Influence of maturity on the oxidation kinetics and nanostructure evolution of soot sampled from a propane coflow diffusion flame

Exploration of soot oxidation kinetics at different soot maturity stages is significant for understanding nucleation mechanisms, controlling growth rates, and improving removal efficiency. However, there is still a limited understanding of the oxidation mechanisms for early-stage soot. This work investigates the oxidation kinetics and nanostructure evolution of soot particles sampled from a laminar co-flow diffusion flame of propane with two distinct residence times. Efficient sampling of both early-stage soot and mature soot are performed by using our recently developed capillary-nozzle-hybrid sampling method, which can achieve high sampling efficiency and reduced perturbance to flames simultaneously. Kinetic parameters are evaluated from non-isothermal thermogravimetric data at different heating rates, revealing that early-stage and mature soot present distinct oxidation reactivity. Within the conversion rates of 20%−90%, the oxidation activation energy of mature soot distributes within 160−185 kJ/mol, which grows monotonically with the conversion rate and has a slower and slower growth rate. The oxidation activation energy of early-stage soot has much lower values, distributing from 128 to 156 kJ/mol with a multi-stage dependence on the conversion rate. The variation in soot nanostructure during oxidation is monitored within a 75% conversion rate. Mature soot presents the combined modes of surface and internal oxidation, which results in declined particle diameters and disordered nanostructure. In contrast, early-stage soot merely presents surface oxidation mode with the nanostructure being more and more organized.