An Experimental Study on Exhaust Emissions Abatement in Marine Diesel Engines with a Wet Scrubber System

Abstract

Experimental studies were performed for the simultaneous removal of SO2 and NOx from simulated exhaust gas using a laboratory-based custom-designed wet scrubber, with detailed analysis on both the gaseous and aqueous components. In the initial phase, a range of chemical compounds with potential for commercial application, namely, seawater, NaOH, NaClO, NaClO2, H2O2 and KMnO4, were studied to establish the various reaction mechanisms at play. It was found that the absorption of SO2 in the aqueous phase was influenced by three prominent factors: pH, ionic concentration and redox potential. Experimental results showed that for NOx removal, effectiveness of the various substances studied can be ranked from least to most effective as follows: Seawater, NaOH, H2O2 < NaClO < KMnO4 < NaClO2. This effectiveness was found to be influenced by the substance’s ability to oxidize NO to NO2, absorb the NO2 that was formed and retaining the nitrogen in the aqueous phase. High oxidation potential promoted the oxidation of NO to NO2 but hindered the absorption of NO2. The initial phase showed that NaClO2 was most viable and subsequent work focused on this compound. When used for NOx removal, it was shown that the first step of the removal mechanism involving the oxidation of NO to NO2 was the fast step while the second step involving the absorption of NO2 was the rate determining step. Alternative pathways to enhance NO2 removal were considered and the usage of a reducing agent was shown to be the most promising. Various sulfur-based reducing agents were reviewed from literature before narrowing to sodium sulfite and sodium thiosulfate. It was shown that although sulfite was around 15% more effective than thiosulfate in NO2 removal, its consumption rate was more by a factor of 100 when compared to thiosulfate – this was because sulfite was unstable in a high oxygen environment and a significant amount was lost through oxidation to form sulfate. Usage of formaldehyde (1% v/v) as a stabilizing agent reduced sulfite consumption significantly, from 14.2 to 3.2 mol of reactant consumed per mol of NOx pollutant removed. However, this was still poor compared to thiosulfate where only around 0.10 to 0.20 mol were consumed per mole of pollutant removed. A novel wet scrubbing system comprising of an oxidation and a reducing section arranged in series proposed in this study demonstrated significant improvement over a conventional wet scrubbing system of equivalent specifications with just oxidation/absorption. At a L/G ratio of 15, NOx removal was improved up to 20%, reactants consumed were reduced by up to 25%, and the formation of nitrite and nitrates in the aqueous phase were lowered by about 30% due to more of the NO2 being reduced to N2. The optimal pH values in the oxidizing and reducing sections of the wet scrubber in order to achieve the least aqueous nitrogen formation, lowest reaction consumption rate and avoidance of precipitation were determined to be pH 10 to 12. At pH 12, the high alkaline condition in the reducing section of the wet scrubber was able to convert CO2 to carbonates through absorption and neutralization, thereby enabling a partial CO2 capture of approximately 4 – 5% being achieved in the system. Finally, results from thermodynamic models derived from the Metso Outotec HSC Chemistry software showed good agreement with the various experimental results shown here.