Molecular-beam mass spectrometry of ethylene and cyclohexane flames
Molecular-beam mass spectrometry (MBMS) is a technique that is used to measure stable and radical species within flames, and thus it is a strong tool for understanding the formation and destruction pathways of precursors to PAH and soot. Using MBMS and modeling techniques, three well-chosen premixed flat flames have been characterized.
Fuel-lean ethylene flames with and without added allene were mapped to help elucidate the C3H3 self-combination route to benzene formation, and a cyclohexane flame was characterized due to the abundance of cyclohexane within real fuels such as gasoline. Benzene is the precursor to PAH and soot whose formation is the rate-limiting step, and thus knowledge of its formation route is paramount.
The flames characterized include an undoped fuel-lean (&phis; = 0.70), C2H4/O2/56.4% Ar flame (30.00±0.01 Torr and ub-300 = 30.6 cm/s), an allene-doped fuel-lean (&phis; = 0.69), 0.19 % C3H4/C2H 4/O2/56.54 % Ar flame (30.00±0.01 Torr and ub-300 = 30.6 cm/s), and a stoichiometric (&phis; = 1.00) cyclohexane/O2/32.5% Ar flame (30.00±0.01 Torr and ub-300 = 35.0 cm/s). Mole fraction profiles of 31, 35, and 70 stable and radical species were measured within the three flames, respectively. They are modeled with overall good agreement between the model and data.
Comparison of both fuel-lean ethylene flames shows that benzene was detected in the allene-doped flame but not in the undoped ethylene flame, strongly suggesting the importance of C3 routes to benzene. Reaction path analysis showed that benzene in the allene-doped flame is mainly formed through propargyl self-combination as described by the kinetics of Miller and Klippenstein (2003). Examination of the cyclohexane flame showed high concentrations of benzene. A reaction path analysis showed that benzene is mainly formed instead by the dehydrogenation of cyclohexane.
Experiments done at the Advanced Light Source of Lawrence Berkeley National Laboratory show that the isomeric C6H6 composition in the allene-doped flame consisted of 20% fulvene, 45% benzene and 35% 1,5-hexadiyne, while the cyclohexane flame consisted of 99.5% benzene and 0.5% fulvene. This difference in isomeric composition strongly points to the difference in benzene formation pathways in the two fuels.
0486: Analytical chemistry