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.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: The authors would like to thank Hitachi Higher Technologies Canada and Norcada Inc. for their continued support with the development of this new technique. The authors would also like to acknowledge NSERC Canada and King Abdullah University of Science and Technology (KAUST) for project funding. We would also like to thank the OCCAM- UofT team, where all the microscopy was performed on the Hitachi HF3300. Finally, the authors would also like to thank Dr. Stas Dogel, Ali Naseri, and Dr. Pál Tóth for their invaluable technical expertise and discussions.