Design Optimization of Urea Injectors and Mixers in a Compact SCR System 2018-28-0025
Stringent emission norms require the implementation of an effective after treatment technology like Selective Catalytic Reduction (SCR). The SCR system converts NOx from exhaust gases into harmless N2. The efficiency of the system lies on two main regions. First, is the full conversion of urea to NH3 and proper uniformity of NH3 over the exhaust gases and next, is complete reaction over the catalyst bed which produces maximum NOx conversion efficiency. The NOx conversion efficiency is variable, not constant, and is dependent on many parameters, including temperature, exhaust mass flow, NOx concentration, NO2:NO ratio and SCR catalyst formulation. This paper deals with few concepts to improve the NH3 uniformity at the upstream of the catalyst. CFD simulation helps in developing and diagnosing the urea water injection systems. Mixers enable fast urea water droplet breakup and mixing with the exhaust gases. The performance of mixers varies regarding blade profiles and blade area. Augmented turbulence improves the mixing process. The design and position of urea injectors facilitate the NH3 uniformity at the SCR inlet. There exists a trade-off between the risk of clogging and pressure drop due to injector and mixer. An investigation is carried out to study the effect of various types of mixer designs along with injection position and their impact on NH3 conversion ratio.