Development and Validation of a Predictive Model for DEF Injection and Urea Decomposition in Mobile SCR DeNOx Systems

Selective catalytic reduction (SCR) of oxides of nitrogen with ammonia gas is a key technology that is being favored to meet stringent NOx emission standards across the world. Typically, in this technology, a liquid mixture of urea and water - known as Diesel Exhaust Fluid (DEF) - is injected into the hot exhaust gases leading to atomization and subsequent spray processes. The water content vaporizes, while the urea content undergoes thermolysis and forms ammonia and isocyanic acid, that can form additional ammonia through hydrolysis. Due to the increasing interest in SCR technology, it is desirable to have capabilities to model these processes with reasonable accuracy to both improve the understanding of processes important to the aftertreatment and to aid in system optimization. In the present study, a multi-dimensional model is developed to simulate DEF spray processes and the conversion of urea to ammonia. The model is then implemented into a commercial CFD code. Multicomponent DEF particles are tracked in the Lagrangian framework and separate laws are defined for the heat and mass exchange between each component and the surroundings to account for water vaporization and urea thermolysis. In addition, hydrolysis of isocyanic acid is modeled as a single step homogenous chemical reaction. The model is validated with experimental data available in literature. It is then used to analyze a decomposition tube with static mixing device to demonstrate its use in analysis-led-design.