Experimental investigation on the characteristics of ammonia storage for heavy-duty diesel engine SCR catalyst

Abstract

The ammonia storage characteristics were studied experimentally in two stages before and after shutting off the urea injection on a full-size Cu-based zeolite SCR catalyst. The experiment was conducted under the engine operating conditions with exhaust temperatures of 180, 220, 260, 300, 340, and 380 ℃. The variations of the ammonia storage amount, NOx conversion efficiency, and ammonia slip were separately investigated within the period of ammonia storage and consumption process. The results indicate that the maximum ammonia storage is only 10 g at 180 ℃, but around 43 g at 220 ℃. The maximum ammonia storage decreases with the increase of exhaust temperature over 220 ℃, which drops from 35 g at 260 ℃ down to below 13 g at 380 ℃. The most extended duration of 8082 s on the ammonia storage process occurred at the exhaust temperature of 220 ℃, mainly due to the deposit formation. The time width of the ammonia storage process is shortened with the exhaust temperature increasing over 220 ℃. The maximum NOx conversion efficiency is 80% at the temperature of 180 ℃ and 100% when the temperature rises to 220 ℃ or above. The higher temperature can improve the catalyst activity to supply more activated ammonia reacting with NOx. However, it promotes further ammonia slip. The ammonia storage and exhaust temperature are the main factors in the NOx conversion performance in the SCR catalyst. When the temperature is below 220 ℃, the NOx conversion efficiency strongly depends on the ammonia storage and increases with its amount. With the temperature further rising to 220℃ and above, the impact of the exhaust temperature on NOx conversion efficiency gradually increases.