GAS TURBINE COMBUSTOR AND COMBUSTION METHOD THEREFOR

Abstract: A gas turbine combustor comprises a head com­ bustion chamber (2) and a rear combustion chamber (1) connected to a downstream side of the head combustion chamber (2), a first stage burner (6) for premixing first stage fuel and air and supplying the resultant fuel and air premixture into the head combustion chamber (2) to effect first stage premix combustion, a second stage burner (7) for premixing second stage fuel and air and the resultant fuel and air premixture into the rear combustion chamber (1) to effect premix combustion in addition to the first stage premix combustion, and a device (9, 25) for regulating flow rates of combustion air to be premixed with first and second stage fuel. The combustor is further provided with a pilot burner (5) in the head combustion chamber (2) to form pilot flame and stabilize the first stage premix combustion.

Description: Background of the Invention: This invention relates to a combustor for an industrial gas turbine and more particularly, to a multi-­ stage combustion type combustor providing a low nitrogen oxides (NO x ) concentration in an exhaust gas. As an example of conventional combustors, Fig. 1 of European Patent Publication No. 0 169 431 illustrates a two-stage combustion type combustor. The NO x concentration in an exhaust gas of this combustor is lower than in a single-stage combustion type combustor. Fig. 1 of US Patent No. 4,112,676 also shows an example of a combustor providing diffusion combustion while controlling the flow rate of a fuel and multi-stage premix combustion on the downstream side thereof. Recently, it is required to extremely strictly restrain the emission of NO x for environmental preservation and it cannot be satisfied sufficiently by merely employing the known systems described above. Therefore, a more precise control of a combustion phenomenon is necessary. Of the two prior arts described above, the former reduces the NO x concentration by the combination of diffusion combustion and premix combustion. Since diffusion combustion is used partially, however, the occurrence of hot spot is unavoidable. In order to further reduce the NO x concentration, an improvement in the diffusion combustion portion is by all means necessary. The latter employs multi-stage premix combustion on the downstream side, but since the diffusion combustion system is employed at the head portion, there is an inevitable limit to the reduction of the NO x concentration. Therefore, practical problems will develop. Japanese Patent Laid-Open No. 57-41524/1982 discloses a gas turbine in which a premixing chamber is provided outside the combustor for premixing fuel with air that an air from a compressor is boosted up and supplies the resultant premixture into a combustion chamber at a head portion to form a pilot flame, and premixed fuel and air is further supplied on a downstream side thereof for main combustion. Summary of the Invention: It is therefore an object of the present invention to provide a premixing multi-stage combustor which econimically minimizes the occurrence of NO x inside the combustor and moreover, can carry out combustion stably within an operation range. In a combustor of the type wherein fuels are supplied into a head combustion chamber and a rear combustion chamber and combustion is effected at multiple stages, the object described above can be accomplished by mixing in advance both of the fuels supplied to the head and rear combustion chambers with combustion air regulated in flow rate so as to strengthen the degree of premixing and to carry out multistage lean premix combustion. Brief Description of the Drawings: Fig. 1 is a sectional view of one embodiment of a turbine combustor according to the present invention; Fig. 2 is a diagram showing the result of measurement of NO x in premix combustion; Fig. 3 is a diagram showing the relation between NO x and a gas turbine load; Fig. 4 is a sectional view of another embodiment of the turbine combustor according to the present invention; Figs. 5 and 6 each are schematic views showing other embodiments of the present invention, respectively; Fig. 7 is a diagram showing the relation between a combustion type and a quantity of resulting NO x ; and Fig. 8 is a characteristic diagram showing conventional first stage and second stage combustion conditions. Detailed Description of the Invention: The combustion phenomenon can be classified broadly into diffusion combustion and premix combustion. The generation quantity of NO x in these combustors is generally such as shown in Fig. 7. It can be understood that lean combustion must be made in order to restrict the generation quantity of NO x . The NO x concentration can be more reduced with an increasing degree of premixing if the fuel-air ratio is kept constant, while NO x concentration increases drastically with increasing fuel air ratio even if premixing is sufficiently effected. From stability of combustion, however, the stable range of the fuel-air ratio becomes narrower with the increasing degree of premixing. On the other hand, one of characterizing features of gas turbine combustors lies in that the operation range of the fuel-air ratio from the start to the rated load is extremely wide. Particularly at the time of the load operation of the gas turbine, the operation is made by adjusting only the fuel flow rate under the condition that the air quantity is substantially constant. For this reason, the fuel quantity becomes small at the time of the low load to establish a lean state and there is the danger that unburnt components increase and dynamic pressure increases thereby causing oscillation. Taking the problems described above into consideration, European Patent Publication No. 0 169 431 employs the system which employs diffusion combustion having a wide stable combustion range at the start and the low load operation, adds premix combustion at the time of the high load operation and thus reduces the NO x concentration. Fig. 8 shows the operation zones of first stage and second stage nozzles (F₁, F₂). In other words, it employs the combination of diffusion combustion using lean combustion (F₁ operational zone) and premix combustion (F₂ operational zone), and the conventional combustor was improved from a combustion system using diffusion combustion alone, which operational zone is shown by C, in order to reduce the NO x concentration. To further reduce the NO x concentration, the degree of premixing must be further improved. In other words, reduction of NO x can be accomplished by employing premixing for the first stage combustion, improving the degree of premixing, inclusive of that of the second stage and effecting lean combustion. The factors that might become necessary when premixing is improved are counter-measures for narrowness of the stable combustion range, the structure and controlling method for effecting combustion under the condition approximate to the optimal condition throughout the full operation range, and the structure for improving premixing. A stable combustion range is made sufficiently wide by providing a pilot flame particularly at the time of low load so as to let a premixed fuel combustion flame burn stably. To effect combustion under the condition approximate to the optimal condition throughout the full operation range, the air-fuel ratio cannot be controlled at only one stage due to the limitation of an actual machine, so that two stage combustion is employed and the fuel-air ratio is controlled at each stage. The structure for improving premixing can be accomplished by employing a structure wherein a premixing distance is sufficiently elongated. Hereinafter, one embodiment of the present invention will be described with reference to Fig. 1, which is a sectional view of one embodiment of the invention. A combustor 15 is shown, wherein a combustor liner 3 consisting of portions of a main chamber 1 or rear combustion chamber and a sub-chamber 2 or head combustion chamber is disposed in an outer cylinder 4. The combustor is of a multi-stage combustion type wherein a pilot burner 5, a first stage burner 6 and a second stage burner 7 are provided. The first stage burner 6 comprises a pilot burner partition 19 fixed to an end plate 4a of the outer cylinder 4. The partition 19, which is formed annular, is fixed to an annular member 21a with an annular space therebetween, a plurality of swirler vanes 21 disposed between and fixed to the annular member 21a and the partition 19 thereby providing a plurality of outlets for premixed fuel and air, and a plurality of first stage fuel nozzles 20 the tips of which are disposed on more upper reaches than the upperstream side of the swirler 21 so that sufficient length for premixing fuel and air is obtained. The plurality of outlets of the first stage burner 6 are annularly arranged adjacent to the inner surface of the sub-chamber 2 and surround the pilot burner 5 disposed at a central axis of the sub-chamber 2. The pilot burner 5 has a swirler made of a plurality of swirler vanes 21 and surrounding a central fuel nozzle. The pilot burner 5 is supplied with combustion air from a line 14a branched from a compressed air line 14. The second stage burner 7 is slidably disposed between an outer surface of a downstream end of the sub-­ chamber 2 and an inner surface of an upstream end of the main chamber 1. The second stage burner 7 comprises an inner annular member 27b, an outer annular member 27a, a plurality of swirler vanes 23 secured thereto thereby providing a plurality of outlets for premixed fuel and air, and a plurality of second stage fuel nozzles 22 the tips of which are disposed on more upper reaches than the swirler vanes 23, so that a sufficient length for premixing fuel and air is obtained. An inlet side of the second stage burner 7 is secured to a partition 8 secured to the outer cylinder 4. The partition 8 has a plurality of air holes 26 communicating with the inlet of the first stage burner 6. A guide ring 9 has a plurality of air holes 25, surrounds the air holes of the partition 8 and the inlet of the second stage burner 7 and is axially movable so as to control flow rates of combustion air to the first and second stage burners. The outer cylinder 4, guide ring 9, the partition 8 and the outer surface of the main chamber 1 define an annular space for air passage communicating with the compressed air line 14. Combustion air to be introduced into the first stage and second stage burners is separated by the partition 8 and the quantity of air inflowing there is controlled by the guide ring 9. The fuel is dividedly supplied as a pilot burner fuel 10, a first stage burner fuel 11 and a second stage burner fuel 12. Hereinafter, the operation sequence of this combustor will be explained. The air leaving a compressor portion 13 of a gas turbine 16 is introduced into the combustor 15 through the line 14 and turned to high temperature gas by the combustor 15 and rotates a dynamo 17 at the turbine portion 16 to produce electric power. At the start, the pilot burner fuel 10 is first supplied to the pilot burner 5 to make diffusion combustion. The fuel is supplied from the center portion and causes combustion by combustion air from the swirler 18 for the pilot burner. This pilot burner 5 generates a stable flame in the sub-chamber 2 and power at the time of start in the gas turbine, and plays the role of the flame for burning stably the premix combustion flame generated by the first stage burner 6. In this embodiment, the combustion air for pilot burner 5 enters the space 19a which is completely partitioned by the partition 19 and the combustion air for first stage burner 6, which quantity is controlled, enters the outside of the space 19a. Therefore, this structure is one that controls completely the combustion air for the first stage burner 6 rather than for the pilot burner 5. The first stage burner 6 is provided with the nozzles 20 disposed upstream of the swirler 21 and the fuel is swirled by the swirler 21 after reaching the premixed state and is supplied into and combusted inside the sub-­ chamber 2. At a time of low load operation of the turbine, a first stage fuel is supplied into the sub-chamber 2 through the first stage burner 6 with combustion air being regulated by an air flow rate regulating device as described later and fired by the pilot flame. As the first stage fuel increases, the combustion air is increased by the air flow rate regulating device so that lean combustion can be effected. Since this flame is premix combustion flame controlled in flaw rate of combustion air so as to effect lean combustion, the range of stable combustion becomes generally narrow but since the fuel is swirled by the swirler 21 and the flame is kept stably by the pilot burner 5, combustion can be made stably and moreover, with a low NO x concentration. The second stage burner 7 is disposed downstream of the first stage burner 6 and effects stable premix combustion with a low NO x concentration in the main chamber 1. Ignition in this case is made by the flame generated in the sub-chamber 2. Next, the explanation will be given on the control of first and second stage fuel-air ratios. To control the fuel-air ratio, the air flow rate must be controlled in response to the increase of the fuel that occurs with the increase of the load. The control is made by the above-mentioned air flow rate regulating device. Namely, the device comprises the guide ring 9 and the guide ring moving mechanism 24, and the guide ring 9 can be moved in the axial direction by the guide ring moving mechanism 24. A plurality of air supply holes 25 are bored in the guide ring 9 and the air can inflow from the portions which can communicate with a partition air introduction hole 26 disposed on the partition 8 and a second stage burner air introduction portion 27. The area of this communication portion can be increased and decreased with the movement of the guide ring 9 in the axial direction. In other words, the air inflowing from the partition air introduction holes 26 is used as the combustion air for the first stage burner 6 and the air from the second burner air introduction holes 27 is used as the combustion air for the second stage burner. According to the structure described above, the air-fuel ratio of the first and second stage burners 6, 7 can be controlled suitably and low NO x concentration can be accomplished. This effect will be explained with reference to Fig. 2. Fig. 2 shows an example of the result of measurement of NO x of premix combustion. There are shown the NO x value corresponding to the equivalent ratio of fuel to combustion air, in case that a multi-diffusion combustion nozzle is used for the first stage burner and a premix combustion nozzle is used for the second stage burner. Two lines A and B in premix combustion represent the results of two cases A and B wherein different structures of the second stage burner are employed. The rightward line which is large in a gradient exhibits a larger degree of premixing. Since the ratio of the air flow rate to the fuel is substantially constant in the gas turbine, the NO x must be as low as possible with respect to a certain equivalent ratio. From this respect, an effective system is one that increases the premixing degree as much as possible but does not provide a high NO x value even when combustion is made at a high equivalent ratio. In other words, it is extremely effective to employ premix combustion for the first and second stage burners and to reduce the diffusion combustion portion as much as possible. According to the embodiment, a amount of fuel can be stably combusted under a state of lean fuel because the combustion air flow rate is regulated to be a suitable fuel air premixture. Therefore, as the turbine comes into a high load operation, an amount of combustion air is increased in addition to increase in fuel amount. In this control, excess combustion air in the annular space enters the combustor through dilution holes (not shown) made in the combustor liner, so that even if the turbine load changes, the stable lean combustion is effected. Fig. 3 shows the estimated relationship between NO x and the gas turbine load when combustion is made as described above. The prior art example represents the case where the first stage burner employs diffusion combustion and the second stage burner does premix combustion. In the case of the present invention, suitable premix combustion is made by reducing the diffusion combustion portion as much as possible and increase the premixing degree at the first and second stage burners. As a result, premix combustion with a substantially constant equivalent ratio can be made by controlling suitably the fuel-air ratio, and NO x can be reduced drastically in comparison with the prior art example. The examples shown in Fig. 3 are of the two-stage type. NO x concentration drops in the step-like form at the point of shift from diffusion combustion to premix combustion and at about intermediate point of premix combustion. This happens when the first stage burner 6 and the second stage burner 7 are ignited sequentially. When the flame is shifted from the pilot burner 5 to the first stage burner 6 and further to the second stage burner 7, the fuel-air ratio must be optimized and set to a suitable value that the shift of flame occurs reliably. For, there is the danger of occurrence of unburnt components if firing is not quickly effected, but the flame can be shifted stably by premix combustion and moreover, by controlling the fuel-air ratio. The gradient of the increase of NO x during the switch of the burners is determined by the proportion of diffusion combustion to the entire combustion and the conditions at the time of switch of the burners. Such operation conditions can be controlled in detail by controlling the fuel-air ratio as in the present invention. Namely, the present invention is characterized in that NO x can be reduced by suitably controlling the combustion phenomenon itself. Next, a modified example of this embodiment will be described. First of all, differently from Fig. 1, a partition is not made completely by a pilot burner partition 19 so that a gap 19b is left, and the pilot burner 5 communicates with the first stage burner 6 in air passage. This example is shown in Fig. 4. The combustion air passes through the air supply ports 25 of the guide ring 9 and the partition air introduction holes 26 of the partition 8 and is supplied into the pilot burner 5 and the first stage burner 6. In this case, the air flow rates of both of the burners are controlled simultaneously, but the same effect can be expected in the sense that the fuel-air ratio of the first stage burner 6 is controlled suitably. The second stage burner 7, and its control and other construction are the same in Fig. 1. Other modified examples include an example where the portion of the pilot burner 5 is replaced by other premixing type burner or an example where the pilot burner 5 is removed completely. In these cases, unstability of premix combustion cannot be covered by other flames but this problem can be solved by setting the fuel-air ratio of the premix combustion flame to a little high value to insure stable combustion. In this sense, these modified examples are expected to exhibit substantially the same effect. Fig. 5 shows another modified example. The construction of this example is somewhat different. Namely, a single or a plurality of pilot burners 28 for the first stage burner and pilot burners 29 for the second stage burner are disposed. Accordingly, the apparatus has somewhat thick main chamber 1 and sub-chamber 2 but exhibits good stability of flame. Fig. 6 shows still another modified example. The first stage burner 6 is disposed in such a manner as to face the pilot burner 5 and the first stage flame 30 is generated as a stable eddy flame inside the sub-chamber 2. Further, the second stage burner 7 sprays the fuel in the radial direction to form second stage flame 31. In this manner, a two-stage combustor is formed which generates the stable flames for both of the burners. Though the embodiment and examples given above all deal with second-stage premix combustion by way of example, the same effect can be expected in the case of multi-stage premixing wherein the number of stage is further increased and such multi-stage premix combustion is also embraced in the scope of the present invention. In accordance with the present invention, it becomes possible to enlarge the load range of premix combustion, to control both of the fuel and air in the respective combustion portions, to control suitably the fuel-air ratio, to reduce NO x and thus to accomplish low NO x concentration.

Claims: 1. A multistage combustion type gas turbine combustor (15) comprising a head combustion chamber (2) at a head of said combustor (15) for effecting first stage premix combustion of fuel premixed with combustion air, a rear combustion chamber (1) connected to a downstream side of said head combustion chamber (2) for effecting second stage combustion of fuel premixed with combustion air, and means for regulating a flow rate of combustion air to be premixed with fuel and introduced into at least one of said head and rear combustion chamber (1, 2) so as to form a suitable fuel and air premixture to effect premixed fuel combustion thereby to reduce NOx production. 2. A gas turbine combustor as defined in claim 1, wherein a means (5) is provided in said head combustion chamber (2) for producing pilot combustion flame and stabilizing premix combustion flame generated in said head combustion chamber (2). 3. A gas turbine combustor as defined in claim 1, wherein a plurality of pilot burners (5) are provided in said head and rear combustion chambers (1, 2). 4. A gas turbine combustor as defined in claim 2, wherein said head combustion chamber (2) has a reduced cross-sectional area portion at a downstream side thereof, and a plurality of first stage burners (6) are provided at said reduced cross-sectional area portion to inject premixed fuel and air into a central portion of said head combustion chamber (2). 5. A gas turbine combustor comprising: a head combustion chamber (2) at a head of said com­ bustor for effecting first stage combustion, a first stage burner (6) provided on an upstream side of said head com­ bustion chamber (2) for introducing premixed fuel and air into said head combustion chamber to effect a first stage premix combustion, a pilot burner (5) provided adjacent to said first stage burner (6) for pilot combustion flame in said head combustion chamber (2), a rear combustion chamber (1) connected to a downstream side of said head combustion chamber (2) for effecting second stage com­ bustion, a second stage burner (7) on an upstream side of said rear combustion chamber (1) for introducing premixed fuel and air into said rear combustion chamber (1) to effect a second stage premix combustion, and an air flow regulating device provided in said combustor to regulate a flow rate of first stage combustion air to be premixed with first stage fuel and a flow rate of a second stage combustion air to be premixed with second stage fuel, thereby providing suitable fuel air premixture to effect lean combustion. 6. A gas turbine combustor as defined in claim 5, wherein said second stage burner (7) is provided with a plurality of premixed fuel and air outlets (23) arranged circumferentially of said rear combustion chamber (1). 7. A gas turbine combustor as defined in claim 6, wherein said first stage burner (6) is provided with a plurality of annularly arranged premixed fuel and air outlets (21) and said pilot burner (5) is located at a center of said first stage burner (6), whereby premix combustion flame produced by said first stage burner (6) is made stable by combustion flame produced by said pilot burner (5). 8. A gas turbine combustor comprising: a combustor liner (3) for defining a cylindrical head combustion chamber (2) at a head of said combustor and a cylindrical rear combustion chamber (1) on a down­ stream side of said head combustion chamber, said rear combustion chamber (1) having a larger diameter than said head combustion chamber (2); an outer casing (4) axially elongated and sur­ rounding said combustor liner (3) with a space therebet­ ween for an air passage; a first stage burner (6) disposed at an upstream side of said head combustion chamber (2) and having annularly arranged outlets for premixed fuel and air; a pilot burner (5) disposed adjacent to said first stage burner (6) for producing combustion flame; a second stage burner (7) having annularly arranged outlets for premixed fuel and air, said outlets disposed between an outer surface of a downstream end portion of said head combustion chamber (2) and an inner surface of an upstream end portion of said rear combustion chamber (1); and a combustion air flow regulating device provided to regulate a flow rate of combustion air to be led to said first stage burner (6) from said space and a flow rate of combustion air to be led to said second stage burner from said space. 9. A gas turbine combustor as defined in claim 8, wherein said combustion air flow regulating device com­ prises a guide (9) ring having a plurality of air holes (25) each communicating with said first and second stage burners (6) to introduce combustion air and a mechanism for axially sliding said ring (9) so that opening areas of said air holes opening to each of said first (6) and second stage burners (7) change. 10. A gas turbine combustor as defined in claim 8, wherein said pilot burner (5) has combustion air supply passages independant of said space for air passage. 11. A combustion method for a gas turbine combustor comprising a head combustion chamber for effecting first stage combustion and a rear combustion chamber for effecting second stage combustion, said method comprising the steps of: premixing first stage fuel with combustion air and supplying the resultant first stage fuel air premixture into said head combustion chamber; regulating a flow rate of the combustion air to be premixed with the first stage fuel so that the first stage fuel air premixture will be suitable to effect lean premix combustion; firing and combusting the first stage fuel air premixture in said head combustion chamber when the turbine is in a low load operation; premixing second stage fuel with combustion air and supplying the resultant second stage fuel air premixture into the rear combustion chamber when the turbine reaches a high load operation, the second stage fuel air premixture being fired by premixed first stage fuel combustion flame, and combusted in addition to the premixed first stage fuel combustion,and regulating a flow rate of the combustion air to be premixed with the second stage fuel so as to be suitable in a fuel air ratio to effect lean premix combustion. 12. A combustion method for a gas turbine combustor comprising a head combustion chamber for effecting first stage combustion and a rear combustion chamber for effecting second stage combustion, said method comprising the steps of; supplying fuel and air into said head combustion chamber and mixing them in said head combustion chamber; igniting and combusting the fuel and air mixture thereby to form pilot flame; supplying first stage fuel into a first stage burner provided in said head combustion chamber; supplying combustion air to said first stage nozzle to premix it with the first stage fuel while regulating a flow rate thereof so as to increase the flow rate with increasing first stage fuel amount at an initial stage and then to be suitable fuel air premixture to effect lean premix combustion; supplying the premixed first stage fuel and air into said head combustion chamber; firing the premixed first stage fuel and air by the pilot flame to combust it at an initial and a low load operation of the turbine; supplying second stage fuel to a second stage burner provided in said rear combustion chamber; supplying combustion air to said second stage burner to premix it with the second stage fuel while regulating a flow rate thereof so as to increase the combustion air with increasing second stage fuel and to be suitable fuel air premixture to effect lean premix combustion; supplying the premixed second stage fuel and air into said rear combustion chamber, the premixed second stage fuel and air being fired by the first stage premixed fuel combustion flame and combusted therein thereby effecting premix combustion in addition to the first stage combustion when the turbine is in a high load operation.

Inventors: HISANO, KATSUKUNI, IIZUKA, NOBUYUKI, ISHIBASHI, YOJI, KIRIKAMI, SEIICHI, KURODA, MICHIO, OHMORI, TAKASHI, SATO, ISAO, URUSHIDANI, HARUO
Applicants: HITACHI, LTD.
Application year: 1988
Main classification: F23R3
IPC codes: F23C99, F23R3