(590j) Internal and External Soot Oxidation

Oxidation of soot and carbon blacks determines their emitted mass concentration and production rate as well as mobility diameter, d_m. Soot oxidation takes place mostly by O₂ and OH radicals at high temperatures, T (> 1100 K) and competes with agglomeration in determining its particle sizes [1]. In diesel engine exhaust and regenerative traps of particulate filters, single or packed beds of soot agglomerates are oxidized at lower temperatures [1]. Even though accurate oxidation kinetics over a wide Τ range 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 oxidation mode index, α, 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 [2]. 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 α > 3. As T increases, surface oxidation becomes dominant, decreasing both d_m and α. The surface oxidation model for spheres underestimates the measured d_m [3] by 50 % on average over the whole T range. Accounting for the fractal-like agglomerate morphology during surface oxidation improves substantially the agreement with data for T > 1100 K, but underestimates soot d_m up to 24 % for lower T, as oxidation takes place mostly within soot primary particles. Accounting for both internal and surface soot oxidation of agglomerates results in good agreement with mature soot data [3] over the whole T range. Coupling this detailed moving sectional model with mobility size distributions of nascent soot [4] yields a specific oxidation rate that is on average more than 50 % smaller than that obtained when neglecting internal oxidation and the fractal-like soot morphology at [O₂] = 0.2 - 0.78 vol %.

References:

[1] Stanmore, B. R., Brilhac, J. F., & Gilot, P. (2001). Carbon, 39, 2247-2268.

[2] Essenhigh, R. H. (1988). Symp. (Int.) Combust., 22, 89-96.

[3] Ma, X. F., Zangmeister, C. D., & Zachariah, M. R. (2013). J. Phys. Chem. C, 117, 10723-10729.

[4] Camacho, J., Tao, Y. J., & Wang, H. (2015). Proc. Combust. Inst., 35, 1887-1894.