Powder-Matrix Systems for Safer

Handling and Storage of Suppression

Agents

Gregory T. Linteris

Harsha K. Chelliah

NISTIR 6766
NISTIR 6766

Powder-Matrix Systems for Safer

Handling and Storage of Suppression

Agents

Gregory T. Linteris

Building and Fire Research Laboratory

National Institute of Standards and Technology

Harsha K. Chelliah

Mechanical and Aerospace Engineering

University of Virginia

August 2001

U.S. Department of Commerce

Norman Y. Mineta, Secretary

Technology Administration

Dr. Cheryl L. Shavers, Under Secretary of Commerce for Technology

National Institute of Standards and Technology

Raymond G. Kammer, Director
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Final Technical Report

Powder – Matrix Systems for Safer Handling and Storage of

Suppression Agents

4A/1/8

Date: 1/31/2001

Gregory T. Linteris

National Institute of Standards and

Technology

Building and Fire Research Laboratory

Fire Science Division

100 Bureau Dr. Stop 8652

Gaithersburg MD 20899-8652

Harsha H. Chelliah

Mechanical and Aerospace Eng.

University of Virginia

122 Engineer's Way

P.O.Box 400746

Charlottesville, VA 22904-4746

July, 2001

Final Technical Report, March 1999 – December 2000

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, National Institute of Standards and

Technology, or any other part of the U.S. Government.

Sponsored by:

The Department of Defense

Strategic Environmental Research and Development Program
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Table of Contents

1. Summary.............................................................................................................................4

2. Bibliography .......................................................................................................................7

3. Detailed Description of the Project......................................................................................8

3.1 Background.......................................................................................................................9

3.2 Absorption Tests of Fe(CO)5 into Matrix Materials .........................................................10

3.2.1. Matrix Materials and Super Agents Considered .......................................................10

3.2.2. Fe(CO)5 Absorption Method....................................................................................11

3.2.3. Fe(CO)5 Absorption Results ....................................................................................12

3.2.4. Quantification of Fe(CO)5 Absorption/Desorption ...................................................14

3.2.5. Surrogate Agents for Absorption/Desorption Testing...............................................16

3.3 Methods of Quantifying Suppression Effects by Particulates...........................................17

3.3.1 Counterflow Diffusion Flames with Inert and NaHCO3 Particles..............................18

3.3.1.1. Experiment .......................................................................................................18

3.3.1.2. Particle Seeder:.................................................................................................18

3.3.1.3. Results..............................................................................................................19

3.3.2 Premixed Flames with Inert and NaHCO3 Particles...................................................21

3.3.2.1. Experiment .......................................................................................................21

3.3.2.2. LDV System.....................................................................................................22

3.3.2.3. Results..............................................................................................................23

3.3.2.4. Conclusions......................................................................................................26

3.3.3 Premixed Flames with Iodine....................................................................................26

3.4. Complexation of Super-Effective Metals........................................................................27

3.4.1. Super Agents and Complexing Ligands Considered.................................................27

3.4.2. Selection of Ferrocene .............................................................................................28

3.4.3. Experiment..............................................................................................................29

3.4.4. Numerical modeling ................................................................................................30

3.4.5. Results and Discussion ............................................................................................30

3.4.5.1. Inhibition by Ferrocene.....................................................................................30

3.4.5.2. Inhibition by Ferrocene and CO2 .......................................................................32

3.4.5.3. Inhibition by Ferrocene and CF3H.....................................................................34

3.4.5.4. Comparison of Individual and Blended Performance.........................................36

3.4.6. Conclusions.............................................................................................................37

3.5. Cup Burner Flame Extinction Tests with Super-Effective Chemical Inhibitors...............37

3.5.1. Introduction.............................................................................................................37

3.5.2. Background .............................................................................................................38

3.5.3. Experiment..............................................................................................................39

3.5.4 Results and Discussion ............................................................................................40

3.5.4.1. Heptane – Air Flames .......................................................................................40

3.5.4.2. Methanol – Air Flames .....................................................................................41

3.5.4.3. Methane – Air Flames.......................................................................................42

3.5.4.4. Phosphorus Flames...........................................................................................42

3.5.4.5. Discussion ........................................................................................................43

3.5.4. Conclusions..........................................................................................................43
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4. Technical Problems...........................................................................................................44

5. Recommendations.............................................................................................................44

6. Conclusions.......................................................................................................................44

7. References.........................................................................................................................45
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1. Summary

1.1. Task Objectives

To assess, and if reasonable develop, the capability for absorbing/adsorbing or

complexing effective but toxic fire suppression agents in inorganic media. The

encapsulated agent can then be safely delivered to the fire where the high temperatures

will release the agent.

1.2. Technical Problems

There exist chemicals that are up to two orders of magnitude more effective than CF3Br

at reducing the overall reaction rate of some flames. They tend, however, to be

organometallic compounds that are highly toxic and flammable. If a means can be

developed for absorbing them in inert carriers, their extraordinary effectiveness might be

harnessed while rendering them safe to handle and use. Alternatively, there may exist

stable compounds that contain the active chemical moiety which can be safely handled

and yet will release the active chemical under the appropriate conditions. The challenge

in this project was originally perceived to be to find agent/matrix combinations which

were stable for storage, but also effective in releasing the active species into the flame

region to suppress the fire. In conducting the research for the project, however, we

discovered that the flame suppression behavior of previously postulated ‘super-effective

agents’ depends quite strongly upon the type of flame in which the tests are conducted.

Hence, the project took a somewhat changed route as we worked to determine the merits

of the super agents for more practical flames.

1.3. General Methodology

As perceived, the project was a combination of various approaches. A literature survey

would determine the complete range of possible 1.) super-effective agents, 2.) agent

support matrices, and 3.) inorganic forms of complexing the active metal-based species.

The most promising of these would be tested in the lab for agent uptake, stability, and

release at elevated temperatures. Finally, the results of the laboratory tests would be

used to guide flame tests of the most promising combinations. Early results showed,

however, that there were quantitative flame inhibition data for only a handful of metal

species (Fe, Cr, and Pb). The latter two were not pursued because of the environmental

toxicity of the element. Thus, the list of known super agents was quickly culled down

those containing one element: iron. We felt that demonstration tests with Fe(CO)5 in

various available support materials were achievable in the time available. The general

methodology then, was one of laboratory tests of agent uptake, stability, and release, and

laboratory flame tests of complexed or encapsulated agent effectiveness. As we

progressed, we obtained dramatic results for the effectiveness of Fe(CO)5 which would

change the interpretation of this as well as other NGP project results.
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1.4. Technical Results

The most important finding of this research is that the behavior of some super-agents

relative to CF3Br is markedly different in cup-burner flames (which more closely

resemble buoyant diffusion flame fires) than it is in either premixed or counterflow

diffusion flames. In the past, the counterflow diffusion flame was believed to provide a

good metric of inhibitor effectiveness. Our present work indicates, however, that the

performance benefit of an agent may be dependent upon the type of flame used in the

test. As a result of our tests of Fe(CO)5 in cup-burner flames, it does not appear that

getting this agent into a flame in any form will provide much benefit (if any) over alkali

metal compounds. Hence, work to absorb it or other super-effective agents may not be

warranted. In addition, other projects that seek to evaluate or discover new, supereffective compounds may need to take these recent findings into consideration. Specific

test results of this project are:

a.) Measured the flame speed of premixed methane-air flames in the presence of

ferrocene (Fec) and Fe(CO)5 at Tin=80 ° C, at φ=0.9, 1.0, and 1.1, and XO2

,ox =0.2,

0.21, and 0.24.

b.) Measured cup-burner extinction conditions of methane-air, heptane-air, and

methanol-air flames in the presence of CO2 and Fe(CO)5 or CF3Br.

c.) Measured the fuel consumption rate of liquid-fueled cup burners in the presence of

suppressants CO2, Fe(CO)5 , and CF3Br.

d.) Measured the Fe(CO)5 absorption rate into zeolite-NaX, zeolite-NaY, silica gel,

aerogel.

e.) Measured the I2 absorption rate into crystalline silica and zeolite-NaX.

f.) Measured the effect of inert particles on the flame speed and extinction strain rate of

methane-air flames.

1.5. Important Findings and Conclusions

a.) An Fe(CO)5 mass fraction of 0.33, 0.26, 0.21, and 0.14 to 2.0 can be absorbed into

zeolite-NaX, zeolite-NaY, porous silica, and aerogel, respectively.

b.) An I2 mass fraction of 0.06 and 0.24 can be absorbed into crystalline silica and

zeolite-NaX, respectively.

c.) Fe(CO)5 added to zeolite-NaX and zeolite-NaY is pyrophoric in air.

d.) Fe(CO)5 decomposes when added to zeolite-NaX (and probably Zeolite-NaY also).

e.) The powerful flame inhibition property of iron is not unique to Fe(CO)5, and there

exists at least one other, much less toxic compound (ferrocene) which is equally as

effective at reducing the burning velocity of premixed methane-air flames.

f.) Combining a highly effective chemical agent (e.g., Fec) with a thermally acting agent

(e.g., CO2) can mitigate the loss of effectiveness of iron compounds in premixed

methane air flames.

g.) Combining iron compounds with HFCs is not an effective combination because stable

iron-halogen species act as sinks for active gas-phase iron compounds.
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h.) Inert particles of SiO2 or Zeolite-NaX added to premixed methane-air flames at mass

fractions of about 1 % reduce the flame speed by about 5 %.

i.) The performance of a super-effective inhibitor can vary drastically depending upon

the type of flame in which it is tested.

1.6. Significant Hardware Developments

We designed, built, tested, and optimized the following:

a.) A system for absorbing Fe(CO)5 into porous particles

b.) A premixed flame burner for use with added particles.

c.) A counterflow diffusion flame burner for use with added particles.

d.) A steady and reliable particle seeding system for premixed or counterflow burners.

e.) A system for measuring the fuel consumption rate of liquid-fueled cup burner flames.

f.) A system for adding ferrocene to flames.

g.) A heated, insulated, premixed burner system which operates at elevated temperatures,

and to which lower vapor pressure agents (such as ferrocene) can be added.

h.) A new experimental protocol for testing super-effective agents in the cup burner.

1.7. Special Comments

It is an unexpected result that the super-effective agent Fe(CO)5 does not appear to work

well in the cup burner. Unless Fe(CO)5 (or any other super agent) itself works to

suppress cup burner flames, it is very unlikely that any encapsulated or complexed form

of these agents will work either. Since the reasons for this poor behavior are not clearly

understood, we believe that it is of value to perform some limited additional tests to

determine the reasons for the lack of effectiveness of this otherwise powerful agent. It is

important to resolve this unexpected result, not only to understand the behavior of these

most effective catalytic agents, but also to help direct efforts to define the most

appropriate test burner for use as an agent screen in future research.

1.8. Implications for Further Research

The present results for Fe(CO)5 in cup burner flames, carried out at one or two Fe(CO)5

concentrations in the fuel or air stream, are intriguing. In order to understand this

unexpected poor performance of the super agents in the cup burner, we recommend the

following additional tests.

A first step is to perform the tests of the effectiveness of blends of CO2 and Fe(CO)5 in

the cup burner over a range of mole fractions of Fe(CO)5. The purpose of these tests is to

search for abrupt changes in the efficacy of Fe(CO)5 in reducing the amount of CO2

necessary to extinguish the cup burner flame. From our previous work in premixed and

counterflow diffusion flames, we found distinctly different behavior of Fe(CO)5 at high

and low mole fraction, which was attributed to condensation of iron compounds. If
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similar behavior was found in cup burners, it would lead us to suspect similar reasons for

the change in effectiveness.

A next step to take to try to understand the unexpected poor performance of the super

agent Fe(CO)5 would be to test super-effective agents other than those of iron in the cup

burner. Other metallic agents may have different mechanisms of radical re-combination

(involving different types of metal-compound intermediates, or different flame radicals).

These other metals (and their oxides and hydroxides) will have different vapor pressures,

thermodynamic equilibrium conditions, and particle formation characteristics as

compared to iron, providing insight into the factors which are important for their

effectiveness, or lack thereof. Since phosphorus compounds are leading candidates as

super agents, it is essential to tests these compounds (alone and in combination with CO2)

in the cup burner to see if they are effective for this type of flame.

It has been hypothesized that the cause of the poor performance of Fe(CO)5 in the cup

burner flames is the formation of particles, which are then prevented from entering the

flame region by flow field and thermophoretic effects. The most direct way to test this

hypothesis is to measure for particles in the air stream of cup burner flames with added

Fe(CO)5 using classical laser scattering and extinction techniques. Finally, experiments

and numerical modeling of the flame structure of cup burner flames inhibited by inert and

catalytic agents are recommended. Clearly, even with particle measurements in Fe(CO)5-

inhibited cup-burner flames, and interpretation of the data based on similar measurements

and modeling in premixed and counterflow diffusion flames, it will not be possible to

truly understand the loss of effectiveness in the cup burner until flame extinction in the

cup burner in general is understood. This could be achieved through comprehensive

modeling and accompanying experiments on cup-burner flame structure. Given the

extensive database of flame suppressant effectiveness in the cup burner, and its wide use,

modeling and experiments to understand this ubiquitous metric seem warranted.

2. Bibliography

1. Babushok, V., Tsang, W., Linteris, G. T. and Reinelt, D., "Chemical Limits to Flame

Inhibition," Combustion and Flame, 115, pp. 551-560, 1998.

2. Rumminger, M., Reinelt, D., Babushok, V., and Linteris, G. T., "Numerical Study of the

Inhibition of Premixed and Diffusion Flames by Iron Pentacarbonyl," Combustion and Flame,

116(1-2), pp. 207-219, 1999.

3. Rumminger, M. D.; Reinelt, D.; Babushok, V. I.; Linteris, G. T., “Inhibition of Flames by Iron

Pentacarbonyl,” Halon Options Technical Working Conference. HOTWC-98. Proceedings.

May 12-14, 1998, Albuquerque, New Mexico, 1-12 pp, 1998.

4. Rumminger, M.D. and Linteris, G.T., "Particle Measurements in Fe(CO)5-Inhibited Flames,"

NIST Annual Conference on Fire Research, Rockville MD, Nov. 2-5, 1998.

5. Rumminger, M.D. and Linteris, G.T., “The Role of Particles in Flame Inhibition by Iron

Pentacarbonyl,” to appear in the proceedings of the Halon Options Technical Working

Conference, Albuquerque, NM, April 1999.
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6. Rumminger, M.D. and Linteris, G.T., “Particle Measurements in Flames Inhibited by Iron

Pentacarbonyl”, Joint Meeting of the United States Sections of the Combustion Institute,

Washington, D.C., March 1999.

7. Linteris, G.T. and Rumminger, M.D., “Flame Inhibition by Ferrocene, Carbon Dioxide, and

Trifluoromethane Blends: Synergistic and Anti-synergistic Effects,” 1999 Fall Technical

Meeting, Eastern States Section of the Combustion Institute, July, 1999.

8. Rumminger, M.D. and Linteris, G.T., “Numerical Modeling of Inhibition of Counterflow

Diffusion Flames by Iron Pentacarbonyl”, in Fire Safety Science Proceedings of the Sixth

International Symposium, International Association for Fire Safety Science (Michel Curtat,

Ed.), Marne-La-Vallee, France, 2000, pp. 289-300.

9. Rumminger, M.D. and Linteris, G.T., “Inhibition of Premixed Carbon Monoxide-HydrogenOxygen-Nitrogen Flames by Iron Pentacarbonyl,” Combustion and Flame, 120(4):451-464,

2000.

10. Linteris, G.T. and Rumminger, M.D., “Premixed Carbon Monoxide/Nitrous Oxide/Hydrogen

Flames: Measured and Calculated Burning Velocities with and without Fe(CO)5,”

Combustion and Flame, 122(1/2):58-75, 2000.

11. Rumminger, M.D. and Linteris, G.T. “The Role of Particles in Flame Inhibition by Iron

Pentacarbonyl,” Combustion and Flame, 123(1/2):82-94, 2000.

12. Linteris, G.T. and Rumminger, M.D., “Flame Inhibition by Ferrocene, Carbon Dioxide, and

Trifluoromethane Blends: Synergistic and Anti-synergistic Effects,” 1999 Fall Technical

Meeting, Eastern States Section of the Combustion Institute, July, 1999.

13. Linteris, G.T. and Rumminger, M.D., “Flame Inhibition by Ferrocene, Alone and with CO2

and CF3H,” Halon Options Technical Working Conference, Albuquerque, NM, May 2-4,

2000, pp. 129-140.

14. Lazzarini, A. K., Krauss, R. H., Chelliah, H. K., and Linteris, G. T., “Extinction of

Counterflow Diffusion Flames with Fine-Water Droplets,” Halon Options Technical Working

Conference, Albuquerque, NM, May 2-4, 2000, pp. 195-203.

15. Linteris, G.T. and Rumminger, M.D., “Flame Inhibition by Thermal and Catalytic Blends:

Synergistic and Antagonistic Effects,” accepted for publication in Proceedings of the

Combustion Institute, Vol. 28, April 2000.

16. Lazzarini, A.M., Krauss, R.H., Chelliah, H.K., and Linteris, G.T, “Extinction Conditions of

Non-premixed Flames with Fine Droplets of Water and Water-NaOH Solutions,” accepted for

publication in Proceedings of the Combustion Institute, Vol. 28, April 2000.

17. Wanigarathne, P.C., Krauss, R. H., Chelliah, H. K., and Davis, R.J., “Fire Suppression by

Particulates Containing Metallic Compounds,” Halon Options Technical Working

Conference, Albuquerque, NM, May 2-4, 2000, pp. 393-402.

3. Detailed Description of the Project

The overall goal of the project is to determine if effective but flammable and/or toxic metalbased compounds can be made safer and easier to handle through either their

absorption/adsorption into inert matrix materials, or through complexation of the active species

in a larger molecule. To this end, we performed several separate tasks, as described below. The

first sub-section below describes helpful background information on the action of the supereffective agents relevant to subsequent discussions. In the next sub-section, we describe the

work to examine absorption/adsorption and desorption of the super agents into the inert matrix
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materials. Of course, the interaction of these matrix particles (containing the super-agent) with

actual flames is an essential part of their performance. Consequently, we developed accurate

methods of quantifying the flame inhibition/suppression performance of particles added to

flames, and describe those methods in a separate sub-section. The other method for releasing

effective super agents into the flame is to complex them into a larger molecule; that is, to build

them into a less toxic and easier to handle molecule which then decomposes in the flame,

releasing the active moiety. A sub-section below describes tests with iron complexed in a safer

form. Finally, it is important to investigate the efficacy of the super agents in flames that closely

resemble fires. The cup burner flame resembles buoyant diffusion flames. It is widely used in

the fire protection community for evaluating the effectiveness of fire suppressants, and there is

an extensive database on the cup-burner extinction values for fire suppressants. Consequently,

we used the cup-burner as an indicator of the effectiveness of super agents in buoyant diffusion

flame fires. The last sub-section describes those tests.

3.1 Background

It is well known that some metallic compounds reduce the burning velocity of premixed flames

one or two orders of magnitude more than does CF3Br. The overall reaction rate, which is

related to the square of the burning velocity [1], is thus greatly reduced by these agents under

premixed conditions. If means could be found to incorporate such agents in a practical fire

suppressant (particularly for unoccupied spaces), very effective agents may be possible. Previous

work has shown this superior performance for iron pentacarbonyl Fe(CO)5, tetraethyllead

(C2H5)4Pb, and chromyl chloride CrO2Cl2 [2]. One of these agents, Fe(CO)5, has been studied in

some detail. It has been found to be an extraordinarily effective flame inhibitor, up to two orders

of magnitude more efficient than CF3Br at reducing the burning velocity of premixed flames

[3,4].

Recent progress has been made in understanding Fe(CO)5‘s mechanism of inhibition [5-11].

Experiments and modeling have quantified its performance and explained its mechanism of

inhibition for a variety of conditions. The strong inhibition is believed to occur from a catalytic

radical recombination cycle involving iron oxides and hydroxides:

FeOH + H ↔ FeO + H2

FeO + H2O ↔ Fe(OH)2

Fe(OH)2 + H ↔ FeOH + H2O

(net: H + H ↔ H2).

This mechanism leads to very strong inhibition for Fe(CO)5 mole fractions below about 100

ppm1; however, above this value, the inhibitor begins to lose its effectiveness. We believe this to

occur from condensation of the active iron-containing intermediates to particles [7,9]. The

reduction in the effectiveness of Fe(CO)5 as the mole fraction increases was shown to be much

more dramatic in premixed flames than in counterflow diffusion flames. Any practical fire

suppressant using these super-effective agents would require some method to overcome their loss

of effectiveness and high toxicity.

 

1

 All references to ppm in the report are on a volume basis, and refer to μL/L.
10

If some means can be found of absorbing/adsorbing Fe(CO)5 into an inert matrix and then

releasing it in the flame zone, or, alternatively, if other non-toxic forms of iron exist which are

also superb inhibitors and which maintain their action up to higher mole fractions, they could

lead to the development of very effective fire suppressants.

Throughout this report, uncertainties are calculated from the individual uncertainty components

and root mean square summation of components [12,13]. All uncertainties are reported as

expanded uncertainties: X ± U, where U is kuc, and is determined from a combined standard

uncertainty (estimated standard deviation) uc, and a coverage factor k = 2 (level of confidence

approximately 95 %). Likewise, when reported, the relative uncertainty is U / X · 100 %, or kuc /

X · 100 %.

3.2 Absorption Tests of Fe(CO)5 into Matrix Materials

The primary objectives of this task were to develop a system for introducing the inhibitor into the

matrix materials, quantify the absorption and desorption processes of the inhibitor into and out of

the matrix, and investigate the thermal stability of the matrix-inhibitor combination. We first

describe the possible matrix materials and super agents considered, and the rational for the tests

cases adopted. Next, we describe the method of absorbing the agent into the matrix. Following

that, we present the results for the amount of Fe(CO)5 which we were able to add to the various

support materials. The next sub-section describes additional methods to study the details of the

Fe(CO)5 absorption. Finally, we present data on tests with I2–impregnated particles which were

used as a surrogate material to test the concept.

3.2.1. Matrix Materials and Super Agents Considered

The selection of matrix material must consider several key factors. Since the primary purpose of

the matrix is storage and transport of the super agent, for ideal testing purposes it should be an

inert compound. In this context, porous silica and aerogel fits these criteria well. In contrast,

zeolite particles considered have Na+ imbedded in the structure and can influence the flame

inhibition. Similarly, porous carbon particles can promote flame propagation through oxidation

of carbon. For this reason, porous carbon particles were excluded from the list of matrix

materials.

Another important criterion is the surface area to mass of the particle. For example, zeolite

particles have a high surface area to mass ratio of about 800 m2/g, while crystalline sodium

bicarbonate particles have a very low value. The porous silica particles considered had an

intermediate value of 550 m2/g. Aerogel particles have high pore volume to mass, but offer

lower agent adsorption surface area.

Seeding of the matrix material in flame experiments is another important criterion to be

considered. For example, submicron particles were hard to seed in a steady, consistent manner

as opposed to particles above 5 μm. In this regard, zeolite particles with a mean diameter of 2

μm were on the borderline of our seeding capabilities. Hence, the maximum mass fraction of

seeded zeolite particles did not exceed 0.6 %. In comparison, porous silica particles in the size
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range of 15 μm to 40 μm were well behaved in our seeder, and a mass fraction as high as 6 %

was attained in flame experiments.

Finally, the desorption rate of the agent from the pore structure must be favorable. Ideally, the

matrix material must be stable during the heating and desorption process. Unfortunately, a

sample of zeolite particles collected after passing through a premixed flame indicated spherical

particles greater than the initial size. This perhaps indicates that the matrix material had melted

and, through agglomeration, formed large spherical particles.

Because of the superior flame inhibition properties of Fe(CO)5, most of the absorption effort was

devoted to this compound. As an alternative, iodine I2 was considered as a surrogate material for

testing the performance of an absorbed agent. Previous studies had indicated a weak chemical

adsorption of iodine in silica, premixed flame inhibition experiments with these iodine absorbed

porous silica particles were performed.

3.2.2. Fe(CO)5 Absorption Method

The system developed for introducing the Fe(CO)5 into the various matrix materials is shown in

Figure 1. The technique is a standard method for absorbing compounds into matrix materials.

The apparatus can be evacuated by a vacuum pump to a pressure of 7 Pa. In the initial

absorption studies, about 1 g of solid support material is first inserted into one of the tubes. This

sample is then heated under vacuum conditions to remove the absorbed gases from the internal

pore structure, which typically takes about 3 h to 4 h. After removal of the absorbed gases, the

support material is allowed to return to room temperature. The Fe(CO)5 to be absorbed is then

poured into the second tube, with the interconnecting valve closed. While keeping the Fe(CO)5

close to its freezing temperature, the valve in the interconnecting tube is opened briefly to

remove the gas-Fe(CO)5 mixture. Finally, the valve to the suction pump is closed and the

interconnecting valve is left open, allowing the Fe(CO)5 vapor to diffuse and absorb into the pore

structure. This vapor-phase absorption process typically takes about 1 to 2 days.

Iron pentacarbonyl was absorbed into particles of zeolite-NaX, zeolite-NaY, silica aerogel, and

porous silica. An overview is presented below, followed by a description of more detailed tests

for the quantity and characteristics of the absorbed iron compounds.
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Figure 1 - Schematic of the experimental setup for Fe(CO)5 absorption.

3.2.3. Fe(CO)5 Absorption Results

For zeolite-NaX with pore volume of 0.3 cm3/g [14], the estimated mass of Fe(CO)5 required to

completely saturate the internal pore volume is about 0.3 g/g of zeolite-X. About 10 % of

additional Fe(CO)5 is generally poured into the glass tube to ensure complete absorption. Based

on the mass increase of the tube containing zeolite-NaX, about 33 % increase in gross mass of

the particles was observed, which is consistent with the estimate based on the internal pore

volume. This implies that the particles of zeolite-NaX absorb the Fe(CO)5 into their pore

structure, as opposed to just adsorbing the agent onto the external surface. This amount of mass

uptake is encouraging, since only 100 ppm of Fe(CO)5 added to the reaction zone should reduce

the overall reaction rate by a factor of four. Thus, adding a mass loading of particles of 1 %

should provide sufficient Fe(CO)5 to yield an observable burning velocity or extinction strain

rate reduction in our apparatus.

After the Fe(CO)5 was absorbed into the zeolite-NaX, the particles changed color to a faint

brown/yellow. Based on the color change, the absorption appeared to be uniform in the zeolite

particle sample. In addition, a very faint coating (presumably Fe(CO)5) is observed on the tube

walls. When the absorption process is allowed to take place with the chemical hood light on, the

color of the Fe(CO)5-absorbed zeolite particles is a darker shade of brown. This is believed to be

caused by the decomposition of Fe(CO)5 to Fe2(CO)9 (iron nonacarbonyl) in the presence of

light.
13

When the Fe(CO)5–absorbed zeolite-NaX was exposed to air by opening the valve, a reaction

front propagated rapidly across the sample, sometimes with a bright orange colored emission

(apparently a flame inside the absorption tube). Hence, it appears that the iron pentacarbonyl

reacted pyrophorically with air. Additional experiments were conducted by exposing the

Fe(CO)5-impregnated zeolite-NaX particles to either pure oxygen or nitrogen. In oxygen, the

particles showed even more violent reaction, whereas in nitrogen, there was no reaction. These

tests show that the decomposition process is related to the oxygen concentration. While these

observations are qualitative, this unexpected result indicates that zeolite-NaX may not be an

appropriate support matrix for delivering unreacted Fe(CO)5 to a flame, and other support

materials are desired.

There is a theoretical basis for believing that zeolite-NaY would be a more desirable support

material than would be zeolite-NaX. The surface physical-chemical properties of the zeolite

likely have an important role in the Fe(CO)5 decomposition process. The general formula for a

unit cells of zeolite is given by Na_xAl_xSi_(192-x)O_384.yH_2O, where x ranges from 48 to

74 for zeolite Y and from 74 to 96 for zeolite X. While the cage structure of zeolite-NaX and -

NaY is similar, the difference between them is in the content of cation Na+ in the solid matrix, so

that zeolite-NaY has less Na+ than -NaX. This leads to weaker internal electric fields in zeoliteNaY than for zeolite-NaX, and hence less propensity for decomposition of the Fe(CO)5 in

zeolite-NaY than in zeolite-NaX. Hence, absorption experiments were also conducted with

zeolite-NaY. When exposed to air, however, the Fe(CO)5-absorbed zeolite-NaY also indicated

similar Fe(CO)5 decomposition characteristics, but the intensity of the observed reaction appears

to be much slower than with zeolite-NaX. In contrast, tests with porous silica particles gave

weak indication of Fe(CO)5 decomposition.

Besides porous silica particles, aerogel particles synthesized at University of Virginia were also

tested as a possible support material. These particles were formed by hydrolysis of tetrathyl

orthosilicate (Si(OC2H5)4) in ethanol, followed by drying to remove extra ethanol and water,

leaving behind a solid matrix which is 96 % porous. Three Fe(CO)5 absorption experiments were

conducted with the aerogel particles produced using the apparatus in Figure 1. Prior to absorbing

Fe(CO)5, the aerogel particles were heated to 200 °C to 250 °C to remove as much moisture as

possible. The first sample indicated a particle mass increase of about 200 %. Two subsequent

absorption experiments with aerogels, however, indicated Fe(CO)5 absorption of 14 % and

101 % of the aerogel particle mass. (Note that the second sample was from the same batch of

aerogel as the first, while the third sample was from a newly synthesized batch of aerogel).

Furthermore, both the second and third samples reacted after about 5-10 minutes exposure to air,

while the first sample with substantially large increase in mass (about 200 %) did not react when

exposed to air for a long period of time.

The decreased uptake of Fe(CO)5 in the second and third aerogel samples and the subsequent

reaction when exposed to air lead us to believe that either water or ethanol in the aerogel (from

the synthesis process described above) was not completely removed. Since Fe(CO)5 is known to

react violently with water, the absorbed water is perhaps the most likely cause for the above

discrepancy in Fe(CO)5 absorption and its decomposition.
14

3.2.4. Quantification of Fe(CO)5 Absorption/Desorption

A standard technique for measuring the desorption or the decomposition properties of materials

is thermogravimetric analysis TGA. In these tests, the sample is continuously weighed, while its

temperature is increased. The TGA analyses required transferring of the Fe(CO)5 absorbed

particles from the absorption flask to the weighing pan of the TGA. During this process, some

fraction of the particles can react with ambient air, especially those with zeolite and alkali

carbonates as support material. Minimizing the time involved in this transfer process, and hence

exposure to air, TGA analysis were performed for all of the absorbed particles. In the case of

Fe(CO)5-absorbed aerogel particles, the sample did not react immediately when exposed to air,

hence the desorption of Fe(CO)5 from the aerogel particles measured via TGA is probably the

most reliable. However, the desorption of Fe(CO)5 from aerogels showed a great variation

depending on the sample used. For example, some aerogel samples showed as little as 5 %

desorption while the maximum detected was 200%. In the case of zeolites, silica and aerogels,

the sample were heated up to about 500 °C in steps of 5 °C/min, while NaHCO3 and Na2CO3

were heated only to 150 °C because of their propensity to decompose. Based on the TGA data,

Table 1 shows, in the second column, the weight loss observed of all the particles considered. It

is not clear whether the observed weight loss is due to desorption of undissociated Fe(CO)5 or

decomposed carbonyl, which may describe the discrepancy with some of the elemental analysis

data presented below.

Table 1 - Summary of the Fe mass fraction in Unheated and Heated particles.

From TGA From Elemental Analysis

Fe % (by mass) Fe % (by mass) Na % (by mass)

Support Material Heated Unheated Heated Unheated Heated

Zeolite-NaX 7.1 ± 0.7 8.9 ± 0.9 7.1 ± 0.7 11.0 ± 1.1 9.8 ± 1.0

Zeolite-NaY 7.1 ± 0.7 9.7 ± 1.0 6.8 ± 0.7

Porous Silica 6.0 ± 0.6 12.8 ± 1.3 6.8 ± 0.7 < 0.8 < 0.1

Silica Aerogel 4 to 57

NaHCO3 5.0 ± 0.5 < 0.1 < 0.2 28.0 ± 2.8 41.0 ± 4.1

Na2CO3 2.0 ± 0.2

Elemental analysis was performed (by Southern Testing and Research Laboratories2) to quantify

the amount of Fe (and Na) in absorbed and reacted particles (after exposing to air). Table 1 lists

the support matrix in the first column, with the mass fraction of Fe in the sample after exposure

to Fe(CO)5, based on the weight loss of the sample in the TGA measurements, in the second

column. The third through fifth columns provide the results of the elemental analysis for Fe and

Na. These data are presented for the two cases: unheated and heated particles. The heated

 

2

 Certain commercial equipment, instruments, materials or vendors are identified in this paper to adequately specify

the procedure. Such identification does not imply recommendation or endorsement by the National Institute of

Standards and Technology, nor does it imply that the materials or equipment are necessarily the best available for

the intended use.
15

particles were raised to 500 °C in air for several hours to ensure that Fe(CO)5 was fully

decomposed.

Comparison of the elemental analyses for the two cases indicates that heating of the reacted

particles had a small effect on the absorbed Fe level for zeolite particles, but a larger effect for

porous silica particles. Hence, it appears that any Fe remaining in the zeolite particles was no

longer intact Fe(CO)5 that could be liberated by heating, but rather, was already decomposed.

For porous silica, about half of the Fe(CO)5 was decomposed or unavailable for liberation by

heating. Interestingly, the mass fractions Fe- and Na-atoms in zeolite are considerable and are

very similar. If the Na could be liberated from the zeolite particles in the flame, they might

provide flame inhibition even without added iron.

In order to determine whether any unreacted Fe(CO)5 remained inside the particle after exposure

to air, infrared (IR) spectroscopy analysis was performed. The reacted zeolite or porous silica

particles were crushed and then mixed with potassium bromide particles. This mix is then

pressed to form a thin wafer and then tested in the IR spectrometer. A search for the Fe-CO

stretching bond (characteristic of Fe(CO)5) indicated that these was little Fe(CO)5 remaining in

the solid support matrices.

The Fe(CO)5-impregnated and reacted zeolite particles were also analyzed using the highresolution transmission electron microscope (HRTEM). Images of zeolite-NaY particles after

absorption of Fe(CO)5 and exposure to air showed that a condensed-phase substance formed on

the exterior surface. For example, Figure 2 shows an image of a zeolite particle after Fe(CO)5

was absorbed and then the sample exposed to air, while for comparison, Figure 3 shows an

image of a pure zeolite particle. In Figure 2, the zeolite-NaX particle is shown to have a

diameter on the order of a micrometer, with pore sizes of the order of 1 nm, while nodules of

condensed–phase material (likely Fe, FeO, or perhaps even Fe2O3 ) have diameters of about 20

nm, and are rather well dispersed on the support material. The main purpose of this investigation

was to obtain qualitative information about the nature of the absorbed agent. The formation of

these condensed-phase nodules implies that Fe(CO)5 may not be available from the matrix to

provide gas-phase Fe, which is required for efficient flame inhibition.
16

Figure 2 - High resolution transmission electron

micrograph image of Zeolite-NaX particle with

condensed phase iron species on surface.

Figure 3 - Pure zeolite-NaX, without Fe2O3

clusters.

3.2.5. Surrogate Agents for Absorption/Desorption Testing

With the unexpected difficulties associated with Fe(CO)5 decomposition within the solid support

material, the possibility of using other chemical compounds to quickly test the encapsulation

concept was pursued during the last year. Because of previous experience in

adsorption/desorption of iodine in zeolite particles, iodine was selected as a suitable, safe

compound to handle in the laboratory. Recent absorption/desorption investigations have

indicated that an iodine mass fraction of about 24 % can be loaded into zeolite-NaX particles,

while about 6 % mass fraction of I2 can be loaded into the milled and sized (10 μm to 20 μm size

range) crystalline silica particles. Here, crystalline silica particles were considered because of

their better fluidization and seeding characteristics. In addition, absorption of iodine in the

porous silica gel particles (used in above flame studies) has been carried out. To quantify the

amount of iodine loaded into these particles, TGA analyses were performed on two samples.

One set of particles was tested immediately after the absorption process and the other, after the

particles had remained in the airflow of the fluidization seeder for several hours. The TGA

analysis on the fresh particles indicated a mass loss of 19 %, while that on the particles out of the

seeder after the experiments indicated a mass loss of about 8 %. We also noted a change in the

particle color (to a lighter brown shade) after they were in the seeder for several hours, a result

consistent with the lower mass loss for the post-seeder particles.
17

3.3 Methods of Quantifying Suppression Effects by Particulates

In order to determine the effectiveness of particles impregnated with super-effective catalytic

inhibitors, it is first necessary to have a method to add particles to the appropriate flame. To this

end, we have designed, built, and tested an effective particle seeding system for introduction of

the NaHCO3, silica, zeolite, or aerogel particles to both counterflow diffusion flames and

premixed Bunsen-type flames. The particle seeder designed was modular in nature and could be

easily adapted to either flame. While development of the seeder took much time and care, it was

critical for the ultimate success of this investigation. Although this delayed some of the other

tasks of the project, the results have been broadly useful to other NGP researchers who are now

basing their designs on that developed here. We first describe the counterflow burner and

particle seeding system, followed by a description of the premixed burner and LDV system and

the conclusions for this section. Sodium bicarbonate and silica particles were used as the test

cases for development of the seeding and burner systems, and the results for those materials are

presented below.

Two burner types were selected as test beds for the particles: a premixed Bunsen-type nozzle

burner, and a gaseous counterflow diffusion flame burner, also with nozzle-generated flows.

These flames were selected for several reasons. First, and foremost, unlike co-flow diffusion

flames burners (such as cup-burner flames), they both allow unambiguous determination of the

amount of agent with reaches the relevant high-temperature reaction region of the flame. In

contrast, it is difficult in cup-burner flames to determine the amount of agent which actually

makes it to the region of the flames where it is need to extinguish the flame. These values are

difficult to obtain for a cup-burner since the transport of agent or particles to the appropriate

region of the cup-burner is complex; moreover, stabilization (and therefore extinction) of cupburner flames is not presently well understood theoretically. Further, cup-burner flames allow

tests at only one strain rate; whereas, counterflow diffusion flames allow tests at variable strain

rates. We originally planned to do tests only in the counterflow diffusion flames. As we gained

experience working with particles, however, we became aware of the danger of Fe(CO)5-

impregnated particles contaminating the laboratory space. Because premixed flames involved

much smaller quantities of gas and particles, and since all of the reactants pass through a flame,

they were believed to provide a safer configuration for the tests. Since premixed flames directly

provide a measure of the overall reaction rate in the flame, results in those flames should be

directly comparable to those in a counterflow diffusion flame. As discussed below, Peters has

shown [15], from a theoretical basis, that the performance of a chemical catalytic inhibitor in

premixed and counterflow diffusion flames should be correlated. Most importantly, either one

should provide a direct measure of the effect of an agent on the overall reaction rate in the flame,

in a configuration which is easily interpreted or modeled. The development of these two test

beds for experiments with particles is described below.
18

3.3.1 Counterflow Diffusion Flames with Inert and NaHCO3 Particles

3.3.1.1. Experiment

A steady, planar, non-premixed flame was established in the mixing layer of counterflowing

methane and air streams. Figure 4 shows the experimental setup of the counterflow burner. The

fuel and air nozzles are made from Pyrex glass. The exit diameter of the inner tubes is 15 mm

with a nozzle area contraction factor of 6.5, producing a nearly plug flow velocity profile at the

nozzle exit. The co-annular nitrogen streams on both fuel and air sides help maintain a stable

planar flame disk. The cylindrical burner chamber includes provision for dilution of the chamber

with room air at the bottom (to minimize the occurrence of secondary flames), water cooling,

and easy adjustment of the nozzle separation distance via o-ring sealed vacuum fittings (MDC).

The typical separation distance between the two nozzles is 10 mm to 12 mm. The exhaust gases

were evacuated using an air-driven mass flow ejector, with the chamber pressure monitored by a

differential pressure gauge. The methane was BOC grade 4.0 with a purity of 99.99 %. The air

was supplied by an oil-free compressor and dried by a series of dessicant beds. The flow rates of

fuel and air were controlled using factory-calibrated mass flow meters (Teledyne HastingsRaydist), with a reported accuracy of ± 1 %.

3.3.1.2. Particle Seeder:

Because of the challenge in verifying the accuracy of previously reported data with NaHCO3

particles [16,17], considerable attention was devoted to the development of a particle seeder that

could provide steady feed rates of particles of various diameters (i.e., from 2 μm to 100 μm).

For the small air flows involved (typical flows of 0.1 L/s), the fluidized bed approach shown in

Figure 4 gave satisfactory seeder performance only for relatively small particles, i.e., less than 30

μm. This fluidized bed seeder consists of a 19 mm diameter glass tube with two porous plugs at

either end. The air flow to the seeder is controlled through a mass flow controller and split into

two streams, which are connected to either end of the glass tube. The flow passing through the

bottom, which controls the fluidization level, is monitored through another flow controller. The

fluidized particles and the air entering the fluidization tube (from the top and bottom) is extracted

through a small tube located near the middle of the tube. By maintaining the total air flow to the

fluidizer tube constant, the particle feed rate is steady, and this is verified by continuously

monitored the light scattered from the particle stream, as shown in Figure 4. The detection

system consists of a laser diode (671nm), a high speed silicon detector (Thor Labs DET1-SI )

with a 3 cm focal length collection lens, laser line filter at 671 nm (Edmund Scientific), and a

strip chart recorder (Hewlett-Packard Model 7132) to monitor the detector output.
19

Figure 4 - Schematic diagram of counterflow diffusion flame burner with provision for addition of particles

to air stream.

3.3.1.3. Results

Figure 5 shows an image of a typical flame disk established without and with NaHCO3 particles.

Experiments have been conducted to establish the effectiveness of particle size on non-premixed

flame suppression. These results are shown in Figure 6 as the mass fraction of NaHCO3 needed

vs. flow strain rate at extinction, for particle sizes 10 μm to 20 μm, 20 μm to 30 μm, 30 μm to 40

Fluidizer air

Co-flow

Ejection

Water 

Nitrogen 

Methane 

Observation 

windows

Fluidizer 

Nitrogen 

Laser 

diode 

and the 

detector
20

μm, and 40 μm to 60 μm. For comparison, the effectiveness of 10 μm to 20 μm crystalline silica

particles are also shown. It is clear that NaHCO3 particles below 20 microns are significantly

more effective in this flow field. As the NaHCO3 particle size increases, the performance

approaches that of the inert silica particles.

Figure 5 - Methane-air counterflow diffusion flame pure (left), and with NaHCO3 particles (right).

 NaHCO3 and Inert Particle Mass Fraction vs. Strain Rate

0

1

2

3

4

5

6

100 150 200 250 300 350 400 450

Extinction Strain Rate (1/s)

Mass Fraction (%)

NaHCO3 0-10 micron (seeder 2)

NaHCO3 10-20 micron (seeder 2)

NaHCO3 20-30 micron (seeder1)

NaHCO3 30-40 micron (seeder 1)

NaHCO3 40-60 micron (seeder 1)

NaHCO3 20-30 micron (seeder 2)

SiO2 10-20 micron

SiO2 10-20 micron 

Silica Gel 15-40 micron

Figure 6 - NaHCO3 and Silica mass fraction vs. methane-air nonpremixed extinction strain rate.

Figure 7 shows two photographs of the crossed laser beams from an LDV system penetrating a

non-premixed counterflow methane–air flame seeded with 40 μm to 60 μm NaHCO3 particles.

In the upper image, the beams are above the flame, in the region between the flame and the

incoming air stream (which is laden with particles), so that the particles have not yet reached the
21

flame. Laser scattering from the particles is clearly visible. In the lower image, the beams are

located below the flame and there is still obvious laser light scattering, indicating these large

particles survive through the flame. This behavior results in the lower suppression effectiveness

as illustrated in Figure 6. These results also indicate that even inert particles tend to reduce the

flame extinction rate with increasing particle loading. Such effects can be related to the thermal

effects associated with heating of these inert particles.

Parallel modeling efforts have shown that by selecting an appropriate value for the particle

decomposition temperature, the experimental results shown in Figure 6 can be mimicked. With a

fixed particle decomposition temperature, the trends observed in Figure 6 for the variation in

effectiveness due to particle size can then be captured. The basic outcome of such modeling

results is that the existence of a critical particle size which is effective in this flow field is

directly related to the flow residence time, hence the heating of the particle and its

decompositions rate, which would ultimately control the chemical effectiveness.

Figure 7 - Laser Doppler velocimetry measurements of NaHCO3 particles in a counterflow diffusion flame of

methane and air, showing that the particles pass through the flame. The two images show laser scattering

(bright green) when the probe beams are directed either above or below the flame (indicated by the bright

orange emission from sodium).

3.3.2 Premixed Flames with Inert and NaHCO3 Particles

Premixed flames afford a clear measure of the effect of an inhibitor on the overall reaction rate

while also providing a safe, convenient system for controlling the release of toxic particles.

Since there is a theoretical basis for relating the performance of an inhibitor in premixed flames

to that in counterflow diffusion flames [15], we pursued flame inhibition tests in premixed

Bunsen-type flames as described below.

3.3.2.1. Experiment

A Mache-Hebra nozzle burner (1.5 cm ± 0.05 cm diameter) produces a premixed Bunsen-type

flame about 1.9 cm tall with a straight sided schlieren image that is captured by a CCD array

camera connected through a USB port to a PC. Digital mass flow controllers hold the
22

equivalence ratio φ and the flame height constant while the mass flow of particles from the

seeder is set at the desired value. The average burning velocity is determined from the reactant

flows and the schlieren image using the flame angle method [18] from the measured velocity

profile. The fuel gas is methane (BOC grade 4.0; 99.99 %), and the oxidizer stream consists of

laboratory air from an oil-free compressor which was dried by a series of desiccant beds. Air

and fuel gas flows are set by PC-controlled mass flow controllers (Sierra, 860).

3.3.2.2. LDV System

A laser Doppler velocimetry (LDV) system was assembled to establish the quality of the top hat

velocity profile leaving the nozzle exit (for both non-premixed and premixed flames) and to

measure the local strain rate in the counterflow field. Because the enclosed counterflow burner

permitted optical access from the two opposing sides, an on-axis forward scattering LDV

configuration was selected. The LDV system consisted of an Argon Ion laser (Lexel Model 95)

operating at 514.5 nm with a beam splitter and 300 mm focal length transmitting lens (DISA) to

obtain two equal intensity beams forming a fringe pattern at the intersection of the focused

beams. The probe volume was 0.151 mm wide and 1.51 mm long, with a fringe spacing of 2.57

μm. The collection optic system (DISA) gathered the forward scattered photons from the

particles and imaged them on a photomultiplier tube (PMT). The PMT current was amplified

(C-Cor Electronics Model 4375A Wideband Video Amplifier) and processed by a timer/counter

(TSI Model 1990B) operated in the single burst mode using 8 cycles/burst. The filtered and

amplified timer/counter signal was monitored by an oscilloscope (Tektronix Model 2212), and

the digital output was interfaced to a personal computer using a digital input card (National

Instruments DAQ pad 6510) and processed using data aquisition software (LabView). Measured

frequencies were related to particle velocity using the known fringe spacing. To facilitate

acquisition of velocity profiles in axial and radial direction of the counterflow field, both fringe

generating and collecting optics were mounted on translation stages (Velmex Unislide Model

MB2500), which have a minimum step size of 1 μm, with a reported accuracy of 5 μm in 100

mm. Figure 8 shows typical velocity profiles at the exit of the premixed burner nozzle, for two

different volume flow rates. For comparison, the calculated top-hat velocity profiles are also

shown as dotted lines.
23

0

1

0 0.5 1 1.5

Horizontal Location (cm)

Upward Veloctiy m/s

9 L/min

7.4 L/min

Figure 8 – Calculated (dotted lines) and LDV-measured (points) velocity profiles at exit of premixed burner

nozzle for total flow rates of 7.4 and 9 L/min.

3.3.2.3. Results

We characterized the performance of the premixed burner through flame speed measurements in

the presence of silica, zeolite, and sodium bicarbonate particles. Unfortunately, few data were

available in the literature for comparison with the present data. Figure 9 shows this burner

operating with methane-air, with and without NaHCO3 particles added to the reactant flow.

Extraction of the burning velocity information from this setup requires obtaining the flame

surface area or the cone angle. A schlieren system allowed determination of the flame area for

conditions with a high background visible radiation (e.g., blackbody radiation with inert particles

or sodium D-line emission with NaHCO3 or zeolite). The burner exit velocity was characterized

using an LDV system. A comparison of the normalized burning velocity obtained for various

particles tested are shown in Figure 23. The results clearly indicate that the inert silica particles

and zeolite-NaX have a minor effect on the measured burning velocity, while NaHCO3 has a

noticeable effect on the burning velocity.
24

Figure 9 - Premixed methane-air nozzle burner flame pure (left) and with NaHCO3 particles (right).

Figure 10 - A comparison of reduction of normalized burning velocity with mass fraction of

silica, NaHCO3, and zeolite particles.
25

Comparison of the relative reduction of normalized, non-premixed flame extinction strain rates

with normalized premixed burning velocities indicates significant differences. However, such

comparisons should take into account the structural differences of the two flames. As shown by

Peters [15] and employed by Chelliah et al. [19] for chlorinated hydrocarbon flames, the

normalized burning rate is related the normalized extinction strain rate as SL/SL,0 = √(aext/aext,0)

Using this correlation, the modified comparison of the non-premixed and premixed flames

inhibited with NaHCO3 is shown in Figure 11. The disagreement between the two is somewhat

greater than the uncertainty in each measurement; however, this disagreement is much less than

one might expect based on the significantly different structures of the premixed and diffusion

flames. Hence, we expect that tests with either flame should be roughly comparable in

demonstrating the effectiveness of chemically acting agents on the global reaction rate in

methane-air flames. On the other hand, the data for inert particles in premixed and counterflow

diffusion flames from Figure 6 and Figure 10, if plotted in Figure 11 would show a significant

discrepancy. Most likely, these differences are related to the thermal effects associated with

varying particle residence time in the two flows considered, and further analysis is required.

Normalized flame strength vs NaHCO3 Mass fraction

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2

Mass Fraction (%)

Normalized flame strength

Premixed

Nonpremixed

Figure 11 - Normalized flame strength as a function of added NaHCO3 mass fraction for premixed and

counterflow diffusion flames.

As discussed below, recent experiments with iron species in cup-burner flames did not indicate

much benefit. Hence, for these chemicals in actual fires, there may not be much utility to their

encapsulation. Another option is to test the technique with known, effective halogens such as I2

or Br2. Experiments of this type were conducted, and are discussed below. The key features,

aside from the rate of release of the agent from the matrix, are the mass of I2 or Br2 required, and

the fractional holding capacity of the matrix.
26

3.3.2.4. Conclusions

The fluidized-bed seeding system described above is effective for introducing particles of less

than 30 μm to the premixed and counterflow burners. The experimental techniques developed

are amenable to testing the efficacy of particles with absorbed super-agents in these flames.

Premixed and counterflow diffusion flames have the advantage in that they can directly provide a

measure of the effect of the particles on the overall reaction rate of the fuel-air mixture. We have

determined, however, that in these flames, the effectiveness of particles (which either decompose

or release an absorbed agent) is dependent upon the residence time for this process. For

example, the effectiveness of the sodium bicarbonate as a flame inhibitor was seen to be

dependent upon the size of the particles. The correlation between the performance of NaHCO3

in premixed and counterflow diffusion flames was close, although there was significant

discrepancy which was likely due to the different residence times for particle decomposition in

each of the two flame configurations.

3.3.3 Premixed Flames with Iodine

Since particles in the laboratory tend to be ubiquitous, there was some concern about safety of

seeding particles embedded with Fe(CO)5 into a counterflow diffusion flame. Premixed flames

have much smaller flows, and all of the reactants pass through the flame, increasing the

likelihood of decomposing Fe(CO)5 in the particles. Hence, premixed flames were selected for

the first tests of agent-impregnated particles. The NGP executive committee recommended tests

with CF3I as a surrogate for Fe(CO)5 in order test the concept. Since iodine I2 would be expected

to be about twice as effective as CF3I, we decided to use it rather than CF3I in these tests.

Selection of the appropriate matrix material for the particles was based on agent mass loading

potential and the seeding properties of the particles in the flow. With the zeolite-NaX, maximum

particle seeding achievable with the present apparatus used in the premix flame configuration is

about 0.5 % by mass (because of very small size, 2 μm). On the other hand, in the same

premixed flame configuration, more than 2 % by mass of sized 10 μm to 20 μm crystalline silica

particles can be introduced, yielding a similar iodine loading. These limitations, together with

the mass fraction of I2 absorbed into the particle, limit the mass fraction of I2 which can be added

to the flame to about 0.12 %. With silica gel particles (15 μm to 40 μm), however, with much

higher particle seeding (up to 6 % particle mass) and about 20 % iodine mass loading, the

resultant theoretical iodine mass delivery can approach as high as 1.2 % in a flame environment.

Therefore, premixed flame inhibition experiments were conducted only with iodine absorbed

silica gel particles. Figure 12 shows the normalized flame speed of a methane-air flame for

increasing particle mass fraction, for pure silica gel and I2–impregnated silica gel. As seen from

this figure, iodine absorbed silica particles do not show any significant reduction in burning

velocity compared to chemically inert silica particles.

Based on literature values of the burning velocity reduction of Br2 in hydrocarbon flames [20],

we estimate that an I2 mass fraction of 10 % would be required to reduce the burning velocity by

a factor of two. Hence, even if all of the I2 in the silica gel particles (estimated to be a mass

fraction of 0.20) were liberated in the flame, the maximum expected burning velocity reduction

would be about 5 % for a 6 %. In the present experiments, with very high particle seeding, such
27

a difference in burning velocity may be difficult to detect; for lower iodine desorption rates,

difference with and without I2 would be even harder to see.

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6

Mass Fraction %

Normalized Flame speed

SiO2

 SiO2 with I2

Figure 12 – Normalized flame speed for a methane-air flame with added silica gel particles (14 to 40)μm, with

and without I2 impregnated in the particles.

3.4. Complexation of Super-Effective Metals

3.4.1. Super Agents and Complexing Ligands Considered

As a first step, the physical and chemical properties of complexes of potential agents were

compiled and surveyed. The complexes involved main-group or metal-centered with different

ligands. Organic ligands included the following: 1. acetate (ac), acetylacetate (acac), oxalate; 2.

trifluoroacetylacetate (TFacac), hexafluoroacetylacetonate (Hfacac); 3. methoxide (OMe),

ethoxide (OEt). The halide ligands: chlorine (Cl), bromine (Br), and iodine (I) were also

considered. The main-group centered compounds included 1. Group III - aluminium (Al); 2.

Group IV - tin (Sb), germanium (Ge), lead (Pb); 3. Group V - antimony (Sb). Metal-centered

compounds included the following classes: 1. copper (Cu); 2. titanium (Ti); 3. chromium (Cr),

molybdenum (Mo); 4. manganese (Mn), nickel (Ni), cobalt (Co); 5. iron (Fe).

For the metal-centered halides, it was found that the three most volatile compounds MX4 were

(in order of volatility): TiX4 < SnX4 < GeX4. The bromides become volatile in the range 70 °C
28

to 150 °C, while the chlorides were higher in the range (180 °C to 240 °C). Germanium iodide

was iodide compound with the lowest boiling point - estimated near 350 °C.

For the organic-complexes, it was found that the order of volatility was generally

oxalate < ac < acac < OMe < OEt < TFacac < Hfacac.

Examples of this include Cr(HFacac)3 and Cr(ac)3 that sublime or decompose near 100 °C to 160

°C and 210 °C to 280 °C, respectively and Cu(HFacac)2, Cu(TFacac)2, and Cu(acac)2 that

sublime or decompose near 130 °C to 180 °C, 180 °C to 210 °C, and 290 °C to 320 °C,

respectively.

The relative volatility of complexes was dependent on the metal- or main-group-center atom.

The order of volatility for compounds with different ligands varies dependent upon the ligand.

Consequently, a direct ordering is not exact. However, in general the volatility was

Al < Cu < Mo < Mn < Ni < Cr < Sn < Ti < Fe < Co < Sb < Pb < Ge

That is, aluminum and copper compounds were generally significantly less volatile than lead and

germanium compounds.

3.4.2. Selection of Ferrocene

As described above, there was a wide range of possible choices for complexed metal agents. For

our first complexed agent, we chose a compound which contained a known, highly active metal.

This limited the metals to Pb, Cr, and Fe. Because of the high toxicity of Pb and Cr in most

states and the low toxicity of Fe in its most common forms, we selected iron as the element to try

to complex. Previous research has shown that iron atom in the gas phase leads to the inhibiting

iron-species intermediates, and that the main property required for the parent molecule is that it

readily decomposes at flame temperatures to release iron atom. A possible alternative source of

Fe, and the one selected as a the first complexed agent is ferrocene (Fe(C5H5)2 or Fec).

Ferrocene modifies the sooting tendency of flames [21-26], is added to materials as a flame

retardant [27], is an antiknock agent, and is used as a source of iron atoms for kinetic studies. It

is also far less toxic than Fe(CO)5.

In this section we present the first measurements of flame inhibition by ferrocene, and compare

its performance with that of Fe(CO)5 and CF3Br in the same flames. We numerically model

Fec’s flame inhibition using the iron-species mechanism developed for studies of Fe(CO)5 flame

inhibition. Finally, we present results on the performance of Fec in combination with other

agents, including CO2, and CF3H. The present research demonstrates that the efficiency of

Fe(CO)5 is not unique, and that there are methods for overcoming its loss of performance. (More

details of the work can be found in ref. [28]).

As described above, flame inhibition by highly effective chemical inhibitors has been described

in the literature. The inhibition is characterized by several gas-phase radical recombination

cycles which proceed at nearly gas-collisional rates. It has been shown that radical
29

recombination on particle surfaces is not likely to be fast enough to produce the observed

burning velocity reductions at low inhibitor mole fraction [7]. Particle formation is important,

however, in that condensation serves as a loss mechanism for the active inhibiting species when

their concentration is above the saturation vapor pressure. Additionally, at high enough inhibitor

mole fractions, a loss in effectiveness is expected for all catalytic inhibitors since eventually,

radical concentrations are lowered to equilibrium levels, and further reduction is not possible.

For Fe(CO)5 however, the loss in effectiveness caused by condensation occurs well before the

approach to equilibrium of radicals.

Although the present tests are performed in premixed flames of methane, they are relevant to the

suppression of practical fires. Methane oxidation is atypical of that of larger hydrocarbons;

however, Babushok and Tsang have recently observed [29] that for a wide variety of

hydrocarbons, including methane, the burning velocity is most sensitive to the rates of the same

reactions. Since these reactions are the ones most influenced by an inhibitor, the trends in

inhibitor effectiveness are the same for most hydrocarbons. As has been discussed previously

[10], burning velocity measurements are an important first step in assessing an inhibitor’s

effectiveness and testing mechanism performance. In future research, it is desirable to test these

highly-effective agents in flames more closely resembling actual fires.

3.4.3. Experiment

The flame speed SL provides a measure of an agent’s reduction of the global reaction rate. The

experimental arrangement, described in detail previously [9,10,30], has been modified to

accommodate a new evaporator for ferrocene and heating of the gas lines and burner tube[28]. A

Mache-Hebra nozzle burner (1.0 cm ± 0.05 cm diameter) produces a premixed Bunsen-type

flame about 1.3 cm tall with a straight sided schlieren image that is captured by a video framegrabber board in a PC. Digital mass flow controllers hold the oxygen mole fraction in the

oxidizer stream XO2,ox , the equivalence ratio φ, and the flame height constant while maintaining

the inlet mole fraction of the inhibitor (Xin) at the desired value. The average burning velocity is

determined from the reactant flows and the schlieren image using the total area method. The fuel

gas is methane (Matheson UHP, 99.9 %), and the oxidizer stream consists of nitrogen (boil-off

from liquid N2) and oxygen (MG Industries, H2O < 50 ppm, and total hydrocarbons < 5 ppm).

The inhibitors used are Fec (Aldrich), Fe(CO)5 (Aldrich), CF3H (DuPont), CF3Br (Great Lakes),

N2, and CO2 (Airgas). The Fe(CO)5 is added to N2 carrier gas using a two-stage saturator in an

ice bath. Because the vapor pressure of Fec is much lower than that of iron pentacarbonyl, Fec

addition at mole fractions up to 650 ppm requires both higher bath temperature (79.1 °C held

within 0.1 °C) and higher nitrogen carrier gas flow rates (up to 2800 cm3/min) relative to

Fe(CO)5. Also, the solid state of Fec requires an evaporator with larger surface areas for heat

and mass transfer. Our evaporator design, based upon that of Megaridis [22], has a (30 ± 5) cm3

packed bed (to provide the bulk of the ferrocene), followed by thirty sublimation stages (to

insure that the carrier gas is saturated with Fec at the bath temperature). Each sublimation stage

consists of a 5 mm layer of ferrocene on a 2.36 cm diameter 60 mesh stainless steel screen. A 4

mm gap separates each stage. The vapor pressure correlation of Pelino et al. [31] is used to

determine the ferrocene mole fraction in the carrier gas. Temperature controllers maintain the

transfer lines at (80 ± 3) °C and the burner tube at (80 ± 1) °C. For all flames, the equivalence

ratio (in the absence of inhibitor) is 1.0, and agent mole fraction is calculated relative to the total
30

reactant flow. The flows of fuel, oxidizer, Fec-N2, and the blended agent (CO2, or CF3H) are

mixed after the Fec evaporator. The inlet reactant stream temperature is (80 ± 1) °C, which

corresponds to a calculated adiabatic flame temperature of 2260 K and 2391 K for uninhibited

flames at XO2

,ox=0.21 and 0.244.

3.4.4. Numerical modeling

The laboratory flames inhibited by Fec and Fec-CO2 blends were numerically modeled as onedimensional freely-propagating flames. Solutions were obtained using the Sandia flame code

Premix [32], and the Chemkin [33] and transport property [34] subroutines. Details of the

calculations are provided elsewhere [28]. Little is known about the chemical kinetic behavior of

ferrocene in flames. A rate constant exists for its thermal decomposition: k = 2.188 ⋅ 1016 exp(-

384 kJ/RT) s-1 [35], but the activation energy is high, leading one to suspect that reactions with

radicals may also be important. Thermodynamic data are available [36,37], and the transport

properties are estimates [38,39]. A reaction set for combustion of methane and larger

hydrocarbon fragments was adopted from Sung et al. [40]. We added iron chemistry from a

mechanism developed for flame inhibition by Fe(CO)5 [5]. Overall, the kinetic model contains

105 species and 677 reactions. Calculations showed that addition of C5H5 at mole fractions up to

400 ppm had negligible effect on the burning velocity, and that the major effect of Fec is from

the iron chemistry. It should be emphasized that the reaction mechanism used for the present

calculations should be considered only as a starting point. Numerous changes to both the rates

and the reactions incorporated may be made once a variety of experimental and theoretical data

are available for testing the mechanism.

3.4.5. Results and Discussion

3.4.5.1. Inhibition by Ferrocene

Figure 13 shows the relative burning velocity reduction with addition of Fec (open symbols) or

Fe(CO)5 (closed symbols) to the present slightly pre-heated (80 °C) methane-air flames. (The

uncertainties in the experimental data, described in detail previously [41], are typically about

± 5 %.) Data are plotted as normalized burning velocity, which is the burning velocity of the

inhibited flame divided by the value for the same flame in the absence of inhibitor. The

uninhibited experimental burning velocities used for the normalizations are (53.7 ± 3) cm/s and

(75.9 ± 6) cm/s for XO2

,ox = 0.21 and 0.244 respectively; for comparison, the calculations for

uninhibited flames using GRI-Mech 1.2 yield 55.5 cm/s and 72.6 cm/s. The effect of the two

agents is essentially the same, with very strong initial inhibition followed by a loss of

effectiveness above a few hundred ppm of agent, as found previously for Fe(CO)5 inhibition in

flames with reactants at 21 °C [9].

Figure 13 shows that for both Fec and Fe(CO)5, the magnitude of the inhibition is strongly

dependent upon the oxygen mole fraction in the oxidizer, with flames with lower mole fraction

of O2 showing more rapid burning velocity reduction. As a result, blends of inert agents with

iron-containing compounds may greatly increase the efficiency over that of the inert alone.
31

The modeling results for the ferrocene-inhibited flames are also shown in Figure 13. The

calculations (dotted lines) predict the flame speed reduction caused by ferrocene reasonably well.

The major difference between the ferrocene reaction scheme and that for Fe(CO)5 is in the

decomposition of the iron precursor. The present mechanism includes only the high-activation

energy thermal decomposition step for Fec consumption: FeC10H10 -> Fe + 2 C5H5, which has a

peak reaction flux at 1800 K in the present flames. In contrast, iron pentacarbonyl

decomposition has a peak reaction flux at about 900 K. Figure 14 shows the normalized burning

velocity for a methane-air flame with 400 ppm of ferrocene as a function of the hypothetical

activation energy Ea of the one-step decomposition reaction. In the temperature range of the

stoichiometric methane-air flame of the figure, the predicted inhibition effect of Fec is

independent of the overall activation energy of the decomposition of Fec for values of Ea less

than about 400 kJ/mol. Hence, the decomposition rate of Fec used in the model (Ea = 384

kJ/mol) does not influence the predicted behavior. For other conditions, however, (for example

non-preheated reactants and highly diluted flames) the decomposition of Fec may need to be

modeled more accurately to provide agreement with experimental data.

0

0.2

0.4

0.6

0.8

1

0 200 400 600

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

Normalized Burning Velocity

Fe(CO)5 Fec

Tin = 80 oC

X = 0.244 O2, ox

0.21

Fec

φ = 1.0

Figure 13 – Normalized burning velocity of

premixed CH4/O2/N2 flames inhibited by ferrocene

(open symbols) and Fe(CO)5 (closed symbols) for

XO2,ox=0.21 and 0.244, together with modeling

predictions (dotted lines).

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500 600

Activation Energy (kJ/mol)

Normalized Burning Velocity

Figure 14 – The normalized burning velocity of

stoichiometric CH4/O2/N2 flames at 400 ppm of

Fec as a function of the activation energy of the

one-step ferrocene decomposition reaction.

Since both the Fec and Fe(CO)5 mechanisms use the same iron sub-mechanism, the mode of

flame speed reduction in the model is the same. Decomposition of the ferrocene molecule

releases iron atom in the gas phase. Iron reacts with O2 to form FeO2, which reacts with O atom

to form FeO. FeO is a long-lived intermediate, which together with Fe(OH)2 and FeOH, enters

into the catalytic cycle for H-atom recombination

FeOH + H ↔ FeO + H2

FeO + H2O ↔ Fe(OH)2

Fe(OH)2 + H ↔ FeOH + H2O

(net: H + H ↔ H2).
32

The modeling results show that the stronger burning velocity reduction for the cooler flames

(XO2

,ox = 0.21) is due to their smaller radical pool; in these flames, a given amount of iron can

remove a larger percentage of the hydrogen radicals.

Ferrocene appears to be an alternative to the highly toxic iron pentacarbonyl for addition of gasphase iron to a flame. Unfortunately, its effectiveness also appears to diminish as the mole

fraction increases. For Fe(CO)5 the loss of effectiveness has been identified to be due to

formation of condensed-phase particulates in the reaction zone [42]. Since addition of nitrogen

clearly increases the rate of burning velocity reduction at low mole fraction (note the results in

Figure 13 for XO2

,ox = 0.21 and 0.244), it is of interest to determine if other thermally acting

agents can be combined with Fec to mitigate the loss of effectiveness, and perhaps enhance the

flame speed reduction at low Fec mole fraction. It is desired to take advantage of the strong

initial flame speed reduction from iron species in the flame, while avoiding the loss of active

species due to condensation. A drawback, however, is that addition of an inert, while reducing

the burning velocity, also increases the residence time for particle formation in the flame, so that

condensation is increased [42]. It is not known a priori if the net effect of combining thermal

and iron-containing agents will reduce the overall reaction rate faster than the increase of the rate

of active-species condensation.

3.4.5.2. Inhibition by Ferrocene and CO2

Figure 15a and Figure 15b present the experimental data for tests at XO2

,ox = 0.21 and 0.244.

Data are shown for pure CO2 as well as for CO2 -Fec blends corresponding to three values of the

mole percentage of Fec in CO2 (0 %, 0.25 %, and 1.5 % in Figure 15a, and 0 %, 0.4 %, and

1.5 % in Figure 15b). For the pure compounds, addition of about 10 % of CO2 (or 25 % N2)

reduces SL by a factor of two at XO2

,ox = 0.21 or 0.244. Adding Fec to CO2 produces a

particularly effective agent. The equivalent of 0.35 % Fec in CO2 reduces the required CO2 for

a 50 % reduction in the flame speed by about a factor of three at XO2

,ox = 0.21 and about a factor

of two at XO2

,ox = 0.244. The combination of 1.5 % Fec reduces the required CO2 by ten, making

this blend about as effective as CF3Br (for which addition of about 1 % halves the burning

velocity). Nonetheless, the curvature in the ends of the data sets (particularly for 1.5 % Fec in

CO2) illustrates condensation can be important. Although one might expect the slightly cooler,

slower flames with added CO2 to always show more condensation of iron species, the greater

efficiency of the catalytic cycle in the diluted flames predominates for most of the conditions for

the flame of Figure 15. Higher effectiveness of iron compounds in diluted flames has been

observed previously for flames with XO2

,ox lowered below 0.21 [9].
33

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10

% CO2 (or % N2/2.5) 

Normalized Burning Velocity

X = 0.21 O2

φ = 1.0

0% Fec

 in CO2

CF3Br 1.5 0.25

[N2]/2.5

15a.)

Tin = 80 oC 

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10 12 14

% CO2 

Normalized Burning Velocity

X = 0.244 O2

Tin = 80 oC

φ = 1.0

0% Fec

 in CO2

1.5

0.4

CF3Br

15b.)

Figure 15 - Normalized burning velocity of CH4/O2/N2 flames, a) XO2

,ox = 0.21, b) XO2,ox = 0.244, inhibited by

CO2, by CO2–ferrocene blends, and by CF3Br. The equivalent percentage of ferrocene in CO2 (which is

constant for each curve) is given. The solid lines are curve fits through the data, and the dotted lines, the

calculated results. Data for nitrogen are included in a.) by dividing % N2 by 2.5.

The condensation behavior of the blends can be discerned from Figure 16a and 7b, which present

additional data for CO2 and ferrocene in stoichiometric flames with XO2

,ox = 0.21 and 0.244,

respectively. In Figure 15, CO2 and ferrocene were added together in proportional amounts,

whereas in Figure 16, CO2 is first added at a constant volume fraction (0 %, 2 %, and 6 % in 7a;

or 0 %, 4 %, and 12 % in 7b), and then the Fec is added. This approach allows a clearer

delineation of the effects of each component of the blend. As the figures show, the curve with

0 % CO2 (pure Fec) has a decreasing slope magnitude as Xin increases (due to increased

condensation). For each of the other curves, the added CO2 reduces the normalized burning

velocity before the Fec is added, so that each curve starts at a value less than unity; addition of

Fec further reduces the flame speed. Below an Fec volume fraction of about 70 ppm, the curves

are all quite linear, showing that there no loss of effectiveness. But for curves extending beyond

this amount of Fec, there is increasing curvature as Xin increases (due to condensation).

Nonetheless, at low mole fractions, the inhibition by Fec is actually stronger for conditions of

higher CO2 mole fraction (note the larger magnitude of the slope for the 12 % CO2 curve in

Figure 16b). Hence, for some conditions, adding CO2 makes Fec more effective. As discussed

below, once the particles start to form, the deterioration of inhibition may be more rapid with

added CO2. These results imply that combinations of non-condensing quantities of several

catalytic agents combined with a thermal agent can be particularly effective.
34

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200

Fec ppm

Normalized Burning Velocity

0% CO2

X = 0.21 O2

Tin = 80 oC

φ = 1.0

16a.)

2% CO2

6% CO2

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600

Fec ppm

Normalized Burning Velocity

0 % CO2

12% CO2

X = 0.244 O2

Tin = 80 oC

φ = 1.0

4% CO2

16b.)

Figure 16 – Normalized burning velocity of CH4 / N2 / O2 flames, a.) XO2

,ox = 0.21, b.) XO2,ox = 0.244, with 0, 2,

and 6, or 0, 6, and 12 volume percent of CO2, respectively, added to the reactant stream, as a function of

added ferrocene (lines are curve fits to the experimental data).

3.4.5.3. Inhibition by Ferrocene and CF3H

Many compounds are candidates for blending with catalytic agents, including thermally acting

and other chemical agents. Hydrofluorocarbons, which are easily stored at moderate pressure,

are of interest since they are presently used as halon replacements. These compounds have been

found to reduce the burning velocity of premixed methane-air flames by reducing peak H-atom

mole fractions by acting as a sink for H atoms through reactions forming HF, and by lowering

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

fractions of radicals in flames lower than expected based on temperature reduction alone [43],

they might be expected to show enhanced performance relative to CO2 when combined with

catalytic agents.

Figure 17 presents the burning velocity reduction caused by pure CF3H addition to the above

flames; a mole fraction of about 5 % is required to reduce SL by two. Data are also presented

for addition of 0.35 % Fec in CF3H. Unlike ferrocene addition to CO2, in which 0.35 % Fec in

CO2 reduces the amount of CO2 required by a factor of about five, this amount of ferrocene in

CF3H reduces the amount of CF3H required only by about a third. This poor performance may

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

of the active iron-species intermediates, effectively poisoning the iron catalyst [44].
35

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5 6

CF3H Volume Fraction % 

Normalized Burning Velocity

Pure CF3H

0.35% Fec in CF3H

CF3Br

X = 0.21 O2

Tin = 80 oC

φ = 1.0

Figure 17 – Normalized burning velocity of

premixed CH4/O2/N2 flames inhibited by pure

CF3H and by CF3H with 0.35 % ferrocene,

together with data for CF3Br. Lines are curve fits

to the data.

0.0

0.2

0.4

0.6

0.8

1.0

1 2 3 4

CF3H Volume Fraction %

Mole fraction x 10

4

FeF2

FeF

Fe(OH)2

Fe

FeOH, 

FeO, 

FeO2,

FeF3 ,

all < 2 x 10-6

Figure 18 – Equilibrium mole fraction of active

inhibiting species (Fe, FeO, FeOH, Fe(OH)2) and

iron-fluorine species with 1 % to 4 % CF3H

(containing 0.35 % ferrocene) added to a

stoichiometric methane-air reaction mixture.

Since there presently exist no experimental data on the rates of reactions of iron species with

fluorine containing species in flames, the poisoning effect of fluorinated hydrocarbons on ironcatalyzed radical recombination reactions is assessed through equilibrium calculations for the

combustion products. The species included in the calculations are those in the mechanisms for

hydrocarbon oxidation, iron-inhibition, and fluorinated hydrocarbon-inhibition [45], as well as

the iron-fluorine species: FeF, FeF2, FeF3, Fe2F4, Fe2F6 [46]. Calculations were performed for

the equilibrium products of a stoichiometric methane-air flame with 1 % to 4 % CF3H containing

0.35 % Fec (the conditions of Figure 17). The results of the calculations, shown in Figure 18,

indicate that FeF and FeF2 are major product species when CF3H has been added. For 1 % to

4 % CF3H, the amount of iron taken up by the sum of FeF and FeF2 increases from 42 % to 84 %,

making less iron available in the form of the active iron intermediate species Fe, FeO, FeOH,

FeO2, and Fe(OH)2. The formation of fluorinated iron species with strong bonds can clearly act

as a sink for iron in the flame, and reduce the mole fractions of active iron-containing species

available to participate in the flame inhibition reactions. While the experiments and calculations

are presented for CF3H, the results are likely to be the similar for larger HFCs such as C2HF5 and

C3HF7 since the decomposition of all proceeds largely through the CF3, CF2, and CFO

intermediates [30,43,47].
36

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10

Inhibitor(s) Mole Fraction

Normalized Burning Velocity

Φa Xa

Φb Xb

Φa Xa + Φb Xb 

Φa+b Xa |actual 

Xa, Xb

Figure 19 – Illustration of linear contribution of

burning velocity reduction from each component

of a two-component blend of inhibitors, together

with the actual reduction from the blend.

0.4

0.6

0.8

1.0

1.2

0 2 4 6

Percent Major Inhibitor Component

Xa Φ|actual / ( Xa Φa + Xb Φb )|pred

0.4% Fec in CO2

1.5% Fec in CO2

0.25% Fec in CO2

0.35% Fec in CF3H

(50% CO2 in N2)/2

Figure 20 - Ratio of the actual reduction in SL

from the two component mix to the predicted

reduction based on linear combination of the

effect from each component. Open and closed

symbols are for XO2

,ox = 0.21 and 0.244; lines are

curve fits to the data. The percent CO2/N2 has

been divided by two to fit on the figure.

3.4.5.4. Comparison of Individual and Blended Performance

The behavior of the blends of agents can be investigated by comparing the actual amount of

flame speed reduction for the blend to the sum of the inhibition which would result from each

agent individually. This approach is illustrated schematically in Figure 19. We adopt the

inhibition index Φ(Xin) of Fristrom and Sawyer [48], where Φ(Xin)={{(Vo-V(Xin

))/Vo}{XO2

,ox/Xin} (and using the oxygen mole fraction in the oxidizer). The index Φ(Xin) is seen

to be the magnitude of the average slope of the normalized burning velocity curve (times XO2

,ox)

evaluated at the mole fraction of interest Xin. For a given blend, we can evaluate the amount of

normalized burning velocity reduction that would have been caused by each individual

component of the blend, say components a and b. The predicted inhibition index is just a linear

combination of the reduction from each component, or Xa ⋅ Φa+b(Xa, Xb)|pred = Xa ⋅

Φa(Xa)+Xa⋅Φb(Xb), in which a is the major component of the blend, (and we have selected it for

defining Φa+b(Xa, Xb)). The actual inhibition index Φa+b(Xa, Xb)|actual is evaluated from the

normalized flame speed of the blend (using Xa in its definition). The ratio of Xa ⋅Φa+b(Xa, Xb)|actual

to Xa⋅Φa(Xa)+Xa⋅Φb(Xb) provides a reasonable indicator of the performance of the blend relative

to the individual components.

As Figure 20 shows, a blend of CO2 and N2 (in the molar ratio of 1:2) provides a performance

index of nearly 1.0 for N2 added up to 12 % (i.e., containing 6 % CO2 ). (The percent amount of

the N2/CO2 mix plotted in Figure 20 is divided by two to allow plotting on the same scale.) In
37

contrast, the poor performance of the Fec-CF3H blend is clearly indicated by a decreasing

performance index as Xin increases. For the CO2/Fec blends, the combination appears to work

slightly better than the individual components at low Xin of the blend, and slightly worse at

higher Xin . The good performance at low Xin is due to the higher radical super-equilibrium

which occurs with flame dilution as observed previously for N2 addition [9], and the degraded

performance at higher Xin is due to the longer residence times for condensation which result from

the lower flame speeds [42]. In Figure 20, the condition at which the curves for Fec/CO2

crossover the unity ratio correspond roughly to the location on the curves in Figure 15, where the

linear behavior has ended and the curvature begins.

3.4.6. Conclusions

This section presents the first data on flame inhibition by ferrocene, and shows it to be as

efficient as Fe(CO)5 at reducing the burning velocity of premixed methane flames. Ferrocene,

like Fe(CO)5, loses its effectiveness at a mole fraction above a few hundred ppm. The

experimental results are reasonably predicted by a gas-phase iron inhibition mechanism. The

results imply that any rapidly decomposing iron-containing agent that releases atomic iron in the

gas phase can act as a precursor for the active iron-species intermediates, and that the results are

not unique to Fe(CO)5. As with Fe(CO)5, the magnitude of the inhibition by ferrocene has a

strong dependence on the oxygen mole fraction. As a result, many combinations of CO2 and Fec

show strong inhibition, mitigating the loss of effectiveness observed for pure Fec or Fe(CO)5.

The results imply that an inert agent, together with multiple catalytic agents (to keep the absolute

mole fraction of each below the saturation point) may prove to be highly effective for all

conditions. In contrast to the results with CO2, blends of CF3H and Fec are not particularly

effective, implying that iron species and halogens may enter into undesired reactions which

poison the catalytic cycles. These results with ferrocene imply that combinations of catalytically

and relatively inert thermally acting inhibitors may prove to be an efficient approach for

developing effective fire suppressants. A main conclusion of this section is that iron, in forms

other than Fe(CO)5, acts as a very powerful flame inhibitor, and other means for introducing it to

premixed flames should prove effective.

3.5. Cup Burner Flame Extinction Tests with Super-Effective Chemical

Inhibitors

3.5.1. Introduction

The effectiveness of the iron compounds Fe(CO)5 and ferrocene ((C5H5)2Fe, Fec) has been

observed in premixed Bunsen-type flames as well as in counterflow diffusion flames

[2,4,6,28,49]. No tests with these agents, however, have been done in flames more typical of

fires. The present work remedies this deficiency by presenting results for addition of these

highly effective agents to cup-burner flames. Not only does the structure of cup burner flames

resemble that of fires, but there also exists a large database of previous cup-burner extinction

results for comparison with the present data.
38

3.5.2. Background

One approach to overcoming the loss of effectiveness is to combine catalytic agents with inert

agents; this is, the overall reaction rate is lowered in part by radical recombination by the

catalytic agent, and in part by the lower temperature from the added diluent. This approach has

been discussed in work since the 1950’s [10,50-54], which suggested that combinations of

thermally acting and catalytic agents might prove beneficial. These predictions have been

confirmed in premixed flames with ferrocene and CO2 as the inhibitors [28,49]and in premixed

and counterflow diffusion flames inhibited by Fe(CO)5 at reduced oxygen mole fraction in the

oxidizer (which is equivalent to adding diluent nitrogen) [6]. Moreover, previous tests with

Fe(CO)5 at reduced oxygen mole fraction, and tests with ferrocene blended with CO2, show that

addition of an inert compound can actually enhance the performance of the catalytic agent.

Nonetheless, it is generally not known a priori if the combination of an inert with the catalytic

agents will be effective in a particular flame configuration: the lower oxygen mole fraction (or

added diluent) makes the catalytic effect stronger, while the lower temperature (and

consequently longer residence in premixed flames) increases the likelihood of condensation [9].

Motivated by these results, tests of ferrocene together with an inert compound generated by a

solid propellant gas generator (SPGG) have recently been conducted in an enclosure containing a

spray flame [55]. Unfortunately, the combination did not have the intended high efficiency, and

failed to extinguish the flame. As described above, the combination of inert and catalytic agents

has been shown to be effective in premixed flames and counterflow diffusion flames.

Notwithstanding, carefully controlled tests like those in the premixed and counterflow diffusion

flames have not been performed in cup-burner flames. In order to explore possible reasons for

the lack of effectiveness of Fec with the SPGG in suppressing the spray flame tested, we

performed tests with catalytic metal-based inhibitors and CO2 added to a cup burner.

Extrapolating from our recently acquired understanding of the action of iron in premixed and

diffusion flames, addition of Fe(CO)5 alone to the air stream of a cup burner would not be

expected to be efficient at extinguishing the flame. This is because particle formation acts as a

sink for the active gas-phase inhibiting species, keeping the maximum inhibiting species mole

fractions at their saturation value. That is, adding more and more Fe(CO)5 to the air stream does

not increase the gas-phase mole fraction of active inhibiting species, so we may never be able to

extinguish a cup-burner flame (or a spray flame) with Fe(CO)5 or Fec alone. However, based on

the premixed results with Fec and CO2, we would expect that added Fec (or Fe(CO)5) should

significantly reduce the amount of CO2 necessary to extinguish the cup-burner flame. This

approach should be effective for two reasons. First, based on premixed flame results presented

previously, we expect that adding 200 ppm or above of iron should reduce the overall reaction

rate by about a factor of four (i.e., about a factor of two reduction in flame speed); and second,

with added inert the effectiveness of the iron in the gas phase should be improved. Hence, we

would expect that adding an effective catalytic agent to the air stream would reduce the amount

of CO2 required to extinguish the flame.

Two approaches are used in the present work. In the first, the catalytic agent Fe(CO)5 is added to

the co-flowing air or fuel stream at two mole fractions, and the amount of added CO2 required to

extinguish the flame is determined. In the second approach, the fuel consumption rate of the

burner (due to heat feedback from the flame) is measured in the presence of increasing amounts
39

of CO2, with and without added Fe(CO)5 in the air stream. An effective fire suppressant would

be expected to reduce the fuel consumption rate, much as do fire retardants added to materials.

We present some data for extinction of cup-burner flames of heptane and methanol, but most of

the extinction tests were performed in flames of methane. Tests with flames of methane are

warranted since they have been shown to provide similar inhibitor rankings for extinction

compared to other larger hydrocarbon fuels [56]. Also, although methane oxidation is atypical of

that of larger hydrocarbons, Babushok and Tsang have recently observed [29] that for a wide

variety of hydrocarbons (including methane) the burning velocity is most sensitive to the rates of

the same reactions. Since these reactions are the ones most influenced by an inhibitor, the trends

in inhibitor effectiveness are the same for most hydrocarbons.

3.5.3. Experiment

A cup burner [57,58] was used for the tests. The cylindrical borosilicate glass cup, 28 mm in

outside diameter with a 45° bevel on the inside edge was positioned with the top at a height of

20.5 cm inside a 53.3 cm tall, 9.5 cm inside diameter glass chimney. A 7.5 cm thick bed of glass

beads (6 mm dia) at the base of the chimney provided uniform laminar flow in the air stream.

The cup burner was operated with methane, methanol, or heptane fuel. For the methane tests, the

fuel cup was filled with 3 mm dia. glass beads, with two 15.8 mesh/cm stainless steel screens

(separated by 1 mm) at the top. For the liquid fuels, a dual syringe-pump (Yale Apparatus model

YA-12) feed system provided fuel at measured rates. In the first tests, the fuel consumption rate

of the burner was determined by adjusting the syringe pumping rate such that the fuel level was

flush at the edge of the burner. This approach, while apparently working well, was time

consuming since it required long wait periods (several minutes) between changes in the feed rate

to insure that the fuel level remained at a constant level at the burner rim. A subsequent method

used an overflow system as in ref. [58]. Gas flows were measured with digitally-controlled mass

flow controllers (Sierra 860) with a quoted repeatability of 0.2 % and accuracy of 1 % of fullscale flow, which have been calibrated with bubble (Gillian Gilibrator), piston (Bios 20K), and

dry (American Meter Co. DTM-200A) flow meters so that their accuracy is 2 % of indicated

flow.

The organometallic inhibitor Fe(CO)5 was added to the air stream using a two-stage saturator in

a controlled temperature bath. The Fe(CO)5 was added at mole fractions up to 950 ppm using

the saturator described previously [10]. The mole fraction of the Fe(CO)5 inhibitor in the air

stream was calculated based on the measured air flow, measured carrier gas flow (N2 or CH4),

and the calculated vapor pressure of the agent at the bath temperature, assuming saturated carrier

gas (which has been verified in previous work). The Antoine equation for Fe(CO)5 is log10(P)=

6.77273-4.0932/(T+7.2283) [59].

To determine the extinction condition for pure CO2, the CO2 was added to the airflow in

increasing amounts (with increments of < 1 %) until lift-off was observed. The test was repeated

at least three times. In the presence of a second inhibitor, that agent was first added to the air

stream at a constant mole fraction, and then the amount of added CO2 required for lift-off was

determined as in the pure CO2 case. The amount of CO2 required for lift-off is reported as its

mole fraction in the burner co-flow stream (i.e., the sum of the air, added CO2, and carrier-gas

CO2). (Note that in some early tests, to be described in Table 2, N2 was used as the carrier gas
40

instead of CO2. In those cases, we included the N2 in the CO2 flow by correcting for differences

in specific heat between the two.) The fuel gas is methane (Matheson UHP, 99.9 %), and the

oxidizer is house compressed air (filtered and dried) which is additionally cleaned by passing it

through an 0.01 μm filter, a carbon filter, and a desiccant bed to remove small aerosols, organic

vapors, and water vapor. The chemicals used were Fe(CO)5 (Aldrich), TMT (Alfa Aesar), MMT

(Alfa Aesar), CH3OH (Aldrich, 99.8 %), Br2 (Aldrich, 99.5 %), CF3Br (Great Lakes), N2 (boiloff), and CO2 (Airgas). The expanded relative uncertainties for the experimentally determined

quantities in this study are 4 % for the CO2 mole fraction, 5 % for the Fe(CO)5 mole fraction, and

otherwise as noted on the figures.

3.5.4 Results and Discussion

3.5.4.1. Heptane – Air Flames

We conducted our first tests in a heptane-fueled cup burner. In the absence of Fe(CO)5, the

extinction condition occurs at a CO2 mole fraction of (19.2 ± 0.8) %. The value is somewhat

low compared to other values in the literature, due to the low air volumetric flow, 25 L/min, in

these tests. With about 450 ppm of Fe(CO)5 added to the air stream, the amount of added CO2

required for extinction is reduced by only about 5 %. This result is unexpected since adding 150

ppm of Fe(CO)5 to a premixed methane-air flame reduces by a factor of eight the amount of CO2

required to halve the burning velocity.

For a cup burner flame, addition of Fe(CO)5 or CO2 might not only change the extinction

condition, but might also change the heat release rate at each agent mole fraction. We measured

the fuel consumption rate as an approximate measure of the heat release in the flame.

Figure 21 shows the heptane consumption rate in a cup burner as a function of the CO2 mole

fraction in the air stream. The upper curve shows the results for pure CO2, while the lower curve

presents the results for addition of 450 ppm of Fe(CO)5 to the air stream prior to addition of CO2

to the air stream. As the figure shows, the presence of Fe(CO)5 causes a 16 % to 38 % reduction

in the fuel consumption rate for CO2 mole fractions up to 75 % of extinction; however, as

described above for methane, the extinction condition changes by only about 5 % with iron

present.
Presented at the Halon Options Technical Working Conference, April 2001, Albuquerque, NM

41

0

0.5

1

1.5

2

0.00 0.05 0.10 0.15 0.20

CO2 Mole Fraction in Air Stream

Heptane Cons. Rate ml/min

Pure CO2

450 ppmv of Fe(CO)5 in air

Figure 21 - Heptane consumption rate in cupburner flame as a function of added CO2 mole

fraction with and without 450 ppm Fe(CO)5 added

to the air stream

0

0.5

1

1.5

2

0.00 0.05 0.10 0.15 0.20 0.25 0.30

CO2 Mole Fraction in Air Stream

Methanol Cons. Rate ml/min

Pure CO2

450 ppmv of Fe(CO)5 in air

Figure 22 - Methanol consumption rate in cupburner flame as a function of added CO2 mole

fraction with and without 450 ppm Fe(CO)5 added

to the air stream.

Unfortunately, the results in Figure 21 are confounded by the sooting tendencies of heptane cupburner flames. Heat transfer to the pool surface is enhanced by soot in the gas phase, as well as

by soot on the hot chimney walls. With addition of Fe(CO)5, soot production was visually

observed to be reduced, which is consistent with its known effects on the sooting tendencies of

flames [21-26]. (At higher mole fractions of added CO2, soot formation is visually observed to

be suppressed, with and without addition of Fe(CO)5). We therefore desire to eliminate the

complicating effects of soot formation in cup-burner flames.

3.5.4.2. Methanol – Air Flames

In order to retain a liquid fuel but virtually eliminate sooting, we ran additional tests with

methanol as the fuel. Tests were conducted to determine the amount of CO2 required for

extinction with and without addition of 450 ppm of Fe(CO)5 to the air stream. As with heptane,

the reduction in CO2 required for extinction is small, here only about 8 %. Tests for the effect of

air-stream addition of Fe(CO)5 on the methanol consumption rate were also conducted. Figure

22 shows the change in the methanol consumption rate with addition of 450 ppm of Fe(CO)5 to

the air stream, as a function of the amount of added CO2. We see that without the confounding

effects of soot formation, the fuel consumption rate is reduced by a nearly constant amount. The

fuel consumption rate is reduced somewhat, but the reduction is not nearly as high as might be

expected based the overall reaction rate reduction in premixed methane-air flames.
42

3.5.4.3. Methane – Air Flames

To further understand the influence of iron in cup-burner flames, we also conducted tests with

methane fuel. In these tests, Fe(CO)5 could be added to either the fuel or the air stream,

removing the uncertainty associated with agent transport to the relevant reaction zone. Table 2

summarizes the results for all of the tests. The top two entries show that addition of Fe(CO)5 to

the air stream at quite high mole fractions (450 ppm and 924 ppm) causes only a 9 % and 13 %

reduction in the amount of CO2 required for extinction. Likewise, addition of Fe(CO)5 to the

fuel stream at either 450 ppm or 4500 ppm causes only a 2 % and 4 % reduction in the amount of

CO2 required. If we add 1 % CH4 to the air stream to move the flame location and hence the

scalar dissipation rate, the reduction in the CO2 required for extinction with addition of 450 ppm

of Fe(CO)5 is slightly greater, but still only 11 %. These results are in contrast to those for

CF3Br with CO2 in the cup burner which are shown at the bottom of the figure. There, we see

that addition of CF3Br to either stream at mole fractions which would approximately halve the

burning velocity leads to a reduction of the CO2 required for extinction by a factor of two to

three. Clearly, CF3Br and Fe(CO)5 behave quite differently in the cup burner with respect to

their ability to reduce the CO2 requirement for extinction.

Table 2 - Extinction volume fraction of CO2 in methane-air cup burner with and without

various amounts of Fe(CO)5 or CF3Br added to the fuel or air stream (air flow =41.6 l/min).

 Catalytic Inhibitor

Vol % CO2

at

Extinction

% Reduction

from

Pure CO2

 None - - 15.7 ± 0.6 -

Fe(CO)5 450 ppm in Air 14.1 ± 0.6 9.6 ± 0.5

“ 924 ppm 13.5 ± 0.5 13.5 ± 0.8

“ 450 ppm in Air w/ 1% CH4 14.0 ± 0.6 10.7 ± 0.6

“ 450 ppm in Fuel 15.4 ± 0.6 1.3 ± 0.1

“ 4500 ppm 15.2 ± 0.6 2.6 ± 0.2

 CF3Br 1.3 % In Air 4.4 ± 0.2 72.0 ± 4.1

“ 11 % In Fuel 8.7 ± 0.3 44.2 ± 2.5

3.5.4.4. Phosphorus Flames

The question naturally arises as to whether the poor performance in the cup burner is specific to

iron, or characteristic of super-effective inhibitors in general. To investigate this, we examined

the cup-burner extinction data of TMP and DMMP from researchers at NMERI [60]. In their

tests, they found that about 5 % of DMMP was required for extinction, or 5.3 % to 6.9 % for

TMP (all volume basis). It is surprising that so much is required to extinguish cup-burner flames

considering that in methane-air flames, these phosphorus compounds have been shown to be
43

about seventeen and three times more effective than CF3Br in premixed [61] and diffusion

flames [62], respectively. In these tests in cup burners, on a molar basis, DMMP and TMP are

less effective than CF3Br, while on a mass basis, they are only about as effective as CO2.

Further, at these concentrations, the agent could be working through oxygen starvation. For

example, for DMMP and TMP which are combustible, the stoichiometric mole fraction in air is

about 4.5 % and 5.0 %. Hence, the actual mole fractions required for extinguishment are close to

those at which the agent would consume all of the available oxygen in the air stream.

3.5.4.5. Discussion

We speculate that particle formation is the cause of the degraded performance of the metal

agents. A visible outer annulus, apparently particles, was observed in all flames with added

metals, and the blackbody radiation from that region increased with higher agent mole fraction.

Finally, as discussed in refs. [9,42], the particle formation rates appear to be linked to their

residence time in the cooler regions of the flame. Since the present cup-burner flames have quite

different flow fields than either premixed or counterflow diffusion flames, it is likely that the

condensation behavior in them is different. It is possible that metal compounds added to the air

stream are sequestered as solid phase particles which are then convected away from the relevant

radical recombination zones of the flame before they can affect the flame chemistry.

3.5.4. Conclusions

We have presented the first data on the extinction characteristics of the highly effective catalytic

agent Fe(CO)5 added to a cup burner flame together with CO2. We have found that for this

agent, its behavior in co-flow diffusion flames is drastically different from that in either

premixed or counterflow diffusion flames. We postulate (but have not demonstrated) that the

loss in effectiveness is due to condensation of the metal, metal oxides, or metal hydroxides. For

methanol, the effect of the Fe(CO)5 on the fuel consumption rate (caused by the heat feedback to

the liquid fuel surface) is small. For heptane, the effect is somewhat larger, and this is

postulated to be due to reduced soot formation. In future research, it is suggested to study a

wider variety of metal compounds to determine if this behavior is specific to Fe(CO)5 or

generally true of super-effective metal compounds. Also, tests over a range of mole fractions of

inhibitor would more help to determine if the indicated behavior is due to particle formation. It

is of high importance to determine if phosphorus compounds, which have also been shown to be

effective in premixed and counterflow diffusion flames, are effective at reducing the amount of

CO2 required to extinguish a cup burner flame. It has been postulated that the reason for the

lower effectiveness in these diffusion flames is particle formation and the subsequent

sequestering of the active gas-phase inhibiting compounds prior to their introduction to the

stabilization regions of the flame. Measurements of particles in these flames via traditional laser

scattering and extinction would serve to determine if this is indeed the reason for loss of

effectiveness.
44

4. Technical Problems

This project uncovered numerous technical problems that were not anticipated at the outset of the

work. While high mass fractions of the Fe(CO)5 could be added to zeolite and porous silica,

some decomposition of the agent was indicated, especially for the zeolites, which were also

pyrophoric on exposure to air. The most significant finding was that the super agents—even

added alone—were not effective in flames more closely resembling fires (e.g., cup burner

flames). This severely limits their utility, whether added alone, as absorbed material in a particle

support matrix, or as a complexed compound.

5. Recommendations

We have clearly determined that Fe(CO)5 does not have the expected high efficiency at flame

suppression when added, together with CO2, to the air stream of a cup burner flame of methane,

heptane, or methanol, or to the fuel stream in a methane-air flame. We have not determined,

however, the reasons for the poor performance. Tests with other potential super-effective agents

in cup burner flames would determine if there was any utility of such compound in flames more

closely resembling fires, and would show if this poor performance in cup burners was unique to

Fe(CO)5 or a general property of the super-effective agents. Tests with the agents over a range

of mole fraction in the air stream will help to determine the cause of the ineffectiveness. We

have postulated that the reason for the lack of effectiveness of Fe(CO)5 in the cup burner flames

is condensation of the active species to particles of iron, iron oxide, or iron hydroxide, caused by

the particular time-temperature history of the cup burner flames. We recommend testing this

hypothesis by performing classical laser scattering and extinction tests in cup-burner flames with

added CO2 and super agents to determine the particle formation properties of the chemical

systems.

Our evaluations of the data of other researchers concerning the performance of the phosphorus

compounds DMMP and TMP added alone in cup burner flames showed them to be fairly

ineffective. This too is surprising considering their strong inhibition of premixed and

counterflow diffusion flames. We recommend that tests be performed with blends of TMP or

DMMP with CO2 added to the air stream of cup burner flames to determine if they too suffer

from the loss of effectiveness experienced by Fe(CO)5. Finally, since the cup burner is a widely

used flame for assessment of suppressant effectiveness, we recommend experiments and

numerical modeling to more fully understand the chemical and flow-field effects which lead to

flame extinction in these flames for a range of inhibitor types.

6. Conclusions

We have shown that an iron pentacarbonyl mass fraction of 0.33, 0.26, 0.21, and 0.14 to 2.0 can

be absorbed into zeolite-NaX, zeolite-NaY, porous silica, and aerogel, respectively. However,

we have also shown that Fe(CO)5 added to zeolites is pyrophoric and decomposes on exposure to

air. Because of the sensitivity of aerogel particles to moisture, our experiments found it a very
45

difficult to work with. Finally, experiments with I2-impregnated silica gel, while perhaps

delivering as much as 1.2 % of I2 (by mass) to the flame, failed to show significant flame speed

reduction. Further, based on literature values of the amount of I2 required for flame inhibition,

an I2 mass fraction of 0.88 % would be required an 10 % reduction in flame speed. To achieve

this I2 loading, the inert carrier would have to be present at about 4.5 %. Extinction mass

loadings would be about nine times this, or about 40 % by mass of the inert carrier. Obviously,

for such conditions, the advantage of the chemically active agent becomes marginal.

We have shown that the powerful flame inhibition properties of iron is not unique to Fe(CO)5,

but is about the same for ferrocene—a much less toxic form of iron. We have shown that

combining ferrocene with CO2 mitigates the loss of effectiveness in premixed flames, but that

mixing ferrocene with HFC’s is nor particularly effective due to the formation of stable FexFy

compounds (especially FeF and FeF2).

The agent Fe(CO)5, alone or combined with CO2, is not effective in cup burner flames. We

speculate that the reason for the lack of effectiveness is the loss of active inhibiting species to

condensed phase particles. Hence, we do not believe that this agent, combined with any support

matrix, would be particularly effective either.

Acknowledgements: We thank Prof. Dino Megarides of UIC for helpful

discussions, and for sending us his ferrocene evaporator, which we studied in

designing our own. The assistance of Anthony Lazzarini and Pasan Wanigarathne

in conducting many of the experiments is gratefully acknowledged. The essential

contributions of Valeri Babushok, Don Burgess, Michael, Zachariah, and Wing

Tsang are gratefully acknowledged. This research is part of the Department of

Defense's Next Generation Fire Suppression Technology Program, funded by the

DoD Strategic Environmental Research and Development Program under

contract number W74RDV83528667. Internal NIST funding is also gratefully

acknowledged.

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