Green, Turquoise, Blue, or Grey? Environmentally friendly Hydrogen Production in Transforming Energy Systems

Hydrogen is a promising energy carrier and feedstock alike for decarbonizing the energy, transport, and chemical sector and mitigating the effects of global warming. Identifying and realizing environmentally friendly hydrogen production pathways is, however, significantly impeded by the need for step-wise transformation of national energy systems. This paper reviews the current level of hydrogen production technology development. Nine process configurations based on four different process technologies were considered comprising steam methane reforming, steam methane reforming with carbon capture and storage, methane pyrolysis and polymer electrolyte membrane electrolysis. Hydrogen from these technologies is often associated with the respective colors grey, blue, turquoise, and green. The critical comparison of the technologies is objectified and quantified based on the methodology of life cycle assessment. For this purpose, the environmental impacts of the hydrogen production technologies are gathered and the most promising solutions with respect to the progressing energy transition identified thereby differentiating the approaches for their short, medium-, and long-term benefit. By considering sixteen impact categories, both, environmental co-benefits and burden shifting resulting from the transition to more climate-friendly hydrogen production technologies were taken into account. The environmental impact of the hydrogen production technologies was found to be determined to large extend by the underlying electricity and natural gas supply chains. Anticipating technology shifts and taking regional differences and future advances in national supply chains into account, technology recommendations deviate substantially for the countries considered.