AU2005256099B2

AU2005256099B2 - Sea water treatment method and sea water treatment system

Info

Publication number: AU2005256099B2
Application number: AU2005256099A
Authority: AU
Inventors: Seiji Furukawa; Itsuo Kouhara; Shozo Nagao; Keisuke Sonoda; Yoshihiko Tsuchiyama
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Power Ltd
Priority date: 2004-08-20
Filing date: 2005-01-27
Publication date: 2007-08-30
Anticipated expiration: 2025-01-27
Also published as: NO20060627L; DE112005000039T5; WO2006018911A1; CN1839101A; CN100447092C; AU2005256099A1; TW200619151A; NZ545182A; JP2006055779A; TWI276609B; NZ583491A; JP4460975B2; MY144625A

Description

DESCRIPTION

SEA WATER TREATMENT METHOD AND SEA WATER TREATMENT SYSTEM TECHNICAL FIELD [0001] The present invention relates to a sea water treatment method and a sea water treatment system, and more specifically to a sea water treatment method and a sea water treatment system in which a pH of a sulfur-absorbing solution produced by sea water desulfurization and the like is adjusted to a level at which the sulfur-absorbing solution may be discharged to the sea.

BACKGROUND ART [0002] Issues of environmental conservation have come to the surface, and the development of clean energy has been pursued. On the other hand, conventional methods for generating electric power such as the one using fossil fuel are, however, still operated widely because of a technical or economical reason.

[0003] When fossil fuel such as coal is burnt, a desulfurizer to remove a sulfur content from exhaust gas and the like is essentially needed. Although there are various desulfurizers, sea water desulfurization in which desulfurization is carried out utilizing sea water has attracted a lot of attention in recent years in view of constructing power plants at a place by the sea in many cases because the large-scale facilities such as power plants need an enormous amount of cooling water and in view of suppressing a running cost of desulfurization treatment, and so forth.

[0004] As an example of a case where fossil fuel is burnt and a generated exhaust gas is desulfurized, an outline of sea water desulfurization is explained. An exhaust gas generated by burning fossil fuel contains a sulfur content in the form of SO 2 In sea water desulfurization, an exhaust gas and sea water are allowed to make gas-liquid contact in a sulfur content absorption tower (desulfurizer), S02 in the exhaust gas is absorbed by the sea water and the gas after the treatment is discharged into the air. It is thought that reactions shown in to below occur by the contact of sea water to an exhaust gas.

S0 2

+H

2 0- HSO 3

-+H

HS0 3 2 -+1/202-+SO 4 2

-+H

C0 2

+H

2 0->HC0 3

-+H

HC03--> C022-+H+ [0005] As shown above, H+ is generated due to the gas-liquid contact between the sea water and the exhaust gas, and this is dissolved in the sea water. Therefore, not only does the concentration of S0 4 in the sea water after the gas-liquid contact of the sea water to the exhaust gas rise but also the pH is lowered due to.the dissolution of H Although the pH varies depending on operational conditions and the like, the pH of the sulfur-absorbing solution produced by absorbing abundant sulfur and CO 2 becomes approximately three. Note that, in the present specification, the sulfur-absorbing solution indicates a solution of the sea water that has absorbed a sulfur content.

[0006] The sea water discharged from the sulfur absorption tower is discharged to the sea or recycled as described above; however, to reduce its influence on the environment, it is necessary for at least the pH to be raised close to that of sea water before discharging to the sea.

Therefore, the sea water containing sulfur at a high concentration is mixed with ordinary sea water, and at the same time subjected to gas-liquid contact with air, thereby causing the following reactions such as and After carrying out the treatment to raise the pH, the sea water is discharged to the sea.

HCO3-+H+--CO 2

CO

3 2 -+2H+->CO 2 [0007] Since sea water desulfurization is simple as an operational method as described above, it attracts a lot of attention. As to the sea water desulfurization, it is disclosed, for example, in Patent document 1 and the like.

[0008] Patent document 1: Japanese Patent Application Laid- Open Publication No. 2001-129352 DISCLOSURE OF INVENTION PROBLEM TO BE SOLVED BY THE INVENTION [0009] With respect to a sulfur-absorbing solution produced by treatments such as an exhaust gas treatment, its pH is raised to pH 6.5 until now. On the other hand, the pH value of general sea water is 8.0 to 8.3, and its alkalinity is approximately 110 milligrams per liter (mg (as CaCO 3 A trend that a requirement to reduce an influence on the environment is growing in recent years is conspicuous, and further raising the pH of the sulfurabsorbing solution has also been required at the time of P:\OPER\PHH\1 2724140 Ist spa.doc 10/3/06 P OPEREFKC( I! PC 2!22 dMe SA,'2rn, -4discharging it to the sea.

[0010] To increase the pH by mixing sea water, it is conceivable that an amount of sea water to be mixed is increased with respect to a solution to be treated of which pH is to be adjusted. However, to further raise the pH of a solution discharged from a sea water desulfurizer (absorption tower) in an amount on an industrial scale to higher than 6.5 by simply mixing sea water, a much larger amount of sea water is required, resulting in necessity of making a facility to adjust the pH enormously large in scale and an increase in initial cost of the facility.

[0011] Although aeration used to be provided at the same time when sea water is mixed, there is no report that the pH could be successfully raised to higher than seven. Further, when aeration is provided to the sea water that has been allowed to absorb sulfur as it is, the amount of harmful sulfur dioxide (S02) discharged from the solution increases all at once, thereby giving off a stench foul enough for man to perceive it as uncomfortable.

[0012] In other words, for a treatment of a sulfur-absorbing solution containing a sulfur content at a high concentration that is produced by sea water desulfurization, it has been required that a hurdle to keep a pH level that is further required strictly is overcome without elevating in vain a size of a facility, an initial cost of the facility, and a running cost of the treatment, and that generation of a foul stench is suppressed. Under such circumstances described above, an object of the present invention is to provide a treatment method for sea water that is industrially P \OPER FI CQ 525( 222 dcc- I 1.

;Zpractical and more friendly to the environment.

MEANS FOR SOLVING PROBLEM [00013] To solve the above problem, the present inventors have achieved the present invention by obtaining ideas that flow rates of respective nozzles and a total aeration flow rate O in aeration are set to predetermined conditions, and that separating adjustment of the pH into two steps is effective.

In other words, the present invention adopts the following structure to solve the above problem.

[00014] [00015] The first invention provides a sea water treatment method comprising: a first step at which a pH is adjusted to higher than and lower than 6.5 by mixing a sulfur-absorbing solution produced by sea water desulfurization with sea water; and a second step at which the pH is further raised by providing aeration to the liquid adjusted at the first step.

[00016] The second invention provides the sea water treatment method according to the first invention, wherein the pH of the liquid adjusted at the first step is raised to higher than 7.0 at the second step.

[00017] The third invention provides the sea water treatment method, wherein, at the second step, the aeration is provided to the sulfur-absorbing solution produced by the sea water desulfurization under the conditions of the nozzle flow rate, and the total aeration flow rate that fulfil the following equations and (III) P \OPER\EFHi(xi525WN sI: 222 doc-lI 5A)2(171 -6y20.526+0.0114x (I) x<35 (II) y£1.32 (III) (in the equations and (III), y represents the total aeration flow rate (Nm 3 /t-sea water), and x represents the nozzle flow rate (Nm 3 [00018] The third invention is characterized by separating the adjustment process of pH into two steps and adjusting the operating conditions of aeration in the second step by setting flow rates of nozzles and a total aeration flow rate in aeration to be predetermined ranges.

[00019] The fourth invention provides a sea water treatment system comprising: a sea water mixing apparatus that allows a sulfurabsorbing solution produced by sea water desulfurization to be mixed with sea water and its pH to be raised up to at least higher than 5.5; and an aeration apparatus that is arranged at downstream of the sea water mixing apparatus, provides aeration to the liquid adjusted by the sea water mixing apparatus, and allows its pH to be raised further.

[00020] The fifth invention provides the sea water treatment system, wherein the aeration apparatus is provided with an air supplying unit having a plurality of nozzles that discharge air, and wherein the air supplying unit is capable of discharging air under conditions of a nozzle flow rate of 5 to 35 Nm 3 and a total aeration flow rate of 0.58 to 1.32 Nm 3 /t-sea water.

[00021] P X0PERFFK2:X,5.'V,'I9" i' -7- The fourth and the fifth aspects provide the preferable system to carry out the sea water treatment method of the present invention described above.

[00022] According to the present invention, the sea water treatment method and the sea water treatment system that are industrially practical and more friendly to the environment are provided.

More specifically, according to the present invention, the pH of the sulfur-absorbing solution produced by sea water desulfurization can be adjusted up to a desirable level at which the solution may be discharged to the sea.

Further, according to the present invention, generation of a foul stench resulted from SO 2 and the like may be suppressed at the time of a pH adjustment of the sulfur-absorbing solution produced by sea water desulfurization.

Furthermore, according to the present invention, the pH of a sulfur-absorbing solution produced by sea water desulfurization may be adjusted within a range of a practical scale for industrial plants.

BRIEF DESCRIPTION OF DRAWINGS [00023] Various embodiments of the water treatment method and the water treatment system according to the present invention will now be described by way of example only with reference to the accompanying drawings.

Fig. 1 is an illustration representing an aeration apparatus; Fig. 2 is an illustration representing a relation between nozzle flow rate and total aeration flow rate; Fig. 3 is an illustration representing an embodiment of the present invention provided with a sea water mixing apparatus and the aeration apparatus; Fig. 4 is an illustration representing a relation between pH of a solution and S02 in equilibrium in gas; and Fig. 5 is an illustration representing one example of the present invention.

EXPLANATIONS OF LETTERS OR NUMERALS [0024] Aeration apparatus 11 Air supplying unit 12 Nozzle Sea water mixing apparatus 30 Desulfurizer (flue gas desulfurizer (FGD)) Sea water Exhaust gas (before desulfurization) 61 Exhaust gas (after desulfurization) Solution to be treated 71 Sea water from FGD outlet (sulfur-absorbing solution) Air 81 Bubbled air BEST MODE(S) FOR CARRYING OUT THE INVENTION [0025] Hereinafter, embodiments and an example of the present invention are explained. Note that the present invention is not limited to the following embodiments and example.

In the structural components in the following embodiments and examples may include other components that those skilled in the art can easily assume and that are essentially identical.

P OPfR\EFI'4)5--) 1P, 2.2 11 -9- [0026] [First Embodiment] An embodiment of an aeration apparatus for use with the present invention is explained with the use of Figs. 1 and 2.

In this embodiment, a solution to be treated 70 is sent to an aeration apparatus 10, and aerated under predetermined conditions, followed by being discharged to the sea. An air supplying unit 11 is provided to the aeration apparatus 10. In the air supplying unit 11, a plurality of nozzles 12 are provided, and finely bubbled air 81 is made contact with the solution to be treated 70 to aerate the solution to be treated As the solution to be treated 70, sea water absorbing a sulfur content by sea water desulfurization is listed as a typical example.

[0027] In this embodiment, a flow rate of air supplied from individual nozzle 12, that is, a nozzle flow rate, and a total flow rate of air supplied from the air supplying unit 11, that is, a total aeration flow rate are set to predetermined conditions. The setting of the nozzle flow rate and the total aeration flow rate fulfils the conditions of the following equations and (III).

[0028] y 0.526+0.0114x (I) 5<x<35

(II)

y 1l.32 (III) (in the equations and (III), y represents the total aeration flow rate (Nm 3 /t-sea water), and x represents the nozzle flow rate (Nm 3 [0029] Nm 3 is normal cubic meter. The total aeration flow rate is represented as a volume that a total volume of air supplied per one ton of the solution to be treated 70 is converted to a state at zero degree C at one atmospheric pressure. Further, the nozzle flow rate is represented by Nm 3 /h/m and as a volume that a volume of bubbled air 81 supplied by each nozzle 12 per one meter per one hour is converted to a state at zero degree C at one atmospheric pressure.

[0030] Fig. 2 represents the conditions of the equations and (III) in a form of graph. The area diagonally shaded is an area where all the conditions of the equations to (III) are fulfilled. By providing aeration according to the conditions fulfilling all the equations to (III), it is possible to raise effectively the pH to a sufficiently high level. Further, since it is possible to raise the pH effectively, suppression of upsizing the facility may be realized.

[0031] The aeration apparatus 10 offers performance to provide aeration that fulfills the conditions of the above equations to (III). Specifically, the aeration apparatus 10 is equipped with an air supplying unit capable of releasing air under the conditions of a nozzle flow rate of 5 to 35 Nm 3 and a total aeration flow rate of 0.58 to 1.32 Nm 3 /t-sea water. To provide aeration to fulfill the conditions shown by the above equations to (III), the number of the nozzles and arrangement distance between the nozzles arranged in the area where aeration is performed may be adjusted appropriately based on the air supply performance of each of the nozzles 12 with respect to the volume of the solution to be treated [0032] The structure of the aeration apparatus 10 is not P \OPER\EF1P2 x $pC 222 d I'21MQ(X17 -11particularly limited as long as the solution to be treated O 70 can be aerated to fulfil the above conditions. For example, a treatment of the flowing solution to be treated may be carried out by providing an area that has an air supplying unit arranged on the bottom surface of the Ndischarge flow path of the solution to be treated 70. In addition to that, as another structure, it may be accepted that the solution to be treated 70 is stored once in a tank, aeration is carried out there, and the pH is adjusted, followed by discharging the solution further downstream.

[0033] [Second Embodiment] An embodiment of the present invention is explained with accompanying Fig. 3 and Fig. 4. The same components as those of the above embodiment are assigned by the same letters and numerals and their explanations are omitted.

[0034] This embodiment of the present invention is characterized by separation of the adjustment of pH into two steps. Since the level of S02 in equilibrium in gas varies depending on pH levels, the pH is raised up to the level at which SO 2 is hardly released into the air by mixing sea water even though aeration is provided, and then aeration is provided as a second step. By carrying out the pH adjustment in two steps in such a way, it is possible to suppress generation of SO 2 as well as raise the pH of the solution to be treated 70 (sulfur-absorbing solution) [0035] In the embodiment, a sea water mixing apparatus 20 is arranged in the upstream portion of the aeration apparatus The sea water mixing apparatus 20 is an apparatus that mixes the solution to be treated 70 and sea water 50. The sea water mixing apparatus 20 may suffice as long as it is provided with an apparatus that supplies a volume of the sea water 50 sufficient to allow raising the pH of the solution to be treated 70 to the level over and with place in which the solution to be treated 70 and the sea water 50 are mixed. Specific examples may include an apparatus that mixes the solution to be treated 70 and the sea water 50 by supplying sea water by a pump (not shown) that pumps up the sea water 50 from the sea to a discharge channel to which the solution to be treated 70 is delivered or to a tank in which the solution to be treated is stored. The sea water mixing apparatus 20 may be structured such that, for example, the sea water 50 is pumped and discharged by a pump not shown in the Figure to the channel for discharging the solution to be treated and that the solution to be treated 70 and the sea water are mixed in the discharge channel. Moreover, the sea water mixing apparatus 20 may be structured such that the solution to be treated 70 is stored once in the tank, the sea water 50 is mixed there, and the pH is adjusted, followed by discharging to the downstream.

[0036] A first pH adjustment is carried out in the sea water mixing apparatus 20. In the first pH adjustment, it is preferred to adjust the pH to higher than 5.5. By adjusting the pH of the solution to higher than 5.5, it is possible to suppress generation of a foul stench due to aeration. In Fig. 4, a relation between the pH of the solution and SO2 in equilibrium in gas is shown. The SO2 in equilibrium in gas that is represented on the vertical axis in Fig. 4 represents a concentration (ppm) at which the amount of SO2 dissolved in the solution and the amount of S02 in gas achieve a state of equilibrium. It is said that the lowest limit of a S02 concentration in a vapor phase at which man generally perceives uncomfortable is 0.3 ppm. In other words, when a SO 2 concentration exceeds 0.3 ppm, man tends to easily perceive an uncomfortable stench.

It is found from Fig. 4 that a state of equilibrium at 0.3 ppm in terms of SO 2 in gas is achieved when the pH of the solution is [0037] Accordingly, the solution may be reformed to a solution that hardly generates a foul stench by adjusting the pH to higher than 5.5 at the first pH adjustment step even though aeration is provided. The volume of sea water that is required to raise the pH of the solution to be treated 70 to higher than 5.5 can be determined with ease corresponding to the volume of the solution to be treated.

[0038] On the other hand, in many cases, the pH of drainage after sea water desulfurization that is a typical example of the solution to be treated 70 is lowered to approximately pH 3. To raise the pH of sea water that has been lowered to approximately pH 3 due to sea water desulfurization to higher than pH 6.5 by simply mixing the sea water with ordinary sea water, a sea water mixing facility of a huge scale that is not industrially practical is necessary. That is, at the first pH adjustment step, the pH is kept to be adjusted to lower than 6.5, thereby making it possible to downsize the sea water mixing apparatus [0039] After the first pH adjustment step has been carried out in the sea water mixing apparatus 20, a second pH adjustment step is carried out in the aeration apparatus Since the solution of which pH has been P:\OPER\PHH\ 2724140 1 st spa.doc 10/3/06 P IOPEREFH,2A6 42' doc -14adjusted to exceed pH 5.5 as described above, generation of a foul stench is suppressed even though aeration is provided.

[0040] As a preferred mode for the second pH adjustment step, the aeration apparatus 10 and the pH adjustment method explained in the above embodiment are exemplified. After the pH has been preferably adjusted to higher than pH 6.5 or more, further preferably to higher than pH 7.0 or more by the second pH adjustment, the solution is discharged to the sea. By adjusting the pH up to above values, the influence on the environment may be reduced.

[0041] By separating the pH adjustment into two steps as described above, it is possible to raise the pH of a sulfur-absorbing solution produced by sea water desulfurization to a sufficiently high level while suppressing generation of a foul stench as well as carry out desulfurization in a facility of an industrially practical scale.

[0042] [Other embodiments] The above embodiments can be employed in modification examples below.

Modification examples, may include that a series of treatments may be carried out in a discharge channel. For example, when a solution that flows continuously in the discharge channel or the like is treated, an area in which the solution and sea water are mixed and an aeration area in which aeration is provided are provided in the discharge channel, and treatments such as a pH adjustment may be completed by the time when the treated solution flows out to the sea from an outlet.

Further, as another modification example, the adjustment may be carried out in a batch method by providing tanks in which each treatment is performed.

EXAMPLE

[0043] An example of the present invention is explained with reference to Fig. 5. The present example shows that the sulfur-absorbing solution discharged from a desulfurizer (FGD) 30 is adjusted and discharged to the sea by the system and the method explained in the above second embodiment. The same components as those in the second embodiment are assigned by the same numerals and their explanations are omitted.

[0044] The sea water 50 was pumped up by a pump from the sea for the sake of sea water desulfurization and sea water treatment. The pH of the sea water pumped up was 8.3 and the sea water was pumped up at a rate of 114,000 t/h.

[0045] The sea water 50 was supplied to the desulfurizer (FGD) 30, and gas-liquid contact between the sea water and an exhaust gas 60 containing a sulfur content generated by burning coal and the like was carried out. The exhaust gas was supplied at 1,650,000 Nm 3 and an exhaust gas 61 after desulfurization was generated at 1,600,000 Nm 3 /h.

The pH of the sea water having absorbed a sulfur content (sulfur-absorbing solution) was pH 3 at the outlet of the FGD 30, and the sea water was discharged at a rate of 21,000 t/h.

[0046] The sulfur-absorbing solution was supplied to the sea water mixing apparatus 20, and the sea water 50 pumped up from the sea and the sulfur-absorbing solution were mixed.

The pH at the time of sufficient mixing was 6.1.

[0047] The solution of which pH was adjusted to pH 6.1 was delivered to the aeration apparatus 10. In the aeration apparatus 10, air in a bubble form was supplied by the air supplying unit 11, followed by aerating the solution. The conditions of the aeration were set to a nozzle flow rate of 13 Nm 3 /h/m and a total aeration flow rate of 0.79 Nm 3 /t-sea water.

The supply volume of the air 80 per one hour was 90,000 Nm 3 /h.

The pH after completion of the aeration was pH 7.2. The solution after completion of the aeration was returned to the sea as sea water.

INDUSTRIAL APPLICABILITY [0048] The present invention is useful for pH adjustment of sea water, and more particularly for a case of adjusting a large volume of sea water. As a specific application example, it is useful for adjusting sea water having been used for sea water desulfurization such that the sea water may be discharged to the sea.

[0049] Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0050] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

P:\OPER\PHH\I2724140 Ist spa.doc- 10/3/06

Claims

1. A sea water treatment method comprising: a first step at which a pH is adjusted to higher than

5.5 and lower than 6.5 by mixing a sulfur-absorbing solution NO produced by sea water desulfurization with sea water; and a second step at which the pH is further raised by Sproviding aeration to the liquid adjusted at the first step. 2. The sea water treatment method according to claim 1, wherein the pH of the liquid adjusted at the first step is raised to higher than 7.0 at the second step. 3. The sea water treatment method according to claim 1 or 2, wherein, at the second step, the aeration is provided to the sulfur-absorbing solution produced by the sea water desulfurization under the conditions of the nozzle flow rate, and the total aeration flow rate that fulfil the following equations and (III): y20.526+0.0114x (I) x!35 (II) y 1.32 (III) (in the equations and (III), y represents the total aeration flow rate, and x represents the nozzle flow rate. 4. A sea water treatment system comprising: a sea water mixing apparatus that allows a sulfur- absorbing solution produced by sea water desulfurization to be mixed with sea water and its pH to be raised up to at least higher than 5.5; and an aeration apparatus that is arranged at downstream of P \OPERfi..F I ;Sl pc 2 22 d-l 5M.:2 x7 -18- the sea water mixing apparatus, provides aeration to the liquid adjusted by the sea water mixing apparatus, and allows its pH to be raised further. 5. The sea water treatment system according to claim 4, wherein the aeration apparatus is provided with an air supplying unit having a plurality of nozzles that discharge air, and wherein the air supplying unit is capable of discharging air under conditions of a nozzle flow rate of to 35 Nm 3 and a total aeration flow rate of 0.58 to 1.32 Nm 3 /t-sea water.

6. A sea water treatment method according to claim 1 and substantially as herein described with reference to the accompanying drawings and/or the Example.

7. A sea water treatment system according to claim 4 and substantially as herein described with reference to the accompanying drawings and/or the Example.