Dictionary of future fuels

What is green hydrogen? What is pink hydrogen? What does Power-to-X technology mean? When it comes to future fuels, there is no shortage of jargon. We made a dictionary of future fuels to explain key terminology understandably.

“Terminology can be perplexing. Why does the energy industry speak of green, blue, grey and pink hydrogen? We put together a dictionary to offer clear-cut explanations of the essential terminology relating to future fuels and decarbonisation,” says Tommi Rintamäki, Senior Manager, Power-to-X at Wärtsilä Energy.

You can check out our dictionary of future fuels further down in the article here. But if you first want to learn more about why there are so many colours of hydrogen, keep on reading.

The hydrogen colour spectrum

Hydrogen is a colourless gas. It’s the most abundant element in the Universe, but it rarely occurs naturally as a gas on Earth. Instead, it combines with many elements to form molecules that are important for life, like water.

All the hydrogen used by the energy industry is always produced in some way. The energy industry uses different colours to describe what source of energy has been used in the hydrogen production.

“You need a primary source of energy to produce hydrogen – for example solar, wind or nuclear power. Some forms of hydrogen production create CO₂ as a by-product, and it makes a world of difference what is done with the CO₂,” Rintamäki explains.

Green hydrogen is the cleanest option. It is produced from water via electrolysis with renewable electricity, without any CO₂ as a by-product. Blue hydrogen is produced by splitting fossil natural gas into hydrogen and CO₂ and then capturing and storing the CO₂. Grey hydrogen is created the same way as blue hydrogen, however the carbon dioxide is not captured but released into the atmosphere. Pink hydrogen is made via electrolysis using nuclear power.

What about brown, black, white and yellow hydrogen?

“Transforming coal into gas is the oldest way of producing hydrogen. The end product is called either brown or black hydrogen. It’s a highly polluting process that releases CO₂ in the atmosphere – and thus not described in our dictionary of future fuels,” Rintamäki says.

“White hydrogen refers to the naturally occurring hydrogen on Earth, while yellow hydrogen is simply one type of green hydrogen. It is called yellow because it is created solely with solar power. At Wärtsilä Energy, we try to keep things simple, and just speak about the four main types of hydrogen: green, blue, grey and pink,” he concludes.


Green hydrogen

Green hydrogen is produced by splitting water via electrolysis, which produces only hydrogen and oxygen. However, the electrolysis process requires electricity. For hydrogen to be labelled green, the electricity used in production must come from renewable energy sources, like solar, wind or hydropower. Green hydrogen is sometimes also called renewable hydrogen.

Blue hydrogen

Blue hydrogen is produced by splitting fossil natural gas into hydrogen and CO₂ – and then capturing, storing, or reusing the CO₂ to mitigate environmental impacts. Blue hydrogen is sometimes also called low carbon hydrogen.

Grey hydrogen

Most hydrogen nowadays comes from fossil natural gas. Grey hydrogen is produced the same way as blue hydrogen: by splitting natural gas into hydrogen and CO₂. Hydrogen is called grey whenever the excess CO₂ is not captured. Grey hydrogen is sometimes also called fossil hydrogen.

Pink hydrogen

Pink hydrogen is made from water via electrolysis just like green hydrogen but using nuclear energy as its source of power. Nuclear-produced pink hydrogen is sometimes also called purple hydrogen or red hydrogen.

Power-to-X technology

‘Power-to-X’ is used as an umbrella term for various emerging technology solutions for electricity conversion, energy storage, and energy reconversion, all of which use renewable electricity to produce for example synthetic fuels.

Power-to-X technology essentially means turning energy into something else. For instance, renewable electricity can be turned into synthetic natural gas by combining CO₂ captured from the air and hydrogen.

Direct air capture

Direct air capture (DAC) is a process that extracts CO₂ directly from the air. The captured CO₂ can then be converted to many products that normally originate from fossil materials.

Currently, frontrunner DAC technology can be used for example in an office building’s ventilation to reduce CO₂ levels inside the building. In the future, DAC may be used on an industrial scale to filter excess carbon dioxide out of the air.


Methanisation is a process that uses renewable electricity to combine hydrogen and CO₂, turning it into methane. Currently most of the methane used for heating, transport and power generation is fossil. Methanisation – also called Power-to-Gas – provides a carbon neutral alternative.

Synthetic fuels (eFuels)

Synthetic fuels are not the same thing as fossil fuels, which are a finite resource. Synthetic fuels, or eFuels, are created by generating fuel from CO₂. In synthetic fuel production, CO₂ is used as a raw material for creating for example gasoline, diesel, and substitute natural gas, together with electricity from renewable sources and hydrogen.

Carbon neutral vs carbon negative

Energy is carbon neutral if the amount of CO₂ emissions released into the atmosphere during energy production is the same as the amount of CO₂ emissions removed from the atmosphere during the process. If the amount of CO₂ emissions removed during energy production is greater than the emissions released, energy is labelled carbon negative.

Net zero

Energy is net zero, if the CO₂ emissions generated during energy production are offset by the same amount of CO₂ elsewhere, for example through reforestation schemes, making the “net total” of emissions zero. Net zero carbon emissions are considered a synonym for carbon neutrality.

Carbon free

When energy sources are labelled carbon free, the energy is produced by a resource that generates no carbon emissions. Examples include solar and wind energy – or when speaking about fuels, hydrogen and ammonia.

Hydrogen blends

Hydrogen blending refers to blending natural gas with hydrogen. The blend can then be used by the conventional end-users of natural gas to generate power and heat, as long as the technology itself allows it. Using hydrogen blends may require some adjustments to the technology – especially when talking about higher shares of hydrogen. At present, hydrogen blending is at a fairly early stage of development, but it is a promising option for expanding the use of hydrogen to reduce the environmental impact of natural gas.


Decarbonisation refers to reducing or removing CO₂ emissions for example through reducing or stopping the use of fossil fuels, switching to cleaner fuels, and using electric vehicles in transportation.

In the energy industry, decarbonisation means reducing ‘carbon intensity’ i.e., cutting or eliminating the emissions per unit of electricity generated (grams of carbon dioxide per kilowatt-hour).

Decarbonisation is needed to mitigate or reverse climate change and lowering the CO₂ levels in the atmosphere.

Hydrogen economy

The hydrogen economy is an envisioned future, in which hydrogen delivers a substantial fraction of a nation’s energy. In a hydrogen economy, hydrogen would be used both as a fuel and an energy carrier to help offset fluctuations in solar and wind power production and consumption.

Future fuels

Future fuels refer to a wide range of promising, carbon-neutral fuels, which can be used to advance decarbonisation. Future fuels include, for example, green hydrogen and hydrogen-based fuels such as synthetic methane, ammonia, and methanol, as well as bio and synthetic liquefied natural gas (LNG).

Text: Joanna Sinclair
Photo:  Wärtsilä

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