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Magnesium Energy Cycle

Potentially Advantageously Wasteful
  [vote for,

In the realm of vehicles, there is an advantage to being wasteful. Let me explain....

You put fuel, usually hydrocarbons like "octane", into the vehicle. The fuel is chemically combined with oxygen in the atmosphere. The resulting chemical compounds are mostly water vapor and carbon dioxide. Do we store those reaction products in the vehicle? NO! We throw them away, and there are two reasons why.

First, CO2 and water vapor are gases. There is typically a 1000:1 ratio (or higher) between the volumes occupied by a gaseous substance and the liquid form of that same substance. We could not FIT all that CO2 and water vapor into any ordinary-sized car!

Second (see link), for every 11 grams of octane we burn, we need 19.3 grams of oxygen. So if we filled our vehicle with 110 kilograms of pure octane, and retained the combustion products after burning it, the overall vehicle would get heavier by 193 more kilograms. These days one of the major goals of automakers is to find ways to REDUCE the weight of cars, not increase it!

This brings us to a Fundamental Problem associated with battery-powered vehicles. A battery is usually a closed device; all its "fuel" and "oxidizer" are fully contained. It's weight never goes up or down to any notice-able degree as it is energized and drained. The vehicle is ALWAYS carrying the MAXIMUM weight AND substances-volume around, associated with its energy-source.

There is an alternative, the "fuel cell". It works like a battery but is "open" in that it can interact with atmospheric oxygen, and the reaction-product can be discarded as waste. This makes the fuel cell much more Naturally equivalent, weight-wise and volume-wise, to the hydrocarbon- burning engine.

The main problem with fuel cells is that the ideal fuel appears to be hydrogen gas. This is very bulky stuff! That is not a good thing, making the vehicle bigger, when we know that smaller vehicles have less air resistance and better fuel economy.


Is there anything besides Carbon and Hydrogen that we can "burn" and SAFELY throw away the chemical-reaction products?

There are a number of common substances that, if we reacted them with oxygen them, we could get a decent amount of energy from them. Beryllium, for example. However, practically all beryllium compounds are quite toxic --it is NOT safe to throw the reaction product, beryllium oxide, away!

Then there are lithium, sodium, and potassium, more substances that yield lots of energy when oxidized, but also are not-so-good to throw away afterward. They dissolve in rain-water and will make soils too "alkaline" for proper plant growth.

Aluminum is another possibility, but because there are fears that aluminum might be linked to Alzheimer's Disease, it might be best to not start dumping lots of aluminum oxide into the environment, where-ever vehicles go.

That just about leaves us with magnesium and nothing else. What do we know about its oxide's toxicity? PRETTY SAFE STUFF. "Milk of magnesia", drinkable, has magnesium oxide as the main active ingredient.

OK, how does magnesium stack up against octane in other respects? Here:

Energy associated with the formation of molecules:
octane . . . . . . . 208.4 kilojoules per mole
carbon dioxide . . 393.5
water . . . . . . . . 241.8 (as vapor)
. . . . . . . . . . . . 285.5 (as liquid; the difference is the "energy of condensation")
magnesium oxide 601.2

The chemical formula for octane is C8H18. One mole of octane consists of 8 moles of carbon and 9 moles of molecular hydrogen. So, 8*393.5 + 9*241.8 = 3139 + 2176.2 = 5315.2 kilojoules per mole, combusted. Except we have to subtract the energy-of-formation of the octane, 208.4 (because it has to be broken apart in order to burn in oxygen), and so the actual final result is 5106.8.

If we burned 8 or 9 moles of magnesium (I'll call it 8.5 here) to imitate the quantites above, for carbon and hydrogen, then 8.5*601.2 = 5110.2 kilojoules/mole. In terms of pure energy, that's very comparable! But things are actually better than that, because I'm comparing 8.5 moles of magnesium to 8 moles of carbon PLUS 9 moles of hydrogen.

Now for some weight stuff:
carbon . . . 12 grams/mole
hydrogen . . 2 (molecular hydrogen)
octane, C8H18 weighs 12*8 + 9*2 = 96 + 18 = 114 grams
magnesium: 24 grams/mole, so 8.5 times that is 204 grams, roughly twice as much. I'm staying with the 8.5 moles of magnesium because of the "comparable energy" thing.

Some density stuff:
octane . . . . 703 kilograms per cubic meter (1000 liters), so .703 kg/liter
magnesium: 1.738 grams per cubic centimeter (1/1000 liter), so 1.738 kilograms/liter

Calculating density in terms of moles:
octane: 1 mole per 114 grams is 1 mole per .114 kilogram. Multiply mol/kg * kg/liter to get mol/liter, so this actually becomes a division problem, .703/.114 = 6.167 moles/liter

magnesium: 8.5 moles per .204 kg means dividing 1.738 kg/liter by .204 = (8.52 blocks of 8.5 moles)/liter --I'm still sticking with the energy-comparability thing; a mere 1 mole of magnesium would only occupy about a 1/72 of a liter.

Now we get to see the "kicker" that has allowed hydrocarbon-fueled vehicles to rule the world. Earlier above we started with 1 mole of octane, and after translating it into moles of hydrogen and carbon, we figured that mole of octane has about the same potential chemical energy as 8.5 moles of magnesium. Now we see that that single mole of octane occupies about 1/6 of a one-liter fuel tank, and that same tank would be roughly 1/8 full of magnesium fuel having the same energy.

Basically, hydrocarbon fuels offer tremendous "energy density" (but not quite as much as magnesium), and that --AND because liquids are so much easier to handle than solids-- is why we'll continue using them even if we have to build nuclear fusion power plants to get the energy to synthesize hydrocarbon fuels!

We would have a smaller but heavier fuel tank, for our magnesium-fueled vehicle. I'm not going to speculate here about just exactly how we would oxidize the magnesium to get the energy to power the vehicle. The purpose of this Idea is simply to explore the possibility of throwing away the chemical-reaction product, after obtaining the energy. It looks very feasible.

Of course, someone will immediately build a special vehicle to vacuum up the MgO dropped all over the roadways, for recycling into more magnesium fuel. That's OK, because the goal here is to give all the OTHER vehicles a route to smaller size than involves batteries. And, of course, we don't really want the roads to become clogged with spent fuel. :) (Note: we are not in any danger of running out of magnesium soon; it is the 7th most-common element in the Earth's crust --and I explicitly mentioned "recycling".)

Vernon, Oct 07 2013

Burning Octane http://answers.yaho...080203222246AAFSKZz
As mentioned in the main text. [Vernon, Oct 07 2013]

About magnesium http://education.jl...emental/ele012.html
Density and abundance, among other things [Vernon, Oct 07 2013]

Energy of formation of various compounds https://en.wikipedi...mation_(data_table)
Lots more compounds listed here than discussed in the Idea. [Vernon, Oct 07 2013]

powder metal fuel http://goyder.net.a...2/Powdered_fuel.htm
[xaviergisz, Oct 07 2013]

Magnesium air fuel cell http://www.mgciv.co...l-cell.html?lang=en
Many companies are doing this Phinergy is just another one. [travbm, Oct 29 2015]

boron hydrogen polymer fuel zipfuel higher energ density https://www.google....9Z92tCBu3dpS4dlSqvQ
[beanangel, Nov 02 2015]


       Uranium is relatively dense, and also pyrophoric. But the caveat about discarding waste products applies.   

       Also, probably not a good idea to store too much of it in the one place …
8th of 7, Oct 07 2013

       How about sodium? Powdered sodium and water could be injected through two nozzles, generating large amounts of steam and hydrogen. Add a little extra air and - gadulka! - multimodal power generation.   

       The alkaline exhaust would keep road drains clear too.
MaxwellBuchanan, Oct 07 2013

       A gadulka does not contain very much energy relative to its size or price, so while a car fuelled by gadulkas would work it would not be very efficient or effective.
pocmloc, Oct 07 2013

       Folks, I made an error in figuring the moles per liter numbers (fixed). Magnesium beats octane there!
Vernon, Oct 07 2013

       [Vern], your formal education, especially in the areas of mathematics and engineering, obviously far exceeds my own, but I've read this twice now and the word 'weaponized' persists in my mind.
Alterother, Oct 07 2013

       /weeps a bitter tear for poor aluminum. And its UK cousin aluminium/
bungston, Oct 07 2013

       From the MSDS for Magnesium Oxide:   

       //May cause irritation of the upper respiratory passages. Inhalation of magnesium oxide fume may cause metal fume fever, however no evidence of metal fume fever resulting from industrial exposure to magnesium oxide fume has been found. Symptoms of metal fume fever include; cough, tightness of chest, sweating, headache, fever, muscle aches, nausea, vomiting and tiredness. //   

       Also the LD50 is //Not available//. Remember, everything is toxic, it's just a matter of quantity. I'd be very careful about declaring MgO safe without a lot more research.
MechE, Oct 07 2013

       Yeah, pretty sure the whole alzheimer's fear with Al has been thoroughly debunked. Considering the enormous ammount of Al2O3 in it's various hydrated forms that we encounter in our daily lives, as well as in our soils, rocks, etc - we're already immersed in the stuff, if it's bad, we're already getting the full effects.   

       ..and I'm pretty sure Al would be highly competitive in terms of energy density.   

       The obvious choice here is a small steam turbine engine.
Custardguts, Oct 07 2013

       Well, if you want to play with aluminum, the place to start is with numbers like these:
Energy released when AL2O3 forms: 1669.8 kilojoules/mole
So, about 3 moles of burned aluminum needed to match the energy of one mole of burned octane.

       The density of aluminum is about 2.700 kilograms/liter, rather more than that of magnesium.   

       [MechE], I didn't say that magnesium oxide was super- safe, only "pretty safe". I will admit I didn't think about other methods of getting MgO internally, besides direct ingestion.
Vernon, Oct 07 2013

       // I didn't say that magnesium was super- safe, only"pretty safe" //   

       Ahhh, if only the Luftwaffe had known that in 1940 … they might not have been so tempted to drop so much of it on London.   

       Not that that was necessarily a bad thing …   

       [Vernon], do you have any direct personal experience of the behaviour of powdered metals ? Microcrystalline powdered Aluminium - theatrical "Flash Powder" is nasty enough, requiring precautions such as ESD grounding, anti-flash garments, and "standing well back and letting the trainee ASM do it".   

       Have you wondered why there's a commercial explosive called Ammonal, but not one called Ammomag ?   

       When did you last see your eyebrows ?   

       We assert that powdered Aluminium, in anything more than gram quantities, is very bad and dangerous, requiring careful handling. Octane fires can be suppressed with foam or dry powder … powdered metal fires are a trifle more challenging. If you doubt us, try it.
8th of 7, Oct 07 2013

       Sulfur +Water open to atmosphere? Sulfur Dioxide formation: 296.84 kilojoules per mole.   

       If then combined with additional O2 and run through a catalyst (ie:V2O5) to form Sulfur Trioxide, combining it with water will form Sulfuric Acid releases 817kj/mol; further dilution with water is also highly exothermic.
acurafan07, Oct 07 2013

       The downside of that is the exhaust is highly corrosive. Burning sulphur in an internal- combustion process will have detrimental effects on lubricants and bearing surfaces. An external combustion cycle, like [Custardguts] steam turbine is going to be heavy and bulky, and you still need a way to reject waste heat (and entropy).   

       A Sterling engine using hydrogen as the working fluid would offer the best energetic solution, but implementation is problematic, as hydrogen motors are still in the lab.
8th of 7, Oct 07 2013

       If anyone seriously believes that they can fuel a viable car with moles, I'm in.
MaxwellBuchanan, Oct 07 2013

       [8th of 7], nowhere in the main text do I specify that the magnesium used is in powdered form. The waste, however, magnesium oxide (quite non- flammable), can be in whatever form is the easiest to dump. (But I do see I left out a word --"oxide"-- in my last anno (fixed).
Vernon, Oct 07 2013

       MgO is a useful commodity although it isn't particularly valuable $400-$800 / tonne depending upon purity. I suspect that you would end up with Mg.2H2O magnesium hydroxide because all it takes is a little bit of moisture. The hydroxide form is less valuable still but some farmers with high sodic but low magnesium soils like here in Oz spread it on their soils to reduce a grazing animal condition called grass tetany.   

       If you can recover the MgO form there are some interesting cements which can be made from it with high tensile, rather than compressive, strength and can act as a carbon sink as well.
AusCan531, Oct 08 2013

       But if you wanted to make fuel your best bet is a methogen or carbide of aluminum or calcium or some metal that makes fuel upon reaction to water. Though I do not know if larger hydrocarbons could be made with something such as organic acids such as vinegar.
travbm, Nov 01 2015

       [travbm] calcium carbide is well known as producing acetylene as well   

       I recently read about boron based zip fuel [link], branched hydroboranes, much research yet the boron microparticles (possibly boron nitride) cause wear   

       [vernon]s idea is that well, hmmm, toss the reaction products to be mass efficient at the moving vehicle   

       perhaps something could be amplified out of context with potassium metal with liquid chlorine. potassium chloride is kind of a nutrient, the energy of their combiantion might be vast.   

       I suppose on oceanic things you could use sodium metal with chlorine gas, then use nuclear to just turn the ocean back to the separate component fuels   

       this idea actually makes all gaseous products from octane look ultra beneficially convenient, particularly with catalytic converters co2 n water are quite mild.   

       [vernons] magnesium version though would cause white atmosphere global cooling!
beanangel, Nov 02 2015

       thermite, binary storage.
FlyingToaster, Nov 02 2015


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