Kinetic modelling of the oxidation of carbon monoxide in flames

Abstract

Implicit solutions of the time-dependent flame equations have been used to calculate, for assumed reaction mechanisms, the properties of a series of hydrogen/carbon monoxide/oxygen/nitrogen flames. Comparison of the predictions with experiment, over a wide range of composition and conditions, has established the probable kinetic mechanism in the flames. The major part of the observed changes in burning velocity from those of hydrogen/air flames can be satisfactorily explained by the addition of the single reaction (xxi): OH + CO ⇄ C O 2 + H ( xxi ) with k21=1.5×107T1.3 exp (+385/T) cm3 mole−1 sec−1, to the mechanism already established for the hydrogen/oxygen/nitrogen system. This applies particularly to fuel-rich mixtures not too far from stoichiometric. 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. However, for fuel-rich flames far from stoichiometric, in which the ratio [OH]/[H] becomes small, 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. These reactions were investigated by measuring the influence of carbon monoxide addition on the burning velocity of a low temperature, fuel-rich hydrogen/oxygen/nitrogen flame. On the assumption that k23.H2=5×1014 exp (−755/T) cm6 mole−2 sec−1, the analysis of the results favours the low heat of dissociation of the formyl radical to H and CO and, on the further assumption of zero activation energies for reactions (xxiv) and (xxv), leads to k25=(4±1)×1013 and k24=3×10(12±1) cm3 mole−1 sec−1. H + C O + M ⇄ H C O + M ( x x i i i ) H C O + O 2 ⇄ H O 2 + C O ( x x i v ) H C O + H ⇄ H 2 + C O ( x x v ) With additional reactions to provide for (a) attack of OH and O on the formyl radical, and (b) conversion of methane (via CH3 and CH2O) to formyl, this same mechanism and rate coefficients are capable of predicting the burning velocities of both lean and slightly rich methane/air flames at atmospheric pressure. A mechanistic interpretation which covers both the carbon monoxide and methane flames gives an optimum value of k24=(3.5±0.5)×1012. Most of the flame simulations have been carried out by means of an adiabatic flame model. In order to match the experimental conditions in one of the carbon monoxide flames studied, the boundary conditions were modified in two calculations to take account of truncation of the flame profiles at the cold end due to heat losses at the burner. For simulations on the basis of the same final temperature in the flame, the results from the two models showed a small reduction in the burning velocity in the truncated cases.