Limits to the Effectiveness of MetalContaining Fire Suppressants

Gregory T. Linteris 

NISTIR 7177
 

NISTIR 7177

Limits to the Effectiveness of MetalContaining Fire Suppressants

Gregory T. Linteris

Fire Research Division

Building and Fire Research Laboratory

October 2004

U.S. DEPARTMENT OF COMMERCE

Donald L. Evans, Secretary

TECHNOLOGY ADMINISTRATION

Phillip J. Bond, Under Secretary of Commerce for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

Arden L. Bement, Jr., Director
 

Final Technical Report 

Limits to the Effectiveness of 

Metal-Containing Fire Suppressants 

Date:10/12/2004 

Gregory Linteris 

Building and Fire Research Laboratory 

Fire Research Division 

100 Bureau Dr. Stop 8652 

Gaithersburg MD 20899-8652 

October 2004 

Final Technical Report, March 1, 2004 – July 31, 2004 

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 
1

Abstract 

This report reviews the literature on metal inhibition of flames and identifies metal species with 

potential as fire suppressant additives. To provide a basis for discussion, the detailed mechanism 

of inhibition of iron is reviewed, and the reasons for its loss of effectiveness are described. The 

demonstrated flame inhibiting properties of other metals is then discussed, followed by a 

description of the potential loss of effectiveness for these other metals. 

Introduction 

The production ban on the widely used and effective halon fire suppressants due to their ozone 

depletion potential, has motivated an extensive search for replacements (Gann, 2004). Metal 

containing compounds have attracted attention—especially for unoccupied spaces—because of 

their extraordinary effectiveness in some configurations. For example, Fe(CO)5 has been found 

to be up to eighty times more effective than CF3Br at reducing the overall reaction rate in 

premixed methane-air flames, when added at low concentration (Reinelt and Linteris, 1996). 

Unfortunately, it has also been found to produce condensed-phase particles which reduce its 

effectiveness for co-flow diffusion flames (Linteris and Chelliah, 2001). Hence, it is of interest 

to identify other metal compounds which may be strong flame inhibitors and then to assess their 

potential for loss of effectiveness through condensation. To achieve this goal, the present report 

provides background on current understanding of metal inhibition of flames, identifying metals 

with fire suppression potential. The inhibition mechanism of the iron is described, and the 

followed by a description of the reasons why it losses its effectiveness in some flame systems. 

The equivalent flame inhibiting species of other metal agents is then discussed, and evidence for 

any potential loss of effectiveness for these other metals is assembled and discussed. 

Background 

There has been much work, in a number of different applications, on the effect of metal 

compounds on the high-temperature oxidation reactions of hydrocarbons. The interest in the 

present work is on their potential as fire suppressants, as well as evidence for and mechanisms of 

a loss of effectiveness that they might demonstrate for certain fire suppression applications. 

Since their inhibition, as well as any loss of effectiveness, may be governed by consistent 

mechanisms between the different applications, we explore all relevant chemical systems that 

might have bearing on their performance as fire suppressants. 

The behavior of metal compounds in flames has been studied with regard to several flame 

phenomena. In the most obviously relevant work, metal compounds have been added to flames 

in a number of screening tests, aimed specifically at assessing their potential as fire suppressants. 

In flames, screening tests have has been performed to understand their influence on flame speed 

(premixed flames) and extinction (diffusion flames), as well as their effect on ignition. Flame 

studies have also been used to understand the detailed mechanism of inhibition of metal agents, 

either by providing direct experimental data on species present in the flame zone, or by 

validating numerical models which are then used to calculate the flame structure; either of these 
2

approaches are then used to develop an understanding of the relevant chemical kinetic 

mechanisms. Studies of engine knock suppression by metals provided much early data on metals 

in flames. Since engine knock is known to occur from the rapid pressure rise (and subsequent 

detonations) inside an engine cylinder caused by too fast reaction of the homogeneous charge of 

fuel and air, the mechanisms of engine knock reduction may have clear relevance to fire 

suppression—in which the goal is again to reduce the overall reaction rate with the addition of 

the suppressing agent. In engines—as well as in heating applications—research has also been 

directed at understanding their efficacy at soot reduction. Although both soot formation and the 

overall reaction rate of flames are known to be related to the location and concentration of 

radicals in the flame, the effects of metals on soot formation are not reviewed here. 

Other systems have been used to understand metal chemistry in flames. A large amount of 

fundamental work has been done with premixed atmospheric-pressure flat flames. In this 

system, the flat flame provides a nearly one-dimensional system, and the region above the flame 

(i.e., downstream of the main reaction zone) provides a long residence time, high temperature 

region for radical recombination. The H-atom concentration typically is measured with the LiLiOH method (described below), and the additive’s effect on the radical recombination rate is 

determined. Ignition studies using flames for screening tests have been reported. In addition, 

much detailed fundamental understanding has come from shock-tube studies and flash-photolysis 

studies in reaction vessels. Some studies of fire retardants are also relevant to fire suppression 

mechanisms of metals. For example, when the fire retardant works by suppressing the gas-phase 

reactions (and the subsequent heat release and heat feedback to the solid sample), the 

mechanisms are directly relevant to fire suppression. Finally, after-burning in rocket nozzles 

provided motivation to understand metal-catalyzed radical recombination reactions, and 

modeling studies have been performed for those systems. 

From these studies, it is clear that metals can have a profound effect on flame chemistry. 

Further, their effectiveness in these varied applications may well be related. In any event, data 

from each of the applications can provide insight into possible metals for application to fire 

suppression as well as provide fundamental data useful for predicting their performance in a 

range of applications. Work investigating the effect of metals for each of the applications is 

described below. 

Engine Knock 

Agents that reduce engine knock may also be effective flame inhibitors, and it is useful to 

examine the literature of engine knock to search for possible moieties. Engine knock is the onset 

of detonation waves in an engine cylinder brought about by the homogeneous ignition of the 

end-gas region of highly compressed and heated fuel and air. The effect of some agents in 

reducing knock has been known since the 1920s, including compounds of bromine, iodine, 

tellurium, tin, selenium, iron, and lead, as well as aniline (Kovarik, 1994). Tetraethyl lead (TEL) 

very early became the anti-knock agent of choice. While much subsequent research was 

performed to understand its mechanism of knock reduction, the exact mechanism for this agent 

remains an unsolved problem in combustion research (perhaps because leaded fuels were later 

banned due to their poisoning effect on exhaust catalytic converters). Although much progress 

was made, the researchers divided into two camps: those endorsing a heterogeneous mechanism 
3

(Walsh, 1954) and those promoting a homogeneous radical recombination mechanism (Erhard 

and Norrish, 1956). 

Several known effects of lead in engines support the heterogeneous mechanism. Muraour (1925) 

appears to have been the first to propose chain-breaking reactions on the surface of a colloidal 

fog formed from TEL. The particle cloud was subsequently shown to be composed of PbO, 

which is the active species (Chamberlain and Walsh, 1952). Since a strong influence of PbO 

coatings on reaction vessel walls has also been observed (Chamberlain and Walsh, 1952), a 

heterogeneous mechanism of PbO was assumed. Although other results support the 

heterogeneous mechanism, the evidence is somewhat circumstantial. The known metallic

antiknock compounds (tetraethyl lead, tellurium diethyl, iron pentacarbonyl, nickel 

tetracarbonyl, (Walsh, 1954)) all produce a fog of solid particles. The alkyls of bismuth, lead, 

and thallium are anti-knocks, but those of mercury (which does not form particles) are not 

(Cheaney et al., 1959). Richardson et al. (1962) showed that carboxylic acids increase the 

research octane number of TEL in engines, and argued that they reduced agglomeration of the 

PbO particles in the engine end-gas, but acknowledged that their arguments were qualitative. 

Zimpel and Graiff (1967) sampled end gases in a fired engine for transmission electron 

microscopy (TEM). They claimed that 1000 nm diameter particles formed prior to the arrival of 

the flame, proving that the effect of TEL was heterogeneous. Commenters to the paper pointed 

out, however, that the particles could be forming in the sampling system, that the effect of lead 

extenders was not captured in the particle morphology, and that even if particles form, the gasphase species can also be present, and it is thus not precluded that they are doing the inhibition. 

Based on the work of Rumminger and Linteris (2000c), even if particles form, they can reevaporate in the hot region of the flame if they are small enough. Finally, as described by 

Walsh (1954), a major shortcoming in the work with TEL is that it is easily absorbed on the way 

to the reaction vessel, so that it is difficult to know how much TEL actually makes it to the 

flame. As subsequently described by Linteris and co-workers, the efficiency (Reinelt and 

Linteris, 1996; Rumminger et al., 1999; Rumminger and Linteris, 2000a; Linteris et al., 2000a; 

Linteris et al., 2002) and particle formation (Rumminger and Linteris, 2000c; Rumminger and 

Linteris, 2002) of organometallic agents are strongly affected by the volume fraction of the metal 

compound. 

In later work, Kuppo Rao and Prasad (1972) claimed to prove the heterogeneous mechanism of 

lead anti-knock agents. They inserted copper fins coated with PbO into the cylinder of an engine, 

or injected 30 μm particles into the air stream, and found antiknock effects. They interpreted 

these results as evidence that the mechanism is heterogeneous. Nonetheless, they did not 

measure for the presence of gas-phase lead compounds, so a homogeneous mechanism cannot be 

ruled out. For the eleven lead compounds tested, they found the effectiveness to vary by a factor 

of about six, and found similarly sized particles of CuO2, CuO, CuCl2, NiCl2, and SnCl2 to have 

equal effectiveness which was less than any of the lead compounds. 

The early and strong evidence for a homogeneous gas-phase inhibition mechanism of TEL was 

developed by Norrish (1956). Using flash photolysis of mixtures of acetylene, amyl nitrite, and 

oxygen in a reaction vessel, with and without TEL, the absorption and emission spectra of amyl 

nitrate, OH, Pb, PbO, NO, CN, CH, and TEL were obtained as a function of reaction progress. 

The reactants were chosen since, in the absence of anti-knock compounds, they showed the 
4

strong homogeneous detonation characteristic of engine knock. The researchers found that the 

induction time increased linearly with TEL addition at low TEL partial pressures, but that the 

effectiveness dropped off at higher pressures (a result described subsequently for the flame 

inhibitors Fe(CO)5 (Jost et al., 1961; Reinelt and Linteris, 1996), SnCl4, (Lask and Wagner, 

1962; Linteris et al., 2002), and MMT (Linteris et al., 2002)). After decomposition of TEL, Pb 

was present in low concentration, followed by large amounts of PbO and OH , which 

subsequently dropped off. With TEL addition, the formation of OH was retarded, and the 

increase in OH emission was smoother and more well-behaved. No particles were reported. 

Norrish et al. described the action of TEL as a two-stage homogeneous gas-phase reaction 

mechanism. In the first stage, TEL reacts with the peroxide and aldehyde intermediates 

(characteristic of the end of the cool flame regime of alkane combustion), thus reducing the 

availability of these species for initiating the well-known second stage of the combustion. Gasphase PbO from the TEL then reacts with the chain-carrying intermediates to reduce the rate of 

heat release, slow the temperature rise, and reduce detonation. It is noteworthy that many of the 

features subsequently described by Linteris and Rumminger (2002) as necessary for flame 

inhibition by iron compounds (effectiveness in the absence of particles, decreasing effectiveness 

with higher inhibitor concentration, key role of the metal monoxide species, and the necessary 

coexistence of OH and metal monoxide species) were shown in the 1950s by Norrish and coworkers to be necessary for effective knock reduction by TEL. Although it was of the highest 

quality, the findings of Norrish and co-workers were discredited by the engine community 

largely because the tests were not done in engines, or with typical fuels. This work predated 

most of the important flame inhibition work of the 1970s by 20 years. 

As a prelude to the description of the flame screening tests, it is useful to describe the different 

terminology often used to describe the extinguishing of a flame. Flame inhibition usually refers 

to a weakening of a flame, that is, a lowering of the overall exothermic reaction rate in the flame. 

This weakening may or may not lead to extinguishment, depending upon the flow field in which 

the flame exists. In contrast, the terms fire suppression, flame extinguishment, or flame 

extinction are often used to refer to the case in which the flame has been weakened to the point 

where it can no longer stabilize in the relevant flow field. Flame quenching refers to flame 

extinguishment for which heat losses to a surface was a precipitating factor. 

Flame Screening Tests 

Papers describing the results of screening tests clearly demonstrated the superior effectiveness of 

some metals as flame inhibitors. The seminal work of Lask and Wagner (1962) investigated the 

efficiency of numerous compounds for reducing the burning velocity of premixed Bunsen-type 

hexane-air flames stabilized on a nozzle burner. They found the metal halides SnCl4 and TiCl4

to be quite effective at low volume fraction (and GeCl4 about a factor of two less then these). In 

unpublished work, cited by others (Morrison and Scheller, 1972), Lask and Wagner provide the 

measured flame speed reduction with addition of SbCl3 to hexane-air flames, showing it to be 

about twice as effective as CF3Br. Further investigating the efficiency of metal compounds, they 

tested iron pentacarbonyl Fe(CO)5, tetraethyl lead TEL Pb(C2H5)4, and chromyl chloride 

CrO2Cl2 and found these to be spectacularly effective, with volume fractions of only 170 μL/L 

and 150 μL/L required for the first two to reduce the burning velocity the hexane-air flames by 
5

30 % (although not quantified, they believed the effectiveness of CrO2Cl2 to be even higher). 

They categorized the compounds they tested into two classes: the halogens, and the much more 

effective transition metals. Miller et al (1963), using a burner which produced conical flames, 

tested eighty compounds for reducing the burning velocity of premixed hydrogen-air flames 

(fuel-air equivalence ration φ of 1.75). They found the most effective to be: tetramethyl lead 

TML Pb(CH3)4, Fe(CO)5, TiCl4, SnCl4, SbCl5, and TEL (in that order), with TEL only slightly 

better than CF3Br, and TML about 11 times better. A limitation of this work, however, is that 

the agents were tested at only a single volume fraction. Since the efficiency of many agents is 

known to vary with their concentration (Reinelt and Linteris, 1996), such an approach can skew 

the relative performance rankings. It should also be noted that in the hotter, faster-burning 

hydrogen flames, the propensity to form particles from the metal oxides and hydroxides will be 

less than in the slower, cooler hydrocarbon-air flames of other studies (see Rumminger et al. 

(2000c)). An additional flame screening test was performed by Miller (Miller, 1969), who tested 

fifteen compounds added to low-pressure (1.01 kPa) premixed and diffusion flames, and found 

tin (SnCl4), phosphorus (POCl3), titanium (TiCl4), iron (Fe(CO)5), tungsten (WF6), and 

chromium (CrO2Cl2) to have some promise. 

Two screening tests involved the inhibition of propagating premixed flames through clouds of 

small solid particles of inhibitor. Rosser et al. (1963) added metal salts as dispersions of fine 

particles (2 μm to 6 μm diameter) to premixed methane-air flames. They used the reduction in 

upward flame propagation rate through a vertical tube as a measure of the inhibition effect. They 

correctly described the effects of flame speed and particle diameter on the particle heating rate, 

and together with the volatilization rate for each compound, assessed the fraction of the particle 

which was vaporized. They quantified the effect of particle size on inhibition effectiveness, and 

showed that while additional agent decreased the flame speed, the effectiveness eventually 

saturated (i.e., beyond a certain additive mass fraction, additional inhibitor had a greatly reduced 

effect on the flame speed). They postulated a homogeneous gas-phase inhibition mechanism 

involving H, O, and OH radical recombination reaction with the metal atom and its hydroxide. 

This mechanism had many of the features of subsequently described mechanisms (Cotton and 

Jenkins, 1971; Bulewicz et al., 1971; Bulewicz and Padley, 1971a; Linteris et al., 2002) which 

are now believed to be correct. They also correctly understood that catalytic radical 

recombination relies upon a super-equilibrium concentration of radicals (commonly present in 

premixed flames and diffusion flames at higher strain), and that this provides an upper limit to 

the chemical effect of catalytically acting agents. Finally, they suggested that adding an inert 

compound as a co-inhibitor can overcome this limitation. A number of alkali metal sulfates, 

carbonates, and chlorides were tested, as well as cuprous chloride CuCl, which was found to be 

about twice as effective as Na2CO3 (after correcting for the larger size of the CuCl particles. 

A later study involving premixed flames with particles was performed by deWitte et al. (1964). 

In it, relatively large particles (100 μm diameter) were electrostatically suspended in a tube and 

then injected into a downward facing premixed Bunsen-type flame, and their effect on the flame 

temperature, burning velocity, and extinction condition was measured. Various barium, sodium, 

and potassium compounds were tested, as well as AlCl2, CuCl2, and PbO. The authors noted a 

thermal and chemical effect of the particles, and assumed that the chemical effect was due to 

recombination of chain-carrying radicals on the particle surfaces. The authors estimated that for 

these large particles, little of the particles could be vaporized. Nonetheless, it is not possible to 
6

separate the heterogeneous and homogeneous inhibition effects from their data. The authors 

found particles of CuCl2 and PbO to be about two and eight times as effective as particles of 

Na2CO3, and surprisingly, found AlCl3 to be about three times as effective. 

A flame screening test incorporating many compounds was also performed by Vanpee and 

Shirodkar (1979) to test the relative effectiveness of metal salts. The metal acetonates and 

acetylacetonates were dissolved in ethanol, and fine droplets of the metal salt solutions were 

sprayed into the air stream of a counterflow diffusion flame over an ethanol pool. The inhibition 

effect was quantified, at a given air flow velocity (i.e., strain rate) as the change in the oxygen 

mole fraction at extinction caused by addition of the inhibitor, normalized by the inhibitor mole 

fraction (Mole merit number = (XO2,ext- XO2,ext,i)/Xi, in which XO2,ext,i and XO2,ext are the oxygen 

mole fractions required for extinction, with and without added inhibitor, and Xi is the mole 

fraction of inhibitor). Their results are depicted in Figure 1, which shows the metal compounds 

tested, listed from most effective to least. The maximum and minimum values of the mole merit 

number are listed for the range of oxidizer velocities of the tests (50 cm/s to 60 cm/s). As the 

figure shows, the metal salts of Pb, Co, Mn, Fe, and Cr all show some inhibition effect. 

Interestingly, Fe(CO)5 was not as effective as iron acetylacetonate, and for lead, the acetonate 

was about twice as effective the acetylacetonate. It should be noted, however, that the 

interpretation of the present data are complicated by several effects. First, changing the oxygen 

mole fraction changes the temperature, which can change the effectiveness of an agent, as 

subsequently described in refs. (Reinelt and Linteris, 1996; Rumminger et al., 1999; Rumminger 

and Linteris, 2000b; Rumminger and Linteris, 2000a; Macdonald et al., 2001). Since the air 

stream velocity is changed while holding the nebulizer flow constant, the ethanol concentration 

changes in these partially premixed diffusion flames. Adding changing amounts of a fuel species 

(i.e., the carrier ethanol) in the air stream, changes the flame location and the scalar dissipation 

rate for a given strain rate (i.e., air flow velocity), so that the extinction condition is modified (as 

discussed in (Fallon et al., 1996) and (MacDonald et al., 1998)). The size of the residual particle 

(which will vary from agent to agent) could affect its ability to vaporize in the flame, affecting 

the indicated efficiency. Finally, for metal agents that condense, their marginal effectiveness is a 

very strong function of the concentration at which they are added. Hence, without knowing what 

the additive mole fraction is, it is difficult to cross compare the effectiveness of the different 

agents. For these reasons, it seems appropriate to consider the results shown in Figure 1 as 

qualitative, rather than quantitative. For example, subsequent studies (Babushok and Tsang, 

2000) have rated iron as about ten times as effective as sodium (in contrast to the results of 

Vanpee and Shirodkar, which show the acetylacetonate of iron to be only about 20 % more 

effective than that of sodium. 
7

0

50

100

150

200

250

300

350

400

Pb Ac

Co Ac Ac

Mn Ac Ac

Fe Ac Ac

Na Ac Ac

Pb Ac Ac

Cr Ac Ac

Fe(CO)5

NiCl2

CCl4

Mole Merit Number

Figure 1 – Mole merit number of metal compounds for the oxidizer velocities in the range of 50-60 cm/s (from 

(Vanpee and Shirodkar, 1979)). 

Radical Recombination above Premixed Flames 

Much of the understanding of the homogeneous gas-phase flame inhibition by metals came from 

studies of H-atom recombination rates above premixed, fuel rich, H2 – O2 – N2 burner-stabilized 

premixed flames. The techniques, pioneered by Sugden and co-workers (James and Sugden, 

1956; Buelwicz et al., 1956; Bulewicz and Sugden, 1956a), involve absorption spectroscopy for 

OH, and the Li/LiOH technique for H atom. (In the latter technique, the strong absorption lines 

of Li and Na are simultaneously measured above a flat flame. With the assumptions that Na is 

present in the flame only in atomic form and that Li is present only as Li and LiOH, the 

measured ratio of the Li to Na absorption together with the known equilibrium constant for the 

reaction Li + H2O = LiOH + H provides [H].) In some of their early relevant work they 

determined the dominant metal species in H2-O2-N2 premixed flames above Meker burners with 

added dilute sprays from aqueous salts of copper and manganese. Copper was found to exist 

mostly as Cu in the flame, and the dissociation constants of CuH (Bulewicz and Sugden, 

1956b)and CuOH (Bulewicz and Sugden, 1956a) were determined from their concentrations 

above the flame at different temperatures. Similar results were subsequently obtained for MnO 

and MnOH (Padley and Sugden, 1959). 

In a detailed study of chromium, Bulewicz and Padley (1971b) added μL/L levels of Cr (from 

either chromium carbonyl or aqueous sprays of chromium salts) to a premixed, fuel rich, flatflame burner of H2+O2+N2, and measured the catalytic radical recombination by the chromium 

compounds. They identified the active chromium species as Cr, CrO, CrO2, and HCrO3 and also 

detected solid particles, which appeared to have equivalent black-body temperatures up to 500 K 

higher than the gas. They estimated an upper limit for the rate of heterogeneous radical 
8

recombination on the particle surfaces, and estimated that, at an upper limit, it was the same 

order as the natural un-catalyzed recombination rate in the flame. They also found that Cr 

showed measurable catalytic radical recombination even when added at volume fractions of 

about 1μL/L—at which heterogeneous particle catalysis cannot be contributing. They cite 

Jenkins (1969, personal communication) as first showing the catalytic effect of metals (Ca, Sr, 

Ba) on radical recombination in flames. Interestingly, their data show a saturation effect in the 

catalytic radical recombination by Cr species, and their analyses show that it is due—not to 

condensation of the active gas-phase Cr-containing species to particles—but to reduction in the 

available radicals to recombine. 

In continuing work, Bulewicz and coworkers (Bulewicz and Padley, 1971a) (Bulewicz et al., 

1971) studied the catalytic effects of twenty-one metal species for recombining chain-carrying 

flame radicals present at super-equilibrium levels above premixed, fuel rich, flat flames of 

H2+O2+N2 at 1860 K. Table 1 shows the ratio of the catalyzed to un-catalyzed recombination 

rate for H atom caused by each of the metals, added at 1.3 μL/L. The cut-off value for this ratio 

was 

Table 1 - Catalytic efficiency of different metals in promoting radical recombination. 

Values of kobs/kuncat (T = 1860°K; X[M] = 1.3 μL/L; H2/O2/N2 = 3/l/6). 

Strong effect Some effect No effect 

Cr 2.8 Co 1.1 V 1 

U 1.82 Pb 1.1 Ni 1 

Ba 1.75 Zn 1.07 Ga 1 

Sn 1.6 Th 1.06 Cl 1 

Sr 1.35 Na 1.04 

Mn 1.3 Cu 1.04 

Mg 1.25 La 1.04 

Ca 1.25 

Fe 1.2 

Mo 1.16 

arbitrarily set to 1.1, and those above that value were described as having a strong catalytic effect 

(Cr, U, Ba, Sn, Sr, Mn, Mg, Ca, Fe, and Mo, in that order). Those species having less but some 

effect were Co, Pb, Zn, Th, Na, Cu, and La, while V, Ni, Ga, and Cl (included for comparison 

purposes) had no effect at these volume fractions. The possibility of heterogeneous 

recombination on particles was admitted, but at these low volume fractions, the authors argued 

primarily for a homogeneous gas-phase mechanism involving H or OH reaction with the metal 

oxide or hydroxide (attributed to Jenkins). 

H + MO + (X) = HMO + (X) 

HMO + OH (or H) = MO + H2O (or H2), 
9

in which M is a metal, and X is a third body. 

In concurrent work with the alkaline earth metals, Cotton and Jenkins (1971) showed that Ca, Sr, 

and Ba added as a fine mist of a salt solution to premixed H2+N2+O2 flat flames catalyzed the 

radical recombination for additive volume fractions in the range of 1 μL/L to 10 μL/L. By 

estimating the rates reaction in possible recombination mechanisms, they recommended the 

radical recombination mechanism to be: 

 MOH + H => MO + H2

 MO + H2O(+X) => M(OH)2(+X) 

 M(OH)2 + H ↔ MOH + H2O 

Jenkins and co-workers extended their studies of metal-catalyzed radical recombination in 

premixed flames to study soot formation in diffusion flames. The strong effect of metals on soot 

formation in flames is well-known (Howard and Kausch, 1980). Cotton et al. (Cotton et al., 

1971) added forty metals to co-flow diffusion flames of propane and N2/O2 mixtures and 

measured their effect on soot emissions and the smoke point. They postulated a mechanism for 

soot reduction whereby the catalytic reaction scheme listed above runs backwards, dissociating 

H2O and H2 into OH and H, which then oxidize the soot particles. 

The question of heterogeneous versus homogeneous gas-phase chemistry was investigated by 

Bulewicz et al (Bulewicz et al., 1969). Through examination the variation of the intensity of 

emitted radiation as a function of wavelength from particles formed above a premixed H2+O2+N2

flat flame with added spray of aqueous uranium salt, they determined that the particles 

(presumably uranium oxide) were up to 500 K above the gas temperature. They interpreted the 

particle temperature rise to be caused by the catalytic recombination of H and OH on the particle 

surfaces. Nonetheless, Tischer and Scheller (1970) pointed out that the spectral variation of the 

particle emissivity is unknown, and the gray-body assumption of Bulewicz is probably 

unjustified. They also argued that the excess temperature may be due to surface reactions other 

than radical recombination. 

In similar work with premixed, fuel rich, flat flames of H2+O2+N2, Jensen and Jones (1975) 

extended the classic Li + H2O ↔ LiOH + H photometric method to include the equilibrium for 

the reaction 

Sr+ + H2O ↔ SrOH+ + H , 

in which [SrOH+

]/[Sr+] is measured mass-spectrometrically. Using both this new technique as 

well as the LiOH photometric method, they studied the catalytic flame radical recombination by 

tungsten and molybdenum, as well as once again confirming the strong effect of tin. Using 

flames at temperatures of 1800 K to 2150 K, and with metal addition at 1 μL/L to 110 μL/L, they 

collected data on the rates of radical recombination in the presence of the metal catalysts (added 

as tetramethyl tin, or hexacarbonyls of tungsten or molybdenum), and measured the major 

species present in flames inhibited by W and Mo. By analogy with the mechanisms they 
10

developed for calcium (Cotton and Jenkins, 1971)and iron (Jensen and Jones, 1974) in flames, 

they then postulated reaction mechanisms for W and Mo, and estimated the rates for the 

reactions in the catalytic cycles. For the conditions of their flames, the radical recombination 

cycles, for either W or Mo, were about five times faster then those of tin. (Using the results of 

Linteris et al., (2002) this translates to an effectiveness about ten times that of CF3Br, or about 

one fifth that of Fe(CO)5). For tungsten and molybdenum, the cycles are: 

 HWO3 + H ↔ WO3 + H2

WO3 + H2O ↔ H2WO4

 H2WO4 + H ↔ HWO3 + H2O 

 ――――――――――――――

 (net: H + H ↔ H2) 

and 

 HMoO3 + H ↔ MoO3 + H2

MoO3 + H2O ↔ H2MoO4

 H2MoO4 + H ↔ HMoO3 + H2O 

 ―――――――――――――――

 (net: H + H ↔ H2). 

In continuing work, Jensen and Jones (1976) used similar techniques to study the radical 

recombination by cobalt added to a premixed, fuel rich, flat flame of H2-N2-O2. With Co added 

as cyclopentadienylcobalt dicarbonyl at volume fractions of about 0.03 μL/L to 145 μL/L, and 

flame temperatures ranging from 1800 K to 2615 K, they spectroscopically identified the 

dominant cobalt-containing species to be Co, CoO, CoOH, and Co(OH)2, with most of the cobalt 

being present in the flame as free Co atoms. Again by analogy with the Ca and Fe mechanisms, 

the Co mechanism was postulated to be: 

 Co + OH => CoO + H 

CoOH + H ↔ Co + H2

CoO + H2O ↔ Co(OH)2

 Co(OH)2 + H ↔ CoOH + H2O 

 ―――――――――――――

 (net: H + H ↔ H2) , 

with the first step added since Co is the dominant Co-containing species. The rates of these 

catalytic steps were again inferred from the experimental data. Cobalt appeared to be about 2/3 

as effective as tin in these flames. 

Hastie (1973a) also used mass spectrometry to study the effect of inhibitors on premixed flames. 

Using a Bunsen-type burner with a premixed CH4-O2-N2 flame, he added SbCl3 or SbBr3 and 

detected the intermediate species through the flame. This work is described below. 
11

Gas-phase Flame Retardants 

Insight into mechanisms of metal flame inhibition can also be gleaned from studies of metal 

species added to materials as fire retardants (when their mode of action has been found to be in 

the gas phase). One such system is the antimony - halogen combination. Although they did not 

unravel the detailed mechanism, Fenimore and coworkers (Fenimore and Martin, 1966a; 

Fenimore and Jones, 1966; Fenimore and Martin, 1966b; Fenimore and Martin, 1972) showed 

that the relevant species act in the gas phase; they believed that the antimony moieties poisoned 

the flame, much as do brominated species. Similarly, Martin and Price (1968)observed that the 

addition of triphenylantimony to certain polymer substrates provided fire retardancy, even in the 

absence of halogen, and believed the mechanism involved antimony species in the gas phase. In 

a series of detailed experiments, Hastie and co-workers determined the mechanism of flame 

inhibition in the antimony-halogen system. Using a Knudsen effusion cell containing Sb2O3 over 

which passed HCl, they showed that SbCl3 was evolved through a series of halogenation steps 

involving successive oxychloride phases (1973b). Using a molecular beam mass spectrometer, 

they studied both the pyrolysis products of polyethylene retarded by antimony-oxide/halogen, as 

well as intermediate species profiles in premixed CH4/O2/N2 flames with added SbCl3 and SbBr3

(1975). For the pyrolysis studies, the major effused species from the polymer were SbCl3 and 

SbOCl. In the flame studies (Hastie, 1973a), they found that SbCl3 reacts readily to SbOCl, 

which then reacts with H to form SbO. They measured the major intermediate species of SbBr3

flame inhibition to be SbO, Sb, HBr, and Br. They also measured the decrease in the hydrogen 

atom volume fraction with addition of SbBr3, and developed a reaction sequence for the 

formation of the intermediate species, as well as for the gas-phase inhibition reactions. They 

argued strongly that the inhibition effect of antimony-oxide/halogen system is predominantly 

from the reaction sequence: 

SbO + H → SbOH* 

SbOH + H → SbO + H2

――――――――――

 net: H + H → H2

with a smaller effect from the usual bromine sequence, and an even smaller contribution from the 

equivalent chlorine cycle. 

Ignition Studies 

The effect of metals on ignition has been studied in both shock tubes and flames. Morrison and 

Scheller (1972) investigated the effect of twenty flame inhibitors on the ignition of hydrocarbon 

mixtures by hot wires, and found that SnCl4 was the most effective inhibitor tested for increasing 

the ignition temperature; whereas the powerful flame inhibitors CrO2Cl2 and Fe(CO)5 had no 

effect on the ignition temperature. Dolan and Dempster (1955) studied the effect of small 

particles (5 μm to 10 μm diameter) of metal compounds on suppressing the spark ignition of 

premixed natural gas-air mixtures in a vertical 7 cm diameter tube. They found that barium 
12

hydroxide octahydrate, barium chloride, and copper acetate monohydrate were each about three 

times less effective than sodium bicarbonate particles, while cuprous oxide and cobaltous 

chloride hexahydrate were each about 20 % less effective than NaHCO3. Several studies have 

shown that metals can actually speed the ignition process in some chemical systems. In shocktube studies, Matsuda and co-workers (1971)found that Cr(CO)6 reduced the initiation time for 

reaction of CO or C2H2 with O2 in shock-heated gases. The metal carbonyl was present at 

0 μL/L to 50 μL/L, so the presence of any particles was precluded. The mode of action was 

believed to be gas-phase reactions involving CrO, CrO2, and CrO3. In later shock-tube work 

with Fe(CO)5 in mixtures of CO-O2-Ar, Matsuda (1972) found that a volume fraction of Fe(CO)5

of few hundred μL/L greatly accelerated the consumption of CO2. They postulated the effect to 

be due to oxidation of CO by metal oxides via: FeO + CO => Fe + CO2, and noted that these 

reactions may be of importance since the mixtures were quite dry (XOH ≈ 5 μL/L). Interestingly, 

such accelerating oxidation pathways were also found to be important in the dry reaction of CO 

with N2O in flames (Linteris et al., 2000b). In recent shock-tube studies of CH4-O2-Ar mixtures, 

Park et al. (2002) found that with 500, 1000, or 2000 μL/L of Fe(CO)5, the ignition time was 

shorter than without the additive, again indicating a promotion effect of the metal additive; 

however, they did not determine the cause of the promotion for this moist system. Finally, in 

flash photolysis studies many years ago, Erhard and Norrish (1956)found that unlike Pb(C2H5)4

and Te(CH3)2 which retarded hydrocarbon combustion, Ni(CO)4, Fe(CO)5, and Cr(CO)6 all 

greatly accelerated the combustion. 

 

Radical Recombination in Rocket Nozzles 

Jensen and co-workers were motivated to study combustion inhibition by metal compounds 

largely by the desire to suppress afterburning in rocket nozzle exhausts. Using mechanisms 

developed for K, Ba, Fe, Mo, W, Cr, and Sn, Jensen and Webb (1976) calculated the amount of 

inhibitor required to suppress afterburning in the exhaust plume of a double-base propellant 

(unspecified composition, but usually a homogeneous mixture consisting of nitrocellulose and 

nitroglycerine). The products of the propellant reactions consist of a fuel rich mixture—

principally CO and H2, much like the recombination region above the fuel-rich premixed H2-O2-

N2 flames laboratory flames used to study the metal-catalyzed radical recombination. Their 

calculations (which do not include the effects of condensation to particles) indicate that W and 

Mo are about a factor of five less effective than Fe, Cr about a factor of six less effective, and Sn 

more than seven times less effective. These results are consistent with those of the flat flame 

measurements described above. 

Particle formation was discussed by Jensen and Webb. Although they did not measure particles 

or calculate the degrading effects of particle formation on the suppression of afterburning in 

rocket motors, they did estimate the upper limit for radical recombination by the heterogeneous 

reactions on the particle surfaces. Their calculations indicated that although significant, the 

heterogeneous reactions could not suppress the afterburning, even for the smallest particle 

diameters assumed (10 nm). They also estimated that although the inhibiting species were 

probably volatilized in the combustion chamber, the characteristic times for condensation were 

probably of the same order as the residence time in the nozzle, indicating the potential for 

condensation. Their conclusions were that the metals held good promise for afterburning 
13

suppression in rocket motors, and that further work was necessary to estimate the rates of the 

catalytic cycle reactions and of the condensation rates of the metal derivatives at the conditions 

of their system. 

Other Relevant Investigations of Metals in Combustion Systems 

There have been a number of recent papers dealing with metals in flames. Crosley and coworkers (Westblom et al., 1994) added MMT to premixed flames with the purpose of studying 

its effect on NO formation. Adding MMT at volume fractions of about 0.5 μL/L to nearstoichiometric premixed low-pressure (522 Pa) propane-air flat flames in a McKenna burner, 

they measured the temperature and the relative concentrations of OH, H, O, CH, NO, and CO 

through the flame. While they observed no discernable effect of the MMT, this may have been 

due either to the low concentration of the additive, or the low pressure, both of which could limit 

the influence (Bonne et al., 1962; Linteris et al., 2002). They did, however, start to develop a 

mechanism for manganese which served as a basis for future efforts (Linteris et al., 2002). 

Chromium reaction in a atmospheric pressure, premixed hydrogen-air flame flat flame was 

studied by Yu et al. (1998). Using microprobe gas sampling in the region downstream from the 

main reaction zone, they collected chromium species in the gas and condensed phase. The 

particle size distribution and the fraction of Cr as Cr(VI) was determined as a function of 

position. In addition, they assembled a kinetic mechanism for Cr reaction in flames through 

analogy with Boron and Aluminum combustion, and used the mechanism, together with the 

measured temperature profile, to calculate the fraction of hexavalent Cr in the downstream 

region from the flame zone. They also modeled the growth (but not the reaction) of the particle 

phase. For both particle growth and Cr(IV) formation, the calculations were able to predict the 

experimental trends. They concluded that further kinetic model development was necessary for 

accurate treatment of Cr speciation in flames. In later work, Kennedy et al. (1999) studied the 

morphology of the particles formed in a hydrogen–air–nitrogen co-flow diffusion flame with 

added chromium nitrate or chromium hexacarbonyl. They found that the morphology of the 

particles varied with the temperature of the flame and the source of the chromium. 

Kellogg and Irikura (1999) performed theoretical calculations to predict the heats of formation as 

well as the enthalpies and free energies of reaction for the FeOxHy species thought to be 

important in iron inhibition. They found that nearly all of the reactions involving these species 

and potentially contributing to flame inhibition are exergonic at 1500 K. Hence, they suggested 

that further refinement of the inhibition mechanisms of iron would require knowledge of the 

actual kinetic rates of the inhibition reactions to improve upon the preliminary estimates of 

Rumminger et al. (1999). 

In a comprehensive review of possible chemicals for use as halon alternatives, Tapscott et al. 

(2001)suggested that of the metals, Cu, Fe, Mn, and Sn, were worthy of further study. Since that 

time, premixed (Linteris et al., 2002) and co-flow diffusion flame (Linteris et al., 2004) studies 

have been performed for Fe, Mn, and Sn. No additional work has been reported for copper as a 

fire suppressant. 
14

Detailed Studies with Iron 

After the potential effectiveness of iron as a flame inhibitor was indicated (Walsh, 1954; 

Cheaney et al., 1959; Lask and Wagner, 1962), the behavior of iron pentacarbonyl was 

investigated in detail in several old investigations. The extraordinary flame inhibiting 

effectiveness of iron was first identified by Lask and Wagner (1962) in their screening study 

involving methane-, hexane-, and benzene-air premixed Bunsen-type flames with numerous 

additives. In continuing work, (Bonne et al., 1962) described the superior effectiveness of 

Fe(CO)5 for reducing the burning velocity of hydrocarbon-air flames in nozzle burners. They 

found that an Fe(CO)5 volume fraction of 100 μL/L reduces the burning velocity by 25 % when 

added to stoichiometric methane-air flames at atmospheric pressure. With oxygen as the 

oxidizer, or at reduced pressure, they found the effectiveness to be lowered. For hydrogen-air 

flames, Fe(CO)5 was again much more effective than Br2. They noted that at low volume 

fractions, the decrease in the burning velocity was proportional to the concentration of Fe(CO)5 , 

whereas for increasing concentrations of Fe(CO)5, the relative influence seems to decrease. They 

postulated a homogeneous radical recombination mechanism at low volume fraction, and a 

heterogeneous one at higher. 

To understand the detailed mechanism of Fe(CO)5 , Bonne et al. (1962) spectroscopically 

investigated premixed flat flames of methane with air or O2. Unfortunately, at the low pressures 

for which the flame zone was expanded sufficiently to optically probe the flame (800 kPa), the 

kinetic effect of Fe(CO)5 was very small. Nonetheless, for Fe(CO)5 volume fractions up to 100 

μL/L they observed that 1.) the peak OH volume fraction XOH was unchanged, but shifted 

slightly downstream from the burner, and 2.) the decay rate of XOH was increased in the presence 

of Fe(CO)5, clearly indicating the effect of Fe(CO)5 on radical concentrations in the flame. They 

measured FeO and Fe emission, as well as Fe absorption, and found that Fe and FeO emission 

peaked in the main high-temperature reaction zone, implicating these species in the radical 

recombination reactions. Although FeO had a double peak, with a minimum at the location 

where the rate of OH recombination was greatest, the authors did not feel that the decrease in 

FeO emission was correlated with a decrease in FeO concentration. They noted that solid 

particles were forming, that they were attempting to measure them, and said that the results of 

these measurements would be reported in the future (but no publications subsequently appeared). 

In later work, the high effectiveness of iron was confirmed for methane-air premixed Bunsentype flames, and extended to counterflow diffusion flames, by Reinelt and Linteris (1996). Their 

work indicated clearly that iron was very effective at low concentration, but that the 

effectiveness leveled off at higher concentration. Further studies with premixed flames of COO2-N2 andCO-N2O-N2 confirmed the effectiveness of Fe(CO)5 in other systems. 
15

Metals with Demonstrated Flame Inhibition Potential 

From the above, we can assemble a list of metals with demonstrated inhibitory effects in flame 

systems. These are presented in Table 2. The metallic elements which have shown flame 

inhibition potential in experimental studies are listed down the left column, and the type of flame 

system used to determine the effectiveness is listed across the top. Under each category of flame 

system, the individual reference is listed. 

Table 2 – Metals which have shown flame inhibiting properties. The type of experiment is shown at the top, 

followed by the specific reference. The elements are listed in the approximate order of demonstrated or 

expected effectiveness. 

Detailed Flame 

Studies 

Engine Knock Flame Screening 

Tests 

Flat-Flame Radical 

Recombination 

Ignition FR 

Element Bonne (1962)

Reinelt and Linteris (1996)

(Linteris et al., 2002)(Linteris et al., 2000a)

(Linteris et al., 2004)

Cheaney (Cheaney et al.,Kovarik (Kovarik, 1994)

(Kuppu Rao and Prasad,(Erhard and Norrish,(Walsh, 1954)(Lask and Wagner, 1962)(Vanpee and Shirodkar,(Miller et al., 1963)(Miller, 1969)(deWitte et al., 1964)(Rosser et al., 1963)(Bulewicz and Padley,(Bulewicz et al., 1971)(Bulewicz and Padley,(Jensen and Jones, 1976)(Jensen and Jones, 1975)(Jensen and Jones, 1974)(Dolan and Dempster,(Morrison and Scheller,(Erhard and Norrish,Hastie (1975). 

 

Cr XX X XXX 

Pb X X X X X X X X X 

Fe X X X X X X X X X X X X X 

Mn X X 

Ni X X 

W X X 

Mo X X 

Sn X X X X X X X X X X X X 

Co X X X X 

Ti X XX 

Ge X 

Sb X X X 

Te X X X 

Tl X 

Bi X 

Cu X X X X X 

U XX 

Zn X 

La X 

Th X 

Se X 
16

Flame Inhibition by Iron 

Recent results provide convincing evidence that the strong flame inhibition by iron is due to a 

homogeneous gas-phase mechanism, which is described below. Following that, the experimental 

data showing iron’s loss of effectiveness is described, along with a discussion of the parameters 

which most affect the particle formation which is believed to cause the loss of effectiveness. 

Gas-Phase Mechanism 

A detailed mechanism for the gas-phase iron-catalyzed radical recombination in flames was 

developed by Jensen and Jones (1974), and later, expanded by Rumminger et al. (1999). In both, 

the main catalytic cycle leading to radical recombination was postulated to be: 

 FeOH + H ↔ FeO + H2 

 FeO + H2O ↔ Fe(OH)2 

 Fe(OH)2 + H ↔ FeOH + H2O 

 ――――――――――――――

 (net: H + H ↔ H2). 

This mechanism is shown schematically in Figure 2. 

Fe(CO)5 

Fe

FeO2 

FeO 

Fe(OH)2 

FeOH 

+O

+H 

+H2O 

+H

+O2 + M 

H-atom cycle

+H

Figure 2 – Main catalytic radical recombination cycle of iron found to be important for methane-air flames. 

Although the mechanism described in Rumminger et al. also included other catalytic cycles, they 

were not found to be particularly important in methane-air flames, premixed or diffusion 

(Rumminger and Linteris, 2000a). In work with premixed CO-N2-O2 flames, however, the 

following additional catalytic cycle was found to be much more important than the H-atom 

cycle: 
17

Fe + O2 +M ↔ FeO2 +M

 FeO2 + O ↔ FeO + O2

 FeO + O ↔ Fe + O2 

 ―――――――――

net: O + O ↔ O2 . 

This new O-atom cycle together with the H-atom cycle are shown in Figure 3. 

Fe(CO)5 

Fe

FeO2 

FeO 

Fe(OH)2 

FeOH 

+O

+H 

+H2O 

+H

+O2 + M 

+O 

O-atom cycle H-atom cycle

Figure 3 - Schematic diagram of radical recombination reaction pathways in found to be important for COH2-O2-N2 flames (Rumminger and Linteris, 2000b). Thicker arrows correspond to higher reaction flux. 

Reaction partners are listed next to each arrow. 

Further, more recent computations of the thermochemistry of iron compounds at flame 

conditions support the possibility of many more radical recombination cycles (Kellogg and 

Irikura, 1999). In that study, seven iron species thought to exist at flame temperature were 

considered: Fe, FeH, FeO, FeOH, FeO2, FeO(OH), Fe(OH)2 (Rumminger et al., 1999), and the 

heat of reaction at 0 K, and the change in Gibbs free energy at 1500 K were calculated. Based on 

the results, however, very few of the considered reactions and cycles could be eliminated based 

on the thermodynamics. The complexity of the situation is illustrated in Figure 4, (from Kellogg 

and Irikura, (1999)), which shows schematically the possible inhibition cycles of iron. (Note that 

in the figure there are on the order of 50 possible cycles since [Fe] can be replaced by Fe, FeH, 

FeO, or FeOH.) 

It is important to observe that even for iron, for which the most research has been performed, the 

mechanism is in very early stages of development. For example, the rates of the most important 

reaction steps in the mechanism were selected (within the uncertainty bounds suggested by 

Jensen and Jones (1974)) so as to provide the best agreement with experiments. The mechanism 
18

has been tested only for near stoichiometric premixed CH4-N2-O2 , CO-N2-O2, and CO-N2O 

premixed flames, CH4-O2-N2 counterflow diffusion flames, and CH4-air cup-burner flames. 

Although at low Fe additive mole fraction the agreement was usually good, there were some 

conditions for which the predicted inhibition was off significantly. These cases include lean 

premixed CH4-air flames (φ=0.9), CH4-O2-N2 flames with an oxygen volume fraction in the 

oxidizer of 0.20, and the cup-burner flames with added CO2 (at very low Fe(CO)5 volume 

fraction). 

Figure 4 - Schematic representation of different classes of reactions which-may contribute to iron's superefficient flame suppression ability through the catalytic recombination of radical species from (Kellogg and 

Irikura, 1999). 

The main ramification of the above gas-phase inhibition mechanism for fire suppression is that 

lowering radical concentrations lowers the overall reaction rate in the flame, weakening it. In 

laboratory flames, radical concentrations typically go above equilibrium levels, to superequilibrium levels. This is particularly true for both premixed and high and moderate strain 

counterflow diffusion flames. At low strains, however, the radical super-equilibrium is less (see 

Figure 5). Radical concentrations in actual fires have not been measured. Nonetheless, recent 
19

research using cup-burner flames (Katta et al., 2003b) (which, to some extent, resemble smallscale fires) has shown that a stabilization region at the flame base, called the flame kernel, 

resembles a near-stoichiometric low-temperature premixed flame. Hence, the gas-phase 

catalytic radical recombination cycles should be as important in the stabilization of fires as they 

are for laboratory flames. This is partially confirmed since other gas-phase catalytic agents (for 

example, CF3Br) are effective in both laboratory flames and full-scale fires. 

One would expect that if the gas-phase inhibition mechanism of iron were the only 

consideration, it should be a very effective agent for extinguishing fires. 

0

100

200

300

400

500

0 50 100 150 200

Strain Rate (1/s)

Superequilibrium H-atom Mole Fraction

1500

1700

1900

2100

2300

Temperature K

Tmax

Figure 5 - Calculated H-atom super-equilibrium ratio and peak temperature in counterflow diffusion flame 

as a function of strain rate. 

Nonetheless, recent work has shown that iron pentacarbonyl is much less effective in cup-burner 

flames than was expected based on the results in premixed or counterflow diffusion flames 

(Linteris et al., 2004). This was partially expected since earlier work with premixed flames 

(Rumminger and Linteris, 2000c) and counterflow diffusion flames (Rumminger and Linteris, 

2002) indicated that particle formation would limit the effectiveness of iron to those situations in 

which it was added at high concentration. Nonetheless, even at low concentrations, Fe(CO)5 was 

much less effective in cup-burner flames than was expected. 

Iron’s Loss of Effectiveness through Particle Formation 

The effectiveness of iron as a flame inhibitor goes down as the mole fraction at which it is added 

goes up. This lower effectiveness was mentioned briefly by Jost and co-workers (1961), who 

said that the effectiveness of Fe(CO)5 was lower at high mole fractions, and surmised that a 

particle inhibition mechanism may be at work at high concentration. They also said that work 

was underway to understand the role of particle formation; however, subsequent papers did not 

appear. Many other flame inhibitors also show lower effectiveness as their concentration 

increases (Noto et al., 1998). Nonetheless, the loss of effectiveness of iron is much more 

dramatic. Reinelt and Linteris (1996) quantified the loss of effectiveness of iron for premixed 
20

and diffusion flames. Rumminger et al. (1999) described a gas-phase kinetic mechanism for iron 

inhibition, and showed that the loss of effectiveness was not predicted by a gas-phase mechanism 

(ruling out low radical concentration as the reason for the loss of effectiveness). Linteris et al. 

(2002) have recently described the performance of iron, tin, and manganese in cup-burner 

flames. The loss-of-effectiveness of iron in each of these flame systems is described below. 

It should also be noted that based on the encouraging results for the powerful flame inhibition 

properties of iron, extinction experiments of ferrocene together with an inert compound 

generated by a solid propellant gas generator (SPGG) were conducted in an enclosure containing 

a spray flame (Fallis et al., 2000). Unfortunately, the combination did not have the intended high 

efficiency. Although it was not possible to extract fundamental information concerning the lack 

of effectiveness for their flame configuration, the results of Holland and co-workers (2000) 

provide important evidence for a loss-of-effectiveness for iron, and motivate a search for an 

explanation. 

Premixed Flames 

For premixed flames, Figure 6 shows that the gas-phase mechanism predicts a continuing 

decrease in the normalized burning velocity (SL/SL, uninhibited) as [Fe(CO)5] increases, whereas the 

experiments show a leveling off. Subsequent measurements of particles in the premixed flames 

showed that the loss of effectiveness of the iron was correlated with formation of particles (as 

illustrated in Figure 7 and Figure 8), and the scattering from the particles was correlated with the 

residence time in the flame (necessary for condensation to occur), as shown in Figure 9. In 

addition to measuring the particles with thermophoretic sampling and quantifying their size and 

agglomeration characteristics with transmission electron microscopy, they also calculated the 

maximum effect that the particles could have on the burning velocity. Constructing a “perfect 

heterogeneous inhibitor” model (in which any collision of a radical with a particle recombines 

the radical to a stable species), they showed that heterogeneous reactions cannot account for the 

measured flame speed reduction of Fe(CO)5. Figure 10 shows the experimental data for 

normalized flame speed reduction by Fe(CO)5 along with the prediction for the perfect gas-phase 

inhibitor and perfect heterogeneous inhibitor (for different assumed particle diameter sizes). As 

shown, the perfect gas-phase inhibitor mechanism shows burning velocity reductions fairly close 

to that of Fe(CO)5; whereas, the perfect heterogeneous mechanism does not show enough 

inhibition, about a factor of eight too low, even for perfect collisions with 10 nm particles. 

Interestingly, the slope of the Fe(CO)5 curve in Figure 10 in the “flattening out” region is of the 

same order as that of the perfect 10 nm diameter particles. Hence, Jost et al.’s (1961) postulate 

that the region of lower effectiveness may be due to radical recombination on particles surfaces, 

may be true. Note also that the slope in the flat region is of the same order as that for CF3Br in 

these premixed flames. 
21

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

Fe(CO)5 Mole Fraction (ppm)

Normalized Burning Velocity

 0.20

X = 0.24 O2,ox

0.21

φ = 1.0

Figure 6 - Calculations and measurements of normalized burning velocity of premixed CH4/O2/N2 flames with 

XO2,ox = 0.20 (squares), 0.21 (circles), and 0.24 (triangles) for φ = 1.0 (source: (Rumminger et al., 1999)) 

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400

Fe(CO)5 (μL/L)

Normalized Burning Velocity

0.E+00

2.E-07

4.E-07

Maximum Qvv (1/cm-sr)

X = 0.21 O2,ox X = 0.24 O2,ox

Figure 7: Normalized burning velocity (Reinelt and Linteris, 1996)and maximum measured scattering signal 

Qvv (Rumminger and Linteris, 2000c)for φ=1.0 CH4 flame with XO2,ox = 0.21 and 0.24. 
22

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400

Fe(CO)5 (ppm)

0.E+00

2.E-07

4.E-07

6.E-07

8.E-07

0.01

Maximum 

Qvv (1/cm-sr)

Normalized Burning Velocity

0.005

X = 0.015 H2

0.01

0.005

0.015

Figure 8: Maximum scattering signal and normalized burning velocity (Rumminger and Linteris, 2000b) for 

CO-H2 flames as Fe(CO)5 concentration varies (Rumminger and Linteris, 2000c). 

0E+00

1E-07

2E-07

3E-07

4E-07

5E-07

6E-07

10 20 30 40 50 60

Burning Velocity (cm/s)

Maximum 

Qvv (1/cm-sr)

300 μL/L 

200

100

l

m

h h h h m m

l

h

h

h

h

h

h

h

h

l

Figure 9 - Maximum Qvv for flames of CH4 (open symbols) and CO (closed symbols) as a function of the 

burning velocity. The letters correspond to the adiabatic flame temperature (low, medium, and high, 2220, 

2350, and 2470 K), while the symbol shape (square, diamond, and circle) corresponds to the loading of 

Fe(CO)5: (100, 200 , and 300) μL/L (Rumminger and Linteris, 2000c). 
23

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600 700 800

Gas-Phase Precursor Volume Fraction (μL/L)

Perfect gas-phase inhibitor

10 

20

dm = 80 nm

Fe(CO)5

40

Normalized Burning Velocity

Perfect heterogeneous inhibitor w/

Figure 10 - Calculated normalized burning velocity for several diameters dm of ideal heterogeneous inhibitor. 

Also shown are Fe(CO)5 data (Reinelt and Linteris, 1996), and calculated normalized burning velocity using 

the perfect gas-phase inhibitor mechanism (Babushok et al., 1998; Rumminger and Linteris, 2000c). 

Counterflow Diffusion Flames 

Similar experimental and modeling results were described for counterflow diffusion flames. As 

shown in Figure 11, the gas-phase model predicts more inhibition at higher [Fe(CO)5] than do 

the experiments. Measured particle scattering signals again showed that the loss of effectiveness 

(and discrepancy between the measured and gas-phase predicted inhibition) increased as particle 

scattering signal increased; i.e., particle formation was correlated with loss of effectiveness of the 

Fe(CO)5. (Figure 12 It is important to note that in the counterflow diffusion flames, the 

propensity to form condensed-phase iron particles in the flame region was not the only 

mechanism shown to be correlated with loss of effectiveness of iron. If particles formed in one 

part of the flame, but were unable (due to entrainment or thermophoresis) to be transported to the 

reaction zone, the active iron species could be sequestered from the region in which they must be 

present to inhibit the flame. By adding the Fe(CO)5 to either the fuel or air stream, and changing 

the flame location relative to the stagnation plane (by diluting the fuel or oxidizer with N2), 

Rumminger et al. clearly showed the effect of drag and thermophoretic forces on reducing 

particle (and hence iron) transport to the region of radical chain branching. 
24

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500

Inhibitor Mole Fraction (ppm)

Normalized aext

Fe(CO)5 in fuel

Fe(CO)5 

in oxidizer

Perfect

 in oxidizer

Perfect in fuel

Figure 11 - Normalized extinction strain rate for counterflow diffusion flames. Closed symbols: 

measurements with the Fe(CO)5 in the oxidizer; open symbols: measurements with Fe(CO)5 in the fuel; solid 

lines: calculations with Fe(CO)5; dashed lines: calculation with perfect inhibitor (source: (Rumminger and 

Linteris, 2000a) ). 

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

Fe(CO)5 (μL/L)

Normalized Extinction Strain Rate

0.E+00

2.E-07

4.E-07

6.E-07

8.E-07

Maximum Qvv in Flame Zone (1/cm-sr)

Figure 12 - Correlation between inhibition effect in counterflow diffusion flames and maximum measured 

scattering signal Qvv . Filled points are experimental normalized aext , solid line is calculated aext (Rumminger 

and Linteris, 2000a). Open symbols connected by dotted lines are maximum measured Qvv. Particle data 

collected at 75 % of aext (Rumminger and Linteris, 2002). 
25

Cup Burner Flames 

Linteris et al. have recently tested Fe(CO)5, TMT, and MMT in cup-burner flames (Linteris and 

Chelliah, 2001; Linteris et al., 2002; Linteris et al., 2004). Based on the loss of effectiveness at 

higher Fe(CO)5 concentrations that was demonstrated in premixed and diffusion flames, it was 

not expected that iron pentacarbonyl alone would be effective in cup-burner flames. 

Notwithstanding, there was much suggestion in the literature that a combination of a good 

catalytic agent and an inert agent would prove to be an effective combination. In this case, the 

overall reaction rate is lowered in part through radical recombination by the catalytic agent, and 

in part through the lower temperature caused by the added diluent. This approach has been 

discussed in work since the 1950s (Rosser et al., 1959; Rosser et al., 1963; Lippincott and Tobin, 

1953; Lott et al., 1996; Noto et al., 1998; Rumminger and Linteris, 2000b) which suggested that 

combinations of thermally acting and catalytic agents might prove beneficial. To test these 

suggestions, Linteris et al. added Fe(CO)5, MMT, or TMT to a cup burner of methane and air, 

and measured the amount of CO2 required for extinction (Linteris et al., 2004). This approach is 

conceptually the same as the classic oxygen index test used for assessing material flammability 

(Fenimore and Jones, 1966). In that test, the oxygen volume fraction in the air stream at blowoff 

(i.e., the oxygen index) is determined for solid, liquid, or gaseous fuels with chemical additives 

in either the fuel or oxidizer. 

Unfortunately, the effectiveness of CO2 combined with any of the metal agents was much less 

than anticipated. Figure 13 shows the measured CO2 volume fraction required for extinction as a 

function of the catalytic agent volume fraction in the air stream. Data are presented for Br2, 

CF3Br, Fe(CO)5, MMT, and TMT. As shown, although the metals are still more effective than 

CF3Br at low concentration, they are not nearly as effective as expected from the results in 

premixed and counterflow diffusion flames. The reasons for this loss of effectiveness are 

described below. 

In order to understand the lower effectiveness of metals in cup-burner flames compared to 

premixed and counterflow diffusion flames, several steps were taken. First, particle 

measurements in the cup-burner flames inhibited by Fe(CO)5 were made. The results (shown in 

Figure 14 to Figure 17) indicate that particles are present both inside and outside (but not 

coincident with) the luminous flame zone, and that higher Fe(CO)5 loadings produced higher 

particle scattering signals. In order to understand the particle formation and chemical inhibition, 

numerical modeling of the cup-burner flames inhibited by Fe(CO)5 were performed (Linteris et 

al., 2004), using the gas-phase only numerical model developed previously. This model has 

predicted the blow-off condition of methane and air cup-burner flames with added CO2 (Katta et 

al., 2003b) and CF3H (Katta et al., 2003a). The temperature field and the velocity vectors for 

methane-air cup-burner flames with 10 % CO2, and 0 and 100 μL/L of Fe(CO)5 are shown in 

Figure 18, while Figure 19 shows the calculated blow-off behavior. As in other flame 

configurations, the loss of effectiveness of iron and a discrepancy between predicted (gas-phase 

model) and measured effectiveness were both correlated with the formation of particles (see 

Figure 20). To understand the propensity for particle formation, the degree of super-saturation 
26

0.00

0.05

0.10

0.15

0.20

0 0.5 1 1.5 2 2.5

Catalytic Inhibitor Volume Fraction in Air (%)

CO2 Volume Fraction Required for Extinction

CF3Br

Sn(CH3)4

Br2

0 0.05 0.1

0.11

0.12

0.13

0.14

0.15

0.16

Sn(CH3)4

CF3Br

Br2

MMT

Fe(CO)5

MMT + Fe(CO)5+ Sn(CH3)4

Figure 13 – Volume fraction of CO2 required for extinction (XCO2,ext) of methane-air cup burner flames as a 

function of the volume fraction of catalytic inhibitor added to the air stream. Inset shows region in dotted 

box with expanded scales (Linteris et al., 2004). 

of some of the iron-containing intermediates was calculated through the flame, using the detailed 

flame structure obtained from the model, together with vapor pressure data available from the 

literature. For a height above the cup burner which passes through the flame kernel (i.e., the 

stabilization region), Figure 21 shows the radial profile of temperature and volume fractions of 

iron species and radicals, as well as the supersaturation ratio (which is the ratio of the calculated 

species partial pressure to the planar vapor pressure at the local conditions). The supersaturation 

ratio is highest for FeO, followed by Fe and Fe(OH)2, and the values decrease as the radial 

location of peak temperature is approached. Note that vapor pressure data for FeOH, FEOOH, 

and FeO2 are not available, so their condensation potential has not been assessed. The 

condensation potential is strong since the temperature of the flame kernel is much lower than the 

relevant regions of premixed or counterflow diffusion flames. 

Finally, the numerical model was extended to include calculation of the particle trajectory for 

inert particles added to the flame, including the effects of gravity, drag, and thermophoretic 

forces. This was done since early estimates (Linteris et al., 2002) were that thermophoresis may 

have been driving the particles away from the flame region. The results of the calculations 

(Figure 22) show that near the flame base, there is some deviation of the particles both up and 
27

down around the reaction kernel; however, examination of the estimated radial and axial 

thermophoretic velocities shows them to be much less than the gas velocity. Consequently, the 

particles still pass directly into the reaction kernel, so the effect of thermophoresis near this 

region is expected to be minor. Nonetheless, the other results described above provide evidence 

that the loss of effectiveness is due to particle formation. 

A review of the results from previous work with particle formation in premixed, counterflow 

diffusion, and cup-burner flames inhibited by Fe(CO)5 outlined the importance of the following 

physical effects with respect to effective chemical inhibition: 

1.) gas-phase transport of the active iron-containing species to the region of high H-atom 

concentration is necessary for efficient inhibition. 

2.) Particle formation near the location of peak [H] can act as a sink for the iron-containing 

intermediate species and reduce the catalytic effect. 

3.) The volume fraction of inhibitor influences condensation since at low values, it may be 

below its saturation value. 

4.) The available residence time affects particle growth. 

5.) If the particles are small enough, they can re-evaporate upon passing into the hightemperature region of the flame. 

6.) Thermophoretic forces can be large in the flame and re-distribute particles away from peak 

[H]. 

7.) Convection and drag forces combined with the existing flow field in the flame can prevent 

particles from reaching the region of peak [H]. 

8.) The flame temperature of the stabilization region of cup-burner flames is much lower than in 

premixed or counterflow diffusion flames, exacerbating the condensation potential. 

In order to assess the condensation potential of other flame inhibiting metals, it is necessary to 

know their concentrations in the flame, as well as their local vapor pressure. The availability of 

these data for the metals listed in Table 2 is discussed below. 
28

147

1013161922252831343740434649

S1

S4

S7

S10

S13

S16

S19

S22

-20 -15 -10 -5 0 5 10 15

Radial Position (cm)

0

5

10

15

20

Ht. Above Burner (cm)

-20-18-15-13-11

-8-6-3-1

1469

111316180

3

6

9

12

15

18

-20 -15 -10 -5 0 5 10 15

Radial Position (cm)

0

5

10

15

20

Ht. Above Burner (cm)

147

1013161922252831343740434649

S1

S4

S7

S10

S13

S16

S19

S22

-20 -15 -10 -5 0 5 10 15

Radial Position (cm )

0

5

10

15

20

Ht. Above Burner (cm)

147

1013161922252831343740434649

S1

S4

S7

S10

S13

S16

S19

S22

-20 -15 -10 -5 0 5 10 15

Radial Position (cm)

0

5

10

15

20

Ht. Above Burner (cm)

Figure 14 to Figure 17 – Scattering cross 

section for laser light at 488 nm as a 

function of radial position and height 

above burner in methane-air cup-burner 

flame with 8 % CO2 and Fe(CO)5 in air at 

specified volume fraction . Dotted lines 

show flame location from a digitized video 

image of the uninhibited flame (Linteris 

et al., 2004).

Figure 14 - Fe(CO)5 in air at 100 μL/L. 

Figure 15 – Fe(CO)5 in air at 200 μL/L. 

Figure 16 – Fe(CO)5 in air at 325 μL/L. 

Figure 17 – Fe(CO)5 in air at 450 μL/L. 
29

Figure 18 – Calculated temperature (color scale) and velocity vectors (arrows) for methane-air cup-burner 

flame with an oxidizer stream CO2 volume fraction of 10 %, with (left) and without (right) an added 

Fe(CO)5 volume fraction of 100 μL/L (Linteris et al., 2004). 
30

a.) b.)

Figure 19 – Two-D color map of calculated temperature in cup-burner methane-air flames with 10 % CO2 in 

the oxidizer stream, and a.) 0.011 and b.) 0.012 % Fe(CO)5 volume fraction in the air stream, illustrating the 

blowoff phenomenon (Linteris et al., 2004). 
31

Figure 20 – Experimental and numerically predicted extinction volume fraction of CO2 (left axis) and peak 

measured scattering cross section (right axis), as a function of the volume fraction of Fe(CO)5 in the air 

stream; from (Linteris and Chelliah, 2001) and (Linteris et al., 2004). 

Methane-Air Cup Burner

0.00

0.05

0.10

0.15

0 500

Fe(CO)5 Volume Fraction in Air (μL/L)

CO2 Volume Fraction at Extinction

0

1

2

3

4

Maxiumu Qvv x 105 1/(cm-sr) 

Calc

Expt.
32

 

10-7

10-6

10-5

10-4

10-3

Mole Fraction, X

XFe(OH)2 XFeO2

XFe(CO)5

XFe XFeO

XFe(CO)5

XFeOH

XOH XO

( a )

i

XH

XFeOOH

400

600

800

1000

1200

1400

1600

1800

10-5

10-3

10-1

101

103

105

2 4 6 8 10 12

Supersaturation Ratio, S

Radial Distance, r (mm)

Temperature, T (K)

i

( b )

SFeO

SFe

SFe(OH)2

T

Figure 21 – a) Calculated iron-containing and major species volume fraction Xi as a function of radial 

position at the height above the burner of 4.8 mm (corresponding to the location of the reaction kernel in the 

flame base); and b) the super-saturation ratio, Si, for Fe, FeO, and Fe(OH)2 (Linteris et al., 2004). 
33

Figure 22 – Calculated particle trajectories for free-molecular-regime particles in a CH4 – air flame with 

10 % CO2 in the oxidizer stream (Linteris et al., 2004). 
34

Equivalent Flame Inhibiting Species of Other Metals 

In general terms, there are two basic approaches for identifying the equivalent flame 

inhibiting species for other metals besides iron. One is through analogy with existing 

known mechanisms (Cotton and Jenkins, 1971; Bulewicz et al., 1971; Jensen and Jones, 

1974; Jensen and Webb, 1976; Jensen and Jones, 1976; Jensen and Jones, 1975), and the 

other is through a comprehensive consideration of all possible radical recombination 

cycles based on estimated thermodynamic properties (Kellogg and Irikura, 1999). Both 

depend upon generation of a list of probable species in the flame, which usually comes 

from spectroscopic identification of the species in flame systems. The latter generates a 

large number of possible species and reactions, which must then be culled down based on 

measured reaction rates of the possible inhibition reactions. Each of these approaches 

can be a research project in itself, for each metal inhibitor. It should also be noted that 

even for iron (for which the most research has been completed) there exist major 

limitations in the mechanistic description. For example, only a sub-set of the possible 

catalytic cycles has been included in the inhibition mechanism; the rates of the key 

reactions have been adjusted to provide agreement with recent burning rate data; some 

experimental results are not predicted well by the mechanism, and the vapor pressure of 

some key intermediates is not available. Nonetheless, the mechanism developed 

represents good progress in understanding flame inhibition by these super-effective 

moieties, and provides a basis for further work. The extension of the work for iron to 

other metals is a challenging but tractable task. As a first step, the following sections 

describe what it known, for other metals, about the species present in flames, the 

inhibition mechanisms, and the possibility for loss of effectiveness due to condensation. 

Discussion is provided for Cr, Pb, Mn, W, Mo, Sn, Co, Cu, and Sb. These metals have 

been selected because they have the most indicated potential for flame inhibition; they 

are listed in decreasing order of the expected inhibition efficiency (based on existing 

experimental results). 

Cr 

Although chromium as a flame inhibitor has been the subject of several studies (Lask and 

Wagner, 1962; Miller, 1969; Bulewicz and Padley, 1971b; Vanpee and Shirodkar, 1979), 

and it is believed to be one of the most powerful catalytic metals for radical 

recombination, very little is known about its mechanism. As described above, Bulewicz 

and Padley (1971a) did a detailed study in which chromium carbonyl or chromium salts 

were added to a flat premixed rich H2-O2-N2 flame. They detected the presence of 

particles, and estimated the upper limit on the radical recombination on the particle 

surfaces to be of the same order as the un-catalyzed radical recombination rate. They 

proposed that the following balanced reactions are important in chromium inhibition, but 

did not develop an explicit mechanism. 

Cr + OH ↔ CrO + H 

CrO + OH ↔ CrO2 + H 
35

CrO2 + H2O ↔ HCrO3 + H 

Further, they speculated that at very low additive concentrations, the following gas-phase 

reactions might be important. 

CrO + H + X => CrOH* + X 

CrOH* + H => CrO + H2

Pb 

Despite it’s known effectiveness as a flame inhibitor (see Table 2), there is very little 

detailed knowledge of the homogeneous gas-phase catalytic mechanism of lead. The 

active species were detected to be Pb and PbO in flash photolysis studies (Erhard and 

Norrish, 1956). 

Mn 

The mechanism for Mn is believed to be very similar to that of iron (Linteris et al., 2002). 

Mn reacts with O2 to form MnO2 , which reacts primarily with radicals to form MnO. 

The catalytic radical recombination cycle consists of: 

MnO + H2O ↔ Mn(OH)2

Mn(OH)2 + H ↔ MnOH + H2O 

MnOH + H (or OH, O) ↔ MnO + H2 (or H2O, OH) 

 ---------------------------------------------------------------- 

 net: H + H ↔ H2 (or: H + OH ↔ H2O) 

Although flame equilibrium calculations show that the species MnH is present at 

relatively large concentrations, the contribution of reactions of this species to the 

inhibition effect is relatively small. 

W 

Jensen and Jones (1975) found the dominant species in tungsten inhibition to be WO3, 

HWO3, and H2WO4. They developed the following mechanism for tungsten inhibition by 

analogy with their mechanisms for Ca and Fe. 

 HWO3 + H ↔ WO3 + H2

WO3 + H2O ↔ H2WO4

 H2WO4 + H ↔ HWO3 + H2O 

 --------------------------------------- 

 (net: H + H ↔ H2) 
36

From their experiments with premixed flat flames, they estimated the rate of the 

cycle to be about ten times that of CF3Br, or about one fifth that of iron. 

Mo 

Using the premixed of H2-N2-O2 flat flame, Jensen and Jones (1975) found the dominant 

species in molybdenum inhibition to be MoO3, H MoO3, and H2MoO4, and they 

developed the following inhibition mechanism : 

 HMoO3 + H ↔ MoO3 + H2

MoO3 + H2O ↔ H2MoO4

 H2MoO4 + H ↔ HMoO3 + H2O 

 ------------------------------------------- 

 (net: H + H ↔ H2). 

Sn 

A mechanism for flame inhibition by tin was developed by Babushok and co-workers 

(Linteris et al., 2002). The tin atom formed as a result of TMT decomposition quickly 

reacts with O2 through the reactions Sn + O2 (+M) ↔ SnO2, and Sn + O2 ↔ SnO + O. 

The former reaction leads to SnO from the reaction of SnO2 with CO, H, or other 

radicals. Conversely, the latter reaction forms SnO directly, and is fast at room 

temperature as compared to the analogous reaction of iron atom. Formation of SnO leads 

to the following reactions with H and HCO radicals: 

SnO+H + M ↔ SnOH + M 

SnO + HCO ↔ SnOH + CO 

which, together with the radical scavenging reactions of SnOH, complete the catalytic 

radical recombination cycle of tin: 

SnOH + H ↔ SnO + H2 

SnOH + OH ↔ SnO + H2O 

SnOH + CH3 ↔ SnO + CH4

SnOH + O ↔ SnO + OH. 

The net effect of the dominant inhibition reactions can be shown as: 

SnO + H + M ↔ SnOH + M 

SnOH + H ↔ SnO + H2

 ------------------------------------ 

net: H + H ↔ H2 
37

Equilibrium calculations show that SnO is the major tin species in the products of a 

stoichiometric methane–air flame with added TMT. 

Co 

Cobalt was added to a premixed, fuel rich, flat flame of H2-N2-O2. by Jensen and Jones 

(1976) who studied the radical recombination rate in the post combustion region having 

temperatures ranging from 1800 K to 2615 K. The dominant cobalt containing species 

were found to be Co, CoO, CoOH, and Co(OH)2, with most of the cobalt being present in 

the flame as free Co atoms. By analogy with the Ca and Fe mechanisms, the Co 

mechanism was postulated to be: 

 Co + OH => CoO + H 

CoOH + H ↔ Co + H2

CoO + H2O ↔ Co(OH)2

 Co(OH)2 + H ↔ CoOH + H2O 

 -------------------------------------- 

 (net: H + H ↔ H2) , 

in which the first step was included since Co is the dominant Co-containing species. The 

rates of these elementary steps were estimated so as to provide agreement with the 

experimental radical decay rate. On this basis, cobalt appeared to be about 2/3 as 

effective as tin in these flames. 

Cu 

Little has been reported on the actual flame inhibition mechanism of copper. The coppercontaining species present above a Meker burner supplied with H2-N2-O2 mixtures with 

added copper salts were found to be Cu, CuO, CuOH, and CuH (Bulewicz and Sugden, 

1956b; Bulewicz and Sugden, 1956a). Their concentrations were related by the balanced 

reactions: 

Cu + H2O ↔ CuOH + H 

Cu + OH + X ↔ CuOH + X 

Cu + H2 ↔ CuH + H 

Cu + H + X ↔ CuH + X 

Sb 

Hastie (1973a) used premixed flames with mass spectrometry to measure intermediate 

species in antimony inhibition. The only antimony species he identified in the flame were 

Sb, SbO, and halogenated products of the additive (SbCl3 or SbBr3). He postulated the 

mechanism to be: 
38

SbO + H → SbOH* 

SbOH + H → SbO + H2

Potential for Loss of Effectiveness of Other Metals 

There exist several approaches for estimating whether other metals will have similar loss 

of effectiveness to that of iron. One approach is to have experimental data for flame 

systems in which the loss of effectiveness is evident. This requires that the inhibitor be 

added at volume fractions high enough to allow the loss of effectiveness (which is 

believed to be due to condensation of the active gas-phase species to particles). For 

example, many of the early studies with metal compounds did not provide data to high 

enough volume fractions to show the loss of effectiveness (Lask and Wagner, 1962) or 

the inhibiting effect was not presented as a function of additive volume fraction (so the 

decreasing effectiveness was not illustrated) (Vanpee and Shirodkar, 1979; Miller et al., 

1963; Miller, 1969; Bulewicz and Padley, 1971a; Bulewicz et al., 1971; Jensen and 

Jones, 1975; Jensen and Jones, 1976). Another clue that a loss of effectiveness may 

occur is a reported presence of particles in some flame system. Although the presence of 

particles will depend upon the temperature of the flame, the concentration at which the 

metal moiety is added, and the residence time for particle formation, the observed 

presence of particles in one flame system is an indication that it may be important in 

other flame systems as well. Finally, the potential for condensation can be assessed by 

considering the local metal species volume fraction as compared to the its local vapor 

pressure in the flame. A limitation of this method is that it relies upon knowledge of the 

metal species present in a flame system, the mechanism of inhibition, as well as the vapor 

pressure (or gas-phase and condensed-phase thermodynamic data). Often, this 

information is incomplete. Further, the kinetic rates of the formation of more stable 

oxides of the metal must be known to assess the contribution of those compounds to 

condensed-phase particles (since often, the vapor pressure of these oxides is very low; 

e.g., Fe2O3). Below, the available information related to the potential for condensation is 

presented for each metal species of interest. 

Cr 

Bulewicz and Padley (1971b) found particles in the post combustion region of premixed 

flat flames inhibited by chromium through visual observation, and by detecting 

continuum radiation from them. They also observed saturation in both the inhibiting 

effect and in the concentration of gas-phase Cr in the flame. In addition, solid deposits 

were observed on the burner in premixed flames inhibited by CrO2Cl2, indicating the 

potential for particle formation (similar deposits have been observed for premixed flames 

inhibited by Sn, and Mn (Linteris et al., 2002) and for Fe (Reinelt and Linteris, 1996). 

Hence, it seems clear that the potential for condensation of Cr species exists and may be 

important in other flames systems. To further examine this potential, the vapor pressure 

of Cr, CrO, CrO2, and CrO3 above Cr2O3, under neutral and oxidizing conditions is 

shown in Figure 23. Note that since Cr2O3 is a particularly stable oxide, these vapor 
39

pressures will be lower than if the equilibrium included other, less stable condensedphase oxides. For example, in Figure 23, the blue curve for Cr is data for Cr vapor above 

Cr solid. Nonetheless, since it is not yet know for any metal inhibitor what the 

composition of the condensed phase oxide actually is, the potential for condensation of 

Cr species is illustrated. That is, many of the chromium oxides appear to have quite low 

vapor pressures. 

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

500 1000 1500 2000

Temperature K

Agent Volume Fraction mL/L

CrO3(ox)

CrO2(ox) CrO(ox)

Cr

Cr(ox)

CrO2

CrO

Cr2O3

Cr

Figure 23 – Volume fraction of Cr, CrO, CrO2, and CrO3 above Cr2O3 at 10133 Pa under neutral 

(closed symbols) and oxidizing (open symbols) conditions above Cr2O3 (reddish hues), and Cr above 

itself (blue); from (Nesmeianov, 1963; Grmiley et al., 1961; Gurvich et al., 1993). 

Pb 

The presence of PbO particles was detected in many engine knock studies (Cheaney et 

al., 1959; Kovarik, 1994; Kuppu Rao and Prasad, 1972; Walsh, 1954), and it was often 

argued that the strong inhibition of knock occurred from heterogeneous radical 

recombination on the particle surfaces. Although lead has been shown to be one of the 

more effective metal agents in flame screening studies (Lask and Wagner, 1962), the 

additive mole fraction (maximum 150 μL/L) was not increased high enough to show the 

loss of effectiveness that might occur. In the flash photolysis studies of Norrish et al. 

(1956), however, the TEL concentration was systematically increased, and a drop-off in 

the effectiveness was observed above a certain volume fraction. Hence, condensation is 

likely to limit lead effectiveness for at least some applications. It should also be noted, 

that the high effectiveness of Pb in premixed flames cannot be from particles since, as 

shown by Rumminger et al. (Rumminger and Linteris, 2000c), the collision rate of 

radicals with solid particles is not high enough to account for the observed inhibition of 

premixed flames by super-effective agents such as iron or lead. The vapor pressure of Pb 

and PbO are shown as a function of temperature in Figure 24. The data for PbO2 are too 
40

low to appear on this figure, and hence, condensation is clearly possible for Pb-inhibited 

flames if PbO2 readily forms in the inhibition cycle, or if other higher oxides readily 

form. 

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

500 1000 1500 2000

Temperature K

Gas-Phase Volume Fraction μL/L

Pb

PbO

PbO2 (10-13 @ 1000 K)

Figure 24 - Pb species gas-phase volume fraction at equilibrium over the condensed-phase (at 1 atm), 

i.e., vapor pressure, Pb and PbO from (Barin and Knacke, 1973), PbO2 from (Gurvich et al., 1993; 

Barin et al., 1977). 

Mn 

The effect of MMT on the flame speed of methane-air mixtures was measured by Linteris 

et al. (2002), as shown in Figure 25. The data clearly show a saturation effect, which was 

beyond that due to lowering of the radical concentrations. Additional tests in cup-burner 

flames showed a much lower effectiveness than expected, as well as a rapid loss of 

effectiveness for MMT as its volume fraction was increased (see Figure 13 inset, above). 

The presence of solid particles was also observed visually as were solid deposits on the 

burner. Finally, the vapor pressure of Mn, MnO, and MnO2 are shown below in Figure 

26. Since the volume fraction necessary for flame inhibition is in excess of 200 μL/L, the 

potential for condensation is apparent. 
41

0.4

0.5

0.6

0.7

0.8

0.9

1

0 200 400 600

MMT (ppm)

Normalized Burning Velocity

φ = 0.9

1.0

1.1

Tin = 353 K

X = 0.21 O2, ox

Figure 25 – Normalized burning velocity of premixed CH4/O2/N2 flames inhibited by MMT with 

XO2,ox=0.21 and φ=0.9, 1.0, and 1.1 (dotted lines: curve fits to data; solid lines: numerical predictions; 

from: (Linteris et al., 2002)). 
42

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

500 1000 1500 2000

Temperature K

Gas-Phase Volume Fraction 

μL/L

MnO2

MnO

Mn

Figure 26 – Mn species gas-phase volume fraction at equilibrium over the condensed-phase (at 1 

atm), i.e., vapor pressure, Mn, MnO, and MnO2 from (Gurvich et al., 1993) . 

W 

There are no data in the literature concerning particle formation or loss of effectiveness of 

flames inhibited by molybdenum compounds. A search for vapor pressure or 

thermodynamic data for molybdenum oxides, hydrides, and hydroxides allowed 

generation of the results in Figure 27. As shown, WO3 has quite high vapor pressure at 

flame temperatures; on the other hand, the vapor pressures of WO2 and W are low 

enough to allow particle formation. Depending upon the flame reactions of W-containing 

compounds and their rates, as well as the vapor pressures of other compounds which 

form in flames, condensation may or may not be important. 

 
43

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

500 1000 1500 2000

Temperature K

Gas-Phase Volume Fraction 

μL/L

WO3

W

 (10-11@ 2000 K)

WO2

Figure 27 - W species gas-phase volume fraction at equilibrium over the condensed-phase (at 1 atm), 

i.e., vapor pressure, W, WO2, and WO3 , from (Nesmeianov, 1963; Gurvich et al., 1993; Stull, 1947). 

Mo 

There are likewise no data in the literature concerning particle formation or loss-ofeffectiveness for flames inhibited by Mo-containing species. The vapor pressure of Mo, 

MoO2, and MoO3 for flame conditions are shown in Figure 28. As for W compounds, the 

only species thought to participate in the catalytic cycle for which vapor pressure data are 

available (WO3 and MoO3) have quite high vapor pressures. 
44

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

500 1000 1500 2000

Temperature K

Gas-Phase Volume Fraction 

μL/L

MoO3

MoO2

Mo

Figure 28 - Mo species gas-phase volume fraction at equilibrium over the condensed-phase (at 1 

atm), i.e., vapor pressure, Mo, MoO2, and MoO3 , from (Nesmeianov, 1963; Gurvich et al., 1993; 

Stull, 1947). 

Sn 

The results for Sn are similar to that of Mn. At the volume fractions necessary for 

effective flame inhibition, a loss of effectiveness was observed in premixed flames, 

although not as dramatic as for Mn (see Figure 29 below); particle deposits were 

observed on the burner. Cup-burner tests showed much lower effectiveness than 

expected, as well as a further loss of effectiveness as the Sn volume fraction increased 

(see Figure 13 inset, above), and solid particles were observed both in the flame and 

deposited on the burner. The vapor pressure of Sn, SnO, and SnO2 is shown in Figure 30. 

Clearly, condensation is possible at the 3000 μL/L volume fraction needed for flame 

inhibition. 
45

0.4

0.6

0.8

1.0

0 1000 2000 3000 4000 5000

Sn(CH3)4 ppm

Normalized Burning Velocity

φ = 0.9

1.0

1.1

X = 0.21 O2,ox

Tin = 298 K

Figure 29 - Normalized burning velocity of premixed CH4/O2/N2 flames inhibited by TMT with 

XO2

,ox=0.21 and φ=0.9, 1.0, and 1.1 (dotted lines: curve fits to data; solid lines: numerical predictions; 

from (Linteris et al., 2002)). 

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

500 1000 1500 2000

Temperature K

Gas-Phase Volume Fraction 

μL/L

Sn

SnO

SnO2

Figure 30 – Sn species gas-phase volume fraction at equilibrium over the condensed-phase (at 1 atm), 

i.e., vapor pressure, Sn, SnO, and SnO2 from (Gurvich et al., 1993) . 
46

Co 

The data for Co effectiveness in various flame systems has not be presented as a function 

of Co concentration. Hence, loss of effectiveness at higher concentrations has not been 

demonstrated. Likewise, particles formation in flames with added Co has also not been 

discussed. The available vapor pressure data for Co oxides and hydroxides is shown 

below. Since CoO is an intermediate species in the proposed inhibition mechanism of Co 

in H2-O2-N2 flames, and total Co loading in the flame is expected to be on the order of 

several thousand μL/L, condensation may occur. On the other hand, depending upon the 

relative rates of the inhibition reactions, the volume fraction of CoO may not build up too 

high, and hence it’s propensity to condense could be lower than Figure 31 might lead on 

to believe. 

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

500 1000 1500 2000

Temperature K

Gas-Phase Volume Fraction μL/L

Co

CoO

Co(OH)2

Figure 31 - Co species gas-phase volume fraction at equilibrium over the condensed-phase (at 1 atm), 

i.e., vapor pressure, Co, CoO, and Co(OH)2 , from (Nesmeianov, 1963; Gurvich et al., 1993; Stull, 

1947). 

Cu 

As illustrated in Table 1, copper salts have been used in experiments on engine knock, 

ignition suppression, radical recombination above flat flames, as well as in flame 

screening tests. The compounds are added either as aqueous sprays or directly as 

particles. There has been no mention in the literature of observed particle formation after 

decomposition of the additive, or of any loss of effectiveness at higher mole fraction. 

Only one of these studies ran experiments over a range of additive mole fraction ((Rosser 

et al., 1963)), and in it, they attributed changes in the performance with additive mole 
47

fraction to heating effects of the particles (not condensation of active species or saturation 

of the radical recombination cycle). Hence, it is difficult to assess from previous work if 

condensation may be possible for copper-based inhibitors. To assess the condensation 

potential, we have assembled the available vapor pressure data for copper compounds 

which may exist in flames (see Figure 32). Although data are not available for other 

possible hydrides, oxides and hydroxides (e.g., CuH, CuOH) CuO and Cu2O have quite 

high vapor pressures, and that of Cu is also relatively high at flame temperatures. 

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

500 1000 1500 2000

Temperature K

Gas-Phase Volume Fraction 

μL/L

Cu

Cu2O

CuO

Figure 32 - Cu species gas-phase volume fraction at equilibrium over the condensed-phase (at 1 atm), 

i.e., vapor pressure, Cu, CuO, and Cu2O, from {Barin, Knacke, et al. 1973 #710}{Barin, Knacke, et 

al. 1977 #720}. 

Sb 

Although the only flame data for antimony inhibition (Lask and Wagner, 1962) did not 

go to high enough concentrations to show loss of effectiveness, antimony may form 

condensed-phase particles in flames. In studies of polymers with added Sb2O3 and 

halogen, Fenimore and Marin (1966a) showed that the fire retardant effect increases 

linearly with Sb2O3 at low mass fractions of Sb2O3, but saturates at some value, above 

which further addition of Sb2O3 is ineffective. Hence, the fire retardancy effect of 

antimony shows a strong saturation, much like that for iron, manganese, and tin in cupburner flames (Linteris, 2002). To illustrate this, the data in Figure 13 (CO2 required for 

extinction with metal species added) is re-plotted below in Figure 33 in terms of the 

limiting oxygen index (based on N2). The top of the figure shows the limiting oxygen 

index for polyethylene/halogen blends as a function of the volume fraction of the metallic 

inhibitor in the gas phase. The curves for Sb2O3 were calculated based on data available 

in Fenimore and Martin (1966b). The effectiveness of the antimony/halogen system 
48

saturates at Sb2O3 volume fractions near 400 μL/L (based on Sb). On the bottom of the 

figure, curves for Fe, Mn, and Sn added to the air stream of heptane-air cup burner flames 

are also shown (these data are obtained from (Linteris, 2002); although those experiments 

were conducted with CO2 added as the diluent, the data were converted to an equivalent 

LOI with nitrogen diluent by correcting for the difference in heat capacity between N2

and CO2). 

0.15

0.20

0.25

0 1000 2000 3000

Catalytic Inhibitor Volume Fraction (μL/L)

Equivalent LOI (N2/O2 mix) at Extinction

Sn(CH3)4

Sb2O3 in C2H3.8Cl0.2

MMT Fe(CO)5

Sb2O3 in C2H3.8Br0.068

Methane-air cup burner

PE / halogen substrate, N2/O2 oxidizer, from 

 Fennimore and Martin, CNF, 10(2):133-139 (1966) 

Figure 33 - Equivalent N2/O2 limiting oxygen index for extinction of polyethylene (PE) halogen 

blends or methane-air cup burner flames with MMT, Fe(CO)5, and Sn(CH3)4 . 

Although the influence of Fe, Mn, and Sn on the LOI when added to the air stream of 

methane-air cup-burner flames is much weaker than that of Sb2O3 in halogenated 

polyethylene, a similar saturation behavior is observed. It would be of value to 

understand why saturation occurs in the antimony-halogen system, since it has not been 

explained in the literature. 

Flame Inhibition and Loss-of-Effectiveness Summary 

The current state of understanding of the inhibition of flames by metal compounds is 

summarized in Table 3. The inhibition potential is summarized in terms of the known 

inhibition behavior, the detection of metal-containing species in flames, and the state of 

development of a kinetic description of the inhibition. The knowledge of the potential 

for loss-of-effectiveness is characterized in terms of the demonstrated loss of 

effectiveness in flame systems, the presence of particles, and the availability of vapor 
49

pressure data for the condensed-phase metal-containing species. The quality of the 

information is rated as high, medium, low, and none. It should be noted however, that 

this is a relative scale. Even for elements such as iron, which is the most extensively 

studied, there are fragmented data on the gas-phase species in flames, the gas-phase 

kinetic mechanism has many estimated rate constants and the dominant inhibition cycles 

were assumed, and vapor pressure data for some of the important intermediates are not 

available. As the table illustrates, most of the information needed to accurately predict 

both the inhibition potential as well as the potential for loss-of-effectiveness is 

incomplete. (Note again that the rating in the Inhibition Potential column is the quality of 

the data about the inhibition potential, not the inhibition ability itself) 
50

Table 3 – Current state of knowledge relevant to inhibition potential of metals, and potential loss of 

effectiveness due to condensation. Key: ▓ - high, ▒ - medium, ░ - low, -none. 

Inhibition 

 Information 

Condensation 

 Information 

Element 

Inhibition Potential Gas-phase Species Identified 

Kinetic Mechanism State of Development 

Demonstrated Loss-of-Effectiveness Experimental Evidence of Particles Experimental Vapor Pressure Data 

 

Cr ▓ ░ ░ ▓ ▓ ▒

Pb ▓ ░ ░ ▓ ▒

Fe ▓ ▓ ▒ ▓ ▓ ▒

Mn ▓ ▓ ▒ ▓ ░ ░

Ni ▒ 

W ▒ ░ ░

Mo ▒ ░ ░

Sn ▒ ▓ ▒ ▓ ░ ░

Co ▒ ▓ ░

Ti ▒ ░ 

Ge ▒ 

Sb ▒ ░ ░ ▒ 

Te ░ 

Tl ░

Bi ░

Cu ░ ░ ░

U ░ 

Zn ░

La ░

Th ░

Se ░
51

Approaches for Ascertaining the Potential of Metal-containing Compounds 

Actually calculating the condensation potential of metal inhibitors in flames is quite 

challenging. To do this in a quantitative way requires knowledge of the species present 

in the flame, the inhibition mechanism with its associated rate constants, the vapor 

pressures of the metal species, the condensation, agglomeration, and re-evaporation rates 

for the particles, and the actual species present in the particles. This is far from being 

achieved even for iron, the most highly studied metal. 

An alternative way to assess the potential for condensation of metal inhibitors in flame 

systems may be simple screening experiments with cup-burners. In order to determine 

both the variation with concentration, as well as the potential benefit of using the metal 

compounds with inert agents, tests need to be conducted with the added metal compound 

and a secondary inert agent added together. In such a test, described above and in the 

literature (Linteris et al., 2002; Linteris and Rumminger, 2002) (Shmakov et al., 2004) 

the amount of a diluent (e.g., CO2, or N2) added to the air stream necessary to cause 

blow-off of the cup-burner flame is determined, both with and without the metal agent 

added at increasing concentration in the oxidizer stream. Since, to some extent, the 

flames resemble the low-strain conditions of fires, there is expected to be much higher 

correlation between the behavior in this system and in actual suppressed fires. By adding 

the metal compounds in the form of organometallic agents (rather than metal salts), the 

complicating effects of particle evaporation and decomposition are avoided. 

A more empirical approach to evaluating metal-containing compounds would be to add 

them to a solid propellant-based extinguisher. There is inert gas in the effluent, as well as 

fragmented metal-containing species, so that by proper application to an appropriate fire, 

the utility of the metal additive could be assessed at larger scale. The challenge would be 

to find a configuration which has the proper sensitivity to the inert gas flow rate so that 

the effects of the additive could be quantified. 

Proposed Approaches for Overcoming Condensation 

There may exist approaches for overcoming the loss of effectiveness of metal compounds 

due to condensation of metal oxides. For example, halogens could be used to attack the 

metal oxide and provide metal-halogen species in the gas phase. As described by Hastie 

and co-workers (Hastie and McBee, 1975), the halogen in the antimony-halogen fire 

retardant system act to release the antimony from the condensed phase through a series of 

halogenation steps involving successive oxychloride phases (1973b). Bromine has been 

used in the past to etch off the lead oxide deposits on engine valves, and halogen is used 

to remove the oxide coating from the incandescent filament in quartz-halogen light bulbs. 

It is clear that halogens can release metals from solid oxides, and it may be possible to 

use this property to re-introduce the metals from the condensed oxide into the gas-phase 

where they can again inhibit the flame. Finally, the use of carboxylic acids as extenders 

of antiknock agents (Richardson et al., 1962) (through the formation of metal salts which 
52

can persist in the gas phase) leads naturally to the question of whether such an approach 

would work for metal-based fire suppressants as well. 

Conclusions 

An extensive review of the literature concerning flame inhibition by metals has been 

performed. The previous work has been considered with respect to the potential of the 

metals for flame inhibition, as well as the potential for loss of effectiveness due to 

condensation of the active species to particles. A list of metals with demonstrated flame 

inhibition potential has been compiled, along with the type of experiment used to 

recommend the metal, and the performance of the metal with respect to that of CF3Br 

(neglecting any potential condensation of metal species) has been estimated. The 

mechanism of inhibition of iron compounds in flames has been described, as well as the 

mechanism of the loss of effectiveness of iron for some flame systems. The equivalent 

flame inhibiting species of the other metals has been suggested (when possible), and 

kinetic mechanisms have been outlined (when they have been proposed in the literature). 

The potential of the metals other than iron to lose effectiveness through condensation has 

been estimated, based on consideration of the performance of the agent as a function of 

its concentration, the observation of particles in the flame, or the vapor pressures of the 

active metal oxides and hydroxides (when the active species are known and the data are 

available). 

This work has uncovered several additional metals which are likely to be effective at low 

concentrations, but which have not been suggested in previous reviews of metal flame 

inhibition. These are tungsten, molybdenum, and cobalt. 

Most of the metals discussed have shown a possibility to condense to particles in some 

flame systems. These metals include Cr, Pb, Mn, Sn, and Sb. Were this condensation to 

occur in practical fires, compounds of these metals would likely not demonstrate fire 

suppression capabilities commensurate with their high flame velocity inhibition 

efficiency that was observed when added at low concentration to laboratory premixed 

flames. For other metals (W, Mo, Co, and Cu), there is no direct evidence of the 

potential for loss of effectiveness, and the vapor pressures of the suggested flamequenching species (for which data are available) are reasonably high. For Co, the 

monoxide CoO has a lower vapor pressure and this is likely to be an important 

intermediate in the inhibition cycle. This can be taken as evidence for the potential for 

condensation. Nonetheless, the potential for condensation of metal species really 

depends upon the local super-saturation ratio in the inhibited flame. Calculation of the 

super-saturation ratio depends upon knowing both the detailed kinetic mechanism of 

inhibition as well as the vapor pressure of all of the intermediate species. Further, the 

kinetics of the condensation (and potential re-evaporation of particles) will be highly 

dependent on the flow-field of the particular flame system to be extinguished. Thus, 

prediction of the potential loss of effectiveness due to condensation is presently difficult. 
53

To assess both the inhibition potential as well as the potential for loss of effectiveness for 

metal-based agents, two experimental approaches have been offered . Finally, to 

overcome the loss of effectiveness, the use of agents (halogens or carboxylic acids), 

which would keep a higher fraction in the metal in the gas phase, has been suggested. 

 

Acknowledgements 

Helpful conversations with Wing Tsang, Kermit Smyth, Fumi Takahashi, Dick Gann, 

George Mulholland, Vish Katta, and Bill Grosshandler are gratefully acknowledged. The 

author is indebted to Tania Ritchie, Marc Rumminger, and Valeri Babushok for their 

previous efforts in this research, upon which many of the current results depend. The 

assistance of Valeri Babushok in providing vapor pressure data for some of the metal 

species is gratefully noted. 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 

W74RDV30663237. 
54

References 

Babushok, V. and Tsang, W. (2000). Inhibitor Rankings for Hydrocarbon Combustion. 

Combustion and Flame, 123, 488. 

Babushok, V., Tsang, W., Linteris, G. T., and Reinelt, D. (1998). Chemical Limits to Flame 

Inhibition. Combustion and Flame, 115, 551. 

Barin, Ihsan. and Knacke, O. (1973). Thermochemical properties of inorganic substances, Berlin, 

New York, Springer-Verlag. 

Barin, Ihsan., Knacke, O., and Kubaschewski, O. (1977). Thermochemical properties of 

inorganic substances : Springer-Verlag, Berlin ; New York . 

Bonne, U., Jost, W., and Wagner, H. G. (1962). Iron Pentacarbonyl in Methane-Oxygen (or Air) 

Flames. Fire Research Abstracts and Reviews, 4, 6. 

Buelwicz, E. M., James, C. G., and Sugden, T. M. (1956). Proceedings of the Royal Society A, 

235, 89. 

Bulewicz, E. M., Jones, G., and Padley, P. J. (1969). Temperature of metal oxide particles in 

flames. Combustion and Flame, 13, 409. 

Bulewicz, E. M. and Padley, P. J. (1971a). Catalytic Effect of Metal Additives on Free Radical 

Recombination Rates in H2+O2+N2 Flames. Proceedings of the Combustion Institute, 13, 73. 

Bulewicz, E. M. and Padley, P. J. (1971b). Photometric investigations of the behavior of 

chromium additives in premixed H2+O2+N2 flames. Proceedings of the Royal Society London 

A., 323, 377. 

Bulewicz, E. M., Padley, P. J., Cotton, D. H., and Jenkins, D. R. (1971). Metal-AdditiveCatalysed Radical-Recombination Rates in Flames. Chemical Physics Letters, 9, 467. 

Bulewicz, E. M. and Sugden, T. M. (1956a). Determination of the dissociation constants and 

heats of formation of molecules by flame photometry; Part 3.-Heat of formation of cuprous 

hydroxide. Transactions of the Faraday Society, 52, 14811488. 

Bulewicz, E. M. and Sugden, T. M. (1956b). Determination of the dissociation constants and 

heats of formation of molecules by flame photometry; Part 2.-Heat of formation of cuprous 

hydride. Transactions of the Faraday Society, 52, 1475. 

Chamberlain, G. H. N. and Walsh, A. D. (1952). Proceedings of the Royal Society A, 215, 175-. 

Cheaney, D. E., Davies, D. A., Davis, A., Hoare, D. E., Protheroe, J., and Walsh, A. D. (1959). 

Effects of Surfaces on Combustion of Methane and Mode of Action of Anti-Knock Containing 

Metals, Seventh Symposium (International) on Combustion, The Combustion Institute, p. 183. 

Cotton, D. H., Friswell, N. J., and Jenkins (1971). The suppression of soot emission from flames 
55

by metal additives. Combustion and Flame, 17, 87. 

Cotton, D. H. and Jenkins, D. R. (1971). Catalysis of Radical-Recombination Reactions in 

Flames by Alkaline Earth Metals. Transactions of the Faraday Society, 67, 730. 

deWitte, M., Vrebosch, J., and van Tiggelen, A. (1964). Inhibition and extinction of premixed 

flames by dust particles. Combustion and Flame, 9, 257. 

Dolan, J. E. and Dempster, P. B. (1955). The suppression of methane-air ignitions by fine 

powders. Journal of Applied Chemistry, 5, 510. 

Erhard, K. H. L. and Norrish, R. G. W. (1956). Studies of knock and antiknock by kinetic 

spectroscopy. Proc. Roy. Soc. (London) A, 234, 178. 

Fallis, S., Reed, R., Lu, Y.-C., Wierenga, P. H., and Holland, G. F (2000). Advanced 

Propellant/Additive Development for Fire Suppressing Gas Generators, Halon Options Technical 

Working Conference, 361. 

Fallon, G. S., Chelliah, H. K, and Linteris, G. T. (1996). Chemical Effects of CF3H in 

Extinguishing Counterflow CO/Air/H2 Diffusion Flames, Proceedings of the Combustion 

Institute, Vol. 26, The Combustion Institute, p. 1395. 

Fenimore, C. P. and Jones, G. W. (1966). Modes of Inhibiting Polymer Flammability. 

Combustion and Flame, 10, 295. 

Fenimore, C. P. and Martin, F. J. (1966a). Flammability of Polymers. Combustion and Flame, 

10, 135. 

Fenimore, C. P. and Martin, F. J. (1966b). Candle-type test for flammability of polymers. 

Modern Plastics, 141. 

Fenimore, C. P. and Martin, F. J. (1972), in The mechamisms of pyrolysis, oxidation and burning 

of organic materials (Wall, L. A., Ed.), National Bureau of Standards, Washington, D.C., pp. 

159-170. 

Gann, R. G. (2004). FY2003 Annual Report -- Next Generation Fire Suppression Technology 

Program (NGP), National Institute of Standards and Technology, NIST Technical Note 1457. 

Grmiley, R. T., Burns, R. P., and Inghram, M. G. (1961). Thermodynamics of the vaporization of 

Cr2O3: dissociation energies of CrO, CrO2, and CrO3. Journal of Chemical Physics, 34, 664. 

Gurvich, L. V., Iorish, V. S., Chekhovskoi, D. V., Ivanisov, A. D., Proskurnev, A. Yu., 

Yungman, V. S., Medvedev, V. A., Veits, I. V., and Bergman, G. A. (1993). IVTHANTHERMO - 

Database on Thermodynamic Properties of Individual Substances, Institute of High 

Temperatures, Moscow. 

Hastie, J. W. (1973a). Mass spectroscopic studies of flame inhibition: analysis of antimony 

trihalides in flames. Combustion and Flame, 21, 49. 

Hastie, J. W. (1973b). Molecular Basis of Flame Inhibition. Journal of Research of the National 

Bureau of Standards - A. Physics and Chemistry, 77A, 733. 
56

Hastie, J. W. and McBee, C. L. (1975), in Halogenated Fire Suppressants (Gann, R. G., Ed.), 

American Chemical Society , Washington, DC, pp. 118-148. 

Howard, J. B. and Kausch, W. J. (1980). Soot Control by Fuel Additives. Progress in Energy 

and Combustion Science, 6, 263. 

James, C. G. and Sugden, T. M. (1956). Proceedings of the Royal Society A, 248, 238. 

Jensen, D. E. and Jones, G. A. (1974). Catalysis of Radical Recombination in Flames by Iron. 

Journal of Chemical Physics, 60, 3421. 

Jensen, D. E. and Jones, G. A. (1975). Mass-Spectrometric Tracer and Photometric Studies of 

Catalyzed Radical Recombination in Flames. Journal of the Chemical Society-Faraday 

Transactions I, 71, 149. 

Jensen, D. E. and Jones, G. A. (1976). Aspects of Flame Chemistry of Cobalt. Journal of the 

Chemical Society-Faraday Transactions I, 72, 2618. 

Jensen, D. E. and Webb, B. C. (1976). Afterburning Predictions for Metal-Modified Propellant 

Motor Exhausts. AIAA Journal, 14, 947. 

Jost, W., Bonne, U., and Wagner, H. G. (1961). Chemical & Engineering News, 39, 76. 

Katta, V. R., Takahashi, F., and Linteris, G. T. (2003a). Extinction Characteristics of Cup-Burner 

Flame in Microgravity, 41st Aerospace Sciences Meeting and Exhibit, AIAA, p. AIAA Paper 

No. 2003. 

Katta, V. R., Takahashi, F., and Linteris, G. T. (2003b). Numerical Investigations of CO2 as Fire 

Suppressing Agent, Fire Safety Science: Proc. of the Seventh Int. Symp., Int. Assoc. for Fire 

Safety Science, p. 531. 

Kellogg, C. B. and Irikura, K. K. (1999). Gas-Phase Thermochemistry of Iron Oxides and 

Hydroxides:Portrait of a Super-Efficient Flame Suppressant. Journal of Physical Chemistry a, 

103, 1150. 

Kennedy, I. M., Zhang, Y., Jones, A. D., Chang, D. P. Y., Kelly, P. B., and Yoon, Y. (1999). 

Morphology of Chromium Emissions From a Laminar Hydrogen Diffusion Flame. Combustion 

and Flame, 116, 233. 

Kovarik, B. (1994). Charles F. Kettering and the Development of Tetraethyl Lead in the Context 

of Alternative Fuel Technologies, Fuels and Lubricants, Society of Automotive Engineers, p. 

Kuppu Rao, V. and Prasad, C. R. (1972). Knock suppression in petrol engines. Combustion and 

Flame, 18, 167. 

Lask, G. and Wagner, H. G. (1962). Influence of Additives on the Velocity of Laminar Flames. 

Proceedings of the Combustion Institute, 8, 432. 

Linteris, G. T. (2002). Extinction of Cup-Burner Diffusion Flames by Catalytic and Inert 

Inhibitors, NRIFD , 

Linteris, G. T. and Chelliah, H. K. (2001). Powder-Matrix Systems for Safer Handling and 
57

Storage of Suppression Agents, National Institute of Standards and Technology, NISTIR 6766. 

Linteris, G. T., Katta, V. R., and Takahashi, F. (2004). Experimental and Numerical Evaluation 

of Metallic Compounds for Suppressing Cup-Burner Flames. Combustion and Flame, 138, 78. 

Linteris, G. T., Knyazev, K., and Babushok, V. (2002). Inhibition of Premixed Methane Flames 

by Manganese and Tin Compounds. Combustion and Flame, 129, 221. 

Linteris, G. T. and Rumminger, M. D. (2002). Particle Formation in Laminar Flames Inhibited by 

Metals, Western States Section Meeting, The Combustion Institute, Paper 030. 

Linteris, G. T., Rumminger, M. D., Babushok, V., Chelliah, H., Lazzarini, A. K., and 

Wanigarathne, P. (2002). Effective Non-Toxic Metallic Fire Suppressants, National Institute of 

Standards and Technology, NISTIR 6875. 

Linteris, G. T., Rumminger, M. D., Babushok, V., and Tsang, W. (2000a). Flame Inhibition by 

Ferrocene and Blends of Inert and Catalytic Agents. Proceedings of the Combustion Institute, 

28, 2965. 

Linteris, G. T., Rumminger, M. D., and Babushok, V. I. (2000b). Premixed Carbon MonoxideNitrous Oxide-Hydrogen Flames:Measured and Calculated Burning Velocities With and Without 

Fe(CO)5. Combustion and Flame, 122, 58. 

Lippincott, E. S. and Tobin, M. C. J. (1953). .Journal of the American Chemical Society, 75, 

4141. 

Lott, J. L., Christian, S. D., Sliepcevich, C. M., and Tucker, E. E. (1996). Synergism Between 

Chemical and Physical Fire-Suppressant Agents. Fire Technology, 32, 260. 

Macdonald, M. A., Gouldin, F. C., and Fisher, E. M. (2001). Temperature Dependence of 

Phosphorus-Based Flame Inhibition. Combustion and Flame, 124, 668. 

MacDonald, M. A., Jayaweera, T. M., Fisher, E. M., and Gouldin, F. C. (1998). Variation of 

Chemically Active and Inert Flame-Suppression Effectiveness with Stoichiometric Mixture 

Fraction, Proceedings of the Combustion Institute, Vol. 27, The Combustion Institute, p. 

Martin, F. J. and Price, K. R. (1968). Journal of Applied Polymer Science, 12, 143. 

Matsuda, S. (1972). Gas Phase Homogeneous Catalysis in Shock Waves. II. The Oxidation of 

Carbon Monoxide by Oxygen in the Presence of Iron Pentacarbonyl. Journal of Physical 

Chemistry, 57, 807. 

Matsuda, S. and Gutman, D. (1971). Shock-tube study of C2H2-O2 reaction. Acceleation of 

reaction in the presence of trace amounts of Cr(CO)6. Journal of Physical Chemistry, 75, 2402. 

Miller, D. R., Evers, R. L., and Skinner, G. B. (1963). Effects of Various Inhibitors on 

Hydrogen-Air Flame Speeds. Combustion and Flame, 7, 137. 

Miller, W. J. (1969). Inhibition of Low Pressure Flames. Combustion and Flame, 13, 210. 

Morrison, M. E. and Scheller, K. (1972). The Effect of Burning Velocity Inhibitors on the 

Ignition of Hydrocarbon-Oxygen-Nitrogen Mixtures. Combustion and Flame, 18, 3. 
58

Muraour, H. (1925). Chem. et Industr., 14, 1911. 

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

halogenated compounds. Combustion and Flame, 112, 147. 

Padley, P. J. and Sugden, T. M. (1959). Determination of the dissociation constants and heats of 

formation of molecules by flame photometry; Part 6.-stability of MnO and MnOH and their 

mechanisms of formation. Transaction of the Faraday Society, 55, 2054. 

Park, K., Bae, G. T., and Shin, K. S. (2002). The addition effect of Fe(CO)5 on methane ignition. 

Bulletin of the Korean Chemical Society, 23, 175. 

Reinelt, D. and Linteris, G. T. (1996). Experimental Study of the Inhibition of Premixed and 

Diffusion Flames by Iron Pentacarbonyl. Proceedings of the Combustion Institute, 26, 1421. 

Richardson, W. L., Ryason, P. R., Kautsky, G. J. , and Barusch, M. R. (1962). Organolead 

antiknock agents--their performance and mode of action . Proceedings of the Combustion 

Institute, 9, 1023. 

Rosser, W. A, Inami, S. H., and Wise, H. (1959). Study of the Mechanisms of Fire 

Extinguishment of Liquid Rocket Propellants, WADC Technical Report 59-206. 

Rosser, W. A, Inami, S. H., and Wise, H. (1963). The Effect of Metal Salts on Premixed 

Hydrocarbon-Air Flames. Combustion and Flame, 7, 107. 

Rumminger, M. D. and Linteris, G. T. (2000a). Numerical Modeling of Counterflow Diffusion 

Flames Inhibited by Iron Pentacarbonyl, Fire Safety Science: Proc. of the Sixth Int. Symp., Int. 

Assoc. for Fire Safety Science, p. 289. 

Rumminger, M. D. and Linteris, G. T. (2000b). Inhibition of Premixed Carbon MonoxideHydrogen-Oxygen-Nitrogen Flames by Iron Pentacarbonyl. Combustion and Flame, 120, 451. 

Rumminger, M. D. and Linteris, G. T. (2000c). The Role of Particles in the Inhibition of 

Premixed Flames by Iron Pentacarbonyl. Combustion and Flame, 123, 82. 

Rumminger, M. D. and Linteris, G. T. (2002). The Role of Particles in the Inhibition of 

Counterflow Diffusion Flames by Iron Pentacarbonyl. Combustion and Flame, 128, 145. 

Rumminger, M. D., Reinelt, D., Babushok, V., and Linteris, G. T. (1999). Numerical Study of 

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

116, 207. 

Shmakov, A. G., Korobeinichev, O. P., Shvartsberg, V. M., Knyazkov, D. A., Bolshova, T. A., 

and Rybitskaya, I. V. (2004). Inhibition of Premixed and Non-Premixed Flames with 

Phosphorus-Containing Compounds. Proceedings of the Combustion Institute, 30, in press. 

Stull, D. R. (1947). Vapor Pressure of Pure Substances Organic Compounds. Ind. Eng. Chem., 

39, 517. 

Suyuan Y., Jones, A. D., Chang, D. P. Y., Kelly, P. B., and Kennedy, I. M. (1998). The 

Transformation of Chromium in a Laminar Premixed Hydrogen-Air Flame. Proceedings of the 
59

Combustion Institute, 27, 1639. 

Tapscott, R. E., Sheinson, R. S., Babushok, V., Nyden, M. R., and Gann, R. G (2001). Alterntive 

Fire Suppressant Chemicals: A Research Review with Recommendations, NIST TN 1443. 

Tischer, R. L. and Scheller, K. (1970). The behavior of uranium oxide particles in reducing 

flames. Combustion and Flame, 15, 199. 

Vanpee, M. and Shirodkar, P. (1979). A Study of Flame Inhibition by Metal Compounds. 

Proceedings of the Combustion Institute, 17, 787. 

Walsh, A. D. (1954), in Six Lectures on the Basic Combustion Process (Ethyl Corp., New York, 

NY, pp. 

Westblom, U., Fernandezalonso, F., Mahon, C. R., Smith, G. P., Jeffries, J. B., and Crosley, D. R. 

(1994). Laser-Induced Fluorescence Diagnostics of a Propane Air Flamewith a Manganese Fuel 

Additive. Combustion and Flame, 99, 261. 

Zimpel. C.F. and Graiff, L. B. (1967). An Electron Microscopic Study of Tetraethyllead 

Decomposition in an Internal Combustion Engine, Eleventh Symposium (International) on 

Combustion, The Combustion Institute, p. 1015.