Catalytic Copyrolysis of Waste Biomass and LLDPE: Insights into Bio-Oils Valorization

Biomass byproducts from agroindustrial crops and plastic waste represent major environmental and health challenges due to their accumulation and associated impacts on land, air, and water pollution. To revalorize these residues, three waste biomasses (Persian lemon peel, bitter orange peel, and coconut shell) were copyrolyzed with linear low-density polyethylene (LLDPE), used either as a reactant-grade or as a commercial-grade sample, under catalytic and noncatalytic conditions. The type of feedstock affected the product distribution, with citrus peels producing higher amounts of bio-oil than the lignin-rich coconut shell. Catalytic copyrolysis was performed using a chemically modified clinoptilolite. The influence of both LLDPE and the catalyst on pyrolytic product distribution was systematically investigated, and liquid products were characterized by GC-MS. Catalytic biomass pyrolysis reduced the fraction of oxygenated compounds in the resulting bio-oils up to 7.4% compared to noncatalytic biomass pyrolysis. During copyrolysis, the presence of LLDPE increased the yield of olefinic/paraffinic hydrocarbons up to 7-fold relative to biomass-alone pyrolysis, leading to improved bio-oil quality. While copyrolysis with reactant-grade LLDPE enhanced the paraffinic+olefinic hydrocarbon formation on the bio-oils, the use of commercial LLDPE resulted in higher oxygen contents and lower paraffinic+olefinic hydrocarbon fractions, attributable to plastic additives. These results demonstrate that biomass composition and the presence of plastic play a critical role in the copyrolysis pathways, influencing bio-oil quality. Importantly, this study provides a systematic assessment of catalytic copyrolysis involving real, additive-containing plastics, addressing a critical knowledge gap in the literature. The findings in this study demonstrate that plastic additives substantially alter the product distribution on the bio-oils, highlighting the importance of considering commercial plastics in copyrolysis studies to provide insights for the development of more realistic and scalable waste-valorization strategies.