Study On Performance And Mechanism Of Nox Removal From Marine Exhaust Gas Via Na₂S₂O₈ Solution Scrubbing Enhanced By Hydrodynamic Cavitation

Shipping plays a pivotal role in the world economy,but the hazards of ship exhaust to people and the environment are becoming more and more serious.In order to alleviate ship exhaust pollution,policies and regulations are introduced and improved by the governments around the world.The large gross,serious harm and difficulties in treatment make nitrogen oxides（NO_x）become one of the focuses of marine exhaust gas,making it one of the hot spots in the current research on ship exhaust gas treatment.Wet oxidation technology is efficient and demands simple mechanical structure,which attracts much attention from scholars.However,there is still great progress can be made by improve the drug utilization.Hydrodynamic cavitation creates extreme reaction conditions,which is can intensify chemical reactions.During the cavitation process,numerous tinny bubbles are produces which increases the gas-liquid contact area;the microjets generated by the bursting of cavitation bubbles facilitate the mass transfer of chemical reactions.the cavitation hot spot effect can create extreme reaction conditions of high temperature and high pressure,Sodium persulfate（Na₂S₂O₈）has following advantages:strong oxidation capacity,low price and stable chemical properties at room temperature.Therefore,in this thesis,the special reaction conditions produced by hydrodynamic cavitation are used to improve the Na₂S₂O₈ wet denitration effect.The steps are as follows:First,a hydrodynamic cavitation enhanced Na₂S₂O₈ denitrification experimental platform is built,and a comparative experiment of denitrification between hydrodynamic cavitation and bubbling methods are carried out.The bubble shape,denitrification effect and chemical reaction mechanism are compared and analyzed.The results show that with same pipeline flow and 0.10mol/L Na₂S₂O₈ solution for denitrification,the bubbles generated by the hydraulic cavitation method are small in size and large in amount,which greatly increases the gas-liquid contact area.Under the same experimental conditions,the NO removal rate of hydrodynamic cavitation denitrification reaches 62.5%,which is much higher than the 40%NO removal rate of bubbling denitrification.In the hydrodynamic cavitation experiment,the faster pH drops prove the feasibility and high efficiency of hydrodynamic cavitation denitrification.Secondly,the Na₂S₂O₈ solution temperature,Na₂S₂O₈ concentration,the inlet and outlet pressure of hydrodynamic cavitation reactor as well as other conditions are studied to find out their influences on hydrodynamic cavitation for Na₂S₂O₈ denitrification enhancement.The experimental result turns out that the NO removal rate increases with the rise of the solution temperature.Keep raising the temperature after the solution exceeds 70℃,and the increase of the NO removal rate slows down.In addition,this method also has a good effect on NO₂absorption.When the temperature is raised to 80℃,the outlet concentration of NO₂ is only 11ppm.As the concentration of Na₂S₂O₈ solution increases,the NO removal rate increases.When the concentration of Na₂S₂O₈ solution exceeds 0.10 mol/L,the NO removal rate increases more slowly.As the inlet pressure rises,the reactor’s suction effect is improved,and the NO removal rate increases first and then decreases.When the inlet pressure is 350 kPa,the most bubbles are produced,and the NO removal effect is the best.The NO removal rate increases first and then decreases as the outlet pressure.When the outlet pressure is 40 kPa,the NO removal rate reaches the highest.Finally,an experiment of Na₂S₂O₈ denitrification enhanced by hydrodynamic cavitation with chloride ion （Cl^-） is carried out to explore the influence of different Cl^- concentration,solution pH,solution temperature and other parameters on the removal effect of NO,and the related mechanism is analyzed and discussed.The results demonstrate that the oxides produced by the reaction between Cl^- and Na₂S₂O₈,such as HOCl and·Cl,enhancing the oxidation ability of the solution.Increasing the concentration of Cl^- can greatly strengthen the NO removal effect.The NO removal effect achieves the best and the NO removal rate can reach 94%when sodium chloride（NaCl）reaches the salinity of seawater（435 g）,the front pressure is 300 kPa,the back pressure is 30 kPa,the solution temperature is 60℃,and the Na₂S₂O₈ concentration is0.10 mol/L.A lower pH is beneficial to enhance the NO removal effect.As the temperature rises,the NO removal rate shows an upward trend.