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

Abstract

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.

Corrosion of Stainless Steel by Urea at High Temperature

“…The IR results of 10 min heating experiment (T air = 200 • C), at which the collected deposit had a temperature of 110 • C, showed that the product consists of urea, this is in agreement with the colourless gel state of the sample I-110 (Table 3). At higher heating temperature (T air = 250 • C; T sample = 181 • C), the collected deposit I-181 has slightly changed its aggregate state to solidified gel and its colour became denser white (Table 3), which can be explained by the beginning of the urea decomposition process (T urea dec. = 133 • C [22,26,27]) and formation of biuret product (T biuret form. = 150 • C [22,27]).…”

“…At higher heating temperature (T air = 250 • C; T sample = 181 • C), the collected deposit I-181 has slightly changed its aggregate state to solidified gel and its colour became denser white (Table 3), which can be explained by the beginning of the urea decomposition process (T urea dec. = 133 • C [22,26,27]) and formation of biuret product (T biuret form. = 150 • C [22,27]). Sample I-208 obtained at 208 • C (T air = 300 • C) as a white powder (Table 3) consisted mostly of biuret with a small amount of urea and cyanuric acid.…”

“…Sample I-208 obtained at 208 • C (T air = 300 • C) as a white powder (Table 3) consisted mostly of biuret with a small amount of urea and cyanuric acid. At this stage, no liquid urea was present in the sample (T urea instable = 210 • C [24]) and biuret reached its first decomposition step (T biuret 1st dec. = 193 • C [22,24,27]), which is explained by the appearance of cyanuric acid. Sample I-258, heated at T air = 350 • C, has a milky colour and crystallized form (Table 3), which can happen when biuret has passed the second decomposition step (T biuret 2nd dec = 230 • C [24]) from solid into the liquid phase.…”

“…The IR analysis confirmed this and showed the presence of thermally stable at this temperature ammelide (T ammelide form. = 250 • C [22,27], T ammelide dec. = 360-410 • C [22,24,26,27]) as the main product with a sign of ammeline (T ammeline form. = 250 • C [22,27], T ammeline dec. = 360-430 • C [22,24,27]).…”

“…= 250 • C [22,27], T ammelide dec. = 360-410 • C [22,24,26,27]) as the main product with a sign of ammeline (T ammeline form. = 250 • C [22,27], T ammeline dec. = 360-430 • C [22,24,27]). Deposit I-301, collected at 301 • C (T air = 375 • C), was the first sample of beige colour (powder-crystalline) (Table 3), which is confirmed by the detection of cyanuric acid (it has a characteristic beige colour) besides ammelide.…”