Structure, chemical mechanism and properties of premixed flames in mixtures of carbon monoxide, nitrogen and oxygen with hydrogen and water vapour

Abstract

Implicit solutions of the time-dependent flame equations have been used to calculate, for assumed reaction mechanisms, the expected structures and properties of a series of hydrogen-carbon monoxide-oxygen-nitrogen flames, some containing traces of added water vapour, at atmospheric and reduced pressures. Predicted burning velocities at atmospheric pressure have been compared with: ( a ) recent measurements, reported here, of the effect of addition of up to 10 % carbon monoxide on the burning velocity of a low temperature hydrogen-oxygennitrogen flame; ( b ) previous measurements by Scholte & Vaags (1959c) on dry hydrogen-carbon monoxide-air mixtures over the whole composition range on the fuel-rich side of stoichiometric; and ( c ) previously reported measurements by Jahn (1934), Badami & Egerton (1955), Scholte & Vaags (1959 b )and Wires et al . (1959) for moist carbon monoxide-air or carbon monoxide-oxygen mixtures, with or without traces of added hydrogen. Additionally, the following comparisons are made: ( d )The mole fraction profile for the decay of a trace of carbon dioxide added to the low temperature hydrogen-oxygen-nitrogen flame has been recalculated with the aid of the full reaction mechanism, for comparison with the previously reported measurements of Dixon-Lewis et al. (1965). ( e ) Computed structures of two hydrogen-carbon monoxide-oxygen-argon flames burning at reduced pressure have been compared with previous measurements by Fenimore & Jones (1959) and Vandooren et al . (1975). ( f ) The mole fraction ratio X co /X CO 2 in the burnt gas from a low temperature, fuel-rich hydrogen-carbon monoxide-oxygen-argon flame at atmospheric pressure was measured by using a mass spectrometer. The measured ratio agreed to within 1 % with that predicted by computation of the complete flame properties. Both the calculated and measured ratios were higher than would correspond with the establishment of the water gas equilibrium in the flame. The major part of the observed changes in burning velocity from those of hydrogen-air mixtures can be satisfactorily explained by the addition of the single reaction (xxi) , OH + CO ⇌ C O 2 + H , ( xxi ) to the mechanism already established for the hydrogen-oxygen-nitrogen flame system (Dixon-Lewis 1979). This applies particularly to fuel-lean flames and to fuel-rich mixtures not too far from stoichiometric. For fuel-rich flames further from stoichiometric, and particularly for the measurements in §(a), agreement between predicted and measured burning velocities is improved by adding to the mechanism a series of chain terminating steps involving the formation and subsequent reactions of the formyl radical. For reasonable values of its rate coefficient, reaction (xxii), O + CO + M ⇌ C O 2 + M , ( xxii ) never exerts more than a minor influence on the burning velocity. The major features of the structure of the flames are: ( a ) a preferential oxidation of hydrogen in the early stages of the reaction zones, leading to overshoot in the water concentration followed by a slow approach to the water gas equilibrium from the carbon monoxide-water side; and ( b ) marked enrichment of the oxygen atom concentration in the radical pool as the hydrogen content of the flames is decreased. In the flames containing only traces of hydrogen, the degree of enrichment is markedly influenced by reaction (xxii).