Keith Schofield

The chemical kinetics and thermodynamics of sodium species in oxygen‐rich hydrogen flames

Measurements of sodium and OH concentrations in ten oxygen‐rich H2/O2/N2 flames by respective saturated and low‐power laser‐induced fluorescence techniques have led to a much improved understanding of the complex flame chemistry of sodium in such oxygen‐rich media. Previous interpretations have been shown to be largely incomplete or in error. The one‐dimensional flame downstream profiles indicate that the amount of free sodium approximately tracks the decay of H atom and as the flame radicals decay sodium becomes increasingly bound in a molecular form. A detailed kinetic model indicates that the sodium is distributed between NaOH, which is dominant, and NaO2. Concentrations of NaO are very small and NaH negligible. The actual distribution is controlled by the temperature, the oxygen concentration, and the degree of nonequilibration of the flames’ basic free radicals. Na, NaO, NaO2, and NaOH are all coupled to one another by fast reactions which can rapidly interconvert one to another as flame conditions v...

A reevaluation of the vaporization behavior of sodium oxide and the bond strengths of NaO and Na2O : implications for the mass spectrometric analyses of alkali/oxygen systems

There has been a long standing disagreement between our flame experiments, which predict a very stable NaO2 molecule, and Na2O(c) vaporization/mass spectrometric studies of Hildenbrand et al., which imply a weak bond strength from an inability to detect such a species. We have now reanalyzed the vaporization experiments and have identified a possible explanation for this frustrating controversy. It appears that on becoming ionized, NaO+2 fragments to Na+ and O2. As a result, mass 23 reflects p(Na)+p(NaO2). This and the changes to the thermochemical data for NaO2 modifies the earlier ion intensity/vapor pressure calibration. As a result, the previously accepted thermochemical values for NaO and Na2O need to be reduced by 18 and 11 kJ mol−1, respectively. Recommended values now become ΔHf298K (NaO)=87±4, D0(NaO)=266±4, ΔHf298K(Na2O) =−36.0±8 and D0(Na–ONa)=228±8 kJ mol−1. It also appears that the I.P.(NaO2)≤739 kJ mol−1 (7.66 eV).The reported Clausius–Clapeyron vapor pressure curves are entirely consistent ...

Sulfur chemistry in flames

Quantitative laser fluorescence measurements of the concentrations of SH, S2, SO, SO2 and OH have been made in the post flame gases of a series of 10 atmospheric pressure, stoichiometric and fuel-rich H2/O2/N2 flames, containing 0, 0.25, 0.5 or 1% mole fraction of sulfur as H2S. The present discussion characterizes the chemistry of sulfur in the fuel-rich flames and also validates this fluorescence monitoring technique. A kinetic rate analysis of all the possible interactions has established that the sulfur chemistry is controlled by 8 fast bimolecular radical reactions. S, S2, SH, H2S, SO and SO2 are all coupled by fast reactions and it is only a result of the imposition of the non-equilibrated H2/O2 flame chemistry that controls their relative proportions. Termolecular reactions, other than providing a catalytic means for recombining excess H and OH concentrations are insignificant. The establishment of the equilibration of the reaction H+SO2=SO+OH provides a new method whereby fluorescence measurements of OH along with SO and SO2 can be used to determine both H and H2 concentrations in stoichiometric flames containing sulfur. This study constitutes the first systematic application of quantitative laser fluorescence measurements to a study of chemistry in a series of flames of varying composition and temperature. It demonstrates an important and powerful new method of great sensitivity and non-perturbing nature for the detailed study of combustion processes.

Laser induced flame chemistry of Li (2 2P1/2,3/2) and Na (3 2P1/2,3/2). Implications for other saturated mode measurements

Saturated laser fluorescence measurements of sodium or lithium in a series of fuel rich, atmospheric pressure H2/O2/N2 flames at 1700–2200 K indicate induced chemical interactions between the excited 2P1/2,3/2 states of the metals and the H2O or H2 flame constituents. A steady state redistribution occurs among the metal’s elemental, hydroxide and hydride forms within the initial fraction of the μs laser pulse duration. A saturated absorption model incorporating these chemical effects illustrates the significant depletion of the free atom concentrations under these conditions and explains previous discrepancies between such measurements and conventional absorption experiments. Estimates of the rates of the reactions between the 2P1/2,3/2 states of sodium or lithium with H2O or H2 indicate that they proceed predominantly via the nonadiabatic physical relaxation channel. For sodium the two chemical channels are relatively inefficient constituting only about 2% and 0.5% of the total interaction cross section ...

The controlling chemistry of surface deposition from sodium and potassium seeded flames free of sulfur or chlorine impurities

Abstract Sodium and potassium salt deposition have been studied in a series of propane and hydrogen flames free of sulfur or halogen impurities. With the collection probe in the 400 to 800 K range, samples of pure carbonate are observed and more importantly the rates of, for example, sodium carbonate deposition measured in terms of alkali metal are identical to those previously reported for sodium sulfate formation and also those observed for dominant NaCl deposition. Moreover, the behavior of Na 2 CO 3 deposition mirrors exactly that of Na 2 SO 4 in this temperature range. It shows a corresponding first order dependence on flame total sodium concentration, a zero order dependence on flame carbon, an insensitivity to fuel type, equivalence ratio, flame temperature, flow rate, probe material, or the nature of the sodium speciation in the flame, be it atomic or the hydroxide, or the state of the flame equilibration. A constant rate of deposition between 330 and 800 K conveys formation kinetics with a zero activation energy and that the surface accommodates atomic sodium equally well, be it below or above its dew point temperature and also at a seemingly approximately equal rate to that of flame NaOH. The fact that Na 2 CO 3 cannot exist in the gaseous state in a flame finally proves irrefutably that these alkali deposition processes producing sulfate, carbonate or halide salts are heterogeneous in nature. The high collection efficiencies of the surface for alkalis have been confirmed by a further independent new calibration method for flame total alkali content. Also deposition rates are seen to be extremely similar in C 3 H 8 /O 2 flames heavily diluted with either He, Ne, or Ar and also in a very fuel rich H 2 or D 2 flame. As with sulfate deposition, the rate of deposition is predominantly controlled by the actual flux of alkali in the flame gases that are intercepted by the collection probe. Moreover, there is an insensitivity to probe geometry and the nature of the flame flowfield, be it laminar or turbulent. The theoretical understanding of the complex boundary layer penetration and deposition mechanism is still inadequate in explaining these observations. The most intriguing results and differences from sulfate deposition have been observed on probes at lower temperatures (330–370 K). Although the formation of NaHCO 3 , and more so KHCO 3 , was expected to compete with that of their carbonates, in the case of sodium under fuel lean conditions only a small competing contribution of NaNO 3 formation was noted. This was very marginal for fuel rich conditions. However, with potassium the effects were enhanced and KNO 3 competes significantly with K 2 CO 3 under fuel lean conditions. However, in fuel rich flames an unexpected dominant formation of potassium oxalate, K 2 C 2 O 4 , was observed, along with some K 2 CO 3 and a small amount of KHCO 3 . Thermodynamic expectations in this lower temperature regime tend to suggest nitrate>bicarbonate>carbonate>oxalate. This is our first clearly observed non-equilibrium deposition behavior where the flame begins to display a pivotal role in controlling the surface molecular distribution. It also raises the possibility that low temperature surfaces in flames may be a new route for synthesizing certain thermodynamically metastable materials.

The kinetic nature of sulfur’s chemistry in flames

Abstract Radi et al. recently presented the first quantitative multiplexed measurements of S2 and OH concentrations in flames using degenerate four-wave mixing (DFWM). Although the absorption-calibrated OH measurements were in agreement with expectations, very severe discrepancies were reported for the indirectly calibrated S2 densities. Magnitudes ranged up to 236-fold larger than predicted. The researchers have questioned the adequacy of previous flame work and the current kinetic model of the mechanisms of sulfur’s flame chemistry. The implications of their suggestion are extensive and severe, and their result unexpected. They have suggested that the kinetic modeling is either incomplete, or that an additional species such as NS may be responsible. As a consequence, a reexamination of their work and all previous studies in H2, C3H8, and CH3OH flames has been made. The resulting conclusions are that it will be very difficult to modify the kinetic model of sulfur in flames to encompass such results. Suggestions that other species such as NS may be playing a significant role are shown to have little merit. In addition, an analysis of potential roles for OCS, CS, CS2, and one recently suggested for HCS in fossil-fueled flames also indicates these to be very minor. A closer examination of the recent DFWM measurements implies various other disquieting aspects. One is that the reported S2 densities are essentially close to or above expected flame equilibrium values. Numerous independent measurements all agree that S2 concentrations are depressed by flame nonequilibrium, and increase with downstream time as flame radical concentrations relax towards their equilibrium values. In the present case, however, measured OH concentrations still are in the range of 42- to 10-fold above their equilibrium values. A second aspect of concern is that the measurements imply an insensitivity to S2, recording levels at several hundreds of ppm with some difficulty. Other DFWM measurements, for example, with CH, C2, CN, NO, and OH, all report sensitivities down to a few ppm 3 , 4 , 5 , 6 , 7 , 8 . Even CH3 can be measured in flames at levels of 65–70 ppm 9 , 10 . The fact that under saturation conditions, DFWM intensities fall off as the square of the concentration indicates the severity of these differences. On the other hand, the degree of theoretical understanding now is quite sound for DFWM, and the levels of approximation involved would not appear to conceivably introduce the magnitude of change that is required to bring this body of data together. The DFWM spectroscopy involved, per se, appears to be reasonable and valid. An indirect calibration method, however, is used to scale the S2 intensities, and has never been validated. One plausible explanation that is proposed herein relates to the very efficient collision free coupling that occurs between the S2(B3Σu−) and S2(B″3Πu) states. There is evidence in the literature that the latter long-lived state can act as a pseudometastable state. Instantaneous depletion of the pumped S2(X3Σg−, v = 2) into this state would modify the data by lowering the resultant values. If correct, this appears to be the first such reported interference with DFWM monitoring. A review of the current status of our understanding of the behavior of the major sulfur species in flames indicates that any possible need for modification should be only minor. Major remaining uncertainties, which cannot noticeably perturb the sulfur chemistry itself, center on the exact nature of the mechanisms by which sulfur modifies NOx formation and, to a lesser extent, to reexamine the validity of the exact mechanisms involved in the catalytic flame radical recombination cycles. Definitive studies of these have yet to be done.

The chemistry of sodium with sulfur in flames

Abstract An experimental and analytical program of sodium/sulfur chemistry has been conducted in a series of fuel rich and lean H2/O2/N2 flames, with and without added sulfur, and covering a wide range of temperatures and stoichiometries. Fluorescence measurements of OH and Na profiles together with sodium line reversal temperature profiles provided a broad data base for kinetic modeling. Analysis indicated NaSO2 to be the only significant sodium/sulfur product formed in the lean flames. NaOS is dominant in the rich flames, coupled with small contributions from NaSO2, NaSH, NaS and NaS2. A bond dissociation energy of D0(NaSO2) = 197 ± 20 kJ mol−1 is derived. Calculations indicate that the linear or triangular structures for NaOS both co-exist in approximately equal proportions in flames. Analyses based on results developed in the study show that Na2SO4 formation is kinetically limited and cannot be a significant gas phase flame product at sodium levels much below 100 ppm. Na2SO4 induced corrosion in combustion systems must result from heterogeneously formed Na2SO4.

Atomic and molecular fluorescence as a stratospheric species monitor

The results of an extensive evaluation are presented assessing the potential of atomic and molecular fluorescence as a stratospheric monitor of the concentrations of any one of eighteen minor species. These include Cl, Cl2, ClO, ClO2, CO, H2, HCHO, HCl, HNO2, HNO3, H2S, NH3, NO, NO2, N2O, O, OH and SO2. All spectral regions from the vacuum u.v. through to the i.r. have been included. Where appropriate, detection limits (signal/noise ratio of unity) are presented for each species under various sample pressure conditions and are based on practical systems that could be constructed using current technology. The most promising systems, with typical detection limits indicated either as parts per million, billion or trillion by volume, are for CO(5ppb), NO2(<1ppb), OH(0.2ppt) and O(50–200ppt). The fluorescence sensitivities for Cl(0.5–1 ppt), H2(0.2 ppm at 10 torr sample pressure) and SO2(1–10 ppb) are marginally insufficient at present for such a stratospheric application. Likewise HCHO(10 ppb) and NO(100 ppb) fluorescence detection may be of interest in other applications where sensitivity demands are not as severe. There are no promising analytical possibilities using direct fluorescence techniques for Cl2, ClO2, HCl, HNO2, HNO3, H2S, NH3 or N2O. ClO fluorescence has not yet been characterized. It has been noted, for various reasons, that i.r. fluorescence techniques in general cannot be exploited in the development of sensitive analyzers. However, by far the most surprising outcome of this study has been the recognition of the analytical potential of vacuum u.v. fluorescence. For some species, under certain conditions, extremely high sensitivities are possible even with samples in air at atmospheric pressure.

Near saturation laser induced chemical reactions of Na(3 2P32,12) in H2/O2/N2 flames☆

Abstract Laser induced chemical reactions of electronically excited Na(3 2 P 3 2 , 1 2 ) have been observed from fluorescence measurements in stoichiometric and rich H 2 /O 2 /N 2 flames under laser saturated conditions. Such removal of excited sodium by H 2 O and H 2 is sufficiently fast relative to the laser pulse duration to significantly modify the free sodium concentration. This possibility, previously overlooked, has significant implications concerning the general interpretation of all laser saturated absorption data. It also provides a new approach for studying the high temperature chemistry of electronically excited states.

A new method for the direct flame calibration of nebulized additive concentrations

Abstract A new technique is described to calibrate the flame delivery flux of an aerosol-producing nebulizer. The method is based on flame deposition. It centers on the nature of deposition onto a cooled surface immersed in the burned flame gases that contain an alkali salt (Na, K, Rb, or Cs) that has been nebulized into the flame. This deposition process has been shown to be controlled solely by the alkali concentration in the flame and for the same alkali flux the rates of deposition are invariant being independent of the nature of the alkali flame speciation, the fuel, equivalence ratio, flame or probe temperature (in the 350-800 K range for the latter), probe material, or the deposit’s molecular composition. In flames containing, for example, sodium and sulfur or chlorine, there is a pronounced preferential ranking of Na 2 SO 4 > NaCl > Na 2 CO 3 > NaOH for the resulting composition of the deposit. This dominance of sulfate formation is so pronounced that it produces a linear titration method for the alkali, and fractional addition of sulfur reflects into a corresponding fractional formation of Na 2 SO 4 . In flames containing solely sodium and sulfur additives, insufficient sulfur for a Na 2 SO 4 formulation will result in a fractional mixture of the sulfate, with the remaining sodium forming carbonate. Collecting a deposit from a flame containing a nebulized alkali salt solution aerosol and a known trace addition of SO 2 , and then analyzing the deposit using an inductively coupled plasma/atomic emission spectrometer (ICP/AES) for its alkali to S ratio, provides a measure of the extent of sulfate formation. This then translates into an absolute calibration of the nebulizer delivery flux of alkali in the burned gases. A variation of the method is to use instead a Raman scattering spectrometer to measure the ratio of sulfate to carbonate in a deposit, in this case for flames free of halogens. The method has the advantage that it can be used in any flame, turbulent or laminar, requires only a trace of deposit for ICP analysis, and is accurate. Moreover, the probe can intercept any partial part of the flame gases. A limited number of other techniques currently are available for nebulizer calibration. However, other than the curve of growth method, these have not been discussed in combustion texts. Their various limitations and accuracies are outlined together with that of the presently suggested method.