Ammonia (NH3) is made of Nitrogen, Hydrogen and one other magic ingredient - energy.
In the Haber Process the reaction is exothermic (it produces heat) :
|
N2 + 3H2 + 450 °C + 200 atmospheres => 2NH3 + 92.22 kJ |
because neither is found in pure form in nature.
Hydrogen is currently made from fossil fuels
but it can also be made by electrolysing water,
although that reaction is endothermic (it requires energy) :
|
2H2O + 571.6 kJ => 2H2 + O2 |
That is, more than six times as much energy is needed to make the Hydrogen as is recovered in the Haber Process.
Nitrogen is obtained by the fractional distillation of liquefied air,
that is, you cool air to -196 °C to liquefy it,
then raise the temperature to -184 °C so that the Nitrogen boils off leaving the Oxygen behind.
For every 1Kg of Ammonia you need to cool 1.1 Kg of air, containing 0.82 Kg of Nitrogen, by about 220 °C.
The resulting Nitrogen has to be purified using hot Copper to remove any traces of Oxygen,
and a dehydration agent like P4O10 to remove water vapour.
(The Copper and the P4O10 requires energy to make .... )
So making Nitrogen takes a lot of energy - I'll spare you the detail.
As you can see, the energy needed to make Hydrogen and pure Nitrogen out of water and air
is way more than the energy given off during the Haber Process,
so Ammonia is an energy intensive product.
This is quite aside from all the energy that gets wasted in the form of heat or cold leaking out of the system,
and friction in the pumps, and resistance is the wiring, and ....
Ammonia itself is not very suitable as a fertiliser,
being a gas that is lighter than air.
Ammonia dissolves in water to produce Ammonium hydroxide,
which is strongly alkaline (caustic), so the plants won't like that for very long.
What we need is a neutral compound, a salt such as Ammonium sulphate or nitrate.
Ammonium nitrate is very soluble in water so gives a fast acting hit of Nitrogen to the plants
(organic farmers don't like these strong hits, but agro-industrialists do)
It is made by reacting Ammonium hydroxide with Nitric Acid :
|
NH3 + H2O => NH4.OH NH4.OH + HNO3 = > NH4.NO3 + H2O |
But where does the Nitric Acid come from ?
It is commercially prepared by the Ostwald Process using Ammonia and Oxygen :
|
4NH3 + 5O2 + 1000 °C + Platinum catalyst => 4NO + 6H2O 2NO + O2 => 2NO2 3NO2 + H2O => 2HNO3 + NO |
Once again, the high temperature and expensive catalyst indicates that this process is going to use a lot of energy,
partly to rearrange the atoms into new molecules,
and partly as waste heat leaking from the hot reaction.
The acid is very corrosive towards metals (and humans),
so the capital equipment needs to be cleverly designed.
But where does the Oxygen come from ?
Well if you were smart, you could have kept the Oxygen from the electrolysis of water that gave us our Hydrogen.
Or you could have kept the Oxygen from the liquefied air that gave us our Nitrogen.
Or you could heat air (3 parts Nitrogen and 1 part Oxygen by weight) to 500 °C
and then zap it with a high voltage electric arc,
to make the Nitrous Oxide (NO) directly, and skip the first step above.
In effect, Ammonium nitrate is water split into Hydrogen and Oxygen,
with the Hydrogen attached to a Nitrogen,
and the Oxygen attached to another Nitrogen.
(There is actually an extra Oxygen atom in there,
hence Ammonium nitrate is called an oxidising agent).
So all you need to maintain the Nitrogen compounds in the soil into the indefinite future is water, air AND ENERGY.
Of course there is more to fertiliser than just Nitrogen compounds,
there is also Phosphorus, Potassium, Calcium, etc etc.
and they can all be made out of other things so long as you have enough ENERGY.
Its a pity we are running out of cheap energy,
because that means we are running out of cheap fertiliser
and that means we are running out of cheap food,
and all the poor people are going to starve.
Well, actually the poorest are starving already.
Malthus was not wrong when he said that population would outstrip the food supply,
although he made his argument sound as if it was very mathematical,
when if fact he was just estimating (see http://home.austarnet.com.au/davekimble/malthus.htm).
The "Green Revolution" saw the world's population soar on the back of a huge increase
in the use of fossil fuels to make fertiliser, to drive tractors and to pump water.
When Peak Oil kicks in and fossil fuels become much more expensive,
agricultural products will have to cost much more or farmers will rapidly go broke.
Either way, the world's population has overshot its sustainable level
and is certain to collapse well within this century.
Dave Kimble April 2006
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