Detailed modeling of the evaporation and thermal decomposition of urea-water solution in SCR systems



This work is aimed to develop a multicomponent evaporation model for droplets of urea-water solution (UWS) and a thermal decomposition model of urea for automotive exhausts by using the selective catalytic reduction systems. In the multicomponent evaporation model, the influence of urea on the UWS evaporation is taken into account using a nonrandom two-liquid activity model. The thermal decomposition model is based on a semidetailed kinetic scheme accounting not only for the production of ammonia (NH3) and isocyanic acid but also for the formation of heavier solid by-products (biuret, cyanuric acid, and ammelide). This kinetics model has been validated against gaseous data as well as solid-phase concentration profiles obtained by Lundstroem et al. (2009) and Schaber et al. (2004). Both models have been implemented in IFP-C3D industrial software to simulate UWS droplet evaporation and decomposition as well as the formation of solid by-products. It has been shown that the presence of the urea solute has a small influence on the water evaporation rate, but its effect on the UWS temperature is significant. In addition, the contributions of hydrolysis and thermolysis to urea decomposition have been assessed. Finally, the impacts of the heating rate as well as gas-phase chemistry on urea decomposition pathways have been studied in detail. It has been shown that reducing the heating rate of the UWS causes the extent of the polymerization to decrease because of the higher activation energy. © 2011 American Institute of Chemical Engineers AIChE J, 2012