Hydrogen is forecast to replace 25% of all oil demand by 2050, as governments around the world push to decarbonise their economies with emissions-free fuel.

But not all hydrogen is created equal. Right now, almost all hydrogen made globally is either ‘brown’ 'grey', and is produced from fossil fuels. This has led to a surge of interest in the production of ‘green’ hydrogen, using renewable energy.

Hydrogen can also be black and blue, but it’s green hydrogen that is generating so much interest and excitement around the world.

Here’s what all the colours of hydrogen mean, and why green hydrogen is the most promising fuel for a low carbon future.

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Brown (or black) hydrogen

Brown or black hydrogen is created through burning coal or lignite. It’s considered the least environmentally friendly, creating as much carbon dioxide as burning the source fuel would have in the first place. For every tonne of brown hydrogen produced, around 10-12 tonnes of CO2 are produced.

Grey hydrogen

Almost all the hydrogen that’s produced for industrial use at the moment is called “grey hydrogen”. This means it’s derived mainly from natural gas through a process called steam reforming that combines natural gas with water (as steam) to create hydrogen gas. The process also produces large volumes of CO2 - about nine parts for every one part of hydrogen it creates.

Blue (and turquoise) hydrogen

Blue hydrogen is created by steam reforming, but the CO2 by product has been captured and stored during the manufacturing process. Generally, carbon capture and storage involves injecting CO2 underground. Blue hydrogen processes can potentially halve the amount of carbon produced, so it’s certainly considered cleaner than brown hydrogen, but it does still emit greenhouse gases. Turquoise hydrogen is similar to blue hydrogen, using a process called methane pyrolysis to produce hydrogen and solid carbon, which can then be permanently stored and used.

Green hydrogen

Green hydrogen is produced from electrolysis powered by electricity generated from renewable sources, mainly hydro, solar or wind generation. This means it doesn’t emit any greenhouse gases, making it a remarkable carbon-zero fuel source. Green hydrogen is produced through water electrolysis, where an electrical current splits water into its components of hydrogen and oxygen. When hydrogen is combusted with air to create energy the emission is water.

Due to its eco-friendly production and its potential for powering heavy industry and transport, many experts think green hydrogen will become an increasingly significant energy source globally in the next five to ten years.