Hydrogen - in Colors

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 decarbonizing our economy. But we don’t want to decarbonize our atmosphere in total, we only want to come back to a CO2 ratio in the atmosphere like is was around 1900. What we really want to achieve is a defossilizing of our planet, which means to eliminate the burning of fossil fuels.

In nature, we find it mostly in gaseous form (H2) and it is colorless. 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 colors are for: each one refers to the energy source and/or process that was used to produce hydrogen

Red 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. If we capture the CO2 completely and if we don’t have other emissions, it is like green hydrogen, but we call it red 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 colors: 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

Green hydrogen, often also called “clean hydrogen”, produced using electricity generated from renewables and currently accounting for a small percentage 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 center of the EU Green Deal (and its conspicuous budget).