Patent Publication Number: US-2017350591-A1

Title: Burner system with a perforated flame holder and a plurality of fuel sources

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Continuation-in-Part of co-pending International Patent Application No. PCT/US2016/018343, entitled “BURNER SYSTEM WITH A PERFORATED FLAME HOLDER AND A PLURALITY OF FUEL SOURCES,” filed Feb. 17, 2016 (docket number 2651-206-04); co-pending International Patent Application No. PCT/US2016/018343 claims priority benefit from U.S. Provisional Patent Application No. 62/117,443, entitled “BURNER SYSTEM WITH A PERFORATED FLAME HOLDER AND A PLURALITY OF FUEL SOURCES,” filed Feb. 17, 2015 (docket number 2651-206-02); each of which, to the extent not inconsistent with the disclosure herein, is incorporated herein by reference. 
    
    
     SUMMARY 
     According to an embodiment, a burner system includes a perforated flame holder and a plurality of fuel and oxidant sources configured to collectively provide a fuel and oxidant mixture to the perforated flame holder. The perforated flame holder is configured to hold a combustion reaction supported by the fuel and oxidant mixture. 
     According to an embodiment, an apparatus for supporting combustion includes a plurality of fuel nozzles configured to emit a corresponding plurality of fuel streams and a perforated flame holder aligned to receive the plurality of fuel streams into respective portions of the perforated flame holder. The perforated flame holder has elongated apertures each having a transverse dimension D passing through the perforated flame holder from an input face to an output face. The perforated flame holder includes a plurality of segments separated from one another by respective gaps of at least two times D. 
     According to an embodiment, a method for supporting combustion includes emitting a plurality of fuel streams from a corresponding plurality of fuel nozzles and receiving the plurality of fuel streams into respective portions of a perforated flame holder. The perforated flame holder has elongated apertures each having a transverse dimension D passing through the perforated flame holder from an input face to an output face. The perforated flame holder includes a plurality of sections separated from one another by respective gaps of at least two times D. Combustion of the respective fuel streams is supported in the portions of the perforated flame holder. 
     According to an embodiment, a burner apparatus includes a plurality of fuel nozzles configured to output respective fuel streams and a perforated flame holder configured to receive the respective fuel streams at respective portions of the perforated flame holder and to support combustion in the respective portions of the perforated flame holder. A second portion of the plurality of fuel nozzles is configured to cooperate with a corresponding second portion of the perforated flame holder to cause an output of at least a portion of heat sufficient to maintain a first portion of the perforated flame holder at an elevated operating temperature when a first portion of fuel nozzles different from the second portion of fuel nozzles is controlled to stop outputting a respective first fuel stream to the first portion of the perforated flame holder. 
     According to an embodiment, a method for operating a burner includes delivering a plurality of fuel streams from a corresponding plurality of fuel nozzles to a perforated flame holder for combustion in the perforated flame holder. At least one first fuel stream from a first fuel nozzle that delivers fuel to a first portion of the perforated flame holder is stopped. An elevated operating temperature of the first portion of the perforated flame holder is maintained at least in part by continued combustion in other portions of the perforated flame holder. 
     According to an embodiment, a burner assembly for a furnace includes fuel nozzles configured to eject n streams of fuel in respective directions and k segments of a perforated flame holder disposed where the fuel streams will impinge on the segments. According to an embodiment, each segment includes one or more perforated flame holder tiles. The term “tile” shall be construed to include a single tile or a plurality of tiles placed in a direct contact with each other, depending on the context of an embodiment. Each segment of the perforated flame holder further includes apertures passing through the perforated flame holder, the apertures being smaller across than a surface of any of the segments, each segment including plural apertures. Each fuel stream impinges substantially on a single segment or on a plurality of segments. 
     According to an embodiment, a combustion method includes aligning a plurality of fuel nozzles to stream fuel toward a perforated flame holder such that each fuel stream impinges substantially on one respective segment of the perforated flame holder, streaming the fuel, and arranging combustion parameters such that combustion takes place substantially inside the perforated flame holder. In an embodiment, the method also includes controlling the streaming to individually vary a rate of fuel streaming from each fuel nozzle. 
     According to an embodiment, a method includes outputting a first fuel stream from a first fuel nozzle onto a first portion of a perforated flame holder positioned in a furnace volume, outputting an oxidant into the furnace volume, and supporting a combustion reaction of the first fuel stream and the oxidant in the first portion of the perforated flame holder. The method includes heating, with the combustion reaction in the first portion of the perforated flame holder, a second portion of the perforated flame holder to a threshold temperature. 
     According to an embodiment an apparatus for supporting combustion includes a first fuel nozzle configured to output a first fuel stream into a furnace volume, a second fuel nozzle configured to selectively output a second fuel stream into the furnace volume, and an oxidant source configured to output an oxidant into the furnace volume. The apparatus for supporting combustion also includes a perforated flame holder positioned in the furnace volume. The perforated flame holder includes a first portion aligned to receive the first fuel stream and configured to support a combustion reaction of the first fuel stream and the oxidant within the first portion. The perforated flame holder includes a second portion aligned to receive the second fuel stream and configured to support a combustion reaction of the second fuel stream and the oxidant within the second portion when the second fuel nozzle outputs the second fuel stream. The perforated flame holder, the first fuel nozzle, and the second fuel nozzle are configured to cooperate to operate the perforated flame holder in a first heat output mode by outputting the first fuel stream and by not outputting the second fuel stream, and to operate in a second heat output mode by outputting both the first and the second fuel streams. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a burner system including a perforated flame holder and a plurality of fuel and oxidant sources, according to an embodiment. 
         FIG. 2  is a simplified perspective view of a burner system including a perforated flame holder, according to an embodiment. 
         FIG. 3  is a side sectional diagram of a portion of the perforated flame holder of  FIGS. 1 and 2 , according to an embodiment. 
         FIG. 4  is a flow chart showing a method for operating a burner system including the perforated flame holder of  FIGS. 1, 2 and 3 , according to an embodiment. 
         FIG. 5  is a simplified perspective view of a burner system including a plurality of perforated flame holders and a plurality of fuel nozzles, according to an embodiment. 
         FIG. 6  is a simplified perspective view of a burner system illustrating segments of a perforated flame holder, according to an embodiment. 
         FIG. 7  is a graphical depiction of a fuel mixture delivered to a perforated flame holder by two fuel nozzles, shown as two negative-exponential functions and their sum. 
         FIG. 8  is an elevational and cross-sectional view of a furnace incorporating a perforated flame holder burner system, according to an embodiment. 
         FIG. 9  is a flow chart of a method for supporting combustion, according to an embodiment. 
         FIG. 10A  is a simplified perspective view of a combustion system including a reticulated ceramic perforated flame holder, according to an embodiment. 
         FIG. 10B  is a simplified side sectional diagram of a portion of the reticulated ceramic perforated flame holder of  FIG. 10A , according to an embodiment. 
         FIG. 11  is a flow chart of a method for operating 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. 
     Perforated flame holders are described in the Applicant&#39;s U.S. patent application Ser. No. 14/762,155, entitled “FUEL COMBUSTION SYSTEM WITH A PERFORATED REACTION HOLDER,” filed Jul. 20, 2015, (agents&#39; docket number 2651-188-03); which to the extent not inconsistent with the disclosure herein, is incorporated by reference. 
       FIG. 1  is a diagram of a burner system  100  including a perforated flame holder  102  and a plurality of fuel and oxidant sources  104   a,    104   b  disposed in a furnace volume  106 , according to an embodiment. Each fuel and oxidant source  104   a,    104   b  is configured to output a respective stream of fuel and oxidant mixture  108   a,    108   b  aligned to impinge upon respective portions of the perforated flame holder  102 , according to an embodiment. Optionally, each fuel and oxidant source  104   a,    104   b  can be formed as a respective paired fuel nozzle  110   a,    110   b  and oxidant (e.g., combustion air) source  112   a,    112   b,  as shown, each respective pair referred to as a fuel and oxidant source  104   a,    104   b  herein. 
     It can be desirable for the perforated flame holder  102  to be spaced away from the fuel nozzles  110   a,    110   b  by a dilution distance D D . As fuel travels through the dilution distance, it entrains more and more oxidant, such as combustion air (and optionally, flue gas or other diluent), until it attains a desired dilution and uniformity when it reaches the perforated flame holder  102 . The dilution distance D D  is a function of fuel nozzle diameter (the inside diameter of the nozzle orifice). As will be described in greater detail below, it can be desirable for the dilution distance D D  to be at least 100 to (preferably) 200 nozzle diameters in distance. 
     According to an embodiment, providing a plurality of fuel nozzles  110   a ,  110   b  allows each individual nozzle diameter to be smaller, while still delivering a desired total fuel flow at a given fuel pressure. For applications where it is desirable to limit the distance at which the perforated flame holder  102  is positioned away from the fuel nozzles  110   a,    110   b ; providing the plurality of fuel nozzles, in and of itself, can create value. For example, this can be useful when retrofitting an existing burner application that may be optimized to receive radiant heat energy at a relatively short distance above a furnace floor (not shown). 
     Optionally, each fuel nozzle  110   a,    110   b  (or subset of fuel nozzles  110   a ,  110   b ) may be separately controlled. For example, a plurality of fuel control valves  114   a,    114   b  may be operatively coupled to a respective nozzle  110   a,    110   b  or group of nozzles. 
     According to another embodiment, providing the plurality of fuel control valves  114   a,    114   b  can be useful for increasing turndown of the burner system  100 . Turndown (also known as turndown ratio) is defined as the maximum firing rate divided by the actual firing rate, and refers to the ability for a furnace to operate at a reduced total heat output, perhaps responsive to a reduced thermal load. Higher turndown means that a furnace can be operated at lower heat release rates and is better able to tolerate changes in thermal load. According to an embodiment, the burner system  100  includes a plurality of fuel control valves  114   a,    114   b  configured to provide separate control of fuel flow to a corresponding plurality of fuel nozzles  110   a,    110   b  or groups of fuel nozzles. Optionally, the plurality of fuel control valves  114   a,    114   b  can be manually actuated valves. In another embodiment, at least some of the plurality of fuel control valves  114   a ,  114   b  are controlled by a valve actuator portion of the valve  114   a,    114   b  responsive to receipt of a control signal. The control signal can be received via electrical, pneumatic, hydraulic, radio, or other control medium. 
     When a fuel nozzle  110   b  is turned off, it stops outputting a flow of fuel and oxidant mixture  108   b  to a portion  102   b  of the perforated flame holder  102 . With no fuel (and optionally, no oxidant) arriving at the portion  102   b  of the perforated flame holder  102 , combustion within the portion  102   b  of the perforated flame holder  102  stops. This reduces the thermal output of the system. However, a neighboring region portion  102   a  of the perforated flame holder  102  that continues to receive a flow of the fuel and oxidant mixture  108   a  continues to support combustion and output heat. 
     The operating portion  102   a  of the perforated flame holder  102  can be arranged to provide heat energy to an idle portion  102   b  of the perforated flame holder  102 . This can cause the idle portion  102   b  to be in a “hot stand-by” mode, and, according to some embodiments, available for substantially instantaneous start-up. 
     According to an embodiment, the burner system  100  includes a controller  116  operatively coupled to each of the plurality of fuel control valves  114   a,    114   b . Responsive to a change in thermal output command received through a data interface  118 , the controller  116  may actuate one or more of the plurality of fuel control valves  114   a,    114   b  to trim the amount of total fuel delivered to the perforated flame holder  102 , and thereby control the total amount of heat or thermal energy output by the burner system  100 . 
     The burner system  100  can include a heater  120  configured to preheat the perforated flame holder  102  to an (start-up) operating temperature. Various heater embodiments are contemplated and described below. 
     According to an embodiment, the burner system  100  is equipped with the heater  120  configured to heat only a portion  102   a  of the perforated flame holder  102 . According to an embodiment, at system start-up, the controller  116  first drives the heater  120  to heat the first portion  102   a  of the perforated flame holder  102 . Upon reaching a start-up temperature (referred to as T s  in conjunction with  FIG. 4 ) at the first portion  102   a  of the perforated flame holder  102 , the controller  116  actuates the heater  120  (which may include a start-up flame holder or an electrical resistance heater, for example) and/or a corresponding fuel nozzle  110   a  to output fuel and oxidant mixture  108   a  to the portion  102   a  of the perforated flame holder that is sufficiently hot to support combustion. Subsequently, after the first portion  102   a  of the perforated flame holder outputs heat energy to a second portion  102   b  of the perforated flame holder  102 , the controller  116  may drive a second fuel nozzle  110   b  to output the corresponding fuel and oxidant mixture  108   b  to the second portion  102   b  of the perforated flame holder  102 . In an embodiment, a relatively small heater  120  can provide sufficient heat energy to enable start-up of a relatively large perforated flame holder  102 , albeit over a sequence of steps. 
     According to an embodiment, the burner system  100  includes one or more sensors  122  configured to measure temperature or other combustion parameter(s) in one or more portions  102   a,    102   b  of the perforated flame holder  102 . According to an embodiment, the controller  116  is driven by computer instructions, carried (at least remotely) by a non-transitory computer readable medium, to take action responsive to receiving a signal or data from the sensor(s)  122  corresponding to a sensed combustion condition. 
     For example, if the portion  102   b  of the perforated flame holder  102  breaks or otherwise loses its ability to maintain stable combustion, the sensor(s)  122  can sense the condition and transmit data or a signal to the controller  116 . The controller  116  can responsively actuate a fuel control valve  114   b  to cause the corresponding fuel nozzle  110   b  to stop outputting fuel to the affected portion  102   b  of the perforated flame holder  102 . 
     According to another example, if a sensor  122  detects a drop in perforated flame holder  102  temperature in a portion  102   b,  and the fuel nozzle  110   b  is outputting a high flow of fuel, then the controller can issue a command to open a fuel control valve  114   a  to provide fuel and oxidant mixture  108   a  to another portion  102   a  of the perforated flame holder  102  to cause combustion to be initiated in the other portion  102   a.  Other things being equal, initiating combustion in another portion  102   a  of the perforated flame holder  102  causes an increase in total heat output by the burner system  100 . Thus, a decrease in perforated flame holder  102  temperature at an operating portion  102   b  temperature caused by an increased cooling load can be responded to by increasing the total heat output of the burner system  100 . 
     According to an embodiment, the fuel nozzle  110   a  is a first fuel nozzle configured to output a first fuel stream  108   a  into a furnace volume  106 . The fuel nozzle  110   b  is a second fuel nozzle configured to selectively output a second fuel stream  108   b  into the furnace volume  106 . The oxidant sources  112   a,    112   b  are configured to output an oxidant into the furnace volume  106 . The perforated flame holder  102  includes the first portion  102   a  and the second portion  102   b . The first portion  102   a  is aligned to receive the first fuel stream  108   a  and is configured to support a combustion reaction of the first fuel stream  108   a  and the oxidant within the first portion  102   a.  The second portion  102   b  is aligned to receive the second fuel stream  108   b  and is configured to support a combustion reaction of the second fuel stream  108   b  and the oxidant within the second portion  102   b  when the second fuel nozzle  110   b  outputs the second fuel stream  108   b.  The perforated flame holder  102 , the first fuel nozzle  110   a,  and the second fuel nozzle  110   b  are configured to cooperate to operate the perforated flame holder  102  in a first heat output mode by outputting the first fuel stream  108   a  and by not outputting the second fuel stream  108   b.  The perforated flame holder  102 , the first fuel nozzle  110   a,  and the second fuel nozzle  110   b  cooperate to operate the perforated flame holder  102  in a second heat output mode by outputting both the first and the second fuel streams  108   a,    108   b.    
     According to an embodiment, in the first heat output mode the combustion reaction is not present in the second portion  102   b  because the second fuel nozzle  110   b  is not outputting the second fuel stream  108   b.  In the first heat output mode, the combustion reaction of the first fuel stream  108   a  and the oxidant in the first portion  102   a  is present. Thus, with only a portion of the perforated flame holder holding a combustion reaction, the perforated flame holder  102  outputs less heat than when both portions  102   a,    102   b  of the perforated flame holder  102  hold the combustion reaction. 
     According to an embodiment, in the second heat output mode, the first fuel nozzle  110   a  outputs the first fuel stream  108   a  onto the first portion  102   a  of the perforated flame holder  102 . In the second heat output mode, the second fuel nozzle  110   b  outputs the second fuel stream  108   b  onto the second portion  102   b  of the perforated flame holder  102 . The first portion  102   a  supports a combustion reaction of the first fuel stream  108   a  and the oxidant within the first portion  102   a.  The second portion  102   b  supports a combustion reaction of the second fuel stream  108   b  and the oxidant within the second portion  102   b.  Thus, with both the first portion  102   a  and the second portion  102   b  holding a combustion reaction in the second heat output mode, the perforated flame holder  102  outputs more heat than when only a single portion of the perforated flame holder  102  supports a combustion reaction. 
     According to an embodiment, because the perforated flame holder  102  is able to selectively operate in the first heat output mode and in the second heat output mode, the burner system  100  has a very high turndown ratio. This enables the burner system  100  to meet a variety of operating conditions for a variety of circumstances. 
     According to an embodiment, in the first heat output mode, second portion  102   b  is configured to be heated by the combustion reaction of the first fuel stream  108   a  and the oxidant, even though the second portion  102   b  does not hold a combustion reaction. The transfer of heat from the first portion  102   a  to the second portion  102   b  can maintain the second portion  102   b  at a threshold temperature. The threshold temperature corresponds to a temperature at which the second portion  102   b  can ignite and support a combustion reaction of the second fuel stream  108   b  and the oxidant when the second fuel nozzle begins outputting the second fuel stream  108   b  onto the second portion  102   b.  Thus, according to an embodiment, the burner system  100  can very quickly transfer from the first heat output mode to the second heat output mode by simply outputting the second fuel stream  108   b  from the second fuel nozzle  110   b  onto the second portion  102   b.  Because the second portion  102   b  has already been heated to the threshold temperature, the second portion  102   b  is ready to support the combustion reaction of the second fuel stream  108   b  and the oxidant. 
     According to an embodiment, the oxidant sources  112   a,    112   b  are configured to selectively output oxidant into the furnace volume  106 . When the first fuel nozzle  110   a  outputs the first fuel stream  108   a,  the oxidant source  112   a  can output oxidant into the furnace volume  106 . When the second fuel nozzle  110   b  outputs the second fuel stream  108   b,  the oxidant source  112   b  can output oxidant into the furnace volume  106 . Alternatively, both oxidant sources  112   a ,  112   b  can output oxidant even if only one of the fuel nozzles  110   a,    110   b  is outputting a fuel stream  108   a,    108   b.  Alternatively, the burner systems  100  may include only a single oxidant source  112  configured to output oxidant into the furnace volume  106 . The single oxidant source  112  can be configured to adjust the flow of oxidant into the furnace volume when one of the fuel streams  108   a ,  108   b  begins or ceases. The single oxidant source  112  can be configured to maintain a constant ratio of the fuel to oxidant in the furnace volume  106  by adjusting the flow of oxidant when fuel streams  108   a,    108   b  are initiated or stopped. 
     According to an embodiment, the burner system  100  can include a damper (not shown) configured to adjust the flow of the oxidant into the furnace volume  106 . Alternatively, the burner system  100  can include another mechanism for adjusting the flow of oxidant into the furnace volume  106 . 
     According to an embodiment, the first portion  102   a  can be a first burner tile. The second portion can be a second burner tile. According to an embodiment, the first portion  102   a  and the second portion  102   b  are separated by a gap. The burner tiles can include reticulated ceramic burner tiles ( FIG. 10 a   ,  FIG. 10 b   ) or other kinds of porous burner tiles. 
     According to an embodiment, burner system  100  can include multiple first fuel nozzles  110   a  configured to selectively output first fuel streams  108   a  onto the first portion  102   a  of the perforated flame holder  102 . According to an embodiment, the burner system  100  can include multiple second fuel nozzles  110   b  configured to selectively output second fuel streams  108   b  onto the second portion  102   b  of the perforated flame holder  102 . Thus, the first portion  102   a  can support a combustion reaction of the multiple first fuel streams  108   a  when the burner system  100  is operated to support a combustion reaction in the first portion  102   a  of the perforated flame holder  102 . The second portion  102   b  can support a combustion reaction of the multiple second fuel streams  108   b  when the burner system  100  is operated to support a combustion reaction in the second portion  102   b  of the perforated flame holder  102 . 
       FIG. 2  is a simplified diagram of a burner system  200  including a perforated flame holder  102  configured to hold a combustion reaction, according to an embodiment. As used herein, the terms perforated flame holder, perforated reaction holder, porous flame holder, porous reaction holder, duplex, and duplex tile shall be considered synonymous unless further definition is provided. 
     Experiments performed by the inventors have shown that perforated flame holders  102  described herein can support very clean combustion. Specifically, in experimental use of systems  200  ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (O 2 ) concentration with undetectable carbon monoxide (CO) at stack temperatures typical of industrial furnace applications (1400-1600° F.). Moreover, these results did not require any extraordinary measures such as selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), water/steam injection, external flue gas recirculation (FGR), or other heroic extremes that may be required for conventional burners to even approach such clean combustion. 
     According to embodiments, the burner system  200  includes a fuel and oxidant source  202  disposed to output fuel and oxidant into a furnace volume  204  to form a fuel and oxidant mixture  206 . As used herein, the terms fuel and oxidant mixture and fuel stream may be used interchangeably and considered synonymous depending on the context, unless further definition is provided. As used herein, the terms combustion volume, combustion chamber, furnace volume, and the like shall be considered synonymous unless further definition is provided. The perforated flame holder  102  is disposed in the furnace volume  204  and positioned to receive the fuel and oxidant mixture  206 . 
       FIG. 3  is a side sectional diagram  300  of a portion of the perforated flame holder  102  of  FIGS. 1 and 2 , according to an embodiment. Referring to  FIGS. 2 and 3 , the perforated flame holder  102  includes a perforated flame holder body  208  defining a plurality of perforations  210  aligned to receive the fuel and oxidant mixture  206  from the fuel and oxidant source  202 . As used herein, the terms perforation, pore, aperture, elongated aperture, and the like, in the context of the perforated flame holder  102 , shall be considered synonymous unless further definition is provided. The perforations  210  are configured to collectively hold a combustion reaction  302  supported by the fuel and oxidant mixture  206 . 
     The fuel can include hydrogen, a hydrocarbon gas, a vaporized hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel can be a single species or can include a mixture of gas(es), vapor(s), atomized liquid(s), and/or pulverized solid(s). For example, in a process heater application the fuel can include fuel gas or byproducts from the process that include carbon monoxide (CO), hydrogen (H 2 ), and methane (CH 4 ). In another application the fuel can include natural gas (mostly CH 4 ) or propane (C 3 H 8 ). In another application, the fuel can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant can include oxygen carried by air, flue gas, and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein. 
     According to an embodiment, the perforated flame holder body  208  can be bounded by an input face  212  disposed to receive the fuel and oxidant mixture  206 , an output face  214  facing away from the fuel and oxidant source  202 , and a peripheral surface  216  defining a lateral extent of the perforated flame holder  102 . The plurality of perforations  210  which are defined by the perforated flame holder body  208  extend from the input face  212  to the output face  214 . The plurality of perforations  210  can receive the fuel and oxidant mixture  206  at the input face  212 . The fuel and oxidant mixture  206  can then combust in or near the plurality of perforations  210  and combustion products can exit the plurality of perforations  210  at or near the output face  214 . 
     According to an embodiment, the perforated flame holder  102  is configured to hold a majority of the combustion reaction  302  within the perforations  210 . For example, on a steady-state basis, more than half the molecules of fuel output into the furnace volume  204  by the fuel and oxidant source  202  may be converted to combustion products between the input face  212  and the output face  214  of the perforated flame holder  102 . According to an alternative interpretation, more than half of the heat or thermal energy output by the combustion reaction  302  may be output between the input face  212  and the output face  214  of the perforated flame holder  102 . As used herein, the terms heat, heat energy, and thermal energy shall be considered synonymous unless further definition is provided. As used above, heat energy and thermal energy refer generally to the released chemical energy initially held by reactants during the combustion reaction  302 . As used elsewhere herein, heat, heat energy and thermal energy correspond to a detectable temperature rise undergone by real bodies characterized by heat capacities. Under nominal operating conditions, the perforations  210  can be configured to collectively hold at least 80% of the combustion reaction  302  between the input face  212  and the output face  214  of the perforated flame holder  102 . In some experiments, the inventors produced a combustion reaction  302  that was apparently wholly contained in the perforations  210  between the input face  212  and the output face  214  of the perforated flame holder  102 . According to an alternative interpretation, the perforated flame holder  102  can support combustion between the input face  212  and output face  214  when combustion is “time-averaged.” For example, during transients, such as before the perforated flame holder  102  is fully heated, or if too high a (cooling) load is placed on the system, the combustion may travel somewhat downstream from the output face  214  of the perforated flame holder  102 . Alternatively, if the cooling load is relatively low and/or the furnace temperature reaches a high level, the combustion may travel somewhat upstream of the input face  212  of the perforated flame holder  102 . 
     While a “flame” is described in a manner intended for ease of description, it should be understood that in some instances, no visible flame is present. Combustion occurs primarily within the perforations  210 , but the “glow” of combustion heat is dominated by a visible glow of the perforated flame holder  102  itself. In other instances, the inventors have noted transient “huffing” or “flashback” wherein a visible flame momentarily ignites in a region lying between the input face  212  of the perforated flame holder  102  and the fuel nozzle  218 , within the dilution region D D . Such transient huffing or flashback is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations  210  of the perforated flame holder  102 , between the input face  212  and the output face  214 . In still other instances, the inventors have noted apparent combustion occurring downstream from the output face  214  of the perforated flame holder  102 , but still a majority of combustion occurred within the perforated flame holder  102  as evidenced by continued visible glow from the perforated flame holder  102  that was observed. 
     The perforated flame holder  102  can be configured to receive heat from the combustion reaction  302  and output a portion of the received heat as thermal radiation  304  to heat-receiving structures (e.g., furnace walls and/or radiant section working fluid tubes) in or adjacent to the furnace volume  204 . As used herein, terms such as radiation, thermal radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody-type radiation of electromagnetic energy, primarily at infrared wavelengths, but also at visible wavelengths owing to elevated temperature of the perforated flame holder body  208 . 
     Referring especially to  FIG. 3 , the perforated flame holder  102  outputs another portion of the received heat to the fuel and oxidant mixture  206  received at the input face  212  of the perforated flame holder  102 . The perforated flame holder body  208  may receive heat from the combustion reaction  302  at least in heat receiving regions  306  of perforation walls  308 . Experimental evidence has suggested to the inventors that the position of the heat receiving regions  306 , or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls  308 . In some experiments, the location of maximum receipt of heat was apparently between ⅓ and ½ of the distance from the input face  212  to the output face  214  (i.e., somewhat nearer to the input face  212  than to the output face  214 ). The inventors contemplate that the heat receiving regions  306  may lie nearer to the output face  214  of the perforated flame holder  102  under other conditions. Most probably, there is no clearly defined edge of the heat receiving regions  306  (or for that matter, the heat output regions  310 , described below). For ease of understanding, the heat receiving regions  306  and the heat output regions  310  will be described as particular regions  306 ,  310 . 
     The perforated flame holder body  208  can be characterized by a heat capacity. The perforated flame holder body  208  may hold thermal energy from the combustion reaction  302  in an amount corresponding to the heat capacity multiplied by temperature rise, and transfer the thermal energy from the heat receiving regions  306  to heat output regions  310  of the perforation walls  308 . Generally, the heat output regions  310  are nearer to the input face  212  than are the heat receiving regions  306 . According to one interpretation, the perforated flame holder body  208  can transfer heat from the heat receiving regions  306  to the heat output regions  310  via thermal radiation, depicted graphically as  304 . According to another interpretation, the perforated flame holder body  208  can transfer heat from the heat receiving regions  306  to the heat output regions  310  via heat conduction along heat conduction paths  312 . The inventors contemplate that multiple heat transfer mechanisms including conduction, radiation, and possibly convection may be operative in transferring heat from the heat receiving regions  306  to the heat output regions  310 . In this way, the perforated flame holder  102  may act as a heat source to maintain the combustion reaction  302 , even under conditions where a combustion reaction  302  would not be stable when supported from a conventional flame holder. 
     The inventors believe that the perforated flame holder  102  causes the combustion reaction  302  to begin within thermal boundary layers  314  formed adjacent to walls  308  of the perforations  210 . Insofar as combustion is generally understood to include a large number of individual reactions, and since a large portion of combustion energy is released within the perforated flame holder  102 , it is apparent that at least a majority of the individual reactions occur within the perforated flame holder  102 . As the relatively cool fuel and oxidant mixture  206  approaches the input face  212 , the flow is split into portions that respectively travel through individual perforations  210 . The hot perforated flame holder body  208  transfers heat to the fluid, notably within thermal boundary layers  314  that progressively thicken as more and more heat is transferred to the incoming fuel and oxidant mixture  206 . After reaching a combustion temperature (e.g., the auto-ignition temperature of the fuel), the reactants continue to flow while a chemical ignition delay time elapses, over which time the combustion reaction  302  occurs. Accordingly, the combustion reaction  302  is shown as occurring within the thermal boundary layers  314 . As flow progresses, the thermal boundary layers  314  merge at a merger point  316 . Ideally, the merger point  316  lies between the input face  212  and output face  214  that define the ends of the perforations  210 . At some position along the length of a perforation  210 , the combustion reaction  302  outputs more heat to the perforated flame holder body  208  than it receives from the perforated flame holder body  208 . The heat is received at the heat receiving region  306 , is held by the perforated flame holder body  208 , and is transported to the heat output region  310  nearer to the input face  212 , where the heat is transferred into the cool reactants (and any included diluent) to bring the reactants to the ignition temperature. 
     In an embodiment, each of the perforations  210  is characterized by a length L defined as a reaction fluid propagation path length between the input face  212  and the output face  214  of the perforated flame holder  102 . As used herein, the term reaction fluid refers to matter that travels through a perforation  210 . Near the input face  212 , the reaction fluid includes the fuel and oxidant mixture  206  (optionally including nitrogen, flue gas, and/or other “non-reactive” species). Within the combustion reaction region, the reaction fluid may include plasma associated with the combustion reaction  302 , molecules of reactants and their constituent parts, any non-reactive species, reaction intermediates (including transition states), and reaction products. Near the output face  214 , the reaction fluid may include reaction products and byproducts, non-reactive gas, and excess oxidant. 
     The plurality of perforations  210  can be each characterized by a transverse dimension D between opposing perforation walls  308 . The inventors have found that stable combustion can be maintained in the perforated flame holder  102  if the length L of each perforation  210  is at least four times the transverse dimension D of the perforation. In other embodiments, the length L can be greater than six times the transverse dimension D. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D. Preferably, the length L is sufficiently long for thermal boundary layers  314  to form adjacent to the perforation walls  308  in a reaction fluid flowing through the perforations  210  to converge at merger points  316  within the perforations  210  between the input face  212  and the output face  214  of the perforated flame holder  102 . In experiments, the inventors have found L/D ratios between 12 and 48 to work well (i.e., produce low NOx, produce low CO, and maintain stable combustion). 
     The perforated flame holder body  208  can be configured to convey heat between adjacent perforations  210 . The heat conveyed between adjacent perforations  210  can be selected to cause heat output from the combustion reaction portion  302  in a first perforation  210  to supply heat to stabilize a combustion reaction portion  302  in an adjacent perforation  210 . 
     Referring especially to  FIG. 2 , the fuel and oxidant source  202  can further include a fuel nozzle  218 , configured to output fuel, and an oxidant source  220  configured to output a fluid including the oxidant. For example, the fuel nozzle  218  can be configured to output pure fuel. The oxidant source  220  can be configured to output combustion air carrying oxygen, and optionally, flue gas. 
     The perforated flame holder  102  can be held by a perforated flame holder support structure  222  configured to hold the perforated flame holder  102  at a dilution distance D D  away from the fuel nozzle  218 . The fuel nozzle  218  can be configured to emit a fuel stream selected to entrain the oxidant to form the fuel and oxidant mixture  206  as the fuel stream and oxidant travel along a path to the perforated flame holder  102  through the dilution distance D D  between the fuel nozzle  218  and the perforated flame holder  102 . Additionally or alternatively (particularly when a blower is used to deliver oxidant contained in combustion air), the oxidant or combustion air source can be configured to entrain the fuel and the fuel and oxidant travel through the dilution distance D D . In some embodiments, a flue gas recirculation path  224  can be provided. Additionally or alternatively, the fuel nozzle  218  can be configured to emit a fuel stream selected to entrain the oxidant and to entrain flue gas as the fuel stream travels through the dilution distance D D  between the fuel nozzle  218  and the input face  212  of the perforated flame holder  102 . 
     The fuel nozzle  218  can be configured to emit the fuel through one or more fuel orifices  226  having an inside diameter dimension that is referred to as “nozzle diameter.” The perforated flame holder support structure  222  can support the perforated flame holder  102  to receive the fuel and oxidant mixture  206  at the distance D D  away from the fuel nozzle  218  greater than  20  times the nozzle diameter. In another embodiment, the perforated flame holder  102  is disposed to receive the fuel and oxidant mixture  206  at the distance D D  away from the fuel nozzle  218  between  100  times and  1100  times the nozzle diameter. Preferably, the perforated flame holder support structure  222  is configured to hold the perforated flame holder  102  at a distance about  200  times or more of the nozzle diameter away from the fuel nozzle  218 . When the fuel and oxidant mixture  206  travels about  200  times the nozzle diameter or more, the mixture is sufficiently homogenized to cause the combustion reaction  302  to produce minimal NOx. 
     The fuel and oxidant source  202  can alternatively include a premix fuel and oxidant source, according to an embodiment. A premix fuel and oxidant source can include a premix chamber (not shown), a fuel nozzle configured to output fuel into the premix chamber, and an oxidant (e.g., combustion air) channel configured to output the oxidant into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the perforated flame holder  102  and be configured to prevent flame flashback into the premix fuel and oxidant source. 
     The oxidant source  220 , whether configured for entrainment in the furnace volume  204  or for premixing, can include a blower configured to force the oxidant through the fuel and oxidant source  202 . 
     The support structure  222  can be configured to support the perforated flame holder  102  from a floor or wall (not shown) of the furnace volume  204 , for example. In another embodiment, the support structure  222  supports the perforated flame holder  102  from the fuel and oxidant source  202 . Alternatively, the support structure  222  can suspend the perforated flame holder  102  from an overhead structure (such as a flue, in the case of an up-fired system). The support structure  222  can support the perforated flame holder  102  in various orientations and directions. 
     The perforated flame holder  102  can include a single perforated flame holder body  208 . In another embodiment, the perforated flame holder  102  can include a plurality of adjacent perforated flame holder sections that collectively provide a tiled perforated flame holder  102 . 
     The perforated flame holder support structure  222  can be configured to support the plurality of perforated flame holder sections. The perforated flame holder support structure  222  can include a metal superalloy, a cementatious, and/or ceramic refractory material. In an embodiment, the plurality of adjacent perforated flame holder sections can be joined with a fiber reinforced refractory cement. 
     The perforated flame holder  102  can have a width dimension W between opposite sides of the peripheral surface  216  at least twice a thickness dimension 
     T between the input face  212  and the output face  214 . In another embodiment, the perforated flame holder  102  can have a width dimension W between opposite sides of the peripheral surface  216  at least three times, at least six times, or at least nine times the thickness dimension T between the input face  212  and the output face  214  of the perforated flame holder  102 . 
     In an embodiment, the perforated flame holder  102  can have a width dimension W less than a width of the furnace volume  204 . This can allow the flue gas circulation path  224  from above to below the perforated flame holder  102  to lie between the peripheral surface  216  of the perforated flame holder  102  and the furnace volume wall (not shown). 
     Referring again to both  FIGS. 2 and 3 , the perforations  210  can be of various shapes. In an embodiment, the perforations  210  can include elongated squares, each having a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations  210  can include elongated hexagons, each having a transverse dimension D between opposing sides of the hexagons. In yet another embodiment, the perforations  210  can include hollow cylinders, each having a transverse dimension D corresponding to a diameter of the cylinder. In another embodiment, the perforations  210  can include truncated cones or truncated pyramids (e.g., frustums), each having a transverse dimension D radially symmetric relative to a length axis that extends from the input face  212  to the output face  214 . In some embodiments, the perforations  210  can each have a lateral dimension D equal to or greater than a quenching distance of the flame based on standard reference conditions. Alternatively, the perforations  210  may have lateral dimension D less then than a standard reference quenching distance. 
     In one range of embodiments, each of the plurality of perforations  210  has a lateral dimension D between 0.05 inch and 1.0 inch. Preferably, each of the plurality of perforations  210  has a lateral dimension D between 0.1 inch and 0.5 inch. For example the plurality of perforations  210  can each have a lateral dimension D of about 0.2 to 0.4 inch. 
     The void fraction of a perforated flame holder  102  is defined as the total volume of all perforations  210  in a section of the perforated flame holder  102  divided by a total volume of the perforated flame holder  102  including body  208  and perforations  210 . The perforated flame holder  102  should have a void fraction between 0.10 and 0.90. In an embodiment, the perforated flame holder  102  can have a void fraction between 0.30 and 0.80. In another embodiment, the perforated flame holder  102  can have a void fraction of about 0.70. Using a void fraction of about 0.70 was found to be especially effective for producing very low NOx. 
     The perforated flame holder  102  can be formed from a fiber reinforced cast refractory material and/or a refractory material such as an aluminum silicate material. For example, the perforated flame holder  102  can be formed to include mullite or cordierite. Additionally or alternatively, the perforated flame holder body  208  can include a metal superalloy such as Inconel or Hastelloy. The perforated flame holder body  208  can define a honeycomb. Honeycomb is an industrial term of art that need not strictly refer to a hexagonal cross section and most usually includes cells of square cross section. Honeycombs of other cross sectional areas are also known. 
     The inventors have found that the perforated flame holder  102  can be formed from VERSAGRID® ceramic honeycomb, available from Applied Ceramics, Inc. of Doraville, S.C. 
     The perforations  210  can be parallel to one another and normal to the input and output faces  212 ,  214 . In another embodiment, the perforations  210  can be parallel to one another and formed at an angle relative to the input and output faces  212 ,  214 . In another embodiment, the perforations  210  can be non-parallel to one another. In another embodiment, the perforations  210  can be non-parallel to one another and non-intersecting. In another embodiment, the perforations  210  can be intersecting. The body  208  can be one piece or can be formed from a plurality of sections. 
     In another embodiment, which is not necessarily preferred, the perforated flame holder  102  may be formed from reticulated ceramic material. The term “reticulated” refers to a netlike structure. Reticulated ceramic material is often made by dissolving a slurry into a sponge of specified porosity, allowing the slurry to harden, and burning away the sponge and curing the ceramic. 
     In another embodiment, which is not necessarily preferred, the perforated flame holder  102  may be formed from a ceramic material that has been punched, bored or cast to create channels. 
     In another embodiment, the perforated flame holder  102  can include a plurality of tubes or pipes bundled together. The plurality of perforations  210  can include hollow cylinders and can optionally also include interstitial spaces between the bundled tubes. In an embodiment, the plurality of tubes can include ceramic tubes. Refractory cement can be included between the tubes and configured to adhere the tubes together. In another embodiment, the plurality of tubes can include metal (e.g., superalloy) tubes. The plurality of tubes can be held together by a metal tension member circumferential to the plurality of tubes and arranged to hold the plurality of tubes together. The metal tension member can include stainless steel, a superalloy metal wire, and/or a superalloy metal band. 
     The perforated flame holder body  208  can alternatively include stacked perforated sheets of material, each sheet having openings that connect with openings of subjacent and superjacent sheets. The perforated sheets can include perforated metal sheets, ceramic sheets and/or expanded sheets. In another embodiment, the perforated flame holder body  208  can include discontinuous packing bodies such that the perforations  210  are formed in the interstitial spaces between the discontinuous packing bodies. In one example, the discontinuous packing bodies include structured packing shapes. In another example, the discontinuous packing bodies include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig ring, ceramic Berl saddles, ceramic Intalox saddles, and/or metal rings or other shapes (e.g. Super Raschig Rings) that may be held together by a metal cage. 
     The inventors contemplate various explanations for why burner systems including the perforated flame holder  102  provide such clean combustion. 
     According to an embodiment, the perforated flame holder  102  may act as a heat source to maintain a combustion reaction even under conditions where a combustion reaction would not be stable when supported by a conventional flame holder. This capability can be leveraged to support combustion using a leaner fuel-to-oxidant mixture than is typically feasible. Thus, according to an embodiment, at the point where the fuel stream  206  contacts the input face  212  of the perforated flame holder  102 , an average fuel-to-oxidant ratio of the fuel stream  206  is below a (conventional) lower combustion limit of the fuel component of the fuel stream  206 —lower combustion limit defines the lowest concentration of fuel at which a fuel and oxidant mixture  206  will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.). 
     The perforated flame holder  102  and systems including the perforated flame holder  102  described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. According to one interpretation, such a performance can be achieved due to a sufficient mixing used to lower peak flame temperatures (among other strategies). Flame temperatures tend to peak under slightly rich conditions, which can be evident in any diffusion flame that is insufficiently mixed. By sufficiently mixing, a homogenous and slightly lean mixture can be achieved prior to combustion. This combination can result in reduced flame temperatures, and thus reduced NOx formation. In one embodiment, “slightly lean” may refer to 3% O 2 , i.e. an equivalence ratio of ˜0.87. Use of even leaner mixtures is possible, but may result in elevated levels of O 2 . Moreover, the inventors believe perforation walls  308  may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperatures and lower NOx. 
     According to another interpretation, production of NOx can be reduced if the combustion reaction  302  occurs over a very short duration of time. Rapid combustion causes the reactants (including oxygen and entrained nitrogen) to be exposed to NOx-formation temperature for a time too short for NOx formation kinetics to cause significant production of NOx. The time required for the reactants to pass through the perforated flame holder  102  is very short compared to a conventional flame. The low NOx production associated with perforated flame holder combustion may thus be related to the short duration of time required for the reactants (and entrained nitrogen) to pass through the perforated flame holder  102 . 
       FIG. 4  is a flow chart showing a method  400  for operating a burner system including the perforated flame holder shown and described herein. To operate a burner system including a perforated flame holder, the perforated flame holder is first heated to a temperature sufficient to maintain combustion of the fuel and oxidant mixture. 
     According to a simplified description, the method  400  begins with step  402 , wherein the perforated flame holder is preheated to a start-up temperature, T S . After the perforated flame holder is raised to the start-up temperature, the method proceeds to step  404 , wherein the fuel and oxidant are provided to the perforated flame holder and combustion is held by the perforated flame holder. 
     According to a more detailed description, step  402  begins with step  406 , wherein start-up energy is provided at the perforated flame holder. Simultaneously or following providing start-up energy, a decision step  408  determines whether the temperature T of the perforated flame holder is at or above the start-up temperature, T S . As long as the temperature of the perforated flame holder is below its start-up temperature, the method loops between steps  406  and  408  within the preheat step  402 . In step  408 , if the temperature T of at least a predetermined portion of the perforated flame holder is greater than or equal to the start-up temperature, the method  400  proceeds to overall step  404 , wherein fuel and oxidant is supplied to and combustion is held by the perforated flame holder. 
     Step  404  may be broken down into several discrete steps, at least some of which may occur simultaneously. 
     Proceeding from step  408 , a fuel and oxidant mixture is provided to the perforated flame holder, as shown in step  410 . The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and oxidant (e.g., combustion air) source, for example. In this approach, the fuel and oxidant are output in one or more directions selected to cause the fuel and oxidant mixture to be received by the input face of the perforated flame holder. The fuel may entrain the combustion air (or alternatively, the combustion air may dilute the fuel) to provide a fuel and oxidant mixture at the input face of the perforated flame holder at a fuel dilution selected for a stable combustion reaction that can be held within the perforations of the perforated flame holder. 
     Proceeding to step  412 , the combustion reaction is held by the perforated flame holder. 
     In step  414 , heat may be output from the perforated flame holder. The heat output from the perforated flame holder may be used to power an industrial process, heat a working fluid, generate electricity, or provide motive power, for example. 
     In optional step  416 , the presence of combustion may be sensed. Various sensing approaches have been used and are contemplated by the inventors. Generally, combustion held by the perforated flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, flame rod, and/or other combustion sensing apparatuses. In an additional or alternative variant of step  416 , a pilot flame or other ignition source may be provided to cause ignition of the fuel and oxidant mixture in the event combustion is lost at the perforated flame holder. 
     Proceeding to decision step  418 , if combustion is sensed not to be stable, the method  400  may exit to step  424 , wherein an error procedure is executed. For example, the error procedure may include turning off fuel flow, re-executing the preheating step  402 , outputting an alarm signal, igniting a stand-by combustion system, or other steps. If, in step  418 , combustion in the perforated flame holder is determined to be stable, the method  400  proceeds to decision step  420 , wherein it is determined if combustion parameters should be changed. If no combustion parameters are to be changed, the method loops (within step  404 ) back to step  410 , and the combustion process continues. If a change in combustion parameters is indicated, the method  400  proceeds to step  422 , wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step  404 ) back to step  410 , and combustion continues. 
     Combustion parameters may be scheduled to be changed, for example, if a change in heat demand is encountered. For example, if less heat is required (e.g., due to decreased electricity demand, decreased motive power requirement, or lower industrial process throughput), the fuel and oxidant flow rate may be decreased in step  422 . Conversely, if heat demand is increased, then fuel and oxidant flow may be increased. Additionally or alternatively, if the combustion system is in a start-up mode, then fuel and oxidant flow may be gradually increased to the perforated flame holder over one or more iterations of the loop within step  404 . 
     Referring again to  FIG. 2 , the burner system  200  includes a heater  228  operatively coupled to the perforated flame holder  102 . As described in conjunction with  FIGS. 3 and 4 , the perforated flame holder  102  operates by outputting heat to the incoming fuel and oxidant mixture  206 . After combustion is established, this heat is provided by the combustion reaction  302 ; but before combustion is established, the heat is provided by the heater  228 . 
     Various heating apparatuses have been used and are contemplated by the inventors. In some embodiments, the heater  228  can include a flame holder configured to support a flame disposed to heat the perforated flame holder  102 . The fuel and oxidant source  202  can include a fuel nozzle  218  configured to emit a fuel stream  206  and an oxidant source  220  configured to output oxidant (e.g., combustion air) adjacent to the fuel stream  206 . The fuel nozzle  218  and oxidant source  220  can be configured to output the fuel stream  206  to be progressively diluted by the oxidant (e.g., combustion air). The perforated flame holder  102  can be disposed to receive a diluted fuel and oxidant mixture  206  that supports a combustion reaction  302  that is stabilized by the perforated flame holder  102  when the perforated flame holder  102  is at an operating temperature. A start-up flame holder, in contrast, can be configured to support a start-up flame at a location corresponding to a relatively unmixed fuel and oxidant mixture that is stable without stabilization provided by the heated perforated flame holder  102 . 
     The burner system  200  can further include a controller  230  operatively coupled to the heater  228  and to a data interface  232 . For example, the controller  230  can be configured to control a start-up flame holder actuator configured to cause the start-up flame holder to hold the start-up flame when the perforated flame holder  102  needs to be pre-heated and to not hold the start-up flame when the perforated flame holder  102  is at an operating temperature (e.g., when T≧T S ). 
     Various approaches for actuating a start-up flame are contemplated. In one embodiment, the start-up flame holder includes a mechanically-actuated bluff body configured to be actuated to intercept the fuel and oxidant mixture  206  to cause heat-recycling and/or stabilizing vortices and thereby hold a start-up flame; or to be actuated to not intercept the fuel and oxidant mixture  206  to cause the fuel and oxidant mixture  206  to proceed to the perforated flame holder  102 . In another embodiment, a fuel control valve, blower, and/or damper may be used to select a fuel and oxidant mixture flow rate that is sufficiently low for a start-up flame to be stream-stabilized; and upon reaching a perforated flame holder  102  operating temperature, the flow rate may be increased to “blow out” the start-up flame. In another embodiment, the heater  228  may include an electrical power supply operatively coupled to the controller  230  and configured to apply an electrical charge or voltage to the fuel and oxidant mixture  206 . An electrically conductive start-up flame holder may be selectively coupled to a voltage ground or other voltage selected to attract the electrical charge in the fuel and oxidant mixture  206 . The attraction of the electrical charge was found by the inventors to cause a start-up flame to be held by the electrically conductive start-up flame holder. 
     In another embodiment, the heater  228  may include an electrical resistance heater configured to output heat to the perforated flame holder  102  and/or to the fuel and oxidant mixture  206 . The electrical resistance heater can be configured to heat up the perforated flame holder  102  to an operating temperature. The heater  228  can further include a power supply and a switch operable, under control of the controller  230 , to selectively couple the power supply to the electrical resistance heater. 
     An electrical resistance heater  228  can be formed in various ways. For example, the electrical resistance heater  228  can be formed from KANTHAL® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstaham mar, Sweden) threaded through at least a portion of the perforations  210  defined by the perforated flame holder body  208 . Alternatively, the heater  228  can include an inductive heater, a high-energy beam heater (e.g. microwave or laser), a frictional heater, electro-resistive ceramic coatings, or other types of heating technologies. 
     Other forms of start-up apparatuses are contemplated. For example, the heater  228  can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the oxidant and fuel. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite the fuel and oxidant mixture  206  that would otherwise enter the perforated flame holder  102 . The electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller  230 , which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the fuel and oxidant mixture  206  in or upstream from the perforated flame holder  102  before the perforated flame holder  102  is heated sufficiently to maintain combustion. 
     The burner system  200  can further include a sensor  234  operatively coupled to the control circuit  230 . The sensor  234  can include a heat sensor configured to detect infrared radiation or a temperature of the perforated flame holder  102 . The control circuit  230  can be configured to control the heating apparatus  228  responsive to input from the sensor  234 . Optionally, a fuel control valve  236  can be operatively coupled to the controller  230  and configured to control a flow of fuel to the fuel and oxidant source  202 . Additionally or alternatively, an oxidant blower or damper  238  can be operatively coupled to the controller  230  and configured to control flow of the oxidant (or combustion air). 
     The sensor  234  can further include a combustion sensor operatively coupled to the control circuit  230 , the combustion sensor being configured to detect a temperature, video image, and/or spectral characteristic of a combustion reaction held by the perforated flame holder  102 . The fuel control valve  236  can be configured to control a flow of fuel from a fuel source to the fuel and oxidant source  202 . The controller  230  can be configured to control the fuel control valve  236  responsive to input from the combustion sensor  234 . The controller  230  can be configured to control the fuel control valve  236  and/or oxidant blower or damper to control a preheat flame type of heater  228  to heat the perforated flame holder  102  to an operating temperature. The controller  230  can similarly control the fuel control valve  236  and/or the oxidant blower or damper to change the fuel and oxidant mixture  206  flow responsive to a heat demand change received as data via the data interface  232 . 
       FIG. 5  shows a burner assembly  500  with multiple fuel nozzles  110   a ,  110   b,    110   c,  and so on up to  110 k, where k refers to an integer number which corresponds to the number of fuel nozzles, configured to output respective fuel streams  108   a,    108   b,    108   c,  according to an embodiment. As used herein, depending on the context, numbers  108   a,    108   b,    108   c,  etc. can refer to a fuel and oxidant mixture or to a fuel stream output by a fuel nozzle  110   a,    100   b,    110   c,  etc. providing the fuel for said mixture. Each fuel stream may propagate along a respective different direction and may impinge substantially on only a single respective segment  516   a ,  516   b ,  516   c  of the perforated flame holder  102 . Additionally, or alternatively, several fuel streams  108   a,    108   b,    108   c,  etc. may impinge on the same respective segment of the perforated flame holder  102 . In an illustrated embodiment, a segment may be taken as referring to a physically-realized division of an entire perforated flame holder. Depending on an embodiment, the segments  516   a ,  516   b ,  516   c  may include individual unitary tiles or tile assemblies (a plurality of segments placed in a direct contact with each other) held in position by the perforated flame holder support structure  222 . In an embodiment, the individual segments in the segment sub-assembly may be fastened together, glued together, or held in a common sub-support. For example, the segment assembly may be formed as an arch made of individual unitary segments. 
     According to an embodiment, a portion of a perforated flame holder  102  (such as portions  102   a  or  102   b  of  FIG. 1 ) is an area impinged by 1 or more fuel streams. A segment of a perforated flame holder (such as segments  516   a - c  of  FIG. 5 , segments  516   m  of  FIG. 6 , or segments  516   a -d of  FIG. 8 ) refer to sections of a perforated flame holder  102  that are separated from each other by air gaps or by an intervening structure or material. According to an embodiment, a single segment can correspond to a plurality of portions. According to an embodiment, a single portion can correspond to a plurality of segments. According to an embodiment a single portion may correspond to a single segment. 
     The flame holder support structure  222  may be embodied as any support structure configured to support the plurality of perforated flame holder segments  516   a ,  516   b ,  516   c . The support structure may, for example include a metal or superalloy. In another embodiment, the support structure  222  may be formed from a refractory cement or fiber reinforced refractory cement. Optionally, the support structure  222  may include a cooling structure. 
     In an embodiment, the segments  516   a ,  516   b , . . . may have air gaps in between them (e.g., if the support structure  222  in the drawing holds only the corners of the segments). These segments may, in an embodiment, be tiles. The inventors have found that with this arrangement, contrary to conventional expectations, little fugitive emissions are produced when the gap is as much as a few inches. Fugitive emissions are defined to mean unburned or partially unburned fuel that exits the combustion zone unreacted. With little fugitive emissions, there is negligible threat to produce CO or unburned hydrocarbon (UHC) emissions; but the segment cooling may be relatively large because the segment edges may be immersed in non-combusting fluid. 
     The potential advantages of a reduced segment (e.g., tile) temperature are many: longer tile life, increased firing capacity and heat density for a given surface area, lower NOx emissions, and reduced or even eliminated upstream flame propagation. Segment separation may be particularly useful in higher-temperature applications (e.g., ethylene cracking, hydrogen reformers) and/or when using fuels with high adiabatic flame temperatures (e.g., hydrogen or hydrogen blends in a refinery process heater). 
     According to an interpretation, substantially all of heat output by the combustion reaction is output between the input surface  212  and the output surface  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 surface  212  and the output surface  214  of the perforated flame holder  102 . In some experiments, the inventors produced a combustion reaction that was wholly contained in the perforations between the input surface  212  and the output surface  214  of the perforated flame holder  102 . 
     The perforated flame holder  102  can be configured to output a portion of the received heat as thermal radiation  304 . The perforated flame holder can transfer another portion of the received heat to the fuel and oxidant mixture  108  received at the input surface  212  of the perforated flame holder  102 . 
     The inventors further contemplate that the various combinations of perforated flame holder portions and fuel nozzles can be used to increase the turndown ratio. The turndown ratio may be defined as the ratio of maximum heat release to minimum heat release with maintenance of a stable flame (see  The John Zink Hamworthy Combustion Handbook,  2nd Ed., Volume 1, ¶10.7.1.3 and FIG. 17.15). 
     The provision (and/or deployment) of multiple nozzles is related to the inventors&#39; novel structure and method of increasing the turndown ratio defined above. The multiple fuel nozzles  110   a,    110   b,  and  110   c  may be individually controllable as to fuel flow rate, feed pressure, or on/off actuation. Such an arrangement may allow individual segments  516   a ,  516   b ,  516   c  of the perforated flame holder  102  to generate heat at different rates (because each of the fuel streams  108   a,    108   b,    108   c  can impinge substantially onto a respective segment  516   a ,  516   b ,  516   c  of the perforated flame holder  102 ). Thus, each combination (such as  110   a,    108   a,  and  516   a ) may be operated independently of the others. 
     However, the operation of individual segments may not be fully independent because heat, generated by combustion (e.g., flame) inside bore-like apertures or other voids of the perforated flame holder  102 , may escape the perforated flame holder in various ways. 
     First, heat can travel by conduction from hotter parts toward colder parts. It may travel in the direction from the output surface  214  (in  FIG. 2 ) toward the input surface  212 . It also may travel laterally. For example, it may travel laterally from a portion of the perforated flame holder under an area where there is greater fuel and oxidant mixture impingement toward a portion under an area where there is less impingement. The same type of heat travel may happen due to varied fuel/oxidant ratios. In the case of physically distinct segments such as in  FIG. 5 , heat can travel between adjoining segments if they are in contact with each other. Heat may also travel through the support structure  222  that touches both segments (by conduction). Additionally, or alternatively, heat may travel through an intervening fluid via conduction and/or convection. 
     Second, heat can leave the perforated flame holder  102  via hot flue gas. Third, heat can leave as electromagnetic radiation at infrared and visible region wavelengths. The perforated flame holder is often hot enough to emit a bright glow. (Heat balance in a perforated flame holder is discussed further in relation to  FIG. 3 .) 
     Once heat leaves the perforated flame holder  102 , it may heat a furnace in which the burner assembly (or just the perforated flame holder) may be installed. Interior walls may not only absorb heat, they may also re-emit it, especially via thermal radiation, which may return heat to a perforated flame holder. 
     These processes can transfer heat from one segment of the perforated flame holder  102  to an adjoining segment, and also to non-adjoining segments. Such heat transfer between segments may make it possible to increase the turndown ratio of a burner assembly or furnace. In one embodiment, various combinations of nozzles can be active, while others are not streaming fuel thereby increasing the turndown ratio. 
     The heat transfer mechanisms mentioned above can provide greater flame stability, because of the segment-to-segment heat transfer. When a segment receives less fuel because its single fuel nozzle stream is diminished or intermittent, then its temperature will tend to decrease, due to the several heat-loss mechanisms mentioned above. If the segment is not isolated, but is a part of the perforated flame holder in which it can receive heat from the surrounding elements, such as segments, perforated flame holder support structure or furnace walls, then its temperature will not decrease as quickly as it would in isolation. 
     The longer it takes the segment&#39;s temperature to decrease, the longer it can maintain a temperature that is viable for flame stability, resulting in a longer viable time to extinction. Time to extinction may be defined as the maximum time interval before which the nominal fuel impingement may resume without producing flame instability. That is, fired segments present in a vicinity of a non-fired tile can transfer heat to it, thus increasing the robustness of the entire perforated flame holder. This, in turn, may lead to stable combustion in the face of fuel flow instabilities (assuming the average heat flow is not affected over a time period less than the extinction time). 
     The viable turn-off time may in some cases be of indefinite length, depending on the combustion parameters and perforated flame holder characteristics. For example, variations in oxidant (e.g. combustion air) temperature, variations in fuel composition, or unknown elevation, altitude, barometric pressure, or furnace insulation (at least upon installation) may make the viable turn-off time indeterminate. Moreover, periods of reduced fuel flow or increased load immediately before a turn-off period may tend to cause a portion  102   a  of a perforated flame holder  102  to start off at a lower temperature upon a stoppage of fuel flow. The system designer may elect not to include Look-Up Table (LUT) or algorithmic support for a fully parameterized determinate viable turn off time calculation. Either situation (environmental or limitations of an implementation that add uncertainty) is referred to as indeterminate viable turn-off time herein. 
     According to an embodiment, a control system can be equipped with a search algorithm selected to infer a viable turn-off time even in the presence of factors that may cause absolute foreknowledge to be indeterminate. For example, a control system may respond to computer instructions carried by a non-transitory computer readable medium to make conservative changes to restart time. Longer delays in full restart may be indicative of approaching or surpassing a viable turn-off time. 
       FIG. 6  illustrates a control system  600  for implementing an increased turndown ratio, according to an embodiment. The perforated flame holder  102  may include numerous segments  516   m  (for convenience, only one is labeled in the figure), which, in an embodiment, can correspond to respective plural fuel nozzles  110 . The segments  516   m  may be separated by contiguous areas roughly corresponding to substantially circular areas, one of which is labeled  602   m . Although seven segment areas  516   m  are shown, there can be any number of areas  516   m  and any number of fuel nozzles  110   m . Each segment can be larger across than any of the apertures  210  (which may cover the entire surface of the perforated flame holder  102 , although only a few are illustrated). 
     Each of the fuel nozzles  110   m  may, in an embodiment, be coupled to a common hydraulic header (not shown) or otherwise supplied with fuel, which may be under pressure sufficient to expel a respective fuel stream  108   m . Pipes leading to tips of the fuel nozzles may include respective valves  602   m , mechanically coupled to step motors or other actuators  604   m , which can be in turn actuated by a controller  116  through wires  606   m . The controller  116  may receive feedback signals from one or more sensors  122 , which may sense flame characteristics, fluid speed(s), temperature(s), species, or any other characteristics that relate to or result from combustion parameters. The sensors may be optical, acoustic, thermal, etc. 
     The illustration is exemplary, and other embodiments are contemplated. For example, the wires  606   m  may be replaced by a wireless link or links; the step motor(s) and valve(s) may be substituted with a DC motor and a pump, and so on. Likewise, the controller  116  can be of various forms, from electro-mechanical timer to digitally controlled systems under the direction of a programmed processor such as a microprocessor or a computer programmed from a non-transitory medium. These examples will suggest other approaches to those skilled in the art. 
     Whatever the exact mechanism, the system of  FIG. 6  should be capable of at least on-off stream switching (fuel rate as a step function of time), producing variable flow rates, or performing other basic functions that vary heat generation. 
     In an embodiment, the system of  FIG. 6  may interrupt the flow of the fuel to selected nozzles  110   m  for particular time intervals, which may be selected so that the temperature of each segment stays above the temperature needed for a stable flame. One example of such temperature may be an auto-ignition temperature of the fuel and oxidant mixture impinging on the respective segment. In general, the temperature needed for a stable flame is a function of several variables and parameters. Such an arrangement may be useful in turndown scenarios in which some portions of the tile(s) may be cooling due to the reduced energy input. In that case, those areas of the perforated flame holder may receive pulsed fuel to maintain a minimum segment temperature. 
     MIXING LENGTH: The inventors&#39; apparatus and method also relates to the “mixing length,” which may be defined as the distance from a fuel nozzle at which the fuel stream is mixed with entrained oxidant (e.g., air) in a manner suitable to burn properly. This is related to the physics and geometry of free fuel streams, which are explained in  The John Zink Hamworthy Combustion Handbook,  2nd Ed., Volume 1, ¶9.11 and FIG. 9.100. The equations presented in that book (which are incorporated herein by reference) show that the characteristics of a fuel stream generally depend on position, and a few other factors. Such factors can include the diameter of the fuel nozzle orifice, the fuel velocity at the nozzle tip (itself a function of fuel nozzle orifice inner diameter and fuel pressure in most fuel nozzle designs), and the densities of the fuel and oxidant (the last two appearing in the equation for mass entrainment rate). 
     The equations show that, as fuel leaves the fuel nozzle, it entrains (draws in) oxidant and any diluent present, such as combustion air. At a distance of about 18 nozzle orifice diameters, the “core” of pure fuel is gone and only a mixture of entrained oxidant and fuel exists downstream of that distance; however, the mixture is not uniform at that distance, nor does it ever become fully uniform no matter what the axial distance from the nozzle orifice for a stream issuing into quiescent fluid. The velocity of the mixture decreases steadily with distance from the nozzle, while at any one distance the velocity “profile” (the variation of the velocity as function of radial distance from the stream centerline or axis) has the form of a bell-shaped curve. Because of the relation between these, the velocity profile becomes flatter (closer to a uniform velocity field) at greater axial distances, but still retains some character of a bell-shaped curve (FIG. 9.100, id.). 
     The fuel concentration transverse to the stream and mass entrainment rate can also vary with axial distance and radial distance. The fuel concentration varies as a function of the axial distance normalized by the fuel nozzle diameter. Therefore, for a fuel stream issuing into quiescent fluid (e.g., air) for a fixed axial distance, a smaller fuel nozzle diameter will result in more thoroughly mixed fuel with oxidant, since the fixed axial distance equates to a greater number of smaller fuel nozzle diameters. A larger fuel nozzle diameter will result in less thoroughly mixed fuel with oxidant, since the fixed axial distance equates to a fewer number of larger fuel nozzle diameters. 
     Thus, for a given size of perforated flame holder, and given required rate of heat generation or fuel burning, and/or a given furnace geometry, there may be cases in which a single fuel nozzle, as in  FIG. 2 , is less than optimum, and in those cases the inventors&#39; plurality of fuel nozzles, as shown as an embodiment in  FIG. 6 , may be an improvement leading to better mixing or other desirable properties. 
     In  FIG. 6 , it can be seen that the fuel streams  108   m  are at least roughly aligned with the circular segments  516   m . Inasmuch as the fuel concentration profile is a function of radial distance from the projected nozzle axis, the fluid velocity and concentration from a single fuel nozzle  110   m  is greatest in the center of the corresponding segment  516   m , and decreases with distance from the center. If the segments  516   m  are, however, arranged near one another as shown, the various fuel streams  108   m  can overlap. If, for example, the stream velocity is monotonically decreasing from the center of a first segment toward the center of a second, but at the same time is also monotonically decreasing from the center of the second segment toward the center of the first, then their summation along the line between the centers can be more uniform than would be the case with only one fuel stream. In other words, with plural fuel streams  108   m  a more uniform flow of mixture to the perforated flame holder  102  can be achieved, which can result in more homogeneous (closer to ideal) concentrations at the perforated flame holder. 
       FIG. 7  illustrates the concentration profile  700  for two overlapping fuel streams issuing into quiescent fluid, such as air. One fuel stream is centered at the left side of the figure at value 0 on the x axis. The other is centered at the right side of the figure at value 2 on the x axis. The curve sloping downward from left to right pictorially represents the fuel concentration as it decays from its maximum value at the center of the fuel nozzle of the first stream to a minimum value as it approaches the other fuel stream. The ratio of the minimum to maximum fuel concentration of this curve is about 7.4. That is, the fuel is about 7.4 times more concentrated along its centerline than at the periphery shown at the right. Likewise, for the fuel nozzle centered at the right side of the figure, the fuel concentration slopes downward from right to left. This curve also represents a fuel concentration that is about 7.4 times more concentrated along its centerline than its periphery. However, the concentration profile resulting from the overlap of the fuel streams, represented by the top curve, has a much more modest ratio of maximum to minimum values of about 1.35. Thus, the fuel concentration has been much homogenized by the addition of an overlapping fuel stream. Additional fuel nozzles along the plane can result in an even more uniform concentration profile. Because of the nature of fuel stream entrainment, the mixture impinging on a perforated flame holder from multiple fuel nozzles can be much more uniform than that from a single fuel nozzle. For example, a hexagonal array of fuel nozzles, such as illustrated in  FIG. 6 , theoretically, can provide fuel concentration profiles whose maximum to minimum ratio is about 1.3. Other arrangements of fuel nozzles  110   m  can also be envisioned including those that are not geometrically regular. 
       FIG. 8  shows a furnace  800 , according to an embodiment. The furnace  800  includes the fuel nozzle  110 , two pilots  802 , the perforated flame holder support  222 , and the perforated flame holder made up of contiguous segments  516   a ,  516   b ,  516   c , and  516   d . The pilots  802  may be ignited by igniters  803 . In this embodiment, the segments form an arch. According to an embodiment, the segments  516   a - d  are tiles. In an embodiment, the structure shown in this figure may be repeated at other locations (in a direction into or out of the paper plane) so that a furnace will have rows of parallel arches inside it. According to an embodiment the gaps between the arches can be useful in reducing the tile temperature. (The benefits of air gaps between segments were discussed above in relation to  FIG. 5 .) 
     Referring to  FIGS. 1 and 2 , an apparatus for supporting combustion  100 , according to an embodiment, includes a plurality of fuel nozzles  110   a,    110   b , configured to emit a corresponding plurality of fuel streams  108   a,    108   b,  and a perforated flame holder  102  aligned to receive the plurality of fuel streams  108   a ,  108   b  into respective portions  102   a,    102   b  of the perforated flame holder  102 . The perforated flame holder  102  can have elongated apertures passing through the perforated flame holder  102  from an input face  212  to an output face  214  (both shown in  FIG. 2 ), and having a transverse dimension D between opposing perforation walls  308  (shown in  FIG. 3 ). 
     Referring to  FIG. 5 , the perforated flame holder  102  may be formed to include a plurality of segments  516   a - c  separated from one another by respective gaps  502   a, b  at a distance at least twice as large as the dimension D. For example, the gaps  502   a,    502   b  between the segments  516   a  and  516   b , and  516   c , respectively, can be formed above the support frame pieces  222 . In some embodiments, the gaps  502   a,    502   b  separating the segments  516   a ,  516   c ,  516   c , etc. can be at least five times the transverse dimension D. In other embodiments, the gaps  502   a,    502   b  can be at least ten times the transverse dimension D. For example, the transverse dimension D can be between 0.1 inch and 0.25 inch. 
     In an embodiment, each segment  516   a ,  516   b , etc. of the perforated flame holder  102  may correspond to a respective portion of the perforated flame holder  102 . The plurality of fuel nozzles  110   a,    110   b,  etc. can be aligned to cause the fuel streams  108   a,    108   b,  etc. to collectively be received by the perforated flame holder segments  516   a ,  516   b , etc. and the gaps  502   a,    502   b,  etc. between them. Furthermore, the perforated flame holder  102  can be configured to combust the fuel passing through the elongated apertures and through the gaps  502   a,    502   b , etc. sufficiently completely that no fugitive emission of fuel may be detected at a flue through which combustion products may pass. 
       FIG. 9  is a flow chart showing a method  900  for supporting combustion, according to an embodiment. The method  900  can include the following steps. At step  902 , a plurality of fuel streams can be emitted from a corresponding plurality of fuel nozzles. 
     At step  904 , the plurality of fuel streams may be received into respective portions of a perforated flame holder. According to an embodiment, the perforated flame holder can have elongated apertures passing through the perforated flame holder from an input face to an output face. Each of the elongated apertures can be characterized with a transverse dimension D, corresponding to a distance between opposing perforation walls. Furthermore, the perforated flame holder can include a plurality of segments separated from one another by respective gaps of at least two times D. 
     Step  906  of the method  900  may include supporting combustion of the respective fuel streams in the portions of the perforated flame holder. 
     According to an embodiment, the perforated flame holder  102  may be formed to include a plurality of segments separated from one another by respective gaps at a distance at least twice as large as the dimension D. In some embodiments, the gaps separating the segments can be at least five times the transverse dimension D. In other embodiments, the gaps can be at least ten times the transverse dimension D. For example, the transverse dimension D can be between 0.1 inch and 0.25 inch. 
     In an embodiment, each segment may correspond to a respective portion of the perforated flame holder. The plurality of fuel nozzles can be aligned to cause the fuel streams to collectively be received by the perforated flame holder segments and the gaps between them. Furthermore, the perforated flame holder can be configured to combust the fuel passing through the elongated apertures and through the gaps sufficiently completely that no fugitive emission of fuel may be detected at a flue through which combustion products pass. 
     Referring to  FIG. 1 , a burner apparatus  100 , according to an embodiment, can include a plurality of fuel nozzles  110   a,    110   b,  etc. configured to output respective fuel streams  108   a,    108   b,  etc., and a perforated flame holder  102  configured to receive the respective fuel streams  108   a,    108   b,  etc. at respective portions  102   a,    102   b  of the perforated flame holder  102  and to support combustion in the respective portions  102   a,    102   b  of the perforated flame holder  102 . 
     According to an embodiment, a second portion of the plurality of fuel nozzles  110   a,    110   b,  etc. can be configured to cooperate with a corresponding second portion  102   b  of the perforated flame holder  102  to cause the output of at least a portion of heat sufficient to maintain a first portion  102   a  of the perforated flame holder  102  at an elevated operating temperature. A first portion of fuel nozzles  110   a,    110   b,  etc. can be different from the second portion of fuel nozzles  110   a,    110   b,  etc., and be configured to be controlled to stop outputting a respective first fuel stream  108   a,    108   b,  etc. to the first portion  102   a  of the perforated flame holder  102 . 
     According to an embodiment of the burner apparatus  100 , the aforementioned portion of heat may include the entirety of heat necessary to maintain the first portion  102   a  of the perforated flame holder  102  at the elevated operating temperature for at least a time greater than a time sufficient to stop and start the first fuel stream  108  from the first portion of fuel nozzles  110 . In other embodiments, such a portion of heat may correspond to a time 10 times greater or a time 100 times greater than the time sufficient to stop and start the first fuel stream  108  from the first portion of fuel nozzles  110 . 
     For example, in an illustrative application, it may require 2 seconds to shut off the first fuel stream from a first portion consisting of three fuel nozzles; and it may require 4 seconds to turn the first fuel stream on from the first portion of fuel nozzles. The time sufficient to stop and start the first fuel stream would thus be 6 seconds, irrespective of how much time elapsed between stopping and starting the first fuel stream. According to this example, a time greater than 10 times the time sufficient to stop and start the first fuel stream would be at least greater than 60 seconds, or 1 minute. The inventors have found that a vertically fired burner apparatus including a perforated flame holder consisting essentially of a 2-inch-thick square mullite honeycomb having about 70% void volume and 64 cells per square inch satisfies this condition when the first portion is about 25% or less of the total perforated flame holder area. Also according to this example, a time greater than 100 times the time sufficient to stop and start the first fuel stream would be at least greater than 600 seconds, or 10 minutes. The inventors contemplate that a perforated flame holder made from 6 inch thick square mullite honeycomb having about 70% void volume and 16 cells per square inch may satisfy this condition when the first portion is about 10% or less of the total perforated flame holder area. 
     According to another embodiment of the burner apparatus  100 , the aforementioned portion of heat may include less than the entirety of heat necessary to maintain the first portion  102   a  of the perforated flame holder  102  at the elevated operating temperature. Additionally, the burner apparatus  100  can include a heating mechanism configured to cause heating of the first portion  102   a  of the perforated flame holder  102  sufficient, in cooperation with the portion of heat output by the second portion of fuel nozzles  108  cooperating with the second portion  102   b  of the perforated flame holder  102 , to maintain the first portion  102   a  of the perforated flame holder  102  at the elevated operating temperature. For example, the heating mechanism can include a conventional flame holder configured to hold a conventional flame upstream from the perforated flame holder  102 . Alternatively, the heating mechanism may include an electrical resistance heater. 
     A method for operating a burner, according to an embodiment, can include a step of delivering a plurality of fuel streams  108  (for example, shown in  FIGS. 1 and 2 ) from a corresponding plurality of fuel nozzles  110  (shown in  FIGS. 1 and 2 ) to a perforated flame holder  102  for combustion in the perforated flame holder  102 . A second step of the method for operating a burner can include stopping at least one first fuel stream  108  from a first fuel nozzle  110  that delivers fuel to a first portion of the perforated flame holder. A third step of the method for operating a burner can include maintaining an elevated operating temperature of the first portion  102   a  of the perforated flame holder  102  at least in part by continued combustion in other portions of the perforated flame holder  102 . 
     According to an embodiment, the step of maintaining the elevated operating temperature of the first portion of the perforated flame holder may include applying heat to the first portion  102   a  of the perforated flame holder with a heater mechanism. For example, this may include exposing the first portion of the perforated flame holder to heat output from a start-up flame. Alternatively, it may include outputting heat to the first portion of the perforated flame holder with an electrical resistance heater. 
     Additionally, the method for operating a burner can include a step of resuming the first fuel stream through the first fuel nozzle, after a first-time delay. In such an embodiment, the maintained elevated operating temperature of the first portion of the perforated flame holder  102  may cause the resumed first fuel stream to support combustion in the first portion of the perforated flame holder  102 . According to and embodiment, the resumed first fuel stream may be configured to support combustion in the first portion  102   a  of the perforated flame holder  102  immediately upon entering the first portion of the perforated flame holder  102 . 
     According to and embodiment, the method for operating a burner can include a step of determining a viable turn-off time, and resuming the first fuel stream only if the first-time delay is less than or equal to the viable turn-off time. The step of determining a viable turn-off time may include receiving, with a computer control system, at least one set of sensor data from at least one sensor configured to sense a parameter that can cause a variation in the viable turn-off time. Additionally, this step may also include calculating the viable turn-off time with the computer control system as a function of the at least one set of sensor data. 
     Alternatively, the step of determining a viable turn-off time may include reading, from a non-transitory computer readable medium, a predetermined viable turn-off time. 
       FIG. 10A  is a simplified perspective view of a combustion system  1000 , including another alternative perforated flame holder  102 , according to an embodiment. The perforated flame holder  102  is a reticulated ceramic perforated flame holder, according to an embodiment.  FIG. 10B  is a simplified side sectional diagram of a portion of the reticulated ceramic perforated flame holder  102  of  FIG. 10A , according to an embodiment. The perforated flame holder  102  of  FIGS. 10A, 10B  can be implemented in the various combustion systems described herein, according to an embodiment. The perforated flame holder  102  is configured to support a combustion reaction of the fuel and oxidant  206  at least partially within the perforated flame holder  102 . According to an embodiment, the perforated flame holder  102  can be configured to support a combustion reaction of the fuel and oxidant  206  upstream, downstream, within, and adjacent to the reticulated ceramic perforated flame holder  102 . 
     According to an embodiment, the perforated flame holder body  208  can include reticulated fibers  1039 . The reticulated fibers  1039  can define branching perforations  210  that weave around and through the reticulated fibers  1039 . According to an embodiment, the perforations  210  are formed as passages through the reticulated ceramic fibers  1039 . 
     According to an embodiment, the reticulated fibers  1039  can include alumina silicate. According to an embodiment, the reticulated fibers  1039  can be formed from extruded mullite or cordierite. According to an embodiment, the reticulated fibers  1039  can include Zirconia. According to an embodiment, the reticulated fibers  1039  can include silicon carbide. 
     The term “reticulated fibers” refers to a netlike structure. According to an embodiment, the reticulated fibers  1039  are formed from an extruded ceramic material. In reticulated fiber embodiments, the interaction between the fuel and oxidant  206 , the combustion reaction, and heat transfer to and from the perforated flame holder body  208  can function similarly to the embodiment shown and described above with respect to  FIGS. 2-4 . One difference in activity is a mixing between perforations  210 , because the reticulated fibers  1039  form a discontinuous perforated flame holder body  208  that allows flow back and forth between neighboring perforations  210 . 
     According to an embodiment, the reticulated fiber network is sufficiently open for downstream reticulated fibers  1039  to emit radiation for receipt by upstream reticulated fibers  1039  for the purpose of heating the upstream reticulated fibers  1039  sufficiently to maintain combustion of a fuel and oxidant  206 . Compared to a continuous perforated flame holder body  208 , heat conduction paths  310  between fibers  1039  are reduced due to separation of the fibers  1039 . This may cause relatively more heat to be transferred from the heat-receiving region  306  (heat receiving area) to the heat-output region  310  (heat output area) of the reticulated fibers  1039  via thermal radiation. 
     According to an embodiment, individual perforations  210  may extend from an input face  212  to an output face  214  of the perforated flame holder  102 . Perforations  210  may have varying lengths L. According to an embodiment, because the perforations  210  branch into and out of each other, individual perforations  210  are not clearly defined by a length L. 
     According to an embodiment, the perforated flame holder  102  is configured to support or hold a combustion reaction or a flame at least partially between the input face  212  and the output face  214 . According to an embodiment, the input face  212  corresponds to a surface of the perforated flame holder  102  proximal to the fuel nozzle  218  or to a surface that first receives fuel. According to an embodiment, the input face  212  corresponds to an extent of the reticulated fibers  1039  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  1039  distal to the fuel nozzle  218  or opposite to the input face  212 . 
     According to an embodiment, the formation of boundary layers  314 , transfer of heat between the perforated reaction holder body  208  and the gases flowing through the perforations  210 , a characteristic perforation width dimension D, and the length L can be regarded as related to an average or overall path through the perforated reaction holder  102 . In other words, the dimension D can be determined as a root-mean-square of individual Dn values determined at each point along a flow path. Similarly, the length L can be a length that includes length contributed by tortuosity of the flow path, which may be somewhat longer than a straight line distance T RH  from the input face  212  to the output face  214  through the perforated reaction holder  102 . According to an embodiment, the void fraction (expressed as (total perforated reaction holder  102  volume−fiber  1039  volume)/total volume)) is about 70%. 
     According to an embodiment, the reticulated ceramic perforated flame holder  102  is a tile about 1″×4″×4″. According to an embodiment, the reticulated ceramic perforated flame holder  102  includes about 10 pores per square inch of surface area. Other materials and dimensions can also be used for a reticulated ceramic perforated flame holder  102  in accordance with principles of the present disclosure. 
     According to an embodiment, the reticulated ceramic perforated flame holder  102  can include shapes and dimensions other than those described herein. For example, the perforated flame holder  102  can include burner tiles that are reticulated ceramic tiles that are larger or smaller than the dimensions set forth above. Additionally, the reticulated ceramic perforated flame holder  102  can include shapes other than generally cuboid shapes. 
     According to an embodiment, the reticulated ceramic perforated flame holder  102  can include multiple reticulated ceramic tiles. The multiple reticulated ceramic tiles can be joined together such that each ceramic tile is in direct contact with one or more adjacent reticulated ceramic tiles. The multiple reticulated ceramic tiles can collectively form a single perforated flame holder  102 . Alternatively, each reticulated ceramic tile can be considered a distinct perforated flame holder  102 . 
       FIG. 11  is a flow diagram of a method  1100  for operating a burner system, according to an embodiment. At  1102 , a first fuel stream is output from a first fuel nozzle onto a first portion of a perforated flame holder positioned in a furnace volume. At  1104  an oxidant is output into the furnace volume. At  1106  a combustion reaction of first fuel stream and the oxidant is supported in the first portion of the perforated flame holder. At  1108  a second portion of the perforated flame holder is heated to a threshold temperature with the combustion reaction in the first portion of the perforated flame holder. At  1110 , a second fuel stream is output from a second fuel nozzle onto the second portion of the perforated flame holder. At  1112 , a combustion reaction of the second fuel stream and the oxidant is supported in the second portion of the perforated flame holder when the second fuel nozzle outputs the second fuel stream onto the second portion of the perforated flame holder. 
     According to an embodiment, the threshold temperature is a temperature at which the second portion of the perforated flame holder can ignite and support the combustion reaction of the second fuel stream and the oxidant in the second portion of the perforated flame holder. 
     According to an embodiment, the method includes operating the perforated flame holder in a first heat output mode by the supporting the combustion reaction of the first fuel stream and the oxidant within the first portion of the perforated flame holder and controlling the second fuel nozzle to not output the second fuel stream. According to an embodiment, controlling the second fuel nozzle to not output the second fuel stream can include stopping, preventing, or otherwise inhibiting the second fuel nozzle from outputting the second fuel stream. According to an embodiment, controlling the second fuel nozzle to not output the second fuel stream can include stopping an upstream fuel source from delivering fuel to the second fuel nozzle. 
     According to an embodiment, operating the perforated flame holder in the first heat output mode includes heating the second portion of the perforated flame holder to the threshold temperature. 
     According to an embodiment, operating the perforated flame holder in the first heat output mode includes maintaining, with the combustion reaction of the first fuel stream and the oxidant in the first portion of the perforated flame holder, the second portion of the perforated flame holder at or above the threshold temperature. 
     According to an embodiment, the method includes operating the perforated flame holder in a second heat output mode by selectively outputting the second fuel stream from the second fuel nozzle and supporting the combustion reaction of the second fuel stream and the oxidant in the second portion of the perforated flame holder when the second fuel nozzle outputs the second fuel stream onto the second portion of the perforated flame holder. 
     According to an embodiment, operating the perforated flame holder in the second heat output mode includes continuing to support the combustion reaction of the first fuel stream and the oxidant in the first portion of the perforated flame holder. 
     According to an embodiment, operating the perforated flame holder in the first heat output mode includes selectively outputting a plurality of first fuel streams from a plurality of first fuel nozzles onto the first portion of the perforated flame holder positioned in a furnace volume and supporting the combustion reaction of the first fuel streams and the oxidant in the first portion of the perforated flame holder. 
     According to an embodiment, operating the perforated flame holder in the second heat output mode includes selectively outputting a plurality of second fuel streams from a plurality of second fuel nozzles onto the second portion of the perforated flame holder and supporting the combustion reaction of the second fuel streams and the oxidant in the second portion of the perforated flame holder. 
     According to an embodiment, the operating the perforated flame holder in the second heat output mode includes outputting the oxidant into the furnace volume at an increased rate with respect to the reduced output mode. 
     According to an embodiment, the method includes adjusting a flow of the oxidant into the furnace volume by operating a damper. 
     According to an embodiment, the method includes adjusting a flow of the oxidant into the furnace volume by selectively controlling each of multiple oxidant sources. 
     According to an embodiment, the method includes adjusting a flow of the oxidant into the furnace volume by selectively controlling an oxidant source. 
     According to an embodiment, the method includes maintaining a selected fuel to oxidant ratio when transferring between the first heat output mode and the second heat output mode by adjusting a flow of the oxidant into the furnace volume. 
     According to an embodiment, the method includes operating the perforated flame holder in a third heat output mode by selectively outputting a third fuel stream from a third fuel nozzle onto a third portion of the perforated flame holder and supporting the combustion reaction of the third fuel stream and the oxidant in the third portion of the perforated flame holder. 
     According to an embodiment, operating the perforated flame holder in the second heat output mode includes supporting the combustion reaction of the first fuel stream and the oxidant in the first portion of the perforated flame holder, the supporting the combustion reaction of the second fuel stream and the oxidant in the second portion of the perforated flame holder, and selectively not outputting the third fuel stream from the third fuel nozzle. 
     According to an embodiment, operating the perforated flame holder in the second heat output mode includes heating, with the combustion reaction in the second portion of the perforated flame holder, the third portion of the perforated flame holder to the threshold temperature. 
     According to an embodiment, operating the perforated flame holder in the third heat output mode includes supporting the combustion reaction of the first fuel stream and the oxidant in the first portion of the perforated flame holder, the supporting the combustion reaction of the second fuel stream and the oxidant in the second portion of the perforated flame holder, and the supporting the combustion reaction of the third fuel stream and the oxidant in the third portion of the perforated flame holder. 
     According to an embodiment, the first portion of the perforated flame holder is a first burner tile and the second portion of the perforated flame holder is a second burner tile. 
     According to an embodiment, the first portion and second portion of the perforated flame holder are separated by gaps. 
     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.