A marine exhaust flue gas desulphurization system for reducing the amount of sulphur oxides present in an exhaust flue gas of one or more diesel engines of a marine vessel, the marine exhaust flue gas desulphurization system being configured for being able to run in an open and in a closed loop mode, comprising: - a wet scrubber system comprising at least one wet scrubber comprising a scrubbing tower having a multiplicity of spray nozzles (3, 3') connected to scrubber liquid pipes (4, 4', 4") arranged for scrubbing the exhaust flue gases originating from the diesel engine of the marine vessel using a scrubber liquid, thereby generating a used scrubber liquid, wherein the wet scrubber comprises one or more used scrubber liquid withdrawal pipes (6, 6') for withdrawing the used scrubber liquid from the wet scrubber via an open loop mode outlet pipe controlled by a valve or via a closed loop mode scrubber liquid recycle pipe controlled by a valve; further comprising when configured to run in closed loop mode; - at least one preparation reservoir arranged for preparing a magnesium hydroxide slurry by hydrolysis of magnesium oxide powder with water, the preparation reservoir comprising water addition means, magnesium oxide powder addition means (31, 31') and agitation means for mixing the water and the magnesium oxide powder; - slurry transporting means (40, 40', 40", 36, 36', 55) for transporting the prepared magnesium hydroxide slurry from the at least one preparation reservoir towards the used scrubber liquid withdrawn from the wet scrubber in the one or more used scrubber liquid withdrawal pipes (6, 6'), configured for contacting the magnesium hydroxide slurry with the used scrubber liquid in the closed loop mode scrubber liquid recycle pipe, in order to clean the used scrubber liquid and/or to neutralize the acidity thereof; wherein the one or more used scrubber liquid withdrawal pipes (6, 6') are configured to withdraw the used scrubber liquid from the wet scrubber via a closed loop mode scrubber liquid recycle pipe towards a used scrubber liquid tank when the wet scrubber system is operating in closed loop mode, wherein valve is closed and valve is open; wherein the system is further configured to recycle the scrubber liquid in the used scrubber liquid tank via the recycle pipe, valve and pump into the scrubber liquid pipes (4, 4', 4") to the spray nozzles; wherein the system, when configured to run in open loop mode wherein valve is open and valve is closed, is further configured to introduce seawater via a seawater inlet, a seawater pipe, a valve, a scrubber liquid pump into the scrubber liquid pipes (4, 4', 4") to the spray nozzles. - a wet scrubber system comprising at least one wet scrubber comprising a scrubbing tower having a multiplicity of spray nozzles (3, 3') connected to scrubber liquid pipes (4, 4', 4") arranged for scrubbing the exhaust flue gases originating from the diesel engine of the marine vessel using a scrubber liquid, thereby generating a used scrubber liquid, wherein the wet scrubber comprises one or more used scrubber liquid withdrawal pipes (6, 6') for withdrawing the used scrubber liquid from the wet scrubber via an open loop mode outlet pipe controlled by a valve or via a closed loop mode scrubber liquid recycle pipe controlled by a valve; further comprising when configured to run in closed loop mode; - at least one preparation reservoir arranged for preparing a magnesium hydroxide slurry by hydrolysis of magnesium oxide powder with water, the preparation reservoir comprising water addition means, magnesium oxide powder addition means (31, 31') and agitation means for mixing the water and the magnesium oxide powder; - slurry transporting means (40, 40', 40", 36, 36', 55) for transporting the prepared magnesium hydroxide slurry from the at least one preparation reservoir towards the used scrubber liquid withdrawn from the wet scrubber in the one or more used scrubber liquid withdrawal pipes (6, 6'), configured for contacting the magnesium hydroxide slurry with the used scrubber liquid in the closed loop mode scrubber liquid recycle pipe, in order to clean the used scrubber liquid and/or to neutralize the acidity thereof; wherein the one or more used scrubber liquid withdrawal pipes (6, 6') are configured to withdraw the used scrubber liquid from the wet scrubber via a closed loop mode scrubber liquid recycle pipe towards a used scrubber liquid tank when the wet scrubber system is operating in closed loop mode, wherein valve is closed and valve is open; wherein the system is further configured to recycle the scrubber liquid in the used scrubber liquid tank via the recycle pipe, valve and pump into the scrubber liquid pipes (4, 4', 4") to the spray nozzles; wherein the system, when configured to run in open loop mode wherein valve is open and valve is closed, is further configured to introduce seawater via a seawater inlet, a seawater pipe, a valve, a scrubber liquid pump into the scrubber liquid pipes (4, 4', 4") to the spray nozzles.