The hydrolysis of metals is a promising technique to produce pure hydrogen without the emission of CO2 and other polluting agents. It is also a safe and efficient way of H2 storage. Nowadays, the most commonly used metal is micrometric aluminum. This material has several advantages such as stable properties with time, low cost, relatively low toxicity and high availability. However, contact between water and aluminum is prevented by its protective oxide layer on the surface: this phenomenon reduces dramatically the reactivity of the powder and, as a consequence, its hydrogen generation capability. The increase of reactivity for aluminum powders plays a fundamental role. Mechanical activation via ball milling is a possible way to enhance metal powder reactivity. In literature, interesting results were achieved by reducing aluminum size, down to nanometric scale. Unfortunately, long activation times are often required. In order to favor the reaction between aluminum and water, different strategies were employed in the past. The use of strong acid or alkaline aqueous solutions instead of pure water was one of the most used. However, such method increases dramatically health and safety hazards. Another promising way is the mechanical activation of aluminum mixed with salts or other metals. Good results were obtained, but they were still insufficient for industrial application on large scale. The other important aspect involving Al-based powders is, beside the increase of reactivity, the control of hydrogen generation. This works explores these two paths. Some strategies to favor the aluminum corrosion in pure water in order to find the best way for an efficient hydrogen generation are investigated. NaCl, Bi, Zn and graphite were employed as additives in the milling process. The powders were obtained through mechanical activation using a particular milling technique (NHEMA). An optimal combination of milling variables was found. The Al/Bi/NaCl powders, with a weight percent of 90/5/5 respectively, shows interesting results (in tap water) such as high hydrogen generation, high yield (> 95%) and short activation time (10 min). The second path starts from a characterization of the batch produced with this powder; then, an innovative technique, based on powder pressing, is proposed and discussed in order to investigate how to control the production of hydrogen during time.
L’idrolisi dei metalli è una promettente tecnica per produrre idrogeno puro senza emissioni di CO2 e altri agenti inquinanti. É anche un modo sicuro ed efficiente per l’immagazzinamento di H2. Attualmente, il metallo più comunemente usato è l’alluminio micrometrico. Questo materiale ha diversi vantaggi tra cui proprietà stabili nel tempo, basso costo, tossicità relativamente bassa e un’elevata disponibilità. Tuttavia, il contatto tra acqua e alluminio è ostacolato dal suo strato protettivo di ossido sulla superficie: questo fenomeno riduce drasticamente la reattività della polvere e, di conseguenza, la sua capacità di generare idrogeno. L’aumento della reattività per le polveri di alluminio ha un ruolo fondamentale. L’attivazione meccanica tramite la macinazione con sfere è un possibile modo per migliorare la reattività della polvere di metallo. In letteratura, risultati interessanti furono ottenuti riducendo le dimensione dell’alluminio, fino a scala nanometrica. Sfortunatamente, sono richiesti lunghi tempi di attivazione. Nel passato, per favorire la reazione tra alluminio e acqua, sono state adottate diverse strategie. L’utilizzo di soluzioni acquose acide o alcaline forti al posto dell’acqua pura fu una delle più usate. Tuttavia, tale metodologia aumenta notevolmente i rischi per la salute e la sicurezza. Un’altro modo promettente è l’attivazione meccanica dell’alluminio miscelato con sali o altri metalli. Sono stati ottenuti ottimi risultati, ma erano ancora insufficienti per applicazioni industriali su larga scala. L’altro aspetto importante riguardo le polveri a basi di alluminio è, oltre all’aumento della reattività, il controllo della generazione di idrogeno. Questo lavoro esplora due strade. Sono state investigate alcune strategie per favorire la corrosione dell’alluminio in acqua pura e con lo scopo di trovare il modo migliore per una efficiente generazione di idrogeno. Sono stati utilizzati come additivi durante il processo di macinazione NaCl, Bi, Zn e grafite. Le polveri furono ottenuti tramite una particolare tecnica di macinazione (nuova attivazione meccanica ad alta energia - NHEMA). É stata trovata una combinazione ottimale tra le variabili di macinazione. L polvere a base di Al/Bi/NaCl, con un peso percentuale rispettivamente di 90/5/5 mostrano risultati interessanti (nell’acqua di rubinetto) tra cui una significativa generazione di idrogeno, un rendimento elevato (> 95%) e un breve periodo di attivazione (10 min). La seconda strada parte da una caratterizzazione del lotto prodotto con questa polvere; successivamente viene proposta e discussa una tecnica innovativa, basata sulla pressatura della polvere, allo scopo di investigare come controllare la produzione di idrogeno nel tempo.
Exploratory investigations of techniques for hydrogen generation from water hydrolysis by aluminum
BRAMANI, NICOLÒ
2019/2020
Abstract
The hydrolysis of metals is a promising technique to produce pure hydrogen without the emission of CO2 and other polluting agents. It is also a safe and efficient way of H2 storage. Nowadays, the most commonly used metal is micrometric aluminum. This material has several advantages such as stable properties with time, low cost, relatively low toxicity and high availability. However, contact between water and aluminum is prevented by its protective oxide layer on the surface: this phenomenon reduces dramatically the reactivity of the powder and, as a consequence, its hydrogen generation capability. The increase of reactivity for aluminum powders plays a fundamental role. Mechanical activation via ball milling is a possible way to enhance metal powder reactivity. In literature, interesting results were achieved by reducing aluminum size, down to nanometric scale. Unfortunately, long activation times are often required. In order to favor the reaction between aluminum and water, different strategies were employed in the past. The use of strong acid or alkaline aqueous solutions instead of pure water was one of the most used. However, such method increases dramatically health and safety hazards. Another promising way is the mechanical activation of aluminum mixed with salts or other metals. Good results were obtained, but they were still insufficient for industrial application on large scale. The other important aspect involving Al-based powders is, beside the increase of reactivity, the control of hydrogen generation. This works explores these two paths. Some strategies to favor the aluminum corrosion in pure water in order to find the best way for an efficient hydrogen generation are investigated. NaCl, Bi, Zn and graphite were employed as additives in the milling process. The powders were obtained through mechanical activation using a particular milling technique (NHEMA). An optimal combination of milling variables was found. The Al/Bi/NaCl powders, with a weight percent of 90/5/5 respectively, shows interesting results (in tap water) such as high hydrogen generation, high yield (> 95%) and short activation time (10 min). The second path starts from a characterization of the batch produced with this powder; then, an innovative technique, based on powder pressing, is proposed and discussed in order to investigate how to control the production of hydrogen during time.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/169983