PERFORATED FLAME HOLDER WITH INTEGRATED SUB-QUENCH DISTANCE LAYER

Abstract

A flame holder assembly includes a flame holder element and a flame shield element. The flame holder element has a first plurality of apertures extending through the flame holder element. The flame shield element has a second plurality of apertures extending through the flame shield element. Each of the second plurality of apertures has a lateral dimension that is no greater than a flame quenching distance. The flame shield element is positioned facing the flame holder element, and the flame holder assembly is configured such that fuel is supplied to the flame holder element via the second plurality of apertures of the flame shield element.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority benefit from U.S. Provisional Patent Application No. 62/274,032, entitled “PERFORATED FLAME HOLDER WITH INTEGRATED SUB-QUENCH DISTANCE LAYER,” filed Dec. 31, 2015 (docket number 2651-269-02); which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

BACKGROUND

The term quenching distance can be defined as the critical diameter of a tube, critical dimension of an opening, or critical distance between two parallel plates, through which a flame will not propagate. In practice, the quenching distance depends upon a number of factors, including the composition of the fuel (e.g., methane, propane, particulate, etc.), the composition of the fuel stream, (e.g., ratio of fuel to O₂, proportion of diluents, etc.), the shape of the passage, the thermal characteristics of the surrounding material, etc. The determination of the quenching distance for a specific application is well understood in the art.

SUMMARY

According to an embodiment, a flame holder assembly is provided, including a flame holder element and a flame shield element. The flame holder element has first and second faces lying opposite each other, and a first plurality of apertures extending through the flame holder element between the first and second faces, each of the first plurality of apertures having lateral dimensions greater than a flame quenching distance. The flame shield element has third and fourth faces lying opposite each other, and a second plurality of apertures extending through the flame shield element between the third and fourth faces. Each of the second plurality of apertures has a lateral dimension that is no greater than the flame quenching distance. The flame shield element is positioned with the third face of the flame shield element facing the second face of the flame holder element.

According to an embodiment, each of the first plurality of apertures extends substantially unobstructed between the first and second faces.

According to an embodiment, the flame holder element has a void fraction of at least 0.50. According to further embodiments, the flame holder element has a void fraction that is, respectively, at least 0.60, at least 0.70, at least 0.80, and equal to about 0.70.

According to an embodiment, an assembly support element is provided, configured to hold the perforated flame holder element and the flame shield element in a spaced-apart relationship.

According to another embodiment, the flame holder element and the flame shield element are positioned with the third face of the flame shield element in direct contact with the second face of the flame holder element.

According to an embodiment, each of the second plurality of apertures has a slot shape, extending laterally in the flame shield element a distance at least equal to a distance between two adjacent ones of the first plurality of apertures.

According to an embodiment, a length of each of the first plurality of apertures is greater than a transverse dimension of the respective one of the first plurality of apertures by a factor of at least 4. According to further embodiments, the length of each of the first plurality of apertures is greater than a transverse dimension of the respective one of the first plurality of apertures by a factor of, respectively, at least 4, 6, 8, 12, 16, 24, and 48.

According to an embodiment, a preheat structure is provided, configured to apply thermal energy to the flame holder element, when activated.

According to an embodiment, a method of operation is provided, in which a fuel stream is introduced to a perforated flame holder via a plurality of passages formed in a shield element that is positioned between the perforated flame holder and a source of the fuel stream, each of the passages having a transverse dimension that is no greater than a flame quenching distance for a fuel component of the fuel stream. A majority of the fuel stream is combusted within a plurality of apertures extending between first and second faces of the perforated flame holder.

According to an embodiment, a quantity of fuel is combusted, sufficient to produce at least 1 MBTUH/ft² of thermal energy. According to further embodiments, the quantity of fuel combusted is sufficient to produce, respectively, at least 1.5, 3, and 5 MBTUH/ft² of thermal energy.

According to an embodiment, the perforated flame holder has a void fraction of at least 0.50. According to further embodiments, the flame holder element has a void fraction that is, respectively, at least 0.60, at least 0.70, at least 0.80, and equal to about 0.70.

According to an embodiment, a length of each of the plurality of apertures in the perforated flame holder is greater than a transverse dimension of the respective one of the plurality of apertures by a factor of at least 4. According to further embodiments, the length of each of the plurality of apertures is greater than a transverse dimension of the respective one of the plurality of apertures by a factor of, respectively, at least 4, 6, 8, 12, 16, 24, and 48. According to an embodiment, each of the plurality of apertures extends without obstruction between the first and second faces of in the perforated flame holder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a burner system including a perforated flame holder configured to hold a combustion reaction, according to an embodiment.

FIG. 2 is a side sectional diagram of a portion of the perforated flame holder of FIG. 1, according to an embodiment.

FIG. 3 is a flow chart showing a method for operating a burner system including the perforated flame holder shown and described herein, according to an embodiment.

FIG. 4 is a side-sectional diagram of a perforated flame holder assembly, according to an embodiment.

FIGS. 5A-5C are diagrams showing a flame holder assembly, according to another embodiment. FIG. 5A is a side sectional view of the flame holder assembly; FIG. 5B is an enlarged view of the portion of the flame holder assembly indicated at 5B in FIG. 5A; and FIG. 5C is a plan view of a portion of the flame holder assembly of FIG. 5A, as viewed from the downstream side of the assembly.

FIGS. 6A and 6B are respective views of a flame holder assembly, according to another embodiment. FIG. 6A is a plan view of a portion of the flame holder assembly, as viewed from the upstream side of the assembly.

FIG. 6B is a perspective view of a portion of the flame holder assembly of FIG. 6A, with additional portions cut-away to show further details.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.

FIG. 1 is a simplified diagram of a burner system 100 including a perforated flame holder 102 configured to hold a combustion reaction, according to an embodiment. As used herein, the terms perforated flame holder, perforated reaction holder, porous flame holder, and porous reaction holder shall be considered synonymous unless further definition is provided. Experiments performed by the inventors have shown that perforated flame holders 102 described herein can support clean combustion. Specifically, in experimental use of systems ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (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 100 includes a fuel and oxidant source 103 disposed to output fuel and oxidant into a combustion volume 104 to form a fuel and oxidant mixture 106. 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 106.

FIG. 2 is a side sectional diagram 200 of a portion of the perforated flame holder 102 of FIG. 1, according to an embodiment. Referring to FIGS. 1 and 2, the perforated flame holder 102 includes a perforated flame holder body 108 defining a plurality of perforations 110 extending substantially unobstructed through the body of the perforated flame holder, aligned to receive the fuel and oxidant mixture 106 from the fuel and oxidant source 103. 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 110 are configured to collectively hold a combustion reaction 202 supported by the fuel and oxidant mixture 106.

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 CO, hydrogen (H₂), and methane (CH₄). In another application the fuel can include natural gas (mostly CH₄) 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 inventors. The oxidant can include O₂ carried by air and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein.

According to an embodiment, the perforated flame holder body 108 can be bound by an input face 112 disposed to receive the fuel and oxidant mixture 106, an output face 114 facing away from the fuel and oxidant source 103, and a peripheral surface 116 defining a lateral extent of the perforated flame holder 102. The plurality of perforations 110, which are defined by the perforated flame holder body 108, extend from the input face 112 to the output face 114. The plurality of perforations 110 can receive the fuel and oxidant mixture 106 at the input face 112. The fuel and oxidant mixture 106 can then combust in or near the plurality of perforations 110 and combustion products can exit the plurality of perforations 110 at or near the output face 114.

According to an embodiment, the perforated flame holder 102 is configured to hold a majority of the combustion reaction 202 within the perforations 110. 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 103 may be converted to combustion products between the input face 112 and the output face 114 of the perforated flame holder 102. According to an alternative interpretation, more than half of the heat output by the combustion reaction 202 may be output between the input face 112 and the output face 114 of the perforated flame holder 102. Under nominal operating conditions, the perforations 110 can be configured to collectively hold at least 80% of the combustion reaction 202 between the input face 112 and the output face 114 of the perforated flame holder 102. In some experiments, the inventors produced a combustion reaction that was apparently wholly contained in the perforations 110 between the input face 112 and the output face 114 of the perforated flame holder 102. According to an alternative interpretation, the perforated flame holder 102 can support combustion between the input face 112 and output face 114 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 114 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 110, but the “glow” of combustion heat is dominated by a visible glow of the perforated flame holder 102 itself, produced by the visible portion of thermal radiation 204 emitted during operation. In other instances, the inventors have noted transient “huffing” wherein a visible flame momentarily ignites in a region lying between the input face 112 of the perforated flame holder 102 and a fuel source—a fuel nozzle 118, in the embodiment shown—within the dilution region D_D. Such transient huffing is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations 110 of the perforated flame holder 102, between the input face 112 and the output face 114. In still other instances, the inventors have noted apparent combustion occurring above the output face 114 of the perforated flame holder 102, but still a majority of combustion occurred within the perforated flame holder 102 as evidenced by the continued visible glow (a visible wavelength tail of blackbody radiation) from the perforated flame holder 102.

The inventors have determined that the perforated flame holder 102 does not require turbulence-producing elements to hold combustion within the apertures 110, even during operation at surprisingly high output loads. The inventors have conducted tests in which output loads of up to about 5 MBTUH/ft² were achieved, while holding the combustion reaction 202 substantially within the apertures 110.

The perforated flame holder 102 can be configured to receive heat from the combustion reaction 202 and output a portion of the received heat as thermal radiation 204 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 thermal radiation, infrared radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody radiation of electromagnetic energy, primarily in infrared wavelengths.

Referring especially to FIG. 2, the perforated flame holder 102 outputs another portion of the received heat to the fuel and oxidant mixture 106 received at the input face 112 of the perforated flame holder 102. The perforated flame holder body 108 may receive heat from the (exothermic) combustion reaction 202 at least in heat receiving regions 206 of perforation walls 208. Experimental evidence has suggested to the inventors that the position of the heat receiving regions 206, or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls 208. In some experiments, the location of maximum receipt of heat was apparently between ⅓ and ½ of the distance from the input face 112 to the output face 114 (i.e., somewhat nearer to the input face 112 than to the output face 114). The inventors contemplate that the heat receiving regions 206 may lie nearer to the output face 114 of the perforated flame holder 102 under other conditions. Most probably, there is no clearly defined edge of the heat receiving regions 206 (or for that matter, the heat output regions 210, described below). For ease of understanding, the heat receiving regions 206 and the heat output regions 210 will be described as particular regions 206, 210.

The perforated flame holder body 108 can be characterized by a heat capacity. The perforated flame holder body 108 may hold heat from the combustion reaction 202 in an amount corresponding to the heat capacity times temperature rise, and transfer the heat from the heat receiving regions 206 to heat output regions 210 of the perforation walls 208. Generally, the heat output regions 210 are nearer to the input face 112 than are the heat receiving regions 206. According to one interpretation, the perforated flame holder body 108 can transfer heat from the heat receiving regions 206 to the heat output regions 210 via thermal radiation, depicted graphically as 204. According to another interpretation, the perforated flame holder body 108 can transfer heat from the heat receiving regions 206 to the heat output regions 210 via heat conduction along heat conduction paths 212. The inventors contemplate that both radiation and conduction heat transfer mechanisms may be operative in transferring heat from the heat receiving regions 206 to the heat output regions 210. In this way, the perforated flame holder 102 may act as a heat source to maintain the combustion reaction 202, even under conditions where a combustion reaction would not be stable when supported from a conventional flame holder.

The inventors believe that the perforated flame holder 102 causes the combustion reaction 202 to occur within thermal boundary layers 214 formed adjacent to walls 208 of the perforations 110. As the relatively cool fuel and oxidant mixture 106 approaches the input face 112, the flow is split into portions that respectively travel through individual perforations 110. The hot perforated flame holder body 108 transfers heat to the fluid, notably within thermal boundary layers 214 that progressively thicken as more and more heat is transferred to the incoming fuel and oxidant mixture 106. 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 202 occurs. Accordingly, the combustion reaction 202 is shown as occurring within the thermal boundary layers 214. As flow progresses, the thermal boundary layers 214 merge at a merger point 216. Ideally, the merger point 216 lies between the input face 112 and output face 114 that defines the ends of the perforations 110. At some point, the combustion reaction 202 causes the flowing gas (and plasma) to output more heat to the body 108 than it receives from the body 108. The heat is received at the heat receiving region 206, is held by the body 108, and is transported to the heat output region 210 nearer to the input face 112, where the heat recycles into the cool reactants (and any included diluent) to raise them to the combustion temperature.

In an embodiment, the plurality of perforations 110 are each characterized by a length L defined as a reaction fluid propagation path length between the input face 112 and the output face 114 of the perforated flame holder 102. The reaction fluid includes the fuel and oxidant mixture 106 (optionally including nitrogen, flue gas, and/or other “non-reactive” species), reaction intermediates (including transition states in a plasma that characterizes the combustion reaction), and reaction products.

The plurality of perforations 110 can be each characterized by a transverse dimension D_S between opposing perforation walls 208. The inventors have found that stable combustion can be maintained in the perforated flame holder 102 if the length L of each perforation 110 is at least four times the transverse dimension D_S of the perforation. In other embodiments, the length L can be greater than six times the transverse dimension D_S. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D_S. Preferably, the length L is sufficiently long for thermal boundary layers 214 formed adjacent to the perforation walls 208 in a reaction fluid flowing through the perforations 110 to converge at merger points 216 within the perforations 110 between the input face 112 and the output face 114 of the perforated flame holder 102. In experiments, the inventors have found L/D_S 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 108 can be configured to convey heat between adjacent perforations 110. The heat conveyed between adjacent perforations 110 can be selected to cause heat output from the combustion reaction portion 202 in a first perforation 110 to supply heat to stabilize a combustion reaction portion 202 in an adjacent perforation 110.

Referring especially to FIG. 1, the fuel and oxidant source 103 can further include the fuel nozzle 118, configured to output fuel, and an oxidant source 120 configured to output a fluid including the oxidant. For example, the fuel nozzle 118 can be configured to output pure fuel. The oxidant source 120 can be configured to output combustion air carrying oxygen.

The perforated flame holder 102 can be held by a perforated flame holder support structure 122 configured to hold the perforated flame holder 102 a distance D_D away from the fuel nozzle 118. The fuel nozzle 118 can be configured to emit a fuel jet selected to entrain the oxidant to form the fuel and oxidant mixture 106 as the fuel jet and oxidant travel along a path to the perforated flame holder 102 through a dilution distance D_D between the fuel nozzle 118 and the perforated flame holder 102. Additionally or alternatively (particularly when a blower is used to deliver oxidant combustion air), the oxidant or combustion air source can be configured to entrain the fuel and the fuel and oxidant travel through the dilution distance D_D. In some embodiments, a flue gas recirculation path 124 can be provided. Additionally or alternatively, the fuel nozzle 118 can be configured to emit a fuel jet selected to entrain the oxidant and to entrain flue gas as the fuel jet travels through a dilution distance D_D between the fuel nozzle 118 and the input face 112 of the perforated flame holder 102.

The fuel nozzle 118 can be configured to emit the fuel through one or more fuel orifices 126 having a dimension that is referred to as “nozzle diameter.” The perforated flame holder support structure 122 can support the perforated flame holder 102 to receive the fuel and oxidant mixture 106 at a distance D_D away from the fuel nozzle 118 greater than 20 times the nozzle diameter. In another embodiment, the perforated flame holder 102 is disposed to receive the fuel and oxidant mixture 106 at a distance D_D away from the fuel nozzle 118 between 100 times and 1100 times the nozzle diameter. Preferably, the perforated flame holder support structure 122 is configured to hold the perforated flame holder 102 about 200 times the nozzle diameter or more away from the fuel nozzle 118. When the fuel and oxidant mixture 106 travels about 200 times the nozzle diameter or more, the mixture is sufficiently homogenized to cause the combustion reaction 202 to output minimal NOx.

The fuel and oxidant source 103 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 air channel configured to output combustion air into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the perforated flame holder 102 and be configured to prevent flame flashback into the premix fuel and oxidant source.

The combustion air source, whether configured for entrainment in the combustion volume 104 or for premixing can include a blower configured to force air through the fuel and air source 103.

The support structure 122 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 support structure 122 supports the perforated flame holder 102 from the fuel and oxidant source 103. Alternatively, the support structure 122 can suspend the perforated flame holder 102 from an overhead structure (such as a flue, in the case of an up-fired system). The support structure 122 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 108. 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 122 can be configured to support the plurality of perforated flame holder sections. The perforated flame holder support structure 122 can include a metal superalloy, a cementatious, and/or ceramic refractory material. In an embodiment, the plurality of adjacent perforated flame holder sections can be joined with a fiber reinforced refractory cement.

The perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 116 at least twice a thickness dimension T between the input face 112 and the output face 114. In another embodiment, the perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 116 at least three times, at least six times, or at least nine times a thickness dimension T between the input face 112 and the output face 114 of the perforated flame holder.

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 circulation path 124 from above to below the perforated flame holder 102 to lie between the peripheral surface 116 of the perforated flame holder 102 and the combustion volume wall (not shown).

Referring again to both FIGS. 1 and 2, the perforations 110 can include elongated squares, each of the elongated squares having a transverse dimension D_S between opposing sides of the squares. In another embodiment, the perforations 110 can include elongated hexagons, each of the elongated hexagons having a transverse dimension D_S between opposing sides of the hexagons. In another embodiment, the perforations 110 can include hollow cylinders, each of the hollow cylinders having a transverse dimension D_S corresponding to a diameter of the cylinders. In another embodiment, the perforations 110 can include truncated cones, each of the truncated cones having a transverse dimension D_S that is rotationally symmetrical about a length axis that extends from the input face 112 to the output face 114. The perforations 110 can each have a lateral dimension D_S equal to or greater than a quenching distance of the fuel based on standard reference conditions.

In one range of embodiments, each of the plurality of perforations 110 has a lateral dimension D_S between 0.05 inch and 1.0 inch. Preferably, each of the plurality of perforations 110 has a lateral dimension D_S between 0.1 inch and 0.5 inch. For example the plurality of perforations 110 can each have a lateral dimension D_S 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 110 in a section of the perforated flame holder 102 divided by a total volume of the perforated flame holder 102 including the body 108 and perforations 110. The perforated flame holder 102 preferably has 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 greater than about 0.50. According to another embodiment, the perforated flame holder 102 can have a void fraction of greater than about 0.60. According to another embodiment, the perforated flame holder 102 can have a void fraction of greater than about 0.70. According to another embodiment, the perforated flame holder 102 can have a void fraction of greater than about 0.80. According to a further embodiment, the perforated flame holder 102 can have a void fraction of about 0.70. In experiments conducted by the inventors, 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 from mullite or cordierite. Additionally or alternatively, the perforated flame holder body 108 can include a metal superalloy such as Inconel® or Hastelloy®. The perforated flame holder body 108 can define a honeycomb.

The inventors have found that the perforated flame holder 102 can be formed from VERSAGRID® ceramic honeycomb, available from Applied Ceramics, Inc. of Doraville, S.C.

The perforations 110 can be parallel to one another and normal to the input and output faces 112, 114. In another embodiment, the perforations 110 can be parallel to one another and formed at an angle relative to the input and output faces 112, 114. In another embodiment, the perforations 110 can be non-parallel to one another. In another embodiment, the perforations 110 can be non-parallel to one another and non-intersecting. In another embodiment, the perforations 110 can be intersecting. The body 108 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 fibers formed from an extruded ceramic material. The term “reticulated fibers” refers to a netlike structure.

In another embodiment, the perforated flame holder 102 can include a plurality of tubes or pipes bundled together. The plurality of perforations 110 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 108 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 108 can include discontinuous packing bodies such that the perforations 110 are formed in the interstitial spaces between the discontinuous packing bodies. In one example, the discontinuous packing bodies include structured packing shapes. In another example, the discontinuous packing bodies include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig ring, ceramic Berl saddles, ceramic Intalox® saddles, and/or metal rings or other shapes (e.g. Super Raschig Rings) that may be held together by a metal cage.

The inventors contemplate various explanations for why burner systems including the perforated flame holder 102 provide such clean combustion.

In one aspect, the perforated flame holder 102 acts as a heat source to maintain a combustion reaction 202 even under conditions where a combustion reaction 202 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 106 contacts the input face 112 of the perforated flame holder 102, an average fuel-to-oxidant ratio of the fuel stream 106 is below a (conventional) lower combustion limit of the fuel component of the fuel stream—lower combustion limit defines the lowest concentration of fuel at which a fuel/air mixture will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.).

According to one interpretation, the fuel and oxidant mixtures supported by the perforated flame holder 102 may be more fuel-lean than mixtures that would provide stable combustion in a conventional burner. Combustion near a lower combustion limit of fuel generally burns at a lower adiabatic flame temperature than mixtures near the center of the lean-to-rich combustion limit range. Lower flame temperatures generally evolve a lower concentration of NOx than higher flame temperatures. In conventional flames, too-lean combustion is generally associated with high CO concentration at the stack. In contrast, 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. In some embodiments, the inventors achieved stable combustion at what was understood to be very lean mixtures (that nevertheless produced only about 3% or lower measured O₂ concentration at the stack). Moreover, the inventors believe perforation walls 208 may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperature.

According to another interpretation, production of NOx can be reduced if the combustion reaction 202 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.

Since CO oxidation is a relatively slow reaction, the time for passage through the perforated flame holder 102 (perhaps plus time passing toward the flue from the perforated flame holder 102) is apparently sufficient and at sufficiently elevated temperature, in view of the very low measured (experimental and full scale) CO concentrations, for oxidation of CO to carbon dioxide (CO₂).

FIG. 3 is a flow chart showing a method 300 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 300 begins with step 302, wherein the perforated flame holder is preheated to a start-up temperature, T_S. After the perforated flame holder is raised to the start-up temperature, the method proceeds to step 304, wherein 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 302 begins with step 306, wherein start-up energy is provided at the perforated flame holder. Simultaneously or following providing start-up energy, a decision step 308 determines whether the temperature T of the perforated flame holder is at or above the start-up temperature, T_S. As long as the temperature of the perforated flame holder is below its start-up temperature, the method loops between steps 306 and 308 within the preheat step 302. In step 308, 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 300 proceeds to overall step 304, wherein fuel and oxidant is supplied to and combustion is held by the perforated flame holder.

Step 304 may be broken down into several discrete steps, at least some of which may occur simultaneously.

Proceeding from step 308, a fuel and oxidant mixture is provided to the perforated flame holder, as shown in step 310. The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and combustion air source, for example. In this approach, the fuel and combustion air are output in one or more directions selected to cause the fuel and combustion air mixture to be received by an input face of the 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 312, the combustion reaction is held by the perforated flame holder.

In step 314, 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 316, the presence of combustion may be sensed. Various sensing approaches have been used and are contemplated by the inventors. Generally, combustion held by the perforated flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, and/or other known combustion sensing apparatuses. In an additional or alternative variant of step 316, 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 318, if combustion is sensed not to be stable, the method 300 may exit to step 324, wherein an error procedure is executed. For example, the error procedure may include turning off fuel flow, re-executing the preheating step 302, outputting an alarm signal, igniting a stand-by combustion system, or other steps. If, in step 318, combustion in the perforated flame holder is determined to be stable, the method 300 proceeds to decision step 320, wherein it is determined if combustion parameters should be changed. If no combustion parameters are to be changed, the method loops (within step 304) back to step 310, and the combustion process continues. If a change in combustion parameters is indicated, the method 300 proceeds to step 322, wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step 304) back to step 310, 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 322. 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 304.

Referring again to FIG. 1, the burner system 100 includes a heater 128 operatively coupled to the perforated flame holder 102. As described in conjunction with FIGS. 2 and 3, the perforated flame holder 102 operates by outputting heat to the incoming fuel and oxidant mixture 106. After combustion is established, this heat is provided by the combustion reaction 202; but before combustion is established, the heat is provided by the heater 128.

Various heating apparatuses have been used and are contemplated by the inventors. In some embodiments, the heater 128 can include a flame holder configured to support a flame disposed to heat the perforated flame holder 102. The fuel and oxidant source 103 can include a fuel nozzle 118 configured to emit a fuel stream and an air source 120 configured to output combustion air adjacent to the fuel stream. The fuel nozzle 118 and air source 120 can be configured to output the fuel stream to be progressively diluted by the combustion air. The perforated flame holder 102 can be disposed to receive a diluted fuel and air mixture 106 that supports a combustion reaction 202 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 rich fuel and air mixture that is stable without stabilization provided by the heated perforated flame holder 102.

The burner system 100 can further include a controller 130 operatively coupled to the heater 128 and to a data interface 132. For example, the controller 130 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 106 to cause heat-recycling vortices and thereby hold a start-up flame; or to be actuated to not intercept the fuel and oxidant mixture 106 to cause the fuel and oxidant mixture 106 to proceed to the perforated flame holder 102. In another embodiment, a fuel control valve, blower, and/or damper may be used to select a fuel and oxidant mixture flow rate that is sufficiently low for a start-up flame to be jet-stabilized; and upon reaching a perforated flame holder 102 operating temperature, the flow rate may be increased to “blow out” the start-up flame. In another embodiment, the heater 128 may include an electrical power supply operatively coupled to the controller 130 and configured to apply an electrical charge or voltage to the fuel and oxidant mixture 106. 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 106. The attraction of the electrical charge was found by the inventors to cause a start-up flame to be held by the electrically conductive start-up flame holder.

In another embodiment, the heater 128 may include an electrical resistance heater 128 configured to output heat to the perforated flame holder 102 and/or to the fuel and oxidant mixture 106. The electrical resistance heater 128 can be configured to heat up the perforated flame holder 102 to an operating temperature. The heater 128 can further include a power supply and a switch operable, under control of the controller 130, to selectively couple the power supply to the electrical resistance heater 128.

An electrical resistance heater 128 can be formed in various ways. For example, the electrical resistance heater 128 can be formed from KANTHAL® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstaham mar, Sweden) threaded through at least a portion of the perforations 110 formed by the perforated flame holder body 108. Alternatively, the heater 128 can include an inductive heater, a high energy (e.g. microwave or laser) beam heater, a frictional heater, or other types of heating technologies.

Other forms of start-up apparatuses are contemplated. For example, the heater 128 can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the air and fuel. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite a fuel and oxidant mixture 106 that would otherwise enter the perforated flame holder 102. An electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller 130, which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the fuel and oxidant mixture 106 in or upstream from the perforated flame holder 102 before the perforated flame holder 102 is heated sufficiently to maintain combustion.

The burner system 100 can further include a sensor 134 operatively coupled to the controller 130. The sensor 134 can include a heat sensor configured to detect infrared radiation or a temperature of the perforated flame holder 102. The controller 130 can be configured to control the heating apparatus 128 responsive to input from the sensor 134. Optionally, a fuel control valve 136 can be operatively coupled to the controller 130 and configured to control a flow of fuel to the fuel and oxidant source 103. Additionally or alternatively, an oxidant blower or damper 138 can be operatively coupled to the controller 130 and configured to control flow of the oxidant (or combustion air).

The sensor 134 can further include a combustion sensor operatively coupled to the control circuit 130, the combustion sensor being configured to detect a temperature, video image, and/or spectral characteristic of a combustion reaction 202 held by the perforated flame holder 102. The fuel control valve 136 can be configured to control a flow of fuel from a fuel source to the fuel and oxidant source 103. The controller 130 can be configured to control the fuel control valve 136 responsive to input from the combustion sensor 134. The controller 130 can be configured to control the fuel control valve 136 and/or oxidant blower or damper 138 to control a preheat flame type of heater 128 to heat the perforated flame holder 102 to an operating temperature. The controller 130 can similarly control the fuel control valve 136 and/or the oxidant blower or damper 138 to change the fuel and oxidant mixture 106 flow responsive to a heat demand change received as data via the data interface 132.

Hereafter, and in the claims, unless otherwise defined or limited, terms such as fuel, fuel stream, fuel supply, etc., are to be construed broadly as reading on a substance that includes fuel, but that can also include additional components, such as oxidant, recirculated combustion products, diluents, inert gases, ambient air, etc.

FIG. 4 is a side-sectional view of a perforated flame holder assembly 400, according to an embodiment. The flame holder assembly 400 includes a flame holder element 402 and a flame shield element 404, held in a spaced-apart relationship by an assembly support element 406. The flame holder element 402 is structured substantially as described with reference to the perforated flame holder 102 of FIGS. 1 and 2, and includes input and output faces 112, 114, and a plurality of apertures 110.

The flame shield element 404 includes an upstream face 412 and a downstream face 414, and a plurality of fuel passages 408 extending between the upstream and downstream faces 412, 414. Lateral dimensions D_P of the fuel passages 408 are selected to be no larger than a quenching distance of a flame. As with the apertures 110 of the flame holder element 402, the fuel passages 408 can have any of a number of shapes—in a transverse sectional view—including, for example, square, rectangular, round, and polygonal. Lateral dimensions of the flame shield element 404 are preferably coextensive with those of the flame holder element 402. In the embodiment of FIG. 4, the upstream face 412 of the flame shield element 404 defines an input face 410 of the flame holder assembly 400.

According to an embodiment, the assembly support element 406 extends around the entire lateral perimeter of the flame holder assembly 400. This serves to prevent the introduction of fuel or other fluids into the assembly except via the fuel passages 408. This is of particular benefit in combustion systems in which the flame holder assembly 400 is configured to be held away from side walls of a combustion volume.

According to an embodiment, the flame holder element 402 can be preheated prior to normal operation, in order to initiate and sustain the combustion reaction 202. For example, an electrically resistive heater 128 can be provided, positioned in close proximity, or in contact with the flame holder element 402. Just prior to the introduction of fuel to the flame holder assembly 400, a voltage is applied to the heater 128, which raises the temperature of at least a portion of the flame holder element 402 to an operating temperature, which is sufficient to initiate combustion. Various other structures and methods for preheating the flame holder element 402 are contemplated, including many of those previously described.

During normal operation of the flame holder assembly 400, according to an embodiment, fuel—including oxidizer—is introduced to the flame holder assembly 400 via the fuel passages 408 of the flame shield element 404. A combustion reaction 202 is held within the apertures 110 of the flame holder 102 as explained in detail above, with reference to FIGS. 1 and 2. Thermal radiation 204 is emitted by the flame holder assembly 400, particularly from the output face 114 of the flame holder element 402. Because the fuel passages 408 of the flame shield element 404 are smaller than the quenching distance, the combustion reaction 202 cannot pass through the flame shield element 404, which reduces or eliminates the danger of flashback, in embodiments where this may be a concern, such as in cases where the fuel-to-oxidant ratio of the fuel stream 106 is above the lower combustion limit as it is introduced to the input face 410 of the flame holder assembly 400.

According to another embodiment, at the point where the fuel stream 106 contacts the flame holder element 402, the average fuel-to-oxidant ratio of the fuel stream 106 is below the lower combustion limit of the fuel component of the fuel stream, and only becomes flammable when it is heated by the combustion reaction 202. Even though flashback may not be an issue in such cases, the flame shield element 404 can be beneficial in other ways. For example, the flame shield element 404 may also serve to substantially block the radiation of thermal energy 204 from the flame holder element 402 in the upstream direction (i.e., toward the fuel source, etc.). This is beneficial, for example, in reducing the heat applied to system components located on the upstream side of the flame holder assembly 400 such as, e.g., fuel nozzles, valves, support structures, enclosures, etc. Furthermore, by substantially preventing the radiation of thermal energy 204 in the upstream direction, a greater proportion of the total thermal energy produced by the combustion reaction 202 is radiated downstream from the flame holder assembly 400. Thus—assuming a thermal load positioned in the downstream direction—for a given expenditure of fuel, more thermal energy is delivered to the load, or, alternatively, for a given value of thermal energy to be delivered, the fuel expenditure can be reduced, as compared to a system in which the flame shield is omitted.

According to an embodiment, the maximum thermal output of the flame holder assembly 400 is between 1 and 5 MBTUH/ft². According to an embodiment, the maximum thermal output of the flame holder assembly 400 is greater than 1.5 MBTUH/ft². According to another embodiment, the maximum thermal output of the flame holder assembly 400 is greater than 3 MBTUH/ft². According to a further embodiment, the maximum thermal output of the flame holder assembly 400 is greater than 5 MBTUH/ft².

It will be understood that the thermal output of the flame holder assembly 400 is directly related to the volume of fuel—including oxidizer—that is supplied to the flame holder element 402. Because the dimensions D_P of the fuel passages 408 are relatively small, i.e., smaller than the quenching distance, the void fraction of the shield element 404 may be lower than that of the flame holder element 402. This can result in a significant pressure drop across the shield element 404, particularly in embodiments in which the number of passages 408 is substantially equal to the number of apertures 110 in the flame holder element 402 (see, e.g., FIG. 5). This, in turn may necessitate an increased fuel pressure on the upstream side of the flame shield element 404, depending upon the desired thermal output.

According to the embodiment shown in FIG. 4, the number of fuel passages 408 per unit of transverse area is greater than the number of apertures 110 per unit of area. This results in an increased void fraction of the flame shield element 404, which reduces the pressure required for a given volume of fuel to pass through the flame shield element 404. According to another embodiment, the number of fuel passages 408 per unit of transverse area is selected to provide a void fraction substantially equal to that of the flame holder element 402. According to a further embodiment, the number of fuel passages 408 per unit of transverse area is selected to provide a void fraction that is greater than that of the flame holder element 402.

FIG. 4 shows an embodiment in which a fuel stream 106 is introduced to the flame holder assembly 400 as from a fuel nozzle, substantially as described with reference to FIG. 1. In embodiments in which a nozzle is employed, the fuel and oxidizer can be premixed prior to passing through the nozzle, or the system can be configured such that the fuel stream 106 entrains the oxidizer as it traverses the distance between the nozzle and the flame holder assembly 400. According to another embodiment, fuel is supplied via a manifold or mixing chamber coupled to the input face 410 of the flame holder assembly 400. In such embodiments, the oxidizer can be separately introduced to a mixing chamber, for example, where it then mixes with the fuel. Alternatively, the fuel can be premixed with the oxidizer prior to introduction into a manifold.

The flame shield element 404 can be manufactured using the same types of materials that can be employed to make the flame holder element 402, including various ceramics and alloys, as previously discussed. Likewise, similar manufacturing processes can also be employed, including, for example, extrusion, casting, sintering, machining, and isostatic pressing. However, it should be noted that it is not essential that the flame holder element 402 and the flame shield element 404 be made of the same material. In some embodiments, it may be advantageous for one or more specific characteristics of the flame shield element 404 to differ from those of the flame holder element 402. Such characteristics may include, for example, thermal conductivity, thermal emissivity, thermal capacity, mechanical strength and toughness, coefficient of thermal expansion, etc.

FIGS. 5A-5C are diagrams showing a flame holder assembly 500, according to an embodiment. FIG. 5A is a side sectional view of the flame holder assembly 500; FIG. 5B is an enlarged view of the portion of the flame holder assembly 500 indicated at FIG. 5B of FIG. 5A; and FIG. 5C is a plan view of a portion of the flame holder assembly 500, as viewed from the downstream side of the assembly.

Referring to FIGS. 5A-5C, the flame holder assembly 500 includes a flame holder element 402 and a flame shield element 404, positioned in face-to-face contact with each other, i.e., with the downstream face 414 of the flame shield element 404 arranged to be very close to, or in direct physical contact with the input face 112 of the flame holder element 402. The flame shield element 404 of the flame holder assembly 500 is configured such that there is a fuel passage 408 in alignment with each of the apertures 110 of the flame holder element 402, as shown in FIG. 5C. Fuel entering one of the fuel passages 408 is introduced directly into a corresponding one of the apertures 110 of the flame holder element 402.

Operation of the flame holder assembly 500 is substantially similar to the operation described with reference to the flame holder assembly 400 of FIG. 4. Because the dimensions D_P of the fuel passages 408 are no greater than the quenching distance, fuel traverses the passages at a relatively high velocity, particularly under high heat output conditions. This helps to cool the flame shield element 404, even in close contact with the flame holder element 402. Accordingly, no more than a small fraction of the thermal energy generated is emitted in the upstream direction.

According to an embodiment, the flame holder element 402 and flame shield element 404 are held in their relative positions by an assembly support element similar to the assembly support element 406 shown and described with reference to FIG. 4. According to another embodiment, a refractory adhesive or cement is used to hold the flame holder element 402 and flame shield element 404 in face-to-face contact. According to a further embodiment, the flame holder element 402 and flame shield element 404 are simply stacked together and supported by a bracket within a furnace, the support bracket being configured to prevent lateral movement of either element during operation, to prevent misalignment of the flame holder element 402 and the flame shield element 404.

According to an embodiment, the flame holder element 402 and flame shield element 404 are formed together in a single piece. This can be achieved by forming the flame holder assembly 500 in a casting operation, or by machining the assembly from a single block of material, etc. In embodiments in which the flame holder element 402 and flame shield element 404 are formed together in a single piece, the downstream face 414 of the flame shield element 404 and the input face 112 of the flame holder element 402 can be considered to be merged, and defined by the positions within the single piece where the apertures 110 transition to fuel passages 408. It should be noted that in some embodiments, each of the transitions may not lie in a common plane, inasmuch as the transitions may be somewhat beveled, and/or the depths of the positions may vary.

FIGS. 6A and 6B are respective views of a flame holder assembly 600, according to an embodiment. FIG. 6A is a plan view of a portion of the flame holder assembly 600, as viewed from the input face 410 of the assembly. FIG. 6B is a perspective view of a portion of the flame holder assembly 600, with portions cut-away to show further details.

Referring to FIGS. 6A and 6B, the flame holder assembly 600 includes a flame holder element 402 and a flame shield element 602, positioned in face-to-face contact with each other. The flame shield element 602 includes a plurality of fuel passages 604 having a slot shape, each extending to admit fuel to a plurality of the apertures 110 in the flame holder element 402. According to an embodiment, the slot-shaped fuel passages 604 have a width D_P that is no greater than the quenching distance.

According to an embodiment, the flame shield element 602 is held in a face-to-face relationship with the flame holder element 402 by an assembly support element 606 similar to the assembly support element 406 shown and described with reference to FIG. 4. According to another embodiment, a refractory adhesive or cement is used to hold the flame holder element 402 and flame shield element 602 in face-to-face contact. According to a further embodiment, the flame holder element 402 and flame shield element 602 are stacked together and held by a support bracket within a furnace. According to another embodiment, the flame holder element 402 and flame shield element 602 are formed together in a single piece.

According to an embodiment, the flame shield element 602 is spaced apart from the flame holder element 402, similar to the configuration of the flame holder assembly 400 described with reference to FIG. 4. In embodiments in which the flame shield element 602 and the flame holder element 402 are spaced apart, the fuel passages 604 can be spaced at a pitch that is different from the pitch of rows of apertures 110 in the flame holder element 402, i.e., there is no necessary correspondence between one of the fuel passages and a row of apertures. Thus, the void fraction of the flame shield element 602 can be increased by increasing the number of slot-shaped fuel passages 604, with a corresponding reduction in the distance between the passages.

One benefit provided by embodiments that include slot-shaped fuel passages 604 like those described above is that, for a given row pitch, the void fraction of the shield element 602 can be much greater than the void fraction of a shield element 602 that has fuel passages 604 like those described above with reference to FIGS. 4 and 5, i.e., square or round, etc. The increased void fraction produces a significantly lower pressure drop in the fuel as it traverses the shield element 602. This, in turn, means that, for a given output, the fuel pressure required on the input side of the flame holder assembly 500 is also reduced. This has an impact on the design parameters and cost of structures such as fuel pumps, blowers, nozzles, etc. Furthermore, in embodiments in which a flame holder assembly 500 is spaced away from side walls defining a combustion volume 104, a lower input pressure reduces the likelihood that some fraction of the fuel stream 106 will pass around, rather than through, the flame holder assembly 500.

Ordinal numbers, e.g., first, second, third, etc., are used in the claims according to conventional claim practice, i.e., for the purpose of clearly distinguishing between claimed elements or features thereof. The use of such numbers does not suggest any other relationship, e.g., order of operation or relative position of such elements. Furthermore, an ordinal number used to refer to an element in the claims does not necessarily correlate to a number used in the specification to refer to an element of a disclosed embodiment on which those claims read, nor to numbers used in unrelated claims to designate similar elements or features.

The abstract of the present disclosure is provided as a brief outline of some of the principles of the invention according to one embodiment, and is not intended as a complete or definitive description of any embodiment thereof, nor should it be relied upon to define terms used in the specification or claims. The abstract does not limit the scope of the claims.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated, including systems in which elements of one or more of the disclosed embodiments are combined with elements that are known in the art, to provide additional embodiments. 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.

Claims

1. A flame holder assembly, comprising:

a flame holder element, including a first face and a second face lying opposite each other, and a first plurality of apertures extending through the flame holder element between the first and second faces, each of the first plurality of apertures having lateral dimensions that are greater than a flame quenching distance, the flame holder element having a void fraction greater than 0.50; and

a flame shield element including a third face and a fourth face lying opposite each other, and a second plurality of apertures extending through the flame shield element between the third and fourth faces, each of the second plurality of apertures having at least one lateral dimension that is no greater than the flame quenching distance, the flame shield element being positioned with the third face facing the second face of the flame holder element.

2. The flame holder assembly of claim 1, wherein the flame holder element is configured to hold a combustion reaction substantially within the first plurality of apertures and between the first and second faces.

3. The flame holder assembly of claim 2, wherein each of the first plurality of apertures extends substantially unobstructed between the first and second faces.

4. The flame holder assembly of claim 1, comprising an assembly support element configured to hold the flame holder element and the flame shield element in a spaced-apart relationship.

5. The flame holder assembly of claim 4, wherein the assembly support element extends around an entire lateral perimeter of the flame holder assembly, enclosing a space between the flame holder element and the flame shield element.

6. The flame holder assembly of claim 1, wherein the flame holder element and flame shield element are positioned with the third face of the flame shield element in direct contact with the second face of the flame holder element.

7. The flame holder assembly of claim 1, wherein each of the second plurality of apertures has a slot shape, extending laterally in the flame shield element a distance at least equal to a distance between two adjacent ones of the first plurality of apertures.

8. The flame holder assembly of claim 1, wherein the flame holder element has a void fraction of greater than 0.50.

9. The flame holder assembly of claim 8, wherein the flame holder element has a void fraction of greater than 0.60.

10. The flame holder assembly of claim 8, wherein the flame holder element has a void fraction of about 0.70.

11. The flame holder assembly of claim 1, wherein a length of each of the first plurality of apertures is greater than a transverse dimension of the respective one of the first plurality of apertures by a factor of at least 4.

12. The flame holder assembly of claim 11, wherein the length of each of the first plurality of apertures is greater than a transverse dimension of the respective one of the first plurality of apertures by a factor of at least 12.

13. The flame holder assembly of claim 12, wherein the length of each of the first plurality of apertures is greater than a transverse dimension of the respective one of the first plurality of apertures by a factor of at least 16.

14. The flame holder assembly of claim 13, wherein the length of each of the first plurality of apertures is greater than a transverse dimension of the respective one of the first plurality of apertures by a factor of at least 24.

15. The flame holder assembly of claim 14, wherein the length of each of the first plurality of apertures is greater than a transverse dimension of the respective one of the first plurality of apertures by a factor of at least 48.

16. The flame holder assembly of claim 1, comprising a preheat structure configured to apply thermal energy to the flame holder element, when activated.

17. The flame holder assembly of claim 16, wherein the preheat structure includes an electrical heating element positioned adjacent to the flame holder element.

18. A method, comprising:

introducing a fuel stream to a perforated flame holder having a void fraction of at least 0.50, via a plurality of passages formed in a shield element positioned between the perforated flame holder and a source of the fuel stream, each of the passages having transverse dimensions that are no greater than a quenching distance for a fuel component of the fuel stream; and

combusting a majority of the fuel within a plurality of apertures extending between first and second faces of the perforated flame holder.

19. The method of claim 18, wherein the introducing a fuel stream to a perforated flame holder having a void fraction of at least 0.50 comprises introducing a fuel stream to a perforated flame holder having a void fraction of at least 0.60.

20. The method of claim 18, wherein the introducing a fuel stream to a perforated flame holder having a void fraction of at least 0.50 comprises introducing a fuel stream to a perforated flame holder having a void fraction of about 0.70.

21. The method of claim 18, wherein the introducing a fuel stream to a perforated flame holder comprises introducing a fuel stream having an average fuel-to-oxidant ratio that is below a lower combustion limit of the fuel component of the fuel stream.

22. The method of claim 18, comprising premixing the fuel stream, including adding a fuel component to an oxidant component.

23. The method of claim 18, wherein the combusting a majority of the fuel within a plurality of apertures extending between first and second faces of the perforated flame holder comprises combusting a majority of the fuel stream within a plurality of apertures that extend without obstruction between the first and second faces of the perforated flame holder.

24. The method of claim 23, comprising combusting a quantity of fuel sufficient to produce at least 1.5 MBTUH/ft2 of thermal energy.

25. The method of claim 24, wherein the combusting a quantity of fuel sufficient to produce at least 1.5 MBTUH/ft2 of thermal energy comprises combusting a quantity of fuel sufficient to produce at least 3 MBTUH/ft2 of thermal energy.

26. The method of claim 25, wherein the combusting a quantity of fuel sufficient to produce at least 3 MBTUH/ft2 of thermal energy comprises combusting a quantity of fuel sufficient to produce at least 5 MBTUH/ft2 of thermal energy.

27. The method of claim 23, comprising preheating at least a portion of the perforated flame holder to a start-up temperature during a start-up procedure.

28. The method of claim 27, wherein the preheating at least a portion of the perforated flame holder comprises applying an electrical current to an electrically resistive element positioned adjacent to the perforated flame holder.

29. A method, comprising:

introducing a fuel stream to a perforated flame holder via a plurality of passages formed in a shield element positioned between the perforated flame holder and a source of the fuel stream, each of the passages having a transverse dimension that is no greater than a quenching distance for a fuel component of the fuel stream; and

combusting a majority of the fuel within a plurality of apertures extending between first and second faces of the perforated flame holder, including combusting a quantity of fuel sufficient to produce at least 1.5 MBTUH/ft2.

30. The method of claim 29, wherein the combusting a quantity of fuel sufficient to produce at least 1.5 MBTUH/ft2 comprises combusting a quantity of fuel sufficient to produce at least 3 MBTUH/ft2.

31. The method of claim 30, wherein the combusting a quantity of fuel sufficient to produce at least 3 MBTUH/ft2 comprises combusting a quantity of fuel sufficient to produce at least 5 MBTUH/ft2.