From: dietz@cs.rochester.edu (Paul Dietz)
Newsgroups: rec.autos.tech,sci.environment,sci.chem
Subject: Re:  Hydrides
Date: 9 Sep 92 15:28:54 GMT
Organization: Computer Science Department University of Rochester

In article <1992Sep9.082509.4474@bb1t.monsanto.com> bjgaed@bb1t.monsanto.com writes:

> The "hydrides" you are speaking of are *transition metal* 
...
> I don't remember the exact numbers, but the metal sponges 
> can hold a surprising amount of hydrogen, more than their 
> own weight, I believe.

No way.  With transition metal hydrides, the weight
of hydrogen is only a few percent of the weight of the
metal.


> To burn fossil fuel to make electricity to make hydrogen 
> from water to put into an internal combustion engine is just 
> plain crazy.  The generating station may run at 35% 
> efficiency, then a 10% transmission loss, then you throw 
> away the energy that goes into the oxygen when you make 
> hydrogen from water, then you have to compress and transport 
> the hydrogen, then burn it in an engine at about 30% 
> efficiency.

This *is* crazy, because it is more efficient and economical to make
hydrogen from fossil fuels and water by *chemical* means.
Specifically, the reaction

    C + H2O -->  CO + H2

followed by

    CO + H2O --> CO2 + H2.

This is, overall, mildly endothermic; the heat input can be supplied
by burning some of the fossil fuel, or by some other high grade heat
source (nuclear, say).  Similar reactions involving hydrocarbons
(specifically, methane) are the primary source of industrial hydrogen
today.

I have no idea what you mean by saying that energy "goes into the
oxygen".  The energy is a property of the combination of the two
products; it is not allocated some to each.

Finally, hydrogen is not going to make much sense in cars unless used
in fuel cells.  It's just too expensive and low in energy density
otherwise.


> automobile engine.  The generating station can achieve much 
> more complete combustion, thus eliminating much of the 
> hydrocarbon and carbon monoxide problem, but the carbon ends 
> up as carbon dioxide no matter what.  If it turns out we do 
> have to control carbon dioxide emissions, the best way to do 
> it in this scenario is to burn the fuel *in the car* because 
> you don't have to burn 3-5 times as much to make up for all 
> of those losses and inefficiencies.

No, because if you convert the fossil fuel at a stationary plant, the
CO2 can be potentially scrubbed out for sequestration, either in
depleted gas wells or via deep sea disposal.

This is rather difficult to do with cars, but a hybrid system has been
proposed.  The idea is to burn both gasoline and hydrogen, with the
hydrogen stored as magnesium hydride.  Water of combustion is
circulated back into the hydride, producing additional hydrogen and
magnesium hydroxide.  CO2 from gasoline combustion is scrubbed out
using the hydroxide, making magnesium carbonate.  The carbonate is
recovered at refueling time and refined back to the hydride, with the
CO2 removed and sequestered.  I doubt this would be, in the end,
either economical or feasible, but it's at least thought provoking,
and has much higher energy density than a pure hydride system.

	Paul F. Dietz
	dietz@cs.rochester.edu