Hydrogen can be produced in large central plants located as far as several hundred miles from the point of end-use; in smaller, semi-central plants within 20-100 miles of the point of end-use; or in small “distributed generation” facilities located very near or at the point of end-use. All three are expected to play a role in the development of a hydrogen economy.
Distributed production may be the most viable approach for introducing hydrogen in the near term because it requires less capital investment for the small volume of hydrogen needed initially, and it does not require a substantial hydrogen transport and delivery infrastructure. Natural gas or renewable liquid fuel reforming and small-scale water electrolysis at the point of end-use are two such distributed technologies with potential for development and commercialization during the transition to a hydrogen economy.
Large central hydrogen production facilities that take advantage of economies of scale will be needed in the long term to meet the expected hydrogen demand. Central production can use a variety of resources –like renewable energy – but an infrastructure will be required to efficiently and cost-effectively deliver the hydrogen to vehicle refueling stations or other points of end-use.
About 95% of the hydrogen produced today in the United States is made via steam methane reforming, a process in which high-temperature steam (700 – 1000°C) is used to produce hydrogen from a methane source, such as natural gas. In steam methane reforming, methane reacts with steam under 3-25 bar pressure in the presence of a catalyst to produce hydrogen and carbon monoxide and a relatively small amount of carbon dioxide (see pressure conversion box). Steam reforming is endothermic, that is, heat must be supplied to the process for the reaction to proceed. Subsequently, in what is called the “water-gas shift reaction,” the carbon monoxide and steam (hot water vapor) are reacted using a catalyst to produce carbon dioxide and more hydrogen. In a final process step called “pressure-swing adsorption,” carbon dioxide and other impurities are removed from the gas stream, leaving essentially pure hydrogen. Steam reforming can also be used to produce hydrogen from other fuels, such as ethanol, propane, or even gasoline.
Pressure Conversion Factors
1 atmosphere (atm) = 14.7 pounds per square inch(psi)
1 atm = 29.92 inches of mercury (in Hg)1 bar = 14.5 psi
1 mega Pascal (MPa) = 10 bar = 145 psi