Experimental study of the effect of CF3I addition on the ignition delay time and laminar flame speed of methane, ethylene, and propane
Introduction
With its high suppression efficiency [1], along with a relatively low toxicity, Halon 1301 (CF3Br) was the most commonly used fire suppressant prior to being phased-out by the Montreal Protocol. Although a large body of work has been performed to find a suitable replacement [2], no component that meets all the required criteria has been found [3]. To date, CF3Br is still the subject of extensive research in order to have a detailed understanding of its combustion chemistry (to identify/design similar reaction pathways for candidate replacements) [4], [5] and to serve as a benchmark for new compounds [3], [6], [7]. However, few -- if any -- fundamental combustion data have been available for the development of the CF3Br chemical kinetics mechanism [1], [8], [9], explaining the discrepancy between the model and recent fundamental combustion data (ignition delay time (τign) and laminar flame speed (SL0)) [5].
Since no single substitute for Halon 1301 has been found, a more likely approach is to identify diverse agents or blends of agents for specific applications. Amongst the possible replacement candidates, CF3I (Halon 13001) displayed some interesting properties. Notably, when released into the lower atmosphere, the fluorinated iodoalkanes degrade by photolysis of the carbon–iodine bond and are expected to have short atmospheric lifetimes (and hence not reaching and depleting the ozone layer) [2]. Numerical studies have shown that it has the same extinction efficiencies, sometimes even superior on a molar basis, than CF3Br for inhibiting CH4, CH3OH, C2H6, and C2H4 [10] and alkanes (C1–C4) burning velocities [6]. Smith and Everest [11] compared OH and soot concentration measurements in propane/air diffusion flames inhibited by CF3Br or CF3I. Overall, under their experimental conditions, CF3I was slightly superior to CF3Br in terms of (i) requiring smaller concentrations at extinction, and (ii) producing less within-flame soot. The reductions in the OH concentration were essentially the same for CF3I and CF3Br. The flame structure was compared between low-pressure flames doped with CF3I or CF3Br by Sanogo et al. [12]. Results showed that the flames are essentially similar, and species involved in the inhibition process are formed in similar amounts. CF3I also has a boiling point that is low enough for in-flight application consideration, even if its release at high altitude can be problematic for the ozone layer [2]. One can however mention a few drawbacks with the use of CF3I such as its cardiac sensitization at levels well below the extinguishing concentration, which restricts its utilization to unoccupied areas [2].
Despite the interesting characteristics of CF3I, there is a noticeable lack of fundamental data to have a good understanding of its flame inhibition mechanisms and to better assess its fire suppressant potential. The aim of the present work was therefore to provide fundamental combustion data, namely ignition delay times (for kinetics model assessment) and laminar flame speeds (suppressant assessment), on the effects of CF3I on methane (main component of natural gas), ethylene (one of the most-used hydrocarbons in the petrochemical industry), and propane (to represent saturated-chain hydrocarbons). Pressure conditions were the same as in Osorio et al. [5], but sub-atmospheric experiments were investigated as well herein using the shock-tube technique to account for possible in-flight applications.
Section snippets
Shock-tube
Ignition delay times were determined behind reflected shock waves (RSW) in a single-diaphragm, stainless-steel shock tube. The driven section is 15.24-cm i.d., 4.72-m long; and the driver section is 7.62-cm i.d., 2.46-m long. A schematic of the shock-tube setup can be found in Aul et al. [13]. Five PCB P113A piezoelectric pressure transducers, equally spaced alongside the driven section and mounted flush with the inner surface, were used to measure the incident-wave velocities. A curve fit of
Shock tube
The ignition delay time was defined as the time between the passage of the RSW and the intersection of lines drawn along the steepest rate-of-change of OH de-excitation (i.e., chemiluminescence) and a horizontal line which defines the zero-concentration level, as documented in Osorio et al. [5]. The chemiluminescence emission from the A2∑+ → X2Π transition of the excited-state hydroxyl radical (OH) was followed at the sidewall location using an interference filter centered at 307 ± 10 nm with a
Discussion
During this study, the effect of CF3I addition on the ignition delay times and laminar flame speeds of methane, ethylene, and propane were investigated experimentally. The addition of CF3I was found to decrease τign for methane. A similar effect was observed with CF3Br over similar conditions by Osorio et al. [5]. A sensitivity analysis points at the same reaction for the two suppressants: CF3 + O2 ⇄ CF3O + O (r1).
For C3H8, no reaction involving CF3I or its products is amongst the 25 most-sensitive
Conclusion
During this study, the effect of CF3I addition on the ignition delay times and laminar flame speeds of methane, ethylene, and propane was studied experimentally for the first time. Overall, the effects on the ignition delay time were found to be fuel dependent: a decrease in τign was observed with methane, whereas an increase in τign was observed at low temperature with C2H4 and, to a lesser extent, with C3H8. Flame speed measurements allow assessing the suppressant effect of CF3I. A
Acknowledgments
Financial support was provided by the Mary Kay O’Connor Process Safety Center at the Artie McFerrin Department of Chemical Engineering, Texas A&M University for the TAMU effort and by CNRS-France for the work performed at ICARE, CNRS-INSIS Orléans.
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