Final Technical Report

PROJECT TITLE: Mechanisms of Ultra-High Efficiency Chemical Suppressants

SERDP PROJECT IDENTIFIER: NGP 2a2

RESEARCH CATEGORY: 6.2 Applied Research

AGENCY: Navy

LAB: Naval Research Laboratory

PI: Dr. James Fleming (NRL)

Code 6185

Naval Research Laboratory

Washington, DC 20375-5342

phone: 202-767-2065

fax: 202-767-1716

fleming@code6185.nrl.navy.mil

co-PI: Dr. Bradley A. Williams (NRL)

Code 6185

Naval Research Laboratory

Washington, DC 20375-5342

phone: 202-767-3583

fax: 202-767-1716

brad@code6185.nrl.navy.mil

THRUST AREA: 2A Mechanisms of Chemical Suppressants

PROJECT DATES: April 1998 – May 2001

REPORT DATE: October 1, 2003

The views and conclusions contained in this document are those of the authors and should

not be interpreted as representing the official policies, either expressed or implied, of the

Strategic Environmental Research and Development Program, U. S. Navy, or any other part

of the U.S. Government.

Sponsored by:

The Department of Defense

Strategic Environmental Research and Development Program
TABLE OF CONTENTS

I. EXECUTIVE SUMMARY

A. TASK OBJECTIVES

B. TECHNICAL PROBLEMS

C. GENERAL METHODOLOGY

D. TECHNICAL RESULTS

E. IMPORTANT FINDINGS AND CONCLUSIONS

F. SIGNIFICANT HARDWARE DEVELOPMENTS

G. SPECIAL COMMENTS

H. IMPLICATIONS FOR FUTURE RESEARCH

II. ARTICLES PUBLISHED

III. DETAILED PROJECT DESCRIPTION

III A. Approach and Techniques

III B. Suitability of CF3Br as a Mechanism Benchmark for Fire Suppression

III C. Suppressant Effect on Burning Velocities and Species Profiles in

Inhibited Premixed Flames

III D. Combined Effects of Mixtures of Agents

III E. Correlation of Burning Velocities, Final Flame Temperature, and Radical

 Concentrations

III F. Changes in Flame Structure Due to Different Agents

III G. Influence of Bond Energies on Suppression Cycles

III G 1. Bromine: H + H and H + OH Recombination

III G 2. Sodium: OH + H Recombination

III G 3. Iron: O + O Recombination

III G 4. Effect of Bond Energies on Suppression Cycles: Conclusions

IV. RECOMMENDATIONS FOR ADDITIONAL RESEARCH

V. CONCLUSIONS

VI. ACKNOWLEDGMENTS

VII. REFERENCES

VIII. LIST OF TABLES

IX. LIST OF FIGURES
I. EXECUTIVE SUMMARY

A. TASK OBJECTIVES

The objective of this project was to determine how chemicals that are as or more efficient than

Halon 1301 (CF3Br) quench the various flames of military interest in order to identify

high-efficiency chemical fire suppression agents as candidates for replacing Halon 1301. To

answer this question, three specific tasks were undertaken, all based on an assessment of the

behavior of prototype "superagent" fire suppressants such as iron pentacarbonyl. These were: (1)

understand key chemical reactions or processes of combustion which are affected by

"superagent" suppressants and how these reactions or processes differ from those important for

fire suppression by Halon 1301; (2) determine the requirements for (and partitioning of) the

contributions of a suppressant's chemical and physical interactions with flames, leading to

guidelines for suppression systems; (3) determine minimum properties for a "superagent"

suppressant.

B. TECHNICAL PROBLEMS

Superagents are typically metal or alkali metal compounds that typically exist as liquids or solids

at room temperature. The evaluation of these compounds under the same conditions for

comparison purposes is often difficult if not impossible. Flame configurations that are very

amenable to the study of gas-phase suppression (e.g cup burner flames) are difficult or totally not

appropriate for studying condensed phase suppression compounds. Also, because of their very

high efficiencies, superagents compounds are required only in very small quantities. Controlling

the delivery of small amounts of condensed phase compounds and the subsequent monitoring of

these compound under flame conditions are challenges.

C. GENERAL METHODOLOGY

Specific plans of the project included the following: develop list of suppressants (current NGP

efforts, scientific literature, mechanistic insight gained from project) for compounds to

investigate; select compounds for (a) physical suppression, (b) efficient chemical suppression,

and (c) combined mechanism of suppression; examine flames in several configurations including

non-premixed counterflow and premixed Bunsen flames; quantify additive effect on extinction

strain rate in non-premixed counterflow flames and burning velocity reduction in premixed

flames; characterize flame structure (flame species and temperature profiles) as function of

additive concentration and strain rate; measure flow fields using (LDV) and/or (PIV); develop

required chemical kinetic mechanisms; model inhibited flames using existing flame codes;

compare flame structure modeling results to experimental measurements; provide systematic

method for identifying new candidate suppressants based on mechanistic knowledge; and

develop list of such chemicals.
D. TECHNICAL RESULTS

• Burning velocity of premixed flames seeded with small (< 0.5 micron diameter) water drops

carried out to determine potential for introducing non-gaseous agents into flames.

• Burning velocity reduction of methane/air premixed flames agreed with theoretical maximum

thermal effect of water. Total evaporation of drops established by light scattering.

• Burning velocity reduction of premixed methane/air flames by added alkali (sodium

hydroxide solution) exhibit either (i) no significant reduction over pure water drops or (ii)

flames could not be stabilized. Behavior consistent with the complete evaporation of drops

in the various flames.

• Suppression effectiveness of efficient chemical inhibitors existing as liquids or solids at room

temperature (requiring vaporization or decomposition) depends strongly on residence time

and thus flame geometry (premixed versus non-premixed, counterflow versus co-flow) even

for very small particle/drop sizes.

• A kinetic model was constructed and computationally evaluated for tin. The calculated

suppression concentrations (based on tetramethyltin) are an order of magnitude higher than

for potassium, iron, or manganese.

• Extinction strain rate calculations for methane/air counterflow non-premixed flames inhibited

by dimethyl methylphosphonate (DMMP) agree with experimental data of Fisher et.

al.(Cornell). Numerical results demonstrate a "strong saturation" effect for DMMP.

• Modeling calculations using assembled manganese kinetic mechanism show similar

effectiveness for MMT(methylcyclopentadienyl manganese tricarbonyl) and

monomanganese-pentacarbonyl. The estimated efficiency is 5-7 times less than that for

Fe(CO)5. Uncertainties do exist in the estimation of the rate constants; the possibility exists

that Mn compound suppression effectiveness could be higher than estimated.

• Modeling studies show that the behavior for bromine (as expressed by CF3Br) behavior is not

typical of highly efficient catalytic suppressants; the use of CF3Br as a "baseline" for

inhibitor mechanistic comparisons must be done with care.

• Modeling studies were carried out to compare Br (as provided by CF3Br) to efficient catalytic

scavengers (iron pentacarbonyl and sodium hydroxide), inert thermal agents (CF4 and N2)

and non-brominated fluorocarbons (CF3CHFCF3, HFC-227ea and CF3CH2F, HFC-134a.

E. IMPORTANT FINDINGS AND CONCLUSIONS

• The use of Halon 1301 as a "baseline" for inhibitor mechanistic studies must be done with

care since bromine's effect is not typical of catalytic suppressants in many respects. For

CF3Br, almost no effect of chemical saturation is observed compared to the “superagents”.

Br only reduces radical concentrations to a significant extent in a narrow region of the flame.

NaOH and Fe(CO)5 reduce radical concentrations wherever they exceed equilibrium values.

• For chemical catalytic “superagents”, flame speed follows the peak H atom concentration

and global activation energy for most agents. For halogen compounds (including HBr and

CF3Br), H atom reduction occurs early in flame zone, not at the location of peak H

concentration.

• For suppression effectiveness, selective depletion of H atoms early in the flame (at lower

flame temperatures) has the greatest effect on burning velocity reduction.
• For catalytic inhibitors, ability to start the catalytic scavenging cycle early in the flame is

critical for maximum effectiveness.

• Combining physical and chemical suppression is key to optimum suppression performance

whether from a single agent or where two agents are combined.

• Experimental and modeling studies show that temperature reduction from physical

suppression greatly enhances extinction effectiveness of "superagents".

• Favorable bond energy of key catalytic players is necessary (but not sufficient) for good

suppression.

• Predictions of required concentrations for tin suggest that it is not a likely “superagent”.

• For combined agents, the formation of stable fluorides limits the usefulness of combining

fluorocarbons with a number of chemical agents. The choice of physical agents is likely to

be limited to inert gases or liquids (e.g. water, if the chemical agent is a water soluble salt).

F. SIGNIFICANT HARDWARE DEVELOPMENTS (none)

G. SPECIAL COMMENTS

The chemistry of suppression has been a topic of intellectual curiosity and active research for a

number of years. Through the efforts of the Next Generation Plan program, the conclusions and

implications of several of these findings have been re-examined (taking advantage of advances in

laboratory diagnostic capabilities and computation resources) and some re-interpreted in light of

more recent data providing key details. Mechanistic studies have more clearly defined the

suppression picture and documented what does not work and why. The NGP program has also

made possible the examination of suppression mechanisms with the express intent of identifying

alternative agents in a concentrated, methodical manner, involving scientists from a number of

backgrounds including physics and chemistry, mechanical and fire protection engineering, and

environmental and health sciences.

H. IMPLICATIONS FOR FUTURE RESEARCH

Superagents identified to date have very differing physical properties. Evaluation of their

effectiveness presents problems when attempting an absolute effectiveness determination. There

are limited options to achieve the sought after high efficiency suppression levels. The best

choice will be a compromise between the physical properties and the specific applications that

must be protected. Thus an evaluation on a case by case basis must be carried out.

II. ARTICLES PUBLISHED

Papers supported in whole or in part by this project

1. “Determination of the Strain in Counterflow Diffusion Flames From Flow Conditions”,

E.M. Fisher, B.A. Williams, and J.W. Fleming, Proceedings of the Eastern States

Section: The Combustion Institute, pp. 191-194, 1997.

2. “Suppression Mechanisms of Alkali Metal Compounds”, B.A. Williams and J.W.

Fleming, Proceedings of the 1999 Halon Options Technical Working Conference, pp.

157-169, 1999.

3. “Flow Field Considerations for Counter Flow Burners”, M.P. Davis, J.W. Fleming, B.A.
Williams, and H.D. Ladouceur, Proceedings of the Fall Technical Meeting of the Eastern

States Section of the Combustion Institute, pp. 200-203, 1999.

4. “Chemical Fire Suppressants: How Can We Replace Halon?”, (invited presentation)

Proceedings of the Fall Technical Meeting of the Eastern States Section of the

Combustion Institute, pp. 16-23, 1999.

5. “Inhibition of Premixed Methane-Air Flames by Water Mist”, S.P. Fuss, D.J. Dye, B.A.

Williams, and J.W. Fleming, Proceedings of the Fall Technical Meeting of the Eastern

States Section of the Combustion Institute, pp. 105-108, 1999.

6. "Equivalence of the Ranking of Inhibitor Effectiveness for Combustion of Hydrocarbon

Fuels", V.Babushok, W.Tsang, W.Grosshandler, Proceedings of the Fall Technical

Meeting of the Eastern States Section of the Combustion Institute, pp. 180-183, 1999.

7. "Controlling the Self-Ignition Behavior of Silane", Tsang, W., Babushok, V.,

Proceedings of the Fall Technical Meeting of the Eastern States Section of the

Combustion Institute, pp. 229-232, 1999.

8. "Influence of Phosphorus-Containing Fire Suppressants on Flame Propagation",

Babushok, V., Tsang, W., Proceedings of the Third International Conference on Fire

Research and Engineering, pp. 257-267, 1999.

9. “Inhibition of Premixed Methane-Air Flames by Submicron Water Mists”, S.P. Fuss, D.J.

Dye, B.A. Williams, and J.W. Fleming, Proceedings of the 2000 Halon Options

Technical Working Conference, pp. 242-250, 2000.

10. “Inhibitor Rankings for Hydrocarbon Combustion", V.Babushok and W.Tsang, Combust.

Flame. 123(4), 488-506, 2000.

11. "Reduction in Burning Velocity of Premixed Methane-Air Flames by Submicron Water

Mist”, S.P. Fuss, D.J. Dye, B.A. Williams, and J.W. Fleming, 28th International

Symposium on Combustion, Abstracts of Work-in-Progress Posters, Poster No. WIP 1-

J13, p. 13, 2000.

12. "Is CF3Br Representative of Efficient Fire Suppressants?", B.A. Williams and J.W.

Fleming, Proceedings of the 2nd Joint Meeting of the US Sections of the Combustion

Institute, (CD) 2001.

13. “Influence of Bond Energies on Catalytic Flame Inhibition: Implications for the Search

for New Fire Suppressants”, Bradley A. Williams and James W. Fleming, NRL

Memorandum Report, 2003.
III. DETAILED PROJECT DESCRIPTION

III A. Approach and Techniques

Compounds or families of compounds were selected from lists of suppressants compiled in other

NGP efforts, the scientific literature, and by mechanistic insight gained as the project progressed.

Compounds were selected that demonstrated essentially physical suppression, compounds that

were characterized as efficient chemical suppressants, and compounds that combined both

mechanisms of suppression. Compounds or combinations of compounds were selected which

could give guidance on the development of suppression systems, and demonstrate and

characterize the combined effects of physical and chemical inhibition.

Both counterflow non-premixed flames and premixed Bunsen flames were considered. To

simulate the typical mode of application, compounds were added to the air side of non-premixed

counterflow flames. For the counterflow flames, the amount of additive to cause extinction was

determined as a function of the extinction strain rate, maximum velocity gradient at extinction on

the air side of the flame. For premixed flames, the effect of the additive concentration on the

burning velocity was used to measure suppression effectiveness.

The additive effect on various aspects of the flame structure including flame species profiles and

temperature as a function of additive concentration and flame strain rate were also examined.

Numerical simulations of inhibited and extinguishing flames using the latest computer codes

were carried out. Kinetic rates and thermochemical data needed by the flame codes were

assembled where available or developed where needed.

A major goal of the project was to determine whether the mechanisms of flame extinction by the

addition of various agents were similar to or distinct from those exhibited by Halon 1301. It was

intended that this insight would guide the search for and/or help to identify additional potential

superagents.

The Combustion Dynamics Section of the Navy Technology Center for Safety and Survivability

at the Naval Research Laboratory (NRL) carried out this project in collaboration with Drs. Wing

Tsang and Valeri Babushok at NIST. Results specific for the NRL effort are detailed below. In

the first year of the project, NRL collaborated with the Ignition and Combustion Branch,

Propulsion and Flight Division, Weapons and Materials Research Directorate, of the Army

Research Laboratory (ARL/WMRD). Details and findings of the Army 2a mechanism effort are

documented in a separate report. Drs. Wing Tsang and Valeri Babushok and colleagues at NIST

also collaborated on several other NGP projects closely related to the mechanism investigation.

Those results are reported under the specific NGP projects.

III B. Suitability of CF3Br as a Mechanism Benchmark for Fire Suppression

To assess whether CF3Br is "typical" of efficient fire suppressants in general, premixed flames

containing representative inhibitors of various types, including inert gases, fluorocarbons, and

catalytic scavengers of flame radicals were modeled. Representatives from various suppression
groups were selected. Those groups included: physical suppressants (such as nitrogen, argon,

CF4, and water) that do not participate in flame chemistry to a significant extent and inhibit

combustion by adding heat capacity and diluting the reactants; chemical suppressants that

participate in the flame chemistry and can be subdivided into catalytic suppressants (including

bromine, iodine, and various metallic elements) which reduce concentrations of flame radicals

through a regenerative cycle (one molecule of suppressant can recombine several radicals) and

noncatalytic suppressants (e. g. fluorocarbons) that 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

[Sheinson, 1989], 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. [Rosser, 1958] and modified by Noto et

al. [Noto, 1998]:

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 [Noto, 1998] 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. [Saso, 1999] 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.

III C. Suppressant Effect On Burning Velocities and Species Profiles in Inhibited

Premixed Flames

In the calculations described below, the PREMIX code [Kee, 1985] was used to compute burning

velocities and flame structures of atmospheric pressure flames. An 85 cm 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

[L’Esperance, 2000; Williams, 2000]. Bromine chemistry used for CF3Br was that developed by

Noto et al. [Noto, 1998]. The kinetic mechanism for sodium was based on that of [Zamansky,

1999], while that for Fe(CO)5 was described by [Rumminger, 2000].

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

superequilibrium, as well as the inhibition parameter Φ as defined by [Noto, 1998] 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 40K for the range of concentrations shown in the Figure 3). 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, 1998] although the modifications to the fluorocarbon

kinetics yield somewhat higher inhibition parameters, in excellent agreement with the

experimental value of 14.0 [Saso, 1999].

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 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 [Westbrook 1980]

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

H + Br2 => HBr + Br (ii)

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

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

has rather slow kinetics. This has two consequences: the increasing importance of Reaction (i)

(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.

III D. 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, it is worth exploring whether there is 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, 2000] 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 that operate through

different pathways.

In Table 3, 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 Table 3, 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 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. [Saso, 1999], 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.

III E. Correlation of Burning Velocity, Final Flame Temperature, and Flame

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

 = A2XHexp(-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

(Figure 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.

Padley and Sugden [Padley, 1958] observed in studies of H2/O2/N2 flames that the burning

velocity correlated with the partial pressure of atomic hydrogen in the reaction zone. More

recently, this correlation has been observed to hold also for hydrogen flames inhibited by CF3Br

[Kim, 2001]. Figure 4 plots the peak H atom concentration against burning velocity for a variety

of inhibited methane/air flames. For methane flames inhibited by N2, CF4, and CF3Br, there is a

nearly identical linear relationship between the peak H atom mole fraction and burning velocity,

as with the hydrogen flames.

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 Equation (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 Equation (2) is within 10% of the actual value using the full

kinetics. This relationship between burning velocity, final 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

Equation (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.

III F. 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 Figure 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 non-brominated 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.

III G. Influence of Bond Energies on Suppression Cycles

For a substance to catalytically 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

XH + H => X + H2 (R1)

XH + OH => X + H2O (R2)

XOH + H => X + H2O (R3

XO + O => X + O2 (R4)

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 (R1)-(R4) 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 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 (R1) and (R2), sodium for (R3), and

iron for (R4). 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.
III G 1. 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. Figure 7 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 that permits an

efficient catalytic cycle (Figure 7). 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, while chlorine has a much smaller effect.

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

+ H cycle, in accord with analysis [Casias, 1998] 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.

III G 2. Sodium: OH + H Recombination

 The effect of hypothetical variation of the Na-OH bond energy is illustrated in Figure 8. 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 [Babushok, 2000]. In view of the similarity of the thermodynamic properties, this

difference may be due to differences in kinetic rates between the two elements, particularly for

the recombination step X + OH +M -> XOH + M.

III G 3. 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 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 FeO, 99 kcal/mole. Results of the calculations are shown in Figure 9. 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.
III G 4. Effect of Bond Energies on Suppression Cycles: Conclusions

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.

IV. RECOMMENDATIONS FOR ADDITIONAL RESEARCH

A great number of compounds (through their individual metal constituents) were considered

and/or tested in this and related mechanism projects. It is clear that after eliminating those

elements that clearly will not work, there are only a limited number of compounds that are

capable of providing the ultra-high efficiency sought after and still satisfy the environmental and

health safety concerns. These compounds have different physical properties and must be

evaluated on a case by case basis for specific applications.

V. CONCLUSIONS

CF3Br is an efficient fire suppression agent that can be used to assess the overall effectiveness of

potential replacement agents. Using the effectiveness of CF3Br as a extinguishment benchmark

has merit. However, as pointed out in this project, the suppression mechanism of CF3Br is not

typical of efficient fire suppressants, and certainly not similar to the ultra-high efficient agents.

The anomalous suppression behavior of CF3Br is specifically related to the flame kinetics of

bromine. Demanding a replacement agent display the same suppression mechanism as CF3Br

essentially requires that the replacement agent also contain bromine. However, the role of

bromine in the destruction of stratospheric ozone is the reason replacements are needed.

Providing the bromine on a compound that will not survive intact as it travels through the

troposphere (tropodegradable) is one approach currently pursued by the NGP.

In order to achieve ultra-high suppression efficiency, a replacement agent (or a species provided

by the agent) must participate in the catalytic removal of the key flame propagation radicals: H,

O, or OH. There are several species whose chemistry permits this behavior including the alkali

metals, iron, phosphorus, and manganese. Of these so-called superagents, iron has been shown

to be the most efficient at flame speed inhibition. Three principles control the efficiency of

catalytic superagents. First, a sufficiently large enough concentration of the catalytic species

must be achievable at the flame condition (fundamental limitation due to thermodynamics and

the system temperature and composition). Second, the catalyst can only take the flame radicals

to their equilibrium concentration at that flame temperature. If the equilibrium concentrations of

the flame radicals are still sufficient to maintain combustion, then in the absence of any other

heat losses, extinguishment will never be achieved. The use of a thermal agent to lower the

flame radical equilibrium concentration amount can be exploited. Extinction can be achieved by

combining the chemical catalytic ultra-efficient agent and a thermal agent. However, the first

principle mentioned above cannot be violated; a sufficient concentration of the catalytic species

must exist at the lower flame temperature. The inability of iron compounds to efficiently achieve
extinction in some non-premixed flames (e.g. cup burner flames) is due to this limitation. The

third controlling principle deals with the reactivity of the catalytic specie. It must significantly

react only with the H, O, and OH flame radicals and not with any other species in the flame. The

reactivity of fluorine (introduced by fluorinated thermal agents) limits the practical use of HFC

thermal agents and most catalytic agents. The optimum ultra-efficient agent thus achieves

extinction through a balance of achievable flame concentration, sufficient catalytic activity, and

sufficient flame temperature reduction. This balance is optimally achieved by CF3Br and can be

met with other systems. In the search for viable, ultra-efficient replacement agents, chemistry is

definitely important but there are thermodynamic limitations that must be assessed on a case by

case basis.

VI. ACKNOWLEDGEMENTS

Valeri Babushok (NIST) was a co-PI on this project and contributed to the mechanistic

understanding of iron, manganese, phosphorus, and tin. Contributions of Paul Fuss (NRL/ASEE

Postdoc) are acknowledged. This project was supported by the Department of Defense NextGeneration Fire Suppressant Technology Program, funded by the DoD Strategic Environmental

Research and Development Program. Partial funding was received from the Office of Naval

Research through the Naval Research Laboratory.

VII. REFERENCES

Babushok, V. and Tsang, W., (2000), Inhibitor rankings for alkane combustion

,Combust. Flame 123:488-506.

Casias, C. R., and McKinnon, J. T., (1998), A Modeling Study of the Mechanisms of Flame

Inhibition by CF3Br Fire Suppressant Agent, Proc. Combust. Inst.. 27:2731-2740.

Kee, R.J., Grcar, J.F., Smooke, M.D., and Miller, J.A., (1985), PREMIX: A Fortran Program for

Modeling Laminar One-Dimensional Premixed Flames, Sandia Report SAND85-8240.

Kim, C. H., Kwon, O. C., and Faeth, G. M., (2001), Second Joint Meeting of the U. S. Sections

of the Combustion Institute, paper #219.

L’Espérance, D., Williams, B. A., and Fleming, J. W., (1999), Intermediate Species Profiles in

Low Pressure Premixed Flames Inhibited by Fluoromethanes, Combust. Flame 117:709-731.

Noto, T., Babushok, V., Hamins, A., and Tsang W., (1998), Inhibition effectiveness of

halogenated compounds, Combust. Flame 112:147-160.

Padley, P. J. and Sugden, T. M., (1958), Proc. Combust. Inst. 7:235-242.

Rosser, W. A.., Wise, H., and Miller, J., (1958), Mechanism of Combustion Inhibition by

Compounds Containing Halogen, Proc. Combust. Inst. 7:175-182.
Rumminger, M. D., and Linteris, G. T., (2000), Inhibition of Premixed Carbon MonoxideHydrogen-Oxygen-Nitrogen Flames by Iron Pentacarbonyl, Combust. Flame 120:451–464.

Saso, Y., Ogawa, Y., Saito, N., and Wang, H., (1999), Binary CF3Br- and CHF3-inert flame

suppressants: Effect of temperature on the flame inhibition effectiveness of

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

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.

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

Williams, B. A., L' Espérance, D. M., and Fleming, J. W., (2000), 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.

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

(1999), Reactions of Sodium Species in the Promoted SNCR Process, Combust. Flame 117:821-

831.
VIII. LIST OF TABLES

Table 1: Inhibition by Two Chemical Agents in Combination

Table 2: Comparison of Calculated Flame Speeds to Values from Equation (2)

Table 3: Kinetic Mechanisms for Suppressants

IX. LIST OF FIGURES

Figure 1: Flame speed, final flame temperature, inhibition parameter (defined in [Noto, 1998]),

and superequilibrium concentrations of flame radicals computed for atmospheric pressure

stoichiometric methane/air mixtures inhibited by nitrogen.

Figure 2: Flame speed, final flame temperature, inhibition parameter (defined in [Noto, 1998]),

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.

Figure 3: Flame speed, final flame temperature, inhibition parameter (defined in [Noto, 1998]),

and superequilibrium concentrations of flame radicals computed for atmospheric pressure

stoichiometric methane/air mixtures inhibited by CF3Br.

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.

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 the

indicated suppression agents.

Figure 6. Relationship between burning velocity and peak concentration of atomic hydrogen for

methane air flames inhibited by various compounds. The N2 + Fe data is that presented in Figure

4, containing a FeO2 as a reactant assumed to operate via an O + O inhibition cycle.

Figure 7. 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 3. The accepted values for the bond energies of H-Br, H-Cl, and H-I are indicated by

vertical lines.

Figure 8. 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 3.

The accepted values of X-OH bond energies for the alkali metals are indicated by vertical lines.
Figure 9. 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 3. The accepted values of the Fe-O and OFe-O bond energies are indicated by vertical

lines.
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.
Table 2

Comparison of Calculated Flame Speeds to Values from Equation (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 ∆Spremix/∆Seqna

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)
Table 3

Kinetic Mechanisms for Suppressants

 k = ATbexp(-Ea/kT)

A(cm3,mol,s) b Ea(cal/mol) Remark.

Mechanism for Na (H + OH cycle only-Figure 8)

(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-Figure 7):

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

Addition for Br (H + OH cycle-Figure 7):

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

Mechanism for Fe (O + O cycle-Figure 9):

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).
Figure 1: Flame speed, final flame temperature, inhibition parameter (defined in [Noto 1998]),

and superequilibrium concentrations of flame radicals computed for atmospheric pressure

stoichiometric methane/air mixtures inhibited by nitrogen.
Figure 2: Flame speed, final flame temperature, inhibition parameter (defined in [Noto 1998]),

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.
Figure 3: Flame speed, final flame temperature, inhibition parameter (defined in [Noto 1998]),

and superequilibrium concentrations of flame radicals computed for atmospheric pressure

stoichiometric methane/air mixtures inhibited by CF3Br.
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.
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 the

indicated suppression agents.
Figure 6: Relationship between burning velocity and peak concentration of atomic hydrogen for

methane air flames inhibited by various compounds. The N2 + Fe data is that presented in Figure

4, containing a FeO2 as a reactant assumed to operate via an O + O inhibition cycle.
Figure 7: 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

3. The accepted values for the bond energies of H-Br, H-Cl, and H-I are indicated by vertical lines.
Figure 8: 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 3. The accepted values of X-OH bond energies for the alkali metals are indicated

by vertical lines.
Figure 9. 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 3. The accepted values of the Fe-O and OFe-O bond energies are indicated by

vertical lines.