Lateral fuel dispersion in a large-scale bubbling fluidized bed

Artikel i vetenskaplig tidskrift, 2012

The lateral fuel dispersion in a large-scale (1.44 m(2) in cross section) bubbling fluidized bed operated under ambient conditions has been investigated by means of particle tracking with video recording and subsequent digital image analysis. Wood chips and bark pellets were used as tracer particles. Characterization of the fuel mixing pattern was made by single tracer particle tracking while tracking of batches of tracer particles was applied to quantify the fuel mixing through lateral dispersion coefficients. The experimental technique shows good repeatability and dispersion coefficients are found to be in the order of 10(-3) m(2)/s. The lateral transport of the wood chips is observed to occur mainly while these are submerged in the dense bed. Increased fluidization velocity accentuates this pattern, i.e. the share of time spent in the dense bed increases. Only for the wood chips the fluidization velocity was found to have an influence on the lateral dispersion coefficient, which is due to that these are of larger size and lower density than the bark particles. Diffusion-type modeling of the horizontal fuel dispersion is discussed, concluding that such an approach is not suitable in cases with low lateral fuel mixing rate or with a characteristic mixing length in the same order as the lateral length scale of the bed. As alternative, this paper proposes a macroscopic modeling approach for the lateral fuel mixing, which includes physical parameters relating to the local mixing mechanisms, operational conditions and fuel particle properties. The proposed model is shown to give an adequate macroscopic description of the lateral fuel mixing.