Uncovering strategies for quantifying thermo-kinetics, reaction heat, and synergistic rates of heat transfer-degradation in the chemical upcycling of polyolefins

Utilizing fluidized bed reactors for the upcycling of polyolefins (polyethylene (PE) and polypropylene (PP)) is a promising recycling approach, while the industrialization of this emerging process is still challenging in reactor design. Herein, thermo-kinetics, reaction heat, and heat transfer-degradation synergistic rates of polyolefin pyrolysis were systematically investigated via Py-GCMS and TGA-DSC. We found that the complete pyrolysis time (3.1–14.6 s) at 550 °C is comparable to the residence time of the waste plastic catalytic cracking process (∼5 s), PP is more easily thermally decomposed than PE under the same conditions. Moreover, we present a reliable method for calculating reaction heat based on thermodynamic principles and detailed hydrocarbon composition, revealing a negative linear relationship between cracking depth and reaction heat over different heat supply rates (temperature at 500–600 °C), with reaction heat ranging from 283.3 to 494.3 kJ kg−1 for PE and 592.2–729.1 kJ kg−1 for PP, respectively. Quantitative results from the coupled heat transfer-degradation model indicate elevated heat transfer limitations at higher temperatures (>550 °C, faster heat supply and reaction rate) or larger particle sizes (>250 μm, slower heat transfer rate and higher Biot number), highlighting optimal pyrolysis efficiency for PP particles of 250 μm. We anticipate this work could guide the design and optimization of reactors for upcycling polyolefin waste.