GAS TURBINE COMBUSTOR

Abstract: A gas turbine combustor (10) comprises a combustion chamber (36), a premixing chamber (23) in which hydrocarbon fuel and air are premixed, a nozzle (25) with a swirler (24) for swirling and injecting the premixed fuel and air into the combustion chamber (36), to effect premixed combustion. A fuel nozzle (32) projects into the combustion chamber and injects fuel at region where the premixed combustion is completed, so as to effect low air-ratio diffusion combustion thereby to produce reducing substance. NOx produced by the premixed combustion is reduced by reducing reaction with the reducing substances, so that the NOx concentration in the combustion gas is reduced. The combustor (10) may further include after-air ports (40) on a peripheral wall (26) defining the combustion chamber (36) to effect combustion of unburnt substances.

Description: The present invention relates to a gas turbine combustor and a method of combustion thereby. It is concerned with reducing NOx emission and, for example, a gas turbine combustor which operates by producing reducing substances for NOx through combustion of fuel supplied to the combustor, and reducing the NOx produced by premixed combustion with the reducing substances. Nitrogen oxides (referred to as NOx) produced by combustion in a gas turbine combustor and called thermal NOx is caused by nitrogen in air. A mechanism for producing thermal NOx is explained by a Zerdovich mechanism. Thus, it is caused by the following elementary reactions. N₂ + O₂ → NO + N N + O₂ → NO + O N + OH → NO + H It can be understood from these reactions that generation of nitrogen atoms by a dissociation reaction of nitrogen molecules in the equation (1) is an initiation reaction. More specifically, nitrogen atoms N produced in the equation (1) are oxidized by oxygen molecules and hydroxyl radicals in the equations (2), (3) respectively to becomes NO. A series of these reactions increase as a flame temperature increases, whereby an amount of thermal NOx produced increase. Therefore, the flame temperature must not be increased in order to reduce the production of thermal NOx. At present, a lean-combustion system described below is primarily employed as a low NOx combustion system using this principle, although there is available a system for supplying steam or combustion gas into a combustor or the like. The lean-combustion system cools a flame temperature with a large amount of air to prevent temperature increase. More specifically, a combustor is controlled so as not to have any area where an air-ratio (a ratio of an amount of really supplied to air to an amount of air necessary for prefect combustion of fuel supplied) is near 1 thereby effecting combustion with a high air-ratio (about 2.0) in all of the areas. Japanese Utility Model Laid-Open No. 57-154853 relating to a lean-combustion system discloses a system for effecting lean-combustion as stably as possible by supplying air into a combustor when an air pressure is low by use of a pressure in a gas turbine casing. Japanese Utility Model Laid-Open No. 57-150373 discloses an air introducing device for a combustor of a gas turbine. The lean-combustion system for preventing an increase in a combustion temperature by supplying a large amount of premixed air and fuel for forming a high air-ratio premixed combustion flame (flame produced when premixed fuel and air are combusted) has a problem of blow-off. This is because a premixed combustion flame is generally most stable when an air ratio is in the vicinity of 1 and blow-off is liable to arise when the air ratio is greater than 1. JP-A-61-41810 discloses a system different from the aforesaid system, in which fuel is separately supplied into two (first and second) regions in a combustor, the fuel in the first region, supplied by a first nozzle is combusted at a high air-ratio of about 1.2 for perfect combustion, and then NOx produced there is reduced by a low air-ratio combustion flame with its fuel supplied by a second nozzle in the second region including a small amount of oxygen and a large amount of reducing combustible gas. The combustible gas remaining in the second region is oxidized and combusted by air from an after-air port in a rear flow for thereby decreasing an amount of NOx. JP-A-61-41810 does not describe whether the flame in the first region is a premixed combustion flame or a diffusion combustion flame and if the flame is the premixed flame, a problem of blow-off arises. In addition, as disclosed in JP-A-61-41810, the first nozzles for high air-ratio combustion and the second nozzle for low air-ratio combustion are located within the combustion adjacent each other. Therefore flame interference should occur between the high air-ratio combustion flame and low air-ratio combustion flame. More specifically, the positional relationship of the flames is such that excessive oxygen is diffused from the side of the high air-ratio combustion flame to the side of the low air-ratio combustion flame, while fuel is diffused from the side of the low air-ratio flame. Because of the flame interference, NOx produced from the high air-ratio combustion flame is not effectively reduced by the low air-ratio combustion flame. As a result, there is the problem that the respective flames cannot achieve their roles sufficiently. The present invention seeks to provide a gas turbine combustor and a combustion method thereby, wherein fuel is separately supplied into two regions in the combustor to provide a premixed combustion flame and a low air-ratio diffusion combustion flame, a large thermal load is produced by the former flame and NOx produced therefrom is reduced by the latter flame for decreasing an amount of NOx, and wherein the premixed combustion flame is prevented from being blown off and a flame interference between both the flames is prevented thereby sufficientyl reducing the amount of NOx. Thus according to a first aspect of the present invention a gas turbine combustor comprising an axially elongate combustion chamber for effecting combustion therein; a fuel injection nozzle having a tip for injecting fuel to effect low air-ratio diffusion combustion; and after-air ports provided in a peripheral wall of said combustion chamber for combusting unburnt combustible substances contained in combustion gas produced by combustion; wherein there is a premixing chamber out of said combustion chamber for premixing fuel and air; there is a premixture injection nozzle having a swirler provided at an upstream side of said combustion chamber for swirling and injecting premixed fuel and air into said combustion chamber to effect premixed combustion therein to produce premixed combustion flame including NOx; and said tip of fuel injection nozzle is at the downstream side within the combustion chamber where the premixed combustion is terminated, for injecting fuel to effect the low air ratio diffusion combustion thereby to produce reducing substances for reducing NOx in combustion through reduction reaction between the reducing substances and the NOx. The above construction of the combustor does not cause the premixed combustion flame and the diffusion combustion flame to interfere with each other, so that NOx is reduced by the reducing substances formed by the diffusion combustion. The position where the premixed combustion has been terminated or completed, that is, a position where not fuel is contained in the combustion gas can be detected by measuring the concentration of unburnt fuel, for example. According to a second aspect of the present invention there is provided a method of combustion by a gas turbine combustor comprising the steps of: premixing fuel and air in a premixing chamber to provide premixed fuel and air; injecting the premixed fuel and air into a combustion chamber while causing the premixed fuel and air to swirl and igniting it to effect premixed combustion during operation of the combustor thereby to produce combustion gas including NOx; injecting fuel into said combustion chamber at a downstream side of a region wherein the premixed combustion is completed, to effect low air-ratio diffusion combustion so as to produce reducing substances; reacting NOx contained in the combustion gas with the reducing substances to lower the concentration of NOx in the combustion gas; and introducing combustion air into said combustion chamber at a downstream side of a region wherein the diffusion combustion is effected, thereby to effect combustion of unburnt combustible substances. An embodiment of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a vertical cross sectional view illustrative of an embodiment of a gas turbine combustor according to the present invention; Fig. 2 is a diagram of a result of a gas analysis explanatory of a point where a high air-ratio premixed combustion terminates; and Fig. 3 is a diagram for comparing the result of a combustion test of a combustor of an embodiment according to the present invention with that of a combustor shown in Fig. 1 of Japanese Patent Laid-Open No. 61-41810. The gas turbine combustor according to the present embodiment includes a premixture injection nozzle with a swirler and produces premixed combustion flame through combustion of premixed fuel and air of a high air-ratio. Although premixed combustion at an air-ratio of about 1 to 1.6 (1 to 1.2 is preferable to obtain a suitable thermal load from the premixed combustion flame) results in a high flame temperature and hence produces a large amount of thermal NOx, it also produces a large thermal load. In general, a premixed combustion at an air-ratio of about 1 is most stable and the flame due to premixed combustion causes blow-off if the air-ratio increases to a value greater than 1. Therefore, to obtain high combustion efficiency, the flame must be protected to be free from blowing off. In the present invention, the swirler is provide on the nozzle for the high air-ratio premixed combustion flame for the purpose. With the arrangement, when a swirl flow is generated in a flame flow, a negative pressure is produced in the swirl to cause the flame flow to be directed to its center so that the flame is held or stabilized more sufficiently. At the same time, a length of the flame is shortened to make the combustor smaller in size. The gas turbine combustor further includes the central nozzle for forming low air-ratio diffusion combustion flame. The diffusion flame reduces NOx produced from the high air-ratio premixed combustion flame. Incidentally, when a so-called flame interference occurs in which a high air-ratio combustion flame is brought into contact with a low air-ratio combustion flame to cause oxygen to diffuse from the side of the high air-ratio combustion flame to the side of the low air-ratio combustion flame and fuel is diffused from the side of the low air-ratio combustion flame, the respective flames cannot perform their roles sufficiently. Therefore, the present invention takes a measure to prevent the flame interference between both flames by disposing the nozzle tip of the central nozzle for forming the low air-ratio diffusion combustion flame for reducing NOx in a region where no fuel remains in the high air-ratio premixed combustion flame and only oxides composed of NOx, oxygen, nitrogen and the like produced by combustion exist, i.e., in the rear flame flow region wherein the combustion process has terminated. With the arrangement, both flames do not cause the flame interference so that the respective flames sufficiently performs their roles such that the high air-ratio premixed combustion flame provides a high thermal load through high combustion efficiency and the low air-ratio diffusion flame reduces NOx. An embodiment of the gas turbine combustor according to the present invention will be described hereunder in detail, referring to Fig. 1. In Fig. 1, a gas turbine combustor 10 comprises an inner cylindrical casing 26 axially elongated for defining a combustion chamber 36 therein. The inner casing 26 has a front end mounted on an end plate 29 of an outer casing 28. The outer casing 28 is disposed coaxially with the inner casing 26 with an annular space therebetween. The annular space communicates with a compressor (not shown) and the combustion chamber 36, whereby air is introduced from the compressor into the combustion chamber 36 through the annular space and many air holes made in the side wall of the inner casing 26. The other end of the inner casing 26 is joined to a larger diameter inner casing 27 defining a dilution zone. The combustor further comprises a premixture injection nozzle 25 and a fuel injection nozzle 32. The nozzle 25 is mounted on the front end of the inner casing 26. The nozzle 25 which is equipped with a swirler 24 is annularly formed along the inner periphery of the front end of the inner casing 26, and a central axis of the nozzle 25 is coincided with a central axis of the inner casing 26. A premixture of fuel and air is formed in a premixing chamber 23 disposed out of the combustion chamber 36 and upstream of the premixture injection nozzle 25. The premixing chamber 23 communicates with an air supply port 21 and a fuel supply port 22 to receive air and fuel therefrom. An air supply nozzle 33 which is hollow and conically cylindrical is mounted at the front end of the inner casing 26 so as to project into the combustion chamber 36 through the nozzle 25. The fuel nozzle 32 is disposed in the air supply nozzle 33 so as to be coaxial with the central axis of the premixture injection nozzle 25. The fuel nozzle 32 has one end forming a nozzle tip disposed in the combustion chamber for injecting fuel thereinto and the other end communicating with a fuel supply port 31. The air supply nozzle 33 communicates with an air supply port 34 for supplying air therein. According to the present invention, fuel (hydrocarbon fuel, e.g., methane in general) is divided into two portions and supplied to form a flame 20 of premixed combustion at a high air-ratio of about 1.2 in the same combustor 10 for obtaining a large thermal load from the flame 20. Further, NOx produced from the flame is reduced by reducing substances such as NH₃, HCN, hydrocarbon compounds existing in a flame 30 of diffusion combustion at a low air-ratio of about 0.8 to decrease an amount of NOx. As a low air-ratio, it is preferable to by 0.6 to 1.0 for obtaining reducing chemical species such as NH₃, ·NH, ·CH, H.C., ·H.C. etc, for example. The high air-ratio premixed combustion flame 20 is formed when fuel supplied from the fuel supply port 22 and air supplied from the air supply port 21 are mixed into a premixture of gas in the premixing chamber 23 and the premixture is ejected through the nozzle 25 for the high air-ratio premixed combustion flame provided with the swirler 24 for swirl flow generation. Since a flow of the flame becomes a swirl flow, a negative pressure region is produced in the vicinity of the center of the swirl flow thereby producing a flow going back inwardly so that a length of the flame is shortened to make the combustor 10 smaller in size and a flame stabilizing capability for the high air-ratio combustion flame, which is otherwise liable to be blown off, is improved, whereby blow-off can be prevented. The low air-ratio diffusion combustion flame 30 is formed by ejecting fuel supplied from the fuel supply port 31 through the nozzle 32 at the center and is used to reduce NOx produced from the high air-ratio combustion flame 20. In order to prevent the occurrence of a flame interference between the aforesaid two flames 20, 30, the nozzle tip of the nozzle 32 is disposed at a rear flow of the high air-ratio premixed combustion flame 20 so that it ejects fuel at a position where a combustion reaction of the flame 20 has terminated. The position where the combustion reaction terminates is determined based on a result of a gas analysis shown below. That is, a gas in the high air-ratio premixed combustion flame 20 is sequentially sampled at respective distances ℓ in the direction of the flame flow to analyze concentration of methane in the gas and a position where the concentration of the methane is 0% is determined to be the position where the combustion reaction terminates, i.e., a flame end. Fig. 2 shows an example of it. The distances ℓ showing that the methane concentrations are greater than 0% represent a flame where combustion reaction goes on. Accordingly, the gas at distances ℓ beyond the aforesaid distances contains no fuel methane and then a flame is not longer formed and no combustion reaction occurs, that is, it is only a high temperature exhaust gas and not called a flame. According to the embodiment, the nozzle 32 for forming the low air-ratio diffusion flame 30 has the nozzle tip just disposed at the position where the combustion reaction of the high air-ratio premixed combustion flame 20 terminates. Although a value of the distance ℓ where the methane concentration becomes 0% slightly changes depending on a condition of combustion or the like, the methane concentration becomes 0% at the point where the combustion terminates, which determines a positional relationship between the nozzle 25 for the high air-ratio premixed combustion flame and the nozzle 32 for the low air-ratio diffusion combustion flame. With the above disposition, the two flames 20, 30 can sufficiently perform their roles, that is, the flame 20 is combusted efficiently to provide a high thermal load and the flame 30 reduces NOx. The fuel nozzle 32 disposed at the center of the combustor 10 is cooled because it may be damaged by the high-temperature premixed combustion flame 20 with the high air-ratio and combustion air is supplied to the low air-ratio diffusion combustion flame 30 so that the air supply nozzle 33 has multi-state air ejecting ports 35 in series and is concentrically disposed around the outer circumference of the nozzle 32. Air is supplied to the nozzle 33 from the air supply port 34. The low air-ratio diffusion combustion flame 30 produces carbon monoxide and excessive hydrocarbon compounds as well as NOx reducing compounds (NH₃, HCN, hydrocarbon compounds and the like). The discharge of them from an outlet of the combustor 10 is not only harmful but also disadvantageous from a view point of energy saving. In the present invention, after-air ports 40 are disposed to solve these problems. The carbon monoxide and the hydrocarbon compounds are combusted by air entering into the combustor 10 from the after-air ports 40 to make them harmless and to produce combustion heat. Some of the after-air ports 40 are disposed around a position where diffusion combustion flame is terminated and the other downstream of the former after-air ports 40, for example. Fig. 3 shows a result of a combustion test effected using the combustor of the present invention in comparison with a result of a combustion test effected using a conventional combustor, i.e., a combustor wherein a high air-ratio premixed combustion flame and a low air-ratio diffusion combustion flame are positioned at the same position and a flame interference arises between both flames. A horizontal axis represents a concentration of an unburnt combustible composition (CO and hydrocarbon compounds) in an exhaust gas from the combustor as one of indexes showing inferior combustibility and a vertical axis represents a concentration of NOx in the exhaust gas from the combustor. Values on the horizontal axis and the vertical axis mean that combustion is effected more efficiently with a less amount of NOx as they are nearer to the origin. A graph 60 shows a better result of an efficient combustion with a less amount of NOx obtained by the embodiment as compared with a graph 50 showing a result of a conventional type combustor. According to the present invention, the nozzle for forming a high air-ratio premixed combustion flame is provided with the swirler. As a result, no blow-off is caused, even if the premixed flame is combustion produced at the high air-ratio because a flame stabilizing capability is improved. Further, a flame length is shortened to make the combustor smaller in size. When NOx produced from the high air-ratio premixed combustion flame is reduced by the low air-ratio diffusion flame or products therefrom, no flame interference occurs between both flames because the nozzle tip of the nozzle for the air-ratio diffusion is disposed at the rear flame flow position where the combustion process of the high air ratio premixed flame combustion has terminated and no fuel is contained in its combustion exhaust gas. Therefore, the high air-ratio premixed combustion flame provides a large thermal load at a high combustion efficiency and the low air-ratio diffusion flame reduces NOx for decreasing an amount of NOx, whereby the respective flames can sufficiently perform their roles.

Claims: 1. A gas turbine combustor comprising an axially elongate combustion chamber (36) for effecting combustion therein; a fuel injection nozzle (32) having a tip for injecting fuel to effect low air-ratio diffusion combustion; after-air ports (40) provided in a peripheral wall of said combustion chamber (36) for combusting unburnt combustible substances contained in combustion gas produced by combustion; wherein there is a premixing chamber (23) of said combustion chamber for premixing fuel and air; there is a premixture injection nozzle (25) having a swirler (24) provided at an upstream side of said combustion (36) chamber for swirling and injecting premixed fuel and air into said combustion chamber (36) to effect premixed combustion therein to produce premixed combustion flame including NOx; and said tip of fuel injection nozzle (32) is at the downstream side within the combustion chamber (36) where the premixed combustion is terminated, for injecting fuel to effect the low air ratio diffusion combustion thereby to produce reducing substances for reducing NOx in combustion through reduction reaction between the reducing substances and the NOx. 2. A gas turbine combustor according to claim 1, further including a tubular member (33) in said combustion chamber passing through the central portion of said premixture injection nozzle (25) said tubular member having fine holes (35) formed in a surface thereof for jetting cooling air into said combustion chamber (36), and said fuel injection nozzle (32) passes through said tubular member (33) to inject fuel from downstream side of said tubular member (33) into said combustion chamber (36). 3. A gas turbine combustor according to claim 1 or 2, wherein said premixture injection nozzle (25) is formed in an annular shape, and said fuel injection nozzle (32) is disposed coaxially of said premixture injection nozzle (25). 4. A method of combustion by a gas turbine combustor comprising the steps of: premixing fuel and air in a premixing chamber to provide premixed fuel and air; injecting the premixed fuel and air into a combustion chamber while causing the premixed fuel and air to swirl and igniting it to effect premixed combustion during operation of the combustor thereby to produce combustion gas including NOx; injecting fuel into said combustion chamber at a downstream side of a region wherein the premixed combustion is completed, to effect low air-ratio diffusion combustion so as to produce reducing substances; reacting NOx contained in the combustion gas with the reducing substances to lower the concentration of NOx in the combustion gas; and introducing combustion air into said combustion chamber at a downstream side of a region wherein the diffusion combustion is effected, thereby to effect combustion of unburnt combustible substances. 5. A method of combustion according to claim 4, wherein fuel and air is premixed at an air-ratio higher than 1. 6. A method of combustion according to claim 5, wherein said region wherein the premixed combustion is completed is detected by measuring fuel concentration in said combustion flame, said region being a region where the measured fuel concentration is zero. 7. A method of combustion according to claim 6, wherein the fuel comprises methane and said region wherein the premixed combustion is completed is one wherein the concentration of methane is zero. 8. A method of combustion according to claim 7, wherein fuel is injected into said combustion chamber at downstream side of said region so that an air ratio is 1 to 1.6.

Inventors: ARASHI, NORIO, AZUHATA, SHIGERU, INADA, TOORU, ISHIBASHI, YOJI, IWAI, YASUO, KOBAYASHI, HIRONOBU, KURODA, MICHIO, MASUTANI, STEPHEN MASAO, MURAKAMI, TADAYOSHI, NARATO, KIYOSHI, SOHMA, KENICHI
Applicants: HITACHI, LTD.
Application year: 1989
Main classification: F23R3
IPC codes: F23R3