Kinetics of C₂ Reactions during High-Temperature Pyrolysis of Acetylene

Abstract

The kinetics of C2 radical reactions during the first stage of acetylene high-temperature pyrolysis was studied by monitoring C, C2 and C3 radicals. Quantitative C2 detection was performed by ring dye laser absorption spectroscopy, C atoms were measured by applying atomic resonance absorption spectroscopy, and C3 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 C2H2 were used. In this very low concentration range, rate coefficients for the following four reactions were determined: C2H2 + M C2H + H + M (R1); C2H + M C2 + H + M (R2); C2 + C2 C + C3 (R3); C2 + M C + C + M (R4); where k1 = 6.96 × 1039 T-6.06 exp(−67 130/T) cm3 mol-1 s-1, k2 = 1.74 × 1035 T-5.16 exp(−57 367/T) cm3 mol-1 s-1, k3 = 3.2 × 1014 cm3 mol-1 s-1, and k4 = 1.5 × 1016 exp(−71 650/T) cm3 mol-1 s-1. Furthermore, this experiments indicate that a modification of the JANAF1 thermodynamic data of either C, C2 or C3 seems to be necessary. In the second part, some experiments with relatively high initial acetylene concentrations up to 500 ppm C2H2 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 H2 to the initial mixtures of Ar with 20 and 50 ppm of C2H2. For the most important perturbation reactions, C2 + H2 C2H + H (R5) and C2H + H2 C2H2 + H (R6), rate coefficients of k5 = 6.6 × 1013 exp(−4000/T) cm3 mol-1 s-1 and k6 = 7.4 × 1014 exp(−3400/T) cm3 mol-1 s-1 were obtained.