Fluoromethane chemistry and its role in flame suppression
- Univ. of Massachusetts, Amherst, MA (United States)
- National Inst. of Standards and Technology, Gaithersburg, MD (United States)
A detailed reaction set is composed for fluoromethanes in flames, and the competing roles of suppression chemistry, oxidation chemistry, and heat capacity are analyzed. The set is constructed using (1) thermo-chemistry from the literature, from group additivity, and from BAC-MP4 ab initio-based calculations and (2) kinetics from the literature, from simple analogies, from thermochemical kinetics, from BAC-MP4 transition-state calculations, and from Quantum-RRK and RRKM/Master Equation calculations. Structures of freely propagating laminar flames are then predicted and analyzed. A 6.4% CH{sub 4}/air flame is the base case with dopant CF{sub 4}, CHF{sub 3}, CH{sub 2}F{sub 2}, or CH{sub 4} to make up 1 ppm to 2 mol% of the feed. CF{sub 4}, which proves to be inert, slows the adiabatic flame speed and reduces the adiabatic flame temperature by dilution and its heat capacity. CHF{sub 3} causes chemical suppression effects, slowing adiabatic flame speed below that with CF{sub 4}, despite increasing adiabatic flame temperature. Adding CH{sub 2}F{sub 2}, CH{sub 3}F, or CH{sub 4} increases both flame speed and temperature. The chemical cause is competition between chain termination, primarily by chemically activated HF elimination, and chain propagation by normal oxidation pathways. Like methane, fluoromethane flame chemistry is dominated by abstraction and by chemically activated reactions. However, abstraction of H is greatly favored over abstraction of F. Thus, OH + CH{sub 3} = CH{sub 3}OH{degree} slowly forms CH{sub 3}OH by third-body stabilization, but OH + CF{sub 3} = CF{sub 3}OH{degree} goes rapidly to CF{sub 2}O + HF. Slow destruction of CF{sub 2}O formed by this reaction and by CF{sub 3} + O helps suppress the CHF{sub 3}-doped flame, but CH{sub 2}F{sub 2} and CH{sub 3}F are accelerants because they are oxidized easily.
- OSTI ID:
- 93302
- Report Number(s):
- CONF-940711--
- Country of Publication:
- United States
- Language:
- English
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