Apparatus for reducing emissions when burning various fuels

An apparatus for burning of a gaseous fuel includes a gas manifold comprising a blast tube with an axis of rotation and an outer wall; a center bluff body disposed inside the blast tube; a plurality of aerodynamic blocks circumferentially distributed in the annular space between the blast tube and the center bluff body, creating passage channels for combustion air between the aerodynamic blocks; two injector nozzles located inside the wake zone of each of the aerodynamic block and are fluidically communicating to the gas manifold; an air control mechanism comprising a center hub and a plurality of air control modules. The control modules fit through the passage channels. Each air control module comprises an air deflector located at the outer edge of a passage channel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to combustion apparatus, and more specifically relates to a burner that is capable of achieving high turndown, high thermal efficiency, and extremely low NOx, CO and hydrocarbon emissions.

2. Description of the Related Art

Boilers are widely used for the generation of hot water and steam. A conventional boiler (excluding Heat Recovery Steam Generator or HRSG) comprises a furnace in which fuel is burned, and surfaces typically in the form of steel tubes to transfer heat from the flue gas to the water. A conventional boiler has a furnace that burns a fossil fuel or, in some installations, waste fuels or biomass derived fuels. Most conventional boilers are classified as either firetube or watertube types. In a firetube boiler, the water surrounds the steel tubes through which hot flue gases from the furnace flow. In a watertube boiler, the water is inside the tubes with the hot flue gases circulating outside the tubes. The current invention can be used in firetube and watertube boilers, as well as in other applications including but not limited to furnaces, incinerators and ovens. NOx is a recognized air pollutant. Regulations on NOx tend to get more stringent in densely populated areas of the world. In some areas, local regulations require low NOx or even ultra low NOx emissions in the exhaust from the combustion processes. Various low NOx and ultra low NOx burners are available in the market to meet these requirements. A review of typical NOx reduction methods can be found in the article “NOx emissions: Reduction Strategy” in “Today's Boiler” magazine Spring 2015 by Jianhui Hong. FGR (Flue gas recirculation) is a commonly used technique for NOx reduction. In one common implementation called “Induced FGR”, flue gas is drawn through a pipe or duct to the inlet of a blower and mixed with the combustion air by using the blower wheel as a mixing device.

According to the Perry's Chemical Engineers' Handbook (7thEdition) Section 10-46, the horsepower requirement for a centrifugal blower is determined by the multiplication of two factors, the volumetric flow rate through the blower in cubic feet per minute, and the blower operating pressure in inches water column. Induced FGR increases both the volumetric flow rate through the blower and the pressure drop through the burner and the boiler (hence increasing the blower operating pressure), and therefore greatly increases the horsepower requirement for the blower motor. Everything else being equal, if the amount of induced flue gas is reduced, the horsepower requirement of the motor can be reduced as well.

U.S. Pat. No. 5,407,347A teaches an apparatus and method for reducing NOx, CO and hydrocarbon emissions when burning gaseous fuels. The advantage of this invention is that ultra low NOx emission can be achieved at relatively low oxygen level (such as 3% dry volume basis) in the flue gas. The shortcoming of this technology is that a large amount of FGR (up to 40% of combustion air by mass) is required to achieve <9 ppm NOx emissions. In addition, the rapid mixing design requires large pressure drops across the swirl vanes in the combustion air pathway near the burner head. Since mixing rate slows down as flow velocity is reduced, this design also has a limited turndown (3:1 or 4:1 in some cases) for ultra low NOx performance. Due to the large amount of FGR and the high pressure drop the air/FGR mixture has to overcome, a markedly larger motor and a larger blower are required compared to a typical burner of the same firing rate. The larger motor means higher initial capital costs, higher electricity consumption and higher noise during the burner's operation. In the state of California in particular, operators of boilers often dislike use of FGR, perhaps due to the concerns of earthquake and the additional mandatory structural inspection related to the field installation of the FGR pipe. U.S. Pat. No. 6,776,609 also discussed the motor size penalty problem in details related to the use of Induced FGR for ultra low NOx performance.

Another commonly used technique for ultra low NOx is called “lean premixed combustion”. U.S. Pat. No. 6,776,609 was intended to teach a method for operating a burner with FGR, but it also discussed the disadvantages of the lean premixed combustion method based on fiber matrix. It disclosed that “Alzeta Corp. of Santa Clara, Calif. sells a burner for use in food processing and other industries that utilizes only excess combustion air (no FGR) to achieve the flame dilution necessary for 9-ppm NOx emissions. A dilution level of 60% on a mass basis is required”. The shortcomings of the “lean premixed combustion” technique are well recognized in the combustion community: low thermal efficiency due to the very high excess air level and the resultant very high oxygen level in the flue gas (9% oxygen is typical), and the extra electricity consumption due to the extra excess air for the dilution effects. The large amount of excess air was intended to reduce the peak flame temperature by dilution effects. The extra dilution air carries additional heat into the atmosphere (wasted heat) when the exhaust is vented, and causes a reduction of thermal efficiency.

In view of the foregoing, there exists a need for an improved method and apparatus for burning a gaseous fuel that can achieve high turndown, extremely low emissions of NOx, CO and hydrocarbons, low electricity consumption for the motor, and high thermal efficiency (low excess oxygen in the flue gas) at the same time.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide an apparatus for burning of a gaseous fuel and producing extremely low emissions of NOx, CO and hydrocarbons in the burning process.

A more specific object of the present invention is to provide an apparatus for burning of a gaseous fuel that achieves high turndown, ultra low NOx emissions, low oxygen level in the flue gas which leads to higher thermal efficiency, low horsepower requirement for the blower motor for the burner.

These objects are achieved by an apparatus for burning of a gaseous fuel, said apparatus comprising a gas manifold10comprising an inlet pipe11, a blast tube13and an outer wall12, wherein said blast tube13is substantially cylindrical with an inside diameter D and an axis of rotation AoR; a center bluff body40with an outside diameter d such that the ratio d/D is in the range of 0.45 to 0.65; a plurality of aerodynamic blocks20circumferentially distributed in the annular space between said blast tube13and said center bluff body40, creating passage channels70for combustion air between said aerodynamic blocks20, said aerodynamic blocks20are affixed to the inside of said blast tube13; each of said aerodynamic block comprising a small and substantially closed leading end21and a large and open trailing end22, forming a wake zone29inside and downstream of said aerodynamic block; Two injector nozzles60located inside wake zone29of each of said aerodynamic block; said nozzles60are fluidically communicating with said gas manifold10; An air control mechanism30comprising a center hub31and a plurality of air control modules33, said air control modules33fitting through said passage channels70, wherein each air control module comprising an air deflector35located at the outer edge of each of said passage channels70, said deflector forming an angle theta equal to or greater than 30 degrees from said axis of rotation.

Additional objects and features of the invention will appear from the following description from which the preferred embodiments are set forth in detail in conjunction with the accompanying drawings.

Identical reference numerals throughout the figures identify common elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purpose of this disclosure, the phrase “combustion air” may be air from the atmosphere supplied through the burner for combustion of the fuel, or may be the mixture of air and flue gas when the technique of FGR is used.

FIG. 1shows a trimetric view of an embodiment of the apparatus in accordance with the present invention.FIG. 2Ashows a front view of the burner inFIG. 1.FIG. 2Bshows a sectional view of the apparatus inFIG. 2A, taken along line A-A. The apparatus inFIGS. 1 and 2is typically referred to as a burner. The burner comprises a burner head1and a burner body100. The burner body100comprises an electric motor101(internal details not shown), a louver box102, a blower housing103, a transition duct104, an air duct105. The louver box102includes two air dampers106and an inlet pipe107for recirculated flue gas. A blower wheel108is located inside the blower housing103, and is driven by the electric motor101. The blower wheel108serves to provide the combustion air for the burner, and also serves to induce FGR (flue gas recirculation) through the inlet pipe107. The air dampers106and optionally a variable frequency drive (VFD, not shown) are used to precisely control the amount of combustion air for the proper combustion of the fuel. As is well known in the art, an FGR damper (not shown) is often used to control the ratio of the recirculated flue gas to the air from the ambient atmosphere.

FIG. 3is a front view of the burner head1.FIG. 4is a section view of the burner head inFIG. 3, taken along section B-B. The burner head1comprises a gas manifold10, four aerodynamic blocks20, an air control mechanism30, and a center bluff body40.

FIG. 5is a front view of the burner head1inFIG. 3with some parts removed for clarity, showing the gas manifold10and the aerodynamic blocks20.FIG. 6is a sectional view of the apparatus inFIG. 5taken along lines B-B.

FIG. 7is the same apparatus inFIG. 5but with the aerodynamic blocks removed.FIG. 8is the section view of the same apparatus inFIG. 7taken along section C-C to show an access port18and fuel gas outlet ports19.

Referring toFIGS. 3 through 8, the gas manifold10comprises a fuel gas inlet pipe11, an outer tube12, a blast tube13, a cone14, and an end cover16. The blast tube13is substantially cylindrical with as axis of rotation AoR. The burner head1also include a diverging cone15, a flange17A that affixes the burner to the front of a boiler, and a flange17B that affixes the burner to the air duct105. The blast tube13is preferably in a substantially cylindrical shape, due to its relationship with components20,30and40. The outer tube12, along with the blast tube13, the cone14and end plate16, forms an annular-shape gas manifold for the fuel gas. In the particular embodiment inFIG. 1 through 8, the outer tube12takes the shape of a cylinder; however, it could have taken other shapes such as rectangular or square in its cross section, and still functions as the outer wall of the gas manifold as well. Changing the external shape of the gas manifold to rectangular or square does not create a new invention outside the scope of the current invention. Similarly, the cone14could have taken the shape of a flat plate like the end plate16, and still functions just as well as a part of the gas manifold.

Referring toFIG. 8, a fuel gas enters the gas manifold10through inlet pipe11, and exits the gas manifold through eight outlet ports19, which are evenly distributed in four groups of two on the blast tube13. Each group of two ports19is housed in an aerodynamic block20. Access port18is an opening on the blast tube13allowing for human observation through sight port51, but it can similarly be used for the flame scanner52. The outlet ports19take the form of tube segments welded to openings on the blast tube13, with set screw ports19A to allow easy attachment and detachment of gas injection spuds60. The outlet ports19are pointing inwardly and radially toward the center axis of the blast tube13.

FIG. 9shows a front view of gas injection spud60.FIG. 10shows a section view of the gas spud60, taken along section E-E. The gas injection spud60consists of three parts, parts61,62and63. Part61has a male end65that goes into outlet port19, a grove66for receiving a set screw in port19A, and a female end that is 90 degree from the male end65. Part62is a cylindrical tube with a grove67to receive a set screw in port64. Part62is welded to part61. Part63is a nozzle with a three gas ports68, a female end and a screw port64. The gas injection spud60can be attached and detached from one of the eight ports19. By loosening the set screw through the port19A, the spud60can also be rotated around the axis of part61to adjust its orientation. Similarly, by loosening the set screw through port64, part63can be rotated around the axis of part62.

The gas injection spud60allows fuel gas to exit from one of the eight ports19into the part61, making a 90 degree turn from the radial and inward direction to the axial direction of the blast tube13, and exit into the combustion air stream through injection ports68. The injection ports68are generally pointing in the direction of the axis of the blast tube13, flowing in substantially the same direction of the combustion air, but it can incorporate a small angle alpha between the direction of fuel gas injection and the axis of the blast tube13. The small angle can allow the fuel gas to point slightly inward toward the center axis of the blast tube13, or outward away from the center axis of the blast tube13, or in any direction that may be advantageous to the shape of the flame and the emission performance of the burner. The number and size of ports68are dependents on the flow rate of the fuel gas and the gas pressure available. The fuel gas jets from ports68are located in the wake zone of the aerodynamic blocks20. It is believed that these fuel gas jets entrain a significant amount of internal flue gas before they are mixed with the combustion air stream (which may contain external flue gas), resulting in low NOx and even ultra low NOx emissions.

FIG. 11shows a schematic illustration of the relationship among different parts of the burner head. The inside diameter of the blast tube13is represented by an uppercase letter D. The outside diameter of the largest part of the cone42of the bluff body40is represented by a lowercase letter d. The shaded area in the center, marked with numeral40, is the projected area taken by the center bluff body40. The center bluff body represents 20-42% (preferably around 33%) of the cross sectional area inside the blast tube13. In other words, the ratio d/D should be in the range of 0.45 to 0.65. The four shaded areas marked with numeral20are the projected areas taken by the four aerodynamic blocks20. These areas together take up roughly another 20-40% (preferably around 33%) of the cross section area inside the blast tube13. The spaces marked with numeral70are passages channels formed between the center bluff body40and the blast tube13, and between the four aerodynamic blocks20. These passage channels70allow combustion air to pass through the burner head. An air control mechanism30is used to control how the combustion air passes through these passage channels70. Each passage channel70allows an air control module33to move back and forth in the axial direction of the blast tube13. Both the aerodynamic blocks20and the center bluff body40are all considered bluff bodies in the combustion community. It can be seen that the burner head design of the current invention uses a large portion of the cross section area of the blast tube as bluff bodies; it uses a relatively small portion of the cross section area of the blast tube for the flow of the combustion air. The bluff bodies act to create recirculation zones in the wake zones downstream of these bluff bodies, allowing an extremely stable flame to establish in the wake zones, and allowing internal flue gas recirculation (IFGR) to help reduce NOx. Due to the internal FGR that is inherent to the geometries of the burner head1, the burner head of the current invention is able to achieve low NOx emissions without external FGR, and ultra low NOx emissions with a reduced amount of external FGR. For example, the burner is able to achieve 25-40 ppm NOx emissions without the use of external FGR. With up to 25% external FGR, the burner is able to achieve NOx emissions as low as 3 ppm, dry volume based, corrected to 3% oxygen in the flue gas. The burner also enjoys a 10:1 or higher turndown for gas firing. It is capable of 8:1 turndown for oil firing. The burner can be operated with low excess air levels, which increases the thermal efficiency of the burner by minimizing the heat loss carried away by the exhaust gas, which is typically at a temperature higher than the ambient air.

FIG. 12shows four views of the aerodynamic block20. The aerodynamic block20comprises two oblique walls25joined at a leading end21, two vertical walls26forming and an open trailing end22, a triangular wall27and a removable wall28. The triangular wall27is welded to the oblique walls25. The removable wall28is attached to the triangular wall27by a set screw. Since the aerodynamic block20is attached to the inside of the blast tube13, a void space is formed inside the walls25and26, the walls27and28, and the blast tube13. This void space, together with the space downstream of the trailing end22, is referred to as a wake zone29. The wake zone29is characterized by relatively low flow velocity, since the approaching combustion air stream is diverted by the walls25,26and28of the aerodynamic block. The wake zone29provides space for two gas injection spuds60. The leading end21is narrow and substantially closed, with a tube23penetrating the leading end21to allow a small amount of combustion air to go into the aerodynamic block20, if it is so desired. The tube23has an open end24facing the air flow approaching the leading end21. The open end24can be threaded and capped off by a pipe cap (not shown). The open end24can be used as an access port for a gas fired pilot igniter (not shown), which is a common requirement in a burner. Referring toFIG. 12B, the combustion air approaches the leading end21, flows around the aerodynamic block20, and creates a wake zone29inside the aerodynamic block20and downstream of the trailing end22. The average flow velocity is reduced in the wake zone, helping to stabilize the flame. The wake zone is also believed to help the formation of internal flue gas recirculation (IFGR). When fuel gas is injected through ports68of spud60, the fuel gas entrains the internal flue gas before it mixes with the combustion air, which helps reduce the peak flame temperature and thermal NOx.

FIG. 13shows the structure of the air control mechanism30, which comprises a center hub31, four arms32, four air control modules33, and a positioning bracket34.FIG. 14shows a side view of the same air control mechanism30shown inFIG. 13. Each arm32connects an air control module33to the center hub31so that when the center hub moves forward or backward in the axial direction of the blast tube13, all four air control modules33move with the center hub in the same direction. Each arm32is welded to the center hub31, and is connected to an air control module33using two fasteners through two ports36. The center hub has the general shape of a pipe section, with its inner diameter machined smoothly to allow the tube41of the center bluff body40to go through. The positioning bracket can be connected to a corresponding bracket44on the center bluff body40through a rod.

FIG. 15shows three views of the air control module33, which comprises the deflector35, the outer wall38, two inner walls37, and two decelerators36. In the particular embodiment shown inFIG. 15, the two inner walls37are substantially parallel to each other, forming a passage channel with a rectangular cross section for the combustion air. The combustion air flows in this passage channel in parallel to the axis of blast tube13, and is directed by the deflector35inward toward the center of the axis of the blast tube13.

FIG. 16shows the rear view of the center bluff body40.FIG. 17are a section view of the same apparatus inFIG. 16. The enter bluff body40comprises a tube41, a diverging cone42, a cover cone43, a positioning bracket44. The diverging cone42has an outside diameter that is represented with a lower case letter d. The diverging cone42has four holes45allowing small amount of air to go through ports46on the cover cone43in order to cool the cover cone43, to avoid mechanical failure due to overheat from the flame. Three nuts47are used with three bolts48(two bolts48shown inFIG. 2) for centering of the tube41relatively to the blast tube13. The cover cone43tends to be subject to high temperatures due to the recirculation pattern formed in the stream of the cone42. In an alternative embodiment, the cover cone43could take the shape of a flat plate. In yet another embodiment, the cover cone43can be eliminated.

The tube41serves multiple purposes. First it provides a conduit for the insertion of an oil gun, where fuel oil or other liquid fuels can be injected for combustion. In many places, it is advantageous to be able to switch from a gaseous fuel to a standby liquid fuel when the supply of the gaseous fuel is in short supply or is interrupted. Second, it serves as a guide for the center hub31. The axis of the tube41substantially coincides with the axis of the blast tube13. When the center hub31slides forward or backward along the axis of the tube41, the entire air control mechanism30moves accordingly. This movement changes the locations of the air deflectors35relative to the cone42of the center bluff body40and the gas injection spuds60. This axial movement changes the flow pattern of the combustion air, which affects the flame shape. The axial movement of the air control mechanism30can be used to shape the flame from a bushy short flame to a narrow long flame, and vice versa.