Production of hydrogen

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 million tonnes were an incidental by-product of the chlor-alkali process.[1] Hydrogen production is an estimated $100 billion industry.[2]

Steam reforming

Main article: Steam reforming

Fossil fuels are the dominant source of industrial hydrogen.[3] 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.[4] At high temperatures (700–1100 °C), steam (H2O) reacts with methane (CH4) in an endothermic reaction to yield syngas.[5]


CH4 + H2O → CO + 3 H2

In a second stage, additional hydrogen is generated through the lower-temperature, exothermic, water gas shift reaction, performed at about 130 °C:

CO + H2O → CO2 + H2

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 without atmospheric release, 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.

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.[6]

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:

CnHm + n/2 O2n 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)[7] 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 (CnHm). 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.[8] 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[9]

Coal

Coal can be converted into syngas and methane, also known as town gas, via coal gasification. Syngas consists of hydrogen and carbon monoxide.[10] Another method for conversion is low temperature and high temperature coal carbonization.[11]

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."[3] High temperature electrolysis of alkaline solutions has been used for the industrial scale production of hydrogen (see Sable Chemicals) and there are now a number of small scale polymer electrolyte membrane (PEM) electrolysis units available commercially.[12][13][14]

Electrolysis

Currently, the majority of hydrogen (∼95%) is produced from fossil fuels by steam reforming or partial oxidation of methane and coal gasification with only a small quantity by other routes such as biomass gasification or electrolysis of water.[15] There are three main types of cells, solid oxide electrolysis cells (SOEC's), polymer electrolyte membrane cells (PEM) and alkaline electrolysis cells (AEC's). SOEC's operate at high temperatures, typically around 800°C. At these high temperatures a significant amount of the energy required can be provided as thermal energy (heat). This energy can be provided from a number of different sources, including waste industrial heat, nuclear power stations or concentrated solar thermal plants. This has the potential to reduce the overall cost of the hydrogen produced by reducing the amount of electrical energy required for electrolysis.[15][16][17][18] PEM electrolysis cells typically operate below 100°C and are becoming increasingly available commercially.[15] These cells have the advantage of being comparatively simple and can be designed to accept widely varying voltage inputs which makes them ideal for use with renewable sources of energy such as solar PV.[19] AEC's optimally operate at high concentrations electrolyte (KOH or potassium carbonate) and at high temperatures, often near 200 °C.

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.

Ferrosilicon method

Ferrosilicon is used by the military to quickly produce hydrogen for balloons. The chemical reaction uses sodium hydroxide, ferrosilicon, and water. The generator is small enough to fit a truck and requires only a small amount of electric power, the materials are stable and not combustible, and they do not generate hydrogen until mixed.[20] The method has been in use since World War I. A heavy steel pressure vessel is filled with sodium hydroxide and ferrosilicon, closed, and a controlled amount of water is added; the dissolving of the hydroxide heats the mixture to about 200 °F and starts the reaction; sodium silicate, hydrogen and steam are produced.[21]

Photobiological water splitting

Main article: Biological hydrogen production

Biological hydrogen can be produced in an algae bioreactor.[22] 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 surpassing the 7–10 percent energy efficiency (the conversion of sunlight into hydrogen) barrier.[23] with a hydrogen production rate of 10-12 ml per liter culture per hour.[24]

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, it can be made more efficient.[25][26]

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).[27]

Biohydrogen routes

[3] 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

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.[28]

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.[29]

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.[30] The stoichiometric reaction is:

C6H10O5 + 7 H2O → 12 H2 + 6 CO2

The key technology is cell-free synthetic enzymatic pathway biotransformation (SyPaB).[31][32] 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.[33] 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.[34]

Synthetic biology[35][36][37]

Biocatalysed electrolysis

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[38] can be used. These include reed sweetgrass, cordgrass, rice, tomatoes, lupines, algae[39]

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.[40]

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 may be used in fuel cells for local electricity generation, making it possible for hydrogen to be used as a transportation fuel for an electric vehicle.

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.[3]

See also

Sustainable development portal

References

External links

  • U.S. DOE 2008-Technical progress in hydrogen production
  • U.S. NREL article on hydrogen production
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