Inhibition and oxidation characteristics of chloromethanes in reacting CO/H₂O/O₂ mixtures

The study of chlorinated hydrocarbon oxidation in an environment controlled by the moist CO oxidation reaction is directly relevant to the post-flame chemistry of hazardous waste incinerators, and thus to the control of pollutant emissions. In the present article, experimental results are presented on the CO/H₂O/O₂ reaction perturbed by trace amounts of CH₃Cl, CH₂Cl₂, CHCl₃, and CCl₄ obtained from a flow reactor at temperatures near 1000 K and at atmospheric pressure. As is well documented in the fire retardent literature, the chloromethanes are observed here to be strong inhibitors of CO oxidation. In the present post-flame like environments, ranking in order of increasing inhibition effectiveness is CHCl₃ < CH₃Cl < CH₂Cl₂ on a molar basis. Inhibition from CCl₄ was greater or less than CH₃Cl at low or high loadings respectively. This ranking correlates well with literature rate constants of reactions controlling the radical pool level in this type of system. Detailed species profiles were obtained and revealed that all but CH₃Cl formed considerable amounts of highly toxic phosgene, with yields ranging from 7 to 26% of the initially added chloromethane on a molar basis. These yields are due in part to the slow oxidation rate of phosgene, slower than all other chlorocarbons observed in this study. As a result, it, survives well into the region of CO oxidation, with potential implications on the toxicity of emissions. Finally, a rate constant for the primary consumption channel for phosgene, COCl₂ + H → COCl + HCl was evaluated from these data to be 1.0 (±0.4) × 10¹² cc/mol/s at 1000 K, which is a factor 2 to 4 slower than current literature estimates.