The Selective Catalytic Reduction (SCR) based on Urea-Water-Solution (UWS) injection is an effective technique to reduce the nitrogen oxides (NOx) emitted from the exhaust gases of diesel engines. The evaporation characteristics obtained for single Urea-Water-Solution (UWS) droplet using CFD code AVL FIRE is compared with experimental values. 3D numerical model of the injection of UWS and its interaction with the exhaust gas flow and exhaust tubing is developed. The model accounts for all relevant processes appearing from the injection point to the entrance of the SCR catalyst, especially during evaporation and thermolysis of droplets, hydrolysis of isocyanic acid in gas phase, heat transfer between wall and droplets and spray/wall-interaction. The spray wall deposition is found to be varying with temperature and residence time. The simulated results of variation of wall deposition, film deposition area, evaporated mass with respect to residence time are compared at temperatures 573K and 623K.