Ammonia Storage and Slip in a Urea Selective Catalytic Reduction Catalyst under Steady and Transient Conditions

This paper aims to investigate quantitatively the characteristics of the storage and slip of ammonia in a full-size vanadium-based selective catalytic reduction (SCR) catalyst under typical steady and transient conditions on a diesel engine test bench. The effect of space velocity (SV) and exhaust temperature on the ammonia storage process and the storage capacity of the catalyst was studied. The ammonia slip in various transient processes was also investigated. The experimental results indicate that the increase of SV causes early appearance of ammonia slip. Then, the ammonia storage time is shorten to avoid the ammonia slip and the ammonia storage amount is consequently reduced which has an effect on the NOx conversion efficiency. The exhaust temperature is the key factor affecting the ammonia storage capacity of the catalyst which sharply decreases with the increase of temperature. The NOx conversion efficiency strongly depends on the amount of ammonia storage when the temperature is lower than 280 degrees C, and it is nearly linear to the amount of ammonia storage at 200 degrees C. If the ammonia storage is not controlled properly ammonia slip will occur under various transient conditions. Rapid heating of the catalyst at low temperature can cause serious ammonia slip. The initial temperature of the heating process is crucial to causing ammonia slip in transient processes while SV is not the key factor. The saturation level of the ammonia storage has a significant effect on ammonia slip in transient processes. There is an appropriate amount of ammonia storage which can be defined as the "slip-preventing limit" that maximizes the NOx conversion efficiency while keeping ammonia slip under 10 ppm to comply with the Euro 6 legislation for heavy-duty diesel engines.