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