According to the Wikipedia, aluminum has a very high energy density as a ratio to its volume.
It seems like if you can get a lot of energy out of it, and its prevalent (which it is), that it would be a good fuel source. So why isn’t it? How does one get aluminum to burn? What all toxins does this process emit?
Also, I would imagine that a liquid would be a preferable medium for an IC engine. Is there any liquidous aluminum-based compounds? Can those be combusted?
There appears to be a wikipedia article on “aluminum batteries”: Aluminium battery - Wikipedia
It doesn’t look like the products of the reaction are particularly harmful - just aluminum oxide is formed, which is pretty inert and can be recycled back into aluminum. However, from what I can tell it doesn’t look like you can easily recharge the batteries, so they would have to be replaced/refurbished every time they ran out. Based on the energy density chart, aluminum has worse energy density per unit mass than gasoline (though much higher per unit volume) so you’d still have to have over a hundred pounds of aluminum in your car to get good range out of it.
I’m not aware of any aluminum alloys or compounds that would be liquid at room temperature, though even if they did exist (and could work as a fuel source), it wouldn’t be the same as a petroleum-fueled car. Hydrocarbons burn to form just CO2 and H2O in gaseous form (primarily), so all the waste products can just be vented into the air. The waste from reacting aluminum will form solute/solids and would require some form of disposal. I think this is why not much effort is generally put into “metal fuel cells” vs. hydrogen or hydrocarbon fuel cells.
One way to get aluminum to burn is to expose it to water. No, really: Pure aluminum will react instantly on exposure to water. The reason you can store liquids in aluminum containers is that when the surface reacts, it forms a thin sapphire-like non-reactive layer that protects the bulk of the material from further reaction. So the first step in getting aluminum to react on a large scale is to powder it.
The quickest way to get it to react is to mix it with rust, and then ignite it with a small amount of high-temperature fuse like magnesium (this is what thermite is). The reaction produces aluminum oxide and molten iron. Neither is toxic (in fact, aluminum oxide is one of the most common compounds in the environment), and both are solids at room temperature, so they won’t be getting out into the air or water supply.
Aluminum is not found in its elemental form in nature; it has to be produced from aluminum oxide ores through application of a great deal of electrical energy. So using aluminum for fuel is comparable to using hydrogen as a fuel: In both cases it’s effectively an energy storage medium, not an energy source.
There may be liquid compounds containing aluminum, but aluminum-containing compounds wouldn’t have nearly the energy density found in the pure element.
Well, no.
No, aluminum is NOT prevalent; aluminum oxide is. Aluminum oxide has the same energy density as most rocks, which is to say “not enough to get excited about.” In fact, we spend a hell of a lot of energy to turn aluminum ores into aluminum (enough to make recycling big business), and burning it for fuel makes no sense (OK, what I really mean is that it’s insane).
No, aluminum is not some great superfuel. It burns very well, when finely divided, but finely dividing it reduces its density from the 2.7 g/cc that the author of that article is using to 0.7 g/cc, reducing its energy density to the ranks of mere mortals like gasoline – which is far easier to handle than powdered aluminum.
No, there’s no “liquid form” of aluminum that you could use as a fuel – aluminum compounds contain oxidized aluminum, which has, as I previously pointed out, the energy density of a rock. And not a cool rock like anthracite or pitchblende, either. The only liquid form that would approach that density is molten aluminum, which has a lower density AND has a melting point over 1200 degrees Fahrenheit. Oh, and it’s not volatile like gasoline, either.
True, but a hundred pounds, so far as a car goes, isn’t all that bad. The Tesla roadster, for instance, weighs about a thousand pounds more than a Lotus Elise and is still considered serviceable.
A number of metals release copious quantities of heat when they oxidize; the challenge is creating enough exposed surface area to allow for a continuing reaction. That’s a rare situation, and so it comes as a surprise sometimes when it happens.
Solid blocks of iron rust very slowly. But steel wool - which has a large surface-area-to-volume ratio - ignites readily; steel wool shorted across a 9-volt battery can be used to get a campfire going. Likewise, steel/iron dust will smolder nicely. I’ve lit off piles of steel dust at the bottom of my belt sander before. Put iron rods in a pure O2 environment, light them, and you’ve got a thermal lance that can cut through concrete.
Magnesium? Same thing. A magnesium flare is made of powdered magnesium, giving it the needed surface area.
Aluminum? Same thing. Will burn readily if powdered. In fact, it’s so hungry for oxygen that it will take the oxygen from iron oxide. Put rust powder together with aluminum powder, and you get thermite, which releases some truly bad-ass heat. Mix aluminum powder with ammonium perchlorate, and you get APCP, the stuff packed into the space shuttle’s solid rocket boosters.
(As an aside, lots of seemingly innocous things will burn vigorously or even explode if ground fine enough and dispersed in air. A log burns slowly in your fireplace; but if you chuck a handful of sawdust into a hot fire in such a way as to disperse it a bit in the air before it hits the flame, you’ll get a satisfying “WHOOMPF” out of it as it all burns very rapidly. Also, see grain elevator explosions.)
So why don’t we use aluminum as a fuel regularly? As has been noted, elemental aluminum is uncommon in nature; when we find it, it’s already been oxidized. We expend energy to de-oxidize it back to a high-energy elemental state. To try to use that for fuel would be like taking carbon dioxide and water, and combining them (plus energy) to make gasoline and oxygen.
That’s a hundred pounds of Al consumed.
I definitely forsee a future where renewable resources are used to create exhaustible fuels. Face it, we can’t create a battery that can store as much energy as fossil fuels by weight.
It really comes down to the economics. Will anyone pay enough for the end result to make it worthwhile? The answer is simple, not as long as there are cheaper viable alternatives.
Powdered Aluminum is also used in thermobaric bombs.
Aluminum (in powder form) is added to many deflagrating explosives and solid propellant grains to increase the energy density; nearly every single modern solid rocket motor composite propellant has some form of powdered metallic fuel to increase energy density; generally spherical aluminum is used, contributing 14-20% by weight, but boron or beryllium have been used in some motors (although the later is extremely toxic). There have been some attempts to develop liquid slurry propellants using very finely powdered aluminum or magnesium to increase energy density (which would substantially decrease the size of tankage for liquid rockets) but the problems of the slurry clogging injectors or clumping up and getting incomplete combustion within the engine chamber have prevented this from being a practical fuel.
Although aluminum is not considered an immediate hazard, it does accumulate in the body from repeated exposure and has been indicated as contributing to many chronic illnesses, including encephalopathy, myocardial dysfunction and microcytic anemia, hypercalcemia, osteomalacia, and renal dysfunction. Dietary absorption of aluminum is minimal (a couple percent of total consumption), mostly through takeup of transferrin and citrate; the primary means of absorption is via inhalation of aluminum oxide fumes. The British did some testing on aluminum armor intended for lightly armored personnel carriers and found that not only did it provide ineffectual defense against modern anti-vehicle weapons, the amount consumed by animals exceeded acceptable thresholds. This didn’t stop the US Army from developing the M2/M3 Bradley Fighting Vehicle using aluminum armor, despite numerous trials that clearly showed it to be inadequate against anticipated battlefield threats. (“Goddamnit! We fought a revolution so we wouldn’t have to pay any attention to the fucking British!”) The BFV was penetrated and destroyed in several occasions during Operation Desert Storm (although admittedly most by friendly fire) and several did have the armor sheathing catch fire.
As far as a fuel, although it is very energy dense, it would be a very undesirable energy storage medium. Aside from the toxic byproducts and difficulty with injection and exhaust systems, aluminum is a scarce resource. Though we’re accustomed to seeing it in all manner of container applications and as a structural material, the fact is that processing bauxite ore is very energy intensive, hence why aluminum is the most profitable material to recycle. Burning away aluminum so that it is not available to recover would be an incredible waste, paramount to disposable gold-plated paper plates. Petrofuels and biofuels use carbon and hydrogens that are easily recovered from the environment (albeit take a lot of energy to put into a robust but volatile form), and hydrogen simply has to be pared away from any of a number of molecules and then combined with oxygen or another oxidizer.
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