| More stringent limitations for nitrogen oxide(NOx)emission from marine diesels will be enforced in 2016 by International Maritime Organization(IMO),and Selective Catalytic Reduction(SCR)technology is the most effective way for removing NOx emission to meet the IMO Tier III standard.However,due to low temperature,high water content and sulfur content of exhaust gas discharged from marine diesel engine,SCR system still has many problems in the application of marine diesel.Catalyst capability is directly related to working performances of SCR system,while reaction pathway is greatly depended by the catalyst activity and selectivity.In the thesis,a SCR system in the marine diesel is selected as research subject,and combination of simulation and experimental method has been used.With the view of catalyst capability,detailed reaction pathway and kinetics mechanism,influencing characteristics of exhaust conditions and key parameters on SCR reaction process have been studied in the thesis,which could provide theoretical guidances for optimization of marine SCR system.In the thesis,the following research works have been carried out:(1)Urea decomposition and their deposits formation,detailed reaction pathway and their kinetic model and numerical simulation of NH3-SCR system have been reviewed,and application problems of marine SCR system also have been discussed.On the basis of Eley-Rideal catalytic mechanism,transport equations for SCR system were built,and fitting equations for kinetic parameters of key SCR reaction pathways were also given.(2)Using catalyst characterization techniques and tools,element composition,element valence,specific surface area,pore structure,surface morphology and other physical parameters of V2O5/TiO2 catalyst have been analyzed in detail.Then,multi-performance experiments of V2O5/TiO2 catalyst samples have been carried out,and influences of space velocity,reaction temperature,NH3/NOx,NO2/NO,O2 concentration and H2O concentration on the catalyst activity were also discussed.It is found that pentavalent vanadium-based oxides-V5+-OH and V5+=O are required activity sites of SCR reaction,while V4+ and V3+vanadium-based constituents are only treated as intermediates of SCR reaction process.And NH3 adsorption/desorption reaction causes the time delaying effect of SCR reaction,while fast SCR reaction can decrease their time delaying effect.(3)On the basis of Topsoe 2-cycle catalytic mechanism,the low-temperature reaction pathway and oxidation reaction pathway have been added into the standard SCR reaction pathway,and detailed SCR reaction kinetic model with V2O5/TiO2 catalyst also has been built in the thesis.Combining heat and mass transfer models,and coupling SCR detailed reaction kinetic model,simulation studies of SCR reaction process have been carried out,of which results were also compared with experimental results of catalyst samples.Influencing factors and characteristics of SCR reaction process have been studied,and influences of space velocity,reaction temperature,NH3/NOx,NO2/NO,O2 concentration and H2O concentration on the SCR reaction kinetics process were also analyzed deeply.It is found that kinetics characteristics of NH3-NO/NO2-SCR reaction can be predicted deeply by the built kinetics model,and detailed SCR reaction pathways amd reaction rates can also be anzlyzed under different conditions,but its application field is 150?500?,NO2/NOx?0.5.At low temperatures(T<250?),NH4NO3 formation rate was found being faster than its decomposition rate,while increasing temperature,NH4NO3 formation and decomposition achieve a quasi-equilibrium state,and NH4NO3 crystals would not significantly exist on the catalyst surface.(4)According to the testing data of SCR system in the marine diesel,coupling simplified overall reaction pathway,three-dimensional transient model of SCR system has been built,by which working processes of the SCR system under 25%,50%,75%and 100%load were siumulated studied.The reductant injection process,gas-liquid mixing process,reductant decomposition process and catalytic reaction process were analyzed in detail,and optimized methods of SCR system were also presented.Uniformity of urea solution in the exhaust gas is found being inmproved efficiently by combination of urea solution high-pressure injection and installing static mixers,but some droplets would be deposited on the surface of exhaust pipe,static mixer and catalyst.At high temperatures,the deposited urea could continue to be decomposited,which can casues difficulties in the development of reductant injection strategy.It is also found that required NH3/NOx can be significantly reduced unde lower loads,and the time delaying effect of SCR reaction can also be limited,while sharp increasing of NH3 escape rate in short time can be decreased in the SCR system. |
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