David Pallarès

Dynamical Modeling of the Gas Phase in Fluidized Bed Combustion-Accounting for Fluctuations

A model for gas phase mixing in fluidized bed boiler furnaces is presented. The model takes its basis in a description of the dynamics of the dense bottom bed which strongly govern the gas mixing up through the furnace. Thus, a time-resolved approach is used to link the modeling to the physics of the underlying processes determining the gas mixing. As output, the model gives the fluctuating flux of gas species, in contrast to the classical modeling approach which is limited to time-averaged gas fluxes. Such a dynamical approach allows assumption of the volatile combustion system as transport-controlled which avoids complete consumption of either oxygen or combustible gases in each modeled cell. Thus, the time-resolved analysis employed enables application of a realistic criterion for the mixing such as that reactants can coincide in both space and time in order to react. While fitting of kinetics is strongly dependent on the system and operational conditions, the present model integrates key system variables such as the bottom bed height and the characteristic pressure-drop constant over the primary air distributor, allowing application of transport-controlled (i.e. infinitely fast kinetics) volatile combustion.

Modeling of fluidized bed combustion processes

This chapter provides an overview of the aims, principles, and limitations of the different types of modeling used for fluidized bed combustion (FBC) systems with a focus on macroscopic modeling, which is currently considered the most suitable modeling approach for FBC units of industrial scale. Sub-models for the different regions characterizing FBC units are described, with emphasis on the most critical in-furnace phenomena. Guidelines and examples are given of how different sub-models can be linked into a comprehensive process model for FBC units.

3-Dimensional Particle Tracking in a Fluid Dynamically Downscaled Fluidized Bed Using Magnetoresistive Sensors

This paper presents a measurement technique for continuous tracking of particles in 3-dimensional bubbling fluidized beds operated according to scaling laws. By applying Glicksman’s full set of scaling laws to both bulk solids and tracer particle, the bed is assumed to be fluid dynamically similar to a combustor operated at 900 °C with the tracer particle corresponding to a fuel particle with properties similar to anthracite coal. Two different gas distributors with varying pressure drop are used to investigate the influence of bed design on fuel mixing. Flow structures formed around rising gas bubbles, the so-called bubble paths, are identified, and the tracer particle traverses the entire bed for a gas distributor yielding a high pressure drop. For a gas distributor yielding a low pressure drop, flow structures are less pronounced, and the tracer particle is not circulating the entire bed.