It is the lightest element in the periodic table and the most widespread in the universe, from the warm and beating heart of the stars to the cold ice crystals of these winter days.

In the gaseous and molecular state it is odorless and colorless, when it is "burned" in the presence of oxygen it produces water.

Hydrogen, its production and its possible uses enter every day more in the discussions of the media and public opinion for the fight against global warming and for the role that hydrogen can have as a clean energy vector since its combustion does not produce greenhouse gases.

You may have noticed that hydrogen is assigned different colors: brown colors are often mentioned, Grey, blue and green.

It is therefore appropriate to clarify the meaning of the kaleidoscope of hydrogen colors.

Brown hydrogen is what is associated with what was called city gas or water gas. The gasometers that we see in many of our cities are the legacy of when this gas was widely used for lighting. The gasification process from which town gas is obtained is now more commonly associated with the terms synthesis gas or syngas. The composition of these types of gas can vary depending on the processes and feeds used but the main constituents are carbon monoxide, hydrogen, carbon dioxide and methane. Historically, coal was reacted with oxygen and steam but the gasification process can convert any carbonaceous material, such as waste, in a gas.

Currently, hydrogen is already produced in large quantities for its extensive use in the oil industry, petrochemical or to produce ammonia. The most widespread and widely used process has been well known for some time and is called "steam reforming". Natural gas is used, preferably methane e, unlike gasification, a catalyst is used to make the water vapor react with the hydrocarbon. As for gasification, carbon dioxide is produced in the reaction. When carbon dioxide is released into the atmosphere, this hydrogen is characterized by the gray color. In fact, gray hydrogen cannot be considered a clean or carbon-free energy carrier. Although its combustion would not lead to the generation of additional greenhouse gases, its production releases an abundant quantity of it into the atmosphere.

If steam reforming is added to the capture and storage of the carbon dioxide produced, the resulting hydrogen is called blue. Carbon dioxide can be removed from the main gas in absorption columns that use amines and then be released during the regeneration phase of the amine itself. Carbon dioxide is then compressed, until it becomes liquid, transported to a suitable location and injected underground into porous and permeable rocks. In this way the CO2 it is subtracted from the natural cycle and the production of hydrogen has no net effect on the climate. Currently, it is above all this type of hydrogen and its problems that we are talking about, precisely because the technologies involved are well known, known and used for some time.

Finally, the green color generally identifies hydrogen produced by electrolysis in which electric current is used in an electrolytic cell to split water into its components, precisely hydrogen and oxygen, to the two opposite electrodes. Although the generation of hydrogen in this way does not include the production of carbon dioxide, it should be considered that even the production of the necessary electricity must not have greenhouse gas emissions to make hydrogen truly green. Furthermore, costs and efficiencies do not yet make the process attractive, practical or simply competitive for large-scale productions.

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