Hydrothermal carbonization of raw and lipid-extracted <i>Nitzschia laevis</i> : evaluation of hydrochar as solid fuel and process water as biochemical source

Microalgal biomass represents a renewable and carbon-neutral resource to produce biofuels and value-added biochemicals. In this study, both raw and lipid-extracted <i>Nitzschia laevis</i> biomass were subjected to hydrothermal carbonization (HTC) in a batch-type SS-316 autoclave reactor (100 mL) to investigate the dual valorization potential of the resulting hydrochar and process water. HTC was conducted at 180-260 °C for 30-60 min using a biomass-to-water ratio of 0.2 (w/v). The operating conditions of 260 °C and 60 min produced the highest carbon efficiency (46.3 wt%) and hydrochar yield (35.9 wt%), while the maximum higher heating value reached 2.95 MJ kg^-1. The process water contained up to 1075 mg C L^-1 total organic carbon and was enriched with acetic acid, formic acid, and furfurals, reflecting extensive hydrolysis and reformation reactions of carbohydrate and protein fractions. Although the high silica and ash contents of <i>Nitzschia laevis</i> limit its suitability as a direct solid fuel, the hydrochars exhibited enhanced porosity and diverse surface functionalities, indicating potential applications in adsorption, catalysis, and soil improvement. Overall, this study establishes a comprehensive framework for microalgal biomass valorization, demonstrating that simultaneous recovery of hydrochar and biochemical-rich process water through HTC can support next-generation, circular, and sustainable microalgae-based biorefineries.