CHEMICAL KINETICS STUDY OF NITROGEN OXIDE SYNTHESIS IN A D.C. PLASMA JET : A PROPOSED MODEL

(1)

HAL Id: jpa-00219152

https://hal.archives-ouvertes.fr/jpa-00219152

Submitted on 1 Jan 1979

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

CHEMICAL KINETICS STUDY OF NITROGEN OXIDE SYNTHESIS IN A D.C. PLASMA JET : A

PROPOSED MODEL

J. Coudert, E. Bourdin, Jean-Marie Baronnet, J. Rakowitz, Pierre Fauchais

To cite this version:

J. Coudert, E. Bourdin, Jean-Marie Baronnet, J. Rakowitz, Pierre Fauchais. CHEMICAL KINETICS STUDY OF NITROGEN OXIDE SYNTHESIS IN A D.C. PLASMA JET : A PROPOSED MODEL.

Journal de Physique Colloques, 1979, 40 (C7), pp.C7-355-C7-356. �10.1051/jphyscol:19797175�. �jpa-

00219152�

(2)

JOURNAL

DE

PHYSIQUE

CoZZoque

C7,

suppZ6ment au n07, Tome

40,

JuiZZet 1979, page

C7- 355

CHEMICAL KINETICS STUDY OF NITROGEN OXIDE SYNTHESIS I N A D.C. PLASMA JET : A PROPOSED MODEL

J.F. Coudert, E. Bourdin, J.M. Baronnet, J. Rakowitz and P. Fauchais.

Laboratoire de Thermodyncmrique,

123,

rue

A.

Thomas 87060 Limoges cgdex, France.

INTXODUCTION. The experimental study of nitrogen oxide synthesis in a D.C. nitrogen-oxygen plasma jet shows that the final products, after quenching, have a concentration higher than the maximum predicted by equilibrium calculations at the same pressure /I/. Departure from equilibrium can be partialy explained by chemical kinetics considerations

:

quenching models from high temperature equili- brium have been proposed by POLAK /2/ from /3/ and /4/ and by AMMAN 151.

I. COMPUTING METHOD. Let us consider a mixture of I chemical species A. reacting in

J

chemical reactions with reaction rate constants k

'

I k. I ^j

^'

(I) V j i A i + J "iiAi

The thermodynamic temperature of the system is as- signed to follow a law T

=

f(t) which is supposed to describe the temperature history of the bulk gas from entrance of the torch to the end of the reac- tor. The pressure is assumed to be constant all along the system.

If diffusion processes are neglected the time depen- dence of the chemical composition of the system is calculated by solving the following differential system.

dyi Y. I Yi IT

(2) x = w i - (2 izl ^wi

^{+ - - )}

^{T dt} _. _.

where yi is the molar concentration of the i

cn

species,

p =

P/%T and wi the production and loss terms for ith specie.

j I

(3)

wi

=

j ~ i kj(vji -

^{V .}.)

vj 1

j r

121 Y1

if the initial concentration y.(O) are known, the differential system can be solved using the appro- priate predictor-corrector method proposed by WINSLOW /6/.

11. REACTIONS AND REACTION RATE CONSTANTS. We have used data gathered by PRUD'HOMME /7/, and these re- commended by BAULCH 181.

At temperature bel,ow 5000 K only the following neu- tral special have been considered

:

N2, 02, NO, N and 0. They are supposed to take part in the follo- wing reactions

(4) 0 2 + M 3 0 + O + M (5) N 2 + M 2 N + N + M (6) NO + M 3 N + 0 + M (7) O2

+

~~z ^NO ⁺ ^NO

(8) 0 + N ~ : NO + N (9) N

+ 02:

NO

+ 0

The selected rate constants are listed in Table I.

M is any one of the five species.

111. RESULTS. From some measurements performed on a nitrogen-oxygen plasma jet we have fitted the expe- rimental time-temperature history along the reactor in the following manner. The temperature at each end of the reactor is 300K, the gas is heated up to 5000K in see. The initial value of the heating rate (d~ldt) is 10' K/s, its mean value is 5 10*~/s, the heating law is parabolic with time. A similar parabolic law is used for the quenching stcp i'aring 9 with an initial rate - lo8 K/sec.

Molar fractions versus time are shown in figure I.

Figures I1 and 111 describe respectively the time evolution of the over all production of NO (d(NO)/

dt) and N, and the production of NO and N by each of the reactions (4) to

( 9 ) .

At the very beginning of the reaction (t<S~sec, T<4000 K) the production of NO is due mainly to reaction (7), a result which is not in agreement with the conclusions of ZEL'DOVICH /3/ and POLAK /2/ for this process. At the same time N is produ- ced by' reaction (8).

The maximum production rate of NO is reached bet- ween 8 and 12 usec (T=4000+5000+4800 K) and proces- ses (7) (8) and (9) produce an equivalent amont of NO. The production rate of N by process

(8)

is equal to the loss rate by process

( 9 )

and

[N]

is maximum.

Between 12 and 18 usec (T=4800+4200 K) the proces- ses (7) (8) and (9) always produce NO but with a lower rate until a zero rate is reached at 18 usec and the process (9) destroy N.

Between 18 and 60 ~sec(T=4200K+1500K) reactions (8) and (9) slightly destroy NO. The destruction of N by the process (9) is higher than the production by (8).

From t=60 usec (731500 K) the system is partially frozen especialy for NO since there is no more ato-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797175

(3)

mic nitrogen and oxygen recombines very slowly by the process (4) which governs [o] during the total reaction time.

Processes (5) and (6) are significant only at high temperature (T 4500

K).

CONCLUSION. At 1 atm. the frozen high temperature equilCbrium predicts a maximum of [NO:] of 7%, the kinetics model shows that it is possible to obtain up to 1 1

%

(a result which is in good agreement with experiment / 1 1) .

The important role of quenching rate is shown with our model, but, as far as we know, it is the only one which points out the equally important role of heating rate. The heating rate controls the maximum concentration of NO as the quenching controls the freezing of the high temperature mixture.

REFERENCES.

/I/ BARONNET J.M. et al, Journal de Chimie Physique 75, (1978), 949.

-

121 POLAK L.S. et al, Kinetics and Thermodynamics of Chemical Reaction in low-Temperature Plasma.

Moscow - Nauka (1965).

/3/ ZEL'DOT.TICI! Ya.E.,

PAIZERX.,

Physics. of SLocks Waves and High Temperature ~ ~ d r o d ~ n a m i c Pheno- mena, Academic Press, (1966).

I41 DUFF R.E. et al, J. Chem. Phys. 31, (1959), 1018.

151

AMMANN

P.R., TIMMINS R.S., A.I. Ch. E. Journal 12, (19661, 956.

-

/61 WINSLOW A.M., J. Phys. Chem., 2, (1977), 25 171 PRUD'HOMME R. et al, Rapport O.N.E.R.A. Paris

(1 969).

181 BAULCH D.L., High Temperature Reaction Rate Data,

N O

4, Leeds (19699.

Table I. Reaction rate constants .k=B T~ exp(-EIRT)

Figure I -5 -4

(4)d (4)r (5)d (5)r (6)d (6)r (7)d

. .

)lg~a[Eu/~)

^*---•

^Figure ^XII

a E