Kinetics of C₂ Reactions during High-Temperature Pyrolysis of Acetylene

Abstract

The kinetics of C 2 radical reactions during the first stage of acetylene high-temperature pyrolysis was studied by monitoring C, C 2 and C 3 radicals. Quantitative C 2 detection was performed by ring dye laser absorption spectroscopy, C atoms were measured by applying atomic resonance absorption spectroscopy, and C 3 radicals were monitored by their emission using a combination of a spectrograph and an intensified CCD camera system. The experiments were performed behind reflected shock waves and cover the temperature range of 2580−4650 K at pressures around 2 bar. In the first part of the study initial mixtures containing Ar with 5−50 ppm C 2 H 2 were used. In this very low concentration range, rate coefficients for the following four reactions were determined: C 2 H 2 + M C 2 H + H + M (R1); C 2 H + M C 2 + H + M (R2); C 2 + C 2 C + C 3 (R3); C 2 + M C + C + M (R4); where k 1 = 6.96 × 10 39 T -6.06 exp(−67 130/ T ) cm 3 mol -1 s -1, k 2 = 1.74 × 10 35 T -5.16 exp(−57 367/ T ) cm 3 mol -1 s -1, k 3 = 3.2 × 10 14 cm 3 mol -1 s -1, and k 4 = 1.5 × 10 16 exp(−71 650/ T ) cm 3 mol -1 s -1 . Furthermore, this experiments indicate that a modification of the JANAF 1 thermodynamic data of either C, C 2 or C 3 seems to be necessary. In the second part, some experiments with relatively high initial acetylene concentrations up to 500 ppm C 2 H 2 in Ar were carried out to check the validity of a more complex mechanism for the acetylene pyrolysis. Finally in a third part, a perturbation study was performed by adding 1000 ppm H 2 to the initial mixtures of Ar with 20 and 50 ppm of C 2 H 2 . For the most important perturbation reactions, C 2 + H 2 C 2 H + H (R5) and C 2 H + H 2 C 2 H 2 + H (R6), rate coefficients of k 5 = 6.6 × 10 13 exp(−4000/ T ) cm 3 mol -1 s -1 and k 6 = 7.4 × 10 14 exp(−3400/ T ) cm 3 mol -1 s -1 were obtained.