Abstract:
The present invention relates to a method of cleaning a process gas containing sulphur dioxide the method including removing sulphur dioxide from the process gas by contacting the process gas with seawater to generate an at least partly cleaned process gas in a first gas cleaning device. In a second gas cleaning device, being arranged in direct fluid connection with the first gas cleaning device, the at least partly cleaned process gas having passed through the first gas cleaning device is cooled to condense water there from, thereby generating a process gas having a reduced content of water vapour. At least a part of the condensed water generated in the second gas cleaning device is passed to the first gas cleaning device. The present invention moreover relates to a gas cleaning system for cleaning of a process gas containing sulphur dioxide.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European application 12182954.3 filed Sep. 4, 2012, the contents of which are hereby incorporated in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a system and a method of cleaning a process gas containing sulphur dioxide by contacting the process gas with seawater for removal of sulphur dioxide from said process gas. 
       BACKGROUND 
       [0003]    Process gases containing sulphur dioxide, SO 2 , are generated in many industrial processes. One such industrial process is the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant. In such a power plant, a hot process gas, often referred to as a flue gas, is generated containing pollutants including acid gases, such as sulphur dioxide, SO 2 . It is necessary to remove as much of the acid gases as possible from the flue gas before the flue gas may be emitted to the ambient air. Another example of an industrial process in which a process gas containing pollutants is generated is the electrolytic production of aluminium from alumina. In that process, a process gas containing sulphur dioxide, SO 2 , is generated within venting hoods of the electrolytic cells. 
         [0004]    WO 2008/105212 discloses a boiler system comprising a boiler, a steam turbine system, and a seawater scrubber. The boiler generates, by combustion of a fuel, high-pressure steam utilized in the steam turbine system for generating electric power. Seawater is collected from the ocean, and is utilized as a cooling medium in a DCC of the steam turbine system. The seawater is then utilized in the seawater scrubber for absorbing sulphur dioxide, SO 2 , from flue gas generated in the boiler. Sulphur dioxide, SO 2 , is absorbed in the seawater and forms sulfite and/or bisulfite ions. Effluent seawater from the seawater scrubber is forwarded to an aeration pond. Air is bubbled through the effluent seawater in the aeration pond for oxidation by means of oxygen gas contained in the air, of the sulfite and/or bisulfite ions to sulphate ions for release back to the ocean together with the effluent seawater. 
       SUMMARY 
       [0005]    An object of the present invention is to provide a seawater based method and a system for cleaning a process gas containing sulphur dioxide, such as a carbon dioxide rich flue gas generated in a boiler combusting a fuel in the presence of a gas containing oxygen, the method being improved with respect to sulphur dioxide removal efficiency and process integration. 
         [0006]    Another object of the present invention is to provide a method and a system for cleaning a process gas containing sulphur dioxide that require less capital expenses. 
         [0007]    The above-noted objects are, in a first aspect, achieved by means of a method comprising: 
         [0008]    in a first gas cleaning device, removing sulphur dioxide from the process gas by contacting the process gas with seawater to generate an at least partly cleaned process gas; 
         [0009]    in a second gas cleaning device being arranged in direct fluid connection with the first gas cleaning device, cooling said at least partly cleaned process gas having passed through said first gas cleaning device to condense water there from, thereby generating a process gas having a reduced content of water vapour, and 
         [0010]    passing at least a part of the condensed water generated in the second gas cleaning device to the first gas cleaning device. 
         [0011]    The method according to the invention may advantageously be used in industrial carbon capture and storage (CCS) applications, such as in an oxyfuel boiler island. The present method may enable improved removal of sulphur dioxide (SO 2 , as well as other sulphur oxides; SOx, such as SO 3 ) as compared to previous seawater based methods, by combining sulphur dioxide removal and cooling operations in devices that are arranged in direct fluid connection as described above. 
         [0012]    In known industrial CCS applications, cooling is performed in an acidic stage separate from the alkaline sulphur dioxide removal. Acidic conditions are generally required in order to avoid carbonate formation in e.g. cooling equipment. Carbonate formation may cause scaling or fouling on e.g. packings and heat-exchanger surfaces. In the present method, cooling may be performed in direct fluid connection with the alkaline stage of the sulphur dioxide removal without increasing the risk of carbonate formation. In addition, seawater carry-over in e.g. ductwork, which may be detrimental in that it causes corrosion, may further be minimized. 
         [0013]    Cooling of the process gas in the gas cleaning device enables condensation of water vapour from the process gas. The process gas leaving the second gas cleaning device thus contains a lower amount of water vapour than the at least partly cleaned process gas having passed through the first gas cleaning device. A process gas containing only a low amount of water vapour is consequently forwarded for further processing, such as for example gas compression operations. 
         [0014]    Passing of at least a part of the condensed water from said second device to said first device may be advantageous in that it allows utilization of the condensed water in other parts of e.g. a CCS process. This may moreover reduce the overall water consumption of the method. 
         [0015]    According to one embodiment, cooling in the second gas cleaning device comprises contacting the at least partly cleaned process gas with a cooling liquid to condense water there from, thereby further generating a used cooling liquid. By use of a cooling liquid, such as for example water, the partly cleaned process gas may be efficiently cooled and the content of water vapour comprised in the process gas may thereby efficiently be reduced by condensation. In addition, much of the remaining content of sulphur dioxide in the partly cleaned process gas may be removed by such direct contact with a cooling liquid. 
         [0016]    According to one embodiment, the method further comprises polishing, in the second gas cleaning device, the partly cleaned process gas to further remove sulphur dioxide there from, thereby generating a cleaned process gas. An advantage of this embodiment is that the content of SOx compounds (and other traces) in the process gas may be further reduced, such that a cleaned process gas is generated. 
         [0017]    According to one embodiment, the method further comprises controlling the pH-value of the cooling liquid to be in the range of 4, 5-7 by supply of an alkaline substance. An advantage of this embodiment is that a pH of 4, 5-7, and more preferably a pH of 5-6, may improve sulphur dioxide removal efficiency. A good gas polishing effect is hence achieved in the second gas cleaning device. The alkaline substance may be chosen from substances having a high pH influence on the cooling liquid. This ensures that the sulphur dioxide uptake ability of the cooling liquid is kept high while having only a small alkaline substance consumption rate. One non-limiting example of an alkaline substance is sodium hydroxide. 
         [0018]    According to one embodiment, cooling of the process gas in the second gas cleaning device reduces the temperature of the process gas by 10-55° C., such as 20-55° C., such as 30-55° C. In this embodiment, the process gas leaving said second device thus has a temperature that is lower than the temperature of the partly cleaned gas having passed through said first device. Having a distinct temperature gradient in the second gas cleaning device may further improve condensation of water vapour from the partly cleaned process gas, and may concurrently further improve sulphur dioxide, and/or SOx, removal. 
         [0019]    According to one embodiment, the method further comprises returning used cooling liquid to the second gas cleaning device; wherein during said returning, the used cooling liquid is subjected to heat-exchanging with the seawater feed prior to providing the seawater feed to the first gas cleaning device. Used cooling liquid is thus, after heat-exchanging, forwarded to the second gas cleaning device for utilization once again for cooling of the process gas. Recirculation of used cooling liquid and heat-exchanging enable efficient control of heat transfer in the method by for example improving temperature control of liquid circulation in the second gas cleaning device. Efficient temperature control moreover prevents drying up of the second gas cleaning device and thus maintenance of the liquid balance. 
         [0020]    According to one embodiment, all surplus water generated in said second device is passed to said first device. In some instances, all of the condensed water generated in the second gas cleaning device is passed to the first gas cleaning device. In other instances, a portion of the condensed water generated in the second gas cleaning device is added to the recirculation of cooling liquid in order to keep the liquid balance, while the surplus water is passed directly to the first gas cleaning device. In this way, the liquid balance of the second gas cleaning device may be optimized. 
         [0021]    There is, in another aspect, provided a gas cleaning system for cleaning a process gas containing sulphur dioxide comprising: 
         [0022]    a first gas cleaning device being arranged for receiving the process gas containing sulphur dioxide, for receiving seawater feed and for contacting the process gas with the seawater for removal of sulphur dioxide from the process gas, thereby generating an at least partly cleaned process gas; 
         [0023]    a second gas cleaning device being arranged in direct fluid connection with the first gas cleaning device, for receiving the at least partly cleaned process gas having passed through the first gas cleaning device, and for removing at least a portion of the water content of the partly cleaned process gas by means of cooling the partly cleaned process gas to condense water there from, thereby generating a process gas having a reduced content of water vapour; 
         [0024]    wherein the first gas cleaning device is arranged for receiving at least part of the condensed water generated in the second gas cleaning device. 
         [0025]    It should be understood that specific embodiments as well as advantages disclosed in respect of the method aspect of the invention are contemplated as equally relevant embodiments and advantages, where applicable, to the system aspect of the invention and vice versa. Specific advantages of equivalent embodiments are thus not further elaborated for a second aspect if they are disclosed in respect of a first aspect. 
         [0026]    An advantage of the present gas cleaning system is that it provides for a cleaning of the process gas which is efficient both with regards to removal of sulphur dioxide and water vapour as well as with regards to operating and investment costs. By arrangement of said first and second device in direct fluid connection the plot space requirements of the system is for example reduced. More specifically, the overall system size, such as the cross-sectional area and the height of the combination of said first and second device, may be reduced. 
         [0027]    According to one embodiment, the second gas cleaning device is arranged for receiving a cooling liquid and for contacting the partly cleaned process gas with the cooling liquid, thereby further generating a used cooling liquid. Said second device may in some instances comprise a direct contact cooler that is arranged for contacting said process gas with said cooling liquid. 
         [0028]    Alternatively, the second gas cleaning device may comprise a tube DCC being arranged for indirect cooling of the partly cleaned process gas. In this embodiment, no cooling liquid for direct contacting with the process gas is utilized in the second gas cleaning device. 
         [0029]    According to one embodiment, the second gas cleaning device is provided with a packing material for bringing the cooling liquid into contact with the at least partly cleaned process gas. Efficient contact between the cooling liquid and the at least partly cleaned process gas can in this way be achieved in a manner which does not generate a large amount of very small liquid droplets that might harm downstream equipment. 
         [0030]    According to one embodiment, the second gas cleaning device is provided with a pH-control device which is arranged for controlling the pH of the cooling liquid by supply of an alkaline substance. This may enable more efficient removal of sulphur dioxide from the partly cleaned process gas and utilization of less expensive steel materials in the second gas cleaning device. 
         [0031]    According to one embodiment, the gas cleaning system further comprises a heat-exchanger being arranged for receiving seawater feed, prior to passing it to the first gas cleaning device, and for receiving used cooling liquid generated in the second gas cleaning device, the heat-exchanger being arranged for exchanging heat between the seawater feed and the used cooling liquid. Apart from providing advantages as disclosed in respect of the method aspect, such a heat-exchanger may function as a lever for creating a desirable temperature gradient in the second gas cleaning device and thus for further promoting condensation of water vapour from the partly cleaned process gas. 
         [0032]    According to one embodiment, a chimney or a duct is provided for directly forwarding the process gas having passed through the first gas cleaning device to the second gas cleaning device. A chimney or duct may thus provide the direct fluid connection between said first and second devices with respect to gases. Having passed the first gas cleaning device, the partly cleaned process gas is directly forwarded, or passed, to the second gas cleaning device. 
         [0033]    According to one embodiment, a liquid collection receptacle is provided between the first and the second gas cleaning device, the liquid collection receptacle being arranged for collecting condensed water generated in the second gas cleaning device and for directly forwarding at least part of the condensed water to the first gas cleaning device. Thus, the liquid collection receptacle provides corresponding direct liquid connection between the second gas cleaning device and the first gas cleaning device. 
         [0034]    According to one embodiment, the first gas cleaning device is arranged for receiving all of the condensed water generated in the second gas cleaning device. In this embodiment, the first gas cleaning device is capable of handling all of the condensed water; i.e. no water has to be passed elsewhere. In some instances, the first gas cleaning device is arranged for receiving all surplus water generated in the second gas cleaning device. 
         [0035]    According to one embodiment, the second gas cleaning device is provided on top of the first gas cleaning device within the same column or tower. This embodiment has the advantage of further reducing the plot space requirements, or the footprint of the system, as well as reducing the overall height of the system. 
         [0036]    Further objects and features of the present invention will be apparent from the description and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    The invention will now be described in more detail with reference to the appended drawings in which: 
           [0038]      FIG. 1  is a schematic side cross-section view of a power plant with a seawater based gas cleaning system of the prior art. 
           [0039]      FIG. 2  is a schematic side cross-section view of a direct contact cooler of the prior art. 
           [0040]      FIG. 3  is a schematic side cross-section view of a gas cleaning system according to one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    As used throughout the description, the terms “cleaned”, “cleaning” and “partly cleaned” should primarily be understood as referring to removal of SOx compounds, such as sulphur dioxide and sulphur trioxide, from a process gas. 
         [0042]    When used to describe connection and relative position between devices, “direct fluid connection” should be understood as a connection providing direct forwarding, or passage, of gas and liquid. One specific example of direct fluid connection is a system wherein the second gas cleaning device is located adjacent, such as on top of, the first gas cleaning device. 
         [0043]      FIG. 1  is a schematic side cross-section view illustrating a power plant  1  according to prior art. Power plant  1  comprises a boiler  2  in which a fuel, such as coal, oil, peat, natural gas, or waste, supplied via feeding pipe  4  is combusted in the presence of oxygen, supplied via oxygen supply duct  5 . Oxygen may, for example, be supplied in the form of air and/or in the form of a mixture of oxygen gas and recirculated gases, in case boiler  2  is a so-called “oxy-fuel” boiler. The combustion of fuel generates a hot process gas in the form of a flue gas. Sulphur species contained in the fuel upon combustion form, at least partly, sulphur dioxide, SO 2 , which forms part of the flue gas. 
         [0044]    The flue gas may flow from boiler  2  via a fluidly connected duct  6 , to an optional dust removal device in the form of an electrostatic precipitator  3 . The electrostatic precipitator  3  (an example of which is described in U.S. Pat. No. 4,502,872), serves to remove dust particles from the flue gas. As an alternative, another type of dust removal device may be used, such as for example, a fabric filter (an example of which is described in U.S. Pat. No. 4,336,035. 
         [0045]    Flue gas, from which most of the dust particles optionally have been removed, flows from the electrostatic precipitator  3  via a fluidly connected duct  7  to a seawater scrubber  18 . Seawater scrubber  18  comprises a wet scrubber tower  10 . An inlet  8  is arranged at a lower portion  9  of wet scrubber tower  10 . Duct  7  is fluidly connected to inlet  8  such that flue gas flowing from electrostatic precipitator  3  via duct  7  may enter interior  11  of wet scrubber tower  10  via inlet  8 . 
         [0046]    After entering interior  11 , flue gas flows vertically upwards through wet scrubber tower  10 , as indicated by arrow F. Central portion  12  of wet scrubber tower  10  is equipped with a number of spray arrangements  13  arranged vertically one above each other. In the system of  FIG. 1 , there are three such spray arrangements  13 , and typically there are 1 to 20 such spray arrangements  13  in a wet scrubber tower  10 . Each spray arrangement  13  comprises a supply pipe  14  and a number of nozzles fluidly connected to each supply pipe  14 . Seawater supplied via supply pipes  14  to the nozzles is atomized and contacts in interior  11  of wet scrubber tower  10 , the flue gas for absorption of sulphur dioxide, SO 2 , there from. 
         [0047]    A pump  15  is arranged for pumping seawater via fluidly connected suction pipe  16  from ocean  17 , and forwarding the seawater via fluidly connected pressure pipe  18  to fluidly connected supply pipes  14 . 
         [0048]    Seawater atomized by means of nozzles in interior  11  of wet scrubber tower  10  flows downwardly within wet scrubber tower  10  and absorbs sulphur dioxide from flue gas F flowing vertically upwards within interior  11  of wet scrubber tower  10 . As a result of such absorption of sulphur dioxide by the seawater, the seawater gradually turns into effluent seawater as it flows downwardly within interior  11  of wet scrubber tower  10 . Effluent seawater is collected in lower portion  9  of wet scrubber tower  10  and is forwarded, via fluidly connected effluent pipe  19 , from wet scrubber tower  10  to the ocean or to an effluent seawater treatment system (not shown). 
         [0049]      FIG. 2  is a schematic cross-section view illustrating a direct contact cooler (DCC)  20 , typically forming part of a CCS system, according to prior art. The DCC  20  comprises a tower  25 , which is filled with a packing material  26  for providing good contact between a flue gas, typically containing carbon dioxide, coming from e.g. a limestone based wet scrubber or a spray dryer absorber, and the cooling liquid being circulated in the DCC  20  by means of the pump  22  in the pipe  24 . A liquid distributor  27  is arranged for evenly distributing the cooling liquid, e.g. water, over the packing material. 
         [0050]    The flue gas is supplied, via the duct  21 , to the lower end of the tower  25  and moves vertically upwards through the tower  25 , being brought into contact, in a counter-current flow manner, with the cooling liquid flowing down through the packing material  26 . At the upper end of the tower  25  a mist eliminator  28  is arranged. The mist eliminator  28  removes liquid droplets from the flue gas. 
         [0051]    A heat exchanger  31  is arranged in the pipe  24 , as illustrated in  FIG. 2 . The heat exchanger  31  cools the cooling liquid being transported in the pipe  24 . A cooling media is supplied to the heat exchanger  31  via a pipe  32 , and leaves the heat exchanger  31  via a pipe  33 . The cooling media may come from a cooling tower. 
         [0052]    The cooling media supplied to the heat exchanger  31  of the DCC  20 , as illustrated in  FIG. 2 , has a temperature adapted for adequate cooling of the cooling liquid circulating in the pipe  24 . In the packing material  26  of the DCC  20  the flue gas is cooled, upon the direct contact with the cooling liquid. As a result of this cooling, generally being a cooling to a temperature below the saturation temperature with respect to water vapour, water condenses from the flue gas inside the DCC  20 . Hence, the flue gas leaving the DCC  20  via the duct  29  will have a lower water content than the flue gas entering the DCC  20 . A fan  30  is arranged for forwarding the flue gas to e.g. a gas processing unit (not shown). 
         [0053]    A pH-sensor  34  is arranged for measuring the pH of the cooling liquid being forwarded in the pipe  24 . A control unit (not shown) is typically arranged for receiving a signal from the pH-sensor  34 . The control unit controls the supply of an alkaline substance, such as NaOH, from an adjacent alkaline substance storage (not shown). Hence, the control unit typically compares the pH as measured by means of the pH sensor  34  to a pH set point. When the pH measured by the pH sensor  34  is below the pH set point the control unit sends a signal to an alkali supply device (e.g. in the form of a pump) to the effect that alkaline substance is to be pumped from the storage via a pipe (not shown) to the pipe  24  in order to increase the pH of the cooling liquid. 
         [0054]    Before leaving the DCC  20 , the flue gas is passed through the mist eliminator  28  which removes liquid droplets entrained with the flue gas flow. In some instances, the flue gas of the duct  29  may be reheated in a heat-exchanger (not shown) in order to increase the temperature of the flue gas of the duct  29 . Reheating may in this way evaporate some of the very small droplets and mist that have passed through the mist eliminator  28 . 
         [0055]    An embodiment of the present invention will now be described with reference to  FIG. 3 . 
         [0056]      FIG. 3  is a schematic cross section view illustrating a combined gas cleaning system  40  according to one embodiment of the present invention. Flue gas containing sulphur dioxide flows e.g. from boiler  2  optionally via a dust removal device  3 , as illustrated in  FIG. 1 , in duct  7  to the first gas cleaning device  42 . The flue gas enters the interior  43  of the first gas cleaning device  42 , e.g. wet scrubber section, via inlet  8 . 
         [0057]    Having entered interior  43  of the wet scrubber section  42 , flue gas flows vertically upwards through wet scrubber section  42 . Central portion  44  of wet scrubber section  42  is equipped with a number of spray arrangements  13  arranged vertically one above each other. In the embodiment of  FIG. 3 , four such spray arrangements  13  are arranged. There may be 2 to 7 such spray arrangements  13  installed in a wet scrubber section  42 . Depending on the process, the number of spray arrangements in operation can be smaller than the number of spray arrangements installed. Each spray arrangement  13  comprises a supply pipe  14  and a number of nozzles fluidly connected to each supply pipe  14 . Seawater supplied via supply pipes  14  to the nozzles is atomized and contacts in interior  43  of wet scrubber tower  42 , the flue gas for absorption of sulphur dioxide, SO 2 , there from. 
         [0058]    A pump  15  is arranged for pumping seawater via fluidly connected suction pipe  16  from ocean  17 , and forwarding the seawater via fluidly connected pressure pipe  18  to fluidly connected supply pipes  14 . in some instances, seawater supplied to pipes  14  may be seawater previously utilized as cooling water in e.g. steam turbine systems associated with boiler  2  prior to such seawater being utilized as scrubbing water in seawater scrubber  42 . 
         [0059]    Atomized seawater flows downwardly in the interior  43  of the wet scrubber section  42  of the system and absorbs sulphur dioxide from the flue gas flowing vertically upwards. Absorption of sulphur dioxide into the seawater generates an at least partially cleaned flue gas and effluent seawater. The effluent seawater is collected in lower portion  41  of the wet scrubber section  42  of the gas cleaning system. Effluent seawater may be forwarded via fluidly connected effluent pipe  19  to the ocean or to an optional effluent seawater treatment system. 
         [0060]    The flue gas forwarded to the wet scrubber section  42  typically has a temperature of 90-180° C. Upon contacting in the interior  43  of the wet scrubber section  42  with the relatively cold seawater deriving from the ocean  17 , the flue gas will be partially cooled. Partial cooling in the wet scrubber section may be controlled by control of the supply temperature of the seawater in supply pipes  14  as well as by the number of spray arrangements in operation. Flue gas may thus be cooled to a temperature of 40-75° C. in the wet scrubber section  42 . The seawater transported in the supply pipes may be partially heated in an optional heat-exchanger  60 , as will be further described below. Depending on the amount of supplied seawater, the liquid to gas ratio in the wet scrubber section may be in the range of 5-20:1. 
         [0061]    The at least partly cleaned flue gas leaves wet scrubbing section  42  via chimney arrangement  46 , which is adapted to forward the flue gas to the second gas cleaning device  45 , e.g. DCC section, of the gas cleaning system  40 . The flue gas is forwarded upwardly through the chimney arrangement  46 , which is connected to liquid collection receptacle  51 , e.g. tray. Above chimney arrangement  46  a top cover arrangement  50  is provided. Flue gas leaves the chimney arrangement  46  below the top cover arrangement  50  and passes between the individual covers of the top cover arrangement  50  into the lower part of the DCC section  45 . 
         [0062]    Alternatively, a bypass duct (not shown) may be arranged for forwarding the partly cleaned flue gas from the wet scrubber section  42  (first gas cleaning device) to the DCC section (second gas cleaning device)  45 . A bypass duct may have an outlet arranged below tray  51 , that separates the wet scrubber section  42  from the DCC section  45 , and an inlet arranged above tray  51  such that flue gas may enter into the interior of the DCC section  45 . 
         [0063]    The DCC section  45  comprises a packing material  49 , being arranged for providing good contact between the flue gas, at least partly cleaned from sulphur dioxide, and the cooling liquid being circulated in the DCC section  45  by means of pump  55  in pipe  47 . A liquid distributor  48  is arranged for distributing the cooling liquid over the packing material. The liquid distributor  48 , which may be, for example, Jaeger Model LD3 or Model LD4, which are available from Jaeger Products, Inc, Houston, USA, or liquid distributors available from Koch-Glitsch LP, Wichita, USA, distributes the liquid evenly over the packing material  49  without causing an undue formation of small liquid droplets. 
         [0064]    The packing material  49  could be of the so-called structured packing type, for example Mellapak Plus, which is available from Sulzer Chemtech USA Inc, Tulsa, USA, or Flexipak, which is available from Koch-Glitsch LP, Wichita, USA. Alternatively, the packing material  49  could be of the so-called random packing type, for example Jaeger Tri-Pack, which is available from Jaeger Products, Inc, Houston, USA, or IMTP, which is available from Koch-Glitsch LP, Wichita, USA. 
         [0065]    The flue gas entering the lower part of the DCC section  45  moves vertically upwards through the DCC  45 , being brought into contact, in a counter-current flow manner, with the cooling liquid flowing down through the packing material  49 . The liquid to gas ratio of the DCC section  45  may for example be 2-6:1, such as 3:1. Optionally, a mist eliminator, principally as illustrated in  FIG. 1 , may be arranged at the upper part of the DCC section. 
         [0066]    A heat exchanger  35  is arranged in the pipe  47 . The heat exchanger  35  is arranged for cooling the cooling liquid being transported in the pipe  52  by means of a cooling media. This cooling media, e.g. water or water containing glycol originating from a cooling tower, is supplied to the heat exchanger  35  via a pipe  56 , and leaves the heat exchanger  35  via a pipe  57 . Cooling of the at least partly cleaned flue gas is enabled in the DCC section  45  by contacting the partly cleaned flue gas with the cooling liquid in the form of, e.g. clean water. This cooling promotes condensation of water vapour from the flue gas inside the DCC section  45 . Hence, the flue gas leaving the DCC section  45  will have a lower water content than the flue gas entering the DCC section  45 . The condensed water generated in the DCC section  45  flows downwardly, together with the cooling liquid successively becoming used, in the DCC section  45  and is collected on tray  51 . Top cover  50  of the chimney arrangement  46  prevents the used cooling liquid and the condensed water from entering the chimney arrangement  46 . 
         [0067]    The partly cleaned flue gas is cooled considerably in the DCC section  45  of the gas cleaning system  40 , as illustrated in  FIG. 3 . The flue gas, depleted in water vapour, leaving the gas cleaning system via upper part  58  of the DCC section  45 , may have a temperature that is 10-55° C. lower than the partly cleaned flue gas entering the DCC section  45 . This creates a distinct temperature gradient in the packing material  49 , leading to significant condensation of water vapour from the flue gas. 
         [0068]    An overflow tube  63  may be arranged, e.g. as a part of tray  51  and extending downwards along the side of the gas cleaning system  40 , for handling overflow of liquid in the DCC section  45 . The overflow tube, having an inlet in the DCC section  45  and an outlet in the lower part  41  of the wet scrubber section  42 , is arranged for forwarding a liquid (volume) portion comprising primarily the water condensed from the flue gas from the DCC section  45  to the wet scrubber section  42 . Preferably, all surplus liquid, comprising principally all condensed water generated in the DCC section  45 , is passed from the DCC section  45  to the wet scrubber section  42 . 
         [0069]    Condensed water generated in the DCC section  45  may consequently be passed to the wet scrubber section  42  independently of any recirculation of cooling liquid in the DCC section  45 . In the embodiment of  FIG. 3 , pump  55  withdraws cooling liquid in pipe  52  up to a normal liquid (volume) level, while the overflow tube handles the liquid above the normal (volume) level. This helps to keep the water balance of the DCC section  45 . 
         [0070]    In addition to cooling, the direct contacting of the cooling liquid and the flue gas in the packing material  49  of the DCC section  45  will also result in further removal of sulphur dioxide. The DCC section  45  thus further generates a cleaned flue gas which may be forwarded to e.g. gas compression. This further increases the sulphur removal capacity of the gas cleaning system, as compared to seawater scrubbers of the prior art. 
         [0071]    The sulphur dioxide becoming dissolved in the cooling liquid of the DCC  45  will result in a decrease in the pH value of the cooling liquid circulating in the pipe  47 . A pH-sensor (not shown) may be arranged for sensing such decrease in pH-value and for ordering a pump (not shown) to supply an alkaline substance from a storage (not shown) to the pipe  47 . pH control may be performed essentially as described with respect to  FIG. 2 . The set point for the pH-value is typically pH 4, 5-7. Such a set point has been found to provide efficient removal of sulphur dioxide, without a large and unwanted removal of carbon dioxide from the flue gas. Controlling the pH value of the cooling liquid circulating in the DCC section  45  will also control the removal efficiency of the sulphur dioxide. Hence, the pH set point is typically set to such a value that at least 70% of the remaining sulphur dioxide content of the partly cleaned flue gas is removed in the DCC section  45 . Examples of suitable alkaline substances include sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium carbonate (Na 2 CO 3 ), and sodium bicarbonate (NaHCO 3 ). Often, the most preferred alkaline substance is sodium hydroxide (NaOH). 
         [0072]    The significant condensation occurring in the packing material  49  of the DCC section  45  moreover provides for an efficient removal of sulphur trioxide, SO 3 , which is present, principally in the form of an aerosol, in the partly cleaned flue gas. It is assumed that the water condensing in the packing material  49  to a large extent condenses on the aerosol particles, making such aerosol particles grow to droplets of such a size that they become captured by the circulating cooling liquid circulating in the packing material  49 . 
         [0073]    By efficient condensation and removal of SOx-substances as described above, carbonate formation may be held at a minimum in the DCC section  45 . 
         [0074]    Optionally, another heat-exchanger  60  is arranged in pipe  47  for further cooling of the cooling liquid circulated in the DCC section  45 . The cooling media utilized in the heat-exchanger  60  is the seawater feed transported in pipe  18 . Seawater feed enters the heat-exchanger  60  via a pipe  61  and leaves the heat-exchanger via a pipe  62 . The pipes  61  and  62  may fluidly connect to pipe  18  or to one or more of the supply pipes  14 . In order to balance the flow rate of the cooling liquid in pipe  47 , the pipe  61  may be connected at various positions of pipe  18  and supply pipes  14 . Heat-exchanger  60  thus generates a cooled cooling liquid, as compared to the cooling liquid upstream of the heat-exchanger  60 , and a heated seawater feed, as compared to the seawater feed upstream of the heat-exchanger  60 . By addition of this heat-exchanger  60  to the gas cleaning system  40 , more efficient cooling, and consequently condensation of water vapour as well as sulphur dioxide removal from the flue gas, is provided for in the DCC section  45 . Alternatively, heat-exchanger  60  may take the place of heat-exchanger  35 . 
         [0075]    The gas cleaning system of the present invention may advantageously be utilized both at atmospheric pressure and at pressure above atmospheric pressure, such as for example in the range of 5-40 bar, such as 8-20 bar. 
         [0076]    While the present invention has been described with reference to a number of preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc do not denote any order or importance, but rather the terms first, second, etc are used to distinguish one element from another.