Coupling of Liquid and Surface Chemistry in Urea SCR Systems

Close-coupled selective catalytic reduction (SCR) systems are one method to deal with tightening emission legislation for NO$_x$ in internal combustion engines. Due to smaller mixing sections and at unfavourable boundary conditions, however, urea-water solution (UWS) droplets can impact on the SCR catalyst itself. To investigate this phenomenon further, this work develops a modeling capability of this process. Established mechanism for NH$_3$-SCR and HNCO hydrolysis from literature is integrated into DETCHEM$^{CHANNEL}$ and a 2D COMSOL model to simulate the influence in the SCR Channel. Simulations are validated against end-of-pipe experiments from literature and spatially resolved concentration profiles from a hot gas test rig with very good agreement. Finally, a channel simulation is coupled with a model to describe the catalytic decomposition of an urea droplet. The coupled simulation is able to simulate the influence of UWS droplet impact onto a catalyst channel. Fast droplet decomposition causes a peak in NH$_3$ and HNCO in the single channel and thus increases NO$_x$ conversion. However, the overall uniformity and efficiency are decreased, which is why droplet impact on the catalyst should be strictly avoided.