Masters Theses

Date of Award

5-2005

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

Ke Nguyen

Committee Members

David Irick, Stuart Daw

Abstract

As lean-burn engines are being introduced in the United States, both advantages and disadvantages arise. Lean-burn engines can operate at a high efficiency, and are developed for a wide range of power supplies. Unfortunately, due to the low temperature at which these engines operate, NOx formation becomes an issue. Forthcoming legislation pertaining to heavy-duty lean-burn engines aimed at reducing both particulate matter emissions and emissions of nitric oxides has brought about a need for a better method for reducing NOx from lean exhaust gases at moderate temperatures. It is generally accepted that current fuel treatment processes alone will be unable to accommodate emission standards proposed for upcoming years. While the current 3-way catalyst is ineffective in reducing NOx under lean conditions, many new strategies are being developed. The Lean NOx Catalyst (LNC), Lean NOx Trap (LNT), and Selective Catalytic Reduction (SCR) catalyst are all viable methods with research underway.

Currently, the selective catalytic reduction (SCR) of nitrogen oxides by N-containing reducing agents is one of the most powerful methods for accomplishing the removal of NOx from an exhaust stream. This technology has been in place in steady state power plants, but has yet to be fully implemented in mobile engines. This is due in part to the problems encountered in the automated control of ammonia addition to the exhaust gas. In steady state operation, a relatively constant amount of NOx is produced over a given amount of time. Thus, to provide a stoichiometric amount of ammonia only the steady state concentration of NOx must be known. In an automotive application the NOx produced is not constant and the addition of ammonia must vary accordingly.

The purpose of this thesis is to explore the SCR process of the reaction between NO and NH3 through an experimental matrix and also through a kinetic study extracted from the results. These results are used in a simple theoretical model of the SCR reaction. The use of NO as the only form of NOx allows for the kinetics of the NO reaction to be studied separately from the NO2 kinetics. This will be a first step in understanding the overall SCR process involving both NO and NO2.

The SCR process for the reaction between NO and NH3, while understood on a global scale, is still under debate at the elementary level. It is currently thought that the reaction occurs according to an Eley-Rideal mechanism, where strongly absorbed ammonia reacts with weakly absorbed or gas phase NO to produce nitrogen and water. It is generally accepted that this reaction proceeds in first order with respect to nitric oxide and zero order with respect to ammonia and oxygen. In this thesis the reaction orders of NO and NH3 are evaluated through an experimental matrix designed to run the SCR reaction to partial completion. The results from these experiments are then used to extract the kinetic data necessary to evaluate the rate order with respect to nitric oxide and ammonia.

In parallel to this study, a theoretical model of the reaction is developed. This model approximates the catalyst as a series of very small continuously stirred tank reactors. The model provides not only the exit concentrations of NO and NH3 to compare to experimental results, but concentration profiles along the length of the catalyst. The assimilation of the two parts of this thesis occurs when the pre-exponential factor and activation energy found from the experiments are used in the theoretical model. A final comparison of the model with the experimental results then takes place and a discussion of these results follows.

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