Lean flame

When [NO] temperature dependence, an activation energy of 316 kj/mol (75.5 kcal/mol). Unfortunately, the rate becomes appreciable just in the range of typical hydrocarbon—ak flame conditions. If it is also assumed that [O] = [O], the observed rate in most lean-flame products in which N2 is roughly 75 mol % of the gas can be approximated by... 

For the sake of brevity, the so-called cool flame techniques based upon the use of an oxidant-lean flame such as hydrogen/nitrogen-air, have not been included. 

The ketenyl radical (HCCO) is a key intermediate in the oxidation of acetylene in flames. It is mainly formed from the O + C2H2 HCCO + H reaction. In lean flames, the HCCO + O2 reaction is the main pathway for decay of HCCO, and this reaction has recently been shown to be the source of prompt CO2 [44, 45]. 

The sensitivity of atomic absorption can often be enhanced by aspirating solutions in organic solvents. The increased sensitivity is due to a number of factors, but can be attributed in large part to the lower viscosity and surface tension as compared to aqueous solutions. The flow rate is increased and smaller droplets are formed which are more efficiently vaporized. When organic solvents are aspirated, a fuel lean flame must be used in order to burn the solvent. 

Droplets entering the flame evaporate then the remaining solid vaporizes and decomposes into atoms. Many elements form oxides and hydroxides in the outer cone. Molecules do not have the same spectra as atoms, so the atomic signal is lowered. Molecules also emit broad radiation that must be subtracted from the sharp atomic signals. If the flame is relatively rich in fuel (a rich flame), excess carbon tends to reduce metal oxides and hydroxides and thereby increases sensitivity. A lean flame, with excess oxidant, is hotter. Different elements require either rich or lean flames for best analysis. The height in the flame at which maximum atomic absorption or emission is observed depends on the element being measured and the flow rates of sample, fuel, and oxidizer.6... 

At very high temperatures in highly oxygen rich (lean) flames ozone possibly could form from ... 

However very lean flames never give high temperatures and the existence of ozone in a C, H, 0, N system is most improbable. 

One point which is often overlooked when optimizing fuel-to-oxidant ratio is that the optimum fuel flow is sometimes matrix-dependent. For example, the determination of calcium in water using an air-acetylene flame is more sensitive if a fuel-rich flame is used. If, however, the samples contain dilute sulfuric acid, a more fuel-lean flame usually gives substantially improved sensitivity. As a general rule, it is best to find optimal conditions for the particular matrix which you are analysing. 

Since Cd, Co, Cr, Ni, and Pb were the metals to be used for the collaborative test, efforts were concentrated toward understanding possible flame-related phenomena for these metals. The various flame conditions studied were all obtained by varying the acetylene flow against a constant air flow of approximately 28 Jt/min. The lean flame corresponds to an acetylene flow of approximately 4.6 Jt/min and produces a very intense, short blue cone. The blue flame is more fuel-rich than the lean flame yet it is slightly leaner than the white flame, and corresponds to an acetylene flow of approx imately 6.0 Jt/min. 

The comparison of single analyte standard curves with mixed analyte standard curves was repeated, adding La flame buffer to all standard solutions. The standard matrix was 0.5 percent La, 10 percent HNO3. These results are presented in Table XV. The only slope ratios which differ from unity by more than one percent are Pb analyzed in a lean flame and Cu analyzed in a lean or rich flame. Therefore, when the analysis of a sample depends on the choice of calibration standards and flame stoichiometry, a La flame buffer added to both samples and standards alleviates the dependence. 

Single factor experiments showed that Cu mixed analyte standards give a 25% higher response than single analyte standards for analysis in a lean flame (see Table XIV). 

Somewhat unexpected is the relatively low consumption of CO in the excess oxygen flame. The CO profile of Figure 1-a shows only about 60 percent of the CO was oxidized to CO2. Other fuel-lean flames probed also showed incomplete combustion of the CO. Equilibrium CO levels were not attained in most of the flames. 

Procedure Use an atomic absorption spectrophotometer equipped with a 4-in., single-slot burner head. Set the instrument to previously determined optimum conditions for organic solvent aspiration (3 to 5 mL/min) and at a wavelength of 283.3 nm Use an air-acetylene flame adjusted for maximum lead absorption with a fuel-lean flame. Aspirate the blanks, the Standard Solutions, and the Sample Solution, flushing with water and then with Aqueous Butyl Acetate between... 

Adjust the instrumental parameters of spectral band pass, wavelength and lamp current in accordance with the manufacturer s recommended conditions. Use an air—acetylene flame and, aspirating the blank solution, adjust the acetylene flow rate to obtain a fuel-lean flame. 

Chemi-ionization. Chemi-ionization is the main and probably the only mechanism responsible for the formation of primary ions in fuel-lean flames. This mechanism, first proposed by Calcote (IS), has received much attention, and there is general agreement that CHO" is the only primary chemi-ion formed in lean flames (see below). 

Ionization by Charge Transfer. Many ion-molecule reactions between small species (< 50 amu) occur in flames (24), As mentioned above, the only primary ion in fuel-lean flames (non-sooting) is CHO", formed by chemi-ionization (13, 24, 25). 

This method integrally employs the quasi-steady state assumptions to relate the concentrations of H, OH and O in the overall radical pool, and can be applied to either fuel-rich or fuel-lean flames. Concentrations of HO2 were also calculated using the quasi-steady state condition, but because these were mostly much smaller than the other radical concentrations they were considered in the same manner as OH and O in the simpler method. Both methods lead to similar results for the low temperature, fuel-rich flames considered at present, indicating that the reverse reactions other than (—i) and (—iii) are relatively unimportant over most of these reaction zones. Three internally consistent sets of rate coefficients on which the more refined treatments may be based are given... 

Using the calculation technique described, with k, fe, 7 and tei g taking the values in Table 30, a survey of a number of published flame recombination investigations in both rich and lean systems leads to the assessment, shown in Table 32, of the relative importance of the net contributions of the three primary recombination steps at approximately the centre of each range of measurement. Clearly, results with sufficiently fuel-rich flames should be capable of providing reliable values of fei7, while in lean flames recombination is principally by way of HO2 formation. On the other hand, fe,g is always more difficult to measure reliably, since reaction (xviii) is not the exclusive recombination step in any system. The recombination in lean flames depends also on the fate of the intermediate HO2. This in turn depends on the rate coefficients fe 4, feg, and k2o k2 2-... 

Reactions (xxi) and (xxii) may a priori be expected to become more important in lean flames, and eventually to overtake reactions (viii) and (xx). The radical concentrations in lean flames are probably such that reaction (xxi) dominates. However, because both reactions (xxi) and (xxii) increase in importance together, their separation is again difficult. 

Fate of hydroperoxyl in lean flames of Tabic 32, at approximate mid-points of range of investigation... 

More revealing in relation to the comparative lack of reactivity of nitric oxide are the observations of Wolfhard and Parker on H2 + NO2 flames. Here nitric oxide is found strongly in absorption in the burnt gas of both rich and lean flames, showing that it does not play a major part in the reaction. This conclusion is supported by measurement of the flame temperature of the stoichiometric mixture for H2 + 5NO2 = H2O + 5N2 [286]. Theoretically this should be 2890 K if the stoichiometry is as quoted. The measured flame temperature by line reversal was 1780 K. 

Burning of the fuel is a stepwise degradation leading to many intermediate products and formation of radicals. Some of these are of interest because of their ability to interact with metallic atoms, notably atomic oxygen and the OH radical. If the available oxygen equals the theoretical amount necessary to bum the fuel completely, such a flame is called stoichiometric. Otherwise, depending on the amount of fuel, we speak of a fuel-rich or a lean flame. Where fuel and metallic atoms compete for O and OH radicals, the formation of metal oxides can be kept down by making the flame fuel-rich. Fassel et al. (F2, F3) have recently shown that satisfactory atomic vapor concentrations can be produced in fuel rich flames with a a iety of metals particularly prone to form refractory oxides in stoichiometric flames. 

The choice of flame appears to be important and best results were obtained with the air-acetylene flame (Fig. 15). For optimal sensitivity, it is necessary to work with a fuel-rich, reducing flame, because of considerable oxide and hydroxide formation in lean flames, an effect even more pronounced with other elements (S4). The concentration of neutral calcium atoms is greatest in a narrow, clear zone of the flame just above... 

The same system was used to record temporal data from a H2/air flame using spectral band-pass filters. Fig. 6 shows water flame spectra obtained under two different flame conditions a near stoichiometric flame and a lean flame. The spectral data match the known theoretical spectra of water. The width of the spectral band is an indicator of water temperature. Thus, we can determine the density and temperature of the water vapor within the flame using a set of three spectral bands corresponding to the center and wing of the water band, and a null band outside of the water radiation band. 

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