AU2011364139A1 - Exhaust gas treatment system and exhaust gas treatment method - Google Patents

Abstract

Combining a desulfurization unit and a fresh water generator that is based on membrane treatment can reduce costs. A gas cooler (11) operates as a heat exchanger that cools exhaust gas (1) that flows into an absorption column (12) using treatment water (21) (treated sea water 20)and also heats the treatment water (21) using the heat of the exhaust gas (1). The water content of the exhaust gas (1) is condensed by gas cooling and fed to the absorption column (12) and the water content of the exhaust gas (1) emitted to outside the system is reduced. Consequently, the amount of water that must be replenished is reduced, the amount of fresh water generated is reduced, and energy consumption is reduced. The increase in the water temperature by the heating of the treatment water (21) is accompanied by a reduction in energy consumption during fresh water production. As a result, the operating cost can be reduced.

Description

DESCRIPTION Title of the Invention: EXHAUST GAS TREATING SYSTEM AND EXHAUST GAS TREATING METHOD Technical Field [0001] The present invention relates generally to exhaust gas treating systems and methods each using a wet desulfurizing apparatus and a water production apparatus. The invention relates to an exhaust gas treating system and method employed to supply make-up water from a water production apparatus to a wet-desulfurizing apparatus. Background Art [0002] The electric power-generating boilers and other equipment that burn fuels containing a relatively large amount of sulfur such as coal have a flue gas desulfurizing apparatus to remove the sulfur oxides (SOx) that become a main cause of air pollution and acid rain, from exhaust gas. A wet limestone gypsum process that is one of desulfurizing methods is used in a wide range of applications since this method features a high desulfurizing rate and enables recovered gypsum to be recycled as a valuable material. However, such a wet-desulfurizing apparatus has a problem in that it requires a large volume of industrial water. 1 [0003] In the wet limestone gypsum process, Sox is removed from exhaust gas which is supplied to an absorption column of the wet-desulfurizing apparatus, by a gas-liquid reaction with a limestone slurry sprayed, and then the exhaust gas are released into the atmosphere. The limestone slurry that has reacted with SOx is air-aerated to generate gypsum, which is then dewatered and recovered. A filtrate left after the gypsum has been dewatered is mixed with limestone once again and recycled. [0004] While the filtrate created by dewatering is cyclically used, chloride ions contained therein are concentrated, and when the chloride ion content reaches or exceeds a certain level, the limestone slurry decreases in desulfurizing efficiency. In order to avoid such concentration, it is necessary to discharge a part of the dewatering filtrate and maintain the internal chloride-ion content of the system at a level below that preplanned. [0005] In addition, since the exhaust gas supplied to the absorption column are as hot as nearly 900C, the limestone slurry has its moisture evaporated and released with treated gases into the atmosphere. Furthermore, the crystallization water in a dewatered cake of the gypsum is separated and recovered with the gypsum. In the wet limestone gypsum process, the water in the desulfurizing apparatus is thus discharged in a plurality of forms from the system. In 2 general, an electric power plant of a 1000-MW scale consumes about 160 tons of water per hour as make-up water for its desulfurizing apparatus. To introduce a desulfurizing apparatus of this scheme into regions lacking in supplies of industrial water, therefore, a water production apparatus that supplies make-up water also needs to be installed together. [0006] In the meantime, the expansion of water shortages in recent years is increasing installation cases of desalination facility for producing/utilizing freshwater by desalinating seawater or brackish water. Major desalinating methods include an evaporation method that uses heat, a membrane method that uses a reverse osmosis membrane or an ion exchange membrane, and electrodialysis intended primarily for brackish water. In the Middle East and others, desalination facility is usually designed as electric power generating/water production plants that supply heat, steam, and/or electric power from annexed thermal power-generating facility to freshwater production facility. Freshwater thus purified by such water production apparatus is also frequently used as drinking water. [0007] As discussed above, wet-desulfurizing apparatuses require a large amount of make-up water and hence a water production apparatus, but it does not suffice just to apply an existing water production apparatus as it is. In consideration of the fact that it is a main purpose of the 3 water production apparatus to supply make-up water, requirements relating to reduction in installation costs, operational costs, and the like, are stringent relative to those of the existing water production apparatus. For example, the amount of water to be supplied from the water production apparatus (amount of water to be produced) can be lowered by reducing make-up water consumption to save operational costs. [0008] Patent Document 1 describes a conventional technique relating to reducing make-up water consumption. At a stage preceding an absorption column, the amount of evaporation of moisture in a limestone slurry can be reduced by cooling exhaust gas with a heat exchanger. Additionally, at a stage following the absorption column, if the exhaust gas are brought into indirect contact with seawater and cooled down, the moisture that has evaporated in the absorption column can be condensed and recovered to be recycled as make-up water. In this way, reduction in make-up water consumption is possible by reducing the amount of evaporation in the absorption column and that of moisture released into the atmosphere. [0009] Furthermore, supplying heated cooling seawater to a nearby water production apparatus based on electrodialysis allows highly efficient production of freshwater since energy consumption of the water production apparatus decreases with increases in water temperature. This 4 freshwater can also be used as make-up water to reduce the amount of freshwater to be supplied from the outside of the system. Prior Art Literature Patent Document [0010] Patent Document 1: JP-1987-30530-A Summary of the Invention Problems to be Solved by the Invention [0011] Such an electrodialytic water-producing method as a conventional technique, however, has posed a problem in that since energy consumption increases with an increase in chloride ion concentration of raw water, use of seawater high in chloride ion concentration also tends to increase costs. [0012] An object of the present invention is to provide an exhaust gas treating system and exhaust gas treating method effective for solving the above-discussed problems associated with the conventional techniques, and reducing costs associated with combination of a desulfurizing apparatus and a water production apparatus based on a membrane treatment method. Means for Solving the Problems 5 [0013] (1) In order to achieve the above object, an exhaust gas treating system includes: a wet-desulfurizing apparatus for removing sulfur oxides contained in boiler exhaust gas; a water production apparatus equipped with a pump for boosting seawater, the water production apparatus having a membrane treatment apparatus for separating the seawater pumped in from the pump, into a permeate and a non-permeate; a make-up water supply line for supplying at least one part of the permeate as make-up water to the desulfurizing apparatus; and a gas cooler having an exhaust gas cooling function which, by means of the seawater used in the water production apparatus, cools the exhaust gas that flow into the desulfurizing apparatus, and a seawater heating function that uses heat of the exhaust gas to heat the seawater during the cooling of the exhaust gas. [0014] Moisture contained in the exhaust gas is condensed by the exhaust gas cooling function and then supplied to the wet-desulfurizing apparatus. This reduces the amount of moisture discharged from the system, and hence the volume of water produced by the water production apparatus, thus also reducing energy consumption. [0015] As the seawater heating function increases the water temperature, the energy consumption associated with water production decreases, whereby operational costs are reduced. [0016] 6 Thus, an exhaust gas treating system with a wet desulfurizing apparatus can be supplied to regions lacking in freshwater resources. [0017] (2) Additionally, the system in above item (1) preferably includes water temperature control means for controlling a temperature of the seawater heated by the seawater heating function, to supply the temperature controlled seawater to the pump. [0018] The water temperature control means permits seawater heating until the make-up water has reached an upper limit of its chloride ion concentration. The energy consumption associated with water production decreases as the water temperature increases. [0019] (3) In item (1), preferably the water production apparatus further includes a first pump for boosting seawater, a first membrane treatment unit for separating the seawater pumped in from the first pump, into permeate and non-permeate, a second pump for boosting a part of the first-stage permeate, and a second membrane treatment unit for separating the first-stage permeate pumped in from the second pump, into permeate and non-permeate; wherein the make-up water supply line supplies the first-stage permeate and the second-stage non-permeate to the desulfurizing apparatus. [0020] 7 The increases in water temperature due to seawater heating somewhat reduce a desalination capability of the membrane treatment apparatus, so that the permeate has its applications limited, except when the water is used as make up water. Forming the membrane treatment apparatus into the dual-stage configuration described above expands the applications of the second-stage membrane permeate, thus expanding applicability of the exhaust gas treating system itself. [0021] (4) In above item (3), preferably the exhaust gas cooling function of the gas cooler uses the first-stage permeate to cool the exhaust gas. [0022] In general, installation of a gas cooler tends to be expensive since the cooler is required to be constructed of a material having corrosion resistance high enough to withstand the high chloride ion concentration of the seawater flowing through the cooler. In the present invention, however, only the first-stage permeate of a low chloride ion concentration is introduced into the gas cooler, so an installation cost of the cooler can be reduced. [0023] (5) in item (1), preferably the system further includes a second gas cooler for cooling the exhaust gas from the wet-desulfurizing apparatus by means of the seawater used in the water production apparatus, and then condensing and removing any moisture contained in the 8 exhaust gas, and a second make-up water supply line for supplying to the desulfurizing apparatus at least one part of the condensate formed by the second gas cooler. [0024] Part of the moisture contained in the exhaust gas released from the system is supplied as the condensate to refill the desulfurizing apparatus, with the result that the amount of water to be produced is significantly reduced, which in turn also reduces related energy consumption. [0025] (6) In item (1), preferably the system further includes a second gas cooler for cooling the exhaust gas from the wet-desulfurizing apparatus by means of the seawater used in the water production apparatus, and then condensing and removing the moisture contained in the exhaust gas, and a condensate mixing line for mixing at least one part of the condensate formed by the second gas cooler, with the seawater supplied to the membrane treatment apparatus. [0026] Mixing the condensate of a low chloride ion concentration into the seawater of a high chloride ion concentration reduces the chloride ion concentration of the water supplied to the membrane treatment apparatus. Thus, water-producing energy consumption also decreases. [0027] (7) In order to achieve the foregoing object, an exhaust gas treating method includes: a water production 9 step in which water is produced by boosting seawater and then separating this seawater into a permeate and a non permeate via a membrane treatment apparatus; a make-up water supply step in which at least one part of the permeate obtained in the water production step is supplied as make-up water to a wet-desulfurizing apparatus; an exhaust gas desulfurizing step in which any sulfur oxides contained in exhaust gas from a boiler are removed using the wet desulfurizing apparatus that sprays a limestone slurry over the exhaust gas while supplying moisture to the slurry in the make-up water supply step; and an exhaust gas cooling/seawater heating step in which, the exhaust gas flowing into the desulfurizing apparatus in the exhaust gas desulfurizing step are cooled by means of the seawater used in the water production step, and during the cooling of the exhaust gas, the seawater is heated using heat energy of the gases. Effects of the Invention [0028] The present invention reduces costs associated with combination of a desulfurizing apparatus and a water production apparatus based on a membrane treatment method. Thus, an exhaust gas treating system with a wet desulfurizing apparatus can be supplied to regions lacking in freshwater resources. Brief Description of the Drawings 10 [0029] Fig. 1 is a block diagram of an exhaust gas treating system (first embodiment); Fig. 2 is a block diagram of an exhaust gas treating system (second embodiment); Fig. 3 is a functional block diagram of a controller; Fig. 4 is a flowchart that shows details of process steps by the controller; Fig. 5 is a conceptual diagram that illustrates advantageous effects; Fig. 6 is a block diagram of an exhaust gas treating system (first modification); Fig. 7 is a block diagram of an exhaust gas treating system (second modification); Fig. 8 is a block diagram of an exhaust gas treating system (third modification); Fig. 9 is a block diagram of an exhaust gas treating system (fourth modification); Fig. 10 is a block diagram of an exhaust gas treating system (third embodiment); Fig. 11 is a block diagram of an exhaust gas treating system (fourth embodiment); Fig. 12 is a conceptual diagram that illustrates advantageous effects; Fig. 13 is a block diagram of an exhaust gas treating system (fifth embodiment); Fig. 14 is a block diagram of an exhaust gas treating system (fifth modification); and 11 Fig. 15 is a block diagram of an exhaust gas treating system (sixth modification). Mode for Carrying Out the Invention [0030] Hereinafter, a plurality of embodiments of the present invention will be described in detail. The same reference numbers in each drawing denote equivalent elements. (First Embodiment) [0031] Basic configuration and basic operation Fig. 1 is a block diagram of an exhaust gas treating system according to a first embodiment. The exhaust gas treating system includes a wet-desulfurizing apparatus and a water production apparatus. [0032] First, the wet-desulfurizing apparatus including an absorption column 12, a related configuration, and basic operation of the apparatus are described below. Exhaust gas 1 from a boiler are cleared of any soot and dust by an electric dust collector 10 and after having been cooled by a gas cooler 11, flow into the absorption column 12. In the absorption column 12, a limestone slurry 36 is sprayed over the exhaust gas 1 and SOx is removed from the gases 1 by a gas-liquid reaction with the limestone slurry 36. The exhaust gas that have exited the absorption column 12 are released from a stack 13 into the atmosphere. 12 [0033] The limestone slurry 36, after absorbing SOx in the absorption column 12, is supplied to a gypsum dewatering machine 15 and separated into gypsum 28 and a dewatering filtrate 29. The gypsum 28 is unloaded from the system and recycled. Part of the dewatering filtrate 29 is sent as desulfurized wastewater 30 to a wastewater treating apparatus not shown. The remainder of the dewatering filtrate 29 is fed into a limestone slurry supply unit 16, then mixed with limestone to form a supplementary limestone slurry 37, and supplied to the absorption column 12. [0034] As can be seen from the above, the wet-desulfurizing apparatus requires a large volume of industrial water, and moisture is discharged from the system in various forms, which include the desulfurized wastewater 30, the moisture contained in the exhaust gas 1 that are released into the atmosphere, crystallization water in the gypsum 28, and so on. Make-up water 31 is therefore supplied from a make-up water tank 18 to the absorption column 12 in appropriate timing when necessary. [0035] Next, a configuration and basic operation of the water production apparatus are described below. Seawater 20, pumped up by an intake pump 6, has its solid/dissoluble substances removed by a pretreating apparatus 7 or conditioned in water temperature, pH value, and the like, to become pretreated water 21. After being heated by heat 13 energy of the exhaust gas 1 in the gas cooler 11, the pretreated water 21 is pumped from a high-pressure pump 8 into a high-pressure reverse osmosis membrane 9 and then separated into brine 22 and permeate 23. [0036] Part of the permeate 23 flows into the make-up water tank 18. The remainder of the permeate 23 is pumped from a low-pressure pump 17 into a low-pressure reverse osmosis membrane 14 and then separated into a brine 24 and a permeate 25. Part of the brine 24 flows into the make-up water tank 18. Part of the permeate 25 also flows into the make-up water tank 18. The remainder of the permeate 25 is used for freshwater 26 various purposes outside the system, such as in the form of a raw material for creation of drinking water or in the form of sealing water for on-site pumps and other auxiliary machines and equipment. The make up water 31 from the make-up water tank 18 is supplied to the absorption column 12 via a make-up supply line 51. Thus, the water production apparatus can supply the make-up water 31. [0037] Characteristic configuration/operation and beneficial effects Configurational and operational features of the present embodiment are set forth below. The gas cooler 11 works with an exhaust gas cooling function 11a to cool the exhaust gas 1 flowing into the absorption column 12 by use of the pretreated water 21 (pretreated seawater 20). The 14 gas cooler 11 also operates with a seawater heating function llb as a heat exchanger to heat the pretreated water 21 with the heat energy from the exhaust gas 1. [0038] Gas cooling by the gas cooler 11 condenses the moisture contained in the exhaust gas 1, and the condensate is supplied to the absorption column 12. This reduces the moisture in the exhaust gas 1 released from the stack 13. The amount of make-up water required, therefore, also decreases, which in turn reduces the amount of water to be produced, and hence, related energy consumption as well. [0039] Increases in water temperature, coupled with the heating of the pretreated water 21, increase a velocity of the permeate passing through the high-pressure reverse osmosis membrane 9. At this time, a motive power requirement per unit flow rate of the pump 8 decreases. That is to say, water production is energy-efficient and operational cost reduction is implemented. Similar effects can also be obtained with the low-pressure pump 17 and the low-pressure reverse osmosis membrane 14, in which case, operational costs are further reduced. [0040] The increases in water temperature, on the other hand, slightly reduce a desalination capability of the high pressure reverse osmosis membrane 9, so the permeate 23 somewhat increases in chloride ion concentration. However, in consideration of the facts that further desalination 15 occurs in the low-pressure reverse osmosis membrane 14 and that a desalination level required of the make-up water 31 is not as high as that of drinking water (for use as drinking water, the make-up water 31 needs to have its chloride ion concentration lowered), no problem arises, even if the desalination capability of the high-pressure reverse osmosis membrane 9 decreases. (Second Embodiment) [00411 New problems As described above, as the water temperature rises, the energy consumption associated with water production decreases, whereas the desalination capability of the high pressure reverse osmosis membrane 9 slightly lowers. In other words, the decrease in energy consumption (improvement of energy efficiency) and the desalination capability are in a relationship of trade-off. [0042] Although not required to be as high as drinking water in desalination level, the make-up water 31 needs to have its chloride ion concentration reduced below a preset level. Heating to a temperature higher than that actually required could cause the chloride ion concentration to exceed the preset level, making the make-up water unuseable for the desulfurizing apparatus. [00431 In addition, while it is a main purpose of the water 16 production apparatus to supply the make-up water 31, there will be a request to utilize the freshwater 26. To further reduce the chloride ion concentration of the freshwater 26, it is also necessary to lower the preset chloride ion concentration of the make-up water 31. [0044] Furthermore, the temperature of seawater 20 is changing throughout the year. That is to say, the seawater temperature is high in the summer and low in the winter. The changes in the temperature of the seawater 20 also change a temperature of the water supplied to the high pressure reverse osmosis membrane 9, and hence the chloride ion concentration of the make-up water 31. If the chloride ion concentration of the make-up water 31 exceeds the preset level, the make-up water will be unsuitable, and thus unuseable, for the desulfurizing apparatus. Conversely if the chloride ion concentration of the make-up water 31 decreases below the preset level, there will arise room to improve energy efficiency. [0045] For these reasons, the temperature of the water supplied to the high-pressure reverse osmosis membrane 9 needs to be controlled to obtain an optimal chloride ion concentration of the make-up water 31. [0046] Configuration Fig. 2 is a block diagram of an exhaust gas treating system according to a second embodiment. In the present 17 embodiment, a bypass valve 40 and a drainage valve 41 are added to the configuration of the first embodiment. [0047] The bypass valve 40 is provided on the line for supplying the pretreated water 21 to the gas cooler 11. Part of the pretreated water 21 is supplied directly to the pump 8, not to the gas cooler 11, via a bypass line branched at the bypass valve 40. A ratio between a supply quantity Ql to the gas cooler 11 and a supply quantity (bypass quantity) Q2 to the bypass line is controlled according to a particular opening angle of the bypass valve 40. [0048] The drainage valve 41 is provided on the line for discharging the pretreated water 21 from the gas cooler 11. Part of the pretreated water 21 which has been heated by the gas cooler 11 is drained from the system into, for example, seawater via a drainage line branched at the drainage valve 41. The quantity of water discharged from the drainage line, Qout, is controlled according to a particular opening angle of the drainage valve 41. [0049] The exhaust gas treating system controls the temperature Tf of the supply water to the high-pressure reverse osmosis membrane 9 by controlling the opening angle of the bypass valve 40 (i.e., the bypass quantity Q2), the opening angle of the drainage valve 41 (i.e., the drainage quantity Qout), and the quantity Qin of seawater 20 taken in by the intake pump 6. 18 [0050] These control actions may be implemented by an operator's manual control, or a controller 50 may control the water temperature as an alternative. In the latter case, the controller 50 and related constituent elements are added to the configuration shown in Fig. 2. [0051] Fig. 3 is a functional block diagram of the controller 50. The controller 50 receives various input signals. These input signals denote a seawater temperature value Tin from a temperature sensor 46 provided at a seawater intake port, an exhaust gas temperature value Tg and exhaust gas flow-rate value Qg from an exhaust gas state sensor 47 provided at an inlet of the gas cooler 11, the chloride ion concentrations Csm of the make-up water 31 and Cpm of the freshwater 26, both measured and detected by a chloride ion concentration sensor 48, and data settings from an input terminal 49. More specifically, the data settings relate to the chloride ion concentration Cst of the make-up water 31, the chloride ion concentration Cpt of the freshwater 26, and make-up water and freshwater volume requirements entered from the input terminal 49. [0052] After receiving the above data, the controller 50 conducts predetermined computations, then outputs the water intake rate Qin, the bypass quantity Q2, and the drainage quantity Qout, to the intake pump 6, the bypass valve 40, and the drainage valve 41, respectively. Thus, the 19 controller 50 controls the intake pump 6, the bypass valve 40, and the drainage valve 41. [0053] Fig. 4 is a flowchart that shows details of process steps by the controller 50. [0054] The make-up water chloride ion concentration setting Cst, the freshwater chloride ion concentration setting Cpt, the make-up water volume requirement, and the freshwater volume requirement are acquired in accordance with input instructions from the operator (step Sl) . The quantity Qf and temperature Tf of water to be supplied to the high pressure reverse osmosis membrane 9 are then computed from the above four values and a characteristics curve (see Fig. 5) that relates to the high-pressure reverse osmosis membrane 9 (step S12). [0055] In the meantime, the temperature Tin of the seawater 20 is input (step S13) and the exhaust gas temperature Tg and the exhaust gas flow rate Qg are next input (step S14). The quantity Q1 of seawater 20 (pretreated water 21) to be supplied to the gas cooler 11, and a temperature Ti of cooled exhaust gas (heated seawater) are then computed in accordance with the above three values and the as-cooled exhaust gas temperature (step S15). [0056] Water volumes satisfy expression 1 at the branching point of the bypass valve 40 and expression 2 at where the 20 intake water line meets the bypass line located downstream relative to the drainage valve 41. Qin = Q1 + Q2 (1) Qf = (Q1 - Qout) + Q2 (2) When the flow of water in the entire system is viewed, expressions 1 and 2 can be rewritten as follows: Qin (= Q1 + Q2) = Qf + Qout (3) At the same time, the water temperature at the meeting point satisfies expression 4. Qf x Tf = (Q1 - Qout) x T1 + Q2 x Tin( = T2) (4) Expression 5 is derived from expressions 2 and 4. (Ti - Tf) x Q1 = (Tl-Tf) x Qout + (Tf-Tin) x Q2 (5) Optimal drainage quantity Qout and bypass quantity Q2 that satisfy expression 5 and minimize a total value of Qout and Q2 are computed and in addition, a water intake rate Qin that satisfies expression 1 is computed (step S16). [0057] The intake pump 6, the bypass valve 40, and the drainage valve 41 are controlled on the basis of calculation results obtained in step S16 (step S17). [0058] The measured chloride ion concentration values Csm of the make-up water and Cpm of the freshwater 26, are input (step S18) and whether the Csm and Cpm values agree with the make-up water chloride ion concentration setting value Cst and the freshwater chloride ion concentration setting value Cpt, respectively, is determined (step S19). If the concentration is of the preset level, control will be 21 completed. If the concentration is not of the preset level, control will be repeated until the preset level has been reached. [0059] Operation For example, to reduce the chloride ion concentration of the make-up water 31 and the chloride ion concentration of the freshwater 26, the system opens the bypass valve 40. The result is that since the flow rate Ql of heated water will decrease and since the flow rate Q2 of non-heated water will increase, the temperature Tf of the supply water to the high-pressure reverse osmosis membrane 9 will decrease and the chloride ion concentration values measured of the make up water 31 and freshwater 26 will also decrease in accordance with the characteristics curve (see Fig. 5) of the high-pressure reverse osmosis membrane 9. [0060] If the flow rate Q1 decreases at a constant water intake rate Qin, however, the gas cooler 11 will be unable to maintain its cooling performance. Accordingly, while maintaining the flow rate Q1, the system will increase the water intake rate Qin and the flow rate Q2 and open the drainage valve 41 to drain a part of the heated water. The water temperature will be controllable with the cooling performance of the gas cooler 11 being kept constant. [0061] In addition, in the summer the temperature of the seawater 20 rises and the chloride ion concentrations of the 22 make-up water 31 and the freshwater 26 also increase. The above operation involved with water temperature control so as to lower the seawater temperature and the concentrations. [0062] In the winter, on the other hand, the temperature of the seawater 20 lowers and the chloride ion concentrations of the make-up water 31 and the freshwater 26 also decrease. The water temperature control operation is inversely implemented to the above so as to raise the seawater temperature and the concentrations. [0063] Advantageous effects Fig. 5 is a conceptual diagram that illustrates advantageous effects of the present embodiment. Changes in the temperature of the supply water to the reverse osmosis membrane are plotted on a horizontal axis, and changes in water-producing energy consumption and in the chloride ion concentration of the permeate are plotted on a vertical axis. As the water temperature rises, the water-producing energy consumption decreases, whereas the desalination capability of the high-pressure reverse osmosis membrane slightly lowers. [0064] The make-up water 31 is not required to be as high as drinking water in desalination level, and the chloride ion concentration of the permeate can be set to equal a maximum permissible level. In other words, heating to point A shown in Fig. 5 is allowed. In the present embodiment, the water 23 temperature can be controlled and heating to point A is possible. On the other hand, the rise in water temperature due to heating reduces the water-producing energy consumption to point B shown in Fig. 5. This allows energy efficient water production, ensuring operation cost reduction. [0065] In the present embodiment, controlling the water temperature to an optimal level prevents heating to a temperature higher than that actually required, and even when the preset chloride ion concentration of the make-up water 31 or freshwater 26 is changed or when the temperature of the seawater 20 changes, allows energy-efficient water production according to the particular change, and hence the reduction of operational costs. [0066] Additionally, the second embodiment that includes water temperature control means added to the configuration of the first embodiment offers the advantageous effects of the first embodiment. [0067] Modifications 1. A block diagram of an exhaust gas treating system according to a first modification is shown in Fig. 6. A pretreatment apparatus 7 may be disposed at a stage that follows the gas cooler 11. In this case, for example if the pretreatment uses a microfiltration (MF) membrane or an ultrafiltration (UF) membrane, energy consumption in the 24 intake pump 6 due to increases in water temperature is expected to decrease. In addition, while the pretreated water 21 in the second embodiment has been discharged from the drainage valve 41, the first modification allows the amount of treatment with the pretreatment apparatus 7 to be reduced by pretreating the seawater 20 left after the discharging operation. Operational costs can therefore be reduced. [0068] In a case of low-quality untreated seawater heavily laden with phytoplankton and organics, for ease in maintenance of the gas cooler 11, the pretreatment apparatus 7 is desirably disposed at a stage that precedes the gas cooler 11 as in the second embodiment. [0069] 2. A block diagram of an exhaust gas treating system according to a second modification is shown in Fig. 7. An electric dust collector 10 may be disposed at a stage that follows the gas cooler 11. In this case, improvement of the electric dust collector 10 in dust-removal performance is anticipated since the exhaust gas 1 temperature flowing into the dust collector 10 decreases. At exhaust gas temperatures below a certain level, however, there will arise problems such as dust sticking to the inside of the electric dust collector 10, so that the amount of cooling water in the gas cooler 11 needs to be controlled to maintain the exhaust gas temperature in a predetermined range. 25 [0070] 3. A block diagram of an exhaust gas treating system according to a third modification is shown in Fig. 8. The brine 24 that has been separated by the low-pressure reverse osmosis membrane 14 may be supplied directly to the absorption column 12 without being passed through the make up water tank 18. In the second embodiment, make-up water 31 from the make-up water tank 18 is supplied to the absorption column 12 via the make-up water supply line 51 by a pressure of a pump 52 not shown. The brine 24, separated by the low-pressure reverse osmosis membrane 14, can be supplied to the absorption column 12 without being passed through the make-up water tank 18 or the make-up water supply line 51, because of a residual pressure applied from the low-pressure pump 17. The result is that a decrease in the flow rate of the water to be passed through the make-up water supply line 51 is expected to reduce the energy consumed in the pump 52. [0071] 4. A block diagram of an exhaust gas treating system according to a fourth modification is shown in Fig. 9. If freshwater 26 is not to be used or if the chloride ion concentration of the freshwater 26 is permitted to be of much the same level as that of the make-up water 31, the low-pressure pump 17 and the low-pressure reverse osmosis membrane 14 may be removed from the system configuration. Not only operational costs associated with energy consumption in the low-pressure pump and with cleaning, 26 replacement, and other labor requirements relating to the low-pressure reverse osmosis membrane, but also related installation costs are reduced in that case. (Third Embodiment) [0072] New problems In the first embodiment and the second embodiment, the gas cooler 11 uses pretreated water 21 (pretreated seawater 20) to cool the exhaust gas 1 that flow into the absorption column 12. The gas cooler 11 also uses the heat from the exhaust gas 1 to heat the pretreated water 21, and supplies the heated water to the high-pressure pump 8 and the high-pressure reverse osmosis membrane 9. That is to say, seawater 20 (more accurately, the pretreated water 21) flows through the gas cooler 11, and materials of sections exposed to the seawater are therefore required to be highly resistant to corrosion. This presents problems associated with an installation cost of the gas cooler 11. [0073] Configuration Fig. 10 is a block diagram of an exhaust gas treating system according to a third embodiment. A gas cooler 11 is disposed at a stage that follows the high-pressure reverse osmosis membrane 9. The gas cooler 11 uses a permeate 23, the water separated by the high-pressure reverse osmosis membrane 9, to cool the exhaust gas 1 that flow into the absorption column 12. The gas cooler 11 also uses the heat 27 from the exhaust gas 1 to heat the permeate 23. The heated permeate 23 is pressurized by the low-pressure pump 17 and flows into the low-pressure reverse osmosis membrane 14. [0074] In addition, part of the permeate 23 which has flown out from the high-pressure reverse osmosis membrane 9 flows into the make-up water tank 18 via a temperature control valve 42. In the second embodiment, the drainage valve 41 has discharged a part of the pretreated water 21 from the system, whereas in the third embodiment, the temperature control valve 42 uses the permeate 23 in the system. The flow rate of the water passed through at the temperature control valve 42 is equivalent to the water drainage quantity Qout in the second embodiment. [0075] Advantageous effects In the first embodiment and the second embodiment, the chloride ion concentration of the seawater 20 passed through the gas cooler 11 is nearly 35,000 mg/l. The permeate 23, passed through the gas cooler 11 in the third embodiment, ranges nearly from 200 to 300 mg/l in chloride ion concentration. Corrosion resistance of the permeate 23, therefore, is not required to be as high as in the first and second embodiments, so an installation cost of the gas cooler 11 can be accordingly reduced. [0076] During water production with the high-pressure pump 8 and the high-pressure reverse osmosis membrane 9, an energy 28 consumption reduction effect associated with an increase in water temperature cannot be obtained. During water production with the low-pressure pump 17 and the low pressure reverse osmosis membrane 14, however, the energy consumption reduction effect associated with the increase in water temperature can be obtained and operational costs can therefore be reduced. [0077] Installation cost reduction and operational cost reduction work together to achieve total cost reduction. [0078] In addition, since the pretreated water 21 is not heated, the chloride ion concentration of the permeate 23 separated by the high-pressure reverse osmosis membrane 9 is low, compared with the chloride ion concentrations of the permeates 23 in the first and second embodiments. As a result, freshwater 26 also has its chloride ion concentration reduced in comparison with the values of the permeates 23 in the first and second embodiments, as shown in Fig. 5. Thus the freshwater 26 becomes expanded in applications and can be used more effectively. (Fourth Embodiment) [0079] Problems In the first embodiment and the second embodiment, the exhaust gas 1 from which SOx has been removed by the absorption column 12 are released from the stack 13 into the 29 atmosphere. The great deal of moisture contained in the exhaust gas 1 released from the system at that time becomes one causative factor that makes it necessary to produce a large volume of make-up water 31. Producing the make-up water 31 in large quantities poses problems associated with operational costs. [0080] Configuration and operation Fig. 11 is a block diagram of an exhaust gas treating system according to a fourth embodiment. In the present embodiment, gas cooler (a second gas cooler) 43 is added to the configuration of the second embodiment. The gas cooler 43 is provided at a stage that immediately follows the absorption column 12. [0081] Operation of the wet-desulfurizing apparatus including the absorption column 12 is described below. Exhaust gas 1 from a boiler are cleared of any soot and dust by the electric dust collector 10 and after having been cooled by the gas cooler 11, flow into the absorption column 12. In the absorption column 12, a limestone slurry 36 is sprayed over the exhaust gas 1 and SOx is removed from the gases 1 by a gas-liquid reaction with the limestone slurry 36. At this time, heat from the exhaust gas 1 evaporates the moisture contained in the limestone slurry 36. The exhaust gas 1 that contains moisture is cooled by the gas cooler 43, and has the moisture condensed and removed during cooling. The exhaust gas 1 that have been cleared of the 30 moisture are released from the stack 13 into the atmosphere. [0082] The condensate 32, the moisture condensed by the gas cooler 43, flows into the make-up water tank 18 via a second make-up water supply line 53. [0083] Advantageous effects Fig. 12 is a conceptual diagram that illustrates advantageous effects of the present embodiment. Cases 1 to 4 are plotted on a horizontal axis. Changes in the amount of moisture drained from the system, changes in a necessary amount of make-up water, changes in the amount of energy consumed during water production, and factors relating to each case are plotted in a top row, upper middle row, lower middle row, and bottom row, respectively, of a vertical axis. [0084] The moisture drained from the system consists of, for example, the moisture contained in the exhaust gas 1, desulfurized wastewater 30, and crystallization water in a gypsum 28. [0085] The necessary amount of make-up water varies with the amount of moisture drained from the system. This means that an increase in the amount of moisture drained from the system increases the necessary amount of make-up water as well and hence the amount of water to be produced. [0086] The energy that a pump consumes during water 31 production depends upon the amount of water produced and upon the temperature of the supply water. That is to say, a decrease in the amount of water production also reduces the energy consumption. In addition, an increase in water temperature increases a velocity of a permeate and reduces the energy consumption. [0087] Based upon these premises, comparisons are conducted between cases 1 to 4. [0088] Case 1 indicates that none of the above three factors, that is, that there occurs neither gas cooling by the gas cooler 11, nor supply water heating by the gas cooler 11, nor replenishment with the condensate 32, are included. Case 2 indicates that only gas cooling by the gas cooler 11 exists. Advantageous effects of gas cooling by the gas cooler 11, as compared between cases 1 and 2, are described below. [0089] The moisture contained in the exhaust gas 1 is condensed by gas cooling with the gas cooler 11, and then supplied to the absorption column 12. The moisture contained in the exhaust gas 1 released from the stack 13 decreases as a result. This also reduces the necessary amount of make-up water and hence the amount of water produced. Thus, the energy consumption also decreases. [0090] Case 3 indicates that gas cooling by the gas cooler 32 11 and supply water heating by the gas cooler 11 exist. Advantageous effects of supply water heating by the gas cooler 11, as compared between cases 2 and 3, are described below. [0091] An increase in water temperature due to supply water heating also causes the chloride ion concentration of the make-up water 31 to increase. Since moisture circulates through the desulfurizing apparatus, the increase in the chloride ion concentration of the make-up water 31 increases a flow rate of the desulfurized wastewater 30 as well to avoid further concentrating of the make-up water. A consequential slight increase in the amount of moisture drained from the system causes a slight increase in the necessary amount of make-up water as well and hence a slight increase in the amount of water produced. The energy consumption reduction effect associated with the increase in water temperature, however, outweighs the energy consumption increase effect associated with the increase in the amount of water produced, and resultantly reduces the energy consumption as well. [0092] Case 4 indicates that all of the three factors, that is, that gas cooling by the gas cooler 11, supply water heating by the gas cooler 11, and replenishment with the condensate 32 are included. Advantageous effects of replenishment with the condensate 32, as compared between cases 3 and 4, are described below. 33 [0093] Part of the moisture in the exhaust gas 1 released from the stack 13 is supplied as the condensate 32 to refill the desulfurizing apparatus, with the result that the amount of water to be produced is significantly reduced, which in turn also reduces the energy consumption and thus reduces operational costs. [0094] Additionally, the effects associated with water production volume reduction and energy consumption reduction allow the high-pressure pump 8 and the high-pressure reverse osmosis membrane 9 to be miniaturized and installation costs of these elements to be reduced. (Fifth Embodiment) [0095] Problems In the first embodiment and the second embodiment, the high-pressure pump 8 pressurizes the pretreated water 21 and supplies this water to the high-pressure reverse osmosis membrane 9. The pretreated water 21, generated by pretreating the seawater 20, has a high chloride ion concentration. The fact that the chloride ion concentration is high accordingly increases water-producing energy consumption, thus causing problems associated with operational costs. [0096] Configuration and operation 34 Fig. 13 is a block diagram of an exhaust gas treating system according to a fifth embodiment. The present embodiment is substantially the same as the fourth embodiment in that the second gas cooler 43 is added to the configuration of the second embodiment. The fifth embodiment, however, differs from the fourth embodiment in that the condensate 32, the moisture condensed by the gas cooler 43, flows into the pretreatment apparatus 7 via a condensate mixing line 54. [0097] Advantageous effects Mixing the condensate 32 of a low chloride ion concentration into the pretreated water 21 of a high chloride ion concentration lowers the chloride ion concentration of the supply water to the high-pressure reverse osmosis membrane 9. Thus, energy consumption also decreases and operational costs can be reduced. [0098] Additionally, even if any trace heavy metals and other substances contained in the exhaust gas 1 are entrained in the condensate 32, these substances such as the heavy metals are separated into a brine 22 by the high pressure reverse osmosis membrane 9 and then separated and disposed of as sludge during draining. Accordingly, an outflow of the condensate into the ocean or other water environments can be avoided. [0099] Furthermore, advantageous effects similar/equivalent 35 to those of the fourth embodiment can be obtained since the condensate 32 is used effectively in the system. [0100] Modifications 5. A block diagram of an exhaust gas treating system according to a fifth modification is shown in Fig. 14. A condensate 32 that is moisture that has been condensed by the gas cooler 43 may be mixed with seawater 20 via the condensate mixing line 54, at a stage immediately preceding the pretreating apparatus 7. [0101] 6. A block diagram of an exhaust gas treating system according to a sixth modification is shown in Fig. 15. In the present modification, the second make-up water supply line 53 of the fourth embodiment is added to the configuration of the fifth embodiment. This configuration of the sixth modification provides advantageous effects similar/equivalent to those of the fourth and fifth embodiments. Description of Reference Numerals [0102] 1... exhaust gas 6... intake pump 7... pretreating apparatus 8... high-pressure pump 9... high-pressure reverse osmosis membrane 10 ... electric dust collector 36 11... gas cooler 12... absorption column 13... stack 14... low-pressure reverse osmosis membrane 15... gypsum dewatering machine 16 ... limestone slurry supply unit 17... low-pressure pump 18... make-up water tank 20... seawater 21... pretreated water 22... brine 23... permeate 24... brine 25 ... permeate 26... freshwater 28... gypsum 29... dewatering filtrate 30... desulfurized wastewater 31... make-up water 32... condensate 36... limestone slurry 37... supplementary limestone slurry 40... bypass valve 41... drainage valve 42... temperature control valve 43 ... gas cooler (a second gas cooler) 46 ... temperature sensor 47... exhaust gas state sensor 37 48... chloride ion concentration sensor 49... input terminal 50... controller 51... make-up water supply line 52... pump 53... make-up water supply line (second make-up water supply line) 54... condensate mixing line 38

Claims (7)

1. An exhaust gas treating system, comprising: a wet-desulfurizing apparatus (12) for removing sulfur oxides contained in boiler exhaust gas (1); a water production apparatus equipped with a pump (8,17) for boosting seawater (20), the water production apparatus having a membrane treatment apparatus (9,14) for separating the seawater pumped in from the pump, into a permeate (23,25) and a non-permeate (22,24); a make-up water supply line (18,51) for supplying at least one part of the permeate as make-up water (31) to the desulfurizing apparatus; and a gas cooler (11) having an exhaust gas cooling function (lla) which, by means of the seawater used in the water production apparatus, cools the exhaust gas that flow into the desulfurizing apparatus, and a seawater heating function (lib) that uses heat of the exhaust gas to heat the seawater during the cooling of the exhaust gas.

2. The exhaust gas treating system according to claim 1, further comprising: water temperature control means (6,40,41,43,50) for controlling a temperature of the seawater heated by the seawater heating function, to supply the temperature controlled seawater to the pump.

3. The exhaust gas treating system according to claim 1, wherein: the water production apparatus further includes a 39 first pump (8) for boosting seawater, a first membrane treatment unit (9) for separating the seawater pumped in from the first pump, into permeate (23) and non-permeate (22), a second pump (17) for boosting a part of the first stage permeate, and a second membrane treatment unit (14) for separating the first-stage permeate pumped in from the second pump, into permeate (25) and non-permeate (24); wherein the make-up water supply line (18,51) supplies the first-stage permeate (23) and the second-stage non-permeate (24) to the desulfurizing apparatus (12).

4. The exhaust gas treating system according to claim 3, wherein: the exhaust gas cooling function (11a) of the gas cooler (11) uses the first-stage permeate (23) to cool the exhaust gas.

5. The exhaust gas treating system according to claim 1, further comprising: a second gas cooler (43) for cooling the exhaust gas from the wet-desulfurizing apparatus (12) by means of the seawater (21) used in the water production apparatus, and then condensing and removing any moisture contained in the exhaust gas; and a second make-up water supply line (53) for supplying to the desulfurizing apparatus at least one part of the condensate (32) formed by the second gas cooler.

6. The exhaust gas treating system according to claim 1, further comprising: a second gas cooler (43) for cooling the exhaust gas 40 from the wet-desulfurizing apparatus (12) by means of the seawater (21) used in the water production apparatus, and then condensing and removing any moisture contained in the exhaust gas; and a condensate mixing line (54) for mixing at least one part of the condensate (32) formed by the second gas cooler, with the seawater (20,21) supplied to the membrane treatment apparatus(9,14).

7. An exhaust gas treating method, comprising: a water production step in which water is produced by boosting seawater (20,21) and then separating the seawater into a permeate and a non-permeate via a membrane treatment apparatus (9,14); a make-up water supply step in which at least one part of the permeate (23,25) obtained in the water production step is supplied as make-up water (31) to a wet desulfurizing apparatus (12); an exhaust gas desulfurizing step in which any sulfur oxides contained in exhaust gas (1) from a boiler are removed using the wet-desulfurizing apparatus (12) that sprays a limestone slurry (36) over the exhaust gas while supplying moisture to the slurry in the make-up water supply step; and an exhaust gas cooling/seawater heating step in which, the exhaust gas (1) flowing into the desulfurizing apparatus in the exhaust gas desulfurizing step are cooled by means of the seawater (20,21) used in the water production step, and during the cooling of the exhaust gas, the seawater (20,21) 41 is heated using heat energy of the exhaust gas (1). 42