Pyrolysis study of CF3I to CF3 + I Using Laser Absorption Spectroscopy in a Miniature Shock Tube

Trifluoroiodomethane (CF3I) has gained attention as a fire suppressant due to its low environmental impact. It is an alternative to fire suppressants containing bromine or chlorine, which have been phased out due to their detrimental effects on the ozone layer. This study focuses on investigating the initial dissociation of CF3I to CF3 + I. The thermal dissociation rate constant was measured from the time resolved concentration of iodine at 1315 nm, measured using laser absorption spectroscopy in a narrow-bore high repetition rate shock tube. The experimental measurements were challenging due to the low signal strength of this relatively weak transition through the short pathlength in the shock tube. Because of the small absorption of iodine atom compared to beam steering, 3 different schemes for ensemble averaging the measurements were tested, and a method was developed. A custom reactor model was implemented with Cantera to assign the rate constants, and the experimental results were compared against the literature. 2454 experiments for CF3I were run at a temperature range between 1050 -1250 K and a pressure range from 2 to 3.5 bar, using a 1% composition of CF3I in argon. After modeling, the thermal dissociation rate constant of CF3I to CF3 + I is 𝑘(𝑇) = 10^(28.81) ∙ 𝑇^(−5.19) ∙ exp (−11016/𝑇) [s^-1], similar to existing values from previous studies, but with slightly different temperature dependence. The developed method enables accurate measurement of rate constants from weak absorption measurements using ensemble averaging, the result facilitates the evaluation and application of CF3I as a fire suppressant.