Pilot-Scale Thermo-Pressure Hydrolysis of Biowaste and Silphium perfoliatum for Efficient Natural Fiber and Pulp Utilization in Paper and Biogas Applications

Residual and waste materials such as biowaste and the perennial energy crop Silphium perfoliatum (cup-plant) contain high fiber contents, which limit their energetic utilization in biogas plants. Pre-separating the fiber fraction can improve overall valorization. The recovered natural fibers can be further used as raw materials, e.g., in paper production or fiber-reinforced composites. This study aimed to optimize fiber extraction from biogenic residues and renewable raw materials using pilot-scale Thermal-pressure hydrolysis (TPH). Biowaste and cup-plant were used as substrates. Process parameters (150, 160, 170 °C; 15, 30, 60 min) were systematically varied to evaluate their influence on process efficiency, chemical composition, and functional properties of the resulting fiber and pulp fractions. Biowaste and cup-plant produced final products with similar dry matter (DM) contents—fibers (~36% DM) and pulp (~3.2% DM)—but differed markedly in chemical composition: biowaste was richer in nutrients, whereas the cup plant contained more fiber. Sugar release from the cup-plant increased by over 1900% during TPH and, like the organic acids, was largely relocated to the pulp fraction. Methane yields of the resulting pulps ranged between 310 and 375 L CH4 kg ODM −1 , significantly higher than those measured in the fiber fractions, which ranged from 180 to 250 L CH4 kg ODM −1 . Approximately 55% of the total energy potential was transferred into the pulp. Despite the formation of organic acids and potential inhibitors during TPH, no critical threshold values were exceeded. The energy balance of the Biowaste fiber processing was neutral (biowaste: energy demand 475 kWh/t, energy yield from biogas 484 kWh/t). For papermaking applications, the cup-plant proved to be significantly more suitable, as the heterogeneity and contamination of biowaste limited its material usability. The results highlight the potential of TPH for the combined energetic and material utilization of biogenic residues.