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

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.

Automated summary

A novel wet scrubber with an oxidation and a reducing section in series was used to remove SO2 and NOx from simulated ship emissions. Sodium chlorite and sodium thiosulfate served as the oxidizing and reducing agents respectively. The results showed that adjusting the pH of the chlorite oxidant affected NO oxidation and nitrogen formation rates, while the efficiency of removing NO2 was higher with sulfite compared to thiosulfate. However, thiosulfate had a significantly lower reactant consumption rate and formed less soluble nitrogen, making it a promising option. Precipitation of sulfur can be prevented by operating at a high pH. Mixing the washwater from both sections allows for adjustment of oxidation potential within nitrate limits.