PAH formation and soot morphology in flames of C4 fuels

Abstract

In this work, we describe experimental studies on the formation of polycyclic aromatic hydrocarbons (PAH’s) in opposed-flow atmospheric-pressure flames of n-butane, i-butane, i-butene, and i-butanol and on the morphology of nascent soot particles sampled from premixed atmospheric-pressure flames of the same fuels. To identify the major contributors to the molecular growth mechanism in opposed-flow flames, we employed flame-sampling molecular-beam mass spectrometry with electron ionization (EI) and in situ gas-chromatography (GC) with mass spectrometric detection. The EI and GC-EI mass spectra indicate that several pathways with different building blocks can contribute to molecular growth. Besides the commonly accepted hydrogen-abstraction-C2H2-addition steps, we found reactions of the methyl radical to be important steps. This observation is also supported by the complexity of the mass spectra which indicates that at least one of the building blocks is rather small. The importance of phenyl radicals as building blocks seems to be limited. We also sampled nascent soot particles from premixed atmospheric-pressure flames of the above mentioned fuels and used helium-ion microscopy to unravel the influence of the fuel structure on the morphology of the sampled particles. While differences in flame temperatures and residence times are known to influence the particle sizes, the observed different morphologies are likely due to slightly different C/O ratios and potentially the chemical nature of the fuel.