Thermal and kinetic study of used tire vacuum pyrolysis

摘要

Tire pyrolysis involves complex interacting chemical and physical phenomena. Since most studies conducted in the past were experimental by nature, the mechanism of tire pyrolysis is still not well understood. To improve the understanding of tire pyrolysis, a systematic fundamental study has been carried out which involved the following three steps: (i) study of tire pyrolysis for a small particle; (ii) study of pyrolysis for a large particle; and (iii) a pyrolysis study for a packed bed of tire particles. A new approach to model tire pyrolysis has been developed which dealt with reaction kinetics, enthalpy, change, heat transfer and mass transfer mechanisms. The computation results of the mathematical models provided more precise and detailed data than that found in previous tire pyrolysis studies.;For the pyrolysis of a small tire particle, the enthalpy change and weight loss behavior have been studied quantitatively by thermal analysis techniques (TG, DTA and DSC). The study revealed a two step enthalpy change during tire pyrolysis. The first step is exothermic and produced by the pyrolysis reactions. The second step is endothermic and is produced by the evaporation of the pyrolysis products. The weight loss behavior of a small tire particle reflects the combined behavior of pyrolysis reactions and evaporation of the pyrolysis products. The rate of weight loss was found to be proportional to the rate of the heat consumption.;A transient, two-dimensional mathematic model which included heat transfer, mass transfer and reaction kinetics phenomena has been developed for the pyrolysis of a large tire particle. There are mainly four steps for the pyrolysis of a large tire particle: the linear temperature increase preceding the pyrolysis itself, the tire exothermic pyrolysis reactions and the flat endothermic period corresponding to the volatile matter evaporation and vapor accumulation. Based on the well known "bubble" mass transfer mechanism, a new mass transfer model was proposed with a novel concept for m$sb{rm c}$, the "critical vapor mass of bubble's breaking". A well controlled pyrolysis experiment involving a large tire particle has been carried out. Comparison of the temperature history data measured experimentally with that of the model prediction exhibited a very good agreement.;Used tire pyrolysis was modelled in a laboratory scale batch reactor, based on the pyrolysis of a small unit cell in the bed of tire particles. The model included the following mechanisms: intra- and inter-particle heat transfers; enthalpy changes; pyrolysis reactions; and thermal property changes during pyrolysis. The computational results of the model provided more precise and detailed data, leading to an improved understanding of the pyrolysis mechanisms for a bed of tire particles. Satisfactory accuracy of the computational results has been demonstrated through a comparison with the experimental data of Labrecque (1987).