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New Energy Insights: The Colours of the H2 Rainbow

This article is part of our New Energy Insights series from our Energy, Infrastructure and Resources team. Stay tuned for regular updates and commentary on topical issues across the sector.


Hydrogen contains roughly 2.4 times as much energy as natural gas, making it a resource that many are eager to capitalise on. However, for hydrogen to be utilised as a source of energy, it must first be broken down into its pure form, requiring processes to extract it from the water, biomass, fossil fuels or minerals that it is commonly a part of. It has been classified using increasingly colourful names as new technological developments produce different combinations of “hydrogen +”. These names given to the different forms of hydrogen correspond to the processes used to extract it. 

With billions of dollars now being invested in the global hydrogen economy, the proper classification – and with that, the divergences in price and carbon content - of the various forms of production, is now more important than ever.

Brown and black hydrogen

Brown hydrogen, made from brown coal methane gas and other fossil fuels, and black hydrogen, made from black coal, are produced via gasification.

A coal to hydrogen gasification plant produces synthetic hydrogen gas generated by partially oxidising brown coal feedstock. The resulting carbon monoxide is then converted into carbon dioxide with steam and the hydrogen is separated out through a refining process.

It is expected that for every 150 tonnes of brown coal used during this process, three tonnes of hydrogen will be produced.

While brown hydrogen has long been the most abundant in use for its affordability, the shift towards environmentally friendly alternatives now means this extraction process is no longer viable in the long term.

For every tonne of brown hydrogen produced, the by-product is between 10 and 12 tonnes of CO2. This environmental impact is worsened when you take into account the CO2 produced by transporting the hydrogen to and from the plant and the CO2 produced in driving the process.

Overall, the production of brown and black hydrogen is inefficient and extremely damaging to the environment and has resulted in significant research and investment into alternative extraction processes.

Grey hydrogen

Grey hydrogen is the most common form of hydrogen production and is extracted from natural gas using a process called steam methane reforming which releases carbon dioxide into the atmosphere.

This form of hydrogen is referred to as “grey” to indicate it was created from fossil fuels without capturing the greenhouse gases. The main difference compared to brown or black hydrogen is the fewer emissions generated in the process.

Blue hydrogen

Blue hydrogen uses the same process as grey hydrogen except that the carbon emissions are captured and stored underground using carbon capture, utilisation and storage (CCUS) technology leaving nearly largely pure hydrogen. Often, the CO2 is then transported by a pipeline and stored deep underground, often in salt caverns or depleted oil and gas reservoirs.

It is often considered a carbon neutral energy source, however, “low carbon” would be more accurate since around 10 – 20% of the generated CO2 cannot be captured. Blue hydrogen is often seen as a stepping-stone from grey to green, however, it has proven to be extremely contentious among stakeholders in the industry.

Green hydrogen

Green hydrogen (GH2) is a relatively new development in the hydrogen fuel industry and involves extracting hydrogen in an environmentally sustainable way via a process called electrolysis.  

Through electrolysis, water is split into hydrogen and oxygen. The electricity that powers the electrolyser comes from renewable energy sources, such as wind and solar. Hydrogen and oxygen are the products of the process, meaning that no greenhouse gas emissions are produced.

Clear hydrogen

Clear hydrogen – a nascent form of hydrogen production - is produced by extracting hydrogen from water, like GH2, but rather than using electrolysis, it does so using water that is subjected to external influences coupled with extremely rapid variations in pressure, temperature and motion. There is no heat, and therefore, no carbon is used or generated in the process.

The goal is that this should enable hydrogen production at a significantly reduced cost, without the carbon impact. In addition, this process is proposed to be much less capital intensive than other hydrogen technologies, further reducing production costs.

Turquoise hydrogen, pink hydrogen, yellow hydrogen and others. 

Turquoise hydrogen is produced by breaking methane down into hydrogen and solid carbon using a process called pyrolysis. While it was initially thought that this process produced relatively low emissions because the carbon by-product can be buried or used for industrial processes, recent research shows that turquoise hydrogen is no more carbon-free than blue hydrogen.

Pink hydrogen is extracted through nuclear-powered electrolysis. Nuclear-produced hydrogen can also be referred to as purple hydrogen or red hydrogen.

Yellow hydrogen is a relatively new process for hydrogen extracted through electrolysis using solar power.

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Promotion of GH2 - certification and standardisation

Global concerns over climate change, air pollution and the environment have pressured governments to reduce their carbon footprint, and GH2 is being coveted as playing an instrumental part in achieving this. GH2 is the only truly carbon-zero extraction process with no carbon footprint. However, it does face its limitations.

Currently, GH2 is more costly to buy per USD / kg than other dirtier forms of hydrogen.

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Demand creation is also unfortunately still lagging behind what is needed to help in reaching a net-zero emissions renewables economy with GH2 as a key component.

There are a variety of measures that still need to be taken to help with its adoption such as policy instruments like auctions, mandates, quotas and hydrogen requirements in public procurement can help de-risk investments and improve the economic feasibility of low-carbon green hydrogen.

Central to this will be establishing appropriate certification, standardisation and regulation regimes internationally to ensure that hydrogen production is truly low carbon. This in turn will give consumers the confidence they require that by investing in GH2, they are getting exactly what they have paid for, ie hydrogen produced using zero greenhouse gases.

We will be considering this certification and regulation issue further in an upcoming article.


The Hamilton Locke team advises across the energy project life cycle – from project development, grid connection, financing, construction, including the buying and selling of development and operating projects.

Matt Baumgurtel leads the New Energy sector team at Hamilton Locke which specialises in renewable energy, energy storage and hydrogen projects and transactions as part of the firm’s Energy, Infrastructure and Resources practice.

David O’Carroll is an Associate in Hamilton Locke’s New Energy sector team and specialises in renewable energy projects including wind, solar, energy storage and hydrogen.

Bridget Hudson is a paralegal in Hamilton Locke’s New Energy sector team and specialises in renewable energy projects including wind, solar, energy storage and hydrogen.