Estimating the internal and surface oxidation of soot agglomerates

Oxidation of soot 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. The oxidation mode index, a, given by the ratio of the characteristic O₂ reaction and diffusion times is used to quantify the contributions of internal and surface oxidation of soot. At low Τ (e.g., < 1100 K), O₂ diffuses into the primary particles and reacts with bulk soot, hardly altering the d_m and yielding a > 3. As T increases, surface oxidation becomes dominant, decreasing both d_m and a. The common assumption that soot agglomerates are spheres underestimates their d_m up 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 3-7 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 %).