Patent Publication Number: US-11391201-B2

Title: Plant and plant operation method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a plant and a plant operation method. 
     BACKGROUND ART 
     Techniques have been developed to improve the output and the efficiency of a plant. 
     For instance, Patent Document 1 discloses a plant configured to obtain high-pressure fume by pressurizing high-temperature and low-pressure fume (combustion gas) discharged from a gas turbine, and to recover expansion energy by expanding fume after separating and recovering CO 2  from the high-pressure fume using an expander. Further, in the above plant, fume is intermediate-cooled by a heat exchanger disposed between a plurality of stages of compressors. As a cooling medium which exchanges heat with fume is heated by the heat exchanger, a cycle is driven where the cooling medium is a working fluid. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Document 1: JP2005-2996A 
       
    
     SUMMARY 
     Problems to be Solved 
     Meanwhile, in order to improve the output and the efficiency of a plant, it is considered desirable to effectively utilize pressure energy of high-pressure fuel gas from a pipe line, for instance. 
     However, Patent Document 1 does not disclose any technique to effectively utilize pressure energy of high-pressure fuel gas. 
     In view of the above, an object of at least one embodiment of the present invention is to provide a plant and a plant operation method whereby it is possible to improve the output and the efficiency of a plant as a whole by effectively utilizing pressure energy of high-pressure fuel gas. 
     Solution to the Problems 
     (1) According to at least one embodiment of the present invention, a plant includes: a fuel supply line for supplying high-pressure fuel gas; and at least one expander disposed in the fuel supply line and configured to extract power from the high-pressure fuel gas by expanding the high-pressure fuel gas. 
     With the above configuration (1), the pressure of the high-pressure fuel gas is utilized and extracted as power, and thus it is possible to improve the output and the efficiency of the plant as a whole. 
     (2) In some embodiments, in the above configuration (1), the plant includes a heater, disposed in the fuel-supply line at an upstream side of an expander of the at least one expander, for heating the high-pressure fuel gas flowing into the expander. 
     With the above configuration (2), by providing the heater at the upstream side of the expander, it is possible to extract more power at the expander. Accordingly, it is possible to improve the output and the efficiency of the plant as a whole even further. 
     (3) In some embodiments, in the above configuration (1) or (2), the at least one expander includes a plurality of expanders disposed so as to be arranged in a flow direction of the high-pressure fuel gas, and the plant includes a plurality of heaters, each disposed in the fuel-supply line at an upstream side of corresponding one of the plurality of expanders, for heating the high-pressure fuel gas flowing into the corresponding expander. 
     With the above configuration (3), by providing the heater at the upstream side of each expander, it is possible to extract more power at each expander. Furthermore, it is possible to use waste heat having a relatively low temperature effectively as a heat source used for the heaters to increase the power to be recovered at the respective expanders. Accordingly, it is possible to improve the output and the efficiency of the plant as a whole even further. 
     (4) In some embodiments, in the above configuration (2) or (3), the plant includes a CO 2  rich gas line through which CO 2  rich gas flows; and at least one compressor, disposed in the CO 2  rich gas line, for pressurizing the CO 2  rich gas. The heater is configured to heat the high-pressure fuel gas by using waste heat of the at least one compressor. 
     With the above configuration (4), by pressurizing the CO 2  rich gas with the compressor, it is possible to use the pressurized CO 2  rich gas in enhanced oil recovery (EOR), or seal and fix CO 2  in the rock ground or under the sea. Further, by heating high-pressure fuel gas in the heater by utilizing waste heat of the compressor for pressurizing CO 2  rich gas, it is possible to recover more power at the expander, and improve the output and the efficiency of the plant as a whole even further. 
     (5) In some embodiments, in the above configuration (4), the plant is configured such that the at least one compressor includes a plurality of compressors disposed in series in the CO 2  rich gas line, and the heater is configured to heat the high-pressure fuel gas through heat exchange with the CO 2  rich gas flowing between a pair of adjacent compressors among the plurality of compressors. 
     With the above configuration (5), by heating high-pressure fuel gas through heat exchange with CO 2  rich gas flowing between a pair of compressors, it is possible to recover waste heat of the compressor to the high-pressure fuel gas and extract more power at the expander. Furthermore, CO 2  rich gas that is cooled through heat exchange with the high-pressure fuel gas in the heater flows into the compressor positioned downstream of the heater in the CO 2  rich gas line. Thus, the heater functions as an intermediate cooler of the compressor, and it is possible to cut power required to operate the compressor. 
     (6) In some embodiments, in the above configuration (5), the at least one compressor includes: a plurality of upstream compressors disposed in the CO 2  rich gas line with the heater interposed between a pair of adjacent compressors; and at least one downstream compressor disposed in the CO 2  rich gas line at a downstream side of the upstream compressors. The plant further includes a heat exchanger disposed in the CO 2  rich gas line between the downstream compressor and a most downstream compressor of the plurality of upstream compressors, or between a pair of the downstream compressors, the heat exchanger being configured to cool the CO 2  rich gas by using a cooling medium other than the high-pressure fuel gas. 
     The CO 2  rich gas tends to have a greater isobaric specific heat Cp near the critical pressure. Thus, when cooling CO 2  rich gas having a pressure level equivalent to the critical pressure through heat exchange with the high-pressure fuel gas, it is difficult to ensure balance between the temperature decrease amount of CO 2  rich gas and the temperature increase amount of high-pressure fuel gas. 
     In this regard, with the above configuration (6), by cooling CO 2  rich gas that is pressurized by the upstream compressor through heat exchange with a cooling medium other than high-pressure fuel gas, it is possible to cool inlet gas of the downstream compressor appropriately, and reduce compression power at the downstream compressor. 
     (7) In some embodiments, in any one of the above configurations (4) to (6), the plant includes a CO 2  separation device, disposed in the CO 2  rich gas line at a downstream side of the at least one compressor, for separating CO 2  from the CO 2  rich gas pressurized by the at least one compressor. 
     With the above configuration (7), by separating CO 2  from CO 2  rich gas using the CO 2  separation device, it is possible to obtain CO 2  with a high purity. Further, in a case where the CO 2  rich gas contains combustible gas as an impurity substance, it is possible to utilize the impurity gas obtained by the CO 2  separation device as a fuel, which may contribute to improvement of the energy efficiency of a plant as a whole. 
     (8) In some embodiments, in any one of the above configurations (4) to (7), the plant includes a fuel cell which includes an anode, a cathode supplied with exhaust gas containing carbon dioxide, and an electrolyte configured to transfer carbonate ion derived from the carbon dioxide contained in the exhaust gas from the cathode to the anode. The at least one compressor is configured to compress the CO 2  rich gas derived from an outlet gas of the anode. The anode of the fuel cell is configured to be supplied with the high-pressure fuel gas from which power has been recovered by the at least one expander. 
     With the above configuration (8), it is possible to recover CO 2  while generating power with the fuel cell, and thus it is possible to suppress reduction of the energy efficiency of a plant as a whole upon CO 2  recovery. Furthermore, by compressing the CO 2  rich gas, it is possible to utilize at least the carbon dioxide recovered with the fuel cell in EOR, or solidify the same in the rock ground or under the sea. Furthermore, while the supply pressure of fuel gas to the anode of the fuel cell does not need to be so high, it is possible to improve the energy efficiency of the plant as a whole by recovering the pressure of high-pressure fuel gas supplied to the anode as power at the expander. 
     (9) In some embodiments, in any one of the above configurations (4) to (8), the at least one compressor is configured to be driven by using the power extracted by the at least one expander. 
     With the above configuration (9), by driving the compressor by using power recovered from high-pressure fuel gas using the expander, it is possible to improve the energy efficiency of the plant as a whole. 
     Furthermore, power may be transmitted from the rotational shaft of the expander to the rotational shaft of the compressor via a power transmission mechanism. Alternatively, the electric motor coupled to the rotational shaft of the compressor may be driven by electric power generated by the generator coupled to the rotational shaft of the expander. 
     (10) According to at least one embodiment of the present invention, a method for operating a plant includes: a step of supplying high-pressure fuel gas via a fuel-supply line; and a step of extracting power from the high-pressure fuel gas by expanding the high-pressure fuel gas by using at least one expander disposed in the fuel-supply line. 
     According to the above method (10), the pressure of the high-pressure fuel gas is utilized and extracted as power, and thus it is possible to improve the output and the efficiency of the plant as a whole. 
     (11) In some embodiments, the above method (10) includes a step of heating the high-pressure fuel gas flowing into the expander by using a heater disposed in the fuel-supply line at an upstream side of an expander of the at least one expander. 
     According to the above method (11), the heater provided at the upstream side of the expander is used to heat the high-pressure fuel gas flowing into the expander, and thereby it is possible to extract more power at the expander. Accordingly, it is possible to improve the output and the efficiency of the plant as a whole even further. 
     (12) In some embodiments, the above method (11) further includes a step of pressurizing the CO 2  rich gas by using at least one compressor disposed in a CO 2  rich gas line through which CO 2  rich gas flows. The step of heating the high-pressure fuel gas includes heating the high-pressure fuel gas using waste heat of the at least one compressor. 
     According to the above method (12), by pressurizing the CO 2  rich gas with the compressor, it is possible to use the pressurized CO 2  rich gas in enhanced oil recovery (EOR), or seal and fix CO 2  in the rock ground or under the sea. Further, by heating high-pressure fuel gas in the heater by utilizing waste heat of the compressor for pressurizing CO 2  rich gas, it is possible to recover more power at the expander, and improve the output and efficiency of the plant as a whole even further. 
     (13) In some embodiments, in the above method (12), the at least one compressor includes a plurality of compressors disposed in series in the CO 2  rich gas line, and the step of heating the high-pressure fuel gas includes heating the high-pressure fuel gas through heat exchange with the CO 2  rich gas flowing between a pair of adjacent compressors of the plurality of compressors. 
     According to the above method (13), by heating high-pressure fuel gas through heat exchange with CO 2  rich gas which flows between a pair of compressors, it is possible to recover waste heat of the compressor to the high-pressure fuel gas and extract more power at the expander. Furthermore, CO 2  rich gas that is cooled through heat exchange with the high-pressure fuel gas in the heater flows into the compressor positioned downstream of the heater in the CO 2  rich gas line. Thus, the heater functions as an intermediate cooler of the compressor, and it is possible to cut power required to operate the compressor. 
     (14) In some embodiments, in the above method (13), the at least one compressor includes: a plurality of upstream compressors disposed in the CO 2  rich gas line with the heater interposed between a pair of adjacent compressors; and at least one downstream compressor disposed in the CO 2  rich gas line at a downstream side of the upstream compressors. The method further includes a step of cooling the CO 2  rich gas by using a cooling medium other than the high-pressure fuel gas at a heat exchanger disposed between the downstream compressor and a most downstream compressor of the plurality of upstream compressors, or between a pair of the downstream compressors in the CO 2  rich gas line. 
     The CO 2  rich gas tends to have a greater isobaric specific heat Cp near the critical pressure. Thus, when cooling CO 2  rich gas having a pressure level equivalent to the critical pressure through heat exchange with the high-pressure fuel gas, it is difficult to ensure balance between the temperature decrease amount of CO 2  rich gas and the temperature increase amount of high-pressure fuel gas. 
     In this regard, according to the above method (14), by cooling CO 2  rich gas pressurized by the upstream compressor through heat exchange with a cooling medium other than high-pressure fuel gas, it is possible to cool inlet gas of the upstream compressor appropriately, and reduce compression power at the upstream compressor. 
     (15) In some embodiments, any one of the above methods (12) to (14) includes a step of separating CO 2  from the CO 2  rich gas pressurized by the at least one compressor by using a CO 2  separation device disposed in the CO 2  rich gas line at a downstream side of the at least one compressor. 
     According to the above method (15), by separating CO 2  from CO 2  rich gas using the CO 2  separation device, it is possible to obtain CO 2  with a high purity. Further, in a case where the CO 2  rich gas contains combustible gas as an impurity substance, it is possible to utilize the impurity gas obtained by the CO 2  separation device as a fuel, which may contribute to improvement of the energy efficiency of a plant as a whole. 
     (16) In some embodiments, any one of the above methods (12) to (15) further includes: a step of supplying at least a part of exhaust gas containing carbon dioxide to a cathode of a fuel cell; a step of transferring carbonate ion derived from the carbon dioxide contained in the exhaust gas from the cathode to an anode of the fuel cell through an electrolyte of the fuel cell; and a step of supplying the anode of the fuel cell with the high-pressure fuel gas from which power has been recovered by the at least one expander. The step of pressurizing the CO 2  rich gas includes compressing the CO 2  rich gas derived from an outlet gas of the anode by using the at least one compressor. 
     According to the above method (16), it is possible to recover CO 2  while generating power with the fuel cell, and thus it is possible to suppress reduction of the energy efficiency of a plant as a whole upon CO 2  recovery. Furthermore, by compressing the CO 2  rich gas, it is possible to utilize at least the carbon dioxide recovered with the fuel cell in EOR, or solidify the same in the rock ground or under the sea. Furthermore, while the supply pressure of fuel gas to the anode of the fuel cell does not need to be so high, it is possible to improve the energy efficiency of the plant as a whole by recovering the pressure of high-pressure fuel gas supplied to the anode as power at the expander. 
     Advantageous Effects 
     According to at least one embodiment of the present invention, it is possible to provide a plant and a plant operation method whereby it is possible to improve the output and the efficiency of a plant as a whole by effectively utilizing pressure energy of high-pressure fuel gas. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of a plant according to an embodiment. 
         FIG. 2  is a schematic configuration diagram of a plant according to an embodiment. 
         FIG. 3  is a schematic configuration diagram of a plant according to an embodiment. 
         FIG. 4  is a schematic configuration diagram of a plant according to an embodiment. 
         FIG. 5  is a schematic configuration diagram of a plant according to an embodiment. 
         FIG. 6  is a schematic configuration diagram of a plant according to an embodiment. 
         FIG. 7  is a schematic configuration diagram of a plant according to an embodiment. 
         FIG. 8  is a schematic configuration diagram of a CO 2  separation device according to an embodiment. 
         FIG. 9  is a schematic configuration diagram of a CO 2  separation device according to an embodiment. 
         FIG. 10  is a schematic configuration diagram of a plant according to an embodiment. 
         FIG. 11  is a schematic configuration diagram of a plant according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly identified, dimensions, materials, shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention. 
       FIGS. 1 to 7, 10, and 11  are each a schematic configuration diagram of a plant according to an embodiment. As depicted in  FIGS. 1 to 7, 10, and 11 , a plant  1  according to some embodiments includes a fuel supply line  2  for supplying high-pressure fuel gas and at least one expander  4  disposed in the fuel supply line  2 . The fuel supply line  2  includes an upstream fuel supply line  2   a  positioned upstream of the at least one expander  4  and a downstream fuel supply line  2   b  positioned downstream of the at least one expander  4 . The expander  4  is configured to expand high-pressure fuel gas that flows in from the upstream fuel supply line  2   a  and extract power from the high-pressure fuel gas. 
     In  FIGS. 10 and 11 , the plant  1  includes a thermal power generation device  102  including a gas turbine  200 , and a CO 2  recovery system  103  configured to recover carbon dioxide (CO 2 ) contained in exhaust gas from the thermal power generation device  102 . In  FIGS. 10 and 11 , the expander  4  is not depicted. The configurations of the thermal power generation device  102  and the CO 2  recovery system  103  will be described later. 
     The high-pressure fuel gas is a fuel gas having a higher pressure than normal pressure, and may have a pressure of approximately 1 MPa to 20 MPa at the inlet of the expander  4  (in a case where the at least one expander  4  includes a plurality of expanders  4 , the inlet of the most upstream expander  4 ). For instance, the fuel gas may be a natural gas, or a syngas containing CO or Hz, for instance, obtained by processing coal or biomass in a gasifier. 
     For instance, in an illustrative embodiment depicted in  FIG. 2 , a fuel tank  10  is connected to the upstream fuel supply line  2   a , and a pump  12  and a heat exchanger  14  are disposed in the upstream fuel supply line  2   a . Further, the liquefied natural gas stored in the fuel tank  10  is gasified by the heat exchanger  14  after being pressurized by the pump  12 , and then flows into the expander  4  as a high-pressure fuel gas. The heat exchanger  14  may be configured to gasify a liquefied natural gas through heat exchange with a heat source such as air and sea water. 
     Further, for instance, in an illustrative embodiment depicted in  FIG. 3 , the upstream fuel supply line  2   a  is connected to a gas discharge port of a gasification furnace  20  for gasification processing of coal or biomass, for instance. Further, a syngas obtained by processing coal or biomass, for instance, in the gasification furnace  20  flows into the expander  4  as a high-pressure fuel gas. 
     Further, the gasification furnace  20  may be supplied with air for combusting coal or biomass, for instance. The air may be supplied to the gasification furnace  20  after being compressed by the compressor  16 , as depicted in  FIG. 3 . The compressor  16  may be driven by an electric motor, or driven by a turbine. The turbine that drives the compressor  16  may be the above described expander  4 . 
     The expander  4  may be a turbine (rotary-type expander) configured to extract expansion work of a gas as rotational motion, or a reciprocation-type expander configured to extract expansion work of a gas as reciprocal motion. 
     Further, in the illustrative embodiment depicted in  FIGS. 1 to 7 , a turbine configured to be rotary-driven by gas that flows in is used as the expander  4 . Further, as depicted in  FIGS. 1 to 7 , the generator  6  may be connected to the rotational shaft  5  of the turbine (expander  4 ), and the generator  6  may be configured to generate electric power by being rotary-driven by the turbine. 
     Further, as depicted in  FIGS. 6 and 7 , a plurality of expanders  4  may be disposed in series in the fuel supply line  2 . The plurality of expanders  4  may be disposed coaxially and configured such that each expander  4  rotary drives a common generator  6 . 
     Fuel gas from which power has been extracted by the expander  4  and whose pressure has decreased is discharged from the expander  4 , and then is supplied to a device or the like that uses the fuel gas via the downstream fuel supply line  2   b . The pressure of fuel gas at the outlet of the expander  4  (in a case where the at least one expander  4  includes a plurality of expanders  4 , the outlet of the most downstream expander  4 ) may be, for instance, a pressure of approximately 5% to 30% of the pressure of the fuel gas at the inlet of the expander  4  (high-pressure fuel gas), for instance. Alternatively, the pressure of fuel gas at the outlet of the expander  4  may be approximately 0.2 MPa to 1.5 MPa, for instance. 
     The fuel gas may be supplied to a fuel cell (various fuel cells such as MCFC, SOFC, PEFC, PAFC, etc.), a boiler, or a micro gas turbine or the like as a fuel, or, may be supplied to houses or various facilities as city gas. 
     As described above, in the plant  1  according to some embodiments, by using the expander  4  disposed in the fuel supply line  2  to effectively use the pressure of the high-pressure fuel gas and extract the pressure as power, it is possible to improve the output and the efficiency of the plant as a whole. 
     In some embodiments, the plant  1  includes a heater  22  disposed in the fuel supply line  2  at the upstream side of one of the at least one expander  4 . The heater  22  is configured to heat high-pressure fuel gas that flows into the above expander  4  (i.e. the expander  4  positioned downstream of the heater  22  in the fuel supply line  2 ). 
     For instance, in an illustrative embodiment depicted in  FIG. 4 , one expander  4  is disposed in the fuel supply line  2 , and the heater  22  is disposed in the fuel supply line at the upstream side of the expander  4 . 
     Further, for instance, in the illustrative embodiment depicted in  FIGS. 5 to 7 , a plurality of expanders  4  are disposed in the fuel supply line  2 , arranged in the flow direction of the high-pressure fuel gas. Further, in the fuel supply line  2 , a plurality of heaters  22  corresponding to the respective expanders  4  are disposed at the upstream side of the respective expanders  4 . 
     As described above, by providing the heater  22  at the upstream side of the expander  4 , it is possible to extract more power at the expander  4 . Accordingly, it is possible to improve the output and the efficiency of the plant as a whole even further. 
     Furthermore, as depicted in  FIGS. 5 to 7 , by providing the plurality of heaters  22  upstream of the respective expanders  4 , it is possible to extract more power per expander  4 . Furthermore, by providing a plurality of heaters  22  corresponding to the respective expanders  4 , it is possible to use waste heat having a relatively low temperature effectively as a heat source for the heaters  22  to increase the power to be recovered at the respective expanders  4 . Accordingly, it is possible to improve the output and the efficiency of the plant as a whole even further. 
     In some embodiments, as depicted in  FIGS. 6, 7, 10, and 11 , the plant  1  includes a CO 2  rich gas line  24  through which CO 2  rich gas flows, and at least one compressor  26 A and/or  26 B (hereinafter, referred to as compressor ( 26 A,  26 B)) disposed in the CO 2  rich gas line  24 . The CO 2  rich gas line  24  includes an upstream CO 2  rich gas line  24   a  positioned upstream of the at least one compressor ( 26 A,  26 B), and a downstream CO 2  rich gas line  24   b  positioned downstream of the at least one compressor ( 26 A,  26 B). The compressor ( 26 A,  26 B) is configured to increase the pressure of CO 2  rich gas that flows through the CO 2  rich gas line  24 . Further, the heater  22  is configured to heat high-pressure fuel gas flowing through the fuel supply line  2  with waste heat of the compressor ( 26 A,  26 B). That is, at the heater  22 , high-pressure fuel gas flowing through the fuel supply line  2  is heated, through heat exchange with CO 2  rich gas that has its pressure increased through compression at the compressor ( 26 A,  26 B) positioned upstream of the heater  22  in the CO 2  rich gas line  24 . 
     In  FIGS. 10 and 11 , the compressor ( 26 A,  26 B) is not depicted. 
     The compressor ( 26 A,  26 B) may be driven by an electric motor  28  connected via a rotational shaft  27 . 
     In the illustrative embodiments depicted in  FIGS. 6 and 7 , a plurality of compressors ( 26 A,  26 B) are disposed in series in the CO 2  rich gas line. As depicted in  FIGS. 6 and 7 , a plurality of compressors ( 26 A,  26 B) may be disposed coaxially, and the respective compressors ( 26 A,  26 B) may be driven by a common electric motor  28 . 
     CO 2  rich gas that flows through the CO 2  rich gas line may be a gas that has a higher CO 2  concentration than high-pressure fuel gas that flows through the fuel supply line  2 . 
     Alternatively, CO 2  rich gas that flows through the CO 2  rich gas line  24  may be CO 2  rich gas that is generated during the process of recovering CO 2  from exhaust gas containing CO 2  discharged from an exhaust gas generation facility. In this case, the CO 2  rich gas may be a gas that has a higher CO 2  concentration than the exhaust gas to be processed. 
     For instance, in the illustrative embodiments depicted in  FIGS. 10 and 11 , CO 2  contained in exhaust gas from the gas turbine  200  is recovered via the cathode  112  and the anode  116  of the fuel cell  110 , and anode outlet gas that flows out from the anode  116  as CO 2  rich gas is guided to the CO 2  rich gas line  24 . 
     By pressurizing the CO 2  rich gas with the compressor ( 26 A,  26 B) as described above, it is possible to use pressurized CO 2  rich gas in enhanced oil recovery (EOR), or seal and fix CO 2  in the rock ground or under the sea. Further, by heating high-pressure fuel gas at the heater  22  by utilizing waste heat of the compressor  26  ( 26 A,  26 B) for pressurizing CO 2  rich gas, it is possible to recover more power at the expander  4 , and improve the output and efficiency of the plant as a whole even further. 
     In some embodiments, for instance, as depicted in  FIGS. 6 and 7 , in the CO 2  rich gas line  24 , the heater  22  is disposed between a pair of adjacent compressors  26 A among a plurality of compressors  26 A disposed in series. At the heater  22 , the high-pressure fuel gas flowing through the fuel supply line  2  is heated through heat exchange with CO 2  rich gas that flows between the above described pair of adjacent compressors  26 A. 
     As described above, by heating high-pressure fuel gas through heat exchange with CO 2  rich gas which flows between a pair of compressors  26 A, it is possible to recover waste heat of the compressor  26 A to the high-pressure fuel gas and extract more power at the expander  4 . Furthermore, CO 2  rich gas that is cooled through heat exchange with the high-pressure fuel gas at the heater  22  flows into the compressor  26 A positioned downstream of the heater  22  in the CO 2  rich gas line, and thus the heater  22  functions as an intermediate cooler of the compressor  26 A, which makes it possible to cut power required to operate the compressor  26 A. 
     In some embodiments, as depicted in  FIG. 6 , the at least one compressor ( 26 A,  26 B) disposed in the CO 2  rich gas line  24  includes a plurality of compressors  26 A (upstream compressors) and at least one compressor  26 B (downstream compressor) positioned downstream of the plurality of compressors  26 A in the CO 2  rich gas line  24 . In the CO 2  rich gas line  24 , a heater  22  is interposed between a pair of adjacent compressors  26 A of the plurality of compressors  26 A (upstream compressors). Further, in the CO 2  rich gas line, a heat exchanger  34  is disposed between the compressor  26 B (downstream compressor) and the compressor  26 A′ positioned most downstream among the plurality of compressors  26 A (upstream compressors), or between a pair of compressors  26 B (downstream compressors). A cooling medium other than high-pressure fuel gas is guided to the heat exchanger  34  via the cooling medium line  32 . The heat exchanger  34  is configured to cool CO 2  rich gas that flows through the CO 2  rich gas line  24  through heat exchange with the cooling medium. 
     Further, in an illustrative embodiment depicted in  FIG. 6 , one compressor  26 B (downstream compressor) is disposed downstream of the plurality of compressors  26 A (upstream compressors) in the CO 2  rich gas line  24 . In the CO 2  rich gas line  24 , a heat exchanger  34  is disposed between the compressor  26 B and the compressor  26 A′ positioned most downstream of the plurality of compressors  26 A. 
     In some embodiments, the plant  1  includes a boiler (not depicted) for generating steam, and the heat exchanger  34  may be configured such that water supplied to the boiler for generating steam is guided to the heat exchanger  34  via a cooling medium line  32  as a cooling medium. 
     Further, the above described boiler may be a waste-heat recovery boiler (HRSG) for recovering heat of waste gas from the gas turbine. 
     The CO 2  rich gas tends to have a greater isobaric specific heat Cp near the critical pressure of CO 2  (approximately 7.4 MPa). Thus, when cooling CO 2  rich gas having a pressure level equivalent to the critical pressure through heat exchange with the high-pressure fuel gas, it is difficult to ensure balance between the temperature decrease amount of CO 2  rich gas and the temperature increase amount of high-pressure fuel gas. 
     In this regard, as described above, by cooling CO 2  rich gas pressurized by the compressor  26 A (upstream compressor) through heat exchange with a cooling medium other than high-pressure fuel gas, it is possible to cool inlet gas of the compressor  26 B (downstream compressor) appropriately, and reduce compression power at the compressor  26 B (downstream compressor). 
     In some embodiments, the pressure of CO 2  rich gas flowing through the heat exchanger  34  for exchanging heat between CO 2  rich gas and a cooling medium other than the high-pressure fuel gas may be equal to or higher than 90% of the critical pressure of CO 2 . 
     In this case, by cooling CO 2  rich gas having a pressure close to the critical pressure through heat exchange with a cooling medium other than high-pressure fuel gas, it is possible to cool inlet gas of the compressor  26 B (downstream compressor) having a comparatively high isobaric specific heat Cp effectively, and reduce compression power at the compressor  26 B (downstream compressor). 
     Although not depicted, the heater  22  and/or heat exchanger  34  may be configured to be capable of separating drain water generated from condensation of moisture contained in CO 2  rich gas. In this way, it is possible to suppress damage due to erosion or the like of the compressor ( 26 A,  26 B) that compresses CO 2  rich gas. 
     In some embodiments, at least one compressor ( 26 A,  26 B) may be configured to be driven by using power extracted by the at least one expander  4 . 
     As described above, by driving the compressor ( 26 A,  26 B) by using power recovered from high-pressure fuel gas using the expander  4 , it is possible to improve the energy efficiency of the plant as a whole. 
     For instance, in an embodiment, the rotational shaft  27  (see  FIG. 6  or  FIG. 7 ) of the compressor ( 26 A,  26 B) and the rotational shaft  5  of the expander  4  (see  FIG. 6  or  FIG. 7 ) may be connected via a power transmission mechanism (e.g. gear; not depicted). Further, the compressor ( 26 A,  26 B) may be driven at least partially through transmission of power from the rotational shaft  5  of the expander  4  to the rotational shaft  27  (see  FIG. 6  or  FIG. 7 ) of the compressor ( 26 A,  26 B) via the power transmission mechanism. 
     In this case, the amount of power for driving the compressor ( 26 A,  26 B) may be, for instance, adjusted by a generator  6  coupled to the rotational shaft  5  of the expander  4 , or an electric motor  28  connected to the rotational shaft  27  of the compressor ( 26 A,  26 B). 
     Further, for instance, in an embodiment, the electric motor  28  coupled to the rotational shaft  27  of the compressor ( 26 A,  26 B) may be driven by electric power generated by the generator  6  coupled to the rotational shaft  5  of the expander  4 . 
     In this case, the electric motor  28  and the generator  6  may be connected to each other via an electric cable (not depicted), and the electric motor  28  and the generator  6  may be connected to a utility grid via the electric cable. Further, the amount of electric power for driving the compressor ( 26 A,  26 B) may be adjusted through power supply to the utility grid and power input from the utility grid. 
     In  FIGS. 6 and 7 , a plurality of expanders  4  are coupled coaxially and directly via the rotational shaft  5 , and a plurality of compressors ( 26 A,  26 B) are coupled coaxially and directly via the rotational shaft  27 , and thereby power is transmitted between the plurality of expanders  4  and the plurality of compressors ( 26 A,  26 B). Nevertheless, the power transmission between the plurality of expanders  4  and the plurality of compressors ( 26 A,  26 B) is not limited to this. 
     For instance, the plurality of expanders  4  and/or the plurality of compressors ( 26 A,  12 B) may have respective rotational shafts connected via a power transmission mechanism such as a gear. Furthermore, a generator and/or an electric motor corresponding to each of the plurality of expanders  4  and/or the plurality of compressors ( 26 A,  12 B) may be provided, and each electric motor may drive the plurality of compressors ( 26 A,  26 B). The above electric motors may be supplied with electric power generated by a generator driven by the expander  4 . 
     In some embodiments, as depicted in  FIG. 7 , the plant  1  includes a CO 2  separation device  40  disposed in the CO 2  rich gas line  24  at a downstream side of the compressor ( 26 A,  27 B). The CO 2  separation device  40  is configured to separate CO 2  from CO 2  rich gas pressurized by the compressor ( 26 A,  26 B). 
     By separating CO 2  from CO 2  rich gas using the CO 2  separation device  40 , it is possible to obtain CO 2  with a high purity. Further, in a case where the CO 2  rich gas contains combustible gas (e.g. H2 or CO) as an impurity substance, it is possible to utilize the impurity gas obtained by the CO 2  separation device  40  as a fuel, which may contribute to improvement of the energy efficiency of a plant as a whole. 
     Herein,  FIGS. 8 and 9  are each a schematic configuration diagram of a CO 2  separation device according to another embodiment. 
     In the illustrative embodiment depicted in  FIGS. 7 to 9 , CO 2  rich gas pressurized by the compressor ( 26 A,  26 B) is guided to the CO 2  separation device  40  via the downstream CO 2  rich gas line  24   b . Further, CO 2  separated from the CO 2  rich gas is discharged from the CO 2  separation device  40  via the CO 2  recovery line  24   c  of the CO 2  rich gas line  24 . Further, the remaining impurity gas obtained by excluding CO 2  from the CO 2  rich gas is discharged from the CO 2  separation device  40  via the discharge line  30 . The impurity gas may contain H 2 , CO, or N 2 , for instance. 
     In some embodiments, as depicted in  FIG. 7  for instance, the CO 2  separation device  40  includes a CO 2  separation membrane  42  configured to separate CO 2  from the above described CO 2  rich gas. 
     In the illustrative embodiment depicted in  FIG. 7 , the CO 2  separation membrane  42  is configured to selectively let CO 2  permeate and separate from CO 2  rich gas containing CO 2  and components other than CO 2  (e.g. H 2  or N 2 ) by utilizing the pressure difference across the CO 2  separation membrane  42  (e.g. CO 2  partial pressure difference). The CO 2  rich gas compressed by the compressor ( 26 A,  26 B) has a high pressure and thus it is possible to separate CO 2  effectively from CO 2  rich gas by using the CO 2  separation membrane  42 . 
     At the CO 2  separation device  40 , CO 2  having been separated from the CO 2  rich gas and permeated the CO 2  separation membrane  42  is discharged from the CO 2  separation device  40  via the CO 2  recovery line  24   c , and the remaining impurity gas that does not permeate the CO 2  separation membrane  42  is discharged from the CO 2  separation device  40  via the discharge line  30 . 
     Further, in some embodiments, as depicted in  FIGS. 8 and 9  for instance, the CO 2  separation device  40  includes a CO 2  liquefaction/solidification device  44  configured to separate CO 2  from the above described CO 2  rich gas. The CO 2  liquefaction/solidification device  44  is configured to cool CO 2  rich gas through heat exchange with a cooling medium from a freezer  46 . Further, after CO 2  contained in the CO 2  rich gas is cooled to be liquefied or solidified, the CO 2  is discharged from the CO 2  separation device  40  via the CO 2  recovery line  24   c . Meanwhile, of the components contained in the CO 2  rich gas, impurity components (e.g. H 2  or CO) that have a lower solidification point or boiling point than CO 2  is discharged from the CO 2  separation device  40  via the discharge line  30  while remaining in the gas state. 
     In the illustrative embodiment depicted in  FIG. 8 , the CO 2  liquefaction/solidification device  44  is configured to cool and liquefy CO 2  contained in CO 2  rich gas. The liquefied CO 2  having a high purity (liquefied CO 2 ) is discharged from the CO 2  separation device  40  via the CO 2  recovery line  24   c.    
     A pump  49  for pressurizing the liquefied CO 2  may be disposed in the CO 2  recovery line  24   c . By pressurizing the liquefied CO 2  with the pump  49 , it is possible to obtain CO 2  having a high purity and a high pressure. 
     In the illustrative embodiment depicted in  FIG. 9 , the CO 2  liquefaction/solidification device  44  is configured to cool and solidify CO 2  contained in CO 2  rich gas. The solidified CO 2  having a high purity (solid CO 2 ) is extracted to the first chamber  50   a  of the gasifier  50  and then transferred to the second chamber  50   b , where the CO 2  receives heat from the heat source  56  and is gasified and pressurized. The gasified and pressurized CO 2  having a high purity is discharged from the CO 2  separation device via the CO 2  recovery line  24   c.    
     As depicted in  FIG. 9 , the gasifier  50  may include a lid part  52  for receiving the solid CO 2  generated in the CO 2  liquefaction/solidification device  44  to the first chamber  50   a , and a lid part  54  for switching the communication state between the first chamber  50   a  and the second chamber  50   b . The lid parts  52 ,  54  may be configured to open and close appropriately when taking out solid CO 2  from the CO 2  liquefaction/solidification device  44  to the first chamber  50   a , and when transferring solid CO 2  from the first chamber  50   a  to the second chamber  50   b . Further, in  FIG. 9 , the lid part  52  is open and the lid part  54  is closed to take out the solid CO 2  from the CO 2  liquefaction/solidification device  44  to the first chamber  50   a.    
     In some embodiments, the CO 2  liquefaction/solidification device  44  may be configured to cool the CO 2  rich gas to liquefy a part of CO 2  contained in the CO 2  rich gas and solidify another part of CO 2 . In this case, the liquefied CO 2  and the solidified CO 2  may be recovered through different recovery lines. 
     The plant  1  (e.g. see  FIGS. 6 to 9 ) including the above described expander  4  and compressor ( 26 A,  26 B) may be applied to a plant including the fuel cell  110  as depicted in  FIG. 10 or 11 , for instance. 
     The plant  1  depicted in  FIGS. 10 and 11  includes a fuel cell  110  that includes a cathode  112 , an anode  116 , and an electrolyte  114  disposed between the cathode  112  and the anode  116 . The cathode  112  of the fuel cell  110  is supplied with exhaust gas containing CO 2 . Further, the electrolyte  114  is configured to transfer carbonate ion (CO 3   2− ) derived from CO 2  contained in exhaust gas from the cathode  112  to the anode  116 . Further, the compressor ( 26 A,  26 B) is configured to compress CO 2  rich gas derived from outlet gas of the anode  116  of the fuel cell  110 , and the anode  116  of the fuel cell  110  is configured to be supplied with high-pressure fuel gas from which power has been recovered by the expander  4 . 
     The configuration of the plant  1  depicted in  FIGS. 10 and 11  will be described below. A 1  to A 4  in  FIG. 10  represent the same sections as A 1  to A 4  in  FIG. 6 , and B 1  to B 5  in  FIG. 11  represent the same sections as B 1  to B 5  in  FIG. 7 . 
     The plant  1  depicted in  FIGS. 10 and 1  is a thermal power generation facility that includes a thermal power generation device  102  and a carbon dioxide recovery system  103 . The carbon dioxide recovery system  103  is configured to recover carbon dioxide (CO 2 ) contained in exhaust gas from the thermal power generation device  102 . 
     The thermal power generation device  102  is a device that generates power by using combustion gas or combustion heat generated by combustion of fuel. For instance, the thermal power generation device  102  may be a power generation device including a boiler or a gas turbine, or a power generation device such as a gas turbine combined cycle power generation device (GTCC) or an integrated coal gasification combined cycle power generation device (IGCC), for instance. In the illustrative embodiment depicted in  FIGS. 10 and 11 , the thermal power generation device  102  is a power generation device that generates power using combustion gas of the gas turbine  200 . 
     The carbon dioxide recovery system  103  is configured to recover CO 2  contained in exhaust gas that contains gas generated from combustion in the thermal power generation device  102 . For instance, in a case where the thermal power generation device  102  includes a boiler or a gas turbine including a combustor, the carbon dioxide recovery system  103  may be configured to recover CO 2  contained in exhaust gas from the boiler or the gas turbine. Alternatively, in a case where the thermal power generation device  102  includes a waste-heat recovery boiler (heat recovery steam generator; HRSG) for recovering heat of exhaust gas from the gas turbine or the like, the carbon dioxide recovery system  103  may be configured to recover CO 2  contained in exhaust gas from the waste-heat recovery boiler. 
     In the illustrative embodiment depicted in  FIGS. 10 and 11 , the thermal power generation device  102  is a power generation device that includes the gas turbine  200 . In the embodiment depicted in  FIGS. 10 and 11 , the carbon dioxide recovery system  103  is configured to recover CO 2  contained in exhaust gas from the gas turbine  200 . 
     The gas turbine  200  depicted in  FIGS. 10 and 11  includes a compressor  202  for producing compressed air, a combustor  204  for producing combustion gas by combusting a fuel (e.g. natural gas), and a turbine  206  configured to be rotary driven by combustion gas. 
     The combustor  204  is supplied with fuel (e.g. natural gas) from a fuel storage part  122 . Further, air compressed by the compressor  202  is sent into the combustor  204 , and the compressed air has a function of an oxidizing agent in combustion of fuel at the combustor  204 . 
     A generator  208  is coupled to the turbine  206  via a rotational shaft  203 . The generator  208  is driven by rotational energy of the turbine  206 , and thereby electric power is generated. The combustion gas having worked at the turbine  206  is discharged from the turbine  206  as exhaust gas. 
     In the illustrative embodiment depicted in  FIGS. 10 and 11 , the carbon dioxide recovery system  103  includes the above described fuel cell  110 , and a CO 2  rich gas line (anode outlet flow passage)  24  connected to the outlet side of the anode  116  of the fuel cell  110  and configured to guide CO 2  rich gas derived from the outlet gas of the anode  116 . The CO 2  contained in exhaust gas from the thermal power generation device  102  is recovered via the fuel cell  110  and the CO 2  rich gas line  24  as described below. 
     In the present specification, the CO 2  rich gas derived from the outlet gas of the anode may be the anode outlet gas itself, or gas after performing a predetermined treatment on the anode outlet gas (e.g. CO shift reaction in a CO shift reactor  120  described below, or membrane separation at the gas separation unit  136 ). Further, the CO 2  rich gas refers to a gas having a higher CO 2  concentration than exhaust gas to be processed. 
     As described above, the fuel cell  110  includes an anode (fuel pole)  116 , a cathode (air pole)  112 , and an electrolyte  114 . The cathode  112  is supplied with exhaust gas (exhaust gas containing CO 2 ) from the thermal power generation device  102 . Furthermore, fuel gas containing hydrogen (H 2 ) is supplied to the anode  116 . The electrolyte  114  is configured to transfer carbonate ion (CO 3   2− ) derived from CO 2  contained in exhaust gas from the cathode  112  to the anode  116 . 
     The fuel cell  110  may be a molten carbonate fuel cell (MCFC) using carbonate as the electrolyte  114 . The carbonate used as the electrolyte  114  may be lithium carbonate, sodium carbonate, potassium carbonate, or combination of the above. 
     The cathode  112  is supplied with exhaust gas containing CO 2  from the thermal power generation device  102  via the cathode inlet flow passage  170 . 
     A fuel storage part  122  storing a fuel (e.g. natural gas) is connected to the anode  116  via an anode inlet flow passage  176  and the fuel supply line  2 . The fuel inside the fuel storage part  122  is reformed into hydrogen (H 2 ) in a pre-reformer  124  disposed in the fuel supply line  2  or the reforming part  118  disposed in the fuel cell  110 , for instance, and is supplied to the anode  116  via the anode inlet flow passage  176 . 
     At the cathode  112  of the fuel cell  110 , CO 2  and oxygen (O 2 ) contained in exhaust gas from the thermal power generation device  102  reacts with electrons and thereby carbonate ion (CO 3   2− ) is produced. The carbonate ion produced at the cathode  112  transfers through the electrolyte  114  toward the anode  116 . 
     On the other hand, at the anode  116  of the fuel cell  110 , the hydrogen (H 2 ) supplied via the anode inlet flow passage  176  reacts with carbonate ion (CO 3   2− ) that has transferred through the electrolyte  114 , and thereby water (H 2 O), CO 2 , and electrons are produced. As described above, CO 2  supplied to the cathode  112  transfers through the electrolyte  114  in the form of cathode ion from the cathode  112  to the anode  116 , and becomes CO 2  after reaction at the anode  116 . 
     CO 2  generated at the anode  116  flows out to the CO 2  rich gas line (anode outlet flow passage)  24  as gas mixture (outlet gas of the anode  116 ) with H 2 O and non-combusted components of the fuel gas (e.g. CO and H 2 ). The anode outlet gas that flows out to the CO 2  rich gas line  24  is a CO 2  rich gas having a higher CO 2  concentration than exhaust gas to be processed. 
     CO 2  contained in the CO 2  rich gas discharged from the anode  116  is recovered via the CO 2  rich gas line  24 . Further, the recovered CO 2  (i.e. CO 2  recovered to the side of the anode  116  by the fuel cell  110 ) may be compressed by the compressor  109  (see  FIG. 11 ). 
     Meanwhile, the reforming reaction of the fuel is an endothermic reaction, and it is normally necessary to add heat from outside. Thus, as depicted in  FIGS. 10 and 11 , at the upstream side of the reforming part  118 , a heat exchanger  126  for heating the fuel to be supplied to the reforming part  118  via the fuel supply line  2  may be disposed. By heating the fuel with the heat exchanger  126  and then supplying the fuel to the reforming part  118 , it is possible to cause the reforming reaction of the fuel efficiently. 
     Further, in the embodiment depicted in  FIGS. 10 and 11 , the heat exchanger  126  is configured to heat the fuel supplied from the fuel supply line  2  to the reforming part  118  through heat exchange with the outlet gas (CO 2  rich gas) of the anode  116 . 
     The molten carbonate fuel cell operates at a high temperature of approximately 600 to 700° C., and gas that flows out from the anode  116  has a high temperature of the same level. Thus, with the above described heat exchanger  126 , it is possible to cause the reforming reaction of the fuel while effectively utilizing the reaction heat generated at the fuel cell  110 . 
     Further, in the embodiment depicted in  FIGS. 10 and 11 , a combustor  119  for combusting a fuel (fuel from the fuel storage part  122 , for instance) is disposed in the cathode inlet flow passage  170 . 
     Furthermore, for appropriate operation of the fuel cell  110 , the temperature of supplied gas should have a high temperature of a certain level in some cases. In such a case, by combusting fuel at the combustor  119  and increasing the temperature of exhaust gas at the inlet side of the cathode  112  with combustion heat, the fuel cell  110  can be operated appropriately. 
     In the illustrative embodiment depicted in  FIGS. 10 and 11 , a CO shift reactor  120  for denaturing CO contained in CO 2  rich gas is disposed in the CO 2  rich gas line  24 . The CO shift reactor  120  is configured to convert CO contained in CO 2  rich gas into CO 2  through reaction with water (H 2 O). 
     By denaturing CO with the CO shift reactor  120 , it is possible to increase the CO 2  concentration in the CO 2  rich gas line  24  downstream of the CO shift reactor  120  compared to that upstream of the CO shift reactor  120 . Accordingly, it is possible to recover purer carbon dioxide. 
     In the illustrative embodiment depicted in  FIG. 10 , a gas separation unit  136  for separating a gas component in the CO 2  rich gas is disposed in the CO 2  rich gas line  24 . 
     The gas separation unit  136  may be configured to separate CO 2  from CO 2  rich gas supplied to the gas separation unit  136 . By separating CO 2  from the CO 2  rich gas with the gas separation unit  136 , it is possible to increase the CO 2  concentration in the CO 2  rich gas line  24  downstream of the gas separation unit  136  compared to that upstream of the gas separation unit  136 . Accordingly, it is possible to recover purer carbon dioxide. 
     The gas separation unit  136  may include a separation membrane configured to separate CO 2  from the CO 2  rich gas. Alternatively, the gas separation unit  136  may be configured to separate CO 2  from CO 2  rich gas by the cryogenic distillation method. 
     Furthermore, as depicted in  FIG. 10 , in the CO 2  rich gas line  24 , a compressor  134  for increasing the pressure of CO 2  rich gas to a pressure suitable for a separation method adapted by the gas separation unit  136  may be disposed upstream of the gas separation unit  136 . 
     In the embodiment depicted in  FIG. 11 , CO 2  rich gas flowing through the CO 2  rich gas line  24  is guided to the compressor ( 26 A,  26 B) (see  FIG. 7 ) via the upstream CO 2  rich gas line  24   a . Further, the CO 2  rich gas having passed through the compressor ( 26 A,  26 B) is guided to the CO 2  separation device  40  (see  FIG. 7 ; corresponding to the above described gas separation unit  136 ) from the downstream CO 2  rich gas line  24   b  (see  FIG. 7 ), and CO 2  is separated from the CO 2  rich gas at the CO 2  separation device  40 . The CO 2  separated from the CO 2  rich gas at the CO 2  separation device  40  is recovered via the CO 2  recovery line  24   c.    
     In some embodiments, as depicted in  FIGS. 10 and 11 , exhaust gas from the thermal power generation device  102  is supplied to the cathode  112  of the fuel cell  110 , and is supplied to the waste-heat recovery boiler  140  via the bypass flow passage  178  branched from the cathode inlet flow passage  170 . 
     The waste-heat recovery boiler  140  depicted in  FIGS. 10 and 11  includes a duct to which exhaust gas from the thermal power generation device  102  is guided (not depicted) and a heat exchanger (not depicted) disposed in the duct. The heat exchanger is configured to generate steam through heat exchange with exhaust gas flowing through the duct. The steam generated by the waste-heat recovery boiler  140  is guided to the steam turbine  142  and rotary drives the steam turbine  142 . Further, a generator  144  is connected to the steam turbine  142 , and the generator  144  is configured to generate electric power by being rotary-driven by the steam turbine  142 . 
     In the illustrative embodiment depicted in  FIG. 11 , exhaust gas having flown through the duct of the waste-heat recovery boiler  140  and passed through the heat exchanger is discharged from a stack  146 . 
     In the illustrative embodiment depicted in  FIG. 10 , exhaust gas having flown through the duct of the waste-heat recovery boiler  140  is discharged from the waste-heat recovery boiler via the duct outlet and guided to the chemical absorption tower  130 . 
     That is, in the illustrative embodiment depicted in  FIG. 10 , in addition to the CO 2  recovery via the fuel cell  110  and the CO 2  rich gas line  24  described above, the chemical absorption tower  130  is used to recover CO 2  contained in exhaust gas from the thermal power generation device  102 . 
     As depicted in  FIG. 10 , exhaust gas from the thermal power generation device  102  is guided to the chemical absorption tower  130  via the cathode outlet flow passage  172  of the fuel cell  110  and/or the bypass flow passage  178 . Furthermore, exhaust gas guided to the chemical absorption tower  130  may contain, as depicted in  FIG. 10 , exhaust gas after heat recovery at the waste-heat recovery boiler  140 . 
     At the chemical absorption tower  130  according to an embodiment, an absorption liquid (e.g. absorption liquid containing amine) makes contact with exhaust gas guided to the chemical absorption tower  130 , and thereby CO 2  contained in exhaust gas is absorbed by the absorption liquid. Accordingly, CO 2  is removed from exhaust gas. The exhaust gas deprived of CO 2  is discharged from the outlet  130   a  of the chemical absorption tower  130  as processed exhaust gas. 
     The absorption liquid having absorbed CO 2  is sent to the regeneration tower  132  from the chemical absorption tower  130 , and is regenerated at the regeneration tower  132 . At the regeneration tower  132 , the absorption liquid having absorbed CO 2  is heated by steam, and thereby CO 2  is separated and removed from the absorption liquid (that is, the absorption liquid is regenerated). 
     The gas containing CO 2  removed from the absorption liquid is discharged from the regeneration tower  132  and deprived of moisture at a gas-liquid separator (not depicted), and then CO 2  is recovered as a gas. 
     On the other hand, the absorption liquid regenerated after removal of CO 2  at the regeneration tower  132  is returned to the chemical absorption tower  130 , and is used again to absorb CO 2  contained in exhaust gas from the thermal power generation device  102 . 
     In the embodiment depicted in  FIGS. 10 and 11 , the above described expander  4  is disposed in the fuel supply line  2  for supplying the fuel from the fuel storage part  122  to the anode  116 , at the A 1  to A 2  section in  FIG. 10  and the B 1  to B 2  section in  FIG. 11  (see  FIGS. 6 and 7 ). High-pressure fuel gas from the fuel storage part  122  flows into the fuel supply line  2 . The expander  4  extracts power from the high-pressure fuel gas by expanding the high-pressure fuel gas. Furthermore, at the upstream side of the expander  4  in the fuel supply line  2 , the heater  22  is disposed. 
     Furthermore, in the embodiment depicted in  FIGS. 10 and 11 , the above described compressor ( 26 A,  26 B) is disposed in CO 2  rich gas line  24  at the outlet side of the anode  116  of the fuel cell  110 , at the A 3  to A 4  section in  FIG. 10  and the B 3  to B 4  section and the B 3  to B 5  section in  FIG. 11  (see  FIGS. 6 and 7 ). The compressor ( 26 A,  26 B) is configured to compress CO 2  rich gas that flows through the CO 2  rich gas line  24 . Further, the heater  22  is configured to heat high-pressure fuel gas flowing through the fuel supply line  2  with waste heat of the compressor ( 26 A,  26 B). 
     Further, in the embodiment depicted in  FIGS. 10 and 11 , the electric motor  28  for driving the compressor ( 26 A,  26 B) may be supplied with at least a part of the electric power generated at the fuel cell  110 . 
     In the embodiment depicted in  FIG. 10 , CO 2  (CO 2  rich gas) pressurized by the compressor ( 26 A,  26 B) is recovered via the downstream CO 2  rich gas line  24   b.    
     Further, in the illustrative embodiment depicted in  FIG. 11 , at the B 3  to B 5  section in  FIG. 11 , a CO 2  separation device  40  is disposed in the CO 2  rich gas line  24  at the downstream side of the compressor ( 26 A,  27 B) (see  FIG. 7 ). Further, from CO 2  separated from the CO 2  rich gas at the CO 2  separation device  40 , CO 2  compressed by the compressor ( 26 A,  26 B) (CO 2  rich gas) is recovered via the CO 2  recovery line  24   c . Further, the impurity gas containing H 2  or CO 2  that is removed from the CO 2  rich gas line at the CO 2  separation device  40  flows into the fuel supply line  2  via the discharge line  30 , and the impurity gas is supplied to the anode  116  of the fuel cell  110  via the pre-reformer  124  and the reforming part  118  of the fuel cell  110 . 
     As depicted in  FIGS. 10 and 11 , by pressurizing the CO 2  rich gas with the compressor ( 26 A,  26 B) as described above, it is possible to use pressurized CO 2  rich gas in enhanced oil recovery (EOR), or seal and fix CO 2  in the rock ground or under the sea. Further, by heating high-pressure fuel gas at the heater  22  by utilizing waste heat of the compressor  26  ( 26 A,  26 B) for pressurizing CO 2  rich gas, it is possible to recover more power at the expander  4 , and improve the output and the efficiency of the plant as a whole even further. 
     Further, as depicted in  FIG. 11 , by separating CO 2  from CO 2  rich gas using the CO 2  separation device  40 , it is possible to obtain CO 2  with a high purity. Further, in a case where the CO 2  rich gas contains combustible gas (e.g. H 2  or CO) as an impurity substance, it is possible to improve the energy efficiency of a plant as a whole by utilizing the impurity gas obtained by the CO 2  separation device  40  as a fuel at the fuel cell  110  or the like. 
     Embodiments of the present invention were described in detail above, but the present invention is not limited thereto, and various amendments and modifications may be implemented. 
     Further, in the present specification, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function. 
     For instance, an expression of an equal state such as “same” “equal” and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function. 
     Further, for instance, an expression of a shape such as a rectangular shape or a cylindrical shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved. 
     On the other hand, an expression such as “comprise”, “include”, “have” and “contain” are not intended to be exclusive of other components. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           1  Plant 
           2  Fuel supply line 
           2   a  Upstream fuel supply line 
           2   b  Downstream fuel supply line 
           4  Expander 
           5  Rotational shaft 
           6  Generator 
           10  Fuel tank 
           12  Pump 
           14  Heat exchanger 
           16  Compressor 
           20  Gasification furnace 
           22  Heater 
           24  CO 2  rich gas line 
           24   a  Upstream CO 2  rich gas line 
           24   b  Downstream CO 2  rich gas line 
           26 A,  26 A′ Compressor 
           26 B Compressor 
           27  Rotational shaft 
           28  Electric motor 
           30  Discharge line 
           30  Cooling medium line 
           32  Heat exchanger 
           34  Solidifier 
           44  Freezer 
           49  Pump 
           50  Gasifier 
           50   a  First chamber 
           50   b  Second chamber 
           52  Lid part 
           54  Lid part 
           56  Heat source 
           102  Thermal power generation device 
           103  Carbon dioxide recovery system 
           109  Compressor 
           110  Fuel cell 
           112  Cathode 
           114  Electrolyte 
           116  Anode 
           118  Reforming part 
           119  Combustor 
           120  CO shift reactor 
           122  Fuel storage part 
           124  Pre-reforming part 
           126  Heat exchanger 
           130  Chemical absorption tower 
           130   a  Outlet 
           132  Regeneration tower 
           134  Compressor 
           136  Gas separation unit 
           140  Waste-heat recovery boiler 
           142  Steam turbine 
           144  Generator 
           146  Stack 
           170  Cathode inlet side flow passage 
           172  Cathode outlet side flow passage 
           176  Anode inlet side flow passage 
           178  Bypass flow passage 
           200  Gas turbine 
           202  Compressor 
           203  Rotational shaft 
           204  Combustor 
           206  Turbine 
           208  Generator