Simulation Analysis Of Spray And Reaction On SCR System In Marine Low Speed Diesel Engine

NOx is one of the main pollutants of the marine diesel engine. With the rapid development of shipping industry, NOx emissions of the marine diesel engine are growing year by year. SCR technology is recognized as the most mature and effective technique, can meet the NOx Tier III requirements.For reason of safety, the reduction agent ammonia added to system normally in the form of aqueous urea, then decomposes to ammonia when injected into the vaporizer. The working performance of SCR depends greatly on urea decomposition, exhaust flow state and reduction reaction. The problems such as low NOx conversion efficiency or too much NH3 slip will occur if the design is not optimized.In this thesis, the SCR system installed on the low-speed two-stroke marine diesel engine6S50ME-Cwas selected as research subject. Simulation studies on spray and reaction process were carried out, and key factors affecting on conversion efficiency were analyzed.The work assisted the design and manufacture of the SCR system. The main study items are as followings:(1) The exhaust flow process was analyzed. The uniformity of the flow velocity was studied. It is found that the non-uniformity was less than 10%, and velocities of the gas would be in the allowable range of design value, all beneficial to the activity of the catalyst.(2) Calculating model of SCR was built, and simulation analysis of atomization process was carried out. It is found that the gas temperature would be reduced slightly owing to injecting of urea solution. It is also found that the air auxiliary nozzle has a good atomization effect. The results showed that the urea decomposed entirely. It also shows the uniformity of the NH3 concentration can meet the design requirements.(3) 1-D simulation of reaction process has been carried out. Design value of the weighted NOx conversion efficiency is 77.9%.It is proved that the SCR technology can meet MARPOL73/78 Tier III limitation.(4) The SCR system has been test .Based on 6S50ME-C marine diesel engine, the test result of the weighted NOx conversion efficiency is 79.8%, close to the results of simulation.The SCR system got the emissions compliance statement.The research has a certain reference and guidance significance to the development of the SCR system equipped on the low-speed two-stroke marine diesel.