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Kinetic and mechanistic study into emission of HCl in fires of PVC

PVC pyrolyses in fires eliminating HCl, which can subsequently participate in the formation of chloroaromatic pollutants. In this study, the Density Functional Theory (DFT) has been deployed to simulate the mechanisms of HCl elimination from pyrolysing PVC. Although PVC consists mainly of polymerised chloroethene, it also contains other structural entities as impurities or defect compounds, which significantly enhance its decomposition. For this reason, we have studied elimination of HCl from seven compounds that represent the defects in PVC. We have found two generic pathways for the elimination of HCl. The first involves a C-Cl fission at an allylic site and a C-H cleavage at a vinylic site, whereas the second entails scissions of allylic Cl and methylenic H. The latter pathway appears more favourable from thermodynamic and kinetic standpoints. We have investigated the effect of the length of carbon chain on reaction and activation enthalpies by considering analogous dehydrochlorination pathways for short chlorinated aliphatics (i.e., C3, C4), discovering the reaction and activation enthalpies required for HCl elimination to be independent of the length of the carbon chain. We then explored the effects of temperature and pressure on rate constants for all possible dehydrochlorination pathways within the formalism of the unimolecular reaction rate theory of RRKM. Pressure fall-off regions extend generally between 0.001 and 1.0 atm, and the dehydrochlorination reactions exhibit pressure-independent behaviour even under ambient pressure. Kinetic parameters presented herein should be useful to model the decomposition of PVC in fires.