Energy & Fuels 1988,2, 494-504
494
Are Ions Important in Soot Formation?? H. F. Calcote,* D. B. Olson, and D. G. Keil AeroChem Research Laboratories, Inc., Princeton, New Jersey 08542 Received November 2, 1987. Revised Manuscript Received April 14, 1988
The postulates of the ionic mechanism of soot formation in flames are summarized, and some of the evidence for the ionic mechanism is reviewed. The mechanism assumes that the chemi-ion C3H3+ reacts progressively with acetylene and diacetylene and other small molecules to produce large ions that ultimately lead to soot particles. The evidence is examined under the following headings: 1. Ion Concentration; 2.Reaction Rates; 3. Confirmation of Ions; 4. Location of Ions in Flame; 5.Changes with Equivalence Ratio; 6. Propensity of Ions to Grow; 7.Fuel Effects; 8. Chemical Additive Effects; 9. Electric Field Effects and Electron Injection; 10. Aesthetics. This evidence leads to an affirmative answer to the question raised in the title.
Introduction Many chemical mechanisms have been proposed for the formation of soot in flames but most of these have been rejected. The ionic mechanism has not been widely embraced; it is considered a competitor to free-radical mechanisms, which have received the major attention. It seems timely to review the evidence for the ionic soot formation mechanism. We first review the postulates of this mechanism and place it in the context of the total process for the formation of soot. The evidence is then discussed under 10 headings. Postulates Figure 1 serves to place the ionic mechanism in the appropriate context with the other steps involved in the formation of soot in flames. This is done to make it clear that there are two distinct stages in the formation of soot where charged species may be important: (1)formation of the initial soqt precursor species, which lead to incipient soot; (2) the final stages where the coagulation of small soot particles is inflhenced by the charge(s) on the particles. This is an important distinction because the two stages of soot formation are frequently confused when the significance of ions is being discussed. Some of the early evidence for the ionic mechanism was certainly based on phenomena involving particle coagulation, and some of the criticism of the ionic mechanism relates to that evidence. In Figure 1, the precursor of soot is the propargylium ion, C3H3+.The source of this ion is not completely clear, but it is the dominant ion in fuel-rich hydrocarbon flames. Two possible sources of this ion are generally considered: CH* C2H2 C3H3++ e(1) or the chemi-ionization reaction, which dominates ion formation in stoichiometric and fuel-lean flames CH + 0 -. CHO+ + e(2) followed by several reactions paths to produce' C3H3+,e.g. CHO+ + CH2 CH3+ + CO (3)
-
+
CH3+ + C2H2
-
-
C3H3'
+ H2
(4)
and/or 'Presented at t h e Symposium on Advances in Soot Chemistry, 194th National Meeting of the American Chemical Society, New Orleans, LA, August 30-September 4, 1987.
CHO+ + HzO H30+ + C3H2
-
H30+ + CO
(5)
C3H3+ + H2O
(6)
Recently Eraslan and Brown2 have demonstrated, in modeling studies of fuel-rich systems, that the chemiionization reaction 2, when the CH radical is electronically excited, is responsible for ionization in very rich hydrocarbon flames. They confirmed the series of reactions 3-6 as important in producing C3H3+from CHO+. Reaction 1 does not seem to be significant in the mechanism for forming C3H3+in fuel-rich flames.2 There are two isomeric structures of C3H3+,a more stable cyclic structure, cyclopropenylium, and a linear structure; propargylium. Measurements of ion-molecule reaction rates for these two isomers near room temperature demonstrate that reactions of the Iinear isomer are fast, generally equal to the Langevin rate, while the rate coefficients for the cyclic isomer are ~ m a l l e r . ~Eyler ? ~ and associates5recently determined that the major low-pressure reaction channel of linear C3H3+with C2Hzis not a condensation reaction, as shown in Figure 1, although they confirmed the rapid condensation reactions of C3H3+with C4H2. However, in a study at a higher pressure (40-100 Pa), Smith and Adam# did observe rapid condensation reactions between C2H2 and linear C3H3+. Thus the question of the products of reaction of the propargylium ion with acetylene remains open. The validity of extrapolating near-room-temperature measurements to high temperatures is not clear. Which isomer of C3H3+ is formed in reactions such as (l),(4), and (6) is also not known. Reactions 1,4,and 6 are exothermic for producing either isomer, 100-150 kJ/mol for propargylium and 200-250 kJ/mol for cyclopropenylium. At flame temperatures the two isomers should be in equilibrium. For (1) Calcote, H. F. In Zon-Molecule Reactions; Franklin, J. L.,Ed.; Plenum: New York, 1972; Vol. 2, p 673. (2) Eraslan, A. N.; Brown, R. C. submitted for publication in Combust. Flame. (3) Eyler, J. R. In The Chemistry of Combustion Processes; Sloane, T. M., Ed.; Advances in Chemistry Series 249, American Chemical Society: Washington, DC, 1984; p 49. (4) Smyth, K. C.; Lias,S. C.; Ausloos, P. Combust. Sci. Technol. 1982, 28, 147. ( 5 ) Ozturk, F.; Baykut, G.; Aoini, M.; Eyler, J. R. J. Phys. Chem. 1987, 91, 4360. (6) Smith, D.; Adams, N. G. Znt. J. Mass Spectrom. Zon Processes 1987, 76,307.
0887-0624/88/2502-0494%01.5QIQ0 1988 American Chemical Societv
Energy & Fuels, Vol. 2, No. 4, 1988 495
Ionic Mechanism of Soot Formation
,".........
TIME, m s
1.
5
0
A \
P I 1
1
I
10 I
I
I
I
I
I
I
I1~2000 la00
Y
w 3
1600
1400
2
a Y a
+ 1200 ClH5+
-CPC-E-CX-
J M,
Figure 1. From chemi-ions to soot.
example, at 2000 K the concentration of the propargylium ion should be =0.2% of the total C3H3+concentration. The rate of isomerization of C3H3+at flame temperatures is clearly important to the establishment of the postulated mechanism. In the ionic soot formation mechanism, the precursor ions react with neutral species, e.g., acetylenes, to produce larger ions: C3H3+ + C2H2 -w C5H3+ + H2
-
C3H3+ + C4H2 C3H3+ + C6H2
+
CgH7+ + C2H2 CgH7+ + C4H2 C13H9+ + C4H2 C17Hll+ + C4H2
-
-
-
C7H5'
(8)
CgHS+
(9)
C17Hll+ + C2H2
CgH7+
(10)
CllHg+
(11)
C13Hg'
(12)
C17Hll+
(13)
C19Hll+ C19Hll+
I
1
*
2
I
I
3
4
5
DISTANCE ABOVE BURNER, c m
Figure 2. Comparison of total ion concentration and soot concentration profiles in low pressure (2.67 kPa), = 3.0 acetylene/oxygen flame (50 cm/s unburned gas velocity). The temperature, ion profiles, and time axis are from AeroChem.14 The soot profiles are from Howard et al." and have been reduced b 50% so the charged soot and ion concentrations agree at 3.5 cm.E
(7)
These ions then sequentially add small neutral species, e.g., acetylenes, to produce increasingly larger ions, e.g.: C7H5+ + C2Hz
0' 0
+ CZHZ
(14)
+ HZ
(15)
Major features of this mechanism are the large rate coefficients for ion-molecule reactions7+' and the ease with which ions Thus the formation of cyclic structures does not represent a significant energy barrier as it does for free-radical mechanisms. In addition to recombination with positive ions, some of the electrons produced in reaction 2 produce negative ions by electron attachment to large molecules; these reactions are favored by low temperature and increasing molecular weight. As the positive ions grow larger, their recombination rate coefficients for reaction with electrons or negative ions increase, thus removing the positive ions (7)Bowers, M. T., Ed. Gas Phase Zon Chemistry; Academic: New York, 1979; Vol. 1 and 2. (8)Auslm, P., Ed. Kinetics of Zon-MoleculeReactions; Plenum: New York, 1979. (9)Lias, G. L.; Ausloos, P. Zon-Molecule Reactions, Their Role in Radiation Chemistry; American Chemical Society: Washington, DC, 1975. (10)Talrose, V. L.; Vinogradov, P. S.; Larin, I. K. In Gas Phase Zon Chemistry; Bowers, M. T., Ed., Academic: New York, 1979;Vol. 1,p 305. (11)Aueloos, P.; Lias, S. G. In Zon-Molecule Reactions; Franklin, J. L., Ed.; Plenum: New York, 1972;Vol. 2,p 707.
and forming either large neutral molecules or small neutral soot particles. There is, in fact, no distinction between a large molecule and a small soot particle.12 The small neutral particles continue to grow, and as they become larger, their work function is assumed to approach that of bulk graphite. At sufficiently high temperatures, these particles become thermally ionized and play a major role in the final steps of the process, Figure 1, of coagulation and agglomeration. In this paper we review the evidence for the sequence of reactions starting with C3H3+and proceeding through what is termed in Figure 1as "incipient soot ions". This set of reactions has been labeled "the ionic mechanism". Evidence 1. Ion Concentration. The most frequently raised criticism of the ionic mechanism of soot formation has been that the concentration of ions is too small to play a major role in soot formation. Ion concentrations have been measured by AeroChem13J4and Delfau and associates15J6 in low-pressure acetylene/oxygen flames. Many people have contributed to our understanding of a premixed, sooting, acetylene/oxygen flame on a flat-flame burner at 2.7 kPa, an equivalence ratio of 3.0, and an unburned gas flow velocity of 50 cm/s. This has come to be known as the standard flame. Ion concentrations measured in this flame are shown in Figure 2 along with neutral soot concentrations, charged soot concentrations, and the flame temperature. The ion concentration in this flame is suf(12)Calcote, H. F. Combust. Flame 1981,42, 215. (13)Calcote, H. F.; Keil, D. G. Combust. Flame, in press. (14)Keil, D. G.; Gill, R. J.; Olson, D. B.; Calcote, H. F. Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1985;p 1129. (15)Delfau, J. L.; Michaud, P.; Barassin, A. Combust. Sci. Technol. 1979,20,165. (16)Michaud, P.; Delfau, J. L.; Barassin, A. Eighteenth Symposium (Znternutional) on Combustion; The Combustion Institute: Pittsburgh, PA, 1981;p 443. (17)Howard, J. B.; Wersborg, B. L.; Williams, G. C. Faraday Symp. 1973,7, 109. Chem. SOC.
496 Energy & Fuels, Vol. 2, No. 4, 1988
ficient to account for the formation of the observed soot. Similar measurements are not generally available in other flames. The magnitude of the ion concentration may, in fact, not be relevant.18 Under conditions where the ions and soot particles overlap in the same region of the reacting system, the important question has more to do with reaction rates than with concentrations. In this situation, in which the observed ion concentration is smaller than the soot concentration, the rate of ion recombination must be large compared to the rate of ion formation. 2. Reaction Rates. Ion-molecule reaction rate coefficients are generally several orders of magnitude greater than those of neutral species, which, of course, means that the ion concentrations need not be as great as the neutral concentrations to generate a product at the same rate. Ion-molecule rate coefficients for observed flame ions have been measured only near ambient t e m p e r a t u r e ~ . ~The J~ Langevin theory,"pZ1 which does not predict a temperature dependence for ions reacting with nonpolar molecules, has been well tested at ambient temperatures and is generally consistent with experiments. It should be recognized, however, that the Langevin theory accounts only for the rate of production of an ion-molecule complex and not for how it breaks up;2w22thus, at higher temperatures the dominant dissociation paths may be different from those at room temperature, leading to a smaller rate of increase in ion-molecule size than indicated by simple application of Langevin theory. Thus room-temperature rate coefficients must be used with caution at flame temperatures. Evidence that ion-molecule reactions are rapid at flame temperatures comes from measurements of ionization in premixed nonsooting hydrocarbon/oxygen or hydrocarbon/air flames.' The large number of different ions all occur nearly at the same position in the flame.' The accepted explanation is that the charge of the original chemi-ion, CHO+, is transferred to neutral species via ionmolecule reactions.l.23 These ion-molecule reactions must be very rapid for this to occur. No alternative mechanism has been proposed. These reactions have, in fact, been used to identify the neutral species p r e ~ e n t . ~ ~ * ~ ~ Ion concentrations in atmospheric pressure flames are t y p i d y 10'o-1012ions/cm3.'* Typical rate constants for dissociative recombination are about cm3/s.'J0 Because of the rapid rate of ion recombination, the rates of ion formation must be relatively high to maintain the observed concentration^.^^ Rates of chemi-ion formation in atmospheric pressure flames have been reported to be 10'3-10'7 ions/(cm3 It is interesting to note that this is similar to the rates reported for nucleation of soot s).28929
(18) Calcote, H. F. In Soot in Combustion Systems and Its Toxic Properties; Lahaye, J., Prado, G., Us.; Plenum: New York, 1983; p 197. (19) Anicich, V. G.; Blake, G. A.; Kim, J. K.; McEwan, J. M.; Huntress, W. T., Jr. J.Phys. Chem. 1984,88,4608. (20) Patrick, R.;Golden, D. M. J. Chem. Phys. 1985, 82, 75. (21) Meot-Ner, M. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic: New York, 1979, Vol. 1, Ch. 6. (22) Chang, J. S.;Golden, D. M. J. Am. Chem. SOC.1981, 103, 496. (23) Calcote, H. F.;Miller, W. J. In Reactions Under Plasma Conditions; Venugopalan, M., Ed.; Wiley: New York, 1971; Vol. 11, p 327. (24) Goodings, J. M.; Tanner, S. D.; Bohme, D. K. Can. J. Chem. 1982, 60, 2766. (25) Bohme, D. K.;Goodings, J. M.; Ng, E. Int. J.Mass Spectrom. Ion Phys. 1977,24, 355. (26) MacLatchy, C. S. Combust. Flame 1979, 36, 171. (27) Calcote, H. F. In Fundamental Studies of Ions and Plasmas; AGARD Conference Proceedings No. 8; NATO: Paris, France, 1965; Vol. 1.
(28) Peeters, J.; Vinckier, C.; Van Tiggelen, A. Orid. Combust. Reu. 1969, 4, 93. (29) Lawton, J.; Weinberg, F. J. Electrical Aspects of Combustion; Clarendon: Oxford, England, 1969.
Calcote et al. NEUTRAL S P E C I E S CHARGED SPECIES
25
/
lot 0
1
2
3
4
5
6
7
D I S T A N C E A B O V E BURNER. c m
Figure 3. Neutral soot particle and charged soot particle diameters in same acetylene/oxygen flame as Figure 2: HWW, ref 17; BHW, ref 32; H, ref 33 and 34; PH, ref 35. Unless noted, mean diameters are number means. Values for ions were taken from ref 14.
particles, iO'4-iO'6 particles/ (cm3 s ) . ~ O ~ ~ ' Figure 2 also shows evidence that the rates of ionmolecule reactions are sufficiently rapid in flames to account for soot formation; as the ion concentration decays by recombination, the soot concentration increases. The rates of ion decay and soot particle increase are comparable within the accuracy of the data. We note, however, that soot particles were identified17only as those that could be detected by using an electron microscope; i.e., their diameter exceeded 1.5 nm. Several available sets of data on neutral soot particle diameters and positively charged soot particle diameters are presented in Figure 3 for the standard CzH2/OZflame. These data have previously presented a dilemma; the neutral particles appear to grow faster than the charged particles, but in the early part of the flame, the charged particles have a larger diameter than the neutrals. This observation seems to relate more to particle growth and thermal ionization of soot particles than to soot nucleation; nevertheless, the phenomenon occurs in the flame at just the position where the initial ion concentration is falling and the concentrations of neutral and charged particles are increasing. Previous calculations'2*36 (see, however, ref 34 and 36) attributed the appearance of charged soot particles at about 2 cm downstream from the burner to thermal ionization of the neutral particles. The assumption was made that the neutral particles were produced first and then thermally ionized to approach equilibrium. This assumption is difficult to rationalize if the concentration of charged particles exceeds the concentration of neutral particles at small distances (Figure 3) where the (30) Harris, S.J. Combust. Flame 1986, 66, 211. (31) Haynes, B. S.;Wagner, H. G.Prog. Energy Combust. Sci. 1981, 7, 229. (32) Bonne, U.;Homann, K. H.; Wagner, H. G. Tenth Symposium (International) on Combustion;The Combustion Institute Pittsburgh, PA, 1965; p 503. (33) Homann, K. H.Ber. Bunsen-Ges. Phys. Chem. 1979, 83, 738. (34) Homann, K.H.; Stroefer, E. In Soot in Combustion S y s t e m and Its Toxic Properties; Lahaye, J., Prado, G., Eds.; Plenum: New York, 1983; p 217. (35) Prado, G.P.;Howard, J. B. In Evaporation-Combustion of Fuels; Zung, J. T. Ed.; Advances in Chemistry Series 166; American Chemical Society: Washington, DC, 1978; p 153. (36) Howard, J. B.; Prado, G. P. Combust. Sci. Technol. 1980,22,189.
Energy & Fuels, Vol. 2, No. 4, 1988 497
Ionic Mechanism of Soot Formation particle diameters are the smallest; the extent of thermal ionization is very sensitive to particle diameter.12 The rate of thermal ionization is also sensitive to particle diameter, so that for the small neutral particles the rate of ionization is slow compared to the experimentally observed rate. The rate law for charging particles with an initial charge ( z 1)e is defined by dN$/dt = k5N6-l
CO H, C2H2
H,O
-
co2
where the rate coefficient, k+, is given by the modified Richardson equation for small p a r t i c l e ~ ~ ~ , ~ * :
k+ = ad2
(
)
4amek2F' h3
X
Here d is the particle diameter, meis the electron mass, k is Boltzmann's constant, T is the absolute temperature, h is Planck's constant, e is the electron charge, 6 is the work function of the material, and C, the electrical capacity of the particle, is given by 27rcod for a spherical particle (to is the dielectric constant of free space). On the basis of neutral soot concentrations, Figure 2, and diameters, Figure 3, we calculate a rate of ionization at 2.25 cm above the burner of 2 X 10" charged particles/(cm3 s). From the slope of the "charged-soot" curve at this point, Figure 2, we obtain a greater rate of 6 X 10" charged particles/ (cm3s). Similar considerations show that between 2.5 and 3.0 cm, however, the calculated rate exceeds the observed rate and equilibrium controls the concentrations. Assuming the data are sufficiently accurate for these calculations to be meaningful, we can conclude that the rate of thermal ionization of neutral particles early in this flame is too small to account for the observed rate of charged particle appearance, leading to the conclusion that ion-neutral equilibrium early in the soot zone is approached from the direction of excess charged particles. This is further evidence that charged species are involved in the formation of soot. It also demonstrates that the large ions are not derived from neutral soot particles. The difference in slopes of the neutral and charged particle diameters in Figure 3 may be due to a faster rate of ion recombination for large ions than for small ions. The equation for the rate coefficient of ion-electron recombin a t i ~ n , ~k,,' is, assuming that all electrons stick
k , = ad2 c,
( + &) I
where E, is the average electron velocity at the electron temperature T,, assumed to be the gas temperature, and the other symbols are defined above. Increasing the particle diameter from 5 to 10 nm increases the rate coefficient of recombination from about cm3/s. Thus the larger charged 2X to about 6 X particles are neutralized more rapidly than the smaller ones. The observation that the neutral particles grow faster than the charged particles is consistent with the size dependence of recombination coefficients and is inconsistent with the size dependence of thermal ionization rate constants, evidence that the observed charged particles are derived from the chemi-ions produced early in the flame (37) Sodha, M.S.;Guha, S. Ado. Plasma Phys. 1971,4, 219. (38) Newman, R.N.;Page, F. M.; Wolley, D. E. In Euaporation-Combustion of Fuels; Zung, J. T., Ed.; Advances in Chemistry Series 166; American Chemical Society: Washington, DC, 1978; p 141.
5 0
5
10 15 DISTANCE, m m
20
25
Figure 4. Concentration profiles of selected neutral and ionic species. The OH profile was adapted from ref 42. Neutral profiles were calculated from published mole fraction profiles. CzHzand O? profiles were adapted from Figure 11 in ref 40 and are indistinguishablefrom those of Delfau and V ~ v e l l e(DV). ~ ~ Other profiles adapted from Bittner and Howard&(BH) included C10H8, COz, HzO, C3H3,C3H4,and Cl2H8,and CI4HB.Profiles of CO, Hz, C&15were adapted from DV. Profdes for C4Hzand C4H4,reported by both BH and DV, are, respectively, similar in shape and magnitude (less than a factor of 2 difference) and were averaged for presentation here. The corresponding profiles for CBHG were also averaged although they were significantly different. From BH, the C6H6 maximum occurs at 7 mm while DV show the maximum at 4 mm. Also, DV found lower mole fractions by as much as a factor of 10. Selected ion profiles are adapted from AeroChem.13 Shading indicates where the yellow emission first appears.
and are not derived from the neutral particles. Not only is this evolution of charged particles further evidence for the ionic mechanism of soot formation but it also makes very awkward any explanation of the source of ions being the thermally ionized particles or large m o l e ~ u l e s . ' ~ ~ ~ ~ ~ ~ ~ We have recently13identified a large number of heavy, polyaromatic hydrocarbon ions in a sooting, C2H2/02/Ar flame (4 = 3.0; 2.7 kPa; 50 cm/s; 3% Ar) similar to that in Figure 2. Ions with carbon numbers up to 45 were observed, and all can be represented as polyaromatic ions similar to those given in Figure 1. Absolute ion concentration profiles were determined for this flame, some of which are shown in Figure 4 along with absolute concentrations of neutral species as measured by Bittner and (39) Homann, K. H. Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1985; p 857.
Calcote et al.
498 Energy & Fuels, Vol. 2, No. 4, 1988 Table I. Rate Constants for Reaction of Neutrals with Ions To Produce Larger IonsI3 10IOk,cm3/s reactants neutral
ionbVc
CzHz
C3H3+
%HZ (%HZ
C14H11+
C4H2 ClHP
C45H17+ C3H3++
C14Hll
C4H2
C4H3t
C3H4 C4H4
CH+ C:Hi:
CBH6
C3H3
ClZHB C3H3’ CloHs C H ” C14H8 (Hit
from characteristic timesd
0.0007 0.0007 0.0007 0.02 0.02
0.02 0.4 0.9 3 300 400 1000
Langevin: est 11 9 9 13 10 12 8 12
16 19 17 19
CARBON ATOMS 0
-
5
10
15
20
25
30
35
40
45
O
experimentally obsd
10,l > 15
1o e 7.4h 1l e 1l h 14,15e*f @,i
7f
6.i
Estimated with Langevin’s theory using polarizabilities calculated from refractive indices and the Lorentz-Lorenz equation. Ion in “estimated” and “observed” reactions. Linear C3H3+assumed. dIndependent of ion.13 “myth e t al.4 f E ~ l e r 8Eyler .~ and associates6 report a much lower rate coefficient for this reaction. T h e higher rate coefficients are supported by recent measurements by Smith and Adams.6 hAnicich e t al.19 ‘For l-methylnaphthalene, CllHlo.
Howard@in the same flame using the same burner design and by Delfau and Vovelle41in a similar flame without the Ar. The OH profile is from L a ~ r e n d e a u . ~ With ~ only a few exceptions, all of the ion concentration profiles exhibited13 peaks at about the same location in the flame, 0.8-1.0 cm. This was true for both odd and even carbon number species, with the profiles for heavier species, >400 amu, being broader than for the lighter ions. This observation indicates either that the ions are in equilibrium with each other or that they are related by a series of rapid reactions, since a bottleneck would tend to delay the formation of heavier ions in the reaction sequence and they would be observed further downstream from the burner. From the shapes of the profiles, we estimated characteristic times for formation of ions,13 T , and related these to a bimolecular rate coefficient and the concentration of the reactant in excess [n], i.e., the neutral reactant, through the expression k[n] = T In 2 Since the concentrations of the neutral reactants were known, the rate coefficients could be calculated. The estimated rate constants derived from the “characteristic times” for various assumed reactions are given in Table I, along with Langevin estimated and some experimentally observed rate constants. The required rate constants involving acetylene and diacetylene as the neutral growth species are considerably lower than the Langevin or experimental values, indicating ion growth reactions involving these species are potentially important in this flame. In fact, both allene and vinylacetylene also appear as viable reactants; the rate coefficients from the characteristic times are smaller than the Langevin or measured rate coefficients. These are the major processes included in the ionic soot formation mechanism (Figure 1). Reactions of larger neutral species such as the polyaromatic hydrocarbons in Table I with small ions to generate large ions would require such large rate constants that this route can be eliminated as contributing signifi~~
(40) Bittner, J. D:; HGardTJ. B I n Particulate Carbon: Formation During Combustion; Siegla, D. C., Smith, G. W., Eds.; Plenum: New York, 1981; p 109. (41) Delfau, J. L.; Vovelle, C. Combust. Sci. Technol. 1984, 41, 1. (42) Laurendeau, N. M.,personal communication, 1986.
Figure 5. Occurrence of carbon a n d hydrogen atoms in flame ions.13
cantly to ion growth in this flame. These observations support the view of a series of rapid ion-molecule growth reactions, involving the addition of small neutral species to ions, taking place in the flame and eventually leading to soot formation. 3. Confirmation of Ions. All of the individual ions involved in the postulated mechanism, up to mass 557, have been observed in sooting f l a m e ~ . ~ ~ The J ~ Jcar~,~~ bon-hydrogen content of these ions is identified in Figure 5. The carbon-hydrogen ratios require that the molecular structures be highly condensed aromatic rings. The presence of larger ions prior to the appearance of soot has also been confirmed, without identification of the individual The data in Figure 5 demonstrate that the hydrogen content remains constant as the number of carbon atoms increases, until at some specific number of carbon atoms the number of hydrogen atoms increases by 2. This observation dictates the possible reactions that can be employed in any ionic mechanism. As expected, the ratio of carbon to hydrogen increases with increasing molecular weight. The highest ratio observed is still less than 3 while the ratio in soot is about 10. Thus the largest observed ions are far from being soot particles. 4. Location of Ions in Flame. The relative position at which ions and soot occur, Figure 2, in a flame is not in itself evidence of the order of production of ions before soot in the reaction sequence; the order of appearance would be reversed if the rate of production of soot from ions were fast compared to the rate of production of ions. However, when the species peak considerably far apart in the flame, such as do ions and neutrals in Figures 2 and 4,it seems safe to assume that the first peak precedes the second in reaction sequence as well as in order of appearance. This assumption is further warranted when there is no clear pathway for proceeding from soot particles to moderately sized ions.ls With these caveats, the appearance of ions in the flame preceding the appearance of soot is evidence for ions being the precursors of soot and is further evidence that the ions are not produced from the soot particles. In this regard, it is interesting to note that both Homann and Wagner44and Bittner and Howard40observed mass spectrometrically a group of large neutral molecules with mass greater than 500 amu occurring at exactly the same position in the flame at which the total ion concentration peaks. Both of these groups identified these species as the precursors of soot formation. Could they have been observing the peak in ion concentration reported in Figures 2 and 4? (43) Olson, D.B.; Calcote, H. F. Eighteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1981; p 453.
(44) Homann, K.H.;Wagner, H. G. Eleuenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1967; p 371.
Ionic Mechanism of Soot Formation
Energy & Fuels, Vol. 2, No. 4,1988 499 Y
2000 1500
J
2
g
1000
24
1700
w U 1000
2100
1500
z
1300
2
500
i w
‘qH2
/1
:
1Ol2
PCAH
0
,
E 108
I
-4
-2
I I 0
2
4
IONS
6
FUEL
1
2 AIR
3
4
DISTANCE FROM F U E L l A l R BOUNDARY, mm
Figure 6. Profiles 15 mm above a Wolfhard Parker slot burner supporting an atmospheric pressure methane/air diffusion flame. Fuel velocity = 9.7 cm/s. Air velocity = 19.4 cm/s. Temperature
and positive ions (concentration measured with Langmuir probe, baaed on lo00 amu and 39 amu assumed ion mass-resulting range indicated by dotted lines) profiles were measured at AeroChem.* Other profiles were adapted from Smyth et al.& The profile for C4Hzrepresents experimentalprofiles at 9 mm scaled to 15 mm by the acetylene ratio at the two heights. PCAH is a visible-laser-induced fluorescence profile approximately scaled with a constant factor based on the ratio of C14HB to C8& concentrations in the low-pressure flame of Figures 2 and 4. For the soot curve, see text. Further evidence of a similar nature comes from observations in diffusion flames at one atmosphere. We have measured46ion concentrations in the same methanelair flame on which Smyth et a1.46 made a number of measurements. Our temperature profiles agree with theirs. The data for this diffusion flame are summarized in Figure 6, and the rationale for the estimates of concentrations is summarized in the caption. The soot concentration profile was not measured by Smyth et al., but they determined the position of the soot maximum in the flame from laser-induced ionization. Similarly, they estimated the concentrations of polycyclic aromatic hydrocarbons, PCAH, from laser-induced fluorescence. We confirmed the radial location of the soot maximum by 633-nm laser extinction measurements at a slightly higher position in the flame. We estimate from our measurements that the maximum volume fraction of soot was about 5 X lo+, which, for 20-nm-diameter particles, would correspond to about 1O1O particles/cm3. We scaled their relative laser induced ionization curve to this estimate of the maximum soot number density in Figure 5. The location of soot, Figure 6, with respect to the possible reactants is more consistent with an ionic mechanism than with a mechanism involving acetylene reacting with a polycyclic aromatic hydrocarbon. To shift the peak acetylene-PCAH rate to correspond to the location of the peak in the soot curve would require the involvement of a third reactive species such as hydrogen atoms30*47*48 or ~~
0
-1
FUEL AIR DISTANCE FROM FUELlAlR BOUNDARY, mm
~
(45) Calcote, H. F.; Keil, D. G., to be submitted for publication in Combust. Flume. (46) Smyth, K . C.; Miller, J. H.; Dorfman, R. C.; Mallard, W. G.; Santoro, R. J. Combust. Flame 1985,62, 157. (47) Frenklach, M.; Clary, D. W.; Gardiner, W. C., Jr.; Stein, S. E. Twentieth Symposium (Intermtiom0 on Combustion; The Combustion Institute: Pittsburgh, PA, 1985; p 887.
Figure 7. Comparison of reactant products and soot (an arbitrary
location in a methane/air diffusion flame. scaling factor of Data were taken from Figure 6. As in Figure 6, the dotted curves represent the extremes based on assumed ion masses of 39 and 1000 amu.
that the limiting reaction have a very high activation energy. Soot is located between the peak concentrations of acetylenes and ions, just where expected if these were the two critical reactants as described by the ionic mechanism, and no unusual phenomena were occurring to distort the picture. This point is better shown in Figure 7, where relative global rates for the proposed processes are approximated as products of reactant concentrations and plotted against lateral distance in the flame for comparison with the observed soot profile. The flow streamlines in this flame are from the air to the fuel suggesting a net transport to the left in Figure 7. An important question with respect to soot formation is, “Why does inception stop?” Harris has recently suggested30 that the falloff in oxygen concentration may be responsible due to its promotion of the formation of high-energy species that are important for soot formation and that disappear as the oxygen disappears; see, e.g., Figure 4. He suggested that the production of excess H atoms may be responsible for the effect. An even more obvious explanation for the termination of soot inception would be the termination of ion formation, which involves 0 atoms in their formation (see reaction 2) and the rapid decrease in ion concentration (see, e.g., Figure 2). It is a long-established fact that ions show a sharp peak in the flame front of hydrocarbon flame^.^^,^^ 5. Changes with Equivalence Ratio. Dramatic changes in ion concentrations occur in premixed flames at the threshold for soot formation (Figure 8). Small flame ions become large ions as the equivalence ratio is increased through the threshold soot point. This simple observation would be consistent with an ionic mechanism of soot formation; there are, however, complications. Why does the concentration of ions increase at higher equivalence ratios beyond the soot threshold? This observation has, in fact, been used to argue against the ionic mechanism; the increase in ion concentration with equivalence ratio (48) Frenklach, M.; Warnatz, J. Combust. Sci. Technol., in press. (49) Calcote, H. F. Combust. Flame 1957,1, 385. (50) Wilson, H.A. Rev. Mod. Phys. 1931, 3, 156.
500 Energy & Fuelsj Vol. 2, No. 4, 1988
Calcote et al. 10.0
PI)
5 0 r
0 c
2
0 U
a
z I-
8
1.0
z
0 0
z
0 3 5
I X 4
I
I
1.5
2.0
2.5
3.0
3.5
4.0
4 Figure 8. Variation of maximum ion concentration with equivalence ratio in a low-pressure acetylene/oxygen flame. Total ions are Langmuir probe measurements. Profiles of individual ions, identified by mass in amu, were measured with flame ion sampling mass spectrometry. The ratio of total ions to the sum of the individual currents a t 4 = 2.0, where mass 39 dominates, provided a calibration of the mass spectrometer against the Langmuir probe result for lighter masses. Similarly, >300 amu, representing all heavy ion masses between 300 amu and the high mass cutoff of the mass spectrometer, was calibrated against the Langmuir probe at = 2.9. Shading indicates the threshold for soot formation.
0.1 1
EQUIVALENCE RATIO,
g
, I 2 3 EQUIVALENCE R A T I O , 6
4
Figure 10. Maximum total ion concentrations as a function of equivalence ratio: circle, maxima closest to burner; square, second maxima. The soot threshold is indicated by shading. C2H2(02 flames a t 2.7 kPa pressure and 50 cm/s unburned gas velocity.
1
2 3 EQUIVALENCE RATIO
4
Figure 11. Variation of recombination coefficients with equivalence ratio. Same flames as Figure 10 (data from ref 14). Figure 9. C1&ll+(239 amu) ion profiles in an acetylene/oxygen flame: 2.1 kPa; 50 cm/s unburned gas velocity; soot threshold equivalence ratio = 2.4.
was attributed to ionization of neutral particle^.^*^^ It has been demonstrated above that thermal ionization of soot particles cannot explain the formation of moderately sized ions, Le., less than mass 1O00, as shown in Figure 8. It has, in fact, been shown that the second rise in total ion concentration with equivalence ratio is due to the formation of a second maximum in the ion concentration vs distance profile14and that this maximum is comprised of the same ions as the first maximum; see, e.g., Figure 9. The source of this second maximum is unknown but appears to be related to soot formation. In Figure 10 the concentrations of total ions in the two maxima are plotted together. The hatched area indicates the equivalence ratio in the flame where soot is first observed. (51) Wagner, H. G. Seventeenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1979; p 3.
Of additional interest is the manner in which the global second-order recombination coefficient, calculated from downstream ion concentration decays,14 varies with equivalence ratio (Figure 11). This rate coefficient is about 2 X lo-' cm3/s in a stoichiometric flame, a typical value for small ions,l decreases to about 0.1 of this value just before soot is observed, rises to a maximum at about the same point in the flame as where the second ion concentration peak exceeds the first peak (Figure lo), and then falls again to an even lower value. Both decreases in recombination coefficient must be due to the formation of negative ions (recombination rate coefficients for positive and negative ions are much smaller than for positive ions and electrons); such ions have, in fact, been observed in sooting flames by Homann and Stroefer.% Multicharged particles may also play a role.52 Exactly why the recombination rate coefficient reaches a minimum at about C#J = (52)Bradley, D.; Jamel, M. A. M. Combust. Flame 1984, 58, 115.
Ionic Mechanism of Soot Formation
2.0 and then increases again is not clear. The answer will have to await further study or computer modeling of the process in which all of the rates are included. Electron attachment increases at lower temperatures and for larger molecular species. Nevertheless, the big changes in ionic character of the flame at the soot threshold, if not the result of soot formation (see above), may very well be the cause of soot formation. 6. Propensity of Ions to Grow. The ionic mechanism assumes that small ions grow to larger ions by the addition of small molecular species such as acetylene, polyacetylenes, and other small hydrocarbon species. This is consistent with the known facts of ion-molecule reaction~.~+J~4- Most of these reactions are exothermic, and the rates of reaction are proportional to the exothermicity of the reaction.57 7. Fuel Effects. Soot is only found in those flames of carbon-containing fuels that also produce ions. Diffusion flames of all organic substances except methyl alcohol, formaldehyde, and formic acid produce soot;5s all organic substances except formaldehyde and formic acid produce ions in flame-ionization detectors, and methyl alcohol flames have very low ion concentration^.^^^^^ Inorganic carbon-containing molecules such as carbon monoxide,58 carbon disulfide, mixtures of carbon disulfide and hydrogenF2and cyanogen61do not produce soot. Of these gases only cyanogen flames produce ions, and they are different from those in hydrocarbon flames.23i62 These observations are consistent with the ionic mechanism of soot formation. Another indication of a correlation between fuel structure effects on soot formation and flame ionization can be gleaned from the correlation made by Takahashi and Glassmane3in premixed flames between the equivalence ratio for soot formation and the number of carbon bonds. They observed that the tendency to soot increases with the number of carbon bonds. This is reminiscent of several observations made on the tendency of compounds to produce ions under various conditions. In flame-ionization detectors for gas chromatography, the magnitude of the signal produced is correlated with the number of carbon atoms in the molecule.59@ Bulewicz and Padley62found in premixed fuel/oxygen flames that the electron current increased with the number of carbon atoms in the molecule. Interestingly acetylene is off their correlation very much as in the correlation of Takahashi and Glassman. B u l e w i ~ has z ~ also ~ ~ ~correlated the response in a flameionization detector to the heat of reaction of the fuel to C02+ Hz, which, of course, also increases with the number
Energy & Fuels, Vol. 2, No. 4, 1988 501 Table 11. Effect of Chemical Additivesn on the Concentration of Small Ions in Flames'O ratios of ion concn with and without additive H,O+ CHO+ C,H,+ C,H,O+ C,H,+ additive Na 1.0 0.9 0.9 1.0 1.0 Ca 0.7 0.7 0.6 0.5 0.7 Ba 0.4 0.4 0.4 0.4 0.5 Additive concentration 25 ppm.
of carbon atoms in the fuel. 8. Chemical Additive Effects. The same additives have been observed to both promote67@or inhibit the formation of soot, and the results have been interpreted as affecting either the nucleation or the coagulation step; see Figure 1. Much of the confusion arises because of the number of possible roles an additive can play69 and the fact that these roles can vary with the experimental conditions, e.g., the concentration of the additive. For our purposes in this paper we present evidence that under some conditions the additive alters the nucleation step where the effect is consistent with the ionic mechanism of soot nucleation. Some of the first evidence for an ionic mechanism of soot formation was the observation of the effect of chemical additives with low ionization potentials on soot formation.68*70,71 Strong correlations are observed between an additive's effect on soot formation and its ionization potential.6s.70Alkaline-earthmetals are a special case because the level of ionization is greater than anticipated by thermal equilibrium; ions are produced by chemi-ionizat i ~ n , e.g. ~* Ba
+ OH
-
BaOH+ + e-
Thus alkaline-earth metals may have a greater effect on flame ionization than predicted from their ionization potential. The effects of additives on soot nucleation are based on two processes as suggested by Addecutt and N ~ t t (1) : ~ ~ the transfer of charge from a chemi-ion, M+, to a metallic atom, A, e.g.
M+ + A + M
+ A+
(17)
where M+ is a hydrocarbon ion, such as C3H3+,or (2) an increase of the concentration of electrons due to thermal ionization of easily ionized metals. This would increase the rate of dissociative recombination of chemi-ions: A + A+
(53)Dheandhanoo, S.;Forte, L.; Fox, A.; Bohme, D. K. Can. J. Chem. 1986,64,641. (54) Schiff, H. I.; Bohme, D. K. Astrophys. J. 1979,232,740. (55)Knight, J. S.;Freeman, G. G.; McEwan, M. J.; Anicich, V. G.; Andress, W. T., Jr. J. Phys. Chem. 1987,91,3898. (56)Kebarle, P.; Haynes, R. M.; Searles, S. In Ion-Molecule Reactions in the Gas Phase, Advances in Chemistry Series 58;American Chemical Society: Washington, DC, 1966;p 210. (57)Ausloca, P.; Jackson, J.-A. A.; Lias, S. G. Int. J.Mass Spectrom. Ion Phys. 1980,33,269. (58)Gaydon, A. G.; Wolfhard, H.G. Flames, Their Structure, Radiation and Temperature, 4th ed.; Chapman and Hall: London, 1979; Chapter VII. (59)Sternberg, J. C.; Galloway, W. S.; Jones, D. T. L. In Gas Chromatography; Brenner, N., Jones, J. E., Weiss, M. D., Eds.; Academic: London, 1962;p 231. (60) Goodings, J. M.; Ng, C.-H.; Bohme, D. K. Int. J.Mass. Spectrom. Ian Phys. 1979,29,57. (61)Abers, E. A.; Homann, K. H. 2.Phys. Chem. 1968,58,220. (62)Bulewicz, E. M.; Padley, P. J. Ninth Symposium (International) on Combustion; Academic: New York, 1963;p 638. (63)Takahashi, F.;Glassman, I. Combust. Sci. Technol. 1984,37,1. (64)Scanlan, J. T.; Willis, D. E. J. Chromat. Sci. 1985,23,333. (65)Bulewicz, E. M. Nature (London) 1966,211,961. (66)Bulewicz, E. M. Combust. Flame 1969,13,214.
(16)
M+ + e-
-
+ e-
products
(18)
(19)
Either of these mechanisms would remove the precursor chemi-ion from the system and thus reduce the number of soot nuclei produced. Reaction 17 will be important under conditions such that the equilibrium concentration of M+ is lower than that of A+. In fact, the rate coefficient for reaction 17 is so much greater than for reaction 18 that (67)Feugier, A. Combustion Institute European Symposium; Weinberg, F. J., Ed.; Academic: London, 1973;p 406. (68)Bulewin, E. M.;Evans, D. G.; Padley, P. J. Fifteenth Symposium (International) on Combustion; Reinhold: New York, 1977;p 1461. (69)Howard, J. B.; Kausch, W. J., Jr. Prog. Energy Combust. Sci. 1980,6,263. (70)Addecutt, K. S. B.; Nutt, C. W. Presented at the Symposium on Deposit, Wear, and Emission Control, 158th National Meeting of the American Chemical Society, New York, Sept 1969;Paper A69. (71)Bartholome, E.; Sacksse, H. 2.Elektrochem. 1949,53,326. (72)Schofield, K.; Sugden, T. M. Tenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1965;p 589.
Calcote et al.
502 Energy & Fuels, Vol. 2, No. 4, 1988 SOOTING
I
'
I
1
'
I I
PRO-SOOT
--50
E
a I-
- 25 0 RADIAL DISTANCE, without CCI,
1
2
3
4
5
cm I
with CCI,
Figure 12. Effect of CC14on cation profiles and soot formation in a C2H2/02spherical diffusion flame: 0.53 kPa, 0.35 mol % of CC14 added. C2H2inlet tube a t 0.0 cm. Figure adapted from Miller.74
-
N0N S00TIN G
SOOTING
I
1
10'
P
- + + - + + -
C12 10-12 3
without CCiq
4
5
0 1 RADIAL DISTANCE, cm
2
3
I 10'2
cs+ CONCENTRATION, cm-3
C12 10-11
2
I
10"
Figure 14. Effect of CsCl additive concentration on soot formation in an acetylene/oxygen diffusion flame. Figure adapted from Bulewicz et a1.68
CC14 + e-
2
1
1
10'0
simultaneously completely altered the ionic structure of the flame (Figures 12 and 13). The ion concentrations were measured with molecular-beam ion mass spectrometry. The chlorine from the additive formed compounds that attached electrons producing large concentrations of negative ions, e.g.
10-8
0
s4
4
5
with CCi4
Figure 13. Effect of CC14on anion profiles and soot formation in a C2H2/02spherical diffusion flame. See Figure 12.
equilibrium concentrations of A+ may be approached via reaction 17 rather than reaction 18.l The metal ion, A+, may also be produced in greater than equilibrium concentrations by reaction 17.l Thus the specific mechanism by which an additive is effective will be determined by the relative equilibrium concentrations of A+ and M+ and the time available to approach equilibrium. Reaction 17 is well documented in nonsooting flames.l Addecutt and N ~ t tin, a~ study ~ of the effect of chemical additives on soot formation, observed, by molecular beam mass spectrometry, the effect of 25 ppm of Na, Ca, and Ba on the small ions in a flame at approximately 1970 K. Under these conditions Na was only slightly ionized while Ca and Ba were both fully ionized. The results, Table 11, indicate an effect of Ca and Ba on the concentrations of small ions. Goodings et al.73did a similar experiment in which they demonstrated that hydrocarbon ion concentrations up to mass 120 amu were suppressed by the addition of CsC1. It is not possible, for either of these sets of data, to distinguish which of the above two mechanisms (e.g., reaction 17 or reactions 18 and 19) is operative; it does demonstrate an additive effect on chemi-ion concentration, for the same additives and in the same order in which they affect soot formation.70 Miller74observed that the addition of carbon tetrachloride to a spherical low-pressure diffusion flame transformed a nonsooting flame into a sooting flame and (73)Goodings, J. M.; Graham, S. M.; Karallas, N. S. Int. J. Mass. Spectrom. Ion Processes 1986,69, 343. (74)Miller, W.J. Eleventh Symposium (International)on Combustion, "he Combustion Institute, Pittsburgh, PA, 1967;p 252;unpublished work, AeroChem TP-151,1967.
CC13
e-
C1-
e-
C1-
C1
C12-
(20) (21) (22)
These anions reduced the rate of recombination of cations, which normally disappear via dissociative reactions such as C12H3+ + e- ---+ products
(23)
When the additive is present, the cation can disappear only by reactions with anions such as C12H3+ + C1-
-
products
(24)
Recombination coefficients for dissociative ion-ion recombination reactions, such as reaction 24, in flames are about an order of magnitude lower than for those ionelectron reactions such as reaction 23.75*7s Thus the positive ion concentration is increased, and this increase can, assuming the ionic mechanism of soot formation, be responsible for the increase in observable soot. Bulewicz et aLB studied the effect of metal additives on soot formation in flames and have given a detailed interpretation of their results which is in complete accord with the ionic mechanism of soot formation. A number of chemical additives were added to the fuel side of an acetylene/oxygen diffusion flame, and soot was collected on a glass-fiber filter and weighed. The soot particle size was determined by electron microscopy of samples taken at 2 cm, for about 1ms, above the burner, and the total positive ion concentration was determined with a Langmuir probe. Flame temperatures were in the range of 1400-1800 K. The same additive acted as a prosoot or an antisoot additive (Figure 14), depending upon the total ion concentration due to the additive. With increasing additive concentration, the total quantity of soot increased and then with a greater concentration of additive the quantity of soot decreased so that at a little over 1O1O cesium ions/cm3 (75)Burdett, N.A.; Hayhurst, A. N. Second European Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1975;p 55. (76)King, I. R. J. Chem. Phys. 1962, 37, 74.
Ionic Mechanism of Soot Formation
the amount of soot produced was the same as without the additive. At this additive concentration the particle number density reached a maximum. Beyond this concentration of additive the number density, total mass of soot, and the soot particle size all decreased, indicating that the dominant effect of the additive was on the nucleation step rather than on the coagulation step. In other flames the dominant effect of additives is on coagulation, see e.g., Haynes et al.I7 Bulewicz et a1.68explain both the prosoot and the antisoot effect by means of an ionic mechanism. The reader is referred to their paper for the detailed arguments. Briefly, the antisoot effect is due to charge transfer from the precursor hydrocarbon ion, M+, to the metal atom as in reaction 17. They point out that in their flame, thermal ionization of the metal atom, reaction 18, would be too slow to produce electron concentrations above the natural flame level so that reaction 19 would not be effective in reducing the concentration of hydrocarbon flame ions. The prosoot effect was explained by arguing that small concentrations of additive may, in fact, maintain the level of M+ at a higher value throughout the flame than when no additive is present. Consider the potassium range of concentration from prosooting to antisooting, similar to the cesium levels used in Figure 14, which vary from about lo1(‘to 1014atoms/cm3. At about 1600 K, the equilibrium concentration of K+ varies from about lo8 to 1O’O ions/cm3 (function of solution molarity). The natural flame ionization concentration is about 1O’O ions/cm3, several orders of magnitude above equilibrium levels. When the concentration of K is 1014 atoms/cm3 and the concentration of M+ is 1O1O ions/cm3, it is clear that reaction 17 can proceed at a significant rate, thus reducing the concentration of M+. At this high level of additive concentration the equilibrium concentration of K+ cannot be exceeded. On the other hand, when the K concentration is only 1Olo atoms/cm3, the M+ concentration can be only negligibly reduced in the time available. However, the equilibrium level of K+, which is only about loEions/cm3, may be significantly exceeded because the recombination rate is slow compared to the rate of charge transfer from chemi-ions, reaction 17 (see, e.g., ref 1). Further downstream the slow decay of K+ maintains a higher level of total ionization, and at this point Bulewicz et al.m suggest that the reverse of reaction 17 maintains the concentrations of M+. In the absence of this reaction, M+ would have decayed because dissociative recombination of molecular ions is about 2 orders of magnitude faster than recombination of atomic ions, which must decay by a three-body process.78 Thus new ionic nuclei are available at later stages in the combustion process to grow to incipient soot particles. The prosoot action of higher ionization elementsm (e.g.: Pb, 7.42 eV; Mg, 7.64 eV; Cr, 6.76 eV; Co, 7.06 eV; Mn, 7.43 eV; and even Li, 5.39 eV) are also explained by the above argument. Their equilibrium ionization levels are very low, and their ionization potentials are somewhat less than those for hydrocarbon ions, so a concentration above the equilibrium concentration is expected. Clearly this analysis merits more detailed modeling in which all of the simultaneous reactions can be accounted for more quantitatively. It is, however, at this stage of development consistent with the ionic mechanism of soot formation. 9. Electric Field Effects and Electron Injection. The sometimes dramatic effects of electric fields on the (77)Haynes, B. S.;Jander, H.; Wagner, H. G. Seventeenth Symposium (International)on Combustion; The Combustion Institute Pittsburgh, PA, 1979;p 1365. (78) Ashton, A. F.; Hayhurst, A. N. Combust. Flame 1973, 21, 69.
Energy & Fuels, Vol. 2, No. 4, 1988 503
quantity of soot produced in a flame or deposited on an electrode are sometimes quoted as evidence of an ionic mechanism of soot nucleation. This evidence is somewhat ambiguous and will be the subject of a future publication; it is mentioned here only for completeness. Weinberg and associates have demonstrated dramatic effects of electric fields on soot f o r m a t i ~ n , ~which *~~@ they have interpreted as due to both an effect on soot nucleation and on coagulation. Both enhancement and suppression of soot concentrations were demonstrated. Similar results have also been obtained by o t h e r ~ . ~ l - ~ ~ Both Bowser and WeinbergE4and Saloojae5have demonstrated that the injection of electrons into flames, from small coated wires placed in the flame, can markedly alter the quantity of soot produced. The effect, enhancement or suppression, is dependent upon the location of the wire in the flame and the quantity of electrons introduced. Whether this effect occurs on the nucleation step or on the coagulation step is not clear. 10. Aesthetics. The ionic mechanism of soot formation is aesthetically pleasing: it is simple; and it does not have any “ad hoc” assumptions. Summary
In responding to the title question, a number of observations have been examined. Some of these have been found to be consistent with an ionic mechanism, albeit not a proof, and others are strongly supportive of an ionic mechanism and in fact difficult to explain without invoking an ionic mechanism of soot nucleation. Clearly much information is missing, and some of the published data used in the arguments are of questionable validity. Below we summarize some of the data and arguments used in examining the title question and then indicate the information that is most needed to place the ionic mechanism on a sound footing. The data and arguments in support of an ionic mechanism of soot formation are as follows: 1. Ion concentrations measured in the “standard” flame, an acetylene/oxygen flame at 2.7 kPa and 50 cm/s unburned gas velocity, are sufficient to account for the soot concentrations observed. 2. The rates of ion formation and soot formation in flames seen to be of the same magnitude; however, there are no really comparable measurements in the same flame, except in the “standard” flame. 3. Ion-molecule reaction rates involved in the ionic mechanism of soot formation have been demonstrated to be fast at the high temperatures of sooting flames by measuring the characteristic times for the ions to appear in the standard flame. 4. Ion-molecule reactions involving large ions reacting with small neutrals are demonstrated to be possible can(79)Weinberg, F. J. In Soot in Combustion Systems and Its Toxic Properties; Lahaye, J., Prado, G., Eds.; Plenum: New York, 1983;p 243. (80)Hardesty, D.R.;Weinberg, F. J. Fourteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1976;p 907. (81)Heinsohn, R.J.; Becker, P. M. In Combustion Technology, Some Modern Deuelopments; Palmer, H. B., Beer, J. M., Eds.; Academic: New York, 1974;Chapter IX. (82)Lester, T. W.; Wittig, S. L. K. Sixteenth Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, PA, 1977; p 671. (83)Wittig, S.L. K.; Lester, T. W. In Euaporation-Combustion of Fuels; Zung, J. T., Ed.; Advances in Chemistry Series 166;American Chemical Society: Washington, DC, 1978; p 167. (84) Bowser, R. J.; Weinberg, F. J. Nature (London) 1974, 249,339. (85)Salooja, K. C. Combustion Institute European Symposium; Weinberg, F. C., Ed.; Academic: London, 1973;p 400.
504 Energy & Fuels, Vol. 2, No. 4, 1988
didates for ion growth, but the opposite reactions, small ions reacting with large neutrals, are eliminated as possibilities for producing large ions. This is largely the result of the concentration of reactive species in sooting flames. 5. Data on neutral and charged soot particle diameters through a sooting flame have in the past presented a dilemma, because the neutral particles seemed to be growing at a faster rate than the charged particles. Within the accuracy of the data, these observations are interpreted as indicating that, for small particles, equilibrium between ionized and neutral particles is approached from the side of excess ionized particle. This is consistent with an ionic mechanism of soot formation. 6. All of the ions proposed in the ionic mechanism, up to mass 557, have been identified in sooting flames. 7. The location of the peak in ion concentration with respect to the peak in soot concentration, in both premixed and diffusion flames, is consistent with an ionic mechanism of soot formation. 8. Soot formation ceases in a premixed sooting flame at about the same location in the flame as that at which ions are no longer being formed by chemi-ionization. 9. The character of the ions observed in flames with various fuel/oxidizer ratios changes at the equivalenceratio at which soot first appears in premixed flames. 10. Ions have a thermodynamic propensity to grow. 11. There is a correlation between the tendency of fuels to produce soot and their tendency to produce ions. 12. The effect of some chemical additives on soot formation in premixed and in diffusion flames can be readily explained by assuming an ionic mechanism of soot nucleation. 13. The ionic mechanism of soot nucleation is aesthetically pleasing; it is simple, and it does not require any “ad hoc” assumptions. The research and information needed to place the ionic mechanism of soot formation on a sounder basis are as follows: 1. A theoretical model should be developed by using the
Calcote et al.
best available reaction rate coefficient information to determine the extent to which such an ionic model is consistent with experimental data and to better define the unknowns in the mechanism. We are currently pursuing this course. 2. The structure of the ion C3H3+,the assumed precursor in the mechanism, should be determined as it is produced in a flame; Le., is the linear or cyclic isomer produced? 3. The rate at which the equilibrium between linear and cyclic C3H3+proceeds needs to be determined at the temperature of sooting flames. 4. The rate coefficienb of ion-molecule reactions at high temperatures are very badly needed. There is a dearth of data on the effect of temperature on ion-molecule reaction rates. 5. The measurement of the concentration and the identification of large negative ions in sooting flames are very important. There is evidence of the presence of large negative ions; they could make a difference in equilibrium considerations of the sources of large hydrocarbon ions in flames. 6. More detailed and quantitative data on the concentration and size distribution of both small neutral and ionized soot particles are very important to an understanding of the mechanism of incipient soot formation. 7. Determination of the effect of chemical additives on soot formation and ionization would help unravel the mechanism of soot formation as well as have practical implications. The answer to the question raised in the title seems to be “yes”.
Acknowledgment. This paper is based on work funded by the Air Force Office of Scientific Research under Contract F49620-83-C-0150and National Science Foundation Grant CBT-8502122. We gratefully acknowledge the assistance of Helen Rothschild in reducing the data and preparing the figures.
