An Infrared and Thermal Decomposition Study on Solid Deposits Originating from Heavy-Duty Diesel SCR Urea Injection Fluids

In this study, we present the effects of time and temperature on the formation of urea deposits and their composition under conditions realistic to mobile heavy-duty SCR applications. To this end, various synthesis times were evaluated (1 h, 4 h and 24 h), as well as temperatures (298–673 K). The formed urea deposits were qualitatively evaluated using ATR FTIR to elucidate their molecular composition. Furthermore, the effect of dry and moist synthesis conditions were evaluated to simulate water rich and deficient exhaust gas composition. This was achieved by conducting the synthesis in open or closed containers to simulate dry and humid conditions, respectively. The findings are presented in this paper in two maps covering dry and moisture conditions. The trend reveals that at low temperatures the deposits mainly consist of urea derived compounds, like biuret and cyanuric acid. Increasing the temperature leads to an increase in more aminated mono-triazine compounds. At the highest synthesis temperature of 673 K, the main constituents are cyamuleric compounds, consisting of fused triazine rings like melem. Humid synthesis conditions hampers the formation of highly aminated compounds up to a synthesis temperature of 573 K, even after a synthesis period of 24 h. Temperatures higher than 573 K, and a long synthesis period of 24 h, results only in minor differences and are observed between samples prepared under dry or humid conditions. The decomposition properties of the synthesized materials are optimal for the samples prepared at 523 K, whereas the lowest decomposition rates were observed for samples prepared at 623 K and 673 K. A humid air gas flow was shown to be beneficial for the decomposition rate under these conditions.