Accurate Urea Consumption Prediction in SCR System Using CFD Simulation

Abstract

<div class="section abstract"><div class="htmlview paragraph">The selective catalytic reduction (SCR) is a technique, which is using in diesel engine to reduce harmful nitrogen oxide (NOx) emissions. SCR technique involves the injection of urea-water-solution (Water-urea solution) into the hot exhaust stream. The water first evaporates and then urea undergoes thermal decomposition. The thermal decomposition of urea produces ammonia, which reacts with the nitrogen oxides inside a SCR catalyst layers and nitrogen and water vapor are the final product. The production of ammonia from urea strongly influenced by the droplet size, residence time of the droplets inside decomposition chamber and exhaust gas temperature.</div><div class="htmlview paragraph">During the combustion process of Diesel engines, Nitrogen oxide (NOx) is produced as a pollutant which is harmful for environment. Acceptance level of (NOx) is made more stringent in BS-VI and subsequent standards to regulate (NOx) levels. In SCR system NH3 reacts with (NOx) and converts N2 and H2O. Ammonia (NH3) concentration and uniformity of flow pattern at SCR catalyst inlet is important, to achieve higher (NOx) conversion rate (to minimize NOx emissions).</div><div class="htmlview paragraph">CFD simulation plays a vital role in design and development of exhaust after-treatment systems. In early development stages evaluating complex designs through physical validation is not feasible in terms of cost and time. In the automotive industry CFD simulations are extensively used to verify performance through iterative process to achieve an optimized design. The exhaust flow interaction with the Water-urea solution spray and evaporation of urea are key factors in the design for a given length of the exhaust system. The thermal response and evaporation time of the droplet is solely depending on the droplet size. The work presented in this paper is focused on developing a CFD methodology to simulate SCR system using commercial CFD software Ansys FLUENT. Current study conducted on a SCR system of a small diesel engine consisting of a decomposition chamber with a static mixer along with SCR catalysts. The numerical analysis deals with the gas phase reactions happening in the SCR system. In this work, transient CFD simulation is conducted to capture the phenomenon of wall film formation and spray break up in detail. The continuous phase (exhaust gases) and discrete phase (Water-urea solution droplets) has been modelled with RNG k-є model and Lagrangian frame respectively. The continuous phase influences discrete phase via drag and turbulence. This single direction influence between continuous and discrete phase is called one way-coupling, which has been considered in present work. Two assessment methods is considered; one is urea to ammonia conversion and other is area weighted uniformity index across SCR catalyst inlet to evaluate mixing efficiency.</div></div>