# Development of a Novel De-NOX Technology for the Aftertreatment of Ship Exhaust Gases
## 3 Results and Discussion  
### Operation of Sea Water Absorption Unit  
The SWA unit was set in operation first (by-passing the oxidative and SCR catalytic reactors) in order to completely remove SO\({}_{2}\) molecules from the ship's exhaust stream. Figure 4 presents the SO\({}_{2}\) concentration profile with time before and after the operation of the SWA unit. As it can be seen in Figure 4, practically complete removal of SO\({}_{2}\) from the feed stream was achieved after less than 60 min of operation of the unit. According to the measurements of the concentration of SO\({}_{2}\) at the inlet and outlet of the SWA unit, the initial concentration of the sulfur dioxide was 380 ppm, while after the unit was set in operation its concentration decreased immediately to less than 50 ppm. It is obvious that SO\({}_{2}\) concentration is reduced drastically (>85%) within the first five minutes (relative time) of the SWA unit's operation (Figure 3). After 30 min of the unit's operation, the concentration of SO\({}_{2}\) was further reduced to 10 ppm. It is well known that sulfur dioxide has the ability to absorb (dissolve) in sea water, reacting with dissolved cations such as Ca\({}^{2+}\) and Na\({}^{+}\) forming soluble salts (CaSO\({}_{4}\) and Na\({}_{2}\)SO\({}_{4}\), respectively) [19]. Based on the above, one can safely claim that the SWA unit was successful in removing SO\({}_{2}\) molecules from the feed stream almost immediately after is set in operation. The unit at hand was continuously operable for more than 15 h; for the trip from Limassol harbor to the Greek island of Rhodes, the concentration of SO\({}_{2}\) was constantly less than 0.5 ppm (not detectable) during the voyage. It was noted that during the trip from Rhodes back to Limassol, the SWA unit was set in operation before the main ship engines were initiated. In the latter case, no measurable concentration of SO\({}_{2}\) was found in the exhaust feed stream at any time during the trip. Based on the above, it becomes obvious that the SWA unit designed and used in the present work was capable of removing efficiently SO\({}_{2}\) from the exhaust gas stream from the moment the ship's engines start until they cease operation. It is important to note at this point that the main target of the present work was not to design and operate the most efficient SO\({}_{2}\) scrubber nor to study in detail the removal of SO\({}_{2}\) by seawater absorption. In this work, the only actual importance of the SWA unit was the protection of the SCR de-NO\({}_{\rm x}\) catalyst from exposure to high concentrations of SO\({}_{2}\) (>20 ppm) which may significantly affect its overall efficiency. It is noted that the operation of the SWA unit also achieved a small reduction of the concentration of other pollutants in the exhaust gas stream, such as NO and PM. As it can be seen in Figure 5, during the operation of the SWA unit, the aforementioned air pollutants were also slightly reduced. Although no variation in CO concentration was measured, NO concentration was found to be slightly reduced by 6% (from 623 ppm to 586 ppm), while the concentration of PMs was reduced by 15% (from 250 mg/m\({}^{3}\) to 212 mg/m\({}^{3}\)). According to the international literature, nitrogen oxides (NO\({}_{\rm x}\)), particularly NO\({}_{2}\), can react with water towards the formation of nitric acid (HNO\({}_{3}\)) or with cations, such as Ca\({}^{2+}\) and Na\({}^{+}\), towards the formation of the respective metallic salts (e.g., Ca(NO\({}_{3}\))\({}_{2}\), NaNO\({}_{3}\)), which are soluble in water [20]. Moreover, it is well known that PM can be partially removed by wet-scrubbing [21]. As a result, a fraction of NO and PM are successfully removed from the feed stream, by the SWA unit. As shown in Figure 5, the concentration of CO was not reduced by the use of the SWA unit, since CO cannot be withheld by sea water cations or dissolved in water.