Kinetic analysis of co formation under oxy-combustion conditions

Abstract

The paper reports the results of numerical computations concerning the formation of CO in the kinetic flame during natural gas oxy-combustion. The effect of temperature, reagent residence time and the composition of an atmosphere containing CO2 with 21 and 29 vol % O2 on the variation of CO concentration in the flame was examined. The oxy-combustion process was conducted with a 25 % excess oxygen. The analysis of reactions in flames at temperatures of 1500 K and 1800 K was performed within the Chemked II program using the combustion mechanism proposed by Mendiara & Glarborg, which includes 779 reactions. The computation results have confirmed that the rate of the key reactions responsible for the production of CO in the flame depends on the flame temperature and the oxy-combustion temperature. The peak CO concentrations are higher for the oxidizing mixture containing 29 vol % O2. After attaining a maximum, the CO flame concentration drops faster for an atmosphere richer in oxygen. The longer the time of reagent residence in the flame region, the lower the CO concentration. In different atmospheres and at different combustion temperatures, an identical CO level can be achieved in wet combustion gas. Irrespective of the temperature and atmosphere of oxy-combustion, most CO is produced as a result of the reaction OH + CO HCO2. The reduction of oxygen in the oxidizing atmosphere at flame temperatures of 1500 and 1800 K lowers the CO production in the dominant reactions responsible for CO formation. The contribution of individual reactions in the CO production for the identical atmospheres is different with varying temperature. In the case of the reaction HCO + O2 HO2 +CO, the temperature increase reduces the CO production. A reverse dependence of CO production on temperature characterizes the reaction H2 + CO + M CH2O + M. In addition, change in temperature changes the order in which the dominant reactions occur. Within the residence time equal to 100 ms, two periods of intensified CO production and consumption can be identified. The peak concentrations of H, OH and O radicals in the flame attain a maximum within the same time; as time goes by, the highest concentration is achieved by OH radicals. The presence of considerable levels of CO2 in the combustion substrates has an inhibiting effect on the natural gas oxidation process.