1

STUDIES OF FUNDAMENTAL PHYSICAL-CHEMICAL MECHANISMS AND PROCESSES OF FLAME-EXTINGUISHING BY POWDER

AEROSOLS

Mikhail Krasnyansky, Independent Scientist

Abstract The theoretical and experimental studies on mechanisms of interaction of the fire-extinguishing aerosol wit flame are carried out. The factors of heterogeneous inhibition of

flame-free-radicals - FFR (О, ОН, Н, СНХ) on a crystal surface of inorganic salts being the basic components of fire-extinguishing powders and aerosols are measured with the use of laboratory equipment. The semi-empirical calculations by methods of quantum chemistry for heterogeneous and homogeneous inhibition reactions of burning are

executed.The thermal mechanism for fire-extinguishing is evaluated. The universal mechanism of influence on flame of a crystal surface of a fire-extinguishing powder particle is offered. Key words: fire-extinguishing aerosol, recombination factor, flame inhibition.

1. Introduction

Fire-extinguishing powders and aerosols are widely used in fire- and explosion

suppression [1,2]. However, the development of new high efficiency fire-

extinguishing compositions requires better understanding of fundamental physical-

chemical mechanisms of their effects on flame.

Spread of flame inside a combustible gaseous mixture is caused by the so-called

“chain” exothermic reactions, i.e. competing reactions between active atoms (or

“molecular fragments”) of the flame (the so-called "flame free radicals" (FFR), such

as O, Н, OH, CHx (by Semenov-Hinshelwood [3]): О2 + Н = О + OH, etc.

Using the Arrhenius equation K=AT[exp(-E/RT)], the rate constants (K) of

these reactions were calculated to be 104-10

6 l/sec·mol at 1000 К, with the average

energy of activation and reaction time being 30-50 kJ/mol and 10-4

- 10-6

sec,

respectively [4,5]. Hence, in order to suppress these reactions it is necessary to find

such processes that would interrupt chain combustion reactions. In terms of time,

they should be compatible with 10-5

sec (or less), and their activation energy should

be competitive, i.e. lower than 40 kJ/mol.

T. Mitani [6] was one of the first who tried "to separate" theoretically flame

inhibition mechanisms based on mass and energy conservation laws.

2

Many authors believe that destruction of particles of ammonium phosphates

and sulfates as well as vaporization of NaCl and KCl particles take place in a high-

temperature flame, and these are their decomposition/evaporation products (together

with a cooling effect), which are responsible for flame extinguishing. That’s why

[7,8], the authors studied such reactions of flame free radicals (H, O and OH) in a gas

phase such as NH3 + O = NH2 + H2О. Jensen [9] has reported that reactions of high-

temperature flame inhibition with participation of vaporous potassium, potassium

chloride and KOH are possible, for example: KCl + H = K + HCl or KOH + H = K +

H2O. Based on the experiments on flame inhibition by Fe-pentacarbonyl [10], the

authors also come to a conclusion that the flame inhibition mechanism is

predominantly homogeneous.

That means that the authors [7, 8, 9, 10] propose homogeneous and thermal

flame suppression mechanisms as the main ones. However, some authors prove that

the thermal mechanism is the only flame suppression mechanism, which can be

considered essential [11].

However, a heterogeneous flame inhibition theory is more and more

recognized among scientists, despite of the fact that its mechanism is the most

difficult for studies. How is it possible to ensure that such chain combustion reactions

as H2 + О = Н + OH do not take place and to make them choose the way of chain

interruption: H2 + O = H2O? The chain interruption reaction H2 + O = H2O is not

possible in the flame, because having collided in a high-temperature gas phase, H2

and O particles will fly away from each other, not entering into any reactions. There

are three reasons for that: high energy barrier, “excessive” kinetic energy and

"excessive" entropy. Three conditions should be fulfilled so that the reaction proceeds

in the flame by a chain interruption path [12, 13, 14].

First of all, it is necessary to “anchor” (adsorb) one of the reacting FFR species

(O, for example) on a solid surface of aerosol particle (as a rule, these are crystal

surface defects that are the centers of adsorption), thus increasing the probability of

collision and decreasing the energy barrier (energy of activation) of the reaction.

Secondly, it is important to take up (absorb) the "excessive" kinetic and thermal

3

energy of the second ("attacking") reactant (for example, Н2). This will be done by a

fire-extinguishing aerosol particle (playing a role of the so-called "third particle"

("M") in a gas-phase reaction) that will dissipate collision energy along its crystal

grid in the form of oscillatory phonons. For example, for hydrogen atom, the time

required for passing through a "reaction" path is 10-11

-10-13

sec, and the velocity of

the oscillatory energy distribution over a crystal grid is of the same order, namely: 10-

12-10

-13 Å /sec [14].

Thus, the scientific literature discusses three main mechanisms of flame suppression

by fire-extinguishing powders (aerosols):

1) Thermal mechanism of flame suppression;

2) Homogeneous flame inhibition by products of decomposition (or

evaporation) of aerosol particles (without participation of an aerosol particle surface);

3) Heterogeneous flame inhibition due to interaction of flame free radicals

(FFR) (such as H, O, OH) with a crystal surface of aerosol particles.

Each of the three factors mentioned above is a complex final process of a great

number of more elementary stages; all of them are in a complex interdependence: in

some cases they sum up, in other cases they compete.

Therefore, we studied the following directions of these mechanisms:

A) Molecular-orbital calculations of homogeneous and heterogeneous

inhibition reactions involving such FFM as O, OH, H, CH3, etc.;

B) Measurements of the coefficients of heterogeneous recombination of FFR

(Y) on salt crystal surfaces by using a chemi-luminescence technique in

high vacuum;

C) Calculation of decomposition/evaporation rates of powder particles and

theoretical and experimental assessment of "thermal" contribution of

aerosol particles in the fire-extinguishing effect.

2. Research methods

2.1. Methods of the semi-empirical quantum-chemical calculations A) For gas phase

4

The semi-empirical presentation of the MO LCAO scheme (molecular orbitals

as a linear combination of atomic orbitals) was used for calculation of electronic and

geometrical structures of molecular systems. The most advanced schemes such as a

Roothan technique involving semi-empirical approximation while calculating

molecular integrals use self-consistency on the density matrix - bond orders [15,16],

and the most accurate heat of formation and molecular geometry of bonds are

described in MINDO approach [17]:

Pμv

ni

Cμi

C i ;i

Cμi μ ,

i μ

where ni is the occupancy of the i-th molecular orbital (МО) (ni = 2, 1, 0),

μ is the atomic orbital;

Cμi is the МО expansion coefficient in atomic orbitals

(AO); is the sum of occupied МО.

The valence approximation was used in most of the cases, i.e. all valence orbitals

of all system atoms are included in the МО as basic AO. The atomic orbitals are

represented as

Xμ (r, , ) Nnl Rnl (r)Ylm ( , ) ,

where Y(θ,φ) and Ylm are the real cubic harmonics and special functions,

respectively.

The radial component of AO was selected using a one-exponential Slater-type

function [18]:

R ne (r) (2 )n

1 1

r n 1

2 [(2n)!] 2

exp( nl r) ,

where n, l are the main orbital quantum numbers, (nl) is the orbital exponent.

One-centered two-electronic integrals were estimated based on the data of

nuclear spectroscopy; two-center two-electronic integrals were calculated as

described in [17].

According to the activated complex theory [18], the reaction rate constant is

defined by the equation:

5

1

Po

So

H o

Kk B h T (

) exp(

) exp(

) ,

RT R RT

where σ is the transmission coefficient (usually σ = 1), Т is the absolute temperature,

kb is the Boltzmann constant, h is the Planck constant, R is the absolute gas constant;

P0 is the standard pressure; ν = (1-2) =-1, H0* is the enthalpy of activation:

HO HO ([ AB] ) [HO ( A) HO (B)] RT .

The entropy of the transition complex S0* can

procedure. The entropy (S) of this system is defined

sum) Q according to the formula (for 1 mole)

be calculated following a similar

through a sum of states (statistic

S R ln Q RT ( ln Q ) 0

T V

Statistic sum Q is the sum looking as follows:

Q g exp[ Ei ]

i RT

i

based on the states of the energy Ei and the multiplicity factor gi.

The variable metrics technique is the most suitable for determination of the

balanced geometry of molecular systems, allowing to find the improved geometry X (n+1)

using data of the initial geometry X(n)

and recurrence equation

X ( n 1) X ( n ) n A( n ) g ( n ) ,

where g(n)

is the total energy gradient, αn is the scalar value, and X is the symmetrical

matrix.

At the minimum, the matrix (X) is positive, i.e. all its proper values are

positive. If the matrix H has a single negative proper value at a certain point, the

latter is a saddle point, which characterizes the transition state of the reaction.

B) For [solid surface – gas] systems

The active centers of cluster surface

clusters, combining orbital-stoichiometrical

cluster approaches [19].

(ACCS) were simulated by using

cluster (ОSC) and uncharged ionic

6

Figure 1. Cluster models

Particular attention was paid to electrical neutrality of the cluster, geometric

optimization of the surface groups, and calculation of the total energy (as well as of

enthalpy of system formation) with consideration of the relevant boundary

conditions. While selecting active centers for clusters (ACCS), the possibility of

"egress" on surface for carbonyl, hydroxide and phosphoric groups and for oxygen

bridges was also taken into account.

Figures 2 and 3 depict the structure of ACCS models optimized by using PM-3

[20] and partially MINDO/3 [21] techniques.

The models are close to zonal approaches [22], but at the same time they are

quasi-molecular (cluster-like) structures. The final fragment (cluster) in the form of

the expanded unit cell (EUC) is singled out from the solid body, with cyclic boundary

conditions being imposed on its molecular orbitals (MO). This corresponds to

consideration of the states not in all points of Brillouin zone (B) within the space of

the reciprocal lattice but only in N points; N, being the number of unit cells in EUC,

is defined as

K m1m2m3 m1

в1 m2

в2 m3

в3 ,

M1 M2 M3

7

where М1, М2 and М3 are the factors for taking into account a vector increase of unit

cell translation along their basic directions (М1 + М2 + М3 = N); т1, т, and т3 are

the integers ranging from 0 to Мі (i=1,2,3).

The set of points Km1,m2,m3 corresponds to high-symmetry points (the so-called

«special points») in Brillouin zone lying along its borders and axes of symmetry.

In fact, the transition from strict zone calculations to cyclic models means that

the systems with translation symmetry will be considered using a theory of special

points of Brillouin. The task becomes “quasi-molecular” and in order to solve it there

might be used calculation procedures, developed in the theory of molecules.

The possibility of application of calculation procedures of the theory of

molecules to the models of solid bodies allows to study within the limits of the

uniform calculation model both the isolated molecules, surface centers and

elementary acts with their participation.

The modification of calculation procedures for the OSC model should allow

[23]:

- to maintain the system of semi-empirical parameters inherent in the method; - to operate both canonical and hybrid basic orbitals (HО); - to have an incomplete AO (or HО) basis on some atoms; - the procedures to stay invariant to rotations of the coordinate system and AO

hybridization of intra-cluster atoms (if the latter is inherent in a non-modified

"molecular" variant); - the procedure to be adjusted to the assessment of an electrostatic field of a solid

body residue.

Taking it into consideration, in approximation of the MINDO chart we shall

receive in the basis of valent orbitals (VO) (i.e. canonical - S, Px,Py and Pz types or HO) the following:

Cvi (F v v Ei ) 0

V

F AI(1 Z 1 P )

A (F )(PZ ) V

A

v

2 V VVV

8

A 1

F AB G (1 ) ( a avU ) P

v AB AB rr

v rr 2

r

where: F is the potential of ionization of μ-th VO; Z is the charge of μ-th VO -

Ψ A ("an atom part") introduced in the ОSC (it is equal to VO electronic occupancy in

an isolated A atom); ß is the resonance integral;ris the hybridization

coefficient:

A a x A

rr

R

( rA - is the canonical AO of A atom); URRis the spanning integral; Gμυ is the bridging

integral of the basis VO such as Ψμ and Ψυ; V A is the contribution of intra-cluster

in out

( ) and off-cluster ( ) electrostatic interactions:

in out

V A

(P Z ) (P Z )

It is necessary to add a term for the boundary A atom with an incomplete basis: d A bound P

,

which compensates interaction VO Ψμ OSC orbitals Ψσ that are not included into the basis.

The Madelung potential for binary links of type AeBm is approximated by

Broughton formula [24] with a good precision:

VmA

ad qA (RAB0 )

1 [1,89 (KB )

1 ] , where:

R 0 is the distance between atoms of the proximate neighbors,

AB

KB is the coordination number (CN) of B atom which is next to A atom.

The total energy of OSC including the energy of interaction with the crystal

fragment is:

E 1 Sp[P(F H ) osk out A B

E cov

0

2 0 A B

A B M

1 osk osk

Eion EOSK (I)

Eion q [V (I) V A

2 AB

A AB A A mad

A B( A) B( A)

VmA

ad

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2.2. Measurement of the heterogeneous recombination coefficient Y

As it is described above (see comments to Figure 1), during heterogeneous

recombination with participation of the superficial center of a crystal lattice there is

an electronic excitation of this center and emission of the "characteristic" phonon

which, getting on luminophor substance, can cause a chemi-luminescent radiation.

It appears that non-stationary methods of measurement of heterogeneous

recombination coefficient Y are the most efficient, being based on the registration of

concentration dependences of flame free radicals (atoms) in the vacuum of the closed

spherical vessel. Walls (or part of the surface) of the vessel are covered by the crystal

(fire-extinguishing) substance in question. As a rule, a chemi-luminescent detector is

used for these purposes [25]. "Y" is calculated on the basis of radiation intensity

changes of the Chl-sensor (IChl) before and after covering the walls of the vessel by

the crystal (fire-extinguishing) substance in question.

The intensity of the heterogeneous chemi-luminescence (І Сhl) looks as follows

1

_

[26]: I

хл ηγChl U n , where:

8

_

nU 4

is an atom collision rate with a unit of surface of the chemi-luminescent crystal

surface (in the presence of a Chl-atom sensor); chl is a coefficient of atom

recombination at its surface; n is the probability of a light quantum emission by the

Chl-crystal surface center in the atom heterogeneous recombination.

The correctness of chemi-luminescent detection of gas atoms is reached when

the following requirements are fulfilled [26]:

1. The velocity of atom destruction at the Chl-detector surface should be small

enough, i.e.

γchl

k chl

Schl γkS, γc k cSc

where k c h l a n d S c h lare the roughness coefficient and the area of the Chl-

detector surface accordingly. This requirement can be easily fulfilled by reduction of

luminescent substance dimensions.

2. Linear dependence IChl (intensity of heterogeneous chemi-luminescence) on

atom concentration n should be fulfilled. It is possible, if YChl coefficient will not

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change during recession of concentration [n(t)]. This condition is achieved if the

interval of kinetic time registration IChl (t) is small.

Thus, provided that the required conditions are fulfilled, the curve ІChl (t) will

repeat precisely the relationship of atom concentration decrease n (t), namely:

__ __ I

chl (t) Ichl exp γ c k cSc U γkS U

t

4(Vc V)

4(Vc V)

0

The Y coefficient on the surface of the examined substance (or on walls of a

vessel) can be defined based on І Chl1 and І Chl2 values at t1 and t2 moments

accordingly. As a result, the final equation for Y will look like:

4(V V) ln(IXA

/I XA )

2 Vc ln(IXA

/I XA )

2

Y = c 1 2 1 2

, __

(t

t1 )

(Vc V)

(t 2 t1 )

kS U 2

justifying the measuring technique for the ACCS heterogeneous recombination

coefficient on the surface of any substances. The proposed technique allows to

measure Y within a wide range of atom flow values which can be registered by Chl–

detector (>1015

cm-2

sec-1

). It is necessary to select for each type of atoms (radicals)

the detector material possessing both the best sensitivity and bringing the least error

into a Y measurement technique.

For the measurement of a flame free radical (FFM) recombination coefficient

(Y) an experimental unit has been designed and constructed (Figure 2) [26]. The

measurement unit consists of a spherical vessel the internal walls of which have an

inert coating (for example, a fluoroplastic film). The researched substance (2) and the

chemical-luminescence-detector (Chl) (3) are placed inside the vessel. Powdered

luminephors ZnCdS-Ag or Zn2SiO4-Mn were used as the detector. In order to receive

atoms dissociation of molecular gas in high-frequency electrodeless discharge created

with the help of high-frequency generator (5) was used. The total (the degree of

dissociation did not exceed 5 %) pressure was kept at the level of

5·10-2

with the help of the pumping system, at the entrance of which there

were placed adsorption and nitrogen (freezing out) traps. The Chl-atom detector

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signals were registered by an electronic system (8-12). The atom flow per sample of

powder was 1018

cm-2

·sec-1

). Heterogeneous atom recombination at the surface of

Chl-detector caused its chemi-luminescent radiation which during 2-3 minutes

reached a stationary value.

Fig. 2. Experimental unit for measurement of factor heterogeneous recombination Y

1 - spherical vessel; 2 - sample of powder (compressed); 3 - chemi-luminescent (Chl)

detector; 4 - modulator; 5 - high-frequency (HF) generator; 6 - adsorption trap; 7 - nitric trap; 8 - photo-electrical amplifier (PhEA); 9 - power unit of PhEA; 10 – signal amplifier; 11 - recorder; 12 - optical filter; 13 - power supply unit of heater for a sample; 14 - system of registration of a sample temperature ; 15 - detector of atom partial pressure.

It is necessary to select for each type of atoms (radicals) (H, O, OH etc.) its

own detector material possessing both the best sensitivity and bringing the least error

into a "Y" measurement technique.

The preliminary preparation technique for Y measurements included:

reception of the supersaturated solution of a powder substance (for example, KCl) in

the water; sputtering of a dense layer of dissolved substance on the walls of an open

vessel (the sample area is 6,3 cm2) and heating of these walls within ten hours under

the temperature of more than 100°С; then there was a "training" in the vacuum of all

the volume (at Р <10-4

torrs) within 2-3 hours.

As the extinguishing powder particles get into the flame from atmosphere, their

surface is covered by adsorbed components of air (most likely О2, СО2, Н2О

molecules) as a multi-layer "fur coat". Therefore, at the initial stage of contacting the

flame the destruction of FFR will depend mainly on the adsorbed “fur coat” rather

than on the crystal powder basis.

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However, coefficient of heterogeneous recombination (Y) for salts which are

used as the basic component of extinguishing powders is not an absolute constant as

its size is influenced by many factors, including experiment conditions. Therefore,

both the measurement and interpretation of results concerning (Y) is a very

complicate task.

2.3. Assessment techniques of flame-extinguishing thermal mechanisms

Theoretical calculations and laboratory measurements of powder evaporation

and/or decomposition as well as gas adsorption were done with the help of traditional

thermodynamic and derivatographic methods (derivatograph model - Q-1500,

Hungary, the charge weight is 0.5 g, the heating speed is 5 degree/min). Gas

chromatography was also used for measurement of gas concentrations

(chromatograph “Model 3700”, Russia, column with molecular sieves, diameter – 3

mm, length – 2 m). The traditional photoelectrocalorimeter (FEK-56, Russia) was

also used for gas concentration measurements. The relative error was 10%.

The adsorption heat was measured with the help of chromatography: time (τ)

of nitrogen desorption at different temperatures (Т) during heating of powder

particles; based on the results of measurements the curve of dependence [Lg (τ) as T-

1 function] was built. The tangent of an inclination angle took in equal Hads.

3. Results and their discussion

3.1. Results of the evaluation of the thermal mechanism of flame-

extinguishing

As mentioned above, the process of burning results from many competing

exothermal chain reactions (by Semenov-Hinshelwood) in a gas phase. Hence, flame

extinguishing is a consequence of suppression (inhibition) of these reactions. Let’s

apply now these speculations to powder aerosols.

Scientific literature provides numerous theoretical calculations of powder

particle heating processes. However, majority of these calculations have not much to

do with the thermal mechanism of fire-extinguishing (if the matter concerns flame

extinguishing by solid-crystal aerosols. The reason is that physical and chemical

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processes relating to interaction [gas-crystal system] are calculated by absolutely

others laws and formulas.

It is also necessary to consider that a powder particle in a "powder cloud" is

screened around by other particles and that’s why it is heated much more slowly than

a single particle; the same concerns the "powder cloud", in contrast to a single particle, it settles much quicker.

For example, it is well-known (see [27]) that passing through narrow channels

the flame is being suppressed. If to consider a powder cloud as a three-dimensional

grid "put" on a flame, then in accordance with our calculations the flame should

extinguish when the distance (dcrit) between powder particles will be the following:

dcrit = cPe

υ

where Рe is a Peclet criterion (Рe = 30...100); c, , stand for thermal capacity, heat

conductivity and gas density; V is a flame speed. For a 9.5% methane-air mixture dcr

= 3.6 mm. It is important to pay attention to the fact that this formula has only gas

parameters and no parameters of grid material.

Indeed, if, for example, it will be a copper grid, the copper neither evaporates

nor decomposes in a usual flame, and the surface of copper is inert. But the flame,

however, is suppressed (provided that the size [dcrit] of copper cells or rings is small

enough). It is obvious, that such a mechanism of flame suppression consists in fast

(and simultaneous!) "thermal" destruction (or recombination) of FFR due to their

collision with a surface of copper grid elements. The term "thermal" means only that

the three-dimensional copper grid serves as an “inert wall” or the “third particle”

which absorbs part of FFR energy, and then distributes it throughout crystal grid in

the form of phonons. This process is “synchronized” thanks to a small size [dcrit] of

copper grid cells. Mass simultaneous destruction of FFR results in the instant

termination of chain reactions of burning (by Semenov-Hinshelwood). As these

reactions are exothermal, there is a sharp decrease of heat supply to the flame - the

flame ceases. This is the so-called "thermal mechanism of flame suppression"! It is

important to note that the temperature increase of the grid itself is low – in

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accordance with calculations and measurements of numerous authors (see [27]) it is 7-15ºC.

This is explained, first of all, by a huge difference in density and mass of gas and copper,

and secondly, by a small kinetic energy of radicals (up to 100 eV), which is transferred to a

grid site in collision point and which cannot be a reason of a considerable "wall" heating.

If we pass from the copper grid to a "powder cloud" of KCl or NaHCO3

particles, then we get the following: A) additional removal of heat due to

decomposition (evaporation) of powder particles; B) homogeneous inhibition due to

interaction with FFR of gaseous decomposition (evaporation) products of powder

matter; C) additional physical-chemical activity of powder particle surface (in

contrast to inert copper) as an additional heterogeneous inhibition effect.

In this connection the important results are received in [28]: the mass-

spectrometer study of the extinguishing effect of a reactor wall on the flame has

shown that the thermal mechanism reveals at the maximum distance of 10-2

mm from

the wall, while recombination – at the distance of 10-3

mm. I.e., each powder particle

in such a "cloud" contributes to destruction of FFR around itself in accordance with

homogeneous and heterogeneous (both thermal and chemical) mechanisms within the

radius of minimum 10-2

mm from edge of the particle.

The fact that the extinguishing efficiency is interrelated with chemical activity

of an extinguishing powder particle surface is confirmed by our experiments on

measurement of heat of adsorption ( Н) confirm (see Table 1).

Table 1. Н2О adsorption heat (100 mono-layers)

Material Нads, kJ/mol Extinguishing dose (g/l)*

NH4NO3 24.8 0,21

KCL 33.6 0,16

NH4H2PO4 47.9 0,10

* These data are received in the experimental chamber (see [1], fig. 1)

We have also calculated heat absorption during the whole process of

decomposition of some fire-extinguishing substances into more simple components

15

using traditional thermo-chemical and thermo-dynamic techniques. After that we

measured (using derivatograph and chromatograph) "the duration of semi-

decomposition" of these substances (i.e. the relative time of a half weight loss during

their heating in at furnace under 1000°С; the "duration of semi-decomposition" of

NaHCO3 was assumed to be 1).

Table 2. Some characteristics extinguishing substance

Substance Heat absorption in “Semi-decomposition”

case of full duration τ (1/2), c

decomposition, kJ/g at 1000˚C

NaHCO3 1.8 1

(NH2)2CO 5.5 14.2

NH4H2PO4 3.6 39

(NH4)2SO4 4.5 467

KCl 0.5

The data in Table 2 clearly demonstrate that the "duration of semi-

decomposition" is a very important characteristic of extinguishing substances. For

example, while decomposing in flame, 1 g of NaHCO3 absorbs 1.8 kJ of heat, and 1

g of (NH4)2SO4 – 4.5 kJ. On the one hand, (NH4)2SO4 cools the flame quicker. On

the other hand, the speed of NaHCO3 decomposition is 467 times higher than the

speed of ammonium sulphate, and hence, the time required for decomposition of 1 g

of ammonium sulphate with absorption of 4,5 kJ from flame is equivalent to the time

required for decomposition of 467 g of sodium bicarbonate, the heat absorption being

467х1.8 = 8400 kJ. It means that during fire suppression the kinetic parameters are

often much more important, than thermodynamic ones!

16

3.2. Results of semi-empirical quantum-chemical calculations of homogeneous inhibition

Table 3. Calculated values of Arrhenius equation parameters for homogeneous inhibition reactions of the second order with participation of

NH3, NH2, PO, KCl, KOH, CO2 at Т=300 ºК and 1000 ºК O * *

Reaction T K * * А К, l/s·mol

H 0 , S0 ,

exp S0 exp H 0

Kcal/ Cal/ R

RT

mol mol

NH3+О → NH2+HO 300 33,63 -23,17 8,1 ·10-6

1,8· 10-25

3,3·109 5,8 ·10

15

NH3+H → NН2+Н2 1000 -28,96 4,7 ·10-8

4,4 ·10-8 2,2 ·10

9 97

NH3+H → NН2+Н2 300 15,79 -27,11 1,2 ·10-6

2,4 ·10-12

5,0 ·108 1,2 10

3

1000 -33,88 3,9· I0-8 3,5 10

-4 1,8 ·10

8 6,5 10

4

NH3+CH3 → 300 19,38 -6,17 4 ·10-2

5,4 ·10-15

l,9·1013

9,9 10-2

NН2+CН4 1000 -7,71 2·10-2

5,8 ·10-5 9,3 ·10

13 5,6 10

9

NH2+О → NH+ОН 300 33,08 -22,94 9,7 ·10 -6

4,5 ·10 -25

4,0 ·10 9 1,7 ·10

-15

1000 -28,68 5,41· 10-7

5,87 ·10-7

2,5 ·109 1,5 ·10

2

NH2+H→ NН+Н2 300 0 -20,86 2,6 ·10-5

1,0 1,1 ·1010

1,1 ·10-10

1000 -26,08 1,9 ·10-6

1,0 8,9 ·109 8,9 ·10

9

NH2+ОH → NН + Н2О 300 16,9 -27,7 9,72 ·10 -7

3,65 ·10 -13

4,01 ·10 8 1,4 ·10

-4

1000 -34,4 3,0 ·10-8

2,01 ·10-4

1,4 ·108 2,9 ·10

4

NH2+СH3 → NН + 300 18,74 -7,84 2,0 ·10-2

1,6· 10-4

8,0 ·1012 1,25 ·10

-1

СН4 1000 -9,8 7,2 ·10-3

8,0 ·10-5

3,4 ·1013

2,7 ·109

NH2+СH3 → NН3 + 300 21,05 -30,3 2,4 ·10 -7

3,2 ·10 -16

9,85 ·107 3,1 ·10

-8

СН2 1000 -37,9 5,3 ·10-9

2,5 ·10-5

2,4 ·107 5,1 ·10

2

PO+H→P+OH 94,07 300 -22,27 1,4 ·10 -5

5,7 ·10 -70

6,0 ·109 3,2 ·10

-60

1000 -27,84 8,2 ·10-7

2,8 ·10-21

3,8 ·109 1,1 ·10

-11

PO + O → P + O2 85,32 300 -29,18 4,2 ·10 -7

1,6 ·10 -63

1,7 ·108 2,5 ·10

-55

1000 -36,48 1,1 ·10-8

2,3 ·10-19

4,9 ·107 1,1 ·10

-11

CO2 + O → CO + O2 34,6 300 -19,36 5,9 ·10 -5

3,4 ·10 -25

2,4 ·1010

8,0 ·10 -16

1000 -24,2 5,1 ·10-6

2,7 ·10-8

2,4 ·1010

6,5 ·10-2

KCl + H → K + OH 19,19 300 -20,72 3,0 ·10 -5

7,5 ·10 -15

1,2 ·1010

8,9 ·10-5

1000 -25,90 2,2 ·10-5

6,4 ·10-5

1,0 ·1010

6,5 ·105

KOH + H → K + H2O 18,6 300 -19,55 5,2 ·10 -5

2,0 ·10 -14

2,1 ·1010

4,3 ·10-4

1000 -24,44 4,6 ·10-6

8,6 ·10-5

2,1 ·1010

1,8 ·105

17

Figure 3 - Models of transition complexes of homogeneous inhibition reactions

in a gas phase for FFR with participation of NH2

a - (NH2 + H); b - (NH2 + O); c - (NH2 + OH); d - (NH2 + CH3); e - (NH2 + CH3);

The data of table 3 show that the inhibition reaction such as (NH2) gas + (H) gas →

NH3 cannot be a direct reaction in a gas phase: the “third particle” or “wall” (M) is

required so that to create conditions for the following reaction: (NH2)gas + (H)gas + M

→ NH3 + M or (NH2)gas + (H)ads = NH3; that’s why only the following "direct" (i.e.

without participation of the " third particle" or "wall") homogeneous inhibition

reactions in a gas phase are possible: (NH2)gas + (H)gas → NH + H2; (KОН)gas +

(H)gas = (K)gas + (Н2О)gas. Besides, we can see clearly that, for instance, such a

homogeneous inhibition reaction as (KОН)gas + (H)gas = (K)gas + (Н2О)gas at 300 ºK

is extremely slowly (K = 4,3 ·10-4

), however, at 1000 ºK its speed will increase

billion times (K = 1.8 ·105)! As far as homogeneous inhibition with participation of

18

phosphate "splinters" is concerned, for reactions PO+O=P+O2 or PO+H=P+OH it is

almost absent even under 1000 ºK (K=1.1·10-11

).

The last conclusion is in line with Twarowski’s conclusions [29], who describes

inhibition reactions with participation of PO2 only at presence of the "third particle"

(M): H + PO2 + M = HPO2 + M; HPO2 + H + M = H2 + PO2 + M (even adsorbed

gas CO2 from atmosphere is less inert under 1000 ºK: K = 6,5 ·10-2

- see Table 3).

Similar conclusions makes in article [30].

3.3. Results of semi-empirical quantum-chemical calculations of heterogeneous inhibition

The models of active centers on cluster surfaces (ACCS) for phosphates received

as a result of their optimization by PM-3 and MINDO/3 techniques are presented at

Figures 4,5. The similar models of transition complex adsorption are developed and

calculated for sulfates and carbonates, etc. (see Figures 4). For example, in case of

surface clusters such as Na2CO3 (or NaНCO3) the hydrogen Н atom forms chemi-

adsorptive bonds with surface centers of carbonyl groups (R [С=O...Н] = 0,95 Å) as

well as with the hydroxyl group (R [HO...Н] = 0,98 Å) and H-atom of the carbonate

group (R [O3C....Н] = 1,39Å.

As shown for the majority of ACCS models (chlorides, sulfates, carbonates,

phosphates), out of many particles participating in the flame the adsorption

complexes are formed only for particles Н and СН3 and, to a smaller extent, for OH

and O. The molecules with the filled electron orbital (CH4, О2, Н2) that form

complexes with the ACCS are either very weak or do not form complexes at all. It

means that chain reactions (i.e. elementary reactions of combustion such as [(O2) ads

+ Н → O + HO; (Н2) ads + O → Н + HO; (СН4) ads + O → СН3 + HO] are not

"supported" by the crystal surface because of weak adsorption of О2, Н2 molecules

and CH3 particle (see Table 4).

19

Figure 4. Models of transition adsorption complexes of heterogeneous inhibition reactions for FFR Н and OH with carbonates of ACCS

Figure 5a. Models of transition adsorption complexes of heterogeneous

inhibition reactions for FFR Н and СН3 with phosphates of ACCS

20

Figure 5b. Models of transition adsorption complexes of heterogeneous

inhibition reactions for FFR Н and СН3 with phosphates of ACCS

Table 4. Results of calculation for heterogeneous recombination elementary reactions (Kcal/mol)

Recombination reaction H Е S300·10-3

S1000·10-3

Crystal ACCS FFR

Na3PO4 Р-ОН Н -788,8 -1368,7 67,0 82,9

Na3PO4 Р-ОН CН3 -937,6 -1542,5 75,2 99,5

Na3PO4 Р-О-Р Н -422,4 -2380,9 72,2 85,0

Na3PO4 Р-О-Р CН3 -400,2 -2529,3 78,8 103,0

NaHCO3 О-С Н -253,2 -1053,7 80,7 91,3

NaHCO3 О-С О -227,0 -1329,1 81,3 92,2

NaHCO3 О-С ОН -288,9 -1347,1 81,4 93,6

NaHCO3 С=О Н -116,7 -1045,5 63,9 79,3

NaHCO3 ОС-ОН Н -335,74 -1057,2 67,6 88,1

Na3PO4 Р=О Н -259,1 -1381,3 66,6 81,1

Na3PO4 Р=О CН3 -251,7 -1510,5 74,9 99,5

Na2SO4 S=О Н -638,9 -1430,0 - -

Na2SO4 S=О CН3 -830,7 -1587,6 - -

Na2SO4 ОSО-Н О -545,5 -1717,9 69,0 81,7

SiO2 ОSi ОН -259,8 -1328,1 73,5 90,6

SiO2 ОSi CН3 -274,4 -1478,1 79,4 106,2

SiO2 Si-О - Si Н -532,7 -2318,9 74,2 89,3

SiO2 Si-О - Si CН3 -525,2 -2467,9 77,9 101,3

21

For example, during interaction of О2 molecule with the atom adsorbed at the

surface, at the moment the О2 molecule approaches Нads the transition state (the

saddle point at the surface of potential energy) corresponding to the chain

development reaction is not localized. Instead of that, there takes place a surface

recombination reaction Нads + О2 → НОО, bringing to formation of an НОO particle

which is less active than a hydrogen atom. In case CH3 is absorbed, the attack on

(CH3)ads by molecules, for instance, by О2, is “sterically” difficult: while O2

approaches (CH3)ads (a hydrogen atom) the reaction CH3+O2 is not observed, instead

of that there is a desorption of the CH3 radical, which only afterwards (without the

surface) can interact in a gas medium with O2. During interaction of O(ads) with

molecular systems (H2 and CH4) the stages of chain development have not been

detected (the corresponding transition stage is not observed). Instead of that, while H2

or CH4 approaches (O)ads the O atom detaches from the surface.

O3C…O + H2 → O3C…O…H-H → O3C + O + H2

O3C…O + CH4 → O3C…O…H-CH3 → O3C + O + CH4,

That’s why afterwards the interaction between CH4 and O can be the same as

in gas phase: CH4 + O → CH3 + OH.

Thus, chain-ramifying reactions such as [(О2) ads+Н (gas)] and/or [(H)ads+О2

(gas)] do not occur on the extinguishing powder particle surface; its surface supports

(catalyzes) only chain-breaking reactions such as [(O)ads + O (gas) → О2], etc.

The processes with participation of the phosphate surface center (О3Р=O)surf

require separate examination. The adsorption of Н and СН3 particles is accompanied

by the thermal emission - 188.6 and 62.9 kJ/mole accordingly. However, beside

catalysis of chain-breaking reactions such as H(ads) + H(gas) = H2 there is a direction

interaction between the crystal phosphorus center with H(ads). The phosphorus atom

changes its quinquevalence for trivalence and comes to the surface in the structure of

a (О3Р)surf group. This process is accompanied by the thermal emission - 287.4 (for

R=H) and 293.3 kJ/mol (for R=СН3). A newly-formed АCCS-center (О3Р)surf is not

a radical and, therefore, it cannot facilitate chain development reactions. On the other

22

hand, it can catch active O and OH particles, taking them away from branching-chain

reactions.

Let’s consider now the role of silica surface centers. They can adsorb H (Q=7.4

Kcal/mole), then in case of a further attack of H there takes place a reaction which

results in detachment of HOH and formation of a new surface radical centre

consisting of a partially coordinated Si atom that comes to the surface (SiOOH).

In contrast with the (O3P)surf. Centre a new intermediary ACCS (O3Si) is a

surface radical, capable of being active in branching-chain reactions, following, for

instance, the scheme provided below:

(O3Si)surf + O2 → (O3Si – O)surf + O;

(O3Si)surf + HOH → (O3Si – OH)surf +

H; (O3Si)surf + H2 → (O3SiH)surf + H;

(O3Si)surf + CH4 → (O3SiH)surf + CH3;

(O3Si)surf + O → (O3Si)surf + HO.

It is evidently that because of that the fine-milling silica (sand) does not reveal

inhibiting properties for combustion reactions and not be considered a good fire-

extinguishing substance.

3.3. Y measurements

We have carried out a series of experiments in order to study a heterogeneous

recombination coefficient (Y) for various components of fire-extinguishing aerosols

using a unit the scheme of which is shown at Figures 6,7,8. NaCl, Na2CO3, Na2SO4

and Na3PO4 were examined.

3.3.1. We have measured Y0 on NaCl samples heated in vacuum (heating

temperature Tprogr = 400°K) after their exposing to the air (30 minutes). The results

are depicted at Figure 6.

As it follows from the dependence of Yox from the time of treatment in the atomic

oxygen, at the initial stage the NaCl sample is very active to make heterogeneous

destruction of O centres ( ox/NaСL[init.]=6•10-2

). Further on its activity decrease,

reaching in 60 minutes the stationary value ( ox /NaСL[ const] =1,2·10-2

), i.e.

23

it becomes 15 times less. We should consider that at the initial stage it is the

adsorption layer, which determines destruction of FFR such as O.

Figure 6. Dependence of the oxygen atom recombination coefficient on a NaCl surface

(t)

ox

3.3.2. As the flame contains simultaneously both free radicals of type O and Н,

we found it interesting to measure Yox on the surface after their treatment by H

atoms. During 4 hours the samples were exposed to a flow of dissociated hydrogen

(concentration [H] =1018

/cm3). After that, the hydrogen was pumped out and the

oxygen was filled. The results are provided at Figure 7.

Figure 7. Dependence of oxygen atom recombination coefficient ox(t)

on a NaCl

surface, preliminary treated by [H] atoms

Based on the data of Figure 7, we might conclude that the efficiency of

destruction of the O-like centres on chloride surfaces increases several times if the

24

surface is preliminary exposed to the action of such radicals as Н. That means that

there takes place a heterogeneous catalysis of O + Н OH reaction. The energy of

activation process of heterogeneous recombination of O atoms on NaCl surface

calculated on the basis of curves was 0,17 eV (~16 kJ/mol 3.8 Kcal/mole), which is

10 times lower (!) than the energy of activation for the same gas-phase reaction. The

analysis of the data obtained allows underlining that the adsorption layer at the

surface of extinguishing powders produces an activating effect on the efficiency of

destruction of FFR of O and H types during the initial stage of contact with the

extinguishing surface.

Figure 8. Values of recombination coefficient on the surface of sodium salts:

(t) ox

1 - Na2SO4; 2 - Na2СО3; 3 - Na3PO4.

Figure 8 shows the curves ox(t)

for various salts of sodium - Na2SO4, Na2СО3

and Na3PO4. They have been preliminary dried in the open air and exposed to

vacuum treatment. As it follows from Figure 8, under the indoor temperature, in

terms of heterogeneous destruction of O atoms the most active is Na3PO4 powder

25

( const

/Na3РО4=1,2·10-2

). This figure is 5 times higher than const

/Nа2СО3, which is

equal -3

and one order more than the values const

/Na2SO4, equal to 1,0·10-3

The highest activity of Na3PO4 powder in comparison with Na2СО3 and

Na2SO4 powders is observed on all kinetic sites ox/Na3PO4(t). It is explained by a

bigger defectiveness of the surface and bigger concentration thereat of ACCS.

A long-term "attack" of Na2СО3 sample by Н atoms under Т>400°C resulted in

destruction ("chemical etching") of its surface: Na2СО3 + Н = 2Na + CO2 + OH.

However, under the influence of H flow even more stable salts (NaCl, Na3PO4)

emitted surface atoms Na+, their flow being j=10

10 – 10

11 unit/cm

3.

Table 5. Coefficients of heterogeneous recombination Yox

Experiment conditions

Warmed up After a long After treatment After treatment After exposing

Sample in the open treatment by H by H atoms at by H atoms at to the air at

air at 3800K atoms at 380

0K 400

0K 600

0K t

0 = tambient *

γH γHstationar. γHstationar. γHstationar. γHstationar.

NaCl 3,5 · 10-3

2,7 · 10-4

1,3 · 10-4

6,0 · 10-4

-

KCl - 1,0 · 10-4

1,1 · 10-4

1,7 · 10-4

8,0 · 10-4

NH4Cl 1,0 · 10-3

2,0 · 10-4

1,2 · 10-4

unstable -

Na2SO4 - 1,0 · 10-3

3,5 · 10-3

unstable 6,0 · 10-4

Na3PO4 - 3,7 · 10-4

2,1 · 10-3

5,0 · 10-3

1,7 · 10-4

Na2CO3 - 1,0 · 10-3

2,0 · 10-3

unstable 1,0 · 10-3

*) An exposure to the air was carried out at t0 = 200K during 10 hours.

Conclusions and recommendations

The results of theoretical and experimental studies provided above let us make

the following conclusions:

1. The coefficient of heterogeneous inhibition (Y) for the salts used as the base

of fire-extinguishing aerosols is not constant. It can be used only conventionally for

qualitative characteristic of their efficiency. In particular, it depends on such factors

as surface adsorption properties, degree of surface "deficiency" (especially for F-

centers), presence Н2О molecules (thickness of "adsorption fur-coat") on the surface

of adsorption layer, surface background (i. е. the time it was in chemical-active gas

conditions), ability of superficial layers for vibrational relaxation (i.e. to energy

dispersion), etc.

26

2. At the surface of fire-extinguishing powder particles there is neither the

adsorption of Н2, О2, СН4 molecules, nor branching-chain reactions such as [О2 + Н =

О + OH] (neither for [Оads + Н2gas], nor for [Нads + О2gas] variants). The preference is

given recombination reactions (breakage of a chain) of type: [Нads + Н = Н2], [Оads +

О = О2], [Hads + ОН = Н2О], [ОHads + Н = Н2О]) with desorption of products (Н2, О2,

Н2О).

3. The surfaces of powder particles of different chemical composition (NaCl,

Na2CO3, Na2HPO4, SiO2) take part in heterogeneous inhibition reactions through

different mechanisms: while for NaCl it is a "pure" chemi-adsorption, the carbonates

and hydro-carbonates are exposed to destruction by the flow of hydrogen atoms, the

scheme of their destruction being the following:

NaHCO3 + H ---> Na + H2O + CO2;

on the surface of phosphates the oxidation-reduction reaction takes place: P+5

---> P+3

---> P+5

. Mechanism of interaction of phosphate crystal with FFR:

4. "The thermal mechanism" of flame-extinguishing by powder aerosols is an

effect of “thermal death” of free flame radicals (FFR) on a cold inert "wall".

5. All reactions of recombination in flame can be divided into “permitted” and

“prohibited” ones (see Table 6).

6. The general scheme of FFR interaction with the surface of solid fire-

extinguishing particle is presented at Figure 9.

27

Figure 9. Scheme of heterogeneous inhibition of burning reactions 1 – powder particle; 2 - surface of the particle; 3 - defect (F-center); 4 - super-balanced desorption

of an "adsorption fur-coat"; 5 - chemi-adsorbed FFR; 6 - attacking FFR; 7 – H2 desorption molecule

(product of recombination reaction); 8 - oscillating relaxation process in powder particle

(accommodation of energy during recombination); 9 - FFR, chemi-adsorbed by adsorption "fur

coat"; 10 - adsorption layer ("fur coat"); 11 - emitted atom (ion) of powder substance.

Table 6. «Permitted» and «prohibited» recombination and branching reactions at the surface of powder particle and in gas phase

№ Reaction Comments

1 (O2) ads+ (H) gas —>O+OH No reaction, O2 desorption

2 (O2) ads+(H) gas—HO2 No reaction, O2 desorption

3 (H2) ads+(O) gas—>H+HO2 No reaction, H2 desorption

4 (H2) ads+(O) gas—>H2O No reaction, H2 desorption

5 (CH4) ads+(O2) gas No reaction, CH4 desorption —>CH3+HO2

6 (CH4) ads+O —>CH3+OH No reaction, CH4 desorption

7 (CH3) ads+(O2) gas No reaction, CH3 desorption —>H2CO+OH

8 (CH3) gas+(H) gas—>CH4 No reaction without the “third particle”

9 (O) ads+(O) gas—>O2 The reaction takes place

10 (H) ads+(O2) gas—>O+OH No reaction

11 (H) ads+(O2) gas—>HO2 The reaction takes place

12 (H) ads+(H) gas —>H2 The reaction takes place

13 (H) ads +(O) gas —>OH The reaction takes place

14 (H)gas+(H) gas —>H2 No reaction without the “third particle”

15 (H)gas+(O) gas —>OH No reaction without the “third particle”

16 (O) ads +(H) gas —>OH The reaction takes place

17 (O) ads +(H2) gas —>OH+H No reaction

18 (O) ads +(CH4) gas No reaction —>CH3+OH

19 (O) ads +(H2) gas —>H2O The reaction takes place

20 (O)gas+(O) gas —>O2 No reaction without the “third particle”

28

7. In order to make a precise assessment of the ratio of different mechanisms in the global effect of fire-extinguishing it is necessary to

carry out a well-thought and coordinated international program of studies.

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***** Note. The full text of article is published: Journal “Fire and Materials”, 32, 2008, p. 27-47