Axial Mixing of Large Solids in Fluidised Beds – Modelling and Experiments

Licentiate thesis, 2019

Fluidisation is a technology commonly found wherever particulate solids are to be transported, mixed and/or reacted with a gas. At present, it is a widespread technology with applications ranging from the production of carbon nanotubes in the manufacturing industry to the conversion of solid fuels in the heat and power sector. As for the latter, fluidised beds are well received for their fuel flexibility (being able to efficiently convert low-grade fuels) and for their ability to control emissions with in-bed methods. In most applications, like solid fuel conversion, the heat and mass transfer between the gas and the solids (e.g. fuel particles) play an important role in the process performance. In turn, these transfer mechanisms are affected by the axial solids mixing, as solids immersed in the dense bed will experience higher heat transfer and lower mass transfer than otherwise.

This work focuses on the axial mixing of large solids in fluidised beds with the aim to advance current knowledge on in-bed mixing with an emphasis on biomass particles. As the latter typically have a high content of moisture, volatile and ash and are larger and lighter than conventional fuels like e.g. coal or lignite, they are even more prone to segregate axially in the bed in a flotsam fashion. Yet, the effect of fuel density and size as well as the effect of fluidisation conditions on the axial mixing of fuel has not been fully understood.

To enhance the understanding of solids mixing, this work combines a one-dimensional semi-empirical model with experiments applying magnetic particle tracking (MPT) in a fluid-dynamically down-scaled fluidised bed. The model is used to identify governing mechanisms and the respective key parameters to be studied with dedicated experiments which, in their turn, contribute to the continuous upgrading of the model.

The key parameters in the axial mixing of larger solids in a fluidised bed are found to be: i) the apparent viscosity of the emulsion, for which MPT measurements confirmed its Newtonian character, and ii) the bubble flow, which experiments revealed to have a higher upwards velocity and fuel-to-bubble velocity ratio than shown in previous literature not accounting for hot conditions.