Date: Tue Apr 13 14:08:03 1999
Posted By: Glen Hutson, Grad student, Geographic Information Systems, University of Western Australia
Area of science: Chemistry
Hi there ,
I noticed in my research for this answer that Urea has become
a popular fertlizer because it contains more available nitrogen
per unit weight and hence saves greatly on transport costs.
This got me thinking that you might want to consider more than
just the consumption of Natural gas in terms of an energy balance
I'm reading between the lines that you are seeking to determine a
cradle to grave analysis of the food production cycle ...
Here are some other factors to think about I'm sure I've missed some.
Some Other Considerations
Cost of transport ( energy )
Cost of plant & production
Sub production units ( catalysts etc )
Energy budget of plant ( yeilds and efficiency )
Cost of calculation ( true energy costs of calculations )
Bagging and storage
In farm losses
Anyhow , lets consider the Fertilizer production :
The natural gas ( methane ) is used in the production of Hydrogen Gas
usually by the Industrial Process known as "Steam Methane Reforming"
( there are others including byproducts of petroleum processing )
CH4 + H2O > 3H2 + CO
Here are some notes I paraphrased and the WEB references, you could read.
Steam-methane reforming is the most common method of producing hydrogen
on-site.Steam-methane reformers (SMR) can produce H2 in quantities
of 20,000 scf per hour and up, at purities up to 99.999%.
SMR uses a multi-step process to crack natural gas and water to create
NG Feed is compressed before entering plant.Approximately 75% of NG is
used for reaction(feed) and 25% for heating (fuel).
Sulfur free NG is mixed with steam as it enters catalyst tubes. Tubes
contain nickel on alumina catalyst. The NG react with steam forming H2
80% of the heat required for the reaction is provided by burning waste
gas from the purification unit and 20% from the NG fuel stream. The
reformed gas exiting the catalyst tubes consists of: H2, CO, CH4, H2O,
and inerts (dependent on feedstock).
Water Shift Reaction Process gas and excess steam from the SMR
catalyst tubes is cooled and sent to the shift converter, where it is
catalytically reacted with additional water. CO will react with H2O to
form CO2 and H2.
Impurities and some hydrogen are recovered in a wastegas surge tank
The waste gas is sent to the reformer burner to provide additional heat
So Methane is combined with steam to produce Hydrogen & Carbon Monoxide.
If we assume no losses and a 100% yield from this process we can assume
that 3 moles of hydrogen gas is produced for each mole of Methane feed.
In the Haber process for the production of Ammonia we are driving the
N2 + 3H2 <> 2NH3
so we can assume our 3 moles of H2 produce 2 moles of ammonia ...
I have ignored catalysis , yeild losses and the energy costs
( since the Haber process is usually conducted at 200atm & 500C)
you can find lots of general references about the HABER process.
The Ammonia is used to neutralize Nitric acid with one mole of
Ammonia producing one mole of Ammonium Nitrate : according to
HNO3 + NH3 > NH4NO3
So lets assume 1 mole of Methane feedstock produces 3 moles of H2
and those 3 moles produce 2 moles of Ammonia, producing 2 moles
of Ammonium Nitrate
Hence: 1 mole of methane produces 2 moles of Ammonium Nitrate
Now I know that at least 25% of the feed stock of methane in the H2
production is used for heating so lets reduce our yeild by say 25%
Now I'll just work out some masses ..
1 Mole CH4 > 1.6 mole NH4NO3
So: 100g of Methane Produces about 1Kg of Fertilizer a weight ratio 1 : 10
So 1 pound of Natural Gas produces about 10lb of Fertilizer
Since 150lbs of fixed nitrogen is needed to produce 150 bushels of
corn per acre ( I haven't checked your figures ) your 150 Bushels of
corn consumes 15lbs of Natural Gas. ( Probably )
My conditional answer is based on my ignorance of the losses in the
systems and the particular processes used (I have chosen typical ones)
I know that Steam Reformation is popular in the US, but possible
processes down the lines are numerous … I think we can safely assume
the 1:10 ratio is typical …
It would be most interesting ( and gain you a PhD ) to produce a
complete energy cycle for food production ( Say Corn ) evaluating
energy consumed by production versus energy liberated in the Biological
process of consumption. It would be a very complicated exercise. One
Good Luck … Glen ..
Reference : Chemical Properties and Reactions ( Cole Watts Bucat )
School of Chemistry , University of Western Australia.
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