Design Optimization of Urea Injectors and Mixers in a Compact SCR System

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.