Laser powered homogeneous pyrolysis of butane initiated by methyl radicals in a quasi-wall-free reactor at 750-1000 K

The pyrolysis of n-butane, initiated by methyl radicals has been studied in the temperature range 750-1000 K and at pressures 0.08-0.13 bar in a quasi-wall-free reactor using laser heating by fast vibrational-translational (V-T) energy transfer. This is a convenient method to study homogeneous high-temperature kinetics since the reactor walls remain cold. The radial temperature distribution in the reactor has been investigated by four different methods: stationary heat balance, optical absorption, pressure rise, and the temperature dependence of the rate of an isomerization reaction. Methyl radicals were produced via the fast thermal dissociation of di-tert-butyl peroxide and product analysis was performed by the use of GC-MS. The main products of the overall reaction of the model system (n-C4H10+CH3) were C2H4, C3H6, C3H8, whereas 1-C4H8, n-C5H12, iso-C5H12 were minor components, all showing a strong dependence on temperature. The product distribution and the temperature dependence were analyzed by a kinetic model of 61 species and 164 reactions developed for the high-temperature butane and the low-temperature n-pentane oxidation. Good agreement was found between our experimental investigations and the modeling. However, we had to slightly adjust the rate constants for the reactions CH3+n-C4H10-->n-C4H9+CH4 (3) CH3+n-C4H10-->sec-C4H9+CH4 (4) At a temperature of 1000 K we found a larger branching ratio of k(3)/(k(3)+k(4))=1/3 compared to 1/8 as extrapolated from low-temperature data. The total rate coefficient was found to be (k(3)+k(4))=8x10(9) cm(3) mol(-1) s(-1) which is about 50% higher than the extrapolated values.