Hydrogen production
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Hydrogen production is commonly completed from hydrocarbon fossil fuels via a chemical path. Hydrogen may also be extracted from water via biological production in an algae bioreactor, or using huge amounts of electricity (by electrolysis) or heat (by thermolysis). These methods are presently not cost effective for bulk generation. Cheap bulk production of hydrogen is a requirement for a healthy hydrogen economy.
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[edit] From hydrocarbons
Hydrogen can be generated from natural gas with approximately 80% efficiency, or other hydrocarbons to a varying degree of efficiency. The hydrocarbon conversion method releases greenhouse gases. Since the production is concentrated in one facility, it is possible to separate the gases and dispose of them properly, for example by injecting them in an oil or gas reservoir (see carbon capture), although this is not currently done in most cases. A carbon dioxide injection project has been started by Norwegian company Statoil in the North Sea, at the Sleipner field.
[edit] Steam reforming
Commercial bulk hydrogen is usually produced by the steam reforming of natural gas. At high temperatures (700–1100 °C), steam (H2O) reacts with methane (CH4) to yield syngas.
The heat required to drive the process is generally supplied by burning some portion of the methane.
[edit] Carbon monoxide
Additional hydrogen can be recovered from the carbon monoxide (CO) through the lower-temperature water gas shift reaction, performed at about 130 °C:
- CO + H2O → CO2 + H2 + 40.4 kJ/mol
Essentially, the oxygen (O) atom is stripped from the water (steam) to oxidize the carbon (C), liberating the hydrogen formerly bound to the carbon and oxygen.
[edit] Coal
Coal can be converted into syngas and methane, also known as town gas, via coal gasification.
[edit] From water
[edit] Biological production
Hydrogen can be produced in an algae bioreactor. In the late 1990s it was discovered that if the algae is deprived of sulfur it will switch from the production of oxygen, i.e. normal photosynthesis, to the production of hydrogen.
It seems that the production is now economically feasible by trespassing the 7-10 percent energy efficiency (the conversion of sunlight into hydrogen) barrier.
[edit] Electrolysis
When the energy supply is chemical, it will always be more efficient to produce hydrogen through a direct chemical path. But when the energy supply is mechanical (hydropower or wind turbines), hydrogen can be made via electrolysis of water. Usually, the electricity consumed is more valuable than the hydrogen produced, which is why only a tiny fraction of hydrogen is currently produced this way.
When the energy supply is in the form of heat (solar thermal or nuclear), the only existing path to hydrogen is currently through high-temperature electrolysis. In contrast with low-temperature electrolysis, high-temperature electrolysis (HTE) electrolysis of water converts more of the initial heat energy into chemical energy (hydrogen), potentially doubling efficiency, to about 50%. Because some of the energy in HTE is supplied in the form of heat, less of the energy must be converted twice (from heat to electricity, and then to chemical form), and so less energy is lost. HTE has been demonstrated in a laboratory, but not at a commercial scale.
[edit] High-temperature electrolysis (HTE)
HTE processes are generally only considered in combination with a nuclear heat source, because the other non-chemical form of high-temperature heat (concentrating solar thermal) is not consistent enough to bring down the capital costs of the HTE equipment. Research into HTE and high-temperature nuclear reactors may eventually lead to a hydrogen supply that is cost-competitive with natural gas steam reforming.
Some prototype Generation IV reactors operate at 850 to 1000 degrees Celsius, considerably hotter than existing commercial nuclear power plants. General Atomics predicts that hydrogen produced in a High Temperature Gas Cooled Reactor (HTGR) would cost $1.53/kg. In 2003, steam reforming of natural gas yielded hydrogen at $1.40/kg. At 2005 gas prices, hydrogen cost $2.70/kg[citation needed]. Hence, just within the United States, a savings of tens of billions of dollars per year is possible with a nuclear-powered supply. Much of this savings would translate into reduced oil and natural gas imports.
One side benefit of a nuclear reactor that produces both electricity and hydrogen is that it can shift production between the two. For instance, the plant might produce electricity during the day and hydrogen at night, matching its electrical generation profile to the daily variation in demand. If the hydrogen can be produced economically, this scheme would compete favorably with existing grid energy storage schemes. What is more, there is sufficient hydrogen demand in the United States that all daily peak generation could be handled by such plants[1].However the Generation IV reactors are not expected until 2030 and its not sure the reactors can compete by then in safety and supply with the distributed generation concept.
[edit] Thermochemical production
Some thermochemical processes, such as the sulfur-iodine cycle, can produce hydrogen and oxygen from water and heat without using electricity. Since all the input energy for such processes is heat, they can be more efficient than high-temperature electrolysis. Thermochemical production of hydrogen using chemical energy from coal or natural gas is generally not considered, because the direct chemical path is more efficient.
None of the thermochemical hydrogen production processes have been demonstrated at production levels, although several have been demonstrated in laboratories.
[edit] Other methods
- Photoelectrochemical cells. - Nanotechnology research on photosynthesis may lead to some other more efficient direct solar production of hydrogen. Or perhaps carbon dioxide neutral synthetic hydrocarbon fuels.
- The radical Hydridic Earth theory suggests that large quantities of hydrogen may exist in the Earth's mantle.
[edit] See also
- Hydrogen economy
- Hydrogen leak testing
- Hydrogen storage
- Hydrogen station
- Hydrogen technologies
- The Hype about Hydrogen
[edit] External links
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