Subproject B05 Deutschmann

Teilprojekt B05 Deutschmann

Modelling and numerical simulation of multiphase chemical reactions in exhaust gas after treatment systems

Motivation

Exhaust gas after treatment systems are an essential part of every combustion system for the chemical conversion of harmful substances, which form unavoidably in combustion processes, in. Besides classical catalytic systems that reduce unburnt hydrocarbons (HC), carbon monoxide (CO) and nitrous oxides (NOx) on solid catalysts, nowadays a liquid reducing agent, a urea-water solution (UWS), is used to reduce nitrous oxides in selective catalytic reduction (SCR) systems for lean-burn engines. The complex preparation of the liquid reducing agent to the actual reducing agent ammonia caused many problems recently. The interaction of urea-water-solution, gaseous intermediate species like isocyanic acid, liquid films and solid deposits with exhaust pipes, catalysts and the flow field in the catalyst has not been investigated fundamentally yet. In addition to the experimental investigation of these processes, it is necessary to develop mathematical models to describe them and integrate these models into numerical simulations of reactive turbulent flows in exhaust systems.

Objectives

After successful investigation of the chemical and physical processes between the dosing of urea-water solution and the catalyst in the first period, in the second period the catalyst downstream is included into the investigations. This is of importance, because experiments have shown that the preparation of urea-water solution is not completed at the beginning of the catalyst and the distance between dosing point and catalyst will be further reduced. The objective of this project is the development of a multiphase reaction mechanism of the system exhaust gas/urea/isocyanic acid/hydrolysis/SCR-catalyst, based on the mechanism of the first period. Furthermore, submodels for all chemical processes and the integration into CFD simulations will be created to be able to describe all processes from dosing until the end of the catalyst. In addition, the influence of the SpaciPro measuring device on the measuring outcome in experiments is evaluated through CFD-simulations.

Visualization of the injection-spray; pool-fire on the piston wall surface
Overview of modeled processes in first and second period.

Previous Findings

Based on gas phase reaction mechanisms, which were established in this project, many experimental observations on a new SCR-catalyst could be understood. It shows that NO in the gas phase oxidizes at low temperatures already, which particular influences the catalytic conversion in a SCR-catalyst. Besides gas phase investigations, a new developed chemical reaction mechanism can now describe the decomposition of urea-water solution, the formation of solid deposits and their decomposition. To simulate this new mechanism the code DETCHEMMPTR was created. With this model, thermogravimetrical experiments from C04 (Deutschmann/Suntz) were successfully simulated with great accordance.

Scheme of the urea decomposition mechanism developed in the first period.
Scheme of the urea decomposition mechanism developed in the first period.

Approach

For the simulations the code DETCHEMMPTR which was developed in the first period, will be used and further expanded. This is a zero-dimensional multiphase batch type reactor model with an arbitrary number of phases. The new developed functions are written in FORTRAN and coupled to the existing code. The platform CaRMeN is helpful to integrate and compare all numerical and experimental results. CFD simulations for the error correction of the SpaciPro measurements will be done with DUO (DETCHEM Und OpenFOAM).

Current Work

Current objectives include the investigation of the hydrolysis of isocyanic acid. Experiments from the first period have shown that the kinetics in literature cannot describe the processes with acceptable quality. Based on new experiments from C04 and theoretical DFT investigations from B04, the kinetic parameter are optimized in CaRMeN. For consideration of chemical species streams from catalyst surface into gaseous, liquid or solid phase of urea decomposition, models will be developed. Therefore, internal transport limitations in the porous structure and the condition of the catalyst, as well as thereby changed reaction kinetics, will be taken into account. This developed model will be implemented into DETCHEMMPTR and the adapted code will be integrated into CaRMeN. With this refined model, the reaction mechanism of urea decomposition will be expanded and the chemical and physical processes between urea or byproducts and the catalyst will be included. Taking into account also the gas phase reactions, a whole SCR catalyst with its laminar flow will be modeled. The comparison of concentrations profiles in the channels with experiments of subproject C04 allows an evaluation of the total mechanism. With the models from the first period and this work, it will be possible to model all chemical processes between urea dosing and the end of the SCR catalyst.

Processes on the catalyst surface.
Processes on the catalyst surface.

Cooperations

The main goal of this subproject is to develop models for the chemical and physical processes between dosing and the end of the SCR-catalyst in the exhaust gas after treatment system. These models are prepared for the use in other subprojects. The chemical source terms, which are calculated by the chemical models using the reaction mechanism, will be integrated into the CFD-Simulations (C04 (Sadiki/Hasse)) directly. Because of high computational costs of turbulent flows using a complex chemical reaction mechanism, suitable reducing strategies (B07 (Bykov/Maas)) shall be applied. Experimental measurements for model development and evaluation will be done in C04 (Deutschmann) and theoretical fundamentals will be delivered by the subproject B04 (Olzmann).

Selected Publications

  • Brack, W., Heine, B., Birkhold, F., Kruse, M., Deutschmann, O.: Formation of Urea-Based Deposits in an Exhaust System. Emiss. Control Sci. Technol. 2(3), 115–123 (2016).
  • 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).
  • Cai, X., Wörner, M., Marschall, H., Deutschmann, O.: CFD Simulation of Liquid Back Suction and Gas Bubble Formation in a Circular Tube with Sudden or Gradual Expansion. Emiss. Control Sci. Technol. 3(4), 289–301 (2017).
  • Stein, M., Bykov, V., Bertótiné Abai, A., Janzer, C., Maas, U., Deutschmann, O., Olzmann, M.: A reduced model for the evaporation and decomposition of urea–water solution droplets. Int. J. Heat Fluid Flow 70, 216–225 (2018).