Publication Date:
2017-07-27
Description:
This paper addresses the selective catalytic reduction of NO x (SCR) in diesel exhaust by using a monolithic V 2 O 5 /WO 3 /TiO 2 catalyst and urea as reducing agent. A special laboratory test bench was used enabling the supply of aqueous urea solution (“AdBlue”) or gaseous NH 3 and admitting realistic flow conditions (Reynolds numbers 〈 230). The residence time from AdBlue dosing nozzle to catalyst was ca. 0.037 s. The tests provided very similar standard and fast SCR performance between 150 and 350 °C when dosing AdBlue and NH 3 . For standard urea-SCR, axial concentration profiles were evaluated by employing catalyst monoliths with different lengths corresponding to space velocities between 85,000 and 425,000 h – 1 . The results evidenced incomplete conversion of urea and isocyanic acid along the catalyst, especially at low temperatures. Despite this, local SCR rates were not significantly affected by the under-stoichiometric concentration of NH 3 . This was substantiated by kinetic analysis showing an activation energy of ca. 68 kJ mol – 1 typical for standard NH 3 -SCR. However, above 250 °C SCR was limited by internal mass transport as indicated by Weisz-Prater criterion and clearly lower activation barrier (ca. 26 kJ mol – 1 ). Furthermore, formation of deposits was investigated at low temperatures (175 °C and below) in 3 h runs. In these tests, some urea was deposited around the nozzle, whereas in front of and behind the catalyst cyanuric acid and biuret were additionally found originated from side reactions of urea and isocyanic acid. The deposits were quantitatively checked by HPLC, while qualitative analysis was made by ATR IR.
Print ISSN:
0930-7516
Electronic ISSN:
1521-4125
Topics:
Chemistry and Pharmacology
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Process Engineering, Biotechnology, Nutrition Technology
