The present application claims priority benefit from U.S. Provisional Patent Application No. 62/218,462, entitled "PARTIALLY TRANSITIONED FLAME START-UP OF A PERFORATED FLAME HOLDER," filed September 14, 2015 (docket number 2651 -252-02); which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

BACKGROUND Combustion systems are employed as components in many commercial, industrial, residential, and consumer-based systems. Some combustion systems are designed for substantially continuous operation, while other systems operate in cycles, or are shut down periodically. Once cooled down, the variety of procedures for restarting a combustion system varies significantly, from the very simple procedures employed in a residential furnace, to extremely complex procedures practiced in the start-up of large industrial furnaces, etc.

Additionally, most combustion systems are provided with safety systems that are configured to detect, for example, the loss of a flame in a system, and to automatically shut down the system, including closing any fuel valves that feed fuel to the affected system. SUMMARY

According to an embodiment, a combustion system is provided that includes a main flame holder configured to hold a main combustion reaction substantially between input and output faces thereof. A main fuel nozzle is positioned and configured to emit a main fuel stream toward the input face. An igniter assembly is provided, configured to ignite a preheat flame, supported by the main fuel stream, between the main fuel nozzle and the main flame holder, and to selectably control a degree of ignition of the fuel stream in the preheat flame.

According to an embodiment, the main flame holder is a perforated flame holder, having a plurality of apertures extending between the input and output faces, and configured to hold a majority of the main combustion reaction substantially within the plurality of apertures.

According to an embodiment, each igniter assembly is configured to selectably shift between igniting the fuel to support the preheat flame and not igniting the fuel to release the preheat flame, thus allowing fuel and combustion air to reach the main flame holder for combustion. Optionally, each igniter assembly or a plurality of igniter assemblies can be configured to cooperate to selectably shift the preheat flame point of ignition between positions along the fuel stream between the main fuel nozzle and the main flame holder.

According to an embodiment, the igniter assembly includes first and second pilot nozzles, each configured to emit a respective pilot fuel stream to support a respective pilot flame. The first and second pilot nozzles are positioned such that while the preheat flame is held by both the first and second pilot flames, the preheat flame fully ignites the main fuel stream. However, while the preheat flame is held by only one of the pilot flames, the preheat flame only partially ignites the main fuel stream.

According to another embodiment, the igniter assembly includes a pilot nozzle configured to emit a pilot fuel stream and to support a pilot flame. The position of the pilot nozzle is selectably switchable between a first position, in which the pilot flame holds the preheat flame so as to fully ignite the main fuel flow, and a second position, in which the pilot flame holds the preheat flame so as to partially ignite the main fuel flow.

According to an embodiment, the combustion system includes a controller configured to control the igniter assembly to hold the preheat flame to fully ignite the main fuel stream while a temperature of the main flame holder is below a threshold temperature, and to hold the preheat flame to partially ignite the main fuel stream while the temperature of the main flame holder is above the threshold temperature. The controller is further configured to control the igniter assembly to fully release the preheat flame while the temperature of the main flame holder is above the threshold temperature such that fuel and combustion air reaches the main flame holder and the combustion reaction is held by the main flame holder.

According to an embodiment, the combustion system includes one or more flame sensors, configured to produce respective sensor signals indicative of the presence or absence of a flame in the combustion system. The controller is configured to receive the sensor signal or signals and to stop emission of the main fuel stream from the main nozzle in the event that no flame is present in the combustion system.

According to an embodiment, a start-up procedure for a combustion system is provided, including emitting a main fuel stream from a main fuel nozzle toward a main flame holder, and preheating the main flame holder by igniting and holding a preheat flame corresponding to full ignition of the main fuel stream. After a temperature of the main flame holder reaches a threshold temperature, the preheat flame is controlled to only partially ignite the main fuel stream, permitting a portion of the main fuel stream to reach the main flame holder. A main combustion reaction is ignited in the main flame holder and supported by the portion of the main fuel stream that reaches the flame holder. Once the main combustion reaction is ignited, the preheat flame is fully released to allow substantially all of the combustion reaction to occur in the main flame holder. BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are simplified diagrammatic views, according to an embodiment, of a combustion system, showing the combustion system in respective modes of operation.

FIG. 2 is a simplified diagram of a burner system including a perforated flame holder configured to hold a combustion reaction, according to an embodiment.

FIG. 3 is a side sectional diagram of a portion of the perforated flame holder of 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 shown and described herein, according to an embodiment.

FIG. 5 is a simplified diagram of a combustion system, according to another embodiment, that is configured to operate according to principles similar to those described herein with reference to FIGS. 1A-1C.

FIG. 6 is a diagrammatic view of a combustion system, according to an embodiment, showing the system in a preheat mode of operation.

FIGS. 7A and 7B are diagrammatic representations of a combustion system while in respective modes of operation, according to an embodiment.

FIG. 8 is a flow chart illustrating a start-up process for a combustion system such as can be practiced, in various configurations of combustion systems, including the systems described herein with reference to FIGS. 1A-1C, 5, 6, 7A, and 7B, 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. As used in the specification and claims, the term fuel stream is to be construed broadly, as reading on a stream of fuel; fuel and oxidizer; fuel, oxidizer, and/or other reactants, diluents, inert fluids, etc. Some or all of the non- fuel components of a fuel stream can be premixed with the fuel and emitted from a nozzle, or can be entrained by a stream of fuel as it exits a nozzle.

FIGS. 1A-1C are diagrammatic views, according to an embodiment, of a combustion system 100, showing the combustion system in respective modes of operation. Many elements of the combustion system 100 that are not necessary for an understanding of the disclosed principles are omitted from this description, including a flame holder support structure and other elements that define a combustion volume within which many of the elements that are disclosed and described would be positioned during operation of the system.

The combustion system 100 includes a perforated flame holder 102, a main fuel nozzle 104, a start-up flame stabilization assembly 106, and, optionally, a controller 108. The start-up flame stabilization assembly 106 may include an igniter assembly 106, according to an embodiment.

The igniter assembly 106 can include first and second pilot nozzles 1 10a, 1 10b, first and second pilot fuel valves 1 18a, 1 18b, and, optionally, first and second flame sensors 1 12a, 1 12b. The main nozzle 104 is coupled to a fuel source via a fuel line 1 14 and a main fuel valve 1 16. The first and second pilot nozzles 1 10a, 1 10b are coupled to the fuel source via fuel lines 1 14 and the first and second pilot fuel valves 1 18a, 1 18b, respectively. For embodiments that use automatic control, respective control terminals of the main fuel valve 1 16 and the first and second pilot fuel valves 1 18a, 1 18b are coupled to the controller 108 via control lines 120, while signal output terminals of the first and second flame sensors 1 12a, 1 12b are coupled to the controller 108 via signal transmission lines 122. The first flame sensor 1 12a is positioned and configured to produce a signal corresponding to the presence or absence of a flame held by the perforated flame holder 102, while the second flame sensor 1 12b is positioned and configured to produce a signal corresponding to the presence or absence of a flame located between the main nozzle 104 and the perforated flame holder 102. The controller 108 is configured, according to an embodiment, to control operation of the combustion system 100, in part, based upon the presence of a detectable flame in the system. Specifically, with respect to the embodiment of FIGS. 1A-1C, if neither of the first or second flame sensors 1 12a, 1 12b produces a signal that indicates the presence of a flame, the controller 108 can be configured to close all fuel supply valves and shut down operation of the system 100.

The first and second flame sensors 1 12a, 1 12b can be any of a large number of known types of flame sensors. A partial listing of known types of flame sensors 1 12a, 1 12b is provided below, with reference to FIG. 2. The ultraviolet sensor is an example of one type of flame sensor 1 12a, 1 12b that is appropriate in the configuration of the embodiment of FIGS. 1A-1C. An ultraviolet sensor produces a voltage signal corresponding to a level of intensity of ultraviolet radiation within a selected wavelength band. By evaluation of the output signal of the sensor, the presence of a flame within the range of the sensor can be inferred.

FIG. 1A shows the combustion system 100 in a normal operating mode. During normal operation, the combustion system 100 is configured to perform according to the design parameters of the particular application. For example, the combustion system 100 may be operated as a heat source in a boiler, furnace or kiln, to drive chemical processes, to burn waste gases, etc.

The main nozzle 104 is configured to receive a flow of fuel from the fuel source and to emit a fuel stream 124 toward the perforated flame holder 102. A combustion reaction 126 is supported by the fuel stream 124 and held by the flame holder 102 (details of the structure and operation of the perforated flame holder 102, according to various embodiments, are described below with reference to FIGS. 2-4). The controller 108 is configured to control the main fuel valve 1 16 to regulate operation of the main nozzle 104, and to control the first and second pilot fuel valves 1 18a, 1 18b to regulate operation of the first and second pilot nozzles 1 10a, 1 10b, as described in more detail with reference to FIGS. 1 B and 1C. During operation in the normal operating mode, the first flame sensor 1 12a responds to the presence of the main combustion reaction 126 by producing the corresponding signal. If the controller 108 fails to detect a flame on the basis of the signal from one or both of the first and second flame sensors 1 12a, 1 12b, the controller 108 closes the main fuel valve 1 16 and the first and second pilot fuel valves 1 18a, 1 18b, which otherwise could accumulate unburned fuel within the combustion system 100, creating a potentially dangerous condition.

While in a normal operating mode, in which the combustion reaction 126 is held by the perforated flame holder 102, the controller 108 receives a signal from the first flame sensor 1 12a indicating the presence of the combustion reaction 126, and thus continues to enable operation of the combustion system 100 by holding the main fuel valve 1 16 open. Meanwhile, the controller 108 can be configured to produce a disable signal at the input terminals of the first and second pilot fuel control valves 1 18a, 1 18b, holding both valves in a closed or a low amplitude flame configuration selected to avoid ignition of the fuel and oxidant 124 between the main fuel nozzle 104 and the perforated flame holder 102.

Typically, prior to operation in a normal operating mode, the perforated flame holder 102 is preheated to a start-up temperature T _s . FIG. 1 B shows the combustion system 100 in a preheat mode, while FIG. 1C shows the system in a transition mode, according to an embodiment.

While in preheat mode, the controller 108 can be configured to control the start-up flame stabilization assembly 106 to hold a preheat flame 132 between the nozzle 104 and the perforated flame holder 102, as shown in FIG. 1 B.

According to an embodiment wherein the start-up flame stabilization assembly 106 includes a start-up igniter assembly 106, the controller 108 is configured to provide signals at the control terminals of the first and second pilot fuel control valves 1 18a, 1 18b, causing the valves to open and admit a flow of fuel to each of the first and second pilot nozzles 1 10a, 1 10b. Accordingly, the first and second pilot nozzles 1 10a, 1 10b can emit first and second pilot fuel streams 128a, 128b, respectively, which, upon ignition, support corresponding high amplitude first and second pilot flames 130a, 130b. Ignition of the first and second pilot fuel streams 128a, 128b can be accomplished by the use of any appropriate means, including any of a number of structures and methods that are known in the art, such as, e.g., electrical spark or arc generators, glow wires, pilot lights, etc. Optionally, the first and second pilot fuel streams 128a, 128b can be manually ignited.

Optionally, control of main fuel stream 124 ignition by the first and second pilot flames 130a, 130b can be performed by controlling the volume of the respective first and second pilot fuel streams 128a, 128b, which in turn controls the amplitude of the first and second pilot flames 130a, 130b. At low pilot fuel stream 128a, 128b flow rates, the resultant pilot flames 130a, 130b are small enough (i.e., of sufficiently low amplitude) to provide insufficient heat within the main fuel stream 124 to cause main fuel stream 124 ignition. At a higher pilot fuel stream flow rate 128a and/or 128b, the resultant pilot flame 130a and/or 130b is of sufficiently high amplitude to ignite a portion of the main fuel stream 124. According to embodiments, neither pilot flame 130a nor 130b alone, can reach sufficient amplitude to ignite the entirety of the main fuel stream 124.

Rather, the pilot flames 130a, 130b, when both are operated at high amplitude, are selected to ignite the entirety of the main fuel stream 124 in combination.

The controller 108 can be further configured to provide a signal at the control terminal of the main fuel control valve 1 16, causing it to open and admit a flow of fuel to the main nozzle 104. When fuel flows via the main control valve 1 16 to the main nozzle 104, the main nozzle 104 emits the fuel stream 124 toward the perforated flame holder 102. Typically, the volume of fuel emitted from the main nozzle 104 in the fuel stream 124 is much greater than the volume of fuel emitted collectively from both of the first and second pilot nozzles 1 10a, 1 10b. Thus, according to an embodiment, in order to reduce the likelihood of the release of a significant quantity of unburned fuel before a flame is ignited, the controller 108 can be configured to control the main fuel control valve 1 16 to open only after the first flame sensor 1 12a detects the presence of one or both of the first and second pilot flames 130a, 130b. While initiating ignition of the pilot flames 130a, 130b, the controller 108 can be configured to allow a selected pilot ignition delay period during which a lack of a positive flame signal corresponding to pilot flame presence from the second flame sensor 1 12b is ignored. If, by the end of the pilot ignition delay period no flame has been detected, the controller 108 can be configured to close the first and second pilot fuel control valves 1 18a, 1 18b. The controller 108 can also be configured to reattempt pilot ignition, and/or to issue a start-up failure alert.

Once the pilot flames 130a, 130b are ignited, the controller can be configured to open the main fuel valve 1 16, producing the fuel stream 124 from the main nozzle 104. As explained in detail below with reference to FIGS. 2-4, according to various embodiments, characteristics of the fuel stream 124 are selected to prevent stable combustion from occurring between the main nozzle 104 and the perforated flame holder 102 during normal operation of the combustion system 100. Such characteristics can include, for example, the composition and velocity of the fuel stream 124. However, while the system 100 operates in the preheat mode, and first and second pilot flames 130a, 130b are present, they act to ignite the fuel stream 124 and hold the resulting preheat flame 132 between the main nozzle 104 and the flame holder 102. The perforated flame holder 102 is thus heated by the preheat flame 132.

In practice, pilot flames 130a and 130b will generally merge with a preheat flame 132 in a fuel stream 124, so that there is no easily distinguishable division or separation between them. However, they are shown in the drawings as separate elements in order to more clearly illustrate the relevant principles.

Once the perforated flame holder 102 has reached a startup temperature, the controller 108 can be configured to proceed to the normal operation mode, as described above.

The inventors have noted that operation of systems configured to transition quickly (or instantaneously) from a start-up state depicted by FIG. 1 B directly to a normal operation state depicted by FIG. 1A can be correlated with some amount of anxiety by burner operators. While the inventors have not observed any instances of non-ignition upon fuel contact with the perforated flame holder 102 when the perforated flame holder 102 is properly raised to an operating temperature before transition, the momentary lack of visible flame may be somewhat disturbing to experienced combustion engineers and plant operators. Accordingly, the inventors propose partial transition from start-up mode (FIG. 1 B) to operating mode (FIG. 1A), wherein a portion of the main fuel progresses unburned to the perforated flame holder 102 while another portion of the main fuel remains ignited between the main nozzle 104 and the perforated flame holder 102. This can result in conditions where there is always a visible flame or infrared radiation characteristic of a stable combustion reaction present during all phases of operation. This partially transitioned mode is depicted by FIG. 1C

According to an embodiment, the controller 108 is configured to control the system 100 to operate briefly in a transition mode, as illustrated in FIG. 1C, before switching from the preheat mode to the normal operation mode. When shifting to the transition mode from the startup mode, the controller 108 can be configured to close one of the first or second pilot fuel valves 1 18a, 1 18b, while holding open the other of the first and second pilot fuel valves 1 18a, 1 18b. Thus, only one of the first or second pilot flames 130a, 130b is extinguished. As shown in FIG. 1C, the second pilot fuel valve 1 18b is closed, so that the second pilot flame 130b is extinguished, and only the first pilot fuel stream 128a continues to flow, supporting the first pilot flame 130a.

Characteristics of at least one of the pilot flames 130 are selected such that, in the absence of the other of the pilot flames 130, the preheat flame 132 cannot fully consume the main fuel stream 124. These characteristics can include, for example, the size, location, and/or orientation of the pilot flame(s) 130. In the embodiment of FIGS. 1A-1C, the first and second pilot flames 130a, 130b are, for practical purposes, substantially identical with respect to their respective flame-holding capacities. Because of the velocity of the main fuel stream 124, a flame that is ignited upstream from the flame holder 102 is carried toward the flame holder 102, even as it propagates laterally across the fuel stream 124. With both pilot flames 130 in operation, so that the main fuel stream 124 is continuously ignited from opposite sides, the preheat flame 132 ignites the entire main fuel stream 124. However, in the absence of one of the pilot flames 130, so that the preheat flame 132 is ignited and held by only one pilot flame 130, the fuel stream 124 is not fully ignited by the preheat flame 132, but a portion 124a of the fuel stream 124 reaches a first region Ri of the perforated flame holder 102. Because the flame holder 102 has been preheated, upon contact with the flame holder 102 the temperature of the portion 124a of the fuel stream 124 is immediately raised to a value that exceeds its own auto-ignition temperature, and the main combustion reaction 126 ignites in the first region R-i . Meanwhile, the reduced preheat flame 132 continues to consume the remainder of the fuel stream 124, preventing unburned fuel from reaching a second region R ₂ of the flame holder 102.

Where a fuel stream 124 is said to be fully ignited by a preheat flame 132, this means that substantially all of the fuel stream 124 passes through, and/or is burned adjacent to the preheat flame 132. Where a portion of a fuel stream 124 supports a flame 132 while other portions of the fuel stream 124 pass outside the flame 132, the fuel stream 124 can be said to be partially ignited by the preheat flame 132. The term should not be construed as requiring that all reactants of the fuel stream 132 be fully reacted by the combustion process.

The first flame sensor 1 12a is positioned and configured to produce a sensor signal corresponding to the presence or absence of a flame held by the perforated flame holder 102. Thus, when the combustion reaction 126 is ignited and held in the first region Ri of the perforated flame holder 102, the first flame sensor 1 12a produces a corresponding signal, and the controller 108 detects the main combustion reaction 126. Upon detection of the main combustion reaction 126, the controller 108 closes the first pilot fuel control valve 1 18a, causing the first pilot flame 130a to cease igniting a portion 124a of the main fuel stream 124. For example, this can be done by extinguishing or reducing the amplitude of the first pilot flame 130a. The remaining preheat flame 132 is thereafter blown out or carried downstream by the main fuel stream 124, so that unburned fuel is able to reach the second region R ₂ of the flame holder 102, permitting the combustion reaction 126 to spread across the entire flame holder 102, as shown in FIG. 1A, as the system 100 moves into the normal operating mode.

If, during the transition state of FIG. 1C, the first flame sensor 1 12a fails to detect combustion present in the region Ri of the perforated flame holder 102, then an error condition occurs. In some embodiments, the controller 108 again enables or increases the amplitude of the pilot flame 130 supported by the igniter 1 10b to cause the system 100 to return to the state of FIG. 1 B.

One benefit of the embodiment described above with reference to

FIGS. 1A-1C is that a flame is continually present in the system 100 from the moment the pilot flames 130 are ignited, and continuing on to operation in the normal operating mode.

Some systems that employ a perforated flame holder 102 are configured to fully extinguish a preheat flame 132 once the flame holder 102 has reached the startup temperature. If the preheat flame 132 is supported in the main fuel stream 124, typically none of the fuel stream 124 reaches the flame holder 102 until the preheat flame 132 is extinguished. This results in a brief delay, between the moment the preheat flame 132 is extinguished and the moment unburned fuel from the fuel stream 124 is caused to auto-ignite. While such systems are extremely reliable, there is, nevertheless, a very short period, typically on the order of less than one second, during which no flame is present. As previously noted, many combustion systems include a safety feature by which a system is configured to shut down if a flame is not present. This is intended to prevent the potentially dangerous condition in which fuel continues to flow from a nozzle after a flame has been unintentionally extinguished, resulting in a buildup of fuel, with potentially devastating consequences if such a buildup is inadvertently ignited. By ensuring that a flame is continually present during operation, the system of the embodiment of FIGS. 1A-1C can incorporate flame detection safety features without the risk of unnecessary shutdowns. 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 jet selected to entrain the oxidant to form the fuel and oxidant mixture 206 as the fuel jet and oxidant travel along a path to the perforated flame holder 102 through the dilution distance D _D between the fuel nozzle 218 and the perforated flame holder 102. Additionally or alternatively (particularly when a blower is used to deliver oxidant contained in combustion air), the oxidant or combustion air source can be configured to entrain the fuel and the fuel and oxidant travel through the dilution distance D _D . In some embodiments, a flue gas recirculation path 224 can be provided. Additionally or alternatively, the fuel nozzle 218 can be configured to emit a fuel jet selected to entrain the oxidant and to entrain flue gas as the fuel jet travels through the dilution distance D _D between the fuel nozzle 218 and the input face 212 of the perforated flame holder 102.

The fuel nozzle 218 can be configured to emit the fuel through one or more fuel orifices 226 having an inside diameter dimension that is referred to as "nozzle diameter." The perforated flame holder support structure 222 can support the perforated flame holder 102 to receive the fuel and oxidant mixture 206 at the distance D _D away from the fuel nozzle 218 greater than 20 times the nozzle diameter. In another embodiment, the perforated flame holder 102 is disposed to receive the fuel and oxidant mixture 206 at the distance D _D away from the fuel nozzle 218 between 100 times and 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 > T _s ).

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

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

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

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

perforated flame holder 102 before the perforated flame holder 102 is heated sufficiently to maintain combustion.

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

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

FIG. 5 is a simplified diagram of a combustion system 500, according to an embodiment, that is configured to operate according to principles like those described above with reference to FIGS. 1A-1C. In particular, the combustion system 500 is shown in a transition mode of operation, substantially as described with reference to FIG. 1C. A first pilot flame 130a is supported by a first pilot fuel stream 128a emitted from a first pilot nozzle 1 10a, so that a reduced preheat flame 132 is supported in the main fuel stream 124, while a portion 124a of the main fuel stream 124 supports a main combustion reaction 126 of a first region Ri of a perforated flame holder 102. As shown in FIG. 5, the main fuel nozzle 104 has a longitudinal axis A _N that lies approximately normal to an input face 212 of the perforated flame holder 102. The first and second pilot nozzles 1 10a, 1 10b have respective longitudinal axes A-i , A ₂ . It can be seen that the

longitudinal axes Ai , A ₂ of the first and second pilot nozzles 1 10a, 1 10b do not lie parallel to the longitudinal axis A _N of the main fuel nozzle 104, but instead are inclined with respect to the axis A _N by respective nozzle angles N-i, N ₂ . In the embodiment shown, the nozzle angles Ni and N ₂ are substantially identical, at least in absolute value, but this is not essential. Nor, in fact, is it essential that the respective longitudinal axes A-i, A ₂ of the first and second pilot nozzles 1 10a, 1 10b lie at angles relative to the axis A _N . Embodiments are contemplated in which the first and second pilot nozzles 1 10a, 1 10b lie parallel to the axis A _N , and other embodiments in which the nozzle angles Ni and N ₂ are significantly different from each other, relative to the axis A _N .

As previously noted, there are a number of factors that affect the size of the preheat flame 132 that the pilot flame 130 can ignite in the main fuel stream 124. For example, the angle at which the pilot flame 130 approaches the main fuel stream 124 affects lateral distance within the fuel stream 124 that receives sufficient heat to be ignited directly by the pilot flame 130, which in turn affects the lateral distance that the preheat flame 132 is able to generate sufficient heat for ignition and propagate across the main fuel stream 124 as it is carried downstream toward the perforated flame holder 102.

The determination of whether a preheat flame 132 can fully ignite the main fuel stream 124 is based on at least two factors: (1 ) the time it would take for the preheat flame 132 to propagate, from its point of origin, laterally across the entire main fuel stream 124— referred to hereafter as flame propagation time, and

(2) the time it will take the fuel stream 124 to travel from the point of origin of the preheat flame 132 to the input face 212 of the perforated flame holder 102— referred to hereafter as remaining fuel stream travel time. Typically, at a location that is optimal for a preheat flame 132, the velocity of the fuel stream 124 exceeds the propagation speed of the preheat flame 132. Thus, if a flame were ignited in the main fuel stream 124, the flame would be blown out or carried downstream to the flame holder 102. However, here, the pilot flame 130a continually reignites the preheat flame 132, holding it at a stable position. From the point closest to the nozzle 104 at which the pilot flame 130 contacts the main fuel stream 124 (the point of origin), the preheat flame 132 begins propagating laterally as it is carried by the fuel stream 124 toward the flame holder 102. If the flame propagation time from the point of origin is less than the remaining fuel stream travel time, the preheat flame 132 will fully ignite the fuel stream 124. If the opposite is the case, the preheat flame 132 will only partially ignite the fuel stream 124.

FIG. 6 is a diagrammatic view of a combustion system 600, according to an embodiment, and shows the system in a preheat mode of operation. The combustion system 600 is similar in many respects to the system 100 of

FIGS. 1A-1C, and operates according to similar principles. However, where the pilot nozzles 1 10a, 1 10b of the system 100 are arranged symmetrically, relative to the main nozzle 104, the corresponding pilot nozzles 1 10a, 1 10b of the system 600 are positioned at different distances from the main nozzle 104, particularly as measured longitudinally, i.e., in a direction parallel to the longitudinal axis A _N of the main nozzle 104. The outlet of the first nozzle 1 10a lies a longitudinal distance D ₃ from the outlet of the main nozzle 104, which is much greater than the distance D ₄ between the outlets of the second pilot nozzle 1 10b and the main nozzle 104.

As a result of this asymmetry, the second pilot flame 130b contacts and ignites the main fuel stream 124 upstream from the first pilot flame 130a. From the point at which the second pilot flame 130b contacts the main fuel stream 124, the preheat flame 132 propagates some distance laterally within the main fuel stream 124 before the main fuel stream 124 contacts the first pilot flame 130a. Thus, the second pilot flame 130b is responsible for igniting and holding a larger portion of the preheat flame 132 than the first pilot flame 130a.

When the combustion system 600 moves into a transition mode of operation, the controller 108 can be configured to close the second pilot fuel control valve 1 18b or deflect the second pilot flame 130b away from the main fuel stream 124, substantially as described with reference to the combustion system 100. However, because the first pilot nozzle 1 10a is closer to the perforated flame holder 102 than in the embodiment of FIGS. 1A-1C, the portion of the preheat flame 132 held by the first pilot flame 130a, in the absence of the second preheat flame 130b, is relatively much smaller, as suggested in FIG. 6.

Accordingly, a larger portion 124a (shown in FIG. 5) of the main fuel stream 124 is able to reach the perforated flame holder 102 during the transition mode of operation, and a combustion reaction 126 occupying a larger region Ri of the flame holder 102 is immediately ignited, improving efficiency of operation.

The combustion system 600 of FIG. 6 also includes a single flame sensor 602, instead of the two flame sensors of the system 100. The flame sensor 602 includes a relatively wider detection angle than the previous sensors, and is positioned so as to be capable of responding to emissions from any of the flames of the system, including the first and second pilot flames 130a, 130b, the preheat flame 132, and a main combustion reaction 126 held by the perforated flame holder 102 (shown in FIGS. 1A-1 C and 5). According to an embodiment, the flame sensor 602 is configured to provide a voltage signal that is related to or corresponds to the degree to which the input field of the sensor 602 is occupied by flame. Thus, while the combustion system 600 is operating in the preheat mode of operation, as shown in FIG. 6, the preheat flame 132 and the first and second pilot flames 130a, 130b occupy a significant portion of the input field. When the system 600 moves to the transition mode and extinguishes the second pilot flame 130b, along with the majority of the preheat flame 132, a much smaller portion of the input field is occupied, resulting in a sensor signal having a different value. When the main combustion reaction 126 is established in the portion Ri of the flame holder 102, the flame present in the input field of the sensor 602 is again different from other values, as it is also, when the system is in its normal operating mode. Accordingly, on the basis of the value of the single sensor signal, the controller 108 is able to distinguish between the flame conditions of the various modes of operation, and to detect when a flame is missing.

According to another embodiment, the controller 108 is configured to detect, on the basis of the sensor signal, only the presence or absence of a flame, for the purpose of controlling a safety procedure in which the system 600 is shut down upon detection of an absence of a flame in the system. In this embodiment, there is no requirement that the sensor 602 be capable of providing different signal values according to the size, distance, or intensity of a flame. Instead, the sensor 602 need only be capable of providing separate signals or ranges of signals corresponding, respectively, to a first condition, in which no flame is present in the system 600, and a second condition, in which any flame is present in the system 600.

FIGS. 7A and 7B are diagrammatic representations of a combustion system 700, according to an embodiment. The combustion system 700 is similar in many respects to the system 100 of FIGS. 1A-1 C, and operates according to similar principles. The system 700 includes an igniter assembly 702 that differs from the igniter assembly 106 of previous embodiments in that it includes, according to an embodiment, a single pilot nozzle 704, and a flame sensor 706 positioned on a side of the perforated flame holder 102 opposite the main fuel nozzle 104. As indicated above, the presence of flame sensors and a controller is optional in that the system 100 can be operated manually. Moreover, the position of the flame sensor 706 may be varied according to system geometry and/or operating conditions. The inventors have successfully monitored operation of the perforated flame holder 102 from both the output (e.g., as shown in FIG. 7A) and input sides (e.g., as shown in FIGS. 1A-1C) of the perforated flame holder 102.

The pilot nozzle 704 is configured to emit a pilot fuel stream 128 along a longitudinal axis A ₃ that lies, with respect to a longitudinal axis A _N of the main nozzle 104, at a nozzle angle N ₃ , which is variable. According to an

embodiment, the pilot nozzle 704 includes a nozzle outlet element 708 that is configured to rotate about a pivot, and further includes an actuator 710 that is configured to control the nozzle angle N ₃ by controlling the position of the nozzle outlet element 708.

FIG. 7A shows the combustion system 700 in a preheat mode of operation. The nozzle outlet element 708 lies at an angle N ₃ , as controlled by the controller 108 via the actuator element 710, and emits a pilot fuel stream 128 that supports a pilot flame 130. The pilot flame 130 contacts the main fuel stream 124 at a point that lies a distance D ₅ , axially, from the outlet of the main nozzle 104. The pilot flame 130 ignites and holds a preheat flame 132 that is supported by the main fuel stream 124, and that fully ignites the main fuel stream 124.

When the perforated flame holder 102 reaches the start-up temperature, the controller 108 is configured to shift the system 700 to a transition mode of operation, as shown in FIG. 7B. The controller 108 is configured to control the actuator element 710 to rotate the nozzle outlet element 708 to a new angle N ₃ , that is smaller than the previous angle N ₃ . As a result, the point at which the pilot flame 130 contacts the main fuel stream 124 moves away from the main nozzle 104, to lie a distance D ₆ from the nozzle 104, where it ignites the preheat flame 132. Primarily because the pilot fuel stream 128 holds the preheat flame 132 much closer to the flame holder 102, the preheat flame 132 cannot propagate laterally across the entire main fuel stream 124, but instead occupies a smaller portion, and only partially igniting the fuel stream 124, permitting a portion 124a of the main fuel stream 124 to reach the flame holder 102.

According to another embodiment, the nozzle angle N ₃ of the pilot nozzle 704 is fixed, but the pilot nozzle 704 itself is configured to translate along a line that is substantially parallel to the longitudinal axis A _N of the main fuel nozzle 104. The controller 108 is configured to control movement of the pilot nozzle 704 along the line between a first position, in which the pilot flame 130 contacts the main fuel stream 124 at a point from which the preheat flame 132 can fully ignite the main fuel stream 124, and a second position, in which the pilot flame 130 contacts the main fuel stream 124 at a point from which the preheat flame 132 cannot fully ignite the main fuel stream.

According to an embodiment, the flame sensor 706 can include a thermal imaging camera positioned and focused so that the thermal imaging camera detects temperatures at the output face 214 of the perforated flame holder 102. The controller 108 is configured to interpret image data from the flame sensor 706 to determine approximate temperatures and temperature changes at the output face 214 of the flame holder 102. Various conditions produce respective different temperature patterns that are detectable at the output face 214. For example, at the beginning of a preheat process, assuming that the combustion system 700 is cold, ignition of the pilot flame 130 manifests as a warm spot at the output face 214 where heated gas from the relatively small pilot flame 130 rises through the flame holder 102.

In embodiments that employ multiple pilot flames 130, such as those described with reference to FIGS. 1A-1C, 5, and 6, corresponding multiple warm spots are produced at the output face 214. Thus, the controller 108 can be configured to confirm ignition of each pilot flame 130 prior to introducing the main fuel stream 124. When the main fuel stream 124 begins to flow, and the preheat flame 132 is ignited, much higher temperatures show at the output face 214, as high volumes of hot gas from the preheat flame 132 pass through the flame holder 102. The temperature continues to rise as a large portion of the flame holder 102 is directly heated by the preheat flame 132. Because the controller 108 is able to monitor the temperature of the output face 214, it can determine when the temperature of the flame holder 102 is at or above the start-up temperature; a separate temperature sensing device is not required. Once the controller 108 moves into the transition mode— as illustrated in FIG. 7B— and the main combustion reaction 126 is ignited in the first region Ri of the flame holder 102, the first region Ri of the flame holder 102 continues to grow hotter, while the second region R ₂ warms more slowly, confirming the presence of the main combustion reaction 126. Finally, when the reduced preheat flame 132 is extinguished, the second region R ₂ of the flame holder 102 comes up to the temperature of the first region R-i, so that the temperature across the output face 214 becomes fairly consistent.

A loss of flame in the system 700 is also readily detectable by the controller 108. In the absence of a flame, the high-velocity flow of the fuel stream 124 as it passes through the flame holder 102 quickly cools the flame holder 102. Even if a flame remains burning in a portion of the flame holder 102 while another portion loses the flame, this is only a danger if unburned fuel actually escapes through the flame holder 102. If for example, contamination or foreign deposits on the flame holder 102 result in sections being blocked, preventing a flow of combustion components and products, the same blockage will likewise prevent the passage of unburned fuel. Without the cooling effect of the passage of gases, the blocked portions will show only a small drop in temperature, relative to the surrounding portions. On the other hand, any sizeable quantity of unburned fuel that escapes through the flame holder 102 will cause a significant drop in temperature at that location, which is instantly detectable by the controller 108. The controller 108 can be configured to shut down the system 700 if, during normal operation, the average temperature at the output face 214 drops below a selected temperature, or if any portion of the output face 214 drops below a lower threshold temperature, such as, e.g., the start-up temperature. Specific temperatures that might be observed at the output face 214 while the system 700 is in the various modes of operation will vary according to factors such as, e.g., volume and velocity of the pilot fuel stream 128 and the main fuel stream 124, the distance between the pilot flame 130 and the input face 212 of the flame holder 102, and likewise between the preheat flame 132 and the input face 212, the relative and absolute sizes of the first and second regions R-i , R ₂ of the flame holder 102, dimensions and material of the flame holder 102, the thermal load of the system, etc. According to an embodiment, the temperatures are determined, and the programming/configuring of the controller 108

established, during initial operation and testing of the system 700. According to another embodiment, empirical data, collected from combustion systems having similar design parameters, are used to program/configure the controller 108 prior to initial operation of the system 700.

FIG. 8 is a flow chart illustrating a start-up process 800 for a combustion system, according to an embodiment. The start-up process 800 can be performed in various configurations of combustion systems, and is particularly suited for use in systems like those described above with reference to FIGS. 1A- 1 C, 5, 6, 7A, and 7B.

Beginning at step 802, a start-up command is issued, after which, at step 804, a flow of fuel is admitted to a nozzle, which emits a main fuel stream toward a main flame holder. At step 806, a preheat flame is ignited and held in the main fuel stream, the heat of which is used to preheat the main flame holder. The preheat flame is controlled to fully ignite the main fuel stream, so that none of the main fuel stream reaches the main flame holder.

In step 808, the temperature T _F H of the main flame holder is compared to a start-up temperature T _s . If the flame holder T _F H is below the start-up

temperature T _s , the process loops back to the previous step 806, and the process loops until the flame holder T _F H is at least equal to the start-up temperature T _s , at which point the process proceeds to step 810. In step 810, the preheat flame is controlled to ignite only a portion of the fuel stream. This permits another portion of the fuel stream to move past the preheat flame and reach the main flame holder, without extinguishing the preheat flame. In step 812, the main combustion reaction is ignited when the unburned fuel reaches the preheated flame holder. In step 814, the presence of a flame at the main flame holder is verified. If no flame is present, the process loops back through the ignite step 812. Once a flame is verified in step 814, the preheat flame is released, in step 816. At this point, substantially the entire main fuel stream reaches the main flame holder, permitting the main combustion reaction to fully ignite across the entire flame holder, in step 818. The start-up procedure is then complete, and, in step 820, normal system operation is begun.

Depending upon the specific configuration of the combustion system, some of the steps of the start-up process 800 can be performed in different ways. For example, in step 806, according to an embodiment, first and second pilot flames are ignited in positions on different sides of the main fuel stream, where they ignite and hold the preheat flame to fully ignite the main fuel stream. Then, in step 810, to control the preheat flame to only partially ignite the main fuel stream one of the first and second pilot flames is extinguished. The position of at least the other of the pilot flames is selected to hold the preheat flame at a location where it cannot propagate all the way across the main fuel stream before it is carried by the fuel stream to the flame holder.

This embodiment of the process 800 can be employed, for example, with combustions systems similar to the embodiments of FIGS. 1A-1C, 5, and 6.

According to another embodiment, in step 806, a pilot flame is held adjacent to the main fuel stream in a position where it ignites and holds the preheat flame at a location where it can fully ignite the main fuel stream. Then, in step 810, the pilot flame is moved to a new position where it continues to hold the preheat flame. However, in the new position, the pilot flame holds the preheat flame at a location where it cannot fully ignite the main fuel stream. This embodiment of the process 800 is suitable for use with the combustion system 700 of FIGS. 7A and 7B, for example.

According to another alternative embodiment, a first pilot flame is positioned to hold the preheat flame at a location where it can fully ignite the main fuel stream, while a second pilot flame is positioned to hold the preheat flame at a location where it cannot fully ignite the main fuel stream. During execution of step 806, the first pilot flame ignited, then, in step 810, the second pilot flame is ignited and the first pilot flame is extinguished.

Referring back to the process 400 of FIG. 4, it will be recognized that the startup procedure 800 of FIG. 8 essentially incorporates the steps of the start-up procedure 402, while ensuring that a flame is always present during start-up. Thus, according to an embodiment, a process is provided for operating a combustion system, which essentially follows the steps of the process 400, but in which the steps of the start-up process 800 are used in place of the start-up procedure 402, and then omitting step 820 and proceeding from step 818 of

FIG. 8 to step 410 of FIG. 4, and thence through the remaining steps of the process 400.

Various elements are described with reference to one or more of the disclosed embodiments. However, in many cases, this is a matter of

convenience. The inventors envision many different embodiments, some including combinations of elements described as parts of separate embodiments. For example, a single flame sensor is described with respect to the embodiment of FIG. 5, which also includes a particular configuration of the igniter assembly. But the inventors also contemplate embodiments in which the single flame sensor is incorporated with a system having an igniter that is configured differently from those described herein. Likewise, an embodiment is

contemplated that includes a structure like that of FIG. 5, except that a different sensor configuration is employed.

For convenience and clarity, embodiments shown in the drawings are similarly oriented and described. For example, in any drawing that shows a fuel nozzle/flame holder combination, the flame holder 102 is positioned above the nozzle in the drawing, with a longitudinal axis of the nozzle oriented vertically. This is not to be construed as suggesting or requiring that a physical embodiment should necessarily have a specific orientation. Many embodiments have been constructed and tested, in all manner of orientations, and found to be fully functional. The claims are not limited by the manner in which particular embodiments are represented.

In the embodiments disclosed and described herein, one or more pilot flames 130 are used as a flame holder, to hold a preheat flame 132 at a selected location, relative to a main fuel stream 124 and a perforated flame holder 102. According to other embodiments, principles disclosed above are practiced using alternative flame holding structures and/or methods.

For example, according to an embodiment, a first electrode is positioned adjacent to the main fuel stream 124 at a first distance, axially, from the main fuel nozzle 104, and a second electrode is positioned adjacent to the main fuel stream 124 at a second distance, greater than the first distance, from the main fuel nozzle 104. While the combustion system is operating in a preheat mode, an electrical charge is applied to the main fuel stream 124, and a complimentary electrical potential is applied to the first electrode. Consequently, a pilot flame 130 is held in the main fuel stream 124 in a position near the first electrode, and fully igniting the main fuel stream 124.

When moving to a transition mode of operation, the system is configured to remove the complimentary electrical potential from the first electrode, and apply the potential to the second electrode, causing the pilot flame 130 to be held in the main fuel stream 124 in a position near the second electrode. From this position, the pilot flame 130 cannot fully ignite the flame, so that a portion 124a of the main fuel stream 124 passes to the perforated flame holder 102, as

described elsewhere. Thus, operation of this embodiment is functionally similar, in many respects, to the operations described with reference to the systems of FIGS. 6 and 7A, 7B in which a degree of ignition of the main fuel stream 124 by the preheat flame 132 is controlled by controlling the position of the pilot flame 130 along the longitudinal axis. 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. 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.