Heat recovery system and heat recovery method of co2 recovery unit

Abstract

A heat recovery system of a CO 2 recovery unit ( 55 ) including an absorber that removes CO 2 in flue gas ( 101 ) discharged from a boiler ( 51 ) by absorbing CO 2 by an absorbent, and a regenerator that emits CO 2 from the absorbent having absorbed CO 2 for reusing the absorbent in the absorber. The heat recovery system further includes a Ljungström heat exchanger ( 57 ) that performs heat exchange between the flue gas ( 101 ) discharged from the boiler ( 51 ) and before reaching the CO 2 recovery unit ( 55 ) and unburned air ( 102 ) supplied to the boiler ( 51 ), and an air preheater ( 58 ) that preheats the unburned air ( 102 ) before reaching the Ljungström heat exchanger ( 57 ) by exhaust heat from the CO 2 recovery unit ( 55 ).

Claims

1 . A heat recovery system of a CO 2 recovery unit including an absorber that removes CO 2 in flue gas discharged from a boiler by absorbing CO 2 by an absorbent, and a regenerator that emits CO 2 from the absorbent having absorbed CO 2 for reusing the absorbent in the absorber, the heat recovery system comprising: a Ljungström heat exchanger that performs heat exchange between flue gas discharged from the boiler and before reaching the CO 2 recovery unit and unburned air supplied to the boiler; and an air preheater that preheats the unburned air before reaching the Ljungström heat exchanger by exhaust heat from the CO 2 recovery unit. 2 . The heat recovery system of a CO 2 recovery unit according to claim 1 , further comprising a heat recovery device that recovers heat of the flue gas before reaching the CO 2 recovery unit through the Ljungström heat exchanger and supplies recovered heat to the CO 2 recovery unit. 3 . The heat recovery system of a CO 2 recovery unit according to claim 1 , wherein the heat recovery system is applied to a power generation plant that drives a steam turbine by superheated steam from the boiler to generate power. 4 . The heat recovery system of a CO 2 recovery unit according to claim 1 , wherein the air preheater indirectly heats the unburned air by circulating a heat medium heated by heat transfer of exhaust heat from the CO 2 recovery unit. 5 . The heat recovery system of a CO 2 recovery unit according to claim 1 , wherein the air preheater directly heats the unburned air by circulating a exhaust heat fluid from the CO 2 recovery unit. 6 . A heat recovery method of a CO 2 recovery unit including a CO 2 recovery step of removing CO 2 in flue gas discharged from a boiler by absorbing CO 2 by an absorbent, emitting CO 2 from the absorbent having absorbed CO 2 , and of reusing the absorbent for absorbing CO 2 , the heat recovery method comprising: a heat exchanging step of performing heat exchange between flue gas discharged from the boiler and before reaching the CO 2 recovery step and unburned air supplied to the boiler; and an air preheating step of preheating the unburned air before reaching the heat exchanging step by exhaust heat from the CO 2 recovery step. 7 . The heat recovery method of a CO 2 recovery unit according to claim 6 , further comprising a heat recovery step of recovering heat of the flue gas before reaching the CO 2 recovery step through the heat exchanging step and of supplying recovered heat to the CO 2 recovery step. 8 . The heat recovery method of a CO 2 recovery unit according to claim 6 , further comprising a power generation step of driving a steam turbine by superheated steam from the boiler to generate power.
FIELD [0001] The present invention relates to a heat recovery system and a heat recovery method of a CO 2 recovery unit that effectively use heat discharged from the CO 2 recovery unit. BACKGROUND [0002] The greenhouse effect due to CO 2 has been pointed out as a cause of global warming, and there has been an urgent need to take measures against the greenhouse effect for protecting the global environment in an international scale. Generation sources of CO 2 extend over all human activities that burn fossil fuel, and demands for emission limitation are further increasing. Under these circumstances, a method of removing and recovering CO 2 in flue gas by bringing flue gas discharged from a boiler into contact with an amine absorbent such as an amine compound solution has been intensively studied. The method is targeted for power generation facilities such as a thermal power plant that uses a large amount of fossil fuel. [0003] As a CO 2 recovery unit that recovers CO 2 from flue gas by using an absorbent, there has been known an apparatus that removes CO 2 in flue gas by bringing flue gas into contact with a CO 2 absorbent in an absorber, heats the absorbent that has absorbed CO 2 in a regenerator, thereby emitting CO 2 and regenerating the absorbent, and returns the absorbent to the absorber and reuses the absorbent (see, for example, Patent Literature 1). CITATION LIST Patent Literature [0004] Patent Literature 1: Japanese Patent Application Laid-open No. 5-245339 SUMMARY Technical Problem [0005] In the CO 2 recovery unit described above, a large amount of low-temperature flue gas at a temperature level of 70° C. to 80° C. is discharged. Generally, however, because the heat of flue gas is low, the flue gas is cooled by cooling water and disposed. [0006] The present invention has been achieved to solve the above problems, and an object of the present invention is to provide a heat recovery system and a heat recovery method of a CO 2 recovery unit that can effectively use low-temperature exhaust heat discharged at the time of recovering CO 2 . Solution to Problem [0007] According to an aspect of the present invention, a heat recovery system of a CO 2 recovery unit include an absorber that removes CO 2 in flue gas discharged from a boiler by absorbing CO 2 by an absorbent, and a regenerator that emits CO 2 from the absorbent having absorbed CO 2 for reusing the absorbent in the absorber. The heat recovery system includes: a Ljungström heat exchanger that performs heat exchange between flue gas discharged from the boiler and before reaching the CO 2 recovery unit and unburned air supplied to the boiler; and an air preheater that preheats the unburned air before reaching the Ljungström heat exchanger by exhaust heat from the CO 2 recovery unit. [0008] According to the heat recovery system of a CO 2 recovery unit, the flow rate of low-pressure steam consumed by the CO 2 recovery unit can be decreased by preheating unburned air before reaching the Ljungström heat exchanger by low-temperature exhaust heat discharged at the time of recovering CO 2 . [0009] Furthermore, because the Ljungström heat exchanger performs heat exchange between flue gas containing sulfur content and unburned air, a heat reservoir may be subjected to sulfuric acid corrosion due to dew condensation generated on a heat transfer surface of the heat reservoir. However, according to the heat recovery system of a CO 2 recovery unit, the gas temperature of unburned air on an inlet side of the Ljungström heat exchanger is raised, and the gas temperature of flue gas on an outlet side of the Ljungström heat exchanger is also raised. As a result, an average cold-end temperature ((the gas temperature on the outlet side of flue gas+the gas temperature on the inlet side of unburned air)/2) of the Ljungström heat exchanger increases. Consequently, dew condensation on the heat transfer surface of the heat reservoir of the Ljungström heat exchanger can be prevented to suppress sulfuric acid corrosion of the heat reservoir. [0010] Advantageously, the heat recovery system of a CO 2 recovery unit further includes a heat recovery device that recovers heat of the flue gas before reaching the CO 2 recovery unit through the Ljungström heat exchanger and supplies recovered heat to the CO 2 recovery unit. [0011] According to the heat recovery system of a CO 2 recovery unit, the flow rate of low-pressure steam consumed by the CO 2 recovery unit can be further decreased by recovering the heat of flue gas after passing through the Ljungström heat exchanger, while preheating unburned air before reaching the Ljungström heat exchanger by low-temperature exhaust heat discharged at the time of recovering CO 2 . [0012] Advantageously, in the heat recovery system of a CO 2 recovery unit, the heat recovery system is applied to a power generation plant that drives a steam turbine by superheated steam from the boiler to generate power. [0013] According to the heat recovery system of a CO 2 recovery unit, because the gas temperature of unburned air supplied to the boiler increases, the flow rate of superheated steam to be supplied from the boiler to the steam turbine increases, thereby enabling to improve the power generation efficiency of a power generation plant. [0014] Advantageously, in the heat recovery system of a CO 2 recovery unit, the air preheater indirectly heats the unburned air by circulating a heat medium heated by heat transfer of exhaust heat from the CO 2 recovery unit. [0015] According to the heat recovery system of a CO 2 recovery unit, exhaust heat fluid can be prevented from flowing out, as compared to a case where unburned air is directly heated by using exhaust heat fluid of the CO 2 recovery unit. [0016] Advantageously, in the heat recovery system of a CO 2 recovery unit, the air preheater directly heats the unburned air by circulating a exhaust heat fluid from the CO 2 recovery unit. [0017] According to the heat recovery system of a CO 2 recovery unit, the heat transfer efficiency can be improved with respect to a case where unburned air is indirectly heated via a heat medium. [0018] According to another aspect of the present invention, a heat recovery method of a CO 2 recovery unit including a CO 2 recovery step of removing CO 2 in flue gas discharged from a boiler by absorbing CO 2 by an absorbent, emitting CO 2 from the absorbent having absorbed CO 2 , and of reusing the absorbent for absorbing CO 2 , includes: a heat exchanging step of performing heat exchange between flue gas discharged from the boiler and before reaching the CO 2 recovery step and unburned air supplied to the boiler; and an air preheating step of preheating the unburned air before reaching the heat exchanging step by exhaust heat from the CO 2 recovery step. [0019] According to the heat recovery method of a CO 2 recovery unit, the flow rate of low-pressure steam consumed by the CO 2 recovery unit can be decreased by preheating unburned air before reaching the heat exchanging step by low-temperature exhaust heat discharged at the time of recovering CO 2 . [0020] Furthermore, because the heat exchange between flue gas containing sulfur content and unburned air is performed at the heat exchanging step, a heat reservoir may be subjected to sulfuric acid corrosion due to dew condensation generated on a heat transfer surface of the heat reservoir. However, according to the heat recovery method of a CO 2 recovery unit, the gas temperature of unburned air on an inlet side of the heat exchanging step is raised, and the gas temperature of flue gas on an outlet side of the heat exchanging step is also raised. As a result, an average cold-end temperature ((the gas temperature on the outlet side of flue gas+the gas temperature on the inlet side of unburned air)/2) at the heat exchanging step increases. Consequently, dew condensation on the heat transfer surface of the heat reservoir at the heat exchanging step can be prevented to suppress sulfuric acid corrosion of the heat reservoir. [0021] Advantageously, the heat recovery method of a CO 2 recovery unit further includes a heat recovery step of recovering heat of the flue gas before reaching the CO 2 recovery step through the heat exchanging step and of supplying recovered heat to the CO 2 recovery step. [0022] According to the heat recovery method of a CO 2 recovery unit, the flow rate of low-pressure steam consumed at the CO 2 recovery step can be further decreased by recovering the heat of flue gas after passing through the heat exchanging step, while preheating unburned air before reaching the Ljungström heat exchanger by low-temperature exhaust heat discharged at the time of recovering CO 2 . [0023] Advantageously, the heat recovery method of a CO 2 recovery unit, further includes a power generation step of driving a steam turbine by superheated steam from the boiler to generate power. [0024] According to the heat recovery method of a CO 2 recovery unit, because the gas temperature of unburned air supplied to the boiler increases, the flow rate of superheated steam to be supplied from the boiler to the steam turbine increases, thereby enabling to improve the power generation efficiency at a power generation step. Advantageous Effects of Invention [0025] According to the present invention, low-temperature exhaust heat discharged at the time of recovering CO 2 can be effectively used. BRIEF DESCRIPTION OF DRAWINGS [0026] FIG. 1 is a schematic diagram of a power generation plant to which a heat recovery system of a CO 2 recovery unit according to a first embodiment of the present invention is applied. [0027] FIG. 2 is a schematic diagram of the heat recovery system of a CO 2 recovery unit according to the first embodiment of the present invention. [0028] FIG. 3 is a chart of a pattern of the heat recovery system of a CO 2 recovery unit according to the first embodiment of the present invention. [0029] FIG. 4 is a schematic diagram of an effect of the heat recovery system of a CO 2 recovery unit according to the first embodiment of the present invention. [0030] FIG. 5 is a schematic diagram of a heat recovery system of a CO 2 recovery unit according to a second embodiment of the present invention. FIG. 6 is a chart of a pattern of the heat recovery system of a CO 2 recovery unit according to the second embodiment of the present invention. [0031] FIG. 7 is a schematic diagram of an effect of the heat recovery system of a CO 2 recovery unit according to the second embodiment of the present invention. [0032] FIG. 8 is a schematic diagram of a power generation plant to which a heat recovery system of a CO 2 recovery unit according to a third embodiment of the present invention is applied. FIG. 9 is a schematic diagram of the heat recovery system of a CO 2 recovery unit according to the third embodiment of the present invention. [0033] FIG. 10 is a chart of a pattern of the heat recovery system of a CO 2 recovery unit according to the third embodiment of the present invention. [0034] FIG. 11 is a schematic diagram of an effect of the heat recovery system of a CO 2 recovery unit according to the third embodiment of the present invention. [0035] FIG. 12 is a schematic diagram of a heat recovery system of a CO 2 recovery unit according to a fourth embodiment of the present invention. [0036] FIG. 13 is a schematic diagram of an effect of the heat recovery system of a CO 2 recovery unit according to the fourth embodiment of the present invention. DESCRIPTION OF EMBODIMENTS [0037] Exemplary embodiments of the present invention will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to these embodiments. In addition, constituent elements in the following embodiments include elements that can be easily replaced by those skilled in the art, or substantially the same elements. First Embodiment [0038] A first embodiment is explained with reference to the drawings. FIG. 1 is a schematic diagram of a power generation plant to which a heat recovery system of a CO 2 recovery unit according to the first embodiment is applied. FIG. 2 is a schematic diagram of the heat recovery system of a CO 2 recovery unit according to the first embodiment. FIG. 3 is a chart of a pattern of the heat recovery system of a CO 2 recovery unit according to the first embodiment. FIG. 4 is a schematic diagram of an effect of the heat recovery system of a CO 2 recovery unit according to the first embodiment. [0039] As shown in FIG. 1 , a power generation plant 50 mainly includes a boiler 51 that heats water in a sealed vessel by thermal energy acquired by burning fuel to acquire high-temperature and high-pressure superheated steam 100 , a steam turbine 52 that acquires rotative power by the superheated steam 100 from the boiler 51 , and a power generator 53 that generates power by the rotative power of the steam turbine 52 . Furthermore, the power generation plant 50 includes a flue gas desulfurizer 54 that removes sulfur content (including a sulfur compound) contained in flue gas 101 discharged from the boiler 51 , and a CO 2 recovery unit 55 that removes CO 2 (carbon dioxide) contained in the flue gas 101 discharged from the boiler 51 . That is, the power generation plant 50 generates power by the superheated steam 100 from the boiler 51 , and discharges decarbonated flue gas 101 a, in which sulfur content and CO 2 discharged from the boiler 51 are removed, from a stack 56 . [0040] In the power generation plant 50 , the boiler 51 is attached with a Ljungström heat exchanger 57 . The Ljungström heat exchanger 57 performs heat exchange between the flue gas 101 discharged from the boiler 51 and unburned air 102 supplied to the boiler 51 . The Ljungström heat exchanger 57 improves the thermal efficiency of the boiler 51 by preheating the unburned air 102 supplied to the boiler 51 . [0041] The heat recovery system of the CO 2 recovery unit 55 according to the present embodiment includes an air preheater 58 that preheats the unburned air 102 before reaching the Ljungström heat exchanger 57 by exhaust heat from the CO 2 recovery unit 55 . [0042] Exhaust heat from the CO 2 recovery unit 55 is specifically explained below. The CO 2 recovery unit 55 includes a cooling column 1 that cools the flue gas 101 discharged from the boiler 51 by cooling water 103 , an absorber 2 that brings a lean solution 104 a of an absorbent 104 , which is an aqueous solution of an amine compound that absorbs CO 2 , into countercurrent contact with the flue gas 101 so that CO 2 in the flue gas 101 is absorbed by the absorbent 104 and discharges the decarbonated flue gas 101 a in which CO 2 has been removed, and a regenerator 3 that emits CO 2 from a rich solution 104 b of the absorbent 104 having absorbed CO 2 to regenerate it to the lean solution 104 a. [0043] In the cooling column 1 , the flue gas 101 containing CO 2 is boosted by a flue gas blower (not shown) and fed into the cooling column 1 , and brought into countercurrent contact with the cooling water 103 , thereby cooling the flue gas 101 . [0044] The cooling water 103 is accumulated in a lower part of the cooling column 1 and is supplied to an upper part of the cooling column 1 by a humidified cooling-water circulating pump 1 a through a cooling water pipe 1 b outside of the cooling column 1 . The cooling water 103 is brought into countercurrent contact with the flue gas 101 moving upward at a position of a packed bed 1 d provided in a process leading to the lower part of the cooling column 1 , while flowing down from a nozzle 1 c provided in the upper part of the cooling column 1 . Furthermore, a cooler 1 e that condenses the cooling water 103 by cooling to acquire condensed water is provided in the cooling water pipe 1 b . The flue gas 101 cooled in the cooling column 1 is discharged from a top of the cooling column 1 through a flue gas pipe 1 f and supplied to the absorber 2 . [0045] The absorber 2 has a CO 2 absorbing unit 21 in a lower half thereof, and a flushing unit 22 in an upper half thereof. The CO 2 absorbing unit 21 brings the flue gas 101 supplied from the cooling column 1 into countercurrent contact with the lean solution 104 a of the absorbent 104 , so that CO 2 in the flue gas 101 is absorbed by the absorbent 104 . [0046] The lean solution 104 a of the absorbent 104 is supplied from the regenerator 3 , and brought into countercurrent contact with the flue gas 101 moving upward at a position of a packed bed 21 b provided in a process leading to the lower part of the absorber 2 , while flowing down from a nozzle 21 a, to become the rich solution 104 b having absorbed CO 2 , and is accumulated at a bottom of the absorber 2 . The rich solution 104 b of the absorbent 104 accumulated at the bottom of the absorber 2 is pumped by a rich-solution discharge pump 21 d installed in a rich solution pipe 21 c outside of the absorber 2 , and supplied to the regenerator 3 . Furthermore, while being supplied to the regenerator 3 through the rich solution pipe 21 c, the rich solution 104 b of the absorbent 104 is heat-exchanged with the lean solution 104 a of the absorbent 104 being supplied to the absorber 2 through a lean solution pipe 33 d described later by a rich-lean heat exchanger 4 . [0047] The flushing unit 22 brings the decarbonated flue gas 101 a with CO 2 being removed by the CO 2 absorbing unit 21 into countercurrent contact with wash water 105 to remove the amine compound entrained with the decarbonated flue gas 101 a by the wash water 105 , and discharges the decarbonated flue gas 101 a with the amine compound being removed, to outside of the absorber 2 . [0048] The wash water 105 is brought into countercurrent contact with the decarbonated flue gas 101 a moving upward at a position of a packed bed 22 b provided in a process going downward, while flowing down from a nozzle 22 a, and is accumulated in a water receptor 22 c. The wash water 105 accumulated in the water receptor 22 c is pumped and circulated by a wash-water discharge pump 22 e installed in a wash water pipe 22 d outside of the absorber 2 , cooled by a cooler 22 f, and caused to flow down from the nozzle 22 a again. [0049] The absorber 3 recovers CO 2 from the rich solution 104 b of the absorbent 104 , regenerates it as the lean solution 104 a, and accumulates the lean solution 104 a at the bottom, thereby emitting CO 2 from the absorbent 104 having absorbed CO 2 . The regenerator 3 is divided into three parts, which are an upper part, a middle part, and a lower part. That is, these are an upper regenerating unit 31 , a middle regenerating unit 32 , and a lower regenerating unit 33 . [0050] In the upper regenerating unit 31 , the absorbent 104 is supplied from the rich solution pipe 21 c of the CO 2 absorbing unit 21 in the absorber 2 , to flow down from a nozzle 31 a, passes through a packed bed 31 b provided in a process going downward, and accumulated in a chimney tray 31 c. The absorbent 104 accumulated in the chimney tray 31 c is supplied to the middle regenerating unit 32 , while being pumped to outside of the regenerator 3 by a semi-lean-solution discharge pump 31 e installed in a semi-lean-solution extracting pipe 31 d. [0051] In the middle regenerating unit 32 , the absorbent 104 is supplied from the semi-lean-solution extracting pipe 31 d of the upper regenerating unit 31 to flow down from a nozzle 32 a, passes through a packed bed 32 b provided in a process going downward, and accumulated in a chimney tray 32 c. The absorbent 104 accumulated in the chimney tray 32 c is supplied to the lower regenerating unit 33 , while being pumped to outside of the regenerator 3 by a semi-lean-solution discharge pump 32 e installed in a semi-lean-solution extracting pipe 32 d. [0052] In the lower regenerating unit 32 , the absorbent 104 is supplied from the semi-lean-solution extracting pipe 32 d of the middle regenerating unit 32 to flow down from a nozzle 33 a, passes through a packed bed 33 b provided in a process going downward, and accumulated at a bottom 33 c of the regenerator 3 . The absorbent 104 accumulated at the bottom 33 c of the regenerator 3 is pumped to outside of the regenerator 3 by a lean-solution discharge pump 33 e installed in a lean-solution extracting pipe 33 d, heat-exchanged with the rich solution 104 b of the absorbent 104 being supplied to the regenerator 3 through the rich solution pipe 21 c by the rich-lean heat exchanger 4 , and then, supplied to the absorber 2 while being cooled by a cooler 33 f. [0053] A regenerating heater 34 that uses low-pressure steam 106 from the steam turbine 52 as a source of heat is provided outside of the regenerator 3 . A part of the absorbent 104 accumulated at the bottom 33 c of the regenerator 3 is heated by the regenerating heater 34 while being extracted to outside of the regenerator 3 via a heating pipe 34 a to become the lean solution 104 a, and returned to the bottom 33 c of the regenerator 3 and accumulated therein. The low-pressure steam 106 supplied to the regenerating heater 34 heats the absorbent 104 while passing through the heating pipe 34 a, to become steam drain 106 a and discharged via a drain pipe 34 b. [0054] A lean-solution/steam-drain heat recovery unit 35 is provided in the semi-lean-solution extracting pipe 31 d of the upper regenerating unit 31 . The lean-solution/steam-drain heat recovery unit 35 performs heat exchange between the absorbent 104 being supplied to the middle regenerating unit 32 through the semi-lean-solution extracting pipe 31 d and the lean solution 104 a of the absorbent 104 being supplied to the absorber 2 via the lean solution pipe 33 d and the steam drain 106 a being discharged via the drain pipe 34 b. [0055] A lean-solution/steam-drain heat recovery unit 36 is provided in the semi-lean-solution extracting pipe 32 d of the middle regenerating unit 32 . The lean-solution/steam-drain heat recovery unit 36 performs heat exchange between the absorbent 104 being supplied to the lower regenerating unit 33 through the semi-lean-solution extracting pipe 32 d and the lean solution 104 a of the absorbent 104 being supplied to the absorber 2 via the lean solution pipe 33 d and the steam drain 106 a being discharged via the drain pipe 34 b. [0056] That is, in the regenerator 3 , the rich solution 104 b of the absorbent 104 to be supplied to the upper regenerating unit 31 flows down from the nozzle 31 a, while being heated by heat exchange with the lean solution 104 a of the absorbent 104 being supplied to the absorber 2 by the rich-lean heat exchanger 4 , in the process of being supplied to the regenerator 3 via the rich solution pipe 21 c. The rich solution 104 b of the absorbent 104 becomes a semi-lean solution 104 c with the most part of CO 2 being emitted by an endothermic reaction, while passing through the packed bed 31 b in the process going downward, and is accumulated in the chimney tray 31 c. [0057] The semi-lean solution 104 c of the absorbent 104 to be supplied to the middle regenerating unit 32 from the upper regenerating unit 31 through the semi-lean-solution extracting pipe 31 d flows down from the nozzle 32 a, while being heated by heat exchange by the lean-solution/steam-drain heat recovery unit 35 in the process of being supplied to the middle regenerating unit 32 via the semi-lean-solution extracting pipe 31 d. CO 2 is further emitted from the semi-lean solution 104 c of the absorbent 104 by the endothermic reaction, while passing through the packed bed 32 b in the process going downward, and the semi-lean solution 104 c is accumulated in the chimney tray 32 c. [0058] The semi-lean solution 104 c of the absorbent 104 to be supplied to the lower regenerating unit 33 from the middle regenerating unit 32 through the semi-lean-solution extracting pipe 32 d flows down from the nozzle 33 a, while being heated by heat exchange by the lean-solution/steam-drain heat recovery unit 36 in the process of being supplied to the lower regenerating unit 33 via the semi-lean-solution extracting pipe 32 d. The semi-lean solution 104 c of the absorbent 104 becomes the lean solution 104 a with substantially whole CO 2 being emitted by the endothermic reaction, while passing through the packed bed 33 b in the process going downward, and is accumulated at the bottom 33 c of the regenerator 3 . The lean solution 104 a accumulated at the bottom 33 c of the regenerator 3 is heated by the low-pressure steam 106 in the regenerating heater 34 . [0059] CO 2 gas 107 removed from the absorbent 104 in the respective regenerating units 31 , 32 , and 33 is recovered by a CO 2 recovering unit 37 . In the CO 2 recovering unit 37 , the CO 2 gas 107 removed from the absorbent 104 moves upward in the regenerator 3 , and is discharged to outside of the regenerator 3 from a top of the regenerator 3 through a condensed water pipe 37 a. After the CO 2 gas 107 is cooled by a regenerator reflux condenser 37 b while passing through the condensed water pipe 37 a, moisture is condensed by a CO 2 separator 37 c and separated from condensed water 108 , and the CO 2 gas 107 is guided to a CO 2 processing step (not shown) by a recovered CO 2 discharge pipe 37 d. The condensed water 108 separated from CO 2 by the CO 2 separator 37 c is pumped by a condensed water pump 37 e installed in the condensed water pipe 37 a and supplied to the regenerator 3 , to flow down from a nozzle 37 f provided at the top in the regenerator 3 . [0060] In the configuration described above, the regenerator 3 includes three stages, which are an upper stage, a middle stage, and a lower stage. That is, these are the upper regenerating unit 31 , the middle regenerating unit 32 , and the lower regenerating unit 33 . However, the regenerator 3 can include two stages, which are upper and lower stages. That is, these are the upper regenerating unit 31 and the lower regenerating unit 33 . Furthermore, the regenerator 3 is not limited to the upper, middle, and lower stages, which are the upper regenerating unit 31 , the middle regenerating unit 32 , and the lower regenerating unit 33 , and can include three or more regenerating units. [0061] In the CO 2 recovery unit 55 , a heat recovery system that effectively uses low-temperature exhaust heat discharged at the time of recovering CO 2 is explained with reference to FIG. 2 . In the lean solution pipe 33 d for supplying the lean solution 104 a of the absorbent 104 from the regenerator 3 to the absorber 2 , a first heat exchanger ex 1 is provided between the rich-lean heat exchanger 4 and the cooler 33 f. The first heat exchanger ex 1 performs heat exchange between the lean solution 104 a of the absorbent 104 before being cooled by the cooler 31 f via the lean solution pipe 33 d and a heat medium (for example, water) 109 , and circulates the heat medium to which the exhaust heat of the lean solution 104 a is transferred. [0062] In the wash water pipe 22 d in the flushing unit 22 of the absorber 2 , a second heat exchanger ex 2 is provided between the wash-water discharge pump 22 e and the cooler 22 f. The second heat exchanger ex 2 performs heat exchange between the heat medium (for example, water) 109 and the wash water 105 before being cooled by the cooler 22 f via the wash water pipe 22 d, and circulates the heat medium to which the exhaust heat of the wash water 105 is transferred. [0063] Furthermore, in the condensed water pipe 37 a in the CO 2 recovering unit 37 of the regenerator 3 , a third heat exchanger ex 3 is provided between the regenerator 3 and the regenerator reflux condenser 37 b. The third heat exchanger ex 3 performs heat exchange between the CO 2 gas 107 before being cooled by the regenerator reflux condenser 37 b via the condensed water pipe 37 a and the heat medium 109 in the first heat exchanger ex 1 or the second heat exchanger ex 2 and circulates the heat medium 109 , while transferring the exhaust heat of the CO 2 gas 107 to the heat medium 109 . [0064] All or at least one of the heat medium 109 heat-exchanged by these heat exchangers ex 1 , ex 2 , and ex 3 is circulated from [a] to [b] of the CO 2 recovery unit 55 so as to pass through the air preheater 58 of the power generation plant 50 shown in FIG. 1 , and used for preheating the unburned air 102 before reaching the Ljungström heat exchanger 57 . [0065] Specifically, respective patterns in which all or at least one of the heat medium 109 heat-exchanged by the respective heat exchangers ex 1 , ex 2 , and ex 3 is used for preheating the unburned air 102 are explained below with reference to FIGS. 1 to 3 . [0066] A pattern 1 shown in FIG. 3 uses the heat medium 109 circulated only to the first heat exchanger ex 1 , in which the heat medium 109 at a temperature of 44.3° C. heat-transferred by the first heat exchanger ex 1 is supplied from [a- 1 ] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 . The heat medium 109 at a temperature of 33.1° C. heat-exchanged via the air preheater 58 is returned to [b- 1 ] (corresponding to [b] in FIG. 1 ) and heat-transferred again through the first heat exchanger ex 1 . [0067] A pattern 2 shown in FIG. 3 uses the heat medium 109 circulated only to the second heat exchanger ex 2 , in which the heat medium 109 at a temperature of 47° C. heat-transferred by the second heat exchanger ex 2 is supplied from [a- 2 ] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 . The heat medium 109 at a temperature of 33.1° C. heat-exchanged via the air preheater 58 is returned to [b- 2 ] (corresponding to [b] in FIG. 1 ) and heat-transferred again through the second heat exchanger ex 2 . [0068] A pattern 3 shown in FIG. 3 uses the heat medium 109 circulated to the first heat exchanger ex 1 and the second heat exchanger ex 2 , in which the heat medium 109 at a temperature of 44.3° C. heat-transferred by the first heat exchanger ex 1 is supplied from [a- 1 ] to [b- 2 ], and then the heat medium 109 at a temperature of 47° C. heat-transferred by the second heat exchanger ex 2 is supplied from [a- 2 ] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 . The heat medium 109 at a temperature of 33.1° C. heat-exchanged via the air preheater 58 is returned to [b- 1 ] (corresponding to [b] in FIG. 1 ) and heat-transferred again through the first heat exchanger ex 1 and the second heat exchanger ex 2 . [0069] A pattern 4 shown in FIG. 3 uses the heat medium 109 circulated to the first heat exchanger ex 1 and the third heat exchanger ex 3 , in which the heat medium 109 at a temperature of 44.3° C. heat-transferred by the first heat exchanger ex 1 is supplied from [a- 1 ] to [x] (corresponding to [a] in FIG. 1 ), and then the heat medium 109 at a temperature of 78.6° C. heat-transferred by the third heat exchanger ex 3 is supplied from [a- 3 ] to the air preheater 58 . The heat medium 109 at a temperature of 33.4° C. heat-exchanged via the air preheater 58 is returned to [b- 1 ] (corresponding to [b] in FIG. 1 ) and heat-transferred again through the first heat exchanger ex 1 and the third heat exchanger ex 3 . [0070] A pattern 5 shown in FIG. 3 uses the heat medium 109 circulated to the second heat exchanger ex 2 and the third heat exchanger ex 3 , in which the heat medium 109 at a temperature of 47° C. heat-transferred by the second heat exchanger ex 2 is supplied from [a- 2 ] to [x], and then the heat medium 109 at a temperature of 79.2° C. heat-transferred by the third heat exchanger ex 3 is supplied from [a- 3 ] (corresponding to [a] in FIG. 1 ) to the air preheater 58 . The heat medium 109 at a temperature of 33.4° C. heat-exchanged via the air preheater 58 is returned to [b- 2 ] (corresponding to [b] in FIG. 1 ) and heat-transferred again through the second heat exchanger ex 2 and the third heat exchanger ex 3 . [0071] A pattern 6 shown in FIG. 3 uses the heat medium 109 circulated to the first heat exchanger ex 1 , the second heat exchanger ex 2 , and the third heat exchanger ex 3 , in which the heat medium 109 at a temperature of 44.3° C. heat-transferred by the first heat exchanger ex 1 is supplied from [a- 1 ] to [b- 2 ], the heat medium 109 at a temperature of 47° C. heat-transferred by the second heat exchanger ex 2 is supplied from [a- 2 ] to [x], and then the heat medium 109 at a temperature of 79.3° C. heat-transferred by the third heat exchanger ex 3 is supplied from [a- 3 ] (corresponding to [a] in FIG. 1 ) to the air preheater 58 . The heat medium 109 at a temperature of 33° C. heat-exchanged via the air preheater 58 is returned to [b- 1 ] (corresponding to [b] in FIG. 1 ) and heat-transferred again through the first heat exchanger ex 1 , the second heat exchanger ex 2 , and the third heat exchanger ex 3 . [0072] In the heat recovery system of the CO 2 recovery unit 55 according to the present embodiment, an effect of a case where the air preheater 58 is provided, assuming the temperature efficiency of the Ljungström heat exchanger 57 to be 90%, and the pattern 6 shown in FIG. 3 is applied is explained below with reference to FIG. 4 . As shown in FIG. 4 , in the case of the gas temperature of the unburned air 102 being 30° C. corresponding to the ambient air, the gas temperature of the unburned air 102 at a position A in an upstream of the air preheater 58 in FIG. 1 was 30° C., and the gas temperature of the preheated unburned air 102 at a position B in an upstream of the Ljungström heat exchanger 57 became 72° C. by passing through the air preheater 58 . The gas temperature of the heat-exchanged unburned air 102 at a position C in an upstream of the boiler 51 via the Ljungström heat exchanger 57 became 322° C. Furthermore, the gas temperature of the flue gas 101 at a position D in the upstream of the Ljungström heat exchanger 57 via the boiler 51 became 350° C. Further, the gas temperature of the heat-exchanged flue gas 101 at a position E in an upstream of the flue gas desulfurizer 54 via the Ljungström heat exchanger 57 became 141° C. At this time, a flow rate G of the low-pressure steam 106 supplied to the regenerating heater 34 of the CO 2 recovery unit 55 was 870 t/hr. [0073] On the other hand, an effect in a conventional case where the air preheater 58 is not provided and the temperature efficiency of the Ljungström heat exchanger 57 is 82.6% is as described below. As shown in FIG. 4 , when the gas temperature of the unburned air 102 is 30° C. corresponding to the ambient air, in FIG. 1 , the gas temperature of the unburned air 102 at the position A is 30° C. Because the air preheater 58 is not provided, the gas temperature of the unburned air 102 at the position B in the upstream of the Ljungström heat exchanger 57 becomes 30° C. The gas temperature of the heat-exchanged unburned air 102 at the position C in the upstream of the boiler 51 via the Ljungström heat exchanger 57 became 294° C. Furthermore, the gas temperature of the flue gas 101 at the position D in the upstream of the Ljungström heat exchanger 57 via the boiler 51 became 350° C. Furthermore, the gas temperature of the heat-exchanged flue gas 101 at the position E in the upstream of the flue gas desulfurizer 54 via the Ljungström heat exchanger 57 became 130° C. At this time, the flow rate G of the low-pressure steam 106 supplied to the regenerating heater 34 of the CO 2 recovery unit 55 was 870 t/hr. [0074] In this manner, in the heat recovery system of the CO 2 recovery unit 55 according to the first embodiment, the air preheater 58 that preheats the unburned air 102 before reaching the Ljungström heat exchanger 57 by exhaust heat from the CO 2 recovery unit 55 is provided. [0075] According to the heat recovery system of the CO 2 recovery unit 55 , low-temperature exhaust heat equal to or lower than a temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 can be used efficiently. Specifically, as described above, a gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 28° C. to be 322° C. A heat input to the boiler 51 increases due to the increase of 28° C., and effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 by 56 t/hr can be acquired. The flow rate of 56 t/hr corresponds to 6% of the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 . As a result, by preheating the unburned air 102 before reaching the Ljungström heat exchanger 57 by low-temperature exhaust heat equal to or lower than a temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 , effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 can be acquired. [0076] Furthermore, because the Ljungström heat exchanger 57 performs heat exchange between the flue gas 101 containing sulfur content and the unburned air 102 , the heat reservoir may be subjected to sulfuric acid corrosion due to dew condensation generated on the heat transfer surface of the heat reservoir. In this regard, according to the heat recovery system of the CO 2 recovery unit 55 of the first embodiment, as described above, the gas temperature of 30° C. of the unburned air 102 at the position B on an inlet side of the Ljungström heat exchanger 57 in the conventional case is raised to 72° C., and the gas temperature of 130° C. of the flue gas 101 at the position E on an outlet side of the Ljungström heat exchanger 57 in the conventional case is also raised to 141° C. As a result, an average cold-end temperature ((the gas temperature on the outlet side of flue gas+the gas temperature on the inlet side of unburned air)/2) of the Ljungström heat exchanger 57 increases from 80° C. to 106.5° C. by 26.5° C. Consequently, dew condensation on the heat transfer surface of the heat reservoir of the Ljungström heat exchanger 57 can be prevented and sulfuric acid corrosion of the heat reservoir can be suppressed. [0077] The heat recovery system of the CO 2 recovery unit 55 according to the first embodiment is applied to the power generation plant 50 that drives the steam turbine 52 by the superheated steam 100 from the boiler 51 to generate power by the power generator 53 . [0078] As described above, according to the heat recovery system of the CO 2 recovery unit 55 , the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 28° C. to be 322° C. Therefore, because the flow rate of the superheated steam 100 supplied from the boiler 51 to the steam turbine 52 increases, the power generation efficiency of the power generation plant 50 can be improved. [0079] In the heat recovery system of the CO 2 recovery unit 55 according to the first embodiment, the air preheater 58 circulates the heat medium 109 , to which exhaust heat from the CO 2 recovery unit 55 is heat-transferred, to heat the unburned air 102 indirectly. [0080] According to the heat recovery system of the CO 2 recovery unit 55 , a exhaust heat fluid such as the absorbent 104 , the wash water 105 , or the CO 2 gas 107 can be prevented from flowing out, with respect to a case where the exhaust heat fluid is used to heat the unburned air 102 directly. [0081] A heat recovery method of the CO 2 recovery unit 55 according to the first embodiment includes a step of removing CO 2 in the flue gas 101 discharged from the boiler 51 by absorbing CO 2 by the absorbent 104 , emitting CO 2 from the absorbent 104 having absorbed CO 2 , and of reusing the absorbent 104 for absorbing CO 2 (CO 2 recovery step). The heat recovery method further includes a step of performing heat exchange between the flue gas 101 discharged from the boiler 51 and before reaching the CO 2 recovery step and the unburned air 102 supplied to the boiler 51 (heat exchanging step), and a step of preheating the unburned air 102 before reaching the heat exchanging step by exhaust heat from the CO 2 recovery step (air preheating step). [0082] According to the heat recovery method of the CO 2 recovery unit 55 , low-temperature exhaust heat equal to or lower than a temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 can be used efficiently. Specifically, the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 28° C. to be 322° C. A heat input to the boiler 51 increases due to the increase of 28° C., and effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 by 56 t/hr can be acquired. The flow rate of 56 t/hr corresponds to 6% of the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 . As a result, by preheating the unburned air 102 before reaching the heat exchanging step by low-temperature exhaust heat equal to or lower than a temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 , effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 can be acquired. [0083] Furthermore, because heat exchange between the flue gas 101 containing sulfur content and the unburned air 102 is performed at the heat exchanging step, the heat reservoir may be subjected to sulfuric acid corrosion due to dew condensation generated on the heat transfer surface of the heat reservoir. In this regard, according to the heat recovery method of the CO 2 recovery unit 55 of the first embodiment, the gas temperature of 30° C. of the unburned air 102 at the position B on the inlet side of the heat exchanging step in the conventional case is raised to 72° C., and the gas temperature of 130° C. of the flue gas 101 at the position E on the outlet side of the heat exchanging step in the conventional case is also raised to 141° C. As a result, an average cold-end temperature ((the gas temperature on the outlet side of flue gas+the gas temperature on the inlet side of unburned air)/2) at the heat exchanging step increases from 80° C. to 106.5° C. by 26.5° C. Consequently, dew condensation on the heat transfer surface of the heat reservoir at the heat exchanging step can be prevented and sulfuric acid corrosion of the heat reservoir can be suppressed. [0084] The heat recovery method of the CO 2 recovery unit 55 according to the first embodiment further includes a power generation step of driving the steam turbine 52 by the superheated steam 100 from the boiler 51 to generate power by the power generator 53 . [0085] According to the heat recovery method of the CO 2 recovery unit 55 , the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 28° C. to be 322° C. Therefore, because the flow rate of the superheated steam 100 supplied from the boiler 51 to the steam turbine 52 increases, the power generation efficiency of the power generation plant 50 can be improved. Second Embodiment [0086] A second embodiment is explained with reference to the drawings. FIG. 5 is a schematic diagram of a heat recovery system of a CO 2 recovery unit according to the second embodiment. FIG. 6 is a chart of a pattern of the heat recovery system of a CO 2 recovery unit according to the second embodiment. FIG. 7 is a schematic diagram of an effect of the heat recovery system of a CO 2 recovery unit according to the second embodiment. [0087] As in the heat recovery system of the first embodiment, the heat recovery system of the CO 2 recovery unit 55 according to the second embodiment includes the air preheater 58 that preheats the unburned air 102 before reaching the Ljungström heat exchanger 57 by exhaust heat from the CO 2 recovery unit 55 , but the mode of exhaust heat in the CO 2 recovery unit 55 is different from that of the first embodiment. Therefore, in the second embodiment, exhaust heat from the CO 2 recovery unit 55 is explained, and elements equivalent to those in the first embodiment described above are denoted by like reference signs and explanations thereof will be omitted. [0088] The heat recovery system that efficiently uses low-temperature exhaust heat discharged at the time of recovering CO 2 in the CO 2 recovery unit 55 is explained with reference to FIG. 5 . The lean solution pipe 33 d for supplying the lean solution 104 a of the absorbent 104 from the regenerator 3 to the absorber 2 is provided between the rich-lean heat exchanger 4 and the cooler 33 f via the air preheater 58 . [0089] The wash water pipe 22 d in the flushing unit 22 of the absorber 2 is provided between the wash-water discharge pump 22 e and the cooler 22 f via the air preheater 58 . [0090] Furthermore, the condensed water pipe 37 a in the CO 2 recovering unit 37 is provided between the regenerator 3 and the regenerator reflux condenser 37 b via the air preheater 58 . [0091] All of the lean solution 104 a of the absorbent 104 passing through the lean solution pipe 33 d, the wash water 105 passing through the wash water pipe 22 d, and the CO 2 gas 107 passing through the condensed water pipe 37 a or at least one of these exhaust heat fluids is circulated from [a] to [b] of the CO 2 recovery unit 55 through the air preheater 58 shown in FIG. 1 of the power generation plant 50 and used for preheating the unburned air 102 before reaching the Ljungström heat exchanger 57 . [0092] Specifically, respective patterns in which all or at least one of the respective exhaust heat fluids (the lean solution 104 a of the absorbent 104 , the wash water 105 , and the CO 2 gas 107 ) is used for preheating the unburned air 102 are explained with reference to FIGS. 1 , 5 , and 6 . [0093] In a pattern 1 shown in FIG. 6 , the lean solution 104 a of the absorbent 104 at a temperature of 47.3° C. heat-exchanged by the rich-lean heat exchanger 4 is supplied from [a- 1 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 . The lean solution 104 a at a temperature of 44.9° C. heat-exchanged via the air preheater 58 is returned to [b- 1 ′] (corresponding to [b] shown in FIG. 1 ), cooled by the cooler 33 f, and used for absorbing CO 2 . [0094] In a pattern 2 shown in FIG. 6 , the wash water 105 at a temperature of 50° C. heat-exchanged with the decarbonated flue gas 101 a is supplied from [a- 2 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 . The wash water 105 at a temperature of 48.5° C. heat-exchanged via the air preheater 58 is returned to [b- 2 ′] (corresponding to [b] shown in FIG. 1 ), cooled by the cooler 22 f, and used for cleaning of the decarbonated flue gas 101 a. [0095] In a pattern 3 shown in FIG. 6 , the CO 2 gas 107 at a temperature of 85.9° C., which has been removed from the rich solution 104 b of the absorbent 104 by the endothermic reaction, is supplied from [a- 3 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 . The CO 2 gas 107 at a temperature of 75.5° C. heat-exchanged via the air preheater 58 is returned to [b- 3 ′] (corresponding to [b] shown in FIG. 1 ), cooled by the regenerator reflux condenser 37 b, and recovered. [0096] A pattern 4 shown in FIG. 6 uses the pattern 1 and the pattern 2 in parallel, in which the lean solution 104 a of the absorbent 104 at a temperature of 47.3° C. is supplied from [a- 1 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 , and the wash water 105 at a temperature of 50° C. is supplied from [a- 2 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 . The lean solution 104 a at a temperature of 44.9° C. heat-exchanged via the air preheater 58 is returned to [b- 1 ′] (corresponding to [b] shown in FIG. 1 ), and the wash water 105 at a temperature of 49.6° C. heat-exchanged via the air preheater 58 is returned to [b- 2 ′] (corresponding to [b] shown in FIG. 1 . [0097] A pattern 5 shown in FIG. 6 uses the pattern 1 and the pattern 3 in parallel, in which the lean solution 104 a of the absorbent 104 at a temperature of 47.3° C. is supplied from [a- 1 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 , and the CO 2 gas 107 at a temperature of 85.9° C. is supplied from [a- 3 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 . The lean solution 104 a at a temperature of 44.9° C. heat-exchanged via the air preheater 58 is returned to [b- 1 ′] (corresponding to [b] shown in FIG. 1 ), and the CO 2 gas 107 at a temperature of 78.9° C. heat-exchanged via the air preheater 58 is returned to [b- 3 ′] (corresponding to [b] shown in FIG. 1 ). [0098] A pattern 6 shown in FIG. 6 uses the pattern 2 and the pattern 3 in parallel, in which the wash water 105 at a temperature of 50° C. is supplied from [a- 2 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 , and the CO 2 gas 107 at a temperature of 85.9° C. is supplied from [a- 3 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 . The wash water 105 at a temperature of 48.5° C. heat-exchanged via the air preheater 58 is returned to [b- 2 ′] (corresponding to [b] shown in FIG. 1 ), and the CO 2 gas 107 at a temperature of 79.6° C. heat-exchanged via the air preheater 58 is returned to [b- 3 ′] (corresponding to [b] shown in FIG. 1 ). [0099] A pattern 7 shown in FIG. 6 uses the pattern 1 , the pattern 2 and the pattern 3 in parallel, in which the lean solution 104 a of the absorbent 104 at a temperature of 47.3° C. is supplied from [a- 1 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 , the wash water 105 at a temperature of 50° C. is supplied from [a- 2 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 , and the CO 2 gas 107 at a temperature of 85.9° C. is supplied from [a- 3 ′] (corresponding to [a] shown in FIG. 1 ) to the air preheater 58 . The lean solution 104 a at a temperature of 44.9° C. heat-exchanged via the air preheater 58 is returned to [b- 1 ′] (corresponding to [b] shown in FIG. 1 ), the wash water 105 at a temperature of 49.6° C. heat-exchanged via the air preheater 58 is returned to [b- 2 ′] (corresponding to [b] shown in FIG. 1 ), and the CO 2 gas 107 at a temperature of 79.8° C. heat-exchanged via the air preheater 58 is returned to [b- 3 ′] (corresponding to [b] shown in FIG. 1 ). [0100] In the heat recovery system of the CO 2 recovery unit 55 according to the present embodiment, an effect of a case where the air preheater 58 is provided, assuming the temperature efficiency of the Ljungström heat exchanger 57 to be 90%, and the pattern 7 shown in FIG. 6 is applied is explained below with reference to FIG. 7 . As shown in FIG. 7 , in the case of the gas temperature of the unburned air 102 being 30° C. corresponding to the ambient air, the gas temperature of the unburned air 102 at the position A in the upstream of the air preheater 58 in FIG. 1 was 30° C., and the gas temperature of the preheated unburned air 102 at the position B in the upstream of the Ljungström heat exchanger 57 became 82° C. by passing through the air preheater 58 . The gas temperature of the heat-exchanged unburned air 102 at the position C in the upstream of the boiler 51 via the Ljungström heat exchanger 57 became 323° C. Furthermore, the gas temperature of the flue gas 101 at the position D in the upstream of the Ljungström heat exchanger 57 via the boiler 51 became 350° C. Furthermore, the gas temperature of the heat-exchanged flue gas 101 at the position E in the upstream of the flue gas desulfurizer 54 via the Ljungström heat exchanger 57 became 148° C. At this time, the flow rate G of the low-pressure steam 106 supplied to the regenerating heater 34 of the CO 2 recovery unit 55 was 870 t/hr. [0101] On the other hand, an effect in the conventional case where the air preheater 58 is not provided and the temperature efficiency of the Ljungström heat exchanger 57 is 82.6% in the conventional case is as described below. As shown in FIG. 7 , when the gas temperature of the unburned air 102 is 30° C. corresponding to the ambient air, in FIG. 1 , the gas temperature of the unburned air 102 at the position A is 30° C. Because the air preheater 58 is not provided, the gas temperature of the unburned air 102 at the position B in the upstream of the Ljungström heat exchanger 57 becomes 30° C. The gas temperature of the heat-exchanged unburned air 102 at the position C in the upstream of the boiler 51 via the Ljungström heat exchanger 57 became 294° C. Furthermore, the gas temperature of the flue gas 101 at the position D in the upstream of the Ljungström heat exchanger 57 via the boiler 51 became 350° C. Further, the gas temperature of the heat-exchanged flue gas 101 at the position E in the upstream of the flue gas desulfurizer 54 via the Ljungström heat exchanger 57 became 130° C. At this time, the flow rate G of the low-pressure steam 106 supplied to the regenerating heater 34 of the CO 2 recovery unit 55 was 870 t/hr. [0102] In this manner, in the heat recovery system of the CO 2 recovery unit 55 according to the second embodiment, the air preheater 58 that preheats the unburned air 102 before reaching the Ljungström heat exchanger 57 by exhaust heat from the CO 2 recovery unit 55 is provided. [0103] According to the heat recovery system of the CO 2 recovery unit 55 , low-temperature exhaust heat equal to or lower than a temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 can be used efficiently. Specifically, as described above, the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 29° C. to be 323° C. A heat input to the boiler 51 increases due to the increase of 29° C., and effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 by 58 t/hr can be acquired. The flow rate of 58 t/hr corresponds to 7% of the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 . As a result, by preheating the unburned air 102 before reaching the Ljungström heat exchanger 57 by low-temperature exhaust heat equal to or lower than the temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 , effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 can be acquired. [0104] Furthermore, because the Ljungström heat exchanger 57 performs heat exchange between the flue gas 101 containing sulfur content and the unburned air 102 , the heat reservoir may be subjected to sulfuric acid corrosion due to dew condensation generated on the heat transfer surface of the heat reservoir. In this regard, according to the heat recovery system of the CO 2 recovery unit 55 of the second embodiment, the gas temperature of 30° C. of the unburned air 102 at the position B on the inlet side of the Ljungström heat exchanger 57 in the conventional case is raised to 82° C., and the gas temperature of 130° C. of the flue gas 101 at the position E on the outlet side of the Ljungström heat exchanger 57 in the conventional case is also raised to 148° C. As a result, an average cold-end temperature ((the gas temperature on the outlet side of flue gas+the gas temperature on the inlet side of unburned air)/2) of the Ljungström heat exchanger 57 increases from 80° C. to 115° C. by 35° C. Consequently, dew condensation on the heat transfer surface of the heat reservoir of the Ljungström heat exchanger 57 can be prevented and sulfuric acid corrosion of the heat reservoir can be suppressed. [0105] The heat recovery system of the CO 2 recovery unit 55 according to the second embodiment is applied to the power generation plant 50 that drives the steam turbine 52 by the superheated steam 100 from the boiler 51 to generate power by the power generator 53 . [0106] As described above, according to the heat recovery system of the CO 2 recovery unit 55 , the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 29° C. to be 323° C. Therefore, because the flow rate of the superheated steam 100 supplied from the boiler 51 to the steam turbine 52 increases, the power generation efficiency of the power generation plant 50 can be improved. [0107] In the heat recovery system of the CO 2 recovery unit 55 according to the second embodiment, the air preheater 58 circulates all or at least one of the respective exhaust heat fluids (the lean solution 104 a of the absorbent 104 , the wash water 105 , and the CO 2 gas 107 ) from the CO 2 recovery unit 55 , thereby directly heating the unburned air 102 . [0108] According to the heat recovery system of the CO 2 recovery unit 55 , the heat transfer efficiency can be improved with respect to the case where the unburned air 102 is indirectly heated via the heat medium. [0109] A heat recovery method of the CO 2 recovery unit 55 according to the second embodiment includes a step of removing CO 2 in the flue gas 101 discharged from the boiler 51 by absorbing CO 2 by the absorbent 104 , emitting CO 2 from the absorbent 104 having absorbed CO 2 , and of reusing the absorbent 104 for absorbing CO 2 (CO 2 recovery step). The heat recovery method further includes a step of performing heat exchange between the flue gas 101 discharged from the boiler 51 and before reaching the CO 2 recovery step and the unburned air 102 supplied to the boiler 51 (heat exchanging step), and a step of preheating the unburned air 102 before reaching the heat exchanging step by exhaust heat from the CO 2 recovery step (air preheating step). [0110] According to the heat recovery method of the CO 2 recovery unit 55 , low-temperature exhaust heat equal to or lower than the temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 can be used efficiently. Specifically, the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 29° C. to be 323° C. A heat input to the boiler 51 increases due to the increase of 29° C., and effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step by 58 t/hr can be acquired. The flow rate of 58 t/hr corresponds to 7% of the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step. As a result, by preheating the unburned air 102 before reaching the heat exchanging step by low-temperature exhaust heat equal to or lower than the temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 , effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step can be acquired. [0111] Furthermore, because heat exchange between the flue gas 101 containing sulfur content and the unburned air 102 is performed at the heat exchanging step, the heat reservoir may be subjected to sulfuric acid corrosion due to dew condensation generated on the heat transfer surface of the heat reservoir. In this regard, according to the heat recovery method of the CO 2 recovery unit 55 of the second embodiment, the gas temperature of 30° C. of the unburned air 102 at the position B on the inlet side of the heat exchanging step in the conventional case is raised to 82° C., and the gas temperature of 130° C. of the flue gas 101 at the position E on the outlet side of the heat exchanging step in the conventional case is also raised to 148° C. As a result, an average cold-end temperature ((the gas temperature on the outlet side of flue gas+the gas temperature on the inlet side of unburned air)/2) at the heat exchanging step is increased from 80° C. to 115° C. by 35° C. Consequently, dew condensation on the heat transfer surface of the heat reservoir at the heat exchanging step can be prevented and sulfuric acid corrosion of the heat reservoir can be suppressed. [0112] The heat recovery method of the CO 2 recovery unit 55 according to the second embodiment further includes a step of driving the steam turbine 52 by the superheated steam 100 from the boiler 51 to generate power by the power generator 53 (power generation step). [0113] According to the heat recovery method of the CO 2 recovery unit 55 , the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 29° C. to be 323° C. Therefore, because the flow rate of the superheated steam 100 supplied from the boiler 51 to the steam turbine 52 increases, the power generation efficiency of the power generation plant 50 can be improved. Third Embodiment [0114] A third embodiment is explained with reference to the drawings. FIG. 8 is a schematic diagram of a power generation plant to which a heat recovery system of a CO 2 recovery unit according to the third embodiment is applied. FIG. 9 is a schematic diagram of the heat recovery system of a CO 2 recovery unit according to the third embodiment. FIG. 10 is a chart of a pattern of the heat recovery system of a CO 2 recovery unit according to the third embodiment. FIG. 11 is a schematic diagram of an effect of the heat recovery system of a CO 2 recovery unit according to the third embodiment. [0115] The heat recovery system of the CO 2 recovery unit 55 according to the third embodiment includes, as shown in FIG. 8 , a heat recovery device 59 that recovers the heat of the flue gas 101 before reaching the CO 2 recovery unit 55 via the Ljungström heat exchanger 57 and supplies the heat to the CO 2 recovery unit 55 in addition to the heat recovery system of the first embodiment. Therefore, in the third embodiment, heat recovery from the heat recovery device 59 to the CO 2 recovery unit 55 is explained, and elements equivalent to those in the first embodiment described above are denoted by like reference signs and explanations thereof will be omitted. [0116] Supply of recovered heat to the CO 2 recovery unit 55 is explained with reference to FIG. 9 . As shown in FIG. 9 , the rich solution pipe 21 c for supplying the rich solution 104 b of the absorbent 104 from the absorber 2 to the regenerator 3 is provided between the rich-lean heat exchanger 4 and the nozzle 31 a of the regenerator 3 via the heat recovery device 59 . [0117] The semi-lean-solution extracting pipe 31 d in the regenerator 3 is provided between the lean-solution/steam-drain heat recovery unit 35 and the nozzle 32 a via the heat recovery device 59 . [0118] The semi-lean-solution extracting pipe 32 d in the regenerator 3 is provided between the lean-solution/steam-drain heat recovery unit 36 and the nozzle 33 a via the heat recovery device 59 . [0119] All or at least one of the exhaust heat fluids of the rich solution 104 b of the absorbent 104 passing through the rich solution pipe 21 c, the semi-lean solution 104 c passing through the semi-lean-solution extracting pipe 31 d , and the semi-lean solution 104 c passing through the semi-lean-solution extracting pipe 32 d is circulated from [c] to [d] of the CO 2 recovery unit 55 via the heat recovery device 59 of the power generation plant 50 shown in FIG. 8 , and used for recovering the heat of the flue gas 101 before reaching the CO 2 recovery unit 55 via the Ljungström heat exchanger 57 . [0120] Specifically, respective patterns in which all or at least one of the respective exhaust heat fluids (the rich solution 104 b and the semi-lean solution 104 c of the absorbent 104 , the wash water 105 , and the CO 2 gas 107 ) is used for recovering the heat of the flue gas 101 are explained with reference to FIGS. 8 to 10 . [0121] In a pattern 1 shown in FIG. 10 , the rich solution 104 b of the absorbent 104 at a temperature of 91.2° C. heat-exchanged by the rich-lean heat exchanger 4 is supplied from [c- 1 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 . The rich solution 104 b at a temperature of 100.8° C. heat-recovered via the heat recovery device 59 is returned to [d- 1 ] (corresponding to [d] shown in FIG. 8 ), caused to flow down from the nozzle 31 a into the regenerator 3 , thereby reacting endothermically. [0122] In a pattern 2 shown in FIG. 10 , the semi-lean solution 104 c of the absorbent 104 at a temperature of 105.0° C. heat-recovered by the lean-solution/steam-drain heat recovery unit 35 is supplied from [c- 2 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 . The semi-lean solution 104 c at a temperature of 112.4° C. heat-recovered via the heat recovery device 59 is returned to [d- 2 ] (corresponding to [d] shown in FIG. 8 ), caused to flow down from the nozzle 32 a into the regenerator 3 , thereby reacting endothermically. [0123] In a pattern 3 shown in FIG. 10 , the semi-lean solution 104 c of the absorbent 104 at a temperature of 117.6° C. heat-recovered by the lean-solution/steam-drain heat recovery unit 36 is supplied from [c- 3 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 . The semi-lean solution 104 c at a temperature of 123.2° C. heat-recovered via the heat recovery device 59 is returned to [d- 3 ] (corresponding to [d] shown in FIG. 8 ), caused to flow down from the nozzle 33 a into the regenerator 3 , thereby reacting endothermically. [0124] A pattern 4 shown in FIG. 10 uses the pattern 1 and the pattern 2 in parallel, in which the rich solution 104 b of the absorbent 104 at a temperature of 91.2° C. is supplied from [c- 1 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 , and the semi-lean solution 104 c of the absorbent 104 at a temperature of 105.0° C. is supplied from [c- 2 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 . The rich solution 104 b at a temperature of 94.1° C. heat-recovered via the heat recovery device 59 is returned to [d- 1 ] (corresponding to [d] shown in FIG. 8 ), and the semi-lean solution 104 c at a temperature of 112.4° C. heat-recovered via the heat recovery device 59 is returned to [d- 2 ] (corresponding to [d] shown in FIG. 8 ). [0125] A pattern 5 shown in FIG. 10 uses the pattern 1 and the pattern 3 in parallel, in which the rich solution 104 b of the absorbent 104 at a temperature of 91.2° C. is supplied from [c- 1 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 , and the semi-lean solution 104 c of the absorbent 104 at a temperature of 117.6° C. is supplied from [c- 3 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 . The rich solution 104 b at a temperature of 95.6° C. heat-recovered via the heat recovery device 59 is returned to [d- 1 ] (corresponding to [d] shown in FIG. 8 ), and the semi-lean solution 104 c at a temperature of 123.2° C. heat-recovered via the heat recovery device 59 is returned to [d- 3 ] (corresponding to [d] shown in FIG. 8 ). [0126] A pattern 6 shown in FIG. 10 uses the pattern 2 and the pattern 3 in parallel, in which the semi-lean solution 104 c of the absorbent 104 at a temperature of 105.0° C. is supplied from [c- 2 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 , and the semi-lean solution 104 c of the absorbent 104 at a temperature of 117.6° C. is supplied from [c- 3 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 . The semi-lean solution 104 c at a temperature of 107.4° C. heat-recovered via the heat recovery device 59 is returned to [d- 2 ] (corresponding to [d] shown in FIG. 8 ), and the semi-lean solution 104 c at a temperature of 123.2° C. heat-recovered via the heat recovery device 59 is returned to [d- 3 ] (corresponding to [d] shown in FIG. 8 ). [0127] A pattern 7 shown in FIG. 10 uses the pattern 1 , the pattern 2 and the pattern 3 in parallel, in which the rich solution 104 b of the absorbent 104 at a temperature of 91.2° C. is supplied from [c- 1 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 , the semi-lean solution 104 c of the absorbent 104 at a temperature of 105.0° C. is supplied from [c- 2 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 , and the semi-lean solution 104 c of the absorbent 104 at a temperature of 117.6° C. is supplied from [c- 3 ] (corresponding to [c] shown in FIG. 8 ) to the heat recovery device 59 . The rich solution 104 b at a temperature of 93.4° C. heat-recovered via the heat recovery device 59 is returned to [d- 1 ] (corresponding to [d] shown in FIG. 8 ), the semi-lean solution 104 c at a temperature of 107.4° C. heat-recovered via the heat recovery device 59 is returned to [d- 2 ] (corresponding to [d] shown in FIG. 8 ), and the semi-lean solution 104 c at a temperature of 123.2° C. heat-recovered via the heat recovery device 59 is returned to [d- 3 ] (corresponding to [d] shown in FIG. 8 ). [0128] In the heat recovery system of the CO 2 recovery unit 55 according to the present embodiment, an effect of a case where the air preheater 58 and the heat recovery device 59 are provided and the pattern 7 shown in FIG. 10 is applied is explained below with reference to FIG. 11 . As shown in FIG. 11 , in the case of the gas temperature of the unburned air 102 being 30° C. corresponding to the ambient air, the gas temperature of the unburned air 102 at the position A in the upstream of the air preheater 58 in FIG. 8 was 30° C., and the gas temperature of the preheated unburned air 102 at the position B in the upstream of the Ljungström heat exchanger 57 became 72° C. by passing through the air preheater 58 . The gas temperature of the heat-exchanged unburned air 102 at the position C in the upstream of the boiler 51 via the Ljungström heat exchanger 57 became 302° C. Furthermore, the gas temperature of the flue gas 101 at the position D in the upstream of the Ljungström heat exchanger 57 via the boiler 51 became 350° C. Further, the gas temperature of the heat-exchanged flue gas 101 at the position E in the upstream of the heat recovery device 59 via the Ljungström heat exchanger 57 became 158° C. Further, the gas temperature of the heat-recovered flue gas 101 at a position F in the upstream of the flue gas desulfurizer 54 via the heat recovery device 59 became 104° C. At this time, the flow rate G of the low-pressure steam 106 supplied to the regenerating heater 34 of the CO 2 recovery unit 55 was 800 t/hr. [0129] On the other hand, an effect in the conventional case where the air preheater 58 and the heat recovery device 59 are not provided is as described below. As shown in FIG. 11 , when the gas temperature of the unburned air 102 is 30° C. corresponding to the ambient air, in FIG. 8 , the gas temperature of the unburned air 102 at the position A is 30° C. Because the air preheater 58 is not provided, the gas temperature of the unburned air 102 at the position B in the upstream of the Ljungström heat exchanger 57 becomes 30° C. The gas temperature of the heat-exchanged unburned air 102 at the position C in the upstream of the boiler 51 via the Ljungström heat exchanger 57 became 294° C. Furthermore, the gas temperature of the flue gas 101 at the position D in the upstream of the Ljungström heat exchanger 57 via the boiler 51 became 350° C. Furthermore, the gas temperature of the heat-exchanged flue gas 101 at the position E in the upstream of the flue gas desulfurizer 54 via the Ljungström heat exchanger 57 became 130° C. Because the heat recovery device 59 is not provided, the gas temperature of the flue gas 101 corresponding to the position F is 130° C. At this time, the flow rate G of the low-pressure steam 106 supplied to the regenerating heater 34 of the CO 2 recovery unit 55 was 870 t/hr. [0130] In this manner, in the heat recovery system of the CO 2 recovery unit 55 according to the third embodiment, the heat recovery device 59 that recovers the heat of the flue gas 101 before reaching the CO 2 recovery unit 55 via the Ljungström heat exchanger 57 and supplies the heat to the CO 2 recovery unit 55 is further provided, with respect to the heat recovery system of the first embodiment. [0131] According to the heat recovery system of the CO 2 recovery unit 55 , recovered heat can be effectively used, while effectively using low-temperature exhaust heat equal to or lower than a temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 . Specifically, as described above, the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 8° C. to be 302° C. A heat input to the boiler 51 increases due to the increase of 8° C., and effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 by 15 t/hr can be acquired. The flow rate of 15 t/hr corresponds to 1.7% of the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 . Furthermore, the flow rate of 870 t/hr of the low-pressure steam 106 supplied to the regenerating heater 34 of the CO 2 recovery unit 55 in the conventional case is decreased to 800 t/hr by 70 t/hr. The flow rate of 70 t/hr corresponds to 8% of the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 . That is, the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 is decreased by 9.7% in total. As a result, by recovering the heat of the flue gas 101 after passing through the Ljungström heat exchanger 57 while preheating the unburned air 102 before reaching the Ljungström heat exchanger 57 by low-temperature exhaust heat equal to or lower than the temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 , the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 can be decreased. [0132] Furthermore, because the Ljungström heat exchanger 57 performs heat exchange between the flue gas 101 containing sulfur content and the unburned air 102 , the heat reservoir may be subjected to sulfuric acid corrosion due to dew condensation generated on the heat transfer surface of the heat reservoir. In this regard, according to the heat recovery system of the CO 2 recovery unit 55 of the third embodiment, the gas temperature of 30° C. of the unburned air 102 at the position B on the inlet side of the Ljungström heat exchanger 57 in the conventional case is raised to 72° C., and the gas temperature of 130° C. of the flue gas 101 at the position E on the outlet side of the Ljungström heat exchanger 57 in the conventional case is also raised to 158° C. As a result, an average cold-end temperature ((the gas temperature on the outlet side of flue gas+the gas temperature on the inlet side of unburned air)/2) of the Ljungström heat exchanger 57 increases from 80° C. to 115° C. by 35° C. Consequently, dew condensation on the heat transfer surface of the heat reservoir of the Ljungström heat exchanger 57 can be prevented and sulfuric acid corrosion of the heat reservoir can be suppressed. [0133] The heat recovery system of the CO 2 recovery unit 55 according to the third embodiment is applied to the power generation plant 50 that drives the steam turbine 52 by the superheated steam 100 from the boiler 51 to generate power by the power generator 53 . [0134] As described above, according to the heat recovery system of the CO 2 recovery unit 55 , the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 8° C. to be 302° C. Therefore, because the flow rate of the superheated steam 100 supplied from the boiler 51 to the steam turbine 52 increases, the power generation efficiency of the power generation plant 50 can be improved. [0135] A heat recovery method of the CO 2 recovery unit 55 according to the third embodiment further includes a step of recovering the heat of the flue gas 101 before reaching the CO 2 recovery step through the heat exchanging step, and of supplying the heat to the CO 2 recovery step (heat recovery step), with respect to the heat recovery method of the first embodiment. [0136] According to the heat recovery method of the CO 2 recovery unit 55 , low-temperature exhaust heat equal to or lower than the temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 can be used efficiently. Specifically, the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 8° C. to be 302° C. A heat input to the boiler 51 increases due to the increase of 8° C., and effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step by 15 t/hr can be acquired. The flow rate of 15 t/hr corresponds to 1.7% of the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step. Furthermore, the flow rate of 870 t/hr of the low-pressure steam 106 supplied to the CO 2 recovery step in the conventional case is decreased to 800 t/hr by 70 t/hr. The flow rate of 70 t/hr corresponds to 8% of the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step. That is, the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step is decreased by 9.7% in total. As a result, by recovering the heat of the flue gas 101 after passing through the heat exchanging step while preheating the unburned air 102 before reaching the heat exchanging step by low-temperature exhaust heat equal to or lower than the temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 , the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step can be decreased. [0137] Furthermore, because heat exchange between the flue gas 101 containing sulfur content and the unburned air 102 is performed at the heat exchanging step, the heat reservoir may be subjected to sulfuric acid corrosion due to dew condensation generated on the heat transfer surface of the heat reservoir. In this regard, according to the heat recovery method of the CO 2 recovery unit 55 of the third embodiment, the gas temperature of 30° C. of the unburned air 102 at the position B on the inlet side of the heat exchanging step in the conventional case is raised to 72° C., and the gas temperature of 130° C. of the flue gas 101 at the position E on the outlet side of the heat exchanging step in the conventional case is also raised to 158° C. As a result, an average cold-end temperature ((the gas temperature on the outlet side of flue gas+the gas temperature on the inlet side of unburned air)/2) at the heat exchanging step is increased from 80° C. to 115° C. by 35° C. Consequently, dew condensation on the heat transfer surface of the heat reservoir at the heat exchanging step can be prevented and sulfuric acid corrosion of the heat reservoir can be suppressed. [0138] The heat recovery method of the CO 2 recovery unit 55 according to the third embodiment further includes a power generation step of driving the steam turbine 52 by the superheated steam 100 from the boiler 51 to generate power by the power generator 53 . [0139] According to the heat recovery method of the CO 2 recovery unit 55 , the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 8° C. to be 302° C. Therefore, because the flow rate of the superheated steam 100 supplied from the boiler 51 to the steam turbine 52 increases, the power generation efficiency of the power generation plant 50 can be improved. [0140] In the configuration described above, the regenerator 3 includes three stages, which are an upper stage, a middle stage, and a lower stage. That is, these are the upper regenerating unit 31 , the middle regenerating unit 32 , and the lower regenerating unit 33 . However, the regenerator 3 can include two stages, which are upper and lower stages. That is, these are the upper regenerating unit 31 and the lower regenerating unit 33 . In this case, the semi-lean-solution extracting pipe 31 d in the regenerator 3 is provided between the lean-solution/steam-drain heat recovery unit 35 and the nozzle 32 a via the heat recovery device 59 , and the pattern 2 and patterns including the pattern 2 shown in FIG. 10 are deleted. Furthermore, the regenerator 3 is not limited to the upper, middle, and lower stages, which are the upper regenerating unit 31 , the middle regenerating unit 32 , and the lower regenerating unit 33 , and can include three or more regenerating units. In this case, a pattern for an increased stage and patterns including this pattern are added. Fourth Embodiment [0141] A fourth embodiment is explained with reference to the drawings. FIG. 12 is a schematic diagram of a heat recovery system of a CO 2 recovery unit according to the fourth embodiment. FIG. 13 is a schematic diagram of an effect of the heat recovery system of a CO 2 recovery unit according to the fourth embodiment. [0142] The heat recovery system of the CO 2 recovery unit 55 according to the fourth embodiment further includes, as shown in FIGS. 8 and 12 , the heat recovery device 59 that recovers the heat of the flue gas 101 before reaching the CO 2 recovery unit 55 via the Ljungström heat exchanger 57 and supplies the heat to the CO 2 recovery unit 55 in addition to the heat recovery system of the second embodiment. Furthermore, heat recovery from the heat recovery device 59 to the CO 2 recovery unit 55 in the fourth embodiment is identical to that of the third embodiment described above. Therefore, in the fourth embodiment, explanations of heat recovery will be omitted because it has been explained in the second and third embodiments described above. [0143] In the heat recovery system of the CO 2 recovery unit 55 according to the present embodiment, an effect of a case where the air preheater 58 and the heat recovery device 59 are provided and the pattern 7 shown in FIG. 10 is applied is explained below with reference to FIG. 13 . As shown in FIG. 13 , in the case of the gas temperature of the unburned air 102 being 30° C. corresponding to the ambient air, the gas temperature of the unburned air 102 at the position A in the upstream of the air preheater 58 in FIG. 8 was 30° C., and the gas temperature of the preheated unburned air 102 at the position B in the upstream of the Ljungström heat exchanger 57 became 82° C. by passing through the air preheater 58 . The gas temperature of the heat-exchanged unburned air 102 at the position C in the upstream of the boiler 51 via the Ljungström heat exchanger 57 became 303° C. Furthermore, the gas temperature of the flue gas 101 at the position D in the upstream of the Ljungström heat exchanger 57 via the boiler 51 became 350° C. [0144] Further, the gas temperature of the heat-exchanged flue gas 101 at the position E in the upstream of the heat recovery device 59 via the Ljungström heat exchanger 57 became 165° C. Further, the gas temperature of the heat-recovered flue gas 101 at the position F in the upstream of the flue gas desulfurizer 54 via the heat recovery device 59 became 104° C. At this time, the flow rate G of the low-pressure steam 106 supplied to the regenerating heater 34 of the CO 2 recovery unit 55 was 791 t/hr. [0145] On the other hand, an effect in the conventional case where the air preheater 58 and the heat recovery device 59 are not provided is as described below. As shown in FIG. 13 , when the gas temperature of the unburned air 102 is 30° C. corresponding to the ambient air, in FIG. 8 , the gas temperature of the unburned air 102 at the position A is 30° C. Because the air preheater 58 is not provided, the gas temperature of the unburned air 102 at the position B in the upstream of the Ljungström heat exchanger 57 becomes 30° C. The gas temperature of the heat-exchanged unburned air 102 at the position C in the upstream of the boiler 51 via the Ljungström heat exchanger 57 became 294° C. Furthermore, the gas temperature of the flue gas 101 at the position D in the upstream of the Ljungström heat exchanger 57 via the boiler 51 became 350° C. Furthermore, the gas temperature of the heat-exchanged flue gas 101 at the position E in the upstream of the flue gas desulfurizer 54 via the Ljungström heat exchanger 57 became 130° C. Because the heat recovery device 59 is not provided, the gas temperature of the flue gas 101 corresponding to the position F is 130° C. At this time, the flow rate G of the low-pressure steam 106 supplied to the regenerating heater 34 of the CO 2 recovery unit 55 was 870 t/hr. [0146] In this manner, in the heat recovery system of the CO 2 recovery unit 55 according to the fourth embodiment, the heat recovery device 59 that recovers the heat of the flue gas 101 before reaching the CO 2 recovery unit 55 via the Ljungström heat exchanger 57 and supplies the heat to the CO 2 recovery unit 55 is further provided, with respect to the heat recovery system of the second embodiment. [0147] According to the heat recovery system of the CO 2 recovery unit 55 , recovered heat can be effectively used, while effectively using low-temperature exhaust heat equal to or lower than a temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 . Specifically, as described above, the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 9° C. to be 303° C. A heat input to the boiler 51 increases due to the increase of 9° C., and effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 by 17 t/hr can be acquired. The flow rate of 17 t/hr corresponds to 2% of the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 . Furthermore, the flow rate of 870 t/hr of the low-pressure steam 106 supplied to the regenerating heater 34 of the CO 2 recovery unit 55 in the conventional case is decreased to 791 t/hr by 79 t/hr. The flow rate of 79 t/hr corresponds to 9.1% of the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 . That is, the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 is decreased by 11.1% in total. As a result, by recovering the heat of the flue gas 101 after passing through the Ljungström heat exchanger 57 while preheating the unburned air 102 before reaching the [0148] Ljungström heat exchanger 57 by low-temperature exhaust heat equal to or lower than the temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 , the flow rate of the low-pressure steam 106 consumed by the CO 2 recovery unit 55 can be decreased. [0149] Furthermore, because the Ljungström heat exchanger 57 performs heat exchange between the flue gas 101 containing sulfur content and the unburned air 102 , the heat reservoir may be subjected to sulfuric acid corrosion due to dew condensation generated on the heat transfer surface of the heat reservoir. In this regard, according to the heat recovery system of the CO 2 recovery unit 55 of the fourth embodiment, the gas temperature of 30° C. of the unburned air 102 at the position B on the inlet side of the Ljungström heat exchanger 57 in the conventional case is raised to 82 ° C., and the gas temperature of 130° C. of the flue gas 101 at the position E on the outlet side of the [0150] Ljungström heat exchanger 57 in the conventional case is also raised to 165° C. As a result, an average cold-end temperature ((the gas temperature on the outlet side of flue gas+the gas temperature on the inlet side of unburned air)/2) of the Ljungström heat exchanger 57 increases from 80° C. to 123.5° C. by 43.5° C. Consequently, dew condensation on the heat transfer surface of the heat reservoir of the Ljungström heat exchanger 57 can be prevented and sulfuric acid corrosion of the heat reservoir can be suppressed. [0151] The heat recovery system of the CO 2 recovery unit 55 according to the fourth embodiment is applied to the power generation plant 50 that drives the steam turbine 52 by the superheated steam 100 from the boiler 51 to generate power by the power generator 53 . [0152] As described above, according to the heat recovery system of the CO 2 recovery unit 55 , the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 9° C. to be 303° C. Therefore, because the flow rate of the superheated steam 100 supplied from the boiler 51 to the steam turbine 52 increases, the power generation efficiency of the power generation plant 50 can be improved. [0153] A heat recovery method of the CO 2 recovery unit 55 according to the fourth embodiment further includes a step of recovering the heat of the flue gas 101 before reaching the CO 2 recovery step through the heat exchanging step, and of supplying the heat to the CO 2 recovery step (heat recovery step), with respect to the heat recovery method of the first embodiment. [0154] According to the heat recovery method of the CO 2 recovery unit 55 , low-temperature exhaust heat equal to or lower than the temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 can be used efficiently. Specifically, the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 9° C. to be 303° C. A heat input to the boiler 51 increases due to the increase of 9° C., and effects equivalent to those of decreasing the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step by 17 t/hr can be acquired. The flow rate of 17 t/hr corresponds to 2% of the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step. Furthermore, the flow rate of 870 t/hr of the low-pressure steam 106 supplied to the CO 2 recovery step in the conventional case is decreased to 791 t/hr by 79 t/hr. The flow rate of 79 t/hr corresponds to 9.1% of the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step. That is, the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step is decreased by 11.1% in total. As a result, by recovering the heat of the flue gas 101 after passing through the heat exchanging step while preheating the unburned air 102 before reaching the heat exchanging step by low-temperature exhaust heat equal to or lower than the temperature level of 70° C. to 80° C. discharged at the time of recovering CO 2 , the flow rate of the low-pressure steam 106 consumed at the CO 2 recovery step can be decreased. [0155] Furthermore, because heat exchange between the flue gas 101 containing sulfur content and the unburned air 102 is performed at the heat exchanging step, the heat reservoir may be subjected to sulfuric acid corrosion due to dew condensation generated on the heat transfer surface of the heat reservoir. In this regard, according to the heat recovery method of the CO 2 recovery unit 55 of the fourth embodiment, the gas temperature of 30° C. of the unburned air 102 at the position B on the inlet side of the heat exchanging step in the conventional case is raised to 82° C., and the gas temperature of 130° C. of the flue gas 101 at the position E on the outlet side of the heat exchanging step in the conventional case is also raised to 165° C. As a result, an average cold-end temperature ((the gas temperature on the outlet side of flue gas+the gas temperature on the inlet side of unburned air)/2) at the heat exchanging step is increased from 80° C. to 123.5° C. by 43.5° C. Consequently, dew condensation on the heat transfer surface of the heat reservoir at the heat exchanging step can be prevented and sulfuric acid corrosion of the heat reservoir can be suppressed. [0156] The heat recovery method of the CO 2 recovery unit 55 according to the fourth embodiment further includes a power generation step of driving the steam turbine 52 by the superheated steam 100 from the boiler 51 to generate power by the power generator 53 . [0157] According to the heat recovery method of the CO 2 recovery unit 55 , the gas temperature of 294° C. of the unburned air 102 at the position C supplied to the boiler 51 in the conventional case is raised by 9° C. to be 303° C. Therefore, because the flow rate of the superheated steam 100 supplied from the boiler 51 to the steam turbine 52 increases, the power generation efficiency of the power generation plant 50 can be improved. [0158] In the configuration described above, the regenerator 3 includes three stages, which are an upper stage, a middle stage, and a lower stage. That is, these are the upper regenerating unit 31 , the middle regenerating unit 32 , and the lower regenerating unit 33 . However, the regenerator 3 can include two stages, which are upper and lower stages. That is, these are the upper regenerating unit 31 and the lower regenerating unit 33 . In this case, the semi-lean-solution extracting pipe 31 d in the regenerator 3 is provided between the lean-solution/steam-drain heat recovery unit 35 and the nozzle 32 a via the heat recovery device 59 , and the pattern 2 and patterns including the pattern 2 shown in FIG. 10 are deleted. Furthermore, the regenerator 3 is not limited to the upper, middle, and lower stages, which are the upper regenerating unit 31 , the middle regenerating unit 32 , and the lower regenerating unit 33 , and can include three or more regenerating units. In this case, a pattern for an increased stage and patterns including this pattern are added. REFERENCE SIGNS LIST [0000] 1 cooling column 2 absorber 3 regenerator 4 rich-lean heat exchanger 50 power generation plant 51 boiler 52 steam turbine 53 power generator 54 flue gas desulfurizer 55 CO 2 recovery unit 56 stack 57 Ljungström heat exchanger 58 air preheater 59 heat recovery device 100 superheated steam 101 flue gas 101 a decarbonated flue gas 102 unburned air 103 cooling water 104 absorbent 104 a lean solution 104 b rich solution 104 c semi-lean solution 105 wash water 106 low-pressure steam 106 a steam drain 107 CO 2 gas 108 condensed water 109 heat medium

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