A simplified two-reaction zone model of magnesium combustion in carbon dioxide

Abstract

A simplified model of magnesium particle combustion in pure carbon dioxide has been developed andutilized for prediction of particle burning times. This model includes two reaction zones—an outer zone (shell) where magnesium reacts at a transport-limited rate with carbon dioxide to form condensed magnesium oxide plus CO and an inner zone, at the particle surface, where CO reacts with liquid magnesium to form solid carbon and solid magnesium oxide, which remain with the particle. The main simplifications utilized are the neglect of reactions in the gaseous regions inside and outside the outer shell and use of average transport properties across these regions. First, two limiting cases, the first with transport-limited reaction rate at the surface and the second with zero reaction rate at the surface, were examined. Next, finite-rate kinetics of the surface reaction, with the reaction rate constant being the product of the collision rate of CO molecules with the surface and a reaction probability expressed in Arrhenius form, was examined, with the activation energy of the Mg+CO reaction being varied parametrically. Model development and resultant burning rate and burning time predictions are presented. For the two limiting cases, predicted burning times were proportional to the square of the initial particle radius, while for the finite kinetics cases, the dependency was predicted to be slightly less than second order. Comparison of predictions with the limited experimental data available indicates reasonable agreement between theory and data.