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Grey to Green: Hydrogen's Carbon Footprint


Image credit: Petrofac

Hydrogen is the most abundant chemical substance in the universe, constituting an estimated 75% of all matter.


Today, our Hydrogen use is dominated by industry and manufacturing, including oil refining, ammonia production, methanol production and steel production. Almost all of our chemical fuels are based on hydrogen and the majority of hydrogen used today is produced from steam reforming of natural gas, oil reforming, or coal gasification.


To tackle the climate change crisis, solutions need to be found for generating carbon-­neutral and more sustainable fuels. Only a small percentage of the hydrogen we use is currently produced using sustainable methods such as the electrolysis of water. These variations in production and sustainability lead to the colour coding system, with three main categories of hydrogen: grey, blue, and green.


Grey Hydrogen

The most common form of hydrogen currently produced, grey hydrogen is derived from natural gas and produced from fossil fuels, making it the most inexpensive but also the least renewable form of hydrogen.


Grey hydrogen is produced by reforming natural gas, a processing technique used to rearrange the molecular structure of hydrocarbons. In this process, methane—the primary element in natural gas—is mixed with steam at a high temperature to yield hydrogen and carbon dioxide through a catalytic chemical reaction.


Unfortunately, almost 10 kg of carbon dioxide is released into the atmosphere for every 1 kg of grey hydrogen produced. This high ratio of CO2 generation gives this form of hydrogen its "grey" designation.

Blue Hydrogen

Blue hydrogen is produced using the same chemical processing technique as grey hydrogen but includes supported by carbon capture and storage technology to manage the CO2 emitted. With blue hydrogen, the majority of CO2 produced does not escape into the environment. Instead, it is captured at the production facility and stored separately, often underground. As the greenhouse gasses are captured, this mitigates the environmental impacts on the planet, and the result is low-carbon hydrogen that produces only around 10-20% of the CO2 emissions of grey hydrogen.


The emissions capture technology is known as Carbon Capture and Storage (CCS) but whilst more attractive than grey hydrogen, the blue hydrogen process does not eliminate carbon emissions into the atmosphere entirely, while storage is costly and has logistical challenges.

Green Hydrogen

Green hydrogen on the other hand is hydrogen generated by renewable energy such as via solar, wind or offshore energy or from low-carbon power.


The technique for producing green hydrogen uses electrolysis, the separation of hydrogen and oxygen molecules, by applying electrical energy to water.

By deploying renewable sources instead of fossil fuels, green hydrogen production is a closed loop of sustainable energy in which no harmful gases are created at any point in the production chain.


The challenges with green hydrogen are primarily cost and availability. Until hydrogen adoption accelerates, electrolysers are prohibitively expensive, while the supply of clean electricity from solar and wind sources is limited and demand for sustainable green hydrogen is high, driving up prices.


Sadly, estimates state that ~0.03% of the hydrogen produced today is green hydrogen, but this is expected to rise as the cost of renewable energy continues to fall, and as investors and governments cotton on to the fact that, truly, green hydrogen is the answer to our energy problems.

ACUA Ocean Hydrogen

At ACUA we are committed to both sustainability and scalable platform solutions. We have sourced green liquid hydrogen for our demonstration projects and we are working towards our ISO14001 Environmental certification. We see a future where as green hydrogen becomes more rapidly adopted, so the cost of the production and storage technology will rapidly fall - making green hydrogen a natural and affordable successor to diesel fuels.