Patent Publication Number: US-2018038588-A1

Title: Burner and  support structure with a perforated flame holder

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. Continuation-in-Part patent application of co-pending U.S. patent application Ser. No. 15/047,557, entitled “BURNER WITH A PERFORATED FLAME HOLDER SUPPORT STRUCTURE,” filed Feb. 18, 2016 (docket number 2651-224-03). Co-pending U.S. patent application Ser. No. 15/047,557 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). The present application also claims priority benefit from and is a continuation-in-part of co-pending International Patent Application No. PCT/US2016/018532, entitled “BURNER WITH A PERFORATED FLAME HOLDER SUPPORT STRUCTURE,” filed Feb. 18, 2016 (docket number 2651-221-04). Co-pending International Patent Application No. PCT/US2016/018532 claims priority benefit from U.S. Provisional Patent Application No. 62/117,943, entitled “BURNER WITH A PERFORATED FLAME HOLDER SUPPORT STRUCTURE,” filed Feb. 18, 2015 (docket number 2651-221-02). Co-pending U.S. patent application Ser. No. 15/047,557, co-pending International Patent Application No. PCT/US2016/018532, U.S. Provisional Patent Application No. 62/117,941, and U.S. Provisional Patent Application No. 62/117,943 are each, to the extent not inconsistent with the disclosure herein, incorporated herein by reference. 
    
    
     SUMMARY 
     According to an embodiment, a combustion system includes a furnace body defining a furnace volume. A fuel and oxidant source and a perforated flame holder are positioned within the furnace 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. 
     According to an embodiment, a combustion system includes a furnace wall defining a furnace volume, a fuel and oxidant source configured to output fuel and oxidant into the furnace volume, and a perforated flame holder disposed within the furnace volume and configured to hold a combustion reaction of the fuel and oxidant. A support structure is configured to hold the perforated flame holder in alignment with the fuel and oxidant output by the fuel and oxidant source. 
     According to an embodiment, the support structure may be a cooled support structure having an interior channel configured to pass a fluid coolant therethrough. 
     According to an embodiment, the support structure may be a movable support structure configured to move the perforated flame holder relative to the fuel and oxidant source. 
     According to an embodiment, a method of operating a combustion system includes outputting fuel and oxidant from a fuel and oxidant source and supporting, with a support structure, a perforated flame holder in alignment with the fuel and oxidant source. The fuel and oxidant are received into the perforated flame holder. The perforated flame holder holds a combustion reaction of the fuel and oxidant within the perforated flame holder. 
     According to an embodiment, a combustion system includes a fuel and oxidant source configured to output fuel and oxidant and a perforated flame holder configured to receive the fuel and oxidant and to hold a combustion reaction of the fuel and oxidant within the perforated flame holder. A support structure is configured to hold the perforated flame holder in alignment to receive the fuel and oxidant source and to adjust a position of the perforated flame holder. At least one sensor is configured to sense at least one parameter of the combustion reaction and to output one or more sensor signals. A controller is configured to receive the one or more sensor signals and to cause actuation of the support structure to adjust the position of the perforated flame holder based on the one or more sensor signals. 
    
    
     
       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 support structure 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. 
         FIG. 11  is an illustrative diagram of a combustion system including a perforated flame holder supported by a support structure, according to an embodiment. 
         FIG. 12  is an enlarged partly cross-sectional view of a portion of a furnace wall having a support structure coupled thereto, according to an embodiment. 
         FIG. 13  is an enlarged cross-sectional view of a portion of a furnace wall having a support structure coupled thereto, according to an embodiment. 
         FIG. 14  is a diagram of a combustion system, according to an embodiment. 
         FIG. 15  is a diagram of a portion of an actuator for a movable support structure for a perforated flame holder, according to an embodiment. 
         FIG. 16  is a diagram of a portion of an actuator for a movable support structure for a perforated flame holder, according to an embodiment. 
         FIG. 17  is a diagram of a combustion system including a movable support structure operatively coupled to a plurality of perforated flame holders, according to an embodiment. 
         FIG. 18  is a diagram of a portion of a combustion system including a perforated flame holder and a movable support structure operatively coupled to a burner tile, according to an embodiment. 
         FIG. 19  is a side-sectional view of a portion of a boiler including an insertable support structure for supporting a perforated flame holder within a combustion pipe, according to an embodiment. 
         FIG. 20  is a diagram of a portion of a support structure for a perforated flame holder, according to an embodiment. 
         FIG. 21  is a diagram of a portion of a movable support structure for a perforated flame holder, according to an embodiment. 
         FIG. 22  is a diagram of a portion of a combustion system including a movable support structure, according to an embodiment. 
         FIG. 23  is a diagram of a portion of an actuator for a movable support structure for a perforated flame holder that is adapted for changing a dilution distance from outside of a boiler, according to an embodiment. 
         FIG. 24  is a diagram of a combustion system including a perforated flame holder mounted on a movable support structure including sensors operatively coupled together, according to an embodiment. 
         FIG. 25  is a diagram of a portion of a cooled support structure for a perforated flame holder, according to an embodiment. 
         FIG. 26  is a cross-section of a portion of a cooled support structure for a perforated flame holder, according to an embodiment. 
         FIG. 27  is a diagram of a portion of a combustion system including a cooled support structure for a perforated flame holder, according to an embodiment. 
         FIG. 28  is an illustration of a portion of cooled support structure for a perforated flame holder, according to an embodiment. 
         FIG. 29  is a cross-section of a portion of a cooled support structure for a perforated flame holder, according to an embodiment. 
         FIG. 30  is a plan view of a support structure for a perforated flame holder, shown in the detail cross-section of  FIG. 29 , according to an embodiment. 
         FIG. 31  is a cross-section of a portion of the cooled support structure for a perforated flame holder of  FIG. 30 , according to an embodiment. 
         FIG. 32  is an illustration of a cooled support structure and a perforated flame holder in a heat pipe configuration, according to an embodiment. 
         FIG. 33  is an enlarged cross-sectional view of a portion of the cooled support structure of  FIG. 32 , according to an embodiment. 
         FIG. 34  is a diagram of a combustion system including a movable support structure and a conical truncated perforated flame holder, according to an embodiment. 
         FIG. 35A  is a simplified perspective view of a combustion system including a reticulated ceramic perforated flame holder, according to an embodiment. 
         FIG. 35B  is a simplified side sectional diagram of a portion of the reticulated ceramic perforated flame holder of  FIG. 35A , 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 furnace volume  106 . A perforated flame holder  102  and a fuel and oxidant source  104  are positioned within the furnace 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 into the furnace volume  106 . 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 furnace 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 Hallstahammar, 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 furnace volume  506 . A perforated flame holder  102  and a fuel nozzle  504  are positioned within the furnace 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 furnace 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 at 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 furnace volume  506 . A perforated flame holder  102  and a fuel nozzle  504  are positioned within the furnace 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  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 oxidant  507  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  507  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  609  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  608  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  by one or more ceramic 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  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 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  600  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 furnace volume  506 . A perforated flame holder  102  and a fuel nozzle  504  are positioned within the furnace 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 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  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 oxidant  507  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  507  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 coupled 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  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 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  716  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 furnace volume  106  within the furnace body  810 . A perforated flame holder  102  and a fuel and oxidant source  104  are positioned within the furnace 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 coolant fluid) 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  507  and supports a combustion reaction of the fuel and oxidant  507  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  812  can be a liquid and/or a gas. The coolant  812  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  808  may vent the coolant  812  to the furnace volume  106 . For example, the support structure  808  can be cooled by air, and the air may be vented to deliver 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  808  can be cooled by water, and the water may be vented downstream from the perforated flame holder  102  to quickly reduce temperature of the combustion products, or upstream from the perforated flame holder  102  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  812  can pass. The coolant source  814  passes the fluid coolant  812  through the tubes  909 . The perforated flame holder  102  rests on the tubes  909 . 
     As the fluid coolant  812  is passed through the interior channels  911  of the tubes  909  the fluid coolant  812  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  908  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  507  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  812  can pass. Alternatively, each tube  909  can be a separate tube coupled to the coolant source  814  and through which the fluid coolant  812  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 furnace 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 furnace 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. 
       FIG. 11  is an illustrative diagram of a combustion system  1100  including a perforated flame holder  102  supported by a support structure  1108 , according to an embodiment. The combustion system  1100  can include a furnace wall  110  having an interior surface  1112  that defines an interior space  1106 , i.e. a furnace volume, of the combustion system  1100  in which combustion takes place. The interior surface  1112  of the furnace wall  110  can include an inner surface of cylindrical shape and end wall portions adjacent to the cylindrical surface. An exhaust vent  1114 , a fuel and oxidant source attachment plate  1116 , and one or more of viewing windows (“sight ports”)  1118  may further define the interior space  1106  of the combustion system  1100 . 
     A fuel and oxidant source  104  provides a flow of fuel and oxidant mixture  206 . Although the flow of the fuel and oxidant mixture  206  is depicted in  FIG. 11  as being horizontal, the direction of the flow of the fuel and oxidant mixture can be upward, downward, or any angle therebetween. As described herein, depending on an embodiment the fuel may include one or more of vaporized liquid, gas, and/or powdered solid. The fuel and oxidant source  104  can include one or more fuel nozzles (not shown) and one or more combustion air sources (not shown) arranged to cause the fuel to entrain the oxidant as the flow of the fuel and oxidant mixture  206  proceeds toward the perforated flame holder  102 . More rigorously, because the mass flow rate of oxidant-carrying fluid is typically much higher than the mass flow rate of fuel, the oxidant typically entrains the fuel. For reasons of simplifying understanding, either explanation will be considered equivalent herein. Alternatively, the fuel and oxidant source  104  can include a pre-mix chamber or a partial pre-mix chamber configured to introduce a premixed or partially premixed fuel and oxidant mixture  206 . The terms oxidant and oxidizer shall be considered synonymous herein. 
     The perforated flame holder  102  is disposed transverse to the direction of flow of the fuel and oxidant mixture  206 . The perforated flame holder is configured to receive the fuel and oxidant mixture  206  and hold the combustion reaction  302  of the fuel and oxidant mixture  206  within the perforated flame holder  102 . According to an embodiment, the perforated flame holder  102  can be configured to support a majority of the combustion reaction  302  within the perforated flame holder  102 . 
     The support structure  1108  can be configured to hold the perforated flame holder  102  at a selected position within the furnace wall  110 . In particular, the support structure  1108  can support the perforated flame holder  102  in alignment with the fuel and oxidant source  104  so that the perforated flame holder receives the fuel and oxidant mixture  206  from the fuel and oxidant source  104  and support the combustion reaction of the fuel and oxidant mixture  206  within the perforated flame holder  102 . The position of the perforated flame holder  102  can be selected based on one or more of a fuel flow rate, fuel nozzle diameter, mixture velocity, fuel type, emissions requirements, and/or other variables. 
     According to an embodiment, the support structure  1108  is coupled to the interior surface  1112  of the furnace wall  110 . Additionally or alternatively, the support structure  1108  can be coupled to the fuel and oxidant source  104 , to the exhaust vent  1114 , and/or to another structure (e.g., a steam tube) inside the combustion system  1100 . In this way, the support structure  1108  can support the perforated flame holder  102  in a selected position relative to the fuel and oxidant source  104  within the interior space  1106 . 
     According to an embodiment, the support structure  1108  can include bolts  1120 , struts  1122 , and/or a metal rim  1124 . Each bolt  1120  may pass into or through the furnace wall to couple a strut  1122  to the furnace wall  110 . The struts  1122  may additionally or alternatively be coupled to the metal rim  1124  at attachment points  1126 . The metal rim  1124  may directly contact the perforated flame holder  102 . The components of the support structure  1108  may support the perforated flame holder  102  at a selected position within the interior space  1106 . 
     According an embodiment, the support structure  1108  can include structures additional or alternative to those shown in  FIG. 11 . For example, the support structure  1108  may include a hanging (tensile) support, a compression member support, a moveable support, and/or a cooled support for the perforated flame holder  102 . 
     According to an embodiment, the combustion system  1100  may include one or more sensors  234  configured to detect one or more parameters of the combustion reaction held by the perforated flame holder  102 . The sensor(s)  234  may, for example, be positioned external to the furnace wall  110  to sense parameter(s) of the combustion reaction  302  through one or more windows or ports  1118 . Additionally or alternatively, one or more sensors  234  may be located within the interior space  1106  defined by the furnace wall  110 . The sensor(s)  234  can include one or more cameras or other image capture devices configured to sense infrared, visible, and/or ultraviolet radiation emitted by the combustion reaction  302  and/or the perforated flame holder  102 . Additionally or alternatively, the sensor(s)  234  can include a temperature sensor, an electrical conductivity, resistance, inductance, or capacitance sensor (e.g. a “flame rod”), or other kinds of sensors capable of sensing parameters related to the combustion reaction  302  of the perforated flame holder  102  that can be usable for feedback to adjust the furnace parameters to maintain the combustion reaction  302  inside the perforated flame holder  102 . Any suitable types of sensors may be included. 
     The combustion system  1100  may also include tubes, fittings, and the like not shown in  FIG. 11 . The furnace may have any suitable interior shape. For example, the furnace interior may have a rectangular or circular cross-section. 
       FIG. 12  is an enlarged partly cross-sectional view of a portion of a furnace wall  110  having a support structure  1108  coupled thereto. The support structure  1108  can include one or more bolts  1120  coupled to the furnace wall  110  at the attachment point(s)  1128 . The support structure  1108  can include one or more struts  1122  attached to the interior surface  1112  of the furnace wall  110  at the attachment point(s)  1128  by the bolt(s)  1120 . 
     According to an embodiment, the furnace wall  110  may include an outer steel shell and an inner lining of firebrick  1212 . The bolts  1120  of the support structure  1108  can fix the firebrick  1212  to the furnace wall  110 . Thus, the bolts  1120  can serve the dual purpose of being a part of the support structure  1108  that supports the perforated flame holder  102 , as well as fixing the firebrick  1212  to the furnace wall  110 . The bolts  1120  can have a length that is sufficiently long to both fix the firebrick  1212  to the furnace wall  110  and to support the strut(s)  1122 . Thus, bolts  1120  can be longer than conventional bolts used for fixing the firebrick  1212  to the furnace wall  110  so that extra nuts may be used to attach the strut  1122  thereto. The bolts of the type shown in  FIG. 12  may also be used with continuous refractory, rather than firebrick. 
       FIG. 13  is an enlarged partly cross-sectional view of a portion of a furnace wall  110  having a support structure  1308  coupled thereto, according to an embodiment. The support structure  1308  can include one or more bolts  1120  fixed to the furnace wall  110  by a weld, and may be used with or without firebrick or refractory material. It may also be used on the inside of a fire tube in a boiler, for example (a fire tube may be characterized as a furnace wall).  FIG. 13  shows two alternate embodiments of the bolt  1120 , a straight bolt and a bolt that ends in a hook. Alternatively, the bolts  1120  may end in other conventional structure used for attaching (not shown). This type of fixture may also be applicable in the case of a furnace with refractory applied in a continuous layer over an inside surface of a steel shell. 
       FIG. 14  is a diagram of a combustion system  1400 , according to an embodiment. The combustion system  1400  can include a furnace wall  110  that defines a furnace volume  106  within the furnace wall  110 . A perforated flame holder  102  and a fuel and oxidant source  104  are positioned within the furnace volume  106 . A movable support structure  1408  can be fixed to the furnace wall  110  and support the perforated flame holder  102  at any one of a plurality of selected distances from the fuel and oxidant source  104 . 
     The fuel and oxidant source  104  can output the fuel and oxidant mixture  206  onto the perforated flame holder  102 . The perforated flame holder  102  receives the fuel and oxidant mixture  206  from the fuel and oxidant source  104  and supports a combustion reaction  302  of the fuel and oxidant mixture  206  within the perforated flame holder  102 . 
     The movable support structure  1408  can be configured to support the perforated flame holder  102  in a stable position at one or more selected distances from the fuel and oxidant source  104 . Characteristics of the combustion reaction within the perforated flame holder  102  may depend in part on the distance between the fuel and oxidant source  104  and the perforated flame holder  102 . The movable support structure  1408  can move the perforated flame holder  102  to adjust parameters of the combustion reaction  302 , for example to improve stability of the combustion reaction  302  or to adjust the temperature of the perforated flame holder  102 . In this way, the movable support structure  1408  can adjust the position of the perforated flame holder  102  to promote desired behavior of the combustion reaction  302 . 
     According to an embodiment, the combustion system  1400  can include an actuator  1410  coupled to the movable support structure  1408 . The actuator  1410  can move the movable support structure  1408 . A controller  230  can control the actuator  1410  through instructions input by a technician and/or by software instructions executed by the controller  230 . 
     According to an embodiment, the movable support structure  1408  can include tension bearing members such as rods, tubes, cables, wires, chains, and/or other structures that can support the perforated flame holder  102  in the selected position and/or adjust the position of the perforated flame holder  102 . The tension bearing members can extend parallel to the direction of output of the fuel and oxidant mixture  206 . This may allow the movable support structure  1408  to easily move the perforated flame holder  102  toward and away from the fuel and oxidant source  104  to affect the action of the perforated flame holder  102 , and/or to preheat the perforated flame holder  102  via a conventional flame. 
     In an embodiment, the perforated flame holder  102  may be suspended in a large, vertically-fired furnace by chains or cables that run through furnace-wall openings, such as the exhaust vent  1114 , to a windlass mechanism or the like. Heavy chains may be advantageous for such use because they may provide some positional stability. Pure tension members such as cables or chains may also support the perforated flame holder  102 , in the manner of a gondola supported on an aerial cable. The perforated flame holder  102  may be supported on one or several such tension members that run parallel to one another (similar to a suspension bridge), and may include a mechanism for moving the perforated flame holder  102  along their length. The chain(s) may be engaged with a mechanism that engages the chain and works along from link to link (not shown). Also, relatively stiff rods or tubes can replace the flexible cables or chains, which may reduce the amount of tension needed to support the perforated flame holder  102  against its weight or against fluid forces. Rods and tubes may alternatively, or in conjunction, be used as rails rather than as suspension members. 
     According to an embodiment, the movable support structure  1408  may include a rail configured to enable moving the position of the perforated flame holder  102 . For example, a strut may run along the interior surface of the furnace wall  110 , extend to a second bolt or stud to which it may be attached in a similar manner, and serve as a rail. Two or more such rails, mutually parallel and spaced around the circumference of the interior surface  1112  of the furnace wall  110 , may be sufficient to constitute a track on which the perforated flame holder  102  can slide or roll, or be clamped into a fixed but adjustable position. A single, wide rail may alternatively be used. 
     According to an embodiment, the movable support structure  1408  can include screws, clamps, sliders, rollers, wheels, gears, etc., for mounting the perforated flame holder  102  to the rail(s) in a movable fashion. In this way, the movable support structure  1408  can enable the perforated flame holder  102  to move along the rail or rails between selected positions. Additionally or alternatively, one or more of the rails can be rotatable and threaded to intercept a female thread or nut fixedly coupled to the perforated flame holder  102 . According to an embodiment, the movable support structure  1408  can include a rack-and-pinion or worm-and-wheel arrangement, with teeth provided on the rail meshing with a worm or pinion gear coupled to the perforated flame holder  102 . Additionally or alternatively, the movable support structure  1408  can include a structure configured to slide on the rail(s) in conjunction with a cable, rod, or other mechanism that is not involved in the contact of the perforated flame holder  102  and the rail(s). 
     According to an embodiment, the movable support structure  1408  can be adapted to furnaces of a type having tubes (e.g., process heaters, water-tube boilers), which are disposed in the interior  1106  and/or form part of the interior surface  1112  of the furnace wall  110 . Such furnaces may have tubes that are horizontal, vertical, and/or inclined. For example, the tube may be of a helical shape and be positioned at a shallow angle. The tubes, which may be separated from a refractory wall and/or each other by some distance, may be disposed in a vertical plane on each side of a row of burners on the floor of the furnace. Alternatively, a burner may fire toward a plane of tubes from one or two sides. Other configurations are also contemplated. The movable support structure  1408  can suspend the perforated flame holder  102  from the tubes. The movable support structure  1408  may engage with tubes via conventional hardware such as hooks, pipe clamps, and the like. The movable support structure  1408  can enable movement of the perforated flame holder  102  to a selected position within the furnace. 
       FIG. 15  is a diagram of a portion of an actuator  1500  for a movable support structure  1508 , according to an embodiment. The movable support structure  1508  can be configured to support a perforated flame holder  102  and to enable movement of the perforated flame holder  102 . The combustion system  1500  can include tubes  1512  adjacent a refractory wall. The tubes  1512  here exemplify an attachment point. The movable support structure  1508  can include a large gear wheel  1514  that configured to mesh with the tubes  1512 , e.g., as a pinion meshes with a rack gear. The large gear wheel  1514  may be unitary with an axle  1516  and a wheel gear  1518  configured to mesh with a worm gear  1520 , which in turn can be fixed to a drive shaft  1522 . Turning the drive shaft  1522  may cause the large gear wheel  1514  to climb or descend the column of tubes  1512 . 
     According to an embodiment, the combustion system  1500  may include horizontal tubes  1512  adjacent two facing refractory walls of a furnace wall  110 . A vertically-fired perforated flame holder  102  may be suspended from the tubes  1512  by the movable support structure  1508 . The movable support structure  1508  may enable the vertically-fired perforated flame holder  102  to be raised and lowered from a floor on which are disposed one or more fuel and oxidant sources  104 . The movable support structure  1508  may include a plurality of large gear wheels  1514 , which may be driven by their respective drive shafts  1522  from a common driver, using linkages, chains, belts, gear trains, and/or individual motors as are known in the art. 
     According to an embodiment, an alternate movable support structure  1508  for suspending the perforated flame holder  102  from horizontal tubes  1512  in the furnace may include a hinged track with a hook on each segment, with the hinged track being moved over the rollers, similar to a tank tread (e.g., see  FIG. 17 ). The track segments may have such a length that the hooks can engage the tubes  1512  sequentially. This arrangement can distribute the weight of the perforated flame holder  102  over several tubes  1512 . 
       FIG. 16  is a diagram of a portion of a combustion system  1600  including a movable support structure  1608 , according to an embodiment. The movable support structure  1608  may include a rolling support  1612  engaging a vertical tube  1512 . The rolling support  1612  may have several rollers (or wheels)  1614  engaging the surface of the tube  1512 . The movable support structure  1608  can be configured to support the perforated flame holder  102  and adjust the position of the perforated flame holder  102  by rolling the wheels  1614  along the tube  1512 . 
     According to an embodiment, there may be three wheels  1614 , one of which may be spring-loaded by a spring  1616  tending to clamp the tube  1512  between the wheels  1614 . One or more of the illustrated rolling supports  1612  may be attached to the perforated flame holder  102  in various ways to serve as a means for adjusting the vertical position of the perforated flame holder  102 . The rolling support  1612  may be modified with rough wheel surfaces for gripping, motors, gears, and so on. Additionally or alternatively, the rolling support  1612  may be used in conjunction with a cable, rod, chain or other pure tension member that may control or adjust the vertical position of the perforated flame holder  102  while the rolling support  1612  serves to locate the perforated flame holder  102  within the horizontal plane. 
       FIG. 17  is a diagram of a combustion system  1700  including a movable support structure  1708  coupled to a perforated flame holder  102 , according to an embodiment. The perforated flame holder  102  may include a chain of perforated flame holder segments linked by hinges  1712  of the movable support structure  1708 . The hinges  1712  can enable movement of the perforated flame holder sections  102  relative to the fuel and oxidant source  104 . 
       FIG. 18  is a diagram of a combustion system  1800  including a perforated flame holder  102  and a movable support structure  1808  coupled to a burner tile  1812 , according to an embodiment. The movable support structure  1808  can enable movement of the perforated flame holder  102  relative to the burner tile  1812  along a vertical axis. 
     According to an embodiment, the movable support structure  1808  includes C-shaped grapples  1816  coupled to the rim  1814  of the burner tile  1812 . The movable support structure  1808  can further include vertical posts  1818  coupled to the grapples  1816 . The vertical posts  1818  can be coupled to the metal rim  1124  by sliders  1822 . The support structure  1808  may grip the metal rim  1124  or may be forced onto the metal rim  1124  by tension and/or compression members, such as springs or struts. The vertical posts  1818  may be aligned generally with the velocity of the fuel and oxidant mixture  206  coming from the fuel and oxidant source  104  inside the burner tile  1812 , on which the perforated flame holder  102  is movable toward or away from the burner tile  1812  on the sliders  1822 . 
     According to an embodiment, the sliders  1822  may include gears or wheels engaging the illustrated serrations or rack-gear teeth along a side of each vertical post  1818 . Other known mechanisms for adjusting the position of the perforated flame holder  102  may also be used. Alternatively, the perforated flame holder  102  may be removably clamped, welded, or otherwise permanently or adjustably fixed in a position by conventional means. 
     According to an embodiment, the perforated flame holder  102  has a diameter that does not allow complete entry of the perforated flame holder  102  into a concave part of the burner tile  1812 . Alternatively, the perforated flame holder  102  can enter the conventional burner tile  1812 . 
     Those skilled in the art will understand, in light of the present disclosure, that the movable support structure  1808  and the perforated flame holder  102  can be configured for a rectangular burner tile, for a row of burners, and so on, merely by changing the shape. All such other shapes for movable support structure  1808 , the perforated flame holder  102 , and a burner tile  1812  fall within the scope of the present disclosure. 
       FIG. 19  is a side sectional view of a boiler  1900  including a movable support structure  1908 , for supporting the perforated flame holder  102  within a combustion pipe  1912 , according to an embodiment. The support structure  1908  can have several functions in support of the perforated flame holder  102 , among which may be physical support (e.g., holding position against gravity and/or fluid forces), deployment in a specific desired location or orientation, and alignment (geometric centering and/or rotation) to an axially aligned orientation. The support structure  1908  may also provide active features such as axial motion to a desired position, rotations, tilting, or the like, which may be actuated from outside the combustion device. 
     In an embodiment, a shell can include an exterior wall  1924  peripheral to the combustion pipe  1912 . A cover plate  1914  may be operatively coupled to the exterior wall  1924 . The perforated flame holder support structure  1908  can be operatively coupled to the cover plate  1914 . The cover plate  1914 , the support structure  1908 , and the perforated flame holder  102  may be configured to be installed in the combustion pipe  1912  as a unit without a mechanical coupling to the combustion pipe  1912 . 
     Additionally, or alternatively, the cover plate  1914 , the fuel and oxidant source  104 , the support structure  1908 , and the perforated flame holder  102  may be configured to be retrofitted to the boiler  1900 . The cover plate  1914 , the fuel and oxidant source  104 , the support structure  1908 , and the perforated flame holder  102  can be configured to be installed in and uninstalled from the boiler  1900  as a unit for purposes of changing the perforated flame holder  102 . The cover plate  1914  can be coupled to the exterior wall  1924  of the shell using threaded fasteners  1916 , for example. 
     The illustrated support structure  1908  of  FIG. 19  may be configured to hold the perforated flame holder  102  away from the fuel nozzle  1922  at a dilution distance D D  sufficient to cause substantially complete mixing of the fuel and oxidant (e.g., air) at a location where the fuel and oxidant mixture  206  can impinge on the perforated flame holder  102 . Thermal insulation  1926  may be operatively coupled to the perforated flame holder support structure  1908 . The thermal insulation  1926  can be supported by the support structure  1908  adjacent to the wall  1924  of the combustion pipe  1912  along at least a portion of the distance D D  between the fuel nozzle  1922  and the perforated flame holder  102 . In some embodiments, the thermal insulation  1926  may be affixed to the wall  1924  of combustion pipe  1912 . Additionally or alternatively, thermal insulation  1926  can be disposed adjacent to the wall  1924  of the combustion pipe  1912  along at least a portion of the dilution distance D D  between the fuel nozzle  1922  and the perforated flame holder  102 . In an embodiment, the thermal insulation  1926  can be formed from a 1 inch thick FIBERFRAX© DURABLANKET© high temperature insulating blanket, available from UNIFRAX I LLC of Niagara Falls, N.Y. 
     As mentioned above, in some embodiments, the perforated flame holder support structure  1908  and the perforated flame holder  102  may be retrofitted to a boiler which already has associated with it a cover plate  1914 , held to the exterior wall  1924  (for example, by threaded fasteners  1916 ) and a fuel and oxidant source  104  held onto the cover plate  1914 . In such a case, the perforated flame holder support structure  1908  may be attached by various methods, such as welding the support structure  1908  to an inside surface of the cover plate  1914  (not illustrated). Alternatively, the support structure  1908  may be attached by drilling holes in the cover plate  1914  and bolting the support structure  1908  to the cover plate  1914  with additional threaded fasteners (such extra holes and fasteners are not shown in the drawing). A distal end of the support structure  1908  (located farthest into the combustion pipe  1912 ) may be mechanically supported and located by such an attachment. The support structure  1908  proximal end may be operatively coupled to the combustion pipe  1912 , in addition or alternative to the cover plate  1914 . The distal end of the support structure  1908 , located closer to the perforated flame holder  102 , may be suspended in a space inside the combustion pipe  1912 , either coaxially or not coaxially, and with an annular space between the support structure  1908  and the inside of the combustion pipe  1912 . 
     According to an embodiment, an existing boiler may be retrofitted with the perforated flame holder support structure  1908  with little downtime and without requiring machine work. Such a support structure may include a flange  1928 . The flange  1928  may be unitary with or attached to the support structure  1908 , and may extend into a space between the outside surface of the exterior wall  1924  of the shell and the inward-facing surface of the cover plate  1914 . The flange  1928  may replace, or augment, a gasket (not shown), which may be provided in that space for sealing purposes. In addition to an ability to seal, the flange  1928  can also provide a mechanical support, and so may be made of strong metal such as steel. 
     The flange  1928  can include three areas: an outer annular portion that can be clamped tightly between the cover plate  1914  and the exterior wall  1924  of the combustion pipe  1912 , such as by the threaded fasteners  1916 ; an inner annular portion that can be fastened to the support structure  1908 ; and an intermediate annular portion that is neither fastened nor clamped. 
       FIG. 20  is an enlarged view of a portion of a movable support structure  2008 , according to an embodiment. The movable support structure  2008  includes a spring  2012  fitted to an end of the support structure  2008 . Such a spring  2012  may be provided to support the distal end of the support structure  2008  if, referring to  FIG. 13 , the flange  1928  does not provide full support against its weight or the weight of the assembly including the perforated flame holder  102 . One or more such springs  2012  can be fastened near the distal end of the support structure  2008  to help support the distal weight of the support structure  2008  and/or to center the support structure  2008 . 
     In the embodiment shown in  FIG. 20 , the spring  2012  is fixed with a screw  2014 , but any permanent or temporary attachment can be used. As shown, the spring  2012  is fixed near the distal end of the support structure  2008  but projects backward toward the proximal end, which may reduce the maximum temperature to which the spring  2012  is exposed. If the combustion pipe  1912  is metal and in contact with water, then it may cool the spring  2012 . The spring  2012  may include an up-turned proximal end so as to avoid catching when the support structure  2008  may be withdrawn. 
     As can be seen in  FIG. 20 , the spring  2012  may extend only a short distance from the side of the support structure  2008  when pressed onto it by a force. This may allow the support structure  2008  to be inserted through a smaller opening. 
       FIG. 21  is a diagram of a movable support structure  2108 , according to an embodiment. The movable support structure  2108  includes a spring  2112  combined with a wheel carriage  2114  that supports a wheel  2116 . As compared to the spring  2012 , the use of the wheel  2116  can reduce friction force applied against the inside surface of the combustion pipe  1912 , which may avoid wear or damage if the combustion pipe  1912  is lined with, or made of, refractory material, rather than being made of metal as in the exemplary illustrated fire tube boiler  1900 . 
     Each wheel  2116  may be made retractable into the body of the support structure  2108 , which may be done to allow the support structure  2108  to be inserted through a proximal opening of the least diameter (or opening width if the space to be inserted into is not cylindrical), and provide the same advantage as mentioned above for the spring  2112 . 
     The spring  2112  can be unitary with or fastened to the wheel carriage  2114  that supports the wheel  2116 ; these two may be combined into one integral piece of metal, by folding a sheet of metal, for example. On the other side of the wheel  2116  an arm  2115  can be attached to carry a weight  2120 . The weight  2120  may be adjusted in size and/or placement to provide an outward or inward weight force on the wheel or wheels  2116  jointly, such that the sum of the weight forces counteracts gravity force acting on the support structure  2108 . 
     It will be apparent that the screw  2014 , also seen in  FIG. 20 , together with the spring portion  2112  of the folded-sheet-metal structure, can be considered as a fulcrum of a lever arm, by which the weight force of the mass  2120  is amplified at the wheel  2116 . If the wheel  2116  faces and bears downwardly as shown, then the leveraged force can push against the inside of the combustion pipe  1912 ; if the wheel  2116  is located above, on the opposite side of the support structure  2108 , then the bearing force of the wheel  2116  may be reduced. Furthermore, at intermediate points, the force may be apportioned; for example, if the axis of the wheel  2116  is vertical, then the weight  2120  may exert no leveraged force. 
     Thus, by properly deploying the wheels  2116  and by properly biasing them with weights  2120 , the weight of the support structure  2108  can be effectively canceled, so that the support structure  2108  may levitate and, as a consequence, automatically center itself under the influence of the forces due solely to the springs  2012 ,  2112  (which, if these forces are equal and equally spaced, can center the support structure  808  inside the combustion pipe  1912 ). 
     A cover (not shown) may be provided to keep flames away from the wheel  2116  and reduce its operating temperature. The cover may also act as a structural reinforcement if, for example, formed as a stiffening rib of bent sheet metal or metal plate. 
       FIG. 22  is a diagram of a combustion system  2200  including a movable support structure  2208 , according to an embodiment. The movable perforated flame holder support structure  2208  can support the perforated flame holder  102  and enable movement of the perforated flame holder  102 . 
     As mentioned above, the dilution distance D D  can extend from the perforated flame holder  102  to the fuel nozzle  1922 . It may be desirable to vary this distance. Therefore, the support structure  2208  may have a more-distal cylindrical portion in which the perforated flame holder  102  can slide (or, a prismatic portion if the perforated flame holder  102  has a non-circular outline, and is to fit snugly and/or slide inside the combustion pipe). Alternatively, the support structure  2208  may include a base portion and a sliding portion, where the perforated flame holder  102  may be fixed to the sliding portion, and the sliding portion may slide on an outside of the base portion, in the manner of an engine cylinder sliding on a piston. These portions may be either cylindrical, as are pistons and cylinders, or prismatic (regardless of the shape of the perforated flame holder  102 ). 
     In an embodiment illustrated in  FIG. 22 , the movable support structure  2208  includes a sandwich of three sheets of metal  2216 ,  2218 , and  2220 , with the outer two sheets  2216  and  2220  being flat. Such an arrangement may offer an advantage of providing a flat-surfaced flange  1928 , which may be sealed with conventional sealing techniques used when the flange  1928  is not present (e.g., in the original “package” unit, which may use a non-metallic gasket, not shown, between the cover plate  1914  and the exterior wall  1924 ). The middle sheet  2218  can be punched or machined to create spaces for drive gears  2224  and rod-end gears  2223 . The outer two sheets  2216  and  2220  can be drilled, punched, or the like to provide bearing holes for axial pins  2314  of the drive gears  2224 , which, being supported on two sides rather than one, can resist skewing forces and work more reliably. 
     In the region of the intermediate annular portion, which, as discussed above, can help to resist torque when the support structure  2208  is horizontal, the three sheets  2216 ,  2218 , and  2220  may be rigidly fastened together by spot-welding, strong adhesives, and/or fasteners, or the like. Most preferably for resisting torque, the rods  2226  can be arranged so that, when viewed axially, the uppermost region of the intermediate portion of the flange  1928  (which can be the region subjected to the most mechanical stress) may be far from the rods  2226 . For example, one rod may be in the lowermost region and two others disposed at 120 degrees on either side. 
     According to an embodiment, gaps between the sheets  2216 ,  2218 , and  2220  may be filled with sealant. Another option is a stuffing box through which the rod  2226  may pass. 
     On the outside, for each of the several gear trains and rods  2226 , there may be provided a drive motor incorporating a motor gear  2312  meshing with the outermost drive gear  2224 . The drive motor may include feet (not shown) with holes positioned so that one or more of the threaded fasteners  1916  may serve to locate the actuator  1410 , such as a drive motor. Actuators  1410  for the several rods  2226  may be simultaneously controlled to move in synchrony so that the perforated flame holder  102  may move without becoming skewed. 
     One advantage of the embodiment shown in  FIG. 22  can be that the adjustable dilution distance D D  is provided, but no modifications are needed if the cover plate  1914  is a pre-existing or given part of the device. The cover plate  1914  may include holes already drilled for passage of the rods  2226  to the outside, which can avoid the complication of gears. For a pre-existing cover plate  1914  lacking such holes, as in a “package boiler” for example, the embodiment can provide for an adjustable dilution distance D D  without any modifications being needed; only disassembly of the parts other than the support structure  2208 , and reassembly with the support structure  2208 , may be needed. A flange with gears may not be unduly expensive, nor require difficult maintenance (the gears can be oiled from the outside with an oil can), nor be difficult or expensive to fix. 
       FIG. 23  is a diagram of an actuator  2308  for a support structure that is adapted for changing the dilution distance D D  from outside of an assembled boiler such as the boiler  1900  shown in  FIG. 19 , according to an embodiment. In an embodiment, a gear train can be embedded in the flange  1928 . A gear in the gear train may have an axial pin  2314 , which is held in a hole in an adjacent sheet-metal ply on at least one side. 
     According to an embodiment, there may be three gear trains (or a different number) embedded in the flange  1928 , and each gear train may rotate the proximal end of a respective rod  2226 . Each of the exemplary three rods  2226  can be rotatable by being fastened to or unitary with a respective rod-end gear  2223 , that is trapped within a respective end-gear cavity formed by: a hole or molded depression in the flange  1928 ; the inside surface of the cover plate  1914 ; and/or a proximal surface of a part of the support structure  1408  lying beyond the distal surface of the flange  1928 . 
     Each rod-end gear  2223  can be engaged with at least one drive gear  2224  in a respective gear train that drives the rod-end gear  2223 . The one or more drive gears  2224  can be engaged with one another successively as needed to the point where one of them extends beyond the outer edge of the cover plate  1914 . At that point, the gear train can be driven by a motor gear  2312  meshing with the outermost drive gear  2224 . (In some cases not shown, due to the geometry of the exterior wall  1924  and the cover plate  1914 , no drive gears may be needed, as the rod-end gear  2223  may protrude from under the edge of the cover plate  1914 .) 
     The three rods  2226 , being simultaneously turned by their respective gear trains, can cause the perforated flame holder  102  to move in an axial direction to vary the dilution distance D D  if the rods  2226  are each similarly threaded and engage female threads  2214  of the perforated flame holder  102 , a peripheral frame  2228  of the perforated flame holder  102 , or a sliding portion of the support structure  2208 , mentioned above. 
       FIG. 24  is a diagram of a combustion system  2400  including a movable support structure  2408  and one or more sensors  234  coupled to a controller  2422 , according to an embodiment. The sensor(s)  234  can include an ultraviolet detector  2412 , a still or video camera  2414 , an infrared detector  2416 , and a gas conductivity detector  2418 , among others. The sensors may be coupled to an electronic controller  2422  that is also coupled to the fuel and oxidant source  104  and/or a movable support structure  2408 . 
     Data or signals from the sensor(s)  2412 ,  2414 ,  2416 , and/or  2418  can be used by the controller  230  to select one of a plurality of positions for the perforated flame holder  102 . The movable support structure  2408  can adjust the position of the perforated flame holder  102  relative to the fuel and oxidant source  104 . 
       FIG. 25  is an illustration of a portion  2508  of a cooled support structure, according to an embodiment. The cooled support structure  2508  can include a wheel  2514  riding on two adjacent parallel water tubes, process tubes, or rails  2516 , with the wheel  2514  being mounted on a bracket  2512  that can be part of a movable perforated flame holder support. The cooled support structure  2508  can be suited to non-cylindrical furnace volumes, such as those having a generally square cross-section defined by parallel tubes, as well as to a cylindrical furnace volume, according to an embodiment. 
     A furnace can become quite hot, and not only a rail, but also a strut or a tensioned or compressed suspending member, can be embodied as a hollow tube or pipe  2516  into which fluid coolant is introduced so as to keep the suspending member cool enough to avoid heat failure. Water or other fluid coolant  812  can be injected, flow, or be pumped into the tube or rail  2516  to keep its temperature safely below a softening temperature, even while the furnace is hot enough to soften the material from which it is made. In an embodiment, the coolant fluid  812  is circulated to a location external to the furnace volume. In another embodiment, the coolant fluid  812  is vented into the furnace. 
       FIG. 26  is a cross-section of a cooled support structure  2608 , according to an embodiment. The cooled support structure  2608  can include bolts or studs  2612  and/or struts  2614 . The bolt(s) or stud(s)  2612  and the strut(s)  2614  can define interior channels  2616 . The bolt(s) or stud(s)  2612  and the strut(s)  2614  can be configured to support the perforated flame holder  102  in a selected position relative to the fuel and oxidant source  104 . Although a single bolt or stud  2612  and a single strut  2614  are shown in  FIG. 26 , in practice the cooled support structure  2608  can include multiple struts  2614  and bolts or studs  2612  to support the perforated flame holder  102 . 
     According to an embodiment, the bolts  2612  or struts  2614  can be bored, or fabricated from pipes or tubes, thus allowing the fluid coolant  812  to be introduced into the furnace through them, and the fluid coolant  812  to be transferred to other members of the cooled support structure  2608 , if they are also hollow. 
     According to an embodiment, a blind bore in the bolt  2612  makes hydraulic contact with a second bore in the strut  2614 , via a conical press-fit as shown. These bores illustrate interior channels  2616 . Nuts are not shown, but a lower nut may hold firebrick in place, while an upper nut may compress the strut  2614  onto the conical portion of the bolt  2612  and thereby hold it in a position with friction, as well as sealing the hydraulic passage. Optionally, the blind bore  2616  can be configured to make hydraulic contact with a second bore  2616  at a location corresponding to threads, with the second bore formed in a nut. 
       FIG. 27  is a diagram of a portion  2700  of a combustion system including a perforated flame holder (not shown) cooled support structure  2708 , according to an embodiment. The cooled support structure  2708  can include a U-shaped rod or tube  2712  defining an internal channel. The U-shaped rod or tube  2712  is configured as a rail on which a perforated flame holder  102  is supported. 
     According to an embodiment, the tube  2712  can be welded to a steel furnace wall  2510 . The tube  2712  can protrude over a continuous layer of refractory material  2716  held in place by anchors  2718 . The tube  2712  can pass through holes in the furnace wall  810 . Couplings  2714  are disposed on ends of the tube  2712  for attaching to a source and drain of fluid coolant. 
       FIG. 28  is an illustration of a cooled support structure  2808 , according to an embodiment. A hollow bolt  2812  can be disposed to hold firebrick adjacent to a wall of a furnace (not shown). An inner bolt  2814  is configured to be inserted through the hollow bolt  2812  and to couple to a perforated flame holder support structure (not shown). The inner bolt  2814  is bored to conduct fluid coolant. The inner bolt  2814  can circulate fluid coolant  812  to the cooled support structure  2808  for the perforated flame holder  102 . 
       FIG. 29  is a cross-section  2900  of a portion of a cooled support structure  2908  and a perforated flame holder  102 , according to an embodiment. The cooled support structure  2908  includes a metal grid  2912  that includes hollow members with interior channels  2916  that can conduct fluid coolant. The hollow members can be shaped to accept and hold sections of a perforated flame holder  102 . The fluid coolant can be passed through the interior channels  2916  to cool the perforated flame holder support structure  2908 . 
       FIG. 30  is a plan view  3000  of the perforated flame holder cooled support structure  2908  of  FIG. 29 , according to an embodiment. The cooled support structure  2908  is configured to support perforated flame holder tiles  102 . Fluid coolant supply pipes  2916  may couple to the metal grid  2912  at corners of the support structure, according to an embodiment. According to an embodiment, vent holes  3018  are formed to vent fluid coolant into the furnace volume. Alternatively, vent holes  3018  may be omitted and heated fluid coolant may be withdrawn through one or more of the pipes  2916 . 
       FIG. 31  is a cross-section of a cooled support structure  3108  for a perforated flame holder  102 , according to an embodiment. A metal grid  3116  includes a descending portion  3112  containing a capillary mesh  3111 . A working fluid inside the metal grid  3116  is vaporized adjacent to the perforated flame holder  102 , travels downward, and is condensed in the descending portion  3112 . The descending portion  3112  is cooled by cool fuel and/or oxidant traveling from a fuel and oxidant source (not shown) to the perforated flame holder  102 . Condensed working fluid travels along the capillary mesh  3111  to return to the evaporator near the upper end, adjacent to the perforated flame holder  102 . 
       FIG. 32  is an illustration of a support structure  3208  configured to receive a perforated flame holder  102 , according to an embodiment. 
       FIG. 33  is an enlarged cross-sectional view of a portion of the cooled support structure  3208 , according to an embodiment. The cooled support structure  3208  can include passages  3212  through which fuel and oxidant travel to a perforated flame holder  102 . A capillary mesh  2111  may be disposed on an interior wall of the support structure  3208 . An interior volume of the support structure can hold a working fluid  3114  configured to cooperate with the capillary mesh  2111  to operate as a heat pipe. Heat received from the perforated flame holder may thus be transferred to cool fuel and oxidant flowing through the passages  3212 . 
       FIG. 34  is a diagram of a combustion system  3400  including a support structure  3408  and a conical truncated perforated flame holder  3402 , according to an embodiment. The support structure  3408  can include a rim  3416  coupled to the perforated flame holder  3402  configured to be rotated on rollers  3412 . 
     According to an embodiment, the cooled support structure  3408  can rotate the perforated flame holder  3402  around its geometrical axis, bringing new areas of the perforated flame holder  3402  into the combustion region above the fuel and oxidant source  104 . 
       FIG. 35A  is a simplified perspective view of a combustion system  3500 , including another alternative perforated flame holder  102 , according to an embodiment. The perforated flame holder  102  is a reticulated ceramic perforated flame holder, according to an embodiment. 
       FIG. 35B  is a simplified side sectional diagram of a portion of the reticulated ceramic perforated flame holder  102  of  FIG. 35A , according to an embodiment. The perforated flame holder  102  of  FIGS. 35A, 35B  can be implemented in the various combustion systems described herein, according to an embodiment. The perforated flame holder  102  is configured to support a combustion reaction  302  of the fuel and oxidant  206  at least partially within the perforated flame holder  102 . According to an embodiment, the perforated flame holder  102  can be configured to support a combustion reaction  302  of the fuel and oxidant  206  upstream, downstream, within, and adjacent to the reticulated ceramic perforated flame holder  102 . 
     According to an embodiment, the perforated flame holder body  208  can include reticulated fibers  3539 . The reticulated fibers  3539  can define branching perforations  210  that weave around and through the reticulated fibers  3539 . According to an embodiment, the perforations  210  are formed as passages through the reticulated ceramic fibers  3539 . 
     According to an embodiment, the reticulated fibers  3539  can include alumina silicate. According to an embodiment, the reticulated fibers  3539  can be formed from extruded mullite or cordierite. According to an embodiment, the reticulated fibers  3539  can include Zirconia. According to an embodiment, the reticulated fibers  3539  can include silicon carbide. 
     The term “reticulated fibers” refers to a netlike structure. According to an embodiment, the reticulated fibers  3539  are formed from an extruded ceramic material. In reticulated fiber embodiments, the interaction between the fuel and oxidant  206 , the combustion reaction  302 , and heat transfer to and from the perforated flame holder body  208  can function similarly to the embodiment shown and described above with respect to  FIGS. 2-4 . One difference in activity is a mixing between perforations  210 , because the reticulated fibers  3539  form a discontinuous perforated flame holder body  208  that allows flow back and forth between neighboring perforations  210 . 
     According to an embodiment, the reticulated fiber network  3539  is sufficiently open for downstream reticulated fibers  3539  to emit radiation  304  for receipt by upstream reticulated fibers  3539  for the purpose of heating the upstream reticulated fibers  3539  sufficiently to maintain combustion of a fuel and oxidant  206 . Compared to a continuous perforated flame holder body  208 , heat conduction paths  312  between fibers  3539  are reduced due to separation of the fibers  3539 . This may cause relatively more heat to be transferred from the heat-receiving region  306  (heat receiving area) to the heat-output region  310  (heat output area) of the reticulated fibers  3539  via thermal radiation  304 . 
     According to an embodiment, individual perforations  210  may extend from an input face  212  to an output face  214  of the perforated flame holder  102 . Perforations  210  may have varying lengths L. According to an embodiment, because the perforations  210  branch into and out of each other, individual perforations  210  are not clearly defined by a length L. 
     According to an embodiment, the perforated flame holder  102  is configured to support or hold a combustion reaction  302  or a flame at least partially between the input face  212  and the output face  214 . According to an embodiment, the input face  212  corresponds to a surface of the perforated flame holder  102  proximal to the fuel nozzle  218  or to a surface that first receives fuel. According to an embodiment, the input face  212  corresponds to an extent of the reticulated fibers  3539  proximal to the fuel nozzle  218 . According to an embodiment, the output face  214  corresponds to a surface distal to the fuel nozzle  218  or opposite the input face  212 . According to an embodiment, the input face  212  corresponds to an extent of the reticulated fibers  3539  distal to the fuel nozzle  218  or opposite to the input face  212 . 
     According to an embodiment, the formation of boundary layers  314 , transfer of heat between the perforated reaction holder body  208  and the gases flowing through the perforations  210 , a characteristic perforation width dimension D, and the length L can be regarded as related to an average or overall path through the perforated reaction holder  102 . In other words, the dimension D can be determined as a root-mean-square of individual Dn values determined at each point along a flow path. Similarly, the length L can be a length that includes length contributed by tortuosity of the flow path, which may be somewhat longer than a straight line distance T RH  from the input face  212  to the output face  214  through the perforated reaction holder  102 . According to an embodiment, the void fraction (expressed as (total perforated reaction holder  102  volume−fiber  3539  volume)/total volume)) is about 70%. 
     According to an embodiment, the reticulated ceramic perforated flame holder  102  is a tile about 1″×4″×4″. According to an embodiment, the reticulated ceramic perforated flame holder  102  includes about 10 pores per square inch of surface area. Other materials and dimensions can also be used for a reticulated ceramic perforated flame holder  102  in accordance with principles of the present disclosure. 
     According to an embodiment, the reticulated ceramic perforated flame holder  102  can include shapes and dimensions other than those described herein. For example, the perforated flame holder  102  can include reticulated ceramic tiles that are larger or smaller than the dimensions set forth above. Additionally, the reticulated ceramic perforated flame holder  102  can include shapes other than generally cuboid shapes. 
     According to an embodiment, the reticulated ceramic perforated flame holder  102  can include multiple reticulated ceramic tiles. The multiple reticulated ceramic tiles can be joined together such that each ceramic tile is in direct contact with one or more adjacent reticulated ceramic tiles. Additionally or alternatively all or a portion of the multiple reticulated ceramic tiles may be held adjacent to one another but not in direct contact with one another by a perforated flame holder support structure, many embodiments of which are disclosed herein. The multiple reticulated ceramic tiles can collectively form a single perforated flame holder  102 . Alternatively, each reticulated ceramic tile can be considered a distinct perforated flame holder  102 . 
     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.