NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/rec/autos/tech/12385

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From: [e-mail]
Newsgroups: rec.autos.tech,sci.environment,sci.chem
Subject: Re:  Hydrides
Message-ID: <[e-mail]>
Date: 9 Sep 92 08:25:09 CST
Organization: Monsanto Company, St. Louis, MO
Lines: 126

[e-mail] (Herman Rubin) writes:

>>In article <[e-mail]> [e-mail] (Michael George Buening) writes:
>>>In article <[e-mail]> [e-mail] (Kevin Carothers) writes:
>>>>In article <[e-mail]> [e-mail] (Craig MacFarlane)
writes:

            ......................

>>I wonder how they liberated the hydrogen from the metal hydride in a reasonable
>>fashion.  In the Merck Index, the following is stated about sodium hydride :
>>     "Reacts explosively with water, violently with lower alcohols, ignites
>>      spontaneously on standing in moist air."
>>For lithium hydride, the index states:
>>     "No solvent known.  Rapidly dec in water to form lithium hydroxide
>>      and hydrogen.  Reacts with the lower alcohols, carboxylic acids,
>>      chlorine, and ammonia at 400 oC to liberate hydrogen."
>>There are also the corresponding metal borohydrides which are more stable
>>on the shelf but are not as reactive.  These things must be stored in
very
>
[Fearsome stuff about borohydrides deleted]

The "hydrides" you are speaking of are *transition metal*
hydrides.  These have the happy property of binding hydrogen
very weakly as hydride.  Simple gas pressure is enough to
force the hydrogen into the metal (which is usually in the
form of a metal sponge), and the release of that pressure
and heating (usually with waste heat from the exhaust) is
enough to reverse the reaction and allow the hydrogen to
come back out.  Because of the nature of the metal sponge
this release is much slower than if you, say, opened the
valve of a hydrogen tank under the same pressure, and if
there is no constant supply of heat the release would
probably stop.  Only in a fire would you have a problem, and
a tank of gasoline is just as much of a problem in that
situation.    We could
go into a long debate on whether the hydrogen really
undergoes reduction to hydride, or whether molecular
hydrogen is really adsorbed on the surface of the metal
sponge, but I don't think you folks would be all that
interested.  Suffice it to say that the hydrogen is bound in
a form that will not undergo catastrophic release if the
tank breaks or is shot at by terrorists or myopic deer
hunters.


>There is also the amount of hydrogen in the hydride.  A gallon of gasoline,
>which is a carbon hydride, is about 16% hydrogen.  Methane is 25% hydrogen,
>and is the highest by weight of any hydrogen compound, and with hydrocarbons,
>it is not necessary to use the packaging material to reform the hydride.
>How much of the energy of com combustion comes from the carbon and how much
>from the hydrogen?  LiH is 14% hydrogen, NaH is 4% hydrogen, boron hydrides
>are 8-20% hydrogen with most on the low side, and except for beryllium (nasty)
>and nitrogen hydride (ammonia gas, not too desirable), only aluminum and
>silicon hydrides even reach 10% hydrogen.

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.

>Also, it is a major problem to get solids conveniently into fuel tanks,
>and the fuel to be burned must either be gaseous or in the form of droplets.
>So one must have some method of getting the hydrogen gas out of the hydride,
>and of course a method of recharging the hydride.  Of the ones listed, only
>boron seems to be a practical method, if indeed it is.  Remember that a
>filling up of 10 gallons of gas is about 60 pounds, which is about 10
>pounds of hydrogen and 50 pounds of carbon.

As you pointed out before, the "50 pounds of carbon" is not
such a bad fuel itself, being burned to (ideally) carbon
dioxide.

I didn't catch how this thread got started, but the point of
using hydrogen fuel is that the combustion product is only
water.  No toxic gasses, no "greenhouse" gasses, no
hydrocarbons to contribute to smog and ozone.  About the
only pollutant to be expected *from the automobile* would be
nitrogen oxides, and those can be largely controlled by
adjusting the combustion temperature.

I emphasize *from the automobile* because so often one sees
media hype about "clean" electric cars or hydrogen-burning
cars.  Well, neither electricity nor hydrogen occur in a
recoverable form in nature; they must be manufactured.  And
they must be manufactured from either fossil fuel, nuclear
power, or some other energy source.

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.  I don't have time to work out the numbers, but
it is very clear that you would be much better off
air-pollution-wise to just burn the fossil fuel in the
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.

Nuclear energy does not emit carbon dioxide, but it has
other problems of which I am sure everyone is aware.  Still,
IMO, if we do have to limit CO2, it is the only short term
choice available.

In the long run, however, it is the "other" energy sources
that show the most promise.  If we can generate hydrogen
from sunlight and water at any sort of efficiency at all, we
will have a good shot at achieving a truly non-polluting,
very long-term (almost inexhaustible) energy source for
vehicular transportation.  Wind power, tidal power, etc.,
also are possibilities.

--
--Electric Monk            (Bruce Gaede);
                            e-mail: [e-mail]

"...and then time started seriously to pass."
--Douglas Adams, _Dirk Gently's Holistic Detective Agency_