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

Abstract

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 vary. NaO2 plays an indispensable role in providing alternate efficient channels by which NaOH can be produced. Its contribution becomes increasingly important at lower temperatures where the flux through the NaO2 intermediate becomes dominant over that for the direct reaction between Na and H2O. As a consequence, the ratio of NaOH to Na can become enhanced by up to two orders of magnitude at lower temperatures over what might have been expected from the Na+H2O direct reaction alone. The dissociation energy D°0(Na–O2) is established to be 39±5 kcal mol−1 and the value of the rate constant for the Na+O2+M reaction of 2×10−28 T−1 cm6 molecule−2 s−1 for the flame gases. The sodium distribution within the highest temperature, low-O2 flame, in which NaOH is dominant and equilibrated, supports a value of D°0(Na–OH) of 78.9±2 kcal mol−1. The rate constants for several reactions of Na, NaOH, NaO2, and NaO with flame species have been established approximately. An analysis of the total kinetic scheme shows that the chemical fluxes are carried predominantly by four reactions only. These considered alone, reproduce the data surprisingly well. An analysis of the implications of the corresponding large rate constants for the termolecular reaction of the other alkali metals with oxygen suggests that these will undoubtedly show to varying degrees similar behavior to sodium. Values for the bond dissociation energies of the other alkali dioxides are discussed. It appears that in practical combustion systems, even at low temperatures, the conversion of alkali metals to the corresponding hydroxide will not be kinetically constrained and its concentration will be at or in excess of the expected equilibrium value.