Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally
in the molecular H2 form. To exist as a liquid, H2 must be cooled below
hydrogen's critical point of 33 K. However, for hydrogen to be in a full liquid state
without evaporating at atmospheric pressure, it needs to be cooled to 20.28 K. One common
method of obtaining liquid hydrogen involves a compressor resembling a jet engine in both
appearance and principle. Liquid hydrogen is typically used as a concentrated form of
hydrogen storage. As in any gas, storing it as liquid takes less space than storing it
as a gas at normal temperature and pressure. However, the liquid density is very low compared
to other common fuels. Once liquefied, it can be maintained as a liquid in pressurized
and thermally insulated containers. Liquid hydrogen consists of 99.79% parahydrogen,
0.21% orthohydrogen.
History
In 1885 Zygmunt Florenty Wróblewski published hydrogen's critical temperature as 33 K; critical
pressure, 13.3 atmospheres; and boiling point, 23 K.
Hydrogen was liquefied by James Dewar in 1898 by using regenerative cooling and his invention,
the vacuum flask. The first synthesis of the stable isomer form of liquid hydrogen, parahydrogen,
was achieved by Paul Harteck and Karl Friedrich Bonhoeffer in 1929.
Spin isomers of hydrogen Room temperature hydrogen consists mostly
of the orthohydrogen form. After production, liquid hydrogen is in a metastable state and
must be converted into the parahydrogen isomer form to avoid the exothermic reaction that
occurs when it changes at low temperatures, this is usually performed using a catalyst
like iron(III) oxide, activated carbon, platinized asbestos, rare earth metals, uranium compounds,
chromium(III) oxide, or some nickel compounds. Uses
It is a common liquid rocket fuel for rocket applications. In most rocket engines fueled
by liquid hydrogen, it first cools the nozzle and other parts before being mixed with the
oxidizer) and burned to produce water with traces of ozone and hydrogen peroxide. Practical
H2/O2 rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen.
This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight
of the exhaust that can actually increase specific impulse despite the incomplete combustion.
Liquid hydrogen can be used as the fuel storage in an internal combustion engine or fuel cell.
Various submarines and concept hydrogen vehicles have been built using this form of hydrogen.
Due to its similarity, builders can sometimes modify and share equipment with systems designed
for LNG. However, because of the lower volumetric energy, the hydrogen volumes needed for combustion
are large. Unless LH2 is injected instead of gas, hydrogen-fueled piston engines usually
require larger fuel systems. Unless direct injection is used, a severe gas-displacement
effect also hampers maximum breathing and increases pumping losses.
Liquid hydrogen is also used to cool neutrons to be used in neutron scattering. Since neutrons
and hydrogen nuclei have similar masses, kinetic energy exchange per interaction is maximum.
Finally, superheated liquid hydrogen was used in many bubble chamber experiments.
Properties
The byproduct of its combustion with oxygen alone is water vapor, which can be cooled
with some of the liquid hydrogen. Since water is considered harmless to the environment,
an engine burning it can be considered "zero emissions." Liquid hydrogen also has a much
higher specific energy than gasoline, natural gas, or diesel.
The density of liquid hydrogen is only 70.99 g/L, a relative density of just 0.07. Although
the specific energy is around twice that of other fuels, this gives it a remarkably low
volumetric energy density, many fold lower. Liquid hydrogen requires cryogenic storage
technology such as special thermally insulated containers and requires special handling common
to all cryogenic fuels. This is similar to, but more severe than liquid oxygen. Even with
thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen
will gradually leak away. It also shares many of the same safety issues as other forms of
hydrogen, as well as being cold enough to liquefy atmospheric oxygen which can be an
explosion hazard. The triple point of hydrogen is at 13.81 K
7.042 kPa. See also
Industrial gas Liquefaction of gases
Hydrogen safety Compressed hydrogen
Cryo-adsorption Expansion ratio
Gasoline gallon equivalent Slush hydrogen
Solid hydrogen Metallic hydrogen
Hydrogen infrastructure Hydrogen-powered aircraft
Liquid hydrogen tank car Liquid hydrogen tanktainer
Liquid hydrogen tank truck References