Energy Valorisation of Fucus serratus via the Integration of Hydrothermal Carbonisation and Anaerobic Digestion: Influence of Seawater as a Reactant Medium

Integrating hydrothermal carbonization (HTC) and anaerobic digestion (AD) has the potential to improve energy conversion efficiency (ECE) of biomass with low energy density and high moisture content. This study aims to assess the influence of alkali metals and chlorides by comparing seawater and distilled water as a HTC reactant medium, treating Fucus serratus across a range of processing temperatures (150 °C, 200 °C and 250 °C). All HTC-AD integration options improved ECE of F. serratus compared to AD alone. ECE of F. serratus was similar across temperatures of 150 °C (84–88%) and 200 °C (75–77%) regardless of seawater or distilled water usage. However, HTC processing at 250 °C yielded a greater ECE from F. serratus using distilled water (78%), compared to seawater (57%), due to a higher hydrochar yield and biomethane generation from the process water. Higher HTC processing temperatures significantly reduced slagging and fouling propensity of hydrochars by selectively removing problematic alkali metals. This creates a compromise between process energetics and favourability of hydrochar properties in large-scale conversion systems. Overall, HTC of F. serratus in seawater at 250 °C produces hydrochar suitable for combustion, process water that generates biomethane during AD (168.4 mL CH₄/g COD) and a net energy-positive process (energy return on energy investment EROI = 1.53).