Model-based analysis of motion and constitution of water/urea droplets in the exhaust gas of SCR-systems

Abstract

The selective catalytic reduction (SCR) of nitrogenous oxides is the most promising technique to meet prospective emission regulations of cars. Due to its toxic potential the reducing agent ammonia cannot be stored in a car but it can be carried in form of a urea-water-solution (UWS) which is injected into the hot exhaust gas and reacts to ammonia. This formation of ammonia before the catalytic converter is responsible for the efficiency of the complete SCR-process-chain.

This paper deals with the numerical analysis and description of interactions between droplets of the urea-water-solution and the exhaust gas.

For the description of the droplets’ trajectories, a numeric model was developed, which considers, in contrast to previous surveys, the loss of the droplets’ mass due to evaporation of water and thermal decomposition of urea. The model for the droplets’ motion was extended by an evaporation model for binary fluids and on a kinetic approach to describe the thermal decomposition. This model is able to determine the exact position of flying droplets, the droplets’ velocity, the composition and its current temperature in a gas flow. These parameters are substantial for further considerations of the interactions between droplets and hot surfaces, e.g. wall wetting.