Soot and Tar Production in a Jet-Stirred/Plug-Flow Reactor System: High and Low C₂H₂ Concentration Environments

ABSTRACT

We demonstrate that carbon molecular weight growth chemistry can continue to occur when polycyclic aromatic hydrocarbons (PAH), produced by localized fuel rich pockets within an incinerator, disperse into the near-stoichiometric bulk flow characteristic of the unit. Naphthalene, a model PAH, was added to a near-stoichiometric (φ = 1.2) post-flame combustion flow produced by premixed C₂H₄/O₂/N₂/Air combustion in a jet-stirred/plug-flow reactor system (JSR/PFR), and the results compared against those from a fuel rich (φ = 2.2) post-flame combustion flow without naphthalene addition. In both experiments, the PFR was sampled for soot (CH₂Cl₂ insolubles), tar (CH₂Cl₂ solubles; PAH), C₂H₂ and other light gases.

The results indicate that naphthalene addition to the φ = 1.2 flow produces PAH in the naphthalene (M W = 128) to cyclopenta[cd]pyrene(MW = 226) fraction at exactly the same total mass concentration as observed in the φ = 2.2 flow without naphthalene addition. Similarly, the rate of growth of soot mass is approximately the same in both cases. In contrast, the concentration of high molecular weight PAH (PAH > MW 226) produced is 3 times less, and that of C₂H₂ is 100 to 600 times less in the φ = 1.2 flow with C₁₀H₈ addition than in the φ = 2.2 flow without C₁₀H₉ addition. These data suggest that under our reactor conditions, PAH in the 128 to 226 amu molecular weight range are more important contributors to the soot growth process than either C₂H₂ or high molecular weight PAH.

Key Words:

• Naphthalene
• PAH
• soot
• tar
• jet-stirred reactor
• plug-flow reactor
• incineration