Subproject C04 Deutschmann/Suntz

Subproject C04 Deutschmann

Experimental studies of multiphase chemical reactions in exhaust gas aftertreatment systems

Motivation

Optimization of technical systems in exhaust gas systems is difficult. Due to interactions of multi-phase chemical reactive flows with the interface, problems arise for the whole system. This holds true particularly for selective catalytic reduction (SCR) systems in which the nitric oxid is converted with the help of urea-water-solution.

Therefore, this subproject is dedicated to investigate the interaction of the urea-water-solution and its derived products with the SCR catalyst. After understanding the spray-wall-interaction, the second funding period addresses the processing of chemical reactions of isocyanic acid with the SCR cata-lyst. This is the next step in order to investigate the entire exhaust gas system.

Objectives

It was shown in previous studies that the conversion of urea-water-solution is not completed at the entrance of the SCR catalyst. Thus, the interaction of the urea-water-solution with the catalyst is paramount to this subproject. This includes expected interactions between the location of injection and the monolith, the entrance to the monolith and interaction within the channels. Experimental in-vestigations aim for elucidating the chemical reactions of urea and its derived products, especially isocyanic acid, with the surface material of the SCR catalyst. With this, an understanding of vapori-zation and condensation reactions is reached and the resulting deposit forming and material damage can be assessed.

During the last years, the focus was on understanding the production of ammonia and the problemat-ic creation of deposits through liquid films. In the second funding period, the focus is on chemical reactions of isocyanic acid and the SCR surface.

regime_map_for_drop_wall_interaction_of a urea_water_solution
Schematic process in the exhaust gas system during urea-water-solution injection.

Previous Findings

A reactor equipped with optical access and high resolution measurement techniques have been used to observe and map a regime for drop-wall-interactions. These measurements reveal four character-istic regimes. Therefore, a base was created for modelling a two-phase flow as well as for further experimental investigations of spray-wall interactions.

Regime map for drop-wall interaction of a urea-water-solution.
Regime map for drop-wall interaction of a urea-water-solution.

Approach

The experiments are performed within a hot gas generator setup which was especially designed for this subproject. The test bench setup is optically accessible through a quartz glass window in the injection area. In this area it is possible to observe liquid film formation and deposits via video cam-era and laser absorption spectroscopy. Downstream, a SCR monolith is installed. By adapting the SpaciPro measurement technology, the channels of the monolith can be accessed by capillaries. With this methodical extension, it is possible to measure axially resolved concentration and tempera-ture profiles in the channels. Concentration are determined by FTIR spectroscopy.

To better understand the chemical process of urea decomposition, ex situ analysis is used. Similar to the first period, the deposits, formed at different conditions on the wall and the catalyst surface, are analyzed via thermogravimetrical analysis (TGA). High-pressure liquid chromatography (HPLC) and nuclear magnetic resonance spectroscopy (NMR) are used to understand not only the composi-tion of deposits but the process of formation.

Current Work

Presently the hot gas generator is improved. Furthermore, adapting the SpaciPro technology needs thorough preparations. Therefore, investigation of concentration and temperature profiles are per-formed with ammonia and a dummy monolith before testing with urea-water-solution and a real SCR monolith. With this, the chemical information acquired can be used for further kinetic models.

Schematic process in the exhaust gas system during urea-water-solution injection
SpaciPro technology: Model monolith and operating principal of the measurement

Cooperations

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

  • Börnhorst, M., Deutschmann, O.: Single droplet impingement of urea water solution on a heated substrate. Int. J. Heat Fluid Flow 69, 55–61 (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 198, 548–557 (2016).