Hydrothermal Treatment of Residual Forest Wood (Softwood) and Digestate from Anaerobic Digestion—Influence of Temperature and Holding Time on the Characteristics of the Solid and Liquid Products

Gregor Sailer, Julian Comi, Florian Empl, Martin Silberhorn, Valeska Heymann, Monika Bosilj, Siham Ouardi, Stefan Pelz and Joachim Müller
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Gregor Sailer: Department of Bioenergy, University of Applied Forest Sciences Rottenburg, Schadenweilerhof, 72108 Rottenburg, Germany
Julian Comi: Department of Bioenergy, University of Applied Forest Sciences Rottenburg, Schadenweilerhof, 72108 Rottenburg, Germany
Florian Empl: Department of Bioenergy, University of Applied Forest Sciences Rottenburg, Schadenweilerhof, 72108 Rottenburg, Germany
Martin Silberhorn: Department of Bioenergy, University of Applied Forest Sciences Rottenburg, Schadenweilerhof, 72108 Rottenburg, Germany
Valeska Heymann: Sustainable Catalytic Materials Group, Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstraße 2, 79110 Freiburg im Breisgau, Germany
Monika Bosilj: Sustainable Catalytic Materials Group, Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstraße 2, 79110 Freiburg im Breisgau, Germany
Siham Ouardi: Sustainable Catalytic Materials Group, Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstraße 2, 79110 Freiburg im Breisgau, Germany
Stefan Pelz: Department of Bioenergy, University of Applied Forest Sciences Rottenburg, Schadenweilerhof, 72108 Rottenburg, Germany
Joachim Müller: Tropics and Subtropics Group, Institute of Agricultural Engineering, University of Hohenheim, Garbenstrasse 9, 70599 Stuttgart, Germany

Energies, 2022, vol. 15, issue 10, 1-26

Abstract: Hydrothermal treatment (HTT) offers the potential to upgrade low-value biomass such as digestate (DG) or forest residue (FR) by producing solids and liquids for material use or energetic utilization. In this study, microwave-assisted HTT experiments with DG and FR as feedstocks were executed at different temperatures (130, 150, 170 °C) and with different holding times (30, 60, 90 min) to determine the influences on product properties (ash and elemental concentrations, calorific values and chemical compounds). In general, DG and FR reacted differently to HTT. For the DG solids, for instance, the ash concentration was reduced to 8.68% DM at 130 °C (initially 27.67% DM ), and the higher heating value increased from 16.55 MJ/kg DM to 20.82 MJ/kg DM at 170 °C, while the FR solids were affected only marginally. Elements with importance for emissions in combustion were leached out in both HTT solids. The DG and FR liquids contained different chemical compounds, and the temperature or holding time affected their formation. Depending on the designated application of HTT, less severe conditions can deliver better results. It was demonstrated that different low-temperature HTT conditions already induce strong changes in the product qualities of DG and FR. Optimized interactions between process parameters (temperature, holding time and feedstock) might lead to better cost–benefit effects in HTT.

Keywords: waste materials; HTC; microwave-assisted hydrothermal carbonization (MAHC); physicochemical characterization; trace elements; chemical compounds; ICP-OES; GC-MS (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2022
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