Abstract:
A method and apparatus for reducing emissions in combustion systems, particularly gas turbines. A mixture of diluent and fuel is created, wherein the diluent and the fuel are at a predetermined diluent-to-fuel ratio. The mixture is homogenized to create a homogenized mixture having a uniform concentration distribution of the diluent and the fuel at the predetermined diluent-fuel ratio. Thereafter, the homogenized mixture is introduced into a flame zone and the homogenized mixture is combusted.

Description:
FIELD 
     The disclosure herein relates to the field of combustion systems, and more particularly, to a system for reducing emissions in combustion systems. 
     BACKGROUND 
     The reduction of harmful emissions has been a longstanding goal in the design of combustion systems, particularly power plants. The predominant emissions from gas turbine power plants are the oxides of nitrogen, or NO x . The most prevalent NO x  emissions are nitric oxide, NO, and nitrogen dioxide, NO 2 . 
     Although many combustion systems use natural gas, which is one of the cleanest-burning fuels, the NOx levels of these combustion systems remain relatively high. For example, in the standard household kitchen stove, the burner flame releases NOx emissions at about 48 parts per million (ppm). Other devices such as gas barbecue stands, hot water heaters, and Bunsen Burners also release NOx emissions at approximately that level. Therefore, there is a need to further reduce NOx emissions for combustion systems, particularly in power plants, but also in other combustion systems. Although electricity is the cleanest energy option, NOx emissions still occur, concentrated at the source where electricity is generated (i.e., at the power plants). 
     NOx emissions are produced by a high-temperature reaction of the nitrogen and oxygen contained in air. Reducing the combustion temperature reduces the level of NOx emissions. However, a reduction of the combustion temperature generally slows down the chemical reaction of carbon combustion, thereby generating high levels of carbon monoxide. For this reason, gas turbine combustion systems and natural gas burning power plants usually use a diluent such as steam or water spray in order to reduce the flame temperature. 
     Mixing steam and water creates turbulence, effectively increasing the diffusivity of the oxygen to be mixed with the fuel for combustion. Water droplets in the flame front evaporate rapidly, creating a phenomena known as “microexplosions.” While the injection of steam or water creates turbulence and reduces the flame temperature, the water vapor becomes an additional inert gas (other than nitrogen) with a high heat capacity. It has been shown that the use of such diluents in a gas turbine significantly reduces NOx emission levels, for example, to lower than 25 ppm. 
     NOx reduction improvements have stagnated, and a need remains to add further reduction means to combustion systems to reduce the NOx emission levels even more. Existing devices can be expensive and difficult to operate, and sometimes even create other emissions themselves. One such device is a selective catalytic reduction system (SCR), which uses ammonia and a catalyst to reduce the NOx emissions. A selective catalytic reduction system can normally reduce NOx emissions by 90% in the flue gas. However, ammonia itself can be a dangerous substance, and under high temperature conditions, ammonia can react violently with water, causing bums and eye injuries. Ammonia also decomposes into nitrogen and hydrogen, which is an undesired and unproductive result. Therefore, there is a need to further reduce NOx emissions of combustion systems through more practical and effective means. 
     FIG. 1 shows the structure of a typical diffusion flame. The gaseous fuel enters through a nozzle  10  and is supported by a diffusion flame such as a fuel injector or a candle. The flame structure can be simplified into a paralysis zone  12  (shown cross-hatched in the middle), a fuel diffusion zone  14 , and a flame surface  16 . Oxygen is diffused from the surrounding area toward the flame surface. Under the diffusion flame structure, the combustion reaction can only take place on the flame surface  16  when the fuel and oxidizer reach the stoichiometric ratio. The temperature at the flame surface therefore remains substantially constant and independent of the rate at which fuel is emitted into the nozzle  10 . The change to a higher fuel emission rate would cause a larger flame surface. 
     The heat from the flame surface transfers back to the center of the fuel supply, causing the fuel to be paralyzed into smaller chemical elements such as carbon and hydrogen. These smaller elements diffuse toward the flame surface to support the combustion process. The combustion heat is divided between the combustion products and ambient inert gas. If the surrounding gas is air, then nitrogen will remove some of the heat without participating in the chemical reaction, thereby lowering the overall flame surface temperature. However, if the gas is pure oxygen, the flame surface will reach its highest possible combustion temperature. A gas that does not react with oxygen also can act as an inert gas, removing heat from the flame temperature without participating in the chemical reaction and thereby further lowering the flame temperature. 
     FIG. 2 a  illustrates a typical mutual diffusion profile of fuel and oxidizer without combustion. That is, FIG. 2 a  represents a diffusion phenomena of fuel and oxidizer as a concentration profile with respect to distance from the centerline (i.e., from the source of the fuel or the middle of the paralysis zone) without combustion. The x-axis represents the distance from the source of the fuel. No chemical reaction has taken place in FIG. 2 a . In FIG. 2 b , when the chemical reaction occurs, in the form of combustion, the concentrations of fuel and oxidizer both approach zero at the flame surface. The concentration of the combustion products is highest at the flame surface. Despite the disappearance of fuel and oxidizer, however, the flame maintains the diffusion rate present when the concentrations of fuel and oxidizer are at the stoichiometric ratio, as illustrated in FIG. 2 a.    
     FIG. 3 illustrates the flame height as a function of turbulence level with an increasing fuel nozzle jet velocity. The left side shows a very long flame having a height that increases along with the fuel jet velocity. The flame is a laminar flame. The right side shows the flame as the fuel jet velocity increases. Although the height of the flame decreases at first, an increase of the fuel jet velocity eventually keeps the turbulent diffusion flame at a constant height. With the laminar flame on the left side, the flame diffusion is strictly molecular. Therefore, the surface area of the flame remains proportional to the fuel ejection rate from the fuel nozzle. When the velocity continues to increase, it induces turbulent mixing which greatly increases the molecular diffusivity. The jet of the fuel nozzle finally reaches a condition known as a similarity flow, which means that the flame is at a constant flame height. The similarity flow occurs when the turbulent mixing profile becomes independent of the magnitude of the velocity. 
     When the chemical reaction rate is slower than the turbulent diffusion rate, the flame will be lifted from the fuel nozzle, creating a blowout condition. FIG. 4 illustrates combustion flame profiles with respect to blowout conditions. FIG. 4 a  illustrates the condition of fuel with an extremely high jet velocity. In order to improve the chemical reaction rates and stabilize the flame, some of the combustion products are recirculated through turbulent mixing as chemical reaction seed material. The bell-shaped profile in FIG. 4 a  illustrates the root of the flame, and the cone-shaped region represents the turbulent combustion of fuel and air. When the velocities of both fuel and air increase, the root of the flame lifts away from the nozzle, leaving certain recirculation chemical species to support the combustion. As illustrated in FIG. 4 b , when the velocity increases, the recirculation is reduced, causing the flame to lift away from the nozzle and creating a neutral condition. FIG. 4 c  illustrates the results of a maximum increase in the velocities of both the jet and air. Chemical species can no longer recirculate, and the flame completely lifts from the nozzle, creating a blowout condition. Candles illustrate this phenomena well: when one blows gently on a candle, the combustion rate of the candle increases. However, as one blows harder on the candle, the combustion rate catches up to the diffusion rate, thereby extinguishing the flame. 
     As mentioned above, a particular need is to reduce the level of NOx emissions in gas turbines. The publication “Fundamentals of Gas Turbines, Second Edition,” William W. Bathie, provides a detailed description of gas turbines, and is hereby incorporated by reference. FIG. 5 illustrates a typical gas turbine combustion system. The outside liner  20  has many dilution holes  30 . A pre-mixing swirler  40  surrounds a fuel nozzle  50 . The dilution holes  30  create a recirculation flow which serves to guide the combustion product back into the primary combustion zone to help accelerate the chemical reaction of combustion. The swirler  40  creates the fundamental turbulent mixing for the fuel jet as the fuel exits the hole  51 . This design uses recirculation and turbulence to establish a similarity flow. The combustion products then mix with dilution air through the dilution holes  30  to reach a final temperature before entering the nozzle of the gas turbine. 
     FIG. 6 illustrates prior art devices used in the industry. A concentric nozzle  61  has fuel and diluent injections for creating a turbulent flame. Specifically, one conduit supplies fuel, while the other supplies steam or water. The concentric nozzle  61  is surrounded by another system  63 . The turbulence of the fuel, and the high velocity of the diluent (such as a steam jet or water spray), usually create the flame mixing region. The steam, fuel, and air are mixed while burning or combusting. A problem with this prior art device is that the length of the mixing depends on the geometry of the nozzle for a turbulent jet; therefore, the concentrations are not homogeneous. Some places have more fuel than other places, which does not ensure that the steam, fuel, and air is a homogenous mixture. As a result, “hot spots” are produced and the NOx level is relatively high. As explained with regard to FIG. 1, the temperature of the flame surface is uncontrollable by bulk mixing. In fuel-rich regions, the flame temperature can still reach a very high level and produce NOx. 
     “Homogenous” as used in this specification means a concentration deviation from the average, with average being 100% homogeneous. For example, if a closed vessel contains on average 50% fuel and 50% air, and in a localized region actually contains 49% fuel and 51% air, then the concentration deviation from the average, or from the overall ratio of components, is 2%, denoting 98% homogeneity. 
     The concentration deviation from the average of prior art devices using turbulent mixing is believed to be in the approximate range of 15%-25%, or, a range of homogeneity from 75%-85%. It is an object of the disclosure herein to significantly improve upon the percentage of homogeneity present in prior art combustion systems. 
     FIG. 7 illustrates a traditional coaxial mixing of a jet of fuel surrounded by another gas (in this case, air). The solid contour lines represent fuel concentration. For example, a fuel concentration of 0.1 represents 10% fuel and 90% air. Although 1.0 is not marked on the figure, it is indicated by the last contour of fuel coming out over the nozzle. The data relating to FIG. 7 showed that even at more than 20 diameters downstream of the fuel nozzle, the homogeneous mixing was nowhere near completion. Therefore, the turbulent flame creates uncertainties in terms of concentration fluctuations as represented by the dash lines in the region containing a 50/50 mixture average. If the surrounding gas is steam, then this mixture represents rich and lean regions of fuel mixed with steam. The turbulent properties and fluctuation intensity of this mixture subject it to different temperature fluctuations. Unfortunately, a region with a higher fuel concentration will have a higher flame temperature, and, consequently, produce a higher level of NOx emissions. Because of this, this prior art nozzle design has not been able to achieve a NOx level below approximately 20 ppm in gas turbines burning natural gas. 
     FIG. 8 shows typical plots of NOx and CO productions based on a well-stirred combustion situation as a function of flame temperature. This graph was generated assuming that the turbulence levels were high enough for combustion to occur at ratios other than the stoichiometric ratio. These plots illustrate the best attempts at reducing NOx productions with a highly turbulent, lean, well-stirred combustion situation. Previously used as the most advanced technology in gas turbines, these systems are called Dry Low NOx Combustion Systems (DLN). The word Dry (D) indicates a lack of mixture with steam or water. It is clear that further NOx reductions are needed. 
     SUMMARY 
     One object of the disclosure herein is to reduce the level of NOx emissions in combustion systems well below that of natural flame processes. To achieve this object, the disclosure herein teaches to homogeneously pre-mix the fuel with a diluent, such as steam, before it enters the diffusion flame system. To eliminate the hot spots in a turbulent flame, the concentration distribution of a turbulent jet using the teachings of the disclosure herein becomes uniform. Another object of the disclosure herein is to simplify combustion systems by using a static mixer to save space in the system. Another object is to sustain lean combustion without flameouts, using homogeneous mixing and a pilot third gas. Ultimately, the disclosure herein greatly reduces NOx emissions in combustion systems at a decreased cost by means of a simplified arrangement. 
     The disclosure herein in a preferred embodiment provides a method for reducing emissions in a combustion system, comprising the steps of creating a mixture of diluent and fuel, wherein the diluent and the fuel are at a predetermined diluent-to-fuel ratio, homogenizing the mixture to create a homogenized mixture having a uniform concentration distribution of the diluent and the fuel at the predetermined diluent-fuel ratio, and, thereafter, introducing the homogenized mixture into a flame zone and combusting the homogenized mixture. 
     The diluent can be steam. The homogenizing step can be performed by a compact mixer. The homogeneity of the homogenized mixture is preferably in the range of 97-99%. A third gas such as air, hydrogen, or hydrogen peroxide may be added to the mixture before the homogenizing step. The predetermined diluent-to-fuel ratio is preferably in the range of 0.2 to 1, or 0.2 to 3. “Ratio” as used in this specification means the ratio by weight of components. 
     The disclosure herein in another embodiment provides a gas turbine. The gas turbine has a compressor and a chamber disposed downstream of the compressor for receiving diluent and fuel at a predetermined diluent-to-fuel ratio to form a mixture. A compact mixer is disposed downstream of the chamber for homogenizing the mixture to create a homogenized mixture having a uniform concentration distribution of the diluent and the fuel at the predetermined diluent-fuel ratio. A combustion section is disposed downstream of the compact mixer for combusting the homogenized mixture after the homogenized mixture leaves the compact mixer to produce a hot energetic flow of gas. A turbine is disposed downstream of the combustion section driven by the hot energetic flow of gas for driving the compressor. 
     Experiments have proven the teachings of the disclosure herein, wherein the mixture of gaseous fuel and diluent is homogenized, to be effective for reducing emissions in combustion systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the structure of a typical diffusion flame; 
     FIG. 2 a  illustrates a typical mutual diffusion profile of fuel and oxidizer without combustion; 
     FIG. 2 b  represents the diffusion of fuel, oxidizer, and combustion products with combustion; 
     FIG. 3 illustrates the flame height as a function of fuel ejection velocity; 
     FIGS. 4 a ,  4   b ,  4   c  illustrate combustion flame profiles with respect to blowout conditions; 
     FIG. 5 illustrates a typical combustion liner structure in a jet engine; 
     FIG. 6 is a typical structure of a concentric nozzle with fuel and diluent injections for creating a turbulent flame; 
     FIG. 7 illustrates a typical mixing of fuel and air using a jet mixing method; 
     FIG. 8 illustrates typical emission products of NOx and CO as a function of flame temperature; 
     FIG. 9 illustrates the system according to a preferred embodiment, including a homogenization of fuel and diluent before they enter the fuel injection tips; 
     FIG. 10 is a steam-to-fuel ratio of natural gas as a function of NOx emission and CO production at a typical gas turbine discharge temperature; 
     FIG. 11 illustrates a control system leading to the control of homogenized fuel diluents; 
     FIG. 12 is a table showing third gases available to act as a pilot; and 
     FIG. 13 is a test configuration for a gas turbine. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As is evident from the above-described background, there is a need to further reduce emissions in combustion systems. A homogeneous mixing of diluent (such as steam) and fuel operates to decrease then the nitrogen concentration of the flame. The disclosure herein uses certain dry low NOx principles, but also uses a well-stirred mixture of steam and fuel to achieve more effective results, since the steam replaces nitrogen as one of the participants in the flame zone. If the nitrogen concentration in the flame zone decreases, then the production of NOx can be expected to decrease. The disclosure herein teaches a homogeneous mixing of steam and fuel first, and then an intermixture with turbulent air so that the concentration remains uniform. 
     FIG. 9 illustrates a preferred embodiment for the homogenization of fuel and diluents before they enter the fuel injection tips. As illustrated, the fuel and steam sometimes mix with another gas through a compact or static mixer  80 . The fuel, steam, and third gas (optional) are injected through a “cross” or “Tee” piping system  70 . This mixture, passing through the static mixer  80 , produces a homogeneous mixing of all fluids involved. The mixture then passes into the gas turbine nozzle  90  for combustion. 
     The disclosure herein significantly improves upon the percentage of homogeneity present in prior art combustion systems, which as mentioned above typically have a range of homogeneity of approximately 75%-85%. The disclosure herein achieves homogeneity of significantly greater than 85%; preferably, the homogeneity is greater than 90%, more preferably is greater than 95%, and most preferably is greater than 97%. 
     The chemical kinetics with steam can be improved by adding a third gas as mentioned above. For example, in a normal kitchen stove, the natural gas and air are pre-mixed before they exit the stove&#39;s burner holes. This pre-mix reduces the need for rapid diffusion of oxygen to the flame front without a turbulent flow. In the combustion of a gas turbine, on the other hand, the velocity of the combustion fluid is very high. The mixing occurs so rapidly that it will sometimes reach a blowout limit, even with recirculation. The disclosure herein teaches the introduction of the third gas, thereby providing a much lower ignition temperature as a pilot to sustain combustion at very lean mixing conditions. Some of the gases available for this purpose, such as hydrogen and hydrogen peroxide, are listed in the table in FIG.  12 . Using such a third gas, or a mixture of such third gases, will ensure stable combustion under leaner conditions than at the stoichiometric ratio and at a lower flame temperature. 
     A flame is ordinarily at the stoichiometric ratio. “Lean” means that there is more air than fuel. In other words, the amount of fuel concentration present is reduced. This lowers the flame temperature, reducing the NOx level, but also causes the flame to be unstable. Adding a third gas in accordance with the disclosure herein accelerates the burning process, thereby stabilizing the flame. 
     The purposes of the pilot gas are therefore to sustain combustion and reduce NOx emissions. Using the configuration of FIG. 9, experiments have proven that the flame can be stabilized at a fuel-to-steam ratio of significantly more than 1/1, for example up to 2/1 or even 3/1; traditional nozzles were limited by fuel-to-steam ratios of close to 1/1. 
     FIG. 10 illustrates the results of experiments using a GE Frame  5  combustion liner transition piece and gas fuel nozzles. The steam is homogeneously mixed by the system of FIG. 9. A NOx level as low as 2 ppm has been obtained. In the region of a 5 ppm NOx level, the flame remains quite stable, with a relatively low CO production and a wide range of turndown ratios. While experimental temperatures covered the range of approximately 600 C to 1000 C, the operating region of GE Frame  5  gas turbines, FIG. 10 in particular shows the results of experiments conducted at an optimal turbine inlet temperature (TIT) of 1800 F, or approximately 982 C. To implement the control systems, the piping systems preferably use metered flows of fuel, steam, and the third gas, if necessary. The disclosure contemplates using a range of diluent-fuel ratios such as 0.2 to 1 or 0.2 to 3 along with its other teachings to both sustain flame stability and maintain low NOx emissions. 
     FIG. 10 is plotted in terms of the weight ratio. The weight ratio is the number of pounds of fuel vs. the number of pounds of steam. Dividing by the molecular weight gives the volume ratio. Steam has a molecular weight of 18. Methane, for example, has a molecular weight of 16. Therefore, the difference between the volume ratio and the weight ratio using methane as the fuel is relatively small (roughly 12%). 
     FIG. 11 is a piping diagram illustrating an embodiment with steam entering at port A through a control valve  101  and fuel entering at port B through a control valve  100 . A third gas, if used, will come through port C, controlled by valve  102 . The static mixer  80  is mounted downstream of all pipe connections and before the fuel nozzles. Each mixer can have a metering system. For instance, meter  105  corresponds to steam, meter  106  corresponds to fuel, and meter  107  corresponds to the third gas. To maintain the mixture ratio, computer controls use the meters as feedback to set the valve positions, providing a correct fuel-steam ratio, with the optional third gas. The objective of this design is to homogeneously mix fuel and steam before they enter combustion system nozzles. The static mixer is a means for shortening the mixing length. Alternatively, if space is available for an adequate length of pipe which can achieve homogeneous mixing, a similar result can be achieved without using static mixers. 
     The disclosure herein thereby teaches to reduce the level of NOx emissions in combustion systems. This is accomplished by homogeneously pre-mixing the fuel with a diluent, such as steam, before it enters the diffusion flame system. Alternative embodiments can use Argon, Helium, or other non-chemical reacting gases instead of steam. A uniform concentration distribution of a turbulent jet operates to eliminate the hot spots in a turbulent flame. The disclosure herein also teaches to simplify combustion systems by using a static mixer to save space. The disclosure herein also teaches to sustain lean combustion without flameouts, using homogeneous mixing and a pilot third gas. Ultimately, NOx emissions in combustion systems are greatly reduced at a decreased cost by means of a simplified mechanical arrangement as taught by the disclosure herein. 
     The teachings of the disclosure herein have been experimentally tested on full-scale gas turbine combustion systems, using test configurations such as illustrated in FIG.  13 . The results indicate that much lower NOx emission levels and stable flames can be obtained compared to previously existing current concentric steam fuel nozzle systems. The results also indicate that a NOx level below that of the Dry Low NOx (DLN) systems used in today&#39;s gas turbines can be achieved without the cost and complication of a DLN system. The disclosure herein teaches a mechanical arrangement of a fuel/diluent system using a homogeneous mixing method to achieve uniform combustion properties of flames before entering the fuel nozzle as a way to reduce NOx emissions. The current design uses a well-stirred mixing principle to achieve the homogeneous combustion property of a diffusion flame. This method both simplifies the combustion system and stabilizes the flame for gas turbine systems, thereby eliminating alternatives which can be expensive such as the Selective Catalytic Reduction system (SCR) or the absorption system. This device is a significant step toward implementing NOx reduction methods for all combustion systems, particularly power plants. 
     The above specific embodiments are illustrative, and a person skilled in the art can introduce many variations on these embodiments without departing from the spirit of the disclosure or from the scope of the appended claims. The embodiments are presented for the purpose of illustration only and should not be read as limiting the claimed invention or its application. Therefore, the claims should be interpreted commensurate with the spirit and scope of the disclosure and its variations.