Hydrogen is seen as the energy carrier of the future, because when it is used - whether to power a car or to generate heat in households and industry - only water is produced. But whether hydrogen is really climate-friendly depends on how it is produced. The ideal case is so-called green hydrogen. It is produced from water by electrolysis, with the electricity for this process coming exclusively from renewable energies such as hydro, wind and solar power. This makes it largely climate-neutral. But at present, hydrogen produced in this way is expensive and not available everywhere - because renewable electricity and electrolysis capacities are lacking.

"The rapidly increasing demand for hydrogen probably cannot be met with this in the foreseeable future," the current international study therefore states. Most hydrogen today is produced from natural gas or other fossil fuels. It is referred to as "gray hydrogen. However, it has no advantages in the life cycle assessment because its production pollutes the climate and energy is lost during conversion.

A kind of compromise is represented by so-called blue hydrogen, whose possible role in the energy transition has been the subject of heated debate among experts and politicians for months. Like gray hydrogen, it is produced from natural gas by heating it and then splitting it into hydrogen and carbon dioxide in a process known as steam reforming. In this case, however, the carbon dioxide is not simply allowed to escape into the atmosphere, but a portion of it is captured and permanently stored underground to reduce the greenhouse effect. This is called carbon capture and storage, or CCS for short. This improves the climate balance.

Leaks reduce the climate footprint

However, a study published in August by researchers from the U.S. universities Cornell and Stanford came to the conclusion that, despite CCS, blue hydrogen for heat generation is not better for the climate, but on balance a good 20 percent worse than natural gas used directly as an energy source. According to the authors, this is mainly due to the fact that gas escapes into the air through leaks along the entire supply chain of the natural gas - from its extraction at the well to its transport by pipeline or ship to hydrogen production. Since natural gas, or its main component methane, has a greenhouse effect around 30 times greater than CO2, leaks of just a few percent can have a huge impact on the climate balance of the hydrogen produced from it. In addition, CO2 emissions occur during natural gas reforming if not all the carbon dioxide is captured and some of it is released into the atmosphere instead of being buried in the ground.

"This study prompted us to analyze the climate effect of blue hydrogen in even greater detail," says Christian Bauer of the Paul Scherrer Institute's Laboratory for Energy System Analysis (PSI), lead author of the current study. Bauer, together with Mijndert van der Spek, professor at the Carbon Solutions Research Centre at Heriot-Watt University, quickly formed an international collaboration of researchers from a wide range of institutes to put the new study on a broad basis. The collaboration was particularly close with colleagues at ETH Zurich, who have special models that allow them to simulate processes such as CO2 capture in detail. "They ran the production of blue hydrogen through their simulation software, and we at PSI fed the results into our life cycle assessment models," Bauer reports. "These map the entire production chain from natural gas extraction to CO2 storage."

The results of the life cycle assessment paint a differentiated picture: According to the study, whether blue hydrogen benefits the climate depends very much on how much methane is lost on the way from the extraction of natural gas to the production of hydrogen and how effective CO2 capture is during natural gas reforming. "Methane emissions are very diffuse because they can occur at many different points in the production chain," Bauer says. "Therefore, they are difficult to determine. Depending on the production technology and the country the natural gas comes from, they vary from a few tenths of a percent to a few percent." And when it comes to CO2 capture, there are processes that can capture and store almost all of the CO2 produced. Others come up with only half. "With modern CO2 capture technologies, virtually all of the CO2 produced in hydrogen production can be captured," van der Spek says. This means blue hydrogen could play a key role in the transition to a carbon-neutral society.

The key lies in a high standard of technology

The key to blue hydrogen that really benefits the climate is therefore to set high standards for the technology: "Countries like Norway can serve as role models," says Bauer. These countries already extract and transport natural gas almost loss-free with emissions of less than 0.5 percent. If almost the entire amount of CO2 emissions is now captured during natural gas reforming and stored in former natural gas fields in the North Sea, for example - something that has proven effective and safe for many years - then this blue hydrogen is almost as climate-friendly as green hydrogen.

The PSI researcher emphasizes that his U.S. colleagues did not necessarily get their calculations wrong. "They just looked quite one-sidedly at the negative side of the spectrum, how the production of blue hydrogen can run. We, on the other hand, show: If you do it right, it does make a valuable contribution to the energy transition." At the very least, he said, it could be a kind of interim solution until green hydrogen becomes widely and cheaply available. "It's also possible that increasing demand will make it necessary for both forms to be used in a complementary way over a longer period of time," Bauer says.

The natural gas industry has already recognized that only production that is as emission-free as possible can ensure its continued existence. It has set itself the goal of bringing technology worldwide up to a standard that allows a maximum of 0.2 percent methane emissions. Thresholds are being discussed at the political level below which blue hydrogen may be considered low-carbon and thus climate-friendly. "It is important that such a value really takes into account emissions along the entire chain," Bauer says. Crucial factors should not be left out of the balancing process.

Original publication: "On the climate impacts of blue hydrogen production",Christian Bauer, Karin Treyer, Cristina Antonini, Joule Bergerson, Matteo Gazzani, Emre Gencer, Jon Gibbins, Marco Mazzotti, Sean T. McCoy, Russell McKenna, Robert Pietzcker, Arvind P. Ravikumar, Matteo C. Romano, Falko Ueckerdt, Jaap Vente, Mijndert van der Spek, Sustainable Energy & Fuels, 11/19/2021, DOI: https://dx.doi.org/10.1039/D1SE01508G

Nomenclature of hydrogen

Depending on how hydrogen is produced, it is given different names in the energy industry. Here is the common "color theory":

Green hydrogen: Made with electricity from renewable sources

Turquoise: Produced by so-called methane pyrolysis: the natural gas is thermally split into hydrogen and solid carbon

Orange/yellow: Produced from organic materials such as biomass, biogas and biomethane

Purple/red: Made with electricity from nuclear energy

Blue: Produced from natural gas with capture and geological storage of carbon dioxide.

White: Naturally occurring hydrogen

Gray: Produced from natural gas (often "gray" also includes all fossil fuels).

Brown: From lignite

Black: From hard coal