Ignition and combustion of metals in a carbon dioxide stream

Abstract

In the prospect of using metals as fuel of breathing combustion engines in carbon dioxide rich planet atmospheres without oxygen such as those of Mars and Venus, a fundamental study was performed experimentally on the ignition and combustion of metals in an impinging pure CO2 gas stream. Metals selected were lithium, magnesium, boron and aluminum, because of their high heats of reaction with CO2. To help the understanding of combustion processes of the metals in CO2, the chemical equilibrium compositions of reaction products and flame temperatures were calculated, and compared with the experimental results. Li and Mg could ignite spontaneously in the CO2 stream. The ignition process of Li in the CO2 stream had two distinct steps, first a surface reaction and then a gas phase reaction. This process was found to be similar to those of Mg and Na in air streams, but was different from that of Li in an air stream. The spontaneous ignition temperature of Li in the CO2 stream was much lower than that in the air stream. It was also found that Li burned in vapor-phase at first and then on the metal surface. At the ignition in the CO2 stream, the Mg surface was covered with a protective film against further oxidation, resulting a higher spontaneous ignition temperature than that in the air stream. Mg could burn in vapor-phase. B could not ignite in the CO2 stream, but the reactions occurred appreciably on the surface. The reaction rate was quite slow, compared with that in an oxygen stream, and the molten boron oxide appeared on its surface. Al could not ignite within the temperature limits of our apparatus. However, the possibility of its ignition and combustion in the CO2 stream was demonstrated in a different experiment allowing the attainment of somewhat higher temperatures.