Thermal decomposition behaviour and kinetics of food waste and low density polyethylene during microwave copyrolysis

Abstract

Microwave (MW) pyrolysis showed a promising and efficient mean of deriving energy from food waste (FW). This study evaluated the thermal decomposition characteristics of FW and commingled FW (FW mixed with low density polyethylene; LDPE (87:13)) using the thermogravimetric analyzer and the MW copyrolysis reactor. Thermograms of commingled FW (up to 892 K) using different MW susceptors (Granular Activated Carbon (GAC), Cement, Silica gel, Flyash, and Biochar) demonstrated complete devolatilization within 700–1100 s of heating time. A maximum weight reduction of 89.3 wt% was achieved for the commingle FW at 753 ± 1 K within 700 s using GAC as the MW susceptor. The MW absorptive capacity of different MW susceptors strongly influenced the thermal decomposition characteristics of FW and LDPE, and the activation energy of the MW copyrolysis; accordingly, the activation energy varied 7.01–12.03 kJ/mol with different MW susceptors. Thermal decomposition of commingled FW in MW copyrolysis could be best represented with the Kissinger–Akahira–Sunose (KAS) method (R² = 0.85–0.95). Gibbs free energy (104.60–148.15 kJ/mol), free entropy (600.520–601.662 J/mol/K), free enthalpy (1.055–6.412 kJ/mol) showed non-spontaneity, low randomness and endothermic behaviour of the process. Overall, the low activation energy of the MW copyrolysis process (7.01 kJ/mol) achieved with the biochar as the MW susceptor showed a promising future for MW copyrolysis in developing efficient, environmental-friendly and sustainable conversion technology for commingled FW processing.