Study On NO_x Removal From Marine Exhaust Gas By Cyclic Scrubbing Using NaClO₂ Buffered Solution

Shipping is playing an irreplaceable role in the transportation of bulk goods,while pollutants such as SO_x,NO_x and PMs emitted from ships also have serious impacts on the atmospheric environment.With the introduction of stringent environmental laws and regulations,various marine exhaust gas emission control technologies are developing rapidly.At present,wet scrubbing desulfurization technology,SCR and EGR denitrification technologies have been applied on ships.However,if single desulfurization technology and denitrification technology are simply combined on the ship,it will inevitably cause problems such as large volume of the exhaust gas treatment device,high installation cost,and complicated operation and maintenance.In this paper,a method for simultaneous desulfurization and denitrification from marine exhaust gas by wet scrubbing using NaClO₂ as oxidant is proposed.The method can simultaneously remove various pollutants such as SO_x and NO_x.The main work is summarized as follows:Firstly,the effect of buffer on NO removal was investigated by non-cyclic scrubbing using NaClO₂ solution.The results showed that high denitrification efficiency could be achieved in pH range of 4.0?9.0 using NaClO₂ non-buffered solution.The solution at gas-liquid interface was rapidly acidified in the NO_x absorption process.The pH of solution was significantly decreased after scrubbing.The NO removal efficiency decreased significantly with increasing of the pH of NaClO₂ buffered solution.This is because the pH value of the gas-liquid reaction interface was well controlled,and remained basically unchanged before and after the reaction.Secondly,the denitrification experiments by cyclic scrubbing using NaClO₂ solution were conducted.The effects of pH value,NaClO₂ concentration of initial solution,NaClO₂ concentration of online injection solution and other coexisting gases on NO_x removal efficiencies were studied.The results showed that the solution pH value and NaClO₂ concentration had a great influence on NO_x romoval.The lower the solution pH value,the higher the ratio of NaClO₂ to HClO₂ and ClO₂ in the solution,which is more efficient to NO removal in the flue gas.The NO removal efficiency was gradually decreased as the NaClO₂ was continuously consumed in the scrubbing process without oxidant online injection.When the concentration of NaClO₂ in online injection solution was 54 mmol·L-1,the amount of oxidant supplemented by online injection per minute nearly equal to the amount of consumed oxidant,and the NO concentration at reactor outlet remained almost unchanged.The absorption of SO₂ from flue gas had a certain competition relationship with NO by consumed a certain amount of NaClO₂.Finallly,the experiments of simultaneous removal of NO and SO₂ by cyclic scrubbing using NaClO₂ bufferd solution were performed.The effects of pH value,NaClO₂ concentration of online injection solution,NO concentration,SO₂ concentration and the doses of ethanol and urea on NO_x and SO₂ removal were studied.The results showed that SO₂ could be completely absorbed by scrubbing solution under different conditions.The increase of NaClO₂ supplementary concentration lead to relatively high NO removal efficiency.The denitrification performance was significantly improved by adding a certain dose ethanol to the initial solution.NO oxidation rate and NO_x removal efficiency gradually decreased with the increase of SO_<sub>2 concentration.The results of ion chromatography showed that the main liquid products of SO_x and NO_x removal by NaClO₂ buffered solution were SO₄²-and NO₃^-.