Modeling and simulation of periodic storage and reaction in nitrogen oxide traps

Lean-burn gasoline and diesel vehicles have higher fuel efficiency than conventional gasoline vehicles, but produce more NOx with a net oxidizing exhaust which makes the traditional three-way converter ineffective for NOx treatment. One of the emerging techniques for NOx reduction is that of periodic NOx Storage and Reduction (NSR), and the device in which NSR is carried out is commonly referred to as the lean NOx trap (LNT). The LNT is a periodically-operated adsorptive reactor and comprises a bifunctional catalyst with deliberate periodic operation in which the air fuel ratio is altered between lean (oxygen excess) and rich (fuel excess) mixtures. During the storage process, NOx is adsorbed on an alkali earth oxide (barium oxide), forming a surface nitrate. The nitrate is then chemically reduced over a noble metal (Pt) by periodic "rich" operation, which increases the level of hydrocarbons in the exhaust, promoting the release and reduction of NOx.;This work simulates the effect of various parameters on the working of the NOx trap by using a generic two-phase, 1-D model describing adsorption, desorption and reaction of different species. The steady state operation is compared with the cyclic operation and the results show that there exists a feed temperature window where cyclic operation gives high NO x conversions. Several design and operating parameters are varied to study how the NOx conversion efficiency of the LNT varies and to find out the best operating strategy. The parameters studied include the feed reactant fraction, cycle timing and feed temperature. The results are compared with the experimental results and found to follow the same trends.;The storage and reaction phases are analyzed at steady-state using a detailed kinetic model. Study of the storage is done by varying parameters like storage time and fluid space velocities. The simulations show the importance of NO oxidation reaction and the NO oxidation equilibrium during storage. The steady-state simulations for the reaction phase show that the NO x oxidation goes through a maximum with feed temperature. The ignition and extinction of the hydrocarbon combustion and NOx reduction reactions are analyzed to identify the region of multiplicity.