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 iso-thermally 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 iso-thermal analysis: 3D 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 iso-thermally 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.