Patent Publication Number: US-2016238242-A1

Title: Burner with a perforated flame holder support structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority benefit from U.S. Provisional Patent Application No. 62/117,941, entitled “BURNER WITH A PERFORATED FLAME HOLDER SUPPORT STRUCTURE,” filed Feb. 18, 2015 (docket number 2651-224-02); which, to the extent not inconsistent with the disclosure herein, is incorporated by reference. 
    
    
     SUMMARY 
     According to an embodiment, a combustion system includes a furnace body defining a combustion volume. A fuel and oxidant source and a perforated flame holder are positioned within the combustion volume. A support structure is fixed to the furnace body and supports the perforated flame holder at a selected distance from the fuel and oxidant source. The fuel and oxidant source outputs fuel and oxidant onto the perforated flame holder. The perforated flame holder supports a combustion reaction of the fuel and oxidant within the perforated flame holder. Because the support structure supports the perforated flame holder at the selected distance from the fuel and oxidant source, the perforated flame holder can stably support the combustion reaction of the fuel and oxidant within the perforated flame holder. 
     According to an embodiment, a method for operating a combustion system includes supporting, with a support structure fixed to a furnace body, a perforated flame holder at a selected distance from a fuel and oxidant source; outputting fuel and oxidant from the fuel and oxidant source; and receiving the fuel and oxidant in the perforated flame holder positioned to receive the fuel and oxidant from the fuel and oxidant source. The method further includes supporting a majority of a combustion reaction of the fuel and oxidant within the perforated flame holder. 
     According to an embodiment, a combustion system includes an enclosure defining an interior volume, a fuel and oxidant source disposed within the enclosure and configured to output fuel and oxidant, and a perforated flame holder disposed to receive the fuel and oxidant from the fuel and oxidant source and to support a combustion reaction of the fuel and oxidant within the perforated flame holder. The combustion system further includes a first support arm coupled between the enclosure and the perforated flame holder and configured to support the perforated flame holder within the enclosure at a selected distance from the fuel and oxidant source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a combustion system including a perforated flame holder supported by a support structure, 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 a 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. 5A  is a diagram of a combustion system including a perforated flame holder supported by a support structure mounted to a floor of a furnace, according to an embodiment. 
         FIG. 5B  is a diagram of the combustion system of  FIG. 5A  in which the support structure includes brackets and a plurality of finger members on which the perforated flame holder rests, according to an embodiment. 
         FIG. 5C  is a top view of the combustion system of  FIG. 5B , according to an embodiment. 
         FIG. 5D  is a diagram of the combustion system of  FIG. 5A  in which the support structure includes brackets on which the perforated flame holder rests, according to an embodiment. 
         FIG. 6A  is a diagram of a combustion system including a perforated flame holder supported by a support structure mounted to a sidewall of a furnace, according to an embodiment. 
         FIG. 6B  is a diagram of the combustion system of  FIG. 6A  in which the support structure includes an array of support rods on which the perforated flame holder rests, according to an embodiment. 
         FIG. 6C  is a top view of the combustion system of  FIG. 6B , according to an embodiment. 
         FIG. 7A  is a diagram of a combustion system including a perforated flame holder supported by a support structure mounted to a ceiling of a furnace, according to an embodiment. 
         FIG. 7B  is a diagram of the combustion system of  FIG. 7A  in which the support structure includes brackets and a plurality of finger members on which the perforated flame holder rests, according to an embodiment. 
         FIG. 8  is a diagram of a combustion system including a perforated flame holder supported by a cooled support structure cooled by a fluid coolant, according to an embodiment. 
         FIG. 9A  is a diagram of a combustion system including a perforated flame holder supported by a plurality of tubes configured to pass a fluid coolant therethrough, according to an embodiment. 
         FIG. 9B  is a top view of the cooled support structure of  FIG. 9A , according to an embodiment. 
         FIG. 10  is a flow diagram of a process for operating a combustion system including a perforated flame holder and a support structure, according to an 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. Other embodiments may be used and/or other changes may be made without departing from the spirit or scope of the disclosure. 
       FIG. 1  is a diagram of a combustion system  100 , according to an embodiment. The combustion system  100  includes a furnace body  110  that defines a combustion volume  106  within the furnace body  110 . A perforated flame holder  102  and a fuel and oxidant source  104  are positioned within the combustion volume  106 . A support structure  108  is fixed to the furnace body  110  and supports the perforated flame holder  102  at a selected distance from the fuel and oxidant source  104 . 
     The fuel and oxidant source  104  outputs fuel and oxidant onto the perforated flame holder  102 . The perforated flame holder  102  receives the fuel and oxidant from the fuel and oxidant source  104  and supports a combustion reaction of the fuel and oxidant within the perforated flame holder  102 . 
     Characteristics of the combustion reaction within the perforated flame holder  102  depend, in part, on the distance between the fuel and oxidant source  104  and the perforated flame holder  102 . The support structure  108  supports the perforated flame holder  102  in a stable position at the selected distance from the fuel and oxidant source  104 . In this way, the combustion reaction of the fuel and oxidant can be stably supported within the perforated flame holder  102 . 
       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, porous reaction holder, duplex, and duplex tile 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 (O 2 ) 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 fuel and oxidant mixture and fuel stream may be used interchangeably and considered synonymous depending on the context, unless further definition is provided. 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 a side sectional diagram  300  of a portion of the perforated flame holder  102  of  FIGS. 1 and 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 (H 2 ), and methane (CH 4 ). In another application the fuel can include natural gas (mostly CH 4 ) or propane (C 3 H 8 ). 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, flue gas, 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 or thermal energy 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 . As used herein, the terms heat, heat energy, and thermal energy shall be considered synonymous unless further definition is provided. As used above, heat energy and thermal energy refer generally to the released chemical energy initially held by reactants during the combustion reaction  302 . As used elsewhere herein, heat, heat energy and thermal energy correspond to a detectable temperature rise undergone by real bodies characterized by heat capacities. 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  302  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 flame 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  102 . Alternatively, if the cooling load is relatively low and/or the furnace temperature reaches a high level, the combustion may travel somewhat upstream of the input face  212  of the perforated flame holder  102 . 
     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” or “flashback” wherein a visible flame momentarily ignites in a region lying between the input face  212  of the perforated flame holder  102  and the fuel nozzle  218 , within the dilution region D D . Such transient huffing or flashback 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 downstream from the output face  214  of the perforated flame holder  102 , but still a majority of combustion occurred within the perforated flame holder  102  as evidenced by continued visible glow from the perforated flame holder  102  that was observed. 
     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 radiation, thermal 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-type radiation of electromagnetic energy, primarily at infrared wavelengths, but also at visible wavelengths owing to elevated temperature of the perforated flame holder body  208 . 
     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 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 ⅓ and ½ 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 thermal energy from the combustion reaction  302  in an amount corresponding to the heat capacity multiplied by temperature rise, and transfer the thermal energy 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 multiple heat transfer mechanisms including conduction, radiation, and possibly convection 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  302 , even under conditions where a combustion reaction  302  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 begin within thermal boundary layers  314  formed adjacent to walls  308  of the perforations  210 . Insofar as combustion is generally understood to include a large number of individual reactions, and since a large portion of combustion energy is released within the perforated flame holder  102 , it is apparent that at least a majority of the individual reactions occur within the perforated flame holder  102 . 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 define the ends of the perforations  210 . At some position along the length of a perforation  210 , the combustion reaction  302  outputs more heat to the perforated flame holder body  208  than it receives from the perforated flame holder body  208 . The heat is received at the heat receiving region  306 , is held by the perforated flame holder body  208 , and is transported to the heat output region  310  nearer to the input face  212 , where the heat is transferred into the cool reactants (and any included diluent) to bring the reactants to the ignition temperature. 
     In an embodiment, each of the perforations  210  is 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 . As used herein, the term reaction fluid refers to matter that travels through a perforation  210 . Near the input face  212 , the reaction fluid includes the fuel and oxidant mixture  206  (optionally including nitrogen, flue gas, and/or other “non-reactive” species). Within the combustion reaction region, the reaction fluid may include plasma associated with the combustion reaction  302 , molecules of reactants and their constituent parts, any non-reactive species, reaction intermediates (including transition states), and reaction products. Near the output face  214 , the reaction fluid may include reaction products and byproducts, non-reactive gas, and excess oxidant. 
     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  to form 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  102 . 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, and optionally, flue gas. 
     The perforated flame holder  102  can be held by a perforated flame holder support structure  222  configured to hold the perforated flame holder  102  at a dilution 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 the 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 contained in 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 the 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 an inside diameter 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 the 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 the distance D D  away from the fuel nozzle  218  between 100 times and 1100 times the nozzle diameter. Preferably, the perforated flame holder support structure  222  is configured to hold the perforated flame holder  102  at a distance about 200 times or more of the nozzle diameter away from the fuel nozzle  218 . When the fuel and oxidant mixture  206  travels about 200 times the nozzle diameter or more, the mixture is sufficiently homogenized to cause the combustion reaction  302  to produce 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 oxidant (e.g., combustion air) channel configured to output the oxidant 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 oxidant source  220 , whether configured for entrainment in the combustion volume  204  or for premixing, can include a blower configured to force the oxidant through the fuel and oxidant 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 the thickness dimension T between the input face  212  and the output face  214  of the perforated flame holder  102 . 
     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 be of various shapes. In an embodiment, the perforations  210  can include elongated squares, each having a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations  210  can include elongated hexagons, each having a transverse dimension D between opposing sides of the hexagons. In yet another embodiment, the perforations  210  can include hollow cylinders, each having a transverse dimension D corresponding to a diameter of the cylinder. In another embodiment, the perforations  210  can include truncated cones or truncated pyramids (e.g., frustums), each having a transverse dimension D radially symmetric relative to a length axis that extends from the input face  212  to the output face  214 . In some embodiments, the perforations  210  can each have a lateral dimension D equal to or greater than a quenching distance of the flame based on standard reference conditions. Alternatively, the perforations  210  may have lateral dimension D less then than a standard reference quenching distance. 
     In one range of embodiments, each of the plurality of perforations  210  has a lateral dimension D between 0.05 inch and 1.0 inch. Preferably, each of the plurality of perforations  210  has a lateral dimension D between 0.1 inch and 0.5 inch. For example the plurality of perforations  210  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  102  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 to include 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. Honeycomb is an industrial term of art that need not strictly refer to a hexagonal cross section and most usually includes cells of square cross section. Honeycombs of other cross sectional areas are also known. 
     The inventors have found that the perforated flame holder  102  can be formed from VERSAGRID® ceramic honeycomb, available from Applied Ceramics, Inc. of Doraville, S.C. 
     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 ceramic material. The term “reticulated” refers to a netlike structure. Reticulated ceramic material is often made by dissolving a slurry into a sponge of specified porosity, allowing the slurry to harden, and burning away the sponge and curing the ceramic. 
     In another embodiment, which is not necessarily preferred, the perforated flame holder  102  may be formed from a ceramic material that has been punched, bored or cast to create channels. 
     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. 
     According to an embodiment, the perforated flame holder  102  may act 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  206 —lower combustion limit defines the lowest concentration of fuel at which a fuel and oxidant mixture  206  will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.). 
     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. According to one interpretation, such a performance can be achieved due to a sufficient mixing used to lower peak flame temperatures (among other strategies). Flame temperatures tend to peak under slightly rich conditions, which can be evident in any diffusion flame that is insufficiently mixed. By sufficiently mixing, a homogenous and slightly lean mixture can be achieved prior to combustion. This combination can result in reduced flame temperatures, and thus reduced NOx formation. In one embodiment, “slightly lean” may refer to 3% O 2 , i.e. an equivalence ratio of ˜0.87. Use of even leaner mixtures is possible, but may result in elevated levels of O 2 . 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 temperatures and lower NOx. 
     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  102  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 . 
       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, T S . After the perforated flame holder is raised to the start-up temperature, the method proceeds to step  404 , wherein the 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 oxidant (e.g., combustion air) source, for example. In this approach, the fuel and oxidant are output in one or more directions selected to cause the fuel and oxidant mixture to be received by the 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, flame rod, and/or other 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  302 ; 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  206  and an oxidant source  220  configured to output oxidant (e.g., combustion air) adjacent to the fuel stream  206 . The fuel nozzle  218  and oxidant source  220  can be configured to output the fuel stream  206  to be progressively diluted by the oxidant (e.g., combustion air). The perforated flame holder  102  can be disposed to receive a diluted fuel and oxidant mixture  206  that supports a combustion reaction  302  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 unmixed fuel and oxidant 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 flame 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 and/or stabilizing 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 a perforated flame holder  102  operating temperature, the flow rate may be increased to “blow out” the start-up flame. In another embodiment, the heater  228  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  102  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 Hallstaham mar, Sweden) threaded through at least a portion of the perforations  210  defined by the perforated flame holder body  208 . Alternatively, the heater  228  can include an inductive heater, a high-energy beam heater (e.g. microwave or laser), a frictional heater, electro-resistive ceramic coatings, 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 oxidant and fuel. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite the fuel and oxidant mixture  206  that would otherwise enter the perforated flame holder  102 . The 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. 5A  is a diagram of a combustion system  500 , according to an embodiment. The combustion system  500  includes a furnace body having a sidewall  512 , a floor  514 , and a ceiling  516 . The sidewall  512 , the floor  514 , and the ceiling  516  collectively define a combustion volume  506 . A perforated flame holder  102  and a fuel nozzle  504  are positioned within the combustion volume  506 . The perforated flame holder  102  is supported above the fuel nozzle  504  by a support structure  508 . The support structure  508  includes support arms  509  fixed to the floor  514  of the furnace body. The support structure  508  holds the perforated flame holder  102  a selected distance above the fuel nozzle  504 . 
     According to an embodiment, the fuel nozzle  504  outputs a stream  507  of fuel and/or a mixture of fuel and oxidant onto the perforated flame holder  102 . The oxidant can be provided to the combustion volume independent of the fuel nozzle  504 . The perforated flame holder  102  supports a combustion reaction of the fuel and oxidant  507  within the perforated flame holder  102 . 
     The characteristics of the combustion reaction within the perforated flame holder  102  depend, in part, on a distance that the fuel and/or fuel and oxidant travel between the fuel nozzle  504  and the perforated flame holder  102 . The perforated flame holder  102  may not support the combustion reaction of the fuel and oxidant if the perforated flame holder  102  is not positioned a proper distance from the fuel nozzle  504 . The support structure  508  is configured to support the perforated flame holder  102  in a stable position at the selected distance from the fuel nozzle  504 . 
     The support structure  508  includes one or more support arms  509  fixed to the floor  514  and coupled to the perforated flame holder  102 . According to an embodiment, the support structure  508  includes two support arms  509  coupled to opposite sides of the perforated flame holder  102 . 
     According to an embodiment, the support structure  508  is fixed to a side of the perforated flame holder  102 . Alternatively, the perforated flame holder  102  can rest on the support structure  508 . 
     According to an embodiment, the support structure  508  is fixed to the floor  514  by one or more screws or bolts. Alternatively, the support structure  508  can be fixed to the floor by a refractory cement material, by fitting into slots or grooves in the floor  514 , or by gravity alone, for example. 
     According to an embodiment, the support structure  508  can include multiple finger members  515  (shown in  FIG. 5C ) on which the perforated flame holder  102  rests. The finger members  515  can be configured to allow the fuel and oxidant  507  to pass between the thin finger members  515  to enter into the perforated flame holder  102  without significantly inhibiting the fuel and oxidant  507  from entering into the perforated flame holder  102 . According to an embodiment, the perforated flame holder  102  can include multiple perforated flame holder sections fixed together. Each perforated flame holder section can be positioned on and supported by at least one of the finger members  515 . 
     According to an embodiment, the support structure  508  can be covered by a thermal insulator and coupled to a structure for extracting heat from the insulated structure. Such structures for extracting heat (not shown) may include the use of a fluid coolant such as air, flue gas, steam, or water. Heat may optionally be extracted from the fluid coolant electronically using a Peltier cooler or by other means known to those skilled in the art. In transient operation, thermal insulation alone may allow the structural material to remain sufficiently cool. These or other methods can help prevent the support structure  508  from overheating to the point of becoming structurally unsound, thereby jeopardizing the stability of the positioning of the perforated flame holder  102 . For example, the inventors have found that ordinary high temperature steel materials may undergo plastic deformation under the influence of furnace temperatures. Providing insulation and/or fluid coolant are contemplated to provide sufficient protection to avoid plastic deformation. 
     According to an embodiment, the support structure  508  can be coupled to the perforated flame holder  102  by one or more of gravity; a refractory cement material; superalloy or ceramic screws, bolts, pins, or clamps; or by fitting into grooves or slots in the perforated flame holder  102 , for example. 
     According to an embodiment, the support structure  508  can include one or more of a metal superalloy (such as Inconel or Hastelloy), a ceramic material, a refractory brick, a refractory material, or a fiber reinforced refractory material. 
     According to an embodiment, the support structure  508  includes support arms coupled between the floor  514  and the perforated flame holder  102 . 
       FIG. 5B  is a diagram of the combustion system  500  of  FIG. 5A  in which the support structure  508  includes brackets  513  fixed to the support arms  509 . The support structure  508  includes a plurality of finger members  515  coupled to the brackets  513 . The perforated flame holder  102  rests on the finger members  515 . 
     According to an embodiment, the brackets  513  can be fixed to the perforated flame holder  102  by gravity; screws, bolts, or pins, refractory cement, or other suitable mechanisms or materials for fixing a bracket to a support arm. The brackets  513  can include a metal or a metal superalloy, a ceramic or refractory material, and/or other materials suitable for being placed in a high temperature combustion environment. The brackets  513  can be of the same material as the support arms  509  and/or continuous with the support arms  509 . 
     According to an embodiment, the finger members  515  are rods, bars, or other relatively long and thin structure suitable for supporting the perforated flame holder  102 . As shown more clearly in a top view of  FIG. 5C , the finger members  515  are spaced apart from each other in such a way as to permit the fuel and oxidant  507  to enter the perforated flame holder  102 . The finger members  515  can be discreet members positioned on the brackets  513  or a unitary grid positioned on the bracket  513 . The finger members  515  can be fixed to the brackets  513  or can merely rest on the brackets  513 . The finger members  515  can include a metal or a metal superalloy, a ceramic or refractory material, and/or other materials suitable for being placed in a high temperature combustion environment. The finger members  515  can be of the same material as the brackets  513  and/or the support arms  509 . 
       FIG. 5C  is the top view of the support structure  508  of  FIG. 5B , according to an embodiment. The support structure  508  includes the support arms  509  positioned on the floor  514  of the furnace, the brackets  513  fixed to the support arms  509 , and the finger members  515  positioned on the brackets  513 . The finger members  515  are positioned in an array or a grid configuration. The perforated flame holder  102  (not shown in  FIG. 5C ) rests on the finger members  515 . The finger members  515  are spaced apart so that fuel and oxidant  507  can enter the perforated flame holder  102 . 
       FIG. 5D  is a diagram of the combustion system  500  of  FIG. 5A  in which the support structure  508  includes brackets  513  fixed to the support arms  509 . The perforated flame holder  102  rests directly on the brackets  513 . 
     According to an embodiment, the brackets  513  can be fixed to the perforated flame holder  102  by a refractory cement material; metal, superalloy, or ceramic screws, bolts, pins, or clamps; or by fitting into grooves or slots in the perforated flame holder  102 , for example. The brackets  513  can include a metal or a metal superalloy, a ceramic or refractory material, and/or other materials suitable for being placed in a high temperature combustion environment. The brackets  513  can be of the same material as the support arms  509 . 
       FIG. 6A  is a diagram of a combustion system  600 , according to an embodiment. The combustion system  600  includes a furnace body having a sidewall  512 , a floor  514 , and a ceiling  516 . The sidewall  512 , the floor  514 , and the ceiling  516  collectively define a combustion volume  506 . A perforated flame holder  102  and a fuel nozzle  504  are positioned within the combustion volume  506 . The perforated flame holder  102  is supported above the fuel nozzle  504  by a support structure  608 . The support structure  608  includes support arms  609  fixed to the sidewall  512  of the furnace body. The support structure  608  holds the perforated flame holder  102  at a selected distance above the fuel nozzle  504 . 
     According to an embodiment, the fuel nozzle  504  outputs a stream  507  of fuel and/or a mixture of fuel and oxidant  507  onto the perforated flame holder  102 . The perforated flame holder  102  supports a combustion reaction of the fuel and oxidant within the perforated flame holder  102 . 
     The characteristics of the combustion reaction within the perforated flame holder  102  depend, in part, on a distance that the fuel and oxidant travel between the fuel nozzle  504  and the perforated flame holder  102 . The perforated flame holder  102  may not support the combustion reaction of the fuel and oxidant if the perforated flame holder  102  is not positioned a proper distance from the fuel nozzle  504 . The support structure  608  is configured to support the perforated flame holder  102  in a stable position at a selected distance from the fuel nozzle  504 . 
     The support structure  608  includes two or more portions each fixed to the sidewall  512  and coupled to the perforated flame holder  102 . According to an embodiment, the support structure  608  includes two support structure portions coupled to opposite sides of the perforated flame holder  102 . 
     According to an embodiment, the support structure  608  is fixed to a side of the perforated flame holder  102 . Alternatively, the perforated flame holder  102  can rest on the support structure  608 . According to an embodiment, the support structure may include two or more layers of support arms  609  arranged in alternating directions, such as in a crisscrossed arrangement. 
     According to an embodiment, the support structure  608  is coupled to the sidewall  512  by gravity. In another embodiment the support structure  608  can be coupled to the sidewall  512  one or more screws, bolts, or pins. Alternatively or additionally, the support structure  608  can be fixed to the sidewall  512  by a refractory cement material, by fitting into slots or grooves in the sidewall  512 . 
     According to an embodiment, the support structure  608  can include multiple finger members  515  (shown and described in relation to  FIG. 5 ) on which the perforated flame holder  102  rests. The finger members  515  can be configured to allow the fuel and oxidant  507  to pass between the finger members  515  to enter into the perforated flame holder  102  without significantly inhibiting the fuel and oxidant from entering into the perforated flame holder  102 . According to an embodiment, the perforated flame holder  102  can include multiple perforated flame holder sections fixed together. Each perforated flame holder section can be positioned on and supported by at least one of the thin finger members  515 . 
     According to an embodiment, the support structure  608  can be covered by a thermal insulator and coupled to a method for extracting heat from the insulated structure. Such means of extracting heat (not shown) may include the use of a fluid coolant such as air, steam, or water. Heat may also be extracted electronically using a Peltier cooler or by other methods or structures known to those skilled in the art. In transient operation, thermal insulation alone may allow the structural material to remain sufficiently cool. These or other methods can help prevent the support structure  608  from overheating to the point of becoming structurally unsound, thereby jeopardizing the stability of the positioning of the perforated flame holder  102 . 
     According to an embodiment, the support structure  608  can be coupled to the perforated flame holder  102  by one or more of gravity; a refractory cement; superalloy or ceramic screws, bolts, clamps, or pins; or by fitting into grooves or slots in the perforated flame holder  102 ; for example. 
     According to an embodiment, the support structure  608  can include one or more of a metal superalloy (such as Inconel or Hastelloy), a ceramic material, a refractory brick, a refractory material, or a fiber reinforced refractory material. 
     According to an embodiment, the support structure  608  includes support arms coupled between the wall  512  and the perforated flame holder  102 . 
       FIG. 6B  is a diagram of the combustion system  500  of  FIG. 6A  in which the arms  609  include a plurality of rods or tubes coupled the wall  512 . The perforated flame holder  102  rests on the rods  609 . 
     According to an embodiment, the support arms  609  are finger members  515  such as rods, tubes, bars, or other relatively long and thin structure suitable for supporting the perforated flame holder  102 . The support arms  609  pass through the walls  512  and are supported thereby. As shown more clearly in a top view of  FIG. 6C , the finger members  515  are spaced apart from each other in such a way as to permit the fuel and oxidant  507  to enter the perforated flame holder  102 . According to an embodiment, the support arms  609  can be fixed to one or more brackets coupled to the walls  512 . The support arms  609  can include a metal or a metal superalloy, a ceramic or refractory material, and/or other materials suitable for being placed in a high temperature combustion environment. 
       FIG. 6C  is the top view of the support structure  608  of  FIG. 5B , according to an embodiment. The support structure  608  includes the support arms  609  extending between the walls  512  of the furnace. The support arms  609  are positioned in an array or a grid configuration. The perforated flame holder  102  (not shown in  FIG. 6C ) rests on the support arms  609 . The support arms  609  are spaced apart so that fuel and oxidant  507  can enter the perforated flame holder  102 . 
       FIG. 7A  is a diagram of a combustion system  700 . The combustion system  700  includes a furnace body having a sidewall  512 , a floor  514 , and a ceiling  516 . The sidewall  512 , the floor  514 , and the ceiling  516  collectively define a combustion volume  506 . A perforated flame holder  102  and a fuel nozzle  504  are positioned within the combustion volume  506 . The perforated flame holder  102  is supported above the fuel nozzle  504  by a support structure  708 . The support structure  708  includes support arms  709  coupled to the ceiling  516  and the perforated flame holder  102 . The support structure  708  holds the perforated flame holder  102  at a selected distance above the fuel nozzle  504 . 
     According to an embodiment, the fuel nozzle  504  outputs a stream  507  of fuel and/or a mixture of fuel and oxidant onto the perforated flame holder  102 . The perforated flame holder  102  supports a combustion reaction of the fuel and oxidant within the perforated flame holder  102 . 
     The characteristics of the combustion reaction within the perforated flame holder  102  depend, in part, on a distance that the fuel and oxidant travel between the fuel nozzle  504  and the perforated flame holder  102 . The perforated flame holder  102  may not support the combustion reaction of the fuel and oxidant if the perforated flame holder  102  is not positioned a proper distance from the fuel nozzle  504 . The support structure  708  is configured to support the perforated flame holder  102  in a stable position at a selected distance from the fuel nozzle  504 . 
     The support structure  708  includes one or more support arms  709  each fixed to the ceiling  516  and coupled to the perforated flame holder  102 . According to an embodiment, the support structure  708  includes two support arms  709  coupled to opposite sides of the perforated flame holder  102 . 
     According to an embodiment, the support structure  708  is fixed to a side of the perforated flame holder  102 . Alternatively, the perforated flame holder  102  can rest on the support structure  708 . 
     According to an embodiment, the support structure  708  is fixed to the ceiling  516  by one or more superalloy or ceramic screws, bolts, or pins. Alternatively, the support structure  708  can pass through the ceiling  516  from outside the furnace body, or can be fitting into slots or grooves in the ceiling  516 . 
     According to an embodiment, the support structure  708  can include multiple finger members  515  on which the perforated flame holder  102  rests. The finger members  515  can be configured to allow the fuel and oxidant  507  to pass between the thin finger members  515  to enter into the perforated flame holder  102  without significantly inhibiting the fuel and oxidant from entering into the perforated flame holder  102 . According to an embodiment, the perforated flame holder  102  can include multiple perforated flame holder sections fixed together. Each perforated flame holder section can be positioned on and supported by at least one of the thin finger members  515 . 
     According to an embodiment, the support structure  708  can be covered by a thermal insulator and coupled to a method for extracting heat from the insulated structure. Such means of extracting heat (not shown) may include the use of a fluid coolant such as air, flue gas, steam, or water. Heat may also be extracted electronically using a Peltier cooler or by other structures or methods known to those skilled in the art. In transient operation, thermal insulation alone may allow the structural material to remain sufficiently cool. These or other methods can help prevent the support structure  708  from overheating to the point of becoming structurally unsound, thereby jeopardizing the stability of the positioning of the perforated flame holder  102 . 
     According to an embodiment, the support structure  708  can be coupled to the perforated flame holder  102  by gravity; a refractory cement; superalloy or ceramic screws, bolts, clamps, or pins; or by fitting into grooves or slots in the perforated flame holder  102 . 
     According to an embodiment, the support structure  708  can include one or more of a metal superalloy (such as Inconel or Hastelloy), a ceramic material, a refractory brick, a refractory material, or a fiber reinforced refractory material. 
     According to an embodiment, the support structure  708  includes support arms coupled between the ceiling  516  and the perforated flame holder  102 . 
       FIG. 7B  is a diagram of the combustion system  700  of  FIG. 7A  in which the support structure  708  includes brackets  716  coupling the support arms  709  to the ceiling  516 . The support structure  708  further includes brackets  713  coupled to lower ends of the support arms  709 . A plurality of finger members  715  are coupled to the brackets  713 . The perforated flame holder  102  rests on the finger members  715 . 
     The brackets  716  can include a metal or a metal superalloy, a ceramic or refractory material, and/or other materials suitable for being placed in a high temperature combustion environment. The brackets can be of the same material as and/or continuous with the support arms  709 . 
     According to an embodiment, the finger members  715  are rods, bars, or other relatively long and thin structure suitable for supporting the perforated flame holder  102 . The finger members  715  are spaced apart from each other in such a way as to permit the fuel and oxidant  507  to enter the perforated flame holder  102 . The finger members  715  can be discreet members positioned on the brackets  713  or a unitary grid positioned on the brackets  713 . The finger members  715  can be fixed to the brackets  713  or can merely rest on the brackets  713 . The finger members  715  can include a metal or a metal superalloy, a ceramic or refractory material, and/or other materials suitable for being placed in a high temperature combustion environment. The fingers  715  can be of the same material as the brackets  713  and/or the support arms  709 . 
       FIG. 8  is a diagram of a combustion system  800 , according to an embodiment. The combustion system  800  includes a furnace body  810  defining a combustion volume  106  within the furnace body  810 . A perforated flame holder  102  and a fuel and oxidant source  104  are positioned within the combustion volume  106 . A cooled support structure  808  is fixed to the furnace body  810  and supports the perforated flame holder  102  at a selected distance from the fuel and oxidant source  104 . The cooled support structure  808  includes a fluid coolant  812  (also referred to as a fluid coolant) within a hollow portion of the support structure  808 . A coolant source  814  is coupled to the cooled support structure  808 . 
     The fuel and oxidant source  104  outputs fuel and oxidant onto the perforated flame holder  102 . The perforated flame holder  102  receives the fuel and oxidant and supports a combustion reaction of the fuel and oxidant within the perforated flame holder  102 . 
     Characteristics of the combustion reaction within the perforated flame holder  102  depend, in part, on a distance between the fuel and oxidant source  104  and the perforated flame holder  102 . The cooled support structure  808  supports the perforated flame holder  102  in a stable position at the selected distance from the fuel and oxidant source  104 . 
     According to an embodiment, the cooled support structure  808  can include an interior channel, such as a tube, a channel, or chamber through which the fluid coolant  812  can pass. In particular, the coolant source  814  can circulate or pass the fluid coolant  812  through the cooled support structure  808 , thereby cooling the cooled support structure  808  and/or the perforated flame holder  102  and maintaining the cooled support structure  808  at a selected temperature or below a failure temperature. 
     According to an embodiment, the fluid coolant can be a liquid and/or a gas. The coolant can include water, flue gas, water vapor, or any other suitable fluid for cooling tubes  909  (shown in  FIG. 9 ) and/or the perforated flame holder  102 . Optionally, the cooled support structure may vent the coolant to the combustion volume  106 . For example, the support structure can be cooled by air, and the air may be vented to deliver oxygen oxidant upstream from the perforated flame holder  102  to combine with fuel and contribute oxidant to the fuel and oxidant mixture  206 . In another example, the support structure can be cooled by water, and the water may be vented downstream from the perforated flame holder to quickly reduce temperature of the combustion products, or upstream from the perforated flame holder to reduce an incidence of flashback. 
       FIG. 9A  is a diagram of a combustion system  900 , according to an embodiment. The combustion system  900  includes a furnace body having a sidewall  512 . The combustion system  900  includes a cooled support structure  908  and a perforated flame holder  102  supported by the cooled support structure  908 . 
     According to an embodiment, the cooled support structure  908  includes tubes  909  passing through the sidewall  512  of the furnace body and coupled to a coolant source  814 . Each tube  909  includes an interior channel  911  through which fluid coolant can pass. The coolant source  814  passes the fluid coolant through the tubes  909 . The perforated flame holder  102  rests on the tubes  909 . 
     As the fluid coolant is passed through the interior channels  911  of the tubes  909  the fluid coolant absorbs heat from the tubes  909 , thereby cooling the tubes  909 . As the tubes  909  are cooled, the perforated flame holder  102  is also cooled. In this way, the temperature of the tubes  909  forming the support structure can be kept within a selected temperature range. 
     According to an embodiment, the tubes  909  can include a refractory material such as quartz, silicon carbide, or another material capable of withstanding a high temperature combustion environment. 
       FIG. 9B  is a top view of the cooled support structure  908  of  FIG. 9A , according to an embodiment. The support structure  908  includes the tubes  909  passing through the walls  512  of the furnace. The tubes  909  are positioned in an array or a grid configuration. The perforated flame holder  102  (not shown in  FIG. 9B ) rests on the tubes  909 . The tubes  909  are spaced apart so that fuel and oxidant can enter the perforated flame holder  102 . 
     According to an embodiment, the tubes  909  are connected with U shaped connectors outside the furnace walls  512  such that the tubes  909  form a single tube through which the fluid coolant can pass. Alternatively, each tube  909  can be a separate tube coupled to the coolant source  814  and through which the fluid coolant passes. 
       FIG. 10  is a flow diagram of a method  1000  for operating a combustion system including a perforated flame holder and a support structure, according to an embodiment. At  1002 , the perforated flame holder is supported within a combustion volume by the support structure. In particular, the support structure holds the perforated flame holder at a selected distance from a fuel and oxidant source. At  1004 , fuel and oxidant is output from the fuel and oxidant source. At  1006 , the fuel and oxidant is received at the perforated flame holder. At  1008 , a combustion reaction of the fuel and oxidant is supported within the perforated flame holder. 
     According to an embodiment, the support structure can be fixed to a sidewall, a ceiling, or a floor of a furnace defining the combustion volume. Because the support structure is fixed to one or more selected portions of the furnace body, the support structure can stably support the perforated flame holder at a selected distance from the fuel and oxidant source. 
     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.