Microwave catalytic pyrolysis of solid digestate for high quality bio-oil and biochar

Microwave-assisted catalytic pyrolysis (MACP) of solid digestate (SD) into value-added products presents a promising solution for waste SD. Both types of catalysts and reactor temperature critically influence the properties of MACP products. This study systematically investigated pyrolysis of SD mixed with different catalysts (K 3 PO 4 , natural zeolite, and mixture of K 3 PO 4 and natural zeolite) at various pyrolysis temperatures (300, 400, and 500 ℃) for bio-oil and biochar production. The results showed that higher temperatures led to reduced bio-oil and biochar yields, favoring gas production. The bio-oil derived from SD with 20 wt% K 3 PO 4 and 20 wt% natural zeolite at 500 ℃ exhibited the largest fraction of aromatic hydrocarbons, reaching 92.43 % and 91.56 %, respectively. Catalytic pyroysis resulted in reduction in bio-oil acidity. Biochar specific surface area is influenced by both heating rate and temperature, with the highest surface area (207 m 2 /g), pore volume (0.2244 cm³/g), and a more regular pore structure being obtained at 500 ℃ and 66.1 °C/min with 20 wt% K 3 PO 4 . This work demonstrated the feasibility of upgrading waste SD into value-added chemicals, materials, and energy-rich fuels by MACP. Notably, SD with 20 wt% K 3 PO 4 at 500 ℃ represents the optimal operating condition for both bio-oil and biochar production. • Catalytic pyrolysis resulted in a reduction in bio-oil acidity. • K 3 PO 4 catalytic pyrolysis yielded more aromatic hydrocarbons in bio-oil. • Biochar specific surface area is influenced by both heating rate and temperature. • Highest surface area (207 m 2 /g) was obtained at 500 ℃ with 20 wt% K 3 PO 4 .