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Prototype version of Alydro reactor, which brings the aluminum into reaction with water vapor. The photo taken in Alchemy Research lab.^[4]

Aluminum is well know^[1][2] for its high energetic containment properties. Aluminum reacts with oxygen exothermically following the reaction:

2Al + 3/2O₂ >> Al₂O₃

The calorimetric heat of reaction at 1 Bar is (ΔH° (298K) - 404 kcal/mol).

The process of aluminum oxidation generates 31 MJ of energy per kilogram – 70% by weight or 240% by volume compared to gasoline. High energy content positions aluminum as appropriate candidate to serve as energy carrier substance, replacing energy sources such as fossil fuels, and energy carriers such as electric batteries. Aluminum, being self-passivating, non-polluting, safe and recyclable material possesses many properties required from optimal energy carrier.

Oxidation methods

However there are several major complications impairing usability of aluminum as practical fuel:

Aluminum quickly forms oxide layer (Al2O3) on its surface, which inhabits further reaction with oxygen. Oxide layer film is extremely strong, both mechanically and thermally. If oxide layer is broken, it immediately reforms, sealing the break. Literature^[5] mentions several methods to tear oxide layer and bring aluminum in contact with oxidizing substance:

• Heating aluminum up to 2030°C melts the oxide layer and enables further oxidation. Oxidation rate dramatically increases if aluminum is brought to vapor phase, by heating over 2800°C. This method however requires very high temperatures and is limited to narrow range of applications, such as solid fuel rocket propulsion.

• Oxide layer can be penetrated by electrical discharge, by processes known as "anodizing" or "plasma electric oxidation". Usually, the propose of these processes is creation of thick protection layer on the metal, by repeated penetrating and deepening of oxide layer with help of local electric discharge. Although widely used in industry, the scope of these processes is limited to the creating of protective layer rather than for transformation of aluminum into useful energy, as most of electric power input is eventually converted to useless mild heat waste.

• Grinding aluminum to powder increasing its surface area and therefore increasing probability of reaction with oxidizer. Furthermore, surface curvature of nanometric powder particle affects oxide layer perfection and increases probability of reaction to the level sufficient for sustained burning. Powdered aluminum, in concentration up to 25%, is used as enhancer in rocket fuel. However it cannot be used in its pure form, because powder requires initial reheating by another, more reactive ingredients. On the other hand, the production of nanometric powder, which is reactive enough to implement sustained burning by itself, is extremely expensive and is not considered as practical solution for any application requiring cost efficiency.

• Addition of certain chemical elements to the reaction environment can create compounds which are replacing or weakening the oxide film and allowing penetration of oxidizer through it. However this results either in complicated chemical compositions that is difficult to separate and recycle, or in reactions which are too slow to be considered a useful energy releasing process.

To conclude, although methods of oxide layer penetration exist, none of them offers practical solution for utilizing aluminum as a fuel.

Direct reaction between aluminum and oxygen gas yields heat and molten aluminum oxide which quickly turns solid. Combination of heat and aluminum oxide is difficult to transform to useful mechanical or electrical power. In contrast, burning regular gasoline results in a mix of hot compressed vapor and carbon dioxide, which tends to expand and therefore is easily converted to a mechanical power output. The energy output of aluminum oxidation cannot be utilized directly, but rather requires conversion process. The conversion process, such as boiling the liquid by combustion heat, intrinsically affects the process efficiency and therefore the attractiveness of aluminum oxidation as energy source.

Reaction with water

The concept of reacting aluminum metal with water to produce hydrogen and thermal energy is not new^[1]. There have been a number of claims^[1][2][3] that such aluminum-water reactions might be employed to power fuel cell devices for portable applications such as emergency generators and laptop computers, and might even be considered for possible use as the hydrogen source for fuel cell-powered vehicles. In the vicinity of room temperature, the reaction between aluminum metal and water to form aluminum hydroxide and hydrogen is the following: 2Al + 6H2O = 2Al(OH)3 + 3H2. The gravimetric hydrogen capacity from this reaction is 3.7 wt.% and the volumetric hydrogen capacity is 46 g H2/L. Although this reaction is thermodynamically favorable, it does not proceed due to the presence of a coherent and adherent layer of aluminum oxide that forms on the surface of aluminum particles which prevents water from coming into direct contact with the aluminum metal. The key to inducing and maintaining the reaction of aluminum with is the continual removal and/or disruption of this coherent/adherent aluminum oxide layer. A number of reaction-promoting approaches have been investigated for the aluminum-water reaction. These include additions of hydroxide promoters such as NaOH, oxide promoters such as Al2O3, and salt promoters such as NaCl. These additions act to disrupt the aluminum oxide layer on the aluminum metal. In addition, the reaction of water with aluminum alloys such as aluminum-lithium and aluminum-gallium has been studied. However, none of these approaches have proven commercially viable. The concept of using the aluminum-water reaction to provide onboard hydrogen for hydrogen-powered vehicles presents a number of difficulties. First, storage systems using this approach will not be able to meet the system targets of 5.5 wt.% hydrogen and 40 grams hydrogen per liter defined by U.S. Department of Energy. Second, based on published aluminum-water reaction rate kinetics, it appears difficult for this approach to meet the DOE minimum hydrogen flow rate target for fuel-cell powered vehicles.

See also

• Aluminum
• Water splitting

References

1. U.S. Department of Energy, Reaction of Aluminum with Water to Produce Hydrogen.
2. U.S. Pattent Office, Aluminum Cycle Energy System, Campbell; Berry (Monrovia, CA).
3. Purdue University, Recent Results of Splitting Water with Al Alloys.
4. Alchemy Research, Alydro Energy System.
5. American Insitute of Aeronautics, INVESTIGATION OF ALUMINUM PARTICLE COMBUSTION FOR UNDERWATER PROPULSION.