WO2012066088A1

WO2012066088A1 - A scrubbing system

Info

Publication number: WO2012066088A1
Application number: PCT/EP2011/070374
Authority: WO
Inventors: Peter Ring Carlsen; Gunnar BJÖRNSSON; Per Hother Rasmussen
Original assignee: AP Moller Maersk AS
Current assignee: AP Moller Maersk AS
Priority date: 2010-11-18
Filing date: 2011-11-17
Publication date: 2012-05-24
Anticipated expiration: 2013-05-18

Abstract

An exhaust gas scrubbing system configured for removal of pollutants from a stream of exhaust gas. The system comprises a wet scrubbing unit (10) through which an exhaust gas stream (11) is passed through. The scrubbing unit (10) is provided with a reservoir (26) for receiving and holding a scrubbing medium (25) and a first primary circulating port (15) through which the scrubbing medium (26) is circulated. The scrubbing unit (10) further comprise means enabling injection (30,35) of the scrubbing medium (25) into the exhaust gas stream (11) passing through the scrubbing unit (10) such that pollutants are removed from the stream of exhaust gas.

Description

A scrubbing system

The present invention relates, according to a first aspect, to a scrubbing system configured for removal of pollutants from a stream of gas. The present invention find its primary application in sites wherein the stream of gas stem from combustion processes taking place in sites having relatively high gas outlets, i.e. on board ships or rigs where the exhaust gas is emitted high above the surface of the sea. According to a second aspect, the present invention relates to a method of removing pollutants from a stream of gas.

The pollutants removed by the system and by the method according to the present invention include sulphur dioxide and other environmentally harmful pollutants incl. particulate material. The removed substances are formed under various combustion processes, e.g. under processes such as the combustion process taking place in an engine.

According to the first aspect, the present invention relates to an exhaust gas scrubbing system configured for removal of pollutants from a stream of exhaust gas. The scrubbing system includes:

- a wet scrubbing unit through which a stream of exhaust gas is passed through. The wet scrubbing unit includes a reservoir for receiving and holding a level or volume of scrubbing medium,

- means configured for injecting the scrubbing medium into the stream of exhaust gas passing through the scrubbing unit such that pollutants are removed from the stream of exhaust gas, - a scrubbing medium exchange tank arranged at a height below the reservoir,

- a first primary circulating conduit configured to enable fluid communi- cation between the scrubbing medium exchange tank and the level of scrubbing medium held by the reservoir.

It is understood that the expressions rig, platform, ship, boat or vessel, throughout this specification, are meant to denote any kind of maritime struc- ture, floating or not, suitable for carrying persons, cargo, equipment or a combination thereof.

Background Diesel engines are almost exclusively used as prime movers in marine traffic as well as in gensets in offshore installations and the like. The fuel used by the engines contains sulphur, and as a result of the combustion taking place in the engines, sulphur oxides and other pollutants incl. particulate pollutants are formed. At the present, there are official regulations setting up maximum emission levels of various pollutants, hereunder sulphur oxides and other pollutants incl. particulate pollutants. The requirements for reducing the sulphur dioxide emissions of flue or exhaust gases are expected to tighten in the near future. One way to reduce the level of pollutants in the exhaust or flue gases created in diesel engines is to use low-sulphur fuels. The price of such fuel is, when compared to currently used high-sulphur fuels, however very high.

Another widely used alternative is to clean the exhaust gases prior to emitting them. For this purpose, it has been proposed to install exhaust gas scrubbers in way of the source of emission, e.g. on board ships, in way of the exhaust gas passageway. One such scrubbing system is disclosed, for example, in publication WO 99/44722. The publication discloses a wet scrubbing system where through the exhaust gas from an engine is conveyed. US 5690899 A discloses an exhaust gas scrubbing system wherein sulphur dioxide is removed from exhaust gas by scrubbing the exhaust gas with sea water. The scrubber housing include an aerated sump configured to convert the removed sulphur dioxide into bisulphate with neutralization in a reaction basin subsequently forming sulphate.

Scrubber systems such as exhaust gas scrubber systems arranged in connection with ships' or platforms' exhaust systems as per the above publication are widely used and recognized as an efficient way of removing various particulate pollutants from exhaust gases of ships and platforms etc.

Scrubber systems installed on board ships or on platforms and other facilities is, as mentioned, typically located in the way of the exhaust gas passage, i.e. in a funnel or a casing high above the place of combustion. Scrubber systems require large volumes of scrubbing medium, such as sea- water, to be applied; hence large volumes of scrubbing medium are today lifted, by means of one or more pumps, up to the place of application. After application, or scrubbing action, the scrubbing medium is discharged and the potential energy invested in the scrubbing water by means of the said pump- ing actions, typically is lost.

The lifting action of the scrubbing medium require a substantial amount of energy, often in the range of as much as 2-5% of a facility's total produced power. Obviously, this power requirement is considered a severe drawback of today's exhaust gas scrubbing systems. An exhaust gas scrubbing system having following characteristics is therefore much desired:

- a system requiring far less energy, optimally by recuperating the po- tential energy stored in the elevated scrubbing medium,

- a system which is simple and easy to maintain,

- a system utilizing well known technologies, and

- a system allowing for a relatively easy retrofit on existing installations. Brief description of the invention

The present invention seeks to significantly reduce power consumption of today's exhaust gas scrubbing systems. According to the invention, there is provided a scrubbing system as per the introductory part of this specification, and in particular, upon configuring the scrubbing system such that the scrubbing medium exchange tank is configured to maintain an internal overpressure at least partially established by a height difference between the scrubbing medium exchange tank and the level of scrubbing medium held by the reservoir, a scrubbing system resolving the abovementioned drawbacks is provided.

In accordance with the present invention, the scrubbing medium is, after scrubbing action, not discharged freely and the potential energy invested in the scrubbing water by means of the said pumping actions, is recuperated.

According to one embodiment, the scrubbing unit may further include a second primary circulating port and a secondary circulating circuit comprised inter alia of a secondary circulating conduit and a pump. According to one embodiment, the means through which the scrubbing medium is circulated may be located at a height below the scrubbing unit. The means through which the scrubbing medium is circulated may further be configured to pressurize the first and second primary circulating conduits such that a predetermined level of scrubbing medium may be maintained in the reservoir of the scrubbing unit.

According to one embodiment, the means for injecting the scrubbing medium into the exhaust gas stream passing through the scrubbing unit may be con- figured for circulating the scrubbing medium through the reservoir via pumping means, a secondary circulating conduit and injectors configured for scrubbing the exhaust gas stream.

According to one embodiment, the means through which the scrubbing me- dium is circulated may constitute at least two reservoirs and one, two or more valve means configured to alternate a flow of circulating scrubbing medium through the first and the second primary circulating conduits between the reservoirs of the means through which the scrubbing medium is circulated. The valve means may include any type of valves such as directional valves and/or individual valves. The valves may be configured to, by means of mechanical and/or electrical means or control means, in any combination of valves, enable circulation and exchange of the scrubbing medium.

According to one embodiment, the means through which the scrubbing me- dium is circulated may be configured to withstand a resulting hydraulic pressure rendered by the scrubbing medium maintained in the reservoir.

According to one embodiment, the means through which the scrubbing medium is circulated may be configured for exchanging the scrubbing medium received from the scrubbing unit with clean scrubbing medium, and supply the clean scrubbing medium to the scrubbing unit. According to one embodiment, the means through which the scrubbing medium is circulated may be located below a surrounding water level, and further, the scrubbing unit may be located above a surrounding water level, e.g. such as in a funnel or an engine casing of a ship.

According to one embodiment, the means through which the scrubbing medium is circulated may be provided with an inlet and an outlet, both of which may be located below a surrounding water level. Further, the inlet may be configured for conveying clean medium into the system, and the outlet may be configured for conveying contaminated scrubbing medium out of the system. The clean medium may constitute seawater taken from a ships sea- water return from the central coolers whereby following advantages are achieved:

- Savings on additional pump energy,

- Savings on purchase of additional pumps,

- Savings on reheat of exhaust gas as the scrubbing medium is warmed up. According to one embodiment, the scrubbing medium may constitute sea water.

According to one embodiment, the potential height energy of the scrubbing medium may be recuperated.

According to one embodiment, a ship comprising an exhaust gas scrubbing system according to the present invention is provided.

According to a second aspect of the present invention, a method of scrubbing a stream of exhaust gas comprising circulating a scrubbing medium by means of the system according to the present invention is provided. The actual scrubbing process is considered well known by the skilled person. As the teachings according to the present invention are not directly related to the actual scrubbing process, this specification will not elaborate further on the actual process of scrubbing exhaust gas.

Brief description of the drawings

Figure 1 schematically illustrates a simplified embodiment of an exhaust gas scrubbing system according to the present invention.

Figure 2 schematically illustrates a simplified embodiment of an exhaust gas scrubbing system according to the present invention. Figure 3 schematically illustrates an embodiment of an exhaust gas scrubbing system according to the present invention.

Figure 4 schematically illustrates an embodiment of an exhaust gas scrubbing system according to the present invention.

Detailed description of the invention with reference to the figures

Figure 1 shows a simplified form of an exhaust gas scrubbing system according to one embodiment of the present invention. As can be seen, a wet scrubbing unit 10 is provided with in and outlets 15, 20 which allows for circulation of a scrubbing medium 25. The scrubbing medium 25 typically constitute seawater; however other specialized liquids may equally be applied without departing from the scope of the present invention. The scrubbing unit 10 is configured such that a level or volume of scrubbing medium 25 may be maintained in a reservoir of the scrubbing unit 10. During scrubbing, scrubbing medium 25 is drawn from the reservoir via a pump 40 and injected, via one or more injectors 30, into the stream of gas 1 1 passing through the scrubbing unit 10 whereby the exhaust gas is cleaned. As can be seen in figure 1 , the scrubbing medium 25 is lifted and falling, or circulating, only within the limited local height of the scrubbing unit 10 and not, as per prior art teachings, lifted and discharged a distance equivalent to a height defined by the surrounding water level and the place of application. Hence, the energy required in order to provide a spray or mist of scrubbing medium is greatly reduced.

The first and second primary circulating conduits 16, 21 connects the in and outlets 15, 20 of the scrubbing unit 10, to the scrubbing medium exchange tank 50 through which the scrubbing medium 25 may by circulated, possibly be means of the pump 41 . The scrubbing medium exchange tank may, in a simplified form, constitute a simple tank configured for resisting a pressure resulting from the height energy of the scrubbing medium 25 maintained in the reservoir of the scrubbing unit 10. The scrubbing medium exchange tank 50, or tank, through which the scrubbing medium 25 is circulated, is in the embodiments according to figures 1 and 2 further provided with an inlet 51 and an outlet 52. The in and outlets are connected to means for exchanging the scrubbing medium circulated through the tank 50. Obviously, the in and outlets of the embodiment accord- ing to figure 1 , may be pressurized in order to allow for a relatively high system pressure.

Figure 2 is an embodiment wherein the circulating pump 40 is omitted.

Scrubbing medium 25 is in this embodiment fed to the scrubbing unit 10 by means of the pump 41 . As the scrubbing medium 25 is collected in the reservoir of the scrubbing unit 10, the potential height energy of the scrubbing me- dium present in the system pressurizes the supply conduit 21 whereby the embodiment according to figure 2 allows for energy savings equivalent to that of the embodiment according to figure 1 . A more sophisticated circulating system will in the below, and with reference to figures 3 and 4, be described. The scrubbing medium exchange tank 50, as described with reference to figures 1 and 2, and through which the scrubbing medium 25 is circulated, constitute in this embodiment a system comprised of two tanks interconnected by directional valves 66, 67. The system further comprises pumping means 68, 69.

According to the embodiment illustrated in figure 3, the flow is alternated between the two tanks 55, 60, and the sequence is:

1 . The lower pump 68 supplies a clean medium such as seawater to tank

60 whereby scrubbing medium 25 in tank 60 is exchanged as directional valve 67 lead surplus scrubbing medium away/overboard, 2. The upper pump 69 supplies scrubbing medium 25 from the tank 55 to the reservoir or basin of the scrubbing unit 10 and as the directional valve 66 is positioned such that return scrubbing medium from the scrubbing unit 10 returns to the tank 55, scrubbing medium circulates an a closed loop,

3 When the concentration of pollutants in the scrubbing medium 25 circulating through the tank 55 reaches a certain level, the directional valves 66, 67 rotate switching the flow such that:

4 The lower pump 68 supplies clean medium such as seawater to tank 55 whereby the scrubbing medium in the tank is exchanged,

5 The upper pump 69 supplies scrubbing medium from tank 60 to the reservoir or basin of the scrubbing unit 10, 6. When the concentration of pollutants in the scrubbing medium 25 circulating through the tank 55 reaches a certain level, the directional valves 66, 67 rotate and the sequence is repeated from 1 . The embodiment according to figure 4 deviates from the embodiment according to figure 3 in that the lower pump 68 is configured such that exchange of scrubbing medium through the tanks 55, 60 is reversed with respect to the embodiment according to figure 3. Such reversal may allow for savings on power consumption of the pump 68 as a direct result of reduced flow re- quirements.

The scrubbing medium incl. clean medium may, as already and briefly mentioned, constitute seawater or, as an equal alternatively, fresh water, grey water or a specialized scrubbing solution, without departing from the scope of the present invention.

Grey water is waste water formed on the ship from washing, cooking, cleaning, dish washing and laundering etc. Grey water is collected to a separate container, from where it can be conveyed to the scrubbing system.

The invention is not intended to be limited to the embodiments presented as examples above. The invention is however intended to be applied widely and inter alia within the scope of the inventive idea as defined in the appended claims.

The term "comprises/comprising/comprised of when used in this specification incl. appended claims is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

Claims

Claims:

1 . An exhaust gas scrubbing system configured for removal of pollutants from a stream of exhaust gas (1 1 ), said scrubbing system includes:

- a wet scrubbing unit (10) through which a stream of exhaust gas (1 1 ) is passed through, said wet scrubbing unit (10) includes a reservoir (26) for receiving and holding a level of scrubbing medium (25), means configured for injecting said scrubbing medium (25) into the stream of exhaust gas (1 1 ) passing through said scrubbing unit (10) such that pollutants are removed from the stream of exhaust gas (1 1 ),

- a scrubbing medium exchange tank (50) arranged at a height below said reservoir (26),

- a first primary circulating conduit (16) configured to enable fluid communication between said scrubbing medium exchange tank (50) and said level of scrubbing medium (25) held by said reservoir (26),

CHARATERIZED IN that said scrubbing medium exchange tank (50) is configured to maintain an internal overpressure at least partially established by a height difference between said scrubbing medium exchange tank (50) and said level of scrubbing medium (25) held by said reservoir (26).

2. An exhaust gas scrubbing system according to claim 1 , wherein said scrubbing system is configured such that said level of scrubbing medium (25) held by said reservoir (26) is determined by said internal overpressure in scrubbing medium exchange tank (50).

3. An exhaust gas scrubbing system according to claim 1 or 2, wherein said scrubbing unit (10) further include a first primary circulating port (15) and a second primary circulating port (20) and through which said scrubbing medium (25) may flow.

4. An exhaust gas scrubbing system according to any one or more of the preceding claims, wherein said scrubbing system further include a second primary conduit (21 ) configured to enable fluid communication between said scrubbing medium exchange tank (50) and said reservoir (26).

5. An exhaust gas scrubbing system according to any one or more of the preceding claims, wherein said means configured for injecting said scrubbing medium (25) into the stream of exhaust gas (1 1 ) passing through said scrubbing unit (10) includes a secondary circulating conduit (35) and a pump (40).

6. An exhaust gas scrubbing system according to any one or more of the claims 4 - 5, wherein said scrubbing medium (25) is circulated through said scrubbing medium exchange tank (50), and wherein in said scrubbing medium exchange tank (50) is configured to maintain pressure in said first and said second primary circulating conduits (16) and (21 ) such that a level of said scrubbing medium (25) may be maintained by said pressure in said reservoir (26) of said scrubbing unit (10).

7. An exhaust gas scrubbing system according to any one or more of the preceding claims, wherein said means configured for injecting said scrubbing medium (25) into the stream of exhaust gas (1 1 ) passing through said scrubbing unit (10) is configured for circulating said scrubbing medium (25) through said reservoir (26) via pumping means (40), a secondary circulating conduit (35) and injectors (30) configured for scrubbing said stream of ex- haust gas (1 1 ).

8. An exhaust gas scrubbing system according to any one or more of the preceding claims, wherein said scrubbing medium exchange tank (50) include at least two reservoirs (55), (60) and valve means (66), (67) configured to alternate a flow of circulating scrubbing medium (25) through said first and said second primary circulating conduits (16), (21 ) between said reservoirs (55), (60).

9. An exhaust gas scrubbing system according to claim 8, wherein at least one of said valve means (66), (67) constitute a directional valve.

10. An exhaust gas scrubbing system according to claim 8 or 9, wherein said valve means (66), (67) constitute, individual valves functionally connected by mechanical and/or electrical means.

1 1 . An exhaust gas scrubbing system according to any one or more of the claims 8 - 10, wherein said valve means (66), (67) is configured such that said valves enable circulation and exchange of said scrubbing medium (25).

12. An exhaust gas scrubbing system according to any one or more of the preceding claims, wherein said scrubbing medium exchange tank (50) is configured to withstand a resulting hydraulic pressure rendered by said scrubbing medium maintained in said reservoir (25).

13. An exhaust gas scrubbing system according to any one or more of the preceding claims, wherein said scrubbing medium exchange tank (50) is configured for exchanging said scrubbing medium (25) received from said scrubbing unit (10) with clean scrubbing medium (25), and supply clean scrubbing medium (25) to said scrubbing unit (10).

14. An exhaust gas scrubbing system according to any one or more of the preceding claims, wherein said scrubbing medium exchange tank (50) is lo- cated below a surrounding water level (45), and wherein said scrubbing unit (10) is located above a surrounding water level (45).

15. An exhaust gas scrubbing system according to any one or more of the preceding claims, wherein said scrubbing medium exchange tank (50) is provided with an inlet (52), (57), (62) and an outlet (51 ), (56), (61 ) both of which are located below a surrounding water level (45), and wherein said inlet (52), (57), (62) is configured for conveying clean medium into said system, and said outlet (51 ), (56), (61 ) is configured for conveying contaminated scrub- bing medium (25) out of said system.

16. An exhaust gas scrubbing system according to any one or more of the preceding claims, wherein said scrubbing medium (25) constitute sea water.

17. An exhaust gas scrubbing system according to any one or more of the preceding claims, wherein potential height energy of said scrubbing medium (25) is recuperated.

18. A ship or a rig or other technical installation including an exhaust gas scrubbing system according to any one or more of the preceding claims

19. A method of scrubbing a stream of exhaust gas comprising circulating a scrubbing medium by means of the system according to any one or more of the preceding claims.