STUDY OF THE FIRE RETARDING MECHANISM OF NITROGEN AND PHOSPHORUS CONTAINING INHIBITORS IN NATURAL COMBUSTIBLE MATERIALS

Abstract

The method of mathematical planning of the experiments was used for determination of the leading factors influencing on the efficiency of nitrogen-phosphorus-containing flame inhibitors for wood, peat. It was found, that the inhibition of free radical reactions in the gaseous phase with volatile nitrogen products is dominating mechanism of burn termination of combustible natural materials (wood, peat).

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ACCOMPLISHMENTS IN THE COMBUSTION SCIENCE IN THE LAST DECADE
STUDY OF THE FIRE RETARDING MECHANISM
OF NITROGEN AND PHOSPHORUS CONTAINING
INHIBITORS IN NATURAL COMBUSTIBLE MATERIALS
V. Bogdanova1, O. Kobets1, and V. Kirlitsa2
1Research Institute for Physical Chemical Problems
Belarusian State University
Minsk 220050, Belarus
e-mail: Bogdanova@bsu.by
2Belarusian State University
Minsk 220030, Belarus
1 Introduction
From an economic point of view, nitrogen and phosphorus containing §ame re-
tardants (FR) are the most perspective ¦re extinguishing agents for natural com-
bustible materials (wood, peat). Due to the fact that the speci¦city of burning
of solid combustible materials (SCM) in the presence of FR consists in occur-
rence of several concurrent transformations of the combustible material, §ame
retardant, products of their decomposition interacting both in the condensed and
gas phases, up to the present time, there is no information in literature about
the mechanism of the synergistic inhibitory action of nitrogenphosphorus FR.
This, in turn, impedes directed development of extinguishing agents and SCM
¦re suppressants meeting the contemporary requirements.
To understand why the developed §ame retardants exhibit di¨erent §ame
protective properties, the present authors have previously investigated evolution
of volatile §ame retardants (nitrogen and phosphorus) into the gaseous phase
and properties of the pre§ame zone formed in the condensed phase melts and
foam protective structures [14].
In order to obtain more information about the role of the factors that in-
troduce the dominant contribution to inhibition of burning natural polymeric
combustible materials (wood, peat), the mathematical models that adequately
describe the FR and ¦re-extinguishing e©ciency have been developed using the
mathematical apparatus of optimal experiment planning [58].
2 Experimental Technique
Fire extinguishing e©ciency of FR with respect to peat was determined by the
method, described in [9], according to which the relative weight loss of a peat
82 V. Bogdanova et al.

Filtration and Heterogeneous Combustion
sample (in %) after ¦ring tests was measured. The ¦re retardant e©ciency with
respect to wood was assessed, following the State Standard 16363, by weight
loss (in %) by a ¦re protected wood sample displaced in a ¤ceramic pipe¥ over
a gas burner §ame. Flame retardant composition (FRC) is considered to be
e©cient when –m≤25%. To assess the adequacy of the obtained observations
to mathematical models, the criterion of adequacy of the models was applied to
repeated observations at each full factorial experiment (FFE) point [8]:
(N−n)mY ′Y−Nk
θk2
(n−p)Y′Y−mY ′Y≤Fα;n−p,N−n(1)
where nis the the number of di¨erent points in the FFE; mis the the number
of repeated observations at each point of FFE; pis the the number of unknown
model parameters; Yis the vector of the mean observed values at each point in
the spectrum of FFE; and Fα;n−p,N −nis the quantile of signi¦cance level αof
Fischer distribution with n−p,N−nbeing the degrees of freedom. In case of
implementation of inequality (1), the model was deemed adequate to obtained
observations at the level of signi¦cance α. Signi¦cance of coe©cients in the
mathematical models was determined with the use of Student£s tcriterion [8].
Coe©cient θjis a signi¦cant factor if


θj
s√cjj
> tα,N−p
where tα,N−pis the quantile of the level αof Student distribution with N−p
degrees of freedom; cj j is the jth diagonal element of the inverse matrix (X′X)−1;
and s2is the unbiased estimate of the variance of equally accurate observations.
For FFE, cj j = 1/N; the value s√cjj , counts on the appropriate aggregate
function Excel; and tα,N −pis the quantile of the αlevel of Student distribution
with N−pdegrees of freedom.
3 Results and Discussion
The synthetic dispersion of ammonium-metalphosphate (bi- and trivalent met-
als) which has a complex ¦re-retarding e¨ect (peat ¦re-extinguishing and wood
¦re-protection) was selected as an object of research. Natural metallosilicate
(bentonite) was used as one of the synthesis starting reagents [10]. Previously,
it was found that during thermolysis of ¦re-retarded wood and peat at tempera-
tures realized in the pre§ame zone of condensed phase (200500 ◦C), formation
of foam structures preventing further SCM pyrolysis and evolution of volatile
combustible products into the gas phase were observed. Therefore, such vari-
able factors as the contents of phosphorus (factor x1), bentonite (factor x2), and
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ACCOMPLISHMENTS IN THE COMBUSTION SCIENCE IN THE LAST DECADE
nitrogen (factor x3) in the formulation of FRC were selected as the main compo-
nents that can signi¦cantly a¨ect the ¦re-retarding properties of extinguishing
and §ame retardant compositions for wood and peat. Numerical values of these
components (g/100 g) for the best FRC e©ciency are as follows: x(0)
1= 6.09,
x(0)
2= 2.8, and x(0)
3= 6.09. The e©ciency of peat extinguishing (vpeat) and
the e©ciency of wood ¦re-protecting (vwood) were selected as response functions
that characterize the e¨ectiveness of ¦re-resistant, ¦re-extinguishing means for
peat and wood burnout (FRC).
In ¦re tests, averaged data on extinguishing and ¦re-protecting properties of
FRC were obtained: y(0)
peat = 1.825% and y(0)
wood = 4.22%. In order to determine
the in§uence of the FRC chemical composition on its ¦re-extinguishing e©ciency
with respect to peat, the FRC analyzed was chosen as the center of the type 23
FFE plan. As a phenomenological model of the FRC ¦re-extinguishing e©ciency
with respect to peat, the regression model with pairwise interaction coe©cients
was chosen:
E{y}=a0+a1x1+a2x2+a3x3+a12x1x2+a13 x1x3+a23x2x3(2)
where E{y}is the average expected value of extinguishing e©ciency y.
In the coded variables Xiwith given variation interval of x(0)
i, equal to 10%,
the regression equation (2) takes the form:
E{y}=b0+b1X1+b2X2+b3X3+b12X1X2+b13 X1X3+b23X2X3.(3)
In order to estimate the unknown parameters of the model (3) in accordance
with the FFE, in each of the eight corners of a cube in coded variables X(1)
= (−1,−1,−1), X(2) = (−1,−1,1), X(3) = (−1,1,−1), X(4) = (−1,1,1),
X(5) = (1,−1,−1), X(6) = (1,−1,1), X(7) = (1,1,−1), and X(8) = (1,1,1)
double observations have been performed, results are presented in Table 1.
In the matrix form, the model of observation (3) can be written as
E{Y}=Xθ
where Yis the observations vector of dimension 16; Xis the experiment plan-
ning matrix of dimension 16 ×7; and θis the vector of unknown parameters of
dimension 7.
Since the experiments were conducted in accordance with the FFE, the ex-
periment planning matrix Xis a matrix with mutually orthogonal columns; in
this case, the e¨ect of multicollinearity factors vanishes and the best linear un-
biased estimator [8] is equal to

θ=X′Y N−1
where X′is the transposed matrix X; and Nis the total number of experiments.
84 V. Bogdanova et al.

Filtration and Heterogeneous Combustion
Table 1 Plan 23FFE and the results of experiments to optimize FRC formulations
for peat
No.
of
Factors in the
natural scale
Factors in the
coded variables Response function, ypeat , %
experiment x1x2x3X1X2X3
1 5,48 2,52 5,48 −1−1−1y11 = 3.62; y12 = 3.98; y13 = 3.80
2 5,48 2,52 6,7 −1−1 1 y21 = 2.71; y22 = 3.06; y23 = 2.89
3 5,48 3,08 5,48 −1 1 −1y31 = 2.68; y32 = 3.13; y33 = 3.10
4 5,48 3,08 6,7 −1 1 1 y41 = 1.35; y42 = 1.17; y43 = 1.18
5 6,7 2,52 5,48 1 −1−1y51 = 3.51; y52 = 2.61; y53 = 2.80
6 6,7 2,52 6,7 1 −1 1 y61 = 2.51; y62 = 2.37; y63 = 2.48
7 6,7 3,08 5,48 1 1 −1y71 = 1.64; y72 = 1.91; y73 = 1.70
8 6,7 3,08 6,7 1 1 1 y81 = 1.06; y82 = 0.83; y83 = 0.94
Using the statistical functions of Excel spreadsheets, the estimates of the
parameters of the model (3) were obtained:
E{y}= 2.3838 −0.3288X1−0.663X2−0.501X3−0.033X1X2
+ 0.1388X1X3−0.118X2X3.(4)
In determining the adequacy of the observation model obtained (4), after
veri¦cation of the signi¦cance of its coe©cients, it has been established that
the model is adequate on the level of signi¦cance α= 0.05 and the coe©cients
X12,X13 , and X23 are not signi¦cant at the 0.05 level. If they are not taken
into account, the model (4) will be very simpli¦ed and poorly describe the real
phenomenological e¨ect of extinguishing e©ciency due to a small number of ex-
periments (N= 16), which does not allow all the seven signi¦cant coe©cients of
model (4) to be evaluated. Therefore, to get more information for estimating the
unknown parameters, at each range of FFE, another additional experiment has
been conducted and data for y13y83 obtained (see Table 1). After revaluation
of model (4) coe©cients, determining the new model adequacy at a signi¦cance
level α= 0.05, and removing the insigni¦cant coe©cient of X12, the adequate
observation model with all relevant coe©cients was received:
E{y}= 2.3763 −0.3463X1−0.6521X2−0.4971X3+ 0.1654X1X3
−0.1388X2X3.(5)
Upon transition to natural variables, model (8) looks as follows:
E{y}= 19.95178 −3.27493x1−4.60345x2−7.31x3+ 0.4455x1x3
−0.81235x2x3.(6)
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ACCOMPLISHMENTS IN THE COMBUSTION SCIENCE IN THE LAST DECADE
Model (6) describes the phenomenological e¨ect of changing the composition
of initial ¦re-resistant, ¦re-extinguishing mixture in the vicinity of points with
values x(0)
1= 6.09, x(0)
2= 2.8, and x(0)
3= 6.09 on changes in the average expected
value of FRC e©ciency in peat extinguishing. Because peat and wood are the
solid combustible materials of di¨erent nature and it is quite di©cult to ¦nd
a combination of values of the in§uencing factors which provide extremums of
both response functions of interest, therefore, to improve the ¦re-extinguishing
and protective properties of FRC simultaneously with respect to peat and wood,
the following approach was used. Initially, an adequate mathematical model was
built that describes the in§uence of the selected factors on the e¨ectiveness of
FRC ¦re extinguishers for peat, then, this response function was minimized by
the BoxWilson method [6, 8], with additional condition of increasing the FR
e©ciency for wood.
Models (5) or (6) were used to increase the ¦re-protective and ¦re-
extinguishing e©ciency of the original FRC with respect to peat and wood
by the BoxWilson method of steepest descent [8]. According to the condi-
tion, the e¨ectiveness of a ¦re-resistant, ¦re-extinguishing FRC is the higher,
the smaller E{y}. To formulate a new FRC composition, more e©cient simul-
taneously to the two combustible materials studied, the present authors have
assumed that antigradient function (5) at the FFE center is three-dimensional
vector g= (0.3463; 0.6521; 0.4971) and made a transition from the plan center to
a new point (X(i)=αigat i= 1,2,3,...where αi>0 is the setting step motion)
in the direction of vector g. In the chosen direction g, a consistent displacement
was performed at step α1= 0.2 and then at step α2= 0.4 with transition to the
¦rst point with coordinates: X(1)
1= 0.0693, X(1)
2= 0.1304, and X(1)
3= 0.0994
(in natural variables: x(1)
1= 6.13, x(1)
2= 2.84, and x(1)
3= 6.15) and then to
a point with coordinates X(2)
1= 0.1385, X(2)
2= 0.2608, and X(2)
3= 0.1988 (in
natural variables: x(2)
1= 6.17, x(2)
2= 2.87, and x(2)
3= 6.21).
For each of the two new FRC pertaining to the coordinates in the nat-
ural variables, ¦ve experiments were conducted to determine the e©ciency of
§ame-retarding and ¦re-extinguishing of peat and wood and which furnished the
experimental results, as well as average values of vectors ypeat and ywood listed
in Table 2. Thus, the new recipe made it possible to improve extinguishing
and ¦re-resisting properties of FRC for both peat and wood. Inasmuch as fur-
ther displacements in the selected direction gat α3= 0.6 has led to an FRC,
¦re tests of which showed lower ¦re-extinguishing and ¦re-protection e©ciencies
(see Table 2), the previous FRC with parameters x(2)
1= 6.17, x(2)
2= 2.87, and
x(2)
3= 6.21 was adopted as the best formulation. Application of the mathemat-
ical method of experiment planning allowed to optimize the FRC and increase
its ¦re-retardant and ¦re-extinguishing e©ciency with respect to both peat and
wood as compared to the initial recipe. The conclusions about the in§uence of the
86 V. Bogdanova et al.

Filtration and Heterogeneous Combustion
Table 2 Values of the vectors of observations (¦re retardant and ¦re-extinguishing
e©ciency of FRC) changing with transition from central plan FFE in direction of
vector gwith increments α
α
Factors
in positive
integer
variables
Values of the vectors of observations
x1x2x3ypeat, % ywood, %
0.2 6.13 2.84 6.15 y(1)
peat = (0.78; 1.55; 0.78; 1.15; 0.85)
y(1)
peat = 1.02
y(1)
wood = (3.09; 3.85; 3.75; 2.5; 5.84)
y(1)
wood = 3.81
0.4 6.17 2.87 6.21 y(2)
peat = (0.40; 0.41; 0.01; 0.31; 0.04)
y(2)
peat = 0.23
y(2)
wood = (2.55; 2.52; 2.25; 3.07; 3.87)
y(2)
wood = 2.85
0.6 6.22 2.91 6.27 y(3)
peat = (3.37; 1.53; 2.89; 2.22; 1.83)
y(3)
peat = 2.37
y(3)
wood = (5.41; 5.92; 4.61; 4.27; 4.67)
y(3)
wood = 4.97
selected factors x1,x2, and x3on ¦re-protection and ¦re-extinguishing e©ciency
for peat and wood were drawn based on the adequate model (6). Coe©cients of
these factors determine how fast the FRC ¦re-extinguishing and ¦re-protective
e¨ectiveness changes. Absolute values of the regression coe©cients (6) indicate
that the nitrogen content in the formulation (x3) exerts the greatest e¨ect on the
FRC ¦re-extinguishing and protective e¨ectiveness. Preemptive e¨ect of nitro-
gen on FRC ¦re-extinguishing and ¦re-protective properties is also con¦rmed by
the fact that the model includes nitrogen additionally in pair interactions with
phosphorus (x1) and bentonite (x2).
This fact con¦rms the present authors£ experimental data [11] that metal
phosphate ammonium-containing FR systems exhibit a complex mechanism of
¦re-¦ghting action, slowing down the thermolysis reaction of material in the
condensed phase and simultaneously inhibiting the combustion processes in the
gas phase. Thus, the dominant role in ¦re-retardant action of basically belongs to
the volatile nitrogenous products of their thermolysis, which correlates with the
experimental data on the quantitative admission of volatile nitrogen compounds
in the gas phase [13].
4 Concluding Remarks
Application of the mathematical experiment planning method to ¦nd the factors
that exert the decisive e¨ect on the ¦re-resistive and ¦re-extinguishing e¨ective-
ness of the synthetic nitrogen and phosphorus containing FR for wood and peat
allowed to con¦rm and re¦ne the mechanism of their action. It is found that the
dominant process in termination of burning of natural materials is the inhibition
of radical processes in the gas phase by volatile nitrogen-containing products. It
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ACCOMPLISHMENTS IN THE COMBUSTION SCIENCE IN THE LAST DECADE
is shown that the synergism of nitrogen-, phosphorus-containing FR is due to
their complex action: phosphorus is mainly involved in formation of organic min-
eral structures in the condensed phase, and nitrogen is an inhibitor for reactions
in the gas phase.
References
1. Bogdanova, V. V. 2003. The ¦re extinguish e¨ect of §ame retardants for synthetic
polymers and some native combustible materials. Chemical problems of develop-
ment of new materials and technologies. Minsk: Com. Art. 2:344375.
2. Bogdanova, V. V., and O. I. Kobets. 2008. Modi¦cation of metallophosphate aque-
ous dispersions for increase of extinguishing e©cioncy on peat. Sviridov Readings.
Minsk: Com. Art. 4:125133.
3. Bogdanova, V. V., and O. I. Kobets. 2009. Research of ¦re-retardant e©ciency of
ammonium metallophosphate compositions on the base of di- and trivalent met-
als depending on conditions of synthesis. Vestnik BSU. Ser. Chemistry. Biology.
Geography 1:3439.
4. Bogdanova, V. V., and O. I. Kobets. 2011. Regulation of physicochemical proper-
ties of metal ammonium phosphatic compositions showing ¦re-retardant and ¦re-
extinguish e¨ect. Sviridov Readings. Minsk: Com. Art. 7:2127.
5. Fedorov, V. V. 1971. Theory of optimal experiments. Moscow: Nauka. 312 p.
6. Sautin, S.N. 1975. Design of experiments in chemistry and chemical technology.
Leningrad. 48 p.
7. Ahnazarova, S. L., and V. V. Kafarov. 1978. Optimization of experiment in chem-
istry and chemical technology. Moscow. 320 p.
8. Asaturian, V.I. 1983. The theory of design of experiments. Moscow. 248 p.
9. Bogdanova, V. V., V. V. Usenya, O. I. Kobets, and V. G. Tishchenko. 1998. Inves-
tigation of the e¨ectiveness of chemical compositions for the extinguishing of peat-
bog ¦res. Problems of Silvics and Silviculture: Com. Scienti¦c w. Gomel. 49:108
114.
10. Bogdanova, V. V., O. I. Kob ets, and A. A. Lyudko. 2011. Fire-retardant properties
of metal phosphate suspensions based on natural raw materials. Chemical Reagents
and Fine Chemical Processes of Low-Tonnage Chemistry: Com. Scienti¦c w. Minsk.
272284.
11. Bogdanova, V. V., O. I. Kobets, and A. A. Lyudko. 2009. New approaches to the
creation of e¨ective and economical ¦re-extinguishing and ¦re-protectiv agents for
natural combustible materials. Emergency Situations: Prevention and Elimination.
5th Scienti¦c-Practical Conference (International). Minsk. 1:148151.
88 V. Bogdanova et al.