title: Impact of Wall Surface Characteristics on Deposit Formation of Evaporating Urea Water Solution
Creators: name: Schumacher, Olaf
Publisher: name: UNSPECIFIED
Year: 2024

description: Health hazards and strict regulatory requirements make the reduction of NOx emissions from indispensable combustion processes, such as those in construction vehicles, waste incineration plants, and industrial boilers, essential. A highly relevant technological solution is the Selective Catalytic Reduction (SCR), in which an aqueous urea solution is injected into the exhaust gas treatment system. The urea decomposes into ammonia, which reduces NOx emissions to nitrogen and water in the presence of a catalyst. A disadvantage of this method is the unavoidable formation of liquid films on the system walls. Upon evaporation, these films leave behind solid deposits of urea and, at higher temperatures, increasingly heat-resistant decomposition products such as biuret, cyanuric acid, and ammelide. These deposits significantly impair the overall process by disrupting spray formation or increasing back pressure. To minimize such deposits, this work focuses on the surface characteristics of the involved system walls. By collecting data from generic experiments and modeling the influencing factors, the understanding of the deposit formation process and the effects of modifiable parameters will be improved. In the experiments, several drops of an aqueous urea solution are evaporated on heated metal samples under precise control of the boundary conditions. The samples are prepared to test the influences of different wall surface characteristics. A statistical experimental design is conducted beforehand to allow for thoroughly analyzing the effects and their interactions. The studied influences include various roughness levels, submillimeter-scale structures, and chemical coatings that vary the wetting behavior. Additionally, the influence of sample temperature and heating duration is investigated. The final deposits are characterized in terms of their mass, volume, and the wall area they cover. In addition to detailed phenomenological observations, the collected data is used for subsequent regression analysis. Models are developed to determine the dependency of the deposit metrics on the investigated influences. The results and analyses show that the preliminary processes of wetting and evaporation are significantly influenced by the surface characteristics. Depending on the structure geometry, capillary forces have a considerable effect on liquid spreading, while in other cases, wall films are noticeably hindered in their spread by the structures. Introduced surface roughness exhibits a clear dependence on the respective surface coating. At roughness values ≥ 1.6 μm, significantly enhanced wetting or non-wettability of the surfaces is observed depending on the chemical pretreatment. These phenomena also affect the subsequent deposit formation. By purposefully maximizing film spreading, deposit mass and volume can be minimized. Wall surface structuring, on the other hand, is particularly suitable for minimizing the deposit area due to its effect on initial film spreading. Overall, the analyses highlight both complex interactions between surface characteristics as well as fundamentally distinct effects of influencing factors at the two considered wall temperatures. The developed models show very good agreement with the experimental data and expand the insights gained by quantifying the relevant effects, leading to a more detailed understanding of the deposit formation process. Additionally, the models form the foundation for an optimization strategy demonstrated in this work. This is necessary because the target variables, like deposit area and mass, respond oppositely with respect to the investigated influences. Thus, model-based optimization that considers individual objectives proves valuable for deriving insights for engineering applications from the findings. This approach is ultimately demonstrated through an application featuring a user-friendly interface.