Waste-tyre pyrolysis and gasification via the reverse Boudouard reaction : derivation of empirical kinetics from TGA data

Abstract

The pyrolysis of scrap tyre rubber crumbs under nitrogen and treatment with pure carbon dioxide was investigated both isothermally and dynamically up to 1100 °C, at heating rates up to 20 °C min−1. The rubber sample was a mixture of industrially representative tread and sidewall material. Workable, but not definitive, models could be derived from the isothermal analysis: Jander D3 diffusion for the first pyrolysis event under nitrogen up to 550 °C; the Mampel mechanism for high-temperature pyrolysis above 550 °C; and shrinking-particle chemical-reaction control as the rate limiting step for the reverse Boudouard reaction. The isothermally derived pre-exponential factors and activation energies were further refined by non-linear fitting to the dynamic data of all heating rates, and by making both parameters functions of the degree of conversion. In addition, the Sestak-Berggren equation was directly fitted to the full data set, i.e., for all heating rates, also using pre-exponential factors and activation energies that are dependent on degree of conversion. Both the approaches yielded workable engineering kinetics, with the Sestak-Berggren performing worse. With single-value pre-exponential factors and activation energies, the models fitted the data less satisfactorily across the range of heating rates. The required numerical analysis is fully implementable on a commercial spreadsheet.