Effect of atomizing air of external-mixing twin fluid atomizer on urea thermolysis

In the urea-SCR system, the conversion efficiency of reductant is defined as how much of the injected urea solu-tion is decomposed into reductant before the SCR catalyst. So, it is closely related with the urea-SCR system’s scale, such as a distance from urea solution injector to the SCR catalyst and a space velocity of SCR catalyst. This is ac-cording to previous studies, the atomization characteristics of urea solution injector are known for an important factor about the conversion efficiency of a reductant. Recently, there has been rising interest in applying urea-SCR systems to large marine diesel engines because the International Maritime Organization (IMO) has decided to en-force NOx reduction regulations. Generally, in the case of urea-SCR of the marine diesel engine, a type of twin fluid atomizer has been used for injection of the urea solution. This study was conducted to investigate an effect of the atomization of external-mixing twin fluid nozzle on the conversion efficiency of reductant. The lab-scaled experi-ment device was installed to mimic the urea-SCR system of the marine diesel engine for this study. In a low tem-perature inflow gas condition which is similar with the exhaust temperature of a large marine diesel engine, this study found that the conversion efficiency of reductant when relative big size urea solution droplets are injected into exhaust gas stream can be larger than that of small urea solution droplets are injected. According to the results from numerical simulation, the reason was associated with the decrease of the reaction rate constant caused from temperature drop of exhaust gas by atomizing air of twin fluid atomizer.