Subproject C04
Holistic investigation of chemical multiphase reactions in exhaust after treatment systems


  Name Contact
Marion Börnhorst, KIT
Picture: KIT
Dr.-Ing. Marion Börnhorst
+49 231 755-2564

At a glance

In the video, Daniel Hodonj presents project C04: Emission control – experiments.


Multiphase, turbulent, chemically reacting flows occur in a variety of technical systems, including emission control with preparation of a liquid reducing agent to convert harmful nitrogen oxides (NOx). In this subproject, the multiphase, chemical reactions of the urea-water spray are investigated in the context of selective catalytic reduction (SCR) of nitrogen oxides.

Previous studies investigated the interaction of urea-water solution with the surfaces of a generic exhaust system, which leads to the formation of wall films and solid deposits. Based on the shown incomplete decomposition of the urea in the mixing section, the investigations are extended to include multiphase phenomena and reactions in the channels of the monolithic SCR catalyst.


Since the investigations of the multiphase reaction processes between the dosing point of the urea-water solution and the SCR monolith had shown that the thermal decomposition of the urea is incomplete, the investigations were extended to the SCR monolith. The focus of the investigations is the interaction of the urea-water solution and its downstream products with the SCR monolith. In addition to the phenomena between the dosing point and the catalyst inlet, the phenomena on the front side of the monolith and inside the channels are considered in particular. The main objectives are (I) to elucidate the chemical reactions of urea and its derivatives, especially isocyanic acid, and the materials of the monolith, (II) to understand the related evaporation and condensation effects including the formation of solid deposits, and (III) to characterize formed deposits by means of chemical and structural analyses. Based on the experimental data, reaction mechanisms for urea decomposition, taking into account the influence of catalyst materials, are developed for modeling the reaction kinetics in B05, B07 and C05. For the detailed investigations, the optically accessible flow test rig was optimized and extended with the in situ capillary technique SpaciPro for the measurement of spatially and temporally resolved concentration and temperature profiles in the individual channels of the monolith.

Figure 1 Schematic of relevant physical and chemical phenomena in the mixing section of SCR systems.

Previous Findings

Considering the results from previous work, the investigations have been expanded to include phenomena inside the monolithic catalyst. For this purpose, a capillary technique for measuring locally resolved concentration profiles in the channels of the monolith was adapted to the existing flow test bench. The results showed the incomplete decomposition of the urea due to the entry of the intermediate product isocyanic acid into the monolith and its accelerated hydrolysis over the catalyst. Through detailed kinetic investigations, new mechanisms for the decomposition of urea were developed, which also take thermodynamic equilibria and the formation of eutectic mixtures as well as catalytic reactions into account. For the first time, the morphology of the resulting solid deposits was analyzed in detail and correlated with the conditions of the solid evolution. Furthermore, spatially and temporally resolved measurements for the adsorption and desorption of nitrogen monoxide on storage catalysts were carried out by means of planar laser-induced fluorescence spectroscopy. The method is used to record the transient reaction kinetics as well as mass transport limitations in transient, catalytic systems.

Figure 2 DSC data of the thermal decomposition of urea and biuret. 1: Melting of urea, 2: Melting of biuret in eutectic mixture with urea, 3: complete decomposition of urea, 4: melting of biuret, 5: sublimation of cyanuric acid. [Tischer et al., Phys Chem Chem Phys 21 (30), 2019]]
Figure 3 2D concentration distribution of NO in a flow channel reactor including a storage catalyst (25mm long catalyst plate, marke das red line). A NO/O2/N2 mixture is dosed at t = 0 s and it is switched to O2/N2 at t = 62.5 s. Gas temperature 723 K, gas flow 1 l/min. [Wan et al., Chemphychem 21 (23), 2021]

Current Work

Further systematic measurements on the reaction kinetics of catalytic urea decomposition and, in particular, isocyanic hydrolysis are performed over a wide range of operating conditions. A large number of experiments are required for a comprehensive description of the reaction kinetics of isocyanic acid and other relevant gas phase species. The experimental results will be submitted to B05 for reaction kinetic modeling of catalytic and non-catalytic HNCO hydrolysis.

Figure 4 Axially resolved concentration profiles for HNCO, NO and NH3 in a single channel of a state-of-the-art VWT catalyst. Gas temperature 300°C, gas flow 1000 L/min, urea mass flow 0.5 g/min, 5% H2O, 700 ppm NO in air.


This subproject takes up a central role in the experimental section in the Collaborative Research Centre. Experimental basics are conceived for other projects and in return, models are validated, including simple reaction kinetics as well as the complete model (B05, C05). Liquid film and gas interfaces are investigated in cooperation with subproject A05 using laser absorption spectroscopy. Furthermore, process conditions and phenomena support other subprojects (B04, B07, and B08).

Selected Publications

  • Wang, S., Rohlfs, P., Börnhorst, M., Schillaci, A., Marschall, H., Deutschmann, O., Wörner, M.: Bubble cutting by cylinder – Elimination of wettability effects by a separating liquid film. Chemie Ingenieur Technik, 10.1002/cite.202100145, 2022.
  • Ates, C., Börnhorst, M., Koch, R., Eck, M., Deutschmann, O., Bauer, H.-J.: Morphological characterization of urea derived deposits in SCR systems. Chemical Engineering Journal, 10.1016/j.cej.2020.128230, 2021.
  • Börnhorst, M., Deutschmann, O.: Advances and challenges of ammonia delivery by urea-water sprays in SCR systems. Progress in Energy and Combustion Science, 10.1016/j.pecs.2021.100949, 2021.
  • Kuhn, C., Schweigert, D., Kuntz, C., Börnhorst, M.: Single droplet impingement of urea water solution on heated porous surfaces. International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2021.121836, 2021.
  • Kuntz, C., Kuhn, C., Weickenmeier, H., Tischer, S., Börnhorst, M., Deutschmann, O.: Kinetic modeling and simulation of high-temperature by-product formation from urea decomposition. Chemical Engineering Science, 10.1016/j.ces.2021.116876, 2021.
  • Börnhorst, M., Kuntz, C., Tischer, S., Deutschmann, O.: Urea derived deposits in diesel exhaust gas after-treatment: Integration of urea decomposition kinetics into a CFD simulation. Chemical Engineering Science, 10.1016/j.ces.2019.115319, 2020.
  • Schweigert, D., Damson, B., Lüders, H., Stephan, P., Deutschmann, O.: The effect of wetting characteristics, thermophysical properties, and roughness on spray-wall heat transfer in selective catalytic reduction systems. International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2020.119554, 2020.
  • Wan, S., Guo, Y., Häber, T., Suntz, R., Deutschmann, O.: Spatially and temporally resolved measurements of NO adsorption/desorption over NOx‐storage catalyst. ChemPhysChem, 10.1002/cphc.202000765, 2020.
  • Wan, S., Torkashvand, B., Häber, T., Suntz, R., Deutschmann, O.: Investigation of HCHO catalytic oxidation over platinum using planar laser-induced fluorescence. Applied Catalysis B: Environmental, 10.1016/j.apcatb.2019.118473, 2020.
  • Börnhorst, M., Cai, X., Wörner, M., Deutschmann, O.: Maximum spreading of urea water solution during drop impingement. Chem. Eng. Technol., 10.1002/ceat.201800755, 2019a.
  • Börnhorst, M., Langheck, S., Weickenmeier, H., Dem, C., Suntz, R., Deutschmann, O.: Characterization of solid deposits from urea water solution injected into a hot gas test rig. Chemical Engineering Journal, 10.1016/j.cej.2018.09.016, 2019b.
  • Schweigert, D., Damson, B., Lüders, H., Börnhorst, M., Deutschmann, O.: Heat transfer during spray/wall interaction with urea water solution: An experimental parameter study. International Journal of Heat and Fluid Flow, 10.1016/j.ijheatfluidflow.2019.108432, 2019.
  • Tischer, S., Börnhorst, M., Amsler, J., Schoch, G., Deutschmann, O.: Thermodynamics and reaction mechanism of urea decomposition. Phys. Chem. Chem. Phys., 10.1039/C9CP01529A, 2019.
  • Börnhorst, M., Deutschmann, O.: Single droplet impingement of urea water solution on a heated substrate. Int. J. Heat Fluid Flow, 10.1016/j.ijheatfluidflow.2017.10.007, 2018.
  • Günter, T., Pesek, J., Schäfer, K., Bertótiné Abai, A., Casapu, M., Deutschmann, O., Grunwaldt, J.-D.: Cu-SSZ-13 as pre-turbine NOx-removal-catalyst. Appl. Catal., B, 10.1016/j.apcatb.2016.06.005, 2016.