A “Hydrogen Economy”
Hydrogen is an enormously attractive fuel because it’s environmentally clean, giving only water as a combustion product. If hydrogen is burned in air, small amounts of nitrogen oxides can be produced because of the high-temperature combination of nitrogen and oxygen, but the combustion products are free of CO, CO2, SO2, unburned hydrocarbons, and other environmental pollutants that result from the combustion of petroleum fuels. In addition, the amount of heat liberated when hydrogen burns is 242 kJ/mol (121 kJ/g), more than twice that of gasoline, oil, or natural gas on a mass basis.
H2 (g) + ½ O2 (g) → H2O (g) ∆H0 = -242 kJ
As a result, some people envision what they call a “hydrogen economy” in which our energy needs are met by gaseous, liquid, and solid hydrogen. For heating homes, gaseous hydrogen could be conveyed through underground pipes, while liquid hydrogen could be shipped by truck or by rail in large vacuum-insulated tanks. Automobiles might be powered by compressed hydrogen gas, liquid hydrogen, or “solid hydrogen” in the form of solid interstitial hydrides or hydrogen stored in the recently discovered tube-shaped molecules called carbon nanotubes. Prototype cars have already been built with their engines modified to run on hydrogen. Current work centers on development of electric vehicles powered by fuel cells, a type of battery that uses hydrogen as an energy source.
What is keeping us from reaching a hydrogen economy? Before a hydrogen economy can become a reality, cheaper ways of producing hydrogen must be found. Since hydrogen is not a naturally occurring energy source like coal, oil, or natural gas, energy must first be expended to produce the hydrogen before it can be used. Current research therefore focuses on finding cheaper methods for extracting hydrogen from its compounds.
At present, hydrogen is produced mainly by the steam–hydrocarbon reforming process. This method can contribute to global warming because it produces CO2 as a by-product. It may be possible, however, to capture the CO2 and sequester it in depleted gas wells or deep saline aquifers, thus avoiding addition of CO2 to the atmosphere.
Where inexpensive electric power is available, for example off-peak hydropower or nuclear power, hydrogen can be produced by electrolysis of water. Another approach is to use solar energy to “split” water into H2 and O2. The feasibility of this scheme depends on the development of catalysts that absorb sunlight and then use the energy to reduce water to hydrogen.
Liquid hydrogen has been used as a fuel in the U.S. space program for many years. Hydrogen powered the Saturn V rocket that carried the first astronauts to the moon, and it fuels the rocket engines of the space shuttle.
Although liquid hydrogen has been handled safely for many years, it is an extremely dangerous substance. The disastrous breakup of the Challenger space shuttle, which took the lives of seven astronauts in 1986, resulted from a leak in the O-ring of the solid-fuel rocket boosters and subsequent explosive burning of massive amounts of hydrogen. Before liquid hydrogen can come into more general use as a fuel, the hazards of storing and distributing this flammable and explosive material must be solved.