Development And Optimization Of High Efficient Marine Urea-SCR Systems

In the past few decades,the emissions from vessels have caused increasingly serious air pollution,especially in port cities and inland river rigions,which is mainly attributed to the lack of emission standards and supervision.This problem has attracted growing public concern and become an urgent issue these days.One of the most pernicious air pollutants in exhausts is NO_x,which has been strictly controled in ECAs.Urea-SCR is the most efficient in limiting NO_x emission to the standard of Tier III.However,there are still several problems to be solved,including the waste of reducing agent（urea）,errosion of equipments and pipe blockages caused by deposits.These problems are mainly resulted from the harsh conditions in vessels,such as limited space,low temperatures of exhausts and fluctuations of working conditions.Efficient decomposition of urea would be helpful in solving these problems.Therefore,research on urea decomposition was conducted in this thesis by studying the effects of different catalysts and environmental parameters.It has been found that catalysts can significantly improved the decomposition rates and reaction selectivity of urea decomposition.The reaction mechanisms and paths were also studied in this thesis by identifying the components of byproducts at different temperatures.Based on the results above,the best catalyst among tested ones was selected.In addition,the effects of typical operating conditions and injection characteristics of different vessels on urea decomposition process were studied.Finally,the engineering application of the marine denitrification system was successfully achieved after the optimization of the Urea-SCR system.Here are the main findings of my research:1.Got a deep insight into urea decomposition and figured out the reaction equations and pathways of this process.Reaction byproducts were identified with FTIR and an optimized HPLC testing method with C18 was utilized to quantify the main byproducts,including biuret,cyanuric acid,melamine,etc.2.Got a good understand of catalytic decomposition mechanisms and paths of urea on V-Ti catalysts and obtained a catalytic activity order of tested catalysts.Pyrolysis reaction characteristics of urea on V-Ti catalysts were studied and catalytic activity declines in the following order:V1Ti>V0.5Ti ≈ TiO₂>V5Ti>V10Ti>pure urea.V1Ti has the highest catalytic activity and urea conversion rate was considerably improved by 40%,while the productivity of NH3 and HNCO was increased by 32%and 40%perspectively.It was found by TG/DTG analysis that urea as well as byproducts could decompose at lower temperatures on catalysts because of the catalytic effects.It was also proved by a FTIR and HPLC combined analysis that side reactions were obviously suppressed under catalytic conditions.3.Conducted research on how environmental parameters could influence urea decomposition process and optimized the SCR denitrification systems.Exhaust treatment experimental platform was designed and set to simulate typical emission characteristics of marine engine.How urea soluton injection parameters and working conditions of diesel engine would affect urea decomposition rates and NH3 production rates was studied in this thesis.Urea-SCR denitrification system was tested combined with this platform and achieved emisssion standards of IMO Tier III within 50-100%load range.Finally,based on above research results,a denitrification system transformation on a fishing vessel in Zhoushan City was successfully completed and achieved Tier III under normal operating conditions.