Hydrogen (H₂) is a colorless, odorless, tasteless, flammable and nontoxic gas at atmospheric temperatures and pressures. It is the most abundant element in the universe, but is almost absent from the atmosphere as individual molecules in the upper atmosphere can gain high velocities during collisions with heavier molecules, and become ejected from the atmosphere. It is still quite abundant on Earth, but as part of compounds such as water.  

Hydrogen burns in air with a pale blue, almost invisible flame. Hydrogen is the lightest of all gases, approximately one-fifteenth as heavy as air.  Hydrogen ignites easily and forms, together with oxygen or air, an explosive gas (oxy-hydrogen).

Hydrogen has the highest combustion energy release per unit of weight of any commonly occurring material. This property makes it the fuel of choice for upper stages of multi-stage rockets. 

Hydrogen has the lowest boiling point of any element except helium.  When cooled to its boiling point, -252.76^o C (-422.93^o F) hydrogen becomes a transparent, odorless liquid that is only one-fourteenth as heavy as water. Liquid hydrogen is not corrosive or particularly reactive. When converted from liquid to gas, hydrogen expands approximately 840 times. Its low boiling point and low density result in liquid hydrogen spills dispersing rapidly.

The most common large-scale process for manufacturing hydrogen is steam reforming of hydrocarbons, in particular, natural gas (mostly methane). Other methods used for hydrogen production methods include generation by partial oxidation of coal or hydrocarbons, electrolysis of water, recovery of byproduct hydrogen from electrolytic cells used to produce chlorine and other products, and dissociation of ammonia. Hydrogen is recovered for internal use and sale from various refinery and chemical streams, typically purge gas, tail gas, fuel gas or other contaminated or low-valued streams. Purification methods include pressure swing adsorption (PSA), cryogenic separation and membrane gas separation. 

Many hydrogen gas users purchase it as a liquid, which can be vaporized as needed, instead of producing it on their own site.  Liquefaction of gaseous hydrogen is a multi-stage process using several refrigerants and compression/ expansion loops to produce extreme cold. As part of the process, the hydrogen passes through "ortho/ para" conversion catalyst beds that convert most of the "ortho" hydrogen to the "para" form.  These two types of diatomic hydrogen have different energy states. In "ortho" hydrogen, which is the most common form at room temperature, the nuclei have "anti-parallel" spins. In "para" hydrogen the nuclei have parallel spins. "Ortho" hydrogen is less stable than "para" at liquid hydrogen temperatures.  It spontaneously changes to the "para" form, releasing energy, which vaporizes a portion of the liquid. By using a catalyst such as hydrous ferric oxide to convert most of the hydrogen to the more stable form during the liquefaction process, the liquid hydrogen product can be stored without excessive vent loss.

Some industrial processes with relatively small hydrogen requirements may choose to produce some or all of their needs using compact generators.  In the past, ammonia dissociation was a common technology choice.  More recently, improvements in small packaged electrolytic and hydrocarbon reforming systems have made these routes to small volume hydrogen production increasingly attractive.  In some cases these systems may be the sole source of hydrogen, while in others they may be used to supplement and/or back-up other supply sources.  Electrolytic production techniques can produce high purity hydrogen at elevated pressure, eliminating the need for supplemental compression.  They can also produce high purity oxygen (at one-half the hydrogen production rate).  The latest generation of highly packaged hydrocarbon reforming units, in particular those which employ an autothermal generation process, which operates at relatively low-temperature and pressure, have made on-site hydrocarbon reforming a viable route to hydrogen production at much lower production rates than were considered commercially feasible just a few years ago.

Much has been said about hydrogen being the "fuel of the future" due to its abundance and its non-polluting combustion products.  Less has been said about the fact that other forms of energy must be used to produce the hydrogen which will be used as fuel.  Most hydrogen is bound up in compounds such as water or methane, and energy is required to break the hydrogen free from these compounds, then separate, purify, compress and/ or liquefy the hydrogen for storage and transportation to usage points. Widespread production, distribution and use of hydrogen will require many innovations and investments to be made in efficient and environmentally-acceptable production systems, transportation systems, storage systems and usage devices. 

Currently, there is a great deal of interest in hydrogen fuel cell technology development and investigations into unconventional or specialized hydrogen storage systems. New technologies and equipment developed to support these applications will undoubtedly find uses in industry as well.