Sustainable production of methanol from waste expanded polystyrene via CO₂-assisted catalytic pyrolysis-based process: Techno-economic analysis and life cycle assessment

Waste expanded polystyrene (EPS) has surged in marine environments, harming ecosystems and human health. Therefore, an appropriate disposal method is urgently required. This study proposes a sustainable approach for converting waste EPS into methanol (MeOH) based on CO₂-assisted catalytic pyrolysis. MeOH is regarded as next-generation low-carbon fuel in the maritime sector. Experimental investigations on catalytic pyrolysis of waste EPS demonstrated that presence of CO₂ enhanced syngas formation via gas-phase reactions with volatile matters. The syngas yield from catalytic pyrolysis of waste EPS under N₂ and CO₂ were 64.5 and 161.1 mmol, respectively. Based on these findings, two waste EPS-to-MeOH conversion processes involving catalytic pyrolysis under N₂ and CO₂ (Strategies A and B, respectively) were developed. Strategy B achieved 11,356 metric tons yr⁻¹ of MeOH productivity, approximately five times that of Strategy A (2,197 metric tons yr⁻¹). The levelized cost of production (LCOP) of MeOH was reduced in Strategy B ($1052 metric ton⁻¹) compared to Strategy A ($2098 metric ton⁻¹). This highlights economic benefits of incorporating CO₂-assisted catalytic pyrolysis in waste EPS-to-MeOH process. To further enhance feasibility, Strategy B was modified by integrating syngas recycling and carbon capture and storage stages (Strategy C). This lowered the LCOP of MeOH to $463 metric ton⁻¹, comparable to those of bio-MeOH and e-MeOH. Strategy C exhibited highest eco-efficiency among three strategies, demonstrating that superior economics and lower environmental burdens. Moreover, Strategy C showed lower impacts than incineration and landfill in various categories, indicating that it is environmentally feasible alternative to conventional EPS disposal methods.