Aluminum use of industrial waste for obtaining hydrogen by controlled aluminum-water reaction

摘要

The emergence in the market of vehicles feeded with PEM (proton exchange membrane) fuel cells leads to the need of supplying high purity hydrogen, in a dosed way and at low pressure. Nowadays it is yielded by means of heavy hydrogen cylinders at high pressure, 350 or 700 bar. These cylinders also require pressure regulators and strict security controls and maintenance. This needed hydrogen comes mostly from natural gas reforming, a nonrenewable gas. A more secure and sustainable alternative to this system consists in the hydrogen production, in situ, from the aluminium-water reaction in alkaline medium, and if possible from aluminium residues. In the aluminium-water reaction system, there are the difficulties of obtaining a constant hydrogen flow of great purity and the avoidance of the Al(OH) precipitation on the aluminium surface, if the metal becomes dry, an event that would delay the reinitiation of the reaction. It is described a new processs for the controlled production of hydrogen, from aluminium residues, by the dosage of sodium hydroxide aqueous solution over alcoholic medium. The alcohol always covers the aluminum surface, thus avoiding the Al(OH) precipitation in its surface, stabilizing the reaction and fiftering the hydrogen gas. The behaviour of different alcohols has been studied: ethanol, isopropylic alcohol, 1,2-propanediol and 1,2-ethanediol. Isopropylic alcohol turned out to be the most adequate. The sodium hydroxide molarity that provides the highest hydrogen flow rates has been determined experimentally, in this isopropylic alcohol medium. It has been compared and analyzed the hydrogen gas flow obtained for pure granulated, 99,7 % aluminum (<1mm) and for different aluminium residues, obtained from different industrial processes. The relationship between the flow of sodium hydroxide solution dosed into the reactor and the hydrogen gas flow produced has been estimated, assuming that there is always enough aluminium active surface for a complete reaction. With all this information, there has analyzed the purity of the hydrogen and of the aluminium hydroxide, Al(OH)(3), after the leaked one and extraction from the generated residue, NaAl(OH)(4.)