AU2011364094A1

AU2011364094A1 - Exhaust gas treatment system and exhaust gas treatment method

Info

Publication number: AU2011364094A1
Application number: AU2011364094A
Authority: AU
Inventors: Koji Harada; Hiroshi Ishizaka; Atsushi Katagawa; Takashi Muramoto; Takanori Nakamoto; Ryota OCHIAI; Takayuki Saito; Misaki SUMIKURA
Original assignee: Babcock Hitachi KK; Sasakura Engineering Co Ltd
Current assignee: Sasakura Engineering Co Ltd; Mitsubishi Power Ltd
Priority date: 2011-03-31
Filing date: 2011-03-31
Publication date: 2013-03-07
Anticipated expiration: 2031-03-31
Also published as: WO2012132003A1; AU2011364094B2

Abstract

[Problem] The purpose of the present invention is to provide an exhaust gas treatment system and exhaust gas treatment method that can reduce the amount of make-up water to a desulfurization means and make a reduction in the heat source required for a desalination means. [Solution] This exhaust gas treatment system (10) is characterized by comprising a wet type desulfurization means (40) that eliminates sulfur oxides in the exhaust gas, a desalination means (80) that produces fresh water from seawater and supplies the same to the wet type desulfurization means (40), a heat exchanger (20) that heats a heating medium by means of the exhaust gas in a stage prior to the wet type desulfurization means (40), a seawater heater (22) for the desalination means (80), and a circulation line (24) that connects the heat exchanger (20) to the seawater heater and circulates the heating medium.

Description

DESCRIPTION EXHAUST GAS TREATMENT SYSTEM AND EXHAUST GAS TREATMENT METHOD TECHNICAL FIELD [0001] This invention relates to an exhaust gas treatment system and an exhaust gas treatment method having wet desulfurization means for removing sulfur oxides and the like in exhaust gas in particular. BACKGROUND ART [0002] Exhaust gas emitted from a boiler and the like installed in thermal power plants, factories, and the like includes acid gases such as sulfur oxides, hydrogen chloride, and hydrogen fluoride. There is a wet desulfurization device as means for removing the acid gas. The wet desulfurization device has higher desulfurization performance than dry desulfurization device, and there is an advantage, e.g., low pollution of wastewater discharged from the device. [00031 Exhaust gas discharged from a conventional boiler installed in thermal power plants and factories has been introduced into the wet desulfurization device at a relatively high gas temperature of 120 to 160'C. Therefore, in an absorption tower of the wet desulfurization device, the hot exhaust gas and slurry 1 come into contact with each other, thereby being vaporized to increase the amount of steam water. Large amount of generated mist (vapor) cannot be captured with a mist eliminator, and is exhausted to the outside of the absorption tower. Therefore, makeup water has to be replenished to compensate for the amount of water equivalent to the vapor discharged to the outside, and a large amount of makeup water is required. [0004] As described above, in the conventional wet desulfurization device, a large amount of makeup water is needed. In an area where it is difficult to obtain fresh water (industrial water), a desalination device for converting seawater into fresh water produces fresh water, which is used as the makeup water in the wet desulfurization device. [00051 Conventional desalination devices include a method using evaporation methods such as Multi-Stage Flash Distillation and Multi-Effect Desalination. In the desalination device using the evaporation method, heat supply is needed from the outside to a seawater heater in the production process of fresh water by means of supply of, e.g., steam and hot water. Therefore, there is a problem in that plants such as a thermal power plant have a problem in that the power generation efficiency decreases. For an example, in order to produce fresh water at about 160 Ton / h, the desalination device requires the amount of heat provided by steam 2 at about 1 to 2 Ton / h and hot water at about 1000 Ton / h at 1100C. [00061 On the other hand, there is a wet desulfurization device using seawater as desulfurization absorbent. In this method, the seawater is supplied directly into the absorption tower, and after removal of, e.g., sulfur oxides and dust in the exhaust gas, seawater is discharged into the sea. However, when using seawater, a large amount of seawater is supplied to the absorption tower and is discharged. Thus, when wastewater treatment is performed, a large amount of wastewater is processed, which requires large facilities. In a simple discharge processing in which only the concentration of dissolved oxygen in the wastewater is adjusted, heavy metals and the like in the wastewater are not treated, and the temperature of the wastewater is also high. Thus there was a problem of secondary pollution of polluted wastewater such as increase of the seawater temperature. [0007] Patent literatures 1 and 2 are techniques for desalinating seawater using discharged heat from a boiler and the like. Patent literature 1 discloses equipment for desalinating seawater using discharged heat from power generators and improving the efficiency of power generation facilities, thereby directly supplying the discharged heat from gas turbines to a seawater heating device of an evaporation-type seawater desalination device to heat the 3 seawater. Patent literature 2 discloses equipment for using discharged heat from a plurality of power generation plants such as discharged heat boilers and directly supplying the heat to a seawater heating device of a seawater desalination device to heat the seawater. [00081 Patent literatures 3, 4 include techniques having a desulfurization device and a desalination device. Patent literature 3 is a system in which exhaust gas and seawater are brought into indirect contact with each other for cooling in a heat exchanger of indirect heat exchanger device at a desulfurization treatment side. Patent literature 3 discloses a system in which moisture is collected by condensing moisture in exhaust gas by cooling and the heated seawater is supplied to an electrodialyzer. [00091 Patent literature 4 is a system in which exhaust gas is treated by a dry flue gas desulfurization device of a boiler for thermal power generation and the sensible heat of the exhaust gas is used as a heat source for a seawater desalination device. The seawater desalination device of patent literature 4 is Multi-Stage Flash, and the obtained freshwater is supplied to the boiler. Citation List Patent Literature 4 [0010] Patent literature 1: Japanese Patent Laid-Open No. 2006-70889 Patent literature 2: Japanese Patent Laid-Open No. 61-15003 Patent literature 3: Japanese Patent Laid-Open No. 62-30530 Patent literature 4: Japanese Patent Laid-Open No. 59-107105 SUMMARY OF INVENTION TECHNICAL PROBLEM [0011] However, the use of the discharged heat disclosed in patent literatures 1, 2 is configured to directly exchange the heat of the exhaust gas discharged from the power generation plant as a heat source of the seawater desalination device. When a heat exchanger malfunctions during plant operation in this kind of method of direct exchanging heat between the exhaust gas and seawater, it is impossible to stop only the malfunctioning device since the treatment step of the exhaust gas and the desalination step are synchronized, and there is no choice but to stop the entire system. [0012] Patent literature 3 performs heat exchange in a stage subsequent to the desulfurization device. Therefore, the temperature difference between the exhaust gas temperature and the seawater is small, and unless the capacity of the heat 5 exchanger is increased, efficient heat exchange cannot be performed, and there is a problem in that the size of the entire device increases. Patent literature 4 uses dry method as the desulfurization, and therefore, expensive activated coke is required, and there is a problem in that the desulfurization performance is lower than the wet method. [0013] In order to solve the above problems of the conventional techniques, it is an object of this invention to provide an exhaust gas treatment system and an exhaust gas treatment method capable of reducing the amount of makeup water of the desulfurization means. Further, it is an object of the present invention to provide an exhaust gas treatment system and an exhaust gas treatment method capable of reducing the heat source required by the desalination means. Further, it is an object of the present invention to provide an exhaust gas treatment system and an exhaust gas treatment method that improves the efficiency of fresh water production by the desalination means. Further, it is an object of the present invention to provide an exhaust gas treatment system and an exhaust gas treatment method that improves the efficiency of the dust collection rate of the electrostatic precipitator means. 6 SOLUTION TO PROBLEM [0014] An exhaust gas treatment system according to the present invention includes wet desulfurization means for removing a sulfur oxide in an exhaust gas, desalination means for producing freshwater from seawater and supplying the freshwater to the wet desulfurization means, a heat exchanger for heating a heat medium with the exhaust gas at a stage before the wet desulfurization means, a seawater heating device of the desalination means, and a circulation line of the heat medium for connecting the heat exchanger with the seawater heating device. [0015] According to the above configuration, the amount of makeup water given to desulfurization means can be reduced. The heat source of the exhaust gas can be used in the desalination means. Therefore, the cost of the entire system can be reduced, and energy-saving can be achieved. [0016] In this case, it is preferable to be provided with a bypass line connecting a feed pipe and a return pipe of the circulation line, a flow amount control valve provided in the circulation line and the bypass line for adjusting an amount of flow of the heat medium, and control means connected to the flow amount control valve for controlling an amount of supply of the heat medium to the seawater heating device. 7 According to the above configuration, the heating temperature of the seawater can be controlled, and the freshwater can be produced efficiently. [0017] In this case, it is preferable to be provided with seawater temperature measuring means for detecting an outlet temperature of the seawater heating device, wherein the control means controls the amount of supply of the heat medium to the seawater heating device on the basis of a measured value measured by the temperature measuring means, so that the outlet temperature of the seawater heating device attains a temperature defined in advance. According to the above configuration, the heating temperature of the seawater can be controlled so that it is maintained at a setting value defined in advance, and the freshwater can be produced efficiently. [0018] In this case, electrostatic precipitator means is preferably provided at a stage before the heat exchanger. According to the above configuration, heat exchange can be performed between the exhaust gas and the heat medium of the heat exchanger without reducing the exhaust gas temperature of the electrostatic precipitator means. [0019] In this case, electrostatic precipitator means is preferably provided between the heat exchanger and the wet 8 desulfurization means. According to the above configuration, this reduces the gas temperature of the high-temperature exhaust gas introduced to the electrostatic precipitator means, so that the dust removing efficiency can be performed efficiently. [0020] In this case, the control means preferably controls the amount of supply of the heat medium to the heat exchanger by connecting with the flow amount control valve. According to the above configuration, the exhaust gas temperature can be controlled, and the electrostatic precipitator means can efficiently perform removing operation. [0021] In this case, it is preferable to be provided with gas temperature measuring means for detecting the temperature of the exhaust gas introduced to the electrostatic precipitator means, wherein the control means preferably controls the amount of supply of the heat medium to the heat exchanger on the basis of the measured value measured by the gas temperature measuring means, so that the temperature of the exhaust gas introduced to the electrostatic precipitator means attains a temperature defined in advance. According to the above configuration, the exhaust gas temperature can be controlled to be at the setting value of the gas temperature defined in advance, and the electrostatic 9 precipitator means can efficiently perform removing operation. [0022] In this case, a second heat exchanger is preferably provided at a liquid holding unit of the wet desulfurization means and is connected to the circulation line, and the heat medium preferably circulates through the heat exchanger and the second heat exchanger. According to the above configuration, the heat medium can be heated by performing heat exchange between the heat medium and the absorption liquid without reducing the removing efficiency of the dust by the electrostatic precipitator means. Therefore, even when the exhaust gas temperature is low, the heat medium can be heated, and the amount of used steam in the desalination means can be reduced. Further, the cost of the entire system can be reduced, and energy-saving can be achieved. [0023] An exhaust gas treatment system according to the present invention includeswet desulfurization means for removinga sulfur oxide in an exhaust gas, desalination means for producing freshwater from seawater and supplying the freshwater to the wet desulfurization means, a heat exchanger for heating a heat medium with the exhaust gas at a stage subsequent to the wet desulfurization means, a seawater heating device of the desalination means, a heat medium circulation line for connecting the heat exchanger with the seawater heating device and 10 circulating the heat medium, and a mist eliminator for removing mist included in the exhaust gas. [0024] According to the above configuration, the exhaust gas in the water saturation state having been subjected to the desulfurization treatment is subjected to the heat exchange in the heat exchanger with the heat medium, whereby the gas temperature is reduced, and the moisture component can be condensed and recovered. Since the heat of the exhaust gas is used to heat the seawater in the desalination means, the amount of used steam can be reduced. Further, the cost of the entire system can be reduced, and energy-saving can be achieved. [0025] An exhaust gas treatment method of the present invention removes a sulfur oxide included in an exhaust gas, and the exhaust gas treatment method includes the steps of heating the heat medium by reducing a gas temperature of the exhaust gas by performing heat exchange between the heat medium and the exhaust gas that has not yet subjected to the desulfurization treatment, performing desulfurization treatment on the exhaust gas of which gas temperature decreased, circulating the heated heat medium to a seawater heating device, performing heat exchange between the heated heat medium and a seawater, producing freshwater from the heated seawater, and measuring the heated seawater temperature to control an amount of circulation of the heat medium. 11 [0026] According to the above configuration, the amount of makeup water given to desulfurization means can be reduced. The heat source of the exhaust gas can be used in the desalination means. Therefore, the cost of the entire system can be reduced, and energy-saving can be achieved. Further, the heating temperature of the seawater can be controlled, and the freshwater can be produced efficiently. ADVANTAGEOUS EFFECTS OF INVENTION [0027] The heat exchanger is provided at the upstream side (prior stage) of the wet desulfurization means, and the exhaust gas temperature at the inlet of the wet desulfurization means can be reduced. Therefore, a large amount of water in the absorption tower is not evaporated and discharged to the outside. Therefore, the amount of water supplied to the absorption tower can be reduced. [0028] Further, the heat exchanger is an indirect-type. Accordingly, by reusing the heat obtained from the heat exchange with the exhaust gas as the heat source for the desalination means, it is not necessary to supply steam required by water purification means, and power-saving can be achieved in the entire system. BRIEF DESCRIPTION OF DRAWINGS 12 [0029] Fig. 1 is a schematic diagram illustrating a configuration of an exhaust gas treatment system according to a first embodiment. Fig. 2 is a schematic diagram illustrating a configuration of the exhaust gas treatment system using Multi-Effect Desalination in desalination means. Fig. 3 is a schematic diagram illustrating a configuration of an exhaust gas treatment system using Multi-Stage Flash method in the desalination means. Fig. 4 is a graph illustrating correlation between an absorption tower inlet gas temperature and an absorption tower evaporated water amount. Fig. 5 is a schematic diagram illustrating a configuration of an exhaust gas treatment system according to a second embodiment. Fig. 6 is a schematic diagram illustrating a configuration of an exhaust gas treatment system according to a third embodiment. Fig. 7 is a schematic diagram illustrating a configuration of an exhaust gas treatment system according to a fourth embodiment. Fig. 8 is a schematic diagram illustrating a configuration of an exhaust gas treatment system according to a fifth embodiment. DESCRIPTION OF EMBODIMENTS [00301 Embodiments of an exhaust gas treatment system and an 13 exhaust gas treatment method according to the present invention will be hereinafter described in detail with reference to attached drawings. Fig. 1 is a schematic diagram illustrating a configuration of an exhaust gas treatment system according to a first embodiment. As shown in the figure, an exhaust gas treatment system 10 according to the first embodiment is mainly configured to include electrostatic precipitator means 12 for removing smoke and dust from exhaust gas, desulfurization means 40 provided at a stage subsequent to the electrostatic precipitator means 12 for removing sulfur component in the exhaust gas, desalination means 80 for generating freshwater from seawater, and a heat exchanger 20 for using heat of the exhaust gas for heating seawater in the desalination means. [0031] The electrostatic precipitator means 12 is mainly configured to include a discharge electrode and a dust collection plate. The electrostatic precipitator means 12 having this configuration charges and removes dust in the exhaust gas discharged from a boiler with corona discharge generated between the discharge electrode and the dust collection plate. An exhaust gas fan 14 is provided at a stage subsequent to the electrostatic precipitator means 12. The exhaust gas fan 14 increases the pressure of the exhaust gas. [0032] 14 The heat exchanger 20 is a multi-tubular type, and is provided at an outlet path of the electrostatic precipitator means 12. The heat exchanger 20 is structured such that exhaust gas flows on the external surface of a heat exchanger tube, and heat medium flows on the inner surface of a heat exchanger tube. The heat exchanger 20 is connected via a circulation line 24 for the heat medium and a seawater heating device 22 provided in the desalination means 80. The circulation line 24 has a circulating pump 26 and serves as a circulation path for the heat medium. With this circulation line 24, the heat exchanger 20 exchanges heat between the exhaust gas and the heat medium, and the seawater heating device 22 exchanges heat between the heat medium and the seawater, whereby the heat of the exhaust gas is used to heat the seawater supplied to the desalination means 80. For example, the temperature of the exhaust gas discharged from boilers and the like installed in thermal powerplants, and the like, is relatively high, e.g., 120 to 1600C in a case of a coal fired boiler. The heat exchanger 20 having this configuration heats the heat medium using the high temperature exhaust gas. The heated heat medium is passed to the seawater heating device 22, where heat is exchanged between the seawater and the heat medium, so that the seawater is heated. On the other hand, due to the heat exchange with the heat medium flowing on the inner surface of the tube, the exhaust gas is cooled and the exhaust gas temperature is reduced. It should be noted that the heat medium is preferably 15 fresh water (industrial water) . In this case, the heat exchanger and the circulation line can be made of low-cost carbon steel materials. When the heat medium is circulated in a closed system such as the circulation line 24, it is not necessary to replenish any heat medium from outside of the system. When the heat exchangers 20 at the exhaust gas side and the seawater side are made as an indirect-type, exhaust gas treatment step and freshwater treatment step can independently be performed even when the heat exchanger 20 malfunctions during plant operation, so that this does not greatly affect the operations of them. Therefore, in terms of plant operation, highly reliable system can be established. [00331 The desulfurization means 40 basically has such configuration including, in an absorption tower 42, a liquid reservoir unit 44 provided at the lower portion thereof, an absorption unit 46 provided in the exhaust gas ascending path, and a mist eliminator 48 provided at the outlet portion. [0034] In the absorption tower 42, a gas inlet portion 41 is provided at the upstream side where the exhaust gas of which pressure is increased by the exhaust gas fan 14 is introduced. The liquid reservoir unit 44 is provided at the bottom of the absorption tower 42, and can temporarily store absorbent slurry to be reacted with the sulfur component in the exhaust gas. 16 The liquid reservoir unit 44 is connected to a supply unit 50 of absorbent slurry (limestone slurry) . Necessary absorbent slurry (limestone slurry) is supplied from the supply unit 50 to the liquid reservoir unit 44 in accordance with the amount of sulfur oxides included in the exhaust gas from the boiler and the like. [00351 The liquid reservoir unit 44 is connected with the absorption unit 46 via an absorption liquid circulation pipe 52. The absorption liquid circulation pipe 52 is provided with an absorption liquid circulating pump 54. In this configuration, the slurry absorption liquid in the liquid reservoir unit 44 is circulated and supplied to the absorption unit 46 via the absorption liquid circulation pipe 52 by the absorption liquid circulating pump 54. [00361 The absorption unit 46 is provided above the liquid reservoir unit 44 in the absorption tower 42. The absorption unit 46 has spray headers 56 provided in multiple stages in a gas flow direction, and absorbent is supplied thereto. Each spray header 56 is provided with a plurality of spray nozzles 58. With the above supply pressure of the absorption liquid circulating pump 54, the absorption liquid is sprayed from the spray nozzles 58 toward the upward flow of the exhaust gas. In this configuration, in the absorption unit 46, as a result of gas-liquid contact between the exhaust gas and the absorption liquid sprayed from 17 the spray nozzles 58, acid gases such as sulfur oxides, hydrogen chloride, and hydrogen fluoride included in the exhaust gas are absorbed on the droplet surfaces of the absorption liquid circulating within the absorption tower 42. At this occasion, due to the temperature of the exhaust gas, a part of the absorption liquid is evaporated and made into mist. [0037] The mist eliminator 48 is provided at a desulfurization means gas outlet portion 59 (a stage subsequent to the absorption unit 46) within the absorption tower 42. The mist eliminator 48 can remove the mist included in the exhaust gas. In this configuration, the mist eliminator 48 removes the mist from the exhaust gas including the mist, and thereafter, the exhaust gas is ultimately discharged from a chimney (not shown). [00381 On the other hand, the sulfur oxides included in the exhaust gas reacts with calcium compound in the absorption liquid to become calcium sulfite as an intermediate product, and flows down to the liquid reservoir unit 44. The liquid reservoir unit 44 is provided with the oxidation air blower 60 and the oxidation agitator 62. The oxidation air blower 60 forcibly supplies air to the liquid reservoir unit 44 so as to cause oxidation reaction between air and calcium sulfite, thereby generating gypsum slurry as reaction product. At this occasion, the oxidization air supplied to the liquid reservoir 18 unit 44 is made into small size by the oxidation agitator 62 agitating the absorption liquid within the liquid reservoir unit 44. Thereby, the efficiency of use of the oxidization air can be improved. [00391 The liquid reservoir unit 44 is also connected to gypsum dehydration means 64. An absorption liquid pulling pump 66 is provided in a pipe connecting with the gypsum dehydration means 64. In the liquid reservoir unit 44 having this configuration, the absorption liquid slurry is withdrawn by the absorption liquid pulling pump 66 from the liquid reservoir unit 44 to the gypsum dehydration means 64 in accordance with the amount of generated gypsum. [0040] The gypsum dehydration means 64 performs dehydration processing, and powder gypsum 65 is collected. On the other hand, the processed water is temporarily stored in the filtrate recovery tank 67, and is discharged by a filtrate pump 68 as makeup water or discharged liquid to the outside. The desalination means 80 is a seawater desalination device for producing freshwater from seawater. The desalination means according to the present embodiment employs an evaporation method such as Multi-Effect Desalination and Multi-Stage Flash methods. [0041] Fig. 2 is a schematic diagram illustrating a configuration 19 of the exhaust gas treatment system using Multi-Effect Desalination in desalination means. The configurations other than the desalination means are the same as the configurations as those shown in Fig. 1, and are denoted with the same reference numerals and detailed description thereabout is omitted. [0042] Desalination means 80a according to Multi-Effect Desalination is mainly configured to include a pre-heater 82 for heating seawater using hot water at a stage before the seawater heating device 22, an effect evaporator 84 including a heat exchanger of steam and a diffuser of seawater, an ejector 86 for reducing the pressure within the system, and a condenser 88 for heating the obtained seawater and condensing freshwater at the final stage of the effect evaporator 84. [0043] The obtained seawater is supplied to the condenser 88, where the seawater is heated. The heated seawater is passed to a seawater return line, and some of the seawater is supplied to the pre-heater 82 where the seawater is heated with hot water. The seawater heated by the pre-heater 82 condenses steam sprayed by the diffuser of each effect evaporator 84 and supplied within the heat exchanger of each effect evaporator 84, and the seawater is further heated and made into steam due to the depression by the ejector 86. In the seawater heating device 22, the seawater from the pre-heater 82 is further heated by the heat medium of the 20 circulation line 24 or the heat of the steam supplied from the outside. Then, it is supplied to a first effect evaporator 84a, where a part of it is made into steam due to the depression by the ejector 86 and the heat given by the hot water of the subsequent stage of the pre-heater 82. The heat of the steam generated here can be used for evaporation of seawater in a second effect evaporator 84b. The steam generated by the first effect evaporator 84a is supplied to the second effect evaporator 84b in the subsequent stage. In the second effect evaporator 84b, the seawater at the subsequent stage side of the pre-heater 82 is heated, and the steam is condensed to become freshwater. This operation is repeatedly performed in the effect evaporators 84 arranged in multiple stages. The freshwater condensed by the condenser 88 of the final stage is discharged by a freshwater pump 90 to the outside of the system, and is used as industrial water. On the other hand, the concentrated water is discharged by a concentrated water pump 92 to the outside of the system. As described above, the amount of heat of hot water and steam required to heat the seawater are given by the exhaust gas heat at the upstream side of the absorption tower 42 of the wet desulfurization means 40, so that more efficient heat exchange is enabled. [0044] Fig. 3 is a schematic diagram illustrating a configuration of an exhaust gas treatment system using Multi-Stage Flash method 21 in the desalination means. The configurations other than the desalination means are the same as the configurations as shown in Fig. 1, and are denoted with the same reference numerals and detailed description thereabout is omitted. Desalination means 80b using the Multi-Stage Flash method is constituted by a heat discharge unit 94 and a heat recovery unit 95 in which evaporation chambers 93 are formed in multiple stages. Each evaporation chamber 93 includes a condenser 96 and a concentrated water storage 97. [0045] In the desalination means 80b, the obtained seawater passes through the condenser 96 of the heat discharge unit 94 and condenses flash steam in the evaporation chamber 93. Thereafter, the seawater is passed to a seawater return line, and some of the seawater is supplied to the concentrated water storage 97 of the heat discharge unit 94. The seawater in the concentrated water storage 97 of the heat discharge unit 94 is supplied to the condenser 96 of the heat recovery unit 95 via a supply pump 98, and condenses the flash steam in each evaporation chamber 93. Thereafter, the seawater is heated by the seawater heating device 22. The heated seawater is supplied to the concentrated water storage 97a of the first stage of the heat recovery unit 95, and the seawater moves from the concentrated water storage 97a of the first stage to the concentrated water storage 97 of the subsequent stage in order, during the time, the seawater flash evaporates 22 in each evaporation chamber 93, and is condensed by the condenser 96. The condensed water generated by each evaporation chamber moves to subsequent stages in order, and the condensed water is discharged from the evaporation chamber 93b of the final stage to the outside of the system by the freshwater pump 90, and is used as industrial water. On the other hand, concentrated water is discharged by a concentrated water pump 92 to the outside of the system. The seawater heated by the seawater heating device 22 generates steam upon depression by the ejector 86 in each stage. The seawater is used before this steam is generated. The seawater used for condensation passes through the condenser 96 and while the seawater is heated in each stage, the seawater is supplied to the seawater heating device 22. The amount of heat of steam required to heat the seawater is compensated by the exhaust gas heat at the upstream side of the absorption tower 42 of the wet desulfurization means 40, so that more efficient heat exchange is enabled. Some of the freshwater produced by the desalination means 80 is used as makeup water for the absorption liquid within the absorption tower 42. In addition, it is used as inlet washing water for the absorption tower 42. [0046] At the mist eliminator 48 provided at the outlet of the absorption tower 42, the absorption liquid circulating in the absorption tower 42 scatters, and attaches to the element of the 23 mist eliminator 48. For this reason, the freshwater is used as cleaning water, and the element is washed with water. The absorption liquid within the liquid reservoir unit 44 is generally about 500C. The outlet temperature of the oxidation air blower 60 is usually 120 to 1500C. If it is directly supplied to the liquid reservoir unit 44, dry state and wet state are alternately repeated in the pipe end portion inserted into the absorption liquid within the liquid reservoir 44, so that slurry is attached thereto. Accordingly, freshwater is sprayed to the air at the outlet of the oxidation air blower 17, so that the air temperature is reduced. Then, it is introduced to the liquid reservoir unit 44. With exhaust gas treatment system 10 having the above configuration according to the present invention, the exhaust gas discharged from the boiler and the like is introduced to the electrostatic precipitator means 12, where dust is removed from the exhaust gas. [0047] Subsequently, the pressure of the exhaust gas is increased by the exhaust gas fan 14, and the exhaust gas is introduced to the heat exchanger 20. The temperature of the exhaust gas is reduced by heat exchange with the heat medium flowing on the inner surface of the pipe of the heat exchanger 20. The heat medium heated by the heat exchanger 20 is passed through the circulation line 24, so that the heat medium is 24 subjected to heat exchange, where heat is exchanged between the heat medium and the seawater of the seawater heating device 22. The desalination means 80 can produce freshwater from the heated seawater. [0048] Subsequently, the exhaust gas of which gas temperature decreases is introduced to the wet desulfurization means 40, and gas-liquid contact occurs between the exhaust gas and the absorption liquid sprayed from the spray nozzles 58 in the absorption unit 46. Then, acid gases such as sulfur oxides, hydrogen chloride, and hydrogen fluoride included in the exhaust gas are absorbed on the droplet surfaces of the absorption liquid circulating within the absorption tower 42. Some of the absorption liquid within the absorption tower 42 is made into mist. Then, the mist eliminator 48 removes the mist from the exhaust gas including the mist, and thereafter, the exhaust gas is ultimately discharged from a chimney (not shown) [0049] Fig. 4 is a graph illustrating correlation between an absorption tower inlet gas temperature and an absorption tower evaporated water amount. The horizontal axis of the graph represents an absorption tower inlet gas temperature (0C), and the vertical axis represents an absorption tower evaporated water amount (t/h) . As shown in the figure, the absorption tower inlet gas temperature and the absorption tower evaporated water amount 25 are in proportional relationship. Therefore, when the absorption tower inlet gas temperature is reduced, the absorption tower evaporated water amount can also be reduced. [00501 According to the exhaust gas treatment system of the present invention, the exhaust gas temperature introduced to the wet desulfurization means is reduced by the heat exchanger, and therefore, steam generated within the absorption tower of the wet desulfurization means can be reduced. Therefore, steam is not discharged to the outside of the wet desulfurization means, and the absorption liquid is not reduced. The amount of makeup water can be greatly reduced. Since the heat of the exhaust gas is used to heat the seawater in the desalination means, the amount of used steam can be reduced. Further, the cost of the entire system can be reduced, and energy-saving can be achieved. In the explanation about the present embodiment, heat exchange is performed upon connecting the heat exchanger and the desalination means via the circulation line. Alternatively, the desalination means may be provided at an upper portion of the heat exchanger, so that space-saving can be achieved. [00511 Fig. 5 is a schematic diagram illustrating a configuration of an exhaust gas treatment system according to a second embodiment. The amount of recovered heat by the heat exchanger 20 varies according to, e.g., the amount of exhaust gas from the boiler and 26 the like and the exhaust gas temperature. A exhaust gas treatment system 100 according to the second embodiment includes seawater temperature measuring means 102 for measuring the outlet temperature of seawater heated by a seawater heating device 22, a first flow amount control valve 104 capable of adjusting the amount of flow of the circulation line 24, a bypass line 106 connecting a feed pipe and a return pipe of a circulation line, a second flow amount control valve 108 for adjusting the amount of flow of the bypass line 106, and control means 110. The seawater temperature measuring means 102 is a temperature sensor attached to the outlet side of the seawater heating device 22 of the circulation line 24 to measure the heated seawater temperature. [0052] The first flow amount control valve 104 is a valve attached to the circulation line 24 to adjust the amount of flow of the heat medium flowing in the pipe. The bypass line 106 is a pipe connecting the feed pipe and the return pipe of the circulation line 24. [00531 The second flow amount control valve 108 is a valve attached to the bypass line 106 to adjust the amount of flow of the heat medium flowing in the pipe. The control means 110 electrically connects the seawater temperature measuring means 102 and the first and second flow 27 amount control valves 104, 108. The control means 110 controls the amount of flow of the heat medium in the circulation line 24 and can control the seawater temperature so that it is maintained at a setting value defined in advance, on the basis of a detection value of the outlet temperature of the seawater heating device 22 measured by the seawater temperature measuring means 102 (heated seawater temperature). [0054] For example, when the heating temperature is 1200C or more, a control target value of the temperature at the seawater outlet side of the seawater heating device 22 of the circulation line 24 is 1050C. When the seawater temperature is increased more than the target value, there is a problem of scaling, i.e., salt component in the seawater attaches to the seawater heating device 22. Therefore, it is necessary to open the bypass line 106 to increase the amount of bypass of the heat medium, so that control is performed to reduce the amount of circulation of the heat medium to the seawater heating device 22. Alternatively, the degree of opening of the first flow amount control valve 104 may be reduced, so that control is performed to reduce the amount of circulation of the heat medium. [00551 On the other hand, when the seawater temperature is less than the target value, the difference in each stage of the evaporation device in the desalination means 80 decreases, and 28 it is less likely to evaporate. Therefore, it is necessary to reduce the amount of flow of the seawater and perform control so as to prevent carry-over of the seawater. When the seawater heating device outlet temperature is low, it is necessary to heat the seawater by introducing steam from the outside in addition to the heat exchanger 20. [00561 According to the exhaust gas treatment system 100 of the second embodiment, the heating seawater temperature can be controlled so that it can be set at the setting value of the seawater temperature defined in advance, and this enables the desalination means to efficiently produce freshwater. [0057] Fig. 6 is a schematic diagram illustrating a configuration of an exhaust gas treatment system according to the third embodiment. As shown in the figure, an exhaust gas treatment system 200 according to the third embodiment is configured such that a heat exchanger 20a is provided upstream of electrostatic precipitator means 12. Further, instead of the seawater temperature measuring means, gas temperature measuring means 102a for exhaust gas is provided between the heat exchanger 20a and the electrostatic precipitator means 12. Control means 110 makes electrical connection with the gas temperature measuring means 102a. The configurations other than the above are the same as the 29 configurations of the exhaust gas treatment system 100 according to the second embodiment, and detailed description thereabout is omitted. [00581 The gas temperature of the exhaust gas discharged from the boiler and the like is relatively high, i.e., 120 to 1600C. The heat exchanger 20a is provided upstream side of the electrostatic precipitator means 12, so that as a result of heat exchange between the exhaust gas and the heat medium, the temperature of the exhaust gas introduced to the electrostatic precipitator means 12 can be reduced. [00591 In this case, the removing performance of dust included in the exhaust gas by the electrostatic precipitator means 12 is determined by multiple factors such as a dust particle size, a dust composition, an electrical resistance value of dust, and the amount of charge at the electrostatic precipitator means 12. In general, as the gas temperature decreases, the electrical resistance value of the dust decreases, and dust removing performance is improved. However, when the inlet gas temperature of the electrostatic precipitator means 12 decreases to a certain value or less, there is a problem in that, e.g., the dust may fix to the inside of the electrostatic precipitator means 12 and it may be difficult to convey the dust collected by the electrostatic precipitator means 12. 30 [00601 Accordingly, in the exhaust gas treatment system 200 according to the third embodiment, gas temperature measuring means 102a is provided at the exit of the heat exchanger 20a. The control means 110 controls the amount of flow of the heat medium in the circulation line 24 and can control the exhaust gas temperature so that it is maintained at a setting value defined in advance, on the basis of a detection value of the outlet temperature of the heat exchanger 20a measured by the gas temperature measuring means 102a (exhaust gas temperature). [0061] For example, when the boiler is a low-load operational state, the exhaust gas / exhaust gas temperature at the boiler outlet also decreases, and the inlet temperature of the electrostatic precipitator means 12 decreases. When the inlet temperature of the electrostatic precipitator means 12 decreases, for example, ashes fix to the internal electrode plates, and the hopper unit of the electrostatic precipitator means 12 is clogged with ashes, which makes it difficult to maintain stable operation. Therefore, when the boiler has low-load, in order to prevent the outlet temperature of the heat exchanger 20a from decreasing too much, the degree of opening of the first flow amount control valve 104 is reduced, and the amount of circulation of the heat medium is reduced, so that the inlet temperature of the electrostatic precipitator means 12 can be maintained at a value equal to or 31 more than a setting value defined in advance (for example, 800C) [0062] According to the exhaust gas treatment system 200 of the third embodiment, the exhaust gas temperature can be controlled so that it is maintained at the setting value of the gas temperature defined in advance, and the electrostatic precipitator means 12 can efficiently perform removing operation. [00631 Fig. 7 is a schematic diagram illustrating a configuration of an exhaust gas treatment system according to a fourth embodiment. As shown in the figure, an exhaust gas treatment system 300 according to the fourth embodiment is based on the configuration of the exhaust gas treatment system 200 according to the third embodiment. In the exhaust gas treatment system 200 according to the third embodiment, the heat exchange between the exhaust gas and the heat medium is performed at a stage before the electrostatic precipitator means 12, and therefore, when the initial temperature of the exhaust gas is low, the exhaust gas temperature decreases too much, and the dust removing performance of the electrostatic precipitator means 12 decreases. Therefore, there is a limitation on the cooling temperature of the exhaust gas. [0064] Accordingly, in the exhaust gas treatment system 300 of the 32 fourth embodiment, the second heat exchanger 21 is provided within the absorption tower 42 of the wet desulfurization means 40 so as to heat the heat medium. The second heat exchanger 21 is connected to the second circulation line 25 branched from the circulation line 24. More specifically, the second heat exchanger 21 is provided in the liquid reservoir unit 44. The oxidation reaction of absorbed SO 2 occurring within the absorption tower 42 is exothermic reaction. Therefore, with the second heat exchanger 21, thermal exchange is performed between the heat medium and the absorption liquid, and the heat medium can be heated. Then, this heat medium is introduced to the circulation line 24 via the second circulation line 25, and the seawater heating device 22 can perform heat exchange with the seawater. [00651 According to the exhaust gas treatment system 300 according to the fourth embodiment having this configuration, the heat medium can be heated by performing heat exchange between the heat medium and the absorption liquid without reducing the removing efficiency of the dust by the electrostatic precipitator means 12. Therefore, even when the exhaust gas temperature is low, the heat medium can be heated, and the amount of used steam in the desalination means can be reduced. Further, the cost of the entire system can be reduced, and energy-saving can be achieved. [00661 Fig. 8 is a schematic diagram illustrating a configuration 33 of an exhaust gas treatment system according to a fifth embodiment. As shown in the figure, an exhaust gas treatment system 400 according to the fifth embodiment is configured such that the heat exchanger 20 according to the first embodiment is provided at stage subsequent to the first mist eliminator 48a. The second mist eliminator 48b is arranged at a stage subsequent to the heat exchanger 20b. The second mist eliminator 48b is connected to the supply line of the makeup water of the wet desulfurization means 40. The configurations other than the above are the same as the configurations of the first embodiment, and detailed description thereabout is omitted. [0067] According to an exhaust gas treatment system 400 according to the fifth embodiment having the above configuration, exhaust gas from which components such as sulfur oxides are removed and mist made into steam by the high-temperature exhaust gas are introduced to the first mist eliminator 48a. The exhaust gas including the mist having passed through the first mist eliminator 48a is subjected to heat exchange with the heat medium in the heat exchanger 20b. The heat medium passes through the circulation line 24, and is used for heat exchange with the seawater in the seawater heating device 22. The exhaust gas exchanges heat with the heat medium in the heat exchanger 20b, and the gas temperature decreases, so that the exhaust gas attains water saturation state. The moisture component in the exhaust gas in the water saturation 34 state is condensed and recovered in the second mist eliminator 48b. The recovered moisture component can be used as the makeup water for the absorption liquid of the absorption tower 42. [00681 According to the exhaust gas treatment system according to the fifth embodiment as described above, the exhaust gas in the water saturation state having been subjected to the desulfurization treatment is subjected to the heat exchange in the heat exchanger with the heat medium, whereby the gas temperature in the water saturation state is cooled, and the moisture component can be condensed and recovered. Since the heat of the exhaust gas is used to heat the seawater in the desalination means, the amount of used steam can be reduced. Further, the cost of the entire system can be reduced, and energy-saving can be achieved. [00691 The heat exchanger may be installed at not only the position described above but also the inlet side or the outlet side of the absorption tower liquid circulating pump 54 of the absorption liquid circulation pipe 52. The seawater heating device 22 may be provided at the inlet side of the seawater of the desalination means 80. INDUSTRIAL APPLICABILITY [0070] The exhaust gas treatment system and the exhaust gas 35 treatment method of the present invention can be applied to exhaust gas treatment for various kinds of plants such as thermal power plants of which exhaust gas includes sulfur oxides. REFERENCE SIGNS LIST [0071] 10, 100, 200, 300, 400.........exhaust gas treatment system, 12.........electrostaticprecipitator means, 14.........exhaust gas fan, 20, 20a, 20b.........heat exchanger, 21.........second heat exchanger, 22.........seawater heating device, 24.........circulation line, 25.........second circulation line, 26.........circulating pump, 40.........desulfurization means, 41.........gas inlet portion, 42.........absorption tower, 44.........liquid reservoir unit, 46.........absorption unit, 48.........mist eliminator, 50.........supply unit, 52.........absorption liquid circulation pipe, 54.........absorption liquid circulating pump, 56.........spray header, 58.........spray nozzle, 59.........desulfurization means gas outlet portion, 60.........oxidation air blower, 62.........oxidation agitator, 64.........gypsum dehydration means, 65.........gypsum, 66.........absorption liquid pulling pump, 67.........filtrate recovery tank, 68.........filtrate pump, 80.........desalination means, 82.........pre-heater, 84.........effect evaporator, 86.........ejector, 88.........condenser, 90.........freshwater pump, 92.........concentrated water pump, 93.........evaporation 36 chamber, 94.........heat discharge unit, 95.........heat recovery unit, 96.........condenser, 97.........concentrated water storage, 98.........supply pump, 102.........seawater temperature measuring means, 102a.........gas temperature measuring means, 104.........first flow amount control valve, 106.........bypass line, 108.........second flow amount control valve, 110.........control means 37

Claims

1. An exhaust gas treatment system comprising: wet desulfurization means for removing a sulfur oxide in an exhaust gas; desalination means for producing freshwater from seawater and supplying the freshwater to the wet desulfurization means; a heat exchanger for heating a heat medium with the exhaust gas at a stage before the wet desulfurization means; a seawater heating device of the desalination means; and a heat medium circulation line for connecting the heat exchanger with the seawater heating device and circulating the heat medium.

2. The exhaust gas treatment system according to claim 1 further comprising: a bypass line connecting a feed pipe and a return pipe of the circulation line; a flow amount control valve provided in the circulation line and the bypass line for adjusting an amount of flow of the heat medium; and control means connected to the flow amount control valve for controlling an amount of supply of the heat medium to the seawater heating device.

3. The exhaust gas treatment system according to claim 2 further comprising seawater temperature measuring means for detecting an outlet temperature of the seawater heating device, 38 wherein the control means controls the amount of supply of the heat medium to the seawater heating device on the basis of a measured value measured by the temperature measuring means, so that the outlet temperature of the seawater heating device attains a temperature defined in advance.

4. The exhaust gas treatment system according to any one of claims 1 to 3, wherein electrostatic precipitator means is provided at a stage before the heat exchanger.

5. The exhaust gas treatment system according to claim 2, wherein electrostatic precipitator means is provided between the heat exchanger and the wet desulfurization means.

6. The exhaust gas treatment system according to claim 5, wherein the control means controls the amount of supply of the heat medium to the heat exchanger by connecting with the flow amount control valve.

7. The exhaust gas treatment system according to claim 6 further comprising gas temperature measuring means for detecting the temperature of the exhaust gas introduced to the electrostatic precipitator means, wherein the control means controls the amount of supply of the heat medium to the heat exchanger on the basis of the measured value measured by the gas temperature measuring means, so that the temperature of the exhaust gas introduced to the electrostatic precipitator means attains a temperature defined in advance.

8. The exhaust gas treatment system according to any one of 39 claims 5 to 7, wherein a second heat exchanger is provided at a liquid holding unit of the wet desulfurization means and is connected to the circulation line, and the heat medium circulates through the heat exchanger and the second heat exchanger.

9. An exhaust gas treatment system comprising: wet desulfurization means for removing a sulfur oxide in an exhaust gas; desalination means for producing freshwater from seawater and supplying the freshwater to the wet desulfurization means; a heat exchanger for heating a heat medium with the exhaust gas at a stage subsequent to the wet desulfurization means; a seawater heating device of the desalination means; a heat medium circulation line for connecting the heat exchanger with the seawater heating device and circulating the heat medium; and a mist eliminator for removing mist included in the exhaust gas.

10. An exhaust gas treatment method for removing a sulfur oxide included in an exhaust gas, the exhaust gas treatment method comprising the steps of: heating the heat medium by reducing a gas temperature of the exhaust gas by performing heat exchange between the heat medium and the exhaust gas that has not yet subjected to the desulfurization treatment; 40 performing desulfurization treatment on the exhaust gas of which gas temperature decreased; circulating the heated heat medium to a seawater heating device; performing heat exchange between the heated heat medium and a seawater; producing freshwater from the heated seawater; and measuring the heated seawater temperature to control an amount of circulation of the heat medium. 41