Abstract:
The carbon dioxide recovery system includes a carbon dioxide absorption tower which absorbs and removes carbon dioxide from the combustion exhaust gas of a boiler by an absorption liquid, and a regeneration tower which heats and regenerates the loaded absorption liquid with carbon dioxide. The regeneration tower includes plural loaded absorption liquid heating means, which heat the loaded absorption liquid and remove carbon dioxide in the loaded absorption liquid. The turbine includes plural lines which extract plural kinds of steam with different pressures from the turbine and which supply the plural kinds of steam to the plural loaded absorption liquid heating means as their heating sources. The plural lines make the pressure of supplied steam increase from a preceding stage of the plural loaded absorption liquid heating means to a post stage of the plural loaded absorption liquid heating means.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a carbon dioxide recovery system for removing and recovering carbon dioxide contained in a combustion exhaust gas of a boiler in a thermal power plant, a power generation system using the carbon dioxide recovery system, and a method for these systems. 
     2. Background Art 
     In a power generation system of a thermal power plant using a large amount of fossil fuel, an amine absorption method is adopted as a method for removing and recovering carbon dioxide (CO 2 ) which is one of the causes of global warming. The amine absorption method has a problem that the large energy consumption at the time of separating and recovering CO 2  from a loaded absorption liquid with CO 2  absorbed therein significantly lowers the power generation output. 
     For example, in Japanese Patent Laid-Open No. 3-193116, as shown in  FIG. 4A , there is proposed a configuration in which a reboiler  41  is provided for a tower bottom part of a regeneration tower  24  for regenerating a loaded absorption liquid with CO 2  absorbed therein, and in which high pressure steam of about 3 kg/cm 2  absolute pressure is extracted from a low pressure turbine  8  and is supplied to the reboiler  41  provided for the bottom part of the regeneration tower as a heating source. This enables the loaded absorption liquid of the tower bottom part to be heated to an absorption liquid regeneration temperature of about 110 to 130° C., and hence, CO 2  in the loaded absorption liquid is separated so that the absorption liquid is regenerated. However, when all thermal energy required for the reboiler  41  of the tower bottom part is supplemented by the steam extracted from the low pressure turbine  8 , the amount of the steam extracted from the low pressure turbine  8  becomes large, which causes a problem that the output of the low pressure turbine  8  is significantly lowered and the power generation output is reduced. 
     SUMMARY OF THE INVENTION 
     Therefore, in view of the above described problem, it is an object of the present invention to provide a carbon dioxide recovery system capable of preventing reduction in turbine output at the time of regenerating the absorption liquid with carbon dioxide absorbed therein, a power generation system using the carbon dioxide recovery system, and a method for these systems. 
     In order to achieve the above described object, according to the present invention, there is provided a carbon dioxide recovery system comprising: a turbine which is driven and rotated by steam; a boiler which generates the steam supplied to the turbine; a carbon dioxide absorption tower which absorbs and removes carbon dioxide from a combustion exhaust gas of the boiler by an absorption liquid; and a regeneration tower which heats and regenerates a loaded adsorption liquid with carbon dioxide absorbed therein, the carbon dioxide recovery system being characterized in that the regeneration tower is provided with plural loaded adsorption liquid heating means in multiple stages, which heat the loaded adsorption liquid and remove carbon dioxide in the loaded adsorption liquid, in that the turbine is provided with plural lines which extract plural kinds of steam with different pressures from the turbine and which supply the extracted plural kinds of steam to the plural loaded adsorption liquid heating means as their heating sources, and in that the plural lines are connected to make the pressure of supplied steam increased from a preceding stage of the plural loaded adsorption liquid heating means to a post stage of the plural loaded adsorption liquid heating means. 
     As a variant, according to the present invention, there is provided a power generation system characterized by including the above described carbon dioxide recovery system and a generator which generates electric power by the rotation of the turbine. 
     Further, as a variant, according to the present invention, there is provided a method for recovering carbon dioxide characterized by including the steps of: generating steam by a boiler; supplying the steam to a turbine; extracting plural kinds of steam with different pressures from the turbine; absorbing and removing carbon dioxide by an absorption liquid from a combustion exhaust gas of the boiler; and removing carbon dioxide in a loaded absorption liquid and regenerating the absorption liquid by heating the loaded absorption liquid which absorbs the carbon dioxide with successive use of the plural kinds of steam from the steam with lower pressure. 
     Further, as a variant, according to the present invention, there is provided a power generation method characterized by including each step of the method for recovering carbon dioxide, and a step of generating electric power by the rotation of the turbine from which the plural kinds of steam with different pressures are extracted. 
     In the case of the regeneration tower in which the loaded absorption liquid heating means (reboiler) is provided only for the tower bottom part, as shown in  FIG. 4B , the temperature of the loaded absorption liquid in the regeneration tower has a distribution formed in such a manner that the temperature is gradually raised from the tower top part to near the tower bottom part and is abruptly raised to the absorption liquid regeneration temperature in the tower bottom part. Thus, according to the present invention, there is provided a configuration in which plural loaded absorption liquid heating means are provided for the regeneration tower in multiple stages, and in which when plural kinds of steam with different pressures are extracted from the turbine and supplied to the plural loaded absorption liquid heating means as their heating sources, the pressure of supplied steam is arranged to be increased from a preceding stage of the plural loaded absorption liquid heating means to a post stage of the plural loaded absorption liquid heating means. As a result, by utilizing the steam with the pressure lower than the pressure of the steam supplied to the loaded absorption liquid heating means of the post stage (tower bottom part), the temperature of the loaded absorption liquid can be increased while the loaded absorption liquid flows down to the tower bottom part in the loaded absorption liquid heating means of the preceding stage. Thereby, the amount of high pressure steam required for heating the loaded absorption liquid by the loaded absorption liquid heating means of the post stage (tower bottom part) can be reduced. Therefore, a part of the high pressure steam extracted from the turbine can be replaced with the steam with the lower pressure, so that it is possible to suppress the reduction in turbine output due to the steam extraction. 
     Further, the power generation system according to the present invention is configured to comprise the above described carbon dioxide recovery system, and a generator which generates electric power by the rotation of the turbine. Thus, as described above, the reduction in turbine output can be suppressed and thereby power generation output of the generator can be improved. 
     Further, according to the present invention, the method for recovering carbon dioxide is configured to extract plural kinds of steam with different pressures from the turbine, and to heat the loaded absorption liquid with successive use of the plural kinds of steam with different pressures from the steam with lower pressure. Thus, as described above, it is possible to eventually reduce the amount of the high pressure steam for heating and regenerating the loaded absorption liquid. As a result, a part of the high pressure steam extracted from the turbine can be replaced with the steam with lower pressure, so that it is possible to suppress the reduction in turbine output due to the steam extraction. 
     Further, according to the present invention, the power generation method is configured by comprising each step of the above described method for recovering carbon dioxide, and a step of generating electric power by the rotation of the turbine from which the plural kinds of steam with different pressures are extracted. Thus, as described above, the reduction in turbine output can be suppressed and thereby power generation output of the generator can be improved. 
     As described above, according to the present invention, it is possible to provide a carbon dioxide recovery system capable of preventing the reduction in turbine output at the time of regenerating the absorption liquid with carbon dioxide absorbed therein, and a power generation system using the carbon dioxide recovery system, and a method for these systems. 
     In the following, an embodiment according to the present invention is described with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an embodiment of a carbon dioxide recovery type power generation system according to the present invention; 
         FIG. 2  is a schematic illustration of an internal structure of a regeneration tower in  FIG. 1 ; 
         FIG. 3  is a schematic illustration of another embodiment of the carbon dioxide recovery type power generation system according to the present invention; and 
         FIG. 4A  is a schematic illustration of a structure in the vicinity of a regeneration tower of a conventional carbon dioxide recovery type power generation system; and  FIG. 4B  is a graph showing a temperature distribution in the regeneration tower. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It is noted that in the accompanying drawings, only main facilities are shown and accessory facilities are omitted. In the drawings, tanks, bulbs, pumps, blowers and heat exchangers are provided as required. Further, two turbines are usually provided in pairs as each of a low pressure turbine, a medium pressure turbine and a high pressure turbine, but each pair of the turbines is also represented by a single reference numeral. 
     As shown in  FIG. 1 , the carbon dioxide recovery type power generation system according to the present invention comprises a boiler  1  having a reheating unit  5 , a high pressure turbine  3  which is driven by steam of the boiler  1 , a medium pressure turbine  7  which is driven by steam discharged from the high pressure turbine  3  and heated by the reheating unit  5 , a low pressure turbine  8  which is driven by steam discharged from the medium pressure turbine  7 , and a generator  13  which generates electric power by the rotation of these turbines. The exhaust side of the low pressure turbine  8  is connected to the boiler  1  via a line  11  provided with a condenser  10  which condenses the exhaust, and an overhead condenser  25  which effects heat exchange between condensed water and recovered CO 2 , in this sequence. 
     Further, on the combustion exhaust gas outlet side of the boiler  1 , a blasting blower  14  which pressurizes of a combustion exhaust gas, a cooler  15  which cools the combustion exhaust gas, and a CO 2  absorption tower  18  which is filled with CO 2  absorption liquid for absorbing and removing CO 2  from the combustion exhaust gas are successively arranged in this sequence from the side of the boiler. It is noted that as the CO 2  absorption liquid, an alkanolamine such as, for example, monoethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, diisopropanolamine, diglycolamine, is preferred, and an aqueous solution of one of these compositions or an aqueous solution obtained by mixing two or more of these compositions can be used. 
     The CO 2  absorption tower  18  is installed in combination with a regeneration tower  24  which regenerates the loaded adsorption liquid with CO 2  absorbed therein. These towers are connected by a line  20  which supplies the loaded absorption liquid to the regeneration tower  24 , and by a line  19  which supplies a reproduced adsorption liquid to the CO 2  absorption tower  18 . A rich/lean solvent heat exchanger  23  which effects heat exchange between the line  20  and the line  19  is provided for the line  20  and the line  19 . Further, a lean solvent cooler  33  which further cools the regenerated adsorption liquid is provided for the line  19  between the CO 2  absorption tower  18  and the heat exchanger  23 . 
     In the regeneration tower  24 , as shown in  FIG. 2 , a nozzle  56  for spraying the loaded adsorption liquid downward from the line  20  is provided. Underneath the nozzle  56 , a lower filling section  52  filled with a filler is provided in order to make the sprayed loaded adsorption liquid easily brought into contact with steam. Further, above the nozzle  56 , an upper filling section  51  filled with a filler is provided in order to remove adsorption liquid steam and mist. 
     A first reboiler  41  for heating the loaded absorption liquid is provided for a bottom part of the regeneration tower  24 . The first reboiler  41  and the regeneration tower  24  are connected by a line  47 , which leads the loaded adsorption liquid stored in the tower bottom part to be heated by the first reboiler and then returns the heated absorption liquid again to the tower bottom part. Further, the first reboiler  41  and the low pressure turbine  8  are connected by a line  44  which supplies steam extracted from the low pressure turbine  8  as a heating source of the first reboiler  41 . 
     Further, in the regeneration tower  24 , a liquid storage section  61  for storing the loaded adsorption liquid which flows down is provided between the nozzle  56  and the tower bottom part. Thus, the lower filling section  52  is vertically divided into two parts which are positioned above and below the liquid storage section  61 . Further, a second reboiler  42  for heating the loaded adsorption liquid is provided for a preceding stage of the first reboiler  41 . The second reboiler  42  and the regeneration tower  24  is connected by a line  48 , which leads the loaded adsorption liquid stored in the liquid storage section  61  to be heated by the second reboiler and then returns to the lower part of the liquid storage section  61 . Further, the second reboiler  42  and the low pressure turbine  8  are connected by a line  45  which supplies, as a heating source of the second reboiler  42 , steam with a pressure lower than the pressure of the steam which is extracted to be supplied to the first reboiler  41 . 
     It is noted that a nozzle  58  for spraying the heated loaded adsorption liquid downward is provided for the line  48 . Further, a vent hole  62  for allowing CO 2  gas ascending from the lower part of the tower to pass upward is provided for the liquid storage section  61 . Above the vent hole  62 , there is provided a top plate  63  for preventing the loaded adsorption liquid, which flows down from the upper part of the tower, from passing to the lower part of the tower. 
     Further, a line  28  is provided for the CO 2  gas outlet side of the tower top part of the regeneration tower  24 , the line  28  being successively provided with an overhead condenser  25  for effecting heat exchange between CO 2  gas and condensed water, an overhead cooler  26  for cooling CO 2  gas, and a separator  27  for separating water content from CO 2  gas, in this sequence. In addition, a line  30  which supplies the water separated by the separator  27  again to the tower top part of the regeneration tower  24  is provided for the separator  27 . A nozzle  57  for spraying the reflux water downward is provided for the line  30 . 
     With the above configuration, steam which is generated and heated to a high pressure and a high temperature (of about 250 kg/cm 2 G, about 600° C.) by the boiler  1  is introduced into the high pressure turbine  3  via a line  2  to drive the high pressure turbine  3 . Steam (of about 40 kg/cm 2 G, about 300° C.) discharged from the high pressure turbine via a line  4  is heated by the reheating unit  5  in the boiler  1 . The steam discharged from the high pressure turbine which is reheated (to about 600° C.), is introduced into the intermediate pressure turbine  7  via a line  6 , to drive the medium pressure turbine  7 . Steam (of about 10 kg/cm 2 G) discharged from the intermediate pressure turbine is introduced into the low pressure turbine  8  via a line  9  to drive the low pressure turbine  8 . In this way, the turbines are driven to enable the generator  13  to generate electric power. 
     Further, a part of the steam is extracted from the low pressure turbine and supplied via the line  44  to the first reboiler  41  provided for the tower bottom part. Further, a part of steam with a pressure lower than the pressure of the steam supplied to the first reboiler is extracted from the low pressure turbine and supplied to the second reboiler  42  via the line  45 . The two kinds of extracted steam are respectively used to heat the loaded absorption liquid in the first reboiler  41  and the second reboiler  42 , so as to be condensed. Further, the two kinds of extracted steam are pressurized by a reboiler condensate pump  32 , and then mixed with boiler feed water of the line  11 . Thereby, the boiler feed water is heated up and transferred to the boiler  1 . 
     Here, the steam which is extracted to be supplied to the first reboiler  41  provided for the tower bottom part, preferably has a temperature which makes it possible to remove almost all CO 2  from the loaded absorption liquid to regenerate the absorption liquid, and which for example preferably ranges from 130 to 160° C., although the temperature may be different depending upon the kinds of CO 2  absorption liquid. It is noted that the absolute pressure of the steam corresponding to this temperature ranges from 2.75 to 6.31 ata. Further, the steam which is extracted to be supplied to the second reboiler  42  preferably has a temperature lower than the above described temperature, that is, an absolute pressure lower than the above described absolute pressure, in order to heat the loaded absorption liquid in stages. It is noted that when supplied into the regeneration tower  24 , the loaded absorption liquid is depressurized to release a part of CO 2  and cooled (for example, by a temperature about 20° C.). Therefore, the lower limit value of the steam is preferably set to a temperature which makes it possible to effect heat exchange with the absorption liquid with the temperature when it is introduced into the tower (for example, a temperature higher by about 10° C. compared with the temperature of the absorption liquid after it is introduced into the tower, or a temperature lower by about 10° C. compared with the temperature of the absorption liquid when it is supplied to the tower), that is, preferably set to an absolute pressure corresponding to the steam temperature. 
     The exhaust (of about 0.05 ata, about 33° C.) of the low pressure turbine  8  is introduced into the condenser  10  via the line  11  and condensed. A boiler feed pump  12  makes the condensed water preheated through the overhead condenser  25  and then transferred to the boiler  1  as the boiler feed water. 
     On the other hand, the boiler combustion exhaust gas containing CO 2  discharged from the boiler  1  is first pressurized by the blasting blower  14 , and then transferred to the cooler  15  so as to be cooled by cooling water  16 . The cooled combustion exhaust gas is transferred to the CO 2  absorption tower  18 , and cooling wastewater  17  is discharged to the outside of the system. 
     In the CO 2  absorption tower  18 , the combustion exhaust gas is brought into contact in counterflow with CO 2  absorption liquid based on the alkanolamine, so that CO 2  in the combustion exhaust gas is absorbed by the CO 2  absorption liquid through a chemical reaction. The combustion exhaust gas  21  with CO 2  removed therefrom is discharged from the tower top part to the outside of the system. The loaded absorption liquid (rich absorption liquid) with CO 2  absorbed therein is pressurized by a rich solvent pump  22  via the line  20  connected to the tower bottom part, and heated by the rich/lean solvent heat exchanger  23 , and thereafter is supplied to the regeneration tower  24 . 
     In the regeneration tower  24 , the loaded absorption liquid is sprayed from the nozzle  56 , and flows downward through the lower filling section  52 B so as to be stored in the liquid storage section  61 . Then, the loaded absorption liquid in the liquid storage section  61  is extracted by the line  48 , and heated by the low pressure steam of the line  45  in the second reboiler  42 , and thereafter returned again to the regeneration tower  24 . The loaded absorption liquid thus heated is sprayed by the nozzle  58 , and a CO 2  gas partially separated from the absorption liquid by the heating operation ascends upward in the tower as shown by a dotted line in  FIG. 2 , while the loaded absorption liquid still containing CO 2  flows down in the tower. 
     Further, the loaded absorption liquid, which passes through the lower filling section  52 A and is stored in the tower bottom part, is extracted by the line  47  to be heated by the higher pressure steam of the line  44  in the first reboiler  41 , and thereafter is returned again to the tower bottom part. The residual CO 2  is almost separated from the absorption liquid by this heating operation in the first reboiler  41  of the tower bottom part. The separated CO 2  gas ascends in the tower in the same way as described above. 
     The CO 2  gas which ascends in the tower is discharged from the tower top part of the regeneration tower  24 . The discharged CO 2  gas passes through the line  28 , to preheat the boiler feed water of the line  11  in the overhead condenser  25 , and is further cooled by the overhead cooler  26 . Thereby, the water content in the CO 2  gas is condensed. The condensed water is removed by the separator  27 . The high purity CO 2  gas with water content removed therefrom is discharged to the outside of the power generation system, so as to be able to be used effectively for other applications. 
     Further, the condensed water separated by the separator  27  is refluxed by a condensed water circulation pump  29  into the regeneration tower  24  through the line  30 . The reflux water is sprayed by the nozzle  57  to wash CO 2  gas ascending through the upper filling section  51 , thereby making it possible to prevent the amine compound contained in the CO 2  gas from being discharged from the tower top part. 
     On the other hand, almost all CO 2  is separated from the loaded absorption liquid by the heating operation in the first reboiler of the tower bottom part, so that the absorption liquid is regenerated. The regenerated absorption liquid (lean absorption liquid) is extracted by the line  19 , and pressurized by a lean solvent pump  31 . Then, the regenerated absorption liquid is cooled by the loaded absorption liquid in the rich/lean solvent heat exchanger  23  and is further cooled by the lean solvent cooler  33  so as subsequently to be supplied to the CO 2  absorption tower  18 . Thus, the CO 2  absorption liquid can be used in circulation in the power generation system. 
     In this way, high pressure steam is extracted from the low pressure turbine  8  as a heating source of the first reboiler  41  of the tower bottom part, and steam with a pressure lower than the pressure of the high pressure steam is extracted from the low pressure turbine  8  as a heating source of the second reboiler  42  between the nozzle  56  and the tower bottom part, as a result of which the loaded absorption liquid can be heated in stages by the steam extracted in the two stages. Thus, instead of a part of the high pressure steam extracted from the low pressure turbine  8 , which part is to be supplied to the first reboiler  41  of the tower bottom part, steam with a lower pressure can be extracted from the low pressure turbine  8 , as a result of which output decrease of the low pressure turbine  8  can be suppressed as a whole and power generation output of the generator  13  can be improved. 
     It is noted that in  FIG. 1  and  FIG. 2 , the reboiler is constituted in two stages by providing the second reboiler  42  between the nozzle  56  and the tower bottom part so as to extract steam from the low pressure turbine  8  in two stages. However, the reboiler provided for the regeneration tower  24  may be constituted in three or more stages to extract steam from the low pressure turbine  8  in three or more stages. In this case, the line which supplies the extracted steam to the reboiler is connected so as to make the pressure of supplied steam increased from the reboiler in the preceding stage of the regeneration tower  24  (the tower top part side) to the reboiler in the post stage of the regeneration tower  24  (the tower bottom part side). 
     For example, as shown in  FIG. 3 , a liquid storage section  66 , a vent hole  67  and a top plate  68  are additionally provided between the nozzle  56  and the liquid storage section  61 , and a third reboiler  43  is also provided in the preceding stage of the second reboiler  42 , so that steam with a pressure further lower than the pressure of the steam supplied to the second reboiler  42  is extracted from the low pressure turbine  8  and is supplied to the third reboiler  43  via a line  46 . Thereby, the loaded absorption liquid in the added liquid storage section  66  is heated by the third reboiler  43  via a line  49 . As a result, the loaded absorption liquid in the regeneration tower  24  can be heated in more stages. Therefore, instead of a part of the high pressure steam supplied to the first reboiler  41  and the second reboiler  42 , the steam with further lower pressure is extracted from the low pressure turbine  8 , so that output decrease of the low pressure turbine  8  can be further suppressed. 
     EXAMPLE 
     A rich absorption liquid with CO 2  absorbed therein is regenerated by using a steam system consisting of the regeneration tower and the low pressure turbine shown in  FIG. 3 . The result is shown in Table 1. Further, a result of the case where the steam system consisting of the conventional regeneration tower and the low pressure turbine shown in  FIG. 4  is used, is also shown in Table 1 as a comparison example. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 CONVENTIONAL 
                 PRESENT 
               
               
                   
                 SYSTEM 
                 INVENTION 
               
               
                   
                 (FIG. 4) 
                 (FIG. 3) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 CO 2  RECOVERY AMOUNT 
                 324 ton/h 
                 324 ton/h 
               
               
                 REBOILER INPUT HEAT 
                 242.41 Gcal/h 
                 243.02 Gcal/h 
               
               
                 AMOUNT 
               
             
          
           
               
                 REBOILER 
                 FIRST 
                 417 ton/h 
                 174 ton/h 
               
               
                 INPUT STEAM 
                 REBOILER 
                 (3.6 ata) 
                 (3.6 ata) 
               
               
                 AMOUNT 
                 SECOND 
                 — 
                 138 ton/h 
               
               
                 (ABSOLUTE 
                 REBOILER 
                   
                 (3.16 ata) 
               
               
                 PRESSURE) 
                 THIRD 
                 — 
                 107 ton/h 
               
               
                   
                 REBOILER 
                   
                 (2.73 ata) 
               
             
          
           
               
                 TURBINE OUTPUT DECREASE 
                 76,330 W 
                 73,756 kW 
               
               
                 DUE TO EXTRACTION OF 
                 (100) 
                 (96.6) 
               
               
                 STEAM SUPPLIED TO 
               
               
                 REBOILER (CONVENTIONAL 
               
               
                 CASE: 100) 
               
               
                 RICH ABSORPTION LIQUID 
                 3824 ton/h 
                 3824 ton/h 
               
               
                 SUPPLY AMOUNT 
               
               
                 LEAN ABSORPTION LIQUID 
                 3500 ton/h 
                 3500 ton/h 
               
               
                 DISCHARGE AMOUNT 
               
               
                 REGENERATION TOWER 
                 112° C. 
                 112° C. 
               
               
                 INLET TEMPERATURE 
               
               
                 OF RICH 
               
               
                 ABSORPTION LIQUID 
               
               
                 REGENERATION TOWER 
                 120° C. 
                 120° C. 
               
               
                 OUTLET TEMPERATURE 
               
               
                 OF LEAN 
               
               
                 ABSORPTION LIQUID 
               
               
                   
               
             
          
         
       
     
     As shown in Table 1, in the conventional system, it is necessary to supply high pressure steam of 3.6 ata to the reboiler of the tower bottom part at a rate of 417 ton/h, in order to make the rich absorption liquid of a predetermined amount heated to 120° C. and regenerated. As a result, the output of the low pressure turbine from which the steam is extracted, is lowered by 76,330 kW. On the other hand, in the system according to the present invention shown in  FIG. 3 , steam with a lower pressure of 2.73 ata and steam with a lower pressure of 3.16 ata are supplied to the third reboiler and the second reboiler at a rate of 107 ton/h and at a rate of 138 ton/h, respectively, so that even when the rate of the high pressure steam of 3.6 ata supplied to the first reboiler of the tower bottom part is reduced to 174 ton/h, the rich absorption liquid can be regenerated similarly to the conventional system. Therefore, the total amount of heat supplied to the first to third reboilers is approximately equal to the amount of heat supplied to the reboiler of the tower bottom part in the conventional system, but the output of the low pressure turbine is lowered only by 73,756 kW. As a result, the turbine output can be improved by about 3.4% in comparison with the conventional system.