Co-Hydrothermal Carbonization of Swine Manure and Soybean Hulls: Synergistic Effects on the Potential Use of Hydrochar as a Biofuel and Soil Improver

The management through co-hydrothermal carbonization (co-HTC) of swine manure (SM) and soybean hulls (SH), a by-product of animal feeding, is established as a strategy for their material and/or energy recovery. The effect of hydrothermal carbonization (HTC) temperature (210–240 °C) and mass ratio (1:0, 1:1, 1:3, 0:1) on hydrochar characteristics revealed that an improved hydrochar (C (51–59%), HHV (21–24 MJ/kg), N (~2%), S (~0.3%), and ash (<9%)) is produced with respect to hydrochar obtained from individually treated wastes. Regarding biofuel characteristics, hydrochar obtained from the SM/SH mass ratio (1:3) at 240 °C complied with the requirements of the ISO/TS 17225-8:2023 (N < 2.5%; S < 0.3%; HHV > 17 MJ/kg; ash < 12%) and showed high energy content (23.2 MJ/kg) and a greater thermal stability than the hydrochar obtained from individual wastes. Hydrochar retained relatively high amounts of nutrients such as phosphorus (6.5–9.7 g/kg), potassium (2.0–3.5 g/kg), and calcium (9–20 g/kg), which supports their use as soil improvers. Moreover, all hydrochar fulfill the standards (Spanish Royal Decrees 1051/2022, 824/2024 and EU Regulation 2019/1009) for sustainable nutrition in agriculture soils in terms of heavy metals concentration. The co-HTC of swine manure and soybean hulls demonstrated a promising transformation of waste materials into biofuel and/or soil improvers.