Modelling of the pyrolysis of large wood particles

Abstract: Wood is an interesting alternative to fossil fuels. It is CO2-neutral and widely available. However it is a difficult fuel to handle which features a low energy content. Thus technologies for wood thermal conversion need to be improved.This work concerns the development of a comprehensive two-dimensional mathematical model describing the pyrolysis of large wood particles and its implementation in a Fortran program. The model has been continuously tested and improved by experimental results obtained in a reactor for single particle pyrolysis (SPAR) at the Division of Physical Chemistry at Göteborg University.The first part of the thesis (Paper I) presents a kinetic study of the pyrolysis of large wood particles, based on experiments carried out in the SPAR. Three pyrolysis kinetic schemes were selected for later inclusion in a model featuring heat and mass transfer.Paper II concerns the addition of a sub-model for heat and mass transfer to the three kinetic schemes. The resulting model for large wood particles has been tested against experiments in the SPAR. A scheme based on two competing reactions developed from experiments at low temperature pyrolysis in the SPAR was found to perform well but its empirical nature limits its validity to the experimental conditions of the SPAR. A scheme from the literature based on TGA experiments appeared promising, especially when planning to enhance it with secondary reactions.Paper III deals with the development of shrinkage models for 2D cylindrical particles. The predicted mass loss, size variation and surface temperature were tested against experiments carried out in the SPAR. The shrinkage does not a?ect the pyrolysis rate or the surface temperature in the conditions prevailing in the SPAR.Paper IV investigates the influence of different shrinkage models and the geometry on the heating rate of a shrinking particle. Shrinkage influences the heating rate positively by increasing the conductiveheat flow and negatively by decreasing the surface area of the particle. Therefore the net effect of shrinkage on the heating rate depends on the particle geometry and the location of shrinkage.Paper V studies three di?erent models for wood drying under pyrolysis conditions.The predicted surface temperature and global drying rate were compared with experimental results from pyrolysis experiments of wet particles in the SPAR. A model based on a first order kinetic evaporation rate was found to be the most interesting because of the quality of the prediction of the drying rate and the ease of implementation.