| Marine exhaust gas emissions have caused more and more adverse impacts on air pollutions.In recent years,more stringent regulations for ship emission control have been implemented by international maritime organization as well as domestic and international environmental protection agencies.Nowadays,many one-to-one emission control technologies have been developed with the ability to remove SOx or NOx from marine exhaust gas.Stringent requirements of both SOx and NOx limits may be achieved by combing desulfurization and denitrification technologies.However,this kind of combined process may suffer from several drawbacks,including high investment cost,large footprint,and increased complexity of operational management.Currently,desulfurization using wet scrubbing process for ship emissions offers advantages of high removal efficiency,low cost,and low secondary pollution,which has achieved commercial applications.If an appropriate oxidant is added to the existing scrubbing system,extra removal of NOx from flue gas could probably be realized in one process.Therefore,in view of characteristics of ship operation and surrounding environment,a wet scrubbing process for NOx removal using UV-irradiated electrolyzed seawater(UV/electrolyzed seawater)was proposed in this paper.This process could solve the emission control problem by simply using seawater and electricity,both of which are available in abundant supply on board.Then,a feasible wet scrubbing process for multi-gas pollutants removal from ship emissions could be developed.Primary works are summarized as follows:(1)Preparation of active chlorine by seawater electrolysis using an undivided cell was investigated and modeled.Effects of electrode material,electrode spacing,current density,and seawater salinity on the performance of seawater electrolysis were studied separately.Based on reaction kinetics of the active chlorine formation and current-voltage characteristics of the electrolysis cell,several multivariable theoretical models were established,including active chlorine concentration(ACC),current efficiency,cell voltage,power consumption,and energy consumption.Results showed that when the diffusion rate of Cl-was the rate-determining step,the formation rate of active chlorine improved substantially as seawater salinity increased,while the energy consumption decreased dramatically.When the mass transfer was controlled by both the diffusion rate of Cl-and the charge transfer,increasing the current density in a certain range could improve the formation rate of active chlorine,but would not have an obvious impact on the current efficiency.Furthermore,results of natural seawater electrolysis verified accuracies of these theoretical models.Then,these models were used to estimate the energy consumption for preparation of the active chlorine(? 500 mg/L[Cl2])by concentrated seawater(S=40.0 ‰)on board,which would consume about 3.2 kWh/kg[Cl2]at seawater temperature of 20?.(2)A simulated marine exhaust gas generation platform and a lab-scale semi-continuous bubble column reactor were designed and built to investigate removal performance of SOx and NOx from simulated marine exhaust gas.The main parameters,kinetics of the reactive mass transfer process,and reaction mechanisms for NOx removal by electrolyzed seawater were systematically investigated.Results showed that four parameters had profound influences on NOx removal efficiencies,including ACC,NO inlet concentration,solution initial pH,and O2 concentration.NOx removal efficiencies increased with increasing of ACC,NO inlet concentration,and O2 concentration.When the solution initial pH increased from 3 to 10,the NOx removal efficiency increased first and then decreased.Then,the reaction mechanisms of NO removal process were revealed by analyzing pH-dependent distribution of the active chlorine species in seawater and material balance of nitrogen in the liquid.Furthermore,mass transfer parameters of the bubble column reactor were determined by classical chemical methods.Effects of the main parameters on NO absorption rate were discussed.According to the macro-reaction kinetic theory,the NO absorption rate equation was established and verified.Four potential enhancement methods for the reactive mass transfer process of NO absorption were proposed.A multivariatenon-linear fitting model for NO absorption reaction rate constant as a function of ACC and NO inlet concentration was established and verified.In addition,an integrated marine exhaust gas treatment method by electrolyzed seawater was proposed and analyzed with cost-benefits.(3)Based on the works of NOx removal by electrolyzed seawater alone,NOx removal by UV/electrolyzed seawater in a custom-made ultraviolet-bubble column reactor was further investigated.This process was thoroughly discussed from the point of experiments and models.Results showed that NOx removal efficiencies increased with increasing of UV power,ACC,and O2 concentration.When the pH value was in a range of 3-10,the NO.removal efficiency was kept relatively high,and the NOx removal enhancement factor increased at elevated pH values.The NOx removal enhancement factor decreased as solution temperature increased.As NO inlet concentration increased,the NOx removal efficiency decreased gradually.After that,the NO absorption pseudo-first-order reaction rate equation was established and verified.A spray tower was suggested to be the suitable reactor type for UV/electrolyzed seawater process.Then,based on the steady-state assumption theory,a simplified reaction kinetic model of NO absorption process was established and verified.These experimental results and models shed light on reaction pathways of UV/electrolyzed seawater,which could also further support numerical simulation and scale-up design.Furthermore,an innovative method for multi-pollutants removal by the UV/electrolyzed seawater advanced oxidation process was proposed,which could inspire new ideas of multi-pollutants removal from ships in term of exhaust gas emissions,ballast water,and domestic sewage. |
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