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Tech brief: Hydrogen classification systems
Hydrogen can be produced in many different ways and from different feedstocks. The choice of method and feedstock gives rise to more or less greenhouse gas emissions. In the general discourse on hydrogen and climate change, it is essential to distinguish between different types of hydrogen. As a result, systems have emerged to classify hydrogen, the most commonly known and used being colors. However, the European Commission has introduced one that focuses more on emissions. This tech brief explains the two systems.
What color does hydrogen have? None at all. It is transparent and cannot be seen with the naked eye. However, hydrogen is called black, brown, gray, blue, green, and so forth. The colors denote the feedstocks and production methods used to produce hydrogen. In this brief, we explain the meaning of the different colors and look at an alternative classification of hydrogen used by the EU.
Tech brief
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Colors of hydrogen
Let’s start by reviewing the color spectrum used to classify hydrogen based on how it is produced and with which feedstock.
- Black hydrogen is produced by coal gasification of black coal (bituminous).
- Brown hydrogen is produced by coal gasification of brown coal (lignite) or biomass gasification.
- Grey hydrogen is produced by steam methane reforming, partial oxidation, or autothermal reforming of natural gas and oil.
- Blue hydrogen is produced as black, brown, or grey hydrogen with the added application of carbon capture, utilization, and storage (CCUS) methods or technologies.
- Turquoise hydrogen is produced by methane pyrolysis.
- Yellow hydrogen is produced by water electrolysis using electricity from the grid regardless of how this electricity has been produced.
- Pink hydrogen is produced by water electrolysis using electricity from nuclear power.
- Purple hydrogen is produced by thermochemical water splitting using energy from nuclear power.
- Red hydrogen is produced by high-temperature catalytic splitting of water using energy from nuclear power.
- Green hydrogen is produced by water electrolysis using renewable energy sources.
- White hydrogen is naturally occurring hydrogen found as a free gas in layers of continental crust, deep in the oceanic crust, or in volcanic gases, geysers, and hydrothermal systems.
The list above can be summarized in this schematic table illustrating the relationships between the different color classes used for hydrogen and the feedstock and process used to produce it.
| Feedstock | Production method | Hydrogen color | ||
|---|---|---|---|---|
| Natural gas | Methane pyrolysis | Turquoise | ||
| Steam reforming | Gray | Blue (if carbon is captured) | Yellow (if used to produce electricity for water electrolysis) | |
| Black coal | Gassification | Black | ||
| Brown coal | Gassification | Brown | ||
| Biomass | ||||
| Renewable electricity | Water electrolysis | Green | ||
| Nuclear electricity | Water electrolysis | Pink | ||
| Water splitting | Purple | |||
| Catalytic splitting | Red | |||
Caveat
The hydrogen industry’s color classification is evolving, and not all terms have universally agreed-upon definitions. This can lead to variations in how terms are used and understood.
Yellow hydrogen is an example that shows variation in how terms are used. Above, it’s described as hydrogen produced by water electrolysis, where the electricity comes from the grid and, therefore, can be produced by anything from coal-fired power stations to wind turbines. But quite a few people have a completely different definition: Yellow hydrogen is produced by water electrolysis using electricity from solar power plants. This is an entirely different definition far from the one used by most international organizations, governments, authorities, and researchers.
Green hydrogen is an example that shows variation in how terms are understood. Above, it’s described as hydrogen produced by water electrolysis, where the electricity comes from any renewable source. Biomass is an example of a renewable source, but it’s not very” green” when used as fuel in power plants. Therefore, the EU Commission and Parliament use a definition of green hydrogen that excludes biomass and also requires at least a 70 percent reduction in greenhouse gas emissions compared to fossil fuels.
In addition, some use violet instead of pink, and others conflate pink, purple, and red hydrogen and use one of these colors to mean all of them.
On top of that, researchers and developers slap new colors on each novel method of creating hydrogen. For example, aqua is used for hydrogen extracted from oil sands and oil fields, and orange is used for hydrogen produced by chemical reactions in iron-rich underground formations.
EU classification
However, as we have seen, there is no consensus on which colors to use and what they stand for. This has prompted the European Commission to introduce its own classification in the commission’s hydrogen strategy for a climate-neutral Europe:
- Electricity-based hydrogen is hydrogen produced through water electrolysis regardless of the electricity source. (Thus, includes both yellow and green hydrogen).
- Renewable hydrogen is hydrogen produced through water electrolysis with electricity from renewable sources. (Thus, it’s a large subset of green hydrogen).
- Clean hydrogen is another name for renewable hydrogen.
- Fossil-based hydrogen is hydrogen produced through various processes using fossil fuels as feedstock. (Thus, this includes black, brown, gray, and blue hydrogen.)
- Fossil-based hydrogen with carbon capture is a subpart of fossil-based hydrogen, but where greenhouse gases emitted as part of the hydrogen production process are captured. (Thus, this is equivalent to blue hydrogen.)
- Low-carbon hydrogen encompasses fossil-based hydrogen with carbon capture and electricity-based hydrogen, with significantly reduced full life-cycle greenhouse gas emissions compared to existing hydrogen production. As of February 7, 2023, this means at least a 70% reduction in greenhouse gas emissions compared to fossil fuels. (Thus, this contains a subset of blue hydrogen and a subset of yellow hydrogen.)
- Hydrogen-derived synthetic fuels refer to various gaseous and liquid fuels based on hydrogen and carbon. (Thus, this has no equivalent color.)
The table below illustrates the relationships between the European Commission classification and the color classification.
| Hydrogen color classification | EU hydrogen classification |
| Black | Fossil-based hydrogen |
| Brown | |
| Gray | |
| Blue | Fossil-based hydrogen with carbon capture |
| Low-carbon hydrogen | |
| Turquoise | Low-carbon hydrogen |
| Red | |
| Purple | |
| Pink | Electricity-based hydrogen |
| Yellow | |
| Green | Renewable hydrogen |
Purpose of classification
The classification of hydrogen helps various groups like politicians, business leaders, and environmentalists to quickly identify its production impact on the environment. This classification separates hydrogen with high greenhouse gas emissions (black, brown, and gray) from low-emission (blue) and completely fossil-free types (pink and green).
Present and future
In 2022, almost 95 million tonnes (Mt) of hydrogen were produced globally, according to Global Hydrogen Review 2023. 99.3% was fossil-based hydrogen (black, brown, or gray). Only 0.6% was fossil-based hydrogen with carbon capture (blue). And pity 0.1% was electricity-based or renewable hydrogen (pink, yellow, or green).
Over the next six years, the Global Hydrogen Review 2023 predicts a sharp increase in demand. The demand is expected to increase by more than 50%, reaching 150 tons of hydrogen by 2030.
Analysts and experts generally believe that virtually all of this increase will consist of hydrogen produced by water electrolysis. Data from the Hydrogen Production and Infrastructure Projects Database shows that in October 2023, there were over 1,500 planned or ongoing constructions of electrolyzer plants to be completed by 2030. Together, they will generate 92 Mt of hydrogen, of which 66 Mt are green, 4 Mt are yellow, and the rest are unspecified.
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