Combustor for gas turbines

Abstract: A gas turbine combustor for reducing generation of NOx by mixing a fuel and air homogeneously and by improving the flame holdability of a pilot fame. At the center of the gas turbine combustor, there is arranged a pilot nozzle 3 which is surrounded by a plurality of main nozzles 4. From the vicinity of the injection port of the pilot nozzle 3, there is projected a diverging cone 5 which improves the flame holding of the main flame by the pilot flame, so that the generation of NOx by the pilot can be reduced. The mixing of the fuel and air is homogenized by arranging the main nozzles upstream of the pilot nozzle and by arranging an annular premixing nozzle having a throttled exit downstream of the main nozzles. From fuel nozzles having a multi-tube structure, moreover, a gas fuel is injected into the main nozzles, and a liquid fuel is vaporized to the injection ports of the annular premixing nozzle so that the liquid fuel is atomized to mix the fuel and air homogeneously.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a gas turbine combustor capable of reducing generation of NOx from a gas turbine. Description of the Relevant Art For higher efficiencies,of cogenerator plants, the gas turbine has its entrance temperature elevated in recent years to have a tendency of increasing its generation of NOx. Serious demands for lower NOx content in the exhaust gases have bee raised and met by various proposals for the NOx emission. One of factors influencing production of NOx is the combustion temperature, and it is revealed that the lower combustion temperature will lead the less NOx generation. At present, therefore, the two-stage combustion combining the diffusion and premixed types is carried out to effect efficient combustions and to suppress the rise of the combustion temperature and accordingly the generation of Nox. In this two-stage combustion system, the diffusion combustion is performed at the first stage for advantages of ignition and flame holdability, and the premixed combustion is performed at the second stage because of its high NOx reducing effect. Fig. 10 is a section showing a premixed type combustor for a gas turbine in the prior art. In Fig. 10, a gas turbine premixed type combustor 01 is arranged at its center with a pilot nozzle 02. A plurality of cylindrical main (or premixing) nozzles 03 are arranged around and on a common circle of the pilot nozzle 02. In this arrangement, each main nozzle 03 has its leading end located substantially in the same plane as that of the leading end of the pilot nozzle 02. Incidentally, reference numeral 04 designates a combustion chamber, and numeral 05 designates swirl vanes. In recent years, as described above, the more NOx is emitted to the atmosphere as the entrance temperature of the gas turbine rises to the high level. Hence, it is essential to achieve the low NOx generation. This raises serious demands for the lower content in the NOx in the exhaust gases, for which the various investigations have been made. In this case, the rise of the gas temperature increases the ratio of the burning air so that the mixing of the fuel and air causes an important factor. In the premixed type combustor for the gas turbine of the prior art shown in Fig. 10, however, the premixing nozzles are wholly formed into a cylindrical shape because of the necessity for the compact structure. Thus, the mixing of the fuel and air is not always sufficient to limit the generation of Nox. A prior art gas turbine combustor for reducing NO x emissions disclosing the features of the preamble of claim 1 is known from EP-0 455 487 A1. In this combustor a pilot nozzle is arranged at the center of the combustor and a plurality of main nozzles are arranged around the pilot nozzle to create a premixture flow. The exit end of the pilot nozzle is provided with a small cone which, however, does not project beyond the nozzle exit end. The premixture flow hits the inner wall of the combustion chamber at a converged wall section 101a at the vicinity of the pilot nozzle exit end. Thereby the flow is directed towards the pilot flame to mix the main premixture and the pilot (primary) flame in the region immediately in front of the pilot nozzle. SUMMARY OF THE INVENTION It is the object of the present invention to provide a gas turbine combustor which is further improved with respect to the reduction of NO x . According to the invention there is provided a gas turbine combustor comprising a pilot nozzle arranged at the center of a gas turbine combustor; and a plurality of main nozzles arranged around said pilot nozzle for creating a premixture flow; and a diverging cone projected from the vicinity of the injection port of said pilot nozzle radially outward toward a downstream combustion chamber to converge a premixture flow passage. According to a preferred embodiment of the gas turbine combustor the plurality of main nozzles are arranged upstream of said pilot nozzle; and an annular premixing nozzle is disposed downstream of said main nozzles and having a throttled exit formed by the radially outward diverging cone structure. According to a further preferred embodiment of the gas turbine combustor each of said main nozzles includes a fuel nozzle having a structure of at least two tubes, one for injecting a gas fuel into said main nozzle and another for atomizing a liquid fuel at the exit of said annular premixing nozzle. In the gas turbine combustor of the invention, the diverging cone is projected from the vicinity of the injection port of the pilot nozzle so that the zone of the circulating flow of the fuel from the pilot nozzle can be enlarged to improve the holding characteristics of the main flame by the pilot flame. As a result, the combustion is stabilized even with a low pilot injection rate, to reduce the Nox generation from the pilot. In the combustor of the preferred embodiment, the fuel and air are mixed at the first stage individually in the plurality of main nozzles arranged around and upstream of the pilot nozzle, and the mixtures then join and are mixed at the second stage in the annular premixing nozzle so that the air and fuel can be further homogeneously mixed to improve their combustion in the combustion chamber to reduce the NOx generation. Moreover, the homogeneous mixture is introduced at a higher velocity into the combustion chamber through the throttled premixture flow passage so that the flash back can be prevented while improving the flame holdability. In the combustor of the further preferred embodiment, the gas fuel is injected into the main nozzles, and the liquid fuel is sprayed at the exit of the annular premixing nozzle, so that the fine liquid vapors are evaporated into the gas phase and premixed with the gas fuel. As a result, the liquid fuel is homogeneously gasified to ensure the combustion with a lower NOx emission. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a section showing a first embodiment of the present invention; Fig. 2 is a section showing a second embodiment of the present invention; Fig. 3 is a diagram illustrating the fuel concendistribution at the nozzle exit of the premixed type combustor according to the second embodiment; Fig. 4 is a diagram illustrating the fuel concentration distribution at the nozzle exit of the premixed type combustor of the prior art: Fig. 5 is a graph plotting the NOx concentrations resulting from the combustion tests of the premixed type combustor according to the second embodiment; Fig. 6 is a graph plotting the NOx concentrations resulting from the combustion tests of the premixed type combustor of the prior art; Fig. 7 is a section showing a third embodiment of the present invention; and Fig. 8 is a graph illustrating the comparison of the NOx emissions between the combustion results of the premixed type combustors of the third embodiment and the prior art. In Fig. 9 showing a fuel nozzle according to a fourth embodiment of the present invention, Fig. 9(a) presents a section of the same, and Fig. 9(b) is a transverse section of the same. Fig. 10 is a section showing the premixed type combustor for a gas turbine of the prior art. DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described in the following with reference to Fig. 1 presenting a section of the first stage of a premixed type combustor. In Fig. 1, a combustor 1 is equipped at its center with a pilot nozzle 3 directed toward the combustion zone of an internal cylinder 2. The pilot nozzle 3 is surrounded by a plurality of main nozzles 4. These main nozzles 4 are arranged to have their injection ports contained in generally the same plane at that of the pilot nozzle 3. From the injection port of the pilot nozzle 3, moreover, there is projected a diverging cone 5 which is also directed toward the combustion zone of the inner cylinder 2, to expand the zone of the circulating flow of a fuel injected from the pilot nozzle 4. As a result, a stable combustion can be established even for a low injection rate of the pilot fuel, to reduce the emission of NOx from the pilot. Incidentally reference numeral 6 designates the fuel pipe of the main nozzles 4. A second embodiment of the present invention will be described with reference to Figs. 2 to 6. In Fig. 2, a premixed type gas turbine combustor 11 is arranged at its center with a pilot nozzle 12. The combustor 11 is arranged with a plurality of cylindrical main (or premixed) nozzles 13 around the pilot nozzle 12 in a common circle. These main nozzles 13 are made shorter than the main nozzles 03 of the prior art shown in Fig. 10 and are arranged upstream of the pilot nozzle 12. Each of main nozzles 13 is equipped therein with swirl vanes 15. Downstream of those cylindrical main nozzles 13, there is extended an annular premixing nozzle 16. As a result, the individual insides of the cylindrical main nozzles 13 provide primary mixing chambers for the fuel and air, and the insides of the annular premixing nozzles provide secondary mixing chambers. These secondary mixing chambers have their inner circumferences defined by an inner cylinder 17, which has its exit end 18 diverged or expanded radially outward toward a downstream combustion chamber 14 so that the premixture flow passage is converged or throttled. Here will be described the operations of the combustor of the second embodiment. The fuel and air are mixed at a first stage in the cylindrical premixing nozzles 13, and these preliminary mixtures join to one another and are subjected to a second-stage mixing in the annular premixing nozzle 16 so that the fuel and air are sufficiently mixed into a homogeneous mixture. Thus, the combustion in the combustion chamber 14 can be improved to reduce the NOx generation. On the other hand, the mixture has its flow velocity accelerated, when it flows into the combustion chamber 14, by the action of the premixture passage which is throttled by the diverging exit end 18 of the inner cylinder 17 defining the inner circumference of the annular premixing nozzle (or the secondary mixing chamber) 16. As a result, the flash back can be prevented, and the circulating flow can be formed without fail in the diverging exit end 18 of the inner cylinder 17 to improve the flame holdability. Fig. 3 is a diagram plotting fuel concentration distributions at the nozzle exit of the premixed type combustor according to the present invention, as shown in Fig. 2, and Fig. 4 is a diagram plotting fuel concentration distributions at the nozzle exit of the premixed type combustor of the prior art, as shown in Fig. 10. In Figs. 3 and 4, moreover, letter x designates the distances from the confluences at which the mixture from the pilot nozzle and the mixtures from the main nozzles join one another. As could be seen by comparing those Figures, the premixed type combustor of the prior art has a dispersion in the fuel concentration distributions at the nozzle exit. In the premixed type combustor according to the present invention, on the contrary, the mixtures from the main nozzles have substantially homogeneous fuel concentration distributions at the confluence. On the other hand, Fig. 5 is a graph plotting the NOx concentrations which are obtained from the combustion experiments of the premixed type combustor according to the present invention, as shown in Fig. 2, and Fig. 6 is a graph plotting the NOx concentrations which are obtained from the combustion experiments of the premixed type combustor of the prior art, as shown in Fig. 10. In Figs. 5 and 6, moreover, solid curves are prepared by joining the points which were decided to give the best results including the CO concentrations. The comparison of these Figures will reveal that the present invention can reduce the NOx concentrations to one half of the prior art under the rated load conditions, as indicated at points A, of the practical combustor. A dual-fuel burning premixed type combustor according to a third embodiment of the present invention will be described with reference to Fig. 7. In a premixed type combustor according to the present embodiment, all the pilot nozzle 12, the main (or premixing nozzles) 13, the swirl vanes 15, the annular premixing nozzle 16, the inner cylinder 17, and the exit end of the inner cylinder are given the same structures as those of the foregoing second embodiment. A plurality of fuel nozzles 24 for feeding the fuel individually to the main nozzles 13 and the annular premixing nozzle 16 are additionally provided to extend through the main nozzles 13 and the annular premixing nozzle 16. The fuel nozzles 24 are arranged to have their leading ends directed at the exit of the premixing nozzle 16 toward the downstream of the premixed combustor 21. These fuel nozzles 24 are composed of dual tubes, one of which is fed with the gas fuel whereas the other is fed with the liquid fuel. The gas fuel is injected just downstream of the swirl vanes 15 into the cylindrical main nozzles 16 so that it is preliminarily mixed with the swirls by the swirl vanes 15 and then injected downstream. The resultant mixture jets atomize the fine liquid fuel vapors, which are sucked and vaporized from the fuel nozzles 24 at the exit of the annular premixing nozzle 16, into a finer and more homogeneous mixture. In short, the fine fuel vapors are preliminarily evaporated and mixed sufficiently with the gas fuel so that they are completely burned with a low NOx emission. Fig. 8 compares the generations of Nox between the combustions of the dual-fuel burning premixed type combustor according to the third embodiment of the present invention and the premixed type combustor of the prior art, as shown in Fig. 10. The generation of NOx from the gas fuel and the liquid fuel are plotted when the individual combustors are run under predetermined loads. For the liquid fuel (or oil), it is found that the combustor of the present invention always emits as low as about 50 % of the conventional one. For the gas fuel, on the other hand, the combustor of the present invention emits about 50 % of NOx as high as that of the prior art under a light load, and this emission is reduced to about 20 % under a high load. A fourth embodiment of the present invention will be described with reference to Fig. 9. In the present embodiment, the dual fuel nozzles 24 of the foregoing third embodiment are replaced by triple fuel nozzles 34, as will be described in the following. Specifically, each triple fuel nozzle 34 is constructed of three tubes: the innermost one providing a liquid fuel passage 34a for the liquid fuel; the outermost one providing an air passage 34b for the air; and the intermediate one providing a gas fuel passage 34c for the gas fuel. The intermediate gas fuel passage 34c is so extended rather downstream of the swirl vanes 15 of the main nozzle 13 that the gas fuel may be injected into the main nozzle 13 through radially formed tubular passages 35. On the other hand, the innermost liquid fuel passage 34a and the outermost air passage 34b are extended together to the vicinity of the injection port of the fuel nozzle 34. In the present embodiment, the gas fuel is injected from just behind of the swirl vanes 15, as indicated by arrow, into the cylindrical main nozzle 13 and is premixed with the air flows by the swirl vanes 15 so that this preliminary mixture is injected into the annular premixing nozzle 16 located downstream thereof. On the other hand, the liquid (or oil) fuel is injected by the two-fluid or air/oil nozzle for atomization with the air, so as to promote the mixing, i.e., to make the injected vapors finer and more homogeneous. The liquid fuel passage 34a and the air passage 34b are extended to the vicinity of the injection port of the fuel nozzle 34 so that the liquid fuel is atomized at the exit of the fuel nozzle 34, which is disposed at the injection port of the annular premixing nozzle 16, by the injection of the air flow from the air passage 34b. At this time, the air flow from the air passage 34b acts to promote the vaporization of the liquid fuel and atomize the fuel vapors. At this time, the gas fuel premixed in the cylindrical main nozzles 13 is injected to promote the atomization of the atomized liquid fuel better so that it can be homogenized as the gas fuel to ensure a complete fuel combustion with a low NOx emission. According to the aforementioned third and fourth embodiments, the fuels can be prevented from any overheat by the multiplex fuel passages. According to the fourth embodiment, moreover, this fuel cooling effect can be improved better by the air passage disposed at the outermost side. As has been described in detail hereinbefore, according to the present invention as defined in Claim 1, the circulation zone of the fuel from the pilot nozzle is expanded to improve the holding characteristics of the main flame by the pilot flame, so that the combustion and the flame holding can be stabilized to reduce the NOx to be emitted from the pilot. Thus, the present invention makes a remarkable contribution to the problem of air pollution. According to the present invention as defined in Claim 2, in the premixed type gas turbine combustor, the mixing of the fuel and air is effected at the two stages so that it can be homogenized to improve the combustion thereby to reduce the NOx generation. Thus, it is possible to provide a premixed type combustor which can make a contribution to the improvement in the efficiency of the gas turbine and can cope with the problem of air pollution sufficiently. According to the present invention, moreover, the homogenized air-fuel mixture is introduced through the throttling premixture passage into the combustion chamber at the accelerated flow velocity so that the flame holdability can be improved while preventing the flash back. According to the present invention as defined in Claim 3, moreover, the combustor can be run with a low NOx generation for the liquid fuel thereby to suppress the air pollution drastically.

Claims: A gas turbine combustor comprising: a pilot nozzle (3,12) arranged at the center of a gas turbine combustor; and a plurality of main nozzles (4,13) arranged around said pilot nozzle (3,12) for creating a premixture flow; and a diverging cone (5,18) projected from the vicinity of the injection port of said pilot nozzle (3,12) radially outward toward a downstream combustion chamber (14) to converge a premixture flow passage. The gas turbine combustor according to claim 1, characterized in that said plurality of main nozzles (13) are arranged upstream of said pilot nozzle (12); and in that an annular premixing nozzle (16) is disposed downstream of said main nozzles (13) and having a throttled exit (18) formed by the radially outward diverging cone structure. The gas turbine combustor according to claim 2, characterized in that each of said main nozzles (13) includes a fuel nozzle (24) having a structure of at least two tubes (34c,34a), one (34c) for injecting a gas fuel into said main nozzle (13) and another (34a) for atomizing a liquid fuel at the exit (18) of said annular premixing nozzle (16).

Inventors: FUKUE ICHIRO, GORA TETSUO, KAWABATA HITOSHI, MANDAI SHIGEMI, NISHIDA HIROYUKI, SATO NOBUO, TANAKA KATSUNORI, FUKUE, ICHIRO, GORA, TETSUO, KAWABATA, HITOSHI, MANDAI, SHIGEMI, NISHIDA, HIROYUKI, SATO, NOBUO, TANAKA, KATSUNORI, Fukue, Ichiro, c/o Takasago Machinery Works, Gora, Tetsuo, c/o Takasago Res.Dev.Center of, Kawabata, Hitoshi, c/o Takasago Machinery Works, Mandai, Shigemi, c/o Takasago Machinery Works, Nishida, Hiroyuki, c/o Takasago Res.Dev.Center of, Sato, Nobuo, c/o Takasago Res.Dev.Center of, Tanaka, Katsunori, c/o Takasago Machinery Works
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1993
Main classification: F23R3
IPC codes: F23R3, F23D17