Modelling of isothermal and dynamic pyrolysis of plastics considering non-homogeneous temperature distribution and detailed degradation mechanism

The kinetics of pyrolysis of plastics are important to predict the formation of gaseous compounds from plastic waste. A common method to determine kinetic parameters is to adapt kinetic models to conversion curves gained from either isothermal or dynamic experiments. Recent studies about the pyrolysis of polystyrene showed considerable discrepancies of parameters derived from isothermal and dynamic experiments. Explanation for this may be supplied by heat transfer limitations or by complex degradation mechanisms that are not in agreement with the simple kinetic model. In the first part of this investigation the non-stationary heat transfer inside the sample is numerically simulated. The comparison of the results from simulating isothermal and dynamic cases reveal no significant shift in the adapted overall kinetic parameters if modest heating rates and sample sizes are applied. Therefore, in the second part a model is considered including a more detailed kinetic scheme which comprises statistic scission of the polymer chain and subsequent depolymerization of the polymer radical. The rate equations for the polydispers species are solved via the method of moments. The parameters derived from isothermal simulations show good correspondence with the rate coefficients of the initiation reactions. However, when simulating dynamic experiments considerable changes in the adapted parameters are found for a certain range of reaction rates. It is demonstrated that degradation mechanisms exist for which dynamic experiments yield kinetic parameters that differ up to 100% from the input parameters due to the inapplicable simplification of the kinetic model. (C) 1999 Elsevier Science B.V. All rights reserved.