Soot is produced from incomplete combustion of hydrocarbon fuels. Because soot is an anthropogenic contaminant in the environment and an important contributor to climate change, it is necessary to understand how it forms. A recent study addressed this question by examining the chemical composition of soot particles sampled from a diffusion flame burning a jet fuel surrogate.
Research findings suggest fundamental models of soot formation are not completely accurate. Molecular processes of soot formation in diffusion flames, when the oxidizer combines with the fuel by diffusion, may be more complex than previously thought. By providing a more complete understanding of the chemical nature of soot particles, this research could ultimately lead to improved climate-model representations of anthropogenic soot’s chemical and physical properties, greater insight into the role of soot in cloud formation and cleaner aviation engines through more accurate soot models.
Researchers from the University of Dayton Research Institute, the Environmental Molecular Sciences Laboratory (EMSL), the University of Toronto and Stanford University examined the chemical composition of young soot particles along the centerline of a diffusion flame of a three-component jet fuel surrogate blend. The researchers conducted high-resolution mass spectrometry coupled with nanospray desorption electrospray ionization at EMSL, a Department of Energy national scientific user facility.
Researchers found these young soot particles differ in mass, size and chemical composition from mature soot particles. In lower positions of the diffusion flame, young particles are composed mainly of peri-condensed polycyclic aromatic hydrocarbons. These particles become enriched with aliphatic hydrocarbon compounds as they grow in mass and size. Prior to carbonization, young soot particles are composed of aliphatic and polycyclic aromatic hydrocarbon compounds. These findings show young soot particles observed in the diffusion flame can be rich in both aliphatic and aromatic hydrocarbon compounds. Moreover, particles dominated by polycyclic aromatic hydrocarbons or mixtures of polycyclic aromatic hydrocarbons and aliphatics can exhibit a liquid-like appearance, as observed by electron microscopy, and can be transparent to visible light.
This research reveals significant variations in the morphology and chemical composition of soot particles as they develop in the flame, indicating molecular processes of soot formation in diffusion flames may be more complex than previously thought. Because current models of soot formation have yet to account for the wide range of variations in particle composition, it will be important for future experimental work to focus on a comprehensive quantitative picture of the mechanisms underlying soot growth and chemical transformation in diffusion flames.
Contact: Jeremy Cain, University of Dayton Research Institute, firstname.lastname@example.org
Funding: Portions of this research were funded by the Chemical Imaging Initiative, Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory(PNNL); Office of Biological and Environmental Research (BER); National Research Council Research Associateship Award at Wright-Patterson Air Force Base; University of Dayton Research Institute Shock Tube Lab; Combustion Energy Frontier Research Center, an Energy Frontier Research Center funded by Office of Basic Energy Sciences (BES); Natural Sciences and Engineering Research Council of Canada; and BioFuelNet Canada.
Publication: Cain J, A Laskin, MR Kholghy, MJ Thomson, and H Wang. 2014, "Molecular Characterization of Organic Content of Soot Along the Centerline of a Coflow Diffusion Flame." Physical Chemistry Chemical Physics 16:25862-75. DOI:10.1039/C4CP03330B