CFD Analysis And Optimization Of Exhaust Flow Field In Marine SCR System

For the past few years, with the rapid development of global shipping industry, NOχ emissions of marine diesel engine are growing year by year. Considering NOx would cause great harm to human health and environment, International Maritime Organization (IMO) has formulated MARPOL73/78Annex VI to restrict it. Facing the imminent implementation of the third stage (Tier III standards) regulations, Selective Catalytic Reduction (SCR) of NOx by ammonia is recognized as the most effective technology for the control of marine NOx emission.Compared to vehicle SCR system, the exhaust flow field of marine SCR system has changed and need much more reducing agent. So, it is necessary to study and optimize the exhaust flow field to obtain high NOχ conversion without increasing the consumption of reducing agent. This paper investigated the flow process in a marine SCR system with the aid of CFD (Computational Fluid Dynamics), then cleared the flow property and proposed some effective techniques for flow optimization. All the results provide theoretical basis for improving NOx conversion.Firstly, the merits of different rules for evaluating the uniformity of flow distribution were compared. Then, this study introduced uniformity index for velocity distribution and ammonia concentration distribution to analyze the exhaust flow distribution quantitatively. At the same time, using pressure drop, NOx conversion and ammonia slip to assess the optimization effects.Secondly, with initial and boundary conditions obtained from Engine bench test, a three-dimensional CFD model was established on AVL FIRE platform for simulating the exhaust flow, urea injection and decomposition, catalytic reactions under different operating conditions. According to the simulation results, the regularities of flow, concentration and pressure distribution were studied, which reveals that the flow field inside the marine SCR system can’t be simplified into a one-dimensional flow and should be optimized because it is unevenly distributed.The effects of structures of flow dresser and mixer, injector arrangement and configuration on the performance of SCR system as well as the flow distribution have been evaluated based on the three-dimensional numerical model. The main conclusions are as follows:(1) Installing porous plate at the nozzle upstream can regulate the flow bias after elbow to some extent and reduce the flow velocity at the center region of catalyst. Besides, porous plate with50%open porosity get more uniform velocity and ammonia concentration distribution at the entrance of catalyst, the system pressure drop only increased by0.6kPa.(2) When choosing four-hole radial nozzle, ammonia concentration distributes a little more even at the entrance of catalyst. With the distance between the injection point and catalyst (mixed distance) increased, NOx conversion will rise and rising rate gradually decrease. When the mixed distance is10d (d is the diameter of the exhaust pipe), using one nozzle can obtain high conversion efficiency. When the mixed distance is4d, using four nozzles can achieve7%higher NOx conversion than that with only one nozzle.(3) When the mixed distance is10d, NOx conversion increased slightly after installing the designed mixer at the nozzle downstream, but in view of the NH3distribution uniformity, mixer with a angle (the acute angle between the blade and cross-section of exhaust pipe) of45°can get better mixing than the others. When the mixed distance is4d, after installing the mixer with a angle of45°make NOx conversion increase by8.2%.