Hydrothermal Carbonization of Marine Biowaste: A Focused Review of Hydrochar Production, Characterization, and Applications

Marine biowaste (fish and crustacean processing residues) is produced in tens of millions of tons annually, yet remains dramatically underutilized as a feedstock. Hydrothermal carbonization offers a technically attractive valorization route for these high-moisture, non-lignocellulosic materials, converting them to carbon-enriched hydrochar without the energy-intensive pre-drying required by pyrolysis. This focused review treats marine animal waste as the primary studies and micro- and macroalgal hydrothermal carbonization as a comparative benchmark to understand how the current research is going, the impact of production parameters, potential application, and possible research gaps to explore. Crustacean waste yields substantially more hydrochar (37–69%) than fish waste (15–34%) under equivalent conditions, driven by calcium carbonate retention in the solid phase. Unactivated hydrochars have low BET surface areas (