Life cycle greenhouse gas emissions of renewable gas technologies: A comparative review

摘要

Natural gas is an energy carrier of predominant significance for today's electricity and heating sectors. However, science heavily discusses the actual environmental burden of natural gas mainly due to the uncertainty in upstream methane losses during its extraction and transportation. In this context, numerous technologies pave the way for the production of renewable methane to replace natural gas: biomethane from anaerobic digestion of biomass, substitute natural gas (bio-SNG) from gasification and Power-to-Gas via water electrolysis and subsequent methanation. In recent years, numerous studies aimed at analysing the life cycle carbon intensity of those renewable gases. Given the high degree of freedom in the methodology of life cycle assessment (LCA) however, the studies are highly dependent on the respective boundary conditions and assumptions. To summarise and discuss the different findings, this review identifies and quantitatively analyses 30 life cycle assessment studies on the greenhouse gas emissions of renewable gases, comparing their results and deriving the main determinants on their environmental friendliness. A comparison between the results for renewable gases and existing literature reviews on the LCA of fossil natural gas shows the considerable emission reduction potential of renewable gases. This however requires the consideration and right implementation of the main influencing factors (inter alia the storage of digestate in closed tanks for biomethane, heat extraction of excess heat for bio-SNG, or the use of renewable electricity for Power-to-Gas) and is not a mere result of the technologies per se.