Characterisation of deposits formed from urea-water-solution droplets under high temperature engine environment

Abstract

The challenge of NOx emissions from diesel engines necessitates the development of cleaner and more efficient combustion processes, along with advanced emission control strategies. The use of Urea-Water Solution (UWS) in Selective Catalytic Reduction (SCR) is the state-of-the-art NOx emission control strategy; however, it also leads to unwanted deposit formation, which can affect system performance. To understand the deposit formation process, optical analysis reveals that at temperatures below 400 °C, rapid water evaporation localizes urea at the droplet core, ultimately resulting in solid crystal deposition. At higher temperatures, accelerated urea dissolution prevents crystallization due to insufficient time for supersaturation. This study further investigates the mechanisms of UWS droplet breakup, nucleation, phase transitions, and the physical characteristics of the resulting deposits. Thermogravimetric analysis (TGA) of UWS indicates that water evaporates between 20 and 100 °C, while urea degrades between 120 and 230 °C, leading to a total weight loss of 98.33 % by 400 °C, leaving a 1.67 % residue. Scanning electron microscopy (SEM) images of the UWS residue reveal a densely packed, crystal-like structure. Additionally, X-ray diffraction (XRD) analysis of the deposits at 400 °C shows a crystallinity of 70 % and an amorphous content of 30 %, indicating that even at high temperatures, a portion of the material remains non-crystalline due to rapid thermal decomposition and evaporation dynamics. Mixing urea directly with fuel, leading to the development of a ternary blend of UWS, Oxymethylene Ether (OME1), and methanol enhances droplet atomization and breakup, promoting the formation of secondary droplets and reducing residual crystallization. However, higher concentrations of UWS can inhibit droplet breakup and increase evaporation, resulting in a higher propensity for deposit formation. Overall, this study highlight that formation of deposits remains a critical challenge.