DESCRIPTION

SEAWATER DESULFURIZATION/OXIDATION TREATMENT APPARATUS, METHOD OF TREATING DESULFURIZATION SEAWATER, AND POWER GENERATING SYSTEM EMPLOYING THE SAME

TECHNICAL FIELD

[1] The present invention relates to a seawater desulfurization/oxidation treatment apparatus, a method of treating desulfurization seawater, and a power generating employing the same those capable of treating sulfur-content absorbing seawater, which is produced by removal of a sulfur content such as sulfur oxide contained in a flue gas emitted from an industrial combustion facility with the use of seawater, to have a pH and a chemical oxygen demand (COD) at dischargeable seawater levels and then discharging the treated seawater.

BACKGROUND ART

[2] Recently, there has been an increase in the number of thermal power plants employing a seawater flue- gas desulfurization apparatus using seawater as absorbent in a flue-gas desulfurization process. The flue-gas desulfurization apparatus is provided to reduce a sulfur content contained in a flue gas produced by the combustion of fossil fuel such as coal. Such a seawater desulfurization technology for desulfurization with the use of seawater as absorbent has attracted attention from the standpoint that power plants and the like are mostly built on the sea to require a large amount of cooling water and also from the standpoint that the running cost for desulfurization is kept low.

[3] The seawater flue-gas desulfurization apparatus can keep the costs lower than a limestone-gypsum flue-gas desulfurization apparatus. Therefore, the seawater flue- gas desulfurization apparatus is used, especially, in a thermal power plant in a coastal area, which has difficulty with stable supply of limestone, and so on. Furthermore, as cooling water, a large amount of seawater is used in a condenser of a boiler. Therefore, a portion of a heated seawater effluent discharged from the condenser is supplied to the desulfurization apparatus so that the desulfurization apparatus reduces SO2 from a flue gas with the use of the portion of heated seawater effluent as absorbent for seawater desulfurization.

[4] Fig. 8 shows an example of a conventional seawater desulfurization/oxidation treatment apparatus. Fig. 8 is a diagram schematically showing a configuration of a power generating system including the conventional seawater desulfurization/oxidation treatment apparatus. As shown in Fig. 8, a power generating system 100 employing the conventional seawater desulfurization/oxidation treatment apparatus is composed of a boiler 12 that causes a burner (not shown) to burn fossil fuel with preheated air 11; a dust collecting apparatus 14 that reduces ash dust contained in a flue gas 13 that is produced by heat exchange in the boiler 12 and emitted from the boiler 12; and a seawater desulfurization/oxidation treatment apparatus 101 that reduces a sulfur content contained in the flue gas 13 by absorbing the sulfur content in absorption seawater 15A, and performs a water-quality restoring treatment of produced sulfur-content absorbing seawater 16A containing the sulfur content in a high concentration. The seawater desulfurization/oxidation treatment apparatus 101 is composed of a flue-gas desulfurization absorber 20 and an oxidation basin 21. In the flue-gas desulfurization absorber 20, SO2 contained in the flue gas 13 is absorbed into the absorption seawater 15A, and recovered as sulfurous acid (H2SO3) and sulfuric acid (H2SO4) . In the oxidation basin 21, a water-quality restoring treatment is performed on the sulfur-content absorbing seawater 16A containing the sulfur content in the high concentration, which is discharged from the flue-gas desulfurization absorber 20.

[5] A generator (not shown) of a steam turbine (not shown) is driven to generate electricity by steam that is generated with the use of the flue gas 13 emitted from the boiler 12 as a heat source for the generation of steam.

[6] The flue gas 13 is fed to a flue-gas denitration apparatus (not shown), and is subjected to denitration. After that, the flue gas 13 is fed to the dust collecting apparatus 14, and ash dust contained in the flue gas 13 is reduced. Then, the flue gas 13 from which the ash dust is reduced by the dust collecting apparatus 14 is supplied into the flue-gas desulfurization absorber 20 by an induction fan 22.

[7] In the flue-gas desulfurization absorber 20, out of seawater 15 pumped from sea 25 with a pump 23, a portion of the seawater 15 pumped with a pump 24 is used as the absorption seawater 15A. The absorption seawater 15A is used for the removal of the sulfur content contained in the flue gas 13. That means the flue gas 13 produced by the combustion of fossil fuel contains the sulfur content that is sulfur oxide (SOx) in the form of SO2 or the like. In the seawater desulfurization, in the flue-gas desulfurization absorber 20, the flue gas 13 and the absorption seawater 15A supplied through a seawater supply line LI are brought into gas-liquid contact with each other, which causes a reaction as indicated by the following formula. The desulfurization is performed by absorbing the sulfur content such as sulfur oxide (SOx) contained in the flue gas 13 in the form of sulfurous acid gas (SO2) or the like into the absorption seawater 15A.
sO2(g)+H2O H2SO3(I) HSO3"+H+ (L)

[8] By the gas-liquid contact between the absorption seawater 15A and the flue gas 13, SO2 is absorbed, and sulfurous acid (H2SC>3) is produced in the absorption seawater 15A. Successively, the sulfurous acid is dissociated, and hydrogen ion (H+) is produced. Therefore, the absorption seawater 15A after the gas-liquid contact with the flue gas 13 decreases in pH along with the absorption of the sulfurous acid gas. A pH of the sulfur- content absorbing seawater 16A as the absorption seawater 15A into which the sulfur content is absorbed is about 3 to 6.

[9] Then, a purged gas 26 that the flue gas 13 is desulfurized in the flue-gas desulfurization absorber 20 is released into the atmosphere from a stack 27 through a purged-gas discharge line L2. The sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is discharged from the flue-gas desulfurization absorber 20 through a sulfur-content absorbing seawater discharge line L3.

[10] Before being discharged into the sea 25 or reused, the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber needs to be reduced in concentration of sulfurous acid to be a COD component, and thereby increasing a pH and a dissolved oxygen level. Therefore, the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber, containing the sulfur content in the high concentration, is supplied to the oxidation basin 21 through the sulfur- content absorbing seawater discharge line L3. In the oxidation basin 21, an oxidation air blower 29 supplies air 30 into the oxidation basin 21 from a nozzle 32 through a diffuser tube 31 to bring the air 30 into gas-liquid contact with sulfur-content absorbing seawater 28 in the oxidation basin, which causes reactions as indicated by the following formulae so as to reduce the concentration of sulfurous acid to be a COD component and increase the pH and the dissolved oxygen level.
O2(g) O2(I) (2)
HSO3"+l/2O2 (I) SO42"+H+ (3)
HCO3"+H+ CO2 (g) +H2O (4)
CO32~+2H+ CO2 (g) +H2O (5)

[11] Incidentally, the oxidation basin 21 is generally placed in an outdoor location. Thus, if a pH of the sulfur-content absorbing seawater 28 in the oxidation basin is low, due to aeration for the supply of oxygen as indicated by the formula (2) , SO2 is unfavorably re-emitted as indicated by the following formula (6).
H2SO3(I) SO2(g)+H2O (6)

[12] Therefore, to prevent SO2 re-emission, as first dilution seawater 15B, a portion of the seawater 15 supplied through the seawater supply line LI is mixed into the sulfur-content absorbing seawater 28 in the oxidation basin through a first dilution seawater supply line L4 to dilute the sulfur-content absorbing seawater 28 in the oxidation basin thereby increasing the pH of the sulfur- content absorbing seawater 28 in the oxidation basin.

[13] While oxidation-basin-inlet sulfur-content absorbing seawater 16B flows from the inlet toward the outlet of the oxidation basin 21, a water quality of the oxidation-basin-inlet sulfur-content absorbing seawater 16B is gradually restored by the oxidation of hydrogen sulfite ion (HSO3") and the decarbonation, and the oxidation-basin- inlet sulfur-content absorbing seawater 16B is recovered as oxidation-basin-outlet sulfur-content absorbing seawater 16C. In this manner, the sulfur-content absorbing seawater 28 in the oxidation basin continuously varies in composition. Thus, the sulfur-content absorbing seawater 28 in the oxidation basin refers to the whole seawater having a relatively-wide composition range.

[14] Furthermore, if further restoration of the water quality is required, before the sulfur-content absorbing seawater is discharged into the sea 25 from the oxidation basin 21, in the oxidation basin 21, as second dilution seawater 15C, the rest of the seawater 15 is mixed into the sulfur-content absorbing seawater 28 in the oxidation basin through a second dilution seawater supply line L5 to dilute the sulfur-content absorbing seawater 28 in the oxidation basin. Because of this, the COD reduction and the pH increase of the discharged oxidation-basin-outlet sulfur- content absorbing seawater 16C can be made efficiently, which leads to the restoration of the water quality. Water-quality restored seawater 33 having the restored water quality is discharged as a seawater effluent into the sea 25 through a seawater discharge line L6.

[15] In this manner, in the conventional power generating system 100, to achieve the prevention of SO2 re- emission and the increase in pH in the oxidation basin 21, the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is mixed with the first dilution seawater 15B in the oxidation basin 21 so as to be diluted, and subjected to aeration, thereby oxidizing hydrogen sulfite ion (HSO3-) to render the hydrogen sulfite ion harmless and simultaneously increasing the dissolved oxygen level, and then subjected to decarbonation, thereby increasing the pH of the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber, and after that, the treated seawater is discharged into the sea 25 (see Patent documents 1 to 3).

[16] Patent document 1: Japanese Patent Application Laid-open No. 2006-055779
Patent document 2: Japanese Patent Application Laid- open No. 2007-125474
Patent document 3: International Publication W0/2008/077430

DISCLOSURE OF INVENTION PROBLEM TO BE SOLVED BY THE INVENTION

[17] As an amount of supply of the absorption seawater 15A to the flue-gas desulfurization absorber 20 increases, a pump capacity gets larger, and the facility cost and the running cost get larger. Therefore, it is preferable that an amount of supply of the absorption seawater 15A to the flue-gas desulfurization absorber 20 is reduced as much as possible.

[18] On the other hand, if an amount of supply of the first dilution seawater 15B in the inlet of the oxidation basin 21 is insufficient, leading to an increase in acid- alkaline equivalent ratio of the sulfur-content absorbing seawater 28 in the oxidation basin and a decrease in pH, even when the air 30 is blown into the sulfur-content absorbing seawater 28 in the oxidation basin, the oxidation reaction of hydrogen sulfite ion does not proceed, and thus the COD of the sulfur-content absorbing seawater 28 in the oxidation basin is not reduced.

Incidentally, the term acid-alkaline equivalent ratio here is a ratio of an acid equivalent of the absorbed sulfur content to an alkaline equivalent of the seawater.

Furthermore, the acid equivalent of the absorbed 'sulfur content means the maximum concentration of hydrogen ion (H+) produced when sulfurous acid (H2SO3) and sulfuric acid (H2SO4) that are produced from the absorbed sulfur content are completely dissociated.

Moreover, the alkaline equivalent of the seawater is equivalent to the alkalinity, and means an acid equivalent consumed when the seawater is titrated with hydrochloric acid until a pH reaches 4.8.

[19] Furthermore, if an amount of supply of the first dilution seawater 15B in the inlet of the oxidation basin 21 is insufficient, and the pH of the sulfur-content absorbing seawater 2 8 in the oxidation basin becomes low, when the air 30 is blown into the sulfur-content absorbing seawater 28 in the oxidation basin, SO2 harmful to the human body is re-emitted, and a bad odor is generated around the oxidation basin 21.

[20] On the other hand, if an amount of supply of the first dilution seawater 15B in the inlet of the oxidation basin 21 is excessively large, and the concentration of sulfurous acid in the sulfur-content absorbing seawater 28 in the oxidation basin becomes too low, an oxidation reaction rate is slowed down, so that it is necessary to lengthen a residence time in the oxidation basin 21. Therefore, it is necessary to enlarge the oxidation basin 21. And also, the oxidation air blower 29 needs to supply a larger amount of the air 30 because of a decrease in CO2 partial pressure due to an increase in pH of the sulfur- content absorbing seawater 28 in the oxidation basin. Therefore, costs such as an oxidation facility cost and a power cost of the oxidation air blower 29 and the like are increased.

[21] In view of the above problems, an object of the present invention is to provide low-cost and safe seawater desulfurization/oxidation treatment apparatus and seawater desulfurization system and a method of treating desulfurization seawater those capable of achieving the prevention of re-emission of SO2 harmful to the human body and the maintenance of a dischargeable water quality of seawater simultaneously by controlling an amount of seawater used for dilution of sulfur-content absorbing seawater subjected to desulfurization with the use of the seawater appropriately.

MEANS FOR SOLVING PROBLEM

[22] According to an aspect of the present invention, a seawater desulfurization/oxidation treatment apparatus includes: a flue-gas desulfurization absorber in which a flue gas is brought into contact- with seawater to reduce sulfur oxide contained in the flue gas, and the seawater is collected as sulfur-content absorbing seawater containing sulfurous acid; an oxidation basin in which a sulfur content contained in the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber is oxidized and decarbonated to restore a water quality of the sulfur-content absorbing seawater; a seawater supply line through which the seawater is supplied as absorption seawater to the flue-gas desulfurization absorber; an oxidation-basin-inlet dilution mixing basin that is provided on the side of an inlet of the oxidation basin, and mixes a portion of the seawater as first dilution seawater with the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber therein; an oxidation-basin-outlet dilution mixing basin that is provided on the side of an outlet of the oxidation basin, and mixes a portion of the seawater as second dilution seawater with water-quality restored oxidation- basin-outlet- sulfur-content absorbing seawater therein; a sulfur-content absorbing seawater discharge line through which the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber is discharged into the oxidation-basin-inlet dilution mixing basin; a first dilution seawater supply line through which the first dilution seawater is supplied to the oxidation-basin-inlet dilution mixing basin; a second dilution seawater supply line through which the second dilution seawater is supplied to the oxidation-basin-outlet dilution mixing basin; and a discharge line through which water-quality restored seawater that the oxidation-basin-outlet sulfur-content absorbing seawater is diluted with the second dilution seawater is discharged into sea.

[23] Advantageously, in the seawater desulfurization/oxidation treatment apparatus, the first dilution seawater is supplied so that just after the first dilution seawater is mixed with the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber, a ratio of an acid equivalent derived from absorbed sulfur content to an alkaline equivalent of the seawater is 0.83 or more but not exceeding 1.2.

[24] Advantageously, the seawater desulfurization/oxidation treatment apparatus further includes a detector for detecting a concentration of hydrogen sulfite ion in the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber. The detector is provided on the sulfur-content absorbing seawater discharge line.

[25] Advantageously, the seawater desulfurization/oxidation treatment apparatus further includes a detector for detecting an alkaline equivalent of the first dilution seawater, the detector being provided on the first dilution seawater supply line.

[26] Advantageously, in the seawater desulfurization/oxidation treatment apparatus, the seawater is an effluent discharged from a condenser.

[27] According to another aspect of the present invention, a power generating system includes: a boiler; a steam turbine that uses a flue gas emitted from the boiler as a heat source for generation of steam, and drives a power generator with generated steam; a condenser that collects water condensed in the steam turbine, and circulates the water; a flue-gas denitration apparatus that denitrates the flue gas emitted from the boiler; a dust collecting apparatus that reduces ash dust contained in the flue gas; any one of the above described seawater desulfurization/oxidation treatment apparatus; and a stack through which a purged gas which is the flue gas after desulfurized in the seawater desulfurization/oxidation treatment apparatus is released into outside.
[28] According to still another aspect of the present invention, in a method of treating desulfurization seawater that washes a flue gas by bringing a sulfur content in the flue gas into contact with seawater, oxidizes and decarbonates sulfurous acid in sulfur-content absorbing seawater absorbing the sulfur content after washing, and discharges the sulfur-content absorbing seawater after restoring a water quality of the desulfurization seawater, after a portion of the seawater as first dilution seawater is mixed into the sulfur-content absorbing seawater in an oxidation-basin-inlet dilution mixing basin, oxidation- basin-inlet sulfur-content absorbing seawater is supplied into an oxidation basin to oxidize sulfurous acid in the oxidation-basin-inlet sulfur-content absorbing seawater, and the oxidation-basin-inlet sulfur-content absorbing seawater is decarbonated, and after oxidation-basin-outlet sulfur-content absorbing seawater discharged from the oxidation basin after oxidation/decarbonation processes is fed to an oxidation-basin-outlet dilution mixing basin, and a portion of the seawater is mixed as second dilution seawater into the oxidation-basin-outlet sulfur-content absorbing seawater in the oxidation-basin-outlet dilution mixing basin, the oxidation-basin-outlet sulfur-content absorbing seawater is discharged.

[29] Advantageously, in the method of treating desulfurization seawater, the first dilution seawater is supplied so that just after the first dilution seawater is mixed with the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber, a ratio of an acid equivalent derived from absorbed sulfur content to an alkaline equivalent of the seawater is 0.83 or more but not exceeding 1.2.

[30] Advantageously, in the method of treating desulfurization seawater, the seawater is an effluent discharged from a condenser.

EFFECT OF THE INVENTION

[31] According to an aspect of the present invention, in the flue-gas desulfurization absorber, a portion of seawater as the first dilution seawater is previously supplied through a first dilution seawater supply line to be mixed into sulfur-content absorbing seawater produced by seawater desulfurization, so that the sulfur-content absorbing seawater is diluted. As a result, an acid- alkaline equivalent ratio of the sulfur-content absorbing seawater is decreased, and a pH of the sulfur-content absorbing seawater is increased. Thus, it is possible to enhance an oxidation reaction rate. Furthermore, SO2 partial pressure is. reduced by the dilution of the sulfur- content absorbing seawater with the first dilution seawater, so that re-emission of SO2 harmful to the human body can be prevented.

[32] Furthermore, an amount of the sulfur-content absorbing seawater to be flown into an oxidation basin in which the sulfur-content absorbing seawater is mixed with the first dilution seawater is minimized without supplying an excessive amount of the first dilution seawater through the first dilution seawater supply line, and the concentration of sulfurous acid in the seawater in the oxidation basin is maintained at a high level, and also the pH is maintained at a low level enough to prevent from the re-emission of SO2 or a decrease in reaction rate constant. Therefore, the restoration of water quality by the oxidation treatment of sulfurous acid and the decarbonation can be efficiently performed with a smaller-scale oxidation basin and the blower while maintaining CO2 partial pressure of the sulfur-content absorbing seawater in the oxidation basin.

[33] Moreover, a portion of the seawater as the second dilution seawater is supplied through the second dilution seawater supply line into the sulfur-content absorbing seawater discharged from the seawater discharge line after being subjected to the water-quality restoring treatment so as to dilute the sulfur-content absorbing seawater. Therefore, a pH of the water-quality restored seawater can be increased efficiently, and a COD can be decreased.

[34] Thus, the oxidation basin can be downsized while maintaining the pH and the COD of the water-quality restored seawater at dischargeable seawater levels, and the oxidation facility cost can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

[35] [Fig..1] Fig._1 is a schematic diagram showing a configuration of a power generating system employing a seawater desulfurization/oxidation treatment apparatus according to an embodiment of the present invention.

[Fig. 2] Fig. 2 is a graph showing a relation between pH and CO2 partial pressure of seawater.

[Fig. 3] Fig. 3 is a graph showing a relation between pH of sulfur-content absorbing seawater and rate constant of oxidation reaction of hydrogen sulfite ion in the sulfur-content absorbing seawater.

[Fig. 4] Fig. 4 is a graph showing a relation between acid-alkaline equivalent ratio of oxidation-basin-inlet sulfur-content absorbing seawater and pH of each of the oxidation-basin-inlet sulfur-content absorbing seawater, oxidation-basin-outlet sulfur-content absorbing seawater, and water-quality restored seawater.

[Fig. 5] Fig. 5 is a graph showing a relation between acid-alkaline equivalent ratio of the oxidation-basin-inlet sulfur-content absorbing seawater and COD concentration of each of the oxidation-basin-inlet sulfur-content absorbing seawater, the oxidation-basin-inlet sulfur-content absorbing seawater, and the water-quality restored seawater.

[Fig. 6] Fig. 6 is a graph showing a relation between acid-alkaline equivalent ratio of the oxidation- basin-inlet sulfur-content absorbing seawater and maximum SO2 partial pressure of sulfur-content absorbing seawater in oxidation basin.

[Fig. 7] Fig. 7 is a graph showing a relation between acid-alkaline equivalent ratio of the oxidation-basin-inlet sulfur-content absorbing seawater and total carbonic acid concentration of each of the oxidation-basin-inlet sulfur- content absorbing seawater, the oxidation-basin-outlet sulfur-content absorbing seawater, and the water-quality restored seawater.

[Fig. 8] Fig. 8 is a diagram schematically showing a configuration of a power generating system including a conventional seawater-based seawater desulfurization/oxidation treatment apparatus.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

[36] An exemplary embodiment of the present invention is explained in detail below with reference to the accompanying drawings. Incidentally, the present invention is not limited to the embodiment. Furthermore, elements used in the embodiment described below may include an element that can be easily arrived at by those skilled in the art or an element substantially identical to that is disclosed in the prior art.

[37] EMBODIMENT

A power generating system employing a seawater desulfurization/oxidation treatment apparatus according to an embodiment of the present invention is explained below with reference to the accompanying drawings.

Fig. 1 is a schematic diagram showing a configuration of the power generating system employing the seawater desulfurization/oxidation treatment apparatus according to the embodiment of the present invention. In the diagram, the same elements as the system shown in Fig. 8 are denoted with the same reference numerals, and the description of those elements is omitted.

[38] As shown in Fig. 1, a power generating system 40 employing a seawater desulfurization/oxidation treatment apparatus 10 according to the present embodiment is composed of the boiler 12 that causes a burner (not shown) to burn fuel with the air 11 preheated by an air preheater (AH) 41; a steam turbine 44 that uses the flue gas 13 emitted from the boiler 12 as a heat source for the generation of steam, and drives a power generator 43 with generated steam 42; a condenser 46 that collects water 45 condensed in the steam turbine 44, and circulates the water 45; a flue-gas denitration apparatus 47 that denitrates the flue gas 13 emitted from the boiler 12; the dust collecting apparatus 14 that reduces ash dust contained in the flue gas 13 emitted from the boiler 12; the seawater desulfurization/oxidation treatment apparatus 10 that reduces a sulfur content contained in the flue gas 13 with the use of the absorption seawater 15A, and performs a water-quality restoring treatment of the sulfur-content absorbing seawater 16A containing the sulfur content in a high concentration, which is produced by the seawater desulfurization; and the stack 27 from which the purged gas 26 that the flue gas 13 is desulfurized in the seawater desulfurization/oxidation treatment apparatus 10 is released into the outside.

[39] The air 11 supplied from the outside is fed to the air preheater 41 by a forced draft fan 48, and preheated by the air preheater 41. Fuel (not shown) and the air 11 preheated by the air preheater 41 are supplied to the burner. The fuel is burned by the boiler 12, and the steam 42 for driving the steam turbine 44 is generated. The fuel (not shown) used in the present embodiment is supplied, for example, from an oil basin or the like.

[40] The flue gas 13 produced by the combustion of the fuel in the boiler 12 is fed to the flue-gas denitration apparatus 47. Incidentally, if there is no regulation of nitrogen oxide (NOx) emission, the installation of the flue-gas denitration apparatus 47 can be eliminated. At this time, the flue gas 13 is subjected to heat exchange with the water 45 discharged from the condenser 46, and is used as the heat source for the generation of the steam 42.

The generated., steam 42 drives the power generator 43 of the steam turbine 44. Then, the water 45 condensed in the steam turbine 44 is again fed back to the boiler 12, and circulates in the boiler 12.

[41] Then, the flue gas 13 that is discharged from the boiler 12 and guided to the flue-gas denitration apparatus 47 is denitrated in the flue-gas denitration apparatus 47, and is subjected to heat exchange with the air 11 in the air preheater 41. After that, the flue gas 13 is fed to the dust collecting apparatus 14, and ash dust contained in the flue gas 13 is reduced. Then, the flue gas 13 from which the dust is reduced in the dust collecting apparatus 14 is supplied into the flue-gas desulfurization absorber 20 by the induction fan 22. At this time, the flue gas 13 is subjected to heat exchange with the purged gas 2 6 emitted after the desulfurization in the flue-gas desulfurization absorber 20 by a heat exchanger 49, and then supplied into the flue-gas desulfurization absorber 20. Alternatively, the flue gas 13 can be directly supplied to the flue-gas desulfurization absorber 20 without being subjected to the heat exchange with the purged gas 26 in the heat exchanger 49.

[42] In the power generating system 40 employing the seawater desulfurization/oxidation treatment apparatus according to the present embodiment, the seawater desulfurization/oxidation treatment apparatus 10 includes the flue-gas desulfurization absorber 20 in which the flue gas 13 is brought into contact with the absorption seawater 15A as a portion of the seawater 15 to reduce sulfur oxide (SOx) contained in the flue gas 13, and the absorption seawater 15A is recovered as the sulfur-content absorbing seawater 16A containing sulfurous acid (H2SO3) ; the oxidation basin 21 in which a sulfur content contained in the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is oxidized and decarbonated to restore a water quality of the sulfur- content absorbing seawater 16A; the seawater supply line LI through which the seawater 15 is supplied as the absorption seawater 15A to the flue-gas desulfurization absorber 20; an oxidation-basin-inlet dilution mixing basin 21A that is provided on the side of the inlet of the oxidation basin 21, and a portion of the seawater 15 as the first dilution seawater 15B is mixed with the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber therein; an oxidation-basin-outlet dilution mixing basin 21C that is provided on the side of the outlet of the oxidation basin 21, and a portion of the seawater 15 as the second dilution seawater 15C is mixed with the water- quality restored oxidation-basin-outlet sulfur-content absorbing seawater 16C therein; the sulfur-content absorbing seawater discharge line L3 through which the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is discharged into the oxidation-basin-inlet dilution mixing basin 21A; the first dilution seawater supply line L4 through which the first dilution seawater 15B is supplied to the oxidation-basin- inlet dilution mixing basin 21A; the second dilution seawater supply line L5 through which the second dilution seawater 15C is supplied to the oxidation-basin-outlet dilution mixing basin 21C; and the seawater discharge line L6 through which the water-quality restored seawater 33 that the oxidation-basin-outlet sulfur-content absorbing seawater 16C is diluted with the second dilution seawater 15C is discharged into the sea 25.

[43] In the present embodiment, out of the seawater 15, a portion of which is fed to the flue-gas desulfurization absorber 20 shall be referred to as the absorption seawater 15A, a portion of which is supplied to the oxidation-basin- inlet dilution mixing basin 21A shall be referred to as the first dilution seawater 15B, and a portion of which is supplied to the oxidation-basin-outlet dilution mixing basin 21C shall be referred to as the second dilution seawater 15C.

[44] In the flue-gas desulfurization absorber 20, the sulfur content contained in the flue gas 13 is reduced with the use of the seawater 15 pumped from the sea 25. The seawater desulfurization is performed in such a manner that the flue gas 13 is brought into gas-liquid contact with the absorption seawater 15A supplied through the seawater supply line LI in the flue-gas desulfurization absorber 20, and SO2 contained in the flue gas 13 is absorbed into the absorption seawater 15A.

[45] The seawater 15 pumped from the sea 25 is used for the heat exchange in the condenser 4 6, and discharged as a seawater discharge effluent. A portion of the discharged seawater 15 as the seawater discharge effluent is fed as the absorption seawater 15A to the flue-gas desulfurization absorber 20 to be used for the seawater desulfurization. Alternatively, the seawater 15 pumped from the sea 25 can be directly used for the seawater desulfurization.

[46] In the flue-gas desulfurization absorber 20, by the gas-liquid contact between the absorption seawater 15A and the flue gas 13 in the seawater desulfurization, sulfurous acid gas (SO2) is absorbed, and sulfurous acid (H2SO3) is produced in the absorption seawater 15A. Successively, the sulfurous acid is dissociated, and produced hydrogen ion (H+) is freed in the absorption seawater 15A. Thus, a pH of the absorption seawater 15A after the gas-liquid contact with the flue gas 13 decreases along with the absorption of the sulfurous acid gas. At this time, a pH of the sulfur-content absorbing seawater 16A is, for example, about 3 to 6.

[47] Furthermore, the seawater desulfurization/oxidation treatment apparatus 10 according to the present embodiment is provided with the first dilution seawater supply line L4 that branches from the seawater supply line LI, and a portion of the seawater 15 as the first dilution seawater 15B is supplied to the oxidation-basin-inlet dilution mixing basin 21A there through. The sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is mixed with the first dilution seawater 15B in the predetermined proportion in the oxidation-basin-inlet dilution mixing basin 21A, so that pre-dilution diluted seawater 16B that an acid-alkaline equivalent ratio of which is adjusted can be fed to the oxidation basin 21.

Incidentally, the pre-dilution diluted seawater 16B means the oxidation-basin-inlet sulfur-content absorbing seawater 16B that the first dilution seawater 15B is mixed with the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber.

Furthermore, the acid-alkaline equivalent ratio means a ratio of an acid equivalent of the absorbed sulfur content as described above to an alkaline equivalent of the seawater.

[48] In the oxidation-basin-inlet dilution mixing basin 21A, the first dilution seawater 15B is previously mixed into the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber thereby diluting the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber,.. having a low pH. Thus, .a pH of the pre-dilution diluted seawater 16B to be flown into the oxidation basin 21 can be increased. As a result, SO2 partial pressure of the sulfur-content absorbing seawater 28 in the oxidation basin can be reduced, and re-emission of harmful SO2 can be prevented, and also generation of a bad odor around the oxidation basin 21 can be prevented.

[49] Furthermore, a detector 35 for detecting the concentration of sulfurous acid in the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is provided on the sulfur-content absorbing seawater discharge line L3 so as to detect the concentration of sulfurous acid. As a means for detecting the concentration of sulfurous acid, a standard oxidation- reduction potential electrode (an ORP sensor) can be used.

[50] Moreover, a detector 36 for detecting an alkaline equivalent of the first dilution seawater 15B is provided on the first dilution seawater supply line L4 so as to detect the alkaline equivalent. The alkaline equivalent can be estimated from the total carbonic acid concentration and a pH of the seawater. Therefore, as a means for detecting the alkaline equivalent, a total organic carbon analyzer (product name: TOC-VCSH manufactured by Shimadzu Corporation) and a pH meter can be used.

[51] The sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is not directly supplied to the oxidation basin 21. The sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is previously mixed with a predetermined amount of the first dilution seawater 15B in the oxidation-basin-inlet dilution mixing basin 21A, and the pre-dilution diluted seawater 16B that an acid-alkaline equivalent ratio of which is adjusted is supplied to the oxidation basin 21.„ Therefore, in the oxidation basin 21, restoration of a water quality of the pre-dilution diluted seawater 16B by the decarbonation can be performed efficiently. Fig. 2 shows a relation between pH and CO2 partial pressure of the sulfur-content absorbing seawater. As shown in Fig. 2, the lower the pH'of the sulfur-content absorbing seawater, the higher the CO2 partial pressure. An amount of decarbonation is proportional to the CO2 partial pressure. Therefore, by maintaining a pH of the sulfur-content absorbing seawater 28 in the oxidation basin at a low level, the water-quality restoration by the decarbonation can be performed efficiently.

[52] Fig. 3 shows a relation between pH of the sulfur- content absorbing seawater and rate constant of oxidation reaction of hydrogen sulfite ion (HSO3") in the sulfur- content absorbing seawater. As shown in Fig. 3, as a pH of the sulfur-content absorbing seawater decreases, a rate constant of the oxidation reaction of hydrogen sulfite ion (HSO3") decreases. Therefore, an amount of the first dilution seawater 15B to be mixed into the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is adjusted, and the pre-dilution diluted seawater 16B that a pH of which is previously adjusted is supplied to the oxidation basin 21. Thus, in the oxidation basin 21, the oxidation reaction of hydrogen sulfite ion (HSO3") in the pre-dilution diluted seawater 16B can be accelerated.

[53] Then, the oxidation-basin-outlet sulfur-content absorbing seawater 16C that the water quality of which is restored by the oxidation reaction of hydrogen sulfite ion (HSO3") in the sulfur-content absorbing seawater 28 in the oxidation basin and the decarbonation in the oxidation basin 21 is discharged via the oxidation-basin-outlet dilution mixing basin 21C and the seawater discharge line L6.

[54] Furthermore, the second dilution seawater supply- line L5 is provided. The second dilution seawater supply line L5 branches from the seawater supply line LI, and lets a portion of the seawater 15 as the second dilution seawater 15C flow into the oxidation-basin-outlet sulfur- content absorbing seawater 16C. The second dilution seawater 15C is fed to the oxidation-basin-outlet dilution mixing basin 21C through the second dilution seawater supply line L5, and mixed with the oxidation-basin-outlet sulfur-content absorbing seawater 16C in the oxidation— basin-outlet dilution mixing basin 21C, so that the oxidation-basin-outlet sulfur-content absorbing seawater 16C can be diluted. By diluting the oxidation-basin-outlet sulfur-content absorbing seawater 16C with the second dilution seawater 15C, the water quality is restored, and the water-quality restored seawater 33 is discharged into the sea 25 as a seawater effluent through the seawater discharge line L6.

[55] The dilution in the oxidation-basin-outlet dilution mixing basin 21C can raise the pH more efficiently than the dilution in the oxidation-basin-inlet dilution mixing basin 21A because the alkalinity is recovered by mixing the second dilution seawater 15C into the oxidation- basin-outlet sulfur-content absorbing seawater 16C that the alkalinity and the total carbonic acid concentration of which are reduced. An amount of supply of the first dilution seawater 15B to be supplied through the first dilution seawater supply line L4 is reduced, and instead, an amount of supply of the second dilution seawater 15C to be supplied through the second dilution seawater supply line L5 is increased. Therefore, a residence time of the pre-dilution diluted seawater 16B in the oxidation basin 21 can be lengthened, so that the oxidation of sulfurous acid and the decarbonation in the oxidation basin 21 can be carried out sufficiently. Furthermore, the oxidation- basin-outlet sulfur-content absorbing seawater 16C is diluted with the second dilution seawater 15C, so that a COD value of the water-quality restored seawater 33 can be reduced.

[56] Furthermore, in the seawater desulfurization/oxidation treatment apparatus 10 according to the present embodiment, in the pre-dilution diluted seawater 16B that the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is diluted by being mixed with the first dilution seawater 15B, the most preferred ratio of an acid equivalent derived from the absorbed sulfur content to an alkaline equivalent of the seawater is 1 to 1. When the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is diluted with the first dilution seawater 15B, it is preferred to supply the first dilution seawater 15B so that in the pre-dilution diluted seawater 16B, a ratio of an acid equivalent derived from the absorbed sulfur content to an alkaline equivalent of the seawater is 0.83 or more but not exceeding 1.2. Moreover, the ratio is preferably 0.9 or more but not exceeding 1.1, and more preferably 0.95 or more but not exceeding 1.05.

[57] Furthermore, in the present invention, the acid equivalent derived from the absorbed sulfur content means the maximum concentration of hydrogen ion (H+) produced by the complete dissociation of sulfurous acid and sulfuric acid that are produced when the sulfur content contained in the flue gas is absorbed in the flue-gas desulfurization absorber.

[58] Moreover, as an index of the alkaline equivalent of the seawater, an acid consumption leading up to a pH of 4.8 or an alkaline equivalent calculated from a total inorganic carbonic acid content and a pH can be used.

[59] If the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is excessively diluted with the first dilution seawater 15B, the oxidation reaction of hydrogen sulfite ion (HSO3") and the decarbonation reaction of bicarbonate ion (HCO3") in the pre-dilution diluted seawater 16B after the dilution are slowed down. Therefore, to maintain a predetermined sulfurous-acid oxidation rate and a predetermined amount of decarbonation in the oxidation basin 21, it is necessary to enlarge the oxidation basin 21 and enhance the oxidation facilities (the oxidation air blower 29, the diffuser tube 31, and the nozzle 32) to secure the residence time. Consequently, the first dilution seawater 15B is previously supplied into the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber so that in the pre-dilution diluted seawater 16B, a ratio of an acid equivalent derived from the absorbed sulfur content to an alkaline equivalent of the seawater can be 0.83 or more but not exceeding 1.2. Thus, an amount of the pre- dilution diluted seawater 16B in the oxidation basin 21 can be minimized without using an excessive amount of the first dilution seawater 15B, and also an oxidation rate of sulfurous acid and a decarbonation rate can be increased by increasing the concentration of sulfurous acid in the pre- dilution diluted seawater 16B.

[60] Consequently, the oxidation reaction of hydrogen sulfite ion (HSO3") and the decarbonation in the pre- dilution diluted seawater 16B can be progressed efficiently. Therefore, it is possible to curb the oxidation facility cost and the running cost without enlarging the oxidation basin 21. Furthermore, the water-quality restored seawater 33 can be discharged as the one having a pH and a COD at dischargeable seawater levels.

[61] Furthermore, the concentration of sulfurous acid in the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is detected by the detector 35 provided on the sulfur-content absorbing seawater discharge line L3. Moreover, the alkaline equivalent of the first dilution seawater 15B is calculated based on an amount of total inorganic carbon and a pH that are detected by the detector 36 provided on the first dilution seawater supply line L4.

[62] A relation of an acid equivalent of the pre- dilution diluted seawater 16B in the oxidation-basin-inlet dilution mixing basin 21A and a seawater property of each of the sulfur-content absorbing seawater 16C in the oxidation-basin-outlet dilution mixing basin 21C and the water-quality restored seawater 33 is concretely explained below.

[63] Figs. 4 to 7 show relations among pH, COD, SO2 partial pressure, and total carbonic acid concentration of each of the pre-dilution diluted seawater 16B, the oxidation-basin-outlet sulfur-content absorbing seawater 16C, and the water-quality restored seawater 33 after being diluted with the second dilution seawater 15C when a ratio of an acid equivalent derived from the absorbed sulfur content in the pre-dilution diluted seawater 16B to an alkaline equivalent of the seawater ("an acid equivalent derived from the absorbed sulfur content in the pre- dilution diluted seawater 16B"/"an alkaline equivalent of the seawater") is changed by changing an amount of the first dilution seawater 15B with a total amount of a desulfurization amount in the flue-gas desulfurization absorber 20, the absorption seawater 15A, the first dilution seawater 15B, and the second dilution seawater 15C kept constant.

[64] Table 1 shows flow condition, water quality of the water-quality restored seawater 33, and maximum SO2 partial pressure of the oxidation basin in practical examples 1 to 12. A total amount of seawater used in the desulfurization/oxidation apparatus shall be 70,000 m3/hr, and other facility conditions are as follows:
Desulfurization amount in absorber: 66 kgmol/hr Alkaline equivalent of seawater: 2.4 meq/L Area of oxidation basin: 2,800 m2
Ventilation volume in oxidation basin: 90,000 m3/hr Seawater temperature:
42 degrees C (summertime)
30 degrees C (wintertime)

Furthermore, discharge standards of the water-quality restored seawater 33 are as follows. pH: 6 to 9 COD: 5 mg/L or less

Moreover, an upper limit of SO2 partial pressure in the oxidation basin is, as an odor-insensitive upper limit, as follows.

SO2 partial pressure in oxidation basin: 1 ppm or less

[65]
Table

[66] Fig. 4 is a graph showing a relation between ratio of an acid equivalent derived from the absorbed sulfur content in the oxidation-basin-inlet sulfur-content absorbing seawater 16B to an alkaline equivalent of the seawater (acid-alkaline equivalent ratio) and pH of each of the oxidation-basin-inlet sulfur-content absorbing seawater 16B, the oxidation-basin-outlet sulfur-content absorbing seawater 16C, and the water-quality restored seawater 33 after being diluted with the second dilution seawater.

Fig. 5 is a graph showing a relation between ratio of an acid equivalent derived from the absorbed sulfur content in the oxidation-basin-inlet sulfur-content absorbing seawater 16B to an alkaline equivalent of the seawater (acid-alkaline equivalent ratio) and COD concentration of each of the oxidation-basin-inlet sulfur-content absorbing seawater 16B, the oxidation-basin-outlet sulfur-content absorbing seawater 16C, and the water-quality restored seawater 33 after being diluted with the second dilution seawater.

Fig. 6 is a graph showing a relation between ratio of an acid equivalent derived from the absorbed sulfur content in the oxidation-basin-inlet sulfur-content absorbing seawater 16B to an alkaline equivalent of the seawater (acid-alkaline equivalent ratio) and maximum SO2 partial pressure of the sulfur-content absorbing seawater 28 in the oxidation basin.

Fig. 7 is a graph showing a relation between ratio of an acid equivalent derived from the absorbed sulfur content in the oxidation-basin-inlet sulfur-content absorbing seawater 16B to an alkaline equivalent of the seawater (acid-alkaline equivalent ratio) and total carbonic acid concentration of each of the oxidation-basin-inlet sulfur- content absorbing seawater 16B, the oxidation-basin-outlet sulfur-content absorbing seawater 16C, and the water- quality restored seawater 33 after being diluted with the second dilution seawater.

[67] Incidentally, in the present invention, the acid equivalent derived from the absorbed sulfur content in the pre-dilution diluted seawater 16B means, as described above, the maximum concentration of hydrogen ion (H+) produced when sulfurous acid and sulfuric acid that are produced by the absorption of the sulfur content contained in the flue gas 13 in the flue-gas desulfurization absorber 20 are completely dissociated.

[68] Furthermore, the total carbonic acid concentration means the sum of carbonic acid (H2CO3) , hydrogen carbonate ion (HCO3~) , and carbonate ion (CO33-) .

[69] Unless the ratio of the acid equivalent derived from the absorbed sulfur content in the pre-dilution diluted seawater 16B to the alkaline equivalent of the seawater ("the acid equivalent derived from the absorbed sulfur content in the pre-dilution diluted seawater 16B"/"the alkaline equivalent of the seawater") is about 1.2 or less, as shown in Fig. 3, the oxidation reaction rate of hydrogen sulfite ion (HSO3~) in the oxidation basin is decreased due to the decrease in- pH of the sulfur- content absorbing seawater in the oxidation basin as shown in Fig. 4. Thus, as shown in Fig. 5, the COD concentration in the water-quality restored seawater 33 increases, and exceeds the discharge standard (the COD concentration of 5 mg/L or less). In addition, as shown in Fig. 6, the maximum SO2 partial pressure of the sulfur-content absorbing seawater 28 in the oxidation basin undesirably exceeds a reference value (1 ppm).

[70] Furthermore, by increasing the ratio of the acid equivalent derived from the absorbed sulfur content in the pre-dilution diluted seawater 16B to the alkaline equivalent of the seawater ("the acid equivalent derived from the absorbed sulfur content in the pre-dilution diluted seawater 16B"/"the alkaline equivalent of the seawater"), as shown in Fig. 7, the total carbonic acid concentration in the oxidation-basin-outlet sulfur-content absorbing seawater 16C can be reduced efficiently because of the enhancement of the decarbonation rate. Therefore, effects of the pH enhancement as shown in Fig. 4 can be obtained efficiently. When the ratio of the acid equivalent derived from the absorbed sulfur content in the pre-dilution diluted seawater 16B to the alkaline equivalent of the seawater is about 0.83 or more, a pH of the water-quality restored seawater 33 after being diluted with the second dilution seawater 15C becomes 6.0 or more (including a measurement error of 0.15). Therefore, it is possible to meet a discharge reference value of pH (6 to 9). [0071] Consequently, as shown in Figs. 4 to 7, in consideration of the pH, COD, and SO2 partial pressure of the water-quality restored seawater 33 after being diluted with the second dilution seawater 15C, it is preferable that when the sulfur-content absorbing seawater 16A is diluted with the first dilution seawater 15B, an amount of the first dilution seawater 15B to be mixed into the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber on the side of the inlet of the oxidation basin 21 to dilute the sulfur-content absorbing seawater 16A is adjusted so that the ratio of the acid equivalent derived from the absorbed sulfur content in the pre-dilution diluted seawater 16B in the inlet of the oxidation basin 21 to the alkaline equivalent of the seawater ("the acid equivalent derived from the absorbed sulfur content in the pre-dilution diluted seawater 16B"/"the alkaline equivalent of the seawater") can be 0.83 or more but not exceeding 1.2, and the rest of the seawater 15 is used as the second dilution seawater 15C on the side of the outlet of the oxidation basin 21. Furthermore, it is preferable that an amount of the first dilution seawater 15B used for the dilution of the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber on the side of the inlet of the oxidation basin 21 is adjusted so that the ratio of the acid equivalent derived from the absorbed sulfur content in the pre- dilution diluted seawater 16B to the alkaline equivalent of the seawater ("the acid equivalent derived from the absorbed sulfur content in the pre-dilution diluted seawater 16B"/"the alkaline equivalent of the seawater") can be 0.9 or more but not exceeding 1.1.

[72] Furthermore, out of the seawater 15, a portion of the seawater 15 is used as the absorption seawater 15A used for the seawater desulfurization in the flue-gas desulfurization absorber 20 and the first dilution seawater 15B used for the dilution of the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber, and the rest of the seawater 15 is used as the second dilution seawater 15C for the dilution of the oxidation-basin-outlet sulfur-content absorbing seawater 16C.
[73] In this manner, an amount of the absorption seawater 15A used for the desulfurization in the flue-gas desulfurization absorber 20, an amount of the first dilution seawater 15B used for the dilution of the sulfur- content absorbing seawater 16A discharged from the flue-gas desulfurization absorber, and an amount of the second dilution seawater 15C used for the dilution of the oxidation-basin-outlet sulfur-content absorbing seawater 16C are adjusted appropriately. Therefore, SO2 re-emission from the oxidation basin 21 can be prevented, and an amount of the pre-dilution diluted seawater 16B in the oxidation basin 21 can be minimized without using an excessive amount of the first dilution seawater 15B, and also decreases in oxidation reaction rate of sulfurous acid and decarbonation rate can be prevented. Consequently, the oxidation facility cost and the running cost can be curbed by the reduction in size of the oxidation basin 21 while maintaining the pH and the COD of the water-quality restored seawater at dischargeable seawater levels. [0074] Incidentally, properties of the seawater (for example, a temperature, the alkalinity, a pH, and an oxidation rate of sulfurous acid) vary seasonally. Furthermore, specifications of the oxidation basin (for example, a residence time and a ventilation volume) need to be changed depending on effluent standards, and the effluent standards conform to standards of a region where the facility is installed. Therefore, optimum specifications of the oxidation basin cannot be determined just based on the properties of the seawater 15 and the properties of the flue gas 13 only. However, a means for minimizing the facility cost of the oxidation equipment (for example, a size of the oxidation basin, a ventilation volume) and the running cost (for example, a ventilation volume) through an effective water-quality restoration (for example, oxidation, decarbonation) depends on a pH setting of the sulfur-content absorbing seawater 28 in the oxidation basin. Therefore, an acid-alkaline equivalent ratio of the pre-dilution diluted seawater 16B determining a pH of the sulfur-content absorbing seawater 28 in the oxidation basin is still preferably within the range mentioned above.

[75] Therefore, the seawater desulfurization/oxidation treatment apparatus 10 according to the present embodiment includes the flue-gas desulfurization absorber 20 in which the flue gas 13 is brought into contact with the absorption seawater 15A to reduce sulfur oxide contained in the flue gas 13, and the absorption seawater 15A is recovered as the sulfur-content absorbing seawater 16A containing sulfurous acid; the oxidation basin 21 in which a sulfur content contained in the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is oxidized and decarbonated to restore a water quality of the sulfur-content absorbing seawater 16A; the seawater supply line LI through which the seawater 15 is supplied as the absorption seawater 15A to the flue-gas desulfurization absorber 20; the oxidation-basin-inlet dilution mixing basin 21A that is provided on the side of the inlet of the oxidation basin 21, and a portion of the seawater 15 as the first dilution seawater 15B is mixed with the sulfur- content absorbing seawater 16A discharged from the flue-gas desulfurization absorber therein; the oxidation-basin- outlet dilution mixing basin 21C that is provided on the side of the outlet of the oxidation basin 21, and a portion of the seawater 15 as the second dilution seawater 15C is mixed with the water-quality restored oxidation-basin- outlet sulfur-content absorbing seawater 16C therein; the sulfur-content absorbing seawater discharge line L3 through which the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is discharged into the oxidation-basin-inlet dilution mixing basin 21A; the first dilution seawater supply line L4 through which the first dilution seawater 15B is supplied to the oxidation-basin-inlet dilution mixing basin 21A; the second dilution seawater supply line L5 through which the second dilution seawater 15C is supplied to the oxiciation-basin- outlet dilution mixing basin 21C; and the seawater discharge line L6 through which the water-quality restored seawater 33 is discharged into the sea 25. Therefore, the first dilution seawater 15B is supplied through the first dilution seawater supply line L4, and previously mixed into the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber, so that the sulfur- content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is diluted, resulting in a decrease in ratio of an acid equivalent derived from the absorbed sulfur content in the pre-dilution diluted seawater 16B to an alkaline equivalent of the seawater and an increase in pH of the pre-dilution diluted seawater 16B, and thus it is possible to enhance the oxidation reaction rate. Furthermore, SO2 partial pressure is reduced, so that the SO2 re-emission can be prevented, and also generation of a bad odor around the oxidation basin 21 can be prevented. Thus, it is possible to secure worker's safety.

[76] Moreover, the second dilution seawater 15C is supplied through the second dilution seawater supply line L5 into the oxidation-basin-outlet sulfur-content absorbing seawater 16C, resulting in an increase in pH and a decrease in COD of the oxidation-basin-outlet sulfur-content absorbing seawater 16C, and thus it is possible to shorten a residence time of the pre-dilution diluted seawater 16B in the oxidation basin 21.

[77] Furthermore, an amount of the first dilution seawater 15B to be mixed into the sulfur-content absorbing seawater 16A discharged from the flue-gas desulfurization absorber is adjusted so that a ratio of an acid equivalent derived from the absorbed sulfur content in the pre- dilution diluted seawater 16B to an alkaline equivalent of the seawater can be 0.83 or more but not exceeding 1.2, so that an amount of the pre-dilution diluted seawater 16B in the oxidation basin 21 can be minimized, and the oxidation reaction rate of sulfurous acid and the decarbonation rate can be enhanced. Consequently, it is possible to curb the oxidation facility cost and the running cost without enlarging the oxidation basin 21. Furthermore, the water- quality restored seawater 33 discharged from the oxidation- basin-outlet dilution mixing basin 21C can be discharged as the one having a pH and a COD at dischargeable seawater levels.

[78] In this manner, in the power generating system 40 employing the seawater desulfurization/oxidation treatment apparatus 10 according to the present embodiment, SO2 re- emission from the oxidation basin 21 is prevented, and thus it is possible to secure the worker's safety. In addition, a seawater effluent can be discharged into the ocean or reused, while meeting the effluent standard of pH, and also costs can be reduced by the reduction of the oxidation facility cost and the running cost.

[79] Furthermore, in the power generating system 40 employing the seawater desulfurization/oxidation treatment apparatus 10 according to the present embodiment, the flue- gas denitration apparatus 47 is provided on the back stream side of the boiler 12 in consideration of the nitrogen oxide (NOx) emission control so that the flue gas 13 from which nitrogen oxide is previously reduced is fed to the flue-gas desulfurization absorber 20. However, the present invention is not limited to this configuration. If there is no particular regulation of nitrogen oxide (NOx) emission control or the like, it can be configured that the flue-gas denitration apparatus 47 is not provided, and the flue gas 13 emitted from the boiler 12 is fed to the flue- gas desulfurization absorber 20 without being subjected to denitration.

[80] Moreover, the seawater desulfurization/oxidation treatment apparatus 10 according to the present embodiment can be used for the removal of a sulfur content contained in sulfur-content absorbing solution that is produced in seawater desulfurization of sulfur oxides contained in flue gases emitted from a factory in any industries, a power plant such as a large or medium-scale thermal power plant, an electric industrial large boiler, a general industrial boiler, or the like.

INDUSTRIAL APPLICABILITY

[81] As described above, the seawater desulfurization/oxidation treatment apparatus according to the present invention makes possible to reduce the oxidation facility cost and the running cost in such a manner that sulfur-content absorbing seawater produced in seawater desulfurization is previously diluted by being mixed with a moderate amount of seawater in the inlet of the oxidation basin, and the sulfur-content absorbing seawater produced in seawater desulfurization is subjected to oxidation of a sulfur content and decarbonation to adjust a pH and a COD. Therefore, the seawater desulfurization/oxidation treatment apparatus according to the present invention is suitable for a seawater desulfurization/oxidation treatment apparatus capable of treating seawater used in seawater desulfurization to be discharged into the ocean.

EXPLANATIONS OF LETTERS OR NUMERALS

[82] 10 Seawater desulfurization/oxidation treatment apparatus

11 Air
12 Boiler
13 Flue gas
14 Dust collecting apparatus
15 Seawater
15A Absorption seawater
15B First dilution seawater
15C Second dilution seawater
16A Sulfur-content absorbing seawater discharged from flue-gas desulfurization absorber
16B Oxidation-basin-inlet sulfur-content absorbing seawater (pre-dilution diluted seawater)
16C Oxidation-basin-outlet sulfur-content absorbing seawater
20 Flue-gas desulfurization absorber
21 Oxidation basin
21A Oxidation-basin-inlet dilution mixing basin
21C Oxidation-basin-outlet dilution mixing basin
22 Induction fan
23, 24 Pump
25 Sea
26 Purged gas
27 Stack
28 Sulfur-content absorbing seawater in oxidation basin
29 Oxidation air blower
30 Air
31 Diffuser tube
32 Nozzle
33 Water-quality restored seawater
34 Flow adjuster
35 Detector
36 Detector
40 Power generating system
41 Air preheater (AH)
42 Steam
43 Power generator
44 Steam turbine
45 Water
4 6 Condenser
47 Flue-gas denitration apparatus
48 Forced draft fan
49 Heat exchanger
LI Seawater supply line
L2 Purged-gas discharge line
L3 Sulfur-content absorbing seawater discharge line
L4 First dilution seawater supply line
L5 Second dilution seawater supply line
L6 Seawater discharge line

CLAIMS

1. A seawater desulfurization/oxidation treatment apparatus comprising:

a flue-gas desulfurization absorber in which a flue gas is brought into contact with seawater to reduce sulfur oxide contained in the flue gas, and the seawater is collected as sulfur-content absorbing seawater containing sulfurous acid;

an oxidation basin in which a sulfur content contained in the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber is oxidized and decarbonated to restore a water quality of the sulfur- content absorbing seawater;

a seawater supply line through which the seawater is supplied as absorption seawater to the flue-gas desulfurization absorber;

an oxidation-basin-inlet dilution mixing basin that is provided on the side of an inlet of the oxidation basin, and mixes a portion of the seawater as first dilution seawater with the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber therein;

an oxidation-basin-outlet dilution mixing basin that is provided on the side of an outlet of the oxidation basin, and mixes a portion of the seawater as second dilution seawater with water-quality restored oxidation-basin-outlet sulfur-content absorbing seawater therein;

a sulfur-content absorbing seawater discharge line through which the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber is discharged into the oxidation-basin-inlet dilution mixing basin;

a first dilution seawater supply line through which the first dilution seawater is supplied to the oxidation- basin-inlet dilution mixing basin;

a second dilution seawater supply line through which the second dilution seawater is supplied to the oxidation- basin-outlet dilution mixing basin; and

a discharge line through which water-quality restored seawater that the oxidation-basin-outlet sulfur-content absorbing seawater is diluted with the second dilution seawater is discharged into sea.
2. The seawater desulfurization/oxidation treatment apparatus according to claim 1, wherein the first dilution seawater is supplied so that just after the first dilution seawater is mixed with the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber, a ratio of an acid equivalent derived from absorbed sulfur content to an alkaline equivalent of the seawater is 0.83 or more but not exceeding 1.2.

3. The seawater desulfurization/oxidation treatment apparatus according to claim 1 or 2, further comprising a detector for detecting a concentration of hydrogen sulfite ion in the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber, the detector being provided on the sulfur-content absorbing seawater discharge line.

4. The seawater desulfurization/oxidation treatment apparatus according to any one of claims 1 to 3, further comprising a detector for detecting an alkaline equivalent of the first dilution seawater, the detector being provided on the first dilution seawater supply line.

5. The seawater desulfurization/oxidation treatment apparatus according to any one of claims 1 to 4, wherein the seawater is an effluent discharged from a condenser.

6. A power generating system comprising: a boiler;

a steam turbine that uses a flue gas emitted from the boiler as a heat source for generation of steam, and drives a power generator with generated steam;

a condenser that collects water condensed in the steam turbine, and circulates the water;

a flue-gas denitration apparatus that denitrates the flue gas emitted from the boiler;

a dust collecting apparatus that reduces ash dust contained in the flue gas;

the seawater desulfurization/oxidation treatment apparatus according to any one of claims 1 to 5; and

a stack through which a purged gas that the flue gas is desulfurized in the seawater desulfurization/oxidation treatment apparatus is released into outside.

7. A method of treating desulfurization seawater that washes a flue gas by bringing a sulfur content in the flue gas into contact with seawater, oxidizes and decarbonates sulfurous acid in sulfur-content absorbing seawater absorbing the sulfur content after washing, and discharges the sulfur-content absorbing seawater after restoring a water quality of the desulfurization seawater, wherein

after a portion of the seawater as first dilution seawater is mixed into the sulfur-content absorbing seawater in an oxidation-basin-inlet dilution mixing basin, oxidation-basin-inlet sulfur-content absorbing seawater is supplied into an oxidation basin to oxidize sulfurous acid in the oxidation-basin-inlet sulfur-content absorbing seawater, and the oxidation-basin-inlet sulfur-content absorbing seawater is decarbonated, and

after oxidation-basin—outlet sulfur-content absorbing seawater discharged from the oxidation basin after

oxidation/decarbonation processes is fed to an oxidation- basin-outlet dilution mixing basin, and a portion of the seawater is mixed as second dilution seawater into the oxidation-basin-outlet sulfur-content absorbing seawater in the oxidation-basin-outlet dilution mixing basin, the oxidation-basin-outlet sulfur-content absorbing seawater is discharged.

8. The method of treating desulfurization seawater according to claim 7, wherein the first dilution seawater is supplied so that just after the first dilution seawater is mixed with the sulfur-content absorbing seawater discharged from the flue-gas desulfurization absorber, a ratio of an acid equivalent derived from absorbed sulfur content to an alkaline equivalent of the seawater is 0.83 or more but not exceeding 1.2.

9. The method of treating desulfurization seawater according to claim 7 or 8, wherein the seawater is an effluent discharged from a condenser.