To exist as a liquid, H₂ must be pressurized and cooled to a very low temperature, 20.27 K (−423.17 °F/−252.87°C). 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. Once liquified it can be maintained as a liquid in pressurized and thermally insulated containers.

Uses

Liquified hydrogen can be used as a fuel in an internal combustion engine or fuel cell. Various concept hydrogen vehicles have been built using this form of hydrogen (see BMW H2R). 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 fumigators. 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 (elastic collision).

Advantages

Hydrogen has one of the highest gravimetric energy densities of all available fuels, which means it has very high energy content per unit mass (143 MJ/kg, 40 percent more than other rocket fuels).

As one of the lightest fuels available, one liter of hydrogen weighs only 0.07 kg. That is a density of 70.99 g/L (at 20 K).

Producing “zero emissions”, the byproducts of its combustion with oxygen alone are mainly water vapor.

Drawbacks

Liquid Hydrogen requires complex storage technology such as the special thermally insulated containers and requires special handling common to all cryogenic substances. 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. (Typically it will evaporate at a rate of 1% per day. )

The low strength of the hydrogen-hydrogen bond results in low minimum ignition energy. For intermittent-combustion engines, this results in a low octane rating. In jet and rocket engines, which are typically ignited once per flight, this is an advantage, making the engine easy to start, and resistant to "flameout."

Hydrogen burns with a very high flame temperature. Typical piston engines burning hydrogen in ambient air (not simply oxygen) thus produce high amounts of NOx pollution.

Under the right conditions, the presence of hydrogen can lead to degradation of mechanical properties in certain metals. (see Hydrogen embrittlement).

According to NASA safety guidelines for dealing with hydrogen:

Ignition:

• "Hydrogen-air mixtures can ignite with very low energy input, 1/10th that required igniting a gasoline-air mixture. For reference, an invisible spark or a static spark from a person can cause ignition."
• "Although the autoignition temperature of hydrogen is higher than those for most hydrocarbons, hydrogen's lower ignition energy makes the ignition of hydrogen–air mixtures more likely. The minimum energy for spark ignition at atmospheric pressure is about 0.02 millijoules."

Mixtures:

• "The flammability limits based on the volume percent of hydrogen in air (at 14.7 psia) are 4.0 and 75.0. The flammability limits based on the volume percent of hydrogen in oxygen (at 14.7 psia) are 4.0 and 94.0."
• "Condensed and solidified atmospheric air, or trace air accumulated in manufacturing, contaminates liquid hydrogen, thereby forming an unstable mixture. This mixture may detonate with effects similar to those produced by trinitrotoluene (TNT) and other highly explosive materials"
• "Explosive limits of hydrogen in air are 18.3 to 59 percent by volume"
• "Flames in and around a collection of pipes or structures can create turbulence that causes a deflagration to evolve into a detonation, even in the absence of gross confinement."

(For comparison: Deflagration limit of gasoline in air: 1.4–7.6%)

Leaks:

• "Leakage, diffusion, and buoyancy: These hazards result from the difficulty in containing hydrogen. Hydrogen diffuses extensively, and when a liquid spill or large gas release occurs, a combustible mixture can form over a considerable distance from the spill location."
• "Hydrogen, in both the liquid and gaseous states, is particularly subject to leakage because of its low viscosity and low molecular weight (leakage is inversely proportional to viscosity). Because of its low viscosity alone, the leakage rate of liquid hydrogen is roughly 100 times that of JP-4 fuel, 50 times that of water, and 10 times that of liquid nitrogen."

As per chapters 4 through 6 of the document, hydrogen collects under roofs and overhangs, where it forms an explosion hazard; any building that contains a potential source of hydrogen should have good ventillation, strong ignition suppression systems for all electric devices, and preferably be designed to have a roof that can be safely blown away from the rest of the structure in an explosion. It also enters pipes and can follow them to their destinations.

See also

• Slush hydrogen

References