Life cycle greenhouse gas emissions of imported renewable hydrogen and hydrogen carriers – a comparative review

The import of renewable gases is considered an essential step towards achieving the climate targets of many countries in Europe and Asia. The EU Commission demands that renewable hydrogen (carriers) must achieve a 70 % reduction in emissions over their life cycle compared to fossil hydrogen. In recent years, numerous studies have therefore been published that evaluate the production and subsequent transportation of gaseous and liquid hydrogen (GH2/LH2), ammonia (NH3), liquid organic hydrogen carriers (LOHC), methanol, dimethylether and substitute natural gas (SNG) from an ecological perspective. Given the high degree of freedom in the methodology of life cycle assessment (LCA), however, their results are highly dependent on the respective system boundaries and assumptions. This review summarizes and discusses the different findings of those studies, aiming at identifying trends and main drivers in greenhouse gas emissions. The results show that renewable gases can contribute to a significant reduction in greenhouse gas emissions compared to natural gas. The main driver of emissions is the electricity mix used for hydrogen production and subsequent processing. This shows that the production of GH2 has lower emissions than other derivatives. Due to the low energy density, this advantage predominates up to a transport distance of 1500 km. Beyond these distances hydrogen (carriers) with high energy densities, such as LH2 or NH3, prove to be advantageous. LOHCs have very high emissions due to the energy-intensive dehydrogenation process. Previous studies have shown great potential for emissions reduction, particularly with hydrogen derivatives that require green carbon. However, further research is needed as many production pathways have not yet been considered.