INHIBITION DUE TO HALOGENATED COMPOUNDS

sive inert additive with a high heat capacity.
The composition of a composite retardant
should be determined by the “saturation”
concentration of the chemical inhibitor and
the peak concentration of the composite
retardant.

. The regeneration coefficient of an inhibi-

tion agent showing the number of regenera-
tion cycles that occurred during combustion
is determined. For atmospheric stoichiomet-
ric methane-air flames with 1% CF,Br ad-
ditive or 1% CF,I additive, the regeneration
coefficients are equal to approximately 7
and 4, respectively.

The authors are grateful to Dr. M. Nishioka,

Nagoya University, Japan, for discussions on the
accuracy of the calculations using the windward
difference and central difference formulae in the
PREMIX flame code.

REFERENCES

1

Rosser, W. A. Wise, H., and Miller, J., Seventh
Symposium (International) on Combustion, Butter-
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Halpern, C., J. Res. Nat. Bur. Standards, Phys. and
Chem. 67A (1):71 (1963).

Fristrom, R. M., and Sawyer, R. F., AGARD Conf.
Proc. 84:12-1 (1971).

Fristrom, R. M., and Van Tiggelen, P. J., Seventeenth
Symposium (International) on Combustion, The Com-
bustion Institute, Pittsburgh, 1979, p. 773.

Da Cruz, F.N., Vandooren, J., and Van Tiggelen, P.
J., Bull. Soc. Chim. Belg. 97:1011 (1988).

Reid, R. C., Prausnitz, J. M., and Poling, B. E., The
Properties of Gases and Liquids, 4th ed., McGraw-Hill,
New York, 1987.

Rosser, W. A., Jr, Inami, S. H., and Wise, H., Com-
bust. Flame 10:287 (1966).

Parks, D. J., Alvares, N. J., and Beason, D. G., Fire
Safety J. 2:237 (1979/1980).

Linteris, G. T., in Halon Replacements, Technology
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American Chemical Society, Washington, DC, 1995,
p. 260.

Sanogo, O., Delfau, J. L. Akrich, R., and Vovelle, C.,
Twenty-Fifth Symposium (International) on Combus-
tion, The Combustion Institute, Pittsburgh, 1994, p.
1489.

Sanogo, O., Etude Expérimentale et Modélisation de la
Structure de Flammes: Applications @ la Recherche de
Substituts Gux Flalons, Ph.D. dissertation, Université
d’Orleans, France, 1993.

12,

21.

22.

23.

27.

29.

31.
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33.

159

Linteris, G. T., and Truett, L., International Confer-
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pp. 153-158,

Noto, T., Babushok, V., Burgess, D. R. F., Jr., Hamins,
A., Tsang, W., and Miziolek, A., Twenty-Sixth Sympo-
sium (International) on Combustion, The Combustion
Institute, Pittsburgh, 1996, in press.

Sheinson, R. S., Penner-Hahn, J. E., and Indritz, D.,
Fire Safety J. 15:437 (1989).

Tucker, D, M., Drysdale, D. D., and Rasbash, D. J.,
Combust, Flame 41:293 (1981).

Kee, R. J., Grear, J. F., Smooke, M. D., and Miller, J.
A,, Sandia National Laboratories Report No., SAND
85-8240, 1985.

Miller, J. A., and Bowman, C. T., Prog. Energy Com-
bust. Sci. 15:287 (1989).

Engolfopoulos, F. N., Du, D. X., and Law, C. K,,
Combust. Sci. Technol. 83:33-75 (1992).
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Twenty-Third Symposium (Intemational) on Combus-
tion, The Combustion Institute, Pittsburgh, 1990, p.
47.

Bowman, C. T., Frenklach, M., Gardiner, W., Golden,
D., Lissianski, V., Smith, G, and Wang, H.,
GRIMECH, Gas Research Institute Report, 1995.
For the detailed C/H/O kinetic mechanism, see
http: / /fluid.nist.gov /ckmech.htm].

Burgess, D. R., Jr., Zachariah, M. R., Tsang, W., and
Westmoreland, P. R., Prog. Energy Combust. Sci.
21:453 (1996).

‘Westmoreland, P. R., Burgess, D. R. F., Jr., Zachariah,
M. R,, and Tsang, W., Twenty-Fifth Symposium (Inter-
national) on Combustion, The Combustion Institute,
Pittsburgh, 1994, p. 1505.

Babushok, V., Noto, T., Hamins, A., and Tsang, W.,
Combust, Flame 107:351 (1996).

Wagman, D. D., Evans, W. H., Parker, V. B., Schumm,
R. H.,, Halow, 1, Bailey, S. M., Churney, K. L., and
Nuttall, R. L., J. Phys. Chem. Ref. Data 11(suppl. 2)
(1982).

. Benson, S. W., Thermochemical Kinetics, John Wiley,

New York, 1976.

Stull, D. R., Westrum, E. F., Jr., and Sinke, G. C., The
Chemical Thermodynamics of Organic Compounds,
John Wiley, New York, 1969.

McMillen, D. F., and Golden, D. M., Ann. Rev. Phys.
Chem. 33:494 (1982).

Chase, M. W., Jr., Davies, C. A. Downey, J. R., Jr.,
Frurip, D. J., McDonald, R. A., and Syverud, A.N., J.
Phys, Chem. Ref, Data 14(suppl. 1) (1985).

|. Glushko, V. P., Ed., Thermodynamic Properties of Indi-

vidual Substances, 3rd ed., Nauka,
1978-1982.

Pedley, J. B., and Rylance, J., Thermochemical Data,
University of Sussex, Brighton, England, 1977, Vol. 1.
Lias, S. G., Bartmess, J. E., Liebman, J. F., Holmes, J.
L, Levin, R. D., and Mallard, W. G., J. Phys. Chem.
Ref. Data \H(suppl. 1) (1988).

Lide, D. R., Ed., CRC Handbook of Chemistry and
Physics, CRC Press, Boca Raton, FL, 1995.

Moscow,



150 T. NOTO ET AL,
TABLE 2
Thermodynamic Properties of Bromine- and lodine-Containing Species
‘AH; (298) §° (298) ¢,(T) {cal (mol K))

Species (kcal/mol) [cal/(mol K)] 300 500-800-1000 1200 1500 2000 -3000_—séRef.
Br 26.73 41.80 497 497 SAL 5.21 $325.42 5.424
Br 7.39 58.64 8.62 8.86 901 (9.04 (9.089.149.2524
HBr -871 41.46 6.96 7.04 172 798 829 863 B98 24
BrO 30.10 56.80 770 7.93 821 832 848 8.70 895 25
BrOH - 19:00 59,20 9.00 9.50 1042 10.73 11.20 11.97 1332 26
HBr -9.01 58.76 10.19 13.56 19.04 20,38 21.77 23.00 23.75 27
CH,Br 41.50 60.30 425 7.78 1162 13.44 1483 1633 17.71 1851 28
C,H,Br -15.30 68.72 15.47 21.97 2861 31.63 3387 35.96 27.16 37.36 27
C,H,Br 18.73, 65.84 13.30 18.06 22.51 24.45 25.84 27.26 «28.45 29.1027
CF,Br — 155.10 TAB 16.78 20.54 23.49 24.13 24.49 2493 25.41 25.62 29
I 25.49 43.15 496 496 496 496 497 5.00 5.09 530 30
I, 14.03 62.22 8.81 894 = 9.03 9.08 S 9.31 966 9.96 29
HI 6.29 49.31 69% 710 759 7.93 817 846 879 913 2
10 31.03 57.17 786 856 930 962 9.81 989 = 9.91 973 30
10H - 18.10 61.10 931 1044 11.34 11.73 12.08 1249 1295 1345 30
CH;I 3.43, 60.61 10.60 14.01 17.49 19.19 2053 21.91 2359 25.12 29
CH,I 54.42 62.05 950 12.70 15.80 17.30 1843 19.31 20.20 2056 2%
C)Hst —2.00 70.83 15.80 22.10 28.70 31.70 33.72 35.25 37.21 3982 33
CHyl 31.03 68.03 13.60 1840 22.70 24.58 26.03 27.56 29.46 3138 32
CFA — 140.60 73.42 16.92 20.42 23.57 24.44 24.70 24.90 25.20 2558 29

TABLE 3 wide range of concentrations. According to
‘Transport Properties of Bromine- and simple theoretical relations [39], the burning
Todine-Containing Species* velocity is proportional to the square root of
(CHEMKIN Format Presentation) the reaction rate, which has an exponential
Species €/ky o - a Ref. dependence on the adiabatic flame tempera-
ture. The main effect of dilution by an inert is

9 4S 27.02 33 . aS

Pe ies on ee 361 O33 to approximately decrease the adiabatic flame

Br 2300 370 '— — ¢ temperature linearly. Thus, for inert additives,

BrO 360.0 3800 1.76 — €,33 an exponential decay of the burning velocity is

BrOH = 440.0 3.950 — — — e expected in contrast to a linear dependence.

pb oa nar 182 5.87 6.33 As shown in Fig. 1a, a linear relationship

HBr 4700 4850 203° «805, 33 between burning velocity and additive concen-

C,HsBr 460.0 4600 142 759 33 tration becomes tenuous when a wide concen-

CF;Br = 270.0 4.100 — - e tration range is considered. A best fit to the

Hs pies ret te ia 6 3 additive concentration for the burning velocity

4 00 3900 : leads to an exponential dependence,

ice} 420.0 4.100 245 — 33 S,/Sy = exp(—b-C. 4

10H 480.0 4200 — — e «/ So = exp ia)s @

ae ee ed 1620 797,33 where 6 is a constant. The numerically ob-
0 4: So ¢ . ee 4

@ u MI 4400 $000 «191 100 033 tained burning velocities, sing jhe PREMDE

C,H] 4200 4800 13 93 ¢,33,34 code, are presented in Fig. 1b. The results

CFI 3200 4900 105 — 6,33 the numerical simulations also are well approx-

“e/ky, the Lennard-Jones well depth (K); 0, the
Lennard-Jones collision diameter (A); 1, the dipole mo-
ment (De); a, the polarizability (A); e, estimation.

imated by an exponential function.
The constant b in Eq. 4 reflects the effi-
ciency of an inhibitor. In dimensionless form,



INHIBITION DUE TO HALOGENATED COMPOUNDS 155
1 100
09 90 ee eee
2os ~ 80
go x 10
B06 ¢ 0
Fos = 50
Fhe * aot
zo gx Lee Toa COURS CAps CEgBe
°° 2» cer ope
oO. 10
; Oo

0 0.02 0.04 0.06 0.08 Ol

Mole fraction of additive
Fig. 7. The physical component Y, as a function of addi-
tive concentration in stoichiometric CH,-air flames.

component of the retardant influence in-
creases with increasing additive concentration.

Figure 8 shows the dependence of the chem-
ical component (Y,) on additive concentration.
As expected, nitrogen has no chemical contri-

CERH C2HFS C2P6 CE3Br
mR

0.04 0.06 01
Mole fraction of additive
Fig. 8. The chemical component Y, as a function of addi-
tive concentration in stoichiometric CH,-air flames.

°

0.02 0.04 0,06
Mole fraction of additive
Fig. 9. The fractional physical inhibition contribution
Y,/(%, + Y,) as a function of additive concentration in
CH,-air flames.

0.08 Ot

bution. Chemical effectiveness is thought to
depend on the ability to reduce the concentra-
tion of chain carriers [44]. The chemical effec-
tiveness of C,F, is almost twice that of CF,H.
One molecule of C,F, produces two CF, radi-
cals during the initial stage of decomposition.
For C,HF;, the decomposition mechanism is
dominated by the reaction C,HF, =» CHF, +
CF;, and the chemical component can be
thought to consist of the sum of the inhibition
effects of CF, and CHF, radicals. The chemi-
cal action of CHF, is less than 2/3 that of the
CF, radical.

Table 8 shows the relative physical, 100 x
Y,/(¥, + Y,.), and the chemical, 100 x ¥,/
(¥, + ¥,), components of inhibition for the
additives in methane-air flames at S, = 5
cm/s (the presumed extinction value [41].
Also, contributions are presented for decreas-
ing S, to half its value, where S,/So = 0.5.

TABLE 8

Physical and Chemical Contributions at $,/Sp = 0.5 and S, = 5 cm/s for CH,-Air Stoichiometric Flames

S,/Sy = 05 S, = Scm/s
Physical Chemical Physical Chemical
Effectiveness Effectiveness Effectiveness Effectiveness
Additive (%) (%) (%) (%)
CRyH 64 36 9 21
CHF, 67 3B 81 19
Cr 59 41 0 31
CF,Br 22 B 35 65
CRI 23 7 36 64
cr, 89 i 95 5



152

the initial decomposition products. CF,H and
C,F, decompose mainly to CF,. Decomposi-
tion of C,HF, generates CHF, and CF, radi-
cals. CHF, decomposes to CHF, and CHF
[22, 40). Table 4 shows that there is reasonable
agreement between the experimentally ob-
served (6,,,) and the numerically obtained ex-
ponential constants (5,,,). A maximum differ-
ence between the experimental and computa-
tional exponential constants occurs for CF,Br
and is approximately 25%. Table 5 compares
the inhibition indices for ¢p and 5, derived
from the experimental measurements and the
modeling, and the linear indices suggested by
Fristrom and Van Tiggelen [4]. It is seen that
the experimental linear indices [4] obtained in
a small concentration range coincide with the
“exponential” indices. For values of ® -
Cin/Co, < 0.3, the linear indices are approxi-
mately equal to the exponential indices.

Figure 2a-c shows calculations of normal-
ized burning velocity for stoichiometric pre-
mixed C,H,-air, C,H,-air, and CH,OH-air
flames with additives. Table 6 shows the con-
stant 6 and the inhibition parameter ® for
stoichiometric C,H,, C,H,, and CH,OH
flames with additives. The values of b and ®
follow similar trends. The constants b and ®
are largest for CF,I and CF,Br followed by
C,F,, C,HF,, CF,H, and CF,, consistently for
the three fuels.

The inhibition indices for F, Br, I, CHF,,
and CE, obtained from numerical modeling
are shown in Table 7. Figure 3 shows a dia-
gram of the inhibitor ranking for different
halogenated additives estimated using inhibi-
tion indices. The index for the chlorine atom

TABLE 5

Comparison of Inhibition Indices of Fluorine and Bromine
Atoms Derived from Experimental Measurements,
Calculation Results, and Those Suggested by Fristrom
and Van Tiggelen [4]

Inhibition Index

‘Atomic Exponential Dependence Linear Dependence

Index Experiment Calculation Calculation Ref. 4
or 09 1.0 1.0 10
op, 11.6 B81 8.6 10.9

T. NOTO ET AL,

0 0.02 0.04 0.06 0.08 O41
Mole fraction of additive

(a)

c) 0.02 0.04 0.06 0.08 O1
Mole fraction of additive
(b)

0 0.02 0.04 0.06 0.08 Ol
‘Mole fraction of additive
©)

Fig. 2. The calculated relative burning velocity as a func-
tion of additive concentration for stoichiometric (a)
C,H,-air, (b) C,H,-air, and (c) CHOH-air flames.

was assumed to be equal to the index given by
Ref. 4. Note that the inhibition efficiencies in
terms of decreases in flame velocity by halo-
genated compounds correspond approximately
to experimental data [1, 9-12].

Figure 4 shows the relative burning velocity
as a function of the ratio ®-C,,/Co, for stoi-


INHIBITION DUE TO HALOGENATED COMPOUNDS 151

CEMBI2) C2HESHI2}

Fag] cHREZHAI

0.8
CH4-air

g 06
04 t
02+
% 002 00h 0.06 0.08 0.1
Mole fraction of additive
@

°% 0.02 0.04 0.06 0.08 Ol
Mole fraction of additive
(b)

Fig. 1. (a) The relative burning velocity and (b) calculated
relative burning velocity as a function of additive concen-
tration for stoichiometric CH ,~air flames. Symbols repre-
sent (a) experimental data and (b) calculation results; lines
are best exponential fits.

using the oxygen concentration Cy, as a nor-
malizing parameter [3], the inhibition parame-
ter ® is

® = (Co,/Cyy) * In $/S,)- ()

Here, ® is modified from the form suggested
by Fristrom and Sawyer (3), taking the expo-
nential concentration dependence of S, into
account. The parameter ® for the halogenated
compounds considered here can also be repre-
sented as a sum of elementary indices

® = nede + Neue, bcur, + Mr, bcr,
+ Na dp, + m4. (6)

It is assumed that ¢- = 0 and ¢, = 0. To
obtain better fits to the burning velocity data,
the groups CHF, and CF, are also used. This
is a reflection of the fact that an atomic fluo-
tine additivity scheme cannot fit the data due
to the high stability of compounds such as CF,
and CF,O.

Table 4 presents both experimental and cal-
culated values of the exponential constant b
and the inhibition parameter ® for different
additives in stoichiometric CH,-air premixed
flames. Nitrogen is also presented as a refer-
ence basis in Table 4. The combination of
inhibition indices for the fluorinated additives
such as CF,H, C,HF,, and C,F, is based on

TABLE 4
Exponential Constant b and the Inhibition Parameter © for CH,—Air Stoichiometric Flame with Halogenated Additives*

Additive Bes? eg’ ba Of Indices for @
CF,H 149 26 16.3 29 3¢p

C, HF, 22.1 40 6 45 365+ deur,
C.F 28.5 5.2 34.6 63 6d
CF,Br 78.6 140 60.9 ud 365+ by,
CFI - _ 58.5 10.7 3b5 + o;
cr, 10.3 18 Md 19 bce,
CH,F, 13.7 24 - - oe + bcur,
C;HF, 33.9 6.2 _ _ -
CF, 3S 58 - _ -

N, 3.64 - 3.6 - _

‘exp, experimental measurement; cal, calculation result.

* The constant b is derived from the equation S,/Sy = exp(—b + C,).
The constant © is derived from ® = (Co,/Ciq)° In(Sa/S,).

“The constant b is derived from data reported in Ref. 4.



154

Inhibition

parameter © additives
24 -CHBr3
20 +

CF2Br2 CF2I2

16 +—CH28r2 CH2I2 Br2

124—
Crier

CH2Brct

Cra

HI, ccw
cnt cFCI3

C2F6 CHCIS CF2Ci2

e+ Hee

a CPHFS cHci2 ciz
CHES
cH3cI

CH3F

ot
Fig. 3. The relative inhibition efficiency of halogenated
additives according to inhibitor indices for stoichiometric
methane~air flames.

component, Y,» is defined as
¥, =1-S'/Sq, mM

where S, is the burning velocity calculated
without additives. The chemical inhibition
component (Y,) is the difference between the

V CF3H C2HFS CF3BR CF3I
co ao

08
ws
oe S,1 Sy exp-® Cn! Co)
02
% rv 08 12 16 2 za 28

®GalCor

Fig. 4. The relative burning velocity as a function of the
ratio ©+C,,/Co, for stoichiometric CH,-air flames with
additives.

T. NOTO ET AL.

0

4 on”
Fig. 5. The relationship between the maximum flame tem-

perature and the normalized burning velocity for
methane-air flames with additives,

physical component and the total inhibition
effect,

¥, = (S’ - S,)/So, (8)

where S, is the burning velocity calculated
with the additive using the kinetic model in-
cluding inhibitor reactions. Figure 7 shows the
behavior of the physical component (Y,) as a
function of additive concentration for a stoi-
chiometric CH,-air flame. The physical influ-
ence of the additives on flame speed is mostly
due to heat capacity and dilution effects. Halo-
genated compounds with approximately equal
heat capacities such as (a) C,HF, and C,F, or
(b) CF,H, CF,Br, CF,I, and CF, have approxi-
mately equal physical effects. The physical

with reaction set
0.2+ of the additive species

°9 0.02 0.04 0.06 0.08 Ot
Mole fraction of additive
Fig. 6. Definition of the physical ¥, and the chemical ¥,
components of inhibitor influence.



INHIBITION EFFECTIVENESS OF HALOGENATED COMPOUNDS

by

T. Noto, V. Babushok,
A. Hamins and W. Tsang
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899 USA

Reprinted from COMBUSTION AND FLAME, Vol. 112, No. 1/2, 147-160 pp., January
1998.

NOTES: This paper is a contribution of the National Institute of Standards and
Technology is not subjected to copyright.


INHIBITION DUE TO HALOGENATED COMPOUNDS 157

TABLE 10

Physical and Chemical Contributions of the Influence of
CF,Br Determined at S, = 5 cm/s for CH,-air
Stoichiometric Flame by Different Measures

Physical Chemical
Effectiveness Effectiveness
Measure (%) (%)
Burning velocity 35 65
Additive concentration "7 83
Maximum flame temperature 15 85

An analysis of reaction pathways for inhibi-
tion of methane flame by CF,Br shows that the
most important radical scavenging reactions
for HBr consumption are [13, 44]

H + HBr = H, + Br, (R1)
OH + HBr = H,O + Br. (R2)

The relative importance of different reactions
is determined through integral contributions of
reaction rates, {w; dt, where w, is the ith reac-
tion rate and ¢ is the reaction time [36]. Inte-
gration is carried out until a maximum H atom
concentration is achieved. The integration in-
terval approximately corresponds to the flame
zone. Figure 10 contains the dependence of
contributions of R1, R2, and the sum R1 + R2
to HBr consumption on CF,Br concentration.
The contributions of R1 and R2 constitute
more than 80% of the total consumption of
HBr. For CF,Br concentrations less than 0.02
mole fraction, HBr is mainly consumed by R1.

0.16

for Rt etme. > HZ Br
for na. Oe HBe-> W206
0.12 H toe ni on

°

&

.
N

Integrated reaction contributions
to HBr consumption
°
8

°% 0.01 0.02 0.03 0.04

Mole fraction of CF3Br
Fig. 10. Contribution of the reactions Ri, H + HBr =
H, + Br, and R2, OH + HBr = H,0 + Br, to HBr con-
sumption as a function of CF,Br mole fraction in stoichio-
metric CH ,-air flames.

For CF,Br concentrations higher than 0.025
mole fraction, the importance of R2 increases.
It is seen that the total effect of R1 and R2
saturates with retardant concentration. Satura-
tion of HBr consumption is observed. The de-
pendence of R1 + R2 contributions has a simi-
lar character as a chemical component of re-
tardant influence (Fig. 8). In addition, an anal-
ysis of flame structure shows that the su-
perequilibrium concentration of chain carriers
[44] is reduced with increasing inhibitor con-
centration, resulting in a reduction in inhibitor
effectiveness with further inhibitor addition.

After the saturation point, increase of addi-
tive concentration leads to preferentially less
additional chemical influence, and thus, it is
possible to consider the inhibitor as an inert
gas. This leads us to suggest the use of a
“composite” inhibitor which contains an effec-
tive chemical retardant and an additive with a
high heat capacity. Its composition is deter-
mined by the saturation concentration of the
retardant and the extinction concentration of
the composite inhibitor. Figure 11 contains
results of numerical modeling of CH,-air
flames when polyfluoroalkylamine (CF,CF,),N
[45] is added to 2.5% CFI which corresponds
toughly to the saturation concentration.
(CF,CF,),N is treated as an inert. To achieve
extinction, it may be advantageous to use a
composite inhibitor consisting of an effective
chemical inhibitor for rapid reduction of S,
and an inexpensive inert compound with a high
heat capacity.

1
O8
0.6

$

O4

02

2.55 CEs :

XS (CFICRIYIN  ——

0 0.01 0.02 0.03 0.04 0.05

Mole fraction of additive

Fig. 11. The burning velocity as a function of inhibitor

concentration for an additive composed of CFjI and
(CR,CF,),N.

0,


156

The physical contributions of the fluorinated
compounds CF,H and C,HF, are 80% for
S, = 5 cm/s. CE, acts mostly through physical
effect. On the other hand, physical inhibition
components for CF,Br and CF;I are approxi-
mately 20% at S/S = 0.5 and about 35% at
extinction, respectively. Figure 9 shows the
physical contribution percentage, 100 x
Y,/(¥, + Y,), as a function of additive concen-
tration. The physical contribution increases
linearly with additive concentration. Figure 9
shows that as the additive concentration in-
creases, dilution and heat capacity effects be-
come relatively more important. The results
indicate that the mode of influence of an in-
hibitor changes with its concentration. The
physical and chemical components of CF,Br
and CF,I for fuels other than CH, are pre-
sented in Table 9. For C,H,, C,H,, and
CH,OH, the percentage of the physical contri-
bution of CF,Br influence at S, = 5 cm/s is
38, 41, and 50%, respectively. For the
CH,OH-air flame, physical and chemical fac-
tors for CF,Br are nearly equal near extinc-
tion.

Different measures of chemical and physical
contributions are possible. The burning veloc-
ity is taken as an appropriate measure for
differentiation of the chemical and physical
contributions to inhibition. In addition, it is
possible to use the additive concentrations cal-
culated with and without reaction kinetics for

T. NOTO ET AL,

the additive that yield the same burning veloc-
ity reduction. Sheinson et al. [14] used the
“concentration” measure to estimate the
chemical and physical contributions to inhibi-
tion. The difference in the maximum flame
temperatures calculated with and without reac-
tion kinetics for the additive yielding the same
burning velocity reduction represents another
means to differentiate physical and chemical
contributions. Table 10 shows the physical and
chemical contributions to inhibition by CF,Br
determined by these different definitions. The
“concentration” and “temperature” measures
for physical and chemical contributions coin-
cide and are approximately consistent with
those of Sheinson et al. [14].

Saturation Effect

For the halogenated compounds studied, when
the additive concentration increases, the chem-
ical contribution initially increases also. When
the relative burning velocity (S,/5S,) is approx-
imately 0.2-0.4, saturation of chemical influ-
ence is observed (Fig. 8). In other words, the
contribution of the chemical factor decreases
with increasing additive concentration. This
tendency is pronounced for CF,Br and CF,I
which are more effective retardants than the
other fluorinated compounds. It is suggested
that the phenomenon arises from the reduc-
tion in radical concentration by the inhibitor.

TABLES
Physical and Chemical Contributions of CF\Br and CF,I at S, = 5em/s for C,-C, Stoichiometric Air Flames

Physical Chemical
Mole Fraction Effectiveness Effectiveness
Additive for S, = Scm/s (%) (%)
CF,Br
C,H-air 0.056 al 59
C,H,-air 0.061 38 62
CH,-air 0.035 35 65
CH,OH-air 0.069 50 50
CF,
C,H g-air 0.057 rr) 37
C,H ,-air 0.070 “4 56
CH,-air 0.036 36 64
CH,OH-air 0.046 37 63



INHIBITION DUE TO HALOGENATED COMPOUNDS 153

TABLE 6

Exponential Constant 6 and Inhibition Parameter for
C,Hg-Air, C,H,~Air, and CH \OH-Air Stoichiometric
Premixed Flames with Additives of Halogenated
Compounds CF,H, C,HFs, C3 F,, CF,Br, CFyl, and CF,

System oo oe
C,Hy-air + CFH 12.1 2.24
+C,HF, 18.7 351

+C,F, 26.4 5.01

+CF,Br 398 722

+CFyI 39.2 7.46

+CF, 891 1.63
C,H,-air + CF, 143 2.60
+C, HF, 24.2 447

+C,F, 313 5.85

+CF,Br 43,2 8.23

+CFI 374 147

+CR 8.02 1.45
CH,OH-air + CF,H 12.0 2.05
+C, HF, 16.7 2:93

+C)F 26.9 4.76

+CF,Br M9 5.67

+CFyI 47.1 837

+CF 9.21 157

* The constant b is derived from the equation S,,/Sy =
exp(—b+ Cin)

°The constant @ is derived from ® = (Coy,/Ciq)*
In(Sp/S,).

chiometric methane~air flames. This figure
represents, in another form, that the burning
velocity can be approximately described by a
single exponential dependence for all additives
using Eq. 5 over a wide range of additive
concentrations. Also, it is seen that the ratio
®-C,,/Co, is a similarity parameter. The re-
sults of calculations for other equivalence ra-
tios show that the inhibition indices are valid
for equivalence ratios from 0.8 to 1.2 with an
accuracy + 15%. For very lean or rich mixtures
(<0.8 and > 1.2), however, the inhibition

efficiency of the additives may differ from near
stoichiometric values.

Physical and Chemical Action of Flame
Retardants

The mechanisms of a retardant’s influence
usually are divided into two categories: physi-
cal mechanisms, where heat capacity and dilu-
tion effects dominate, and chemical mecha-
nisms, where radical scavenging is important.
The burning velocity is reduced due to both
physical and chemical influences of an addi-
tive. Figure 5 shows the calculated adiabatic
flame temperature as a function of the relative
burning velocity for different additives. The
ratio S,/Sy = 1 corresponds to a flame with-
out additives. A value of 5 cm/s is assumed as
the critical flame speed at extinction [41]. This
velocity is attained as the nitrogen concentra-
tion increases, at a flame temperature of ap-
proximately 1600 K. This temperature has been
thought to approximately represent the extinc-
tion limits for diffusion flames [14, 42]. Use of
chemical retardants such as CF,Br, CF,I, and
other halogenated additives leads to larger val-
ues of the calculated adiabatic flame tempera-
ture (7;) in comparison with inert additives for
the same amount of S$, reduction. Large values
of 7; represent increasing chemical influence
of an additive [43].

A procedure for determination of the physi-
cal and chemical contributions of an additive is
shown in Fig. 6. The physical influence of a
retardant can be characterized by treating the
additive as an inert compound and by calculat-
ing the burning velocity (S‘), by turning off all
reactions involving the inhibitor. The physical

TABLE 7

Inhibition Indices of Halogen Atoms ($¢, bp, $1) and Groups (deur. $ce,) Composing the Inhibition Parameter
for C;-C, Hydrocarbon-Air Stoichiometric Flames with Halogenated Compounds

Fuel oe cur, dcr, dor o
CH, 10 15 19 8.1 WW
C,H, 08 MW 16 53 $A
C,H, 10 15 15 5.2 45
(CH,OH 07 08 1.6 36 63



INHIBITION DUE TO HALOGENATED COMPOUNDS 149

simple predictive capability. The correlations
derived here are based on both experimental
and simulation results. An analysis of chemical
and physical contributions of additive influence
is also presented.

MODELING TECHNIQUES AND KINETIC
DATA BASE

Computations were conducted mainly for stoi-
chiometric laminar flames burning mixtures of
air with methane, ethylene, ethane, and
methanol at atmospheric pressure. The initial
temperature of the mixtures was 298 K. The
following halogenated retardants were consid-
ered: CF,H, C,HF,, C,F,, CF,, CF;Br, and
CFI. For the calculations, the PREMIX code
was employed [16]. Therma! diffusion has been
considered. The calculated burning velocities
of stoichiometric premixed methane-air,
ethane~air, ethylene-air, and methanol-air
flames were 42.1, 46.1, 69.1, and 44.4 cm/s,
respectively. Burning velocities were calculated
by using the first order windward difference
formula (WDIF) for the convective terms, in
the PREMIX flame code [16], with the param-
eters GRAD (= 0.2) and CURV (= 0.3), and
fewer than 100 grid points. Using the central
difference formula (CDIF) with almost the
same number of grid points, the accuracy of
the calculations was improved and the values
of the burning velocity were 40.9, 44.6, 67.1,
and 40.6 cm/s, respectively. It is possible to
obtain the same values by using WDIF with
increasing the number of grid points to approx-
imately 200. Since the calculations were per-
formed with a large kinetic model (approxi-
mately 700 reactions and 80 species), central
processing unit (CPU) time for calculations
using CDIF were much longer than those using
WDIF. Thus, we used WDIF to reduce the
CPU time. Analyses of the numerical results
were completed using the National Institute of
Standards and Technology (NIST) interactive
graphics postprocessor Senkplot.

The data base used for the C/H/O system
was based on existing kinetic models [17~20]
and has been employed in an earlier study [13,
21). A comprehensive set [22, 23] of elementary
reactions for fluorine-containing C,—C, species
was used. The kinetic submodels for bromine-

and iodine-containing species were the same as
that in our earlier work [13, 24]. Tables 2 and 3
contain the thermodynamic and_ transport
properties of the bromine- and iodine-contain-
ing species, Comparison of our numerical re-
sults showed that the burning velocities were
in agreement with measurements for C.-C,
hydrocarbon flames over a wide range of
equivalence ratios [18, 19, 35, 36]. Favorable
comparisons with experimental measurements
were also made for the burning velocities of
methane-air flames with the additives CF,H,
C,HF;, CFs, CF,, and CF,Br [9-12]. Good
agreement is found between the measured and
calculated burning velocities which differed less
than 15% on average, as we showed in Fig. 1 of
Ref. 13. In addition, our computational results
were in close agreement with measurement of
the burning velocity of methane-air flames
with CFI addition [37] and ethylene-air flames
with CH,Br and CH,I addition (38] at atmo-
spheric pressure. Thus, the data base can be
expected to predict with reasonable accuracy
the influence of inhibitors on flame propaga-
tion. Although these comparisons cannot be
considered a complete quantitative validation
of the data base, they do indicate that the
simulation results are compatible with existing
experimental results and that confidence can
be placed in their reliability.

RESULTS AND DISCUSSION

Burning Velocity Dependence on Additive
Concentration

Figure 1 shows the relative burning velocity of
a stoichiometric CH,~air premixed flame as a
function of additive concentration for CF,H,
CF,, CH, F,, C,HF;, C, Fy, C;Fy, C,HF,, and
CF,Br. Figure 1a shows the experimental re-
sults [9-12] where the lines are least-squares
best exponential fits to the experimental data.
The mixture stoichiometry (the equivalence ra-
tio) is determined without taking into account
the properties of the inhibitor.

As previously mentioned, S$, was found to
have a linear dependence on retardant concen-
tration over a small range of agent concentra-
tion [1-3]. For inert compounds, a linear de-
pendence is usually obtained over a relatively


160

35.

37.

39.

Nelson, R. D. Jr., Lide, D, R., Jr., and Maryott, A. A.,
NSRDS-NBS Report No. 10, 1967.

Law, C.K. in Reduced Kinetic Mechanisms for Appli-
cation in Combustion Systems (N., Peter and B. Rogg,
Eds.), Springer-Verlag, Berlin, 1993, p. 15.

Warnatz, J., Eighteenth Symposium (International) on
Combustion, The Combustion Institute, Pittsburgh,
1981, p. 369.

Sanogo, O., personal communication.

Homann, K. H., and Poss, R., Combust. Flame 18:300
(1972).

Glassman, I., Combustion, Academic Press, New York,
1987, p. 119.

Linteris, G. T., and Truett, L., Combust. Flame 105:15
(1996).

4.

42.

43.

45.

T. NOTO ET AL,

Westbrook, C. K., Combust. Sci. Technol. 34:201
(1983).

Roberts, A. F., and Quince, B. W., Combust, Flame
20:245 (1973).

Hamins, A., Trees, D., Seshadri, K., and Chelliah, H.
K., Combust. Flame 99:221 (1994).

. Dixon-Lewis, G., and Simpson, R. J., Sixteenth Sympo-

sium (Intemational) on Combustion, The Combustion
Institute, Pittsburgh, 1976, p. 1111.

Takahashi, K., Sekiuji, Y., Inomata, T., Abe, T.,
Fukaya, H., Hayashi, E., and Inoue, G., Combust. Sci.
Technol. 102:213 (1994).

Received 19 July 1996; accepted 21 February 1997



148

fuel systems. Table 1 shows atomic inhibition
indices obtained by Fristrom and Van Tiggelen
{4]. In addition to atomic indices, Da Cruz et
al, [5] introduced some groups which improved
the correlation. This procedure is similar to
the group additivity method which is widely
used for estimation of thermodynamic and
other properties of chemical substances [6].
The group additivity procedure makes it possi-
ble to estimate the inhibition efficiencies of
different compounds. Its application is re-
stricted to small additive concentrations due to
the assumption of a linear dependence of S,.
Another restriction is the absence of some
atomic and group indices for fuel systems other
than methane.

The assumption of a linear dependence of
the burning velocity on inhibitor concentration
(Eqs. 1 and 2) is, however, not consistent with
other studies, Later, Rosser et al. [7] reported
that the reciprocal of the square of the burning
velocity was proportional to the inhibitor con-
centration for methane flames with additives
such as BBr,, PCl,, POCI;, and others. Parks
et al. [8] showed that for CF,Br addition over a
wide range of additive concentrations and mix-
ture compositions, the flame speed of pre-
mixed methane-air and propane-air flames
decreased exponentially.

Some recent studies [9-12] have shown the
nonlinear dependence of the burning velocity
on additive concentration for methane-air
premixed flames over a wide concentration
tange for CF,, CF,H, CF,H,, and other addi-

T. NOTO ET AL,

tives. We have computed the burning velocity
for C,-C, hydrocarbon-air premixed flames
with halogenated additives, showing that the
burning velocity decreases with additive con-
centration in a nonlinear fashion (13). Specifi-
cally, to decrease S, until flame extinction,
relatively large amounts of additive are neces-
sary.

The mechanisms of combustion inhibition
and suppression are a long standing question.
The mechanisms of retardant action can be
split into two categories: physical mechanisms,
when heat capacity and dilution effects domi-
nate, and chemical mechanisms, where radical
scavenging is important. Flame retardants are
usually considered to behave both chemically
and physically. Sheinson et al. [14] estimated
the chemical and physical contributions of
CF;Br effect on heptane diffusion flames
through an enthalpy balance. The chemical
contribution of CF,Br to obtain suppression
was estimated as approximately 80%. Tucker
et al. [15] studied flame extinction by the addi-
tion of inert gases and CF,Br, and inferred
that the chemical effect of the retardant de-
creased with increasing additive concentration.

The main focus of this work is to take into
account the nonlinear dependence of burning
velocity on additive concentration and to re-
consider the group additivity procedure [6] for
the evaluation of flame retardant efficiency.
This is important because in actual practice a
large concentration of inhibitor is usually ap-
plied. In addition, it is important to develop a

TABLE 1

Inhibition Indices Obtained by Fristrom and Van Tiggelen (4}

Fuel (A)" ou bc de ba ar C7
H, (1.75) 034 0.33 0.25, 0.25 1,22 b
cow) c 0.60 12 10 b
CH, (1.06) 0.25 1 1 19 109 M2
CH, (WD 05 0 b 1.6 10.6 88
CoH) 03 04 b Mt 44 b
C,H; (1) -03 16 b 13 $1 b
C,H, (1) da ad 6 25 6.1 68
CoHy (1) b b 6 13 5 b

* ) is the equivalence ratio.
” No data available,

© $¢ is assumed to equal 10.
“deny = beuy


Inhibition Effectiveness of Halogenated Compounds*

T. NOTO,' V. BABUSHOK;' A. HAMINS,' and W. TSANG
National Institute of Standards and Technology, Geithersburg, Maryland 20899, USA

‘A numerical study of the inhibition efficiency of halogenated compounds was carried out for C,-C;
hydrocarbon-air laminar premixed flames. The inhibition efficiency of CF,Br, CF1, CF/H, C,HFs, C2F,,
and CF, additives was interpreted using an additive group method. In agreement with measurements, the
calculated burning velocity decreased exponentially with increasing additive concentration over a wide
concentration range. The inhibition parameter proposed by Fristrom and Sawyer indicating inhibition
efficiency was modified to take into account the exponential dependence of burning velocity on inhibitor
concentration. The inhibition indices for halogen atoms and groups important in the inhibition process were
determined for stoichiometric conditions. The physical and chemical effects of the additives were studied.
With increasing additive concentration, the chemical influence of an inhibitor saturates and the physical
influence increases. Therefore, use of a composite inhibitor composed of a mixture of an effective chemical
inhibitor with a high heat capacity diluent may be beneficial. The contribution of physical and chemical
components on inhibitor influence are estimated near extinction. A procedure for determination of a

regeneration coefficient, which indicates an effective number of catalytic cycles involving inhibitor during the
combustion process, is suggested. The regencration coefficient of HBr in stoichiometric methane~air flame
with 1% CF,Br added is approximately 7. © 1998 by The Combustion Institute

INTRODUCTION

The burning velocity, S,, is an important pa-
rameter which characterizes the inhibition ef-
ficiency of halogen-containing flame retar-
dants. As the inhibitor concentration increases,
the burning velocity decreases due to increased
inhibitor influence. Rosser et al. [1] studied
experimentally the inhibition effect of different
additives on methane flames. The burning ve-
locity was found to decrease linearly with in-
creasing additive concentration for concentra-
tions less than 0.5% by volume, leading to the
expression

S$, =S)—-O-Cigs a)

* Certain commercial materials and equipment are identi-
fied in this paper in order to specify adequately the proce-
dure. In no case does such identification imply recommen-
dation or endorsement by the National Institute of Stan-
dards and Technology nor does it imply that the material
‘or equipment is necessarily the best for the purpose.

* Corresponding author: A. Hamins, Building 224, Room
8258, NIST, Gaithersburg, MD 20899.

© On leave from NKK Corporation, Kawasaki-Ku, Kawasaki
210, Japan,

‘On leave from Institute of Chet
bustion, Novosibirsk 630090, Russi

COMBUSTION AND FLAME 112: 147-160 (1998)
‘© 1998 by The Combustion Institute
Published by Elsevier Science Inc.

) Kinetics and Com-

where S, and S, are the burning velocities for
CH,~air flames without and with additives,
respectively, C,, is the additive mole fraction,
and ® is a constant. Rosser et al. [1) used the
parameter ® (= dS,/dC,,) as a measure of
inhibition efficiency. Halpern [2] reported that
the flame speed is a linear function of inhibitor
concentration for methane flames for small
concentrations of CF,Br, CF,Br,, CH;Cl,
CF,Cl, and CHFC!, addition. Based on the
linear dependence of S, on additive concen-
tration, Fristrom and Sawyer [3] suggested us-
ing a dimensionless inhibition parameter ®,;
for evaluation of the inhibition efficiency of
additives:

gs = [(Sp - $,)*Co,]/AS0° Cin] @)

Here, Co, is the initial concentration of oxy-
gen in the mixture. Fristrom and Van Tiggelen
[4] suggested that the parameter @,, could be
written as the sum of the inhibition indices ¢;
for each atom constituting the additive
molecule:

Ops = Md t Mode + Linde @)

Here, the subscript x indicates a halogen atom.
Atomic indices were derived for a number of

0010-2180/98/$19.00
PIT SO010-2180(97)001 14-4



158

Regeneration Coefficient

The scavenging of chain carriers by a flame
inhibitor is increased due to reactions which
regenerate the scavenging agent, or equiva-
lently due to an increase of the “acting” con-
centration of an inhibitor [44]. The superior
efficiency of CF,Br and CF,] as flame in-
hibitors, as compared to fluorinated hydrocar-
bons, is connected with the regeneration cycle
associated with HBr and HI, respectively. It is
possible to define a regeneration coefficient
which indicates the effective number of cat-
alytic cycles involving the inhibitor during the
combustion process. The regeneration coeffi-
cient is determined by the ratio of the total
concentration of scavenging agent (HBr, HI)
consumed (or produced) to the initial concen-
tration of this species which formed the agent:

regeneration coefficient
= [HBr)ott/(CF; Br] yiciar- (9)

The total HBr consumed is determined by
integration of the rates of reactions in which
HBr is consumed until a maximum H atom
concentration

THBrhowt = f Capes a. (0)

The term wyp,,; is the reaction rate of the ith
reaction consuming HBr. Figure 12 contains
the concentration dependence of the regenera-
tion coefficients of HBr and HI. The regenera-

10

3 8

0

0 0.01 0.02 0.03 0.04
Mole fraction of CF3Br, CF31

Fig. 12. Dependence of regeneration coefficients of HBr

and HI on CF,Br and CF,{ concentration in stoichiomet-

ric CH -air flames.

T. NOTO ET AL,

tion coefficient varies from 7 to S for CF,Br
concentrations from 0.5 to 4%. The regenera.
tion coefficient of HI is approximately 4. The
additional inhibition efficiency of CF,I is due
to an additional termination step, CH, + I +
M = CH,I + M, with the reaction H + CH,I
= HI + CH, [13].

CONCLUSIONS

In this work, the inhibition effectiveness of
halogenated flame retardants is analyzed by a
modified additive group procedure (3, 4] taking
into account the exponential dependence of
burning velocity. Premixed methane, ethylene,
ethane, and methanol-air flames are consid-
ered. Inherent in this treatment are assump-
tions regarding the nature of the decomposi-
tion products of the inhibitor during combus-
tion. The chemical and physical contributions
of retardant effect on flame propagation are
determined. The main results are the follow-

ing:

1. For the halogenated additives CF,Br, CFI,
CF,H, C,HF;, C,F,, and CF,, the burning
velocity of C,-C, hydrocarbons decreases
exponentially with increasing additive con-
centration over a wide range of additive
concentrations.

2. The inhibition parameter ® proposed by
Fristrom and Sawyer indicating inhibition
efficiency is modified to the expression ® =
(Co,/Cin) + In(Sy/S,) to take into account
the nonlinear dependence of burning veloc-
ity when a wide range of additive concentra-
tions is considered. The inhibition indices
for halogen atoms and groups were ob-
tained for stoichiometric premixed C,-C,
hydrocarbon~air flames.

3. A procedure for the differentiation of physi-
cal and chemical contributions of additive
effect is suggested. Increasing additive con-
centrations lead to saturation of the chemi-
cal contribution of additive effect. The sub-
sequent increase of additive action is due to
physical influence (heat capacity and dilu-
tion effects).

4. The utility of a “composite” inhibitor is
suggested. Its constituents would be an ef-
fective chemical inhibitor and an inexpen-