Session chaired by Pr. Epaminondas Mastorakos
A fundamental understanding of the chemical and physical characteristics of particles is essential for accurate bed modelling in large-scale furnaces for waste and biomass combustion. In recent years, the pyrolysis of biomass particles has been studied extensively, mostly by means of one-dimensional models. Even though these models have been successful in describing the main chemical and physical phenomena, they have also addressed the need for developing a multi-dimensional single particle model that can account for the anisotropy of the solid fuel and for the gas phase movement in the interior and exterior of the particle. This paper presents a fully transient two-dimensional CFD (computational fluid dynamics) model for simulating the pyrolysis of thermally thick particles. The solid conversion process is programmed via a C++ routine and embedded into the CFD code, allowing the gas flow field and the solid conversion process to be resolved simultaneously. The complete model was validated with experimental data from the literature, and a good agreement has been obtained. The established model was used to study the anisotropy of woody particles (near-spherical and cylindrical particles, diameters from 9.5 to 40 mm) and the dynamics of particle conversion at different pyrolysis temperatures (600K-1300K). The anisotropy of thermal conductivity was investigated for radial and longitudinal samples, which have grain direction coinciding with the particle radial and axial direction, respectively. It was found that the radial sample is converted faster than the corresponding longitudinal sample, which is explained by the effect of anisotropic thermal conductivity on the heating profile and product yields. The permeability, even though it has little impact on the overall conversion process, can determine where and when overpressure occurs. As the permeability is anisotropic, the pressure peaks are observed at the outer layer of the particle during the initial heating and when reaction rates are maximum. The pyrolysis temperature can be used to primarily assess the interaction between the heat transfer and the conversion process of thermally thick particles. At pyrolysis temperatures below about 650K, the conversion time is much longer than the heating time, the thermal degradation process is purely kinetically controlled. At a temperature of 700-800K, not only the internal heat transfer and the conversion process are important, but also the convective external heat transfer should be taken into account. At higher temperatures (800-1300K), the coupling between heat transfer and thermal conversion is prominent as the conversion time and the heating time are in the same range. The simulation results demonstrate that the developed two-dimensional model is capable of capturing the internal and external flow field and of including the anisotropy of the solid fuels. The model and its results will be of interest for future research into the combustion of biomass/waste for packed and fluidized-bed applications.