H – What does the color spectrum of Hydrogen mean?

Before to elaborate on the topic in the headline of this article, let me just remind that the Hydrogen element is an odorless and colorless gas. If observed in the Universe in the interstellar space it can give a pinkish red glow, this being only a visual effect created when Hydrogen is projected by the radiation from nearby stars, which is known as alpha hydrogen. But again as element, Hydrogen is totally colorless.

As fuel, Hydrogen is a versatile power source, by itself hydrogen doesn’t generate any greenhouse gas emissions, yet its production processes do. And here is where colors come in. They’re essentially colour codes, or nicknames, used within the energy industry to differentiate between hydrogen production processes. But there is no universal naming convention and these colour definitions may change over time, and even between countries. New colors could be added as well, based on future development ideas for hydrogen production. What we have today are the following 12 types:

1. BROWN;
2. BLACK;
3. GREY;
4. BLUE;
5. GREEN;
6. DARK GREEN;
7. RED;
8. PINK;
9. PURPLE;
10. TURQUOISE;
11. YELLOW;
12. WHITE.

And despite their colourful descriptions, there is no visible difference in the final product. The subdivision of hydrogen into different colours is only intended to provide information about how the hydrogen is produced, the energy sources used and the climate neutrality of the hydrogen. From the 12 codes as just listed only the “Green Hydrogen” is 100% clean of greenhouse emisions. All the rest are more or less “dirty” versions. In certain conditions Red, Pink, Yellow and White could be easily comparable with or even considered new versions of “green”.

In the future, some hydrogen colours may fade in importance and others burn brighter. What’s certain is that the hydrogen rainbow will play a significant role in reaching  net zero as we reduce our historical reliance on fossil fuels and look to green alternatives to power our homes, businesses and transport. From these 12 types, the most popular forms of hydrogen nowadays are:

• Grey,
• Blue
• Green.

This is mainly due to the high availability of the source of hydrogen and the accumulated knowledge in the production of hydrogen leading to these 3 types. The description of hydrogen by color is a way for those active in the hydrogen industry to quickly communicate with each other about the production processes that create the hydrogen. The color distinctions are important as they not only reveal the source, but also the associated carbon emissions that come with the production method.

Hydrogen is considered to be a futuristic fuel with a promise to be free of carbon emissions, that’s why “Green hydrogen” is the most environmentally friendly. As there are many different types of production methods. It is important for producers to monitor the hydrogen quality to be contamination-free, in order to ensure that the fuel can be usable for end-consumers. This kaleidoscope of color codes helps differentiate between the various production methods. Here’s a roundup up of what each one means:

BLACK & BROWN

BLACK and BROWN HYDROGEN represent the traditional process for making hydrogen. Both types are produced from coal through a method called coal gasification. This is an established process that uses heat to turn carbon-rich coal into gas.

• Black hydrogen is produced from bituminous coal
• Brown hydrogen is produced from lignite coal.

However the gasification of coal is a very high pollution process, cabon dioxide (CO₂) and carbon monoxide (CO) are likewise produced as by-products which are then released into the atmosphere. From all the hydrogen spectrum, black and brown hydrogen are the most environmentally damaging. This production process emits more than 20kg of CO₂ for every 1kg of Hydrogen produced. To date fortunately it is no longer very common. However according to the International Energy Agency (IEA). In 2020, around a 5th of hydrogen was still made using coal gasification.

GREY

Currently, this is the most common and cheapest form of hydrogen production. Estimates show that more than 90% of all hydrogen produced worldwide today is classified as GREY HYDROGEN. Grey hydrogen is produced from fossil fuel through a process called Steam Methane Reforming (SMR) of natural gas. In SMR – natural gas is maintained in steam (water vapor) at high temperatures (700°-1000°C) at 3-25 bar pressure. Under these conditions using a catalyst the methane reacts with steam and produce hydrogen as outcome. Steam reforming is an endothermic process – that is, heat must be supplied to the process for the reaction to proceed. Subsequently as result of water-gas shift reaction, besides hydrogen the process releases other greenhouse emissions too (Carbon monoxide and carbon dioxide). However Grey hydrogen generates significantly lower CO₂ emissions than the black and brown versions, SMR emits 8-12 kg CO2 per  1kg of H versus the 20kg CO2/1kg H by coal gasification. In 2020, it was estimated that 2% of global carbon emissions come from grey hydrogen.

BLUE

BLUE HYDROGEN is essentially produced in the same way like grey hydrogen. The key difference is that the blue hydrogen include the use of Carbon Capture Storage Technology (CCS) to trap and store most (albeit not all) the by-product carbon dioxide underground or to bound it in a solid product (such as bricks) for later use. Also as an improvement of SMR process, Auto-Thermal Reforming (ATR) used for blue hydrogen combines the steam reforming reaction and fuel oxidation into a single unit. This process is more efficient and is able to capture more of the CO₂ emitted in course of production. It is estimated that for every 1 kg of hydrogen produced, carbon emissions fall to 1.5 – 5 kg of CO₂, depending on how much CCS is used. Blue hydrogen is sometimes called ‘low-carbon hydrogen’ as the production process doesn’t avoid the creation of greenhouse gases, just stores them away.

GREEN

GREEN HYDROGEN is produced through water electrolysis using electricity generated from renewable energy sources (RES). This is an electrochemical process in which the water molecule is split into its constituent elements hydrogen (H₂) and Oxygen (O) without any subsequent greenhouse emissions. The energy input as electricity required to power the process relies on renewable sources (such as solar, wind or hydropower) and since neither the process not the final products (hydrogen gas and oxygen gas) are harmful for the environment, the green hydrogen is 100% Eco-Friendly. When used in a fuel cell, the only by-product of its use is the pure water that was originally used in its production.

Although “green” hydrogen often refers to electrolytic hydrogen produced using electricity generated from renewable energy sources, it can also refer to hydrogen produced via different methods using other renewable sources such as biogas, biomethane, bio-waste (often refered as Dark Green) – and other renewable sources- such as the recently experimented Quantum Hydrogen which is a promising technique for the direct conversion of solar energy (a.k.a. light) into chemical fuels – these methods are less common than water electrolysis but also result in either very low or zero emissions.

However, due to its currently high cost (for instance the electrolyzes required for the process are made from expensive materials – such as platinum-  and are still not produced in large scale) green hydrogen currently makes up only a small percentage of the overall produced hydrogen. It’s a great alternative to grey and blue, but for now the main challenge for green is in reducing the production costs of green hydrogen to make it a truly obtainable renewable and environmentally friendly alternative. By “reducing cost” meaning being able to produce green hydrogen at 1$/kg or even less.

YELLOW

YELLOW HYDROGEN is essentially a form of green hydrogen. Is a relatively new phrase for hydrogen made through electrolysis directly powered by solar energy. However it is sometimes used to describe electolized hydrogen created from a mix of renewables and fossil fuel power. Yellow hydrogen uses solar panels to collect energy from the sun and convert it into electricity. That electricity is then used to power the electrolyzers that split water molecules into hydrogen and oxygen. The renewable power allows this process to occur without greenhouse gas emissions. The resulting H₂ is also used without producing greenhouse gas emissions, meaning that it is often viewed as one of the cleanest alternatives to using fossil fuels.

DARK GREEN

This type of hydrogen is also sometimes labelled “green” because it comes from steam reforming of biomethane, or gasification of solid biomass but those are not likely to be major sources. In principle is similar with grey hydrogen, the key difference is that DARK GREEN HYDROGEN is produced from biomethane extracted from agricultural waste-streams; from existing farm-based resources and from the establishments of new agroforestry systems. It also uses the carbon capture technology similar with blue hydrogen, It is of course a better alternative to both grey and blue; the system to produce dark green hydrogen is designed around the integration of anaerobic digestion, micro-scaled pyrolysis and biomass gasification technologies to produce hydrogen fuel. But it still produce negative emissions, so it’s not really 100% Eco-friendly.

TURQUOISE

This is a new entry in the hydrogen colour charts and production has yet to be proven at scale. TURQUOISE HYDROGEN can be extracted by using the thermal splitting of methane via methane pyrolysis. The process, though at the experimental stage, remove the carbon in a solid form (aslo known as ”carbon black“) instead of CO₂ gas. If the carbon remains permanently bound and is not combusted during further processing, this process is also CO₂-neutral. The reactors or blast furnaces used to split the methane would also have to be powered by renewable energies. Another factor in the analysis of turquoise hydrogen is that the extraction of the raw material- natural gas-, often also produces emissions. As a result, turquoise hydrogen is usually not completely climate-neutral when it comes to the entire production process and the downstream processing of carbon as a by-product.

In the future, turquoise hydrogen may be valued as a low-emission hydrogen, dependent on the thermal process being powered with renewable energy and the carbon being permanently stored or used. Turquoise hydrogen is very new and is still in the process of discovering if it can be used well at a large scale.  If proven to be effective, turquoise may join blue as a ‘low-carbon hydrogen’ if the carbon can be permanently stored in an environmentally safe way.

RED

For the moment, this type of hydrogen is subject of research projects which are currently on going in Japan. As we know a nuclear power plant, it is not just producing electricity but also generating a lot of heat. This heat instead of being wasted is harnessed to create high temperature steam. This steam is then directed to a device a steam methane reformer, here water in the form of steam is exposed to a catalyst, the high temperatures cause the water molecules to break apart into their elemental components, hydrogen and oxygen. This process is known as thermochemical water splitting, it may sound complicated but it’s essentially breaking down water using heat, and like that we can easily produce hydrogen. Therefore, RED HYDROGEN is produced through the high-temperature catalytic splitting of water using nuclear power thermal as an energy source. Even though the production process doesn’t result in carbon emissions, it does still create nuclear waste, which isn’t always considered ideal.

PINK

Red an pink, 2 colors close on the spectrum/ Red H and Pink H are both produced using nuclear power. While Red is produced at high temperature Pink is produced through low temperature electrolysis, this process involves passing an electric current through water to separate the hydrogen and oxygen atoms, it’s a straightforward process but it’s energy intensive. PINK HYDROGEN is therefore generated through electrolysis of water by using electricity from a nuclear power plant. Pink hydrogen production is similar to green but with one difference: the electrolysis is powered not by renewable but by nuclear energy, which is experiencing a revival as a carbon-free energy source.

PURPLE

PURPLE HYDROGEN is produced through nuclear power & heat through combined chemo-thermal water electrolysis and thermolysis processes.

WHITE

WHITE HYDROGEN refers to naturally occurring hydrogen in its most natural state. It’s the geological hydrogen found in underground deposits in  Earth’s crust. Surprisingly naturally occurring white hydrogen can sometimes be seen seeping out of the ground, as well. These can often be accessed relatively easily by drilling a well. The white hydrogen can be extracted through fracking, a process which involves drilling through geological layers and injecting a mixture of water, sand, and chemicals under high pressure to release the gas from the rocks. The advantage of white hydrogen versus other colors is its natural availability and it can become another new clean and renewable energy source.  Its production does not require direct use of fossil fuels or electricity. In other words, simply extracting natural hydrogen out of the Earth. At present, however, it isn’t viable to extract it, and there is little known about this type of hydrogen.