Modelling wood combustion under fixed bed conditions ☆

Abstract

A computer model describing the conversion of wood under packed-bed conditions is presented. The packed bed is considered to be an arrangement of a finite number of particles, typically sized between 5 and 25 mm, with a void space left between them. Each particle is undergoing a thermal conversion process, which is described by a one-dimensional and transient model. Within the single-particle model, heating, drying, pyrolysis, gasification and combustion are considered, whereby each particle exchanges energy due to conduction and radiation with its neighbours. Because of the one-dimensional discretization of the particles, heat transfer and mass transfer is taken into account explicitly. Therefore, no macrokinetic data are needed within the model. For ease of implementation and access, kinetic data and property data are stored in a database. The global conversion of the packed bed is represented by the contributions of single particles, where each particle is coupled to the surrounding gas phase by heat and mass transfer. For gas phase flow through the porous bed, the conservation equations for mass, momentum and energy are solved on a Cartesian mesh by a Finite Volume method. Experiments have been performed to validate the single particle model for the conversion of beech wood during pyrolysis and char combustion. Agreement between experimental and predictions obtained by the model is very satisfactory. However, for wet wood, changes in structure seem to enhance the heat transfer to the solid which is not yet covered in the model.