Nano-Scale Soot Particle Formation During the High-Temperature Pyrolysis of Waste Plastics in an Entrained Flow Reactor

Advanced carbon material production via pyrolysis is an economical process for the recycling-utilization of waste plastics. The purpose of this study is to demonstrate the characteristics and formation mechanism of nano-scale soot particles from waste plastic pyrolysis at high temperatures. Waste rigid polyurethane (PU) was adopted to decompose in an entrained flow reactor at 1000, 1100, 1200, and 1300 °C. The yield, morphology, elemental composition, chemical and crystal structure, and reactivity of soot particles, and the yield and composition of gas products were measured and characterized. Results show that with the temperature increasing from 1000 to 1200 °C, the soot yield increases from 12.2 to 26.5 wt%, while with the temperature increasing to 1300 °C, the change on soot yield is slight. The morphology analysis by scanning transmission electron microscopy indicates that the inception of soot particles starts at 1100 °C, when the onion layered structure of spherical particles (20–50 nm) is observed. With increased temperature, the soot particle size becomes smaller and more uniform. Fourier transform infrared analysis and X-ray diffraction characterization present more C=C functional groups and better graphite structures at higher temperatures, agreeing with the increased carbon content in soot. The oxidation reactivity of soot particles generally increased with the increase of pyrolysis temperature. NaCl is observed in soot particles when the pyrolysis temperature is higher than 1100 °C, as an efficient catalyst, significantly enhances the ignition and burnout of soot particles. The soot formation pathway during plastic pyrolysis is finally proposed based on the measurement of representative gases.