Soot emissions from gas-turbine engines are a concern for the military for a variety of reasons including environmental and human health. A better understanding of the process of soot formation and oxidation can lead to different strategies to mitigate the release of soot into the atmosphere. Studies to date have concentrated on soot formation. The oxidation process has not been as well characterized. Soot oxidizes by reactions with molecular oxygen (O₂), oxygen radicals (O*), and hydroxyl radicals (OH*). The reactions of O* and OH* with soot are relatively well understood, but the reactions of soot with O₂ are not as well understood, particularly under the conditions applicable to gas-turbine engines.

This project focused on the mechanisms of soot oxidation by O₂ for jet fuels used in military engines. The technical objectives were to (1) determine the effect of the structure of soot, as influenced by the fuel composition, on the rate of soot oxidation by O₂; (2) quantify the role of internal surface area on the soot reactivity; and (3) develop power-law kinetic correlations for soot/O₂ oxidation as a function of temperature, O₂, and time for soots of different structures and porosity.

To separate the soot oxidation mechanisms from the formation steps, a two-stage burner was used. The two-stage system consists of an initial premixed burner where soot was generated under a variety of conditions for ethylene/air, JP-8 surrogate (n-dodecane/m-xylene)/air, m-xylene/air, and n-dodecane/air premixed flames. Downstream, the soot-laden combustion gases were passed through a secondary flat-flame burner where soot was burned out under fuel lean or slightly fuel rich conditions. The process of soot oxidation in the secondary burner was followed by the evolution of particle size distribution (PSD), flame temperature, gas-phase composition, soot surface area, and soot morphology and nanostructure as a function of the height above the second burner (HAB).

Measurements of soot size distribution and number concentration as a function of the HAB under fuel lean (Φoverall = 0.8) and slightly rich (Φoverall = 1.14) conditions showed particle fragmentation, evidenced by the decrease in particle mean diameter and a significant increase in number concentration in the region where O₂ concentration decreased. Higher in the burner, soot was burned as a result of the increase in OH* concentration, which produced higher soot oxidation rates. Analysis of the results, in terms of the changes in soot surface area, soot morphology and nanostructure, and the effectiveness factor suggested soot fragmentation at a low burnout percentage with internal burning, which in turn caused both the breakup of the bridges cementing primary particles and the rupture of the primary particles.

Experimental information from PSDs, temperature, gas-phase composition was used to develop an oxidation kinetic expression that accounts for the effects of temperature, O2, and OH* on the rate of oxidation of soots of different structure. The kinetic expression was able to reproduce the major features of the experimental results relative to the other predictions using current models proposed in the literature.

Results of this project enhance fundamental combustion science and over the long term will help enable military gas-turbine engines to continue to meet their performance and operating requirements with reduced PM emissions by opening new paths to evaluate different strategies for reducing the emissions of soot into the atmosphere.