Drying and pyrolysis of wood particles: experiments and simulation

The objective of this study is to develop a flexible and stable numerical method to predict the thermal decomposition of large wood particles due to drying and pyrolysis. At a later stage, this model is applied to each particle of a packed bed and thus, forms the entire packed bed process as a sum of individual particle processes. Therefore, this approach can deal with particles of different sizes, shapes and properties. A general formulation of the conservation equations allows the geometry of a fuel particle to be treated as a plate, cylinder or sphere. The various processes such as heat-up, drying and pyrolysis are described by a set of one-dimensional and transient conservation equations for mass and energy. This allows for simultaneous processes e.g. reactions in time and covers the entire range between transport-limited (shrinking core) and kinetically limited (reacting core) reaction regimes. The particles interact with a gas phase by heat and mass transfer taking into account the Stefan correction due to the gas outflow during conversion. Experiments carried out span a temperature range between T=300 and 900 °C for particle sizes varying between 8 and 17 mm. A comparison between measurements and predictions of drying models yielded satisfactory agreement only for the constant evaporation temperature model and thus, indicating, that the drying process is transport limited by heat transfer for large wood particles. Likewise, predicted results of pyrolysis for the above-mentioned range of temperatures and sizes agreed satisfactorily with measurements.