Date of Award

Fall 1997

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering - (Ph.D.)

Department

Mechanical Engineering

First Advisor

Robert Benedict Barat

Second Advisor

Pasquale J. Florio

Third Advisor

Bernard Koplik

Fourth Advisor

John Vincent Droughton

Fifth Advisor

John G. Stevens

Abstract

The feasibility of combining electrostatic precipitation and use of a catalytic wall in a straight tube reactor as a means of destroying soot particles was investigated. Enhanced particle diffusion to the wall by an applied electric field provided the means of particle capture for subsequent catalytic oxidation at the active surface in a small length tube.

Soot particles flowing in a gas stream are influenced by the following transport mechanisms: convective flux as a result of bulk flow, diffusion flux as a result of particle concentration or number density gradient, and an electrostatic flux from the coulombic attraction as charged particles move to an electrically grounded wall. When an external electric field is applied, the resulting electrostatic flux dominates the particle transport mechanism. Soot capture on a catalyst wall is by adsorption onto a catalytically active site. With sufficient oxygen present and surface temperatures near 400 °C, catalytic oxidation of soot is evident by heat released due to exothermic reactions, and increased CO and CO2 (COx) concentrations.

The experimental results indicated increased catalytic activity under light sooting conditions by raising the applied voltage in stepwise increments. A voltage of -2.5 kV was found to yield the maximum COx levels and highest catalytic surface temperatures (30-60 °C). Increased oxygen concentration (> 0.40 mole fraction) was the most important factor in promoting soot oxidation. Heavy sooting conditions, or a high voltage quickly applied caused rapid accumulation of particle deposition on the surface resulting in fouling the catalyst and decreasing the catalytic activity. The particle size fraction of soot flowing into the catalytic reactor from the combustor indicated a bimodal distribution. The largest peak occurred at 1.4 μm, while a smaller peak was found at 3.0 μm.

A mathematical model to simulate electrostatic precipitation was developed to incorporate the use of a distribution of particle size fractions. The predicted penetration from modeling was compared with experimental results of reactor outlet soot loadings for increased voltage. Under light sooting conditions, model predictions agreed well with the trends exhibited by the experimental data for a particle satuation charge level of 35%. Additionally, the mathematical model was able to predict particle penetration along the axial tube length. The modeling was found to be in good agreement with the experimental results.

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