Multi-response optimization using novel metrics for accurate biochar quantification in microwave pyrolysis of agroforestry byproducts

초록

Microwave assisted pyrolysis offers a rapid, energy efficient route to convert agroforestry residues into high value biochar but is hindered by hotspots, temperature non-uniformity, imprecise measurements, partial carbonization, and poor replicability. This study combines constant-power operation with new metrics to overcome these challenges and accurately quantify biochar production. A central-composite response-surface design (30 runs at 300–700 W and 20–60 min) was applied to oak sawdust, perilla stem and rice straw to model seven responses, including two novel metrics (carbonized amount and absolute yield), as well as energy-specific yield, higher heating value, energy efficiency, energy yield and process temperature. Quadratic regressions demonstrated excellent fits (R2 up to 0.96; adequate precision >20) and confirmed that both power and time exert highly significant effects (p < 0.001) on all responses. Carbonized amount and HHV increased with power and time until optimal thresholds, while absolute yield peaked at 500 W/40 min. Energy efficiency was maximized at moderate condition (500 W/20 min)– 70.8 %, 54.1 % and 58.4 % for oak, perilla and rice straw, respectively. Multi-response optimization identified global optimum at 616 W/26 min for oak (Derringer's desirability of 0.547), with feedstock-specific conditions at 590 W/20 min for rice straw and 594 W/29 min for perilla. Oak sawdust offered the widest operational window and highest energy metrics; rice straw achieved near-complete carbonization at minimal energy; perilla delivered intermediate performance with rapid carbonization. This tunable framework enables energy-efficient biochar production for energy recovery and fuel applications. © 2025 Elsevier Ltd

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