In marine air pollution control, only SCR (Selective Catalytic Reduction) is estimated at a variable method to control NOx, but on the other hand, seawater scrubber applying the basic characteristic that is naturally alkaline (pH typically around 8.1) is viewed as an economical SOx removal system at present. Especially, seawater scrubber would not be necessary to follow any of the various land based flue gas desulfurization methods, i.e. wet, dry or alkali scrubbing. However, these methods are not readily adaptable to marine conditions due to the quantities of consumables required, i.e. lime or limestone, the means of operation and the commercial availability.
This research is undertaken to develop a new method as the main target of eliminating all exhaust emissions, particularly onboard, because the vessel can easily use seawater and be apt to apply a wet scrubber system. First, with using only seawater, the SOx are absorbed by relatively high solubility compared to other components of exhaust pollutants, and PM (Particulate Matter) is removed through direct contact with sprayed seawater droplets. Besides, the electrolyzed alkaline seawater by electrolysis, which contains mainly NaOH together with alkali metal ions (i.e. Na+, K+, Mg2+, Ca2+), is used as the absorption medium of NOx and CO2. Especially, to increase NOx absorption rate into the alkaline seawater, nitric monoxide (NO) is adequately oxidized to nitric dioxide (NO2) in the acidic seawater, which means both volume fractions are adjusted to identical proportion. The results found that the SOx removal could be achieved nearly perfectly and also led to a sufficient reduction of PM without regard to the pH values of reaction seawater. However, it may be also necessary to do a trial for NOx and CO2absorption with using the electrolyzed seawater.
