(585bp) Global Kinetic Modelling and Reactor Analysis of Lean NOx Traps Catalysts(LNT)

Nishithan Balaji, Preeti Aghalayam, Niket Kaisare*

Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai - 600036, India.

Corresponding Author’s e-mail: nkaisare@iitm.ac.in

Lean NOx traps (LNT) are one of the commercialised after-treatment techniques used for NOx abatement in lean burn gasoline engines. The challenge in the exhaust gas treatment of lean burn engines is to selectively reduce the ppm levels of NOx in the presence of percentage levels of oxygen. Traditional catalytic converters like TWC fail to yield good NOx reductions at these conditions. The reactions that take place in LNT are not well established. The aim of this work is to develop a generic kinetic model for LNT and use it to design and analyse the LNT reactor for NOx after-treatment. In this work, we propose an overall reaction scheme consisting of 16 global reactions that are modelled using LHHW Kinetics.

They can be categorised as oxidation reactions, NOx reduction reactions, Barium oxide-NOx storage reactions and Ceria-oxygen storage reactions. They are a simplified version of the model proposed by [1]. The reactor considered is a monolith channel for which a 1D Heterogeneous model is used. The proposed model is validated against experimental data in literature [1-3]. It is demonstrated that the model is capable of predicting the performance of several catalyst formulations including Pt/BaO/Al₂O₃, Pt/Rh/BaO/Al₂O₃ and Pt/Rh/BaO/Al₂O₃/Ce₂O₃ with only a slight modification of parameters. Next the contribution of each of the proposed reactions in the scheme to the overall observed phenomena is analysed in order to determine the important steps.

A complete reactor level analysis is performed to look into the ways of extending the time scale of lean phase, reducing the time scale of rich phase and reducing the dimensions of the reactor. The major aspects taken into consideration for analysis includes improving the fuel efficiency, reducing NOx emissions to meet the upcoming stringent emission standards and cost reduction by reducing the dimensions of the reactor which in turn reduces the amount of catalyst used. Analysis is done by varying the dimensions (length, diameter and cpsi) and operating conditions (temperature and flow rate) from the base set of conditions within a certain range.

 References

[1] P. Koci, F. Plata, J. Stepanek, M. Marek, M. Kubicek, V. Schmeiber, D. Chatterjee and M. Weibel, “Global Kinetic Model for the regeneration of NOx storage catalyst with CO, H₂ & C₃H₆ in the presence of CO₂ and H₂O”, Catalysis Today, pp. S257 – S264, 2009.

[2] W.S. Epling, J.E. Parks, G.C. Campbell, A. Yezerts, N.W. Currier and L.E. Campbell, “Further Evidence of multiple NOx sorption sites on NOx storage/reduction catalysts”,Catalysis Today, pp. 21 - 30, 2004.

[3] L. Olsson, R.J.Blint, E.Friedell, “Global Kinetic Model for Lean NOx Traps”,Ind. Eng. Chem., pp.3021 - 3032, 2005.