Performance And Mechanism Of Simultaneous Removal Of SO_x And NO_x By Wet Scrubbing Using VUV/UV/NaClO Solution

Ship transportation is playing an irreplaceable role in the international trade,while pollutants such as SOx and NOx emitted from ships also cause serious damage to the atmospheric environment.In order to control ship exhaust pollution,the International Maritime Organization has made strict regulations on SOx and NOx emission.Therefore,how to simultaneously remove SOx and NOx from marine exhaust gas has become a huge challenge for the shipping business.At present,relatively mature desulfurization technology(such as wet desulfurization)and denitration technology(such as SCR and EGR)have good removal effects.Although single desulfurization combining with single denitrification(such as wet desulfurization combined with SCR)may meet the emission control regulations on SOx and NOx from marine exhaust gas,this is obviously limited by installation space,operating costs and others,and is difficult to apply on ship.In this paper,a method for simultaneous desulfurization and denitrification from marine exhaust gas by VUV/UV/NaClO advanced oxidation process is proposed.Vacuum ultraviolet light(VUV)and ultraviolet light(UV),which are generated by the low-pressure mercury lamp,irradiate NaClO solution to produce various active free radicals to enhance the removal of SOx and NOx.The main work is summarizied as follows:Firstly,the experiments of simultaneous removal of SOx and NOx were performed by cyclic scrubbing using NaClO solution without ultraviolet light irradiation.The effects of inlet S02 concentration,NaClO concentration,initial solution pH value,inlet NOx concentration,coexisting gases and solution temperature were investigated.The results showed SO2 could be completely absorbed.Increasing NaCIO solution concentration,inlet NOx concentration and solution temperature could enhance the removal of NOx.The initial solution pH had a significant effect on denitration by affecting the existence form of active chlorine in NaClO solution.When initial solution pH value was below 8,NOx removal ability of the solution was better and NOx removal efficiency could be higher than 60%.Secondly,the experiments of simultaneous removal of SOx and NOx were performed by VUV/UV online irradiating NaClO solution to increase removal capacity of NOx.The effects of VUV/UV intermitant irradiation,different reaction systems,active chlorine concentration,VUV/UV irradiation intensity,initial solution pH value,solution temperature,nlet NOx concentration,coexisting gases and inlet SO2 concentration were investigated.The results showed SO2 could be completely absorbed by scrubbing solution under different experimental conditions.Compared with NOx removal efficiency using NaClO solution alone,VUV/UV-irradiated NaCIO solution could generate strong oxidizing material such as ·OH,·Cl and O3 to increase the NOx removal efficiency by about 41%.Free radicals oxidating denitrification was an important mechanism.Increasing active chlorine concentration,VUV/UV irradiation intensity and inlet O2 concentration could enhance the removal of NOx.Inlet CO2 concentration and SO2 concentration have almost no effect on NOx removal.NOx removal efficiency changed smaller within the initial solution pH value of 3-11.Finally,the performance experiments of simultaneous removal of SOx and NOx were performed by VUV/UV-irradiated NaClO solution adding radical-trapping agents(such as methanol,tert-butanol or nitrobenzene)to indirectly check the role of main active free radicals in SOx and NOx removal.The final products and intermediates of the reaction process were measured by Ion chromatography,electron spin resonance spectrometer and ozone analyzer.The results showed NO3-and SO42-were the main liguid products,nitrogen element in NOx and sulphur element in SO2 almost completely transformed into NO3-and SO42-.·Cl,·OH and O3 were the main intermediates and participated in desulfurization and denitrification processes.