Electrically heated burner

ELECTRICALLY HEATED BURNER

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority benefit from U.S. Provisional Patent Application No. 62/104,028, entitled "ELECTRICALLY HEAT ANY FLAME HOLDER", filed January 15, 2015, (Docket No. 2651 -267-02); and claims priority to and is a Continuation-in-Part of PCT Patent Application No.

PCT/US2014/016632, entitled "FUEL COMBUSTION SYSTEM WITH A

PERFORATED REACTION HOLDER", filed February 14, 2014, (Docket No. 2651 -188-04); each of which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

SUMMARY

According to an embodiment, an electrically heated burner includes a burner portion configured for contact with at least one of a fuel stream or a flame and an electrically powered heater operatively coupled to the burner portion. A power supply can operatively coupled to the electrically powered heater. A controller can be configured to control a flow of current from the power supply to the electrically powered heater. The electrically powered heater can be configured to output heat energy to the burner portion to cause a temperature of the burner portion to be controlled proportional to the controlled current flow.

According to an embodiment, a burner includes a fuel and oxidant source configured to cooperate to produce a fuel and oxidant stream that includes a range of fuel concentrations. A body is disposed to receive the fuel and oxidant stream, configured to maintain combustion within perforations of the body, and to exchange heat energy with the combustion reaction therein. An electrically powered heater is operatively coupled to the body. A current source powers the electrically powered heater. A thermostatic control circuit selectively couples the current source to the electrically powered heater to hold the body at a selected temperature.

According to another embodiment, a conventional flame holder body is disposed adjacent to a fuel and oxidant stream and is configured to initiate and maintain vortices in the fuel and oxidant stream. An electrically powered heater is configured to raise the conventional flame holder body to an elevated temperature. The conventional flame holder body is configured to exchange heat energy with an adjacent combustion reaction supported by the fuel and oxidant stream.

According to another embodiment, a method of combustion includes outputting fuel into a combustion volume, admitting air into the combustion volume, and allowing the fuel and air to mix in the combustion volume to form a fuel and air jet having a range of mixtures. An igniter ignites the fuel and air jet to initiate combustion. A flame holder is supported adjacent to the fuel and air jet or to receive the fuel and air jet so has to hold the combustion reaction in a predetermined position. The flame holder is electrically heated. Ignition of the fuel and air mixture is maintained by heat exchange with the electrically heated flame holder.

According to another embodiment, a control system controls dynamics of a flame. The control system includes a source of electric current and an electrically-heated flame holder positioned to be in contact with the flame and operatively coupled to the source of electric current. The source of electric current induces electric current flow in an electrical heater operatively coupled to the electrically-heated flame holder. A temperature-responsive current controller coupled to the current source and the electrically-heated flame holder adjusts the electric current flow through the electrically-heated flame holder to drive the electrically-heated flame holder to a predetermined temperature. According to an embodiment, a method for operating an electrically heated burner includes supporting combustion adjacent to at least a portion of an electrically heated industrial or commercial burner, outputting electrical current from a power supply to an electrically powered heater, causing the electrically powered heater to dissipate heat energy responsive to outputting the electrical current, and raising a temperature of the portion of the electrically heated burner with the heat energy.

According to another embodiment, a method for operating an electrically heated burner includes receiving start-up command data into a controller via a data interface, enabling a relay to output a drive voltage on at least one node of a power supply, receiving the drive voltage at an electrically powered heater, outputting heat energy from the electrically powered heater to a component of an industrial or commercial burner to cause a sensible temperature rise in the burner component and contacting a fuel stream or a flame supported by the fuel stream with the burner component.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of an electrically heated burner, according to an embodiment.

FIG. 2 is a simplified perspective view of a burner system including a perforated flame holder, according to an embodiment.

FIG. 3 is side sectional diagram of a portion of the perforated flame holder of FIGS. 1 and 2, according to an embodiment.

FIG. 4 is a flow chart showing a method for operating a burner system including the perforated flame holder of FIGS. 1, 2 and 3, according to an embodiment.

FIG. 5 is a partially schematic, partially cross-sectional diagram of a system for controlling electric heating of a flame holder, according to an embodiment. FIG. 6 is an elevational, partially cross-sectional diagram of a flame- holding burner tile with controllable fuel and air feeds, according to an

embodiment.

FIG. 7 is a cross-sectional diagram of a flame holder with an electric heater element inserted therethrough, according to an embodiment.

FIG. 8 is a diagrammatic perspective view of a combustion system in a furnace with feedback control of fuel flow and electric heating, according to an embodiment.

FIG. 9 is a flow chart showing a method for operating an electrically heated burner, according to an embodiment.

FIG. 10 is a flow chart showing a method for operating an electrically heated burner, according to another embodiment.

DETAILED DESCRIPTION

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. FIG. 1 is a diagram of an electrically heated burner 100, according to an embodiment. Various embodiments of the electrically heated burner 100 are contemplated. For ease of understanding, FIG. 1 depicts a sectional view, augmented by a block diagram, of an electrically heated burner 100 including an otherwise conventional burner tile 104 with an added electrically powered heater 106, according to an embodiment. The burner tile 104 can be a type that can be used to heat a furnace, duct, oven, or boiler volume 108 arranged to receive combustion heat from the electrically heated burner. The burner tile 104 can be of a type that is typically formed from a refractory material such as a fiber-reinforced refractory or cementatious material formed as a bluff body flame holder configured to hold a combustion reaction 1 10, shown as a flame, in the furnace, duct, oven, or boiler volume 108. According to embodiments, primary fuel nozzles 1 12 and

secondary fuel nozzles 1 14 are arranged to respectively impinge at least partially on interior and exterior surfaces 1 16, 1 18 of an axially symmetric burner tile 104. The burner may receive combustion air through a central natural or forced draft air source 120 and/or from other sources of combustion air. As is known to the art, the conventional aspects of the burner 100 support staged combustion as an attempt to control output of undesirable combustion products such as oxides of nitrogen (NOx) and carbon monoxide (CO).

One function of the burner tile 104 can be to provide flame holding for the combustion reaction 1 10. The inventors note that, especially in low NOx burners, flame holding can be somewhat tenuous and tricky to maintain. Other

shortcomings are apparent to those skilled in the art.

To address the shortcomings and design trade-offs of burners 100 used in commercial and industrial applications, the inventors believe that electrically heating a portion of the burner 100 such as the burner tile 104 offers

performance, capital cost, and/or operating cost advantages. The inventors contemplate use of the electrically powered heater 106 during system start-up, for example, to reduce time during which the burner 100 must be held at a low heat output. The inventors contemplate use of the electrically powered heater 106 during steady-state operation, for example, to improve stability of flame 1 10 holding by the burner tile 104. For example, the inventors have noted a tendency for flame lift-off and other asymmetries in existing burners under some operating conditions. For another example, the inventors contemplate use of the

electrically powered heater 104 to improve system turn-down range.

Conventional burners can exhibit a limitation with respect to the minimum amount of heat output compared to maximum heat output, especially in "low NOx" and "ultra low NOx" burners (both of which typically output far more NOx and/or more CO than a burner 200 with perforated flame holder 102 described in FIGS. 2-3 below). In an embodiment, electrically heating the burner tile 104 can help to maintain stable combustion even when fuel flow rate is reduced to a level insufficient to maintain a preferred operating temperature of the burner tile 104. In some embodiments, the inventors contemplate use of the electrically powered heater 106 to maintain the burner 100 in a "warm stand-by" mode wherein fuel flow is stopped during periods of low demand for heat release. This approach can afford the ability to start up the burner 100 more quickly to respond to an increase in power demand, process throughput, etc.

During start up, an igniter 122, such as a hot surface igniter, electrical discharge igniter, pilot flame, etc. can be used to ignite the primary fuel stream from the primary fuel nozzlesl 14. In typical prior art systems, only primary fuel is output for a time sufficient to heat the surface, and particularly a flame-holding surface 124 sufficiently to hold a larger flame supported by secondary fuel from the secondary fuel nozzles 1 14. During this time, the burner 100 cannot respond as quickly as may be desired to surge demand and/or the concentration of undesirable combustion products such as NOx and CO can be elevated.

According to an embodiment, during start up, a controller 126 controls an electrical current source 128 to energize a primary fuel igniter 122 and to energize the electrically powered heater 106. The additional heat output by the electrically powered heater 106 causes extra heat energy to be dissipated to the burner tile, and allows for faster start up. In an embodiment, the primary fuel igniter 122 and the electrically powered heater 106 can be energized at different and/or overlapping periods of time. In an embodiment, the controller 126 causes the electrical current source 128 to energize the electrically powered heater 106 upon receipt of a start up command via a data interface 130. A timer and/or optional one or more sensor(s) 132 operatively coupled to the controller can determine when the burner tile 104, and especially a flame holding surface 124 of the burner tile 104 is heated sufficiently for optimum ignition of the primary flame. Upon reaching a start up temperature, the controller 126 can cause the current source 128 to energize the igniter 122 and can cause a primary fuel valve 134 to open to supply fuel flow to the primary fuel nozzles 1 14 and ignite a primary flame. At a later time or simultaneously, the controller 126 can cause a secondary fuel valve 136 to open to supply fuel flow to the secondary fuel nozzles 1 14. Owing to the electrical heating of the burner tile 104 by the electrically powered heater 106, the time delay between receiving a start up command via the data interface 130 and igniting primary and secondary combustion 1 10 can be decreased, compared to a non electrically heated burner.

According to an embodiment, during steady-state operation, the controller can receive a temperature signal from temperature sensor(s) 132 (which may optionally be embedded in the burner tile 104 and/or embodied as an electrical resistance measurement across the power supply nodes 138, 140) and cause output of current from the current source 128 to maintain a desired temperature. In this way, the controller 126 and sensor(s) 132 can operate as a thermostat that maintains a temperature of at least the flame holding surface 124 of the burner tile 104.

According to an embodiment, a thermostat is configured to sense the temperature of the burner portion(s) and control current delivered from the power supply to the electrically powered heater to maintain a range of temperatures near a temperature set point. In one embodiment the thermostat has a precision of plus or minus 5 degrees F. In another embodiment, the thermostat has a precision of plus or minus 2 degrees F. In another embodiment, the thermostat has a precision that is poorer than plus or minus 2 degrees F.

According to an embodiment, the temperature sensor is operatively coupled to the burner portion and the controller, and configured to measure a temperature of the burner portion, and to output a temperature signal or data corresponding to the measured temperature to the controller.

According to another embodiment, the controller includes a timer circuit, a non-transitory computer readable medium carrying data corresponding to a duty cycle set point, and a signal generator operatively coupled to the timer circuit and configured to generate an electrical heater control signal having a duty cycle corresponding to the duty cycle set point. The controller can thus be configured to control the current flow from the power supply to the electrically powered heater according to the electrical heater control signal.

While the description corresponding to FIG. 1 was directed to an embodiment wherein the electrically powered heater 106 was operatively coupled to an otherwise conventional burner tile, the inventors contemplate additionally or alternatively providing an electrically powered heater to a perforated flame holder 102, described in more detail below in conjunction with FIGS. 2-4.

FIG. 2 is a simplified diagram of a burner system 200 including a perforated flame holder 102 configured to hold a combustion reaction, according to an embodiment. As used herein, the terms perforated flame holder, perforated reaction holder, porous flame holder, and porous reaction holder shall be considered synonymous unless further definition is provided. Experiments performed by the inventors have shown that perforated flame holders 102 described herein can support very clean combustion. Specifically, in

experimental use of systems 200 ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (O₂) concentration with undetectable carbon monoxide (CO) at stack temperatures typical of industrial furnace applications (1400 - 1600 °F). Moreover, these results did not require any extraordinary measures such as selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), water/steam injection, external flue gas recirculation (FGR), or other heroic extremes that may be required for conventional burners to even approach such clean combustion.

According to embodiments, the burner system 200 includes a fuel and oxidant source 202 disposed to output fuel and oxidant into a combustion volume 204 to form a fuel and oxidant mixture 206. As used herein, the terms

combustion volume, combustion chamber, furnace volume, and the like shall be considered synonymous unless further definition is provided. The perforated flame holder 102 is disposed in the combustion volume 204 and positioned to receive the fuel and oxidant mixture 206.

FIG. 3 is side sectional diagram 300 of a portion of the perforated flame holder 102 of FIG. 2, according to an embodiment. Referring to FIGS. 2 and 3, the perforated flame holder 102 includes a perforated flame holder body 208 defining a plurality of perforations 210 aligned to receive the fuel and oxidant mixture 206 from the fuel and oxidant source 202. As used herein, the terms perforation, pore, aperture, elongated aperture, and the like, in the context of the perforated flame holder 102, shall be considered synonymous unless further definition is provided. The perforations 210 are configured to collectively hold a combustion reaction 302 supported by the fuel and oxidant mixture 206.

The fuel can include hydrogen, a hydrocarbon gas, a vaporized

hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel can be a single species or can include a mixture of gas(es), vapor(s), atomized liquid(s), and/or pulverized solid(s). For example in a process heater application, the fuel can include fuel gas or byproducts from the process that include carbon monoxide (CO), hydrogen (H₂), and methane (CH ). In another application the fuel can include natural gas (mostly CH ) or propane (C3H8). In another application, the fuel can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant can include oxygen carried by air and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein.

According to an embodiment, the perforated flame holder body 208 can be bounded by an input face 212 disposed to receive the fuel and oxidant mixture 206, an output face 214 facing away from the fuel and oxidant source 202, and a peripheral surface 216 defining a lateral extent of the perforated flame holder 102. The plurality of perforations 210 which are defined by the perforated flame holder body 208 extend from the input face 212 to the output face 214. The plurality of perforations 210 can receive the fuel and oxidant mixture 206 at the input face 212. The fuel and oxidant mixture 206 can then combust in or near the plurality of perforations 210 and combustion products can exit the plurality of perforations 210 at or near the output face 214.

According to an embodiment, the perforated flame holder 102 is

configured to hold a majority of the combustion reaction 302 within the

perforations 210. For example, on a steady-state basis, more than half the molecules of fuel output into the combustion volume 204 by the fuel and oxidant source 202 may be converted to combustion products between the input face 212 and the output face 214 of the perforated flame holder 102. According to an alternative interpretation, more than half of the heat output by the combustion reaction 302 may be output between the input face 212 and the output face 214 of the perforated flame holder 102. Under nominal operating conditions, the perforations 210 can be configured to collectively hold at least 80% of the combustion reaction 302 between the input face 212 and the output face 214 of the perforated flame holder 102. In some experiments, the inventors produced a combustion reaction that was apparently wholly contained in the perforations 210 between the input face 212 and the output face 214 of the perforated flame holder 102. According to an alternative interpretation, the perforated fame holder 102 can support combustion between the input face 212 and output face 214 when combustion is "time-averaged". For example, during transients, such as before the perforated flame holder 102 is fully heated, or if too high a (cooling) load is placed on the system, the combustion may travel somewhat downstream from the output face 214 of the perforated flame holder.

While a "flame" is described in a manner intended for ease of description, it should be understood that in some instances, no visible flame is present.

Combustion occurs primarily within the perforations 210, but the "glow" of combustion heat is dominated by a visible glow of the perforated flame holder 102 itself. In other instances, the inventors have noted transient "huffing" wherein a visible flame momentarily ignites in a region lying between the input face 212 of the perforated flame holder 102 and the fuel source 218, within the dilution region D_D. Such transient huffing is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations 210 of the perforated flame holder 102, between the input face 212 and the output face 214. In still other instances, the inventors have noted apparent combustion occurring above the output face 214 of the perforated flame holder 102, but still a majority of combustion occurred within the perforated flame holder as evidenced by the continued visible glow (a visible wavelength tail of blackbody radiation) from the perforated flame holder 102.

The perforated flame holder 102 can be configured to receive heat from the combustion reaction 302 and output a portion of the received heat as thermal radiation 304 to heat-receiving structures (e.g., furnace walls and/or radiant section working fluid tubes) in or adjacent to the combustion volume 204. As used herein, terms such as thermal radiation, infrared radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody radiation of electromagnetic energy, primarily in infrared wavelengths.

Referring especially to FIG. 3, the perforated flame holder 102 outputs another portion of the received heat to the fuel and oxidant mixture 206 received at the input face 212 of the perforated flame holder 102. The perforated flame holder body 208 may receive heat from the (exothermic) combustion reaction 302 at least in heat receiving regions 306 of perforation walls 308. Experimental evidence has suggested to the inventors that the position of the heat receiving regions 306, or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls 308. In some experiments, the location of maximum receipt of heat was apparently between 1/3 and 1/2 of the distance from the input face 212 to the output face 214 (i.e., somewhat nearer to the input face 212 than to the output face 214). The inventors contemplate that the heat receiving regions 306 may lie nearer to the output face 214 of the perforated flame holder 102 under other conditions. Most probably, there is no clearly defined edge of the heat receiving regions 306 (or for that matter, the heat output regions 310, described below). For ease of understanding, the heat receiving regions 306 and the heat output regions 310 will be described as particular regions 306, 310.

The perforated flame holder body 208 can be characterized by a heat capacity. The perforated flame holder body 208 may hold heat from the combustion reaction 302 in an amount corresponding to the heat capacity times temperature rise, and transfer the heat from the heat receiving regions 306 to heat output regions 310 of the perforation walls 308. Generally, the heat output regions 310 are nearer to the input face 212 than are the heat receiving regions 306. According to one interpretation, the perforated flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via thermal radiation, depicted graphically as 304. According to another interpretation, the perforated flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via heat conduction along heat conduction paths 312. The inventors contemplate that both radiation and conduction heat transfer mechanisms may be operative in transferring heat from the heat receiving regions 306 to the heat output regions 310. In this way, the perforated flame holder 102 may act as a heat source to maintain the combustion reaction, even under conditions where a combustion reaction would not be stable when supported from a conventional flame holder.

The inventors believe that the perforated flame holder 102 causes the combustion reaction 302 to occur within thermal boundary layers 314 formed adjacent to walls 308 of the perforations 210. As the relatively cool fuel and oxidant mixture 206 approaches the input face 212, the flow is split into portions that respectively travel through individual perforations 210. The hot perforated flame holder body 208 transfers heat to the fluid, notably within thermal boundary layers 314 that progressively thicken as more and more heat is transferred to the incoming fuel and oxidant mixture 206. After reaching a combustion temperature (e.g. the auto-ignition temperature of the fuel), the reactants continue to flow while a chemical ignition delay time elapses, over which time the combustion reaction 302 occurs. Accordingly, the combustion reaction 302 is shown as occurring within the thermal boundary layers 314. As flow progresses, the thermal boundary layers 314 merge at a merger point 316. Ideally, the merger point 316 lies between the input face 212 and output face 214 that defines the ends of the perforations 210. At some point, the combustion reaction 302 causes the flowing gas (and plasma) to output more heat to the body 208 than it receives from the body 208. The heat is received at the heat receiving region 306, is held by the body 208, and is transported to the heat output region 310 nearer to the input face 212, where the heat recycles into the cool reactants (and any included diluent) to raise them to the combustion temperature.

In an embodiment, the plurality of perforations 210 are each characterized by a length L defined as a reaction fluid propagation path length between the input face 212 and the output face 214 of the perforated flame holder 102. The reaction fluid includes the fuel and oxidant mixture 206 (optionally including nitrogen, flue gas, and/or other "non-reactive" species), reaction intermediates (including transition states in a plasma that characterizes the combustion reaction), and reaction products.

The plurality of perforations 210 can be each characterized by a

transverse dimension D between opposing perforation walls 308. The inventors have found that stable combustion can be maintained in the perforated flame holder 102 if the length L of each perforation 210 is at least four times the transverse dimension D of the perforation. In other embodiments, the length L can be greater than six times the transverse dimension D. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D. Preferably, the length L is sufficiently long for thermal boundary layers 314 formed adjacent to the perforation walls 308 in a reaction fluid flowing through the perforations 210 to converge at merger points 316 within the perforations 210 between the input face 212 and the output face 214 of the perforated flame holder. In experiments, the inventors have found L/D ratios between 12 and 48 to work well (i.e., produce low NOx, produce low CO, and maintain stable combustion).

The perforated flame holder body 208 can be configured to convey heat between adjacent perforations 210. The heat conveyed between adjacent perforations 210 can be selected to cause heat output from the combustion reaction portion 302 in a first perforation 210 to supply heat to stabilize a combustion reaction portion 302 in an adjacent perforation 210.

Referring especially to FIG. 2, the fuel and oxidant source 202 can further include a fuel nozzle 218, configured to output fuel, and an oxidant source 220 configured to output a fluid including the oxidant. For example, the fuel nozzle 218 can be configured to output pure fuel. The oxidant source 220 can be configured to output combustion air carrying oxygen.

The perforated flame holder 102 can be held by a perforated flame holder support structure 222 configured to hold the perforated flame holder 102 a distance D_D away from the fuel nozzle 218. The fuel nozzle 218 can be configured to emit a fuel jet selected to entrain the oxidant to form the fuel and oxidant mixture 206 as the fuel jet and oxidant travel along a path to the perforated flame holder 102 through a dilution distance D_D between the fuel nozzle 218 and the perforated flame holder 102. Additionally or alternatively

(particularly when a blower is used to deliver oxidant combustion air), the oxidant or combustion air source can be configured to entrain the fuel and the fuel and oxidant travel through the dilution distance D_D. In some embodiments, a flue gas recirculation path 224 can be provided. Additionally or alternatively, the fuel nozzle 218 can be configured to emit a fuel jet selected to entrain the oxidant and to entrain flue gas as the fuel jet travels through a dilution distance D_D between the fuel nozzle 218 and the input face 212 of the perforated flame holder 102.

The fuel nozzle 218 can be configured to emit the fuel through one or more fuel orifices 226 having a dimension that is referred to as "nozzle diameter". The perforated flame holder support structure 222 can support the perforated flame holder 102 to receive the fuel and oxidant mixture 206 at a distance D_D away from the fuel nozzle 218 greater than 20 times the nozzle diameter. In another embodiment, the perforated flame holder 102 is disposed to receive the fuel and oxidant mixture 206 at a distance D_D away from the fuel nozzle 218 between 100 times and 1 100 times the nozzle diameter. Preferably, the perforated flame holder support structure 222 is configured to hold the perforated flame holder 102 about 200 times the nozzle diameter or more away from the fuel nozzle 218. When the fuel and oxidant mixture travels about 200 times the nozzle diameter or more, the mixture is sufficiently homogenized to cause the combustion reaction to output minimal NOx.

The fuel and oxidant source 202 can alternatively include a premix fuel and oxidant source, according to an embodiment. A premix fuel and oxidant source can include a premix chamber (not shown), a fuel nozzle configured to output fuel into the premix chamber, and an air channel configured to output combustion air into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the perforated flame holder 102 and be configured to prevent flame flashback into the premix fuel and oxidant source.

The combustion air source, whether configured for entrainment in the combustion volume 204 or for premixing can include a blower configured to force air through the fuel and air source 202.

The support structure 222 can be configured to support the perforated flame holder 102 from a floor or wall (not shown) of the combustion volume 204, for example. In another embodiment, the support structure 222 supports the perforated flame holder 102 from the fuel and oxidant source 202. Alternatively, the support structure 222 can suspend the perforated flame holder 102 from an overhead structure (such as a flue, in the case of an up-fired system). The support structure 222 can support the perforated flame holder 102 in various orientations and directions.

The perforated flame holder 102 can include a single perforated flame holder body 208. In another embodiment, the perforated flame holder 102 can include a plurality of adjacent perforated flame holder sections that collectively provide a tiled perforated flame holder 102.

The perforated flame holder support structure 222 can be configured to support the plurality of perforated flame holder sections. The perforated flame holder support structure 222 can include a metal superalloy, a cementatious, and/or ceramic refractory material. In an embodiment, the plurality of adjacent perforated flame holder sections can be joined with a fiber reinforced refractory cement.

The perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least twice a thickness dimension T between the input face 212 and the output face 214. In another embodiment, the perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least three times, at least six times, or at least nine times a thickness dimension T between the input face 212 and the output face 214 of the perforated flame holder.

In an embodiment, the perforated flame holder 102 can have a width dimension W less than a width of the combustion volume 204. This can allow the flue gas circulation path 224 from above to below the perforated flame holder 102 to lie between the peripheral surface 216 of the perforated flame holder 102 and the combustion volume wall (not shown).

Referring again to both FIGS. 2 and 3, the perforations 210 can include elongated squares, each of the elongated squares has a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations 210 can include elongated hexagons, each of the elongated hexagons has a transverse dimension D between opposing sides of the hexagons. In another embodiment, the perforations 210 can include hollow cylinders, each of the hollow cylinders has a transverse dimension D corresponding to a diameter of the cylinders. In another embodiment, the perforations 210 can include truncated cones, each of the truncated cones has a transverse dimension D that is rotationally symmetrical about a length axis that extends from the input face 212 to the output face 214. The perforations 210 can each have a lateral dimension D equal to or greater than a quenching distance of the fuel based on standard reference conditions.

In one range of embodiments, each of the plurality of perforations has a lateral dimension D between 0.05 inch and 1 .0 inch. Preferably, each of the plurality of perforations has a lateral dimension D between 0.1 inch and 0.5 inch. For example the plurality of perforations can each have a lateral dimension D of about 0.2 to 0.4 inch.

The void fraction of a perforated flame holder 102 is defined as the total volume of all perforations 210 in a section of the perforated flame holder 102 divided by a total volume of the perforated flame holder including body 208 and perforations 210. The perforated flame holder 102 should have a void fraction between 0.10 and 0.90. In an embodiment, the perforated flame holder 102 can have a void fraction between 0.30 and 0.80. In another embodiment, the perforated flame holder 102 can have a void fraction of about 0.70. Using a void fraction of about 0.70 was found to be especially effective for producing very low NOx.

The perforated flame holder 102 can be formed from a fiber reinforced cast refractory material and/or a refractory material such as an aluminum silicate material. For example, the perforated flame holder 102 can be formed from mullite or cordierite. Additionally or alternatively, the perforated flame holder body 208 can include a metal superalloy such as Inconel or Hastelloy. The perforated flame holder body 208 can define a honeycomb.

The inventors have found that the perforated flame holder 102 can be formed from VERSAGRID ® ceramic honeycomb, available from Applied

Ceramics, Inc. of Doraville, South Carolina.

The perforations 210 can be parallel to one another and normal to the input and output faces 212, 214. In another embodiment, the perforations 210 can be parallel to one another and formed at an angle relative to the input and output faces 212, 214. In another embodiment, the perforations 210 can be non- parallel to one another. In another embodiment, the perforations 210 can be non-parallel to one another and non-intersecting. In another embodiment, the perforations 210 can be intersecting. The body 308 can be one piece or can be formed from a plurality of sections.

In another embodiment, which is not necessarily preferred, the perforated flame holder 102 may be formed from reticulated fibers formed from an extruded ceramic material. The term "reticulated fibers" refers to a netlike structure. In another embodiment, the perforated flame holder 102 can include a plurality of tubes or pipes bundled together. The plurality of perforations 210 can include hollow cylinders and can optionally also include interstitial spaces between the bundled tubes. In an embodiment, the plurality of tubes can include ceramic tubes. Refractory cement can be included between the tubes and configured to adhere the tubes together. In another embodiment, the plurality of tubes can include metal (e.g., superalloy) tubes. The plurality of tubes can be held together by a metal tension member circumferential to the plurality of tubes and arranged to hold the plurality of tubes together. The metal tension member can include stainless steel, a superalloy metal wire, and/or a superalloy metal band.

The perforated flame holder body 208 can alternatively include stacked perforated sheets of material, each sheet having openings that connect with openings of subjacent and superjacent sheets. The perforated sheets can include perforated metal sheets, ceramic sheets and/or expanded sheets. In another embodiment, the perforated flame holder body 208 can include discontinuous packing bodies such that the perforations 210 are formed in the interstitial spaces between the discontinuous packing bodies. In one example, the discontinuous packing bodies include structured packing shapes. In another example, the discontinuous packing bodies include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig ring, ceramic Berl saddles, ceramic Intalox saddles, and/or metal rings or other shapes (e.g. Super Raschig Rings) that may be held together by a metal cage.

The inventors contemplate various explanations for why burner systems including the perforated flame holder 102 provide such clean combustion.

In one aspect, the perforated flame holder 102 acts as a heat source to maintain a combustion reaction even under conditions where a combustion reaction would not be stable when supported by a conventional flame holder. This capability can be leveraged to support combustion using a leaner fuel -to- oxidant mixture than is typically feasible. Thus, according to an embodiment, at the point where the fuel stream 206 contacts the input face 212 of the perforated flame holder 102, an average fuel-to-oxidant ratio of the fuel stream 206 is below a (conventional) lower combustion limit of the fuel component of the fuel stream— lower combustion limit defines the lowest concentration of fuel at which a fuel/air mixture will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C (77° F).

According to one interpretation, the fuel and oxidant mixtures supported by the perforated flame holder may be more fuel-lean than mixtures that would provide stable combustion in a conventional burner. Combustion near a lower combustion limit of fuel generally burn at a lower adiabatic flame temperature than mixtures near the center of the lean-to-rich combustion limit range. Lower flame temperatures generally evolve a lower concentration of oxides of nitrogen (NOx) than higher flame temperatures. In conventional flames, too-lean combustion is generally associated with high CO concentration at the stack. In contrast, the perforated flame holder 102 and systems including the perforated flame holder 102 described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. In some embodiments, the inventors achieved stable combustion at what was understood to be very lean mixtures (that nevertheless produced only about 3% or lower measured O2 concentration at the stack). Moreover, the inventors believe perforation walls 308 may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperature.

According to another interpretation, production of NOx can be reduced if the combustion reaction 302 occurs over a very short duration of time. Rapid combustion causes the reactants (including oxygen and entrained nitrogen) to be exposed to NOx-formation temperature for a time too short for NOx formation kinetics to cause significant production of NOx. The time required for the reactants to pass through the perforated flame holder is very short compared to a conventional flame. The low NOx production associated with perforated flame holder combustion may thus be related to the short duration of time required for the reactants (and entrained nitrogen) to pass through the perforated flame holder 102.

Since CO oxidation is a relatively slow reaction, the time for passage through the perforated flame holder (perhaps plus time passing toward the flue from the perforated flame holder 102) is apparently sufficient and at sufficiently elevated temperature, in view of the very low measured (experimental and full scale) CO concentrations, for oxidation of CO to carbon dioxide (CO₂).

FIG. 4 is a flow chart showing a method 400 for operating a burner system including the perforated flame holder shown and described herein. To operate a burner system including a perforated flame holder, the perforated flame holder is first heated to a temperature sufficient to maintain combustion of the fuel and oxidant mixture.

According to a simplified description, the method 400 begins with step 402, wherein the perforated flame holder is preheated to a start-up temperature, Ts. After the perforated flame holder is raised to the start-up temperature, the method proceeds to step 404, wherein fuel and oxidant are provided to the perforated flame holder and combustion is held by the perforated flame holder.

According to a more detailed description, step 402 begins with step 406, wherein start-up energy is provided at the perforated flame holder.

Simultaneously or following providing start-up energy, a decision step 408 determines whether the temperature T of the perforated flame holder is at or above the start-up temperature, T_s. As long as the temperature of the perforated flame holder is below its start-up temperature, the method loops between steps 406 and 408 within the preheat step 402. In step 408, if the temperature T of at least a predetermined portion of the perforated flame holder is greater than or equal to the start-up temperature, the method 400 proceeds to overall step 404, wherein fuel and oxidant is supplied to and combustion is held by the perforated flame holder.

Step 404 may be broken down into several discrete steps, at least some of which may occur simultaneously. Proceeding from step 408, a fuel and oxidant mixture is provided to the perforated flame holder, as shown in step 410. The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and combustion air source, for example. In this approach, the fuel and combustion air are output in one or more directions selected to cause the fuel and

combustion air mixture to be received by an input face of the perforated flame holder. The fuel may entrain the combustion air (or alternatively, the combustion air may dilute the fuel) to provide a fuel and oxidant mixture at the input face of the perforated flame holder at a fuel dilution selected for a stable combustion reaction that can be held within the perforations of the perforated flame holder.

Proceeding to step 412, the combustion reaction is held by the perforated flame holder.

In step 414, heat may be output from the perforated flame holder. The heat output from the perforated flame holder may be used to power an industrial process, heat a working fluid, generate electricity, or provide motive power, for example.

In optional step 416, the presence of combustion may be sensed. Various sensing approaches have been used and are contemplated by the inventors. Generally, combustion held by the perforated flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, and/or other known combustion sensing apparatuses. In an additional or alternative variant of step 416, a pilot flame or other ignition source may be provided to cause ignition of the fuel and oxidant mixture in the event combustion is lost at the perforated flame holder.

Proceeding to decision step 418, if combustion is sensed not to be stable, the method 400 may exit to step 424, wherein an error procedure is executed. For example, the error procedure may include turning off fuel flow, re-executing the preheating step 402, outputting an alarm signal, igniting a stand-by combustion system, or other steps. If, in step 418, combustion in the perforated flame holder is determined to be stable, the method 400 proceeds to decision step 420, wherein it is determined if combustion parameters should be changed. If no combustion parameters are to be changed, the method loops (within step 404) back to step 410, and the combustion process continues. If a change in combustion parameters is indicated, the method 400 proceeds to step 422, wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step 404) back to step 410, and combustion continues.

Combustion parameters may be scheduled to be changed, for example, if a change in heat demand is encountered. For example, if less heat is required (e.g., due to decreased electricity demand, decreased motive power requirement, or lower industrial process throughput), the fuel and oxidant flow rate may be decreased in step 422. Conversely, if heat demand is increased, then fuel and oxidant flow may be increased. Additionally or alternatively, if the combustion system is in a start-up mode, then fuel and oxidant flow may be gradually increased to the perforated flame holder over one or more iterations of the loop within step 404.

Referring again to FIG. 2, the burner system 200 includes a heater 228 operatively coupled to the perforated flame holder 102. As described in conjunction with FIGS. 3 and 4, the perforated flame holder 102 operates by outputting heat to the incoming fuel and oxidant mixture 206. After combustion is established, this heat is provided by the combustion reaction; but before combustion is established, the heat is provided by the heater 228.

Various heating apparatuses have been used and are contemplated by the inventors. In some embodiments, the heater 228 can include a flame holder configured to support a flame disposed to heat the perforated flame holder 102. The fuel and oxidant source 202 can include a fuel nozzle 218 configured to emit a fuel stream and an air source 220 configured to output combustion air adjacent to the fuel stream. The fuel nozzle 218 and air source 220 can be configured to output the fuel stream to be progressively diluted by the combustion air. The perforated flame holder 102 can be disposed to receive a diluted fuel and air mixture 206 that supports a combustion reaction that is stabilized by the perforated flame holder 102 when the perforated flame holder 102 is at an operating temperature. A start-up flame holder, in contrast, can be configured to support a start-up flame at a location corresponding to a relatively rich fuel and air mixture that is stable without stabilization provided by the heated perforated flame holder 102.

The burner system 200 can further include a controller 230 operatively coupled to the heater 228 and to a data interface 232. For example, the controller 230 can be configured to control a start-up flame holder actuator configured to cause the start-up flame holder to hold the start-up flame when the perforated flame holder 102 needs to be pre-heated and to not hold the start-up flame when the perforated flame holder 102 is at an operating temperature (e.g., when T > T_s).

Various approaches for actuating a start-up flames are contemplated. In one embodiment, the start-up flame holder includes a mechanically-actuated bluff body configured to be actuated to intercept the fuel and oxidant mixture 206 to cause heat-recycling vortices and thereby hold a start-up flame; or to be actuated to not intercept the fuel and oxidant mixture 206 to cause the fuel and oxidant mixture 206 to proceed to the perforated flame holder 102. In another embodiment, a fuel control valve, blower, and/or damper may be used to select a fuel and oxidant mixture flow rate that is sufficiently low for a start-up flame to be jet-stabilized; and upon reaching an perforated flame holder 102 operating temperature, the flow rate may be increased to "blow out" the start-up flame. In another embodiment, the heater may include an electrical power supply operatively coupled to the controller 230 and configured to apply an electrical charge or voltage to the fuel and oxidant mixture 206. An electrically conductive start-up flame holder may be selectively coupled to a voltage ground or other voltage selected to attract the electrical charge in the fuel and oxidant mixture 206. The attraction of the electrical charge was found by the inventors to cause a start-up flame to be held by the electrically conductive start-up flame holder. In another embodiment, the heater 228 may include an electrical resistance heater configured to output heat to the perforated flame holder and/or to the fuel and oxidant mixture 206. The electrical resistance heater can be configured to heat up the perforated flame holder 102 to an operating

temperature. The heater 228 can further include a power supply and a switch operable, under control of the controller 230, to selectively couple the power supply to the electrical resistance heater.

An electrical resistance heater 228 can be formed in various ways. For example, the electrical resistance heater 228 can be formed from KANTHAL ® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstahammar, Sweden) threaded through at least a portion of the perforations 210 defined formed by the perforated flame holder body 208. Alternatively, the heater 228 can include an inductive heater, a high energy (e.g. microwave or laser) beam heater, a frictional heater, or other types of heating technologies.

Other forms of start-up apparatuses are contemplated. For example, the heater 228 can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the air and fuel. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite a fuel and oxidant mixture 206 that would otherwise enter the perforated flame holder 102. An electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller 230, which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the fuel and oxidant mixture 206 in or upstream from the

perforated flame holder 102 before the perforated flame holder 102 is heated sufficiently to maintain combustion.

The burner system 200 can further include a sensor 234 operatively coupled to the control circuit 230. The sensor 234 can include a heat sensor configured to detect infrared radiation or a temperature of the perforated flame holder 102. The control circuit 230 can be configured to control the heating apparatus 228 responsive to input from the sensor 234. Optionally, a fuel control valve 236 can be operatively coupled to the controller 230 and configured to control a flow of fuel to the fuel and oxidant source 202. Additionally or alternatively, an oxidant blower or damper 238 can be operatively coupled to the controller 230 and configured to control flow of the oxidant (or combustion air).

The sensor 234 can further include a combustion sensor operatively coupled to the control circuit 230, the combustion sensor being configured to detect a temperature, video image, and/or spectral characteristic of a combustion reaction held by the perforated flame holder 102. The fuel control valve 236 can be configured to control a flow of fuel from a fuel source to the fuel and oxidant source 202. The controller 230 can be configured to control the fuel control valve 236 responsive to input from the combustion sensor 234. The controller 230 can be configured to control the fuel control valve 236 and/or oxidant blower or damper to control a preheat flame type of heater 228 to heat the perforated flame holder 102 to an operating temperature. The controller 230 can similarly control the fuel control valve 236 and/or the oxidant blower or damper to change the fuel and oxidant mixture 206 flow responsive to a heat demand change received as data via the data interface 232.

FIG. 5 is a schematic diagram of a fuel and oxidant source 202 in conjunction with an electrically powered heater 502 combined with a flame holder body 208, according to an embodiment. The body 208 may be made of a refractory material, such as for example ceramic, fire brick, etc., and may be part of an industrial furnace or heater. The body 208 may, according to an

embodiment, exclude metals such as stainless steel, except possibly as attachments or auxiliary parts (e.g., swirlers, vanes, supports, bolts, etc.) that are not part of the body 208 proper, which may be less than 0.5, 0.4, 0.3, 0.2, 0.1 , 0.05, or 0.01 of the body structure or the body proper by weight; or in other words, the body may substantially comprise a non-metal or consist substantially of non-metallic material. The fuel and oxidant source 202 comprises a fuel jet nozzle 218 configured to emit a jet of fuel and an oxidant 206 as a combined stream, for example, by the jet of fuel entraining air (or other oxidant). The fuel and oxidant source 202 may optionally be configured so as not to comprise or constitute a fuel/air mixer, i.e., a device or system that premixes fuel and air for delivery to the region of a combustion reaction 302.

The flame holder body 208 may be disposed adjacent to the fuel and oxidant stream 206 at a predetermined position at which it may be configured to initiate and maintain vortices of turbulence T in the fuel and oxidant stream 206. Optionally, to induce more local turbulence and/or vortices, and thereby assist in flame holding, the body 208 may includes a vortex generator 502, that may be shaped as a ramp or projection as illustrated, or may be shaped in some other way that will produce additional turbulence, or vortices, or otherwise assist with flame holding; or, the body 208 may act as a bluff body that induces additional turbulence without being shaped specifically for that purpose (e.g., without a specific vortex generator), but rather does so by being placed in a predetermined position in the flow of mixture and/or in the combustion reaction or flame 302. The body may act either by disrupting streamlines that would otherwise be present, or by the Coanda effect, or by its motion, or by some other mechanism.

A body that generates vortices and/or turbulence thereby promotes heat transfer between the body and fluid with which it interacts. The body 208 may also increase such heat transfer by having a larger surface area. For example, if the vortex generator 502 were perforated with through-holes, then it might create more turbulence, but its heat transfer rate would be increased even if the amount of turbulence did not increase, due to the larger surface area. Heat transfer between a body and fluid is a concern of the applicants.

While not shown in FIG. 5, it will be understood that the illustrated portion of flame-holder body 208 may be merely representative of a differently shaped or longer body, and in particular might schematically represent a curved flame- holder body that curves circumferentially around the combustion reaction 302, and or an axis of the nozzle 218.

The body 208 receives heat from an adjacent diffusion-limited combustion reaction or flame 302 supported by the fuel and oxidant stream 206. The heat of the flame raises the body 208 to an elevated temperature, after the flame is well established and the body 208 has time to heat up. However, in the early stages of combustion, especially, the body 208 may not have reached its steady state or equilibrium temperature, and as a result the dynamics of the flame or reaction 302 are affected. To more quickly bring the body 208 to an operating

temperature, and/or to maintain that temperature during steady-state operation (or, to maintain a more-elevated temperature), an electrically powered heater 502 is arranged and deployed to conduct at least a majority of heat electrically generated in the heater 502 into the flame holder body and, at most, a minority of the generated heat to the fuel and oxidant stream 206.

The oxidant source 220 may be illustrated as a mere pipe conducting the oxidant. The oxidant may be air, although any oxidizer, such as gaseous oxygen or liquid oxidants may be used. The oxidant source 220 may include a natural- draft or forced-draft arrangement, such as a blower 506 configured to provide a flow of combustion air. Conventional devices may be used for the fuel and oxidant source 202.

In an embodiment, the electrically powered heater 502 constitutes an electrical resistance heating element, and may be made from conventional materials suitable for low or high-temperature devices, such as tungsten for example; it may have insulation, like the burner of an electric-stove element, or it may be bare or otherwise covered. In FIG. 5, the two ends of the heater 502 are coupled, by a schematically-illustrated wire 508, to a voltage supply, power supply, or current source 510 (shown schematically by a battery symbol).

A control system 512 may include a controller 514 (which may be, for example, a processor, relay arrangement, or other device), a temperature sensor 234 that is coupled to the controller 514, and a device 516 for varying the electric current through the heater element 502; in FIG. 5, this device is shown

schematically as a variable resistance, but any other current-varying device, such as a transistor, can be used. The control system may use input signals from the temperature sensor 234 for feedback to control the current-control device 516; thus, in one embodiment the control system 512 comprises a thermostat. The temperature sensor 234 may detect the temperature directly (as by infrared detector, thermocouple, thermopile, or the like) or indirectly (as by combustion products).

A schematically-illustrated known electrically powered hot-surface igniter 518 is shown in FIG. 51 , with a hot end thereof disposed in the flow of fuel and oxidant. The hot-surface igniter is not a flame holder and is used only for ignition of the fuel in the flow of entrained air (e.g., starting a flame). Because it only needs to heat a local region to above the flammability point, a hot-surface igniter is often small and does not affect the flow or create substantial vortices.

The flame holder body 208 may be disposed in relation to the heater 502 in such a way that a majority of the heat generated by electric current in the heater 502 flows into the flame holder body, rather than flowing directly into the environment of the flame holder body and the electrically powered heater 502, and therefore bypassing the flame holder body. When a majority of the electric- current heat flows into the body 208, then the body 208 will more quickly reach its desired operating temperature, and a steady-state dynamic of the combustion reaction 302 will more quickly be reached. If heat is not being removed from the combustion reaction 302 into the body 208, as it is when the body 208 is cold, then the flame dynamic is not disturbed and, e.g., flame "lifting" or even extinguishing of the flame, is made less likely.

Heat, as is well known in physics, moves from a place of higher temperature to a place of lower temperature via three primary mechanisms: conduction, convection, and radiation. In conduction, heat is transferred via molecular impacts; in convection, heat is transferred via bulk motion of hot or cold fluid; and in radiation, heat is transferred via photons (electromagnetic radiation).

In all three types, the rate of heat transfer is substantially dependent on surface area. In radiation, for example, a hot object such as a glowing ember in a fire emits a certain amount of radiation, carrying a corresponding certain amount of heat energy away from the ember; and two such embers, having twice the surface area, will emit two times the amount of heat. Similarly, two similar cold objects in the path of the ember's heat radiation will absorb twice the heat that one would. In conduction, a similar correspondence exists, because heat transfer through a slab of material is directly proportional to the area of that slab (the other factors are the thickness, the heat conductivity of the slab material, and the temperature difference between the two sides). Convection is more complicated, because fluid flow is itself more complicated— the flows are usually deterministically chaotic, and heat transfer from a moving fluid to a surface depends on local flow conditions near the surface. However, the same general dependence on area exists for convection also, in part because the heat path of convection usually starts and stops on respective solid surfaces that bound the convective fluid.

FIG. 5 typifies that an electrically powered heater 502 may be disposed substantially within a flame holder body 208, which may be one way of causing heat to flow from the hotter body (the heated wire) to the colder body (the flame holder), rather than to flow somewhere else (i.e., the environment, which may be the fuel and oxidant stream 206, ambient air, nearby or attached objects, etc.). If a majority of the area of the heater is in contact with, or close to, the flame holder body 208, this will tend to cause a majority of the heat generated in the heater to flow directly into the flame holder body 208, by any or all of the three heat- transfer mechanisms.

However, this is not a necessary condition. In the case where an electrically powered heater wire or elongated element is in contact with a planar part of the surface of the flame holder body (not shown in FIG. 5), the body may not substantially surround the heater, but it may still absorb most of the heat generated in the heater because conduction through the contact region will pass more energy than radiation and convection will pass to the environment.

One of the inventors' objects is to pre-heat a flame holder body, which may for example be done to quickly normalize a combustion process. In an ongoing combustion process, the flame holder body has achieved a certain overall or average temperature by absorbing heat generated in a flame, and this temperature may be a steady-state temperature if the fuel and oxidant flows are steady-state. However, when the flame holder body is cold, for example when combustion is started, the flame conditions are different from these steady-state conditions; and then, adjustment of the flame conditions (for example by adjusting the fuel or oxidant flow) cannot be performed directly for steady-state conditions. The steady-state conditions corresponding to mixture flow settings will not result from the adjustment, due to the cold flame holder; the operator must guess. Thus, operation of a boiler, furnace, jet engine, or other fuel-burning device, may be simplified by electrically heating a flame holder.

FIG. 6 illustrates an embodiment in which the flame-holding body may be associated with a burner tile 602 that has an internal ridge or ledge 604. The burner tile 602 is shown in cross section, as if sawn down its axis. Curved arrows around the ridge or ledge 604 indicate toroidal vortices which may comprise a substantially toroidal pattern of vortices (i.e., there is an overall toroidal swirl pattern regardless of non-toroidal turbulence superimposed on the more-prevalent toroidal swirl pattern). The ridge or ledge 604 comprises a ramp shape, which is one shape for a projection into the moving portion of a flame; the vortex generator 502 is another projection shape.

A burner tile can be described as a flame holder, since it affects the flow of fuel/oxidant mixture and because a flame may be attached to a rim of the burner tile, except during the undesirable condition of flame "lift-off' (which a flame holder can prevent). A burner tile may also include specific flame-holding structures such as the illustrated internal ledge 604 that acts as a flame holder, and may also include a separate flame stabilizer or flame holder disposed within, that may be considered as a part of the burner tile (see, e.g., The John Zink Hamworthy Combustion Handbook, 2^nd ed., Vol. 2, Figs. 6-10 and 6-1 1 ).

An electrically powered heater wire 502 is shown embedded inside the burner tile 602, in a position near to the ledge 604. In the illustrated

predetermined position, a majority of the heat generated in the heater wire 502 will appear at the internal surface of the burner tile 602 and cause an elevated temperature at the ledge 604. The wire 502 may be cast in place in the burner tile 602. If the burner tile 602 is assembled from segments, then the embedded section may be electrically connected at their ends to form a complete circuit, or, a partial circuit may have its ends connected to a current source. If the wire 502 forms a complete circuit, then current may be magnetically induced in it. The figure shows a multi-turn induction coil 606 on the outside of the burner tile 602. If the material of the burner tile may be non-conductive (e.g., non-metallic), then magnetic fields generated by the induction coil 606 will induce current in the wire 502 when an AC voltage is impressed across it. As in any transformer, more current will be induced in the coil with fewer turns (wire 502 in FIG. 6), and a higher current causes more electric heating. Thus, a multi-turn induction coil may be capable of inducing large heating effects in a single-turn coil such as wire 502.

It may be noted that at least a portion of the body of the burner tile 602 forms a conical surface, and the electrical heater wire 502 is wrapped around the conical interior surface, though separated from it (it may also be wrapped around an exterior conical surface, as are the wires of the coil 606 in FIG. 6). In alternative embodiments (not illustrated) the heater wire 502 may be deployed in contact with a conical surface of a refractory body, either on the outside of an exterior or interior conical surface, in a groove formed in an otherwise conical surface, or against a ridge protruding from a conical surface.

According to various embodiments, the burner tile 602 may comprise a dielectric body formed by casting refractory material a mold cavity (not shown), and the wire or wires 502 may be supported in the mold cavity during the formation of the dielectric body. During such formation, refractory material can flow or pack around the wire 502, causing the wire 502 to be cast into the body of the burner tile 602 when the refractory material is hardened. The mold cavity can include at least one via for wire supports and/or, when the burner tile 602 is assembled from segments, for portions of the wire 502 that extend to an outer or inner surface for electrical connection to the wire 502 of an adjacent burner tile segment (or to a power supply or current source even when there are no segments). In such a case, the wire 502 in each segment might be shaped as a partial arc of a circle, with radial extensions at either end that protrude through the outer surface of the burner tile 602 for electrical connection to the wire 502 of the adjacent segment when the refractory material is hardened (not illustrated).

The heater wire 502 may be configured to provide tensile reinforcement of the dielectric body of the burner tile 602. According to various embodiments, burner tile 602 segments can be formed in a mold cavity with one or more inserts (not shown) configured to establish mechanical fastener locations, as well as electrical connections. According to various other embodiments, the burner tile 602 or burner tile segments can be formed by sand casting the refractory material. The cast refractory material can include a cement-bonded material, phosphate-bonded materials, fiber reinforcement, and/or an aggregate particle distribution.

In another embodiment, the refractory tile can include cast passages for allowing electrodes to be inserted, taken out, or interchanged as needed. Cast passages can be formed during manufacturing of refractory tile according to the dimensions, shapes, and desired applications of electrodes within the structure. Cast passages can allow metal parts to be inserted and taken out of the burner tile 602. In general, castable materials can be used for a refractory body in order to minimize cost when manufacturing complex shapes that can integrate one or more electrodes, resistance heating elements, or other parts.

Forming a refractory tile, with or without cast passages, can involve known refractory manufacturing processes which can include mixing raw materials and forming into desired shapes and dimensions under wet or moist conditions;

followed by heating the refractory material to high temperatures in a periodic or continuous tunnel kiln to form the ceramic bond that gives the refractory tile its refractory properties; and concluding with a final processing stage that can involve milling, grinding, and sandblasting of the refractory tile.

In FIG. 6, the oxidant source or pipe 220 includes an air-flow control, shown as a damper plate 608 for example, while the fuel source or jet-forming pipe 218 includes a fuel-concentration adjustment (a device for varying the ratio of fuel and oxidant flowing into the combustion reaction 302, for example by varying the flow rate of fuel, of oxidant, or of both). In FIG. 6, the fuel- concentration adjustment is shown as an exemplary adjustable valve 610 (a variable-fuel-flow-rate pump/motor is an alternative; such a device can use a variable-speed motor and/or a variable-flow pump). The valve 610 can adjust the fuel concentration even without providing any airflow control such as the plate 608. These adjustment devices may, individually or in a combined manner, be coupled to respective actuators 612 and 614, which may be step motors, for example, and the actuators 612 and 614 may in turn be coupled to a control system (they may also be manual, or both manual and automatic). Only one motor may be needed if the adjustment devices are combined, or is only one of them is provided. For example, they may be coupled by wires 616, 618 to the controller 514 of FIG. 5 (or some other automatic controller). The characteristics of the combustion reaction 302 are determined in part by the proportion and flow rates of the fuel and oxidant.

FIG. 7 is a cross-sectional diagram of a flame holder with an electric heater element inserted therethrough, according to an embodiment. FIG. 7 relates to a flame holder body 208 that may be of arbitrary shape in cross section and has an elongated central passage 702 through which the heater wire 502 passes. If the heater 502 passes along a central passage, then virtually all of the heat generated in the heater will pass into the body 208. FIG. 7 illustrates the fact that the flame holder body 208 is not limited to any specific shapes.

A particular example of such an arrangement might be in a gas fireplace of the type in which a gas flame impinges on non-inflammable (e.g., ceramic) "logs". These fireplaces require a warm-up period both to achieve mimicry of a wood fire, and also to begin to throw radiant heat, most of which comes from the heated "logs" once they are hot. If the "logs" include electric heaters according to the Applicants' construction, then the fireplace will more rapidly reach the desired condition of resembling a wood fire and giving off radiant heat. (The "logs" act as a flame holder because they give heat back to the fuel/air mixture, and thereby maintain complete combustion, and also because their irregular shape causes turbulence. Such a "log" body 208, though not commonly denoted as a "flame holder" in the art, is so termed herein by the applicants and the phrase covers such a "log" in the following claims.)

In the case of the fireplace, and also in other cases such as metallurgical and industrial applications, the flame holder may eventually reach a temperature at which even high-temperature heating elements deteriorate, oxidize, or even melt; the flame holder may be constructed of ceramic, refractory material, or other non-metallic materials that can withstand temperatures higher than those tolerable to metals such as stainless steels. If so, then the heater element can be surrounded by a layer of insulation, anti-oxidant, or other covering 704 as shown, but can also be exposed to a force flow of outside air. For example, if there is an air gap between the heater 502 and the passage wall, then air can be pumped through that passage, and keep the temperature of the heater 502 below that of the flame holder body 208. The air can then be released into the environment, or recycled to an air pump. In a fireplace, a small air pump (not illustrated) can send air into the "logs" by metal pipes to pass through the air gap passage and thence out into the fire.

FIG. 8 is a diagrammatic perspective view of a combustion system or burner system, according to an embodiment in which the flame holder body 208 is embodied as a perforated flame holder, a flame holder including perforations 210 that allow the passage of fuel and oxidant and/or combustion reactants and/or combustion products from an input face 212 of the perforated flame holder 102 to an output face 214 thereof, and which may support flames inside the perforations 210 of the perforated flame holder 102. The perforations 210, and also the edges of the perforated flame holder 102, may induce turbulence in a flow of mixture or combustion products.

In FIG. 8, the electrically powered heater 502 is exemplified by an electrically resistive heating element in the form of a wire that is interleaved in and out through some of the plurality of perforations 210 (which may include through-bores or through-passages. The electrically powered heater 502 may, for example, be formed from KANTHAL ® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstahammar, Sweden) threaded through at least a portion of elongated perforations 210 formed of the perforated flame holder 102. (Alternatively, the flame holder 102 can be heated by an inductive heater, a high energy (e.g. microwave or laser) beam heater, a frictional heater, or other types of heating technologies.)

The heating element 502 is, in an embodiment, operatively coupled to a voltage supply 510 via wires 508, and the voltage supply 510 may be coupled to a controller or control unit 514 by a bus, wire, or wireless means. The control unit 514 may control the current flowing through the heating element 502 so as to vary the temperature of the perforated flame holder 502, or portions thereof.

Although only a single heating element 502 is illustrated, it will be understood that plural heating elements 502 with associated plural pairs of wires 508 may be arranged from the voltage supply 510 (or from respective plural voltage supplies 510) all under the control of the controller 514, or, of plural controllers. In such an arrangement, the control unit 514 may control the different heating units differentially or individually according to an algorithm, sequence, or timing schedule, and can also determine a distribution pattern of heat in the perforated flame holder 102. The voltage supply 510 may be replaced by a current supply or some other electrical heat driver.

The nozzle 218 is shown located in an aperture 806 in a plate 808, which schematically represents a furnace, or a furnace wall or wall portion. The nozzle 218 may, in an embodiment, be coaxial with an axis A of the perforated flame holder as shown. The nozzle 218 emits a fuel jet or stream 206 and is supplied from a fuel supply 802, a fluid line 804, and a fuel control valve 610, which may in an embodiment be under the control of the control unit 514. (In an alternative embodiment, the valve 610 includes a sensor that informs the control unit 514 of the rate at which fuel is being delivered to the nozzle 218.) The control unit 514 may further be coupled to a temperature sensor 234 that is capable of detecting a temperature or temperature distribution of the upper second face 214 of the perforated flame holder 102 (for example, by infrared detection or imaging, although any other temperature or heat-measuring sensor may be used). Because of these connections, the control unit 514 is capable, in an embodiment, of adjusting both the flow of fuel and the electrical heating of the perforated flame holder 102 in such as way as to maintain a predetermined temperature and/or temperature distribution of the perforated flame holder, or to make other adjustments related to the fuel rate and the temperature, such as controlling a rate of temperature change. Such adjustment can extend the turndown ratio, and/or compensate for lower flame-holder temperature during cold starting.

During a startup procedure, the system control unit 514 may control the voltage supply 510 to apply a voltage potential (or current) across the ends of the heating element 502. The resistance value of the heating element 502 and the magnitude of the voltage potential may be selected to generate sufficient heat to raise the temperature of the portion of the flame holder 102 in the vicinity of the heating element 502 to beyond a startup threshold within a few seconds, after which the system control unit 514 controls valve 610 to open, while controlling the voltage supply 510 to remove the voltage potential from the heating element 502. When the fuel stream 206 contacts the heated portion of the flame holder 502, auto-ignition occurs, and a stable flame is established in the flame holder 502.

The stream of fuel 206 ejected from the nozzle 218 toward the first face

212 of the flame holder 102 disperses from the nozzle 218 in a conical spray at an angle that is typically about 7.5 degrees from the longitudinal axis A, resulting in a solid conical angle of about 15 degrees. As the fuel stream 206 disperses, it entrains air, and eventually reaches a flammable proportion of fuel and air. By selection of the nozzle orifice diameter and the pressure at which fuel is ejected, the velocity at which the fuel stream 206 ejected from the nozzle 218 is preferably selected to be much higher than the flame propagation speed of the particular type of fuel employed, so that, on the one hand, the fuel stream is prevented from supporting a flame near the nozzle, and on the other hand, by the time the dispersing fuel stream 206 has slowed to near the flame propagation speed, the fuel stream has entrained enough air that the mixture is too lean for combustion at the temperature of the fuel stream.

A nozzle-produced fuel jet such as 206 may have greater velocity near the axis A and may have fuel/oxidant ratios changing with the radial distance from the axis A. Therefore, a wire heating element 502 may be placed generally at a selected distance from the axis A where an ignition temperature might be reached more quickly due to jet characteristics, or, where it might cause adjacent areas to more quickly reach ignition temperature, or where some combination of flame characteristics are optimized. As mentioned above, plural heating elements can be used, and these plural elements can be placed at generally different radial distances from the axis A.

After ignition and some time of combustion, the flame holder 102 is held at a much higher temperature because of the ongoing combustion. The higher temperature of the flame holder 102 is sufficient to maintain combustion of a lean fuel mixture. A stable flame can thus be maintained by the flame holder 102. The flame is held primarily within the perforations 210, although the flame may extend a short distance beyond either or both faces 212, 214 of the flame holder 102. The fuel stream 206 is able to continually feed the combustion, and the flame holder 102 is able to support a leaner flame than could be maintained in a conventional burner system.

The inventors have recognized that, although the flame holder 102 is able to support combustion with a very lean fuel mixture during steady-state

operation, startup of combustion is problematic. The fuel mixture at any particular distance of the perforated flame holder 102 from the nozzle 218 is flammable only at elevated temperatures, and because the flame holder 102 may be at ambient temperature at startup, conventional ignition methods or devices are not generally effective for startup of combustion.

The applicants' apparatus and method are, however, also useful for steady-start operation, following the start-up phase. This is because their heating can not only more quickly achieve steady-state operation, but also can widen the so-called "turndown ratio". The turndown ratio may be defined as the ratio of maximum heat release to minimum heat release, with maintenance of stable flames (see The John Zink Hamworthy Combustion Handbook, 2nd Ed., Volume 1 , 0.7.1 .3 and Figure 17.15).

Loss of flame stability can result from too much or from too little fuel flow to a flame holder. Depending on the geometry of the flame holder and other factors such as the type of fuel, the flame will go out when there is not enough fuel to support the combustion (this may occur at the lower end of the range of heat that defines the turndown ratio); and when there is too much flow, the flame will move up away from the flame holder (which may correspond to the upper end of the range of heat that defines the turndown ratio). In general, a flame will move "upstream" as the fuel rate or flow rate decreases, and will move

"downstream" as the fuel rate or flow rate increases.

As mentioned, too little fuel will hinder combustion. Aside from fuel, the other two main requirements of fire are oxidant and high-enough temperature. In a flame holder, the temperature of the fuel/air mixture must be sufficient to reach combustion if the flame is to be stable. If the flow is too great, then the temperature of the flame holder will be lowered by the relatively cool un-burned mixture, and eventually the flame will leave the flame holder. Once that happens, flame stability is greatly decreased. In general, a flame will move "upstream" as the fuel rate or flow rate decreases, and will move "downstream" as the fuel rate or flow rate increases.

The applicants electrically heat the flame holder at both ends of the range which determines the turndown ratio.

If, for example, the valve 610 informs the controller 514 that the fuel flow rate is too low to support a stable flame (or, when the controller shuts the valve to a certain point), then the controller may in response send electric current to the heating element 502, which will have the effect of producing flame stability. As explained above, a leaner mixture can be burned when the flame holder is heated.

If, on the other hand, the sensor 234 detects a drop in flame holder temperature, perhaps due to too-rapid flow of unburned mixture, then the control unit 514 might electrically heat the flame holder so as to bring the combustion "upstream" and thereby prevent the flame from "lifting". Or, the controller might electrically heat the flame holder whenever it has instructed the valve 610 to open beyond a predetermined threshold. (The controller 514 may also detect fuel pressure and/or air flow and use them as factors in determining when to initiate flame-holder heating.)

The discussion above is equally applicable to a perforated flame holder and a non-perforated flame holder, or to a flame holder that includes a vortex generator 210 that is not a perforation.

FIG. 9 is a flow chart showing a method 900 for operating an electrically heated burner, such as the electrically heated burner 100 of FIG. 1 , according to an embodiment. Beginning at step 902, start-up command data is received into a controller via a data interface. The controller processes the instruction and, in step 904, enables a relay to output a drive voltage on at least one node of a power supply. Proceeding to step 906, the output drive voltage is coupled to an electrically powered heater configured to cause a sensible temperature rise in a component of an industrial or commercial burner, the component being disposed so as to contact a fuel stream and/or a flame supported by the fuel stream.

In step 908, the electrically powered heater causes a temperature rise in at least a portion of a burner. Optionally, the method 900 can include step 910 to sense the temperature rise in the portion(s) of the burner. The method 900 can the proceed to optional step 912 to enable an ignition source in the burner, and then to optional step 914 to enable a fuel valve to output fuel at a fuel nozzle.

Proceeding to step 916, the electrically heated burner supports

combustion adjacent to the portion(s) of the burner.

FIG. 10 is a flow chart showing a method 1000 for operating an electrically heated burner, such as the electrically heated burner 100 of FIG. 1 , according to another embodiment.

In step 916, combustion is supported adjacent to at least a portion of an electrically heated industrial or commercial burner operatively coupled to an electrically powered heater. In step 1002, current is output from a power supply to the electrically powered heater. The output current causes the electrically powered heater to dissipate energy, such as by electrical resistance or electrical inductance. The dissipated energy enters the portion(s) of the electrically heated burner.

Proceeding to step 1004, a temperature of the burner portion(s) is sensed.

Proceeding to step 1006, the current output to the electrically powered heater is controlled to maintain the temperature. Additionally or alternatively, the controller can operate a timer and cause application of electrical current to the electrically powered heater according to a duty cycle set point.

Proceeding to step 1008, burner temperature command data is received via a data interface. Responsive to the command data, an electronic controller, in step 1010, changes a temperature set point for the burner portion(s). In step 1012, the controller controls the current output from the power supply to the electrically powered heater to change the temperature of the burner portion to the new set point. Additionally or alternatively, the controller can change a heater duty cycle set point and control the current output according to the new duty cycle set point. The new duty cycle set point can change the temperature of the burner portion(s). Above, and in the following claims, "substantially within" indicates that, at room temperature or some other start-up temperature, most (a majority of) heat generated by the electrically powered heater due to electric current flowing therethrough passes into the flame holder body, rather than passing directly into the environment (e.g., fuel/air mixture) while by-passing the flame holder body; or, conversely, that at most a minority of electrically-generated heat passes directly to the fuel and oxidant stream.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.