What are the different types of Hydrogen Production Techniques?
Today, a growing consensus is building up again on the potential of hydrogen, mostly due to a stronger climate agenda with challenging targets. Clean hydrogen is part of a group of technologies that need to be deployed across final uses to ensure a transition towards climate-friendly energy sources. Hydrogen technologies are also being considered as an opportunity to develop national industrial sectors, in a recovery perspective after the COVID-19 pandemic.
Hydrogen production technologies are increasingly being codified by referring to a scheme based on different colors. The main colors that are being considered are the following:
Grey (or brown/black) hydrogen, produced by fossil fuels (mostly natural gas and coal), and causing the emission of carbon dioxide in the process;
Blue hydrogen, through the combination of grey hydrogen and carbon capture and storage (CCS), to avoid most of the GHG emissions of the process;
Turquoise hydrogen, via the pyrolysis of a fossil fuel, where the by-product is solid carbon;
Green hydrogen, when produced by electrolyzers supplied by renewable electricity (and in some cases through other pathways based on bioenergy, such as biomethane reforming or solid biomass gasification);
Yellow (or purple) hydrogen, when produced by electrolyzers supplied by electricity from nuclear power plants.
In addition to these colors, different nomenclatures are often in use when referring to groups of hydrogen pathways, including “clean hydrogen”, “low-carbon hydrogen”, “renewable hydrogen”. These definitions may sometimes be confusing since there is no unique standard to provide a common reference. In this paper, the term low-carbon hydrogen includes green, blue, turquoise, and yellow hydrogen. Yet, it is important to remember that also within each “color”, there may be significant variability of carbon intensity, due to a large number of parameters. In some cases, hydrogen maybe even carbon-negative, such as with pathways that involve bioenergy and CCS together.
A scheme of the main different pathways is reported in Figure 1. Additional pathways exist, but they are still at the research stage and have not been included.
While each technological pathway presents opportunities and limitations, it is important to remember that the choice of a specific solution is often related to additional aspects, including geopolitical choices based on national strategies driven by the availability of resources, energy security concerns, or the support to specific industrial sectors. Moreover, cross-border hydrogen trade, due to the need of very strong decarbonization of energy systems in the next decades, can become a potential game-changer in global energy geopolitics.
Widespread and effective development of green hydrogen requires a notable amount of renewable electricity, which may be a problem in the short term since RES are already needed to decarbonize existing electricity demand. For this reason, blue hydrogen can represent a useful option in the short and medium-term, by helping in paving the way for green hydrogen at a later stage.
