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Background: The proportion of biogas in the mix of renewable energies is still remarkably high. The process of anaerobic digestion (AD) provides the basis of biogas production but often leads to excessive foaming. Identifying the reasons for foaming is difficult for biogas plant operators because many factors may play a role. It is therefore difficult for laboratory research to give answers to this specific problem, as the consistency of the digestate itself plays a crucial part in the foam formation process. Hence, careful investigation of foaming in full-scale biogas plants is important in order to identify the main causes and to develop strategies for the prevention of foaming.

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Background: The use of biogas as renewable resource of energy is of growing interest. To increase the efficiency and sustainability of anaerobic biogas reactors, process failures such as overacidification, foaming, and floating layers need to be investigated to develop sufficient countermeasures and early warning systems to prevent failure.

While a large part of traffic volume can be electrified to avoid local pollutant emissions, some applications will still require an internal combustion engine as an energy converter in the future. Construction machines such as vibratory plates, excavators, or emergency generators are examples of such applications. In order to reduce or avoid the pollutants of the remaining internal combustion engines, exhaust aftertreatment systems or synthetic fuels can be implemented. One category of these literature-discussed fuels is poly(oxymethylene) dimethyl ether (OME). In this work, neat OME as well as four OME− diesel blends and conventional diesel fuel as reference were investigated according to the material compatibility of two elastomers (NBR and FKM) and long-term storage stability of the fuel itself. The material compatibility of OME according to both elastomers NBR and FKM is difficult. The tensile stress test (DIN 53504), the Shore hardness (ISO 7619-1), and the compression set (ISO 815-1) show significant changes for neat OME compared to the diesel used elastomers. In addition, the change of mass and volume of the elastomers (DIN ISO 1817) are up to 75 wt % and 140 vol % higher compared to diesel fuel. Only FKM specimens, which were submerged in an OME−diesel blend with 5 vol % OME content, show comparable properties to the diesel reference. Based on the findings of this work, it is concluded that engines with NBR or FKM seals should replace their seals with OME resistant seals. In contrast, the long-term storage stability (ASTM D4625-14) of OME has much better properties than diesel fuel. No deposits were detectable after 24 weeks accelerated fuel aging at an ambient temperature of 43 °C.

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A new approach for biomass liquefaction was developed and evaluated in a joint research project. Focus of the project, called FEBio@H2O, lies on a two-step hydrothermal conversion. Within step 1, the input biomass is converted employing a hydrothermal degradation without added catalyst or by homogeneous catalysis. Within step 2, the hydrogen accepting products of step 1, e.g., levulinic acid (LA) are upgraded by a heterogeneously catalyzed hydrogenation with hydrogen donor substances, e.g., formic acid (FA). As a result, components with an even lower oxygen content in comparison to step 1 products are formed; as an example, γ-valerolactone (GVL) can be named. Therefore, the products are more stable and contained less oxygen as requested for a possible application as liquid fuel. As a hydrothermal process, FEBio@H2O is especially suitable for highly water-containing feedstock. The evaluation involves hydrothermal conversion tests with model substances, degradation of real biomasses, transfer hydrogenation or hydrogenation with hydrogen donor of model substances and real products of step 1, catalyst selection and further development, investigation of the influence of reactor design, the experimental test of the whole process chain, and process assessment.

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