The effect of metallic additives on ionization and soot formation in oxyacetylene flames

COMBUSTION

A N D F L A M E 7 8 : 3 3 9 - 3 5 6 (1989)

339

The Effect of Metallic Additives on Ionization and Soot Formation in Oxy-Acetylene Flames A. N. H A Y H U R S T and H. R. N. JONES Department of Chemical Engineering, Cambridge University, Pembroke Street, Cambridge CB2 3RA

A quadrupole mass spectrometer was used to measure individual and total ion concentrations in premixed atmospheric-pressure flames of acetylene, oxygen and argon, to which small quantitiesof sodium, strontium, or manganese had been added. Large increases in total flame ionization were noted. Although chemi-ionization mechanismscan account for such an increasewith strontiumand manganeseaddition, some other mechanism(e.g., collisional ionizationby "hot" electrons) is needed to explain the rapid formationof sodium ions. All three metals accelerated the rate of disappearanceof natural flame ions of mass < 170 ainu. This was because of charge exchange reactions between such flame ions and neutral metallic species, as well as enhanced recombinationof these flame ions with free electrons. Sodiumand strontiumalso reduced the average size of charged soot particles, whereas manganesehad no observableeffect. The observationsstronglysuggestthat smallchemi-ions, such as C3H3~, play only a very minor role in soot nucleationand that it is larger primarychemi-ionswith massesof a few hundred ainu, which are the key nuclei in an ionic mechanismof soot formation. These big chemi-ionsare typicallyformedby reactionslike C,6Ht0 + C6H "-* C22H. * + e- ; they are removedby charge exchangewith Na or Sr species, but not ones of Mn with their relativelyhigh ionizationpotentials. This explainshow Na or Sr addition (ionizationpotentials relativelylow) can reduce the rates of soot nucleationand consequentparticle growth.

INTRODUCTION So long as soot remains a perennial unwanted sideproduct of combustion, methods will be devised to prevent or suppress its formation. There are a number of physical methods available, e.g., redesign of the combustion chamber, which will affect the physical conditions of combustion and can have a marked effect on the way in which the fuel burns. An alternative method is to use an additive to reduce the amount of soot present. Such a material may be added to the unburned fuel mixture with the intention of influencing the chemical rates of formation and destruction of, for example, various soot precursors. It is the role played by the addition of metals to the unburnt gases, which is the prime concern of this paper. The first study of flame additives suppressing soot formation is apparently by Bartholome and Sachse [1], who seeded a fuel-rich hydrocarbon flame with a variety of metals. They reported that many Copyright © 1989 by The CombustionInstitute Published by Elsevier Science PublishingCo., Inc. 655 Avenueof the Americas, New York, NY 10010

metals markedly reduced the amount of soot present and that the soot particles seemed smaller than without the additive. Subsequently, there has been much work on metallic additives; however, the picture presented in the literature is one of confusion, with a number of apparently contradictory results. It seems evident that the observed action of an additive is critically dependent on the precise conditions of combustion, such as whether the flame is premixed or not, burner geometry, fuel-oxygen ratio, and how the additive is physically added to the flame. It is worth mentioning at the outset that soot formation is believed [2] to proceed via some nucleus (or nuclei), which is attacked repeatedly by a growth species (usually thought to be C2H2), in, e.g., C xHy + C2H 2--*C x + 2Hy + H 2.

O)

It is not totally agreed whether the nucleus and

0010-21801891503.5(

340 subsequently growing species are charged or not. In fact, it is quite possible [3, 4] that both ionic and neutral species act as nuclei. Once the resulting species reach a diameter of ~ l0 nm, corresponding to a mass of ~ 10 6 amu, they either coalesce or coagulate. The production of soot nearly always involves a competition between growth and oxidation of a particle. There are basically two ways in which an additive can reduce soot emissions. Firstly, it can inhibit the formation of soot, e.g., by removing the original nuclei or growth species, or by decelerating the rates of coalescence and coagulation of soot particles. Alternatively, an additive can remove soot after it has formed by accelerating its subsequent oxidation, e.g., by stimulating the generation of oxidizing species, such as free O atoms or OH radicals. As for metal additives, their mode of action is still not well understood. Some experiments have shown soot enhancement, others soot suppression for the same metal. Salooja [5] even reported that a metal can show both pro- and anti-soot behaviour in the same flame, depending on where exactly in the flame the metal is introduced. Despite the contradictions, some general mechanistic trends have emerged. The suppression of soot formation (in contrast to the enhancement of oxidation) is generally believed to involve an ionic mechanism and is accordingly considered to be more important for those metals that have low ionization potentials. Metals that have been included in this category are the alkalis, together with barium and strontium. Haynes et al. [6] reported that their addition results in a shift towards smaller particle sizes, an increase in soot particle number density, and only a small decrease in the soot volume fraction (SVF). This was explained [6] by the metal ions transferring their charge to incipient uncharged soot particles. This increases the electrostatic repulsion between the resulting charged soot particles, which in turn reduces the rates of coalescence and coagulation. Hence, there are more particles, but of smaller average size. The decrease in SVF would then be due to the smaller particles having a shorter burnout time. The chief problem with this interpretation is that, for charge exchange to occur, the metal must have a higher ionization potential (IP) than a soot particle.

A . N . HAYHURST and H. R. N. JONES Although this may indeed be the case for a metal like sodium (IP = 5.14 eV = 496 kJ/mol), such a mechanism will probably not operate for, say, cesium (IP = 3.89 eV = 375 kJ/mol). Bulewicz et al. [71, working with diffusion flames, also reported a shift to lower particle sizes on the addition of alkali metals. They also, however, reported a decrease in number density and SVF. This can be accommodated by the scheme of Haynes et al. [6], if the rate of burnout of the smaller particles is increased such that fewer particles are actually observed. Bulewicz et al. [7] prefer, however, an alternative explanation, namely that nucleation is slower when a metal is added. Nucleation, they believe, involves small ions, so removal of natural flame ions by the metal via charge exchange would decrease the rate of nucleation and hence the amount of soot formed. Charge exchange between a metal atom and a small flame ion is likely to be favorable, since all the relevant flame ions possess much higher ionization potentials than an alkali metal. There is evidence for such charge exchange reactions which remove flame ions. Calcote [8] and Salooja [5] have reported a decrease in "natural" flame ionization, although total ionization (i.e., including the metal) is increased. Hayhurst and Telford [9] have measured the rates of processes, such as H3O÷ + N a ~ N a ÷ + H 2 0 + H

(II)

for several metals in flames. More recently, Goodings and colleagues [10, 11] and Bonczyk [12] have suggested that the reduction in natural ionization is due to enhanced ion-electron recombination, such as by C3H3 ~ + e- --' C3H2 + H,

(III)

rather than charge transfer. In contrast to the above, both Feugier [13, 14] working with premixed flames and Marinescu and Danescu [15] with diffusion flames, have reported that there is soot enhancement with alkali metals, probably by metal ions increasing the rate of nucleation. Both sets of workers considered that any inhibiting effects observed with the alkali metals are due to increased rates of oxidation and that both a soot

METALLIC ADDITIVES IN SOOTING C2H2 FLAMES enhancement mechanism and a soot removal mechanism may operate simultaneously. Salooja [5] concluded that, for both diffusion and premixed flames, if metals were introduced into the unburned gases, soot suppression was observed; by contrast, injection of a metal into the carbon-forming zone increased the amount of soot produced. Salooja considered that the anti-soot behavior was due to removal of natural flame ions that reduced the rate of ionic nucleation. The pro-soot behavior was explained by charge exchange between ionized soot and neutral metal atoms. This reduces electrostatic repulsion between particles, thereby increasing the rate of coalescence and coagulation, i.e., the reverse of the soot reduction mechanism of Haynes et al. [6]. The position, then, is quite confused and uncertain, although ions definitely seem to play some sort of role, either at the nucleation or coagulation stage. One aim of this present work was to establish the influence, if any, of small quantities of metal additives on the abundances of natural flame ions. This in turn should provide useful information regarding the early stages of soot formation. The plan of this study was to add a number of metals to sooting and near-sooting oxyacetylene flames. Three metals were chosen-sodium, strontium, and manganese--with ionization potentials of 5.14, 5.69, and 7.43 eV (496, 549, and 717 kJ/mol), respectively. This gives a wide spread of metal type and ionization level. Padley and Sugden [ 16] have studied the formation of metallic compounds in flames in great detail. From their work, it is evident that an alkali metal is present in a flame in three forms, namely free metal atoms, free metal ions, and molecules of the hydroxide. These hydroxides are formed by the balanced equilibrium [17, 18] A + H20 = AOH + H, where A is the alkali metal. In fuel-rich flames, sodium forms negligible quantities of NaOH; in O2-rich flames the situation is more complicated [19, 201. The rate of ionization of alkali metals in hydrogen and carbon monoxide flames is very slow [16, 21] and is by A+M~A ~+e-+M,

(IV)

341

where M is an inert third body. The kinetics of such thermal ionization reactions have been measured and reviewed [22]. In hydrocarbon flames, where natural flame ionization is higher, ionization of alkali metals is much faster, so a different mechanism has been invoked. Charge exchange with natural flame ions (in, e.g., II) has been a popular explanation, because all small molecular flame species possess higher ionization potentials than the alkali metal atoms. Thus, reactions of the general form HB ÷ + N a ~ N a - + B + H

(V)

are exothermic, where HB ÷ represents a flame ion such a s H 3 0 ÷ , CHO ÷, o r C 3 H 3 * . Hayhurst and Sugden [23] found that reactions of this type were not sufficient to account for the levels of sodium ionization in their fuel-rich hydrogen flames with added traces of acetylene. They indicated that the increase in sodium ionization on acetylene addition was possibly linked in some way with the chemi-ionization reactions always associated with hydrocarbon addition. It was therefore postulated that " h o t " electrons ( e - * ) were formed during chemi-ionization, so that ionization via e *+Na--,2e- + N a ÷

(VI)

could operate. They calculated that in a flame at 2000 K, the rate of collisional ionization would be increased by as much as 1000 times if the electron temperature were raised to 2500 K. There is some evidence [2, 24] that free electrons can be "hotter" than the flame gases by ---200-300 K.

EXPERIMENTAL Two laminar premixed flames of C 2 H 2 + 0 2 + Ar were burned horizontally at atmospheric pressure on a piece of quartz tubing (i.d. 2.5 mm; o.d. 5 mm). These flames have been described before [3, 25], where they were referred to as flames 3 and 5. In appearance they are somewhat similar to flames on a Bunsen burner. Flame 3 (C2H2/O2/Ar = 1.02/1.00/5.25) has a thin luminous turquoise reaction zone, almost conical in shape, downstream of which is a paler greyish-blue mantle, the acetylene feather. This extends for about 4-5 mm

342 from the reaction zone and is followed by the burned gas region. There is no visual evidence of soot particles in flame 3, which is just on the fuellean side of the sooting point (C/O = 1.04). The richer flame 5 (C2HE/OE/Ar = 1.09/1.00/5.84) has a more extensive feather, in which soot is seen, from its characteristic yellow emission. These soot particles survive beyond the feather into the burned gases. A metal was added to the flame gases by atomizing into the unburned gases an aqueous solution (strength 0.002 M) of the appropriate chloride. The sum of the mole fractions of all the metallic species in a flame is ~- 6 × 1012 molecules/cm 3. The burned gases in each flame have an axial velocity close to 8 m/s. Calculations show that for such a flow rate, Oz diffuses from the laboratory air through to the flame's axis after ~-0.2 m. Since the observations described below were of axial ion concentrations over only the first 20 mm of the burned gases, it can be concluded that any effects of the surrounding atmosphere diffusing into the flame can be ignored. A tiny fraction ( - 1 cm3/s) of the gases from a point on the axis of one of these flames was sampled into a quadrupole mass spectrometer. This instrument has been described in detail already [25]. It measures the abundance of each ionic mass for positively and negatively charged species at selected points along a flame. Its mass range extends up to 170 amu. Total ion concentrations for one sign of charge are obtained in two ways--firstly, by summing all the individual concentrations for each peak in a spectrum up to 170 amu. Alternatively, the spectrometer can be operated without the d.c. supply to the quadrupole rods. In this case a true total of the concentrations of all ions of one chosen sign can be obtained for masses up to infinity. Any difference between these two measures of total ion abundance indicates the presence of ions heavier than 170 amu. In unseeded flames, such species have only been found [25] in sooting flames, such as flame 5. RESULTS AND DISCUSSION Natural Flame Ionization The concentration profiles of positive and negative ions in flames 3 and 5 without additives have been

A . N . HAYHURST and H. R. N. JONES discussed previously [3, 25, 26]. A brief summary is given here to facilitate comparisons with the systems discussed below. It has been argued [25, 26] that pyrolytic reactions of hydrocarbon species dominate the ionic and neutral chemistry within the acetylene feather. Extensive polymerization of hydrocarbon species can occur in the reaction zone and feather, resulting in the formation of both aromatic and aliphatic heavy species. All ions in flame 3 have masses less than 170 ainu; the sooting flame 5 contains large concentrations of charged molecules and particles of mass up to at least 104 amu [3]. In the reaction zone and feather, where oxidation reactions are largely absent, C3H3 ÷ appears to be the primary chemi-ion; CHO ÷ and other oxygen-containing ions are only important well downstream in the burned gases [25, 26]. ADDITION OF SODIUM Positive Ions with Sodium Present Covcentration profiles of some of the positive ions in flames 3 and 5 with sodium added are shown in Figs. 1 and 2. Figure la refers to flame 3 and shows the oxygen-containing ions H30 ÷ (mass 19), CHO + (29) and mass 47, which is protonated ethanol, dimethylether, or formic acid. Comparison with previously published results [25] shows that the presence of sodium does not alter the rate of production of these oxygen--containing ions in the reaction zone and feather. Also, their peak concentrations are unchanged, but their disappearance is accelerated. In fact, the removal of these ions from the burned gases conforms to a linear plot in Fig. la, the slope of which is increased 3.0 times by the addition of Na. This is discussed later. Figure lb shows the polyacetylenic [25] ions CxH3 ~ , with x varying from 3 to 8. Again, rates of ion production are unaffected by sodium, but their lifetimes are shortened, except for mass 39, which is longer-lived. This is almost certainly due to potassium impurities in the sodium source being responsible for the generation of K + at mass 39. Figure lc shows concentration profiles along flame 3 for the more hydrogenated aliphatic ions C4H5 + (53) and C6H5 + (77), which persist throughout the feather in the absence of Na [25].

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Fig. I. Measured positive ion concentrationsalong flame 3 with sodium added to the burner supplies. Part (a) gives the profiles of some oxygen-containingions, part (b) refers to polyacetylenicions CxH3", and part (c) is for some of the more hydrogenated CxH/ ions. The mass of each ion in a.m.u, is as shown. In addition, the locationof the reactionzone (r.z.) and the extents of the acetylenefeather and burned gas regionare as shown.

along the axis of the sooting flame 5. The position of the reaction zone (r.z.) is as shown, as are the extents of the feather, the burned gases (b.g.), and the region where soot particles are visibly present. Again part (a) refers to some of the oxygenatedions. part (b) to the polyacetylenicions C,H3'. and part (c) to a few of the more hydrogenated C, H, ÷ ions. Ionic masses are as shown.

Both ions actually disappear at the beginning of the feather (with Na added), as also does the aromatic ion C7H7 ÷ (91), as shown in Fig. lc. Profiles of the same ions along the sooting flame 5 are shown in Fig. 2. Comparison with previously published observations [25] shows that all the natural positive flame ions are formed at about the same rate as without Na. Also, they reach approximately the same maximum concentration at the same point in the flame. The rate of removal of every natural ion is, however, much faster when sodium is present. In fact, no hydrocarbon positive ion is observed beyond 5 mm from the reaction zone. Also, unlike the natural flame, the

aromatic hydrocarbon ions show no signs of persisting into the feather. There is again the clear suggestion that the addition of sodium has no effect on ion formation, but increases their rate of removal. This is discussed further below. The total ion currents (as obtained by summing the individual positive ion signals) for seeded flames 3 and 5 show roughly a 30-fold increase when sodium is added. This huge increase is shown in Fig. 3 for flame 3 and is entirely due to the appearance of appropriate quantities of sodium ions. Examination of Fig. 3 shows that with sodium present, ions are produced earlier, but at a

Fig. 2. Concentration profiles for some of the positive ions

344

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reduced rate, just upstream of the reaction zone. Maximum ion concentration is reached near the end of the feather. There is evidently little ionelectron recombination in the burned gases, whether or not sodium is added. It should also be noted that because the total positive ion concentration is much greater than total negative ion concentration (see below), nearly all the negative charge is in the form of free electrons. Thus Fig. 3 is, effectively, also a plot of free electron concentration against axial distance. The only sodium-containing ions observed in either flame were Na ÷ and its monohydrate, which is formed during sampling as well as being a genuine flame ion [27]. The Na ÷ concentration profile along flame 3 is shown in Fig. 4; it is clear that [Na ÷ ] becomes about 30 times larger than the most abundant natural flame ions in the absence of an additive. (The profiles of other metallic ions shown in Fig. 4 will be discussed in the relevant sections below.) Production of Na ÷ is slower (see Figs. I and 4) than that of the hydrocarbon ions, a maximum being reached about 5 mm from the reaction zone in both flames 3 and 5. Removal of Na ÷ is slow, presumably by the three-body recombination:

high levels of Na ÷ , well above the natural ion density of these flames, is worth examining. If all Na ÷ were formed by charge exchange between natural flame ions and sodium atoms, then there would be no apparent increase in total ionization, provided the rate of recombination was not changed. Thus it seems that some other ionization process must be occurring. Thermal ionization (IV) can be ruled out (see below), since both the rate of production and the peak magnitude of Na + are too high. Also, Na ÷ is first seen 2 mm upstream of the reaction zone. In this much cooler part of the flame, no thermal ionization could be envisaged. To justify some of these conclusions it should be noted that Fig. 3 shows that there is a maximum in the total ion concentration with sodium added to flame 3 at about 5 mm downstream of the reaction zone. In fact, Fig. 3 also shows this is the location for a maximum in the total ion concentration without a metal added. It was deduced above that the addition of Na does not appear to change the rates of production of the natural flame ions. In this case it is worth examining if the increase (by a factor of 30) in the maximum at 5 mm downstream is entirely due to ion recombination changing from

Na ÷ + e - + M - - , N a + M,

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(-IV)

i.e., the reverse of (IV). The origin of these very

(VII)

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M E T A L L I C ADDITIVES IN SOOTING C2H2 F L A M E S

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smaller rate. At any maximum in the total ion concentration, the rates of production and recombination of ions must equal each other. Therefore, at 5 mm downstream, the overall rate of production of natural flame ions (rp) is k7 [P+ 102, where [P+]0 is the maximum total concentration of positive ions (assumed equal to electron concentration) in the absence of Na and k7 is the rate coefficient of reaction (VII). It will now be assumed that (a) all the Na + ions arise by charge exchange with natural flame ions in processes like (II) and (V), i.e., no new process generates ions from neutral species when a flame is seeded with Na, and (b) all ionic recombination occurs via reaction (-IV). In this situation the overall rate of production of ions at 5 mm downstream is still given by rn (because ion production rates were seen above to be unaffected by the addition of Na), which now equals k_4[M][P+]w 2. Here, [P+]~ is the maximum in the total positive ion concentration shown in Fig. 3 at 5 mm from the reaction zone for Na added, k_ 4 is the rate constant for the reverse of (IV). This scenario requires kT[P+]02 = k_4[M][P+]w 2 = rp, which yields [ P + ] ~ / [P+]0 = ( k T / k - 4 l M l ) 1/2. Using k7 = 4.1 ± 1.0 x 10- 7 ion- 1 cm 3 s 1 [28] and k_4 = 4.1 ± 1.1

x 1 0 - 2 4 / T i o n -2 cm 6 s -1 [22], with [M] = 3.2 x 1018 molecules/cm 3 at 2300 K, gives [P+]w/ [P+]0 = 8.5 _+ 4.3. In fact Fig. 3 shows [ P + ] J [P+]0 = 30. We accordingly conclude that the observed 30-fold increase in ion concentration at 5 mm, when Na is added, is not solely due to recombination changing from the two-body process (VII) to the inherently slower three-body step (-IV). It is thus likely that Na + ions are produced by some other process, in addition to those analogous to (II) and (V). Also, as shown below, ion recombination occurs via both reactions (VII) and (-IV) when Na is present. Thermal ionization of sodium in (IV) should first be considered in more detail. The rate constant is given [22] by k4 = (9.9 + 2.7) x 10 -9 T 1/2 e x p ( - E / R T ) molecule -1 c m 3 S - l , where E is the ionization potential of Na. This gives the maximum rate of thermal ionization as k4[Na][M] = (2.6 x 10 -18 ) x (6 x 1012) x (3.2 x 1018) = 5.0 x 1013 ions cm -s s -1 for [Na +] ,~ [Na]. In fact, [Na + ] = 7.9 × l0 l' ions/ cm 3 and [Na] = 6.0 × 1012 - 7.9 x 10 H = 5.2 x 10 ~2 atoms/cm 3, so the condition is just satisfied. The rate of production of ions in the absence o f N a e q u a l s kv[P+]o 2 = (4.1 x 10 - 7 ) X

346 (3.2 x 101°) 2 = 4.2 x 1014 ions cm -3 s - i at 5 mm downstream. With Na added to flame 3 the rate of recombination of ions at this point is actually [e-]{k_4[Na+][M] + k7[H30+]} .~ 30 × 3.2 x 101°{(5.7 x 10 -9) × (7.9 × 1011) + (4.1 × 10 -7) × (1.6 X 101°)} -.~ 1.0 × 1016ionscm -3 s- i. However, the total rate of production of ions in the presence of Na is only equal to (5.0 x 1013) + (4.2 x 1014) = 4.7 × 1014 ions cm -3 s ' l by the two mechanisms considered so far. Clearly this overall rate of ion production is less than the ionic recombination rate of 1.0 × 10 t6 ions c m 3 s i . Thus it is clear that thermal ionization is not fast enough to explain the 30-fold rise in maximum ion concentration at 5 mm downstream in flame 3. It might also be noted from the above that the ratio of the rates of recombination via (-IV) and (VII) is 0.69, so that reaction (-IV) does not totally replace (VII) as the recombination step. A likely explanation seems to be some form of collisional ionization. Hayhurst and Sugden [23] have postulated the formation of " h o t " electrons (see above) during the combustion of the hydrocarbon fuel. The notion is that if electrons have a temperature above that of the neutrals, then the rate of ionization of sodium atoms would be increased. Such electrons would be very mobile and could diffuse upstream of the reaction zone into the preheat zone, thereby explaining the very early appearance of Na*. Fixing attention again on the maximum at 5 mm downstream in Fig. 3 for sodium added, the production of Na + by electrons in (VI) requires its rate to be 1.0 × 1016 - 4.7 × 10 ~4 -~ 1.0 x 1016 ions cm -3 s - i . Equating this tok6[e-][Na]givesk~ = 1.0 × 1016/(5.2 × 1012 x 7.9 x l0 II ) = 2.4 × 1 0 - 9 c m 3 i o n - l s - l . At this stage it is worth noting that the logarithmic plot of Na + against distance (or time) in Fig. 4 rises linearly, thereby showing that the growth in [Na + I is exponential in the early part of the flame. This is consistent with electrons causing ionization of Na, as can be seen as follows. Reaction (VI) leads to d[Na+]/dt = k 6 [ N a ] [ e - ] k6[Nal[Na+]. Integration yields ln[Na +] = ke[Na]t + constant, i.e., ln[Na+] increases linearly with time t, as observed in Fig. 4. The slopes of the lines for sodium in Figs. 3 and 4 are identical and yield k6 = 3.1 × 10-9 cm 3 ion-i

A . N . HAYHURST and H. R. N. JONES s- 1. This value of k6 from the slope of the early linear rise in Fig. 4 for Na ÷ agrees remarkably well with the value of about 2.4 × 10 -.9 cm 3 ion- 1 s -1 for k6 deduced above from the measured maximum ion concentration at 5 mm downstream in Fig. 3 with Na added. This approach does not constitute proof for the existence of hot electrons, but it does provide quantitative evidence for their presence in the very fuel-rich flame 3. Interestingly, these observations suggest that electrons are hotter than the neutrals over a large distance, i.e., from a point 2 mm upstream of the reaction zone to roughly where the ions in Fig. la display maxima in the burned gases. Finally, it is worth noting that at 5 mm downstream the ratio of the rate of ionization of Na atoms by collisions with electrons to the thermal rate is (1.0 x 1016)/(5.0 x 1013) = 200. This would correspond to a rise in the bulk gas temperature of very roughly 500600 K above the flame temperature of 2300 K, or, taking into account the much greater collision frequencies of free electrons, an excess of the electron temperature over that of the neutrals equal to about 200 K. In fact, very little is known about electron temperatures in these rich flames. As well as electrons, molecular species in excited states (such as vibrationally excited OH, and electronically excited C2, C2H, or CH, all of which are present in these flames) could in principle transfer enough energy during collisions to cause ionization. All these species are, however, much less mobile than electrons, and very probably are an unlikely cause of the very early Na t signals. Additionally, they cannot explain the exponential rise in [Na*] near the reaction zone. However, it is worth noting that there is evidence [24] that the electron temperature is coupled with vibrationally excited OH radicals, so that a possible alternative ionization mechanism would be of the form Na + OH*---' Na + + OH - , followed by O H - + H--'H20 + e - . It should be noted that the increase in ionization rate on the addition of metals has been considered

M E T A L L I C A D D I T I V E S IN S O O T I N G C2H2 FLAMES by Bulewicz and Padley [29]. Neither of the two explanations examined by them was concluded to be free of objections. Also, both of their mechanisms and that above depending on OH* are incapable of explaining the exponential growth kinetics for Na ÷ ions seen in Fig. 4, which does indicate participation by electrons in the production of Na" ions.

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I 6

1 8

distancelmm

Fig. 6. Concentration profiles for some of the negative ions in flame 5. The reaction zone (r.z.), feather, burned gases (b.g.), and region where soot is visibly present are shown. Part (a) refers to oxygenated ions, part (b) to those of the types Cx and C~H- and also C2H2-, and part (c) is for some of the more hydrogenated C,H r- ions.

I 10

12

axial distance I m m Fig. 5. Some of the individual negative ion profiles along flame 3. Part (a) is for oxygenated ions, part (b) for ions of the type Cx " and C x H - , but includes C2H2-, and part (c) gives a few of the more hydrogenated hydrocarbon ions. Ionic masses are as shown.

ion concentrations (for all masses < 170 amu) show a similar tenfold increase to the positive ions with sodium added. This is not unexpected, since any fresh production of positive ions must be balanced by formation of negative charges, either as ions or free electrons. Only a trace of Cl (from the added NaCI) is seen in these flames. This possibly surprising absence is due [30] to the removal of halogen ions during sampling by the reaction H+CI

~HCI+e

.

(VIII)

The relaxation time for (VIII) has been shown to be sufficiently short for the reaction to proceed

348 both in the relatively cool boundary layers just before flame gases pass through the sampling orifice and also during the subsequent supersonic expansion. As a result, the observed ion current for CI- may be low. Figures 5a and 6a show concentration profiles for some oxygen-containing negative ions, Figs. 5b and 6b refer to ions of the type Cx- and CxH-, but include C2 H2- (mass 26), and Figs. 5c and 6c depict three of the more hydrogenated ions, viz. C4H3 - (mass 51), C5H5- (65), and C7H5- (89), the last of which is most probably aromatic. Masses 63 (H3CO3 -) and 79 (H3CO4-) are the only long-lived oxygen-containing negative ions in these flames. Figures 5a and 6a show their concentrations remaining fairly steady, as the flame gases pass from the feather into the burned gases. In the flame without additive, their concentrations are smaller [25] by about a factor of 10, as well as showing a decline in the burned gases of flame 3. In fact, the addition of sodium increases the concentrations of all natural negative ions in these flames, but the slopes of the plots in Figs. 5 and 6 for their removal are, by and large, the same as in the unseeded flames. This is consistent with Na ÷ only affecting the positive ion chemistry, apart from the increase in free electron concentration. Collisions between positive and negative ions resulting in ion-ion neutralization are fairly rare in flames and so a direct effect from Na ÷ is not expected. The oxygen-containing ions are in higher concentrations than in the unseeded flames, such that C2HO- (mass 41), which was detected only in traces in unseeded flames, is quite a longlived ion in the seeded flame. The increase in the concentration of negative hydrocarbon ions is sufficiently large for these ions to be almost as abundant as the positive hydrocarbon ions. They also seem to be formed slightly earlier in the flame than previously, so that, in the unburned gases, the total concentration of negative ions appears higher than that of positive ions. If free electrons are being produced in large quantities as a result of sodium ionization near the reaction zone, then, as they are very mobile and highly diffusive, one could expect them to diffuse upstream from the reaction zone into the preheat zone, where electron attachment to neutrals may occur [31].

A . N . HAYHURST and H. R. N. JONES Because the rate of removal of negative ions is apparently the same in the seeded and unseeded flames, the negative hydrocarbon ions are longerlived than the positive hydrocarbon ions in the seeded flame 5. Two " n e w " negative ions are observed in the seeded flames, viz. C2H2- and C2H20- (masses 26 and 42). They are virtually absent from the reaction zone where C2H- and C2HO- are more plentiful, but they are present in moderate amounts in the feather, peaking in concentration near the feather tip. The presence of these two species in the seeded flame indicates that the increase in free electron concentration may quite considerably enhance reactions involving electron attachment to stable neutral flame molecules, e.g., C2H2 + e --~C2H2-, C2H20 + e- ~,C2H20-. Their detection also strongly suggests that neutral C2H2 and C2H20 (ketene) are both present throughout the feather and disappear only on entering the burned gases. Total Ionization with Sodium Present

Discussion in the previous two sections with regard to the apparent increase in total ionization when sodium is added refers to measurement of small ( < 170 amu) ions. This approach yields a measure of real total ionization in nonsooting flames (such as flame 3), where there are no ions of mass above 170 amu. However, it takes no account of any effects of metal addition on the very heavy (i.e., mass >170 amu) charged particles known to be present [3] in sooting flames such as flame 5. Figures 7 and 8 compare the real total positive and negative ion concentrations in unseeded and seeded flame 5, as measured directly by switching off the d.c. supply to the quadrupole rods (see above). It should be noted that the vertical scales in these two figures are not the same. Clearly, the positive ion profiles in Fig. 7 show a reduction in the number of charged species present by a factor of 2 to 3 when sodium is added, but the general features of a sharp rise in ionization at about 3 mm from the reaction zone and a

METALLIC ADDITIVES IN SOOTING C2H2 FLAMES

349

g ~o 100 CL

"6

,,

8O

g

6O

F1

,.N.

-

~ 2o "6 0

l,

8 axial

12

16

20

distancelmm

Fig. 7. The relative values of the sums of the concentrations of all positive ions measured along the sooting flame 5. The curves refer to the natural unseeded flame and also to it seeded with Na, St, and Mn, in turn.

broad maximum at 20 mm remain. The rise in ion concentration at about 3 mm coincides with the visual appearance of soot and has been attributed [3] to the production of heavy chemi-ions of mass greater than 170 amu. One illustrative example [32] is the exothermic reaction

by the mass spectrometer. The heaviest charged particles (mass > 104 amu) originate therefore by two routes: (1) an ionic route using chemi-ions generated in (IX), and (2) a neutral route, where a proportion of the uncharged soot particles ionize thermally when they are sufficiently large. An alternative ionic route using small chemi-ions (e.g., C3H3+) produced in the reaction zone is mechanistically feasible, but appears unlikely to be a major contributor (probably less than 10%) given the relatively low concentration of those

(IX)

C 16n i0 + C6 a ~ C22HII + + e - .

The slower rise in ion concentration from 5 to 18 mm is thought [3] to be due to thermal ionization of soot particles, which are subsequently detected cm 100 o >

'

r.z.

I

' feather

1

80

I ' burnt gases

I

60-

8 ,-

'

-

I

~

-

-

-

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,

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,

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Fig. 8. Plots of the relative value of the sum of the concentration of every negative ion versus axial distance along flame 5. Curves are given for the unseeded flame and also for it seeded with Na, Sr or Mn. Note that the overall ion signal changes sign for the natural flame and also with Mn added.

350

A . N . HAYHURST and H. R. N. JONES

small ions compared with the ultimate charged panicle concentration. The negative ion profile gives a signal always above zero when sodium is added (Fig. 8), whereas in the unseeded flame 5, a subzero total negative ion signal was recorded in the sooting part of the flame. The subzero signal was interpreted [31 as being due to transmission of very heavy ( > 104 amu) positively charged soot particles, i.e., even though the spectrometer is set up to measure negative ion concentrations, these heavy positively charged panicles have sufficient kinetic energy to overcome the voltages normally repelling positive ions. Because the subzero signal is no longer observed on the addition of sodium, it may be concluded that sodium prevents the formation of these very heavy positively charged particles. Calcote [321 has demonstrated that the ionization potential of a large molecule (or soot panicle) decreases as its physical size increases. In fact, molecules of mass less than = 3 × 104 amu possess a higher ionization potential than a sodium atom. It follows that any positive species of mass less than 3 x 104 amu is likely to exchange charge with a sodium atom in a seeded flame. This will apply both to small flame ions (e.g., H30 +, C3H3 +) and to the heavier gaseous chemi-ions formed by reactions such as (IX). Such a removal of charge from chemi-ion soot precursors reduces the rate of soot growth in CxHy + + C2H2--+Cx.2Hy ~ + H,..

(X)

This reaction represents the ionic contribution to soot particle growth--the neutral contribution was represented in reaction (1). A general expression for the rate of growth of soot particles can be written combining both routes: Rate = kl[S][S] + kl0[S +]IS1. The first term on the right-hand side represents a contribution to the rate from interactions between two neutral species, S, while the second term is a contribution from interactions between neutral S and ionic S +. Any charge exchange between S + and a neutral metal atom will lower [S +] and the ionic growth rate considerably. However, because [S] ~, [S ÷ ], such conversion of S + to S will have

minimal effect on either [S] or the neutral growth rate. Consequently, one may expect a decrease in total growth rate only if the ionic route is a major contributor. The corollary to this is that if a decrease in soot growth rate is observed (as indeed it is in this study), then the ionic route must play a major role in soot formation.

Removal of Natural Flame lons One question remains from this study of the addition of Na. It was noted above that the disappearance of an ion such as H30 + from flame 3 is three times faster in the logarithmic plot of Fig. la when Na is added. In the absence of Na, oxygen-containing ions have concentrations that are interrelated by equilibria like H30* + CO = HCO + + H20.

(XI)

In addition, ions are still being produced [26] in the burned gases by CH + O--*CHO* + e - .

(XII)

However, some recombination will occur in the reverse of (XII), as well as in [281 H30 + + e - ~ 2 H + OH.

(VII)

Thus, with proton transfer, ion recombination, and ion production occurring simultaneously in the burned gases, the kinetics of disappearance of H 3 0 + a r e complicated. With sodium present, ions are generated by the new mechanism (VI), as well as by the slower thermal process (IV). The question arises as to whether a natural flame ion such as H30 + disappears faster when Na is added because of increased recombination in (VII), resulting from a larger [e-], or because of proton or charge transfer in, e.g., (II). The ratio of the rates of these two reactions in flame 3 (see Fig. 1) is r o u g h l y k T [ e - ] / k 2 [ N a ] ~ (4.1 x 10 -7) x (9.5 x 1011)/(6 X 10 -9) X (6 × 1012) = 11 usingk 2 = 6 × 10 -9 cm 3 ion-t s-X [9]. Thus a natural flame ion like H30 * seems most likely to be disappearing in Fig. la via recombination with electrons in (VII). It is worth noting that both reactions (II) and (VII) would give a linear plot for [H30 +] in Fig. la, because [ e - ] is constant (see

METALLIC ADDITIVES IN SOOTING C2H2 FLAMES Fig. 3), as also is [Na], being equal to the total concentration of added sodium. If the experimental slope for [I-I30 +] in Fig. la is equated to kT[e-]/2.303 V, where V(= 8 m/s) is the velocity of the burned gases, then k7 ~ 4.9 x 10 -9 cm 3 ion-1 s-~. This is much less than the expected value of 4.1 × 10 -7 cm 3 ion -t s -1 quoted above [28] and suggests that not as much ion recombination is occurring as might have been expected. Indeed, Fig. 3 shows little net recombination in the burned gases. It appears, therefore, that ions are still being produced in the burned gases roughly at the same rate as they are disappearing by recombination. The slope of Fig. la for [H30 ÷] can also be equated to k2[Na]/2.303 V to obtain k2 ~ 8 × 10-~0 cm 3 molecule-l s-t, which is less than the value [9] of 6 x 10 -9 cm 3 molecule -I s -t used above. Thus removal by charge exchange is also less than might have been expected. One can only conclude that H30 + may be reacting with both Na atoms (reaction II) and electrons (reaction VII), as well as being generated by the reverse of (XI) and (VII). This conclusion compares with Goodings and colleagues [10, 11] and Bonczyk [12], who are more definite in their support of ion-electron recombination. In any event Fig. 1a shows identical slopes for the disappearance of masses 19, 47, and 29. This indicates that the disappearance of any of these ions is affected by rapid equilibria, such as (XI), which couple their concentrations.

A D D I T I O N OF S T R O N T I U M

The chemistry of alkaline earths in flames is more complex than that of the alkali metals owing to the greater number of flame species that are formed. This is due in part to the great stability of their oxides, which can condense to give solid oxide particles. Also, stable hydroxides and dihydroxides are formed by most alkaline earths, with the dihydroxide, M(OH)2, usually the most abundant species. A comprehensive account of alkaline earth chemistry has been given by Jensen and Jones [33]. As well as the neutrals mentioned, two ions have commonly been observed [34, 35], viz. Sr ÷ and SrOH ÷ , plus their hydrates. The

351

neutral species of an alkaline earth M are thought [33] to be formed by a sequence of balanced equilibria: M + H20 = MOH + H,

(xm)

M + OH = MO + H,

(XIV)

MOH + H20 = M(OH)2 + H.

(xv)

Ionization of alkaline earth metals is much higher than might be expected, in that they give more ions than sodium, despite their atoms having higher ionization potentials. A chemi-ionization reaction is now generally accepted [34-36]: M +OH~MOH + +e-,

(XVI)

MO + H--*MOH + + e - .

(xvII)

One cannot distinguish between these two possibilities because reaction (XIV) is thought to be rapid and balanced [36]. The free metal ion is produced by the rapidly balanced equilibrium: MOH + + H = M + + H20.

Positive Ion with Strontium Added

Figure 9 shows the results of adding strontium to flame 3, which is also typical of flame 5. The strontium-containing ions detected were Sr + and SrOH ÷ plus their hydrates; profiles are shown for Sr + and SrOH + along flame 3 in Fig. 4. As with sodium, the overall level of ionization in flame 3 (all ionic masses < 170 amu) has increased on addition of strontium (see Fig. 3), which may be attributed (see below) to the operation of chemiionization reactions (XVI) and (XVII). The ions Sr + and SrOH + are formed at a much faster rate than is Na ÷ (see Fig. 4); they reach peak concentration at about 2 mm downstream from the reaction zone (cf. about 5 mm for Na÷). This rapid production of ions by chemi-ionization, which depends on [H] and [OH] being larger than their equilibrium values, tends to obscure any evidence that may or may not be present for hot electrons or related excited species in this system. However, it should be noted that the absence of any strontium ions far upstream in the preheat

352

A . N . HAYHURST and H. R. N. JONES

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Fig. 9. Plots of some individual positive ion concentrations with strontium added to flame 3. Ionic masses are as shown. Part (a) is for some oxygenated ions and part Co) for some of the

acetylenic ions CxH3+. zone suggests that a hot electron mechanism is not operating here. A second much broader maximum is seen in Fig. 4 at about 12 mm from the reaction zone. This is most probably due to the known increase in [OH] at the tip of the feather [37], which would raise the rate of production of SrOH ÷ via reaction (XVI). For both flames, [SrOH+I is higher than [Sr+l, an observation which has been made previously for all alkaline earth metals [33-35]. As with Na, removal of both ions is fairly slow. The natural flame ions (Fig. 9) show similar characteristics to the sodium-seeded flames (cf. Fig. 1). Hydrocarbon ions all reach approximately the same maximum concentration in the same part of the flame as in the unseeded flames, and they all decay at the same rate as in the sodium-seeded flames. This is in spite of [ e - ] being higher in flame 3 (Fig. 3). It is unfortunate that, because of the great variety and abundance of strontiumcontaining ions, many hydrocarbon ion signals are masked by the metal. It seems logical, though, that the charge exchange reactions that remove the lighter polyacetylenic ions also remove the heavier

hydrocarbon ions in exactly the same way as in the sodium-seeded flames. This means that recombination with free electrons is probably also playing a significant role in the enhanced rate of disappearance of natural flame ions. No great difference between the rate constants for charge exchange or recombination reactions with Na and Sr present is to be expected. In contrast to both the unseeded and sodiumseeded flames, Fig. 9 shows there is a definite double-peaked profile for the oxygen containing ions of mass 19 and 47, i.e., as well as the broad maximum just downstream from the feather tip, anew peak has appeared in the reaction zone. If these profiles are a pointer towards the neutral chemistry, then one must conclude that quantities of H20, CO, etc. are being formed much earlier in the flame than in either the unseeded or sodiumseeded flames. A likely explanation seems to be as follows. Jensen and Jones [33] have listed a number of reaction schemes, involving the species SrOH and Sr(OH)2, which together catalyze the recombination of radicals in H + OH -~ H20. Thus a buildup of H20 and hence H30 + (plus related ions such as mass 47 via charge exchange) is likely in or near the reaction zone. Because sodium is a relatively poor catalyst for the recombination, no H30 ÷ signal peak is observed in the reaction zone of a sodium-seeded flame. It is important to check if the chemi-ionization reactions (XVI) and (XVII) are capable of explaining the rapid production of ions in Figs. 3 and 4 for flame 3 doped with strontium. Jensen and Jones's [33] value for the equilibrium constant of reaction (XIV) indicates that [SrO][H]/[Sr][OH] = 1.4 at 2300 K, in which case both chemiionization reactions should be considered. Reaction (XVII) has a rate constant [35] with an upper value ofkl7 = 1.3 X 10 -H e x p ( - 7 3 3 7 / T ) c m 3 ion-~ s-~. If it is assumed that only reaction (XVII) operates, then the rate of production of ions equals kl7[SrO][H] = k173,[SrO][H]m, where 3' = [H]/[H]~q and [H]~ is the equlibrium concentration of free hydrogen atoms in flame 3. With [H]m -- 6.0 × 1015 atoms/cm 3, [SrO] ~ 1 × 1012 molecules/cm 3 and k]7 ~ 5.4 × 10-13 cm 3 ion -~ s -~, the rate of production of ions becomes ~<3.2 × 10 ~53' ions cm -3 s- 1. Examina-

METALLIC ADDITIVES IN SOOTING C2H2 FLAMES tion of the first maximum in total ion concentration in Fig. 3 with strontium added gives [P+] = 2.0 × 1012 ions/cm 3. The rate coefficient for SrOH ÷ and free electrons recombining is 7.0 x 10 -8 ( ___50%) cm 3 ion- 1 s- 1 [35]. Since the dominant positive ion is SrOH ÷ (see Fig. 4), we can equate the rates of ion production and recombination at almost 2 mm downstream of the reaction zone. The latter rate is (7.0 × 10-8) × (2.0 × 1012)2 -~ 2.8 X 1017 ions cm-3 s- 1, which is higher than that calculated above for Na by a factor of 28. This gives 3' t> 88. Such a value for 3' is possibly on the high side. However, given the uncertainties in concentrations and rate constants and also that reaction (XVI) has been ignored, there does not appear to be any need to invoke the participation of hot electrons in the ionization of Sr species. In fact, the plots in Figs. 3 and 4 with Sr added are quite different from those with Na present, where a prolonged linear region was observed in Fig. 4. Careful analysis [351 shows that the rate of production of strontium ions is proportional to 3'3, i.e., the chemi-ionization reactions (XVI) and (XVII) have a rapidity that both depends on flame radicals exceeding their equilibrium concentrations and also masks any enhanced collisional ionization caused by hot electrons.

Negative Ions with Strontium Added As was the case for sodium addition, total negative ionization ( < 170 ainu) increases by about ten times on the addition of strontium to flames 3 and 5. More Ci- is detected, possibly because strontium is divalent, and so twice as much chlorine is added for the same molarity of solution. However, observed CI- concentrations are still well below those of the metal-containing ions, so reaction (VIII), which removes chloride ions in the sampling process, still appears to be operative. The natural flame negative ions all show increased concentrations when strontium is added. The oxygen-containing ions are all in higher abundances, so that mass 41 (C2HO-) is again detected for some way downstream. The ions C2H20- and C2H2- are again detected in the latter part of the feather. Also, O H - is observed in significant concentrations in the seeded flame 3 in the latter

353

part of the feather, but not in flame 5, where the negative ion concentrations are generally somewhat less. Therefore, it may be present, but only below the limit of detection of the mass spectrometer. The presence of O H - so far down flame 3 is probably a reflection of reduced [H] (owing to Srcatalyzed radical recombination), leading to less O H - being lost during sampling. The negative hydrocarbon ions are seen in roughly the same concentrations as in the sodium-seeded flame. They are also removed at about the same rate as in the unseeded and sodium-seeded flames. They also appear to be formed somewhat earlier than in the unseeded flame, presumably owing to backdiffusion from the reaction zone of electrons formed in chemi-ionization processes (XVI) and (xviI).

Total Ionization with Strontium Present Results of the two measurements of total ionization in the strontium-seeded flame 5 appear very similar to those for sodium addition. This is shown in Figs. 7 and 8. The positive ion profile (Fig. 7) shows a clear reduction in charged particles by a factor of 2 to 3 in the burned gases, with the general features of a broad maximum about 20 mm downstream remaining. As with sodium addition, the negative total ionization profile (Fig. 8) produces a signal always above zero, thereby showing that strontium also has the effect of reducing the rate of soot growth and particle size. The striking similarity of the results for both metals suggests that their mode of action in these flames may be very similar. It seems quite likely that the conclusions above for sodium are also broadly applicable for the action of strontium, i.e., neutral strontium species may undergo charge exchange with natural flame ions. The calculations of Calcote [32] show that Sr and SrOH can charge exchange with ionic species with a mass of less than about 5000 amu. Consequently, all natural gas-phase ionic species, including those from reactions like (IX), are likely to be removed at a faster rate when strontium is added. This in turn again leads to a lower rate of ionic nucleation and hence a lower rate of soot growth.

354

A . N . HAYHURST and H. R. N. JONES

ADDITION OF MANGANESE Most work on metal additives in flames has concentrated on alkali and alkaline earth metals. Some transition metals are known to be efficient soot suppressants [38], but very little systematic work seems to have been carried out on their chemistry in combustion systems. Padley and Sugden [39] have looked at emission spectra from hydrogen flames to which traces of manganese had been added. The two major species present are free Mn atoms (about 75%) and MnO (about 25%); the only other compound observed was a trace ( < 1%) of the hydroxide MnOH. The neutral chemistry therefore appears somewhat similar to that of the alkaline earths. Hayhurst and Telford [9] investigated the formation of ions, and duly found Mn ÷ and MnOH ÷ in the ratio of about 50:1. Both sets of workers have postulated reaction schemes that are very similar to those for the alkaline earths, i.e., Mn + OH = MnO + H, Mn + H20 = MnOH + H, Mn ÷ + H 2 0 = M n O H ÷ + H , MnO + H30 ÷ = MnOH ÷ + H20.

(XVIII) (XIX) (XX) (XXI)

Positive Ions with Manganese Present Only two manganese-containing ions were observed, viz. Mn ÷ and MnOH ÷ (see Fig. 4); the ratio [Mn+]/[MnOH +] was found to be roughly constant at a value of about 100 throughout both flames. Reaction (XX) is therefore probably a balanced equilibrium. The ratio of the two ions is dependent on temperature, [H] and [H20], so it is difficult to conclude anything about the variation of [H], since neither temperature nor [H20] are constant, especially in the feather. The overall positive ion concentration ( < 1 7 0 amu) has increased only slightly with manganese added (Fig. 3), which reflects the higher ionization potentials of manganese compounds. The shapes of the Mn + and MnOH ÷ concentration profiles are very similar to those of Sr ÷ and SrOH ÷, i.e., a peak concentration at about 2 mm from the reaction

zone and a second broader maximum further downstream. This second peak is very unlikely to be thermal ionization of Mn or MnOH, because their ionization potentials are too high. A probable explanation is that mentioned earlier for Sr ÷ and SrOH +, viz. an increase in ionization owing to [OH] rising towards the end of the feather region of the flame. This together with the known neutral chemistry [reactions (XVIII) to (XXI)] suggests a similar chemi-ionization mechanism for manganese to that found for the alkaline earths: Mn+ OH~MnOH + +e-.

(XXII)

The rate of this (Fig. 3) is sufficient to avoid invoking the participation of hot electrons in flame 3. The changes observed in the individual natural flame ion profiles when manganese is added are very similar to those seen with strontium. Hydrocarbon ions are still being removed at a faster rate than in the unseeded flames. This is to be expected, since the small hydrocarbon species still possess higher ionization potentials than the added metal. The rate of decay of these small ions is identical to that found with both sodium and strontium addition. The profiles for the oxygencontaining ions continue to show the double-peak noted in the strontium-seeded flame. The new but smaller reaction zone peak can be attributed to a catalytic reaction scheme analogous to that for strontium leading to the formation of H20. The latter then is transferred a proton by other ions to yield quantities of H30 ÷ in the reaction zone. The ultimate rate of removal of the oxygen-containing ions is the same as for sodium and strontium addition, i.e., charge exchange between ions and neutral metal species, but complicated by the participation of equilibria like (XI), as well as electron-ion recombination in, e.g., (VII).

Negative Ions with Manganese Present The effect of the addition of manganese on the negative ion concentration profiles is consistent with the observations made on the positive ions. The overall concentration of negative ions ( < 170 amu) is virtually unchanged from the unseeded

METALLIC ADDITIVES IN SOOTING C2H2 FLAMES flame. The individual profiles also show no great differences. The ions OH- and CI- are seen only in trace quantities; the hydrocarbon ions appear to have identical prof'des to the unseeded flames, except for their earlier appearance noted above for sodium and strontium seeding. Otherwise, C2H2and C2H20 - are present in significant concentrations, indicating that free electrons are still present in higher numbers than in the unseeded flame.

Total Ionization with Manganese Present The total positive ionization signal in flame 5 with added manganese (Fig. 7) is only slightly less than in the unseeded flame 5. This contrasts markedly with about a 60% reduction when either sodium or strontium is added. The total negative ion signal (Fig. 8) is subzero in the sooting part of the flame both with and without addition of manganese. This indicates that manganese addition has very little effect on the size and number of soot particles present. This is entirely consistent with its higher ionization potential. Calculations by Calcote [32] show that only particles of mass less than about 350 amu will possess ionization potentials greater than that of manganese. (It may be recalled that for sodium and strontium, the values were about 30,000 and 5,000 amu, respectively.) Thus, any charged species of mass greater than about 350 amu is unlikely to charge exchange with Mn. The result of this is that Mn can only remove small flame ions, such as C3H3÷, and is unable to remove those large chemi-ions produced by reactions such as (IX). This, coupled with the apparent inability of manganese to reduce soot particle size, strongly supports the view that it is large chemiions that are the nuclei for the ionic contribution to the rate of soot growth. CONCLUSIONS The observed effects of these additives can be explained solely on the grounds of them influencing an ionic nucleation mechanism for soot formation. Sodium and strontium both appear to reduce the average size of soot particles. This is a direct consequence of the removal of large chemi-ions by charge exchange with neutral metal species. This

355

reduces both the rate of ionic nucleation and the rate of soot particle growth. In addition, less ionization of soot particles is observed with Na and Sr added. This is mainly because the soot particles now formed are smaller and consequently have higher ionization potentials, which in turn reduces the extent of thermal ionization. Manganese, which has a much higher ionization potential than both sodium and strontium, can remove only small charged gaseous molecules ( < 350 amu) by charge exchange; it is unable to charge exchange with chemi-ions of the type formed by reaction (IX). The observation that neither soot particle size nor total final ionization is affected by manganese addition strongly suggests that large chemi-ions (rather than smaller hydrocarbons such as C3H3 ÷) are the nuclei in an ionic mechanism of soot formation. The question of enhanced soot oxidation in these flames has not been raised. An ionic scheme for the prevention of soot formation adequately accounts for the observations, but that is not to say that enhanced oxidation cannot occur in other systems. One by-product of this work is that Na atoms in these fuel-rich flames appear to be ionized mainly by collisions with electrons. These are most probably " h o t " in that their temperature is somewhat above that of the neutrals present. No such effects were observed with Sr and Mn present.

British Gas plc is to be thanked for the award of a Research Scholarship to H.R.N.J. and for other support of this work. REFERENCES 1.

Bartholome, E., and Sachse, H.. Z. Elektrochem. 53:326 (1949). 2. Gaydon, A. G., and Wolfhard, H. G., Flames--Their Structure, Radiation and Temperature, 4th ed., Chapman and Hall, London, 1979. 3. Hayhurst, A. N., and Jones, H. R. N., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1985, p. 1121. 4. Homann, K. H., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1985, p. 857. 5. Salooja, K. C., First European Symposium on Combustion, Academic, London, 1973, p. 400.

356 6.

7.

8. 9. 10. I1. 12. 13. 14. 15. 16.

17. 18. 19. 20. 21. 22.

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Received 18 February 1988; revised 10 October 1988