New Experimental Results of NO_x Removal from Marine Diesel Engines by the Ozone/Urea System: Removal Efficiency and the NO_x Conversion Ratio

摘要

Ship exhaust gas greatly threatens human health and the environment. The ozone/urea system has the advantages of strong oxidability, excellent reducibility, and low reductant consumption. This paper thoroughly studied the effects of urea concentration, O_3 concentration, initial pH, temperature, O_2 concentration, SO_2 concentration, and initial NO concentration on NO_x removal efficiency using the ozone/urea system. Moreover, this paper conducted a systematic study on the conversion proportion of NO_x into nitrate and nitrite, which is very important for the ship NO_x wet method. Urea concentration and O_3 concentration played important roles in NO_x removal efficiency. Weak acid and weak alkali media could effectively improve NO_x removal efficiency, but the reasons were completely different. As the temperature increased, the NO_x removal efficiency initially decreased and subsequently increased. Besides, the increase of temperature and initial pH could decrease the conversion proportions of NO_x, into nitrate and nitrite. The increase of SO_2 concentration could not only reduce the NO_x removal efficiency but also increase the conversion proportion of NO_x into nitrate. However, increasing SO_2 could significantly decrease the conversion proportion of NO_x, into nitrite. Under the optimal conditions of this paper, the NO_x removal efficiency reached 97.8% and the conversion proportion of NO_x into nitrate was only 20.39%. In addition, the urea content was just 0.3 mol/L, which was far lower than that in the prior study. This illustrated that the ozone/urea system had excellent application potential in marine exhaust gas purification.