Types of hydrogen fuel

It takes energy to produce molecular hydrogen. The source of energy and the production method used to make molecular hydrogen determines whether it’s classified as grey hydrogen, blue hydrogen or green hydrogen. Hydrogen can be made from natural gas, coal, or biomass, but these energy sources have associated greenhouse gas emissions. Hydrogen can also be made using an electrolysis process to split water into oxygen and hydrogen.[1]

Figure 1: Depiction of grey, blue and green hydrogen production.[2]

Grey hydrogen

Grey hydrogen is hydrogen produced using fossil fuels such as natural gas or coal. Grey hydrogen accounts for roughly 95% of the hydrogen produced in the world today.[3] The two main production methods are steam methane reforming and coal gasification.[4] Both of these processes release carbon dioxide (CO2). If the carbon dioxide is released into the atmosphere, then the hydrogen produced is referred to as grey hydrogen.[1] Grey hydrogen is not considered a low-carbon fuel.

Blue hydrogen

Blue hydrogen is similar to grey hydrogen, except that most of the CO2 emissions are sequestered (stored in the ground) using carbon capture and storage (CCS).[4] Capturing and storing the carbon dioxide instead of releasing it into the atmosphere allows blue hydrogen to be a low-carbon fuel. The two main production methods are steam methane reforming and coal gasification, both with carbon capture and storage. Blue hydrogen is a cleaner alternative to grey hydrogen, but is expensive since carbon capture technology is used.[1]

Green hydrogen

Green hydrogen is hydrogen produced using electricity from clean energy sources.[4] Green hydrogen is considered low or zero-emission hydrogen because it uses energy sources such as wind and solar which don’t release greenhouse gases when generating electricity. Green hydrogen is made when water (H2O) is split into hydrogen (H2) and oxygen (O2). Water splitting is also known as electrolysis, and requires an energy input. The method of supplying electricity to split water is an expensive process, but much more environmentally-friendly compared to the production of grey hydrogen.[5]

Other Colours

In the energy industry, other colours may be used to differentiate between the types of hydrogen. Although grey, blue and green are most common colours, black, brown, red, pink, yellow, turquoise and white exist as colours for molecular hydrogen.

Colours such as black and brown are more specific types of grey hydrogen. Black hydrogen is when black coal is used (usually in a gasification process) to make hydrogen. Brown hydrogen is when lignite (brown coal) is used to make hydrogen (also a gasification process).[13]

Red hydrogen is made from biomass. Biomass can be transformed to produce hydrogen via gasification. Depending on the type of biomass and the use of carbon capture and storage technologies, red hydrogen can have lower CO2 emissions than grey hydrogen. If the CO2 is completely captured and there are no other emissions, it can be considered green hydrogen.[14]

Pink hydrogen refers to hydrogen generated through electrolysis powered by nuclear energy. Pink hydrogen is usually considered green because it does not produce and CO2 emissions during operation.[14]

The colour yellow sometimes indicates hydrogen produced via electrolysis through solar energy. However, it is also used to indicate that the electricity used for the electrolysis comes from mixed sources. This mix of sources comes from the electrical grid and is based on availability and can range from renewables to fossil fuels.[14]

An upcoming colour of hydrogen is turquoise hydrogen, although it has yet to be proven at a large scale. Turquoise hydrogen is made using a process called methane pyrolysis to produce molecular hydrogen and solid carbon. In the future, turquoise hydrogen may be valued as a low-emission hydrogen, dependent on the thermal process being powered with renewable energy and the carbon being permanently stored or used.[13]

In addition, there is what's called white hydrogen. In nature, we find hydrogen in gaseous form (H2) and it is colorless. That is why, when you hear about “white hydrogen”, it refers to naturally occurring geological hydrogen that might be (rarely) found in underground deposits. There aren't any viable strategies to use these deposits now, which is why we apply different processes to generate it artificially (gasification, electrolysis, methane pyrolysis, etc).[14][15]

Further Reading


  1. 1.0 1.1 1.2 "Hydrogen: grey, blue, and BC green," Resource Works, 2020. [Online]. Available: https://www.resourceworks.com/hydrogen-grey-blue. [Accessed: 11-May-2021]
  2. "What is hydrogen and how is it made?" The World of Hydrogen. [Online]. Available: https://www.theworldofhydrogen.com/gasunie/what-is-hydrogen/. [Accessed: 11-May-2021]
  3. "What is the difference between Grey, Blue and Green?" CertifHy Canada Inc. [Online]. Available: https://www.certifhy.ca/Green%20and%20Blue%20H2.html. [Accessed: 11-May-2021]
  4. 4.0 4.1 4.2 "New Hydrogen Economy - Hope or Hype?" World Energy Council, 2019. [Online]. Available: https://www.worldenergy.org/assets/downloads/WEInsights-Brief-New-Hydrogen-economy-Hype-or-Hope-ExecSum.pdf. [Accessed: 11-May-2021]
  5. "From Grey and Blue to Green Hydrogen," TNO. [Online]. Available: https://www.tno.nl/en/focus-areas/energy-transition/roadmaps/towards-co2-neutral-industry/hydrogen-for-a-sustainable-energy-supply/. [Accessed: 11-May-2021]
  6. Wikimedia Commons, [Online] Available: https://commons.wikimedia.org/wiki/File:Bituminous_coal_(Vandusen_Coal,_Lower_Pennsylvanian;_Irish_Ridge_East_roadcut,_near_Trinway,_Ohio,_USA)_3_(33236930615).jpg
  7. Wikimedia Commons. (May 13, 2015). Lignite [Online]. Available: http://commons.wikimedia.org/wiki/File:Lignite_Klingenberg.jpg
  8. Wikimedia Commons, [Online] Available: https://commons.wikimedia.org/wiki/File:U.S._Department_of_Energy_-_Science_-_394_051_002_(9443064499).jpg
  9. Wikimedia Commons. (July 7, 2015). Nuclear Power Plant [Online]. Available: https://commons.wikimedia.org/wiki/File:Nuclear_Power_Plant_Cattenom.jpg#/media/File:Nuclear_Power_Plant_Cattenom.jpg
  10. Wikimedia Commons. Renewable Energy on the Grid [Online]. Available: https://commons.wikimedia.org/wiki/File:Renewable_Energy_on_the_Grid.jpg
  11. "The quest for CO2-free hydrogen – methane pyrolysis at scale," William Daloz, Frederik Scheiff, Kai Ehrhardt, Dieter Flick, Andreas Bode. BASF, 2019. [Online]. Available: https://arpa-e.energy.gov/sites/default/files/1%20Scale%20up%20BASF.pdf
  12. Wikimedia Commons. Anticline trap [Online]. Available: https://commons.wikimedia.org/wiki/File:Anticline_trap.svg
  13. 13.0 13.1 "The hydrogen colour spectrum," National Grid, 2021. [Online] Available: https://www.nationalgrid.com/stories/energy-explained/hydrogen-colour-spectrum. [Accessed: July 22, 2021]
  14. 14.0 14.1 14.2 14.3 "Hydrogen - in Colors," H2 Industries. [Online] Available:https://h2-industries.com/en/hydrogen. [Accessed: July 22, 2021]
  15. Prinzhofer, Alain, Tahara Cissé, Cheick Sidy, & Diallo, Aliou Boubacar. (2018). Discovery of a large accumulation of natural hydrogen in Bourakebougou (Mali). International Journal of Hydrogen Energy, 43(42), 19315–19326. https://doi.org/10.1016/j.ijhydene.2018.08.193.