Re: [JFA2840@idahopower.com: Ammonia from electricity]



Note: I'm CC:'ing this to the floating-cities list, as one of the major
industries of a floating city will probably be ammonia production. Also,
floating cities will be able to easily use the great pressures and cooling
capabilities of the ocean to help along the process in an economical way.

If there are any chemists reading this, please check my math, as I'm still
learning this stuff. 

-Date: Fri, 01 May 1998 09:15:18 -0600
-From: Jim Ashworth <JFA2840 at idahopower dot com>
-Reply-To: JFA2840 at idahopower dot com
-Organization: Idaho Power Company
-To: salsbury at sculptors dot com
-Subject: Ammonia from electricity
-
-Could you tell me who I can talk to about comparing the cost of
-producing ammonia from natural gas verses using a hydrogen generating
-fuel cell powered by electricity through electolysis.
-jashworth at idahopower dot com
-
	This is a good question, and as I dig through my chemistry book,
("Chemical Principles" by Steven S. Zumdahl, 2nd Edition, (c) 1995 by
D.C. Heath and Company) I'm not positive of the answer. I would think, on a
gut level, that natural gas would win out on this one, but there seem to be
a bunch of variables.

	First off, a fuel cell doesn't produce hydrogen, it consumes
it. (Combining with oxygen to produce electricity and H20.) Using a fuel
cell to run an electrolysis reaction will get you hydrogen, but the
electrolysis reaction isn't terribly efficient, so production may be lower
than other methods could yield. 

	As I understand it, the ammonia production reaction, as done 
in the Haber process, requires both a great amount of heat, and of
pressure. The reaction is carried out at about 250 atm of pressure, and
about 400 degrees C, and this is even while using catalysts. (It would
require higher temperatures, otherwise.)

	When it comes to heat generation, methane wins out over
electricity. Burning methane is much more efficient than heating a resistor
coil with electricity.

	Interestingly enough, the chem book discusses methane combustino on
pages 364-366 (Standard Enthalpies of Formation).

	Combustion of Methane (with Oxygen) produces Carbon Dioxide and
liquid Water:
	CH4(g) + 2O2(g) -> CO2(g) +2H2O(l)

	They then trace through the 4 parts of the above reaction,
determining the amount of energy released by the whole process. It comes
out to -891 kJ of energy, which is a fair amount of heat.

	Ammonia is made by combining nitrogen from the air with hydrogen
according to the equation: 

	N2(g) + 3H2(g) -> 2NH3(g)

	The standard enthalpy of formation of ammonia is -46 kJ/mol, so it
releases 46 kJ of energy for every 17.034 grams of NH3 produced. The above
reaction would release 96 kJ, because you're producting 2 moles of ammonia at
the end. 

	Now using CH4 as a hydrogen *donor* to create the raw materials for
NH3 production is a different question, but I think that methane would win
over electrolysis there, too. 

	The hydrogen can be (and apparently usually is) produced from the
reaction of methane with gaseous water: 

	CH4(g) + H2O(g) -> 3H2(g) + CO(g)

	The standard enthalpy of formation of this reaction is 206.5
kJ/mol, so it requires 206.5 kJ of energy to do this, giving 1 mole of
carbon monoxide (28.01 grams) and 3 moles of hydrogen gas (6.048 grams). 

	So it would seem that by burning the methane, we'd get lots and
lots of heat, which we could use to keep the ammonia production reaction at
high temperature (although ammonia production also seems to release heat,
so we may be able to get a sustaining reaction, I'm not sure about
this...) Also, using the methane mixing with water will give us lots of
hydrogen, provided we can give it the excess heat from the other
reactions. 

To compare with electrolysis, the net reaction is: 
	6H2O -> 2H2 + O2 + 4(H+ + OH-) 
		with an electrical potential of -2.06 Volts (V)
or:	2H20 -> 2H2 + O2
			
	That means (if I understand my book correctly) that we need to put
2.06 V of electricity into the water to force the electrolysis reaction. 

	You're still left with "cost" of electricity vs. "cost" of methane,
and that may be determined by whether we have huge banks of electrical
generating capacity, huge bio-waste reactors to produce methane, or both. I
don't have a clear answer for that.

	Another thing to consider is that many or all of the "waste
products" from these reactions are desirable factors for other
reactions. The "waste heat" from combusting methane can be used to drive
other endothermic reactions, while the CO2 can be pumped into greenhouses
to enrich the atmosphere for the plants, while the H2O can be collected for
drinking. The H2 and O2 produced by electrolysis can be used in fuel cells,
as could the H2 from heating methane with water, although the carbon
monoxide would need to be further combusted to make it CO2. 

	The electricity required to drive the electrolysis can be produced
by wind, solar, or fuel cells (or even wave activity!) and the "byproducts"
of the electrodeposition process for making hulls for a floating city give
off the same H2 and O2. It's electrolysis, but done in seawater, we're able
to extract valuable minerals from the ocean which we can use in structure. 

	It all fits together amazingly well, doesn't it? :-)

-- 
Pat
	   ___________________Think For Yourself____________________
	   Patrick G. Salsbury - http://www.sculptors.com/~salsbury/
      Check out the Reality Sculptors Project: http://www.sculptors.com/
	   ---------------------------------------------------------
One of these days, the world may get used to the fact that there is
a wide variety of people around.  It's been an awfully long time in
coming, and personally Miss Manners can't wait.   --Judith Martin




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