Chapter 9 - NOx reduction in IC engines through adsorbing technique

Abstract

The three-way catalytic converter (TWCC), diesel particulate filter (DPF), selective catalytic reduction (SCR) under urea injection, and diesel oxidation catalyst (DOC) are used for the efficient removal of PM, NOx, HC, and CO from the engine exhaust. However, these after-treatment systems demand a considerable amount of time—nearly 60 to 180s—to warm up the catalyst. During this period, nearly 80% of the total vehicle emissions are released because of the nonfunction of the catalyst. Another issue of these systems is the complexity in the removal of NOx in lean burn applications. Adsorption is one of the recent emerging after-treatment techniques to handle NOx emissions in the lean burn as well as cold start conditions. For lean burn application, an active NOx adsorber or lean NOx trap (LNT) is used to adsorb the NOx. Once the adsorber has reached its maximum point, LNT is regenerated with the help of a fuel-rich condition for both lean burn gasoline and diesel engines. For the cold start problem, a passive NOx adsorber is used to store the NOx during the low-temperature period. Once the adsorber has reached its saturation point, the stored NOx is reduced into N2 at high temperature in a gasoline engine. Whereas in a diesel engine, the adsorbed NOx is reduced by sending released NOx into the SCR once it is operational. Sulfur poisoning is the main limitation for both LNT and the passive NOx adsorber. It drastically affects the NOx conversion efficiency of the adsorber. However, the doping of titanium dioxide (TiO2) and the use of zeolite-based material have increased the sulfur tolerance level of the adsorber.