(375l) Oxidation of Fractal-like Soot Agglomerates

Oxidation is the last soot formation stage and determines the mass, mobility, d_m, and primary particle diameter, d_p, of particulate emissions. The fractal-like soot agglomerates produced by nucleation,¹ surface growth² and agglomeration³ react with molecular oxygen and shrink producing gaseous CO and CO₂ pollutants. Accurate soot oxidation kinetics are essential to optimize combustion and aftertreatment devices.

Here, an analytical model for the evolution of soot d_mand d_p during internal and external oxidation is derived. External oxidation takes place on soot primary particle surface reducing d_p and d_m, while internal burning decreases the soot bulk density, ρ. The unoxidized soot ρ_o and d_p,o is related to ρ and d_p by ρ=ρ_o(d_p/d_p,ο)³, finding excellent agreement with thermogravimetric and microscopic data of carbon black, diesel and ethylene soot. The common assumption that soot nanoparticles are spheres (i.e. d_m = d_p) results in 35 % overprediction of the d_m measured during oxidation. Thus, a relationship for the d_m of agglomerates formed by coagulation and surface growth³ is used to account for the ramified soot morphology. The evolution of soot mobility size distribution during internal and external oxidation dynamics of fractal-like soot agglomerates is in excellent agreement with oxidation measurements of ethylene soot. The analytical model developed here from first principles can be coupled with mobility size measurements to probe accurately soot oxidation kinetics or with population balance models to design efficient and clean combustion processes.

References:

[1] Kholghy, M. R.; Kelesidis, G.A.; Pratsinis, S.E., Reactive polycyclic aromatic hydrocarbon dimerization drives soot nucleation. Phys Chem Chem Phys 2018, in press.

[2] Kelesidis, G. A.; Goudeli, E.; Pratsinis, S. E., Flame synthesis of functional nanostructured materials and devices: Surface growth and aggregation. Proc Combust Inst 2017, 36, 29-50.

[3] Kelesidis, G. A.; Goudeli, E.; Pratsinis, S. E., Morphology and mobility diameter of carbonaceous aerosols during agglomeration and surface growth. Carbon 2017, 121, 527-535.