Laboratory and Engine Study of Urea-Related Deposits in Diesel Urea-SCR After-Treatment Systems

Diesel exhaust systems equipped with selective catalytic reduction (SCR) catalysts based on urea were subjected to an aging process where the exhaust gas temperature was below 300°C. Solid deposits related to urea injection were found on the wall of the exhaust pipe down stream of the urea injector and on a urea mixer in front of the SCR catalyst. In laboratory tests, an aqueous solution of urea (1.5wt%) was dripped onto an SCR catalyst core in a simulated lean gas mixture at a rate corresponding to a 1:1 NH₃-to-NOx ratio (NOx = 350ppm) and a space velocity (SV) of 15,000 h^-1 at various temperatures. At 300°C and below, urea-related deposits appeared on the SCR catalyst surface and totally plugged the SCR catalyst monolith within 250 hours. When the aging temperature was 350°C or above, no deposits were observed on the SCR catalyst core. The deposits from engine dynamometer and laboratory aging experiments were found to be similar, based on analyses by FTIR spectroscopy and powder X-ray diffraction. Thermal gravimetric analysis (TGA) demonstrated that most of the deposits vaporized at temperatures between 310 to 325°C when heated slowly, although the deposits did not vaporize quickly until the temperature was above 350°C when heated in a static oven. The temperature impact on the formation of the various deposits was also studied in the laboratory.