Complications of one-step kinetics for moist CO oxidation

Abstract

The one-step reaction mechanism, CO+1/2O2→CO2 with d [ C O ] / d t = − k o υ [ C O ] a [ H 2 O ] b [ O 2 ] c which is frequently used in combustion problems when simplified chemistry is necessary, is numerically studied in order to (i) define its limitations (and therefore usage) and (ii) understand the chemical and physical reasons for these limitations. The analysis is carried out with the aid of a validated comprehensive, elementary reaction mechanism for moist CO oxidation and by specialized sensitivity coefficients which correlate the parameters of the global model to the parameters of the elementary model. The results confirm many of the previous, empiricially derived, literature models and show the overall rate constant, as a function of temperature, to exhibit non-Arrhenius kinetics and to be dependent on pressure and mixture equivalence ratio. More importantly, models derived from temporally reacting systems are shown to be improper for use in modeling systems reacting with transport phenomena. The specialized sensitivity coefficients are used to explain these complex behaviors in the overall model. For the temporal system, these coefficients show that the global model must be able to account for dissociation and equilibration at high temperatures, for explosion phenomena in the intermediate temperatures, and for reaction of carbon monoxide with both the hydroxyl radical and hydroperoxy radical at low temperatures. Lastly, methods for modifying the existing model or for developing a new model are suggested.