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  Combustion system with high turn down ratioUSPTO Application #: 20060183068Title: Combustion system with high turn down ratio Abstract: A low NOx combustion system for reducing the production of nitrogen oxides in its emissions while allowing unusually high turndown ratios in operation is shown. The system includes an improved burner which has a firing head with an outlet end which can be mounted on a sidewall of a heat exchanger such as a boiler. The burner includes a fan, a windbox, a burner drawer assembly and a gas manifold. The windbox has an internal scroll-shaped passageway which is contoured to improve air flow from the fan to the firing head. An improved air straightener and diffuser head in the burner drawer assembly cooperate with improvements in the air supply system to provide improved combustion characteristics and a higher than usual turndown ratio for the burner assembly. (end of abstract) Agent: Charles D. Gunter, Jr. Whitaker, Chalk, Swindle & Sawyer, LLP - Forth Worth, TX, US Inventors: Theodore J. Wrona, Gene A. Tompkins USPTO Applicaton #: 20060183068 - Class: 431354000 (USPTO) Related Patent Categories: Combustion, Mixer And Flame Holder The Patent Description & Claims data below is from USPTO Patent Application 20060183068. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates generally to a combustion system, such as a fire tube boiler or furnace combustion system. More specifically, the invention relates to such a combustion system that reduces the NO.sub.x emissions while allowing an unusually high turn down ratio during operation. [0003] 2. Description of the Prior Art [0004] In the operation of heat exchangers, such as furnaces or boilers, various gases are produced as by-products, including the so called nitrogen oxides (NO.sub.x). Such oxides of nitrogen, when produced in combination with hydrocarbons present in the atmosphere, constitute a major source of pollution in the environment. Depending on the type of fuel being burned, there are generally two types of nitrogen oxides which can be formed. Fuel bound NO.sub.x is formed as a result of nitrogen being present in the fuel itself, i.e., in fuel oils. During combustion, the nitrogen is released and quickly reacts with the oxygen in the combustion air to form NO.sub.x. Thermal NO.sub.x is formed, on the other hand, when high combustion temperatures break down the nitrogen gas in the combustion air, resulting in the formation of atomic nitrogen. When this occurs, the atomic nitrogen will very quickly react with oxygen to form thermal NO.sub.x. If natural gas is employed as the furnace or boiler fuel, only thermal NO.sub.x should be formed, because clean natural gas generally does not contain any significant nitrogen containing compounds. On the other hand, both thermal and fuel bound NO.sub.x are formed when burning fuel oils. [0005] Since the production of NO.sub.x by the burning of fuels in the operation of boilers and furnaces is potentially damaging to the environment, various environmental emissions standards have been imposed by various governmental authorities and agencies to regulate and to suppress the formation of nitrogen oxides during operation of boilers and furnaces. Various techniques have been utilized in the design and operation of boilers and furnaces to meet those regulations. [0006] For example, it is known that burning a hydrocarbon fuel in less than a stoichiometric concentration of oxygen will increase CO production. This concept is utilized in a staged air type low NO.sub.x burner where the fuel is first burned in a deficiency of air in one zone to produce an environment that suppresses NO.sub.x formation, and then the remaining portion of the air is added in a subsequent zone. The use of staged fuel has also has been suggested for suppressing the NO.sub.x formation. In staged fuel, the air and some of the fuel is burned in the first zone and then the remaining fuel is added in the second zone. The subsequent lowering of the combustion temperature in the first zone is thought to suppress NO.sub.x formation. Another widely used technique to reduce NO.sub.x emissions is to recirculate flue gas to one or more of the combustion zones to lower the flame temperature and reduce NO.sub.x formation. [0007] Despite the success resulting from the use of such techniques as flue gas recirculation, certain of these prior art processes have exhibited deficiencies and associated problems which have led to limited commercial acceptance. For example, flame stability can be a critical factor when operating a burner at significantly sub-stoichiometric conditions. Moreover, many of the prior processes and systems have been complicated and expensive to build, install, use and maintain and require extensive modifications of standard furnaces, boiler and fuel burners. [0008] In the case of a boiler burner or other industrial heat exchanger burner, the purpose of the burner is to mix molecules of fuel with molecules of air. A boiler will run only as well as the burner performs. For this reason, a poorly designed boiler with an efficient burner may perform better than a well designed boiler with a poor burner. Burners are designed to maximize combustion efficiency while minimizing the generation of emissions. Thus, a particularly efficient burner design may reduce or eliminate many of the emissions problems associated with NO.sub.x and other undesirable by-products of the combustion process. [0009] A power burner mechanically mixes fuel and combustion air and injects the mixture into the combustion chamber. All power burners provide essentially complete combustion while maintaining flame stabilization over a range of firing rates. Different burners, however, require different amounts of excess air and have different "turndown ratios." The turndown ratio can be defined as the maximum inlet fuel or firing rate divided by the minimum firing rate. Turndown ratio can also be used to compare the maximum to minimum heat output. For example, a turndown ratio of 25:1 means that the burner can modulate from 4% to 100% of full fire. On the other hand, a turndown ratio of 2.5:1 would limit the heat output from 40% to 100% of full fire. [0010] Most gas burners of the type under consideration in the marketplace exhibit turndown ratios in the range from about 5:1 to about 8:1, with little or no loss in combustion efficiency. A higher turndown ratio reduces burner starts, provides better load control, saves wear and tear on the burner, reduces refractory wear, reduces purge-air requirements, and provides fuel savings. It would thus be advantageous to provide a burner of the type under consideration with a higher turndown ratio without increasing NO.sub.x emissions. [0011] It is an object of the present invention to provide a particularly efficient combustion system for burning oil or gas, which combustion system utilizes a burner with an exceptionally high turndown ratio. SUMMARY OF THE INVENTION [0012] Accordingly, a primary object of the invention is to provide a low NO.sub.x combustion system that reduces the amount of NO.sub.x formed during combustion. [0013] It is a further object of this invention to provide a burner system which will provide low NO.sub.x burning for a wide range of fuel burning rates and corresponding air or oxidant supply rates. [0014] Another object of the invention is to provide a low NO.sub.x combustion system which provides turndown ratios much higher than those conventionally achieved by commercially available equipment. [0015] Another object of the invention is to provide a low NO.sub.x combustion system which is relatively inexpensive to manufacture, install, use, and maintain, and requires no significant heat exchanger modification. [0016] The foregoing objects are basically obtained by providing a low NO.sub.x combustion system comprising a heat exchanger having sidewalls defining a closed interior containing a medium to be heated and a burner having a firing head with an outlet end, the outlet end being mounted on a selected sidewall of the heat exchanger. A fuel supply means is fluidly coupled to the burner for conveying a combustible fuel to the burner. An air supply means is fluidly coupled to the burner for conveying combustion supporting air to the burner. An igniting means is positioned adjacent the outlet end of the burner for igniting the combustible fuel to thereby heat the medium contained in the heat exchanger. A windbox forms a part of the air supply means which connects with the firing head, the windbox having an exterior comprised of opposing sidewalls connected by a mid wall and having an interior including a scroll-shaped interior passageway and inlet and outlet openings. Air from the air supply means travels at a right angle to the windbox opposing sidewalls as it enters the inlet opening and as it passes out the outlet opening in passing from the air supply means to the burner. The scroll-shaped passageway of the windbox is defined by lateral edges which fit at right angles to the windbox opposing sidewalls, the lateral edges being flush with the outlet opening without forming a lip region with respect to the opposing sidewalls, thereby providing more uniform air flow though the windbox to the burner. [0017] The firing head includes a burner drawer assembly located within a head extension which connects the windbox to the sidewall of the heat exchanger. The burner drawer assembly includes a diffuser, a pilot, a scanner and an air straightener all carried on a longitudinal support tube which extends perpendicular to a back plate. The preferred air straightener comprises a single plate having a length and a width and opposing side edges which define opposing planar surfaces. The plate is mounted on the longitudinal support tube along a selected opposing side edge thereof. Preferably, the longitudinal support tube comprises an oil gun tube, the oil gun tube being slidably received within an opening provided in the back plate, whereby the position of the air straightener can be varied longitudinally by sliding the oil gun tube within the opening provided in the back plate. [0018] The firing head diffuser provides directional control of combustion air for mixing and combustion stability. The diffuser includes an manifold plate with a first and second annular wall regions. The first annular region forms a collar-like region surrounding a central opening in the manifold plate. The second annular wall region is made up of a series of overlapping fins which are separated by slits. The slits provide a swirling action to combustion air passing through the diffuser. A plurality of gas orifices are located about the outer periphery of the second annular wall region and are arranged in a circular array for conveying and communicating natural gas outward and into a combustion region of the burner. [0019] The air supply means also includes a combustion air fan which attaches to the windbox, whereby the windbox routes combustion air from a fan inlet to the firing head. The combustion air fan has an associated air damper which communicates with an inlet to the combustion air fan. The air damper comprises a box-like enclosure which preferably houses a single damper blade. The damper blade has upper and lower longitudinal sealing edges and opposing side edges. The upper and lower longitudinal sealing edges and side edges are provided with resilient sealing strips which provide ease in adjustment for leakage. [0020] In the preferred embodiment of the invention, the head extension connects to a cylindrical gas manifold which, in turn, connects to the heat exchanger sidewall. The gas manifold has radial gas ports used to direct gas fuel to the burner outlet. The radial gas ports may have gas spuds installed therein to improve the distribution of the gas. The gas manifold holds the outlet end of the burner assembly. The gas manifold also has an outer face which is protected from flame temperatures at the burner outlet by a refractory front plate. Preferably, a ceramic blanket is used between the outer face of the gas manifold and the refractory plate to further prevent the transfer of heat. [0021] Additional objects, features and advantages will be apparent in the written description which follows. 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