The use of energy vectors with reduced or zero net greenhouse gas emissions is one of the tools available to man to decarbonize the energy mix. Vectors are “secondary” sources obtained from an energy transformation process. Correctly assessing the emissions associated with them therefore allows the impact of the source over the entire life cycle to be measured. The greenhouse gas emissions requiring measurement therefore include both those associated with the energy transformation process and those associated with the use of the energy vector itself. Some of these vectors are biofuels produced from biomass and energy sources produced from certain types of waste. In recent months, however, another energy vector - hydrogen - has attracted renewed attention from the international community of experts and, in particular, the European Union, which has assigned hydrogen an important role in its strategic transition towards a zero environmental impact economy by 2050 (the Green New Deal).
A vector for sustainability
Hydrogen, in fact, occurs in gaseous form at ambient pressure and temperature and can generate energy either through the simple combustion process or when it is used in special “machines” called “fuel cells”, which allow electricity to be generated. In both cases, when it is being used, hydrogen produces energy without emitting carbon dioxide or pollutants, but rather water vapor. For this reason, many believe it to be one of the ideal solutions for decarbonizing the energy system. Not everyone, however, is aware of the fact that hydrogen is the product of an energy transformation and that this feature has some implications precisely in terms of net carbon dioxide emissions. In order to evaluate them, we need to examine the individual production chains in their entirety. There are many technologies - already available or under study - to produce hydrogen, but currently the most used are steam reforming of natural gas and electrolysis of water. With steam reforming - the most widespread process used today to produce hydrogen for industrial purposes - natural gas, the molecules of which contain carbon and hydrogen, is used as the primary source. Through a particular technology, natural gas is combined with water vapor at high temperatures, producing separate hydrogen and carbon dioxide. Hydrogen is used as an energy vector and does not generate greenhouse gases, while the CO₂ generated in the production process is either emitted into the air or captured and stored (using technologies - CCS - which we will examine in more detail in one of the next articles). In the first case, hydrogen is called “gray hydrogen” and the associated carbon dioxide emissions are produced by the production process, essentially equal to those resulting from the combustion of natural gas. In the second case, the hydrogen is called “blue hydrogen” (an energy vector that we encountered in the last article dedicated to natural gas) and the associated carbon dioxide emissions are very low or zero, depending on the percentage of CO₂ coming from the steam reforming process, which is captured and stored. Hydrogen is also produced by the electrolysis of water. In this process, passing electric current through water (H₂O) causes its molecule to break down into oxygen and hydrogen in the gaseous state. In this case, no carbon dioxide is generated even in the hydrogen production process. But if the electricity used is generated by burning fossil sources, the emission of CO₂ moves upstream and the hydrogen generation process is not emission-free. Only if the carbon dioxide produced by electricity generation is captured and stored or if the electricity used is produced from renewable sources such as wind or solar power do the overall emissions drop to zero (in the latter case, this is known as “green hydrogen”).
Costs and infrastructure
The use of hydrogen, therefore, does not in itself ensure zero carbon dioxide emissions: it depends on the supply chain from which it comes. This is not the only problem as production costs remain high, particularly if zero emission technologies are used. Finally, hydrogen is a gas with particular chemical and physical characteristics, including low energy density (one cubic meter of hydrogen contains a third of the energy of one cubic meter of natural gas) which requires high pressure to be used or means it has to be converted into a liquid state (at -250 °C) to be transported and stored. Precisely because of these particular characteristics, special precautions and dedicated infrastructure and combustion plants are needed to ensure their safety, transportation, distribution, storage and use by the final customer. For example, hydrogen can only be transported using the natural gas transport and distribution network if it is mixed with it in low percentages (between 5% and 20% of the total). If you want to transport and distribute it in its pure form (without mixing it), a dedicated network must be created using pipes built with special materials. There are many obstacles to the rapid spread of hydrogen, therefore, but the world of research and the energy industry have been working for some time to try to overcome them. Eni is also involved in this activity: for more details about hydrogen and Eni's projects and strategies for the production of blue hydrogen or other low-emission supply chains, visit our website.
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