Feedstock-Informed Technology Selection for Chemical Recycling of Plastic Rejects: An Integrated Physicochemical Characterization and Multi-Criteria Decision Framework

Rapid urbanization in the Global South has intensified plastic waste management challenges, and limited empirical data on plastic waste characteristics continue to hinder the adoption of appropriate recovery technologies in Africa. This study characterized non-recyclable plastic rejects from the Libegreen Innovation Material Recovery Facility (MRF) in Dar es Salaam, Tanzania, and evaluated suitable chemical recycling pathways. An integrated analytical approach combining material flow analysis, Fourier-transform infrared spectroscopy (FTIR), proximate and ultimate analysis, calorific value determination, X-ray fluorescence (XRF), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) was used to determine the physicochemical and thermal properties. A hybrid Analytic Hierarchy Process and Technique for Order of Preference by Similarity to Ideal Solution (AHP-TOPSIS) multi-criteria decision framework was used to compare pyrolysis, gasification, and depolymerization technologies. The results showed that the Material Recovery Facility achieved a high material recovery efficiency of 95.5%, with only 4.5% (181 tons/year) rejected, resulted in an estimated annual economic loss of USD 34,774–62,632. The rejects were dominated by polypropylene (51.58%), polyethylene terephthalate (28.42%), and polyvinyl chloride (20%). Polypropylene exhibited the highest energy recovery potential (28.82 MJ/kg HHV), whereas polyvinyl chloride presented processing challenges due to high chlorine content (36.7 wt%) and dehydrochlorination at approximately 280 °C. AHP-TOPSIS ranked gasification as the most suitable pathway (Ri = 0.718), followed by pyrolysis (Ri = 0.517) and then depolymerization (Ri = 0.196). These findings provide empirical evidence for advancing chemical recycling strategies and transforming plastic rejects into valuable resources within emerging circular economies.