Pretreatment of Plastic Waste and Optimization of the Pyrolysis Process for Liquid Fuel Production

Abstract

This study investigates the conversion of polypropylene (PP), polyethylene terephthalate (PET), and high-density polyethylene (HDPE) into liquid fuel through pyrolysis, emphasizing the mechanistic effects of mechanical and chemical pretreatment. As global plastic waste generation reaches critical levels, sustainable upcycling technologies are urgently required to support the circular economy and mitigate escalating environmental contamination. Therefore, this study aimed to optimize pyrolysis parameters and maximize liquid-fuel yield by comparing untreated plastic feedstocks with ozonated, crushed, and combined ozonated-crushed samples. Pretreatment methods included ozonation and crushing, followed by pyrolysis under varying conditions of temperature (300 to 400 °C), heating rate (5 to 15 °C/min), holding time (1 to 3 hours), and ozonation duration (0.5 to 1.5 hours). For the first main objective, the highest yield (46.6%) was achieved using ozone-treated PP at 400°C, a heating rate of 5 °C/min, a holding time of 1 hour, and an ozonation time of 1 hour. Mechanistically, ozonation increased liquid fuel yield by introducing oxygenated functional groups that facilitate chain scission at lower activation energies, whereas crushing plastic reduced liquid recovery by accelerating volatilization into non-condensable gases. Furthermore, FTIR analysis revealed that liquid fuel from treated plastics shared chemical similarities with diesel, gasoline, and kerosene, particularly in the C-H and aromatic C=C bands, although PP-derived fuels exhibited unique olefinic signatures. A Box-Behnken design and ANOVA were used to optimize the second main objective. A maximum yield of 38.8% was achieved under the following conditions: 400 °C, a heating rate of 10 °C/min, a holding time of 2 hours, and an ozonation time of 0.5 hours. The PP-derived liquid fuel contained hydrocarbons similar to commercial fuels, indicating its potential as an alternative fuel source. This study concludes that chemical pretreatment significantly enhanced fuel yield and quality, making PP-derived liquid fuel a potential substitute for conventional petroleum fuels and thereby advancing the technological readiness of plastic-to-fuel biorefineries. ABSTRAK: Kajian ini mengkaji penukaran polipropilena (PP), polietilena tereftalat (PET), dan polietilena berketumpatan tinggi (HDPE) kepada bahan api cecair melalui pirolisis, dengan penekanan terhadap kesan mekanistik prarawatan mekanikal dan kimia. Memandangkan penjanaan sisa plastik global telah mencapai tahap kritikal, teknologi kitar tinggi nilai sisa secara mampan amat diperlukan bagi menyokong ekonomi kitaran dan mengurangkan pencemaran alam sekitar yang semakin meningkat. Oleh itu, kajian ini bertujuan mengoptimum parameter pirolisis dan memaksimum hasil bahan api cecair dengan membanding bahan suapan plastik tanpa rawatan dengan sampel rawatan menggunakan ozon, dihancurkan, serta gabungan ozonasi dan penghancuran. Kaedah prarawatan melibatkan ozonasi dan penghancuran, diikuti oleh pirolisis pada keadaan suhu berbeza (300-400 °C), kadar pemanasan (5-15 °C/min), masa pegangan (1-3 jam), dan tempoh ozonasi (0.5-1.5 jam). Melalui objektif pertama, dapatan tertinggi sebanyak 46.6% diperoleh daripada PP rawatan ozon pada suhu 400 °C, kadar pemanasan 5 °C/min, masa pegangan 1 jam, dan tempoh ozonasi 1 jam. Secara mekanistik, ozonasi meningkatkan hasil bahan api cecair dengan memperkenal kumpulan oksigen yang memudahkan pemutusan rantai pada tenaga pengaktifan yang lebih rendah, manakala penghancuran plastik mengurangkan perolehan cecair akibat pemeruapan yang dipercepatkan kepada gas tidak terkondensasi. Selain itu, analisis inframerah transformasi Fourier (FTIR) menunjukkan bahawa bahan api cecair daripada plastik terawat mempunyai persamaan kimia dengan diesel, petrol, dan kerosin, khususnya pada jalur C–H dan C=C aromatik, walaupun bahan api terbitan PP menunjukkan ciri olefin tersendiri. Reka bentuk Box-Behnken dan analisis varians (ANOVA) digunakan bagi mengoptimum objektif kedua. Hasil maksimum sebanyak 38.8% dicapai pada suhu 400 °C, kadar pemanasan 10 °C/min, masa pegangan 2 jam, dan tempoh ozonasi 0.5 jam. Bahan api cecair terbitan PP mengandungi hidrokarbon serupa bahan api komersial, sekaligus menunjukkan potensinya sebagai sumber bahan api alternatif. Kajian ini menyimpulkan bahawa prarawatan kimia meningkatkan hasil dan kualiti bahan api dengan ketara, menjadikan bahan api cecair terbitan PP berpotensi sebagai pengganti kepada bahan api petroleum konvensional serta meningkatkan kesiapsiagaan teknologi biopenapisan plastik kepada bahan api.