50 shades of (grey and blue and green) hydrogen

Puzzled by all the colours & jargon? This article is for you


Publication date

November 13, 2020

When discussing the future of energy, hydrogen is on everyone’s lips. You might have heard of some of the colours associated to hydrogen, and wondered about their meaning. We have put together a short hydrogen glossary for you!

White: naturally occurring

Let’s start from the basics: hydrogen (H) is the lightest element of the periodic table and the most common substance in the universe. It can be used as feedstock, fuel or energy carrier and does not emit CO2 when burnt, that is why you often hear about its high potential for decarbonising our economy.

In nature, we find it mostly in gaseous form (H2) and it is colourless. That is why, when you hear about “white hydrogen”, we refer to the naturally occurring one that might be (rarely) found in underground deposits. We don’t have any viable strategy to use these deposits now, so we apply different processes to generate it artificially. That is what the colours are for: each one refers to the energy source and/or process that was used to produce hydrogen.

Brown/black: the name says it all

The oldest way of producing hydrogen is by transforming coal into gas. Gasification processes convert organic or fossil-based carbonaceous materials into carbon monoxide, hydrogen, and carbon dioxide. Gasification is achieved at very high temperatures (more than 700°C), without combustion, with a controlled amount of oxygen and/or steam. The carbon monoxide then reacts with water to form carbon dioxide and more hydrogen via a water-gas shift reaction.

The gas generated via coal gasification is called syngas and the hydrogen can be separated from the other elements using adsorbers or special membranes. This hydrogen is known as brown or black depending of the type of coal used: brown (lignite) or black (bituminous) coal. It is the result of a highly polluting process since both CO2 and carbon monoxide cannot be reused and are released in the atmosphere.

Hydrogen from biomass

Biomass can also be transformed to produce hydrogen via gasification. Depending on the type of biomass but also on the use of carbon capture and storage technologies, the net carbon emissions can be lower than brown or grey hydrogen.

Grey: the most common one

Most hydrogen nowadays comes from natural gas: it is bonded with carbon and can be separated from it via a process involving water called “steam reforming”, but the excess carbon generates CO2. This hydrogen is called grey whenever the excess CO2 is not captured. Grey hydrogen accounts for most of the production today and emits about 9.3kg of CO2 per kg of hydrogen production. Sometimes, hydrogen is referred to as “grey” to indicate it was created from fossil fuels without capturing the greenhouse gases and the difference with brown or black hydrogen is just in the smaller amount of emissions generated in the process.

Blue…if you put the emissions underground

Hydrogen is considered blue whenever the emission generated from the steam reforming process are captured and stored underground via industrial carbon capture and storage (CSS), so that it is not dispersed in the atmosphere. That is why Blue hydrogen is often considered a carbon neutral energy source, even though “low carbon” would be more accurate since around 10-20% of the generated CO2 cannot be captured.

Turquoise: solid carbon as a by-product

A new way of extracting hydrogen from natural gas is currently in experimentation phase. The gas can be decomposed at very high temperatures generating hydrogen and solid carbon thanks to a process called methane pyrolysis. This hydrogen is then referred to as “turquoise” or low carbon-hydrogen.

If the hydrogen is the result of a process called water electrolysis – that is using electricity to decompose water into hydrogen gas and oxygen, then we have a palette of 3 colours: pink, yellow and green. In this case, the full life-cycle emissions of this electricity-based hydrogen production, depends on how the electricity is generated.

Pink: from nuclear energy

The colour pink is often used for hydrogen obtained from electrolysis through nuclear energy.  

Yellow: using a mix of whatever is available

The colour yellow sometime indicates hydrogen produced via electrolysis through solar power, but often is also used to indicate that the electricity used for the electrolysis comes from mixed sources based on availability (from renewables to fossil fuels).

Green: from renewables

Last but not least comes green hydrogen, often also called “clean hydrogen”, produced using electricity generated from renewables and currently accounting for around 1% of the overall hydrogen production. The European Commission intends to change that and built an entire strategy to support hydrogen, highlighting its potential for a climate neutral Europe and putting it right at the centre of the EU Green Deal (and its conspicuous budget).

There are almost as many types of electrolyser technologies for green hydrogen as there are colours of hydrogen. Davine Janssen from Euractiv.com has a thorough look at the different approaches in China and the EU.

Many of the big energy players and companies welcomed the EU strategy, but it also generated questions around resiliency, economic feasibility, and more importantly for our members, around the potential roles of hydrogen in cities. Keep an eye on our website to learn more in the following weeks!