FIRE SUPPRESSION BY PARTICULATES CONTAINING METALLIC COMPOUNDS 

P.C. Wanigarathne, R.H. KrauTc, H.K. Chelliah, R.J., and Davi,, R.J 

University of Virginia 

UNIVERSITY OF VIRGINIA INTRODUCTION 

Fire suppression mechanisms by chemically active particulates (especially those due to alkali 

metal bicarbonates) have been investigated for a considerable period of time [ 141, but have 

received renewed interest because of the ban on production of Halon 1301 [S-91. Some recent 

studies have indicated that on a mass basis, fine sodium bicarbonate (NaHCOi) powder is about 

2-10 times more effective in suppressing fires than the now banned Halon 1301, while iron pentacarbonyl vapor is known to be about 60 times more effective [ IO]. The exact chemical and 

physical models that describe the suppression mechanism of such compounds are not well established and development of a comprehensive method of testing these detailed models is the focus 

of the present investigation. This effort has been pursued along two paths, (a) model development effort based on relatively well studied sodium bicarbonate particles and (b) development of 

super effective fire suppressing particles, where a highly effective metallic compound (e.g., iron 

pentacarbonyl) is encapsulated in a porous solid particle (e.,".. zeolite X). The details of these 

two efforts are described below. 

FIRE SUPPRESSION WITH NaHC03 

As mentioned above, fine NaHC03 powder (<IO pm) is known to be far more effective than any 

other gaseous chemical agent being considered to replace Halon 130 I, but the exact physical and 

chemical mechanism of NaHCO, is yet to be quantified. Early studies with NaHCO? have 

hypothesized that the surface area of the particles plays a key role in determining its effectiveness. However, a recent experimental investigation by Trees and Seshadri [6] aimed at characterizing the particle size effects of NaHC03 has indicated a non-monotonic variation of particle 

mass fraction required to extinguish a flame as a function of the particle size (for the size range 

0-30 pm). Although counterflow flame extinction experiments, similar to those in Trees and 

Seshadri [6], were performed by Fleming and co-workers [8,9], to our knowledge, no attempt 

has been made to verify the above counter intuitive nonmonotonic size effects of NaHC03 for the 

size range of 0-30 pm. In addition, no detailed modeling effort has been undertaken to describe a 

physical mechanism for the existence of such a phenomenon. A better understanding of the 

underlying fire suppression mechanism of NaHCOj particles is expected not only to clarify the 

above results, but also assist future efforts on (a) development of' new fire suppre 

superior chemical, thermal, and physical properties and (b) design of optimum delivery methods 

of such condensed phase agents. 

When sodium bicarbonate particles are heated, they are known to decompose into oxides of Nn 

in two stages. In the first stage, NaHCO? decomposes to solid sodium carbonate (Na2CO3). H20. 

and Cor around 543 K (note that the global decomposition temperature can vary depending on 

the particle heating rate. e.&, smaller particles are expected to heat rapidly when exposed to the 

flame environment because of their higher surface areaivolume ratios than the larger particles). 

In the second stage, NarCOj formed is known to decompose at I I70 K, leading to formation of 

Nn-.O and CO?. In the presence of water, however, Na2C03 and Na20 are known to react IO form 

sodium hydroxide (NaOH) [21. The NaOH formed promotes catalytic recombination of the 
radicals species needed for flame propagation, causing the flame to extinguish readily [ 1 I]. The 

present experimental and theoretical study was undertaken to address uncertainties regarding 

effect of sodium bicarbonate particle size on flame extinction, and more importantly, to develop 

a rigorous method of quantifying various physical, thermal, and chemical contributions. 

EXPERIMENTS WITH NaHC03 

In the present flame extinction experiments, a counterflow of nonpremixed methane and air with 

a steady laminar flame established within the mixing layer is considered (Figure 1). The sodium 

bicarbonate particles, which were separated into size ranges of <lo, 10-20,20-30,30-40, and 4& 

60 pm using a vortex separation technique, are introduced with the air stream at a relatively 

steady rate. Obtaining a steady particle feed rate and its calibration have been the most challenging aspect of the present experimental investigation. Typical particle mass fraction introduced 

with the air stream was about I%, corresponding to a particle flow rate of about 0.1 gm/min. For 

a selected sodium bicarbonate loading, the flame extinction effectiveness of sodium bicarbonate 

particles was characterized by the extinction flow strain rate, measured by two methods (a) global 

formula based on the nozzle exit velocities and nozzle separation distance [I21 and (b) local 

velocity field measurements using laser Doppler velocimetry (LDV). 

PARTICLE SEEDER 

Because of the need to establish the accuracy of previously reported data with NaHCOx particles, 

considerable attention was devoted to the development of a particle seeder that could deliver 

steady feed rates of particles of various sizes (Le., from 0 to 100 pm). For small airflow rates 

involved (typical flow rates of 0.1 liter/s), the fluidized bed approach shown in Figure 1 gave 

satisfactory seeder performance only for relatively small particles, Le., less than 30 pm. This 

fluidized bed seeder consists of a 19 mm diameter glass tube with two porous plugs connected at 

either end using Swagelock fittings. The air inflow to the seeder, controlled through a mass flow 

controller, was split into two streams and connected to either end of the glass tube. The flow 

passing through the bottom, which controls the fluidization level, was monitored through another 

flow controller. The fluidized particles and the total air entering the fluidization tube were ejected through a small tube located towards the middle of the tube. By maintaining the total air 

flowing through the fluidizer tube constant, the mass fraction of particles was expected to attain a 

steady state. The particle feed rate was continuously monitored using a Mie scattering detection 

system located at the exit of the seeder air (Figure 1). This detection system consists of a laser 

diode operating at 67 1 nm, a Thor Labs DETl -SI High Speed Silicon Detector with collection by 

a 3-cm focal length lens, background radiation filtering using an Edmund Scientific 67 1 nm 

interference filter, and a Hewlett-Packard Model 7 132 strip chart recorder. Because the seeder 

air flow rate was held fixed, scattering signal was independent of the total air flow rate exiting 

through the air nozzle and was primarily dependent on the particle delivery rate of the seeder. 

The scattered signal was observed to decay slightly over 20-30 min periods, but could be controlled by shifting part of the bottom fluidizing air to the top. Different particle flow rates were 

obtained by controlling the fraction of seeder air flowing through the top and the bottom. The 

actual mass fraction of the particles in the air stream was determined by calibrating the recorded 

scattering signal by a separate gravimetric analysis, usually performed before and after each 

flame extinction experiment. 

394 Halon Options Technical Working Conference 2-4 May Zoo0 
1 Fluidizer 

I 

'1- _- I Nitrogen 

Figure 1. Schematic of the experimental configuration, with the particle seeder. 

For particles greater than 20 pm, a positive feed auger system (not shown) was employed. This 

seeder arrangement consisted of a multi-start Teflon screw (diameter of 7 mm OD and S mm ID) 

in a high-precision stainless steel tube where the screw was driven by speed-locked variable 

speed motor. This seeder was vibrated pneumatically to assist the particle flow, but the particle 

feed rate was primarily controlled by the motor rpm. Part of the total airflow was diverted 

through a screw-driven particle seeder and then mixed with the remaining air prior to exiting 

through the air nozzle. As before, airflow through the seeder held constant while the total airflow through the nozzle was varied to attain different flame strain rates. For particle sizes above 

20 pm, the seeder described above performed extremely well with variation of the Mie scattering 

signal less than +/- 10%. In this positive feed auger system, particles below 20 pm had a tendency to clump, and consequently significant variation of the scattering signal was observed. To 

overcome such clumping, in the previous work by Hamins et al. [SI, about I\% by mass of IO nm 

silica particles had been added to the sodium bicarbonate particles. Because of the uncertainty of 

the effect of adding such fine silica particles and also the associated safety issues, this approach 

was not pursued here. 

Halon Option\ Technicdl Wvrkinz Conference 2-4 May 2000 395 
MEASURED FLAME EXTINCTION RESULTS 

In counterflow flame experiments, the flame extinction condition can be directly related to the 

extinction flow strain rate (which is related to the axial velocity gradient along the axis of symmetry) and is typically reported based on (a) global strain rate formula derived by Seshadri and 

Williams [ 121, or (b) local velocity gradients along the axis of symmetry measured using LDV. 

For the methane and air nonpremixed flames considered, without particles, the measured global 

flame extinction strain rate was about 470 s-', while the measured local LDV strain rate was 

about 390 s-'. These values are consistent with those reported previously by Chelliah et al. [13]. 

As sodium bicarbonate particles are added to the air stream, the flame extinction strain rate is 

expected to decrease because of the physical, thermal, and chemical effects of the particles. For 

different particle size groups (<10,10-20, 20-30, 30-40 and 40-60 pm), using the particle seeders 

developed, the measured variation of flame extinction strain rate vs. the sodium bicarbonate 

particle mass fraction in the air stream is shown in Figure 2. All particle size groups demonstrated a clear decrease in the extinction strain with increasing particle loading. For a given 

particle mass fraction, 10-20 pm particles indicate the lowest extinction strain rate indicating its 

higher effectiveness as a suppressant. 

C 

g '1 

Y 

I. 

Ae 

Aee 

eo 

0 

0 

0 

. 

0 d 

00 0 

.. 

100 150 200 250 300 350 400 

Extinction Strain (11s) 

Figure 2. Comparison of the sodium bicarbonate mass fraction as a function of extinction strain 

rate, for different size groups of particles. 

Seshadri and co-workers [5,6] have previously reported a similar trend in variation of the mass 

of particle required with varying extinction strain rate, for each size group. However, between 

size groups, they observed non-monotonic effectiveness, namely 0-10 pm the most effective, 10- 

20 pm the least effective, and 20-30 pm with an intermediate effectiveness. It should be pointed 

out that there are a few key differences in the two studies. For example, the fuels used were 

different and could have an impact on the results (in Trees and Sheshadri [6] a liquid heptane 

fuel pool was used, while in the present experiments gas methane was used). Second, in 

396 Halon Options Technical Working Conference 2-4 May 2lx)o 
Seshadri's experiments, about 1 %) by mass of IO nm silica particles was added to minimize the 

clumping of particles and help fluidize the smaller particles considered. Although experiments 

were performed in the present system using 0-10 pm particles without addition of silica particles, 

the particle feed rate fluctuations were too large (- +/-30%) with potential inaccuracies of the 

particle feed rate calibrations. 

NUMERICAL SIMULATION OF NaHC03 PARTICLE EFFECTS 

A numerical model, based on a hybrid Eulerian-Lagrangian formulation for the gas and condensed phase, has been developed recently to describe the interaction of fine-water droplets with 

countertlow nonpremixed laminar flames and its extinction phenomena [ 141. This model was 

extended to include the effects of NaHC03 particle transport, heating, decomposition, and subsequent interaction of decomposed gaseous species with the homogeneous flame chemistry. The 

detailed gas-phase kinelic model for methane-air was augmented with a 20-step elementary 

mechanism for Na from Jensen and Jones [l I]. Although there are considerable uncertainties in 

the global particle decomposition model, the theoretical approaches are expected to provide detailed information about the rate controlling physical, thermal, and chemical effects of NaHCOj 

particles. 

NaHC03 DECOMPOSITION MODEL 

Unlike the analysis of fire suppression by water droplets, the model developed for NaHC03 

particles involves considerable uncertainties. These include the non-spherical nature of the 

particles, the transient heating effect of the particles. the heterogeneous decomposition 

mechanism of NaHC03, and the gas-phase chemical inhibition mechanism by intermediate 

species formed. 

Previous studies have indicated that sodium bicarbonate decomposes at a low temperature of 

about 543 K to form sodium carbonate through the reaction [I] 

2 (NzIHCO~), -> (NazCO) ~ +COz+H?O. 

For particle temperatures above I124 K, the sodium carbonate formed can decompose further to 

form other reaction intermediates of Na. Based on these observations, Friedman and Levy [2] 

had postulated that NaOH is the primary inhibiting compound responsible for flame extinction 

and proposed the following reaction pathways leading to formation of NaOH 

(Na?COj), -> (Na2O) ij, +COz, 

(Na?Oji,,+H~0 -> 2NaOH, 

or 

(Na2O) I(,, -> NazO, 

followed by the homogeneous reaction 

NazO+H?O -> 2NaOH. 

Once the alkali hydroxide is formed, the following catalytic radical scavenging cycle is known to 

be responsible for reduction ol'thc radical pool 

NaOH+H -> H*O+Na, 

Halon Options Technical Working Conlcrcncc 2-4 May 2000 397 
Na +OH+M -> NaOH+M, 

NaOH+OH -> H20+NaO. 

The relative importance of these reactions is not well established and the literature indicates 

some conflicting results [2,4,9, 1 I]. 

In the initial simulations presented here, a fast global decomposition reaction of the form, 

2(NaHC03) -> 2NaOH +2C02 

is employed here. The particle temperature at which the decomposition occurs (defined here as 

T,.,,,) is varied to obtain the sensitivity of the particle size. Although such a global model can 

be fine tuned to a very narrow range of conditions, e.g., for a narrow particle size range, it should 

be cautioned here that extension of such models to a wider range of conditions may lead to physically unrealistic results. For the homogeneous chemistry associated with NaOH, a detailed redction model involving 6 species in 20 elementary reactions proposed by Jensen and Jones [ 1 11 was 

employed, in conjunction with the detailed chemistry model for methane oxidation [14]. 

PREDICTED FLAME EXTINCTION RESULTS 

Figure 3 shows the predicted variation of NaHCO3 mass fraction as a function of flame extinction strain rate, for selected sodium bicarbonate particle size groups with mass fraction of 1% in 

the air stream. For comparison, a plot of equivalent NaOH mass fraction for different extinction 

strain rates is also shown. The predicted curve with NaOH indicates the absolute maximum inhibition possible, based on the homogeneous model employed. Different NaOH reaction mechanisms are expected to modify this curve, but the predicted trends are not expected to change. 

The results clearly indicate that particles above 20 pm have a small effect on flame extinction, 

but as the particle size decreases below 20 pm, the extinction strain rate decreases significantly, 

indicating a remarkable increase in the effectiveness. This effect is attributed to the rapid heating 

of the particle and fast decomposition process. 

G13bal Exlinctlon Strain Rate. s"(-1) 

Figure 3. Numerical prediction of sodium bicarbonate mass fraction as a 

function of extinction strain rate, for selected sizes of particles. 

398 Halon Options Technical Working Conference 2-4 May Zoo0 
By varying the T,,.,,,,,,!,described earlier, the location at which these particles are rapidly decomposing in the flow field can be varied. As a consequence, the amount of Na released to the 

homogeneous phase is affected, leading to a varying degree of chemical inhihition. To quantify 

the contributing thermal and chemical effects. the source terms can be selectively turned “oft” in 

the model. Unlike previously reported fine-water droplet simulations, these results indicate a 

significant chemical effect in the presence of NaHCOj. For dilute particle loading considered 

here, the physical effects are found to be negligible. Methods of improving the particle heating 

and subsequent heterogeneous particle decomposition models are currently being analyzed and 

such efforts are expected to provide far more accurate tlame extinction results. 

ENCAPSULATION OF Fe(C0)j 

Iron pentacarbonyl, Fe(CO)5, known to be one of the most effective fire suppression compounds, 

is extremely toxic and flammable [ 15,161. Hence, direct utilization of Fe(CO)> in fire suppression applications is impractical. However, an extremely effective fire suppressing compound can 

he developed if this toxic compound can be encapsulated or immobilized in a benign solid matrix 

(or support material), from which it can be released when exposed to the high temperature region 

of a flame front. Because of our previous experience in developing and working with zeolite 

compounds, the encapsulation of Fe(C0I5 in zeolite was considered first in this investigation. 

ABSORPTION METHOD 

Figure 4 shows a schematic of the ahsorption/impregnation setup. Two glass tubes were coiinected to a vacuum pump, capable of achieving pressure of50 mtorr. In the initial absorption 

studies, about I gm of solid support material was first inserted into one of the tubes. This sample 

was then heated under vacuum conditions to remove the absorbed gases from the internal pore 

structure, which typically took about 3-4 hours. After removal of the absorbed gases, the support 

material was allowed to return to room temperature. The Fe(CO)5 to he absorbed was then 

poured into the second tube, with the interconnecting valve closed. While keeping the Fe(C0)5 

close to freezing temperature, the valve in the interconnecting tube was opened brietly to remove 

the gas-Fe(CO)5 mixture. Finally, the valve to the suction pump was closed and the interconnecting tube valve opened, allowing the Fe(CO)5 vapor to diffuse and xbsorb into the pore structure. 

This vapor-phase absorption process typically took about 1-2 days. At the end of the absorption 

process. zeolite particles were observed to change to a faint brown/yellow color. Bascd on the 

change in color of zeolite, the absorption was observed to he uniform across the entire sample. 

When Fe(C0)5 absorbed zeolite was exposed to air. a rapid propagation of a reaction front across 

the sample is observed. To determine the effect of oxygen concentration on the decomposition/ 

oxidation process, separate experiments were conducted by exposing the encapsulated zeolite-X 

particles to pure oxygen. The latter investigation has indicated a dramatic increase iii the 

decomposition process. Since Fe(CO)j is also known to decompose readily when exposed to 

light. several investigations were performed with different light intensities. Besides the physical 

factors indicated above, the surface properties of the support material can have an important role 

in the Fe(CO)5 decomposition process. Experiments were conducted with zeolite-Y and silica 

particles with mesopores to address this issue. The basic difference between zeolite->( and 

zeolite-Y is in the content of Na in the solid matrix. The Fe(CO)5 absorbed zeolite-Y also 

indicated similar Fe(C0)5 decomposition characteristics when exposed to ail-, but the intensity of 
Figure 4. Schematic of the Fe(C0)5 absorption method. 

the observed reaction rate is much slower than in zeolite->(. In contrast, porous silica particles 

gave no visible indications of Fe(CO)5 decomposition. 

Because of the breakdown of Fe(CO)5 within the pore structure of zeolite, TGA analysis with 

absorbed zeolite could not be performed. Instead, elemental analyses (performed by Southern 

Testing & Research Laboratories, Inc.) of Fe(C0)5 encapsulated particles have revealed very 

interesting information about the level of Fe absorption (Table 1). 

TABLE 1. SUMMARY OF THE ELEMENTAL ANALYSIS. 

Suooort Material Na mass fraction (70'0) Fe mass fraction (%) 

Zeolite-X 10.6 8.85 

Zeolite-Y 6.79 9.71 

Si02 <U.8 12.8 

As seen in Table I, both zeolite X and Y have a significant amount of Na embedded in their 

structure. The absorbed iron mass fraction is also significant and comparable to the Na loading. 

Considering the superior fire suppression ability of Fe (which is about 5-25 times more effective 

than Na), these particles may show a superior fire suppression ability. The exact size of condensed iron or oxides of iron is not known yet, but high-resolution microscopic studies to be undertaken shortly will clarify this issue. Premixed flame studies with these dissociated Fe(C0)s in 

zeolite will be conducted in the near future to quantify the effectiveness of these encapsulated 

particles. The motivation for switching to premixed flame suppression studies is its simplicity, 

and more importantly, safety issues associated with Fe(C0)s. 

400 Halon Options Technical Working Conference 2-4 May 2NH) 
CONCLUSIONS 

The goal of the present investigation was (a) to develop a comprehensive model that can identify 

the leading mechanism of fire suppression of sodium bicarbonate particles and (b) to investigate 

the concept of developing super effective iron pentacarbonyl encapsulated particles. 

Designing a steady particle seeder for sizes ranging from 0-60 pm proved to he a rather challenging task. Two independent approaches of particle seeding were developed for the size range 

considered, and the flame extinction experiments indicated that both approaches yielded identical 

results for the overlapping particle size range of 20-30 pm, providing considerable confidence in 

the measured results. All the particle sizes considered (Le., <0,10-20, 20-30, and 40-60 pm) 

indicated a decrease in flame extinction strain rate with increasing particle mass fraction, a result 

consistent with previous work. However, the effectiveness of flame suppression (quantified here 

based on the amount of NaHCOi mass fraction needed) is shown to vary monotonically with 

particle size, with the smallest size group being the most effective. 

Numerical predictions performed taking into account the detailed homogeneous chemistry and 

global particle decomposition model indicate a trend in particle effectiveness very similar to that 

obtained from the experiments. By varying the temperature at which the particles decompose 

rapidly to gaseous products, the predictions have shown that the smallest particles are more 

effective because of the rapid heating, leading to complete decomposition of the particles. By 

manipulating various source terms in the model, the superior effectiveness of sodium bicarbonate 

particles was attributed to the homogeneous catalytic radical scavenging by NaOH formed, while 

thermal effects associated with particle heating play a minor role. 

Results with iron pentacarbonyl encapsulated in porous zeolite particles indicate that Fe(CO)I is 

not stable and reacts when exposed to air/oxygen. The elemental analysis performed indicates 

that the Fe mass fraction in zeolite is about 10% by mass. The effectiveness of these dissociated 

Fe(C0)S absorbed zeolite particles will be tested in the near future using a premixed flame setup. 

ACKNOWLEDGMENTS 

This work was supported by National Institute of Standards and Technology, Gaithersburg, MD. 

1. 

2. 

3. 

4. 

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402 Halon Options Technical Working Conference 2-4 May 2UK) 

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