A comparison of different methods for predicting coal devolatilisation kinetics

Abstract

Knowledge of the coal devolatilisation rate is of great importance because it exerts a marked effect on the overall combustion behaviour. Different approaches can be used to obtain the kinetics of the complex devolatilisation process. The simplest are empirical and employ global kinetics, where the Arrhenius expression is used to correlate rates of mass loss with temperature. In this study a high volatile bituminous coal was devolatilised at four different heating rates in a thermogravimetric analyser (TG) linked to a mass spectrometer (MS). As a first approach, the Arrhenius kinetic parameters (k and A) were calculated from the experimental results, assuming a single step process.Another approach is the distributed-activation energy model, which is more complex due to the assumption that devolatilisation occurs through several first-order reactions, which occur simultaneously. Recent advances in the understanding of coal structure have led to more fundamental approaches for modelling devolatilisation behaviour, such as network models. These are based on a physico-chemical description of coal structure.In the present study the FG-DVC (Functional Group-Depolymerisation, Vaporisation and Crosslinking) computer code was used as the network model and the FG-DVC predicted evolution of volatile compounds was compared with the experimental results.In addition, the predicted rate of mass loss from the FG-DVC model was used to obtain a third devolatilisation kinetic approach. The three methods were compared and discussed, with the experimental results as a reference.

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