Chapter 3: FLAME SUPPRESSION

CHEMISTRY

Gregory T. Linteris, Ph.D. 

National Institute of Standards and 

Technology

TABLE OF CONTENTS

3.1 Introduction......................................................................................................................120 

3.2 Halogen-containing Compounds .....................................................................................121 

3.2.1 Previous Understanding of the Inhibition Mechanism of CF3Br (as of 1995)......121 

3.2.2 Suppression of Nonpremixed Flames by Fluorinated Ethanes and Propanes.......124 

3.3 Metal-containing Compounds..........................................................................................142 

3.3.1 Background ...........................................................................................................142 

3.3.2 Metals with Demonstrated Flame Inhibition Potential..........................................155 

3.3.3 Flame Inhibition by Iron-containing Compounds.................................................156 

3.3.4 Flame Inhibition by Tin and Manganese...............................................................178 

3.3.5 Complexation/Matrix Absorption of Super-effective Agents...............................199 

3.3.6 Potential for Loss of Effectiveness of Other Metals .............................................224 

3.3.7 Flame Inhibition and Loss-of-effectiveness Summary .........................................232 

3.3.8 Conclusions...........................................................................................................235

3.4 Phosphorus.......................................................................................................................236 

3.4.1 Introduction ...........................................................................................................236 

3.4.2 Inhibition of Non-premixed Flames by Phosphorus-containing Compounds.......238 

3.4.3 Effects of Dimethyl Methylphosphonate on Premixed Methane Flames..............267 

3.4.4 Summary: Phosphorus-containing Compounds in Flames ...................................281 

3.5 Comparative Flame Inhibition Mechanisms of Candidate Moieties ...............................282 

3.5.1 Introduction ...........................................................................................................282 

3.5.2 Spectroscopic Studies of Inhibited Opposed Flow Propane/Air Flames ..............282 

3.5.3 CF3Br and Other Suppressants: Differences in Effects on Flame Structure .........291 

3.5.4 Influence of Bond Energies on Catalytic Flame Inhibition...................................299 

3.5.5 Temperature Regions of Optimal Chemical Inhibition of Premixed Flames........307 
120 Flame Suppression Chemistry

3.6 Insights on Flame Inhibition ............................................................................................316

3.7 References........................................................................................................................318 

3.1 INTRODUCTION 

This chapter describes NGP research agents which work primarily through chemical means. As a prelude 

to this, it is useful to delineate the different terminology often used to describe the extinguishing of a 

flame. Flame inhibition usually refers to a weakening of a flame, i.e., a lowering of the overall 

exothermic reaction rate in the flame. This weakening may or may not lead to extinguishment, depending 

upon the flow field in which the flame exists. In contrast, the terms fire suppression, flame 

extinguishment, or flame extinction are often used to refer to the case in which the flame has been 

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

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

The first three sections of the chapter group the research according to the active moiety in the chemical 

agent: halogens, metals, or phosphorus. 

• The section on halogens starts with a description of what was known about the flame inhibiting 

action of CF3Br prior to NGP research, and follows with research describing some alternative 

agents based on fluorine. 

• The section on metals also first gives some background, and then provides extensive data on 

flame inhibition by iron-containing compounds - primarily because these agents have been shown 

to be the strongest flame inhibitors ever found. Detailed results for the effect of iron (and other 

agents) on flame radicals are also presented, followed by results for tin and manganese. Since 

most of the metals in these previous studies were added as toxic organometallic agents, the next 

section describes work to investigate metal moieties in various forms which are non-toxic. Since 

it was discovered fairly early in NGP research that metal compounds lose their effectiveness due 

to the formation of particles, additional research was aimed at finding other metals which were 

effective but did not suffer that same consequence. The remaining parts of that section describe 

the potential of other metals to be effective yet not to lose their effectiveness. 

• The third section describes extensive research to characterize the potential and effectiveness of 

phosphorus-containing compounds. It starts with a description of a new experimental approach in 

counterflow diffusion flames which is helpful with aspirated samples of trimethyl phosphate 

(TMP) and dimethyl methylphosphonate (DMMP). Following that is a description of how 

varying the agent transport rates into the flame (through changes to the stoichiometric mixture 

fraction and the location of agent addition) changes the effectiveness of DMMP. In this section, a 

temperature dependence of the effectiveness is also described, which is further explored in the 

following section which describes spectroscopic measurements of OH and numerical modeling of 

counterflow diffusion flames of methane and air with DMMP added. The numerical model is 

further validated in the final subsection, where experiments and calculations are performed for 

premixed flat flames of methane and air with added DMMP. 

The final section of the present chapter discusses studies aimed at finding unified principles to describe 

the fire suppression effectiveness of alternative chemical moieties: 
Halogen-containing Compounds 121

• Comparison of spectroscopically measured reductions in the concentration of the chain-carrying 

radical OH and in peak temperature in opposed flow, atmospheric pressure, propane/air flames 

brought to near-extinction conditions through addition of several agents (CF3Br, N2, Fe(CO)5, 

C3F7H, C3F6H2, DMMP, and PN (cyclo-P3N3F6). 

• Description of additional spectroscopic tests to delineate the reduction in [OH] with addition of 

Fe(CO)5 (and, for comparison, CF3Br) to low-pressure methane-air flames. 

• Discussion of the merits of using CF3Br as a benchmark for comparison of other fire 

suppressants. 

• A discourse on the use of inhibitor molecule bond energies as a criterion by which the flame 

inhibition efficiency can be predicted and understood. 

• Examination of the regions of a premixed flame which are most susceptible to chemical

inhibition. 

The NGP research described here has moved the science of flame inhibition and suppression chemistry 

forward in very large measure. Entirely new, detailed, comprehensive chemical kinetic mechanisms were

developed specifically for the NGP, and the use of these kinetic mechanisms in detailed numerical

simulations for the present investigations has led to great insight into the action of chemically acting 

agents in fire suppression. New experimental techniques were developed, as well as new methods of 

using existing approaches. In fact, much of the present work has been cited in fields external to fire

research due to their utility beyond the present needs. The work presented below will provide an 

outstanding basis for those future researchers who will continue the search for new fire-fighting materials. 

The new unified insights on fire suppression developed through NGP research are summarized at the end 

of this chapter. 

3.2 HALOGEN-CONTAINING COMPOUNDS 

3.2.1 Previous Understanding of the Inhibition Mechanism of CF3Br (as of 1995) 

In previous studies, CF3Br (and related compounds) have been the subject of much fundamental research, 

for example in flow reactors,1,2

 premixed flames,3-11 co-flow diffusion flames,12-14 and counterflow 

diffusion flames.9,15-20 Much recent research has been aimed at finding both short-21,22 and long-term23

replacements for CF3Br. As a result, CF3Br itself has been the continuing subject of many studies24-27

since an improved understanding of its mechanism of inhibition will help in the search for alternatives, 

and nearly all assessments of new agents use CF3Br as a baseline for comparison of the new agents. 

As outlined above, the flame configurations used to study flame inhibition mechanisms have tended to be 

premixed and counterflow diffusion flames. Premixed flames have been selected mainly because the 

overall reaction rate, heat release, and heat and mass transport in these flames can be described with a

single fundamental parameter - the laminar burning velocity, and because over certain regions, the flow

field can be considered one-dimensional (greatly simplifying data collection and numerical simulation).

Similarly, counterflow diffusion flames can be considered one-dimensional along the centerline, and the 

extinction strain rate has been commonly used as the characteristic suppression parameter. In principle, 

such fundamental parameters can ultimately be used to relate the behavior of the agent in the laboratory 

flame to its behavior in suppressing a large-scale fire28, although this scaling is difficult in practice. 
122 Flame Suppression Chemistry

Premixed Systems 

Early studies of the inhibitory effects of halogenated hydrocarbons on flames were conducted in premixed 

systems. Numerous research groups12,29-31 determined the influence of halogenated methanes on the 

flammability limits of hydrocarbon-air mixtures. The magnitude of the inhibitory effect and its 

dependence on halogen type and stoichiometry generally indicated a chemical rather than thermal 

mechanism. Simmons and Wolfhard12 studied the flammability limits of methane-air mixtures with 

bromine and methyl bromide inhibitors and showed that methyl bromide influenced the limits of 

flammability in the same manner as equivalent amounts of bromine and methane, indicating that the 

decomposition products of the halogenated methanes were responsible for the action. Garner et al.,32

Rosser et al.,33 Lask and Wagner34 and Edmondson and Heap35 measured the reduction in burning velocity 

of various burner − stabilized hydrocarbon-air flames with addition of several halogenated methanes, 

halogen acids and halogen dimers. In later work, Niioka et al.

36 experimentally investigated the effect of 

CF3Br on the extinction velocity of opposed premixed C2H4 flames. The inhibitory effect was found to be 

more effective in rich flames than in lean flames. Mitani37 used experimental results on the decrease of 

flame propagation speeds of premixed H2-O2-N2 and C2H4-O2-N2 flames with CF3Br addition to derive 

overall activation energies for the inhibition reactions, which correlated well with measured energies for 

key inhibition steps. All studies were in consensus that the magnitude of the inhibition was related to the 

number and type of halogen atoms present in the reactants, the concentration of the inhibitor, and the 

equivalence ratio, φ, and that the effect was generally too large to be accounted for by thermal dilution 

effects. This conclusion was based on supporting calculations or measurements showing that the final 

temperatures in the inhibited flames were not low enough to account for the burning velocity reductions 

observed. 

Detailed information on the inhibition mechanism of brominated species has been provided by flame 

structure measurements. Mass spectrometric measurements of stable species concentrations in premixed 

flames of methane and air were conducted by several investigators: Levy et al.38 used HBr to inhibit a 

Bunsen flame and found that HBr inhibited methane consumption but not CO oxidation; Wilson4

 and 

Wilson et al.39 performed low-pressure inhibited premixed flame studies with methyl bromide, HCl, HBr, 

Cl2, and concluded that both CH3Br and HBr act to reduce radical concentrations in the preheat region of 

the flame by competing with the chain branching reaction H + O2 = OH + O, and that CH3Br lowers the 

peak OH concentration to near equilibrium values, while uninhibited flames have the characteristic 

super-equilibrium OH concentrations. Biordi et al.5,40 used a molecular-beam mass spectrometer to obtain 

stable and radical species concentrations as a function of position in low-pressure premixed flat flames of 

methane and air with and without addition of CF3Br, and Safieh et al.7

 performed similar experiments 

with a low-pressure CO-H2-O2-Ar flame. The data and analyses of these researchers provided detailed 

kinetic information on the mechanisms of Br and CF3 inhibition under low-pressure premixed, 

burner-stabilized conditions. Day et al.41 and Dixon-Lewis and Simpson42 studied the burning velocities 

and the rich flammability limits of H2-O2-N2 and H2-O2-N2O flames. Their experiments and numerical 

modeling showed that the thermodynamic equilibrium relationship for the reaction H + HBr = H2 + Br 

together with recombination steps involving bromine atoms are primarily responsible for the inhibition 

effect of HBr compared to HCl and HF, rather than competition of the H atom-hydrogen halide reaction 

with the branching reaction H + O2 = OH + O. Westbrook8,43 developed a detailed chemical kinetic 

mechanism for HBr, CH3Br, and CF3Br inhibition and performed numerical calculations of flame 

structure, testing the mechanism using experimental burning velocity reductions and, to some extent, 

using measured species concentration profiles. 
Halogen-containing Compounds 123

Diffusion Flames 

Early investigations of the effects of chemical inhibitors on diffusion flames used the co-flow 

configuration. Creitz44 determined the blow-off limits with CH3Br and CF3Br added to either the fuel or 

oxidizer stream for various oxygen volume fractions, and found that oxygen volume fractions 

approaching 0.30 required over 20 % CF3Br in the oxidizer for flame detachment. Simmons and

Wolfhard12 found the blow-off limits for CH3Br addition to the fuel and air streams for alkane and

hydrogen flames, and showed that blow-off required different amounts of inhibitor for different burner

geometries. Their measurements, as well as more recent ones with CF3Br,45,46 have shown that lower gas 

velocities require greater amounts of inhibitor. Simmons and Wolfhard12 also performed accompanying 

spectroscopic measurements in a slot-burner with CH3Br added to the air stream and found enhanced C2

formation as well as a secondary reaction zone (observed via Br2 emission) on the air side with either

CH3Br or ethane and bromine added to the air stream. 

The above jet diffusion flame studies have all shown that a higher volume fraction of the brominated 

agent was required when it was added to the fuel stream rather than the air stream. Friedman and Levy15

were the first to use the extinction of counterflow diffusion flames to assess the inhibitory effect of CF3Br 

and CH3Br added to the methane stream. Milne et al.16 added CF3Br to either the fuel or air stream of

several hydrocarbon-air flames, again showing that addition to the air stream extinguished the flames at 

lower volume fraction than addition to the fuel stream. Ibiricu and Gaydon47 made spectroscopic

measurements with addition of CH3Br and other inhibitors to flames of various fuels. They found OH 

emission to be decreased, while C2 and HCO emission were increased. Niioka et al.36 correlated the 

extinction conditions of counterflow diffusion flames of ethylene inhibited by CF3Br in the air or fuel

stream with the estimated mass flux of inhibitor to the reaction zone. 

The first measurements of inhibited counterflow flames with liquid fuels were made by Kent and 

Williams

17,48 for CF3Br in the oxidizer stream against a heptane pool. These were followed by extensive

measurements for the same agent with numerous condensed-phase fuels by Seshadri and Williams.49 

Among other results and analyses, these investigations characterized the extinction flow velocity as a 

function of the oxygen mass fraction in the oxidizer stream, found the extinction flow with added 

inhibitor, and measured stable species mass fractions with gas chromatography. The amount of inhibitor 

required for flame extinction was found to increase with decreasing air flow velocity (i.e. strain) and the 

results were highly fuel dependent. In later work, Seshadri and co-workers 50 determined the extinction 

strain rate for a large number of fluorinated agents in a similar counterflow heptane-air pool burner. 

Some of the above diffusion flame studies also show increased soot production with addition of

halogenated inhibitors. The laser-induced incandescence and fluorescence measurements of Smyth and

Everest51 have quantified both the increased soot and decreased peak OH in such flames, although the 

mechanism of higher soot formation for CF3Br was not resolved. 

Mechanism of Inhibition 

The basic mechanism of halogen flame inhibition was suggested by Rosser et al.33 and further justified

and refined by Butlin and Simmons,31 Dixon-Lewis and co-workers,41,42

 Westbrook,8,13,43 and Babushok 

and co-workers.52 The reaction mechanism is: 

Hα + X → HX + α
124 Flame Suppression Chemistry

β + HX → Hβ + X 

in which X is a halogen, α is a hydrocarbon, and β is a reactive radical such as H, O, or OH. The 

hydrogen atom is typically affected most by the catalytic radical recombination cycles above, and its 

decrease leads to a lowering of the chain-branching reaction H + O2 → OH + O and the CO consumption 

reaction CO + OH → CO2 + H. It was demonstrated fairly early41 that for Br and Cl, the equilibrium 

condition of the first reaction was key, as was the necessity of the regeneration steps (in the form of the 

second reaction or some alternatives). 

As described above, there has been fairly extensive past research on CF3Br itself, as well as some simple 

small hydrofluorocarbon (HFC) short-term replacements. These include phenomenological tests with cup 

burners as well as fundamental studies. Nonetheless, tests with larger HFCs appear less frequently in the 

literature, with very little fundamental data from diffusion flames. The following work remedies that 

deficiency. 

3.2.2 Suppression of Nonpremixed Flames by Fluorinated Ethanes and Propanes 

Introduction 

The need for environmentally acceptable substitutes for bromine-containing halons (halogenated 

hydrocarbons) has led to a number of studies on alternative fire suppression agents encompassing a 

variety of experimental geometries and scales,21,53 as well as to the development of chemical kinetic 

models for these agents.54-56 Models are instrumental in guiding the search for effective halon 

replacements in that they facilitate the identification of mechanisms key to suppression. Such models 

must be validated using experimental data. Data sets that can directly be compared to numerical results 

are therefore particularly valuable. For models of suppression in particular, the usefulness of 

experimental and numerical studies depends on how well they capture the characteristics of real fire 

scenarios. Most experimental studies on hydrofluorocarbons (HFCs), used as near-term halon 

replacements, have focused on inhibited premixed flames, usually with methane fuel, measuring 

parameters such as flame speed57,58 or profiles of intermediate species.59-61 However, most fire-fighting 

scenarios involve higher hydrocarbon fuels, non-premixed flames, and aim to achieve extinction rather 

than inhibition. In order to extend laboratory results to more realistic situations, a broad range of data sets 

is needed to provide adequate model validation for suppression by both HFCs and other classes of 

alternative agents now being identified. 

Research by Williams and co-workers62 focused on the suppression effectiveness of HFCs in 

extinguishing non-premixed flames. Although less effective fire suppressants than brominated halons, 

these agents are currently used as near-term replacements. They possess many of the same favorable 

physical properties as brominated halons, but do not destroy the stratospheric ozone layer.21,50 Previously, 

the Naval Research Laboratory (NRL) investigated non-premixed counterflow methane/air and 

propane/air flames suppressed by CF4, CHF3, and CF3Br.63 The NGP extended the methane/air and 

propane/air extinction studies to include all fluorinated ethanes containing at least four fluorine atoms, 

and ten fluorinated propanes containing at least five fluorine atoms. The fluorinated ethane and 

fluorinated propane data sets were generated to provide additional input to validate chemical kinetic 

models such as those developed recently for HFCs.54-56, 59 Also tested were sulfur hexafluoride (SF6, 
Halogen-containing Compounds 125

largely inert13), nitrogen, three bromine-containing halons, and one iodine-containing halon. All 

compounds investigated are listed in Table 3-1. 

Table 3-1. Agents Tested. 

Agent Chemical Formula

Cp

300 K

(J/mol.K)

a

H1600 K-H300 K

(kJ/mol)b Flamese

HFC-134 CHF2CHF2 90.9 183[31] M,P,H

HFC-134a CF3CH2F 87.2 186[31] M,P,H

HFC-125 CF3CHF2 95.9 195[31] M,P,H

FC-116 CF3CF3 107.7 206[31] M,P,H

HFC-245ca CHF2CF2CH2F 123.7 244c M,P

HFC-245cb CF3CF2CH3 123.0 242c M,P

HFC-245eb CF3CHFCH2F 123.0c 242c M

HFC-245fa CF3CH2CHF2 122.0[35] 241[35] M

HFC-236cb CF3CF2CH2F 130.0c 256c M,P,H

HFC-236ea CF3CHFCHF2 129.8 256c M,P,H,A

HFC-236fa CF3CH2CF3 128.1 253c M,P,H,A

HFC-227ca CF3CF2CHF2 140.0c 276c M,P,H

HFC-227ea CF3CHFCF3 139.8 275[38] M,P,H,A

FC-218 CF3CF2CF3 150.6 289c M,P,H

Halon 1301 CF3Br 70.5 125[36] P,H,A

Halon 13001 CF3I 61.3[36] 126[36] P,H

Halon 1211 CF2ClBr 74.6d 133[39] P,H

Halon 1201 CHF2Br 58.7d 120[39] P,H

Sulfur Hexafluoride SF6 97.4[36] 185[36] M,P,H

Tetrafluoromethane CF4 61.2 122[36] H,A

Nitrogen N2 29.1[36] 42[36] M,P,H,A

a

 Specific heat at 300 K. The numerical superscripts in square brackets indicate reference numbers. If no superscript is present,

the value is from Reference 64. b

 Sensible enthalpy required to raise the agent’s temperature from 300 K to 1600 K. The sources for the data used to derive the 

sensible enthalpy values are indicated by the numerical superscripts in square brackets. c

 Estimated. d

 From Reference 65, evaluated at 298 K. e

 Flames in which given agent was tested: M, methane counterflow; P, propane counterflow; H, n-heptane cup burner; A, 

methanol cup burner. 

An experimental geometry that captures many characteristics of pool fire threats is that of the cup burner,

which is described more fully in Chapter 6. Bench-scale co-flow burners operated with a liquid fuel, cup

burners are widely used for estimating agent concentrations required to combat fires.66 Hamins et al. 50

compared a number of HFC agents in cup burner tests using five different fuels: n-heptane, two types of

jet fuel, and two types of hydraulic fluid. For nearly all agents tested, the concentrations required to 
126 Flame Suppression Chemistry

extinguish the n-heptane flame exceeded those required for the other fuels by less than 10 %. Thus, 

n-heptane cup burner studies provide extinction data applicable to many hydrocarbon flames. 

For validation of kinetic models, treating detailed chemical mechanisms in the two-dimensional model 

required to describe cup burner flames is computationally difficult. An alternative to cup burners for 

suppression mechanism studies is provided by non-premixed counterflow flames. This configuration is 

computationally tractable since it can be modeled quasi-one-dimensionally, along the axis of symmetry of 

the flame, by invoking a similarity transform.67 Experimentally, it is also advantageous in that it allows 

control of the strain rate imposed on the flame. In this paper, the strain rate in a counterflow burner is 

defined as the maximum axial gradient in axial velocity on the oxidizer side of the flame zone. The strain 

rate is easily adjusted by changing the gas flows. The effects of suppression agents can then be quantified 

over a range of flame conditions by measuring the extinction strain rate as a function of agent 

concentration. Hamins et al.50 and Saso et al.68 found that HFC extinction concentrations for low strain 

rate n-heptane counterflow flames were similar to those obtained in n-heptane cup burners. Since the cup 

burner configuration is relevant to many fire scenarios, the results of Hamins et al. and Saso et al. suggest 

that agent extinction concentrations in low strain rate non-premixed counterflow flames are pertinent to 

these same scenarios. 

The choice of a model fuel compound for suppression studies involves a trade-off between representative 

extinction properties and computational tractability. Gaseous fuels including methane, ethane, and 

propane are attractive to use from both experimental and modeling standpoints, but their relevance to fires 

involving long chain hydrocarbons needs to be determined. Furthermore, no model fuel is representative 

of all fire threats. Indeed, alcohol fires, methanol fires in particular, are very difficult to extinguish, even 

with CF3Br.69 The extinction behavior of methane/air counterflow, propane/air counterflow, n-heptane 

cup burner and methanol cup burner flames are compared in this study to identify systematic trends in the 

suppression effectiveness of selected agents as a function of the fuel used. In addition to providing 

experimental data for HFC model validation, another objective of the current investigation is to assess 

whether methane or propane counterflow burner extinction studies are appropriate models for HFC 

suppression of liquid hydrocarbon fires. 

Tanoff et al. 70 recently computed the extinction strain rate of methane/air counterflow flames suppressed 

by several C1 and C2 HFCs. One of their major conclusions was that most HFCs investigated, with the 

exception of CHF3, would be better suppressants in methane flames if they were chemically inert. Also, 

agents predicted to form CF2O in the counterflow flame, principally from CF3 radicals, were found to be 

better suppressants than those that do not. The chemical contribution to suppression of the CF3 group was 

reported by Sheinson et al.,13 who attributed 25 % of CF3Br’s suppression ability to this moiety. The 

present study further investigates the impact of CF3 groups in an agent on suppression by comparing the 

performance of four sets of HFC isomers. 

In counterflow burners, flows at extinction are apparatus dependent. Comparisons between results 

obtained from different experimental and numerical counterflow flame studies must therefore be made 

through extinction strain rates. While flows are easily determined, velocity profile measurements are 

required to determine local extinction strain rates. Here, a burner-specific relationship between local 

strain rate and flow conditions was obtained. All strain rates quoted herein are therefore traceable to an 

experimental determination of the local velocity field. 
Halogen-containing Compounds 127

Experiment 

The counterflow burner and experimental methodology have been described previously.

63 The burner

consisted of two vertically-mounted open straight tubes, approximately 50 cm long, having an inner 

diameter of 1 cm. The tube exits were spaced 10.0 mm ± 0.5 mm apart. The gap size was determined by 

translating the burner assembly with respect to a laser beam perpendicular to the tube axis, and measuring 

the height at which the beam was blocked by the tubes. The top tube was equipped with a cooling jacket

to prevent the hot combustion products from preheating the upper reactant stream. The tubes were

housed in a Plexiglas chamber, which was continuously purged with nitrogen to dilute combustion

products and unburned reactants before they entered the exhaust. 

For experiments with methane, the fuel was introduced through the top tube while air/agent mixtures were 

introduced through the bottom. For experiments with propane, air/agent mixtures were delivered from the 

top, fuel from the bottom. These configurations were used because they produced the most stable flames

at low strain rates. For propane, inverting the reactant streams did not significantly affect the extinction

strain rate of the uninhibited flame. Flows of all gases were regulated by mass flow controllers (Sierra

Instruments), which were calibrated using a piston flowmeter (DC-2, Bios International). All flow 

conditions were laminar, with tube Reynolds numbers never exceeding 2100. 

During extinction measurements, the flows of fuel and air were set to obtain a flame midway between the

tubes. The agent flow was then slowly increased at intervals of 30 s to 45 s, with periodic adjustments of

the fuel and air flows to maintain the flame’s position, until the flame extinguished. The flame’s position 

was determined by monitoring its image on a video screen. The stagnation plane was always slightly 

toward the fuel tube from the flame zone. In experiments with methane, the flame typically extinguished

completely. For propane, local extinction typically occurred along the centerline, leaving a ring-shaped

flame with a hole along the burner axis, similar to that described by Potter and Butler.71 In all tests, the 

agent was added to the air stream. This mode of application most closely resembled actual situations, in 

which an agent is introduced into the air around the fire, with the fuel burning in a non-premixed flame. 

The cup burner apparatus and methodology have been described in detail elsewhere.13 The design was

virtually identical to that of Hamins et al.50 The piston flow meter was used to measure the agent and air 

flows after each extinction measurement. The total flow of air + agent was 20 L/min ± 1 L/min for all 

extinction measurements, giving a flow velocity, averaged over the 105 mm inner diameter chimney, of 

4.1 cm/s ± 0.2 cm/s past the 28 mm outer diameter cup. Experiments were conducted on both n-heptane

and methanol flames. The fuel level was kept within 0.5 mm of the top of the cup without overflowing, 

as low liquid levels led to attachment of the flame to the heated rim of the Pyrex cup, resulting in

anomalously high extinction values. 

The agents listed in Table 3-1 were obtained from the following suppliers: HFC-245eb and halon 13001,

Flura; HFC-245fa, AlliedSignal; HFC-236ea and HFC-236fa, Dupont; HFC-227ea and halon 1201, Great

Lakes; FC-218, Scott Specialty Gases; halon 1301 and CF4, Matheson. The nitrogen was obtained from 

the Naval Research Laboratory’s in-house supply. The remaining agents were from PCR. The stated

purity of the agents was at least 97 %. All agents were used as supplied, with the exception of HFC245ca, HFC-245eb and HFC-245fa. These pentafluoropropanes have boiling points close to room 

temperature: 25 °C, 23 °C, and 15 °C, respectively. They were therefore premixed with air in a 1:3 ratio 

before being tested. The pressure increase ensured that stable flows of these agents were delivered to the 
128 Flame Suppression Chemistry

burners. Extinction data was also collected with HFC-245fa unmixed with air. These data were 

consistent with those obtained using the agent/air mixture. 

All of the agents in Table 3-1 were tested in the propane counterflow flame with the exception of HFC245eb, HFC-245fa, and CF4. All of the agents were tested in the methane counterflow flame with the 

exception of halon 1301, halon 13001, halon 1211, halon 1201, and CF4. All of the agents with the 

exception of the pentafluoropropanes were tested with n-heptane in the cup burner. HFC-236ea, 

HFC-236fa, HFC-227ea, halon 1301, CF4, and nitrogen were tested with methanol in the cup burner. 

Extinction Strain Rate Determination 

Many researchers performing studies in counterflow burners have characterized extinction conditions in 

terms of global or imposed parameters, such as flow rates and gap size,72 or global strain rates calculated 

from these parameters.50,73 These global parameters representing the imposed flow conditions can be 

compared to the same parameters obtained from numerical calculations of extinction conditions.74 

Because the local flow field near the flame determines extinction, the comparison of global parameters is 

valid only as long as the relationship between the local strain rate and the global conditions are the same 

for the experiment and the calculations. Recent measurements67,75 have cast doubts on the adequacy of 

representing counterflow experimental boundary conditions by either plug or potential flow in numerical 

calculations. Comparisons of measured and numerical local strain rates in the vicinity of the flame should 

show less sensitivity to boundary conditions than comparisons of measured and numerical global strain 

rates. 

The most direct approach in determining strain rate is to measure the local velocity fields. As velocities 

cannot be measured at extinction, determination of local extinction strain rate involves a series of velocity 

measurements in increasingly strained flames, and extrapolation to extinction.63 This sequence of 

measurements must be repeated for each fuel/oxidizer/additive system and each additive loading, 

requiring large numbers of velocity profile measurements. 

In the present study, air stream local strain rates were determined by measuring the centerline axial 

velocity as a function of axial position by an LDV system (QSP Digital). The oxidizer stream was seeded 

with 0.3 μm alumina particles (Buehler). The velocity profile on the oxidizer side of the flame was fit to 

a 6th order polynomial, and the relevant local strain rate was taken to be the maximum value of the 

derivative of this function. Velocity profiles were measured for propane/air, methane/air, methane/(air + 

HFC-227ea), and propane/(air + SF6) flames. 

Based on these measurements, an empirical, burner-specific relationship to convert global parameters into 

local strain rates was determined. Figure 3-1 shows the data used in obtaining this correlation. Measured 

local strain rate is plotted versus the plug flow global strain rate parameter derived by Seshadri and 

Williams76, and restated by Chelliah et al. 67 in Equation 3-1. 
Halogen-containing Compounds 129

Global Strain Rate (s-1)

0 50 100 150 200 250 300

Measure

d Strain Rate (s-1

)

0

100

200

300

400

500

Methane(top)/Air

Propane(top)/Air

Propane(bottom)/Air

Propane(bottom)/Air + HFC-227ea

Propane(bottom)/Air + SF6 Figure 3-1. Comparison of 

Local Strain Rates Measured 

by Laser Doppler Velocimetry 

in Non-premixed Counterflow

Flames and Global Strain Rates 

Calculated from Equation 3-1. 

(The solid line corresponds to 

the linear regression given in 

Equation 3-2.) 

(3-1) ) ),

V

V

(1+( L

2 |V | K = 0.5

o

2

o

f

2

o f

global,plug

ρ

ρ

where L is the gap size (1 cm), and ρf,o and Vf,o are the densities and velocities of the fuel and oxidizer 

streams, respectively. V is as the average velocity across the tube exit. Measured local strain rates for all

flame conditions, including those with suppressants present, were proportional to Kglobal, plug from Equation

3-1, 

Klocal = (1.57 ± 0.02) Kglobal,plug , (3-2)

where the velocities used in the global expression are averages based on measured volumetric flows. The

local extinction strain rates discussed in the present study were obtained using Equations 3-1 and 3-2. 

Seshadri and Williams' plug flow global strain rate expression is for the axial gradient in axial velocity at 

the stagnation plane calculated for inviscid non-reacting flow, with plug flow boundary conditions 

imposed at the burner exits. The current apparatus, with straight burner tubes, has velocity boundary

conditions significantly different from plug flow. Measurements near the tube exits indicated a nearly

parabolic profile for an individual tube in isolation, with some flattening of the profile when the opposing 

flow from the second tube was introduced. The centerline velocity of a parabolic profile in tube flow was 

twice the average velocity determined from the volumetric flow and the cross-sectional area. At a given

flow, the tube exit centerline velocities in the current apparatus were therefore expected to be between 

once and twice the plug flow values. Hence, the coefficient relating Klocal and Kglobal,plug was predicted to 

lie between 1 and 2. The factor of 1.57 obtained was consistent with this prediction. Pellett et al. 77 also 

determined that local and global strain rates were linearly related in their opposed-tube burners: local 
130 Flame Suppression Chemistry

strain rates were found to be 3 times larger than their global counterparts. Pellett et al. used burners with 

a ratio of gap size to tube diameter equal to 2. For the burner considered here, this ratio was equal to 1. 

Non-Premixed Counterflow Flame Experiments 

Figure 3-2 and Figure 3-3 show how local extinction strain rate varied with agent molar concentration for

methane and propane non-premixed counterflow flames, respectively. Figure 3-2a and Figure 3-3a show 

the suppression effects of the fluorinated ethanes, SF6 and N2. The N2 extinction concentrations have

been divided by two for presentation in the figures. Figure 3-2b and Figure 3-3b show results for the 

fluorinated propanes. Figure 3-3c shows the effects of halon 1301, halon 13001, halon 1211 and halon

1201 on the extinction strain rate of a propane flame. The error bars shown in Figure 3-2a and Figure

3-3a for the HFC-134 data are at the 99.7 % (3σ) confidence level. Horizontal error bars represent

uncertainties in air and agent flows. Vertical error bars represent uncertainties in air, agent and fuel 

flows, gap size, room temperature and pressure, and the constant of proportionality between local and 

global strain rates (Equation 3-2). At comparable concentrations or extinction strain rates, uncertainties in

the data for the other agents are similar to those shown for HFC-134. The N2 extinction concentration 

uncertainties are similar to those for HFC-134 at comparable loadings. They increase to ± 1.7 mole % at 

the highest concentrations. 

Figure 3-2. Extinction Strain Rate vs. Agent Molar Concentration for Non-premixed 

Methane-air Flames Suppressed by: a) Fluorinated Ethanes, SF6, and N2; and 

b) Fluorinated Propanes. 
Halogen-containing Compounds 131

Figure 3-3. Extinction Strain Rate vs. Agent 

Molar Concentration for Non-premixed 

Propane-air Flames Suppressed by: 

a) Fluorinated Ethanes, SF6, and N2; 

b) Fluorinated Propanes; and c) Bromineor Iodine-containing Methanes. 

Extinction of Uninhibited Methane and Propane Flames 

The average uninhibited non-premixed counterflow methane/air and propane/air flame local extinction

strain rates were compared to values found in the literature. Pellett et al.77 used two different opposednozzle burners to obtain extinction strain rates of 384 s-1 and 396 s-1 for methane/air flames. Furthermore,

Chelliah et al.

67 measured the local extinction strain rate of their non-premixed counterflow methane/air 

flame to be 380 s-1. Yang and Kennedy78 measured theirs to be 340 s-1. Both of these values were

obtained for flames having strain rates below that required for extinction. They correspond to lower
132 Flame Suppression Chemistry

bounds for the values that would be calculated from an extrapolation to extinction, and are consistent with 

the results of the present study. Yang and Kennedy indicated that the strain rate they report is 

approximately 10 % lower than that which would be obtained through extrapolation; and that the 

uncertainty in their critical strain rate is approximately +10 % and -5 %. 

Global extinction strain rates for uninhibited counterflow non-premixed methane/air and propane/air 

flames have also been reported. MacDonald et al.79 reported a global extinction strain rate of 296 s-1 for 

methane. The corresponding global value obtained in the present study was 273 s-1. Since the 

experimental geometries in both studies are similar, the values are expected to be comparable. 

MacDonald et al. indicate that variations of up to 80 s-1 were observed in their uninhibited global 

extinction strain rates, due to variations in their air’s oxygen content. The global methane extinction 

number collected in the present study is thus consistent with theirs. Puri and Seshadri80 report global 

extinction values, but for a different experimental geometry. The results cannot be directly compared. 

Agent Suppression Effectiveness 

Figure 3-2 and Figure 3-3 show that, in general, the extinction strain rate of both methane and propane 

flames decreased as agent loading increases. Thus, the agents inhibited the flames. For HFC-245ca, 

however, the extinction strain rates at concentrations below 2 % are higher than those for the uninhibited 

flames. At these loadings, this agent increased flame strength. Nevertheless, above 2 %, extinction 

occurred at strain rates below the uninhibited values, and extinction strain rates decreased with increases 

in concentration: in larger quantities, HFC-245ca acted like a suppressant. The extinction data presented 

in Figure 3-2 and Figure 3-3 are limited to strain rates above 70 s-1. Below this value, considerable scatter 

was observed and, for several agents, extinction concentrations began to decrease with diminishing strain 

rate. At low strain rates, the flames became more susceptible to both conductive and radiative heat losses 

to the burner, and to fluctuations in the flow currents present in the surrounding chamber gases. 

A secondary flame zone about 1 mm on the air side of the principal non-premixed flame was observed in 

tests with HFC-245ca, particularly at high loadings. Hamins et al.50 also found separate agent and fuel 

consumption zones in their numerical investigation of the structure of halon 1301 inhibited n-heptane/air 

non-premixed counterflow flames. In the current study, the secondary flame zone appeared to be caused 

by the agent burning in the air in a premixed fashion. Yang et al.81 predicted this phenomenon for CH3Cl 

addition to the oxidizer stream of a methane/air counterflow flame. Although HFC-245ca is nonflammable at ambient temperature and pressure, heat release from the non-premixed flame was apparently 

sufficient to support its premixed combustion. Grosshandler et al.82 investigated the flammability of 

HFC-245ca, and concluded that a stoichiometric mixture of HFC-245ca and air could sustain combustion 

if preheated 100 K above ambient temperature. A secondary flame zone was also observed in tests with a 

more effective suppressant, HFC-245cb, but was less pronounced than in the HFC-245ca case. 

The relative suppression effectiveness of the fluorinated propanes can be assessed from Figure 3-2b and 

Figure 3-3b. The most effective fluorinated propanes were FC-218, HFC-227ea, and HFC-236fa, all of 

which contain two CF3 groups. HFC-227ca, HFC-236ea, HFC-236cb, and HFC-245cb, along with 

HFC-245fa and HFC-245eb in the methane case, were less effective and contain one CF3 group. 

HFC-245ca has no CF3 groups and was clearly less effective than the other fluorinated propanes tested. 

The extinction strain rate curves clustered together in terms of the number of CF3 groups present in the 

agent. 
Halogen-containing Compounds 133

Among agents with the same number of CF3 groups, compounds with higher fluorine to hydrogen atom

(F:H) ratios were slightly more effective agents, particularly at low concentrations: FC-218 was slightly 

more effective than HFC-227ea and HFC-236fa; and HFC-227ca, HFC-236ea and HFC-236cb were more 

effective than the pentafluoropropanes. At higher concentrations, agent suppression effectiveness was not 

as affected by the F:H ratio. The impact on suppression of substituting an F atom for an H atom in the 

agent molecule is both chemical and physical in nature. F atoms can scavenge H atoms in flames to form

HF, reducing the number of H’s that participate in flame propagation reactions.61 Furthermore, as shown

in Table 3-1, substituting one F for one H in a fluorinated propane led to a 5 % to 8 % increase in its 

specific heat at 300 K, enhancing the molecule’s sensible enthalpy. Higher F:H ratios were therefore 

expected to increase the physical suppression effectiveness of fluorinated propanes. However, as the data 

show, the effect of substituting one H for an F was minor compared to that of arranging the fluorine atoms 

to provide a CF3 group: HFC-236fa was more effective than HFC-227ca, which has one more fluorine

atom but one less CF3 group. 

The impact of CF3 groups on suppression is further illustrated by the fluorinated ethane results shown in

Figure 3-2a and Figure 3-3a. FC-116, which has two CF3 groups, was more effective than HFC-125 and

HFC-134a, which only have one. HFC-134, which contains no CF3 groups, was the least effective of the

fluorinated ethanes for most flame conditions. The presence of additional fluorine atoms in FC-116 and 

HFC-125 could explain their enhanced effectiveness relative to HFC-134a and HFC-134. However, the 

tetrafluoroethanes have the same number of fluorine atoms. At strain rates above 100 s-1, HFC-134a was 

more effective than HFC-134, which correlates with the presence of a CF3 group in the former. At lower 

strain rates, the extinction concentrations of these two agents were comparable. 

The presence or absence of a CF3 group also influenced the suppression effectiveness of the halons shown 

in Figure 3-3c. This figure shows that halon 1201 (CHF2Br) was less effective than both halon 1301 

(CF3Br) and halon 13001 (CF3I), particularly at higher loadings; and halon 1211 (CF2ClBr) was slightly 

less effective than halon 1301. Sheinson et al.13 investigated the chemical suppression effectiveness of

the CF3Y and SF5Y (Y = F, Cl, Br, I) series in an n-heptane cup burner. They concluded that, for CF3Cl,

the CF3 moiety made a greater chemical contribution to suppression than the Cl atom. This conclusion is

consistent with the greater effectiveness of halon 1301 relative to halon 1211 observed here. 

Fluorinated Ethanes vs. Fluorinated Propanes 

The effectiveness of the fluorinated ethanes relative to that of the fluorinated propanes can be assessed by 

comparing Figure 3-2a to Figure 3-2b, and Figure 3-3a to Figure 3-3b. At strain rates below 100 s-1, 

fluorinated ethanes extinguished methane flames at concentrations between 5 % and 10 %. Loadings 

between 4 % and 8 % were required for the fluorinated propanes. In the case of propane flames, below 

100 s-1, extinction concentrations for the fluorinated ethanes ranged from 6 % to 10 %. For the 

fluorinated propanes, they ranged from 4 % to 8 %. The steeper extinction strain rate curves for the

fluorinated propanes indicate that, as a group, they were more effective agents on a molar basis than the 

fluorinated ethanes. Table 3-1 shows that the specific heats at 300 K of the fluorinated propanes range 

from 122.0 to 150.6 J-mol

-1-K-1, as opposed to 87.2 to 107.7 J-mol-1-K-1 for the fluorinated ethanes. The

greater effectiveness of the fluorinated propane agents is consistent with their greater sensible enthalpy 

relative to the fluorinated ethanes. 
134 Flame Suppression Chemistry

Methane Flames vs. Propane Flames 

Figure 3-2 and Figure 3-3 can also be compared to assess the differences between suppression of methane 

and propane non-premixed counterflow flames. The uninhibited propane flame was more difficult to 

extinguish, requiring an average strain rate of 583 s-1 vs. 415 s-1 for methane. However, the extinction 

strain rates of flames fueled with propane decreased more rapidly with increases in suppressant loading 

such that, at strain rates below 100 s-1, methane and propane flames were extinguished by similar 

fluorinated agent loadings. If the extinction strain rates are normalized by the corresponding uninhibited 

strain rates and compared, the sensitivity of both methane and propane flames to agent addition is found 

to be similar. The propane flames were slightly more sensitive to the suppressants. Figure 3-2 and show 

that extinction strain rate curve shapes are different for the two fuels. 

Numerical Investigation of Tetrafluoroethane Isomer Effectiveness 

The kinetic model developed at NIST83 for fluorinated hydrocarbons has been used to numerically 

investigate the effects of halogenated compounds on C1 and C2 organic flames. 52,54,58,70 With this model, 

Tanoff et al.70 predicted extinction strain rates for methane/air counterflow flames suppressed by CF4, 

CHF3, CF3CF3 (FC-116), CF3CHF2 (HFC-125), and CF3CH2F (HFC-134a). The NIST model was also 

used by Linteris et al.57, who reported both experimental and computational flame speeds for methane/air 

mixtures containing FC-116, HFC-125, and HFC-134a. Reaction pathway analyses of agent 

decomposition showed that CF2O was an important intermediate for FC-116, which initially decomposes 

to form two CF3 radicals. The numerical calculations predicted that as the hydrogen content of the 

fluorinated ethane increased, less CF2O was produced. To our knowledge, no previous modeling studies 

have compared different isomers. In the present form of the kinetic mechanism, only fluoroethane 

isomers may be compared, because the fluorocarbon kinetic mechanism, in its original form, does not 

include C3 species. Recently, the mechanism has been extended to cover HFC-227ea. 55,56,84 Kinetic 

mechanisms for other fluoropropanes have not yet been developed. 

In the present study, calculations were conducted using the NIST model to predict flame speeds of 

stoichiometric methane/air mixtures doped with 3.81 mole % of either HFC-134 or HFC-134a. The goal 

of these calculations was to determine if the current kinetic model predicts isomeric differences between 

the two agents, and why these differences may exist. Although extinction strain rate predictions, such as 

those performed by Tanoff et al., are more relevant to the measurements of Figure 3-2a and Figure 3-3a, 

they are computationally more demanding. It is generally acknowledged that flame speed is closely 

related to extinction strain rate.71 The current calculations, which predict isomeric differences in 

suppression, should therefore carry over to non-premixed flames. 

The flame speed calculations were performed on a domain extending 25 cm from the flame into the cold 

reactants, and 60 cm into the hot products. The calculations used multicomponent viscosities, thermal 

diffusivities for H and H2, and windward differencing for the convective terms. The initial temperature of 

the reactants was set to 298.2 K. The ultimate solutions were obtained on meshes having 150 grid points. 
Halogen-containing Compounds 135

The kinetic mechanism used was essentially that developed by NIST, modified according to L’Espérance 

et al.61 The reactions 

FCO + CHF2 ⇔ CF2:CO + HF (R3-1) 

CF3CHF + H ⇔ CHF:CF2 + HF (R3-2) 

CF3CHF + H ⇔ CH2F + CF3 (R3-3)

HCO + CF3 ⇔ CF2:CO + HF (R3-4) 

were taken from Linteris et al. 57 Furthermore, the reactions 

CH2F + CF3 + M ⇔ CH2FCF3 + M (R3-5) 

CH2F + CF3 ⇔ CHF:CF2 + HF (R3-6) 

were also added. These latter two reactions were not present in the NIST mechanism, although the 

analogous reactions for all other pairs of fluorinated methyl radicals were included. The kinetic rates of

reactions R3-5 and R3-6 were estimated by taking the geometric mean of the rates of the analogous 

reactions for CH3 + CF3 and CHF2 + CF3. R3-5 represents an additional destruction pathway for 

HFC-134a not considered in the modeling of Reference 57. 

Finally, the thermal decomposition reaction 

CF3-CHF (+ M) ⇔ CF2:CHF + F (+ M) (R3-7) 

was added using the high pressure kinetic expression from Reference 56 and estimated low pressure 

kinetics from Reference 84. This reaction proved very important in modeling suppression by HFC-227ea, 

in which the CF3-CHF radical is formed from the parent agent by C-C bond dissociation. From

HFC-134a, this radical is produced by hydrogen abstraction. 

The predicted flame speeds for the mixtures containing HFC-134 and HFC-134a were 18.9 cm/s and

15.5 cm/s, respectively, or 48 % and 39 % of the calculated uninhibited methane/air flame speed of 

39.4 cm/s. For the HFC-134a case, the experimental measurement and numerical prediction of Linteris et 

al.57 were 38 % and 29 %, respectively. Flame speed measurements for methane/air/HFC-134 mixtures 

have not been reported. Thus, the kinetic model predicted a substantial difference in inhibition

effectiveness between the two tetrafluoroethane isomers. The refinements in the kinetic mechanism 

greatly improved agreement with the experimental flame speed measurement for HFC-134a. 

Reaction pathway diagrams for HFC-134 and HFC-134a are shown in Figure 3-4a and Figure 3-4b. The 

major removal pathway for CHF2-CHF2 is thermal dissociation of the C-C bond. For CH2F-CF3, the 

major removal pathways are hydrogen abstraction by H and OH, and HF elimination. Sensitivity analysis 

indicated that these initial reaction steps had the greatest influence on the difference in flame speed

between the two isomers. This may be a consequence of the major removal pathways for HFC-134a 

consuming H or OH and generating less reactive species, while the dominant HFC-134 pathway generates 

radicals. 
136 Flame Suppression Chemistry

Figure 3-4. Reaction 

Pathways for 

Premixed 

Methane/air Flames 

Doped with 3.81 

Volume % of: 

(a) HFC-134 and 

(b) HFC-134a. 

The reaction pathways associated with HFC-227ea should be representative of those of other fluorinated 

propanes. HFC-227ea, under most conditions, is consumed by thermal decomposition, not radical attack.

The decomposition pathways are HF elimination and C-C bond removal. The latter process dominates in 

near-stoichiometric, high temperature flames.84 For asymmetric isomers, more than one product channel

exists for most agent destruction reactions. If a fluorinated ethyl radical is produced by thermal

decomposition, it is likely to decompose through H atom elimination from the methyl group, if one is 

present. Otherwise, F atom elimination will be the dominant process.84 H and F are both reactive species. 

However, H has a higher diffusivity and participates in the H + O2 chain branching reaction, while F 

reacts primarily in a chain propagating reaction with H2O. These considerations provide one possible 

explanation for the observed isomeric differences among fluoropropanes. Kinetic mechanisms for these 

agents must be developed and validated before a more definitive analysis can be conducted. 
Halogen-containing Compounds 137

Cup Burner Experiments 

The n-heptane and methanol cup burner extinction concentrations obtained in Reference 62 are provided

in Table 3-2. Concentrations based on both volumetric and molar proportions are included. The two sets

of values are not identical as several of the agents deviate from ideal gas behavior at ambient conditions.

Each concentration represents the average of at least 3 runs. At the 99.7 % confidence level (3σ), the 

uncertainties associated with the cup burner values were estimated to be ± 0.6 mole %, with the exception 

of those for CF4 and N2, estimated to be ± 1.5 mole %. Table 3-2 also shows the n-heptane cup burner

data (vol. %) of Hamins et al.50 Extinction concentrations from the two data sets are within 7 % of each 

other for all agents except HFC-236fa, for which they differ by 11.5 %.

Table 3-2. Cup Burner Extinction Concentrations. 

n-Heptane 

Cup Burner 

(Present Study) 

n-Heptane 

Cup Burner 

(Reference 50) 

Methanol 

Cup Burner 

(Present Study) 

Agent Vol. % Mol. % Vol. % Vol. % Mol. %

Halon 1301 3.1a 3.1 3.0 6.3 6.4

Halon 13001 3.2a 3.2

Halon 1211 3.6 3.7 

Halon 1201 4.1 4.1 

FC-218 6.1b 6.3 6.4

HFC-236fa 6.1 6.3 6.8 8.0 8.2

HFC-227ea 6.4 6.6 6.3 8.9 9.2

HFC-227ca 6.9 7.1

HFC-236ea 7.2 7.5 9.9 10.3

HFC-236cb 7.4 7.7

FC-116 7.9a 8.0 8.4

HFC-125 8.8 8.9 8.8

HFC-134a 10.0 10.2 10.2

Sulfur Hexafluoride 10.6 10.9 

HFC-134 10.9 11.1

Tetrafluoromethane 16.0a 16.0 22.0 22.1

Nitrogen 30.0a 30.0 31.8 41.6 41.6

a

 From Reference 13. b

 From Reference 13, but corrected for typographical error. 

Hamins et al.

50 and Saso et al.68 compared cup burner values to extinction data collected in liquid 

n-heptane counterflow flames. Both studies found that counterflow extinction concentrations at low

strain rates were similar to those obtained in a cup burner. To assess the impact of fuel type on extinction 

loadings, methane and propane counterflow values at low strain rates were compared in the present study
138 Flame Suppression Chemistry

to n-heptane and methanol cup burner values. An appropriate counterflow extinction strain rate must be 

selected to make the comparison. Hamins et al. used a global strain rate of 50 s-1. Saso et al. used a 

global strain rate of 30 s-1, but defined their global strain rate as half that used by Hamins et al. In this 

investigation, a local strain rate of 80 s-1 was chosen. 

Figure 3-5a, b, c, and d show n-heptane cup burner, methanol cup burner, methane counterflow, and 

propane counterflow extinction results, respectively. For each fuel, the agents are presented in decreasing 

order of effectiveness, with the most effective agent being the top bar. The methane and propane 

counterflow extinction values for N2 were divided by two for presentation in the figure. The counterflow 

extinction concentrations for agents not tested at strain rates as low as 80 s-1 were extrapolated from the 

curves of Figure 3-2 and Figure 3-3. The predicted agent extinction concentrations represent the loadings 

that would be required to suppress the given flames assuming that the agents were inert, and that they 

extinguished the flames at the same sensible enthalpy per mole of oxygen as CF4 and SF6. 

Physical Versus Chemical Contributions to Suppression 

Physical versus chemical contributions to suppression for different agents are compared following the 

formalism of Tucker et al.69 and Sheinson et al.13 Physical contributions attributed to agent addition 

include increased thermal mass, oxygen dilution, and increased conductive heat losses for agents with 

high thermal conductivities such as helium. Sheinson et al. found that the dominant physical effect of 

most fluorinated agents is to add thermal mass. CF4 and SF6, essentially inert, both extinguish n-heptane 

cup burner flames when the amount of agent added is such that the energy required to raise the 

temperature of the nitrogen and agent present, from 300 K to 1600 K, exceeds a critical value. This 

critical value is proportional to the amount of oxygen supplied, and is similar for CF4 and SF6. It is lower 

for non-fluorinated agents such as Ar, He, N2, and CO2, due to greater contributions of thermal 

conductivity and oxygen dilution to suppression for these agents. 

In the present study, the extinction concentration for CF4 was used to estimate the sensible enthalpy per 

mole of oxygen required to extinguish a methanol flame. The estimated extinction sensible enthalpy for 

this flame was found to be 322 kJ/mol O2. Similarly, SF6 extinction concentrations were used to obtain 

extinction sensible enthalpies for methane and propane flames of 254 kJ/mol O2 and 258 kJ/mol O2, 

respectively. In the case of n-heptane, both CF4 and SF6 extinction concentrations were available. The 

SF6 data led to a sensible enthalpy value 1 % lower than that estimated from the CF4 data. The average of 

the two values, 267 kJ/mol O2, was chosen as the extinction sensible enthalpy for the n-heptane flame. 

Physical contributions to suppression of other compounds can be estimated by assuming that they are 

inert and that they extinguish a given flame at the same sensible enthalpy per mole of oxygen as CF4 and 

SF6. The method is valid for compounds with physical contributions to suppression dominated by 

thermal mass addition, as they are for SF6 and CF4. Physical predictions for the extinction concentrations 

of the agents tested in this study are shown in Figure 3-5. The sensible enthalpies required to raise agent 

temperature from 300 K to 1600 K are provided in Table 3-1. A comparison of the measured and 

predicted extinction data reveals that the bromine- and iodine-containing compounds suppress flames 

mostly through chemical means, with physical contributions to suppression not exceeding 30 % 

regardless of fuel type. For all of the other agents, the chemical contribution to suppression does not 

exceed 35 %. The differences between the measured and predicted values for N2 are consistent with the 

findings of Sheinson et al. For HFC-134 and HFC-245ca in methane, the actual extinction concentrations 
Halogen-containing Compounds 139

are larger than the sensible enthalpy predictions. The net effect of fluorocarbon chemistry for these 

agents, which do not contain CF3 groups, is to promote combustion. 

Figure 3-5. Extinction Concentrations for Agents Suppressing: (a) n-heptane/air Cup 

Burner Flames; (b) Methanol/air Cup Burner Flames; (c) Methane/air Counterflow Flames 

at a Strain Rate of 80 s-1; (d) Propane/air Counterflow Flames at a Strain Rate of 80 s-1. 
140 Flame Suppression Chemistry

Effect of Fuel Type on Suppression Effectiveness Ranking 

The extinction data of Figure 3-5 show that agent effectiveness rankings were similar for the fuels tested. 

The bromine- or iodine-containing agents formed the most effective group of suppressants. FC-218, 

HFC-227ea, HFC-227ca and HFC-236fa constituted the next most effective group, followed by the 

remaining hexafluoropropanes and the pentafluoropropanes. HFC-245ca was the least effective of the 

fluorinated propanes tested, but was more effective than FC-116 and HFC-125. The tetrafluoroethanes 

and SF6 were only more effective than CF4 and N2. The effectiveness ranking for methane, although 

similar to that of the other fuels, did exhibit some peculiarities. First, FC-116 appeared more effective 

than most of the fluorinated propanes. Second, within the 3σ experimental uncertainty of ± 0.8 mole %, 

all of the fluorinated propanes tested containing less than two CF3 groups were equal in their ability to 

suppress methane flames at 80 s-1. 

Although the relative effectiveness of the various suppressant groups was similar for all the fuels tested, 

the exact ranking within a group depended on the fuel. Furthermore, the rankings extracted from the 

counterflow data were somewhat strain rate dependent. The position of the SF6 extinction strain rate 

curve relative to those of the fluorinated ethanes illustrates this dependence. 

Figure 3-3a shows that at strain rates between 400 s-1 and 500 s-1, in a propane flame, SF6 was more 

effective than HFC-125. Below 100 s-1, it was less effective than both HFC-134 and HFC-134a. These 

extinction concentrations are all expressed on a molar basis. On a mass basis, the bromine- and iodinecontaining agents were still the most effective suppressants, with all of the other agents tested, including 

N2, exhibiting similar effectiveness. 

Effect of Fuel Type on Agent Extinction Concentrations 

Figure 3-5 shows that fuel type had an impact on the magnitude of the extinction concentrations, but not 

on the effectiveness ranking of the agents tested. The extinction concentrations obtained in the four 

different fuels are compared for several agents in Figure 3-6. Included are all of the agents tested in the 

n-heptane cup burner, with the exception of halon 13001, halon 1211, and halon 1201. The agents are 

ranked in order of decreasing n-heptane effectiveness, with the most effective agent as the top bar. 

Figure 3-6 shows that, for the agents tested, methanol flames were more difficult to extinguish than the 

alkane flames. This was due in part to methanol’s high sensible enthalpy of extinction relative to that of 

the alkanes. For all agents except halon 1301, between 30 % and 40 % more agent was required to 

suppress methanol flames relative to the n-heptane flames. For halon 1301, the required amount doubled. 

The greater reduction in the effectiveness of halon 1301 was attributed to a decrease in its chemical 

contribution to suppression. For n-heptane, chemical factors contribute 80 % to halon 1301’s 

effectiveness. For methanol, they contribute 70 %. 

Extinction concentrations for methane and propane non-premixed counterflow flames at 80 s-1 were 

similar to those for n-heptane cup burner flames. All 12 of the agents tested in both n-heptane and 

methane flames were more effective against the latter. The largest deviations between the two fuels 

occurred with N2, 28 %, and HFC-116, 20 %. For 8 out of the 12 agents, the deviations were less than 

10 %. In the case of propane, the largest deviations from the n-heptane values occurred with HFC-236cb, 

25 %, and HFC-134, 22 %. For 4 out of the 13 agents tested in both fuels in Figure 3-6, the deviations 

were less than 10 %, with all 13 agents more effective in propane. Halon 1201 and halon 13001 were the 
Halogen-containing Compounds 141

only agents more effective in n-heptane: 10 % more halon 1201 and 3 % more halon 13001 were required

for propane flames. Counterflow experiments with both methane and propane at low strain rates yield 

extinction concentrations, in addition to effectiveness rankings, applicable to n-heptane cup burners. 

Figure 3-6. Effect of Fuel Type on 

Agent Extinction Concentration. 

The n-heptane and Methanol Data 

were Collected in a Cup Burner. 

The Methane and Propane Data 

were Collected in a Counterflow

Burner at a Strain Rate of 80 s-1. 

Conclusions 

Curves relating extinction strain rate to agent molar concentration were obtained for a variety of 

suppressants in methane/air and propane/air non-premixed counterflow flames. The list included four

fluorinated ethanes, ten fluorinated propanes, four bromine- or iodine-containing halons, CF4, SF6 and N2. 

• The relative effectiveness of the fluorinated propanes correlated with the number of CF3 groups in

the agent’s molecular structure. Effectiveness did not correlate significantly with any other 

structural feature. 
142 Flame Suppression Chemistry

• For agents with the same number of CF3 groups, compounds with higher fluorine to hydrogen 

atom ratios were slightly more effective, particularly at low concentrations. 

• The presence of CF3 groups also correlated with an enhanced suppression performance of agents 

that contain bromine. 

• Numerical predictions of the flame speed of doped methane/air mixtures indicated that 

HFC-134a, with one CF3 group, caused a greater reduction in flame speed than HFC-134. 

Chemical kinetic analysis indicated that a difference in the major pathway for removal of the 

parent compound was primarily responsible. 

• Overall, the fluorinated propanes were found to be more effective on a molar basis than the 

fluorinated ethanes. This was consistent with the greater sensible enthalpy of the former. For all 

the HFCs studied, the chemical contributions to suppression did not exceed 35 %. 

• Bromine- or iodine-containing agents were more effective than compounds that contained 

fluorine as the sole halogen. These bromine- and iodine-containing agents suppressed flames 

mainly through chemical means, with physical contributions to suppression not exceeding 

30 %. 

• Fuel type did not affect agent effectiveness ranking, but did affect the magnitude of the extinction 

concentrations. Methanol flames were more difficult to extinguish than either methane, propane, 

or n-heptane flames, which were similar to each other. For halon 1301, the agent concentration 

required to extinguish a methanol cup burner was more than double the amount required to 

extinguish an n-heptane flame. For HFC-236fa, HFC-227ea, HFC-236ea, CF4, and N2, 30 % to 

40 % more agent was required. 

3.3 METAL-CONTAINING COMPOUNDS 

3.3.1 Background 

The NGP also conducted research to identify new, non-halocarbon suppressants and to understand the 

mechanisms of inhibition of known, effective agents.85-87 Metal-containing compounds were of particular 

interest. Prior to the NGP, the behavior of metal compounds in flames had been studied with regard to 

several flame phenomena and had been found to be up to several orders of magnitude more effective 

flame inhibitors than the halogens.34 While research had been conducted to understand metals in 

hydrocarbon fames, and progress had been made, a thorough description of the mechanisms of inhibition 

provided by these agents had yet to be delineated. 

In the most obviously relevant work, metal-containing compounds have been added to flames in a number 

of screening tests, aimed specifically at assessing their potential as fire suppressants. Results from these 

tests have helped understand the compounds’ influence on flame speed (premixed flames) and extinction 

(diffusion flames), as well as their effect on ignition. Flame studies have also been used to understand the 

detailed mechanism of inhibition by metal agents, either by providing direct experimental data on species 

present in the flame zone, or by validating numerical models which are then used to calculate the flame 

structure; either of these approaches are then used to develop an understanding of the relevant chemical 

kinetic mechanisms. 
Metal-containing Compounds 143

Studies of engine knock suppression by metal-containing compounds provided much early data on metals

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

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

air, the mechanisms of engine knock reduction may have clear relevance to fire suppression—in which 

the goal is again to reduce the overall reaction rate with the addition of the suppressing agent. In 

engines—as well as in heating applications—research has also been directed at understanding their 

efficacy at soot reduction. Although both soot formation and the overall reaction rate of flames are 

known to be related to the location and concentration of radicals in the flame, the effects of metals on soot 

formation are not reviewed here. 

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

work has been done with premixed, atmospheric pressure flat flames. The flat flame provides a nearly

one-dimensional system, and the region above the flame (i.e., downstream of the main reaction zone) 

provides a long residence time, high temperature region for radical recombination. The H-atom

concentration typically is measured with the Li-LiOH method (described later in this chapter), and the 

additive’s effect on the radical recombination rate is determined. In addition, much detailed fundamental 

understanding has come from shock tube studies and flash photolysis studies in reaction vessels. Some 

studies of fire retardants are also relevant to fire suppression mechanisms of metals, such as when the fire

retardant works by suppressing the gas phase reactions (and the subsequent heat release and heat feedback

to the solid sample). Finally, after-burning in rocket nozzles provided motivation to understand metalcatalyzed radical recombination reactions, and modeling studies have been performed for those systems. 

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

effectiveness in these varied applications may well be related. In any event, data from each of the

applications can provide insight into possible metals for application to fire suppression as well as provide 

fundamental data useful for predicting their performance in a range of applications. Work investigating

the effect of metals for each of the applications is described in the remainder of this subsection. 

The remainder of this section provides background on the current understanding of metal inhibition of 

flames, identifying metals with fire suppression potential, and presents fundamental data which can be 

used in future comparison with modeling results. 

In particular, iron pentacarbonyl, Fe(CO)5, has been found to be one of the strongest inhibitors, up to two

orders of magnitude more effective than CF3Br at reducing the burning velocity of premixed 

hydrocarbon-air flames.88,89

 Despite past research, a thorough description of its mechanism of inhibition 

had not been delineated. Iron pentacarbonyl forms particulates upon passing through a flame. 

Interestingly, other very effective inhibitors also involve a condensed phase. These inhibitors include 

those which form the particulates after passing through the flame as well as those which are initially

added as a condensed phase. The former category includes other organometallics compounds such as lead

tetraethyl and nickel carbonyl and the halometallic compounds TiCl4 and SnCl4,

34 as well as a new class

of fire suppressants, pyrotechnically generated aerosols,90 which may work similarly. The latter category 

includes the widely used alkali metal salt powders NaHCO3 and KHCO3,

91 other metal salts,92 and a new 

type of suppressant, non-volatile organic precursors.93 These condensed phase agents have many

similarities, in particular, their strong inhibiting action and the lack of a complete understanding of their 

modes of inhibition. 
144 Flame Suppression Chemistry

Because Fe(CO)5 is so effective, it was selected first for further study under the NGP. Although its 

toxicity prevents it from being used as a flame suppressant, an understanding of its inhibition mechanism 

was expected to help in developing new agents, and the experimental and analytical tools being developed 

would be useful for the study of other heterogeneous inhibitors. The approach was to use simple 

laboratory flames, both premixed Bunsen-type and counterflow diffusion, to obtain fundamental 

information on the action of iron pentacarbonyl. The burning velocity and extinction strain rate, both of 

which provide a measure of the overall reaction rate, were determined with addition of Fe(CO)5, while 

varying the stoichiometry, oxygen mole fraction, flame temperature, and flame location. These 

experiments allowed control of the chemical environment, the location where the metal-containing 

species were formed, and the transport of these species to the reaction zone. 

Studies of the inhibition mechanism of the iron are described, and are followed by a description of the 

reasons why it losses its effectiveness in some flame systems. The equivalent flame inhibiting species of 

other metal agents are then discussed, and evidence for any potential loss of effectiveness for these other 

metals is assembled. 

Engine Knock 

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

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

in an engine cylinder brought about by the homogeneous ignition of the end-gas region of highly 

compressed and heated fuel and air. The effect of some agents in reducing knock has been known since 

the 1920s, including compounds of bromine, iodine, tellurium, tin, selenium, iron, and lead, as well as 

aniline.94 Tetraethyl lead (TEL) very early became the anti-knock agent of choice. While much 

subsequent research was performed to understand its mechanism of knock reduction, the exact 

mechanism for this agent remains an unsolved problem in combustion research (perhaps because leaded 

fuels were later banned due to their poisoning effect on exhaust catalytic converters). Although much 

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

mechanism95 and those promoting a homogeneous radical recombination mechanism.96

Several known effects of lead in engines support the heterogeneous mechanism. Muraour97 appears to 

have been the first to propose chain-breaking reactions on the surface of a colloidal fog formed from TEL. 

The particle cloud was subsequently shown to be composed of PbO, which is the active species.98 Since a 

strong influence of PbO coatings on reaction vessel walls has also been observed,98 a heterogeneous 

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

evidence is somewhat circumstantial. The known metallic antiknock compounds (tetraethyl lead, 

tellurium diethyl, iron pentacarbonyl, nickel tetracarbonyl95) all produce a fog of solid particles. The 

alkyls of bismuth, lead, and thallium are anti-knocks, but those of mercury (which does not form 

particles) are not.99 Richardson et al.100 showed that carboxylic acids increase the research octane number 

of TEL in engines, and argued that they reduced agglomeration of the PbO particles in the engine endgas, but acknowledged that their arguments were qualitative. Zimpel and Graiff101 sampled end gases in a 

fired engine for transmission electron microscopy (TEM). They claimed that 1000 nm diameter particles 

formed prior to the arrival of the flame, proving that the effect of TEL was heterogeneous. Commenters 

to the paper pointed out, however, that the particles could be forming in the sampling system, that the 

effect of lead extenders was not captured in the particle morphology, and that even if particles form, the 

gas phase species can also be present, and it is thus not precluded that they are doing the inhibition. 
Metal-containing Compounds 145

Based on the work of Rumminger and Linteris,102 even if particles form, they can re-evaporate in the hot 

region of the flame if they are small enough. Finally, as described by Walsh,95 a major shortcoming in the 

work with TEL is that it is easily absorbed on the way to the reaction vessel, so that it is difficult to know 

how much TEL actually makes it to the flame. As subsequently described by Linteris and 

co-workers, the efficiency103-107

 and particle formation102,108 of organometallic agents are strongly affected

by the volume fraction of the metal compound. 

In later work, Kuppo Rao and Prasad109

 claimed to prove the heterogeneous mechanism of lead antiknock agents. They inserted copper fins coated with PbO into the cylinder of an engine, or injected 30 μm 

particles into the air stream, and found antiknock effects. They interpreted these results as evidence that 

the mechanism is heterogeneous. Nonetheless, they did not measure for the presence of gas phase lead

compounds, so a homogeneous mechanism cannot be ruled out. For the eleven lead compounds tested, 

they found the effectiveness to vary by a factor of about six, and found similarly sized particles of CuO2, 

CuO, CuCl2, NiCl2, and SnCl2 to have equal effectiveness which was less than any of the lead

compounds. 

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

by Norrish.96 Using flash photolysis of mixtures of acetylene, amyl nitrite, and oxygen in a reaction

vessel, with and without TEL, the absorption and emission spectra of amyl nitrate, OH, Pb, PbO, NO,

CN, CH, and TEL were obtained as a function of reaction progress. The reactants were chosen since, in 

the absence of anti-knock compounds, they showed the strong homogeneous detonation characteristic of

engine knock. The researchers found that the induction time increased linearly with TEL addition at low

TEL partial pressures, but that the effectiveness dropped off at higher pressures. This result was 

described subsequently for the flame inhibitors Fe(CO)5,

88,103

 SnCl4

34,107 and CH3C5H4Mn(CO)3

(MMT).107

 After decomposition of TEL, Pb was present in low concentration, followed by large amounts 

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

and the increase in OH emission was smoother and better behaved. No particles were reported. Norrish

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

stage, TEL reacts with the peroxide and aldehyde intermediates (characteristic of the end of the cool

flame regime of alkane combustion), thus reducing the availability of these species for initiating the wellknown second stage of the combustion. Gas phase PbO from the TEL then reacts with the chain-carrying 

intermediates to reduce the rate of heat release, slow the temperature rise, and reduce detonation. It is 

noteworthy that many of the features subsequently described by Linteris and Rumminger110

 as necessary

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

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

coexistence of OH and metal monoxide species) were shown in the 1950s by Norrish and co-workers to

be necessary for effective knock reduction by TEL. Although it was of the highest quality, the findings of 

Norrish and co-workers were discredited by the engine community largely because the tests were not

done in engines or with typical fuels. This work predated most of the important flame inhibition work of 

the 1970s by 20 years. 

Flame Screening Tests 

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

metal-containing compounds as flame inhibitors. The seminal work of Lask and Wagner34 investigated 

the efficiency of numerous compounds for reducing the burning velocity of premixed Bunsen-type
146 Flame Suppression Chemistry

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

effective at low volume fraction (and GeCl4 about a factor of two less then these). In unpublished work, 

cited by others,111 Lask and Wagner provide the measured flame speed reduction with addition of SbCl3

to hexane-air flames, showing it to be about twice as effective as CF3Br. They tested Fe(CO)5, Pb(C2H5)4, 

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

150 μL/L required for the first two to reduce the burning velocity by 30 %. (Although not quantified, they 

believed the effectiveness of CrO2Cl2 to be even higher.) They categorized the active species in the 

compounds they tested into two classes: the halogens, and the much more effective transition metals. 

Skinner and coworkers,112 using a burner which produced conical flames, tested 80 compounds for 

reducing the burning velocity of premixed hydrogen-air flames (fuel-air equivalence ratio φ of 1.75). 

They found the most effective to be: tetramethyl lead TML Pb(CH3)4, Fe(CO)5, TiCl4, SnCl4, SbCl5, and 

TEL (in that order), with TEL only slightly better than CF3Br, and TML about 11 times better. A 

limitation of this work, however, was that the agents were tested at only a single volume fraction. Since 

the efficiency of many agents is known to vary with their concentration,103 such an approach can skew the 

relative performance rankings. Also, in the hotter, faster-burning hydrogen flames, the propensity to form 

particles from the metal oxides and hydroxides is less than in the slower, cooler hydrocarbon-air flames of 

other studies (see Rumminger et al.102). An additional flame screening test was performed by Miller,113

who tested 15 compounds added to low-pressure (1.01 kPa) premixed and diffusion flames, and found 

SnCl4, POCl3, TiCl4, Fe(CO)5), WF6, and CrO2Cl2 to have some promise. 

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

particles of inhibitor. Rosser et al.91 added metal salts as dispersions of fine particles (2 μm to 6 μm 

diameter) to premixed methane-air flames. They used the reduction in upward flame propagation rate 

through a vertical tube as a measure of the inhibition effect. They correctly described the effects of flame 

speed and particle diameter on the particle heating rate, and together with the volatilization rate for each 

compound, assessed the fraction of the particle which was vaporized. They quantified the effect of 

particle size on inhibition effectiveness, and showed that while additional agent decreased the flame 

speed, the effectiveness eventually saturated (i.e., beyond a certain additive mass fraction, additional 

inhibitor had a greatly reduced effect on the flame speed). They postulated a homogeneous gas phase 

inhibition mechanism involving H, O, and OH radical recombination reaction with the metal atom and its 

hydroxide. This mechanism had many of the features of subsequently described mechanisms107,114-116

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

relies upon a super-equilibrium concentration of radicals (commonly present in premixed flames and 

diffusion flames at higher strain), and that this provides an upper limit to the chemical effect of 

catalytically acting agents. Finally, they suggested that adding an inert compound as a co-inhibitor can 

overcome this limitation. A number of alkali metal sulfates, carbonates, and chlorides were tested, as 

well as cuprous chloride CuCl, which was found to be about twice as effective as Na2CO3 (after 

correcting for the larger size of the CuCl particles). 

A later study involving premixed flames with particles was performed by deWitte et al.117 In it, relatively 

large particles (100 μm diameter) were electrostatically suspended in a tube and then injected into a 

downward facing premixed Bunsen-type flame, and their effect on the flame temperature, burning 

velocity, and extinction condition was measured. Various barium, sodium, and potassium compounds 

were tested, as well as AlCl2, CuCl2, and PbO. The authors noted a thermal and chemical effect of the 

particles, and assumed that the chemical effect was due to recombination of chain-carrying radicals on the 

particle surfaces. The authors estimated that for these large particles, little of the particles could be 
Metal-containing Compounds 147

vaporized. Nonetheless, it is not possible to separate the heterogeneous and homogeneous inhibition 

effects from their data. The authors found particles of CuCl2 and PbO to be about two and eight times as 

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

Flame screening tests of many compounds were performed by Vanpee and Shirodkar92 to test the relative 

effectiveness of metal salts. The metal acetonates and acetylacetonates were dissolved in ethanol, and 

fine droplets of the metal salt solutions were sprayed into the air stream of a counterflow diffusion flame 

over an ethanol pool. The inhibition effect was quantified, at a given air flow velocity (i.e., strain rate) as 

the change in the oxygen volume fraction at extinction caused by addition of the inhibitor, normalized by 

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

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

volume fraction of inhibitor in the oxidizer stream). Their results are depicted in Figure 3-7, which shows 

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

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

0

50

100

150

200

250

300

350

400

Pb Ac

Co Ac Ac

Mn Ac Ac

Fe Ac Ac

Na Ac Ac

Pb Ac Ac

Cr Ac Ac

Fe(CO)5

NiCl2

CCl4

Mole Merit Number

Figure 3-7. Mole Merit 

Number of Metalcontaining Compounds 

for the Oxidizer Velocities 

in the Range of 50 cm/s to 

60 cm/s.92

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

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

twice as effective the acetylacetonate. It should be noted, however, that the interpretation of the present 

data are complicated by several effects. First, changing the oxygen mole fraction changed the 

temperature, which can change the effectiveness of an agent, as subsequently described in References

103-105, 118, and 119. Since the air stream velocity was changed while holding the nebulizer flow 

constant, the ethanol concentration changed in these partially premixed diffusion flames. Adding 

changing amounts of a fuel species (i.e., the carrier ethanol) in the air stream, changed the flame location

and the scalar dissipation rate for a given strain rate (i.e., air flow velocity), so that the extinction 

condition was modified (as discussed in References 73 and 120). The size of the residual particle (which

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

efficiency. Finally, for metal agents that condense, their marginal effectiveness was a very strong 
148 Flame Suppression Chemistry

function of the concentration at which they are added. Hence, without knowing what the additive mole 

fraction was, it is difficult to cross compare the effectiveness of the different agents. For these reasons, 

the results shown in Figure 3-7 are considered qualitative, rather than quantitative. For example, 

subsequent studies121 have rated iron as about 10 times as effective as sodium, in contrast to the results of 

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

that of sodium. 

Radical Recombination above Premixed Flames 

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

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

The techniques, pioneered by Sugden and co-workers,122-124 involve absorption spectroscopy for OH, and 

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

simultaneously measured above a flat flame. With the assumptions that Na is present in the flame only in 

atomic form and that Li is present only as Li and LiOH, the measured ratio of the Li to Na absorption 

together with the known equilibrium constant for the reaction Li + H2O = LiOH + H provides [H].) In 

some of their early relevant work they determined the dominant metal species in H2-O2-N2 premixed 

flames above Meker burners with added dilute sprays from aqueous salts of copper and manganese. 

Copper was found to exist mostly as Cu in the flame, and the dissociation constants of CuH125 and 

CuOH124 were determined from their concentrations above the flame at different temperatures. 

The copper-containing species present above a Meker burner supplied with H2-N2-O2 mixtures with added 

copper salts were found to be Cu, CuO, CuOH, and CuH.124,125 Their concentrations were related by the 

balanced reactions: 

Cu + H2O ↔ CuOH + H 

Cu + OH + X ↔ CuOH + X 

Cu + H2 ↔ CuH + H 

Cu + H + X ↔ CuH + X . 

In a detailed study of chromium, Bulewicz and Padley126 added μL/L levels of Cr (from either chromium 

carbonyl or aqueous sprays of chromium salts) to a premixed, fuel-rich, flat-flame burner of H2+O2+N2, 

and measured the catalytic radical recombination by the chromium compounds. They identified the 

active chromium species as Cr, CrO, CrO2, and HCrO3 and also detected solid particles, which appeared 

to have equivalent black-body temperatures up to 500 K higher than the gas. They estimated an upper 

limit for the rate of heterogeneous radical recombination on the particle surfaces, and estimated that, at an 

upper limit, it was the same order as the natural un-catalyzed recombination rate in the flame. They also 

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

about 1μL/L, at which heterogeneous particle catalysis cannot be contributing. They cited Jenkins (1969, 

personal communication) as first showing the catalytic effect of metals (Ca, Sr, Ba) on radical 

recombination in flames. Interestingly, their data showed a saturation effect in the catalytic radical 

recombination by Cr species, and their analyses showed that it is due, not to condensation of the active 

gas phase Cr-containing species to particles, but to reduction in the available radicals to recombine. 

Bulewicz and Padley proposed that the following balanced reactions are important in chromium 

inhibition, but did not develop an explicit mechanism: 
Metal-containing Compounds 149

Cr + OH ↔ CrO + H 

CrO + OH ↔ CrO2 + H 

CrO2 + H2O ↔ HCrO3 + H . 

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

be important: 

CrO + H + X = > CrOH* + X 

CrOH* + H = > CrO + H2 . 

In continuing work, Bulewicz and coworkers115,116

 studied the catalytic effects of 21 metal species for 

recombining chain-carrying flame radicals present at super-equilibrium levels above premixed, fuel rich, 

flat flames of H2+O2+N2 at 1860 K. Table 3-3 shows the ratio of the catalyzed to uncatalyzed 

recombination rate for H atom caused by each of the metals, added at 1.3 μL/L. 

Table 3-3. Catalytic Efficiency of Different Metals in Promoting Radical Recombination. 

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

Strong Effect Some Effect No Effect 

Cr 2.8 Co 1.1 V 1

U 1.82 Pb 1.1 Ni 1

Ba 1.75 Zn 1.07 Ga 1

Sn 1.6 Th 1.06 Cl 1

Sr 1.35 Na 1.04

Mn 1.3 Cu 1.04

Mg 1.25 La 1.04

Ca 1.25

Fe 1.2

Mo 1.16

The cut-off value for this ratio was arbitrarily set to 1.1, and those above that value were described as 

having a strong catalytic effect. The possibility of heterogeneous recombination on particles was

admitted, but at these low volume fractions, the authors argued primarily for a homogeneous gas phase 

mechanism involving H or OH reaction with the metal oxide or hydroxide (attributed to Jenkins). 

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

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

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

Concurrent with the alkaline earth metals, Cotton and Jenkins114 showed that Ca, Sr, and Ba added as a 

fine mist of a salt solution to premixed H2+N2+O2 flat flames catalyzed the radical recombination for

additive volume fractions in the range of 1 μL/L to 10 μL/L. By estimating the reaction rates in possible

recombination mechanisms, they recommended the radical recombination mechanism to be: 
150 Flame Suppression Chemistry

MOH + H => MO + H2

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

M(OH)2 + H ↔ MOH + H2O. 

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

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

flames is well-known.127 Cotton et al.128

 added 40 metals to co-flow diffusion flames of propane and 

N2/O2 mixtures and measured their effect on soot emissions and the smoke point. They postulated a

mechanism for soot reduction whereby the catalytic reaction scheme listed above ran backwards, 

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

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

al.129

 Through examination of the variation of the intensity of emitted radiation as a function of

wavelength from particles formed above a premixed H2+O2+N2 flat flame with added spray of aqueous

uranium salt, they determined that the particles (presumably uranium oxide) were up to 500 K above the 

gas temperature. They interpreted the particle temperature rise to be caused by the catalytic

recombination of H and OH on the particle surfaces. Nonetheless, Tischer and Scheller130 pointed out 

that the spectral variation of the particle emissivity was unknown, and the gray-body assumption of 

Bulewicz was probably unjustified. They also argued that the excess temperature may have been due to

surface reactions other than radical recombination. 

In similar work with premixed, fuel rich, flat flames of H2+O2+N2, Jensen and Jones131

 extended the 

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

Sr+

 + H2O ↔ SrOH+ + H ,

in which [SrOH+

]/[Sr

+

] was measured mass spectrometrically. Using both this new technique as well as

the LiOH photometric method, they studied the catalytic flame radical recombination by tungsten and

molybdenum, as well as once again confirming the strong effect of tin. Using flames at temperatures of

1800 K to 2150 K, and with metal addition at 1 μL/L to 110 μL/L, they collected data on the rates of 

radical recombination in the presence of the metallic catalysts (added as tetramethyl tin, or hexacarbonyls 

of tungsten or molybdenum), and measured the major species present in flames inhibited by W and Mo. 

By analogy with the mechanisms they developed for calcium114 and iron132 in flames, they then postulated 

reaction mechanisms for W and Mo, and estimated the rates for the reactions in the catalytic cycles. For 

the conditions of their flames, the radical recombination cycles, for either W or Mo, were about five times 

faster then those of tin. (Using the results of Linteris et al.,107

 this translates to an effectiveness about

10 times that of CF3Br, or about one-fifth that of Fe(CO)5). For tungsten and molybdenum, the cycles 

are: 

HWO3 + H ↔ WO3 + H2

WO3 + H2O ↔ H2WO4

H2WO4 + H ↔ HWO3 + H2O 

(net: H + H ↔ H2) 
Metal-containing Compounds 151

and 

HMoO3 + H ↔ MoO3 + H2

MoO3 + H2O ↔ H2MoO4

H2MoO4 + H ↔ HMoO3 + H2O 

(net: H + H ↔ H2). 

Jensen and Jones131

 found the dominant species in tungsten and molybdenum inhibition to be WO3, 

HWO3, and H2WO4, and MoO3, H MoO3, and H2MoO4. 

In continuing work, Jensen and Jones133

 used similar techniques to study the radical recombination by

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

dicarbonyl at volume fractions of about 0.03 L/L to 145 L/L, and flame temperatures ranging from

1800 K to 2615 K, they spectroscopically identified the dominant cobalt-containing species to be Co,

CoO, CoOH, and Co(OH)2, with most of the cobalt being present in the flame as free Co atoms. Again by 

analogy with the Ca and Fe mechanisms, the Co mechanism was postulated to be: 

Co + OH => CoO + H 

CoOH + H ↔ Co + H2

CoO + H2O ↔ Co(OH)2

Co(OH)2 + H ↔ CoOH + H2O 

(net: H + H ↔ H2) , 

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

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

in these flames. 

Gas phase Flame Retardants 

Insight into mechanisms of metal flame inhibition was gleaned from studies of metal species added to

materials as fire retardants (when their mode of action has been found to be in the gas phase). One such

system is the antimony - halogen combination. Although they did not unravel the detailed mechanism, 

Fenimore and coworkers134-137

 showed that the relevant species act in the gas phase; they believed that the 

antimony moieties poisoned the flame, much as do brominated species. Similarly, Martin and Price138

observed that the addition of triphenylantimony to certain polymer substrates provided fire retardancy, 

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

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

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

HCl, they showed that SbCl3 was evolved through a series of halogenation steps involving successive 

oxychloride phases.139 Using a molecular beam mass spectrometer, they studied both the pyrolysis 

products of polyethylene retarded by antimony-oxide/halogen, as well as intermediate species profiles in 

premixed CH4/O2/N2 flames with added SbCl3 and SbBr3.

140 For the pyrolysis studies, the major effused

species from the polymer were SbCl3 and SbOCl. In the flame studies,141

 they found that SbCl3 reacted

readily to SbOCl, which then reacted with H to form SbO. They measured the major intermediate species
152 Flame Suppression Chemistry

of SbBr3 flame inhibition to be SbO, Sb, HBr, and Br. They also measured the decrease in the hydrogen

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

intermediate species, as well as for the gas phase inhibition reactions. They argued strongly that the 

inhibition effect of antimony-oxide/halogen system was predominantly from the reaction sequence: 

SbO + H → SbOH* 

SbOH + H → SbO + H2

net: H + H → H2

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

equivalent chlorine cycle. 

Ignition Studies 

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

Scheller111 investigated the effect of twenty 20 inhibitors on the ignition of hydrocarbon mixtures by hot 

wires. They found that SnCl4 was the most effective inhibitor tested for increasing the ignition 

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

temperature. Dolan and Dempster142 studied the effect of small particles (5 μm to 10 μm diameter) of 

metal compounds on suppressing the spark ignition of premixed natural gas-air mixtures in a vertical

7 cm diameter tube. They found that barium hydroxide octahydrate, barium chloride, and copper acetate 

monohydrate were each about three times less effective than sodium bicarbonate particles, while cuprous 

oxide and cobaltous chloride hexahydrate were each about 20 % less effective than NaHCO3. Several 

studies have shown that metals can actually speed the ignition process in some chemical systems. In

shock-tube studies, Matsuda and co-workers143

 found that Cr(CO)6 reduced the initiation time for reaction 

of CO or C2H2 with O2 in shock-heated gases. The metal carbonyl was present at 0 μL/L to 50 μL/L, so 

the presence of any particles was precluded. The mode of action was believed to be gas phase reactions

involving CrO, CrO2, and CrO3. In later shock-tube work with Fe(CO)5 in mixtures of CO-O2-Ar, 

Matsuda144

 found that a volume fraction of Fe(CO)5 of few hundred μL/L greatly accelerated the 

consumption of CO2. They postulated the effect to be due to oxidation of CO by metal oxides via: FeO + 

CO => Fe + CO2, and noted that these reactions may be of importance since the mixtures were quite dry

(XOH ≈ 5 μL/L). Interestingly, such accelerating oxidation pathways were also found to be important in 

the dry reaction of CO with N2O in flames.145 In recent shock tube studies of CH4-O2-Ar mixtures, Park et 

al.146

 found that with (500, 1000, or 2000) μL/L of Fe(CO)5, the ignition time was shorter than without the

additive, again indicating a promotion effect of the metal additive; however, they did not determine the 

cause of the promotion for this moist system. Finally, in flash photolysis studies many years ago, Erhard 

and Norrish96 found that unlike Pb(C2H5)4 and Te(CH3)2 which retarded hydrocarbon combustion, 

Ni(CO)4, Fe(CO)5, and Cr(CO)6 all greatly accelerated the combustion. 

Radical Recombination in Rocket Nozzles 

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

desire to suppress afterburning in rocket nozzle exhausts. Using mechanisms developed for K, Ba, Fe, 

Mo, W, Cr, and Sn, Jensen and Webb147

 calculated the amount of inhibitor required to suppress 

afterburning in the exhaust plume of a double-base propellant (unspecified composition, but usually a 
Metal-containing Compounds 153

homogeneous mixture consisting of nitrocellulose and nitroglycerine). The products of the propellant

reactions consisted of a fuel rich mixture, principally CO and H2, much like the recombination region 

above the fuel-rich premixed H2-O2-N2 laboratory flames used to study the metal-catalyzed radical

recombination. Their calculations (which did not include the effects of condensation to particles) 

indicated that W and Mo were about a factor of five less effective than Fe, Cr was about a factor of six 

less effective, and Sn was more than seven times less effective. These results are consistent with those of

the flat flame measurements described above. 

Jensen and Webb did not measure particles or calculate the degrading effects of particle formation on the

suppression of afterburning in rocket motors, but they did estimate the upper limit for radical

recombination by the heterogeneous reactions on the particle surfaces. Their calculations indicated that 

although significant, the heterogeneous reactions could not suppress the afterburning, even for the 

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

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

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

Their conclusions were that the metals held good promise for afterburning suppression in rocket motors, 

and that further work was necessary to estimate the rates of the catalytic cycle reactions and of the

condensation rates of the metal derivatives under the conditions in their system. 

Other Relevant Investigations of Metal Compounds in Combustion Systems 

There have been a number of recent papers dealing with metals in flames. Crosley and co-workers148

added MMT to premixed flames with the purpose of studying its effect on NO formation. Adding MMT

at volume fractions of about 0.5 μL/L to near-stoichiometric premixed low-pressure (522 Pa) propane-air 

flat flames in a McKenna burner, they measured the temperature and the relative concentrations of OH, 

H, O, CH, NO, and CO through the flame. While they observed no discernable effect of the MMT, this 

may have been due either to the low concentration of the additive, or the low pressure, both of which 

could limit the influence.89,107

 They did, however, start to develop a mechanism for manganese which

served as a basis for future efforts.107

Chromium reaction in a atmospheric pressure, premixed hydrogen-air flat flame was studied by Yu et 

al.149

 Using microprobe gas sampling in the region downstream from the main reaction zone, they

collected chromium species in the gas and condensed phase. The particle size distribution and the 

fraction of Cr as Cr(VI) was determined as a function of position. In addition, they assembled a kinetic 

mechanism for Cr reaction in flames through analogy with boron and aluminum combustion, and used the 

mechanism, together with the measured temperature profile, to calculate the fraction of hexavalent Cr in

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

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

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

treatment of Cr speciation in flames. In later work, Kennedy et al.150

 studied the morphology of the 

particles formed in a hydrogen–air–nitrogen co-flow diffusion flame with added chromium nitrate or

chromium hexacarbonyl. They found that the morphology of the particles varied with the temperature of

the flame and the source of the chromium. 

Kellogg and Irikura151 performed theoretical calculations to predict the heats of formation as well as the

enthalpies and free energies of reaction for the FeOxHy species thought to be important in iron inhibition. 
154 Flame Suppression Chemistry

They found that nearly all of the reactions involving these species and potentially contributing to flame 

inhibition are exergonic at 1500 K. Hence, they suggested that further refinement of the inhibition 

mechanisms of iron would require knowledge of the actual kinetic rates of the inhibition reactions to 

improve upon the preliminary estimates of Rumminger et al.104

In a comprehensive review of possible chemicals for use as halon alternatives, Tapscott et al.152 suggested 

that of the metals, Cu, Fe, Mn, and Sn, were worthy of further study. Since that time, premixed107 and 

co-flow diffusion flame153 studies have been performed for compounds containing Fe, Mn, and Sn. No 

additional work has been reported for copper compounds as fire suppressants. 

Detailed Studies with Iron 

After the potential effectiveness of iron as a flame inhibitor was indicated,34,95,99 the behavior of iron 

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

effectiveness of iron was first identified by Lask and Wagner34 in their screening study involving 

methane-, hexane-, and benzene-air premixed Bunsen-type flames with numerous additives. In 

continuing work,89 Wagner and co-workers described the superior effectiveness of Fe(CO)5 for reducing 

the burning velocity of hydrocarbon-air flames in nozzle burners. They found that an Fe(CO)5 volume 

fraction of 100 μL/L reduced the burning velocity by 25 % when added to stoichiometric methane-air 

flames at atmospheric pressure. With oxygen as the oxidizer, or at reduced pressure, they found the 

effectiveness to be lowered. For hydrogen-air flames, Fe(CO)5 was again much more effective than Br2. 

They noted that at low volume fractions, the decrease in the burning velocity was proportional to the 

concentration of Fe(CO)5, whereas for increasing concentrations of Fe(CO)5, the relative influence 

seemed to decrease. They postulated a homogeneous radical recombination mechanism at low volume 

fraction, and a heterogeneous one at higher. 

To understand the detailed mechanism of Fe(CO)5, Bonne et al.89 spectroscopically investigated premixed 

flat flames of methane with air or O2. Unfortunately, at the low pressures for which the flame zone was 

expanded sufficiently to optically probe the flame (800 kPa), the kinetic effect of Fe(CO)5 was very small. 

Nonetheless, for Fe(CO)5 volume fractions up to 100 μL/L they observed that (1) the peak OH volume 

fraction XOH was unchanged, but shifted slightly downstream from the burner, and (2) the decay rate of 

XOH was increased in the presence of Fe(CO)5, clearly indicating the effect of Fe(CO)5 on radical 

concentrations in the flame. They measured FeO and Fe emission, as well as Fe absorption, and found 

that Fe and FeO emission peaked in the main high-temperature reaction zone, implicating these species in 

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

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

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

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

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

In later work, the high effectiveness of iron was confirmed for methane-air premixed Bunsen-type flames, 

and extended to counterflow diffusion flames, by Reinelt and Linteris.103 Their work indicated clearly that 

iron was very effective at low concentration, but that the effectiveness leveled off at higher concentration. 

Further studies with premixed flames of CO-O2-N2 andCO-N2O-N2 confirmed the effectiveness of 

Fe(CO)5 in other systems. 
Metal-containing Compounds 155

3.3.2 Metals with Demonstrated Flame Inhibition Potential 

Based on the above, Table 3-4 presents a list of metals whose compounds have demonstrated inhibitory

effects in flame systems. The type of flame system used to determine the effectiveness is listed across the

top (Note: FR denotes Fire Retardant). Under each category of flame system, the individual reference is

listed. The elements are listed in the approximate order of demonstrated or expected effectiveness. Note, 

however, that there is large uncertainty in this ranking. 

Table 3-4. Metals Which Have Shown Flame Inhibiting Properties. 

Detailed Flame 

Studies Engine Knock

Flame Screening 

Tests 

Flat Flame Radical 

Recombination Ignition FR 

Element 89 103 107 106 153 99 94 109 96 95 34 92 112 113 117 91 116 115 126 133 131 132 142 111 96 139 

Cr x x x x x x

Pb x x x x x x x x x

Fe x x x x x x x x x x x x x

Mn x x

Ni x x

W x x

Mo x x

Cu x x x x x

Te x x x

Tl x

Bi x

U x x

Zn x

La x

Th x

Se x

Sn x x x x x x x x x x x x

Ti x x x

Co x x x x

Ge x

Sb x x x
156 Flame Suppression Chemistry

3.3.3 Flame Inhibition by Iron-containing Compounds 

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

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

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

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

Gas Phase Mechanism 

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

Jensen and Jones,132

 and later, expanded by Rumminger et al.104 In both, the main catalytic cycle leading 

to radical recombination was postulated to be: 

FeOH + H ↔ FeO + H2

FeO + H2O ↔ Fe(OH)2

Fe(OH)2 + H ↔ FeOH + H2O 

(net: H + H ↔ H2). 

This mechanism is shown schematically in Figure 3-8. 

Figure 3-8. Main Catalytic Radical 

Recombination Cycle of Iron 

Found to be Important for 

Methane-air Flames. 

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

not found to be particularly important in methane-air flames, premixed or diffusion.105

 In work with

premixed CO-N2-O2 flames, however, the following additional catalytic cycle was found to be much more

important than the H-atom cycle: 

Fe + O2 +M ↔ FeO2 +M 

FeO2 + O ↔ FeO + O2

FeO + O ↔ Fe + O2

Fe(CO)5 

Fe

FeO2 

FeO 

Fe(OH)2

FeOH 

+O

+H

+H2O 

+H

+O2 + M 

H-atom cycle

+H 

(net: O + O ↔ O2) . 
Metal-containing Compounds 157

This new O-atom cycle together with the H-atom cycle are shown in Figure 3-9. Thicker arrows 

correspond to higher reaction flux. Reaction partners are listed next to each arrow. 

Fe(CO)5 

Fe

FeO2 

FeO 

Fe(OH)2

FeOH

+O

+H

+H2O 

+H

+O2 + M 

+O

O-atom cycle H-atom cycle

Figure 3-9. Radical 

Recombination Reaction 

Pathways Found to be Important 

for CO-H2-O2-N2 Flames.118

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

the possibility of many more radical recombination cycles.151

 In that study, seven iron species thought to

exist at flame temperature were considered: Fe, FeH, FeO, FeOH, FeO2, FeO(OH), Fe(OH)2,

104

 and the 

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

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

thermodynamics. The complexity of the situation is illustrated in Figure 3-10, (Kellogg and Irikura151),

which shows schematically the possible inhibition cycles of iron. (Note that in the figure there are on the

order of 50 possible cycles since [Fe] can be replaced by Fe, FeH, FeO, or FeOH.) 

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

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

steps in the mechanism were selected (within the uncertainty bounds suggested by Jensen and Jones132

) so 

as to provide the best agreement with experiments. The mechanism has been tested only for near 

stoichiometric premixed CH4-N2-O2 , CO-N2-O2, and CO-N2O premixed flames, CH4-O2-N2 counterflow 

diffusion flames, and CH4-air cup burner flames. Although at low Fe additive mole fraction the 

agreement was usually good, there were some conditions for which the predicted inhibition was off

significantly. These cases include lean premixed CH4-air flames (φ=0.9), CH4-O2-N2 flames with an 

oxygen volume fraction in the oxidizer of 0.20, and the cup burner flames with added CO2 (at very low 

Fe(CO)5 volume fraction). 
158 Flame Suppression Chemistry

Figure 3-10. Different Classes of Reactions 

Which May Contribute to Iron's Super-efficient 

Flame Suppression Ability through the Catalytic 

Recombination of Radical Species.151

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

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

radical concentrations typically go above equilibrium levels, to super-equilibrium levels. This is 

particularly true for both premixed and high and moderate strain counterflow diffusion flames. At low 

strains, however, the radical super-equilibrium is less. (See Figure 3-11.) Radical concentrations in actual

fires have not been measured. Nonetheless, recent research using cup burner flames

154 (which, to some

extent, resemble small-scale fires) has shown that a stabilization region at the flame base, called the flame

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

radical recombination cycles should be as important in the stabilization of fires as they are for laboratory

flames. This is partially confirmed since other gas phase catalytic agents (for example, CF3Br) are 

effective in both laboratory flames and full-scale fires. 

0

100

200

300

400

500

0 50 100 150 200

Strain Rate (1/s)

Superequilibrium H-atom Mole Fraction

1500

1700

1900

2100

2300

Temperature K

Tmax

Figure 3-11. Calculated 

H-atom Superequilibrium Ratio and 

Peak Temperature in a 

Counterflow Diffusion

Flame as a Function of 

Strain Rate. 
Metal-containing Compounds 159

Inhibition of Low Pressure Opposed-flow Methane-air Flames by Fe(CO)5 

Introduction 

For premixed and non-premixed flames, measurements of radical concentrations (O, H, OH) serve as

useful indicators of the chemical rates that are being affected, because chemical inhibiting agents act to

reduce the super-equilibrium concentration of radicals. Radical concentrations reach their maximum 

concentrations in the flame reaction zone and then decay toward equilibrium values. Thus, if an inhibitor 

were present, it would reduce the maximum radical concentrations toward equilibrium values and show

the amount of chemical inhibition present.155 Radical measurements are thus directly related to burning 

velocity and extinction strain rate measurements. 

Prior to the NGP, there had been no laboratory measurements of the change in the concentration of radical 

species with increasing addition of Fe(CO)5 to non-premixed flames. In fact, only limited measurements 

of radical species in non-premixed flames inhibited by CF3Br existed.36,51,156

 McNesby and co-workers157

examined the influence that Fe(CO)5 had on the structure of a low pressure, opposed flow, CH4-air

flames. OH radical profiles were measured as the concentration of inhibitor was increased, approaching

extinguishment. Flames inhibited by N2 or CF3Br were included for reference. 

Apparatus 

Figure 3-12 presents a schematic diagram of the experimental arrangement. The opposed flow burner 

apparatus was contained within a 200 L combustion chamber. The combustion chamber was a six-armed

stainless steel cross, with the four horizontal arms being 25.4 cm in diameter and the two vertical arms 

being 30.48 cm in diameter. The opposed jet ducts were mounted in the vertical arms with each head 

having two axes of motion under computer control. The opposed jet duct diameters were 7.68 cm and the 

active burner surface was approximately 7.6 cm in diameter. Laminar flow was established in the jet 

ducts, with the initial gases impinging on a 0.32 cm thick, porous, stainless steel, sintered disk. The gases 

flowed through a 2.54 cm region before passing through a second, 0.32 cm thick, porous, stainless steel,

sintered disk. The second porous disk opened to the combustion chamber. Schlieren images of the gases 

showed laminar flow from both ducts. The gases and cooling water were fed to the burner ducts via 

stainless tubing. 

Optical access to the chamber was provided on the vertical arms of the chamber through 10.16 cm

diameter windows. There were also four additional ports in the horizontal plane, each located between

two of the window arms. These ports served as access for pressure monitoring, thermocouple 

manipulation, and electrical connections. Also mounted on the top and bottom of each window arm were

vacuum ports to remove the combustion gases. These eight ports were joined to a single 5.08 cm I.D.

tube which fed the gas to a scrubber mounted on a high volume vacuum pump. The scrubbed exhaust 

gases were then vented to the atmosphere. All flames were studied at a pressure of 6.6 kPa (50 torr), the

oxidizer flow consisted of 10 L/min synthetic air (79 % N2 + 21 % O2, Matheson UHP Grade) flowing 

from the upper duct, and the fuel flow was 10 L/min of methane (Matheson UHP Grade) flowing from the 

lower duct. The gases were regulated and monitored by a gas handling manifold system constructed from 

a series of flow controllers (Tylan General). 
160 Flame Suppression Chemistry

PC

Boxcar

D/A

Low-Pressure

CFDF PMT

Excim

er/Dye

Las

er

CCD 

Camera

Spectrograph

Slits

Lens

Turning Prism

Iris

Lens

Dye

Cell

Photodiode

Photodiode

Thermocouple

Pulse

Generator

Electrical Signal

Optical Signal

Trigger Signal

312 nm

Filter

Figure 3-12. 

Schematic 

Diagram of the 

Low Pressure 

Burner and 

Optical 

Measurement 

Arrangement.

The oxidizer and fuel ducts were separated 3.8 cm. For the flow conditions and flow duct separation, the

luminous flame zone was located on the oxidizer side of the stagnation plane, and the global strain rate 

was calculated to be 50.2 sec-1.

76 The inhibitors were added to the oxidizer flow for delivery to the flame 

zone. 

Fe(CO)5 is a liquid at room temperature and its introduction into the flame was accomplished by bubbling

argon (Ar) through a flask containing 25 ml of Fe(CO)5 that was immersed in a constant temperature bath 

maintained at 13 °C. The resulting Ar/ Fe(CO)5 mixture consists of ≈ 2 % Fe(CO)5 and ≈ 98 % Ar. The 

maximum flow of Ar through the bubbler was 0.41 L/min. At this flow, the effect of Ar on the flame was 

minimal. The gaseous output of the bubbler apparatus passed through a 5 L/min mass flow meter to

monitor the Ar/Fe(CO)5 mixture being delivered to the oxidizer stream. The mass flow meter was

calibrated for the Ar/Fe(CO)5 mixture using a standard soap bubble technique. 

Diagnostics 

Flame emission spectra were measured using a Princeton Instruments ICCD camera (Model 120) coupled 

to a 0.75 m SPEX spectrograph (Model 1702) with a 1200 gr/mm grating controlled by an external 

Compudrive. A 50 cm focal length lens collected light from the center of the flame and focused it onto

the entrance slits (0.05 mm) of the spectrograph. The determined field of view of this optical arrangement

was 1 cm2

. The ICCD camera, which had an active area of 384 x 576 pixels, was operated in a CW 

manner and each image recorded was acquired with 50 total accumulations on the camera. 

Laser induced fluorescence excitation spectra in the flame were measured using a Lambda Physik 

excimer/dye laser system. The system consists of a Lambda Physik Compex 102 XeCl excimer laser, a 

Scanmate 2 dye laser (Rhodamine 6g), and a Second Harmonic Generator (SHG). The fundamental

output of the dye laser (580 nm wavelength) was frequency doubled in the SHG unit with a BBO crystal 

to around 290 nm. The UV laser radiation was tuned to the peak of the P2(8.5) transition at 286.566 nm 
Metal-containing Compounds 161

((1,0) A2Σ+←X2Π) (References 158-160). The intensity of this transition was slightly temperature 

dependent,161 varying by approximately 12 % over the range of maximum temperatures for the flames

studied here. The selected transition varied linearly with input energy. Low laser energies were used and

the laser was operated in the linear regime. The UV light output of the SHG unit was focused to the center

of the burner chamber using a 50 cm focal length fused silica lens and had a vertical and horizontal beam 

waist of 

0.4 mm and 0.5 mm, respectively. Fluorescence was collected at 90 o to the direction of the excitation 

laser beam, focused through 0.75 mm horizontal slits to define the collection volume, passed through a 

band pass filter centered at 312 nm with an 11 nm bandwidth, and detected by a photomultiplier tube

(PMT) (Phillips Model XP2018B). Fluctuations in laser power were detected by a photodiode that 

monitored laser induced fluorescence from a cell containing Rhodamine 6g dye. 

The output signals from the PMT and monitor photodiode were directed to gated integrator/boxcar

averagers (SRS Model SR-250) operating in a 10 shot average mode. The boxcar gate widths were set to 

3 ns. The trigger pulses to the excimer laser and boxcars were supplied by a digital delay pulse generator

(SRS Model DG535) at a rate of 10 Hz. Spatially resolved OH LIF profiles were measured by tuning the 

excitation laser to the peak of the P2(8.5) transition and, with the beam location fixed, vertically

translating the burner assembly. 

Results 

Table 3-5 lists the inhibitor agent concentrations at extinction and the average uncertainties due to 

measurement variance. 

Table 3-5. Inhibitor Concentrations (μL/L) and Uncertainty (± μL/L) at Flame Extinction. 

Inhibitor N2 CF3Br Fe(CO)5 

Extinction Concentration 9426 3735 451

Uncertainty 566 672 77

The large degree of variance was due to changes in the burner cooling. The flow of cooling water to the 

burner ducts was not monitored precisely, and thus the burner performance probably changed slightly

from day-to-day operation. This deficiency was perhaps significant because the burner was operated in a

quasi-adiabatic condition, and thus heat losses from the flame to the burner ducts caused decreases in

extinction concentrations observed experimentally. Preliminary numerical calculations of extinction 

concentrations for CF3Br and Fe(CO)5 were 4000 μL/L and 271 μL/L, respectively.162

The numerical calculations for both CF3Br and Fe(CO)5 were performed using kinetic models.52,104

Preliminary numerical calculations by Babushok163

 using the kinetic model for Fe(CO)5 inhibition of 

Rumminger et al.,104

 but corrected for the pressure dependence of the FeO + H2O ↔ Fe(OH)2 reaction, 

demonstrated an extinction concentration of 612 μL/L. 

All three inhibited flames extinguished in the flat region directly between the opposed flow fuel and

oxidizer ducts. Addition of N2 or CF3Br to the 6.6 kPa CH4-air flame did not significantly alter the visual

appearance of the flat, axisymmetric, blue disk flame. Addition of Fe(CO)5 to the flame caused an

orange/yellow luminous zone to appear above the blue luminous zone, similar to what was observed by

Reinelt and Linteris.164 As the Fe(CO)5 inhibited flame approached extinction, the blue and the 
162 Flame Suppression Chemistry

orange/yellow luminous zones gradually merged, and the flame assumed a uniform bright orange/yellow 

color. The orange/yellow luminosity in the Fe(CO)5 inhibited flame was believed to be caused by iron 

oxide emission.89 The dual flame zone observation, which has been observed in previous Fe(CO)5

studies,89 was further characterized using emission spectroscopy. 

For the compositional analysis of the solid particles formed in the flame, particle samples were collected 

and analyzed using a scanning electron microscope (SEM) and X-ray fluorescence (XRF) techniques. 

Particle samples were taken from the upper cooled burner duct, where large deposits were found, and 

from an uncoated, 0.2 mm diameter Pt/Pt-10 % Rh wire thermocouple inserted into the visible center of a 

Fe(CO)5 inhibited flame. The collected particulate material appeared as a ruddy orange powder. The XRF 

showed the elemental composition of the particles to be iron and oxygen. An exact ratio of iron to oxygen 

was not determined, but the SEM analysis indicated that the size of the particles ranged from 1 μm to 

10 μm, with most being 1μm to 2 μm and quasi-spherical in morphology. 

Emission Spectroscopy 

Emission spectroscopy was used to qualitatively examine the behavior of OH for each inhibited flame as 

well as the decomposition of Fe(CO)5 upon introduction into the flame. For OH, images were collected at 

increasing concentrations of inhibitor. Images indicated that for approximately equal inhibitor 

concentrations added to the flame, the flame to which Fe(CO)5 had been added demonstrated the smallest 

relative OH emission intensity. These observations illustrated that addition of Fe(CO)5 to the flame had 

the greatest effect on the OH population. However further quantification of the results was risky because 

ground state populations and temperatures calculated from OH emission in flames using a Boltzmann 

distribution can be misleading. That is, nascent OH that is responsible for the chemiluminescence may not 

be in thermal equilibrium with ground electronic state OH and other combustion gases. 

Figure 3-13 shows a representative emission image collected in the 554 nm wavelength region from the 

centerline of a flame with 10 % of the experimentally determined extinction concentration of Fe(CO)5

added to the oxidizer flow. The addition changed the molecular emission spectra to include iron species 

such as FeO and FeOH as well as atomic iron lines.165 These modifications to the molecular emission 

spectra altered the flame’s emission intensity, which is illustrated in Figure 3-13. The spatial mapping 

indicated that some of the Fe(CO)5 had decomposed prior to the luminous flame region at approximately 

10.4 mm ± 1 mm from the oxidizer duct with a majority of the emission in this spectral region attributed 

to FeO.166 The lower continuum, at approximately 14.8 mm ± 1 mm from the oxidizer duct, was due to 

the broadband luminosity of the hydrocarbon flame. Earlier emission images165 of CH emission from 

similar low pressure CH4-air flames seeded with Fe(CO)5 showed an additional region of emission 

intensity closer to the oxidizer duct which was not observed in both uninhibited and inhibited flames 

seeded with N2 or CF3Br. The additional intensity was identified as iron lines not present in the 

uninhibited flame. 

Recognition of iron oxide particle formation invoked suggestions of heterogeneous chemistry occurring. 

Work by Babushok et al.,167 who modeled the effect of an ideal gas phase flame inhibitor using gas 

kinetic rate constants for a set of radical scavenging and inhibitor-regenerating reactions, indicated 

otherwise. Calculations were conducted for a premixed flame containing Fe(CO)5 and a premixed flame 

with the ideal gas phase flame inhibitor. The calculations showed that the burning velocities, which were 

very non-linear as inhibitor concentrations increased, for both flame systems were nearly equal for the 
Metal-containing Compounds 163

same inhibitor quantities. This result showed that the Fe(CO)5 inhibition mechanism was dominated by

homogeneous gas phase chemistry when the inhibitor is added in small concentrations.167

7

8

9

10

11

12

13

14

15

16

17

18

546 548 550 552 554 556 558 560 562

Iron Emission

Flame Emission

Distance From Oxidizer Duct, mm

Wavelength, nm

Figure 3-13. Emission Image 

Collected from the Centerline of 

the Low Pressure, Opposed 

Flow, CH4-air flame with 10 % of 

the Fe(CO)5 Concentration 

Required to Extinguish the 

Flame. 

Laser Induced Fluorescence 

Figure 3-14 shows representative OH 

LIF profiles measured in flames to 

which Fe(CO)5, CF3Br, or N2 had

been added. 

N2

CF3Br

Fe(CO)5

0 5 10 15 20 25 30 35 40

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Normalized P

MT Voltage

Distance From Oxidizer Duct, mm

Figure 3-14 LIF [OH] Profiles 

Collected from Inhibited CH4-air 

Flames Containing Nominally

Half of the Concentration 

Required for Extinguishment. 

N2: О; CF3Br: ∆; Fe(CO)5: □. 

The inhibitor concentrations and percents of the concentration required for extinguishment were N2 

(4721 μL/L, 50 %), CF3Br (1996 μL/L, 53 %), and Fe(CO)5 (229 μL/L, 51 %). The data had been

normalized to the peak intensity of the OH LIF profile in an uninhibited flame. The N2 doped flame had

the smallest effect (leaving the highest intensity), followed by CF3Br and Fe(CO)5. 
164 Flame Suppression Chemistry

The addition of an inhibitor to the flame can also change the nature of the flame’s reaction zone. 

• Location: For the uninhibited flame, the location of the average OH LIF maximum from the 

oxidizer duct was 14.34 mm ± 0.05 mm. The addition of Fe(CO)5 had a negligible effect on the 

OH LIF maximum position. The addition of CF3Br or N2 to the flame slightly shifted the position 

of the OH LIF maximum toward the oxidizer duct to locations of 14.12 mm and 13.65 mm, 

respectively. The shifts were small and were largest for the weakest inhibitor, each added at 50 

% of its extinguishing concentration. Preliminary numerical calculations163 using the kinetic 

model for Fe(CO)5 inhibition of Rumminger et al.,104 but corrected for the pressure dependence of 

the FeO + H2O ↔ Fe(OH)2 reaction, predicted the position of the OH maximum concentration at 

13.0 mm from the oxidizer duct. 

• Width: Previous studies168-173 had shown that a decrease in the flame’s reaction zone width 

indicated increased localized strain, which could cause local quenching or flame extinction. The 

width of the flame’s reaction zone may be characterized by the width of a radical profile (Miller, 

1996). For the analysis of the radical profile shape modifications, the profiles were fit to a 

Gaussian function. The width of the flame zone was defined as the distance of one half of the 

maximum intensity of the Gaussian OH profile, i.e., Full Width Half Maximum (FWHM). The 

determined [OH] profile width for the uninhibited flame was 5.67 ± 0.08 mm. Addition of N2 did 

not change the profile width. The addition of CF3Br or Fe(CO)5 narrowed the profile width to 

5.50 mm and 5.22 mm respectively. Thus, reduction of the profile widths was small and was 

largest for the strongest inhibitor, the opposite of the location effect described above. 

• Intensity Trend: Figure 3-15 presents normalized OH LIF peak intensities vs. inhibitor 

concentrations. For normalization, the OH maximum intensity from the uninhibited flame was 

used. The presented data were uncorrected for collisional quenching and temperature dependence, 

and the major source for error in the peak OH LIF intensities was the laser shot-to-shot variation. 

The shot-to-shot variance occurred as the laser energy was scanned over the spectral region of 

interest. The shot-to-shot uncertainty averaged ± 1.5 % which contributed to the overall 

measurement uncertainty of 4 %. 

Flames inhibited by Fe(CO)5 and CF3Br demonstrated decreases in OH populations that were 

quasiproportional to the inhibitor concentrations. Flames inhibited by N2 demonstrated non-linear 

behavior, with initially small changes in [OH] and more rapid decreases as extinction was approached. 

The final measurable [OH] data points, which were near each flame’s extinction point concentration, were 

approximately the same for all three inhibitors: 0.73 to 0.67. Thus, there was an approximately 30 % 

decrease in the normalized OH intensity at extinction for each inhibitor. The same model used to calculate 

the peak OH location gave a normalized OH maximum concentration at extinction of 0.73 for Fe(CO)5. 

The ratio of each inhibitors’ concentration relative to CF3Br near extinction was N2:CF3Br:Fe(CO)5 = 

2.5:1:0.11, indicating that the Fe(CO)5 flame inhibition effectiveness was approximately a factor of 10 

greater than CF3Br. These results were similar to Reinelt and Linteris103 who observed that 500 μL/L of 

Fe(CO)5 added to the air stream of an atmospheric pressure, CH4/-air, counterflow flame reduced the 

strain by ≈ 30 %, while addition of 5000 μL/L of CF3Br reduced the strain about 20 %. 
Metal-containing Compounds 165

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

N

ormalize

d Peak OH LIF Intensity

Inhibitor Agent Concentration, ppm

Figure 3-15. 

Dependence of 

Normalized 

Maximum OH 

LIF Intensity on 

Inhibitor 

Concentration. 

◊: N2; ∆: CF3Br; 

□: Fe(CO)5. The 

lowest 

experimental 

measurements 

of OH intensity

correspond to 

near-extinction 

conditions. 

[OH] measurements

156 in non-premixed atmospheric pressure hydrocarbon flames have also shown

decreasing OH concentrations with increasing CF3Br concentrations.51 The observed quasimonotonic 

decrease of [OH] in the current flames with increasing of CF3Br and Fe(CO)5 concentrations was similar

to decreases of chain carrier concentrations in the reaction zone of simulated premixed C1-C2 flames with

CF3Br, CF3I, CF4, C2F6, and C2F5H additives.52 Uncorrected OH absorption profile measurements by 

Bonne and co-workers89 showed a slight decrease in the integrated OH concentrations, i.e., area under

curves, with increasing Fe(CO)5 concentrations for a low pressure (8 kPa), premixed, stoichiometric 

methane/air flame. 

The saturation effect (leveling), which was observed for burning velocities in atmospheric pressure,

premixed methane flames seeded with Fe(CO)5,

103,104,174

 was not observed in this work. Two types of 

saturation have been discussed in the literature: (1) saturation of chemical influence52, and (2) saturation 

due to condensation processes.103 Both result in a decrease in inhibitor efficiency with increased inhibitor 

concentration, but the latter was believed to dominate for Fe(CO)5 inhibited flames. 

The current low pressure, non-premixed flame results show some similarities to the premixed CH4-O2

flame of Bonne et al.89 at pressures from 101 kPa to 10 kPa. At 101 kPa , addition of 0.03 % Fe(CO)5 to 

the Bonne flame decreased the burning velocity ≈ 7.5 %, while at 50 kPa, addition of 0.03 % Fe(CO)5

decreased the burning velocity ≈ 6 %. In premixed CH4-air flames, addition of 0.01 % Fe(CO)5 at 101 kPa 

caused the burning velocity to decrease 24 % while at 50 kPa the decrease in burning velocity was only

8 % to 9 %. Obviously, Fe(CO)5 had a more pronounced effect in the CH4-air flames, but for both 

systems the decreases in burning velocities were observed to be linear at all pressure conditions and the 

inhibitor influence of Fe(CO)5 diminished with decreasing pressure. 
166 Flame Suppression Chemistry

Conclusions 

The first LIF measurements of [OH] in the reaction zone of inhibited low pressure, opposed flow flames 

approaching extinction showed: 

• There was a two-zone (blue and orange) luminous flame structure for a Fe(CO)5 inhibited flame. 

• [OH] levels decreased with increasing Fe(CO)5 and CF3Br concentrations added to the flames. 

• Fe(CO)5 was about 10 times as effective a suppressant as CF3Br. 

• Addition of both chemically active and physically active inhibitors had relatively small effects on 

the locations of the OH maximum intensities and on the width of the [OH] distribution in the 

flame front. The weakest inhibitor had the largest effect. 

• The addition of Fe(CO)5 and CF3Br slightly narrows the width of the flame reaction zone. The 

strongest inhibitor had the largest effect. 

• Close to extinguishment, flames inhibited with both physically active and chemically active 

agents showed a 30 % reduction in peak OH concentration. 

Iron’s Loss of Effectiveness through Particle Formation 

If the gas phase inhibition mechanism of iron were the only consideration, it should be a very effective 

agent for extinguishing fires. However, NGP research with premixed flames102 and counterflow diffusion 

flames108 indicated that particle formation would limit the effectiveness of iron to those situations in 

which it was added at high concentration. Subsequent work showed that iron pentacarbonyl at low 

concentrations was much less effective in cup burner flames than was expected based on the results in 

premixed or counterflow diffusion flames.153

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

increased. This lower effectiveness had been mentioned briefly by Jost and co-workers,88 who surmised 

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

underway to understand the role of particle formation; however, subsequent papers did not appear. Many 

other flame inhibitors also showed lower effectiveness as their concentration increased.175 However, the 

loss of effectiveness of iron is much more dramatic. Reinelt and Linteris103 quantified the loss of 

effectiveness of iron for premixed and diffusion flames. Rumminger et al.104 described a gas phase 

kinetic mechanism for iron inhibition, and showed that the loss of effectiveness was not predicted by a 

gas phase mechanism (ruling out low radical concentration as the reason for the loss of effectiveness). 

Linteris et al.107 have recently described the performance of iron, tin, and manganese in cup burner 

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

Based on the encouraging results for the powerful flame inhibition properties of iron compounds, 

extinction experiments of ferrocene together with an inert compound generated by a solid propellant gas 

generator (SPGG) were conducted in an enclosure containing a spray flame.176 The combination did not 

have the expected high efficiency. Although it was not possible to extract fundamental information 

concerning the lack of effectiveness for their flame configuration, the results of Holland and co-workers 

provided evidence for a loss of effectiveness for iron, and motivated a search for an explanation. 
Metal-containing Compounds 167

Premixed Flames 

For premixed flames, Figure 3-16 shows that the gas phase mechanism predicts a continuing decrease in

the normalized burning velocity (SL/SL, uninhibited) as [Fe(CO)5] increases, whereas the experiments show a

leveling off. (Uncertainties in any experimental data cited throughout this chapter are discussed in the

source publication.) 

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

Fe(CO)5 Volume Fraction (μL/L)

N

orm

alized Bur

ning Vel

o

city

 0.20

X = 0.24 O2,ox

0.21

φ = 1.0

Figure 3-16. Calculated and Measured Normalized Burning Velocity of Premixed 

CH4/O2/N2 Flames with XO2,ox = 0.20, 0.21, and 0.24 for φ = 1.0.104

Subsequent measurements of particles in the premixed flames showed that the loss of effectiveness of the

iron was correlated with formation of particles (as illustrated in Figure 3-17 and Figure 3-18), and the 

scattering from the particles was correlated with the residence time in the flame (necessary for 

condensation to occur), as shown in Figure 3-19. In addition to measuring the particles with

thermophoretic sampling and quantifying their size and agglomeration characteristics with transmission 

electron microscopy, they also calculated the maximum effect that the particles could have on the burning

velocity. Constructing a “perfect heterogeneous inhibitor” model (in which any collision of a radical with

a particle recombines the radical to a stable species), they showed that heterogeneous reactions cannot 

account for the measured flame speed reduction of Fe(CO)5. 
168 Flame Suppression Chemistry

1.0 8.E-07

X = 0.015 H2

0.01

Figure 3-17. Normalized Burning Velocity103 and Maximum Measured Scattering Signal 

Qw

102 for φ=1.0 CH4 Flame with XO2,ox = 0.21 and 0.24. 

Figure 3-18. Maximum Scattering Signal and Normalized Burning Velocity118 for CO-H2

Flames as Fe(CO)5 Concentration Varies.102

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400

Fe(CO)5 (μL/L)

Normalized Burning Velocity

0.E+00

2.E-07

4.E-07

Maximum Qvv (1/cm-sr)

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

0.0

0.2

0.4

0.6

0.8

Normalized BurningVelocity

0.005

6.E-07 Maximum

4.E-07 Q vv (1/cm-sr)

0.005

2.E-07

0.01

0.015

0.E+00

0 100 200 300 400

Fe(CO)5
Metal-containing Compounds 169

0E+00

1E-07

2E-07

3E-07

4E-07

5E-07

6E-07

10 20 30 40 50 60

Burning Velocity (cm/s)

Maximum 

Qvv (1/cm-sr)

300 μL/L

200

100

l

m

h h h h m m

l

h

h

h

h

h

h

h

h

l

Figure 3-19. Maximum

y Fe(CO)5 along with the

igure 3-20. Calculated

l

,

Qvv for Flames of CH4

(open symbols) and 

CO (closed symbols) 

as a Function of the 

Burning Velocity. The

letters correspond to 

the adiabatic flame 

temperature (l = 2220 

K, m = 2350 K, h = 

2470 K); the symbol 

shape (square, 

diamond, and circle) 

corresponds to the 

loading of Fe(CO)5.

102

Figure 3-20 shows the experimental data for normalized flame speed reduction b

prediction for the perfect gas phase inhibitor and perfect heterogeneous inhibitor (for different assumed 

particle diameter sizes). As shown, the perfect gas phase inhibitor mechanism shows burning velocity

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

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

Interestingly, the slope of the Fe(CO)5 curve in Figure 3-20 in the “flattening out” region is of the same 

order as that of the perfect 10 nm diameter particles. Hence, Jost et al.’s

88 postulate that the region of 

lower effectiveness may be due to radical recombination on particles surfaces may be true. Note also that

the slope in the flat region is of the same order as that for CF3Br in these premixed flames. 

0

0.2

0.4

0.6

0.8

0 100 200 300 400 500 600 700 800

Gas-Phase Precursor Volume Fraction (μL/L)

Perfect gas-phase inhibitor

10

20

dm = 80 nm

Fe(CO)5

40

Normalized Burning Velocity

Perfect heterogeneous inhibitor w/

1

F

Normalized Burning 

Velocity for Several 

Diameters, dm, of Idea

Heterogeneous Inhibitor

Fe(CO)5 data103, and 

Calculated Normalized 

Burning Velocity using

the Perfect Gas Phase 

Inhibitor 

Mechanism.102,167
170 Flame Suppression Chemistry

Counterflow Diffusion Flames 

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

Figure 3-21, the gas phase model predicts more inhibition at higher [Fe(CO)5] than do the experiments. 

Measured particle scattering signals again showed that the loss of effectiveness (and discrepancy between 

the measured and gas phase predicted inhibition) increased as particle scattering signal increased; i.e.,

particle formation was correlated with loss of effectiveness of the Fe(CO)5 (Figure 3-22). It is important 

to note that in the counterflow diffusion flames, the propensity to form condensed-phase iron particles in

the flame region was not the only mechanism shown to be correlated with loss of effectiveness of iron. If 

particles formed in one part of the flame, but were unable (due to entrainment or thermophoresis) to be 

transported to the reaction zone, the active iron species could be sequestered from the region in which 

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

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

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

hence iron) transport to the region of radical chain branching. 

te for Counterflow Diffusion Flames. Closed 

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

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500

Inhibitor Mole Fraction (ppm)

N

ormalized aext

Fe(CO)5 in fuel

Fe(CO)5 

in oxidizer

Perfect

 in oxidizer

Perfect in fuel

Figure 3-21. Normalized Extinction Strain Ra

s

with Fe(CO)5 in the fuel; solid lines: calculations with Fe(CO)5; dashed lines: calculation 

with perfect inhibitor.105 
Metal-containing Compounds 171

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

Fe(CO)5 (μL/L)

Normalized Extinction Strain Rate

0.E+00

2.E-07

4.E-07

6.E-07

8.E-07

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

Figure 3-22. Correlation between Inhibition Effect in Counterflow Diffusion Flames and 

Maximum Measured Scattering Signal Qvv . Filled points are experimental normalized 

aext, solid line is calculated aext.

105 Open symbols connected by dotted lines are maximum 

measured Qvv. Particle data were collected at 75 % of aext.

108

Cup Burner Flames 

Linteris et al. tested Fe(CO)5, TMT, and MMT in cup burner flames.85,86,153 Based on the loss of 

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

was not expected that iron pentacarbonyl alone would be effective in cup burner flames. However, 

several publications had suggested that a combination of a good catalytic agent and an inert agent would

prove to be effective.14,91,118,175,177,178

 The premise was that the overall reaction rate would be lowered in

part through radical recombination by the catalytic agent, and in part through the lower temperature 

caused by the added diluent. To test these suggestions, Linteris et al. added Fe(CO)5, MMT, or TMT to a 

cup burner of methane and air, and measured the amount of CO2 required for extinction.153

 This approach

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

135 In 

that test, the oxygen volume fraction in the air stream at blowoff (i.e., the oxygen index) is determined for 

solid, liquid, or gaseous fuels with chemical additives in either the fuel or oxidizer. 

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

anticipated. Figure 3-23 shows the measured CO2 volume fraction required for extinction as a function of 

the catalytic agent volume fraction in the air stream. Data are presented for Br2, CF3Br, Fe(CO)5, MMT,

and TMT. As shown, although the metals are still more effective than CF3Br at low concentration, they

are not nearly as effective as expected from the results in premixed and counterflow diffusion flames. 

The reasons for this loss of effectiveness are described below. 
172 Flame Suppression Chemistry

0.00

0.05

0.10

0.15

0.20

0 0.5 1 1.5 2 2.5

Catalytic Inhibitor Volume Fraction in Air (%)

CO2 V

olu

m

e Fraction R

equired for Extinction

CF3Br

Sn(CH3)4

Br2

0 0.05 0.1

0.11

0.12

0.13

0.14

0.15

0.16

Sn(CH3)4

CF3Br

Br2

MMT

Fe(CO)5

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

Figure 3-23. Volume Fraction of CO2 Required for Extinction (XCO2,ext) of Methane-air Cup 

Burner Flames as a Function of the Volume Fraction of Catalytic Inhibitor Added to the 

Air Stream. Inset shows region in dotted box with expanded scales.153

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

counterflow diffusion flames, several steps were taken. First, particle measurements were made in cup

burner flames inhibited by Fe(CO)5. Shown in Figure 3-24 are scattering cross sections for laser light at 

488 nm as a function of radial position and height above the burner in a methane-air cup burner flame 

with 8 % CO2 and Fe(CO)5 in air at four volume fractions.153 The dotted lines show flame location from a

digitized video image of the uninhibited flame. The figures indicated that particles were present both 

inside and outside (but not coincident with) the luminous flame zone, and that higher Fe(CO)5 loadings 

produced higher particle scattering signals. 
Metal-containing Compounds 173

147

1013161922252831343740434649

S1

S4

S7

S10

S13

S16

S19

S22

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

Radial Position (cm )

0

5

10

15

20

Ht. Above Burner (cm )

-20-18-15-13-11

-8-6-3-1

1469

111316180

3

6

9

12

15

18

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

Radial Position (cm)

0

5

10

15

20

Ht. Above Burner (cm)

147

1013161922252831343740434649

S1

S4

S7

S10

S13

S16

S19

S22

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

Radial Position (cm )

0

5

10

15

20

Ht. Above Burner (cm )

147

1013161922252831343740434649

S1

S4

S7

S10

S13

S16

S19

S22

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

Radial Position (cm )

0

5

10

15

20

Ht. Above Burner (cm )

Figure 3-24. Particle Light Scattering as a Function of Radial Position and Height above 

Cup Burner for Four Loadings of Fe(CO)5. 153 Top left: 100 μL/L, top right: 200 μL/L, 

bottom left: 325 μL/L, bottom right: 450 μL/L. 

In order to understand the particle formation and chemical inhibition, numerical modeling of the cup

burner flames inhibited by Fe(CO)5 were performed,153

 using the gas phase only numerical model

developed previously. This model has predicted the blow-off condition of methane and air cup burner

flames with added CO2

154

 and CF3H.179 The temperature field and the velocity vectors for methane-air

cup burner flames with 10 % CO2, and 0 μL/L and 100 μL/L of Fe(CO)5 are shown in Figure 3-25, while 

Figure 3-26 shows the calculated blow-off behavior. 
174 Flame Suppression Chemistry

Figure 3-25. Calculated 

Temperature (color scale) and 

Velocity Vectors (arrows) for 

Methane-air Cup Burner Flame 

with an Oxidizer Stream CO2

Volume Fraction of 0.1, with 

(left) and without (right) an 

Added Fe(CO)5 Volume Fraction 

of 100 μL/L.153

a.) b.) 

Figure 3-26. Map of Calculated Temperatures

in Cup Burner Methane-air Flames with 10 %

CO2 in the Oxidizer Stream and (a) 0.011 and 

(b) 0.012 Fe(CO)5 Volume Fraction in the Air 

Stream, Illustrating the Blowoff 

Phenomenon.153
Metal-containing Compounds 175

As in other flame configurations, the loss of effectiveness of iron and a discrepancy between predicted 

(gas phase model) and measured effectiveness were both correlated with the formation of particles, as

shown in Figure 3-27. 

Methane-Air Cup Burner

0.00

0.05

0.10

0.15

0 500

Fe(CO)5 Volume Fraction in Air (μL/L)

CO2 V

olume Fracti

on at Extincti

on

0

1

2

3

4

Maxiu

mu Qvv x 105 1/(cm-sr) 

Calc

Expt.

Figure 3-27. Experimental and 

Calculated Extinction Volume 

Fraction of CO2 (left axis) and 

Peak Measured Scattering 

Cross Section (right axis), as a 

Function of the Volume 

Fraction of Fe(CO)5 in the Air 

Stream.85,153

To understand the propensity for particle formation, the degree of supersaturation of some of the ironcontaining intermediates was calculated through the flame, using the detailed flame structure obtained

from the model, together with vapor pressure data available from the literature. For a height above the 

cup burner which passes through the flame kernel (i.e., the stabilization region), Figure 3-28 shows the 

radial profile of temperature and volume fractions of iron species and radicals, as well as the

supersaturation ratio (which is the ratio of the calculated species partial pressure to the planar vapor 

pressure at the local conditions). The supersaturation ratio is highest for FeO, followed by Fe and 

Fe(OH)2, and the values decrease as the radial location of peak temperature is approached. Note that 

vapor pressure data for FeOH, FEOOH, and FeO2 are not available, so their condensation potential has 

not been assessed. The condensation potential is strong since the temperature of the flame kernel is much

lower than the relevant regions of premixed or counterflow diffusion flames. 

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

particles added to the flame, including the effects of gravity, drag, and thermophoretic forces. This was 

done since early estimates180 were that thermophoresis may have been driving the particles away from the 

flame region. The results of the calculations (Figure 3-29) show that near the flame base, there was some

deviation of the particles both up and down around the reaction kernel; however, examination of the 

estimated radial and axial thermophoretic velocities shows them to have been much less than the gas 

velocity. Consequently, the particles still passed directly into the reaction kernel, so the effect of

thermophoresis near this region is expected to be minor. Nonetheless, the other results described above

provided evidence that the loss of effectiveness was due to particle formation. 
176 Flame Suppression Chemistry

10-7

10-6

10-5

10-4

10-3

M

ole Fr

a

ctio

n, X

XFe(OH)2 XFeO2

XFe(CO)5

XFe XFeO

XFe(CO)5

XFeOH

XOH XO

( a )

i

XH

XFeOOH

400

600

800

1000

1200

1400

1600

1800

10-5

10-3

10-1

101

103

105

2 4 6 8 10 12

S

upers

atur

ation R

atio, S

Radial Distance, r (mm)

T

e

m

p

erat

ure, T (K)

i

( b )

SFeO

SFe

SFe(OH)2

T

Figure 3-28. (a) Calculated Volume Fractions, Xi, of Iron-containing and Other Major 

Species as a Function of Radial Position at 4.8 mm above the burner (corresponding to 

the location of the reaction kernel in the flame base); and (b) the Supersaturation Ratio, 

Si, for Fe, FeO, and Fe(OH)2.

153
Metal-containing Compounds 177

Figure 3-29. Calculated Particle Trajectories 

for Free Molecular Regime Particles in a CH4–

air Flame with 10 % CO2 in the Oxidizer 

Stream.153

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

and cup burner flames inhibited by Fe(CO)5 outlined the importance of the following physical effects 

with respect to effective chemical inhibition: 

• Gas phase transport of the active iron-containing species to the region of high H-atom

concentration is necessary for efficient inhibition. 

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

intermediate species and reduce the catalytic effect. 

• The volume fraction of inhibitor influences condensation, since at low values, it may be below its 

saturation value. 

• The available residence time affects particle growth. 

• If the particles are small enough, they can re-evaporate upon passing into the high temperature 

region of the flame. 

• Thermophoretic forces can be large in the flame and re-distribute particles away from peak [H].
178 Flame Suppression Chemistry

• Convection and drag forces combined with the existing flow field in the flame can prevent 

particles from reaching the region of peak [H]. 

• The flame temperature of the stabilization region of cup burner flames is much lower than in 

premixed or counterflow diffusion flames, exacerbating the condensation potential. 

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

concentrations in the flame, as well as their local vapor pressure. The availability of these data for the 

metals listed in Table 3-4 is discussed below. 

3.3.4 Flame Inhibition by Tin and Manganese 

Introduction 

Faced with the finding that the highly effective flame inhibitor iron pentacarbonyl loses its effectiveness 

due to condensation of the active species to particles, it became important for the NGP to determine if 

other metal-containing compounds caused similar strong flame inhibition while not suffering from the 

loss of effectiveness. Since there had been evidence that manganese- and tin-containing compounds were 

of high flame inhibition efficiency and since these compounds typically have higher vapor pressures than 

the corresponding iron compounds, these two families became a subject of NGP research. 

Tin compounds are used as fire retardant additives for polymers, and as agents to reduce smoke and CO 

formation.127,181 The mechanism of flame inhibition has been attributed to both the promotion of 

condensed phase char and gas phase flame inhibition.181,182 Lask and Wagner34 found SnCl4 to be about 

1/34 as effective as Fe(CO)5 at reducing the burning velocity of premixed n-hexane–air flames by 30 %, 

and Miller et al.112 found it to be about 2/3 as effective as Fe(CO)5 at reducing the flame speed of 

hydrogen-air flames by 80 %. Miller113 measured the amount of inhibitor required to lift-off a premixed 

CH4/O2/N2 flat flame at low pressure, and found that tetramethyltin (Sn(CH3)4, TMT) and SnCl4 required 

a mole fraction of 1.7 % and 1.1 %, respectively; whereas Fe(CO)5 and Br2 required 0.23 % and 2.3 %, 

respectively. Morrison and Scheller111 investigated the effect of 20 flame inhibitors on the ignition of 

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

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

effect on the ignition temperature. As a result of these studies, SnCl4 was recommended as a compound 

deserving further study.183

The effects of manganese compounds on flames had also been studied. Vanpee and Shirodkar92

investigated the influence of many metal chlorides and metal acetates and acetylacetonates on the 

limiting oxygen index at extinction in a partially premixed counterflow pool burner of ethanol and air. In 

their experiments, the inhibitor was dissolved in ethanol, which was aspirated into the air stream. They 

found manganese acetylacetonate to be more effective than acetylacetonates of iron or chromium. 

Westblom et al.148 analyzed the consequence of trace amounts of methylcyclopentadienylmanganese 

tricarbonyl (CH3C5H4Mn(CO)3, MMT) on the flame structure of a premixed propane-air flame at 

5.33 kPa, but found no measurable effect. They suggested a kinetic model for the influence of MMT on 

those flames.148 In a review article, Howard and Kausch127 reported that manganese-containing 

compounds were among the most effective soot-reducing fuel additives. Finally, MMT is a known 

antiknock agent for gasoline,184 and an NGP panel185 suggested manganese compounds as agents for 

further consideration in studies of fire suppression performance. 
Metal-containing Compounds 179

This section presents NGP data on flame inhition by manganese- and tin-containing compounds. The

additive influence was analyzed through the effects on the laminar burning velocity of methane-air 

mixtures for different equivalence ratios and oxygen mole fractions. The kinetic mechanisms of flame 

inhibition were analyzed by comparing simulation results with experimental data. The relative inhibition

efficiencies of TMT, MMT, Fe(CO)5, and CF3Br were deduced and analyzed. While the manganese and 

tin compounds tested are too toxic to be used directly as fire suppressants, they provide convenient means 

for introducing Mn and Sn to a flame, so that the inhibition mechanisms of these elements can be studied. 

Experimental Apparatus 

Premixed flames were used for the first tests of new metallic inhibitors. The premixed burning velocity is

a fundamental parameter describing the overall reaction rate, heat release, and heat and mass transport in

the flame. Furthermore, the inhibitor reaches the reaction zone by convection, so that the amount which

enters is unambiguous. The experiments are rapid and relatively straightforward, allowing tests over a 

wide range of conditions. The flames are easily modeled with existing numerical codes, so that detailed

information on the mechanism of chemical inhibition can be determined. There is also a very large data 

base on the effectiveness of other chemical and inert inhibitors in premixed flames for comparison. 

The laminar flame speed SL in a premixed flame provides a measure of an agent’s reduction of the global 

reaction rate. While good techniques exist which allow measurement of burning velocity under

conditions of controlled stretch rates,186 they require seeding with particles for determination of the local 

gas velocity. Since the presence of particles would influence the condensation rates of metallic species,

these tests instead employed the total area method with a Bunsen-type flame.187 The experimental 

arrangement, described in detail elsewhere,57,102,106,118,164

 was modified to accommodate new evaporators

for TMT and MMT. A Mache-Hebra nozzle burner (1.0 cm ± 0.05 cm diameter) produced a premixed

Bunsen-type flame about 1.3 cm tall with a straight-sided schlieren image that was captured by a video

frame-grabber board in a PC. Digital mass flow controllers held the oxygen mole fraction in the oxidizer 

stream XO2,ox , the equivalence ratio φ, and the flame height constant while maintaining the inlet mole 

fraction of the inhibitor (Xin) at the desired value. The average burning velocity was determined from the 

reactant flows and the schlieren image using the total area method. The fuel gas was methane (Matheson

UHP, 99.9 %), and the oxidizer stream consisted of nitrogen (boil-off from liquid N2) and oxygen (MG 

Industries, H2O less than 50 μL/L, and total hydrocarbons less than 5 μL/L). The inhibitors used were 

Fe(CO)5 (Aldrich), TMT (Alfa Aesar), MMT (Alfa Aesar), CF3Br (Great Lakes), N2 (boil-off), and CO2

(Airgas). The catalytic agents were liquids at laboratory conditions. Since they were required in low mole 

fraction, they were added to the flame in gaseous form rather than as droplets. The Fe(CO)5 was added to

the carrier gas using a two-stage saturator in an ice bath.118

 Nitrogen was the carrier gas for all agents.

The TMT was added using an identical two-stage saturator, with a volume of liquid TMT in each stage

greater than 50 cm3

 for all tests. The ice bath was maintained at (0 ± 0.2) °C with a maximum carrier gas

flow 0.40 L/min. Because of the toxicity of the agents, the Fe(CO)5 and TMT saturators, as well as the 

premixed flame burner, were located in fume hoods. For the MMT, the saturator had three stages, each 

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

temperature bath (Neslab), and vented. The bath temperature was typically (79.2 ± 0.1) °C, and the 

carrier gas flow for this saturator was always <0.5 L/min. The mole fraction of the organometallic 

inhibitors in the air stream was calculated based on the measured air flow, measured carrier gas flow, and 

vapor pressure of the agent at the bath temperature, assuming saturated carrier gas. The parameters in the 

Antoine equation, log10(P)=A-B/(T+C) (T in °C, P in bar), were (A,B,C): (6.77273, 4.0932, 7.2283), 
180 Flame Suppression Chemistry

(1258.22, 1286.16, 1882), and (211.587, 235.846, 200) for Fe(CO)5,

188 TMT, 189 and MMT, 190

respectively. Since the vapor pressure of MMT is much lower than that of the other agents, experiments 

with MMT were conducted at a slightly elevated temperature, with the transfer lines and inlet gases 

maintained at (80 ± 3) °C and the burner tube maintained at (80 ± 1) °C. For the experiments with TMT 

and Fe(CO)5, the inlet gas temperature Tin was (294.2 ± 1) K. Although the absolute value of the burning 

velocity is quite sensitive to the inlet temperature, comparisons of agent performance across this range of 

differing gas inlet temperatures is valid, since the reduction in the normalized burning velocity with agent 

addition was relatively insensitive in Tin. For example, calculations for inhibition by TMT (discussed 

below), and calculations and experiments with CO2 (unpublished data of Reference 106) showed that 

changing the inlet gas temperature from 294 K to 353 K provided nearly identical curves of normalized 

burning velocity versus inhibitor mole fraction, differing from each other by less than 2 % for the two 

inlet temperatures. 

Tests were performed for ranges of equivalence ratio and oxygen mole fraction in the oxidizer stream 

XO2,ox. The agent mole fraction was calculated relative to the total reactant flow. The test conditions are 

listed in Table 3-6. The inhibitor for which each of the test conditions was used is included for reference; 

the values in the table are for uninhibited flames only. Note that while the inlet temperature for the 

Fe(CO)5 and TMT experiments was 294 K, the experimental and numerically calculated burning 

velocities in the table have been converted to equivalent values at 298 K to facilitate comparison with 

other values available in the literature. 

Table 3-6. Uninhibited Laminar Burning Velocities SL and Adiabatic Flame Temperatures 

TAFT from 1-D Planar Numerical Calculations, Together with the Average Burning Velocity 

Measured in the Bunsen-type Flames. 

Inhibitor(s) φ XO2,ox

Tin

K 

TAFT 

K 

SL,calc. 

cm/s 

SL,exp

cm/s 

0.9 0.21 298 2159 35.3 33.9 ± 1.3 

1.0 “ “ 2235 39.6 38.0 ± 2.3 

1.1 “ “ 2193 39.8 38.0 ± 1.5 

1.0 0.20 “ 2185 34.7 33.6 ± 1.4 

“ 0.244 “ 2377 57.0 58.0 ± 3.4 

TMT 

0.9 0.21 353 2177 48.0 47.2 ± 1.5 

1.0 “ “ 2264 53.2 52.9 ± 2.9 

1.1 “ “ 2251 53.6 52.8 ± 2.0 

1.0 0.19 “ 2167 41.3 39.9 ± 1.6 

“ 0.2 “ 2220 47.4 45.5 ± 1.7 

“ 0.244 “ 2396 74.3 74.7 ± 4.1 

MMT 

MMT and Fe(CO)5 1.0 0.244 353 2396 74.3 75.9 ± 4.9 
Metal-containing Compounds 181

The burning velocity in Bunsen-type flames is known to vary at the tip and base of the flame; however,

these effects are most important over a small portion of the flame. Several steps were taken to minimize

the influence of curvature and stretch on interpretation of the action of the chemical inhibitor.

• Use of the schlieren image for the flame area, which was maintained at a constant size (1.3 cm

tall) for all tests. 

• Normalization of the burning velocity of inhibited flames, i.e., the burning velocity of the 

inhibited flame divided by the burning velocity of the uninhibited flame, in order to reduce the

error caused by flame curvature and stretch. 

• Limiting interpretation of the data to inhibitor loading which produced less than 40 % reduction

in flame speed. 

Determination of the uncertainties in the experimental data using the present apparatus has been described

in detail elsewhere.118 For the present data, the uncertainty (expanded uncertainties with a coverage factor

of 2) in the normalized burning velocity were less than ± 5 % for all cases. The uncertainty in the

equivalence ratio was 1.4 %. Neglecting the uncertainties (unspecified) in the vapor pressure correlation 

for Fe(CO)5, TMT, and MMT, uncertainties in the bath temperature, ambient pressure and carrier gas

flow rate yielded an inhibitor mole fraction uncertainty of 6.5 %.

Kinetic Mechanisms and Numerical Modeling 

There are few data on the chemical kinetics of tin compounds at flame temperatures, although kinetics 

studies of tin have been conducted for chemical vapor deposition. Studies with H2-O2-N2 flames by

Bulewicz and Padley191

 indicated that tin was present as Sn, SnO, and SnOH, with SnO overwhelmingly 

predominant. Recent spectroscopic data also indicated that tin was present in flames as Sn, SnO, and 

SnOH,192

 with SnO accounting for approximately 97 % by volume of all tin species. 

The present kinetic model for flame inhibition by tin compounds contains reactions of the species Sn, 

SnO2, SnO, SnH and SnOH. The reaction set was based on the consideration of possible reactions of tincontaining species with the radical pool and with the main species of methane combustion. The 

mechanism, listed in Table 3-7, consists of 37 reactions of tin-containing species. Rate constants were

obtained from the literature when available, or otherwise estimated using empirical procedures and 

analogies with similar reactions. The reverse rates of the reactions in Table 3-7 were calculated from the

forward rate and the equilibrium constant. It was assumed that tin (and manganese) species are nonreactive with hydrocarbon molecules. In the model development, estimates of rate constants were first

made, and then the rates of the most important reactions were adjusted (based on sensitivity analysis) to

provide agreement with the experimental results. The decomposition of TMT was described by the 

overall step listed in Table 3-7, but using the rate constant for the reaction Sn(CH3)4 → Sn(CH3)3 + 

CH3.

193

 Enthalpies of formation for the tin-containing (and manganese-containing) species are presented

in Table 3-8. Enthalpies of formation for SnOH and SnH were estimated based on bond energies from 

References 191 and 194. 
182 Flame Suppression Chemistry

Table 3-7. Reaction Rate Data for Tin Inhibition of Premixed Methane-air Flames. 

(k = A Tb exp(-E/RT), mole, s, cm, kJ) 

No. Reaction A b E Reference 

1 SnC4H12 => Sn+C2H6+C2H6 7.94E+13 0.0 230. 193 

2 Sn+H+M = SnH+M 1.00E+15 0.0 0.0 a 

3 Sn+OH+M = SnOH+M 5.36E+18 -0.45 0.0 * 

4 Sn+OH = SnO+H 1.00E+13 0.0 27.2 a 

5 Sn+O+M = SnO+M 1.00E+17 0.0 0.0 a 

6 Sn+O2 = SnO+O 3.07E11 0.79 3.63 195 

7 Sn+O2 (+M) = SnO2 (+M) 2.00E+13 0.0 0.0 a 

 Low pressure limit: 1.5E+18 0.0 16.7 

8 Sn+HCO = SnH+CO 3.00E+13 0.0 0.0 a 

9 Sn+CH3O = SnO+CH3 2.00E+13 0.0 0.0 a 

10 Sn+CO2 = SnO+CO 1.39E+14 0.0 75.6 195 

11 Sn+HO2 = SnO+OH 1.00E+13 0.0 0.0 a 

12 SnO+H+M = SnOH+M 5.50E+17 0.0 0.0 a 

13 SnO+O+M = SnO2+M 1.00E+20 -1.0 0.0 a 

14 SnO+HCO = SnOH+CO 9.30E+13 0.0 0.0 a 

15 SnO+HO2 = SnOH+O2 3.00E+13 0.0 29.3 a 

16 SnO2+H = SnO+OH 1.00E+14 0.0 8.37 a 

17 SnO2+OH = SnOH+O2 3.00E+12 0.0 31.4 a 

18 SnO2+OH = SnO+HO2 3.00E+12 0.0 46.0 a 

19 SnO2+O = SnO+O2 3.00E+13 0.0 8.37 a 

20 SnO2+CH3 = SnO+CH3O 3.00E+12 0.0 18.8 a 

21 SnO2+CO = SnO+CO2 2.00E+12 0.0 20.9 a 

22 SnOH+H = Sn+H2O 1.20E+12 0.0 12.6 a 

23 SnOH+H = SnO+H2 7.10E+13 0.0 4.18 a 

24 SnOH+OH = SnO+H2O 6.30E+13 0.0 0.0 a 

25 SnOH+O = SnO+OH 3.00E+13 0.0 0.0 a 

26 SnOH+O = SnO2+H 5.00E+12 0.0 37.7 a 

27 SnOH+CH3 = SnO+CH4 2.00E+13 0.0 4.18 a 

28 SnH+H = Sn+H2 5.00E+13 0.0 4.18 a 

29 SnH+OH = Sn+H2O 3.00E+13 0.0 0.0 a 

30 SnH+OH = SnOH+H 5.00E+12 0.0 20.9 a 

31 SnH+O+M = SnOH+M 1.00E+15 0.0 0.0 a 

32 SnH+O = Sn+OH 5.00E+13 0.0 4.18 a 

33 SnH+O = SnO+H 8.00E+12 0.0 4.18 a 

34 SnH+CH3 = CH4+Sn 5.00E+13 0.0 4.18 a 

35 SnH+HCO = Sn+CH2O 2.00E+12 0.0 18.8 a 

36 SnH+O2 = SnO+OH 3.00E+12 0.0 29.3 a 

a Estimates 

* By analogy with reactions of K species in Reference 196 
Metal-containing Compounds 183

Table 3-8. Thermodynamic Properties of Tin- and Manganese-containing Species (298 K). 

Species 

Enthalpy of 

Formation 

kJ/mol 

Entropy

J/(mol K)

Heat Capacity 

J/(mol K) Reference 

Mn 283.6 173.6 20.8 197

MnO 161.7 236.0 31.7 197

MnO2 23.01 269.3 42.2 197

MnOH 17.32 250.3 45.8 197

Mn(OH)2 -373.2 291.2 67.1 e, 198

MnOOH -116.3 283.3 53.9 e, 198

MnH 197.9 213.6 29.6 197

MMT -439.3 401.7 149.9 198

Sn 301.2 168.4 21.3 197

SnO 21.91 232.1 31.8 197

SnO2 11.69 251.5 49.5 197

SnOH -15.06 244.8 46.0 e, 191

SnH 268.2 214.7 29.7 e, 194

Sn(CH3)4 -17.70 361.2 137.8 199,178

Sn2 425.4 267.2 42.1 197

a Estimates

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

is presented in Table 3-9. The list of possible Mn-containing species participating in inhibition reactions

includes Mn, MnH, MnO, MnOOH, MnHOH, MnOH, MnO, MnO2 and Mn(OH)2. All species except

MnH and MnHOH were considered in the mechanism of Smith.198

 The role of MnOH and MnO in 

radical recombination was discussed by Bulewicz and Padley

116

, and the species MnO and Mn were

recently measured in a low pressure propane flame doped by MMT.

148

 Hildenbrand and Lau200 used mass

spectrometry to identify the species MnO2, MnOH, and Mn(OH)2. MnH was included in the current 

model since equilibrium calculations showed it to be present in significant quantities in MMT-inhibited 

flames. 

The manganese inhibition reaction set was reduced from an initial list of approximately 160 reactions to

61 reactions based on thermochemical considerations and preliminary calculations. The decomposition of 

MMT followed the overall description suggested in References 148 and 198, and used their rate constants 

for the Mn-species reactions whenever possible. Rate constants for the remaining reactions were 

estimated by analogy and based on thermochemical estimates. The main assumptions were the formation 

of MnO2 through the reaction of Mn atom with oxygen molecule and the formation of Mn(OH)2 via 

reaction of MnO with water (both by analogy to reactions of iron-containing species104). 

The transport parameters of tin- and manganese-containing species were estimated through analogy with 

similar metallic species, or based on molecular weight correlations for similar species. 
184 Flame Suppression Chemistry

Kinetic models for highly effective flame inhibitors can be considered to consist of two sub-models. The 

first sub-model includes reactions for the agent decomposition and formation of the active inhibiting 

species, and the second includes the inhibition reactions. In previous work, it was shown that for the 

phosphorus-containing compound DMMP and for ferrocene, the decomposition reactions have a small 

influence on the predicted inhibitor efficiency as long as the overall activation energy of decomposition is 

less than 250 kJ/mol to 335 kJ/mol.106,201 In the present work, this was also found to be true for TMT and 

MMT decomposition.

Table 3-9. Reaction Rate Data for Manganese Inhibition of Premixed Methane-air Flames. 

(k = A Tb exp(-E/RT), mole, cm, s, kJ) 

No. Reaction A b E Reference 

1 Mn+H+M = MnH+M 1.00E+15 0.0 0.0 a 

2 Mn+OH+M = MnOH+M 8.00E+22 -2.2 0.0 198 

3 Mn+O+M = MnO+M 1.00E+15 0.0 0.0 a 

4 Mn+O2 = MnO+O 2.50E+14 0.0 125.5 198 

5 Mn+O2 (+M) = MnO2 (+M) 2.00E+13 0.0 0.0 a 

 Low pressure limit: 1.50E+18 0.0 12.6 

6 Mn+HCO = MnH+CO 3.00E+13 0.0 0.0 a 

7 MnO+H+M = MnOH+M 7.00E+15 0.0 0.0 198 

8 MnO+O+M = MnO2+M 2.00E+19 -1.0 0.0 198 

9 MnO+H = Mn+OH 1.00E+14 0.0 16.7 a 

10 MnO+OH+M = MnOOH+M 3.00E+17 0.0 0.0 a 

11 MnO+CH3 = Mn+CH3O 1.00E+14 0.0 29.3 a 

12 MnO+H2 = Mn+H2O 3.00E+12 0.0 20.9 202 

13 MnO+H2O = MnO2H2 5.40E+12 0.0 0.0 202 

14 MnO+CO = Mn+CO2 3.00E+11 0.0 0.0 198 

15 MnO+HCO = MnOH+CO 2.40E+13 0.0 0.0 198 

16 MnO+CH2OH = MnOH+CH2O 2.40E+13 0.0 0.0 198 

17 MnO2+H+M = MnOOH+M 2.00E+22 -1.5 0.0 198 

18 MnO2+H = MnO+OH 1.00E+14 0.0 29.3 a 

19 MnO2+OH = MnOH+O2 3.00E+12 0.0 29.3 a 

20 MnO2+O = MnO+O2 5.00E+13 0.0 8.37 a 

21 MnO2+CO = MnO+CO2 2.00E+12 0.0 20.9 a 

22 MnOH+H = Mn+H2O 1.20E+12 0.0 2.09 198 

23 MnOH+H = MnO+H2 3.00E+13 0.0 4.18 a 

24 MnOH+OH+M = MnO2H2+M 1.00E+23 -2.0 0.0 198 

25 MnOH+OH = MnO+H2O 1.00E+13 0.0 6.28 198 

26 MnOH+O+M = MnOOH+M 1.00E+18 0.0 0.0 a 

27 MnOH+O = Mn+HO2 3.00E+13 0.0 71.1 a 

28 MnOH+O = MnO+OH 3.00E+13 0.0 0.0 198 
Metal-containing Compounds 185

No. Reaction A b E Reference 

29 MnOH+O = MnO2+H 5.00E+12 0.0 37.7 a

30 MnOH+CH3 = MnO+CH4 2.00E+13 0.0 12.6 a

31 MnOOH+H+M = MnO2H2+M 1.00E+16 0.0 0.0 a

32 MnOOH+H = MnO+H2O 2.00E+13 0.0 0.0 a

33 MnOOH+H = MnOH+OH 3.00E+13 0.0 20.9 a 

34 MnOOH+H = MnO2+H2 1.00E+13 0.0 16.7 a

35 MnOOH+OH = MnO2+H2O 6.00E+12 0.0 6.28 198

36 MnOOH+O = MnOH+O2 2.00E+13 0.0 10.5 a

37 MnOOH+O = MnO+HO2 3.00E+12 0.0 66.9 a

38 MnOOH+O = MnO2+OH 3.00E+13 0.0 18.8 a

39 MnOOH+CH3 = MnO+CH3OH 3.00E+12 0.0 31.4 a

40 MnOOH+CH3 = MnOH+CH3O 1.00E+13 0.0 46.0 a

41 MnOOH+CO = MnOH+CO2 2.00E+12 0.0 20.9 a

42 MnO2H2+H = MnOH+H2O 6.60E+13 0.0 4.18 a

43 MnO2H2+H = MnOOH+H2 5.00E+13 0.0 79.5 a

44 MnO2H2+OH = MnOOH+H2O 1.00E+13 0.0 37.7 a

45 MnO2H2+O = MnOOH+OH 2.00E+13 0.0 83.7 a 

46 MnO2H2+CH3 = MnOOH+CH4 1.00E+13 0.0 87.9 a

47 MnH+H = Mn+H2 5.00E+13 0.0 12.6 a

48 MnH+OH = Mn+H2O 1.00E+14 0.0 0.0 a

49 MnH+O+M = MnOH+M 1.00E+15 0.0 0.0 a 

50 MnH+O = Mn+OH 1.00E+14 0.0 8.37 a 

51 MnH+CH3 = CH4+Mn 1.00E+14 0.0 8.37 a

52 MnH+O2+M = MnOOH+M 1.00E+16 0.0 0.0 a 

a Estimates 

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

freely-propagating flames. Solutions were obtained using the Sandia flame code Premix,203 and the 

CHEMKIN204 and transport property205

 subroutines. (Solutions were obtained for values of GRAD and

CURV of 0.17 and 0.25 in PREMIX.) The kinetic mechanism for methane combustion was GRIMech

3.0, 206

 with the nitrogen chemistry removed. The methane sub-mechanism contained 36 species and

219 reactions.

i

 The calculations were for 1-D planar flames, while the experiments determined the 

average flame speed of Bunsen-type flames which can be influenced by curvature and stretch. To

minimize these effects, both the experimental and calculated data are presented as normalized flame 

speed reduction. 

 

i

 The reaction mechanisms used for the present calculations for flames with manganese or tin compounds should be considered

only as a starting point. Numerous changes to both the rates and the reactions incorporated may be made once a variety of

experimental and theoretical data are available for testing the mechanism. 
186 Flame Suppression Chemistry

Results 

Observations

The appearance of the flames with added organometallic inhibitors is shown in Figure 3-30. Flames with

iron pentacarbonyl were bright orange, with tetramethyltin they were bright pale blue, and with MMT, 

yellow-green. The intensity of the visible emission increased with inhibitor mole fraction. As the loading 

of metallic inhibitor increased, there became visible a luminous outer shroud as seen clearly in the last 

two images on the right in Figure 3-30. These were regions of high particle concentration from inhibitor 

condensation, leading to broad-band black body radiation, visible here in the orange part of the spectrum.

For the TMT-inhibited flames, a white-colored powder (presumably tin oxide) formed on the rim of the 

quartz burner tube during the tests, especially at high TMT loading. This deposit was removed between 

collection of each data point. For MMT and Fe(CO)5, a dark red or an orange deposit was formed, 

respectively. The rate of deposition for these inhibitors, however, was much lower than for the TMT,

which was added to the flames at mole fractions about ten times higher. 

Figure 3-30. Visible Images of Methane-air Premixed Flames. (From left to right: no 

inhibitor, 50 μL/L of Fe(CO)5, 4000 μL/L of TMT, and 400 μL/L of MMT). 

Inhibition by Tetramethyltin 

Figure 3-31 shows the relative burning velocity reduction with addition of TMT to methane-air flames 

(XO2,ox =0.21) for values of equivalence ratio of 0.9, 1.0, and 1.1. The dotted lines are curve fits to the 

experimental data, and the solid lines are the results of the numerical calculations described above (and 

discussed below). Data are plotted as normalized burning velocity, which is the burning velocity of the 

inhibited flame divided by the value for the same flame in the absence of inhibitor. The calculated and 

experimental burning velocities, along with the calculated adiabatic flame temperatures of the uninhibited

flames used for the normalization are listed in Table 3-6. For the uninhibited flames, the experimentally

determined average burning velocities for the Bunsen-type flames were within about 4 % of the 

calculated values for 1-D planar flames. In Figure 3-31 (as well as Figure 3−32, Figure 3−33, Figure 

3−34, and Figure 3−36 described below), the last data point shown represents the highest volume fraction 

of inhibitor for which flames could be stabilized on the present burner (i.e., they extinguished at higher 

volume fractions). The experimental results in Figure 3-31 demonstrate that for stoichiometric flames, 

3000 μL/L of TMT reduced the flame speed by about 41 %, which is about a factor of two better than

CF3Br. The data also showed that, unlike Fe(CO)5, the richer flames were inhibited more strongly by

TMT than the leaner flames. (Additional numerical tests with SnO as the inhibitor showed that the poorer 
Metal-containing Compounds 187

inhibition of lean flames was due to the fuel effect from the hydrocarbon portion of the relatively large

amounts of Sn(CH3)4 added to the flames.) 

Figure 3-31. Normalized 

MT

s described below, the burning velocity reduction caused by tin species was most sensitive to the rates 

Figure 3-32 shows the measured and calculated flame speeds for TMT in stoichiometric flames XO2,ox

0.4

0.6

0.8

1.0

0 1000 2000 3000 4000 5000

Sn(CH3)4 ppm

Normalized Burning Velocity

φ = 0.9

1.0

1.1

X O = 0.21 2,ox

Tin = 298 K

Burning Velocity of 

Premixed CH4/O2/N2

Flames Inhibited by T

for XO2,ox=0.21 and φ=0.9, 

1.0, and 1.1 (Dotted lines:

curve fits to data; solid 

lines: numerical 

predictions.) 

A

of the reactions: SnO + H + M ↔ SnOH + M, and SnOH +H ↔ SnO + H2. Consequently, the preexponential factors were adjusted in those rates to provide agreement with the experimental results for

stoichiometric mixtures of methane with air. Note, however, the relatively high level of the rate constant

for the reaction H + SnO + M ↔ SnOH + M. Bulewicz and Padley116

 also found that a high rate was 

required for this process to provide agreement with their experimental data on hydrogen atom

recombination in the products of a hydrogen flames. As Figure 3-31 shows, the numerical model 

predicted the amount of inhibition well for stoichiometric flames. For rich and lean flames, however, the 

model over- and under-predicted the burning velocity, respectively. For the reaction set considered, 

reasonable adjustments to the rate constants did not provide better agreement. 

equal to 0.20, 0.21, and 0.244. The experimental data and calculations (with adjusted rate constants) 

show good agreement for the three values of XO2,ox; however, for the hottest flames (XO2,ox = 0.244), the

mechanism slightly overpredicted the inhibition at low mole fraction, and underpredicted it for higher

mole fraction. The experimental data show that for the slower and cooler flames (e.g., equivalence ratio 

is 0.9 or 1.0 and XO2,ox = 0.20 or 0.21), the TMT started to lose its effectiveness above a certain value. For 

Fe(CO)5 inhibited flames, such behavior was shown to be due to condensation of the iron-containing

intermediates.102
188 Flame Suppression Chemistry

0.4

0.6

0.8

1.0

0 1000 2000 3000 4000 5000

Sn(CH3)4 ppm

Normalized Burning Velocity

0.21

0.20

XO ,ox = 0.244 2

φ = 1.0

T = 298 K

Figure 3-32 – Normalized 

Burning Velocity of 

Premixed CH4/O2/N2

flames Inhibited by TMT 

for φ=1.0 and XO2,ox=0.20, 

0.21, and 0.244. (Dotted 

lines: curve fits to the 

data; solid lines: 

numerical predictions) 

Inhibition by MMT 

The premixed flames inhibited by the Mn-containing compound MMT were slightly preheated 

(Tin = 80 °C). The values of the calculated and experimental uninhibited burning velocities, and the 

adiabatic flame temperatures are shown in Table 3-6. The normalized burning velocities of MMTinhibited flames with variation in equivalence ratio and XO2,ox are shown in Figure 3−33 and Figure 3−34. 

MMT was about 13 times more efficient at flame inhibition than TMT; however, it too started to lose its 

effectiveness for flame speed reductions near 50 %. Based on the sensitivity of the burning velocity to

the reaction rates, the pre-exponential factors of the reactions Mn(OH)2 + H ↔ MnOH + H2O and MnO + 

H2O ↔ Mn(OH)2 were adjusted to provide agreement with experimental data for these methane-air 

flames at φ=1.0. Using the rates shown in Table 3-9, the model predicted the burning velocity reduction 

quite well. Nonetheless, for the hottest flames (XO2,ox = 0.244) this mechanism also overpredicted the 

inhibition slightly at low inhibitor mole fraction, and underpredicted the inhibition somewhat at 

intermediate mole fractions. Also, this gas phase mechanism did not capture the decrease in inhibitor 

effectiveness which occurred with increasing inhibitor initial mole fraction, likely a result of condensation 

of Mn-containing species. 
Metal-containing Compounds 189

0.4

0.5

0.6

0.7

0.8

0.9

1

0 200 400 600

MMT (ppm)

Normalized Burning Velocity

φ = 0.9

1.0

1.1

Tin = 353 K

X = 0.21 O2, ox

Figure 3-33. Normalized 

Burning Velocity of 

Premixed CH4/O2/N2

Flames Inhibited by MMT 

with XO2,ox=0.21 and 

φ=0.9, 1.0, and 1.1. 

(Dotted lines: curve fits 

to data; solid lines: 

numerical predictions) 

0.4

0.6

0.8

1

0 100 200 300 400 500 600

MMT (ppm)

Normalized Burning Velocity

0.21 0.20

0.19

XO , ox =0.244 2

Tin = 353 K

φ = 1.0

Figure 3-34. Normalized 

Burning Velocity of 

Premixed CH4/O2/N2

Flames Inhibited by MMT, 

with φ=1.0 and 

XO2,ox=0.19, 0.20. 0.21, a

0.244. (Dotted lines: 

curve fits to the data; 

solid lines: numerical 

predictions) 

nd 
190 Flame Suppression Chemistry

Comparative Performance of Metals 

Figure 3-35 compares the inhibition effectiveness of Fe(CO)5, MMT, TMT, SnCl4, CF3Br, and CO2. The 

data for SnCl4 inhibition from Reference 34 show tin tetrachloride was as effective in n-hexane/air flames 

as TMT was in methane/air flames. Although the experimental data shown for TMT and SnCl4 are not

for an elevated inlet temperature of 80 °C, numerical calculations showed that the reduction in the 

normalized flame speed caused by TMT addition with Tin=298 K differs from that with Tin=353 K by less 

than 1 %. Fe(CO)5 is significantly more effective than any of the other agents, and all of the metal-based

inhibitors had greatly reduced effectiveness for burning velocity reductions greater than 50 %. When the

inhibitor mole fractions are re-scaled in Figure 3-35 to provide overlap at 30 % reduction in burning 

velocity, the normalized burning velocity curves of all inhibitors are nearly coincident for flame speed 

reductions less than 40 % (i.e., the curves are roughly linear up to this degree of normalized flame speed

reduction). Such re-scaling of the experimental data shows that at low mole fraction, Fe(CO)5 was about

80 times, MMT 40 times, and TMT three times as effective as CF3Br at reducing the overall reaction rate 

of stoichiometric, premixed methane-air flames. For flame speed reductions greater than 40 %, the curves 

for these five agents diverged. As discussed previously,

175,207

 most inhibitors lose their marginal

effectiveness at higher mole fractions, but the decrease in inhibition effectiveness is much more dramatic 

for the organometallic compounds, as was found previously for Fe(CO)5. 

igure 3-35. Normalized

Burning Velocity of 

O2, 

l4 

34, 

0.4

0.6

0.8

1

0 2000 4000 6000 8000 10000

Inhibitor (ppm)

Normalized Burning Velocity

CF3Br

Sn(CH3)4

Fe(CO)5

MMT

φ = 1.0

SnCl4

Tin = 353 K

(except Sn)

CO2

X = 0.21 O2,ox

F

Premixed CH4/O2/N2

Flames Inhibited by C

CF3Br, Sn(CH3)4, SnC

MMT, and Fe(CO)5. Tin = 

353 K for all data except 

Sn(CH3)4 and SnCl4 which

are at 298 K. Lines are 

curve fits to data. 
Metal-containing Compounds 191

Blends of Metals 

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

inhibitors. To test this approach in premixed flames, Linteris and co-workers performed tests with a

blend of MMT and Fe(CO)5, added at a molar ratio of 2:1, respectively. Data for pure MMT, pure 

Fe(CO)5, and their combination are shown in Figure 3-36 (Tin = 353 K, phi=1.0, and XO2,ox=0.244). The 

experimental data are represented by the points with a curve fit (dotted line), while the results of the

numerical calculations are shown by the solid lines. (The data for the combination of MMT and Fe(CO)5

are plotted as a function of the mole fraction of the abundant agent, MMT). The numerical model, which 

includes both the reactions of manganese-containing species and the iron-containing species from 

Reference 104 predicts well the normalized flame speed reduction. As the figure shows, adding 0.5

moles of Fe(CO)5 for each mole of MMT added did provide additional flame speed reduction over that 

from MMT alone. This is significant since, as discussed previously,102 the loss of effectiveness of the

metals at higher concentration could be caused either by condensation of active species, or by the loss of 

radical population by catalytic recombination. Since the addition of iron to the manganese-inhibited 

flame caused significant additional inhibition, the strong loss of effectiveness in the MMT-inhibited

flames was likely due to condensation rather than radical depletion. If the cause was radical depletion, 

addition of Fe(CO)5 to the flames already inhibited by high amounts of MMT would have yielded no

additional inhibition, since few radicals would be left to recombine. The gas phase kinetic model

captured the mild reduction of effectiveness of either agent or their blend acceptably well (for flame speed

reductions of less than 40 %). 

Figure 3-36. Normalized 

Burning Velocity of 

Premixed CH4/O2/N2

Flames Inhibited by Pure 

MMT and Fe(CO)5 , and b

a Blend of the Two. 

Dotted lines are curve fits 

to the data; solid lines are 

numerical model 

calculations. 

y

0.4

0.6

0.8

1

0 100 200 300 400 500 600

Inhibitor (ppm)

Normalized Burning Velocity

MMT

Fe(CO)5 MMT + Fe(CO)5

Tin = 353 K

φ = 1.0

X = 0.244 O2,ox
192 Flame Suppression Chemistry

Discussion 

One of the benefits of numerical modeling of the flame structure of inhibited flames is that the results of

the calculations provide detailed insight into the actual reasons for the strong inhibition. Further, the 

results for different inhibitors can be used to understand why they each demonstrate a different 

effectiveness. In the sections below, the inhibition mechanisms of tin and manganese are discussed and 

compared to that for iron. The difference in efficiency for these systems are shown to be related to the 

formation reaction and equilibrium relationships for the intermediate metal di-hydroxide species Fe(OH)2

and Mn(OH)2. 

Inhibition Mechanisms of Tin and Manganese 

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

predicted flame structure allows investigation of the mechanisms of inhibition of these metallic 

compounds. The calculations show that TMT decomposes quickly in the flame, with 90 % consumption 

at 1000 K. A diagram showing the important reactions for tin inhibition is shown in Figure 3-37 (which

also shows in parallel format the reaction paths for MMT, and Fe(CO)5 inhibition). In the calculations 

used to prepare Figure 3-37, TMT, MMT, or Fe(CO)5 were present at (1963, 128, or 105) μL/L,

respectively, in the premixed methane-air flames (φ=1.0, Tin=353 K, and Xox =0.21); these volume

fractions produced a 30 % reduction in flame speed. 

SnO 

Sn

SnO2

+ O2 +(M) (4) 

+ H (63)

 CO (16)

 O (5) 

+ OH (46)

 H (43)

 CH3 (5) 

 O (5)

SnOH

+ H + M (86)

 HCO (9)

+ CO (4) 

+ O2 (95)

+ H (15)

FeO 

Fe

FeO2

+ O2 +(M) (92)

+ O (94)

 H (4)

+ H2O (95) 

+ H (100)

+ H (76)

 O (13)

 CH3 (4) 

 OH (3) 

Fe(OH)2

FeOH

+ H (2) 

 O (1) 

 H2 (1)

MnO

Mn

MnO2

+ O2 +(M) (96)

+ H (43)

 O (37)

 CO (15)

 OH (5)

 H +M (3) 

+ H2O (95)

+ H (100) 

+ H (66)

 O (14)

 OH (13)

 CH3 (3)

Mn(OH)2

MnOH

Figure 3-37. Reaction Pathways for Sn, Mn, and 

Fe in a Premixed Methane-air Flame (φ=1.0, 

XO2,ox =0.21, Tin=353 K). The numbers in 

parentheses are the percent consumption of t

reactant molecule to a specific product.

he
Metal-containing Compounds 193

The determination of the important reactions was based on consideration of both the reaction flux and

sensitivities. The reaction flux represents the production or consumption of a species by chemical

reaction. For a particular reaction, it is defined as the integral of that reaction rate per unit volume over 

the entire flame domain. The total reaction flux for a species is defined as the sum of the reaction fluxes

for the individual reactions which produce or consume it. In Figure 3-37, the pathway for consumption of

each species is shown, with arrows connecting the relevant reactant and product species. The number

next to each arrow represents the fraction of the total consumption flux for that species which proceeds

through that particular reaction. 

The tin atom formed as a result of TMT decomposition quickly reacts with O2 through the reactions Sn + 

O2 (+M) ↔ SnO2, and Sn + O2 ↔ SnO + O. The former reaction leads to SnO from the reaction of SnO2

with CO, H, or other radicals. Conversely, the latter reaction forms SnO directly, and is fast at room

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

reactions with H and HCO radicals: 

SnO + H + M ↔ SNOH + M 

SnO + HCO ↔ SnOH + CO

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

recombination cycle of tin: 

SnOH + H ↔ SnO + H2

SnOH + OH ↔ SnO + H2O 

SnOH + CH3 ↔ SnO + CH4

SnOH + O ↔ SnO + OH . 

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

SnO + H + M ↔ SnOH + M 

SnOH + H ↔ SnO + H2 

net: H + H ↔ H2

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

methane–air flame with added TMT. Figure 3-38 presents the sensitivity coefficients of the burning 

velocity to the rate constants (after adjustment) of tin-containing species for methane/air mixtures with 

equivalence ratios of 0.9, 1.0, and 1.1. In general, the burning velocity was sensitive to the rates of the 

catalytic cycle reactions with high fluxes: SnO reaction with H or HCO, and SnOH reaction with H, OH,

CH3, or O. The burning velocity was most sensitive to the rate of the reaction SnO + H + M ↔ SnOH +

M, which had a sensitivity about four times less than the chain-branching reaction H + O2 ↔ OH + O. As

was found for DMMP and ferrocene additives,106,201

 the burning velocity of flames inhibited by TMT was 

not particularly sensitive to the rate of the decomposition reaction. Numerical tests showed that changes 

in the overall activation energy of TMT decomposition in the range 170 kJ/mol to 335 kJ/mol had little

effect on the burning velocity with up to 2000 μL/L of TMT. Hence, the inhibition effectiveness of tin 

compounds is likely to be independent on the parent molecule, as long as rapid decomposition occurs. 
194 Flame Suppression Chemistry

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

SNC4H12=>

SNO+H+M=SNOH+M

SNO+HCO=SNOH+CO

SNO+HO2=SNOH+O2

SNOH+H=SNO+H2

SNOH+OH=SNO+H2O

SNOH+CH3=SNO+CH4

SNOH+O=SNO+OH

SN+CO2=SNO+CO

Sensitivity Coefficient

0.9

1

1.1

Figure 3-38. First-order Sensitivity Coefficient of the Burning Velocity to the Specific 

Reaction Rate Constant for Reactions with Tin-containing Species (1963 μL/L of TMT). 

The reaction SnO + CO ↔ Sn + CO2 (which is followed by Sn+O2=SnO+O) is an additional route for CO

consumption, and is chain-branching (net: CO+O2=CO2+O), which reduced the inhibiting effect of SnO 

in these flames. Similar behavior was found for CO – N2O flames inhibited by Fe(CO)5.

145

 Changes in 

these rates affected the calculated inhibition efficiency of TMT; nevertheless, the mechanism was 

dominated by the rate of reaction SnO + H + M ↔ SnOH + M. 

Analysis of the numerical results for inhibition by MMT showed that the behavior of manganese in 

premixed methane-air flames was similar in many details to that of iron pentacarbonyl. The reaction

pathway for manganese species is also shown in Figure 3-37, and the pathway for iron species from 

Fe(CO)5 is shown for comparison. As with iron pentacarbonyl, Mn reacts with O2 to form MnO2 , which 

reacts primarily with radicals to form MnO. The catalytic radical recombination cycle consists of: 

MnO + H2O ↔ Mn(OH)2

Mn(OH)2 + H ↔ MnOH + H2O 

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

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

net: H + H ↔ H2

(or: H + OH ↔ H2O, H + O ↔ OH) 
Metal-containing Compounds 195

Although flame equilibrium calculations showed that the species MnH was present at relatively large 

concentrations, the contribution of reactions of this species to the inhibition effect was relatively small.

Figure 3-39 shows the highest absolute values of the sensitivity coefficients of burning velocity to the rate 

constants (after adjustment) for reactions of manganese-containing species. The burning velocity was

sensitive to the rates of the three reactions in the catalytic cycle above, with the rate of the reaction 

Mn(OH)2 + H ↔ MnOH + H2O having the greatest absolute value. The burning velocity was also 

somewhat sensitive to the rates of the reactions forming MnO2 and MnO. The reaction MnO + H ↔ Mn

+ OH has a positive sensitivity; i.e., increasing its rate increased the burning velocity. This chain

propagating reaction temporarily removed MnO from the catalytic cycle above, thus weakening the 

inhibition. 

-0.08

-0.06

-0.04

-0.02

0

0.02

Mn+O2+M=MnO2+M

MnO2+H=MnO+OH

MnO2+O=MnO+O2

MnO+H2O=Mn(OH)2

Mn(OH)2+H=MnOH+H2

O

MnOH+H=MnO+H2

MnOH+O=MnO+OH

MnO+H=Mn+OH

Sensitivity Coefficient

0.9

1

1.1

Figure 3-39. First-order Sensitivity Coefficient of the Burning Velocity to the Specific 

Reaction Rate Constant for Reactions with Manganese-containing Species (150 μL/L of 

MMT). 

Comparison of inhibition by Fe(CO)5, MMT, and TMT 

In order to evaluate the inhibition mechanisms under comparable conditions, the numerical calculations 

(for φ=1.0, XO2,ox =0.21, and Tin=353 K) were performed, as above, for initial values of TMT, MMT, and 

Fe(CO)5 which provided an equivalent reduction (30 %) in the normalized burning velocity. These 

volume fractions were found to be 1963 μL/L, 128 μL/L, and 105 μL/L, respectively. While suppression 

of a fire would probably require higher agent concentration than that which provides a 30 % flame speed
196 Flame Suppression Chemistry

decrease, this degree of flame speed reduction was selected for two reasons. It provided a significant 

reduction in overall reaction rate (a factor of two) and kept the agent concentration far enough above the 

values at which the model and experiments start to diverge (possibly due to condensation). For these 

flames, the structures were quite similar and the flame speed was the same (37.3 cm/s), allowing 

straightforward comparison of the inhibition mechanisms. 

Based on the calculated results, TMT was required at a mole fraction which was about 17 times greater 

than iron or manganese compounds for a similar reduction in overall reaction rate. This occurred because 

the reactions to form SnOH from SnO were rate-limiting and slow. Further, the reverse of the reaction 

SnO + H + M ↔ SnOH + M was relatively fast at the location of peak catalytic cycle activity because 

SnO was a dominant equilibrium product in the high temperature region. 

In order to further compare the performance of TMT, MMT, and Fe(CO)5, it was useful to plot the 

relevant species mole fractions as a function of temperature through the flame. Figure 3-40 shows the 

mole fractions of the metal species in the inhibition cycles, and the radical species H, OH, and O. The 

vertical line shows the location of the maximum rate of the H+O2 branching reaction. Note that the 

locations of the peak fluxes of the inhibition reactions (not shown) were very close to the peak flux of the 

H+O2 reaction and to the maximum concentrations of H, OH, and O. The bottom figure for TMT 

inhibition clearly shows the preponderance of SnO as the sink for tin atoms (note the rescaling), hence 

requiring a large initial TMT mole fraction to achieve both fast reaction of SnO with H atom, and 

appreciable mole fraction of SnOH for radical scavenging. In Figure 3-40, consider the Mn-containing 

species at the location of peak H-atom flux. MnO was present in higher mole fraction than FeO. This 

occurred from the significant backward reaction of MnO + H2O ↔ Mn(OH)2, which provided a lower 

Mn(OH)2 concentration for the rate-limiting step Mn(OH)2 + H ↔ MnOH + H2O. 

The importance of constraints on equilibrium concentrations as they relate to inhibitor efficiency is 

illustrated in Figure 3-41, which shows the fraction of all metal species in the flame. For these 

equilibrium calculations, the metallic element (Sn, Mn, or Fe) was present at a mole fraction or 1.0 x 10-4, 

and methane and air were present at stoichiometric proportions. For TMT inhibition, the Sn appeared 

overwhelmingly as SnO (note the scale change on Sn and SnOH), thus higher levels of TMT were 

required to yield the required levels of SnOH reaction with H atom, and for the required rates of the slow 

reaction SnO + H + M ↔ SnOH + M. Note that since the flames of Figure 3-40 all had equivalent levels 

of inhibition, the flux of each radical recombining catalytic cycle was about the same; e.g., SnOH, 

MnOH, and FeOH must have been present at about the same mole fraction since their rates of reaction 

with H-atom were close, and the rates of reactions forming the hydroxide were approximately the same. 

Comparing Mn(OH)2 and Fe(OH)2 in Figure 3-41, in the MMT-inhibited flames, the concentration of 

Mn(OH)2 for temperatures above 1800 K dropped off rapidly, whereas in the Fe(CO)5-inhibited flames, 

Fe(OH)2 did not. Since the reaction of Mn(OH)2 with H atom was rate-limiting (Figure 3-39), decreases 

in the Mn(OH)2 mole fraction made MMT less effective as an inhibitor than Fe(CO)5. 
Metal-containing Compounds 197

0

50

100

150

500 1000 1500 2000 2500

V

olu

me Fracti

on (x 10-6)

MnOH

Mn(OH)2

MnO MnO2

O /33

OH /100

H /50

ω H|max

.

0

50

100

150

500 1000 1500 2000 2500

V

olume Fraction (x 10-6)

Fe(OH)2

FeO

FeO2

OH /100

H /50

O /33

ωH| m ax

.

FeOH

0

50

100

150

500 1000 1500 2000 2500

Volum e Fraction (x 10-6)

SnO2 SnO /100

ωH| m ax

.

OH /100

H /50

O /33

SnOH

Figure 3-40. Volume Fractions of O, and OH Radicals and Metal Species Intermediates as 

a Function of Temperature in Flame for Fe(CO)5, MMT, and TMT Added at 105 μL/L, 128 

μL/L, and 1963 μL/L (corresponding to a 30 % reduction in flame speed). Tin=353 K, φ=1.0, 

XO2,ox =0.21.
198 Flame Suppression Chemistry

0 .0 0 1

0 .0 1 0

0 .1 0 0

1 .0 0 0

1500 2000

Fraction of

Sn-Species

S nO

S nO H (x100 )

Figure 3-41 Fractions of 

Sn-, Mn-, and Fe-species 

at Equilibrium in 

Methane-air Flames as a 

Function of Temperature. 

S n (x100 )

0 .0 0 1

0 .0 1 0

0 .1 0 0

1 .0 0 0

1500 2000

Fraction of Mn-Species

M nO

M n

M n(O H )2

M nO 2

M nO O H

M nO H

0 .0 0 1

0 .0 1 0

0 .1 0 0

1 .0 0 0

1 500 2 000

T e m p e ra tu re (K )

Fraction of Fe-Species

F e O

F e

F e (O H )2

F e O 2

F e O O H

F e O H
Metal-containing Compounds 199

Conclusions 

This section described the first experimental measurements of the influence of manganese- and tincontaining compounds (MMT, TMT) on the burning velocity of methane/air flames. Comparisons with

the agents Fe(CO)5 and CF3Br demonstrated that manganese and tin-containing compounds are effective 

inhibitors. The inhibition efficiency of MMT was about a factor of two less than that of iron 

pentacarbonyl. TMT was about 26 times less effective, although it was still about twice as effective as 

CF3Br. There were conditions for which both MMT and TMT showed a loss of effectiveness beyond that 

expected due to radical depletion, and the cause is believed to be particle formation. Kinetic models 

describing the inhibition mechanisms of MMT and TMT additives were suggested. Simulations of 

MMT- and TMT-inhibited flames showed reasonable agreement with experimental burning velocity data. 

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

effect on the inhibition properties for the range of concentrations used in this work. Calculations

confirmed that the main tin-containing species in the flame zone was SnO, while the concentrations of 

SnO2, SnOH and Sn were relatively small. The inhibition effect of TMT was determined mostly by the 

rate of the association reaction H+SnO + M ↔ SnOH + M, and the catalytic recombination cycle was

completed by the reactions SnOH + H ↔ SnO + H2 and SnOH + OH ↔ SnO + H2O. The inhibition

mechanism of manganese-containing compounds was similar, in many details, to the inhibition 

mechanism for iron pentacarbonyl: MnO + H2O ↔ Mn(OH)2; Mn(OH)2 + H ↔ MnOH + H2O, and 

MnOH + OH (or H) ↔ MnO + H2O (or H2). The burning velocity was most sensitive to the rate of

Mn(OH)2 + H ↔ MnOH + H2O reaction. Comparison of the mechanisms of inhibition of TMT and 

MMT to Fe(CO)5 showed that the manganese was not as efficient an inhibitor as iron: due to equilibrium 

constraints, the mole fraction of the intermediate species Mn(OH)2 drops off at higher temperature (in

comparison to Fe(OH)2), slowing its rate-limiting reaction with H atom in the catalytic cycle. This result

illuminated the role of equilibrium constraints on species concentrations in the efficiency of catalytic 

cycles. 

3.3.5 Complexation/Matrix Absorption of Super-effective Agents 

Introduction 

While the high flame inhibition effectiveness of these types of compounds would appear to encourage 

their pursuit as potential fire suppressants, their high toxic potency is a severe deterrent. (See Chapter 6.)

To overcome this limitation, the NGP examined means to absorb or adsorb such compounds into an inert 

matrix for transport to and release in the flame zone. Alternatively, perhaps other, non-toxic compounds

of these metals existed which are also superb inhibitors and which maintain their action up to higher mole

fractions. Iron-containing compounds, and in particular Fe(CO)5, were the initial focus of the research.

They had been found to be the most efficient metallic flame inhibitors, and thus would be most able to

overwhelm the additional mass of the matrix material or complexing moiety and contend with halon 1301 

in flame suppression efficiency. 

Absorption Tests of Fe(CO)5 into Matrix Materials 

The objectives of this research85 were to develop a system for introducing the inhibitor into matrix

materials, quantify the absorption and desorption processes of the inhibitor into and out of the matrix, and

investigate the thermal stability of the matrix-inhibitor combination. 
200 Flame Suppression Chemistry

Matrix Materials and Super Agents Considered 

The selection of matrix material had to consider several key factors: 

• Since the primary purpose of the matrix was safe storage and transport of the superagent, it 

needed to be inert. Porous silica and aerogel fit these criteria well. Zeolite particles have Na+

imbedded in the structure and could influence the flame inhibition. Porous carbon particles were 

not considered, since they could promote flame propagation through oxidation of carbon. 

• For efficient loading of agent, the matrix needed to have a high surface area to mass ratio. Zeolite 

particles have a high ratio of about 800 m2

/g, while crystalline sodium bicarbonate particles have 

a very low value. The porous silica particles considered had an intermediate value of 550 m2

/g. 

Aerogel particles have high pore volume to mass, but offer lower agent adsorption surface area. 

• The matrix particles had to be seedable into flames. Submicron particles were hard to seed in a 

steady, consistent manner as opposed to particles above 5 μm. In this regard, zeolite particles 

with a mean diameter of 2 μm were on the borderline, and the maximum mass loading of seeded 

zeolite particles did not exceed 0.6 %. In comparison, porous silica particles in the size range of 

15 μm to 40 μm were well behaved in the seeder, and a mass loading as high as 6 % was attained 

in flame experiments. 

• The desorption rate of the agent from the pore structure had to be favorable. 

• The matrix material had to be stable during the heating and desorption process. A sample of 

zeolite particles collected after passing through a premixed flame indicated spherical particles 

greater than the initial size. This perhaps indicated that the matrix material had melted and, 

through agglomeration, formed large spherical particles. 

Because of the superior flame inhibition properties of Fe(CO)5, most of the absorption effort was devoted 

to this compound. For safety, some experiments were conducted with molecular iodine as a surrogate 

material for testing the performance of an absorbed agent. 

Fe(CO)5 Absorption Method 

Iron pentacarbonyl was absorbed into particles of zeolite-NaX, zeolite-NaY, silica aerogel, and porous 

silica, as well as NaHCO3 and Na2CO3. The system developed for introducing the Fe(CO)5 into the 

various matrix materials is shown in Figure 3-42. The technique is a standard method for absorbing 

compounds into matrix materials. The apparatus was evacuated to a pressure of 7 Pa. In the initial 

absorption studies, about 1 g of solid support material was first inserted into one of the tubes. This sample 

was then heated under vacuum for 3 h to 4 h to remove the absorbed gases from the internal pore 

structure. The support material was then allowed to return to room temperature. The Fe(CO)5 to be 

absorbed was poured into the second tube, with the interconnecting valve closed. While keeping the 

Fe(CO)5 close to its freezing temperature, the valve in the interconnecting tube was opened briefly to 

remove the gas-Fe(CO)5 mixture. Finally, the valve to the suction pump was closed and the 

interconnecting valve was left open, allowing the Fe(CO)5 vapor to diffuse and absorb into the pore 

structure. This process typically took about one to two days. 
Metal-containing Compounds 201

Figure 3-42. Schematic of the Experimental Setup for Fe(CO)5 Absorption. 

Fe(CO)5 Absorption Results 

For zeolite-NaX with pore volume of 0.3 cm3

/g,208 the estimated mass of Fe(CO)5 required to completely

saturate the internal pore volume was about 0.3 g/g of zeolite-X. About 10 % of additional Fe(CO)5 was 

generally poured into the glass tube to ensure complete absorption. Based on the mass increase of the tube

containing zeolite-NaX, about 33 % increase in gross mass of the particles was observed, which was 

consistent with the estimate based on the internal pore volume. This implied that the particles of zeoliteNaX absorb the Fe(CO)5 into their pore structure, as opposed to just adsorbing the agent onto the external 

surface. This amount of mass uptake was encouraging, since only 100 μL of Fe(CO)5 added to one L of

reaction zone volume should reduce the overall reaction rate by a factor of four. Thus, adding a mass 

loading of particles of 1 % should provide sufficient Fe(CO)5 to yield an observable burning velocity or 

extinction strain rate reduction in our apparatus. 

After the Fe(CO)5 was absorbed into the zeolite-NaX, the particles changed color to a faint 

brown/yellow. Based on the color change, the absorption appeared to be uniform in the zeolite particle

sample. In addition, a very faint coating (presumably Fe(CO)5) was observed on the tube walls. When the

absorption process was allowed to take place with the chemical hood light on, the color of the Fe(CO)5-

absorbed zeolite particles was a darker shade of brown. This was believed to be caused by the

decomposition of Fe(CO)5 to Fe2(CO)9 (iron nonacarbonyl) in the presence of light. 
202 Flame Suppression Chemistry

When the Fe(CO)5-absorbed zeolite-NaX was exposed to air by opening the valve, a reaction front 

propagated rapidly across the sample, sometimes with a bright orange-colored emission (apparently a 

flame inside the absorption tube). Hence, it appears that the iron pentacarbonyl reacted pyrophorically 

with air. Additional experiments were conducted by exposing the Fe(CO)5-impregnated zeolite-NaX 

particles to either pure oxygen or nitrogen. In oxygen, the particles showed even more violent reaction, 

whereas in nitrogen, there was no reaction. These tests showed that the decomposition process was 

related to the oxygen concentration. While these observations were qualitative, this unexpected result 

indicated that zeolite-NaX may not be an appropriate support matrix for delivering unreacted Fe(CO)5 to 

a flame, and other support materials were pursued. 

There is a theoretical basis for believing that zeolite-NaY would be a more desirable support material than 

would be zeolite-NaX. The surface physical-chemical properties of the zeolite likely have an important 

role in the Fe(CO)5 decomposition process. The general formula for a unit cell of zeolite is given by 

NaxAlxSi192-xO384·yH2O, where x ranges from 48 to 74 for zeolite Y and from 74 to 96 for zeolite X. 

While the cage structures of zeolite-NaX and -NaY are similar, the difference between them is in the 

content of cation Na+

 in the solid matrix. The lower NA+ content of zeolite-NaY leads to weaker internal 

electric fields than in zeolite-NaX, and hence a lower propensity for decomposition of the Fe(CO)5 in 

zeolite-NaY. When exposed to air, the Fe(CO)5-absorbed zeolite-NaY also indicated Fe(CO)5

decomposition, but the intensity of the observed reaction was much slower than with zeolite-NaX. Tests 

with porous silica particles gave weak indication of Fe(CO)5 decomposition. 

Besides porous silica particles, aerogel particles synthesized at University of Virginia were also tested as 

a possible support material. These particles were formed by hydrolysis of tetraethyl orthosilicate 

(Si(OC2H5)4) in ethanol, followed by drying to remove extra ethanol and water, leaving behind a solid 

matrix which was 96 % porous. Three Fe(CO)5 absorption experiments were conducted with the aerogel 

particles produced using the apparatus in Figure 3-42. Prior to absorbing Fe(CO)5, the aerogel particles 

were heated to 200 °C to 250 °C to remove as much moisture as possible. The first sample indicated a 

particle mass increase of about 200 %. Two subsequent absorption experiments with aerogels, however, 

indicated Fe(CO)5 absorption of 14 % and 101 % of the aerogel particle mass. (Note that the second 

sample was from the same batch of aerogel as the first, while the third sample was from a newly 

synthesized batch of aerogel.) Furthermore, both the second and third samples reacted after about 5 min 

to 10 minutes exposure to air, while the first sample with substantially larger increase in mass did not 

react when exposed to air for a long period of time. 

The decreased uptake of Fe(CO)5 in the second and third aerogel samples and the subsequent reaction 

when exposed to air led to the hypothesis that either water or ethanol in the aerogel (from the synthesis 

process described above) was not completely removed. Since Fe(CO)5 is known to react violently with 

water, the absorbed water was perhaps the most likely cause for the above discrepancy in Fe(CO)5

absorption and its decomposition. 

Quantification of Fe(CO)5 Absorption/Desorption 

A standard technique for measuring the desorption or the decomposition properties of materials is 

thermogravimetric analysis TGA. In these tests, the sample is continuously weighed, while its 

temperature is increased. Zeolite, silica, and aerogel samples were heated to about 500 °C in steps of 

5 °C/min, while NaHCO3 and Na2CO3 were heated only to 150 °C because of their propensity to 

decompose. The TGA analyses required transferring of the Fe(CO)5 absorbed particles from the 
Metal-containing Compounds 203

absorption flask to the weighing pan of the TGA. During this process, some fraction of the particles could 

react with ambient air, so the transfer time was strongly minimized. TGA analyses were performed for all

of the absorbed particles. The Fe(CO)5-absorbed aerogel particles did not react immediately when 

exposed to air, hence the desorption of Fe(CO)5 from these particles measured via TGA was probably the

most reliable. However, the desorption from aerogels showed a great variation from sample to sample,

ranging from 5 % to 200 %. 

The data in Table 3-10 help understand the desorption loss from the various types of Fe(CO)5-laden 

particles following exposure to air. The elemental analyses (by Southern Testing and Research

Laboratories) quantify the amount of Fe and Na remaining in the particles. 

• The second column lists the Fe remaining in the sample, based on the weight loss of the sample in 

the TGA measurements. 

• The third and fifth columns provide the Fe and Na remaining in the samples following exposure 

of the specimens to air at ambient temperature. 

• The fourth and sixth columns show the Fe and Na remaining following heating at 500 °C in air 

for several hours to ensure that Fe(CO)5 was fully decomposed. 

Table 3-10. Summary of the Fe and Na Mass Loadings in Unheated and Heated Particles. 

From TGA From Elemental Analysis 

Fe (% by Mass) Fe (% by Mass) Na (% by Mass) Support 

Material Heated Unheated Heated Unheated Heated 

Zeolite-NaX 7.1 ± 0.7 8.9 ± 0.9 7.1 ± 0.7 11.0 ± 1.1 9.8 ± 1.0

Zeolite-NaY 7.1 ± 0.7 9.7 ± 1.0 6.8 ± 0.7

Porous Silica 6.0 ± 0.6 12.8 ± 1.3 6.8 ± 0.7 < 0.8 < 0.1 

Silica Aerogel 4 to 57

NaHCO3 5.0 ± 0.5 <0.1 < 0.2 28.0 ± 2.8 41.0 ± 4.1 

Na2CO3 2.0 ± 0.2 

• It was not resolved whether the observed weight loss was due to desorption of undissociated

Fe(CO)5 or decomposed carbonyl. 

• Comparison of columns 3 and 4 indicates that heating of the reacted particles had a small effect

on the absorbed Fe(CO)5 for zeolite particles, but a larger effect for porous silica particles.

Hence, it is possible that most of the Fe(CO)5 remaining in the unheated zeolite particles was no 

longer intact Fe(CO)5 that could be liberated by heating, but rather was already decomposed. 

• For porous silica, about half of the Fe(CO)5 was decomposed or unavailable for liberation by

heating. 

• The Fe and Na mass fractions remaining in the zeolites were considerable and similar. If the Na

could be liberated from the zeolite particles in the flame, they might provide flame inhibition 

even without added iron. 

In order to determine whether any unreacted Fe(CO)5 remained inside the particle after exposure to air, 

infrared (IR) spectroscopy analysis was performed. The reacted zeolite or porous silica particles were 
204 Flame Suppression Chemistry

crushed and then mixed with potassium bromide particles. This mix was then pressed to form a thin 

wafer and then tested in the IR spectrometer. The absence of significant absorption of the Fe-CO

stretching bond (characteristic of Fe(CO)5) indicated that there was little Fe(CO)5 remaining in the solid 

support matrices. 

The Fe(CO)5-impregnated and reacted zeolite particles were also analyzed using the high-resolution

transmission electron microscope (HRTEM). Images of zeolite-NaX particles after absorption of 

Fe(CO)5 and exposure to air showed that a condensed phase substance formed on the exterior surface. 

For example, the left frame of Figure 3-43 shows an image of a zeolite particle after Fe(CO)5 was

absorbed and then the sample exposed to air. For comparison, the right frame shows an image of a pure 

zeolite particle. In the left frame, the zeolite-NaX particle is shown to have a diameter on the order of a 

micrometer, with pore sizes of the order of 1 nm, while nodules of condensed–phase material (perhaps Fe, 

FeO, or Fe2O3 ) have diameters of about 20 nm and are rather well dispersed on the support material. The 

formation of these condensed-phase nodules implies that Fe(CO)5 may not be available from the matrix to 

provide gas phase Fe, which is required for efficient flame inhibition. 

Figure 3-43. High Resolution 

Transmission Electron 

Micrograph Images of 

Zeolite-NaX Particles. Left: 

with condensed phase iron 

species on surface; right: 

without Fe2O3 clusters. 
Metal-containing Compounds 205

Methods of Quantifying Suppression Effects by Particulates 

Test Burners

Two burner types were selected as test beds for the particles: a premixed Bunsen-type nozzle burner, and 

a gaseous counterflow diffusion flame burner, also with nozzle-generated flows. These flames were

selected for several reasons: 

• Both types, unlike co-flow diffusion flame burners (such as a cup burner), allowed unambiguous 

determination of the amount of agent that reaches the relevant high-temperature reaction region 

of the flame. This is difficult in cup burner flames, since the transport of agent or particles to the 

appropriate region of the cup burner is complex; moreover, stabilization (and therefore 

extinction) of cup burner flames is not presently well understood theoretically. 

• Cup burner tests allowed flames at only one strain rate, whereas counterflow diffusion flames 

allowed tests at variable strain rates. 

• The premixed flames mitigated the hazard from Fe(CO)5-impregnated particles contaminating the 

laboratory space. Premixed flames involved much smaller quantities of gas and particles, and all 

of the reactants passed through a flame. 

• Both counterflow diffusion flames and premixed flames directly provide a measure of the overall 

reaction rate in the flame, in a configuration which is easily interpreted or modeled. Peters209 has

shown, on a theoretical basis, that for the performance of a chemical catalytic inhibitor, the two 

burners should be correlated. 

Particle Seeder 

Chelliah and co-workers designed, built, and tested an effective particle seeding system for introduction 

of the NaHCO3, silica, zeolite, or aerogel particles to both counterflow diffusion flames and premixed

Bunsen-type flames. The design became the basis for particle experiments discussed in Chapter 4. 

Because of the challenge in verifying the accuracy of previously reported data with NaHCO3 

particles,210,211

 considerable attention was devoted to the development of a particle seeder that could 

provide steady feed rates of particles of various diameters. For the small air flows involved (typically

0.1 L/s), the fluidized bed approach shown in Figure 3-44 gave satisfactory seeder performance for

relatively small particles, i.e., less than 30 μm in diameter. This fluidized bed seeder consisted of a

19 mm diameter glass tube with two porous plugs at either end. The air flow to the seeder was controlled 

through a mass flow controller and split into two streams, which were connected to either end of the glass 

tube. The flow passing through the bottom, which controlled the fluidization level, was monitored 

through another flow controller. The fluidized particles and the air entering the fluidization tube (from 

the top and bottom) were extracted through a small tube located near the middle of the tube. By

maintaining the total air flow to the fluidizer tube constant, the particle feed rate was steady, as verified 

by continuously monitored the light scattered from the particle stream (Figure 3-44). The detection 

system consisted of a laser diode (671 nm), a high speed silicon detector (Thor Labs DET1-SI ) with a 

3 cm focal length collection lens, a laser line filter at 671 nm (Edmund Scientific), and a strip chart

recorder (Hewlett-Packard Model 7132) to monitor the detector output. 
206 Flame Suppression Chemistry

Figure 3-44. Schematic Diagram of C

Diffusion Flame Burner with Provision for

Addition of Particles to the Air Stream. 

ounterflow

Flow Profile Measurement 

For both burners, a laser Doppler velocimetry (LDV) system was used to establish the quality of the top 

Fluidizer air

Co-flow

Ejection

Water 

Nitrogen 

Methane 

Observation

windows

Fluidizer 

Nitrogen 

Laser 

diode 

and the 

detector

hat velocity profile leaving the nozzle exit and to measure the local strain rate in the counterflow field. 

Because the enclosed counterflow burner permitted optical access from the two opposing sides, an onaxis forward scattering LDV configuration was selected. The LDV system consisted of an Argon Ion

laser (Lexel Model 95) operating at 514.5 nm with a beam splitter and 300 mm focal length transmitting 

lens (DISA) to obtain two equal intensity beams forming a fringe pattern at the intersection of the focused

beams. The probe volume was 0.151 mm wide and 1.51 mm long, with a fringe spacing of 2.57 μm. The

collection optic system (DISA) gathered the forward scattered photons from the particles and imaged

them on a photomultiplier tube (PMT). The PMT current was amplified (C-Cor Electronics Model

4375A Wideband Video Amplifier) and processed by a timer/counter (TSI Model 1990B) operated in the 

single burst mode using 8 cycles/burst. The filtered and amplified timer/counter signal was monitored by

an oscilloscope (Tektronix Model 2212), and the digital output was interfaced to a personal computer 

using a digital input card (National Instruments DAQ pad 6510) and processed using data acquisition

software (LabView). Measured frequencies were related to particle velocity using the known fringe

spacing. To facilitate acquisition of velocity profiles in axial and radial direction of the counterflow field, 

both fringe generating and collecting optics were mounted on translation stages (Velmex Unislide Model 

MB2500), which had a minimum step size of 1 μm, with a reported accuracy of 5 μm in 100 mm. Figure

3-45 shows typical velocity profiles at the exit of the premixed burner nozzle for two different volume 

flows. For comparison, the calculated top-hat velocity profiles are also shown as dotted lines. 
Metal-containing Compounds 207

0

1

0 0.5 1 1

Horizontal Location (cm)

Upward Veloctiy m/s

9 L/min

7.4 L/min

.5

Figure 3-45. Calculated (dotted lines) and LDV-measured (points) Velocity Profiles at Exit 

of Premixed Burner Nozzle for Total Flows of 7.4 and 9 L/min. 

Counterflow Diffusion Flames with Inert and NaHCO3 Particles 

Prior to testing agent-loaded particles, sodium bicarbonate and pure silica particles were used as the test 

cases for development of the seeding and burner systems. 

Figure 3-44 shows the experimental setup of the counterflow burner. A steady, planar, non-premixed 

flame was established in the mixing layer of counterflowing methane and air streams. The fuel and air 

nozzles were made from Pyrex glass. The exit diameter of the inner tubes was 15 mm with a nozzle area

contraction factor of 6.5, producing a nearly plug flow velocity profile at the nozzle exit. The co-annular 

nitrogen streams on both fuel and air sides helped maintain a stable planar flame disk. The cylindrical 

burner chamber included provision for dilution of the chamber with room air at the bottom (to minimize 

the occurrence of secondary flames), water cooling, and easy adjustment of the nozzle separation distance

via o-ring sealed vacuum fittings (MDC). The typical separation distance between the two nozzles was 

10 mm to 12 mm. The exhaust gases were evacuated using an air-driven mass flow ejector, with the 

chamber pressure monitored by a differential pressure gauge. The methane was BOC grade 4.0 with a

purity of 99.99 %. The air was supplied by an oil-free compressor and dried by a series of desiccant beds. 

The flows of fuel and air were controlled using factory-calibrated mass flow meters (Teledyne HastingsRaydist), with a reported accuracy of ± 1 %. Figure 3-46 shows an image of a typical flame disk

established without and with NaHCO3 particles. 
208 Flame Suppression Chemistry

Figure 3-46. Methane-air 

Counterflow Diffusion

Flames: Pure (left), and 

with NaHCO3 Particles

(right). 

Experiments were conducted to test the system and to establish the effectiveness of particle size on nonpremixed flame suppression. These results are shown in Figure 3-47 as the mass fraction of NaHCO3

needed vs. flow strain rate at extinction, for particle sizes 10 μm to 20 μm, 20 μm to 30 μm, 30 μm to

40 μm, and 40 μm to 60 μm. For comparison, the effectiveness of 10 μm to 20 μm crystalline silica 

particles are also shown. It is clear that NaHCO3 particles below 20 μm were significantly more effective

in this flow field. As the NaHCO3 particle size increased, the performance approached that of the inert

silica particles. 

 NaHCO3 and Inert Particle Mass Fraction vs. Strain Rate

0

1

2

3

4

5

6

100 150 200 250 300 350 400 450

Extinction Strain Rate (1/s)

Mass Fraction (%)

NaHCO3 0-10 micron (seeder 2)

NaHCO3 10-20 micron (seeder 2)

NaHCO3 20-30 micron (seeder1)

NaHCO3 30-40 micron (seeder 1)

NaHCO3 40-60 micron (seeder 1)

NaHCO3 20-30 micron (seeder 2)

SiO2 10-20 micron

SiO2 10-20 micron

Silica Gel 15-40 micron

Figure 3-47. 

NaHCO3 and Silica 

Mass Fraction vs. 

Methane-air 

Nonpremixed 

Extinction Strain 

Rate. 
Metal-containing Compounds 209

Figure 3-48 shows two photographs of the crossed laser beams from an LDV system penetrating a nonpremixed counterflow methane–air flame seeded with 40 μm to 60 μm NaHCO3 particles. In the upper 

image, the beams are above the flame, in the region between the flame and the incoming air stream 

(which is laden with particles), so that the particles have not yet reached the flame. Laser scattering from 

the particles is clearly visible. In the lower image, the beams are located below the flame and there is still

obvious laser light scattering, indicating these large particles survive through the flame. This behavior 

results in the lower suppression effectiveness as illustrated in Figure 3-47. These results also indicate that 

even inert particles tend to reduce the flame extinction rate with increasing particle loading. Such effects

can be related to the thermal effects associated with heating of these inert particles. (See Chapter 4.) 

igure 3-48 – Laser Scattering

O3 Particles in a 

that

arallel modeling efforts have shown that by selecting an appropriate value for the particle decomposition

temperature, the experimental results shown in Figure 3-47 can be replicated. With a fixed particle 

es with Inert and NaHCO3 Particles 

iameter) produced a premixed Bunsen-type flame 

about 1.9 cm tall with a straight sided schlieren image that was captured by a CCD array camera 

F

of NaHC

Counterflow Diffusion Flame of 

Methane and Air, Showing

the Particles Pass Through the 

Flame. 

P

decomposition temperature, the trends observed in Figure 3-47 for the variation in effectiveness due to 

particle size were then captured. The outcome of such modeling results was that the existence of a critical 

particle size which was effective in this flow field was directly related to the flow residence time, hence 

the heating of the particle and its decompositions rate, which would ultimately control the chemical

effectiveness.

Premixed Flam

A Mache-Hebra nozzle burner (1.5 cm ± 0.05 cm d

connected through a USB port to a PC. Digital mass flow controllers held the equivalence ratio φ and the 

flame height constant while the mass flow of particles from the seeder was set at the desired value. The

average burning velocity was determined from the reactant flows and the schlieren image using the flame

angle method187

 from the measured velocity profile. The fuel gas was methane (BOC grade 4.0; 

99.99 %), and the oxidizer stream consisted of laboratory air from an oil-free compressor which was 

dried by a series of desiccant beds. Air and fuel gas flows were set by PC-controlled mass flow

controllers (Sierra, 860). Figure 3-49 shows this burner operating with methane-air, with and without 

NaHCO3 particles added to the reactant flow. 
210 Flame Suppression Chemistry

Figure 3-49. Premixed Methane-air 

ozzle Burner Flames: Pure (left)

and with NaHCO3 Particles (right). 

zed in the presence of silica, zeolite, and

sodium bicarbonate particles (Figure 3-50). The results indicate that the inert silica particles and zeoliteNaX have a minor effect on the measured burning velocity, while NaHCO has a noticeable effect. 

Figure 3-50. 

omparison of 

Normalized 

les.

N

Using the LDV system, the burner exit velocity was characteri

3

C

Burning 

Velocities with

Added Partic
Metal-containing Compounds 211

Comparison of the relative reduction of normalized, non-premixed flame extinction strain rates with

normalized premixed burning velocities indicated significant differences. However, such comparisons

hould take into account the structural differences of the two flames. As shown by Peters209 and

employed by Chelliah et al.

212

 for chlorinated hydrocarbon flames, the normalized burning rate is related

h less expected based on the significantly

different structures of the premixed and diffusion flames. The results suggested that tests with either 

Fraction 

summary, this fluidized-bed seeding system described was effective for introducing particles smaller

t

amenable to testing th these flames. The

effectiveness of particles (which either decompose or release an absorbed agent) was expected to be

s

the normalized extinction strain rate as SL/SL,0 = √(aext/aext,0) 

Using this correlation, the comparison of the non-premixed and premixed flames inhibited with NaHCO3

is shown in Figure 3-51. The disagreement between the two was somewhat greater than the uncertainty in 

each measurement; however, this disagreement was muc

flame should be roughly comparable in demonstrating the effectiveness of chemically acting agents on the

global reaction rate in methane-air flames. On the other hand, the data for inert particles in premixed and 

counterflow diffusion flames from Figure 3-47 and Figure 3-50, if plotted in Figure 3-51, would show a

significant discrepancy. Most likely, these differences are related to the thermal effects associated with 

varying particle residence time in the two flows considered, and further analysis is required. 

Normalized flame strength vs NaHCO3 Mass fraction

1.2

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2

Mass Fraction (%)

Normalized flame strength

Premixed

Nonpremixed

Figure 3-51. Normalized Flame Strength as a Function of Added NaHCO3 Mass

for Premixed and Counterflow Diffusion Flames. 

In

han 30 μm to the premixed and counterflow burners. The experimental techniques developed were

e efficacy of particles with absorbed super-agents in

dependent upon the residence time for this process. 
212 Flame Suppression Chemistry

Premixed Flames with Iodine 

Due to the unexpected decomposition of Fe(CO)5 within the solid support material and the potential 

hazard from desorption of this toxic compound, molecular iodine was used as a surrogate to assess the 

aterial was selected to be silica gel, based on agent mass loading potential 

and the seeding properties of the particles in the flow. 

e mass loading of the air stream was about 0.12 %. 

•

Figu

fraction

signific

r a Methane-air Flame with

les (14 to 

0)μm, with and without I2

storage concept. The matrix m

• Absorption/desorption investigations indicated that an iodine mass fraction of about 24 % could 

be loaded into zeolite-NaX particles. However, the maximum particle seeding achievable with 

the present apparatus used in the premix flame configuration was about 0.5 % by mass (because 

of very small size, 2 μm). Thus, the total iodin

• About 6 % mass fraction of I2 could be loaded into the milled and sized (10 μm to 20 μm size 

range) crystalline silica particles. Here, crystalline silica particles were considered because of

their better fluidization and seeding characteristics. In the same premixed flame configuration, 

more than 2 % by mass of sized 10 μm to 20 μm crystalline silica particles could be introduced. 

The total iodine mass loading of the air stream was thus 0.12 %, similar to that for zeolite-NaX. 

Selection of the appropriate matrix material for the particles was based on An iodine mass

loading of about 20 % was measured for porous silica gel particles (15 μm to 40 μm).

ii Particle

seeding of up to 6 % by mass was possible, for a theoretical iodine mass delivery of 1.2 %. 

re 3-52 shows the normalized flame speed of a methane-air flame for increasing particle mass

, for pure silica gel and I2-impregnated silica gel. Iodine-absorbed silica particles do not show any 

ant reduction in burning velocity compared to chemically inert silica particles. 

Figure 3-52. Normalized Flame 

Speed fo

Added Silica Gel Partic

0

0.2

0.4

0.6

0.8

1

012 3 4 5

1.2

6

Mass Fraction %

Normalized Flame speed

SiO2

 SiO2 with I2

4

Impregnated in the Particles. 

 

ii For porous silica gel particles, TGA analyses of the absorption of iodine were performed on two samples: one tested 

immediately after the absorption process and the other after the particles had remained in the airflow of the fluidization seeder

for several hours. The mass loss from the fresh particles was 19 %, while that from the “aged” particles was about 8 %. There 

was also a change in the particle color (to a lighter brown shade) after being in the seeder for several hours, a result consistent 

with the lower mass loss for the post-seeder particles. 
Metal-containing Compounds 213

This result was not surprising. Based on literature values of the burning velocity reduction of Br2 in 

hydrocarbon flames,186

 an estimated I2 mass loading of 10 % in the air stream would be required to reduce 

e burning velocity by a factor of two. Hence, even if all of the I2 in the tested silica gel particles were

berated in the flame, the maximum expected burning velocity reduction would be about 5 % for a 6 %

mass loading. In these experiments, such a difference in burning velocity would have been comparable to 

-effective metals and inert additives. 

As a first step, the physical and chemical properties of complexes of potential super-effective metal

omplexes involved main group metals or transition group 

metals with different ligands. 

Table 3-11. Components of Metallic Complexes. 

th

li

the experimental uncertainty. For lower iodine desorption rates, the difference between the I2-laden and 

unladen particles would have been undetectable. 

Nonetheless, this result was not discouraging. From Table 3-11, the mass loading of Fe(CO)5 can be

about half that of I2. However, the flame inhibition effectiveness of Fe(CO)5 is far higher than I2. Thus, 

further experiments were conducted on alternative means of introducing iron atoms into the flames and on 

elucidating the potential synergism between super

Complexes of Super-effective Metals 

Super Agents and Complexing Ligands Considered 

agents were compiled and surveyed. The c

Ligands Metals 

Organic Halide Main Group Metals 

Transition Group

Metals 

acetonate (ac) 

acetylacetonate (acac) 

oxalate (ox) 

trifluoroacetylacetonate 

ate

Et)

iodide (I) Group IV - tin (Sb),

germanium (Ge), lead

(P

b) 

copper (Cu) 

nium (Ti) 

chr

mo

n), nickel

)

chloride (Cl) 

omide (Br) 

Group III - aluminum

(Al) br

b)

Group V - antimony (S

tita

omium (Cr), 

lybdenum (Mo)

manganese (M

(Ni), cobalt (Co

iron (Fe) 

(TFacac) 

hexafluoroacetylaceton

(Hfacac) 

methoxide (OMe) 

ethoxide (O

For the metal-centered halides, it was found that the three most volatile compounds MX4 were (in order of 

v SnX4 < GeX4. The bromides become volatile in the range 70 °C to 150 °C, while the 

chlorides were higher, in the range (180 °C to 240 °C). Germanium iodide was the iodide compound with 

e lowest boiling point, estimated near 350 °C. 

olatility): TiX4 <

th

For the organic-complexes, it was found that the order of volatility was generally 

ox < ac < acac < OMe < OEt < TFacac < Hfacac. 
214 Flame Suppression Chemistry

Examples of this include Cr(HFacac)3 and Cr(ac)3, which sublime or decompose near 100 °C to 160 °C 

and 210 °C to 280 °C, respectively, and Cu(HFacac) , Cu(TFacac) , and Cu(acac)2, which sublime or 

decompose near 130 °C to 1 pectively. 

cantly less volatile than lead and 

germanium compounds. 

The selection of a metal complex for initial study was based on the following: 

e inhibition effectiveness. This narrowed the field to Pb, Cr, and Fe. 

ates, while Fe compounds 

d that the 

also far less toxic than Fe(CO)5. 

Exp m

Linteris by ferrocene, and compared 

its performance with that of Fe(CO)5 and CF3Br in the same flames.106 They numerically modeled Fec’s 

 iron-species mechanism developed for studies of Fe(CO)5 flame inhibition. 

nsisted of nitrogen (boil-off from liquid 

2 2

80 °C, 180 °C to 210 °C, and 290 °C to 320 °C, res

The relative volatility of complexes was dependent on the metal- or main-group-center atom. The order 

of volatility for compounds with different ligands varied, dependent upon the ligand. Consequently, a 

direct ordering was not exact. However, in general the volatility was 

Al < Cu < Mo < Mn < Ni < Cr < Sn < Ti < Fe < Co < Sb < Pb < Ge 

That is, aluminum and copper compounds were generally signifi

• Known high flam

• Low toxicity. Pb and Cr compounds are generally toxic in most st

(other than the pentacarbonyl) are generally of low toxicity. 

• The iron atom in the gas phase leads to the inhibiting iron-species intermediates, an

main property required for the parent molecule is that it readily decomposes at flame 

temperatures to release iron atom. 

• Ferrocene (Fe(C5H5)2 (or Fec) was known to modify the sooting tendency of flames,127,213-217 was 

added to materials as a flame retardant,218 was an antiknock agent, and was used as a source of 

iron atoms for kinetic studies. It is 

eri ental Apparatus 

 and coworkers performed the first measurements of flame inhibition

flame inhibition using the

They also measured the performance of Fec in combination with other agents, including CO2, and CF3H. 

The tests were performed in premixed flames of methane. Nonetheless, they are relevant to the 

suppression of practical fires. Babushok and Tsang 219 observed that for a wide variety of hydrocarbons, 

including methane, the burning velocity is most sensitive to the rates of the same reactions. Since these 

reactions are the ones most influenced by an inhibitor, the trends in inhibitor effectiveness are the same 

for most hydrocarbons. While burning velocity measurements are an important first step in assessing an 

inhibitor’s effectiveness and testing mechanism performance, future research should test these highlyeffective agents in flames more closely resembling actual fires. 

The premixed flame burner was the same as that described in the previous section, modified to 

accommodate a new evaporator for ferrocene and heating of the gas lines and burner tube.106 The fuel gas 

was methane (Matheson UHP, 99.9 %), and the oxidizer stream co

N2) and oxygen (MG Industries, H2O < 50 L/L, and total hydrocarbons < 5 L/L). The inhibitors used 

were Fec (Aldrich), Fe(CO)5 (Aldrich), CF3H (DuPont), CF3Br (Great Lakes), N2, and CO2 (Airgas). The 

Fe(CO)5 was added to N2 carrier gas using a two-stage saturator in an ice bath. Because the vapor 
Metal-containing Compounds 215

pressure of Fec is much lower than that of iron pentacarbonyl, Fec addition at up to 650 μL/L required 

both higher bath temperature (79.1 °C held within 0.1 °C) and higher nitrogen carrier gas flows (up to

2800 cm3

/min) relative to Fe(CO)5. Also, the solid state of Fec required an evaporator with larger surface

areas for heat and mass transfer. The evaporator design, based upon that of Megaridis213

, had a (30 ± 5) 

cm3

 packed bed (to provide the bulk of the ferrocene), followed by 30 sublimation stages (to insure that

the carrier gas was saturated with Fec at the bath temperature). Each sublimation stage consisted of a

5 mm layer of ferrocene on a 2.36 cm diameter 60 mesh stainless steel screen. A 4 mm gap separated 

each stage. The vapor pressure correlation of Pelino et al.220

 was used to determine the ferrocene fraction

in the carrier gas. Temperature controllers maintained the transfer lines at (80 ± 3) °C and the burner tube

at (80 ± 1) °C. For all flames, the equivalence ratio (in the absence of inhibitor) was 1.0, and agent mole

fraction was calculated relative to the total reactant flow. The flows of fuel, oxidizer, Fec-N2, and the 

blended agent (CO2, or CF3H) were mixed after the Fec evaporator. The inlet reactant stream temperature

was (80 ± 1) °C, which corresponded to a calculated adiabatic flame temperature of 2260 K and 2391 K

for uninhibited flames at XO2,ox=0.21 and 0.244. 

Numerical modeling 

The laboratory flames inhibited by Fec and Fec-CO2 blends were numerically modeled as onedimensional freely-propagating flames. Solutions were obtained using the Sandia flame code Premix,203

 and transport property205 subroutines. Details of the calculations are provided

elsewhere. Little was known about the chemical kinetic behavior of ferrocene in flames. A rate 

(closed symbols) to the present slightly pre-heated (80 °C) methane-air flames. (The uncertainties in the 

etail previously,

227 were typically about ± 5 %.) The data are plotted as 

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

upon the oxygen mole fraction in the oxidizer, with flames with lower mole fraction of O2 showing more 

and the CHEMKIN204

106

constant existed for its thermal decomposition: k = 2.188 ⋅ 1016 exp(-384 kJ/RT) s-1,

221 but the activation

energy is high, leading one to suspect that reactions with radicals may also be important. Thermodynamic

data were available,222,223

 and the transport properties were estimates.224,225 A reaction set for combustion 

of methane and larger hydrocarbon fragments was adopted from Sung et al.226

 The iron chemistry was 

from a mechanism developed for flame inhibition by Fe(CO)5.

104 Overall, the kinetic model contained 

105 species and 677 reactions. Calculations showed that addition of C5H5 at mole fractions up to

400 μL/L had negligible effect on the burning velocity, and that the major effect of Fec was from the iron

chemistry. It should be emphasized that the reaction mechanism used for the present calculations should 

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

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

Flame Inhibition by Ferrocene 

Figure 3-53 shows the relative burning velocity reduction with addition of Fec (open symbols) or Fe(CO)5

experimental data, described in d

the same flame in the absence of inhibitor. The uninhibited experimental burning velocities used for the 

normalizations were (53.7 ± 3) cm/s and (75.9 ± 6) cm/s for XO2,ox = 0.21 and 0.244, respectively. For

comparison, the calculations for uninhibited flames using GRI-MECH 1.2 yielded 55.5 cm/s and 72.6 

cm/s. The effect of the two agents was essentially the same, with very strong initial inhibition followed 

by a loss of effectiveness above a few hundred μL/L of agent, as found previously for Fe(CO)5 inhibition 

in flames with reactants at 21 °C.96

Figure 3-53 shows that for both Fec and Fe(CO)5, the magnitude of the inhibition is strongly dependent
216 Flame Suppression Chemistry

rapid burning velocity reduction. As a result, blends of inert agents with iron-containing compounds may

greatly increase the efficiency over that of the inert alone. 

igure 3-53. Normalized

urning Velocity of 

remixed CH4/O2/N2

y

errocene (open

ymbols) and Fe(CO)5

ng

shown in Figure 3-53. The calculations 

major difference between

position of the iron precursor. The

position step for Fec consumption: 

10 10 5 5 K in the present flames. In contrast, 

iron pentacarbonyl decomposition had a peak reaction flux at about 900 K. 

a = 384 kJ/mol) did not

influence the predicted behavior. For other conditions, however, (for example non-preheated reactants

0

0.2

0.4

0.6

0.8

0 200 400 600

Ferrocene or Fe(CO)5 Volume Fraction (ppm)

Normalized Burning Velocity

Fe(CO)5

1

Fec

X = 0.244 O2, ox

0.21

Fec

Tin = 80 o

C

φ = 1.0

F

B

P

Flames Inhibited b

F

s

(closed symbols) for 

XO2,ox=0.21 and 0.244, 

Together with Modeli

Predictions (dotted 

lines). 

The modeling results for the ferrocene-inhibited flames are also 

predicted the flame speed reduction caused by ferrocene reasonably well. The 

the ferrocene reaction scheme and that for Fe(CO)5 was in the decom

mechanism included only the high activation energy thermal decom

FeC H -> Fe + 2 C H , which had a peak reaction flux at 1800 

Figure 3-54 shows the normalized burning velocity for a methane-air flame with 400 μL/L of ferrocene as 

a function of the hypothetical activation energy Ea of the one-step decomposition reaction. In the 

temperature range of the stoichiometric methane-air flame of the figure, the predicted inhibition effect of

Fec was independent of the overall activation energy of the decomposition of Fec for values of Ea less 

than about 400 kJ/mol. Hence, the decomposition rate of Fec used in the model (E

and highly diluted flames) the decomposition of Fec may need to be modeled more accurately to provide 

agreement with experimental data. 
Metal-containing Compounds 217

Figure 3-54. Normalized

Burning Velocity of 

toichiometric CH4-O2-N2

Flames at 400 μL/L of 

errocene as a Function of

the Activation Energy of 

tion.

Since both the Fec and Fe(CO)5 mechanisms used the same iron sub-mechanism, the mode of flame speed

reduction in the model was the same. Decomposition of the ferrocene molecule released iron atom in the 

as phase. Iron reacted with O2 to form FeO2, which reacted with O atom to form FeO. FeO is a longlived intermediate, which together with Fe(OH)2 and FeOH, entered into the catalytic cycle for H-atom

combination: 

(net: H + H ↔ H2). 

The modeling results showed that the s eduction for the cooler flames (XO2,ox = 

0.21) was due to their smaller radical iven amount of iron removed a larger

percentage of the hydrogen radicals. 

Ferrocene appears to be an alternative to th ntacarbonyl for addition of gas phase iron 

to a flame. Unfortunately, its effectiveness also appeared to diminish as the mole fraction increased. For 

S

F

the One-step Ferrocene 

Decomposition Reac

g

re

FeOH + H ↔ FeO + H2 

FeO + H2O ↔ Fe(OH)2 

Fe(OH)2 + H ↔ FeOH + H2O 

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500 600

Activation Energy (kJ/mol)

Normalized Burning Velocity

tronger burning velocity r

pool. In these flames, a g

e highly toxic iron pe

Fe(CO)5, the loss of effectiveness was due to the formation of condensed-phase particulates in the 

reaction zone, and it was presumed that the same mechanism held for ferrocene. 
218 Flame Suppression Chemistry

Inhibition by Ferrocene and CO2

It is desirable to take advantage of the strong initial flame speed reduction from iron species in the flame, 

while avoiding the loss of active species due to condensation. Since addition of nitrogen clearly increased 

the rate of burning velocity reduction at low mole fraction (note the results in Figure 3-53 for XO2,ox = 0.21 

and 0.244), it was logical to determine whether other thermally acting agents could be combined with Fec 

to mitigate the loss of effectiveness, and perhaps enhance the flame speed reduction at low Fec mole 

fraction. It was known, however, that addition of an inert, while reducing the burning velocity, also 

increases the residence time for particle formation in the flame, so that condensation would be increased. 

It was not known a priori if the net effect of combining thermal and iron-containing agents would reduce 

the overall reaction rate faster than the increase of the rate of active-species condensation. 

Therefore, Linteris and coworkers conducted experiments to resolve this. Using the same apparatus, they 

added CO2 to the methane-air-Fec system. Figure 3-55a and Figure 3-55b present the experimental data 

for tests at XO2,ox = 0.21 and 0.244. Data are shown for pure CO2 as well as for CO2 -Fec blends 

corresponding to three values of the mole percentage of Fec in CO2 (0 %, 0.25 %, and 1.5 % in Figure 

3-55a, and 0 %, 0.4 %, and 1.5 % in Figure 3-55b). 

For the pure compounds, addition of about 10 % of CO2 (or 25 % N2) reduced SL by a factor of two at 

XO2,ox = 0.21 or 0.244. Adding Fec to CO2 produced a particularly effective agent. The equivalent of 

0.35 % Fec in CO2 reduced the required CO2 for a 50 % reduction in the flame speed by about a factor of 

three at XO2,ox = 0.21 and about a factor of two at XO2,ox = 0.244. The combination of 1.5 % Fec reduced 

the required CO2 by 10, making this blend about as effective as CF3Br (for which addition of about 1 % 

halved the burning velocity). Nonetheless, the curvature in the ends of the data sets (particularly for 

1.5 % Fec in CO2) illustrates condensation was important. Although one might expect the slightly cooler, 

slower flames with added CO2 to always show more condensation of iron species, the greater efficiency 

of the catalytic cycle in the diluted flames predominated for most of the conditions for the flame of Figure 

3-55. Higher effectiveness of iron compounds in diluted flames has been observed previously for flames 

with XO2,ox lowered below 0.21.96

The condensation behavior of the blends can be discerned from Figure 3-56a and Figure 3-56b, which 

present additional data for CO2 and ferrocene in stoichiometric flames with XO2,ox = 0.21 and 0.244, 

respectively. In Figure 3-55, CO2 and ferrocene were added together in proportional amounts, whereas in 

Figure 3-56, CO2 is first added at a constant volume fraction (0 %, 2 %, and 6 % in 7a; or 0 %, 4 %, and 

12 % in 7b), and then the Fec is added. This approach allows a clearer delineation of the effects of each 

component of the blend. As the figures show, the curve with 0 % CO2 (pure Fec) has a decreasing slope 

magnitude as Xin increases (due to increased condensation). For each of the other curves, the added CO2

reduces the normalized burning velocity before the Fec is added, so that each curve starts at a value less 

than unity; addition of Fec further reduces the flame speed. Below an Fec volume fraction of about 70 

L/L, the curves are all quite linear, showing that there no loss of effectiveness. But for curves extending 

beyond this amount of Fec, there is increasing curvature as Xin increases (due to condensation). 

Nonetheless, at low mole fractions, the inhibition by Fec is actually stronger for conditions of higher CO2

mole fraction (note the larger magnitude of the slope for the 12 % CO2 curve in Figure 3-56b). Hence, for 

some conditions, adding CO2 makes Fec more effective. As discussed below, once the particles start to 

form, the deterioration of inhibition may be more rapid with added CO2. These results imply that 

combinations of non-condensing quantities of several catalytic agents combined with a thermal agent can 

be particularly effective. 
Metal-containing Compounds 219

1

Burning Velocity of 

CH4/O2/N2 Flames, a) 

calculated results. Data 

for nitrogen are included 

X = 0.21 O2

Tin = 80 oC 

0.4

0.6

0.8 φ = 1.0

[N2]/2.5

Normalized Burning Velocity

Figure 3-55. Normalized 

0% Fec

 in CO2

CF3Br 1.5 0.25

0.2

XO2,ox = 0.21, b) XO2,ox = 

0.244, Inhibited by CO2, 

by CO2–ferrocene 

Blends, and by CF3Br. 

(The equivalent 

percentage of ferrocene 

in CO2 (which is constant 

for each curve) is given. 

The solid lines are curve 

fits through the data, and 

the dotted lines, the 

0

0246 8 10

% CO2 (or % N2

in the upper plot by

dividing % N2 by 2.5.) 

15a.) /2.5)

0

0.2

0.4

0 2 4 6 8 10 12 14

% CO2 

Normalized Burning Velocity

0.6

0.8

1

X = 0.244 O2

Tin = 80 oC

φ = 1.0

0% Fec

 in CO2

1.5

0.4

CF3Br

15b.)
220 Flame Suppression Chemistry

,ox = 

f

d

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200

Fec ppm

Normalized Burning Velocity

0% CO2

X = 0.21 O2

Tin = 80 oC

φ = 1.0

16a.)

2% CO2 6% CO2

0

0.2

0.4

0.6

0.8

1

Normalized Burning Velocity

0 % CO2

12% CO2

X = 0.244 O2

Tin = 80 oC

φ = 1.0

4% CO2

Figure 3-56. Normalized 

Burning Velocity of CH4 /

N2 / O2 Flames, a.) XO2

0.21, b.) XO2,ox = 0.244, 

with 0, 2, and 6, or 0, 6, 

and 12 Volume Percent o

CO2, Respectively, Adde

to the Reactant Stream,

as a Function of Added 

Ferrocene. (The lines are 

curve fits to the 

experimental data.) 

0 100 200 300 400 500 600

16b.) Fec ppm
Metal-containing Compounds 221

Inhibition by Ferrocene and CF3H 

Many compounds, both thermally and chemically acting, are candidates for blendin

agents. Hydrofluorocarbons, which are easily stored at moderate

were being used as halon replacements. These compounds had been found to r

of premixed methane-air flames by reducing peak H-atom mole fractions by

through reactions forming HF, and by lowering the temperature of the flam

shown to reduce the equilibrium mole fractions of radicals in flam

temperature reduction alone,

58 they might show enhanced performance relative to CO

with catalytic agents. 

Figure 3-57 presents the burning velocity reduction caused by pure CF3H 

mole fraction of about 5 % was required to reduce SL by two. Data are al

0.35 % Fec in CF3H. Unlike ferrocene addition to CO2, in which 0.35 %

of CO2 required by a factor of about five, this amount of ferrocene in CF3

g with catalytic

 pressure, were of interest since they 

educe the burning velocity

 acting as a sink for H atoms 

e. Since they had also been 

es lower than expected based on

2 when combined

addition to the above flames; a 

so presented for addition of

 Fec in CO2 reduced the amount 

H reduc H 

required only by about one-third. This poor performance may have be due to reactions between iron

species and fluorine, which would have reduced the gas phase mole frac

intermediates, effectively poisoning the iron catalyst.228

remixed CH4/O2/N2

y Pure 

F3H and by CF3H with 

Ferrocene, 

ogether with Data for

F3Br. (Lines are curve 

ts to the data.)

Since there existed no experimental data on the rates of reactions of iron species with fluorine containing 

species in flames, the poisoning effect of fluorinated hydrocarbons on iron-catalyzed radical 

recombination reactions was assessed through equilibrium calculations for the combustion products. The 

species included in the calculations were those in the mechanisms for hydrocarbon oxidation, ironinhibition, and fluorinated hydrocarbon-inhibition,229 as well as the iron-fluorine species: FeF, FeF2, FeF3, 

ed the amount of CF3

tion of the active iron-species

Figure 3-57. Normalized

Burning Velocity of 

P

Flames Inhibited b

C

0.35 %

T

C

fi

0

0.2

0.4

0.6

0.8

1

012 3 456

CF3H Volume Fraction % 

Normalized Burning V

elocity

Pure CF3H

0.35% Fec in CF3H

CF3Br

X = 0.21 O2

Tin = 80 oC

φ = 1.0
222 Flame Suppression Chemistry

Fe2F4, Fe2F6.

197

 Calculations were performed for the equilibrium products of a stoichiometric methane-air 

flame with 1 % to 4 % CF H containing 0.35 % Fec (the conditions of Figure 3-57). The results of the

f these proceeds largely through the CF3, CF2, and CFO intermediates.55,57,58

FeO, FeOH, Fe(OH)2) and 

on-fluorine Species with

% to 4 % CF3H 

ontaining 0.35 %

errocene) Added to a

toichiometric Methaneir Reaction Mixture.

aring the actual amount of flame speed

ach agent individually. This

approach is illustrated schematically in Figure 3-59. This uses the inhibition index Φ(Xin) of Fristrom and

Sawyer,230

 where Φ(Xin)={{(Vo-V(Xin ))/Vo}{XO2,ox/Xin} (and using the xygen mole fraction in the

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

velocity curve (times XO2,ox) evaluated at the mole fraction of interest Xin For a given blend, one can 

evaluate the amount of normalized burning velocity reduction that would have been caused by each 

3

calculations, shown in Figure 3-58, indicated that FeF and FeF2 were major product species when CF3H 

was added. For 1 % to 4 % CF3H, the amount of iron taken up by the sum of FeF and FeF2 increased 

from 42 % to 84 %, making less iron available in the form of the active iron intermediate species Fe, FeO,

FeOH, FeO2, and Fe(OH)2. The formation of fluorinated iron species with strong bonds clearly acted as a 

sink for iron in the flame, and reduced the mole fractions of active iron-containing species available to 

participate in the flame inhibition reactions. While the experiments and calculations were performed for

CF3H, the results are likely to be the similar for larger HFCs such as C2HF5 and C3HF7 since the

decomposition of all o

Figure 3-58. Equilibrium 

Mole Fraction of Active 

Inhibiting Species (Fe, 

Ir

1

(c

F

S

a

0.0

0.2

0.4

0.6

1 2 3 4

CF3H Volume Fraction %

Mole fraction x 10

FeF

Fe(OH)2

Fe

FeOH, 

FeO,

FeO2,

FeF3 ,

all < 2 x 10-6

0.8

1.0

4

FeF2

Comparison of Individual and Blended Performance 

The behavior of the blends of agents was investigated by comp

reduction for the blend to the sum of the inhibition which would result from e

o

.

individual component of the blend, e.g., components a and b. The predicted inhibition index is just a 

linear combination of the reduction from each component, or Xa ⋅ Φa+b(Xa, Xb)|pred = Xa ⋅ Φa(Xa)+Xa⋅Φb(Xb),

in which a is the major component of the blend, (selected for defining Φa+b(Xa, Xb)). The actual inhibition 

index Φa+b(Xa, Xb)|actual was evaluated from the normalized flame speed of the blend (using Xa in its
Metal-containing Compounds 223

definition). The ratio of Xa ⋅Φa+b(Xa, Xb)|actual to Xa⋅Φa(Xa)+Xa⋅Φb(Xb) provided a reasonable indicator of the 

performance of the blend relative to the individual components. 

Figure 3-59. Linear 

Contribution of Burning 

elocity from Each 

woomponent Blend of

d a performance index of 

ount of the N2/CO2 mix

e scale.) In contrast, the poor

performance of the Fec-CF3H blend is clearly indicated by a decreasing performance index as Xin

increases. For the CO2/Fec blends, the combination worked slightl better than the individual

X Xin . The good performance at low Xin

hich occurred with flame dilution, as observed

96

V

Component of a t

C

Inhibitors, Together with 

the Actual Reduction 

from the Blend. 

0.0

0.2

0.4

0.6

024 6 8 10

Inhibitor(s) Mole Fraction

Normalized Burning Velocity0.8

1.0

Φa Xa

Φb Xb

Φa Xa + Φb Xb 

Φa+b Xa |actual

Xa, Xb

As Figure 3-60 shows, a blend of CO2 and N2 (in the molar ratio of 1:2) provide

nearly 1.0 for N2 added up to 12 % (i.e., containing 6 % CO2 ). (The percent am

plotted in Figure 3-60 was divided by two to allow plotting on the sam

y

components at low in of the blend, and slightly worse at higher 

was due to the higher radical super-equilibrium w

previously for N2 addition , and the degraded performance at higher Xin was due to the longer residence

times for condensation which resulted from the lower flame speeds.102

 In Figure 3-60, the condition at

which the curves for Fec/CO2 crossover the unity ratio correspond roughly to the location on the curves in 

Figure 3-55, where the linear behavior has ended and the curvature begins. 

Conclusions 

The first data on flame inhibition by ferrocene showed that the efficiency of Fe(CO)5 is not unique, and

that there may be methods for overcoming its loss of performance. 

• Ferrocene was as efficient as Fe(CO)5 at reducing the burning velocity of premixed methane 

flames, but, like Fe(CO)5, lost its effectiveness at a mole fraction above a few hundred μL/L. The

experimental results were reasonably predicted by a gas phase iron inhibition mechanism. Thus, 
224 Flame Suppression Chemistry

similar results can be expected for any rapidly decomposing, iron-containing agent that releases

atomic iron in the gas phase 

• As with Fe(CO)5, the magnitude of the inhibition by ferrocene had a strong dependence on the 

d Fec showed strong inhibition, 

O . 

• These results suggest that an inert agent, together with multiple catalytic agents (to keep the 

absolute mole fraction of each below the saturation point) may prove to be highly effective for all

conditions. 

• In contrast to the results with CO2, blends of CF3H and Fec were n

species and halogens may enter into undesired reactions which pois

Figure 3-60. Ratio of the 

ctual Reduction in SL

from the Two-component 

ix to the Predicted

inear Combination of

Component. (Open and

ymbols are for 

O2,ox = 0.21 and 0.244; 

nes are curve fits to the

loss of effectiveness similar to

that of iron: 

stems in which the loss of effectiveness is evident. This 

pounds did not provide data to high

34

oxygen mole fraction. As a result, many combinations of CO2 an

mitigating the loss of effectiveness observed for pure Fec or Fe(C )5

ot effective, implying that iron

on the catalytic cycles.

A

M

Reduction Based on 

L

the Effect from Each 

closed s

X

li

data. The percent 

CO2/N2 has been divided 

by two to fit on the 

figure.) 

3.3.6 Potential for Loss of Effectiveness of Other Metals

There are multiple approaches for determining whether a metal will have a

0.4

0.6

024 6

Percent Major Inhibitor Component

Xa 0.8

1.0

1.2

Φ|actual / ( Xa Φa + Xb Φb )|pred

0.4% Fec in CO2

1.5% Fec in CO2

0.25% Fec in CO2

0.35% Fec in CF3H

(50% CO2 in N2)/2

• Obtain experimental data for flame sy

requires that the inhibitor be added at volume fractions high enough to allow the loss of 

effectiveness. Many of the early studies with metal com

enough volume fractions to show the loss of effectiveness or the inhibiting effect was not 

presented as a function of additive volume fraction (so the decreasing effectiveness was not 

illustrated).92
Metal-containing Compounds 225

•

e of the flame, the concentration at which the metal moiety is added, 

• cal vapor pressure in the 

of the formation of more stable oxides of the metal must be known to assess the 

these oxides is very low; e.g., Fe2O3).

pre

species of interest. 

Cr 

Bulewicz and Padley126 found particles in the post combustion region of premixed flat flames inhibited by

chromium through visual observation and by detecting continuum radiation from them. They also 

observed saturation in both the inhibiting effect and in the concentration of gas phase Cr in the flame. In 

addition, solid deposits were observed on the burner in premixed flames in

the potential for particle formation (similar deposits have been observed fo

Sn, and Mn 107

 and for Fe.

103

 Hence, the potential for condensation of

important in other flame systems. To further examine this potential, th CrO,

CrO2, and CrO3 above Cr2O3, under neutral and oxidizing conditions are sh

Figure 3-61. Volume Fraction of 

Cr-containi

CrO2, a 3

10.133 kPa under Neutral 

(closed s

(open symbols). Conditions 

above Cr2O3 (reddish hues), and 

Cr above itself (blue). Data from 

References 197, 231, and 232 

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

depends upon the temperatur

and the residence time for particle formation, the observed presence of particles in one flame

system is an indication that it may be important in other flame systems as well. 

Consider the local metal species volume fraction compared to the its lo

flame. A limitation of this method is that it relies upon knowledge of the metal species present in 

a flame system, the mechanism of inhibition, as well as the vapor pressure (or gas phase and 

condensed-phase thermodynamic data). Often, this information is incomplete. Further, the 

kinetic rates

contribution of those compounds to condensed-phase particles (since often, the vapor pressure of 

Below, the available information related to the potential for condensation is sented for each metal

hibited by CrO2Cl2, indicating

r premixed flames inhibited by

Cr species exists and may be

e vapor pressures of Cr,

own in Figure 3-61. 

ng Species. Cr, CrO, 

nd CrO3 above Cr2O at 

ymbols) and Oxidizing 

1.E

CrO3(ox) -08 Cr(ox)

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

Agent Volume Fraction mL/L

CrO2(ox) CrO(ox)

Cr

CrO2

CrO

Cr

1.E-10

1.E-09

Cr2O3

500 1000 1500 2000

Temperature K
226 Flame Suppression Chemistry

Note th

equilibr

curve for Cr is for data for Cr vapor above Cr solid. Nonetheless, since it is not yet know for any metal

inh o

Cr s c

Pb 

The e

argued

particle

screenin

show th

howeve

observe condensation is likely to limit lead effectiveness for at 

least some applications. It should also be noted, that the high effectiveness of Pb in premixed flames 

enough to account for the observed inhibition of premixed flames by super-effective

agents such as iron or lead. The vapor pressure of Pb and PbO are shown as a function of temperature in

re 3-62. The data for PbO2 are too low to appear on this figure, and hence, condensation is clearly

possible for Pb-inhib flames if PbO readily forms in the inhibition cycle, or if other higher oxides 

hase Volume Fractions at

quilibrium over the Condensed

hase (at 101.3 kPa). (Vapor

ressure of Pb and PbO from

at since Cr2O3 is a particularly stable oxide, these vapor pressures will be lower than if the 

ium included other, less stable condensed-phase oxides. For example, in Figure 3-61, the blue 

ibit r what the composition of the condensed phase oxide actually is, the potential for condensation of

pe ies is illustrated. That is, many of the chromium oxides appear to have quite low vapor pressures. 

pr sence of PbO particles was detected in many engine knock studies,94,95,99,109 and it was often

that the strong inhibition of knock occurred from heterogeneous radical recombination on the

surfaces. Although lead has been shown to be one of the more effective metal agents in flame 

g studies,

34 the additive mole fraction (maximum 150 μL/L) was not increased high enough to

e loss of effectiveness that might occur. In the flash photolysis studies of Norrish et al.,96

r, the TEL concentration was systematically increased, and a drop-off in the effectiveness was 

d above a certain volume fraction. Hence, 

cannot be from particles since, as shown by Rumminger et al.,102 the collision rate of radicals with solid

particles is not high

Figu

ited 2

readily form. 

Figure 3-62. Pb Species Gas 

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

500 1000 15

Temperature K

Gas-Phase Volume Fraction 

PbO2 (10-13 @ 1000 K)

1.E-02

1.E-01

1.E+00

μL/

L

Pb

PbO

P

E

P

p

Reference 233, PbO2 from 

References 197 and 234.) 

00 2000
Metal-containing Compounds 227

Mn

he effect of MMT on the flame speed of methane-air mixtures was measured by Linteris et al.,107

 as 

shown in Figure 3-63. The data clearly show a saturation effect, which was beyond that due to lowering

f the radical concentrations. Additional tests in cup burner flames showed a much lower effectiveness 

T

o

than expected, as well as a rapid loss of effectiveness for MMT as its volume fraction was increased. (See

Figure 3-23 inset.) The presence of solid particles was also observed visually, as were solid deposits on 

the burner. The vapor pressure of Mn, MnO, and MnO2 are shown in Figure 3-64. Since the volume

fraction necessary for flame inhibition exceeded 200 μL/L, the potential for condensation was apparent. 

Figure 3-63. Normalized 

Burning Velocity of Premixed 

CH4/O2/N2 Flames Inhibited by

MMT with XO2,ox = 0.21 and φ = 

0.9, 1.0, and 1.1. 107 (Dotted 

lines: curve fits to data; solid 

lines: numerical predictions.) 

Figure 3-64. Mn Species Gas

Phase Volume Fraction at 

Equilibrium over the 

Condensed-phase at 

101.3 kPa. 107

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

500 1000 1500 2000

Temperature K

G

a

s-Pha

s

e V

olu

m

e Fra

ction μL/

L

MnO2

MnO

Mn

0.4

0.5

0.6

0.7

0.8

0.9

0

Normalized Burning Velocity

φ = 0.9

1.0

1.1

1

200 400 600

MMT (μL/L)

Tin = 353 K

X = 0.21 O2, ox
228 Flame Suppression Chemistry

here are no published data concerning particle formation or loss of effectiveness of flames inhibited by

tungsten compounds. A search for vapor pressure or thermodynamic data for tungsten oxides, hydrides,

nd hydroxides allowed generation of the results in Figure 3-65. As shown, WO3 has a quite high vapor 

at flame temperatures; on the other hand, the vapor pressures of WO2 and W are low enough to

particle formation. Depending upon the flame reactions of W-containing compounds and their 

F

containing Species Gas Phase 

Volume Fraction at Equilibrium 

over the Condensed Phase at 

101.3 kPa).189,197,231

Mo 

There are likewise no published data concerning particle formation or loss of effectiveness for flames

inhibited by Mo-containing species. The vapor pressures of Mo, MoO2, and MoO3 for flame conditions 

are shown in Figure 3-66. As for W compounds, the only species thought to participate in the catalytic 

cycle for which vapor pressure data are available (WO3 and MoO3) have quite high vapor pressures. 

re are no published data concerning particle formation or loss of effectiveness of flames inhibited by

tungsten compounds. A search for vapor pressure or thermodynamic data for tungsten oxides, hydrides,

nd hydroxides allowed generation of the results in Figure 3-65. As shown, WO3 has a quite high vapor 

at flame temperatures; on the other hand, the vapor pressures of WO2 and W are low enough to

particle formation. Depending upon the flame reactions of W-containing compounds and their 

F

containing Species Gas Phase 

Volume Fraction at Equilibrium 

over the Condensed Phase at 

101.3 kPa).189,197,231

Mo 

There are likewise no published data concerning particle formation or loss of effectiveness for flames

inhibited by Mo-containing species. The vapor pressures of Mo, MoO2, and MoO3 for flame conditions 

are shown in Figure 3-66. As for W compounds, the only species thought to participate in the catalytic 

cycle for which vapor pressure data are available (WO3 and MoO3) have quite high vapor pressures. 

W

T

a

pressure 

allow

pressure 

allow

rates, as well as the vapor pressures of other compounds which form in flames, condensation may or may 

not be important. 

igure 3-65. Tungstenrates, as well as the vapor pressures of other compounds which form in flames, condensation may or may 

not be important. 

igure 3-65. Tungsten1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

500 1000 1500 2000

Temperature K

Gas-Phase Volume Fraction 

μL/L

W

 (10-11@ 2000 K)

1.E-02

1.E-01

1.E+00

WO3

WO2
Metal-containing Compounds 229

Figure 3-66. Mo-containing 

Species Gas Phase Volume 

Fraction at Equilibrium over the 

Condensed Phase at 

101.3 kPa.189,197,231

Sn 

The results for Sn are similar to that of Mn. At the volume fractions necessary for effective flame 

inhibition, a loss of effectiveness was observed in premixed flames, although not as dramatic as for Mn 

(Figure 3-67); particle deposits were observed on the burner. Cup burner tests showed much lower 

effectiveness than expected, as well as a further loss of effectiveness as the Sn volume fraction increased

(Figure 3-23 inset), and solid particles were observed both in the flame and deposited on the burner. The 

vapor pressures of Sn, SnO, and SnO2 are shown in Figure 3-68. Clearly, condensation is possible at the 

3000 μL/L volume fraction needed for flame inhibition. 

Figure 3-67. Normalized Burning 

Velocity of Premixed CH4/O2/N2

Flames Inhibited by TMT with 

XO2

,ox = 0.21 and φ = 0.9, 1.0, and 

1.1.107 (Dotted lines: curve fits to 

data; solid lines: numerical 

predictions.) 

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

500 1000 1500 2000

Temperature K

Gas-Phase Volume Fraction 

μL/L

MoO3

MoO2

Mo

1.E+00

0.4

0.6

0.8

1.0

0 1000 2000 3000 4000 5000

Sn(CH3)4 μL/L

Normalized Burning Velocit

y

φ = 0.9

1.0

1.1

X = 0.21 O2,ox

Tin = 298 K
230 Flame Suppression Chemistry

Condensed Phase at 101.3 kPa.197

s has not been measured as a function of Co 

gher concentrations has not been demonstrated. 

een reported. The available vapor 

w. Since CoO is an intermediate species in the

and total Co loading in the flame is expected to

u On the other hand, depending upon

the relative rates of the inhibition reactions, the volume fraction of CoO may not build up too high, and 

hence it’s propensity to condense could be lower than Figure 3-69 might lead one to believe. 

Figure 3-69 – Co-containing 

Species Gas Phase Volume 

Fraction at Equilibrium over the 

Condensed Phase at 

101.3 kPa.189,197,231

Figure 3-68. Sn-containing 

Species Gas Phase Volume 

Fractions at Equilibrium over the 

Co 

The data for Co effectiveness in various flame system

concentration. Hence, a loss of effectiveness at hi

Likewise, particle formation in flames with added Co has also not b

pressure data for Co oxides and hydroxides are shown belo

proposed inhibition mechanism of Co in H2-O2-N2 flames, 

be on the order of several thousand μL/L, condensation may occ

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E+00

500 1000 1500 2000

Temperature K

Gas-Phase Volu

SnO

SnO2

1.E-04

1.E-03

1.E-02

1.E-01

me Fraction

μL/L

Sn

r.

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E+00

500 1000 1500 2000

Temperature K

Ph Vo e F ti

CoO

Co(OH)2

1.E-03

1.E-02

1.E-01

on

μL/L

Co

rac lu

m ase Gas-
Metal-containing Compounds 231

periments on engine knock, ignition

e screening tests. The compounds

rticles. There has been no mention in the literature 

 the additive, or of any at 

higher mole fractions. Only one of these studies ran experim mole 

fractions,91 and in it, they attributed changes in the performance 

effects of the particles (not condensation of active species or sa

cycle). Hence, it was difficult to assess from previous work if c e for copperbased inhibitors. To assess the condensation potential, Figure 3-70 assembles the available vapor

pressure data for copper compounds which may exist in flames. Al ough data are not available for other

possible hydrides, oxides and hydroxides (e.g., CuH, CuOH) CuO and Cu2O have quite high vapor

pressures, and that of Cu is also relatively high at flame temperatur

Figure 3-70. Cu-containing 

Species Gas Phase Volume 

Sb 

Although the only flame data for antimony inhibition34 did not go

loss of effectiveness, antimony may form condensed-phase partic

with added Sb2O3 and halogen, Fenimore and Martin134 showed

linearly with Sb2O3 at low mass fractions of Sb2O3, but saturate

addition of Sb2O3 is ineffective. Hence, the fire retardancy effect

much like that for iron, manganese, and tin in cup burner flames.1 his, the data in Figure

3-23 (CO2 required for extinction with metal species added) are re-plotted below in Figure 3-71 in terms 

of the limiting oxygen index (based on N2). The top of the figure shows the limiting oxygen index for

polyethylene/halogen blends as a function of the volume fraction of the metallic inhibitor in the gas 

phase. The curves for Sb2O3 were calculated based on data available in Fenimore and Martin.136

 The

effectiveness of the antimony/halogen system saturated at Sb2O3 volume fractions near 400 μL/L (based

Cu 

As shown in Table 3-3, copper salts have been used in ex

suppression, radical recombination above flat flames, as well as in flam

were added either as aqueous sprays or directly as pa

of observed particle formation after decomposition of loss of effectiveness 

ents over a range of additive

with additive mole fraction to heating

turation of the radical recombination

ondensation were possibl

th

es. 

1.E-10

1.E-09

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

500 1000 1500 2000

Temperature K

 Fr n μL/L

Cu

ac

CuO

tio

Fractions at Equilibrium over the 

Condensed Phase at 

101.3 kPa.233,234

1.E-08

1.E-07

1.E-06

1.E-05

Gas-Phase Volume

Cu2O

to high enough concentrations to show 

les in flames. In studies of polymers

that the fire retardant effect increases

s at some value, above which further

of antimony shows a strong saturation, 80 To illustrate t
232 Flame Suppression Chemistry

m of heptane-air cup

burner flames are also shown.180

 Although those experiments were conducted with CO2 added as the 

diluent, the data were converted to an equivalent LOI with trogen diluent by correcting for the 

difference in heat capacity between N2 and CO2. 

Figure 3-71. Equivalent N2/O2 Limiting Oxygen Index for Extinction of Polyethylene 

(PE)-halogen- Sb2O3 Blends136 and Methane-air Cup Burner Flames with MMT, 

Fe(CO)5, and Sn(CH3)4. 

Although the influence of Fe, Mn, and Sn on the LOI when added to the air stream of methane-air cup

burner flames was much weaker than that of Sb2O3 in haloge ration

behavior was observed. It would be of value to understand w -

halogen system, since it has not been explained in the literature. 

3.3.7 Flame Inhibition and Loss-of-effectiveness Summary

The current state of understanding of the inhibition of flames by metal compounds is summarized in

Table 3-12. 

on Sb). On the bottom of the figure, curves for Fe, Mn, and Sn added to the air strea

ni

0.15

0 2000 4000 6000

Catalytic Metal Inhibitor Mole Fraction (μL/L)

Eq

ui

Sn(CH3)4

Fe(CO) MMT 5

Methane-Air Cup Burner

0.20

0.25

valent (vi

a N2) L

OI at E

xti

n

ction

Sb2O3 in C2H3.

Sb2O3 in C2H3.8Br0.068

8Cl0.2

Flame-retarded

polymer with

O2/N2 oxidizer

nated polyethylene, a similar satu

hy saturation occurs in the antimony
Metal-containing Compounds 233

Table 3-12. Current State of Knowledge Relevant to Inhibition Potential of Metals, and 

Loss of Effectiveness due to Condensation. ▓: high, ▒ : medium, ░: low, -none. 

Inhibition 

Information 

Condensation

Information 

Element 

Inhibition Potential 

Gas phase Species Identified 

Kinetic Mechanism St

Demonstrated Loss-of-Effectiveness 

Experimental Evidence of 

Particles 

Experimental 

ate of Development

Vapor Press

ure Data 

Cr ▓ ░ ░ ▓ ▓ ▒

Pb ▓ ░ ░ ▓ ▒

Fe ▓ ▓ ▒ ▓ ▓ ▒

Mn ▓ ▓ ▒ ▓ ░ ░

Ni ▒ 

W ▒ ░ ░

Mo ▒ ░ ░

Cu ░ ░ ░

Te ░ 

Tl ░ 

Bi ░ 

U ░ 

Zn ░ 

La ░ 

Th ░ 

Se ░ 

Sn ▒ ▓ ▒ ▓ ░ ░

Ti ▒ ░ 

Co ▒ ▓ ░

Ge ▒ 

Sb ▒ ░ ░ ▒ 

P ▓ ▓ ▓ ▒ ░ 
234 Flame Suppression Chemistry

The inhibition potential is summarized in terms of the known inhibition behavior, the detection of metalcontaining species in flames, and the state of development of a kinetic description of the inhibition. The 

knowledge of the potential for loss of effectiveness is characterized in terms of the demonstrated loss of

effectiveness in flame systems, the presence of particles, and the availability of vapor pressure data for the 

condensed-phase metal-containing species. In the “Inhibition Potential” column, the quality of the 

information is rated as high, medium, low, and none, not the inhibition ability itself. Even for elements 

such as iron, 

species inant

inhibition c tes were not 

available.

The last row in t us

compounds. l, the active 

species for flam n. However,

there is not 

condensation

potential for New Mexico 

Engineering Rese

other flame sy on 

Approache

Actually ing. To do

this in a e inhibition 

mechanism condensation, 

agglomer

This is far from

An alternativ may be 

imple screening experiments with cup burners. In order to determine both the variation with

con ed

to b a

test, de 2 2 e air

stream necessary to cause blow-off of ined, both with and without the 

ing effects of particle evaporation and 

decomposition are avoided.

e em uld be to add them to a solid 

which has been the most extensively studied, there were fragmented data on the gas phase

 in flames, the gas phase kinetic mechanism had many estimated rate constants and the dom

ycles were assumed, and vapor pressure data for some of the important intermedia

As the table illustrates, most of the information needed to accurately predict both the inhibition 

potential as well as the potential for loss-of-effectiveness were incomplete. 

he column shows the state of information relevant to flame inhibition by phosphor

Due largely to NGP research, the state of understanding of the inhibition potentia

e inhibition, and the kinetics of the inhibition cycles are fairly well know

nearly as complete an understanding of the potential for loss of effectiveness due to

 of intermediate species formed in a flame. The only data which indicate, indirectly, a 

 condensation are the cup burner extinguishment data from researchers at 

arch Institute,235

 in which the volume fraction for extinguishment is high relative to

stems. In a section below, the NGP work to understand the gas phase flame inhibiti

mechanism of phosphorus compounds is described in detail. 

s for Ascertaining the Potential of Metal-containing Compounds

calculating the condensation potential of metal inhibitors in flames is quite challeng

quantitative way requires knowledge of the species present in the flame, th

with its associated rate constants, the vapor pressures of the metal species, the 

ation, and re-evaporation rates for the particles, and the actual species present in the particles. 

 being achieved even for iron, the most highly studied metal. 

e way to assess the potential for condensation of metal inhibitors in flame systems 

s

centration, as well as the potential benefit of using the metal compounds with inert agents, tests ne

e conducted with the added metal compound and a secondary inert agent added together. In such

scribed above and in the literature86,110,236

 the amount of a diluent (e.g., CO , or N ) added to th

the cup burner flame is determ

metal agent added at increasing concentration in the oxidizer stream. Since, to some extent, the flames

resemble the low-strain conditions of fires, there is expected to be much higher correlation between the

behavior in this system and in actual suppressed fires. By adding the metal compounds in the form of

organometallic agents (rather than metal salts), the complicat

A mor pirical approach to evaluating metal-containing compounds wo

propellant-based extinguisher. There is inert gas in the effluent, as well as fragmented metal-containing

species, so that by proper application to an appropriate fire, the utility of the metal additive could be 
Metal-containing Compounds 235

as y

to th

Approaches for Overcoming Conden

There may exist approaches for overcoming the loss of effectiveness of metal compounds due to 

condensation of metal oxides. For example, halogens could used attack the m oxide and pr e

metal-halogen species in the gas phase. escribed by stie co-workers, he halogen in the 

antimony-halogen fire retardant system acts to release the on m the condensed phase through a

series of halogenation steps involving successive oxychlo pha Bromine has been used in the 

past to etch off the lead oxide deposits on engine valves, and halogens are used to remove the oxide 

coating from the incandescent filament in quartz-halogen light bulbs. It is clear that halogens can release 

metals from solid oxides, and it may be possible to use thi oper o re-introduc e metals fro e 

condensed oxide into the gas phase where they can again inhibit th me. Finally use of carb c 

acids as extenders of antiknock agents100 (through the formation of lts which persist in t s

phase) leads naturally to the question of whether such an approach would work for metal-base re

suppressants as well. 

3.3.8 Conclusions 

Extensive analysis of published data and new da obta from experiments and calculations has 

enabled new understanding of the flame-inhibiting m of metal-containing compoun s and

potential as successors to halon 1301. 

• There are several metal atom compounds ha show very h h effe ivenes at reducing 

the rates of the che al reactions that propagate fl iron, ma ganese, chrom ,

an ny, lead, and tin. NG resea uncovered additional metals whose compounds 

are likely to be eff ive at low concentrations, but which ad not een su gested n pre us

reviews of metal flame inhibition. These are tungsten, molybdenum, nd cob t.

• Most of the metals discussed above have the potential to lose their effectiveness at concentrations

below those needed to extinguish flames, due to condensation of the active species. Were 

condensation to occur in practical fires, com ounds these etals would likely not demons ate 

fire suppression capabilities commensurate e velocity inhibition efficiency

that was observed when added at low concentration to laboratory premixed f mes. n the o er 

hand, for the metals W, Mo, Co, and Cu, there is no direct evidence of the 

effectiveness, and the vapor pressures of the suggested flam -quenc ing spe ies (fo which ata

are ailable) are r onably gh. In general, prediction of the potential loss of eff is

beyond the current state of the art. mpirical testing in a cup burner or a solid propellant fire 

extinguisher (Chapter 9) is needed. 

sessed at a larger scale. The challenge would be to find a configuration which has the proper sensitivit

e inert gas flow so that the effects of the additive could be quantified. 

sation

be

Ha

antim

ride

s pr

to

and

y fro

ses.

139

ty t

e fla

metal sa

etal

140 t

e th

, the

can

ovid

m th

oxyli

he ga

d fi

As d

ta ined

echanisms d their 

s whose ve n ig ct s

mic ames. These are n ium

timo P rch has also

ect h b

a

g

al

i vio

this 

p of m tr

with their high flam

la O th

potential for loss of

e h c r d

av eas hi ectiveness

 E
236 Flame Suppression Chemistry

3.4 PHOSPHORUS 

3.4.1 Introduction 

Reasons for Interest 

Phosphorus-containing compounds (PCCs) are known to be effective at reducing flammability, with some 

ambiguity 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 plastics237,238

 has increased

dramatically239

 in recent years, with the 1998 estimated U.S. market for organophosphorus fire retardants 

at 80 million lb.240

 In this application, the relative importance of gas phase chemistry and solid-phase 

nds in cup burner and streaming tests.249,250

lames, with the 

intent of using this knowledge to identify and screen candidate chemicals as successors to halon 1301. 

oxy groups attached to the

phosphorus atom. 

 investigated the flame structure of low-pressure, premixed stoichiometric 

less rapid oxygen consumption as flame inhibition. Both DMMP and TMP acted as inhibitors in 

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

the two mechanisms, depending on the specific PCC chemistry.

238,241-244

 Monoammonium phosphate has 

been used for many years as a dry chemical fire-extinguishing agent,245

 and has also been found to be 

very effective in preventing dust explosions.246,247 Since it is applied as a powder, it is possible that 

physical properties may contribute substantially to its effectiveness. (See Chapter 4.) 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.248 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 compou

The NGP was thus interested in developing further understanding of how PCCs affect f

Such foreknowledge was necessary because of potential obstacles to their use as flame suppressants: 

• Many, if not all, of these compounds, have low vapor pressures at ambient temperature.251 (See 

Chapter 7.) 

• Some PCCs have a significant heating value due to alkyl and alk

• There are diverse bonding states of the phosphorus atom and a variety of ligand groups within the 

realm of PCCs. The import of these to flame suppression effectiveness was uncertain. 

• PCCs range in their toxicity from non-toxic, essential food constituents and FDA approved 

additives to some of the most toxic compounds (chemical warfare agents) known.249,252

Chemical Effects 

Well-characterized investigations of the flame inhibition properties of PCCs have been sparse, and in 

some cases ambiguous or contradictory.

• Korobeinichev et al.

H2/O2/Ar flames with the non-halogenated PCC additives trimethyl phosphate (TMP)253 and 

dimethyl methylphosphonate (DMMP).254

 They compared oxygen profiles in flames with and 

without the PCC additive, interpreting more rapid oxygen consumption as flame promotion and 
Phosphorous 237

the low-temperature zone near the burner surface and as promoters in the higher-temperature

region downstream. 

• Fenimore and Jones255

 found that TMP addition produced no change in the oxygen consumption

In a more dilute, rich flame at atmospheric pressure,

for flame suppression by PCCs was as follows. Phosphorus-containing radicals, 

such as HOPO and HOPO (formed from PCCs during combustion253,254

) catalyze the recombination of 

, H and OH, slowing the overall reaction rate and thus inhibiting the

256

rent compounds that might be less toxic and more amenable for 

deli y

Several 

suggest

reactor product gases

incr e

that e

halogen

halogen

low-pre

dilute co

Transp

A key f

several d oxidant-side doping of various fuel/inert vs. oxygen/inert 

ames have found dramatic changes in additive effectiveness with these changing conditions. When

drocarbons are burned in air, the stoichiometric contour, where the flame is assumed to be, is situated 

n the oxidant side of the stagnation plane. Fuel-side additives must diffuse across the stagnation plane to 

reach the flame, while air-side additives are convected through the flame. The result is that fuel-side 

dditive mole fractions at the flame are substantially reduced from their fuel-stream values, while 

oxidizer-side additive mole fractions are only slightly reduced from their oxidizer-stream values. The 

rate in a low-pressure, rich, H2/O2/Ar flame.

they observed a reduction in the oxygen consumption rate with increasing TMP loading. 

• A more comprehensive study was conducted by Hastie et al.256 with TMP, triphenylphosphine 

oxide and POCl3 additives in premixed and non-premixed atmospheric-pressure propane,

methane, and hydrogen flames. PCC effects were evaluated from measurements of H radical 

concentrations and burning velocities. Inhibition was observed for rich and lean flames with 

temperatures up to 2350 K. At higher temperatures, flame promotion was observed. Inhibition in

premixed flames has been observed in terms of reduced laminar flame speeds for several 

halogenated PCC additives and one non-halogenated PCC additive, TMP.34

A more complete literature review of the influence of PCCs on flames can be found in References 79 and 

257. 

The emerging premise

2

the important combustion radicals

flame. Since it is the phosphorus-containing radicals that participate in the recombination mechanism, 

and not the parent PCC, the form of the parent compound is relatively unimportant.19 If supporting

evidence were found for this hypothesis, then further studies of flame inhibition by PCCs could

investigate alternative forms of the pa

ver to the flame.

mechanisms for catalytic cycles resulting in H + OH → H2O and H + H → H2, have been 

ed.

140,256,258-260 Partial validation of the radical recombination hypothesis was provided by flow

experiments showing that the addition of PH3 (phosphine) to H2-O2 combustion

260 eas d H+OH recombination rates following laser photolysis. Flame experiments have also shown

th presence of PCCs affects radical levels. The addition of small quantities of the POCl3, a 

ated PCC, produced lower OH levels in non-premixed methane and ethene flames than did other 

ated compounds not containing phosphorus.47 The addition of TMP lowered H levels in premixed 

ssure H2/O2/N2 flames under rich, dilute conditions, and raised H levels under stoichiometric, less 

nditions.256

ort Effects

actor affecting suppression is the amount of the additive reaching the non-premixed flame,261 and 

studies

 36,45,51,103 of both fuel-side an

fl

hy

o

a
238 Flame Suppression Chemistry

extent of this additive loading reduction at the flame location varies with the stoichiometric mixture 

fraction (Zst), defined as the mass fraction of material originating from the fuel stream at the location 

nts are in stoichiometric proportions. For hydrocarbon/air flames, Zst is 

st can be achieved by varying the compositions of the 

either via dilution or enrichment. For Zst>0.5, the flame is on the fuel side of 

additive loading reduction at the flame is more severe for oxidant-side additives 

was 

first suggested by Hastie and Bonnell in 1980, but since then there has been little investigation of this

beh o

there ha which flame temperature has been systematically varied. 

The infl perature on inhibition effectiveness is an important consideration in evaluating 

the feasibility of new phosphorus-based flame suppressants, and has direct bearing on how to maximize

thei f

phospho the flame,

thus c

of the c concerns

and possible health impacts. A similar temperature dependence has been demonstrated for CF3Br, 

ation of synergistic interaction with the inert suppressants N2, CO2 and H2O.14,262

 

at lower flame temperatures has also been observed for the iron-based suppressant 

3.4

Fisher

(DMMP

P(=O)(O

flame. 

where the concentrations of reacta

typically < 0.1. Considerably different values of Z

fuel and oxidant streams,

the stagnation plane, and

than for fuel-side additives. 

In several experimental studies of flame extinction by CF3Br, the effect of additive loading reduction at

the flame location has been quantified, assuming unity Lewis number. Results have been mixed. For 

methane/air opposed-jet diffusion flames, Trees et al.261 found that CF3Br was roughly twice as effective 

as an air-side additive than as a fuel-side additive, when the quantity of additive at the stoichiometric

contour is compared for equal extinction strain rates. Work by Masri45, on the other hand, found that 

oxidizer-side and fuel-side addition of CF3Br had very similar effectiveness in extinguishing turbulent

natural-gas/air piloted jet flames, when corrected for additive concentrations at the flame location. Niioka 

et al.36 reported comparable effectiveness for fuel-side and oxidizer-side addition in extinguishing various 

ethene/nitrogen vs. oxygen/nitrogen opposed-jet diffusion flames, including one with Zst >0.5. Close 

examination of the Niioka data showed that the fuel-side addition was generally less effective than 

oxidizer-side addition. 

Temperature Effects 

Extinction studies under a variety of flame conditions found a significant temperature dependence for 

DMMP effectiveness, with higher effectiveness being observed at lower stoichiometric adiabatic flame

temperatures.

120

 The existence of a temperature dependence for phosphorus-based flame inhibition

256

avi r. There is one computational study on premixed flame speeds,257 but aside from Reference 79, 

s been no experimental work in 

uence of flame tem

r e fectiveness in practical applications. The indicated trend implied that a mixture of inert and

rus-based suppressants could interact synergistically as the inert component cooled

in reasing the efficiency of the chemically active component. This would lower the needed quantity

hemically active component, thus reducing system weight and decreasing environmental

including the observ

Increased effectiveness 

Fe(CO)5. 103

.2 Inhibition of Non-premixed Flames by Phosphorus-containing Compounds 

and co-workers

79 examined the inhibiting effects of two PCCs: dimethyl methylphosphonate

) [CAS #756-79-6, P(=O)(CH3)(OCH3)2], and trimethyl phosphate (TMP) [CAS #512-56-1,

CH3)3], along with other additives for comparison, on a non-premixed methane-air diffusion

Their results have led to greater understanding of how a phosphorus-containing fire suppressant
Phosphorous 239

would a

purpose

•

Experi

Appara

 cm ID glass tubes. The sheath tube exits were offset by approximately 1 cm, 

ant tube exits, to minimize the impact of the sheath flow on the development of the

flame was produced when the oxidant and fuel stream velocities were such that the 

the central portion of the space between the burner nozzles. The entire burner was

ct to quench flames and developed some guidance for the selection of candidate PCCs for this 

. Their research: 

• Established a novel and sound protocol for conducting such experiments. 

Measured the effectiveness of the two PCCs relative to other, reference suppressants and to each 

other. 

• Determined how the flame inhibition effectiveness of the two additives changed as a function of 

the agent loading and the adiabatic flame temperature, using the reduction in [OH] as a marker. 

mental

tus and Materials 

The opposed-jet burner is shown schematically in Figure 3-72. 

Figure 3-72. 

Schematic of 

Opposed-jet Burner. 

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 were

provided through 2.22

upstream of the react

reactant flows. A flat

flame was situated in

isolated in a glass enclosure for control of exhaust gases. This enclosure was purged with nitrogen and 

maintained slightly below atmospheric pressure. 

The burner was aligned vertically with the lower tube used as the fuel source and the upper tube as the 

oxidizer source. The 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 extreme sensitivity of the extinction strain rate to the mass fraction of oxygen in the oxidizer stream

necessitated very precise metering of reactant flows in order to achieve repeatable results. Precision mass
240 Flame Suppression Chemistry

flow controllers, supplied by MKS Industries, were used to measure reactant flows. Even so, to minimize

variability, measurements comparing doped and undoped flames were conducted by first evaluating the

undoped extinction strain and then adding the dopant to one reactant stream without otherwise altering the 

composition of either stream. 

Most of the extinction measurements were conducted using premixed oxygen and nitrogen (21 % ± 0.2%

oxygen by volume) or Ultra Zero Grade air (21 % ± 2% oxygen by volume, total hydrocarbon content 

< 0.1 μL/L) as the oxidizer and methane (99 % pure) as the fuel. All gases were supplied by MG 

e control of the sheath flow temperature. These final reactant temperatures were

measured by sheathed thermocouples in direct contact with the gas flow. The liquid PCC dopant was 

ams via a syringe pump that provided a constant volume flow. The gas flow at

ately 130 °C for a rapid vaporization of the incoming liquid 

it n was selected due to the weak temperature dependence of the rotational ground state 

population in the temperature region of interest (< 10 % variation for 1400 K < T < 2500 K), which helps 

7 ns

industries. For the extinction measurements with flame and stagnation plane coincident, oxygen

(99.994 % pure) was mixed with nitrogen (99.998 % pure) in measured proportions to form the oxidizer

stream, and the methane fuel was diluted with nitrogen (99.998 % pure). 

The chemically active flame inhibitors, DMMP and TMP, were obtained from Aldrich Chemical 

Company (each 97 % pure). They were liquids at room temperature with low vapor pressures (less than

100 Pa 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 of the heated lines were measured with adhesive thermocouples. The 

temperature of the reactant streams 10 cm upstream from the exit of the nozzles was maintained at

100 oC ± 1 °C via activ

added to the reactant stre

the injection site was preheated to approxim

dopant. The liquid density, syringe size, and motor speed fixed the mass flows of the dopants. Previous

pyrolysis experiments with DMMP and TMP 263,264

 have shown that thermal decomposition is negligible

below 1000 K; thus the preheating of the reactant streams should have had no effect on their chemical

composition.

Measurements 

The enclosure had 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. Flame 

position was measured relative to the oxidizer nozzle exit plane using the separation gap between the

nozzles for reference. 

OH concentration measurements were made using pointwise LIF. Figure 3-73 shows a schematic of the 

experimental set up. Profile measurements were made by traversing the laser beam and collection optics

simultaneously across the flame in 100 μm steps. The translation stages used had a resolution of 10 μm. 

Excitation of the P1(9) transition of the A←X (1,0) band near 286 nm was made using a frequencydoubled Quanta Ray PDL 2 pulsed dye laser, nominal linewidth of 0.2 cm-1, with rhodamine 590 dye.

This trans io

maintain a strong LIF signal across the entire OH profile. The dye laser was pumped with a 6 ns to

pulse at 532 nm provided by a frequency-doubled Spectra Physics DCR 2A Nd:YAG laser operating at

10 Hz. 
Phosphorous 241

Nd:YAG

Figure 3-73. Schematic of LIF Apparatus. 

Detection of OH fluorescence was performed at 90°

photomultiplier tube and color glass filters (Schott WG305 and UG11) which transm

305 nm to 371 nm range. The photomultiplier signal was amplified by a S

 to the laser beam axis using an RCA 8850 

it light in the 

amplifier 

(Stanford Research Systems). The filter combination used blocks elastically ght (and

near-elastic resonant fluorescence) while passing fluorescence emission from the

and emission from the overlapping (0,0) band (306-320 nm) which can occur following collisionally 

driven Vibrational Energy Transfer (VET) from the ν’ = 1 to the ν’ = 0 states. At atmospheric

pressure, VET rates can be significant265,266

 with more than a third of the total measured fluorescence

originating from the (0,0) band when using excitation/detection schemes similar to this.267

 Fluorescence 

trapping of emission from the (1,1) band was negligible due to the small population of the electronic 

ground state with ν’’ = 1 over the temperature range under consideration. Some trapping occurred for

uorescence emission originating from the (0,0) band, but the effect was small due to the short path 

268,269

search Systems), and recorded on a personal computer 

sulting changes in the spectral

overlap of the laser and molecular absorption line shapes, the output wavelength of the dye laser was

Signal to Fast

Preamp and

Boxcar

Integrator

RS240 Fast Pre

scattered laser li

 (1,1) band (312-326 nm)

fl

length through the flame (less than 1 cm) and is not expected to influence the measurements. PAH 

fluorescence268

 was expected to be negligible because the moderately strained laminar flames under

investigation were virtually soot-free. This was confirmed by measurements taken with the dye laser

tuned off resonance. The combination of the focused (ƒ = 30cm quartz focusing lens) UV laser beam and 

slits in the detection optics give a probe volume of less than 100 μm in diameter and 1.5 mm in length. 

The opposed-jet burner flame was locally planar near the centerline with the long axis of the probe 

volume aligned in the plane of the flame surface. Laser pulse energy was monitored using a UV-sensitive 

photodiode. Both the LIF and laser energy signals were integrated on a shot-by-shot basis using an SRS 

250 Boxcar Integrator and Averager (Stanford Re

using a GW Instruments data acquisition card. Operation within the linear LIF regime was ensured by 

checking the linearity of LIF signal strengths with pulse energy for all measured data points.270

To avoid errors associated with wavelength drift of the dye laser and the re

Laser

(532 nm)

Dye

Laser

(572 nm)

Frequency

Doubling

Crystal

Visible 572 nm

Beam Blocked

Prism

286 nm UV beam

Laser Pulse Energy

Photodiode

Beam Traverse

with Focussing Lens

Oppos

and H

ed-Jet Burner

ousing

PMT, Filters and

Collection Optics

on Traverse

Signal to

Boxcar Integrator

OH

Fluorescence
242 Flame Suppression Chemistry

scanned across the P1(9) transition at each measurement location using a stepper motor to adjust the

grating in the dye laser cavity. 100 measurements of OH fluorescence and laser pulse energy were

acquired at each half-step of the motor (corresponding to a wavelength change of approximately

0.0012 nm), and the half-step motor position with the maximum average ratio of fluorescence signal to

laser pulse energy was taken to be the absorption line center for the transition. LIF data for that half-step

motor position were used for the subsequent evaluation of OH concentration. The repeatability of this 

procedure was found to be within ± 5 % of the mean observed ratio. The average ratio of OH

ot noise and electrical noise, the 

statistical uncertainty in the measurements reflected fluctuations in flame location. For these 

otal fluorescence signal; C is a constant that incorporates geometric factors such as

probe volume as well as detector efficiencies and gain characteristics of the 

ound state rotational level excited by the laser; and Φ is the fluorescence quantum

yield, which depends on the local quenching and VET rates. 

fluorescence signal to laser pulse energy was found by fitting the 100 measurements to a straight line 

using linear regression analysis. The statistical uncertainty (one standard deviation) in the fitted slopes

was less than ± 3 %. Additional uncertainty in these slopes arose from statistical uncertainty in the 

measurement of the zero level in the presence of electrical noise from the Nd:YAG laser. Statistical

scatter in the zero measurement (± one standard deviation) corresponded to an uncertainty of ± 3 % to 

5 % of the peak slope for all data points in the profile. In addition to sh

aerodynamically stabilized flames, perturbations in the reactant flow can cause small, momentary flame 

displacements. The 100 shot LIF averaging process largely compensated for these oscillations. The total 

uncertainty for each measured data point in a given profile due to random errors was found by summing

the contribution of the uncertainties described above (assuming statistical independence) and is estimated

to be less than ± 10 % of the peak value in the profile. 

In order to quantify the fluorescence measurements, corrections for collisional quenching and the fraction 

of the OH population that is in the pumped rotational level must be made. However, the overall effect of 

these corrections on the profile shape is small, as is expected for our excitation/detection scheme in nonpremixed atmospheric-pressure flames.

271-273

 Equation 3-3 relates the fluorescence signal to the

concentration of OH assuming low laser energy (i.e. negligible saturation of the excited transition): 

(3-3)

F = CBIfNΦ

Here F is the t

collection solid angle and 

integrators, amplifiers etc.; B is the Einstein coefficient for stimulated absorption; I is the spectral

overlap integral between the laser intensity and the molecular absorption line shape, temporally integrated 

over the duration of the laser pulse; N is the total OH population, and f is the fraction of the population 

that is present in the gr

Species and temperature profiles from the undoped-flame calculations were used to estimate the required

corrections to the linear LIF data for both doped and undoped flames. The fraction of the ground state 

population in the pumped rotational level is determined using the local gas temperature given by the

calculations and assuming rotational thermal equilibrium (Boltzmann distribution), as implemented in the 

spectral simulation software LIFBASE.274

The total fluorescence yield, Φ, was estimated using:267
Phosphorous 243

where A1 and A0 are the spontaneous emission rates for the ν’ = 1 and ν’ = 0 vibrational levels, 

respectively. Equation 3-4 has been simplified by assuming equal transmission and detection efficiencie

of the collection optics and PMT for both the (1,1) and (0,0) bands. Q1 and Q0 are the total quenching

rates for the ν’ = 1 and ν’ = 0 levels, and V10 is the total VET rate from ν’ = 1 to ν’ = 0. 

influence of VET rates on the fluorescence quantum yield is primarily due to the higher spontaneous 

emission rate of the ν’ = 0 state (A1/A0 ≈ 0.575) 275. The quenching and VET rates were evaluated using 

Equation 3-5.267,276

where P is pressure; T is temperature; kb is Boltzmann’s constant; χi is the mole fraction of OH collision 

(3-4)

s

The

(3-5)

artner species i; <ν>i-OH is the mean relative collision velocity of species i and OH given by <ν>i-OH = 

(8kbT/πμi-OH)

1/2

, where μi-OH is the red σi

Q,V(T) is the value of the 

quenching or VET (superscr ns with species i, averaged

es. This

variation was moderated by the presence of a large concentration of N which has a relatively small

p

uced mass of species i and OH; and 

ipt Q or V respectively) cross section for collisio

over all rotational levels in a given vibrational level, assuming rotational thermal equilibrium. 

Substitution of Equation 3-5 into Equation 3-4 followed by some algebraic manipulation yielded:267

(3-6)

where <σQ>1, <σQ>0 and <σV>10 represent the sum over all collision partners of the thermally averaged 

cross sections for quenching from ν’ = 1, quenching from ν’ = 0, and VET from ν’ = 1 to ν’ = 0,

respectively. Quenching cross-section measurements for the ν’ = 1 and the ν’ = 0 state indicated

similar values.265,275

 The ratio of averaged VET cross sections to the averaged quenching cross sections

was assumed to be independent of position in the flame (e.g., <σV>10/<σQ>0 ≈ constant). As a result, the 

quantity in square brackets in Equation 3-6 became a constant, and the fluorescence yield was simply

inversely proportional to the quenching rate for the ν’ = 0 state. The error in the assumption 

<σV>10/<σQ>0 ≈ constant was less than 5 %. Values for the quenching rate cross-sections used in this 

work were taken from temperature-dependent quenching cross-section data compiled by Tamura et al.277

from the extensive measurements that have been made for the ν’ = 0 state of OH. 

The calculated quenching rate varied by 15 % across the OH containing regions in N2-O2 flam

1 10

0

0

10

1

Q V

A

Q

V A

+

+

Φ =

v ( ) T k T Q V P Q V

i

i

i i OH b

, , ∑χ σ − ⎟

⎠

⎞ ⎜

⎝

⎛ =

⎥

⎥

⎥

⎥

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎢

⎢

⎢

⎢

⎣

⎡

+

+

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

Φ =

0

10

0

10

1

0

0

1

0

0

1

Q

V

Q

V

Q

Q

A

A

Q

A

σ

σ

σ

σ

σ

σ

2
244 Flame Suppression Chemistry

gradient in the flame zone. For flames where Ar (a highly inefficient quencher) fully replaced N2, the 

variation was substantially higher at 30 %.

The same quenching environment was assumed for flames both with and without DMMP loading. The 

maximum phosphorus loading was only 715 μ/L. Since the mole fractions of phosphorus-containing 

species were small in this case, it was expected that they would not contribute significantly to quenching 

of OH. Changes in major species profiles due to the presence of DMMP were not expected to be large

enough to significantly change the overall quenching environment. Changes in flame temperature due to 

the addition of these small loadings of DMMP were also found to be negligible (< 10 K). 

After correcting for the fluorescence yield and the Boltzmann fraction of the pumped rotational level, the 

corrected LIF profile was directly proportional to OH concentration. An estimate of the scaling constant 

relating the corrected LIF measurements to absolute OH concentrations was made by matching the peak 

of the measured profile from the undoped flame to the peak OH concentration predicted by the flame 

calculations. This matching did not affect observations of PCC effects on OH, which were all presented

normalized to the undoped flame measurements. For profiles measured in flames doped with

phosphorus-containing additives, the scaling constant was determined from undoped profiles taken on the 

same day with the same instrument settings. Because the same constant was used for both doped and

undoped flames, quantitative comparisons could be made between the two cases. Determination of the 

magnitude of the scaling constant was performed by minimizing the mean-square-error between

to be

constant for a given set of instrument settings. This scaling procedure affected only the magnitude and

osition of the observed fluorescence pr

subscripts O and F refer to oxygen and fuel respectively. This

of the stagnation plane under certain 

s ffects must be confined to a narrow region around 

the stagnation plane, and plug flow boundary conditions must be imposed at the nozzles. Under less 

restrictive conditions, this expression has been determined experimentally to be nearly proportional to the 

local oxidant-side velocity gradient: Recent measurements performed in an open-tube burner with 

calculated and measured profiles (via a least squares technique). For this minimization, only data points 

above 50 % of the measured peak value were considered. Minimization of error was performed via

adjustment of the scaling constant and a position offset (i.e., the offset between the measured flame

center, as indicated by the translation stage micrometers, and the computational flame center).

Determination of this scaling constant calibrated the constant C from Equation 3-3, assumed

p ofile and not the shape or width. 

Determination of Extinction Strain Rates 

The flame strength can be characterized by the strain rate at extinction,278,279

 and here, the effectiveness of

additives as flame inhibitors was 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.280 The strain rate at extinction was approximated using the following 

expression involving global flame parameters:76

(3-7)

where L refers to the separation distance between the nozzles, V is the average stream velocity at 

extinction, ρ is the stream density and the

⎟

⎟

⎠ ⎜

⎜

⎝

= +

O O

O F F

q V

V

L

V

a

ρ

ρ 1 2 ⎛ ⎞

expression is equal to the velocity gradient on the oxidant side 

restrictive conditions: reactions, diffusion, and viscou e
Phosphorous 245

geometry very similar to our own early proportional relationship between the aq of 

Equation 3-7 and the maximu nt on the oxidant side of the flame, as measured by 

laser-Doppler velocimetry.

281

y was found to hold for methane/air and 

propane/air flames, with and withou e-inhibiting additives in the air stream. Thus, Equation 3-7 can 

be used to determine relative changes in extinction strain rates obtained on the same apparatus. In

ion studies with opposed-jet burners adjusted the strain rate by varying both the 

oxidizer and fuel streams simultaneously so that the flame position remained constant,63 or so that a 

omentum balance between the two streams was is required varying the mass flows of

both reactant streams, and resulted in ch ations of the inhibiting agent when the agen

delivered via a constant-mass-flow syringe pump. The resulting transients in the adsorption/desorption of 

t time for 

f the

ame varied during the extinction expe y a minor influence on the 

measured extinction strain except whe very close to one of the 

nozzles. 

To validate this method, experim large range of methane and air f

achieving extinction with the flame in a variety of positions. Figure 3-74 shows the resulting global 

extinction strain rates (aq) as a function of the observed flame position. 

-1

d

ons in extinction strain

 have found a n

m local velocity gradie

 The same proportionalit

t flam

particular, the ratio of doped to undoped aq measured using Equation 3-7 should be equal to the same ratio

of local oxidant-side velocity gradients. The validity of this approach was borne out by the good 

agreement between the current data and those of other researchers for inert additives.

In determining extinction conditions for use in calculating aq, Fisher and co-workers developed a novel

method for approaching extinction, motivated by a need to minimize transient effects of dopant loading.

Commonly, extinct

m maintained.80 Th

anging concentr t was

the agent on walls made it difficult to predict the concentration of agent at the exit plane of the doped

reactant stream at extinction. 

The new method circumvented the difficulties associated with dopant loading transients. The

concentration of dopant in the relevant reactant stream (fuel or oxidizer) was fixed by maintaining 

constant flows of all the constituents of the doped reactant stream. After waiting a sufficien

equilibrium conditions to be established, extinction was approached by varying only the flow o

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

fl riment. This was found to have onl

n the flame or stagnation plane was 

ents were performed over a lows,

The extinction strain of the undoped flame varied only ± 2 % (from a mean value of 359 s ) for flame 

positions between 0 mm and 7mm. This region of consistent global extinction strain rates was denoted as 

the acceptable region. For experiments in which extinction occurred with the flame outside the 

acceptable region, large deviations in aq occur. 

Also shown in Figure 3-74 are sets of data from experiments conducted with a 300 μL/L doping of 

DMMP and 50,000 μL/L doping of N2, both in the oxidizer stream. (The limited number of data points in

the DMMP series was a result of the coarsely quantized mass flows of dopant available from the syringe

pump.) Because of the inhibitory effect of the DMMP or N2 additive, these data sets had markedly lower 

mean global extinction strain rates than the data for the undoped flames. However, both data sets showe

the same systematic trends with flame position, and both had only small variati

over the acceptable region. 
246 Flame Suppression Chemistry

niently characterized using the 

ichiometric mixture fraction, Zst, is the fraction of the material

in the fuel stream. Zst was evaluated from reactant

(3-8)

where Y is mass fraction, MW is mo stoichiometric coefficient for com

combustion, the subscripts O and F refer to oxygen and fuel, respectively, and the subscripts ±∞ refer to 

Figure 3-74. Variation of Global 

Extinction Strain, aq, with

Observed Flame Position at 

Extinction. 

The existence of the same acceptable region for a range of flame extinction strain rates validated this 

method for approaching extinction. Within this region, the results should be within experimental 

uncertainty of those obtained using more conventional methods that maintain a nearly constant flame 

position. The nearly constant extinction strain rates over the central region were consistent with the

previous computational findings of Nishioka et al.282

The asymmetrical dependence of extinction strain rate on flame position was due to the offsetting of the 

flame to the oxidant side of the stagnation plane, which was conve

stoichiometric mixture fraction, Zst. The sto

present at the stoichiometric contour that originated 

compositions in the nozzles and the stoichiometry of the overall combustion reaction, using Equation 3-8: 

⎟

⎟

⎝ ⎠

+ ⎟

⎟

⎠

⎞

⎝ F,+∞ O,−∞

F F

O Y Y

MW ν

ν ⎞

⎜

⎜

⎛

⎜

⎜

⎛ = ,−∞

O

O

st

MW

Y

Z

lecular weight, ν is the plete

conditions at the fuel and oxidizer nozzles. The undiluted flame used for the first series of experiments in 

Figure 3-74 had Zst = 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.67,283

The importance of this displacement was investigated by measurements in which the flame and the

stagnation plane were coincident. By varying the dilution of the fuel and oxidizer streams it is possible to 

move the location of the stoichiometric contour (the presumed flame location), relative to the stagnation
Phosphorous 247

plane. For reactants of equal diffusivities, setting Zst = 0.50 placed the flame at the stagnation plane.

Since Zst depended on two independent variables, YF and YO, fixing Zst left one degree of freedom on

reactant concentrations. Unique conditions for the Zst = 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 flame. Results from these conditions, Zst = 0.50 with YO = 0.387 and YF = 

0.097, are shown in Figure 3-75. During these experiments, in which the oxidizer stream was mixed from 

separate O2 and N2 bottles, it was found that the global extinction strain rate was very sensitive to the 

concentration of O2, consistent with experimental results from Puri and Seshadri for diluted methane-air

flames.

80 This sensitivity contributed to the greater scatter in the Z = 0.50 tests than in the Z = 0.054 

Figure 3-75. Variation of Global 

Extinction Strain, aq, with

roduced which can be calculated from the reactant stream 

ance from the oxidizer nozzle to the stagnation plane

Figure 3-76 shows a plot of observed flame location for both Zst = 0.50 and Zst = 0.054 against the

arameter S. The observed flame location did not vary linearly with the predicted location of the 

stagnation plane, but instead followed a sigmoidal curve, being much more sensitive to flow variations 

st st

tests, which were conducted with premixed air. Nonetheless, the global extinction strain rate varied less 

than 3 % of the mean value of 336 s-1 for flame locations between 0 and 9mm. Within experimental 

scatter, results with Zst = 0.5 were symmetrical about the center of the region between the two burners. 

Observed Flame Position at 

Extinction for Non-premixed 

CH4/N2-O2/N2 Flames, with Z st = 

0.50. 

Although the acceptable region of Figure 3-74 is defined in terms of the observed flame position, in

practice it is not necessary to determine the experimental observation of flame location at extinction for 

all measurements. Rather, a parameter S is int

properties at the nozzles. S approximates the dist

and is given by the expression in Equation 3-9, again derived by Seshadri and Williams for plug flow

boundary conditions:76

(3-9) 1 .

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = +

O O

F F

V

V

S L

ρ

ρ

p
248 Flame Suppression Chemistry

when it was near the middle of the separation gap. The data

positions for only high strain cases (275 s-1 to 360 s-1). It was fou

sigmoidal curve increased with increasing strain rates.

new method, it sufficed to observe that the flame lo

to-one function of this parameter S. Therefore, one can 

region in terms of S, and direct observation of flame position beca

 presented in Figure 3-76 show flame

nd that, in general, the steepness of the

 For purposes of conducting experiments with the

cation, for each condition, was a well-correlated onemap the experimentally determined acceptable

m

region is 3.50 mm < S < 5.25 mm. All extinction results repor

acceptable region. 

Figure 3-76. Observed Flame 

Positions as a Function of 

Predicted Distance to the 

Stagnation Plane for Nonpremixed Flames of CH4-air (Zst = 

0.054), and CH4/N2-O2/N2 Flames, 

(Z = 0.50) Near Extinction. 

ose of other researchers. For undiluted methane burning in air, the measured global extinction strain 

rate was 296 s-1. When the reactant °C, the measured value for aq

increased to 360 s-1. Extinction m using the same oxidizer source were 

consistent to within ± 5 s-1. Variation i erent bottles of high p

compressed air, and was attributed to slightly different mass fractions of oxygen. All the results for 

doped flames were normalized to undoped experime ts using the same oxidizer source. 

fferent

boundary conditions and aspect ratio of the burner used by Puri and Seshadri. Agreement with literature 

e unnecessary. For Zst = 0.0544, this

ted below were for flames inside this

 st

Flame Extinction Results 

Initial tests were conducted with undoped reactant streams at ambient temperature to compare results with 

th

streams were preheated to 100 

easurements on undoped flames

n aq up to 80 s

-1 was observed with diff urity

n

The unheated global extinction strain value was comparable to the value found by Puri and Seshadri80 of 

280 s-1 and to the value of 255 s-1 computed from flows at extinction reported by Papas et al.63 using

Equation 3-7. All three numbers fell within the range of variation attributed to differences in composition 

of high purity air batches. This agreement was surprisingly good in light of the significantly di

values of local extinction strain rate was not as good; measurements and calculations ranged from 340 s-1

to 400 s-1 63,67,78 and from 354 s-1 to 544 s-1 67, 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 
Phosphorous 249

strictly valid. Comparable differences between global and local strain rates have been observed by other 

researchers.63

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 Figure 3-77. 

Figure 3-77. Inhibition of Nonpremixed Methane-air Flames,

Based on Reduction in the 

lobal Extinction Strain Rate by

Oxidizer-side Addition of 

itrogen, with and without 

Preheated Reactants. 

en malized by the extinction 

strain rate for a pure methane/air flame. The extinction strain rate dropped linearly with increasing 

loading of this additive. The results were independent of whether or not the reactants are preheated to

100 °C, consistent with the findings of Trees et al.284 The nitrogen dilution data can be compared to 

room-temperature data of Puri and Seshadri,80 also shown in Figur ent is good, with leastmean-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 Zst = 0.0544. This required some nitrogen

addition to the fuel flow. Here, nitrogen was added only to the oxidizer stream. Previous experiments by 

isher and co-workers

285 indicated that correcting for the effect of this fuel-side addition would lower the

present measured extinction strain rates by about 3 %, resulting in an overall discrepancy of 10 % with

hat at a volume fraction of 1500 μL/L, aq was reduced by 35 % of the undoped

value. Detectable inhibition (≈3 % reduction in aq) was found at loadings as low as 100 μL/L. For

olume fractions below 100 μL/L, preheating of react was not necessary to prevent condensation of 

DMMP. This allowed for am esting of low DMMP loadings which demonstrated 

inhibition effectiveness similar to that preheated experiments. Figure 3-78 also shows the 

results from TMP-doped flames. TMP exhibited similar inhibitory properties to DMMP over the 

common range of loadings. This was consistent with the hypothesis19 that it is phosphorus radicals 

G

N

Each data point represents the average of at least three measurem ts, and is nor

e 3-77. Agreem

F

the Puri and Seshadri data. 

Figure 3-78 shows the results for PCC addition to the air stream of methane-air flames. Again, points

represent averages of at least three measurements, and were normalized by the undoped extinction strain. 

For DMMP addition, the global extinction strain rate decreases linearly with additive mole fraction. 

Experiments indicated t

v ants 

bient temperature t

found in the 
250 Flame Suppression Chemistry

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.63,254

 A possible explanation is that the 

relatively low peak temperatures typical of highly strained methane vs. air flames67 were below the

2230 K transition between inhibition and promotion observed by Hastie and Bonnell.256

Figure 3-78. Inhibition of Nonpremixed Methane-air flames, 

Based on Reduction in the Global 

Extinction Strain Rate, by Oxidizerside Addition of trimethyl 

phosphate (TMP), and 

m lso 

s

v

or CF3Br.16,45,261

 The flame inhibition 

n a molar or mass basis, than those

, one of the most effective flame inhibitors

nhibition at 200 μL/L103 is comparable

s an inhibitor plateaus for mole

fractions between 200 μL/L and 500 μL/L, the highest gas phase concentration achievable with ambient 

temperature reactants.103 The PCCs’ inhibition performance can also be compared to the N2 data 

P and TMP are approximately 40 times more effective than N2 on a molar 

erved to inhibit

286,287

dimethyl

ethylphosphonate (DMMP). A

hown are reported literature

alues for inhibition by CF3Br. 

Also shown in Figure 3-78 are published flame inhibition data f

efficiencies of DMMP and TMP are two to four times better, o

reported for CF3Br. Comparisons of these PCCs with Fe(CO)5

known, show that Fe(CO)5 is a stronger inhibitor at low loadings: i

to the PCC’s effectiveness at 1500 μL/L. However, Fe(CO)5’ effect as 

presented in Figure 3-77. DMM

basis, ≈10 times as effective on a 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.21 Extending the experimental range to much lower strains 

would require a different method of introducing the PCC. 

Fisher and co-workers also performed an experiment to determine whether the hydrocarbon content of the

PCCs contributed significantly to the inhibition effectiveness. In certain chemical systems, such as 

carbon monoxide-air combustion, trace amounts of hydrocarbons had been obs

combustion. It is thought that the inhibition occurred because the hydrocarbon acted as a radical

sink. Hastie et al.140,256

 attributed 8 % of TMP’s inhibition effectiveness in hydrogen flames to this effect. 

In order to determine whether radical scavenging 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 
Phosphorous 251

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 μL/L, indicating that the effect of the hydrocarbon was to

promote the flame, not to inhibit it. This result provided further support for the hypothesis that the 

phosphorus atom was crucial to the flame inhibition effects measured in this study. 

Effects of Stoichiometric Mixture Fraction 

Experiments

Fisher and co-workers120 investigated how the effectiveness of the two additives changed as a function of 

ixture fraction, Zst. DMMP is mainly

ion strain rate when the additive was 

7 For low loadings, the effectiveness

should vary linearly with dopant loading. Tad and Zst were chang he

fuel and oxidizer streams. 

For tests of chemically suppressed flames, a constant loading of 50

the reactant streams. Argon additive testing was conducted with a loading of 25000 μL/L. The disparity 

between DMMP and argon concentrations was required because ect of

DMMP. Experiments were performed for stoichiometric mixtur 055 to 0.7. 

Adiabatic flame temperatures for the various values of Zst and overall dilution of the flames were 

evaluated for the undoped case using the STANJAN code.9

To accurately estimate the quantity of dopant at the flame surf e, numerical calculations using the 

OPPDIF code211

 were made with plug flow velocity boundary conditions, multicomponent diffusion but

no thermal diffusion, and detailed chemistry (excluding phos horus compounds), using the GRI 

s

Two types of experiments were performed. The first varied Z from 0.055 to 0.7. At each Z , a was 

the adiabatic flame temperature, Tad, and the stoichiometric m

chemically active; argon is inert. 

Suppressant effectiveness was defined as the change in extinct

introduced, normalized by the undoped extinction strain rate, aqo. 9,288

ed by varying the dilution level of t

0 μL/L of DMMP was added to one of 

of the stronger suppression eff

e fractions ranging from 0.

ac

p

mechanism289 with nitrogen chemistry omitted. The mole fraction of dopant was evaluated at the

maximum temperature in the reaction zone, taken to represent the flame location. The estimated 

diffusivity of DMMP was based on the interaction potentials of similar molecular weight compounds. 

The numerical calculations were required because thi diffusivity was significantly different from the rest 

of the reactant stream components, making the effect of preferential diffusion important.261

In these transport calculations for estimating relevant dopant loadings, the chemically reactive compound 

DMMP was treated as inert because complete chemical kinetic and transport data were not available for it 

or its combustion products. This treatment gave a reasonable estimate of the quantity of phosphorus

present in all chemical forms. 

st st qo

matched to 350 s-1 ± 10 s-1, by trial-and-error variation of the dilution of both reactants with nitrogen. 

This process entailed changing the overall dilution, a parameter which is independent of Zst. Due to the

high sensitivity of aqo to reactant composition, this narrow range in allowed aqo resulted in an even 

narrower range of allowed reactant compositions at a given Zst. Experimentally determining those 

allowed compositions accurately became difficult at high Zst, where the low methane flows resulted in a 

higher percentage uncertainty in their values. 
252 Flame Suppression Chemistry

As Zst was increased with fixed aqo, Tad declined steadily, as seen in Figure 3-79. Calculations by Grudno 

et al.280

 for a fixed Tad, were qualitatively consistent with this result, finding that aqo increased

monotonically as Zst was increased from 0.055 to 0.4. This variation in aqo and Tad with Zst was attributed

to the changing relationship between the strain rate and the appropriate time scale for diffusion of 

reactants. While the strain rate is always considered to be a good measure of that time scale, the

relationship between the two depends on Zst.

168,280,290,291

 The error bars in Figure 3-79 represent standard

deviations of the measurements. The large error at high Zst represent the methane flow uncertainty

described above. 

Figure 3-79. Adiabatic Flame 

Temperature vs. Stoichiometric 

Mixture Fraction for a Methaneme

e

ert dopant’s effectiveness would depend solely on the quantity of dopant 

present at the flam ious experiments had demonstrated that, for a range of loadings greater than 

ert suppressant nitrogen increased linearly with dopant

loading, at fixed Zst.

79,80,280

 Because flame temperature is reduced by the addition of dopant, this implies 

mperature invariance of suppression effectiveness over that loading range. 

es as the flame moves towards the fuel side

(increasing Z ). The converse is true with fuel-side doping. Argon has a diffusivity similar to that of the 

nitrogen-oxygen-nitrogen Fla

with the Extinction Strain Rat

Held Constant at 350 ±10 s-1. 

It was expected that the in

e. Prev

that considered here, the effectiveness of the in

te

The second type of test varied only the adiabatic flame temperature at a fixed Zst. This was accomplished 

in two ways: changing the overall dilution of the flame at a fixed Zst, and manipulating the temperature of

the reactant streams. These experiments were intended to isolate the temperature dependence of the

suppression from other effects. 

Results of Tests with Argon 

Results from the first series of tests are shown in Figure 3-80. The effectiveness of the argon, both

oxidizer and fuel side addition, is illustrated. Each data point represents the average of four extinction 

measurements. The error bars on the figures represent the standard deviation of the observed values,

which in this case is larger than the systematic error due to the method of approaching extinction. As 

expected, effectiveness with oxidant-side doping decreas

st

reactants; thus, the quantity present at the stoichiometric contour, which determines its effectiveness, will

vary linearly with Zst. 
Phosphorous 253

Figure 3-80. Flame Suppression 

Effectiveness of 25,000 μL/L 

Argon as an Oxidant-side or 

Fuel-side Additive vs. 

Stoichiometric Mixture Fraction. 

anufacturer’s stated repeatability of the mass flow controller. The 

ad

al uncertainty, between the effectiveness of argon with oxidizer- or fuel-side 

ad. These results

vs.

Stoichiometric Mixture Fraction 

Figure 3-81 depicts the argon effectiveness results. The uncertainty in the calculated mole fraction of

dopant at the flame was dominated by the uncertainty in the experimental flow of the dopant, which was 

± 10 %, as determined by the m

smooth trends in the data indicated a significantly better repeatability. The effectiveness curves show no 

trend with Zst, indicating that the concomitant changes in Tad and flame structure, collectively, had no

significant influence on the argon effectiveness. Other tests, described below, further demonstrated that 

the argon effectiveness was independent of T . In addition, Figure 3-81 shows no significant difference, 

in view of the experiment

addition, except perhaps at the lowest value of Zst. 

To further clarify the influence of temperature on this system, a series of tests were performed for a fixed 

Zst = 0.055. Tad was manipulated by varying the reactant stream temperature from 22 °C to 112 °C 

(±1 °C). This resulted in a range of Tad from 2237 K to 2283 K. There was a significant influence on the 

overall flame strength, as seen in the variation of aqo from 254 s

-1 to 375 s-1. However, the effectiveness

for argon at different temperatures varied less than 4 % and showed no trend with T

confirmed that argon’s effectiveness displayed no dependence on temperature, as well as none on Zst. 

Figure 3-81. Normalized Flame 

Suppression Effectiveness of 

Argon-doped Flames

at Fixed aqo. 
254 Flame Suppression Chemistry

Results of Tests with DMMP 

Figure 3-82 shows the results of extinction measurements performed with DMMP for Zst between 0.055

and 0.7. The observed effectiveness variation with Zst was somewhat more complicated than for the inert

tests, although it still followed the same general trend. Each data point in Figure 3-82 represents the 

the high observed effectiveness. The result was an estimated overprediction between 3 % and 15 % of 

DMMP effectiveness for this range of conditions due to this systematic uncertainty. The effect was small

Mixture Fraction. 

- and fuel-side addition are

offset significantly with respect to one another. This increased oxidizer-side effectiveness has also been

ence in these curves. It is possible that since the chemically active 

suppressant operated by interfering with the radical chemistry rather than as a heat sink within the flame,

erature contour may not have been the most relevant location for evaluating

average of 8 extinction measurements, with error bars representing the standard deviation of the measured 

values. Differences between individual data points at a given Zst showed that tests repeated at identical

conditions yielded slightly different results. This variation was limited largely by the repeatability of the 

dopant injection system. The dopant effectiveness may also have been influenced by small changes in

flame temperature resulting from lack of repeatability of the reactant stream compositions. The resulting 

overall experimental scatter was significantly higher than in the inert tests. The systematic uncertainty

due to the method of approaching extinction became significant for the oxidizer-side addition tests due to 

for the fuel-side addition data. 

Figure 3-82. Flame Suppression

Effectiveness of 500 μL/L DMMP

as an Oxidant-side or Fuel-side

Additive vs. Stoichiometric 

The normalized effectiveness results for DMMP, seen in Figure 3-83, have some notable features.

Firstly, for a given mole fraction, the DMMP effectiveness is roughly 100 times that of argon, indicating 

strong chemical suppression by DMMP. Secondly, the curves for oxidizer

observed for the chemically active agent, CF3Br.261

 The oxidizer-side curve in Figure 3-83 displays an 

effectiveness roughly triple that of the fuel-side curve. The slight systematic overprediction of the

oxidizer-side addition effectiveness, described above, would reduce the magnitude of this disparity, but

would not reconcile the differ

that the maximum flame temp

the mole fraction of dopant. Rather, the appropriate normalization depended on the quantities present in

the regions of the flame where the most important radical chemistry occurred. Since key radical reaction

rates peaked on the oxidizer side of the flame, a more appropriate normalization might have involved

mole fractions at some location to the oxidizer side of the maximum temperature contour. Such an

assessment was performed using mole fractions at the point of the maximum reaction rate for the chain

branching reaction O2 + H Æ OH + O. Because the concentration gradients of oxidizer- and fuel-side 
Phosphorous 255

e

closer together. However, oxidant-side doping remained more effe

uppression Effectiveness of

MMP-doped Flames vs. 

toichiometric Mixture Fraction 

t Fixed aqo. 

he disparity between oxidizer- and fuel-side addition may also be associated with the types and 

uantities of radicals and products that are formed from the parent molecule as it approaches the flame 

om different directions and their relevance to the suppression action.45 Assuming that the important

e, then additives to the fuel stream must

of DMMP should decrease with increasing flame

temperature. Figure 3-83 is consistent with this expectation, showing decreasing suppressant

below 80 °C, control of the reactant temperatures to vary Tad ove

Instead, the overall dilution of the flame, at a fixed value of Zst

experiment, the mass fraction of oxygen in the oxidizer stream rang

ranging from 2132 K to 2261 K and an aqo range of 294 s-1 to 572 s-1. Unfortunately, the variation in 

reactant stream compositions increased the susceptibility of the results to the sensitivity to random 

uncertainties in relative flows, producing considerable scatter. this experiment are 

shown in Figure 3-84, along with the data for oxidizer-side addition at fixed aqo, but varying Zst values; all 

are plotted against Tad. Although the large experimental scatter makes it difficult to draw quantitative 

conclusions, there is a clear trend of decreasing effectiveness with increasing Tad, comparable to that seen

oxidizer side brought the two curves

ctive.

Figure 3-83. Normalized Flame 

dopants had opposite directions, this shift in location toward th

S

D

S

a

T

q

fr

suppression action takes place on the oxidizer side of the flam

travel through the flame to reach this point. This may have a significant effect on what phosphoruscontaining species are present in the region of important radical chemistry for different modes of addition, 

i.e., oxidizer- or fuel-side doping. 

A third feature of Figure 3-83 is that for both the oxidizer- and fuel-side tests, the normalized

effectiveness’ increases with Zst. The existence of this slope implies that the suppressant effectiveness 

varied with either temperature, structure of the flame, or both. Studies in a variety of premixed flames 

had indicated that at high temperatures (above 2350 K),254,256

 DMMP can become a flame promoter.

Thus, presumably, the suppression effectiveness

effectiveness with increasing adiabatic flame temperature (lower Zst values). However, it is possible that

changes in flame structure, which occur at different Zst values, may also have influenced suppressant

effectiveness.

Further tests were conducted to clarify this temperature dependence. Due to condensation of the dopant 

r a significant range was not possible. 

= 0.5, was used to vary Tad. In this

ed from 0.39 to 0.44, resulting in a Tad

The results from
256 Flame Suppression Chemistry

in the constant aqo data. However, these results did not conclusively eliminate the possibility of fla

structure and the detailed chemistry of dopant action i

me

nfluencing dopant effectiveness. 

Figure 3-84. Normalized 

effectiveness of DMMP-doped 

Flames as a Function of 

Adiabatic Flame Temperature. 

The symbol ∆ represents 

changing Zst from 0.1 - 0.7, fixed 

-1

The temperatures of laboratory flames and the flames from real fires can differ

used above to quantify the temperature

he inhibition mechanism. The 

tration ([OH]) profiles, as measured 

using LIF. Only a limited number of measurements of [OH] in the counterflow configuration have been

reported.271,292-294 A few studies have utilized LIF to observe ch

halogenated suppressants 156, 51,61

work with flames doped with ph

limited.294,295

Here, a series of four non-premixed, atmospheric-pressure flame r mixtures were 

studied (Table 3-13). 

Table 3-13. Flame Conditions. 

aqo = 350 ±10 s , while 

represents a fixed Zst = 0.5, 

changing aqo from 294 s-1 to 572 

s-1 by varying mass fraction of 

oxygen. Error bars represent 

standard deviation of data for 

each set of data runs. 

Effect of Flame Temperature 

Experiments and Calculations 

 significantly. Fisher and 

co-workers119 conducted experiments in the same burner 

dependence for flame inhibition by DMMP and to gain further insight into t

indicator of inhibition was the degree of decrease in the OH concen

anges in [OH] in flames doped with

osphorus-containing agents is far more 

s of methane-O2/N2/A

Flame 

# Oxidizer Composition 

Tadiabatic 

[K] 

Tcalculated peak 

[K]

DMMP

[μL/L] 

VO

[cm/s] 

VF

[cm/s] 

1 21 % O2 – 79 % N2 2261 1967 0 to 572 71 96 

2 21 % O2 – 53 % N2 – 26 % Ar 2353 2080 0 to 715 71 101 

3 21 % O2 – 26 % N2 – 53 % Ar 2450 2189 0 to 715 71 106 

4 21 % O2 – 79 % Ar 2547 2286 0 to 715 71 110 

Flame temperature was varied by changing the proportions of N2 and Ar in the oxidizer side diluent,

while maintaining 21 % (by volume) O2 and thus a constant stoichiometric mixture fraction of 0.055. The 
Phosphorous 257

experimental results were compared to predictions from numerical calculations using a kinetic 

mechanism294 for DMMP decomposition and phosphorus-containing radical chemistry. The key reactions 

in the mechanism responsible for the observed inhibition of OH were identified. 

Calculations of the undoped flames were made using the OPPDIF code version 8.5 from the CHEMKIN 

III Suite.296

 The full GRI-MECH 3.0 mechanism, including nitrogen chemistry, (53 species,

325 reactions)206

 was employed. Mixture-averaged diffusion velocities were used, and thermal diffusion

was neglected. Opposed-jet burners are typically modeled as having either potential flow (linear axial 

velocity gradient) or plug flow (zero axial velocity gradient) boundary conditions, although measurements 

indicate that the actual velocity field lies somewhere between the two approximations.67 Calculations 

boundary conditions of potential flow

ged velocities for the fuel and oxidizer

were performed for several different boundary conditions. The

with centerline axial velocities of twice the measured volume avera

most closely agreed with the 

s appropriate because the flow in the

ose to Poiseuille flow, in which the 

mperature profiles predicted by 1D calculations have 

under a variety of conditions using spontaneous Raman scattering ,

thin SiC filament pyrometry in CH4/O2/N2 flames,298 and CARS in

Calculations with Phosphorus Kinetics 

To investigate the cause of the OH concentration changes observed in the LIF experiments, a set of flame 

calculations was performed using a proposed mechanism for DMMP decomposition and phosphorusradical chemistry.

294

 This mechanism was based on (1) the kin sis of the

influence of PH3 on radical recombination rates in the combustion products of a hydrogen flame

258,259

 and

(2) on kinetic models used for describing DMMP and TMP de ruction in a low pressure hydrogen 

flame.300,301

 Reactions of phosphorus-containing species with radicals and intermediate species of 

ydrocarbon combustion were included.294 Reactions were added to the scheme to complete destruction 

ersion of the mechanism had also been used to predict the effect of

DMMP on premixed flame speeds.257

streams were selected for use in this study, because the resulting OH profiles 

observed profiles. The factor of two for the centerline velocities wa

straight-tube reactant nozzles (L/D > 30) was expected to be cl

centerline velocity is twice the volume average. Te

been found by other researchers to be in good agreement with measurements made in opposed-jet burners 

in CH4 /O2/N2

297 and H2/air292 flames

propane/air flames.299

etic data used for the analy

st

h

pathways for some species. The used version of this phosphorus mechanism included 27 species and

166 reactions. Thermochemical data compiled by Babushok302

 for phosphorus-containing species were 

mostly taken from References 197, 303, 304, and 304 {Gurvich & Glushkov 1998 #2420}. Methane-O2

chemistry was modeled using GRI Mech 3.0 with nitrogen chemistry removed (36 species, 219 reactions 

retained). The phosphorus mechanism predicted reductions in extinction stain rates for non-premixed

methane-air flames due to the presence of DMMP that were in agreement with experimental results from 

MacDonald et al.79 An earlier v

Calculations were performed for all four flames using the combined mechanisms (phosphorus and GRI 

Mech), both with and without the addition of 572 μL/L of DMMP. Calculations with the combined

mechanisms required a higher degree of mesh refinement (both GRAD and CURV parameters for 

OPPDIF script = 0.1, yielding 250-300 grid points) than the GRI Mech calculations in order to produce

[OH] profiles for the doped flames that were independent of grid size. Figure 3-85 illustrates the major 

species profiles from these calculations for flames 1 and 4 without dopant addition. 
258 Flame Suppression Chemistry

r Profiles for Flame 1 (left) and 

Flame 4 (right), both undop

Measurements 

Figure 3-86 shows a comparison between measured and calculated d 4, the

coolest and hottest conditions respectively. Multiple profiles, all undoped, are shown to illustrate the 

repeatability of these measurements. 

The agreement between measured and calculated profiles was qu

flames 2 and 3 were in similar agreement between measured and c k

[OH] was expected since calculated profiles were used to he

measurements for each flame. There was also good agreement ot 

affected by the matching procedure, including the degree of asym . The peak

concentration, as determined by the calculations, increased by 66 % from 0.041 mol/m3

 for flame 1 to 

0.069 mol/m3

 for flame 4, which had a stoichiometric adiabatic flame temperature that was 286 K hotter 

atures (Table 3-13), for these moderately strained flames were

e stoichiometric adiabatic flame temperatures, although the difference 

s for flames 1 and 4 (319 K) were comparable to the change in the

computations; filled symbols - three 

measured profiles. Error bars 

measured values. 

Figure 3-85. Calculated Major Species and Temperatu e

ed.

 [OH] profiles for flames 1 an

ite good. The intermediate cases of

omputed profiles. Agreement in pea

estimate peak concentrations in t

in the profile shape, which was n

metry, and width (FWHM)

than flame 1. The peak temper

approximately 300 K cooler than th

in calculated peak temperature

stoichiometric adiabatic flame temperature (286 K). 

Figure 3-86. Calculated and Measured 

OH Concentration Profiles for Flames 1 

and 4. Flame 1: dashed line – 

computations; open symbols - four 

measured profiles. Flame 4: solid line - 

represent statistical uncertainty in the 
Phosphorous 259

Figure 3-87 illustrates the variation le widths (FWHM) with adiabatic

fl perature. All flame conditions are u B la meas OH profile

widt are observ roximately %, from mm to 0.55mm, across the range of 

temp atures invest ement betw lculated measu le w s is observed 

for a undoped

Figure 3-87. [OH] Profile Widths 

(FWHM). Open circles: 

d circles: potential

flow B.C. with velocities of twice 

v w s motivated by

made using Laser-Doppler Velocimetry in a similar burner 

 higher local strain rate (axial velocity 

in computed and measured OH profi

ame tem

hs

ndoped.

40

oth the calcu

0.4

ted and ured

ed to increase by app

er igated. Good agre een ca and red profi idth

ll flames. 

measured values for all

undoped flames. Lines with 

filled symbols: computational 

values for three different 

velocity boundary conditions 

(B.C.). Fille

the volume average (Poiseuille 

flow). Filled triangles: plug flow

B.C. with velocities equal to the 

volume average. Filled 

diamonds: potential flow B.C. 

with local strain rate equal to 

1.60 times the global strain rate 

given by Equation 3-7. 

The velocity boundary conditions in the calculations had a noticeable effect on the calculated width of the 

[OH] profile. In order to investigate the influence of these boundary conditions, two additional sets of 

calculations were performed, one for plug flow boundary conditions with the measured volume averaged

velocities and the other for potential flow while matching the maximum local strain rate to 480 s-1, or 

1.60 times the global strain rate (Equation 3-1) (which required increasing the centerline velocity for the 

calculations to 2.8 times the volume averaged alue). The higher strain case a

measurements of axial velocity profiles

configuration.

281

 These measurements indicated a substantially

gradient on the oxidizer side, evaluated just upstream of the heat release region) of approximately

1.60 times the global strain rate. The original calculations, with boundary conditions of potential flow

and velocities of twice the volume average, gave a computed velocity profile with a peak local strain rate

of approximately 1.18 times the global strain rate or 355 s-1. The calculated [OH] profile widths for the 

two additional sets of boundary conditions are also shown in Figure 3-87. Both additional sets of 

boundary conditions predicted similar variation in profile width with flame temperature. However, the 

plug flow boundary condition overestimated the profile widths by 10 % to 15 %, and the higher strain rate 

case underestimated the widths by 5 % to 8 %. Finite spatial resolution in the measurements (100 μm)

limited the accuracy with which the experimental width could be determined. Heat and radical losses to 

the burner as well as radiation, which were ignored in these calculations, also contributed to the

discrepancy between measured and calculated profiles. However, these physical loss mechanisms were

significant in these well isolated, non-sooting flames. Uncertainties associated with the chemical
260 Flame Suppression Chemistry

mechanism used influenced the computed profile width and also affected the level of agreement between

computed and measured profile widths.

boundary conditions 

to correct the LIF data changed the observed reductions in total [OH] by less than 1 %, well within the

uncertainty of the measurements. 

Figure 3-88 illustrates the effect of 572 μL/L of DMMP on OH concentrations in flame 1. With this 

loading, flame 1 is close to extinction. 

Figure 3-88. Effect of 572 μL/L 

of DMMP on [OH] in Flame 1. 

Filled symbols: undoped 

profiles. Open symbols: doped 

profile. Thick line: 

computational prediction for the 

undoped profile. Thin line: 

doped profile. Phosphorus 

kinetic mechanism from 

Wainner et al.294

A substantial reduction, approximately 20 %, in OH concentration was observed. The doped profile was 

smaller in magnitude, but the width was not significantly affected. In order to determine changes in the

total flame OH concentration, the profiles were numerically integrated, using a simple trapezoidal rule

approximation, across the width of the flame, and then the integrals from the doped and undoped flames 

were compared. The integration technique was particularly useful for quantifying changes in the higher

temperature flames, where the [OH] profile was less affected by the phosphorus-containing additive. 

There was a 23 % ± 3 % reduction in total OH (by this definition) for the data shown in Figure 3-88. The 

uncertainty resulted from the estimated statistical uncertainty of the individual data points from the 

profile, and was closely comparable to the repeatability of the measurement. 

OH concentration profiles were measured for all four flames with DMMP loadings up to 715 μL/L, with 

the exception of flame 1. A loading of 715 μL/L of DMMP in flame 1 caused extinction at the global

strain rate used in this study. Therefore, the maximum loading used for that flame was 572 μL/L, the next

The good agreement between the measured profiles and the calculations made with potential flow 

boundary conditions with centerline velocities of twice the volume average supported the selection of this

boundary condition for use in the quenching and Boltzmann fraction corrections to the LIF data. Because 

of the relatively small impact of the temperature and quenching correction, the choice of the appropriate

boundary conditions did not substantially influence the conclusions below about the influence of

phosphorus additives on [OH]. Using profiles calculated with either of the alternate
Phosphorous 261

highest loading readily achieved experimentally. Figure 3-89 illustrates the ratio of doped to undoped

total [OH] for the four flames as a function of loading. Raw data for profiles for all conditions studied in

this work can be found in the appendix of Reference 305. Within experimental uncertainty, total [OH]

decreased linearly with DMMP addition over this range of loadings. Results from the calculations 

performed with the phosphorus kinetic mechanism, discussed below, for a range of DMMP loadings from 

0 μL/L to 720 μL/L, confirmed the linear variation in the [OH] reduction with loading. The linear trends 

methane-air flames which indicated a 

he global extinction strain rate,

ss with loading was observed with most

 data for each flame are also shown in 

he increase in inhibition effectiveness

duction 

total [OH] at the maximum loading of 715 μL/L, compared to 23 % reduction in flame 1 for only 

57

are linear regression fits to data. 

ccessful because the high extinction strain rates for 

velocities that were well into the turbulent flow 

e effect of DMMP on global extinction strain rate 

f

stoichiometric adiabatic flame temperatures.120 Effectiven

work was defined as the fractional reduction in global ext le fraction

of phosphorus in the flame: 

The subscript q indicates that the measurement was made at extinction conditions, and the subscript 0 

denotes the value for the undoped flame. Flame calculati te the mole fraction of 

were consistent with extinction measurements on non-premixed 

linear variation in DMMP effectiveness, defined in terms of reduction in t

for loadings up to 1500 μ L/L.79 Linear variation in effectivene

flame suppressants at low loadings. Linear regression fits to the

Figure 3-89. The most striking observation from Figure 3-89 is t

with decreasing flame temperature. The highest temperature case, flame 4, shows less than 5 % re

in

2 μL/L.

Figure 3-89. Effect of DMMP 

Loading for All Flames. Lines 

Flame 1: solid line and circles. 

Flame 2: dot-dash line and 

squares. Flame 3: dashed line 

and diamonds. Flame 4: dotted 

line and triangles. Error bars 

represent uncertainty in

measurements. 

Extinction measurements for the flames were not su

flames 2 through 4 required dramatically higher flow 

regime. However, an earlier study had investigated th

for a variety of different flame conditions with di ferent stoichiometric mixture fractions and

ess in terms of extinction strain rates for that 

inction strain rate divided by the mo

(3-10)

( )

P

q

χ

1

q

q

q a

a a

Effectiveness

,0

,0 − ≡

ons were used to estima
262 Flame Suppression Chemistry

phosphorus at the flame surface, χp, taken to be the maximum temperature contour. There was a similar 

trend of decreasing effectiveness with increasing temperature. The stoichiometric adiabatic flame

temperature in that study was varied in two ways. The first was by changing the stoichiometric mixture

fraction from 0.05 to 0.7 while maintaining a fixed undoped global extinction strain rate. The second was 

by decreasing the overall dilution of the flame while maintaining a stoichiometric mixture fraction of 0.5. 

The range in adiabatic flame temperatures from that work (2130 K to 2260 K) was substantially lower 

than that investigated here. Comparing the temperature dependence from the previous investigation to

that of the current work requires a measure of effectiveness, anal

study, in terms of [OH] reduction as the fractional reduction in total

1)

Again, the subscript 0 denotes the measured value for the undope e

with the combined GRI Mech and phosphorus mechanisms were

fraction of phosphorus (sum of mole fractions of all phosphorus-co e

surface, taken to be the maximum temperature contour. Because t

fixed for all flames, changes in χp were negligible. The use of thi

trends and was introduced solely to provide a definition for effecti in 

Reference 120 to facilitate a comparison to previous extinction stu is

essentially a measure of the slope of the linear trends indicated in F

Figure 3-90 shows a comparison of this effectiveness based on OH with the 

measurements from the previous study. The trends observed in the two data sets are in good qualitative 

ogous to the one used in the previous

 [OH] concentration: 

(3-1

d flame. The flame calculations mad

used to determine χp, the total mole

ntaining species) predicted at the flam

he stoichiometric mixture fraction was

s parameter did not affect the observed

veness that is analogous to that used

dies. This effectiveness definition

igure 3-89.

one based on the extinction 

(

agreement, given that different measures of effectiveness were used. When comparing the two data sets,

it should also be noted that for a given stoichiometric adiabatic flame temperature, measurements 

performed at extinction conditions, such as those from Reference 120, had a peak flame temperature that 

was cooler than that seen in the LIF measurements, which were performed at a fixed global strain rate of

300 s-1, below the value required to extinguish the flame. The effectiveness data in Figure 3-90 show a 

substantial decrease in DMMP effectiveness with increasing temperature. The unambiguous trends in the

LIF data confirmed the hypothesis of Hastie and Bonnell256

 of decreasing inhibition with increasing flame 

temperature, with nearly a 90 % decrease in effectiveness for a 300 K increase in temperature.

Figure 3-90. Temperature 

Dependence of Effectiveness. 

Open circles: effectiveness data 

from Reference 9 defined in terms 

of reduction of global extinction 

strain. Filled squares: 

effectiveness data from the current 

work defined in terms of reduction 

in OH concentration. Error bars 

represent uncertainty in the 

measurements. 

)

P

H

χ

1

OH TotalOH

TotalOH TotalO Effectiveness

0

0 − ≡
Phosphorous 263

Calculations with Phosphorus Kinetics: 

The combined mechanism of phosphorus kinetics by Wainner et al.294

 and CH4-O2 kinetics from GRI 

was significantly greater than the reduction for OH, indicating a larger

effect of phosphorus chemistry on the concentrations of those species. 

Figure 3-91. Effect of 572 μL/L of 

DMMP on Calculated OH, H, and O 

o

(CH

undoped profiles and thin lines are 

wit

After performing the spatial integration of the computed profile across the flame, as described above, the 

change in total OH between the doped and undoped cases was evaluated. Figure 3-92 shows a plot of

effectiveness, as defined in terms [OH] reduction (3-11), as a function of the stoichiometric adiabatic

ot sufficient to explain the 

Mech206

 was used to calculate species and temperature profiles for all four flames with and without the

addition of 572 μL/L of DMMP. The predicted temperature profiles showed no significant effect from 

the addition of DMMP (< 10 K change in peak temperature). However, significant changes were

observed in OH, H and O profiles. Figure 3-91 illustrates the doped and undoped profiles of OH, H, and

O for flame 1. Relative to the undoped profile, the OH profile for the doped flame was lower in 

magnitude. The change was similar to that observed in the measured data, but not as large. A comparison 

of computational predictions to measured data is shown in Figure 3-88. The reduction in H and O atom

concentrations seen in Figure 3-91

C ncentration Profiles in Flame 1 

4/O2/N2). Dark lines are 

h DMMP addition. 

flame temperature for both the experiments and the computations. The computational and experimental 

results in Figure 3-92 showed the same trend of decreasing effectiveness with increasing temperature,

although the temperature dependence of the computational results was weaker. The magnitude of

effectiveness from the flame calculations agreed well with LIF data from the highest temperature case,

flame 4, but the calculations underpredict effectiveness for flames 1 through 3. In these calculations the

magnitude of the reduction in OH (i.e., total OHundoped - total OHdoped) was nearly constant for all flames.

For these flame calculations, changes in the effectiveness (which was a fractional measure of the

reduction of total OH) with temperature were due mainly to the increases in the total OHundoped, which was

larger by a factor of 2.25 for flame 4 (CH4/O2/Ar) compared to flame 1 (CH4/O2/N2) in the calculations. 

This estimate of the increase of total OH for the undoped flames was n

variation in effectiveness observed in the LIF measurements (reduced by a factor of 10 in flame 4

compared to flame 1). 
264 Flame Suppression Chemistry

Figure 3-92. Temperature 

Dependence of Effectiveness Defined 

as in Terms of Reduction in total 

[OH]. Filled circles with the dashed 

line: computational predictions. 

Filled squares: measured data. Error 

bars represent uncertainty in the 

measurements. 

 were determined by analysis of individual

ental and computational results both

MMP, it was expected that the reactions involving

m tion

rates for OH over all reactions involving phosphorus in the mech e 

[OH] profile, indicating that these reactions directly produced OH r

in [OH] upon introduction of phosphorus kinetic sub-mechanism 

an effect on rates of reactions not involving phosphorus-containi ct

recombination of OH by reactions involving suppressant species h -

based104

 and halogen-based55,57 suppressants. The net production

by all reactions involving phosphorus in flames 1 and 4 are shown

figures that the action of the phosphorus-containing species was s e regions of 

radical production/destruction in flame 4 were somewhat wider, in keeping with the wider flame (see 

Figure 3-87), but the peak magnitudes of the production rates for H, O, and OH were 5 % to 10 % lower. 

The processes responsible for the observed reductions in [OH]

reaction rates from the phosphorus mechanism. Since the experim

showed a reduction in [OH] with the addition of D

phosphorus removed OH from the flame. In fact, for all four fla es, the sum of the chemical produc

anism was positive across most of th

ather than destroyed it. Thus, the drop 

must have occurred indirectly, through

ng species. Poor efficiency for dire

as been observed previously for iron

rates for flame radicals OH, H, and O

in Figure 3-93. It is clear from these

imilar in both flames. Th

Figure 3-93. OH, H, and O Production Rates in Flame 1 (left) and Flame 4 (right), Summed 

over All Reactions Involving Phosphorus. 
Phosphorous 265

m (sum of OH 

production rates over reactions not involving P) due to the addition of 572 μL/L of DMMP to flame 1. 

eactions: 

O + H2 ↔ OH +H (R3-8) 

H + HO2 ↔ OH + OH. (R3-9) 

The third reaction that underwent significant changes due to add

OH + H2 ↔ H2O + H. 

This reaction has been identified by Twarowski258,259,303 as ed

recombination rates OH + H → H2O due to the presence of p

current non-premixed flames, the overall effect of R3-10 in the

clear, as the reaction had regions of increased OH production as well as regions of decreased OH 

production across the flame width. 

The cause of the observed reduction in OH was the fast removal of H and O atoms by the reactions

involving phosphorus. Smyth et al.306

 have studied the interrelationships of H, O, and OH concentrations 

in non-premixed methane-air flames and found that while these species were generally not in equilibrium,

their respective populations were still interdependent on one another through a variety of fast radical 

shuffle reactions. Because of this interdependence, the reductions in H and O atom levels created by the 

reactions involving phosphorus, observed in Figure 3-91 for flame 1, caused the radical shuffle reactions 

not involving phosphorus to respond by reducing OH. 

Figure 3-94 illustrates the shift in OH production rates by the GRI sub-mechanis

Figure 3-94. Total OH Production Rates 

in Flame 1 by Reactions Not Involving 

Phosphorus. Dark line: Undoped Rate. 

Thin line: Rate with the Addition of 

572 μL/L of DMMP. 

The OH production in the doped flame was generally lower than that in the undoped case, particularly on 

the oxidizer side of the flame. Three individual reactions that were primarily responsible for the shift 

seen in Figure 3-94 were identified by comparing the shift in doped and undoped reaction rates for all

reactions not involving phosphorus. Two of these reactions, R3-8 and R3-9, underwent a substantial

decrease in OH production for the doped flame. R3-8 is one of the important chain branching r

ition of phosphorus is R3-10: 

(R3-10)

playing an important role in increas

hosphorus containing compounds. In the 

presence of phosphorus addition was not
266 Flame Suppression Chemistry

Figure 3-95 shows the contribution of individual reactions involving phosphorus to OH production rates 

and compares them to the changes in the reaction rates of R3-8 R1 and R3-9 R2 (doped rate minus 

- P3

and changes (∆R3-8 and ∆ R3-9) 

in OH production (doped rate - 

undoped rate) for nonphosphorus containing 

reactions R3-8 and R3-9. The 

inated by R3-11 : 

. (R3-11) 

ed OH, but the rates were small (15 % to 

OH + PO2 + M ↔ HOPO2 + M (R3-12) 

H + HOPO2 ↔ H2O + PO2 (R3-15) 

H + HOPO ↔ H2 + PO2 (R3-16) 

undoped rate) that occurred when DMMP was added to the flame 1. 

Figure 3-95. Influence of the 

Addition of 572 μL/L of DMMP 

on OH Production Rates in 

Flame 1. Rates of OH 

production by reactions

involving phosphorus P1

net effect (∆NET) of these 

reactions is shown in bold. 

OH production by reactions involving phosphorus was dom

O + HOPO ↔ OH + PO2

There were two phosphorus containing reactions that consum

30 %) relative to 3-11: 

OH + HOPO ↔ H2O + PO2. (R3-13)

Figure 3-95 also shows the net change in OH production for all reactions in that figure (R3-11, R3-12,

R3-13, R3-8 and R3-9). The net rate curve shows overall destruction of OH, indicating that the shift 

towards decreased production by R3-8 and R3-9 was more than sufficient to balance the direct production 

of OH by R3-11. 

Removal of H and O atoms by reactions involving phosphorus clearly played an important role in the 

observed inhibition. Calculation results showed that removal of H atoms by reactions involving

phosphorus was done primarily by: 

H + PO2 + M ↔ HOPO + M (R3-14) 

and to a lesser extent by: 
Phosphorous 267

Figure 3-96 shows the contribution of R3-14 to the to

phosphorus. 

b

tal H destruction rate by all reactions involving

Figure 3-96. H and O Production Rates 

y

(s

Reactions (dashed lines) in Flame 1. 

 for 

com or

organop ntal 

data. 

hydro lecular

bea

flam

by the other. Each group 

proposed destruction mechanisms for DMMP that formed a basis for further mechanism development.

 All Reactions Involving Phosphorus 

olid lines) and by Individual 

Removal of O atoms by reactions involving phosphorus was done primarily by the OH-producing

reaction R3-11. The contribution of R3-11 to O removal was also shown in Figure 3-95. Significant 

uncertainties remained for the rate coefficients of several of the key reactions involving phosphorus 

species that have been identified here (R3-11, R3-12 and R3-14).302 Suggestions that the rate coefficients

for the recombination reactions R3-12 and R3-14 might need to be increased by a factor of 2 or more 302

could help reconcile the observed differences in magnitude of effectiveness between the measurements

and the computational predictions. 

3.4.3 Effects of Dimethyl Methylphosphonate on Premixed Methane Flames 

some respects: certain intermediates observed by each group were not seen

Introduction 

Fire suppression science would benefit from a reliable detailed chemical kinetic mechanism

bustion of organophosphorus compounds. Two mechanisms have recently been proposed f

hosphorus combustion294,307,307 but they have been validated against only limited experime

Most previous DMMP combustion experiments yielding species data have involved low pressure 

gen/ oxygen/argon flames. Korobeinichev et al. 254,308

 and Werner and Cool300 both used mo

m mass spectrometry to study the structure of premixed stoichiometric H2/O2/Ar flat flames doped 

with DMMP, between 50 kPa and 100 kPa. They were able to measure DMMP and various stable and

unstable products and intermediates. Temperature was measured by thermocouple. DMMP loading was

between 1000 μL/L and 11,000 μL/L. Both sets of investigators observed substantial increases in poste temperature (approximately 300 K) with the addition of DMMP. Species measurements differed in 
268 Flame Suppression Chemistry

Aside from species profiles, Korobeinichev et al. also inferred changes in adiabatic flame speed by

observing how maximum flame temperature varied with DMMP loading. They found that adding under 

d other organophosphorus compounds have been 

observed in non-premixed opposed-jet hydrocarbon flames, both through extinction measurements79,120

measuring the heat loss from the flame to a Bunsen burner.

as temperature increased,119,120

 and appeared to be greatest

 flame inhibition by DMMP: phosphorus-containing

atures participate in catalytic cycles resulting in

H and OH.258,260,294,303

mental data under flame conditions that

pressure. Only stable species not containing phosphorus we

flame was studied by comparing the undoped with doped p

Experimental 

Apparatus 

Experiments were conducted using a brass flat-flame burner resembling the one used by Korobeinichev et 

al.254,308-311,314,315 In the burner, the premixed reactants ent ed a plenum from the bottom of the burner,

flowed upward through a bed of 3 mm diameter glass beads, and then passed through an array of

hexagonally spaced 0.58 mm diameter holes with center-to-center spacing of 0.95 mm in the upper 

 burner. The use of a perforated plate, rather than a sintered metal plug, made it possible to 

lean phosphorus-containing products from the burner. The diameter of the flow region was 53.4 mm, and

r

relative to stationary probes. The ed on a motorized linear motion feedthrough,

was controlled by a programmable step This device allowed vertical movement with steps of 

0.008 mm over a range of 100 m returning to a position was about 0.08 m

vacuum chamber. 

5000 μL/L of DMMP increased the flame speed, and above this concentration, the opposite held in lowpressure H2/O2/Ar flames. More recently, Korobeinichev et al.309 performed experiments in a premixed 

CH4/O2/Ar flame at 10 kPa with a different organophosphorus compound, TMP. In contrast to their

experiments with TMP310,311

 and DMMP254,308 in hydrogen flames, these experiments showed an 

inhibitory effect on the flame. 

Flame inhibition and suppression effects of DMMP an

and through measurements of flame radical levels.119

 They have also been seen in premixed flames by 

312 Flame suppression effectiveness decreased 

under near stoichiometric conditions.312 There 

has been some agreement on the likely mechanism of

radicals that are produced from DMMP at flame temper

the recombination of standard flame radicals such as 

Nogueira and Fisher313

 tested these mechanisms against experi

were realistic for flame suppression: a CH4/O2/N2/Ar premixed flame doped with DMMP at near ambient 

re measured. The impact of phosphorus on the 

rofiles of the observed species.

er

surface of the

c

the thickness of the brass plate was 6.35 mm. Surrounding the plenum was an annular passage through 

which cooling water flowed. A centrifugal pump and resistance heater kept the cooling water, at 91°C,

flowing at a rate slightly above 0.3 mL/min through a closed circuit including the water channel of the 

burner, a rotameter, and an annulus surrounding the reactants gas flow line. 

Gas samples and temperature measurements at different heights were obtained by moving the burne

burner was mount which

ping motor. 

m. The repeatability of m. The 

burner and the traverse were sealed within a vertical 146 mm ID stainless steel cylinder that served as a 

Gaseous reactants (CH4, O2, N2, and Ar) were metered with calibrated mass flow controllers and meters. 

Upstream of the location where the nitrogen mixed with the other gases, liquid DMMP was introduced

via a syringe pump (model A with 25 ml Hamilton Syringe; Razel, Stanford, CT). Downstream of the
Phosphorous 269

injection point, a chamber with a residence time of 12 s smoothed out fluctuations in the DMMP loading.

The electrical heating and cooling water jacket maintained all reactant streams containing DMMP above 

85 °C, high enough to avoid DMMP condensation. 

manometer. For safety reasons, the chamber was maintained at a

dry ice/acetone (200 K) cold trap removed PCCs and water from th

icroprobe. The

probe was 200 mm long and constructed of 6.0 mm OD, 3.8 mm ID quartz tubing, tapering to an orifice. 

Repeatability tests involved probes of different diameters. The spe red

before each experiment, are listed in Table 3-14. Downstream of t ly

oriented section diverging with an angle of 11o. In the following 25 mm, the probe underwent a smooth 

90o bend to a horizontal orientation, allowing it to be mounted e of the

vacuum chamber. 

The gases flowed through the probe, lengths of perfluoroalkoxy tub

into a vacuum pump. The pressure downstream of the probe was k d

flow at the probe’s orifice. A 50 SFM bubble meter (SGE Internati as used to

measure the volumetric flow through the probe. The bubble meter was located downstream of the small

vacuum pump, where the flow pressure and temperature were clos g upon 

the gas temperature and orifice size, between 15 min and 45 min w th the

gas sample. 

Table 3-14. Probe Orifice Diameters (μm) Used in Specific Experiments. 

The vacuum chamber pressure was measured at the exhaust line downstream of the vacuum chamber by a 

capacitance pressure gauge (model PDR-C-1C; MKS, Andover, MA), calibrated using a “U” mercury 

pressure slightly below ambient and a

e exhaust stream. 

Sampling 

Sampling for species and temperature measurements made use of a stationary quartz m

cific probe orifice diameters, measu

he orifice was a 20 mm long vertical

in a sampling port in the sid

ing, a gas cell, and a needle valve, and

ept under 20 kPa to guarantee choke

onal PTY LTD, Australia) w

e to room conditions. Dependin

as needed to fill the FTIR cell wi

DMMP Fuel-rich Flames Near-stoichiometric Flames

Doped 61, 66 60, 65 

Undoped 56, 67 59, 64 

Fuel and product gases were FTIR spectrometer, operating

wavelength range from 500 cm-1 1 cm-1 and signal gain of 4. The

gases flowed continuously thro

n interactive subtraction technique that compared

ral regions and sensitivities.

Species Measurements 

measured with a Mattson RS-1 in the

 to 4000 cm-1with a resolution of sample

ugh an 884 mL multi-pass White cell (Infrared Analysis, Madison, WI) 

with gold-coated mirrors, zinc selenide windows, and a path length of approximately 5.4 m. The cell was 

maintained near 15 kPa during data acquisition, and was purged and conditioned with gas flow between

samples. The scanning time for the FTIR to obtain a sample spectrum was 7.2 min. 

Compound concentrations were quantified using a

sample spectra to calibration spectra with known species concentrations. Spectral regions were chosen to 

minimize overlap, but in some cases interfering species had to be subtracted from the spectrum before

quantification. Table 3-15 lists spect
270 Flame Suppression Chemistry

Table 3-15. FTIR Detection Limits and Wavenumber Ranges. 

Species 

Wavenumber Range Used for 

Quantification (cm-1) Detection limit (μL/L)a

CH4 3176–3133 400

CH2O 2920–2870 100b

CH3OH 1060–1010 10

CO 2220–2030 1350

CO2 715–670 1200

C H 3000–2950 70b 2 6

C2H4 960–905 60

C2H2 740–710 10

a

 Lower detection limit could be achieved with a different choice of spectr b

 Quantified after subtraction of CH4 and CH3OH. 

Radicals could not be measured with this technique be

al region for many species. 

c

through the sampling system. Although spectral features associated with DMMP and some of its 

phosphorus-containing products were observed, their leve ith

extractive FTIR of organic phosphates and phosphonates ed

that the bulk of the lower-vapor-pressure products were lost by deposition on the surfaces upstream of the 

FTIR cell. DMMP itself should have reached the gas cell, but at these low flows, transients associated

with wall adsorption and desorption resulted in a prohibitively long conditioning time requirement for 

reliable measurements. Reactions of DMMP and other species in the probe also posed problems in

obtaining quantitative species profiles. O2, N2, and H2 are not detectable via IR absorption. 

Probe effects 

on between calculated and experimental profiles, as the calculated

profiles were based on temperatures measured using a choked-probe technique that entailed the same

er surface,

e probe.319

een in the overrepresentation of products and under representation of methane in samples 

rner surface.

, the main distortion introduced by the probe was in the post-flame zone, where some conversion of

2

ause their lifetimes were too short to persist

ls were not repeatable. Previous research w

and their pyrolysis products263,316,317 suggest

The choking at the probe orifice was experimentally verified by measuring flows of post-flame gases 

through the probe while varying the back pressure in the sampling system. The flow did not vary as long 

as the back pressure was below 25 kPa, confirming that flow was choked under sampling conditions. 

Literature correlations318

 indicated that the location from which gases were sampled should be

approximately 3 orifice diameters, or 0.18 mm, upstream of the probe tip location. However, this was 

largely irrelevant to the comparis

location error, as described below. Within 10 orifice diameters, or 0.6 mm, of the burn

surface/probe interactions led to sampling of gas from both downstream and upstream of th

This effect was s

taken near the bu

Experiments with varying back pressure and with different probe designs were performed to assess the

extent of reactions inside the probe.320

 In these experiments, dependence of species concentrations on the

gas residence time in the hot region of the probe was seen as evidence for probe reactions. For undoped 

flames

CO to CO2 was seen. In the flame zone, C2H2 was the only species affected by a factor of two reduction in 

residence time: its concentration dropped by a factor of two. For the preheat and flame zones of doped 

flames, there was evidence of very significant probe reaction involving CH O. All other species changed
Phosphorous 271

concentrations by less than a factor of 1.5 when the probe residence time was lowered by a factor of two 

(flame zone) or three (preheat zone). Many species concentrations were unchanged, and only CH3OH 

changed by more than a factor of 1.3. Under the same conditions, CH2O concentrations dropped by

factors of 4 and 11, respectively, in the flame and preheat zones when residence time was reduced.

DMMP conversion to CH2O by probe reactions was significant in the pre-heat zone. Although CH2O 

measurements in this region were not representative of flame concentrations, they were used as an

indicator of the presence of DMMP. 

Temperature Measurements 

Temperatures in the flame zone were measured using the choked probe technique described by Kaiser et

al.319,321

 In this method, measured flows through the choked probe were compared to those obtained under

reference conditions with known upstream temperature. Approximating the flow as one-dimensional and

inviscid, the ratio of unknown

were used: (1) room conditions, and (2) 

to reference temperature was obtained. Two different reference conditions 

post-flame conditions, where the temperature was measured with

2 

Co. and used as is. Calibration curves to 

ons used in the current work. Four flames in all were studied: fueld undoped, and near-stoichiometric doped and undoped. Doping refers to the addition of a 

a platinum/platinum-rhodium thermocouple and corrected for radiative losses. Calculations based on the 

two different reference conditions yielded temperatures that differed by less than 4 %. Approximations to 

fluid properties were estimated to introduce errors between 1% and 2%, depending on the location in the 

flame.320

Materials 

The reactants, bought from MG industries, were: CH4 (99.99 %), O2 (99.994 %), N (99.998 %), and argon

(99.998 %). DMMP (97 %) was bought from Aldrich Chemical 

quantify the FTIR results used the following gases, bought from MG Industries: CO (99.5 %), CO2 

(99.99 %), C2H2 (99.6 %), C2H4 (99.9 %), and C2H6 (99.0 %). Certified methanol (99.9 %) was bought

from Fisher Scientific for methanol calibration. A solution of formaldehyde (37 %) with methyl alcohol 

(14 %) and water (47 %) was bought from Mallinckrodt Baker for formaldehyde calibration.

Operating Conditions 

Table 3-16 shows the operating conditi

rich doped an

small quantity of DMMP. As can be seen from the table, reactant flows were chosen to match or nearly 

match several parameters: the reactants’ cold flow velocity, the loading of DMMP for the doped cases,

the ratio of inerts (N2 and Ar) to O2, and the adiabatic flame temperature. In addition, the chamber was at

the same pressure and essentially the same inlet gas temperature for all measurements. Within these 

constraints, conditions were chosen by trial and error to provide a significantly lifted, yet non-cellular 

flame when DMMP was present. 
272 Flame Suppression Chemistry

Table 3-16. Operating Conditions. 

Fuel-rich Flames Near-stoichiometric Flames

Equivalence ratio 1.13 0.95 

Flows (L/min): CH4 1.61 1.37 

 O2 2.84 2.88 

 N2 12.48 10.80 

 Ar 0 1.94 

DMMP 9.18E-3 (doped) or 0 (undoped) 9.18E-3 (doped) or 0 (undoped)

Ratio of Ar/N2 to O2 4.39 4.42 

Cold flow velocity (cm/s) 12.6 12.6

Mole fraction DMMP in doped cases 

(L/L) 

540 540

Adiabatic flame temperature (K) 2093 2139

Pressure (kPa) 0.92 0.92 

Inlet gas temperature (K) 376 373 

Computational Method 

Premixed flame calculations were performed using the CHEMKIN suite of programs.296 The steady

laminar flame calculations used detailed chemical kinetics with mixture-averaged species diffusivities.

Thermal diffusion was neglected. Temperature profiles were specified as their measured values. 

Two chemical kinetic mechanisms for DMMP combustion were used, referred to in this work as the 

Babushok294 and Glaude307

 mechanisms. The mechanisms had in common the mechanism for destruction

mixed flame at 66 kPa,300 and the Babushok mechanism was validated with data from 

opposed flo 294

43 species erner and

Cool, but included ial reactions for DMM was va ev’s

low pressure prem me results.254,308,310,311

 Korobeinichev and coworkers314 had also proposed an

updated kinetic m using the mechanisms of wski and Werner and their starting

oint. This mechanism was not evaluated in the present work. 

mbustion mechanisms was combined with the GRI 3.0 methane combustion 

for small phosphorus compounds developed by Twarowski258,260,303

 but they differed somewhat in the 

paths for DMMP conversion into the small phosphorus-containing species. The Babushok mechanism had

32 species and 199 elementary reactions. Many Arrhenius parameters came from Werner and Cool’s 

work. In the Werner-Cool mechanism, there some were estimates by the authors, and some heats of 

formation and activation energies were evaluated by BAC-MP4 calculation. The Werner-Cool mechanism 

for DMMP destruction was validated with relative species profile measurements from a low-pressure

H2/O2/Ar pre

w propane/air flames at 85 kPa. Glaude’s mechanism for DMMP combustion had

and 537 elementary reactions. It too was based on the work of Twarowski, Melius, W

new init P destruction. It lidated against Korobeinich

ixed fla

echanism Twaro Cool as

p

Each of the DMMP co

mechanism,206,206

 from which nitrogen chemistry had been removed. Nogueira and Fisher320 describe a

series of calculations for which GRI 3.0 results were compared to those of a different methane

combustion mechanism.322

 GRI 3.0 was selected because it was more successful in reproducing the 

experimental results for undoped flames, especially for methanol profiles. 
Phosphorous 273

Results and Discussion 

Temperature profiles 

nce. Post-flame temperatures measured

by thermocouple are presented for comparison. In all cases, the lowest measured temperature was about

250 K higher tha interaction near 

the burner surface, resulting in sam downstream of the probe orifice. 

Both profiles had the same trend. Near the ace, doped and undoped tem tures were nearly

the same, wit d slightly higher than u dly than the

undoped one in the early part of the flame, creased more rapidly to achi approximately the

same post-flame value as the undoped case

The shift of DMMP-doped flames away fro ner surface, seen in Figure 3-97, was interpreted by 

considering the heat transfer interaction me and the burner sur s there was a

negligible change in cold velocity with dopi ange in position indicated that less heat transfer was 

required to stabilize the doped flame at a giv ty than was required for the corresponding undoped 

as lower than that of

n rate was slowed by the addition of DMMP. This conclusion 

as consistent with observations of flame suppression effects of organophosphorus compounds in nonthe results of

Korobeinichev et al. for TMP in a CH4/O2/Ar flame at 10 kPa.309

 The lack of significant effect of the 

l temperature, however, was different from Korobeinichev’s observations, which indicated 

that doped final temperatures exceed the corresponding undoped ones by about 200 K. Korobeinichev et 

0 K

254,300,310

Figure 3-97 shows the temperature profiles obtained with and without DMMP doping. Measurements 

included duplicate data points taken on different days to ascertain repeatability. Choked flow

measurements were processed using room conditions as a refere

n that of the reactant stream. This was likely a result of the probe/burner

pling from both upstream and

burner surf pera

h dope ndoped. The doped temperature increased less rapi

and then in eve

.

m the bur

between the fla face. A

ng, the ch

en veloci

flame. This difference implied that the adiabatic flame speed of the doped mixture w

the undoped one, or that the overall reactio

w

premixed methane flames.

79,119,120

 In the absence of a change in adiabatic flame temperature with doping,

the flame shift away from the burner should have lead to lower heat loss and thus a higher post-flame 

temperature in the doped case. Under these conditions, thermocouple measurements suggested that this 

was a small effect. Temperatures measured by the choked-orifice method were similar for doped and

undoped cases at the highest sampling location, but the undoped flame was actually slightly hotter.

However, the shape of the curves suggests that the final post-flame temperature had not been reached for

some profiles at that location, leading to uncertainty in the final temperatures. 

The shift away from the burner, or broadening of the reaction zone, was consistent with

additive on fina

al. attributed this higher temperature to the lower heat loss and more complete combustion in the doped

case. As observed by Korobeinichev,309

 organophosphorus additives had a distinctly different effect on

lower-pressure hydrogen flames, narrowing the reaction zone and raising the final temperature by 35

to 450 K. This change from promotion to inhibition behavior was predicted by low pressure flame

speed calculations performed by Korobeinichev and coworkers,309

 who offered a qualitative explanation 

for it based on the effect of radical termination reactions on the heat release rate. Korobeinichev and coworkers309 observed that the inhibitory behavior of organophosphorus compounds increased with

pressure, and was greater in methane than in hydrogen flames. The current experiments, performed at still

higher pressures, confirmed this trend. 
274 Flame Suppression Chemistry

Figure 3-98 through Figure 3-105 are profiles for the eight measured species. Each profile shows undoped 

experimental results (symbols), along with computational results (lines). In each figure, the 

dashed line is the undoped calculation using the GRI 3.0 mechanism, while the dotted and solid lines are 

Figure 3-97. Temperature Profiles Measured in (a) Rich and (b) Near-stoichiometric 

Flames. Filled symbols: doped flames. Open symbols: undoped flames. Squares: data 

from larger probe. Circles: data from smaller probe. Triangles were taken with 

thermocouples. 

Comparison of Doped and Undoped Profiles 

and doped

the doped calculations with Glaude and Babushok mechanisms, respectively. Each profile includes some 

data points acquired on a different day, with a different probe orifice diameter, as indicated in the figure

captions. 
Phosphorous 275

Figure 3-98. CH4 in Dope

GRI (undop

d and Undoped (a) Rich and (b) Near-stoichiometric Flames, with 

ed) and Babushok and Glaude (doped) Predictions. 

Figure 3-99. CO2 in Doped and Undoped (a) Rich and (b) Near-stoichiometric Flames, with 

GRI (undoped) and Babushok and Glaude (doped) Predictions. 
276 Flame Suppression Chemistry

Figure 3-100. CO in Doped and Undoped (a) Rich and (b) Near-stoichiometric Flames, 

with GRI (undoped) and Babushok and Glaude (doped) Predictions. 

Figure 3-101. CH2O in Doped and Undoped (a) Rich and (b) Near-stoichiometric Flames, 

with GRI (undoped) and Babushok and Glaude (doped) Predictions.
Phosphorous 277

Fi ,

Figure 3-103. C2H4 in Doped and Undoped (a) Rich and (b) Near-stoichiometric Flames, 

with GRI (undoped) and Babushok and Glaude (doped) Predictions. 

gure 3-102. C2H2 in Doped and Undoped (a) Rich and (b) Near-stoichiometric Flames

with GRI (undoped) and Babushok and Glaude (doped) Predictions. 
278 Flame Suppression Chemistry

Figure 3-105. CH3OH in Doped and Undoped (a) Rich and (b) Near-stoichiometric Flames, 

with GRI (undoped) and Babushok and Glaude (doped) Predictions. 

Figure 3-104. C2H6 in Doped and Undoped (a) Rich and (b) Near-stoichiometric Flames, 

with GRI (undoped) and Babushok and Glaude (doped) Predictions. 
Phosphorous 279

Doping the flame shifted all species profiles downstream, similar to the behavior of the temperature as 

described above. The magnitude of this shift was best evaluated from the monotonic CH4 or CO2 profiles,

as the axial spacing of sampling locations was too large to allow reliable comparisons of peak locations

for intermediate species. These profiles were fitted to a scaled Weibull cdf form to reduce the impact of

noise. Defining the shift as the change in position in the curve fit where half the initial methane was 

consumed, the experimental values were 0.33 and 0.49 mm for the rich and near-stoichiometric flames, 

respectively. Similarly, using the location where CO2 reached half its maximum value, the experimental 

shifts were 0.34 and 0.53 mm, respectively. Thus, DMMP consistently had a larger effect on flame 

location for the near-stoichiometric than for the rich flame under these conditions, and defining flame 

position in terms of fuel or product profiles yielded similar shifts. 

The sampling technique produced artifacts in some profiles, most notably CH2O. For this species, two 

peaks can be seen in each of the experimental doped profiles, while the undoped experimental profiles

and all the computational profiles show single peaks. The earlier doped peak occurs 0.25 mm from the 

burner for both equivalence ratios, and was likely due to probe reactions. The location of the first peak 

close to the burner surface suggested that the upstream peak came directly from DMMP destruction, but 

the low temperatures (700 K) and presumed low radical levels there implied that destruction of significant 

quantities of DMMP in the free stream was very y.

264

 The magnitude of this peak increased

trongly with probe residence time, as shown in the back pressure experiments described in the previous 

se n

Figure 3-101 e 

other data points. The points taken with the smaller probe orifice (and corresponding longer residence 

me in the probe with choked flow) had much higher CH2O levels than the points taken with the larger 

probe orifice. Although it was a sampling artifact, this upstream CH2O peak provided information about 

e DMMP profile. The drop in CH2O levels at 0.25 mm above the burner indicated that DMMP levels in 

the gas stream entering the probe began to drop at this location. 

For most intermediates, there was no clear change in peak height due to doping. The measured peak 

hrough Figure 3-105 also show how successful the two chemical kinetic mechanisms for

p

(The first ex onsidered a

sampling artifact.) Using the GRI-MECH predictions of undoped species profiles as a benchmark, the 

 unlikel

s

ction. The current data set showed further evidence of this dependence: The highest values of CH2O i

 were repeat data points measured using slightly smaller probe orifice diameters than th

ti

th

height changed with doping by about a factor of two for C2H2, about a factor of 1.5 for CO and for the 

second peak of CH2O, and by smaller amounts for C2H2 and C2H6. In each of these cases, a narrow peak

along with limited spatial resolution made it hard to determine whether the true height had been sampled. 

CH3OH levels, on the other hand, showed a dramatic change in peak height with doping, increasing by

factors of seven and four for the rich and near-stoichiometric cases, respectively. The broad peak shape

(see Figure 3-105 greatly reduced uncertainty due to limited spatial resolution of sampling. Accounting 

for probe reactions would have increased the difference between doped and undoped profiles, as can be 

seen from the doped data points taken with smaller probe orifice size (longer residence time). Werner and

Cool300

 did not measure methanol and formaldehyde because the energy needed for their ionization was 

above the capability of their apparatus. Korobeinichev and co-workers254,308,310 did not report profiles for

these species.

Comparison between Experimental and Computational Profiles 

Figure 3-98 t

hosphorus313 combustion (Babushok and Glaude) were at duplicating experimental data in doped flames.

perimental CH2O peak was excluded from the comparison below as it was c
280 Flame Suppression Chemistry

phosphorus mechanisms were as successful as the GRI mechanism. All mechanisms successfully predict 

the direction of change in peak height when the equivalence ratio changed from rich to nearstoichiometric for CH2O, CO, C2H2, and C2H4, and all mechanisms failed for C2H6. For CH2OH, GRI

failed while the Babushok and Glaude mechanisms succeeded. None of the mechanisms succeeded in 

predicting the ranking of peak heights in a particular doped or undoped flame. 

More quantitatively, in predicting peak heights, the Glaude mechanism for doped flames was roughly

equally successful to the GRI mechanism for undoped flames. Glaude’s fractional differences in peak

height between experiment and calculation were smaller than or about the same magnitude as the 

corresponding GRI values for all species except CH3OH. Babushok’s mechanism was less successful, 

producing larger fractional differences than GRI for all cases except C2H6 in the near-stoichiometric

flame and CO in both flames. In the case of over-predictions, these discrepancies must be viewed with 

some caution because true peaks may not be sampled experimentally. Another explanation for some

differences was probe reactions, which produced changes of 30 % or so in most species concentrations in 

the flame zone. The discrepancies between experiment and calculations with the Babushok mechanism 

clearly exceeded errors due to probe reactions (i.e., exceeded the observed sensitivity to probe residence 

time) only for C2H2, C2H4, and CH3OH. 

Methanol computational results showed large discrepancies with experiment for both phosphorus

combustion mechanisms. Qualitatively, the mechanisms succeeded in predicting that the doped CH3OH 

peak was higher than the undoped peak. However, they predicted the peak to be too narrow and located

too far downstream, and they substantially underpredicted its height. Babushok and Glaude mechanisms 

predicted that the CH3OH peak height increased, respectively, by factors of 1.4 to 1.8 and 2.2 to 3.2 over 

the undoped calculated values. The experimental increase was by factors of 4 (near-stoichiometric) and

7 (rich). This difference indicated an area in which improvements to both mechanisms are needed. The 

early location of the start of the doped CH3OH peaks suggested that CH3OH was an important 

intermediate in the early decomposition process for DMMP. 

Both phosphorus mechanisms agree with experiments in predicting that doping produced a downstream 

shift of species profiles. The magnitude of the shift was similar for the two mechanisms, an expected

result, as calculations with both mechanisms made use of the same specified experimental temperature

profiles. The two calculations predicted almost identical shifts in the rich case for all species. In the nearstoichiometric flame, the shift was somewhat larger for Babushok than for Glaude, especially for CH2O. 

With the measures of shift size based on the position where CH4 concentration was half of the initial

concentration as described in the previous section, Glaude and Babushok gave almost identical shifts for 

the rich case, while Babushok’s shift was 30 % larger in the near-stoichiometric flame. In comparison to

experimental shifts, calculations were about 20 % too large in the rich flame, and about 45 % too small in

the near-stoichiometric flame. Both mechanisms failed to predict that the shift in the near-stoichiometric 

flame was greater than the one in the rich flame. The magnitudes of the shifts were predicted slightly

better by Babushok than by Glaude. 

It is important to note that in the current work Glaude’s and Babushok’s phosphorus mechanisms were 

combined with the same methane combustion mechanism (GRI 3.0). It is possible that either mechanism 

would make more successful predictions when used with the methane combustion mechanism with which

it was developed. 
Phosphorous 281

3.4.4 Summary: Phosphorus-containing Compounds in Flames 

A novel method for performing repeatable extinction measurements was developed to maintain constant 

loadings of phosphorus-containing compounds (PCCs) in the oxidizer stream using syringe pump

delivery. This method requires that the locations of the flame and stagnation plane relative to the nozzles 

change during an extinction measurement. The global extinction strain rate was invariant (within ± 2 %)

over a broad range of flame locations for methane-air non-premixed flames. Findings using these flames 

and this technique included:

• Extinction measurement and OH reduction were equivalent markers of agent effectiveness. 

• Phosphorus-containing compounds can be 40 times more effective than nitrogen and 2 to

4 times more effective than CF3Br at quenching non-premixed hydrocarbon flames. 

• DMMP and TMP were roughly equivalent in suppression efficiency, independent of the 

differences in their ligands. 

• DMMP was more effective when added to the air side – as in fire suppression. 

• The effectiveness of DMMP has a strong temperature dependence, with lower effectiveness at

on fires. 

o A mixture of inert and phosphorus-based suppressants could interact synergistically

as the physical agent cools the flame, thus increasing the efficiency of the chemically

active component. 

3

shift was over 50 % greater for a near-stoichiometric flame than for a rich flame. 

• Flame calculations again were reasonably successful in predicting the effects of DMMP on the

higher adiabatic flame temperatures. 

o These very high temperatures were well above those in ordinary hydrocarb

• Current computational models adequately (but somewhat under-) predicted the effect of DMMP 

on flame extinguishment. This may have been due to the significant uncertainties in several of 

the reaction rate coefficients in the phosphorus mechanism.

o The direct role of the phosphorus-containing species in the flame was the fast 

removal of H and O atoms, leading indirectly to a decrease in OH. 

o Inhibition was due to the phosphorus-containing radicals, PO2, HOPO, and HOPO2, 

formed after the decomposition of the parent compound. 

o PCC flame inhibition does not depend on the form of the parent compound, provided 

that the parent breaks down in the flame. Thus, a broad class of potential PCC fire

suppressants should exist, from which an alternative to halons with minimal health 

and environmental impact may be selected. 

Additional experiments with methane-nitrogen-oxygen-argon premixed flames showed: 

• DMMP addition caused a downstream shift of temperature profiles and all species profiles except 

CH OH. This shift was a consequence of the flame inhibition properties. The magnitude of the 

flame. 
282 Flame Suppression Chemistry

3.5 COMPARATIVE FLAME INHIBITION MECHANISMS OF CANDIDATE 

MOIETIES 

3.5.1 Introduction 

A goal of NGP research was to develop a unified understanding of flame inhibition and extinguishment, 

so that basic principles could then be used to interpret data from a wide range of inhibitors, as well as a 

wide range of fire scales. The following sections present work to that end. Since the mechanisms of

chemically acting fire suppressants were widely believed to involve recombination of free radicals in the 

flame, radical concentrations measurements were made for flames with addition of various inhibitors,

including CF3Br. Since CF3Br was often used as the benchmark for comparison of new agents, an

analytical study was undertaken to determine if its mechanism is representative of chemically acting 

inhibitors in general, or a special case. Further work sought to correlate the effectiveness of agents with 

the bond strength of the active intermediates. Finally, an idealized inhibitor was used to investigate the

properties of the flames themselves which lead to the most effective inhibition. 

3.5.2 Spectroscopic Studies of Inhibited Opposed Flow Propane/Air Flames 

Introduction 

Skaggs and co-workers93 applied optical diagnostics and chemical modeling to a propane-air, opposed

flow, non-premixed flame to investigate the similarities and differences among the characteristics of

flames inhibited and extinguished by a diversity of additives: 

• N2 represented purely physical influence on flame inhibition since it has no chemical inhibition 

capability. 

• HFC-227ea (C3F7H) and HFC-236 (C3F6H2). Hydrofluorocarbons, although they typically 

increase the heat release rate in the flame, also remove H atoms from the reaction zone and 

reduce the overall reaction rate.323

• Fe(CO)5 is a highly efficient flame inhibitor. It is chemically active and representative of a class

of metal-containing compounds. 

• DMMP (CH3P(O)(OCH3)2) and PN (cyclo-P3N3F6) are also efficient chemically active inhibitors, 

representative of phosphorus-containing compounds. 

• Halon 1301 (CF3Br) is both an efficient halogenated fire suppressant and the reference agent for 

the NGP. 

Experimental 

The flame imaging experiments were conducted using the arrangement presented in Figure 3-106. All 

flames were at atmospheric pressure and consisted of 7.0 L/min synthetic air (79 % N2 + 21 % O2) 

flowing from the lower duct, and 5.6 L/min of propane flowing from the upper duct. The oxidizer and

fuel ducts were separated by a distance of 1.2 cm, and the duct diameters were each 2.54 cm. The

inhibitors were added to the oxidizer flow in gaseous form at room temperature with the exception of 

Fe(CO)5, which was cooled to 11°C and DMMP which was heated to 70°C. Based on the flow conditions
Comparative Flame Inhibition Mechanisms of Candidate Moieties 283

and duct separation, the luminous flame zone was located on the oxidizer side of the stagnation plane, and

the global strain rate was calculated to be 72.5 sec-1.

76 The burner apparatus was located inside a stainless

steel hood to contain any toxic fumes that were exhausted from the burner. 

PC

Excim

er/Dye

Laser

ICCD 

Camera

Cylindrical

Plano-Convex Lens

Turning 

312 nm

Filter

Prism

Iris

ATM-Pressure

Opposed Flow

Horizontal Burner

Slits

Delay Pulse

Generator

Electrical Signal

Optical Signal

Pulse Signal

Mobile Exhaust Hood

UV Sheet

Figure 3-106. Schematic Diagram of the Experimental Apparatus. 

revious studies of non-premixed propane/air flames had experimentally determined global extinction 

train rates of 489 s

-1.324

 A decrease in the extinction strain rate was used as an indicator of an inhibitor’s 

et was apertured just before the burner to produce a vertically uniform intensity that 

as 1.2 cm in height, allowing passage through the entire burner flow field. The OH fluorescence passed

Instruments spect to the

mera, which had an active area of 384 x 576 pixels, had a field of view with this 

ptical arrangement of approximately 33 cm2

 and each image recorded was acquired with 25 total 

P

s

efficiency. 

Planar laser induced fluorescence (PLIF) and laser induced fluorescence (LIF) were used to measure the

H atom, O atom, and OH radical species concentrations as flame extinction was approached. These 

measurements complemented the extinction strain rate by illustrating an inhibitor’s influence on the 

radical concentration profiles in the flame zone which indicated if the flame’s radical chemistry was being 

perturbed by agent addition. 

PLIF images were measured using a Lambda Physik excimer/dye laser system. This system consisted of a 

Lambda Physik Compex 102 XeCl excimer laser, a Scanmate 2 dye laser (Coumarin 153) and a Second 

Harmonic Generator (SHG). The fundamental output of the dye laser (560 nm wavelength) was frequency 

doubled in the SHG unit with a BBO crystal to approximately 281 nm. The UV laser radiation was tuned

to the peak of the R2(9.5) transition at 281.8 nm ((1,0) A2

Σ+←X2Π).158-160 The UV light output of the 

SHG unit entered an optical train where the beam is turned 90°, apertured by a sub mm iris, projected 

through a cylindrical plano-convex lens to form the UV beam into a vertical sheet. To create a uniform 

sheet width, the sheet was apertured with 0.5 mm vertical slits as it was projected toward the center of the 

burner. The UV she

w

through a band pass filter centered at 312 nm with a 11 nm bandwidth and was detected with a Princeton

ICCD camera (Model 120) coupled with a Nikon UV lens located at 90° with re

UV sheet. The ICCD ca

o
284 Flame Suppression Chemistry

accumulations on the camera. With this arrangement, the entire relative OH concentration profile was

obtained. 

Laser induced fluorescence excitation spectra were measured in the flame using the same excimer/dye 

laser system. This arrangement has been utilized before for similar measurements.157 The UV laser

radiation was scanned from 281.5 nm to 282 nm.

158-160

 Low laser energies were used, and the laser was

operated in the linear regime. The UV light output of the SHG unit was focused to the center of the burner

30 cm focal length fused silica lens and had a vertical and horizontal beam waist of 0.4 and 0.5 mm, 

respectively. Fluorescence was collected at 90o to the direction of the excitation laser beam, focused

through an 0.5 mm iris to define the collection volume, passed through a band pass filter centered at

312 nm with an 11 nm bandwidth, and detected by a photomultiplier tube (PMT) (Phillips Model

sponding OH Intensity Profiles 

% (by volume) CF3Br and 1.5 %

XP2018B). 

Before inhibitor addition, the uninhibited flame was profiled using LIF between the fuel and oxidizer

ducts to obtain a profile of the uninhibited temperature values. To expedite measurements upon addition

of an inhibitor, the burner was translated about ± 1 mm around the OH maximum, and excitation spectra 

were collected. Each excitation spectrum was fit using a non-linear least squares algorithm to obtain the

OH rotational temperature for the spectral measurement.160

Results 

Figure 3-107 presents two representative, two-dimensional images of OH fluorescence for an uninhibited 

propane-air flame and for a propane-air flame to which CF3Br was added (1.5 % by volume). 

5000

0 % CF Br 3

Figure 3-107. Representative PLIF Images and the Corre

from an Opposed Flow Propane-air Flame Seeded with 0

(by volume) CF3Br. The fuel and air ducts are at the top and bottom of each PLIF image. 

Both images, which were uncorrected for laser energy fluctuations and local quenching rates, illustrated

the presence of two luminous zones as the UV sheet passed through the flame. The lower, thicker zone 

1.5 % CF3Br

0 1 234 5 6 789 10 11 2500

3000

3500

4000

4500

5000

Intensity, A.U.

Distance From Fuel Duct, mm

4000

4500

0 1 234 5 6 789 10 11 2500

3000

3500

Intensity, A.U.

Distance From Fuel Duct, mm
Comparative Flame Inhibition Mechanisms of Candidate Moieties 285

was the fluorescence from the OH transition while the upper, thinner zone was the broadband

fluorescence due to derivative fuel species such as polycyclic aromatic hydrocarbons. To construct a

spatially resolved OH LIF profile from an OH PLIF image, as shown on the right hand side of 

Figure 3-107, the pixel intensity corresponding to a given height between the fuel and oxidizer ducts 

(spatial resolution approximately 0.149 mm/pixel) was summed and averaged over a 1 mm horizontal 

width. The two-dimensional images and LIF profiles illustrated that addition of CF3Br to the propane 

flame caused a decrease in the OH fluorescence signal while the broadband fluorescence increased just

slightly. Similar results had been observed previously for CF3Br addition to hydrocarbon diffusion 

flames.

51,156

Table 3-17 lists the measured inhibitor concentrations in the air stream at extinction and their estimated

uncertainties.

Table 3-17. Inhibitor Concentrations (% by volume) and Uncertainty (± % by volume) at 

Flame Extinction. 

Inhibitor 

N2 CF3Br HFC-236 HFC-227ea PN DMMP Fe(CO)5 

Extinction Concentration 23.1 2.3 6.1 5.3 2.7 0.3 0.2 

Estimated Uncertainty 8.20 0.93 1.29 1.08 1.00 0.04 0.03

• The extinction concentration for CF3Br was similar to cup burner values, 2.90 %325, but slightly

lower than values obtained in a co-flowing propane-air flame and a co-flowing propane-air cup

burner, 4.1 % and 4.3 %, respectively.

21,51

• HFC-227ea and HFC-236 had extinction concentrations that were above double that for CF3Br, 

consistent with cup burner values of 6.3 % and 6.6 %, respectively.

325

• For the phosphorus compounds, PN had an extinction concentration similar to CF3Br, while 

chemical structure, similar extinction concentrations would be 

ins the results of the analyzed OH profile areas vs. each agent’s concentration as the

flames were stepped towards extinction. The reported OH profile areas were averaged over three or more

DMMP’s value was seven to eight times lower than CF3Br. Previous studies by MacDonald et 

al.79,120

 had shown DMMP to be two to four times more effective than CF3Br. However, Fisher et

al.324

 reported that for an opposed flow propane-air flame with DMMP added to the air stream, a 

25 % decrease in the normalized extinction strain rate corresponded to a DMMP concentration ≈

1200 μL/L. Linear extrapolation of the cited Fisher et al. data324 to the strain rate used for the

opposed flow, propane-air flame studied here gave a DMMP concentration of 4080 μL/L to 6500 

μL/L or 0.4 % to 0.65 % by volume. The DMMP concentration obtained from the extrapolated

strain rate data supported the DMMP extinction concentration determined in this study. For PN, 

cup burner experiments had found an extinction concentration of 1.08 % by volume.

249 The

results reported here for PN and DMMP were of concern for several reasons. First, the obtained 

value for PN was higher, while the DMMP value was lower than other cited experiments. Second, 

it was assumed prior to these experiments, that if a given compound contained a phosphorus 

atom, then regardless of its

observed. A possible explanation for the contrasting behavior between the two phosphorus agents 

is that the resonant P-N ring structure of PN could be very stable and thus less efficient at

delivering phosphorus to the flame.326

Figure 3-108 conta
286 Flame Suppression Chemistry

separate inhibitor extinction experiments, where the data for each experiment were normalized to the 

[OH] profile area measured in the uninhibited flame acquired prior to each inhibitor extinction experiment 

f the other inhibitors showed precipitous decreases in the measured [OH]

valu u d for

DMMP and Fe(CO)

Figure -

Concentration

Fe(CO)5. Insert:

n zone. Previous 

stud

localize

modific ssian function.

The e

give l

profile.1

width of the flame’s reaction zone was defined here as the distance of one half of the 

maximum intensity of the Gaussian OH profile which was similar to previous studies170

 that estimated the

width of a laminar flame reaction zone using one half of the maximum value of a temperature profile. 

to account for changes in burner and camera conditions. The data indicated that there were both physical

and chemical modes of inhibition being observed for the agents studied. That is, N2 which is chemically

inert, had the least impact on [OH] with respect to the other agents studied. For the concentration range 

plotted in Figure 3-108, the flame was not even extinguished by N2. Similar results were observed for the

two fluorinated propanes which showed initially small declines in [OH], but more rapid decreases just

before extinction. Addition o

es p to the extinction concentrations where the data decreased more gradually, as highlighte

5 with the inserted graph in Figure 3-108. 

3 108. Normalized OH LIF Profile Areas vs. Inhibitor Agent Delivery

s. □: N2, О: HFC-227ea, ∆: HFC-236, : PN, ◊: CF3Br, ■: DMMP, and ●: 

Expanded data for PN, CF3Br, DMMP, and Fe(CO)5 Concentrations up to 

0.75 % by Volume. 

0 1 6

0 .0

0 .2

0 .4

0 .6

.8

1 .0

0

Normali

zed OH Profile Area

0 .8

0 .2

0 .4

0 .6

0 .0 0 0 .2 5 0 .5 0 0 .7 5

0 .0

1 .0

2 3 4 5

A g ent C o n c entra tion, % V o lu m e

The addition of an inhibitor can change the position and width of the flame’s reactio

ies had shown168-170,172,173 that a decrease in the flame’s reaction zone width indicated increased

d strain, which can cause local quenching or flame extinction.171

 For the analysis of reaction zone

ations and relative OH concentrations, each OH intensity profile was fit to a Gau

ar a under the Gaussian profile curve provided a general indicator of the entire OH population for a 

n f ame condition. The width of the flame’s reaction zone was characterized by the width of a radical

73

 The
Comparative Flame Inhibition Mechanisms of Candidate Moieties 287

Table 3-18 lists the measured flame widths determined from the relative OH concentration profil

situation studie

es for

each flame d. For the inhibited flames, the widths were measured at 50 % of each agent’s

determined extinction concentration. The uncertainty in the reported widths due to measurement variance

was 11 %. 

Table 3-18. Measured OH Profile Widths (FWHM, mm) for the Uninhibited Flame and 

Inhibited Flames at 50 % of the Inhibitor Extinction Concentrations. 

Agent OH Profile Width, mm 

Uninhibited 1.30

N2 1.24

FE-36 1.31

FM-200 1.26 

CF3Br 0.96 

PN 0.96 

DMMP 1.04

Fe(CO)5 0.83 

The Table 3-18 width values indicated that the widths for N2, FE-36, and FM-200 were not significantly 

different from the uninhibited flame. By contrast, CF3Br, PN, DMMP, and Fe(CO)5 exhibited width

changes that were at least 20 % of the uninhibited width value. The OH width trends suggested that 

inhibitor agents with m

inh o

Sim

measure

tempera etween 2125 K and 2200 K. The obtained temperature values 

for N and F

esti

uninhib on concentrations were added. Previous studies of

an a o

flow, fo with the uninhibited flame. By contrast, 

Mas e

exis 3

near extinction. With mixed results from previous investigations and the large degree of uncertainty in 

nts, the only creditable temperature values are those close to extinction. For Fe(CO)5 and 

ture decreases with respect to the uninhibited flame were not observed until proximal

ore physical inhibition capabilities exhibited less effect on the flame structure than

ibit rs with enhanced chemical inhibiting capabilities. 

ilar trends were expected for the peak flame temperatures. Figure 3-109 presents a plot of peak LIF 

d flame temperatures vs. agent delivery concentrations for each inhibited flame. The peak flame

ture for the uninhibited flame was b

2 M-200 were not statistically different from those measured in the uninhibited flame with an 

mated uncertainty of ± 300 K. For CF3Br and PN, temperature differences with respect to the

ited flame were not observed until near extincti

tm spheric pressure, axisymmetric propane/air flame inhibited by addition of CF3Br to the oxidizer 

und only small temperature differences in comparison 36,51

156 ri t al. reported for a non-premixed atmospheric pressure CH4-air flame, that higher temperatures

ted in the reaction zone of a CF Br inhibited flame than in the reaction zone of an uninhibited flame

these measureme

DMMP, tempera

extinction concentrations were reached as well. On a concentration basis, Fe(CO)5 and DMMP had

decreased flame temperatures, T ≈ 1700 K, at agent concentrations lower than the other agents studied. 

For Fe(CO)5, small decreases in flame temperatures had been observed by Brabson et al.327

 in studies of

low-pressure premixed flames inhibited by Fe(CO)5. 
288 Flame Suppression Chemistry

Figure 3-109. Peak LIF 

Measured 

umerical Modeling

Numerical mo P, Fe(CO)5, 204

- 2

calculations, a simplified model was used to decrease computational time. The kinetic

5

3 developed mechanisms for these two species42,104

 were added to the 

2100

2200

Temperatures (K) vs. 

Inhibitor Delivery

Concentrations. Data 

Legend: □: N2, О: HFC227ea, ∆: HFC-236, : 

PN, ◊: CF3Br, ■: DMMP,

and ●: Fe(CO)5. 

0 1 2 3 4 5 6

1600

1700

1800

1900

2000

Pea

k LIF T

e

m

p

eratur

e, K

Agent Concentration, % Volume

N

deling of a stoichiometric, premixed, propane-air flame inhibited by DMM

CF3Br, and N2 flame was carried out using the CHEMKIN suit of programs. For the simulations, a

kinetic model for propane combustion developed by Marinov et.al.328-330 was slightly modified and

combined with a C1 C hydrocarbon kinetic model52 that had been employed in earlier inhibition studies.

For routine

mechanism for phosphorus containing species was based on the model suggested for the analysis of the 

influence of PH3 products on the recombination of hydroxyl and hydrogen atoms in a hydrogen flame.331

and kinetic models254,300,301 developed to simulate destruction of DMMP and TMP in low pressure

hydrogen flames. Additional reactions were added to the phosphorus mechanism to complete the reaction

pathways for the consumption of some of the P-containing species. For the modeling of Fe(CO) and

CF Br inhibition, previously

hydrocarbon model. 

Computations of the propane flame inhibited by DMMP demonstrated that the consumption of DMMP

led via a sequence of reactions to the formation of CH3PO2. Reactions of CH3PO2 with H and OH created 

HOPO and HOPO2. At this stage, reactions of HOPO, HOPO2, and PO2 with chain carriers formed the

following two inhibition cycles: 

H + PO2 + M = HOPO + M (R3-17) 

OH + HOPO = H2O + PO2

H + HOPO = H2 + PO2

O + HOPO = PO2 + OH 

OH + PO2 +M = HOPO2 +M (R3-18) 

H + HOPO2 = H2O + PO2
Comparative Flame Inhibition Mechanisms of Candidate Moieties 289

These inhibition cycles represent the catalytic scavenging cycles that accelerate radical recombination in 

combustion products containing phosphorus compounds.260

 It is well known that the addition of an 

inhibitor decreases the burning velocity for premixed flames. Numerical results for burning velocity

kinetics for the decomposition to PO2 to HOPO species by varying the overall activation 

energy for the decomposition reaction. Calculations showed that for the compounds with global activation 

/s 332

). The calculations were conducted for a

gas phase model without taking into account possible condensation processes. Calculation results, Figure 

in reducing the burning velocity in comparison with Fe(CO)5, but 

relative to CF3Br both were more effective. For increases in the concentration of Fe(CO)5 and DMMP,

ndensation

processes.104

Delivered 

ta. Solid 

0.00 0.25 0.50 0.75 1.00

0.0

0.2

0.4

0.6

0.8

1.0

decreases of 20 % to 30 % using the original rate constants given by Twarowski,259

 indicated that DMMP 

decreased the flame’s burning velocity by a factor of 1.5 to 2 relative to CF3Br in a methane-air flame.

Sensitivity analysis revealed that the burning velocity was receptive to changes in the rate constants for

the reactions of PO2 radical: H + PO2 + M = HOPO + M and OH + PO2 +M = HOPO2 +M. Reasonable

adjustment of rate constants led to agreement with experimental data. 

Phosphorus compounds have a wide range of thermal stability. Activation energies of decomposition 

reactions are in the range 15 kcal/mol to 90 kcal/mol. The influence of the decomposition rate was studied

using global

energies less than 50 kcal/mol, the burning velocity was not affected by the stability of the phosphorus 

compounds. 

Suppression calculations were carried out with increasing additive loadings until suppression

concentration levels were achieved (burning velocity ≤ 5 cm

3-110, show that DMMP had less effect 

both agents exhibited increasing saturation effects. Typically, two types of saturation have been discussed

in the literature: (1) saturation of chemical influence,52 and (2) saturation due to co

Figure 3-110. Calculated 

Burning Velocities vs. 

Inhibitor Concentration for a 

Numerical, Stoichiometric, 

Premixed, Propane-air Flame. 

Dashed line: DMMP da

line: Fe(CO)5 data. Dashed dot 

dashed line: CF3Br data. Nearhorizontal Dashed Dot Dot 

Normalize

Dashed Line: N2 data trend. 

d Burning Velocity

Agent Concentration, % Volume
290 Flame Suppression Chemistry

Both processes resulted in a decrease in inhibitor efficiency with increased inhibitor concentration. For 

example, to decrease the burning velocity to 10cm/s required a DMMP loading of approximately 0.9 %,

but an additional 1.2 % of DMMP was needed to decrease the burning velocity to the extinction level of 

5 cm/s. Such a strong saturation effect led to a substantial increase in extinction concentrations and a 

decrease in inhibitor efficiency relative to CF3Br. The calculated extinction concentrations, in units of %

volume, for the numerical propane/air flame were: DMMP: 2.1; CF3Br: ≈ 3.5; Fe(CO)5: 0.4 to 0.5; and 

N2: ≈ 40. The modeling results supported DMMP and Fe(CO)5 exhibiting superior inhibition capabilities 

relative to CF3Br. 

Finally, comparison of the normalized OH concentrations dependency on inhibitor concentrations

nt propane flames inhibited by the same agents had normalized OH concentrations that

tracked more or less with one another. At the experimental OH extinction level, i.e., 0.3 to 0.1, both data sets 

(e

ure 3- . Norm zed O

ibitor cent ns. D

legend: experimental (●) and 

numerical (dashed line) DMMP 

ed dot dot dashed

Conclu

The exp

was app

was a g

the deg

activity

and Fe(

width.

Contrar difference among PCCs in their flame extinguishment 

concentrations, on a per-phosphorus atom basis. PN, with the phosphorus atoms in its resonancedemonstrated a correlation between experimental and calculated OH concentrations. Figure 3-111 illustrates 

that two differe

xperimental/computational) had similar normalized OH reductions. 

Fig 111 ali H

Concentrati

Inh

ons vs. Delivered 

Con ratio ata

numerical (solid line) Fe(CO)5

data; experimental (■) and 

data; experimental ( ) and 

numerical (dash-dot line) CF3Br 

data; dash

line, N2 data trend. 

sions

erimental results presented here showed for the first time changes in [OH] profiles as extinction 

roached in a series of inhibited, atmospheric pressure, non-premixed, propane/air flames. There 

ood relationship between the narrowing and peak intensity of [OH] profiles in these flames with 

ree of chemical activity of added inhibitors. N2, HFC-236, and HFC-227ea, with little chemical 

, showed no peak narrowing relative to the uninhibited flame. By contrast, CF3Br, PN, DMMP 

CO)5 demonstrated chemical inhibition capabilities and significant decreases in the [OH] peak 

y to prior hypothesis, there was a

0.0

0.0

0.6

1.0

0.8

stabilized ring structure, was less efficient than DMMP. 

0 0.25 0.50 0.75 1.00

0.2

0.4

Normalized O

H

Agent Concentration, % Volume
Comparative Flame Inhibition Mechanisms of Candidate Moieties 291

3.5.3 CF3Br and Other Suppressants: Differences in Effects on Flame Structure 

Introduction 

With some exception, the working hypothesis of the NGP was that significant catalytic scavenging of 

flame-propagating free radicals was necessary to match or exceed the efficiency of halon 1301. A logical

issue was the extent to which the knowledge of the flame inhibition mechanism of CF3Br, itself a catalytic

scavenger, was applicable to identifying alternative suppressants. Was CF3Br “typical” of efficient fire

suppressants in general? Would all promising replacements for CF3Br show similar behavior? Was 

commonality of properties with CF3Br a useful guide in the search for alternatives? 

action zone on the hot boundary. The mesh extension and refinement were such that the spacing

between species

gra e

gradient (GRAD key

all calculations. The final meshes typically contained 90 to 125 grid points, depending on the number of 

To this end, Williams and co-workers26 performed modeling studies of premixed flames containing

substances representative of different classes of inhibitors, including inert gases, fluorocarbons, and

catalytic scavengers of flame radicals. While suppression of nonpremixed flames is the typical scenario 

in fire extinguishment, the behavior of suppression agents in premixed flames is important in mitigating 

deflagrations and protecting against re-ignition. Furthermore a large body of suppression data in different

types of flames compiled by Babushok and Tsang333 had shown that for many agents, extinction of

nonpremixed flames correlated with reduction of burning velocity in premixed flames. 

Williams and co-workers compared the effects on burning velocity and flame structure of CF3Br with

those of the catalytic inhibitor NaOH, the non-catalytic hydrofluorocarbons CF3CHFCF3 (HFC-227ea)

and CF3CH2F (HFC-134a), and the inert agents N2 and CF4. (CF4 undergoes little decomposition in the 

reaction zone of most flames,61 and is thus predominantly a physical agent). For exploratory purposes, 

they also considered an idealized, constrained version of Fe(CO)5 inhibition which includes only the 

much less effective iron catalytic cycles for O-atom recombination. 

Computational Methodology 

The PREMIX code 203 was used to compute burning velocities and flame structures of atmospheric 

pressure, stoichiometric methane-air flames with and without suppressants. A domain of 85 cm was used 

in the calculations, extending 25 cm from the reaction zone on the cold boundary and 60 cm from the 

re

the outermost grid points on each boundary was 1 cm, to minimize any temperature or

dients at the boundaries. Multicomponent transport formulations, as well as the Soret effect for th

species H and H2, were used for all calculations. Refinement tolerances of 0.1 for the normalized species 

word) and 0.2 for the normalized second derivative (CURV keyword) were used for 

additional species introduced by the inhibitor chemistry. 

Comparison of the PREMIX results with thermal equilibrium calculations for several inhibited and

uninhibited flames showed that, at the final grid point, temperatures were within 5 K and that radical

concentrations of H, OH, and O were within a few percent of their equilibrium values. Thus, the radical

equilibrium values and adiabatic temperatures were assumed to be those calculated at the hot boundary. 

All flames were stoichiometric, with the inhibitors listed above added in various concentrations. For the

hydrocarbon chemistry, the GRIMech 2.11 334 (excluding nitrogen chemistry) was used as the chemical
292 Flame Suppression Chemistry

kinetic mechanism. For the uninhibited, stoichiometric methane air flame, a predicted burning velocity of 

39.5 cm/s ± 0.2 cm/s was obtained. 

The fluorocarbon chemistry was based on the mechanism developed by Westmoreland et al., as refined in

previous studies performed at the Naval Research Laboratory 61,335 The bromine chemistry used in the 

odeling of CF3Br was that developed by Noto et al. 175

 The kinetic mechanism for NaOH was based on 

that of Zamansky et al.,335

 while that for Fe(CO) was described by Rumminger and Linteris.118 NaOH

mu o

provision fo the sodium

hydroxide was therefore as to initially consist of the gas phase d as required

to dissociate (∆Hf = 42 fore inhibition chemistry could proceed. N CF4 were assumed

inert: no nitrogen or fluorine chemistry was included in the respective calculatio

Inhibition Parameter 

The effectiveness of an inhibitor at reducing burning velocity in a fuel/oxidizer mixture may be placed on 

a quantitative basis in terms of the inhibition parameter, Φ, proposed by Rosser et al.33 and modified by 

Noto et al.:175

Φ = ln(U0/Ui) (XO /Xi) (3-12)

tant value of Φ with inhibitor concentration 

indicated that the exponential behavior found in Reference 175 held. 

m

5

st vaporize before being able to participate in the flame chemistry, but the PREMIX code had n

r inclusion of condensed phase species. To simulate the evaporation effect,

sumed imer Na2O2H2, which w

kcal/mol) be 2 and

ns.

2

where U0 and Ui are the burning velocities of the uninhibited and inhibited flames, respectively, and XO2

and Xi are the reactant mole fractions of oxygen and inhibitor. In Reference 175, the burning velocity of

a given fuel/air mixture was found to exhibit an exponential dependence on concentrations of several inert 

and fluorocarbon inhibitors, including CF3Br. A cons

Catalytic radical scavengers necessarily manifest saturation effects, because catalysis cannot reduce 

concentrations of flame radicals below the thermal equilibrium level at the local flame temperature. The 

existence of a similarity transform, obtained by solving Equation 3-12 for Ui

175

, which caused the flame

speed reduction curves for CF3Br and non-catalytic agents to coincide, was thus somewhat unexpected. 

Changes in burning velocity, adiabatic flame temperature, radical super-equilibrium, and Φ are plotted as 

a function of (NaOH)2 concentration in Figure 3-112 and CF3Br concentration in Figure 3-113. The error 

bars for Φ were determined by propagating a change of 0.2 cm/s in the inhibited flame speed (the

estimated computational uncertainty for the mesh refinement tolerances used) to determine the change in 

Φ in Equation 3-12. The points at low inhibitor concentrations had high uncertainties in Φ due to the

small reductions in burning velocities. The radical super-equilibrium was defined as the ratio of the peak

concentrations of the radical to its concentration at thermal equilibrium at the adiabatic flame

temperature. The value for this ratio is always unity or greater; a value greater than unity means the

radical exists in super-equilibrium concentration somewhere in the flame. 
Comparative Flame Inhibition Mechanisms of Candidate Moieties 293

Figure 3-112. Burning Velocity nal 

Flame Temperature, Inhibition 

Parameter, and Superequilibrium 

Concentrations of Flame Radicals 

Computed for Atmospheric Pressure 

Stoichiometric Methane-air Mixtures 

Inhibited by Na H.

Computed for Atmospheric Pressure 

Stoichiometric Methane-air Mixtures 

d by CF3Br. 

, Fi

O

Figure 3-113. Burning Velocity, Final 

Flame Temperature, Inhibition 

Parameter, and Superequilibrium 

Concentrations of Flame Radicals 

Inhibite
294 Flame Suppression Chemistry

The behavior of the inhibition parameter in Figure 3-112 was different from that in Figure 3-113. For 

sodium, the inhibition parameter was not constant as a function of inhibitor concentration, but varied by

more than a factor of three over the range of sodium concentrations considered here. The similarity

relationship identified in Reference 336 did not hold for sodium. For CF3Br, by contrast, the inhibition

parameter was nearly constant as a function of inhibitor concentration. For comparison, the inhibition 

parameter of N2, up to a 50 % reduction in burning velocity maintained an almost constant value of

0.5 (data not shown). The results for CF3Br agreed with the findings of Noto et al.336

 although the 

modifications to the fluorocarbon kinetics yielded better agreement with the experimental value of 

14.0 for the inhibition parameter, compared to the value of 11.1 reported in Reference.336

 The value of

12, the radical super-equilibrium was drastically reduced with increasing NaOH. For inert 

ium by nearly 80 % at a 50 % burning velocity reduction. This finding supported the 

conclusion of Saso et al.337

 that saturation was a minor effect in the suppression effectiveness of

3Br inhibition because the peak radical

r substantial reductions in burning velocity. 

o catalytic 

cycle involved the sequence of reactions332

Br + Br + M => Br2 + M 19)

H + Br2 => HBr + Br 0)

as an important pathway in regeneration of HBr. The direct reactio

H + Br + M => HBr + M 1)

has slow kinetics. This has two consequences: the second order the

bromine concentration compensates for the saturation effect with increasing CF3Br concentration. Also, a

high concentration of bromine is required for this reaction to be significant, leading to a much smaller

inhibition parameter for bromine than for iron and sodium,

333

 whose scavenging cycles are not thought to

depend on second order kinetics. 

approximately 100 for the inhibition parameter of (NaOH)2 per sodium atom at low concentrations is 

comparable to measured values for various sodium compounds ranging from 114 to 200.

336

Radical Super-equilibrium 

In Figure 3-1

agents, by contrast, the radical super-equilibrium was increased as the flame is inhibited. When the flame 

speed was reduced by 40 % by the addition of N2, for example, the H atom super-equilibrium increased

from 17 to 70. Although peak radical concentrations decreased as an inert agent was added, the reduction

in flame temperature reduced the equilibrium concentrations proportionately more. 

For inhibition by CF3Br (Figure 3-113), the degree of radical super-equilibrium was also reduced with

increasing agent concentration, although not as dramatically as for sodium. CF3Br reduced the H atom

super-equilibrium by only 30 % for a 50 % reduction in burning velocity; NaOH reduced the H atom

super-equilibr

CF3Br/inert mixtures. Saturation was not as pronounced for CF

concentrations remained far from equilibrium fo

Another factor contributing to the lack of a saturation behavior f r CF3Br was that the bromine

(R3-

(R3-2

n 

(R3-2

dependence of reaction R3-19 on
Comparative Flame Inhibition Mechanisms of Candidate Moieties 295

Synergism Between Catalytic and Physical Agents 

Several experimental and modeling studies had observed that the effectiveness of CF3Br 91,337 as well as

other chemical agents14,106 could be enhanced by the addition of a physical agent. Saso et al. 337 attributed 

synergism in mixtures of CF3Br and inert inhibitors to a temperature effect on the inhibition effectiveness

of CF3Br, rather than a saturation phenomenon. Over a range of adiabatic flame temperatures, Saso et al. 

found CF3Br to have virtually identical inhibition parameters at loadings of 0.5 % and 1 % (by volume) in

methane/oxygen/inert mixtures, indicating the absence of significant saturation effects. 

To extend the computational investigation of synergy to other combinations of agents, inhibition by iron

was modeled in combination with nitrogen. For this modeling, iron was considered to participate in a 

three step mechanism involving only O + O recombination. This pathway was identified by Rumminger 

and Linteris118

 but was usually secondary in importance to an H + H recombination pathway. This model 

was not intended as an accurate description of iron's combustion chemistry, but rather to consider whether

synergism occurred in the absence of a temperature dependence on the inhibition kinetics. 

R

FeO2 + O => FeO + O2 (R

FeO + O => Fe + O2 (R

R3-23 and R3-24 were assumed to have rate coefficients indep n

R3-22 had a slight increase in rate with increasing temperature, bu o t and

the number density of third body colliders varied by less than 1

1400K and 2500K. 

The iron + nitrogen combination exhibited synergy, as seen in Figure 3-114, which plots burning velocity, 

flame temperature, and differential inhibition parameter of the O + O cycle of 500 μL/L FeO2 as a

function of nitrogen addition. 

air Mixtures Inhibited by N2 and 

FeO2 in Combination. 

Of the three reactions making up the O + O catalytic cycle of iron: 

Fe + O2 + M => FeO2 + M ( 3-22)

3-23) 

3-24)

endent of temperature. The reactio

t the product of the rate c efficien

0 % over the temperature range from

Figure 3-114. Burning Velocity and 

Differential Inhibition Parameter 

computed for Atmospheric 

Pressure Stoichiometric Methane-
296 Flame Suppression Chemistry

The differential inhibition parameter for inhibitor B in the presence of inhibitor A is 

Φ = ln(UA/UA+B) (XO2/XB) (3-13)

While the findings of synergism agreed qualitatively between CF Br/inert and Fe/inert mixtures, the final

338

general one. The fluorocarbon agents CF3CHFCF3 and CF3CH2F both reduced burning velocity more 

where UA and UA+B are the burning velocities in the flames inhibited by inhibitor A alone and A and B in

combination, respectively. The differential inhibition parameter of 500 μL/L FeO2 increased by about

23 % as the flame temperature was lowered by nitrogen addition from 2230 K to 2010 K. Saso et al.337

found that the inhibition parameter of CF3Br increased by some 45 % at loadings of both 0.5 volume % 

and 1.0 % volume % over the same range of final flame temperatures. 

3

flame temperature influenced the inhibition parameter of CF3Br twice as much as that of the O + O

recombination cycle of iron, due to the lack of explicit temperature dependence of the Fe kinetic cycle.

Thus, these results showed that synergism in catalytic/inert mixtures did not require temperature

dependence of the inhibition cycle, although this could influence its magnitude. 

Correlation of Burning Velocities with Changes in Flame Structure 

Padley and Sugden observed in studies of H2-O2-N2 flames that the burning velocity correlated with the 

partial pressure of atomic hydrogen in the reaction zone. More recently, this correlation had been

observed to hold also for hydrogen flames inhibited by CF3Br. 339 Figure 3-115 plots the peak H atom

concentration against burning velocity for a variety of inhibited methane/air flames. For methane flames

inhibited by N2, CF4, and CF3Br, there was a nearly identical linear relationship between the peak H atom

mole fraction and burning velocity, as with the hydrogen flames. 

Figure 3-115. Relationship between 

Burning Velocity and Peak 

Concentration of Atomic Hydrogen 

for Methane-air Flames Inhibited by

Various Compounds. 

Examination of the effect of other inhibitors, however, demonstrated that this correlation was not a
Comparative Flame Inhibition Mechanisms of Candidate Moieties 297

than would be expected based on the peak H atom concentrations. Inhibition by sodium or iron produced

the opposite effect. Noteworthy in the sodium data was the curvature which coincided with the onset of 

saturation. The coincidence of the CF Br data points with those of the inert agents appears to have been

(3-14)

urning velocity, XH the peak mole fraction of atomic hydrogen, and Tf the final

flame temperature. The fitted parameters A and Ea, obtained by considering the flame inhibited by

H + OH recombination rate. For catalytic agents which did not significantly change the

perature, the burning velocity correlated with the square root of the H atom peak mole 

action, but not with those of atomic ox he effect of catalytic agents on reducing 

burning velocity proceeded through reduction of the atom drogen concentration, whether or not the 

3

accidental. The calculations for the fluorocarbons and catalytic agents lay on opposite sides of those of 

the inert agents, and CF3Br was in some sense a combination of the two. 

An alternate correlation between burning velocity and flame structure was based on the proportionality 

between the burning velocity and the square root of the overall reaction rate.337 In modeling the burning 

velocity of a large number of inhibited flames, the burning velocity correlated in almost all cases with the

product of the peak H atom mole fraction and a global activation energy: 

SL

2

 = A2XHexp(-Ea/kTf) . 

where SL is the laminar b

nitrogen, were A=7940 cm/s, Ea=103 kJ/mol. The same empirical correlation held for inhibition by other

inert gases, by iron and sodium, by nitrogen and iron in combination (Table 3-19), and by artificially

increasing the

final flame tem

fr ygen or OH radicals. T

ic hy

scavenging cycle directly involved H atom recombination (as in the O + O cycle of iron). 

Table 3-19. Comparison of Calculated Burning Velocities. 

Flame Condition Tadiabatic XHmax x 103 Spremix Seqn ∆Spremix/ ∆Seqna 

CH4-air (uninhibited) 2234 6.64 39.8 39.8 --- 

+9.09 % N2 2121 5.12 30.2 30.2 1.00 

+16.67 % N2 2015 3.94 22.6 22.7 1.01

+8.26 % CF4 (inert) 1968 3.50 18.4 19.9 1.07

+200 μL/L Fe(CO)5 2223 1.46 18.4 18.6 1.01

+1.0 % CF3Br 2203 3.42 18.9 27.6 1.71 

+0.5 % HBr 2221 5.36 30.3 35.2 2.08 

+3.8 % CF3CH2F 2031 4.11 15.5 23.9 1.54

+2.9 % CF3CHFCF3 2161 3.86 16.3 26.1 1.73

+0.05 % (NaOH)2 2219 2.97 28.0 26.2 0.87

+0.40 % (NaOH)2 2177 0.33 9.4 8.3 0.97

Increase H+OH+M rate x 100 2234 1.35 19.8 18.0 0.92 

a

 39.6-Spremix)/(39.6-Seqn) 

Table 3-19 compares the burning velocities estimated using Equation 3-14 to the calculated values for a 

ariety of flame inhibitors. In 6 of the 11 cases, the reduction in burning velocity relative to the

bited flame, predicted by Equation 3-14 was within 13 % of the result of the PREMIX calculation. 

owever, the correlation stated in Equation 3-14 greatly underpredicted flame speed reduction by the

fluorocarbons CF3CH2F and C3HF7, as well as CF3Br and HBr. Flame structure modeling indicated that 

v

uninhi

H
298 Flame Suppression Chemistry

the breakdown of this relationship involving the peak H at concentration was a consequence of these 

agents reducing the H atom concentration mostly in the early part of the flame, prior to the peak 

concentration, as discussed below. 

Changes in Flame Structure Due to Different Agents 

Insight into why the correlation between burning velocity, H atom concentration, and temperature did not

hold for either fluorocarbons or bromine-containing compounds was gained from examination of these 

compounds’ effects on flame structure. In Figure 3-116, th mole fraction of atomic hydrogen is plotted

against the local temperature for flames inhibited by a variety of agents. All the inhibited flames had

burning velocities approximately 50 % that of the uninhib r

comparison. For all these flames, the temperature monot from

reactants to products. 

Figure 3-116. Mole Fraction of 

Atomic Hydrogen as a Function of 

Local Flame Temperature for an 

Uninhibited and Uninhibited 

Atmospheric Pressure Methane-air 

Flame. 

The relationship between H atom mole fraction and local temperature shown in Figure 3-116 was 

characteristic of each type of agent except for CF3Br. The phy e

temperature, but the H atom mole fraction at a given i s

changed very little from its value in the uninhibited flame atom

mole fraction by a relatively constant factor throughout the d the

H atom mole fraction early in the flame (in the region below t

on the peak concentration. This diminished effectiveness m

concentration also occurred for CF3Br; in this respect CF nce to nonbrominated

fluorocarbons than it did to other catalytic agents such as iron and sodium. Since Babushok et al. 167

 found 

that iron approached the performance of an “ideal” cataly c inhibitor, the different effect of CF3Br on 

flame structure from that of iron and sodium indicated a departure from ideality, in particular it only

scavenged radicals significantly in low temperature regions of the flame. All the flames inhibited by 

chemically active suppressants showed nearly equal reductions in H atom mole fraction at 1700 K. In the 

flames inhibited by inert agents, the H atom concentration at this isotherm was essentially the same as in 

the uninhibited flame. Therefore, the H atom mole fraction at 1700 K (or any other fixed temperature)

om

e

ited flame, whose structure is also plotted fo

onically increased with position passing

sical agents reduced the final flam

sotherm (above approximately 1300 K) wa

. Both iron and sodium reduced the H

reaction zone. The fluorocarbons reduce

about 1200 K) but had relatively little impac

at the location of maximum hydrogen ato

3Br bore more resembla

ti
Comparative Flame Inhibition Mechanisms of Candidate Moieties 299

cannot be used as a predictor of burning velocity which is applicable to all inhibited flames. Given the 

different influences of the suppressants on flame structure, it seemed unlikely that an empirical

relationship between flame speed and some other quantity can be found which applies to all suppressed 

flames. 

The depletion of radical species early in the flame had a marked influence on the burning velocity. It is 

for this reason that fluorocarbons and bromine compounds are better inhibitors than the changes in

temperature and peak H atom concentrations predicted. This observation suggested that agents which 

deplete radicals in high temperature regions but not early in the flame were likely to be less effective 

inhibitors than would otherwise be expected. This may be the case for condensed-phase agents which 

must vaporize before inhibition can begin. 

Conclusions 

In several respects, CF3Br is not representative of catalytic fire suppressants in general. Several features 

of its kinetics, including the strong temperature dependence of the catalytic suppression cycle and the 

preferential reduction of atomic hydrogen concentrations in low temperature regions of the flame, were

not shared by other catalytic suppressants. In these regards, sodium and iron exhibited behavior much

more typical of this class of suppressants. For these reasons, caution is required in using specific kinetic

properties observed for CF3Br as a basis for identifying other efficient fire suppressants. 

free radicals

several chem

l sues have limited their implementation. Among the halogens,

iodine is a good catalytic scav logens, good suppression efficiency h

demonstrated for alkali metals, etals including iron,10

chromium,

126 and manganese.1 prising this list vary greatly in their c

able (e.g., nitrogen, fluorine) do not

cause efficient catalytic suppression of flames. 

ing and co-workers343 broadened the scope of the previous work by 

focusing on a common thermodynamic basis for catalytic suppression by different families of elements,

and different types of catalytic cycles. The rationale for revisiting the effect of bond energies at the 

present time was threefold:

3.5.4 Influence of Bond Energies on Catalytic Flame Inhibition 

Introduction 

The suppression effectiveness of CF3Br and related compounds is primarily due to the chemical activity 

of bromine,13,332,340

 which participates in a catalytic cycle and reduces concentrations of the

involved in flame propagation. As documented earlier in this Chapter, ical elements besides 

bromine have been shown to exhibit catalytic radical scavenging in flames, sometimes much stronger 

than that of bromine,333 although practica is

enger.

20,167

 Among non-ha as been

341

 phosphorus,79 and several transition m 4 lead,92

07

 The elements com hemical

properties, while other elements in the same groups of the periodic t

Finding a common basis for efficient chemical suppressants, whose results can be extrapolated to other

elements with combustion chemistry which has not yet been thoroughly explored, was of importance to 

the NGP. Previous work had pointed to the binding energy of a suppressant (or one of its decomposition 

products) to one or more flame radicals as an important parameter.42,342

 This approach has been used

previously to explain, for instance, the greater suppression effectiveness of bromine compared to

chlorine.42 Under the NGP, Flem
300 Flame Suppression Chemistry

the effect of thermodynamics directly, rather 

than resorting to simplifying approximations such as zone modeling of flame structure342 or

• A great deal of additional chemical kinetic data had become available, including provisional 

are much higher than the thermal

equilibrium concentrations at the adiabatic flame t kinetic rates 

of the chain branching reactions

(R3-25) 

2 (R3-26) 

O + O + M => O2 + M (R3-28) 

participate in a catalytic cycle whose net effect is equivalent to ions.

For a substance to scavenge flame radicals, there must exist some species X derived from the inhibitor 

which can bond to H, O, or OH. The species XR, where R is one of the flame radicals, must react with

another flame radical R' to form X and RR', where RR' is usually a stable molecule such as H2, H2O, or

O2. Possible scavenging reactions include. 345

XH + H => X + H2 (R3-30) 

XH + OH => X + H2O (R3-31) 

XOH + H => X + H2O (R3-32) 

XO + O => X + O2 (R3-33) 

To complete the cycle, X must then be converted back to XR For significant

inhibition to occur, the kinetic rate of the scavenging cycle ust exceed that of the equivalent direct

recombination reaction. 

• Computational tools had become available to model

partial equilibrium assumptions of species concentrations.42

• The NGP objective was to assess the fire suppression potential of elements that had not yet been 

extensively investigated, rather than to explain the behavior of elements whose properties were 

already (empirically) known. By contrast, the work which elucidated the suppression kinetics of

bromine was performed after halons were identified and implemented as fire suppressants. 

suppression mechanisms for a number of non-halogen-containing species. 

Simplified but representative catalytic cycles for known inhibitors (bromine, sodium, iron) were used in 

conjunction with detailed kinetic modeling of the hydrocarbon flame structure. 

General Features of Catalytic Scavenging Cycles 

In most flames, the peak concentrations of the radicals H, O, and OH 

emperature. This is a consequence of the 

, for example 

H + O2 => OH + O 

O + H => OH + H, 

exceeding the rates of chain termination reactions such as 

OH + H + M => H2O + M (R3-27) 

H + H + M => H2 + M, (R3-29) 

which require a third body for stabilization of the product.344 Chemical fire suppressants generally 

one of the recombination react

through one or more steps.

m
Comparative Flame Inhibition Mechanisms of Candidate Moieties 301

Inhibition by bromine occurs primarily through (R3-30) and to a lesser extent (R3-31),

346

 while alkali

metals operate primarily through (R3-32).

347

 Reaction (R3-33) is likely to occur for metallic elements.118 

ome elements, including iron118 and p ar participate in more than one cyc

X were set as follows: GRAD =

0.10, CURV = 0.20, up to a maximum of 150 grid points. Multicomponent viscosities and thermal 

s used for modeling the 

uninhibited flames. Calculations were carried out using bromine, sodium, and iron as examples of

added to the hydrocarbon 

is thought to be the primary scavenging reaction, with the reaction with OH 

O (R3-35) 

es several pathways. The direct recombination 

(R3-36) 

n

hydrogen atoms from such species as HO2, HCO and CH2O; th t flames. 

Two simplified kinetic mechanisms describing inhibition by b

Table 3-20. Both mechanisms included Br and HBr as the o ies. For these

calculations, HBr was chosen as the bromine-containing reactant. In the first mechanism, HBr was

assumed to react only with atomic hydrogen to form Br, which was converted back to HBr by a direct

recombination with H, as well as abstraction of H from HO2, HCO, and CH2O. The second mechanism 

was identical, except that HBr was allowed to react only with OH, rather than H. 

S hosphorus,258 e thought to le.

Computational Methodology

The computer code PREMIX203

 was used to model freely propagating atmospheric pressure premixed

flames. The baseline condition consisted of stoichiometric methane/air, with an initial reactant

temperature of 298.2 K. Tolerances for mesh refinements in PREMI

diffusion (Soret effect) were used. The computational domain extended 25 cm from the flame into the 

reactants gases, and 60 cm into the exhaust gases. The domain was extended in such a way that on both 

boundaries, the second last grid point was 1 cm from the boundary. 

The reaction set given in GRI-MECH 2.11289

 (without nitrogen chemistry) wa

chemical inhibitors. Kinetics of chemical inhibition for these elements were

mechanism as described below. 

Kinetic Mechanisms of Specific Inhibitors 

The inhibition cycle of bromine is believed to proceed by H atom scavenging.42,342 The reaction 

HBr + H => Br + H2 (R3-34) 

HBr + OH => Br + H2

also contributing. The conversion of Br to HBr involv

H + Br + M => HBr + M

has rather slow kinetics and is typically not the predomina t pathway. Atomic bromine can abstract

ese play an important role in mos

romine were constructed and are listed in

nly bromine-containing spec
302 Flame Suppression Chemistry

Table 3-20. Kinetic Mechanisms for Suppressants (k = ATbexp(-Ea/kT)). 

Reaction

A 

(cm3/mol-s) b 

Ea

(kcal/mol) Source 

Mechanism for Na (H + OH cycle only) 

 (NaOH)2 + M = NaOH + NaOH + M a 3.0E14 0.0 48 

NaOH + H = Na + H2O 1.0E13 0.0 1.97 

Na + OH + M = NaOH + M a 1.8E21 -1.0 0

Mechanism for Br (reactions in common for all cycles) 

H + Br + M = HBr + M a 1.92E21 -1.86 0 348 

Br + CH2O = HBr + HCO 1.02E13 0.0 1.59 349 

 Br + HO2 = HBr + O2 8.43E12 0.0 1.17 349

Br + HCO = HBr + CO 1.69E14 0.0 0 350 

Addition for Br (H + H cycle): 

HBr + H = H2 + Br 6.25E13 0.0 2.40 348 

Addition for Br (H + OH cycle): 

HBr + OH = H2O + Br 6.62E12 0.0 0 351 

Mechanism for Fe (O + O cycle): 

 Fe + O2 + M = FeO2 + M a 1.57E18 0.0 4.05 352 

 FeO2 + O = FeO + O2 1.73E13 0.0 0.0 a 

FeO + O = Fe + O2 1.73E13 0.0 0.0 b a

 No reaction rate repor b

 Based on reverse reac

F mpounds, R ger and L s

118 identified catalysis of H atom reco ination 

a mportant scavenging in hydrocarbon flames. In addition, t ound that an O atom

r cycle could be sign moist ames. Th e-step is shown in the last

t 3-20. This m nism was tended a ccurat iction of the overall 

i of iron, only ntribution e O + O nging (which or 

h els). Analogous O + cles may exist for other tr on metal elements as well. 

F nd other alkali metals n and Jone ropose the arily c

X + OH + M = > X + M 3-37)

XO H = > X + 3-38)

T chanism for so consisted R3-37) an 3-38), ther with action

c speci the reacta the mono to simu n evaporation step.26

ris al equilibrium calculations with NaOH(s) and (NaOH)2 as reactants indicate that 

e very minor reduction in the adiabatic flame temperature caused by the addition of NaOH dimer was 

ted; assumed equal to the rate in the next row. 

tion of Reference 353. 

or iron-containing co ummin interi mb

s the most i cycle hey f

ecombination ificant in CO fl e thre cycle

hree rows of Table echa not in s an a e dep

nhibition chemistry the co of th scave cycle is small f

ydrocarbon fu O cy ansiti

or sodium a , Jense s

347 p prim ycle:

OH (R

H + H2O. (R

he simplified me dium of ( d (R toge a re

onverting the NaOH dimer ( fied as nt) to mer late a

Compa ons of therm

th

about 70 % that of adding an equal amount of solid NaOH. 

Using the simplified mechanism, the predicted flame speed of a stoichiometric methane/air mixture

inhibited by 0.1 % mole fraction of (NaOH)2 was 25.0 cm/s, compared to 23.1 cm/s using a more 

comprehensive mechanism26 adapted from Zamansky et al.,335

 and 39.4 cm/s for the uninhibited flame.
Comparative Flame Inhibition Mechanisms of Candidate Moieties 303

This result confirmed that the OH + H cycle identified by Jensen and Jones was indeed the most

important for suppression of near stoichiometric hydrocarbon flames. 

Influence of Bond Energies on Suppression Cycles 

For an efficient scavenging cycle to exist, the bond energy between the scavenging atom or radical X and

hing cycle if it ran backwards. 

of bond energies for which an efficient

guments on a quantitative basis by 

ges in bond energies on suppression 

compatible with efficient suppression. 

c factors, so appropriate bond energies by

e ever,

were known to have good suppression properties, so they posses

Bromine was used as the example element for Reactions (R3-3 nd

iron for (R3-33). During the combustion process, each of th

partners and thus could have several bond energies hypothetic

simplify the situation so that sensitivity to bond energy can be determined in a relatively straightforward 

way, it was assumed that only one catalytic cycle involving one type of bond existed for each element. 

For each set of calculations, the thermodynamic functions for the species XR were not altered, but the

heat of formation of the species X at 298 K was varied, while keeping the heat capacity and entropy 

unchanged. This had the effect of changing the dissociation energy of X-R. In the kinetic mechanisms, 

the chemical reactions for the scavenging cycle were written in the exothermic direction, and the

a flame radical R must satisfy certain conditions. If it is too high, the scavenging reaction (R3-30) to 

(R3-33) would be endothermic. This is the case for fluorine, which binds irreversibly to hydrogen and 

thus cannot sustain a catalytic cycle. On the other hand, if the bond were too weak, the equilibrium

between XR and X + R would be so far toward dissociation that there would be insufficient XR to 

participate in the scavenging reactions. Stated another way, the rates of the reverse reactions must be

small compared to those of the forward reactions, since any catalytic recombination cycle would become

a catalytic chain branc

These considerations indicated that there would be a limited range 

scavenging cycle could exist. Putting these qualitative ar

computational investigation of the effect of hypothetical chan

efficiencies allowed the determination of the bond energies 

Suppression depends on kinetic as well as thermodynami

themselves do not guarantee good suppression properties. Th elements chosen as examples, how

s kinetics favorable to scavenging.

0) and (R3-31), sodium for (R3-32), a

ese elements can have multiple binding

ally relevant to combustion. In order to 

Arrhenius parameters of the forward reaction were unchanged. All reactions were assumed to be

reversible, however, so changing the heat of formation altered the activation energy of the reverse

reaction, even without any explicit changes to the kinetic parameters. 

In the PREMIX code, the facility existed for determining the sensitivity of the solution vector, including 

the flame speed eigenvalue, to changes in the heats of formation of species (HSEN keyword). This was 

distinct from sensitivities with respect to changes in the kinetic rates (ASEN keyword) which have been 

more commonly reported in modeling studies. Sensitivity analysis, however, only provided the first

derivative of the flame structure with respect to thermodynamic or kinetic quantities. The dependence of 

suppression efficiency on heats of formation is non-monotonic. A small sensitivity coefficient for the

enthalpy of a particular species on the flame speed may mean that the species in question is unimportant 

to suppression, or it could mean that its enthalpy is very close to the optimal value, leading to a vanishing 

first derivative. 
304 Flame Suppression Chemistry

Bromine: H + H and H + OH Recombination 

For this series of calculations HBr was chosen as the bromine-containing reactant. The kinetic

mechanism was chosen to model the effect of a single catalytic mechanism, with Br and HBr as the only 

bromine species. Two sets of calculations were performed, the difference being whether HBr was 

ycles of 

Table 3-20. The accepted values 

energies of H-Br, Hre indicated by

vertical lines. 

ct.

assumed to react with H or with OH. Figure 3-117 shows the predicted flame velocity of a stoichiometric

methane/air mixture inhibited by 0.5 volume % of HBr. The catalytic cycle was found to be most

efficient for H-Br bond energies (which were artificially varied in the calculations) between 65 kcal/mole

and 90 kcal/mole. 

Figure 3-117. Calculated Flame 

Velocity as a Function of 

Hypothetical Variation of the HBr Bond Energy for Premixed 

Methane-air Flames Inhibited by

0.5 % HBr Using the H + H and H 

+ OH Scavenging C

for the bond

Cl, and H-I a

The accepted value of the H-Br bond energy lies near the upper limit of the range, which permits an

optimally efficient catalytic cycle (Figure 3-117). The bond energy of H-I is near the lower limit of the 

optimal range, while the H-Cl bond is too strong for good suppression. The bond energy of H-F 

(135 kcal/mole) is so high that the molecule is completely inert in this environment. Although the

kinetics of the analogous reactions are somewhat different for the other halogens, the bond energy factor

by itself led to the correct prediction that the measured suppression effect of iodine was nearly equal that 

of bromine20, while chlorine had a much smaller effe 354

Sodium: OH + H Recombination 

The effect of hypothetical variation of the Na-OH bond energy is illustrated in Figure 3-118. There was a

more pronounced falloff in suppression efficiency with bond energy than for either of the bromine 

scavenging cycles. The Na-OH bond energy was close to the optimal value for good suppression, as were 

the bond energies for all the other alkali metals except lithium. Experimental studies have found that

potassium is a considerably more efficient flame suppressant than sodium.

333 In view of the similarity of

the thermodynamic properties, this difference may be due to differences in kinetic rates between the two

elements, particularly for the recombination step (R3-37). 
Comparative Flame Inhibition Mechanisms of Candidate Moieties 305

by 0.1 % (NaOH)2

using the H + OH 

The accepted values

of X-OH bond energies for 

the alkali metals are 

indicated by vertical lines. 

Iron: O + O Recombination 

two relevant bond energies, Fe-O and OFe-O, 

y keeping the 

standard state enthalpy of FeO f Fe and FeO2 in opposite directions fr

“normal” values. For FeO2, the enthalpy used was not the actual value,

chosen such that the bond energy that of Fe-O, 99 kcal/mole. 

timal value for efficiency of the catalytic cycle. 

Figure 3-118. Calculated 

Flame Velocity as a 

Function of Hypothetical 

Variation of the Na-OH

Bond Energy for Premixed 

Methane-air Flames 

Inhibited

scavenging cycle of Table 

3-20.

The three step O + O catalytic cycle modeled for iron had

which differed by about 3 kcal/mole. The bond energies were varied in the calculation b

ixed, while varying those of om their 

“normal” standard state but was

of OFe-O was equal to

In the calculation, FeO2 was chosen as the reactant. For most iron-containing compounds such as 

Fe(CO)5, the parent agent was quickly converted to Fe,104

 which then combined with oxygen through 

R3-22, making the original identity of the iron compound unimportant. Because of iron's high 

suppression efficiency, a smaller concentration of suppressant was used (500 μL/L) than for the

calculations with sodium or bromine. 

Results of the calculations are shown in Figure 3-119. The accepted bond energies of both Fe-O 

(99 kcal/mol) and OFe-O (96 kcal/mol) are close to the op
306 Flame Suppression Chemistry

Figure 3-119. Calculated Flame

Velocity as a Function of 

Hypothetically Variation of Fe-O 

Bond Energy for Premixed 

catalytic

al value 

varying somewhat for different n ions. This range of values was consistent

expectation that the bond must be strong enou odynamically stable at flame temperatures, 

l. Table 3-21 shows that the

more efficient a scavenging cy 175

changes in bond dissociation ene

Table 3-21. Efficiencies and Sensitivities to Bond Energies of Catalytic Cycles.

Methane-air Flames Inhibited by

500 μL/L FeO2 Using the Threestep O + O Scavenging Cycle of 

Table 3-20. The accepted values 

of the Fe-O and OFe-O bond 

energies are indicated by

vertical lines. 

Discussion 

These calculations indicated that for an efficient scavenging cycle, the bond energy between a

scavenger and a flame radical must lie in the range of 70 kcal/mol to 100 kcal/mol, the optim

et recombination react with the

gh to be therm

but not so strong as to prevent regeneration of the active scavenging radica

cle (in terms of the inhibition parameter ), the more sensitive it is to 

rgies.

Element Net Cycle 

Inhibition 

Parameter175 ∆D(kcal/mol)

a Dmax(kcal/mol)

b ∆D/Dmax

Br (0.5%) H + OH 6.9 39 77 0.51 

Br (0.5%) H + H 11.5 33 77 0.43 

Na (0.2%) H + OH 46 29 85 0.34 

Fe (0.05%) O + O 66 14 99 0.14 a

 Range of bond energies for which the suppression index is at least 75 % of its maximum value. b

 Bond energy at which the maximum value of the suppression index is achieved. 

Other factors also play a role in suppression efficiency, although less definitive than does the bond

energy. In most catalytic cycles, there is likely to be at least one recombination step, e.g., Na + OH =

> NaOH, Fe + O2 = > FeO2. In many cases, the recombination reaction may be the rate limiting step in 

the catalytic cycle. This recombination step must have a faster rate than the direct radical-radical

recombination for the catalyst to cause inhibition. In general, the recombination complex is more likely

to be stabilized by third body colliders if it is polyatomic, rather than diatomic. Also, if the recombination 

reaction involves a stable, rather than a radical flame species, it is likely to proceed faster due to the

higher reactant concentrations. This is the case for the Fe + O2 recombination step. 
Comparative Flame Inhibition Mechanisms of Candidate Moieties 307

Many tran than one 

stable valence. This did not appear to be a fundamental requirem t for efficient suppre n, since alkali

metals only exhibit one t are efficient suppressants. The existence of mu

i ies, increasing the kelihood that a efficient catalytic c cle may

o itate recombina tions. reactant and product of a 

recom spon ed to two stable valences of the inhibitor element, then the kinetics of 

the reaction might be m le.

3.5.5 

I tro

U wa rably in the spatial distributions and peak values of their tem res.

H perature sensitivity of chemical inhibition can aid in the search for halon

a ern mes 4,39,150,332,346,355

 are a useful medium to resolve any perature effects.

T een divided into various zones or regions, and prior to the NGP there was

so in these zones.356- 9

A most sensitive to chemical or thermal inhibition has been an active area

o fo many years.4,26,38,39,41- 346,360 veral investigations have 

i region of action for halogenated agents is the low-tem ure reg of the

flame. rthwhile to examine this claim termine if it applied either to CF3Br or 

umerical simulations of flame inhibition chemistry using complex kinetics and model inhibitors would

Numerical Technique 

Perturbation Mechanics 

CO2 formation reactions), or a portion of a

chemical mechanism (e.g., reactions involving Br-containing species). 

sition metals whose compounds showed good suppression properties exhibit more

en ssio

valence ye ltiple valences greatly

ncreases the number of possible spec li n y

ccur. Multiple valences may also facil tion reac If the

bination reaction corre d

ore favorab

Temperature Regions of Optimal Chemical Inhibition of Premixed Flames 

n duction

n nted fires vary conside peratu

ence, understanding the tem

lt atives. Premixed fla tem

hese flames have commonly b

me knowledge of the flame chemistry 35

nalysis of the region of a flame 

f flame inhibition research r 43,150,230,304, Se

ndicated that the important perat ion

360 It was deemed wo and de

to other chemicals. 

N

allow simulations of inhibitors which had not yet been identified or synthesized. Linteris and co-workers 

perturbed the reaction rates of selected chemical reactions in a region that was defined by temperature,

species concentration, time, and space.361 Using model inhibitors introduced at concentrations below

those typically used to suppress fires, they calculated the magnitude of the burning velocity reduction for 

two types of perturbations: impulsive or stepwise. The perturbations were induced by addition of CF3Br, 

addition of a perfect inhibitor, variation of the rate of chain branching, or variation of the heat release rate

from CO oxidation. 

The reacting system was perturbed by modifying selected reaction rates in a region or regions, also called 

bands. In some cases, reactions were turned “on” in the band, and in others, they were turned “off” in the 

band. In this study, a band was defined by a temperature range. The perturbations were designed to limit 

the chemical action of an inhibitor in one or more of these bands. A perturbation could affect a single

reaction (e.g., H+O2↔OH+O), a class of reactions (e.g.,
308 Flame Suppression Chemistry

A perturbation factor, Bi, was defined for the selected group of reactions as, e.g.: 

 Bi = 1 T < Tb 

0 < Bi < 1 Tb ≤ T ≤ (Tb + ∆) 

 Bi = 1 T > (Tb + ∆) 

Figure 3-120b. For modeling of activation or

Figure 3-120. Shapes of the 

where T is the gas temperature, Tb is the temperature at the low edge of the band, and ∆ is the width of the

band. Figure 3-120a illustrates the band shape for this example. The reaction rates of the selected 

reactions were multiplied by Bi, and the rest of the reactions were not directly disturbed (i.e., Bi=1). To 

study different types of perturbations, the band depth (the magnitude of Bi), width (∆), and shape (the 

variation of Bi with T) were varied. 

For the study of flame inhibition, the pertinent reactions were turned off outside the band (Bi=0) and 

turned on inside the band (Bi=1), as illustrated in

deactivation of an inhibitor, a forward-facing or backward-facing step function was used; the inhibition 

reactions were turned off (or on) below a certain temperature and turned on (or off) above that 

temperature, as shown in Figure 3-120c and d. 

Reaction Rate Profile Bi Used to 

Describe: a) Perturbation of 

Chain Branching and Heat 

Release Rates, b) Addition of 

Perfect Inhibitor or CF3Br, c) 

Rapid Inhibitor Activation, and 

d) Rapid Inhibitor Deactivation. 

The band was smoothed at the edges to avoid numerical convergence problems caused by infinite 

gradients. The smoothing function was Bi = a - b erf(-c T*

 + d), where T* = (T - Tb) / ∆ at the low

temperature edge and T*

 = (Tb + ∆ - T) / ∆ at the high temperature edge. The constants a, b, c, and d were

selected such that the width of the smoothed portion of the band was approximately 0.1⋅∆ at each end. 

Bi

∆

1

Temperature

0

Tb

Temperature

Bi

∆

1

0

Tb

b.) a.)

Temperature

Bi

0

1

Tb

Temperature

Bi

0

1

Tb

c.) d.)
Comparative Flame Inhibition Mechanisms of Candidate Moieties 309

Flame Modeling Approach 

The Sandia flame code Premix203, with the kinetic204

 and transport205 subroutines, was used to simulate

the freely-propagating premixed flame. The CHEMKIN subroutine CKRAT was modified to

accommodate the chemical behavior of the model inhibitors. Part of the post-processing was performed

using a graphical postprocessor.362 For all calculations, the initial reactant temperature was 300 K and the 

pressure was 0.10133 MPa. The “GRAD” parameter was set to 0.15, and “CURV” was set to 0.35 for all

of the calculations except those involving CF3Br, where higher values of GRAD (0.35) and CURV (0.55) 

were used to reduce the calculation time. The former values of GRAD and CURV yielded 90 to 125 

active grid points in the calculation, and the latter, 65 grid points. As discussed in Reference 57, the 

% relative to a calculation with a 

(computationally more intensive) infinite number of grid points. 

The kinetic and thermodynamic data of GRI-MECH 1.2334

 (32 species and 177 

as a basis for describing the methane combustion, except when the

mechanism of Babushok et al.52 described the C1/C2 hydrocarbon

(70 species and 595 reactions). Reference 52 also provided ex r

mechanism. 

The perfect inhibitor model

167

 represented an upper limit to the 

model assumed that the inhibitor-containing species In and InX (i , O, or 

OH), reacted with radicals at gas-kinetic rates. All reactions were 

and temperature exponent of 0, and a pre-exponential factor 

properties of Ar were used for the species In and InX. The perfec re present at low

concentrations, so their contribution to the energy balance was sm ns for these

species were listed as forward and reverse rates, the choice of therm namic properties for these species

did not affect their reaction rates. The perfect inhibitor reactions167 were combined with those of GRIMECH 1.2. Since the perfect inhibitor mechanism was a theoretical construct, it could not be 

O2↔OH+O (referred to as “H+O2” below), 

because it provided a majority of the chain branching at high temperature. 

by CO+OH↔CO2+H (referred to as “CO+OH” below), 

sistently showed that the location of maximum 

inhibition was independent of the width of the band. The minimum burning velocity decreased as the

calculated burning velocity was estimated to be reduced about 6

chemical reactions) served

inhibitor was CF3Br. In that case, the

chemistry and the halogen chemistry

perimental verification of the CF3B

catalytic action of an inhibitor. The

n which X is a flame radical H

bimolecular, with an activation energy

of 1 x 1014 cm3

/mol-s; the transport

t inhibitor species we

all. Also, since the reactio

ody

experimentally verified. However, it had been compared with the experimental results for inhibition by 

the very powerful flame inhibitor Fe(CO)5, and found to give burning velocity reductions reasonably

close to that agent.167

A stoichiometric, freely-propagating premixed CH4-air flame was perturbed in temperature bands of

∆ = 300 K. The following four types of perturbation were used: 

• Reduction of the rate of chain branching by H+

• Reduction of the rate of heat release 

because of its importance in determination of the final flame temperature. 

• Catalytic scavenging of radicals (O, OH, and H) by a perfect inhibitor,167

 to investigate the 

maximum inhibition effect. 

• Inhibition by CF3Br, because it was a benchmark in flame inhibition studies. 

The band width of 300 K was selected after conducting extensive numerical experiments with different

band widths ranging from 50 K to 2000 K. The results con
310 Flame Suppression Chemistry

band became wider, but the band-center temperature of minimum

constant for the perturbation considered. The choice of a 300 K 

between having too little effect (a 100 K band only decreased burning velocit

wide to provide insight into the inhibition mechanisms. Selected input

are listed in Table 3-22. 

Table 3-22. Input Parameters and Calculated Temperatu

 burning velocity remained nearly

bandwidth was made as a compromise 

y by 15 %) and being too 

 parameters and calculated results 

res and Flame Speeds.

Perturbation 

 

φ

Bi

Inside 

Band 

Bi

Outside 

Band 

Inhibitor 

Volume

Fraction 

Tmax

(K) 

SL,0 a

(cm/s) 

SL,all b

(cm/s) 

SL,min 

c

(cm/s) 

Tc for 

SL,min

(K) 

H+O2 0.7 0.45 1.0 n.a. 1850 20.7 n.c. 15.5 1550

H+O2 1.0 0.45 1.0 n.a. 2235 41.2 n.c. 31.0 1800

H+O2 1.3 0.45 1.0 n.a. 2056 25.2 n.c. 18.0 1850

CO+OH 1.0 0.1 1.0 n.a. 2235 41.2 n.c. 29.1 1850

Perfect 0.7 1.0 0.0 2.0⋅10-5 1850 20.7 13.8 16.2 1650

Perfect 1.0 1.0 0.0 4.0⋅10-5 2235 41.2 27 30.9 1950

Perfect 1.3 1.0 0.0 2.6⋅10-5 2056 25.2 16.8 18.3 2000

CF3Br 1.0 1.0 10-4 0.00755 2235 41.2 d 26.4 29.2 1750 a

 Flame speed of the unperturbed or uninhibited flame.

roughout flame.

c Minimum flame speed with perturbation in a bandwidth of 300 K.

For the perfect inhibitor, the smoothing function constants are a=0.5, b=0.5, c=60, d=57.

ns involving the perfect inhibitor were turned off outside the band

(B = 0), and turned on inside the band (B =1). The volume fraction of the perfect inhibitor in the unburned 

or inhibition by CF3Br, the reactant stream consisted of a stoichiometric mixture of methane and air a

olume fraction of added CF3Br of 0.0755, resulting in a one-third reduction in flame speed (to 26.4 cm/s)

when the inhibitor was active throughout the flame. Ideally, when simulating the model inhibitors, the

reactions involving Br- and F-containing species would be turned on (Bi = 1) in the band and turned off 

outside the band (Bi = 0). Because of convergence problems compounded by the size of the mechanism, a

very small value of Bi (= 10-4) for the halogen chemistry was used outside the band. 

b Flame speed with perturbation th

d

 The flame speed is 39.0 cm/s when the halogen chemistry is turned off throughout the flame 

 because of the physical effects of CF3Br. 

n.a. = not applicable. 

n.c. = not calculated. 

For H+O2 and CO+OH, the smoothing function constants are a=0.55, b=0.45, c=60, d=57. 

For the H+O2 and CO+OH perturbation, the reaction of interest was unaffected outside the band (Bi = 1)

and damped inside the band (Bi = 0.45 for H+O2, Bi = 0.1 for CO+OH). Thus, the reduction in heat

release for the CO+OH reaction was achieved through a reduction in the rate of this reaction within the 

band. For the perfect inhibitor, reactio

i i

gas was 4.0⋅10-5, which resulted in a flame speed (SL) of 27 cm/s when the inhibitor was active 

throughout the flame. (For comparison, SL=41 cm/s for the uninhibited flame.) 

F

v
Comparative Flame Inhibition Mechanisms of Candidate Moieties 311

Results and Discussion 

Figure 3-121 shows the response of the flame to each of the fou

0

10

20

00 1500 00 2300

Flame Speed (cm/s)

0.00

0.01

Volume Fraction

CO+OH

O

30

40

0.02

Perfect

H+O2

300 7 1100 19

Temperature at Center of Band, Tc (K) 

O

H

H

CF3Br

CF3Br baseline

r

fractions of OH, H, and O (right axis) are shown for an uninhib

The minimum flame speed occurred at a band-center temperature

H+O2, 1850 K for CO+OH, and 1950 K for the perfect inhibitor

possible to infer that the burning velocity of the flames would

temperatures: for H+O2 and CO+OH, these temperatures correspon

affected reaction (as shown in Figure 3-122). Similarly, for the pe

the temperature corresponding to the peak volume fractions of O, H ).

In this figure, 

• “CF3Br baseline” refers to the flame speed when the haloge mped by 10-4. 

• The band width ∆ is 300 K, so the band extends 150 K below and above the temperature shown 

on the x-axis.

• The kinks in some of the curves in Figure 3-121, as well as in Figure 3-123 and Figure 3-125,

were caused by a somewhat coarse progression of calculated band-center temperatures, and

would not be present for a finer series of calculations. 

duction in flame speed for H+O2 and CO+OH can only be compared qualitatively to results 

for the perfect inhibitor and CF Br, because of the different value of B in the bands. 

The calculated volume 

fraction of OH, H, and O 

or 

stoichiometric methaneai

perturbations. The calculated volume 

ited stoichiometric methane-air flame.

(Tc) of 1750 K for CF3Br, 1800 K for

. Using reaction flux analyses, it was

be most strongly perturbed at these

d to the maximum reaction rate of the

rfect inhibitor, the temperature is near

, and OH (as shown in Figure 3-121

n chemistry is da

• The re

3 i

Figure 3-121. Variation of 

Flame Speed for Four 

Types of Perturbation. 

(right axis) is shown f

an uninhibited 

r flame.

The difference in location of peak effectiveness of CF3Br vs. the perfect inhibitor was caused by the shift 

of the partial equilibrium of the scavenging reactions. For CF3Br at higher temperature, the equilibrium 

for the inhibition reaction H+HBr ↔ H2+Br shifted to the left,41 reducing the effectiveness. 
312 Flame Suppression Chemistry

Figure 3-122. Net H-atom

Reaction Rate for Three 

Important Reactions in 

Stoichiometric Methaneair Flames. Solid lines: 

dashed lines: CF3Brinhibited flame (agent 

volume fraction of 

These results show that, although some of the inhibition effect of HBr and CF Br occurred upstream of

methane-oxygen flames inhibited by CH3Br and HBr. Based on flame structure

ylene-air with 1 % CF3Br) were somewhat different. 

ect of chain branching perturbation (∆ = 300 K) on normalized flame speed for 

three equivalence ratios: lean (φ = 0.7), stoichiometric (φ = 1.0), and rich (φ = 1.3). Table 3-22 lists 

eratures and flame speeds for uninhibited flames at each equivalence ratio. The

uninhibited flame; 

0.00755). 

3

the locations of the maximum rates for the chain branching and heat release reactions, the most sensitive 

temperature region (1600 K to 1900 K) was higher than had been found in prior work. In studies of 

inhibition of premixed flames by CH3Br, CF3Br and HBr, it had been argued that the primary inhibition

occurred upstream of the main reaction zone. Wilson4

 and Wilson et al.39 investigated inhibition of low 

pressure (5.1 kPa)

measurements and calculations, they suggested that inhibition occurred through reduction of radical 

generation in the “pre-ignition” part of the flame. In a numerical modeling study, Westbrook332 argued 

that HBr had its maximum effect in a temperature range between about 1000 K and 1400 K, where the 

reaction of H atom with HBr competed with its reaction with CH4 or O2. Williams and Fleming 26 also 

described CF3Br as reducing H-atom concentrations significantly in the lower temperature regions of the

flame (< 1200 K); nonetheless, their data showed that the largest absolute reduction in the H-atom volume 

fraction occurred near 1700 K. Casias and McKinnon346

 also contended that reactions in the low

temperature region were of highest importance for inhibition by CF3Br, although the conditions of their

calculated flames (eth

Figure 3-123 shows the eff

calculated maximum temp

normalized flame speed was defined as the ratio of the flame speed of a band-inhibited flame to the flame

speed of the corresponding uninhibited flame. The temperature of minimum flame speed of the different

flames followed the relative position peak of the H-atom volume fraction (Figure 3-124a). The

temperature range of maximum influence varied with φ, however, because the final temperature of the

flame changed with φ (Table 3-22). 

-1600

300 700 1100 1500 1900 2300

Temperature (K)

-1200

-80

H + O2 = OH + O

0

H + HBr = H2 + Br

-400

0

400

800

Net H-atom Reaction Rate 

(volume fraction per second)

CO + OH = CO2 + H
Comparative Flame Inhibition Mechanisms of Candidate Moieties 313

1.0

Figure 3-123 Effect of 

damping the rate of the 

Figure 3-124. Calculated Volume 

Fraction Profiles for H, OH and O in

Uninhibited Methane-air Flames for 

Three Stoichiometric Ratios. 

0.2

0.4

Nor

malized Flame Speed

0.6

0.8

1.0

φ = 0.7

H+O2 reaction on 

normalized flame speed 

for φ=0.7, 1.0, and 1.3. 

0.0

300 700 1100 1500 1900 2300

Temperature at Center of Band (K), Tc (= Tb+∆/2)

1.3

0.000

0.006

0.008

300 700 1100 1500 1900 2300

Temperature (K)

me Fraction

c.) 

0.000

0.002

0.004

0.006

0.008

300 700 1100 1500 1900 2300

H-Atom Volume Fraction

1.0

φ = 0.7

1.3

a.) 

0.000

0.002

0.004

0.006

0.008

300 700 1100 1500 1900 2300

OH Volu

me Fraction

b.) 1.0

Ξ⏐= 0.7

1.3

0.002

0.004

O-Ato

m V

olu

1.0

φ = 0.7

1.3
314 Flame Suppression Chemistry

The flame speeds for stoichiometric, rich, and lean flames inhibited by the perfect inhibitor are shown in 

Figure 3-125. The initial volume fraction of perfect inhibitor used for the calculations was the amount 

me domain. The equivalent results for 

stoichiometric and rich flames were 72 % and 82 %.

Figure 3-125. Effect of the 

t calculations for the perfect inhibitor was that perturbation near the region of high 

ame speed. In other words, re

volume fraction and high temperature.

The s

certain r

A s i

that cou via decomposition or 

eva a

which d e scavenging species into the gas phase. To simulate such a

system, ion in the previous 

section). The shape of the i perature,” 

needed for a one-third reduction of flame speed (with the inhibiting reactions active throughout the flame) 

from the uninhibited condition (see Table 3-22). For band inhibition at the three values of φ, the perfect

inhibitor showed flame speed reductions qualitatively similar to the results for H+O2 suppression (as 

indicated in Table 3-22): the temperature range for maximum effect was lowest for the lean flame, and 

was higher for stoichiometric and rich flames, which were nearly the same. The ordering followed that of 

the peak radical volume fractions that are shown in Figure 3-124a-c. The calculations generally

demonstrated that inhibition in only a small portion of the flame was responsible for most of the 

inhibitory effect. For example, as shown in Figure 3-125, when the inhibiting reactions in the lean flame

were turned on between 1500 K and 1800 K (Tc = 1650 K), the reduction in flame speed was 65 % as 

much as when the inhibiting reactions occurred over the entire fla

Location of Perfect 

Inhibition on Normalized 

Flame Speed for φ=0.7, 

1.0, and 1.3. The 

horizontal line marks the 

normalized flame speed 

when the flames are 

inhibited throughout the 

flame. 

A general result of he

radical volume fraction had the strongest effect on fl moving H atoms from 

the high temperature region before they could diffuse upstream had the strongest effect on flame speed

because inhibition reactions are faster in regions of high radical

re ults for CF3Br however, showed that the shift of the partial equilibrium at high temperature for

eactions could influence the region of maximum influence of the agent. 

pec al, pertinent case evaluated by Linteris and co-workers was that of a potential new fire suppressant 

ld be inert at storage temperature and release an effective fire suppressant

por tion when added to a fire.85 This would be analogous to the commonly used powder NaHCO3, 

ecomposes to release the activ

they used a step function perturbation (as opposed to the impulse funct

B function is illustrated in Figure 3-120c. Below the “activation tem

the inhibition reactions were turned off, and above the activation temperature, the inhibition reactions 

were turned on. For ease of calculation, they used the perfect inhibitor model (with inhibitor volume

fraction of 4.0⋅10-5). 

0.0

0.2

0.4

0.6

300 700 1100 1500 1900 2300

 Temperature at Center of Band (K), Tc

Normalized Flame Speed 0.8

1.0

1.0

φ

1.3

= 0.7
Comparative Flame Inhibition Mechanisms of Candidate Moieties 315

The curve labeled “activation” in Figure 3-126 shows the flame speed resulting from inhibitor activation

at different temperatures between 300 K and 2200 K. The flame speed was fairly constant up to about

1700 K, but then started to increase. The location of the increase coincided with the peak radical 

concentration. As the activation occurred later in the flame, the inhibitor had less time to scavenge 

radicals. 

Figure 3-126. Variation of 

wis ibito

ati

tiv for

hio ic M eixt Also n

on H, H O 

n un ited

hio ic m eA in temperature and lost its

in d by Fe(CO)5 inhibition, in which the active gas phase species were

lo densation to particles at some point in the flame. Since the particle formation was 

related to the residence ti peratures. The stepwise

d 120d) was a mirror image of 

h tion reactions were turned off, 

g upon the kinetic 

mechanism of the particular inhibitor. 

Flame Speed for 

Step

Activ

e Inh

on or

r

Deac ation a

Stoic metr ethan

air M ure. show

are calculated volume 

fracti s of O , and

in a inhib

stoic metr ethan

air flame. 0

10

Fl 20

S

m30

40

1900 2300

emperature (K)

a

p

c /s)

de ctiva

atio

on

m

e

e

e

d (

0.00

0.01

Volume Fraction

a tion at Tb

activ n at Tb

off

on off

O

H

O

H

300 700 1100 1500

Activation or Deactivation T

second special case was that of an active inhibitor that decomposed at a certa

hibition power. This was exemplifie

st because of con

me, the deactivation might occur over a range of tem

eactivation was simulated with a band in which the Bi function (Figure 3-

t e previous example. Above the “deactivation” temperature, the inhibi

and below the deactivation temperature, the inhibition reactions were turned on. As in the stepwise

activation calculations, the effect of the inhibitor was more or less constant until about 1700 K. The very

small reduction in flame speed when the inhibitor is active between 300 K and 1700 K reaffirmed the 

importance of the high temperature reactions. The most important region for the perfect inhibitor was 

above 1700 K for a strong effect on the flame speed; hence, particle formation for Fe(CO)5 inhibition

must be retarded until after the active species have reached the region of about 1700 K. 

Conclusions 

Numerical simulations of perturbed, premixed CH4-air flames using model inhibitors with reaction rates 

spatially varied in the flame demonstrated the following: 

• For agents which catalytically recombine radicals, the flame speed was most reduced when the

inhibitor was allowed to act near (a) the region of maximum radical volume fraction or (b) the 

region of maximum rates of the radical scavenging reactions, dependin
316 Flame Suppression Chemistry

flames, the temperature of maximum influence was near 1750 K,

significantly higher than previously suggested. The perturbations had a negligible effect in the 

ometric, or lean. 

en the

ess until about 2150 K if they are to retain their maximum potential effectiveness. 

It is imp

3.6

Flame inhibition research h

d a unified view of chemical flame 

s

concentrations of flame propagating radicals (H, O, and O

concentrations. This can result from (a) reaction of the inhibitor he

radicals, forming stable product species which are relatively r

(b) catalytic radical recombination, in which intermediate speci r

breakdown products form more stable molecules from the radicals ytic cycle. 

• An effective catalytic inhibitor must: 

o Break down in the flame with a relatively low activation energy so that the proper 

intermediates for the catalytic cycle can be present with sufficient time to interact

with the radical pool. NGP research has shown that alternate forms of parent 

molecules can be used as long as they readily br kdown and form the same 

intermediate species. 

o Form intermediates that regenerate themselves in the catalytic cycle and undergo

radical volume fractions). This is generally in the temperature regime between

1700 K and 2100 K. 

• Fuels with larger radical pools will be less affected by addition of a catalytic inhibitor. 

• For CF3Br-inhibited

region of the flame temperature below 1200 K. 

• Inhibition in a relatively small portion of the flame caused a significant reduction in flame speed. 

In some cases, inhibition in a band spanning only 300 K caused nearly as much reduction in 

flame speed as when the inhibiting reactions were turned on everywhere. 

• Perturbation of the H+O2↔OH+O reaction rate or addition of the perfect inhibitor had a

maximum burning velocity reduction in a temperature region which followed the relative position 

of the peak H-atom volume fraction. This was independent of whether the flame was rich, 

stoichi

• The effect of sudden activation and deactivation of the perfect inhibitor was small wh

activation or deactivation temperature was above or below 1700 K, respectively. Hence, new 

inhibitors that may be developed need not become active until near 1700 K, and must not lose 

their effectiven

ortant to note that these conclusions might be different for other fuels, inhibitors, or flame types. 

INSIGHTS ON FLAME INHIBITION 

has been conducted for more than 50 years, providing a technical base on whic

the NGP was able to build. As a result, the NGP has generate

inhibition. 

• Efficient chemical inhibition and suppression of flames re ults from reduction of the

H) toward their equilibrium 

breakdown products with t

 inert in the flame, and/o

es formed from the inhibito

in a gas phase catal

ea

reactions that are nearly thermally neutral and fast. 

o Break down to intermediate species which can then make it to the right part of the 

flame, which is the location of the peak chain branching reactions (and hence, peak
Insights on Flame Inhibition 317

• Flow-field effects can limit effectiveness, for example throug

phase, followed by entrainment or thermophoresis which limit th

pool location. 

• Thermodynamic constraints can keep the active moiety from form

h the formation of a condensed 

e agent’s transport to the radical

at the radical pool location by: 

o Driving the active moiety into less active chemical form

the distribution of species in the catalytic cycle. 

o Causing the intermediates to form a less active co liquid 

NaOH, or particles of metal oxides. 

o Forming non-active stable species, such as the convers

NGP research has also developed solid understanding of the flame quenching activity of chemicals built 

around certain highly effective atoms. 

• Extinction measurement and OH reduction were equivalent marker of agent effectiveness.

• Compounds of iron, manganese, chromium, antimony, lead, and tin were very effective at

ant uncertainties in several of 

ing the necessary intermediates

s, or creating an imbalance in

ndensed form, such as

ion of Fe to iron fluorides.

s

reducing the rates of the chemical reactions that propagate flames. 

• Additional metals whose compounds are likely to be effective at low concentrations, but which 

had not been suggested in previous reviews of metal flame inhibition, are tungsten, molybdenum,

and cobalt. 

• Phosphorus-containing compounds can be 40 times more effective than nitrogen and 2 to

4 times more effective than CF3Br at quenching non-premixed hydrocarbon flames. 

• DMMP and TMP were roughly equivalent in suppression efficiency, independent of the

differences in their ligands. PN was decidedly less effective, likely due to the resonant bonding 

of the phosphorus atoms in the parent molecule. 

• Catalytic inhibitors in general have a strong temperature dependence, with lower effectiveness at 

higher adiabatic flame temperatures. 

• Current computational models adequately (but somewhat under-) predicted the effect of DMMP 

on flame extinguishment. This may have been due to the signific

the reaction rate coefficients in the phosphorus mechanism. 
318 Flame Suppression Chemistry

3.7 REFERENCES 

1. Battin-Leclerc, F., Walravens, B., Come, G.M., Baronnet, F., Sanogo, O., Delf

“The Chemical Inhibiting Effect of Some Fl

Substitutes for Halons,” in Miziolek, A.W. and Tsang, W., eds., Halon

au, J.L., and Vovelle, C., 

uorocarbons and Hydrofluorocarbons Proposed As 

R and

Science ACS Symposium Series 611, American Chemical Society, Washin

2. Li, K., Kennedy, E.M., Moghtaderi, B., and Dlugogorski, B.Z., “Exp l 

Studies on the Gas-Phase Reaction of CF3Br With Hydrogen,” Environmen , 

584-590 (2000). 

3. Simmons, R.F. and Wolfhard, H.G., “The Influence of Methyl Bromide on Flames, Part 1.-Pre-Mixed 

Flames ,” Combustion and Flame 1, 155-161 (1957). 

4. Wilson, W.E., “Structure, Kinetics, and Mechanism of a Methane-Oxygen Flame Inhibited With

Methyl Bromide ,” Proceedings of the Combustion Institute 10, 47-54 (1965). 

5. Biordi, J.C., Lazzara, C.P., and Papp, J.F., “Flame-Structure Studies of CF3Br-Inhibited Methane 

Flames,” Proceedings of the Combustion Institute 14, 367-381 (1973). 

6. Hayes, K.F. and Kaskan, W.E., “Inhibition by CH3Br of CH4-Air Flames Stabilized on a Porous

urner,” Combustion and Flame 24, 405-407 (1975).

7. Safieh, H.Y., Vandooren, J., and Van Tiggelen, P.J., “Experimental Study of Inhibition Induced by

CF3Br in a CO-H2-O2-Ar Flame,” Proceedings of the Combustion Institute 19, 117-126 (1982). 

8. Westbrook, C.K., “Inhibition of Hydrocarbon Oxidation in Laminar Flames and Detonations by 

Halogenated Compounds,” Proceedings of the Combustion Institute 19, 127-141 (1982). 

9. Reynolds, W.C., “The Element Potential Method for Chemical Equilibrium Analysis: Implementation 

in the Interactive Program STANJAN,” ME 270 HO no 7, Stanford University , Stanford, CA, (1986). 

10. Sanogo, O., Delfau, J.L., Akrich, R., and Vovelle, C., “A Comparative Study of the Structure of

CF3Br- and CF3I-Doped Methane Flames,” Journal De Chimie Physique Et De Physico-Chimie 

Biologique 93, 1939-1957 (1996). 

11. Kim, C.H., Kwon, O.C., and Faeth, G.M., “Effects of Halons and Halon Replacements on Hydrogeninar Premixed Flames,” Journal of Propulsion and Power 18, 1059-1067 (2002).

6).

13. Sheinson, R.S., Penner-Hahn, J.E., and Indritz, D., “The Physical and Chemical Action of Fire 

Suppressants,” Fire Safety Journal 15, 437-450 (1989). 

14. Lott, J.L., Christian, S.D., Sliepcevich, C.M., and Tuc nd 

Physical Fire-Suppressant Agents,” Fire Technology 32, 26

15. Friedman, R. and Levy, J.B., “Inhibition of Opposed-Jet Methane-Air Diffusion Flame. The Effects

of Alkali Metal Vapours and Organic Halides,” Combustion and Flame 7, 195 (1963). 

16. Milne, T.A., Green, C.L., and Benson, D.K., “The Use of the Counter-Flow Diffusion Flame in 

Studies of Inhibition Effectiveness of Gaseous and Powdered Agents,” Combustion and Flame 15, 255-

264 (1970). 

17. Kent, J.H. and Williams, F.A., “Effect of CF3Br on Stagnation-Point Combustion of a Heptane Pool,” 

Proceedings of the Western States Section Meeting, pp. 73-24, Combustion Institute, Pittsburgh, PA,

1973. 

eplacements: Technology

gton, D.C., 1995. 

erimental and Computationa

tal Science & Technology 34

B

Fueled Lam

12. Simmons, R.F. and Wolfhard, H.G., “The Influence of Methyl Bromide on Flames, Part 2.-Diffusion

Flames,” Transactions of the Faraday Society 52, 53-59 (195

ker, E.E., “Synergism Between Chemical a

0-271 (1996). 
References 319

18. Seshadri, K. and Williams, F.A., “Effect of CF3Br on Counterflow Combustion of Liquid Fuel With

Diluted Oxygen,” in Gann, R.G., ed., Halogenated Fire Suppressants ACS Symposium Series 16, 

American Chemical Society, Washington, D.C., 1975. 

19. Marolewski, T.A. and Weil, E.D., “A Review of Phosphate Ester Fire Resistance Mechanisms and

Their Relevance to Fluid Testing, Vol. ASTM STP 1284,” in Totten, G.E. and Jürgen, R., Eds., Fire 

Resistance of Industrial Fluids American Society for Testing and Materials, Philadelphia, 1996. 

20. Babushok, V., Noto, T., Burgess, D.R.F., Hamins, A., and Tsang, W., “Influence of CF3I, CF3Br, and 

CF3H on the High-Temperature Combustion of Methane,” Combustion and Flame 107, 351-367 (1996). 

21. Grosshandler, W.L., Gann, R.G., and Pitts, W.M., “Evaluation of Alternative In-Flight Fire

Suppressants for Full-Scale Testing in Simulated Aircraft Engine Nacelles and Dry Bays,” NIST SP 861,

National Institute of Standards and Technology, Gaithersburg, MD, (1994). 

gents in Aircraft Engines and

nal Institute of Standards and

22. Gann, R.G., “Fire Suppression System Performance of Alternative A

Dry Bay Laboratory Simulations,” NIST SP 890, vols. I and II, Natio

eration Fire Suppression Technology Program 

y, Gaithersburg, MD, 

K the 

Combustion Institute 29, 337-344 (2003). 

25. Vora, N. and Laurendeau, N.M., “Analysis of CF3Br Flame Suppressi

Laser-Induced Fluorescence Measurements of the Hydroxyl Radical nd

Technology 166, 15-39 (2001 ). 

26. Williams, B.A. and Fleming, J.W., “CF3Br and Other Suppressants: Differences in Effects on Flame 

Structure,” Proceedings of the Combustion Institute 29, 345-351 (2003). 

27. Bundy, M., Hamins, A., and Lee, K.Y., “Suppression Limits of Lo

Methane Flames,” Combustion and Flame 133, 299-310 (2003). 

28. Williams, F.A., “A Unified View of Fire Suppression,” Journal of Fire and Flammability 5, 54-63

(1974). 

29. Burgoyne, J.H. and Williams-Lier, G., “The Influence of Incombustible Vapours on the Limits of 

Bromide,” Combustion and Flame 13, 472-478 (1969). 

Technology, Gaithersburg MD, (1995). 

23. Gann, R.G., “ FY2003 Annual Report -- Next Gen

(NGP),” NIST Technical Note 1457, National Institute of Standards and Technolog

(2004 ).

24. Saso, Y., “Roles of Inhibitors in Global Gas-Phase Combustion inetics,” Proceedings of

on Activity Using Quantitative

,” Combustion Science a

w Strain Rate Non-Premixed

Inflammability of Gases and Vapours in Air,” Proceedings of the Royal Society A193, 525-539 (1948). 

30. Belles, F.E. and O'Neal, C., Jr., “Effects of Halogenated Extinguishing Agents on Flame Quenching

and a Chemical Interpretation of Their Action,” Proceedings of the Combustion Institute 6, 806-813 

(1957). 

31. Butlin, R.N. and Simmons, R.F., “The Inhibition of Hydrogen-Air Flames by Hydrogen Bromide,” 

Combustion and Flame 12, 447-456 (1968). 

32. Garner, F.H., Long, R., Graham, A.J., and Badakhshan, A., “Effect of Certain Halogenated Methanes 

on Premixed and Diffusion Flames,” Proceedings of the Combustion Institute 6, 802 (1957). 

33. Rosser, W.A., Wise, H., and Miller, J., “Mechanism of Combustion Inhibition by Compounds

Containing Halogen,” Proceedings of the Combustion Institute 7, 175-182 (1959).

34. Lask, G. and Wagner, H.G., “Influence of Additives on the Velocity of Laminar Flames,”

Proceedings of the Combustion Institute 8, 432-438 (1962). 

35. Edmondson, H. and Heap, M.P., “The Burning Velocity of Methane-Air Flames Inhibited by Methyl
320 Flame Suppression Chemistry

36. Niioka, T., Mitani, T., and Takahashi, M., “Experimental Study on Inhibited Diffusion and Premixed

Flames in a Counterflow System,” Combustion and Flame 50, 89-97 (1983). 

n, W.E., O’Donavan, J.T., and Fristrom, R.M., “Flame Inhibition by Halogen Compounds,”

Proceedings of the Combustion Institute 12, 929-942 (1969). 

3Br on a CO-H2-O2-Ar

294 (1975). 

41. Day, M.J., Stamp, D.V., Thompson, K., and Dixon-Lewis, G., “In ir and

Hydrogen-Nitrous Oxide Flames by Halogen Compounds,” Proceedings of the Combustion Institute 13, 

705-712 (1971). 

42. Dixon-Lewis, G. and Simpson, R.J., “Aspects of Flame Inhibitio

Proceedings of the Combustion Institute 16, 1111-1119 (1977). 

43. Westbrook, C.K., “Numerical Modeling of Flame Inhibition by CF3

Technology 34, 201-225 (1983). 

44. Creitz, E.C., “Inhibition of Diffusion Flames by Methyl Bromide e 

Applied to the Fuel and Oxygen Sides of the Reaction Zone,” Journal of Research of the National Bureau

of Standards A. Physics and Chemistry 65A, 389-396 (1961). 

45. Masri, A.R., “Chemical Inhibition of Nonpremixed Flames of Hydrocarbon Fuels With CF3Br,”

Combustion Science and Technology 96, 189-212 (1994). 

51. Smyth, K.C. and Everest, D.A., “The Effect of CF3I Compared to CF3Br on OH Soot Concentrations 

ropane/Air Diffusion Flames,” Proceedings of the Combustion Institute, 1385-1393, The 

” Proceedings of the Combustion Institute 26,

137 3

53. Miz

Chemical Society, Washington, D.C., 1995. 

37. Mitani, T., “Flame-Retardant Effects of CF3Br and NaHCO3,” Combustion and Flame 50, 177-188

(1983). 

38. Levy, A., Droege, J.W., Tighe, J.J., and Foster, J.F., “The Inhibition of Lean Methane Flames,”

Proceedings of the Combustion Institute 8, 524-533 (1960). 

39. Wilso

40. Biordi, J.C., Lazzara, C.P., and Papp, J.F., R.G. Gann, “The Effect of CF

Diffusion Flame,” Halogenated Fire Suppressants, American Chemical Society, Washington, D.C., 256-

hibition of Hydrogen-A

n by Halogen Compounds,”

Br,” Combustion Science and

and Trifluoromethyl Bromid

46. Masri, A.R. and Clarke, T.C., “Lifted Flames and the Effects of Inhibitors,” Combustion Science and

Technology 105, 345-355 (1995). 

47. Ibiricu, M.M. and Gaydon, A.G., “Spectroscopic Studies of the Effect of Inhibitors on Counterflow 

Diffusion Flames,” Combustion and Flame 8, 51-61 (1964). 

48. Kent, J.H. and Williams, F.A., “Extinction of Laminar Diffusion Flames for Liquid Fuels,”

Proceedings of the Combustion Institute 15, 315-325 (1974). 

49. Seshadri, K. and Williams, F.A., “Effect of CF3Br on Counterflow Combustion of Liquid Fuel With

Diluted Oxygen,” in Gann, R.G., ed., Halogenated Fire Suppressants ACS Symposium Series 16,

American Chemical Society, Washington, D.C., 1975. 

50. Hamins, A., Trees, D., Seshadri, K., and Chelliah, H.K., “Extinction of Nonpremixed Flames With

Halogenated Fire Suppressants,” Combustion and Flame 99, 221-230 (1994). 

in Co-Flowing P

Combustion Institute, Pittsburgh, 1996. 

52. Noto, T., Babushok, V., Burgess Jr., D.R., Hamins, A., Tsang, W., and Miziolek, A.W., “Effect of

Halogenated Flame Inhibitors on C1-C2 Organic Flames,

7-1 83 (1996). 

iolek, A.W. and Tsang, W., eds., Halon Replacements, ACS Symposium Series 611, American
References 321

54. We

Chemis he Combustion Institute 25, 1505 (1994). 

55. n

Heptafl

56. Hyn ock-Tube Study of the Pyrolysis of the Halon 

Replace

57. Lin

Combus

58. n

“Inh it

Flame 1

59. San rich, R., and Vovelle, C., “Experimental and Computational Study of the 

Effect of C 25, 

60. V n

of a Sto

61. L'es

Premixed Flames Inhibited by Fluoromethanes,” Combustion and Flame 117, 709-731 (1999).

Nonpremixed Flames by Fluorinated Ethanes and Propanes,” Combustion and Flame 121, 471-487

).

63. Papas, P., Fleming, J.W., and Sheinson, R.S., “Extinction of Non-Premixed Methane- and PropaneM., Gallagher, J., McLinden, M., and Morrison, G., NIST Thermodynamic Properties of

Ref er

Techno

65. Wag

and Nu

Inorgan

Data 1

66. o R., “Critical Concentration Measurements for Flame 

Extinguishment of Diffusion Flames ory “Cup Burner” Apparatus,” ICI Mond Division,

Winnington Lab t

67. Chelliah, H.K.,

Theoretical Investig

Methane-Air-Nitrog

68. Saso, Y., Saito, N., Li “Extinction of Counterflow Diffusion Flames With 

Halon Replacem

69. Tucker, D.M., D of Diffusion Flames Burning in

Various Oxygen o

293-300 (1981).

70. Tanoff, M.A., D

Westmoreland, P.R ated Hydrocarbon Effects on the Extinction Characteristics of 

stmoreland, P.R., Burgess, D.R.F. Jr., Zachariah, M.R., and Tsang, W., “Fluoromethane 

try and Its Role in Flame Suppression,” Proceedings of t

Hy es, R.G., Mackie, J.C., and Masri, A.R., “Inhibition of Premixed Hydrogen-Air Flames by 2-H 

uoropropane,” Combustion and Flame 113, 554-565 (1998).

es, R.G., Mackie, J.C., and Masri, A.R., “Sh

ment Molecule CF3CHFCF3,” Journal of Physical Chemistry a 103, 54-61 (1999). 

teris, G.T. and Truett, L., “Inhibition of Premixed Methane-Air Flames by Fluoromethanes,”

tion and Flame 105, 15-27 (1996 ). 

Li teris, G.T., Burgess, D.R., Babushok, V., Zachariah, M., Tsang, W., and Westmoreland, P.,

ib ion of Premixed Methane-Air Flames by Fluoroethanes and Fluoropropanes,” Combustion and

13, 164-180 (1998). 

ogo, O., Delfau, J.L., Ak

2F6 on Premixed Methane Flames,” Proceedings of the Combustion Institute 1489 (1994). 

a dooren, J., da Cruz, F.N., and Van Tiggelen, P.J., “The Inhibiting Effect of CF3H on the Structure 

ichiometric H2/CO/O2/Ar Flame,” Proceedings of the Combustion Institute 22, 1587-1595 (1988). 

perance, D., Williams, B.A., and Fleming, J.W., “Intermediate Species Profiles in Low Pressure

62. Zegers, E.J.P., Williams, B.A., Fisher, E.M., Fleming, J.W., and Sheinson, R.S., “Suppression of

(2000

Air Counterflow Flames Inhibited With CF4, CF3H, and CF3Br,” Proceedings of the Combustion Institute

26, 1405-1411 (1996). 

64. Huber,

rig ants and Refrigerant Mixtures Database (REFPROP), National Institute of Standards and 

logy, Gaithersburg, MD, 1996. 

man, D.D., Evans, W.H., Parker, V.B., Schumm, R.H., Halow, I., Bailey, S.M., Churney, K.L., 

ttall, R.L., “The NBS Tables of Chemical Thermodynamic Properties: Selected Values for 

ic and C1 and C2 Organic Substances in SI Units.,” Journal of Physical Chemistry Reference

1, 1982). 

B oth, K., Melia, B.J., and Hirst,

Using Laborat

ora ory, (1973). 

Law, C.K., Ueda, T., Smooke, M.D., and Williams, F.A., “An Experimental and 

ation of the Dilution, Pressure and Flow-Field Effects on the Extinction Condition of

en Diffusion Flames,” Proceedings of the Combustion Institute 23, 503-511 (1991). 

ao, C.H., and Ogawa, Y.,

ents,” Fire Safety Journal 26, 303-326 (1996). 

rysdale, D.D., and Rasbash, D.J., “The Extinction

C ncentrations by Inert-Gases and Bromotrifluoromethane,” Combustion and Flame 41, 

obbins, R.R., Smooke, M.D., Burgess, D.R., Jr., Zachariah, M.R., Tsang, W., and 

., “C1 and C2 Fluorin
322 Flame Suppression Chemistry

Met n .C., and

Ren e,

CD

71. Pott Butler, J.N., “A Novel Combustion Measurement Based on the Extinguishment of

Diffusion Flames, can Rocket Society Journal 29, 54-56 (1959). 

72.

Nonadia mes,” Combustion and Flame 105, 308-320 (1996). 

73. Fallon, G.S C

Counterflow CO/A ombustion Institute 26, 1395-1403

(1996). 

74. Balakrishnan, G Experimental Investigation of Strain-Induced

Extinction of Di e 126

(1994). 

76. h ant at

High Reynol

77. l

Northam Extinction of 14 to 100%

Hyd

78. Yan

Hydroc ustion and Flame 92, 187-192 (1993). 

79. c

Flames ). 

80. r

Propane

81. Yan ture of Inhibited Counterflowing Nonpremixed

Flam ,

82. Gro eldorf, C., “Lean Flammability Limit As a Fundamental

Refrigerant Property. Final Technica

Ene ,

83. Bu

Chemic 412, National Institute of Standards 

nd Technology, Gaithersburg, MD, (1995). 

84. Williams, B.A., L'esperance, D.M., and Fleming, J.W., “Intermediate Species Profiles in Lowressure Methane/Oxygen Flames Inhibited by 2-H Heptafluoropropane: Comparison of Experimental 

Data With Kinetic Modeling,” Combustion and Flame 120, 160-172 (2000). 

85. Linteris, G.T. and Chelliah, H.K., “Powder-Matrix Systems for Safer Handling and Storage of

Suppression Agents,” NISTIR 6766, National Institute of Standards and Technology, Gaithersburg, MD, 

(2001). 

86. Linteris, G.T., Rumminger, M.D., Babushok, V., Chelliah, H., Lazzarini, A.K., and Wanigarathne, P.,

“Effective Non-Toxic Metallic Fire Suppressants,” NISTIR 6875, National Institute of Standards and 

Technology, Gaithersburg MD, (2002). 

ha e Vs. Air Counterflow Diffusion Flames,” in Gann, R.G., Burgess, S.R., Whisner, K

ek P.A., eds., Papers From 1991-2006 Halon Options Technical Working Conferences (HOTWC), 

-ROM, NIST SP 984-4, National Institute of Standards and Technology, Gaithersburg, MD, (2006). 

er, A.E., Jr. and

” Ameri

Konnov, A.A., Idir, M., Delfau, J.L., and Vovelle, C., “Experimental Study of Extinction of

batic Counterflow Premixed Fla

., helliah, H.K., and Linteris, G.T., “Chemical Effects of CF3H in Extinguishing

ir/H2 Diffusion Flames,” Proceedings of the C

., Trees, D., and Williams, F.A., “An

lut d Hydrogen-Air Counterflow Diffusion Flames,” Combustion and Flame 98, 123-

75. Rolon, J.C., Veynante, D., Martin, J.P., and Durst, F., “Counter Jet Stagnation Flows,” Experiments in

Fluids 11, 313-324 (1991).

Ses adri, K. and Williams, F.A., “Laminar Flow Between Parallel Plates With Injection of React

ds Number,” Int. J. of Heat and Mass Transfer 21, 137-150 (1978).

Pe lett, G.L., Isaac, K.M., Humphreys, W.M., Gartrell, L.R., Roberts, W.L., Dancey, C.L., and 

, G.B., “Velocity and Thermal Structure, and Strain-Induced

rogen-Air Counterflow Diffusion Flames,” Combustion and Flame 112, 575-592 (1998). 

g, G.S. and Kennedy, I.A., “The Effect of Strain on Laminar Diffusion Flames of Chlorinated

arbons,” Comb

Ma donald, M.A., Jayaweera, T.M., Fisher, E.M., and Gouldin, F.C., “Inhibition of Nonpremixed

by Phosphorus-Containing Compounds,” Combustion and Flame 116, 166-176 (1999

Pu i, I.K. and Seshadri, K., “Extinction of Diffusion Flames Burning Diluted Methane and Diluted 

in Diluted Air ,” Combustion and Flame 65, 137-150 (1986). 

g, M.H., Hamins, A., and Puri, I.K., “The Struc

es ” Combustion and Flame 98, 107-122 (1994).

sshandler, W., Donnelly, M., and Wom

l Report,” Report No. DOE/CE/23810-98, US Department of 

rgy Washington, DC, (1998). 

rgess, D.R., Zachariah, M.R., Tsang, W., and Westmoreland, P.R., “Thermochemical and

al Kinetic Data for Fluorinated Hydrocarbons,” NIST TN 1

a

P
References 323

87. Linteris, G.T., “Limits to the Effectiveness of Metal-Containing Fire Suppressants,” NISTIR 7177, 

National Institute of Standards and Technology, Gaithersburg MD, (2004). 

ir) Flames,”

emical Characteristics of SFE Fire Suppressant Atmospheres: Comparison of Smalland Technology, Gaithersburg, MD, (2006). 

pposed-Flow Propane-Air Flames,” in Gann, R.G., 

ards and

the Context of

.G.W., “Studies of Knock and Antiknock by Kinetic Spectroscopy,”

isopropyl Ether and on the Ignition of

.E., Protheroe, J., and Walsh, A.D., “Effects of 

ger, M.D. and Linteris, G.T., “The Role of Particles in the Inhibition of Premixed Flames by 

 by Iron Pentacarbonyl,” Proceedings of the Combustion Institute 26, 1421-1428 (1996). 

88. Jost, W., Bonne, U., and Wagner, H.G., “Iron Carbonyl Found to Be Powerful Flame Inhibitor,”

Chemical & Engineering News 39, 76 (1961). 

89. Bonne, U., Jost, W., and Wagner, H.G., “Iron Pentacarbonyl in Methane-Oxygen (or A

Fire Research Abstracts and Reviews 4, 6-18 (1962).

90. Kimmel, E.C., Smith, E.A., Reboulet, J.E., Black, B.H., Sheinson, R.S., and Carpenter, R.L.,

“Physical and Ch

With Large-Scale Laboratory Atmospheres,” in Gann, R.G., Burgess, S.R., Whisner, K.C., and Reneke, 

P.A., eds., Papers From 1991-2006 Halon Options Technical Working Conferences (HOTWC), CDROM, NIST SP 984-4, National Institute of Standards

91. Rosser, W.A., Inami, S.H., and Wise, H., “The Effect of Metal Salts on Premixed Hydrocarbon-Air 

Flames,” Combustion and Flame 7, 107-119 (1963). 

92. Vanpee, M. and Shirodkar, P., “A Study of Flame Inhibition by Metal Compounds,” Proceedings of 

the Combustion Institute 17, 787-795 (1979). 

93. Skaggs, R.R., Daniel, R.G., Miziolek, A.W., McNesby, K.L., Babushok, V.I., Tsang, W., and

Smooke, M.D., “Spectroscopic Studies of Inhibited O

Burgess, S.R., Whisner, K.C., and Reneke, P.A., eds., Papers From 1991-2006 Halon Options Technical 

Working Conferences (HOTWC), CD-ROM, NIST SP 984-4, National Institute of Stand

Technology, Gaithersburg, MD, (2006).

94. Kovarik, B., “Charles F. Kettering and the Development of Tetraethyl Lead in

Alternative Fuel Technologies,” Fuels and Lubricants, Society of Automotive Engineers, New York, 

1994. 

95. Walsh, A.D., “The Mode of Action of Tetraethyllead As an Antiknock,” in Six Lectures on the Basic

Combustion Process Ethyl Corp., New York, NY, 1954. 

96. Erhard, K.H.L. and Norrish, R

Proceedings of the Royal Society (London) A 234, 178-191 (1956). 

97. Muraour, H., “La Theorie Des Anti-Detonants,” Chem. et Industr. 14, 1911 (1925). 

98. Chamberlain, G.H.N. and Walsh, A.D., “The Inhibiting Effect of Lead Tetraethyl. 1. The Effect of

Lead Compounds on the Vapour Phase Slow Oxidation of Di

Diethyl Ether.,” Proceedings of the Royal Society A 215, 175-186 (1952). 

99. Cheaney, D.E., Davies, D.A., Davis, A., Hoare, D

Surfaces on Combustion of Methane and Mode of Action of Anti-Knock Containing Metals,”

Proceedings of the Combustion Institute 7, 183-187 (1959). 

100. Richardson, W.L., Ryason, P.R., Kautsky, G.J., and Barusch, M.R., “Organolead Antiknock AgentsTheir Performance and Mode of Action ,” Proceedings of the Combustion Institute 9, 1023-1033 (1962). 

101. Zimpel, C.F. and Graiff, L.B., “An Electron Microscopic Study of Tetraethyllead Decomposition in

an Internal Combustion Engine,” Proceedings of the Combustion Institute 11, 1015-1025 (1967). 

102. Rummin

Iron Pentacarbonyl,” Combustion and Flame 123, 82-94 (2000). 

103. Reinelt, D. and Linteris, G.T., “Experimental Study of the Inhibition of Premixed and Diffusion

Flames
324 Flame Suppression Chemistry

104. Rumminger, M.D., Reinelt, D., Babushok, V.I., and Linteris, G.T., “Numerical Study of the 

Inhibition of Premixed and Diffusion Flames by Iron Pentacarbonyl,” Combustion and Flame 116, 207-

219 (1999). 

105. Rumminger, M.D. and Linteris, G.T., “Numerical Modeling of Counterflow Diffusion Flames 

Inhibited by Iron Pentacarbonyl,” Fire Safety Science: Proceedings of the Sixth International Symposium, 

289-300, Int. Assoc. for Fire Safety Science, Marne-La-Vallee, France, 2000. 

106. Linteris, G.T., Rumminger, M.D., Babushok, V.I., and Tsang, W., “Flame Inhibition by Ferrocene 

and Blends of Inert and Catalytic Agents,” Proceedings of the Combustion Institute 28, 2965-2972

2).

 Combustion and Flame

ngs of the Western States Section Meeting, Paper 030, Combustion Institute, Pittsburgh, PA,

he Effect of Burning Velocity Inhibitors on the Ignition of 

7, 137-142 (1963). 

mminger, M.D. and Linteris, G.T., “Inhibition of Premixed Carbon Monoxide-Hydrogensher, E.M., “Temperature Dependence of Phosphorus-Based

Chemically

(2000). 

107. Linteris, G.T., Knyazev, V.D., and Babushok, V.I., “Inhibition of Premixed Methane Flames by

Manganese and Tin Compounds,” Combustion and Flame 129, 221-238 (200

108. Rumminger, M.D. and Linteris, G.T., “The Role of Particles in the Inhibition of Counterflow

Diffusion Flames by Iron Pentacarbonyl,” Combustion and Flame 128, 145-164 (2002). 

109. Kuppu Rao, V. and Prasad, C.R., “Knock Suppression in Petrol Engines,”

18, 167-172 (1972). 

110. Linteris, G.T. and Rumminger, M.D., “Particle Formation in Laminar Flames Inhibited by Metals,” 

Proceedi

2002. 

111. Morrison, M.E. and Scheller, K., “T

Hydrocarbon-Oxygen-Nitrogen Mixtures,” Combustion and Flame 18, 3-12 (1972). 

112. Miller, D.R., Evers, R.L., and Skinner, G.B., “Effects of Various Inhibitors on Hydrogen-Air Flame 

Speeds,” Combustion and Flame

113. Miller, W.J., “Inhibition of Low Pressure Flames,” Combustion and Flame 13, 210-212 (1969). 

114. Cotton, D.H. and Jenkins, D.R., “Catalysis of Radical-Recombination Reactions in Flames by 

Alkaline Earth Metals,” Transactions of the Faraday Society 67, 730-739 (1971). 

115. Bulewicz, E.M., Padley, P.J., Cotton, D.H., and Jenkins, D.R., “Metal-Additive-Catalysed RadicalRecombination Rates in Flames,” Chemical Physics Letters 9, 467-468 (1971).

116. Bulewicz, E.M. and Padley, P.J., “Catalytic Effect of Metal Additives on Free Radical

Recombination Rates in H2+O2+N2 Flames,” Proceedings of the Combustion Institute 13, 73-80 (1971). 

117. deWitte, M., Vrebosch, J., and van Tiggelen, A., “Inhibition and Extinction of Premixed Flames by

Dust Particles,” Combustion and Flame 9, 257-266 (1964). 

118. Ru

Oxygen-Nitrogen Flames by Iron Pentacarbonyl,” Combustion and Flame 120, 451-464 (2000). 

119. Macdonald, M.A., Gouldin, F.C., and Fi

Flame Inhibition,” Combustion and Flame 124, 668-683 (2001). 

120. MacDonald, M.A., Jayaweera, T.M., Fisher, E.M., and Gouldin, F.C., “Variation of

Active and Inert Flame-Suppression Effectiveness With Stoichiometric Mixture Fraction,” Proceedings of 

the Combustion Institute 27, 2749-2756 (1998). 

121. Babushok, V.I. and Tsang, W., “Inhibitor Rankings for Hydrocarbon Combustion,” Combustion and

Flame 123, 488-506 (2000). 
References 325

122. James, C.G. and Sugden, T.M., “Photometric Investigations of Alkali Metals in Hydrogen Flame

Gases. 3. The Source of the Alkali Metal Continuum,” Proceedings of the Royal Society A 248, 238-247

lewicz, E.M., James, C.G., and Sugden, T.M., “Photometric Investigations of Alkali Metals in 

Transactions of the Faraday Society 52, 1475-1480 (1956). 

me 17, 87-98 (1971). 

Burning of Polymers,” in Wall, L.A., Ed., The Mechanisms of 

d Price, K.R., “Flammability of Epoxy Resins,” Journal of Applied Polymer Science

e Inhibition,” Journal of Research of the National Bureau of 

Standards - A. Physics and Chemistry 77A, 733-754 (1973). 

(1958). 

123. Bu

Hydrogen Flame Gases. 2. The Study of Excess Concentrations of Hydrogen Atoms in Burnt Gas

Mixtures,” Proceedings of the Royal Society A 235, 89-106 (1956). 

124. Bulewicz, E.M. and Sugden, T.M., “Determination of the Dissociation Constants and Heats of

Formation of Molecules by Flame Photometry; Part 3.-Stability of Gaseous Cuprous Hydroxide,”

Transactions of the Faraday Society 52, 1481-1488 (1956). 

125. Bulewicz, E.M. and Sugden, T.M., “Determination of the Dissociation Constants and Heats of

Formation of Molecules by Flame Photometry; Part 2.-Heat of Formation of Gaseous Cuprous

Hydroxide,”

126. Bulewicz, E.M. and Padley, P.J., “Photometric Investigations of the Behavior of Chromium

Additives in Premixed H2+O2+N2 Flames,” Proceedings of the Royal Society London A. 323, 377-400 

(1971). 

127. Howard, J.B. and Kausch, W.J., “Soot Control by Fuel Additives,” Progress in Energy and

Combustion Science 6, 263-276 (1980).

128. Cotton, D.H., Friswell, N.J., and Jenkins, “The Suppression of Soot Emission From Flames by Metal 

Additives.,” Combustion and Fla

129. Bulewicz, E.M., Jones, G., and Padley, P.J., “Temperature of Metal Oxide Particles in Flames,”

Combustion and Flame 13, 409-412 (1969). 

130. Tischer, R.L. and Scheller, K., “The Behavior of Uranium Oxide Particles in Reducing Flames,”

Combustion and Flame 15, 199-202 (1970). 

131. Jensen, D.E. and Jones, G.A., “Mass-Spectrometric Tracer and Photometric Studies of Catalyzed 

Radical Recombination in Flames,” Journal of the Chemical Society-Faraday Transactions I 71, 149-160

(1975). 

132. Jensen, D.E. and Jones, G.A., “Catalysis of Radical Recombination in Flames by Iron,” Journal of

Chemical Physics 60, 3421 (1974). 

133. Jensen, D.E. and Jones, G.A., “Aspects of Flame Chemistry of Cobalt,” Journal of the Chemical

Society-Faraday Transactions I 72, 2618-2630 (1976). 

134. Fenimore, C.P. and Martin, F.J., “Flammability of Polymers,” Combustion and Flame 10, 135-139

(1966). 

135. Fenimore, C.P. and Jones, G.W., “Modes of Inhibiting Polymer Flammability,” Combustion and

Flame 10, 295-301 (1966).

136. Fenimore, C.P. and Martin, F.J., “Candle-Type Test for Flammability of Polymers,” Modern Plastics

12, 141-192 (1966). 

137. Fenimore, C.P. and Martin, F.J., “

Pyrolysis, Oxidation and Burning of Organic Materials National Bureau of Standards, Washington,

D.C., 1972. 

138. Martin, F.J. an

12, 143 (1968). 

139. Hastie, J.W., “Molecular Basis of Flam
326 Flame Suppression Chemistry

140. Hastie, J.W. and McBee, C.L., “Mechanistic Studies of Halogenated Flame Retardants: The

Antimony-Halogen System,” in Gann, R.G., ed., Halogenated Fire Suppressants American Chemical

: Analysis of Antimony Trihalides in

ine Powders,”

ube Study of C2H2-O2 Reaction. Acceleration of Reaction in 

resence of Iron Pentacarbonyl,” Journal of Physical Chemistry 57, 807-812 

-

stblom, U., Fernandezalonso, F., Mahon, C.R., Smith, G.P., Jeffries, J.B., and Crosley, D.R.,

A.D., Chang, D.P.Y., Kelly, P.B., and Kennedy, I.M., “The Transformation of 

A.D., Chang, D.P.Y., Kelly, P.B., and Yoon, Y., “Morphology of

it of a Super-Efficient Flame Suppressant,” Journal of Physical Chemistry a 103, 1150-

ch Review With Recommendations,” NIST TN 1443, National Institute

tional Symposium, 531-542, Int. Assoc. 

ve Development for Fire Suppressing Gas Generators: Hybrid Systems,” in Gann, R.G.,

Society , Washington, DC, 1975. 

141. Hastie, J.W., “Mass Spectroscopic Studies of Flame Inhibition

Flames,” Combustion and Flame 21, 49-54 (1973). 

142. Dolan, J.E. and Dempster, P.B., “The Suppression of Methane-Air Ignitions by F

Journal of Applied Chemistry 5, 510-517 (1955). 

143. Matsuda, S. and Gutman, D., “Shock-T

the Presence of Trace Amounts of Cr(CO)6,” Journal of Physical Chemistry 75, 2402-2404 (1971). 

144. Matsuda, S., “Gas Phase Homogeneous Catalysis in Shock Waves. II. The Oxidation of Carbon 

Monoxide by Oxygen in the P

(1972). 

145. Linteris, G.T., Rumminger, M.D., and Babushok, V.I., “Premixed Carbon Monoxide-Nitrous Oxide

Hydrogen Flames: Measured and Calculated Burning Velocities With and Without Fe(CO)5,”

Combustion and Flame 122, 58-75 (2000). 

146. Park, K., Bae, G.T., and Shin, K.S., “The Addition Effect of Fe(CO)5 on Methane Ignition,” Bulletin 

of the Korean Chemical Society 23, 175-176 (2002). 

147. Jensen, D.E. and Webb, B.C., “Afterburning Predictions for Metal-Modified Propellant Motor 

Exhausts,” AIAA Journal 14, 947-954 (1976). 

148. We

“Laser-Induced Fluorescence Diagnostics of a Propane Air Flame With a Manganese Fuel Additive,”

Combustion and Flame 99, 261-268 (1994). 

149. Suyuan Y., Jones,

Chromium in a Laminar Premixed Hydrogen-Air Flame,” Proceedings of the Combustion Institute 27, 

1639-1645 (1998). 

150. Kennedy, I.M., Zhang, Y., Jones,

Chromium Emissions From a Laminar Hydrogen Diffusion Flame,” Combustion and Flame 116, 233-242

(1999). 

151. Kellogg, C.B. and Irikura, K.K., “Gas-Phase Thermochemistry of Iron Oxides and

Hydroxides:Portra

1159 (1999).

152. Tapscott, R.E., Sheinson, R.S., Babushok, V., Nyden, M.R., and Gann, R.G., “Alternative Fire

Suppressant Chemicals: a Resear

of Standards and Technology, Gaithersburg MD, (2001). 

153. Linteris, G.T., Katta, V.R., and Takahashi, F., “Experimental and Numerical Evaluation of Metallic

Compounds for Suppressing Cup-Burner Flames,” Combustion and Flame 138, 78-96 (2004). 

154. Katta, V.R., Takahashi, F., and Linteris, G.T., “Numerical Investigations of CO2 As Fire Suppressing 

Agent,” Fire Safety Science: Proceeding of the Seventh Interna

for Fire Safety Science, Boston, MA, 2003. 

155. Fallis, S., Holland, G.F., McCormick, J.L., Reed, R., and Wilson, K.A., “Advanced

Propellant/Additi

Burgess, S.R., Whisner, K.C., and Reneke, P.A., eds., Papers From 1991-2006 Halon Options Technical 

Working Conferences (HOTWC), CD-ROM, NIST SP 984-4, National Institute of Standards and 

Technology, Gaithersburg, MD, (2006).
References 327

156. Masri, A.R., Dally, B.B., Barlow, R.S., and Carter, C.D., “The Structure of Laminar Diffusion 

Flames Inhibited With CF3Br,” Combustion Science and Technology 114, 17-34 (1996). 

157. Skaggs, R.R., McNesby, K.L., Daniel, R.G., Homan, B., and Miziolek, A.W., “Spectroscopic

lar, A., Personal Communication, 1998. 

rsonal Communication, 1999. 

eds., Papers From 1991-

troscopy of

hibition,”

remixed Turbulent Combustion,”

etched Laminar Flamelet Model of Turbulent NonScience and Technology 60, 287-318 (1988). 

.C., and Ratcliffe, J.W., “OH Fluorescence Images of The

mistry,” Proceedings of the

n Flames by Iron Pentacarbonyl,” Proceedings of the Eastern States Section Meeting , pp. 273-

2, 147-160 (1998). 

Studies of Low-Pressure Opposed Flow Methane/Air Flames Inhibited by Fe(CO)5, CF3Br, or N2,”

Combustion Science and Technology 162, 1-17 (2001). 

158. Chidsey, I.L. and Crosley, D.R., “Calculated Rotational Transition Probabilities for the A-X System 

of OH ,” Journal of Quantitative Spectroscopy and Radiative Transfer 23, 187 (1980). 

159. Dieke, G.H. and Croswhite, H.M., “The Ultraviolet Bands of OH Fundamental Data,” Journal of

Quantitative Spectroscopy and Radiative Transfer 2, 97 (1962). 

160. Kot

161. Eckbreth, A.C., Laser Diagnostics for Combustion Temperatures and Species, Abacus Press,

Cambridge MA, 1998. 

162. Smooke, M.D., Yale University, Personal Communication, 1997. 

163. Babushok, V., NIST, Pe

164. Rumminger, M.D., Reinelt, D., Babushok, V., and Linteris, G.T., “Inhibition of Flames by Iron 

Pentacarbonyl,” in Gann, R.G., Burgess, S.R., Whisner, K.C., and Reneke, P.A.,

2006 Halon Options Technical Working Conferences (HOTWC), CD-ROM, NIST SP 984-4, National 

Institute of Standards and Technology, Gaithersburg, MD, (2006).

165. Daniel, R.G., McNesby, K.L., Skaggs, R.R., Sagear, P., and Miziolek, A.W., “Spec

Inhibited Counterflow Diffusion Flames Proceedings,” 1997 JANNAF Combustion Subcommittee

Meeting, 1997. 

166. Gaydon, A.G., The Spectroscopy of Flames, John Wiley and Sons, New York, NY, 1972. 

167. Babushok, V.I., Tsang, W., Linteris, G.T., and Reinelt, D., “Chemical Limits to Flame In

Combustion and Flame 115, 551-560 (1998). 

168. Peters, N., “Local Quenching Due to Flame Stretch and Non-P

Combustion Science and Technology 30, 1-17 (1983). 

169. Liew, S.K., Bray, K.N.C., and Moss, J.B., “A Str

Premixed Combustion ,” Combustion and Flame 56, 199 (1984). 

170. Haworth, D.C., Drake, M.C., and Blint, R.J., “Stretched Laminar Flamelet Modeling of a Turbulent

Jet Diffusion Flame,” Combustion 

171. Bilger, R.W., “The Structure of Turbulent Non-Premixed Flames,” Proceedings of the Combustion

Institute 22, 450-488 (1988). 

172. Roberts, W.L., Driscoll, J.F., Drake, M

Quenching of a Premixed Flame During an Interaction With a Vortex 169.,” Proceedings of the

Combustion Institute 24, 169 (1992). 

173. Miller, J.H., “Applications of Conserved Scalars to Combustion Che

Eastern States Section Meeting, pp. 1-10, Combustion Institute, Pittsburgh, PA, 1996. 

174. Reinelt, D., Babushok, V., and Linteris, G.T., “Numerical Study of the Inhibition of Premixed and 

Diffusio

276, Combustion Institute, Pittsburgh, PA, 1996. 

175. Noto, T., Babushok, V., Hamins, A., and Tsang, W., “Inhibition Effectiveness of Halogenated

Compounds,” Combustion and Flame 11
328 Flame Suppression Chemistry

176. Fallis, S., Reed, R., Lu, Y.-C., Wierenga, P.H., and Holland, G.F., “Advanced Propellant/Additive 

Development for Fire Suppressing Gas Generators,” Halon Options Technical Working Conference, 361-

., “Study of the Mechanisms of Fire Extinguishment of 

003.

d Standards for Fire Suppression Systems, Tokyo,

organic Tin-Compounds As Flame, Smoke, and Carbonn-Compounds,” Polymer Degradation and Stability 32, 299-312 

atory List of Chemicals to Initiate the Search for Halon Alternatives,” NIST Technical Note 1279, 

knock compounds, Nauka, Moscow, 1971. 

y of New Mexico, Albuquerque, NM, 

elocities: a Critical Review,”

remixed

hermodynamics of Tin Ionization in H2-

370, Albuquerque, NM, 2000. 

177. Rosser, W.A., Inami, S.H., and Wise, H

Liquid Rocket Propellants,” WADC Technical Report 59-206, ASTIA Document No. AD 216355,

1959). 

178. Lippincott, E.S. and Tobin, M.C.J., “The Vibrational Spectra and Thermodynamic Functions of Lead 

Tetramethyl, Tin Tetramethyl and Germanium Tetramethyl,” Journal of the American Chemical Society

75, 4141-4147 (1953). 

179. Katta, V.R., Takahashi, F., and Linteris, G.T., “Extinction Characteristics of Cup-Burner Flame in

Microgravity,” 41st Aerospace Sciences Meeting and Exhibit, AIAA Paper No. 2003-1150, AIAA, 2

180. Linteris, G.T., “Extinction of Cup-Burner Diffusion Flames by Catalytic and Inert Inhibitors,” 

Second NRIFD Symposium - Science, Technology an

Japan, 2002. 

181. Cusack, P.A. and Killmeyer, A.J., “In

Monoxide Suppressants for Synthetic-Polymers,” in Nelson, G.L., ed., Fire and Polymers, Hazards

Identification and Prevention ACS Symposium Series 425, American Chemical Society, Washington,

DC, 1990. 

182. Hornsby, P.R., Mitchell, P.A., and Cusack, P.A., “Flame Retardation and Smoke Suppression of

Polychloroprene Containing Inorganic Ti

(1991). 

183. Pitts, W.M., Nyden, M.R., Gann, R.G., Mallard, W.G., and Tsang, W., “Construction of an 

Explor

National Institute of Standards and Technology, Gaithersburg MD, (1990). 

184. Nesmeyanov, A.N., Ed., Manganese containing anti

185. Tapscott, R.E., Heinonen, E.W., and Brabson, G.D., “Advanced Agent Identification and

Preliminary Assessment,” NMERI 95/38/32350, NMERI, Universit

(1996). 

186. Chao, B.H., Egolfopoulos, F.N., and Law, C.K., “Structure and Propagation of Premixed Flame in

Nozzle-Generated Counterflow,” Combustion and Flame 109, 620-638 (1997). 

187. Andrews, G.E. and Bradley, D., “Determination of Burning V

Combustion and Flame 18, 133-153 (1972). 

188. Gilbert, A.G. and Sulzmann, K.G.P., “The Vapor Pressure of Iron Pentacarbonyl,” Journal of the

Electrochemical Society 121, 832 (1974). 

189. Stull, D.R., “Vapor Pressure of Pure Substances Organic Compounds,” Industrial Engineering

Chemistry 39, 517-540 (1947). 

190. Hollrah, D., Ethyl Corp., Personal Communication, 2001 

191. Bulewicz, E.M. and Padley, P.J., “Photometric Observations on the Behaviour of Tin in P

H2-O2-N2 Flames,” Transactions of the Faraday Society 67, 2337-2347 (1971).

192. Goodings, J.M. and Chen, Q.-F., “Chemical Kinetics and T

O2-N2 Flames and the Proton Affinity of SnO,” Canadian Journal of Chemistry 76, 1437-1446 (1998). 
References 329

193. Taylor, J.E. and Milazzo, T.S., “Synthesis of Nanoscale Metal-Oxide Particles Using Laser

Vaporization Condensation in a Diffusion Cloud Chamber,” Journal of Physical Chemistry 82, 847-852

(1978). 

194. Gurvich, L.V., Karachevtsev, G.V., Kondratiev, V.N., Lebedev, Yu., A., Medvedev, V.A., Potapov, 

V.K., and Hodeev, Ya.S., Bond energies. Ionization Potentials and Electron Affinities , Nauka, Moscow,

1974. 

195. Fontijn, A. and Bajaj, P.N., “Kinetics of the SN(P-3(0)) Reactions With CO2 and O2 Over Wide 

Temperature Ranges,” Journal of Physical Chemistry 100, 7085-7089 (1996). 

196. Husain, D., Plane, J.M., and Xiang, C.C., “A Direct Kinetic Study of the Reaction K + OH + He

orescence Spectroscopy, 

stances,” NIST

s., NIST Chemistry WebBook, NIST Standard Reference Database Number 69 National Institute

ganese Oxides and

of Phosphorus-Containing Additives on Methane 

tion

a National Laboratories,

r

86-8246, 

Eiteneer, B., Goldenberg, M., Bowman, 

.W. and Tsang, W., eds., Halon Replacements ACS

ons of Calculated Intracrystalline Void Volumes and Limiting Absorption 

Volumes in Zeolite,” in Meier, W.M. and Uytterhoeven, J.B., Eds., Molecular Sieves American Chemical

Society, Washington, D.C., 1973. 

=3D KOH + He by Time-Resolved Molecular Resonance-Fluorescence Spectroscopy,

OH(A2=ABSIGMA=BB+-X 2=ABPI=BB), Coupled With Steady Atomic Flu

K(5 2PJ-4 2S1/2),” Journal of the Chemical Society, Faraday Transactions II 80, 1465-1483 (1984). 

197. Gurvich, L.V. and Glushkov, V.P., “Thermodynamic Properties of Individual Sub

special database 5, IVTHANTHERMO-PC, National Institute of Standards and Technology,

Gaithersburg, MD, (1998).

198. Smith, G.P., SRI, Personal Communication, 1999 

199. J.A. Martinho Simőes, “Organometallic Thermochemistry Data,” in Mallard, W.G. and Linstrom,

P.J., Ed

of Standards and Technology, Gaithersburg, MD 20899 (http://webbook.nist.gov), 2000. 

200. Hildenbrand, D.L. and Lau, K.H., “Thermochemistry of Gaseous Man

Hydroxides,” Journal of Chemical Physics 100, 8377-8380 (1994).

201. Babushok, V.I. and Tsang, W., “Influence

Flames,” Proceedings of the Joint Meeting of the United States Sections, pp. 587-590, The Combus

Institute, Pittsburgh, PA, 1999. 

202. Jensen, D.E., “Condensation Modeling for Highly Supersaturated Vapours: Application to Iron,”

Journal of the Chemical Society, Faraday Transactions 76, 1494 (1980). 

203. Kee, R.J., Grcar, J.F., Smooke, M.D., and Miller, J.A., “A Fortran Computer Program for Modeling

Steady Laminar One-Dimensional Premixed Flames,” SAND85-8240, Sandi

Livermore, CA, (1991). 

204. Kee, R.J., Rupley, F.M., and Miller, J.A., “CHEMKIN-II: A Fortran Chemical Kinetics Package fo

the Analysis of Gas Phase Chemical Kinetics,” SAND89-8009B, Sandia National Laboratories,

Livermore, CA, (1989). 

205. Kee, R.J., Dixon-Lewis, G., Warnatz, J., Coltrin, R.E., and Miller, J.A., “A Fortran Computer

Package for the Evaluation of Gas-Phase, Multicomponent Transport Properties,” SAND

Sandia National Laboratories, Livermore, CA, (1986). 

206. Smith, G.P., Golden, D.M., Frenklach, M., Moriarty, N.W.,

C.T., Hanson, R.K., Song, S., Gardiner, Jr., W.C., Lissianski, V.V., and Qin, Z., “GRI Mech 3.0,” 2000. 

207. Linteris, G.T., “Effect of Inhibitor Concentration on the Inhibition Mechanism of Fluoromethanes in 

Premixed Methane-Air Flames,” in Miziolek, A

Symposium Series 611, American Chemical Society, Washington, D.C., 1995. 

208. Breck, D.W., “Correlati
330 Flame Suppression Chemistry

209. Andersen, S.O., “Halons and the Stratospheric Ozone Issue,” Fire Journal 81, 56-62 (1987). 

210. Reed, M.D., Williams, B.A., Sheinson, R.S., and Fleming, J.W., “Behavior of Bicarbonate Powders 

in Counterflow Diffusion Flames,” Proceedings of the Eastern States Section Meeting, pp. 83-86,

andia National Labs., Albuquerque,

Flames of Chloromethane, Methane, Oxygen

 Journal of Polymer Science Part a-Polymer Chemistry 29,

n of Laminar Premixed Methane-Air Counterflow Flames,” Combustion Science and

iacente, V., and D'Ascenzo, G., “Vapor Pressure Measurements of

Energies in Ferrocene,” Journal of the American

nbull, A.G., “Thermochemistry of Biscyclopentadienyl Metal Compounds. 1.) Preparation and

rez, J.A.G., “Contribution a L'Etude Thermochique De Ferrocene,”

snitz, Y.M., and Sherwood, T.K., The Properties of Gases and Liquids, McGrawand Flame 96, 163-170 (1994). 

d Propane/Air Diffusion Flames From 1 to 5 Atmospheres,” Proceedings of the 

Combustion Institute, Pittsburgh, PA, 1997. 

211. Lutz, A.E., Kee, R.J., Grcar, J.F., and Rupley, F.M., “OPPDIF: A Fortran Computer Program for

Computing Opposed-Flow Diffusion Flames,” SAND96-8243, S

NM, (1996). 

212. Chelliah, H.K., Yu, G., Hahn, T.O., and Law, C.K., “An Experimental and Numerical Study on the

Global and Detailed Kinetics of Premixed and Nonpremixed

and Nitrogen,” Proceeding of the Combustion Institute 24, 1083-1090 (1993). 

213. Zhang, J. and Megaridis, C.M., “Iron/Soot Interaction in a Laminar Ethylene Nonpremixed Flame,”

Proceedings of the Combustion Institute 25, 593-600 (1994). 

214. Zhang, J. and Megaridis, C.M., “Soot Suppression by Ferrocene in Laminar Ethylene/Air

Nonpremixed Flames,” Combustion and Flame 105, 528-540 (1996). 

215. Carty, P., Grant, J., and Metcalfe, E., “Flame-Retardancy and Smoke-Suppression Studies on 

Ferrocene Derivatives in PVC,” Applied Organometallic Chemistry 10, 101-111 (1996). 

216. Kasper, M. and Siegmann, K., “The Influence of Ferrocene on PAH Synthesis in Acetylene and 

Methane Diffusion Flames,” Combustion Science and Technology 140, 333-350 (1998). 

217. Kasper, M., Sattler, K., Siegmann, K., Matter, U., and Siegmann, H.C., “The Influence of Fuel 

Additives on the Formation of Carbon During Combustion,” Journal of Aerosol Science 30, 217-225 

(1999). 

218. Kishore, K., Kannan, P., and Iyanar, K., “Synthesis, Characterization, and Fire Retardancy of 

Ferrocene Containing Polyphosphate Esters,”

1039-1044 (1991). 

219. Zheng, R., Bray, K.N.C., and Rogg, B., “Effect of Sprays of Water and NaCl-Water Solution on the 

Extinctio

Technology 126, 389-401 (1997). 

220. Pelino, M., Tomassetti, M., P

Ferrocene, Mono- and 1,1 Di-Acetyl Ferrocene,” Thermochimica Acta 44, 89-99 (1981). 

221. Lewis, K.E. and Smith, G.P., “Bond Dissociation

Chemical Society 106, 4650-4651 (1984). 

222. Tur

Thermal Stability,” Australian Journal of Chemistry 20, 2059-2067 (1967). 

223. Sabbah, R. and Pe

Thermochemica Acta 297, 17-32 (1997).

224. Reid, R.C., Prau

Hill, New York, 1987. 

225. Wang, H. and Frenklach, M., “Transport Properties of Polycyclic Aromatic Hydrocarbons,”

Combustion

226. Sung, C.J., Li, B., Law, C.K., and Wang, H., “Structure and Sooting Limits in Counterflow

Methane/Air an

Combustion Institute 27, 1523-1529 (1998). 
References 331

227. Zachariah, M.R., Westmoreland, P.R., Burgess, D.R., and Tsang, W., “BAC-MP4 Predictions of 

Thermochemical Data for C-1 and C-2 Stable and Radical Hydrofluorocarbons and Oxidized

Hydrofluorocarbons,” Journal of Physical Chemistry 100, 8737-8747 (1996). 

., and Westmoreland, P.R., “Thermochemical and 

tlantic

mley, R.T., Burns, R.P., and Inghram, M.G., “Thermodynamics of the Vaporization of Cr O : 

ts: Combustion Suppression Chemistry and Cup-Burner 

(1954).

rman, L.M., “Flame Retardants Update: Phosphorus Expands Its Niche,” Plastics Technology

, S.K., “Phosphorus Fire retardance in Polymers. I. General Mode of Action,” Journal of

FRCA Fall

79). 

n and Flame 90, 344-345 (1992). 

228. Tsang, W., NIST, Personal Communication, 1999 

229. Burgess, D.R., Zachariah, M.R., Tsang, W

Chemical Kinetic Data for Fluorinatedhydrocarbons,” Progress in Energy and Combustion Science 21,

453-529 (1995). 

230. Fristrom, R.M. and Sawyer, R.F., “Flame Inhibition Chemistry,” AGARD Conference Proceedings

No. 84 on Aircraft Fuels, Lubricants, and Fire Safety, AGARD-CP 84-71, 12-2 to 12-17, North A

Treaty Organization, Neuilly-sur-Seine, France, 1971.

231. Nesmeianov, A.N., Vapour Pressure of the Elements., New York, Academic Press, 1963. 

232. Gri 2 3

Dissociation Energies of CrO, CrO2, and CrO3,” Journal of Chemical Physics 34, 664-667 (1961). 

233. Barin, I. and Knacke, O., Thermochemical Properties of Inorganic Substances, Springer-Verlag,

Berlin, New York, 1973. 

234. Barin, I., Knacke, O., and Kubaschewski, O., Thermochemical Properties of Inorganic Substances,

Springer-Verlag, Berlin, New York , 1977. 

235. Tapscott, R.E., Mather, J.D., Heinonen, E.W., Lifke, J.L., and Moore, T.A., “Identification and

Proof Testing of New Total Flooding Agen

Testing,” NMERI 97/6/33010, Albuquerque, NM, (1998). 

236. Fay, J.A., “The Distributions of Concentration and Temperature in a Laminar Jet Diffusion Flame,”

Journal of Aeronautical Sciences 21, 681-689

237. Staendeke, H. and Scharf, D.J., “Halogen-Free Flame-Retardant Based on Phosphorus-Compounds,”

Kunststoffe-German Plastics 79, 1200-1204 (1989). 

238. Avondo, G., Vovelle, C., and Delbourgo, R., “Role of Phosphorus and Bromine in Flame

Retardancy,” Combustion and Flame 31, 7-16 (1978).

239. She

38, 102-105 (1992). 

240. Shelley, S., “Keeping Fire at Bay,” Chemical Engineering 100, 71-73 (1993). 

241. Brauman

Fire Retardant Chemistry 4, 18-37 (1977). 

242. Green, J., “Mechanisms for Flame Retardancy and Smoke Suppression: A Review,” Journal of Fire 

Sciences 14, 426-442, 1996). 

243. Kashiwagi, T., Gilman, J.W., McGrath, J.E., and Wan, I.-Y., “Flammability Properties of Phosphine 

Oxide Copolymers and of Commodity Polymers With New Flame Retardant Additives,” 1996

Meeting, 1996. 

244. Peters, E.N., “Flame-Retardant Thermoplastics .1. Polyethylene-Red Phosphorus,” Journal of

Applied Polymer Science 24, 1457-1464 (19

245. Haessler, W., “Dry Chemical Agents and Application Systems,” in Cote, A.E. and Linville, J.L.,

Eds., Fire Protection Handbook National Fire Protection Association, Quincy, MA, 1986. 

246. Kordylewski, W. and Amrogowicz, J., “Comparison of NaHCO3 and NH4H2PO4 Effectiveness As

Dust Explosion Suppressants,” Combustio
332 Flame Suppression Chemistry

247. Amrogowicz, J. and Kordylewski, W., “Effectiveness of Dust Explosion Suppression by Carbonates 

and Phosphates,” Combustion and Flame 85, 520-522 (1991). 

248. Song, C. E., Kim, I. O., Lee, J. K., Kim, T. K., and Lee, S. G., “Liquid Fire Extinguishing

Composition,” U.S. Patent 5219474 (1990). 

249. Kaizerman, J.A. and Tapscott, R.E., “Advanced Streaming Agent Development, Volume II:

Mexico Engineering Research Institute,

Engel, R., Ed., Handbook of

V.M., “ Destruction Chemistry of Trimethyl

olecular-Beam Mass Spectrometry,”

stion and Flame 8, 133-137 (1964). 

ms. Final

ce of Phosphorus-Containing Fire Suppressants on Flame 

stion Products,” Combustion and Flame 94, 341-348 (1993). 

ustion and Flame

997.

Phosphorus Compounds,” NMERI 96/5/32540, New Mexico Engineering Research Institute,

Albuquerque, NM, (1996).

250. Lifke, J. L., Moore, T. A., and Tapscott, R. E., “Advanced Streaming Agent Development, Volume V: 

Laboratory-Scale Streaming Tests,” NMERI 96/2/32540, New

Albuquerque, NM, (1996).

251. Jordan, T.E., “Metal Organic Compounds,” in Vapor Pressure of Organic Compounds, 1st Ed. 

Interscience Publishers, New York, 1954. 

252. Weil, E.D., “Phosphorus-Based Flame Retardants,” in

Organophosphorus Chemistry Marcel Dekker, Inc., New York, 1992. 

253. Korobeinichev, O.P., Chernov, A.A., and Shvartsberg,

Phosphate in H2/O2/Ar Flame Studied by Molecular Beam Mass-Spectrometry,” 3rd Asia-Pacific 

International Symposium on Combustion and Energy Utilization, 1995. 

254. Korobeinichev, O.P., Ilyin, S.B., Mokrushin, V.V., and Shmakov, A.G., “Destruction Chemistry of 

Dimethyl Methylphosphonate in H2/O2/Ar Flames Studied by M

Combustion Science and Technology 116, 51-67 (1996). 

255. Fenimore, C.P. and Jones, G.W., “Phosphorus in the Burnt Gas From Fuel-Rich Hydrogen-Oxygen 

Flames,” Combu

256. Hastie, J.W. and Bonnell, D.W., “Molecular Chemistry of Inhibited Combustion Syste

Report.,” NBSIR 80-2169, National Bureau of Standards, Gaithersburg, MD, (1980). 

257. Babushok, V.I. and Tsang, W., “Influen

Propagation,” International Conference on Fire Research and Engineering, Third, 257-267, Society of 

Fire Protection Engineers, Boston, MA, 1999. 

258. Twarowski, A., “Reduction of a Phosphorus Oxide and Acid Reaction Set,” Combustion and Flame

102, 41-54 (1995). 

259. Twarowski, A., “The Effect of Phosphorus Chemistry on Recombination Losses in a Supersonic 

Nozzle,” Combustion and Flame 102, 55-63 (1995). 

260. Twarowski, A., “Photometric-Determination of the Rate of H2O Formation From Hand OH in the 

Presence of Phosphine Combu

261. Trees, D., Grudno, A., and Seshadri, K., “Experimental and Numerical Studies on Chemical 

Inhibition of Nonpremixed Methane Flames by CF3Br,” Combustion Science and Technology 124, 311-&

(1997). 

262. Saso, Y., Zhu, D.L., Wang, H., Law, C.K., and Saito, N., “Laminar Burning Velocities of

Trifluoromethane-Methane Mixtures: Experiment and Numerical Simulation,” Comb

114, 457-468 (1998). 

263. Zegers, E.J.P. and Fisher, E.M., “Gas-Phase Pyrolysis of Diethyl Methylphosphonate,” Combustion

Science and Technology 116, 69-89 (1996). 

264. Zegers, E.J., “Flow Reactor Pyrolysis of Alkyl Phosphates and Phosphonates, Ph.D. Thesis,

Mechanical Engineering,” Cornell University, Ithaca, NY, 1
References 333

265. Beaud, P., Radi, P.P., Franzke, D., Frey, H.M., Mischler, B., Tzannis, A.P., and Gerber, T.,

“Picosecond Investigation of the Collisional Deactivation of OH A(2)Sigma(+)(V ' = 1, N ' = 4, 12) in an

Atmospheric-Pressure Flame,” Applied Optics 37, 3354-3367 (1998). 

llen, M.G., McManus, K.R., Sonnenfroh, D.M., and Paul, P.H., “Planar Laser-Induced-

., Santoro, R.J., and Smyth, K.C., “Laser-Induced Fluorescence Measurements of

1997).

hen Are Quenching Corrections Unnecessary,” 29th Aerospace Sciences Meeting, AIAA 91-

n Flames at Atmospheric Pressure,” Applied Physics B-Lasers and Optics 61, 499-503 (1995). 

SRI

mperatures in a Shock Tube,” Journal of Physical Chemistry 99, 8472-8476 (1995). 

NO(a) in Low-Pressure Hydrocarbon Flames,” Combustion

rble, F.E., “Effect of Strain Rate on Diffusion Flames,” Siam 

otic Structure of Counterflow Diffusion Flames for Large Activation 

1-

194, Combustion Institute, Pittsburgh, PA, 1997. 

266. Smith, G.P. and Crosley, D.R., “Vibrational-Energy Transfer in A2-Sigma+ OH in Flames,” Applied

Optics 22, 1428-1430 (1983). 

267. A

Fluorescence Imaging Measurements of OH and Hydrocarbon Fuel Fragments in High-Pressure SprayFame Combustion,” Applied Optics 34, 6287-6300 (1995). 

268. Puri, R., Moser, M

OH Concentration in the Oxidation Region of Laminar, Hydrocarbon Diffusion Flames,” Proceedings of 

the Combustion Institute 24, 1015-1022 (1996). 

269. Arnold, A., Bombach, R., Kappeli, B., and Schlegel, A., “Quantitative Measurements of OH 

Concentration Fields by Two-Dimensional Laser-Induced Fluorescence,” Applied Physics B-Lasers and

Optics 64, 579-583 (

270. Kelman, J.B. and Masri, A.R., “Quantitative Imaging of Temperature and OH in TurbulentDiffusion Flames by Using a Single Laser Source,” Applied Optics 33, 3992-3999 (1994). 

271. Barlow, R.S. and Collignon, A., “Linear LIF Measurements of OH in Nonpremixed Methane-Air 

Flames: W

0179 , Reno, 1991. 

272. Kollner, M. and Monkhouse, P., “Time-Resolved LIF of OH in the Flame Front of Premixed and

Diffusio

273. Norton, T.S., Smyth, K.C., Miller, J.H., and Smooke, M.D., “Comparison of Experimental and

Computed Species Concentration and Temperature Profiles in Laminar, 2-Dimensional Methane Air

Diffusion Flames,” Combustion Science and Technology 90, 1-34 (1993). 

274. Luque, J. and Crosley, D.R., “LIFBASE: Database and Simulation Program (v 1.6),”

International Report MP 99-009, 1999).

275. Paul, P.H., “Vibrational-Energy Transfer and Quenching of OH A(2)Sigma(+)(V'=1) Measured at 

High Te

276. Paul, P.H., “A Model for Temperature-Dependent Collisional Quenching of OH A(2)Sigma+,”

Journal of Quantitative Spectroscopy & Radiative Transfer 51, 511-524 (1994).

277. Tamura, M., Berg, P.A., Harrington, J.E., Luque, J., Jeffries, J.B., Smith, G.P., and Crosley, D.R.,

“Collisional Quenching of CH(a), OH(a), and

and Flame 114, 502-514 (1998). 

278. Carrier, G.F., Fendell, F.E., and Ma

Journal on Applied Mathematics 28, 463-500 (1975).

279. Linan, A., “The Asympt

Energy,” Acta Astronautica 1, 1007-1039 (1974). 

280. Grudno, A. and Seshadri, K., “Characteristic Residence Times of Laminar Nonpremixed Flames at 

Extinction,” Combustion Science and Technology 112, 199-210 (1996). 

281. Fisher, E.M., Williams, B.A., and Fleming, J.W., “Determination of the Strain in Counterflow

Diffusion Flames From Flow Conditions,” Proceedings of the Eastern States Section Meeting, pp. 19
334 Flame Suppression Chemistry

282. Nishioka, M., Law, C.K., and Takeno, T., “A Flame-Controlling Continuation Method for 

Generating S-Curve Responses With Detailed Chemistry,” Combustion and Flame 104, 328-342 (1996). 

283. Otsuka, Y. and Niioka, T., “ Combustion and Flame 21, 163-176 (1973). 

logy and Science ACS Symposium Series 611, American Chemical Society, Washington, D.C., 

nhibited Counterflow Nonmptotic Analysis of Inhibition of Nonpremixed

112, 418-437 (1998). 

. GRI-95/0058, 

Theoretical Studies of Counterflow

toichiometric Mixture Fractions,” Siam Journal on Applied Mathematics

“Experimental and 

, H., Hemberger, R., Hentschel, W., Ketterle, W., Kollner, M., Meienburg, W.,

esby, K.L., Daniel, R.G., Miziolek, A.W., and Babushok, V.I., “Experimental 

(HOTWC), CD-ROM, NIST SP 984-4, National 

se 3.5, Reaction Design,

n and Flame 102, 481-492 (1995). 

284. Trees, D., Seshadri, K., and Hamins, A., “Experimental Studies of Diffusion Flame Extinction With

Halogenated and Inert Fire Suppressants,” in Miziolek, A.W. and Tsang, W., eds., Halon Replacements: 

Techno

1995. 

285. MacDonald, M.A., Jayaweera, T.M., Fisher, E.M., and Gouldin, F.C., “I

Premixed Flames With Variable Stoichiometric Mixture Fractions,” Proceedings of the Eastern States 

Section, pp. 91-94, Combustion Institute, Pittsburgh, PA, 1997. 

286. Baldwin, R.R., Hopkins, D.E., and Walker, R.W., “Addition of Ethane to Slowly Reacting Mixtures 

of Hydrogen and Oxygen at 500°C,” Transaction of the Faraday Society 66, 189-203 (1970). 

287. Yetter, R.A. and Dryer, F.L., “Inhibition of Moist CO Oxidation by Trace Amounts of

Hydrocarbons,” Proceedings of the Combustion Institute 24, 757 (1992). 

288. Grudno, A. and Seshadri, K., “Rate-Ratio Asy

Methane-Air Flames by CF3Br,” Combustion and Flame

289. Bowman, C.T., Hanson, R.K., Davidson, D.F., Gardiner, W.C., Lissianski, V., Smith, G.P., Golden,

D.M., Frenklach, M., and Goldenberg, M., “GRI-MECH: Ver. 2.11 - an Optimized Detailed Chemical

Reaction Mechanism for Methane Combustion and NO Formation and Reburning,” No

http://www.me.berkeley.edu/gri_mech, The Gas Research Institute, Chicago, IL, (1995 ). 

290. Li, S.C., Libby, P.A., and Williams, F.A., “Experimental and

Diffusion Flames,” Proceedings of the Combustion Institute 24, 1503-1512 (1996). 

291. Kim, J.S. and Williams, F.A., “Structures of Flow and Mixture-Fraction Fields for Counterflow 

Diffusion Flames With Small S

53, 1551-1566 (1993). 

292. Brown, T.M., Tanoff, M.A., Osborne, R.J., Pitz, R.W., and Smooke, M.D.,

Numerical Investigation of Laminar Hydrogen-Air Counterflow Diffusion Flames,” Combustion Science 

and Technology 129, 71-88 (1997). 

293. Arnold, A., Becker

Monkhouse, P., Neckel, H., Schafer, M., Schindler, K.P., Sick, V., Suntz, R., and Wolfrum, J., “Laser inSitu Monitoring of Combustion Processes,” Applied Optics 29, 4860-4872 (1990). 

294. Wainner, R.T., McN

and Mechanistic Investigation of Opposed-Flow Propane/Air Flames by Phosphorus-Containing

Compounds,” in Gann, R.G., Burgess, S.R., Whisner, K.C., and Reneke, P.A., eds., Papers From 1991-

2006 Halon Options Technical Working Conferences

Institute of Standards and Technology, Gaithersburg, MD, (2006).

295. Skaggs, R.R., Daniel, R.G., Miziolek, A.W., and McNesby, K., “Spectroscopic Studies of Inhibited

Opposed Flow Propane/Air Flames,” Proceedings of the First Joint Meeting of the U.S. Sections, pp. 575-

578, Combustion Institute, Pittsburgh, PA, 1999. 

296. Kee, R.J., Rupley, F.M., and Miller, J.A., CHEMKIN Collection, Relea

Inc., San Diego, CA, 1999. 

297. Sung, C.J., Liu, J.B., and Law, C.K., “Structural Response of Counterflow Diffusion Flames to

Strain-Rate Variations,” Combustio
References 335

298. Ravikrishna, R.V. and Laurendeau, N.M., “Laser-Induced Fluorescence Measurements and

ersaevel, P., Lacas, F., and Rolon, J.C., “Temperature and 

, J.H. and Cool, T.A., “Kinetic Model for the Decomposition of DMMP in a 

ation, 1998. 

ides and Acids on the Rate of H+OH 

bBook, NIST Standard Reference

0). 

, O.P., Ilyin, S.B., Shvartsberg, V.M., and Chernov, A.A., “The Destruction 

H2 in O2 and

V.V., “Hydrogen-Oxygen 

lame 118, 727-732 (1999). 

Suppressants,” in Gann, R.G., Burgess, S.R., Whisner, K.C., and Reneke, P.A.,

ds and Technology, Gaithersburg, MD, (2006). 

Modeling of Nitric Oxide in Methane-Air and Ethane-Air Counterflow Diffusion Flames,” Combustion 

and Flame 120, 372-382 (2000). 

299. Magre, P., Aguerre, F., Collin, G., V

Concentration Measurements by Cars in Counterflow Laminar Diffusion Flames,” Experiments in Fluids

18, 376-382 (1995). 

300. Werner

Hydrogen/Oxygen Flame,” Combustion and Flame 117, 78-98 (1999). 

301. Mokrushin, V.V., Bol'shova, T.A., and Korobeinichev, O.P., “A Kinetic Model for the

Decomposition of DMMP in a Hydrogen/Oxygen Flame”, Personal Communic

302. Babushok, V., NIST, Personal Communication, 2000 

303. Twarowski, A., “The Influence of Phosphorus Ox

Recombination,” Combustion and Flame 94, 91-107 (1993). 

304. Linstrom, P.J. and Mallard, W.G., Eds., NIST Chemistry We

Database Number 69 National Institute of Standards and Technology, Gaithersburg MD, 20899

(http://webbook.nist.gov), 2001. 

305. MacDonald, M.A., “ Inhibition of Non-Premixed Flames by Phosphorus-Containing Compounds,” , 

Ph.D. Dissertation, Cornell University, Ithaca, NY, 2000. 

306. Smyth, K.C., Tjossem, P.J.H., Hamins, A., and Miller, J.H., “Concentration Measurements of OH. 

and Equilibrium Analysis in a Laminar Methane Air Diffusion Flame,” Combustion and Flame 79, 366-

380 (199

307. Glaude, P.A., Curran, H.J., Pitz, W.J., and Westbrook, C.K., “Kinetic Study of the Combustion of 

Organophosphorus Compounds,” Proceedings of the Combustion Institute 28, 1749-1756 (2000). 

308. Korobeinichev

Chemistry of Organophosphorus Compounds in Flames - I: Quantitative Determination of Final

Phosphorus-Containing Species in Hydrogen-Oxygen Flames,” Combustion and Flame 118, 718-726 

(1999). 

309. Korobeinichev, O.P., Bolshova, T.A., Shvartsberg, V.M., and Chernov, A.A., “Inhibition and 

Promotion of Combustion by Organophosphorus Compounds Added to Flames of CH4 or 

Ar,” Combustion and Flame 125, 744-751 (2001). 

310. Korobeinichev, O.P., Shvartsberg, V.M., Chernov, A.A., and Mokrushin, 

Flame Doped With Trimethyl Phosphate, Its Structure and Trimethyl Phosphate Destruction Chemistry.,”

Proceedings of the Combustion Institute 26, 1035-1042 (1996). 

311. Korobeinichev, O.P., Shvartsberg, V.M., and Chernov, A.A., “The Destruction Chemistry of

Organophosphorus Compounds in Flames - II: Structure of a Hydrogen-Oxygen Flame Doped With

Trimethyl Phosphate,” Combustion and F

312. Riches, J., Grant, K., and Knutsen, L., “Laboratory Testing of Some Phosphorus-Containing

Compounds As Flame

eds., Papers From 1991-2006 Halon Options Technical Working Conferences (HOTWC), CD-ROM,

NIST SP 984-4, National Institute of Standar

313. Nogueira, M.F.M. and Fisher, E.M., “Effects of Dimethyl Methylphosphonate on Premixed Methane 

Flames,” Combustion and Flame 132, 352-363 (2003). 
336 Flame Suppression Chemistry

314. Korobeinichev, O.P., Ilyin, S.B., Bolshova, T.A., Shvartsberg, V.M., and Chernov, A.A., “The

Chemistry of the Destruction of Organophosphorus Compounds in Flames - III: The Destruction of 

DMMP and TMP in a Flame of Hydrogen and Oxygen,” Combustion and Flame 121, 593-609 (2000). 

va, T.A., “The Chemistry of the Destruction of 

.P. and Fisher, E.M., “Pyrolysis of Triethyl Phosphate,” Combustion Science and 

am Massio and

e of the CF Radical,” in Gann, R.G. , Burgess, S.R.,

National Institute of Standards and Technology,

Gouldin, F.C., Jayaweera, T.M., and MacDonald, M.A., “Flame Inhibition by

e Defense Advanced

ane Flames,”

315. Korobeinichev, O.P., Chernov, A.A., and Bolsho

Organophosphorus Compounds in Flames - IV: Destruction of DIMP in a Flame of H2+O2+Ar,”

Combustion and Flame 123, 412-420 (2000). 

316. Zegers, E.J

Technology 138, 85-103 (1998). 

317. Zegers, E.J.P. and Fisher, E.M., “Gas-Phase Pyrolysis of Diisopropyl Methylphosphonate,”

Combustion and Flame 115, 230-240 (1998). 

318. Yi, A.C. and Knuth, E.L., “Probe-Induced Concentration Distortions in Molecular-Be

Spectrometer Sampling,” Combustion and Flame 63, 369-379 (1986). 

319. Kaiser, E.W., Rothschild, W.G., and Lavoie, G.A., “Effect of Fuel-Air Equivalence Rat

Temperature on the Structure of Laminar Propane-Air Flames,” Combustion Science and Technology 33, 

123-134 (1983). 

320. Nogueira, M.F.M., “Effects of Dimethyl Methylphosphonate of Premixed Methane Flames, Ph.D. 

Thesis, Mechanical Engineering,” Cornell University, Ithaca, NY, 2001. 

321. Kaiser, E.W., Wailington, T.J., Hurley, M.D., Platz, J., Curran, H.J., Pitz, W.J., and Westbrook,

C.K., “Experimental and Modeling Study of Premixed Atmospheric-Pressure Dimethyl Ether-Air 

Flames,” Journal of Physical Chemistry a 104, 8194-8206 (2000).

322. Karbach, V. and Warnatz, J., “Http://Nathan.Ca.Sandia.Gov/Tdf/3rdWorkshop/Ch4mech. Html. “ 

Proceedings of the 3rd International Workshop on Measurement and Computation of Turbulent

Nonpremixed Flames, Boulder, CO, 1998. 

323. Williams, B.A. and Fleming, J.W., “Extinction Studies of Hydrocarbons in Methane/Air and

Propane/Air Counterflow Diffusion Flames: The Rol 3

Whisner, K.C., and Reneke, P.A., eds., Papers From 1991-2006 Halon Options Technical Working

Conferences (HOTWC), CD-ROM, NIST SP 984-4,

Gaithersburg, MD, (2006).

324. Fisher, E.M.,

Phosphorus-Containing Compounds,” Final Technical Report Distributed by th

Research Projects Agency, Arlington, VA, (1998). 

325. Cup Burner Flame Extinguishment Concentrations,” NMERI/CGET Technical Update Series 

(CGET-a) 1995 20/a, 1998). 

326. Gann, R.G., NIST, Personal Communication, 1999 

327. Brabson, G.D., Walters, E.A., Gennuso, A.R., Owen, J.P., and Tapscott, R.E., “Molecular Beam

Mass Spectroscopy of Low-Pressure Flames Seeded With Iron Pentacarbonyl,” Journal of Physical 

Chemistry (submitted), 1998). 

328. Marinov, N.M., Castaldi, M.J., Melius, C.F., and Tsang, W., “Aromatic and Polycyclic Aromatic 

Hydrocarbon Formation in a Premixed Propane Flame,” Combustion Science and Technology 128, 295-

342 (1997). 

329. Marinov, N.M., Pitz, W.J., Westbrook, C.K., Castaldi, M.J., and Senkan, S.M., “Modeling of

Aromatic and Polycyclic Aromatic Hydrocarbon Formation in Premixed Methane and Eth

Combustion Science and Technology 116, 211-287 (1996). 
References 337

330. Marinov, N.M., Pitz, W.J., Westbrook, C.K., Vincitore, A.M., Castaldi, M.J., Senkan, S.M., and 

Melius, C.F., “Aromatic and Polycyclic Aromatic Hydrocarbon Formation in a Laminar Premixed NH.W., “The Extinction of Hydrocarbon Flames Based on the 

191-202 (1980). 

M., Bowman, C.T., Hanson, R.K., Davidson, D.F.,

for Methane Combustion,” Gas Research Institute Topical 

s Precursors for Polychlorinated

ary CF3Br- and CHF3-Inert Flame Suppressants: 

bustion, Academic Press, Orlando, FL, 1987. 

of the Combustion Institute 27, 2731-2739 (1998). 

Butane Flame,” Combustion and Flame 114, 192-213 (1998). 

331. Ewing, C.T., Hughes, J.T., and Carhart,

Heat-Absorption Processes Which Occur in Them,” Fire and Materials 8, 148-156 (1984). 

332. Westbrook, C.K., “Inhibition of Methane-Air and Methanol-Air Flames by HBr,” Combustion 

Science and Technology 23, 

333. Simmons, R.F. and Wolfhard, H.G., “The Influence of Methyl Bromide on Flames, Part 1.-PreMixed Flames ,” Transactions of the Faraday Society 1, 1211-1217 (1955). 

334. Frenklach, M., Wang, H., Yu, C.-L., Goldenberg, 

Chang, E.J., Smith, G.P., Golden, D.M., Gardiner, W.C., and Lissianski, V., “GRI-MECH: an Optimized

Detailed Chemical Reaction Mechanism

Report No. GRI-95/0058, http://www.me.berkeley.edu/gri_mech, The Gas Research Institute, Chicago, 

IL, (1995). 

335. Zamansky, V.M., Lissianski, V.V., Maly, P.M., Ho, L., Rusli, D., and Gardiner, W.C., “Reactions of 

Sodium Species in the Promoted SNCR Process,” Combustion and Flame 117, 821-831 (1999). 

336. Babushok, V., Tsang, W., and Noto, T., “Propargyl Type Radicals A

Aromatic Hydrocarbons During Incineration,” Proceedings of the Combustion Institute, Vol. 28, (to 

appear), The Combustion Institute, Pittsburgh, PA, 2000. 

337. Saso, Y., Ogawa, Y., Saito, N., and Wang, H., “Bin

Effect of Temperature on the Flame Inhibition Effectiveness of CF3Br and CHF3,” Combustion and 

Flame 118, 489-499 (1999). 

338. Padley, P.J. and Sugden, T.M., “Chemiluminescence and Radical Re-Combination in Hydrogen

Flames,” Proceedings of the Combustion Institute 7, 235-244 (1959). 

339. Kim, C.H., Kwon, O.C., and Faeth, G.M., “Effects of Fire Suppressants on Hydrogen-Fueled

Laminar Premixed Flames,” Proceedings of the Joint Meeting of the U.S. Sections, paper #219,

Combustion Institute, Pittsburgh, PA, 2001. 

340. Westbrook, C.K., “Numerical Modeling of Flame Inhibition by CF3Br,” Combustion Science and

Technology 34, 201-225 (1983). 

341. Hamins, A., “Flame Extinction by Sodium Bicarbonate Powder in a Cup Burner.,” Proceedings of 

the Combustion Institute 29, 2857-2864 (1998). 

342. Fristrom, R.M. and Van Tiggelen, P., “An Interpretation of the Inhibition of C-H-O Flames by C-HX Compounds,” Proceedings of the Combustion Institute 17, 773-785 (1978). 

343. Williams, B.A. and Fleming, J.W., “Influence of Bond Energies on Catalytic Flame Inhibition:

Implications for the Search for New Fire Suppressants,” NRL/MR/618003-8728, Naval Research

Laboratory, 2003). 

344. Griffiths, J.F. and Barnard, J.A., Flame and Combustion, 3rd Edition, Blackie Academic and

Professional, Glasgow, 1995. 

345. Glassman, I., Com

346. Casias, C.R. and McKinnon, J.T., “A Modeling Study of the Mechanisms of Flame Inhibition by

CF3Br Fire Suppressant Agent,” Proceedings

347. Jensen, D.E. and Jones, G.A., “Kinetics of Flame Inhibition by Sodium,” Journal of the Chemical 

Society-Faraday Transactions I 78, 2843-2850 (1982). 
338 Flame Suppression Chemistry

348. Baulch, D.L., Duxbury, J., Grant, S.J., and Montague, D.C., “Evaluated Kinetic Data for HighTemperature Reactions, Vol. 4, - Homogeneous Gas-Phase Reactions of Halogen-Containing and

Cyanide-Containing Species,” Journal of Physical and Chemical Reference Data 10, 1-721 (1981). 

mical and Heterogeneous Data for Atmospheric Chemistry .5. IUPAC 

the Reactions of Atomic Bromine With HO2

J., “Chemical Kinetics and Photochemical Data for Use 

al of the Chemical Society, Faraday Transactions 90, 395-401 (1994). 

8).

and Fleming, J.W., “On the Suitability of CF3Br As a Benchmark for Replacement 

-4, 

nce 10, 1-57 (1984). 

359. Williams, F.A., Combustion Theory, Benjamin/Cummings Publishing Company, Menlo Park, 

California, 1985. 

360. Pownall, C. and Simmons, R.F., “The Effect of Hydrogen Bromide on the Structure of PropaneOxygen Flames Diluted With Argon.,” Proceedings of the Combustion Institute 13, 585-592 (1971). 

361. Rumminger, M.D., Babushok, V.I., and Linteris, G.T., “Temperature Regions of Optimal Chemical 

Inhibition of Premixed Flames,” Proceedings of the Combustion Institute 29, 329–336 (2002). 

362. Burgess, D.R., XSenkplot, an Interactive, Graphics Postprocessor for Numerical Simulations of

Chemical Kinetics, http://Www.Nist.Gov/Cstl/Div836/Xsenkplot, National Institute of Standards and

Technology, Gaithersburg, MD, 1997. 

349. Atkinson, R., Baulch, D.L., Cox, R.A., Hampson, R.F., Kerr, J.A., Rossi, M.J., and Troe, J.,

“Evaluated Kinetic, Photoche

Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry,” Journal of Physical and

Chemical Reference Data 26, 521-1011 (1997). 

350. Poulet, G., Laverdet, G., and Lebras, G., “Kinetics of

and HCO at 298 K,” Journal of Chemical Physics 80, 1922-1928 (1984). 

351. DeMore, W.B., Sander, S.P., Golden, D.M., Hampson, R.F., Kurylo, M.J., Howard, C.J.,

Ravishankara, A.R., Kolb, C.E., and Molina, M.

in Stratospheric Modeling. Evaluation Number 12,” JPL Publication 97-4, 1997). 

352. Helmer, M. and Plane, J.M.C., “Experimental and Theoretical Study of the Reaction

Fe+O2+N2=FeO2+N2,” Journ

353. Akhmadov, U.S., Zaslonko, I.S., and Smirnov, V.N., “ Mechanism and Kinetics of Interaction of Fe,

Cr, Mo, and Mn Atoms With Molecular-Oxygen,” Kinetics and Catalysis 29, 251-257 (198

354. Fuss, S.P. and Hamins, A., “Determination of Planck Mean Absorption Coefficients for HBr, HCl,

and HF,” Journal of Heat Transfer-Transactions of the ASME 124, 26-29 (2002 ). 

355. Williams, B.A.

Fire Suppressants.,” in Gann, R.G., Burgess, S.R., Whisner, K.C., and Reneke, P.A., eds., Papers From

1991-2006 Halon Options Technical Working Conferences (HOTWC), CD-ROM, NIST SP 984

National Institute of Standards and Technology, Gaithersburg, MD, (2006). 

356. Fristrom, R.M. and Westenberg, A.A. , Flame Structure, McGraw-Hill, New York, 1965. 

357. Glassman, I., Combustion, Academic Press, New York, NY, 1977. 

358. Westbrook, C.K. and Dryer, F.L., “Chemical Kinetic Modeling of Hydrocarbon Combustion,”

Progress in Energy and Combustion Scie