Synergetic Effects of Copyrolysis of HDPE and Various Biomass Feedstocks

Sustainable fuels produced from biomass have drawn significant attention due to the low cost and abundance of biomass resources. Catalytic fast pyrolysis (CFP) of biomass can produce high bio-oil yields. However, bio-oils have a high oxygen content and cannot be used directly as fuels. Additionally, CFP suffers from rapid catalyst deactivation due to coke formation. Catalytic copyrolysis of biomass and hydrogen-rich feedstocks, such as waste plastics, has shown encouraging results in improving bio-oil quality. Utilizing plastics to enhance biomass pyrolysis can also be viewed as a promising waste management solution. Nevertheless, the synergetic effects of biomass and plastics during pyrolysis vary significantly depending on feedstock properties and mixing ratios. Thus, the optimal mixing ratio and combination of plastics and biomass have yet to be determined. This study investigates the synergetic effects between high-density polyethylene (HDPE) and various biomass feedstocks at different mixing ratios during catalytic fast copyrolysis. Two real biomass feedstocks, miscanthus × giganteus and food waste, and four basic biomass components─cellulose, hemicellulose (xylan), lignin, and protein (egg whites)─were evaluated for copyrolysis using ZSM-5 as a catalyst. The results showed that miscanthus, cellulose, and hemicellulose have strong synergy with HDPE, resulting in a significant reduction of oxygenates and an increase in hydrocarbon production, with miscanthus with HDPE leading to 80% reduction in phenols and 64% increase in monoaromatic hydrocarbons. Conversely, food waste, lignin, and protein showed relatively weak or no synergy with HDPE. The optimal mixing ratio varied with different combinations of feedstocks. Additionally, significant reductions in solid yields were observed in all copyrolysis experiments, with a maximum reduction of 40.5% in copyrolysis of miscanthus and HDPE. These results demonstrate the positive effects of using plastics as hydrogen donors. The reaction pathways and mechanisms for different feedstocks during pyrolysis and copyrolysis are proposed and provide guidance for future studies.