Shock-tube and modeling study of trichloromethane pyrolysis and trichloromethane/methane oxidation using a HCl laser absorption diagnostic

Abstract

Chlorinated hydrocarbon burning characteristics when involved in incineration processes and when formed during rocket propellant combustion are not well understood, and evaluations of their chemical kinetics mechanisms at high temperatures are limited by the scarce experimental measurements. The pyrolysis of trichloromethane (CHCl₃) and the oxidation of CHCl₃/methane (CH₄) at ϕ = 1.0, both highly diluted in 99.5 % Ar, were investigated behind reflected shock waves near atmospheric pressure using a new H³⁵Cl spectroscopic laser diagnostic. The ability to monitor the isotope H³⁵Cl was possible using a laser source centered at 3045.06 cm^-1 aiming at the H³⁵Cl R(8) transition line in the fundamental band of the spectrum. A large span of temperatures was investigated, i.e. ranging from 1068 to 1444 K for CHCl₃ pyrolysis, where the most sensitive reaction CHCl₃ $\rightleftarrows$ CCl₂ + HCl (R1) produces the H³⁵Cl in correlation to the natural abundance proportions, namely $\left[H^{35}Cl\right]\approx 3\times \left[H^{37}Cl\right]$. Similarly, the oxidation of CHCl₃/CH₄ was recorded at temperatures between 1471 and 2094 K, and the interactions of the active Cl radicals provided by the dichlorocarbene (CCl₂) via the reaction 2CCl₂ $\rightleftarrows$ C₂Cl₃ + Cl (R2) with CH₄ are observed and driven by the reaction CH₄ + Cl $\rightleftarrows$ CH₃ + HCl (R3). Numerical predictions from available detailed kinetics mechanisms for chlorinated hydrocarbons in the literature are compared against this comprehensive set of experimental results, and significant discrepancies are observed. Routes for improvements toward predicting this major intermediate species, i.e. HCl, are suggested. By strengthening the fundamental database for the combustion kinetics of chlorinated hydrocarbons, strategies to reduce the dominant release of extremely toxic chemicals could be discovered.