Flame experiments at the advanced light source: new insights into soot formation processes

Area of Science:

• Chemical Dynamics
• Combustion Science
• Mass Spectrometry

Background:

• Soot formation is a complex chemical process involving the assembly of large carbon structures from simple fuel molecules.
• Understanding combustion intermediates is crucial for controlling soot formation and emissions.
• The H-abstraction-C2H2-addition (HACA) mechanism is a widely accepted model for polycyclic aromatic hydrocarbon (PAH) growth.

Purpose of the Study:

• To demonstrate the analysis of complex chemical structures in model flames using flame-sampling mass spectrometry with synchrotron-generated vacuum-ultraviolet (VUV) radiation.
• To investigate the gas-phase intermediates involved in soot formation processes.
• To study the chemical composition of combustion-generated soot particles and evaluate existing growth mechanisms.

Main Methods:

• Utilized burner-stabilized, reduced-pressure laminar premixed flames with a small hydrocarbon fuel.
• Employed tunable VUV synchrotron radiation for isomer-resolving, high-sensitivity flame-sampling mass spectrometry.
• Applied synchrotron-based aerosol mass spectrometry to analyze soot particles from an opposed-flow diffusion flame.

Main Results:

• Acquired species profiles as a function of distance from the burner surface.
• Identified key combustion intermediates, including resonance-stabilized radicals like C3H3, C3H5, and i-C4H5, based on their ionization energies.
• Found that the HACA mechanism alone does not fully explain the formation of large PAHs observed in combustion-generated soot.

Conclusions:

• Tunable VUV laser-based mass spectrometry is a powerful technique for detailed chemical analysis of flames.
• Specific radicals play significant roles as intermediates in soot formation pathways.
• The formation of large PAHs in soot may involve additional or alternative growth mechanisms beyond HACA.