Microwave co-pyrolysis of oily sludge and corn stalk under different microwave absorbents

This study systematically investigated the microwave co-pyrolysis of oily sludge (OS) and corn stalk (CS) using several microwave absorbents, including silicon carbide (SiC), biochar and OS derived residue. Thermal degradation behaviors were initially analyzed by thermogravimetry-mass spectrometry (TG-MS). Product yields and synergistic interactions at different OS/CS blend ratios were evaluated in a microwave pyrolysis reactor, followed by an assessment of biochar and OS derived residue as alternative microwave absorbents. The results showed that co-pyrolysis significantly enhanced the pyrolysis performance, with the comprehensive pyrolysis index (CPI) of the OS/CS mixture exceeding the calculated value by a factor of 8.36. Under microwave co-pyrolysis, increasing the blend ratio of CS evidently reduced the solid residue yield while enhancing the pyrolysis gas yield and H₂ production rate (from 0.67 to 0.95 L/(kg·min)). An optimal OS/CS ratio of 5:5 was determined for subsequent investigations. Compared with conventional SiC, both biochar and OS derived residue as microwave absorbents exhibited superior performance in terms of waste volume reduction and pyrolysis gas production. Notably, biochar achieved an aromatics selectivity of over 90 area% in the liquid products via enhanced decarbonylation and decarboxylation reactions. This was attributed to the formation of localized hot spots on the biochar surface under microwave irradiation, which promoted heterogeneous cracking and reforming reactions. The synergistic interactions further promoted the cracking of heavy components and the decomposition of oxygenates. This work demonstrates that integrating biochar into the microwave co-pyrolysis of OS and biomass not only enables high-value waste valorization, but also offers a promising strategy for sustainable resource recovery and the implementation of a circular economy.