Improved sooting tendency measurements for aromatic hydrocarbons and their implications for naphthalene formation pathways

Sooting tendencies have been determined for aromatic hydrocarbons using a new definition: the maximum soot volume fraction fv,max measured in a coflow methane/air nonpremixed flame whose fuel is doped with 400 ppm of the test hydrocarbon. These fv,max were converted into apparatus-independent yield sooting indices (YSIs) by the equation YSI=C×fv,max+D, where C and D are apparatus-specific parameters chosen so that YSI-benzene = 30 and YSI-1,2-dihydronaphthalene = 100. The dopants were added to the fuel mixture with a syringe pump and fv,max was measured with laser-induced incandescence. YSI was determined for 6 cycloaliphatics and for 62 aromatics, which included 28 alkylbenzenes, 10 alkenylbenzenes, 10 alkynylbenzenes, 25 multiply substituted benzenes, 6 two-ring aromatics, and 6 substituted benzenes with heteroatoms in the side chains. The YSIs correlate well with literature values of threshold sooting index (TSI), which is a more traditional sooting tendency based on the height of pure-fuelled flames at the smoke point. This agreement indicates that fv,max and smoke height are equivalent measures of sooting tendency and that YSI is largely apparatus-independent. However, the YSIs have a total uncertainty of ±3%, which is substantially better than the TSIs, and the number of aromatic YSIs reported here is more than double the number of aromatic TSIs in the literature. The YSIs depend strongly on molecular structure; thus they provide information about the chemical kinetic reaction mechanisms responsible for fuel decomposition and hydrocarbon growth from a broad cross section of one-ring aromatics. Important naphthalene formation pathways appear to include acetylene addition to ethynylphenyl, propargyl addition to benzyl, and methyl addition to indenyl. This last pathway is particularly significant because it converts indene quantitatively to naphthalene and because the side chains in many alkenylbenzenes and alkynylbenzenes cyclize to form five-membered rings.