Patent Publication Number: US-2016238277-A1

Title: Box heater including a perforated flame holder

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority benefit from U.S. Provisional Patent Application No. 62/117,429, entitled “BOX HEATER INCLUDING A PERFORATED FLAME HOLDER,” filed Feb. 17, 2015 (docket number 2651-272-02); which, to the extent not inconsistent with the description herein, is incorporated by reference. 
    
    
     SUMMARY 
     According to an embodiment, an enclosed heater includes a plurality of walls forming an enclosure. The enclosure defines an interior volume. A plurality of tubes are (or a continuous serpentine tube is) positioned along the walls within the interior volume. A working fluid flows through the tubes. A perforated flame holder is also positioned within the interior volume. A fuel and oxidant source outputs fuel and oxidant onto the perforated flame holder. The perforated flame holder supports a combustion reaction of the fuel and oxidant within the perforated flame holder. The perforated flame holder absorbs heat from the combustion reaction and radiates heat to the tubes, thereby transferring heat from the combustion reaction to the working fluid within the tubes. According to an embodiment, the enclosed heater can include a box heater, a cabin heater, or a vertical cylinder heater. 
     According to an embodiment, a method includes outputting fuel and oxidant onto a first perforated flame holder positioned within an interior of an enclosed heater. The enclosed heater includes one or more walls defining the interior. The method further includes supporting a majority of a combustion reaction of the fuel and oxidant within the first perforated flame holder, passing one or more working fluids through a plurality of tubes lining one or more walls within the interior, and transferring heat from the first perforated flame holder to the one or more working fluids. 
     According to an embodiment, a box heater including an enclosure and a tube including a first segment positioned external to the enclosure, a second segment positioned within the enclosure, and a third segment positioned external to the enclosure, the second segment connecting the first segment to the third segment. The tube is configured to pass a working fluid from the first segment, through the second segment, to the third segment. The box heater further includes a fuel and oxidant source positioned within the enclosure and configured to output fuel and oxidant, and a perforated flame holder positioned to receive the fuel and oxidant, to sustain a combustion reaction of the fuel and oxidant within the perforated flame holder, and to transfer heat from the combustion reaction to the working fluid as the working fluid passes through the second segment positioned within the enclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an enclosed heater, according to one embodiment. 
         FIG. 2  is a simplified perspective view of a burner system including a perforated flame holder, according to an embodiment. 
         FIG. 3  is a side sectional diagram of a portion of the perforated flame holder of  FIGS. 1 and 2 , according to an embodiment. 
         FIG. 4  is a flow chart showing a method for operating a burner system including the perforated flame holder of  FIGS. 1, 2 and 3 , according to an embodiment. 
         FIG. 5A  is perspective view of a cabin heater, according to an embodiment. 
         FIG. 5B  is a top view of the cabin heater of  FIG. 5A , according to an embodiment. 
         FIG. 5C  is a cross-section of the cabin heater of  FIG. 5A , according to an embodiment. 
         FIG. 5D  is a side view of an interior of the cabin heater of  FIG. 5A , according to an embodiment. 
         FIG. 6  is a side view of an interior of a box heater including multiple perforated flame holders, according to an embodiment. 
         FIG. 7  is a side view of the interior of a box heater including multiple perforated flame holders, according to an embodiment. 
         FIG. 8  is a flow diagram of a process for operating an enclosed heater including a perforated flame holder, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Other embodiments may be used and/or other changes may be made without departing from the spirit or scope of the disclosure. 
       FIG. 1  is a block diagram of an enclosed heater burner system  100 , according to one embodiment. The enclosed heater burner system  100  includes a wall  110  enclosing an enclosed heater interior volume  112 . A perforated flame holder  102  is positioned within the enclosed heater interior  112 . A fuel and oxidant source  104  is positioned to output fuel and oxidant onto the perforated flame holder  102 . Tubes  106  carry a working fluid  108 . 
     According to an embodiment, the fuel and oxidant source  104  outputs fuel and oxidant onto the perforated flame holder  102 . The perforated flame holder  102  supports a combustion reaction of the fuel and oxidant within the perforated flame holder  102 . The perforated flame holder  102  absorbs heat from the combustion reaction within the perforated flame holder  102 . The perforated flame holder  102  radiates heat to the tubes  106  lining the wall  110  of the enclosed heater burner system  100 . The working fluid  108  is passed through the tubes  106 . Heat is transferred from the tubes  106  to the working fluid  108 . In this way, heat is transferred from the perforated flame holder  102  to the working fluid  108 . The working fluid  108  is passed via the tubes  106  to the exterior of the enclosed heater burner system  100  and is used to provide heat to an external apparatus. According to an embodiment, the working fluid  108  transfers heat to crude oil flowing through a crude oil pipeline. In another embodiment the crude oil itself can be used as the working fluid  108 . Even and predictable radiative heating provided by the perforated flame holder  102  can substantially prevent coking and other infirmities of conventional flame heaters. This may be used to simplify the design to allow even an inconsistent working fluid, such as crude oil with an asphaltene component, to avoid coking in the tubes  106 . Moreover, whether or not a conventional 2-step working fluid-to-heat sink transfer is used, the perforated flame holder  102  provides very low output of oxides of nitrogen (NOx) and carbon monoxide (CO). 
     According to an embodiment, the enclosed heater defines an interior volume that is between 5 and 30 feet in height, length, and width. 
     According to an embodiment, the enclosed heater burner system  100  is a box heater such as a cabin heater including a peaked roof. According to an alternative embodiment, the enclosed heater burner system  100  incorporates a vertical cylindrical heater. Those of skill in the art will recognize, in light of the present disclosure, that a perforated flame holder  102  can be utilized in a large variety of types of enclosed heaters in accordance with principles of the present disclosure. All types of enclosed heaters fall within the scope of the present disclosure. 
       FIG. 2  is a simplified diagram of a burner system  200  including a perforated flame holder  102  configured to hold a combustion reaction, according to an embodiment. As used herein, the terms perforated flame holder, perforated reaction holder, porous flame holder, porous reaction holder, duplex, and duplex tile shall be considered synonymous unless further definition is provided. 
     Experiments performed by the inventors have shown that perforated flame holders  102  described herein can support very clean combustion. Specifically, in experimental use of systems  200  ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (O 2 ) concentration with undetectable carbon monoxide (CO) at stack temperatures typical of industrial furnace applications (1400-1600° F.). Moreover, these results did not require any extraordinary measures such as selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), water/steam injection, external flue gas recirculation (FGR), or other heroic extremes that may be required for conventional burners to even approach such clean combustion. 
     According to embodiments, the burner system  200  includes a fuel and oxidant source  202  disposed to output fuel and oxidant into a combustion volume  204  to form a fuel and oxidant mixture  206 . As used herein, the terms fuel and oxidant mixture and fuel stream may be used interchangeably and considered synonymous depending on the context, unless further definition is provided. As used herein, the terms combustion volume, combustion chamber, furnace volume, and the like shall be considered synonymous unless further definition is provided. The perforated flame holder  102  is disposed in the combustion volume  204  and positioned to receive the fuel and oxidant mixture  206 . 
       FIG. 3  is a side sectional diagram  300  of a portion of the perforated flame holder  102  of  FIGS. 1 and 2 , according to an embodiment. Referring to  FIGS. 2 and 3 , the perforated flame holder  102  includes a perforated flame holder body  208  defining a plurality of perforations  210  aligned to receive the fuel and oxidant mixture  206  from the fuel and oxidant source  202 . As used herein, the terms perforation, pore, aperture, elongated aperture, and the like, in the context of the perforated flame holder  102 , shall be considered synonymous unless further definition is provided. The perforations  210  are configured to collectively hold a combustion reaction  302  supported by the fuel and oxidant mixture  206 . 
     The fuel can include hydrogen, a hydrocarbon gas, a vaporized hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel can be a single species or can include a mixture of gas(es), vapor(s), atomized liquid(s), and/or pulverized solid(s). For example, in a process heater application the fuel can include fuel gas or byproducts from the process that include carbon monoxide (CO), hydrogen (H 2 ), and methane (CH 4 ). In another application the fuel can include natural gas (mostly CH 4 ) or propane (C 3 H 8 ). In another application, the fuel can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant can include oxygen carried by air, flue gas, and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein. 
     According to an embodiment, the perforated flame holder body  208  can be bounded by an input face  212  disposed to receive the fuel and oxidant mixture  206 , an output face  214  facing away from the fuel and oxidant source  202 , and a peripheral surface  216  defining a lateral extent of the perforated flame holder  102 . The plurality of perforations  210  which are defined by the perforated flame holder body  208  extend from the input face  212  to the output face  214 . The plurality of perforations  210  can receive the fuel and oxidant mixture  206  at the input face  212 . The fuel and oxidant mixture  206  can then combust in or near the plurality of perforations  210  and combustion products can exit the plurality of perforations  210  at or near the output face  214 . 
     According to an embodiment, the perforated flame holder  102  is configured to hold a majority of the combustion reaction  302  within the perforations  210 . For example, on a steady-state basis, more than half the molecules of fuel output into the combustion volume  204  by the fuel and oxidant source  202  may be converted to combustion products between the input face  212  and the output face  214  of the perforated flame holder  102 . According to an alternative interpretation, more than half of the heat or thermal energy output by the combustion reaction  302  may be output between the input face  212  and the output face  214  of the perforated flame holder  102 . As used herein, the terms heat, heat energy, and thermal energy shall be considered synonymous unless further definition is provided. As used above, heat energy and thermal energy refer generally to the released chemical energy initially held by reactants during the combustion reaction  302 . As used elsewhere herein, heat, heat energy and thermal energy correspond to a detectable temperature rise undergone by real bodies characterized by heat capacities. Under nominal operating conditions, the perforations  210  can be configured to collectively hold at least 80% of the combustion reaction  302  between the input face  212  and the output face  214  of the perforated flame holder  102 . In some experiments, the inventors produced a combustion reaction  302  that was apparently wholly contained in the perforations  210  between the input face  212  and the output face  214  of the perforated flame holder  102 . According to an alternative interpretation, the perforated flame holder  102  can support combustion between the input face  212  and output face  214  when combustion is “time-averaged.” For example, during transients, such as before the perforated flame holder  102  is fully heated, or if too high a (cooling) load is placed on the system, the combustion may travel somewhat downstream from the output face  214  of the perforated flame holder  102 . Alternatively, if the cooling load is relatively low and/or the furnace temperature reaches a high level, the combustion may travel somewhat upstream of the input face  212  of the perforated flame holder  102 . 
     While a “flame” is described in a manner intended for ease of description, it should be understood that in some instances, no visible flame is present. Combustion occurs primarily within the perforations  210 , but the “glow” of combustion heat is dominated by a visible glow of the perforated flame holder  102  itself. In other instances, the inventors have noted transient “huffing” or “flashback” wherein a visible flame momentarily ignites in a region lying between the input face  212  of the perforated flame holder  102  and the fuel nozzle  218 , within the dilution region D D . Such transient huffing or flashback is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations  210  of the perforated flame holder  102 , between the input face  212  and the output face  214 . In still other instances, the inventors have noted apparent combustion occurring downstream from the output face  214  of the perforated flame holder  102 , but still a majority of combustion occurred within the perforated flame holder  102  as evidenced by continued visible glow from the perforated flame holder  102  that was observed. 
     The perforated flame holder  102  can be configured to receive heat from the combustion reaction  302  and output a portion of the received heat as thermal radiation  304  to heat-receiving structures (e.g., furnace walls and/or radiant section working fluid tubes) in or adjacent to the combustion volume  204 . As used herein, terms such as radiation, thermal radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody-type radiation of electromagnetic energy, primarily at infrared wavelengths, but also at visible wavelengths owing to elevated temperature of the perforated flame holder body  208 . 
     Referring especially to  FIG. 3 , the perforated flame holder  102  outputs another portion of the received heat to the fuel and oxidant mixture  206  received at the input face  212  of the perforated flame holder  102 . The perforated flame holder body  208  may receive heat from the combustion reaction  302  at least in heat receiving regions  306  of perforation walls  308 . Experimental evidence has suggested to the inventors that the position of the heat receiving regions  306 , or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls  308 . In some experiments, the location of maximum receipt of heat was apparently between ⅓ and ½ of the distance from the input face  212  to the output face  214  (i.e., somewhat nearer to the input face  212  than to the output face  214 ). The inventors contemplate that the heat receiving regions  306  may lie nearer to the output face  214  of the perforated flame holder  102  under other conditions. Most probably, there is no clearly defined edge of the heat receiving regions  306  (or for that matter, the heat output regions  310 , described below). For ease of understanding, the heat receiving regions  306  and the heat output regions  310  will be described as particular regions  306 ,  310 . 
     The perforated flame holder body  208  can be characterized by a heat capacity. The perforated flame holder body  208  may hold thermal energy from the combustion reaction  302  in an amount corresponding to the heat capacity multiplied by temperature rise, and transfer the thermal energy from the heat receiving regions  306  to heat output regions  310  of the perforation walls  308 . Generally, the heat output regions  310  are nearer to the input face  212  than are the heat receiving regions  306 . According to one interpretation, the perforated flame holder body  208  can transfer heat from the heat receiving regions  306  to the heat output regions  310  via thermal radiation, depicted graphically as  304 . According to another interpretation, the perforated flame holder body  208  can transfer heat from the heat receiving regions  306  to the heat output regions  310  via heat conduction along heat conduction paths  312 . The inventors contemplate that multiple heat transfer mechanisms including conduction, radiation, and possibly convection may be operative in transferring heat from the heat receiving regions  306  to the heat output regions  310 . In this way, the perforated flame holder  102  may act as a heat source to maintain the combustion reaction  302 , even under conditions where a combustion reaction  302  would not be stable when supported from a conventional flame holder. 
     The inventors believe that the perforated flame holder  102  causes the combustion reaction  302  to begin within thermal boundary layers  314  formed adjacent to walls  308  of the perforations  210 . Insofar as combustion is generally understood to include a large number of individual reactions, and since a large portion of combustion energy is released within the perforated flame holder  102 , it is apparent that at least a majority of the individual reactions occur within the perforated flame holder  102 . As the relatively cool fuel and oxidant mixture  206  approaches the input face  212 , the flow is split into portions that respectively travel through individual perforations  210 . The hot perforated flame holder body  208  transfers heat to the fluid, notably within thermal boundary layers  314  that progressively thicken as more and more heat is transferred to the incoming fuel and oxidant mixture  206 . After reaching a combustion temperature (e.g., the auto-ignition temperature of the fuel), the reactants continue to flow while a chemical ignition delay time elapses, over which time the combustion reaction  302  occurs. Accordingly, the combustion reaction  302  is shown as occurring within the thermal boundary layers  314 . As flow progresses, the thermal boundary layers  314  merge at a merger point  316 . Ideally, the merger point  316  lies between the input face  212  and output face  214  that define the ends of the perforations  210 . At some position along the length of a perforation  210 , the combustion reaction  302  outputs more heat to the perforated flame holder body  208  than it receives from the perforated flame holder body  208 . The heat is received at the heat receiving region  306 , is held by the perforated flame holder body  208 , and is transported to the heat output region  310  nearer to the input face  212 , where the heat is transferred into the cool reactants (and any included diluent) to bring the reactants to the ignition temperature. 
     In an embodiment, each of the perforations  210  is characterized by a length L defined as a reaction fluid propagation path length between the input face  212  and the output face  214  of the perforated flame holder  102 . As used herein, the term reaction fluid refers to matter that travels through a perforation  210 . Near the input face  212 , the reaction fluid includes the fuel and oxidant mixture  206  (optionally including nitrogen, flue gas, and/or other “non-reactive” species). Within the combustion reaction region, the reaction fluid may include plasma associated with the combustion reaction  302 , molecules of reactants and their constituent parts, any non-reactive species, reaction intermediates (including transition states), and reaction products. Near the output face  214 , the reaction fluid may include reaction products and byproducts, non-reactive gas, and excess oxidant. 
     The plurality of perforations  210  can be each characterized by a transverse dimension D between opposing perforation walls  308 . The inventors have found that stable combustion can be maintained in the perforated flame holder  102  if the length L of each perforation  210  is at least four times the transverse dimension D of the perforation. In other embodiments, the length L can be greater than six times the transverse dimension D. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D. Preferably, the length L is sufficiently long for thermal boundary layers  314  to form adjacent to the perforation walls  308  in a reaction fluid flowing through the perforations  210  to converge at merger points  316  within the perforations  210  between the input face  212  and the output face  214  of the perforated flame holder  102 . In experiments, the inventors have found L/D ratios between 12 and 48 to work well (i.e., produce low NOx, produce low CO, and maintain stable combustion). 
     The perforated flame holder body  208  can be configured to convey heat between adjacent perforations  210 . The heat conveyed between adjacent perforations  210  can be selected to cause heat output from the combustion reaction portion  302  in a first perforation  210  to supply heat to stabilize a combustion reaction portion  302  in an adjacent perforation  210 . 
     Referring especially to  FIG. 2 , the fuel and oxidant source  202  can further include a fuel nozzle  218 , configured to output fuel, and an oxidant source  220  configured to output a fluid including the oxidant. For example, the fuel nozzle  218  can be configured to output pure fuel. The oxidant source  220  can be configured to output combustion air carrying oxygen, and optionally, flue gas. 
     The perforated flame holder  102  can be held by a perforated flame holder support structure  222  configured to hold the perforated flame holder  102  at a dilution distance D D  away from the fuel nozzle  218 . The fuel nozzle  218  can be configured to emit a fuel jet selected to entrain the oxidant to form the fuel and oxidant mixture  206  as the fuel jet and oxidant travel along a path to the perforated flame holder  102  through the dilution distance D D  between the fuel nozzle  218  and the perforated flame holder  102 . Additionally or alternatively (particularly when a blower is used to deliver oxidant contained in combustion air), the oxidant or combustion air source can be configured to entrain the fuel and the fuel and oxidant travel through the dilution distance D D . In some embodiments, a flue gas recirculation path  224  can be provided. Additionally or alternatively, the fuel nozzle  218  can be configured to emit a fuel jet selected to entrain the oxidant and to entrain flue gas as the fuel jet travels through the dilution distance D D  between the fuel nozzle  218  and the input face  212  of the perforated flame holder  102 . 
     The fuel nozzle  218  can be configured to emit the fuel through one or more fuel orifices  226  having an inside diameter dimension that is referred to as “nozzle diameter.” The perforated flame holder support structure  222  can support the perforated flame holder  102  to receive the fuel and oxidant mixture  206  at the distance D D  away from the fuel nozzle  218  greater than 20 times the nozzle diameter. In another embodiment, the perforated flame holder  102  is disposed to receive the fuel and oxidant mixture  206  at the distance D D  away from the fuel nozzle  218  between 100 times and 1100 times the nozzle diameter. Preferably, the perforated flame holder support structure  222  is configured to hold the perforated flame holder  102  at a distance about 200 times or more of the nozzle diameter away from the fuel nozzle  218 . When the fuel and oxidant mixture  206  travels about 200 times the nozzle diameter or more, the mixture is sufficiently homogenized to cause the combustion reaction  302  to produce minimal NOx. 
     The fuel and oxidant source  202  can alternatively include a premix fuel and oxidant source, according to an embodiment. A premix fuel and oxidant source can include a premix chamber (not shown), a fuel nozzle configured to output fuel into the premix chamber, and an oxidant (e.g., combustion air) channel configured to output the oxidant into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the perforated flame holder  102  and be configured to prevent flame flashback into the premix fuel and oxidant source. 
     The oxidant source  220 , whether configured for entrainment in the combustion volume  204  or for premixing, can include a blower configured to force the oxidant through the fuel and oxidant source  202 . 
     The support structure  222  can be configured to support the perforated flame holder  102  from a floor or wall (not shown) of the combustion volume  204 , for example. In another embodiment, the support structure  222  supports the perforated flame holder  102  from the fuel and oxidant source  202 . Alternatively, the support structure  222  can suspend the perforated flame holder  102  from an overhead structure (such as a flue, in the case of an up-fired system). The support structure  222  can support the perforated flame holder  102  in various orientations and directions. 
     The perforated flame holder  102  can include a single perforated flame holder body  208 . In another embodiment, the perforated flame holder  102  can include a plurality of adjacent perforated flame holder sections that collectively provide a tiled perforated flame holder  102 . 
     The perforated flame holder support structure  222  can be configured to support the plurality of perforated flame holder sections. The perforated flame holder support structure  222  can include a metal superalloy, a cementatious, and/or ceramic refractory material. In an embodiment, the plurality of adjacent perforated flame holder sections can be joined with a fiber reinforced refractory cement. 
     The perforated flame holder  102  can have a width dimension W between opposite sides of the peripheral surface  216  at least twice a thickness dimension T between the input face  212  and the output face  214 . In another embodiment, the perforated flame holder  102  can have a width dimension W between opposite sides of the peripheral surface  216  at least three times, at least six times, or at least nine times the thickness dimension T between the input face  212  and the output face  214  of the perforated flame holder  102 . 
     In an embodiment, the perforated flame holder  102  can have a width dimension W less than a width of the combustion volume  204 . This can allow the flue gas circulation path  224  from above to below the perforated flame holder  102  to lie between the peripheral surface  216  of the perforated flame holder  102  and the combustion volume wall (not shown). 
     Referring again to both  FIGS. 2 and 3 , the perforations  210  can be of various shapes. In an embodiment, the perforations  210  can include elongated squares, each having a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations  210  can include elongated hexagons, each having a transverse dimension D between opposing sides of the hexagons. In yet another embodiment, the perforations  210  can include hollow cylinders, each having a transverse dimension D corresponding to a diameter of the cylinder. In another embodiment, the perforations  210  can include truncated cones or truncated pyramids (e.g., frustums), each having a transverse dimension D radially symmetric relative to a length axis that extends from the input face  212  to the output face  214 . In some embodiments, the perforations  210  can each have a lateral dimension D equal to or greater than a quenching distance of the flame based on standard reference conditions. Alternatively, the perforations  210  may have lateral dimension D less then than a standard reference quenching distance. 
     In one range of embodiments, each of the plurality of perforations  210  has a lateral dimension D between 0.05 inch and 1.0 inch. Preferably, each of the plurality of perforations  210  has a lateral dimension D between 0.1 inch and 0.5 inch. For example the plurality of perforations  210  can each have a lateral dimension D of about 0.2 to 0.4 inch. 
     The void fraction of a perforated flame holder  102  is defined as the total volume of all perforations  210  in a section of the perforated flame holder  102  divided by a total volume of the perforated flame holder  102  including body  208  and perforations  210 . The perforated flame holder  102  should have a void fraction between 0.10 and 0.90. In an embodiment, the perforated flame holder  102  can have a void fraction between 0.30 and 0.80. In another embodiment, the perforated flame holder  102  can have a void fraction of about 0.70. Using a void fraction of about 0.70 was found to be especially effective for producing very low NOx. 
     The perforated flame holder  102  can be formed from a fiber reinforced cast refractory material and/or a refractory material such as an aluminum silicate material. For example, the perforated flame holder  102  can be formed to include mullite or cordierite. Additionally or alternatively, the perforated flame holder body  208  can include a metal superalloy such as Inconel or Hastelloy. The perforated flame holder body  208  can define a honeycomb. Honeycomb is an industrial term of art that need not strictly refer to a hexagonal cross section and most usually includes cells of square cross section. Honeycombs of other cross sectional areas are also known. 
     The inventors have found that the perforated flame holder  102  can be formed from VERSAGRID® ceramic honeycomb, available from Applied Ceramics, Inc. of Doraville, S.C. 
     The perforations  210  can be parallel to one another and normal to the input and output faces  212 ,  214 . In another embodiment, the perforations  210  can be parallel to one another and formed at an angle relative to the input and output faces  212 ,  214 . In another embodiment, the perforations  210  can be non-parallel to one another. In another embodiment, the perforations  210  can be non-parallel to one another and non-intersecting. In another embodiment, the perforations  210  can be intersecting. The body  308  can be one piece or can be formed from a plurality of sections. 
     In another embodiment, which is not necessarily preferred, the perforated flame holder  102  may be formed from reticulated ceramic material. The term “reticulated” refers to a netlike structure. Reticulated ceramic material is often made by dissolving a slurry into a sponge of specified porosity, allowing the slurry to harden, and burning away the sponge and curing the ceramic. 
     In another embodiment, which is not necessarily preferred, the perforated flame holder  102  may be formed from a ceramic material that has been punched, bored or cast to create channels. 
     In another embodiment, the perforated flame holder  102  can include a plurality of tubes or pipes bundled together. The plurality of perforations  210  can include hollow cylinders and can optionally also include interstitial spaces between the bundled tubes. In an embodiment, the plurality of tubes can include ceramic tubes. Refractory cement can be included between the tubes and configured to adhere the tubes together. In another embodiment, the plurality of tubes can include metal (e.g., superalloy) tubes. The plurality of tubes can be held together by a metal tension member circumferential to the plurality of tubes and arranged to hold the plurality of tubes together. The metal tension member can include stainless steel, a superalloy metal wire, and/or a superalloy metal band. 
     The perforated flame holder body  208  can alternatively include stacked perforated sheets of material, each sheet having openings that connect with openings of subjacent and superjacent sheets. The perforated sheets can include perforated metal sheets, ceramic sheets and/or expanded sheets. In another embodiment, the perforated flame holder body  208  can include discontinuous packing bodies such that the perforations  210  are formed in the interstitial spaces between the discontinuous packing bodies. In one example, the discontinuous packing bodies include structured packing shapes. In another example, the discontinuous packing bodies include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig ring, ceramic Berl saddles, ceramic Intalox saddles, and/or metal rings or other shapes (e.g. Super Raschig Rings) that may be held together by a metal cage. 
     The inventors contemplate various explanations for why burner systems including the perforated flame holder  102  provide such clean combustion. 
     According to an embodiment, the perforated flame holder  102  may act as a heat source to maintain a combustion reaction even under conditions where a combustion reaction would not be stable when supported by a conventional flame holder. This capability can be leveraged to support combustion using a leaner fuel-to-oxidant mixture than is typically feasible. Thus, according to an embodiment, at the point where the fuel stream  206  contacts the input face  212  of the perforated flame holder  102 , an average fuel-to-oxidant ratio of the fuel stream  206  is below a (conventional) lower combustion limit of the fuel component of the fuel stream  206 —lower combustion limit defines the lowest concentration of fuel at which a fuel and oxidant mixture  206  will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.). 
     The perforated flame holder  102  and systems including the perforated flame holder  102  described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. According to one interpretation, such a performance can be achieved due to a sufficient mixing used to lower peak flame temperatures (among other strategies). Flame temperatures tend to peak under slightly rich conditions, which can be evident in any diffusion flame that is insufficiently mixed. By sufficiently mixing, a homogenous and slightly lean mixture can be achieved prior to combustion. This combination can result in reduced flame temperatures, and thus reduced NOx formation. In one embodiment, “slightly lean” may refer to 3% O 2 , i.e. an equivalence ratio of ˜0.87. Use of even leaner mixtures is possible, but may result in elevated levels of O 2 . Moreover, the inventors believe perforation walls  308  may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperatures and lower NOx. 
     According to another interpretation, production of NOx can be reduced if the combustion reaction  302  occurs over a very short duration of time. Rapid combustion causes the reactants (including oxygen and entrained nitrogen) to be exposed to NOx-formation temperature for a time too short for NOx formation kinetics to cause significant production of NOx. The time required for the reactants to pass through the perforated flame holder  102  is very short compared to a conventional flame. The low NOx production associated with perforated flame holder combustion may thus be related to the short duration of time required for the reactants (and entrained nitrogen) to pass through the perforated flame holder  102 . 
       FIG. 4  is a flow chart showing a method  400  for operating a burner system including the perforated flame holder shown and described herein. To operate a burner system including a perforated flame holder, the perforated flame holder is first heated to a temperature sufficient to maintain combustion of the fuel and oxidant mixture. 
     According to a simplified description, the method  400  begins with step  402 , wherein the perforated flame holder is preheated to a start-up temperature, T S . After the perforated flame holder is raised to the start-up temperature, the method proceeds to step  404 , wherein the fuel and oxidant are provided to the perforated flame holder and combustion is held by the perforated flame holder. 
     According to a more detailed description, step  402  begins with step  406 , wherein start-up energy is provided at the perforated flame holder. Simultaneously or following providing start-up energy, a decision step  408  determines whether the temperature T of the perforated flame holder is at or above the start-up temperature, T S . As long as the temperature of the perforated flame holder is below its start-up temperature, the method loops between steps  406  and  408  within the preheat step  402 . In step  408 , if the temperature T of at least a predetermined portion of the perforated flame holder is greater than or equal to the start-up temperature, the method  400  proceeds to overall step  404 , wherein fuel and oxidant is supplied to and combustion is held by the perforated flame holder. 
     Step  404  may be broken down into several discrete steps, at least some of which may occur simultaneously. 
     Proceeding from step  408 , a fuel and oxidant mixture is provided to the perforated flame holder, as shown in step  410 . The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and oxidant (e.g., combustion air) source, for example. In this approach, the fuel and oxidant are output in one or more directions selected to cause the fuel and oxidant mixture to be received by the input face of the perforated flame holder. The fuel may entrain the combustion air (or alternatively, the combustion air may dilute the fuel) to provide a fuel and oxidant mixture at the input face of the perforated flame holder at a fuel dilution selected for a stable combustion reaction that can be held within the perforations of the perforated flame holder. 
     Proceeding to step  412 , the combustion reaction is held by the perforated flame holder. 
     In step  414 , heat may be output from the perforated flame holder. The heat output from the perforated flame holder may be used to power an industrial process, heat a working fluid, generate electricity, or provide motive power, for example. 
     In optional step  416 , the presence of combustion may be sensed. Various sensing approaches have been used and are contemplated by the inventors. Generally, combustion held by the perforated flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, flame rod, and/or other combustion sensing apparatuses. In an additional or alternative variant of step  416 , a pilot flame or other ignition source may be provided to cause ignition of the fuel and oxidant mixture in the event combustion is lost at the perforated flame holder. 
     Proceeding to decision step  418 , if combustion is sensed not to be stable, the method  400  may exit to step  424 , wherein an error procedure is executed. For example, the error procedure may include turning off fuel flow, re-executing the preheating step  402 , outputting an alarm signal, igniting a stand-by combustion system, or other steps. If, in step  418 , combustion in the perforated flame holder is determined to be stable, the method  400  proceeds to decision step  420 , wherein it is determined if combustion parameters should be changed. If no combustion parameters are to be changed, the method loops (within step  404 ) back to step  410 , and the combustion process continues. If a change in combustion parameters is indicated, the method  400  proceeds to step  422 , wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step  404 ) back to step  410 , and combustion continues. 
     Combustion parameters may be scheduled to be changed, for example, if a change in heat demand is encountered. For example, if less heat is required (e.g., due to decreased electricity demand, decreased motive power requirement, or lower industrial process throughput), the fuel and oxidant flow rate may be decreased in step  422 . Conversely, if heat demand is increased, then fuel and oxidant flow may be increased. Additionally or alternatively, if the combustion system is in a start-up mode, then fuel and oxidant flow may be gradually increased to the perforated flame holder over one or more iterations of the loop within step  404 . 
     Referring again to  FIG. 2 , the burner system  200  includes a heater  228  operatively coupled to the perforated flame holder  102 . As described in conjunction with  FIGS. 3 and 4 , the perforated flame holder  102  operates by outputting heat to the incoming fuel and oxidant mixture  206 . After combustion is established, this heat is provided by the combustion reaction  302 ; but before combustion is established, the heat is provided by the heater  228 . 
     Various heating apparatuses have been used and are contemplated by the inventors. In some embodiments, the heater  228  can include a flame holder configured to support a flame disposed to heat the perforated flame holder  102 . The fuel and oxidant source  202  can include a fuel nozzle  218  configured to emit a fuel stream  206  and an oxidant source  220  configured to output oxidant (e.g., combustion air) adjacent to the fuel stream  206 . The fuel nozzle  218  and oxidant source  220  can be configured to output the fuel stream  206  to be progressively diluted by the oxidant (e.g., combustion air). The perforated flame holder  102  can be disposed to receive a diluted fuel and oxidant mixture  206  that supports a combustion reaction  302  that is stabilized by the perforated flame holder  102  when the perforated flame holder  102  is at an operating temperature. A start-up flame holder, in contrast, can be configured to support a start-up flame at a location corresponding to a relatively unmixed fuel and oxidant mixture that is stable without stabilization provided by the heated perforated flame holder  102 . 
     The burner system  200  can further include a controller  230  operatively coupled to the heater  228  and to a data interface  232 . For example, the controller  230  can be configured to control a start-up flame holder actuator configured to cause the start-up flame holder to hold the start-up flame when the perforated flame holder  102  needs to be pre-heated and to not hold the start-up flame when the perforated flame holder  102  is at an operating temperature (e.g., when T≧T S ). 
     Various approaches for actuating a start-up flame are contemplated. In one embodiment, the start-up flame holder includes a mechanically-actuated bluff body configured to be actuated to intercept the fuel and oxidant mixture  206  to cause heat-recycling and/or stabilizing vortices and thereby hold a start-up flame; or to be actuated to not intercept the fuel and oxidant mixture  206  to cause the fuel and oxidant mixture  206  to proceed to the perforated flame holder  102 . In another embodiment, a fuel control valve, blower, and/or damper may be used to select a fuel and oxidant mixture flow rate that is sufficiently low for a start-up flame to be jet-stabilized; and upon reaching a perforated flame holder  102  operating temperature, the flow rate may be increased to “blow out” the start-up flame. In another embodiment, the heater  228  may include an electrical power supply operatively coupled to the controller  230  and configured to apply an electrical charge or voltage to the fuel and oxidant mixture  206 . An electrically conductive start-up flame holder may be selectively coupled to a voltage ground or other voltage selected to attract the electrical charge in the fuel and oxidant mixture  206 . The attraction of the electrical charge was found by the inventors to cause a start-up flame to be held by the electrically conductive start-up flame holder. 
     In another embodiment, the heater  228  may include an electrical resistance heater configured to output heat to the perforated flame holder  102  and/or to the fuel and oxidant mixture  206 . The electrical resistance heater can be configured to heat up the perforated flame holder  102  to an operating temperature. The heater  228  can further include a power supply and a switch operable, under control of the controller  230 , to selectively couple the power supply to the electrical resistance heater. 
     An electrical resistance heater  228  can be formed in various ways. For example, the electrical resistance heater  228  can be formed from KANTHAL® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstahammar, Sweden) threaded through at least a portion of the perforations  210  defined by the perforated flame holder body  208 . Alternatively, the heater  228  can include an inductive heater, a high-energy beam heater (e.g. microwave or laser), a frictional heater, electro-resistive ceramic coatings, or other types of heating technologies. 
     Other forms of start-up apparatuses are contemplated. For example, the heater  228  can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the oxidant and fuel. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite the fuel and oxidant mixture  206  that would otherwise enter the perforated flame holder  102 . The electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller  230 , which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the fuel and oxidant mixture  206  in or upstream from the perforated flame holder  102  before the perforated flame holder  102  is heated sufficiently to maintain combustion. 
     The burner system  200  can further include a sensor  234  operatively coupled to the control circuit  230 . The sensor  234  can include a heat sensor configured to detect infrared radiation or a temperature of the perforated flame holder  102 . The control circuit  230  can be configured to control the heating apparatus  228  responsive to input from the sensor  234 . Optionally, a fuel control valve  236  can be operatively coupled to the controller  230  and configured to control a flow of fuel to the fuel and oxidant source  202 . Additionally or alternatively, an oxidant blower or damper  238  can be operatively coupled to the controller  230  and configured to control flow of the oxidant (or combustion air). 
     The sensor  234  can further include a combustion sensor operatively coupled to the control circuit  230 , the combustion sensor being configured to detect a temperature, video image, and/or spectral characteristic of a combustion reaction held by the perforated flame holder  102 . The fuel control valve  236  can be configured to control a flow of fuel from a fuel source to the fuel and oxidant source  202 . The controller  230  can be configured to control the fuel control valve  236  responsive to input from the combustion sensor  234 . The controller  230  can be configured to control the fuel control valve  236  and/or oxidant blower or damper to control a preheat flame type of heater  228  to heat the perforated flame holder  102  to an operating temperature. The controller  230  can similarly control the fuel control valve  236  and/or the oxidant blower or damper to change the fuel and oxidant mixture  206  flow responsive to a heat demand change received as data via the data interface  232 . 
       FIG. 5A  is a perspective view of a cabin heater  500 , according to an embodiment. The cabin heater  500  includes sidewalls  510   a - 510   d , though only sidewalls  510   a ,  510   b  are visible in  FIG. 5A . The cabin heater  500  further includes sloped roof portions  514   a - d  (only  514   a  and  514   b  are visible), and a flat roof peak portion  516 . A flue vent  518  is positioned on roof peak  516 . The flue vent  518  vents flue gases from an interior of the cabin heater  500  to an exterior of the cabin heater  500 . A first tube  506   a  passes through the sidewall  510  into the interior of the cabin heater  500 . A second tube  506   b  also passes from the exterior of the cabin heater  500  to the interior of the cabin heater  500 . A working fluid  108  passes into the tube  506   a  via an input  520   a . A working fluid  108  passes into the tube  506   b  via a second input  520   b.    
       FIG. 5B  is a top view of the cabin heater  500 , according to one embodiment. All four of the sloped roof portions  514   a - 514   d  and the roof peak  516  are visible in the top view of  FIG. 5B . Additionally, output portions  522   a ,  522   b  of the tubes  506   a ,  506   b  are visible. In particular, the working fluid  108  is passed from the tube  506   a  via an output  522   a . The working fluid  108  is passed from the tube  506   b  via the output  522   b.    
       FIG. 5C  is a cross-section of the cabin heater  500  taken along cross-section lines  5 C from  FIG. 5B , according to an embodiment.  FIG. 5C  illustrates that the tube  506   a  lines the sidewall  510   d  and the sloped roof portion  514   d  within the interior volume  512  of the cabin heater  500 . Multiple portions of the tube  506   a  are visible in  FIG. 5C  because the tube  506   a  runs in a serpentine manner along the sidewall  510   d  and the sloped roof portion  514   d , as is further apparent with reference to  FIG. 5D . The tube  506   b  lines the sidewall  510   b  and the sloped roof portion  514   b . Multiple portions of the tube  506   b  are visible in  FIG. 5C  because the tube  506   b  runs in a serpentine manner along the sidewall  510   b  and the sloped roof portion  514   b  as is further apparent with reference to  FIG. 5D . A perforated flame holder  102  is positioned in the interior volume  512  between the tubes  506   a  and  506   b  and above a fuel and oxidant source  524 . 
     The fuel and oxidant source  524  outputs fuel and oxidant  526  onto the perforated flame holder  102 . The perforated flame holder  102  supports a combustion reaction of the fuel and oxidant  526  within the perforated flame holder  102 . The perforated flame holder  102  absorbs heat from the combustion reaction of the fuel and oxidant  526 . The perforated flame holder  102  radiates heat to the tubes  506   a ,  506   b . As the working fluid  508  passes through the multiple serpentine portions of the tubes  506   a ,  506   b , heat from the perforated flame holder  102  is transferred to the working fluid  508  in the tubes  506   a ,  506   b . When the working fluid  508  passes from the outputs  522   a ,  522   b  the working fluid has absorbed a large amount of heat. In this way, the working fluid  508  transfers heat from the combustion reaction of the fuel and oxidant  526  to the exterior of the cabin heater  500 . 
     According to an embodiment, the working fluid  508  is crude oil. Alternatively, the working fluid  508  can include natural gas, other hydrocarbon products, water, glycol water, air, or regeneration gas. Those skilled in the art will recognize, in light of the present disclosure, that the working fluid  508  can include many other fluids. 
     According to an embodiment, the fuel and oxidant source  524  protrudes through the floor of the cabin heater  500 . Only the portion of the fuel and oxidant source  524  that is positioned within the cabin heater  500  is shown in  FIG. 5C  and  FIG. 5D . 
       FIG. 5D  is a side view of the interior volume  512  of the cabin heater  500  viewed from the cross-section lines  5 D of  FIG. 5B . The perforated flame holder  102  and the fuel and oxidant source  524  are visible in the foreground. The tube  506   a  is visible in the background. In particular, the serpentine layout of the tube  506   a  is more readily apparent in the side view of  FIG. 5D . 
     Although a cabin heater  500  has been shown with respect to  FIGS. 5A-5D , those of skill in the art will recognize, in light of the present disclosure, that many other types of box heaters can implement a perforated flame holder  102 . For example, a box heater can have a cylindrical outer wall with one or more tubes lining the inside of the wall and a perforated flame holder  102  positioned in an interior volume of the box heater. 
     According to an embodiment, multiple tubes  506  can be positioned on each wall of the cabin heater  500 . The multiple tubes  506  can be wound about each other. Those skilled in the art will recognize, in light of the present disclosure, that many other configurations for tubes  506  within the cabin heater  500  are possible. All such other configurations fall within the scope of the present disclosure. 
       FIG. 6  is a side view of an interior volume  612  of a box heater  600 , according to an embodiment. Multiple perforated flame holders  102   a - 102   c  are positioned in the interior volume  612  of the box heater  600 . A tube  606  carrying a working fluid  608  is positioned in a serpentine manner along the wall of the box heater  600 . Each perforated flame holder  102   a - 102   c  is positioned above a respective fuel and oxidant source  624   a - 624   c . The perforated flame holders  102   a - 102   c  support a combustion reaction of the fuel and oxidant output from the fuel and oxidant sources  624   a - 624   c . The perforated flame holders  102   a - 102   c  absorb heat from the combustion reactions and radiate heat to the tube  606 . A working fluid  608  within the tube  606  receives heat from the tube  606 . Thus, as the working fluid  608  progresses through the tube  606  it is heated by the heat radiated from the perforated flame holders  102   a - 102   c . The working fluid  608  is then passed from the interior volume  612  of the box heater  600  to an exterior of the box heater  600 . 
     According to an embodiment, the box heater  600  includes one or more partitions (not shown) positioned within the interior volume  612  between the first and second perforated flame holders  102   a ,  102   b , and/or between the second and third perforated flame holders  102   b ,  102   c . According to an embodiment, the one or more partitions can separate the first perforated flame holder  102   a  from the second perforated flame holder  102   b  along a line of sight between the first and second perforated flame holders  102   a ,  102   b . According to an embodiment, the partition is configured to receive heat from the first and second perforated flame holders  102   a ,  102   b , reach an elevated temperature, and radiate heat energy to the tube  606 . The partition can be a partial partition that does not define two completely separated portions of the interior volume  612 . Alternatively, the partition can be a full partition that defines two completely separated portions of the interior volume  612 . 
       FIG. 7  is a side view of an interior volume  712  of a box heater  700 , according to one embodiment. Perforated flame holders  102   a ,  102   b  are positioned laterally from respective fuel and oxidant sources  724   a ,  724   b . The fuel and oxidant sources  724   a ,  724   b  output fuel and oxidant onto the perforated flame holders  102   a ,  102   b . The perforated flame holders  102   a ,  102   b  support a combustion reaction of the fuel and oxidant within the perforated flame holders  102   a ,  102   b . A tube  706  is positioned along a wall of the box heater  700  in a serpentine fashion. A working fluid  708  passes through the tube  706 . The perforated flame holders  102   a ,  102   b  radiate heat to the tube  706 , thereby heating the working fluid  708  within the tube  706 . The working fluid  708  therefore absorbs heat as it passes through the tube  706  within the interior  712  of the heater  700 . 
     According to an embodiment, the box heater  700  includes a partition (not shown) positioned within the interior volume between the first and second perforated flame holders  102   a ,  102   b . According to an embodiment, the partition separates the first perforated flame holder  102   a  from the second perforated flame holder  102   b  along a line of sight between the first and second perforated flame holders  102   a ,  102   b . According to an embodiment, the partition is configured to receive heat from the first and second flame holders  102   a ,  102   b , reach an elevated temperature, and radiate heat energy to the tube  706 . The partition can be a partial partition that does not define two completely separated portions of the interior volume  712 . Alternatively, the partition can be a full partition that defines two completely separated portions of the interior volume  712 . 
       FIG. 8  is a flow diagram of a process  800  for operating a box heater, according to an embodiment. At  802  fuel and oxidant are output from a fuel and oxidant source onto a perforated flame holder. At  804 , the perforated flame holder supports a combustion reaction of the fuel and oxidant within the perforated flame holder. At  806 , working fluid is passed through one or more tubes within an interior of the box heater. At  808  heat is transferred from the perforated flame holder to the working fluid within the one or more tubes. 
     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.