Overcoming the HHV–Energy Recovery Tradeoff in Hydrothermal Carbonization of Water Hyacinth via Co-Biomass Selection and Citric Acid Catalysis

Hydrothermal carbonization (HTC) of wet biomass faces a fundamental tradeoff between higher heating value (HHV) and energy recovery (ER), where conditions that enhance carbon densification often reduce solid-phase energy retention. This study investigates whether co-biomass selection combined with citric acid (CA) catalysis can overcome this tradeoff in HTC of water hyacinth (WH), an invasive aquatic feedstock. WH was co-processed with wheat straw (WS), rice husk (RH), and chicken manure (CM) at 240–270 °C, with CA-assisted experiments performed at 240 °C. Individual feedstock HTC confirmed the HHV–ER tradeoff, and co-HTC without catalysis failed to resolve it. CA addition improved carbon densification but reduced ER when applied to WH alone. The WH–CM–CA system uniquely achieved a concurrent HHV of 21.3 MJ kg−1 and ER of 95.8%, with synergistic effects of 50.0% and 29.7%, respectively. FTIR and elemental analysis indicated that Maillard-type condensation between WH-derived sugars and CM-derived amino acids drove preferential solid-phase carbon retention. These findings demonstrate that resolving the HHV–ER tradeoff requires coupling CA catalysis with biochemical complementarity between carbohydrate-rich and protein-rich feedstocks. This approach provides a practical route for hydrochar production with high energy density and recovery for waste-to-energy applications, supporting circular and low-carbon valorization of invasive aquatic biomass and livestock waste streams.