A model for the devolatilisation of large thermoplastic particles under co-firing conditions

Thermoplastic materials are utilised in large quantities for packaging purposes and thus make up a significant share of municipal solid waste. Due to their high calorific value, plastic solid waste (PSW) is suitable for co-firing as a refuse derived fuel in high temperature processes such as industrial furnaces and boilers. In order to achieve a high substitution ratio of fossil fuels and a high degree of burnout, a characterisation of the PSW properties and a prediction of the decomposition behaviour are crucial for process optimisation. However, for PSW particles e.g. polyethylene in the size range of several millimetres combustion models are scarcely published nor are they available in commercial simulation packages. Therefore a simplified devolatilisation model is presented in this paper, transferring findings from the fire protection literature to combustion modelling. The model is capable of calculating the conversion rate of thermoplastic particles under co-firing conditions, considering aspects like particle size, particle shape and related heat and mass transfer phenomena. The modelling results are compared to laboratory experiments in a bench scale reactor and an extrapolation to co-firing conditions is carried out for particles of generic shape.