Patent Application
20170307212
Combustion systems are widely employed throughout society. There is a continual effort to improve the efficiency and reduce harmful emissions of combustion systems.
In an embodiment, a combustion system includes a fuel nozzle, an oxidant supply, and a perforated flame holder. The fuel nozzle is configured to emit fuel in a fuel stream via at least one fuel orifice. The oxidant supply is configured to output oxidant in an oxidant flow adjacent to the fuel nozzle. The perforated flame holder is disposed a distance from the fuel nozzle and the oxidant source, and is oriented to receive mixed fuel and oxidant from the fuel nozzle and the oxidant supply. The perforated flame holder is configured to hold a combustion reaction of the mixed fuel and oxidant when the perforated flame holder is at an operating temperature. The fuel nozzle is defined by a tapered tip that decreases in width from a nozzle base to a nozzle end, where the decrease in width limits mixing of the fuel and the oxidant proximate the fuel nozzle.
According to an embodiment, a burner system includes a perforated flame holder and a fuel nozzle assembly. The perforated flame holder has an input face, an output face opposite the input face, and a plurality of perforations. The perforations are arranged between the input face and the output face. Each perforation is arranged to receive a portion of a fuel and oxidant mixture at the input face and may support a combustion reaction while the perforated flame holder is at an operating temperature. The fuel nozzle assembly includes a tapered fuel nozzle, which in turn includes an airfoil section and fuel orifices. The airfoil section has a toroidal structure arranged substantially perpendicular to an airflow direction. The airfoil section also has an acute trailing edge that is oriented in the airflow direction. The fuel orifices are arranged along the acute trailing edge.
According to another embodiment, a burner system includes a perforated flame holder and one or more tapered fuel nozzles. The perforated flame holder is arranged to receive a mixture of fuel and oxidant respectively from a fuel source and an oxidant source. The tapered fuel nozzles each have a circumferentially symmetric body that tapers from an attachment region to an acicular tip. The acicular tip is oriented toward the perforated flame holder, and at least one of the one or more tapered fuel nozzles is structured to contribute to a swirl number no greater than about 0.6 for oxidant flowing past. This limits formation of a heat-recirculating vortex proximate the respective tapered fuel nozzle.
According to an embodiment, a method for operating a burner system includes preheating a perforated flame holder, and delivering oxidant and fuel via structure that limits formation of fuel and oxidant mixing vortices proximate at least one main fuel nozzle. The oxidant is delivered to the perforated flame holder in an oxidant flow. The fuel is delivered to the perforated flame holder via a fuel stream from a fuel delivery orifice of the at least one main fuel nozzle, with the fuel stream being adjacent to the oxidant flow at least proximate to the at least one main fuel nozzle. The main fuel nozzle has a tapered structure that includes an acute trailing edge or tip, and the tapered structure limits generation of fuel and air vortices proximate the main fuel nozzle.
According to an embodiment a method for operating a burner system includes delivering oxidant and fuel to a perforated flame holder. The oxidant is delivered via an oxidant conduit, while the fuel is delivered via a tapered fuel nozzle, where the tapered fuel nozzle has one or more angled fuel orifices. The fuel becomes swirled by the angled fuel orifices. The fuel nozzle may include an oxidant swirling feature that engages oxidant flowing past the nozzle from the oxidant conduit.
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. Other embodiments may be used and/or other changes may be made without departing from the spirit or scope of the disclosure.
Fuel nozzles in a burner of a combustion system conventionally have a blocky structure. According to an interpretation, this blocky structure may contribute to formation of vortices downstream of the fuel nozzle. For example, according to an interpretation, oxidant flowing past the fuel nozzle may provide a pressure differential between the oxidant flow and a region directly downstream from the conventional, blocky fuel nozzle. The pressure differential may result in movement of fluid from a high pressure area to a low pressure area, which may result in vortices. These vortices may cause earlier than desired mixing of the fuel and oxidant, and may slow flow of the fuel and oxidant mixture downstream of the fuel nozzle. These effects can contribute to initiation or maintenance of a combustion reaction (e.g., a flame) in close proximity to the fuel nozzle. In conventional combustion systems, formation of a combustion reaction close to the fuel nozzle may be desirable or may be tolerated with minimal or no significant penalty.
However, combustion systems that implement a flame holder disposed a distance away from the fuel nozzle, such as a perforated flame holder (PFH), are intended to hold the combustion reaction substantially within the flame holder. Combustion of fuel and oxidant between the fuel nozzle and flame holder may decrease efficiency of the combustion system and is thus typically undesirable. Embodiments of fuel nozzles are described below that address the above-described problematic formation of vortices at the fuel nozzle(s).
A perforated flame holder 102 must be heated to an operating temperature range before its own radiant heat can maintain a stable combustion reaction. Various methods and structures are contemplated to initially attain such operating temperature. In an embodiment, a burner system may include multiple flame holders. A proximal flame holder may hold a flame between a fuel orifice and a distal flame holder. The proximal flame holder may be selectively engaged to hold a flame during a finite period until the distal flame holder reaches the operating temperature, and then may be removed when the operating temperature of the distal flame holder is reached. The proximal flame holder may be removed physically or virtually. For example, the proximal flame holder may be a physical object, such as a bluff body (not shown) that may be mechanically removed. Alternatively, the proximal flame holder may be implemented by an electrical interaction with the fuel and oxidant mixture 112 or products of pilot combustion and may thus be removed by simply changing the electrical characteristics or the fuel and oxidant mixture makeup. In an implementation that includes a dedicated start-up or pilot fuel supply, the pilot fuel nozzle may constitute a proximal flame holder, and may be made inoperative simply by cutting off or reducing fuel and/or oxygen supplied to the proximal flame holder. Although this embodiment describes use of two flame holders (i.e., distal, proximal), it is recognized that additional flame holder stages may be implemented. An alternative preheating configuration is described below with reference to a heater 228 illustrated in
The burner system 100 illustrated in
As shown in
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 100 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 extremes that may be required for conventional burners to even approach such clean combustion.
According to embodiments, the burner systems 100 and 200 include the fuel and oxidant source 202 disposed to output fuel via the fuel nozzle assembly 104 and oxidant via the conduit 110 (shown in
A burner system may include a device for preheating the perforated flame holder 102. For example, in some embodiments, as illustrated in
The ignition and start-up flame source 120 may include a fuel nozzle 122 the shape of which, according to an interpretation, conventionally causes fuel and/or oxidant vortices that mix fuel and oxidant sufficient to support a flame, and may impede release of a flame held proximate the fuel nozzle 122. In other embodiments, the burner system 100, 200 may include between the fuel nozzle assembly 104 and perforated flame holder 102 a permanent or retractable flame holder (not shown) that holds a pilot or start-up flame 206 as described above. The pilot or start-up flame 206 may emit heat from below the perforated flame holder 102 to heat the perforated flame holder 102 to a preheat threshold temperature.
Alternatively, e.g., as described with respect to
The fuel can include hydrogen, a hydrocarbon gas, a vaporized hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel can be a single species or can include a mixture of gas(es), vapor(s), atomized liquid(s), and/or pulverized solid(s). For example in a process heater application, the fuel can include fuel gas or byproducts from the process that include CO, hydrogen (H2), and methane (CH4). In another application the fuel can include natural gas (mostly CH4) or propane (C3H8). In another application, the fuel can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant can include oxygen carried by air and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein.
According to an embodiment, the perforated flame holder body 208 can be bounded by an input face 212 disposed to receive the fuel and oxidant mixture 112, 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 112 at the input face 212. The fuel and oxidant mixture 112 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 during sustained operation. For example, on a steady-state basis, more than half the molecules of fuel output into the combustion volume 150 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 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. 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 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.
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 in or at the perforated flame holder 102. 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 flashback, or “huffing” wherein a visible flame momentarily ignites in a region lying between the input face 212 of the perforated flame holder 102 and a fuel nozzle assembly 104 or, in other embodiments, between the input face 212 of the perforated flame holder 102 and one or more fuel nozzles 218, within the dilution region DD.
Such transient 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 above 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 (a visible wavelength tail of blackbody radiation) from the perforated flame holder 102.
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 150. As used herein, terms such as thermal radiation, infrared radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody radiation of electromagnetic energy, primarily in infrared wavelengths.
Referring especially to
The perforated flame holder body 208 can be characterized by a heat capacity. The perforated flame holder body 208 may hold heat from the combustion reaction 302 in an amount corresponding to the heat capacity multiplied by temperature rise, and transfer the heat 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 both radiation and conduction heat transfer mechanisms 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 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 occur within thermal boundary layers 314 formed adjacent to walls 308 of the perforations 210. As the relatively cool fuel and oxidant mixture 112 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 112. 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 defines the ends of the perforations 210. At some point, the combustion reaction 302 causes the flowing gas (and plasma) to output more heat to the body 208 than it receives from the body 208. The heat is received at the heat receiving region 306, is held by the body 208, and is transported to the heat output region 310 nearer to the input face 212, where the heat recycles into the cool reactants (and any included diluent) to raise them to the combustion temperature.
In an embodiment, the plurality of perforations 210 are each 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. The reaction fluid includes the fuel and oxidant mixture 112 (optionally including nitrogen, flue gas, and/or other “non-reactive” species), reaction intermediates (including transition states in a plasma that characterizes the combustion reaction), and reaction products.
The plurality of perforations 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 formed 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 (e.g., to 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.
According to an interpretation, the transient flashback or huffing phenomenon is at least in part a result of fuel and oxidant mixing in the dilution region DD sufficient to support combustion. The nozzles described herein limit the mixing of fuel and oxidant near the fuel nozzles by limiting the formation of fuel and/or oxidant vortices that can result from conventional non-aerodynamic fuel nozzles. For example, a flat-topped fuel nozzle provides a low pressure area between a central orifice and a boundary such that when oxidant flows past the nozzle the oxidant is drawn toward the low pressure area, thus causing vortices that can mix the fuel and oxidant. In systems that do not utilize a perforated flame holder this mixing may be desirable. However, in systems that implement a perforated flame holder, the mixing near the fuel nozzles undesirably can support combustion if the mixture is ignited (such as with flashback).
Referring especially to
The perforated flame holder 102 can be held by a perforated flame holder support structure 222 configured to hold the perforated flame holder 102 a distance DD away from the fuel nozzle assembly 104. The fuel nozzle assembly 104 can be configured to emit fuel jets selected to entrain the oxidant to form the fuel and oxidant mixture 112 as the fuel jet and oxidant travel along a path to the perforated flame holder 102 through the dilution distance DD between the fuel nozzle assembly 104 and the perforated flame holder 102. Additionally or alternatively (particularly when a blower or damper 238 is used in delivering oxidant or combustion air), the oxidant or combustion air conduit 220 can be configured to entrain the fuel as the fuel and oxidant travel through the dilution distance DD. In some embodiments, a flue gas recirculation path 224 can be provided. Additionally or alternatively, the fuel nozzle assembly 104 can be configured to emit one or more fuel jets selected to entrain the oxidant and to entrain flue gas as the fuel jet travels through the dilution distance DD between the fuel nozzle assembly 104 and the input face 212 of the perforated flame holder 102. The fuel nozzle 105 may be configured to emit the fuel through one or more fuel orifices 108 having a 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 112 at a distance DD away from the tapered fuel nozzle 105 greater than 20 times the nozzle diameter. In another embodiment, the perforated flame holder 102 is disposed to receive the fuel and oxidant mixture 112 at a distance DD away from the tapered fuel nozzle 105 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 about 200 times the nozzle diameter or more away from the tapered fuel nozzle 105. When the fuel and oxidant mixture 112 travels about 200 times the nozzle diameter or more, the mixture is sufficiently homogenized to cause the combustion reaction 302 to output 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 air channel configured to convey combustion air into the premix chamber. A flame arrestor (not shown) can be disposed between the premix fuel and oxidant source and the perforated flame holder 102 and be configured to prevent flame flashback into the premix fuel and oxidant source.
The combustion air conduit 220, whether configured for entrainment in the combustion volume 150 or for premixing can include a blower or damper 238 configured to force air through the fuel and air source 202 and/or to control an amount of the air being forced through the fuel and air source 202.
The perforated flame holder support structure 222 can be configured to support the perforated flame holder 102 from a floor or wall (not shown) of the combustion volume 150, 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 cementitious, 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 a 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 150. This can allow the flue gas recirculation 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 a combustion volume wall (not shown).
Referring again to
In one range of embodiments, each of the plurality of perforations has a transverse dimension D between 0.05 inch and 1.0 inch. Preferably, each of the plurality of perforations has a transverse dimension D between 0.1 inch and 0.5 inch. For example the plurality of perforations can each have a transverse 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 the 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 from 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.
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 208 can be one piece or can be formed from a plurality of sections.
Referring to
According to an embodiment, the reticulated fibers 339 can include alumina silicate. According to an embodiment, the reticulated fibers 339 can be formed from extruded mullite or cordierite. According to an embodiment, the reticulated fibers 339 can include Zirconia. According to an embodiment, the reticulated fibers 339 can include silicon carbide.
The term “reticulated fibers” refers to a netlike structure. According to an embodiment, the fibers 339 are formed from an extruded ceramic material. In reticulated fiber embodiments, the interaction between the fuel and oxidant, the combustion reaction, and heat transfer to and from the perforated flame holder body 208 functions similarly to the embodiment shown and described above with respect to
According to an embodiment, the network of reticulated fibers 339 is sufficiently open for downstream fibers 339 to emit radiation for receipt by upstream fibers 339 for the purpose of heating the upstream fibers sufficiently to maintain combustion of a lean fuel and oxidant mixture. Compared to a continuous perforated flame holder body 208 (e.g., as in
It will be acknowledged that the fuel nozzle assembly 104 illustrated in
According to an embodiment, individual perforations 210 may extend from an input face 212 to an output face 214 of the perforated flame holder 102. Perforations 210 may have varying lengths L. According to an embodiment, because the perforations 210 branch into and out of each other, individual perforations 210 may not be clearly defined by a length L.
According to an embodiment, the perforated flame holder 102 is configured to support or hold a combustion reaction or a flame at least partially between the input face 212 and the output face 214. According to an embodiment, the input face 212 corresponds to a surface of the perforated flame holder 102 proximal to the fuel nozzle 218 or to a surface that first receives fuel. According to an embodiment, the input face 212 corresponds to an extent of the reticulated fibers 1239 proximal to the fuel nozzle 218. According to an embodiment, the output face 214 corresponds to a surface distal to the fuel nozzle 218 or opposite the input face 212. According to an embodiment, the input face 212 corresponds to an extent of the reticulated fibers 1239 distal to the fuel nozzle 218 or opposite to the input face 212.
According to an embodiment, the formation of boundary layers 314, transfer of heat between the perforated reaction holder body 208 and the gases flowing through the perforations 210, a characteristic perforation width dimension, and the length L can be regarded as related to an average or overall path through the perforated reaction holder 102. In other words, the width dimension can be determined as a root-mean-square of individual width dimension values determined at each point along a flow path. Similarly, the length L can be a length that includes length contributed by tortuosity of the flow path, which may be somewhat longer than a straight line distance from the input face 212 to the output face 214 through the perforated reaction holder 102. According to an embodiment, the void fraction (expressed as (total perforated reaction holder 102 volume−fiber volume)/total volume)) is about 70%.
According to an embodiment, the reticulated ceramic perforated flame holder 102 is a tile about 1″×4″×4″. According to an embodiment, the reticulated ceramic perforated flame holder 102 includes about 10 pores per square inch of surface area. Other materials and dimensions can also be used for a reticulated ceramic perforated flame holder 102 in accordance with principles of the present disclosure.
According to an embodiment, the reticulated ceramic perforated flame holder 102 can include shapes and dimensions other than those described herein. For example, the perforated flame holder 102 can include reticulated ceramic tiles that are larger or smaller than the dimensions set forth above. Additionally, the reticulated ceramic perforated flame holder 102 can include shapes other than generally cuboid shapes.
According to an embodiment, the reticulated ceramic perforated flame holder 102 can include multiple reticulated ceramic tiles. The multiple reticulated ceramic tiles can be joined together such that each ceramic tile is in direct contact with one or more adjacent reticulated ceramic tiles. The multiple reticulated ceramic tiles can collectively form a single perforated flame holder 102. Alternatively, each reticulated ceramic tile can be considered a distinct perforated flame holder 102. In another embodiment (not shown), 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 connected openings forming the perforations 210. 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 may include structured packing shapes. In another example, the discontinuous packing bodies may include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig rings, ceramic Berl saddles, ceramic Intalox saddles, and/or metal rings or other shapes (e.g. Super Raschig Rings) that may be held together by a metal cage.
The inventors contemplate various explanations for why burner systems including the perforated flame holder 102 provide such clean combustion.
In one aspect, the perforated flame holder 102 acts as a heat source to maintain a combustion reaction even under certain conditions where a combustion reaction would not be stably 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 and oxidant mixture 112 contacts the input face 212 of the perforated flame holder 102, an average fuel-to-oxidant ratio of the fuel and oxidant mixture 112 is below a (conventional) lower combustion limit of the fuel component of the fuel stream—lower combustion limit defines the lowest concentration of fuel at which a fuel/air mixture will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.).
According to one interpretation, the fuel and oxidant mixture(s) 112 supported by the perforated flame holder 102 may be more fuel-lean than mixtures that would provide stable combustion in a conventional burner. Combustion near a lower combustion limit of fuel generally burns at a lower adiabatic flame temperature than mixtures near the center of the lean-to-rich combustion limit range. Lower flame temperatures generally evolve a lower concentration of NOx than higher flame temperatures. In conventional flames, too-lean combustion is generally associated with high CO concentration at the stack. In contrast, the perforated flame holder 102 and systems including the perforated flame holder 102 described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. In some embodiments, the inventors achieved stable combustion at what was understood to be very lean mixtures (that nevertheless produced only about 3% or lower measured O2 concentration at the stack). Moreover, the inventors believe perforation walls 308 may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperature.
According to another interpretation, production of NOx can be reduced if the combustion reaction 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.
Since CO oxidation is a relatively slow reaction, the time for passage through the perforated flame holder (perhaps plus time passing toward the flue from the perforated flame holder 102) is apparently sufficient and at sufficiently elevated temperature, in view of the very low measured (experimental and full scale) CO concentrations, for oxidation of CO to carbon dioxide (CO2).
According to a simplified description, the method 400 begins with step 402, wherein the perforated flame holder is preheated to a start-up temperature, TS. After the perforated flame holder is raised to the start-up temperature, the method proceeds to step 404, wherein 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 sub-step 406, wherein start-up energy is provided at the perforated flame holder. Simultaneously or following providing start-up energy, a decision sub-step 408 determines whether the temperature T of the perforated flame holder is at or above the start-up temperature, TS. As long as the temperature of the perforated flame holder is below its start-up temperature, the method loops between sub-steps 406 and 408 within the preheat step 402. In sub-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 step 404, wherein fuel and oxidant are supplied to and combustion is held by the perforated flame holder.
Step 404 may be broken down into several discrete sub-steps, at least some of which may occur simultaneously.
Proceeding from sub-step 408, a fuel and oxidant mixture is provided to the perforated flame holder, as shown in sub-step 410. The fuel and oxidant may be provided by a fuel and oxidant source that may include a fuel nozzle and combustion air source that are distinct from a fuel nozzle and combustion air source(s) used for the start-up, for example. In this approach, the fuel and combustion air are output in one or more directions selected to cause a mixture of the fuel and combustion air to be received by an input face (e.g., 212) of the perforated flame holder 102. 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 sub-step 412, the combustion reaction is held by the perforated flame holder.
In sub-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 an optional sub-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, and/or other known combustion sensing apparatuses. In an additional or alternative variant of sub-step 416, a pilot flame or other ignition source (e.g., ignition and start-up flame source 120) 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 sub-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, adjusting fuel and/or air flow rates or direction, outputting an alarm signal, igniting a stand-by combustion system, or other steps. If, in sub-step 418, combustion in the perforated flame holder is determined to be stable, the method 400 proceeds to decision sub-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 sub-step 410, and the combustion process continues. If a change in combustion parameters is indicated, the method 400 proceeds to sub-step 422, wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step 404) back to sub-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 sub-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.
As described in conjunction with
Various preheating apparatuses have been used and are contemplated by the inventors. In some embodiments, the heater 228 can include a proximal flame holder configured to support a flame disposed to heat the perforated flame holder 102, e.g., by radiant or convective heating depending on fuel type and/or other parameters. The fuel and oxidant source 202 can connect to the fuel nozzle assembly 104 configured to emit a fuel stream and connect to an oxidant conduit 220 configured to convey combustion air adjacent to the fuel stream. The fuel nozzle assembly 104 can be configured to output the fuel stream to be progressively diluted by the combustion air. The perforated flame holder 102 can be disposed to receive a diluted fuel and air mixture 112 that supports a combustion reaction (such as 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 rich fuel and air mixture 112, 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 (such as the proximal flame holder described above) to hold the start-up flame 206 when the perforated flame holder 102 needs to be preheated and to not hold the start-up flame when the perforated flame holder 102 is at an operating temperature (e.g., when T≧TS).
Various alternative approaches for actuating a start-up flame are contemplated. In one embodiment, a start-up flame holder includes a mechanically-actuated bluff body configured to be actuated to intercept the fuel and oxidant mixture 112 to cause heat-recycling vortices and thereby hold a start-up flame; or to be actuated to not intercept the fuel and oxidant mixture 112 to cause the fuel and oxidant mixture 112 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 an operating temperature of the perforated flame holder 102, the flow rate may be increased to “blow out” the start-up flame, thus permitting the combustible materials to enter the perforated flame holder 102. In another embodiment, the heater 228 may include an electrical power supply operatively coupled, via, e.g., a control output of the heater 228, to the controller 230 and configured to apply an electrical charge or voltage to the fuel and oxidant mixture 112. 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 112. 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 112. The electrical resistance heater 228 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 via a control input of the heater 228, to selectively couple the power supply to the electrical resistance heater.
The 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 Hallstaham mar, Sweden) threaded through at least a portion of the perforations 210 defined formed by the perforated flame holder body 208. Alternatively, the heater 228 can include an inductive heater, a high energy (e.g. microwave or laser) beam heater, a frictional heater, or other types of heating technologies.
Other forms of start-up apparatuses are contemplated. For example, the heater 228 can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the air and fuel. Additionally or alternatively, a start-up apparatus can include a pilot flame apparatus (e.g., at ignition and start-up flame source 120 shown in
The burner system 200 can further include the sensor 234 operatively coupled to the controller 230.
The sensor 234 can include a heat sensor configured to detect infrared radiation or a temperature of the perforated flame holder 102 and convey data indicating a characteristic of the infrared radiation or temperature via a temperature indication output of the sensor 234. The controller 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 controller 230, the combustion sensor being configured to detect or measure a temperature, video image, and/or spectral characteristic of a combustion reaction held by the perforated flame holder 102. Although presence of combustion may be detected proximate a downstream side of the perforated flame holder 102, it will be recognized that any combination of combustion effects may be measured upstream, within, and/or downstream of the perforated flame holder 102 in order to evaluate and control the flame and its effects. The combustion sensor may be implemented to detect or measure temperature by means of infrared sensor, thermocouple, and/or thermopile.
In some embodiments, the sensor 234 may detect or measure a particulate species concentration and/or an ionization level. Concentration of particulate species in the combustion products may by measured by analyzing concentrations of, e.g., OH* radicals, OH— ions, CH* radicals, and/or other particulate species at predetermined location(s). In some instances, the concentrations may be measured spectroscopically, e.g., using one or more spectrometers arranged to analyze a spectral characteristic of such particular species. Ionization level may be determined in some embodiments by detecting or measuring conductivity of the combustion products using, e.g., one or more flame rods.
The fuel control valve 236 can be configured to control one or more flows of fuel from a fuel source to the fuel and oxidant source 202. For example, the controller 230 may be configured to control at least one of fuel supply to the fuel nozzle 105, 218 and the ignition and start-up flame source 120. 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 238 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 238 to change the fuel and oxidant mixture 112 flow responsive to a heat demand change received as data via the data interface 232.
Embodiments of the tapered fuel nozzle 105 may further include a pilot or start-up fuel connection 504 for delivery of pilot or startup fuel from a pilot fuel line 512 to an ignition and start-up flame source 120 (such as a pilot burner). The pilot fuel line 512 is illustrated as being coaxial with fuel riser 511. Alternatively, the pilot fuel line 512 may be disposed parallel to the fuel riser 511 or supplied from a different direction altogether. The pilot or start-up fuel connection 504 may be operably connected to the ignition source 120. The ignition source or pilot burner 120 may include a nozzle (e.g., conventional nozzle 122 in
The fuel nozzle assembly 104 is configured to allow airflow F to travel in the longitudinal direction L, airflow F combining with the fuel from the fuel orifices 108 to form the fuel oxidant mixture 112. The fuel orifices 108 are equally spaced at or along the acute trailing edge 106 to aid homogenous mixture of fuel expelled therefrom with an oxidant (e.g., the air in airflow F) for the fuel and oxidant mixture 112. The leading edge 514 may be rounded or otherwise formed with a reduced angle of attack with respect to the airflow F.
While the illustrated fuel nozzle assembly 104 is arranged with the airfoil section 502 forming a toric shape when viewed in the longitudinal direction L, (see
As described herein, a fuel nozzle assembly 104 according to an embodiment may be used to accomplish the preheating step 402 of the associated method shown in
In some embodiments the ignition source 120 may comprise a plurality of igniters disposed at various locations about the fuel nozzle assembly 104 to allow a preheat flame 206 to consume a portion of the fuel and oxidant mixture 112 expelled from orifices 108, while an unconsumed portion of the fuel and oxidant mixture 112 reaches the perforated flame holder 102. That is, fuel expelled from orifices 108 may be ignited during a start-up period to heat the perforated flame holder 102. A combustion reaction at the perforated flame holder 102 then ignites the unconsumed portion of the fuel and oxidant mixture 112. Partitioning the fuel and oxidant mixture 112 to include both a preheat flame or partial preheat flame and an unconsumed portion may also be accomplished by moving the ignition source 120 or igniters thereof from a first position to a second position. The first position allows the entire fuel and oxidant mixture 112 to be consumed by the preheat flame 206 and the second position allows the unconsumed portion of the fuel and oxidant mixture 112 to reach the perforated flame holder 102. When the fuel nozzle 105 is used alternately for preheat and normal operating conditions, the perforated flame holder 102 or the nozzle 105 itself may be moved from one position to another to effect various changes in characteristics or results. For example, such position movement may effect a change in flame size, shape, and/or intensity; may change oxidant flow characteristics (e.g., by utilizing the airfoil section 502 to guide air in a desired manner), and/or may otherwise permit efficient use of fuel and/or oxidant for the preheat flame 206 for preheating the perforated flame holder 102.
The fuel nozzle assembly 104 may be configured to allow the fuel and oxidant mixture 112 to reach the perforated flame holder 102 without being consumed by a preheat flame upon the disabling or partial disabling of the ignition source 120. Both the acute trailing edge 106 and rounded leading edge 514 of the airfoil section 502 can be configured to reduce the angle of trajectory for the fuel as it leaves the fuel orifices 108 and/or configured to reduce the turbulence of the airflow as it passes the airfoil section 502. The shape of the airfoil section 502, including the acute trailing edge 106 and rounded leading edge 514, thereby reduces the ability of the flame to propagate through the fuel and oxidant mixture 112 when the ignition and start-up flame source 120 is disabled or partially disabled.
The airfoil section 502 may be formed from a piece of tubing that may be elongated by rolling or some other method in order to form an elongated (e.g., oblong) internal channel 506, where the trailing edge 106 may be machined or otherwise formed into a tapered shape as illustrated in
The fuel nozzles 218 may also have an oblate or flattened section 604 formed thereon that can be utilized in handling the fuel nozzles 218 (e.g., for accepting a conventional wrench for installation and removal). Each of the fuel nozzles 218 may include a connection portion such as threads 606 for connection to one or more fuel supply lines, such as fuel riser 511. The connection portion may alternatively (or additionally) include structure for pressure fit, snap fit, or other attachment mechanism. An array of the fuel nozzles 218 may be arranged in, e.g., a two-dimensional array (not shown) that may correspond to the shape of a perforated flame holder 102.
The fuel nozzle 218 of
In some embodiments, it may be desirable for the fuel nozzle 218 to have an external diameter that is substantially the same as the diameter of the fuel riser 511 in order to best approximate laminar flow of oxidant past the fuel nozzle 218. Thus the fuel nozzle 218 may have an attachment structure, such as threads 606 as shown in
Those having skill in the art will acknowledge that various features of the fuel nozzles 218 described above may be implemented in various combinations. For example, a fuel nozzle 218 may have an outside diameter larger than the fuel riser 511 and may include a plurality of orifices 226. As the fuel nozzles 218 may be arrayed in any pattern to facilitate delivery of fuel and oxidant to the perforated flame holder 102, the fuel riser 511 may be formed having a lateral portion either remote from the fuel nozzles 218 or proximate the fuel nozzles 218. For example, each fuel nozzle 218 in an array of nozzles may have a respective fuel riser 511 that extends a distance behind the fuel nozzles in a direction away from the perforated flame holder 102 and culminating in a manifold (not shown) that may deliver the fuel to the respective fuel risers 511. This structure, in conjunction with individual fuel control valves (e.g., fuel control valve 236) may facilitate individual control of fuel delivery to each fuel nozzle 218. Alternatively, fuel may be delivered to the fuel nozzles 218 via a primary fuel riser 511 which may be divided proximate the fuel nozzles 218 via lateral fuel tubes (not shown).
The fuel nozzle(s), as described above, may have a structure, shape, and/or orientation that limits an amount of fuel and oxidant mixture near the fuel nozzle(s). For example, a tapered structure that decreases in width from a nozzle base to a nozzle tip (such as an acicular tip or acute trailing edge or tip) reduces an area that in conventional flat-ended fuel nozzles may provide a low pressure region adjacent the fuel delivery orifice. According to an interpretation, oxidant flowing past that low pressure region is drawn by the lower pressure and thus interrupts laminar flow. Resulting vortices cause mixture of oxidant and fuel near the orifice. In some instances the fuel-oxidant mixture proximate the fuel nozzle can support combustion. Thus, the tapered nozzle structure is provided to limit generation of such vortices—and thus shifts mixture of the fuel and oxidant to a region closer to the perforated flame holder.
The adjacent fuel stream and oxidant flow eventually mix proximate the perforated flame holder to provide a fuel and oxidant mixture (step 710) for receipt by the perforated flame holder. The perforated flame holder, preheated and/or maintained at an operating temperature, ignites the fuel and oxidant mixture for combustion (step 712).
The disengagable flame holder may, as described above, be a physically movable flame holder, an electrical charge introduced into the fuel and oxidant mixture between the nozzles and perforated flame holder, or may be vortices formed by aerodynamic characteristics of the burner and controllable by managing the flow characteristics (e.g., rate, direction, spread) of fuel and/or oxidant. It will be appreciated that the precise order of steps represented in
Turning again to
Once the combustion reaction is taking place at the perforated flame holder and is stable, the burner system may be operated according to the method described in connection with
Each fuel orifice 1026 may be connected to the fuel chamber 1008 via an orifice path 1010. The orifice path 1010 may be selected to affect a desired swirl of fuel as it is emitted from the fuel nozzle 1018. The orifice path 1018 and fuel orifice 1026 may be arranged to generate fuel distribution with a swirl number of 0.6 or less. In some embodiments each orifice path 1010 fuel orifice 1026 are arranged to result in a swirl number (in aggregate with oxidant swirl) that is much lower than 0.6. Swirl number is a dimensionless ratio of angular to axial momentum, e.g., as described by Chigier and Beer (N. A. Chigier, and J. M. Beer. J. Basic Eng. 788-796, 1964). While an actual swirl number is difficult to measure, a “geometric swirl number” may, in some instances, be based on the geometric angles of swirl generators.
Thus, a compound angle of an orifice path 1010 and fuel orifice 1026 can be provided that creates swirl in the fuel. Compound angle may be defined using at least two angles α and β. In
Similarly, depending on swirl of associated oxidant introduced to the burner, the fuel nozzle 1018 can be configured to emit the fuel in a manner that enhances or that limits mixing of the fuel and oxidant. For example, fuel swirl introduced by the compound angle of the fuel orifice 1026 and its orifice path 1010 may be complementary to an oxidant swirl (thus limiting mixture initially) or may be opposed to an oxidant swirl (enhancing initial mixture). Because there are losses in the system, the geometric swirl number is always higher than the real swirl number.
The oxidant swirl features described above may be combined with the fuel swirl features described with respect to
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.
The present application claims priority benefit from U.S. Provisional Patent Application No. 62/327,779, entitled “FUEL NOZZLE ASSEMBLY FOR A BURNER INCLUDING A PERFORATED FLAME HOLDER,” filed Apr. 26, 2016 (docket number 2651-296-02); which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62327779 | Apr 2016 | US |
