System and method for reducing the amount of sulfur oxides in exhaust gas

Abstract

The present application relates to a spray tower for cleaning the exhaust gas from marine engines, wherein the spray tower, in installed position, is vertically positioned and comprises a central exhaust gas pipe for introducing exhaust gas into the spray tower through an exhaust gas inlet positioned at a proximal end of the central exhaust gas pipe, the central exhaust gas pipe being positioned at a bottom end of the spray tower, an exhaust gas outlet at a top end of the spray tower for withdrawing purified exhaust gas from the spray tower, one or more spray devices for providing scrubber liquid within the spray tower counter-current to the general exhaust gas flow, and at least two engine exhaust gas pipes being in fluid connection with the distal end of the central exhaust gas pipe. Furthermore, a method for reducing the amount of SOx in exhaust gas is described.

Claims

1. A marine exhaust gas cleaning system for reducing the amount of SO.sub.x in exhaust gas from marine engines, comprising a) a spray tower (1) arranged to be vertically positioned and having an inner space (100), wherein the spray tower (1) comprises, 1) a central exhaust gas pipe (7) in fluid connection with the inner space (100) of the spray tower (1) through an exhaust gas inlet (2) positioned at a proximal end (7b) of the central exhaust gas pipe (7) arranged for introducing exhaust gas into the inner space (100), wherein said central exhaust gas pipe (7) is positioned at a bottom end (1a) of the spray tower (1) in installed position; and 2) at least one exhaust gas outlet (3) in fluid connection with, the inner space (100) of the spray tower (1) arranged for withdrawing purified exhaust gas from the inner space (100), wherein said exhaust gas outlet (3) is positioned at a top end (lb) of the spray tower (1) in installed position, wherein from the exhaust gas inlet (2) to the exhaust gas outlet (3), a general exhaust gas flow is passing through the inner space (100) of the spray tower; and 3) at least one scrubber liquid outlet (5) arranged to drain off the scrubber liquid out from the spray tower (1); b) one or more spray devices (41, 42) configured to provide a stream of scrubber liquid within the inner space (100) of the spray tower (1) counter current to the general exhaust gas flow; characterized in that at least two engine exhaust gas pipes (6) are connected to the distal end (7a) of the central exhaust gas pipe (7).

2. A system according to claim 1, wherein the stream of scrubber liquid is fresh water, salt water or an alkaline aqueous solution.

3. A system according to claim 1, wherein the one or more spray devices (41, 42) are configured to provide scrubber liquid droplets when being in operation, wherein more than 50% of the volume of the droplets have a diameter of 0.35 mm to 4 mm.

4. A system according to claim 1, wherein all spray devices (42) are arranged to spray the scrubber liquid upwards or all spray devices (41) are arranged to spray the scrubber liquid downwards.

5. A system according to claim 1, wherein part of the spray devices (42) are arranged to spray the scrubber liquid upwards and part of the spray devices (41) are arranged to spray the scrubber liquid downwards.

6. A system according to claim 1, wherein the central exhaust gas pipe (7) passes through the bottom end (1a) of the spray tower (1) and extends into the inner space (100) of the spray tower (1); and the scrubber liquid outlet (5) is positioned below the proximal end (7b) of the central exhaust gas pipe (7).

7. A system according to claim 1, wherein the spray tower (1) comprises at least one deflection body (8) reducing or preventing used scrubber liquid from entering the central exhaust gas pipe (7).

8. A system according to claim 1, wherein the system comprises a single spray tower (1).

9. A system according to claim 1, wherein each of the engine exhaust gas pipes (6) comprises an exhaust gas valve (15) arranged to prevent backflow of the exhaust gas into the exhaust gas pipes (6) when the exhaust gas valve (15) is closed.

10. A system according to claim 1, wherein each of the engine exhaust gas pipes (6) are arranged to be connected to a separate engine to convey the exhaust gas from each of the single engines to the central exhaust gas pipe (7).

11. A system according to claim 1, wherein the spray tower (1) comprises an upper (14) and a lower scrubber chamber (13) fluidly connected to each other, wherein a) both scrubber chambers (13, 14) comprise at least one spray device (41) configured to provide a stream of scrubber liquid counter current to the general exhaust gas flow through each of the scrubber chambers (13, 14); b) the upper scrubber chamber (14) comprises at least one deflection body (8) located below the spray device (41, 42) situated lowest in the upper scrubber chamber (14), and configured to prevent scrubber liquid from entering the lower scrubber chamber (13); c) the upper scrubber chamber (14) comprises at least one scrubber liquid outlet (11) arranged to drain off the used scrubber liquid out of the upper scrubber chamber (14); d) the lower scrubber chamber (13) comprises at least one deflection body (8) located below the spray device (41, 42) situated lowest in the lower scrubber chamber (13), and configured to prevent scrubber liquid from entering the central exhaust gas pipe (7); e) the lower scrubber chamber (13) comprises at least one scrubber liquid outlet (5) arranged to drain off the used scrubber liquid out of the lower scrubber chamber (13).

12. A system according to claim 11, wherein the spray devices (41) in the upper chamber (14) are arranged to spray the scrubber liquid downwards and the spray devices (42) in the lower chamber (13) are arranged to spray the scrubber liquid upwards.

13. A marine vessel comprising a system according to claim 1.

14. A method for purifying exhaust gas from at least two engines, located in a marine vessel, by reducing the amount of SO.sub.x in the exhaust gas, the method comprising the steps of: a) conveying exhaust gas from the engines via two or more engine exhaust gas pipes (6) to the distal end of a central exhaust gas pipe (7) arranged for introducing exhaust gas into the inner space (100) of a spray tower (1) through an exhaust gas inlet (2) positioned at the proximal end (7b) of the central exhaust gas pipe (7), the central exhaust gas pipe (7) being located at a bottom end (1a) of a vertically positioned spray tower (1), the two or more engine exhaust gas pipes (6) being arranged for introducing exhaust gas into the distal end (7a) of the central exhaust gas pipe (7); b) generating a general exhaust gas flow from the exhaust gas inlet (2) to an exhaust gas outlet (3), for withdrawing purified exhaust gas out from the spray tower (1), said exhaust gas outlet (3) located at a top end (1b) of the spray tower (1), c) injecting scrubber liquid into the spray tower (1) using one or more spray devices (41, 42), thereby providing a stream of scrubber liquid counter current to the general exhaust gas flow.

15. The method according to claim 14 wherein the scrubber liquid is fresh water, salt water or an alkaline aqueous solution.

16. A method according to claim 14 using a marine exhaust gas cleaning system for reducing the amount of SO.sub.x in exhaust gas from marine engines, comprising a spray tower (1) arranged to be vertically positioned and having an inner space (100), wherein the spray tower (1) comprises, 1) a central exhaust gas pipe (7) in fluid connection with the inner space (100) of the spray tower (1) through an exhaust gas inlet (2) positioned at a proximal end (7b) of the central exhaust gas pipe (7) arranged for introducing exhaust gas into the inner space (100), wherein said central exhaust gas pipe (7) is positioned at a bottom end (1a) of the spray tower (1) in installed position; and 2) at least one exhaust gas outlet (3) in fluid connection with, the inner space (100) of the spray tower (1) arranged for withdrawing purified exhaust gas from the inner space (100), wherein said exhaust gas outlet (3) is positioned at a top end (1b) of the spray tower (1) in installed position, wherein from the exhaust gas inlet (2) to the exhaust gas outlet (3), a general exhaust gas flow is passing through the inner space (100) of the spray tower; and 3) at least one scrubber liquid outlet (5) arranged to drain off the scrubber liquid out from the spray tower (1); one or more spray devices (41, 42) configured to provide a stream of scrubber liquid within the inner space (100) of the spray tower (1) counter current to the general exhaust gas flow; characterized in that at least two engine exhaust gas pipes (6) are connected with the distal end (7a) of the central exhaust gas pipe (7).

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a simplified representation of a longitudinal section of a system according to the application comprising a vertically placed spray tower that is provided with a central exhaust gas pipe located at and penetrating the bottom end of the spray tower, in which two engine exhaust gas pipes are penetrating the bottom end of the central exhaust gas pipe and in which the spray tower comprises 2 spray devices spraying scrubber liquid upwards and two spray devices spraying scrubber liquid downwards in the spray tower;

(2) FIG. 2 shows a simplified representation of a longitudinal section of a system according to the application comprising a vertically positioned spray tower with a lower and an upper scrubber chamber, in which the lower scrubber chamber comprises a central exhaust gas pipe located at and penetrating the bottom end of the spray tower, in which two engine exhaust gas pipes are penetrating the bottom end of the central exhaust gas pipe, and in which the lower and the upper scrubber chamber are provided with a spraying nozzle that is arranged to spray scrubber liquid in a downward direction;

(3) FIG. 3 shows a simplified representation of a longitudinal section of a scrubber system, comprising a vertically positioned spray tower with an upper and a lower scrubber chamber, in which the lower scrubber chamber comprises a central exhaust gas pipe located at and penetrating the bottom end of the spray tower, in which three engine exhaust gas pipes that are penetrating the bottom end of the central exhaust gas pipe and that are placed under an angle versus the bottom end, and in which the lower and the upper scrubber chamber are provided with two spray devices of which one is arranged to spray scrubber liquid in an upward direction and one is arranged to spray scrubber liquid in a downward direction;

(4) FIG. 4 shows a simplified representation of a longitudinal section of a scrubber system, comprising a vertically positioned spray tower with an upper and a lower scrubber chamber, in which the lower scrubber chamber comprises a central exhaust gas pipe located at and penetrating the bottom end of the spray tower, in which three vertically positioned engine exhaust gas pipes are penetrating the bottom end of the central exhaust gas pipe, and in which the lower scrubber chamber is provided with three spray devices that are arranged to spray scrubber liquid in an upward direction and the upper scrubber chamber is provided with two spray devices that are arranged to spray scrubber liquid in a downward direction;

(5) FIG. 5 shows a 3D view of three engine exhaust gas pipes that are connected with the bottom end of a central exhaust gas pipe;

(6) FIG. 6a shows the drop size distribution from a spiral spray device injecting 2180 liters water/minute at 2 bar pressure.

(7) FIG. 6b shows the drop size distribution from a spiral spray device injecting 3380 liters water/minute at 2 bar pressure.

DETAILED DESCRIPTION

(8) The different embodiments of a system according to the application, as shown in FIGS. 1 to 5, all comprise a spray tower (1) for removing SO.sub.x out of an exhaust gas produced by engines (not shown on the figures), more specifically marine engines, that is arranged to be essentially vertically positioned. In its simplest form, the spray tower (1) consists of a cylinder having an inner space (100). The spray tower (1) further has a bottom end (1a) and a top end (1b) in the installed position thereof. The bottom end (1a) more in particular lies at the opposite side of the top end (1b) of the spray tower (1). The cylinder more specifically has a rotational symmetry about a common length axis. It is convenient to have an exhaust gas outlet (3) coaxially arranged through the upper end (1b) of the spray tower (1). At the bottom end (1a) of the spray tower (1), a central exhaust gas pipe (7) is located. This central exhaust gas pipe (7) has an open proximal end (7b) defining an exhaust gas inlet (2) and a partially closed off distal end (7a). The distal end (7a) of the central exhaust gas pipe (7) more in particular is located at the opposite side of the proximal end (7b) of the central exhaust gas pipe (7). The central exhaust gas pipe (7) is in fluid connection with the inner space (100) of the spray tower (1). The central exhaust gas pipe (7) more specifically passes through the bottom end (1a) of the spray tower (1) and extends in the inner space (100) of the spray tower (1). The cross section of the central exhaust gas pipe (7) can have any shape. It is however advantageous to have a circular or oval cross-section. If the cross section of the central exhaust gas pipe (7) has the shape of a polygon with angles between adjacent walls, especially right and sharp angles, the flow properties could be negatively affected.

(9) At the distal end (7a) of the central exhaust gas pipe (7), at least two engine exhaust gas pipes (6), that are fluidly connected with the engines expelling the exhaust gas, are connected to the spray tower (1). These engine exhaust gas pipes (6) are thus arranged to convey the exhaust gas generated by the engines to the spray tower (1) via the central exhaust gas pipe (7). The engine exhaust gas pipes (6) more specifically passes through the distal end (7a) of the central exhaust gas pipe (7) and thus extend into the inner space (100) of the spray tower (1). It is however also possible that they are in connection with the distal end (7a) of the central exhaust gas pipe (7) without passing through this distal end (7a). By connecting the engine exhaust gas pipes (6) to the central exhaust gas pipe (7), the flow rate of the exhaust gas in the spray tower (1) is lowered and can facilitate a desired turbulent flow of the exhaust gas throughout the spray tower (1).

(10) This central exhaust gas pipe (7) allows connecting more than two engines to the spray tower (1). More in particular, it is also possible to connect 3, 4, 5 or 6 engines to the central exhaust gas pipe (7). It is possible to connect one engine to one engine exhaust gas pipe (6). Consequently, it is possible to connect 3, 4, 5 or 6 engines to the central exhaust gas pipe (7) via 3, 4, 5 or 6 engine exhaust gas pipes (6). In FIGS. 3 to 5, 3 engine exhaust gas pipes (6) are connected to the central exhaust gas pipe (7) allowing three engines to be connected to the spray tower (1) via the 3 engine exhaust gas pipes (6) that are in their turn connected to the central exhaust gas pipe (7).

(11) Backflow of exhaust gas to an engine which is not in operation, e.g. due to maintenance, can be avoided by an exhaust gas valve (15) in each of the engine exhaust gas pipes (6) (as shown in FIGS. 3 and 4).

(12) The engine exhaust gas pipes (6) can be positioned essentially vertical, as shown in FIGS. 2 and 4. This set-up will have the lowest backpressure. However, when exhaust gas valves (15) are provided in the engine exhaust gas pipes (6), there has to be space to fit these exhaust gas valves (15), through which in most cases, the engine exhaust gas pipes (6) are to be placed under an angle versus the distal end (7a) of the central exhaust gas pipe (7) and diverting away from each other starting at the distal end (7a), as can be seen in FIG. 3, resulting in a slightly higher backpressure.

(13) In the spray tower (1), a general exhaust gas flow moves throughout the spray tower (1) from the exhaust gas inlet (2) to the exhaust gas outlet (3) and comes therewith into contact with a stream of scrubber liquid that is moving counter current to the general exhaust gas flow. As used herein, the general exhaust gas flow is the average direction by which the exhaust gas moves throughout the spray tower (1). Even if the exhaust gas flow is turbulent, and in local areas may move in any direction, the general exhaust gas flow in general occurs upwards in the vertically placed spray tower (1) from the exhaust gas inlet (2) to the to the exhaust gas outlet (3).

(14) In order to produce a stream of scrubber liquid counter current to the general exhaust gas flow in order to scrub the exhaust gas flow to remove the SO.sub.x, one or more spray devices are provided in the inner space (100) of the spray tower (1). There exist spray devices (41) that are arranged to spray scrubber liquid in a downward direction and spray devices (42) that are arranged to spray scrubber liquid in an upward direction. The spray devices (41, 42) are more in particular in the form of spray nozzles (41, 42). A spray nozzle (41, 42) is more specifically mounted on an injector line (4), typically at the end thereof, and directed in an upward, respectively a downward direction to spray the scrubber liquid in an upward, respectively a downward direction. It is possible to arrange all spray devices (41) in the spray tower (1) to spray scrubber liquid in a downward direction (as shown in FIG. 2), as well to arrange all spray devices (42) to spray scrubber liquid in an upward direction (not shown in the figures). It is however also possible to provide a number of spray devices (41) to spray scrubber liquid in a downward direction and to provide a number of spray devices (42) to spray scrubber liquid in an upward direction (as shown in FIGS. 1, 3 and 4).

(15) The spray devices (41, 42) create falling scrubber liquid droplets (not shown on the figure). The stream of scrubber liquid may be any aqueous liquid or aqueous solution including, but not limited to, fresh water, salt water and an alkaline aqueous solution. For marine vessels, the stream of scrubber liquid may conveniently be salt water, in particular sea water. Addition of an alkaline aqueous solution comprising dissolved MgO or Mg(OH).sub.2 to the scrubber liquid can increase the SO.sub.x absorption capacity. Such increase in capacity is especially important if the scrubber liquid is recycled back into the spray tower (closed loop system). This is because the absorption of SO.sub.x will reduce the pH of and thus reduce the capacity to further absorb additional SO.sub.x contaminants.

(16) The spray devices (41, 42) can be of any kind e.g. spray nozzles comprising a plain orifice or a shaped orifice. In particular, the spray devices (41, 42) can be a surface impingement spray nozzle or a flat fan spray pattern nozzle. In particular, it is often desired to utilize spray devices comprising an impingement surface forming a spiral (spiral spray device) which generates a full cone spray pattern or a hollow-cone spray pattern. The spiral design generally produces smaller drop size than pressure swirl type nozzle design, for a given pressure and flow rate. This spiral design can also be more clogging resistant due to a larger free passage. Suitable, and often desired, spiral spray devices can provide a full cone spray with a drop size distribution as shown in FIG. 6a or 6b or smaller when injecting 2180 liters water/minute or 3380 liters water/minute respectively at 2 bar pressure.

(17) The spray devices (41, 42) are typically configured to provide scrubber liquid droplets when being in operation wherein more than 50% of the volume of the droplets have a diameter of 0.35 mm to 4 mm, more specifically 0.5 to 2 mm, most specifically around 1 mm. As used herein, the diameter of the droplets, also called drop size is the Sauter Mean Diameter. This is the diameter of a particle whose ratio of volume to surface area is equal to that of the complete spray sample. It is defined as the cube of the Volume Mean Diameter divided by the square of the Surface Mean Diameter. The Sauter Mean Diameter is typically about 80% of the Volume Median Diameter.

(18) As used herein, scrubber liquid droplets are scrubber liquid particles with a size sufficient to fall downwards in the spray tower during exhaust purification. The sufficient size will depend on the exhaust flow and the exhaust temperature and can be readily determined by a skilled person. Such scrubber liquid droplets will fall counter current to the general exhaust flow. Scrubber liquid droplets may evaporate to become smaller and even scrubber liquid vapour depending on several parameters like their initial size, the exhaust temperature and the exhaust flow. As used herein, scrubber liquid droplets are scrubber liquid particles with a size insufficient to fall downwards in the spray tower during exhaust purification. Thus, scrubber liquid droplets will essentially move along with the general exhaust flow. The scrubber liquid droplets may collide with each other and thus increase their size to become bigger. The injected water volume can be adjusted according to the engine load to optimize performance of the scrubber. For example, the injected water volume may be in the range of 10 000 to 15 000 litres per minute for a 10-15 meter high spray tower (1) with a diameter of about 3 meters, when purifying 30 kg exhaust per second.

(19) The spray tower (1) according to the present application can exist out of only one scrubber chamber (13) (as can be seen in FIG. 1), but can also exist out of multiple scrubber chambers (13, 14), as can be seen in FIGS. 2 to 4. In the system as shown in FIGS. 2 to 4, two scrubber chambers (13, 14) are provided. When the spray tower (1) comprises two or more scrubber chambers (13, 14), they are partly separated from each other but also partly fluidly connected to each other by means of a connection (9). The connection (9) can for example be arranged as a coaxial constriction, conveying partly scrubbed exhaust gas from the lower scrubber chamber (13) to the upper scrubber chamber (14).

(20) The number of spray devices (41, 42) and the direction in which they spray the scrubber liquid can vary within one scrubber chamber (13) or throughout different scrubber chambers (13, 14). In FIG. 1, in which the spray tower (1) only has one scrubber chamber (13), four spray devices (41, 42) are provided of which the upper two spray devices (41) are arranged to spray scrubber liquid in a downward direction while the lower two spray devices (42) are arranged to spray scrubber liquid in an upward direction. In FIGS. 2 to 4, the spray tower (1) has a lower and an upper scrubber chamber (13, 14). In FIG. 2, the lower scrubber chamber (13) as well as the upper scrubber chamber (14) are provided with one spray device (41) that is arranged to spray scrubber liquid in a downward direction. In FIG. 3, the lower scrubber chamber (13) as well as the upper scrubber chamber (14) has two spray devices (41, 42) of which one spray device (41) is arranged to spray scrubber liquid in a downward direction and one spray device (42) is arranged to spray scrubber liquid in an upward direction. In FIG. 4, the lower scrubber chamber (13) is provided with three spray devices (42) all arranged to spray scrubber liquid in an upward direction while the upper scrubber chamber (14) is provided with two spray devices (41) that are arranged to spray scrubber liquid in a downward direction.

(21) The spray tower (1) can furthermore be provided with one or more deflection bodies (8) that are arranged to reduce, or in the optimal case prevent the scrubber liquid from entering the central exhaust gas pipe (7) or entering the connection (9) between the upper and the lower scrubber chamber (13, 14) or in other words the exhaust outlet from the lower scrubber chamber (13) if present. The deflection bodies (8) can furthermore direct the exhaust towards the walls of the spray tower (1) and in this way create a turbulent gas flow. The shape of the deflection bodies (8) may also affect the backpressure and can be easily modified by a person skilled in the art accordingly. For example, the longitudinal section of the deflection bodies (8) can be roughly diamond shaped or have any other shape compatible with low backpressure. Similarly, the cross section of the deflection bodies (8) may be circular, oval or any other shape compatible with low backpressure. The deflection bodies (8) typically are placed below the spray device(s) (41, 42).

(22) In order to drain off the used scrubber liquid out of the spray tower (1), per scrubber chamber (13, 14), a scrubber liquid outlet (5, 11) is arranged. The connection (9) between the upper and the lower scrubber chamber (13, 14) typically has a shape to take care that the used scrubber liquid is easily directed towards the scrubber liquid outlet pipe(s) (5, 11) to be drained off out of the spray tower (1). When there is one scrubber chamber (13), the scrubber liquid outlet (5) is typically placed in the bottom end (1b) of the spray tower (1) in order to optimally drain off the used scrubber liquid out of the spray tower (1). If the central exhaust gas pipe (7) passes through the bottom end (1b) of the spray tower (1), as is the case in the system as shown in FIG. 1, the scrubber liquid outlet (5) is typically placed below the proximal end (7b) of this central exhaust gas pipe (7). When the spray tower (1) has a multiplicity of scrubber chambers (13, 14), as is the case in the systems as shown in FIGS. 2 to 4, then in the lower scrubber chamber (13), the scrubber liquid outlet (5) is also placed in the bottom end (1b) of the spray tower (1), and if the central exhaust gas pipe (7) passes through the bottom end (1b) of the spray tower (1), the scrubber liquid outlet (5) is placed below the proximal end (7b) of the central exhaust gas pipe (7). In the upper chamber (14), the scrubber liquid outlet (11) is arranged in one of the side walls (1c) of the spray tower (1) above the bottom end (9b) of the connection (9) but below the open top end (9a) of this connection (9). This allows an efficient draining off of used scrubber liquid out of the upper scrubber chamber (14).

(23) The spray tower may optionally comprise a droplet separator (not shown on the figures) able to prevent scrubber liquid droplets from leaving the spray tower (1) along with the purified exhaust gas. The droplet separator can be mounted close to the exhaust gas outlet (3).

(24) Tests with a system according to the application as shown in FIG. 4, having a spray tower (1) with a height of 12 meter, have demonstrated that exhaust gas from fuel oil comprising 3,3 w/w % of sulfur was purified to contain only 5 to 8 ppm SO.sub.2. These test results were obtained when the exhaust gas temperature in the central exhaust gas pipe (7) was up to 280 C. and the temperature of the purified exhaust gas leaving the spray tower (1) at the exhaust gas outlet (3) was about 40 C.