Patent Publication Number: US-2022228532-A1

Title: Combustion control device for gas turbine, combustion control method, and program

Description:
TECHNICAL FIELD 
     The present disclosure relates to combustion control for a gas turbine in load rejection. 
     BACKGROUND 
     In a gas turbine plant for power generation, load rejection of cutting off a load may be performed during a load operation. An operation after the load rejection is regarded as success if there is no flame off in a combustor and a rotation speed of a gas turbine is, for example, not greater than 110% of an overspeed trip (to be referred to as OST, hereinafter) prescribed value. In order to make the operation after load rejection succeed, it is necessary to prevent flame off by increasing the air-fuel ratio by, for example, supplying much fuel. However, the above-described rotation speed is increased in accordance with the supply amount of the fuel, increasing possibility of OST. Meanwhile, although OST is reliably avoided as the supply amount of the fuel decreases, flame off occurs if the air-fuel ratio is too low due to the small supply amount of the fuel, which may stop the gas turbine. 
     For example, Patent Document 1 discloses suppressing a decrease in rotation speed of a gas turbine by controlling an opening degree of an inlet guide vane (IGV) and a valve opening degree of a bleed valve for bleeding compressed air from a compressor in load rejection. Further, Patent Document 2 discloses that in load rejection, a combustion control device controls an injection amount (fuel supply flow rate) from a fuel nozzle of a combustor to not greater than a threshold for preventing the rotation speed of the gas turbine from exceeding 110% of the OST prescribed value, and controls a bypass valve (bleed valve) on a bleed pipe for bleeding compressed air from a casing from a closed state to an open state. Thus, since the amount of the compressed air supplied to the fuel nozzle (air supply flow rate) is decreased by bleeding of the compressed air from the casing performed in load rejection, it is possible to increase the air-fuel ratio in the combustor without increasing the fuel supply flow rate into the combustor. Therefore, it is possible to prevent flame off by increasing the air-fuel ratio without consuming wasteful fuel, while preventing occurrence of OST by supplying fuel of the amount that does not cause OST. Patent Document 3 discloses an estimation method for a flame temperature on the basis of an operating condition of a gas turbine. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Document 1: JP5147766B 
         Patent Document 2: JP2017-106324A 
         Patent Document 3: JP2018-178803A 
       
    
     SUMMARY 
     Technical Problem 
     As described above, in load rejection of the gas turbine, prevention of flame off is required while preventing an excessive increase in rotation speed of the gas turbine. In Patent Document 2 described above, the air-fuel ratio, which is the ratio of the fuel supply flow rate to the air supply flow rate, is used as a control reference for preventing flame off, and in order to more accurately determine combustion stability in the combustor as well as to prevent damage to equipment due to the overhigh flame temperature, a method is required which performs combustion control in load rejection based the flame temperature. 
     In view of the above, an object of at least one embodiment of the present invention is to provide a combustion control device capable of appropriately performing an operation of the gas turbine in load rejection while preventing damage to equipment due to flame. 
     Solution to Problem 
     A combustion control device for a gas turbine according to at least one embodiment of the present invention is a combustion control device for a gas turbine for supplying, to a combustor, compressed air by a compressor flowing into a casing, the device including a bleed valve control unit configured to control a bleed valve disposed on a bleed pipe for performing bleeding so that a part of the compressed air flowing into the casing is not used as combustion air in the combustor, a fuel control unit configured to control a fuel regulating valve for regulating a fuel flow rate of fuel supplied to the combustor, and a temperature acquisition unit configured to acquire a flame temperature of flame caused by combustion of the fuel in the combustor. Upon reception of a load rejection signal for cutting off a load from the gas turbine, the bleed valve control unit controls a valve opening degree of the bleed valve from a closed state to an open state with a prescribed opening degree, and the fuel control unit controls the fuel regulating valve such that the acquired flame temperature falls within a predetermined temperature range defined by an upper limit value and a lower limit value. 
     A combustion control method for a gas turbine according to at least one embodiment of the present invention is a combustion control method for a gas turbine for supplying, to a combustor, compressed air by a compressor flowing into a casing, the method including a bleed valve control step of controlling a bleed valve disposed on a bleed pipe for performing bleeding so that a part of the compressed air flowing into the casing is not used as combustion air in the combustor, a fuel control step of controlling a fuel regulating valve for regulating a fuel flow rate of fuel supplied to the combustor, and a temperature acquisition step of acquiring a flame temperature of flame caused by combustion of the fuel in the combustor. Upon reception of a load rejection signal for cutting off a load from the gas turbine, the bleed valve control step includes controlling a valve opening degree of the bleed valve from a closed state to an open state with a prescribed opening degree, and the fuel control step includes controlling the fuel regulating valve such that the acquired flame temperature falls within a predetermined temperature range defined by an upper limit value and a lower limit value. 
     A combustion control program for a gas turbine according to at least one embodiment of the present invention is a combustion control program for a gas turbine for supplying, to a combustor, compressed air by a compressor flowing into a casing, the program including in a computer a bleed valve control unit configured to control a bleed valve disposed on a bleed pipe for performing bleeding so that a part of the compressed air flowing into the casing is not used as combustion air in the combustor, a fuel control unit configured to control a fuel regulating valve for regulating a fuel flow rate of fuel supplied to the combustor, and a temperature acquisition unit configured to acquire a flame temperature of flame caused by combustion of the fuel in the combustor. Upon reception of a load rejection signal for cutting off a load from the gas turbine, the program causes the computer to implement such that the bleed valve control unit controls a valve opening degree of the bleed valve from a closed state to an open state with a prescribed opening degree, and the fuel control unit controls the fuel regulating valve such that the acquired flame temperature falls within a predetermined temperature range defined by an upper limit value and a lower limit value. 
     Advantageous Effects 
     According to at least one embodiment of the present invention, a combustion control device is provided which is capable of appropriately performing an operation of a gas turbine in load rejection while preventing damage to equipment due to flame. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a system diagram of a gas turbine plant according to an embodiment of the present invention. 
         FIG. 2  is a functional block diagram of a combustion control device according to an embodiment of the present invention. 
         FIG. 3A  is a graph showing a transition of an air supply flow rate to a first cylindrical portion of a combustor in load rejection according to an embodiment of the present invention. 
         FIG. 3B  is a graph showing a transition of a valve opening degree of a bleed valve in load rejection according to an embodiment of the present invention. 
         FIG. 3C  is a graph showing a transition of a fuel supply flow rate from a pilot fuel system in load rejection according to an embodiment of the present invention. 
         FIG. 3D  is a graph showing a transition of a flame temperature in load rejection according to an embodiment of the present invention. 
         FIG. 4A  is a functional block diagram of a bleed valve control unit according to an embodiment of the present invention, where the bleed valve is controlled based on a load of the gas turbine. 
         FIG. 4B  is a functional block diagram of the bleed valve control unit according to an embodiment of the present invention, where the bleed valve is controlled based on the flame temperature. 
         FIG. 5  is a functional block diagram of the bleed valve control unit according to an embodiment of the present invention, where the bleed valve is controlled based on the number of used main nozzles in load rejection. 
         FIG. 6  is a functional block diagram of a fuel control unit according to an embodiment of the present invention. 
         FIG. 7  is a flowchart showing a combustion control method for the gas turbine according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Some embodiments of the present invention will be described below with reference to the accompanying drawings. It is intended, however, that unless particularly identified, dimensions, materials, shapes, relative positions and the like of components described or shown in the drawings as the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention. 
     For instance, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function. 
     For instance, an expression of an equal state such as “same”, “equal”, and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function. 
     Further, for instance, an expression of a shape such as a rectangular shape or a tubular shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved. 
     On the other hand, the expressions “comprising”, “including”, “having”, “containing”, and “constituting” one constituent component are not exclusive expressions that exclude the presence of other constituent components. 
       FIG. 1  is a system diagram of a gas turbine plant  6  according to an embodiment of the present invention. As shown in  FIG. 1 , the gas turbine plant  6  of the present embodiment includes a gas turbine  7  and a generator  61  driven by the gas turbine  7  to generate power. The gas turbine  7  and the generator  61  are coupled by a rotor  75 , and the generator  61  is driven by the gas turbine  7  via the rotor  75 , thereby generating power. In the embodiment shown in  FIG. 1 , the generator  61  is coupled to the rotor  75  (to be described later) on a side of a turbine  74  (to be described later). However, in some other embodiments, the generator  61  may be coupled to the rotor  75  (to be described later) on a side of a compressor  71  (to be described later). 
     (Description of Gas Turbine) 
     Describing about the gas turbine  7 , as shown in  FIG. 1 , the gas turbine  7  includes the compressor  71  for compressing air A flowing in from an air inlet system  62  to generate compressed air Ac, a casing  72  into which the air compressed by the compressor  71  (compressed air Ac) flows, at least one (plural in  FIG. 1 ) fuel system  8 , a combustor  73  for combusting fuel F (fuel gas; the same applies hereafter) supplied from the fuel system  8  by mixing the fuel F and the compressed air Ac flowing in from the casing  72  described above to generate a high-temperature combustion gas E, the turbine  74  for rotating the rotor  75  by the combustion gas E to drive the generator  61 , a bleed pipe  9   p  for bleeding the compressed air Ac from the casing  72  described above, a bleed valve  9  disposed on the bleed pipe  9   p,  for regulating a bleed air flow of the compressed air Ac bled from the casing  72 , and a combustion control device  1  for the gas turbine to be described later. 
     In the embodiment shown in  FIG. 1 , the gas turbine  7  includes an IGV (inlet guide vane)  71   a  for regulating the flow rate of the air A flowing in from the air inlet system  62 , and the air having passed through the inlet guide vane  71   a  flows into the compressor  71 . 
     The gas turbine  7  further includes a plurality of fuel systems  8 . That is, the gas turbine  7  includes five fuel systems  8  in total, namely, a diffusion pilot system  8 A for stabilizing flame by performing diffusion combustion, a premix pilot system  8 B for improving a reduction in NOx of the combustor  73  by performing premix combustion, a main A system  8 C and a main B system  8 D which are major fuel systems  8  for supplying premixed fuel (premixed gas) according to an output of the gas turbine  7 , and a top hat system  8 E for injecting the fuel F from upstream of the combustor  73  (a side of the casing  72 ) in order to improve combustion efficiency and stabilize flame. 
     More specifically, the diffusion pilot system  8 A includes a fuel regulating valve  83 A for regulating a fuel flow rate from a fuel tank  86 , and a fuel nozzle  81 A connected to a diffusion pilot manifold  82 A. The premix pilot system  8 B includes a fuel regulating valve  83 B for regulating the fuel flow rate from the fuel tank  86 , and a fuel nozzle  81 B connected to a premix pilot manifold  82 B. The main A system  8 C includes a fuel regulating valve  83 C for regulating the fuel flow rate from the fuel tank  86 , and a fuel nozzle  81 C (main nozzle) connected to a main A manifold  82 C. The main B system  8 D includes a fuel regulating valve  83 D for regulating the fuel flow rate from the fuel tank  86 , and a fuel nozzle  81 D (main nozzle) connected to a main B manifold  82 D. The top hat system  8 E includes a fuel regulating valve  83 E for regulating the fuel flow rate from the fuel tank  86 , and a fuel nozzle  81 E connected to a top hat manifold  82 E. 
     Further, the respective fuel systems  8  are connected to the fuel tank  86 . The fuel tank  86  is connected to a pressure system  87  for applying a fuel supply pressure to the fuel tank  86 , and the fuel supply pressure is applied from the pressure system  87  to the fuel tank  86  via two fuel pressure regulating valves  88  ( 88   a,    88   b ) for controlling the fuel supply pressure supplied from the pressure system  87  to the fuel tank  86 . Thus, the fuel of the fuel tank  86  can be supplied to the combustor  73  via the respective fuel systems  8 . 
     The combustor  73  includes a first cylindrical portion  73   a  (such as a combustion liner) for generating the combustion gas E by combusting the fuel and the compressed air Ac, and a second cylindrical portion  73   b  (such as a transition piece) located downstream of the first cylindrical portion  73   a  and connecting the first cylindrical portion  73   a  and the turbine  74 . Then, the above-described fuel nozzles  81  ( 81 A to  81 E) inject the fuel to the first cylindrical portion  73   a  of the combustor  73 . Meanwhile, the fuel nozzle  81 E of the top hat system  8 E injects the fuel more upstream of the combustor  73 . 
     At least one of the diffusion pilot system  8 A and the premix pilot system  8 B is often used, such as only the premix pilot system  8 B is used. Further, the following description includes all the main systems used for fuel supply to the combustor  73 , such as the main A system  8 C and the main B system  8 D, when referred to as a main fuel system  8   m,  and includes all the pilot systems used for fuel supply to the combustor  73 , such as at least one of the diffusion pilot system  8 A and the premix pilot system  8 B, when referred to as a pilot fuel system  8   p.    
     Further, in the embodiment shown in  FIG. 1 , a downstream end of the above-described bleed pipe  9   p  is connected to an exhaust system  63  for exhausting the combustion gas E from the turbine  74 , and the compressed air Ac bled from the casing  72  flows into the exhaust system  63 . The flow rate of the compressed air Ac flowing through the bleed pipe  9   p  is regulated by the bleed valve  9 . The bleed valve  9  is fully closed in a load operation of the gas turbine  7 , but is opened in load rejection. 
     However, the present invention is not limited to the present embodiment. It is only necessary that the bleed pipe  9   p  is disposed to be able to bleed a part of the compressed air Ac from the casing  72 , so that the part of the compressed air Ac in the casing  72  is not used as combustion air As in the combustor  73 . Thus, in some other embodiments, the compressed air Ac bled from the casing  72  may be flowed to the second cylindrical portion  73   b  of the combustor  73  by connecting one end of the bleed pipe  9   p  to the casing  72  and connecting the other end (downstream end) of the bleed pipe  9   p  to, for example, the second cylindrical portion  73   b.  Thus, since the downstream end of the bleed pipe  9   p  is connected downstream of the first cylindrical portion  73   a  of the combustor  73 , it is possible to flow the part of the compressed air Ac, which flows into the first cylindrical portion  73   a  from the casing  72  inherently (when the bleed valve  9  is fully closed), to bypass the first cylindrical portion  73   a,  and it is possible to prevent the bled compressed air Ac from being used as the combustion air As in the combustor  73 . A cooling pipe  64  is a pipe for bleeding cooling air for cooling the compressor  71 . 
     Further, the combustion control device  1  for the gas turbine  7  (to simply be referred to as the combustion control device  1 , hereinafter) is a device for controlling combustion of the gas turbine  7  described above, and at the time of a normal operation (in the normal operation) other than in load rejection of the gas turbine  7 , controls a fuel flow rate Gf (fuel supply flow rate) supplied to the combustor  73  and a flow rate of the combustion air As (the compressed air Ac; the same applies hereafter) (air supply flow rate Ga) supplied to the combustor  73  as well, in accordance with the output and operating condition of the gas turbine  7 . On the other hand, in load rejection where load rejection of cutting off a load from the gas turbine  7  is performed, the combustion control device  1  controls the fuel regulating valves  83  and the bleed valve  9  while checking a temperature of flame (to be referred to as a flame temperature Tf, hereinafter) in the first cylindrical portion  73   a  caused by combustion of the fuel F injected from the fuel nozzles  81  ( 81 A to  81 E), with the rotation speed of the gas turbine  7  being not greater than a rotation speed threshold, such as the rotation speed being set not greater than 110% of the OST prescribed value, and without causing damage due to flame off and flame. 
     (Description of Combustion Control Device  1  for Gas Turbine  7 ) 
     Hereinafter, combustion control performed by the combustion control device  1  for the gas turbine  7  in load rejection will be described in detail with reference to  FIGS. 2 to 3D .  FIG. 2  is a functional block diagram of the combustion control device  1  according to an embodiment of the present invention.  FIG. 3A  is a graph showing a transition of the air supply flow rate Ga to the first cylindrical portion  73   a  of the combustor  73  in load rejection according to an embodiment of the present invention.  FIG. 3B  is a graph showing a transition of a valve opening degree V of the bleed valve  9  in load rejection according to an embodiment of the present invention.  FIG. 3C  is a graph showing a transition of the fuel supply flow rate from the pilot fuel system  8   p  in load rejection according to an embodiment of the present invention.  FIG. 3D  is a graph showing a transition of the flame temperature Tf in load rejection according to an embodiment of the present invention. 
     The combustion control device  1  is a device for performing combustion control for the gas turbine  7  for supplying, to the combustor  73 , the compressed air Ac by the compressor  71  flowing into the casing  72  described above. As shown in  FIG. 2 , the combustion control device  1  includes a fuel control unit  3 , a bleed valve control unit  2 , and a temperature acquisition unit  4 . The above-described functional units will be described by taking, as an example, a case where the gas turbine  7  performs load rejection from performing combustion control in the main fuel system  8   m  including the main A system  8 C and the main B system  8 D described above, and in the pilot fuel system  8   p  which is the premix pilot system  8 B. 
     The combustion control device  1  may be constituted by a computer. The computer includes, for example, a processor  11  such as CPU (not shown), a memory (storage device  12 ) such as ROM and RAM. Then, the processor  11  performs an operation (such as computation of data) in accordance with an instruction of a program (combustion control program  10 ) loaded to a main storage device, thereby implementing each of the above-described functional units. In other words, the above-described combustion control program  10  is software for causing the computer to implement the above-described respective functional units, not a temporary signal, and may be stored in the above-described storage medium which is computer-readable and portable. 
     The bleed valve control unit  2  is the functional unit configured to control the valve opening degree V of the bleed valve  9  disposed on the bleed pipe  9   p  described above. The bleed valve control unit  2  sets the bleed valve  9  in the closed state in the normal operation other than in load rejection, so that the compressed air Ac of the casing  72  is not bled through the bleed pipe  9   p.    
     The fuel control unit  3  is the functional unit configured to control the each fuel regulating valve  83  described above. Controlling the fuel regulating valve  83  (the valve opening degree of the fuel regulating valve  83 ), the fuel flow rate (fuel amount) supplied into the combustor  73  (into the first cylindrical portion  73   a ) is controlled. In load rejection, the fuel control unit  3  controls the fuel regulating valves  83  ( 83 C and  83 D in  FIG. 1 ) of the main fuel system  8   m,  and the fuel regulating valve  83  (at least one of  83 A or  83 B in  FIG. 1 ) of the pilot fuel system  8   p,  as will be described later. 
     The temperature acquisition unit  4  is the functional unit configured to acquire the flame temperature Tf of flame caused by combustion of the fuel F in the combustor  73 . In some embodiments, the temperature acquisition unit  4  may estimate the flame temperature Tf based on various references indicating the operating conditions of the gas turbine  7 . In some other embodiments, the temperature acquisition unit  4  may acquire a measurement value measured by a temperature measurement unit (such as a temperature sensor or the like) capable of directly or indirectly measuring the flame temperature Tf. Although the details will be described later, in the embodiments shown in  FIGS. 1 to 3D , the temperature acquisition unit  4  acquires the flame temperature Tf by calculating an estimate value of the flame temperature Tf. 
     Then, in the combustion control device  1  having the above-described configuration, upon receiving a load rejection signal S for cutting off the load from the gas turbine  7 , in load rejection, the fuel control unit  3  increases the supply flow rate of the fuel F from the pilot fuel system  8   p  (to be referred to as a pilot fuel flow rate Fp, hereinafter) while decreasing the supply amount of the fuel F from the main fuel system  8   m  in decreasing the total supply amount (total amount) of the fuel F to the combustor  73 . That is, while decreasing the valve opening degree of the fuel pressure regulating valve  88  at which the fuel is supplied to the fuel tank  86 , the valve opening degree of the fuel regulating valve  83  ( 83 C,  84 D) of the main fuel system  8   m  is decreased, and the valve opening degree of the fuel regulating valve  83  in the pilot fuel system  8   p  (the fuel regulating valve  83 B in the premix pilot system  8 B in the present embodiment) is increased. 
     Then, the fuel control unit  3  and the bleed valve control unit  2 , respectively, control the valves to be controlled as follows. That is, the bleed valve control unit  2  controls the valve opening degree V of the bleed valve  9 , which is fully closed before the load rejection signal S is received, from the fully closed state to the open state with a prescribed opening degree Vc (see  FIG. 3B  to be described later). The prescribed opening degree Vc may be full opening as will be described later, or may be an intermediate opening degree where the valve opening degree is low relative to full opening and the valve opening degree is high relative to full closing which is a completely closed state. Further, after the bleed valve  9  is set at the prescribed opening degree Vc in the control in load rejection, the valve opening degree V of the bleed valve  9  may or may not be changed from the prescribed opening degree Vc. 
     The flame temperature Tf rises as the air-fuel ratio is high, decreasing the possibility of flame off. Thus, if the bleed valve  9  is changed from the closed state to the open state in load rejection, a part of the compressed air Ac flows from the casing  72  so as to bypass the first cylindrical portion  73   a  of the combustor  73 . Consequently, the amount of the combustion air As supplied from the casing  72  to the combustor  73  is relatively decreased (see  FIG. 3A  to be described later), and the air-fuel ratio in the first cylindrical portion  73   a  increases, raising the flame temperature Tf (see  FIG. 3D  to be described later). That is, setting the bleed valve  9  in the open state in load rejection, it is possible to raise the flame temperature Tf without increasing the fuel flow rate. However, if the flame temperature Tf is raised too much by the increase in air-fuel ratio, the gas turbine  7 , such as the first cylindrical portion  73   a  of the combustor  73 , may be damaged due to flame. 
     Thus, the above-described fuel control unit  3  controls the fuel regulating valve  83  such that the flame temperature Tf acquired by the temperature acquisition unit  4  described above falls within a predetermined temperature range defined by an upper limit value Lu and a lower limit value Ll (see  FIG. 3C  to be described later). The above-described upper limit value Lu is set at a temperature at which damage to the gas turbine  7  due to flame is preventable, and the above-described lower limit value Ll is set at a temperature at which disappearance of flame (flame off) in the combustor  73  is preventable. Thus, the damage to the gas turbine  7  due to flame off in load rejection and flame having the excessively high flame temperature Tf is to be prevented. 
     In the embodiments shown in  FIGS. 1 to 3D , the above-described combustion control device  1  further includes a load rejection signal reception unit  50  configured to receive the load rejection signal S during the operation of the gas turbine  7 . Then, with reception of the load rejection signal S by the load rejection signal reception unit  50 , the IGV  71   a  is controlled to be closed, the flow rate of the air A flowing into the compressor  71  is decreased, and the overall flow rate of the compressed air Ac flowing into the casing  72  is decreased. More specifically, in the embodiments shown in  FIGS. 3A to 3D , upon receiving the load rejection signal S, the combustion control device  1  starts control in load rejection from time t 0 , and decreases the overall flow rate of the compressed air Ac by a certain proportion as shown in  FIG. 3A . 
     Thus, as indicated by a dashed line of  FIG. 3A , although the flow rate of the combustion air As flowing into the first cylindrical portion  73   a  of the combustor  73  decreases, at the same time, as shown in  FIG. 3B , the bleed valve control unit  2  increases the valve opening degree V of the bleed valve  9  toward the above-described prescribed opening degree Vc over time (in  FIG. 3B , increases the valve opening degree by a certain proportion). Thus, the flow rate of the combustion air As flowing into the first cylindrical portion  73   a  of the combustor  73  is rapidly decreased as indicated by a solid line of  FIG. 3A  relative to the case of the dashed line, due to the decrease in overall flow rate of the compressed air Ac and the decrease caused by setting the bleed valve  9  in the open state. In  FIG. 3A , inflow of the air A to the compressor  71  is still decreased after the valve opening degree V of the bleed valve  9  reaches the prescribed opening degree Vc at time t 1 , the decrease proportion (inclination of the graph) of the air flow rate indicated by the solid line between the time t 1  and time t 2  is identical to that by the dashed line. 
     Meanwhile, regarding the fuel amount supplied into the first cylindrical portion  73   a  of the combustor  73 , the rotation speed of the gas turbine  7  increases as the supplied fuel amount (fuel flow rate) is large. Thus, with reception of the load rejection signal S by the load rejection signal reception unit  50 , the fuel control unit  3  increases the pilot fuel flow rate Fp over time as shown in  FIG. 3C  (increases by a certain proportion in  FIG. 3C ) in order to prevent flame off, while decreasing the fuel flow rate from the main fuel system  8   m  to the first cylindrical portion  73   a  over time as described above. As a result, as indicated by a solid line of  FIG. 3D , in the pilot fuel system  8   p,  with the increase in fuel flow rate and the decrease in the combustion air As by opening control of the bleed valve  9 , the flame temperature Tf rises over time, as indicated by the solid line of  FIG. 3D . 
     More specifically, in  FIG. 3D , until the time t 1 , the flame temperature Tf rises due to the above-described two factors. Since the valve opening degree V of the bleed valve  9  becomes the prescribed opening degree Vc at the time t 1  (see  FIG. 3B ), between the time t 1  and the time t 2  thereafter, the flame temperature Tf rises by the contribution of only the increase in the pilot fuel flow rate Fp described above. Thus, the increase proportion (inclination of the graph) of the flame temperature Tf after the time t 1  is moderate. 
     With such combustion control, the flame temperature Tf becomes greater than the lower limit value Ll, avoiding flame off. Further, the fuel control unit  3  performs feedback control on the fuel supply flow rate while monitoring the flame temperature Tf so that the flame temperature Tf does not exceed the upper limit value Lu, and thus the flame temperature Tf is not greater than the upper limit value Lu. A dashed line of  FIG. 3D  indicates a case where the bleed valve  9  remains closed, and indicates a case where the air-fuel ratio is low relative to the case where the bleed valve  9  is opened and the flame temperature Tf goes below the lower limit value Ll. 
     In the embodiments shown in  FIGS. 1 to 3D , as shown in  FIG. 2 , the fuel control unit  3  includes a fuel flow rate decision unit  31  for deciding the fuel flow rate Gf supplied to the combustor  73  such that the flame temperature Tf input from the temperature acquisition unit  4  becomes a set temperature having any value between the above-described upper limit value Lu and the above-described lower limit value Ll (Ll≤set temperature≤Lu), and a fuel valve opening degree decision unit  32  for controlling the fuel regulating valves  83  such that the fuel flow rate Gf decided by the fuel flow rate decision unit  31  is supplied. Then, the valve opening degree V of the bleed valve  9  is fed back and input to the temperature acquisition unit  4 , and the fuel regulating valves  83  (the fuel regulating valves  83  of the main fuel system  8   m  and the pilot fuel system  8   p ) are controlled while checking the flame temperature Tf that changes in accordance with the valve opening degree V of the bleed valve  9  or the like. 
     In the embodiments shown in  FIGS. 1 to 3D , as shown in  FIG. 2 , the combustion control device  1  includes a prescribed opening degree decision unit  5  for deciding the prescribed opening degree Vc of the bleed valve  9  described above, and the decided prescribed opening degree Vc is stored in the storage device  12 . Then, the bleed valve control unit  2  acquires the prescribed opening degree Vc from the storage device  12 . The prescribed opening degree decision unit  5  may store a value input from an operator in the storage device  12 , or may decide a value in accordance with a predetermined logic as will be described later, and then may store the value in the storage device  12 . Further, the storage device  12  also stores the upper limit value Lu and the lower limit value Ll of the above-described flame temperature Tf, and the above-described fuel control unit  3  acquires such information from the storage device  12 . 
     With the above configuration, the gas turbine  7  includes the bleed pipe and the bleed valve  9  capable of decreasing the supply amount of the combustion air As to the combustor  73  by bleeding the compressed air Ac by the compressor  71  from the casing  72 . In such load rejection of the gas turbine  7 , when the excessive increase in rotation speed of the gas turbine  7  and flame off in the combustor  73  are prevented, the valve opening degree of each of the bleed valve  9  and the fuel regulating valves is further controlled to control the fuel flow rate and the amount of the combustion air As supplied to the combustor  73 , such that the flame temperature Tf in the combustor  73  falls within the predetermined temperature range defined by the upper limit value Lu and the lower limit value Ll. Thus, it is possible to prevent damage to equipment due to the excessive rise in the flame temperature Tf, while preventing the excessive increase in rotation speed of the gas turbine  7  and flame off 
     Next, an embodiment where the flame temperature Tf is estimated will be described. 
     In some embodiments, as shown in  FIG. 2 , the temperature acquisition unit  4  may include an air flow rate calculation unit  41  configured to calculate, based on the valve opening degree V of the bleed valve  9 , the flow rate of the combustion air As (the above-described air supply flow rate Ga) supplied into the first cylindrical portion  73   a,  and a temperature calculation unit  42  configured to calculate an estimate value of the flame temperature Tf based on the air supply flow rate Ga and the fuel flow rate Gf of the fuel each being supplied into the first cylindrical portion  73   a.  That is, the temperature calculation unit  42  calculates the estimate value of the flame temperature Tf in consideration of the valve opening degree V of the bleed valve  9 . 
     At this time, the air flow rate calculation unit  41  may theoretically or experimentally obtain, in advance, a relationship among respective pressures upstream and downstream of the bleed valve  9  in the bleed pipe  9   p  or a pressure difference between the respective pressures when the bleed valve  9  is fully closed, the valve opening degree V of the bleed valve  9 , and the amount of the bled compressed air Ac (to be referred to as a bleed flow rate Ge, hereinafter), for example, and based on a function defining the relationship, may calculate the air supply flow rate Ga when the bleed valve  9  is open at the optional valve opening degree V. That is, the bleed flow rate Ge is obtained through arithmetic calculation of the function by assigning the two upstream and downstream pressures or the pressure difference therebetween and the valve opening degree V of the bleed valve  9  described above to the above-described function. Further, the flow rate of the combustion air As supplied from the casing  72  to the first cylindrical portion  73   a  when the bleed valve  9  is fully closed (to be referred to as a normal air flow rate Gg, hereinafter) is calculated based on, for example, the valve opening degree of the IGV  71   a , the atmospheric pressure, the atmospheric temperature, or the like. The normal air flow rate Gg may be calculated by using a differential pressure type flow rate measurement method. Then, the air supply flow rate Ga when the bleed valve  9  is open at the optional valve opening degree V may be calculated by deducting the bleed flow rate Ge from the calculation result of the normal air flow rate Gg (Ga=Gg−Ge). 
     Meanwhile, in calculating the estimate value of the flame temperature Tf, the temperature calculation unit  42  may further consider a temperature of the compressed air Ac in the casing  72  (casing air temperature T cs ) obtained by, for example, measurement by a temperature sensor (not shown), in addition to the air supply flow rate Ga and the fuel flow rate Gf of the fuel F supplied into the first cylindrical portion  73   a.  Furthermore, the temperature calculation unit  42  may consider a lower heating value Hf of the fuel. That is, the flame temperature Tf may be calculated by any one of Tf=f(Gf, Ga), Tf=(Gf, Ga, T cs ), or Tf=(Gf, Ga, T cs , Hf), where f is an estimate function for estimating the flame temperature Tf, and estimate accuracy improves as elements to be considered increase. 
     In the embodiment shown in  FIG. 2 , the estimate value of the flame temperature Tf is calculated based on the air supply flow rate Ga and the fuel flow rate Gf supplied into the first cylindrical portion  73   a,  the casing air temperature T cs , and the lower heating value Hf. More specifically, the air flow rate calculation unit  41  receives the normal air flow rate Gg and the valve opening degree V of the bleed valve  9 , and outputs the air supply flow rate Ga. Further, the temperature calculation unit  42  receives the fuel flow rate Gf, the air supply flow rate Ga, the casing air temperature T cs  in the casing  72 , and the lower heating value Hf of the fuel calculated in the fuel control unit  3 , and outputs the estimate value of the flame temperature Tf calculated based on the above. Thus, calculating the air supply flow rate Ga in consideration of the valve opening degree V of the bleed valve  9 , it is possible to obtain the air supply flow rate Ga more accurately, and to accurately estimate the flame temperature Tf without actually measuring the same. 
     The above-described lower heating value Hf may be measured in real time by installing a calorie meter (not shown) on the fuel tank  86  or upstream thereof, and in the embodiment shown in  FIG. 2 , a measurement result of the lower heating value Hf by the calorie meter is input to the temperature calculation unit  42 . 
     With the above configuration, the temperature acquisition unit calculates the flow rate of the combustion air As (compressed air Ac) supplied to the inside (combustion space) of the first cylindrical portion  73   a  of the combustor  73  in consideration of the valve opening degree V of the bleed valve  9 , and estimates the flame temperature Tf based on the calculated flow rate of the combustion air As and the fuel flow rate. Thus considering the valve opening degree of the bleed valve, it is possible to obtain the flow rate of the combustion air As more accurately, and to accurately estimate the flame temperature Tf without actually measuring the same. 
     Next, a method for deciding the above-described prescribed opening degree Vc set for the bleed valve  9  by the bleed valve control unit  2  in load rejection will be described with reference to  FIGS. 4A to 5 .  FIG. 4A  is a functional block diagram of the bleed valve control unit  2  according to an embodiment of the present invention, where the bleed valve  9  is controlled based on the load of the gas turbine  7 .  FIG. 4B  is a functional block diagram of the bleed valve control unit  2  according to an embodiment of the present invention, where the bleed valve  9  is controlled based on the flame temperature Tf.  FIG. 5  is a functional block diagram of the bleed valve control unit  2  according to an embodiment of the present invention, where the bleed valve  9  is controlled based on the number of used main nozzles in load rejection. 
     In some embodiments, the above-described prescribed opening degree Vc may be full opening. The valve is in a region where a change in flow rate is extremely small and the flow rate is rarely changed, even if the valve is opened by not less than a certain valve opening degree. Thus, the above-described full opening may include not only a valve opening degree in a case where the valve is opened 100%, but also a valve opening degree which is less than 100% and obtains the same result as the case where the valve is opened 100%. Thereby, it is possible to maximize the increase in air-fuel ratio by the bleed valve  9 . Thus, it is possible to minimize the fuel amount supplied to prevent flame off, and to improve fuel efficiency. 
     The flame temperature Tf is associated with the load (output; the same applies hereafter) of the gas turbine  7 , and in general, the flame temperature Tf is also low when the load of the gas turbine  7  is low and on the contrary, the flame temperature Tf is also high when the load is high. For example, since the flame temperature Tf is inherently low when the above-described load is low, it is easy to perform control so that the flame temperature Tf does not exceed the upper limit value Lu even if the supply amount of the combustion air As to the combustor  73  is decreased by, for example, fully opening the bleed valve  9 . However, since the flame temperature Tf is inherently high when the above-described load is high, it is predicted that the rise of the flame temperature Tf is likely to be excessive if, for example, the bleed valve  9  is fully opened, which may make it difficult to keep the flame temperature Tf within the predetermined range by controlling the fuel regulating valves  83 . 
     Thus, in some embodiments, as shown in  FIGS. 4A and 4B , the prescribed opening degree Vc of the bleed valve  9  described above may be decided based on the above-described flame temperature Tf or a load reference value Ld (a load value, an output value, or the like) of the gas turbine  7  upon reception, such as immediately before reception, of the load rejection signal S. As a result, the above-described prescribed opening degree Vc is decided as full opening, the intermediate opening degree, or the like. For example, if the load reference value Ld or the flame temperature Tf are monitored before reception of the load rejection signal S, and then the load rejection signal S is received, the prescribed opening degree Vc may be decided based on the load reference value Ld or the flame temperature Tf immediately before the reception. 
     In the embodiments shown in  FIGS. 4A and 4B , the above-described prescribed opening degree decision unit  5  further includes a first decision unit  51  for deciding the above-described prescribed opening degree Vc of the bleed valve  9  based on the load reference value Ld ( FIG. 4A ) of the gas turbine  7  or the flame temperature Tf ( FIG. 4B ) when the load rejection signal S is received. In the embodiment shown in  FIG. 4 , the combustion control device  1  further includes a load reference value acquisition unit  52  configured to acquire the load reference value Ld of the gas turbine  7  when the load rejection signal S is received. Then, the above-described first decision unit  51  decides the above-described prescribed opening degree Vc of the bleed valve  9  based on the load reference value Ld acquired by the load reference value acquisition unit  52 . On the other hand, in the embodiment shown in  FIG. 4B , the above-described first decision unit  51  acquires the flame temperature Tf from the above-described temperature acquisition unit  4 , and decides the above-described prescribed opening degree Vc of the bleed valve  9  based on the acquired flame temperature Tf. For example, the prescribed opening degree decision unit  5  may acquire a valve opening degree according the load reference value Ld or the flame temperature Tf acquired by using, for example, a function where a correspondence relationship between the optional load reference value Ld or the optional flame temperature Tf and a valve opening degree according thereto is determined in advance, and may have the acquired valve opening degree as the prescribed opening degree Vc. 
     With the above configuration, the prescribed opening degree Vc of the bleed valve  9  is decided based on the flame temperature Tf or the load reference value Ld of the gas turbine  7  in load rejection. Thus, after the bleed valve  9  is set at the prescribed opening degree Vc, it is possible to more reliably prevent the flame temperature Tf from exceeding the upper limit value Lu by control of the fuel regulating valves  83 . Further, in estimating the flame temperature Tf, it is possible to predict, in advance, a case such as where the flame temperature Tf stays high, and in such a case, it is possible to, for example, set the bleed valve  9  at the intermediate opening degree. 
     Further, in load rejection, in accordance with characteristics of equipment, such as the occurrence status of combustion oscillation, the number of used fuel nozzles  81  in the main fuel system  8   m  (to be referred to as main nozzles, hereinafter) may be changed (changed from eight to three, for example). If the number of used main nozzles is changed, the fuel flow rate per main nozzle used in the main fuel system  8   m  is changed, and thus the flame temperature Tf can also be changed. For example, as shown in  FIG. 1 , five main nozzles are connected to the main A system  8 C, and three main nozzles are connected to the main B system  8 D. In this case, the number of used main nozzles can be changed, such as the number of used main nozzles becomes three in total by, for example, stopping fuel supply from the main A system  8 C after reception of the load rejection signal S and performing fuel supply only from the main B system  8 D from the operation where a total of eight main nozzles is used by fuel supply from the main A system  8 C and the main B system  8 D before reception of the load rejection signal S. 
     Thus, in some embodiments, as shown in  FIG. 5 , the bleed valve control unit  2  may acquire the number of main nozzles used after reception of the load rejection signal S, and may decide the above-described prescribed opening degree Vc of the bleed valve  9  based on the acquired number of used main nozzles. For example, in a case where the bleed valve  9  is set at a high opening degree, such as full opening, when the number of used main nozzles is eight, if the fuel is concentrated by each of the used main nozzles by decreasing the number of used main nozzles to five or three, the fuel per main nozzle increases. Thus, promoting further drop in the flame temperature Tf by further decreasing the air-fuel ratio per main nozzle by further decreasing the prescribed opening degree Vc of the bleed valve  9  relative to the case where the number of used main nozzles is larger, the flame temperature Tf may be prevented from exceeding the upper limit value Lu. 
     In the embodiment shown in  FIG. 5 , the above-described prescribed opening degree decision unit  5  includes a nozzle number acquisition unit  53  configured to acquire the number of fuel nozzles  81  in the main fuel system  8   m  used after reception of the load rejection signal S, and a second decision unit  54  configured to decide the above-described prescribed opening degree Vc of the bleed valve  9  based on the above-described number of used fuel nozzles  81  acquired by the nozzle number acquisition unit  53 . 
     With the above configuration, when the fuel F is supplied from the main fuel system  8   m  to the combustor  73  by using the plurality of fuel nozzles  81  (main nozzles), the prescribed opening degree Vc of the bleed valve  9  is decided based on the number of main nozzles used in load rejection. For example, in the case where the prescribed opening degree Vc of the bleed valve  9  is set at the relatively high opening degree, such as full opening, when the number of used main nozzles is eight, if the fuel F is concentrated by decreasing the number of main nozzles to three, five, the fuel F per main nozzle increases, further decreasing the prescribed opening degree Vc of the bleed valve  9 . Thus, it is possible to more reliably prevent the flame temperature Tf from exceeding the upper limit value Lu. 
     Next, an embodiments relating to the fuel control unit  3  will be described with reference to  FIG. 6 .  FIG. 6  is a functional block diagram of the fuel control unit  3  according to an embodiment of the present invention. 
     The embodiment where the prescribed opening degree Vc of the bleed valve  9  is decided based on the load reference value Ld and the like has already been described. In some embodiments, as shown in  FIG. 6 , the valve opening degree of the fuel regulating valve in the pilot fuel system  8   p  may also be decided based on the load reference value Ld. As already described, in general, the combustor  73  is connected to the pilot fuel system  8   p  and the main fuel system  8   m.  Then, supply of the fuel F to the combustor  73  in load rejection is controlled such that supply from the main fuel system is decreased and supply from the pilot fuel system  8   p  is increased, in decreasing the supply amount of the fuel F as a whole. Thus, the fuel control unit  3  controls the fuel regulating valve ( 81 A or  81 B) of the pilot fuel system  8   p  based on the load reference value Ld. 
     In the embodiment shown in  FIG. 6 , the combustion control device  1  includes the above-described load reference value acquisition unit  52 . Then, the fuel control unit  3  includes a reference opening degree decision unit  31   a  configured to decide a reference opening degree Vb of the fuel regulating valve  83  in the pilot fuel system  8   p  in accordance with the operating condition of the gas turbine  7 , upon reception of the load rejection signal S by the load rejection signal reception unit  50  described above, and an opening degree correction unit  31   b  configured to add an additional opening degree Vp to the reference opening degree Vb in a low load where the load reference value Ld of the gas turbine  7  is lower than the load reference value Ld in rating such as rated load (rated load reference value). That is, by adding the additional opening degree Vp to the reference opening degree Vb in low load, the fuel flow rate is increased relative to low load to increase the air-fuel ratio, thereby less causing flame off. 
     With the above configuration, if the operating condition of the gas turbine  7  is in low load in load rejection, the valve opening degree of the fuel regulating valve in the pilot fuel system  8   p  is further increased relative to that in rated load and high load to further increase the fuel flow rate to be supplied. If the operating condition of the gas turbine  7  is in low load in load rejection, the valve opening degree of the fuel regulating valve  83  described above is low, resulting in relatively low air-fuel ratio and the low flame temperature Tf. Thus, if load rejection is performed in the low load condition, further increasing the fuel flow rate of the fuel regulating valve  83  in the pilot fuel system  8   p,  it is possible to improve responsiveness to the rise in the flame temperature Tf, and to prevent flame off more reliably, such as to prevent the flame temperature Tf from going below the lower limit value Ll. 
     Hereinafter, a combustion control method corresponding to the process performed by the above-described combustion control device  1  will be described with reference to  FIG. 7 .  FIG. 7  is a flowchart showing the combustion control method for the gas turbine  7  according to an embodiment of the present invention. 
     As shown in  FIG. 7 , the combustion control method for the gas turbine  7  (will simply be referred to as the combustion control method, hereinafter) includes a bleed valve control step of controlling the valve opening degree V of the bleed valve  9  described above, a fuel control step of controlling the above-described fuel regulating valves  83 , and a temperature acquisition step of acquiring the flame temperature Tf of the flame described above. These bleed valve control step (S 1 ) and fuel control step (S 2 ), and temperature acquisition step (S 3 ) are, respectively, the same as processing contents performed by the bleed valve control unit  2 , the fuel control unit  3 , and the temperature acquisition unit  4  that have already been described, and thus details of which will be omitted. 
     In the embodiment shown in  FIG. 7 , step S 0  includes receiving the load rejection signal S. Subsequently, step S 1  includes performing the bleed valve control step to control the valve opening degree V of the bleed valve  9  from the closed state to the open state with the prescribed opening degree Vc. Step S 2  includes performing the temperature acquisition step to calculate the estimate value of the flame temperature Tf. Step S 3  includes performing the fuel control step to control the fuel regulating valves  83  by feedback control such that the above-described flame temperature Tf falls within the predetermined temperature range defined by the upper limit value Lu and the lower limit value Ll. 
     For example, the order of step S 2  and step S 3  may be reversed. That is, for example, the fuel flow rate may be decided first which is not greater than the fuel flow rate at which the rotation speed of the gas turbine  7  is the rotation speed threshold, the decided fuel flow rate may be supplied to the combustor  73 , and then the flame temperature Tf may be acquired to control the fuel flow rate. Alternatively, the flame temperature Tf in the case where the valve opening degree V of the bleed valve  9  is set at the prescribed opening degree Vc may be estimated first, and then the fuel flow rate may be controlled based on the estimated flame temperature Tf. 
     Further, in some embodiments, the combustion control method may further include a prescribed opening degree decision step of deciding the above-described prescribed opening degree Vc. The prescribed opening degree decision step is the same as the processing contents performed by the prescribed opening degree decision unit  5  that have already been described, and thus details of which will be omitted. As shown in  FIG. 7 , the prescribed opening degree decision step may be performed between step S 0  and step S 1  of  FIG. 7 . 
     The present invention is not limited to the above-described embodiments, and also includes an embodiment obtained by modifying the above-described embodiments and an embodiment obtained by combining these embodiments as appropriate. For example, the above-described prescribed opening degree Vc may be decided based on the load reference value Ld or the flame temperature Tf and the number of used main nozzles. 
     (Appendix) 
     (1) A combustion control device ( 1 ) for a gas turbine ( 7 ) according to at least one embodiment of the present invention is a combustion control device ( 1 ) for a gas turbine ( 7 ) for supplying, to a combustor ( 73 ), compressed air (Ac) by a compressor ( 71 ) flowing into a casing ( 72 ), the device including a bleed valve control unit ( 2 ) configured to control a bleed valve ( 9 ) disposed on a bleed pipe ( 9   p ) for performing bleeding so that a part of the compressed air (Ac) flowing into the casing ( 72 ) is not used as combustion air (As) in the combustor ( 73 ), a fuel control unit ( 3 ) configured to control a fuel regulating valve ( 83 ) for regulating a fuel flow rate of fuel (F) supplied to the combustor ( 73 ), and a temperature acquisition unit ( 4 ) configured to acquire a flame temperature (Tf) of flame caused by combustion of the fuel (F) in the combustor ( 73 ). Upon reception of a load rejection signal (S) for cutting off a load from the gas turbine ( 7 ), the bleed valve control unit ( 2 ) controls a valve opening degree of the bleed valve ( 9 ) from a closed state to an open state with a prescribed opening degree (Vc), and the fuel control unit ( 3 ) controls the fuel regulating valve ( 83 ) such that the acquired flame temperature (Tf) falls within a predetermined temperature range defined by an upper limit value (Lu) and a lower limit value (Ll). 
     With the above configuration (1), the gas turbine ( 7 ) includes the bleed pipe ( 9   p ) and the bleed valve ( 9 ) capable of decreasing the supply amount of the combustion air (As) to the compressor ( 73 ) by bleeding the compressed air (Ac) by the compressor ( 71 ) from the casing ( 72 ). In such load rejection of the gas turbine ( 7 ), the combustion control device ( 1 ) conventionally performs combustion control so as to prevent the excessive increase (such as the increase exceeding 110% of the OST prescribed value) in rotation speed of the gas turbine ( 7 ) and flame off in the combustor ( 73 ), and further controls the valve opening degree of each of the bleed valve ( 9 ) and the fuel regulating valve ( 83 ) to control the fuel flow rate and the flow rate of the combustion air (As) supplied to the combustor ( 73 ), such that the flame temperature (Tf) in the combustor ( 73 ) falls within the predetermined temperature range defined by the upper limit value (Lu) and the lower limit value (Ll). 
     The rotation speed of the gas turbine ( 7 ) increases as the supplied fuel amount (fuel flow rate) is large, and the flame temperature (Tf) increases as the air-fuel ratio is high. Thus, if the bleed valve ( 9 ) is changed from the closed state to the open state in load rejection, a part of the compressed air (Ac) flows from the casing ( 72 ) so as to bypass the combustor ( 73 ) (a first cylindrical portion ( 73   a ) to be described later), decreasing the amount of the combustion air (As) supplied from the casing ( 72 ) to the combustor ( 73 ) and increasing the air-fuel ratio. That is, setting the bleed valve ( 9 ) in the open state in load rejection, it is possible to raise the flame temperature (Tf) without increasing the fuel flow rate. Thus, by setting the upper limit value (Lu) of the flame temperature (Tf) in the combustor ( 73 ) at, for example, a temperature at which damage to the gas turbine ( 7 ) by flame is preventable, installing the lower limit value (Ll) at a temperature at which flame off of the flame is preventable, and controlling the fuel flow rate and air amount (air flow rate) to be supplied such that the flame temperature (Tf) falls within the temperature range, it is possible to prevent damage to equipment due to the excessive rise of the flame temperature (Tf) while preventing the flame off and the excessive increase in rotation speed of the gas turbine ( 7 ). 
     (2) In some embodiments, in the above configuration (1), the combustor ( 73 ) includes a first cylindrical portion ( 73   a ) for generating a combustion gas by combusting the fuel and the compressed air (Ac), and the temperature acquisition unit ( 4 ) includes an air flow rate calculation unit ( 41 ) configured to calculate a flow rate of the combustion air (As) supplied into the first cylindrical portion ( 73   a ), based on the valve opening degree of the bleed valve ( 9 ), and a temperature calculation unit ( 42 ) configured to calculate an estimate value of the flame temperature (Tf), based on the flow rate of the combustion air (As) and a fuel flow rate of the fuel each of which is supplied into the first cylindrical portion ( 73   a ). 
     With the above configuration (2), the temperature acquisition unit ( 4 ) calculates the flow rate of the combustion air (As) (compressed air (Ac)) supplied to the inside (combustion space) of the first cylindrical portion ( 73   a ) (such as a combustor liner) of the combustor ( 73 ) in consideration of the valve opening degree of the bleed valve ( 9 ), and estimates the flame temperature (Tf) based on the calculated flow rate of the combustion air (As) and the fuel flow rate. Thus considering the valve opening degree of the bleed valve ( 9 ), it is possible to obtain the flow rate of the combustion air (As) more accurately, and to accurately estimate the flame temperature (Tf) without actually measuring the same. 
     (3) In some embodiments, in the above configurations (1) and (2), the combustion control device ( 1 ) for the gas turbine ( 7 ) further includes a prescribed opening degree decision unit ( 5 ) for deciding the prescribed opening degree (Vc). The prescribed opening degree decision unit ( 5 ) includes a first decision unit ( 51 ) for deciding the prescribed opening degree (Vc) of the bleed valve ( 9 ) based on the flame temperature (Tf) or a load reference value (Ld) of the gas turbine ( 7 ) when the load rejection signal (S) is received. 
     With the above configuration (3), the prescribed opening degree (Vc) of the bleed valve ( 9 ) is decided based on the flame temperature (Tf) or the load reference value (Ld) of the gas turbine ( 7 ) in load rejection. The flame temperature (Tf) is associated with the load of the gas turbine ( 7 ), and in general, the flame temperature (Tf) is also low when the load (output) of the gas turbine ( 7 ) is low and on the contrary, the flame temperature (Tf) is also high when the load is high. For example, since the flame temperature (Tf) is inherently low when the above-described load is low, it is easy to perform control so that the flame temperature (Tf) does not exceed the upper limit value (Lu) even if the supply amount of the combustion air (As) to the combustor ( 73 ) is decreased by, for example, fully opening the bleed valve ( 9 ). However, since the flame temperature (Tf) is inherently high when the above-described load is high, it is predicted that the rise of the flame temperature (Tf) is likely to be excessive if, for example, the bleed valve ( 9 ) is fully opened, which may make it difficult to keep the flame temperature (Tf) within the predetermined range by controlling the fuel regulating valves ( 83 ). 
     Thus, by controlling the valve opening degree of the bleed valve ( 9 ) at full opening or an intermediate opening degree which is less than full opening based on the flame temperature (Tf) or the load reference value (Ld) of the gas turbine ( 7 ) upon reception (such as immediately before reception) of the load rejection signal (S), it is possible to more reliably prevent the flame temperature (Tf) from exceeding the upper limit value (Lu) by control of the fuel regulating valves ( 83 ), after the bleed valve ( 9 ) is set at the prescribed opening degree (Vc). Further, in estimating the flame temperature (Tf), it is possible to predict, in advance, a case such as where the flame temperature (Tf) stays high, and in such a case, it is possible to, for example, set the bleed valve ( 9 ) at the intermediate opening degree. 
     (4) In some embodiments, in the above configuration (3), the combustor ( 73 ) is connected to a plurality of fuel systems ( 8 ) including a pilot fuel system ( 8   p ), and the fuel control unit ( 3 ) includes a reference opening degree decision unit ( 31   a ) configured to decide a reference opening degree (Vb) of the fuel regulating valve ( 83 ) in the pilot fuel system ( 8   p ) in accordance with an operating condition of the gas turbine ( 7 ), upon the reception of the load rejection signal (S), and an opening degree correction unit ( 31   b ) configured to add an additional opening degree (Vp) to the reference opening degree (Vb), in a low load where the load reference value (Ld) of the gas turbine ( 7 ) when the load rejection signal (S) is received is lower than a rated load reference value. 
     In general, the combustor ( 73 ) is connected to the pilot fuel system ( 8   p ) for stabilizing flame, and the major main fuel system ( 8   m ) for supplying premixed fuel in accordance with the output of the gas turbine ( 7 ). Then, supply of the fuel to the combustor ( 73 ) in load rejection is controlled such that supply from the main fuel system ( 8   m ) is decreased and supply from the pilot fuel system ( 8   p ) is increased, in decreasing the supply amount of the fuel as a whole. 
     With the above configuration (4), if the operating condition of the gas turbine ( 7 ) is in low load in load rejection, the valve opening degree of the fuel regulating valve ( 83 ) in the pilot fuel system ( 8   p ) is further increased relative to that in rated load and high load to further increase the fuel flow rate to be supplied. If the operating condition of the gas turbine ( 7 ) is in low load in load rejection, the valve opening degree of the fuel regulating valve ( 83 ) described above is low, resulting in relatively low air-fuel ratio and the low flame temperature (Tf). Thus, if load rejection is performed in the low load condition, further increasing the fuel flow rate of the fuel regulating valve ( 83 ) in the pilot fuel system ( 8   p ), it is possible to improve responsiveness to the rise in the flame temperature (Tf), and to prevent flame off more reliably, such as to prevent the flame temperature (Tf) from going below the lower limit value (Ll). 
     (5) In some embodiments, in any one of the above configurations (1) to (4), the combustor ( 73 ) is connected to a plurality of fuel systems ( 8 ) including a main fuel system ( 8   m ) for supplying premixed fuel of the fuel and the combustion air (As) at a flow rate according to a load of the gas turbine ( 7 ) to the combustor ( 73 ) by using a plurality of fuel nozzles ( 81 ), the combustion control device ( 1 ) for the gas turbine ( 7 ) further comprises a prescribed opening degree decision unit ( 5 ) for deciding the prescribed opening degree (Vc), and the prescribed opening degree decision unit ( 5 ) includes a nozzle number acquisition unit configured to acquire the number of fuel nozzles ( 81 ) in the main fuel system ( 8   m ) used after the reception of the load rejection signal (S), and a second decision unit configured to decide the prescribed opening degree (Vc) of the bleed valve ( 9 ) based on the number of used fuel nozzles ( 81 ). 
     In load rejection, in accordance with characteristics of equipment, such as the occurrence status of combustion oscillation, the number of used fuel nozzles ( 81 ) in the main fuel system ( 8   m ) (main nozzles) may be changed (changed from eight to three, for example). If the number of used main nozzles is changed, the fuel flow rate per main nozzle used in the main fuel system ( 8   m ) is changed, and thus the flame temperature (Tf) can also be changed. 
     With the above configuration (5), when the fuel is supplied from the main fuel system ( 8   m ) to the combustor ( 73 ) by using the plurality of fuel nozzles ( 81 ) (main nozzles), the prescribed opening degree (Vc) of the bleed valve ( 9 ) is decided based on the number of main nozzles used in load rejection. For example, in the case where the prescribed opening degree (Vc) of the bleed valve ( 9 ) is set at the relatively high opening degree, such as full opening, when the number of used main nozzles is eight, if the fuel is concentrated by decreasing the number of main nozzles to three, five, the fuel per main nozzle increases, further decreasing the prescribed opening degree (Vc) of the bleed valve ( 9 ). Thus, it is possible to more reliably prevent the flame temperature (Tf) from exceeding the upper limit value (Lu). 
     (6) In some embodiments, in the above configuration (1) or (2), the prescribed opening degree (Vc) is full opening. 
     With the above configuration (6), in load rejection, the valve opening degree of the bleed valve ( 9 ) is set at full opening. Thereby, it is possible to maximize the increase in air-fuel ratio by the bleed valve ( 9 ). Thus, it is possible to minimize the fuel amount supplied to prevent flame off, and to improve fuel efficiency. 
     (7) In some embodiments, in any one of the above configuration (1) to (6), the upper limit value (Lu) is a temperature at which damage to the gas turbine ( 7 ) due to the flame is preventable, and the lower limit value (Ll) is a temperature at which flame off of the flame is preventable. 
     With the above configuration (7), defining the upper limit value (Lu) and the lower limit value (Ll) of the flame temperature (Tf) as described above, it is possible to prevent flame off and damage to the gas turbine ( 7 ) in load rejection. 
     (8) A combustion control method for a gas turbine ( 7 ) according to at least one embodiment of the present invention is a combustion control method for a gas turbine ( 7 ) for supplying, to a combustor ( 73 ), compressed air (Ac) by a compressor ( 71 ) flowing into a casing ( 72 ), the method including a bleed valve ( 9 ) control step of controlling a bleed valve ( 9 ) disposed on a bleed pipe ( 9   p ) for performing bleeding so that a part of the compressed air (Ac) flowing into the casing ( 72 ) is not used as combustion air (As) in the combustor ( 73 ), a fuel control step of controlling a fuel regulating valve ( 83 ) for regulating a fuel flow rate of fuel supplied to the combustor ( 73 ), and a temperature acquisition step of acquiring a flame temperature (Tf) of flame caused by combustion of the fuel in the combustor ( 73 ). Upon reception of a load rejection signal (S) for cutting off a load from the gas turbine ( 7 ), the bleed valve ( 9 ) control step includes controlling a valve opening degree of the bleed valve ( 9 ) from a closed state to an open state with a prescribed opening degree (Vc), and the fuel control step includes controlling the fuel regulating valve ( 83 ) such that the acquired flame temperature (Tf) falls within a predetermined temperature range defined by an upper limit value (Lu) and a lower limit value (Ll). 
     With the above configuration (8), the same effect as the above configuration (1) is achieved. 
     (9) A combustion control program ( 10 ) for a gas turbine ( 7 ) according to at least one embodiment of the present invention is a combustion control program ( 10 ) for a gas turbine ( 7 ) for supplying, to a combustor ( 73 ), compressed air (Ac) by a compressor ( 71 ) flowing into a casing ( 72 ), the program including in a computer a bleed valve control unit ( 2 ) configured to control a bleed valve ( 9 ) disposed on a bleed pipe ( 9   p ) for performing bleeding so that a part of the compressed air (Ac) flowing into the casing ( 72 ) is not used as combustion air (As) in the combustor ( 73 ), a fuel control unit ( 3 ) configured to control a fuel regulating valve ( 83 ) for regulating a fuel flow rate of fuel supplied to the combustor ( 73 ), and a temperature acquisition unit ( 4 ) configured to acquire a flame temperature (Tf) of flame caused by combustion of the fuel in the combustor ( 73 ). Upon reception of a load rejection signal (S) for cutting off a load from the gas turbine ( 7 ), the program causes the computer to implement such that the bleed valve control unit ( 2 ) controls a valve opening degree of the bleed valve ( 9 ) from a closed state to an open state with a prescribed opening degree (Vc), and the fuel control unit ( 3 ) controls the fuel regulating valve ( 83 ) such that the acquired flame temperature (Tf) falls within a predetermined temperature range defined by an upper limit value (Lu) and a lower limit value (Ll). 
     With the above configuration (9), the same effect as the above configuration (1) is achieved. 
     REFERENCE SIGNS LIST 
     
         
           1  Combustion control device 
           10  Combustion control program 
           11  Processor 
           12  Storage device 
           2  Bleed valve control unit 
           3  Fuel control unit