Abstract:
The invention addresses the problem of providing a power generation system capable of effectively using the heat of the exhaust air discharged from an SOFC and also providing a method for operating the power generation system. The power generation system has a gas turbine, a fuel cell, an exhaust air circulation line, an exhaust fuel gas supply line, a turbine, an exhaust heat recovery boiler, and at least one exhaust air heat exchanger. The turbine is equipped with a high-pressure turbine, a medium-pressure turbine, and a low-pressure turbine. The exhaust heat recovery boiler is equipped with a high-pressure steam circulation mechanism, a medium-pressure steam circulation mechanism, and a low-pressure steam circulation mechanism. The exhaust air heat exchanger exchanges heat between the steam exchanging heat with the exhaust gas in the high-pressure steam circulation mechanism or the medium-pressure steam circulation mechanism and flowing toward the turbine and the exhaust gas flowing through the exhaust air circulation line, thereby increasing the temperature of the steam and decreasing the temperature of the exhaust gas.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a power generation system combining a solid oxide fuel cell, a gas turbine, and a steam turbine, and to a method for operating the power generation system. 
       BACKGROUND ART 
       [0002]    A solid oxide fuel cell (hereinafter, referred to as SOFC) is known as a highly efficient fuel cell having a wide range of applications. Such an SOFC has a high operating temperature in order to increase ionic conductivity. Thus, air that has been discharged from a compressor of a gas turbine is usable as the air to be supplied to an air electrode (as an oxidant). The SOFC also enables unused high-temperature fuel to be used as fuel for a combustor of the gas turbine. 
         [0003]    Thus, for example, as described in Patent Literature 1 listed below, various combinations of an SOFC, a gas turbine, and a steam turbine have been proposed as a power generation system that achieves highly efficient power generation. The combined system described in Patent Literature 1 is provided with an SOFC, a gas turbine combustor that combusts exhaust fuel gas and exhaust air discharged from the SOFC, and a gas turbine having a compressor that compresses air and supplies it to the SOFC. 
         [0004]    In addition, Patent Literature 2 describes a technique in which exhaust air discharged from the SOFC exchanges heat with air to be supplied to the SOFC, followed by heat exchange with plumbing of a heat recovery steam generator, so that the heat of the exhaust air is used for power generation of the heat recovery steam generator. 
       CITATION LIST 
     Patent Literature 
       [0005]    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2009-205930A 
         [0006]    Patent Literature 2: Japanese Unexamined Patent Application Publication No. H11-297336A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    The conventional power generation system described above discharges air that has been superheated to a high temperature as the exhaust air from the SOFC. In Patent Literature 1, various heat exchanges are performed with the exhaust air to recover the heat within the exhaust air. Here, improvement of efficiency is desired for the power generation system, which is the impetus for improving the method in which the exhaust air is used. 
         [0008]    In order to solve the above-described problem, an object of the present invention is to provide a power generation system and an operating method for a power generation system that make more efficient use of the heat of the exhaust air discharged from the SOFC. 
       Solution to Problem 
       [0009]    A power generation system of the invention to achieve the above-described object includes: a gas turbine having a compressor and a combustor; a fuel cell having an air electrode and a fuel electrode; an exhaust air circulation line supplying exhaust air discharged from the fuel cell to the gas turbine; an exhaust fuel gas supply line supplying exhaust fuel gas discharged from the fuel cell to the gas turbine; a turbine provided with a high-pressure turbine, an intermediate-pressure turbine, and a low-pressure turbine; a heat recovery steam generator provided with a high-pressure steam circulation mechanism recovering heat of exhaust gas discharged from the gas turbine to generate steam and supplying generated steam to the high-pressure turbine, an intermediate-pressure steam circulation mechanism recovering heat of exhaust gas discharged from the gas turbine to generate steam with pressure lower than that of the high-pressure steam circulation mechanism and supplying generated steam to the intermediate-pressure turbine, and a low-pressure steam circulation mechanism recovering heat of exhaust gas discharged from the gas turbine to generate steam with pressure lower than that of the intermediate-pressure steam circulation mechanism and supplying generated steam to the low-pressure turbine; and at least one exhaust air heat exchanger performing heat exchange between exhaust gas flowing through the exhaust air circulation line and steam that has undergone heat exchange with the exhaust gas in one of the high-pressure steam circulation mechanism and the intermediate-pressure steam circulation mechanism and flowing toward the turbine, so as to raise the temperature of the steam and to lower the temperature of the exhaust gas. 
         [0010]    Therefore, heat exchange is performed with steam superheated in one of the high-pressure steam circulation mechanism and the intermediate-pressure steam circulation mechanism so as to lower the temperature of exhaust air, thereby enabling the temperature of the exhaust air to be lowered while suppressing the temperature of the exhaust air from being excessively lowered. Thus, it is possible for the exhaust air to be supplied to the gas turbine with the temperature of the exhaust air maintained at a relatively high temperature, and it is possible for the heat of the exhaust air to be recovered in both the gas turbine and the heat recovery steam generator. This can improve usage efficiency. Furthermore, the temperature of the exhaust air is lowered, so that the load on the exhaust air circulation line can be reduced. 
         [0011]    In the power generation system of the invention, the high-pressure steam circulation mechanism includes a high-pressure superheater. The exhaust air heat exchanger performs heat exchange between steam superheated by the high-pressure superheater and the exhaust air. Steam flowing through the high-pressure steam circulation mechanism undergoes heat exchange with the exhaust air heat exchanger and is then supplied to the high-pressure turbine. 
         [0012]    Therefore, it is possible for the temperature of the exhaust air to be lowered while suppressing the temperature of the exhaust air from being excessively lowered. 
         [0013]    The power generation system of the invention includes a fuel gas supply line supplying fuel gas to the gas turbine; and at least one fuel gas heat exchanger performing heat exchange between the fuel gas flowing through the fuel gas supply line and the steam that has undergone heat exchange with the exhaust gas in the high-pressure steam circulation mechanism and flowing toward the turbine, so as to lower the temperature of the steam and to raise the temperature of the fuel gas. The steam flowing through the high-pressure steam circulation mechanism undergoes heat exchange with the fuel gas heat exchanger, heat exchange with the exhaust air heat exchanger, and is then supplied to the high-pressure turbine. 
         [0014]    Therefore, it is possible for the temperature of the exhaust air to be lowered while suppressing the temperature of the exhaust air from being excessively lowered. Furthermore, the temperature of the fuel gas is raised, so that the heat thereof can be recovered in both the gas turbine and the turbine. 
         [0015]    In the power generation system of the invention, the heat recovery steam generator further includes a reheat steam circulation mechanism provided with a reheater raising the temperature of recovered steam with exhaust gas discharged from the gas turbine. The reheat steam circulation mechanism recovers steam passing through the high-pressure turbine, raises the temperature of the recovered steam in the reheater, and supplies the steam with the temperature thereof raised to the intermediate-pressure turbine. The exhaust air heat exchanger performs heat exchange between steam superheated by the reheater and the exhaust air, and steam flowing through the reheat steam circulation mechanism undergoes heat exchange with the exhaust air heat exchanger, and is then supplied to the intermediate-pressure turbine. 
         [0016]    Therefore, it is possible for the temperature of the exhaust air to be lowered while suppressing the temperature of the exhaust air from being excessively lowered. 
         [0017]    A method for operating a power generation system including a gas turbine having a compressor and a combustor, a fuel cell having an air electrode and a fuel electrode, an exhaust air circulation line supplying exhaust air discharged from the fuel cell to the gas turbine, an exhaust fuel gas supply line supplying exhaust fuel gas discharged from the fuel cell to the gas turbine, a turbine provided with a high-pressure turbine, an intermediate-pressure turbine, and a low-pressure turbine, and a heat recovery steam generator provided with a high-pressure steam circulation mechanism recovering heat of exhaust gas discharged from the gas turbine to generate steam and supplying generated steam to the high-pressure turbine, an intermediate-pressure steam circulation mechanism recovering heat of exhaust gas discharged from the gas turbine to generate steam with pressure lower than that of the high-pressure steam circulation mechanism and supplying generated steam to the intermediate-pressure turbine, and a low-pressure steam circulation mechanism recovering heat of exhaust gas discharged from the gas turbine to generate steam with pressure lower than the intermediate-pressure steam circulation mechanism and supplying generated steam to the low-pressure turbine. The method involves performing heat exchange between the exhaust gas and steam flowing through one of the high-pressure steam circulation mechanism and the intermediate-pressure steam circulation mechanism, and after performing the heat exchange, performing heat exchange between steam flowing toward the turbine and exhaust gas flowing through the exhaust air circulation line, so as to raise the temperature of the steam and to lower the temperature of the exhaust gas. 
         [0018]    Therefore, heat exchange is performed with steam superheated in one of the high-pressure steam circulation mechanism and the intermediate-pressure steam circulation mechanism so as to lower the temperature of exhaust air, thereby enabling the temperature of the exhaust air to be lowered while suppressing the temperature of the exhaust air from being excessively lowered. Thus, it is possible for the exhaust air to be supplied to the gas turbine with the temperature of the exhaust air maintained at a relatively high temperature, and it is possible for the heat of the exhaust air to be recovered in both the gas turbine and the heat recovery steam generator. This can improve usage efficiency. Furthermore, the temperature of the exhaust air is lowered, so that the load on the exhaust air circulation line can be reduced. 
       Advantageous Effects of Invention 
       [0019]    According to the power generation system and the method for operating the power generation system of the invention, heat exchange is performed with steam superheated in one of the high-pressure steam circulation mechanism and the intermediate-pressure steam circulation mechanism so as to lower the temperature of exhaust air, thereby enabling the temperature of the exhaust air to be lowered while suppressing the temperature of the exhaust air from being excessively lowered. Thus, it is possible for the exhaust air to be supplied to the gas turbine with the temperature of the exhaust air maintained at a relatively high temperature, and it is possible for the heat of the exhaust air to be recovered in both the gas turbine and the heat recovery steam generator. This can improve usage efficiency. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  is a schematic configuration diagram illustrating a power generation system of the embodiment. 
           [0021]      FIG. 2  is a schematic configuration diagram illustrating a heat recovery steam generator and turbine of the power generation system pertaining to an embodiment of the invention. 
           [0022]      FIG. 3  is a schematic configuration diagram illustrating a heat exchange unit of the power generation system of the embodiment. 
           [0023]      FIG. 4  is a schematic configuration diagram illustrating another example of the heat exchange unit of the power generation system of the embodiment. 
           [0024]      FIG. 5  is a schematic configuration diagram illustrating another example of the heat exchange unit of the power generation system of the embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0025]    A preferred embodiment of a power generation system and a method for operating the power generation system pertaining to the invention are described in detail below, with reference to the accompanying drawings. Note that the invention is not limited by the embodiment, and when a plurality of embodiments are present, the invention is intended to include a configuration combining these embodiments. 
       Embodiment 
       [0026]    The power generation system of the embodiment is a Triple Combined Cycle (registered trademark) system combining a solid oxide fuel cell (hereinafter, referred to as SOFC), a gas turbine, and a steam turbine. This Triple Combined Cycle system is able to extract power in the three stages of the SOFC, the gas turbine, and the steam turbine by disposing the SOFC on the upstream side of a gas turbine combined cycle (GTCC) power generation, and is thus able to realize extremely high power generation efficiency. Note that in following description, a solid oxide fuel cell is employed as the fuel cell of the invention; however, fuel cells are not limited to this type. 
         [0027]      FIG. 1  is a schematic configuration diagram illustrating the power generation system pertaining to the embodiment. In the embodiment, as illustrated in  FIG. 1 , a power generation system  10  includes a gas turbine  11  and a power generator  12 , an SOFC  13 , and a steam turbine  14  and power generator  15 . The power generation system  10  is configured to combine power generation by the gas turbine  11 , power generation by the SOFC  13 , and power generation by the steam turbine  14  to achieve high power generation efficiency. The power generation system  10  is also provided with a control device  62 . The control device  62  controls the operation of each component of the power generation system  10  in accordance with input settings, input instructions, results detected in a detection unit, and the like. 
         [0028]    The gas turbine  11  includes a compressor  21 , a combustor  22 , and a turbine  23 . The compressor  21  and the turbine  23  are coupled in an integrally rotatable manner by a rotating shaft  24 . The compressor  21  compresses air A taken in through an air intake line  25 . The combustor  22  mixes and combusts compressed air A 1  supplied from the compressor  21  through a first compressed air supply line  26  and fuel gas L 1  supplied from a first fuel gas supply line  27 . The turbine  23  is rotated by exhaust gas (combustion gas) G supplied from the combustor  22  through an exhaust gas supply line  28 . Although not illustrated in the drawings, the turbine  23  is supplied with the compressed air A 1  compressed by the compressor  21  through a casing, and the compressed air A 1  as cooling air cools blades and the like. The power generator  12  is provided coaxially with the turbine  23  and is capable of generating power through the rotation of the turbine  23 . Note that the fuel gas L 1  supplied to the combustor  22  is, for example, liquefied natural gas (LNG). 
         [0029]    The SOFC  13  is supplied with high-temperature fuel gas as a reductant and with high-temperature air (oxidizing gas) as an oxidant, so as to react at a predetermined operating temperature to generate power. This SOFC  13  is constituted of an air electrode, a solid electrolyte, and a fuel electrode that are housed in a pressure container. A portion of compressed air A 2  compressed by the compressor  21  is supplied to the air electrode and fuel gas is supplied to the fuel electrode, so that power is generated. Note that, as fuel gas L 2  supplied to the SOFC  13 , for example, liquefied natural gas (LNG), hydrogen (H 2 ) and carbon monoxide (CO), hydrocarbon gas such as methane (CH 4 ), or gas produced in a gasification facility for carbonaceous materials such as coal is used. Also, the oxidizing gas supplied to the SOFC  13  is a gas containing approximately 15% to 30% oxygen. Typically, air is suitable. However, in addition to air, mixed gas of combustion exhaust gas and air, mixed gas of oxygen and air, or the like may be used (hereinafter, the oxidizing gas supplied to the SOFC  13  will be referred to as “air”). 
         [0030]    This SOFC  13  is connected with a second compressed air supply line  31  that branches off from the first compressed air supply line  26  and is capable of supplying a portion of the compressed air A 2  compressed by the compressor  21  to an inlet of the air electrode. This second compressed air supply line  31  is provided with a control valve  32  that is capable of adjusting the volume of air to be supplied, and a blower (booster)  33  that is capable of boosting the pressure of the compressed air A 2 , along the air-flow direction. The control valve  32  is provided on the upstream side in the air-flow direction of the second compressed air supply line  31 . The blower  33  is provided on the downstream side of the control valve  32 . The SOFC  13  is connected with an exhaust air line  34  expelling exhaust air A 3  used by the air electrode. This exhaust air line  34  branches into an exhaust line  35  that discharges the exhaust air A 3  used by the air electrode to the outside, and a compressed air circulation line  36  that is connected to the combustor  22 . The exhaust line  35  is provided with a control valve  37  that is capable of adjusting the volume of air to be discharged. The compressed air circulation line  36  is provided with a control valve  38  that is capable of adjusting the volume of air to be circulated. 
         [0031]    The SOFC  13  is also provided with a second fuel gas supply line  41  that supplies the fuel gas L 2  to the inlet of the fuel electrode. The second fuel gas supply line  41  is provided with a control valve  42  that is capable of adjusting the volume of fuel gas to be supplied. The SOFC  13  is connected with an exhaust fuel line  43  expelling an exhaust fuel gas L 3  used by the fuel electrode. This exhaust fuel line  43  branches into an exhaust line  44  that discharges to the outside, and an exhaust fuel gas supply line  45  that is connected to the combustor  22 . The exhaust line  44  is provided with a control valve  46  that is capable of adjusting the volume of fuel gas to be discharged. The exhaust fuel gas supply line  45  is provided with a control valve  47  that is capable of adjusting the volume of fuel gas to be supplied, and with a blower  48  that is capable of boosting the pressure of fuel, along the flow direction of the exhaust fuel gas L 3 . The control valve  47  is provided on the upstream side in the flow direction of the exhaust fuel gas L 3  in the exhaust fuel gas supply line  45 , and the blower  48  is provided on the downstream side of the control valve  47 . 
         [0032]    The SOFC  13  is also provided with a fuel gas recirculation line  49  that connects the exhaust fuel line  43  and the second fuel gas supply line  41 . The fuel gas recirculation line  49  is provided with a recirculation blower  50  that recirculates the exhaust fuel gas L 3  from the exhaust fuel line  43  into the second fuel gas supply line  41 . 
         [0033]    The steam turbine  14  rotates a turbine  52  with steam generated by a heat recovery steam generator (HRSG)  51 . This heat recovery steam generator  51  is connected with an exhaust gas line  53  extending from the gas turbine  11  (turbine  23 ), and produces steam S through heat exchange between air and high-temperature exhaust gas G. A steam supply line  54  and a water supply line  55  are provided between the steam turbine  14  (turbine  52 ) and the heat recovery steam generator  51 . Also, the water supply line  55  is provided with a condenser  56  and a water supply pump  57 . The power generator  15  is provided coaxially with the turbine  52  and is capable of generating power through the rotation of the turbine  52 . Note that the exhaust gas G recovered in the heat recovery steam generator  51  is released into the atmosphere after removal of any toxic materials. 
         [0034]    The operation of the power generation system  10  pertaining to the embodiment will now be described. When the power generation system  10  is started, the gas turbine  11 , the steam turbine  14 , and the SOFC  13  are started in the stated order. 
         [0035]    First, in the gas turbine  11 , the compressor  21  compresses the air A, the combustor  22  mixes the compressed air A 1  with the fuel gas L 1  and combusts the mixture, and the turbine  23  rotates by the exhaust gas G. Thus, the power generator  12  begins to generate power. Next, in the steam turbine  14 , the turbine  52  rotates by the steam S generated by the heat recovery steam generator  51 . Thus, the power generator  15  begins to generate power. 
         [0036]    Subsequently, in the SOFC  13 , the compressed air A 2  is first supplied to boost the pressure, and heating begins. With the control valve  37  of the exhaust line  35  and the control valve  38  of the compressed air circulation line  36  closed and with the blower  33  of the second compressed air supply line  31  stopped, the control valve  32  is opened to a predetermined lift. Then, a portion of the compressed air A 2  compressed by the compressor  21  is supplied from the second compressed air supply line  31  toward the SOFC  13 . Accordingly, the pressure is raised on the SOFC  13  side as the compressed air A 2  is supplied thereto. 
         [0037]    Meanwhile, in the SOFC  13 , the fuel gas L 2  is supplied to the fuel electrode side to boost the pressure. With the control valve  46  of the exhaust line  44  and the control valve  47  of the exhaust fuel gas supply line  45  closed and with the blower  48  stopped, the control valve  42  of the second fuel gas supply line  41  is opened and the recirculation blower  50  of the fuel gas recirculation line  49  is driven. Then, the fuel gas L 2  is supplied from the second fuel gas supply line  41  toward the SOFC  13 , and the exhaust fuel gas L 3  is recirculated by the fuel gas recirculation line  49 . Accordingly, the pressure is raised on the SOFC  13  side as the fuel gas L 2  is supplied thereto. 
         [0038]    Next, once the pressure on the air electrode side of the SOFC  13  reaches an outlet pressure of the compressor  21 , the control valve  32  is fully opened, and the blower  33  is driven. The control valve  37  is simultaneously opened and the exhaust air A 3  from the SOFC  13  is discharged from the exhaust line  35 . Then, the compressed air A 2  is supplied to the SOFC  13  side by the blower  33 . The control valve  46  is simultaneously opened, and the exhaust fuel gas L 3  from the SOFC  13  is discharged from the exhaust line  44 . Next, once the pressure on the air electrode side and the pressure on the fuel electrode side of the SOFC  13  reach target pressures, the boost in the pressure of the SOFC  13  is complete. 
         [0039]    Afterward, once the reaction (power generation) of the SOFC  13  stabilizes and the components of the exhaust air A 3  and the exhaust fuel gas L 3  stabilize, the control valve  37  is closed and the control valve  38  is opened. Then, the exhaust air A 3  from the SOFC  13  is supplied to the combustor  22  through the compressed air circulation line  36 . The control valve  46  is closed, while the control valve  47  is opened and the blower  48  is driven. Then, the exhaust fuel gas L 3  from the SOFC  13  is supplied to the combustor  22  through the exhaust fuel gas supply line  45 . At this time, the fuel gas L 1  supplied to the combustor  22  through the first fuel gas supply line  27  is reduced. 
         [0040]    Here, the power generation by the power generator  12  through the driving of the gas turbine  11 , the power generation by the SOFC  13 , and the power generation by the power generator  15  through the driving of the steam turbine  14  are all active, so that the power generation system  10  is in a steady operation state. 
         [0041]    The configuration of the steam turbine of the embodiment, specifically the configurations of the heat recovery steam generator  51  and the turbine  52 , will now be described, with reference to  FIG. 2 .  FIG. 2  is a schematic configuration diagram illustrating the heat recovery steam generator and the turbine in the power generation system pertaining to an embodiment of the invention. The turbine  52  includes a high-pressure turbine  52 H, an intermediate-pressure turbine  52 I, and a low-pressure turbine  52 L. The high-pressure turbine  52 H is driven by high-pressure steam supplied from the heat recovery steam generator  51 . The intermediate-pressure turbine  52 I is driven by steam supplied from the heat recovery steam generator  51  that is lower in pressure than the steam supplied to the high-pressure turbine  52 H. The low-pressure turbine  52 L is driven by steam supplied from the heat recovery steam generator  51  that is lower in pressure than the steam supplied to the intermediate-pressure turbine  52 I. 
         [0042]    The heat recovery steam generator  51  includes a high-pressure steam circulation mechanism  70  recovering heat of exhaust gas discharged from the turbine  23  of the gas turbine  11  to generate steam and supplying generated steam to the high-pressure turbine  52 H; an intermediate-pressure steam circulation mechanism  72  recovering heat of exhaust gas discharged from the gas turbine  11  to generate steam with pressure lower than that of the high-pressure steam circulation mechanism  70  and supplying generated steam to the intermediate-pressure turbine  52 I; a low-pressure steam circulation mechanism  74  recovering heat of exhaust gas discharged from the gas turbine  11  to generate steam with pressure lower than that of the intermediate-pressure steam circulation mechanism  72  and supplying generated steam to the low-pressure turbine  52 L, and a reheat steam circulation mechanism  79  raising again the temperature of the steam discharged from the high-pressure turbine  52 H with the exhaust gas and supplying the steam with the temperature thereof raised to the intermediate-pressure turbine  52 I. 
         [0043]    The heat recovery steam generator  51  also includes a pre-heater  76 , a high-pressure pump  78 H, an intermediate-pressure pump  78 I, and a low-pressure pump  78 L. The pre-heater  76  pre-heats water supplied from the condenser  56  through the water supply line  55 . The high-pressure pump  78 H supplies water pre-heated by the pre-heater  76  to the high-pressure steam circulation mechanism  70 . The intermediate-pressure pump  78 I supplies water pre-heated by the pre-heater  76  to the intermediate-pressure steam circulation mechanism  72 . The low-pressure pump  78 L supplies the water pre-heated by the pre-heater  76  to the low-pressure steam circulation mechanism  74 . 
         [0044]    The high-pressure steam circulation mechanism  70  is a mechanism for raising the temperature of the water supplied from the pre-heater  76  with the exhaust gas so as to generate steam and then supplying the steam to the high-pressure turbine  52 H. The high-pressure steam circulation mechanism  70  includes a high-pressure drum  70 D, a high-pressure economizer  70 EC, a high-pressure steam evaporator  70 EV, a high-pressure superheater  70 SHa, and a high-pressure superheater  70 SHb. The high-pressure economizer  70 EC, the high-pressure steam evaporator  70 EV, the high-pressure superheater  70 SHa, and the high-pressure superheater  70 SHb are heat exchangers provided with heat transfer pipes and disposed within a pipeline through which the exhaust gas G flows. Heat exchange is performed between the exhaust gas and water or steam flowing within the heat transfer pipes, so that the temperature of the water or steam is raised. Each of the later-described intermediate-pressure steam circulation mechanism  72  and low-pressure steam circulation mechanism  74  also includes an economizer, a steam evaporator, and two superheaters, so being heat exchangers in a similar manner. The components of the high-pressure steam circulation mechanism  70  are each connected by lines (pipes), from the pre-heater  76  to the high-pressure turbine  52 H, in the order of the high-pressure economizer  70 EC, the high-pressure drum  70 D, the high-pressure superheater  70 SHa, and the high-pressure superheater  70 SHb. Further, the high-pressure steam circulation mechanism  70  is disposed having the above-stated order from the downstream side to the upstream side in the flow direction of the exhaust gas of the heat recovery steam generator  51 . In the high-pressure steam circulation mechanism  70 , water pre-heated by the pre-heater  76  is sent to the high-pressure economizer  70 EC by the high-pressure pump  78 H, superheated by the high-pressure economizer  70 EC, and then supplied to the high-pressure drum  70 D. The high-pressure steam evaporator  70 EV is connected with the high-pressure drum  70 D. The high-pressure steam evaporator  70 EV has both ends connected to the high-pressure drum  70 D so as to circulate water accumulated in the high-pressure drum  70 D while raising the temperature of the water with the exhaust gas, thereby generating steam. The steam generated by the high-pressure steam evaporator  70 EV is supplied from the high-pressure drum  70 D to the high-pressure superheater  70 SHa, so as to be further superheated. The steam superheated by the high-pressure superheater  70 SHa is supplied to the high-pressure superheater  70 SHb so as to be further superheated, and then supplied to the high-pressure turbine  52 H. The high-pressure turbine  52 H is driven by the steam supplied from the high-pressure steam circulation mechanism  70 . 
         [0045]    The intermediate-pressure steam circulation mechanism  72  is a mechanism for raising the temperature of the water supplied from the pre-heater  76  with the exhaust gas so as to generate steam and then supplying the steam to the intermediate-pressure turbine  52 I. The intermediate-pressure steam circulation mechanism  72  includes an intermediate-pressure drum  72 D, an intermediate-pressure economizer  72 EC, an intermediate-pressure steam evaporator  72 EV, and an intermediate-pressure superheater  72 SH. The components of the intermediate-pressure steam circulation mechanism  72  are each connected by lines (pipes), from the pre-heater  76  to the intermediate-pressure turbine  52 I, in the order of the intermediate-pressure economizer  72 EC, the intermediate-pressure drum  72 D, and the intermediate-pressure superheater  72 SH. Further, the intermediate-pressure steam circulation mechanism  72  is disposed having the above-stated order from the downstream side to the upstream side in the flow direction of the exhaust gas of the heat recovery steam generator  51 . In the intermediate-pressure steam circulation mechanism  72 , water pre-heated by the pre-heater  76  is sent to the intermediate-pressure economizer  72 EC by the intermediate-pressure pump  78 I, superheated by the intermediate-pressure economizer  72 EC, and then supplied to the intermediate-pressure drum  72 D. The intermediate-pressure steam evaporator  72 EV is connected with the intermediate-pressure drum  72 D. The intermediate-pressure steam evaporator  72 EV has both ends connected to the intermediate-pressure drum  72 D so as to circulate water accumulated in the intermediate-pressure drum  72 D while raising the temperature of the water with the exhaust gas, thereby generating steam. The steam generated by the intermediate-pressure steam evaporator  72 EV is supplied from the intermediate-pressure drum  72 D to the intermediate-pressure superheater  72 SH so as to be further superheated. The steam superheated by the intermediate-pressure superheater  72 SH is supplied to the intermediate-pressure turbine  52 I. The intermediate-pressure turbine  52 I is driven by the steam supplied from the intermediate-pressure steam circulation mechanism  72 . The components of the intermediate-pressure steam circulation mechanism  72  are disposed further on the downstream side in the flow direction of the exhaust gas, relative to the corresponding components of the high-pressure steam circulation mechanism  70 . Accordingly, the temperature and pressure of the steam are lower than those of the high-pressure steam circulation mechanism  70 . 
         [0046]    The low-pressure steam circulation mechanism  74  is a mechanism for raising the temperature of the water supplied from the pre-heater  76  with the exhaust gas so as to generate steam and then supplying the steam to the low-pressure turbine  52 L. The low-pressure steam circulation mechanism  74  includes a low-pressure drum  74 D, a low-pressure economizer  74 EC, a low-pressure steam evaporator  74 EV, and a low-pressure superheater  74 SH. The components of the low-pressure steam circulation mechanism  74  are each connected by lines (pipes), from the pre-heater  76  to the low-pressure turbine  52 L, in the order of the low-pressure economizer  74 EC, the low-pressure drum  74 D, and the low-pressure superheater  74 SH. Further, the low-pressure steam circulation mechanism  74  is disposed having the above-stated order from the downstream side to the upstream side in the flow direction of the exhaust gas of the heat recovery steam generator  51 . In the low-pressure steam circulation mechanism  74 , the water pre-heated by the pre-heater  76  is sent to the low-pressure economizer  74 EC by the low-pressure pump  78 L, superheated by the low-pressure economizer  74 EC, and then supplied to the low-pressure drum  74 D. The low-pressure steam evaporator  74 EV is connected with the low-pressure drum  74 D. The low-pressure steam evaporator  74 EV has both ends connected to the low-pressure drum  74 D so as to circulate water accumulated in the low-pressure drum  74 D while raising the temperature of the water with the exhaust gas, thereby generating steam. The steam generated by the low-pressure steam evaporator  74 EV is supplied from the low-pressure drum  74 D to the low-pressure superheater  74 SH so as to be further superheated. The steam superheated by the low-pressure superheater  74 SH is supplied to the low-pressure turbine  52 L. The low-pressure turbine  52 L is driven by the steam supplied from the low-pressure steam circulation mechanism  74 . The components of the low-pressure steam circulation mechanism  74  are disposed further on the downstream side in the flow direction of the exhaust gas, relative to the corresponding components of the intermediate-pressure steam circulation mechanism  72 . Accordingly, the temperature and pressure of the steam are lower than those of the intermediate-pressure steam circulation mechanism  72 . 
         [0047]    The reheat steam circulation mechanism  79  is a mechanism for heating again the steam that has passed through the high-pressure turbine  52 H with the exhaust gas and supplying the steam to the intermediate-pressure turbine  52 I. The reheat steam circulation mechanism  79  includes reheaters  80   a ,  80   b . The reheaters  80   a ,  80   b  are heat exchangers provided with heat transfer pipes and disposed within a pipeline through which the exhaust gas G flows. Heat exchange is performed between the exhaust gas and steam flowing within the heat transfer pipes, so that the temperature of the steam is raised. The reheaters  80   a ,  80   b  are disposed in an area on the upstream side within the heat recovery steam generator  51 , which is close to the high-pressure superheaters  70 SHa, SHb. Further, the reheater  80   a  is disposed on the downstream side of the reheater  80   b  in the flow direction of the exhaust gas. The reheat steam circulation mechanism  79  superheats the steam that has passed through the high-pressure turbine  52 H with the reheater  80   a , then further superheats the steam with the reheater  80   b , and then supplies the steam to the intermediate-pressure turbine  52 I. The heat recovery steam generator  51  is configured as described above. 
         [0048]      FIG. 3  is a schematic configuration diagram illustrating a heat exchange unit of the power generation system of the embodiment. In the meantime, in a typical power generation system, high-temperature exhaust air is discharged from an air electrode  13   a  of the SOFC  13 . Thus, the compressed air supply line through which the exhaust air flows is exposed to high temperatures. As such, supposing that the exhaust air is supplied as-is to the combustor  22 , the compressed air supply line needs to be manufactured from a heat-resistant material. This raises costs for power generation plants (and power generation systems). 
         [0049]    Therefore, as illustrated in  FIG. 3 , the power generation system  10  of the embodiment is provided with a heat exchange unit  90  performing heat exchange between the exhaust air directed from the air electrode  13   a  of the SOFC  13  toward the gas turbine  11  and the exhaust fuel gas directed from the fuel electrode  13   b  of the SOFC  13  toward the gas turbine  11 , and the steam flowing through the heat recovery steam generator  51  and the turbine  52 . The heat exchange unit  90  includes an exhaust air heat exchanger  91 , an exhaust fuel heat exchanger  92 , a fuel heat exchanger  94 , and a fuel heat exchanger  96 . 
         [0050]    The exhaust air heat exchanger  91  is provided in one of the exhaust air line  34  and the compressed air circulation line  36 , that is, in an exhaust air circulation line circulating the exhaust air discharged from the SOFC  13 . The exhaust air heat exchanger  91  performs heat exchange between the exhaust air flowing through the exhaust air circulation line and the steam superheated by the heat recovery steam generator  51  and directed toward the turbine  52 , so as to lower the temperature of the exhaust air and to raise the temperature of the steam. The exhaust air heat exchanger  91  has flow into it steam which has been superheated by the high-pressure superheater  70 SHa and had its temperature lowered by the fuel heat exchanger  96 , and supplies this steam that has undergone heat exchange to the high-pressure turbine  52 H. Note that in the embodiment, the steam which has been superheated by the high-pressure superheater  70 SHa and had its temperature lowered by the fuel heat exchanger  96  flows into the exhaust air heat exchanger  91 . However, the steam which has been superheated by the high-pressure superheater  70 SHb and had its temperature lowered by the fuel heat exchanger  96  may be used instead. 
         [0051]    The exhaust fuel heat exchanger  92  is provided in one of the exhaust fuel line  43  and the exhaust fuel gas supply line  45 , that is, in an exhaust fuel gas circulation line circulating the exhaust fuel gas discharged from the SOFC  13 . The exhaust fuel heat exchanger  92  performs heat exchange between the exhaust fuel gas flowing through the exhaust fuel gas circulation line and the steam superheated by the heat recovery steam generator  51  and directed toward the turbine  52 , so as to lower the temperature of the exhaust fuel gas and to raise the temperature of the steam. The exhaust fuel exchanger  92  has flow into it steam which has been superheated by the intermediate-pressure superheater  72 SH, and supplies this steam that has undergone heat exchange to the reheater  80   a . The steam supplied to the reheater  80   a  is further superheated by the superheater  80   b  and then supplied to the intermediate-pressure turbine  52 I. 
         [0052]    The fuel heat exchanger  94  is provided in the first fuel gas supply line  27 . The fuel heat exchanger  94  performs heat exchange between the fuel gas flowing through the first fuel gas supply line  27 , that is the fuel gas supplied to the combustor  22 , and the water that has had its temperature raised by the heat recovery steam generator  51 . The fuel heat exchanger  94  raises the temperature of the fuel gas and lowers the temperature of the steam. The fuel heat exchanger  94  has flow into it water which has been superheated by the intermediate-pressure economizer  72 EC, and supplies this water which has undergone heat exchange to the water supply line  55 . 
         [0053]    The fuel heat exchanger  96  is provided further on the downstream side of the first fuel gas supply line  27  in the flow direction of the fuel gas, relative to the fuel heat exchanger  94 , that is, the fuel heat exchanger  96  is provided closer to the combustor  22 . The fuel heat exchanger  96  performs heat exchange between the fuel gas flowing through the first fuel gas supply line  27 , that is the fuel gas supplied to the combustor  22 , and the steam superheated by the heat recovery steam generator  51 . The fuel heat exchanger  96  raises the temperature of the fuel gas and lowers the temperature of the steam. The fuel heat exchanger  96  further raises the temperature of the fuel that has been raised by the fuel heat exchanger  94 . The fuel heat exchanger  96  has flow into it steam which has been superheated by the high-pressure superheater  70 SHa, and supplies, to the exhaust air heat exchanger  91 , this steam with a temperature lowered through heat exchange. The heat exchange unit  90  is configured as described above. 
         [0054]    The power generation system  10  is provided with the exhaust air heat exchanger  91  and performs heat exchange between the steam superheated in the high-pressure steam circulation mechanism  70  and the exhaust air so as to lower the temperature of the exhaust air. This enables the load on the exhaust air circulation line circulating the exhaust air to be reduced. Further, having steam superheated in the high-pressure steam circulation mechanism  70  undergo heat exchange with the exhaust air enables the temperature of the exhaust air to be maintained at or above a high fixed, temperature. Here, the exhaust air is supplied to the combustor  22 , mixed with the exhaust fuel gas and the fuel gas, and heated by the combustor, then passes through the turbine  23  and further passes through the heat recovery steam generator  51 . Therefore, the exhaust air is usable to extract energy for power generation in two places including the gas turbine  11  and the steam turbine  14 . Therefore, maintaining the temperature of the exhaust air higher enables more efficient energy extraction. Accordingly, the power generation system  10  performs heat exchange between the exhaust air and the steam so as to enable the load on the exhaust air circulation line circulating the exhaust air to be reduced, and, in addition, makes the steam to be used for heat exchange serve as a high-temperature steam and maintains the exhaust air at or above a fixed, high temperature, so as to enable energy to be extracted more efficiently. 
         [0055]    The power generation system  10  is also provided with the exhaust fuel heat exchanger  92  so as to enable heat to be recovered from the exhaust fuel gas and to enable the load on the exhaust fuel gas circulation line to be reduced like the exhaust air heat exchanger  91 . The exhaust fuel heat exchanger  92  also makes the steam to be used for heat exchange serve as the steam superheated by the intermediate-pressure superheater  72 SH of the intermediate-pressure steam circulation mechanism  72 , so as to enable the exhaust fuel gas to be maintained at or above a fixed, high temperature. Accordingly, the power generation system  10  performs heat exchange between the exhaust fuel gas and the steam so as to enable the load on the exhaust fuel gas circulation line circulating the exhaust fuel gas to be reduced, and, in addition, makes the steam to be used for heat exchange serve as a high-temperature steam and maintains the exhaust air at or above a fixed, high temperature so as to enable energy to be extracted more efficiently. 
         [0056]    The power generation system  10  is also provided with the fuel heat exchanger  94  and the fuel heat exchanger  96  to raise the temperature of the fuel gas, so as to enable the heat to be extracted as energy for power generation in two places including the gas turbine  11  and the steam turbine  14 . Accordingly, this enables more efficient energy extraction. 
         [0057]    Here, the power generation system  10  may include a path other than the path of the steam flowing into the heat exchange unit. Note that the exhaust air heat exchanger may use the steam that has undergone heat exchange with the exhaust gas in the high-pressure steam circulation mechanism or the intermediate-pressure steam circulation mechanism and flowing toward the turbine. 
         [0058]      FIG. 4  is a schematic configuration diagram illustrating another example of the heat exchange unit of the power generation system of the embodiment. As illustrated in  FIG. 4 , the power generation system  10   a  is provided with a heat exchange unit  90   a  performing heat exchange between the exhaust air and exhaust fuel gas directed from the SOFC  13  toward the gas turbine  11 , and the steam flowing through the heat recovery steam generator  51  and the turbine  52 . The heat exchange unit  90   a  includes an exhaust air heat exchanger  98 , the exhaust fuel heat exchanger  92 , the fuel heat exchanger  94 , and a fuel heat exchanger  99 . 
         [0059]    The exhaust air heat exchanger  98  is provided in the exhaust air circulation line. The exhaust air heat exchanger  98  performs heat exchange between the exhaust air flowing through the exhaust air circulation line and the steam superheated by the heat recovery steam generator  51  and directed toward the turbine  52 , so as to lower the temperature of the exhaust air and to raise the temperature of the steam. The exhaust air heat exchanger  98  has flow into it steam which has been superheated by the reheater  80   b , and supplies this steam that has undergone heat exchange to the intermediate-pressure turbine  52 I. Note that in the embodiment, steam superheated by the reheater  80   b  flows into the exhaust air heat exchanger  98 ; however, steam superheated by the reheater  80   a  may be used instead. The exhaust air heat exchanger  98  which serves to lower the temperature of the steam passing therethrough to a temperature adequate for supply to the intermediate-pressure turbine  52 I may have flow into it the steam which is to be supplied to the intermediate-pressure turbine  52 I and has been superheated by the intermediate-pressure superheater  72 SH like the steam superheated by the reheater  80   b.    
         [0060]    The exhaust fuel heat exchanger  92  is provided in the exhaust fuel gas circulation line. The exhaust fuel heat exchanger  92  performs heat exchange between the exhaust fuel gas flowing through the exhaust fuel gas circulation line and the steam superheated by the heat recovery steam generator  51  and directed toward the turbine  52 , so as to lower the temperature of the exhaust fuel gas and to raise the temperature of the steam. The exhaust fuel heat exchanger  92  has flow into it steam which has been superheated by the intermediate-pressure superheater  72 SH, and supplies this steam that has undergone heat exchange to the fuel heat exchanger  99 . The steam supplied to the fuel heat exchanger  99  is then supplied to the reheater  80   a . The steam supplied to the reheater  80   a  is further superheated by the reheater  80   b  and then supplied to the intermediate-pressure turbine  52 I. 
         [0061]    The fuel heat exchanger  94  is configured similarly to the fuel heat exchanger  94  of the heat exchange unit  90 . 
         [0062]    The fuel heat exchanger  99  is provided further on the downstream side of the first fuel gas supply line  27  in the flow direction of the fuel gas, relative to the fuel heat exchanger  94 , that is, the fuel heat exchanger  99  is provided closer to the combustor  22 . The fuel heat exchanger  99  performs heat exchange between the fuel gas flowing through the first fuel gas supply line  27 , that is the fuel gas supplied to the combustor  22 , and the steam that has passed through the exhaust fuel heat exchanger  92 . The fuel heat exchanger  99  raises the temperature of the fuel gas and lowers the temperature of the steam. The fuel heat exchanger  99  further raises the temperature of the fuel that has been raised by the fuel heat exchanger  94 . The fuel heat exchanger  99  has flow into it steam that has passed through the exhaust fuel heat exchanger  92 , and supplies the flowed steam with temperature lowered through heat exchange to the reheater  80   a . The heat exchange unit  90   a  is configured as described above. 
         [0063]    The power generation system  10   a  is provided with the exhaust air heat exchanger  98 , and performs heat exchange between the exhaust air and the steam superheated by the reheat steam circulation mechanism  79  after being superheated by the high-pressure steam circulation mechanism  70  and supplied to the high-pressure turbine  52 H, so as to lower the temperature of the exhaust air. This also enables the load on the exhaust air circulation line circulating the exhaust air to be reduced. Further, having steam superheated in the reheat steam circulation mechanism  79  undergo heat exchange with the exhaust air enables the temperature of the exhaust air to be maintained at or above a high fixed, temperature. Accordingly, the power generation system  10   a  performs heat exchange between the exhaust air and the steam, so as to enable the load on the exhaust air circulation line circulating the exhaust air to be reduced and, in addition, makes the steam to be used for heat exchange serve as a high temperature steam and maintains the exhaust air at or above a fixed, high temperature, so as to enable energy to be extracted more efficiently. 
         [0064]    The power generation system  10   a  also performs heat exchange between the steam superheated by the high-pressure steam circulation mechanism  70  and directed toward the high-pressure turbine  52 H, and the exhaust air and the steam superheated by the reheat steam circulation mechanism  79  and directed toward the intermediate-pressure turbine  52 I, so as to enable the temperature of the exhaust air to be lowered to a more appropriate temperature range; thus it is preferable, but, as described above, the steam superheated by the intermediate-pressure steam circulation mechanism  72  and directed toward the intermediate-pressure turbine  52 I may be used. Note that it is preferable that the steam serve as steam superheated by at least one stage superheater or reheater. 
         [0065]    Next,  FIG. 5  is a schematic configuration diagram illustrating another example of the heat exchange unit of the power generation system of the embodiment. As illustrated in  FIG. 5 , the power generation system  10   b  is provided with a heat exchange unit  90   b  performing heat exchange between the exhaust air and exhaust fuel gas directed from the SOFC  13  toward the gas turbine  11 , and the steam flowing through the heat recovery steam generator  51  and the turbine  52 . The heat exchange unit  90   b  is provided with the exhaust air heat exchanger  91 , the exhaust air heat exchanger  98 , the exhaust fuel heat exchanger  92 , the fuel heat exchanger  94 , and the fuel heat exchanger  96 . That is, the power generation system  10   b  is constituted of the power generation system  10  with the addition of the exhaust air heat exchanger  98 . Note that into the exhaust air heat exchanger  98  flows steam that has been superheated by the reheater  80   a.    
         [0066]    The power generation system  10   b  uses both the exhaust air heat exchanger  91  and the exhaust air heat exchanger  98 , so as to enable the temperature of the exhaust air to be lowered with the both heat exchangers. Such a heat exchanger is not limited to one stage and may also be provided in plurality. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10 ,  10   a ,  10   b  Power generation system 
           11  Gas turbine 
           12  Power generator 
           13  Solid Oxide Fuel Cell (SOFC) 
           14  Steam turbine 
           15  Power generator 
           21  Compressor 
           22  Combustor 
           23  Turbine 
           25  Air intake line 
           26  First compressed air supply line 
           27  First fuel gas supply line 
           31  Second compressed air supply line 
           32  Control valve 
           33 ,  48  Blower 
           34  Exhaust air line 
           36  Compressed air circulation line 
           41  Second fuel gas supply line 
           42  Control valve 
           43  Exhaust fuel line 
           44  Waste line 
           45  Exhaust fuel gas supply line 
           47  Control valve 
           49  Fuel gas recirculation line 
           50  Recirculation blower 
           51  Heat recovery steam generator 
           52  Turbine 
           52 H High-pressure turbine 
           52 I Intermediate-pressure turbine 
           52 L Low-pressure turbine 
           53  Exhaust gas line 
           54  Steam supply line 
           55  Water supply line 
           56  Condenser 
           57  Water supply pump 
           62  Control device 
           70  High-pressure steam circulation mechanism 
           70 D High-pressure drum 
           70 EC High-pressure economizer 
           70 EV High-pressure steam evaporator 
           70 SHa,  70 SHb High-pressure superheater 
           72  Intermediate-pressure steam circulation mechanism 
           72 D Intermediate-pressure drum 
           72 EC Intermediate-pressure economizer 
           72 EV Intermediate-pressure steam evaporator 
           72 SH Intermediate-pressure superheater 
           74  Low-pressure steam circulation mechanism 
           74 D Low-pressure drum 
           74 EC Low-pressure economizer 
           74 EV Low-pressure steam evaporator 
           74 SH Low-pressure superheater 
           76  Pre-heater 
           78 H High-pressure pump 
           78 I Intermediate-pressure pump 
           78 L Low-pressure pump 
           79  Reheat steam circulation mechanism 
           80   a ,  80   b  Reheater 
           90  Heat exchange unit 
           91 ,  98  Exhaust air heat exchanger 
           92  Exhaust fuel heat exchanger 
           94 ,  96 ,  99  Fuel heat exchanger