Modelling of the selective catalytic NOx reduction in diesel exhaust including ammonia storage

A mathematical model for the simulation of the selective catalytic reduction of NO by ammonia in diesel exhaust is presented. The simulation of heat and mass transfer, and flow phenomena in a monolithic catalytic converter is performed by a previously published mathematical model. The chemical phenomena are described by global reactions for the oxidation of ammonia and the reduction of NOx, using Langmuir-Hinshelwood reaction rate expressions. Ammonia storage phenomena are taken into account, based on the adsorption isotherm proposed by Dubinin and Radushkevich. Both the steady state chemical reaction scheme included in the mathematical model and the ammonia adsorption and desorption model are validated against experimental data previously published in the literature. The effect of ammonia storage phenomena under an oscillating NH3/NOx ratio in the feed gas is investigated. The results indicate that for the application examined, ammonia storage plays an important role at intermediate temperatures, where conversion is controlled by reaction kinetics, provided that the ammonia adsorption capacity is sufficiently high.