Study of evaporation and decomposition of Urea-Water-Solution

Reducing Nitrogen Oxides (NOx) concentration in the exhaust gas from e.g. diesel engines is crucial for pollutant reduction of combustion processes. Selective Catalytic Reduction(SCR) systems are well-known and developed methodology for reducing NOx. SCR system use Ammonia (NH3) as reduction agent. However, due to safety concerns, Urea Water Solution (UWS) – the so-called AdBlue is used in exhaust-gas treatment systems as a source for Ammonia.

Task:
• Computational, parameter study of the evaporation and decomposition of UWS before reaching the catalyst (see Figure).

Objective:
• Better understanding and prediction of the Ammonia release by decomposition of the Urea with control and mitigation of residuals formation, which might caulk the catalyst monolith shown in Figure.
For SCR systems, uniform distribution of Ammonia on the catalyst is an important factor. Furthermore, experimental results show forming of residuals as a result of UWS decomposition. These residuals can poison the catalyst surface slowing down the NOx regeneration process and even block the monolith. In this respect considering the evaporation and decomposition of the UWS becomes crucial to increase the efficiency of SCR systems and improve situation with pollutants reduction. In the after-treatment systems, UWS is injected as droplets to the exhaust gas. Coupled chemical reactions, evaporation with molecular diffusion phenomena in multi-component multiphase flows (i.e. spray of the droplets that eventually might land on the wall and form a film, Fig. 1) make modeling of the process challenging. Therefore, two generic configurations shown in Fig. 2 are suggested. Droplet (spherical) and film (planar) geometries are considered for simplicity. Then, the detailed model can be treated as a spatial 1D model,
which is implemented in in-house code INSFLA. Parameter study as well as analysis of the results obtained from detailed simulation are required for better understanding the process and developing a reduced
model of the urea decomposition process.