Lateral mixing of solids in bubbling fluidized beds – experimental quantification
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. Fuels such as biomass and waste, which are commonly used in fluidized beds, are characterized by volatile contents much higher than what is found in fossil fuels such as coal. In combustion, with volatile matter being released relatively fast, it is critical to predict and control fuel mixing in order to avoid maldistribution and thus optimize the process performance; while in indirect gasification fuel mixing governs the fuel residence time and thus fuel conversion in the gasifier. Furthermore, mixing of the bulk material has its importance in that it governs the variations of the temperature field across the bed. Thus there is a need to be able to quantify solids mixing to optimize the operation of existing units and make prediction about the mixing when designing new units.
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 a number of methods are presented to evaluate lateral dispersion coefficients for both the bulk solids and fuel particles. The experimental work is conducted in fluid-dynamically downscaled fluidized beds and the results are thus relevant for large scale fluidized beds operated at temperature levels typical for commercial operation.
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 (in which wall effects have less influence). Values of the lateral dispersion coefficients obtained for fuel particles are of the same order of magnitude as data obtained in industrial scaled equipment operating at elevated temperatures. This suggests 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 fuel particles are found to follow to different extents. By introducing internals into the fluidized bed it is possible to decrease the impact of such convective cross flow on fuel mixing and thus reduce the fuel mixing rate, which could be used to improve fuel conversion in gasification applications.