Hydrogen production rates of aluminum reacting with varying densities of supercritical water.

Aluminum particles, spanning in size from 10 μm to 3 mm, were reacted with varying densities of water at 655 K. The density of the water is varied from 50 g L to 450 g L in order to understand the effect of density on both reaction rates and yields. Low-density supercritical water is associated with properties that make it an efficient oxidizer: low viscosity, high diffusion, and low relative permittivity. Despite this, it was found that the high-density (450 g L) supercritical water was the most efficient oxidizer both in terms of reaction rate and hydrogen yield. The 10 μm powder had a peak reaction rate of approximately 675 cm min g in the high-density water, and a peak reaction rate below 250 cm min g in the low- and vapour-density water. A decline in peak reaction rate with decreasing water density was also observed for the 120 μm powder and the 3 mm slugs. These findings imply that the increased collision frequency, a property of the high-density water, outpaces reduction in the reaction enhancing properties associated with low-density supercritical water. Hydrogen yield was minimally affected by decreasing the oxidizer density from 450 g L to 200 g L, but did drop off significantly in the vapour-density (50 g L) water.