Modeling of Pulverised Wood Flames

Abstract: Popular Abstract in Swedish The aim of the current work was at development and validation of modeling tools for simulation of pulverised wood flames in furnaces and study how different factors influence on such flames. The numerical model involves different sub-models for the physo-chemical processes, such as, two-phase flow motion, drying, devolatilization and shrinkage of particles, the formation and oxidation of volatile, tar and char, turbulence-chemistry interaction, turbulence-radiation interaction, etc. Because of the complexity of such flames, the work here is divided into different stages. In the first stage, modeling the motion of pulverised wood particles in turbulent flows is carried out. This includes addressing the following open questions: how do highly anisotropic and typically non-spherical particles behave in turbulent flows; how are the particle projected area and orientations changed during particle tracing; how is the interaction between the particles and the flow field (turbulence). In the second stage, the conversion of pulverised wood particles in combustion conditions is extensively studied. Modelling of different processes during the conversion is investigated. More attention is paid upon the devolatilization process since it is the most dominating one during the thermal degradation processes. The devolatilization kinetics for pulverised wood particles in combustion conditions is studied, including the evaluation and determination of devolatilization mechanism and devolatilization rate constants. In the third stage, the effect of moisture and volatile releases during thermal degradation on the motion of pulverised wood particles is studied. Since the fiber structure in the wood particles is anisotropic; the release of gasses is not isotropic along the particle surface. The particle is then subjected to a force that we may refer to ?rocket? force, since the physical process resembles that of rocket propulsion. This new phenomenon is modeled and validated during this stage. In addition, the effect of rocket force on the particle velocity, particle distribution and the combustion processes is investigated. In the fourth stage, the sub-models studied in the previous stages are coupled together with supplementary sub-models, such as oxidation of volatile, tar and char, turbulence-chemistry interaction and turbulence-radiation interaction, to simulate the pulverised wood flames in a vertical furnace. Different flame sub-models together with required governing equations are all integrated to a CFD code, developed previously at LTH. The solid-gas coupling is done here using the Eulerian/Lagrangian coupling approach. From this work the basic structures of pulverised wood flames are identified. It was shown that pulverised wood flames have strong similarity and difference as compared to gaseous flames. Similar to the gaseous premixed flames, the pulverised wood flames consist of distinct zones ? preheat zone, drying-devolatilization zone, oxidation zone and post-flame zone. The mechanism of heat transfer to the preheat zone of pulverised wood flames is however different from the gas flames. The structure of pulverised wood flames is not only a property of the fuel/air mixture itself, but also it depends on the combustor configuration. It was shown that pulverised wood flames are rather sensitive to the physical parameters such as particle size and shape, heat capacity of particles, fuel feeding rate and shrinkage process.