Hydrogen production

Hydrogen production is usually the term for the industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Hydrogen is also produced as a by product in other processes and managed with hydrogen pinch^[1]. Many other methods are known including electrolysis and thermolysis. The discovery and development of less expensive methods of production of bulk hydrogen is relevant to the establishment of a hydrogen economy.^[2]

Contents 1 Hydrogen waste stream 2 From hydrocarbons 2.1 Steam reforming 2.2 Kværner-process 2.3 Coal 3 From water 3.1 Electrolysis of water 3.2 High pressure electrolysis 3.3 Photoelectrochemical water splitting 3.4 Photobiological water splitting 3.5 Thermal decomposition of water 3.5.1 High-temperature electrolysis 3.5.2 Nuclear-thermal 3.5.3 Solar-thermal 3.6 Thermochemical production 3.7 Chemical production 4 Biohydrogen routes 4.1 Fermentative hydrogen production 4.2 Biocatalysed electrolysis 5 Other methods 6 See also 7 References 8 External links

Hydrogen waste stream

Hydrogen is used for the creation of ammonia for fertilizer via the Haber process, and converting heavy petroleum sources to lighter fractions via hydrocracking. It was common to vent the surplus of hydrogen, nowadays the plants are balanced with hydrogen pinch which creates the possibility of collecting the hydrogen.

From hydrocarbons

Steam reforming

Hydrogen can be generated from natural gas with approximately 80% efficiency, or from other hydrocarbons to a varying degree of efficiency. Specifically, bulk hydrogen is usually produced by the steam reforming of methane or natural gas^[3] At high temperatures (700–1100 °C), steam (H₂O) reacts with methane (CH₄) to yield syngas.

CH₄ + H₂O → CO + 3 H₂ + 191.7 kJ/mol^[4]

In a second stage, further hydrogen is generated 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 maintain the reaction. Additional heat required to drive the process is generally supplied by burning some portion of the methane.

Steam reforming generates 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 a 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.)

Kværner-process

The Kværner-process or Kvaerner carbon black & hydrogen process (CB&H)^[5] is a method, developed in the 1980s by a Norwegian company of the same name, for the production of hydrogen and carbon black from liquid hydrocarbons (C_nH_m). 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.^[6] CO₂ is not produced in the process.

A variation of this proces is presented in 2009 using plasma arc waste disposal technology for the creation of hydrogen, heat and carbon from methane and natural gas in a plasma converter^[7]

Coal

Coal can be converted into syngas and methane, also known as town gas, via coal gasification. Syngas consists of hydrogen and carbon monoxide.^[8]

From water

Production of hydrogen from water requires large amounts of energy and is uncompetitive with methods reliant of fossil fuels. Potential electrical energy supplies include hydropower, wind turbines, or photovoltaic cells. Usually, the electricity consumed is more valuable than the hydrogen produced so this method has not been widely used. Other potential energy supplies include heat from nuclear reactors and light from the sun. Hydrogen can also be used to store renewable electricity when it is 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 aspect helps make hydrogen an enabler of the wider use of renewables, ^[9].

Electrolysis of water

Electrolysis of water is the decomposition of water (H₂O) into oxygen (O₂) and hydrogen gas (H₂) due to an electric current being passed through the water.

High pressure electrolysis

When the electrolysis is conducted at high pressures, the produced hydrogen gas is compressed at around 120–200 Bar (1740–2900 psi).^[10] 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%.^[11]

Photoelectrochemical water splitting

Using electricity produced by photovoltaic systems potentially offers the cleanest way to produce hydrogen. Again, water is broken down into hydrogen and oxygen by electrolysis, but the electrical energy is obtained by a photoelectrochemical cell (PEC) process. The system is also named artificial photosynthesis.^[12][13][14].

Photobiological water splitting

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^[15].

Thermal decomposition of water

At elevated temperatures water molecules split into their atomic components hydrogen and oxygen. For example at 2200 °C about three percent of all H₂O molecules are dissociated into various combinations of hydrogen and oxygen atoms, mostly H, H₂, O, O₂, and OH. Other reaction products like H₂O₂ or HO₂ remain minor. At the very high temperature of 3000 °C more than half of the water molecules are decomposed, but at ambient temperatures only one molecule in 100 trillion dissociates by the effect of heat. However, catalysts can accelerate the dissociation of the water molecules at lower temperatures.

Thermal water splitting has been investigated for hydrogen production since the 1960s ^[16]. The high temperatures needed to obtain substantial amounts of hydrogen impose severe requirements on the materials used in any thermal water splitting device. For industrial or commercial application, the material constraints have limited the success of applications for hydrogen production from direct thermal water splitting and with few exceptions most recent developments are in the area of catalytic and two step processes.

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-thermal

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^[update] 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.^[17] 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.

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 has been 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.^[18]

An interesting approach to solar thermal hydrogen production is proposed by H2 Power Systems ^[19]. Material constraints due to the required high temperatures above 2200°C are reduced by the design of a membrane reactor with simultaneous extraction of hydrogen and oxygen that exploits a defined thermal gradient and the fast diffusion of hydrogen. With concentrated sunlight as heat source and only water in the reaction chamber, the produced gases are very clean with the only possible contaminant being water. A "Solar Water Cracker" with a concentrator of about 100 m² can produce almost one kilogram of hydrogen per sunshine hour.

Thermochemical production

More than 352 thermochemical cycles have been described for water splitting or thermolysis.^[20][21], These cycles promise to produce hydrogen and oxygen from water and heat without using electricity.^[22] 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.

Some of the most promising thermochemical cycles include:

• Cerium(IV) oxide-cerium(III) oxide cycle (CeO₂/Ce₂O₃)
• Iron oxide cycle (Fe₃O₄/FeO)
• Sulfur-iodine cycle (S-I)
• cerium-chlorine cycle (Ce-Cl)
• iron-chlorine cycle (Fe-Cl)
• magnesium-iodine cycle (Mg-Cl),see also solar-pumped laser
• vanadium-chlorine (V-Cl)
• copper-sulfate (Cu-SO₄)
• Zinc zinc-oxide cycle (Zn/ZnO)

Also "hybrid" variants have been described, wherein cycles are tied to 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:

All other reagents are recycled. None of the thermochemical hydrogen production processes have been demonstrated at production levels, although several have been demonstrated in laboratories.

There is also research into the viability of nanoparticles and catalysts to lower the temperature at which water splits.^[23][24]

Chemical production

A variety of materials react with water or acids to release hydrogen. Such methods are non-sustainable. In terms of stoichiometry, these methods resemble the steam reforming process. The great difference between such chemical methods and steam reforming (which is also a "chemical method"), is that the necessary reduced metals do not exist naturally and require considerable energy for their production. For example, in the laboratory strong acids react with zinc metal in Kipp's apparatus.

In the presence of sodium hydroxide, aluminium and its alloys react with water to generate hydrogen gas.^[25][26] Unfortunately, due to its energetic inefficiency, aluminium is expensive and usable only for low volume hydrogen generation. Also high amounts of waste heats must be disposed.

Although other metals can perform the same reaction, aluminium is among the most promising materials for future development^[27] because it is safer, cheaper 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

Overall:

Al + 3 H₂O → Al(OH)₃ + 1.5 H₂

In this process, aluminium functions as a compact hydrogen storage material because 1 kg of aluminum can produce up to 0.111 kg of hydrogen (or 11.1%) from water. When employed in a fuel cell, that hydrogen can also produce electricity, recovering half of the water previously consumed.^[28] The U.S. Department of Energy has outlined its goals for a compact hydrogen storage device^[29] and researchers are trying many approaches, such as by using a combination of aluminum and NaBH₄, to achieve these goals.^[30]

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 back-up, remote or marine applications. While the passivation of aluminum would normally slow this reaction considerably,^[31] its negative effects can be minimized by changing several experimental parameters such as temperature, alkali concentration, physical form of the aluminum, and solution composition.

Biohydrogen routes

Biomass and waste streams can be converted into biohydrogen with biocatalysed electrolysis or fermentative hydrogen production:

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.^[32]

Biohydrogen can be produced in bioreactors that utilize feedstocks, 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 (U.S.).

Biocatalysed electrolysis

Besides dark fermentation, electrohydrogenesis (electrolysis using microbes) is another possibility. Using microbial fuel cells, wastewater or plants such as can be used to generate power. Biocatalysed electrolysis should not be confused with biological hydrogen production, as the latter only uses algae and with the latter, the algae itself generates the hydrogen instantly, where with biocatalysed electrolysis, this happens after running through the microbial fuel cell and a variety of aquatic plants^[33] can be used. These include reed sweetgrass, cordgrass, rice, tomatoes, lupines, algae ^[34]

Other methods

• The radical Hydridic Earth theory suggests that large quantities of hydrogen may exist in the Earth's mantle.
• Synthetic biology ^[35][36][37]

See also

References

External links

Wikimedia Commons has media related to: Hydrogen production

• U.S. DOE 2008-Technical progress in hydrogen production
• U.S. NREL article on hydrogen production
• Genetically engineered blood protein can be used to produce hydrogen gas from water