Synergistic effect of mixer and mixing chamber on flow mixing and NOx reduction in a marine urea-SCR system

Abstract

For marine diesel engines, the selective catalytic reduction (SCR) system can be tailored to ensure high nitrogen oxides (NOx) reduction performance, low ammonia (NH3) slip, and low urea consumption. A sufficient mixing zone of urea water solution (UWS) and exhaust gas are required because of the limited installation space. In this study, therefore, mixing units such as a swirl-type mixer and an additional mixing chamber were adopted to improve the flow mixing performance with a low-pressure drop in a marine urea-SCR system. The application of mixing units gave a positive influence on the flow turbulent characteristics and urea decomposition into NH3. The mixer caused flow recirculation and increased turbulent intensity. Moreover, the additional mixing chamber was able to achieve an enhanced residence time for the UWS injections. When mixing units both were combined, it showed the highest conversion efficiency (urea to NH3). In comparison to the SCR system without any mixing units, improvement in the efficiency of deNO(x) was observed of 61.3 % by the application of the mixing chamber, 76.2 % by the application of the mixer, and 86.3 % by the application of the mixer and the mixing chamber combined.