Contribution of Urea and its Derivatives to Particulate Emission of Diesel Vehicles

The formation of solid by-products in urea SCR systems continues to challenge the automotive industry today. The deposit formation on wetted surfaces due to undesirable chemical reactions after the injection of urea-water-solution (UWS) into the exhaust tract has been extensively studied both experimentally and by numerical simulations [1, 2].
However, reactions known from deposit formation can also occur within the small UWS-droplets flowing in the hot exhaust gas, and small solid particles can form. Due to their low mass, these particles can also pass through the SCR catalyst and are emitted directly into the environment, because there is usually no system that would separate the particles after the SCR catalyst in the exhaust tract. In line with this, some recent measurements report an increase in particulate emission from diesel vehicles equipped with urea-SCR technology.
In the current numerical study, the properties of solid particles formed from UWS droplets are investigated. After water evaporation and urea decomposition, a mixture of biuret and triuret remains, whose diameter is 22-37% of the original droplet diameter, depending on the exhaust gas temperature. ... mehrIn addition to the modeling of the formation of "UWS particles", a possibility to minimize the emission of UWS-sourced particles by capturing them in a filter is also investigated. It is shown how the particle properties, such as diameter and composition, change when the filter is heated - analogous to DPF regeneration. Even though the particle diameter decreases up to 80% within 120 seconds after heating, depending on the regeneration temperature, complete elimination in a technically relevant time scale is not possible because the species in the particle react to thermally stable products.
The simulations are performed with the MPTR tool of DETCHEM. The chemical reactions are modeled with the mechanism of Kuntz et al. [3], which also includes the formation of high-temperature by-products of urea. The simulation is validated with experiments by Surendran [4], who studied the change in diameter of UWS droplets in hot gas flow as a function of temperature and urea concentration in the solution.

[1] Stein, M., et al., "A reduced model for the evaporation and decomposition of urea–water solution droplets" International Journal of Heat and Fluid Flow, 2018. 70: p. 216-225.
[2] Börnhorst, M., "Urea-Water Sprays in NOx Emission Control Systems: Interaction with Solid Walls and Deposit Formation" 2020: Karlsruher Institut für Technologie (KIT).
[3] Kuntz, C., et al., "Kinetic modeling and simulation of high-temperature by-product formation from urea decomposition" Chemical Engineering Science, 2021. 246: p. 116876.
[4] Surendran, M., T.N.C. Anand, and S. Bakshi, "Experimental investigation of the evaporation behavior of urea-water-solution droplets exposed to a hot air stream" AIChE Journal, 2020. 66(2): p. e16845.