Effect of Spray-Exhaust Gas Interactions on Ammonia Generation in SCR Mixing Sections 04-11-02-0009

The selective catalytic reduction (SCR) of nitrogen oxides with ammonia is a promising solution to meet upcoming emission regulations for lean-burning combustion engines. Due to the toxicity of ammonia, exclusively SCR systems with precursor substances, e.g., a urea-water solution (UWS), are available or being developed. The determining factors for the efficiency of SCR systems are sufficient ammonia generation and homogenization upstream of the catalytic converter. In the first part, this study presents an experimental investigation of the occurring mechanisms during ammonia generation from UWS droplets; including the evaporation of water, the thermal decomposition of urea, and droplet-wall interactions. In the second part, the observed physical and chemical phenomena are mathematically described and constitute the basis for the development of a simulation model. For this purpose, experiments by means of TGA were conducted to thoroughly investigate the UWS decomposition. In addition, droplet-wall interactions in SCR mixing sections were analyzed. The developed simulation model is able to determine the droplets’ trajectories, their mass loss due to evaporation and decomposition of urea, and the change in the droplets’ temperature. In contrast to previous studies, the created numerical model additionally considers the change in temperature of the exhaust gas and mixing section due to the intermittent UWS injections, heat losses to the environment, and convective and conductive heat transport phenomena. In a final step, the numerical model was applied to determine the necessary level of complexity for the simulation by varying the degrees of detail for a simplified case study. This allowed the composition of a mathematical framework, which could serve as a strong basis for future investigations.