A rainbow of possibilities: Hydrogen colours explained

Hydrogen is rapidly building a reputation as a fuel of the future. The UK Government’s Ten Point Plan for a Green Industrial Revolution, published back in November 2020, outlined a commitment to the creation of a hydrogen economy, confirming that the gas would play a crucial role in the pursuit of a net zero nation. And while 2023 was a frustrating year for the hydrogen sector, characterised by slow policy decisions and delayed investment, the recent Hydrogen Storage and Hydrogen Transport Business Models have begun to give shape to the emerging hydrogen marketplace, setting a path towards future growth.

Outside the gas sector, opinions remain split on how useful hydrogen is as a fuel, with questions around sustainability, practicality and efficiency yet to be properly addressed. A major factor adding to the confusion is that not all types of hydrogen are created equal when it comes to carbon emissions. Where hydrogen comes from, or the method used to produce, it will have a huge impact on how ‘green’ the hydrogen is - and green is just one of the colours it comes in.

Here, we take a closer look at hydrogen in all of its colours and aim to provide an understanding of where different types of hydrogen come from and what production methods mean for the gas’ sustainability credentials.

The hydrogen rainbow

While hydrogen is an invisible gas, any conversations about using hydrogen as a fuel must necessarily include references to its colour. This is the fastest way to understand how the hydrogen was produced and how far it potentially qualifies as a renewable fuel source.

Before we dive into the different hydrogen colours however, it’s important to note that colour classification is only a starting point for the conversation around hydrogen and net zero. Several factors will influence the carbon intensity of hydrogen and the practicality of its production, and this isn’t always as straightforward as choosing green. The UK Government has already recognised this fact by choosing to focus on carbon intensity at the point of production, setting a 4 kg of CO2 per kilogramme threshold, rather than dictating the means of production. Oversimplifying the classification of hydrogen could lead to missed opportunities or production shortages - something we shouldn’t risk during the already challenging transition to a low carbon energy system.

Green hydrogen

Green hydrogen is made by using renewable energy for electrolysis of water. This splits the water into hydrogen and oxygen, emitting zero carbon dioxide in the process. In our future energy system, this production method could potentially make use of excess renewable energy, helping to build in efficiencies and balance the grid. While hydrogen production from electrolysis is not the most efficient way to produce energy, it could provide an essential heat source for hard to abate industries, in place of fossil fuels.

Production costs for green hydrogen are currently high, but this would be likely to quickly reduce as production increases.

The future may also see us re-term some green hydrogen as ‘yellow hydrogen’ where the fuel source is specifically solar power.

Pink (or sometimes purple) hydrogen

Pink hydrogen uses the same process of electrolysis as used for green hydrogen, but relies on nuclear energy rather than wind, solar or other renewable sources.

Blue hydrogen

Blue hydrogen is produced using natural gas and heated water, through a long-established process called steam reforming or sometimes steam methane reforming (SMR). The hydrogen produced is usually referred to as ‘low carbon hydrogen’ as some carbon dioxide is also produced during the process. By definition, the blue hydrogen production process must include carbon capture and storage (CCS). This reduces carbon emissions by moving the carbon for permanent storage underground. A better option is Carbon capture, use and storage (CCUS), which would see the captured carbon reused in industrial processes.

Grey hydrogen

Like blue hydrogen, grey hydrogen is produced using natural gas and steam reforming. However, greenhouse gases are not captured and stored during the process. Grey hydrogen still dominates the marketplace globally, accounting for around 95% of all production. Approximately 90 millions tonnes of grey hydrogen is consumed each year but this is expected to decline as the production of green hydrogen ramps up.

Black and brown hydrogen

Black and brown hydrogen is produced in the least environmentally-friendly way with black coal or lignite (brown coal) used to power the electrolysis process.

Turquoise hydrogen

This is a new technology that is yet to be scaled up. A process called methane pyrolysis is used to produce hydrogen and solid carbon. It is possible that turquoise hydrogen may be a valuable low-carbon fuel source in the future, provided that the process can be powered using renewables and that the carbon produced can be used or stored.

White (or sometimes gold) hydrogen

White hydrogen, or geologic hydrogen, exists as a naturally occurring gas found in underground deposits. While white hydrogen exploration activities are underway in some countries and recent news articles have suggested that this untapped resource could give rise to a ‘hydrogen gold rush’, there is much scepticism surrounding its usefulness as a future fuel source. Crucially, there are environmental concerns related to the extraction of white hydrogen, while most of the available resource is likely to be too deep or too far offshore to be economically recovered. There is currently no plan to exploit this source of hydrogen here in the UK.

The future of hydrogen production

Hydrogen is one of the most abundant elements in the universe, estimated to contribute 75% of the mass of the universe. When burned, it produces no greenhouse gases, meaning it has the potential to replace natural gas and help us protect the future of our planet. However, it rarely exists as a gas here on Earth, so the challenge that remains is to make production of hydrogen practical, efficient and cost-effective. While it’s certain that the old ways are no longer the best, and that black and brown hydrogen production methods need to be replaced with more Earth-friendly alternatives, hydrogen certainly seems to hold a place in our net zero future.

When considering future production, it will also be important to think from a ‘whole system’ angle; careful siting of hydrogen production plants could ensure that the water needed as feedstock puts no additional strain on fresh water supplies, and that the water produced can be recycled as an important resource. In this way, hydrogen production can become an integral part of the energy system for generations to come.