Potentials of hydrothermal carbonization to turn animal excreta and manure-derived streams into energy carriers, and for nutrient recovery

This review critically examines hydrothermal carbonization (HTC) of animal excreta as an efficient pathway for energy recovery and nutrient redistribution. The effects of key process parameters — temperature, solid-to-liquid ratio, and residence time — on hydrochar yield, fuel properties, carbon stability, and aqueous-phase composition are systematically assessed. In general, increasing HTC temperature (180–280 ◦C) reduces hydrochar yield (by up to ~20%) but significantly improves fuel quality, raising the higher heating value (HHV) from 12 to 17 MJ/kg for untreated livestock to 20–26 MJ/kg in hydrochar, comparable to lignite and sub- bituminous coal. Carbon yields mostly reach 57–72%, while volatile matter decreases, enhancing combustion stability. A solid-to-liquid ratio of approximately 1:10 optimizes energy densification (up to 1.49) and promotes nutrient solubilization. Longer residence times (≥4h) favor secondary char formation and carbon stabilization, with limited additional improvement in HHV. Hydrochar yields can reach ~85%, with energy recovery exceeding 60%. When integrated with anaerobic digestion of the aqueous phase, overall energy recovery may increase to ~85%. Furthermore, hydrochar activation can increase surface area beyond 2000 m2/g, while phosphorus recovery efficiencies up to 98% have been reported. The nutrient-rich aqueous phase may support nutrient recovery or controlled fertilizer use, although direct agricultural application requires dilution, treatment, and crop-safety validation. Compared with conventional drying-based treatments, HTC offers lower energy demand and can improve the suitability of the solid product as a fuel additive or nutrient carrier. In practical terms, HTC represents a promising but site-dependent option within circular bioeconomy strategies for managing livestock residues.