SOLIDS MIXING IN BUBBLING FLUIDIZED BEDS
Doctoral thesis, 2016
For solid fuels, thermochemical conversion processes such as combustion and gasification are especially suitable to be carried out in fluidized bed units because of the relatively high mixing rates, fuel flexibility and the possibility to use active bed material to enhance process efficiency. Mixing of fuel is important to ensure complete conversion and reduced emissions of unburnt material. Furthermore, mixing of the bulk material has its importance in that it governs the variations of the temperature field across the bed. Thus, in order to optimize the operation of existing units and the design of new units, there is a need to understand and quantify solids mixing.
Despite solids mixing in fluidized beds having been investigated for several decades, there is still lack of knowledge in the area. A common approach is to quantify mixing using a dispersion coefficient. However, values for the dispersion coefficients published in literature are scattered over several orders of magnitude and have often been derived from small units operated at ambient conditions, i.e. are not representative for industrial scale fluidized beds.
In the present work methods are presented to evaluate lateral mixing for both the bulk solids and fuel particles and vertical mixing of fuel. The experimental work is conducted in fluid-dynamically downscaled units and a large-scale unit, i.e. the results are relevant for commercial conditions. A novel 3-dimensional particle tracking system based on anisotropic magneto resistance sensors has been implemented which provides detailed information about flow structures of the solids found within the bed. Furthermore a novel camera probe has been used to enable tracking of several fuel particles in an industrial scale bed operated at elevated temperature.
The lateral dispersion coefficients for bulk solids obtained in the present work are two orders of magnitude larger than what has been published in literature previously, which is explained by the application of fluid dynamic scaling to study large-scale units. Since wall effects are of less importance in large scale equipment the data provided in the present work is of more relevance to industrial units than previous data found in literature. Values of the lateral dispersion coefficients obtained for fuel particles in fluid-dynamically downscaled units in this work are of the same order of magnitude as data obtained in industrial scaled equipment operating at elevated temperatures. This shows that application of fluid-dynamic downscaling can be applied to predict fuel mixing in large scale fluidized beds.
Lateral solids dispersion is generally found to increase with fluidization velocity and bed height, with an enhanced effect for beds with a gas distributor providing a high pressure drop. The presence of a continuous flow of bulk solids across the fluidized bed is found to create a convective contribution to solids mixing which fuel particles are found to follow to different extents. Mathematical models are applied to account for such convective flow of bulk and fuel particles. Furthermore this continuous flow of bulk solids is found to decrease the vertical segregation of fuel particles in an industrial scale fluidized bed.