3D NUMERICAL ANALYSIS FOR OPTIMIZATION OF UREA-SCR PERFORMANCE IN MARINE DIESEL ENGINE

Due to the increased awareness of negative impacts of nitrogen oxides on health and environment, IMO has issued stringent regulations to control NOx in diesel exhaust emitted from operating marine vessels. In order to comply with these regulations, a large amount of research was performed to develop NOx after-treatment technologies and SCR is one of the most proven mainstream techniques to reduce NOx emission in heavy duty diesel engines. This paper presents a numerical optimization approach for SCR system performance in order to achieve higher NOx reduction efficiency in marine diesel engine. The performance of SCR system is analyzed using ANSYS Fluent 15. The geometry is created in ANSYS workbench and mesh is generated in ANSA. Two parameters are examined for evaluating SCR performance namely urea conversion efficiency, and Stoichiometric Area Index (SAI) in terms of two injection specifications which are not possible to experimentally detect namely droplet size (SMD), and injection velocity at different exhaust gas velocities. In this work, we proposed a new evaluation parameter SAI, for projecting the active range area of NH3/NO molar ratio (0.8-1.2) that helps in understanding the process development. Based on this, a blade static mixer is installed upstream of SCR entrance at different exhaust gas velocities in order to promote urea to ammonia conversion for further improvement in NOx conversion efficiency. It is observed that relatively small-sized droplets are required to achieve the maximum urea conversion efficiency and SAI due to better UWS droplet atomization and enhanced evaporation. Evaluation parameters are also enhanced at elevated injection velocities due to better mixing with exhaust flow. However, higher injection velocity will have negative effect on spray distribution due to small residence time leading to less chemical interaction between ammonia and oxides of nitrogen in exhaust gas. Installation of a blade static mixer enhances the performance of SCR system due to big interaction surface area between exhaust gas flow and mixers' plates so that more ammonia and flow come in touch with each other leading to better mixing quality. The comparison between numerical results obtained by our model and the experimental data of Kim et al. shows that urea conversion efficiency is enhanced by 6% at an exhaust velocity of 8.3 m/s. Based on this, our SCR model proves to be effective and able to maintain a very good NOx reduction efficiency by considering the researched optimization approach.