Mechanistic insights into the thermal pyrolysis of high-density polyethylene and polypropylene: towards sustainable hydrogen production and carbon valorization

Abstract

High-temperature pyrolysis of plastic waste presents a viable approach for concurrent hydrogen production and carbon material synthesis, contributing to energy recovery and environmental remediation. This study investigates the pyrolysis behavior of high-density polyethylene (HDPE), polypropylene (PP), and their mixtures in a high-temperature laboratory-scale reactor across a temperature range of 700 °C to 1600 °C. The objective was to assess how polymer composition and temperature influence the distribution of gaseous, liquid, and solid products. Gas phase analysis revealed a strong temperature dependence in hydrogen and hydrocarbon yields, with distinct variations between individual polymers and their mixtures. This underscores the relevance of investigating the pyrolysis of mixtures, as plastic wastes typically contain multiple polymer materials. Liquid products, primarily hydrocarbons, exhibited increased aromaticity and decreased yields at higher temperatures, indicating enhanced secondary cracking. Solid carbonaceous residues were characterized using Raman spectroscopy and X-ray diffraction (XRD). Raman spectroscopy revealed a decline in graphitization degree with increasing temperature, suggesting higher defect density and reduced structural (graphitic) ordering. Conversely, XRD data demonstrated increased crystallite sizes and decreased interlayer spacing with temperature, implying the formation of more ordered crystalline domains. PP pyrolysis resulted in carbon with the highest graphitization degree and crystallinity at moderate temperatures. The HDPE/PP mixture exhibited a synergistic effect that improves graphitic ordering and crystallite growth, surpassing the simple average of the pure components, particularly between 1200 °C and 1400 °C. At 1600 °C, thermal effects dominate, minimizing the influence of specific polymers and leading to converge all samples toward similar carbon structural characteristics. These findings underscore the critical role of polymer composition and temperature in optimizing pyrolysis processes for thermochemical hydrogen generation and carbon capture from mixed plastic waste.