Not only green hydrogen: there is a whole palette of colors which specifies which is the most and the least polluting

We often talk about green hydrogen , the production of which comes from renewable energies. But  there is a whole “color palette”  that claims to give information about how hydrogen is produced, the energy sources used and its climate neutrality. We are reviewing it.

From green to grey: the different "colors" of hydrogen

Hydrogen is the most abundant chemical element in nature and  a crucial resource for the energy transition. Under normal conditions it is in a gaseous state, it is tasteless, colorless and odorless and it is not found freely in the environment, but is associated with other elements such as water (H₂O)  forming organic compounds .

 

As it is not a fuel that can be taken directly from nature, to be used in energy applications it must first be freed or separated from the rest of the elements,  so it must be "produced" or made.

Hydrogen can be produced from different raw materials, different energy sources and by different processes. Depending on the raw material and the source of energy used to produce it, we can speak of  100% renewable, 100% fossil or hybrid processes  in a certain percentage. We see it in the following diagram:

The color assigned to hydrogen is nothing more than a simple way to  classify it based on where it comes from  and the amount of carbon dioxide released when it is generated.

In other words, it's a simple way to know "how clean your production is", even if the color scale isn't completely homogenized yet. The most common is the following:

Gray hydrogen, the most common

Currently, 70 to 75 million tons  of hydrogen, mainly gray (from methane or coal),  are produced each year in the world. To obtain this type of hydrogen, methane CH₄ or natural gas is used using the “steam reforming” technique.

It is a cheap process, but it consumes a lot of energy because it requires very high pressures and temperatures of 800 to 900 ºC and, in addition, it releases many greenhouse gases (to produce a ton of 'gray H₂, 9 to 12 tons of H₂ CO₂ are emitted).

Green, cleanest and most expensive hydrogen

Green hydrogen is hydrogen that is produced with almost no  harmful greenhouse gas emissions  by using electricity from renewable energy sources such as  solar or wind power. This is possible thanks to electrolyzers which use an electrochemical reaction to split water molecules into hydrogen and oxygen.

It is the most expensive process,  but its price is expected to drop with the falling costs of renewable energy and electrolysers. In addition, we are investing more and more in its development and the infrastructures necessary to produce it abundantly.

Another variety of green hydrogen is that produced  from biogas  from livestock, agricultural and/or urban waste.

Blue hydrogen, from natural gas

Blue hydrogen is produced primarily from natural gas by a process similar to that of gray hydrogen, although unlike the latter some or all of the CO₂ emissions emitted are stored and reused through a system of carbon capture.

Later, this obtained carbon can be used for other purposes such as  the production of synthetic fuels . It is considered a viable method to reduce emissions in the short to medium term.

Black and brown hydrogen, the most polluting

Brown hydrogen  is the most opposite of green , because it is obtained from the process of heating black coal, coal or lignite to more than 700 ºC. A gas is thus obtained from which the hydrogen is extracted. 

It is the oldest form of production and, due to its CO₂ emissions, it is  the most harmful to the environment.

Pink, turquoise, yellow or white hydrogen, the most experimental

 Pink  hydrogen is generated by electrolysis fueled by nuclear energy. It can also be called purple hydrogen or red hydrogen.

As for  hydrogen turquoise , it is a more or less recent and experimental means of obtaining hydrogen, since it has not yet been tested on a large scale. Essentially, hydrogen turquoise is made by a process called “methane pyrolysis,” which requires temperatures of around 1,000 degrees Celsius.

Instead of generating a CO₂ by-product like gray hydrogen, it produces a solid carbon waste that does not contribute to the greenhouse effect. In the future, Turquoise Hydrogen could be upgraded as low-emission hydrogen, depending on whether the thermal process is powered by renewable energy and the carbon is permanently stored or used.

On the other hand, when the electricity used to carry out the hydrolysis comes from a mix of various sources – renewable or not –, we speak of  yellow hydrogen.

Finally,  white hydrogen  is a  natural geological hydrogen  found in underground deposits and which is created by fracturing (injecting a fluid composed mainly of water, at high pressure, into rock formations in order to open cracks for the gas to escape).

Currently, there is no strategy to exploit this method of hydrogen production at present

The different ways to produce hydrogen

As we can see, hydrogen can be produced from different feedstocks, different energy sources and by different processes.

Thus, depending on the raw material and the source of energy used to produce it, we can speak of  100% renewable, 100% fossil or hybrid processes  in a certain percentage.

According to the  latest reports from the International Renewable Energy Agency  (IRENA), in order to meet the emission reduction targets of the Paris agreements, a massive expansion of hydrogen production will be necessary in the coming years.

Moreover, it must be sustainable, since by 2050 the production of hydrogen  must increase to 614 million tons  . According to IRENA, two-thirds of this hydrogen (409.3 million tonnes) would be produced from renewable energies and the remaining third would be blue, from natural gas with carbon capture and storage.

Although in the last decade it has started to gain importance, the relationship between hydrogen and industry is old: this gas has been used as a fuel since the beginning of the 19th century for cars, airships and even spacecraft.

And it is that it is a clean source of energy that only emits water vapor and does not leave any residue in the air,  unlike coal and oil  .

Since 1975, its global demand as a fuel has increased exponentially, reaching  110 million tonnes per year recorded in 2019,  according to the International Energy  Agency (IEA) .

 Global  decarbonization will promote the development of new forms of green hydrogen production, undoubtedly one of the fuels of the future and one of the major alternatives to electrification (not only in terms of mobility, but also for certain  industrial processes or for   heavy transport ).