Combustion of Aluminum Particles in Carbon Dioxide

Abstract

Recently, it has been determined that Martian soil is rich in metallic elements such as Al, Mg, Si, and Fe. Because Martian atmosphere consists largely of CO2, a propulsion system using metals as fuel and CO2 as oxidizer would enable one to utilize Mars's resources very efficiently. Currently, experimental data on metal combustion in CO2 are scarce and an experimental study of aluminum particle combustion in CO2 is presented in this work. Uniform initial size and temperature metal particles are produced and ignited using a pulsed microarc. The free-falling particles bum in the atmospheric pressure CO2. Flame radiation is monitored in real time, the temperature is measured using a three-color optical pyrometer coupled to a computer-based data acquisition system. Particles are quenched at different burn times and their surface morphology and internal compositions are studied using electron microscopy techniques. Preliminary experiments with particles of commercially available aluminum-rich Al-Mg and Al-Si alloys are also performed. The size of the burning particles was observed to decrease and more than 94 % of aluminum was consumed by combustion in CO2. It was found that the rate of aluminum combustion in CO2 is faster than that in air and the dependence of particle burn time from particle size is best described by t ∼ d 2.5 rather than by t ∼ d 2-law. Flame radiation measurements showed that the vapor-phase reaction is significant only during the initial period of combustion. The measured temperature was steady and around 3000 °C during most of the combustion time. The estimated adiabatic flame temperature is reasonably close to the experimental value. Particles quenched on Si wafers were surrounded by spherical smoke clouds and the particle surfaces were coated with a layer of partially coalesced oxide nano-spheres. Surfaces of the completely burnt particles did not have a continuous oxide coating, instead, a number of oxide spheres in the size range of 1 −5 μm were attached to the particle surface. Interiors of the particles rapidly quenched on aluminum foil were uniform and contained significant amounts of carbon and oxygen. Some increase in the overall carbon and oxygen content was observed at longer combustion times. Interiors of the completely burnt particles contained several mixed phases, including layers of two non-stoichiometric aluminum oxycarbides and pure aluminum inclusions. None of the oxycarbide phases detected in the quenched or burnt particles was similar to the aluminum oxycarbides described in literature. Analyses of the experimental results have indicated possible importance of the thermophoretic flows in the transport of the vapor-phase reaction products. It has also been suggested that the internal phase changes affect significantly the temperature and rate of aluminum combustion in CO2.