Thermal cracking of polyolefin waste: Primary and secondary product formation as a function of gas phase residence time in bubbling fluidized bed

The effect of gas phase residence time on the product slate during the thermal cracking of polyolefins was experimentally investigated in a bubbling fluidized bed reactor at a temperature of 750 °C and a steam-to-carbon ratio of 0.8. Three feedstocks were investigated: polyethylene (PE), polypropylene (PP) and a real-world sorted waste stream (DKR-350). Product gases were simultaneously measured at three different heights above the fluidized bed, corresponding to residence times of 1.6–10 s. All feedstocks yielded a product gas rich in CH 4 , C 2 H 4 and C 3 H 6 . The increasing yield of CH 4 and H 2 with residence time indicated the progression of secondary reactions, as the primary products decreased. C 3 H 6 was the most reactive primary product, with yields decreasing significantly (37–47 % within three seconds) with residence time, whereas the C 2 H 4 yield remained relatively stable. The higher C 3 H 6 reactivity was related to reduced thermal stability, caused by the additional methyl group. For the aromatics, the benzene yields increased notably, due to the cyclization reactions of C 3 H 6 . The largest effect of residence time was seen from 1.6 to 5 s, after which the effect decreased due to a reduction in the free radicals concentration which promotes secondary reactions via different pathways, including radical propagation and recombination. The results demonstrated the role of gas phase residence time in the formation of secondary products during thermal cracking of polyolefins. The findings assist with process optimization towards target components, by adjusting the residence time and controlling the occurrence of secondary reactions.