Can Fire Suppressants Promote Ignition? A Study Of HFC-125 and HFC-227

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 ozone-depleting 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), C₃HF₇ and C₂HF₅, although less effective than bromo-containing 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 at 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 C₂HF₅ and C₃HF₇ on the high-temperature chemistry of methane and propane flames at different conditions. Ignition delay time measurements are obtained using a shock tube with reaction monitoring capability. Theoretical analysis is conducted using a detail chemical kinetics mechanism that includes the hydrocarbon and fire suppressants set of reactions. Results show that C₂HF₅ and C₃HF₇ can actually act as ignition promoters at 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 is carried out to identify the most significant reactions responsible for such behavior, and the results are compared with CF₃Br over a similar range of conditions.