Decomposition Behavior of Urea in High Temperature Atmosphere

Recently, Urea-SCR system is used for reducing NOx emission from diesel engine. In the Urea-SCR system, aqueous urea is injected into the exhaust gas flow where it evaporates and decomposes to NH3. It is necessary to clarify the generation process of NH3 in high temperature atmosphere for the improvement of NOx reduction performance. In this study, the generation behavior of NH3 and the mass reduction of solid urea and 32.5wt% urea solution were measured in a high temperature vessel. As a result, the start of NH3 generation became early and the maximum NH3 concentration increased with increasing temperature in the decompositions of both samples. The start of NH3 generation in the urea solution decomposition was later than that in solid urea decomposition owing to the evaporation of water. Moreover, the amount of mass reduction rate was increased when N2 was passed through the vessel. It seemed that the reaction of urea was accelerated because reaction products were removed by the N2 flow. As for the residue after decomposition, the mass of residue decreased with increasing temperature in the decompositions of both samples. The residues of them were decreased when N2 was passed through the vessel under lower atmospheric temperature conditions.