M.Sc. Anna Schmidt


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The energy transition to renewable energies is proceeding rapidly. At the same time, there is still a huge demand for traditional liquid fuels. By 2035 an increase in global demand is predicted. The transport sector will continue to be the largest consumer of liquid fuels in the future [1]. On the other hand, there is an increasingly stringent regulation of emissions by the national as well as the EU legislator. A major goal here is the reduction of nitrogen oxide emissions in diesel vehicles. Internal engine modifications such as exhaust gas recirculation or lowering of the combustion temperature alone can no longer guarantee compliance with the corresponding limits. This can only be achieved by a combination of in-engine and downstream measures [2]. Selective catalytic reduction (SCR) has been established as an exhaust gas after treatment system for diesel engines. SCR uses a liquid reagent in form of a 32.5 % aqueous urea solution to reduce NOx in exhaust gases. The urea solution is injected into the process and then gradually converts to ammonia at the high temperatures given. The ammonia is then available to the process in a subsequent step and catalytically reduces the NOx contained in the exhaust gas to form molecular nitrogen and pure water. These measures have allowed the regulations for NOx to be met so far. However, a tightening of the emission limits is to be expected [2]. In order to be able to realize the optimization of the selective catalytic reduction, a basic understanding of the running processes must be generated. An important parameter in this context is the thickness of the film which forms on the walls of the exhaust system during the injection of the urea solution [3]. Wall wetting and the formation of liquid films with subsequent temperature reduction and solid urea agglomeration on the wall are undesirable processes that must be controlled. In order to examine these processes in a minimally invasive way, laser-optical methods based on absorption spectroscopy appear promising.

The measurement of the absorption of radiation in the ultraviolet and in the visible range is described by the Beer-Lambert law. It indicates the attenuation of light through the absorbing substance as a function of concentration c and path length δ [4]. With knowledge of the intensity of the laser light before and after traversing the film, which is to be examined, a proportionality to the path length δ and thus to the layer thickness can be inferred. In order to make the measurement method robust, the Beer-Lambert law is extended by a second wavelength, so that wavelength-independent transmission losses do not affect the results. Furthermore, a preselection of the wavelengths ensures that cross-sensitivities to temperature and urea concentration have no influence on the measurement. A correlation between the attenuation of the laser light and the film thickness can then be established via calibration measurements. The design of the sensor is based on real SCR systems, which is why a monostatic transceiver design, which only requires a single optical access, has been selected. In addition, the sensor is very robust and compact. Within the scope of the collaborative research centre 150, the sensor was tested on a generic film generator of the Institute of Fluid Dynamics and Aerodynamics (SLA). A validation against the commercially available Chromatic Line Sensor (CLS) was successful. Figure 1 shows the sensor above the film generator as well as the test head of the CLS.

[1] B. p.l.c, „BP Energy Outlook 2035,“ 2016.

[2] A. Schmitt, „Beitrag zur NOx Emissionsminderung für Niedrig-Emissions-Fahrzeuganwendungen mittels Selektiver-Katalytischer-Reduktion,“ Dissertation, Darmstadt, 2013.

[3] Schütte, Ablagerungs- und Alterungsverhalten wässriger Harnstofflösung bei selektiver katalytischer Reduktion von Stickoxidemissionen, Lüneburg, 2010.

[4] W. Demtröder, Laserspektroskopie 1: Grundlagen, Berlin: Springer Verlag, 2011