A study of the effects of NH₃ maldistribution on a urea-selective catalytic reduction system

Abstract

The maldistribution of urea/NH3 at the selective catalytic reduction inlet and the resulting NH3/NOx ratio variation have been reported to have negative effects on the NOx conversion efficiency and NH3 slip in heavy-duty diesel applications. Maldistribution is caused by incomplete mixing and decomposition of the injected urea water solution within the exhaust flow in the aftertreatment system. How NH3 and the NH3/NOx ratio are distributed at the selective catalytic reduction inlet in an engine aftertreatment system and how the resultant maldistribution affects the system performance and the kinetics are presented in this combined experimental and modeling study. The distribution profiles of the selective catalytic reduction inlet NH3 and NH3/NOx ratio were determined through engine experiments. The NH3 maldistribution effects on the selective catalytic reduction performance and kinetics were quantified by using a multi-channel one-dimensional model that takes into consideration the NH3 maldistribution. It was determined that the NH3 maldistribution is a major factor causing the difference in catalyst performance in the engine exhaust as compared to the laboratory reactor environment, and these results explain specific differences in the model calibrations determined from the laboratory reactor and engine data. Improving the selective catalytic reduction inlet NH3 uniformity can improve the NOx conversion efficiency by up to 7% and reduce NH3 slip by 10–20 ppm. A single-channel one-dimensional selective catalytic reduction model calibrated to the engine data that accounts for the NH3 maldistribution phenomena can be applied for the development of on-board diagnostic and control strategies.

Ammonia Concentration Distribution Measurements on Selective Catalytic Reduction Catalysts

“…Moreover, the slow SCR reaction (3) with a low reaction rate is active at the NO2/NO x shares over 50%. The NO x reduction of the slow SCR reaction mechanism is limited by kinetic factors other than the inlet NH3 maldistribution [10]. Furthermore, the possible formation of urea solid deposits inside the exhaust system decreases the amount of the reducing agent available for NO x conversion [14,15].…”

“…As a result, a dependable methodology of urea uniformity evaluation is required. An investigation into the distribution of ammonia in SCR systems has already been investigated and characterized, as published by Song, Naber and Johnson in [10].…”

Numerical and Experimental Investigation of Ammonia Uniformity in a Compact Selective Catalytic Reduction System

“…In this technology, typically the eutectic urea–water solution (UWS) is injected in the exhaust stream, which undergoes evaporation, thermolysis, and hydrolysis reactions to generate gaseous ammonia. , Ammonia then reduces NO x to nontoxic compounds over a vanadia- or zeolite-based catalytic substrate placed downstream of UWS injection location. As the worldwide NO x emission regulations have become more stringent, it is critical to deliver a uniform distribution of ammonia to the catalyst to minimize tailpipe NO x and ammonia slip. − Static mixers play a key role in achieving the desired ammonia uniformity by promoting decomposition on their hot surfaces and creating highly turbulent flow in the mixing section. However, the recent push toward the development of compact urea mixers to meet stringent space-claim requirements makes the impingement of UWS spray on the mixer walls unavoidable.…”

Simulation study on improving the selective catalytic reduction efficiency by using the temperature rise in a non-road transient cycle

“…This improved mixing technology allows compact packaging of the SCR system together with a high deNO x efficiency as well as a low NH 3 slip. 28 Also, control of the urea injection is a key function in the urea SCR system to address some important issues such as formation of a urea deposit, NH 3 slip and in-use compliance requirements. However, challenges in urea injection control are caused by the cost and the accuracy in measurement or estimation of the urea mass injected.…”