Increasing sensibilities regarding environmental pollution and the health hazards due to emission from diesel or gasoline engines require advanced and further optimized pollutant reduction strategies. Particularly interesting is the reduction of nitrogen oxide NOx emissions based on Selective Catalytic Reduction (SCR) as a state-of-the-art method for the minimization of NOx in combustion exhaust gas. In SCR systems, nitrogen oxides are catalytically reacting with ammonia to form molecular nitrogen and water. Ammonia is chosen as reducing agent because it selectively reacts with nitrogen oxide instead of being oxidized by oxygen. It must be supplied to the process adjusted to the flow rate of the exhaust gas and its nitrogen oxide content. For safety considerations, however, ammonia is not carried in vehicles as pure liquid. Instead, it is generated from urea within the exhaust gas system. For this purpose, an urea-water solution is injected into the hot exhaust gas flow. By thermal decomposition and subsequent hydrolysis, it is processed to ammonia and carbon dioxide. Although de-NOx SCR has been widely adopted, it still has significant shortcomings. Especially wetting of the walls of the exhaust system during the injection of the liquid urea solution is an undesirable and efficiency-reducing process that can compromise its robustness. The liquid film can flow into the catalytic converter and block it upon the formation of solid by-products under given adequate temperature and residence time. Furthermore, as the film formation influences the amount of ammonia supplied to the catalytic converter, it affects the effectiveness of the urea dosing strategy. Not only the individual effects but also the correlation between spray properties, gas phase parameters and film formation need to be investigated. To investigate these thermochemical processes, on the one hand a generic hot gas test rig is required on which reproducible experiments with known boundary conditions can be performed. On the other hand, suitable sensors and measurement systems must be available with which it is possible to perform in situ investigations.
To be able to observe and measure the thermochemical phenomena that take place within an SCR system, a generic hot gas test rig was set up in close cooperation with numerical projects that aims for closing the gap between laboratory-scale spray chambers and real exhaust aftertreatment systems. The test rig is designed to provide a fully developed turbulent velocity profile in the optically accessible measurement region. By the well-defined turbulent boundary conditions at the inlet to the measurement section, the numerical domain can be reduced. The gas flow velocities can be set between 1 m/s and 15 m/s and gas temperatures of up to 700 K within 5 K increments can be reached in the measurement section. Two measuring chambers in front of and behind the catalyst allow optical accessibility from four sides due to built-in glass panels. Furthermore, catalysts of variable length can be installed in order to investigate the influence of these on the flow and gas phase chemistry. The measurements include film thickness, urea concentration in the film and film temperature, as well as gaseous water, ammonia, isocyanic acid and carbon dioxide (and many more).
Figure 1: Schematic layout of the generic exhaust gas test bench, starting with the air supply unit (bottom left). The compressed air is heated until the limit of about 500 °C at the optical section. The plenum and the inflow nozzle are designed with integrated turbulence grids to prepare the flow. At the optical access 1 the flow will be a fully developed turbulent block profile. The injection is placed close to the optical access 1 were the investigation of the SCR phenomena takes place. Finally, the hot gas flow will be released in the exhaust section.
Laser optical methods based on absorption spectroscopy are suitable for the analysis of liquid films. The absorption of radiation in the ultraviolet and visible range is described by Beer-Lambert law. It gives the attenuation of the radiation intensity when passing through an absorbing substance as a function of the concentration c, the temperature T and the path length δ. After selecting three suitable wavelengths and calibrating, the film thickness, urea concentration, and film temperature can then be calculated by knowing the intensity of the laser light before and after passing through the film. For robustness of the measurement methodology, the wavelength selection is additionally extended by a fourth wavelength, so that wavelength-independent transmission losses do not affect the measurement. The design of the sensor is based on boundary conditions of a real SCR systems, which is why a monostatic transceiver design was selected that only requires a single optical access. Additionally, the sensor is designed to be highly robust and compact.
Figure 2. (a) Measurement section with location of film thickness measurement and thermocouples; (b) Film thickness sensor as a schematic view and mounted on top of the optical access of the SCR test rig.
Anna Schmidt, Benjamin Kühnreich, Hannah Kittel, Cameron Tropea, Ilia V. Roisman, Andreas Dreizler, Steven Wagner (Juni, 2018): Laser based measurement of water film thickness for the application in exhaust after-treatment processes. In: International Journal of Heat and Fluid Flow, Volume 71, June 2018, Pages 288-294. https://doi.org/10.1016/j.ijheatfluidflow.2018.04.013
Anna Schmidt, Sani van der Kley, Steven Wagner (August, 2020): Optically accessible generic exhaust gas test bench for the investigation of fundamental SCR-relevant processes. In: Applied Optics, Vol. 59, No.23. https://doi.org/10.1364/AO.397574
Schmidt, Anna, Matthias Bonarens, Ilia V. Roisman, Kaushal Nishad, Amsini Sadiki, Andreas Dreizler, Jeanette Hussong, and Steven Wagner. 2021. “Experimental Investigation of AdBlue Film Formation in a Generic SCR Test Bench and Numerical Analysis Using LES” Applied Sciences 11, no. 15: 6907. https://doi.org/10.3390/app11156907
Sani van der Kley, Johannes Emmert, Anna Schmidt, Andreas Dreizler, Steven Wagner,. 2021. “Tomographic spectrometer for the temporally-resolved 2D reconstruction of gas phase parameters within a generic SCR test rig” Proceedings of the Combustion Institute 38, no. 1: 1703-1710. https://doi.org/10.1016/j.proci.2020.09.009
van der Kley, Sani, Gabriele Goet, Anna Schmidt, Valentina Einspieler, and Steven Wagner. 2021. “Multiparameter Determination of Thin Liquid Urea-Water Films” Applied Sciences 11, no. 19: 8925. https://doi.org/10.3390/app11198925
Anna Schmidt, Benjamin Kühnreich, Hannah Kittel, Cameron Tropea, Ilia V. Roisman, Andreas Dreizler, Steven Wagner (Januar, 2018): Diode Laser Based Film Thickness Measurement of DEF. Conference: Laser Applications to Chemical, Security and Environmental Analysis. DOI: 10.1364/LACSEA.2018.LM3C.3
Anna Schmidt, Benjamin Kühnreich, Matthias Jacobs, Steven Wagner (Januar, 2019): Diode Laser-based Film Thickness Measurement of DEF in a generic exhaust gas test bench for the investigation of SCR-relevant processes. Conference: CLEO: Applications and Technology. DOI: 10.1364/CLEO_AT.2019.ATh4K.4
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