INFLUENCE OF PHOSPHORUS-CONTAINING ADDITIVES ON METHANE FLAME

V.Babushok, W.Tsang

National Institute of Standards and Technology, Gaithersburg, MD 20899

(vbabushok@nist.gov, wing. tsang@nist.gov)

Comparison of relative inhibitor efficiencies in terms of the reduction in burning velocities or

extinction strain rates [1-4] demonstrates that phosphorus containing compounds may have efficiencies 3-8

times higher than CF3Br(Halon 1301). Phosphorus additives are used as retardants in fire-resistant polymers

and fire suppressants (e.g., dihydrogen ammonium phosphate). Due to the need for replacing currently used

Halon 1301, studies of the fire suppression efficiency of phosphorus compounds and the mechanism for its

action are of interest.

In this work we report on results of numerical studies on the inhibition of methane flames by

phosphorus-containing additives (DMMP, TEP). The dependencies of burning velocity on inhibitor

concentration and additive decomposition rate have been analyzed and the influence of flame temperature

on inhibition efficiency is considered.

Experimental Data on the Influence of Phosphorus Compounds on Flame Propagation. There

are considerable experimental results on the influence of the addition of phosphorus compounds on burning

velocity, suppression concentrations of compounds (cup burner tests), influence of additives on an extinction

strain rate for opposed diffusion flames and the results of streaming laboratory tests. The phosphorus

compounds studied include POCI3, PSCI3, PSBr3, PCI3, PBr3, TMP, DMMP, triphenylphosphine,

monoammonium phosphate, diammonium phosphate, P3N3F6, P3N3ClFs, P3N3C12F4, P3N3(OCH3)6 and

fluroalkoxyphosphazenes.

Experimental data can be divided on the basis of inhibitor loading. At the lowest loadings comparisons

can be made on the basis of burning velocity reductions. This can be considered the maximum chemical effect

of the suppressants. With increasing additive concentrations, the contributions of heat capacity and dilution

increase. At the same time, the chemical influence of the additive decreases (saturation effect, [5]) due to the

lowering of chain carrier concentration down to equilibrium level.

Analysis of scattered data on the burning velocity of inhibited hydrocarbon flames (methane, hexane,

n-heptane, hydrogen) using a procedure described earlier[3] shows that it is possible to separate studied

compounds according inhibitor efficiency into two groups: 1) PSBr3, PBr3, PSCI3, PCI3, POCI3 which have

inhibition efficiencies higher than CF3Br 6-7 times more, and 2) OPCHiOCH3)2 (DMMP), (CH3)3PO4 (TMP)

and (C2HsO)3PO (TEP) which have a coefficient of efficiency relative to CF3Br of3-4. The scattered data

on suppression concentrations of phosphorus compounds are however not in agreement with these results.

It is possible to expect that extinction concentrations are in the range 0.5-5% by volume.

Calculational procedure and kinetic model. The modeling results described were derived from the

CHE1v1KINsuite of programs. Compiled kinetic model is based on the data suggested for the analysis of the

influence ofPH3 products on the recombination of hydrogen atoms in products of hydrogen flame [6], and

kinetic models [7,8] used for describing destruction ofDMMP and TMP in low pressure hydrogen flames.

Reactions of simple P-containing species with radicals and some intermediate species of methane system to

reflect methane combustion have been included. It has been assumed that P-species are non-reactive to the

stable hydrocarbon species. There exist some indications on low reactivity of phosphorus-containing species.

Some reactions have been added to the scheme to complete the reaction pathways for the consumption of

some of the species. The model has been adjusted to take into account recent data for P-species.

587
i 1.0 '\

OJ 0.9

~ 0.8 ~~ c • ~~ 0.7 ~~ . -

N- 0.6 0

EL

0.20

0.15

0.5 0.05

0.10

Additive concentration, molar %

Fig. 1 Burning velocity dependence on inhibitor

concentration (stoichiometric air/methane flame,

1 atm). Open circles - DMMP additive, rate

constants of[6]; closed circles - DMMP, modified

model; triangles - TEP additive.

DMMP

]

CH3P02 = CH3 + 1'02

HOP02/, HOPO

,~~1'02

Fig.2 The abbreviated inhibition cycle of DMMP

influence.

Thermochemical data are mostly from [9,10,11,6]. For the modeling of the TEP influence the overall

decomposition kinetics has been used [12].

Additive influence on burning velocity. Inhibition mechanism. Fig. 1 contains calculated burning

velocities as a function of the concentration of DMMP and TEP. These results show that application of

original rate constants in the model'ofTwarowski[6] leads to efficiencies ofDMMP relative to CF3Br that

are about a factor of 1.5 larger. Following the decomposition of DMMP in the flame zone the main Pcontaining species are P02' ROPO and ROP02. Sensitivity and reaction pathway analysisshow two main

inhibition cycles with P02 (Fig.2) leading todecreases in concentrations of radicals:

1) R + P02 + M = ROPO + M

OR + HOPO = R20 + P02

R + ROPO = R2 + P02 °

+ ROPO = P02 + OR

2) OR + P02 +M = ROP02 +M

R + ROP02 = H20 + P02

Analysis of the influence of rates of different reactions shows that reasonable adjustment of the rate constants

of addition reactions H + P02 + M = ROPO + M and OR + P02 +M = ROP02 +M leads to approximate

agreement with experimental data on the efficiency ofDMMJ> and TEP relative to CF3Br.

Influence of decomposition rate on inhibition efficiency. It is known that phosphorus compounds

have a wide ra.ri.geof thermal stability. Overall activation energies of decomposition are in the range, 15 - 90

kcallmol. The influence of the decomposition rate has been studied using global kinetics for decomposition

to P02 or to ROPO through varying the activation energy of decomposition. Fig.3 demonstrates that, for an

assumed kinetic model and using experimental initial conditions, activation energies less than 60 kcallmol

show no effect on the burning velocity. Further increase in the activation energy leads to decrease of inhibitor

efficiency. At approximately 90-100 kcallmole the influence of additive disappears. For values in the 20-30

kcallmol range the inhibitor decomposes in the low temperature region of the flame zone or before 1000 K

is achieved. For 80 kcaVmol, the degree of decomposition at the position of maximum H-atom concentration

constitutes 60-70%.

588

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!

I

i

i

I

i

,I

J1 I
1.0 >- -

~0

CD

>

01

0.9 c ..a

-CL:J

-0

Q)

0.8

~ -

0

ELz0 0.7 20

100 46080

Activation Energy, keal/mol

Fig.3 Dependence of burning velocity on the

decomposition rate. Closed circles - HOPO

product; open circles - P02 product

(stoichiometric air/methane flame, 1 atm,

DMMP 0.09%).

VI

.......••

15

E 0- Q) L 0

<1J CDVI 10 -0

>-

-00 - Q) 5 >

01

C :J illL

C 0

220026008 2400 3000

Flame Temperature, K

FigA Dependence of decrease of burning velocity on

flame temperature. Stoichiometric methane/air flame,

0.09% DMMP.

Influence of flame temperature. In earlier works [13,14,15] the possible promotion effect of

phosphorus compounds has been discussed. According to Hastie and Bonnell [13], the increase of flame

temperature may lead to decrease of inhibitor efficiency. The estimated crossover flame temperature point

is near 2350K. Numerical calculations do not show any promotion effect ofD.tvfMP in the temperature range

2100-2900K. FigA contains the dependence of burning velocity decrease on flame temperature. For these

calculations, the decrease of nitrogen content in mixture 2y02+xN2+yCH4+0.09%D.tvfMP is used to provide

the increase in the flame temperature. It can be seen that flame temperatures higher than 2500 K are required

before inhibitor influence is lessened. In fact, the crossover point, where promotion becomes dominating, was

not observed until 3000K. The decrease of the inhibitor effect is mostly due to the relative increase of rates

of formation of species in the radical pool in comparison with recombination rates ofP-species with chain

carners.

Saturation of inhibition effect. Calculations have been carried out across the entire range of additive

loadings or until the level of suppression. These are all based on a gas phase model without taking into

account the condensation processes. The modeling results show "strong" saturation effect with the increase

of the inhibitor concentration, e.g., decrease of burning velocity down to 10cm/s requires loading less than

0.9 %; an additional 1.4 % ofD'MMP is needed to decrease burning velocity from 10 cm/s down to the

extinction level. Such strong saturation effect leads to a substantial increase in extinction concentration

(2.3%) and to a decrease of inhibitor efficiency relative to CF3Br.

Additionally, Table 1 contains extinction concentrations for iron pentacarbonyl (gas phase model) and

for a perfect inhibitor [16], showing the possible minimum level of inhibitor concentration required for flame

extinction. The analysis of saturation effect strongly supports the use of composite inhibitors [5].

589
Table 1.

DMMP

Perfect Fe(CO)s CF3Br* inhibitor * *

0.3 0.05 -3.5.5

* Calculated values depend on assumed rate constants.

* * Rate constants of an inhibition cycle are 5.E 13 cm3/( mol s).

Summary. A first cut kinetic model for inhibition by phosphorus compounds on methane flames has

been developed. The main contribution to inhibition effect is due to reactions with simple P-containing

species: POb HOPO, HOP02. The important reactions of the inhibition cycle appear to involve reactions

of species HOPO<=>P02<=>HOP02 and their scavenging reactions with the chain carriers. To obtain the

agreement with experimental data on relative inhibition efficiency in comparison with CF3Br, the increase of

the rates of P02 radical recombination reactions in 5-10 times is required. For studied initial conditions

(stoichiometric methane mixture) the decomposition processes of pho,sphorus compounds do not show

influence until the level of overall activation energy reaches 60 kcal/mol. The calculations do not demonstrate

the possible promotion effect ofDMMP on methane flame for the flame temperature range 2200-2900K.

Calculations for additive level of flame suppression concentrations demonstrate the strong saturation effect.

Acknowledgments. We wish to thank E.M.Fisher and V.V.Mokrushin for helpful discussions and

lH. Werner for the kinetic model [7]. The financial support of Navy Research Laboratory for VB is gratefully

acknowledged.

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