PREMIXED FLAME INHIBITION BY MANGANESE AND TIN COMPOUNDS* 

Gregory T. Linteris, Vadim Knyazev, and Valeri Babushok 

National Institute of Standards and Technology 

TK 

1500 

2000 

2500 

INTRODUCTION 

The production ban on the effective, bromine-based fire suppressants due to their ozone depletion potential has motivated the search for alternative compounds. Iron has been shown to be up to two orders of 

magnitude more effective than Bras a flame inhibitor; however, it loses its effectiveness due to condensation of the active species to particles. Consequently, it is of interest to determine whether other metals 

cause similar strong flame inhibition while not suffering from the loss of effectiveness. Since manganese 

and tin have higher vapor pressures than iron (Table I), these compounds are potential additives for 

effective fire suppression. The present work seeks to determine the gas-phase flame inhibition properties 

of tin and manganese through experiments and modeling of their reduction in the premixed burning 

velocity of methaneair flames. 

Fe Mn Sn FeO MnO SnO 

2.E-6 1.04E-3 1.14E-5 1.53E-10 3.75E-11 4.7E-3 

3.69E-4 0.126 4.34E-3 2.3E-6 1.3E-6 0.639 

3.09E-2 0.153 5.7E-4 4.26E-4 

TABLE 1. SATURATED VAPOR PRESSURES FOR SOME FE-, MN-, 

AND SN-CONTAINING SPECIES. 

Partial Pressure, atm I 

It is well known that metals can catalyze the recombination of radicals in the post combustion region of 

hydrogen -air flames [ 141. Bulewicz and Padley demonstrated that metallic compounds of Cr, Mn, Sn, 

U, Mg, and Ba accelerate hydrogen atom recombination at ppmtlevels. Nonetheless, careful studies of 

flame inhibition, with the goal of assessing the metals’ effects on the overall reaction rate, are limited. 

Tin compounds are widely used in semiconductor industry. They are fire retardant additives for polymers and are used to reduce smoke and CO formation [S, 61. The mechanism of flame inhibition has 

been attributed to both condensed phase char promotion and gas-phase flame inhibition [6,7]. Lask and 

Wagner [8] found SnCI, to be about 1/34 as effective as Fe(CO), at reducing the burning velocity of premixed n-hexane-air flames by 30%, and Miller et al. [9] found it to be about 2/3 as effective as Fe(CO), 

at reducing the flame speed of hydrogen-air flames by 80%. Miller [IO] measured the amount of inhibitor required to lift-off a premixed, CH,IO,M, flat flame at low pressure, and found that Sn(CH,), (TMT) 

and SnCI, required a mole fraction of 1.7% and 1.1%, whereas Fe(CO), and Brz required 0.23% and 

2.3%. Morrison and Scheller [ 1 1 J investigated the effect of 20 flame inhibitors on the ignition of hydrocarbon mixtures by hot wires, and found that SnCI, was the most effective inhibitor for increasing the 

ignition temperature; whereas CrO,CI, and Fe(CO),, powerful flame inhibitors, had no effect on the ignition temperature. As a result of these studies, tin tetrachloride SnCI, was recommended as a compound 

deserving further study [12]. 

~ ~~ 

* Official contribution ofNIST, not subject to copyright in the United States. 

t All references to ppm in the present paper are on a volume basis, and refer to pLIL. 

72 Halon Options Technical Working Conference 24-26 April 2001 
Manganese compounds have also been studied. Vanpee and Shirodkar [13] investigated the effect of 

inany metal chlorides and metal acetates and acetylacetonates on the limiting oxygen index at extinction 

in a partially premixed counterflow pool burner of ethanol and air. In their experiment, the inhibitor was 

dissolved into ethanol, which was aspirated into the air stream. They found manganese acetylacetonate to 

be more effective than acetylacetonates of iron or chromium. Westblom et al. [I41 analyzed the effect of 

trace amounts of methylcyclopentadienylmanganese tricarbonyl (C,H,MnO,, MMT) on the flame structure of a premixed propane - air flame at 40 torr, but found no measurable effect. They did, however, 

suggest a kinetic model for the influence MMT on those flames. In a review article, Howard and Kausch 

(51 reported that manganese-containing compounds are among the most effective soot-reducing fuel additives. Finally, MMT is a known antiknock agent in gasoline [IS]. In the report ofNMERl (Albuquerque, 

NM) manganese compounds were suggested as the agents for further consideration and studies offire 

suppression performance [I 61. 

EXPERIMENT 

The laminar flame speed S, provides a measure of an agent's reduction of the global reaction rate. The 

experimental arrangement, described in detail previously [ 17-20], has been modified to accoininodate 

new evaporators for TMT and MMT. A Mache-Hebra nozzle burner (1 .O cm + 0.05 cm diameter) produces a premixed Bunsen-type flame about I .3 cm tall with a straight-sided schlieren image that is captured by a video frame-grabber board in a PC. Digital mass flow controllers hold the oxygen mole fraction in the oxidizer stream XO,~~, the equivalence ratio 4, and the flame height constant while maintaining the inlet mole fraction of the inhibitor (&) at the desired value. The average burning velocity is 

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

methane (Matheson* UHP, 99.9%), and the oxidizer stream consists of nitrogen (boil-off from liquid N,) 

and oxygen (MG Industries, H,O < 50 ppm, and total hydrocarbons < 5 ppm). The inhibitors used are 

Fe(CO), (Aldrich), TMT (Alfa Aesar), MMT (Alfa Aesar), CF,Br (Great Lakes), N, (boil-off), and CO, 

(Airgas). The Fe(CO), is added to the carrier gas using a two-stage saturator in an ice bath, described 

previously [20]. Nitrogen is the carrier gas for all agents. The TMT was added using an identical twostage saturator, with >50 cm' of TMT in each stage. The water bath was maintained at (0 ? 0.2) "C with 

a maximum carrier gas flow 0.40 L/min. For the MMT, the saturator had three stages, each a 20 cm long, 

2.36 cm I.D. stainless steel tube, and the entire apparatus was submerged in a controlled temperature bath 

(Neslab). The bath temperature was typically (79.2 ? 0.1) "C, and the carrier gas flow for this saturator 

was always <OS L/min. The mole fraction of the organometallic inhibitors in the air stream was calculated based on the measured air flow, measured carrier gas flow, and vapor pressure of the agent at the 

bath temperature, assuming saturated carrier gas. The parameters in the Antoine equation log,,(P)=AB/(T+C), T in OC, P in bar, are (A,B,C): (6.77273,4.0932, 7.2283), (1258.22, 1286.16, 1882), and 

(21 1.587,235.846,200) for Fe(CO), [21], TMT [22], and MMT.t Since the vapor pressure of MMT is 

much lower than that of the other agents, any tests with MMT used heaters and temperature controllers to 

maintain the transfer lines and inlet gases at (80 k 3) "C and the burner tube at (80 + 1) "C. Tests were 

performed for a range of stoichiometry @ and oxygen mole fraction in the oxidizer stream Xo,,. The 

agent mole fraction is calculated relative to the total reactant flow. 

KINETlC MECHANISMS AND NUMERICAL MODELING 

There is little data on the chemical kinetics of tin compounds at flame temperatures. Kinetic studies of tin 

have been conducted for chemical vapor deposition. Studies in hydrogen-oxygen-nitrogen flames by 

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

procedure. Such identification does not imply recommendation or endorsement by NIST, nor does it imply that 

the materials or equipment are necessarily the best available for the intended use. 

7 Hollrah, D., Ethyl Corp., personal communication, Jan. 2001. 

Halon Options Technical Working Conference 24-26 April 2001 73 
Bulewicz and Padley [23] indicate that tin is present as Sn, SnO, and SnOH, with SnO the overwhelmingly predominant species. Recent spectroscopic data also indicate that tin is present in flames as SnO, 

SnOH, and Sn [24], with SnO and SnOH accounting for >97% and <3% of the tin, and Sn accounting for 

very little. 

The present kinetic model for tin flame inhibition contains reactions of Sn, SnO,, SnO, SnH and SnOH. 

The reaction set is based on the consideration of possible reactions of tin-containing species with the 

radical pool and the main species of methane combustion. The model* consists of 37 reactions with tincontaining species, and includes TMT decomposition to Sn atom. Enthalpies of formation for these 

species are presented in Table 2. 

TABLE 2. ENTHALPY OF FORMATION FOR SOME MANGANESE- AND 

TIN-CONTAINING SPECIES (298 K). 

Estimation 

Estimationn 

The kinetic mechanism for studying the influence of manganese additives in premixed methane-air 

flames is presented by Linteris et a].* The list of possible Mn-containing species participating in inhibition reactions includes Mn, MnH, MnO, MnOOH, MnHOH, MnOH, MnO, MnO, and Mn(OH),. All of 

these except MnH and MnHOH were considered in a mechanism of Smith.tThe role of MnOH and MnO 

in radical recombination was discussed by Bulewitz and Padley [I], and the species MnO and Mn were 

recently measured in a low pressure propane flame doped by MMT [14]. Hildenbrand and Lau [27] used 

mass spectrometry to identify the species MnO,, MnOH, Mn(OH),. We included the species MnH in the 

model since equilibrium calculations showed it to be a significant compound of manganese in flames. 

Table 2 contains enthalpies of formation for manganese-containing species considered in kinetic model. 

For the manganese inhibition reaction set, we generated a comprehensive list of approximately 160 reactions of Mn-containing species with radical pool species and the main species of methane combustion. 

This list was reduced to 61 reactions based on thermochemical considerations and preliminary calculations. This reaction set does not contain a decomposition route for the Mn-containing parent molecule. 

For MMT decomposition, we adopt the description of Smith? and use their rates for other Mn-species 

reactions where possible. Rate constants for the remaining reactions were estimated by analogy with 

reactions for iron containing species [28]. The main assumptions are the formation of MnO, through the 

reaction of Mn atom with oxygen molecule and formation of Mn(OH), in reactions similar to reactions of 

iron-containing species. 

* Linteris, G. T., Knyazev, K., and Babushok, V., “Methane-Air Premixed Flame Inhibition by Manganese and Tim 

t Smith, G. P., personal communication, June 1999. 

74 Halon Options Technical Working Conference 24-26 April 2001 

Compounds,” in preparation, 2001. Copy on tile with the author. 
Kinetic models for highly effective flame inhibitors can he considered to consist of two submodels. The 

first includes reactions for agent decomposition and formation of the active inhibiting species; the second 

includes the inhibition reactions. In previous work, it has been shown that for the phosphorus-containing 

compound DMMP and for ferrocene, the decomposition reactions have a small influence on the predicted 

inhibitor efficiency as long as the overall activation energy of decomposition is less than 250-335 kJ/mol. 

In the present work, this was also found to be true for TMT and MMT decomposition. 

The laboratory flames inhibited by TMT and MMT were numerically modeled as one-dimensional freely 

propagating flames. Solutions were obtained using the Sandia flame code Premix [29], and the Cbcnikin 

[30] and transport property [31] subroutines. The kinetic mechanism for methane combustion was from 

GRIMech 3.0 [32], with the nitrogen chemistry removed; this submechanism contains 36 species and 219 

reactions. It should he emphasized that the reaction mechanisms used for the present calculations should 

be considered only as a starting point. Numerous changes to both the rates and the reactions incorporated 

may he made once a variety of experimental and theoretical data are available for testing the mechanism. 

RESULTS 

OBSERVATIONS 

The appearance of the flames with added organometallic inhibitors is shown in Figure 1. Flames with 

iron are bright orange, tin are bright pale blue, and manganese, yellow-green. The intensity of all flames 

increases with increasing inhibitor mole fraction. As the loading of the metallic inhibitor increases, a 

luminous outer shroud becomes visible, as seen clearly in the last two images on the right in Figure 1. 

We believe these are regions of high particle concentration from inhibitor condensation, leading to broadband black body radiation, visible here in the orange part of the spectrum. 

Figure 1. Visible image of methane-air premixed flame: from left to right, no inhibitor, 50 ppm of 

Fe(CO),, 4000 ppm of TMT, and 400 ppm of MMT. 

INHIBITION BY TETRAMETHYL TIN 

Figure 2 shows the relative burning velocity reduction with addition of TMT to methane-air flames 

(XQ~,~~ = 0.21) for values of 

data, and the solid lines are the results of the numerical calculations described above. Data are plotted as 

normalized burning velocity, which is the burning velocity of the inhibited flame divided by the value for 

the same flame in the absence of inhibitor. The calculated and experimental burning velocities, along 

with the calculated adiabatic flame temperatures of the uninhibited flames used for the normalizations are 

shown in Table 3. (Note that the calculations are for 1-D planar flames, while the experiments determine 

the average flame speed of Bunsen-type flames.) The experimental results in Figure 2 show that for 

stoichiometric flames, 3000 ppm of TMT reduces that flame speed by about 41%, which is about a factor 

of 0.9, 1 .O, and 1.2. The dotted lines are curve fits to the experimental 

llalon Options Technical Working Conferencc 24-26 April 2001 75 
1.0 

i? 

5 

= 0.0 s 

m 

= 0.6 

D 

.- w 

€ 

0 

z 

0.4 

a 

Figure 2. Normalized burning velocity of premixed Figure 3. Normalized burning velocity of premixed 

CHJOJN, flames inhibited by TMT with CH,/OJN2 flames inhibited by TMT, with 

X0~,~~=0.21 and +=0.9, 1.0, and 1.1 +=l.O andX~~,~~= 0.20. 0.21, and 0.244 

(dotted lines: curve fits to data; solid (dotted lines: curve fits to the data; solid 

lines: numerical predictions). lines: numerical predictions). 

TABLE 3. UNINHIBITED LAMINAR BURNING VELOCITIES S, AND ADIABATIC FLAME 

TEMPERATURE T, FROM I-D PLANAR NUMERICAL CALCULATIONS AND 

FROM AVERAGE BURNING VELOCITY IN BUNSEN-TYPE FLAME EXPERIMENT. 

4 X02,0x Ti”K TAFT K SL,+ cm/s S,,,,cm/s 

TMT 

0.9 0.21 298 2159 35.3 33.9 f 1.3 

1 .0 0.2 1 298 2235 39.6 38.0 f 2.3 

1.1 0.21 298 2193 39.8 38.0 f 1.5 

1 .o 0.20 298 2185 34.7 33.6 f 1.4 

1 .o 0.244 298 2377 57.0 58.0 -I 3.4 

______ 

MMT 

0.9 0.21 353 2177 48.0 47.2 f 1.5 

1 .0 0.2 1 353 2264 53.2 52.9 f 2.9 

1.1 0.21 353 2251 53.6 52.8 f 2.0 

1 .o 0.19 353 2167 41.3 39.9 f 1.6 

1 .o 0.2 353 2220 47.4 45.5 f 1.7 

1 .o 0.244 353 2396 74.3 74.7 f 4.1 

-__ -.___ _____ .. 

76 Halon Options Technical Working Conference 24-26 April 2001 

~~- ~ 
of two better than CF,Br. The data also show that, unlike that case for Fe(CO),, the richer flames are 

inhibited more strongly by TMT than the leaner flames. The numerical model predicts the amount of 

inhibition well for stoichiometric flames; however, for lean flames the inhibition is underpredicted, while 

for rich flames, it is overpredicted. Figure 3 shows the measured and calculated flame speeds for TMT in 

stoichiometric flames with values ofXo2,0x equal to 0.20, 0.21, and 0.244. Here, the model predictions 

of the burning velocities are excellent. Nonetheless, for the slower, cooler flames of either figure (e.g., 

$=0.9 or 1 .O with Xo2,0x = 0.20 or 0.21), the TMT starts to lose its effectiveness above a certain value, as 

did Fe(CO), (the cause of which was shown to be condensation of the iron-containing intermediates [ 191). 

INHIBITION BY MMT 

The premixed flames inhibited by manganese are slightly preheated (T," = 80 "C), and the values of the 

calculated and experimental uninhibited burning velocities, and the adiabatic flame temperatures are 

shown in Table 3. The normalized burning velocity of MMT-inhibited flames with variation in $ and 

Xo2,0x are shown in Figures 4 and 5. MMT is seen to be about 15 times more efficient at flame inhibition than TMT; however, it too starts to lose its effectiveness for flame speed reductions near 50%. The 

model predicts the burning velocity reduction quite well, although it underpredicts the inhibition somewhat in all cases, and the discrepancy gets larger as the value ofXo2,, (and the flame temperature) 

increases. 

09 

I 

0 0 1W 2W 3W 400 500 600 200 400 6W 8W 

MMT Ippml MMT IPP~I 

Figure 4. Normalized burning velocity of premixed Figure 5. Normalized burning velocity of premixed 

CHJOJN, flames inhibited by MMT 

with Xo ,=0.2 1 and $=0.9, 1 .O, and 1.1 

(dotted lines: curve fits to data; solid 

lines: numerical predictions). 

CHJ02/N2 flames inhibited by MMT, 

with$=1.0andXo~,ox'0.19, 0.20. 0.21, 

and 0.244 (dotted lines: curve fits to the 

data; solid lines: numerical predictions), 

2. 

Figure 6 compares the performance of Fe(CO),, MMT, TMT, SnCI,, and CF,Br. The data for SnCI, in 

n-hexane-air flames [SI show if to be as effective as TMT is in these methane-air flames. As the figure 

also shows, Fe(CO), is significantly more effective than any of the other agents, and all appear to have 

greatly reduced effectiveness at burning velocity reductions of about 50%. 

One approach for overcoming the loss of effectiveness is to add non-condensing amounts of several 

inhibitors. To test this approach in premixed flames, we performed tests with a blend of MMT and 

Fe(CO), , added at a molar ratio of 2:1, respectively. For the flame of this test, Tin = 80 "C, @=I .O, and 

llalon Options Technical Working Conference 24-26 April 2001 77 
, = 1.0 1 

xq=0.21 I I 

i 

I 

'?i 

1 

04C. , . , I -- 0 ZWO 4wO 6oW Bwo lOOW 0 1W ZW 3W 4W 5W 6W 

lnhlbkor (ppm) lnhlbitar (ppm) 

Figure 6. Normalized burning velocity of premixed Figure 7. Normalized burning velocity of premixed 

CH,/02/N2 flames inhibited by pure 

MMT and Fe(CO),, and by a blend of the 

two (lines are curve fits to the data). 

CH4/OJN, flames inhibited by CF,Br, 

TMT, SnCI, [8], MMT, and Fe(CO), 

(T," = 80 'C for all data except TMT and 

SnCI, (lines are curve fits to data). 

X02,,=0.244; data for pure MMT or Fe(CO), and the combination are shown in Figure 7 (the data for 

the combination are plotted as a function of the mole fraction of the more abundant agent, MMT). As the 

figure shows, adding 0.5 moles of Fe(CO), for each mole of MMT added does provide additional flame 

speed reduction over that from MMT alone. 

BLENDS OF MMT AND FE(CO), 

The behavior of the blends of agents can be investigated by comparing the actual amount of flame speed 

reduction for the blend to the sum of the inhibition that would result from each agent individually. This 

approach is illustrated schematically in Figure 8. We adopt the inhibition index @(Xi,) of Fristrom and 

Sawyer [33], where @(X;,)={ {(Vo-V(X;,))/V,,}{ XO~,~~/~,} (and using the oxygen mole fraction in the 

oxidizer). The index @(Xin) is seen to be the magnitude of the average slope of the normalized burning 

velocity curve (times evaluated at the mole fraction of interest Xi,. For a given blend, we can 

evaluate the amount of normalized burning velocity reduction that would have been caused by each individual component of the blend, say components a and b. The predicted inhibition index is just a linear 

combination of the reduction from each component, or X. * Q.+b(X,, X,)I,,, = X, @.(XJ+ X, mb(Xb), in 

which a is the major component of the blend, (and we have selected it for defining @.+, (X,, X,)). The 

actual inhibition index Qeb(Xn, Xb)lamN.I is evaluated from the normalized flame speed of the blend (using 

X, in its definition). The ratio of X. (X, Xb)lanua, to X,. @,(X.)+X,* mb(Xb) provides a reasonable 

indicator of the performance of the blend relative to the individual components. 

As Figure 9 shows, a blend of CO, and N, (in the molar ratio of 1 :2) provides a performance index of 

nearly 1.0 for N, added up to 12% (i.e., containing 6% CO,). (The percent of the NJCO, mix in Figure 9 

is divided by 20 to allow placement on the same scale.) The blends of CF,Br and ferrocene (Fec) also 

perform approximately like the linear combination of the performance of each (a performance ratio near 

1.0 (Figure 9). Note that in the Fec/CF,Br blend, the maximum amount of Fec is 23 ppm and 66 ppm for 

the curves marked 0.36 and 1.33% Fec, respectively. For similarly low levels of metallic inhibitors, the 

78 Halon Options Technical Working Conference 24-26 April 2001 
10 

02 04 06 

Percent Major Component (a) Inhibitor(s) Mole Fraction 

Figure 8. Illustration of linear contribution of Figure 9. Ratio of the actual reduction in S, from 

the two component mix to the predicted 

reduction based on linear combination 

of the effect from each component. The 

percent COJN, has been divided by 20 

to tit on the figure. 

burning velocity reduction from each 

component of a two-component blend of 

inhibitors, together with the actual 

reduction from the blend. 

blend of MMT and Fe(CO), also behaves like sum of the individual contributions; however, as the 

amount of inhibitor increases, the performance ofthe blend is much less. This is likely due to condensation of mixed oxide species,* showing that this approach may have limitations. 

DISCUSSION 

Examination of species profiles, reaction tlux, and sensitivity coefficients from the numerically predicted 

flame allows investigation of the mechanisms of inhibition of these metallic compounds. The calculations show that TMT decomposes quickly in the flame, with 90% consumption at 1000 K. The tin atom 

formed as a result of Th4T decomposition quickly reacts with 0, to form tin oxide through the reaction 

Sn+O,=SnO+O. This reaction is fast at room temperature, as compared to the analogous reaction of iron 

[34]. 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, completes the catalytic radical recombination cycle: 

SnOH+H=SnO+H, 

SnOH+OH=SnO+H,O 

SnOH+CH,=SnO+CH,. 

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

methane-air flame. Of the reactions of tin compounds, the burning velocity is most sensitive to the rate 

of the reaction SnO+H+M=SnOH+M, which has a sensitivity about four times less than the chain-branching reaction H+O,=OH+O. The burning velocity is next most sensitive to the tin reactions: Sn+CO,= 

* Gilman, J., personal communication, April, 2001. 

Halon Options ‘Technical Working Conferencc 24-26 April 2001 79 
SnO+CO, SnO+HCO=SnOH+CO, SnOH+H=SNO+H,, and SnOH+OH=SNO+H,O, for which the sensitivity is about 40 times less than the chain branching reaction. 

As was also found for DMMP and ferrocene, the burning velocity of flames inhibited by TMT is not 

sensitive to the rate of the decomposition reaction. Numerical tests showed that changes in the overall 

activation energy of TMT decomposition in the range I70 kJ/mol to 335 kJ/mol yield little effect on the 

burning velocity with up to 2000 ppm of TMT. Hence, the inhibition effectiveness of tin compounds is 

likely to be independent of the parent molecule, as long as rapid decomposition occurs. The rate selected 

for the pre-exponential factor of the association reaction SnO+H+M=SnOH+M has a significant effect on 

the predicted burning velocity (in the present mechanism, A=5.5 x IO" cm6 s2 * mol-') 

Note that similar to what was found for CO -N,O flames inhibited by Fe(CO),, the reactions SnO+CO= 

Sn+CO, and Sn+O,=SnO+O can promote the CO consumption and decrease the inhibition effect of tin. 

Changes in these rates can also affect the inhibition efficiency of tin, although the mechanism is dominated by the rate of reaction SnO+H+M=SnOH+M. 

Analysis of the numerical results for inhibition by MMT shows that its behavior in this premixed methane-air flame is similar to that Fe(CO),, with the catalytic radical recombination cycle consisting of: 

MnO + H,O = Mn(OH), 

Mn(OH), + H = MnOH + H,O 

MnOH +OH (or H) = MnO + H,O (or H,), 

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. Sensitivity 

analysis demonstrates that burning velocity is most sensitive to the reaction Mn(OH),+H=MnOH+H,O, 

Mn+O,(+M)=MnO,(+M), MnOH+O+M=MnOOH+M, MnOOH+H=MnO+H,O, MnOH+OH=MnO+ 

H,O, MnO+H=Mn+OH, and MnO+H,= Mn+H,O (in decreasing order of importance), and is less sensitive to the reaction MnO+H,O=Mn(OH),. 

CONCLUSIONS 

In this work we presented the first experimental measurements of the influence of manganese- and tincontaining compounds (MMT, TMT) on burning velocity of methanelair flames. We have also developed kinetic models describing the inhibition mechanisms. Comparisons with Fe(CO), and CF,Br demonstrate that manganese- and tin-containing agents are effective inhibitors. The inhibition efficiency of 

MMT is about a factor of two less than that of iron pentacarbonyl, and that of TMT is about 26 times less 

effective, although TMT is about twice as effective as CF,Br. There exist conditions for which both 

MMT and TMT show a loss of effectiveness beyond that expected due to radical depletion, and the cause 

is believed to be particle formation. 

Simulations of MMT- and TMT-inhibited flames show reasonable agreement with experimental data. 

The decomposition of the parent molecule for the tin and manganese species is found to have a small 

effect on the inhibition properties for the concentrations in this work. Calculations confirmed that the 

main species of tin compounds in the flame zone is SnO, while the concentrations of SnO,, SnOH, and Sn 

are relatively small. The inhibition effect of TMT is determined mostly by the rate of the association 

reaction H+SnO+M=SnOH+M, and the catalytic recombination cycle is completed by the reactions 

SnOH+H=SnO+H, and SnOH+OH=SnO+H,O. The manganese inhibition mechanism is the same as that 

for iron, namely: MnO + H,O = Mn(OH),; Mn(OH), + H = MnOH + H,O, and MnOH + OH (or H) = 

MnO + H,O (or H2), and the burning velocity is most sensitive to the rate of the second of these. 

80 Halon Options Technical Working Conference 24-26 April 2001 
ACKNOWLEDGMENTS 

We thank Tania Richie for help with the experiments, and W. Tsang, J.W. Fleming, Marc Rumminger, 

and G.P. Smith for helpful discussions. We also thank G.P. Smith for providing his MMT kinetic mechanism. This research was supported by the Department of Defense Next-Generation Fire Suppression 

Technology Program, funded by the DoD Strategic Environmental Research and Development Program 

under contract number W74RDV83528667, and by the Office of Biological and Physical Research, 

National Aeronautics and Space Administration, Washington, DC. 

I. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

18. 

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82 Halon Options Technical Working Conference 24-26 April 2001