Microwave co-pyrolysis of Chlorella vulgaris and cow dung with composite additives: Characteristics, bio-oil analysis and reaction mechanism

The catalytic co-pyrolysis of microalgae and waste materials for sustainable energy production has emerged as a promising strategy. This study investigated the catalytic effects of Al2O3/activated carbon (noted as AxCy) and ZSM-5/activated carbon (noted as ZxCy) composite additives with different addition amounts (5 %, 10 %, 15 % and 20 %) and mixing ratios (7:3, 5:5, and 3:7) on microwave co-pyrolysis of CV and CD. The catalytic co-pyrolysis characteristics, product yield, and bio-oil composition of CV and CD were further analyzed by microwave thermogravimetry and GC-MS. Results demonstrated that adding ZxCy improved the co-pyrolysis characteristics of CV and CD, with the shortest reaction time (Ts, 1701 s) and the highest average reaction rate (Rv, 0.03780 wt%/s) achieved at 20 % Z5C5. The maximum liquid yield (23.30 wt%) was obtained at 15 % Z5C5. Furthermore, both composite additives elevated the hydrocarbon content in bio-oil and effectively removed O-containing and N-containing compounds by facilitating the Diels-Alder reaction, deoxygenation, dehydration, decarboxylation and cyclization. Specifically, the maximum hydrocarbon content (49.65 %) and deoxygenation efficiency (54.96 %) were found at 20 % A7C3 while the maximum denitrogenation efficiency (30.25 %) was achieved at 15 % Z3C7. These findings offer new insights into the valorization of CD and high-quality bio-oil produced under composite additives.