Flame Experiments at the Advanced Light Source: New Insights into Soot Formation Processes

Abstract

The following experimental protocols and the accompanying video are concerned with the flame experiments that are performed at the Chemical Dynamics Beamline of the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory [1][2][3][4] . This video demonstrates how the complex chemical structures of laboratory-based model flames are analyzed using flame-sampling mass spectrometry with tunable synchrotrongenerated vacuum-ultraviolet (VUV) radiation. This experimental approach combines isomer-resolving capabilities with high sensitivity and a large dynamic range 5,6 . The first part of the video describes experiments involving burner-stabilized, reduced-pressure (20-80 mbar) laminar premixed flames. A small hydrocarbon fuel was used for the selected flame to demonstrate the general experimental approach. It is shown how species' profiles are acquired as a function of distance from the burner surface and how the tunability of the VUV photon energy is used advantageously to identify many combustion intermediates based on their ionization energies. For example, this technique has been used to study gas-phase aspects of the soot-formation processes, and the video shows how the resonance-stabilized radicals, such as C 3 H 3 , C 3 H 5 , and i-C 4 H 5 , are identified as important intermediates 7 . The work has been focused on soot formation processes, and, from the chemical point of view, this process is very intriguing because chemical structures containing millions of carbon atoms are assembled from a fuel molecule possessing only a few carbon atoms in just milliseconds. The second part of the video highlights a new experiment, in which an opposed-flow diffusion flame and synchrotron-based aerosol mass spectrometry are used to study the chemical composition of the combustion-generated soot particles 4 . The experimental results indicate that the widely accepted H-abstraction-C 2 H 2 -addition (HACA) mechanism is not the sole molecular growth process responsible for the formation of the observed large polycyclic aromatic hydrocarbons (PAHs).

Combustion Chemistry of Fuels: Quantitative Speciation Data Obtained from an Atmospheric High-temperature Flow Reactor with Coupled Molecular-beam Mass Spectrometer

“…While a mass resolution of Δm = 3,000 is sufficient for most combustion species, isomeric species cannot be separated or identified. Highly advanced experiments like synchrotron based approaches 28,33,34,35,36,37,38 or generic theoretical studies on reaction kinetics are needed in the future to provide more detailed knowledge and improve upon the identification capabilities.…”

Fulvenallenyl Radical (C7H5·)-Mediated Gas-Phase Synthesis of Bicyclic Aromatic C10H8 Isomers: Can Fulvenallenyl Efficiently React with Closed-Shell Hydrocarbons?

“…The quasi-continuous tunable synchrotron VUV light crossed the supersonic molecular beam and photoionized a fraction of the entrained species. The ions were then collected by a microchannel plate (MCP) detector in a Re-TOF-MS. − Control experiments were also performed under identical experimental conditions by simply replacing allene or methylacetylene with nonreactive helium gas. Photoionization efficiency (PIE) curves, which report ion counts of well-defined mass-to-charge ( m / z ) ratios versus the photon energy, were extracted from the mass spectra recorded over the photon energy range from 7.20 to 9.00 eV in an interval of 0.05 eV.…”