The Effect of Evaporation Models on Urea Decomposition from Urea-Water-Solution Droplets in SCR Conditions

Selective catalytic reduction using urea-water-solution to reduce emissions of NO_x from diesel engines is commonly used in the automotive industry. For a high efficiency of this process, a good understanding of the formation of ammonia from urea-water-solution droplets is required. There are two main variants for the description of urea decomposition into ammonia and isocyanic acid from droplets of urea-water-solution based on an evaporation model: direct decomposition at the interface and decomposition in the gas phase by a chemical reaction. These variants have been compared using detailed one-dimensional simulations with a detailed model for the gas-liquid interface. In addition, the influence of gas phase chemistry and varying ambient conditions on the decomposition of urea was determined. It is shown that water evaporation and urea decomposition cannot be completely separated. Direct decomposition overestimates the production of ammonia due to the varying gas phase properties of ammonia and isocyanic acid. Decomposition in the gas phase correctly calculates the mass of ammonia produced by a droplet but the gas phase reaction couples strongly with the evaporation process. Especially at lower ambient temperatures, the evaporation rate is increased and it is more sensitive to changes of the ambient conditions and initial droplet diameter. Of the known relevant gas phase chemistry, only the hydrolysis of isocyanic acid happens in a time-scale similar to that of the droplet variation at temperatures typical for selective catalytic reduction.