Can fire suppressants promote ignition? A study of HFC-125 and HFC-227
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abstract
C2HF5(HFC-125,) and C3HF7(HFC-227ea or FM-200) have gained considerable acceptance in the field of fire protection, especially after the restriction of several ozonedepleting substances under the Montreal Protocol. These two substitutes are currently included in the list of the Significant New Alternatives Policy (SNAP) Program of the U.S Environmental Protection Agency (EPA). Their desired properties such as low Ozone Depleting Potential (ODP), relatively low toxicity, electrical non-conductivity, high dispersion capabilities, and low flammability, make them two of the most-used total flooding fire suppressants at the moment. In terms of Minimum Extinguishing Concentration (MEC), C3HF7and C2HF5, although less effective than brominecontaining compounds, are more efficient than other clean alternatives in the market. Nevertheless, the combustion properties of these agents are not well understood. In fact, previous numerical studies have shown that these compounds can act as combustion promoters under certain conditions, but there are scarce experimental data of HFCs that can be used to certify such predictions. This lack of data shows the necessity of having accurate measurements that can be used for model validation but also to better understand the role of different fire suppressants on the combustion initiation stage towards an optimal and safer application. To this end, this paper examines the effect of C2HF5and C3HF7on the high-temperature chemistry of methane and propane flames at different conditions. Ignition delay time measurements were obtained using a shock tube with reaction monitoring capability. Theoretical analysis was conducted using a detailed chemical kinetics mechanism that includes the hydrocarbon and fire suppressants set of reactions. Results indicate that C2HF5and C3HF7can actually act as ignition promoters under certain conditions. These results suggest that these fire suppressants may not be good alternatives to prevent flames, however they can still be used as fire extinguishers after the flame is well established. Finally, sensitivity analysis was carried out to identify the most significant reactions responsible for such behavior, and the results were compared with CF3Br over a similar range of conditions.
