Combustor and method of operating same

Abstract: A combustor includes a first premixture supply device (4,16) provided in a central portion of a combustion chamber (5) disposed generally concentric with a combustion cylinder (2), and a second premixture supply means (8, 32) provided adjacent to an outer periphery of the first premixture supply device (4, 16). The first premixture supply device (4, 16) is operable when the combustor is under a high load, and the second premixture supply device (8, 32) is operable when the combustor is under a low load. The method of operating the combustor is such that the outer premixture supply device (8, 32) is operable in a low-load range while the inner and outer premixture supply devices (4, 16; 8, 32) are operable in a high-load range at above a predetermined load. The first and second premixture supply devices include first and second burners, respectively. The first and second burners are operated in such a manner that the ratio of the load borne by the first burner to the load borne by the second burner is 1:1 in the above-mentioned high-load range.

Description: The present invention relates to a method of operating a combustor and to a combustor according to the preambles of the independent claims. Such a method and such a combustor are known from EP-A-0 281 961. The combustor may e.g. be used in a gas turbine. More particularly, the invention relates to a combustor of the premix combustion type and to a method for operating such a combustor. Most of conventional combustors of the general type employ a two-stage combustion system to reduce production of NOx. More specifically, in such a combustor, a diffusion combustion is effected at one end of a combustion cylinder, at the head of the combustor, for the purpose of stabilizing a flame, whereas a premix combustion highly effective in reducing NOx is effected downstream of the one end of the combustion cylinder. Such a combustor is disclosed, for example, in U. S. Patent No. 4,292,801. More specifically, this conventional combustor comprises a first-stage diffusion combustion burner mounted on a head of the combustor, and a second-stage diffusion combustion burner extending from the combustor head toward a central portion of a combustion chamber. The diameter of the combustion chamber is reduced or constricted in the vicinity of an outlet of the second-stage burner. At the start of combustion, fuel is supplied to the first-stage burner and is ignited so as to effect a diffusion combustion in a first-stage combustion chamber. Then, as the combustion load increases, fuel is introduced into the second-stage burner so as to effect a diffusion combustion in a second-stage combustion chamber. Then, simultaneously when the first-stage fuel is transferred to the second-stage burner, the first-stage burner is extinguished (turned off), and then fuel is again charged into the first-stage burner. At this time, the first-stage combustion chamber serves as a premix chamber for mixing the fuel and the air together. Therefore, at the time of a high-load combustion, a fuel-air premixture from the first-stage combustion chamber is burned by the heat source of the second-stage burner so as to continue the combustion. In the combustor of such a construction, premix combustion is carried out mainly when the combustor is operated under a load higher than a predetermined level, and therefore this combustor is very effective in reducing NOx. This type of combustor is satisfactory in that during a high-load operation, it exhausts small amounts of NOx and unburned substances such as CO, which means that the combustion is sufficiently effected in the combustion chamber, and therefore the combustible fuel-air mixture is hardly discharged in an unburned condition from the combustor. However, during a low-load operation of the combustor, that is, under a condition in which the fuel-air mixture is lean, the cooling air enters about the wall surface of the combustion chamber, so that the amount of discharge of unburned substances such as CO tends to increase. EP-A-0 281 961 discloses a gas turbine combustor and a combustion method therefor. The combustor comprises a head combustion chamber and a rear combustion chamber connected to a downstream side of the head combustion chamber, a first stage burner for premixing first stage fuel and air, a second stage burner for premixing second stage fuel. The combustor is further provided with a pilot burner in a central position of the combustor. The pilot burner forms a pilot flame, at the start the pilot burner is first supplied with fuel to make diffusion combustion. It is the object of the present invention to solve the above problems of the prior art and to provide a combustor and an operating method which will not exhaust CO and other substances even under a low-load operation of the combustor, that is, when a combustible fuel-air mixture is lean. This object is solved in accordance with the features of the independent claims. Dependent claims are directed on preferred embodiments of the invention. A combustor comprises premixture supply means which comprises first premixture supply means disposed about a central portion of a combustion chamber generally coaxial with a combustion cylinder and operable when the combustor is under a high load, and second premixture supply means provided adjacent to an outer periphery of the first premixture supply means and operable when the combustor is under a low load. There is also provided a method of operating a combustor comprising a combustion cylinder having a combustion chamber therein, and premixture supply means provided at one end of the combustion cylinder to supply a combustible mixture to the combustion chamber, the premixture supply means including at least two inner and outer premixture supply means, the outer premixture supply means being operable in a low-load range of the combustor whereas the inner and outer premixture supply means are operable in a high-load range of the combustor at above a predetermined load, the first and second premixture supply means including first and second burners, respectively, the method comprising the step of: operating the first and second burners in such a manner that the ratio of the load borne by the first burner to the load borne by the second burner is 1:1 in the high-load range. With such an arrangement, the burner of the second premixture supply means, that is, the burner disposed near the outer periphery of the combustion chamber is operated to enable maintaining the temperature of combustion gas high at the outer periphery of the combustion chamber even in a low load condition of the combustor, so that in a low load condition of the combustor, unburned substances such as CO tending to be produced at the outer periphery of the combustion chamber can be prevented from being produced. Fig. 1 is a vertical cross-sectional view of a combustor in accordance with the present invention; Fig. 2 is a cross-sectional view of an essential portion of the combustor of Fig. 1, showing a fuel supply system; Fig. 3 is a graph representative of the relationship of fuel flow rates and operating burners with respect to a turbine load; Fig. 4 is a schematic view of the combustor, showing patterns of flames formed by burners; Fig. 5 is a graph showing gas temperature and NOx concentration and CO concentration in a combustor of the prior art; Fig. 6 is a graph showing gas temperature and NOx concentration and CO concentration in the combustor of the present invention; Fig. 7 is a graph showing the relation between the fuel ratio and the NOx exhaust concentration in the present invention and the prior art; Fig. 8 is a graph showing the stability with respect to the fuel-air ratio; and Figs. 9 to 14 are fragmentary, vertical cross-sectional views, showing modified auxiliary burners of the invention. Fig. 1 shows one preferred embodiment of a combustor of the present invention. The combustor comprises an outer cylinder 1, an inner cylinder (or combustion cylinder) 2 mounted within the outer cylinder 1, a tail cylinder 3 mounted within the outer cylinder 1 and connected at one end to the inner cylinder 2 and directed at the outer end toward a turbine, a premix chamber 4 provided at a head of the inner cylinder 2, a combustion chamber 5 for receiving a combustible fuel-air mixture, a premixture supply means provided at one side of the combustion chamber 5, an auxiliary burner 13 provided in the vicinity of the premixture supply means to be supplied with auxiliary fuel 12 and air, and an ignition plug 14 provided downstream of the auxiliary burner 13 to ignite the auxiliary burner 13. The premixture supply means comprises a first (inner) premixture supply means operable during a high-load operation of the combustor, and a second (outer) premixture supply means operable during a low-load operation of the combustor. The first premixture supply means includes a fuel nozzle 16 which is fixedly mounted on an end plate 15 fixedly mounted to one end of the outer cylinder 1 opposite to the turbine to supply fuel for a first premixture to the premix chamber 4. Fuel 18 is supplied to the premix chamber 4 from the fuel nozzle 16, and air 19 is supplied to the premix chamber 4 through a space between the fuel nozzle 16 and the inner peripheral surface of the inner cylinder 2. Therefore, the first premixture is formed by the fuel 18 and the air 19 within the premix chamber 4, and this combustible fuel-air mixture is supplied to the combustion chamber 5. The second premixture supply means includes a premix burner 8 provided adjacent to the outer periphery of the premix chamber 4, that is, adjacent to the inner peripheral surface of the inner cylinder 2. The premix burner 8 is supplied with fuel 6 and air 7 so as to supply a second fuel-air premixture to the combustion chamber 5. As shown in Fig. 2, the premix burner 8 includes a fuel chamber 32 provided in a flange 31 fixedly mounted on the outer cylinder 1, an annular flow passage 33, and fuel nozzles 34 projecting into the annular flow passage 33. The auxiliary burner 13 is disposed around the outer periphery of the premix burner 8 in the vicinity of an injection port of the premix burner 8, and is disposed adjacent to an abruptly-expanded portion 9 of the combustion chamber 5. Arrow 20 denotes the flow of air supplied from a compressor (not shown) to the combustor. A fuel supply system for supplying fuel to the combustor is shown in Fig. 2. This fuel supply system comprises a fuel pressure control valve 24 for controlling the flow rate of fuel 23 fed from a fuel source (not shown), three fuel supply pipes 25, 26 and 27 provided downstream of the control valve 24 in a branched manner to pass the auxiliary fuel 12, the first premixture fuel 18 and the second premixture fuel 6 therethrough, respectively, three fuel flow rate control valves 28, 29 and 30 mounted respectively on the three fuel supply pipes 25, 26 and 27 to control the flow rates of the respective fuels through these three fuel supply pipes, respectively, and a control 35 for producing control signals in accordance with the load level of the turbine to control the above valves 24, 28, 29 and 30. The operation of the above combustor as well as a method of operating the combustor will now be described with reference to Figs. 1 and 2. The fuel 23 from the fuel source, after passing past the fuel pressure control valve 24, is divided into the auxiliary fuel 12, the first premixture fuel 18 and the second premixture fuel 6. These fuels are supplied to the auxiliary burner 13 and the first and second premixture supply means, respectively, via the respective fuel flow rate control valves 28, 29 and 30 and the respective fuel supply pipes 25, 26 and 27. The auxiliary fuel 12 and the second premixture fuel 6 pass through the flange 31, and the auxiliary fuel 12 is supplied to the auxiliary burner 13 disposed adjacent to the abruptly-expanded portion 9 of the combustion chamber 5 while the second premixture fuel 6 reaches the fuel nozzles 34 via the fuel chamber 32 and is injected into the premix burner 8 from injection ports provided respectively at distal ends of these nozzles. The first premixture fuel 18 is supplied to the fuel nozzle 16 via the flow rate control valve 30. These fuels are not always supplied, but are supplied in the following manner. At the start of combustion, the ignition plug 14 is operated, and at the same time the flow control valve 28 is operated to allow the auxiliary fuel 12 to inject from the auxiliary burner 13 to form a flame at a recess portion 11. Then, the flow rate control valve 29 is operated to flow a predetermined amount of the second premixture fuel 6 to form the second premixture, that is, a combustible fuel-air mixture, and this premixture is injected from the premix burner 8. This combustible mixture is burned by a heat source of the flame formed by the auxiliary burner 13. Further, at the time of a high-load combustion, the flow control valve 30 is operated to feed the first premixture fuel 18 via the fuel nozzle 16 to the premix chamber 4 provided at the head of the combustor, thereby forming the first premixture, that is, a combustible fuel-air mixture, and this combustible mixture is supplied to the combustion chamber 5. These operation controls are automatically carried out by the signals from the control 35 in accordance with the load level of the turbine. Fig. 3 shows one example of a fuel control operation method within an operating range of the gas turbine. Fig. 3 shows the flow rate of the fuel 12 for the auxiliary burner 13, the flow rate of the second premixture fuel 6 for the premix burner 8 and the flow rate of the first premixture fuel 18 for the premix chamber 4 with respect to the turbine load. In Fig. 3, a solid line represents the flow rate of the total fuel introduced into the combustion chamber 5 in accordance with the turbine load, a chain line represents the flow rate of the auxiliary fuel 12 fed to the auxiliary burner 13, and a one-dot chain line represents the flow rate of the second premixture fuel 6 fed to the premix burner 8, and a two-dot chain line represents the flow rate of the first premixture fuel 18 fed to the premix chamber 4. As is clear from Fig. 3, the flow rate of the auxiliary fuel 12 increases from the start of combustion, and decreases in a stepwise manner about a point a of the turbine no-load, without changing the load level, and then gradually decreases to the turbine no-load. The second premixture fuel 6 is supplied to the premix burner 8 at the point a of the turbine no-load, and its flow rate increases, and then decreases in a stepwise manner at 25% of the full turbine load, and then increases at a rate half of the flow rate of the total fuel in accordance with the turbine load. The first premixture fuel 18 is supplied in a stepwise manner to the premix chamber 4 provided at the head of the inner cylinder 2 at 25% of the full turbine load, and its flow rate increases at a rate half of the flow rate of the total fuel in accordance with the turbine load. Therefore, the flow rate of the first premixture fuel, as well as the flow rate of second premixture fuel, amounts to 25% to 50% of the total fuel in the range of 25% to 100% of the full turbine load. Patterns of flames formed in the combustion chamber 5 by the above fuel control are shown in Fig. 4. At the start of combustion, the auxiliary burner 13 is operated for a time period from the ignition of the auxiliary fuel 12 to the turbine no-load to form a flame 101 in the combustion chamber 5. The premix burner 8 forms a premix flame 102 in the combustion chamber 5 in the range from around the point a of the turbine no-load to 100% of the full turbine load, and solely effects combustion in the range from the turbine no-load to 25% of the full turbine load. The premixture fuel 18, supplied to the premix chamber 4 at the head of the inner cylinder 2, forms a combustion flame 103 in the range of 25% to 100% of the full turbine load. Namely, each of the first and second premixture fuels is supplied at a rate of 50% of the total fuel in the rated range in accordance with the turbine load. The fuel control operation method may be carried out in the following manner. In order to achieve a great reduction of NOx by the premix combustion, fuel is supplied to the auxiliary burner throughout the entire turbine load to enhance flame stabilization at the outer periphery side of the combustion chamber, and at the same time the combustible mixtures from the premix burner and the premix chamber are burned in a more lean condition, thereby reducing NOx. Further, in the case where a fuel-air ratio control function is applied to the premix burner, the combustible mixtures from the premix burner and the premix chamber can be burned in a more lean condition, using the flame of the auxiliary burner as a basis. Particularly where the premix burner is subjected to a fuel-air ratio control, the transfer of flame from the auxiliary burner, as well as a combustion pattern of the combustible mixture fed from the premix chamber, can be freely determined, thereby achieving a stable premix combustion and a very advantageous operation control. Figs. 5 and 6 show combustion conditions (i.e., gas temperature, NOx concentration and CO concentration) within the combustion chambers in the prior art and the present invention, respectively. Fig. 5 shows the condition of the upstream side of the combustion chamber during the diffusion-premix combustion. The gas temperature is high at the radially-central portion of the combustion chamber, and a low temperature region is formed at the outer peripheral portion of the combustion chamber. Therefore, the premix mixture flowing at the outer peripheral portion of the combustion chamber is not burned satisfactorily, thereby producing unburned substances such as CO. Particularly, since a cooling air layer is present about the wall surface of the combustion chamber, it is considerably difficult to remove such unburned substances once they are produced. Further, the high temperature region at the radially-central portion of the combustion chamber constitutes a source of production of NOx, thus failing to achieve a great reduction of NOx. On the other hand, according to the present invention, the first and second premixture supply means are provided inside and outside the combustion chamber, respectively. The outside second premixture supply means is operated during the low-load operation of the turbine, thus providing a complete premix construction, and as shown in Fig. 6, the gas temperature is relatively averaged to thereby reduce NOx and CO at the same time. In other words, in the present invention, since the flow or movement of air, etc., on the outer peripheral portion of the combustion chamber at its upstream side portion is relatively small, the flame from the auxiliary burner can be relatively easily formed, and also an excessive cooling at the outside of the combustion chamber is prevented during the low-load operation of the turbine, thereby adequately preventing the production of unburned substances. Further, for increasing the combustion in accordance with the turbine load, since the surrounding heat sources (contact ratio, etc.) are large, the thermal diffusion movement (flame propagation and combustibility) at the downstream side of the combustion chamber is rapid. This enables suppressing the production of unburned substances at those portions of the combustion chamber where the mixture is more lean and also at the outlet side of the combustion chamber. Therefore, the present invention provides an improved combustor which can suppress the production of NOx and CO over a range from the turbine no-load to the rated load. Fig. 7 shows a comparison in NOx characteristics in combustion between the prior art and the present invention. Even in the prior art, a region of low production of NOx exists in a certain range where the ratio of premix combustion amount to the diffusion combustion amount has a specified value. In the complete premix combustion according to the present invention, an amount of NOx can be reduced to less than 1/2 of that achieved by the prior art. Further, as shown in Fig. 8, in the combustor of the present invention, even when the fuel-air ratio is shifted 30% toward the lean side with respect to the prior art, there can be achieved a stable combustion free from the production of CO and other substances. Figs. 9 to 14 show various modified auxiliary burners. In an auxiliary burner 13a shown in Fig. 9, a plurality of holes or slits 41 are formed through an inner wall of a fuel tank 40 adjacent to a distal end thereof, the fuel tank 40 receiving the auxiliary fuel 12. A recess portion 11 is provided immediately adjacent to the distal end of the fuel tank 40, and is defined by an abruptly-expanded portion 9 and an annular portion 10. When the auxiliary burner 13a is to be ignited, the auxiliary fuel 12 is injected through the holes 41 toward the air 7 fed from the premix burner 8, and this fuel-air mixture is ignited by an ignition plug to form a flame 101. This flame 101 is kept stable by vortexes 42 induced in the recess portion 11 by a jet of the air 7. Fig. 11 shows a modification in which holes 45 are formed through a wall of the premix burner 8 adjacent to a proximal end of a fuel tank 40, so that the auxiliary fuel 12 can be injected into the premix burner 8. Fig. 12 shows another modification in which holes 46 are formed through the abruptly-expanded portion 9 of the recess portion 11. As shown in Figs. 11 and 12, a constricted portion 47 or 48 is formed at the outlet of the premix chamber 4 provided at the upstream side of the combustion chamber 5 at its radially-central portion. With this construction, the velocity of flow of the combustible fuel-air mixture is increased, thereby effectively preventing the flame from being directed reversely. Fig. 10 shows a premix-and-auxiliary burner 43 which mixes air 44 and the fuel 12 together so as to form a flame 101 in the combustion chamber 5. In order to effectively transfer the flame of the premix burner 8 to the combustible fuel-air mixture fed from the premix chamber 4, the premix burner 8a may be inclined at an angle 49 relative to the premix chamber 4 as shown in Fig. 13, or the premix burner 8b may be disposed perpendicular to the premix chamber 4 as indicated by reference numeral 50 in Fig. 14. With this arrangement, the combustible fuel-air mixture from the premix chamber 4 can be effectively burned. As described above, in the present invention, the premix burner provided at one end (head side) of the combustion cylinder is disposed generally concentric with the combustion cylinder. The first premix burner is operable in a high-load range of the combustor. The second premix burner is provided adjacent to the outer periphery of the first premix burner, and is operable at least a low-load range of the combustor. With this construction, even when the combustor is under a low load, an unburned substance (i.e., CO), which would tend to develop adjacent to the inner peripheral surface of the combustion cylinder, that is, at the outer peripheral portion of the combustion chamber, can be sufficiently prevented from being produced. Further, as the load increases, the combustible fuel-air mixture is supplied to the combustion chamber from the upstream side of the combustion chamber at its radially-central portion, and therefore the surrounding heat source is large when the combustible mixture is burned, so that the thermal diffusion movement is rapid at the downstream side, thereby effectively suppressing the development of the unburned substance at the outlet of the combustion chamber. Although the present invention has been specifically described with reference to the above embodiments, the invention itself is not to be restricted to such embodiments, and various modifications can be made without departing from the scope of appended claims.

Claims: Method of operating a combustor comprising the steps of supplying an auxiliary burner (13) with fuel (12) at the starting of a gas turbine and igniting said auxiliary burner (13), characterized in further comprising the steps of supplying an outer premix burner (8) provided inside of said auxiliary burner (13) with fuel (6) and igniting it when the turbine is operated with low load, and supplying an inner premix burner (4) provided inside of said outer premix burner (8) with fuel (18) and igniting it when the turbine is operated above a predetermined load. Method according to claim 1, characterized in that the fuel supply to the outer premix burner (8) is started when the turbine runs unloaded and the fuel amount supplied to the auxiliary burner (13) is simultaneously reduced, and the fuel supply to the inner premix burner (4) is started above the predetermined turbine load, and the fuel amount supplied to the outer premix burner (8) is simultaneously reduced. Method according to claim 1, characterized in that the fuel-air ratio for the auxiliary burner (13) is increased at the starting of the gas turbine and the auxiliary burner (13) is ignited, the fuel-air ratio for the outer premix burner (8) is increased above the unloaded state of the turbine and the outer premix burner (8) is ignited and the fuel-air ratio for the auxiliary burner (13) is simultaneously reduced, and the fuel-air ratio for the inner premix burner (4) is increased above a predetermined turbine load to a rated turbine load, and the inner premix burner (4) is ignited, and the fuel-air ratio for the outer premix burner (8) is simultaneously reduced. Method according to claim 3, characterized in that the inner premix burner (4) is ignited after the ignition of said outer premix burner (8). Method according to claims 1 to 4, characterized in that the amount of fuel supplied to the outer premix burner (8) is increased before the turbine reaches the unloaded state. Combustor comprising a combustion cylinder (2) having a combustion chamber (5) therein, a first burner (4, 16) disposed along the axis of the combustion cylinder (2), a second burner (8, 32) disposed adjacent to an outer periphery of the first burner (4, 16) and a third burner (13) disposed radially outside the second burner (8, 32), characterized in that the third burner (13) is disposed around the outer periphery of the second burner (8, 32) in the vicinity of its injection port and is adapted to be ignited at the starting of the combustor, the first and the second burner being premix burners. Combustor according to claim 6, characterized in that the third burner (13) comprises a plurality of holes (41, 45) formed through an inner wall of a fuel tank (40). Combustor according to claim 7, characterized in that the holes (45) are formed through a wall of the second burner (8) adjacent to a proximal end of the fuel tank (40).

Inventors: AKATSU SHIGEYUKI, HASHIMOTO TAKASHI, INOUE HIROSHI, ISHIBASHI YOJI, KATO FUMIO, KIRIKAMI SEIICHI, KURODA MICHIO, OHMORI TAKASHI, AKATSU, SHIGEYUKI, HASHIMOTO, TAKASHI, INOUE, HIROSHI, ISHIBASHI, YOJI, KATO, FUMIO, KIRIKAMI, SEIICHI, KURODA, MICHIO, OHMORI, TAKASHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1990
Main classification: F23R3
IPC codes: F23R3, F23C99