DESCRIPTION

SEAWATER FLUE-GAS DESULFURIZATION APPARATUS AND METHOD OF TREATING DESULFURIZATION SEAWATER

TECHNICAL FIELD

[1] The present invention relates to a seawater flue- gas desulfurization apparatus that reduces a sulfur content such as sulfur oxide in a flue gas emitted from an industrial combustion facility with the use of seawater, a seawater flue-gas desulfurization system, and a method of treating desulfurization seawater.

BACKGROUND ART

[2] In recent years, the number of thermal power plants equipped with a seawater flue-gas desulfurization apparatus is increasing. Seawater desulfurization that is desulfurization using seawater as an absorbent has attracted attention, for example, from the following standpoints: most of power plants are built on coastal sites because they require a large amount of cooling water, and costs of facilities for a desulfurization process can be kept lower than that of limestone-gypsum method.

[3] Generally, in a thermal power plant employing the seawater desulfurization, a large amount of seawater is used as cooling water in a condenser of a boiler. Therefore, a portion of heated seawater effluents discharged from the condenser is supplied to a seawater flue-gas desulfurization apparatus so that the seawater flue-gas desulfurization apparatus reduces SO2 in flue gases with the use of the portion of seawater effluents as an absorbent for seawater flue-gas desulfurization.

[4] FIG. 9 is a flow diagram of an example of a thermal power generation system including a conventional seawater flue-gas desulfurization apparatus. As shown in FIG. 9, a thermal power generation system 100 employing the conventional seawater flue-gas desulfurization apparatus is composed of a boiler 102 that causes a burner (not shown) to burn fossil fuel with preheated air 101; a dust collecting apparatus 104 that reduces ash dust in a flue gas 103 emitted from boiler 102; and a seawater flue-gas desulfurization apparatus 107A that reduces a sulfur content in the flue gas 103 by the contact with seawater absorbent 105, and performs a water-quality restoring treatment of the sulfur-content absorbent seawater 106 containing the sulfur content in a high concentration, which is produced by the desulfurization. The seawater flue-gas desulfurization apparatus 107A is composed of a flue-gas desulfurization absorber 108 that recovers sulfurous acid (H2SO3) from SO2 in the flue gas 103 and an oxidation tank 109 that performs a water-quality restoring treatment of the sulfur-content absorbent seawater 106 containing the sulfur content in the high concentration, which is discharged from the flue-gas desulfurization absorber 108 (Patent documents 1, 2).

[5] The flue gas 103 produced by the combustion of fossil fuel in the boiler 102 is fed to a flue-gas denitration apparatus (not shown), and denitrated by the flue-gas denitration apparatus. After that, the flue gas 103 is fed to the dust collecting apparatus 104, and ash dust in the flue gas 103 is reduced by the dust collecting apparatus 104. Then, the flue gas 103 from which the ash dust is reduced by the dust collecting apparatus 104 is supplied into the flue-gas desulfurization absorber 108 of the seawater flue-gas desulfurization apparatus 107A by an induced draft fan 110.

[6] In the flue-gas desulfurization absorber 108, the
sulfur content in the flue gas 103 is reduced by the contact with the absorbent seawater 105A as a portion of the seawater 105 pumped from sea 111. Namely, in the flue gas 103 produced by the combustion of fossil fuel, the sulfur content that is sulfur oxide (SOx) in the form of SO2 or the like is contained. In the seawater desulfurization, in the flue-gas desulfurization absorber 108, the flue gas 103 and the absorbent seawater 105A supplied through a seawater supply line 112 are brought into gas-liquid contact, and thereby absorbing SO2 in the flue gas 103 into the absorbent seawater 105A. Then, a purged gas 113 that the flue gas 103 is desulfurized in the flue-gas desulfurization absorber 108 is released into the atmosphere from a stack 115 through a purged-gas discharge passageway 114. In the flue-gas desulfurization absorber 108, a reaction as indicated by the following formula is caused by the contact with the absorbent seawater 105A and the flue gas 103.

SO2 (g)+H2O H2SO3 (I)→ HSO; +H+ (D

[7] →By the gas-liquid contact with the absorbent seawater 105A and the flue gas 103, SO2 in the flue gas 103 is absorbed into the absorbent seawater 105A, and H2S03 is produced, and then H2S03 is dissociated in the absorbent seawater 105A. Therefore, the absorbent seawater 105A after the gas-liquid contact with the flue gas 103 increases in concentration of HS03~, and also decreases in pH because H+ is dissociated. The sulfur-content absorbent seawater 106 produced by the absorption of a large amount of the sulfur content has a pH of about 3 to 6.

[8] Before the sulfur-content absorbent seawater 106 discharged from the flue-gas desulfurization absorber 108 is discharged into the sea 111 or reused, the pH of the
sulfur-content absorbent seawater 106 needs to be increased to 6.0 or more. Therefore, in the oxidation basin 109, the sulfur-content absorbent seawater 106 containing the sulfur content is mixed with a portion of the seawater 105 supplied through a secondary seawater supply line 116 as dilution seawater 105B. At the same time, an oxidation air blower 117 supplies air 118 into the oxidation basin 109 through a nozzle 120 of a diffuser tube 119 to bring the air 118 into gas-liquid contact with the sulfur-content absorbent seawater, and thereby causing reactions as indicated by the following formula. After that, the rest of the seawater 105 is supplied into the oxidation basin 109 through a tertiary seawater supply line 121, and mixed/diluted as dilution seawater 105C with the sulfur- content absorbent seawater so as to decrease the concentration of sulfurous acid as a COD source and increase in dissolved oxygen level and pH thereby restoring a water quality of the sulfur-content absorbent seawater, and then the sulfur-content absorbent seawater is discharged as water-quality restored seawater 122 into the sea 111.

O2(g)→O2(I) (II)
HSO3 +1/202 (I)→ +SO42' +H+ (III)
HCO3 + H+→ H2CO3(7)→ CO2(g)↑ t +H2O (IV)
CO3~ + 2H+ H2CO3 (I)→ CO2 (g)↑ t +H2O (V)

[9] In this manner, in the thermal power system 100 employing the conventional seawater flue-gas desulfurization apparatus, to prevent SO2 from being emitted in the oxidation basin 109 and to increase the pH of the sulfur-content absorbent seawater 106, the sulfur- content absorbent seawater 106 is mixed/diluted with the seawater 105 discharged from a condenser (not shown) in the oxidation basin 109, and subjected to oxidation and aeration in the oxidation basin 109 thereby oxidizing sulfurous acid to render the sulfur content harmless and increasing the dissolved oxygen level, and then subjected to decarbonation thereby increasing the pH of the sulfur- content absorbent seawater 106 so that the pH of the water- quality restored seawater 122 can meet a discharge standard (a pH of 6.0 or more in general), and the water-quality restored seawater 122 is discharged (Patent documents 1, 2).

[10] Furthermore, FIG. 10 shows another configuration of the conventional seawater flue-gas desulfurization apparatus. FIG. 10 is a diagram schematically showing another configuration of the seawater flue-gas desulfurization apparatus applied to a conventional seawater desulfurization system. As shown in FIG. 10, another conventional seawater flue-gas desulfurization apparatus 107B is composed of a flue-gas desulfurization absorber 131 in which SO2 in the flue gas 103 is converted to sulfurous acid (H2SO3) by desulfurization reaction; a dilution mixing tank 132 that is provided on the underside of the flue-gas desulfurization absorber 131, and sulfur- content absorbent seawater 106A containing a sulfur content is diluted/mixed with the dilution seawater 105B therein; and an oxidation basin 133 that is provided on the downstream side of the dilution mixing tank 132, and a water-quality restoring treatment of sulfur-content absorbent seawater 106 is performed therein (Patent document 3).

[11] In the seawater flue-gas desulfurization apparatus 107B, the absorbent seawater 105A as a portion of the seawater 105 supplied through the seawater supply line 112 is brought into gas-liquid contact with the flue gas 103 in the flue-gas desulfurization absorber 131, and thereby absorbing SO2 in the flue gas 103 into the absorbent seawater 105A. Then, the sulfur-content absorbent seawater 106A absorbing the sulfur content thereinto in the flue-gas desulfurization absorber 131 is mixed with the dilution seawater 105B supplied into the dilution mixing tank 132 provided on the underside of the flue-gas desulfurization absorber 131. Then, sulfur- content absorbent seawater 106B mixed/diluted with the dilution seawater 105B is fed to the oxidation basin 133 provided on the downstream side of the dilution mixing tank 132, and the oxidation air blower 117 supplies the air 118 through the oxidation air nozzle 120 of the diffuser tube 119. After a water quality of the sulfur-content absorbent seawater 106B is restored, the water-quality restored sulfur-content absorbent seawater 106B is discharged. [Prior art references] [Patent documents]

[12] 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

[13] However, the conventional seawater flue-gas desulfurization apparatus 107B has the following problem: at the dilution mixing tank 132, due to entrainment of bubbles by the sulfur-content absorbent seawater 106A, bubbles, each containing a gas with a high concentration of SO2, are entrained into the dilution seawater 105B in the dilution mixing tank 132, so that in a state where the sulfur-content absorbent seawater 106B mixed with the dilution seawater 105B contains the bubbles, if the sulfur- content absorbent seawater 106B flows into the oxidation basin 133 open to outdoors, SO2 could be emitted in the oxidation basin 133, i.e., a pungent odor could be emitted.

[14] Furthermore, in such an apparatus as the conventional seawater flue-gas desulfurization apparatus 107B that the flue-gas desulfurization absorber 131 is provided on the upper side of the dilution mixing tank 132 as a passageway of effluents from a condenser (not shown) where the seawater 105 used for the dilution flows, and performs seawater desulfurization and an oxidation treatment of seawater used in the seawater desulfurization in a unified manner, there is a problem that the sulfur- content absorbent seawater 106A and the seawater 105B, which are supplied into the dilution mixing tank 132, may not be sufficiently mixed with each other due to a temperature difference between the two.

[15] In addition, there is a further problem that the sulfur-content absorbent seawater 106A flowing down through the flue-gas desulfurization absorber 131 is fallen into a passageway of the dilution seawater, so that the bubbles containing SO2 are accumulated in the flue-gas desulfurization absorber 131, and the bubbles containing the high concentration of SO2 flow out of the flue-gas desulfurization absorber 131.

[16] In view of the above problems, an object of the present invention is to prevent SO2 collected into seawater used in desulfurization from being emitted when the seawater is subjected to an oxidation treatment and to provide safe and highly-reliable seawater flue-gas desulfurization apparatus and method of treating desulfurization seawater.

MEANS FOR SOLVING PROBLEM

[17] According to an aspect of the present invention, a seawater flue-gas desulfurization apparatus includes: a flue-gas desulfurization absorber in which a sulfur content in a flue gas is brought into contact with seawater thereby purifying the flue gas; a dilution mixing basin that is integrally provided on underside of the flue-gas desulfurization absorber, and in which sulfur-content absorbent seawater produced by seawater desulfurization in which the sulfur content in the flue gas is reduced by the contact with the seawater in the flue-gas desulfurization absorber is mixed/diluted with seawater fed into a main body thereof; and a gas retaining unit that includes a cover portion and a first weir, the cover portion being provided on lower-end side of a sidewall of the flue-gas desulfurization absorber so as to cover the dilution mixing tank, the first weir being hung from side of a rear surface of the cover portion, and an end of the first weir being submerged under a surface of seawater in the dilution mixing tank.

[18] Advantageously, in the seawater flue-gas desulfurization apparatus, a length LI from the sidewall of the flue-gas desulfurization absorber to an inner wall of the first weir meets either one of following in equations (1) and (2) and following equation (3):

dG1 < τ 1,Ut(dp) (1)
Cc > C0 exp(-6Kg /dpTt) (2)
T1= τ 1/UL (3)

in the above in equations and equation, dG1 denotes a height of an opening from the first weir at an outlet of the gas retaining unit to a bottom of the dilution mixing tank; T1 denotes a retention time of seawater in the gas retaining unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in seawater; Cc denotes an SO2 environmental standard concentration; Co denotes an SO2 concentration of the flue gas at an inlet of the flue-gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of SO2 gases at a gas- liquid interface of bubbles; dp denotes a bubble diameter; and UL denotes an outlet flow rate at the bottom of the dilution mixing tank.

[19] A terminal rise velocity Ut of a single bubble in a static fluid is obtained by the following Stokes' equation (4). The bigger the bubble, the higher the terminal rise velocity.

Ut=gxdp2x(pL-pG)/18 μ (4)

Incidentally, g denotes a gravitational acceleration; dp denotes a bubble diameter; pL denotes a seawater density; pG denotes a gas density; and n denotes a seawater viscosity.

[20] Although the bubble does not have a spherical shape by friction with the fluid if the bubble diameter exceeds 1 mm, and the rise velocity of the bubble swarm is not strictly consistent because the bubble swarm differs in behavior from a single bubble, the bubble diameter in the seawater is generally about 0.5 to 1.0 mm, and it is rare that the bubble diameter exceeds 5.0 mm at most, and the terminal rise velocity Ut of the bubble swarm in the seawater is 200 to 300 mm/s, and about 400 mm/s at a maximum.

[21] Advantageously, in the seawater flue-gas desulfurization apparatus, a second weir is provided on the bottom of the dilution mixing tank.

[22] Advantageously, in the seawater flue-gas desulfurization apparatus, a length L2 from the sidewall of the flue-gas desulfurization absorber to an inner wall of the second weir meets either one of following in equations (5) and (6) and following equation (7):

D C0 exp(~6Kg / dpT2) (6)
τ 2=L2/(ULxD/dG2) (7)

in the above in equations and equation, D denotes a liquid depth of seawater in the dilution mixing tank; x2 denotes a retention time of seawater in the gas retaining unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in seawater; Cc denotes an SO2 environmental standard concentration; C0 denotes an SO2 concentration of the flue gas at the inlet of the flue-gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of SO2 gases at a gas- liquid interface of bubbles; dp denotes a bubble diameter; UL denotes an outlet flow rate at the bottom of the dilution mixing tank; and dG2 denotes a liquid height between a surface of seawater and the second weir.

[23] Advantageously, in the seawater flue-gas desulfurization apparatus, a third weir is provided on inner side of the gas retaining unit.

[24] Advantageously, in the seawater flue-gas desulfurization apparatus, a length L3 from an outer wall of the third weir to an inner wall of the first weir meets either one of following in equations (8) and (9) and following equation (10).

Dg1< τ,Ut{dp) (8)
Cc > C0 exp(-6Kg / dpr2) (9)
τ 3 = L3/(UL xD/MIN(dG1, dG2)) (10)

in the above in equations and equation, dG1 denotes a height of an opening from the first weir at the outlet of the gas retaining unit to the bottom of the dilution mixing tank; T3 denotes a retention time of seawater in the gas retaining unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in seawater; Cc denotes an SO2 environmental standard concentration; C0 denotes an SO2 concentration of the flue gas at the inlet of the flue-gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of SO2 gases at a gas- liquid interface of bubbles; dp denotes a bubble diameter; UL denotes an outlet flow rate at the bottom of the dilution mixing tank; D denotes a liquid depth of seawater in the dilution mixing tank; dG2 denotes a height of an opening from the second weir at the outlet of the gas retaining unit to the bottom of the dilution mixing tank; and MIN(dG1, dG2) denotes a minimum value of dG1 and dG2.

[25] Advantageously, the seawater flue-gas desulfurization apparatus further includes: a second weir, the second weir being provided on a bottom of the dilution mixing tank; and a third weir, the third weir being provided on inner side of the gas retaining unit.

[26] Advantageously, in the seawater flue-gas desulfurization apparatus, a length L4 from an outer wall of the third weir to an inner wall of the second weir meets either one of following in equations (11) and (12) and following equation (13):

D< τ 4Ut(dp) (11)
Cc > CQ exp(-6Kg / dpt A) (12)
τ 4 = L4/(Ul *D/MN(dG1, dG2, dG3)) (13)

in the above in equations and equation, D denotes a liquid depth of seawater in the dilution mixing tank; t4 denotes a retention time of seawater in the gas retaining
unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in seawater; Cc denotes an SO2 environmental standard concentration; Co denotes an SO2 concentration of the flue gas at an inlet of the flue-gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of SO2 gases at a gas- liquid interface of bubbles; dp denotes a bubble diameter; UL denotes an outlet flow rate at the bottom of the dilution mixing tank; dG1 denotes a height of an opening from the first weir to the bottom of the dilution mixing tank; dG2 denotes a liquid height between a surface of seawater and the second weir; dG3 denotes a height of an opening from the third weir to the bottom of the dilution mixing tank; and MIN (dG1, dG2, dG3) denotes a minimum value of dG1, dG2, and dG3.

[27] Advantageously, in the seawater flue-gas desulfurization apparatus, a ventilation hole connecting a space formed between the gas retaining unit and the seawater to the flue-gas desulfurization absorber is formed on the third weir.

[28] Advantageously, in the seawater flue-gas desulfurization apparatus, the seawater is effluent discharged from a condenser.

[29] Advantageously, in the seawater flue-gas desulfurization apparatus further includes an oxidation tank that is provided on downstream of the dilution mixing tank, and in which a sulfur content in the seawater mixed with the sulfur-content absorbent seawater in the dilution mixing tank is oxidized and decarbonated, and thereby restoring a water quality of the seawater.

[30] According to another aspect of the present invention, a seawater desulfurization system includes: a boiler; a steam turbine that uses a flue gas emitted from the boiler as a heat source for generating steam to drive a power generator with generated steam; a condenser that collects water condensed in the steam turbine to circulate the water; a flue-gas denitration apparatus that denitrates the flue gas emitted from the boiler; a dust collecting apparatus that reduces ash dust in the flue gas; the seawater flue-gas desulfurization apparatus described above; and a stack from which a purged gas that the flue gas is desulfurized by the flue-gas desulfurization apparatus is emitted to the outside.
[31] According to still another aspect of the present invention, a method of treating desulfurization seawater for preventing SO2 gas contained in seawater used in desulfurization from being emitted to the outside, using the seawater flue-gas desulfurization apparatus described above.

EFFECT OF THE INVENTION

[32] According to an aspect of the present invention, a gas retaining unit is provided at a connection between a flue-gas desulfurization absorber in which a sulfur content in a flue gas is brought into contact with seawater thereby purifying the flue gas and a dilution mixing tank that is integrally provided on the underside of the flue-gas desulfurization absorber, and in which sulfur-content absorbent seawater flowing down through the flue-gas desulfurization absorber is mixed with seawater fed into a main body thereof. The gas retaining unit includes a cover portion having a certain length and a first weir. The cover portion is provided so as to cover the dilution mixing tank. The first weir is hung from the side of a rear surface of the cover portion, and an end of the first weir is submerged under the surface of seawater in the dilution mixing tank. Bubbles containing a high SO2 concentration of gas in the sulfur-content absorbent seawater, which are entrained in the seawater as the sulfur-content absorbent seawater flows down from the flue- gas desulfurization absorber, are dissipated in a space in the gas retaining unit, which is formed by the cover portion and the first weir, so that it is possible to prevent the SO2 gas from leaking to the outside.

[33] As a result, when the seawater is subjected to an oxidation treatment thereby restoring a water quality of the seawater, the seawater containing SO2 can be prevented from flowing into an oxidation tank, i.e., SO2 can be prevented from being emitted in the outdoor open oxidation tank, and thus it is possible to prevent emission of a pungent odor. Consequently, it is possible to provide a safe and highly-reliable seawater flue-gas desulfurization apparatus.

BRIEF DESCRIPTION OF DRAWINGS

[34] [FIG. 1] FIG. 1 is a schematic diagram showing a seawater flue-gas desulfurization apparatus according to a first embodiment of the present invention.

[FIG. 2] FIG. 2 is a schematic diagram schematically showing a part of the seawater flue-gas desulfurization apparatus according to the first embodiment of the present invention.

[FIG. 3] FIG. 3 is a schematic diagram schematically showing a part of a conventional seawater flue-gas desulfurization apparatus.

[FIG. 4] FIG. 4 is a schematic diagram schematically showing a part of a seawater flue-gas desulfurization apparatus according to a second embodiment of the present invention.

[FIG. 5] FIG. 5 is a schematic diagram schematically showing a part of a seawater flue-gas desulfurization apparatus according to a third embodiment of the present invention.

[FIG. 6] FIG. 6 is a partial enlarged view of the seawater flue-gas desulfurization apparatus according to the third embodiment of the present invention.

[FIG. 7] FIG. 7 is a schematic diagram schematically showing a part of a seawater flue-gas desulfurization apparatus according to a fourth embodiment of the present invention.

[FIG. 8] FIG. 8 is a conceptual diagram of a seawater desulfurization system.

[FIG. 9] FIG. 9 is a flow diagram of an example of a thermal power system including a conventional seawater flue-gas desulfurization apparatus using seawater. [FIG. 10]

FIG. 10 is a diagram schematically showing another seawater flue-gas desulfurization apparatus applied to a conventional seawater desulfurization system. BEST MODE(S)

FOR CARRYING OUT THE INVENTION

[35] Exemplary embodiments of the present invention is explained in detail below with reference to the accompanying drawings. Incidentally, the present invention is not limited to the embodiments. Furthermore, elements used in the embodiments 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.

[36] [First embodiment]

A seawater flue-gas desulfurization apparatus according to a first embodiment of the present invention is explained with reference to the accompanying drawings.
FIG. 1 is a schematic diagram showing a configuration of the seawater flue-gas desulfurization apparatus according to the first embodiment of the present invention.
FIG. 2 is a schematic diagram schematically showing a part of the configuration of the seawater flue-gas desulfurization apparatus shown in FIG. 1.

[37] As shown in FIG. 1, a first seawater flue-gas desulfurization apparatus 10-1 according to the present embodiment includes a flue-gas desulfurization absorber 13, a dilution mixing tank 16, and a gas retaining unit 20A.

In the flue-gas desulfurization absorber 13, a sulfur content in a flue gas 11 is brought into contact with absorbent seawater 12A as a portion of seawater 12, and thereby purifying the flue gas 11. The dilution mixing tank 16 is integrally provided on the underside of the flue-gas desulfurization absorber 13. Sulfur-content absorbent seawater 14A produced by seawater desulfurization in which the sulfur content in the flue gas 11 is reduced by the contact with the absorbent seawater 12A in the flue- gas desulfurization absorber 13 flows down to the dilution mixing tank 16 through the flue-gas desulfurization absorber 13, and is mixed/diluted with dilution seawater 12B fed into a main body 15 in the dilution mixing tank 16. The gas retaining unit 20A includes a cover portion 18 and a first weir 19. The cover portion 18 is provided on the lower-end side of a sidewall 17 of the flue-gas desulfurization absorber 13 to extend along a long side of the dilution mixing tank 16 so as to cover the dilution mixing tank 16. The first weir 19 is hung from the side of a rear surface of the cover portion 18, and an end 19a of the first weir 19 is submerged under the surface of seawater in the dilution mixing tank 16.

Furthermore, in the diagram, a reference numeral 16a denotes a bottom of the dilution mixing tank 16.

[38] In the present embodiment, out of the seawater 12, seawater to be fed to the flue-gas desulfurization absorber 13 and used for purification of the flue gas 11 shall be referred to as the absorbent seawater 12A, and seawater to be fed to the flue-gas desulfurization absorber 13 and used for dilution shall be referred to as the dilution seawater 12B. Furthermore, seawater that the dilution seawater 12B and the sulfur-content absorbent seawater 14A flowing down through the flue-gas desulfurization absorber 13 are mixed in the flue-gas desulfurization absorber 13 shall be referred to as sulfur-content absorbent seawater 14B.

[39] The absorbent seawater 12A used in the flue-gas desulfurization absorber 13 is a portion of the seawater 12 pumped by a pump 24 out of the seawater 12 pumped from sea 21 to a seawater supply line 23 with a pump 22. The absorbent seawater 12A is fed to the flue-gas desulfurization absorber 13. Furthermore, as the seawater 12, seawater pumped directly from the sea 21 with the pump 22 is used. However, the present invention is not limited to this. Alternatively, effluent of the seawater 12 discharged from a condenser (not shown) or the like can be used.

[40] In the flue-gas desulfurization absorber 13, the flue gas 11 is brought into gas-liquid contact with the absorbent seawater 12A, and thereby reducing the sulfur content in the flue gas 11. Namely, by bringing the flue gas 11 into gas-liquid contact with the absorbent seawater 12A in the flue-gas desulfurization absorber 13 thereby causing a reaction as indicated by the following formula, the sulfur content such as sulfur oxide (SOx) in the form of SO2 or the like in the flue gas 11 is reduced with the use of the absorbent seawater 12A.

SO2 (g) + H2O H2SO3 (7) HSO3- +H+ (I)

[41] By the seawater desulfurization, H2SO3 produced by the gas-liquid contact between the absorbent seawater 12A and the flue gas 11 is dissociated, and H+ is released into the absorbent seawater 12A. As a result, a pH of the sulfur-content absorbent seawater 14A is decreased, and a large amount of the sulfur content is absorbed into the sulfur-content absorbent seawater 14A flowing down through the flue-gas desulfurization absorber 13. At this time, the pH of the sulfur-content absorbent seawater 14A flowing down through the flue-gas desulfurization absorber 13 is, for example, about 3. Then, the sulfur-content absorbent seawater 14A flows down through the flue-gas desulfurization absorber 13, and is impounded in the dilution mixing tank 16 integrally-provided on the underside of the flue-gas desulfurization absorber 13. Furthermore, a purged gas 25 that the flue gas 11 is desulfurized in the flue-gas desulfurization absorber 13 is released into the atmosphere through a purged-gas discharge passageway 26.

[42] Moreover, a portion of the seawater 12 from the seawater supply line 23 is fed as the dilution seawater 12B to the dilution mixing tank 16 through a dilution seawater supply line 27. Then, in the dilution mixing tank 16, the dilution seawater 12B is mixed with the sulfur-content absorbent seawater 14A flowing down through the flue-gas desulfurization absorber 13 to dilute the sulfur-content absorbent seawater 14A. Seawater that the sulfur-content absorbent seawater 14A flowing down through the flue-gas desulfurization absorber 13 and the dilution seawater 12B are mixed shall be referred to as the sulfur-content absorbent seawater 14B. The sulfur-content absorbent seawater 14A is diluted by being mixed with the dilution seawater 12B, so that a pH of the sulfur-content absorbent seawater 14B in the dilution mixing tank 16 is increased, and thus it is possible to prevent SO2 re-emission.

[43] FIG. 2 is a schematic diagram schematically- showing a part of the configuration of the seawater flue- gas desulfurization apparatus according to the present embodiment. As shown in FIG. 2, the first seawater flue- gas desulfurization apparatus 10-1 according to the present embodiment includes the gas retaining unit 2OA including the cover portion 18 and the first weir 19. The cover portion 18 is provided on the lower-end side of the sidewall 17 of the flue-gas desulfurization absorber 13 on the downstream side of the dilution mixing tank so as to cover the dilution mixing tank 16. The first weir 19 is hung from the side of the rear surface of the cover portion 18, and the end 19a of the first weir 19 is submerged under the surface of seawater in the dilution mixing tank 16.

The first weir 19 is hung from the gas retaining unit 20A, and serves to hold a portion of the sulfur-content absorbent seawater 14B, as the mixture of the dilution seawater 12B and the sulfur-content absorbent seawater 14A flowing down through the flue-gas desulfurization absorber 13, in the flue-gas desulfurization absorber 13.

[44] As the sulfur-content absorbent seawater 14A flows down through the flue-gas desulfurization absorber 13, bubbles 28, each containing a gas with a high concentration of SO2, of the flue-gas desulfurization absorber 13 are entrained in the sulfur-content absorbent seawater 14B. The bubbles 28 containing the SO2 gas, which are entrained in the sulfur-content absorbent seawater 14B, are dissipated in a space S1 formed by the cover portion 18 and the first weir 19 of the gas retaining unit 20A. Therefore, the SO2 gas entrained in the sulfur-content absorbent seawater 14B can be retained in the space S1 formed by the cover portion 18 and the first weir 19 of the gas retaining unit 2OA.

[45] FIG. 3 is a schematic diagram schematically showing a part of the configuration of the conventional seawater flue-gas desulfurization apparatus shown in FIG. 10. In the conventional seawater flue-gas desulfurization apparatus 107B shown in FIG. 3, a wall plate of the flue- gas desulfurization absorber 131 on the downstream side of the dilution mixing tank directly extends toward the sulfur-content absorbent seawater 106B in the dilution mixing tank 132, and an end of the wall plate is submerged under the surface of seawater in the dilution mixing tank 132. Therefore, in the conventional seawater flue-gas desulfurization apparatus 107B, due to entertainment of bubbles by the sulfur-content absorbent seawater 106A flowing down through the flue-gas desulfurization absorber 131, the bubbles containing a gas with a high concentration of SO2, which are entrained in the dilution seawater 105B, come up, and may be dissipated out of the flue-gas desulfurization absorber 131, i.e., the SO2 gas may leak to the outside.

[46] On the other hand, in the present embodiment, there is provided the gas retaining unit 2OA including the cover portion 18 and the first weir 19. The cover portion 18 is provided on the lower-end side of the sidewall 17 of the flue-gas desulfurization absorber 13 so as to cover the dilution mixing tank 16. The first weir 19 is hung from the side of the rear surface of the cover portion 18, and the end of the first weir 19 is submerged under the surface of seawater in the dilution mixing tank 16. The end 19a of the first weir 19 is submerged under the surface of the seawater in the dilution mixing tank 16, i.e., partially immersed in the sulfur-content absorbent seawater 14B in the main body 15. Therefore, even when the bubbles 28 containing the gas with the high concentration of SO2 are entrained in the sulfur-content absorbent seawater 14B by the entertainment of the bubbles 28 by falling of the sulfur- content absorbent seawater 14A flowing down through the flue-gas desulfurization absorber 13, the bubbles 28 containing the SO2 gas, which are entrained in the sulfur- content absorbent seawater 14B, can be dissipated in the space S1 formed by the cover portion 18 and the first weir 19 of the gas retaining unit 20A. Therefore, the SO2 gas can be retained in the space S1 formed by the cover portion 18 and the first weir 19 of the gas retaining unit 20A, and thus it is possible to prevent outside leakage of the SO2 gas.

[47] As a result, as will be described later, when the sulfur-content absorbent seawater 14B flows into an outdoor open oxidation basin 29, it is possible to prevent the bubbles 28 containing the SO2 gas, which are entrained at the dilution mixing tank 16, from flowing into the oxidation basin 29 and prevent SO2 from being emitted in the oxidation basin 29, i.e., prevent outside leakage of SO2, and thus it is possible to prevent emission of a pungent odor.

[48] Furthermore, in the present embodiment, a length LI from the sidewall 17 of the flue-gas desulfurization absorber 13 to an inner wall 19b of the first weir 19 is set to meet either one of the following in equations (1) and (2) and the following equation (3).

dG1 < τ xUt{dp) (1)
Cc > C0xx p(-6Kg!dprx) (2)
τ 1=L\/U1 (3)

Incidentally, dG1 denotes a height of an opening from the first weir at an outlet of the gas retaining unit to the bottom of the dilution mixing tank; T1 denotes a retention time of the seawater in the gas retaining unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in the seawater; Cc denotes an SO2 environmental standard concentration; Co denotes an SO2 concentration of the flue gas at an inlet of the flue- gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of the SO2 gas at a gas-liquid interface of bubbles; dp denotes a bubble diameter; and UL denotes an outlet flow rate at the bottom of the dilution mixing tank.

[49] A terminal rise velocity Ut of a single bubble in a static fluid is obtained by the following Stokes' equation (4). The bigger the bubble, the higher the terminal rise velocity.
Ut=gxdp2x(pL-pG)/18μ (4)

Incidentally, g denotes a gravitational acceleration; dp denotes a bubble diameter; pL denotes a seawater density; pG denotes a gas density; and μ denotes a seawater viscosity.

[50] Although the bubble does not have a spherical shape by friction with the fluid if the bubble diameter exceeds 1 mm, and the rise velocity of the bubble swarm is not strictly consistent because the bubble swarm differs in behavior from a single bubble, the bubble diameter in the seawater is generally about 0.5 to 1.0 mm, and it is rare that the bubble diameter exceeds 5.0 mm at most, and the terminal rise velocity Ut of the bubble swarm in the seawater is 200 to 300 mm/s, and about 400 mm/s at a maximum.

[51] As in the present embodiment, by meeting the above in equations and equations, out of the bubbles 28 containing the SO2 gas, which are entrained in the sulfur- content absorbent seawater 14B, relatively-large bubbles having a high floating rate are dissipated in the space SI formed by the cover portion 18 and the first weir 19 of the gas retaining unit 2 OA more reliably, and the SO2 gases can be emitted in the space SI. Furthermore, the SO2 gases in relatively-small bubbles having a low floating rate, out of the bubbles 28 containing the SO2 gas, which are entrained in the sulfur-content absorbent seawater 14B, can be absorbed into the sulfur-content absorbent seawater 14B.

[52] Therefore, it is possible to prevent such a situation that the bubbles 28 containing the SO2 gas, which are entrained at the dilution mixing tank 16, flow into the oxidation basin 29 along with the sulfur-content absorbent seawater 14B, and SO2 is emitted in the oxidation basin 29, i.e., a pungent odor is emitted in the oxidation basin 29.

[53] Thus, as will be described later, gases produced when a water quality of the sulfur-content absorbent seawater 14B is restored in the oxidation basin 29 can be emitted in the oxidation basin 29 while meeting the SO2 environmental standard concentration.

[54] Therefore, it is possible to provide such a safe and highly-reliable seawater flue-gas desulfurization apparatus capable of preventing the bubbles 28 containing the high concentration of the SO2 gas, which are entrained at the dilution mixing tank 16, from being dissipated in the oxidation basin 29 provided on the downstream side of the dilution mixing tank 16 thereby preventing outside leakage of the SO2 gas.

[55] After the bubbles 28 in the sulfur-content absorbent seawater 14B are dissipated in the dilution mixing tank 16, the sulfur-content absorbent seawater 14B is fed to the oxidation basin 29 provided on the downstream side of the dilution mixing tank 16. Furthermore, in the present embodiment, the dilution mixing tank 16 and the oxidation basin 29 are integrally configured as one basin. However, the present invention is not limited to this configuration. Alternatively, the dilution mixing tank 16 and the oxidation basin 29 can be separate basins, and connected to each other.

[56] The oxidation basin 29 is integrally provided on the downstream side of the dilution mixing basin 16. In the oxidation basin 29, the sulfur content in the sulfur- content absorbent seawater 14B is oxidized, and the sulfur- content absorbent seawater 14B is decarbonated, and thereby restoring a water quality of the sulfur-content absorbent seawater 14B. An air supply unit 30 is provided in the oxidation basin 29. The air supply unit 30 is composed of an oxidation air blower 32 that supplies air 31; a diffuser tube 33 that feeds the air 31; and an oxidation air nozzle 34 from which the air 31 is supplied to the sulfur-content absorbent seawater 14B in the oxidation basin 29. In the oxidation basin 29, the oxidation air blower 32 blows the air 31 into the oxidation basin 29 from the oxidation air nozzle 34 through the diffuser tube 33, and the sulfur content is brought into contact with the air 31 in the oxidation basin 29, and dissolution of oxygen, an oxidation reaction of sulfurous acid, and a decarbonation reaction as indicated by the following formulae (II) to (V) are caused.

O2(g)→O2(I) (II)
HSO3~ + 1/2o2→ SO,2' +H+ (III)
HCO~ + H+→ CO2 (g) + H2O (IV)
CO2' + 2 H+ → CO2 {g) + H2O (V)

[57] Then, in the oxidation basin 29, the water quality of the sulfur-content absorbent seawater 14B is restored by the oxidation reaction of hydrogen sulfite ion (HSO3~) in the sulfur-content absorbent seawater 14B and the decarbonation reaction of bicarbonate ion (HC03~) . The sulfur-content absorbent seawater 14B after the water quality of which is restored shall be referred to as water- quality restored seawater 35.

[58] Then, the water-quality restored seawater 35 is discharged as seawater effluent into the sea 21 through a seawater discharge line 36. The water-quality restored seawater 35 can increase in pH and decrease in COD, and the water-quality restored seawater 35 having a pH, a dissolved oxygen level, and a COD at dischargeable seawater levels can be discharged.

[59] In this manner, the first seawater flue-gas desulfurization apparatus 10-1 according to the present embodiment includes the flue-gas desulfurization absorber 13 in which the flue gas 11 is purified by bringing the sulfur content in the flue gas 11 into contact with the absorbent seawater 12A; the dilution mixing basin 16 that is integrally provided on the underside of the flue-gas desulfurization absorber 13, and in which the sulfur- content absorbent seawater 14A produced by the seawater desulfurization in the flue-gas desulfurization absorber 13 is mixed/diluted with the dilution seawater 12B fed into the main body 15 thereof; and the gas retaining unit 20A including the cover portion 18, which is provided on the lower-end side of the sidewall 17 of the flue-gas desulfurization absorber 13 so as to cover the dilution mixing basin 16, and the first weir 19, which is hung from the side of the rear surface of the cover portion 18 and the end 19a of the first weir 19 is submerged under the surface of seawater in the dilution mixing basin 16. The first weir 19 is hung from the side of the rear surface of the cover portion 18, and the end 19a of the first weir 19 is submerged under the surface of seawater in the dilution mixing basin 16 to hold a portion of the sulfur-content absorbent seawater 14B in the flue-gas desulfurization absorber 13. Therefore, the bubbles 28 containing a high SO2 concentration of gas, which are entrained in the sulfur-content absorbent seawater 14B as the sulfur-content absorbent seawater 14A flows down from the flue-gas desulfurization absorber 13, are dissipated in the space SI formed by the cover portion 18 and the first weir 19 of the gas retaining unit 20A, and thus it is possible to prevent outside leakage of the SO2 gas.

[60] As a result, it is possible to provide such a safe and highly-reliable seawater flue-gas desulfurization apparatus capable of preventing the bubbles 28 containing the SO2 gas, which are entrained at the dilution mixing basin 16, from flowing into the oxidation basin 29 and being dissipated in the oxidation basin 29 when the sulfur- content absorbent seawater 14B flowing into the outdoor open oxidation basin 29 is subjected to an oxidation treatment to restore a water quality thereof, and thereby preventing outside leakage of the SO2 and also preventing emission of a pungent odor.

[61] Furthermore, in the present embodiment, there is described the seawater flue-gas desulfurization apparatus that treats seawater, which is used in seawater desulfurization in the flue-gas desulfurization absorber 13, in the oxidation basin 29. However, the present invention is not limited to this. The oxidation basin 29 can be used for reduction of the sulfur content in the sulfur-content absorbent seawater 14A produced in seawater desulfurization of sulfur oxide contained in flue gases emitted, for example, from a factory in any industries, a power plant such as a large or medium-scale thermal power plant, an electric industrial large boiler, or a general industrial boiler, and also used for desulfurization seawater.

[62] [Second embodiment]

Subsequently, a seawater flue-gas desulfurization apparatus according to a second embodiment of the present invention is explained with reference to FIG. 4.

A configuration of the seawater flue-gas desulfurization apparatus according to the second embodiment is identical to that of the seawater flue-gas desulfurization apparatus according to the first embodiment of the present invention, and a configuration diagram of the entire seawater flue-gas desulfurization apparatus according to the second embodiment is omitted. The portions identical to those in the seawater flue-gas desulfurization apparatus according to the first embodiment are denoted with the same reference numerals, and the description of those portions is omitted.

FIG. 4 is a schematic diagram schematically showing a part of the configuration of the seawater flue-gas desulfurization apparatus according to the present embodiment. As shown in FIG. 4, a second seawater flue-gas desulfurization apparatus 10-2 according to the present embodiment includes a gas retaining unit 2OB that a second weir 42 is provided on the bottom 16a of the dilution mixing basin 16 in the first seawater flue-gas desulfurization apparatus 10-1 according to the first embodiment shown in FIGS. 1 and 2.

[63] As in the present embodiment, by providing the second weir 42 on the bottom 16a of the dilution mixing basin 16, a liquid height dG2 between the surface of the sulfur-content absorbent seawater 14B and an end 42a of the second weir 42 gets smaller. Thus, a flow rate of the sulfur-content absorbent seawater 14B flowing into the oxidation basin 29 is accelerated, and the bubbles 28 in the sulfur-content absorbent seawater 14B can be concentrated in the portion between the surface of the sulfur-content absorbent seawater 14B and the end 42a of the second weir 42. As a result, the bubbles 28 can be dissipated in the space S1 formed by the cover portion 18 and the first weir 19 of the gas retaining unit 20B.

[64] Consequently, it is possible to prevent the bubbles 28 containing the SO2 gas, which are entrained at the dilution mixing basin 16, from flowing into the oxidation basin 29, and also prevent SO2 from being emitted in the oxidation basin 29, i.e., it is possible to prevent outside leakage of the SO2 gas, and thus it is possible to prevent emission of a pungent odor.

[65] Furthermore, when the sulfur-content absorbent seawater 14A, as a desulfurization falling liquid, is mixed with the dilution seawater 12B in the dilution mixing basin 16, if the sulfur-content absorbent seawater 14A simply flows into the dilution mixing basin 16, it is difficult for the sulfur-content absorbent seawater 14A and the dilution seawater 12B to be mixed with each other uniformly because a temperature of the sulfur-content absorbent seawater 14A is high due to the contact with the flue gas 11, and a temperature of the dilution seawater 12B is low. In the present embodiment, the second weir 42 is provided on the bottom 16a of the dilution mixing basin 16, so that the liquid height dG2 between the surface of the sulfur- content absorbent seawater 14B and the end 42a of the second weir 42 is small, and thus the flow rate of the sulfur-content absorbent seawater 14B flowing into the oxidation basin 29 can be accelerated. Therefore, it is possible to facilitate the mixture of the sulfur-content absorbent seawater 14A and the dilution seawater 12B.

[66] Furthermore, in the present embodiment, a length L2 from the sidewall 17 of the flue-gas desulfurization absorber 13 to an inner wall 42b of the second weir 42 in a flow direction of the sulfur-content' absorbent seawater 14B is set to meet either one of the following in equations (5) and (6) and the following equation (7).

D< τ 2Ut(dp) (5)
Cc > COex p(-6Kg/dpT2) (6)
τ 2=L2/(ULxD/dG2) (7)

Incidentally, D denotes a liquid depth of seawater in the dilution mixing basin; x2 denotes a retention time of seawater in the gas retaining unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in seawater; Cc denotes an SO2 environmental standard concentration; Co denotes an SO2 concentration of the flue gas at the inlet of the flue-gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of the SO2 gas at a gas-liquid interface of bubbles; dp denotes a bubble diameter; UL denotes an outlet flow rate at the bottom of the dilution mixing basin; and dG2 denotes a liquid height between the surface of seawater and the end of the second weir.

[67] As in the present embodiment, by meeting the above in equations and equation, the bubbles 28 containing the SO2 gas in the sulfur-content absorbent seawater 14B can be dissipated in the space S1 formed by the gas retaining unit 20B and the first weir 19 more reliably, i.e., the SO2 gas can be emitted in the space S1 more reliably. Consequently, it is possible to prevent the bubbles 28 containing the SO2 gas, which are entrained at the dilution mixing basin 16, from flowing into the oxidation basin 29 along with the sulfur-content absorbent seawater 14B, also prevent SO2 from being emitted in the oxidation basin 29, and thus it is possible to prevent emission of a pungent odor.

[68] Consequently, as described above, gases produced at the time of the water-quality restoration of the sulfur- content absorbent seawater 14B in the oxidation basin 29 can be emitted in the oxidation basin 29 while meeting the SO2 environmental standard concentration.

[69] In this manner, in the second seawater flue-gas desulfurization apparatus 10-2 according to the present embodiment, the second weir 42 is provided on the bottom 16a of the dilution mixing basin 16, so that a flow rate of the sulfur-content absorbent seawater 14B flowing into the oxidation basin 29 is accelerated; the bubbles 28 are concentrated on the upper side of the liquid; and the mixture of the sulfur-content absorbent seawater 14A and the dilution seawater 12B is facilitated in the dilution mixing basin 16, and also, the bubbles 28 in the sulfur- content absorbent seawater 14B can be dissipated in the space S1 formed by the cover portion 18 and the first weir 19 of the gas retaining unit 20B. Therefore, it is possible to provide such a safe and highly-reliable seawater flue-gas desulfurization apparatus capable of preventing the bubbles 28 containing the SO2 gas, which are entrained at the dilution mixing basin 16, from flowing into the oxidation basin 29 thereby preventing outside leakage of the SO2 gas in the oxidation basin 29.

[70] [Third embodiment]

Subsequently, a seawater flue-gas desulfurization apparatus according to a third embodiment of the present invention is explained with reference to FIGS. 5 and 6.
A configuration of the seawater flue-gas desulfurization apparatus according to the third embodiment is identical to that of the seawater flue-gas desulfurization apparatus according to the first embodiment of the present invention, and a configuration diagram of the entire seawater flue-gas desulfurization apparatus according to the third embodiment is omitted. The portions identical to those in the seawater flue-gas desulfurization apparatuses according to the above embodiments are denoted with the same reference numerals, and the description of those portions is omitted.

FIG. 5 is a schematic diagram schematically showing a part of the seawater flue-gas desulfurization apparatus according to the present embodiment. FIG. 6 is a partial enlarged view of the seawater flue-gas desulfurization apparatus according to the present embodiment.

[71] Here, it is recognized that a lot of bubbles are produced because the sulfur-content absorbent seawater 14A, as a large amount of seawater flowing down through the flue-gas desulfurization absorber 13 as a seawater desulfurization absorber, falls into the dilution mixing basin 16 as a dilution seawater passageway. Furthermore, it is recognized that an amount of bubbles to be produced depends on a water quality of the seawater or a concentration of the SO2 gas in the flue gas 11. When a lot of bubbles are produced, the dilution mixing basin 16 in the flue-gas desulfurization absorber 13 is covered with the bubbles, and the bubbles may flow out of the dilution mixing basin 16 along with the flow of the sulfur-content absorbent seawater 14B.

[72] As shown in FIG. 5, a third seawater flue-gas desulfurization apparatus 10-3 according to the present embodiment includes a gas retaining unit 20C that a third weir 43 is provided inside the gas retaining unit 20C. The third weir 43 is hung from the side of the rear surface of the cover portion 18, and an end 43a of the third weir 43 is submerged under the surface of seawater in the dilution mixing basin 16. The third weir 43 holds a portion of the seawater 12C, as the mixture of the seawater 12B and the sulfur-content absorbent seawater 14 in the flue-gas desulfurization absorber 13, and prevents outflow of the bubbles produced in the flue-gas desulfurization absorber 13 as an absorber.

[73] By submerging the end 43a of the third weir 43 hung from the side of the rear surface of the cover portion 18 under the surface of seawater in the dilution mixing basin 16, the flow of a portion of the sulfur-content absorbent seawater 14B is blocked, so that the mixture of the sulfur-content absorbent seawater 14A and the dilution seawater 12B is facilitated in the dilution mixing basin 16, and the bubbles 28 are dissipated in a space S2 formed by the cover portion 18, the first weir 19, and the third weir 43 of the gas retaining unit 20C, and thereby preventing outside leakage of the SO2 gas and also preventing outflow of the bubbles produced in the flue-gas desulfurization absorber 13. Consequently, it is possible to prevent the bubbles 28 containing the SO2 gas, which are entrained at the dilution mixing basin 16, from flowing into the oxidation basin 29 thereby preventing outside leakage of the SO2 gas and also preventing emission of a pungent odor.

[74] Furthermore, in the present embodiment, as shown in FIGS. 5 and 6, a ventilation hole 44 connecting the space S2 formed by the cover portion 18, the first weir 19, and the third weir 43 of the gas retaining unit 20C to the flue-gas desulfurization absorber 13 is formed on the third weir 43. Therefore, the SO2 gas filling up the space S2 formed by the cover portion 18, the first weir 19, and the
third weir 43 of the gas retaining unit 20C can be dispersed into the side of the flue-gas desulfurization absorber 13.

[75] Furthermore, in the present embodiment, a height dG3 between the end 43a of the third weir 43 and the bottom 16a of the dilution mixing basin 16 is equal to a height dG1 between the end 19a of the first weir 19 and the bottom 16a of the dilution mixing basin 16, but not limited to be equal to the height dG1. The height dG3 can be different from the height dG1-

[76] Furthermore, in the present embodiment, a length L3 from ah outer wall 43b of the third weir 43 to an inner wall 41a of the first weir 19 in a flow direction of the seawater 12C is set to meet either one of the following in equations (8) and (9) and the following equation (10).

dG1< τ 3Ut(dp) (8)
Cc>C0exp(-6Kg/dpT3) (9)
τ 3 = L3/(UL xD/MIN(dG1 ,dG2)) (10)

Incidentally, dG1 denotes a height of an opening from the first weir at the outlet of the gas retaining unit to the bottom of the dilution mixing basin; t3 denotes a retention time of seawater in the gas retaining unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in seawater; Cc denotes an SO2 environmental standard concentration; Co denotes an SO2 concentration of the flue gas at the inlet of the flue-gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of the SO2 gas at a gas-liquid interface of bubbles; dp denotes a bubble diameter; UL denotes an outlet flow rate at the bottom of the dilution mixing basin; D denotes a liquid depth of seawater in the dilution mixing basin; dG2 denotes a height of an opening from the second weir at the outlet of the gas retaining unit to the bottom of the dilution mixing basin; and MIN(dG1, dG2) denotes a minimum value of dG1 and dG2.

[77] As in the present embodiment, by meeting the above in equations and equations, the bubbles 28 containing the SO2 gas in the sulfur-content absorbent seawater 14B can be dissipated in the space S2 formed by the cover portion 18, the first weir 19, and the third weir 43 of the gas retaining unit 20C more reliably. Thus, it is possible to prevent the bubbles 28 containing the SO2 gas, which are entrained at the dilution mixing basin 16, from flowing into the oxidation basin 29 along with the sulfur-content absorbent seawater 14B thereby preventing SO2 from being emitted in the oxidation basin 29, and thus it is possible to prevent outside leakage of the SO2 gas and also prevent emission of a pungent odor.

[78] Consequently, as described above, gases produced at the time of the water-quality restoration of the sulfur- content absorbent seawater 14B in the oxidation basin 29 can be emitted in the oxidation basin 29 while meeting the SO2 environmental standard concentration.

[79] In this manner, in the third seawater flue-gas desulfurization apparatus 10-3 according to the present embodiment, the third weir 43 is provided on the inner side of the gas retaining unit 20C in the flow direction of the sulfur-content absorbent seawater 14B, and a liquid height between the bottom 16a of the dilution mixing basin 16 and the third weir 43 is lessened. Thus, the mixture of the sulfur-content absorbent seawater 14A and the dilution seawater 12B is facilitated in the dilution mixing basin 16, and the bubbles 28 are dissipated in the space S2 formed by the cover portion 18, the first weir 19, and the third weir 43 of the gas retaining unit 20C, and thereby preventing outside leakage of the SO2 gas and also preventing outflow of the bubbles produced in the flue-gas desulfurization absorber 13. Therefore, it is possible to provide such a safe and highly-reliable seawater flue-gas desulfurization apparatus capable of preventing the bubbles 28 entrained at the dilution mixing basin 16 from flowing into the oxidation basin 29 thereby preventing outside leakage of the SO2 gas in the oxidation basin 29.

[80] [Fourth embodiment]

Subsequently, a seawater flue-gas desulfurization apparatus according to a fourth embodiment of the present invention is explained with reference to FIG. 7.
A configuration of the seawater flue-gas desulfurization apparatus according to the fourth embodiment is identical to that of the seawater flue-gas desulfurization apparatus according to the first embodiment of the present invention, and a configuration diagram of the entire seawater flue-gas desulfurization apparatus according to the fourth embodiment is omitted. The portions identical to those in the seawater flue-gas desulfurization apparatuses according to the above embodiments are denoted with the same reference numerals, and the description of those portions is omitted.

FIG. 7 is a schematic diagram schematically showing a part of the seawater flue-gas desulfurization apparatus according to the present embodiment. As shown in FIG. 7, a fourth seawater flue-gas desulfurization apparatus 10-4 according to the fourth embodiment is a combination of the first seawater flue-gas desulfurization apparatus 10-1 according to the first embodiment of the present invention shown in FIGS. 1 and 2, the second seawater flue-gas desulfurization apparatus 10-2 according to the second embodiment of the present invention shown in FIG. 4, and the third seawater flue-gas desulfurization apparatus 10-3 according to the third embodiment of the present invention shown in FIG. 5.

[81] Namely, as shown in FIG. 7, the fourth seawater flue-gas desulfurization apparatus 10-4 according to the present embodiment includes the gas retaining unit 20D that the first weir 19, the second weir 42, and the third weir 43 are provided therein. The first weir 19 is hung from the side of the rear surface of the cover portion 18 that is provided on the lower-end side of the sidewall 17 of the flue-gas desulfurization absorber 13 to extend along the long side of the dilution mixing basin 16 so as to cover the dilution mixing basin 16. The end 19a of the first weir 19 is submerged under the surface of seawater in the dilution mixing basin 16. The second weir 42 is provided on the bottom 16a of the dilution mixing basin 16. The third weir 43 is provided on the inner side of the cover portion 18, and hung from the side of the rear surface of the cover portion 18. The end 43a of the third weir 43 is submerged under the surface of seawater in the dilution mixing basin 16. The first weir 19 and the third weir 43 are hung from the cover portion 18, and serve to hold a portion of the sulfur-content absorbent seawater 14B, as the mixture of the dilution seawater 12B and the sulfur- content absorbent seawater 14A, in the flue-gas desulfurization tower 13.

[82] The first weir 19 is provided to be hung from the side of the rear surface of the cover portion 18 of the gas retaining unit 20D, the second weir 42 is provided on the bottom 16a of the dilution mixing basin 16, and the third weir 4 3 is provided to be hung from the side of the rear surface of the cover portion 18, so that, as described above, the mixture of the sulfur-content absorbent seawater 14A and the dilution seawater 12B is facilitated, and a flow rate of the sulfur-content absorbent seawater 14B flowing into the oxidation basin 29 is accelerated. Thus, the bubbles 28 containing the SO2 gas in the sulfur-content absorbent seawater 14B in the dilution mixing basin 16 can be dissipated in the space S2 formed by the first weir 19, and the third weir 43 of the gas retaining unit 20D.

[83] Furthermore, in the present embodiment, a length L4 from the outer wall 43b of the third weir 43 to the inner wall 42b of the second weir 42 is set to meet either one of the following in equations (11) and (12) and the following equation (13).

D< τ AUt(dp) (11)
Cc > C0exp(-6Kg/dprA) (12)
τ, = L4/{ULxDIMIN{dG1,dG2,dG3)) (13)

Incidentally, D denotes a liquid depth of seawater in the dilution mixing basin; t4 denotes a retention time of seawater in the gas retaining unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in seawater; Cc denotes an SO2 environmental standard concentration; Co denotes an SO2 concentration of the flue gas at the inlet of the flue-gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of the SO2 gas at a gas-liquid interface of bubbles; dp denotes a bubble diameter; UL denotes an outlet flow rate at the bottom of the dilution mixing basin; dG1 denotes a height of an opening from the first weir to the bottom of the dilution mixing basin; dG2 denotes a liquid height between the surface of seawater and the second weir; dG3 denotes a height of an opening from the third weir to the bottom of the dilution mixing basin; and MIN(dG1, dG2, dG3) denotes a minimum value of dG1, dG2, and dG3.

[84] As in the present embodiment, by meeting the above in equations and equations, the bubbles 28 containing the SO2 gas, which are entrained in the sulfur-content absorbent seawater 14B, can be dissipated in the space S2 formed by the first weir 19 and the third weir 43 of the gas retaining unit 20D more reliably. Thus, it is possible to prevent the bubbles 28 containing the SO2 gas, which are entrained at the dilution mixing basin 16, from flowing into the oxidation basin 29 along with the sulfur-content absorbent seawater 14B thereby preventing SO2 from being emitted to the outside at the oxidation basin 29, i.e., preventing outside leakage of the SO2 gas, and thus it is possible to prevent emission of a pungent odor.

[85] Consequently, as described above, gases produced at the time of the water-quality restoration of the sulfur- content absorbent seawater 14B in the oxidation basin 29 can be emitted in the oxidation basin 2 9 while meeting the SO2 environmental standard concentration.

[86] In this manner, in the fourth seawater flue-gas desulfurization apparatus 10-4 according to the present embodiment, the mixture of the sulfur-content absorbent seawater 14A and the dilution seawater 12B is further facilitated in the dilution mixing basin 16, and the bubbles 28 in the sulfur-content absorbent seawater 14B can be dissipated in the space S2 formed by the first weir 19 and the third weir 43 of the gas retaining unit 20D. Therefore, it is possible to provide such a safe and highly-reliable seawater flue-gas desulfurization apparatus capable of preventing the bubbles 28 containing the SO2 gas, which are entrained at the dilution mixing basin 16, from flowing thereby preventing SO2 from being emitted to the outside at the oxidation basin 29, i.e., preventing outside leakage of the SO2 gas in the oxidation basin 2 9 more reliably.

[87] [Fifth embodiment]

Subsequently, a seawater desulfurization system according to a fifth embodiment with the seawater flue-gas desulfurization apparatus according to the present invention is explained with reference to FIG. 8.

FIG. 8 is a conceptual diagram of the seawater desulfurization system. A configuration of the seawater flue-gas desulfurization apparatus is identical to that of the seawater flue-gas desulfurization apparatuses according to the first to fourth embodiments of the present invention, and the description of the configuration of the seawater flue-gas desulfurization apparatus is omitted.

[88] As shown in FIG. 8, a seawater desulfurization system 50 according to the present embodiment is composed of a boiler 53 that causes a burner (not shown) to burn fuel with air 52 preheated by an air preheater (AH) 51; a steam turbine 57 that uses a flue gas 54 emitted from the boiler 53 as a heat source for generating steam, and drives a power generator 56 with generated steam 55; a condenser
59 that collects water 58 condensed in the steam turbine 57, and circulates the water 58; a flue-gas denitration apparatus 60 that denitrates the flue gas 54 emitted from the boiler 53; a dust collecting apparatus 61 that reduces ash dust in the flue gas 54 emitted from the boiler 53; a flue-gas desulfurization absorber 71 that reduces a sulfur content in the flue gas 54 with the use of seawater 62; a seawater flue-gas desulfurization apparatus 64 that performs a water-quality restoring treatment of sulfur- content absorbent seawater 63A containing the sulfur content in a high concentration, which is produced in the flue-gas desulfurization absorber 71; and a stack 66 from which a purged gas 65 that the flue gas 54 is desulfurized in the flue-gas desulfurization absorber 71 is emitted to the outside.

[89] The air 52 supplied from the outside is fed to the air preheater 51 by a forced draft fan 67, and preheated by the air preheater 51. Fuel (not shown) and the air 52 preheated by the air preheater 51 are supplied to the burner. The fuel is burned in the boiler 53, and the steam 55 for driving the steam turbine 57 is generated. The fuel (not shown) used in the present embodiment is supplied, for example, from an oil basin or the like.

[90] The flue gas 54 produced by the combustion of the fuel in the boiler 53 is fed to the flue-gas denitration apparatus 60. At this time, the flue gas 54 is subjected to heat exchange with the water 58 discharged from the condenser 59, and used as the heat source for generating the steam 55. The power generator 56 of the steam turbine 57 is driven by the generated steam 55. Then, the water 58 condensed in the steam turbine 57 is again fed back to the boiler 53, and circulated in the boiler 53.

[91] The flue gas 54 discharged from the boiler 53 is guided to the flue-gas denitration apparatus 60, and denitrated in the flue-gas denitration apparatus 60, and then subjected to heat exchange with the air 52 in the air preheater 51. After that, the flue gas 54 is fed to the dust collecting apparatus 61, and ash dust in the flue gas 54 is reduced by the dust collecting apparatus 61.

[92] The flue gas 54 from which the dust is reduced by the dust collecting apparatus 61 is fed to the seawater flue-gas desulfurization apparatus 64. As the seawater flue-gas desulfurization apparatus 64, the seawater flue- gas desulfurization apparatus according to the present invention is used. The seawater flue-gas desulfurization apparatus 64 includes the flue-gas desulfurization absorber 71 in which the sulfur content in the flue gas 54 is brought into contact with absorbent seawater 62A as a portion of the seawater 62 thereby purifying the flue gas 54; a dilution mixing basin 73 that is integrally provided on the underside of the flue-gas desulfurization absorber 71, and in which sulfur-content absorbent seawater 63A, which is produced by bringing the sulfur content in the flue gas 54 into contact with the absorbent seawater 62A to desulfurize the flue gas 54 in the flue-gas desulfurization absorber 71, is mixed/diluted with dilution seawater 62B fed into a main body 72 thereof; and a gas retaining unit 77 including a cover portion 75, which is provided on the lower-end side of a sidewall 74 of the flue-gas desulfurization absorber 71 to extend along a long side of the dilution mixing basin 73 so as to cover the dilution mixing basin 73, and a first weir 76, which is hung from the side of a rear surface of the cover portion 75 and an end of the first weir 76 is submerged under the surface of seawater in the dilution mixing basin 73. Furthermore, an oxidation basin 78 is integrally provided on the downstream side of the dilution mixing basin 73. In the oxidation basin 78, a sulfur content in sulfur-content absorbent seawater 63B is oxidized, and the sulfur-content absorbent seawater 63B is decarbonated, and thereby restoring a water quality of the sulfur-content absorbent seawater 63B. The sulfur-content absorbent seawater 63B is the seawater that the dilution seawater 62B and the sulfur-content absorbent seawater 63A flowing down through the flue-gas desulfurization absorber 71 are mixed in the flue-gas desulfurization absorber 71. In the oxidation basin 78, an oxidation air blower 80 that supplies air 79, a diffuser tube 81 that feeds the air 79, and an oxidation air nozzle 82 from which the air 79 is supplied to seawater 62C in the oxidation basin 78 are provided.

[93] Specifically, the flue gas 54 is supplied into the flue-gas desulfurization absorber 71 by an induced draft fan 83. At this time, the flue gas 54 is subjected to heat exchange with the purged gas 65, which is emitted from the flue-gas desulfurization absorber 71 by desulfurization of the flue gas 54 in the flue-gas desulfurization absorber 71, in a heat exchanger 84, and then supplied into the flue-gas desulfurization absorber 71.

[94] In the flue-gas desulfurization absorber 71, seawater desulfurization of a sulfur content contained in the flue gas 54 is performed with the use of the absorbent seawater 62A as a portion of the seawater 62 pumped from sea 85. The flue gas 54 produced by the combustion of fossil fuel contains a sulfur content that is sulfur oxide (SOX) in the form of SO2 or the like. The seawater desulfurization is performed in such a manner that in the flue-gas desulfurization absorber 71, the flue gas 54 is brought into gas-liquid contact with the absorbent seawater 62A supplied through a seawater supply line 8 6, and thereby absorbing SO2 in the flue gas 54 into the absorbent seawater 62A. After the seawater 62 pumped from the sea 85 with a pump 87 is subjected to heat exchange in the condenser 59, the absorbent seawater 62A, which is a portion of the seawater 62 and seawater effluent discharged from the condenser 59, is fed to the flue-gas desulfurization absorber 71 by a pump 88. The purged gas 65 that the flue gas 54 is desulfurized in the flue-gas desulfurization absorber 71 is released into the atmosphere from the stack 66.

[95] The sulfur-content absorbent seawater 63A is collected in the dilution mixing basin 73 integrally- provided on the underside of the flue-gas desulfurization
absorber 71. Furthermore, a portion of the seawater 62 is fed as the dilution seawater 62B to the dilution mixing basin 73 through a dilution seawater supply line 89. Thus, the sulfur-content absorbent seawater 63A can be diluted with the dilution seawater 62B, and a pH of the sulfur- content absorbent seawater 63B can be increased.

[96] Furthermore, there is provided the gas retaining unit 77 including the cover portion 75 and the first weir 76. The cover portion 7 5 is provided on the lower-end side of the sidewall 74 of the flue-gas desulfurization absorber 71 to extend along the long side of the dilution mixing basin 73 so as to cover the dilution mixing basin 73. The first weir 76 is hung from the side of the rear surface of the cover portion 75, and the end of the first weir 7-6 is submerged under the surface of seawater in the dilution mixing basin 73. Bubbles 90 containing a high concentration of SO2 gas, which are entrained at a bottom 73a of the dilution mixing basin 73, are dissipated in a space Sll in the gas retaining unit 77, and retained in the space Sll. Thus, it is possible to prevent the SO2 gas from leaking to the oxidation basin 78 on the downstream of the dilution mixing basin 73.

[97] Furthermore, in the present embodiment, as the seawater flue-gas desulfurization apparatus 64, the first seawater flue-gas desulfurization apparatus according to the first embodiment is used. However, it is not limited to the first seawater flue-gas desulfurization apparatus. Alternatively, any of the second to fourth seawater flue- gas desulfurization apparatuses according to the second to fourth embodiments can be used as the seawater flue-gas desulfurization apparatus 64.

[98] The oxidation air blower 80 supplies the air 79 into the oxidation basin 7 8 from the oxidation air nozzle 82 through the diffuser tube 81, and thereby oxidizing hydrogen sulfite ion in the sulfur-content absorbent seawater 63B and desorbing carbon dioxide from bicarbonate ion. As a result, a water quality of the sulfur-content absorbent seawater 63B is restored, and the sulfur-content absorbent seawater 63B becomes water-quality restored seawater 91.

[99] After that, the water-quality restored seawater 91 treated with the water-quality restoration in the oxidation basin 78 is discharged as seawater effluent into the sea 85 through a seawater discharge line 92.

[100] In this manner, in the seawater desulfurization system 50 according to the present embodiment, the sulfur- content absorbent seawater 63A produced by seawater desulfurization in the the oxidation basin 78 is collected, and mixed/diluted with the dilution seawater 62B in the dilution mixing basin 73. Also, the seawater desulfurization system 50 can prevent the bubbles 90 containing the high concentration of the SO2 gas, which are produced when the sulfur-content absorbent seawater 63A falling into the dilution mixing basin 73 are entrained in the dilution seawater 62B at the bottom of the dilution mixing basin 73, from being dissipated in the outdoor open oxidation basin 7 8 on the downstream of the dilution mixing basin 73 thereby preventing SO2 from leaking out. Therefore, it is possible to provide such a safe and highly-reliable seawater desulfurization system.

INDUSTRIAL APPLICABILITY

[101] As described above, the seawater flue-gas desulfurization apparatus according to present invention can prevent SO2, which is entrained in seawater when sulfur-content absorbent seawater produced in seawater desulfurization is mixed with dilution seawater, from being emitted to the outside when an oxidation treatment is performed. Therefore, the seawater flue-gas desulfurization apparatus according to present invention is suitable for use in a seawater flue-gas desulfurization apparatus capable of adjusting seawater used in seawater desulfurization so as to be discharged into the ocean.

EXPLANATIONS OF LETTERS OR NUMERALS

[102] 10-1 to 10-4 first seawater flue-gas desulfurization apparatus to fourth seawater flue-gas desulfurization apparatus

11 flue gas
12, 12A, 62, 62A seawater (absorbent seawater)
12B, 62B dilution seawater
13, 71 flue-gas desulfurization absorber
14A, 14B sulfur-content absorbent seawater
15, 72 main body
16, 73 dilution mixing basin 16a bottom
17, 74 sidewall
18, 75 cover portion 18a end
19, 76 first weir 19a end
19b inner wall
20 A to 20D, 77 gas retaining unit
21, 85 sea
22, 24 pump
23, 8 6 seawater supply line
25 purged gas
26 purged-gas discharge passageway
27, 8 9 dilution seawater supply line
28, 90 bubbles
29, 78 oxidation basin 30 air supply unit
31, 79 air
32, 80 oxidation air blower
33, 81 diffuser tube
34, 82 oxidation air nozzle
35, 91 water-quality restored seawater
36, 92 seawater discharge line
42 second weir 42a end
42b inner wall
43 third weir 43a end
43b outer wall
44 ventilation hole
50 seawater desulfurization system
51 air preheater (AH) 52, 79 air
53 boiler
54 flue gas
55 steam
56 power generator
57 steam turbine
58 water
59 condenser
60 flue-gas denitration apparatus
61 dust collecting apparatus
63 sulfur-content absorbent seawater
64 seawater flue-gas desulfurization apparatus
65 purged gas
66 stack
67 forced draft fan
83 induced draft fan
84 heat exchanger
87, 88 pump S1 to S2, S11 space

CLAIMS

1. A seawater flue-gas desulfurization apparatus comprising:

a flue-gas desulfurization absorber in which a sulfur 3 content in a flue gas is brought into contact with seawater thereby purifying the flue gas;

a dilution mixing basin that is integrally provided on underside of the flue-gas desulfurization absorber, and in which sulfur-content absorbent seawater produced by seawater desulfurization in which the sulfur content in the flue gas is reduced by the contact with the seawater in the flue-gas desulfurization absorber is mixed/diluted with seawater fed into a main body thereof; and

a gas retaining unit that includes a cover portion and a first weir, the cover portion being provided on lower-end side of a sidewall of the flue-gas desulfurization absorber so as to cover the dilution mixing basin, the first weir being hung from side of a rear surface of the cover portion, and an end of the first weir being submerged under a surface of seawater in the dilution mixing basin.

2. The seawater flue-gas desulfurization apparatus according to claim 1, wherein

a length L1 from the sidewall of the flue-gas desulfurization absorber to an inner wall of the first weir 5 meets either one of following in equations (1) and (2) and following equation (3):

dG1< τ 1,Ut(dp) (1)
Cc > C0 exp(-6Kg / dpt1) (2)
τ 1 = L1/UL (3)

in the above in equations and equation, dG1 denotes a
height of an opening from the first weir at an outlet of the gas retaining unit to a bottom of the dilution mixing basin; T1 denotes a retention time of seawater in the gas retaining unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in seawater; Cc denotes an SO2 environmental standard concentration; C0 denotes an SO2 concentration of the flue gas at an inlet of the flue-gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of SO2 gases at a gas- liquid interface of bubbles; dp denotes a bubble diameter; and UL denotes an outlet flow rate at the bottom of the dilution mixing basin.

3. The seawater flue-gas desulfurization apparatus according to claim 1, wherein a second weir is provided on the bottom of the dilution mixing basin.

4. The seawater flue-gas desulfurization apparatus according to claim 3, wherein

a length L2 from the sidewall of the flue-gas desulfurization absorber to an inner wall of the second weir meets either one of following in equations (4) and (5) and following equation (6):

D< τ 2Ut{dp) (4)
Cc > C0 exp(~6Kg / dpz2) (5)
τ 2=L2/{ULxD/dG2) (6)

in the above in equations and equation, D denotes a liquid depth of seawater in the dilution mixing basin; x2 denotes a retention time of seawater in the gas retaining unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in seawater; Cc denotes an SO2 environmental standard concentration; C0 denotes an SO2 concentration of the flue gas at the inlet of the flue-gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of SO2 gases at a gas- liquid interface of bubbles; dp denotes a bubble diameter;
UL denotes an outlet flow rate at the bottom of the dilution mixing basin; and dG2 denotes a liquid height between a surface of seawater and the second weir.

5. The seawater flue-gas desulfurization apparatus according to claim 1, wherein a third weir is provided on inner side of the gas retaining unit.

6. The seawater flue-gas desulfurization apparatus according to claim 5, wherein

a length L3 from an outer wall of the third weir to an inner wall of the first weir meets either one of following in equations (7) and (8) and following equation (9).

dG1<τ3U,(dp) (7)
Cc > C0 exp(-6Kg / dpT3) (8)
τ3 = L3/(Ul xD/MN(dG1, dG2)) (9)

in the above in equations and equation, dG1 denotes a height of an opening from the first weir at the outlet of the gas retaining unit to the bottom of the dilution mixing basin; T3 denotes a retention time of seawater in the gas retaining unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in seawater; Cc denotes an SO2 environmental standard concentration; C0 denotes an SO2 concentration of the flue gas at the inlet of the flue-gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of SO2 gases at a gas- liquid interface of bubbles; dp denotes a bubble diameter; UL denotes an outlet flow rate at the bottom of the dilution mixing basin; D denotes a liquid depth of seawater in the dilution mixing basin; dG2 denotes a height of an opening from the second weir at the outlet of the gas retaining unit to the bottom of the dilution mixing basin; and MIN(dG1, dG2) denotes a minimum value of dG1 and dG2.

7. The seawater flue-gas desulfurization apparatus according to claim 1, further comprising:

a second weir, the second weir being provided on a bottom of the dilution mixing basin; and

a third weir, the third weir being provided on inner j side of the gas retaining unit.

8. The seawater flue-gas desulfurization apparatus according to claim 7, wherein

a length L4 from an outer wall of the third weir to an inner wall of the second weir meets either one of following ) in equations (10) and (11) and following equation (12):

D C0 exp(~6Kg / dpT4) (11)
T4 = L4/(UL x D/MIN(Dg1, dG2, dG3)) (12)

in the above in equations and equation, D denotes a j liquid depth of seawater in the dilution mixing basin; T4 denotes a retention time of seawater in the gas retaining unit; Ut(dp) denotes a terminal rise velocity of a bubble swarm having a bubble diameter of dp in seawater; Cc denotes an SO2 environmental standard concentration; C0 ) denotes an SO2 concentration of the flue gas at an inlet of the flue-gas desulfurization absorber; Kg denotes an overall coefficient of mass transfer of SO2 gases at a gas- liquid interface of bubbles; dp denotes a bubble diameter; UL denotes an outlet flow rate at the bottom of the ) dilution mixing basin; dG1 denotes a height of an opening from the first weir to the bottom of the dilution mixing basin; dG2 denotes a liquid height between a surface of seawater and the second weir; dG3 denotes a height of an opening from the third weir to the bottom of the dilution mixing basin; and MIN(dG1, dG2, dG3) denotes a minimum value of dG1, dG2, and dG3.

9. The seawater flue-gas desulfurization apparatus according to any one of claims 5 to 8, wherein a ventilation hole connecting a space formed between the gas retaining unit and the seawater to the flue-gas desulfurization absorber is formed on the third weir.

10. The seawater flue-gas desulfurization apparatus according to any one of claims 1 to 9, wherein the seawater is effluent discharged from a condenser.

11. The seawater flue-gas desulfurization apparatus according to any one of claims 1 to 10, further comprising an oxidation basin that is provided on downstream of the dilution mixing basin, and in which a sulfur content in the seawater mixed with the sulfur-content absorbent seawater in the dilution mixing basin is oxidized and decarbonated, and thereby restoring a water quality of the seawater.

12. A seawater desulfurization system comprising: a boiler;

a steam turbine that uses a flue gas emitted from the boiler as a heat source for generating steam to drive a power generator with generated steam; a condenser that collects water condensed in the steam turbine to circulate the water;

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

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

the seawater flue-gas desulfurization apparatus according to any one of claims 1 to 11; and

a stack from which a purged gas that the flue gas is desulfurized by the flue-gas desulfurization apparatus is ) emitted to the outside.

13. A method of treating desulfurization seawater for preventing SO2 gas contained in seawater used in desulfurization from being emitted to the outside, using the seawater flue-gas desulfurization apparatus according to any one of claims 1 to 11.