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Hydrogen production
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Hydrogen is commonly produced by extraction 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), chemicals (by chemical reduction) or heat (by thermolysis); these methods are less developed 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 hydrogen economy.
ogen can be generated from natural gas with approximately 80% efficiency, or from other hydrocarbons to a varying degree of efficiency.
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Hydrogen is commonly produced by extraction 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), chemicals (by chemical reduction) or heat (by thermolysis); these methods are less developed 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 hydrogen economy.
From hydrocarbons
Hydrogen can be generated from natural gas with approximately 80% efficiency, or from other hydrocarbons to a varying degree of efficiency. The hydrocarbon conversion method releases carbon dioxide (CO2). Since the production is concentrated in one facility, it is possible to separate the CO2 and dispose of it properly, for example by injecting it 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. However, even if the carbon dioxide is not sequestered, overall producing hydrogen from natural gas and using it for a hydrogen vehicle only emits half the carbon dioxide that a gasoline car would. (This is disputed in The Hype about Hydrogen: Fact and Fiction in the Race to Save the Climate, a book by Joseph J. Romm, published in 2004 by Island Press and updated in 2005. Romm says that directly burning fossil fuels generates less CO2 than hydrogen production.)
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 by adding more water 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 additional water (steam) to oxidize CO to CO2. This oxidation also provides energy to keep the reaction going.
Kværner-process
The Kværner-process or Kvaerner carbon black & hydrogen process (CB&H) is a method, developed in the 1980s by a Norwegian company of the same name, for the production of hydrogen and carbon black from hydrocarbons (CnHm), such as methane, natural gas and biogas.
Of the available energy of the feed, approximately 48% is contained in the Hydrogen, 40% is contained in activated carbon and 10% in superheated steam. CO2 is not produced in the process.
Coal
Coal can be converted into syngas and methane, also known as town gas, via coal gasification. Syngas consists of hydrogen and carbon monoxide.
Fermentative hydrogen production
Fermentative hydrogen production is the fermentative conversion of organic substrate to biohydrogen manifested by a diverse group bacteria using multi enzyme systems involving three steps similar to anaerobic conversion. Dark fermentation reactions do not require light energy, so they are capable of constantly producing hydrogen from organic compounds throughout the day and night. Photofermentation differs from dark fermentation because it only proceeds in the presence of light. For example photo-fermentation with Rhodobacter sphaeroides SH2C can be employed to convert small molecular fatty acids into hydrogen. Electrohydrogenesis is used in microbial fuel cells.
From water
Biological production
Biological hydrogen can be produced in an algae bioreactor. In the late 1990s it was discovered that if the algae are 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.
Biological hydrogen can be 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
Hydrogen can also be produced through a direct chemical path using electrolysis. With a renewable electrical energy supply, such as hydropower, wind turbines, or photovoltaic cells, electrolysis of water allows hydrogen to be made from water without pollution. Usually, the electricity consumed is more valuable than the hydrogen produced so this method has not been widely used in the past, but the importance of high pressure electrolysis is increasing as human population and pollution increase, and electrolysis will become more economically competitive as non-renewable resources (carbon compounds) dwindle and as governments remove subsidies on carbon-based fuels. Hydrogen can also be used to store renewable electricity when it's not needed (like the wind blowing at night) and then the hydrogen can be used to meet power needs during the day or fuel vehicles. This helps make hydrogen an enabler of the wider use of renewables, .
High pressure electrolysis
High pressure electrolysis (HPE) is the electrolysis of water by decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) with a compressed hydrogen output around 120–200 Bar (1740–2900 psi). By pressurising the hydrogen in the electrolyser the need for an external hydrogen compressor is eliminated, the average energy consumption for internal compression is around 3%.
High-temperature electrolysis
When the energy supply is in the form of heat (solar thermal, or nuclear), the best path to hydrogen is through high-temperature electrolysis (HTE). In contrast with low-temperature electrolysis, HTE 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 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. HTE has been demonstrated in a laboratory, but not at a commercial scale.
Nuclear
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 gas prices, hydrogen cost $2.70/kg. 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. However, Generation IV reactors are not expected until 2030 and it is uncertain if they can compete by then in safety and supply with the distributed generation concept.
Concentrating Solar Thermal
The high temperatures necessary to split water can be achieved through the use of concentrating solar power. Hydrosol-2 is a 100-kilowatt pilot plant at the Plataforma Solar de Almería in Spain which uses sunlight to obtain the required 800 to 1,200 °C to split water. Hydrosol II is in operation since 2008. The design of this 100-kilowatt pilot plant is based on a modular concept. As a result, it may be possible that this technology could be readily scaled up to the megawatt range by multiplying the available reactor units and by connecting the plant to heliostat fields (fields of sun-tracking mirrors) of a suitable size.
Chemical production
By using sodium hydroxide as a catalyst, aluminum and its alloys can react with water to generate hydrogen gas. Although other metals can perform the same reaction, aluminum is among the most promising materials for future development because it is safer and easier to transport than some other hydrogen storage materials like sodium borohydride.
The initial reaction (1) consumes sodium hydroxide and produces both hydrogen gas and an aluminate byproduct. Upon reaching its saturation limit, the aluminate compound decomposes (2) into sodium hydroxide and a crystalline precipitate of aluminum hydroxide. This process is similar to the reactions inside an aluminium battery.
(1) Al + 3 H2O + NaOH ? NaAl(OH)4 + 1.5 H2
(2) NaAl(OH)4 ? NaOH + Al(OH)3
The overall reaction is described by Reaction (3).
(3) Al + 3 H2O ? Al(OH)3 + 1.5 H2
In this process, aluminum acts as a compact hydrogen storage device because 1 kg of aluminum can theoretically produce up to 0.111 kg of hydrogen (or 11.1%). When employed in a fuel cell, that hydrogen can also produce electricity. The U.S. Department of Energy has outlined its goals for a compact hydrogen storage device and researchers are trying many approaches, such as by using a combination of aluminum and NaBH4, to achieve these goals.
Since the oxidation of aluminum is exothermic, these reactions can operate under mild temperatures and pressures, providing a stable and compact source of hydrogen. This chemical reduction process is specially suitable for remote, mobile or marine applications. While the passivation of aluminum would normally slow this reaction considerably, its negative effects can be minimized by changing several experimental parameters such as temperature, alkali concentration, physical form of the aluminum, and solution composition.
Thermochemical production
There are more than 352 thermochemical cycles which can be used for water splitting or thermolysis, around a dozen of these cycles are under research and in testing phase to 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. This is because the efficiency of electricity production is inherently limited. Thermochemical production of hydrogen using chemical energy from coal or natural gas is generally not considered, because the direct chemical path is more efficient.
Hundreds of thermochemical cycles have been pre-screened. Some of the most promising ones include:
There are also "hybrid" variants, which are thermochemical cycles with an electrochemical step:
For all the thermochemical processes, the summary reaction is that of the decomposition of water:
All other chemicals used are recycled.
None of the thermochemical hydrogen production processes have been demonstrated at production levels, although several have been demonstrated in laboratories.
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 cell (PEC) process which is also named artificial photosynthesis. Research aimed toward developing higher-efficiency multijunction cell technology is underway by the photovoltaic industry.
Other methods
See also
External links
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