Wet scrubbing process with oxidation and reduction in series for removal of SO2 and NO from marine diesel engine exhaust

•Wet scrubber with oxidation and reduction sections effective for SO2 and NO removal.•Sodium chlorite oxidant formed less nitrates at higher pH.•Sodium thiosulfate reductant was consumed slowly and formed little soluble nitrogen.•High pH of reductant avoided formation of sulfur precipitates.•Partial ocean discharge of wastewater possible without exceeding nitrate limit. A novel wet scrubber comprising of an oxidation and a reducing section in series was used for the removal of SO2 and NOx from simulated ship emissions. Sodium chlorite (NaClO2) and sodium thiosulfate (Na2S2O3) were used for the oxidizing and reducing sections, respectively. Results showed that although increasing the pH of the chlorite oxidant led to less NO being oxidised, lower rates were achieved for both soluble nitrogen formation (chiefly in the form of nitrates) and reactant consumption per mole of pollutant removed. On the reduction side, it was shown that although the more well-known sulfite (Na2SO3) was about 15% more efficient in removing NO2 compared to thiosulfate, the latter had a very low reactant consumption rate, about 118 and 27 times less compared with sulfite and formaldehyde-stabilised sulfite. The formation of soluble nitrogen was also very low for thiosulfates (less than 0.18 mols formed per mole of NOx removed) or between 1.6 and 1.8 times less when compared with sulfites, as most of the pollutant was reduced to harmless N2. Although thiosulfates formed sulfur precipitates during reaction, this could be circumvented by operating at a high pH. A certain percentage of washwater from the scrubbers can also be discharged, with mixing from the oxidation and reduction sections for the adjustment of oxidation potential, within the allowable nitrate limits.