Life-Cycle Greenhouse Gas Emissions Of Biomethane And Hydrogen Pathways In The European Union

Gaseous fuels with low life-cycle emissions of greenhouse gases (GHG) play a prominent role in the European Union’s (EU) decarbonization plans. Renewable and low-GHG hydrogen are highlighted in the ambitious goals for a cross-sector hydrogen economy laid out in the European Commission’s Hydrogen Strategy. Renewable hydrogen and biomethane are given strong production incentives in the Commission’s proposed revision to the Renewable Energy Directive (REDII). The EU uses life-cycle analysis (LCA) to determine whether renewable gas pathways meet the GHG reduction thresholds for eligibility in the REDII. This study aims to support European policymakers with a better understanding of the uncertainties regarding gaseous fuels’ roles in meeting climate goals. Life-cycle GHG analysis is complex, and differences in methodology as well as data inputs and assumptions can spell the difference between a renewable gas pathway qualifying or not for REDII eligibility at the 50% to 80% GHG reduction level. It is thus important for European policymakers to use robust LCA to ensure that policy only supports gas pathways consistent with a vision of deep decarbonization. For this purpose, we conduct sensitivity analysis of the life-cycle GHG emissions of a number of low-GHG gas pathways, including biomethane produced from four feedstocks: wastewater sludge, manure, landfill gas (LFG), and silage maize; and hydrogen produced from eight sources: natural gas combined with carbon capture and storage (CCS), coal with CCS, biomass gasification, renewable electricity, 2030 EU grid electricity, wastewater sludge biomethane, manure biomethane, and LFG biomethane. For each pathway, we estimate the life-cycle GHG intensity using a default central case, identify key parameters that strongly affect the fuel’s GHG intensity, and conduct a sensitivity analysis by changing these key parameters according to the range of possible values collected from the literature. Figure ES1 summarizes the full range of possible GHG intensities for each gaseous pathway we analyzed in this study—biomethane is depicted in the top figure and hydrogen is shown in the bottom. The bars represent the GHG intensity of the central case and vertical error bars indicate the maximum and minimum GHG intensity of each pathway, according to our sensitivity analysis. The dotted orange horizontal line illustrates the fossil comparator, which is 94 grams of carbon dioxide equivalent per megajoule (gCO2e/MJ) for transport fuels in the REDII. The dotted yellow line represents the GHG intensity of a 65% GHG reduction goal for biomethane used in the transportation sector, or 70% GHG reduction for hydrogen. Pathways are situated from left to right in increasing order of GHG intensity of the central case. Comparing the central cases of the four biomethane pathways, the waste-based biomethane pathways generally have negative GHG intensity. However, considering the uncertainty in these GHG intensities, manure biomethane might have more limited carbon reduction potential in the 100-year timeframe if methane leakage from its production process is high. In contrast, wastewater sludge biomethane and LFG biomethane, even after accounting for uncertainties, retain relatively low GHG emissions. On the other hand, biomethane produced from silage maize can have much higher emissions; in the central case, we find that silage maize biogas only reduces GHG emissions by 30% relative to the fossil comparator—the low carbon reduction potential is due to the significant emissions emerging from direct and indirect land use change involved in growing maize. Taking into account the variation in assumptions, silage maize biomethane can be worse for the climate than fossil fuels.