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Comparative lifecycle study of different modes of hydrogen production

Researchers from LUT University in Finland have compared the climate change impacts and the main factors affecting them for different categories of hydrogen production, including grey hydrogen (SMR), blue hydrogen (SMR-CCS), turquoise hydrogen (TDM), and green hydrogen (PEM electrolysis). The comparative lifecycle analysis is published as an open-access paper in the RSC journal Green Chemistry.

Grey hydrogen is produced by steam methane reforming (SMR), a high-temperature process in which steam reacts with a hydrocarbon to produce hydrogen with carbon dioxide as a by-product. Hydrogen produced by SMR with carbon capture and storage (SMR-CCS) to reduce emissions is called blue hydrogen. Turquoise hydrogen is produced by thermal decomposition of methane (TDM) leading to hydrogen and solid carbon.


System boundary conditions for all systems. Patel et al.

Grey hydrogen, blue hydrogen, and turquoise hydrogen are produced using natural gas; green hydrogen is produced from water and renewable electricity sources. The natural gas feedstock is sourced from the pipeline route connected to Russia and through the liquefied natural gas (LNG) route from the USA.

The life cycle assessment (LCA) result showed that:

  • Grey hydrogen had the highest emissions, with the LNG route showing higher emissions, 13.9 kg CO2 eq. per kg H2, compared to the pipeline route, 12.3 kg CO2 eq. per kg H2.

  • Blue hydrogen had lower emissions due to the implementation of carbon capture technology (7.6 kg CO2 eq. per kg H2 for the pipeline route and 9.3 kg CO2 eq. per kg H2 for the LNG route)

  • Turquoise hydrogen had the lowest emissions (6.1 kg CO2 eq. per kg H2 for the pipeline route and 8.3 kg CO2 eq. per kg H2 for the LNG route). Emissions of turquoise hydrogen production can also be reduced if a renewable methane source is used, for example, biogas or methane produced from biomass.

  • The climate change impact showed a 12–25% increase for GWP20 compared to GWP100 for grey, blue, and turquoise hydrogen.

  • The production of green hydrogen using wind energy resulted in the lowest emissions (0.6 kg CO2 eq. per kg H2), while solar energy resulted in higher emissions (2.5 kg CO2 eq. per kg H2).



LCA results of examined hydrogen production technologies (Climate change, GWP 100). Patel et al.

While the green hydrogen path seems preferable in the future, there might be a need for bridging technology or a backup plan for future fluctuating renewable energy. Blue and turquoise hydrogen production could be a middle-term solution until the long-term solution is achieved, that is, the production of 100 percent green sustainable hydrogen. Existing SMR plants could be retrofitted to add CCS to produce blue hydrogen, which would result in a reduction in climate change impact of 25–40% compared to grey hydrogen. Besides the CCS technology, storage locations that are suitable for the captured CO2 are necessary. Ultimately, the outcomes suggest that only green hydrogen aligns with the RED II climate impact limit for hydrogen production.

—Patel et al.


  • Gulam Husain Patel, Jouni Havukainen, Mika Horttanainen, Risto Soukkaa and Mari Tuomaalaa (2024) “Climate change performance of hydrogen production based on life cycle assessment” Green Chem. doi: 10.1039/D3GC02410E



I believe that the cleanest and lowest cost method of making hydrogen is high temperature electrolysis using nuclear power. This would be especially true using high temperature fast reactors.

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