Environmental Engineering Reference
In-Depth Information
These adsorption-based hydrogen storagemethods, if the oxygen poisoning aspect
[118b] can be overcome, seemadaptable to small-scale applications, such as hydrogen
that might be produced by a rooftop system along the lines envisioned by Nocera.
9.8
Economics of Hydrogen
One scenario for future distribution of energy is in the form of hydrogen, which
might be compressed or lique
ed. The hydrogen economy scenario has been
promoted along with the idea that automobiles may switch to electric motors for
which the electricity comes fromfuel cells run on hydrogen gas stored in the cars fuel
tank. A scenario of this sort is being tested in Iceland, which has renewable energy
resources and no oil, coal, or gas.
Hydrogen as a fuel is attractive because it burns to produce water, with no adverse
effect on the atmosphere. Hydrogen is not abundant in its free form; for example, the
availability of hydrogen as a byproduct of commercial liquefaction of air to produce
nitrogen and oxygen is limited. Commercial (nonrenewable carbon-based) produc-
tion of hydrogen is extensive. It is typically based on methane (natural gas), and
approximately half is used to make ammonia for agriculture, the other main use
being in oil re
ning. Worldwide production of ammonia [121] was 109millionmetric
tons in 2004, where a metric ton or tonne is 1000 kg. The
final reaction is direct
stoichiometric combination of hydrogen and nitrogen, to form NH
3
in a catalytic
process called the ammonia synthesis loop (Haber
-
Bosch process).
9.8.1
Further Aspects of Storage and Transport of Hydrogen
Commercial storage and transport of hydrogen is conventionally at high pressure in
steel cylinders, or aluminum-lined containers reinforced with carbon
bers, or as
liquid in a cryogenic container. The network of hydrogen
lling stations in
California dispenses hydrogen at 5000 psi and at 10 000 psi (
330 bar to 660 bar).
There are signi
cant energy costs in pressurizing hydrogen and also in liquefying it.
(In principle, hydrogen can also be stored at high density adsorbed to high surface
area substrates, such as graphitic planes decorated with hydrogen adsorbing centers,
such as Ti atoms, as mentioned above.) Finally, ammonia, NH
3
, is easily lique
ed,
and then contains hydrogen at high density. Liquid ammonia, in spite of its toxicity, is
routinely piped over large distances in connection with its use as a fertilizer. As
mentioned above, the
final step in ammonia formation from hydrogen and nitrogen
is theHaber
-
Bosch process, a well-known commercial catalytic process carried out at
pressures on the order of 100 atm.
It is reported that Iceland produces 2000 tons of hydrogen per year by electrolysis,
using spare electrical capacity. Most of it is used indeed in ammonia production,
some in running fuel cell buses and boats. Eventually, the price of natural gasmay rise
enough that a renewable route to ammonia production will be competitive. (The
