Approved for public release; distribution is unlimited.

Naval Research Laboratory

Washington, DC 20375-5320

December 19, 2003

NRL/MR/6180--03-8728

Influence of Bond Energies on

Catalytic Flame Inhibition: Implications

for the Search for New Fire Suppressants

BRADLEY A. WILLIAMS

JAMES W. FLEMING

Navy Technology Center for Safety and Survivability

Chemistry Division
i

REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

3. DATES COVERED (From - To)

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39.18

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and

maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including

suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway,

Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of

information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE

4. TITLE AND SUBTITLE

6. AUTHOR(S)

8. PERFORMING ORGANIZATION REPORT

NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR / MONITOR’S ACRONYM(S)

11. SPONSOR / MONITOR’S REPORT

NUMBER(S)

12. DISTRIBUTION / AVAILABILITY STATEMENT

13. SUPPLEMENTARY NOTES

14. ABSTRACT

15. SUBJECT TERMS

16. SECURITY CLASSIFICATION OF:

a. REPORT

19a. NAME OF RESPONSIBLE PERSON

19b. TELEPHONE NUMBER (include area

code)

b. ABSTRACT c. THIS PAGE

18. NUMBER

OF PAGES

17. LIMITATION

OF ABSTRACT

December 19, 2003

Influence of Bond Energies on Catalytic Flame Inhibition: Implications for the

Search for New Fire Suppressants

Bradley A. Williams and James W. Fleming

Naval Research Laboratory, Code 6180

4555 Overlook Avenue, SW

Washington, DC 20375-5320

We have conducted computational investigations of the conditions necessary for a substance to be an efficient flame suppressant. Of particular

interest is identification of additional chemical elements, which might be expected to have promising suppression properties, particularly by

catalytic scavenging of flame radicals. Bromine, sodium, and iron are used as examples of prototypical H + H, H + OH, and O + O scavenging

cycles, respectively. For each element, a simplified kinetic mechanism involving a single scavenging cycle was employed. The effect of hypothetical changes in bond dissociation energies on reduction of burning velocity of a premixed methane/air flame by these elements is determined.

Efficient radical scavenging is possible only if a suppressant atom or radical binds to one of the primary flame radicals (H, O, or OH) with a bond

energy in the range of 70-100 kcal/mole.

Unclassified Unclassified Unclassified

UL 20

Bradley A. Williams

(202) 767-3583

Halon; Suppression; Fire; Catalytic; Inhibition; Kinetic mechanism

Office of Naval Research Tactical Technology Office

800 North Quincy Street Defense Advance Research Projects Agency

BCT 1 301 North Fairfax Drive

Arlington, VA 22217-5660 Arlington, VA 22203-1714

NRL/MR/6180--03-8728

Approved for public release; distribution is unlimited.
CONTENTS

INTRODUCTION .................................................................................................................................... 1

GENERAL FEATURES OF CATALYTIC SCAVENGING CYCLES ................................................... 3

COMPUTATIONAL METHODOLOGY ................................................................................................ 4

KINETIC MECHANISMS OF SPECIFIC INHIBITORS ....................................................................... 5

INFLUENCE OF BOND ENERGIES ON SUPPRESSION CYCLES ................................................... 8

Bromine: H + H and H + OH Recombination .................................................................................... 10

Sodium: OH + H Recombination ....................................................................................................... 12

Iron: O + O Recombination ................................................................................................................ 13

DISCUSSION........................................................................................................................................... 14

REFERENCES ......................................................................................................................................... 16

iii
INTRODUCTION 

A principal goal of fire suppression research in recent years has been the 

identification of environmentally benign fire suppressants having efficiency comparable

to the stratospheric ozone depleting Halon 1301 (CF3Br). The suppression effectiveness 

of CF3Br and related compounds is primarily due to the chemical activity of bromine [1-

3], which participates in a catalytic cycle and reduces concentrations of radical species 

involved in flame propagation. 

Comparison of CF3Br to agents that extinguish fires through physical effects, 

indicates that, with the unique exception of liquid water [4,5] suppression properties must 

be as good or better than Halon 1301) are likely to possess chemical suppression 

properties. Furthermore, high chemical suppression efficiency is most easily achieved 

via a catalytic cycle [6], whereby each molecule of suppressant recombines several flame 

radicals. 

Many chemical elements besides bromine have been shown to exhibit catalytic 

radical scavenging in flames; several show exhibit catalytic suppression behavior much 

stronger than that of bromine [7], although practical issues have limited their 

implementation. Among the halogens, iodine as well as bromine is a good catalytic 

scavenger [8,9]. Among non-halogens, good suppression efficiency has been 

demonstrated for alkali metals [10], phosphorus [11], and several transition metals 

including iron [12], lead [13], chromium [14], and manganese [15]. The elements 

comprising this list vary greatly in their chemical properties, while other elements in the

same group of the periodic table (e.g. nitrogen, fluorine) do not cause efficient catalytic 

suppression. 

Manuscript approved Nov. 10, 2003.

 1
Finding a common basis for efficient chemical suppressants, whose results can be 

extrapolated to other elements whose combustion chemistry has not yet been thoroughly

explored, is desirable. Previous work has pointed to binding energy of a suppressant to 

one or more flame radicals as an important parameter [16-18]. This approach has been 

used previously to explain, for instance, the greater suppression effectiveness of bromine 

compared to chlorine. [16]. The present paper broadens the scope of the previous work 

by focusing on a common thermodynamic basis for catalytic suppression by different 

families of elements, and different types of catalytic cycles. The rationale for revisiting

the effect of bond energies at the present time is threefold: 

1) Computational tools now exist to model the effect of thermodynamics directly, 

rather than resorting to simplifying approximations such as zone modeling of flame

structure [17] or partial equilibrium assumptions of species concentrations [16]. 

2) The current research need is to assess the fire suppression potential of elements 

that have not yet been extensively investigated, rather than to explain the behavior of 

elements whose properties are already (empirically) known. By contrast, the work which 

elucidated the suppression kinetics of bromine was performed after halons were 

identified and implemented as fire suppressants. 

3) A great deal of additional chemical kinetic data is now available, including

provisional suppression mechanisms for a number of non-halogen species.

In the calculations presented here, simplified but representative catalytic cycles 

for known inhibitors (bromine, sodium, iron) are used in conjunction with detailed kinetic 

modeling of the hydrocarbon flame structure. 

 2
GENERAL FEATURES OF CATALYTIC SCAVENGING CYCLES

In most flames, the peak concentrations of the radicals H, O, and OH are much 

higher than the thermal equilibrium concentrations at the adiabatic flame temperature. 

This is a consequence of the kinetic rates of the chain branching reactions, for example 

H + O2 => OH + O (R1) 

O + H2 => OH + H, (R2) 

exceeding the rates of chain termination reactions such as 

OH + H + M => H2O + M (R3) 

O + O + M => O2 + M (R4) 

H + H + M => H2 + M, (R5) 

which require a third body for stabilization of the product [19]. Chemical fire 

suppressants generally participate in a catalytic cycle whose net effect is equivalent to 

one of the recombination reactions. 

For a substance to scavenge flame radicals, there must exist some species X 

derived from the inhibitor which can bond to H, O, or OH. The species XR, where R is 

one of the flame radicals, must react with another flame radical R' to form X and RR', 

where RR' is usually a stable molecule such as H2, H2O, or O2. Possible scavenging

reactions include [20]: 

XH + H => X + H2 (R6) 

 XH + OH => X + H2O (R7) 

 XOH + H => X + H2O (R8) 

XO + O => X + O2 (R9) 

 3
To complete the cycle, X must then be converted back to XR through one or more 

steps. For significant inhibition to occur, the kinetic rate of the scavenging cycle must 

exceed that of the equivalent direct recombination reaction. 

Inhibition by bromine occurs primarily through (R6) and to a lesser extent (R7) 

[21], while alkali metals operate primarily through (R8) [22]. Reaction (R9) is likely to 

occur for metallic elements [23]. Some elements, including iron [23] and phosphorus 

[24], are thought to participate in more than one cycle. The object of the present study is 

to investigate the influence of dissociation energies of the species XR on suppression 

behavior. 

 

COMPUTATIONAL METHODOLOGY 

The computer code PREMIX [25] was used to model freely propagating

atmospheric pressure premixed flames. The baseline condition consisted of 

stoichiometric methane/air, with an initial reactant temperature of 298.2 K. Tolerances 

for mesh refinements in PREMIX were set as follows: GRAD = 0.10, CURV = 0.20, up 

to a maximum of 150 grid points. Multicomponent viscosities and thermal diffusion 

(Soret effect) were used. The computational domain extended 25 cm from the flame into

the reactants gases, and 60 cm into the exhaust gases. The domain was extended in such a 

way that on both boundaries, the second last grid point was 1 cm from the boundary. 

The reaction set given in GRI-Mech 2.11 [26] (without nitrogen chemistry) was 

used for modeling the uninhibited flames. Calculations were carried out using bromine, 

sodium, and iron as examples of chemical inhibitors. Kinetics of chemical inhibition for 

these elements were added to the hydrocarbon mechanism as described below. 

 4
 

KINETIC MECHANISMS OF SPECIFIC INHIBITORS 

Inhibition by Halon 1301 (CF3Br) and the associated radical scavenging involving

bromine has been the object of a number of both experimental and modeling studies. 

Mechanisms have recently been proposed for alkali metals, and for iron, although the 

current validation status of these models is more tentative than for bromine. 

The inhibition cycle of bromine is believed to proceed by H atom scavenging

[16,17]. The reaction 

HBr + H => Br + H2 (R10) 

is thought to be the primary scavenging reaction, with the reaction with OH

 HBr + OH => Br + H2O (R11) 

also contributing. The conversion of Br to HBr involves several pathways. The direct 

recombination 

H + Br + M => HBr + M (R12) 

has rather slow kinetics and is typically not the predominant pathway. Atomic bromine

can abstract hydrogen atoms from such species as HO2, HCO and CH2O; these play an 

important role in most flames. 

We constructed two simplified kinetic mechanisms describing inhibition by

bromine. Both mechanisms include Br and HBr as the only bromine-containing species. 

For these calculations HBr was chosen as the bromine containing reactant. The 

mechanisms are listed in Table 1. In the first mechanism HBr is assumed to react only

with atomic hydrogen to form Br, which is converted back to HBr by a direct 

recombination with H, as well as abstraction of H from HO2, HCO, and CH2O. The 

 5
Table 1 

Kinetic mechanisms for suppressants

k = ATbexp(-Ea/kT) A(cm3,mol,s) b Ea(cal/mol) Remark. 

Mechanism for Na (H + OH cycle only-Fig. 9) 

(NaOH)2+M=NaOH+NaOH+M a 3.0E14 0.0 48000 

NaOH+H=Na+H2O 1.0E13 0.0 1970 

Na+OH+M=NaOH+M a 1.8E21 -1.0 0 

Mechanism for Br (reactions in common for all cycles) 

H + Br + M = HBr + M a 1.92E21 -1.86 0 b

Br + CH2O = HBr + HCO 1.02E13 0.0 1590 c

Br + HO2 = HBr + O2 8.43E12 0.0 1172 c 

Br + HCO = HBr + CO 1.69E14 0.0 0 d 

Addition for Br (H + H cycle-Fig. 7): 

HBr + H = H2 + Br 6.25E13 0.0 2405 b 

Addition for Br (H + OH cycle-Fig. 8): 

HBr +OH = H2O+ Br 6.62E12 0.0 0 e 

Mechanism for Fe (O + O cycle-Fig. 10):

Fe + O2 + M = FeO2 + M a 1.57E18 0.0 4050 f

FeO2 + O = FeO + O2 1.73E13 0.0 0 estimated 

FeO + O =Fe + O2 1.73E13 0.0 0 g

a

 Third body efficiencies: H2:2 H2O:6 CH4:2 CO:1.5 CO2:2 C2H6:3 

b

 Baulch, D.L., Duxbury, J., Grant, S.J., and Montague, D.C., J. Phys. Chem. Ref. Data 10: 

Suppl (1981). c

 Atkinson, R., Baulch, D.L., Cox, R.A., Hampson, R.F., Jr., Kerr, J.A., Rossi, M.J., and 

Troe, J. J. Phys. Chem. Ref. Data 26:521-1011 (1997). 

d

 Poulet, G., Laverdet, G., and LeBras, G., J. Chem. Phys. 80:1922 (1984). e

 DeMore, W.B., Sander, S.P., Golden, D.M., Hampson, R.F., Kurylo, M.J., Howard, C.J., 

Ravishankara, A.R., Kolb, C.E., and Molina, M.J., Chemical kinetics and photochemical 

data for use in stratospheric modeling. Evaluation number 12, JPL Publication 97-4: 1-266 

(1997). 

f

 Helmer, M. and Plane, J.M.C., J. Chem. Soc. Faraday Trans. 90:395-401 (1994). 

g based on reverse reaction: Akhmadov, U.S., Zaslonko, I.S., and Smirnov, V.N., Kinet. 

Catal. 29:251 (1988). 

 6
second mechanism is identical, except that HBr is allowed to react only with OH, rather

than H. 

The inhibition chemistry of iron has been investigated by Rumminger and Linteris 

[23], who identified an H + H =>H2 scavenging cycle as the most important one in 

hydrocarbon flames. In addition, they identified an O + O => O2 cycle, which can be 

significant in moist CO flames. We use the O + O cycle of iron identified by Rumminger

and Linteris as an example. The cycle has three steps:

 Fe + O2 + M => FeO2 + M, (R13) 

 FeO2 + O => FeO + O2, (R14) 

FeO + O => Fe + O2. (R15) 

This mechanism is not intended as an accurate depiction of the overall inhibition 

chemistry of iron, only the contribution of the O + O scavenging cycle. Analogous O + 

O cycles may exist for other transition metal elements as well. In our modeling, FeO2 is 

considered as the reactant. We have assumed that the kinetic rate of (R14), which has not 

been reported, is equal to that of (R15). 

Sodium and other alkali metals, were proposed by Jensen and Jones [22] to react 

primarily through the cycle:

X + OH + M => XOH + M (R16) 

XOH + H => X + H2O. (R17) 

The simplified mechanism for sodium consisted of (R16) and (R17), together with 

a reaction converting the NaOH dimer (specified as the reactant) to the monomer to 

simulate an evaporation step [27]. Comparisons of thermal equilibrium calculations with 

NaOH(s) and (NaOH)2 as reactants indicate that the very minor reduction in the adiabatic

 7
flame temperature caused by the addition of NaOH dimer is about 70% that of adding an 

equal amount of solid NaOH. 

Using the simplified mechanism, the predicted flame speed of a stoichiometric

methane/air mixture inhibited by 0.1% mole fraction of (NaOH)2 was 25.0 cm/s, 

compared to 23.1 cm/s using a more comprehensive mechanism [27] adapted from 

Zamansky et al. [28], and 39.4 cm/s for the uninhibited flame. This result indicates that 

the OH + H cycle identified by Jensen and Jones [22] is indeed the most important for 

suppression of near stoichiometric hydrocarbon flames. 

INFLUENCE OF BOND ENERGIES ON SUPPRESSION CYCLES

For an efficient scavenging cycle to exist, the bond energy between the 

scavenging atom or radical X and a flame radical R must satisfy certain conditions. If it 

is too high, the scavenging reaction (R6)-(R9) will be endothermic. This is the case for 

fluorine, which binds irreversibly to hydrogen and thus cannot sustain a catalytic cycle. 

On the other hand, if the bond is too weak, equilibrium between XR and X + R will be so 

far toward dissociation that there will be insufficient XR to participate in the scavenging

reactions. Stated another way, the rates of the reverse reactions should be small 

compared to those of the forward reactions, since any catalytic recombination cycle will 

become a catalytic chain branching cycle if it runs backwards. 

These considerations indicate that there will be a limited range of bond energies 

for which an efficient scavenging cycle can exist. Putting these qualitative arguments on 

a quantitative basis by computational investigation of the effect of hypothetical changes 

in bond energies on suppression efficiencies allows the determination of the bond 

 8
energies compatible with efficient suppression. Suppression depends on kinetic as well 

as thermodynamic factors, so appropriate bond energies by themselves do not guarantee 

good suppression properties. The elements chosen here as examples, however, are 

known to have good suppression properties, so they possess kinetics favorable to 

scavenging. 

Bromine was used as the example element for Reactions (R6) and (R7), sodium 

for (R8), and iron for (R9). All of these elements have several bond energies which may

be hypothetically relevant to combustion. In order to simplify the situation so that 

sensitivity to bond energy can be determined in a relatively straightforward way, it was 

assumed that only one catalytic cycle involving one type of bond existed for each 

element. 

For each set of calculations, the thermodynamic functions for the species XR

were not altered, but heat of formation of the species X at 298K was varied, while 

keeping the heat capacity and entropy unchanged. This has the effect of changing the 

dissociation energy of X-R. In the kinetic mechanisms, the chemical reactions for the 

scavenging cycle were written in the exothermic direction, and the Arrhenius parameters 

of the forward reaction were unchanged. All reactions are assumed to be reversible, 

however, so changing the heat of formation alters the activation energy of the reverse 

reaction, even without any explicit changes to the kinetic parameters. 

In the PREMIX code, the facility exists for determining the sensitivity of the

solution vector, including the flame speed eigenvalue, to changes in the heats of 

formation of species (HSEN keyword). This is distinct from sensitivities with respect to 

changes in the kinetic rates (ASEN keyword) which are more commonly reported in 

 9
modeling studies. Sensitivity analysis, however, only provides the first derivative of the

flame structure with respect to thermodynamic or kinetic quantities. As seen below, the

dependence of suppression efficiency on heats of formation is non-monotonic. A small 

sensitivity coefficient for the enthalpy of a particular species on the flame speed may

mean that the species in question is unimportant to suppression, or it could mean that its 

enthalpy is very close to the optimal value, leading to a vanishing first derivative.

Bromine: H + H and H + OH Recombination 

For this series of calculations HBr was chosen as the bromine containing reactant. 

The kinetic mechanism was chosen to model the effect of a single catalytic mechanism, 

with Br and HBr as the only bromine species. Two sets of calculations were performed, 

the difference being whether HBr was assumed to react with H or with OH. Fig. 1 shows 

the predicted flame speed of a stoichiometric methane/air mixture inhibited by an 0.5%

mole fraction of HBr. The catalytic cycle is most efficient for H-Br bond energies 

between 65 and 90 kcal/mole. 

The accepted value of the H-Br bond energy lies near the upper limit of the range 

which permits an efficient catalytic cycle (Fig. 1). The bond energy of H-I is near the 

lower limit of the optimal range, while the H-Cl bond is too strong for good suppression. 

The bond energy of H-F (135 kcal/mole) is so high that it is completely inert in this 

environment. Although kinetics of the analogous reactions are somewhat different for 

the other halogens, the bond energy factor by itself leads to the correct prediction that 

iodine has a suppression effect nearly equal that of bromine [9], while chlorine has a 

much smaller effect [3].

 10
The peak efficiency of the cycle assuming an H + OH net recombination is lower 

than for the H + H cycle, in accord with analysis [21] of detailed modeling of the 

pathways of CF3Br, in which the reaction of HBr with H is found to be more important 

than the reaction with OH. It is noteworthy that at the value of the bond energy of H-Cl, 

the H + OH cycle is more efficient than the H + H cycle, due to the higher dissociation 

energy of water compared to molecular hydrogen. This indicates that the (small)

catalytic effect of HCl is primarily due to its reaction with OH rather than with H. 

Fig. 1. Calculated flame speed as a function of H-Br bond energy (see text) for 

premixed methane/air flames inhibited by 0.5% HBr using the H + H and H + 

OH scavenging cycles of Table 1. The accepted values for the bond energies of HBr, H-Cl, and H-I are indicated by vertical lines. 

 11
Sodium: OH + H Recombination 

 The effect of hypothetical variation of the Na-OH bond energy is illustrated in 

Fig. 2. There is a more pronounced falloff in suppression efficiency with bond energy

than for either of the bromine scavenging cycles. It is seen that the Na-OH bond energy

is close to the optimal value for good suppression, as are the bond energies for all the 

other alkali metals except lithium. Experimental studies have found that potassium is a 

considerably more efficient suppressant than sodium [7]. In view of the similarity of the 

Fig. 2. Calculated flame speed as a function of Na-OH bond energy (see text) 

for premixed methane/air flames inhibited by 0.1% (NaOH)2 using the H + 

OH scavenging cycle of Table 1. The accepted values of X-OH bond

energies for the alkali metals are indicated by vertical lines. 

 12
thermodynamic properties, this difference may be due to differences in kinetic rates 

between the two elements, particularly for the recombination step (R16). 

Iron: O + O Recombination 

The three step O + O catalytic cycle modeled for iron has two relevant bond 

energies, Fe-O and OFe-O, which differ by about 3 kcal/mole. The bond energies were 

Fig. 3. Calculated flame speed as a function of Fe-O bond energy (see text) for 

premixed methane/air flames inhibited by 500 ppm FeO2 using the three 

step O + O scavenging cycle of Table 1. The accepted values of the Fe-O

and OFe-O bond energies are indicated by vertical lines.

 13
varied in the calculation by keeping the standard state enthalpy of FeO fixed, while 

varying those of Fe and FeO2 in opposite directions from their "normal" values. For 

FeO2, the "normal" standard state enthalpy used was not the actual value, but was chosen 

such that the bond energy of OFe-O was equal to that of Fe-O, 99 kcal/mole. 

In the calculation, FeO2 was chosen as the reactant. For most iron containing

compounds such as Fe(CO)5, the parent agent is quickly converted to Fe [12], which can 

then combine with oxygen through (R13), making the original identity of the iron 

compound unimportant. Because of iron's high suppression efficiency, a smaller 

concentration of suppressant was used (500ppm) than for the calculations with sodium or 

bromine. 

Results of the calculations are shown in Fig. 3. The accepted bond energies of 

both Fe-O (99 kcal/mol) and OFe-O (96 kcal/mol) are close to the optimal value for

efficiency of the catalytic cycle. 

DISCUSSION 

For efficient scavenging cycles, it appears that the bond energy between a 

catalytic scavenger and a flame radical must lie in the range of 70-100 kcal/mol, the 

optimal value varying somewhat for different net recombination reactions. This range of

values is consistent with the expectation that the bond must be strong enough to be 

thermodynamically stable at flame temperatures, but not so strong as to prevent

regeneration of the active scavenging radical. In Table 2, there is a trend that the more 

efficient a scavenging cycle (in terms of the inhibition parameter [6]) the more sensitive it 

 14
is to changes in bond dissociation energies. 

Table 2 

Efficiencies and Sensitivities to Bond Energies of Catalytic Cycles

 Inhibition ∆D

Element Net Cycle Parameter[6] ∆D(kcal/mol)a Dmax(kcal/mol)b Dmax

Br (0.5%) H + OH 6.9 39 77 0.51 

Br (0.5%) H + H 11.5 33 77 0.43 

Na (0.2%) H + OH 46 29 85 0.34 

Fe (0.05%) O + O 66 14 99 0.14 

a

 range of bond energies for which the suppression index is at least 75% of its 

maximum value. b

 bond energy at which the maximum value of the suppression index is achieved. 

Other factors also play a role in suppression efficiency, although a less definitive 

one than does the bond energy. In most catalytic cycles, there is likely to be at least one 

recombination step, e.g., Na + OH => NaOH, Fe + O2 => FeO2. In many cases, the 

recombination reaction may be the rate limiting step in the catalytic cycle. Clearly this 

recombination step must have a faster rate than the direct radical-radical recombination 

for the catalyst to cause inhibition. In general, the recombination complex is more likely

to be stabilized by third body colliders if it is polyatomic, rather than diatomic. Also, if

the recombination reaction involves a stable, rather than a radical flame species, it is 

likely to proceed faster due to the higher reactant concentrations. This is the case for the 

Fe + O2 recombination step. 

Many transition metals whose compounds show good suppression properties 

exhibit more than one stable valence. This does not appear to be a fundamental 

requirement for good suppression, since alkali metals only exhibit one valence yet are 

good suppressants. The existence of multiple valences greatly increases the number of 

 15
possible species, increasing the likelihood that an efficient catalytic cycle may occur. 

Multiple valences may also facilitate recombination reactions. If the reactant and product 

of a recombination reaction correspond to two stable valences of the inhibitor element, 

then the kinetics of the reaction may be more favorable. 

We thank Valeri Babushok for helpful discussions. This work is part of the 

Department of Defense Next-Generation Fire Suppressant Technology Program, funded 

by the DoD Strategic Environmental Research and Development Program. 

REFERENCES

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

2. Westbrook, C. K., Combust. Sci. Technol. 34:201-225 (1983). 

3. Sheinson, R. S., Penner-Hahn, J. E., and Indritz, D., Fire Safety Journal, 15:437-450 

(1989). 

4. "Inhibition of Premixed Methane-Air Flames by Water Mist," Fuss, S. P., Chen, E. F., 

Yang, W., Kee, R. J., Williams, B. A., and Fleming, J. W., Proc. Combust. Inst. 

29:361-368 (2002). 

5. Pitts, W. M., Yang, J. C., Huber, M. L., and Blevins, L. G., Characterization and 

Identification of Super-Effective Thermal Fire Extinguishing Agents NIST Report 

NISTIR 6414, National Institute of Standards and Technology, Gaithersburg, MD, 

1999. 

6. Noto, T., Babushok, V., Hamins, A., and Tsang, W., Combust. Flame 112:147-160 

(1998). 

7. Babushok, V. and Tsang, W., Combust. Flame 123:488-506 (2000). 

8. Babushok, V., Tsang, W., Linteris, G. T., and Reinelt, D., Combust. Flame 115:551-

560 (1998). 

9. Babushok, V., Noto, T., Burgess, D. R. F., Hamins, A., Tsang, W., Combust. Flame 

107:351-367 (1996). 

10. Hamins, A., Proc. Combust. Inst. 27:2857-2864 (1998). 

 16
11. MacDonald, M. A., Jayaweera, T. M., Fisher, E. M., and Gouldin, F. C., Combust. 

Flame 116:166-176 (1999). 

12. Rumminger, M. D., Reinelt, D., Babushok, V., and Linteris, G. T., Combust. Flame

116:207-219 (1999). 

13. Vanpee, M. and Shirodkar, P. P., Proc. Combust. Inst. 17:787-795 (1978). 

14. Bulewicz, E. M. and Padley, P. J., Proc. R. Soc. London A 323:377-400 (1971). 

15. Linteris, G. T., Knyazev, V. D., and Babushok, V. I., "Inhibition of premixed 

methane flames by manganese and tin compounds," Combust Flame 129: 221-238 

(2002). 

16. Dixon-Lewis, G. and Simpson, R. J., Proc. Combust. Inst. 16:1111 (1976). 

17. Fristrom, R. M. and Van Tiggelen, P, Proc. Combust. Inst. 17:773-785 (1978). 

18. The Effect of Metal Salts on Premixed Hydrocarbon Air Flames, Rosser, W. A., 

Inami, S. H., and Wise, H., Combust. Flame 7:107-119 (1963). 

19. Griffiths, J. F. and Barnard, J. A., Flame and Combustion, 3rd Edition, Blackie 

Academic and Professional, Glasgow, 1995, p. 33. 

20. Glassman, I., Combustion, 2nd Edition, Academic Press, Orlando, FL, 1987, pp.145-

146. 

21. Casias, C. R., and McKinnon, J. T., Proc. Combust. Inst. 27:2731-2740 (1998). 

22. Jensen, D. E. and Jones, G. A., J. Chem. Soc. Faraday Trans. 1 78:2843 (1982). 

23. Rumminger, M. D., and Linteris, G. T., Combust. Flame 120:451-464 (2000). 

24. Twarowski, A., Combust. Flame 102:41-54 (1995). 

25. 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. 

26. Bowman, C. T., Hanson, R. K., Gardiner, W. C., Lissianski, V., Frenklach, M., 

Goldenberg, M., and Smith, G. P., GRI-Mech 2.11 - An Optimized Detailed Chemical 

Reaction Mechanism for Methane Combustion and NO Formation and Reburning,

GRI Report GRI-97/0020, 1997, Gas Research Institute, Chicago. 

www.ME.Berkeley.edu/gri_mech 

27. Williams, B. A. and Fleming, J. W., "CF3Br and other Suppressants: Differences in 

Effects on Flame Structure" Proc. Combust. Inst. 29:345-351 (2002). 

 17
28. Zamansky, V. M., Lissianski, V. V., Maly, P. M., Ho, L., Rusli, D., and Gardiner, Jr., 

W. C., Combust. Flame 117:821-831 (1999).

 18