Effects of Urea Injection Timing on Predicting NoX Conversion In SCR Systems

The most undesired diesel engine pollutants are nitride oxide (NOX), particulate matter (PM), carbon monoxide (CO), and unburned hydrocarbon (HC). Selective catalytic reduction (SCR) with ammonia (NH3) has been successfully employed to remove NOX from diesel emissions via chemical reactions to yield steam (H2O) and nitrogen (N2). However, the ammonia homogenization process is usually incomplete and difficult to mix with NOX. This reaction affected from the urea injection process in the system when the condition is not suitable for urea decomposition process. Therefore, a study on the effects of urea injection timing to predict the NOX reduction efficiency was needed in order to identification the suitable condition based on temperature, flow mass, pressure, etc. In this study, the experiments and simulations were conducted to obtain constant and stable conditions in the SCR system, as constant conditions can optimize the uniformity of ammonia distribution in the system. The engine conditions in the experiment were computed from turning an engine on to determine the desired temperature and airflow, and the simulation was performed using a commercial code of STAR-CCM+. The temperatures used for processing ammonia uniformity in diesel SCR systems were investigated with operation at ambient temperature (298 K) and inlet gas temperature (686 K). Numerical results were validated using the experimental results of ammonia concentration distribution.