FLARE STACK WITH PERFORATED FLAME HOLDER

Abstract

A flare stack burner includes a fuel stack housing in which a fuel nozzle and a perforated flame holder are positioned. The fuel nozzle is configured to emit a fuel stream toward the 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 (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 (H₂), and methane (CH₄). In another application the fuel can include natural gas (mostly CH₄) or propane (C₃H₈). In another application, the fuel can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant can include 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% 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, 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 (CO₂), water vapor (H₂O), 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.

Claims

1. A device, comprising:

a housing including an inlet configured to be coupled to a waste fuel supply as part of a flare stack, and an outlet configured to release products of combustion to the atmosphere;

a perforated flame holder positioned inside the housing and 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; and

a nozzle configured to receive a flow of fuel from the inlet, and to emit a fuel stream toward the first face of the perforated flame holder.

2. The device of claim 1, wherein the housing is configured to be freestanding, supported only by a coupling at the inlet.

3. The device of claim 1, wherein the housing is configured to be coupled to the flare stack and to be supported thereby.

4. The device of claim 1, wherein the nozzle is coupled to receive the flow of fuel via a fuel line extending through the inlet into the housing.

5. The device of claim 1, wherein the nozzle is one of a plurality of nozzles, each configured to receive a flow of fuel from the inlet, and to emit a fuel stream toward a respective portion of the first face of the perforated flame holder.

6. The device of claim 5, wherein each of the plurality of nozzles is coupled to receive the respective flow of fuel via a common fuel line extending through the inlet into the housing.

7. The device of claim 6, further comprising a plurality of fuel valves operatively coupled between the common fuel line and a respective one of the plurality of nozzles and configured to independently control operation of the respective nozzle.

8. The device of claim 1, further comprising:

a start-up burner positioned within the housing; and

a retrofit burner positioned within the housing, the retrofit burner including the perforated flame holder and the nozzle.

9. The device of claim 8, wherein the start-up burner includes a plurality of burner nozzles arranged in an array within the housing.

10. The device of claim 9, wherein the nozzle is one of a plurality of nozzles of the retrofit burner, interleaved among the array of burner nozzles of the start-up burner.

11. The device of claim 10, wherein the start-up burner is a fin-tube burner, and wherein the plurality of nozzles of the retrofit burner is interleaved among fin plates of the fin-tube burner.

12. The device of claim 8, wherein the start-up burner is configured to support a flame between the start-up burner and the perforated flame holder of the retrofit burner.

13. The device of claim 1, wherein the nozzle includes an aperture having a size that is variable.

14. The device of claim 13, wherein the nozzle is configured to regulate a velocity of the fuel stream.

15. The device of claim 13, further comprising an actuator operatively coupled to the nozzle and configured to control the size of the aperture.

16. The device of claim 13, wherein the nozzle includes a nozzle outlet and a control element, the control element being positioned to occlude some portion of the nozzle outlet, and wherein movement of the control element varies a degree to which the nozzle outlet is occluded by the control element.

17. A method, comprising:

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.

18. The method of claim 17, wherein the combusting waste gas comprises emitting a fuel stream that includes the waste gas from a nozzle positioned within a flare stack and toward the perforated flame holder.

19. The method of claim 18, further comprising, prior to performing the combusting waste gas substantially within a plurality of perforations extending through the perforated flame holder, preheating the perforated flame holder by operating a start-up burner positioned within the flare stack.

20. The method of claim 19, comprising, after performing the preheating the flame holder:

shutting off a flow of fuel to the start-up burner; and

introducing a flow of fuel to the nozzle.

21. The method of claim 18, wherein the emitting a fuel stream from the nozzle positioned within the flare stack is comprised by emitting a fuel stream from each of a plurality of nozzles positioned within the flare stack, each toward a respective portion of the perforated flame holder.

22. The method of claim 21, wherein the emitting a fuel stream from each of the plurality of nozzles positioned within the flare stack comprises selecting a number of the plurality of nozzles based upon a volume of waste gas to be combusted.

23. The method of claim 22, wherein the selecting a number of the plurality of nozzles based upon the volume of waste gas to be combusted comprises varying the number of the plurality of nozzles in response to changes in the volume of waste gas.

24. The method of claim 18, comprising varying an aperture size of the nozzle in response to changes in a volume of waste gas to be combusted.

25. The method of claim 24, wherein the varying an aperture size of the nozzle comprises varying a degree of occlusion of an outlet of the nozzle.

26. The method of claim 17, wherein the perforated flame holder is positioned within a flare stack; and

further comprising venting products of the combustion to the atmosphere.