A novel method based on hydrodynamic cavitation to effectively remove NO2

Abstract

Marine ships’ exhaust causes serious air pollution, among which NO is the most difficult to control. However, when a wet scrubber is used to oxidize and remove NO, a large amount of NO2 is produced, which makes it difficult to improve the NOx removal efficiency. We propose a new method for NO2 removal based on hydrodynamic cavitation (HC). Compared with the bubbling method, the HC method could be used to improve mass transfer by increasing the gas–liquid contact area and generating microjets, thereby improving NO2 removal efficiency. A 12-h long continuous experiment was conducted which revealed that HC could delay but not prevent NO production. Experiments using HCl, NaOH, and HNO3 to adjust the pH of the solution showed that the change in pH or the accumulation of NO3- had little effect on the formation of NO. The results obtained using ion chromatography analysis further revealed that the accumulation and decomposition of HNO2 were the primary causes of NO formation. Following this, we carried out experiments to investigate possible enhancement of NO2 removal efficiency by H2O2, Na2S2O8, NaClO2, and ClO2 under conditions of HC. The results showed that ClO2, H2O2, and Na2S2O8 increased the NO2 removal efficiency by only 2.29%–4.79% compared to water. However, the NO2 removal efficiency of NaClO2 was 0.54% lower than that of water, and it gradually decreased. The results of this study provide new ideas for improving NO removal efficiency using oxidants.