Patent Application
20180080647
This invention relates generally to combustion processes and, more particularly, to devices and methods that combust oxidant and fuel in a burner to provide or result in low or reduced levels of NOx formation such as through the proper recirculating and mixing products of at least partial combustion with fuel and/or oxidant.
The use of converging-diverging nozzles to draw a first fluid into a second fluid to form a mixture is known. The use of recirculated cooled products of combustion to reduce flame temperature and NOx formation is also known.
A number of technologies exist, such as disclosed in U.S. Pat. Nos. 5,350,293; 6,663,380; and 6,672,859, for example, wherein partially cooled products of partial combustion are recirculated into the root of the flame(s) of the burner to improve temperature and composition uniformity for NOx reduction. While the recirculated gases mix with the flame near its periphery, these gases are not able to mix well with the central portion of the flame (s), especially in larger burners or combustion devices. Since the flames are generally hottest in a zone proximate their axis, this lack of penetration and mixing of recirculated gases into the middle of the flame prevents effective reductions in peak flame temperatures and NOx formation.
As detailed further below, in accordance with one embodiment a two stage mixing approach with multiple secondary mixing conduits is utilized to improve the mixing of oxidant, combustibles and cooled combustion products prior to combustion. The use of multiple secondary mixing conduits helps aspirate generally an amount of fluid that is proportional to the flow rate of the motive gas flowing through individual secondary mixing conduits. The use of multiple aspirating secondary conduits therefore can be helpful in distributing the oxidant, combustibles and recirculated gases uniformly through the conduits and helps mixing these gases well prior to combustion. The use of multiple secondary mixing conduits can also reduce the required length for uniform secondary mixing. The uniform flow and composition profile of gases across these conduits and the enhanced mixing of these gas mixtures within these conduits can desirably lead to one or more of improved flame temperature and composition uniformity, reduced peak flame temperatures and peak oxygen concentrations, even in the middle of the flames proximate its axis, which conventional systems are unable to do effectively, thereby reducing NOx formation and improving combustion efficiency. Further, the use of venturi-type of design for at least one of primary and secondary mixing conduits allows pressure drop recovery and minimizes overall pressure drop for effective mixing. The use of multiple secondary mixing conduits also increases the amount of surfaces that can remove heat to minimize potential for flame flashback into these conduits.
The subject development is suitable for use in combusting a variety of combustible gases and oxidants and mixtures thereof in a variety of applications, including, for example, in or with residential, commercial, industrial, power generation equipment. The concept is especially suitable in applications requiring low NOx emissions.
In one aspect of the subject development, a method for reducing NOx formation upon combusting oxidant with fuel to form products of combustion is provided. In one embodiment, such a method involves mixing at least first portions of at least two fluids selected from the group of oxidant, fuel and recirculated products of combustion in at least one first conduit to form a first mixture. The first mixture is subsequently supplied to a plurality of second conduits each in direct fluid communication with the first conduit. Motive energy in the first mixture is utilized to aspirate at least second portions of one or more fluids selected from the group of oxidant, fuel and recirculated products of combustion in at least one second conduit to form a combustible mixture that can be subsequently burned.
In one embodiment, the motive energy is provided, supplied, or results from or in the first mixture.
In one embodiment, the mixing step may suitably involve aspirating, via motive energy in at least the first portion of a first of the at least two selected fluids the first portion of a second of the at least two selected fluids.
In another aspect of the subject development, there is provided a combustion device such as to produce or result in combustion of oxidant and fuel with reduced NOx formation. In accordance with one embodiment, such a combustion device includes a burner for combusting oxidant with fuel to form products of combustion. The device also includes at least one first conduit wherein at least first portions of at least two fluids selected from the group of oxidant, fuel and recirculated products of combustion are mixed to form a first mixture. The device further includes a plurality of second conduits each in direct fluid communication with the first conduit and in fluid discharge communication with the burner. In the second conduits motive energy aspirates at least second portions of one or more fluids selected from the group of oxidant, fuel and recirculated products of combustion to form a combustible mixture.
In one embodiment, the motive energy is provided, supplied, or results from or in the first mixture.
In accordance with at least some preferred embodiments, combustion devices include or incorporate portions or regions whereat or wherein motive energy is utilized or employed to aspirate and mix two or more different materials together.
Objects and features of this invention will be better understood from the following description taken in conjunction with the drawings, wherein:
A process and apparatus for recirculating and mixing products of partial combustion with fuel and/or oxidant is described. The proposed approach will provide improved mixing of fuel, oxidant and recirculated products of partial combustion, thereby improving uniformity and reducing peak levels of temperature and oxygen concentration within the flame for reduced NOx formation while maintaining stable combustion. The impacts of flame temperatures and flame oxygen levels on NOx formation are well known.
In accordance with one preferred aspect of the subject development, the basic principle of using the kinetic energy in one fluid to aspirate and mix another fluid is purposefully utilized.
As will be appreciated by those skilled in the art and guided by the teachings herein provided, a wide range of basic design approaches for using the kinetic energy in one fluid to aspirate and mix another fluid are known and can be selected and utilized in implementing the subject development and such as depending on the properties, conditions and desired flow rates of the motive and the suction fluid (s) as well as the desired characteristics and conditions of the resulting mixture.
Those skilled in the art and guided by the teachings herein provided will understand and appreciate that while the following embodiment is described using converging-diverging conduits, other types or forms of aspirating approaches can also be used effectively.
Turning to
As the first fluid moves through a converging section 215 of the conduit 214, its velocity increases and pressure decreases. The decrease in pressure draws a second fluid, represented by the arrow 218 (e.g., Fluid B), through at least one opening 220, preferably located proximate the smallest cross section plane 221 of the converging section 215. A mixture of the first and second fluids, such a mixture represented by the arrow 226 goes through a diverging section 224 of the conduit 214 where its pressure is at least partially recovered to minimize pressure drop losses. The fluid mixture 226 then enters a second set of converging-diverging conduits 230 such as are, for example, substantially oriented parallel to each other. The secondary conduits 230 may be identical to each other or different in one or more of parameter including: length, cross section, shape and the size, shape, orientation and number of holes 232 to achieve desired downstream results. The holes can be of a variety of shapes including annular, continuous, or discontinuous slot(s) such as shown in
This top view also shows the inclusion of a screen mesh 245 such as disposed downstream and proximate the plurality of secondary conduits 230. The inclusion or presence of such a screen mesh or the like may be desired such as to assist to stabilize combustion and/or increase radiation. Further, the screen mesh or the like may be coated with or otherwise contain or include catalyst, as may be desired for a particular application such as a catalyst to promote combustion reactions, for example.
As will be appreciated, the view shown in
The water heater assembly 510 is effective to heat a supply of water, generally designated as 526. The burner assembly produces or forms combustion exhaust such as represented by the arrow 530 that can be appropriately vented through an outlet 532.
As shown in the water heater assembly 510, the burner device 512 can, if desired, be supplied with or via a motive force supplied or resulting from application of an incorporated fan device 539 or the like.
Furthermore, as the conduits of the burner device 512 are radially distributed or disposed such an embodiment illustrates that the conduits of burner assemblies in accordance to the subject development are not necessarily required to be either parallel or straight and the broader practice of the subject development is not necessarily so limited.
Those skilled in the art and guided by the teachings herein provided will further understand and appreciate that the above-described embodiments can be suitably modified or altered for particular applications.
The ratio of oxidant used to oxidant needed for complete combustion can desirably be between 0.4 to 20, preferably between 0.7 to 1.7 and more preferably between 1 and 1.5.
The ratio of the amount of recirculated products of partial combustion to total products of combustion can desirably be between 0.05 to 2, preferably between 0.1 to 1, and more preferably between 0.2 to 0.5.
The ratio of number of secondary conduits to number of primary conduits can desirably be from 2 to 200, preferably from 3 to 60, more preferably from 3 to 20 and even more preferably from 5 to 10.
The number of primary conduits can desirably be from 1 to 20 and, in some embodiments from 1 to 10, preferably from 1 to 6 and more preferably from 1 to 3.
In an embodiment, one or more of the conduits is or are replaced with at least 2 substantially parallel oriented and substantially similar conduits.
In another embodiment, the conduit 214 in
In another embodiment, the conduit 314 in
In another embodiment, the diverging section in at least one of the converging-diverging conduits is replaced by a straight conduit section.
In another embodiment, a continuous or segmented annular opening is used proximate the tip of the converging section in at least one of the secondary converging-diverging conduits to draw in the Fluid B and Fluid C, respectively.
In another embodiment, at least a portion of the Fluid C is drawn through openings in one or more of the primary conduits.
In another embodiment, at least a portion of the Fluid B is drawn through openings in the plenum.
In another embodiment, two or more of the mixers 2 can be arranged to form a larger capacity mixer for fuel, oxidant and products of at least partially cooled products of at least partial combustion
In one embodiment, at least one of the Fluids A and B could be preheated.
Increasing the number of secondary conduits may and desirably will improve the uniformity of the mixture across the secondary conduits such as to improve one or more of uniformity of temperature and oxidant concentration within the combustion zone downstream of the secondary conduits for NOx reduction.
Increasing the number of secondary conduits may and desirably will improve at least one or more of uniformity of mixing and stability and reduce peak temperatures within the combustion zone downstream of the secondary conduits for at least one of enhanced turndown and lower emissions of NOx and unburned combustibles.
The mixer concept described can be used as a component to introduce a mixture of the Fluids A, B and C in a more complex combustion device.
In one preferred embodiment, at least one of the Fluid A and the Fluid B contains a mixture of oxygen and mostly inert gases such as nitrogen, moisture and carbon dioxide.
In one preferred embodiment, at least one of the Fluid A and the Fluid B contains a mixture of combustible gases and mostly inert gases such as nitrogen, moisture and carbon dioxide.
In one preferred embodiment, at least one of the Fluid A and the Fluid B is air.
In one preferred embodiment, at least one of the Fluid A and the Fluid B is a gaseous hydrocarbon fuel.
In one preferred embodiment, at least one of the Fluid A and the Fluid B is natural gas.
In a preferred embodiment, the number of secondary conduits is at least 3.
In a preferred embodiment, the number of secondary conduits is at least 7.
In a preferred embodiment, the number of secondary conduits is at least 14.
In one embodiment, the secondary conduits are supplied with at least one of an oxidant and combustible gas and additionally one of an oxidant and combustible gas is supplied through the holes or openings to form a combustible mixture in the secondary conduits which collectively form a radiant permeable or porous membrane combustion device. Alternatively, a premixed mixture of an oxidant and a combustible may be supplied directly to the secondary conduits. This type of device can be used as an infrared radiant burner that typically use, metal, ceramic or composite permeable membranes. These include ported, porous perforated, foam or fiber mesh ceramic, refractory, metallic, composite, or sintered metal radiant devices with or without surface coatings.
The use of secondary mixing conduits increases pressure drop and adds complexity to the design. There is also potential for combustion to initiate within the secondary mixing conduits requiring the use of more expensive materials.
In one embodiment of the invention, the inside of at least one secondary conduit is embedded or coated with catalyst to initiate a reaction between the combustible and the oxidation components in the fluid mixture.
In one embodiment of the invention, the exit surface of at least one of the secondary conduits is made rough or non-uniform to perturb the exiting flow stream and promote formation of recirculating eddies to improve flame stability by incorporating at least one of at least one notch, depression or projection or a continuous depression or projection or a sudden shape change.
In another embodiment, a metallic, ceramic or composite screen or mesh is placed downstream and proximate the secondary conduits to stabilize combustion and increase radiation. The screen or mesh being with or without catalyst, as may be desired for a particular application.
In another embodiment, the screen or mesh is embedded or coated with catalyst to promote combustion reactions.
In some preferred embodiments, the velocity of mixtures at the exits of first and second conduits in devices and methods of the subject development are desirably at least 1 ft/s and are desirably no more than about 500 ft/s. In some preferred embodiments, the velocity of mixtures at the exits of first and second conduits in devices and methods of the subject development are desirably at least 10 ft/s and are desirably no more than about 250 ft/s.
The opening(s) in secondary conduits can be of a wide range of shapes, including round, oval, rectangular, square, elliptical, star, slot, annular, etc., for example.
The proposed mixing and combustion technology reduces NOx formation such as by uniformly mixing and distributing fuel and oxidant, with and without recirculated partially cooled products of combustion across the flame cross section to improve uniformity of flame temperatures and oxygen concentrations across the flame front that promote a reduction in NOx formation.
The use of multiple secondary conduits also reduces the secondary zone mixing length and increases heat dissipation because of increased conduit surface area to reduce potential for flashback.
While the use of converging-diverging nozzles to draw a Fluid A into a Fluid B to form a mixture is known and the use of recirculated cooled products of combustion to reduce flame temperature and NOx formation is also known, the use of multiple small converging-diverging or alternate type mixing conduits to uniformly mix fuel, oxidant and recirculated partially cooled products of combustion in a short distance and uniformly distribute the mixture across the combustion device exit for stable combustion with low NOx formation is not known.
The use of secondary mixing conduits may increase pressure drop and add complexity to the design. There is also potential for combustion to initiate within the secondary mixing conduits requiring the use of more expensive materials.
While combustion device in accordance with the subject development are not necessarily limited by or to specific manufacturing techniques, in one embodiment, application of Additive Manufacturing (AM) methods or techniques are believed to be particularly conducive or desirable to the manufacture of combustion devices in accordance with the subject development. As will be appreciated, suitable such additive manufacturing methods typically may involve technologies and equipment such as used to make 3D objects such as by repetitively adding (i.e. depositing) layer-upon-layer of material until the object achieves the shape defined by a pre-determined computer-aided design (CAD) model. Materials deposited during additive manufacturing processes may include plastic resin, concrete or metal, the latter material being used in one preferred embodiment of the subject low NOx Combustion Device. Currently, there are four primary metal additive manufacturing processes: Powder Bed Fusion (PBF), Binder Jetting (BJ), Direct Energy Deposition (DED), and Sheet Lamination. PBF is most commonly used for metal printing and involves laser sintering or melting or electron beam melting with each having its benefits and limitations. Binder Jetting works by depositing binder on metal powder, curing the binder to hold the powder together, sintering or consolidating the bound powder, and (optionally) infiltrating with a second metal. Sheet lamination uses laminate object manufacturing. Sheet Lamination uses stacking of precision cut metal sheets to form a 3D object. After stacking, these sheets are either adhesively joined or metallurgically bonded using brazing, diffusion bonding, laser welding, resistance welding or ultrasonic consolidation uses indirect 3D printing. DED employs a laser engineered net shaping method that is used for metal 3D printing. Thus it is to be appreciated that in one embodiment subject combustion devices can be suitable prepared or manufactured by additive manufacture such as involving repetitively depositing layer-upon-layer of material, sintering or consolidating bound powder material, or metallurgically bonding metal sheets using brazing, diffusion bonding, laser welding, resistance welding or ultrasonic consolidation, for example.
It is also well-known in the art of atmospheric gas burners that the primary air oxidant can be drawn into the burner by the motive force created by gas entering an orifice opening, such as the fluid ejector principle shown in
In accordance with one embodiment, the fluid ejector principle is used in methods and devices to both create the motive force and to aspirate the fuel in the oxidant. In this embodiment, the motive force can be provided by the fuel itself rather than through an external source. Such methods and devices improves over existing atmospheric burner designs and processing that use the fluid ejector principle because the plurality of ports provides more distributed mixing and uniformity compared to a single port or few ports. In addition, the subject development incorporates individual secondary conduits to distribute and more uniformly entrain the recirculated mixed products to each burner in order to reduce emissions. The subject development, by permitting employment of multiple burners and allowing burners to be oriented in many ways and positions, allows designers of burner assemblies much more flexibility to better control the amount of secondary air entrained, as compared to single burner design.
The present invention is described in further detail in connection with the following examples which illustrate or simulate various aspects involved in the practice of the invention. It is to be understood that all changes that come within the spirit of the invention are desired to be protected and thus the invention is not to be construed as limited by these examples.
In this testing of a burner device in accordance with the subject development, a burner device similar to that shown in
The state-of-the-art burner shown was a steel cylindrical premix burner design that was tested in the commercial water heater burner environment with the controls for the water heater.
As identified above, the testing of the subject burner device was performed with variable excess air levels. However, the thermal NOx emissions are also a function of flame temperature and hence are compared in the current case.
As shown by
While in the foregoing specification the subject development has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.
This application claims the benefit of U.S. Provisional Patent Application, Ser. No. 62/397,985, filed on 22 Sep. 2016. The co-pending Provisional Patent Application is hereby incorporated by reference herein in its entirety and is made a part hereof, including but not limited to those portions which specifically appear hereinafter.
| Number | Date | Country | |
|---|---|---|---|
| 62397985 | Sep 2016 | US |
