[FoRK] Magnesium / Water cycle hydrogen production

J. Andrew Rogers andrew
Tue Oct 25 22:27:38 PDT 2005


On 10/25/05 9:50 PM, "Kevin Elliott" <k-elliott at wiu.edu> wrote:
> You can't put a metal fire out with
> water.  The metal puts off so much heat so fast that you can't push
> enough water onto it to pull enough energy out of it to stop the
> reaction. 


There is a more serious problem.  The issue is that once the burn gets
going, the metal component that makes up the vast majority of thermite
compositions (typically aluminum, but sometimes magnesium) will burn water
as though it were an oxidizer and give off goodly quantities of hydrogen gas
that is prone to exploding when mixed with the air and near a heat source
(say, white hot thermite slag).  It is very, very difficult to put out a
thermite reaction once it starts, and your best bet is to contain it.


> Check out the thermodynamic tables.  The
> reaction energy is impressive to say the least.


The reaction energies are not particularly unusual.  The value of thermite,
in the places it is used is that most of that thermal energy concentrates in
the thermite mixture since it produces little gas or other high entropy
energy dumps.  It stews in its own exothermic output, producing the
extremely hot (~2500 Celsius), dense metallo-ceramic liquid that can eat its
way through most materials by virtue of the fact that the slag retained most
of the reaction energy rather than pissing it into the environment.


> It's interesting to consider an aluminum fueled car were the rust got
> a little TOO pervasive...


You would get better mileage with gasoline.


J. Andrew Rogers




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