COMBUSTOR FOR GAS TURBINE

Abstract: A combustor of two-stage combustion type in which fuel is supplied only from primary fuel nozzles (25) for combustion only in a primary combustion chamber (17) under the operating condition where gas turbine load is low, and premixed mixture is supplied into a secondary combustion chamber (18) provided on the downstream side of the primary combustion chamber (17) for cumbustion in both the primary and secondary combustion chambers under the operating condition where the load is high, wherein there is provided an air-bleed passageway (38, 39, 41, 100, 101) which opens on the downstream side of air holes for intaking secondary combustion air to be mixed with secondary fuel and communicates with the outside of the combustor, this passageway is provided with a regulating value and the secondary combustion air is bled at the minimum point of the load where the secondary fuel is supplied so that the fuel to air ratio of secondary premixed mixture may not be lean excessively, thereby reducing the production of non-burned components.

Description: FIELD OF THE INVENTION: The present invention relates to a combuster for supplying working gas of high temperature and high pres­ sure to a gas turbine, and particularly relates to a gas turbine combustor of two-stage combustion type wherein such a gas fuel as natural gas (LNG) is used. There has been disclosed, for example, in British Patent No. 2146425 published on April 17, 1985 a method wherein a combustor has a primary fuel nozzle for forming pilot flame at the head of the combustor, a secondary fuel nozzle is provided for supply of premixed air on the downstream side of the flow of combustion gas, and fuel is supplied only to the primary fuel nozzle or to both the primary and secondary fuel nozzles depending on the load of a gas turbine so that low temperature combus­ tion is conducted over the whole range of load of the gas turbine. Such premixed stage combustion has an advantage that the production of nitrogen oxide (NOx) is reduced because the low temperature combustion can be conducted. It also has a problem, however, that secondary fuel is difficult to be burned completely and such un-burned components as CO, HC are exhausted when the fuel begins to be supplied from the secondary fuel nozzle in the process of the gas turbine load's varying from low load to high load. Also, in a gas turbine for driving a generator, although the gas turbine is driven at a certain speed regardless of the load thereof when the generator reaches a certain synchronous speed, the amount of fuel supply increases generally in proportion to the increase of the load. Accordingly, there is a problem that the mixture ratio of fuel to air changes depending on the load. That is to say, because the air supplied to the combustor of the gas turbine is compressedly delivered by a compressor which is directly coupled to a rotor of the gas turbine, a generally constant amount of air is given to the combustor when the rotational speed of the rotor is constant. In a combustor for conducting two-stage combus­ tion, it is necessary to regulate and control fuel and air, particularly by supplying secondary fuel, so as to maintain the ratio of air flow rate to fluid flow rate constant in accordance with the load increase and always conduct stable combustion. With reference to a combustor having a fuel nozzle, so-called a single stage combustor, it has been disclosed in Japanese Patent Application Laid-Open No. 45739/80 that the fuel ratio is maintained constant even though the fuel supply amount changes according to the turbine load. In this known art, a part of the whole combustion air is bled to be discharged into exhaust gas or the atmosphere over the wide range from low load operation to rated load operation as a measure of increasing the fuel ratio of combustible zone during the low load, and the amount of bleed air is contrarily decreased or reduced to zero during the high load, whereby the combustion air is controlled according to the respec­ tive loads. However, the control of flow rate by the air-bleeding disclosed in this known art is the regulation of air flowing into the whole combustor. The balance of distribution of the overall air flow rate is broken due to the air bleeding. Further, when this measure is applied to a two-stage combustor, the air of the primary stage is reduced and the air-fuel ratio at the primary stage is too rich, even though combustibility of the secondary stage is improved by the air-bleeding, because the overall air flow rate is regulated. As the result, the amount of nitrogen oxide (NOx) in the exhaust gas is increased and the amount of air for cooling the wall of the combustor is reduced. Consequently, there exists a problem in the life span of the combustor. Also, a large amount of air flow has to be bled from the whole air for maintaining the secondary air flow at its desired rate since the secondary air flow rate is controlled by bleeding a certain amount of air from the overall air. As the air pressurized by the compressor is expelled out of the system for the above end, there is a disadvantage that a considerable reduction in efficiency occurs in the whole system of the gas turbine. Thus, it is difficult to accomplish desirable performance even if the prior art is merely applied to the combustor of the two-stage combustion type, and there exist such disadvantages as mentioned above. A method of regulating the amount of air mixed with secondary fuel by a mechanical vane has been proposed by the same inventors as those of the present application in U.S. Patent Application Serial No. 752680 (filed July 8, 1985). In the invention of this earlier application, the movable vane is provided close to a secondary fuel nozzle to influence the reliability. SUMMARY OF THE INVENTION It is an object of the present invention to provide a combustor of two-stage combustion type which is capable of conducting low-temperature combustion for con­ siderable reduction of NOx and maintaining the mixture ratio of secondary fuel to air can a desired extent for stable combustion. Further, another object of the invention is to provide a gas turbine combustor of two-stage combustion type in which the occurrence of NOx is reduced owing to low temperature diluted combustion at both the first and the second stage to attain high reduction of NOx and stable combustion can be maintained from low load opera­ tion to high load operation. According to the invention, the flow rate of premixed air is regulated so as to maintain the ratio of air to fuel constant in accordance with the change of the fuel flow rate corresponding to turbine load, with respect to secondary combustion where the mixture of the air and the fuel premixed before combustion is supplied into a combustor for restraint of the occurrence of NOx. That is, the amount of air introduced to the secondary stage is controlled by bleeding the air effectively only from an air flow passageway to the secondary stage. During supply of the fuel to the secondary stage i.e. when the rate of the fuel flow is low, a large amount of air is bled, and when the fuel flow rate is increased in accordance with the increasement of load, the amount of bleed air is decreased, so that the control of the air bleeding is conducted to keep the mixture ratio of the fuel to the premixed air constant. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of a combustor for a gas turbine of two-stage combustion type in which the present invention is executed; Figs. 2 (a), (b) and (c) are respectively characteristic diagrams showing relations between gas turbine load versus secondary fuel ratio, air flow rate and fuel flow rate; Fig. 3 is a characteristic diagram showing a relation between the gas turbine load and air-bleeding rate; Fig. 4 is a detailed cross-sectional view of the air-bleed port portion shown in Fig. 1; Fig. 5 is a detailed cross-sectional view of an air-bleed port portion of another embodiment of the invention; Fig. 6 is a detailed cross-sectional view of an air-bleed port portion of still another embodiment of the invention; Fig. 7 is a control characteristic diagram in the embodiment of Fig. 6; Fig. 8 is a characteristic diagram of NOx, CO and un-burned hydrocarbon UHC according to the prior art; Fig. 9 is a characteristic diagram of NOx, CO and UHC according to the present invention; Fig. 10 is a characteristic diagram of the air-bleed rate, NOx and CO according to the prior art; Fig. 11 is a characteristic diagram of the air-bleed rate, NOx and CO according to the present invention; and Fig. 12 is a detailed cross-sectional view of an air-bleed port portion according to still another embodi­ ment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS An embodiment of the present invention is shown in Fig. 1. Fig. 1 illustrates a two-stage combustor to which the present invention is applied, whereby low NOx combustion can be achieved. A gas turbine consists of such major components as a compressor 1, a turbine 2, a combustor 3 and a generator which is not shown. The combustor 3 comprises an inner pipe 4 for defining a combustion chamber, an outer pipe 5 encirculing the inner pipe 4, a casing 7 for defining a cylinder 13 and a tail pipe 8 through which combustion gas is introduced to turbine stator blades 6. There is attached at the closed side end of the outer pipe 5 a cover 10 on which a primary fuel nozzle body 9 is mounted. Air 11 compressed by the compressor 1 is introduced into the cylinder 13 through a diffuser 12 and further into the inner pipe 4 in the form of combustion air flows 15 a , 15 b through dilution air holes 14, wall cooling air holes and combustion air openings all formed in the inner pipe 4. The inner pipe 4 is constituted by a front combustion chamber 17 having an inner cylinder cone 16 mounted therein and a rear combus­ tion chamber 18 of a larger diameter than the front com­ bustion chamber. Secondary air supply holes 20 is provided in a connecting section 19 between the front combustion chamber 17 and the rear combustion chamber 18. Fuel is spouted out in the connecting section from secondary fuel nozzles 21 to be fed into the rear combus­ tion chamber 18 together with secondary air 23 so as to form premixed combustion flame 24. Further, apertures 41 (Fig. 4) for bleeding a part of the secondary air are provided between the secondary air holes 20 and a secondary fuel mixture portion 33. A pipe line 39 leads from the apertures 41 to connect the same to an exhaust duct 42, the turbine at a position 43 before the turbine blades on and after the second stage or a midway stage 44 of the compressor, whereby an air flow rate flowing into a secondary stage is regulated. The air-bleed pipe line 39 is provided with a flow-rate regulating valve 100 which is controlled by a controller 101. Although, ignition of primary fuel 26 spouted from primary fuel nozzles 25 is not shown, primary combustion flame 27 is formed by an ignition plug of piezoelectric compression type. The ignition of the secondary premixed mixture is carried out by applying fire thereto by means of the primary combus­ tion flame 27. In Figs. 2(a), 2(b) and 2(c), the axes of abscissas indicate the gas turbine load and the axes of ordinate respectively exhibit the secondary fuel ratio, the air flow rate and the fuel flow rate. The combustor of two-stage combustion is operated with primary fuel G from the ignition to about 25% rate of the gas turbine load, and the primary fuel is reduced as shown by G ' when the gas turbine load has reached 25% rate while secondary fuel H ' is supplied simultaneously. Subsequently, the primary and secondary fuels G , H are gradually increased for a rise of the load. On the other hand, the air flow rate is increased corresponding to a rise of the rotational speed of the turbine, until the generator reaches a synchronous speed or while the gas turbine is operated with no-load, but the air flow rate becomes constant when the gas turbine is operated with load so that both of the primary and secondary air flow rate C , D are constant. The weight ratio of the secondary fuel to the air is shown in Fig. 2(a). In Fig. 2(a), there are shown an upper limit A above which the production of NOx is increased although the combustion state becomes better, and a lower limit A ' below which the combustion state is not adequate and non-burned components increase too much although the production of NOx is reduced. The secondary combustion must be carried out within the range X bounded by the lower and upper limits. When the secondary air flow rate is not controlled after the fuel is supplied to the secondary stage, that is, when the secondary air flow rate is kept constant from 0 to 100% of the gas turbine load as shown by the E line of Fig. 2(b), the ratio of the secondary fuel to the air results in the dashed line B' to become out of the range X during loaded period from B1' to B2' immediately after the supply of the secondary fuel. Thus, combustion cannot be continued and the supplied primary fuel is discharged in the form of non-burned component of hydrocarbon HC. Therefore, it is necessary to regulate the secondary air flow rate. According to the present invention, the air flow rate is controlled as shown by the F line of Fig. 2(b) with the air flow to the two-stage being bled after the supply of the secondary fuel. More specifically, the controller 101 in Fig. 1 receives load signals of the gas turbine and controls the amount of bleed air in accordance with the load as shown by the solid line in Fig. 3. The amount of bleed air is controlled by changing the extent to which the valve 100 is opened. Then, the ratio of the secondary fuel to the air results in the solid line B and is within the range X where the production of NOx is reduced and the combustion state is preferable so that the stable combustion can be maintained. The details of means for regulating and control­ ling the secondary air flow will be described with refer­ ence to Fig. 4. Secondary fuel and air supply means are provided at the connecting section 19 of the front combustion chamber 17 and the rear combustion chamber 18. The fuel is first introduced from a plurality of secondary fuel introducing portions 28 into respective reservers 29 to be injected from a plurality of spouting holes 31 which open at the top ends of a plurality of secondary fuel nozzles 21 and be mixed with a secondary air flow 32. When the fuel and the air pass a swirler 33 which constitutes a mixing chamber, they are changed into a premixed mixture and introduced into the rear combustion chamber 18 to form premixed flame. Air flow 34 flowing between the inner pipe 4 and the outer pipe 5 branches into primary combustion air flow 35 and secondary combustion air flow 36. After the secondary air flow 36 passes through the combustion air holes 20, it branches into flow 32 toward the rear combustion chamber 18 and air flow 37 to be bled. The bleed air flow 37 is expelled into exhaust gas or the atomosphere while being regulated by way of the air-bleed valve 100 from air-bleed openings 41 through passageways 38 and the pipe line 39. Alternatively, the air flow 37 may be introduced to the turbine at a position before the turbine blades on and after the second stage on the midway stage 44 (Fig. 1) of the compressor. On the other hand, it is preferable that the air-bleed openings 41 are pro­ vided to open in the passageway from the combustion air holes 20 to the fuel nozzles 21. It is not preferable that each air-bleed opening 41 is located in close proximity of the swirler 33 because the mixture is some­ times bled simultaneously with the bleed of air so that the fuel is also discharged out of the system. Also, it is not preferable that the openings are located on the upstream side of the air holes 20 because a part of the combustion air 35 to be introduced into the primary stage is also bled. Fig. 5 shows a modification wherein the air-bleed openings are provided on the reverse side as compared with those in the embodiment of Fig. 4 or the side of the rear combustion chamber 18 relative to the second combustion air holes 20. However, the secondary air flow regulating function of this modification is substantially the same as the first embodiment. Thus, when the secondary air flow rate is regulated from the secondary air flow passageway, the influence on the balance of distribution of the overall air can be suppressed to a low level unlike the prior art. Fig. 3 shows the bleed air rate according to the invention in comparison with that according to the prior art. This figure represents a case where the secondary fuel is supplied from the point of 25% of the load, and the present invention is illustrated by the solid line and the prior art is by the dotted line. In case such fuel supply as shown in Fig. 2(c) is carried out, it is necessary to reduce the secondary air flow rate to about half of that at the time of rated load upon supplying of the secondary fuel so that the secondary fuel may be supplied to be mixed with the secondary air and then ignited to be bruned completely. As the air flow rate for secondary combustion at the time of rated load is about 30% of the overall air flow rate, it is required to reduce the secondary air flow rate to 15% when the secondary fuel is supplied. According to the present invention, there­ fore, it is necessary to bleed approximately 15% of all the air flow rate. If such an effect as mentioned above is attempted to be attained by the known art, all the air rate must be reduced to half level in order to decrease only the secondary air amount from the overall air. Then, it is required to bleed about 45% of the air even if it is taken into account that the gas turbine load is covered by the increase of the fuel flow rate compensating the decrease of the air amount. In this way, according to the prior art, the bleed air amount is required about three times so as to reduce the secondary air flow rate to half. Further, the occurrence of NOx at the primary stage is increased because the fuel ratio in the primary combustion becomes large due to 45% decrease of the overall air amount, and the balance of the combustion air distribution is broken since the amount of air flow for cooling the inner pipe of the combustion is reduced. Moreover, the turbine efficiency is decreased because the amount of bleed air is increased. When the amount of bleed air is increased, the turbine efficiency decreases owing to an increase of the fuel flow rate, and the whole efficiency of the turbine is decreased by about 0.3% responsively to the air bleeding from the overall air flow at 1% rate thereof. Accordingly, the turbine efficiency in the invention is decreased by about 5% in contrast to the decrease of about 13% in case of the prior art, so that the decrease of the turbine efficiency can be sup­ pressed to about one third as compared that of the prior art. As a further preferable method, there is shown in Fig. 6 another embodiment wherein the bleed air amount 48 may be further reduced by attaching a covering 47 which is capable of changing the opening area of each secondary combustion air hole 20 gradually. Fig. 7 shows a result of the case where the combustor is operated to reduce the opening area of each air hole 20 under the operating condition in which the bleed air amount 48 is large, and to increase the opening area of each air hole 20 by slid­ ing the covering 47 when the bleed amount is small. In Fig. 7, the case where the opening area is adapted to be phasedly changeable is shown by the solid line and the case where only the air-bleeding is carried out is represented by the dotted line. When the opening areas are controlled so as to open continuously or intermittent­ ly in accordance with the increase of load over than 25% of load, as shown by the solid line in Fig. 7, the bleed air amount and the fuel flow rate are reduced and the thermal efficiency is increased to result in further improvement of the efficiency of the combustor. That is, when the opening area is reduced by about 50%, the bleed air amount is reduced to about half and the decrease of thermal efficiency is reduced by half. Therefore, a sufficient result can be attained when both the air-bleed­ ing method and the method of changing the opening areas are employed. In this case, the control of the opening areas of the air holes 20 by the covering 47 may be rough and thus, a design may be made giving priority to the reliability. In addition, according to this embodiment, a performance can be obtained at the approximately same degree as the prior art by regulating and controlling the opening areas of the air holes to reduce the bleed air amount to about 1/3 as compared that of the prior art. The decrease of the turbine efficiency resulted from the air-bleeding can be suppressed to a minimum by controlling only secondary air flow rate. Figs. 8 and 9 show a case where the secondary air flow rate is regulated by bleeding the air and a case where the secondary air flow rate is not regulated, respectively, and in these figures the axes of ordinate and abscissa are indicated at the same scale. Fig. 8 shows the case where the air flow rate is not regulated. The experiment is conducted under the actual loading condition of a 70 MW class gas turbine plant and the result is indicated in Fig. 8. The emission of nitrogen monoxide CO, hydrocarbon HC which are un-burned components is increased after the supplying of secondary fuel. This is for the reason that the air amount is large and the fuel ratio is low or lean so that the fuel to air ratio is out of the lower limit A ' since the air flow amount is not regulated as described with reference to Fig. 2, and the emission of the un-burned components are increased. Accordingly, it is understood that the regulation of the secondary air flow rate at time of supplying the secondary fuel is required in order to reduce the un-burned com­ ponents. There is shown in Fig. 9 the experimental result according to the present invention where the flow rate is regulated by bleeding a part of the air passed through the combustion air holes. The amount of the air flow for forming the premixed mixture is reduced since the secondary air flow is exhausted by bleeding a part thereof when the secondary fuel is supplied, and the condition that the air is slightly in excess of a small amount of fuel can be maintained so that the un-burned components as shown in Fig. 8 are scarcely exhausted. Thus, the combus­ tion of high efficiency in which the production of NOx is reduced and the occurrence of the un-burned components is restrained can be conducted over a wide range from the supplying of the secondary fuel to the rated load opera­ tion, by bleeding the secondary air flow at the time of the supply of the secondary fuel and subsequently controlling the air flow rate for reduction of the bleed air rate in accordance with the increase of the fuel. Fig. 10 shows a relation between bleed air rate, and NOx and CO according to the prior art disclosed in Japanese Patent Application Laid-Open No. 45739/80. An experimental result at the time of gas turbine load of 25% rate is shown in Fig. 10. In this case, unless the bleed air amount (the air bleed is carried out from the overall combustion air) is in excess to a certain extent, the density of CO which is an un-burned component is not reduced and the effect is not satisfactory. This is resulted from the fact that the air-bleed is effected on the overall air amount for reduction of the secondary com­ bustion air. As the overall air flow amount is required to be reduced to half so that the flow amount of the secondary air which corresponds to about 25% of the overall air flow amount may be reduced to about half, the efficiency of the secondary combustion cannot be improved unless a great amount of air is bled. Also, the combustion in the primary stage changes to a high-temperature condition wherein the fuel is too rich, because the overall air amount is reduced by the air bleed to decrease particularly the air flow to the primary stage. For this reason, there exists a disadvan­ tage that the occurrence of NOx at the primary stage is increased. Fig. 11 is a characteristic diagram of NOx and CO shown at the same scale as Fig. 10, and represents the result according an embodiment of the present invention in which air is bled from a secondary air passage. As seen in the figure, the secondary combustion efficiency is improv­ ed by a small amount of bleed air and the density of NOx in the exhaust can be reduced. Fig. 12 shows still another embodiment of the invention, in which each air-bleed hole is separated into two sections that are respectively provided on the left and right sides of each secondary combustion air hole 20. When air is bled from an air-bleed opening 41a, the mixture flows in proximity to the outer peripheral wall of the rear combustion chamber 18 as shown by the dashed line 32a, and when the air is bled from an air-bleed opening 41b, the mixture flows on the inner side of the rear combustion chamber as shown by the dashed line 32b. Accordingly, in case that the secondary fuel is ignited by the primary flame 27, the ignition efficiency can be improved when the air is bled from the air-bleed opening 41b and the mixture is supplied around the center of the combustion chamber 18. When the air is bled from the air-bleed opening 41a after the ignition of the secondary fuel, the mixture flows along the inner wall of the inner pipe 4 so that the flame is cooled by the wall surface of the inner pipe 4 and lower temperature combustion can be effected. Although an bleed air rate regulating value is required for each of air-bleed pipe lines 39a, 39b in the embodiment of Fig. 12, an effect of reduction of NOx can be achieved because of low temperature combustion which is resulted from the ignition of the secondary fuel improved as mentioned above. According to the present invention, since only the secondary air to be controlled can be regulated by the air-bleed, the improvement of the secondary combustion efficiency can be attained with about one third of the bleed air amount as compared with the case where the air is bled from the overall air in a combustor as shown in the prior art. The change of the characteristics of the primary combustion does not occur and desirable combustion can be maintained, because the balance of the primary and overall air flow amount distribution is not broken. Therefore, a large reduction of NOx can be achieved over a wide range from ignition to the rated load without lower­ ing the combustion efficiency.

Claims: 1. A gas turbine combustion comprising a primary, combustion chamber (17) which includes at least one primary nozzle (25) opening thereto, through said primary nozzle (25) fuel is always supplied during operation of a gas turbine (1, 2), a secondary combustion chamber (18) provided on the downstream side of said primary combustion chamber (17) with respect to the flow of combustion gas and including on its circumferential wall premixed fuel supply holes (20) for premixing fuel from at least one secondary fuel nozzle (21) with secondary combustion air to supply the mixture, and tail pipe (8) introducing the combustion gas produced in both said combustion chambers (17, 18) into the gas turbine, wherein fuel is supplied only from the primary fuel nozzle (25) to conduct combus­ tion only in the primary combustion chamber (17) under the operating condition where gas turbine load is low and the fuel is supplied from both said fuel nozzles (25, 21) to conduct combustion in both of the combustion chambers (17, 18) under the operating condition where the gas turbine load is high, characterized in that there is provided air-bleed means (38, 39, 41, 100, 101) for bleeding a part of said secondary combustion air alone and said air-bleed means is adapted to bleed air under the operating condi­ tion where the secondary fuel is supplied and the gas turbine load is low. 2. A gas turbine combustor according to Claim 1, characterized in that said air-bleed means includes a regulating valve (100) for regulating the bleed air rate and a controller (101) for controlling said regulating valve (100) and controls the bleed air rate so that the bleed air rate may be maximized at the minimum point of load where the secondary fuel is supplied and may be decreased in accordance with an increase of the load from said point to be reduced to zero at the time of the full load.

Inventors: HIROSE, FUMIYUKI, HISANO, KATSUKUNI, IIZUKA, NOBUYUKI, INOSE, HIROSHI, ISHIBASHI, YOJI, KIRIKAMI, SEIICHI, KURODA, MICHIO, SATO, ISAO, UCHIYAMA, YOSHIHIRO, WADA, KATSUO
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: F02C9
IPC codes: F23R3, F02C9