Patent Publication Number: US-2021190310-A9

Title: Burner system including a distal flame holder and a non-reactive fluid source

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority benefit from co-pending U.S. Provisional Patent Application No. 62/798,934, entitled “BURNER SYSTEM INCLUDING A PERFORATED FLAME HOLDER AND A NON-REACTIVE FLUID SOURCE,” filed Jan. 30, 2019 (docket number 2651-232-02); which application, to the extent not inconsistent with the disclosure herein, is incorporated by reference. 
    
    
     SUMMARY 
     In some embodiments, a burner system may include a distal flame holder, a fuel supply, an oxidant source, and/or a non-reactive fluid source. The distal flame holder may be disposed in a combustion volume and configured to receive and ignite a fuel and oxidant mixture. The fuel supply may be configured to contribute a fuel to the fuel and oxidant mixture. The oxidant source may be configured to contribute an oxidant to the fuel and oxidant mixture. The non-reactive fluid source is configured to deliver a non-reactive fluid in a dilution distance between the distal flame holder and the non-reactive fluid source. 
     In some embodiments, a burner system may include a distal flame holder, an oxidant conduit, a first fuel nozzle, and/or a non-reactive fluid source. The distal flame holder is configured to hold a combustion reaction of a fuel and an oxidant. The oxidant conduit is configured to direct the oxidant toward the distal flame holder. The first fuel nozzle is oriented to direct a first flow of the fuel into a combustion volume for mixture with the oxidant in a dilution region between the first fuel nozzle and the distal flame holder when a temperature of the distal flame holder is above a predetermined temperature. The non-reactive fluid source is oriented to emit a non-reactive fluid into the dilution region when the distal flame holder is at an operating temperature. 
     In some embodiments, a method for inhibiting flashback in a burner system may include supplying an oxidant to a combustion volume, directing a fuel via a first fuel nozzle to a dilution region of the combustion volume between the first fuel nozzle and a distal flame holder, mixing the oxidant with the fuel from the first fuel nozzle in the dilution region to provide a mixture of the fuel and the oxidant, and/or providing a non-reactive fluid to the mixture of the fuel and the oxidant in a portion of the dilution region proximate the distal flame holder. 
     In some embodiments, a multi-stage burner system may include a fuel and oxidant source, a distal flame holder, and/or at least one intermediate flame holder. The oxidant source is configured to emit fuel and oxidant into a combustion volume. The distal flame holder is oriented to receive and ignite a first mixture of the fuel and the oxidant downstream of the fuel and oxidant source. The at least one intermediate flame holder is disposed between the fuel and oxidant source and the distal flame holder and oriented to receive a second mixture of the fuel and the oxidant. 
     In some embodiments, a method of utilizing a multi-stage burner system may include directing an oxidant into a combustion volume, directing a fuel via a first fuel nozzle toward an intermediate flame holder disposed proximate the first fuel nozzle in a dilution region between the first fuel nozzle and a distal flame holder, and/or holding a flame at the intermediate flame holder supported by a mixture of the oxidant and the fuel from the first fuel nozzle. When the distal flame holder is at a predetermined temperature, a second fuel nozzle may direct the fuel toward the distal flame holder via the dilution region, mix the oxidant and the fuel from the second fuel nozzle to provide a second mixture of the fuel and the oxidant for combustion at the distal flame holder, dilute the second mixture of the fuel and the oxidant with combustion products of the flame within the dilution region and burn the second mixture of the fuel and the oxidant substantially at the distal flame holder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a burner system, according to an embodiment. 
         FIG. 2  is a simplified perspective view of a burner system including a distal flame holder, according to an embodiment. 
         FIG. 3  is a side sectional diagram of a portion of the distal flame holders of  FIGS. 1 and 2 , according to embodiments. 
         FIG. 4  is a flow chart showing a method for operating a burner system including the distal flame holder of  FIGS. 2 and 3 , according to embodiments. 
         FIG. 5  illustrates a flashback phenomenon that may occur in a burner system, according to an embodiment. 
         FIG. 6  is a block diagram of a burner system, according to an embodiment. 
         FIG. 7  is a block diagram of a burner system, according to an embodiment. 
         FIGS. 8A-8B  are flow charts showing a method for operating a burner system including the distal flame holder according to  FIGS. 6 and 7 , according to embodiments. 
         FIG. 9  is a block diagram of a multi-stage burner system, according to an embodiment. 
         FIGS. 10A-10B  are flow charts showing a method for operating a multi-stage burner system including an intermediate flame holder and a distal flame holder, according to an embodiment. 
         FIG. 11A  is a simplified perspective view of a combustion system, including another alternative distal flame holder, according to an embodiment. 
         FIG. 11B  is a simplified side sectional diagram of a portion of the reticulated ceramic distal flame holder of  FIG. 11A , according to an embodiment. 
         FIG. 12  is an illustration of a burner system, 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. 
     The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. 
     Certain burner systems utilize a distal flame holder disposed downstream from a fuel and oxidant source. Fuel and oxidant in sufficiently flammable proportion enters perforations at an input side of the distal flame holder, is ignited and burned therein. Ideally the resulting combustion occurs at a speed and temperature that minimizes undesirable combustion products such as nitrogen oxides (NOx) while providing sufficient thermal energy to the distal flame holder to sustain combustion of the continuously received fuel and oxidant mixture and to provide heat for the relevant burner application. 
     The inventors have observed, in a variety of furnace applications, undesirable combustion oscillations occurring between the distal flame holder and the fuel and oxidant source. (Although not necessarily restricted to a confined furnace configuration—e.g., a water heater, boiler, or once-through steam generator (OTSG)—such applications are representative environments that can permit such combustion oscillations.) 
     When fuel and oxidant are in sufficiently combustible proportion and exposed to sufficient heat for ignition, they can undesirably ignite upstream of the distal flame holder. This phenomenon tends to oscillate and is referred to herein as “flashback,” and is sometimes colloquially referred to as “huffing.” In some implementations, insufficiently and/or non-uniformly cooled oxidant, e.g., flue gas, can be recirculated from downstream of the distal flame holder, resulting in a fuel-oxidant mixture with a sufficiently high temperature that the mixture may ignite prior to reaching the distal flame holder. The flashback reduces the efficiency of the burner at least in part because heat from this premature combustion is not (in a gas-fired burner) radiant heat, is not sufficiently absorbed by the distal flame holder, and is thus wasted. Combustion products from the flashback can dilute the mixture and thus temporarily snuff the flashback combustion. Hence the oscillating nature of flashback. 
     As described below with respect to  FIG. 3 , the distal flame holder  102  may be structured to absorb energy from a combustion reaction  302  substantially contained within a perforation  210  of the distal flame holder  102 . The distal flame holder  102  can emit that energy as radiant heat, something a gas-fueled flame alone does poorly due to its low emissivity. Thus, when a gas-fueled combustion reaction  302  occurs outside the distal flame holder  102 , as during flashback, heat is wasted. Moreover, such combustion reaction  302  may be incomplete, resulting in undesired combustion products which may result in unacceptable emissions levels (e.g., NOx) that may adversely affect downstream structures such as the distal flame holder  102  due to fluctuations in temperature, and may present sonic disturbances. 
     It is acknowledged that the term “flashback” is often used to refer to systems in which pre-mixed fuel and oxidant are supplied directly and a premature ignition can result in a flame traveling into the premix supply. In the present disclosure, flashback can also be used in reference to non-premix applications, where fuel (e.g.,  106   b,    506   b ) and oxidant (e.g.,  106   a,    506   a ) become mixed (e.g.,  206 ,  506 ) in a dilution region (e.g.,  510 ) between the fuel and oxidant source(s) (e.g.,  202 ) and the distal flame holder  102 , such as in a space upstream of the distal flame holder  102 , as discussed below with relation to  FIG. 5 . 
     Disclosed herein are mechanisms and methods for preventing flashback, including structures and processes for delay of sufficient fuel and oxidant mixing, and reduction of fuel and oxidant mixture temperature in a dilution region. The burner configurations disclosed in detail below are directed to reduce or eliminate such flashback by reducing the possibility of the fuel and oxidant mixture igniting prior to reaching the distal flame holder  102 . According to an interpretation, disclosed embodiments dilute the fuel and oxidant mixture to prevent ignition upstream of the distal flame holder  102 . According to another interpretation, disclosed embodiments create a hot temperature region upstream of the distal flame holder  102 , which hot temperature region alters oxidant circulation/recirculation patterns and thus prevents premature ignition of the fuel and oxidant mixture. 
     The fuel and oxidant mixture may be diluted by introducing a non-reactive or non-combustible fluid (e.g., gas) upstream of the distal flame holder  102 . As used herein, the term “non-reactive fluid” may include fluids that are reactive under certain conditions but are non-reactive in conditions relevant to the embodiment. Likewise, the term “non-combustible fluid” as used herein may include fluids that are not combustible in conditions relevant to the disclosed embodiment(s), but could be combustible under other conditions. Naturally, fluids, such as certain inert or noble gases, that are not reactive under any conditions are also contemplated by the inventors as being non-reactive and/or non-combustible. 
     A non-reactive or non-combustible fluid may include, for example, a dedicated non-reactive gas, or may include combustion products from burning a portion of fuel upstream of the distal flame holder  102 . In an implementation utilizing combustion products as a non-reactive fluid, a dedicated first-stage flame holder may be utilized to hold a flame. Alternatively, a fuel nozzle may itself act as a first-stage flame holder sufficient to support such combustion, e.g., in a flame-stabilized implementation. Use of two combustion regions (first-stage combustion at/near the first-stage flame holder and second-stage combustion at the distal flame holder  102 ) may permit a low NOx burner to achieve even lower emissions by utilizing the distal flame holder  102  for combustion while ensuring combustion temperatures sufficiently low to avoid creation of NOx. 
     These and other embodiments are described in detail below with reference to the drawings.  FIG. 1  is a block diagram of a burner system  100 , according to an embodiment. The burner system  100  includes a distal flame holder  102 , a fuel and oxidant source  202 , and a mixing tube  110 . The fuel and oxidant source  202  may include an oxidant conduit  104  for delivery of an oxidant  106   a,  and one or more fuel nozzle(s)  118  for main delivery of a fuel  106   b.  The fuel  106   a  and the oxidant  106   b  mix in the mixing tube  110  en route to the distal flame holder  102 , creating a fuel and oxidant mixture  206 . The distal flame holder  102  is disposed and oriented to receive and (when at an operating temperature) to ignite the fuel and oxidant mixture  206 . The oxidant conduit  104  provides a pathway for the oxidant  106   a  (e.g., air), and directs the oxidant  106   a  toward the distal flame holder  102 . The fuel nozzle(s)  118  direct the fuel  106   b  toward the distal flame holder  102 . The fuel nozzle(s)  118  may receive the fuel  106   b  from a fuel reservoir or pipeline (not shown, each or both referred to herein as a fuel supply) via a fuel supply line  108 . The burner system  100  may include a single fuel nozzle  118  or a plurality of the fuel nozzle(s)  118 , each disposed and configured as described herein. The fuel  106   b  emitted by the fuel nozzle(s)  118 , and the oxidant  106   a  emitted by the oxidant conduit  104  become mixed as they travel toward the distal flame holder  102 . The fuel  106   b  and the oxidant  106   a  achieve a sufficiently uniform fuel and oxidant mixture (see element  206  in  FIG. 2 ) to permit efficient and uniform combustion within the distal flame holder  102  at the operating temperature. 
     The burner system  100  may include one or more second nozzle(s)  112 . The one or more second nozzle(s)  112  may be oriented and placed to directly or indirectly supply a pilot flame to the fuel and oxidant mixture  206 . In some implementations, the second nozzle(s)  112  may receive fuel from a pilot fuel supply line  114  or may be in fluid connection with the fuel supply line  108 . In some implementations, the second nozzle(s)  112  may be disposed at the fuel dump plane proximate to the fuel nozzles(s)  118 . Alternatively, the second nozzle(s)  112  may be disposed distally, proximate the distal flame holder  102 . Elements of the burner system  100  (and  600 ,  700 ,  900 ,  1100 ,  1200 ) are described in greater detail with respect to  FIGS. 2-3 and 5-7  below.  FIGS. 2-3  illustrate a simplified burner system which is described generally. Those of skill in the art will acknowledge that any or all of the features described below with respect to  FIGS. 2-3  may apply to above- and later-described implementations of a burner system, such as the burner systems described in relation to  FIG. 1  and  FIGS. 5-12 . 
       FIG. 2  is a simplified diagram of a burner system  200  including a distal flame holder (such as distal flame holder  102 ) configured to hold a combustion reaction, according to an embodiment. It is noted that the distal flame holder  102  may have different configurations. For example, the distal flame holder  102  may be circular, annular, rectangular as illustrated in  FIGS. 2-3 , or other reasonably fabricated shape. It will be appreciated by those having skill in the art, for example, that other configurations may be implemented, such as oblong, semi-circular, triangular, etc. As used herein, the terms distal flame holder (DFH), distal reaction holder, perforated reaction holder (PFH), perforated reaction holder, porous flame holder, and porous reaction holder shall be considered synonymous unless further definition is provided. Experiments performed by the inventors have shown that burner systems described herein can support very clean combustion. Specifically, in experimental use of the burner systems 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 or internal flue gas recirculation (FGR), or other 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 (e.g.,  106   b ) and oxidant (e.g.,  106   a ) into a combustion volume  204  to form a fuel and oxidant mixture  206 . As used herein, the terms combustion volume, combustion chamber, furnace volume, and the like shall be considered synonymous unless further definition is provided. The distal flame holder  102  is disposed in the combustion volume  204  and positioned to receive the fuel and oxidant mixture  206 .  FIG. 3  is side sectional diagram  300  of a portion of a type of distal flame holder  102  of  FIG. 2  that includes a perforated reaction holder, according to an embodiment. Referring to  FIGS. 2 and 3 , the distal flame holder  102  includes a distal 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 distal 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 ( 106   b ) can include hydrogen, a hydrocarbon gas, a vaporized hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel ( 106   b ) 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 ( 106   b ) 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 ( 106   b ) can include natural gas (mostly CH 4 ) or propane (C 3 H 8 ). In another application, the fuel ( 106   b ) can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant ( 106   a ) can include oxygen carried by air and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein. 
     According to an embodiment, the distal 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 distal flame holder  102 . The plurality of perforations  210  which are defined by the distal 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, such a perforated flame holder  102  is configured to hold a majority of the combustion reaction  302  within the perforations  210 . For example, on a steady-state basis, more than half the molecules of fuel output into the combustion volume  204  by the fuel and oxidant source  202  may be converted to combustion products between the input face  212  and the output face  214  of the perforated flame holder  102 . According to an alternative interpretation, more than half of the heat output by the combustion reaction  302  may be output between the input face  212  and the output face  214  of the perforated flame holder  102 . Under nominal operating conditions, the perforations  210  can be configured to collectively hold at least 80% of the combustion reaction  302  between the input face  212  and the output face  214  of the distal 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 distal flame holder  102 . According to an alternative interpretation, the distal flame holder  102  can support combustion between the input face  212  and the output face  214  when combustion is “time-averaged.” For example, during transients, such as before the distal 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 distal 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 distal flame holder  102  itself. In instances, the inventors have observed combustion oscillations (referred to herein as “flashback” and informally as “huffing”) in a variety of burner applications. Flashback includes momentary ignition of fuel ( 106   b ) and oxidant ( 106   a ) in a region lying between the input face  212  of the distal flame holder  102  and a fuel nozzle  218  (described below), within the dilution distance DD. Such transient flashback is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations  210  of the distal flame holder  102 , between the input face  212  and the output face  214 . In still other instances, the inventors have noted apparent combustion occurring above the output face  214  of the distal flame holder  102 , but still a majority of combustion occurred within the distal flame holder  102  as evidenced by the continued visible glow (a visible wavelength tail of blackbody radiation) from the distal flame holder  102 . A perforated flame holder  102  can be configured to receive heat from the combustion reaction  302  and output a portion of the received heat as thermal radiation  304  to heat-receiving structures (e.g., furnace walls and/or radiant section working fluid tubes) in or adjacent to the combustion volume  204 . As used herein, terms such as thermal radiation, infrared radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody radiation of electromagnetic energy, primarily in infrared wavelengths. 
     Referring especially to  FIG. 3 , the distal 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 distal flame holder  102 . The distal flame holder body  208  may receive heat from the (exothermic) combustion reaction  302  at least in heat receiving regions  306  of perforation walls  308 . Experimental evidence has suggested to the inventors that the position of the heat receiving regions  306 , or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls  308 . In some experiments, the location of maximum receipt of heat was apparently between 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 distal flame holder  102  under other conditions. According to an interpretation there is no clearly defined edge of the heat receiving regions  306  (or, for that matter, of 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 distal flame holder body  208  can be characterized by a heat capacity. The distal flame holder body  208  may hold heat from the combustion reaction  302  in an amount corresponding to the heat capacity multiplied by temperature rise, and transfer the heat from the heat receiving regions  306  to the 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 distal 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 distal flame holder body  208  can transfer heat from the heat receiving regions  306  to the heat output regions  310  via heat conduction along heat conduction paths  312 . The inventors contemplate that both radiation and conduction heat transfer mechanisms may be operative in transferring heat from the heat receiving regions  306  to the heat output regions  310 . In this way, the distal flame holder  102  may act as a heat source to maintain the combustion reaction  302 , even under conditions where the combustion reaction  302  would not be stable when supported from a conventional flame holder. 
     The inventors believe that the distal flame holder  102  causes the combustion reaction  302  to occur within thermal boundary layers  314  formed adjacent to the walls  308  of the perforations  210 . As the relatively cool fuel and oxidant mixture  206  approaches the input face  212 , the flow is split into portions that respectively travel through individual perforations  210 . The hot distal flame holder body  208  transfers heat to the fluid, notably within the 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 ( 106   b )), 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 . The merger point  316  may lie between the input face  212  and the output face  214  that defines the ends of the perforations  210 . At some point, the combustion reaction  302  causes the flowing gas (and plasma) to output more heat to the distal flame holder body  208  than it receives from the distal flame holder body  208 . The heat is received at the heat receiving region  306 , is held by the distal flame holder body  208 , and is transported to the heat output region  310  nearer to the input face  212 , where the heat recycles into the cool reactants (and any included diluent) to raise them to the combustion temperature. 
     In an embodiment, the perforations  210  are each characterized by a length L defined as a reaction fluid propagation path length between the input face  212  and the output face  214  of the distal flame holder  102 . The reaction fluid includes the fuel and oxidant mixture  206  (optionally including nitrogen, flue gas, and/or other “non-reactive” species), reaction intermediates (including transition states in a plasma that characterizes the combustion reaction  302 ), and reaction products. 
     The 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 distal flame holder  102  if the length L of each perforation  210  is at least four times the transverse dimension D of the perforation  210 . In other embodiments, the length L can be greater than six times the transverse dimension D. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D. Preferably, the length L is sufficiently long for thermal boundary layers  314  formed adjacent to the perforation walls  308  in a reaction fluid flowing through the perforations  210  to converge at merger points  316  within the perforations  210  between the input face  212  and the output face  214  of the distal 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 distal 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 the fuel nozzle  218 , configured to output fuel (e.g.,  106   b ), and an oxidant source  220  configured to output a fluid including the oxidant (e.g.,  106   a ). For example, the fuel nozzle  218  can be configured to output pure fuel. The oxidant source  220  can be configured to output combustion air carrying oxygen. 
     The distal flame holder  102  can be held by a distal flame holder support structure  222  configured to hold the distal flame holder  102  a distance DD away from the fuel nozzle  218 . The fuel nozzle  218  can be configured to emit a fuel jet selected to entrain the oxidant ( 106   a ) to form the fuel and oxidant mixture  206  as the fuel jet and oxidant ( 106   a ) travel along a path to the distal flame holder  102  through a dilution distance DD between the fuel nozzle  218  and the distal flame holder  102 . Additionally or alternatively (particularly when a blower is used to deliver oxidant combustion air ( 106   a )), the oxidant or combustion air source  220  can be configured to entrain the fuel ( 106   b ), and the fuel ( 106   b ) and oxidant ( 106   a ) travel through the dilution distance DD. 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 ( 106   a ) and to entrain flue gas as the fuel jet travels through a dilution distance DD between the fuel nozzle  218  and the input face  212  of the distal flame holder  102 . 
     The fuel nozzle  218  can be configured to emit the fuel ( 106   b ) through one or more fuel orifices  226  having a dimension that is referred to as “nozzle diameter.” The distal flame holder support structure  222  can support the distal flame holder  102  to receive the fuel and oxidant mixture  206  at a distance DD away from the fuel nozzle  218  greater than  20  times the nozzle diameter. In another embodiment, the distal flame holder  102  is disposed to receive the fuel and oxidant mixture  206  at a distance DD away from the fuel nozzle  218  between  100  times and  1100  times the nozzle diameter. Preferably, the distal flame holder support structure  222  is configured to hold the distal flame holder  102  about  200  times the nozzle diameter or more away from the fuel nozzle  218 . 
     When the fuel and oxidant mixture  206  travels about  200  times the nozzle diameter or more, the fuel and oxidant mixture  206  is sufficiently homogenized to cause the combustion reaction  302  to output minimal NOx. 
     The fuel and oxidant source  202  can alternatively include a premix fuel and oxidant source, according to an embodiment. A premix fuel and oxidant source (not shown) can include a premix chamber, a fuel nozzle configured to output fuel into the premix chamber, and an air channel configured to output combustion air into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the distal flame holder  102  and be configured to prevent flame flashback into the premix fuel and oxidant source. 
     The combustion air source, whether configured for entrainment in the combustion volume  204  or for premixing, can include a blower configured to force air through the fuel and oxidant source  202 . 
     The distal flame holder support structure  222  can be configured to support the distal flame holder  102  from a floor or wall (not shown) of the combustion volume  204 , for example. In another embodiment, the distal flame holder support structure  222  supports the distal flame holder  102  from the fuel and oxidant source  202 . Alternatively, the distal flame holder support structure  222  can suspend the distal flame holder  102  from an overhead structure (such as a flue, in the case of an up-fired system). The distal flame holder support structure  222  can support the distal flame holder  102  in various orientations and directions. 
     The distal flame holder  102  can include a single distal flame holder body  208 . In another embodiment, the distal flame holder  102  can include a plurality of adjacent distal flame holder sections (not shown) that collectively provide a tiled distal flame holder  102 . The distal flame holder support structure  222  can be configured to support the plurality of distal flame holder sections. The distal flame holder support structure  222  can include a metal superalloy, a cementatious, and/or ceramic refractory material. In an embodiment, the plurality of adjacent distal flame holder sections can be joined with a fiber reinforced refractory cement. The distal 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 distal flame holder  102  can have a width dimension W between opposite sides of the peripheral surface  216  at least three times, at least six times, or at least nine times a thickness dimension T between the input face  212  and the output face  214  of the distal flame holder  102 . 
     In an embodiment, the distal flame holder  102  can have a width dimension W less than a width of the combustion volume  204 . This can allow the flue gas recirculation path  224  from above to below the distal flame holder  102  to lie between the peripheral surface  216  of the distal flame holder  102  and the combustion volume wall (not shown). 
     Referring again to both  FIGS. 2 and 3 , the perforations  210  can include elongated squares, each of the elongated squares has a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations  210  can include elongated hexagons, each of the elongated hexagons has a transverse dimension D between opposing sides of the hexagons. In another embodiment, the perforations  210  can include hollow cylinders, where each of the hollow cylinders has a transverse dimension D corresponding to a diameter of the cylinders. In another embodiment, the perforations  210  can each be a frustrum of a cone, each having a transverse dimension D that is rotationally symmetrical about a length axis that extends from the input face  212  to the output face  214 . The perforations  210  can each have a transverse dimension D equal to or greater than a quenching distance of the fuel based on standard reference conditions. 
     In one range of embodiments, each of the plurality of perforations  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 perforations  210  can each have a lateral dimension D of about 0.2 to 0.4 inch. 
     The void fraction of a distal flame holder  102  is defined as the total volume of all perforations  210  in a section of the distal flame holder  102  divided by a total volume of the distal flame holder  102  including distal flame holder body  208  and the perforations  210 . The distal flame holder  102  should have a void fraction between 0.10 and 0.90. In an embodiment, the distal flame holder  102  can have a void fraction between 0.30 and 0.80. In another embodiment, the distal 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 distal 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 distal flame holder  102  can be formed from mullite or cordierite. Additionally or alternatively, the distal flame holder body  208  can include a metal superalloy such as Inconel or Hastelloy. The distal flame holder body  208  can define a honeycomb. 
     The inventors have found that the distal 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 the 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 the 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 distal flame holder body  208  can be one piece or can be formed from a plurality of sections. 
     In another embodiment, which is not necessarily preferred, the distal flame holder  102  may be formed from reticulated fibers formed from an extruded ceramic material. The term “reticulated fibers” refers to a netlike structure. 
     In another embodiment, the distal 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 distal 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 distal 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 distal flame holder  102  provide such clean combustion. In one aspect, the distal flame holder  102  acts as a heat source to maintain a combustion reaction  302  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 and oxidant mixture  206  contacts the input face  212  of the distal flame holder  102 , an average fuel-to-oxidant ratio of the fuel and oxidant mixture  206  is below a (conventional) lower combustion limit of the fuel component of the fuel and oxidant mixture  206 —lower combustion limit defines the lowest concentration of fuel at which a fuel and air mixture will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.). 
     According to one interpretation, the fuel and oxidant mixtures  206  supported by the distal flame holder  102  may be more fuel-lean than mixtures that would provide stable combustion in a conventional burner. Combustion near a lower combustion limit of fuel generally burn at a lower adiabatic flame temperature than mixtures near the center of the lean-to-rich combustion limit range. Lower flame temperatures generally evolve a lower concentration of oxides of nitrogen (NOx) than higher flame temperatures. In conventional flames, too-lean combustion is generally associated with high CO concentration at the stack. In contrast, the distal flame holder  102  and systems including the distal flame holder  102  described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. In some embodiments, the inventors achieved stable combustion at what was understood to be very lean mixtures (that nevertheless produced only about 3% or lower measured O 2  concentration at the stack). Moreover, the inventors believe the perforation walls  308  may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperature. 
     According to another interpretation, production of NOx can be reduced if the combustion reaction  302  occurs over a very short duration of time. Rapid combustion causes the reactants (including oxygen and entrained nitrogen) to be exposed to NOx-formation temperature for a time too short for NOx formation kinetics to cause significant production of NOx. The time required for the reactants to pass through the distal flame holder  102  is very short compared to a conventional flame. The low NOx production associated with distal flame holder  102  combustion may thus be related to the short duration of time required for the reactants (and entrained nitrogen) to pass through the distal flame holder  102 . 
     Since CO oxidation is a relatively slow reaction, the time for passage through the distal flame holder  102  (perhaps plus time passing toward the flue from the distal flame holder  102 ) is apparently sufficient and at sufficiently elevated temperature, in view of the very low measured (experimental and full scale) CO concentrations, for oxidation of CO to carbon dioxide (CO 2 ). 
       FIG. 4  is a flow chart showing a method  400  for operating a burner system including the distal flame holder (e.g.,  102 ) shown and described herein. To operate a burner system including a distal flame holder, the distal flame holder is first heated to a temperature sufficient to maintain combustion of the fuel and oxidant mixture (referred to herein as an operating temperature). 
     According to a simplified description, the method  400  begins with step  402 , wherein the distal flame holder is preheated to a start-up temperature, T S . After the distal flame holder is raised to the start-up temperature, the method proceeds to step  404 , wherein fuel and oxidant are provided to the distal flame holder and combustion is held by thereby. 
     According to a more detailed description, step  402  begins with step  406 , wherein start-up energy is provided at the distal flame holder. Simultaneously or following providing start-up energy, a decision step  408  determines whether the temperature T of the distal flame holder is at or above the start-up temperature, T S . As long as the temperature of the distal flame holder is below its start-up temperature, the method loops between steps  406  and  408  within the preheat step  402 . In decision step  408 , if the temperature T of at least a predetermined portion of the distal 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 distal flame holder. 
     Step  404  may be broken down into several discrete steps, at least some of which may occur simultaneously. 
     Proceeding from decision step  408 , a fuel and oxidant mixture is provided to the distal flame holder, as shown in step  410 . The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and combustion air source, for example. In this approach, the fuel and combustion air are output in one or more directions selected to cause the fuel and combustion air mixture to be received by an input face of the distal 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 distal flame holder at a fuel dilution selected for a stable combustion reaction that can be held within the perforations of the distal flame holder. 
     Proceeding to step  412 , the combustion reaction is held by the distal flame holder. In step  414 , heat may be output from the distal flame holder. The heat output from the distal 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 distal flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, and/or other known combustion sensing apparatuses. In an additional or alternative variant of step  416 , a pilot flame or other ignition source may be provided to cause ignition of the fuel and oxidant mixture in the event combustion is lost at the distal 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 decision step  418 , combustion in the distal 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 distal flame holder over one or more iterations of the loop within step  404 . 
     Referring again to  FIG. 2 , the burner system  200  may include a heater  228  operatively coupled to the distal flame holder  102 . As described in conjunction with  FIGS. 3 and 4 , the distal 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 distal flame holder  102 . The fuel and oxidant source  202  can include a fuel nozzle  218  configured to emit a fuel stream, and an oxidant source  220  configured to output combustion air (oxidant) adjacent to the fuel stream. The fuel nozzle  218  and air source  220  can be configured to output the fuel stream to be progressively diluted by the combustion air. The distal flame holder  102  can be disposed to receive a diluted fuel and air mixture  206  that supports a combustion reaction  302  that is stabilized by the distal flame holder  102  when the distal flame holder  102  is at an operating temperature. A start-up flame holder, in contrast, can be configured to support a start-up flame at a location corresponding to a relatively rich fuel and air mixture that is stable without stabilization provided by the heated distal 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 distal flame holder  102  needs to be pre-heated and to not hold the start-up flame when the distal 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 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 distal 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 distal 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 distal flame holder  102  and/or to the fuel and oxidant mixture  206 . The electrical resistance heater  228  can be configured to heat up the distal 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  228 . 
     An electrical resistance heater  228  can be formed in various ways. For example, the electrical resistance heater  228  can be formed from KANTHAL® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstahammar, Sweden) threaded through at least a portion of the perforations  210  defined formed by the distal flame holder body  208 . Alternatively, the heater  228  can include an inductive heater, a high energy (e.g. microwave or laser) beam heater, a frictional heater, or other types of heating technologies. 
     Other forms of start-up apparatuses are contemplated. For example, the heater  228  can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the air and fuel. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite a fuel and oxidant mixture  206  that would otherwise enter the distal flame holder  102 . An electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller  230 , which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the fuel and oxidant mixture  206  in or upstream from the distal flame holder  102  before the distal 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 distal 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 (e.g.,  106   b ) 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) (e.g.,  106   a ). 
     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  302  held by the distal flame holder  102 . The fuel control valve  236  can be configured to control a flow of fuel ( 106   b ) 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 the oxidant blower or damper  238  to control a preheat flame type of heater  228  to heat the distal 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  238  to change the fuel and oxidant mixture  206  flow responsive to a heat demand change received as data via the data interface  232 . 
       FIG. 5  illustrates the flashback phenomenon that may occur in a burner system  500  that does not employ the structures and methods disclosed herein for addressing flashback. An oxidant conduit  504  and fuel nozzle(s)  518  respectively emit oxidant  506   a  and fuel  506   b  which are mixed in a dilution region  510  (corresponding to the dilution distance DD between the fuel nozzles  518  and the distal flame holder  102 ) to provide a fuel and oxidant mixture  506 . The distal flame holder  102  is oriented to receive the fuel and oxidant mixture  506  and is configured to, when at an operating temperature, ignite the fuel and oxidant mixture  506 . However, as described above, transient “flashback” may occur wherein the fuel and oxidant mixture  506  momentarily ignites in the dilution region  510  between the distal flame holder  102  and the fuel nozzle(s)  518 , producing a visible flame  502 . According to one interpretation, the fuel  506   b  and the oxidant  506   a  become sufficiently well mixed to ignite from heat radiating from the distal flame holder  102  before the mixture  506  reaches the distal flame holder  102 . The flame  502  depicted is not necessarily representative of flame shape or relative size in an implementation. 
       FIG. 6  is a block diagram of a burner system  600 , according to an embodiment. The burner system  600  may include a distal flame holder  102  configured to receive and ignite a fuel and oxidant mixture  206 . As in  FIG. 1 , the fuel and oxidant mixture  206  may include an oxidant  106   a  (partially omitted from  FIG. 6  for clearer presentation of other features) and a fuel  106   b.  A fuel source may be configured to contribute the fuel  106   b  to the fuel and oxidant mixture  206  when the distal flame holder  102  is at an operating temperature above a predetermined threshold temperature. An oxidant source such as an oxidant conduit  104  that conducts air or another oxidant is configured to contribute the oxidant  106   a  to the fuel and oxidant mixture  206 . A second fuel nozzle  610  constitute a portion of a non-reactive fluid source  110  to provide a non-reactive fluid (i.e., combustion products)  616  in a dilution region (e.g.,  510 ) corresponding to the dilution distance (DD) between the distal flame holder  102  and second nozzle  610 . 
     More specifically, the non-reactive fluid source  110  may include the second fuel nozzle  610 , fuel  614  emitted by the second fuel nozzle  610 , and an intermediate flame  618 . Substantially non-reactive combustion products  616  from the intermediate flame  618  may constitute at least part of the non-reactive fluid  116  described with respect to  FIG. 1 . As discussed above, the non-reactive combustion products  616  may dilute the fuel and oxidant mixture  206  such that the fuel and oxidant mixture  206  is insufficiently mixed to auto-ignite in conditions present upstream of the distal flame holder  102 . The second fuel nozzle  610  may operate as a flame holder for purposes of holding the intermediate flame  618  at a low fuel flow rate. 
     It will be understood by those having skill in the art that the embodiment illustrated in  FIGS. 5-7 and 9  may additionally include a mixing tube, such as the mixing tube  110  illustrated in  FIG. 1 . 
     The fuel source may include a fuel supply (not shown), such as a fuel reservoir or pipeline configured to store the fuel  106   b,  a fuel supply line  108 , and/or a first fuel nozzle  118  that is in fluid connection with the fuel supply via the fuel supply line  108 , and is positioned to deliver the fuel  106   b  for the fuel and oxidant mixture  206 . In some implementations, the fuel supply line  108  (and in some embodiments a separate fluid supply line  112  as shown in  FIG. 1 ) may each attach to a fuel supply manifold (not shown) that receives fuel  106   b  from such fuel reservoir(s). 
     In some embodiments, the second fuel nozzle  610  may operate as a pilot fuel nozzle, indirectly providing the non-reactive fluid (combustion products  616 ) as described above. 
     The burner system  600  may further include a fuel flow control mechanism  620  in fluid connection with the second fuel nozzle  610 . The fuel flow control mechanism  620  may include a valve and may be configured to permit a first fuel flow rate to the second fuel nozzle  610  during a startup period. The fuel flow control mechanism  620  may be further configured to permit a second fuel flow rate to the second fuel nozzle  610  after the startup period (e.g., during an operational period when the distal flame holder  102  is at an operating temperature). The burner system  600  may include an ignition source  630  (e.g., an igniter) configured to ignite a fuel  614  and the oxidant  106   a  proximate the second fuel nozzle  610  to produce the intermediate flame  618 . Specifically, the fuel  614  emitted by the second fuel nozzle(s)  610  may be flame stabilized. Combustion of the fuel  614  and the oxidant  106   a  may result in the combustion products  616  as a non-reactive fluid for supply to the fuel and oxidant mixture  206 . Such dilution prevents flashback as discussed above. The fuel and oxidant mixture  206  when appropriately diluted by the non-reactive fluid  616  may permit combustion to be substantially limited to the distal flame holder  102 , with some amount of the combustion flame  103  visible in certain circumstances. 
     In some embodiments, the second fuel nozzle  610  may operate to preheat the distal flame holder  102  to a predetermined temperature, for example an operating temperature at which the fuel and oxidant mixture  206  will auto-ignite within the perforations  210  of the distal flame holder  102 . The fuel flow control mechanism  620  may thus provide fuel at a first fuel flow rate that permits such preheating, and then change the fuel rate to the second fuel flow rate in order to provide the intermediate flame  618  sufficient to generate combustion products  616  as a non-reactive fluid. 
     The combustion products  616  from the intermediate flame  618  may constitute at least a portion of the non-reactive fluid  616 . In some embodiments, for example, the non-reactive fluid  616  may additionally include other non-reactive elements, such as an inert gas provided from a dedicated inert gas source. 
     Returning briefly to  FIG. 1 , in some embodiments, the non-reactive fluid source  110  may include a non-reactive fluid supply (not shown) such as a non-reactive fluid reservoir, and a second nozzle  110   a  in fluid connection with the non-reactive fluid supply via a non-reactive fluid supply line  112 . The second nozzle  110   a  may be positioned to deliver the non-reactive fluid  116  directly to the fuel and oxidant mixture  206 . 
     The non-reactive fluid  116  may be an inert gas at temperatures encountered proximate the distal flame holder  102 . Some inert gases include helium and argon. Other inert gases are contemplated by the inventors, including, for example, compounds of elements that, while reactive under certain conditions, are not reactive in conditions encountered upstream of the distal flame holder  102  (e.g., in the dilution region  510 ) during normal operation. In some implementations, the non-reactive fluid  116  may be an otherwise reactive fluid rendered non-reactive upstream of the distal flame holder  102  by a temperature of the otherwise reactive fluid and/or by the presence of other elements. For example, air, recirculated flue gas or even fuel (e.g.,  106   b ), if sufficiently cool, may act as a non-reactive fluid  116  to delay auto-ignition of the fuel and oxidant mixture  206  until the mixture reaches the distal flame holder  102 . 
     Turning now to  FIG. 7 , we see a block diagram of a burner system  700 , according to another embodiment. In addition to the structural elements described with relation to  FIG. 6 , the burner system  700  may include a non-reactive fluid source  110  that includes, in addition to the second fuel nozzle  610 , the fuel  614  and flame  718 , an intermediate flame holder  702  disposed between the second fuel nozzle  610  and the distal flame holder  102 . The intermediate flame holder  702  of the non-reactive fluid source  110  may be configured to hold the intermediate flame  618  supported by the oxidant  106   a  and the fuel  614  delivered from the second fuel nozzle  610 . The burner system  700  may also include an ignition source  730  configured to ignite the fuel  614  and the oxidant  106   a  at the intermediate flame holder  702  to produce the flame  718 . In one embodiment, the intermediate flame holder  702  is a flame holding feature disposed to hold a pilot flame  718 . The non-reactive fluid source  110  is configured to deliver the non-reactive fluid  616  in a region disposed between the pilot flame  718  (supported by the pilot flame holding feature  702 ) and an ignitable portion of the fuel and air mixture  206 . For example, the fuel and air mixture  206  may be characterized by a surface defining a portion of the fuel and air mixture  206  that is within flammability limits. The non-reactive fluid  616  may provide a buffer between the pilot flame  718  and the surface defining the portion of the fuel and air mixture  206  that is within flammability limits such as to prevent the ignition source  730  from interacting with the fuel and air mixture  206  and causing it to ignite prematurely. 
     In one embodiment, the non-reactive fluid  616  is a substantially non-reactive product of pilot flame combustion. More specifically, the pilot flame  718  held by the pilot flame holding feature  702  may generate at least a portion of the substantially non-reactive fluid  616  in the form of a fuel depleting combustion product. The components of the substantially non-reactive fluid  616  are, at least partly, selected from the group consisting of nitrogen, carbon dioxide, nitrogen oxide, nitrogen dioxide, sulfur oxide, sulfur dioxide, oxygen, and argon. 
     In one embodiment, the ignition source  730  in the burner system  700  includes an ignitor configured to selectably ignite the fuel and oxidant mixture  206 . The ignitor  730  is disposed adjacent to the pilot flame  718  and the fuel and oxidant mixture  206 . 
     In one embodiment, the non-reactive fluid  616  includes flue gas drawn into the fuel and air mixture  206 . For example, the non-reactive fluid  616  may comprise flue gas from downstream of the distal flame holder  102  and combustion reaction products generated by the pilot flame  718 . In another embodiment, the burner system ( 100 ,  500 ,  600 ,  700 ) may include a flue gas recirculation (FGR) element (e.g., path  224 ) that may recirculate a portion of combustion products as flue gas resulting from combustion at or in the distal flame holder  102 . The FGR further may cause the collected flue gas to be directed (or, recirculated) to an area between the fuel nozzle(s)  118  and the distal flame holder  102  to act, at least in part, as a non-reactive fluid  616 . In some embodiments, the recirculated flue gas may include elements that dilute (mix with) the fuel and oxidant mixture. The resulting flue gas-containing fuel and oxidant mixture is then subjected to combustion at the distal flame holder  102 . Consequently, pollutants originally found in the flue gas may be reduced by the further combustion thereof. According to an embodiment, the FGR may include a dedicated flue gas conduit (not shown) having a recirculated flue gas output oriented to provide the recirculated flue gas directly to a position between the distal flame holder  102  and the fuel nozzle(s)  118  where the fuel and oxidant mixture  206  is most likely (from observation) to prematurely ignite. For example, the recirculated flue gas output may direct the recirculated flue gas laterally adjacent (i.e., parallel) to the input face  212  of the distal flame holder  102 . According to an embodiment, (e.g., burner system  100  in  FIG. 1 ), a mixing tube  110  may be disposed between the distal flame holder  102  and the fuel nozzles  118 , and the flue gas recirculation path may direct flue gas at a gap between an end of the mixing tube  110  and a floor of the combustion chamber. Alternatively, the recirculated flue gas output may direct the recirculated flue gas at an angle oblique to an axis of fuel flow from the fuel nozzle(s)  118 . According to another embodiment, the recirculated flue gas output may introduce the recirculated flue gas alongside oxidant or combustion air as part of the fuel and oxidant source ( 202 , in  FIG. 2 ). 
     The burner system  700  may further include the fuel flow control mechanism  620  in fluid connection with the second fuel nozzle  610 . As described with respect to  FIG. 6 , the fuel flow control mechanism  620  may be configured to permit a first fuel flow rate to the second fuel nozzle  610  during a startup period and is configured to permit a second fuel flow rate to the second fuel nozzle  610  after the startup period. The first fuel flow rate of the fuel  614  may be higher than the second fuel flow rate. 
     The intermediate flame holder  702  may be a bluff body or an electrically conductive flame holder disposed across the fuel path between the second fuel nozzle(s)  610  and the distal flame holder  102 . The intermediate flame holder  702  may support a combustion reaction (e.g., intermediate flame  718 ) fueled by a low-rate (compared to a fuel rate of fuel emitted from the first fuel nozzle(s)  118 ) fuel flow emitted from second fuel nozzle(s)  610  and oxidant  106   a  emitted from the oxidant conduit  104 . 
     In some embodiments, the intermediate flame holder  702  may be disposed a distance from the distal flame holder  102  sufficient, at the first fuel flow rate, to cool the combustion products  616  below an auto-ignition temperature of the fuel and oxidant mixture  206 . 
     Another embodiment is described with reference to  FIGS. 1, 6, and 7 . A burner system  100 ,  600 ,  700  may include a distal flame holder  102  configured to hold a combustion reaction ( 302 ) of a fuel  106   b  and an oxidant  106   a.  An oxidant conduit  104  may direct the oxidant  106   a  toward the distal flame holder  102 . A first fuel nozzle  118  may be oriented to direct a first flow of the fuel  106   b  into a combustion volume (e.g.,  204 ) for mixture with the oxidant  106   a  in a dilution region (e.g.,  510 ) between the first fuel nozzle  118  and the distal flame holder  102  when a temperature of the distal flame holder  102  is above a predetermined temperature. A non-reactive fluid source  110  may be oriented to emit a non-reactive fluid  116 ,  616  into the dilution region  510  when the distal flame holder  102  is at an operating temperature. 
     In some embodiments, the non-reactive fluid source  110  may include a second fuel nozzle  610  oriented to emit a second flow of the fuel  614  into the combustion volume (e.g.,  204 ) and/or toward an intermediate flame holder  702  disposed between the second fuel nozzle  610  and the distal flame holder  102 . The intermediate flame holder  702  may be configured to support a flame  718  in the dilution region  510 , the flame  718  produced by combustion of a mixture of the oxidant  106   a  and the second flow of the fuel  614  from the second fuel nozzle  610 . 
     In some embodiments, the non-reactive fluid  616  provided by the non-reactive fluid source  110  may include combustion products  616  of the flame  618 ,  718 . The burner system  700  may further include an ignition source  730  disposed proximate the intermediate flame holder  702  and configured to ignite the mixture of the oxidant  106   a  and the second flow of the fuel  614 . 
     In some embodiments, such a burner system may further include a first valve  619  configured to control a flow rate of the first flow of the fuel  106   b.  In some embodiments, a second valve  620  may be configured to control a flow rate of the second flow of the fuel  614 . The valves  619 ,  620  may be manually adjustable and/or may be electromechanically adjustable. For example, a burner system may further include a controller  622  (possibly included in controller  230  of  FIG. 2 ) having outputs configured to adjust at least one of the first valve  619  and the second valve  620 . The controller  622  may be configured to cause a flow rate of the second flow of the fuel  614  to be lower than a flow rate of the first flow of the fuel  106   b  when the distal flame holder  102  is at the operating temperature. 
     The second fuel nozzle  610  and the first fuel nozzle  118  may be configured to emit the same fuel (e.g.,  106   b  and  614  may be the same fuel). However, the inventors contemplate the possibility that the first and second fuel nozzles  118 ,  610  may deliver different fuels (e.g.,  106   b  and  614  may be different fuels). 
     In some embodiments, such a burner system may further include a heating apparatus, such as heater  228  of  FIG. 2 , configured to heat the distal flame holder  102  to the predetermined temperature and/or a controller, such as controller  230  or  622 , operatively coupled to the heating apparatus  228 . The controller  230 ,  622  may be configured to receive an indication of the temperature of the distal flame holder  102  and to control the heating apparatus  228  based on the indication of the temperature. 
     In some embodiments, the heating apparatus  228  may include the at least portions of the non-reactive fluid source  110 . As suggested above with specific reference to  FIGS. 6 and 7 , the non-reactive fluid source  110  may have multiple modes, e.g., a startup mode and an operating mode. In the startup mode, the fuel  614  may be provided to the second fuel nozzle  610  at a relatively high rate and ignited (e.g., by ignition source  630 ,  730 ) to produce a startup/preheat flame (not shown), thermal energy from the startup/preheat flame may be received by the distal flame holder  102 , heating it to the predetermined temperature. 
     More specifically, with respect to the heating apparatus  228  discussed with respect to  FIG. 2 , in some embodiments, the heating apparatus  228  may include at least the second fuel nozzle  610  configured to direct a second flow of the fuel  614  for mixture and combustion with a portion of the oxidant  106   a  in the dilution region  510 , the combustion of the second flow of the fuel  614  and the oxidant  106   a  may provide thermal energy that heats the distal flame holder  102 . 
     The burner system  600 ,  700  may further include an oxidant supply controller (e.g., as part of controller  230 ,  622 ) configured to control a flow rate of the oxidant  106   a.  The oxidant supply controller  622  may include a first oxidant flow control mechanism  624  configured to control flow of the oxidant  106   a  for mixture with the fuel  106   b,    614  alternately at the first fuel flow rate and the second fuel flow rate and/or a second oxidant flow control mechanism (not shown separately) configured to control flow of the oxidant  106   a  for mixture of the oxidant  106   a  with the fuel  106   b  from the first fuel nozzle  118 . 
     The oxidant supply controller (part of controller  230 ,  622 ) may be further configured to control flow and/or direction of a recirculated flue gas in concert with an external flue gas recirculation element. For example, the oxidant supply controller may control a damper or blower in a recirculated flue gas conduit (not shown) through which the flue gas flows. In another example, the oxidant supply controller may control a shutter or movable duct configured to adjust a direction of output of the recirculated flue gas. 
     A method  800  for inhibiting flashback in a burner system is illustrated in  FIGS. 8A, 8B . In step  810 , an oxidant (e.g.,  106   a ) is supplied to a combustion volume (e.g.,  204 ). A fuel (e.g.,  106   b ) is directed, in step  820 , via a first fuel nozzle (e.g.,  118 ) to a dilution region (e.g.,  510 ) of the combustion volume between the first fuel nozzle and a distal flame holder (e.g.,  102 ). In another step  830 , the oxidant is mixed with the fuel from the first fuel nozzle in the dilution region to provide a mixture (e.g.,  206 ) of the fuel and the oxidant. The method  800  may further include, in step  840  of  FIG. 8A , providing a non-reactive fluid (e.g.,  116 ,  616 ) to the mixture of the fuel and the oxidant in a portion of the dilution region proximate the distal flame holder (e.g.,  102 ). 
     The step  840  of providing the non-reactive fluid (e.g.,  116 ,  616 ) may include, as shown in  FIG. 8B , a step  842  of emitting the fuel from a second fuel nozzle (e.g.,  610 ) at a velocity different from the fuel supplied via the first fuel nozzle. In step  844 , the fuel from the second nozzle may be ignited (e.g., by the ignition source  630 ,  730 ) to combust the oxidant and fuel emitted from the second fuel nozzle in a portion of the dilution region proximate the second fuel nozzle. In step  846 , the combustion reaction (e.g., flame  718 ) supported by the oxidant and fuel from the second fuel nozzle may be held at an intermediate flame holder (e.g.,  702 ). The intermediate flame holder may be disposed proximate the second fuel nozzle. However, in some embodiments the inventors recognize there may be a motivation to position the intermediate flame holder a distance from the second fuel nozzle, or to provide a mechanism for relocating the intermediate flame holder between a plurality of positions. In step  848 , combustion products (e.g.,  616 ) of the flame may be provided to the fuel and oxidant mixture (e.g.,  206 ), changing characteristics of the mixture to prevent flashback. 
     In some embodiments, providing the non-reactive fluid toward the distal flame holder may include emitting a non-reactive gas from a second nozzle (e.g.,  110   a ) into the dilution region (e.g.,  510 ). 
     In some embodiments, step  848  of providing the non-reactive fluid to the mixture of the fuel and the oxidant may include recirculating a flue gas, as the non-reactive fluid, sourced from beyond the distal flame holder. 
     The above described embodiments focus on addressing flashback. In some of the embodiments, an intermediate flame holder (e.g.,  702 ) is implemented. The use of an intermediate flame holder  702  in addition to a distal flame holder  102  is a novel implementation that merits the separate description below. 
     A multi-stage burner according to the disclosure includes two or more flame holders in succession. This differs from a conventional staged burner that introduces fuel and/or oxidant at different stages of the burner. For example, a conventional staged burner may introduce fuel, transport air, and secondary air together for combustion in a primary combustion zone. Staged fuel may be introduced for combustion at a secondary combustion (or reburning) zone and staged air may be introduced for combustion at a tertiary combustion zone. The “stages” therefore conventionally refer to stages of combustion within a particular flame as a result of introduced fuel and/or oxidant. 
     In contrast to a conventional staged combustion, combustion stages in the multi-stage burner of the present disclosure refer to distinct flames supported by distinct flame holders. In an example,  FIG. 9  illustrates a multi-stage burner system  900  having a first or intermediate flame holder  902  and a distal flame holder  102  (e.g., as described above), each configured to simultaneously support combustion during an operation period. The first flame holder  902  may receive fuel (e.g.,  614 ) from at least one fuel nozzle  910 , whereas the distal flame holder  102  may receive fuel (e.g.,  106   b ) from at least one fuel nozzle  118 . 
     In some embodiments, the multi-stage burner system  900  may include a fuel and oxidant source  202  configured to emit fuel (e.g.,  106   b ) and oxidant (e.g.,  106   a ) into a combustion volume, a distal flame holder  102  oriented to receive and ignite a first mixture (e.g.,  206 ) of the fuel and the oxidant downstream of the fuel and oxidant source  202 . At least one intermediate flame holder  902  may be disposed between the fuel and oxidant source  202  and the distal flame holder  102 , and may be oriented to receive a second mixture of the fuel and the oxidant. One or more additional intermediate flame holders  902  may be disposed on a same plane between the fuel nozzle(s)  910  and the distal flame holder  102 , or may be disposed serially. Disposition on a same plane may permit certain regions of the burner  900  to address local combustion inconsistencies of the distal flame holder  102 , such as localized flashback phenomena. Alternatively, multiple intermediate flame holders  902  on a same plane can be used to provide a plurality of intermediate flames (e.g.,  718 ) for preheating with corresponding plural fuel nozzles  910  configured for startup. 
     In some embodiments, a first set of fuel nozzles  118  and a second set of fuel nozzles  910  may each be associated in common with a same oxidant delivery structure, such as oxidant conduit  104 , as illustrated in  FIG. 7 . In other embodiments (not shown), corresponding oxidant delivery structures may be associated with each of the first fuel nozzle(s)  118  and the second nozzle(s)  910 . 
     In still another embodiment, each of plural subsets of the fuel nozzles  118  may respectively be associated with a corresponding oxidant delivery structure. 
     Serially disposed intermediate flame holders (not shown) may be used in implementations that include fuel nozzles  118  disposed at different respective distances from the distal flame holder  102 . 
     In some embodiments, a multi-stage burner system  900  may further include an ignition source  930  disposed proximate the intermediate flame holder  902  and configured to ignite the mixture ( 206 ) of the oxidant and the fuel from the second fuel nozzle  910 . The intermediate flame holder  902  may be configured to hold a flame (not shown in  FIG. 9 ) resulting from the ignited second mixture ( 614 ) of the fuel and the oxidant. 
     The fuel and oxidant source  202  may in some embodiments include a fuel output configured to emit the fuel ( 106   b ) for inclusion in at least one of the first mixture of the fuel and the oxidant ( 614 ) and the second mixture of the fuel and the oxidant ( 206 ) and/or an oxidant output configured to emit oxidant ( 106   a ) for inclusion in the first mixture ( 614 ) of the fuel and the oxidant and the second mixture of the fuel and the oxidant ( 206 ). 
     The fuel output may include two or more fuel nozzles  118 ,  910  each in fluid connection with a fuel supply, e.g., via a fuel supply line  108 . 
     In some embodiments, at least one of the two or more fuel nozzles (e.g., fuel nozzle  910 ) is oriented to direct a portion of fuel toward the at least one intermediate flame holder  902 , and a remaining one or more fuel nozzles (e.g., fuel nozzle(s)  118 ) are oriented to direct fuel toward the distal flame holder  102 . 
     A multi-stage burner system  900  may further include a controller  922  configured to control a rate of the fuel ( 106   b ) directed by the fuel nozzle(s)  910  toward the intermediate flame holder  902 . The controller  922  may control a rate of fuel flow by controlling operation of valves  919 ,  920 . In some embodiments the controller  922  may control flow to the fuel nozzle  910 , supplying fuel to the intermediate flame holder  902  independently from control of fuel supply to the fuel nozzle(s)  118 , supplying fuel to the distal flame holder  102 . In some embodiments, the controller  922  may alternatively or additionally control a flow of oxidant through the oxidant conduit  104  by controlling operation of a damper or blower,  924 . 
     In some embodiments, the intermediate flame holder  902  may be oriented to direct thermal energy released by combustion of the fuel ( 106   b ) and the oxidant ( 106   a ) at the intermediate flame holder  902  toward the distal flame holder  102  to heat the distal flame holder  102  to a predetermined temperature. 
     In some embodiments, the intermediate flame holder  902  may be oriented to direct combustion products released by combustion of the fuel ( 106   b ) and oxidant ( 106   a ) at the intermediate flame holder  902  as a non-reactive fluid for dilution of the first mixture of the fuel and the oxidant ( 206 ) in a region (e.g., dilution region  510 ) between the intermediate flame holder  902  and the distal flame holder  102 . 
     In some embodiments, the intermediate flame holder  902  may include an electrode  950  configured to provide an electrical charge to the second mixture of the fuel ( 106   b ) and the oxidant ( 106   a ). The electrical charge may control a flame conformance characteristic. In some embodiments (not shown), an electrical charge produced by the electrode  950  may constitute the intermediate flame holder  902 . 
       FIG. 10A  shows a method  1000  for utilizing a multi-stage burner system according to the disclosure. The method may include a step  1010  of directing an oxidant (e.g.,  106   a ) into a combustion volume (e.g.,  204 ), and step  1020  of directing a fuel (e.g.,  106   b ) via a first fuel nozzle (e.g.,  910 ) toward an intermediate flame holder (e.g.,  902 ) disposed in a dilution region (e.g.,  510 ) between the first fuel nozzle ( 910 ) and a distal flame holder (e.g.,  102 ). Step  1030  provides holding a flame at the intermediate flame holder (e.g.,  902 ) supported by a mixture of the oxidant and the fuel (e.g.,  206 ) from the first fuel nozzle. If the distal flame holder ( 102 ) is at or above a predetermined temperature T P , (step  1040 ), the fuel is directed (step  1050 ) via a second fuel nozzle (e.g.,  118 ) toward the distal flame holder  102  through the dilution region (e.g.,  510 ). In step  1060 , the oxidant is mixed with the fuel from the second fuel nozzle to provide a second mixture of the fuel and the oxidant for combustion at the distal flame holder ( 102 ). At step  1070 , the second mixture of the fuel and the oxidant is diluted with substantially non-reactive combustion products (e.g.,  616 ) of the intermediate flame or within the dilution region. The substantially non-reactive combustion products (e.g.,  616 ) may be released by the intermediate flame as a non-reactive fluid for dilution of the mixture of the oxidant and the fuel from the first fuel nozzle in the dilution region. At step  1080 , the diluted second mixture of the fuel and the oxidant are burned substantially at the distal flame holder  102 . 
     Certain elements of the method  1000  of  FIG. 10A  are presented in further detail in  FIG. 10B . For example, the step  1020  of directing the fuel via the first fuel nozzle may include, at step  1022 , emitting the fuel from the first fuel nozzle (e.g.,  910 ) at a first fuel flow rate during a startup period in which the distal flame holder ( 102 ) is heated by the flame to the predetermined temperature. At step  2024 , when the distal flame holder ( 102 ) is at the predetermined temperature the method  1000  may include emitting the fuel from the first fuel nozzle at a second fuel flow rate. In some embodiments, the second fuel flow rate may be lower than the first fuel flow rate. Directing the fuel via the first fuel nozzle may include controlling a rate of the fuel directed toward the intermediate flame holder (e.g.,  902 ). 
     The method  1000  may further include a step (not shown) of igniting, proximate the intermediate flame holder, the mixture of the oxidant and the fuel from the first fuel nozzle to produce the flame. Igniting the mixture of the oxidant and the fuel from the first fuel nozzle (e.g.,  910 ) includes providing and using an ignition source (e.g.,  930 ) proximate the intermediate flame holder (e.g.,  902 ), where the ignition source is configured to ignite the mixture of the oxidant and the fuel from the first fuel nozzle. 
     The method  1000  may further include directing thermal energy from the flame toward the distal flame holder to heat the distal flame holder to the predetermined temperature. 
     In some embodiments, the intermediate flame holder may include one or more electrodes configured to produce an electrical charge proximate to and across the intermediate flame holder. 
       FIG. 11A  is a simplified perspective view of a combustion system  1100 , including another alternative distal flame holder  102 , according to an embodiment. The distal flame holder  102  may include a reticulated ceramic distal flame holder, according to an embodiment.  FIG. 11B  is a simplified side sectional diagram of a portion of the reticulated ceramic distal flame holder  102  of  FIG. 11A , according to an embodiment. The distal flame holder  102  having reticulated ceramic, of  FIGS. 11A, 11B , can be implemented in the various combustion systems described herein, according to an embodiment. The distal flame holder  102  may be configured to support a combustion reaction (e.g., combustion reaction  302  of  FIG. 3 ) of the fuel and oxidant mixture  206  received from the fuel and oxidant source  202  at least partially within (e.g., amongst the fiber of the reticulated ceramic distal flame holder  102 . According to an embodiment, such distal flame holder  102  can be configured to support a combustion reaction of the fuel and oxidant mixture  206  upstream, downstream, within, and adjacent to the reticulated ceramic distal flame holder  102 . 
     According to an embodiment, the distal flame holder body  208  can include reticulated fibers  1139 . The reticulated fibers  1139  can define branching perforations  210  that weave around and through the reticulated fibers  1139 . According to an embodiment, the perforations  210  are formed as passages between the reticulated fibers  1139 . 
     According to an embodiment, the reticulated fibers  1139  are formed as a reticulated ceramic foam. According to an embodiment, the reticulated fibers  1139  are formed using a reticulated polymer foam as a template. According to an embodiment, the reticulated fibers  1139  can include alumina silicate. 
     According to an embodiment, the reticulated fibers  1139  can be formed from extruded mullite or cordierite. According to an embodiment, the reticulated fibers  1139  can include Zirconia. According to an embodiment, the reticulated fibers  1139  can include silicon carbide. 
     The term “reticulated fibers” refers to a netlike structure. According to an embodiment, the reticulated fibers  1139  are formed from an extruded ceramic material. In reticulated fiber embodiments, the interaction between the fuel and oxidant mixture  206 , the combustion reaction, and heat transfer to and from the distal 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  1139  form a discontinuous distal flame holder body  208  that allows flow back and forth between neighboring perforations  210 . 
     According to an embodiment, the network of reticulated fibers  1139  is sufficiently open for downstream reticulated fibers  1139  to emit radiation for receipt by upstream reticulated fibers  1139  for the purpose of heating the upstream reticulated fibers  1139  sufficiently to maintain combustion of a fuel and oxidant mixture  206 . Compared to a continuous distal flame holder body  208 , heat conduction paths (such as heat conduction paths  312  in  FIG. 3 ) between reticulated fibers  1139  are reduced due to separation of the reticulated fibers  1139 . This may cause relatively more heat to be transferred from a heat-receiving region or area (such as heat receiving region  306  in  FIG. 3 ) to a heat-output region or area (such as heat output region  310  of  FIG. 3 ) of the reticulated fibers  1139  via thermal radiation (shown as element  304  in  FIG. 3 ). 
     According to an embodiment, individual perforations  210  may extend between an input face  212  to an output face  214  of the distal flame holder  102 . 
     The 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 distal flame holder  102  is configured to support or hold a combustion reaction (see element  302  of  FIG. 3 ) 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 distal 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  1139  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  1139  distal to the fuel nozzle  218  or opposite to the input face  212 . 
     According to an embodiment, the formation of thermal boundary layers  314 , transfer of heat between the distal flame holder body  208  and the gases flowing through the perforations  210 , a characteristic perforation width dimension D, and the length L can each be regarded as related to an average or overall path through the distal flame 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 TRH from the input face  212  to the output face  214  through the distal flame holder  102 . According to an embodiment, the void fraction (expressed as (total distal flame holder  102  volume−reticulated fiber  1139  volume)/total volume)) is about 70%. 
     According to an embodiment, the reticulated ceramic distal flame holder  102  is a tile about 1″×4″×4″. According to an embodiment, the reticulated ceramic distal flame holder  102  includes about 10 pores per square inch, which is to say that line laid across the surface of the tile would cross about 10 pores per inch of line. Other materials and dimensions can also be used for a reticulated ceramic distal flame holder  102  in accordance with principles of the present disclosure. 
     According to an embodiment, the reticulated ceramic distal flame holder  102  can include shapes and dimensions other than those described herein. For example, the distal flame holder  102  can include reticulated ceramic tiles that are larger or smaller than the dimensions set forth above. Additionally, the reticulated ceramic distal flame holder  102  can include shapes other than generally cuboid shapes. 
     According to an embodiment, the reticulated ceramic distal 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. The multiple reticulated ceramic tiles can collectively form a single distal flame holder  102 . Alternatively, each reticulated ceramic tile can be considered a distinct distal flame holder  102 . 
     Turning now to  FIG. 12 , a burner system  1200  may include a distal flame holder  102 , a plurality of main fuel nozzles  118 , one or more distal pilot fuel nozzles  1204 , and a mixing tube  1212 . The main fuel nozzles  118  are arranged in fluid connection with a main fuel source  1232 . According to an embodiment, flow of main fuel from the main fuel source  1232  may be controlled via a main fuel control valve  1236 . The one or more distal pilot fuel nozzles  1204  may be arranged in fluid connection with a pilot fuel source  1230 . According to an embodiment, flow of pilot fuel from the pilot fuel source  1230  may be controlled via a pilot fuel control valve  1234 . 
     The pilot fuel nozzle  1204  is configured to support a pilot flame by outputting a pilot fuel received via a pilot fuel pipe  1210  from the pilot fuel source  1230 . The pilot fuel pipe  1210  may be disposed inside the mixing tube  1212  or—advantageously for maintenance, temperature regulation, etc.—outside the mixing tube  1212 . In some embodiments, the pilot fuel pipe  1210  may form a portion of a support for the mixing tube  1212 . According to an embodiment, the pilot fuel nozzle  1204  is supported by and receives fuel via the fuel pipe  1220 . 
     The fuel pipe  1220  extends into the furnace volume  1201  via the opening  1240  in the floor  1238  of the furnace. The pilot fuel nozzle  1204  may be formed in any of several shapes. For example, in  FIG. 12 , the pilot fuel nozzle  1204  is formed in a Y shape 
     According to an embodiment, the pilot fuel nozzle  1204  defines a plurality of fuel orifices  1218  having a large collective area to collectively support a low momentum pilot flame (not shown). In an embodiment, the main fuel output by the main fuel nozzles  118  and combustion air form a combustible mixture that expands in breadth as it flows from a proximal position of the main fuel nozzles  118  to the distal position of the pilot fuel nozzle  1204 . The plurality of fuel orifices  1218  may be disposed across the furnace volume  1201  sufficiently broadly to cause contact of the pilot flame with the main fuel and combustion air mixture across the breadth of the combustible mixture. In another embodiment, the main fuel nozzles  118  may be configured to output fuel in co-flow with the air. 
     According to an embodiment, the primary fuel nozzle  1204  includes a fuel manifold having a plurality of segments  1219  joined together, each segment  1219  having a plurality of fuel orifices  1218  configured to pass fuel from inside the fuel manifold to the furnace volume  1101 . The plurality of segments  1219  may be formed as respective tubes configured to freely pass the fuel delivered from the fuel pipe  1210  into the fuel manifold. In one embodiment (e.g., as in  FIG. 12 ), at least a portion of the tubes is arranged as spokes radiating from a center disposed substantially at a centerline along the axis. In another embodiment, at least a portion of the tubes is arranged as an “X”, a rectangle, an “H”, a wagon wheel, or a star. 
     According to an embodiment, the pilot fuel nozzle  1204  includes a manifold including a curvilinear tube. In an embodiment, the curvilinear tube is arranged as a spiral, “ ”, “ ”, or “ ”. 
     According to an embodiment, the mixing tube  1212  may be arranged about a longitudinal axis of flow between the main fuel nozzles  118  and the distal flame holder  102 . According to an embodiment, the mixing tube  1212  may include a bell-shaped or flared portion  1214  at an end proximate the main fuel nozzles  118 . The bell-shaped or flared portion  1214  may be disposed a predetermined distance from a floor  1238  of the burner system, and may be configured to receive at least the combustion air via an opening  1240  in the floor  1238 . As described earlier in this disclosure, a source of non-reactive fluid in contemplated. The inventors have observed that the introduction of a mixing tube facilitates a recirculation of flue gas—as a substantially non-reactive fluid—from downstream of the distal flame holder  102 , and/or including combustion products of a pilot flame held at the pilot fuel nozzle. The flue gas is educed to a proximal end of the mixing tube  1212  by a flow of main fuel and combustion oxidant between the floor  1238  and the distal flame holder  102  through the mixing tube  1212 . The recirculated flue gas, and mixes with the fuel and the combustion air before reaching the distal flame holder  102 . The non-reactive elements of the resulting mixture minimize a potential for flashback upstream from the distal flame holder  102  while permitting additional combustion of the reactive elements of the flue gas, thus reducing, e.g., NOx and other potential pollutants. 
     Those having skill in the art will recognize the  FIG. 12  should not be relied upon as representing appropriate scale, relative dimensions, shapes, etc. For example, the mixing tube  1212  may have a diameter appropriate for providing a mixture of fuel and oxidant (e.g., fuel and oxidant mixture  206 ) to at least most of the input face (e.g., input face  212 ) of the distal flame holder  102 . The opening at the end of the mixing tube  1212  closest to the main fuel nozzles  118  may have a largest diameter sized in correspondence to either the opening  1240  in the floor  1238  or sufficient to receive fuel input from each of the main fuel nozzles  118 . For example, in an embodiment that includes the bell-shaped or flared portion  1214 , the largest diameter of the bell-shaped or flared portion  1214  may correspond to either the opening  1240  in the floor  1238  or may correspond to at least the farthest distance between main fuel nozzles  118 . A length of the mixing tube may be selected to permit sufficient time and/or distance for appropriate mixing of the fuel and the oxidant before reaching the distal 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.