Influence of reactant ratio and preheating conditions on aluminum oxidation in supercritical water for energy recovery

Characterizing the rate of reaction of aluminum with supercritical water is crucial to the design of future reactors enabling the production of hydrogen and heat on demand. The goal of this study was to reconcile the differences in oxidation rates obtained from two distinct studies that reported significantly different rates while using the same samples reacting at similar temperatures and pressures. Two experimental configurations were used to test how preheating the reactants, either in contact or separately, affects the reaction. Additionally, aluminum samples of varying sizes and quantities were tested at 410 ∘ C and three different densities, to investigate how the ratio between surface area and the amount of water available would affect the oxidation rate. Changing the preheating conditions did not show an effect on reaction rates that could be quantified independently of variations in water availability caused by differences in water density. However, varying the reactant ratio induced up to a threefold reduction in the bulk oxidation rate observed. Increasing the amount of loosely packed aluminum slugs in the same reactor volume led to a reduction in overall linear oxidation rate assuming all surfaces reacted uniformly. These results highlight the importance of packing factor and water availability on the bulk oxidation rate of aluminum. These parameters should be considered in future work studying the kinetics of metal-water reactions, and for the design of practical systems aimed at the continuous production of heat and hydrogen. • Preheating the reactants did not have a significant effect on the oxidation rate. • Increasing the packing factor can reduce the bulk oxidation rate by up to threefold. • The oxidation rate of an individual slug can differ significantly from the bulk average.