The invention relates to a method according to the preamble of the appended claim 1 for reducing the sulphur dioxide emissions of a marine engine. The invention also relates to an equipment for implementing the aforementioned method in accordance with the preamble of the appended claim 11.

Currently, diesel engines are almost solely used as motors of ships in marine traffic. The fuel used in them is oil, which contains sulphur. As a result of combustion, sulphur oxides that are harmful to the environment are created. The current environmental protection regulations allow the usage of fuel containing a maximum of 4% sulphur in marine engines. The requirements for reducing the sulphur dioxide emissions of flue gases of ships will tighten in the near future.

One way to reduce the sulphur dioxide content of flue gases created in diesel engines is to use low-sulphur fuels. Their price when compared to the currently used high-sulphur fuels is, however, very high. In addition, raising the degree of processing of fuel increases sulphur dioxide emissions.

Another alternative is to clean the flue gases before emitting them. For this purpose, it has been proposed to install flue gas scrubbers in ships. One such scrubber is disclosed, for example, in publication WO 99/44722 . The publication discloses a wet scrubber, where the flue gas from an engine is conveyed to a scrubber, where it is scrubbed with sea water. A problem of this solutions is that the salty sea water causes corrosion in the scrubber and flue gas ducts. The problems of corrosion can be solved by using corrosion-resistant materials, but their price is so high that the material expenses of the equipment and flue gas ducts will rise unreasonably high. In addition, the amount of scrubbing solution to be removed is large, because in order to reach a sufficient sulphur removal result, a large amount of scrubbing solution has to be used in the scrubbing.

It is also known to reduce the sulphur dioxide content of flue gases in power plants situated on land by scrubbing them with a NaOH-containing scrubbing solution. The scrubbing solution is formed by mixing NaOH with fresh water, i.e. water from a lake or a river, or with tap water purified of them, which water does not include salts, as sea water does.

The purpose of the present invention is to disclose a new solution utilizing wet scrubber technique for reducing sulphur dioxide emissions of a marine engine.

To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.

The equipment according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 11.

The other, dependent claims will present some preferred embodiments of the invention.

The invention is based on the idea that the sulphur dioxide emissions of flue gases of a marine engine used in a ship are reduced by scrubbing the flue gases in a scrubber, where fresh water having sulphur removal reagent added to it is used as scrubbing solution.

The fresh water used in the scrubbing solution is either the used fresh water in the ship, or it is made specifically for the scrubber on the ship.

Pure, fresh water brought to the ship can be mixed with the scrubbing solution, but advantageously the fresh water used in the scrubbing solution consists of the so-called grey water formed on the ship and/or of fresh water made on the ship.

The so-called grey water formed on the ship and used in the scrubber is the water that has been used on the ship for washing, cooking, cleaning, dish washing and laundering. Grey water is collected to a separate container, from where it can be conveyed to the scrubber.

Fresh water can also be made by condensing moisture contained in flue gases in the scrubber. For this purpose the scrubbing solution or cooling solution circulating in the scrubber is cooled in an external heat exchanger.

The fresh water used as scrubbing solution can naturally also be made on the ship from sea water by utilizing methods known as such.

A suitable sulphur removal reagent, such as sodium hydroxide (NaOH), is added to the fresh water.

By means of the invention the sodium dioxide content of flue gases can be reduced easily and efficiently. The sulphur removal reagent boosts the sulphur removal results. Since the scrubbing solution is formed of fresh water, no corrosion problems exist in the equipment and flue gas ducts. The amount of scrubbing solution circulating in the scrubber and the concentration of sulphur removal reagent contained in it and the pH are optimized so that no excess scrubbing solution circulates in the scrubber circulation, but only so much as is required by efficient removal of sulphur oxides. This creates savings in the energy costs of pumps pumping the scrubbing solution. Further, because the amount of scrubbing solution to be removed is small, the cleaning equipment of the scrubbing solution coming from the scrubber can be made small. In addition, the pure, fresh water received from the scrubber process can be utilized for other needs of the ship as well.

Further, the equipment according to the invention including the scrubber is formed so compact in size that it is easy to arrange in the narrow spaces of a ship.

In addition to the above-described sulphur oxide removal the scrubber also efficiently removes small particles from the flue gases, resulting in a decrease in the particle emissions of marine engines decrease.

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1

shows schematically an equipment suitable for cleaning flue gases of a ship,

Fig. 2

shows schematically another equipment suitable for cleaning flue gases of a ship,

Fig.3

shows schematically a third equipment suitable for cleaning flue gases of a ship, and

Fig. 4

shows schematically an arrangement suitable for warming the flue gases freed from the scrubber shown in the equipment according to figures 1 to 3.

In Figs. 1 to 4, the same numerals refer to corresponding parts, and they will not be explained separately later on, unless required by the illustration of the subject matter.

In this description the term fresh water refers to water that comprises no salts. The term grey water refers to water that is created on the ship as a result of washing, cooking, cleaning, dish washing and laundering.

The term line refers to any pipe, connection or duct suitable for transferring liquid, gas, or suspension.

Figure 1 shows a scrubber 1 used in scrubbing flue gases of a diesel engine used on a ship. The scrubber 1 has an elongated, substantially vertically installed shell 2, inside which a packed bed 3 has been mounted. Nozzles 4 distributing scrubbing solution to the scrubber have been arranged above the packed bed, after the packed bed in the flow direction of flue gases. The nozzles 4 are arranged at even intervals over the cross-sectional area of the scrubber so that they spray scrubbing solution substantially evenly over the entire cross-sectional area of the scrubber. After the nozzles 4, a droplet separator 5 has been installed in the scrubber, which separator separates the droplets that have been swept along with the flue gases before the cleaned flue gases are directed out via a connection 6 arranged at the upper end of the scrubber.

The flue gases created in a ship's diesel engine or engines, if there are several engines in the ship, are conveyed to the scrubber 1 via a connection 7 installed in its lower part. The diesel engine or engines are not shown in the figure. The flow direction of flue gases in the scrubber is upwards, from the bottom to the top.

Scrubbing solution is conveyed to the flue gas scrubber 1 from the pumping container 9 by means of a pump 10 via line 11. The line 11 is connected to the nozzles 4, which spray the scrubbing solution to the scrubber. The scrubbing solution consists of fresh water directed to the pumping container 9 via line 12, i.e. in this embodiment of the grey water of the ship and the sulphur removal reagent mixed with fresh water. The sulphur removal reagent, i.e. NaOH is conveyed from a sulphur removal reagent container 13 via line 14 to the pumping container 9, to the scrubbing solution. In directing the sulphur removal reagent to the pumping container, a pump 15 arranged to the line 14 is utilized, which also controls the amount of sulphur removal reagent added to the scrubbing solution.

The NaOH fed to the flue gas scrubber reacts with the sulphur dioxide in the flue gases by forming sodium sulphite in accordance with the following formula:



         SO₂ + 2NaOH→ Na₂SO₃ + H₂O     (1)

The sodium sulphite oxidizes in the scrubber at least partly into sodium sulphate. NaOH also reacts with other sulphur oxides in the flue gases and forms other compounds to be removed from the flue gases by scrubbing. The reactions of NaOH and sulphur oxides in the flue gas scrubber are known as such by a person skilled in the art, and therefore they are not described in more detail in this context.

The flue gases are scrubbed with the NaOH-containing scrubbing solution sprayed via nozzles 4. The sodium sulphite and sulphate containing scrubbing solution formed as a result of the reaction of NaOH and sulphur oxides is removed from the scrubber 1 via line 16 arranged in its lower part and is returned to the pumping container 9. From the pumping container 9 the scrubbing solution is recycled to the scrubber 1.

The lower part of the scrubber and the connection point of line 16 to it is arranged so that no scrubbing solution layer is formed in the lower part of the scrubber, as in conventional scrubbers used in power plants operating on land. Either a container in connection with the scrubber or a separate pumping container is used for recycling scrubbing solution. Thus, the scrubber can be built lightweight, because it does not need to carry the weight of the scrubbing solution mass. The pumping container can be placed on the ship so that the mass center of the ship is as close to the bottom of the ship as possible. This is a great advantage. The used scrubbing solution containing sulphur removal products is removed from the scrubbing solution circulation via line 17. The scrubbing solution containing sulphur removal products can be led either directly to the waters surrounding the ship or to a waste water treatment phase, if the ship has one. It is to be noted that the amount of used scrubbing water removed from the scrubbing circulation is significantly small, because the concentration of the scrubbing solution in the scrubbing circulation is great.

Figure 2 shows an embodiment, where the fresh water used in the scrubbing solution is formed by condensing the water vapor in the flue gases by means of cooled scrubbing solution. For this purpose the scrubbing solution used in the scrubber is cooled with an external heat exchanger. In this embodiment the scrubber 1 and its operation, as well as the pumping container 9 and the sulphur removal reagent container 13 are substantially the same as in the embodiment according to figure 1. For cooling the scrubbing solution a heat exchanger 18 has been added to the scrubbing solution circulation. The hot scrubbing solution received from the scrubber 1 is pumped with a pump 10 from the pumping container 9 via line 19 to the heat exchanger 18, where the scrubbing solution cools. From the heat exchanger 18 the cooled scrubbing solution is conveyed to the scrubber via line 20. Sea water is used as a cooling medium for cooling the scrubbing solution in the heat exchanger 18. It is led to the heat exchanger 18 via line 21 and removed from there via line 22. The scrubbing solution is removed from the scrubber 1 via line 16 arranged in the lower part of the scrubber and returned to the pumping container 9. From the pumping container 9 the scrubbing solution is recycled again to the heat exchanger 18 and from there to the scrubber 1. The used scrubbing solution containing sulphur removal products is removed from the scrubbing solution circulation via line 17.

Figure 3 shows an embodiment, where the fresh water used in the scrubbing solution is made by condensing wet flue gases from the sulphur removal phase. Flue gases are condensed separately from the sulphur removal phase by scrubbing them again after the actual sulphur removal scrubbing stage. As a result, the temperature of the flue gases cleaned of sulphur oxides decreases and the water vapor in them condenses into water. The means used in condensing flue gases are arranged in the same scrubber with the means used in sulphur removal. The equipment includes a scrubber 23, which has two scrubbing stages, a sulphur removal stage A and a condensing stage B. The elongated, substantially vertically installed shell 2 of the scrubber embeds both stages, which are installed so that the flue gases coming to the scrubber 23 from the connection 7 are scrubbed first in the sulphur removal stage A. The flow direction of flue gases in the scrubber is upwards, from the bottom to the top.

The packed bed 3 included in stage A and the nozzles 4 spraying scrubbing solution that contains sulphur removal reagent are substantially the same as in the embodiment shown hereinabove in figures 1 and 2. Similarly, the operation of stage A is the same as described above. Scrubbing solution is conveyed to the nozzles 4 from the pumping container 9 by means of a pump 10 via line 11. NaOH is conveyed to the scrubbing solution in the pumping container 9 with a pump 15 from a container 13 via line 14. The sodium sulphite and sulphate containing scrubbing solution formed in the scrubbing stage A is removed form the scrubber 23 via a line 16 arranged in its lower part and returned to the pumping container 9 and recycled from there back to the scrubbing stage A. The used scrubbing solution that contains sulphur removal products and is removed from the scrubbing solution circulation via line 17. The fresh water needed in forming the scrubbing solution is conveyed to the pumping container 9 from the condensing stage B via line 24.

From the sulphur removal stage A the cleaned flue gases flow to the condensing stage B. In the flow direction of flue gases, before the condensing stage B, an intermediate floor 25 separating stages A and B from each other has been installed in the scrubber.

In the condensing stage B, the packed bed 26 is first in the flow direction of flue gases. Nozzles 27 distributing scrubbing solution for condensing flue gases have been arranged above the packed bed 26, after it in the flow direction of flue gases. The nozzles 27 are arranged at even intervals over the cross-sectional area of the scrubber so that they spray cooling solution substantially evenly over the entire cross-sectional area of the scrubber. After the nozzles 27, a droplet separator 5 has been installed in the scrubber 23, which separator separates the droplets that have been swept along with the flue gases before the cleaned flue gases are directed out via a nozzle 6 arranged at the upper end of the scrubber.

In the condensing stage B the flue gases coming from the sulphur removal stage A, whose sulphur oxide content has decreased, are scrubbed with a cooling solution. The fresh water used as cooling solution is formed in this same stage by condensing the flue gases received from the sulphur removal stage. The cooling solution cools the flue gases and makes the water vapor in them to condense to water.

The cooling solution received from the condensing stage B is conveyed from the lower part of the condensing stage via line 30 to a storage container 31 and from there further via line 32 to be cooled in the heat exchanger 28. A pump 33 arranged in line 32 is utilized in transferring the solution to the heat exchanger. Sea water is used as a cooling medium for cooling the cooling solution in the heat exchanger. The sea water is conveyed to the heat exchanger 28 via line 34 and removed from there via line 35. From the heat exchanger the cooled 28 scrubbing solution is recycled to the nozzles 27 of the cooling stage of the scrubber via line 29.

As presented above, the fresh water received from the condensing stage B is used not only in the condensing stage itself, but also in forming the scrubbing solution used in sulphur removal. The cooling solution received from the heat exchanger and removed via line 36, is possible to use for other needs of the ship as well.

The flue gases from the scrubber can still, if desired, be heated before they are released to the environment. When flue gases are scrubbed in a wet scrubber, the result is completely wet, water-vapor-containing, visible flue gas. In some situations the visibility of flue gases is seen as a drawback. This drawback can be overcome by increasing the temperature of the flue gases.

The visibility of water-vapor-containing flue gases cleaned according to the invention can be decreased by using, for example, an arrangement according to figure 4. The arrangement is based on that the above-described method and equipment for decreasing the sulphur dioxide emissions of a marine engine is so effective, that all the flue gases produced by ship engines does not necessarily need to be cleaned in order to realize environmental regulations. Often it is enough that only half or less than a half of the flue gases are scrubbed in the scrubber. The remaining hot and un-cleaned flue gases can be mixed with the cleaned flue gases. Thus, the visibility problem typical for water-vapor-containing flue gases can be overcome.

Figure 4 shows a new type of a droplet separator 5 located last in the scrubber 1 in the flow direction of flue gases, where the flue gas duct 5a of the droplet separator connected to it has been elongated. In addition, the flue gas duct 5a of the droplet separator is made of heat conductive material, which enhances the warming of flue gases before the mixing of them.

Hot, uncleaned flue gases are conveyed to the upper part of the scrubber via a connection 37. The hot flue gases heat the moist flue gases coming from the scrubber. Warming of the cleaned flue gases also continues in the flue gas duct 6 of the scrubber, where the cleaned flue gases discharged from the flue gas duct 5a of the droplet separator are mixed with the hot flue gases.

In addition, the above-described scrubber has the ability to attenuate the noise caused by the engine. Thus, it can be used to replace the silencer after an engine in a ship. The scrubber can be installed in the space freed from a conventional silencer in the machine shaft of the ship. In this case the scrubber can be placed in a place advantageous from the point of view of stability, that is, low, close to the mass center of the ship.

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