Production of monocyclic aromatic hydrocarbons from microwave co-pyrolysis of polyethylene terephthalate and low-density polyethylene using coconut husk carbon as microwave absorbent

The application of biochar as microwave absorbers in microwave-assisted catalytic fast pyrolysis (MACFP) has been widely studied. It has been reported that coconut husk carbon (CHC) showed good process characteristics and product distribution as microwave absorbers. However, the existing studies did not pay much attention to the reusability of CHC absorbers. In this study, the CHC absorbers prepared from agricultural and forestry wastes were reused in multiple cycles in the MACFP of polyethylene terephthalate (PET) and low-density polyethylene (LDPE). Compared with typical microwave absorbers such as spherical silicon carbide (SiC) and granular activated carbon (GAC), CHC demonstrated favorable process characteristics with relatively high heating rate (29 °C/min) and short heating time (within 12 min). In terms of product properties, the highest liquid yield of 51.67 wt% and the highest content of monocyclic aromatic hydrocarbons (MAHs) of 82.82 % were obtained at the appropriate pyrolysis temperature (550 °C). After five consecutive uses, the CHC bed still produced comparable heating profiles and product distributions, compared to that of SiC. At the same time, a positive synergistic effect between PET and LDPE was observed. The co-feeding of PET and LDPE (50L50P) appropriately increased the liquid yield (42.50 wt% vs 38.75 wt%) and the relative content of MAHs (77.11 % vs 72.96 %) in comparison to their theoretical value. Finally, this study supplies a new strategy for utilizing recycled CHC as a microwave absorber for the treatment of hydrogen-rich and hydrogen-deficient mixed plastic wastes to produce MAHs products, which also reduces waste and realizes energy conversion.