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Fluoromethane chemistry and its role in flame suppression

1994; Elsevier BV; Volume: 25; Issue: 1 Linguagem: Inglês

10.1016/s0082-0784(06)80795-2

1878-027X

Phillip R. Westmoreland, Donald R. Burgess, Michael R. Zachariah, Wing Tsang,

Atmospheric chemistry and aerosols

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) thermochemistry 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% CH4/air flame(equivalence ratio 0.65) is the base case with dopant CF4, CHF3, CH2F2, or CH4 to make up 1 ppm to 2 mol% of the feed. CF4, which proves to be inert, slows the adiabatic flame speed and reduces the adiabatic flame temperature by dilution and its heat capacity. CHF3 causes chemical suppression effects, slowing adiabatic flame speed below that with CF4, despite increasing adiabatic flame temperature. Adding CH2F2, CH3F, or CH4 increases both flame speed and temperature. The chemical cause is competition between chain termination, primarily by chemically activated HFelimination, 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, due to the higher bond strength of C−F, and chemically activated species containing H and F can rapidly eliminate HF. Thus, OH+CH3→CH3OHo slowly forms CH3OH by third-body stabilization, but OH+CF3→CF3OHo goes rapidly to CF2O+HF. Slow destruction of CF2O formed by this reaction and by CF3+O helps suppress the CHF3-doped flame, but CH2F2 and CH3F are accelerants because they are oxidized easily.