The present application claims priority benefit from U.S. Provisional Patent Application No. 62/521 ,649, entitled "DUPLEX BURNER PILOT," filed June 19, 2017 (docket number 2651 -322-02); which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

BACKGROUND

Combustion systems are employed in many industrial applications and to perform many different operations. For example, combustion systems are used for generation of electricity, chemical and fuel refining and other chemical processes, smelting, home and office heating, locomotion, etc.

SUMMARY

According to an embodiment, a combustion system is provided that includes a flame holder with first and second faces and a plurality of perforations extending between the first and second faces. A main fuel nozzle is positioned and configured to emit a fuel stream toward the first face of the flame holder and thereby support a combustion reaction substantially between the first and second faces. A pilot nozzle is positioned over the second face of the flame holder and configured to emit a pilot fuel stream along a pilot axis lying parallel to the second face.

According to an embodiment, the pilot nozzle is positioned adjacent to the second face of the flame holder. According to an embodiment, the pilot nozzle is positioned less than two inches from the second face of the flame holder. According to an embodiment, the pilot nozzle is positioned no more than one inch from the second face of the flame holder.

According to an embodiment, a mouth of the pilot nozzle is positioned within one inch of a side plane defined by a side face of the flame holder, while according to another embodiment, the mouth of the pilot nozzle extends over the second face about half an inch beyond the side plane.

According to an embodiment, the pilot nozzle is a first one of a plurality of pilot nozzles, each positioned adjacent to the second face and configured to emit a respective pilot fuel stream along a corresponding pilot axis across to the second face. According to an embodiment, each of the plurality of pilot nozzles is positioned to extend over the second face with a mouth positioned within one inch of a side plane defined by a respective side face of the flame holder.

According to an embodiment, a method of operation is provided, including supporting a combustion reaction substantially within a plurality of apertures extending between first and second faces of a perforated flame holder by emitting a fuel stream from a main nozzle toward the first face of the flame holder. Meanwhile, a pilot flame is continuously supported in a position over the second face of the flame holder by emitting a pilot fuel stream along an axis lying parallel to, and within two inches of, the second face.

According to an embodiment, if the combustion reaction is

unintentionally extinguished, the pilot flame maintains a portion of the flame holder at an elevated temperature, which permits the combustion reaction to relight without a complete restart of the system.

According to an embodiment, the method includes supporting a plurality of pilot flames over the second face by emitting a respective plurality of pilot fuel streams along corresponding axes lying within a common plane, parallel to the second face.

According to an embodiment, a combustion system includes a flame holder having a first face, a second face opposite the first face, and a plurality of perforations extending through the flame holder between the first and second faces. The combustion system also includes a main fuel nozzle positioned and configured to emit a fuel stream toward the first face of the flame holder. The combustion system also includes a first pilot nozzle positioned adjacent to the second face and configured to emit a pilot fuel stream along a first pilot axis adjacent to the second face.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A is a schematic diagram of a combustion system, according to an embodiment, that includes a perforated flame holder and a pilot burner system.

FIG. 1 B is an enlarged view of a portion of the combustion system of FIG. 1A, taken from a region indicated at 1 B, according to an embodiment.

FIG. 2 is a simplified diagram of a burner system including a perforated flame holder similar to the flame holder of FIGS. 1 A and 1 B, configured to hold a combustion reaction, according to an embodiment.

FIG. 3 is a side sectional diagram of a portion of the perforated flame holder of FIG. 2, according to an embodiment.

FIG. 4 is a flow chart showing a method for operating a burner system including a perforated flame holder, according to an embodiment.

FIG. 5 is a diagrammatic top plan view of a furnace, according to an embodiment, that includes a perforated flame holder and a pilot burner system.

FIGS. 6 and 7 are plan views of pilot nozzles, illustrating examples of pilot nozzle configurations, according to respective embodiments.

FIG. 8A is a side elevation of a portion of a pilot burner, according to an embodiment.

FIG. 8B is a top sectional view of the pilot burner of FIG. 8A, according to an embodiment.

FIG. 9A is a simplified perspective view of a combustion system, including another alternative perforated flame holder, according to an embodiment.

FIG. 9B is a simplified side sectional diagram of a portion of the reticulated ceramic perforated flame holder of FIG. 9A, 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.

Where directional terms such as vertical, horizontal, above, below, etc., are used, these are for convenience and clarity in describing features and elements as they appear in the drawings. Such terms do not limit the claims, or the orientation at which embodiments may be employed. Where the term over is used in the context of a position relative to a face of a flame holder, this refers to a position that is on the same side of the flame holder as the referenced face but not directly on that face, i.e., that the position is at least a minimal distance from the referenced face.

FIG. 1A is a schematic diagram of a combustion system 100, according to an embodiment. The combustion system 100 includes a perforated flame holder 102, positioned within a furnace volume 104 defined, in part, by a floor 106 of a furnace. A nozzle 108 is positioned and configured to supply a fuel stream 1 10 to support a combustion reaction 1 12 held by the perforated flame holder 102. A main fuel supply 1 14 is coupled to the nozzle 108 and configured to supply a flow of fuel thereto. The combustion system 100 further includes elements such as fuel lines, valves, controllers, sensors, etc. Such elements and their operation are well known in the art, and so are not shown or described in detail here. Operating principles of the perforated flame holder 102 are described in detail below, with reference to FIGS. 2-4.

The combustion system 100 also comprises a pilot burner system 120 that includes one or more pilot burners 122 positioned and configured to emit pilot flames 124 across an output face 126 of the perforated flame holder 102. A pilot fuel supply 128 is configured to supply fuel to an input port 130 of each of the pilot burners 122. The fuel is emitted into a mixing tube 132 at high velocity from an orifice 134, drawing a flow 136 of air into an air input port 138 of the pilot burner 122 via the venturi effect. The mixing tube 132 includes mixing elements such as, e.g., mixing baffles 140, which combine the incoming fuel and air. The fuel-air mixture is finally supplied to a pilot nozzle 142, from which it is emitted to support one of the pilot flames 124.

According to an embodiment, the mixing tube 132 has a diameter of between about half an inch and three inches. According to another embodiment, the mixing tube 132 has a diameter of between about one and two inches.

According to an embodiment, the pilot nozzle 142 has a diameter of between about one and two inches.

FIG. 1 B is an enlarged view of a portion of the combustion system 100 taken from a region indicated in FIG. 1A at 1 B, showing additional detail of the combustion system 100. FIG.1 B shows a portion of the perforated flame holder 102 and a portion of one of the pilot burners 122, including a portion of the mixing tube 132 and the pilot nozzle 142. The inventor has found that the position of the pilot nozzle 142 over the perforated flame holder 102 affects the operation and effectiveness of the pilot burner 122. In the embodiment shown, the vertical distance Dv of the pilot nozzle 142 above the output face 126 of the perforated flame holder 102 is about one inch, and the horizontal distance DH that the mouth of the pilot nozzle 142 extends over the edge of the perforated flame holder 102 is about half an inch. Optimum values of these distances can vary according to factors such as, for example, the size and average output of the perforated flame holder 102, the type of fuel being supplied to the perforated flame holder 102, and the pilot fuel. According to an embodiment, the distance Dv is between about half an inch and about two inches, and the distance DH is between about minus one inch (i.e., the mouth of the pilot nozzle 142 does not extend over the perforated flame holder 102, but stops short about one inch) and one inch. According to another embodiment, the distance Dv is less than twice the diameter of the pilot nozzle 142 and the distance DH is within one diameter of the pilot nozzle 142 in either direction.

As will be discussed further below with reference to FIGS. 2-4, the perforated flame holder 102 is extremely clean and efficient during normal operation. However, in some types of systems, such as, e.g., refineries, and other systems in which there is some possible fluctuation in supplies of fuel and/or oxidant, the combustion reaction 1 12 can also fluctuate, and even be occasionally extinguished, in response to a sudden change in the flow of the fuel stream 1 10 or availability of oxidizer. When this occurs, if the perforated flame holder 102 cools to a temperature that is below the auto-ignition temperature of the fuel, the combustion reaction 1 12 will not reignite spontaneously, and a restart operation must be performed to reignite the combustion reaction 1 12.

The pilot nozzle 142 is configured to produce a long, narrow pilot flame 124 that extends more or less parallel to the output face 126, and imparting thermal energy, by radiation and/or convection, to the portion of the perforated flame holder 102 over which it passes, and holding at least that portion at a temperature sufficient to initiate auto-ignition.

In the embodiment of FIGS. 1A and 1 B, the pilot burners 122 are provided with a separate pilot fuel supply 128 that is configured to be continuously available, such as, for example, propane, natural gas, etc. The pilot burners 122, which operate continuously during normal operation of the combustion system 100, are positioned and configured to heat respective regions of the output face 126 of the perforated flame holder 102. In the event a fluctuation in the main fuel or air supply causes the combustion reaction 1 12 to become completely extinguished, the pilot burners 122 keep portions of the perforated flame holder 102 above the auto-ignition

temperature of the main fuel, such that when the fuel and air flows return to nominal, the fuel stream 1 10 auto ignites as it enters the perforated flame holder 102, restarting the combustion reaction 1 12, which quickly brings the entire perforated flame holder 102 up to its normal operating temperature.

The separate pilot fuel supply 128 ensures that the pilot burners 122 are provided with an uninterrupted flow of fuel, while the premix configuration of the pilot burners 122 ensures that the supply of oxidizer will also be continuous, and that the pilot flame 124 will be substantially insensitive to variations in the amount of oxygen available downstream from the combustion reaction 1 12. In the embodiment of FIGS. 1 A and 1 B, a flow 136 of oxidizer- ambient air, in this example e, is drawn into the mixing tube 132 by a venturi effect produced as the pilot fuel exits the orifice 134. According to another embodiment, a compressed oxidizer is supplied directly to the mixing tube 132. According to a further embodiment, the pilot burners 122 burn the same fuel as the combustion reaction 1 12. A portion of the main fuel is transferred to a pilot fuel storage, from which it is pumped to the pilot burners 122. When the main fuel flow is interrupted, the volume of the pilot fuel storage is drawn down to provide a continuing supply to the pilot burners 122. When the main flow is restored, the pilot fuel storage is replenished, either before fuel flows again to the main nozzle 108, or by diverting a larger portion of the fuel to the pilot fuel storage until the storage is topped off.

The pilot burner systems 120 of the embodiments disclosed herein are configured to produce between about 35k BTU and 90k BTU of heat.

FIG. 2 is a simplified diagram of a burner system 200 including a perforated flame holder 102 configured to hold a combustion reaction 1 12, 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 inventor have shown that perforated flame holders 102 described herein can support very clean combustion.

Specifically, in experimental use of burner 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 (O2) 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 104 to form a fuel and oxidant mixture 1 10. 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 104 and positioned to receive the fuel and oxidant mixture 1 10.

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 1 10 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 1 12 supported by the fuel and oxidant mixture 1 10.

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 (H2), and methane (CH4). In another application the fuel can include natural gas (mostly CH ₄ ) or propane (C3H8). 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 inventor. 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 1 10, an output face 126 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 126. The plurality of perforations 210 can receive the fuel and oxidant mixture 1 10 at the input face 212. The fuel and oxidant mixture 1 10 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 126.

According to an embodiment, the perforated flame holder 102 is configured to hold a majority of the combustion reaction 1 12 within the perforations 210. For example, on a steady-state basis, more than half the molecules of fuel output into the combustion volume 104 by the fuel and oxidant source 202 may be converted to combustion products between the input face 212 and the output face 126 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 1 12 may be output between the input face 212 and the output face 126 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 1 12. 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 1 12 between the input face 212 and the output face 126 of the perforated flame holder 102. In some experiments, the inventor produced a combustion reaction 1 12 that was apparently wholly contained in the perforations 210 between the input face 212 and the output face 126 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 126 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 126 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 inventor has 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 108 within the dilution region Dp. 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 126. In still other instances, the inventor has noted apparent combustion occurring downstream from the output face 126 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 1 12 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 104. 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 1 10 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 1 12 at least in heat receiving regions 306 of perforation walls 308. Experimental evidence has suggested to the inventor 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 1/3 and 1/2 of the distance from the input face 212 to the output face 126 (i.e., somewhat nearer to the input face 212 than to the output face 126). The inventor contemplates that the heat receiving regions 306 may lie nearer to the output face 126 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 1 12 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 inventor contemplates 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 1 12, even under conditions where a combustion reaction 1 12 would not be stable when supported from a conventional flame holder.

The inventor believes that the perforated flame holder 102 causes the combustion reaction 1 12 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 1 10 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 1 10. 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 1 12 occurs. Accordingly, the combustion reaction 1 12 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 126 that define the ends of the perforations 210. At some position along the length of a perforation 210, the combustion reaction 1 12 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 126 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 1 10 (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 1 12, molecules of reactants and their constituent parts, any non-reactive species, reaction intermediates (including transition states), and reaction products. Near the output face 126, 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 Dp between opposing perforation walls 308. The inventor has 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 Dp of the perforation 210. In other embodiments, the length L can be greater than six times the transverse dimension Dp. 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 Dp. 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 126 of the perforated flame holder 102. In experiments, the inventor has 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 1 12 in a first perforation 210 to supply heat to stabilize a combustion reaction portion 1 12 in an adjacent perforation 210.

Referring especially to FIG. 2, the fuel and oxidant source 202 can further include a fuel nozzle 108, configured to output fuel, and an oxidant source 220 configured to output a fluid including the oxidant. For example, the fuel nozzle 108 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 Dp away from the fuel nozzle 108. The fuel nozzle 108 can be configured to emit a fuel jet selected to entrain the oxidant to form the fuel and oxidant mixture 1 10 as the fuel jet and oxidant travel along a path to the perforated flame holder 102 through the dilution distance Dp between the fuel nozzle 108 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 Dp. In some embodiments, a flue gas recirculation path 224 can be provided. Additionally or alternatively, the fuel nozzle 108 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 Dp between the fuel nozzle 108 and the input face 212 of the perforated flame holder 102.

The fuel nozzle 108 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 1 10 at the distance Dp away from the fuel nozzle 108 greater than 20 times the nozzle diameter. In another embodiment, the perforated flame holder 102 is disposed to receive the fuel and oxidant mixture 1 10 at the distance Dp away from the fuel nozzle 108 between 100 times and 1 100 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 108. When the fuel and oxidant mixture 1 10 travels about 200 times the nozzle diameter or more, the fuel and oxidant mixture 1 10 is sufficiently

homogenized to cause the combustion reaction 1 12 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 104 or for premixing, can include a blower configured to force the oxidant through the fuel and oxidant source 202. The perforated flame holder support structure 222 can be configured to support the perforated flame holder 102 from a floor or wall (not shown) of the combustion volume 104, for example. In another embodiment, the perforated flame holder support structure 222 supports the perforated flame holder 102 from the fuel and oxidant source 202. Alternatively, the perforated flame holder 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 perforated flame holder 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 102. 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 102 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 126. 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 126 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 104. This can allow the flue gas recirculation 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 Dp between opposing sides of the squares. In another embodiment, the perforations 210 can include elongated hexagons, each having a transverse dimension Dp between opposing sides of the hexagons. In yet another embodiment, the perforations 210 can include hollow cylinders, each having a transverse dimension Dp 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 Dp radially symmetric relative to a length axis that extends from the input face 212 to the output face 126. In some embodiments, the perforations 210 can each have a lateral dimension Dp equal to or greater than a quenching distance of the flame based on standard reference conditions. Alternatively, the perforations 210 may have lateral dimension Dp less than a standard reference quenching distance.

In one range of embodiments, each of the plurality of perforations 210 has a lateral dimension Dp between 0.05 inch and 1 .0 inch. Preferably, each of the plurality of perforations 210 has a lateral dimension Dp between 0.1 inch and 0.5 inch. For example the plurality of perforations 210 can each have a lateral dimension Dp 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 perforated flame holder 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 inventor has found that the perforated flame holder 102 can be formed from VERSAGRID ® ceramic honeycomb, available from Applied Ceramics, Inc. of Doraville, South Carolina.

The perforations 210 can be parallel to one another and normal to the input and output faces 212, 126. 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, 126. 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 perforated flame holder body 208 can be one piece or can be formed from a plurality of sections.

In another embodiment, which is not necessarily preferred, the 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 inventor contemplates 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 1 12 even under conditions where a combustion reaction 1 12 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 1 10 contacts the input face 212 of the perforated flame holder 102, an average fuel-to-oxidant ratio of the fuel stream 1 10 is below a (conventional) lower combustion limit of the fuel component of the fuel stream 1 10— lower combustion limit defines the lowest concentration of fuel at which a fuel and oxidant mixture 1 10 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% O2, i.e., an equivalence ratio of -0.87. Use of even leaner mixtures is possible, but may result in elevated levels of O2. Moreover, the inventor believes 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 1 12 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, according to an embodiment. 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, Ts. 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, Ts. 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 decision 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 decision 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 inventor. 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 decision 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 1 10. After

combustion is established, this heat is provided by the combustion reaction 1 12; but before combustion is established, the heat is provided by the heater Various heating apparatuses have been used and are contemplated by the inventor. 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 108 configured to emit a fuel stream 1 10 and an oxidant source 220 configured to output oxidant (e.g., combustion air) adjacent to the fuel stream 1 10. The fuel nozzle 108 and oxidant source 220 can be configured to output the fuel stream 1 10 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 1 10 that supports a combustion reaction 1 12 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 1 10 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 1 10 to cause the fuel and oxidant mixture 1 10 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 1 10 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 1 10. 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 1 10. The attraction of the electrical charge was found by the inventor 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 228 configured to output heat to the perforated flame holder 102 and/or to the fuel and oxidant mixture 1 10. The electrical resistance heater 228 can be configured to heat up the perforated flame holder 102 to an operating temperature. The electrical resistance heater 228 can further include a power supply and a switch operable, under control of the controller 230, to selectively couple the power supply to the electrical resistance heater 228.

An electrical resistance heater 228 can be formed in various ways. For example, the electrical resistance heater 228 can be formed from KANTHAL® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstahammar, Sweden) threaded through at least a portion of the

perforations 210 defined 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 fuel and oxidant mixture 1 10. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite the fuel and oxidant mixture 1 10 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 1 10 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 1 12 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 238 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 238 to change the fuel and oxidant mixture 1 10 flow responsive to a heat demand change received as data via the data interface 232.

FIG. 5 is a diagrammatic top plan view of a furnace 500, according to an embodiment. The furnace 500 includes a perforated flame holder 102 that is made up of a plurality of flame holder tiles 502 arranged in an array within a furnace volume 104 defined in part by furnace walls 504. A plurality of heat exchange tubes 506 within the furnace volume 104 are positioned to receive thermal energy from a combustion reaction 1 12 held by the perforated flame holder 102. A burner system 508 includes a plurality of pilot burners 122 is arranged with respective pilot nozzles 142 positioned over the output face 126 of the perforated flame holder 102, substantially as described above with reference to the combustion system 100 of FIGS. 1A and 1 B. The pilot burners 122 are configured to operate continuously during normal operation of the furnace, and to maintain an elevated temperature of portions of the perforated flame holder 102.

The inventor has found that when using pilot burners 122 as described herein, the perforated flame holder 102 can be maintained in condition to reignite in the event the combustion reaction 1 12 is spontaneously

extinguished. This is surprising for at least a couple of reasons. First, that the pilot burners 122 can hold a large enough portion of the perforated flame holder 102 at a re-ignition temperature when the combustion reaction 1 12 is extinguished— the actual area covered by the pilot flames 124 is much smaller than pictured, inasmuch as the drawings are not to scale.

Second, in view of the arrangement of the pilot nozzles 142 over the output face 126 the perforated flame holder 102, that following a loss of combustion, the incoming fuel stream 1 10 is able to auto-ignite at the input face 212 of the perforated flame holder 102, either when fuel begins to flow again, or shortly thereafter. As explained above, for proper operation of the perforated flame holder 102, the perforated flame holder 102 is preheated prior to ignition to a temperature at which the fuel and oxidant mixture 1 10 auto-ignites as it enters the input face 212 of the perforated flame holder 102. It has been the general understanding that unless at least a portion of the input face 212 is heated beyond the auto-ignition temperature of the fuel, the combustion reaction 1 12 will not reliably settle into the operation described, in which most of the combustion occurs between the input and output faces 212, 126. Instead, a flame may be held above the output face 126, but it will not have the same burn characteristics as described above, and will have a much lower maximum output capacity.

When the combustion reaction 1 12 is extinguished for longer than some threshold time period, the perforated flame holder 102 cools beyond the point at which it can be restarted without first preheating the perforated flame holder 102. It has been understood that a flame or other heat source positioned over the output face 126 cannot heat the input face 212— from the opposite side— sufficient to ignite the fuel, nor can it prevent the input face 212 from cooling excessively, following a shutdown. Nevertheless, the inventor has found that pilot burners 122 can do exactly that. FIGS. 6-8B illustrate examples of various pilot nozzle configurations, according to respective embodiments. FIG. 6 is a plan view of a pilot nozzle 600, according to an embodiment. The pilot nozzle 600 includes a single orifice 602, and is configured to produce a long, narrow "pencil flame 124," similar to the pilot nozzle 142 shown previously.

FIG. 7 is a plan view of a pilot nozzle 700, according to an

embodiment. The pilot nozzle 700 includes a plurality of orifices 602, each configured to emit a respective stream of fuel so as to produce a fan-shaped flame array 702. The diameter of the flame array 702 is determined, in part, by the size of the orifices 602 and the pressure at which the premix fuel is ejected. Where the pilot nozzles 142 and 600 shown and described above are configured to produce long, narrow pilot flames 124, and thus to heat a correspondingly long and narrow region of the perforated flame holder 102, flame array 702 of the pilot nozzle 700 will heat a more compact region. The individual pilot flames 124 of the flame array 702 may not be wider than those of the other nozzles, but the larger number of orifices 602 means that fuel pressure from each orifice, or port, will be lower, for a given total flow of fuel, so the pilot flames 124 will be much shorter, but the heated region of the perforated flame holder 102 may be larger. Additionally, if fuel flow is increased, the area under the flame array 702 can be increased, while a greater fuel flow through the pilot nozzle 700 will not increase the area appreciably.

In the embodiment shown, the pilot nozzle 700 includes eleven orifices 602, with 15 degrees of separation between each adjacent pair.

Embodiments having pilot nozzles 142 with different numbers of orifices 602 and distributed differently are contemplated.

FIG. 8A is a side elevation of a portion of a pilot burner 800, according to an embodiment, extending through an aperture 802 on a perforated flame holder 102. FIG. 8B is a top sectional view of the pilot burner 800, according to an embodiment. The pilot burner 800 includes a mixing tube 804 and a pilot nozzle 806. The pilot nozzle 806 includes a plurality of orifices 602 distributed around the circumference of the pilot nozzle 806. Where the pilot nozzle 806 is configured to be positioned near an edge of a perforated flame holder 102 substantially as shown in FIG. 5, the pilot burner 800 is configured to be positioned away from the edges, and to heat a large circular region of the perforated flame holder 102.

FIG. 9A is a simplified perspective view of a combustion system 900, including another alternative perforated flame holder 102, according to an embodiment. The perforated flame holder 102 is a reticulated ceramic perforated flame holder, according to an embodiment. FIG. 9B is a simplified side sectional diagram 900 of a portion of the reticulated ceramic perforated flame holder 102 of FIG. 9A, according to an embodiment. The perforated flame holder 102 of FIGS. 9A, 9B can be implemented in the various combustion systems described herein, according to an embodiment. The perforated flame holder 102 is configured to support a combustion reaction of the fuel and oxidant 1 10 at least partially within the perforated flame holder 102 between an input face 212 and an output face 126. According to an embodiment, the perforated flame holder 102 can be configured to support a combustion reaction of the fuel and oxidant 1 10 upstream, downstream, within, and adjacent to the reticulated ceramic perforated flame holder 102.

According to an embodiment, the perforated flame holder body 208 can include reticulated fibers 939. The reticulated fibers 939 can define branching perforations 210 that weave around and through the reticulated fibers 939. According to an embodiment, the perforations 210 are formed as passages between the reticulated fibers 939.

According to an embodiment, the reticulated fibers 939 can include alumina silicate. According to an embodiment, the reticulated fibers 939 can be formed from extruded mullite or cordierite. According to an embodiment, the reticulated fibers 939 can include Zirconia. According to an embodiment, the reticulated fibers 939 can include silicon carbide.

The term "reticulated fibers" refers to a netlike structure. According to an embodiment, the reticulated fibers 939 are formed from an extruded ceramic material. In reticulated fiber embodiments, the interaction between the fuel and oxidant 206, the combustion reaction, and heat transfer to and from the perforated flame holder body 208 can function similarly to the embodiment shown and described above with respect to FIGS. 2-4. One difference in activity is a mixing between perforations 210, because the reticulated fibers 939 form a discontinuous perforated flame holder body 208 that allows flow back and forth between neighboring perforations 210.

According to an embodiment, the network of reticulated fibers 939 is sufficiently open for downstream reticulated fibers 939 to emit radiation for receipt by upstream reticulated fibers 939 for the purpose of heating the upstream reticulated fibers 939 sufficiently to maintain combustion of a fuel and oxidant 206. Compared to a continuous perforated flame holder body 208, heat conduction paths 312 between reticulated fibers 939 are reduced due to separation of the reticulated fibers 939. This may cause relatively more heat to be transferred from the heat-receiving region 306 (heat receiving area) to the heat-output region 310 (heat output area) of the reticulated fibers 939 via thermal radiation.

According to an embodiment, individual perforations 210 may extend between an input face 212 to an output face 126 of the perforated flame holder 102. Perforations 210 may have varying lengths L. According to an embodiment, because the perforations 210 branch into and out of each other, individual perforations 210 are not clearly defined by a length L.

According to an embodiment, the perforated flame holder 102 is configured to support or hold a combustion reaction 1 12 or a flame at least partially between the input face 212 and the output face 126. According to an embodiment, the input face 212 corresponds to a surface of the perforated flame holder 102 proximal to the fuel nozzle 108 or to a surface that first receives fuel. According to an embodiment, the input face 212 corresponds to an extent of the reticulated fibers 939 proximal to the fuel nozzle 218.

According to an embodiment, the output face 126 corresponds to a surface distal to the fuel nozzle 108 or opposite the input face 212. According to an embodiment, the input face 212 corresponds to an extent of the reticulated fibers 939 distal to the fuel nozzle 108 or opposite to the input face 212.

According to an embodiment, the formation of boundary layers 314, transfer of heat between the perforated flame holder body 208 and the gases flowing through the perforations 210, a characteristic perforation 210 width dimension D, and the length L can be regarded as related to an average or overall path through the perforated flame holder 102. In other words, the dimension D can be determined as a root-mean-square of individual Dn values determined at each point along a flow path. Similarly, the length L can be a length that includes length contributed by tortuosity of the flow path, which may be somewhat longer than a straight line distance TRH from the input face 212 to the output face 126 through the perforated flame holder 102. According to an embodiment, the void fraction (expressed as (total perforated flame holder 102 volume - fiber 939 volume)/total volume)) is about 70%.

According to an embodiment, the reticulated ceramic perforated flame holder 102 is a tile about 1 " x 4" x 4". According to an embodiment, the reticulated ceramic perforated flame holder 102 includes about 100 pores per square inch of surface area. Other materials and dimensions can also be used for a reticulated ceramic perforated flame holder 102 in accordance with principles of the present disclosure.

According to an embodiment, the reticulated ceramic perforated flame holder 102 can include shapes and dimensions other than those described herein. For example, the perforated flame holder 102 can include reticulated ceramic tiles that are larger or smaller than the dimensions set forth above. Additionally, the reticulated ceramic perforated flame holder 102 can include shapes other than generally cuboid shapes.

According to an embodiment, the reticulated ceramic perforated flame holder 102 can include multiple reticulated ceramic tiles. The multiple reticulated ceramic tiles can be joined together such that each ceramic tile is in direct contact with one or more adjacent reticulated ceramic tiles. The multiple reticulated ceramic tiles can collectively form a single perforated flame holder 102. Alternatively, each reticulated ceramic tile can be considered a distinct perforated flame holder 102.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.