Biofuel production from food waste and rice straw via microwave co-pyrolysis: Product characterization and optimization using Box-Behnken design and response surface methodology

• Microwave co-pyrolysis of FW and RS produced biohydrogen and biochar. • RSM optimized key process parameters for high product quality. • Optimal conditions yielded biochar with favourable fuel properties. • Hydrogen-rich gas production improved under optimal conditions. • Biochar exhibited low –OH groups and favourable fixed carbon. The accelerating negative impacts of climate change driven by overreliance on fossil fuel use and food waste accumulation in landfills underscore the urgent need for sustainable energy solutions. This study presents the first systematic optimization of microwave co-pyrolysis of food waste and rice straw using combined ZnCl 2 and biochar additive to simultaneously maximize hydrogen-rich syngas yield and fuel-quality biochar. Unlike prior works relying on trial-and-error or single-factor variation, this study employed response surface methodology based on Box -Behnken Design to evaluate and optimize the combined effects of three key process variables, viz., mixing ratio (75:25–25:75), microwave power (1300–1700 W), and additive quantity (10–20 g), on both gas composition and biochar quality. This multi-factor statistical approach enabled precise identification of synergistic interactions and minimized the need for extensive experimental runs. Under optimal conditions (49 % food waste, 1460 W, and 19 g additive), hydrogen yield reached 27.83±1.2 vol∙% (predicted: 27.24±0.6 vol∙%), and fixed carbon in biochar reached 71.87±2 wt∙% (predicted: 72.38±0.4 wt∙%). Maximum yield conditions (50 % food waste, 1500 W, 15 g additive) achieved 26.93 vol∙% hydrogen and 72 wt∙% fixed carbon, while the minimum yield conditions (75 % food waste, 1700 W, 15 g additive) produced only 13.21 vol∙% hydrogen and 56 wt∙% fixed carbon. A quadratic polynomial model showed excellent predictive capability, highlighting strong synergistic interactions. Beyond statistical optimization, integrating response surface methodology with microwave-assisted catalysis can significantly improve energy efficiency and product quality in biomass co-pyrolysis, providing a replicable pathway for valorizing diverse organic wastes.