Patent Publication Number: US-10760784-B2

Title: Burner including a perforated flame holder spaced away from a fuel nozzle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. Continuation application of co-pending U.S. patent application Ser. No. 14/763,271, entitled “PERFORATED FLAME HOLDER AND BURNER INCLUDING A PERFORATED FLAME HOLDER,” filed Jul. 24, 2015; co-pending U.S. patent application Ser. No. 14/763,271 is a U.S. National Phase application under 35 U.S.C. 371 of International Patent Application No. PCT/US2014/016628, entitled “PERFORATED FLAME HOLDER AND BURNER INCLUDING A PERFORATED FLAME HOLDER,” filed Feb. 14, 2014; which application claims the benefit of U.S. Provisional Patent Application No. 61/765,022, entitled “PERFORATED FLAME HOLDER AND BURNER INCLUDING A PERFORATED FLAME HOLDER,” filed Feb. 14, 2013; each of which, to the extent not inconsistent with the disclosure herein, are incorporated by reference. 
     The present application is related to International Patent Application No. PCT/US2014/016626, entitled “SELECTABLE DILUTION LOW NOx BURNER,” filed Feb. 14, 2014; International Patent Application No. PCT/US2014/016632, entitled “FUEL COMBUSTION SYSTEM WITH A PERFORATED REACTION HOLDER,” filed Feb. 14, 2014; and International Patent Application No. PCT/US2014/016622, entitled “STARTUP METHOD AND MECHANISM FOR A BURNER HAVING A PERFORATED FLAME HOLDER,” filed Feb. 14, 2014; each of which, to the extent not inconsistent with the disclosure herein, are incorporated by reference. 
    
    
     SUMMARY 
     According to an embodiment, a burner includes at least one fuel nozzle configured to output a diverging fuel stream and a perforated flame holder disposed away from the fuel nozzle(s). The perforated flame holder has a proximal side and a distal side disposed toward and away from the fuel nozzle, respectively. The perforated flame holder defines a plurality of elongated apertures extending from the proximal side of the flame holder, through the flame holder, to the distal side of the flame holder. The fuel nozzle and the perforated flame holder are arranged to provide at least partial premixing of the diverging fuel stream with a fluid containing an oxidizer, such as air or flue gas in a premixing region between the fuel nozzle and the flame holder. The flame holder is configured to support a flame in the plurality of elongated apertures and in regions immediately above the distal side of the flame holder and/or immediately below the proximal side of the flame holder. 
     According to an embodiment, a perforated flame holder for a combustion reaction includes a high temperature-compatible material having a distal surface and a proximal surface, and a plurality of elongated apertures formed to extend through the high temperature compatible material from the proximal surface to the distal surface. The perforated flame holder is configured to be supported in a combustion volume, aligned with a diverging fuel stream provided by at least one fuel nozzle, and separated from the at fuel nozzle by a distance selected to provide at least partial premixing of the diverging fuel stream with a surrounding gas. A flame holder support structure is configured to maintain a selected alignment between the flame holder proximal surface and the fuel nozzle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of a burner including a flame holder having orifices, according to an embodiment. 
         FIG. 2  is a cutaway view of the burner of  FIG. 1 , according to an embodiment. 
         FIG. 3A  is a partial side sectional view of the burner of  FIGS. 1 and 2 , taken along lines  3 - 3  of  FIG. 1  during a startup phase of operation, according to an embodiment. 
         FIG. 3B  shows the same view of the burner of  FIG. 3A  during normal operation, according to an embodiment. 
         FIGS. 4-10  are plan views of flame holders, according to respective embodiments. 
         FIGS. 11-14  are sectional views showing details of elongated apertures of flame holders, according to respective embodiments. 
     
    
    
     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 view of a burner  100  including a flame holder  102  having orifices  104 , according to an embodiment.  FIG. 2  is a cutaway view of the burner  100  including the flame holder  102  of  FIG. 1 , according to an embodiment.  FIGS. 3A and 3B  are partial side sectional views of the burner  100  of  FIGS. 1 and 2  during respective phases of operation, according to an embodiment. Referring to  FIGS. 1, 2, 3A, and 3B , the burner  100  includes at least one fuel nozzle  106 , and can include a plurality of fuel nozzles  106 . The fuel nozzles  106  are configured to output a diverging fuel stream  302 . A flame holder  102  is disposed away from the fuel nozzles  106 . In the embodiment shown, the flame holder  102  is disk-shaped, and has an X:Z aspect ratio that is greater than about 6:1. In other words, a dimension of the flame holder  102  in the X axis, i.e., its diameter, is more than about six-times its dimension in the Z axis, i.e., its thickness. According to other embodiments, the X:Z aspect ratio is greater than about 4:1. 
     The flame holder  102  has a proximal side  108  and a distal side  110 . The proximal side  108  and the distal side  110  are disposed toward and away from the fuel nozzles  106 , respectively. The flame holder  102  defines a plurality of elongated orifices or apertures  104 . The plurality of elongated apertures  104  extend from the proximal side  108  of the flame holder  102 , through the flame holder  102 , to the distal side  110  of the flame holder  102 . 
     In the embodiment shown, the fuel nozzles  106  and the flame holder  102  are separated a distance sufficient to provide at least partial premixing of the diverging fuel stream  302  with a fluid containing an oxidizer, such as air or flue gas, in a premixing region R 1  between the fuel nozzles  106  and the flame holder  102 . The flame holder  102  is configured to support a flame  304  within the plurality of elongated apertures  104 . Under some conditions, the flame can also extend through the distal side  110  of the flame holder  102  into a region R 2  above the distal side  110  of the flame holder  102 . Under some conditions, the flame can also extend through the proximal side  108  of the flame holder  102  into a region R 3  just below the proximal side  108  of the flame holder  102 . 
     According to an embodiment, the burner  100  includes a burner tile  116  disposed adjacent to the fuel nozzles  106  and can occupy a portion of a distance D 1  between the fuel nozzles  106  and the flame holder  102 . 
     As shown in particular in  FIG. 3A , the burner tile  116  defines an intermediate flame support surface  118  disposed along the diverging fuel stream  302 , part way between the fuel nozzles  106  and the proximal surface  108  of the flame holder  102 , and can be configured to support a secondary flame  304  during at least one of start-up, low fuel flow, or ignition by a primary flame  306 . The burner tile  116  can thus define an intermediate flame support surface  118  part way between the fuel nozzles  106  and the proximal surface  108  of the flame holder  102 . The intermediate flame support surface  118  also substantially defines a proximal end of the premixing region R 1 . The proximal side  108  of the flame holder  102  can substantially define a distal end of the premixing region R 1 . 
     In the embodiment shown, in which a plurality of fuel nozzles  106  are provided, the plurality of fuel nozzles  106  includes a plurality of primary fuel nozzles  202  and a corresponding plurality of secondary fuel nozzles  120 . The primary fuel nozzles  202  are configured to selectably support a primary flame (or flames)  306 . The diverging fuel stream  302  includes secondary fuel streams  303  supported by the secondary fuel nozzles  120 . The primary fuel nozzles  202  and the secondary fuel nozzles  120  are separated by the burner tile  116 . The primary flames  306  preferably have a trajectory selected to ignite the secondary fuel streams  303  at or near the intermediate flame support surface  118  of the burner tile  116 . 
     Premixing of the secondary fuel streams  303  in the premixing region R 1  can be viewed as being associated with the formation of vortices  308 , in the premixing region R 1 . The vortices  308  cause entrainment of air or flue gas into the cores of the vortices, which can be viewed as well-stirred tank reactors (see  FIG. 3B ). 
     If the vortices  308  receive sufficient thermal energy from the primary flames  306 , then the resultant heating of the vortex cores (if mixing is provided at a Damkohler Number (Da) greater than or equal to 1) will also cause ignition of the secondary fuel streams  303 , as shown in  FIG. 3A . The action of the vortices  308  then recirculates the heat to cause the resultant secondary flame  304  to be held by the intermediate flame support surface  118  of the burner tile  116 . Under these conditions, holding the flame  304  at the intermediate flame support surface  118  substantially stops premixing in the region R 1  because the ignition causes the combustion reaction to occur at the edges of the vortices  308 , creating a barrier that prevents air from reaching unburnt fuel inside the flame front. Accordingly, supporting the secondary flame  304  at the intermediate flame support surface  118  can be viewed as significantly reducing or preventing premixing of the secondary fuel streams  303  with air or flue gas. 
     If the vortices  308  do not receive heat from the primary flames  306 , then there can be substantially no ignition of the secondary fuel streams  303 . This can be viewed as a prevention of heat recirculation to the intermediate flame support surface  118  of the burner tile  116 . This was found by the inventors to cause the secondary flame  304  to be held by the flame holder  102  above the premixing region R 1 , as shown in  FIG. 3B . In the case where the vortices  308  do not receive heat from the primary flames  306 , then there can be substantially no flame front at the edges of the vortices  308 . In particular, if heat from the primary flames  306  is withdrawn from the vortices  308 , either by being redirected or shut down, the secondary flame  304  alone cannot produce sufficient heat to sustain combustion at the intermediate flame support surface  118 , and goes out or rises into the flame holder  102 , which eliminates the flame front that had acted to isolate the fuel. Having no flame front at the edges of the vortices  308  typically allows dilution of the fuel mixture in the vortex cores, which causes ignition that occurs later at the flame holder  102  to operate under leaner burning conditions. 
     While the premixing region R 1  is described as extending from the intermediate flame support surface  118  and the proximal surface  108  of the flame holder  102 , it will be understood that this is an approximation made for ease of understanding. The inventors have found that the secondary flame  304  can occasionally and briefly extend downward from the proximal surface  108  of the flame holder  102 . Under this instantaneous condition, vortices  308  in the premixing region R 1  can be temporarily bounded by a flame front and premixing may temporarily diminish or stop. However, such flame extensions were found to be transient, and on a time-averaged basis the premixing region R 1  can still be considered to support premixing of the secondary fuel stream  302  with air or flue gas. 
     Another effect found by the inventors was a subtle extension of the secondary flame  304  to a flow stagnation region R 3  adjacent to the proximal surface  108  of the flame holder  102  (as illustrated in  FIG. 3B ). The tertiary flame extension to the stagnation region proved to be more-or-less continuous under stable conditions, and therefore the premixing region R 1  can be considered to extend from the intermediate flame support surface  118  to the edge of the secondary flame  304  in the stagnation region R 3  just below the proximal surface  108  of the flame holder  102 . 
     The inventors found that the extension of the secondary flame  304  into the stagnation region adjacent to the proximal surface  108  of the flame holder  102  may be desirable. The presence of the secondary flame  304  in the stagnation region appeared to be associated with somewhat more stable operation of the burner  100  compared to cases where visible ignition occurred in the elongated apertures  104 . 
     Ignition of the secondary fuel stream  302  by the primary flames  306 , as shown in  FIG. 3A , can be selected to substantially prevent premixing of the secondary fuel stream  302  with air or flue gas in the premixing region R 1 . 
     In other words, premixing of the secondary fuel stream  302  with an oxidizing fluid, such as air or flue gas, in the premixing region R 1  is substantially prevented when the secondary fuel ignites near and is held by the intermediate flame support surface  118 . The flame front acts to stop mixing of the air or flue gas with the fuel. Accordingly, supporting the secondary flame  304  at the intermediate flame support surface  118  caused a richer fuel to air mixture. A richer burning mixture may be associated with a somewhat more stable flame (notwithstanding additional flame stability caused by the elongated aperture  104  structures of the flame holder  102 ) but also a hotter burning flame compared to a leaner burning mixture caused by additional premixing of the secondary fuel stream  302  with air or flue gas in the premixing region R 1 , as shown in  FIG. 3B . A hotter flame is associated with higher oxides of nitrogen (NOx) output than a cooler flame. 
     Selectable attenuation or stopping of the primary flames  306  can be configured to substantially prevent ignition of the secondary fuel stream  302  at or near the intermediate flame support surface  118  of the burner tile  116 . The substantial preventing of ignition of the secondary fuel stream  302  at or near the intermediate flame support surface  118  of the burner tile  116  can cause the secondary flame  304  to be supported by the flame holder  102 , as will be explained in more detail below. 
     In the embodiment of  FIGS. 1-3B , the primary fuel nozzles  202  and the secondary fuel nozzles  120  are aligned with one another radially, with respect to the burner tile  116 . 
     According to an embodiment, a primary fuel control valve  312  is arranged to control fuel flow from a fuel source  314  to the primary fuel nozzles  202 . The primary fuel control valve  312  can include, for example, a manually actuated valve, an electrically actuated valve, a hydraulically actuated valve, or a pneumatically actuated valve. The primary fuel control valve  312  can be configured to control a characteristic of the primary flames  306  independently from a flow rate of fuel in the secondary fuel streams  303 . 
     A primary fuel pressure valve or pressure control fitting  316  is configured to control pressure of fuel flowing to the primary fuel nozzles  202 . The primary fuel pressure valve  316  can be configured to control fuel pressure delivered to the primary fuel nozzles  202  independently from fuel pressure delivered to the secondary fuel nozzles  120 . 
     A secondary fuel control valve  318  is arranged to control fuel flow from the fuel source  314  to the secondary fuel nozzles  120 . The secondary fuel control valve  318  can include, for example, a manually actuated valve, an electrically actuated valve, a hydraulically actuated valve, or a pneumatically actuated valve. The secondary fuel control valve  318  can be configured to control a characteristic of the secondary flame  304  independently from a flow rate of fuel to the primary fuel nozzles  202 . 
     A secondary fuel pressure valve or pressure control fitting  320  is configured to control pressure of fuel flowing to the secondary fuel nozzles  120 . The secondary fuel pressure valve  320  can be configured to control fuel pressure delivered to the secondary fuel nozzles  120  independently from fuel pressure delivered to the primary fuel nozzles  202 . 
     Alternatively or additionally the primary fuel control valve  316 , a primary fuel stream or primary flame  306  deflector can be provided, configured to control a trajectory of the primary flames  306 . The primary fuel stream or primary flame deflector is configured to control exposure of the secondary fuel stream  302  to heat at or near the intermediate flame support surface  118  of the burner tile  116 . According to an embodiment, the burner tile  116  is disposed peripheral to or surrounding a combustion air passage  204  formed in a combustion volume floor, wall, or ceiling  122 . The flame holder  102 , in the embodiment of  FIGS. 1-3B , includes a central opening  124  disposed axially to the combustion air passage  204 . The opening  124  in the flame holder  102  can have a diameter of between 0.10 and 1.0 times a diameter of the combustion air passage  204 . According to another embodiment, the opening  124  in the flame holder  102  can have a diameter of between 0.4 and 0.8 times the diameter of the combustion air passage  204 . 
     According to various embodiments, the flame holder  102  is between 1 inch and 4 inches in thickness between the proximal  108  and distal  110  sides. For example, the flame holder  102  can be about 2 inches in thickness between the proximal  108  and distal  110  sides. 
     The proximal side  108  of the flame holder  102  can be positioned, for example, between 3 inches and 24 inches away from the intermediate flame support surface  118  of the burner tile  116 . For example, the proximal side  108  of the flame holder  102  can be disposed between 4 inches and 9 inches away from the intermediate flame support surface  118  of the burner tile  116 . 
     According to an embodiment, the plurality of elongated apertures  104  extending through the flame holder  102  are less than about 1.0 inch in transverse dimension orthogonal to axes of the elongated apertures. For example, the plurality of elongated apertures  104  extending through the flame holder  102  can be between 0.25 inch and 0.75 inch in transverse dimension orthogonal to axes of the elongated apertures. In particular examples, the plurality of elongated apertures  104  defined by the flame holder  102  can be between 0.375 inch and 0.50 inch in transverse dimension orthogonal to axes of the elongated apertures  104 . 
     The flame holder  102  is preferably formed from a refractory material such as a material including a high temperature ceramic fiber. For example, the material can be formed from alumina-silica fibers and binders. In experiments performed by the inventors, the flame holder  102  was formed from a Fiberfrax® Duraboard® product available from Unifrax Corporation, having a principal place of business at 2351 Whirlpool Street; Niagara Falls, N.Y. (USA). The flame holder  102  can be formed by cutting a disk of the appropriate diameter from a material that includes a high temperature ceramic fiber, and by drilling the elongated apertures  104  through the disk. According to another embodiment, the flame holder is cast substantially in its final form from a refractory material. 
     The flame holder  102  is preferably electrically insulating. However, in other embodiments, the flame holder  102  can be electrically conductive. 
     A flame holder support structure  126  can be configured to support the flame holder  102  in a furnace, boiler, or other combustion volume aligned to receive the secondary fuel stream  302 . The flame holder support structure  126  can be configured to support the flame holder  102  substantially completely around the periphery of the flame holder  102 . The flame holder support structure  126  can be formed from steel, for example. In some embodiments, the flame holder support structure  126  is formed integrally with the flame holder  102 . For example, the flame holder  102  can be formed by casting the flame holder  102  over a portion of the flame holder support structure  126 . According to another embodiment, the flame holder  102  and the flame holder support structure  126  are cast together as a monolithic structure. The flame holder support structure  126  can be configured to couple the flame holder  102  to the burner tile  116 , as shown in  FIGS. 1 and 2 , or can be configured to couple the flame holder  102  to some other mounting substrate, such as, for example, the combustion floor  122 . 
     The fuel nozzles  106  are configured to output a gaseous fuel. In experiments, the inventors used natural gas to test performance and evolve the design. Alternatively or additionally, the fuel nozzles  106  can be configured to output an aerosol of a liquid fuel or a powdered solid fuel. 
     According to an embodiment, the proximal surface  108  of the flame holder  102  is hardened or includes a hard component configured to resist erosion from the diverging fuel stream. 
     According to some embodiments, the proximal and distal surfaces  108 ,  110  are substantially planar. The distal surface  110  and proximal surface can be non-parallel. For example, a thickness of the flame holder  102  can be varied to correspond to an optimal length of the elongated apertures  104 , dependent upon fuel flow and lateral divergence distance of the fuel flow across the proximal surface. 
     Alternatively, the distal surface  110  and the proximal surface  108  can be parallel to one another. The distal surface  110  and proximal surface  108  can define a flame holder thickness. According to an embodiment, the flame holder thickness is about 4 inches. 
     A method of operation of the burner  100  is described hereafter, according to an embodiment. In operation, and in particular, during start up of the burner  100 , as depicted in  FIG. 3A , the primary valve  316  is opened to permit a flow of fuel from the primary nozzles  202 . As fuel flows from the nozzle  202  in a diverging stream  302 , an oxidizing fluid such as air is introduced via the combustion air passage  204 , a portion of which is entrained by the fuel stream  302 . Primary flames  306  are ignited in a known manner. A trajectory of the primary flames  306  is controlled to be directed primarily toward the intermediate flame support surface  118  of the burner tile  116 . Once the primary flames  306  are ignited, the secondary valve  320  is opened and secondary fuel streams  303  flow from the secondary nozzles  120 . 
     Because the burner tile  116  separates the secondary nozzles  120  from the primary nozzle  202  and in particular from the combustion air passage  204 , there is not sufficient oxidizer to support a flame in the vicinity of the secondary nozzles  120 . The secondary fuel streams  303  therefore rise until they clear the intermediate flame support surface  118  of the burner tile  116  and begin to form vortices  308  above the burner tile  116 , and to entrain air from the air passage  204 . As soon as sufficient air has been entrained into the vortex cores, heat from the primary flame  306  ignites the secondary fuel streams  303 , producing a secondary flame  304  that is supported or held by the flame support surface  118  of the burner tile  116 . In addition to the heat supplied by the primary flames  306 , a portion of the heat generated by the secondary flames  304  is recirculated by the vortices  308 , which enables continued combustion at the flame support surface  118 . Heat from the secondary flame  304  also preheats the flame holder  102 . While the secondary flame  304  is present at the flame support surface  118 , its flame front acts as a barrier to prevent air from reaching the remaining fuel, which is substantially enclosed within the secondary flame  304 . 
     Once the flame holder  102  has reached a minimum operating temperature, the primary valve  316  is partially or completely closed, reducing or extinguishing the primary flame  306 , as shown in  FIG. 3B . Alternatively, the trajectories of the primary flames  306  can be redirected away from the area directly above the flame support surface  118 . Deprived of heat from the primary flame  306 , the secondary flame  304  cannot maintain ignition, and eventually goes out. As the secondary flame  304  is extinguished, the secondary fuel streams  303  are no longer prevented from additional premixing in the vortex cores. The premixed fuel then reaches the flame holder  102 , which, having been preheated by the secondary flame  304  is sufficiently hot to cause auto-ignition of the premixed fuel, producing a secondary flame  304  held by the flame holder  102 . The secondary flame  304  is self-sustaining for as long as sufficient fuel and oxidizer are provided. Because of the action of the vortices  308  in the premix region R 1 , the fuel of the secondary fuel streams  303  is significantly diluted by entrained air, resulting in a lean fuel mixture. 
     The flame holder  102  can be configured to be aligned with a diverging fuel stream from a single fuel nozzle. For example, the embodiments of  FIGS. 6, 8 , and  10 , described below, illustrate embodiments configured to be aligned with a single fuel nozzle. Alternatively, the flame holder  102  can be configured to be aligned with diverging fuel streams from a plurality of fuel nozzles. For example, the embodiments of  FIGS. 1-4, 5, 7, and 9  illustrate embodiments formed to be aligned with a plurality of fuel nozzles. 
     The perforated flame holder can be formed as an overall toric shape having a central opening  124  and an outer rim  402 . The plurality of elongated apertures  104  can be positioned or arranged in a plurality of coaxial circles as shown, for example, in  FIGS. 1, 2, 4, 6, 8, and 10 . The plurality of elongated apertures  104  can be formed to be substantially identical in diameter to one another, as in  FIGS. 1-3B . Alternatively, the plurality of elongated apertures  104  can be formed to have a plurality of diameters, as shown in  FIG. 4 . 
       FIG. 4  is a view of a distal surface  110  of a perforated flame holder  400 , according to an embodiment. The plurality of elongated apertures  104  are positioned in a plurality of coaxial circles  404 ,  406   408 ,  410 ,  412 ,  414  with each of the plurality of coaxial circles having elongated apertures  104  of a respective single diameter. For example, according to an embodiment, the diameters of the elongated apertures  104  in each of the coaxial circles  404 ,  406   408 ,  410 ,  412 ,  414  are between 0.375 inches and 1 inch. 
     In the embodiment of  FIG. 4 , for example, the elongated apertures  104  in the innermost circle  404  and the outermost circle  414  have diameters of 1.0 inch, elongated apertures  104  in the two middle circles  408 ,  410  have diameters of 0.375, and elongated apertures  104  in the two intermediate circles  406 ,  412  have diameters of 0.5 inch. 
       FIG. 5  is a view of a distal surface  110  of a perforated flame holder  500 , according to an embodiment. The perforated flame holder  500  is formed in a toric shape having an outer rim  402  and a central opening  124 , and is configured to be aligned with a plurality of diverging fuel streams from a plurality of nozzles of a burner assembly. The plurality of elongated apertures  104  are arranged in a plurality of aperture patterns  502 . Each aperture pattern  502  is configured to align with a corresponding one of the diverging fuel streams and has a diameter D 2  selected to correspond to an approximate diameter of a respective one of the plurality of diverging fuel streams. Each aperture pattern  502  includes a pattern of elongated apertures  104  having a plurality of diameters. In the embodiment shown, each aperture pattern  502  includes a plurality of elongated apertures positioned in concentric circles  506 ,  508 ,  510 . 
     The concentric circles  506 ,  508 ,  510  are positioned around a central aperture  512 , as shown. According to an embodiment, the elongated apertures  104  arranged in the concentric circles  506 ,  508 ,  510  are, respectively, 0.375 inch, 0.5 inch, and 0.75 inches in diameter. 
     Placing the elongated apertures in aperture patterns  502  serves to maximize mechanical robustness of the flame holder  500  in areas where the elongated apertures  104  are not needed to support a combustion reaction. This approach is believed to be advantageous. 
     Moreover, in experiments conducted by the inventors using a half-scale experimental burner with flame holders in configurations similar to those of many of the embodiments disclosed herein, the smaller size of the largest apertures  104 , i.e., those of the concentric circles  506 ,  508 ,  510  described with reference to  FIG. 5 , compared to the largest apertures  104  described with reference to  FIG. 4 , was believed to result in less unburned fuel and was believed to be advantageous. The inventors believe the optimum elongated aperture size can be representative of larger scale burners owing to relatively consistent fluid dynamics that do not change very much with scale. 
     The inventors also tested flame holder geometries where a single flame holder would be aligned with a single or each of a plurality of fuel nozzles and corresponding fuel streams. 
       FIG. 6  is a view of a distal surface  110  of a flame holder  600  having elongated apertures  104 , according to another embodiment. The flame holder  600  is formed as a disk having a diameter D 3  that is selected for alignment with a diverging fuel stream from a single fuel nozzle. The plurality of elongated apertures  104  can be arranged in an aperture pattern. The aperture pattern can include a pattern of elongated apertures having a plurality of diameters or a same diameter. As shown in  FIG. 6 , the aperture pattern includes a plurality of elongated apertures positioned in concentric circles  506 ,  508 ,  510 . In an embodiment, the elongated apertures formed in the concentric circles  506 ,  508 ,  510  are, respectively, 0.375 inch, 0.5 inch, and 0.75 inches in diameter. 
     As indicated above, in experiments conducted with a perforated flame holder similar to the flame holder  600  of  FIG. 6 , it was found that reducing the maximum size of the elongated apertures reduced the amount of unburned fuel. Accordingly, the inventors evolved the designs further to arrive at the patterns illustrated in  FIGS. 7 and 8 . 
       FIG. 7  is a view of a distal surface  110  of a flame holder  700  having orifices  104 , according to an embodiment.  FIG. 8  is a view of a distal surface  110  of a flame holder  800  having elongated apertures  104 , according to a further embodiment. In the embodiments shown in  FIGS. 7 and 8 , each of the elongated aperture patterns  502  includes apertures each having one of two diameters. Apertures  702 ,  704  and  710  have diameters of 0.375 inch, while apertures  706 ,  708  have diameters of 0.5 inch. It is believed by the inventors that changing aperture diameter in a way that increases from the middle toward the outside of a diverging fuel stream can provide a greater turn-down ratio for the burner, i.e., the ratio of the maximum heat output capacity of the burner relative to the minimum required to maintain ignition of the secondary flame  304 . In experiments, observation of tertiary flames held by flame holders having the aperture patterns corresponding to the embodiments of  FIGS. 7 and 8  led at least some of the inventors to conclude that the smaller maximum aperture size (compared to the embodiments of  FIGS. 5 and 6 ) resulted in a more stable flame and/or less unburned fuel.) 
       FIG. 9  is a view of a distal surface  110  of a flame holder  900  having orifices  104 , according to an embodiment.  FIG. 10  is a view of a distal surface  110  of a flame holder  1000  having elongated apertures  104 , according to an embodiment.  FIGS. 9 and 10  illustrate embodiments in which the elongated apertures  104  in each pattern  502  are of a single diameter of 0.375 inch. 
     In the embodiments shown in  FIGS. 8 and 10  above, the flame holder includes a rim  802  of solid material around the hole patterns  502 . The rim  802  of solid material serves to increase mechanical robustness of the respective flame holder. Rim widths can vary, and, according to an embodiment, can range from about 0.5 inch up to about 2 inches. Additionally, it has been found that mechanical robustness is further enhanced by supporting the perforated flame holder around substantially the entirety of its periphery. Accordingly, in some embodiments the flame holder support structure  126  includes a support rim, made from steel or some other material having sufficient heat tolerance and toughness, that supports the flame holder around its entire periphery. 
       FIG. 11  is a longitudinal sectional view of a perforated flame holder  102  having elongated apertures  104 , according to an embodiment. The plurality of elongated apertures  104  defined by the flame holder  102  are cylindrical in shape. In other words, as viewed in a transverse cross section, the elongated apertures  104  of  FIG. 11  are circular along their entire lengths or a portion thereof. Alternatively, the elongated apertures  104  can have any shape that is appropriate, according to the requirements of a particular embodiment. For example, as viewed in a transverse cross section, the elongated apertures  104  can be square, hexagonal, etc. 
       FIG. 12  is a longitudinal sectional view of a perforated flame holder  102  having orifices  104 , according to another embodiment. The plurality of elongated apertures  104  defined by the flame holder  102  of  FIG. 12  are in the shape of tapered cylinders, i.e., are frusto-conical or frusto-pyramidal in shape.  FIG. 13  is a longitudinal sectional view of a perforated flame holder  102  having orifices  104 , according to an embodiment. The plurality of elongated apertures  104  defined by the flame holder  102  of  FIG. 13  are in the shape of stepped and tapered cylinders.  FIG. 14  is a longitudinal sectional view of a perforated flame holder  102  having orifices  104 , according to a further embodiment. In  FIG. 14 , the plurality of elongated apertures  104  defined by the flame holder  102  include vertical portions  1402  and tapered or stepped and tapered portions  1404 . 
     The shape of the elongated aperture  104  can affect the optimum thickness of the flame holder  102 , the flame holding characteristics of the flame holder, the combustion efficiency realized with the flame holder, and/or the mechanical and thermal robustness of the flame holder. A cylindrical elongated aperture may be the most simple to make. For example, the taper can be particularly advantageous in economical manufacturing processes, inasmuch as it can provide for the relief required in a casting operation to permit the removal of a cast part from a mold. Additionally, a tapered elongated aperture (more specifically, an elongated aperture that increases in area from the proximal side to the distal side of the flame holder) can allow for thermal expansion without causing “sonic choke” within the elongated aperture. A tapered elongated aperture may operate in a manner akin to a ramjet, where thermal expansion through the elongated aperture produces “thrust” that enhances flow. A stepped and tapered elongated aperture may additionally provided enhanced flame holding owing to vortices formed adjacent to the step(s). A flame holder including a vertical portion and a tapered or stepped and tapered portion may enhance flame holding owing to enhanced vortex formation adjacent to the distal surface of the flame holder proximate to the vertical edge. 
     An optimal shape of the flame holder, the elongated aperture pattern shape, the thickness of the flame holder, and/or the elongated aperture sectional shape can vary with burner design parameters. For example, a fuel that undergoes combustion with a reduction in moles of products compared to reactants reduce an amount of area increase in a cross sectional shape optimized for thermal expansion. For example, longer chain hydrocarbons have relatively fewer hydrogen atoms and produce less water vapor than methane and other shorter chain hydrocarbons. Similarly, a fuel that is introduced as a powdered solid or as an aerosol has reactants that occupy less volume than a gaseous fuel. A phase change between reactants and products can increase an optimum taper angle of elongated apertures, decrease optimal flame holder thickness, change optimal elongated aperture size, and/or change optimal elongated aperture pattern. 
     In tests conducted by the inventors, using natural gas, significant improvements in reduction of oxides of nitrogen (NOx) were achieved. In an experiment using a flame holder having the elongated aperture pattern shown in  FIG. 8 , at a premix region height of 13.5 inches (about 192 secondary nozzle diameters), the tertiary flame appeared unsteady at start-up, but became steady after the furnace warmed up. After warm-up, NOx was reduced by 50% to 65% compared to a secondary flame held at the intermediate flame holding surface shown in  FIGS. 1-3B . Throughout testing, carbon dioxide (CO2) concentration was held constant at about 10%. No carbon monoxide (CO) was detected. Heat release from the flame held constant between flame holding locations. In the scale model, the heat release was 130,000 to 140,000 BTU/hour.) 
     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.