The particulate emissions of solid fuel fired furnaces typically exhibit a bimodal distribution: a small peak in the range of 0.1 mm and a larger one above 10 mm. The particles with sizes above 10 mm are formed by a mechanical process like disintegration of the fuel after combustion, or erosion, while the smaller particles (aerosols) are formed by phase transition. In a turbulent flow the particles in the two size ranges have manifestly different transport properties: the smaller particles acquire a diffusivity close to the turbulent diffusivity and follow the turbulent flow, while the larger particles are hardly affected by the turbulent eddies. The aerosols are formed by homogeneous or heterogeneous condensation in the boiler section. Away from the wall, far beyond the viscous boundary layer, turbulence is the main dispersing mechanism, while close to the wall, in the viscous boundary layer Brownian motion is the dominating mechanism. Furthermore eddy diffusion-impactation and thermophoresis may contribute to aerosol deposition. In typical boiler situations however their combined effect is small compared to direct gas-phase wall condensation of the condensable species. The coarse particles are deposited due to inertial forces and it is shown that knowledge of the streamlines within the furnace suffices to predict the path of the coarse particles. For both classes of particles it is sufficient to have a general description of the main flow. This can be done by a CFD model, but a potential flow model in combination with a detailed boundary layer model also gives satisfactory results. Especially in the region near the wall the results indicate that integration of a model describing the aerosol formation with the transport model is necessary.
|Name||Series thermal biomass utilization|
|Conference||conference; International Workshop Aerosols in Biomass Combustion ; (Graz) : 2005.05.18|
|Period||1/01/05 → …|
|Other||International Workshop Aerosols in Biomass Combustion ; (Graz) : 2005.05.18|