Modeling the decomposition of urea-water-solution in films and droplets under SCR conditions with chemistry in the liquid phase

Due to strict environmental regulations in the automotive sector selective catalytic reduction with urea-water-solution is often used to reduce nitrogen oxide emissions. To improve and solve technical problems, like the formation of solid residuals, detailed simulations of the exhaust gas system are used. Such simulations have high computational needs due to the multitude of different processes involved at strongly differing spatial and time scales. For this reason, simplified or reduced models are applied and urea decomposition is often modeled with a vapor pressure curve fitted to experimental results. This method is incapable of describing the formation of solid residuals. A recently developed chemical mechanism for urea decomposition in the liquid phase is used in this work to simulate the decomposition of spherical droplets and planar wall films of urea-water-solution in exhaust gas. For droplets, the overall behavior with liquid chemistry is similar for the whole range of ambient temperatures. Less ammonia and nearly no isocyanic acid is produced compared to the evaporation model and residuals of liquid biuret and solid triuret remain. For wall films, the behavior as well as the composition of the residuals strongly depends on the temperature. The mechanism predicts the production of a similar amount of ammonia but less isocyanic acid compared to the evaporation model. The remaining mass loss is mostly composed of cyanuric acid. It is found that the process of urea decomposition is much slower with liquid chemistry and complete decomposition only happens at 673 K or above. The analysis of the chemical time scales results in a skeletal mechanism for the droplet decomposition with 6 out of 13 reactions among 7 out of 13 species that can describe the whole process with good accuracy, where the mass of the produced gases deviates by less than 5%.