Multi-objective optimization of ZSM-5-assisted microalgal torrefaction: Enhancing biochar fuel properties and energy performance under inert and oxidative conditions

Oxidative torrefaction is a cost-effective and energy-efficient thermochemical process for converting microalgae into solid biofuels. However, partial oxidation can increase ash content, adversely impacting thermal degradation. Catalysts help mitigate these issues by enhancing carbon retention, promoting aromatic upgrading, and improving the energy density of biofuels. This study utilizes zeolite ZSM-5-assisted torrefaction to produce biochar from Chlorella sp. under inert and oxidative atmospheres. A multi-objective optimization approach, combining the Taguchi method, composite desirability function, and principal component analysis, is applied to optimize biomass-to-zeolite ratio, oxygen concentration, temperature, and residence time. Key performance indicators include solid yield, higher heating value (HHV), and electricity input. Results show that temperature and catalyst concentration significantly affect solid yield and HHV, while oxygen concentration primarily influences HHV. Optimal conditions (1:3 biomass–zeolite ratio, 12% O2, 220 °C, 15 min) result in an HHV of 23.08 MJ kg−1, a solid yield of 65.01%, and a low electricity input (0.049 kWh). Compared to non-catalytic torrefaction, ZSM-5 improves HHV by 5.24% but reduces yield by 23.63%, reflecting a trade-off between energy content and mass retention. While ZSM-5 enhances the biochar's energy density, the overall energy yield decreases, highlighting a limitation in process energy efficiency. Lastly, ZSM-5 demonstrates stable reusability over five cycles. These findings highlight ZSM-5-assisted torrefaction as a practical approach for producing high-quality biochar from microalgae, supporting the development of sustainable renewable energy systems.