Analysis of the Structure and Slow-Release Performance of Seaweed Biochar Treated by Hydrothermal Carbonization Pretreatment Coupled with Medium–High Temperature Pyrolysis

Seaweed biomass is a kind of marine waste carbon resource with rapid growth and rich elements, but the problem of high-water content makes seaweed face high energy consumption in the process of development and utilization. In this study, a new process of hydrothermal carbonization pretreatment coupled with medium–high temperature pyrolysis was proposed, in which seaweed biomass was preliminarily hydrolyzed by liquid bubble water. Cancel the filtration and drying of hydrothermal intermediate products, and complete pyrolysis in medium–high temperature anaerobic environment. The existence of moisture in hydrothermal intermediate products may promote the water–gas reaction during pyrolysis, resulting in the formation of hollow carbon spheres on the surface of biochar and their rupture into “gourd” pore structure (with small opening but large internal space). Compared to traditional pyrolytic biochar, the biochar produced in this study exhibits similar surface functional groups and atomic bonding states, along with a hierarchical porous structure featuring micropores, mesopores, and macropores. While retaining the basic structure and characteristics of traditional pyrolytic biochar, the slow-release amount of readily available nutrients was reduced by 2.76–45.65%, demonstrating improved slow-release performance. Additionally, the total energy consumption during production decreased by 6.18–6.25%, the overall yield increased by 2.19–2.35 times, and the energy cost per gram was reduced to less than 6 ¥/g. This study demonstrates that batch carbonization of seaweed feedstock using the hydrothermal carbonization pretreatment coupled with medium–high temperature pyrolysis process not only achieves lower total energy consumption but also produces biochar with superior structural and slow-release characteristics. The findings aim to provide theoretical and data support for the high-value conversion and utilization of biomass energy.