Flame Inhibition by Ferrocene, Carbon Dioxide, and Trifluoromethane Blends:

Synergistic and Antagonistic Effects

G.T. Linteris and M.D. Rumminger

 National Institute of Standards and Technology

 Gaithersburg, MD 20899-8651; linteris@nist.gov; mrum@asme.org

(current address for MDR: Sandia National Lab; MS 9052; Livermore, CA 94551)

The production of CF3Br has been banned. As a flame inhibitor, iron pentacarbonyl (Fe(CO)5) is about two

orders of magnitude more efficient than CF3Br, but it is flammable and highly toxic, and its addition to premixed

flames at mole fractions above a few hundred ppm does not further reduce the burning velocity. If other iron

compounds can be identified which show the same strong inhibition but are less toxic and don’t lose their

effectiveness, they may find use in fire suppressants. Ferrocene (Fe(C5H5)2 or Fec) modifies the sooting tendency

of flames, is added to materials as a flame retardant and is an antiknock agent. It is far less toxic than Fe(CO)5,

and it may produce the same iron-containing intermediates. Here, we present the first measurements of flame

inhibition by ferrocene, compare it with Fe(CO)5 and CF3Br, and present data showing how combining it with

other compounds can overcome the loss in effectiveness experienced by both it and Fe(CO)5.

Flame inhibition by bromine, alkali metals, and transition metals have been described in the literature, and their

reduced effectiveness at higher mole fractions has been discussed in detail [1-3]. Since the chemicals are believed

to act through homogeneous gas-phase catalytic radical recombination cycles, the inhibition mechanism requires a

radical superequilibrium to be most effective. Consequently, once radicals are reduced to equilibrium levels via

inhibitor addition, further reduction in the overall reaction rate from additional inhibitor is minimal. This loss in

effectiveness due to radicals approaching their equilibrium values has been demonstrated for increasing mole

fraction of inhibitor in calculations employing both an idealized “perfect” inhibitor [2] and Fe(CO)5 [3], and has

been discussed by Rosser et al. [4] and Hastie [5]. Nonetheless, the measured loss in effectiveness of Fe(CO)5

occurs at a lower mole fraction than expected based on these calculations; the actual cause appears to be

condensation of iron compounds and the resulting limit to the gas-phase iron-species concentrations [6].

Regardless of the cause of the loss of effectiveness, previous researchers [4,5,7-10] have suggested that

combinations of thermally acting and catalytic agents might prove beneficial. The effectiveness of iron

pentacarbonyl in premixed flames has been shown to be greatly increased at lower oxygen mole fraction, and the

effect has been attributed to the larger superequilibrium ratio (the peak radical mole fraction divided by the

equilibrium value) which exists at lower oxygen mole fraction [1]. In contrast, the effectiveness of relatively inert

agents such as N2 and CO2 is not a strong function of the oxygen mole fraction in the oxidizer stream, XO2,ox . (In

the present work we find that addition of 10% CO2 to flames with XO2,ox equal to either 0.21 or 0.244 reduces the

normalized burning velocity by about half, whereas Fe(CO)5 is more than twice as effective for XO2,ox = 0.21 than

for 0.244 [1]). Since adding a thermal agent raises the superequilibrium ratio, the possibility exists for very

effective blends of catalytically and thermally acting agents. Consequently, we report here the burning velocity of

premixed flames in the presence of combinations of a thermal agent CO2, (which does not lose its effectiveness at

higher mole fraction) and catalytic agents (which are superbly effective at low inhibitor mole fraction).

The premixed laminar flame speed SL provides a measure of the effect of the inhibitor on the overall reaction rate.

The experimental arrangement, described in detail previously [1,10,11], has been modified only to accommodate

a new evaporator for ferrocene and heating of the gas lines and burner tube. A Mache-Hebra nozzle burner (1.0

cm ± 0.05 cm diameter) produces a premixed Bunsen-type flame about 1.3 cm tall with a straight sided schlieren

image which is captured by a frame-grabber board in a PC. Digital mass flow controllers hold the equivalence

ratio φ, XO2,ox , and the flame height constant while maintaining the inlet mole fraction of the inhibitor Xin at the

desired value. The inhibitors are N2, CF3H, CO2, Fec, and Fe(CO)5. The average burning velocity is determined

from the reactant flows and the schlieren image using the total area method. As in earlier research, the fuel gas is
methane (Matheson1

 UHP, 99.9%), and the oxidizer stream consists of nitrogen (boil-off from liquid N2) and

oxygen (MG Industries, H2O < 50 ppm, and total hydrocarbons < 5 ppm). The inhibitors used are Fec (Aldrich,

98%), Fe(CO)5 (Aldrich), CF3Br (Great Lakes), N2, and CO2 (Airgas). The Fe(CO)5 is added to N2 carrier gas

using a two-stage saturator in an ice bath. Because the vapor pressure of Fec is much lower than that of iron

pentacarbonyl, Fec addition at mole fractions up to 500 ppm requires both higher bath temperature (70 to 80 C,

held within 0.01 C) and higher nitrogen carrier gas flow rates (up to 350 cm3/min) relative to Fe(CO)5. Also, the

solid state of Fec requires an evaporator with larger surface areas for heat and mass transfer. Our evaporator

design, based upon that of Megaridis [12], has a 30 ± 5 cm3 packed bed (to provide the bulk of the ferrocene),

followed by thirty sublimation stages (to insure that the carrier gas is saturated at the bath temperature); each stage

consists of a 5 mm layer of ferrocene, 2.36 cm in diameter, on a 60 mesh stainless screen and a 4 mm gap.

Temperature controllers maintain the transfer lines at 353 K ± 3 K and the burner tube at 353 K ± 1 K. For all

flames, the equivalence ratio (in the absence of inhibitor) is 1.0, and agent mole fraction is calculated relative to

the reactant total without the inhibitor.

The experimental burning velocity reduction caused by the addition of Fec and Fe(CO)5 to the stoichiometric

methane flames are presented in Figure 1. Data are plotted as normalized burning velocity, which is the burning

velocity of the inhibited flame divided by the value for the same flame in the absence of inhibitor. (For Figs. 1 to

4, the solid lines are fits to the data.) As Figure 1 shows, Fec (closed symbols) reduces the burning velocity as

effectively as does Fe(CO)5 (open symbols), and it does not appear to lose its effectiveness until higher mole

fraction. As with Fe(CO)5, the magnitude of the inhibition is strongly dependent upon the oxygen mole fraction

in the oxidizer, with oxygen-deprived flames inhibited much more strongly.

Figure 2 presents experimental data for addition of pure CO2 (or pure N2) and CO2/Fec blends at four values of

the equivalent percentage of Fec in CO2: 0, 0.053%, 0.2%, and 0.54%. The last data point in a series represents

the maximum value of Xin at which a flame could be stabilized. The blends of the inert and chemically acting

agents are shown to be highly effective. Addition of about 10% of pure CO2 (or 25% pure N2) reduces SL by a

factor of two. Addition of the equivalent of 0.1% Fec to CO2 reduces the required CO2 by about a factor of two,

and 0.6% Fec reduces the required CO2 by ten, making this blend about as effective as CF3Br (for which addition

of about 1% halves the burning velocity). It is important to note that the flames with CO2/Fec combinations in

Figure 2 do not display the large loss in effectiveness, i.e. the plateau region, occurring in most of the curves in

Figure 1. Although one might expect the slightly cooler, slower flames with added CO2 to have more

condensation of iron species, the greater efficiency of the catalytic cycle in the diluted flames predominates.

Other compounds may be convenient for blending with catalytic agents. Hydrofluorocarbons are of interest since

they are presently used as halon replacements. These compounds have been found to reduce the burning velocity

of premixed methane-air flames by reducing peak H-atom mole fractions through reactions forming HF and by

lowering the temperature of the flame. Since they have also been shown to reduce the equilibrium mole fractions

of radicals in flames lower than expected based on temperature reduction alone [13], they might be expected to

show enhanced performance relative to CO2 when combined with catalytic agents. Figure 3 presents the burning

velocity reduction caused by pure CF3H addition to the above flames; a mole fraction of about 5% is required to

reduce SL by two. Data are also presented for addition of the equivalent of 0.35% Fec in CF3H. Unlike ferrocene

addition to CO2, in which 0.35% Fec in CO2 reduces the amount of CO2 required by a factor of about five, this

amount of ferrocene in CF3H reduces the amount of CF3H required only by about 25%. This poor performance

may be due to reactions between iron species and fluorine which reduce the gas-phase mole fraction of the active

iron-species intermediates, effectively poisoning the iron catalyst [5,14].

To further illustrate the synergistic behavior of Fec with CO2 , and the antagonistic behavior of Fec with CF3H,

we examine the performance of the blend relative to individual contributions from each component. We define

the normalized flame speed at each inhibitor mole fraction Xin as NSL(Xin), and the reduction in the normalized

 

1

 Certain commercial equipment, instruments, or materials are identified in this paper to adequately specify the procedure.

Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology,

nor does it imply that the materials or equipment are necessarily the best available for the intended use.
flame speed as RSL (Xin ) = 1 – NSL(Xin). Assuming that the flame speed reduction of a two-component blend

RSL(Xin)|blend is a linear combination of the contribution from each component, RSL(Xin,1 , Xin,2 )|blend = Xin,1RSL,1 +

Xin,2RSL,2 , the predicted normalized burning velocity of the blend is NSL(Xin,1,Xin,2)|blend= 1 – RSL|blend . The ratio of

the predicted to the actual normalized flame speed for several blends is shown in Figure 4. A value greater than

unity shows that the blend provides a reduction in the normalized burning velocity more than expected based on a

linear combination of the contribution from each component, while a value less than one shows the converse. In

Figure 4, the curve near unity shows the result for a blend of 2/1 molar ratio of N2 to CO2; that is, for thermally

acting agents in these flames and in this range of inhibitor mole fraction, the reduction in flame speed from CO2

and N2 is additive. As the figure shows, CO2 with 0.54% Fec is significantly more effective than would be

expected based on a linear combination of the performance of Fec and CO2 alone, while CF3H and Fec is

significantly less effective than the sum of each individually.

We have presented the first data on ferrocene as a flame inhibitor, and shown it to be as efficient as Fe(CO)5 at

reducing the burning velocity of premixed methane flames. The results imply that either agent acts as an effective

precursor for the active iron-species intermediates. For the data presented here, ferrocene does not appear to lose

its effectiveness as drastically as does Fe(CO)5 , and its inhibition has just as strong a dependence on XO2,ox . As a

result, blends of CO2 and Fec show very strong inhibition, overcoming the loss of effectiveness observed for

pure Fec or Fe(CO)5. Further, blends of CO2 and Fec reduce the burning velocity of premixed methane-air flames

more strongly than expected based on the performance of each individually. Blends of CF3H and Fec, however,

have been found to be much less effective than expected based on the performance of each individually, implying

that iron species and halogens may enter into undesired reactions which poison the catalytic cycles. Nonetheless,

if means can be identified to safely introduce gas-phase iron compounds into fires, combinations of catalytically

and relatively inert thermally acting inhibitors may prove to be an efficient approach to solving a problem which

threatens the global environment.

We thank Prof. Dino Megarides of UIC for helpful discussions, and for sending us his ferrocene evaporator. We

also thank Dr. Wing Tsang for stimulating conversations and suggestions throughout this research. This research

was supported in part by NGP MIPR No. W74RDV83528667.

[1]. Reinelt, D. and Linteris, G.T., Twenty-Sixth Symposium (International) on Combustion, The Combustion

Institute, Pittsburgh, 1996, pp. 1421-1428.

[2]. Babushok, V., Tsang, W., Linteris, G.T., and Reinelt, D., Combust. Flame 115:551 (1998).

[3]. Rumminger, M.D., Reinelt, D., Babushok, V., and Linteris, G.T., Combust. Flame 116:207 (1999).

[4]. Rosser, W.A., Inami, S.H., and Wise, H., Combust. Flame 7:107 (1963).

[5]. Hastie, J. W., High Temperature Vapors, Academic Press, New York, 1975.

[6]. Rumminger, M.D. and Linteris, G.T., "An Experimental Study Of The Role Of Particles In Flame

Inhibition By Iron Pentacarbonyl", in preparation for submission to Combustion and Flame. 1999.

[7]. Rosser, W. A, Inami, S. H., and Wise, H., Study of the Mechanisms of Fire Extinguishment of Liquid

Rocket Propellants, WADC Technical Report 59-206, 1959.

[8]. Lott, J.L., Christian, S.D., Sliepcevich, C.M., and Tucker, E.E., Fire Technology 32:260 (1996).

[9]. Noto, T., Babushok, V., Hamins, A., and Tsang, W., Combust. Flame 112:147 (1998).

[10]. Rumminger, M.D. and Linteris, G.T., "Inhibition of Premixed Carbon Monoxide-Hydrogen-OxygenNitrogen Flames by Iron Pentacarbonyl", submitted to Combustion and Flame, March 1999.

[11]. Linteris, G.T. and Truett, L., Combust. Flame 105:15 (1996).

[12]. Zhang, J. and Megaridis, C.M., Twenty-Fifth Symposium (International) on Combustion, The Combustion

Institute, Pittsburgh, 1994, pp. 593-600.

[13]. Linteris, G.T., Burgess, D.R., Babushok, V., Zachariah, M., Tsang, W., and Westmoreland, P., Combust.

Flame 113:164 (1998).

[14]. Tsang, W., "Personal Communication, April 1999.
0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600

Ferrocene or Fe(CO)5 Mole Fraction (ppm)

Normalized Burning Velocity

X = 0.214 0.245

Fe(CO)5

Ferrocene

0.21

0.244

O2

φ = 1.0

Tin = 353 K

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10

% CO2 or % N2/2.5

Normalized Burning Velocity

Pure CO2

0.2 % 0.053% Fec in CO2

0.54 %

X = 0.21

CF3Br

O2

Tin = 353 K

φ = 1.0

Pure N2 / 2.5

Figure 1 – Normalized burning velocity of premixed

CH4/O2/N2 inhibited by ferrocene and Fe(CO)5 for

several values of the oxygen mole fraction in the

oxidizer XO2,ox .

Figure 2 - Normalized burning velocity of CH4/O2/N2

flames inhibited by pure CO2 and pure N2, and by

blends of CO2 and ferrocene. The equivalent

percentage of ferrocene in CO2 is given, and data for

CF3Br are shown for comparison.

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10

% CF3H

Normalized Burning Velocity

Pure CF3H

0.35% Fec in CF3H

CF3Br

X = 0.21 O2

Tin = 353 K

φ = 1.0

0.0

0.5

1.0

1.5

0 2 4 6

% CO2 or CF3H

Actual / Predicted Performance

0.54% Fec in CO2

0.35% Fec in CF3H

200% N2 in CO2

Figure 3 - Normalized burning velocity of premixed

CH4/O2/N2 flames inhibited by pure CF3H and by

CF3H with 0.35% ferrocene, together with data for

CF3Br.

Figure 4 – Index of actual performance of inhibitor

blend as compared to the predicted performance

based on a linear sum of the burning velocity

reduction caused by each component.