NOx abatement – 1D/3D simulation of urea dosing and selective catalytic reduction
Johann Wurzenberger, AVL List GmbH
European emission certification tests are currently changing from standard, repeatable, chassis dyno measurements to random on-read tests assessing real driving emissions (RDE). This change challenges especially the abatement of NOx emissions as studies on NEDC-certified diesel passenger cars reveal that on-road emissions higher by one order of magnitude. This is mainly attributed to the comparably more dynamic real driving, higher engine load points featuring higher exhaust mass flows and engine-out NOx fractions, and the corresponding sizing and operation of the aftertreatment system.
Urea SCR is a well-established technology route to reduce NOx emissions of diesel powered engines. The SCR aftertreatment consists of a dosing unit, injecting aqueous urea mixtures, and a honeycomb catalyst supporting the selective catalytic reduction of NOx in the presence of NH3. Between the injection nozzle and the catalyst inlet, the urea-water solution transforms into gaseous NH3 involving a cascade of phenomena. These are droplet transport, evaporation of water, thermolysis of urea, droplet wall interaction, wall film formation, film crystallization, film evaporation, wall cooling etc. A proper handling of these phenomena is key to ensure high operating efficiencies and also to reach sustainable system life-times.
This work presents a simulation study on NOx reduction considering both, the urea-dosing unit and the SCR catalyst. 3D-CFD methods are used to address the urea spray, wall impingement, wall film build-up and film crystallization in highly resolved 3D simulations. The film crystallization is a key information for the exhaust system design on one the hand and it is very demanding for the simulation regarding its modelling complexity and long simulation time, on the other hand. Here dedicated frozen flow field methods are presented helping to speed-up the computationally demanding simulations of transient injection events. A comparison of calculated wall film patterns with data from a test-rig demonstrates the applicability of the model. It catches the split of water-urea impinging at the wall and evaporating in the gas stream, the temporal evolvement of the film composition and the NH3 fraction and distribution at the SCR catalyst inlet. The crystallization reactions used in the 3D simulations are investigated upfront by a 1D system-level model featuring real-time capable computation. The reaction model is presented and it is discussed in comparison to experimental data. The 1D exhaust aftertreatment system model is also used to calculate a transient drive cycle where the wall impingement fractions gained from the CFD simulations are used. Thus, the usage of identical reaction models in the wall film and also in the SCR catalyst ties up the used 3D and 1D simulations in an efficient workflow.