Steam reforming – gray or blue Hydrogen is industrially produced from
steam reforming (SMR), which uses natural gas. The energy content of the produced hydrogen is around 74% of the energy content of the original fuel, as some energy is lost as excess heat during production. In general, steam reforming emits carbon dioxide, a greenhouse gas, and is known as gray hydrogen. If the carbon dioxide is captured and stored, the hydrogen produced is known as blue hydrogen. Steam methane reforming (SMR) produces hydrogen from natural gas, mostly methane (CH4), and water. It is the cheapest source of industrial hydrogen, being the source of nearly 50% of the world's hydrogen. The process consists of heating the gas to in the presence of steam over a
nickel catalyst. The resulting
endothermic reaction forms carbon monoxide and molecular hydrogen (H2). In the
water-gas shift reaction, the carbon monoxide reacts with steam to obtain further quantities of H2. The WGSR also requires a catalyst, typically over
iron oxide or other
oxides. The byproduct is CO2. For this process, high temperature steam (H2O) reacts with methane (CH4) in an endothermic reaction to yield
syngas. :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 : :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.
Electrified Steam Methane Reforming In May 2019, Science published the results of a Danish study in which the tin catalyst is heated electrically. This reduces natural gas consumption (and CO2-emission) by a third, while the improved heating increases the overall efficiency. SMR needs 4,2 kWh/Nm3 H2, eSMR 3,6 kWh/Nm3 H2 (2,6 kWh natural gas and 1.0 kWh electricity).
From water Methods to produce hydrogen without the use of fossil fuels involve the process of
water splitting, or splitting the water molecule (H2O) into its components oxygen and hydrogen. When the source of energy for water splitting is renewable or low-carbon, the hydrogen produced is sometimes referred to as
green hydrogen. The conversion can be accomplished in several ways, but all methods are currently considered more expensive than fossil-fuel based production methods.
Electrolysis of water – green, pink or yellow Hydrogen can be made via
high pressure electrolysis, low pressure electrolysis of water, or a range of other emerging electrochemical processes such as high temperature electrolysis or carbon assisted electrolysis. However, current best processes for water electrolysis have an effective electrical efficiency of 70-80%, so that producing 1 kg of hydrogen (which has a
specific energy of 143 MJ/kg or about 40 kWh/kg) requires 50–55 kWh of electricity. In parts of the world, steam methane reforming is between $1–3/kg on average excluding hydrogen gas pressurization cost. This makes production of hydrogen via electrolysis cost competitive in many regions already, as outlined by Nel Hydrogen and others, including an article by the IEA examining the conditions which could lead to a competitive advantage for electrolysis. A small part (2% in 2019) is produced by electrolysis using electricity and water, consuming approximately 50 to 55 kilowatt-hours of electricity per kilogram of hydrogen produced. Water electrolysis is using electricity to split water into hydrogen and oxygen. As of 2020, less than 0.1% of hydrogen production comes from water electrolysis.
Electrolysis of water is 70–80% efficient (a 20–30% conversion loss) while
steam reforming of natural gas has a
thermal efficiency between 70 and 85%. The electrical efficiency of electrolysis is expected to reach 82–86% before 2030, while also maintaining durability as progress in this area continues apace. Water electrolysis can operate at , while steam methane reforming requires temperatures at . The difference between the two methods is the primary energy used; either electricity (for electrolysis) or natural gas (for steam methane reforming). Due to their use of water, a readily available resource, electrolysis and similar water-splitting methods have attracted the interest of the scientific community. With the objective of reducing the cost of hydrogen production, renewable sources of energy have been targeted to allow electrolysis. Traditionally, alkaline electrolysers are cheaper in terms of investment (they generally use nickel catalysts), but less-efficient; PEM electrolysers, conversely, are more expensive (they generally use expensive
platinum group metal catalysts) but are more efficient and can operate at higher
current densities, and can therefore be possibly cheaper if the hydrogen production is large enough. SOECs operate at high temperatures, typically around . At these high temperatures, a significant amount of the energy required can be provided as thermal energy (heat), and as such is termed
high-temperature electrolysis. The heat 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. PEM electrolysis cells typically operate below . AECs optimally operate at high concentrations of electrolyte (KOH or
potassium carbonate) and at high temperatures, often near .
Efficiency and economics Efficiency of modern hydrogen generators is measured by
energy consumed per standard volume of hydrogen (MJ/m3), assuming
standard temperature and pressure of the H2. The lower the energy used by a generator, the higher would be its efficiency; a 100%-efficient electrolyser would consume of hydrogen, . Practical electrolysis typically uses a rotating electrolyser, where centrifugal force helps separate gas bubbles from water. Such an electrolyser at 15 bar pressure may consume , and a further if the hydrogen is compressed for use in hydrogen cars. Conventional alkaline electrolysis has an efficiency of about 70%, however advanced alkaline water electrolysers with efficiency of up to 82% are available. Industrial PEM efficiency is expected to increase to approximately 86% before 2030. In 2022, a Nature publication described a capillary-fed electrolysis cell, which reached 98% energy efficiency due to various design optimizations minimizing
overpotentials. As of 2020, the cost of hydrogen by electrolysis is around $3–8/kg. Considering the industrial production of hydrogen, and using current best processes for water electrolysis (PEM or alkaline electrolysis) which have an effective electrical efficiency of 70–82%, producing 1 kg of hydrogen (which has a
specific energy of 143 MJ/kg or about 40 kWh/kg) requires 50–55 kWh of electricity. At an electricity cost of $0.06/kWh, as set out in the Department of Energy hydrogen production targets for 2015, the hydrogen cost is $3/kg. The US DOE target price for hydrogen in 2020 was $2.30/kg, requiring an electricity cost of $0.037/kWh, which is achievable given recent PPA tenders for wind and solar in many regions. In 2021, the US DOE established the Hydrogen Energy Earthshot (Hydrogen Shot) with a target of $1 (USD) for 1 kg of hydrogen in 1 decade, i.e., $1/kg by 2031 (known as "1 1 1"). This low price was selected to be competitive with the price of hydrogen from natural gas in the United States which is approximately $1.50/kg. In comparison, the cost baseline for hydrogen from electrolysis in 2020 was approximately $5/kg, requiring an 80% cost reduction to meet the Hydrogen Shot goal. The report by IRENA.ORG is an extensive factual report of present-day industrial hydrogen production consuming about 53 to 70 kWh per kg could go down to about 45 kWh/kg . The thermodynamic energy required for hydrogen by electrolysis translates to 33 kWh/kg, which is higher than steam reforming with carbon capture and higher than methane pyrolysis. One of the advantages of electrolysis over hydrogen from steam methane reforming (SMR) is that the hydrogen can be produced on-site, meaning that the costly process of delivery via truck or pipeline is avoided.
Chemically assisted electrolysis In addition to reduce the voltage required for electrolysis via the increasing of the temperature of the electrolysis cell it is also possible to electrochemically consume the oxygen produced in an electrolyser by introducing a fuel (such as carbon/coal,
methanol,
ethanol,
formic acid, glycerol, Carbon/hydrocarbon assisted water electrolysis (CAWE) has the potential to offer a less energy intensive, cleaner method of using chemical energy in various sources of carbon, such as low-rank and high sulfur coals, biomass, alcohols and methane (Natural Gas), where pure CO2 produced can be easily sequestered without the need for separation.
Hydrogen from biomass – green Biomass is converted into
syngas by gasification and syngas is further converted into hydrogen by
water-gas shift reaction (WGSR).
Hydrogen as a byproduct of other chemical processes The industrial production of
chlorine and
caustic soda by electrolysis generates a sizable amount of Hydrogen as a byproduct. In the port of Antwerp a 1MW demonstration fuel cell power plant is powered by such byproduct. This unit has been operational since late 2011. The excess hydrogen is often managed with a
hydrogen pinch analysis. Gas generated from
coke ovens in steel production is similar to
Syngas with 60% hydrogen by volume. The hydrogen can be extracted from the coke oven gas economically. When an existing blast furnace is modified to use biomass as its fuel, production of both green steel and green hydrogen/urea are feasible. A fuel-air or fuel-oxygen mixture is partially
combusted, resulting in a hydrogen- and carbon monoxide-rich syngas. More hydrogen and carbon dioxide are then obtained from carbon monoxide (and water) via the water-gas shift reaction. is a
plasma pyrolysis 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.
Coal For the production of hydrogen from coal,
coal gasification is used. The process of coal gasification uses steam and oxygen to break molecular bonds in coal and form a gaseous mixture of hydrogen and carbon monoxide. Carbon dioxide and pollutants may be more easily removed from gas obtained from coal gasification versus coal combustion. Another method for conversion is low-temperature and high-temperature
coal carbonization.
Coke oven gas made from pyrolysis (oxygen free heating) of coal has about 60% hydrogen, the rest being methane, carbon monoxide, carbon dioxide, ammonia, molecular nitrogen, and
hydrogen sulfide (H2S). Hydrogen can be separated from other impurities by the
pressure swing adsorption process.
Japanese steel companies have carried out production of hydrogen by this method.
Petroleum coke Petroleum coke can also be converted to hydrogen-rich
syngas via coal gasification. The produced syngas consists mainly of hydrogen, carbon monoxide and H2S from the sulfur in the coke feed. Gasification is an option for producing hydrogen from almost any carbon source.
Radiolysis Nuclear radiation can break water bonds through
radiolysis. In the
Mponeng gold mine,
South Africa, researchers found bacteria in a naturally occurring high radiation zone. The bacterial community which was dominated by a new
phylotype of
Desulfotomaculum, was feeding on primarily
radiolytically produced hydrogen.
Thermolysis Water spontaneously dissociates at around 2500 °C, but this
thermolysis occurs at temperatures too high for usual process piping and equipment resulting in a rather low commercialization potential.
Pyrolysis on biomass Pyrolysis can be divided into different types based on the pyrolysis temperature, namely low-temperature slow pyrolysis, medium-temperature rapid pyrolysis, and high-temperature flash pyrolysis. The source energy is mainly solar energy, with help of
photosynthetic microorganisms to decompose water or biomass to produce hydrogen. However, this process has relatively low hydrogen yields and high operating cost. It is not a feasible method for industry.
Nuclear-assisted thermolysis The
high-temperature gas-cooled reactor (HTGR) is one of the most promising CO2-free nuclear technique to produce hydrogen by splitting water in a large scale. In this method,
iodine-sulfur (IS) thermo-chemical cycle for splitting water and high-temperature steam electrolysis (HTSE) were selected as the main processes for nuclear hydrogen production. The S-I cycle follows three chemical reactions: Bunsen reaction: I2+SO2+2H2O→H2SO4+2HI HI decomposition: 2HI→H2+I2
Sulfuric acid decomposition: H2SO4→SO2+1/2O2+H2O The hydrogen production rate of HTGR with IS cycle is approximately 0.68 kg/s, and the capital cost to build a unit of power plant is $100 million.
Thermochemical cycle Thermochemical cycles combine solely
heat sources (
thermo) with
chemical reactions to split water into its hydrogen and
oxygen components. The term
cycle is used because aside from water, hydrogen and oxygen, the chemical compounds used in these processes are continuously recycled. If electricity is partially used as an input, the resulting thermochemical cycle is defined as a
hybrid one. The
sulfur-iodine cycle (S-I cycle) is a thermochemical cycle processes which generates hydrogen from water with an efficiency of approximately 50%. The sulfur and iodine used in the process are recovered and reused, and not consumed by the process. The cycle can be performed with any source of very high temperatures, approximately 950 °C, such as by
Concentrating solar power systems (CSP) and is regarded as being well suited to the production of hydrogen by
high-temperature nuclear reactors, and as such, is being studied in the
High-temperature engineering test reactor in Japan. There are other hybrid cycles that use both high temperatures and some electricity, such as the
Copper–chlorine cycle, it is classified as a hybrid
thermochemical cycle because it uses an
electrochemical reaction in one of the reaction steps, it operates at 530 °C and has an efficiency of 43 percent.
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. 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 93 °C and starts the reaction;
sodium silicate, hydrogen and steam are produced. The process is called silicol process and the overall reaction of the process is: : 2NaOH +
Si + H2O → Na2SiO3 + 2H2 The production method is economical to produce fuel cell quality hydrogen since low quality fesrrosilicon can be used to obtain high quality sodium silicate which commands good pricing.
Photobiological water splitting for hydrogen 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 surpassing the 7–10 percent
energy efficiency (the conversion of sunlight into hydrogen) barrier. with a hydrogen production rate of 10–12 ml per liter culture per hour.
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. Current systems, however have low performance for commercial implementation.
Biohydrogen routes Biomass and waste streams can in principle be converted into
biohydrogen with biomass
gasification, steam reforming, or biological conversion like biocatalysed electrolysis An
enzyme-catalyzed process convert the common sugar
xylose into hydrogen with nearly 100% of the theoretical
yield. The process employs 13 enzymes, including a novel
polyphosphate xylulokinase (XK).
Fermentative hydrogen production Fermentative hydrogen production converts organic substrates to hydrogen. A diverse group of
bacteria promote this transformation.
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 some 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.
Enterobacter aerogenes is another hydrogen producer.
Enzymatic hydrogen generation Diverse enzymatic pathways have been designed to generate hydrogen from sugars.
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 can be used. These include
reed sweetgrass, cordgrass, rice, tomatoes, lupines and algae.
Nanogalvanic aluminium alloy powder Aluminium alloy powder reacts with water to produce hydrogen gas upon contact with water. It reportedly generates hydrogen at 100 percent of the theoretical yield. The process is not economical. ==Natural hydrogen==