Patent Publication Number: US-8974582-B2

Title: CO2 recovery system and CO2 recovery method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a divisional of U.S. application Ser. No. 12/901,051 filed Oct. 8, 2010, now U.S. Pat. No. 8,398,758 issued Mar. 19, 2013. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a CO 2  recovery system and a CO 2  recovery method for reducing the concentrations of residual basic amine compounds emitted together with decarbonated flue gas in which the amount of CO 2  has been reduced by contact with an absorbent. 
     BACKGROUND ART 
     The greenhouse effect due to CO 2  has been pointed out as one of the causes of global warming, and there is an urgent need to take global measures against the greenhouse effect to protect the global environment. The sources of CO 2  can be found in various fields of human activities in which fossil fuels are burnt, and there is an increasing tendency to tighten CO 2  emission regulations. Accordingly, extensive studies have been conducted on CO 2  recovery methods applicable to power generation facilities, such as thermal power plants, which use a large amount of fossil fuels. In these methods, flue gas from a boiler is brought into contact with an amine-based absorbent such as an aqueous solution of an amine compound to collect and recover CO 2  in the flue gas. 
     When such an absorbent is used to recover CO 2  from flue gas, the amine compound is entrained in the decarbonated flue gas in which the amount of CO 2  has been reduced. Therefore, to prevent air pollution by the amine compound, the amount of the amine compound emitted together with the decarbonated flue gas must be reduced. 
     PTL 1 discloses a conventional amine recovery system. This system includes a plurality of stages of water-washing units for collecting an amine compound entrained in decarbonated flue gas. In each water-washing unit stage, wash water is brought into gas-liquid contact with the decarbonated flue gas in which the amount of CO 2  has been reduced by gas-liquid contact with an absorbent that absorbs CO 2 . The amine entrained in the decarbonated flue gas is collected sequentially in the plurality of stages of the water-washing units. As the wash water used in PTL 1, condensed water is used which is produced by condensing and separating water contained in CO 2  during a process of regenerating the amine-based absorbent by removing CO 2  from the CO 2 -absorbed amine-based absorbent. 
     PTL 2 discloses a conventional decarbonation system including: a cooling unit for cooling decarbonated flue gas in which the amount of CO 2  has been reduced by gas-liquid contact with an absorbent; and a contact unit in which condensed water condensed in the cooling unit is brought into countercurrent contact with the decarbonated flue gas. PTL 2 discloses another decarbonation system that includes a water-washing unit for collecting the amine compound entrained in decarbonated flue gas by bringing wash water into gas-liquid contact with the decarbonated flue gas in which the amount of CO 2  has been reduced by gas-liquid contact with an absorbent. The wash water used is condensed-water condensed in a cooling tower for cooling the flue gas from which CO 2  has not been recovered. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] JP 2002-126439A 
         [PTL 2] JP H08-80421A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In recent years, from the viewpoint of environmental protection, there is a demand to further reduce the concentrations of residual absorbent components emitted together with decarbonated flue gas. It is expected in the future that a CO 2  recovery system is applied to flue gas from, for example, a thermal power plant in which the flow rate of processed gas is high. In this case, the emission amounts of residual absorbent components emitted together with the decarbonated flue gas tend to increase because the emission amount of the flue gas is high. Therefore, the concentrations of the emitted absorbent components must be further reduced. 
     The present invention solves the foregoing problems, and it is an object of the invention to provide a CO 2  recovery system and a CO 2  recovery method that can further reduce the concentration of residual basic amine compounds emitted together with decarbonated flue gas. 
     Solution to Problem 
     According to an aspect of the present invention, a CO 2  recovery system includes: an absorber including a CO 2  absorbing section and a water-washing section, the CO 2  absorbing section allowing flue gas to come into contact with a basic amine compound absorbent so that the basic amine compound absorbent absorbs CO 2  in the flue gas, the water-washing section allowing the decarbonated flue gas from which CO 2  has been absorbed in the CO 2  absorbing section to come into contact with circulating wash water and to be washed with the wash water so that basic amine compounds entrained in the decarbonated flue gas are reduced in amount; a regenerator releasing the CO 2  from the basic amine compound absorbent containing the CO 2  absorbed into; and a concentrating unit for concentrating the wash water from the water-washing section, the concentrating unit returning condensed water generated during concentration to the water-washing section. 
     In this CO 2  recovery system, since the condensed water generated when the wash water from the water-washing section is concentrated is returned to the water-washing section, the concentrations of the basic amine compounds in the wash water circulating in the water-washing section are reduced, and the efficiency of washing with the wash water is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas can be reduced, and the concentrations of the residual basic amine compounds emitted together with the decarbonated flue gas can thereby be further reduced. 
     Advantageously, in the CO 2  recovery system, the water-washing section includes a plurality of water-washing sections disposed in a path through which the decarbonated flue gas flows upward, and the concentrating unit concentrates part of the wash water discharged from an uppermost one of the plurality of water-washing sections to form a concentrated solution, the concentrated solution being delivered to a lower one of the plurality of water-washing sections, the condensed water being returned to the uppermost water-washing section. 
     In this CO 2  recovery system, since the condensed water generated when the wash water from the uppermost water-washing section is concentrated is returned to the uppermost water-washing section, the concentrations of the basic amine compounds in the wash water circulating in the uppermost water-washing section are reduced, and the efficiency of washing with the wash water in the uppermost water-washing section is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas can be further reduced, and the concentrations of the residual basic amine compounds discharged together with the decarbonated flue gas can thereby be further reduced. 
     According to another aspect of the present invention, a CO 2  recovery method includes the steps of: bringing flue gas into contact with a basic amine compound absorbent so that the basic amine compound absorbent absorbs CO 2  contained in the flue gas; washing the decarbonated flue gas in which an amount of CO 2  has been reduced with circulating wash water by bringing the decarbonated flue gas into contact with the wash water so that amounts of basic amine compounds entrained in the decarbonated flue gas are reduced; releasing CO 2  from the basic amine compound absorbent containing the CO 2  absorbed therein; and concentrating the wash water and delivering condensed water generated during concentration to the step of washing with the wash water. 
     In this CO 2  recovery method, the decarbonated flue gas is washed with wash water by bringing the decarbonated flue gas into contact with the wash water so that the amounts of basic amine compounds entrained in the decarbonated flue gas are reduced. The wash water used in the step of washing is concentrated, and the condensed water generated during concentration is returned to the step of washing with the wash water. In this manner, the concentrations of the basic amine compounds in the circulating wash water are reduced, and the efficiency of washing with the wash water is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas can be reduced, and the concentrations of the residual basic amine compounds discharged together with the decarbonated flue gas can thereby be further reduced. 
     Advantageously, in the CO 2  recovery method, the step of washing the decarbonated flue gas in which an amount of CO 2  has been reduced with circulating wash water by bringing the decarbonated flue gas into contact with the wash water so that amounts of basic amine compounds entrained in the decarbonated flue gas are reduced is performed at a plurality of stages in a flow path through which the decarbonated flue gas flows upward, and wherein, in the step of concentrating the wash water, the wash water from the step of washing performed at an uppermost stage is concentrated to form a concentrated solution, the concentrated solution being delivered to the step of washing performed at a lower stage in the flow path, the condensed water being returned to the step of washing performed at the uppermost stage. 
     In this CO 2  recovery method, since the condensed water generated when the wash water is concentrated is delivered to the washing step performed at the uppermost position, the concentrations of the basic amine compounds in the wash water circulated in the washing step performed at the uppermost position are reduced, and the efficiency of washing with the wash water in the washing step performed at the uppermost position is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas can be further reduced, and the concentrations of the residual basic amine compounds discharged together with the decarbonated flue gas can thereby be further reduced. 
     Advantageous Effects of Invention 
     According to the present invention, the concentrations of the residual basic amine compounds emitted together with the decarbonated flue gas can be further reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of a CO 2  recovery system according to a first embodiment of the present invention. 
         FIG. 2  is a schematic diagram of a CO 2  recovery system according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the embodiments. Components in the following embodiments include those which can be easily replaced by persons skilled in the art and also include substantially equivalent components. 
     A first embodiment will be described with reference to the drawings.  FIG. 1  is a schematic diagram of a CO 2  recovery system according to the first embodiment. 
     As shown in  FIG. 1 , this CO 2  recovery system includes a cooling unit  1 , an absorber  2 , and a regenerator  3 . The cooling unit  1  cools flue gas  101  discharged from an industrial facility (not shown) such as a boiler with cooling water  102 . The absorber  2  allows a lean solution  103   a  of a basic amine compound absorbent  103 , which is an aqueous solution of a basic amine compound that absorbs CO 2 , to come into countercurrent contact with the flue gas  101  so that the basic amine compound absorbent  103  absorbs CO 2  in the flue gas  101 . The flue gas  101  in which the amount of CO 2  has been reduced is discharged from the absorber  2 . The regenerator  3  releases CO 2  from a rich solution  103   b  of the basic amine compound absorbent  103  that contains the CO 2  absorbed therein, so that the lean solution  103   a  is regenerated. 
     The flue gas  101  containing CO 2  is increased in pressure by a flow gas blower (not shown) and is delivered inside the cooling unit  1 . In the cooling unit  1 , the flue gas  101  comes into countercurrent contact with the cooling water  102  and is thereby cooled. 
     The cooling water  102  is accumulated in the lower portion of the cooling unit  1 , and is supplied by a cooling water circulation pump  1   a  to the upper portion of the cooling unit  1  through a cooling water tube  1   b  disposed outside the cooling unit  1 . Then the cooling water  102  flows downward from nozzles  1   c  disposed in the upper portion of the cooling unit  1  while the cooling water  102  comes into countercurrent contact with the rising flue gas  101  at the position of a packed bed  1   d  disposed between the nozzles  1   c  and the lower portion of the cooling unit  1 . The cooling water tube  1   b  is provided with a cooling unit  1   e . The cooling water  102  is thereby cooled to a temperature lower than the temperature of the flue gas  101 , so that part of water in the flue gas  101  is condensed in the cooling unit  1  to form condensed water. The flue gas  101  cooled in the cooling unit  1  is discharged from the cooling unit  1  through a flue gas tube  1   f  and is then supplied to the absorber  2 . 
     The absorber  2  includes a CO 2  absorbing section  21  disposed in the lower half thereof, a water-washing section  22  disposed in the upper half, and a concentrating unit  24  disposed outside the absorber  2 . In the CO 2  absorbing section  21 , the flue gas  101  supplied from the cooling unit  1  comes into countercurrent contact with the lean solution  103   a  of the basic amine compound absorbent  103 , so that the basic amine compound absorbent  103  absorbs CO 2  in the flue gas  101 . 
     The lean solution  103   a  of the basic amine compound absorbent  103  is supplied from the regenerator  3  and flows downward from nozzles  21   a . After coming into countercurrent contact with the rising flue gas  101  at the position of a packed bed  21   b  disposed between the nozzles  21   a  and the lower portion of the absorber  2 , the lean solution  103   a  becomes the rich solution  103   b  containing CO 2  absorbed therein, and the rich solution  103   b  is accumulated in the bottom portion of the absorber  2 . The rich solution  103   b  of the basic amine compound absorbent  103  accumulated in the bottom portion of the absorber  2  is pumped by a rich-solution discharge pump  21   d  disposed in a rich-solution tube  21   c  disposed outside the absorber  2  and is then supplied to the regenerator  3 . The decarbonated flue gas  101 A in which the amount of CO 2  has been reduced flows upward and passes through a demister  21   e , and the basic amine compound absorbent  103  in a mist form that is entrained in the decarbonated flue gas  101 A is thereby collected. 
     The water-washing section  22  allows the decarbonated flue gas  101 A in which the amount of CO 2  has been reduced in the CO 2  absorbing section  21  to come into countercurrent contact with wash water  104 , so that the amounts of the basic amine compounds entrained in the decarbonated flue gas  101 A are reduced through the wash water  104 . The decarbonated-deaminated flue gas  101 B in which the amounts of the basic amine compounds have been reduced is discharged to the outside of the CO 2  recovery system (the absorber  2 ). 
     The wash water  104  flows downward from nozzles  22   a , while the wash water  104  comes into countercurrent contact with the rising decarbonated flue gas  101 A at the position of a packed bed  22   b  disposed between the nozzles  22   a  and a water-washing section water receiver  22   c . Then, the wash water  104  is accumulated in the water-washing section water receiver  22   c . The wash water  104  accumulated in the water-washing section water receiver  22   c  is pumped and circulated by a wash water circulation pump  22   e  disposed in a wash water tube  22   d  disposed outside the absorber  2 , while the wash water  104  is cooled in a cooling unit  22   f , and again flows downward from the nozzles  22   a . More specifically, the wash water  104  is circulated and comes into countercurrent contact with the decarbonated flue gas  101 A so that the basic amine compounds in the decarbonated flue gas  101 A are reduced in amount. After the basic amine compounds in the decarbonated flue gas  101 A are reduced in amount through the wash water  104 , the resultant decarbonated-deaminated flue gas  101 B further flows upward and passes through a demister  22   g , and the wash water  104  in a mist form that is entrained in the decarbonated-deaminated flue gas  101 B is thereby collected. The basic amine compounds include, in addition to the basic amine compound used as absorbent, low-molecular weight basic amine compounds generated through decomposition. 
     In the water-washing section  22 , part of condensed water (* 1 ) generated when the CO 2  gas separated from the basic amine compound absorbent  103  in the regenerator  3  is cooled or part of condensed water (* 2 ) generated when the flue gas  101  is cooled in the cooling unit  1  is supplied, as the wash water  104 , to the wash water tube  22   d  at a position upstream of the cooling unit  22   f . The wash water  104  spilled over the water-washing section water receiver  22   c  of the water-washing section  22  is discharged to an overflow tube  22   h  disposed outside the absorber  2  and is then supplied to the bottom of the CO 2  absorbing section  21 . 
     The concentrating unit  24  discharges part of the wash water  104  circulating in the water-washing section  22  to the outside of the absorber  2  for concentration. The concentrating unit  24  is connected between a wash water discharge pump  22   e  and the cooling unit  22   f  disposed in the wash water tube  22   d  of the water-washing section  22  through a discharge tube  24   a  and is also connected to the water-washing section water receiver  22   c  of the water-washing section  22  through a return tube  24   b  and to the CO 2  absorbing section  21  (the overflow tube  22   h ) through a return tube  24   c . The return tube  24   c  may be directly connected to the bottom portion of the CO 2  absorbing section  21 . 
     Preferably, the concentrating unit  24  is a multiple-effect evaporator or a vapor compression condenser. The multiple-effect evaporator includes a plurality of evaporators. The wash water  104  is accumulated in a first one of the evaporators and is then heated and evaporated. The concentrated wash water  104  is supplied to the next evaporator, and the generated vapor is used as the heat source for the next evaporator. 
     In the vapor compression condenser, vapor generated in an evaporator is pressurized by a compressor to increase the temperature and is used as a heat source for heating. With the vapor compression condenser, the power consumption during concentration can be reduced. 
     In such a concentrating unit  24 , the wash water  104  pumped through the discharge tube  24   a  by the wash water discharge pump  22   e  is concentrated, and the concentrated wash water  104  is returned to the CO 2  absorbing section  21  through the return tube  24   c . Condensed water generated during concentration is delivered to the water-washing section water receiver  22   c  through the return tube  24   b , and the concentrations of basic amines in the wash water  104  can thereby be reduced. 
     The regenerator  3  includes an absorbent regenerating unit  31  disposed in the lower half thereof. In the absorbent regenerating unit  31 , CO 2  is recovered from the rich solution  103   b  to regenerate the basic amine compound absorbent  103  as the lean solution  103   a , thereby releasing CO 2  from the basic amine compound absorbent  103  containing the CO 2  absorbed therein. 
     The rich solution  103   b  of the basic amine compound absorbent  103  is supplied from the rich-solution tube  21   c  of the CO 2  absorbing section  21  in the absorber  2  and flows downward from nozzles  31   a . Then the rich solution  103   b  passes through a lower packed bed  31   b  disposed between the nozzles  31   a  and the lower portion of the regenerator  3  and is thereby converted to the lean solution  103   a  from which substantially the entire amount of CO 2  has been released through endothermic reaction caused by a regenerating heater  31   c  connected to the lower portion of the regenerator  3 . The resulting lean solution  103   a  is accumulated in the bottom portion of the regenerator  3 . A sub-regenerator  31   d  is connected to the lower portion of the regenerator  3 . In the sub-regenerator  31   d , part of the lean solution  103   a  is heated. Therefore, degraded products, and the like are concentrated and collected as sludge, and the generated vapor is returned to the lower portion of the regenerator  3 . The lean solution  103   a  accumulated in the lower portion of the regenerator  3  is pumped by a lean solution discharge pump  31   f  disposed in a lean solution tube  31   e  and is supplied to the absorber  2 . During this process, the lean solution  103   a  is heat-exchanged in a rich-lean heat exchanger  4  with the rich solution  103   b  supplied to the regenerator  3  through the rich-solution tube  21   c  and is cooled in a cooling unit  31   g.    
     The released CO 2  flows upward in the regenerator  3 , passes through an upper packed bed  31   h , and is discharged from the top portion of the regenerator  3 . Since the discharged CO 2  contains water, it is cooled in a condenser  32   b  disposed in a CO 2  discharge line  32   a . The water contained in the CO 2  is thereby condensed, and the condensed water  106  is separated from CO 2  in a CO 2  separator  32   c . The high-purity CO 2  separated from the condensed water  106  is emitted to the outside of the decarbonation system through a CO 2  emission line  32   d  and is used or disposed of in the subsequent process. The condensed water  106  is delivered by a condensed water pump  32   e , and part of the condensed water  106  is supplied to the regenerator  3  from nozzles  32   g  disposed in the top portion of the regenerator  3  through a regenerator reflux water line  32   f . The condensed water  106  has a very low amine concentration and therefore can be used as the replenishing water for the water-washing section  22 . 
     As described above, the CO 2  recovery system of the first embodiment includes the absorber  2  and the regenerator  3 . The absorber  2  includes the CO 2  absorbing section  21  and the water-washing section  22 . The CO 2  absorbing section  21  allows the flue gas  101  containing CO 2  to come into contact with the basic amine compound absorbent  103  so that the basic amine compound absorbent  103  absorbs CO 2  in the flue gas  101 . The water-washing section  22  allows the decarbonated flue gas  101 A in which the amount of CO 2  has been reduced in the CO 2  absorbing section  21  to come into contact with the circulating wash water  104  and to be washed with the wash water  104  so that the amounts of the basic amine compounds entrained in the decarbonated flue gas  101 A are reduced. The regenerator  3  release the CO 2  from the basic amine compound absorbent  103  containing the CO 2  absorbed in. The CO 2  recovery system further includes the concentrating unit  24  for concentrating the wash water  104  discharged from the water-washing section  22 . The concentrated solution is delivered to the CO 2  absorbing section  21 , and the condensed water generated during concentration is returned to the water-washing section  22 . 
     In this CO 2  recovery system, since the wash water  104  from the water-washing section  22  is concentrated and the condensed water is returned to the water-washing section  22 , the concentrations of the basic amine compounds in the wash water  104  circulating in the water-washing section  22  are reduced by a factor of about  10 , and the efficiency of washing with the wash water  104  is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas  101 A can be reduced by a factor of about  10 , and the concentrations of the residual basic amine compounds discharged together with the decarbonated-deaminated flue gas  101 B can be further reduced. 
     A CO 2  recovery method of the first embodiment includes the steps of: bringing the flue gas  101  containing CO 2  into contact with the basic amine compound absorbent  103  to reduce the amount of CO 2  contained in the flue gas  101 ; washing the decarbonated flue gas  101 A in which the amount of CO 2  has been reduced with the circulating wash water  104  by bringing the decarbonated flue gas  101 A into contact with the wash water  104  so that the amounts of the basic amine compounds entrained in the decarbonated flue gas  101 A are reduced; and releasing CO 2  from the basic amine compound absorbent  103  that has absorbed the CO 2 . The CO 2  recovery method further includes the step of concentrating the wash water  104  used in the step of washing with the wash water  104  to form a concentrated solution, and delivering the concentrated solution to the CO 2  absorbing step, and returning the condensed water generated during concentration to the step of washing with the wash water  104 . 
     In this CO 2  recovery method, the decarbonated flue gas  101 A in which the amount of CO 2  has been reduced is brought into contact with the wash water  104  to reduce the amounts of the basic amine compounds entrained in the decarbonated flue gas  101 A, and the wash water  104  used in this step of reducing the amounts of the basic amine compounds is concentrated. The concentrated solution is delivered to the CO 2  absorbing step, and the condensed water is returned to the step of washing with the wash water. In this manner, the concentrations of the basic amine compounds in the wash water  104  are reduced by a factor of about  10 , and the efficiency of washing with the wash water  104  is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas  101 A can be reduced by a factor of about  10 , and the concentrations of the residual basic amine compounds discharged together with the decarbonated-deaminated flue gas  101 B can be further reduced. 
     [Second Embodiment] 
     A second embodiment will be described with reference to the drawing.  FIG. 2  is a schematic diagram of a CO 2  recovery system according to embodiment  2 . 
     As shown in  FIG. 2 , the CO 2  recovery system of the second embodiment includes, in addition to the components in the first embodiment described above, a water-washing section  22  including a plurality of sections disposed in a path through which the flue gas (decarbonated flue gas)  101  flows upward. The concentrating unit  24  is configured such that wash water  104 B from an uppermost second water-washing section  22 B is concentrated. The concentrated solution is delivered to a lower first water-washing section  22 A, and the condensed water generated during concentration is returned to the uppermost second water-washing section  22 B. The other components of the CO 2  recovery system of the second embodiment are the same as those of the CO 2  recovery system of the first embodiment. Therefore, the same components are designated by the same reference numerals, and their description will be omitted. 
     In the present embodiment, the water-washing section  22  includes the first water-washing section  22 A and the second water-washing section  22 B that are disposed in the path through which the decarbonated flue gas  101 A flows upward. The lower first water-washing section  22 A allows the decarbonated flue gas  101 A in which the amount of CO 2  has been reduced in the CO 2  absorbing section  21  to come into countercurrent contact with wash water  104 A, so that the amounts of the basic amine compounds entrained in the decarbonated flue gas  101 A are reduced through the wash water  104 A. 
     The wash water  104 A flows downward form nozzles  22 Aa, while the wash water  104 A comes into countercurrent contact with the rising decarbonated flue gas  101 A at the position of a packed bed  22 Ab disposed between the nozzles  22 Aa and a water-washing section water receiver  22 Ac. Then, the wash water  104 A is accumulated in the water-washing section water receiver  22 Ac. The wash water  104 A accumulated in the water-washing section water receiver  22 Ac is pumped and circulated by a wash water discharge pump  22 Ae disposed in a wash water tube  22 Ad disposed outside the absorber  2 , while the wash water  104 A cooled in a cooling unit  22 Af, and again flows downward from the nozzles  22 Aa. More specifically, the wash water  104 A is circulated and comes into countercurrent contact with the decarbonated flue gas  101 A so that the basic amine compounds in the decarbonated flue gas  101 A are reduced in amount. After the basic amine compounds are reduced in amount through the wash water  104 A, the resultant decarbonated flue gas  101 A further flows upward and passes through a demister  22 Ag, and the wash water  104 A in a mist form that is entrained in the decarbonated flue gas  101 A is thereby collected. 
     The upper second water-washing section  22 B allows the rising decarbonated flue gas  101 A that has passed through the first water-washing section  22 A to come into countercurrent contact with wash water  104 B so that the basic amine compounds entrained in the decarbonated flue gas  101 A are further reduced in amount through the wash water  104 B. The resultant decarbonated-deaminated flue gas  101 B in which the amounts of the basic amine compounds has been reduced is discharged to the outside of the CO 2  recovery system (the absorber  2 ). 
     The wash water  104 B flows downward from nozzles  22 Ba, while the wash water  104 B comes into countercurrent contact with the rising decarbonated flue gas  101 A at the position of a packed bed  22 Bb disposed between the nozzles  22 Ba and a water-washing section water receiver  22 Bc. Then, the wash water  104 B is accumulated in the water-washing section water receiver  22 Bc. The wash water  104 B accumulated in the water-washing section water receiver  22 Bc is pumped and circulated by a wash water circulation pump  22 Be disposed in a wash water tube  22 Bd disposed outside the absorber  2 , while the wash water  104 B is cooled by a cooling unit  22 Bf, and again flows downward from the nozzles  22 Ba. More specifically, the wash water  104 B is circulated and comes into countercurrent contact with the decarbonated flue gas  101 A so that the basic amine compounds in the decarbonated flue gas  101 A are reduced in amount. After the basic amine compounds are reduced in amount through the wash water  104 B, the resultant decarbonated-deaminated flue gas  101 B further flows upward and passes through a demister  22 Bg, and the wash water  104 B in a mist form that is entrained in the decarbonated-deaminated flue gas  101 B is thereby collected. 
     In the second water-washing section  22 B, part of condensed water (* 1 ) generated when the CO 2  gas separated from the basic amine compound absorbent  103  in the regenerator  3  is cooled or part of condensed water (* 2 ) generated when the flue gas  101  is cooled in the cooling unit  1  is supplied, as the wash water  104 B, to the wash water tube  22 Bd at a position upstream of the cooling unit  22 Bf. The wash water  104 B spilled over the water-washing section water receiver  22 Bc of the second water-washing section  22 B is discharged to an overflow tube  22 Bh disposed outside the absorber  2  and then supplied to the water-washing section water receiver  22 Ac of the first water-washing section  22 A. The wash water  104 A spilled over the water-washing section water receiver  22 Ac of the first water-washing section  22 A is discharged to an overflow tube  22 Ah disposed outside the absorber  2  and then supplied to the bottom of the CO 2  absorbing section  21 . The water-washing section  22  may include two or more sections. 
     In the concentrating unit  24 , part of the wash water  104 B circulating in the uppermost second water-washing section  22 B is discharged from the absorber  2  and then concentrated. The concentrating unit  24  is connected between the wash water circulation pump  22 Be and the cooling unit  22 Bf disposed in the wash water tube  22 Bd of the second water-washing section  22 B through the discharge tube  24   a  and is also connected to the water-washing section water receiver  22 Bc of the second water-washing section  22 B through the return tube  24  and to the water-washing section water receiver  22 Ac of the first water-washing section  22 A through the return tube  24   c.    
     Preferably, the concentrating unit  24  is a multiple-effect evaporator or a vapor compression condenser. The multiple-effect evaporator includes a plurality of evaporators. The wash water  104 B is accumulated in a first one of the evaporators and is then heated and evaporated. The concentrated wash water  104 B is supplied to the next evaporator, and the generated vapor is used as the heat source for the next evaporator. 
     In the vapor compression condenser, vapor generated in an evaporator is pressurized by a compressor to increase the temperature and is used as a heat source for heating. With the vapor compression condenser, the power consumption during concentration can be reduced. 
     In such a concentrating unit  24 , the wash water  104 B supplied through the discharge tube  24   a  by the wash water discharge pump  22 Be is concentrated. The concentrated solution is delivered to the first water-washing section  22 A through the return tube  24   c , and the condensed water is returned to the second water-washing section  22 B through the return tube  24   b.    
     As described above, in the CO 2  recovery system of the second embodiment, the water-washing section  22  includes a plurality of sections disposed in the path through which the decarbonated flue gas  101 A flows upward. In the concentrating unit  24 , part of the wash water  104 B discharged from the uppermost second water-washing section  22 B is concentrated. The concentrated solution is delivered through the return tube  24   c  to the first water-washing section  22 A disposed on the upstream side of the flow of the decarbonated flue gas  101 A, and the condensed water is returned to the uppermost second water-washing section  22 B through the return tube  24   b.    
     In this CO 2  recovery system, the wash water  104 B used in the uppermost second water-washing section  22 B is concentrated, and the condensed water is returned to the second water-washing section  22 B through the return tube  24   b . In this manner, the concentrations of the basic amine compounds in the wash water  104 B circulating in the second water-washing section  22 B are reduced, and the efficiency of washing in the second water-washing section  22 B is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas  101 A can be further reduced, and the concentrations of the residual basic amine compounds emitted together with the decarbonated-deaminated flue gas  101 B can be further reduced. Since the concentrated solution is delivered to the first water-washing section  22 A through the return tube  24   c , the flow rate of water discharged from the first water-washing section  22 A is increased. Therefore, the concentrations of the basic amine compounds in the circulation water in the first water-washing section  22 A are reduced, and the concentrations of the basic amine compounds in the decarbonated flue gas supplied to the second water-washing section  22 B are reduced. The concentrated solution may be returned to the CO 2  absorbing section  21 . This is also effective to reduce the concentrations of the basic amine compounds in the decarbonated flue gas. 
     In a CO 2  recovery method of the second embodiment, the step of washing the decarbonated flue gas  101 A in which the amount of CO 2  has been reduced with the wash water  104  by bringing the decarbonated flue gas  101 A into contact with the wash water  104  so that the amounts of the basic amine compounds entrained in the decarbonated flue gas  101 A are reduced is performed at a plurality of positions in the path through which the decarbonated flue gas  101 A flows upward. In the step of concentrating the wash water  104 , the wash water  104 B from the water washing step performed at the uppermost position (the second water-washing section  22 B) is concentrated. The concentrated solution is delivered to the water washing step performed at a lower position (the first water-washing section  22 A disposed on the upstream side of the flow of the decarbonated flue gas  101 A), and the condensed water is returned to the water washing step performed at the uppermost position (the second water-washing section  22 B). The recovery method of the present embodiment includes this step. 
     In this CO 2  recovery method, since the condensed water is returned to the water washing step performed at the uppermost position, the concentrations of the basic amine compounds in the wash water  104  circulated in the water washing step performed at the uppermost position are reduced, and the washing efficiency of the water washing step performed at the uppermost position is improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas  101 A can be further reduced, and the concentrations of the residual basic amine compounds discharged together with the decarbonated-deaminated flue gas  101 B can be further reduced. 
     INDUSTRIAL APPLICABILITY 
     As described above, the CO 2  recovery system and method according to the present invention are suitable for further reducing the concentrations of the basic amine compounds remaining in the decarbonated flue gas. 
     REFERENCE SIGNS LIST 
     
         
           1  cooling unit. 
           2  absorber. 
           21  CO 2  absorbing section. 
           22  water-washing section. 
           22   a  nozzle. 
           22   b  packed bed. 
           22   c  water-washing section water receiver. 
           22   d  wash water tube. 
           22   e  wash water discharge pump. 
           22   f  cooling unit. 
           22   g  demister. 
           22   h  overflow tube. 
           22 A first water-washing section. 
           22 Aa nozzle. 
           22 Ab packed bed. 
           22 Ac water-washing section water receiver. 
           22 Ad wash water tube. 
           22 Ae wash water discharge pump. 
           22 Af cooling unit. 
           22 Ag demister. 
           22 Ah overflow tube. 
           22 B second water-washing section. 
           22 Ba nozzle. 
           22 Bb packed bed. 
           22 Bc water-washing section water receiver. 
           22 Bd wash water tube. 
           22 Be wash water discharge pump. 
           22 Bf cooling unit. 
           22 Bg demister. 
           22 Bb overflow tube. 
           24  concentrating unit. 
           24   a  discharge tube. 
           24   b  return tube. 
           24   c  return tube. 
           3  regenerator. 
           4  rich-lean heat exchanger. 
           101  flue gas. 
           101 A decarbonated flue gas. 
           101 B decarbonated-deaminated flue gas. 
           102  cooling water. 
           103  basic amine compound absorbent. 
           103   a  lean solution. 
           103   b  rich solution. 
           104  ( 104 A,  104 B) wash water.