Torrefaction of microalgal biomass for enhanced fuel properties and sustainability

The growing demand for renewable energy and the urgent need to reduce carbon emissions have intensified interest in sustainable bioresources. Among them, microalgal biomass stands out as a promising third-generation feedstock due to its rapid growth, high biochemical content, and ability to grow in non-arable and wastewater environments. However, large-scale utilization is hindered by challenges such as high moisture content, low energy density, and poor storage stability. This review explores torrefaction, a mild thermochemical pretreatment—as a promising method to enhance the energy and functional properties of microalgal biomass, an area still less explored compared to lignocellulosic biomass. Torrefaction improves stability, reduces moisture, and increases energy density, aiding in the production of biochar, solid fuels, and value-added chemicals. The review examines various torrefaction techniques, including conventional, microwave-assisted, oxidative, wet, and catalytic torrefaction. It examines how each approach affects important traits such as elemental composition, morphology, and application potential. Key findings reveal significant improvements in heating value (up to 31.89 MJ/kg), reduced O/C and H/C ratios, enhanced hydrophobicity, and improved porosity. Consequently, torrefied microalgae exhibit significant promise for applications in energy, agriculture, and environmental remediation. The study also addresses reactor configurations, process modeling, torrefaction severity indices, and kinetic models to support process optimization and scalability. Furthermore, it incorporates Life Cycle Assessment (LCA) and circular economy principles, showing that microalgal torrefaction, especially when integrated with waste heat and nutrient recovery, can achieve net-negative GHG emissions and improved energy efficiency. Also, this review emphasizes the commercial viability of torrefied microalgae as an eco-friendly solid biofuel with enhanced energy density and stability. It also discusses significant challenges and future pathways, providing insights for improving process efficiency, reactor design, and catalytic upgrading. Overall, this review offers a strategic framework for advancing microalgal torrefaction technology and its integration into sustainable biorefinery systems.