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Hydrogen production



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 electricity (by electrolysis) or heat (by thermolysis); these methods are less efficient for bulk generation in comparison to chemical paths derived from hydrocarbons. The discovery and development of less expensive methods of bulk production of hydrogen will accelerate the establishment of a healthy hydrogen economy.

Additional recommended knowledge

Contents

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 StatoilHydro in the North Sea, at the Sleipner field.

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.

CH4 + H2O → CO + 3 H2 - 191.7 kJ/mol

The heat required to drive the process is generally supplied by burning some portion of the methane.

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.

Coal

Coal can be converted into syngas and methane, also known as town gas, via coal gasification.

From water

Biological production

Biohydrogen 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.

Biohydrogen can and is produced in bioreactors that utilize feedstocks other than algae, the most common feedstock being waste streams. The process involves bacteria feeding on hydrocarbons and exhaling hydrogen and CO2. The CO2 can be sequestered successfully by several methods, leaving hydrogen gas. A prototype hydrogen bioreactor using waste as a feedstock is in operation at Welch's grape juice factory in North East, Pennsylvania.

Electrolysis

Main article: Electrolysis of water

  It is more efficient to produce hydrogen through a direct chemical path than by electrolysis, but the chemical feedsource will always produce pollution or toxic byproducts as hydrogen is extracted. With electrolysis, when the energy supply is mechanical (hydropower or wind turbines), or photovoltaic from sunlight, hydrogen can be made via electrolysis of water. Usually, the electricity consumed is more valuable than the hydrogen produced so this method has not been widely used in the past, but with electrolysis production of hydrogen, there is virtually no pollution or toxic byproducts, and the feed sources are fully renewable, so the importance of electrolysis is increasing as human population and pollution increase, and electrolysis will become more economically competitive as non-renewable resources (carbon-based compounds) dwindle and as government removes subsidies on carbon-based energies.

When the energy supply is in the form of heat (solar thermal or nuclear), the path to hydrogen is 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.

Irrespective of efficiency, hydrogen production by electrolysis is a clean and renewable agent for storing electrical and mechanical energy for retrieval on demand.

Photoelectrochemical Water Splitting

Using electricity produced by photovoltaic systems offers the cleanest way to produce hydrogen. Water is broken into hydrogen and oxygen by electrolysis--a photoelectrochemical (PEC) process. Research aimed toward developing higher-efficiency multijunction cell technology is underway by the Photovoltaic industry.

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 it's not sure the reactors can compete by then in safety and supply with the distributed generation concept.

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.

Other methods

See also

Sustainable development Portal
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Hydrogen_production". A list of authors is available in Wikipedia.
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