Impact of SCR Integration on N ₂ O Emissions in Diesel Application

Significant reduction in Nitrogen Oxide (NO_x) emissions will be required to meet LEV III/Tier III Emissions Standards for Light Duty Diesel (LDD) passenger vehicles. As such, Original Equipment Manufacturers (OEMs) are exploring all possible aftertreatment options to find the best balance between performance, durability and cost. The primary technology adopted by OEMs in North America to achieve low NO_x levels is Selective Catalytic Reduction (SCR). The critical parameters needed for SCR to work properly are: an appropriate reductant such as ammonia (NH₃) provided as Diesel Exhaust Fluid (DEF), which is an aqueous urea solution 32.5% concentration in weight with water (CO(NH₂)₂ + H₂O), optimum operating temperatures, and optimum nitrogen dioxide (NO₂) to NO_x ratios (NO₂/NO_x). The NO₂/NO_x ratio is most influenced by Precious Group Metals (PGM) containing catalysts upstream of the SCR catalyst. Different versions of zeolite based SCR technologies are available on the market today and these vary in their active metal type (iron, copper, etc.), and/or zeolite type. To select an appropriate SCR type, the application's operating conditions as well as environmental factors must be considered. In order to fundamentally understand these differences, a study was conducted where various factors were investigated for the Tail Pipe (TP) N₂O emissions. They include 1) aftertreatment architectural differences, 2) individual component contribution to the overall N₂O make and 3) choice of specific component such as Ammonia Slip Catalyst (ASC) or its omission on the TP N₂O emissions. Results highlight the advantages and disadvantages of each factor to the overall TP N₂O emissions.