Investigation of Urea-SCR at Low Temperatures

This paper addresses the NO_x removal from diesel exhaust by using urea-SCR technique with focus on critical operating conditions, particularly low temperatures. The investigations were performed on a special laboratory test bench enabling the supply of aqueous urea solution (“AdBlue”) or gaseous NH₃ to practical model exhaust. The SCR tests were made with a commercial V₂O₅/WO₃/TiO₂ honeycomb catalyst showing very similar performance between 250 and 400 °C when dosing AdBlue and NH₃. However, below 200 °C the activity of urea-SCR was clearly lower than that of NH₃-SCR due to incomplete conversion of urea and formation of deposits covering the reactor wall. ATR-IR analyses evidenced that these residues consist of urea as well as cyanuric acid originated from side-reactions of isocyanic acid. Furthermore, ramping the temperature upon urea-SCR forced the deposits towards consecutive reactions mainly resulting in cyanuric acid. Consequently, our study provides insights into the dynamic decomposition behavior or urea-related residues relevant for reliable SCR process in diesel exhaust.