Aluminum combustion in wet and dry CO2: Consequences for surface reactions

The combustion of aluminum droplets in wet (3 mol% H2O) and dry CO2 is studied in order to identify the influence of the two atmospheres on the surface processes. Millimeter-sized samples are maintained contactless in an aerodynamic levitation system and are heated continuously during burning by a laser. Ignition and combustion of the aluminum droplet are observed with a high-speed camera, the Al surface temperature is measured by an optical pyrometer, and unburnt residues are analyzed by X-ray diffraction. The determination of the burning rates and of the droplet temperatures reveals no differences between wet and dry CO2 (β=1.28±0.05 mm2/sβ=1.28±0.05 mm2/s, T=2600±50 KT=2600±50 K), which shows that the gas-phase combustion regime is not affected by the presence of water vapor. However, the oxide cap, initially formed by the oxide coating breakdown at ignition, is progressively removed in wet CO2, whereas it is unvarying in dry CO2. Comparison between Al burning in CO2/H2 and in CO2/(Ar or He) demonstrates that the oxide cap regression in a wet atmosphere is related to a chemical effect of hydrogen produced in the flame, and then diffusing and reacting at the droplet surface. It is suggested that the adsorption mechanism of H2 on the Al surface may slow down the contribution of adsorbed oxygen-containing species (CO) to the oxide cap, which would consequently promote its decomposition (removal). Furthermore, the carbon dissolution process is observed in wet and dry CO2. When the carbon concentration reaches the saturation limit in the burning Al droplet (xC=0.23xC=0.23 at T=2600 KT=2600 K), the excess of carbon is ejected at the surface and forms a solid coating. In the absence of the oxide cap (wet CO2), the refractory carbon coating prevents strong surface oxidation, and the combustion definitely stops. In the presence of the oxide cap (dry CO2), the carbon coating reacts and produces an oxycarbide phase which is melted by the laser heating; a new burning regime occurs mainly controlled by direct surface reactions, and leading to the slow oxidation of the droplet and the expulsion of dissolved carbon into CO. Finally, a qualitative model of the combustion of aluminum in CO2 atmospheres is proposed.