22 October 2005

A Quick Swipe at "Free" Hydrogen

I am busy with my thesis and marking at the moment but rather than issue corrections to my transport posts I just wanted to take a quick swipe at some specious hydrogen production claims.

A company called Alternate Energy Corporation claims to have some process that reacts water with metals to produce hydrogen gas. A quick browse of their website should set off alarm bells for anyone -- the distinct lack of any details on their system for example. Over at theWatt.com we've found some references to systems that burn aluminium to produce hydrogen. Aluminium won't react with water; you must introduce some acid or base to make the process run. Still, let's just imagine that we have the following irreversible reaction with zero enthalpy.

2 Al + 3H2O → Al2O3 + 3 H2

Given that you input 2 moles of Al for 6 moles of H it would require 9 kg of Aluminium to produce 1 kg of hydrogen. But the manufacture of aluminium is extremely energy intensive!!! Running the Aluminum Association's numbers I come up with 147.7 MJ/kg. This means you need 1329.3 MJ to produce one kilogram of hydrogen with an energy content of 142 MJ. This gives me an ERR of about 10 %. Whee.

If I ran a similarly fictitious reaction with iron I would need 18.6 kg of iron to make a kilogram of hydrogen. You can guess where this is going.

Now if we just burned the aluminium with oxygen in air... (we would have rocket fuel)


GRLCowan said...

It's not quite correct to say aluminum won't react with pure water. Put a piece of it under water and scratch it with a file. The reaction doesn't go very far before the corundum film stops it, but you do get bubbles. As I recall they were too small to detach spontaneously but could easily be shaken loose.

Burning aluminum in water yields 57 percent as much heat as burning it in oxygen. All that 57 percent is wasted if all you use is the freed hydrogen.

Burning aluminum in pure oxygen would be an interesting way of getting vehicle heat engine heat, but boron is better. (It really won't react with water at or near room 'T', and a file won't do for scratching it, unless for some reason you have a diamond file.)

--- Graham Cowan, former hydrogen fan
boron as energy carrier: real-car range, nuclear cachet

Robert McLeod said...

Heh, I really wasn't lying. You need a non-neutral pH to react water with aluminium. You can burn either protons or hydroxyl:

2Al(s) + 6H(+) → 2Al(3+) + 3H2(g)


2Al(s) + 2OH(-) + 6H2O → Al(OH)4(-) + 3H2(g)

It is pretty obvious that because of the extremely low atomic weight of hydrogen you need to find a similarly low atomic number atom to carry it. Of all the low z elements I happen to think that z=6 is the best hydrogen carrier.

GRLCowan said...

The sixth element is a good hydrogen carrier, but it is not obvious that large amounts of hydrogen will continue to need to be carried, by any means at all, except in the cabin.

If those ionic reactions occur, this would suggest an oxide-free aluminum surface, in neutral water, would react with the autoionization-derived solvated protons and the autoionization-derived hydroxide ions, both; this would explain the bubbles.

--- Graham Cowan, former hydrogen fan
boron as energy carrier: real-car range, nuclear cachet