Study On Ship Exhaust Denitrification With Hydrodynamic Cavitation Enhancing Chlorine Dioxide

Ships carry 80%of the world's trade,and meanwhile,they cause severe pollution to the atmosphere.Nitrogen oxides?NOx?is one of the most difficult substances to be treated among ship's air pollutants.In order to meet the requirements of various laws and regulations on ship NOx emissions,scholars have conducted in-depth research on ship exhaust gas treatment.Wet denitration technology is one of the research hotspots at home and abroad.In this paper,a novel wet denitrification technology of hydrodynamic cavitation enhancing chlorine dioxide?ClO₂?is proposed.Hydrodynamic cavitation can create high-temperature and high-pressure reaction condition,which can speed up the chemical reaction.Plus,the large number of micro-bubbles and strong micro-jets generated by hydrodynamic cavitation can effectively improve gas-liquid mass transfer efficiency and greatly help the reaction between the gas and liquid phases.In addition,the oxidant ClO₂,internationally recognized as the green disinfectant,has strong oxidation,contributing to the oxidation of NOx.Firstly,in order to verify the denitrification effect of hydrodynamic cavitation enhancing ClO₂,denitrification experiments were investigated.The denitrification results of hydrodynamic cavitation enhancing ClO₂ proved the effectiveness of the technology.When 0.2 mg/L ClO₂ solution was used,NOx removal rate exceeded 90%.In order to further prove the high efficiency of hydrodynamic cavitation denitrification,a comparative experiment of bubbling denitrification was carried out.Comparison of denitrification effects between the hydrodynamic cavitation reactor and the bubbling reactor confirmed the high efficient of hydrodynamic cavitation denitration.Hydrodynamic cavitation caused rapid consumption of the oxidant in the solution,which could be reflected in the comparison of the pH value of the solution and the duration of the high denitration rate during hydrodynamic cavitation and bubbling denitrification experiments.A high-speed camera was used to photograph the bubble morphology generated by the hydrodynamic cavitation reactor and the bubble morphology in the bubbling reactor.It was found that the hydrodynamic cavitation reactor could produce more and smaller bubbles,which intuitively explained why hydrodynamic cavitation had high denitration efficiency.Secondly,a systematic and in-depth study was carried out after verifying the effect of hydrodynamic cavitation enhancing ClO₂ denitration.Effects of inlet pressure of the hydrodynamic cavitation reactor,outlet pressure of the hydrodynamic cavitation reactor and ClO₂ concentration on the denitrification effect were studied separately.Results showed that the increase of inlet pressure promoted or inhibited denitration.On the one hand,the higher inlet pressure reduced the bubble size,increasing the liquid-gas ratio,which promoted the denitrification effect.On the other hand,the higher inlet pressure increased the bubble velocity,decreasing the reaction time,which inhibited the denitration effect.Similar to the inlet pressure,the outlet pressure also affected the size and velocity of the bubbles,and both have an optimal pressure.What's more,the increase of ClO₂ concentration prolonged the duration of the high denitration rate,but as more ClO₂ escaped,it also led to an increase in the concentration of nitrogen dioxide.Therefore,it is a better choice to maintain low concentration of ClO₂ for continuous denitrification.Furthermore,the mechanism of denitration of hydrodynamic cavitation enhancing ClO₂ was discussed based on the detection results of the ion-chromatographic analyzer and ultraviolet spectrophotometer on the solution after the reaction.Finally,the cost calculation of the hydrodynamic cavitation denitration system was carried out.Results showed that the inlet pressure of 300 kPa and the outlet pressure of 30 kPa were the best choices for the hydrodynamic cavitation enhancing ClO₂ denitration considering both cost benefit and practical application requirements.