Chapter 1. Review of deNOx Technology for Mobile Applications

Abstract

This chapter summarizes the state-of-the-art of emission control technologies for both diesel and gasoline engines. It covers current and emerging regulations in the major markets; lean NOx control using SCR (selective catalytic reduction) and NOx trap catalysts (NTCs); diesel and gasoline particulate filters; three-way catalysts (TWCs); and lean gasoline NOx control. The LD and HD regulations are briefly reviewed. Real driving emissions are becoming increasingly important. India and China are adopting Euro 6 LD and HD standards in 2020. Diesel emission control technologies are covered first. The various SCR catalyst types are compared with copper zeolite being the preferred family of catalysts for most applications. N2O formation, NH3 storage, and system control are discussed. SCR catalysts are now being added to diesel particulate filters, enabling faster heat-up due to closer placement to the engine, and space savings. NTCs are improving incrementally with better low- and high-temperature conversion, sulphur tolerance, and reduced precious metal loadings. Combination NTC+SCR systems are now emerging for LD diesel applications, and passive NOx adsorbers are described that will collect NOx at low temperatures and release them at higher-temperatures when the downstream SCR is operative. Diesel particulate filters fall into two categories described by the soot regeneration method – low temperature (with high NOx : soot ratio) and high-temperature (lower NOx : soot ratio). SCR filter regeneration issues are also discussed. Improvements in TWC coating technology, precious metal utilization, and washcoat and substrate improvements. Gasoline particulate filters (GPFs) are new and just now starting widespread implementation in Europe and China, and updates are provided on performance. Lean burn gasoline NOx control uses combination TWC, NTC, and SCR systems, wherein the NH3 for the SCR is generated in the upstream components during periodic rich excursions. Finally, future perspectives are shared. Exhaust gas temperatures will continue to decrease as GHG reductions dominate future trends. Further criteria emissions reductions will also be needed. Advanced combustion engines utilizing gasoline compression ignition and low-temperature combustion will emerge, enabled by hybridization. Goals for all emissions control systems are 90% conversion at 150 °C.