(227a) Combustion-Generated Carbonaceous Nanoparticles: The Impact of Oxidation on Their Morphology

Carbon black is the largest nanostructured material with annual production rate of just over 10 Mt/y worldwide. At the same time, soot emissions from fossil fuel and biomass combustion are comparable, just over 8 MT/y worldwide. As a result, there is renewed interest in understanding the detailed structure and optical properties of soot as it is the second largest contributor (after CO₂) to radiative forcing with the largest uncertainty of all such contributors. Furthermore, this understanding is still sought for reliable monitoring of carbon black manufacture and fire detection as well for mitigation of air quality impact on health and visibility reduction. Here, soot surface growth, agglomeration and oxidation are elucidated by discrete element modeling (DEM) and compared to experimental data at various combustion conditions from different laboratories. Emphasis now is placed on soot oxidation that takes place inside and on the surface of its constituent primary particles at a rate that depends on temperature, T, and O₂ concentration. Even though accurate oxidation kinetics are essential in both industrial uses and environmental impact of soot, they are often derived neglecting internal particle oxidation and the structure of such soot agglomerates. Here, the detailed evolution of the fractal-like agglomerate soot mass, m, and mobility diameter, d_m, during both internal and surface oxidation is determined by a moving sectional model. The model predictions are in excellent agreement with oxidation data of mature ethylene soot d_m for T = 900 – 1200 K. Then O₂ diffuses into the primary particles and reacts with bulk soot, hardly altering the d_m while at higher T, surface oxidation becomes dominant, decreasing d_m. The common assumption that soot agglomerates are spheres underestimates their d_mup to 50 % during oxidation. Coupling this detailed moving sectional model with soot mobility size distributions can yield realistic soot oxidation rates. Accounting for soot morphology and internal oxidation shows that the classic NSC rate increasingly underestimates (by 3 to 7 times) the oxidation rate of soot (from ethylene and toluene flames) with decreasing temperature (900-1800 K) and/or oxygen concentration (0.2 – 21 vol. %).

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