Flame inhibition by ferrocene and blends of inert and catalytic agents

The production of the fire suppressant CF₃Br has been banned, and finding a replacement with all of its desirable properties is proving difficult. Iron pentacarbonyl has been found to be up to several orders of magnitude more effective than CF₃Br, but it is flammable and highly toxic. Ferrocene [Fe(C₅H₅)₂], which is much less toxic and flammable than Fe(CO)₅, can also be used to introduce iron into a flame. We present the first experimental data and numerical modeling for flame inhibition by ferrocene and find it to behave similarly to Fe(CO)₅. A ferrocene mole fraction of 200 ppm reduced the burning velocity of slightly preheated premixed methane/air flames by a factor of two, and the effectiveness dropped off sharply at higher mole fractions. For air with a higher oxygen mole fraction, the burning velocity reduction was less. We also present experimental data and modeling for flames with ferrocene blended with CO₂ or CF₃H. The combination of the thermally acting agent CO₂ with ferrocene mitigated the loss of effectiveness experienced by ferrocene alone at higher mole fractions. An agent consisting of 1.5% ferrocene in 98.5% CO₂ performed as effectively as CF₃Br in achieving a 50% reduction in burning velocity. Likewise, four times less CO₂ was required to achieve the 50% reduction if 0.35% ferrocene was added to the CO₂. In contrast, addition of 0.35% ferrocene to the hydrofluorocarbon CF₃H reduced the CF₃H required to achieve the 50% reduction in burning velocity by only about 25%. Thermodynamic equilibrium calculations predict that the formation of iron/fluoride compounds can reduce the concentrations of the iron-species oxide and hydroxide intermediates which are believed to be responsible for the catalytic radical recombination cycles.