Self-Sustaining Solar Reactor Creates Clean Hydrogen 406
An anonymous reader writes "A mechanical engineer working out of the University of Delaware has come up with a way to produce hydrogen without any undesirable emissions such as carbon dioxide. The solar reactor is capable of using sunlight to increase the heat inside its cylindrical structure above 3,000 degrees Fahrenheit. Zinc oxide powder is then gravity fed through 15 hoppers into the ceramic interior where it converts to a zinc vapor. At that point the vapor is reacted with water separately, which in turn produces hydrogen. If the prototype gets through 6 weeks of testing at the Swiss Federal Institute of Technology located in Zurich, we could see it scaled up to industrial size, producing emission-free hydrogen."
Re:so... (Score:2, Informative)
That's what I was thinking.. What's the carbon output of obtaining, refinement and purification of the Zinc Oxide?
Re:Darn that dirty hydrogen (Score:5, Informative)
One reason storing it is such a big deal is because generating it can be expensive. Make hydrogen easier to produce and it lowers the demands on storage.
Re:so... (Score:5, Informative)
TFAs: Read one today!
Re:Darn that dirty hydrogen (Score:5, Informative)
Re:But is it really emissions-free? (Score:5, Informative)
You burn zinc metal. Really. The zinc oxide and tower are not the interesting part. That is simply an alternative method of smelting a source of zinc to obtain zinc metal.
The deeper linked articles say "the hoppers will feed zinc oxide powder (a benign substance resembling baking soda) onto the ceramic layer, causing a reaction that decomposes the powder into pure zinc vapor. In a subsequent step, the zinc will be reacted with water to produce solar hydrogen."
Ok.
Zn(s) + 2H+ -> Zn2+(aq) + H2(g)
but
Zn2+ + 2OH- -> Zn(OH)2(s)
So the water that's left over will contain a zinc hydroxide particulate (or sludge).
The zinc hydroxide is an emission. Might be better than a gaseous emission, but it's still a waste product. If this system is truely closed with respect to zinc, then the zinc hydroxide has to be converted into zinc oxide or somehow directly smelted back into zinc vapor. That's the missing element from the article in my opinion.
Other questions: how fast is the aquoeous reaction (toss zinc in a glass of water -- it's slow at standard temperature and pressure); what is the equilibrium pressue of H2 above the liquid (if it's a low partial pressure, then you need to both maintain a vacuum over the liquid and compress the drawn-off gas); what is the net energy output of H2 versus the input of heat (assuming that you close the system with respect to zinc by drying and converting the sludge back to zinc metal).
Re:But is it really emissions-free? (Score:5, Informative)
Nope. Zn(OH)2. You have to do something else to convert the hydroxide into an oxide.
I agree that you can't simply condense the Zn vapor into a liquid or solid. In normal thermal smelting the metal is chemically reduced to draw off the oxygen using a reducing agent such as carbon monoxide. At very high temperatures, you can force a metal oxide to form a plasma of dissociated ions, but as you indicated something has to draw off or separate the oxygen, and something also has to donate electrons to the zinc ion plasma. Might be a set of high temperature electrodes?
Re:Darn that dirty hydrogen (Score:5, Informative)
You start with zink oxide. Apparently (not a chemist here) you de-ogygenate it via heat making zink vapor (releasing O2, which is vented) and that zink vapor grabs oxygen from the water, leaving you with your H2 product, and a clean supply of Zink Oxide again.
The byproduct is Oxygen.
Re:Darn that dirty hydrogen (Score:5, Informative)
Electrolysis tends to eat away at the materials causing you to have to get more. This is fairly closed loop in that the only ingredient is water and heat. Instead of mining zinc for the rectors that will run indefinitely (supposedly) you would have to continuously mine the metals used for electrodes in electrolysis. Cleaning and replacing these parts can be "dirty" where this reactor seems to eliminate that component. The only thing I think you'd have to do with this is grind the zinc back into powder. (as I assume it probably melts and coagulates?)
Re:But is it really emissions-free? (Score:5, Informative)
Just to be clear, chemically this is not hard:
Zn(OH)2 -> ZnO + H2O at about 800 C (this is a calcination reaction)
It's a materials handling issue. Dewatering a sludge, drying a dewatered sludge, and, if necessary, calcining the zinc hydroxide separately from forming the zinc metal, all involve some technically complicated additional steps.
Re:so... (Score:5, Informative)
The recovery could be pretty close to 100%; the reactor's only products are oxygen and hydrogen, both of which are gases, so capturing zinc should be simple enough.
Zinc is usually found in conjunction with other metals like copper, so we get most of it "for free". Zinc oxide is actually a lot easier to produce than pure zinc, so refinement costs should be relatively low. The most common ore of Zinc, Sphalerite, is ZnS, and converting it to ZnO just involves adding oxygen and heat:
ZnS + (3)O2 = ZnO + SO2
The sulphur dioxide can be converted to sulphuric acid (H2SO4).
No carbon involved.
Besides, we already use >10 million tons of the stuff per year, and have at least a couple centuries more deposits to mine (to say nothing of recycling), so using a bit for this solar plant wouldn't even be noticed.
Re:Darn that dirty hydrogen (Score:5, Informative)
The biggest hurdle would seem to be infrastructure. It's catch 22, mass production and distribution of H2 requires a H2 market to sell to, and vica-versa. Petrol did not really have this problem, the first generation of car owners bought their fuel in cans from the local pharmacy. The car and car fueling infrastructure evolved together, by the second generation of car owners we had two new major industries led by companies such as Ford and Standard Oil.
So here we are in the 21st century and FF transport is ubiquiotous, the no way a competing technology such as H2 will never gain a foothold with current market fources. It would have to be a cooperative effort between government and industry to deliberatly kill off FF cars, that's already happened with lightbulbs but transport is a much bigger challenge and (for some people) it beccomes as personal as a cowboy's horse.
Re:Darn that dirty hydrogen (Score:4, Informative)
The leaks may be dangerous, but a hydrogen leak simply escapes to atmosphere and dissipates. A gasoline leak collects on the ground and acts as both a poison if you touch it and worse if the stuff finds a source of ignition. It also pollutes ground water, streams, etc.
Dump out a gallon of gasoline and a gallon of hydrogen and see which one causes a bigger problem.
Re:But is it really emissions-free? (Score:2, Informative)
The ZnO [wikipedia.org] bond is primarily ionic. It is generally insoluble in water, but it is most certaily a salt [wikipedia.org].
And Zn when added to water will most certainly form zinc hydroxide [lenntech.com], particularly when powdered or added as a vapor. It may not form zinc hydroxide in supercritical steam above 800 C, but you did not specify that and I clearly referred to aqueous systems. Water spontaneously disscociates to yield the hydronium and hydroxide that forms the zinc hydroxide skin on bulk zinc metal. It's not a rapid process, as I already suggested, but your blanket statement is wrong.
Re:Darn that dirty hydrogen (Score:5, Informative)
Re:Darn that dirty hydrogen (Score:5, Informative)
Hydrogen exceeding ~4.5% in air is explosive, so a slow leak in a ventilated area just escapes into the atmosphere. A faster leak, or a poorly ventilated area presents a tremendous explosion potential. Remember the reactor buildings in Fukushima? Those were from hydrogen building up inside the building.
Re:Darn that dirty hydrogen (Score:4, Informative)
Re:Sustainable? Not really. (Score:4, Informative)
Re:Darn that dirty hydrogen (Score:4, Informative)
Note that while it has the highest energy by mass, by volume it's actually pretty low, though it does beat LiIon. This also doesn't count the vessel needed to contain it - not a big deal for a massive cylinder at a factory, but it's a substantial mass/volume hit to include a 700 bar pressure vessel in a car.