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| Fig. 1: Hydrogen production pathways: grey, blue, turquoise and green. [3] (Image Source: A. Panda) |
Hydrogen plays a crucial role in various industrial processes, including steel production, ammonia-based fertilizer manufacturing, and oil refining. In 2018, 115 million tonnes of hydrogen were used across these industries globally. Out of the 115 million tonnes, about 60% of the hydrogen demand was in its pure form, while the other 40% was used in mixtures with other gases. [1] In recent years, hydrogen has garnered significant attention as a promising fuel in the energy economy due to its potential as a zero-carbon-emitting energy carrier, as its combustion produces only water. However, a key challenge lies in its production, as hydrogen is not naturally abundant on Earth. Instead, it is typically produced through the pyrolysis of hydrogen-containing molecules, predominantly hydrocarbons.
Grey hydrogen is produced by steam reforming. In steam reforming, hydrocarbons, usually in the form of natural gas, are catalytically pyrolyzed to form carbon monoxide and hydrogen in the presence of steam at high temperatures (700-900°C) and high pressures (3 - 35 bar). This chemical reaction is shown in Equation 1. Since this reaction is endothermic, it requires an external energy source in the form of steam or other thermal inputs. The next step in the process is an exothermic water-gas shift reaction, shown in Equation 2, in which carbon dioxide and hydrogen are produced as products. Hydrogen yield from reforming is usually increased by raising the temperature
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(2) |
The need for a thermal source, the fossil fuel-derived feedstock, and the production of carbon monoxide and carbon dioxide make this process substantially carbon-intensive. Of the 70 million tonnes of pure hydrogen produced, 76% comes from natural gas and about 23% from coal. [2] Hence, in 2018, hydrogen production led to 830 million tonnes of carbon dioxide emissions globally. [1]
Blue hydrogen follows the same production process as grey hydrogen but includes the sequestration of carbon emissions. The sequestration of carbon dioxide can occur at various stages in the reforming process and is usually carried out by chemical or physical absorption of the carbon dioxide stream with solvents such as methyl diethanolamine. Capture efficiency can range from 50-95%, typically coming at the cost of reduced efficiency of the reforming plants. However, capture efficiency is not the only challenge faced by this process. Post-capture, these emissions must be stored long-term for blue hydrogen to be a truly low-carbon option. Since carbon dioxide is usually transported as a liquid or solid, this can only occur at low temperatures ( < 78°dC) or high pressures ( > 73 bar). [3] Consequently, this storage requires sophisticated transportation logistics and has high energy demands. Due to the various challenges associated with blue hydrogen, it only contributed about 0.5 million tonnes of hydrogen in 2018. [1]
Methane pyrolysis has received significant attention and funding recently as a potential alternative to the challenges faced by blue hydrogen. This process relies on Equation 3, where methane decomposes to form carbon and hydrogen. Since this is an endothermic reaction, it requires energy in the form of heat, which, in turn, requires electricity. If renewable electricity is used, no carbon dioxide is produced in the entire process.
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(3) |
However, since it still requires fossil fuel-derived natural gas as the feedstock, it falls between blue and green hydrogen, hence the moniker "turquoise hydrogen." Nebraska-based Monolith Materials has developed and is operating a commercial-scale methane-splitting reactor that uses high- temperature plasma. Their current hydrogen production capacity is 5 kilotonnes per annum. The by-product in this process is solid carbon, which Monolith Materials markets as Carbon Black. Carbon Black has a crystalline structure similar to graphite and has potential applications in many industries such as inks, plastics, industrial rubber, and tires. The challenge with turquoise hydrogen is finding a market for this valuable carbon. The current market demand for Carbon Black is about 16.4 million tonnes. [4] With the efficiency of Monolith reactors, producing only 5.3 million tonnes of hydrogen would meet this demand. If the global hydrogen demand were met with methane pyrolysis, storage and usage of gigatonnes of carbon byproducts would require extensive research and development.
Electrolysis of water to produce hydrogen and oxygen can be an entirely carbon- and fossil fuel-free option if the electricity used is also renewable. Water electrolysis currently contributes to less than 0.1% of total hydrogen demand, whereas about 2% of global hydrogen demand is met by chlor-alkali electrolysis, where hydrogen is produced as a by-product. [1,2] The challenges faced by green hydrogen include the cost and availability of renewable electricity as well as fresh water. As electrolysis capacities increase globally, economies of scale are reducing costs associated with the process. Research into the use of seawater for electrolysis as well as advanced electrode materials has also gained significant momentum in recent years.
Hydrogen holds immense promise as a versatile energy carrier and industrial feedstock, but production challenges vary significantly across different pathways (see Fig. 1). Grey hydrogen dominates current production but comes with a heavy carbon footprint, while blue hydrogen offers a low-carbon alternative, albeit with technical and logistical hurdles. Turquoise hydrogen presents a potentially sustainable middle ground, though its scalability is limited by market constraints for solid carbon byproducts. Green hydrogen, the ultimate goal for a zero-carbon future, remains constrained by the availability and cost of renewable electricity and electrolysis infrastructure. Addressing these challenges through innovation, policy support, and infrastructure development will be key to unlocking hydrogens potential in a sustainable global energy economy.
© Alka Panda. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "The Future of Hydrogen," International Energy Agency, June 2019.
[2] R. B. Laughlin, S. W. Freund, "Economics of Hydrogen Fuel," inMachinery and Enegy Systems for the Hydrogen Economy, ed. by K. Brun and T. Allison (Elsevier, 2022).
[3] M. Hermesmann et al., "Green, Turquoise, Blue, or Grey? Environmentally Friendly Hydrogen Production in Transforming Energy Systems," Prog. Energy Combust. Sci. 90, 100996 (2022).
[4] M. R. G. Pangestu, et al., "Comprehensive Review on Methane Pyrolysis for Sustainable Hydrogen Production," Energy Fuels 38, 13514 (2024).