M. A. MACDONALD, T. M. JAYAWEERA, E. M. FISHER,* and F. C. GOULDIN

Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York 14853, USA

Inhibition of Nonpremixed Flames by

Phosphorus-Containing Compounds

Phosphorus-containing compounds (PCCs) are proposed as viable alternatives to current, ozone-destroying,

flame-inhibiting agents. An opposed-jet burner apparatus was used to study the effectiveness of two

low-vapor-pressure PCCs, dimethyl methylphosphonate (DMMP) and trimethyl phosphate (TMP), in extinguishing a nonpremixed methane-air flame. The global extinction strain rate was determined as a function of

dopant loadings. Tests were also conducted using nitrogen as an inert additive for reference. Results

demonstrate that these phosphorus-containing compounds are significant inhibitors of nonpremixed methaneair flames when introduced into the oxidizer stream, 40 times more effective than nitrogen on a molar basis. A

novel technique for measuring the extinction strain rate while maintaining a constant dopant level in one gas

stream was developed. © 1998 by The Combustion Institute

[20]

POC

mIX~

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. as to whether their effectiveness in these applications can be attributed to gas-phase reactions

involving phosphorus. The use of various PCCs

as fire retardant additives to plastics [3, 4] has

increased dramatically [5] in recent years, with

the 1998 estimated U.S. market for organophosphorus fire retardants at 80 million pounds per

year [6]. In this application, the relative importance of gas-phase chemistry and solid-phase

effects such as char promotion has been debated, with recent work suggesting comparable

importance for the two mechanisms [4, 7-10].

The PCC mono ammonium phosphate has been

used for many years as a dry chemical fireextinguishing agent [11], and has also been

found to be very effective in preventing dust

explosions [12, 13]. Since it is applied as a

powder, it is possible that physical properties

may contribute substantially to its effectiveness.

Finally, phosphorus is often applied in combination with halogens, making it difficult to

assess the contribution of the phosphorus. For

example, a mixture of trialkyl phosphates and

CF3Br for fire-fighting applications has been

patented [14]. Phosphorus and halogens often

appear in the same molecule as well: a recent

evaluation of phosphorus compounds as halon

replacements focused on fluorinated phosphazenes and phosphines, showing good flame suppression properties for these compounds in cup

burner and streaming tests [15, 16].

Well-characterized laboratory-scale investigations of the gas-phase flame inhibition properties of PCCs are sparse, and in some cases

COMBUSTION AND FLAME 116:166-176 (1999)

© 1998 by The Combustion Institute

Published by Elsevier Science Inc.

*Corresponding author.

0010-2180/99/$19.00

PH SOOlO-2180(98)00034-0

INTRODUCTION

The manufacture of widely used fire-extinguishing agents CF3Br (Halon 1301) and CBrCIF2

(Halon 1211) has recently been banned in the

United States because of these compounds'

deleterious effect on the ozone layer. Consequently, there is great interest in finding highly

effective, ozone-friendly alternative agents [1].

At first, much effort was directed toward investigating ozone-friendly alternatives among halogenated compounds with physical properties

similar to those of CF3Br and CBrClF2. However, the relatively poor flame-suppression performance of these alternatives [2] has led to

interest in other families of chemicals, despite

less favorable physical or toxicological properties in many cases. These other families of

candidates include: metal compounds such as

iron pentacarbonyl, powders such as sodium

bicarbonate, and phosphorus-containing compounds (PCCs). In this paper we report on an

experimental investigation of the gas-phase

flame-suppression effectiveness of two PCCs.

This is, to our knowledge, the first study in

which PCCs' effects on the extinction of a

counterflow flame have been determined. We

also report a novel method of approaching

extinction that is convenient for evaluating the

effectiveness of low-vapor-pressure additives.

PCCs are used in several practical applications in fire fighting, but there is some ambiguity
167

applictions

PCCs

1.] has

, with

phosIs per

nporphase

n delrable

7-10].

been

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as a

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been

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(1999)

FLAME INHIBITION BY PHOSPHORUS COMPOUNDS

ambiguous or contradictory. Korobeinichev et [27]. Partial validation of the radical recombial. investigated the flame structure of low-pres- nation hypothesis is provided by flow reactor

sure, premixed stoichiometric Hz/Oz/Ar flames experiments showing that the addition of PH3

with the nonhalogenated PCC additives trim- (phosphine) to Hz--Gz combustion product

ethyl phosphate (TMP) [17] and dimethyl meth- gases increases H + OH recombination rates

ylphosphonate (DMMP) [18]. They compared following laser photolysis [26]. Flame experimolecular oxygen profiles in flames with and ments have also shown that the presence of

without the PCC additive, interpreting more PCCs affects radical levels. The addition of

rapid oxygen consumption as flame promotion small quantities of POCI3, a halogenated PCC,

and less rapid oxygen consumption as flame produced lower OH levels in nonpremixed

inhibition. Both DMMP and TMP acted as methane and ethene flames than did other

inhibitors in the low-temperature zone near the halogenated compounds not containing phosburner surface and as promoters in the higher- phorus [28]. The addition of TMP lowered H

temperature region downstream. In contrast, levels in premixed low-pressure Hz/OzlNz

Fenimore and Jones [19] found that TMP addi- flames under rich conditions with high levels of

tion produced no change in the oxygen con- nitrogen dilution, and raised H levels under

sumption rate in a low-pressure, rich, Hz/Oz/Ar stoichiolIletric, less dilute conditions [20]. This

flame. However, in an atmospheric-pressure result agrees qualitatively with temperature deflame with higher argon levels, they observed a pendence of inhibition observed by Hastie and

reduction in the oxygen consumption rate with Bonnell, described above [20].

increasing TMP loading. A more comprehen- A significant obstacle to the use of PCCs as a

sive study was conducted by Hastie and Bonnell flame-inhibiting agent is that many of these

[20] with TMP, triphenylphosphine oxide, and compounds, including the two investigated

POCl3 additives in premixed and non pre- herein, have low vapor pressures «1 torr) at

mixed atmospheric-pressure propane, meth- ambient temperature [29]. Some PCCs also

ane, and hydrogen flames. PCC effects were have a significant heating value due to alkyl and

evaluated from measurements of H radical alkoxy groups attached to the phosphorus atom,

concentrations and burning velocities. Inhibi- and some, especially halogenated PCCs, are

tion was observed for rich and lean flames with highly toxic [15]. However, if supporting evitemperatures up to 2350 K. At higher temper- dence is found for the hypothesis that the only

)I atures, well above those typically of concern in important contribution of the PCC to flame

fire suppression applications [21], flame promo- inhibition is the central phosphorus atom, then

tion was observed. Lask and Wagner reported further studies of flame inhibition by PCCs

inhibition of premixed hexane/air flames by can investigate alternative forms of the parent

air-side addition of several halogenated PCCs compounds that might be less toxic and more

and one nonhalogenated PCC, TMP, on the amenable to delivery to the flame. It is imporbasis of their observations of laminar flame tant to note that not all PCCs exhibit high

speeds [22]. toxicity. PCCs range in their toxicity from

It is thought that phosphorus-containing rad- nontoxic, essential food constituents and FDAicals, such as HOPO and HOPOz (formed from approved additives, to some of the most toxic

PCCs during combustion [17, 18]) catalyze the compounds (chemical warfare agents) known

recombination of the important combustion [30].

radicals, Hand OH, slowing the overall reaction In this study, a nonpremixed methane-air flame

rate and thus inhibiting the flame [20]. Several was used to study the inhibiting effects of two

mechanisms for catalytic cycles resulting in H + PCCs: dimethyl methylphosphonate (DMMP)

OH -7 HzO and H + H -7 Hz, have been [CAS #756-79-6, P(=O)(CH3)(OCH3h], and

suggested [20, 23-26]. Since it is the phospho- trimethyl phosphate (TMP) [CAS #512-56-1,

rus-containing radicals that participate in the P(=O)(OCH3h]' Experiments were performed

recombination mechanism and not the parent using an opposed-jet burner apparatus. This

PCC, it has been hypothesized that the form of configuration has been used extensively to evalthe parent compound is relatively unimportant uate the performance of flame inhibitors using
168

Exhaust

+-- Measured

Oxidizer

Flow

Measured

-Fuel Flow

Fig. 1. Schematic of opposed-jet burner.

experimental, numerical, and analytical techniques, (e.g. [2, 31-33]). The counterflow configuration is useful for studying flame-suppressing agents because the flame is thermally

isolated and quasi-one-dimensional along the

centerline [34, 35]. The flame strength can be

characterized by the strain rate at extinction [36,

37].

The usual method of performing extinction

measurements is to slowly increase both reactant flowrates until the flame abruptly extinguishes [38]. This approach poses practical difficulties in establishing and maintaining

constant inhibitor loadings for low-vapor-pressure flame-inhibiting compounds. An alternative, novel method for performing extinction

measurements is presented herein which provides a convenient method for achieving constant inhibitor loadings.

EXPERIMENTAL

Apparatus and Materials

Experiments were conducted with an opposedjet burner, shown schematically in Fig. 1. Massflow controllers, supplied by MKS Industries,

were used to measure reactant flow rates. Most

of the extinction measurements were conducted

using premixed oxygen and nitrogen (21 ± 0.2%

oxygen mole fraction) or Ultra Zero Grade air

(21 ± 2% oxygen mole fraction, total hydrocarbon content <0.1 ppm) as the oxidizer and

methane (99% pure) as the fuel. All gases were

supplied by MG Industries. For the extinction

measurements with flame and stagnation plane

coincident, oxygen (99.994% pure) was mixed

M. A. MACDONALD ET AL.

with nitrogen (99.998% pure) in measured proportions to form the oxidizer stream, and the

methane fuel was diluted with nitrogen

(99.998% pure). Thc burner was aligned vertically with the lower tubc used as the fuel Source

and the upper tube as the oxidizer source.

Results of a few experiments conducted with

the reverse orientation to study the effect of

buoyancy showed no significant change in extinction strain rate. The burner was constructed

from straight, open glass tubes 30 cm long with

an ID of 0.98 cm, and a separation distance of

0.95 cm between opposing nozzles. Annular

sheath flows of nitrogen are provided through

2.22-cm-ID glass tubes. The sheath tube exits

are offset by approximately 1 cm, upstream of

the reactant tube exits, to minimize the impact

of the sheath flow on the development of the

reactant flows. A flat flame is produced when

the oxidant and fuel stream velocities are such

that the flame is situated in the central 6 mm of

the space between the burner nozzles. When

the flame is close to either' nozzle, it exhibits

curvature toward that nozzle; curvature is

pronounced when the flame is within 0.2 mm

of the oxidizer nozzle or within 1.9 mm of the

fuel nozzle. The entire burner is isolated in a

glass enclosure for control of exhaust gases.

This enclosure is purged with nitrogen and

maintained slightly below atmospheric pressure.

The enclosure has a glass window on one end,

approximately 25 cm from the centerline of the

flow. A video camera, located a few centimeters

from this window, was used to measure flame

position. Measurements of flame position were

made from magnified images from the camera,

displayed on a television screen. Flame position

was measured relative to the oxidizer nozzle exit

plane using the separation gap between the

nozzles for reference.

The chemically active flame inhibitors used

during this investigation, DMMP and TMP,

supplied by Aldrich Chemical Company (each

97% pure), are liquids at room temperature

with low vapor pressures (less than 1 torr at

ambient temperature). In order to maintain

sufficient concentrations of the PCC in the

vapor phase, the reactant lines were heated to

approximately 100°C with electrical heating

tapes. Wall temperatures on the outside surface

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af

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st
TFLAME INHIBITION BY PHOSPHORUS COMPOUNDS 169

il of the heated lines were measured with adhesive ditions: reactions, diffusion, and viscous effects ~ thermocouples. The temperatures of the reac- must be confined to a narrow region around the

tant streams 10 cm upstream from the exit of stagnation plane, and plug flow boundary conthe nozzles were maintained at 100 ± 1°C via ditions must be imposed at the nozzles. Even

active control of the sheath flow temperature. under conditions where these restrictions do not

These gas temperatures were measured by apply, the global strain rate expression is useful.

sheathed thermocouples in direct contact with Recent measurements [43] performed in an

the gas flow. The liquid PCC dopant was added open-tube burner with geometry very similar to

to the reactant streams via a syringe pump that our own have found a nearly proportional relaprovided a constant volume flow. The gas flow tionship between the aq of Eq. 1 and the

at the injection site was preheated to approxi- maximum local velocity gradient on the oxidant

mately BO°C for a rapid vaporization of the side of the flame, as measured by laser-Doppler

incoming liquid dopant. The liquid density, sy- velocimetry~ The same proportionality was

ringe size, and motor speed fixed the mass flow found to hold for methane-air and propane-air

rates of the dopants. Previous pyrolysis experi- flames, with and without flame-inhibiting addiments with DMMP and TMP [39, 40] have tives in the air stream. Thus it appears that Eq.

shown that thermal decomposition is negligible 1 may be used to determine relative changes in

below 1000 K; thus the preheating of the reac- extinction strain rates obtained on the same

tant streams should have no effect on their apparatus. In particular, the ratio of doped to

chemical composition. undoped global strain rate computed using Eq.

1 should be equal to the ratio of doped to

undoped local oxidant-side velocity gradients.

The validity of this approach is borne out by the

good agreement between our normalized results

and those of other researchers for inert additives, as described in the Results and Discussion

section.

In determining extinction conditions for use

in calculating aq, we made use of a novel

method of approaching extinction, motivated by

a need to minimize transient effects of dopant

loading. Commonly, extinction studies with opposed-jet burners adjust the strain rate by varying both the oxidizer and fuel streams simultaneously so that the flame position remains

constant [31], or so that a momentum balance

between the two streams is maintained [38].

This requires varying the mass flow rates of both

reactant streams, and results in changing concentrations of the inhibiting agent when the

agent is delivered via a constant-mass-flowrate

syringe pump. The resulting transients in the

adsorption/desorption of the agent on walls

make it difficult to predict the concentration of

agent at the exit plane of the doped reactant

stream at extinction.

The novel method presented here is an alternative technique to circumvent the difficulties

associated with dopant loading transients. During our experiments, the concentration of dopant in the relevant reactant stream (fuel or

In Eq. 1, L refers to the separation distance

between the nozzles, V is the average stream

velocity at extinction, P is the stream density,

and the subscripts 0 and F refer to oxygen and

fuel respectively. This expression, derived by

Seshadri and Williams [42], will be referred to

as the global strain rate. It is equal to' the

velocity gradient on the oxidant side of the

stagnation plane under certain restrictive con-

(1)

Determination of Extinction Strain Rates

The effectiveness of additives as flame inhibitors is determined by measuring the dependence of extinction strain rate on additive loading. The inverse of the. strain rate is one of

several related measures of the characteristic

diffusion time of the flame [41]. In many experiments, the value of the strain rate is evaluated

experimentally as the velocity gradient on the

oxidant side of the flame, measured using laserDoppler velocimetry [31]. As our primary interest is to determine the relative reduction of

extinction strain rate due to the additive, we

chose to approximate the strain rate at extinction using the following expression involving

global flame parameters [42]:

aq _- 2Va L ( 1+ VaVPa VF JP;.) ~ '

ed promd the

itrogen

d vertisource

source.

:d with

'fect of

III extructed

I1gwith

mce of

.nnular

hrough

Ie exits

earn of

impact

of the

j when

:e such

mmof

When

xhibits

:ure IS

1.2mm

of the

~din a

gases.

:n and

: pres-

:::T AL.

1e end,

of the

meters

flame

n were

amera,

osition

de exit

~n the

s used

TMP,

(each

. :rature

:orr at

tin tain

in the

lted to

.eating

urface
10

n n

rfJ

lJ

u

...

• Undoped

300 ppm DMMP

n SOOOO ppm N2

I

M. A. MACDONALD ET AL.

2 468

Observed Flame Location

[mm from oxidizer nozzle]

-'. ..1 ... . __ , __ l , .._ ..•.. L. __i._ 0

380 !

r

360f

340

3201

i

300

280 ,.

U,l!) 1I

260 ,h D'lJ nil [J

240 ~-'-'

o

result of the coarsely quantized mass flow rates

of dopant available from the syringe pump.

Because of the inhibitory effect of the DMMP

or Nz additive, these data sets have markedly

lower mean global extinction strain rates than

the data for undoped methane vs. air flames.

However, both data sets show very similar systematic trends with flame position, and both

have only small variations in extinction strain

over the central acceptable region between a

and 7 mm.

The existence of the same acceptable region

for a range of flame extinction strain rates

establishes the validity of our method for approaching extinction. Provided that the flame

position at extinction lies within this region, the

results should be consistent to within 2% of

those obtained using more conventional methods that maintain a nearly constant flame position. Caution should be exercised in using this

method to measure small changes in strain rate

because of the systematic variation of aq within--

the acceptable region.

The nearly constant extinction strain rates

over the central region are consistent with previous computational findings. Nishioka et al.

compared two methods of approaching extinction: increasing air and fuel stream velocities

Fig. 2. Variation of global extinction strain, u,,' with observed flame position at extinction. Results shown are for

undoped nonpremixed methane-air flames, with air-side

loadings of 300 ppm DMMP and with 50,000 ppm N2. 2s1 = 0.054.

RESULTS AND DISCUSSION

We took an empirical approach to validating

our novel method of approaching extinction.

Experiments were performed over a large range

of flow rates of the methane and air streams,

achieving extinction with the flame in a variety

of positions. To our knowledge, this is the first

experimental extinction study in which flame

position is varied systematically over a broad

range. Figure 2 shows the resulting global extinction strain rates (aq) as a function of the

observed flame position. In this figure, flame

positions inside either nozzle are plotted at the

appropriate nozzle location (0 or 9.5 mm from

oxidizer nozzle). Extinction strain varies systematically with flame position, but the variation is

small over a large central portion of the region

between the burner nozzles. The global extinction strain rate varies less than ±2% from a

mean value of 359 S-1 for flame positions

between 0 and 7 mm from the oxidizer nozzle.

This region of nearly con~tant global extinction

strain rates will be referred to as the acceptable

region. For experiments in which extinction

occurs with the flame outside the acceptable

region, large deviations in aq are observed. Also

shown in Fig. 2 are sets of data from two series

of experiments conducted with a 300 ppm doping of DMMP and 50,000 ppm doping of Nz,

both added to the oxidizer stream. The limited

number of data points in the DMMP series is a

oxidizer) was fixed by maintaining corlstant

flowrates of all the constituents of the doped

reactant stream. After waiting a sufficient time

for equilibrium conditions to be established,

we approached extinction by varying only the

flowrate of the undoped stream. One consequence of using this method is that the position of both stagnation plane and flame vary

during the extinction experiment. As described in the following section, this was

found to have only a minor influence on the

measured extinction strain except when the

flame or stagnation plane was very close to

one of the nozzles.

Validation of Method of Approaching

Extinction

170
Fig. 3. Variation of global extinction strain, Qq, with observed flame position at extinction. Results shown are for

nonpremixed methane/nitrogen-oxygen/nitrogen flames,

with ZSI = 0.50.

10

171

4 6 8

Observed Flame Location

[mm trom oxidizer nozzle]

o 2

280

380

~

~ 360

<U

c <a ••••

340 j J ~

.~ -v:' • It c if] .S .. ,: .~ ...•. I .. t)

• •

.5 ~ 320 ~Ci3

.J::J0

(3 300

location of the stoichiometric contour (the presumed flame location), relative to the stagnation plane. If the reactants have equal diffusivities, setting ZSI = 0.50 places the flame at the

stagnation plane. Since the value of ZSI depends

on two independent variables, YF and Yo, fixing

ZSI leaves one degree of freedom on reactant

concentrations. Unique conditions for the ZSI =

0.50 tests were established by choosing the mass

fraction of oxygen to give approximately the

same undoped global extinction strain rate as

was obtained for the undiluted methane-air

flame. Results from these conditions, ZSI =

0.50 with Yo = 0.387 and YF = 0.097, are

shown in Fig. 3. During these experiments, in

which the oxidizer stream was mixed from separate oxygen and nitrogen source bottles, it was

found that the global extinction strain rate was

very sensitive to the concentration of oxygen,

consistent with experimental results from Puri

and Seshadri for diluted methane-air flames

[38]. This sensitivity contributed to the greater

scatter in the ZSI = 0.50 tests than in the ZSI =

0.054 tests, which were conducted with a premixed oxidizer source. Nonetheless, the global

extinction strain rate varies less than 3% of the

mean value of 336 S-l for flame locations

between 0 and 9 mm from the oxidizer nozzle.

FLAME INHIBITION BY PHOSPHORUS COMPOUNDS

(2)

while keeping them equal to each other, and

increasing air stream velocity while holding fuel

stream velocity constant [44]. The resulting calculated global extinction strain rates for H2/N2

vs air flames differed by less than 1%. Our

larger (±2%) experimentally observed variation

of extinction strain rates with flame position can

tentatively be attributed to effects not modeled

in the flame calculations of Nishioka et aI.,

notably the effects of the flame on velocity

boundary conditions and heat losses to the

burner nozzles. Of these, effects on the flow

field appear to be more important. Laminar

convective heat transfer losses are estimated to

amount to only a few percent of the chemical

enthalpy released during combustion, even if

the flame is inside the nozzle.

The asymmetrical dependence of extinction

strain rate on flame position appears to be due

to the displacement of the flame to the oxidant

side of the stagnation plane, which is conveniently characterized using the stoichiometric

mixture fraction, ZSI' The stoichiometric mixture fraction, Zsp is the fraction of the material

present at the stoichiometric contour that originated in the fuel stream. ZSI can be evaluated

from reactant compositions in the nozzles and

the stoichiometry of the overall combustion

reaction, using Eq. 2:

ZSI = Yo,-oo/( (~~~:;)YF'+OO + Yo,-oo),

where Y is mass fraction, MW is molecular

weight, v is the stoichiometric coefficient for

complete combustion, the subscripts 0 and F

refer to oxygen and fuel respectively, and the

subscripts ±oo refer to conditions at the fuel and

oxidizer nozzles. The undiluted methane-air

flame used for the first series of experiments in

Fig. 2 has ZSI = 0.054. Other researchers have

found that, near extinction, this corresponds to

a displacement of the flame of approximately 1

mm to the oxidizer side of the stagnation plane

[34, 35].

The importance of this displacement was

investigated by performing a series of measurements in which flame and stagnation plane were

coincident. By varying the dilution of the fuel

and oxidizer streams it is possible to move the

h obre for

r-side

ZSI =

'AL.

~

I·

rates

Imp.

iMP

:edly

than

meso

sysboth

train

~n a

10

gion

:ates

apame

, the

b of

leth-

)OSIthis

rate

ithin

'ates

pret aI.

tincities
Fig. 4. Observed flame positions as a function of predicted

distance to the stagnation plane, S (based on plug flow

model [42]). Results shown are for nonpremixed flames of

methane-air (ZSl = 0.054), and methane/nitrogen-oxygen/

nitrogen flames, (ZSl = 0.50), near extinction.

c:

.~

ii5

c:

.9

t5

c:

~

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ro

.0

o

(5

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.t:!

ro

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i

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Fig.

bas, OXH

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hea

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for

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ra!

fre wi1 att ex] str se( re~ an

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M. J\. MACDONALD ET AL. ~. FL

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j

Initial tests were conducted with undoped reactant streams at ambient temperature to compare results from our experimental apparatus

with those of other researchers. For undiluted

methane burning in air, our measured global

extinction strain rate was 296 S-I. When the

reactant streams were preheated to 100°C, the

measured value for aq increased to 360 S-I.

Variations in aq up to 80 S-I were observed with

different bottles of high purity compressed air,

and are attributed to slightly different batch

mass fractions of oxygen. However, extinction

measurements on undoped flames using the

same oxidizer source are consistent to within ±5

s-1. All results for doped flames presented

herein are normalized to undoped experiments

under identical conditions, i.e., using the same

oxidizer source.

The unheated global extinction strain value

we measured is comparable to the value found

by Puri and Seshadri [38] of 280 s- I and to the

value of 255 S-I, computed from flow rates at

extinction reported by Papas et al. [31] using

Eq. 1. All three numbers fall within the range

of variation attributed to differences in composition of high purity air batches. This agreement is surprisingly good in light of the

significantly different boundary conditions

and aspect ratio of the burner used relative to

that of PuIi and Seshadri. Agreement with

literature values of local extinction strain rate

is not as good; measurements and calculations

when it is near the middle of the separation gap.

The data presented in Fig. 4 shows flame positions for only high strain.cases (275-360 S-I). It

was found that, in general, the steepness of the

sigmoidal curve increased with increasing strain

rates. For purposes of conducting experiments

with the new method, it suffices to observe that

the flame location, for each condition, is a

well-correlated one-to-one function of this parameter S. Therefore, one can map the experimentally determined acceptable region in terms

of S, and direct observation of flame position

becomes unnecessary. For Z,t = 0.0544, this'

region is 3.50 mm < S < 5.25 mm. All extinction results reported below were for flames

inside this acceptable region.

Extinction Results

6

(3)

DO

••

o

o

•

•

o o •

0-1· •

• • 0

~ 0

~ ~

o

4 5

S [mm from oxidizer nozzle]

• z~= 0.054

o Z" =0.50

o

o • • •

3

8

o

S =L/( 1 + Vo~ Vp {P;).

Figure 4 shows a plot of observed flame location

for both Zst = 0.50 and Zst = 0.054 against the

parameter S. It is interesting to note that the

observed flame location does not vary linearly

with the predicted location of the stagnation

plane, but instead follows a sigmoidal curve,

being much more sensitive to flow variations

Within experimental scatter, results with Zst =

0.5 are symmetrical about the center of the

region between the two burners, as would be

expected for a flame coinciding with the stagnation plane.

Although the acceptable region of Fig. 2 is

defined in terms of the observed flame position,

in practice it is not necessary to determine by

experimental observation the flame location at

extinction for all measurements. Rather, a parameter S is introduced which can be calculated

from the reactant stream properties at the nozzles. S approximates the distance from the

oxidizer nozzle to the stagnation plane and is

given by the expression in Eq. 3, again derived

by Seshadri and Williams for plug flow boundary conditions [42]:

172
FLAME INHIBITION BY PHOSPHORUS COMPOUNDS

Fig. 6. Inhibition of nonpremixed methane-air flames,

based on reduction in the global extinction strain rate, by

oxidizer-side addition of the phosphorus-containing compounds TMP and DMMP. Also shown are reported literature values for inhibition by CF3Br.

173

500 1000 1500 2000

Dopant Loading [ppm]

00:- ___ • 0 -":- -- -. 00 '-

- 0 ---- - o 0 - • 0 0

8u 0

010 0

00

0 o ~ 0 0

TMP 0

0 0

DMMP

- - CF 3Br (ref 49) - - --CF3Br (ref 50)

0

CF3Br (ref 31)

a

0.9

0.8

0.7

0.6

0.0544. This requires some nitrogen addition to

the fuel-side flow in contrast to our experiments

in which nitrogen was added only to the oxidizer

stream. Previous experiments by the authors

[47] indicate that correcting for the effect of this

fuel-side addition would lower our measured

extinction strain rates by about 3%, resulting in

an overall discrepancy of 10% with the Puri and

Seshadri data.

Figure 6 shows our results for PCC addition

to the air stream of methane-air flames. Again,

points represent averages of at least three

measurements, and are normalized by the

undoped extinction strain. For DMMP addition, the global extinction strain rate is observed to decrease linearly with additive mole

fraction. Experiments indicate that at a mole

fraction of 1500 ppm, aq is reduced by 35% of

the undoped value. Detectable inhibition

(~3% reduction in aq) is found at loadings as

low as 100 ppm. For mole fractions below 100

ppm, preheating of reactants was not necessary to prevent condensation of DMMP. This

allowed for ambient-temperature testing of

low DMMP loadings which demonstrated inhibition effectiveness similar to that found in

the preheated experiments. Figure 6 also shows

the results from TMP-doped flames. TMP is

seen to exhibit similar inhibitory properties to

Dopant Loading [ppm]

,....----,--~-- 

, <I) 

ro 0', ro

o. a::

, c:

0' .~

O· 0 i'i5

0.9

, c:

0 0

, 

'0 c:

0 , ~ 

, w 

o '

co 

, ..c 

0 

0.8

0

Heated N2 ,

(3 o. 

0 -c , Unheated N2 <I) 

.t:! 

,

- - N2 (ref 38) co 

E 0 0

z

,

15000 30000 45 60000

Fig. 5. Inhibition of non premixed melhane-air flames,

based on reduction in the global extinction strain rate, by

oxidizer-side addition of nitrogen with and without preheated reactants. Also shown are reported literature values

for nonpremixed methane-air flames diluted with nitrogen.

range from 340 to 400 S-:I [31, 35, 45] and

from 354 to 544 S-I [35], respectively. We

attribute the discrepancy of our measurements

with these values to our use of the global strain

expression under conditions where it is not

strictly valid, as discussed in the Experimental

section. Comparable differences between global

and local strain rates are observed by other

researchers [31].

Further validation experiments were conducted with nitrogen as a chemically inert

flame-inhibiting additive. Normalized extinction

strain rates for nitrogen addition to the air side

of a methane-air flame are shown in Fig. 5. Each

~data point represents the average of at least

three measurements, and is normalized by the

extinction strain rate for a pure methane-air

flame. Extinction strain rate drops linearly with

increasing loading of this additive. Results are

the same regardless of whether or not the

reactants are preheated to 100°C, consistent

with the findings of Trees et al. [46]. The

nitrogen dilution data can be compared to

room-temperature data of Puri and Seshadri

[38], also shown in Fig. 5. Agreement is good,

with least-mean-square slopes agreeing within

7%. However, it should be noted that the

. diluted flame experiments of Puri and Seshadri

were conducted at a fixed value of ZSI =

T'

.~

~,..

1gap.

posi -

-I). It

)f the

strain

nents

~ that

IS a

is paI{penterms

sition

, this

xtinclames

r AL.

reacIratus com- iluted ~lobal )n:, the -I

s . j with batch d air, lction g the ented innents same±5

value

:ound0tes using theat

f !,4 

"

J 

gree- r the ..•

"

ivewithto

¥

ltions
174

DMMP over the common range of loadings.

This is consistent with the hypothesis [27] that it

is phosphorus radicals formed in the flame that

are important and not the parent compound.

Consistent flame inhibition was observed, with

no sign of the flame promotion noted by others

[18, 20]. A possible explanation is that the

relatively low peak temperatures typical of

highly strained methane-air flames [48] are below the 2350 K transition between inhibition

and promotion observed by Hastie and Bonnell

[20).

Also shown in Fig. 6 are literature flame

inhibition data for CF3Br, the current agent in

many fire-suppression systems [31, 49, 50]. The

flame inhibition properties of DMMP and TMP

are two to four times better, on a molar or mass

basis, than those reported in the literature for

CF3Br. Comparisons of these PCCs with

Fe(CO)s, one of the most effective flame inhibitors known, show that Fe(CO)s is a stronger

inhibitor at low loadings: inhibition at 200 ppm

[51] is comparable to the PCe's effectiveness at

1500 ppm. However, the effect of Fe(CO)s as an

inhibitor plateaus for mole fractions between

200 and 500 ppm, the highest gas-phase concentration achievable with ambient temperature

reactants [51]. The PCCs' inhibition performance can also be compared to the Nz data

presented in Fig. 5: DMMP and TMP are

approximately 40 times more effective than Nz

on a molar basis (~10 times as effective on a per

mass basis). It is important to note that the

range of extinction strain rates achieved in these

PCC experiments represents only the upper

portion of the range of local strain rates encountered in a realistic, turbulent fire [52]. Extending

the experimental range to much lower strains

would require a different method of introducing

the PCe.

We performed an experiment to determine

whether the hydrocarbon content of the PCCs

contributes significantly to the inhibition effectiveness. In certain chemical systems, such

as carbon monoxide-air combustion, trace

amounts of hydrocarbons have been observed

to inhibit combustion [53, 54). It is thought

that the inhibition occurs because the hydrocarbon acts as a radical sink. Hastie et al. [20,

25] attribute 8% of TMP's inhibition effectiveness in hydrogen flames to this effect. In

M. A. MACDONALD ET AL.

order to determine whether radieal seaveng_

ing by the methyl groups, found in the PCCs

under investigation, may be responsible in

part for the PCCs' inhibition observed here,

tests were conducted with isooctane as the

dopant. Isooctane has a similar structure to

the PCCs but lacks the central phosphorus

atom. It is expected that any contribution from

methyl groups to flame inhibition by the PCCs

would be observed during the isooctane tests.

However, these tests showed a 4% increase in

the global extinction strain at a loading of 400

ppm, indicating that the effect of the hydrocarbon was to promote the flame, not to inhibit it.

This result provides further support for the

hypothesis that the phosphorus atom is crucial

to the flame inhibition effects of the PCCs

studied here.

SUMMARY

The present study used an 'opposed-jet burner

configuration to investigate the effectiveness of

two PCCs-DMMP and TMP-as flame inhibitors. The global extinction strain rate, used as a

measure of flame strength, was estimated in

terms of the flow rates of fuel and oxidizer. A

novel method for performing extinction measurements was developed to maintain constant

PCC loadings in the oxidizer stream with syringe pump PCC delivery. This method causes

the locations of the flame and stagnation plane

relative to the nozzles to change during an

extinction measurement. It has been found that

the global extinction strain rate is approximately

invariant (within ::'::2%) over a broad range of

flame locations for methane-air nonpremixed

flames. This technique for performing extinction measurements is shown to give consistent

and repeatable results.

Global extinction strain rates were measured

for flames doped with PCC loadings from

0-1500 ppm. As a flame inhibitor on a molar

basis, PCCs were found to be 40 times more

effective than nitrogen and two to four times

more effective than the literature values for

CF3Br. The high level of effectiveness of the

PCCs (35% reduction in global extinction

strain rate at 1500 ppm) encourages the investigation of other phosphorus-eontaining
175

Mass-Spectrometry, presented at 3rd Asia-Pacific International Symposium on Combustion and Energy Utilization, 1995.

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V. V., Combust. Sri. Technol. 116-117:51-67 (1996).

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8:133-137 (1964).

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Chemistry of Inhibited Combustion Systems. National

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21. Audouin, L., Kolb, G., Torero, J. L., and Most, J. M.,

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(International) on Combustion, The Combustion Institute, Pittsburgh, 1960, pp. 432-438. '

23. Twarowski, A, Combust. Flame 102:55-63 (1995).

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25. Hastic, J. W., and McBee, C. L. (1975) Mechanistic Studies of Triphenylphosphine Oxide-Poly(Ethylene-terephthalate) and Related Flame Retardant Systems. National Bureau of Standards, Final NBSIR

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of Industrial Fluids, vol. ASTM STP 1284 (G. E. Totten

and R. Jurgen, Eds.), American Society for Testing

and Materials, Philadelphia, 1996, pp. 102-109.

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1st ed., Interscience Publishers, New York, 1954, pp.

248-253.

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Combustion Institute, Pittsburgh, 1997, pp. 1405-1412.

32. Masri, A R., Combust. Sci. Technol. 96:189-212

(1994).

33. Seshadri, K., and Hinde, N., Combust. Flame 101:271-

294 (1995).

34. Otsuka, Y, and Niioka, T., Combust. Flame 21:163-

176 (1973).

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and Williams, F. A, Twenty-Third (International) Sym- .

posium on Combustion, The Combustion Institute,

Pittsburgh, 1990, pp. 503-511.

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FLAME INHIBITION BY PHOSPHORUS COMPOUNDS

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 DARPA Contract MDA972-97-M0013. Additional support was provided by the

Anny Research Office under Contract DAAL03-

92-G-0113. The authors wish to thank 1. Fleming

and B. Williams at Naval Research Laboratory for

useful discussions. Further appreciation is extended to W McKeon from the University of

Buffalo for his experimental assistance.

compounds as alternatives to halogenated

flame inhibitors.

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\.LD ET AL.

~d-jet burner

'ectiveness of

flame inhibate, used as a

estimated in

j oxidizer. A

inction meatain constant

~am with syethod causes

:nation plane

e during an

:n found that

pproximately

Jad range of

nonpremixed

ming extincve consistent

ical scavengin the PCCs

sponsible in

'served here,

:tane as the

structure to

phosphorus

~ibution from

by the PCCs

loctane tests.

o increase in

lading of 400

:he hydrocart to inhibit it.

port for the

om is crucial

)f the PCCs

:re measured

adings from

r on a molar

times more

o four times

e values for

eness of the

II extinction

ages the ins-containing
176

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Received 2 September 1997; accepted 9 March 1998

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