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Chemi-ionization induced by cyanogen in carbon monoxide-oxygen-nitrogen flames
CHEMI-IONIZATION INDUCED BY CYANOGEN IN CARBON MONOXIDE-OXYGEN-NITROGEN FLAMES A. VAN TIGGELEN, t J. PEETERS, AND C. VINCKIER
Laboratory of Inorganic Chemistry, University of Louvain, Louvain, Belgium The primary and secondary chemi-ionization processes in the burning of cyanogen were investigated by means of a "trace technique." To a stoichiometric carbon monoxide-oxygennitrogen flame, characterized by a complete absence of ions, small and varying amounts (up to 2%) of cyanogen were added. The seeding did not significantly affect the main flame parameters, such as the burning velocity and the final temperature (=t=2500~ Using a saturation-current method, the chemi-ionization rate was found to be directly proportional to the initial cyanogen content; the ion yield per cyanogen molecule amounts to 10-s. A mass-spectrometric study revealed that N O + is by far the most abundant ion; in a flame with an initialcyanogen content of 2%, [NO+]~az is 2 X 10 u ions c m -8. The "order" of each ionic species with respect to the added cyanogen was determined from the dependence of the m a x i m u m ion concentrations on the initialcyanogen content. O n the basis of these "orders" and of the ionizationrate and ion concentration data, it is concluded that N O + is the major primary ion, formed by the reaction C N % 2 0 -~ C O -{- N O + -{- e-. The rate constant is of the order of 10-s3 c m ~ molecule -2 sec-I. The N O + ions disappear mainly by dissociative recombination with electrons;the recombination coefficientwas equal to 1.6 • 10-7 c m s s e c -1.
Another primary ion, CO +, is formed by CN*(B s~) -{- 2 O --~ NO ~- CO + -I- e-. This reaction accounts only for a small fraction of the total observed ionization rate. However CO + is responsible for the production of the secondary ions: 02 +, CO2+, NO~+, and O8+. A set of reactions, fitting the observed "order" dependences, is proposed for the formation and destruction of most secondary ions, are kept sufficiently small, the physico-chemical properties of the CO/02 flames are not significantly affected by the added C2N2. On the other hand, when the fuel concentration is modified in pure C2N2/O2 flames, changes also occur in many other parameters, such as the flame temperature and the O-atom concentration. I t appears, therefore, that the "trace technique" offers the distinct advantage of enabling one to isolate the influence of a single parameter, i.e., the initial C2N: concentration on the rates of the primary and secondary ionization processes.
Introduction The chemi-ionization phenomena in hydrocarbon flames have been studied in great detail. H I t was shown that the primary ion in such flames is CHO +, and that the most abundant ion, I~O +, is formed by transfer of a proton from CHO + to
H20. Chemi-ionization occurs to an even greater extent in cyanogen flames. Evidently, the iongeneration mechanism in such flames, characterized by the absence of hydrogen, is entirely different from the mechanism in hydrocarbon flames. In the present work, an investigation was carried out of the ehemi-ionization processes in atmospheric-pressure carbon monoxide-oxygen flames to which small and varying amounts of cyanogen were added. (In pure C0/02 flames, ionization does not occur). If the trace amounts t Deceased
Experimental All measurements were carried out on a stoichiometric C 0 / 0 2 flame diluted with 25% N2, to which traces (0.3% to 2 % ) of C2N2 were added. The final flame temperature was approximately 2500~ Two experimental techniques were used: a
311
312
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
saturation-current method and flame-ion mass spectrometry. I n order to measure the characteristic saturation current, a fiat flame was stabilized on a Powling-Egerton type burner (with surface area S = 0.23 cm2); a potential difference was applied between the burner (cathode) and a brass plate (anode) located parallel to the burner surface at a distance of 5 ram. With a sufficiently high potential difference, the current was equal to the saturation value i, which was solely determined by the total amount of chemi-ions produced per unit time in the flame front, and which was therefore related to the kinetics of the chemiionization process. 4 The flame-ion mass spectrometer has been described in detail elsewhere,s In a flame containing 2% C2N2, all ionic species were identified, and their absolute concentrations were recorded as a function of the axial distance. In addition, data were obtained on the variation of the maximum concentration of each ionic species upon changing the amount of added C2N2 from 0.3 % to 2%.
(b ) Recombination Coefficient of NO + A mass-spectrometric study of the stoichiometric CO/O2/N2 flame, with 1.2% C2N: added, revealed that NO + is by far the most a b u n d a n t chemi-ion. I n the burned gases, where U + - - 0, and where the stream velocity v8 -- 350 cm/s, the NO + ion disappears only by recombination with electrons: N O + d- e---~ N d- O.
[i]
The ion-electron recombination coefficient a can be derived from the NO + decay curve by plotting 1/[-NO+] versus the" distance x, where x is the distance from the [-NO+']max point (see Fig. 2); a value ~No+ = 1.6 :t: 0.1 X 10-7 ion-1 cms sec-1 was obtained. The recombination coefficient of NO + derived here is about three times higher than the value found by BiondP at the same temperature. Using the same method as described above, the recombination coefficient of the HaO+ ion was measured6 in the same CO/02/N2 flame to which 1.8% CH4 had been added (instead of C2N2): a~,o+ = 4.47 X 10-7 ion-1 cm 3 see-1.
Results
In CH4/O2/N2 flames, however, the authors a
(a ) Chemi-Ionization Reaction Rate The chemi-ionization rate U+ i s directly proportional to the measured saturation current i, according to Eq. (1):
U+ = i/Seoe,
----
kCC2N2]01.
iO
o
'C3
(1)
where S = flame area (0.23 cm2), e0 = thickness of ionization zone, and e = electron charge. By varying only the percentage of C~N2 added to the CO/O2/N2 flame, the influence of the fuel concentration on the chemi-ionization rate can be isolated. Experimentally a linear relationship is found between the saturation current i and the initial conceutration of cyanogen I-C~N~]0 (Fig. 1). Since e0 can justifiably be assumed to be independent of ['C2N2]0, one therefore has U+
O
)
(2)
According to Eq. (2), only one C2N2 molecule or C2N: fragment is involved in the primary chemiionization reaction. From the proportionality constant, the ion yield per fuel molecule can be easily evaluated: it amounts to about 10-5, which is an order-of-magnitude larger than the corresponding ion yield for methane, s
9 15
Inltiat mote fraction of C2Nz
Fro. I. Saturation current versus initialcyanogen mole fraction in a stoichiometric CO/Oz flame, diluted with 2 5 % N2. Flame temperature, TI 2500~
CHEMI-IONIZATION INDUCED BY CYANOGEN found aH30+ to be equal to 1 . 9 • 0.1X 10-z ion-1 cma sec-1; about the same value could be derived 6 for H.~/02/N2 flames to which traces of CH4 were added. The latter value of a~ao+ agrees very well with the results of other workers. L2 The higher values of the recombination coefficients in C0/0.2/N2 flames possibly can be explained by the high electron temperature T~ in the burned gases. Indeed, in a CO/02/N~ flame containing traces of CH4, a T~ value of 30,000~ was measured, while in a H2/02/N2 flame seeded with methane, T, was about equal to the gas temperatureY The higher electron velocity associated with the
313
NO + ..~
-10
"~
§
9
§
..03
0
'8
~
- ~ . n+
CC NC)~
,.
"///
f
o
gS
I
distance x (mm)
Fro. 3. Concentration profiles of the chemi-ions in a stoichiometric CO/O~ flame, diluted with 25% N~, and to which 2% CaN2 is added.
~No']
E
~.o*
......
could be ruled out as real chemi-ions; they are clusters formed near the sampling orifice of the spectrometer as a result of a nucleation process between CO+ and NO + ions and CO molecules.
: 1.6-'0.1 1 0 7 c m ~ s e d ~
o
1 i
2 i
distance x (ram)
3 J
Fro. 2. Determination of the NO+electron recombination coefficient in the burned gases of a stoichiometric CO/O~ flame diluted with 25% Ns, and containing 1.2% C2N2. The flow velocity v, --350 cm sec-z.
elevated Te value could account for the observed increase of the recombination coefficient.
(c) Identification and Concentration Profiles of the Chemi-Ions I n addition to the NO + ion, the following ion masses could be detected: M = 28: N.~+ or CO+; M = 32: 02+; M = 44: C02+; M = 46: NO2+; M = 48:C4 + or 08+; M = 56: C202+; M = 58: N C 0 2 +. O n the basis of theoretical considerationss and experimental results,e the ions C20~ + and N C O ~ +
i
:/ -2.25 '
-2.00 -1.75 i !.Og,oOfthe initial. C2N~mole fraction
FIG. 4. Determination of the "order" dependence of O~+ on the initial C~N~ mole fraction in a stoichiometric C0/02 flame, diluted with 25% N2.
314
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES TABLE I
Discussion
"Order" dependences of flame-ions on the initial cyanogen content
Ion
NO + CO +
"Order" dependence on [C~N~]0 (•
(a ) The Primary Chemi-Ionization Process Several reactions can be considered as possible primary ionization steps: AH (kcal mole-1)
0.5 1
O2 §
1
CO2 § NO~+ O~+
0.7 0.7 0.5
Because of the very high enthalpy of formation of N2+,9 the ion with mass 28 is most probably CO + . On the basis of the experimentally determined "order" dependence [-see Sec. ( d ) l the ion with mass 48 cannot possibly be C4+. The alternative, O3+, has also been observed during electron bombardment of 02.1~ Figure 3 shows a semilogarithmic plot of the chemi-ion concentrations l-CO+-], ~NO+-], [C02+-l, l-NO2+-], and l-On+-] versus the axial distance in the stoichiometric CO/O_,/N2 flame containing 2% C_,N2. The origin of the distance scale x --- 0 is taken a t the point where [NO +] maximizes. Figure 3 illustrates clearly t h a t {-NO+'] represents 98% of the total ion concentration somewhat downstream of the flame front (x 0.1 mm).
(d) Dependence of the Maximum Ion Concentrations on the Initial C2N2 Content. A t the point where each ion reaches its maximum concentration, the rate of formation is equal to the rate of disappearance. Defining the "order" dependence of the maximum ion concentration ['X+']max on the initial C2N2 content [-C2N2-]0 as the exponent m in the expression [-X+Jmax ~'~ [-C~N2-]om, that "order" is obviously related to the number of C~N2 fragments directly or indirectly involved in the formation and disappearance of the considered species X +. As an example, a logarithmic plot of the relative maximum 02+ current versus [-C2N~-]o is shown in Fig. 4. The "order" dependence is equal to the slope of such a plot. The results for all identified flame-ions are listed in Table I.
CN -{- O -~ 0---~ CO ~ NO+-~- eC~* -~- 02--~ CO -~- CO+-~- eCN* ~- 0 2 - ~ CO -~ NO + -{- eNO -{- N -b N - ~ N~ + NO+-~- eNO ~- C + O---) CO + NO+-}- e-
--12 -[-16 432 --11 --44
[-2] [-3] [-4] [5-] [-6]
Because the N and C concentrations are very low in comparison with the O concentration, one can neglect the reaction rates of [ 5 ] and [6]. Indeed, the O-atom concentration amounts to more than 1% of the total particle concentration in CO/O~/N2 flames. II In hydrogen flames, where the ion yield per fuel molecule is of the order of 10-8, the primary chemiionization reaction CH ~ 0--~ CHO + ~- e- is almost thermoneutral. The ion yield in the flames studied in this work is somewhat larger than 10- 5, so t h a t the highly endothermic and, therefore, rather slow reaction E4-] can be ruled out as the major chemi-ionization process. Further, in a spectroscopic study of the C O / O : / N : flame with 1% C2N~ added, the C2" emission intensity (Swan bands) was found to be extremely weak and, thus, Reaction E3"] can also be rejected. On the other hand, i t is well known t h a t termolecular reactions may be responsible for the excitation of added metals and of radicals in flames characterized by a high radical concentration. 1: A similar type of termolecular process, Reaction [-2], appears to be the major primary chemi-ionization reaction in the investigated flame. Experimental evidence in its favor is now presented: 1. Since CN is formed by reaction of C2N2 with an O atom and reacts mainly with 02, and further since the physico-chemical properties (such as the mean particle concentrations) of the C0/O2 flame are not significantly affected by the addition of C2N2 traces, application of the steady-state assumption yields [-CN-]~-~ [C2N2-]0. Reasoning along the same lines, one would expect on the basis of Reaction [-2] that U+ ~'~ [C2N2]0, as was experimentally verified, 2. At the maximum NO + concentration, where
CHEMI-IONIZATION INDUCED BY CYANOGEN
315
U + ~ a E N O + ] [ e - ] = aENO+] 2, it was found that ENO +] ~ EC2N2]$ "5. If NO + were not a primary ion, but the product of a secondary process, involving an ion listed in Table I and an NO radical, the "order" dependence of [NO +] on [C2H2]0 would necessarily be larger than the observed value, since [NO] in the reaction zone is proportional to [C2N2~0, and since the "orders" of the other ions are larger than 0.5. Likewise, on the basis of the observed "order" dependences, the ionization of NO by "hot" electrons can be excluded. 3. Applying the steady-state assumption to the combustion mechanism of C2N2, the estimated CN concentration in the investigated flame containing 2 % C2N2 is of the order of 3 X 1015em-3. About the same value is obtained by extrapolating the estimate of Bulewiez for low-pressure C2N2/O2 flames,is Furthermore, if the measured value u is taken for the O-atom concentration, and if the rate constant ks is assumed equal to the rate constant ofIS
2 0 + M--~ 02 + M,
glSJ
one finds for the rate of Reaction F2J that ( U + ) e a l c - ~- - 1016 ion cm--a see-1. Estimating the thickness of the ionization zone eo to be 2 X 10-2 cm, i.e., the approximate thickness of tLe luminescent region, the value of U+ inferred from the measured saturation current i by means of Eq. (1) is (U+)exp----2.2X 1016 ion cm-s sec-1. Moreover, at the maximum NO + concentration, a[-NO+]I-e- ] = 6.8 X 1015 ion cm-s sec-1. The TABLE II Ion-molecule reactions occurring in CO(+C2N2)/Os/N~ flames AH Reaction (kcal mole-i) No. CO + + NO --* NO + -t- CO CO + -t- 02 --* 02 + -{- CO CO + -{- COs --* CO2+ -I- CO COs+ + NO --* NO + -{- CO2 CO2+ -t- 02 ~ 02 + -{- COs C02 + + NO --* NO2+ -{- CO O~+ -t- O --* 03 + 02 + + O~ --* 03 + + 0 02 + + NO --* NO + + 02 NO2+ + NO-* NO+ + NO2 03 + + CO --* CO2+ + O5
-- 109 -44 -3 - 106 -41 --33 ? ?
--65 -16 ?
[7] [S] [9] [101 [11] [12] [131 [14] [15] [16] [17]
/,/;
o u.~ [c,.j'; ~
d
-7.8
-5.4
9t.6
/
"
/-2~s
,
-8.0 -~
,
-2;,
tog,oof themotefractionof~N Fro. 5. Dependence of the saturation current and of the CN*(B ~2:) emission intensity on the initial C~N2 mole fraction in a lean flame with composition: 24.@% CO, 35.4% 05, 38% N~, and 2% H2.
agreement between theoretical and experimental values of the chemi-ionization rate U+ can be considered as satisfactory. The above constitutes sufficient evidence that Reaction [-2-]is most probably the major primary chemi-ionization process. From Table II, however, it appears that none of the other observed chemi-ions can be formed in ion-molecule reactions involving NO + , because of the low ionization potential of NO. One is forced to conclude, therefore, that in addition to NO +, at least one of the other flameions must be a primary ion. Reasonable primary chemi-ionization reactions can be written only for CO +, namely Reactions [-3] and F19J: CN*(B 22~) -t- 2 0---+ NO ~- C O + + eAH = 19 kcal/mole.
[-19]
I n the stoichiometric C0/O2 flame, containing only traces of C2N2, the CN* (B 2~) emission intensity could not be distinguished from the NO-O continuum. For this reason, the "order" dependence of the CN emission intensity on [C2N2]0 was investigated in a lean flame (24.6% CO, 35.4% O2, 38% N2, and 2% H2) with a markedly weaker continuum. The observed dependence of the CN* emission intensity on
316
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
ICoN]J0 is shown in Fig. 5. It follows that: [ C N * ] ~ [C2Ns]01'2.
(3)
Because of the very high CO2 concentration, the CO+ ion very probably disappears mainly by charge transfer to COs. Consequently, at the [CO + ] maximum where steady state applies, one should have, according to Reaction [19],
At the point where [CO2+] reaches its peak value, [ C O + ] > [O3+]; furthermore, since the rate constants of charge transfer reactions involving 02 molecules are rather low, 28 Eq. (5) can be rewritten as follows:
[C0s
=
kgl-CO+][COs] ~- kg[CO] + k~o[NO] + k121110]"
[CN*][O] s,--, [CO+][CO2],
(s)
or, using Eq. (3),
[co+i,,,,,,,-,., ([csNs]o,.SEO]b/[cos].
(4)
Since it is reasonable that in lean flames the ratio FO]2/[CO2] exhibits a slight negative dependence on [C]Ne]0, Eq. (4) appears to be in good agreemeat with the experimentally obtained relation [CO +] ~ [-CsNs]0L If Reaction [3] were responsible for the formation of CO+ , one should find [CO+] ~-~ ([C2N2]d[02])/[C02], since the formation of C2" most likely involves two CsN2 fragments: CN + CN ~ C2" + Ns.
[20]
It can readily be shown that, in the flames considered, the order with respect to [C]Ns]0 of the CN* formation rate is identical to the "order" of the CN* emission intensity. In lean flames, where Eq. (3) applies, the total ehemi-ionization rate U+ also obeys the relationship U + ~ [-CsN]]02"2 (see Fig. 5). Hence, one may assume that CN* is formed in a process similar to the primary chemi-ionizatioa reaction [2]: CN + O + 0 ~ CN* + O2
[-21]
Reaction [21] has already been proposed for C]Ns/O2 flames15 and C2N2/0 systems. 2~
(b ) The Secondary Chemi-Ionization Processes At the respective maxima of the CO2+, NO2+, and 02+ concentrations, the following equations may be written (see Table II) :
[C02'+]max --,., k9[00+][C02] + k17[O3+][CO]
Further, one may assume that [NO] is proportional to [CsNs]o from steady-state arguments, and of course that [CO] as well as [CO2] are independent of the initial cyanogen content. As [ C O ] > [NO], the "order" dependence of [C02 +] on [C]Ne]0 should approach unity. The experimentu[ vatue is 0.7 (see Table I). From Eq. (6) one deduces
[NO2+],~a~ = klsECO2+]/kle.
(9)
Equation (9) implies that the "order" dependence of NOs+ should be the same as that of C02 +, as the experimental data have shown (see Table I). From Fig. 3, it follows that kls/kt~- 2. At the point where [O2+] peaks, FCO+])) [COs+], and the value of {k23[-O]+ kt,[Os]} is probably higher than k16[NO]. Therefore, [O]]ma~ can be approximated by
EOs+],... ~ k,ECO+]EOs]/{h3[O] + k2,[o~]l.
(lo) Hence, the same "order" dependence as for CO+ should be obtained: [ O ] + ] m ~ [-C2Ns]02 (see Table I).
Conclusion
The much higher ionization level in cyanogen flames as compared to hydrocarbon flames must be ascribed to the fact that, in the former, one of the chain-carrier radicals (CN) is involved in the primary ionization step:
(--- kg[CO] + klo[NO] + k~2[NO] + k~1[O2]
(5) ENO]+Jm~ = kl]ECO]+]FNOJ/k16[NO] (6) ks[CO+J[02] + k,lECO]+][O2] [O2+]m~ = k13[0] + k.[O]] + klsENO] " (7)
CN + O + 0 --~ CO + NO + + e-. Using the experimental value of the rate U+ = 2 X 10TM ions cm-a see-2, the measured value of [O] ~ 6 X 1016cm-s, and the estimated value of [-CN] "~ 3 X 1025 cm-s, the rate constant of this process is found to be approximately 10-38 cm6
CHEMLIONIZATION INDUCED BY CYANOGEN molecule-2 sec-1. This value seems to be somewhat high for such a termolecular process. I t is not impossible that the reaction proceeds by a two-step process involving a short-lived intermediate. As was also observed in the burning of hydrocarbons, 8,4 excited species do not appear to be precursors of the major primary ion in cyanogen flames. Chemiluminescence and chemi-ionization should be considered as parallel, rather than consecutive, phenomena. The following process can be taken as an example of this parallelism:
CN ~ 0 ~- 0 --* CN* ~ 02. Acknowledgments This research has been sponsored in part by the Aerospace Research Laboratories through the European Office of Aerospace Research, O.A.R., United States Air Force, under contract F 61052-70-C-0016. Support has also been received from the Fonds de la Recherche Fondamentale et Collective. One of the authors (C.V.) is much indebted to the Institut pour la Recherche Scientifique dans rIndustrie et l'Agriculture for the allocation of a postgraduate fellowship, and another (J.P.) expresses his thanks to the Fonds National de la Recherche Scientifique for a postdoctoral research fellowship. REFERENCES 1. GREEN, J. A. AND SUGDEN, T. M.: Ninth Symposium (International) on Combustion, p. 607, Academic Press, 1963. 2. C~COTE, H. F.: Ionization in Hydrocarbon Flames, paper presented at the 26th Meeting of Propulsion and Energetics Panel, AGARD, Pisa, Italy, Sept. 1965.
317
3. I~ET~.RS, J., VINCKI~R, C., INn VAN T m O ~ N , A.: Oxidation Combust. Revs. 4, 93 (1969). 4. PEETERS, J. AND VAN TIGGELEN, A. : Twelfth Symposium (International) on Combustion, p. 437, The Combustion Institute, 1969. 5. BIoNnI, M. A.: Can. J. Chem. ~7, 1711 (1969). 6. VINCKIER, C.: Ph.D. thesis, University of Louvain, 1969. 7. BRADLEY, T. ANn MATrHEWS, K. J.: Eleventh Symposium (International) on Combustion, p. 539, The Combustion Institute, 1967. 8. MILNE, T. A. AND Gm~.~r~, F. T.: Tenth Symposium (International) on Combustion, p. 153, The Combustion Institute, 1965. 9. FIELD, F. H. AND FRANKLIN, J. L. : Electron Impact Phenomena, p. 271, Academic Press, 1957. 10. FRANKLIN, J. L..,_ND MUNSON, M. S.: Tenth Symposium (International) on Combustion, p. 561, The Combustion Institute 1965. 11. KILm~, J. K. AND DUNH~, P. G.: Eleventh Symposium (International) on Combustion, p. 899, The Combustion Institute, 1967. 12. ZE~G~RS, P. J. Tm AND ALKAMXDE,C. Tm J. : Tenth Symposium (International) on Combustion, p. 33, The Combustion Institute, 1965. 13. AILLmT, M. AND VAN TIGGELEN, A.: Bull. Soe. Chim. Belg. 77, 433 (1968). 14. BAsco, N.: Proc. Roy. Soc. (London) A283, 302 (1965). 15. Bu~wIcz, E. M. : Twelfth Symposium (International) on Combustion, p. 957, The Combustion Institute, 1969. 16. WISE, I-I. ANDROSSER, W. A. : Ninth Symposium (International) on Combustion, p. 733, Academic Press, 1963. 17. SETSER, D. W. AND TRUSH, B. A.: Proc. Roy. Soc. (London) A~88, 275 (1965). 18. FEHSENFELD, F. C., FERGUSON, E. E., AND SCHm~.LTEKOFF, A. L.: J. Chem. Phys. 441, 3022 (1966).
COMMENTS T. D. Wilkerson, University of Maryland. W h a t is the precision attached to the order numbers given for the different species? Authors' Reply. The precision was about 0.1. A. Fontijn, AeroChem Research Laboratories, Inc. Have the authors considered two-body ionization reactions as an alternative to the three-body reactions discussed? For example, reactions involving 02" in concentrations proportional to [0] ~ would maintain the same reactant order dependence as the authors' report. Such reactions appear a priori somewhat more likely
than the three-body four-center processes of the paper. Are three-body reactions fast enough to explain the observed ionization rates? Reaction (III) involving CN*, a short-lived species, cannot yield very large ion-formation rates. Threebody chemi-ionization reactions have from time to time been proposed. However, further study has usually shown the ionization occurs in a sequence of steps involving a two-body reaction as the ion-production process.
Authors' Reply. In the conclusion of our paper, we state that the reaction CN ~ 0 ~- 0 --* NO + ~ CO -{- e- m a y perhaps proceed in two
318
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
steps via a short-lived intermediary, which could be 02* as well as (CNO)*. We consider CN* responsible for only a small fraction of the total amount of ions produced. With a view to the determination of the reaction order of the chemi-ionization process, we have recently started an investigation of the dependence of ion-formation rates and emission intensities on the total pressure. A t pressures near 1 arm, O2" is deactivated mostly by collisions; if 02* were involved in the formation of ions, one would therefore expect a p2 dependence of the ion formation rate, whereas t h a t dependence would more likely be Pa if two O atoms react directly with a CN radical to form an ion.
E. M. Bulewicz, University College of Swansea. A. On the Experiment. (a) Was the use of a stoichiometric C 0 / 0 2 / N 2 flame the best choice? At stoichiometry, most radical concentrations will vary rapidly for only a small change in the equivalence ratio. 1 When 1.8% C2N2 was added, this could put the system over to the fuel-rich side, with possibly a considerable change in [O]. (b) W h a t tests were made to see if the C2N2 added had any influence on the CO/O2/N2 flame used? (c) Were the gases dried? (d) I n the experiment on CO +, 2% H2 was added. We have found that the addition of such quantities of H2 to C2N2/O2 flames at reduced pressure can have a very marked effect on both ionization2 and CN emission. ~
B. On Interpretation. At the Ninth Symposium, Padley and myself presented a paper on the ionization in C2N2/02 flames at reduced pressure, 2 and we considered the over-all reaction CN -[- O -4- O - ~ CO -~- NO + -~- e- as one of the possibilities. Both this reaction and NO -4- N -{- N --~ NO + ~- N2 ~ e- would agree with our results. We favored the latter, particularly as it is considered to be responsible for NO + production in flow systems. At the Twelfth Symposium, I presented some results on the emission spectrum of C2N2/O2 flames at reduced pressure. 1 In these flames, CN emission reached a maximum well on the fuelrich side (at -~h = 0.5, h being defined as O2 present/02 required for combustion to CO N~) while ionization is at maximum slightly on the 02-rich side. If, as it is suggested, both CN* and NO + were produced from CN -~ O ~ O, we would expect CN* and ions to run very much in parallel. I t is possible, but not very probable, that chemi-ionization and excitation mechanisms are different in C2N2/02 and in C0/O2/N2 ~ C2N2
flames. Could one, therefore, propose an alternative mechanism? In C2N2/02 flames not CN* and NO +, but CN* (violet system) and NO* (3" bands) run in parallel, at all compositions, there being apparently no link between NO chemiluminescence and chemi-ionization. Could the reaction (over all, but not necessarily in one step) CN q- O -k O lead to the production of CN* and NO*, NO + being produced by NO* N + N, with possibly a contribution from CN -4- O q- O only under 02-rich conditions? REFERENCES 1. E. M. BULEWICZ: Twelfth Symposium (International) on Combustion, p. 957, The Combustion Institute, 1969. 2. E. M. BULEWICZ AND P. J. PADLEY: Ninth Sym-
posium (International) on Combustion, p. 647, Academic Press, 1963. 3. E. M. BULEWICZ, F. J. PADLEY, AND R. E. SMITH: To be published.
Authors' Reply. A. The fact that the saturation current increases linearly with the added C~N2, up to 1.8e/v C2N2 (see Fig. 1), indicates that the parameters affecting chemi-ionization do not vary appreciably in that C2N2 concentration range. Moreover, the burning velocity changed by only about 10% in that range. The gases were not dried. However, no traces of HaO+ or of any other H-containing ion were detected. We found that the addition of a few percent H2 to the C O ( ~ C2N2)/O2/N2 flame did not significantly alter the ion yield per C2N2 molecule. B. The reaction NO -[- N q- N --~ NO + qN2 q- e- cannot possibly be a primary ionization step in our system; on the basis of such a reaction, one would expect the ion-formation rate to vary as the third power of the C2N2 content, whereas a linear relationship was actually observed. As mentioned in the paper, recent experiments of ours have shown that, in CO/CeN:/Oe/N2 systems, the ratio of the saturation current to the CN* emission intensity (violet system) remains constant over an extended range of flame composition and flame temperature. For flames with total equivalence ratio ~t = {[COl0 -44[C2N2]0}/2[0210 between 0.6 and 1.2, final flame temperature between 2300 ~ and 2550~ and C2N2 content between 1% and 3%, the mentioned ratio was constant to within 10% (which is about equal to the experimental error). Both the saturation current and the CN* emission intensity showed a rather flat maximmn near stoichiometric (~t = 1).
CHEMI-IONIZATION INDUCED BY CYANOGEN It follows from the above that, in the flames investigated here, the chemi-ions and the excited CN* radicals are formed from the same reaction partners--most likely CN + O + O. In view of the results of Bulewicz, 1 therefore, one is forced to conclude that the chemi-ionization and/or the CN* production mechanisms are not the same in pure C2N~/O2 flames as in CO(-~ C2N2)/O2/N2 flames, at least if the electron concentration in a flame can be considered to be a meaningful indicator of ion-formation rates (see authors' reply to comment by Padley
319
on paper by J. Peeters et al., following paper This Symposium). In fact, it should not surprise us that the mechanisms are different, because of the much higher temperatures of pure C2N2/0~ flames as compared to CO/Oe/N2 flames containing about 1% C~N2. REFERENCE 1. BULEWICZ, E. M. : Twelfth Symposium (International) on Combustion, p. 957, The Combustion Institute, 1969.
Laboratory of Inorganic Chemistry, University of Louvain, Louvain, Belgium The primary and secondary chemi-ionization processes in the burning of cyanogen were investigated by means of a "trace technique." To a stoichiometric carbon monoxide-oxygennitrogen flame, characterized by a complete absence of ions, small and varying amounts (up to 2%) of cyanogen were added. The seeding did not significantly affect the main flame parameters, such as the burning velocity and the final temperature (=t=2500~ Using a saturation-current method, the chemi-ionization rate was found to be directly proportional to the initial cyanogen content; the ion yield per cyanogen molecule amounts to 10-s. A mass-spectrometric study revealed that N O + is by far the most abundant ion; in a flame with an initialcyanogen content of 2%, [NO+]~az is 2 X 10 u ions c m -8. The "order" of each ionic species with respect to the added cyanogen was determined from the dependence of the m a x i m u m ion concentrations on the initialcyanogen content. O n the basis of these "orders" and of the ionizationrate and ion concentration data, it is concluded that N O + is the major primary ion, formed by the reaction C N % 2 0 -~ C O -{- N O + -{- e-. The rate constant is of the order of 10-s3 c m ~ molecule -2 sec-I. The N O + ions disappear mainly by dissociative recombination with electrons;the recombination coefficientwas equal to 1.6 • 10-7 c m s s e c -1.
Another primary ion, CO +, is formed by CN*(B s~) -{- 2 O --~ NO ~- CO + -I- e-. This reaction accounts only for a small fraction of the total observed ionization rate. However CO + is responsible for the production of the secondary ions: 02 +, CO2+, NO~+, and O8+. A set of reactions, fitting the observed "order" dependences, is proposed for the formation and destruction of most secondary ions, are kept sufficiently small, the physico-chemical properties of the CO/02 flames are not significantly affected by the added C2N2. On the other hand, when the fuel concentration is modified in pure C2N2/O2 flames, changes also occur in many other parameters, such as the flame temperature and the O-atom concentration. I t appears, therefore, that the "trace technique" offers the distinct advantage of enabling one to isolate the influence of a single parameter, i.e., the initial C2N: concentration on the rates of the primary and secondary ionization processes.
Introduction The chemi-ionization phenomena in hydrocarbon flames have been studied in great detail. H I t was shown that the primary ion in such flames is CHO +, and that the most abundant ion, I~O +, is formed by transfer of a proton from CHO + to
H20. Chemi-ionization occurs to an even greater extent in cyanogen flames. Evidently, the iongeneration mechanism in such flames, characterized by the absence of hydrogen, is entirely different from the mechanism in hydrocarbon flames. In the present work, an investigation was carried out of the ehemi-ionization processes in atmospheric-pressure carbon monoxide-oxygen flames to which small and varying amounts of cyanogen were added. (In pure C0/02 flames, ionization does not occur). If the trace amounts t Deceased
Experimental All measurements were carried out on a stoichiometric C 0 / 0 2 flame diluted with 25% N2, to which traces (0.3% to 2 % ) of C2N2 were added. The final flame temperature was approximately 2500~ Two experimental techniques were used: a
311
312
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
saturation-current method and flame-ion mass spectrometry. I n order to measure the characteristic saturation current, a fiat flame was stabilized on a Powling-Egerton type burner (with surface area S = 0.23 cm2); a potential difference was applied between the burner (cathode) and a brass plate (anode) located parallel to the burner surface at a distance of 5 ram. With a sufficiently high potential difference, the current was equal to the saturation value i, which was solely determined by the total amount of chemi-ions produced per unit time in the flame front, and which was therefore related to the kinetics of the chemiionization process. 4 The flame-ion mass spectrometer has been described in detail elsewhere,s In a flame containing 2% C2N2, all ionic species were identified, and their absolute concentrations were recorded as a function of the axial distance. In addition, data were obtained on the variation of the maximum concentration of each ionic species upon changing the amount of added C2N2 from 0.3 % to 2%.
(b ) Recombination Coefficient of NO + A mass-spectrometric study of the stoichiometric CO/O2/N2 flame, with 1.2% C2N: added, revealed that NO + is by far the most a b u n d a n t chemi-ion. I n the burned gases, where U + - - 0, and where the stream velocity v8 -- 350 cm/s, the NO + ion disappears only by recombination with electrons: N O + d- e---~ N d- O.
[i]
The ion-electron recombination coefficient a can be derived from the NO + decay curve by plotting 1/[-NO+] versus the" distance x, where x is the distance from the [-NO+']max point (see Fig. 2); a value ~No+ = 1.6 :t: 0.1 X 10-7 ion-1 cms sec-1 was obtained. The recombination coefficient of NO + derived here is about three times higher than the value found by BiondP at the same temperature. Using the same method as described above, the recombination coefficient of the HaO+ ion was measured6 in the same CO/02/N2 flame to which 1.8% CH4 had been added (instead of C2N2): a~,o+ = 4.47 X 10-7 ion-1 cm 3 see-1.
Results
In CH4/O2/N2 flames, however, the authors a
(a ) Chemi-Ionization Reaction Rate The chemi-ionization rate U+ i s directly proportional to the measured saturation current i, according to Eq. (1):
U+ = i/Seoe,
----
kCC2N2]01.
iO
o
'C3
(1)
where S = flame area (0.23 cm2), e0 = thickness of ionization zone, and e = electron charge. By varying only the percentage of C~N2 added to the CO/O2/N2 flame, the influence of the fuel concentration on the chemi-ionization rate can be isolated. Experimentally a linear relationship is found between the saturation current i and the initial conceutration of cyanogen I-C~N~]0 (Fig. 1). Since e0 can justifiably be assumed to be independent of ['C2N2]0, one therefore has U+
O
)
(2)
According to Eq. (2), only one C2N2 molecule or C2N: fragment is involved in the primary chemiionization reaction. From the proportionality constant, the ion yield per fuel molecule can be easily evaluated: it amounts to about 10-5, which is an order-of-magnitude larger than the corresponding ion yield for methane, s
9 15
Inltiat mote fraction of C2Nz
Fro. I. Saturation current versus initialcyanogen mole fraction in a stoichiometric CO/Oz flame, diluted with 2 5 % N2. Flame temperature, TI 2500~
CHEMI-IONIZATION INDUCED BY CYANOGEN found aH30+ to be equal to 1 . 9 • 0.1X 10-z ion-1 cma sec-1; about the same value could be derived 6 for H.~/02/N2 flames to which traces of CH4 were added. The latter value of a~ao+ agrees very well with the results of other workers. L2 The higher values of the recombination coefficients in C0/0.2/N2 flames possibly can be explained by the high electron temperature T~ in the burned gases. Indeed, in a CO/02/N~ flame containing traces of CH4, a T~ value of 30,000~ was measured, while in a H2/02/N2 flame seeded with methane, T, was about equal to the gas temperatureY The higher electron velocity associated with the
313
NO + ..~
-10
"~
§
9
§
..03
0
'8
~
- ~ . n+
CC NC)~
,.
"///
f
o
gS
I
distance x (mm)
Fro. 3. Concentration profiles of the chemi-ions in a stoichiometric CO/O~ flame, diluted with 25% N~, and to which 2% CaN2 is added.
~No']
E
~.o*
......
could be ruled out as real chemi-ions; they are clusters formed near the sampling orifice of the spectrometer as a result of a nucleation process between CO+ and NO + ions and CO molecules.
: 1.6-'0.1 1 0 7 c m ~ s e d ~
o
1 i
2 i
distance x (ram)
3 J
Fro. 2. Determination of the NO+electron recombination coefficient in the burned gases of a stoichiometric CO/O~ flame diluted with 25% Ns, and containing 1.2% C2N2. The flow velocity v, --350 cm sec-z.
elevated Te value could account for the observed increase of the recombination coefficient.
(c) Identification and Concentration Profiles of the Chemi-Ions I n addition to the NO + ion, the following ion masses could be detected: M = 28: N.~+ or CO+; M = 32: 02+; M = 44: C02+; M = 46: NO2+; M = 48:C4 + or 08+; M = 56: C202+; M = 58: N C 0 2 +. O n the basis of theoretical considerationss and experimental results,e the ions C20~ + and N C O ~ +
i
:/ -2.25 '
-2.00 -1.75 i !.Og,oOfthe initial. C2N~mole fraction
FIG. 4. Determination of the "order" dependence of O~+ on the initial C~N~ mole fraction in a stoichiometric C0/02 flame, diluted with 25% N2.
314
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES TABLE I
Discussion
"Order" dependences of flame-ions on the initial cyanogen content
Ion
NO + CO +
"Order" dependence on [C~N~]0 (•
(a ) The Primary Chemi-Ionization Process Several reactions can be considered as possible primary ionization steps: AH (kcal mole-1)
0.5 1
O2 §
1
CO2 § NO~+ O~+
0.7 0.7 0.5
Because of the very high enthalpy of formation of N2+,9 the ion with mass 28 is most probably CO + . On the basis of the experimentally determined "order" dependence [-see Sec. ( d ) l the ion with mass 48 cannot possibly be C4+. The alternative, O3+, has also been observed during electron bombardment of 02.1~ Figure 3 shows a semilogarithmic plot of the chemi-ion concentrations l-CO+-], ~NO+-], [C02+-l, l-NO2+-], and l-On+-] versus the axial distance in the stoichiometric CO/O_,/N2 flame containing 2% C_,N2. The origin of the distance scale x --- 0 is taken a t the point where [NO +] maximizes. Figure 3 illustrates clearly t h a t {-NO+'] represents 98% of the total ion concentration somewhat downstream of the flame front (x 0.1 mm).
(d) Dependence of the Maximum Ion Concentrations on the Initial C2N2 Content. A t the point where each ion reaches its maximum concentration, the rate of formation is equal to the rate of disappearance. Defining the "order" dependence of the maximum ion concentration ['X+']max on the initial C2N2 content [-C2N2-]0 as the exponent m in the expression [-X+Jmax ~'~ [-C~N2-]om, that "order" is obviously related to the number of C~N2 fragments directly or indirectly involved in the formation and disappearance of the considered species X +. As an example, a logarithmic plot of the relative maximum 02+ current versus [-C2N~-]o is shown in Fig. 4. The "order" dependence is equal to the slope of such a plot. The results for all identified flame-ions are listed in Table I.
CN -{- O -~ 0---~ CO ~ NO+-~- eC~* -~- 02--~ CO -~- CO+-~- eCN* ~- 0 2 - ~ CO -~ NO + -{- eNO -{- N -b N - ~ N~ + NO+-~- eNO ~- C + O---) CO + NO+-}- e-
--12 -[-16 432 --11 --44
[-2] [-3] [-4] [5-] [-6]
Because the N and C concentrations are very low in comparison with the O concentration, one can neglect the reaction rates of [ 5 ] and [6]. Indeed, the O-atom concentration amounts to more than 1% of the total particle concentration in CO/O~/N2 flames. II In hydrogen flames, where the ion yield per fuel molecule is of the order of 10-8, the primary chemiionization reaction CH ~ 0--~ CHO + ~- e- is almost thermoneutral. The ion yield in the flames studied in this work is somewhat larger than 10- 5, so t h a t the highly endothermic and, therefore, rather slow reaction E4-] can be ruled out as the major chemi-ionization process. Further, in a spectroscopic study of the C O / O : / N : flame with 1% C2N~ added, the C2" emission intensity (Swan bands) was found to be extremely weak and, thus, Reaction E3"] can also be rejected. On the other hand, i t is well known t h a t termolecular reactions may be responsible for the excitation of added metals and of radicals in flames characterized by a high radical concentration. 1: A similar type of termolecular process, Reaction [-2], appears to be the major primary chemi-ionization reaction in the investigated flame. Experimental evidence in its favor is now presented: 1. Since CN is formed by reaction of C2N2 with an O atom and reacts mainly with 02, and further since the physico-chemical properties (such as the mean particle concentrations) of the C0/O2 flame are not significantly affected by the addition of C2N2 traces, application of the steady-state assumption yields [-CN-]~-~ [C2N2-]0. Reasoning along the same lines, one would expect on the basis of Reaction [-2] that U+ ~'~ [C2N2]0, as was experimentally verified, 2. At the maximum NO + concentration, where
CHEMI-IONIZATION INDUCED BY CYANOGEN
315
U + ~ a E N O + ] [ e - ] = aENO+] 2, it was found that ENO +] ~ EC2N2]$ "5. If NO + were not a primary ion, but the product of a secondary process, involving an ion listed in Table I and an NO radical, the "order" dependence of [NO +] on [C2H2]0 would necessarily be larger than the observed value, since [NO] in the reaction zone is proportional to [C2N2~0, and since the "orders" of the other ions are larger than 0.5. Likewise, on the basis of the observed "order" dependences, the ionization of NO by "hot" electrons can be excluded. 3. Applying the steady-state assumption to the combustion mechanism of C2N2, the estimated CN concentration in the investigated flame containing 2 % C2N2 is of the order of 3 X 1015em-3. About the same value is obtained by extrapolating the estimate of Bulewiez for low-pressure C2N2/O2 flames,is Furthermore, if the measured value u is taken for the O-atom concentration, and if the rate constant ks is assumed equal to the rate constant ofIS
2 0 + M--~ 02 + M,
glSJ
one finds for the rate of Reaction F2J that ( U + ) e a l c - ~- - 1016 ion cm--a see-1. Estimating the thickness of the ionization zone eo to be 2 X 10-2 cm, i.e., the approximate thickness of tLe luminescent region, the value of U+ inferred from the measured saturation current i by means of Eq. (1) is (U+)exp----2.2X 1016 ion cm-s sec-1. Moreover, at the maximum NO + concentration, a[-NO+]I-e- ] = 6.8 X 1015 ion cm-s sec-1. The TABLE II Ion-molecule reactions occurring in CO(+C2N2)/Os/N~ flames AH Reaction (kcal mole-i) No. CO + + NO --* NO + -t- CO CO + -t- 02 --* 02 + -{- CO CO + -{- COs --* CO2+ -I- CO COs+ + NO --* NO + -{- CO2 CO2+ -t- 02 ~ 02 + -{- COs C02 + + NO --* NO2+ -{- CO O~+ -t- O --* 03 + 02 + + O~ --* 03 + + 0 02 + + NO --* NO + + 02 NO2+ + NO-* NO+ + NO2 03 + + CO --* CO2+ + O5
-- 109 -44 -3 - 106 -41 --33 ? ?
--65 -16 ?
[7] [S] [9] [101 [11] [12] [131 [14] [15] [16] [17]
/,/;
o u.~ [c,.j'; ~
d
-7.8
-5.4
9t.6
/
"
/-2~s
,
-8.0 -~
,
-2;,
tog,oof themotefractionof~N Fro. 5. Dependence of the saturation current and of the CN*(B ~2:) emission intensity on the initial C~N2 mole fraction in a lean flame with composition: 24.@% CO, 35.4% 05, 38% N~, and 2% H2.
agreement between theoretical and experimental values of the chemi-ionization rate U+ can be considered as satisfactory. The above constitutes sufficient evidence that Reaction [-2-]is most probably the major primary chemi-ionization process. From Table II, however, it appears that none of the other observed chemi-ions can be formed in ion-molecule reactions involving NO + , because of the low ionization potential of NO. One is forced to conclude, therefore, that in addition to NO +, at least one of the other flameions must be a primary ion. Reasonable primary chemi-ionization reactions can be written only for CO +, namely Reactions [-3] and F19J: CN*(B 22~) -t- 2 0---+ NO ~- C O + + eAH = 19 kcal/mole.
[-19]
I n the stoichiometric C0/O2 flame, containing only traces of C2N2, the CN* (B 2~) emission intensity could not be distinguished from the NO-O continuum. For this reason, the "order" dependence of the CN emission intensity on [C2N2]0 was investigated in a lean flame (24.6% CO, 35.4% O2, 38% N2, and 2% H2) with a markedly weaker continuum. The observed dependence of the CN* emission intensity on
316
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
ICoN]J0 is shown in Fig. 5. It follows that: [ C N * ] ~ [C2Ns]01'2.
(3)
Because of the very high CO2 concentration, the CO+ ion very probably disappears mainly by charge transfer to COs. Consequently, at the [CO + ] maximum where steady state applies, one should have, according to Reaction [19],
At the point where [CO2+] reaches its peak value, [ C O + ] > [O3+]; furthermore, since the rate constants of charge transfer reactions involving 02 molecules are rather low, 28 Eq. (5) can be rewritten as follows:
[C0s
=
kgl-CO+][COs] ~- kg[CO] + k~o[NO] + k121110]"
[CN*][O] s,--, [CO+][CO2],
(s)
or, using Eq. (3),
[co+i,,,,,,,-,., ([csNs]o,.SEO]b/[cos].
(4)
Since it is reasonable that in lean flames the ratio FO]2/[CO2] exhibits a slight negative dependence on [C]Ne]0, Eq. (4) appears to be in good agreemeat with the experimentally obtained relation [CO +] ~ [-CsNs]0L If Reaction [3] were responsible for the formation of CO+ , one should find [CO+] ~-~ ([C2N2]d[02])/[C02], since the formation of C2" most likely involves two CsN2 fragments: CN + CN ~ C2" + Ns.
[20]
It can readily be shown that, in the flames considered, the order with respect to [C]Ns]0 of the CN* formation rate is identical to the "order" of the CN* emission intensity. In lean flames, where Eq. (3) applies, the total ehemi-ionization rate U+ also obeys the relationship U + ~ [-CsN]]02"2 (see Fig. 5). Hence, one may assume that CN* is formed in a process similar to the primary chemi-ionizatioa reaction [2]: CN + O + 0 ~ CN* + O2
[-21]
Reaction [21] has already been proposed for C]Ns/O2 flames15 and C2N2/0 systems. 2~
(b ) The Secondary Chemi-Ionization Processes At the respective maxima of the CO2+, NO2+, and 02+ concentrations, the following equations may be written (see Table II) :
[C02'+]max --,., k9[00+][C02] + k17[O3+][CO]
Further, one may assume that [NO] is proportional to [CsNs]o from steady-state arguments, and of course that [CO] as well as [CO2] are independent of the initial cyanogen content. As [ C O ] > [NO], the "order" dependence of [C02 +] on [C]Ne]0 should approach unity. The experimentu[ vatue is 0.7 (see Table I). From Eq. (6) one deduces
[NO2+],~a~ = klsECO2+]/kle.
(9)
Equation (9) implies that the "order" dependence of NOs+ should be the same as that of C02 +, as the experimental data have shown (see Table I). From Fig. 3, it follows that kls/kt~- 2. At the point where [O2+] peaks, FCO+])) [COs+], and the value of {k23[-O]+ kt,[Os]} is probably higher than k16[NO]. Therefore, [O]]ma~ can be approximated by
EOs+],... ~ k,ECO+]EOs]/{h3[O] + k2,[o~]l.
(lo) Hence, the same "order" dependence as for CO+ should be obtained: [ O ] + ] m ~ [-C2Ns]02 (see Table I).
Conclusion
The much higher ionization level in cyanogen flames as compared to hydrocarbon flames must be ascribed to the fact that, in the former, one of the chain-carrier radicals (CN) is involved in the primary ionization step:
(--- kg[CO] + klo[NO] + k~2[NO] + k~1[O2]
(5) ENO]+Jm~ = kl]ECO]+]FNOJ/k16[NO] (6) ks[CO+J[02] + k,lECO]+][O2] [O2+]m~ = k13[0] + k.[O]] + klsENO] " (7)
CN + O + 0 --~ CO + NO + + e-. Using the experimental value of the rate U+ = 2 X 10TM ions cm-a see-2, the measured value of [O] ~ 6 X 1016cm-s, and the estimated value of [-CN] "~ 3 X 1025 cm-s, the rate constant of this process is found to be approximately 10-38 cm6
CHEMLIONIZATION INDUCED BY CYANOGEN molecule-2 sec-1. This value seems to be somewhat high for such a termolecular process. I t is not impossible that the reaction proceeds by a two-step process involving a short-lived intermediate. As was also observed in the burning of hydrocarbons, 8,4 excited species do not appear to be precursors of the major primary ion in cyanogen flames. Chemiluminescence and chemi-ionization should be considered as parallel, rather than consecutive, phenomena. The following process can be taken as an example of this parallelism:
CN ~ 0 ~- 0 --* CN* ~ 02. Acknowledgments This research has been sponsored in part by the Aerospace Research Laboratories through the European Office of Aerospace Research, O.A.R., United States Air Force, under contract F 61052-70-C-0016. Support has also been received from the Fonds de la Recherche Fondamentale et Collective. One of the authors (C.V.) is much indebted to the Institut pour la Recherche Scientifique dans rIndustrie et l'Agriculture for the allocation of a postgraduate fellowship, and another (J.P.) expresses his thanks to the Fonds National de la Recherche Scientifique for a postdoctoral research fellowship. REFERENCES 1. GREEN, J. A. AND SUGDEN, T. M.: Ninth Symposium (International) on Combustion, p. 607, Academic Press, 1963. 2. C~COTE, H. F.: Ionization in Hydrocarbon Flames, paper presented at the 26th Meeting of Propulsion and Energetics Panel, AGARD, Pisa, Italy, Sept. 1965.
317
3. I~ET~.RS, J., VINCKI~R, C., INn VAN T m O ~ N , A.: Oxidation Combust. Revs. 4, 93 (1969). 4. PEETERS, J. AND VAN TIGGELEN, A. : Twelfth Symposium (International) on Combustion, p. 437, The Combustion Institute, 1969. 5. BIoNnI, M. A.: Can. J. Chem. ~7, 1711 (1969). 6. VINCKIER, C.: Ph.D. thesis, University of Louvain, 1969. 7. BRADLEY, T. ANn MATrHEWS, K. J.: Eleventh Symposium (International) on Combustion, p. 539, The Combustion Institute, 1967. 8. MILNE, T. A. AND Gm~.~r~, F. T.: Tenth Symposium (International) on Combustion, p. 153, The Combustion Institute, 1965. 9. FIELD, F. H. AND FRANKLIN, J. L. : Electron Impact Phenomena, p. 271, Academic Press, 1957. 10. FRANKLIN, J. L..,_ND MUNSON, M. S.: Tenth Symposium (International) on Combustion, p. 561, The Combustion Institute 1965. 11. KILm~, J. K. AND DUNH~, P. G.: Eleventh Symposium (International) on Combustion, p. 899, The Combustion Institute, 1967. 12. ZE~G~RS, P. J. Tm AND ALKAMXDE,C. Tm J. : Tenth Symposium (International) on Combustion, p. 33, The Combustion Institute, 1965. 13. AILLmT, M. AND VAN TIGGELEN, A.: Bull. Soe. Chim. Belg. 77, 433 (1968). 14. BAsco, N.: Proc. Roy. Soc. (London) A283, 302 (1965). 15. Bu~wIcz, E. M. : Twelfth Symposium (International) on Combustion, p. 957, The Combustion Institute, 1969. 16. WISE, I-I. ANDROSSER, W. A. : Ninth Symposium (International) on Combustion, p. 733, Academic Press, 1963. 17. SETSER, D. W. AND TRUSH, B. A.: Proc. Roy. Soc. (London) A~88, 275 (1965). 18. FEHSENFELD, F. C., FERGUSON, E. E., AND SCHm~.LTEKOFF, A. L.: J. Chem. Phys. 441, 3022 (1966).
COMMENTS T. D. Wilkerson, University of Maryland. W h a t is the precision attached to the order numbers given for the different species? Authors' Reply. The precision was about 0.1. A. Fontijn, AeroChem Research Laboratories, Inc. Have the authors considered two-body ionization reactions as an alternative to the three-body reactions discussed? For example, reactions involving 02" in concentrations proportional to [0] ~ would maintain the same reactant order dependence as the authors' report. Such reactions appear a priori somewhat more likely
than the three-body four-center processes of the paper. Are three-body reactions fast enough to explain the observed ionization rates? Reaction (III) involving CN*, a short-lived species, cannot yield very large ion-formation rates. Threebody chemi-ionization reactions have from time to time been proposed. However, further study has usually shown the ionization occurs in a sequence of steps involving a two-body reaction as the ion-production process.
Authors' Reply. In the conclusion of our paper, we state that the reaction CN ~ 0 ~- 0 --* NO + ~ CO -{- e- m a y perhaps proceed in two
318
CHEMI-IONIZATION AND ELECTRICAL PROPERTIES OF FLAMES
steps via a short-lived intermediary, which could be 02* as well as (CNO)*. We consider CN* responsible for only a small fraction of the total amount of ions produced. With a view to the determination of the reaction order of the chemi-ionization process, we have recently started an investigation of the dependence of ion-formation rates and emission intensities on the total pressure. A t pressures near 1 arm, O2" is deactivated mostly by collisions; if 02* were involved in the formation of ions, one would therefore expect a p2 dependence of the ion formation rate, whereas t h a t dependence would more likely be Pa if two O atoms react directly with a CN radical to form an ion.
E. M. Bulewicz, University College of Swansea. A. On the Experiment. (a) Was the use of a stoichiometric C 0 / 0 2 / N 2 flame the best choice? At stoichiometry, most radical concentrations will vary rapidly for only a small change in the equivalence ratio. 1 When 1.8% C2N2 was added, this could put the system over to the fuel-rich side, with possibly a considerable change in [O]. (b) W h a t tests were made to see if the C2N2 added had any influence on the CO/O2/N2 flame used? (c) Were the gases dried? (d) I n the experiment on CO +, 2% H2 was added. We have found that the addition of such quantities of H2 to C2N2/O2 flames at reduced pressure can have a very marked effect on both ionization2 and CN emission. ~
B. On Interpretation. At the Ninth Symposium, Padley and myself presented a paper on the ionization in C2N2/02 flames at reduced pressure, 2 and we considered the over-all reaction CN -[- O -4- O - ~ CO -~- NO + -~- e- as one of the possibilities. Both this reaction and NO -4- N -{- N --~ NO + ~- N2 ~ e- would agree with our results. We favored the latter, particularly as it is considered to be responsible for NO + production in flow systems. At the Twelfth Symposium, I presented some results on the emission spectrum of C2N2/O2 flames at reduced pressure. 1 In these flames, CN emission reached a maximum well on the fuelrich side (at -~h = 0.5, h being defined as O2 present/02 required for combustion to CO N~) while ionization is at maximum slightly on the 02-rich side. If, as it is suggested, both CN* and NO + were produced from CN -~ O ~ O, we would expect CN* and ions to run very much in parallel. I t is possible, but not very probable, that chemi-ionization and excitation mechanisms are different in C2N2/02 and in C0/O2/N2 ~ C2N2
flames. Could one, therefore, propose an alternative mechanism? In C2N2/02 flames not CN* and NO +, but CN* (violet system) and NO* (3" bands) run in parallel, at all compositions, there being apparently no link between NO chemiluminescence and chemi-ionization. Could the reaction (over all, but not necessarily in one step) CN q- O -k O lead to the production of CN* and NO*, NO + being produced by NO* N + N, with possibly a contribution from CN -4- O q- O only under 02-rich conditions? REFERENCES 1. E. M. BULEWICZ: Twelfth Symposium (International) on Combustion, p. 957, The Combustion Institute, 1969. 2. E. M. BULEWICZ AND P. J. PADLEY: Ninth Sym-
posium (International) on Combustion, p. 647, Academic Press, 1963. 3. E. M. BULEWICZ, F. J. PADLEY, AND R. E. SMITH: To be published.
Authors' Reply. A. The fact that the saturation current increases linearly with the added C~N2, up to 1.8e/v C2N2 (see Fig. 1), indicates that the parameters affecting chemi-ionization do not vary appreciably in that C2N2 concentration range. Moreover, the burning velocity changed by only about 10% in that range. The gases were not dried. However, no traces of HaO+ or of any other H-containing ion were detected. We found that the addition of a few percent H2 to the C O ( ~ C2N2)/O2/N2 flame did not significantly alter the ion yield per C2N2 molecule. B. The reaction NO -[- N q- N --~ NO + qN2 q- e- cannot possibly be a primary ionization step in our system; on the basis of such a reaction, one would expect the ion-formation rate to vary as the third power of the C2N2 content, whereas a linear relationship was actually observed. As mentioned in the paper, recent experiments of ours have shown that, in CO/CeN:/Oe/N2 systems, the ratio of the saturation current to the CN* emission intensity (violet system) remains constant over an extended range of flame composition and flame temperature. For flames with total equivalence ratio ~t = {[COl0 -44[C2N2]0}/2[0210 between 0.6 and 1.2, final flame temperature between 2300 ~ and 2550~ and C2N2 content between 1% and 3%, the mentioned ratio was constant to within 10% (which is about equal to the experimental error). Both the saturation current and the CN* emission intensity showed a rather flat maximmn near stoichiometric (~t = 1).
CHEMI-IONIZATION INDUCED BY CYANOGEN It follows from the above that, in the flames investigated here, the chemi-ions and the excited CN* radicals are formed from the same reaction partners--most likely CN + O + O. In view of the results of Bulewicz, 1 therefore, one is forced to conclude that the chemi-ionization and/or the CN* production mechanisms are not the same in pure C2N~/O2 flames as in CO(-~ C2N2)/O2/N2 flames, at least if the electron concentration in a flame can be considered to be a meaningful indicator of ion-formation rates (see authors' reply to comment by Padley
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on paper by J. Peeters et al., following paper This Symposium). In fact, it should not surprise us that the mechanisms are different, because of the much higher temperatures of pure C2N2/0~ flames as compared to CO/Oe/N2 flames containing about 1% C~N2. REFERENCE 1. BULEWICZ, E. M. : Twelfth Symposium (International) on Combustion, p. 957, The Combustion Institute, 1969.