Abstract:
This fuel-preheating device is provided with the following: a cooling-steam line that supplies steam, for the purposes of cooling, to a hot part of a gas turbine, namely a combustion liner of a combustor; a superheated-steam line through which superheated steam that is steam having passed through the combustion liner of the combustor flows; and a preheater that receives the superheated steam from the superheated-steam line and preheats fuel to be supplied to the combustor by exchanging heat between the superheated steam and said fuel.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a preheating device for gas turbine fuel of a gas turbine that includes a plurality of combustors that combusts fuel to generate combustion gas, and a turbine driven by combustion gas, a gas turbine plant provided therewith, and a preheating method for gas turbine fuel. This application claims priority based on Japanese Patent Application No. 2013-062276 filed in Japan on Mar. 25, 2013, of which the contents are incorporated herein by reference. 
       BACKGROUND ART 
       [0002]    A gas turbine includes a compressor that compresses air, a plurality of combustors that combusts fuel in the air compressed by the compressor to generate combustion gas, and a turbine that is driven by the combustion gas. The fuel supplied to the combustors is often preheated before being supplied to the combustors to increase the combustion efficiency in the combustors. 
         [0003]    Patent Document 1 below discloses a gas turbine plant that preheats fuel supplied to a combustor. This gas turbine plant includes a gas turbine, a heat recovery boiler that generates steam by the heat of exhaust gas from this gas turbine, a steam turbine that is driven by steam generated by this heat recovery boiler, and a preheater that heats the fuel to be supplied to the combustor. A first steam line that supplies the steam generated by the heat recovery boiler as a fuel heat source of the preheater is connected to the preheater. Furthermore, a second steam line that introduces steam that heated the fuel into a transition piece of the combustor is connected to this preheater. In addition, a third steam line that introduces the steam that cooled this transition piece into the steam turbine is connected to the transition piece of the combustor. This gas turbine plant further includes an attemperation spray line that sprays water on the second steam line to cool the steam that flows through the second steam line, that is, the steam that heated the fuel. 
       CITATION LIST  
     Patent Document 
       [0004]    Patent Document 1: Japanese Unexamined Patent Application Publication No. H10-131719A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    According to the technique described in Patent Document 1 listed above, since the steam that heated the fuel is cooled by water and then supplied to the transition piece of the combustor, the heat of the steam that heated the fuel cannot be utilized effectively and the thermal efficiency of the gas turbine plant overall is not very good. 
         [0006]    In addition, in the stage when steam is not being generated in the heat recovery boiler at the time of gas turbine activation in the gas turbine plant, the fuel is expected to be heated by steam from an external source. In this case as well, the transition piece of the combustor is cooled by the steam that heated the fuel, and then the high-temperature steam is supplied to the steam turbine. Nevertheless, in the stage when the steam turbine is not being driven, this small amount of high-temperature steam does not contribute to the driving of the steam turbine even when supplied to the steam turbine. As a result, the heat held by the high-temperature steam is wasted. Thus, according to the technique described in the above Patent Document 1, the thermal efficiency is not good from this perspective as well. 
         [0007]    Therefore, an object of the present invention is to provide a preheating device for gas turbine fuel capable of increasing the thermal efficiency of the gas turbine plant overall, a gas turbine plant provided therewith, and a preheating method for gas turbine fuel. 
       Solution To Problem 
       [0008]    According to an aspect of the present invention for resolving the above-described problems, a preheating device for gas turbine fuel includes: 
         [0009]    a cooling-steam line that supplies steam, for the purpose of cooling, to a hot part of either a combustor that combusts fuel to generate combustion gas or a turbine that is driven by the combustion gas, the hot part being a part that contacts the combustion gas; a superheated-steam line through which superheated steam flows, the superheated steam being the steam that has passed through the hot part; and a preheater that preheats the fuel by exchanging heat between the superheated steam supplied from the superheated-steam line and the fuel supplied to the combustor. 
         [0010]    According to the preheating device, fuel is heated by the superheated steam superheated by the cooling of the combustor, making it possible to effectively utilize the heat of the combustor. Thus, according to the preheating device, it is possible to increase the thermal efficiency of the gas turbine plant overall. 
         [0011]    Here, in the preheating device for gas turbine fuel, the preheater may condense the superheated steam by heat exchange between the superheated steam and the fuel. 
         [0012]    According to the preheating device, the superheated steam is condensed when the fuel is heated, making it possible to thoroughly utilize the heat held by the superheated steam. In addition, according to the preheating device, the superheated steam is condensed by heat exchange with the fuel, making it possible to increase the heat exchange rate between the superheated steam and fuel and reduce the size of the preheater. 
         [0013]    In addition, the preheating device for gas turbine fuel according to any one of the above may further include a plurality of the preheaters and a fuel line that connects in series the plurality of preheaters so that the fuel sequentially flows into the plurality of preheaters, the superheated-steam line supplying the superheated steam to each of the plurality of preheaters. 
         [0014]    Furthermore, the preheating device for gas turbine fuel according to any one of the above may further include a first-stage steam line through which steam that does not pass through the hot part flows, a first-stage preheater that preheats the fuel by heat exchange between the steam supplied from the first-stage steam line and the fuel supplied to the combustor, and a fuel line that connects the first-stage preheater and a second-stage preheater, the second-stage preheater being the preheater, so that the fuel preheated by the first-stage preheater is supplied to the second-stage preheater.  
         [0015]    Thus, according to the preheating device that includes a plurality of the preheaters, fuel is heated in two stages, making it possible to supply higher-temperature fuel to the combustor and increase the fuel combustion efficiency in the combustor. 
         [0016]    In addition, the preheating device for gas turbine fuel according to any one of the above may further include a branch steam line that branches from the superheated-steam line and introduces a portion of the superheated steam that flows through the superheated-steam line to the outside of the preheating device; a cooling-steam regulating valve that regulates a flow rate of the steam supplied to the hot part via the cooling stream line; and a superheated-steam regulating valve that controls a flow rate of the superheated steam supplied to the preheater via the superheated-steam line. 
         [0017]    According to the preheating device, although the superheated-steam line is connected in series to the cooling-steam line via the hot part, it is possible to respectively regulate a flow rate of the steam supplied to the hot part via the cooling-steam line, and a flow rate of the superheated steam that flows into the preheater via the superheated-steam line. 
         [0018]    According to an aspect of the present invention for resolving the above-described problems, a gas turbine plant includes: 
         [0019]    the preheating device for gas turbine fuel according to any one of the above; a gas turbine that includes the combustor and the turbine; a steam generating source that generates steam; a condenser that turns the steam generated by the steam generating source back into water; a water supply line that supplies water generated by the condenser to the steam generating source; and a water recovery line that supplies the water condensed from the superheated steam by heat exchange between the superheated steam and the fuel in the preheater to the condenser or the water supply line, the cooling-steam line supplying steam generated by the steam generating source to the hot part. 
         [0020]    The gas turbine plant includes the preheating device for gas turbine fuel according to any one of the above, thereby making it possible to increase the thermal efficiency of the gas turbine plant overall.  
         [0021]    Here, in the gas turbine plant, the steam generating source may include a heat recovery boiler that generates steam by the heat of the exhaust gas from the turbine. 
         [0022]    In addition, according to another aspect of the present invention for resolving the above-described problems, a gas turbine plant includes: 
         [0023]    the preheating device for gas turbine fuel that includes the branch steam line; a gas turbine that includes the combustor and the turbine; a heat recovery boiler that generates steam by the heat of exhaust gas from the turbine; a steam turbine that is driven by the steam generated by the heat recovery boiler; a condenser that turns the steam that drove the steam turbine back into water; a water supply line that supplies the water generated by the condenser to the heat recovery boiler; and a water recovery line that supplies the water condensed from the superheated steam by heat exchange between the superheated steam and the fuel in the preheater to the condenser or the water supply line. 
         [0024]    The heat recovery boiler includes a steam generating part that generates steam by the heat of the exhaust gas, and a reheating part that superheats the steam generated by the steam generating part and sends the superheated steam to the steam turbine, the cooling-steam line supplying the steam generated by the steam generating part, before being superheated by the reheating part, to the hot part, and the branch steam line introducing the superheated steam into the steam turbine. 
         [0025]    According to the gas turbine plant, excess superheated steam not used for preheating fuel is supplied to the steam turbine via the branch steam line, making it possible to effectively utilize excess superheated steam as well. 
         [0026]    Here, in the gas turbine plant according to any one of the above, the steam generating source may include an auxiliary boiler that generates steam by a heat source different from the exhaust gas from the turbine. 
         [0027]    According to the gas turbine plant, it is possible to preheat fuel while cooling the hot part even in a case where the gas turbine plant does not include a heat recovery device or a case where the gas turbine plant includes a heat recovery device but the heat recovery device is not steadily operating since the gas turbine is in an activation stage.  
         [0028]    In addition, according to yet another aspect of the present invention for resolving the above-described problems, a gas turbine plant includes: 
         [0029]    the preheating device for gas turbine fuel that includes the branch steam line; a gas turbine that includes the combustor and the turbine; an auxiliary boiler that generates steam by a heat source different from exhaust gas from the turbine; a condenser that turns the steam generated by the auxiliary boiler back into water; a water supply line that supplies water generated by the condenser to the auxiliary boiler; and a water recovery line that sends water condensed from the superheated steam by heat exchange between the superheated steam and the fuel in the preheater to the condenser or the water supply line, 
         [0030]    the cooling-steam line supplying steam generated by the auxiliary boiler to the hot part, and the branch steam line introducing the superheated steam into the condenser. 
         [0031]    In addition, according to an aspect of the present invention for resolving the above-described problems, a preheating method for gas turbine fuel includes: 
         [0032]    a cooling steam supplying step for supplying steam, for the purpose of cooling, to a hot part of either a combustor that combusts fuel to generate combustion gas or a turbine that is driven by the combustion gas, the hot part being a part that contacts the combustion gas; and a fuel preheating step for preheating the fuel by exchanging heat between superheated steam, the superheated steam being the steam that has passed through the hot part, and the fuel supplied to the combustor. 
         [0033]    According to the preheating method, fuel is heated by superheated steam superheated by the cooling of the combustor, making it possible to effectively utilize the heat of the combustor. Thus, according to the preheating method, it is possible to increase the thermal efficiency of the gas turbine plant overall. 
         [0034]    Here, in the preheating method for gas turbine fuel, the superheated steam may be condensed by heat exchange with the fuel in the fuel preheating step. 
         [0035]    According to the preheating method, the superheated steam is condensed when the fuel is heated, making it possible to thoroughly utilize the heat held by the superheated steam. Furthermore, according to the preheating method, the superheated steam is condensed by heat exchange with the fuel, making it possible to increase the heat exchange rate between the superheated steam and the fuel. 
       Advantageous Effect of Invention 
       [0036]    According to an aspect of the present invention, the fuel is heated by the superheated steam superheated by the cooling of the combustor, making it possible to effectively utilize the heat of the combustor. Thus, according to an aspect of the present invention, it is possible to increase the thermal efficiency of the gas turbine plant overall. In addition, according to an aspect of the present invention, it is possible to suppress an increase in running costs while suppressing a decrease in output of the gas turbine. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0037]      FIG. 1  is a system diagram illustrating a gas turbine plant of a first embodiment according to the present invention. 
           [0038]      FIG. 2  is a schematic cross-sectional view illustrating the gas turbine of the first embodiment according to the present invention. 
           [0039]      FIG. 3  is a system diagram illustrating a gas turbine plant of a second embodiment according to the present invention. 
           [0040]      FIG. 4  is a system diagram illustrating a gas turbine plant of a third embodiment according to the present invention. 
           [0041]      FIG. 5  is a system diagram illustrating a fuel-preheating device of a first modification according to the present invention. 
           [0042]      FIG. 6  is a system diagram illustrating a fuel-preheating device of a second modification according to the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0043]    The following describes various embodiments and various modifications of a gas turbine plant according to the present invention, with reference to the drawings. 
       First Embodiment 
       [0044]    First, a first embodiment of a gas turbine plant according to the present invention will be described with reference to  FIG. 1  and  FIG. 2 .  
         [0045]    The gas turbine plant of this embodiment, as illustrated in  FIG. 1 , includes a gas turbine  10 , a generator  15  that generates power by the driving of the gas turbine  10 , a heat recovery device  100  that generates steam by the heat of an exhaust gas EG exhausted from the gas turbine  10 , a chimney  40  that releases the exhaust gas EG that has passed through the heat recovery device  100  into the atmosphere, a fuel-preheating device  50  that preheats the fuel supplied to the gas turbine  10 , and a control device  90  that controls the gas turbine  10 , the fuel-preheating device  50 , and the like. 
         [0046]    The gas turbine  10  includes a compressor  11  that compresses air, a plurality of combustors  21  that combusts a fuel F in the air compressed by the compressor  11  to generate combustion gas, and a turbine  31  that is driven by the high-temperature, high-pressure combustion gas. A turbine rotor of the turbine  31  and a compressor rotor of the compressor  11  rotate around the same axial line, and are connected to each other to form a gas turbine rotor. A rotor of the generator  15  is connected to this gas turbine rotor. 
         [0047]    The combustor  21 , as illustrated in  FIG. 2 , includes a combustion liner (or transition piece)  23  where the fuel F is combusted in air A from the compressor  11  to generate combustion gas, and an injector  22  that injects the fuel F and the air A from the compressor  11  into the interior of this combustion liner  23 . A fuel line  29  that supplies the fuel F from an external fuel supply source to this injector  22  is connected to the injector  22 . In addition, a steam flow path  24  through which steam passes is formed on a member that forms the combustion liner  23  in order to cool this member. 
         [0048]    The turbine  31  includes a turbine rotor (hereinafter simply referred to a “rotor”)  32  that rotates around an axial line Ar by the combustion gas from the combustor  21 , and a casing  35  which covers the rotor  32  allowing the rotor  32  to rotate. The rotor  32  includes a rotor main body  33  that extends in an axial direction parallel with the axial line Ar, and a plurality of blades  34  fixed to the outer periphery of the rotor main body  33 . In addition, a plurality of vanes  36  is fixed on the inner peripheral surface of the casing  35 . A combustion gas flow path  37  through which the combustion gas from the combustor  21  flows is formed between the inner peripheral surface of the casing  35  and the outer peripheral surface of the rotor main body  33 . The plurality of combustors  21  is arranged around the axial line Ar side-by-side in the peripheral direction, and fixed to the casing  35  of the turbine  31 . 
         [0049]    The heat recovery device  100 , as illustrated in  FIG. 1 , includes a heat recovery boiler  110  that generates steam by the heat of the combustion gas that drove the turbine  31 , that is, the exhaust gas EG exhausted from the gas turbine  10 , steam turbines  121   a,    121   b,    121   c  that are driven by the steam generated by the heat recovery boiler  110 , a generator  122  that generates power by the driving of the steam turbines  121   a,    121   b,    121   c,  a condenser  123  that turns the steam that drove the steam turbine  121   a  back into water, and a water supply pump  124  that returns the water in the condenser  123  to the heat recovery boiler  110 . 
         [0050]    The heat recovery device  100  includes the low-pressure steam turbine  121   a,  the intermediate-pressure steam turbine  121   b,  and the high-pressure steam turbine  121   c  as the steam turbines  121   a,    121   b,    121   c.  In addition, the heat recovery boiler  110  includes a low-pressure steam generating part  111   a  that generates low-pressure steam LS, an intermediate-pressure steam generating part  111   b  that generates intermediate-pressure steam IS, a high-pressure steam generating part  111   c  that generates high-pressure steam HS, and a reheater  115  that heats the steam that drove the high-pressure steam turbine  121   c.  The low-pressure steam generating part  111   a,  the intermediate-pressure steam generating part  111   b,  and the high-pressure steam generating part  111   c  all include economizers  112   a,    112   b,    112   c  that heat water, evaporators  113   a ,  113   b,    113   c  that turn the water heated by the economizers  112   a,    112   b,    112   c  into steam, and superheaters  114   a,    114   b,    114   c  that superheat the steam generated by the evaporators  113   a,    113   b,    113   c.  The intermediate-pressure steam generating part  111   b  and the high-pressure steam generating part  111   c  further include pumps  116   b,    116   c  that send the water heated by the economizer  112   a  of the low-pressure steam generating part  111   a  to their own economizers  112   b,    112   c , in addition to the economizers  112   b,    112   c,  the evaporators  113   b,    113   c,  and the superheaters  114   b,    114   c.    
         [0051]    In the following, the economizer  112   c  of the high-pressure steam generating part  111   c,  the evaporator  113   c  of the high-pressure steam generating part  111   c,  and the superheater  114   c  of the high-pressure steam generating part  111   c  are respectively referred to as the high-pressure economizer  112   c,  the high-pressure evaporator  113   c,  and the high-pressure superheater  114   c.  In addition, the economizer  112   b  of the intermediate-pressure steam generating part  111   b,  the evaporator  113   b  of the intermediate-pressure steam generating part  111   b,  and the superheater  114   b  of the intermediate-pressure steam generating part  111   b  are respectively referred to as the intermediate-pressure economizer  112   b,  the intermediate-pressure evaporator  113   b,  and the intermediate-pressure superheater  114   b.  In addition, the economizer  112   a  of the low-pressure steam generating part  111   a , the evaporator  113   a  of the low-pressure steam generating part  111   a,  and the superheater  114   a  of the low-pressure steam generating part  111   a  are respectively referred to as the low-pressure economizer  112   a,  the low-pressure evaporator  113   a,  and the low-pressure superheater  114   a.  In addition, the pump  116   b  of the intermediate-pressure steam generating part  111   b  and the pump  116   c  of the high-pressure steam generating part  111   c  are respectively referred to as the intermediate-pressure pump  116   b  and the high-pressure pump  116   c.    
         [0052]    The reheater  115 , the high-pressure superheater  114   c,  the high-pressure evaporator  113   c,  the high-pressure economizer  112   c,  the intermediate-pressure superheater  114   b,  the intermediate-pressure evaporator  113   b,  the intermediate-pressure economizer  112   b,  the low-pressure superheater  114   a,  the low-pressure evaporator  113   a,  and the low-pressure economizer  112   a  are arranged side-by-side in this order toward the downstream side of the exhaust gas EG flowing from the turbine  31  toward the chimney  40 . 
         [0053]    The condenser  123  and the low-pressure economizer  112   a  are connected by a water supply line  131 . The aforementioned water supply pump  124  is provided on this water supply line  131 . The low-pressure superheater  114   a  and the steam inlet of the low-pressure steam turbine  121   a  are connected by a low-pressure steam line  132  that sends the low-pressure steam LS from the low-pressure superheater  114   a  to the low-pressure steam turbine  121   a . The steam outlet of the low-pressure steam turbine  121   a  and the condenser  123  are interconnected so that the low-pressure steam LS that drove the low-pressure steam turbine  121   a  is supplied to the condenser  123 . The high-pressure superheater  114   c  and the steam inlet of the high-pressure steam turbine  121   c  are connected by a high-pressure steam line  138  that sends the high-pressure steam HS from the high-pressure superheater  114   c  to the high-pressure steam turbine  121   c.  The steam outlet of the high-pressure steam turbine  121   c  and the steam inlet of the reheater  115  are connected by a high-pressure steam recovery line  139  that sends the high-pressure steam HS from the high-pressure steam turbine  121   c  to the reheater  115 . The steam outlet of the reheater  115  and the steam inlet of the intermediate-pressure steam turbine  121   b  are connected by a reheat steam line  136  that sends the high-pressure steam HS superheated by the reheater  115  to the intermediate-pressure steam turbine  121   b  as a reheat steam RHS. An intermediate-pressure steam recovery line  137  is connected to the steam outlet of the intermediate-pressure steam turbine  121   b.  This intermediate-pressure steam recovery line  137  merges into the low-pressure steam line  132 . An intermediate-pressure steam line  133  is connected to the steam outlet of the intermediate-pressure superheater  114   b.  This intermediate-pressure steam line  133  merges into the high-pressure steam recovery line  139 . 
         [0054]    The condenser  123  is, for example, a water-cooled condenser that condenses steam by heat exchange between seawater or the like and the steam. 
         [0055]    This fuel-preheating device  50  includes a preheater  51  that heats the fuel F supplied to the combustor  21 , a cooling-steam line  55  that supplies the steam generated by the heat recovery boiler  110  to the combustor  21  as cooling steam CS, a superheated-steam line  57  that supplies superheated steam SS superheated by passing through the combustor  21  to the preheater  51 , a water recovery line  61  that sends the water condensed from the superheated steam SS by the preheater  51  to the water supply line  131  of the heat recovery device  100 , and a branch steam line  65  that branches from the superheated-steam line  57 . 
         [0056]    The preheater  51  is provided in the fuel line  29  that supplies the fuel F to the combustor  21 . This preheater  51  is a multitubular heat exchanger and includes a casing  52  and a plurality of tubes  53  provided inside this casing  52 . The fuel F that has passed through the fuel line  29  passes through these tubes  53 . 
         [0057]    The cooling-steam line  55  branches from the high-pressure steam recovery line  139  that sends the high-pressure steam HS from the high-pressure steam turbine  121   c  to the reheater  115 . A cooling-steam regulating valve  56  that regulates the flow rate of the steam supplied from this cooling-steam line  55  to the combustor  21  is provided on this cooling-steam line  55 . This cooling-steam line  55  is connected to one end portion of the steam flow path  24  formed in the combustion liner  23  of the combustor  21 , as illustrated in  FIG. 2 . In addition, one end of the superheated-steam line  57  is connected to the other end of the steam flow path  24 . The other end of this superheated-steam line  57  is connected to the casing  52  of the preheater  51 , as illustrated in  FIG. 1 . The superheated steam SS that has passed through this superheated-steam line  57  flows to the outside of the tubes  53  of the preheater  51 , inside the casing  52  of the preheater  51 . A superheated-steam regulating valve  58  that regulates the flow rate of the superheated steam SS that flows into the preheater  51  via this superheated-steam line  57  is provided on this superheated-steam line  57 . One end of the water recovery line  61  is connected to the casing  52  of the preheater  51 . The other end of this water recovery line  61  is connected to a position on the water supply line  131  of the heat recovery device  100 , the position being upstream (on the condenser side) of the position where the water supply pump  124  is provided. The branch steam line  65  branches from the superheated-steam line  57  at a position in the superheated-steam line  57  that is upstream (on the combustor  21  side) of the position where the superheated-steam regulating valve  58  is provided. This branch steam line  65  is connected to the reheat steam line  136  of the heat recovery device  100 . 
         [0058]    The cooling-steam regulating valve  56  provided on the cooling-steam line  55  and the superheated-steam regulating valve  58  provided on the superheated-steam line  57  both open and close in accordance with instructions from the control device  90 . 
         [0059]    It should be noted that while the other end of the water recovery line  61  is here connected to the water supply line  131  of the heat recovery device  100 , the other end of this water recovery line  61  may be connected to the condenser  123  of the heat recovery device  100 . In addition, while the superheated-steam regulating valve  58  that regulates the flow rate of the superheated steam SS is provided on the superheated-steam line  57  here, the superheated-steam regulating valve  58  may be provided on the branch steam line  65 . 
         [0060]    Next, the operation of the gas turbine plant of this embodiment described above will be described. 
         [0061]    The compressor  11  of the gas turbine  10  compresses the air A that is in the atmosphere and supplies the compressed air A to the combustor  21 , as illustrated in  FIG. 2 . In addition, the fuel F from the fuel line  29  is also supplied to the combustor  21 . Inside the combustion liner  23  of the combustor  21 , the fuel F is combusted in the compressed air A to generate high-temperature, high-pressure combustion gas. This combustion gas is sent from the combustion liner  23  to inside the combustion gas flow path  37  of the turbine  31 , and rotates the rotor  32  of this turbine  31 . With the rotation of this rotor  32 , the generator  15  connected to the gas turbine  10  generates power. 
         [0062]    The combustion gas that rotated the rotor  32  of the turbine  31  is exhausted from the gas turbine  10  as the exhaust gas EG, and released to the atmosphere from the chimney  40  via the heat recovery boiler  110 . The heat recovery device  100  recovers the heat contained in this exhaust gas EG during a process in which the exhaust gas EG from the gas turbine  10  passes through the heat recovery boiler  110 . 
         [0063]    In the heat recovery boiler  110 , the water from the condenser  123  is supplied to the low-pressure economizer  112   a  furthest downstream (on the chimney  40  side) via the water supply line  131 . The low-pressure economizer  112   a  heats the water by heat exchange between the water and the exhaust gas EG. A portion of the water heated by the low-pressure economizer  112   a  is further heated by the low-pressure evaporator  113   a  and turns into steam. This steam is further superheated by the low-pressure superheater  114   a  and supplied as the low-pressure steam LS to the low-pressure steam turbine  121   a  via the low-pressure steam line  132 . The steam that drove the low-pressure steam turbine  121   a  is turned back into water by the condenser  123 . This water is supplied from the condenser  123  to the low-pressure economizer  112   a  once again via the water supply line  131 . 
         [0064]    Another portion of the water heated by the low-pressure economizer  112   a  is pressurized by the intermediate-pressure pump  116   b  and sent to the intermediate-pressure economizer  112   b,  and the remaining water heated by the low-pressure economizer  112   a  is pressurized by the high-pressure pump  116   c  and sent to the high-pressure economizer  112   c.    
         [0065]    The high-pressure economizer  112   c  heats the water that has been sent from the high-pressure pump  116   c  by heat exchange between the water and the exhaust gas EG. The water heated by the high-pressure economizer  112   c  is further heated by the high-pressure evaporator  113   c  and turns into steam. This steam is further superheated by the high-pressure superheater  114   c  and supplied as the high-pressure steam HS to the high-pressure steam turbine  121   c  via the high-pressure steam line  138 .  
         [0066]    The intermediate-pressure economizer  112   b  heats the water that has been sent from the intermediate-pressure pump  116   b  by heat exchange between the water and the exhaust gas EG. The water heated by the intermediate-pressure economizer  112   b  is further heated by the intermediate-pressure evaporator  113   b  and turns into steam. This steam is further superheated by the intermediate-pressure superheater  114   b,  and a portion thereof is sent to the reheater  115  furthest upstream (on the gas turbine  10  side) in the heat recovery boiler  110  via the intermediate-pressure steam line  133  and the high-pressure steam recovery line  139  as the intermediate-pressure steam IS. 
         [0067]    The high-pressure steam HS that drove the high-pressure steam turbine  121   c  and the intermediate-pressure steam IS from the intermediate-pressure superheater  114   b  are sent to the reheater  115  furthest upstream (on the gas turbine  10  side) in the heat recovery boiler  110  via the high-pressure steam recovery line  139 . The reheater  115  superheats this steam by heat exchange between this steam and the exhaust gas EG, and supplies the steam as the reheat steam RHS to the intermediate-pressure steam turbine  121   b  via the reheat steam line  136 . 
         [0068]    The reheat steam RHS that drove the intermediate-pressure steam turbine  121   b  is supplied to the low-pressure steam turbine  121   a  via the intermediate-pressure steam recovery line  137  and the low-pressure steam line  132 . 
         [0069]    A portion of the high-pressure steam HS that drove the high-pressure steam turbine  121   c  is supplied to the combustor  21  via the high-pressure steam recovery line  139  and the cooling-steam line  55  as the cooling steam CS, as described above. At this time, the flow rate of the cooling steam CS supplied to the combustor  21  is regulated by the cooling-steam regulating valve  56  provided on the cooling-steam line  55 . 
         [0070]    The cooling steam CS from the cooling-steam line  55  flows into the steam flow path  24  formed in the combustion liner  23  of the combustor  21 . The cooling steam CS, during the process of passing through this steam flow path  24 , exchanges heat with the combustion liner  23  heated by flame and combustion gas, cooling the combustion liner  23  (cooling steam supplying step). As a result, the cooling steam CS is superheated and turns into the superheated steam SS. 
         [0071]    This superheated steam SS flows into the casing  52  of the preheater  51  via the superheated-steam line  57 . At this time, the flow rate of the superheated steam SS that flows into the casing  52  of the preheater  51  is regulated by the superheated-steam regulating valve  58  provided on the superheated-steam line  57 . The portion of the superheated steam SS from the combustor  21  that does not flow into the casing  52  of the preheater  51 , that is, the excess superheated steam SS not used for preheating the fuel F, is supplied to the intermediate-pressure steam turbine  121   b  via the branch steam line  65  and the reheat steam line  136 . 
         [0072]    The superheated steam SS that flowed into the casing  52  of the preheater  51  exchanges heat with the fuel F that flows inside the tubes  53  of the preheater  51 , heating the fuel F (fuel preheating step). This fuel F is supplied to the injector  22  of the combustor  21  as previously described, via the fuel line  29 . Meanwhile, the superheated steam SS is cooled and condensed by heat exchange with the fuel F, and turns into water. This water is returned to the condenser  123  or the water supply line  131  of the heat recovery device  100  via the water recovery line  61 . 
         [0073]    As described above, according to this embodiment, the fuel F is heated by the superheated steam SS superheated by the cooling of the combustor  21 , thereby making it possible to effectively utilize the heat of the combustor  21 . Furthermore, according to this embodiment, when the fuel F is heated, the superheated steam SS is condensed, making it possible to thoroughly utilize the heat held by the superheated steam SS. In addition, according to this embodiment, the excess superheated steam SS not used for preheating the fuel F is supplied to the intermediate-pressure steam turbine  121   b,  making it possible to utilize the excess superheated steam SS as well. Thus, according to this embodiment, it is possible to increase the thermal efficiency of the gas turbine plant overall. 
         [0074]    In addition, according to this embodiment, the superheated steam SS is condensed by heat exchange with the fuel F, making it possible to increase the heat exchange rate between the superheated steam SS and the fuel and decrease the size of the preheater  51 .  
         [0075]    Incidentally, in many cases the gas turbine plant includes an auxiliary boiler that generates steam by a heat source different from the exhaust gas EG from the gas turbine  10  such as, for example, heat separately generated by fuel combustion. In this case, a portion of the steam generated by this auxiliary boiler is introduced into the combustor  21  via the cooling-steam line  55  as the cooling steam CS, thereby making it possible to cool the combustor  21  and preheat the fuel F even in a stage where the heat recovery boiler  110  is not generating steam, for example, in the activation stage of the gas turbine  10 . In the preheater  51 , the superheated steam SS exchanges heat with the fuel, condenses, and turns into water. This water is sent to the condenser  123  or the water supply line  131  via the water recovery line  61 . Thus, according to this embodiment, even in a case where the heat recovery boiler  110  is not operating steadily and steam from an auxiliary boiler is used, this steam is condensed by heat exchange with the fuel F, making it possible to very effectively utilize the heat of this steam. 
       Second Embodiment 
       [0076]    Next, a second embodiment of a gas turbine plant according to the present invention will be described with reference to  FIG. 3 . 
         [0077]    The gas turbine plant of this embodiment includes the gas turbine  10 , the generator  15  that generates power by the driving of the gas turbine  10 , the chimney  40  that releases the exhaust gas EG exhausted from the gas turbine  10  into the atmosphere, an auxiliary steam generating device  200  that generates steam, a fuel-preheating device  50   a  that preheats the fuel F supplied to the combustor  21 , and the control device  90  that controls the gas turbine  10 , the fuel-preheating device  50   a,  and the like. 
         [0078]    That is, the gas turbine plant of this embodiment includes the auxiliary steam generating device  200  in place of the heat recovery device  100  of the gas turbine plant of the first embodiment. This auxiliary steam generating device  200  includes an auxiliary boiler  201 , a condenser  202  that turns the steam generated by this auxiliary boiler  201  back into water, a degassing tank  203  that removes dissolved gasses in the water from the condenser  202 , and a water supply pump  204  that supplies the water of the degassing tank  203  to the auxiliary boiler  201 . The auxiliary boiler  201  is a boiler that generates steam by a heat source different from the exhaust gas EG from the gas turbine  10 , for example, heat separately generated by fuel combustion. 
         [0079]    An auxiliary steam line  211  that supplies the steam generated in the auxiliary boiler  201  to various equipment and the like is connected to the auxiliary boiler  201 . A steam recovery line  212  that sends the steam utilized by various equipment and the like to the condenser  202  is connected to the condenser  202 . This condenser  202  is, for example, an air-cooled condenser that cools the steam by air. The condenser  202  and the auxiliary boiler  201  are connected by a water supply line  213 . The aforementioned degassing tank  203  and the water supply pump  204  downstream therefrom are provided on this water supply line  213 . 
         [0080]    The fuel-preheating device  50   a  of this embodiment, similarly to the fuel-preheating device  50  of the first embodiment, includes the preheater  51  that heats fuel supplied to the combustor  21 , a cooling-steam line  55   a  that supplies the steam generated by the auxiliary boiler  201  to the combustor  21  as the cooling steam CS, the superheated-steam line  57  that supplies the superheated steam SS superheated by passing through the combustor  21  to the preheater  51 , a water recovery line  61   a  that sends the water condensed from the superheated steam SS by the preheater  51  to the degassing tank  203  of the auxiliary steam generating device  200 , and a branch steam line  65   a  that branches from the superheated-steam line  57 . 
         [0081]    The preheater  51 , similarly to the preheater  51  of the first embodiment, is provided on the fuel line  29  that supplies the fuel F to the combustor  21 . The cooling-steam line  55   a  branches from the auxiliary steam line  211  of the auxiliary steam generating device  200 . The cooling-steam regulating valve  56  that regulates the flow rate of the steam supplied from this cooling-steam line  55   a  to the combustor  21  is provided on this cooling-steam line  55   a.  This cooling-steam line  55   a,  similarly to the first embodiment, is connected to one end portion of the steam flow path  24  formed in the combustion liner  23  of the combustor  21 . In addition, one end of the superheated-steam line  57  is connected to an end portion of the steam flow path  24  formed in this combustion liner  23 . The other end of this superheated-steam line  57  is connected to the casing  52  of the preheater  51 . The superheated-steam regulating valve  58  that regulates the flow rate of the superheated steam SS that flows into the preheater  51  is provided on this superheated-steam line  57 . One end of this water recovery line  61   a  is connected to the casing  52  of the preheater  51 . The other end of this water recovery line  61   a  is connected to the degassing tank  203  of the auxiliary steam generating device  200 . The branch steam line  65   a  branches from the superheated-steam line  57  at a position in the superheated-steam line  57  upstream (on the combustor  21  side) of the position where the superheated-steam regulating valve  58  is provided. This branch steam line  65   a  is connected to the steam recovery line  212  of the auxiliary steam generating device  200 . 
         [0082]    The cooling-steam regulating valve  56  provided on the cooling-steam line  55   a  and the superheated-steam regulating valve  58  provided on the superheated-steam line  57  both open and close in accordance with instructions from the control device  90 , similarly to the first embodiment. 
         [0083]    It should be noted that while the other end of the water recovery line  61   a  is connected to the degassing tank  203  of the auxiliary steam generating device  200  here, it may be connected to the condenser  202  of the auxiliary steam generating device  200 . 
         [0084]    Next, the operation of the gas turbine plant of this embodiment described above will be described. 
         [0085]    In this embodiment as well, the gas turbine  10  operates in the same manner as in the first embodiment. 
         [0086]    A portion of the steam generated by the auxiliary boiler  201  of the auxiliary steam generating device  200  is sent as the cooling steam CS to the combustor  21  via the auxiliary steam line  211  and the cooling-steam line  55   a . At this time, the flow rate of the cooling steam CS supplied to the combustor  21  is regulated by the cooling-steam regulating valve  56  provided on the cooling-steam line  55   a.    
         [0087]    The cooling steam CS from the cooling-steam line  55   a,  similarly to the first embodiment, flows into the steam flow path  24  formed in the combustion liner  23  of the combustor  21 . The cooling steam CS, during the process of passing through this steam flow path  24 , exchanges heat with the combustion liner  23  heated by flame and combustion gas, cooling the combustion liner  23 . As a result, the cooling steam CS is superheated and turns into the superheated steam SS.  
         [0088]    This superheated steam SS, similarly to the first embodiment, flows into the casing  52  of the preheater  51  via the superheated-steam line  57 . At this time, the flow rate of the superheated steam SS that flows into the casing  52  of the preheater  51  is regulated by the superheated-steam regulating valve  58  provided on the superheated-steam line  57 . The portion of the superheated steam SS from the combustor  21  that does not flow into the casing  52  of the preheater  51  is supplied to the condenser  202  of the auxiliary steam generating device  200  via the branch steam line  65   a  and the steam recovery line  212  of the auxiliary steam generating device  200 . 
         [0089]    The superheated steam SS that flowed into the casing  52  of the preheater  51  exchanges heat with the fuel F that flows inside the tubes  53  of the preheater  51 , heating the fuel F. This fuel F is supplied to the combustor  21  via the fuel line  29 . Meanwhile, the superheated steam SS is cooled and condensed by heat exchange with the fuel F, and turns into water. This water is returned to the condenser  202  or the degassing tank  203  of the auxiliary steam generating device  200  via the water recovery line  61   a.    
         [0090]    As described above, according to this embodiment, as with the first embodiment, the fuel F is heated by the superheated steam SS superheated by the cooling of the combustor  21 , thereby making it possible to effectively utilize the heat of the combustor  21 . Furthermore, according to this embodiment as well, when the fuel F is heated, the superheated steam SS is condensed, making it possible to thoroughly utilize the heat held by the superheated steam SS. Thus, according to this embodiment as well, it is possible to increase the thermal efficiency of the gas turbine plant overall. 
       Third Embodiment 
       [0091]    Next, a third embodiment of a gas turbine plant according to the present invention will be described with reference to  FIG. 4 . 
         [0092]    The gas turbine plant of this embodiment includes the gas turbine  10 , the generator  15  that generates power by the driving of the gas turbine  10 , the heat recovery device  100  that generates steam by the heat of the exhaust gas EG exhausted from the gas turbine  10 , the chimney  40  that releases the exhaust gas EG that has passed through the heat recovery device  100  into the atmosphere, the auxiliary steam generating device  200  that generates steam, a fuel-preheating device  50   b  that preheats the fuel supplied to the gas turbine  10 , and the control device  90  that controls the gas turbine  10 , the fuel-preheating device  50   b,  and the like. 
         [0093]    That is, the gas turbine plant of this embodiment has the auxiliary steam generating device  200  of the second embodiment added to the gas turbine plant of the first embodiment. 
         [0094]    The fuel-preheating device  50   b  of this embodiment, similarly to the first embodiment, includes the preheater  51  that heats fuel supplied to the combustor  21 , the cooling-steam line  55  that supplies the steam generated by the heat recovery boiler  110  to the combustor  21  as the cooling steam CS, the superheated-steam line  57  that supplies the superheated steam SS superheated by passing through the combustor  21  to the preheater  51 , the water recovery line  61  that sends the water condensed from the superheated steam SS by the preheater  51  to the water supply line  131  of the heat recovery device  100 , and the branch steam line  65  that branches from the superheated-steam line  57 . Furthermore, the fuel-preheating device  50   b  of this embodiment further includes an auxiliary cooling-steam line  55   b  that supplies the steam generated by the auxiliary boiler  201  to the combustor  21  as the cooling steam CS, an auxiliary water recovery line  61   b  that branches from the water recovery line  61 , and an auxiliary branch steam line  65   b  that branches from the branch steam line  65 . 
         [0095]    The auxiliary cooling-steam line  55   b  branches from the auxiliary steam line  211  of the auxiliary steam generating device  200 , and merges into the cooling-steam line  55 . An auxiliary cooling-steam regulating valve  56   b  that regulates the flow rate of the steam supplied from this auxiliary cooling-steam line  55   b  to the combustor  21  via the cooling-steam line  55  is provided on this auxiliary cooling-steam line  55   b.  The auxiliary water recovery line  61   b  branches from the water recovery line  61  and is connected to the degassing tank  203  of the auxiliary steam generating device  200 . An auxiliary recovered water regulating valve  62   b  that regulates the flow rate of the water that flows into the degassing tank  203  is provided on this auxiliary water recovery line  61   b.  In addition, a recovered water regulating valve  62  is provided on the water recovery line  61  at a position that is on the condenser  123  side of the heat recovery device  100  of the position where the auxiliary water recovery line  61   b  branches. The auxiliary branch steam line  65   b  branches from the branch steam line  65  and is connected to the steam recovery line  212  of the auxiliary steam generating device  200 . An auxiliary recovered steam regulating valve  66   b  that regulates the flow rate of the steam that flows through the auxiliary branch steam line  65   b  is provided on this auxiliary branch steam line  65   b.  In addition, a recovered steam regulating valve  66  that regulates the flow rate of the steam that flows through the branch steam line  65  is provided on the branch steam line  65  at a position on the intermediate-pressure steam turbine  121   b  side of the heat recovery device  100  of the position where the auxiliary branch steam line  65   b  branches. 
         [0096]    Each of the regulating valves  56 ,  56   b,    58 ,  62 ,  62   b,    66 ,  66   b  of the fuel-preheating device  50   b  of this embodiment opens and closes in accordance with instructions from the control device  90 . 
         [0097]    Next, the operation of the gas turbine plant of this embodiment described above will be described. 
         [0098]    In this embodiment as well, the gas turbine  10  and the heat recovery device  100  operate in the same manner as in the first embodiment. 
         [0099]    In addition, in this embodiment, when the gas turbine  10  and the heat recovery device  100  are operating steadily, the fuel-preheating device  50   b  operates in the same manner as in the first embodiment. That is, the fuel-preheating device  50   b  of this embodiment operates as follows. 
         [0100]    A portion of the high-pressure steam HS that drove the high-pressure steam turbine  121   c  is supplied to the combustor  21  via the high-pressure steam recovery line  139  and the cooling-steam line  55  as the cooling steam CS, and a portion of the steam from the intermediate-pressure superheater  114   b  of the heat recovery device  100  is supplied to the combustor  21  via the intermediate-pressure steam line  133  and the cooling-steam line  55  as the cooling steam CS. At this time, the flow rate of the cooling steam CS supplied to the combustor  21  is regulated by the cooling-steam regulating valve  56  provided on the cooling-steam line  55 . In addition, at this time, the auxiliary cooling-steam regulating valve  56   b  is fully closed. 
         [0101]    The cooling steam CS from the cooling-steam line  55  cools the combustion liner  23  of the combustor  21  by heat exchange with this combustion liner  23 . Meanwhile, this cooling steam CS is superheated and turns into the superheated steam SS. 
         [0102]    This superheated steam SS flows into the casing  52  of the preheater  51  via the superheated-steam line  57 . At this time, the flow rate of the superheated steam SS that flows into the casing  52  of the preheater  51  is regulated by the superheated-steam regulating valve  58  provided on the superheated-steam line  57 . The portion of the superheated steam SS from the combustor  21  that does not flow into the casing  52  of the preheater  51  is supplied to the intermediate-pressure steam turbine  121   b  via the branch steam line  65  and the reheat steam line  136 . In addition, at this time, the recovered steam regulating valve  66  is fully open, and the auxiliary recovered steam regulating valve  66   b  is fully closed. 
         [0103]    The superheated steam SS that flowed into the casing  52  of the preheater  51  exchanges heat with the fuel F that flows inside the tubes  53  of the preheater  51 , heating the fuel F. This fuel F is supplied to the combustor  21  via the fuel line  29 . Meanwhile, the superheated steam SS is cooled and condensed by heat exchange with the fuel F, and turns into water. This water is returned to the condenser  123  or the water supply line  131  of the heat recovery device  100  via the water recovery line  61 . At this time, the recovered water regulating valve  62  is fully open, and the auxiliary recovered water regulating valve  62   b  is fully closed. 
         [0104]    In addition, in this embodiment, when the gas turbine  10  is in an activation stage or the like and the heat recovery device  100  is not operating steadily, the fuel-preheating device  50   b  operates in the same manner as in the second embodiment. That is, the fuel-preheating device  50   b  of this embodiment operates as follows. 
         [0105]    A portion of the steam generated by the auxiliary boiler  201  of the auxiliary steam generating device  200  is, as the cooling steam CS, sent to the combustor  21  via the auxiliary steam line  211 , the auxiliary cooling-steam line  55   b,  as well as the cooling-steam line  55 . At this time, the flow rate of the cooling steam CS supplied to the combustor  21  is regulated by the auxiliary cooling-steam regulating valve  56   b.  In addition, at this time, the cooling-steam regulating valve  56  is fully closed.  
         [0106]    The cooling steam CS from the auxiliary steam line  211 , the auxiliary cooling-steam line  55   b,  as well as the cooling-steam line  55  exchanges heat with the combustion liner  23  of the combustor  21 , and cools the combustion liner  23 . Meanwhile, this cooling steam CS is superheated and turns into the superheated steam SS. 
         [0107]    This superheated steam SS flows into the casing  52  of the preheater  51  via the superheated-steam line  57 . At this time, the flow rate of the superheated steam SS that flows into the casing  52  of the preheater  51  is regulated by the superheated-steam regulating valve  58  provided on the superheated-steam line  57 . The portion of the superheated steam SS from the combustor  21  that does not flow into the casing  52  of the preheater  51  is supplied to the condenser  202  of the auxiliary steam generating device  200  via the branch steam line  65 , the auxiliary branch steam line  65   b,  as well as the steam recovery line  212  of the auxiliary steam generating device  200 . At this time, the auxiliary recovered steam regulating valve  66   b  is fully open, and the recovered steam regulating valve  66  is fully closed. 
         [0108]    The superheated steam SS that flowed into the casing  52  of the preheater  51  exchanges heat with the fuel F that flows inside the tubes  53  of the preheater  51 , heating the fuel F. This fuel F is supplied to the combustor  21  via the fuel line  29 . Meanwhile, the superheated steam SS is cooled and condensed by heat exchange with the fuel F, and turns into water. This water is returned to the condenser  202  or the degassing tank  203  of the auxiliary steam generating device  200  via the water recovery line  61  and the auxiliary water recovery line  61   b.  At this time, the auxiliary recovered water regulating valve  62   b  is fully open, and the recovered water regulating valve  62  is fully closed. 
         [0109]    Thus, according to this embodiment, it is possible to use the steam from the heat recovery device  100  to cool the combustor  21  and preheat the fuel F when the gas turbine  10  and the heat recovery device  100  are operating steadily, and use the steam from the auxiliary steam generating device  200  to cool the combustor  21  and preheat the fuel F when the gas turbine  10  is in an activation stage or the like and the heat recovery device  100  is not operating steadily. Moreover, according to this embodiment, it is possible to increase the thermal efficiency of the gas turbine plant overall when the gas turbine  10  and the heat recovery device  100  are operating steadily as well as when the gas turbine  10  is in an activation stage or the like and the heat recovery device  100  is not operating steadily. 
       First Modification of Fuel-Preheating Device 
       [0110]    Next, a first modification of the fuel-preheating device of each of the above embodiments will be described with reference to  FIG. 5 . 
         [0111]    A fuel-preheating device  50   c  of this modification includes two preheaters  51   a,    51   b,  the cooling-steam line  55 , the superheated-steam line  57 , the water recovery line  61 , and the branch steam line  65  that branches from the superheated-steam line  57 . That is, the fuel-preheating device  50   c  of this modification is basically the same as the fuel-preheating device of the above embodiments, except that it includes the two preheaters  51   a,    51   b.  The two preheaters  51   a,    51   b  are provided in series on the fuel line  29 . Of these two preheaters  51   a,    51   b,  the preheater  51   a  provided upstream of the fuel line  29  forms the first preheater  51   a,  and the preheater  51   b  provided downstream of the fuel line  29  forms the second preheater  51   b.    
         [0112]    The cooling-steam line  55  of this modification branches from a steam supply system  70 . This steam supply system  70  is the intermediate-pressure steam line  133  of the heat recovery device  100  in the first and third embodiments, or the auxiliary steam line  211  of the auxiliary steam generating device  200  of the second and third embodiments. This cooling-steam line  55 , similarly to each of the above embodiments, is connected to the combustion liner  23  of the combustor  21 . In addition, the cooling-steam regulating valve  56  is provided on this cooling-steam line  55 . 
         [0113]    The superheated-steam line  57  connected to the combustion liner  23  of the combustor  21  branches into two lines midway, with one line forming a first superheated-steam line  57   a  that is connected to the casing  52  of the first preheater  51   a,  and the other line forming a second superheated-steam line  57   b  that is connected to the casing  52  of the second preheater  51   b.  A first superheated-steam regulating valve  58   a  that regulates the flow rate of the superheated steam SS that flows through the first superheated-steam line  57   a  is provided on the first superheated-steam line  57   a,  and a second superheated-steam regulating valve  58   b  that regulates the flow rate of the superheated steam SS that flows through the second superheated-steam line  57   b  is provided on the second superheated-steam line  57   b.    
         [0114]    The water recovery line  61  is connected to the casing  52  of the first preheater  51   a  and the casing  52  of the second preheater  51   b,  respectively. Both of the water recovery lines  61  are connected to a condensate/water supply system  71 . This condensate/water supply system  71  is the condenser  123  or the water supply line  131  of the heat recovery device  100  in the first and third embodiments, or the condenser  202  or the degassing tank  203  of the auxiliary steam generating device  200  in the second and third embodiments. 
         [0115]    The branch steam line  65  that branches from the superheated-steam line  57  is connected to a steam recovery system  72 . This steam recovery system  72  is the reheat steam line  136  of the heat recovery device  100  in the first and third embodiments, or the steam recovery line  212  of the auxiliary steam generating device  200  in the second and third embodiments. 
         [0116]    In this embodiment as well, the steam from the steam supply system  70  is sent as the cooling steam CS to the combustor  21  via the cooling-steam line  55 . The cooling steam CS from the cooling-steam line  55  cools the combustion liner  23  of the combustor  21  by heat exchange with this combustion liner  23 . Meanwhile, this cooling steam CS is superheated and turns into the superheated steam SS. 
         [0117]    This superheated steam SS flows into the casing  52  of the first preheater  51   a  via the first superheated-steam line  57   a,  and flows into the casing  52  of the second preheater  51  b via the second superheated-steam line  57   b.  At this time, the flow rate of the superheated steam SS that flows into the casing  52  of the first preheater  51   a  is regulated by the first superheated-steam regulating valve  58   a,  and the flow rate of the superheated steam SS that flows into the casing  52  of the second preheater  51   b  is regulated by the second superheated-steam regulating valve  58   b.  The portion of the superheated steam SS from the combustor  21  that does not flow into the casing  52  of the first preheater  51   a  or the casing  52  of the second preheater  51   b  is supplied to the steam recovery system  72  via the branch steam line  65 . 
         [0118]    The superheated steam SS that flowed into the casing  52  of the first preheater  51   a  heats the fuel F. This fuel F is sent to the second preheater  51   b  via the fuel line  29 . In addition, the superheated steam SS that flowed into the casing  52  of the second preheater  51   b  further heats the fuel F heated by the first preheater  51   a.  This fuel F is supplied to the combustor  21  via the fuel line  29 . Meanwhile, the superheated steam SS is cooled and condensed by heat exchange with the fuel F in each of the preheaters  51   a ,  51   b,  and turns into water. This water is supplied to the condensate/water supply system  71  via the water recovery line  61 . 
         [0119]    Thus, according to this modification, the fuel F is heated in two stages, making it possible to supply the fuel F at a higher temperature than in the above embodiments, thereby increasing the combustion efficiency of the fuel F in the combustor  21 . 
       Second Modification of Fuel-Preheating Device 
       [0120]    Next, a second modification of the fuel-preheating device of each of the above embodiments will be described with reference to  FIG. 6 . 
         [0121]    A fuel-preheating device  50   d  of this modification, similarly to the first modification, also includes two preheaters  51   d,    51   b,  the cooling-steam line  55 , the superheated-steam line  57 , the water recovery line  61 , and the branch steam line  65  that branches from the superheated-steam line  57 . Furthermore, in this modification as well, similarly to the first modification, the two preheaters  51   d ,  51   b  are provided in series in the fuel line  29 . Of these two preheaters  51   d,    51   b , the preheater  51   d  provided upstream of the fuel line  29  forms the first preheater (first-stage preheater)  51   d,  and the preheater  51   b  provided downstream of the fuel line  29  forms the second preheater (second-stage preheater)  51   b.    
         [0122]    The cooling-steam line  55  of this modification, similarly to the first modification, also branches from the steam supply system  70  and is connected to the combustion liner  23  of the combustor  21 . In addition, the cooling-steam regulating valve  56  is also provided on this cooling-steam line  55 . This cooling-steam line  55  branches at a position on the steam supply system  70  side of the position of the cooling-steam regulating valve  56 , and this branched line forms a first preheater steam line (first-stage steam line)  68 . This first preheater steam line  68  is connected to the casing  52  of the first preheater  51   d . A first preheater steam regulating valve  69  is provided on this first preheater steam line  68 . 
         [0123]    The superheated-steam line  57  connected to the combustion liner  23  of the combustor  21  is connected to the casing  52  of the second preheater  51   b . The superheated-steam regulating valve  58  that regulates the flow rate of the superheated steam SS that flows through the superheated-steam line  57  is provided on this superheated-steam line  57  as well. 
         [0124]    The water recovery line  61  is connected to the casing  52  of the first preheater  51   d  and the casing  52  of the second preheater  51   b,  respectively, similarly to the first modification. Both of the water recovery lines  61  are connected to the condensate/water supply system  71 . In addition, the branch steam line  65  that branches from the superheated-steam line  57 , similarly to the first modification, is also connected to the steam recovery system  72 . 
         [0125]    According to this modification, a portion of the steam from the steam supply system  70  flows into the casing  52  of the first preheater  51   d  via the first preheater steam line  68 . At this time, the flow rate of the steam that flows into the casing  52  of the first preheater  51   d  is regulated by the first preheater steam regulating valve  69 . The steam that flowed into the casing  52  of the first preheater  51   d  heats the fuel F. This fuel F is supplied to the second preheater  51   b  via the fuel line  29 . Meanwhile, the steam is cooled and condensed by heat exchange with the fuel F in the first preheater  51   d,  and turns into water. This water is supplied to the condensate/water supply system  71  via the water recovery line  61 . 
         [0126]    In addition, in this modification, the remaining portion of steam from the steam supply system  70  is sent as the cooling steam CS to the combustor  21  via the cooling-steam line  55 . The cooling steam CS from the cooling-steam line  55  cools the combustion liner  23  of the combustor  21  by heat exchange with this combustion liner  23 . Meanwhile, this cooling steam CS is superheated and turns into the superheated steam SS. 
         [0127]    This superheated steam SS flows into the casing  52  of the second preheater  51   b  via the superheated-steam line  57 . At this time, the flow rate of the superheated steam SS that flows into the casing  52  of the second preheater  51   b  is regulated by the superheated-steam regulating valve  58 . The portion of the superheated steam SS from the combustor  21  that does not flow into the casing  52  of the second preheater  51   b  is supplied to the steam recovery system  72  via the branch steam line  65 .  
         [0128]    The superheated steam SS that flowed into the casing  52  of the second preheater  51   b  further heats the fuel F heated by the first preheater  51   d.  This fuel F is supplied to the combustor  21  via the fuel line  29 . Meanwhile, the superheated steam SS is cooled and condensed by heat exchange with the fuel F in the second preheater  51   b,  and turns into water. This water is supplied to the condensate/water supply system  71  via the water recovery line  61 . 
         [0129]    Thus, according to this modification as well, similarly to the first modification, the fuel F is heated in two stages, making it possible to supply the fuel F to the combustor  21  at a higher temperature than in the above embodiments, thereby increasing the combustion efficiency of the fuel F in the combustor  21 . 
         [0130]    It should be noted that, according to the first modification, the cooling steam CS superheated by passing through the combustion liner  23  of the combustor  21  is supplied as the superheated steam SS to the interior of the casing  52  of the first preheater  51   a  and the interior of the casing  52  of the second preheater  51   b,  and therefore the flow rate of the cooling steam CS supplied to the combustion liner  23  of the combustor  21  is higher than in each of the above embodiments. Thus, the fuel-preheating device  50   c  of the first modification is effective in a case where a high flow rate is required to cool the combustion liner  23 . 
         [0131]    On the other hand, according to the second modification, the cooling steam CS superheated by passing through the combustion liner  23  of the combustor  21  is supplied as the superheated steam SS to only the interior of the casing  52  of the second preheater  51   b,  and therefore the flow rate of the cooling steam CS supplied to the combustion liner  23  of the combustor  21  is the same as in the above embodiments. Nevertheless, according to the second modification as well, the fuel F is heated in two stages as described above, making it possible to supply the fuel F at a higher temperature than in the above embodiments to the combustor  21 . Thus, the fuel-preheating device  50   d  of the second modification is effective in a case where the flow rate of the steam for cooling the combustion liner  23  is preferably suppressed while supplying the fuel F at a high temperature to the combustor  21 . 
       Other Modifications  
       [0132]    While each of the above embodiments and modifications is an example in which the combustion liner  23  of the combustor  21  is cooled by steam, the hot part of either the combustor  21  or the turbine  31  that contacts the combustion gas may be cooled by steam, and the fuel may be preheated using the steam that cooled the hot part. It should be noted that examples of the hot part include the vane  36  or the blade  34  (illustrated in  FIG. 2 ) of the turbine  31 , a ring segment of the inner peripheral surface of the casing  35  of the turbine  31  that constitutes the section facing the blade  34 , and the like. 
       INDUSTRIAL APPLICABILITY 
       [0133]    According to an aspect of the present invention, it is possible to increase the thermal efficiency of the gas turbine plant overall. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  Gas turbine 
           11  Compressor 
           21  Combustor 
           22  Injector 
           23  Combustion liner 
           24  Steam flow path 
           31  Turbine 
           32  Turbine rotor (or simply rotor) 
           34  Blade 
           35  Casing 
           36  Vane 
           40  Chimney 
           50 ,  50   a,    50   b,    50   c,    50   d  Fuel-preheating device 
           51  Preheater 
           51   a  First preheater 
           51   b  Second preheater 
           51   d  First preheater (first-stage preheater) 
           55 ,  55   a  Cooling-steam line 
           55   b  Auxiliary cooling-steam line 
           56  Cooling-steam regulating valve 
           56   b  Auxiliary cooling-steam regulating valve 
           57  Superheated-steam line 
           57   a  First superheated-steam line 
           57   b  Second superheated-steam line 
           58  Superheated-steam regulating valve 
           58   a  First superheated-steam regulating valve 
           58   b  Second superheated-steam regulating valve 
           61 ,  61   a  Water recovery line 
           61   b  Auxiliary water recovery line 
           65 ,  65   a  Branch steam line 
           65   b  Auxiliary branch steam line 
           100  Heat recovery device 
           110  Heat recovery boiler 
           111  a Low-pressure steam generating part 
           111   b  Intermediate-pressure steam generating part 
           111   c  High-pressure steam generating part 
           121   a  Low-pressure steam turbine 
           121   b  Intermediate-pressure steam turbine 
           121   c  High-pressure steam turbine 
           123  Condenser 
           131  Water supply line 
           133  Intermediate-pressure steam line 
           136  Reheat steam line 
           200  Auxiliary steam generating device 
           201  Auxiliary boiler 
           202  Condenser 
           203  Degassing tank 
           211  Auxiliary steam line 
           212  Steam recovery line 
           213  Water supply line