|
An energy currency is a means of reserving and transmitting energy and
anhydrous ammonia (NH3) is the ideal energy currency for green
electricity. In the human body, ATP (Adenosine TriPhosphate) is
often considered a biological energy currency because it can either be
consumed or stored, depending on the body’s needs. With respect to
renewable power, an energy currency is basically the ability to put
electricity into a bottle so that it can be stored when there is a surplus
and used with there a shortage.
Using ammonia as energy currency is essentially what the US Army was
trying to do with their Mobile
Energy Depot program and it considered hydrogen (H2), ammonia (NH3),
hydrazine (N2H4), and hydrogen peroxide (H2O2) – all compounds of
hydrogen. The authors of THE
THEORY OF OPERATION OF AN AMMONIA BURNING INTERNAL COMBUSTION ENGINE
(Charles G. Garabedian and John H. Johnson) wrote “Factors concerned with
physical and chemical properties, handling, storage, and dispensing of the
four fuels led to the choice of ammonia as the fuel with the greatest
potential.
The holy grail of "green" fuels has traditionally been hydrogen, an element
that is also very scarce in its pure form on earth. Green in the sense
that it is produced from renewable sources, the most common being the
electrolytic cracking of water. Hydrogen may also be produced from
"brown" sources such as the refining of petroleum. Brown in the sense
that by-products of this production are greenhouse gases and other forms of
pollution. Almost all of the world's H2 is produced by steam reforming
of natural gas, or as by-products of petroleum refining. Very little is
currently produced by electrolysis although there is no technical reason
that it can't be produced in large-scale wind farms in Ontario, the US
Midwest, or even Patagonia.
p>Why would anyone consider using anhydrous ammonia rather than
hydrogen? Hydrogen, after all, contains more LHV (lower heating
value) energy than ammonia (51,500 BTU/lb vs 7,987 BTU/lb or 119.93 kJ/g
vs 18.577 kJ/g) on a weight basis. However, on a volume basis
ammonia is a much better hydrogen carrier than even liquefied
hydrogen. The energy density of liquefied hydrogen is 8,491 kJ/litre
compared to ammonia's 11,308 kJ/litre. Although ammonia
contains 17.65% of hydrogen by weight, the fact that there are 3 hydrogen
atoms attached to a single nitrogen atom allows ammonia to contain about
48% more hydrogen by volume than even liquefied hydrogen. That is to
say, a cubic meter of liquid hydrogen contains 71 kg of hydrogen compared
with 105 kg for liquid anhydrous ammonia.
Often, people look to hydrogen as an energy currency but it is extremely
impractical for the following reasons:
- The low energy density of compressed hydrogen gas makes storage and
transport very expensive. Transporting compressed hydrogen gas any
significant distance by truck can consume more energy in diesel fuel
than what is contained in hydrogen. Liquefied hydrogen is
obviously more energy dense than compressed hydrogen gas but a
significant amount of energy must be expended to liquefy hydrogen and
keep it refrigerated because its boiling point is –423 ºF (–253
ºC). Liquefaction requires about 30% of the energy content of
liquid hydrogen while compression to 800 bar requires about 10-15% of
energy carried by the hydrogen.
- Hydrogen's molecules are very small and difficult to contain.
Hydrogen will slowly leak out from hoses and its rate of leakage is much
higher than larger molecule gases like ammonia and propane.
Hydrogen also causes embrittlement in metals which requires periodic
replacement of metallic tubing, valves, and tanks.
- Hydrogen is typically transported as a compressed gas and a 40 ton
truck that can carry 26 tons of gasoline can only carry about 400 kg
(0.4 tonnes) of compressed hydrogen due to the weight of the high
pressure hydrogen tanks.
On the other hand, ammonia makes more sense:
- Ammonia, in comparison, stores and handles very much like LPG.
Its boiling point is -33.35 °C (-28.03 °F). Propane, the main
constituent of LPG, has a boiling point of -42.07 °C (-43.73 °F).
- Extremely small rates of leakage are easily detectible by the human
nose and the risk of explosion or fire is so low that MSDS labels show
it as a non-flammable gas with an NFPA flammability rating of 1.
- Ammonia has no embrittlement issues and therefore no need for periodic
replacement of metallic tubing, valves, and tanks.
- Any leakage dissipates into the atmosphere where it is eventually
destroyed through photodissociation.
An excellent discussion of ammonia as an energy currency is The
Dual-Fuel Strategy: An Energy Transition Plan (Dr William L. Ahlgren,
Vol. 100, No. 11, November 2012 | Proceedings of the IEEE). In this
document, Dr Ahlgren concludes:
Business as done now relies
predominantly on fossil energy sources with fossil fuels and electric
power as vectors. This is unsustainable; fossil sources must be replaced
by renewable and perhaps nuclear sources. We have three choices for our
global energy future, differentiated by energy vectors, as follows:
- the hydrogen economy: electric
power and hydrogen fuel as vectors;
- the electron economy: electric
power the sole vector – no fuel;
- the dual-fuel strategy: electric
power and two (or few) liquid renewable fuels as vectors.
The dual-fuel strategy is the best
choice. In this plan electric power plays a central but not exclusive
role. Fossil fuels are replaced with low-carbon (ultimately,
zerocarbon) alternatives: ammonia (or nitrofuel) and methanol (or
carbofuel). Because they are liquids, both of these renewable fuels
are compatible with existing infrastructure. This enables the
transition to be triggered using lowcost fossil sources (probably
natural gas, perhaps coal) at the beginning; they are phased out
later. The transition is gradual at first, but accelerates as market
feedback kicks in. Innovation is encouraged by enabling competition
between all energy sources. Entrepreneurs adopting the dualfuel
strategy harness the same market feedback that creates economic
inertia to overcome it and drive the transition to a new global
economy. They will supplant their fossil competitors by developing a
more efficient energy market. This will enable them to offer a
superior product: carbon-free energy with more stable supply and at
lower cost. They will be able to do so because they are more agile.
They can take advantage of low-cost fossil sources, including
petroleum, when available; and they can equally well turn to
alternative fossil and nonfossil sources of energy when petroleum is
not available. The dual-fuel strategy provides a hedge, not only
against the risk of unstable oil supply, but also against the risk
ofglobal warming. Because it uses the existing energy infrastructure,
the transition to a post-petroleum system can be accomplished in a
relatively short time, a few decades. The dual-fuel strategy
enables an order of magnitude reduction in global carbon emissions
early in the transition period, perhaps by 2030, and zero net carbon
at its completion, perhaps by 2050. The dual-fuel strategy is a
feasible plan to make the transition to a postpetroleum zero-carbon
global energy system as rapidly as possible, perhaps by midcentury.
See also Dr Ahlgren’s document Planning for Hundred-Fold Increase in Global
Ammonia Production. For more online references to ammonia’s use as an
energy currency, let us suggest the following:
|