Hydrogen production

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 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

gasification

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 steam from hydrocarbons (C_nH_m), 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.

Coal

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

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

Main article: Biological hydrogen production (Algae)

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

Electrolysis of water ship Hydrogen Challenger

Electrolyser front with electrical panel in foreground

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 a renewable electrical energy supply, such as hydropower, wind turbines, or photovoltaic cells, electrolysis of water allows hydrogen to be made 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 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.

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.

High-temperature electrolysis (HTE)

Main article: 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.

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 . 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, 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.

Chemical production

Among the different metals that can react with aqueous solutions to generate hydrogen, aluminum and its alloys are among the most suitable materials for the development of future processes of hydrogen production . Aluminum can be stored and transported in a simpler, safer and cheaper way than hydrogen. It is stable under usual conditions and much cheaper than sodium borohydride (3 $·kg-1) . The reported reactions of aluminum with aqueous solutions of sodium hydroxide are the following  :

2Al+6H2O+2NaOH → 2NaAl(OH)4+3H2 (1).

NaAl(OH)4 → NaOH + Al(OH)3 (2).

2Al + 6H2O → 2Al(OH)3 + 3H2 (3).

Initially, the hydrogen generation reaction (1) consumes sodium hydroxide, but when the aluminate concentration exceeds the saturation limit, aluminate undergoes a decomposition reaction (2) that produces a crystalline precipitate of aluminum hydroxide with the regeneration of the alkali. Reaction (2) has been studied in depth concerning the aluminium battery. The overall hydrogen-generating reaction of aluminum in an aqueous solution is described by reaction (3). It has been shown that this process is able to produce hydrogen gas from Al with regeneration of hydroxyl ions . A major hurdle to the production of hydrogen via this corrosion reaction is that the aluminum surface is easily passivated , but the passivation can be minimized optimizing several experimental parameters such as temperature, alkali concentration, aluminum raw material form and solution composition.

Hydrogen generation systems based on Al corrosion do not need to be warmed up externally, since Al corrosion is an exothermic reaction. This reaction can be achieved under mild conditions of temperature and pressure, providing a stable and compact source of hydrogen. This chemical reduction process is specially suitable for remote, mobile or marine applications. Each kilogram of aluminum produces approximately 4 kWh of energy in the form of hydrogen and, for an achievable 100% hydrogen generation efficiency, it is possible to reach a global gravimetric hydrogen capacity of 11.2 wt% H2, which is a significant value to accomplish the U.S. DOE research targets . This gravimetric hydrogen capacity can be increased using a combination of Al and NaBH4 to produce hydrogen ..

Thermochemical production

Some thermochemical processes 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. 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:

• sulfur-iodine cycle (S-I)
• cerium-chlorine cycle (Ce-Cl)
• iron-chlorine cycle (Fe-Cl)
• magnesium-iodine cycle (Mg-Cl)
• vanadium-chlorine (V-Cl)
• copper-sulfate (Cu-SO₄)

There are also "hybrid" variants, which are thermochemical cycles with an electrochemical step:

• hybrid sulfur cycle
• copper-chlorine cycle (Cu-Cl)

For all the thermochemical processes, the summary reaction is that of the decomposition of water:

$H_{2}O{\text{ }}{\stackrel {Heat}{\rightleftharpoons }}{\text{ }}H_{2}+{1 \over 2}O_{2}$

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.

Other methods

• Nanotechnology research on photosynthesis may lead to more efficient solar production of hydrogen, such as with photoelectrochemical cells.
• The radical Hydridic Earth theory suggests that large quantities of hydrogen may exist in the Earth's mantle.

See also

• Ammonia production
• Hydrogen economy
• Hydrogen leak testing
• Hydrogen storage
• Hydrogen station
• Hydrogen technologies
• The Hype about Hydrogen
• Next Generation Nuclear Plant (partly for hydrogen production)

External links

• U.S. DOE 2007-Technical progress in hydrogen production
• U.S. DOE Hydrogen from Coal Research
• U.S. NREL article on hydrogen production
• Article advocating the use of nuclear power to produce hydrogen
• Genetically engineered blood protein can be used to produce hydrogen gas from water
• Biohydrogen