Abstract
In recent years, regulations on exhaust emission have become more stringent as a result of increased environmental pollution and the need to conserve energy. To meet stringent regulations with regard to exhaust emissions for diesel vehicles, after-treatment systems, such as LNT (Lean NOx Trap), LNC (Lean NOx Catalyst), and urea-SCR (Selective Catalytic Reduction), have been developed for diesel exhaust gas. SCR that uses urea (reductant) is a powerful technique for reducing the NOx that is emitted from diesel engines. However, a problem with SCR is that the NOx conversion efficiency is low in low-temperature regions when a non-precious metal catalyst, such as Fe-zeolite, is used. The purpose of this study is to understand the fundamental characteristics of the LNT and SCR catalysts for improving the NOx conversion efficiency and the width of the temperature window through a combination of the LNT and SCR systems. At 2000C, the NOx storage capacity of the LNT catalyst saturates at about 1200s. The NOx desorption peaks were high at temperatures of 330℃ and 480℃. From the viewpoints of fuel penalties and the NOx conversion efficiency, a rich-spike duration of five seconds is suitable for the LNT catalyst system. Hydrogen, which was used in the LNT system as a reductant, increased the NOx conversion efficiency and the NH3 that formed over the LNT was useful for the SCR catalyst. The LNT+SCR combination without the additional injection of NH3 improved the NOx reduction by about 10-30% at temperatures below 4000C.
Keywords: Diesel engine, LNT(Lean NOx Trap), SCR(Selective Catalytic Reduction), NH3, Reductant, Adsorption, Desorption