for powder samples. Although a nominal particle size can be determined by sieving, obtaining
an in situ particle size or surface area is not possible. Optical techniques assume a spherical
particle (phase Doppler Anemometry measures the local radius of curvature) or provide a
generalized shape (diffraction-based or imaging methods). The assumption that an off-line size
determination is the size delivered to the flame is questionable, especially for very small particles
where several factors including humidity and static charges can affect the particle size
distribution. Despite this limitation, results on sodium and potassium bicarbonate flame
extinction effectiveness based on sieving size show that effectiveness is controlled by particle
size [22]. These studies suggest that there is a limiting small size below which there is no
increase in effectiveness based on mass of the alkali metal compounds. It should be possible to
quantify the chemical effects of various powders if sufficiently small particle samples are
studied.

Studies are underway in our lab to assess the mechanistic reasons for the higher
effectiveness observed for potassium over sodium and the generally observed increase in
effectiveness as one goes down the row of alkali metals (Li < Na <K <Rb). Preliminary flame
speed calculations have been carried out for sodium inhibited flames [23]. Only the sodium
chemistry currently has a sufficient number of reported reaction rate coefficients. In general
alkali metal atoms and metal hydroxide molecules can combine H and OH flame radicals
according to

M+0OH > MOH (1)

MOH +H>M+H,0 (2)
where M represents an alkali metal atom. Calculations assuming similar chemistry and kinetics
for lithium suggest that thermochemistry alone does not account for the higher effectiveness of
potassium relative to sodium. However, thermodynamics does limit the amount of Li relative to
LiOH which in turn greatly diminishes the effectiveness of the Li+ OH +M ~> LiOH / LiOH
+H Li+H,0 catalytic cycle.

Flame speed versus added sodium plots show the typical limiting behavior at high agent
concentration characteristic of very efficient catalytic agents. Calculations predict that
methane/air flames inhibited with an “ideal” agent, one which causes the H + OH recombination
rate to equal the collision rate with no added thermal mass, can reduce the flame speed from ~ 39
cm/s to ~ 8 cm/s. This is not enough to extinguish the flame in the absence of any other heat
losses. For agent addition beyond the chemical saturation point, even an ultra-high efficient
chemical agent begins to act more like a physical agent, essentially only adding thermal mass.
The resulting heat loss is more effectively accomplished using an efficient physical agent such as
water mist.

CONCLUSIONS

Finding a true, drop-in replacement for Halon 1301 with identical properties yet
environmentally acceptable is not likely. However, there are environmentally acceptable
compounds that hold real promise as potential retrofitable replacements. Exactly what chemical
form is best and how to get it to or in the flame are key issues. As we decide on halon


15.

16.

17.

18.

19.

20.

21.

22.

23.

“Water Spray Cooling System — A Gaseous Suppression System Enhancer”, A. Maranghides,
RS. Sheinson, B.A. Williams, and B.H. Black, Proceedings of the Eight International
Interflam Conference, Edinburgh, Scotland, pp. 627-637 (1999).

“Relative Flame Inhibition Effectiveness of Metallic Compounds”, V. Babushok and W.
Tsang, Proceedings of the 1997 Fall Technical Meeting: The Eastern States Section of the
Combustion Institute, Hartford, CT, pp. 79-82 (1997).

“Numerical Study of the Inhibition of Premixed and Diffusion Flames by Iron
Pentacarbonyl”, M.D. Rumminger, D. Reinelt, V. Babushok, and G.T. Linteris, Combust.
Flame 116: 207-219 (1999).

“Chemical Limits to Flame Inhibition,” V. Babushok, W. Tsang, G.T. Linteris, and D.
Reinelt, Combust. Flame 115: 551-560 (1998).

“Role of Particles in Flame Inhibition by Iron Pentacarbonyl”, M.D. Rumminger and G.T.
Linteris, Proceedings of the 1999 Halon Options Technical Working Conference,
Albuquerque, NM, in press.

“Behavior of Bicarbonate Powders in Counterflow Diffusion Flames”, M.D. Reed, B.A.
Williams, R.S. Sheinson, and J.W. Fleming, Proceedings of the Eastern States Section: The
Combustion Institute, pp. 83-86, 1997; “Extinction Studies of Propane/Air Counterflow
Diffusion Flames: The Effectiveness of Aerosols”, J. W. Fleming, M.D. Reed, E.J.P. Zegers,
B.A. Williams, and R.S. Sheinson, Proceedings of the 1998 Halon Options Technical
Working Conference, pp. 403-414, 1998.

“Flame Extinction by Sodium Bicarbonate Powder in a Cup Burner”, A. Hamins,
Proceedings of the Twenty-Seventh (International) on Combustion/The Combustion Institute,
Pittsburgh, PA, 2857-2864 (1998).

Flame Extinction Properties of Dry Chemicals; Extinction Concentrations for Small
Diffusion Pan Fires”, C.T. Ewing, F.R. Faith, J.T. Hughes, and H.W. Carhart, Fire
Technology 25, 134 (1989).

“Suppression Mechanisms of Alkali Metal Compounds”, B.A. Williams and J.W. Fleming,
Proceedings of the 1999 Halon Options Technical Working Conference, Albuquerque, NM,
in press.


distribution of fluorine atoms on the HFC that maximizes the number of CF,-groups leads to
better suppression across a range of strain rates [12]. Extinction strain rate curves for
hydrofluorocarbon agents cluster into groupings corresponding to the number of CF,-groups on
the molecules. The similarity of the thermochemical properties of these compounds suggests
that the higher extinction effectiveness associated with the CF,-groups might be due to
differences in chemical kinetics.

Our laboratory has investigated the chemical kinetic pathways relevant to inhibition by
HFCs in low pressure flat flames [13]. Flame structure information using laser induced
fluorescence was compared to flame modeling calculations; a kinetic sub-model was used for
HFP that was developed by Hynes et al [14]. Reaction pathway analysis points to a combination
of events rather than one specific source to explain the CF,-group enhanced effectiveness. For
HFP there is a strong suppression dependence on agent decomposition, whether thermal or via
flame radical attack. Regardless of the initiation step, the thermodynamically favored by-product
for HFP or any partially fluorinated hydrocarbon will be HF. The human health hazard HF
presents is cause for concern when using fluorine containing agents in occupied spaces or ones
that must be entered after application of the agent.

A method to reduce the amount of HF has been patented by NRL [15] and combines the
use of water spray with the fluorinated agent. Large reductions in HF can be achieved, resulting
from the significantly lower temperature and oxygen dilution due to the water spray, which
greatly weakens the flame and thus reduces the HFP decomposition.

ULTRA-HIGH EFFICIENCY AGENTS

Compounds containing elements other than the halogens have shown potential for
chemical fire suppression many times greater than bromine. Babushok points out the very high
flame speed reduction potential for a number of compounds, including iron and alkali metals
[16]. Recent studies at NIST show that iron pentacarbonyl, Fe(CO),, is some 70 times more
effective than Halon 1301 in reducing the flame speed of a methane/air flame by 25% [17]. The
mechanism responsible is postulated to be a catalytic removal of both H and O atoms.
Unfortunately, above ~ 200 ppm of added Fe (in the form of Fe(CO), ) there is a leveling off of
effectiveness with increased Fe addition. This diminished effectiveness comes at much lower
concentrations than the expected slowdown for catalytic agents [18] due to the flame H, O, and
OH radical concentrations approaching their equilibrium values. Early onset of decreased
effectiveness has been attributed to the condensation of gas phase iron atoms at the higher
concentrations [19]. It is likely that other iron compounds will be impacted by the
thermodynamic limit of the gas phase Fe or FeO concentration.

The effectiveness of sodium and potassium bicarbonate powders was measured in
counterflow diffusion flames [20]; Hamins reported results for sodium bicarbonate in a cup
burner [21]. Potassium bicarbonate powders are at least twice as effective as sodium bicarbonate
for similar particle sizes and flame strain rate; small particles of both compounds are more
effective than Halon 1301. Determining the mechanism for particle suppression requires particle
size and surface area information. Although these studies confirmed the enhanced effectiveness
as particle size decreases, quantifying the effectiveness as a function of particle size is difficult


replacement options, there will also be the need for more critical assessment of the areas
requiring protection and other means of minimizing fire damage. It is apparent that future
choices of which agent to use will be based as much on the fire protection requirements and the
area to be protected as on the agent itself. Choices will obviously result in tradeoffs and
engineering compromises.

Combining a highly efficient chemically acting agent with a physically acting agent holds
the greatest promise for future systems. This might be done in a single designer molecule or
perhaps in a dual system. Combining the superior chemical suppression capability of additives
such as iron or potassium at low concentrations with the large heat extraction capabilities of fine
water mist is a combination worthy of further study. Such systems will present storage and
delivery issues and engineering challenges to meet both reliability requirements and weight and
volume restrictions. The need for environmentally acceptable replacements for halons will
continue to drive the creativity and ingenuity of both chemists and engineers to meet these
challenges.

ACKNOWLEDGMENTS

I would like to thank the members of the Combustion Dynamics Section at NRL whose
work is presented here: Ron Sheinson, Brad Williams, and Alex Maranghides. Others
contributing to the NRL halon replacement efforts have included: Paul Papas (NRC Postdoc,
counterflow flame extinction studies), Drew L’Esperance (NRC Postdoc, fluorinated agents in
low pressure flames), Eric Zegers (NRC Postdoc, water mists and fluorinated agents in
counterflow flames), Paul Fuss (ASEE Postdoc, flame speed inhibition by water mist), Mark
Reed (WPI MS Fire Protection Engineering, work on dry powders), Mike Davis (BS M.E. Notre
Dame, modeling calculations and velocity field measurements), Mike Kozma (MS E.E, Lehigh
U., instrumentation assistance) and Professor Betta Fisher (Cornell U., NRL Summer ASEE
Faculty, counterflow flame studies).

We acknowledge the financial support of the U.S. Naval Sea Systems Command and the
Department of Defense's Next Generation Fire Suppression Technology Program, funded by the
DoD Strategic Environmental Research and Development Program (SERDP).

REFERENCES

1. Halon Replacements — Technology and Science, ACS Symposium Series 611, A.W.
Miziolek and W. Tsang, eds., American Chemical Society, Washington, DC (1995). The
USS. Navy’s program is reviewed by R.S. Sheinson in Ch. 16, “The U.S. Navy Halon Total
Flooding Replacement Program: Laboratory Through Full-Scale”.

2. Information can be found at the NGP web site: www.dtic.mil/ngp/.

3. “Dynamics of Water Droplets in a Counterflow Field and their Effect on Flame Extinction”,
“A.M. Lentati and H.K. Chelliah, Combust. Flame 115: 158-179 (1998).

4. “Water Mist Suppression of Non-Premixed Counterflow Flames”, E.J.P. Zegers, B.A.
Williams, R.S. Sheinson, and J.W. Fleming, Eastern States Section: The Combustion
Institute, 1999 Fall Technical Meeting, Raleigh, NC, 11-13 October 1999.



Fleming, J.W., “Chemical Fire Suppressants: How Can We Replace Halon?,” Proceedings of the Fall
Technical Meeting of the Eastern States Section of the Combustion Institute, pp. 16-23, Raleigh, NC,
1999.

Chemical Fire Suppressants: How Can We Replace Halon?

James W. Fleming
Navy Technology Center for Safety and Survivability, Code 6185
Naval Research Laboratory, Washington, DC 20375-5342
fleming@code6185.nrl.navy.mil
phone: 202-767-2065; fax: 202-767-1716

INTRODUCTION

Halons, halogenated hydrocarbons containing bromine, appeared on the scene for fire
suppression in the late 1940’s following a U.S. Army study to identify effective agents. Halon
1301, bromotrifluoromethane, with its low boiling point is extremely effective in total flooding
applications, which rely on the agent completely filling the space to be protected. Halons that
exhibit good fire suppression properties but have boiling points closer to room temperature
(Halon 1211, bromochlorodifluoromethane, and Halon 2402, dibromotetrafluoroethane) are more
suitable for streaming agents which can be directed toward the fire threat as a liquid stream,
typically from hand-held units. Interestingly, Halon 1301 was not the most effective total
flooding compound on the list. However, Halon 1301 provides a near optimum combination of
good fire suppression effectiveness, low toxicity, suitably low boiling point, and a reasonably
low density and molecular weight. The combined effectiveness and desirable properties quickly
accelerated the use of halons to a wide range of fire suppression applications, including movable
platforms such as ships and planes. Dependence on halons increased as more and more fire
suppression systems were engineered around these compounds.

In 1974, Rowland and Molina called the world community’s attention to the fact that
chlorofluorocarbons (CFCs), although very unreactive at ground level, have the potential to
catalytically destroy stratospheric ozone once the chlorine is released through uv photolysis.
Subsequent research including work done at NRL in 1976 showed that bromine containing
halons are more deleterious to ozone than CFCs. When released in the stratosphere, bromine
catalytically destroys O,: Br+O, > BrO+ O,; BrO + O, > BrO, + O, > Br. CF,Br is very
stable to attack by OH and uv wavelengths present at ground level, the primary removal
mechanisms of compounds in the troposphere. Thus, Halon 1301 has an undesirably long
atmospheric lifetime that allows transport of the problematic bromine into the stratosphere.

The search for environmentally acceptable fire suppression halon replacements requires
identifying compounds that have: good suppression efficiencies compared to Halon 1301, very
low potential for destroying the stratospheric ozone, minimal impact on global warming, low
human toxicity, and desirable boiling points. Significant efforts are underway to find halons
replacements to protect both new construction as well as retrofitting current systems [1].
Different applications require different criteria. Suppression system weight and space constraints
on retrofitting current systems make replacement particularly challenging. The search for
acceptable replacements must be more than a random walk through an extremely large number of
possibilities. Replacement choices demand compromises and systematic methodologies for


10.

11.

12.

13.

14.

“Inhibition of Premixed Methane-Air Flames by Water Mist”, S. P. Fuss, D. J. Dye, B. A.
Williams, J. W. Fleming, Eastern States Section: The Combustion Institute, 1999 Fall
Technical Meeting, Raleigh, NC, 11-13 October 1999.

“Quantification of Physical Fire Suppression of Heptane Pool Fires”, R.S. Sheinson, J.E.
Hahn, K. Geary, and F.W. Williams, Eastern Section of The Combustion Institute, Chemical
and Physical Process in Combustion, Proceedings of the 1977 Fall Technical Meeting, East
Hartford, CT, IV/30-35 (1977). ,

“Quantification of Fire Suppressant Action on Liquid Pool Fires”, R.S. Sheinson, G.I.
Gellene, F.W. Williams, and J.E. Hahn, Eastern Section of The Combustion Institute,
Chemical and Physical Process in Combustion, Proceedings of the 1978 Fall Technical
Meeting, Miami Beach, FL, 18/1-4 (1978); “The Physical and Chemical Action of Fire
Suppressants”, Fire Safety Journal, Vol. 15, 437-450 (1989).

“Fire Suppression Agent Effectiveness Prediction and Implications for Fire Extinguishment
Tests", R.S. Sheinson and §.P. Baldwin, Proceedings of the Eastern States Section: The
Combustion Institute, Princeton, NJ, pp. 483-486, 1993.

“The Cup Burner as a Suppression Mechanism Research Tool: Results, Interpretations, and
Implications”, R.S. Sheinson and A. Maranghides, Proceedings of the 1997 Halon Options
Technical Working Conference, pp. 19-30 (1997).

“Binary CF,Br- and CHF,-Inerst Flame Suppressants: Effect of Temperature on the Flame
Inhibition Effectiveness of CF, and CHF,”, Y. Saso, Y. Ogawa, N. Saito, and H. Wang,
Combust. Flame 118: 489-499 (1999).

“A Holistic Approach to Halon Replacement System Design: A Case Study”, A.
Maranghides, R.S. Sheinson, T. Friderichs, B.H. Black, M Peatross, and W.D. Smith,
Proceedings of the International Conference on Ozone Protection Technologies, Washington,
DC, pp. 578-582, (1996).

“Effect of Fluorinated Ethanes and Propanes and Sulfur Hexafluoride on the Extinction
Strain Rate of Methane/Air and Propane/Air counterflow Diffusion Flames”, B.A. Williams,
E.M. Fisher, E.J.-P Zegers, J.W. Fleming, R.S. Sheinson, 27th International Symposium on
Combustion, Abstracts of Work-in-Progress Posters, W1A10, p. 10 (1998).

“Intermediate Species Profiles in Low Pressure Premixed Flames Inhibited by
Fluoromethanes”, D. L’Esperance, B. A. Williams, and J.W. Fleming, Combust. Flame 117:
709-731 (1999); “Intermediate Species Profiles in Low Pressure Methane/Oxygen Flames
Inhibited by 2-H Heptafluoropropane: Comparison of Experimental Data with Kinetic
Modeling”, B.A. Williams, D. L’Esperance, and J.W. Fleming, Combust. Flame, in press.
“Inhibition of Premixed Hydrogen-Air Flames by 2-H Heptafluoropropane”, R.G. Hynes,
J.C. Mackie, and A.R. Masri, Combust. Flame 113: 554-565 (1998); “Shock-tube Study of
the Pyrolysis of the Halon Replacement Molecule CF,CHFCF,”, R.G. Hynes, J.C. Mackie,
and A.R., Masri, J. Phys. Chem. A, 103: 54-61 (1999); “Sample Probe Measurements on a
Hydrogen/Ethane/Air/2-H Heptafluoropropane Flame”, R.G. Hynes, J.C. Mackie, and A.R.
Masri, Energ. Fuel. 13: 485-492 (1999).


identifying suitable replacements based on fire suppression mechanistic understanding. An
understanding of the suppression mechanism can effectively guide the search in promising
directions. Modeling predictions are valuable for providing insight. Models must be validated
against experimental data for a range of conditions and for a number of compounds. This paper
covers current and past activities related to our efforts in halon replacement in a variety of flame
environments. Some of the projects presented are part of the current Department of Defense’s
Next-Generation Fire Suppression Technology Program which seeks retrofit halon replacements
for military applications [2].

PHYSICAL SUPPRESSION

Flame suppression agents can be generally categorized into chemical agents, such as
those containing bromine or iodine, and physical agents such as nitrogen. However, an agent can
only be a purely physical agent if it suffers no degradation in the flame, either via chemical
attack or thermal decomposition. Molecular nitrogen and other physical agents are effective at
flame suppression through heat extraction and dilution of oxygen. The amount of N, required for
adequate protection is very costly in terms of total system volume and weight, including storage
containers and dispersal hardware. Both volume and weight are critical parameters in movable
platforms such as vehicles, ships, and planes. Although CO, can be used in some applications,
the concentration required for flame suppression exceeds the human toxicity levels. In occupied
spaces, the potential of accidental release is a very serious threat to human safety.

In terms of sensible enthalpy per unit mass, liquid water is the best physical agent. A key
issue is how to take advantage of the latent heat of vaporization (> 40% of the sensible enthalpy)
in an actual fire suppression application, including dissemination and distribution of the optimum
droplet size. If the drops are too small they will not make it to the fire; droplets which are too
large will not completely evaporate. Water spray typically discharged from overhead water
sprinklers is characterized by very large (> 500 p: diameter) water drops. Although water spray
can control a fire, smaller droplets (< 200 4 diameter) characteristic of water mist, are much
more effective at heat abstraction due to their higher evaporation. The increased evaporation rate
of water mist also leads to higher oxygen dilution in the air stream.

Lentati and Chelliah [3] predicted that ~ 25 water droplets should be near the optimum
size to extract the greatest amount of heat yet large enough to just survive traveling through a
low strain rate counterflow diffusion methane/air flame when seeded from the air side. Studies at
NRL on water mist suppressed propane/air flames are consistent with these predictions and are
reported at this meeting [4]. In these studies, water droplets are monitored throughout the
flowfield using phase Doppler anemometry to determine the optimum droplet size as well as the
quantity of mist required to extinguish the counterflow diffusion flames. Recent results also
presented at this meeting on the flame speed inhibition by very fine water mist, nominal 0.3
diameter droplets, show that the theoretical heat extraction potential of water can be achieved [5].

The combination of the heat extraction by fine water mist and a chemical suppression
component by an additive has the potential to impact a significant chemical suppression
component to the overall flame extinction mechanism. Our laboratory is currently assessing the
effectiveness of water additives for suppressing both counterflow diffusion flames and premixed


Bunsen flames.

CHEMICAL SUPPRESSION

All agents have a physical component to suppression. In addition, they can have a
chemical component that significantly increases effectiveness. Halon 1301 is effective at fire
suppression because the bromine can chemically combine H radicals and remove them from the
flame: H + Br > HBr, HBr + H > H, +Br. Catalytic removal is possible because the H, OH,
and O radicals controlling the flame chemistry are at superequilibrium concentrations in the
flame front. Designing bromine containing molecules that do not make it to the stratosphere
(e.g. tropodegradable compounds) is one area of current study. Inclusion of chemical structures
that promote oxidation or photolysis leads to compounds with higher boiling points, toxicity
concerns, and less suitability as “drop-in” Halon 1301 replacements. Iodine containing
compounds are also tropodegradable and have good chemical suppression efficiency but also
suffer from the problem of toxicity and low volatility concerns, Although these compounds are
less likely to be used as total flooding agents, many of them have potential for use in other
applications.

In the mid 1970's Sheinson and others at NRL began studies on the extinction of small
liquid pool fires in a cup bumer [6]. Extinction concentration data provided suppression
effectiveness ranking for a large number of compounds (exclusively gases). Real scale tests show
that the cup burner extinction concentration (plus a small margin) is usually adequate to
extinguish most fires due to the very stable cup burner flame. Cup burner data have been used to
derive recommended agent concentrations for suppression systems. However, realistic scale
results are still needed to evaluate non-scaling effects such as agent mixing, by-product
formation, and fire extinguishment times.

From the large number of compounds measured in the NRL Cup Bumer, Sheinson
derived a suppression predictive capability based on physical and chemical properties [7]. These
studies also provide a great deal of understanding concerning the interplay of chemical and
physical properties of various compounds [8]. They reported the enhanced suppression effect of
combining chemically acting and physically acting agents [9] which has been further explored by
Saso et al. [10]. The source for this enhancement results from the impact of the agent on the
temperature field and the flame radical concentrations.

An outgrowth of the NRL Cup Bumer studies on hydrofluorocarbon compounds (HFCs)
was the recognition that there was an increased chemical contribution to suppression if the
fluorines are arranged on the hydrocarbon in CF,-groups [7]. The increased effectiveness
observed was significantly greater than that predicted based on the heat capacity of the CF,-
group. This observation led to the commercialization of CF,CHFCF, (HFC-227ea,
heptafluoropropane noted here as HFP). In addition, of the HFCs that have multiple isomeric
forms, those that have been commercialized as fire suppression agents have the fluorines and
hydrogens arranged to maximize the number of CF,-groups. NRL suppression testing and full
scale evaluation of HFP resulted in the implementation of HFP by the U.S. Navy as one of the
replacements for Halon 1301 in the next class of ships to be constructed [11].

NRL extinction strain rate studies in counterflow diffusion flames showed that the