Hydrogen production

Hydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis.

In 2006, the United States was estimated to have a production capacity of 11 million tonnes of hydrogen. 5 million tonnes of hydrogen were consumed on-site in oil refining, and in the production of ammonia (Haber process) and methanol (reduction of carbon monoxide). 0.4 tonnes were an incidental by-product of the chlor-alkali process.

Steam reforming

Main article: Steam reforming

Fossil fuels are the dominant source of industrial hydrogen. 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 At high temperatures (700–1100 °C), steam (H₂O) reacts with methane (CH₄) to yield syngas.

CH₄ + H₂O → CO + 3 H₂ + 191.7 kJ/mol

Gasification

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.

CO2 sequestration

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. 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.^{[citation needed]}

Integrated steam reforming / co-generation - It is possible to combine steam reforming and co-generation of steam and power into a single plant. This can deliver benefits for an oil refinery because it is more efficient than separate hydrogen, steam and power plants. Air Products recently built an integrated steam reforming / co-generation plant in Port Arthur, Texas.

Other production methods from fossil fuels

Partial oxidation

The partial oxidation reaction occurs when a substoichiometric fuel-air mixture is partially combusted in a reformer, creating a hydrogen-rich syngas. A distinction is made between thermal partial oxidation (TPOX) and catalytic partial oxidation (CPOX). The chemical reaction takes the general form:

C_nH_m + ⁿ/₂ O₂ → n CO + ^m/₂ H₂

Idealized examples for heating oil and coal, assuming compositions C₁₂H₂₄ and C₂₄H₁₂ respectively, are as follows:

C₁₂H₂₄ + 6 O₂ → 12 CO + 12 H₂

C₂₄H₁₂ + 12 O₂ → 24 CO + 6 H₂

Plasma reforming

The Kværner-process or Kvaerner carbon black & hydrogen process (CB&H) is a plasma reforming 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. CO₂ is not produced in the process.

A variation of this process is presented in 2009 using plasma arc waste disposal technology for the production of hydrogen, heat and carbon from methane and natural gas in a plasma converter

Coal

Coal can be converted into syngas and methane, also known as town gas, via coal gasification. Syngas consists of hydrogen and carbon monoxide. Another method for conversion is low temperature and high temperature coal carbonization.

From water

Main article: Water splitting

Many technologies have been explored but it should be noted that as of 2007 "Thermal, thermochemical, biochemical and photochemical processes have so far not found industrial applications." Only high temperature electrolysis of alkaline solutions finds some applications.

Electrolysis

Approximately 5% of industrial hydrogen is produced by electrolysis. Two types of cells are popular, solid oxide electrolysis cells (SOEC's) and alkaline electrolysis cells (AEC's) . These cells optimally operate at high concentrations electrolyte (KOH or potassium carbonate) and at high temperatures, often near 200 °C. Typical catalysts are yttrium-stabilized zirconium together with nickel.

Thermolysis

Water spontaneously dissociates at around 2500 C, but this thermolysis occurs at temperatures too high for usual process piping and equipment. Catalysts are required to reduce the dissociation temperature.

Photocatalytic water splitting

Main article: Photocatalytic water splitting

The conversion of solar energy to hydrogen by means of water splitting process is one of the most interesting ways to achieve clean and renewable energy systems. However if this process is assisted by photocatalysts suspended directly in water instead of using photovoltaic and an electrolytic system the reaction is in just one step, therefore it can be more efficient.

Sulfur-iodine cycle

The sulfur-iodine cycle (S-I cycle) is a thermochemical process which generates hydrogen from water, but at a much higher efficiency than water splitting. The sulfur and iodine used in the process are recovered and reused, and not consumed by the process. It is well suited to production of hydrogen by high-temperature nuclear reactors or by concentrating solar power systems (CSP).

Biohydrogen routes

Although of no industrial significance, biomass and waste streams can in principle be converted into biohydrogen with biomass gasification, steam reforming or biological conversion like biocatalysed electrolysis or fermentative hydrogen production.

Fermentative hydrogen production

Main articles: fermentative hydrogen production and dark fermentation

Fermentative hydrogen production is the fermentative conversion of organic substrate to biohydrogen manifested by a diverse group of 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.

Fermentative hydrogen production can be done using direct biophotolysis by green algae, indirect biophotolysis by cyanobacteria, photo-fermentation by anaerobic photosynthetic bacteria and dark fermentation by anaerobic fermentative bacteria. For example studies on hydrogen production using H. salinarium, an anaerobic photosynthetic bacteria, coupled to a hydrogenase donor like E. coli, are reported in literature.

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.).

Enzymatic hydrogen generation

Due to the Thauer limit (four H₂/glucose) for dark fermentation, a non-natural enzymatic pathway was designed that can generate 12 moles of hydrogen per mole of glucose units of polysaccharides and water in 2007. The stoichiometric reaction is:

C₆H₁₀O₅ + 7 H₂O → 12 H₂ + 6 CO₂

The key technology is cell-free synthetic enzymatic pathway biotransformation (SyPaB). A biochemist can understand it as "glucose oxidation by using water as oxidant". A chemist can describe it as "water splitting by energy in carbohydrate". A thermodynamics scientist can describe it as the first entropy-driving chemical reaction that can produce hydrogen by absorbing waste heat. In 2009, cellulosic materials were first used to generate high-yield hydrogen. Furthermore, the use of carbohydrate as a high-density hydrogen carrier was proposed so to solve the largest obstacle to the hydrogen economy and propose the concept of sugar fuel cell vehicles.

Synthetic biology

Biocatalysed electrolysis

A microbial electrolysis cell

Main articles: electrohydrogenesis and microbial fuel cell

Besides dark fermentation, electrohydrogenesis (electrolysis using microbes) is another possibility. Using microbial fuel cells, wastewater or plants 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 can be used. These include reed sweetgrass, cordgrass, rice, tomatoes, lupines, algae

Renewable hydrogen

Currently there are several practical ways of producing hydrogen in a renewable industrial process. One is to use landfill gas to produce hydrogen in a steam reformer, and the other is to use renewable power to produce hydrogen from electrolysis. Hydrogen fuel, when produced by renewable sources of energy like wind or solar power, is a renewable fuel.

Use of hydrogen

Hydrogen is mainly used for the conversion of heavy petroleum fractions into lighter ones via the process of hydrocracking and other petroleum fractions (dehydrocyclization and the aromatization process). It is also required for cleaning fossil fuels via hydrodesulfurization.

Hydrogen is mainly used for the production of ammonia via Haber process. In this case, the hydrogen is produced in situ. Ammonia is the major component of most fertilizers.

Earlier it was common to vent the surplus hydrogen off, nowadays the process systems are balanced with hydrogen pinch to collect hydrogen for further use.

Hydrogen is also produced as a by-product of industrial chlorine production by electrolysis. Although requiring expensive technologies, hydrogen can be cooled, compressed and purified for use in other processes on site or sold to a customer via pipeline, cylinders or trucks. The discovery and development of less expensive methods of production of bulk hydrogen is relevant to the establishment of a hydrogen economy.

See also

• Ammonia production
• Biological hydrogen production
• Hydrogen
• Hydrogen analyzer
• Hydrogen compressor
• Hydrogen economy
• Hydrogen embrittlement
• Hydrogen leak testing
• Hydrogen pipeline transport
• Hydrogen piping
• Hydrogen purifier
• Hydrogen purity
• Hydrogen safety
• Hydrogen sensor
• Hydrogen storage
• Hydrogen station
• Hydrogen tank
• Hydrogen tanker
• Hydrogen technologies
• Hydrogen valve
• Industrial gas
• Liquid Hydrogen
• Next Generation Nuclear Plant (partly for hydrogen production)
• The Phoenix Project: Shifting from Oil To Hydrogen (book)
• Renewable energy
• The Hype about Hydrogen
• Lane hydrogen producer
• Linde-Frank-Caro process
• Liquid nitrogen production
• Underground hydrogen storage

External links

• 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