heat pipe 302 may form an electrically conductive path of the electrode 104. The electrode 104 and the heat pipe 302 may include an electrical lug 314 configured for operative coupling to the electrode lead 120 from the electrode controller 118. The electrode 104 and the heat pipe 302 may also include an electrically insulating coating 316 configured to reduce or prevent communication of the voltage placed on the electrode 104 to ground, to another voltage, or to an electrically conductive cooling fluid received from a cooling fluid inlet 320 and output to a cooling fluid outlet 322. The wall 312 of the heat pipe may include one or more smooth contours 318 configured to reduce or prevent charge concentration and arcing to or through the flame 106.

[0030] FIG. 4 is a diagram showing the thermally coupled electrode 104 of FIG. 1, wherein the electrode 104 includes a flow channel 404 for a cooling fluid and an aperture 322 for outputting the heated cooling fluid to the flame 106 or combustion gas, according to an embodiment. The thermally coupled electrode 104, 401 may include a wall 402 forming an electrical conductor and defining a fluid flow channel 404 and at least one aperture 322 formed in the wall 402. The fluid flow channel 404 may be configured to convey a cooling fluid from a cooling fluid inlet 408 to the aperture 322 to transfer heat from the wall 402 to the cooling fluid and to output the heated cooling fluid to the flame 106 or to combustion gas 108 produced by the flame.

[0031] An electrical lug 314 may be configured for operative coupling between the electrically conductive wall 402 and the electrode lead 120 from the electrode controller 118. An electrically insulating coupling 410 to the fluid flow channel 404 may be configured to reduce or prevent communication of the voltage placed on the electrode 104 to ground, to another voltage, or to an electrically conductive secondary cooling fluid (not shown).

[0032] The wall 402 may include one or more smooth contours 318 configured to reduce or prevent charge concentration and arcing to or through the flame 106. Optionally, an electrically insulating coating 412 may be formed over at least a portion of the wall 402 adjacent to the flow channel 404 to reduce or eliminate current flow to the cooling fluid.

[0033] The cooling fluid may include a gas such as air. For example, the aperture 322 may form an overtire air port. According to other embodiments, the cooling fluid may include a liquid.

[0034] FIG. 5 is a diagram showing the electrode 104 of FIG. 1, wherein the electrode 104 includes first and second fluid flow channels 504, 506 for carrying a cooling fluid through the electrode 104, according to an embodiment. A wall 502 may define an electrical conductor for carrying electrode voltage. A first fluid flow channel 504 may be formed within the wall 502 and may be configured to convey received cooling fluid. A second fluid flow channel 506 may be formed within the wall and may be configured to convey output cooling fluid.

[0035] The fluid flow channels 504, 506 may be configured to respectively convey the cooling fluid at least a portion of a flow distance from a cooling fluid inlet port 508 to a cooling fluid outlet port 510. At least one of the fluid flow channels 504, 506 may be configured to transfer heat from the wall 502 to the cooling fluid. At least one fitting 512 may be configured to couple the fluid flow channels 504, 506 respectively to the cooling fluid inlet port 508 and the cooling fluid outlet port 510. The fitting 512 may form the cooling fluid inlet port 508 and the cooling fluid outlet port 510. The at least one fitting 512 may be substantially electrically insulating.

[0036] The fluid flow channels (e.g., 504, 506) may be arranged in various ways. For example, the fluid flow channels 504, 506 may be coaxial. A tube or integrally formed wall 514 may define the inner flow channel 506. The indicated flow directions may be reversed. Alternatively, the fluid flow channels may include parallel lumens that are not coaxial.

[0037] An electrical lug 314 may be configured for operative coupling between the electrically conductive wall 502 to the electrode lead 120, for coupling the electrode 104 to the electrode controller 118.

[0038] The cooling fluid may be electrically conductive or electrically non-conductive. According to embodiments, the cooling fluid may include a gas such as air or a gaseous fuel. In other embodiments, the cooling fluid may be electrically conductive or potentially electrically conductive. The cooling fluid may include a liquid such as water or a liquid fuel.

[0039] Some cooling fluids, such as water and/or some fuels, may be at least partially electrically conductive. Other cooling fluids, such as air that may carry humidity or insulating oil that may contain water, may be potentially conductive. As described above, relatively high voltages may be placed on the electrode 104. Accordingly, it may be advisable at least in some embodiments, to ensure electrical isolation of the electrode 104 from a cooling fluid or to ensure electrical isolation of the cooling fluid from ground or other voltages.

[0040] According to an embodiment, an electrically insulating coating 412 may be formed over at least a portion of surfaces of the wall 502 or walls 502, 514 defining the fluid flow channels 504, 506. The electrically insulating coating 412 may be configured to reduce or eliminate current flow to the cooling fluid. For example, the electrically insulating coating may include a ceramic coating. For example, the electrically insulating coating may include a glass formed by crosslinking a silane to form silicone and pyrolyzing the silicone.

[0041] As indicated above, some cooling fluids may be electrically conductive or at least potentially electrically conductive. Even in cases where electrical conductivity is not anticipated, it may be desirable to provide one or more extra levels of electrical isolation such as for fail-safe protection.

[0042] FIG. 6 is a diagram showing an electrically isolated cooling fluid source 601 configured to provide a cooling fluid to a thermally coupled electrode 104, according to an embodiment. An electrically insulating tank or pool 602 may be configured to hold a reservoir of cooling fluid 604. A cooling fluid supply system 606 may be configured to convey the cooling fluid from the reservoir 602 of cooling fluid 604 to a cooling fluid inlet 320, 408, 508 (respectively seen in FIGS. 3-5) operatively coupled to the thermally coupled electrode 104. The electrically isolated or electrically insulating cooling fluid supply system 601 may include an electrically isolated or electrically isolating pump 608 configured pump the cooling fluid. Additionally or alternatively, the electrically isolated or electrically insulating cooling fluid supply system 601 may be configured to deliver the cooling fluid responsive to a thermal siphon.

[0043] A return line 610 may be configured to return heated cooling fluid from the cooling fluid outlet port 322, 510 (see FIGS. 3 and 5).

[0044] A cooling fluid supply 612 may be configured to provide cooling fluid to the electrically insulating tank or pool 602 through an antisiphon arrangement 614 configured to prevent electrical conduction to the fluid supply 612. The electrically isolated cooling fluid source 601 may include a valve 616 configured to cause the cooling fluid to be supplied across the antisiphon arrangement 614 in a non-continuous stream that prevents electrical conduction from the cooling fluid reservoir 604 to the cooling fluid supply 612.

[0045] A secondary coolant tank 618 may be configured to hold a secondary coolant 620. The secondary coolant 620 may be arranged to receive heat from the cooling fluid reservoir 604 through the electrically insulating tank or pool 602.

[0046] FIG. 7 is a flowchart illustrating a method 701 for cooling an electrode subject to heating by a flame or a combustion gas produced by the flame, according to an embodiment. In step 704 an electric field may be applied to a flame or combustion gas produced by the flame with an electrode. Proceeding to step 706, the electric field may cause a detectable response in the flame or the combustion gas. As described elsewhere herein, it may be desirable or necessary to place the electrode where, in step 708, the electrode receives heat from the flame or the combustion. Proceeding to step 712, the electrode may be cooled to remove the heat received from the flame or the combustion gas.

[0047] According to some embodiments, a step (not shown) of generating heat in the electrode by Joule heating may be included. However, under such cases, the majority of heat removed by cooling in step 712 typically corresponds to heat received from the flame. In many if not all cases, substantially all the heat removed by cooling corresponds to heat received from the flame.

[0048] The method 701 may include supplying fuel and an oxidizer to a burner (not shown) and supporting the flame with the burner (not shown). For example, an electrode system including at least one thermally coupled electrode may be integrated with or sold with a burner such that a single vendor product performs these additional steps. In other cases, different vendors may supply the electrode system and the burner.

[0049] The method 701 may further include electrically isolating the fuel source from ground or voltages other than voltages corresponding to the electrode (not shown).

[0050] The method 701 may include step 702, wherein a time-varying voltage to the electrode. Step 702 may include generating a waveform with a waveform generator and amplifying the waveform to the time-varying voltage. The time-varying voltage applied to the electrode corresponds to the electric field applied to the flame or the combustion gas. The waveform and an amount of amplification applied to the waveform may be selected to cause the detectable response in the flame or the combustion gas. According to embodiments, the waveform and the time-varying voltage are selected not to cause Joule heating of the electrode. The waveform and the time-varying voltage may be selected to avoid causing arcing between the flame or other structures and the electrode; and to cause no inductive or resistive heating of the flame or the combustion gas.

[0051] Optionally, the method 701 may include controlling a cooling apparatus operatively coupled to the electrode with a controller that also generates the waveform for the electrode.

[0052] Cooling the electrode in step 712 may include operating a thermo-electric cooler.

[0053] Optionally, the method 701 may include step 710, including providing at least one of an electrically isolating cooling fluid or an electrically isolated heat sink to receive heat from the electrode. Step 710 may include transferring the heat to the electrically isolating cooling fluid or electrically isolated heat sink. For example, providing an electrically isolating cooling fluid or heat sink may include providing an electrically non-conducting gas. Providing the electrically non-conducting gas may include providing primary air or oxidizer for the flame. Transferring the heat to the electrically isolating cooling fluid in step 710 may include preheating the primary air or oxidizer with the heat removed from the electrode before mixing with a fuel or the flame. Additionally or alternatively, providing the electrically non-conducting gas may include providing overfire air or oxidizer for the flame. Transferring the heat to the electrically isolating cooling fluid may include preheating the overfire air or oxidizer with the heat removed from the electrode. The preheated overfire air or oxidizer may be injected into the flame or the combustion gas. For example, this approach may be used in conjunction with an electrode formed to correspond to the diagram of FIG. 4.

[0054] Additionally or alternatively, preheating the overfire air or oxidizer with heat removed from the electrode may include passing the overfire air or oxidizer through one or more lumens formed in the electrode and convectively receiving the heat into the overfire air or oxidizer from one or more walls of the one or more lumens. Injecting the preheated air or oxidizer into the flame or the combustion gas may include passing the convectively heated air or oxidizer from the one or more lumens through one or more apertures formed in the electrode and into the flame or combustion gas. Providing the electrically non-conductive gas may include providing atmospheric air.

[0055] Transferring the heat to the electrically isolating cooling fluid in step 712 may include transferring heat from the heat sink to the electrically non-conductive gas through cooling fins.

[0056] Optionally, providing an electrically isolating cooling fluid or heat sink in step 710 may include providing an electrically non-conductive liquid coolant. For example, providing a non-conductive liquid coolant may include providing a liquid fuel. Transferring the heat to the electrically isolating cooling fluid in step 712 may include transferring the heat to the liquid fuel to preheat the liquid fuel. The method 701 may include conveying the preheated liquid fuel to a burner and fueling the flame with the preheated liquid fuel.

[0057] Alternatively, step 710 may include providing an electrically conductive liquid coolant and electrically isolating the electrically conductive liquid coolant from ground and from voltages other than a voltage applied to the electrode. For example, providing the electrically conductive liquid fuel may include providing an electrically conductive liquid fuel, water, or a liquid metal. The method 701 may then include transferring heat from the electrically conductive liquid coolant to a secondary coolant or heat sink through an electrically non-conductive wall, heat exchanger, or tank (for example, see the block diagram of FIG. 6). Providing the electrically isolating cooling fluid or an electrically isolated heat sink in step 710 may include pumping the electrically conductive liquid coolant from an electrically isolating coolant reservoir and past a heat sink operatively coupled to the electrode. Additionally or alternatively, providing the electrically isolating cooling fluid or an electrically isolated heat sink in step 710 may include pumping the electrically conductive liquid coolant from an electrically isolating coolant reservoir and or through at least one fluid channel in the electrode. Electrically isolating the electrically conductive liquid coolant may further include providing a pump that is electrically isolating or electrically isolating the pump from a pump drive motor. For example, a peristaltic pump may be electrically isolating by using a non-conductive flexible tube to carry the liquid through the pump. Alternatively, a vane, centrifugal, positive displacement, or other pump may be electrically isolated from the pump drive motor by cutting a conductive shaft, and providing power transmission between ends of the conductive shaft through an insulating universal joint or shaft.

[0058] Electrically isolating the electrically conductive liquid coolant from ground and from voltages other than a voltage applied to the electrode may include providing the electrically conductive liquid coolant to a reservoir from a cooling fluid supply through an antisiphon arrangement that prevents electrical conduction between the fluid supply and the reservoir. For example, providing the electrically conductive liquid coolant to the reservoir from the cooling fluid supply through an antisiphon arrangement that prevents electrical conduction may include modulating a liquid coolant flow to prevent a continuous stream of the electrically conductive liquid coolant from bridging the antisiphon arrangement.

[0059] Additionally or alternatively, electrical isolation may be provided intrinsic to or in conjunction with the electrode. For example, an electrical insulator may be provided between the electrode and one or more cooling fluid flow channels. For example, see FIG. 3, 316, FIG. 4, 412, or FIG. 5, 412.

[0060] Step 712 may be performed in a range of ways described above in conjunction with apparatus diagrams. For example, cooling the electrode to remove heat received from the flame may include passing a cooling fluid through the electrode. Additionally or alternatively, cooling the electrode to remove heat received from the flame may include operating a heat pipe to remove the heat from the electrode. Heat from the heat pipe may be transferred to a cooling fluid. For example, transferring heat from the heat pipe to the cooling fluid may include passing primary combustion oxidizer, overfire oxidizer, or fuel across a condenser portion of the heat pipe to preheat the primary combustion oxidizer, overfire oxidizer, or fuel. Electrical insulation (e.g., FIG. 3, 316) may be provided over at least a condenser portion of the heat pipe to prevent conduction of an electrode voltage to a cooling fluid passing across the condenser.

[0061] Other approaches may also be used to cool a thermally coupled electrode and may fall within the scope of claims. Phase change materials that absorb heat during phase change may be circulated through a thermally coupled electrode or may be positioned in a solid form to respond to transients in heat input from the flame or hot gas. For example, certain metal alloys or salt mixtures (eutectics) may additionally or alternatively be used to provide at least temporary protection of the electrode 104. For example, Pb/Sn solder is a eutectic mixture that melts at 360 F or so. Eutectic mixtures may be provided for nearly any temperature using salts; e.g., between about 200 and 1600 F. One or more cavities in the electrode 104 may be filed with a eutectic mixture to provide passive overheat protection, at least for a time, because a melting salt maintains its melting point or melt range temperature until the last bit of salt is liquefied. The heat of fusion of a eutectic cavity may provide a substantial heat sink to protect a thermally coupled electrode 104, in the event of failure of a cooling apparatus, or in lieu of a separate cooling apparatus.

[0062] According to an embodiment, a eutectic may be circulated, such as in slurry form. One may also use a liquid/vapor equilibrium to provide overtemperature protection. According to an embodiment, a liquid may be adulterated to form a range of vaporization temperatures between a bubble point and a dew point rather than a single boiling point. If the electrode receives sufficient heat from the flame or combustion gas to reach a boiling point or boiling range of a circulated coolant, additional heat of vaporization may provide higher heat transfer rates than can be provided by a liquid responding only with a sensible temperature rise.

[0063] In circulated coolant systems, the circulation may optionally be passive, requiring no pumps or other moving parts. For example, in a system where the cooled phase is more dense, an upper end of a sealed tube containing a eutectic may be allowed to exchange heat with an ambient environment while a lower end of the sealed tube exchanges heat with the furnace. In this case, the device may circulate the cooler dense phase to the high temperature region and the hotter less dense phase to the cooling region. This approach may be used in the form of a heat pipe, such as the cooling system illustrated by FIG. 3, for example. By using materials with juxtaposed densities (e.g., bismuth etc.), the tube may work in the opposite orientation.







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