Combined LNT-SCR Concepts

The combination of lean NO trap (LNT), also referred to as NO storage and reduction catalyst (NSRC), and selective catalytic reduction (SCR) systems provides the potential to increase the efficiency of single LNT systems. During the lean-phase operation, NO emissions are being adsorbed in the LNT while the stored NO are reduced during the short rich phase operation. In the latter phase, NH3 is produced as a secondary product of the NO reduction and then adsorbed in the SCR where it is utilized to further reduce the NO emissions. The NH3-SCR reactor can be either located downstream of the LNT (double-bed LNT-SCR) or merged within the same substrate with the LNT (dual-layer LNT-SCR). 

A simulation study of a dual-bed LNT-SCR system (noted by the authors as NSRC -I- SCR) has been performed by Chatteijee et al. [54]. The performance of the system was evaluated, in terms of NO conversion and NH3 release, in defined periodic lean/rich operation for various temperatures and at different rich phase durations (Fig. 13.22). 

At the short regeneration phase of 3 s (Fig. 13.22a) the de-NO efficiency is low and NH3 is generated over the NSRC only below 350 °C. All NH3 is consumed in the SCR leading to improvement of NO conversion at low temperatures. At the longer rich phase of 10 s, higher NO conversions are obtained and more NH3 is generated. Above 350 °C the NO reduction in the NSRC alone is so high that the NOx concentration in the SCR inlet is very low. Consequently, NH3 is in excess and part of it remains unused. 

A similar modeling study was presented by Koltsakis et al. [55]. In the latter case, three different configurations have been simulated including a dual-layer 

All three catalyst configurations have been tested in a scenario comprising of consecutive lean-rich cycles at different temperatures and space velocities in the range of practical interest for diesel engine applications. Comparative results for the three systems, in terms of NO conversion efficiency, are presented in Fig. 13.24. 

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