In Situ Imaging Studies of Combustor Pressure Effects on Soot Oxidation

Airborne soot is a product of incomplete combustion from engines and industrial processes. Unburnt soot is carcinogenic, a major contributor to climate change, and detrimental to combustor lifespan and efficiency. An understanding of how high-pressure combustion affects the oxidation properties of soot is crucial for the design of clean-burning, high-pressure engines and downstream soot filtration technologies. This paper presents the first real-time look at the oxidation of soot particles formed at high pressure and demonstrates that the oxidation pathway changes as combustor pressures increase. Soot particles were formed in an ethylene-fueled diffusion flame, with pressures ranging from 1 to 25 bar, and were subsequently sampled and oxidized inside an ETEM allowing for the nanoscale, real-time observation of oxidation pathways. The high-pressure generated soot grew larger in diameter, formed larger aggregates, and developed graphitic outer shells, protecting the reactive amorphous carbon core. The graphitic shell structure fundamentally changed the oxidation pathway of soot from diffusion driven internal oxidation at lower pressures to highly nonreactive surface oxidation reactions at high pressures. This work demonstrates that as combustor pressures increase to achieve higher thermodynamic efficiencies, highly resilient oxidation-resistant soot is produced.