Soot and Tar Production in a Jet-Stirred/Plug-Flow Reactor System: High and Low C₂H₂Concentration Environments

Abstract

We demonstrate that carbon molecular weight growth chemistry can continue to occur when polycyclic aromatic hydrocarbons (PAH), produced by localized fuel rich pockels within an incinerator, disperse into the near-stoichiometric bulk flow characteristic of the unit. Naphthalene, a model PAH, was added to a near-stoichiometric (¢ = 1.2) post-flame combustion flow produced by premixed C,H./O,/N,/Air combustion in a jet-stirred/plug-flow reactor system (JSR/PFR), and the results compared against those from a fuel rich (¢ = 2.2) post-flame combustion flow without naphthalene addition. In both experiments, the PFR was sampled for soot (CH,C1, insolubles), tar (CH,CI, solubles; PAH), C,H, and other light gases.The results indicate that naphthalene addition to the ¢ = 1.2 flow produces PAH in the naphthalene (MW = 128)to cyclopenta[cd]pyrene(MW = 226) fraction at exactly the same total mass concentration as observed in the ¢ = 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 ofC,H, is 100 to 600 times less in the ¢ = 1.2flow with C,oH, addition than in the ¢ = 2.2 flow without CIOH, 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 C,H, or high molecular weight PAH.

Analysis of soot surface growth pathways using published plug-flow reactor data with new particle size distribution measurements and published premixed flame data

“…1). The previous soot mass data were duplicated here for the 1620 K condition [14], using a suction probe with filter collection, the same method used before [9,10], with an average agreement to within ‫.%51ע‬ The present probe was water-cooled and had a 3.2 mm i.d. porous-wall inner flow tube.…”

“…The measured particle size distributions were combined with the soot mass concentration data of Marr [9,13] to give number density distributions (Fig. 1).…”

“…diameter PFR [8][9][10] under two sets of conditions (1 atm, ‫ס‬ 2.2, and 1620 K and 1520 K) for which concentration profiles of light gas species, total PAH mass, and soot mass had already been measured [9,13]. The soot particles were deposited on TEM grids inserted into the PFR at different axial distances or residence times, for flame exposure times of 100-200 ms.…”

“…However, the literature does not include soot particle size measurements for any of the data sets, thus leaving the soot inadequately characterized and hampering the study of elementary processes of soot formation. To address this need, soot particle size distributions were measured in the PFR in the present study under two of the previously studied sets of conditions and the results were used with the previous PFR [7][8][9][10] and other flame [1][2][3][4][5][6] data to analyze pathways of soot surface growth.…”

“…Soot formation in a plug-flow reactor (PFR) has been studied extensively under sooting conditions [7][8][9][10], and concentration data for light gas species, individual polycyclic aromatic hydrocarbons (PAH), total PAH (dichloromethane [DCM] soluble compounds) mass, and soot (DCM insolubles) mass have been obtained for various equivalence ratios ( ), temperatures (T), and fuel additives. The PFR has a simple reactor geometry, permitting computations with large kinetics models (500-1000 reactions, 100-300 species) to be run in minutes, as opposed to hours for premixed flames, and without the uncertainty of diffusion parameters.…”