Patent Publication Number: US-2016245509-A1

Title: Flare stack with perforated flame holder

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority benefit from U.S. Provisional Patent Application No. 62/117,887, entitled “FLARE STACK WITH PERFORATED FLAME HOLDER,” filed Feb. 18, 2015 (docket number 2651-257-02); which, to the extent not inconsistent with the disclosure herein, is incorporated by reference. 
    
    
     BACKGROUND 
     Flare stacks are used to burn off vented volatile organic compounds. For example, in an oil refinery, a flare stack may be used to provide emergency burning of volatile compounds, or provide for a safe way to relieve high sudden pressure events of flammable materials. In an oil field, a flare stack may be used to burn off natural gas that is produced as a byproduct of crude oil production. In a landfill, a flare stack may be used to burn off methane released by decomposition processes. Because volatile compounds are considered pollutants and are often flammable, it is generally considered preferable to burn the volatile compounds, rather than to vent the volatile compounds directly to the atmosphere. In flare stack applications, it can be important to control the height of a flame envelope created by the burner. In some applications, especially those known by the term of art “enclosed flares,” it may be required or desired that the flame not exceed the height of the flare stack itself. By keeping the flame inside the flare stack, safety may be improved. Moreover, aesthetics may be improved sufficiently to avoid complaints about a visible flame. 
     Enclosed flare stacks or ground flares can be used for burning off unusable waste field gas in a variety of oil and gas production applications, for example. Waste gases may be released during over-pressuring of plant equipment. The waste gases may be transported to a corresponding ground flare. Some ground flares are enclosed. By “enclosed” it is meant that a flame envelope is substantially blocked from view by persons outside a controlled access area. 
     Flame length may determine a required height, girth, or other dimensions of the ground flare structure. A problem may arise when the flame becomes visible (e.g., is too high). Excessively high flame length may substantially halt operation, and/or may result in fines or be expressed as greater capital cost, increased operating expenses, and/or other remediation expenses. 
     SUMMARY 
     According to an embodiment, a device includes a housing including an inlet configured to be coupled to a waste gas supply as part of a flare stack, and an outlet configured to release products of combustion to the atmosphere. A perforated flame holder is positioned inside the housing, the perforated flame holder having a first face, a second face lying opposite the first face, and a plurality of perforations extending through the perforated flame holder between the first and second faces. A nozzle is configured to receive a flow of waste gas from the inlet and emit a waste gas stream toward the first face of the perforated flame holder. The perforated flame holder is configured to support combustion of the waste gas substantially within the plurality of perforations. 
     According to an embodiment, a method includes outputting a waste gas and supplemental fuel sufficient to raise a heating value of the waste gas plus supplemental fuel to about 100 BTU per cubic foot or less toward a perforated flame holder; and combusting the waste gas and supplemental fuel substantially within a plurality of perforations extending through the perforated flame holder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a flare stack with a perforated flame holder, according to an embodiment. 
         FIG. 2  is a simplified perspective view of a burner system including a perforated flame holder, according to an embodiment. 
         FIG. 3  is a side sectional diagram of a portion of the perforated flame holder of  FIGS. 1 and 2 , according to an embodiment. 
         FIG. 4  is a flow chart showing a method for operating a burner system including the perforated flame holder of  FIGS. 1, 2 and 3 , according to an embodiment. 
         FIG. 5  is a diagrammatic side-sectional view of a portion of a flare stack that includes a perforated flame holder substantially as described with reference to  FIGS. 2 and 3 , according to an embodiment. 
         FIG. 6  is a diagrammatic side-sectional view of a portion of a flare stack, according to another embodiment. 
         FIG. 7  is a diagrammatic side-sectional view of a portion of a flare stack, according to an embodiment, that includes a retrofit burner installed in a pre-existing flare stack. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. 
       FIG. 1  is a diagram of a flare stack  100  with a perforated flame holder  102 , according to an embodiment. The flare stack  100  includes a stack structure  104  configured to support a flare. A flare is a combustion reaction that burns off volatile compounds to control pressure in systems such as oil production, oil refining, and other chemical processing systems. A volatile compound source  106  at least intermittently outputs a flow of high vapor pressure flammable compounds. Optionally, a pressure control valve  108  may provide a constant pressure sink to the volatile compound source  106 , and determine a constant pressure flow of volatile compounds to a volatile compound nozzle  110 . For some systems, this can cause a variable flow rate of volatile compounds to the volatile compound nozzle  110 . When volatile compound flow is sufficient, the volatile compound nozzle outputs a stream of volatile compounds. A combustion air source  112 , such as a damper or a blower, provides combustion air. The stream of volatile compounds flows and entrains combustion air to form a volatile compound mixture  114 . A perforated flame holder  102  is supported by a perforated flame holder support structure  222  at a position selected to receive the volatile compound mixture  114 . As described elsewhere herein, a combustion reaction supported at least partially by the volatile compound mixture can be held by the perforated flame holder  102 . 
     According to an embodiment, a temperature-maintenance fuel nozzle  116  is configured to output a start-up flame or a temperature-maintenance fuel and air mixture  118  to establish or maintain an operating temperature of the perforated flame holder  102  using fuel from a fuel source  120 . 
     Because flow from the volatile compound source  106  can be intermittent or at least non-steady, the temperature-maintenance fuel nozzle  116  can be configured to cooperate with the fuel source  120  to provide a relatively high fuel and air mixture  118  flow rate when the volatile compound mixture  114  flow rate is low and provide a relatively low or zero fuel and air mixture  118  flow rate when the volatile compound mixture  114  flow rate is high. 
     According to an embodiment, a controller  122  is operatively coupled to a flow sensor  124  configured to measure flow of volatile compounds from the volatile compound source  106 . The controller can use digital logic to determine a corresponding flow rate appropriate for the fuel and air mixture  118 , and control a fuel flow valve  126  to provide a selected flow rate of fuel from the fuel source  120  to the temperature-maintenance fuel nozzle  116 . 
     The temperature-maintenance fuel nozzle  116  can include a fuel riser  128  and an ignition source  129  configured to ignite a start-up flame near the temperature-maintenance fuel nozzle. The ignition source  129  can include a hot surface igniter, a spark-discharge igniter, or a pilot flame, for example. Additionally or alternatively, the ignition source  129  may include a flame holder operable to hold a flame at a location proximate to the fuel riser  128 . The flame holder may be configured to be actuated to selectively hold a flame at the location proximate to the fuel source or to allow fuel from the fuel source to travel to the perforated flame holder  102  for combustion. In such an embodiment, the ignition source  129  may additionally include a separate igniter or alternatively the fuel riser  128  may be manually ignited at start-up. 
     According to an embodiment, when the ignition source  129  or actuatable flame holder is enabled, a start up flame is supported between the temperature-maintenance fuel nozzle  116  and the perforated flame holder  102 . When the ignition source  129  or flame holder is not enabled, the temperature-maintenance fuel nozzle  116  outputs a flow of the fuel and air mixture  118  to the perforated flame holder  102  for combustion in the perforated flame holder  102 . The controller  122  can be operatively coupled to the ignition source  129  to determine whether a start-up flame is supported or whether the fuel and air mixture  118  is delivered to the perforated flame holder  102  for combustion. 
     According to an embodiment, the temperature-maintenance fuel nozzle  116  can be configured to add a relatively high BTU-content fuel, such as propane or natural gas, to a relatively low BTU-content fuel from the volatile compound nozzle  110 . For continuous flow operations, the “temperature maintenance” performed by the temperature-maintenance fuel nozzle  116  may consist essentially of increasing the BTU content of the combustible materials (fuel plus volatile compound) delivered to the perforated flame holder  102 . 
     In one experiment, it was found that use of the perforated flame holder  102  could reduce the necessary BTU content of methane fuel plus volatile compound mixture from 300 BTUs per cubic foot to below 100 BTUs per cubic foot while maintaining steady combustion, compared to burning the volatile compound in a conventional flame. The capabilities of the perforated flame holder  102  can thus be used to advantage in many waste gas burn-off applications, whether or not in a contained flame flare stack, and can result in significant fuel cost savings. 
     During times when substantially no volatile compounds are output from the volatile compound source  106 , the controller  122  can cause the flare stack  100  to operate in a “cold standby” state, where minimal or no fuel from the fuel source  120  is consumed, and the fuel control valve  126  is maintained in an “off” position. Optionally, the system  100  can include a volatile compound flow valve  130  operatively coupled to the controller  122 . The controller can hold the volatile compound flow valve  130  in an off state whenever the flare stack  100  is in a cold standby state. 
     When an imminent volatile compound flow is detected (e.g., by a pressure sensor (not shown) or the volatile compound flow sensor  124 , the controller can convert the flare stack  100  to a “warm standby” state, wherein the fuel control valve  126  is opened sufficiently, and the ignition source  129  enabled to support a start-up flame. The system  100  can change from a warm standby state to a “hot standby” state when the temperature of the space between the volatile compound nozzle  110  and the perforated flame holder  102  is warmed by the start-up flame to a sufficiently hot temperature to ensure complete combustion of the volatile compound. In the hot standby state, the flare stack  100  can operate as a normal flare stack with volatile compound flaring occurring in a conventional flame below the perforated flame holder. In some cases, the volatile compound is itself a fuel of sufficient heating value to provide a continual flame without any additional or supporting fuel. In other cases, a supplemental fuel is required to raise the heat value of the (supplemental) fuel plus volatile compound. In still other cases, it is more proper to call the fuel an ignition fuel inasmuch as the volatile compound has sufficient heating value to maintain the combustion reaction, but for reasons of safety or convenience, it is preferable to have a fuel of known composition and pressure available as an ignition source. Unless noted, the term fuel may function in any of these senses. 
     When the perforated flame holder  102  is warmed to a start-up temperature, the controller can disable the ignition source  129  to lift from the start-up flame location and cause the fuel/air mixture  118  to impinge on the perforated flame holder  102 , wherein combustion is held. Simultaneously, with no ignition by the start-up flame, the volatile compound/air mixture  114  travels to the perforated flame holder  102  wherein the volatile compound is combusted. 
     When the volatile compound flow rate is sufficiently high to maintain combustion in the perforated flame holder  102 , the controller  122  can reduce the fuel flow rate or stop fuel flow using the fuel control valve  126 . Optionally, the controller  122  can include a proportional controller configured to maintain a fuel mixture  118  flow rate that is inversely proportional to the volatile compound mixture  114  flow rate. 
     Optionally, the controller can be operatively coupled to control the combustion air source  112 . Optionally, the controller can be operatively coupled to a sensor  132  configured to sense combustion, temperature, or other parameter related to performance of the flare stack  100 . 
       FIG. 2  is a simplified diagram of a burner system  200  including a perforated flame holder  102  configured to hold a combustion reaction, according to an embodiment. As used herein, the terms perforated flame holder, perforated reaction holder, porous flame holder, porous reaction holder, duplex, and duplex tile shall be considered synonymous unless further definition is provided. Experiments performed by the inventors have shown that perforated flame holders  102  described herein can support very clean combustion. Specifically, in experimental use of systems  200  ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (O 2 ) 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 (C 3 H 8 ). 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 ⅓ and ½ 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  1100  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, S.C. 
     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% O 2 , i.e. an equivalence ratio of ˜0.87. Use of even leaner mixtures is possible, but may result in elevated levels of O 2 . 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, T S . 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 diagrammatic side-sectional view of a portion of a flare stack  500 , according to an embodiment, that includes a perforated flame holder  102  substantially as described with reference to  FIGS. 2-3 . The flare stack  500  can include a housing  502  in which a flare burner  504  is positioned. The housing can enclose the combustion volume  204 , and includes an inlet  506 , an outlet  508 , and vent louvers  510 . 
     The flare burner  504  can include the perforated flame holder  102  and a plurality of fuel nozzles  218  configured to produce fuel streams  206  directed toward respective portions of the input face  212  of the flame holder. A fuel line  512  can extend into the housing  502  via the inlet  506 , and is coupled to the plurality of fuel nozzles  218  and configured to deliver fuel to the nozzles. 
     During operation, fuel, such as, for example, waste natural gas from an oil well, may be introduced via the fuel line  512  to the plurality of fuel nozzles  218 , which emit respective fuel streams  206  toward the perforated flame holder  102 . A combustion reaction  302  can be supported by the fuel streams  206  and held substantially within perforations  210  of the flame holder  102 . Products of the combustion, such as, for example, heated air, carbon dioxide (CO 2 ), water vapor (H 2 O), etc., exit the housing  502  via the outlet  508 , whence they are dispersed in the atmosphere. Because the combustion reaction  302  is substantially contained within the perforations  210  of the flame holder  102 , no flames are visible outside the housing. 
     As shown in  FIG. 5 , the flare stack  500  can be positioned at the top of a pole or stack, which serves to distribute the combustion products into the atmosphere at a height that allows them to dissipate 
     Two fuel nozzles  218  are shown in the embodiment of  FIG. 5 . However, this is provided merely as an example. According to an embodiment, a flare stack  500  is provided, employing a single fuel nozzle. According to another embodiment, a flare stack is provided that includes a larger number of fuel nozzles. For example,  FIG. 7  shows a retrofit flare stack that includes an array of fuel nozzles, as described below in detail. According to another embodiment, the flare stack  500  of  FIG. 5  includes a similar array of fuel nozzles. 
       FIG. 6  is a diagrammatic side-sectional view of a portion of a flare stack  600 , according to an embodiment, that is similar in many respects to the embodiment of  FIG. 5 . The flare stack  600  includes a flare burner  602  that includes a fuel nozzle  218  with a variable aperture  606 . The fuel nozzle  218  can include a control element  604  and a nozzle outlet  610 . The control element  604  can be coupled to an actuator element  608  that can be configured to move the control element vertically, thereby regulating the degree to which the control element occludes the nozzle outlet  610 . 
     The size of the fuel nozzle aperture  606  may correspond to the area of the opening, as viewed in transverse section, through which the fuel stream  206  exits the fuel nozzle  218 . In embodiments that include conventional fuel nozzles, the size of fuel nozzle aperture  606  is typically substantially equal to the area of the corresponding opening. However, in the embodiment shown in  FIG. 6 , the size of fuel nozzle aperture  606  is equal to the area of the nozzle outlet  610  minus the area of the control element  604  bisected by a plane defined by the smallest diameter of the fuel nozzle outlet  610 . As the actuator  608  moves the control element  604  upward, a larger area of the control element  604  may be bisected, reducing the size of the fuel nozzle aperture  606 . Conversely, as the control element is moved downward, the size of the fuel nozzle aperture  606  may increase. 
     In applications where the fuel supply to a flare stack may vary over time, a fixed fuel nozzle aperture may be problematic. A reduction in the fuel supply can result in a corresponding reduction in fuel stream velocity. As discussed with reference to  FIG. 2 , according to an embodiment, the velocity of the fuel stream  206  is preferably such that it cannot independently support a stable flame between the fuel nozzle and the flame holder. Under certain conditions, if the velocity of the fuel stream is too low, a flame can begin to burn in the fuel stream before it reaches the flame holder, which would interfere with proper operation of the flame holder, and would tend to increase undesirable emissions, such as NO x . In such situations, it would be desirable to increase the velocity of the fuel stream  206 , in order to cause the flame to be held in the perforated flame holder  102 . 
     It is well understood that the velocity of a fluid passing through an opening is a function of the volume of fluid passing per unit of time, and the size of the opening through which it passes. Velocity rises in direct relation to fluid volume, and in inverse relation to the opening size. Thus, with reference to the embodiment of  FIG. 6 , if during operation, the fuel supply drops, tending to reduce velocity of the fuel stream, a corresponding reduction in the size of the fuel nozzle aperture  606  will produce an increase in fuel stream velocity, and vice-versa. 
     During operation, a fuel stream  206  may exit the fuel nozzle  218  and support a combustion reaction  302  within the perforated flame holder  102 , substantially as previously described. If an increase in velocity of the fuel stream is required, such as when a drop in the fuel supply to the fuel nozzle  218  causes a reduction in velocity, the actuator  608  can be controlled to reduce the size of the fuel nozzle aperture  606 , thereby increasing velocity. Likewise, where desired or required, the actuator  608  can be controlled to increase the size of the fuel nozzle aperture  606  to reduce fuel stream velocity. 
     According to an embodiment, the actuator  608  is controlled by a pressure regulator feedback mechanism, in which changes in the fuel supply produce corresponding changes in fuel pressure. The regulator feedback mechanism is configured to respond to these changes by increasing the size of the aperture  218  as fuel pressure increases, and by reducing the size of the aperture as fuel pressure decreases. 
     According to another embodiment, a controller is provided, configured to control the size of the fuel nozzle aperture  606  in response to changes in one or more of fuel pressure, fuel stream velocity, flame temperature, flame position, emission composition, etc. 
     According to a further embodiment, the actuator  608  is configured to be controlled by an operator during operation of the flare stack  600 . 
     As previously noted, the perforated flame holder  102  is typically preheated prior to normal operation. According to another embodiment, the actuator  608  is controlled to reduce fuel stream velocity during a start-up procedure to permit a flame to be supported by the fuel stream  206  between the fuel nozzle  218  and the flame holder  102 , in order to heat the flame holder. Once a portion of the flame holder reaches a selected temperature, the actuator may be controlled to reduce the fuel nozzle aperture  606  and increase fuel stream velocity, causing the flame to rise to the flame holder  102 . 
     In  FIG. 6 , the fuel nozzle  218  is shown as having a separate housing  612  that is configured to be coupled to a stack or pipe, and to which the housing  502  is in turn coupled. According to other embodiments, the fuel nozzle  218  is enclosed within the housing  502  or within the stack, just upstream of the housing. 
     In  FIG. 6 , the perforated flame holder  102  is shown occupying substantially all of the cross sectional area of the flare stack. According to other embodiments, as shown, for example, in  FIG. 5 , the perforated flame holder  102  occupies less than the entire cross sectional area of the flare stack. In some cases, it may be beneficial to configure a flare stack system such that no circulation of gases around the perforated flame holder is permitted, while in other cases, such circulation may be advantageous. Accordingly, the determination of the size and shape of the perforated flame holder, in relation to the housing, is a design consideration. 
     According to an embodiment, the perforated flame holder occupies between ⅔ and 100% of the cross sectional area of the flare stack. According to another embodiment, the perforated flame holder occupies approximately ⅔ of the cross sectional area of the flare stack. According to a further embodiment, the perforated flame holder occupies between ⅓ and ⅔ of the cross sectional area of the flare stack. According to an embodiment, the perforated flame holder occupies the minimum cross sectional area of the flare stack necessary to maintain sufficient combustion of the volatile compound. 
       FIG. 7  is a diagrammatic side-sectional view of a portion of a flare stack  700 , according to an embodiment, that includes a retrofit burner  702  installed in a pre-existing flare stack. In the example shown, the pre-existing flare stack includes a fin-tube burner  704 , which in turn includes a plurality of fuel tubes  706  extending substantially parallel to each other—along axes that lie perpendicular to the plane of the drawing—through transverse-oriented fin plates  708 , one of which is shown. Each of the plurality of fuel tubes  706  can have a respective plurality of fuel nozzles  710  interleaved with the fin plates  708 . In operation, as fuel is ejected from the fuel nozzles  710 , it can entrain air passing between the fin plates  708 , and a gas flare is supported inside the housing  502  and close to the fin-tube burner  704 . 
     According to an embodiment, the retrofit burner  702  includes a plurality of fuel nozzles  218  coupled to a common fuel line  512 . The fuel nozzles  218  are interleaved between fuel tubes  706  of a start-up fin-tube burner  704 . Each of the plurality of fuel nozzles  218  can be configured to provide a fuel stream  206  to a respective portion of the perforated flame holder  102 . Four fuel nozzles  218  are shown in the view of  FIG. 7 , but the plurality of fuel nozzles can include an array of fuel nozzles extending beyond the plane represented in the drawing. 
     According to an embodiment, during a start-up procedure of the flare stack  700 , the fin-tube burner  704  is operated in a mode in which fuel is ejected from the fuel nozzles  710  and a flame is supported below the perforated flame holder  102 , which serves to pre-heat the flame holder  102 . The fuel supply to the fuel tubes  706  is then cut off, and a fuel supply is supplied to the fuel line  512 . Fuel streams  206  are emitted from each of the plurality of fuel nozzles  218 , and a combustion reaction  302  is ignited and held in the perforated flame holder  102 . 
     As discussed above with reference to  FIG. 6 , in some applications, the fuel supply can vary. Thus, according to an embodiment, valves are provided, and configured to individually control flows of fuel to each of the plurality of fuel nozzles  218 . As the fuel supply increases or decreases, a corresponding number of the plurality of fuel nozzles  218  may be brought online or shut down, as necessary. According to an embodiment, the fuel supply to each of the nozzles is controlled so that, when additional fuel nozzles are to be brought online, only fuel nozzles that are immediately adjacent to currently operating nozzles are activated. Heat from combustion supported by the adjacent fuel nozzles will enable a newly activated fuel nozzle to come up to normal operation very quickly, avoiding extended warm-up time during which unburned fuel might pass through the flame holder. 
     Embodiments are described and shown in a stack configuration, i.e., a configuration in which the respective systems are supported some distance above the ground. However, other embodiments are envisioned, in which similar structures are positioned on the ground. 
     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.