(134d) Comparing Ways to Incorporate Biomass into E-Fuel Production

E-fuels produced with renewable electricity as main energy input are receiving attention as sustainable alternative to fossil fuels, with multiple potential target fuels being discussed [1]. While the first step in e-fuel production is usually water electrolysis to produce hydrogen [2], the production of organic fuels requires a carbon source. The choice of carbon source can have a noticeable impact on the energetic, economic, and environmental performance of the production process. The most common carbon source considered for e-fuel production is carbon dioxide, which can be captured from various point sources or air [3]. However, another promising carbon source is biomass, which will also be available in a low-fossil future supply system, and – unlike carbon dioxide – also constitutes an additional energy input into the system. Conversely, incorporating hydrogen from renewable electricity has been proposed as a way to improve the carbon efficiency and economics of biofuel production pathways [4,5].

There is a variety of ways in which biomass and renewable electricity can be combined to produce a given e-fuel. In particular, biomass can be converted to an intermediate stream suitable for the synthesis of downstream products via different conversion processes. The most versatile of these in terms of allowable feedstock are thermochemical conversion [6], i.e., either combustion (combined with an energy recovery process for utilizing the generated heat) or gasification to produce streams rich in carbon dioxide or carbon monoxide, which can then be combined with green hydrogen for e-fuel production. Similarly, for most downstream products, there are multiple options for producing them from hydrogen, carbon monoxide, and carbon dioxide. For example, synthetic kerosene can be produced via the Fischer-Tropsch pathway or the methanol-to-olefins pathway [7], each of which could be based either on biomass gasification or combustion. In this contribution, we thus compare pathways for producing synthetic kerosene from renewable electricity and biomass in terms of efficiency and economics and discuss inherent thermodynamic differences via exergy analysis. We find that gasification-based pathways benefit (i) from lower exergy destruction in the biomass conversion step than combustion-based pathways, and (ii) from the lower hydrogen demand that requires smaller water electrolyzers, which (at least in case of the more mature low-temperature electrolyzers) are the main contributors to exergy destruction. The latter also has a significant impact on production cost. Furthermore, the methanol-to-olefins pathway is found to have advantages over Fischer-Tropsch pathway for producing kerosene regardless of the biomass conversion technology.

Acknowledgments

The authors gratefully acknowledge the financial support by the German Federal Ministry of Education and Research (BMBF) within the H2Giga project DERIEL (grant number 03HY122D).

References

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