A flexible and stable numerical method for simulating the thermal decomposition of wood particles

The objective of this paper is to present a flexible and stable simulation method to predict the thermal conversion of wood particles. A combination of several subprocesses such as heating-up, drying, pyrolysis, gasification and combustion of fuel particles of different properties and sizes represents the global process of thermal conversion. This approach 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. Thus, the model is applicable to simulate sufficiently accurate the thermal decomposition of each particle in a packed bed, of which the entire conversion is regarded as the sum of all particle processes. Effects such as fragmentation, swelling, homogeneous reactions e.g. ignition outside a particle are excluded as a tradeoff between complexity and calculation time. However, a description of the particle processes by one-dimensional and transient differential conservation equations for mass and energy is feasible to represent the above mentioned subprocesses. The particles are coupled to the gas phase by heat and mass transfer taking into account the Stefan correction due to the gas outflow during conversion. A general formulation of the conservation equations allows the geometry of a fuel particle to be treated as a plate, cylinder or sphere. In order to achieve a high degree of flexibility, the method distinguishes between data, such as kinetics or material properties and the conversion process, for which relevant data are stored in a data base for easy access and extension. The resulting modules of this subdivision are encapsulated into separate software units cast in a hierarchy of well-defined classes in Tools of Object-oriented Software for Continuum-Mechanics Applications (TOSCA) by object-oriented techniques.