Nascent tar formation during polyvinylchloride (PVC) pyrolysis

Pyrolysis experiments of polyvinylchloride (PVC) were performed to investigate the effects of peak temperature, holding time, and heating rate on the formation of nascent tar. The nascent tar samples were collected using a wire-mesh reactor where the secondary reactions of the evolved volatiles were minimized. The small compounds, such as benzenes and alkanes, were not detected in nascent tar in wire-mesh reactor, whose components are quite different from those of other tars in tube type reactor and vacuum reactor. At a heating rate of 1000 K/s, the quasi-3 rings and 3 rings group aromatics were the major components in nascent tar; while the content of 2 rings group aromatics increased from 7.02% to 31.75% with increasing peak temperature from 500 to 800 °C. At a longer holding time of 300 s, an increase of 2 rings group aromatics from 7.02% to 50.33% was also observed for the nascent tar at 500 °C, indicating that the tar composition significantly changed at different stages of PVC pyrolysis. It seems that 3–4 rings compounds form in the early stage and then 2 rings compounds release in the later stage of PVC pyrolysis. Based on the experimental results in this work, a new four-stage mechanism, including (1) dechlorination accompanied with inner cyclization, (2) aromatic chain scission, (3) release of quasi-3 rings or 3 rings group, and (4) release of 2 rings group, of the PVC tar formation was proposed.