(673e) Inferring Fuel-Rich Toluene Flame Chemistry From Photo-Ionization MBMS Analysis and Modeling

Toluene is often the highest-concentration single-ring species in gasoline, and its chemistry captures key features common to xylenes, ethylbenzene, and other substituted aromatics. The present work describes our measurements and modeling of a fuel-rich premixed laminar flat flame (30 torr, phi=1.5, toluene/O2/36.1% Ar, 45.0 cm/s feed velocity at 298 K). Proposed mechanistic pathways are tested with detailed kinetics, revealing the steps by which this chemistry occurs.

Results and Discussion

More than 50 stable and radical species mole fraction profiles were measured as functions of the axial distance from the burner, including many common lower-mass intermediates along with the higher-mass hydrocarbons and oxygenates. The key technique was Photo-ionization Molecular Beam Mass Spectrometry (PI-MBMS) using photons of precise energies (within 0.05 eV) obtained from the Advanced Light Source at Lawrence Berkeley National Laboratory [1]. High-resolution energy scans were used for identifying flame species by their molecular masses and ionization thresholds, allowing even isomers to be resolved and identified. Scans of signal along the flame axis (?burner scans?) were converted to mole fraction profiles, at using photoionization energies selected to avert both fragmentation of species of interest and potential interferences from fragmentation of higher-mass species. The temperature profile was measured with Laser-Induced Fluorescence (LIF).

A detailed mechanism model was composed using 117 species and 945 reactions and was used to simulate the flame data using the measured temperature profile. Agreement was quite good, in general. The predictions were analyzed by Reaction Pathway Analysis (RPA) to investigate the oxidation, molecular-weight growth, and benzene-formation chemistry in the flame.

Toluene was mainly destroyed by abstraction to form benzyl, which was predicted to decompose primarily to acetylene and cyclopentadienyl (C5H5). C5H5 can be oxidized to form cyclopentadienone C5H4O, which provided the main basis for C4 chemistry. C5H5 can also abstract H or combine with H to produce cyclopentadiene C5H6, which can be attacked by H to generate allyl, the main source for all other C3 species. Finally, all these C3, C4, and C5 species will lead to C2 species, which will react to form the final products H2, H2O, CO and CO2.

Also, benzene was very abundant, reaching nearly 1% at its maximum, almost 100 times higher than its usual abundance in small-hydrocarbon flames. Benzene was dominantly generated from H-addition/CH3-elimination from toluene, and the contribution from small-alkyl chemistry (such as C3+C3 or C2+C4 chemistry) was negligible. These findings show how alkyl-aromatic fuels have a much larger tendency for benzene formation than small-alkyl hydrocarbons due to its aromatics ring structure, and benzene should be mainly generated from the elimination of side chains.

References

1.N. Hansen, T. A. Cool, K. Kohse-Höinghaus, P. R. Westmoreland, ?Recent Contributions of Flame-Sampling Molecular-Beam Mass Spectrometry to a Fundamental Understanding of Combustion Chemistry,? Progress in Energy and Combustion Science 35(2) (2009) 168-191; online publication 12/14/2008 at http://dx.doi.org/10.1016/j.pecs.2008.10.001