Experimental study on hydrogen production characteristics of millimeter aluminum spheres in sub/supercritical water

Aluminum is a high-energy-density, plentiful, recyclable metal material that releases hydrogen and heat when reacted with water, and the process and by-products of the reaction are green and non-polluting. Previous studies have focused on the micron aluminum powders-water reaction using the activating additives. In this work, the hydrogen production characteristics of millimeter aluminum spheres in sub/supercritical water without any additives and catalysts are studied experimentally, investigating the effects of different reaction times, reaction temperatures, and aluminum sphere sizes on the hydrogen production characteristics. The results show that the reaction of a millimeter aluminum sphere and water is divided into four stages, and the hydrogen production increases with reaction time. For the 6.35 mm aluminum sphere, the peak of hydrogen yield can be as high as 86.7 % with a temperature of 550–600 °C, and the morphology evolution of the unreacted aluminum core is not a nearly spherical uniform shrinkage, finally showing as a near-pear shape particle. The by-product powder is converted from aluminum oxyhydroxide (AlOOH) to the most stable alumina (α-Al2O3) with the temperature increase. The smaller the particle size of the aluminum spheres, the higher the reaction ratio. The hydrogen yield and the reaction ratios of the 2.38 mm aluminum are up to 95 % and 91.71 %, respectively. The developed millimeter Al-sub/supercritical water reaction is promising to achieve integrated utilization of hydrogen-electricity-heat.