A Shock‐Tube Study of the HCl + M → H + Cl + M (M = Ar) Rate Constant Using HCl Laser Absorption Near 3.3 µm

ABSTRACT Hydrogen chlorine (HCl) is an important combustion intermediate during plastic (municipal waste) and composite propellant (aerospace) combustion, but the understanding of its high‐temperature chemistry is still evolving. In this study, the pyrolysis of 0.5% HCl in Ar was investigated behind reflected shock waves in a shock tube at temperatures ranging from 2290 to 3490 K and pressures from 0.7 to 3.6 atm. A laser source was set to access the R(8) transition line of H 35 Cl in the fundamental band near 3.3 µm at the specific wavelength of 3045.06 cm −1 . Numerical predictions using chemical kinetics mechanisms from the literature did not capture well the decomposition behavior of HCl in our conditions. According to these models, the most sensitive reaction HCl + M H + Cl + M (M = Ar) could be isolated, and its rate constant was retrieved from the experimental measurements. A pressure dependence was observed over the range of pressures covered. The present study measured the bimolecular reaction rate of the title reaction ( k 1.5 atm in cm 3 ·mol −1 ·s −1 , T in K, R = 1.987 cal·mol −1 ·K −1 ) as: valid near 1.5 atm, as well as ( k <1 atm in cm 3 ·mol −1 ·s −1 ): and ( k >2 atm in cm 3 ·mol −1 ·s −1 ): suitable for conditions below 1 atm and above 2 atm, respectively. A generalized overall equation for the unimolecular reaction HCl H + Cl was obtained and is described as k all in (s −1 ). Finally, the results indicate that the measurements at pressures up to 3.6 atm in the present study were sufficient to determine that the title reaction is in its high‐pressure limit.