A chemical kinetic mechanism for combustion and flame propagation of CH₂F₂/O₂/N₂ mixtures

Abstract

In this work, we compiled and evaluated rate expressions for reactions relevant to the decomposition and combustion of CH 2 F 2 (difluoromethane, refrigerant R-32) in CH 2 F 2 /O 2 /N 2 flames. The recommended values have been used in premixed flame calculations, reported elsewhere, to model experimentally derived burning velocities determined using a constant volume spherical flame method. In this work, we also provide a detailed description of the reaction pathways for decomposition and combustion of CH 2 F 2 . This work is part of a larger effort at NIST to characterize and predict the flammability of new refrigerant working fluids and their blends for consideration as replacements of current refrigerants with high global warming potentials.

Initiation and Carbene Induced Radical Chain Reactions in CH2F2 Pyrolysis

“…Such reactive blends can have lowered tropospheric lifetimes with reduced GWP's but are typically associated with increased flammability. Recent studies from the flame and kinetics groups at UTRC and NIST [6,11,12,40,41] have chosen CH 2 F 2 as a prototype HFC species to characterize flammability in refrigerants.…”

“…This is a global property of premixed mixtures of fuel and oxidizer resulting from the synergistic influence [42] of diffusivity, exothermicity, and chemical reactivity that can be used to constrain chemical kinetic models. [43] Recent studies have developed detailed kinetics models [11,12] for the decomposition and combustion of CH 2 F 2 and to predict its flammability over a range of conditions. The modeling work [12] indicates that simulations of CH 2 F 2 flame speeds were noticeably influenced by the initiation reaction (R1) with reaction of CHF + O 2 !CHFO + O being the radical propagating step with reactive O-atoms from this reaction being the subsequent source for chain branching.…”

“…This was quite unexpected as acetylene formation was not considered in prior studies of CH 2 F 2 pyrolysis or oxidation. [8][9][10]22,23] However, the rapid formation of reactive radicals and atoms from reactions (R4) and (R5) facilitates direct reactions that form C 2 H 2 . These include the fast reaction of HCCF with H-atoms via reaction (R6) or F-atom β-scission of the CHFCH radical in reaction (R5).…”

“…In the experimental work acetylene (C 2 H 2 ) was found to be formed in similar concentrations as fluoroacetylene (HCCF). The formation of C 2 H 2 as an intermediate has not been considered in recent models [11,12] describing the pyrolysis and oxidation of CH 2 F 2 . Our calculations also reveal that H-atoms formed in reaction R4 can facilitate the rapid conversion of HCCF to C 2 H 2 via reaction R6 which are supported by the experimental observations.…”