Grey, Blue, Green, Turquoise — The Colors of Hydrogen
The pathways to green hydrogen production
Same same but different…Although hydrogen is always hydrogen (the same structure and characteristics), the input materials and the process technology, determines, whether H2 is a sustainable energy carrier or not.
As an element, hydrogen combines with every other element in the periodic table apart from the nonmetals group (He, Ne, Ar, Kr, Xe, Rn). That is why, technically, pure hydrogen can be derived from a huge variety of molecules through various chemical reactions.
Up to now, only 2 processes have proven to be somewhat economically suitable for industrial-scale hydrogen production: electrolysis & thermochemical conversion. The most promising future addition in regards to sustainability will most likely be the biochemical pathway. So where do the colors come into play? The answer is, that the color is linked to the process used to generate hydrogen, therefore it is ultimately related to the environmental impact of hydrogen production.
Today, the most common process, called steam methane reforming (SMR), utilizes a fossil carbon source (usually natural gas aka methane) to produce hydrogen. Unfortunately, CO2 is produced as a byproduct during this process. From this point, 2 pathways are emerging:
The CO2 is emitted into the atmosphere → grey hydrogen.
The CO2 is captured with a CCS (carbon dioxide capture and storage) system → blue hydrogen.
Electrolysis is the second well-established process to create pure Hydrogen. This process requires electrical energy to split water into hydrogen and oxygen. Only if the electricity was generated by renewable energy sources, the use of the term green hydrogen for this process is legitimate. A second pathway to green hydrogen is currently under development. Green hydrogen can also be produced by microalgae, which convert water, CO2 & sunlight into hydrogen through a biochemical process. Unfortunately, this biochemical pathway, which is extremely promising from an ecological point of view is not even close to being economically suitable at the moment.
Turquoise hydrogen is a proposed alternative pathway for hydrogen from fossil sources. Essentially, a different method (methane pyrolysis) is used to produce hydrogen. This method has one key advantage compared with SMR: Solid carbon is produced as a by-product instead of CO2. The solid carbon could be utilized as raw material to create various goods.
We strongly believe that a circular h2-based economy will majorly build on green hydrogen. Nevertheless, there should remain space for blue and turquoise hydrogen to occupy an important niche. On the one hand, we identify significant potential to generate hydrogen out of bio-methane, on the other hand, an economy 100% free of any fossil synthetics is hardly imaginable within the next 50 years, even under the most positive forecasting scenarios. Therefore, it is very likely, that natural gas production will not be totally terminated by the end of the century.