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Tri-sectional meeting, Combustion Institute, Oakland, CA March 2001

Is CF3Br Representative of Efficient Fire Suppressants?

Bradley A. Williams and James W. Fleming

Chemistry Division, Code 6185

Naval Research Laboratory

Washington, DC 20375-5342

ABSTRACT

CF3Br (Halon 1301) is often used as a standard for measuring the effectiveness of

chemical fire suppressants, since its inhibition properties have been known for many

years, and the chemical kinetics relevant to its suppression of combustion have been

extensively studied. Because of the phaseout of CF3Br due to its contribution to the

depletion of stratospheric ozone, new classes of efficient fire suppressants must be

identified. In recent years, CF3Br has been extensively studied, although its manufacture

has been prohibited in developed countries since 1994. One issue not often addressed is

whether CF3Br is "typical" of efficient fire suppressants in general. To this end, we have

modeled premixed flames containing representative inhibitors of various types, including

inert gases, fluorocarbons, and catalytic scavengers of flame radicals. Calculations show

that although CF3Br is itself a catalytic scavenger, its effect on flame structure and

burning velocity is in many respects more similar to that of the noncatalytic

hydrofluorocarbons than to other catalytic scavengers such as iron and sodium. Details

of the suppression behavior of CF3Br and other representative compounds support the

conclusion that in many combustion environments, knowledge of the behavior of CF3Br

is not necessarily relevant to gaining understanding of efficient fire suppressants in

general.

INTRODUCTION

CF3Br is often used as a benchmark for other chemical fire suppressants, since its

inhibition properties have been known for many years, and the chemical kinetics relevant

to its suppression of combustion have been extensively studied. Because of the phaseout

of CF3Br due to its contribution to the depletion of stratospheric ozone, new classes of

efficient fire suppressants must be identified. In recent years, CF3Br has been studied

both experimentally and computationally, although its manufacture has been prohibited

in developed countries since 1994, with the ban scheduled to take effect worldwide by

2005.

One issue which has not generally been considered is whether CF3Br is "typical"

of efficient fire suppressants in general. In other words, will all promising replacements

for CF3Br have properties which are "similar" in some respect? Is commonality of

properties with CF3Br a useful guide in the search for environmentally acceptable

alternatives? To this end, we have performed modeling studies of premixed flames

containing substances representative of different classes of inhibitors, including inert

gases, fluorocarbons, and catalytic scavengers of flame radicals.
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TYPES OF SUPPRESSANTS

Suppressants may be grouped into various categories. Physical suppressants (such

as nitrogen, argon, CF4, and water) do not participate in flame chemistry to a significant

extent and inhibit combustion by adding heat capacity and diluting the reactants.

Chemical suppressants also participate in the flame chemistry and may be subdivided into

two groups: catalytic suppressants (including bromine, iodine, and various metallic

elements) reduce concentrations of flame radicals through a regenerative cycle (one

molecule of suppressant can recombine several radicals). Noncatalytic suppressants (e. g.

fluorocarbons) also reduce concentrations of flame radicals by scavenging flame radicals,

at least in certain regions of the flame, but do not exhibit a catalytic cycle and are

generally less effective. CF3Br is primarily a catalytic suppressant [1], due to an H + H

recombination cycle involving the bromine atom, although the CF3 moiety adds some

noncatalytic suppression as well.

The effectiveness of an inhibitor may be placed on a quantitative basis by

considering an "inhibition parameter" Φ first proposed by Rosser et al. [2] and modified

by Noto et al. [3]:

Φ = ln(U0/Ui) x (XO2/Xi) (1)

where U0 and Ui are the burning velocities of the uninhibited and inhibited flames,

respectively, and XO2 and Xi are the reactant mole fractions of oxygen and inhibitor. In

ref. [3] the burning velocity of a given fuel/air mixture was shown to exhibit an

exponential dependence on the inhibitor concentration. This exponential dependence, by

which the inhibition parameter was defined, was shown to hold for the inert agents N2

and CF4, several hydrofluorocarbons, and CF3Br. Since catalytic scavengers intrinsically

exhibit saturation effects (the concentrations of flame radicals, which typically exist in

superequilibrium, cannot be reduced below the thermal equilibrium values at the local

flame temperature) the existence of a similarity relation describing inhibition by CF3Br as

well as noncatalytic agents is somewhat unexpected. Saso et al. [4] modeled the

combined effect of CF3Br/inert inhibitors and found synergism (the two agents in

combination had a greater inhibiting effect than the additive effect of the agents in

isolation). The behavior was attributed to a temperature effect on the inhibition

effectiveness of CF3Br, rather than a saturation phenomenon. Over a range of adiabatic

flame temperatures, Saso et al. found CF3Br to have virtually identical inhibition

parameters at concentrations of 0.5% and 1% in methane/oxygen/inert mixtures, once

again suggesting the absence of significant saturation effects.

MODELING RESULTS: BURNING VELOCITIES AND SPECIES PROFILES IN

INHIBITED PREMIXED FLAMES

In the calculations described below, the PREMIX code [5] was used to compute

burning velocities and flame structures of atmospheric pressure flames. An 85 cm
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domain was used in the calculations. Comparison with thermal equilibrium calculations

showed that at the final grid point, temperatures were generally within 5K and radical

concentrations within a few percent of their equilibrium values. Flames were

stoichiometric, atmospheric pressure methane/oxygen, with various inhibitors added as

indicated below.

For the hydrocarbon chemistry, GRIMech2.11 was used as the kinetic

mechanism. Fluorocarbon chemistry was based on mechanism refinement previously

performed in our laboratory [6,7]. Bromine chemistry used for CF3Br was that developed

by Noto et al. [4]. The kinetic mechanism for sodium was based on that of Zamansky et

al. [8], while that for Fe(CO)5 was described by Rumminger and Linteris [9].

In Figs. 1-3, changes in burning velocity, adiabatic flame temperature, radical

superequilibrium, as well as the inhibition parameter Φ as defined by Noto et al., are

plotted as a function of inhibitor concentration. Note that a constant value of Φ indicates

that the exponential dependence of flame speed on inhibitor concentration is well

observed. The three plots compare the changes in the flame structure caused by CF3Br

with that of nitrogen (an inert agent) and sodium hydroxide (a catalytic agent). Nitrogen

has a modest inhibition parameter (approximately 0.5) nearly independent of its

concentration. The adiabatic flame temperature must be lowered by nearly 300K for the

burning velocity to be reduced by 50%. While the peak concentrations of the flame

radicals H, O, and OH decrease as the flame is inhibited, the peak concentrations

normalized by the equilibrium values at the adiabatic flame temperature increase. This

phenomenon provides a basis for the synergy observed between a catalytic and a physical

agent, since a flame inhibited by a physical agent has a greater radical superequilibrium

which the catalytic agent can exploit.

In the flame inhibited by NaOH, by contrast, the flame speed is reduced by 50%

with virtually no change in the adiabatic temperature. The radical superequilibrium is

drastically reduced as the inhibitor is added, eventually reaching unity for all primary

flame radicals for a sufficiently large inhibitor concentration (meaning that the radical

concentrations never overshoot their equilibrium values). Most notably, the inhibition

parameter is not constant as a function of inhibitor concentration, but varies by more than

a factor of three over the range of sodium concentrations considered here. The similarity

relationship identified by Noto et al. does not hold for sodium.

For inhibition by CF3Br, the flame speed is again reduced with little change in the

adiabatic temperature (about 30K for the range of concentrations shown in the plot). The

degree of radical superequilibrium is reduced with increasing agent concentration, though

not as dramatically as for sodium. By both of these measures, CF3Br shows behavior

typical of catalytic agents. The inhibition parameter, unlike for sodium, is nearly

constant as a function of inhibitor concentration (except for the initial data point at an

inhibitor concentration of 0.1% which has a high uncertainty due to the small differences

in flame speeds). This nearly constant inhibition parameter was found by Noto et al.

although the modifications to the fluorocarbon kinetics yield somewhat higher inhibition

parameters, in excellent agreement with the experimental value of 14.0 [4].

This raises the question of why the exponential relationship between burning

velocity and inhibitor concentration is observed for CF3Br but not for other chemical

inhibitors. The present results support the conclusion of Saso et al. that saturation is a

minor effect in the suppression effectiveness of CF3Br/inert mixtures. One point is that
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for inhibition by CF3Br, the H atom superequilibrium is only reduced by some 30% for a

50% reduction in flame speed, whereas for NaOH the H atom superequilibrium is

reduced by nearly 80% for the same reduction in flame speed. Thus saturation is not as

pronounced for CF3Br in part because the peak radical concentrations are still far out of

equilibrium even when the burning velocity has been substantially reduced.

The other important point for CF3Br is that the bromine catalytic cycle involves

the sequence of reactions [10]

Br + Br + M => Br2 + M (1)

H + Br2 => HBr + Br (2)

as an important pathway in regeneration of HBr, since the direct reaction

H + Br + M => HBr + M (3)

has rather slow kinetics. This has two consequences: the increasing importance of

Reaction (1) (since it has a second order dependence on the inhibitor concentration)

compensates for the saturation effect as the inhibitor concentration is increased. Also, a

much higher concentration of bromine is required for this reaction to be significant.

Compared to sodium and iron, whose suppression kinetics are not thought to depend

significantly on any second order pathways, bromine is a relatively inefficient scavenger.

COMBINED EFFECTS OF MIXTURES OF AGENTS

Since various studies have described synergism of combinations of CF3Br and

inert agents, it is worth exploring to what extent the observations carry over to other

combinations of agents. Furthermore, is there any advantage to combining different

chemical scavengers which act independently (i. e. recombine different flame radicals)?

To this end, inhibition by iron was modeled in combination with both nitrogen

and sodium. For this modeling, iron was considered to participate in a three step

mechanism involving only O + O recombination. This pathway was identified by

Rumminger and Linteris [9] but is usually secondary in importance to an H + H

recombination pathway. Using this simplified model, sodium and iron do not directly

compete for the same flame radicals, since sodium primarily undergoes an H + OH

recombination cycle. This model is not intended as an accurate description of iron's

combustion chemistry but to investigate the combined effect of scavengers which operate

through different pathways.

In Table 1, the effects of iron and sodium both separately and in combination, are

listed. The effectiveness of the inhibitors when combined are described by their

"differential inhibition parameters." The differential inhibition parameter of substance A

is determined by comparing the burning velocity of a mixture inhibited by A and B in

combination, to that of the mixture inhibited by B alone. As seen in the Table, the

differential inhibition parameters of iron and sodium in combination are much less than

those of the two elements taken separately. Even though the two elements do not directly
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compete for the same flame radicals, the rapid exchange between H, O, and OH means

that the two suppressants reduce each other's effectiveness.

Iron, in combination with nitrogen, does exhibit synergy. Figure 4 shows the

burning velocity, flame temperature, and differential inhibition parameter of iron (using

the O + O recombination mechanism) as a function of nitrogen addition to a methane/air

mixture. The differential inhibition parameter of 500 ppm FeO2 increases by some 23%

as the flame temperature is lowered by nitrogen addition from 2230K to 2010K. These

results may be compared to those of Saso et al. [4], who found that the inhibition

parameter of CF3Br increased by some 45% (at both 0.5% and 1.0% mole fractions of

agent) over the same range of final flame temperatures. While the findings of synergism

agree qualitatively, the final flame temperature influences the inhibition parameter of

CF3Br twice as much as that of the O + O recombination cycle of iron. Therefore while

synergism in catalytic/inert mixtures may be a general characteristic, its magnitude can

vary considerably depending on the kinetics of the catalytic component.

CORRELATION OF BURNING VELOCITIES, FINAL FLAME

TEMPERATURE, AND RADICAL CONCENTRATIONS

According to laminar flame theory, the burning velocity of a premixed flame is

proportional to the square root of the overall reaction rate. In modeling the burning

velocity of a large number of inhibited flames, the burning velocity correlates in almost

all cases with the product of the peak H atom mole fraction and a global activation

energy:

SL

2

 = A

2XHexp(-Ea/kTf). (2)

where SL is the laminar burning velocity, XH the peak mole fraction of atomic hydrogen,

and Tf the final flame temperature. The fitted parameters A and Ea, obtained by

considering the flame inhibited by nitrogen, are A=7940 cm/s, Ea=24.7 kcal/mol. The

same empirical correlation holds for inhibition by other inert gases, by iron and sodium,

by nitrogen and iron in combination (Fig. 4), and by artificially increasing the H + OH

recombination rate. For the catalytic agents, which do not cause appreciable changes in

the final flame temperature, the flame speed correlates with the peak mole fraction of

atomic hydrogen, but not with that of atomic oxygen or OH radicals.

The observation that atomic hydrogen controls the burning velocity is not

unexpected, due to the high diffusivity and reactivity of this species. Nevertheless, some

noteworthy conclusions can be made: 1) this correlation can quantitatively describe the

effect of a wide variety of inhibitors, both individually and in combination; 2) catalytic

agents reduce the flame speed by reducing the atomic hydrogen concentration, whether or

not the scavenging cycle directly involves H atom recombination. The partial

equilibrium between H, O, and OH is not disrupted in most inhibited flames.

Table 2 compares the burning velocities estimated using Eqn. (2) to the calculated

values for a variety of flame inhibitors. In almost all cases, the reduction in burning

velocity relative to the uninhibited flame, predicted by Eqn (2) is within 10% of the

actual value using the full kinetics. This relationship between burning velocity, final
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flame temperature, and peak H atom mole fraction holds for many agents (both catalytic

and inert) and also when one artificially increases the H + OH recombination rate. At the

same time, there are a few notable exceptions: the fluorocarbons CHF3 and C3HF7 are

much better inhibitors (by nearly a factor of two) than Eqn (2) would predict. So are

both CF3Br and HBr. Flame structure modeling indicates that the breakdown of this

relationship involving the peak H atom concentration is a consequence of these agents

reducing the H atom concentration mostly in the early part of the flame, as demonstrated

below.

CHANGES IN FLAME STRUCTURE DUE TO DIFFERENT AGENTS

The explanation for why the correlation between flame speed, H atom

concentration, and temperature does not hold for either fluorocarbons or bromine

containing compounds can be gained from examination of these compounds’ effects on

flame structure. In Fig. 5 the mole fraction of atomic hydrogen is plotted against the

local temperature for flames inhibited by the inert agent N2, the catalytic agent Fe(CO)5,

the noncatalytic fluorocarbon CF3CHFCF3, and CF3Br. All the inhibited flames have

burning velocities approximately 50% that of the uninhibited flame, whose structure is

also plotted for comparison. For all the flames considered here, temperature overshoot

does not occur; the temperature monotonically increases with position passing from

reactants to products.

The way in which the relationship between H atom mole fraction and local

temperature is altered is characteristic of each type of agent. Other agents modeled (not

shown on the plot for clarity) produce H atom profiles which closely resemble each other

within the same class: CF4 and N2, NaOH and Fe(CO)5, CH2FCF3 and CF3CHFCF3. The

physical agents reduce the final flame temperature, but the H atom mole fraction at a

given isotherm (above approximately 1300K) is changed very little from its value in the

uninhibited flame. Both iron and sodium reduce the H atom mole fraction by a relatively

constant factor throughout the reaction zone, in other words the inhibition occurs

throughout the flame. The fluorocarbons, on the other hand, reduce the H atom mole

fraction early in the flame (in the region below about 1200K) but have relatively little

impact on the peak concentration. The same situation occurs for CF3Br, in this respect

CF3Br bears more resemblance to nonbrominated fluorocarbons than it does to other

catalytic agents such as iron and sodium.

The depletion of radical species early in the flame has a marked influence on the

flame speed. It is for this reason that fluorocarbons and bromine compounds are better

inhibitors than the changes in temperature and peak H atom concentrations would predict.

This observation implies that agents which deplete radicals in high temperature regions

but not early in the flame are likely to be less effective inhibitors than would otherwise be

expected. This may be the case, for instance, for condensed-phase agents which must

undergo a vaporization process before inhibition chemistry can begin.

CONCLUSIONS
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Is CF3Br representative of efficient fire suppressants? The short answer is that it

depends on what CF3Br is to be used for. From a practical perspective, using CF3Br as a

benchmark makes sense, since the fire protection community has widespread experience

with this agent and stating the efficiency of a proposed replacement on a relative basis to

that of CF3Br has merit. At the same time, not all other efficient suppression agents are

“like CF3Br” in either the details of their suppression chemistry or in many cases in the

qualitative aspects of their behavior, particularly for condensed phase agents.

We thank V. Babushok and G. Linteris for helpful discussions and for providing

kinetic mechanisms used here. This work is part of the Department of Defense NextGeneration Fire Suppressant Technology Program, funded by the DoD Strategic

Environmental Research and Development Program.

REFERENCES

1. Sheinson, R. S., Penner-Hahn, J. E., and Indritz, D., (1989). The Physical and

Chemical Action of Fire Suppressants, Fire Safety J. 15:437-450 (1989).

2. Rosser, W. A.., Wise, H., and Miller, J., Mechanism of Combustion Inhibition by

Compounds Containing Halogen, Proc. Combust. Inst. 7:175-182 (1958).

3. Noto, T., Babushok, V., Hamins, A., and Tsang W., Inhibition effectiveness of

halogenated compounds, Combust. Flame 112:147-160 (1998).

4. Saso, Y., Ogawa, Y., Saito, N., and Wang, H., Binary CF3Br- and CHF3-inert flame

suppressants: Effect of temperature on the flame inhibition effectiveness of

CF3Br and CHF3, Combust. Flame 118:489-499 (1999).

5. Kee, R.J., Grcar, J.F., Smooke, M.D., and Miller, J.A., PREMIX: A Fortran Program

for Modeling Laminar One-Dimensional Premixed Flames, Sandia Report SAND85-

8240 (1985).

6. L’Espérance, D., Williams, B. A., and Fleming, J. W., Intermediate Species Profiles in

Low Pressure Premixed Flames Inhibited by Fluoromethanes, Combust. Flame 117:709-

731 (1999).

7. Williams, B. A., L' Espérance, D. M., and Fleming, J. W., Intermediate Species

Profiles in Low Pressure Methane/Oxygen Flames Inhibited by 2-H Heptafluoropropane:

Comparison of Experimental Data with Kinetic Modeling, Combust. Flame 120:160-172

(2000).

8. Zamansky, V. M., Lissianski, V. V., Maly, P. M., Ho, L., Rusli, D., and Gardiner, Jr.,

W. C., Reactions of Sodium Species in the Promoted SNCR Process, Combust. Flame

117:821-831 (1999).
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9. Rumminger, M. D., and Linteris, G. T., Inhibition of Premixed Carbon MonoxideHydrogen-Oxygen-Nitrogen Flames by Iron Pentacarbonyl, Combust. Flame 120:451–

464 (2000).

10. Westbrook, C. K., Combust. Sci. Technol. 23:191-202 (1980).

Table 1

Inhibition by Two Chemical Agents in Combination

Flame: stoichiometric methane/air, atmospheric pressure

Agent Flame Speed (cm/s) Inhibition Parameter Φ [Eqn. (1)]

none 39.6 ---

0.2% (NaOH)2 17.0 40.1a

0.1% FeO2 29.5 56.1

0.1% FeO2 + 0.2%(NaOH)2 14.9 24.9(Fe), 32.4(Na)b

a

 per sodium atom

b

 differential effect of specified inhibitor relative to other inhibitor alone.
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Table 2

Comparison of Calculated Flame Speeds to Values from Eqn. (2)

Seqn = A(XH,max*exp(-Ea/kTad))1/2 A=7940 cm/s Ea=24.7 kcal/mol

flame condition Tadiabatic XHmax Spremix Seqn ΔSpremix/ΔSeqn

a

CH4/air (uninhibited) 2234 6.64e-3 39.8 39.8 ---

+3.85% N2 2187 5.96e-3 35.5 35.6 1.01

+9.09% N2 2121 5.12e-3 30.2 30.2 1.00

+13.04% N2 2065 4.45e-3 26.1 26.0 0.99

+16.67% N2 2015 3.94e-3 22.6 22.7 1.01

+8.26% CF4 (inert) 1968 3.50e-3 18.4 19.9 1.07

+500ppm FeO2 (O+O cycle) 2220 2.52e-3 33.3 32.7 0.92

+0.5%HBr (H+H cycle) 2221 5.36e-3 30.3 35.2 2.08

+0.5%HBr (H+OH cycle) 2220 5.47e-3 33.3 35.5 1.52

+5.2%CHF3 (phi=1.1) 2036 4.04e-3 16.3 23.7 1.46

+3%C3HF7 (phi=0.9) 2161 3.86e-3 17.8 27.7 1.81

+0.05% (NaOH)2 2219 2.97e-3 28.0 26.2 0.87

+0.40%(NaOH)2 2177 3.31e-4 9.4 8.3 0.97

increase H + OH + M rate:

x10 2234 4.04e-3 32.0 31.1 0.90

x100 2234 1.35e-3 19.8 18.0 0.92

x1000 2234 3.64e-4 11.2 9.3 0.94

a

 (39.8-Spremix)/(39.8-Seqn)
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Figure 1: Flame speed, final flame temperature, inhibition parameter (defined in Ref. [3]),

and superequilibrium concentrations of flame radicals computed for atmospheric pressure

stoichiometric methane/air mixtures inhibited by nitrogen.
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Figure 2: Flame speed, final flame temperature, inhibition parameter (defined in Ref. [3]),

and superequilibrium concentrations of flame radicals computed for atmospheric pressure

stoichiometric methane/air mixtures inhibited by sodium hydroxide. To simulate the

evaporation process, NaOH was added to the reactants as a dimmer and required to

undergo an endothermic decomposition process before inhibition chemistry could occur.
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Figure 3: Flame speed, final flame temperature, inhibition parameter (defined in Ref. [3]),

and superequilibrium concentrations of flame radicals computed for atmospheric pressure

stoichiometric methane/air mixtures inhibited by CF3Br.
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Figure 4: Flame speed, differential inhibition parameter (see text), and superequilibrium

concentrations of atomic hydrogen computed for atmospheric pressure stoichiometric

methane/air mixtures inhibited by nitrogen and FeO2 in combination.
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Figure 5: Mole fraction of atomic hydrogen as a function of local flame temperature for

an uninhibited atmospheric pressure methane/air flame, and the same flame inhibited by

various classes of suppression agents.