In many countries, sewage sludge is directly used for energy and agricultural purposes after dewatering or digestion and dewatering. In recent years, there has been a growing interest in additional upstream hydrothermal carbonization (HTC), which could lead to higher yields in the energetic and agricultural use. Twelve energetic and agricultural valorization concepts of sewage sludge are defined and assessed for Germany to investigate whether the integration of HTC will have a positive effect on the greenhouse gas (GHG) emissions. The study shows that the higher expenses within the HTC process cannot be compensated by additional energy production and agricultural yields. However, the optimization of the HTC process chain through integrated sewage sludge digestion and process water recirculation leads to significant reductions in GHG emissions of the HTC concepts. Subsequently, nearly the same results can be achieved when compared to the direct energetic use of sewage sludge; in the agricultural valorization, the optimized HTC concept would be even the best concept if the direct use of sewage sludge will no longer be permitted in Germany from 2029/2032. Nevertheless, the agricultural valorization concepts are not generally advantageous when compared to the energetic valorization concepts, as it is shown for two concepts.

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Renewable transport fuels stem either from renewable electricity or biomass. We perform a model-based systems analysis of the usage of electricity, biomass and carbon for fuel production, focusing on greenhouse gas abatement and cost.

The aim of this study was the experimental demonstration and assessment of a novel lignocellulose biorefinery (LCB) for the integration of beech wood-based products as platform and fine chemicals. The process sequence included organosolv pulping followed by pulp bleaching, hydrothermal conversion of hemicellulose to xylose and its purification, fermentation of xylose to malic acid, and base-catalyzed lignin depolymerization (BCD). The resulting products were dissolving pulp, phenolic BCD-oligomers, and malic acid. The state of the art for these technologies is their experimental proof of concept and validation at a laboratory- and pilot-scale and has a technology readiness level (TRL) of 3–4. By integrating and optimizing the single-process steps into one LCB, the TRL could be increased to 5. Based on the findings of the experimental studies, a LCB converting 50,000 dry metric tonnes ($$ \hat{=} $$=̂ 38.7 MW) of beech wood annually was simulated with Aspen Plus. Mass and energy balances showed that 14,616 dry metric tonnes of dissolving pulp, 5174 dry metric tonnes of BCD-oligomers, and 4077 dry metric tonnes of malic acid annually could be produced. The total energy efficiency is 40.3%. The calculation of specific production costs demonstrated the marketability of dissolving pulp (1350 €/t) and BCD-oligomers (2180 €/t), whereas malic acid (4750 €/t) is not yet competitive. Environmental assessment showed reduced greenhouse gas (GHG) emissions from the production of BCD-oligomers and malic acid and higher GHG emissions from the production of dissolving pulp compared with the reference products. In total, the examined LCB would contribute to the mitigation of global warming.

The transformation from a fossil-based economy to a sustainable and circular bioeconomy is urgently needed to achieve the climate targets of the Paris Agreement, reduce air pollution and ensure a long-term competitive economy. Due to its carbonaceous and aromatic basic components, lignin has the potential for material valorization within bioeconomy. So far, lignin produced in the pulp and paper industry has mainly been used internally to generate thermal process energy, as it is difficult to extract it from biomass in a pure and unaltered form. The valorization of lignin to improve the economics of pulp mills is a current aim of the industry. Hydrothermal treatment (HTT) of a partial flow from the lignin stream to produce a functional filler for use in polymer blends is one valorization option. The environmental assessment of the lignin-based HTT filler, conducted using life cycle assessment (LCA), shows that substitution of the conventional fillers carbon black and silica could be associated with significant reductions in greenhouse gas emissions and air pollutants. Depending on the allocation methodology and the reference filler considered, approx. 5 kg CO2 eq./kg filler, 80–93% SO2 emissions, 27–79% PM emissions, and 88–98% PAH emissions can be saved.