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  Method for control of nox emissions from combustors using fuel dilutionUSPTO Application #: 20070031768Title: Method for control of nox emissions from combustors using fuel dilution Abstract: A method of controlling NOx emission from combustors. The method involves the controlled addition of a diluent such as nitrogen or water vapor, to a base fuel to reduce the flame temperature, thereby reducing NOx production. At the same time, a gas capable of enhancing flame stability and improving low temperature combustion characteristics, such as hydrogen, is added to the fuel mixture. The base fuel can be natural gas for use in industrial and power generation gas turbines and other burners. However, the method described herein is equally applicable to other common fuels such as coal gas, biomass-derived fuels and other common hydrocarbon fuels. The unique combustion characteristics associated with the use of hydrogen, particularly faster flame speed, higher reaction rates, and increased resistance to fluid-mechanical strain, alter the burner combustion characteristics sufficiently to allow operation at the desired lower temperature conditions resulting from diluent addition, without the onset of unstable combustion that can arise at lower combustor operating temperatures. (end of abstract) Agent: Sandia Corporation - Albuquerque, NM, US Inventors: Robert W. Schefer, Jay O. Keller USPTO Applicaton #: 20070031768 - Class: 431004000 (USPTO) Related Patent Categories: Combustion, Process Of Combustion Or Burner Operation, Feeding Flame Modifying Additive The Patent Description & Claims data below is from USPTO Patent Application 20070031768. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0002] Not applicable. FIELD OF THE INVENTION [0003] This invention is directed to a method for the reduction of NOx emissions from combustors. BACKGROUND OF THE INVENTION [0004] The development of low emission, high performance combustors is an area of much current interest. In particular, NOx emissions from numerous major combustion sources such as gas turbines for power generation and aircraft propulsion as well as a variety of boilers, furnaces and heaters are a major environmental problem. Proposed future reduction of allowable NOx emission levels will only increase the need for effective control strategies. Consequently, the reduction of these emissions in an efficient and cost effective manner will have a major economic impact. [0005] It is well known in the art that NOx emissions from combustors are largely determined by combustion temperature, i.e., lower combustion temperatures result in a exponential decrease in NOx emission levels. Coupled with this is the fact that the extent of reduction of gas temperatures in the combustor can be limited by the onset of combustion instabilities. These instabilities generally lead to incomplete combustion of the fuel, unstable flames, the release of higher quantities of carbon monoxide (CO) and unburned hydrocarbons (UHC), and in the limit, flame extinction. High amplitude pressure oscillations in the combustion chamber, driven by combustion heat release, can also be present. Under the right conditions, the amplitude of these pressure fluctuations increases and can, at a minimum, result in a degradation of combustor performance. In the limit, the amplitude of the pressure fluctuations can be sufficient to cause significant damage to combustor hardware and burner components. Whether the combustor operates in a stable mode or an unstable mode is determined by numerous factors. These can include, but are not limited to, fuel type, fuel/air ratio, inlet pressure, combustor geometry, combustor throughput, and the coupling between combustion chamber design and flame heat release. [0006] Combustion consists of a chemical reaction between a mixture of fuel and air to release heat. The term equivalence ratio is often used to identify the actual quantities of fuel and air provided. As used herein, the term is defined as the ratio of fuel to air provided divided by the stoichiometric ratio of fuel to air. The stoichiometric ratio is achieved when the proper amount of air is provided to completely consume all the fuel. Thus, an equivalence ratio of unity corresponds to an amount of air exactly equal to that needed to consume all the fuel while an equivalence ratio less than unity indicates excess air, i.e., a fuel lean condition. Typically maximum combustion temperatures occur at near stoichiometric conditions (near an equivalence ratio of unity). As the equivalence ratio exceeds or becomes less than unity the combustion temperature decreases with a concomitant decrease in NOx emissions since these emissions are a strong function of temperature, increasing exponentially with increasing temperature. [0007] Most current combustors operate in a non-premixed mode where the fuel and air are introduced separately. An advantage of this mode is that potential safety problems such as flame flashback, which can occur when the fuel and air are premixed prior to combustion, are eliminated. In the non-premixed mode combustion occurs predominately at stoichiometric conditions where the maximum temperatures are produced. As discussed above, this high temperature combustion maximizes the production of NOx. The addition of diluents, such as nitrogen or water vapor, can be an effective control strategy for NOx emissions since they tend to lower the combustion temperature. However, this emissions control strategy is limited by the finite operating range of a combustor. At high dilution levels the flame temperature becomes sufficiently low that the heat loss rate exceeds the combustion heat release and the flame can no longer sustain itself. This condition is referred to as the "flame blowout limit" leads to flame extinction and provides an upper boundary for the amount of diluent addition. It is known in the art that because of the wide flammability limits and faster burning rates of hydrogen, the addition of hydrogen to a conventional hydrocarbon fuel, such as methane or natural gas, significantly improves the low temperature combustion characteristics and extends the lean fuel blowout limit so that lower temperature operation can be achieved. A more detailed discussion of the effects of hydrogen on combustion characteristics can be found in co-pending application Ser. No. 10/091,044, filed Mar. 4, 2002, entitled "Method for Controlling Lean Combustion Stability". [0008] Several approaches are currently used in gas turbine combustion systems to reduce NOx emissions. These are typically passive control approaches that include changes in combustion chamber design, variable geometry designs, lean-premixed combustion, staged combustion designs selective catalytic reduction (SCR) with ammonia addition and modification of the injected fuel distribution pattern by modification of the fuel injector design or the air inlet pattern, among others. These approaches are often costly and limited in terms of their effectiveness. SUMMARY OF THE INVENTION [0009] Accordingly, the invention is directed generally to a low cost and easily installed method for simultaneously reducing combustor flame temperature, thereby reducing NOx emissions, while simultaneously enhancing low temperature flame stability in order to reduce or eliminate undesirable effects associated with unstable combustion, as described above. In particular, the invention is directed to the controlled addition of a diluent such as nitrogen or water vapor, to a base fuel to reduce the flame temperature, while a the same time adding to the fuel mixture a gas capable of enhancing flame stability and improving low temperature combustion characteristics, such as hydrogen. The method described herein is equally applicable to base fuels such as natural gas, coal gas, biomass-derived fuels, methane, and other common hydrocarbon fuels. The unique combustion characteristics associated with the use of hydrogen, particularly faster flame speed, higher reaction rates, and increased resistance to fluid-mechanical strain, alter the burner combustion characteristics sufficiently to allow operation at the desired lower temperature conditions resulting from diluent addition, without the onset of unstable combustion that can arise at lower combustor operating temperatures. [0010] The exact nature of unstable combustion is dependent on combustor geometry and operating conditions. The onset of unstable combustion resulting from reduced flame temperature is illustrated graphically for a generalized combustor geometry in FIGS. 1a and 1b where the combustor flow rate is plotted versus the flame temperature. In the combustor geometry illustrated in FIGS. 1a and 1b, the unstable operating region is located just to the right of the flame blowout limit. For a fixed combustor flow rate, unstable combustion occurs over a finite range of flame temperatures; combustion cannot be sustained at temperatures to the left of the flame blowout limit line. FIG. 1a illustrates the case for no hydrogen addition. Operation of a particular combustor for NOx control within the unstable operating region could lead to degradation in combustor performance and eventually to flame blowout or extinction. [0011] The effect of hydrogen addition on the unstable operating region is shown in FIG. 1b. Here, the faster chemical reaction times resulting from hydrogen addition result in a shift in the unstable region and the flame blowout line to lower temperatures. This shift allows operation at the desired combustor flow rate and flame temperature. BRIEF DESCRIPTION OF THE DRAWINGS [0012] Figures 1a and 1b illustrate the relationship between combustor operating point and unstable operation without hydrogen addition (1a) and with hydrogen addition (1b). [0013] FIGS. 2a and 2b show the effect of inlet gas velocity on maximum flame temperature T.sub.max with no H.sub.2 addition (FIG. 2a) and 50% H.sub.2 (FIG. 2b). [0014] FIGS. 3a and 3b show the effect of inlet gas velocity on NO emission with no H.sub.2 addition (FIG. 3a) and 50% H.sub.2 (FIG. 3b). [0015] FIG. 4 illustrates one embodiment of the invention wherein combustion product gas is used as a diluent. [0016] FIG. 5 illustrates an embodiment wherein external diluents are added. DETAILED DESCRIPTION OF THE INVENTION [0017] The invention is directed to a method of controlling NOx emission from combustors. The method comprises, generally, providing a inlet fuel mixture to a combustor, wherein the inlet fuel mixture comprises a base hydrocarbon fuel that can be a natural gas, methane, coal gas, biomass-derived fuel or other hydrocarbon fuel materials, a diluent gas, such as water vapor, nitrogen, or combustion product gas, and a gas capable of promoting flame stability and improve low temperature combustion characteristics, such as hydrogen. [0018] The notation "NOx" as used herein represents all nitrogen oxides. The value of "x" can be at least one and can have non-integer values. [0019] In order to demonstrate the efficacy of the invention, calculations were undertaken to demonstrate reduction in NOx emissions with diluent gas and flame stability gas additions to the inlet gas (CH.sub.4). In the cases illustrated here, the diluent gas was N.sub.2 and the gas used to provide flame stability was H.sub.2. Continue reading... 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