Abstract:
A method for recovering CO 2  from gas to be processed containing CO 2 , includes bringing the gas to be processed containing CO 2  and CO 2  absorbent into contact with each other to absorb and remove CO 2  from the gas to be processed; cleaning the treated gas from which CO 2  has been removed with washing fluid at least once; heating the absorbent which has absorbed CO 2 , separating and removing CO 2  gas from the absorbent and regenerating the absorbent; cooling the separated CO 2  gas to condense moisture contained in the gas to obtain condensed water; and monitoring changes in concentration of the CO 2  absorbent contained in the condensed water and depending on the value of the measured concentration, controlling supply of the condensed water so that the condensed water is reused as a part of the washing fluid or a part of the CO 2  absorbent.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a system configured to recover carbon dioxide (CO 2 ) and a method for recovering CO 2 . 
     The greenhouse effect due to CO 2  has been regarded as one of the causes of global warming. Accordingly, much research and development has been carried out on technologies for preventing or suppressing release of CO 2  into the atmosphere. Because CO 2  is generated mainly by combustion of fossil fuels, it is desired that exhaust gas generated by the combustion of fossil fuel be emitted into the atmosphere after CO 2  contained in the gas is appropriately reduced or removed therefrom. 
     Japanese Patent Application Publication No. 2011-115724 discloses a system including: a CO 2  absorption apparatus configured to absorb and remove CO 2  contained in exhaust gas from combustion of fossil fuel by bringing the exhaust gas into contact with CO 2  absorbent such as basic amine compound; a regeneration apparatus configured to regenerate an absorbent which has absorbed CO 2 ; and a recovery apparatus configured to recover CO 2  separated from the absorbent by the regeneration apparatus. This patent literature also discusses provision of a plurality of washing units configured to clean treated gas by removing CO 2  with washing fluid. Furthermore, according to the above literature, the washing fluid is circulated at each washing unit, and by measuring the concentration of basic amine compound contained in the gas after washing, the circulation rate of washing fluid is adjusted to an appropriate level for collecting the basic amine compound. 
     SUMMARY OF THE INVENTION 
     In a CO 2  absorption apparatus having such a configuration, the concentration of basic amine compound which is a principal component of CO 2  absorbent accompanied by exhaust gas is demanded to be reduced as much as possible. Consequently, a large amount of washing fluid is used at a plurality of washing units. The washing fluid used in the washing units is circulated. In addition to this idea, the inventor of the present invention has considered use of condensed water of CO 2  gas discharged from a regeneration apparatus. However, when an operating condition of the regeneration apparatus is unstable like at a startup time of the system or at a shut down of the system, the concentration of basic amine compound in condensed water changes. Thus, if condensed water containing the basic amine compound at a high concentration is supplied to washing units of the CO 2  absorption apparatus, its washing effect is reduced, so that an amount of basic amine compound accompanied by exhausted gas is increased, which is a problem which the present invention intends to solve. 
     Accordingly, the present invention is directed to providing a CO 2  recovery system and a method therefore, capable of, even when condensed water of CO 2  gas exhausted from a regeneration apparatus for CO 2  absorbent liquid is used as a washing fluid in the washing units of the CO 2  absorption apparatus, preventing the amount of basic amine compound accompanied by gas exhausted from the CO 2  absorption apparatus from being increased 
     According to an aspect of the present invention, a system for recovering CO 2  from gas to be processed containing CO 2 , comprises: a CO 2  absorption apparatus having an absorption unit which brings the gas to be processed containing CO 2  into contact with CO 2  absorbent to absorb and remove CO 2  from the gas to be processed; a regeneration apparatus configured to heat the absorbent that has absorbed CO 2 , separates and removes CO 2  from the absorbent, exhausts CO 2  and regenerates the absorbent; a condensation apparatus configured to cool CO 2  gas exhausted from the regeneration apparatus to condense moisture in the gas; and a condensed water distribution apparatus configured to monitor changes in concentration of the CO 2  absorbent in the condensed water obtained by the condensation apparatus and depending on the value of a measured concentration, supply the condensed water as a part of washing fluid of the washing unit in the CO 2  absorption apparatus or a part of the CO 2  absorbent of the absorption unit. As an analyzer for monitoring changes in concentration of the CO 2  absorbent in the condensed water, a pH meter, an electric conductivity meter, or resistivity meter may be used. 
     The CO 2  absorption apparatus may contain a plurality of the washing units arranged in series for the treated gas from which CO 2  has been removed by the absorption unit. The condensed water distribution apparatus may be constructed to, when the measured pH value is higher than a first threshold, supply condensed water to a washing unit located upstream with respect to a flow of the treated gas, of the plurality of the washing units, and when the measured value is lower than the first threshold, supply condensed water to the washing unit located downstream. The condensed water distribution apparatus may be constructed to, when the measured value is higher than a first threshold and a second threshold, supply condensed water to the absorption unit, and when the measured value is higher than the first threshold and lower than the second threshold, supply condensed water to a washing unit located upstream with respect to a flow of the treated gas, of the plurality of the washing units, and when the measured value is lower than the first threshold and the second threshold, supply condensed water to a washing unit located downstream. 
     According to another aspect of the present invention, a method for recovering CO 2  from gas to be processed containing CO 2 , comprises: a step of bringing the gas to be processed containing CO 2  and CO 2  absorbent into contact with each other to absorb and remove CO 2  from the gas to be processed; a step of washing the processed gas from which CO 2  has been removed with washing fluid at least once; a step of heating the absorbent which has absorbed CO 2 , separating and removing CO 2  gas from the absorbent and regenerating the absorbent; a step of cooling the separated CO 2  gas to condense moisture contained in the gas to obtain condensed water; and a step of monitoring a concentration of the CO 2  absorbent contained in the condensed water and depending on the value of the measured concentration, controlling supply of the condensed water so that the condensed water is reused as a part of the washing fluid or a part of the CO 2  absorbent. To monitor changes in concentration of the CO 2  absorbent in the condensed water, it is permissible to measure pH, electric conductivity, or resistivity of the condensed water. 
     The step of washing the process gas may include cleaning the treated gas with washing fluid multiple times. In this case, the control step may include, when the measured pH value is higher than the first threshold, supplying condensed water so that the condensed water is reused as a part of washing fluid for washing on an upstream side (i.e., initial washing) with respect to the flow of the treated gas of the plurality of the washings, and when the measured pH value is lower than the first threshold, supplying the condensed water so that the condensed water is reused as a part of washing fluid for washing on a downstream side (i.e., second washing). Alternatively, the control step may include: when the measured value is higher than the first threshold and the second threshold, supplying the condensed water so that the condensed water is reused as a part of the absorbent; when the measured value is higher than the first threshold and lower than the second threshold, supplying the condensed water so that the condensed water is reused as a part of washing fluid for washing on an upstream side with respect to the flow of the treated gas of the plurality of the washing; and when the measured value is lower than the first and second thresholds, supplying the condensed water as a part of the washing fluid for the second washing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically illustrating a CO 2  recovery system according to an exemplary embodiment of the present invention. 
         FIG. 2  is a diagram schematically illustrating an improvement of the exemplary embodiment illustrated in  FIG. 1 . 
         FIG. 3  is a diagram schematically illustrating a CO 2  recovery system according to another exemplary embodiment of the present invention. 
         FIG. 4  is a diagram schematically illustrating an improvement example of the exemplary embodiment illustrated in  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an exemplary embodiment of a CO 2  recovery system and method therefor according to the present invention will be described with reference to the accompanying drawings. 
     Referring to  FIG. 1 , a CO 2  recovery system according to the present exemplary embodiment includes, as main components thereof, a desulfurization tower  10  configured to remove sulfur oxide from gas to be processed containing the sulfur oxide and carbon dioxide, such as combustion exhaust gas of fossil fuel, a CO 2  absorption tower  20  configured to remove CO 2  from the desulfurization gas from which sulfur oxide has been removed by the desulfurization tower, using CO 2  absorbent, and a regeneration tower  40  configured to desorb CO 2  from the CO 2  absorbent which has absorbed CO 2  (hereinafter referred to as “rich absorbent”) to regenerate CO 2  absorbent (hereinafter referred to as “lean absorbent”). 
     The desulfurization tower  10  includes a desulfurization unit  12  for removing sulfur oxide from gas to be processed at a high level in a bottom portion of the tower with respect to a chimney tray  14  provided in a central area of the tower and a desulfurization gas cooling unit  13  for cooling desulfurization gas which has passed the desulfurization unit at approximately 50° C. or less. This is also called the high-level desulfurization cooling tower. The desulfurization tower  12  further includes a demister  17  which is located nearer a top portion of the cooling tower  13  to remove droplets contained in passing gas. 
     The desulfurization tower  10  further includes a gas introduction line  11  provided at a bottom portion of the desulfurization unit  12  to introduce gas to be processed into the tower, a cooling fluid circulation line  16  which connects an upper portion of the cooling tower  13  with a lower portion thereof to circulatively supply the cooling fluid accumulated in the chimney tray  14  to the cooling tower  13 , a cooling tower  16  for cooling the cooling fluid in the cooling fluid circulation line  16 , and a gas exhaust line  18  which is provided on the top portion of the absorption tower  10  to exhaust the desulfurization gas which has passed the desulfurization unit  12  and the cooling unit  13 . 
     Preferably, a desulfurization absorbent for use in the desulfurization unit  12  contains a compound of, for example, calcium carbonate, calcium hydroxide, magnesium hydroxide, sodium hydroxide or a mixture of two or more of the compounds. The concentration of the compound contained in the desulfurization absorbent is preferred to be 0.1 to 30% by weight. 
     The CO 2  absorption tower  20  includes a CO 2  absorption unit  21  in a lower portion of the absorption tower  20  and a plurality of washing units  22 ,  23  in an upper portion of the absorption tower  20 . The plurality of the washing units  22 ,  23  are arranged in series with respect to a flow of desulfurized carbon dioxide gas after CO 2  is removed in the CO 2  absorption unit in the lower portion. Additionally, the CO 2  absorption tower  20  includes chimney trays  24 ,  25  between the CO 2  absorption tower  20  and the plurality of the washing units  22 ,  23 , and further includes a demister  27  for removing droplets contained in passing gas on the side of the top portion of the washing unit  23 . A gas introduction line  18  for introducing desulfurized carbon dioxide into the tower is provided below the CO 2  absorption unit  21 . 
     The CO 2  absorption tower  20  further includes a lean absorbent line  52 , which is provided above the CO 2  absorption unit  21  to supply CO 2  absorbent to the CO 2  absorption unit  21 , and a rich absorbent line  41 , which is provided in the bottom portion of the absorption tower  20  to exhaust the rich absorbent which has absorbed CO 2 . The CO 2  absorption tower  20  further includes washing fluid circulation lines  31 ,  33  which connect an upper portion of each of the washing units  22 ,  23  with a lower portion thereof to circulatively supply the washing fluid accumulated in the chimney trays  24 ,  25  to the washing units  22 ,  23 , and a gas exhaust line  28 , which is provided on the top portion of the absorption tower  20  to exhaust the gas which has passed the CO 2  absorption unit  21  and the washing units  22 ,  23 . The washing fluid circulation line  31  on the side of the CO 2  absorption unit  21  is provided with a cooler  32  configured to cool washing fluid supplied circulatively. 
     The CO 2  absorbent is not limited to any particular type absorbent. However, it is useful to use a CO 2  absorbent containing a basic amine compound as a main component thereof. The basic amine compound includes, for example, top-grade amines containing alcoholic hydroxyl, such as monoethanol-amine or 2-amino-2-methyl-1-propanol, second-grade amines containing alcoholic hydroxyl, such as diethanolamine, 2-methyl aminoethanol, or 2-ethylamino ethanol, third-grade amines containing alcoholic hydroxyl, such as triethanolamine, N-methyldiethanolamine, 2-dimethyl aminoethanol, or 2-diethylaminoethanol, polyethylene polyamines, such as ethylenediamine, triethylenediamine, or diethylenetriamine, cyclic amines, such as piperazines, piperidines, or pyrrolidines, polyamines, such as xylylenediamine, or amino acids, such as methylamine carboxylic acid. The CO 2  absorbent may contain one or a plurality of the compounds described above. The concentration of the basic amine may be 10-70% by weight. The CO 2  absorbent can contain a CO 2  absorption accelerator and a corrosion inhibitor. In addition, the CO 2  absorbent can include a medium other than those described above, such as methanol, polyethylene glycol, or sulfolane. 
     The washing fluid circulation line  33  of the washing unit  23  is provided with a washing fluid replenishment line  38  configured to supply fresh washing fluid. The washing fluid used in the washing units  22 ,  23  is not limited to any particular type of washing fluid. However, it is useful if running water, industrial water or the like is used. 
     The regeneration tower  40  includes a CO 2  desorption unit  42 , which is provided in a portion of the regeneration tower  40  from the center to a lower portion thereof. In addition, the regeneration tower  40  includes a washing unit  43 , which is provided above the CO 2  desorption unit  42  and a chimney tray  44 , which is provided below the CO 2  desorption unit  42 . In the regeneration tower  40 , the rich absorbent line  41  for introducing the rich absorbent which has absorbed CO 2  in the absorption tower  20  into the regeneration tower  40  is provided between the CO 2  desorption unit  42  and the washing unit  43 . In addition, in the regeneration tower  40 , the lean absorbent line  52  for supplying the regeneration-processed lean absorbent to the CO 2  absorption tower  20  is provided in the bottom portion of the regeneration tower  40 . Furthermore, in the regeneration tower  40 , a heat exchanger  53 , which exchanges heat between the rich absorbent line  41  and the lean absorbent line  52 , is provided. In addition, a heat exchanger  54 , which further recovers heat from the lean absorbent, is provided between the heat exchanger  53  and the CO 2  absorption tower  20 . 
     The regeneration tower  40  includes an absorbent regeneration line  45  for extracting a part of the lean absorbent from the bottom portion of the regeneration tower  40  and for supplying the extracted lean absorbent to a portion above the chimney tray  44 . The absorbent regeneration line  45  includes a reboiler  46 , which heats the lean absorbent. In addition, the regeneration tower  40  includes a CO 2  gas exhaust line  47  for exhausting CO 2  gas which has been desorbed from the rich absorbent, from the top portion of the regeneration tower  40 . The CO 2  gas exhaust line  47  includes a condenser  48  which condenses steam entrained in the CO 2  gas and a separator drum  49  which separates condensed water, which results from the condensation by the condenser  48  from the gas. The condenser  48  may use cooling water to cool the gas. A condensed water return line  34 , which is a line for supplying the separated condensed water as washing fluid for the washing unit  43  of the regeneration tower  40 , is provided on the separator drum  49 . The condensed water return line  34  is provided with a condensed water return line  35  for supplying a part of the condensed water to the washing units  22 ,  23  as the washing fluid. 
     The condensed water transfer line  35  contains a pH meter  37  configured to measure a pH of the condensed water in the same line. The condensed water transfer line  35  is branched to a first branch line  35 A, which supplies the condensed water to an upstream side with respect to a flow of decarbonized gas within the CO 2  absorption tower  20  or the first washing unit  22  on the side of the bottom of the tower, and a second branch line  35 B, which supplies the condensed water to the downstream with respect to the flow of decarbonized gas or the second washing unit  23  on the side of the top portion of the tower. More specifically, the branch lines  35 A,  35 B are connected to the washing fluid circulation lines  31 ,  33 . In a case in which the cooler  32  is provided on the washing fluid circulation line  31  like in the first washing unit  22 , the branch line  35 A is connected to an outlet of the cooler  32 . The branch lines  35 A,  35 B are provided with valves  36 A,  36 B for adjusting the flow rate of the condensed water. In addition, although  FIG. 1  illustrates two washing units  22 ,  23 , the present invention is not restricted to this example, and the CO 2  absorption tower  20  may include three or more washing units. In this case, the same structure as the first washing unit  22  may be provided repeatedly. 
     The separator drum  49  is provided with a CO 2  gas line  50  configured to supply separated CO 2  gas to a CO 2  gas pressurization system (not illustrated). The CO 2  gas line  50  includes a valve  51  for adjusting the flow rate of CO 2  gas. The CO 2  gas pressurization system compresses CO 2  gas to a predetermined pressure using a plurality of compressors. 
     With this structure, first, the gas to be processed containing sulfur oxide and carbon dioxide is introduced into the desulfurization tower  10  via the gas introduction line  11 . By bringing this gas into contact with desulfurization absorbent and absorbing and removing the sulfur oxide contained in the gas, the desulfurization unit  12  may carry out high-level desulfurization processing so that the concentration of the sulfur oxide is 5 ppm or less, preferably 1 ppm or less. When the concentration of the sulfur oxide in the gas exceeds 5 μm, the sulfur oxide is accumulated in the CO 2  absorbent for use in the CO 2  absorption tower  20  so that the frequency of reclaiming the CO 2  absorbent increases, which is a problem. 
     The desulfurization gas surpasses the chimney tray  14  and flows into the desulfurization gas cooling unit  13  in the top portion of the desulfurization tower  10 . As a result, the temperature of the desulfurization gas is reduced to 50° C. or less, preferably 45° C. or less, more preferably 30 to 45° C. When the temperature of the gas exceeds 50° C., the amount of basic amine which is a main component of the CO 2  absorbent accompanied by gas increases so that the basic amine compound is consumed in waste, which is a problem. By installing the high-grade desulfurization cooling apparatus upstream of the gas to be processed in the CO 2  absorption tower  20 , the CO 2  absorption tower  20  may remove CO 2  from the gas to be processed and collect the CO 2  easily. 
     Next, the desulfurization gas containing CO 2  from the desulfurization tower  10  is introduced into the CO 2  absorption tower  20  via the gas exhaust line  18 . The CO 2  absorbent is supplied to the CO 2  absorption tower  20  from the lean absorbent line  52  and the desulfurization gas and the CO 2  absorbent are brought into gas-liquid contact with each other in the CO 2  absorption unit  21  to absorb and remove CO 2  in the desulfurization gas using the CO 2  absorbent. After CO 2  is removed, the gas flows into the first washing unit  22  across the chimney tray  24 . The gas is cleaned with the washing fluid and flows into the second washing unit  23  across the chimney tray  25 . After that, the gas is further cleaned. After it is cleaned in the second washing unit  23 , the decarbonized gas passes the demister  27  and is exhausted through the gas exhaust line  28  in the top portion of the CO 2  absorption tower  20 . The washing fluid used in the first and second washing units  22 ,  23  is accumulated in the chimney trays  24 ,  25  and never flows down into the CO 2  absorption unit  21 . The washing fluid accumulated here is circulatively used in the washing units  22 ,  23  via the washing fluid circulation lines  31 ,  33 . 
     The rich absorbent which has absorbed CO 2  in the CO 2  absorption tower  20  is discharged from the bottom portion of the CO 2  absorption tower  20  via the rich absorbent line  41 , heated by the heat exchanger  53 , and then, it is fed to the regeneration tower  40 . In the regeneration tower  40 , the rich absorbent is diffused into the CO 2  desorption unit  42  from the rich absorbent line  41 . The rich absorbent flows down in the CO 2  desorption unit  42  while at the same time heated. After most CO 2  is exhausted, the rich absorbent flows down to the chimney tray  44  near the bottom portion of the regeneration tower  40 . The absorbent accumulated in the chimney tray  44  is heated by the reboiler  46  via the absorbent regeneration line  45  and remaining CO 2  is exhausted. Then, the absorbent is regenerated and brought back to the bottom portion of the regeneration tower  40 . The regenerated lean absorbent is fed via the lean absorbent line  52  on the bottom portion of the regeneration tower  40  and heats the rich absorbent at the heat exchanger  53 . After heat is further collected at the heat exchanger  54 , the rich absorbent is supplied to the CO 2  absorption tower  20 . 
     The CO 2  gas which has been desorbed from the rich absorbent passes through the chimney  44  and the CO 2  desorption unit  42 , ascending to the washing unit  43 . In the washing unit  43 , washing fluid is diffused from the condensed water return line  34  to remove the CO 2  absorbent accompanied by the CO 2  gas. The CO 2  gas which has been cleaned by the washing unit  43  is exhausted from the CO 2  gas exhaust line  47  in the top portion of the regeneration tower  40 . Steam entrained in the CO 2  gas which has been exhausted from the regeneration tower  40  is condensed by the condenser  48  and the condensed water is separated by the separation drum  49 . After the condensed water is removed, the CO 2  gas is supplied to the CO 2  gas pressurization system (not illustrated) via the CO 2  gas line  50 , compressed to a predetermined pressure and then collected. 
     Of the separated condensed water, a part thereof is supplied to the regeneration tower  40  via the condensed water return line  34  and the other part thereof is supplied to the washing fluid circulation lines  31 ,  33  of the washing units  22 ,  23  via the condensed water transfer line  35  and reused. The condensed water which passes through the condensed water transfer line  35  is measured in pH by the pH meter  37 . When the measured pH value is higher than a first threshold, a valve  36  is opened/closed to supply the condensed water to the first washing unit  22 , and when the measured pH value is lower than the first threshold, the valve  36  is opened/closed to supply the condensed water to the second washing unit  23 . In this case, from the viewpoint of maintaining cleaning effect in the washing unit, the first threshold is preferred to be 6 to 10 and more preferred to be 7 to 9. 
     When an operating condition of the regeneration tower  40  is unstable at the startup time or stop time of the system, the concentration of basic amine of condensed water obtained from the separator drum  49  may change. When condensed water having a high concentration of the basic amine is supplied to the second washing unit  23  in the top portion of the CO 2  absorption tower  20 , the amount of basic amine compound accompanied by gas exhausted from the gas exhaust line  28  increases, so that a diffused amount of basic amine compound increases. Thus, when the pH value of washing fluid in the condensed water transfer line  35  is higher than the first threshold as described above or the concentration of basic amine compound is high, the condensed water is supplied to the first washing unit  22  far from the gas exhaust line  28  in the top portion of the CO 2  absorption tower  20  to suppress the diffused amount of the basic amine compound. 
       FIG. 2  illustrates an improvement of the embodiment illustrated in  FIG. 1 . Like reference numerals are attached to the same components as the embodiment of  FIG. 1  and description of those components is omitted. 
     As illustrated in  FIG. 2 , a pH meter  37 A for measuring pH of condensed water in the condensed water transfer line  35  is provided on the condensed water transfer line  35  from the regeneration tower  40 . The cleaning water circulation line  33  of the second washing unit  23  in the CO 2  absorption tower  20  is provided with a pH meter  37 B for measuring pH of washing fluid flowing through the condensed water transfer line  35 . The pH meters  37 A,  37 B are connected to the control unit  30  which controls the flow rate of each of the valves  36 A,  36 B according to pH values measured by the pH meters  37 A,  27 B so that the pH meters  37 A,  37 B are capable of communicating with the control unit  30 . 
     With such a structure, of condensed water separated by the separator tank  49 , a part thereof is supplied to the regeneration tower  40  via the condensed water return line  34 , where the condensed water is reused. A part of the condensed water is supplied to the first and second washing units  22 ,  23  via the condensed water transfer line  35 , where the condensed water is reused. Condensed water passing through the condensed water transfer line  35  is measured in pH by the pH meter  37 A. Then, based on the pH values measured by the pH meters  37 A,  37 B, the control unit  30  opens/closes the valves  36 A,  36 B provided in the first and second branch lines  35 A,  35 B of the condensed water transfer line  35 . More specifically, assuming that the pH value of condensed water in the condensed water transfer line  35  which the pH meter  37 A measures is pH1 and that the pH value of washing fluid in the washing fluid circulation line  33  of the second washing unit  23  which the pH meter  37 B measures is pH2, when pH1/pH2 exceeds the first threshold of 1 (that is, pH1/pH2&gt;1), the control unit  30  opens the valve  36 A and closes the valves  36 B to supply the condensed water to the first washing unit  22  located on the side of the bottom portion of the CO 2  absorption tower  20 . On the other hand, when the pH1/pH2 is less than the first threshold or pH/pH&lt;1, the control unit  30  closes the valve  36 A and opens the valve  36 B to supply the condensed water to the second washing unit  23  located on the side of the top portion of the CO 2  absorption tower  20 . 
     When with respect to the pH value (pH2) of washing fluid in the washing fluid circulation line  33  of the second washing unit  23 , the pH value (pH1) of condensed water in the condensed water transfer line  35  is high, that is, the concentration of basic amine compound is high, the diffused amount of the basic amine compound can be suppressed by supplying the condensed water to the first washing unit  22  located far from the gas exhaust line  28  in the top portion of the CO 2  absorption tower  20 . 
     Although the case of controlling the pH value with respect to pH1/pH2=1 has been described, the first threshold of pH1/pH2 is not restricted to 1, but may be set in a range of 0.9 to 1. 
       FIG. 3  illustrates another exemplary embodiment of the CO 2  recovery system of the present invention. Like reference numerals are attached to the same components as the exemplary embodiment of  FIG. 1  and description of those components is omitted. 
     As illustrated in  FIG. 3 , the condensed water transfer line  35  is branched to a first branch line  35 A which supplies condensed water to the washing fluid circulation line  31  of the first washing unit  22 , the second branch line  35 B which supplies condensed water to the washing fluid circulation line  33  of the second washing unit  23 , and a third branch line  35 C which supplies condensed water to the rich absorbent line  41  of the CO 2  absorption unit  21 . The respective branch lines  35 A,  35 B,  35 C are provided with valves  36 A,  36 B,  36 C for adjusting the flow rate of the condensed water. 
     With such a structure, of condensed water separated by the separator tank  49 , a part thereof is supplied to the regeneration tower  40  via the condensed water return line  34 , where the condensed water is reused. A part of the condensed water is supplied to the first and second washing units  22 ,  23  and the CO 2  absorption unit  21  via the condensed water transfer line  35 , where the condensed water is reused. Condensed water passing through the condensed water transfer line  35  is measured in pH by the pH meter  37 . When the measured pH value is higher than the first threshold and the second threshold, the valve  36  is opened/closed to supply the condensed water to the CO 2  absorption unit  21 . When the measured pH value is higher than the first threshold and lower than the second threshold, the valve  36  is opened/closed to supply the condensed water to the first washing unit  22 . When the measured pH value is lower than the first threshold and the second threshold, the valve  36  is opened/closed to supply the condensed water to the second washing unit  23 . In this case, the first threshold may be in the same range as described above, and in viewpoints of maintaining cleaning effect in the washing unit, the second threshold is preferred to be in a range of 9 to 11, and more preferred to be in a range of 10 to 11. 
       FIG. 4  illustrates an improvement of the exemplary embodiment illustrated in  FIG. 3 . Like reference numerals are attached to the same components as the exemplary embodiment of  FIG. 3  and description thereof is omitted. 
     As illustrated in  FIG. 4 , a pH meter  37 A for measuring pH of condensed water in the condensed water transfer line  35  is provided in the condensed water transfer line  35  from the regeneration tower  40 . The pH meter  37 B for measuring the pH of washing fluid flowing through the washing fluid circulation line  33  is provided in the washing fluid circulation line  33  of the second washing unit  23  in the CO 2  absorption tower  20 . In the washing fluid circulation line  31  of the first washing unit  22 , a pH meter  37 C for measuring the pH of condensed water in the first branch line  35 A is provided downstream of the first branch line  35 A. The valves  36 A to  36 C and the pH meters  37 A to  37 C are connected to the control unit  30  which controls the flow rate of the valves  36 A to  36 C based on the pH values measured by the pH meters  37 A to  37 C. 
     With such a structure, of condensed water separated by the separator tank  49 , a part thereof is supplied to the regeneration tower  40  via the condensed water return line  34 , where the condensed water is reused. A part of the separated condensed water is supplied to the first, second washing units  22 ,  23  and the CO 2  absorption tower  21  via the condensed water transfer line  35 , where the condensed water is reused. The condensed water which passes the condensed water transfer line  35  is measured in pH by the pH meter  37 A. The pH of the cleaning water flowing through the cleaning water circulation lines  31 ,  33  of the first, second washing units  22 ,  23  is measured by the pH meters  37 C,  37 B. Based on the pH values measured by the pH meters  37 A to  37 C, the control unit  30  opens/closes the valves  36 A to  36 C provided on the first, second and third branch lines  35 A to  35 C of the condensed water transfer line  35 . 
     More specifically, assuming that the pH value of condensed water in the condensed water transfer line  35  which the pH meter  37 A measures is pH1 and that the pH value of washing fluid in the washing fluid circulation line  33  of the second washing unit  23  which the pH meter  37 B measures is pH2, and that the pH value of cleaning water in the cleaning water circulation line  31  of the first washing unit  22  which the pH meter  37 C measures is pH3, when pH1/pH3&gt;1, the control unit  30  closes the valves  36 A,  36 B and opens the valve  36 C to supply the condensed water to the CO 2  absorption tower  21 . Further, when pH/pH&lt;1 and pH1/pH&gt;1, the control unit  30  opens the valve  36 A and closes the valve  36 B,  36 C to supply the condensed water to the first washing unit  22  located on the side of the bottom portion of the CO 2  absorption tower  20 . When pH1/pH2&lt;1, the control unit  30  closes the valves  36 A,  36 C and opens the valve  36 B to supply the condensed water to the second washing unit  23  located on the side of the top portion of the CO 2  absorption tower  20 . Under such a control, even when the pH value of the condensed water is extremely high, the diffused amount of basic amine compound from the gas exhaust line  28  located on the top portion of the CO 2  absorption tower  20  can be suppressed. 
     Although a case of controlling the pH value with respect to pH1/pH2=1 and pH1/pH=1 has been described, the first threshold of pH1/pH2 is not restricted to 1, but may be set in a range of 0.9 to 1 and the second threshold of pH1/pH3 is not restricted to 1, but may be set in a range of 0.9 to 1. 
     Although  FIGS. 1 to 4  express the pH meter  37 , the present invention is not restricted to this example as long as the meter is capable of monitoring changes in concentration of the basic amine compound in the condensed water, that is, CO 2  absorbent, but it is permissible to install an electric conductivity meter or a resistivity meter instead of the pH meter. 
     The preferred embodiments of the present invention have been described above. However, it is not intended to restrict the scope of the present invention to any particular exemplary embodiments described previously, but a variety of modifications, improvements or equivalent examples of the present invention may be carried out without departing from the spirit and scope of the present invention specified by accompanying claims.