The present application claims priority benefit from U.S. Provisional Patent Application No. 62/1 17,443, entitled "BURNER SYSTEM WITH A PERFORATED FLAME HOLDER AND A PLURALITY OF FUEL SOURCES," filed February 17, 2015 (docket number 2651 -206-02); which, to the extent not inconsistent with the description herein, is incorporated by reference.

SUMMARY

According to an embodiment, a burner system includes a perforated flame holder and a plurality of fuel and oxidant sources configured to collectively provide a fuel and oxidant mixture to the perforated flame holder. The perforated flame holder is configured to hold a combustion reaction supported by the fuel and oxidant mixture.

According to an embodiment, an apparatus for supporting combustion includes a plurality of fuel nozzles configured to emit a corresponding plurality of fuel streams and a perforated flame holder aligned to receive the plurality of fuel streams into respective portions of the perforated flame holder. The perforated flame holder has elongated apertures each having a transverse dimension D passing through the perforated flame holder from an input face to an output face. The perforated flame holder includes a plurality of segments separated from one another by respective gaps of at least two times D. According to an embodiment, a method for supporting combustion includes emitting a plurality of fuel streams from a corresponding plurality of fuel nozzles and receiving the plurality of fuel streams into respective portions of a perforated flame holder. The perforated flame holder has elongated apertures each having a transverse dimension D passing through the perforated flame holder from an input face to an output face. The perforated flame holder includes a plurality of sections separated from one another by respective gaps of at least two times D. Combustion of the respective fuel streams is supported in the portions of the perforated flame holder.

According to an embodiment, a burner apparatus includes a plurality of fuel nozzles configured to output respective fuel streams and a perforated flame holder configured to receive the respective fuel streams at respective portions of the perforated flame holder and to support combustion in the respective portions of the perforated flame holder. A second portion of the plurality of fuel nozzles is configured to cooperate with a corresponding second portion of the perforated flame holder to cause an output at least a portion of heat sufficient to maintain a first portion of the perforated flame holder at an elevated operating temperature when a first portion of fuel nozzles different from the second portion of fuel nozzles is controlled to stop outputting a respective first fuel stream to the first portion of the perforated flame holder.

According to an embodiment, a method for operating a burner includes delivering a plurality of fuel streams from a corresponding plurality of fuel nozzles to a perforated flame holder for combustion in the perforated flame holder. At least one first fuel stream from a first fuel nozzle that delivers fuel to a first portion of the perforated flame holder is stopped. An elevated operating temperature of the first portion of the perforated flame holder is maintained at least in part by continued combustion in other portions of the perforated flame holder.

According to an embodiment, a burner assembly for a furnace includes fuel nozzles configured to eject n streams of fuel in respective directions and k segments of a perforated flame holder disposed where the fuel streams will impinge on the segments. According to an embodiment, each segment includes one or more perforated flame holder tiles. Term tile shall be construed to include a single tile or a plurality of tiles placed in a direct contact with each other, depending on the context of an embodiment. Each segment of the perforated flame holder further includes apertures passing through the perforated flame holder, the apertures being smaller across than a surface of any of the segments, each segment comprising plural apertures. Each fuel stream impinges

substantially on a single segment or on a plurality of segments and k/n>1 .

According to an embodiment, a combustion method includes aligning a plurality of fuel nozzles to stream fuel toward a perforated flame holder such that each fuel stream impinges substantially on one respective segment of the perforated flame holder, streaming the fuel, and arranging combustion

parameters such that combustion takes place substantially inside the perforated flame holder. In an embodiment, the method also includes controlling the streaming to individually vary a rate of fuel streaming from each fuel nozzle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a burner system including a perforated flame holder and a plurality of fuel and oxidant sources, according to an embodiment.

FIG. 2 is a simplified perspective view of a burner system including a perforated flame holder, according to an embodiment.

FIG. 3 is a side sectional diagram of a portion of the perforated flame holder of FIGS. 1 and 2, according to an embodiment.

FIG. 4 is a flow chart showing a method for operating a burner system including the perforated flame holder of FIGS. 1, 2 and 3, according to an embodiment. FIG. 5 is a simplified perspective view of a burner system including a plurality of perforated flame holders and a plurality of fuel nozzles, according to an embodiment.

FIG. 6 is a simplified perspective view of a burner system illustrating segments of a perforated flame holder, according to an embodiment.

FIG. 7 is a graphical depiction of a fuel mixture delivered to a perforated flame holder by two fuel nozzles, shown as two negative-exponential functions and their sum.

FIG. 8 is an elevational and cross-sectional view of a furnace

incorporating a perforated flame holder burner system, according to an embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the

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

Perforated flame holders are described in the Applicant's U.S. patent Application No. 14/762, 155, entitled "FUEL COMBUSTION SYSTEM WITH A PERFORATED REACTION HOLDER," filed July 20, 2015, (agents' docket number 2651 -188-03); which to the extent not inconsistent with the disclosure herein, is incorporated by reference.

FIG. 1 is a diagram of a burner system 100 including a perforated flame holder 102 and a plurality of fuel and oxidant sources 104a, 104b disposed in a combustion volume 106, according to an embodiment. Each fuel and oxidant source 104a, 104b is configured to output a respective stream of fuel and oxidant mixture 108a, 108b aligned to impinge upon respective portions of the perforated flame holder 102, according to an embodiment. Optionally, each fuel and oxidant source 104a, 104b can be formed as a respective paired fuel nozzle 1 10a, 1 10b and oxidant (e.g., combustion air) source 1 12a, 1 12b, as shown, each respective pair referred to as a fuel and oxidant source 104a, 104b herein.

It can be desirable for the perforated flame holder 102 to be spaced away from the fuel nozzles 1 10a, 1 10b by a dilution distance D _D . As fuel travels through the dilution distance, it entrains more and more oxidant, such as combustion air (and optionally, flue gas or other diluent), until it attains a desired dilution and uniformity when it reaches the perforated flame holder 102. The dilution distance D _D is a function of fuel nozzle diameter (the inside diameter of the nozzle orifice). As will be described in greater detail below, it can be desirable for the dilution distance D _D to be at least 100 to (preferably) 200 nozzle diameters in distance.

According to an embodiment, providing a plurality of fuel nozzles 1 10a, 1 10b allows each individual nozzle diameter to be smaller, while still delivering a desired total fuel flow at a given fuel pressure. For applications where it is desirable to limit the distance at which the perforated flame holder 102 is positioned away from the fuel nozzles 1 10a, 1 10b; providing the plurality of fuel nozzles, in and of itself, can create value. For example, this can be useful when retrofitting an existing burner application that may be optimized to receive radiant heat energy at a relatively short distance above a furnace floor (not shown).

Optionally, each fuel nozzle 1 10a, 1 10b (or subset of fuel nozzles 1 10a, 1 10b) may be separately controlled. For example a plurality of fuel control valves 1 14a, 1 14b may be operatively coupled to a respective nozzle 1 10a, 1 10b or group of nozzles.

According to another embodiment, providing the plurality of fuel control valves 1 14a, 1 14b can be useful for increasing turndown of the burner system 100. Turndown (also known as turndown ratio) is defined as the maximum firing rate divided by the actual firing rate, and refers to the ability for a furnace to operate at a reduced total heat output, perhaps responsive to a reduced thermal load. Higher turndown means that a furnace can be operated at lower heat release rates and is better able to tolerate changes in thermal load. According to an embodiment, the burner system 100 includes a plurality of fuel control valves 1 14a, 1 14b configured to provide separate control of fuel flow to a corresponding plurality of fuel nozzles 1 10a, 1 10b or groups of fuel nozzles. Optionally, the plurality of fuel control valves 1 14a, 1 14b can be manually actuated valves. In another embodiment, at least some of the plurality of fuel control valves 1 14a, 1 14b are controlled by a valve actuator portion of the valve 1 14a, 1 14b responsive to receipt of a control signal. The control signal can be received via electrical, pneumatic, hydraulic, radio, or other control medium.

When a fuel nozzle 1 10b is turned off, it stops outputting a flow of fuel and oxidant mixture 108b to a portion 102b of the perforated flame holder 102. With no fuel (and optionally, no oxidant) arriving at the portion 102b of the perforated flame holder 102, combustion within the portion 102b of the perforated flame holder 102 stops. This reduces the thermal output of the system. However, a neighboring region portion 102a of the perforated flame holder 102 that continues to receive a flow of the fuel and oxidant mixture 108a continues to support combustion and output heat.

The operating portion 102a of the perforated flame holder 102 can be arranged to provide heat energy to an idle portion 102b of the perforated flame holder 102. This can cause the idle portion 102b to be in a "hot stand-by" mode, and, according to some embodiments, available for substantially instantaneous start-up.

According to an embodiment, the burner system 100 includes a controller

1 16 operatively coupled to each of the plurality of fuel control valves 1 14a, 1 14b. Responsive to a change in thermal output command received through a data interface 1 18, the controller 1 16 may actuate one or more of the plurality of fuel control valves 1 14a, 1 14b to trim the amount of total fuel delivered to the perforated flame holder 102, and thereby control the total amount of heat or thermal energy output by the burner system 100. The burner system 100 can include a heater 120 configured to preheat the perforated flame holder 102 to an (start-up) operating temperature. Various heater embodiments are contemplated and described below.

According to an embodiment, the burner system 100 is equipped with the heater 120 configured to heat only a portion 102a of the perforated flame holder 102. According to an embodiment, at system start-up, the controller 1 16 first drives the heater 120 to heat the first portion 102a of the perforated flame holder 102. Upon reaching a start-up temperature (referred to as T _s in conjunction with FIG. 4) at the first portion 102a of the perforated flame holder 102, the controller 1 16 actuates the heater 120 (which may include a start-up flame holder or an electrical resistance heater, for example) and/or a corresponding fuel nozzle 1 10a to output fuel and oxidant mixture 108a to the portion 102a of the perforated flame holder that is sufficiently hot to support combustion.

Subsequently, after the first portion 102a of the perforated flame holder outputs heat energy to a second portion 102b of the perforated flame holder 102, the controller 1 16 may drive a second fuel nozzle 1 10b to output the corresponding fuel and oxidant mixture 108b to the second portion 102b of the perforated flame holder 102. In an embodiment, a relatively small heater 120 can provide sufficient heat energy to enable start-up of a relatively large perforated flame holder 102, albeit over a sequence of steps.

According to an embodiment, the burner system 100 includes one or more sensors 122 configured to measure temperature or other combustion

parameter(s) in one or more portions 102a, 102b of the perforated flame holder 102. According to an embodiment, the controller 1 16 is driven by computer instructions, carried (at least remotely) by a non-transitory computer readable medium, to take action responsive to receiving a signal or data from the sensor(s) 122 corresponding to a sensed combustion condition.

For example, if the portion 102b of the perforated flame holder 102 breaks or otherwise loses its ability to maintain stable combustion, the sensor(s) 122 can sense the condition and transmit data or a signal to the controller 1 16. The controller 1 16 can responsively actuate a fuel control valve 1 14b to cause the corresponding fuel nozzle 1 10b to stop outputting fuel to the affected portion 102b of the perforated flame holder 102.

According to another example, if a sensor 122 detects a drop in perforated flame holder 102 temperature in a portion 102b, and the fuel nozzle 1 10b is outputting a high flow of fuel, then the controller can issue a command to open a fuel control valve 1 14a to provide fuel and oxidant mixture 108a to another portion 102a of the perforated flame holder 102 to cause combustion to be initiated in the other portion 102a. Other things being equal, initiating combustion in another portion 102a of the perforated flame holder 102 causes an increase in total heat output by the burner system 100. Thus, a decrease in perforated flame holder 102 temperature at an operating portion 102b temperature caused by an increased cooling load can be responded to by increasing the total heat output of the burner system 100.

FIG. 2 is a simplified diagram of a burner system 200 including a perforated flame holder 102 configured to hold a combustion reaction, according to an embodiment. As used herein, the terms perforated flame holder, perforated reaction holder, porous flame holder, porous reaction holder, duplex, and duplex tile shall be considered synonymous unless further definition is provided.

Experiments performed by the inventors have shown that perforated flame holders 102 described herein can support very clean combustion. Specifically, in experimental use of systems 200 ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (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 204 to form a fuel and oxidant mixture 206. As used herein, the terms fuel and oxidant mixture and fuel stream may be used interchangeably and considered synonymous depending on the context, unless further definition is provided. As used herein, the terms combustion volume, combustion chamber, furnace volume, and the like shall be considered synonymous unless further definition is provided. The perforated flame holder 102 is disposed in the combustion volume 204 and positioned to receive the fuel and oxidant mixture 206.

FIG. 3 is a side sectional diagram 300 of a portion of the perforated flame holder 102 of FIGS. 1 and 2, according to an embodiment. Referring to FIGS. 2 and 3, the perforated flame holder 102 includes a perforated flame holder body 208 defining a plurality of perforations 210 aligned to receive the fuel and oxidant mixture 206 from the fuel and oxidant source 202. As used herein, the terms perforation, pore, aperture, elongated aperture, and the like, in the context of the perforated flame holder 102, shall be considered synonymous unless further definition is provided. The perforations 210 are configured to collectively hold a combustion reaction 302 supported by the fuel and oxidant mixture 206.

The fuel can include hydrogen, a hydrocarbon gas, a vaporized

hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel can be a single species or can include a mixture of gas(es), vapor(s), atomized liquid(s), and/or pulverized solid(s). For example, in a process heater application the fuel can include fuel gas or byproducts from the process that include carbon monoxide (CO), hydrogen (H ₂ ), and methane (CH ₄ ). In another application the fuel can include natural gas (mostly CH ₄ ) or propane (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 oxygen carried by air, flue gas, and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein. According to an embodiment, the perforated flame holder body 208 can be bounded by an input face 212 disposed to receive the fuel and oxidant mixture 206, an output face 214 facing away from the fuel and oxidant source 202, and a peripheral surface 216 defining a lateral extent of the perforated flame holder 102. The plurality of perforations 210 which are defined by the perforated flame holder body 208 extend from the input face 212 to the output face 214. The plurality of perforations 210 can receive the fuel and oxidant mixture 206 at the input face 212. The fuel and oxidant mixture 206 can then combust in or near the plurality of perforations 210 and combustion products can exit the plurality of perforations 210 at or near the output face 214.

According to an embodiment, the perforated flame holder 102 is configured to hold a majority of the combustion reaction 302 within the

perforations 210. For example, on a steady-state basis, more than half the molecules of fuel output into the combustion volume 204 by the fuel and oxidant source 202 may be converted to combustion products between the input face 212 and the output face 214 of the perforated flame holder 102. According to an alternative interpretation, more than half of the heat or thermal energy output by the combustion reaction 302 may be output between the input face 212 and the output face 214 of the perforated flame holder 102. As used herein, the terms heat, heat energy, and thermal energy shall be considered synonymous unless further definition is provided. As used above, heat energy and thermal energy refer generally to the released chemical energy initially held by reactants during the combustion reaction 302. As used elsewhere herein, heat, heat energy and thermal energy correspond to a detectable temperature rise undergone by real bodies characterized by heat capacities. Under nominal operating conditions, the perforations 210 can be configured to collectively hold at least 80% of the combustion reaction 302 between the input face 212 and the output face 214 of the perforated flame holder 102. In some experiments, the inventors produced a combustion reaction 302 that was apparently wholly contained in the perforations 210 between the input face 212 and the output face 214 of the perforated flame holder 102. According to an alternative interpretation, the perforated flame holder 102 can support combustion between the input face 212 and output face 214 when combustion is "time-averaged." For example, during transients, such as before the perforated flame holder 102 is fully heated, or if too high a (cooling) load is placed on the system, the combustion may travel somewhat downstream from the output face 214 of the perforated flame holder 102. Alternatively, if the cooling load is relatively low and/or the furnace temperature reaches a high level, the combustion may travel somewhat upstream of the input face 212 of the perforated flame holder 102.

While a "flame" is described in a manner intended for ease of description, it should be understood that in some instances, no visible flame is present.

Combustion occurs primarily within the perforations 210, but the "glow" of combustion heat is dominated by a visible glow of the perforated flame holder 102 itself. In other instances, the inventors have noted transient "huffing" or "flashback" wherein a visible flame momentarily ignites in a region lying between the input face 212 of the perforated flame holder 102 and the fuel nozzle 218, within the dilution region D _D . Such transient huffing or flashback is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations 210 of the perforated flame holder 102, between the input face 212 and the output face 214. In still other instances, the inventors have noted apparent combustion occurring downstream from the output face 214 of the perforated flame holder 102, but still a majority of combustion occurred within the perforated flame holder 102 as evidenced by continued visible glow from the perforated flame holder 102 that was observed.

The perforated flame holder 102 can be configured to receive heat from the combustion reaction 302 and output a portion of the received heat as thermal radiation 304 to heat-receiving structures (e.g., furnace walls and/or radiant section working fluid tubes) in or adjacent to the combustion volume 204. As used herein, terms such as radiation, thermal radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody-type radiation of electromagnetic energy, primarily at infrared wavelengths, but also at visible wavelengths owing to elevated temperature of the perforated flame holder body 208.

Referring especially to FIG. 3, the perforated flame holder 102 outputs another portion of the received heat to the fuel and oxidant mixture 206 received at the input face 212 of the perforated flame holder 102. The perforated flame holder body 208 may receive heat from the combustion reaction 302 at least in heat receiving regions 306 of perforation walls 308. Experimental evidence has suggested to the inventors that the position of the heat receiving regions 306, or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls 308. In some experiments, the location of maximum receipt of heat was apparently between 1/3 and 1/2 of the distance from the input face 212 to the output face 214 (i.e., somewhat nearer to the input face 212 than to the output face 214). The inventors contemplate that the heat receiving regions 306 may lie nearer to the output face 214 of the perforated flame holder 102 under other conditions. Most probably, there is no clearly defined edge of the heat receiving regions 306 (or for that matter, the heat output regions 310, described below). For ease of understanding, the heat receiving regions 306 and the heat output regions 310 will be described as particular regions 306, 310.

The perforated flame holder body 208 can be characterized by a heat capacity. The perforated flame holder body 208 may hold thermal energy from the combustion reaction 302 in an amount corresponding to the heat capacity multiplied by temperature rise, and transfer the thermal energy from the heat receiving regions 306 to heat output regions 310 of the perforation walls 308. Generally, the heat output regions 310 are nearer to the input face 212 than are the heat receiving regions 306. According to one interpretation, the perforated flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via thermal radiation, depicted graphically as 304. According to another interpretation, the perforated flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via heat conduction along heat conduction paths 312. The inventors contemplate that multiple heat transfer mechanisms including conduction, radiation, and possibly convection may be operative in transferring heat from the heat receiving regions 306 to the heat output regions 310. In this way, the perforated flame holder 102 may act as a heat source to maintain the combustion reaction 302, even under conditions where a combustion reaction 302 would not be stable when supported from a conventional flame holder.

The inventors believe that the perforated flame holder 102 causes the combustion reaction 302 to begin within thermal boundary layers 314 formed adjacent to walls 308 of the perforations 210. Insofar as combustion is generally understood to include a large number of individual reactions, and since a large portion of combustion energy is released within the perforated flame holder 102, it is apparent that at least a majority of the individual reactions occur within the perforated flame holder 102. As the relatively cool fuel and oxidant mixture 206 approaches the input face 212, the flow is split into portions that respectively travel through individual perforations 210. The hot perforated flame holder body 208 transfers heat to the fluid, notably within thermal boundary layers 314 that progressively thicken as more and more heat is transferred to the incoming fuel and oxidant mixture 206. After reaching a combustion temperature (e.g., the auto-ignition temperature of the fuel), the reactants continue to flow while a chemical ignition delay time elapses, over which time the combustion reaction 302 occurs. Accordingly, the combustion reaction 302 is shown as occurring within the thermal boundary layers 314. As flow progresses, the thermal boundary layers 314 merge at a merger point 316. Ideally, the merger point 316 lies between the input face 212 and output face 214 that define the ends of the perforations 210. At some position along the length of a perforation 210, the combustion reaction 302 outputs more heat to the perforated flame holder body 208 than it receives from the perforated flame holder body 208. The heat is received at the heat receiving region 306, is held by the perforated flame holder body 208, and is transported to the heat output region 310 nearer to the input face 212, where the heat is transferred into the cool reactants (and any included diluent) to bring the reactants to the ignition temperature. In an embodiment, each of the perforations 210 is characterized by a length L defined as a reaction fluid propagation path length between the input face 212 and the output face 214 of the perforated flame holder 102. As used herein, the term reaction fluid refers to matter that travels through a perforation 210. Near the input face 212, the reaction fluid includes the fuel and oxidant mixture 206 (optionally including nitrogen, flue gas, and/or other "non-reactive" species). Within the combustion reaction region, the reaction fluid may include plasma associated with the combustion reaction 302, molecules of reactants and their constituent parts, any non-reactive species, reaction intermediates

(including transition states), and reaction products. Near the output face 214, the reaction fluid may include reaction products and byproducts, non-reactive gas, and excess oxidant.

The plurality of perforations 210 can be each characterized by a

transverse dimension D between opposing perforation walls 308. The inventors have found that stable combustion can be maintained in the perforated flame holder 102 if the length L of each perforation 210 is at least four times the transverse dimension D of the perforation. In other embodiments, the length L can be greater than six times the transverse dimension D. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D. Preferably, the length L is sufficiently long for thermal boundary layers 314 to form adjacent to the perforation walls 308 in a reaction fluid flowing through the perforations 210 to converge at merger points 316 within the perforations 210 between the input face 212 and the output face 214 of the perforated flame holder 102. In experiments, the inventors have found L/D ratios between 12 and 48 to work well (i.e., produce low NOx, produce low CO, and maintain stable combustion).

The perforated flame holder body 208 can be configured to convey heat between adjacent perforations 210. The heat conveyed between adjacent perforations 210 can be selected to cause heat output from the combustion reaction portion 302 in a first perforation 210 to supply heat to stabilize a combustion reaction portion 302 in an adjacent perforation 210. Referring especially to FIG. 2, the fuel and oxidant source 202 can further include a fuel nozzle 218, configured to output fuel, and an oxidant source 220 configured to output a fluid including the oxidant. For example, the fuel nozzle 218 can be configured to output pure fuel. The oxidant source 220 can be configured to output combustion air carrying oxygen, and optionally, flue gas.

The perforated flame holder 102 can be held by a perforated flame holder support structure 222 configured to hold the perforated flame holder 102 at a dilution distance D _D away from the fuel nozzle 218. The fuel nozzle 218 can be configured to emit a fuel stream selected to entrain the oxidant to form the fuel and oxidant mixture 206 as the fuel stream and oxidant travel along a path to the perforated flame holder 102 through the dilution distance D _D between the fuel nozzle 218 and the perforated flame holder 102. Additionally or alternatively (particularly when a blower is used to deliver oxidant contained in combustion air), the oxidant or combustion air source can be configured to entrain the fuel and the fuel and oxidant travel through the dilution distance D _D . In some embodiments, a flue gas recirculation path 224 can be provided. Additionally or alternatively, the fuel nozzle 218 can be configured to emit a fuel stream selected to entrain the oxidant and to entrain flue gas as the fuel stream travels through the dilution distance D _D between the fuel nozzle 218 and the input face 212 of the perforated flame holder 102.

The fuel nozzle 218 can be configured to emit the fuel through one or more fuel orifices 226 having an inside diameter dimension that is referred to as "nozzle diameter." The perforated flame holder support structure 222 can support the perforated flame holder 102 to receive the fuel and oxidant mixture 206 at the distance D _D away from the fuel nozzle 218 greater than 20 times the nozzle diameter. In another embodiment, the perforated flame holder 102 is disposed to receive the fuel and oxidant mixture 206 at the distance D _D away from the fuel nozzle 218 between 100 times and 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 218. When the fuel and oxidant mixture 206 travels about 200 times the nozzle diameter or more, the mixture is sufficiently homogenized to cause the combustion reaction 302 to produce minimal NOx.

The fuel and oxidant source 202 can alternatively include a premix fuel and oxidant source, according to an embodiment. A premix fuel and oxidant source can include a premix chamber (not shown), a fuel nozzle configured to output fuel into the premix chamber, and an oxidant (e.g., combustion air) channel configured to output the oxidant into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the perforated flame holder 102 and be configured to prevent flame flashback into the premix fuel and oxidant source.

The oxidant source 220, whether configured for entrainment in the combustion volume 204 or for premixing, can include a blower configured to force the oxidant through the fuel and oxidant source 202.

The support structure 222 can be configured to support the perforated flame holder 102 from a floor or wall (not shown) of the combustion volume 204, for example. In another embodiment, the support structure 222 supports the perforated flame holder 102 from the fuel and oxidant source 202. Alternatively, the support structure 222 can suspend the perforated flame holder 102 from an overhead structure (such as a flue, in the case of an up-fired system). The support structure 222 can support the perforated flame holder 102 in various orientations and directions.

The perforated flame holder 102 can include a single perforated flame holder body 208. In another embodiment, the perforated flame holder 102 can include a plurality of adjacent perforated flame holder sections that collectively provide a tiled perforated flame holder 102.

The perforated flame holder support structure 222 can be configured to support the plurality of perforated flame holder sections. The perforated flame holder support structure 222 can include a metal superalloy, a cementatious, and/or ceramic refractory material. In an embodiment, the plurality of adjacent perforated flame holder sections can be joined with a fiber reinforced refractory cement.

The perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least twice a thickness dimension T between the input face 212 and the output face 214. In another embodiment, the perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least three times, at least six times, or at least nine times the thickness dimension T between the input face 212 and the output face 214 of the perforated flame holder 102.

In an embodiment, the perforated flame holder 102 can have a width dimension W less than a width of the combustion volume 204. This can allow the flue gas circulation path 224 from above to below the perforated flame holder 102 to lie between the peripheral surface 216 of the perforated flame holder 102 and the combustion volume wall (not shown).

Referring again to both FIGS. 2 and 3, the perforations 210 can be of various shapes. In an embodiment, the perforations 210 can include elongated squares, each having a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations 210 can include elongated hexagons, each having a transverse dimension D between opposing sides of the hexagons. In yet another embodiment, the perforations 210 can include hollow cylinders, each having a transverse dimension D corresponding to a diameter of the cylinder. In another embodiment, the perforations 210 can include truncated cones or truncated pyramids (e.g., frustums), each having a transverse dimension D radially symmetric relative to a length axis that extends from the input face 212 to the output face 214. In some embodiments, the perforations 210 can each have a lateral dimension D equal to or greater than a quenching distance of the flame based on standard reference conditions. Alternatively, the perforations 210 may have lateral dimension D less then than a standard reference quenching distance.

In one range of embodiments, each of the plurality of perforations 210 has a lateral dimension D between 0.05 inch and 1 .0 inch. Preferably, each of the plurality of perforations 210 has a lateral dimension D between 0.1 inch and 0.5 inch. For example the plurality of perforations 210 can each have a lateral dimension D of about 0.2 to 0.4 inch.

The void fraction of a perforated flame holder 102 is defined as the total volume of all perforations 210 in a section of the perforated flame holder 102 divided by a total volume of the perforated flame holder 102 including body 208 and perforations 210. The perforated flame holder 102 should have a void fraction between 0.10 and 0.90. In an embodiment, the perforated flame holder 102 can have a void fraction between 0.30 and 0.80. In another embodiment, the perforated flame holder 102 can have a void fraction of about 0.70. Using a void fraction of about 0.70 was found to be especially effective for producing very low NOx.

The perforated flame holder 102 can be formed from a fiber reinforced cast refractory material and/or a refractory material such as an aluminum silicate material. For example, the perforated flame holder 102 can be formed to include mullite or cordierite. Additionally or alternatively, the perforated flame holder body 208 can include a metal superalloy such as Inconel or Hastelloy. The perforated flame holder body 208 can define a honeycomb. Honeycomb is an industrial term of art that need not strictly refer to a hexagonal cross section and most usually includes cells of square cross section. Honeycombs of other cross sectional areas are also known.

The inventors have found that the perforated flame holder 102 can be formed from VERSAGRID ® ceramic honeycomb, available from Applied

Ceramics, Inc. of Doraville, South Carolina.

The perforations 210 can be parallel to one another and normal to the input and output faces 212, 214. In another embodiment, the perforations 210 can be parallel to one another and formed at an angle relative to the input and output faces 212, 214. In another embodiment, the perforations 210 can be non- parallel to one another. In another embodiment, the perforations 210 can be non-parallel to one another and non-intersecting. In another embodiment, the perforations 210 can be intersecting. The body 308 can be one piece or can be formed from a plurality of sections.

In another embodiment, which is not necessarily preferred, the perforated flame holder 102 may be formed from reticulated ceramic material. The term "reticulated" refers to a netlike structure. Reticulated ceramic material is often made by dissolving a slurry into a sponge of specified porosity, allowing the slurry to harden, and burning away the sponge and curing the ceramic.

In another embodiment, which is not necessarily preferred, the perforated flame holder 102 may be formed from a ceramic material that has been punched, bored or cast to create channels.

In another embodiment, the perforated flame holder 102 can include a plurality of tubes or pipes bundled together. The plurality of perforations 210 can include hollow cylinders and can optionally also include interstitial spaces between the bundled tubes. In an embodiment, the plurality of tubes can include ceramic tubes. Refractory cement can be included between the tubes and configured to adhere the tubes together. In another embodiment, the plurality of tubes can include metal (e.g., superalloy) tubes. The plurality of tubes can be held together by a metal tension member circumferential to the plurality of tubes and arranged to hold the plurality of tubes together. The metal tension member can include stainless steel, a superalloy metal wire, and/or a superalloy metal band.

The perforated flame holder body 208 can alternatively include stacked perforated sheets of material, each sheet having openings that connect with openings of subjacent and superjacent sheets. The perforated sheets can include perforated metal sheets, ceramic sheets and/or expanded sheets. In another embodiment, the perforated flame holder body 208 can include discontinuous packing bodies such that the perforations 210 are formed in the interstitial spaces between the discontinuous packing bodies. In one example, the discontinuous packing bodies include structured packing shapes. In another example, the discontinuous packing bodies include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig ring, ceramic Berl saddles, ceramic Intalox saddles, and/or metal rings or other shapes (e.g. Super Raschig Rings) that may be held together by a metal cage.

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

According to an embodiment, the perforated flame holder 102 may act as a heat source to maintain a combustion reaction even under conditions where a combustion reaction would not be stable when supported by a conventional flame holder. This capability can be leveraged to support combustion using a leaner fuel-to-oxidant mixture than is typically feasible. Thus, according to an embodiment, at the point where the fuel stream 206 contacts the input face 212 of the perforated flame holder 102, an average fuel-to-oxidant ratio of the fuel stream 206 is below a (conventional) lower combustion limit of the fuel component of the fuel stream 206— lower combustion limit defines the lowest concentration of fuel at which a fuel and oxidant mixture 206 will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C (77° F).

The perforated flame holder 102 and systems including the perforated flame holder 102 described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. According to one interpretation, such a performance can be achieved due to a sufficient mixing used to lower peak flame temperatures (among other strategies). Flame temperatures tend to peak under slightly rich conditions, which can be evident in any diffusion flame that is insufficiently mixed. By sufficiently mixing, a

homogenous and slightly lean mixture can be achieved prior to combustion. This combination can result in reduced flame temperatures, and thus reduced NOx formation. In one embodiment, "slightly lean" may refer to 3% 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 inventors believe perforation walls 308 may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperatures and lower NOx. According to another interpretation, production of NOx can be reduced if the combustion reaction 302 occurs over a very short duration of time. Rapid combustion causes the reactants (including oxygen and entrained nitrogen) to be exposed to NOx-formation temperature for a time too short for NOx formation kinetics to cause significant production of NOx. The time required for the reactants to pass through the perforated flame holder 102 is very short compared to a conventional flame. The low NOx production associated with perforated flame holder combustion may thus be related to the short duration of time required for the reactants (and entrained nitrogen) to pass through the perforated flame holder 102.

FIG. 4 is a flow chart showing a method 400 for operating a burner system including the perforated flame holder shown and described herein. To operate a burner system including a perforated flame holder, the perforated flame holder is first heated to a temperature sufficient to maintain combustion of the fuel and oxidant mixture.

According to a simplified description, the method 400 begins with step 402, wherein the perforated flame holder is preheated to a start-up temperature, 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, T _s . As long as the temperature of the perforated flame holder is below its start-up temperature, the method loops between steps 406 and 408 within the preheat step 402. In step 408, if the temperature T of at least a predetermined portion of the perforated flame holder is greater than or equal to the start-up temperature, the method 400 proceeds to overall step 404, wherein fuel and oxidant is supplied to and combustion is held by the perforated flame holder. Step 404 may be broken down into several discrete steps, at least some of which may occur simultaneously.

Proceeding from step 408, a fuel and oxidant mixture is provided to the perforated flame holder, as shown in step 410. The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and oxidant (e.g., combustion air) source, for example. In this approach, the fuel and oxidant are output in one or more directions selected to cause the fuel and oxidant mixture to be received by the input face of the perforated flame holder. The fuel may entrain the combustion air (or alternatively, the combustion air may dilute the fuel) to provide a fuel and oxidant mixture at the input face of the perforated flame holder at a fuel dilution selected for a stable combustion reaction that can be held within the perforations of the perforated flame holder.

Proceeding to step 412, the combustion reaction is held by the perforated flame holder.

In step 414, heat may be output from the perforated flame holder. The heat output from the perforated flame holder may be used to power an industrial process, heat a working fluid, generate electricity, or provide motive power, for example.

In optional step 416, the presence of combustion may be sensed. Various sensing approaches have been used and are contemplated by the inventors. Generally, combustion held by the perforated flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, flame rod, and/or other combustion sensing apparatuses. In an additional or alternative variant of step 416, a pilot flame or other ignition source may be provided to cause ignition of the fuel and oxidant mixture in the event combustion is lost at the perforated flame holder.

Proceeding to decision step 418, if combustion is sensed not to be stable, the method 400 may exit to step 424, wherein an error procedure is executed. For example, the error procedure may include turning off fuel flow, re-executing the preheating step 402, outputting an alarm signal, igniting a stand-by

combustion system, or other steps. If, in step 418, combustion in the perforated flame holder is determined to be stable, the method 400 proceeds to decision step 420, wherein it is determined if combustion parameters should be changed. If no combustion parameters are to be changed, the method loops (within step 404) back to step 410, and the combustion process continues. If a change in combustion parameters is indicated, the method 400 proceeds to step 422, wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step 404) back to step 410, and combustion continues.

Combustion parameters may be scheduled to be changed, for example, if a change in heat demand is encountered. For example, if less heat is required (e.g., due to decreased electricity demand, decreased motive power requirement, or lower industrial process throughput), the fuel and oxidant flow rate may be decreased in step 422. Conversely, if heat demand is increased, then fuel and oxidant flow may be increased. Additionally or alternatively, if the combustion system is in a start-up mode, then fuel and oxidant flow may be gradually increased to the perforated flame holder over one or more iterations of the loop within step 404.

Referring again to FIG. 2, the burner system 200 includes a heater 228 operatively coupled to the perforated flame holder 102. As described in conjunction with FIGS. 3 and 4, the perforated flame holder 102 operates by outputting heat to the incoming fuel and oxidant mixture 206. After combustion is established, this heat is provided by the combustion reaction 302; but before combustion is established, the heat is provided by the heater 228.

Various heating apparatuses have been used and are contemplated by the inventors. In some embodiments, the heater 228 can include a flame holder configured to support a flame disposed to heat the perforated flame holder 102. The fuel and oxidant source 202 can include a fuel nozzle 218 configured to emit a fuel stream 206 and an oxidant source 220 configured to output oxidant (e.g., combustion air) adjacent to the fuel stream 206. The fuel nozzle 218 and oxidant source 220 can be configured to output the fuel stream 206 to be progressively diluted by the oxidant {e.g., combustion air). The perforated flame holder 102 can be disposed to receive a diluted fuel and oxidant mixture 206 that supports a combustion reaction 302 that is stabilized by the perforated flame holder 102 when the perforated flame holder 102 is at an operating temperature. A start-up flame holder, in contrast, can be configured to support a start-up flame at a location corresponding to a relatively unmixed fuel and oxidant mixture that is stable without stabilization provided by the heated perforated flame holder 102.

The burner system 200 can further include a controller 230 operatively coupled to the heater 228 and to a data interface 232. For example, the controller 230 can be configured to control a start-up flame holder actuator configured to cause the start-up flame holder to hold the start-up flame when the perforated flame holder 102 needs to be pre-heated and to not hold the start-up flame when the perforated flame holder 102 is at an operating temperature (e.g., when T≥Ts).

Various approaches for actuating a start-up flame are contemplated. In one embodiment, the start-up flame holder includes a mechanically-actuated bluff body configured to be actuated to intercept the fuel and oxidant mixture 206 to cause heat-recycling and/or stabilizing vortices and thereby hold a start-up flame; or to be actuated to not intercept the fuel and oxidant mixture 206 to cause the fuel and oxidant mixture 206 to proceed to the perforated flame holder 102. In another embodiment, a fuel control valve, blower, and/or damper may be used to select a fuel and oxidant mixture flow rate that is sufficiently low for a start-up flame to be stream-stabilized; and upon reaching a perforated flame holder 102 operating temperature, the flow rate may be increased to "blow out" the start-up flame. In another embodiment, the heater 228 may include an electrical power supply operatively coupled to the controller 230 and configured to apply an electrical charge or voltage to the fuel and oxidant mixture 206. An electrically conductive start-up flame holder may be selectively coupled to a voltage ground or other voltage selected to attract the electrical charge in the fuel and oxidant mixture 206. The attraction of the electrical charge was found by the inventors to cause a start-up flame to be held by the electrically conductive start-up flame holder.

In another embodiment, the heater 228 may include an electrical resistance heater configured to output heat to the perforated flame holder 102 and/or to the fuel and oxidant mixture 206. The electrical resistance heater can be configured to heat up the perforated flame holder 102 to an operating temperature. The heater 228 can further include a power supply and a switch operable, under control of the controller 230, to selectively couple the power supply to the electrical resistance heater.

An electrical resistance heater 228 can be formed in various ways. For example, the electrical resistance heater 228 can be formed from KANTHAL® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstahammar, Sweden) threaded through at least a portion of the perforations 210 defined by the perforated flame holder body 208. Alternatively, the heater 228 can include an inductive heater, a high-energy beam heater (e.g. microwave or laser), a frictional heater, electro-resistive ceramic coatings, or other types of heating technologies.

Other forms of start-up apparatuses are contemplated. For example, the heater 228 can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the oxidant and fuel. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite the fuel and oxidant mixture 206 that would otherwise enter the perforated flame holder 102. The electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller 230, which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the fuel and oxidant mixture 206 in or upstream from the perforated flame holder 102 before the perforated flame holder 102 is heated sufficiently to maintain combustion.

The burner system 200 can further include a sensor 234 operatively coupled to the control circuit 230. The sensor 234 can include a heat sensor configured to detect infrared radiation or a temperature of the perforated flame holder 102. The control circuit 230 can be configured to control the heating apparatus 228 responsive to input from the sensor 234. Optionally, a fuel control valve 236 can be operatively coupled to the controller 230 and configured to control a flow of fuel to the fuel and oxidant source 202. Additionally or alternatively, an oxidant blower or damper 238 can be operatively coupled to the controller 230 and configured to control flow of the oxidant (or combustion air).

The sensor 234 can further include a combustion sensor operatively coupled to the control circuit 230, the combustion sensor being configured to detect a temperature, video image, and/or spectral characteristic of a combustion reaction held by the perforated flame holder 102. The fuel control valve 236 can be configured to control a flow of fuel from a fuel source to the fuel and oxidant source 202. The controller 230 can be configured to control the fuel control valve 236 responsive to input from the combustion sensor 234. The controller 230 can be configured to control the fuel control valve 236 and/or oxidant blower or damper to control a preheat flame type of heater 228 to heat the perforated flame holder 102 to an operating temperature. The controller 230 can similarly control the fuel control valve 236 and/or the oxidant blower or damper to change the fuel and oxidant mixture 206 flow responsive to a heat demand change received as data via the data interface 232.

FIG. 5 shows a burner assembly 500 with multiple fuel nozzles 1 10a,

1 10b, 1 10c, and so on up to 1 10k, where k refers to an integer number which corresponds to the number of fuel nozzles, configured to output respective fuel streams 108a, 108b, 108c, according to an embodiment. As used herein, depending on the context, numbers 108a, 108b, 108c, etc. can refer to a fuel and oxidant mixture or to a fuel stream output by a fuel nozzle 1 10a, 100b, 1 10c, etc. providing the fuel for said mixture. Each fuel stream may propagate along a respective different direction and may impinge substantially on only a single respective segment 516a, 516b, 516c of the perforated flame holder 102).

Additionally or alternatively, several fuel streams 108a, 108b, 108c, etc. may impinge on the same respective segment of the perforated flame holder 102. In an illustrated embodiment, a segment may be taken as referring to a physically- realized division of an entire perforated flame holder. Depending on an

embodiment, the segments 516a, 516b, 516c may include individual unitary tiles or tile assemblies (a plurality of segments placed in a direct contact with each other) held in position by the perforated flame holder support structure 222. In an embodiment, the individual segments in the segment sub-assembly may be fastened together, glued together, or held in a common sub-support. For example, the segment assembly may be formed as an arch made of individual unitary segments.

According to an embodiment, a portion of a perforated flame holder 102 (such as portions 102a or 102b of FIG. 1 ) is an area impinged by 1 or more fuel streams. A segment of a perforated flame holder (such as segments 516a-c of FIG. 5, segments 516m of FIG. 6, or segments 516a-d of FIG. 8) refer to sections of a perforated flame holder 102 that are separated from each other by air gaps or by an intervening structure or material. According to an embodiment, a single segment can correspond to a plurality of portions. According to an embodiment, a single portion can correspond to a plurality of segments. According to an embodiment a single portion may correspond to a single segment.

The flame holder support structure 222 may be embodied as any support structure configured to support the plurality of perforated flame holder segments 516a, 516b, 516c. The support structure may, for example include a metal or superalloy. In another embodiment, the support structure 222 may be formed from a refractory cement or fiber reinforced refractory cement. Optionally, the support structure 222 may include a cooling structure.

In an embodiment, the segments 516a, 516b, ... may have air gaps in between them (e.g., if the support structure 222 in the drawing holds only the corners of the segments). These segments may, in an embodiment, be tiles. The inventors have found that with this arrangement, contrary to conventional expectations, little fugitive emissions are produced when the gap is as much as a few inches. Fugitive emissions are defined to mean unburned or partially unburned fuel that exits the combustion zone unreacted. With little fugitive emissions, there is negligible threat to produce CO or unburned hydrocarbon (UHC) emissions; but the segment cooling may be relatively large because the segment edges may be immersed in non-combusting fluid.

The potential advantages of a reduced segment (e.g., tile) temperature are many: longer tile life, increased firing capacity and heat density for a given surface area, lower NOx emissions, and reduced or even eliminated upstream flame propagation. Segment separation may be particularly useful in higher- temperature applications (e.g., ethylene cracking, hydrogen reformers) and/or when using fuels with high adiabatic flame temperatures (e.g., hydrogen or hydrogen blends in a refinery process heater).

According to an interpretation, substantially all of heat output by the combustion reaction is output between the input surface 212 and the output surface 214 of the perforated flame holder 102. Under nominal operating conditions, the perforations 210 can be configured to collectively hold at least 80% of the combustion reaction 302 between the input surface 212 and the output surface 214 of the perforated flame holder 102. In some experiments, the inventors produced a combustion reaction that was wholly contained in the perforations between the input surface 212 and the output surface 214 of the perforated flame holder 102.

The perforated flame holder 102 can be configured to output a portion of the received heat as thermal radiation 304. The perforated flame holder can transfer another portion of the received heat to the fuel and oxidant mixture 108 received at the input surface 212 of the perforated flame holder 102.

The inventors further contemplate that the various combinations of perforated flame holder portions and fuel nozzles can be used to increase the turndown ratio. The turndown ratio may be defined as the ratio of maximum heat release to minimum heat release with maintenance of a stable flame (see The John Zink Hamworthy Combustion Handbook, 2nd Ed. , Volume 1 , i|10.7.1 .3 and Figure 17.15).

The provision (and/or deployment) of multiple nozzles is related to the inventors' novel structure and method of increasing the turndown ratio defined above. The multiple fuel nozzles 1 10a, 1 10b, and 1 10c may be individually controllable as to fuel flow rate, feed pressure, or on/off actuation. Such an arrangement may allow individual segments 516a, 516b, 516c of the perforated flame holder 102 to generate heat at different rates (because each of the fuel streams 108a, 108b, 108c can impinge substantially onto a respective segment 516a, 516b, 516c of the perforated flame holder 102). Thus, each combination (such as 1 10a, 108a, and 516a) may be operated independently of the others.

However, the operation of individual segments may not be fully

independent because heat, generated by combustion (e.g., flame) inside borelike apertures or other voids of the perforated flame holder 102, may escape the perforated flame holder in various ways.

First, heat can travel by conduction from hotter parts toward colder parts. It may travel in the direction from the output surface 214 (in FIG. 2) toward the input surface 212. It also may travel laterally. For example, it may travel laterally from a portion of the perforated flame holder under an area where there is greater fuel and oxidant mixture impingement toward a portion under an area where there is less impingement. The same type of heat travel may happen due to varied fuel/oxidant ratios. In the case of physically distinct segments such as in FIG. 5, heat can travel between adjoining segments if they are in contact with each other. Heat may also travel through the support structure 222 that touches both segments (by conduction). Additionally or alternatively, heat may travel through an intervening fluid via conduction and/or convection.

Second, heat can leave the perforated flame holder 102 via hot flue gas. Third, heat can leave as electromagnetic radiation at infrared and visible region wavelengths. The perforated flame holder is often hot enough to emit a bright glow. (Heat balance in a perforated flame holder is discussed further in relation to FIG. 3 )

Once heat leaves the perforated flame holder 102, it may heat a furnace in which the burner assembly (or just the perforated flame holder) may be installed. Interior walls may not only absorb heat, they may also re-emit it, especially via thermal radiation, which may return heat to a perforated flame holder. These processes can transfer heat from one segment of the perforated flame holder 102 to an adjoining segment, and also to non-adjoining segments. Such heat transfer between segments may make it possible to increase the turndown ratio of a burner assembly or furnace. In one embodiment, various combinations of nozzles can be active, while others are not streaming fuel thereby increasing the turndown ratio.

The heat transfer mechanisms mentioned above can provide greater flame stability, because of the segment-to-segment heat transfer. When a segment receives less fuel because its single fuel nozzle stream is diminished or intermittent, then its temperature will tend to decrease, due to the several heat- loss mechanisms mentioned above. If the segment is not isolated, but is a part of the perforated flame holder in which it can receive heat from the surrounding elements, such as segments, perforated flame holder support structure or furnace walls, then its temperature will not decrease as quickly as it would in isolation.

The longer it takes the segment's temperature to decrease, the longer it can maintain a temperature that is viable for flame stability, resulting in a longer viable time to extinction. Time to extinction may be defined as the maximum time interval before which the nominal fuel impingement may resume without producing flame instability. That is, fired segments present in a vicinity of a non- fired tile can transfer heat to it, thus increasing the robustness of the entire perforated flame holder. This, in turn, may lead to stable combustion in the face of fuel flow instabilities (assuming the average heat flow is not affected over a time period less than the extinction time).

The viable turn-off time may in some cases be of indefinite length, depending on the combustion parameters and perforated flame holder characteristics. For example, variations in oxidant (e.g. combustion air) temperature, variations in fuel composition, or unknown elevation, altitude, barometric pressure, or furnace insulation (at least upon installation) may make the viable turn-off time indeterminate. Moreover, periods of reduced fuel flow or increased load immediately before a turn-off period may tend to cause a portion 102a of a perforated flame holder 102 to start off at a lower temperature upon a stoppage of fuel flow. The system designer may elect not to include Look-Up Table (LUT) or algorithmic support for a fully parameterized determinate viable turn off time calculation. Either situation (environmental or limitations of an implementation that add uncertainty) is referred to as indeterminate viable turn- off time herein.

According to an embodiment, a control system can be equipped with a search algorithm selected to infer a viable turn-off time even in the presence of factors that may cause absolute foreknowledge to be indeterminate. For example, a control system may respond to computer instructions carried by a non-transitory computer readable medium to make conservative changes to restart time. Longer delays in full restart may be indicative of approaching or surpassing a viable turn-off time.

FIG. 6 illustrates a control system 600 for implementing an increased turndown ratio, according to an embodiment. The perforated flame holder 102 may include numerous segments 516m (for convenience, only one is labeled in the figure), which, in an embodiment, can correspond to respective plural fuel nozzles 1 10. The segments 516m may be separated by contiguous areas roughly corresponding to substantially circular areas, one of which is labeled 602m. Although seven segment areas 516m are shown, there can be any number of areas 516m and any number of fuel nozzles 1 10m. Each segment can be larger across than any of the apertures 210 (which may cover the entire surface of the perforated flame holder 102, although only a few are illustrated).

Each of the fuel nozzles 1 10m may, in an embodiment, be coupled to a common hydraulic header (not shown) or otherwise supplied with fuel, which may be under pressure sufficient to expel a respective fuel stream 108m. Pipes leading to tips of the fuel nozzles may include respective valves 602m, mechanically coupled to step motors or other actuators 604m, which can be in turn actuated by a controller 1 16 through wires 606m. The controller 1 16 may receive feedback signals from one or more sensors 122, which may sense flame characteristics, fluid speed(s), temperature(s), species, or any other characteristics that relate to or result from combustion parameters. The sensors may be optical, acoustic, thermal, etc.

The illustration is exemplary, and other embodiments are contemplated. For example, the wires 606m may be replaced by a wireless link or links; the step motor(s) and valve(s) may be substituted with a DC motor and a pump, and so on. Likewise, the controller 1 16 can be of various forms, from electro-mechanical timer to digitally controlled systems under the direction of a programmed processor such as a microprocessor or a computer programmed from a non- transitory medium. These examples will suggest other approaches to those skilled in the art.

Whatever the exact mechanism, the system of FIG. 6 should be capable of at least on-off stream switching (fuel rate as a step function of time), producing variable flow rates, or performing other basic functions that vary heat generation.

In an embodiment, the system of FIG. 6 may interrupt the flow of the fuel to selected nozzles 1 10m for particular time intervals, which may be selected so that the temperature of each segment stays above the temperature needed for a stable flame. One example of such temperature may be an auto-ignition temperature of the fuel and oxidant mixture impinging on the respective segment. In general, the temperature needed for a stable flame is a function of several variables and parameters. Such an arrangement may be useful in turndown scenarios in which some portions of the tile(s) may be cooling due to the reduced energy input. In that case, those areas of the perforated flame holder may receive pulsed fuel to maintain a minimum segment temperature.

MIXING LENGTH: The inventors' apparatus and method also relates to the "mixing length," which may be defined as the distance from a fuel nozzle at which the fuel stream is mixed with entrained oxidant (e.g., air) in a manner suitable to burn properly. This is related to the physics and geometry of free fuel streams, which are explained in The John Zink Hamworthy Combustion

Handbook, 2nd Ed. , Volume 1 , U 9.1 1 and Figure 9.100. The equations presented in that book (which are incorporated herein by reference) show that the characteristics of a fuel stream generally depend on position, and a few other factors. Such factors can include the diameter of the fuel nozzle orifice, the fuel velocity at the nozzle tip (itself a function of fuel nozzle orifice inner diameter and fuel pressure in most fuel nozzle designs), and the densities of the fuel and oxidant (the last two appearing in the equation for mass entrainment rate).

The equations show that, as fuel leaves the fuel nozzle, it entrains (draws in) oxidant and any diluent present, such as combustion air. At a distance of about 18 nozzle orifice diameters, the "core" of pure fuel is gone and only a mixture of entrained oxidant and fuel exists downstream of that distance;

however, the mixture is not uniform at that distance, nor does it ever become fully uniform no matter what the axial distance from the nozzle orifice for a stream issuing into quiescent fluid. The velocity of the mixture decreases steadily with distance from the nozzle, while at any one distance the velocity "profile" (the variation of the velocity as function of radial distance from the stream centerline or axis) has the form of a bell-shaped curve. Because of the relation between these, the velocity profile becomes flatter (closer to a uniform velocity field) at greater axial distances, but still retains some character of a bell-shaped curve (Figure 9.100, id.).

The fuel concentration transverse to the stream and mass entrainment rate can also vary with axial distance and radial distance. The fuel concentration varies as a function of the axial distance normalized by the fuel nozzle diameter. Therefore, for a fuel stream issuing into quiescent fluid (e.g., air) for a fixed axial distance, a smaller fuel nozzle diameter will result in more thoroughly mixed fuel with oxidant, since the fixed axial distance equates to a greater number of smaller fuel nozzle diameters. A larger fuel nozzle diameter will result in less thoroughly mixed fuel with oxidant, since the fixed axial distance equates to a fewer number of larger fuel nozzle diameters.

Thus, for a given size of perforated flame holder, and given required rate of heat generation or fuel burning, and/or a given furnace geometry, there may be cases in which a single fuel nozzle, as in FIG. 2, is less than optimum, and in those cases the inventors' plurality of fuel nozzles, as shown as an embodiment in FIG. 6, may be an improvement leading to better mixing or other desirable properties.

In FIG. 6, it can be seen that the fuel streams 108m are at least roughly aligned with the circular segments 516m. Inasmuch as the fuel concentration profile is a function of radial distance from the projected nozzle axis, the fluid velocity and concentration from a single fuel nozzle 1 10m is greatest in the center of the corresponding segment 516m, and decreases with distance from the center. If the segments 516m are, however, arranged near one another as shown, the various fuel streams 108m can overlap. If, for example, the stream velocity is monotonically decreasing from the center of a first segment toward the center of a second, but at the same time is also monotonically decreasing from the center of the second segment toward the center of the first, then their summation along the line between the centers can be more uniform than would be the case with only one fuel stream. In other words, with plural fuel streams 108m a more uniform flow of mixture to the perforated flame holder 102 can be achieved, which can result in more homogeneous (closer to ideal) concentrations at the perforated flame holder.

FIG. 7 illustrates the concentration profile 700 for two overlapping fuel streams issuing into quiescent fluid, such as air. One fuel stream is centered at the left side of the figure and the other is centered at the right side of the figure. The curve sloping downward from left to right pictorially represents the fuel concentration as it decays from its maximum value at the center of the fuel nozzle of the first stream to a minimum value as it approaches the other fuel stream. The ratio of the minimum to maximum fuel concentration of this curve is about 7.4. That is, the fuel is about 7.4 times more concentrated along its centerline than at the periphery shown at the right. Likewise, for the fuel nozzle centered at the right side of the figure, the fuel concentration slopes downward from right to left. This curve also represents a fuel concentration that is about 7.4 times more concentrated along its centerline than its periphery. However, the concentration profile resulting from the overlap of the fuel streams has a much more modest ratio of maximum to minimum values of about 1 .5. Thus, the fuel concentration has been much homogenized by the addition of an overlapping fuel stream. Additional fuel nozzles along the plane can result in an even more uniform concentration profile. Because of the nature of fuel stream entrainment, the mixture impinging on a perforated flame holder from multiple fuel nozzles can be much more uniform than that from a single fuel nozzle. For example, a hexagonal array of fuel nozzles, such as illustrated in FIG. 6, theoretically, can provide fuel concentration profiles whose maximum to minimum ratio is about 1 .3. Other arrangements of fuel nozzles 1 10m can also be envisioned including those that are not geometrically regular.

FIG. 8 shows a furnace 800, according to an embodiment. The furnace

800 includes the fuel nozzle 1 10, two pilots 802, the perforated flame holder support 222, and the perforated flame holder made up of contiguous segments 516a, 516b, 516c, and 516d. In this embodiment, the segments form an arch. According to an embodiment, the segments 516a-d are tiles. In an embodiment, the structure shown in this figure may be repeated at other locations (in a direction into or out of the paper plane) so that a furnace will have rows of parallel arches inside it. According to an embodiment the gaps between the arches can be useful in reducing the tile temperature. (The benefits of air gaps between segments were discussed above in relation to FIG. 5.)

Referring to FIGS. 1 and 2, an apparatus for supporting combustion 100, according to an embodiment, includes a plurality of fuel nozzles 1 10a, 1 10b, configured to emit a corresponding plurality of fuel streams 108a, 108b, and a perforated flame holder 102 aligned to receive the plurality of fuel streams 108a, 108b into respective portions 102a, 102b of the perforated flame holder 102. The perforated flame holder 102 can have elongated apertures passing through the perforated flame holder 102 from an input face 212 to an output face 214 (both shown in FIG. 2), and having a transverse dimension D between opposing perforation walls 308 (shown in FIG. 3).

Referring to FIG. 5, the perforated flame holder 102 may be formed to include a plurality of segments 516a-c separated from one another by respective gaps 502a, b at a distance at least twice as large as the dimension D. For example, the gaps 502a, 502b between the segments 516a and 516b, and 516c, respectively, can be formed above the support frame pieces 222. In some embodiments, the gaps 502a, 502b separating the segments 516a, 516c, 516c, etc. can be at least five times the transverse dimension D. In other embodiments, the gaps 502a, 502b can be at least ten times the transverse dimension D. For example, the transverse dimension D can be between 0.1 inch and 0.25 inch.

In an embodiment, each segment 516a, 516b, etc. of the perforated flame holder 102 may correspond to a respective portion of the perforated flame holder 102. The plurality of fuel nozzles 1 10a, 1 10b, etc. can be aligned to cause the fuel streams 108a, 108b, etc. to collectively be received by the perforated flame holder segments 516a, 516b, etc. and the gaps 502a, 502b, etc. between them. Furthermore, the perforated flame holder 102 can be configured to combust the fuel passing through the elongated apertures and through the gaps 502a, 502b, etc. sufficiently completely that no fugitive emission of fuel may be detected at a flue through which combustion products may pass.

FIG. 9 is a flow chart showing a method 900 for supporting combustion, according to an embodiment. The method 900 can include the following steps. At step 902, a plurality of fuel streams can be emitted from a corresponding plurality of fuel nozzles.

At step 904, the plurality of fuel streams may be received into respective portions of a perforated flame holder. According to an embodiment, the

perforated flame holder can have elongated apertures passing through the perforated flame holder from an input face to an output face. Each of the elongated apertures can be characterized with a transverse dimension D, corresponding to a distance between opposing perforation walls. Furthermore, the perforated flame holder can be comprised of a plurality of segments separated from one another by respective gaps of at least two times D.

Step 906 of the method 900 may include supporting combustion of the respective fuel streams in the portions of the perforated flame holder.

According to an embodiment, the perforated flame holder 102 may be formed to include a plurality of segments separated from one another by respective gaps at a distance at least twice as large as the dimension D. In some embodiments, the gaps separating the segments can be at least five times the transverse dimension D. In other embodiments, the gaps can be at least ten times the transverse dimension D. For example, the transverse dimension D can be between 0.1 inch and 0.25 inch.

In an embodiment, each segment may correspond to a respective portion of the perforated flame holder. The plurality of fuel nozzles can be aligned to cause the fuel streams to collectively be received by the perforated flame holder segments and the gaps between them. Furthermore, the perforated flame holder can be configured to combust the fuel passing through the elongated apertures and through the gaps sufficiently completely that no fugitive emission of fuel may be detected at a flue through which combustion products pass.

Referring to FIG. 1 , a burner apparatus 100, according to an embodiment, can include a plurality of fuel nozzles 1 10a, 1 10b, etc. configured to output respective fuel streams108a, 108b, etc., and a perforated flame holder 102 configured to receive the respective fuel streams 108a, 108b, etc. at respective portions 102a, 102b of the perforated flame holder 102 and to support

combustion in the respective portions 102a, 102b of the perforated flame holder 102.

According to an embodiment, a second portion of the plurality of fuel nozzles 1 10a, 1 10b, etc. can be configured to cooperate with a corresponding second portion 102b of the perforated flame holder 102 to cause the output of at least a portion of heat sufficient to maintain a first portion 102a of the perforated flame holder 102 at an elevated operating temperature. A first portion of fuel nozzles 1 10a, 1 10b, etc. can be different from the second portion of fuel nozzles 1 10a, 1 10b, etc., and be configured to be controlled to stop outputting a respective first fuel stream 108a, 108b, etc. to the first portion 102a of the perforated flame holder 102.

According to an embodiment of the burner apparatus 100, the

aforementioned portion of heat may comprise the entirety of heat necessary to maintain the first portion 102a of the perforated flame holder 102 at the elevated operating temperature for at least a time greater than a time sufficient to stop and start the first fuel stream 108 from the first portion of fuel nozzles 1 10. In other embodiments, such a portion of heat may correspond to a time 10 times greater or a time 100 times greater than the time sufficient to stop and start the first fuel stream 108 from the first portion of fuel nozzles 1 10.

For example, in an illustrative application, it may require 2 seconds to shut off the first fuel stream from a first portion consisting of three fuel nozzles; and it may require 4 seconds to turn the first fuel stream on from the first portion of fuel nozzles. The time sufficient to stop and start the first fuel stream would thus be 6 seconds, irrespective of how much time elapsed between stopping and starting the first fuel stream. According to this example, a time greater than 10 times the time sufficient to stop and start the first fuel stream would be at least greater than 60 seconds, or 1 minute. The inventors have found that a vertically fired burner apparatus including a perforated flame holder consisting essentially of a 2 inch thick square mullite honeycomb having about 70% void volume and 64 cells per square inch satisfies this condition when the first portion is about 25% or less of the total perforated flame holder area. Also according to this example, a time greater than 100 times the time sufficient to stop and start the first fuel stream would be at least greater than 600 seconds, or 10 minutes. The inventors contemplate that a perforated flame holder made from 6 inch thick square mullite honeycomb having about 70% void volume and 16 cells per square inch may satisfy this condition when the first portion is about 10% or less of the total perforated flame holder area.

According to another embodiment of the burner apparatus 100, the aforementioned portion of heat may comprise less than the entirety of heat necessary to maintain the first portion 102a of the perforated flame holder 102 at the elevated operating temperature. Additionally, the burner apparatus 100 can include a heating mechanism configured to cause heating of the first portion 102a of the perforated flame holder 102 sufficient, in cooperation with the portion of heat output by the second portion of fuel nozzles 108 cooperating with the second portion 102b of the perforated flame holder 102, to maintain the first portion 102a of the perforated flame holder 102 at the elevated operating temperature. For example, the heating mechanism can include a conventional flame holder configured to hold a conventional flame upstream from the perforated flame holder 102. Alternatively, the heating mechanism may include an electrical resistance heater.

A method for operating a burner, according to an embodiment, can include a step of delivering a plurality of fuel streams 108 (for example, shown in FIGS. 1 and 2) from a corresponding plurality of fuel nozzles 1 10 (shown in FIGS. 1 and 2) to a perforated flame holder 102 for combustion in the perforated flame holder 102. A second step of the method for operating a burner can include stopping at least one first fuel stream 108 from a first fuel nozzle 1 10 that delivers fuel to a first portion of the perforated flame holder. A third step of the method for operating a burner can include maintaining an elevated operating temperature of the first portion 102a of the perforated flame holder 102 at least in part by continued combustion in other portions of the perforated flame holder 102.

According to an embodiment, the step of maintaining the elevated operating temperature of the first portion of the perforated flame holder may include applying heat to the first portion 102a of the perforated flame holder with a heater mechanism. For example, this may include exposing the first portion of the perforated flame holder to heat output from a start-up flame. Alternatively, it may include outputting heat to the first portion of the perforated flame holder with an electrical resistance heater.

Additionally, the method for operating a burner can include a step of resuming the first fuel stream through the first fuel nozzle, after a first time delay. In such an embodiment, the maintained elevated operating temperature of the first portion of the perforated flame holder 102 may cause the resumed first fuel stream to support combustion in the first portion of the perforated flame holder 102. According to and embodiment, the resumed first fuel stream may be configured to support combustion in the first portion 102a of the perforated flame holder 102 immediately upon entering the first portion of the perforated flame holder 102. According to and embodiment, the method for operating a burner can include a step of determining a viable turn-off time, and resuming the first fuel stream only if the first time delay is less than or equal to the viable turn-off time. The step of determining a viable turn-off time may include receiving, with a computer control system, at least one set of sensor data from at least one sensor configured to sense a parameter that can cause a variation in the viable turn-off time. Additionally, this step may also include calculating the viable turn-off time with the computer control system as a function of the at least one set of sensor data.

Alternatively, the step of determining a viable turn-off time may include reading, from a non-transitory computer readable medium, a predetermined viable turn-off time.

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.