Integrated Thermochemical Conversion of Microalgae for Sustainable Biofuel Production: An Overview

Abstract

Microalgae have emerged as promising third-generation biomass feedstocks for renewable fuel production due to their high productivity, photosynthetic efficiency, carbon fixation capacity, and ability to grow on non-arable land and wastewater. However, conventional lipid-extraction-based biodiesel production utilizes only part of the biomass and requires energy-intensive processing. This review evaluates integrated thermochemical conversion of whole microalgal biomass, focusing on hydrothermal liquefaction (HTL), pyrolysis, and gasification. The effects of biochemical composition, moisture, ash, temperature, residence time, heating rate, reactor configuration, and catalysts on conversion performance and product quality are examined. HTL is particularly suitable for wet microalgae because it eliminates energy-intensive drying and produces bio-crude, aqueous products, hydrochar, and gases. Pyrolysis can further valorize hydrochar and residual biomass into bio-oil, biochar, and combustible gases, while gasification enables additional syngas and hydrogen production. Integration of these pathways with heat recovery, nutrient recycling, CO₂ utilization, catalytic upgrading, and co-product valorization can improve carbon utilization, energy efficiency, environmental performance, and economic viability. Nevertheless, catalyst deactivation, high-pressure operation, feedstock variability, product upgrading, and scale-up remain critical challenges. Future research should prioritize digitally optimized and demonstration-scale integrated algae biorefineries supported by advanced catalysts, process modelling, artificial intelligence, and comprehensive techno-economic and life-cycle assessment.

Keywords: microalgae; thermochemical conversion; hydrothermal liquefaction; pyrolysis; gasification; biofuel; integrated biorefinery.