John M. Goodings

Detailed ion chemistry in methaneoxygen flames. I. Positive ions

Abstract The detailed positive ion chemistry is discussed for two premixed methaneoxygen flames of fuel-lean and fuel-rich composition burning at atmospheric pressure. For each flame, a complete family of concentration profiles for natural, positive flame ions has been measured mass spectrometrically below 55 amu (atomic mass units), sampled along the axis through the flame front. The ion chemistry is explained with reference to published data on neutral concentration profiles, rate constants for ion-molecule reactions, and thermochemical values when available. The fuel-lean profiles form groups in essentially three regions along the flame axis. Commencing upstream, the first involves initiation of CHO+ by chemi-ionisation. The main second group is dominated by proton transfer to a wide variety of neutral species present in the reaction zone, with minor evidence for charge-transfer processes. In the third region downstream, H3O+ persists through the burnt gas with detectable signals of O2+ and O+. For the fuel-rich flame, the first two regions are essentially similar. Just downtream of the reaction zone, however, a new group of CnHx+ ions (n ⩾ 1, x ⩾ 0) are formed in a series of condensation reactions in which acetylene plays a major role. The fourth region, persisting through the burnt gas, is again dominated by H3O+ but with measurable equilibrium CHO+, which together initiate another proton-transfer regime leading to the detection of additional ionic species. These mechanisms are summarised in reaction flow diagrams showing the ion-molecule reactions that dominate in different flame regions. This knowledge of the ion chemistry reveals many aspects of the underlying neutral chemistry of combustion in a fairly detailed and sequential manner.

A new flame-ion mass spectrometer: chemi-ionizatino of lanthanum observed in hydrogen—oxygen—argon flames

A new flame-ion mass spectrometer has been designed and constructed primarily for kinetic studies employing relatively large, pseudo-one-dimensional (flat) flames in the range 1700–2500 K at atmospheric pressure. Ions present in a flame are sampled through a nozzle into a vacuum system having two stages of differential pumping, and are analyzed by a quadrupole mass filter. Data are obtained as profiles of ion signal, either positive or negative, versus distance (or time) along the flame axis, achieved by moving the flame and burner with respect to the sampling nozzle. Details of the design, construction and special features of the apparatus are discussed. The performance of the mass spectrometer is illustrated by the chemi-ionization of lanthanum, observed for the first time in hydrogen—oxygen—argon flames. The same four ions were observed in both a fuel-lean (oxygen-rich) and a fuel-rich flame at 2150 K as members of a hydrate series LaO+·nH2O, (n = 0–3). The ions are believed to arise from the chemi-ionization reaction of La with atomic O(3P) followed by three-body association reactions with water.

Recombination coefficients for H3O+ ions with electrons e− and with Cl−,Br− and I− at flame temperatures 1820–2400 K

Abstract Rate coefficients were measured for gas-phase electron–ion recombination of H 3 O + and for ion–ion recombination of H 3 O + with Cl − ,Br − and I − in five H 2 –O 2 –N 2 flames spanning a temperature range 1820–2400 K. A novel, simple and accurate technique was employed involving total positive ion collection at an electrode located a variable distance downstream of the flame reaction zone; a bias voltage was applied between the collection electrode and the metallic burner. The measured values of the recombination coefficients in cm 3 molecule −1 s −1 units are: (0.0132±0.0004) T −1.37±0.05 for H 3 O + /e − ;(20.8±0.7) T −2.52±0.16 for H 3 O + /Cl − ;(1.01±0.04) T −2.14±0.07 for H 3 O + /Br − ; and (8.82±0.35) T −2.40±0.12 for H 3 O + /I − .

Metallic ions in hydrocarbon flames. II. Mechanism for the reduction of C3H3+ by metals in relation to soot suppression

Abstract The ion chemistry of the cyclopropenium ion, C3 H3+, viewed as a soot precursor, has been probed in a fuel-rich, CH4-C2H2-O2 flame by the addition of metals (alkali: Li, Na, K, Rb, Cs; alkaline earth: Mg, Ca, Sr, Ba). For each metal, ion concentration profiles observed with a mass spectrometer as a function of distance along the flame axis are presented for the major metallic ion, C3H3+, H3O+ and total ionization. The existence of neutral C3H2 having a high proton affinity is invoked in this flame to the extent of ∼ 5× 10−6 mol fraction. The behaviour of C3H3+, formed initially by proton transfer to C3H2 from natural flame ions, is closely related to that of H3O+; presumably, a fast proton transfer reaction involving C3H2 and H2O is responsible. Metallic additives serve to increase the ionization level and thus, the concentration of free electrons. The primary mechanism for the reduction observed for C3H3+ and H3O+ with added metals is enchanced electron-ion recombination. As a secondary effect, ion/molecule reactions involving charge and proton transfer to metallic atoms and compounds reduce H3O+ even more than C3H3+. For soot suppression, the effectiveness of a given metal in reducing C3H3+ depends on the chemical kinetic rate of the metals ionization reaction early in the flame.

Heat release and radical recombination in premixed fuel-lean flames of H2+ O2+ N2. Rate constants for H + OH + M → H2O + M and HO2+ OH → H2O + O2

The temperatures along the burnt gases of several oxygen-rich flames of H2+ O2+ N2 have been measured. They show there is a rise of some 300–1200 K in the early part of the burnt gases. A detailed evaluation of all the chemical reactions proceeding in such fuel-lean flames has been carried out. In addition, the kinetics of these reactions, especially of radical recombination, have been formulated mathematically. In these flames an equilibrated pool of the minor species O, OH, H and H2 is rapidly established. Early in the burnt gases radical recombination proceeds via H + OH + M → H2O + M (V) and removes species from this pool. Later in the burnt gases, recombination also occurs in the series of reactions: H + O2+ M ⇄ HO2+ M (VI), OH + HO2→ H2O + O2(X) the net overall effect of which is identical to reaction (V) . The fact that heat is released by reaction (V) proceeding either directly or by this catalytic sequence of reaction (VI) followed by reaction (X) enables the total concentration of species in the pool of recombining radicals to be deduced from the measured temperature profiles along each flame. In addition, concentrations of each individual member (i.e. O, OH, H and H2) of the pool are obtained, both directly and in terms of Sugdens disequilibrium parameters. The measured rates of disappearance of species from the pool are consistent with rate constants for reactions (V) and (X), respectively, of k5= 1.0 × 10–28/T cm6 molecule–2s–1 and k10= 1.0 × 10–10 cm3 molecule–1 s–1. In each case there are associated errors of 50%.

Current–voltage characteristics in a flame plasma: analysis for positive and negative ions, with applications

Abstract A flat, cylindrical, laminar, H 2 –O 2 –N 2 flame in plug flow with velocity v f = 19.8 m s −1 and cross-sectional area A = 1.05 × 10 −4 m 2 , at 1 atm and 2400 K, was doped with appropriate additives to give a weak, continuum, quasi-neutral plasma involving Cs + / e − or H 3 O + / e − or H 3 O + /Cl − / e − . The flame impinged on a planar, normal, water-cooled, conducting electrode designated N (for nozzle) located a variable distance z downstream from the flame’s luminous reaction zone which is separated by a dark space or gap δ ≃ 1 mm from a water-cooled metallic burner B, the second electrode. Neither electrode has the properties of a Langmuir probe. Two types of data were measured: i – V characteristics with the current i (N) collected by the nozzle versus the applied voltage Δϕ between N and B at a fixed value of z (e.g. 20 mm); and profiles of i (N) versus z at a fixed value of Δϕ. For Δϕ i + (N) = eAn + (N) v f . This provides a wonderfully simple and accurate measurement of the absolute density n + (N). Alternatively, if a weak solution of a Cs salt (e.g. 10 −4 M CsCl) is sprayed using a pneumatic atomizer such that Cs is completely ionized, the delivery factor of the atomizer can be calibrated. Furthermore, if the delivery factor giving n + (N) is known, v f can be determined. With Δϕ fixed (e.g. −50 V), a profile of n + (N) versus z can be obtained throughout the flame gas downstream; the density distribution is not affected by the application of the applied voltage. For Δϕ > 0 (e.g. +50 V, N is positive), the current through the flame i + (B) = − i e (N) − i − (N) (the latter term is included if negative ions are present), is controlled by the flow of positive ions of mobility μ + to the burner across the potential gradient in the burner gap g: i + (B) = eAn + (B)μ + ∇ϕ g . When Δϕ is sufficiently positive to achieve a constant saturation current, n + (B) can be determined; it represents the total ion production in the flame reaction zone. When negative ions are present replacing even a large fraction of the electrons, the effect on the i – V characteristic is relatively minor; it does not appear possible to provide a separate analysis for n e and n − . However, profiles through the flame plasma clearly show the effects of negative ion processes such as ion–ion recombination, for example. For a wide range of the applied voltage Δϕ, both positive and negative, it is possible to calculate the potential distribution in the burner gap, the bulk flame plasma and in the positive ion sheath at the nozzle. This provides a quantitative understanding of the ion and electron behaviour throughout the flame.

Metallic ions in hydrocarbon flames. I. Ion chemistry involving proton transfer reactions

Abstract Small amounts ( −6 mol fraction) of alkali and alkaline earth metals are added to a conical, premixed, fuel-rich, CH 4 -C 2 H 2 -O 2 flame at atmospheric pressure. Ion concentration profiles are measured as a function of distance along the flame axis by sampling the flame into a mass spectrometer. Metallic ion profiles for all of the metals include A + and AOH 2 + (= A + ·H 2 O) where A is the metal atom and, in addition, for the alkaline earths, AOH + and A(OH) 2 H + (= AOH + ·H 2 O). The metallic ion signals are made comparable with, or greater than, those of the natural C x H y O z H + ions present in any hydrocarbon flame whose ion chemistry is dominated by proton transfer reactions involving C x H y O z neutral flame intermediates. Apart from A + , the metallic ions can be interpreted as protonated forms of the metallic neutral compounds AOH, AO and A(OH) 2 which are present in the flame in appreciable amounts for most of the metals. Moreover, most of the metallic compounds have proton affinities higher than those of the natural C x H y O z flame intermediates. Experimental observations and a discussion of the ion chemistry are presented to show that proton transfer reactions can be expected to dominate the metal ion chemistry in the reaction zone and the early downstream region of a hydrocarbon flame.

Flame-ion probe of intermediates leading to NOx in CH4O2N2 flames☆

Abstract A detailed investigation of the nitrogenous ions naturally occurring in both fuel-lean and fuel-rich CH 4 O 2 N 2 flames at atmospheric pressure was conducted by sampling the ions directly into a mass spectrometer. The flames were diluted interchangeably with about 10% N 2 and Ar at constant temperature to determine the extent of nitrogen involvement. This diluent switching technique provided ion concentration profiles along the flame axis for CN − , NCO − , H 2 CN + , NH 2 + , NH 3 + , NH 4 + , NO + , and NO 2 − . The spatial resolution was sufficient to show distinct regions of the flames where a given ion is involved. The ions can be linked to their corresponding nitrogenous neutrals through chemical ionization (CI) processes, many of which are known from room temperature studies. A detailed discussion of the relevant CI processes is presented, providing strong evidence for the existence and sequential behavior of the neutrals CN, HCN, NCO, HNCO, N, NH, NH 2 , NH 3 , NO, and NO 2 . The fuel-lean results are consistent with the (extended) Zeldovich mechanism for NO x formation. The fuel-rich data clearly show the involvement, both in the reaction zone and further downstream, of the variety of neutral intermediates invoked by Fenimore and others to explain “prompt NO” formation.

Mass-Spectrometric Sampling of Ions from Flames at Atmospheric Pressure: The Effects of Applied Electric Fields and the Variation of Electric Potential in a Flame

Abstract Electrically charged species in flames can be studied by sampling a flame through a metallic nozzle into a mass spectrometer. Here, a voltage has been applied between the sampling nozzle, together with its mounting plate, and a metallic burner to measure current-voltage characteristics, i.e., plots of a sampled ion current (as registered by the mass spectrometer) vs the applied voltage. These characteristics are different for a large, wide, multi-tube (Meker) burner and a small, single-tube (Bunsen) burner. For diagnostic purposes, net current-voltage characteristics were also measured for the total current, positive or negative, collected by the burner or the sampling system. The major problem is to characterize, even qualitatively, the distribution of electrical potential between the burner and sampling system for a flame of H 2 + O 2 + diluent, doped with Cs, at atmospheric pressure. It is confirmed that such flames must be treated as a weakly ionized, flowing, collision-dominated (continuum) plasma, in which diffusion of charged species can be ignored. With the sampling system at a negative potential with respect to the burner, the flame conforms to a sheath-convection model, so that convection to a sheath of positive ions covering the sampling nozzle dominates, while electrons are repelled. With the nozzle, etc. at a positive potential, the applied voltages employed are too low to stop the positive ions and thereby create an electron sheath, but the bulk flame plasma can sustain a substantial ohmic potential difference, and charge collection is dominated by mobility considerations. Of key importance for different burners is the burner-flame plasma impedance, which can greatly affect the distribution of potential between the burner and the nozzle. Elucidation of these phenomena provides a better understanding of the electrical aspects of exactly what is being sampled, when ions are extracted this way from a flame.

A simple method for measuring positive ion concentrations in flames and the calibration of a nebulizer/atomizer

Abstract A simple and accurate ( f for a nebulizer or pneumatic atomizer used to dope a flame with a metal ( −6 mole fraction); the metal is introduced by spraying an aerosol of an aqueous salt solution into the gas mixture feeding the burner. When a dilute solution (∼10 −4 molar) of a Cs salt is sprayed into a fairly hot flame (>2300 K), the Cs added is almost completely ionized (>99%); accordingly, f can be determined by measuring the absolute ion density of Cs + . Calibration difficulties can be encountered if a more concentrated solution is employed, because Cs + can, quite surprisingly, achieve a superequilibrium concentration early in the flame. However, the method was used to show that the delivery of a pneumatic atomizer is essentially linear when the strength of the salt solution varies by at least three orders of magnitude. Of great utility is the ease with which ion concentration profiles can be measured along a flat flame for studying the kinetics of reactions involving ions. Such ion profiles have revealed the very rapid production of Cs + ions and free electrons in the reaction zone of a flame doped with Cs; two possible chemi-ionization reactions are discussed to explain this phenomenon. In addition, the rate constant of the recombination reaction H 3 O + + e - → H + H + OH is confirmed to be (3.2 ± 0.3)×10 −7 cm 3 ion −1 s −1 at 2400 K. Confirmation of this rate coefficient provides verification of the simple method presented here for the measurement of absolute concentrations of positive ions in flat flames.