This research makes a new attempt to apply the activated seawater by electrolysis in the development of two-stage wet scrubber system to control the exhaust gas of large marine diesel engines. First, with using only seawater that is naturally alkaline (pH typically around 8.1), the SO2 and SO3 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+, Mg2+, Ca2+), is used as the absorption medium of NOx and CO2. Especially, to increase NOx absorption rate into the alkaline seawater, nitric oxide (NO) is adequately oxidized to nitrogen dioxide (NO2) in the acidic seawater, which means both volume fractions are adjusted to identical proportion.
The results found that the strong acidic seawater was a valid oxidizer from NO to NO2 and the strong alkaline seawater was effective in CO2 absorption. In the scrubber test, the SOx reduction of nearly 100% could be achieved and also led to a sufficient PM reduction. Hence, the authors believe that applying seawater and its electrolyte would bring the marine air pollution control system to an economical measure. Additionally, it is well known that NOx and SOx concentration has a considerable influence on the N2O emission of green house gas. Although the N2O concentration exhausted from diesel engines is not as high, the green house gas effect is around 300 times greater than an equivalent volume of CO2. So, we investigated the N2O removal efficiency with using the electrolyzed seawater too. Finally, this research would also plan to treat the effluent by applying electro-dialysis and electro-flotation techniques in the future.