Ignition and combustion of magnesium in carbon dioxide streams

Abstract

The prospects for using Mg as fuel in a CO 2 -breathing engine in Mars atmosphere without oxygen led to this experimental study on the ignition and combustion of Mg performed in pure CO 2 and CO 2 /CO streams over a wide range of pressure, velocity, and CO concentration of the streams. The critical ignition temperature of Mg in CO 2 streams decreased with decreasing ambient pressure, and was insensitive to the stream velocity. The ignition temperature in CO 2 /CO streams varied little at first, but increased with decreasing CO 2 concentration. Ignition occurred in two distinct stages, the first being surface reactions controlled by chemical kinetics and the second being gas-phase reactions controlled by CO 2 diffusion. The breaking of a thin protective film formed in the first stage played a crucial role in the ignition process, leading to the beginning of the gas-phase reactions and then to ignition. During combustion, several flames appeared sporadically and intermittently over the swelling samplesurface, which was coated with a thick porous layer. The flames produced CO in the gas phase, while, on the nonflame surface, the reactions of Mg with both CO diffusing from the flames and CO 2 /CO in the streams generated condensed MgO and C that built the porous layer. The pulsating manner of the flames can be explained in terms of the abrupt breaking of the layer followed by its blockage due to the surface reaction with CO.