Background Synthetic fuels based on renewable hydrogen and CO2 are a currently highly discussed piece of the puzzle to defossilize the transport sector. In this regard, CO2 can play a positive role in shaping a sustainable future. Large potentials are available as a product of biogas production, however occurring in small scales and in thin spatial distributions. This work aims to evaluate suitable synthetic fuel products to be produced at farm sites. Methods A thermodynamic analysis to assess the energetic efficiency of synthesis pathways and a qualitative assessment of product handling issues is carried out. Results Regarding the technical and safety-related advantages in storage, liquid products are the superior option for fuel production at decentralized sites. Due to the economy of scale, multi-stage synthesis processes lose economic performance with rising complexity. A method was shown which covers a principle sketch of all necessary reaction, separation steps, and all compression and heat exchanger units. The figures showed that methanol and butanol are the most suitable candidates in contrast to OME3-5 for implementation in existing transportation and fuel systems. These results were underpin by a Gibbs energy analysis. Conclusions As long as safety regulations are met and the farm can guarantee safe storage and transport, farm-site production for all intermediates can be realized technically. Ultimately, this work points out that the process must be kept as simple as possible, favoring methanol production at farm site and its further processing to more complicated fuels in large units for several fuel pathways.

This paper describes a highly integrated catalytic burner for auxiliary power units based on PEM-fuel cells.

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A complete fuel cell-based auxiliary power unit in the 7.5 kWe power class utilizing diesel fuel was developed in accordance with the power density and start-up targets defined by the U.S. Department of Energy. The system includes a highly-integrated fuel processor with multifunctional reactors to facilitate autothermal reforming, the water-gas shift reaction, and catalytic combustion. It was designed with the help of process analyses, on the basis of which two commercial, high-temperature PEFC stacks and balance of plant components were selected. The complete system was packaged, which resulted in a volume of 187.5 l. After achieving a stable and reproducible stack performance based on a modified break-in procedure, a maximum power of 3.3 kWe was demonstrated in a single stack. Despite the strong deviation from design points resulting from a malfunctioning stack, all system functions could be validated. By scaling-up the performance of the functioning stack to the level of two stacks, a power density of 35 We l−1 could be estimated, which is close to the 40 We l−1 target. Furthermore, the start-up time could be reduced to less than 22 min, which exceeds the 30 min target. These results may bring diesel-based fuel cell auxiliary power units a step closer to use in real applications, which is supported by the demonstrated indicators.

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