Morphological characterization of urea derived deposits in SCR systems

Selective catalytic reduction (SCR) systems based on the injection of urea–water solution (UWS) are the state-of-the-art exhaust gas after treatment systems reducing the (NOx) emissions of diesel engines. In such systems, the formation of solid urea by-products is an undesired phenomenon and reduces the overall efficiency. In order to minimize this deposit build-up, it is essential to understand the deposit formation process in the presence of spray–wall contact and how this process is influenced by the surface morphology on which the deposit grows. In this regard, the objective of this study is to gain a profound understanding of the morphology of urea-derived deposits in SCR systems and to establish a methodology to characterize the surface topology of such surfaces. For that purpose, characteristic urea deposits are created in a laboratory test bench at two different temperature regimes. Optical and confocal microscopy are used together to investigate the variations in the surface texture and to measure the surface topology. The morphology of the deposit samples is then characterized by using the root mean square roughness, power spectral density (PSD) and fractal dimension analysis. ... mehrResults suggest that the form of the 3D deposit structure is composed of the underlying urea-derivative crystal blocks, which display a finite size range in all cases. Furthermore, all deposit samples exhibit self-affine surfaces and yield the same power law distribution, indicating that the surface morphology is governed by the same height cascade. In a similar manner, all surfaces converge to the same fractal dimension of 2.3, suggesting that the deposit formation process evolves into a unique fractal structure for the tested operating conditions. Accordingly, it can be used to quantitatively describe the complex morphology of the urea-derived deposits.