engleski

Numerical investigation of wallfilm formation for selective catalytic reduction applications

The urea-water-solution (UWS) based selective catalytic reduction (SCR) is currently most promising method to fulfil stringent requirements of upcoming Euro 6 norm on NOx emissions from diesel engines. Once urea-water- solution spray is injected into hot exhaust gas stream before SCR catalyst, water content evaporates from UWS. Afterwards, ammonia is generated through thermal decomposition of urea and hydrolysis of isocyanic acid. Generated ammonia takes part in various deNOx reactions as a reductant. The resulting spatial distribution of the reducing agent before the catalyst is a crucial factor for the conversion of NOx. The uniform distribution and the degree of processing of the reducing agent upstream of the SCR catalyst can be, besides the evaporation and decomposition, also influenced by the spray/wall interaction. The focus of this paper is therefore the investigation of the spray interaction with both the hot air stream and wall by means of numerical simulation and the validation of mathematical models integrated in commercial CFD code Fire. To predict the generation and distribution of the reducing agent a detailed three-dimensional numerical simulation was performed. UWS droplets were treated with Lagrangian particle tracking, which solves the equation of motion, mass and enthalpy for parcels of droplets with identical properties. Deposition of droplets leads to the formation of a wallfilm which is modelled with a 2D finite volume method in the Fire wallfilm module. To evaluate the influence of varying parameters, well-known spray parameters as an initial condition are essential for a CFD simulation. As modelling of primary breakup is still insufficiently understood, the first step is spray calibration. The single calibration steps of spray were verified with digital CCD (charge- coupled device) camera images, patternator measurements and laser light scattering measurements. After spray calibration the spray/wall-interaction and wallfilm formation were investigated in a rectangular flow channel geometry and obtained simulation results were compared to experimental data.

gas aftertreatment; selective catalytic reduction; wallfilm; urea water solution; NOx reduction; computational fluid dynamics

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