Current production routes and life cycle analysis of “Green Energy-Dense Fuels” from biogas: A comprehensive review

Abstract

Biogas-derived “green energy-dense” fuels such as biomethane, Fischer-Tropsch (FT) fuels, methanol and dimethyl ether (DME) are promising alternatives to fossil fuels for transport applications. This review provides a technical description of production pathways and a critical analysis of life cycle assessments (LCA) of “green energy-dense” fuels, with a focus on how methodological choices influence results.

Across the papers analyzed, biomethane outperforms natural gas in environmental impacts. Fuels such as FT-fuels, methanol and DME show mixed results and are pathway dependent. These results are highly sensitive to methodological choices such as functional units (impacts per unit fuel produced, per unit feedstock processed, or per unit transport work), allocation procedures used to distribute impacts among co-products (energy based, mass based, or cost based), and LCIA methodology used (CML, ReCiPe, Traci etc.). Even within ReCiPe, the perspective used (individualist, hierarchist, egalitarian) can significantly impact final results. It can be seen that different methodological choices can lead to up to 50% changes in final results, often leading to contradicting conclusions. System boundaries and scope definitions also contribute to incomparability of results, with many studies stating cradle-to-gate or cradle-to-grave approaches, but leaving out upstream and downstream processes.

Another key limitation is the emphasis on greenhouse gas emissions, with less focus given to other impact categories such as acidification, eutrophication, resource use, and land use. Emerging technologies such as electrochemical syngas production, waste heat utilization and tail gas reforming and recirculation are underexplored in LCAs. The use of multi-criteria decision analysis (MCDA) along with LCAs will give a broader perspective of these fuels beyond just environmental impacts.

This review demonstrates that methodological choices in LCA can have a significant impact on the sustainability of “green energy-dense” fuels. There is a requirement to harmonize LCA frameworks and standards as well as a need for more detailed and transparent reporting along with the integration of other tools like MCDA to provide clear results to enable sustainable energy transition.