Performance Optimization of High-Pressure SCR System in a Marine Diesel Engine. Part I: Flow Optimization and Analysis

Selective catalytic reduction (SCR) systems for marine low-speed diesel engine can be categorized into two types—the after-turbine low-pressure SCR (LP-SCR) system and the pre-turbine high-pressure SCR (HP-SCR) system. The exhaust pressure in the HP-SCR system is higher, which requires a more compact structural arrangement. According to the exhaust conditions of marine low-speed diesel engine, the vaporizer/mixer, the bend pipe, and the sudden-changed pipe of HP-SCR system were optimized in this paper. Based on a 3D-CFD model coupled with reaction kinetics, the flow fields of a HP-SCR system with various flow-guiding devices were calculated, as well as the impact of the structural shapes and sizes of the flow-guiding devices on the exhaust flow being comprehensively analyzed. From the perspectives of the flow field, the concentration field, and the pressure loss, both the disturbance and mixing effects on the two kinds of the structural flow-guiding devices with vaporizer/mixer were evaluated. Likewise, the axial pressure distribution along the exhaust flow direction in the vaporizer/mixer was also obtained. Moreover, from the perspectives of velocity uniformity and pressure loss, diversion effects on 10 kinds of flow-guiding devices with a bent pipe and a sudden-changed pipe were evaluated—thereby even obtaining the optimization design of the HP-SCR system. By installing the flow-guiding devices in the vaporizer/mixer, the bend pipe, and the sudden-changed pipe of the HP-SCR system, the upwind velocity uniformity in front of the SCR catalyst was enhanced. In the optimization design for the HP-SCR system, the total pressure loss was 4491.3 Pa, and the standard deviation of the upwind velocity in front of the SCR catalyst was 10.17%, which can meet with the design requirement of the velocity uniformity for the marine diesel engine HP-SCR system.